A method and apparatus for efficiently transporting DS-X traffic in packet form over an ATM or other packet network. Specifically, virtual connection or slot provisioning and/or cell concentration techniques are used to compact the amount of DS-X traffic broadcast between communications system devices such as the access mux and the communications switch and spare bandwidth. In provisioning, a configured DS-X loading of an end node supporting DS-X traffic is ascertained. In turn, a minimum number of virtual connections for bearing packetized DS-X traffic are established, either at communications system configuration or as needed. In concentration processing, a dynamic association between the virtual connections and the DS-X traffic is established, typically on a per frame basis. Only those digital channels entering the node within a given time frame which actually bear DS-X traffic will be assigned a slot within packet. Concentration may be augmented by provisioning to further reduce bandwidth requirements. In other disclosed aspects, permanent virtual channels or PNNI soft permanent virtual connections may be used for the virtual connections where ATM is chosen as the transport protocol.

1. Field of the Invention
This invention relates to streaming data transmission (e.g. voice) over packet networks, and is particularly concerned with bandwidth-efficient and reliable DS-X transmission within distributed communication systems utilizing a packet network.
2. Description of the Related Art
Transferring streaming media, particularly voice and video, over a packet network presents challenges not faced with traditional time-switched systems. Traditional packet transport technologies and network/application protocols, such as TCP/IP FTP, and Ethernet are optimized for reliable data transfer, rather than the timeliness of the transfer. While such characteristics are great for transferring a data file or an application that will not work even if a small piece of data is corrupt or missing, connection and delivery confirmation delays make the transmission of time-sensitive streaming media less desirable.
Switched-packet Asynchronous Transfer Mode (ATM) does represent a convenient standard for transmitting voice, data, and video signals at very high speeds (25 Mbits/sec and higher) over packet networks. The increasing deployment of ATM-based networks, particularly on customer premises, has created a demand to provide high quality transport, switching, and call processing of voice communications over a packet-oriented network with minimal delay. The delay must not exceed that currently permitted for traditional centralized Private Branch Exchanges (PBXs), and the voice quality must be at least as good as such systems to warrant upgrading distributed ATM voice services.
A known distributed PBX architecture leveraging an ATM-based network is shown in FIG. 1. This PBX architecture includes several end node devices (e.g. access multiplexors 100, communications switch 120) distributed at the edges of a packet network transport. Access multiplexors 100 (access mux) are coupled to the ATM network (or single switch) 180 via links 150 carrying ATM cell traffic at STS-1/OC-1 speeds or higher. These muxes 100 convert locally-originated, digital channels representing e.g. PCM-encoded voice/data traffic at DS-X speeds (i.e. the US plesiochronous digital hierarchy DS-0, DS-1, DS-2, DS-3, DS-4, etc. or the European E-1, E-2, . . . , hereinafter referred to as xe2x80x9cDS-X trafficxe2x80x9d) into packets, receive and reconstruct remotely-originated voice/data packets back into native DS-X form, and route each to their intended destinations. High speed link 170 (e.g. SONET STS-3c, OC-12) couples the communications switch 120 to the packet network 180. Logical or virtual connections, known in ATM parlance as virtual channels, are established between the access muxes 100 and the communications switch 120 within the packet network 180 to permit transfer, call processing and switching of the packetized DS-X traffic.
As with the case with traditional centralized PBX systems, synchronous Access Devices (ADs) 110, 115 and 118 are coupled to these access muxes 100 to collect voice/data information (such as a 64 Kbps DS-0 voice channel) originating from telephony devices such as digital/analog extensions 130/132 or videophone 134, as well as T1/E1 trunks 136. Once collected using known time-division multiplexing techniques, the ADs relay synchronous DS-X traffic to the access mux 100 using signal lines 120. Of course, ADs 110, 115, 118 are also used to distribute outgoing DS-X traffic extracted by the access mux 100 over the signal lines 120 to the appropriate destination telephony device or trunk.
In order to leverage the ATM network for voice communications, each access mux 100 converts DS-X traffic into ATM cells. According to the ATM standard, each cell is a fixed fifty-three bytes or octets long including a forty-eight byte payload and a five byte header. For outgoing calls, each access mux 100 packs the DS-X traffic into the 48-byte payload using conventional methods for multiplexing voice DS0s into ATM virtual channels, such as T1/E1 Circuit Emulation Service (CES) based on ATM Adaptation Layer 1(AAL 1) or other ATM adaptation layers for carrying voice. For incoming calls, each access mux determines the destination of the DS-X traffic encapsulated within each received cell based in part on the virtual channel or path information contained in the cell as well as its location within the cell payload. The access mux 100 then converts the packets to the carrier format for the signal line(s) 120 corresponding to the intended destination device.
However, without further refinement, this architecture is prone to cell delay (especially where the customer""s packet network is complex or geographically diverse), which can seriously degrade the quality of service for voice and streaming data it is slated to carry. To minimize cell-fill delay here, which can form a major component of the overall architecture delay, it has been proposed that each DS-0 channel serviceable by the access mux 100 contributes one byte to the ATM cell(s) and the so-filled cells are transported within a 125 xcexcsec frame, corresponding to the well-established 8 KHz sampling rate for voice. To reduce communication switching setup, complexity and delay, as well as simplify cell composition/decomposition logic within each access mux 100, each available DS-0 is statically assigned to a unique octet slot within one of the cells filled and transported within the 125 xcexcsec frame. To further simplify network complexity, along with minimizing dynamic connection setup and tear down delays associated with ATM switched virtual channels (SVCs), permanent virtual channels (PVCs) are established within the packet network 180 at system configuration through management action to transport the DS-0 packed cells between the access mux 100 and the switch 120.
In applying these techniques to the architecture of FIG. 1, a number of Constant Bit Rate (CBR) PVCs capable of bearing the simultaneous DS-0 capacity of each access mux 100 is established within the packet network 180. A PVC will be used to bear one ATM cell per 125 xcexcsec frame, and cell payload will contain a byte of data from each of 48 DS-0 channels statically mapped into its 48 octet slots. Since the placement of the DS-0 byte within a particular PVC and octet slot identifies the DS-0 connection (originating/terminating device), all cells must be broadcast within a given 125 xcexcsec frame, whether or not traffic actually exists on the connection. Typically, cell octet slots corresponding to DS-0 channels not bearing activity within a given frame are stuffed with pre-established xe2x80x9cdon""t carexe2x80x9d data.
In the above-stated combination, these design choices seriously impact bandwidth efficiency. For example, if the access muxes 100 could simultaneously service sixteen (16) ADs, each defining thirty-two (32) DS-0 channels (which is typical for such units), a minimum of 512 DS-0 channel bytes would accordingly need to be transferred every 125 xcexcsec frame, whether or not traffic actually exists on those channels. Eleven PVCs would be required to fully transport these bytes with padding left over, and accordingly eleven ATM cells would need to be transmitted every frame. FIG. 3 graphically indicates the DS0 mapping required for the 16 32 DS-0 channel ADs plotted against the payload portions of the cells 330-380 to be filled, with xe2x80x9cXxe2x80x9d representing 16 empty octet slots at the end of the last cell 380 which are padded out in order to complete the payload. Thus, in this example, regardless of the actual DS-0 loading (meaning the number of DS-0 channels actually allocated or reserved for use) or active DS-0 traffic experienced, approximately 37.3 Mbits/sec of bandwidth would be required, exclusive of any control signaling. In view of the fact that this represents almost a quarter of the available bandwidth of an STS-3c link, and that the access mux 100 almost never experiences full loading (and is actually quite sparse in the typical PBX application), this configuration appears to be an undesirable choice where the packet network 180 is not devoted exclusively for PBX transport.
Such inefficiencies are carried over and magnified at the communications switch 120. According to the architecture of FIG. 1, communications switch 120 will be responsible for handling switching and call management responsibilities for the packetized DS-X traffic generated by and destined for each access mux 100. Assuming that each access mux 100 contributes 37.3 Mbits/sec of bandwidth, the link 170 must be able to bear A*37.3 Mbits/sec, exclusive of control and management signals, where A represents the number of access muxes 100 serviced by the communication switch 120. Thus, link 170 must typically support a magnitude higher simultaneous bandwidth than links 150 (e.g. OC-12 vs. STS-3c), which adds to the cost and complexity to communications switch 120.
Moreover, it should be realized that the enormous number of traffic bearing cells entering the communications switch 120 packed according to the above-described techniques and criteria require commensurately enormous and expensive computing resources within the switch 120 if switching and call management responsibilities are to be completed within the required 125 xcexcsec frame.
Yet another problem arises from the use of PVCS under the current ATM standards. PVCs used to bear the packetized DS-X traffic must be established and maintained by the PBX-specific components (either the access muxes 100 or the communications switch 120), despite the limited ongoing control and management they can exert over the packet network 180. This, coupled with the aforementioned static mapping criteria, makes it difficult for the PBX of FIG. 1 to recover from a lost PVC without dropping all connections and reinitializing.
Therefore, it would be desirable to develop an improved distributed PBX architecture which more efficiently utilizes available packet network bandwidth without materially sacrificing cell delay performance. It would also be desirable to develop a distributed PBX architecture which is more tolerant of connection failures within the packet network.
In accordance with these and related desires, a novel and nonobvious method for transporting DS-X traffic over a packet network is proposed. Specifically, according to the present invention, virtual connection or slot provisioning and/or cell concentration techniques are used to compact the amount of DS-X traffic broadcast between communications system devices such as the end node and the communications switch to spare bandwidth. In the case of provisioning, a configured DS-X loading of an end node supporting DS-X traffic is ascertained. This configured DS-X loading sets forth the actual loading in terms of digital channels being simultaneously supported and/or number of number of access devices coupled to and being serviced by the end node. In turn, preferably a minimum number of virtual connections for bearing packetized DS-X traffic are established, either at communications system configuration or as needed.
