An image receptor medium is disclosed as having a non-porous base medium and an imaging layer. The imaging layer includes a solvent-soluble multivalent cationic salt coated from an organic solvent. A water-insoluble binder holds the imaging layer together. The imaging layer also includes organic-solvent insoluble particulates. The image receptor medium can be backed with an adhesive/release liner combination or mechanical fasteners to provide securing means or can be left without such means for xe2x80x9cdrop-inxe2x80x9d backlit uses.

This invention relates to image receptor media for thermal or piezo inkjet printing wherein the media has a porous coating that contains a multivalent cationic salt.
Image graphics are omnipresent in modern life. Images and data that warn, educate, entertain, advertise, etc. are applied on a variety of interior and exterior, vertical and horizontal surfaces. Nonlimiting examples of image graphics range from advertisements on walls or sides of trucks, posters that advertise the arrival of a new movie, warning signs near the edges of stairways.
The use of thermal and piezo inkjet inks have greatly increased in recent years with accelerated development of inexpensive and efficient inkjet printers, ink delivery systems, and the like.
Thermal inkjet hardware is commercially available from a number of multinational companies, including without limitation, Hewlett-Packard Corporation of Palo Alto, Calif., USA; Encad Corporation of San Diego, Calif., USA; Xerox Corporation of Rochester, N.Y., USA; LaserMaster Corporation of Eden Prairie, Minn., USA; and Mimaki Engineering Co., Ltd. of Tokyo, Japan. The number and variety of printers changes rapidly as printer makers are constantly improving their products for consumers. Printers are made both in desk-top size and wide format size depending on the size of the finished image graphic desired. Nonlimiting examples of popular commercial scale thermal inkjet printers are Encad""s NovaJet Pro printers and H-P""s 650C, 750C, and 2500CP printers. Nonlimiting examples of popular wide format thermal inkjet printers include H-P""s DesignJet printers, where the 2500CP is preferred because it has 600xc3x97600 dots/inch (dpi) resolution with a drop size in the vicinity of about 40 picoliters.
3M markets Graphic Maker Inkjet software useful in converting digital images from the Internet, ClipArt, or Digital Camera sources into signals to thermal inkjet printers to print such image graphics.
Inkjet inks are also commercially available from a number of multinational companies, particularly 3M which markets its Series 8551; 8552; 8553; and 8554 pigment-based inkjet inks. The use of four principal colors: cyan, magenta, yellow, and black (generally abbreviated xe2x80x9cCMYKxe2x80x9d) permit the formation of as many as 256 colors or more in the digital image.
Media for inkjet printers are also undergoing accelerated development. Because inkjet imaging techniques have become vastly popular in commercial and consumer applications, the ability to use a personal computer to digitally print a color image on paper or other receptor media has extended from dye-based inks to pigment-based inks. And the media must accommodate that change. Pigment-based inks provide more durable images because pigment particles are contained in a dispersion before being dispensed using a thermal inkjet print head.
Inkjet printers have come into general use for wide-format electronic printing for applications such as, engineering and architectural drawings. Because of the simplicity of operation and economy of inkjet printers, this image process holds a superior growth potential promise for the printing industry to produce wide format, image on demand, presentation quality graphics.
Therefore, the components of an inkjet system used for making graphics can be grouped into three major categories:
1 Computer, software, printer.
2 Ink.
3 Receptor medium.
The computer, software, and printer will control the size, number and placement of the ink drops and will transport the receptor medium through the printer. The ink will contain the colorant which forms the image and carrier for that colorant. The receptor medium provides the repository which accepts and holds the ink. The quality of the inkjet image is a function of the total system. However, the composition and interaction between the ink and receptor medium is most important in an inkjet system.
Image quality is what the viewing public and paying customers will want and demand to see. From the producer of the image graphic, many other obscure demands are also placed on the inkjet media/ink system from the print shop. Also, exposure to the environment can place additional demands on the media and ink (depending on the application of the graphic). Most common, durability of the image graphic is required in humid indoor or outdoor environments, especially locations capable of being soaked with rain or melting snow or ice.
Current inkjet receptor media are direct coated with a dual layer receptor medium according to the disclosure contained in U.S. Pat. No. 5,747,148 (Warner et al.) and are marketed by 3M under the brands 3M(trademark) Scotchcal(trademark) Opaque Imaging Media 3657-10 and 3M(trademark) Scotchcal(trademark) Translucent Imaging Media 3637-20. Other products marketed by 3M include Nos. 8522CP and 8544CP Imaging Media, the former having a coating on the imaging surface for controlling dot gain and the latter having a pigment management system and a fluid management system in pores of the membrane. With the rapid rise in usage of inkjet printing systems to create wide format graphics having digitally-produced images thereon, more and better inkjet receptor media are needed, especially those which rise to the level of precision and lighting requirements that are used for photographically-created image graphics.
These media have coatings provided by water-borne systems, either for entirely water-soluble or water-dispersible ingredients. Water-soluble ingredients are susceptible to loss of durability of the image graphic when encountering humid or wet environments. Most often, the image is created by printing of a water-based ink needs to be fixed to prevent ink migration and loss of precision of the image graphic. Water-dispersible ingredients are particularly difficult to handle during manufacturing to provide reproducible image receptive layers on substrates; working with emulsion-based delivery of coatings introduces a number of additional manufacturing factors that can affect efficiency and productivity.
An image receptor medium, comprising a non-porous base medium having on one major surface an imaging layer. The imaging layer comprises a) water insoluble binder, b) water insoluble and organic-solvent insoluble particles having a mean particle size of about 1 xcexcm to 25 xcexcm, and c) organic-solvent soluble multivalent cationic salt. The imaging layer comprises a plurality of pores capable of imbibing a liquid ink.
The present invention provides a way to create a very ink receptive coating on a non-porous medium. Thus, any non-porous medium can be provided with a porous image receptive layer that provides excellent ink imbibing properties, in combination with excellent rapid ink fixing properties. Because the binder is insoluble in water, the medium is highly water and humidity resistant.
This invention has particular utility for the production of image graphics using wide format inkjet printers and pigment-based ink. This invention solves the problem of obtaining precise digitally-produced image graphics with ink migration inhibitors on inkjet receptor media to endure water-laden environments that would otherwise cause the image graphic to lose precision.
With such precise, durable image graphics capable of being produced with inkjet receptor media of the present invention, one skilled in the art can replace photographically-created image graphics with digitally-created image graphics using inkjet inks. In other words, image graphics of the present invention have the precision and lighting requirements essentially consistent with image graphics prepared from photographic techniques. But digitally-created image