Through provisioning, only the virtual connections needed to support all the digital channels in actual service or the DS-X capacity of all actually-serviced access devices need be established, and packetized DS-X traffic need only flow on those so-established virtual connections. By contrast, conventional virtual connection establishment methods teach that the full capacity of the end node define how many virtual connections to establish and maintain. Thus, through provisioning according to the present invention, bandwidth is spared where the configured capacity of the end node is less than its full capacity such that less total virtual connections are needed to bear the DS-X traffic, even where static slot mapping is utilized.
Compaction of the DS-X traffic bearing cells themselves may be accomplished through concentration processing according to the present invention. In such concentration processing, a dynamic association between the virtual connections and the DS-X traffic is established, typically on a per frame basis. According to this invention, only those digital channels entering the node within a given time frame which actually bear DS-X traffic will be assigned a slot within packet. A DS-X trafficxe2x80x94packet correspondence, such as a slot map, may be compiled and updated as necessary based on the active DS-0 channels within the given frame, and appropriate slot assignment data, such as the updated slot map or any changed assignment data may be exchanged between communication system devices in order to properly compose and decompose the DS-traffic packed in the packets and preserve intended channel connections therebetween.
According to the present invention, concentration processing may be augmented by provision processing to further reduce bandwidth requirements for transporting the packetized DS-X traffic.
In other aspects of the present invention, permanent virtual channels or soft permanent virtual connections established through Private Network-Network Interface management and signalling may be used for the virtual connections where ATM is chosen as the transport protocol. Soft permanent virtual connections are seen as being especially advantageous since their ongoing management and re-routing is performed by the ATM packet network being leveraged by the communications system, rather than the communications system or its components itself.
Also, the present invention contemplates an end node apparatus incorporating these provisioning, concentration processing, and/or virtual connection establishment features to reduce the bandwidth requirements of packetized DS-X traffic flowing into or out of the node.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of the specific preferred embodiments of the invention in conjunction with the accompanying figures.

Method and apparatus for transporting DS-X signals through a packet network


To provide an antireflection film which is easily and inexpensively producible and which has an antireflection property, a scratch resistance and a stain resistance in a sufficient level, and a polarizing plate and a liquid crystal display device utilizing the antireflection film of such excellent ability, the antireflection film includes a transparent support and a low-refractive index layer having a lower refractive index than the transparent support, wherein the low-refractive index layer is an outermost layer, and the low-refractive index layer contains a hollow silica particle and a compound that reduces a surface free energy of the outermost layer.

In general, an antireflection film is disposed on the outermost surface of image display devices such as cathode-ray tube display devices (CRT), plasma display panels (PDP), electroluminescent display devices (ELD) and liquid crystal display devices (LCD). This is for preventing contrast reduction or image reflection owing to external light reflection on the displays, generally having a function of reflectivity reduction owing to light scattering on surface protrusions or to light interference of multi-layered thin films.
The antireflection film is generally produced by forming, on a transparent support, a film of a low-refractive index layer having a suitable thickness and having a lower refractive index than the transparent support. For realizing its low refractive index, the material for the low-refractive index layer is desired to have a refractive index as low as possible. Since the antireflection film is disposed on the outermost surface of displays, it is desired to have high scratch resistance. In order to realize high scratch resistance of thin films having a thickness of 100 nm or so, the films must have high mechanical strength and must be adhesive to underlying layers.
The antireflection layer of an antireflection film formed of multi-layered thin films generally has a layer constitution consisting essentially of a high-refractive index layer and a low-refractive index layer for enhancing the antireflectivity of the film. For attaining efficient antireflection, it is said that the refractive index difference between the high-refractive index layer and the low-refractive index layer must fall within a specific range (see, for example, JP-A-59-50401). For this, various methods have been investigated. MgF2 and silica are used as low-refractive materials for the low-refractive index layer (see, for example, JP-A-2-245702); a fluorine-containing compound is used (see, for example, JP-A-2003-121606); or 2 or more inorganic particles are piled up to form micro-voids (see, for example, JP-A-11-6902). In fact, however, all these methods are problematic in that the materials are difficult to obtain and are unstable and their films are not strong and are not resistant to staining, and sufficiently satisfactory low-refractive index materials could not as yet obtained.
For increasing the mechanical strength of films in some degree so as to make them have increased scratch resistance, a method of using fluorine sol-gel films is proposed in JP-A-2002-265866 and JP-A-2002-317152. However, this has significant limitations in that (1) curing the films requires long-time heating and the production load is therefore great, and (2) the films are not resistant to saponification solution (alkali-processing solution), and saponification of the surface of transparent plastic film substrates after the formation of antireflection film thereon could not be carried out. In addition, the films are not satisfactorily resistant to staining. For satisfying all of low-refractive index and good scratch resistance and good stain resistance, low-surface free energy compounds that have a low refractive index and have a high mechanical strength and are hardly stained on their surface are needed. However, most of such compounds are often in a trade-off relation, or that is, when one of the requirements is improved, then the other is worsened. Accordingly, it is difficult to satisfy all the requirements of refractive index reduction and improvement of scratch resistance and stain resistance.
For the intended refractive index reduction, JP-A 7-287102 discloses a technique of increasing the refractive index of a hard-coat layer to thereby reduce the refractive index of the antireflection film. However, the high-refractive index hard-coat layer causes film color mottles since the refractive index difference between the layer and the support is large, and the wavelength dependency of the refractive index of the film greatly increases.
On the other hand, in JP-A-11-189621, JP-A-11-228631 and JP-A-2000-313709, there is proposed a method of improving the scratch resistance of films by introducing a polysiloxane structure into a fluorine-containing polymer so as to lower the friction coefficient of the film surface. The method may be effective in some degree for the improvement of scratch resistance of the films, but is still unsatisfactory in that films essentially not having high strength and high interfacial adhesiveness could not be improved to have sufficient scratch resistance by the method.
JP-2003-292831 discloses a low-refractive index coating agent and an antireflection film. In this, the agent comprises at least an active energy ray-curable resin that contains a compound having a (meth)acryloyloxy group in the molecule, and hollow particles having a mean particle size of from 0.5 to 200 nm. However, this is also problematic in that, when such a thin-film system is cured in an atmosphere in which the oxygen concentration is near to that in air (oxygen concentration: about 20% by volume), the reactive group in the film could not well react owing to curing failure and therefore the scratch resistance of the cured film is not good. In addition, since the low-refractive index layer in JP-2003-292831 is not processed to be resistant to staining, this is still problematic in that stains such as finger marks or water marks may readily adhere to the films and the stains adhered to them are difficult to wipe away. These days in particular, applications such as TVs and monitors are required to have good scratch resistance and good stain resistance.
In JP-A-7-133105 and JP-A-2001-233611, hollow silica particles are proposed as a low-refractive index material. These are excellent as a low-refractive index material, but it has been found that, when they are used in a low-refractive index layer of antireflection films, their adhesiveness is poor and they detract from the commercial value of the outermost film of displays, and therefore, their amount to be used must be limited.
JP-A-7-333404 describes an antiglare antireflection film having good gas-barrier properties, antiglare properties and antireflection properties. However, since this requires a silicone oxide film to be formed in a mode of CVD, its producibility is low when compared with wet-coated films.
The recent tendency in the art is toward large panels in image display devices (e.g., LCD, PDP, CRT), especially toward liquid crystal display devices with an antiglare antireflection film disposed therein. For protecting such expensive large-panel image display devices, it is desired to further improve the protective films for antiglare films and antireflection films for them. Concretely, the films are required to have better visibility (with neither glare look nor external light reflection in display images, with high image sharpness and transparency), the display panels are not soiled by finger marks or by dust adhesion thereto, and they are highly resistant to scratching. In liquid crystal display devices (LCD), a polarizing plate is an indispensable optical material. The polarizing plate generally comprises a polarizing sheet protected by two protective films. For reducing the number of the constitutive members of liquid crystal display devices and for increasing the producibility thereof with reducing the production costs thereof, it is desired to make the protective films for the polarizing plate have an antireflection function so as to make the resulting polarizing plate enjoy better weather resistance, physical protection and antireflection. This is especially for realizing the increase in producibility, the cost reduction and the thickness reduction of the devices.
As a material of the polarizing sheet, polyvinyl alcohol (hereinafter referred to as PVA) is principally used. Briefly, a PVA film is monoaxially stretched, then colored with iodine or a dichroic dye, or it is first dyed and then stretched, and this is crosslinked with a curable compound to form a polarizing sheet. In general, the polarizing sheet is stretched in the running direction (machine direction) of a long film (stretching along machine direction), and therefore, the absorption axis of the polarizing sheet is nearly parallel to the machine direction.
The protective film is an optically-transparent film having a small birefringence, for which cellulose triacetate is mainly used. Heretofore, the protective film is stuck to the polarizing sheet in such a manner that the slow axis of the protective film could be perpendicular to the transmission axis of the polarizing sheet (or that is, in such a manner that the slow axis of the protective film could be parallel to the absorption axis of the polarizing sheet).
antireflection treatment is attained by the use of an antireflection film formed as a multi-layered product of multiple thin films of different materials having a different refractive index, and the antireflection film is so planned that it could reduce the reflection in a range of visible light. However, in the antireflection film, the thickness of the constitutive layers of the multi-layered product is constant in each layer, and therefore, in principle, the antireflection film could not completely attain its function of antireflection in the entire region of visible light.
Accordingly, in general, the antireflection film is so planned that its antireflection to light of high visibility at around 550 nm is emphasized and that its antireflection could cover a wavelength range as broad as possible. For the reasons in planning the film, the antireflection effect of the film is at present not sufficient in the other ranges than the specific wavelength range, and the reflectivity of the film in a part of a short wavelength region of visible light and in a part of a long wavelength region thereof is larger than that in the other wavelength region of the visible light region. As a result, this is problematic in that the reflected light exhibits a specific color hue and it detracts from the display quality of devices.
As opposed to it, a different investigation has been made of integrating a polarizing plate and an antireflection film to construct an integrated and stuck product so as increase the quality of display images. Concretely disclosed is an antireflection film-combined polarizing plate having a reflectivity of at most 3.5% in a range of from 380 to 700 nm (see, for example, JP-A-2003-270441).
In reflection-type or semi-transmission reflection-type liquid crystal display devices, the backlight used generally has a bright line peak at three wavelengths of 440 nm, 550 nm and 610 nm. Therefore, it is an important point to make the transmittance at these three wavelengths the same for improving the color reproducibility of the devices. An antireflection film-combined polarizing plate is proposed, of which the parallel transmittance and the cross transmittance at wavelength 440 nm, 550 nm and 610 nm are specifically defined (JP-A-2002-22952 and JP-A-2003-344656).
On the other hand, for improving display quality of devices, various optical layers in addition to polarizing plate are provided on the outermost surface of liquid crystal panels. For example, retardation plates (including λ plates such as ½ wavelength plate and ¼ wavelength plate), optical compensation films and brightness-improving films are employed. In particular, elliptically-polarizing plates or circularly-polarizing plates constructed by superposing a retardation plate on a polarizing plate that comprises a polarizing sheet and a protective layer; polarizing plates constructed by superposing an optical compensation film on a polarizing plate that comprises a polarizing sheet and a protective layer; and polarizing plates constructed by superposing a brightness-improving film on a polarizing plate that comprises a polarizing sheet and a protective layer are put into practical use.
The polarizing plate constructed by superposing a brightness-improving film on a polarizing plate that comprises a polarizing sheet and a protective layer is generally provided on the back side (lower side) of a liquid crystal cell, and it acts to increase the brightness of the image display panel of LCD (see, for example, "2003's Views and Strategies in High-Function Film Market", p. 91 (2003), by Yano Keizai Kenkyu-jo).
On the other hand, an antiglare and/or reflection reduction-processed film is proposed for the visible-side protective film of a polarizing plate that is to be disposed on the visible side (upper side) of a liquid crystal cell (see, for example, JP-A-2003-149634).

Antireflection film, polarizing plate, method for producing them, liquid crystal display element, liquid crystal display device, and image display device


Disclosed are epoxy resins prepared by reacting p-hydroxystyrene and epichlorohydrin in the presence of one or more alkali agents. Also disclosed are epoxy resins prepared from reacting a p-hydroxystyrene polymer and one or more di-hydric phenol agents with epichlorohydrin in the presence of alkali. These epoxide resins have a low melt viscosity and when cured exhibit glass transition temperatures above 300xc2x0 C.

The present invention relates to a new class of epoxy resins prepared from one or more p-hydroxystyrene polymers.
Epoxy resins are characterized by the presence of a three-membered ring known as the epoxy, epoxide, oxirane, or ethoxyline group. Commercial epoxy resins contain aliphatic, cycloaliphatic, or aromatic backbones. The capability of the epoxy ring to react with a variety of substrates imparts versatility to the resins. Treatment with curing agents gives insoluble and intractable thermoset polymers. In order to facilitate processing and modify cured resin properties, other constituents may be included in the compositions such as for example, fillers, solvents, diluents, plasticizers, accelerators, curatives, and tougheners.
One type of epoxy resin is made from epichlorohydrin and bisphenol A. Aliphatic polyols such as glycerol may be used instead of the aromatic bisphenol A.
Another type of epoxy resin is made from polyolefins oxidized with peracetic acid. These have more epoxide groups, within the molecule as well as in terminal positions, and can be cured with anhydrides, but require high temperatures. Many modifications of both types are made commercially. Halogenated bisphenols can be used to add flame-retardant properties.
Yet another type of epoxy resin is produced from novolak compounds. Such epoxy novolak resins are typically made by the reaction of epichlorohydrin with a novolak resin, i.e a phenol-formaldehyde type resin. These have a repeating epoxide structure which offers better resistance to high temperatures than the epichlorohydrin-bisphenol A type, and are especially useful as adhesives.
Upon curing, most epoxy resins form a tight cross-linked polymer network and are characterized by toughness, good adhesion, corrosive-chemical resistance, and good dielectric properties.
The manufacture of epoxy resins and processes for their production by the reaction of di-, tri-, and tetra-phenols and epichlorohydrin in the presence of a condensing agent such as caustic soda are well known.
These epoxy resins vary in physical state from liquids to solids. They are cured into a thermoset state by cross linking when reacted with tertiary, secondary or primary amines, anhydrides, lewis acids, lewis bases, amides, imidizoles, ureas, melamines, cyanate esters and other commonly used curing agents and catalysts.
The production of epoxy resins utilizing p-hydroxystyrene is known in the art. Due to the large quantity of reactive sites, this polymer, when cured, produces a relatively high crosslink density. Generally, the higher the crosslink density, the higher the chemical resistance and the higher the heat resistance.
High functionality epoxies, i.e. epoxy resins having a relatively high number of reactive sites, can be used in the manufacture of high performance coatings, adhesives, and fiber reinforced composites. One property of a cured epoxide polymer which is extremely desirable and useful is a high glass transition temperature, Tg. Generally, high Tg values impart high chemical resistance and excellent electrical properties. High functionality epoxies, upon curing, often exhibit high Tg values.
The use of epoxy resins as a matrix material for reinforced composites, molding compounds, casting resins, etc. is known. In such applications, it is desirable that the resins typically exhibit certain properties such as relatively high temperature resistance and high chemical resistance. These properties may be obtained by the use of high functionality resins such as novolak resins.
Unfortunately, conventional epoxy resins having high functionality, such as those based upon currently available p-hydroxystyrene and novolak agents, are difficult to work with due to their high melting points and relatively high viscosity prior to cure. Accordingly, there is a need for high functionality epoxy resins having a pre-cure viscosity that ranges from a low water-like liquid to a high honey-like material depending on the amount of functionality. In particular, it would be desirable to obtain a high functionality epoxy resin that has a low melt viscosity and a high Tg. Further, it would be desirable for such resins to be curable to a thermoset state by crosslinking upon reaction with tertiary, secondary or primary amines, anhydrides, acids, lewis acids, lewis bases, amides, imidizoles, ureas, meiamines, cyanate esters and other commonly used curing agents and catalysts.
It is an object of this invention to provide new high functionality epoxide polymers having low melt viscosity prior to curing, high glass transition temperatures upon curing, and a process for producing them. In a first aspect of the present invention, a process for producing such an epoxy agent is provided. The process involves providing at least one of three specific poly-p-hydroxystyrene polymers, and reacting such with epichlorohydrin.
In another aspect of the present invention, a process is provided for producing a high molecular weight epoxy agent by reacting a certain poly-p-hydroxystyrene polymer with one or more di-hydric phenol agents, and reacting such with epichlorohydrin.
The present invention also provides new classes of epoxy compounds having improved and superior properties as compared to conventional epoxy agents.
In accordance with a preferred aspect of the present invention, high molecular weight epoxy resins with high functionality, low epoxide equivalent weight, and which exhibit low melt viscosity prior to cure and high glass transition temperatures after cured, may be prepared by reacting certain poly-p-hydroxystyrene polymers and epichlorohydrin in the presence of one or more alkali agents.
Before turning attention to the details of the preferred embodiments of the present invention, it is instructive to address certain terminology utilized herein. Epoxide equivalent weight, as that term is used herein, refers to molecular weight divided by the number of reactive epoxy sites per molecule. High functionality epoxy resins, i.e. epoxy resins having a relatively high number of reactive epoxy sites per molecule, typically have relatively low epoxide equivalent weight. Preferably, the epoxy resins according to the present invention have an average functionality of from about 5 to about 70, and most preferably up to about 100 or higher. The term glass transition temperature, as used herein, is accorded its conventional and well understood definition. Generally, this is the temperature at which an amorphous material changes from a brittle vitreous state to a plastic state. The term melt viscosity, as used herein, refers to the viscosity of the material or resin prior to curing.
The preferred embodiment epoxy agents are formed by reacting one or more particular p-hydroxystyrene polymers and epichlorohydrin. Epichlorohydrin (also known as chloropropylene oxide, CAS: 106-89-8), is an epoxide having the structure:
In reacting epichlorohydrin and p-hydroxystyrene agents(s), it is desirable to epoxidize all, or at least a majority of, phenol groups in the p-hydroxystyrene agents. Accordingly, an excess of epichlorohydrin for each equivalent of phenol is preferably employed. Thus, in a preferred process of the present invention, an excess of about 2 to about 10 moles of epichlorohydrin for each mole of total phenol groups is used.
The alkali or alkaline agent present in the reaction mixture is preferably an alkali metal hydroxide such as sodium, potassium or lithium hydroxide, and is present in an amount sufficient to neutralize the hydrochloric acid produced during the reaction, as well as to transform the chlorohydrin formed in the initial reaction of phenol and epichlorohydrin, to an epoxide-containing molecule. Preferably this amount is between about 1 mole and about 4 moles of alkali per mole of total phenolic groups that are present. In order to improve yields the alkali solution may include one or more alcohols rather than water.
The preferred poly-p-hydroxystyrene polymers that may be used in preparing the preferred embodiment epoxy resins are represented by the following structural formulas (I), (II), and (III).
Poly-p-hydroxystyrene-PG (polymer I) is a combination of linear styrenic and branched novolak type structures:
This polymer (I) is generally in the form of a tan powder, having a molecular weight of from about 4,500 to about 6,000. The variable n in the above structure typically ranges from about 18 to about 30, and preferably from about 20 to about 28. Polymer (I) preferably has a polydispersity of from about 2.0 to about 2.6. The glass transition temperature of polymer (I) is from about 135xc2x0 to about 145xc2x0 C.
Poly-p-hydroxystyrene novolak (polymer II) is a 100% branched polymer with no styrenic backbone character:
This polymer (II) is generally in the form of a white free-flowing powder, and is not derived from an aldehyde. Polymer (II) preferably has a molecular weight of from about 5,400 to about 7,000, and a polydispersity of from about 1.5 to about 1.9. The variable n in the above structure typically ranges from about 10 to about 20, and preferably from about 13 to about 17. The glass transition temperature of polymer (II) is preferably from about 1400 to about 145xc2x0 C.
Poly-p-hydroxystyrene-PG-L (homopolymer of 4-ethenyl phenol) (polymer III) is a 100% linear styrenic backbone polymer having a wide range of molecular weights from 5,000 to 60,000:
The variable n for polymer (III) ranges from about 30 to about 600, and preferably from about 40 to about 500.
Upon the completion of the reaction of one or more of the polymers (I) (II), and/or (III), with epichlorohydrin in the presence of an alkali, the resulting epoxized polymer is represented by the following theoretical structure (IV).
This resulting epoxy novolak polymer (IV), as described in the accompanying examples, when suitably cured, provides a resin having significantly improved temperature and chemical resistance, strength, and durability. As previously noted, the term xe2x80x9cnovolakxe2x80x9d as used herein generally refers to a phenol-formaldehyde type resin. The variable n in formula (IV) represents a number preferably ranging from about 10 to about 26. It is to be understood, however that the above structural formula (IV) represents an idealized version, and its exact composition may vary depending on the reaction conditions of the process. For example, a hindered hydroxyl could crosslink with an epoxy end cap.
The reaction mixture obtained contains salts, unreacted epichlorohydrin, excess alkali, and water, all of which should be removed to isolate the product (IV). This may be accomplished by dissolving the resin in a suitable solvent such as benzene, toluene, xylene, alcohol, etc., washing several times with water and separating the aqueous and organic layers. The solvent is preferably distilled off. The resin may be further purified by dissolving in acetone or alcohol, filtering off any solid impurities and evaporating the solvent. The final epoxy polymer has a relatively low epoxide equivalent weight, preferably within the range of from about 150 to about 260, and more preferably from about 200 to about 260.
An epoxy resin based upon the polymer (II) P-Hydroxystyrene-N is a solid at room temperature, with a high molecular (20,000-30,000) weight, a low epoxide equivalent (200-260) and a high functionality (100-110) polymer (IV). Accordingly, the preferred embodiment resin can be processed and may be subjected to subsequent mixing and other operations.
The epoxy resins prepared in accordance with this preferred method can be cured with tertiary, secondary or primary amines, anhydrides, acids, lewis acids, lewis bases, amides, imidizoles, ureas, melamines, cyanate esters, and other commonly used curing agents and catalysts. Upon curing, the resulting epoxy materials exhibit Tg""s of at least about 300xc2x0 C.
In another aspect, the present invention provides another group of high functionality epoxy resins whose pre-cure viscosity ranges from low water-like liquids to high honey-like liquid depending on the amount of functionality. This group of epoxy resins preferably have an average functionality of from about 5 to about 70, and most preferably up to about 100 or higher. These new multi-functional resins can be cured into a thermoset state by crosslinking when reacted with tertiary, secondary or primary amines, anhydrides, acids, lewis acids, lewis bases, amides, imidizoles, ureas, melamines, cyanate esters and other commonly used curing agents and catalysts.
The epoxidation of p-hydroxystyrene-N generally produces a polymer (IV) with a high molecular weight, a relatively low epoxide equivalent weight and a high functionality, but this polymer is a solid. Therefore, such polymers are typically difficult to work with when trying to mix with reactive curing agents or catalysts. The epoxidized p-hydroxystyrene must be heated to liquefy it in order to achieve dispersion and promote mixing of the curing agents and catalysts.
The present inventor has surprisingly discovered that these processing difficulties can be overcome by cross epoxidation of low molecular weight di-hydric-phenols with high molecular weight (i.e. 4500-7000) p-hydroxystyrene.
Preferred low molecular weight di-hydric-phenols for use in this aspect of the present invention include hydroquinone and resorcinol, each having a molecular weight of 110:
Hydroquinone
The preferred high molecular weight p-hydroxystyrene is the previously noted polymer (II).
When di-hydric-phenols are reacted with p-hydroxystyrene (polymer (II)), and during the epoxidation process with epichlorohydrin in the presence of an alkali, polymeric products having various functionalities and molecular weights can be obtained. If the di-hydric-phenol and p-hydroxystyrene are present in a molar ratio between about 1 to 0.01 (100: 1) and about 2 to 1 of di-hydric-phenol to p-hydroxystyrene, an average functionality from about 5.2 to about 70 can be obtained.
As previously noted, in order to produce an epoxy resin having a low epoxide equivalent weight, it is desirable to epoxidize all phenol groups. Accordingly, an excess of epichlorohydrin for each equivalent of phenol should be employed. In accordance with a preferred aspect of this invention, an excess of about 2 to about 10 moles of epichlorohydrin for each mole of total phenol reactant is used.
The alkali present in the reaction mixture is preferably an alkali metal hydroxide such as sodium, potassium or lithium hydroxide, and is present in amount sufficient to neutralize the hydrochloric acid produced during the reaction, as well as to transform the chlorohydrin formed in the initial reaction of phenol and epichlorohydrin to an epoxide-containing molecule. Preferably this amount is between about 1 mole and about 4 moles of alkali per mole of total phenolic compounds. In order to improve yields the alkali solution may include an alcohol rather than water.
While the exact structural formulas for the various epoxide polymers of this aspect of the invention are not known, they may be represented as having the following theoretical structural formula (V):
One of the major advantages of this coepoxidation, besides producing a product having high functionality at low viscosities, is the formation of ether linkage between one aromatic group of the p-hydroxystyrene and the di-hydric-phenol. Ether groups impart high toughness and flexibility without degrading the polymer. That is, incorporation of ether groups generally does not result in a decrease in the polymer""s chemical resistance or its thermal resistance. The final epoxy polymer has a relatively low epoxide weight, preferably in the range from about 150 to about 260, and more preferably, from about 200 to about 260.
Epoxy resins prepared by this aspect of the present invention can be cured to a thermoset condition with tertiary, secondary or primary amines, anhydrides, lewis bases, amides, imidizoles, ureas, melamines, cyanate esters, and other commonly used curing agents and catalysts. Upon curing, the resulting materials exhibit Tg""s above 300xc2x0 C.
In both of the previously described preferred processes for forming epoxy polymers IV and V, it may be desirable to control the temperature of the reaction medium. Prior to, or during the early phase of the reaction, it may be desirable to heat the system to a temperature above ambient temperature. However, since the reaction is generally exothermic, it is preferred to provide a cooling system to ensure that the system does not reach an excessively high temperature.
The epoxy products produced in accordance with the present invention exhibit superior chemical resistance, higher temperature resistance, greater toughness, greater durability and higher degrees of cross-linking than conventional epoxy products. Although the epoxy products of the present invention, when cured, typically exhibit Tg""s above 300xc2x0 C., the present invention also includes epoxy materials having Tg""s greater than 200xc2x0 C. The preferred embodiment epoxy agents produced in accordance with the present invention typically have molecular weights of at least about 10,000, and usually at least about 20,000. However, the present invention includes epoxy agents having molecular weights less than 10,000.
The present invention epoxy resins and the processes for their production have a wide range of applications. The largest single use of epoxy resins is in the protective coatings market where high chemical resistance and adhesion is important. As a result of new ecology and energy requirements, increasing amounts of waterborne, high solids, and solventless systems (powder and liquid) are used. Cure mechanisms can be either one-package, heat-curing (baking) finishes, or room temperature-curing systems (including air-dried esters). It is contemplated that the preferred embodiment resins could be utilized in conjunction with or instead of the following systems.
Ambient-cure systems are often based on lower molecular weight liquid epoxy resins cured with aliphatic polyamines or polyamides. Curing normally occurs at ambient temperatures with a working life (pot life) of 15 to 60 minutes, depending on the formulation. Epoxy-polyamine systems are typically used for maintenance coatings in oil refineries, petrochemical plants, and in many marine applications. Such coverings are applied by spray or brush. These are used widely where water immersion is encountered, particularly in marine applications.
The higher molecular weight solid epoxy resins are used in formulations that usually consist of a resin, high solvents, hardener, reinforcing filler, pigments, flow control agents, and other modifiers. In addition to using conventional hardeners in these formulations, epoxy resins can also be hardened with other resins, ie, acrylics or polyesters.
Ester-type air dried coating systems provide better performance than alkyd coatings but lesser performance than straight epoxies. These coatings are simple esterification products of the epoxy resin with a fatty acid, resin acid, or a tall oil acid. They are available in short, medium, and long oil lengths similar to alkyds. Long oil length esters have limited chemical resistance, whereas the short oil length esters provide improved chemical resistance. The short oil length esters promote surface wetting similar to drying oils; therefore, they are often used when optimum surface preparation is not possible.
Heat curing finishes generally are based on the higher molecular-weight solid epoxy resins cross-linked with phenol, melamine, or urea-formaldehyde condensation products (phenoplasts and aminoplasts). Normal epoxy-aminoplast weight ratios in resins are epoxy-urea, 70:30; epoxy-benzoguanamine, 70:30; epoxy-melamine, 80:20, 90:10. Increasing the epoxy-aminoplast ratio provides: (1) improved flexibility, toughness, adhesion, and gloss; (2) reduced gloss retention, solvent resistance, and hardness; and (3) higher temperature baking schedules. In conjunction with the need to reduce energy consumption, accelerators are often used.
Epoxy-phenolic systems offer chemical resistance along with excellent mechanical properties. Resins of 4000-5000 molecular weight are used in conjunction with a phenolic that has a low degree of condensation. Part of the methylol groups may be etherified with butanol. Generally, the ratio of epoxy-phenolic ranges from 70:30 to 80:20, by weight, depending on the properties desired. Acid catalysts such as phosphoric acid or the morpholine salt of p-toluenesulfonic acid are often used to accelerate the cure. Such systems are used for chemically resistant coatings on process equipment, tank and drum linings, and in pipe lining.
Acid catalysts such as phosphoric acid or the morpholine salt of p-toluenesulfonic acid are often used to accelerate the cure. Such systems are used for chemically resistant coatings on process equipment, tank and drum linings, and in pipe lining.
Another application of the present invention epoxy resin is in waterborne coatings. These coatings utilize either liquid or solid epoxy resins that have been modified to allow their use with water. They are usually in the form of emulsions, suspensions, dispersions, or water-dilutable resins that can be heat- or RT-cured. They are applied by convenient methods such as roller-coating, dipping, spray, or electrodeposition.
Owing to governmental regulations, considerable research has been expended to develop systems suitable for substitution of solvent-based systems, particularly for automobile and container applications. In the switch from solvent-based to waterborne systems, epoxies are successfully bridging the gap largely by adaptation of conventional resins.
A waterborne system for container coatings was developed based on a graft copolymerization of an advanced epoxy resin and an acrylic. The acrylic-vinyl monomers are grafted onto preformed epoxy resins in the presence of a free radical initiator; grafting occurs mainly at the methylene group of the aliphatic backbone on the epoxy resin. The polymeric product is a mixture of methacrylic acid-styrene copolymer, slid epoxy resin, and graft copolymer of the unsaturated monomers onto the epoxy resin backbone. It is dispersible in water upon neutralization with an amine before cure with an amino-formaldehyde resin. It is contemplated that the preferred embodiment resins could be employed in waterborne systems.
Another application relates to solventless coatings. Ecological concerns have led to increasing uses of these materials. The advantages include high buildup in a single application, minimization of surface defects owing to the absence of solvents, excellent heat and chemical resistance, and lower overall application costs. Disadvantages include poor impact resistance and flexibility, short pot life, and increased sensitivity to humidity.
Attempts to overcome the short pot life have focused on development of special equipment in which the components can be mixed in the proper proportion. Another approach has been to develop curing agents that give long pot life compared to conventional systems.
The present invention epoxy resins may also be utilized in high solids coatings. High solids coatings resemble the technology of solvent-free coatings but the compositions contain approximately 70% by volume of solid resin and are modified by reactive diluents, low viscosity multifunctional resins, or backbone structures other than the bisphenol A moiety.
Yet another application for the present invention epoxy resin is for powder coatings. Epoxy-based powder coatings exhibit useful properties such as excellent adhesion, abrasion resistance, hardness, and chemical resistance. The application possibilities are diverse, including refrigerator liners, oil filters, hospital equipment, primers, shelving, automobile springs, and fire extinguishers.
Interest in powders is primarily related to meeting increasingly stringent antipollution regulations. The development of highly reactive powder systems which cure at low energy (150xc2x0 C.) and the possibility of economical thin films (30-40 xcexcm) or electrostatic spray (thin films, 30-40 xcexcm).
In powder coatings, epoxies are continuing to grow at rates exceeding other technology segments mainly because of the 100% solids feature, improved coverage, and recyclability of material. Pipeline projects, important because of world-wide energy problems, are significant users of epoxy resins. The maintenance coating segment, although not spectacular in its overall growth, still provides a steady and solid base for epoxies. The value of improved service life is being increasingly accepted even at the somewhat higher material cost of epoxy systems.
To improve the weatherability of epoxies, which normally chalk and yellow, epoxy-polyester alloys or hybrids are used. These powders with improved overbake resistance cure at temperatures as low as 130xc2x0 C. They have film flexibility similar to epoxy resins, but their hardness is slightly decreased. Corrosion resistance is equivalent to epoxy powders in most cases, although solvent and alkali resistance is inferior.
Another approach to weatherable systems is the use of triglycidyl isocyanurate as a curing agent for carboxyl-containing oil-free polyesters. The trifunctionality is responsible for its high reactivity with carboxyl-containing oil-free polyesters. These types of powder coatings are used in the protection of metal window frames, as well as for exterior siding.
The present invention epoxy resins are also useful in forming structural composites. Because of their excellent adhesion, good mechanical, humidity, and chemical resistance properties, epoxy resins are used extensively in combination with glass, graphite, boron, and Kevlar fibers in the manufacture of high performance composites.
An important application is for filament-wound glass-reinforced pipe used in oil fields, chemical plants, water distribution, and as electrical conduits. A most preferred application is in reinforced composite tank or container construction. Low viscosity liquid systems having good mechanical properties (elongation at break) when cured are preferred. These are usually cured with liquid anhydride or aromatic-amine hardeners. Similar systems are used for filament-winding pressure bottles and rocket motor casings.
In the aerospace industry, the use of graphite fiber-reinforced composites is growing rapidly because of high strength to weight ratios. High performance polyfunctional resins, such as the tetraglycidyl derivative of methylenedianiline in combination with diaminodiphenysulfone, are extensively used to provide good elevated temperature properties and humidity resistance. Handling characteristics are well suited to the autoclave molding technique primarily used in the manufacture of such components.
Improved versions of the high performance resin systems continue to be developed. Toughening of epoxies has emerged as an important area for investigation using both rubber and thermoplastics.
The present invention also finds wide use in electrical laminates. A significant use for epoxy resins is in the manufacture of copper-clad epoxy-glass printed circuit boards. Systems are available that meet the National Electrical Manufacturers Association (NEMA), G10, G11, FR3, FR4, FR5 specifications. Currently the majority of boards are manufactured to the flame-retardant FR4 specification. The flame retardance is achieved by the use of a solid epoxy resin based on tetrbromobisphenol. In combination with dicyandiamide as the hardener, stable impregnation varnishes can be formulated. The impregnated glass fabric can be dried and xe2x80x9cB-stagedxe2x80x9d in a controlled manner by treating in a heated tunnel or tower. The B-staged fabric (prepreg) is cut into sheets and laid up into laminates with a copper foil top and bottom, and pressed at high pressure and elevated temperature to form a well-consolidated fully cured board. Identical procedures are used for making structural and electrical laminates usually based on nonflame-retardant resins.
Photoactive polymers containing epoxy groups are used in the manufacture of high performance printed wiring boards. A polymer with a UV-sensitive group allowing photocross-linking and an epoxy group for thermal cross-linking has been used as a photopolymerizable solder mask and in photoresists.
Furthermore, the present invention epoxy resins may be utilized in electrical, electronic, and structural applications. Thermoset epoxy molding compounds are multicomponent mixtures usually based on solid epoxy resins, hardeners, and various fillers and reinforcements. The exact nature of the compounds is usually dictated by the application and the molding procedure to be used, ie, transfer, compression, or injection molding. Because of the use of narrow gates in transfer and injection molding processes, the use of long-fiber reinforcement ( greater than 3 mm) is usually avoided because of the danger of excessive orientation of the fibers. In compression-molding procedures, this limitation does not apply and long-fiber reinforced molding compounds are frequently used. Because of the good adhesive properties and low shrinkage on cure of epoxy resins, internal release agents are frequently incorporated into the molding compound formulation. Care must be taken in designing the mold to allow sufficient draft to facilitate easy ejection of the part on demolding. The main use of epoxy molding compounds is for the encapsulation via transfer molding of solid-state devices such as diodes, transistors, and integrated circuits. The molding compounds are formulated to meet stringent requirements regarding flow, reactivity, electrical properties, humidity, and thermal resistance. Extensive testing and experience have shown that the best characteristics are achieved using a solid, mutifunctional, epoxidized cresol-novolak resin, and a phenol or cresol novolak as a hardener.
Low ionic impurity levels are imperative. In order to reduce the coefficient of thermal expansion of the final molding, and hence minimize stresses on the encapsulated silicon chip, the highest possible filler loading is desired. This has to be balanced against the need to maintain as low a melt viscosity as possible to minimize the possibility of damage to the device during the encapsulation process. Accordingly, the present invention epoxy resins are particularly well suited for such uses.
Epoxy molding compounds based on solid bisphenol A resins, frequently improved with solid epoxy phenol or cresol novolaks, and cured with novolaks or occasionally with solid anhydrides or aromatic amines, are used for encapsulating passive electronic components, ie, coils capacitors, transformers, ignition coils, etc. In such applications small amounts of short glass fiber in addition to particulate filler may be used to increase impact strength. Similar compounds frequently are used for molding high voltage electrical bushings and connectors used in pad-mounted distribution transformers.
Compression molding is mainly used in the manufacture of large, long-glass-fiber (up to 25 mm) reinforced parts. Chopped prepreg molding compounds, in which glass roving has been impregnated with a solid resin-hardener formulation and chopped to give strands of the desired length (usually 12-25 mm), and epoxy sheet-molding compounds are mainly used. These are characterized by excellent mechanical strength, rigidity, and good thermal or chemical resistance properties, or both. Components such as pump housings, impellers, pipe fittings, and valves are compression molded.
The superior epoxy resins of the present invention may also be used in casting and encapsulation applications. Cast components are made by filling a mold cavity with liquid epoxy system and then cross linking the resin to give an infusible solid part. The resin system used may be formulated to cure either at RT or at elevated temperatures. A problem with coating large parts is that during cure, the excess heat of reaction must be dissipated to avoid large locked-in-thermal stresses in the casting. This is achieved by skillful formulation but results in relatively long mold cycles and poor productivity.
Nevertheless, casting processes have found extensive uses where the good electrical and mechanical properties of epoxy resins offer many design advantages. The most commonly used systems are based on bisphenol A epoxy resins cured with anhydride hardeners and usually heavily (60-65 wt %) filled with silica fillers. Components using from 50-100 kg of mix are cast on a routine basis. Typical components manufactured by casting techniques are post insulators, busbar supports, switchgear components, instrument transformers, distribution transformers, high and low voltage bushings, encapsulated coils, etc. Poor wet arc-track resistance and uv resistance limit bisphenol A-based resins to indoor applications. For outdoor applications, the use of cycloaliphatic resins cured with a nonaromatic anhydride hardener, eg, hexahydrophthalic anhydride, is mandatory. Such systems have increasing use for outdoor insulators, switchgear components, and instrument transformers.
A casting process, pressure gelation, was developed that overcomes problems associated with the high heat of reaction of an epoxy resin. It allows large parts to be gelled rapidly, reducing the mold cycle from hours to minutes. The increased productivity is responsible for growing worldwide use in the electrical industry. The preferred embodiment resins described herein are well suited for such applications.
In addition to electrical uses, epoxy casting resins are utilized in the manufacture of tools, ie, contact and match molds, stretch blocks, vacuum-forming tools, and foundry patterns, as well as bench tops and kitchen sinks. Systems consist of a gel coat formulation designed to form a thin coating over the pattern which provides a perfect reproduction of the pattern detail. This is backed by a heavily filled epoxy system which also incorporates fiber reinforcements to give the tool its strength. For moderate temperature service, a liquid bisphenol A epoxy resin with an aliphatic amine is used. For higher temperature service, a modified system based on an epoxy phenol novolak and an aromatic diamine hardener may be used.
A common application for epoxies is for use in adhesive formulations. The present invention epoxy resins are particularly suited for such use. Because of excellent adhesion to many substrates, epoxy resins are extensively used for high performance adhesives. These can be categorized into high temperature curing systems (solids and liquids) and room temperature curing systems (liquids).
For the high temperature systems, dicyandiamide and aromatic-amine hardeners are frequently used. For room temperature systems, polyamide aminoamide, and aliphatic-amine hardeners are preferred.
Depending on the characteristics and performance requirements, adhesives systems are frequently modified with diluents (reactive and nonreactive) and polyfunctional high performance resins, as well as with fillers of various types.
In demonstrating procedures for preparing the preferred embodiment polymeric products described herein, a series of trials were carried out as follows:


Negative-working, water-developable imageable elements, useful as printing plate precursors, and methods for their use, are disclosed. The elements can be imaged with ultraviolet radiation, with infrared radiation, or with heat. The elements contain an imageable composition that contains a latent Brxc3x6nsted acid, a water-soluble or water-dispersible binder, and an acid-activated cross-linking agent.

This invention relates to lithographic printing. In particular, this invention relates to negative-working, water-developable, infrared and ultraviolet imageable elements useful as lithographic printing plate precursors.
In lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. Typically, the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
Imageable elements useful as lithographic printing plates, also called printing plate precursors, typically comprise an imageable layer applied over the surface of a hydrophilic substrate. The imageable layer includes one or more radiation-sensitive components, which may be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material.
To obtain a printing plate with imagewise distribution of printable regions, it is necessary to remove regions of an imaged imageable element. The most common method for removing the undesired regions is to contact the imaged element with a developer, which is typically an aqueous alkaline solution. If after exposure to radiation the exposed regions of the layer are removed by the developer revealing the underlying hydrophilic surface of the substrate, the element is a positive-working element. Conversely, if the unexposed regions are removed, the element is a negative-working element. In each instance, the regions of the imageable layer (i.e., the image areas) that remain after development are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water and aqueous solutions, typically a fountain solution, and repel ink.
During the development process, the developer becomes loaded with components of the imageable layer that have been removed during development. Once the developer becomes fully loaded, scum will form on the developed printing plates and on the components of the processor. The developer then must be replaced with fresh developer and the loaded developer disposed of.
Because of environmental considerations, the loaded alkaline developers, many of which also contain organic solvents, must be treated before disposal to render them non-hazardous. If the imageable element could be developed with water, rather with an alkaline developer, it would be unnecessary to treat alkaline developers before disposal. Thus, a need exists for imageable elements that can be developed with water, rather than with alkaline developers.
In one aspect the invention is a method for forming an image using a water-developable negative-working ultraviolet and infrared imageable element. The method comprises the steps of:
(a) imaging the imageable element and forming an imaged imageable element comprising imaged regions and unimaged regions in an imageable layer;
in which:
the imageable element is imaged either with ultraviolet radiation, with infrared radiation, or with heat,
the imageable element comprises:
a substrate comprising a hydrophilic surface, and
the imageable layer over the hydrophilic surface, and
the imageable layer comprises an imageable composition that comprises:
a latent Brxc3x6nsted acid,
a water-soluble or water-dispersible binder, and
an acid-activated cross-linking agent;
(b) heating the imaged imageable element; and
(c) developing the imaged imageable element with water and removing the unimaged regions.
When the element is imaged with infrared radiation, the imageable composition typically additionally comprises a photothermal conversion material.
Imageable Elements
The imageable elements comprise an imageable layer over the hydrophilic surface of a substrate. Preferably the imageable layer is on the hydrophilic surface of the substrate. Preferably no other layers are present.
Substrate
The hydrophilic substrate, i e., a substrate that comprises at least one hydrophilic surface, comprises a support, which may be any material conventionally used to prepare imageable elements useful as lithographic printing plates. The support is preferably strong, stable and flexible. It should resist dimensional change under conditions of use so that color records will register in a full-color image. Typically, it can be any self-supporting material, including, for example, polymeric films such as polyethylene terephthalate film, ceramics, metals, or stiff papers, or a lamination of any of these materials. Metal supports include aluminum, zinc, titanium, and alloys thereof.
Typically, polymeric films contain a sub-coating on one or both surfaces to modify the surface characteristics to enhance the hydrophilicity of the surface, to improve adhesion to subsequent layers, to improve planarity of paper substrates, and the like. The nature of this layer or layers depends upon the substrate and the composition of subsequent coated layers. Examples of subbing layer materials are adhesion-promoting materials, such as alkoxysilanes, amino-propyltriethoxysilane, glycidoxypropyltriethoxysilane and epoxy functional polymers, as well as conventional subbing materials used on polyester bases in photographic films.
The surface of an aluminum support may be treated by techniques known in the art, including physical graining, electrochemical graining, chemical graining, and anodizing. The substrate should be of sufficient thickness to sustain the wear from printing and be thin enough to wrap around a printing form, typically from about 100 to about 860 xcexcm. Typically, the substrate comprises an interlayer between the aluminum support and the imageable layer. The interlayer may be formed by treatment of the support with, for example, silicate, dextrine, hexafluorosilicic acid, phosphate/fluoride, polyvinyl phosphonic acid (PVPA) or polyvinyl phosphonic acid copolymers.
The back side of the substrate (i.e., the side opposite the underlayer and imageable layer) may be coated with an antistatic agent and/or a slipping layer or matte layer to improve handling and xe2x80x9cfeelxe2x80x9d of the imageable element.
Imageable Layer
The imageable layer comprises an imageable composition. The imageable composition comprises a latent Brxc3x6nsted acid which is capable of forming a Brxc3x6nsted acid by the action of either heat or light, a water-soluble or water-dispersible binder, an acid-activated cross-linking agent, and optionally and typically a photothermal conversion material.
Useful latent Brxc3x6nsted acids are precursors that form a Brxc3x6nsted acid both by thermally initiated decomposition and by photochemically initiated decomposition. Latent Brxc3x6nsted acids include, for example, onium salts in which the cation is iodonium, sulphonium, diazonium, phosphonium, oxysulphoxonium, oxysulphonium, sulphoxonium, selenonium, arsonium, or ammonium, and the anion is a non-nucleophilic anion selected from, for example, tetrafluoroborate, hexafluorophosphate, hexafluoroantimonate, triflate, tetrakis(pentafluorophenyl)-borate, pentafluoroethyl sulfonate, p-methyl-benzyl sulfonate, ethyl sulfonate, trifluoromethyl acetate, and pentafluoroethyl acetate. Non-ionic latent Brxc3x6nsted acids include, for example, haloalkyl-substituted s-triazines, which are described, for example, in Smith, U.S. Pat. No. 3,779,778. Examples of onium salts are diphenyliodonium hexafluorophosphate; triphenylsulfonium hexafluoroantimonate; and phenylmethyl-ocyanobenzylsulfonium trifluoromethane sulfonate. Water-soluble latent Brxc3x6nsted acids, such as water-soluble onium salts, are preferred.
Water soluble diazonium salts that comprise one diazonium group (i.e., non-polymeric diazonium salts), such as 2-methoxy-4-aminophenyl diazonium hexafluorophosphate, 4-(dimethylamino)benzenediazonium tetrafluoroborate, and 4-(diethylamino)benzenediazonium tetrafluoroborate, which provide equivalent sensitivity to other latent Brxc3x6nsted acids in the infrared region and higher sensitivity in the ultraviolet region, are particularly preferred onium salts. Diazonium polycondensation products, such as those prepared, for example, by condensation of a diazo monomer, such as is described in Toyama, U.S. Pat. No. 4,687,727 with a condensation agent, such as formaldehyde, acetaldehyde, propionaldehyde, butyraldehyde, isobutyraldehyde or benzaldehyde may also be used. Typical diazonium polycondensation products are the reaction products of diphenylamine-4-diazonium salts, optionally having a methoxy group on the phenyl group bearing the diazo group, with formaldehyde or with 4,4xe2x80x2-bis-methoxymethyl diphenyl ether, such as the 4-(phenylamino)benzenediazonium phosphate (1:1) formaldehyde polymer and the 4-(phenylamino)benzenediazonium sulfate (1:1) formaldehyde polymer.
The imageable composition comprises a water-soluble or water-dispersible binder. The water-soluble or water-dispersible binder produces an imageable composition that is removable with water prior to imaging. Water-soluble or water-dispersible binders that contain functional groups that can be cross-linked by the acid-activated cross-inking agent, such as xe2x80x94OH, xe2x80x94NH, xe2x80x94COOH and/or xe2x80x94COOxe2x88x92, can be used. However, it is not necessary that the binder be cross-linkable. The binder should be of sufficient molecular weight, have a sufficiently high glass transition temperature, and be present in sufficient amount that the imageable layer is a substantially solid layer.
Suitable water-soluble or water-dispersible binders include, for example, (1) alginic acid and its water-soluble derivatives, propylene glycol alginates, propylene glycol esters of alginic acid, amine alginates, algin acetate, algin sulfate esters, the sodium, potassium, and/or ammonium salts of alginic acid; (2) water-soluble cellulose ethers, such as methyl cellulose, hydroxypropylmethyl cellulose, hydroxybutylmethyl cellulose, hydroxyethylmethyl cellulose, ethylhydroxyethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, carboxymethyl cellulose, sodium carboxymethylcellulose, (3) carrageenan and its sodium, potassium and/or ammonium salts; (4) water-soluble biopolymers, such as guar gum, agar, gum arabic, gum ghatti, gum karaya, gum tragacanth, locust bean gum, (5) pectins and partially methoxylated polygalacturonans; (6) polyacrylamides; (7) polymers and copolymers of acrylic acid and/or methacrylic acid and their salts, including poly(methacrylic acids)and polyacrylic acid and their salts, such as their sodium, potassium, and/or ammonium salts; copolymers of methacrylic acid with maleic anhydride, itaconic acid and/or acrylamide and their salts, such as their sodium, potassium, and/or ammonium salts, and copolymers of methacrylic acid with methyl vinyl ether and their salts, such as their sodium, potassium, and/or ammonium salts; (8) polyethylene glycol and poly(ethylene oxide); (9) polyvinyl alcohol, partially hydrolyzed polyvinyl acetate, and vinyl alcohol copolymers; (10) poly(2-ethyl-oxazoline); (11) polyvinyl pyrrolidone; and (12) vinylpyrrolidone copolymers, such as vinylpyrrolidone/vinyl acetate copolymers. Preferred binders are vinylpyrrolidone/vinyl acetate copolymers. Phenolic resins, such as resole resins, novolac resins, and polyvinyl phenols, can not be used in the imageable elements of the invention because they do not produce water-developable compositions.
The imageable composition comprises an acid-activated cross-linking agent or cross-linking resin. When the imaged element is heated, the cross-linking agent crosslinks with itself in the exposed regions. If a cross-linkable binder is present, it also cross-links with the cross-linkable binder in the exposed regions. This xe2x80x9chardensxe2x80x9d or insolubilizes the exposed regions so that they are not removed during development. The cross-linking agent may cross-link to itself and form a matrix that entraps and insolubilizes the binder and/or it may cross-link the binder.
The crosslinking agent must be acid-activated so that it insolubilizes the binder only in the exposed regions of the imageable layer during the heating step. Cross-linking agents that form cross-links on heating in the absence of acid, such as those that contain epoxy and/or imino groups, can not be used in the imageable compositions of the invention.
Acid-activated cross-linking agents include, for example, (1) hexamethoxymethyl melamines, (2) ethoxymethoxy melamine formaldehydes, (3) melamine resins, and (4) high solids urea resins. Preferred acid-activated cross-linking agents are hexamethoxymethyl melamines.
When the element is to be thermally imaged with infrared radiation, the imageable composition comprises an absorber referred to as a photothermal conversion material. Photothermal conversion materials absorb radiation and convert it to heat. Photothermal conversion materials may absorb ultraviolet, visible, and/or infrared radiation and convert it to heat. Preferably, the photothermal conversion material has a high extinction coefficient in the range of 750 nm to 1200 nm. More preferably, the infrared absorption maximum of the photothermal conversion closely matches the wavelength of the infrared radiation used for thermal imaging, typically about 830 nm or about 1056 nm.
The photothermal conversion material may be either a dye or pigment, such as a dye or pigment of the squarylium, merocyanine, indolizine, pyrylium, or metal diothiolene class. Examples of absorbing pigments are Projet 900, Projet 860 and Projet 830 (all available from the Zeneca Corporation), and carbon black. Absorbing dyes are disclosed in numerous publications, for example, Nagasaka, EP 0,823,327; Van Damme, EP 0,908,397; DeBoer, U.S. Pat. No. 4,973,572; Jandrue, U.S. Pat. No. 5,244,771; and Chapman, U.S. Pat. No. 5,401,618. Examples of useful absorbing dyes include, ADS-830A and ADS-1064 (American Dye Source, Montreal, Canada), EC2117 (FEW, Wolfen, Germany), Cyasorb IR 99 and Cyasorb IR 165 (Glendale Protective Technology), Epolite IV-62B and Epolite III-178 (Epoline), PINA-780 (Allied Signal), SpectraIR 830A and SpectralR 840A (Spectra Colors), and Trump IR Dye (Eastman Kodak, Rochester, N.Y.).
Other ingredients that are conventional components of imageable compositions may be present in the imageable composition provided they do not adversely affect the water developability of the imageable element. These include, for example, surfactants, inert colorants and plasticizers. Colorants include, for example, Oil Yellow #101, Oil Pink #312, Oil Black BY, Crystal Violet (C.l. 42555), Methylene Blue (C.l. 52015), and Rhodamine B (C.l. 145170B). When present, typically about 0.01 to about 10 wt %, more typically about 0.1 to about 5 wt %, based on the weight of the imageable composition, of colorant is present in the imageable layer. Plasticizers include, for example, polyethylene glycol, tributyl phosphate, dihexyl phthalate, dioctyl phthalate, and tricresyl phosphate. When present, typically about 0.1 to about 10 wt %, more typically about 1 to 5 wt %, based on the weight of the imageable layer, of plasticizer is present in the imageable composition. Surfactants include, for example, sorbitan tristearate, sorbitan monopalmitate, sorbitan triolate, and di(aminoethyl)glycine.
When present, typically about 0.05 to about 15 wt %, more typically about 0.1 to 5 wt %, based on the weight of the imageable layer, of surfactant is present in the imageable composition. In addition, sensitizers that increase the sensitivity of the latent Brxc3x6nsted acid to ultraviolet and/or visible radiation may be present. These sensitizers are well known. Sensitizers for onium salts are described, for example, in Crivello, U.S. Pat. No. 4,026,705; Smith, U.S. Pat. No. 4,069,054; Smith, U.S. Pat. No. 4,250,053; Smith, U.S. Pat. No. 4,394,403; and Devoe, U.S. Pat. No. 4,792,506.
The imageable composition typically comprises about 20 to 70 wt %, preferably about 30 to 55 wt % of the binder; about 1 to 40 wt %, preferably 5 to 25 wt % of the latent Brxc3x6nsted acid; about 1 to 15 wt %, preferably 3 to 10 wt % of the photothermal conversion material, about 20 to 50 wt %, preferably 25 to 45 wt % of the crosslinking resin, and 0 to about 15 wt % of other ingredients. The imageable layer typically has a coating weight of about 0.5 to 2.5 g/m2, preferably coating weight of about 1.0 to 2.0 g/m2.
The imageable element may be prepared by methods well known in the art. The imageable layer may be applied to the substrate by, for example, coating, lamination, or extrusion. Typically the ingredients of the imageable layer are dispersed or dissolved in a suitable coating solvent or mixture of solvents, and the resulting mixtures coated by conventional methods, such as spin coating, bar coating, gravure coating, or roller coating.
Imaging and Processing
Thermal imaging may be carried out with a laser or an array of emitting modulated near infrared or infrared radiation in a wavelength region that is absorbed the element. Infrared radiation, especially infrared-radiation in the range of about 800 nm to about 1200 nm, is typically used for imaging a thermally imageable element. Imaging is conveniently carried out with a laser emitting at about 830 nm or at about 1056 nm. Suitable commercially available imaging devices include image setters such as the CREO(copyright) Trendsetter (CREO Corp., Bumaby, Britsh Columbia, Canada) and a Gerber Crescent 42T (available from the the Gerber Corporation). Preheatng of the imageable element is not-required.
Alternatively, the element may be imaged using a conventional apparatus containing a heated stylus or thermal printing head. A suitable commercially available hot stylus imaging device is the GS 618-400 Thermal Plotter (Oyo Instruments, Houston, Tex., USA). When exposure is carried out with a thermal head, it is unnecessary that the element comprise a photothermal conversion material. However, elements that absorb infrared radiation can be imaged with a thermal head.
The element can also be imaged to form continuous or halftone images by ultraviolet exposure through a suitable imaging master or photomask, such as a silver halide film. Any convenient source or sources of actinic radiation providing wavelengths in the region of the spectrum that overlap the absorption bands of the latent Brxc3x6nsted acids can be used to activate the photoreaction. Conventional light sources, which are used with a photomask, include fluorescent lamps, mercury vapor lamps, metal additive lamps, carbon arc lamps, tungsten filament lamps and photoflood lamps. Alternatively the element may be digitally imaged with a laser emitting in the ultraviolet, such as a xenon, argon ion, or ionized neon laser, a tunable dye laser, or a frequency doubled neodymium: YAG laser.
Because the imageable element is sensitive to both infrared and ultraviolet radiation, the same element can be used in equipment intended for input of data in digital form by imaging with a laser and in equipment that is used for analog imaging by ultraviolet imaging through a photomask. Thus, both digital and analog imaging can be used on the same imageable element. For example, information not available in an electronic format can be added to a digitally imaged element using ultraviolet imaging through a photomask.
Following imaging, the imaged element is heated at a temperature of from about 75xc2x0 C. to about 150xc2x0 C. for a period of from about 15 to about 300 seconds. Preferably, the imaged element is heated for about 30 to about 90 seconds at a temperature from about 90xc2x0 C. to about 110xc2x0 C.
Imaging of the imageable element produces an imaged element, which comprises a latent image of imaged and unimaged regions. Developing the exposed element to form a developed element converts the latent image to an image by removing the unexposed regions of the imageable layer, revealing the hydrophilic surface of the underlying substrate. The element is negative working, in that the imageable layer is removed in the unexposed regions. The exposed regions become the ink accepting regions in the resulting lithographic printing plate.
The element is developed with water. Deionized water or distilled water may be used, but is not required provided that the water does not contain ingredients that interfere with the development process or react with the ingredients of the imageable layer to form scum on the printing plate. Although the imaged element may be developed in warm water, cold water is preferred. Development is carried for a long enough time to dissolve or otherwise remove the unexposed regions of the imageable layer, but not long enough to remove the exposed regions of the imageable layer. Typically water is applied to the imaged element by rubbing or wiping the top layer with an applicator containing water. Alternatively, the imaged element may be brushed with water or water may be applied to the element by spraying the top layer with sufficient force to remove the exposed regions. In either instance, a developed element is produced.
The elements and methods of the invention can be used to produce lithographic printing plates. Printing can then be carried out by applying a fountain solution and then a lithographic ink to the image on the surface of the imaged and developed element, i.e., the lithographic printing plate. The unimaged regions, i.e., the hydrophilic surface of the substrate exposed by the imaging and development process, take up the fountain solution. The imaged regions take up the ink.
The ink is then transferred to a suitable receiving material (such as cloth, paper, metal, glass or plastic) either directly or indirectly through the use of an intermediate offset printing blanket to produce an impression of the image on the receiving material. The printing plate and/or the blanket can be cleaned between impressions, if desired, using conventional cleaning means.
The advantageous properties of this invention can be observed by reference to the following examples, which illustrate but do not limit the invention.

Water-developable negative-working ultraviolet and infrared imageable element


A method for plasma jet welding by means of a free radio frequency-induced plasma beam wherein, the rf-induced plasma beam is generated by a procedure involving a second process gas into a tube so that it has a tangential flow component and the introduction of the plasma through a metal expansion jet at the outlet.

This application claims the priority of German Application No. 101 40 298.8-34, filed Aug. 16, 2001, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method for plasma jet welding.
In recent years, a variety of efforts have been undertaken to particularly increase and refine the performance capacity of conventional plasma jet welding processes, such as tungsten inert gas welding (TIG) or active-gas metal arc welding (MAG).
In TIG welding, an electric arc burns between a non-melting tungsten electrode and the subject, so that the subject is melted open. The electric arc has an angle of divergence of about 45xc2x0. This means that the distance between the TIG welding torch and the subject significantly affects the power density, which on the whole is relatively low. Due to the high heat conductivity of the metals, a substantial portion of the heat escapes into the area surrounding the weld seam. A power level that is limited by the life of the electrode and the resulting limited electric arc output leads to relatively slow welding speeds.
The plasma beam can be restricted in various plasma jet welding processes by means of water-cooled expansion jets. This can reduce electric arc divergence to about 10xc2x0 (visual). As a result, a higher power density and, at identical electric arc power, a resulting faster welding speed can be achieved when working with technically conventional distances between the plasma welding torch and the subject. In addition, the more stable and less divergent plasma beam, as compared with the conventional TIG process, reduces the impact of the welding parameters on the shape of the electric arc.
If significantly more energy is applied to the electric arc by increasing the intensity of current, given a suitable array of electrodes, the so-called plug effect is achieved. If the subject is of the appropriate thickness, it is melted open in a perforated manner and, when the plasma welding torch is continuously advanced, the molten metal flows around the plasma beam and back together behind it.
A disadvantage of the method described above is that the possible intensity of current, and therefore the welding speed, is limited by the life of the electrodes. This results in high thermal stress on the component and broad thermal impact zones, as well as considerable lag in the subject.
The technical options for further increasing welding speed have been essentially exhausted. In addition to the resulting economic consequences, this means that in the future it will not be possible to substantially lower the limits currently reached in terms of section energy, lag, and the adverse effects on properties of a relatively broad thermal impact zone. This is especially disadvantageous in that the inherent potential of modern, high-strength materials, whose properties can only be attained with specific thermal treatments, are largely underutilized at the current level of development of conventional welding methods.
Another disadvantage of conventional plasma jet welding methods lies in the limited accessibility of and opportunity to observe the welding site, due to the relatively large jet diameter and the small distance from the subject (about 5 mm).
The object of the invention is to provide a new method for plasma jet welding in which the disadvantages of the state of the art are avoided.
According to the invention, a free radio frequency- rf-)induced plasma beam is used, which is generated during a hybrid welding torch process by using the following steps:
generation of a stationary high-pressure plasma, referred to in the following as pilot plasma, by igniting a first process gas with a pilot plasma welding torch;
introduction of the pilot plasma into an rf-transparent working tube including a gas inflow and a gas outflow opening, with the working tube being wrapped in a coupling coil;
introduction of a second process gas into the rf-transparent tube at a pressure of pxe2x89xa71 bar, with the second process gas being introduced into the tube in such a way that it exhibits a tangential flow component in the tube;
generation of an rf-plasma in the rf-transparent tube by electrode-free ignition of the gas mixture comprising the pilot plasma and the second process gas; and
generation of a plasma beam by introduction of the rf plasma into a working space through a metal expansion jet arranged at the gas outflow opening of the tube.
In this process, the ignition of the gas mixture takes place especially by absorption of electromagnetic radiation in the radio frequency range. However, it is also possible to ignite the gas mixture by absorption of electromagnetic radiation in the microwave range. The incorporation of the radio frequency energy into the gas mixture is accomplished inductively by means of the coupling coil wrapped around the rf-transparent tube. The coupling coil can be configured in such a way so as to ensure optimal incorporation of the electromagnetic energy into the gas mixture.
The pilot plasma can advantageously be generated in a peak current arc discharge or in an electrode-free microwave discharge.
Through the pilot plasma, an already ionized gas is introduced into the rf-transparent tube, where the ionized gas is mixed with the second process gas. As a result of the interaction between the electromagnetic radiation, which is coupled into the tube through coupling coil, and the ionized gas, the ignition threshold for ignition of the gas mixture from the pilot plasma gas and the second process gas is reduced. As a result, an energy-rich plasma is generated into which virtually the entire radio frequency energy can be incorporated.
The rf-transparent tube is advantageously a tube with dielectric properties. In particular, a tube made of SiO2 or Al2O3, both in pure form and without dopant, is used as the rf-transparent tube.
Especially advantageous plasma properties result from the plasma jet welding method of the invention. For example, the specific enthalpy of the rf plasma and the related enthalpy flow density of the rf plasma are increased. Consequently, the plasma temperature of the rf plasma and of the plasma beam is also increased. This leads to advantages over the plasma jet welding methods known in the art with respect to increased welding speed and lower weld seam costs. Thus, the plasma jet welding method of the invention provides a welding method that offers considerable economic and application-related advantages while at the same allowing for a wide range of application for the welding method.
The properties of the plasma beam are also improved in terms of reduced diameter and reduced beam angle divergence. In addition, the cylindrically symmetrical plasma beam expands in parallel form in the method of the invention, which reduces the effects of changing the distance between the welding torch and the subject on the fusion shape of the plasma beam in the subject. Another advantage is the improved accessibility to the plasma beam, because it allows for a greater possible distance between the welding torch and the subject. Consequently, distances of 30 mm to 100 mm between the welding torch and the subject, at a plasma beam diameter of 1 mm to 3 mm on the subject, can be achieved with the method of the invention. Thus, power densities above 1.5xc3x97105 W/cm2 can be generated.
The tangential introduction of the second process gas supports the generation, according to the invention, of a plasma beam with a small beam angle divergence. Due to the radial acceleration caused by the tangential introduction of the second process gas, which is further amplified by the cross-sectional narrowing of the expansion jet in the direction of the jet opening, the unevenly accelerated free charged particles move on increasingly narrow spiral paths in the direction of the expansion jet opening, which causes the centripetal acceleration of the charged particles to increase. The charged particles retain this movement, even after exiting the expansion jet and entering the working space. As there is no local charge neutrality, due to variations in ion and electron mobility, the plasma beam is induced in an axially oriented magnetic field, which leads to a curtailment in the flow of the plasma once it exits the jet.
Another advantage of the method of the invention is that the plasma beam of the invention can be generated by means of cost-effective and robust radio frequency systems, such as resonant circuit systems with a frequency of approx. 300 kHz in the typical UHF range (approx. 1-150 MHz).
In the plasma jet welding method of the invention, energy efficiency is also increased in comparison to conventional plasma jet welding systems. As a result, it is possible to generate radio frequency-induced plasmas in which power utilization is greater than 90%. This leads to energy efficiency that is 1.5 times greater than that of welding methods involving high-performance diodes and 20 times higher than that of laser welding methods.
Through the appropriate choice of process-capable gases or gas mixtures, it is possible to increase the specific enthalpy of the plasma in conjunction with improved heat transfer between the plasma and the subject. In the plasma jet welding method of the invention, it is also possible to use a broader spectrum of process gases than is possible with known plasma jet welding methods.
In an advantageous embodiment of the invention, it is possible to add powder to the second process gas before it enters the inductive coupling segment, i.e., before it enters the rf-transparent working tube. This makes it possible, for example, to apply the method of the invention as a powder application welding method. Of course, it is also possible to add powder to the plasma beam after it has exited the expansion jet.
Another advantage of the plasma jet welding method of the invention is that the thermal impact zone of the plasma beam on the subject is considerably reduced, which results in reduced heat incorporation, reduced subject lag, and a reduction in damage to the material. Furthermore, error-free welding, in terms of smaller edge notches and a lesser porosity of the weld seam, can be achieved with the plasma jet welding method of the invention.
In an advantageous embodiment of the invention, the second process gas is introduced into the inductive coupling zone in such a way that, by means of one or more jets, for example, the second process gas flowing into the tube exhibits a tangential axial flow component oriented toward the gas outflow opening of the tube.
In another advantage embodiment of the invention, the metal expansion jet, as viewed in the flow direction of the plasma, features a convergent inlet on the plasma side and a free or divergent outlet on the plasma beam side. This increases the flow velocity of the charged particles of the plasma from the convergent inlet to the divergent outlet.
It is also possible to improve the properties of the plasma beam in terms of reducing beam angle divergence. In addition, beam diameter can be limited by means of the opening cross-sections of the expansion jet. Due to the high temperatures of the plasma, the metal expansion jets can be cooled in an advantageous embodiment of the invention.
It is also possible, given suitable pressure conditions between the pressure in the working space and the pressure in the interior of the rf-transparent tube, and provided that the outflow opening of the expansion jet is suitably dimensioned and the inflow area and the outflow area of the expansion jet are suitably designed, to obtain a freely expanding plasma beam that flows into the working space at supersonic speed.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.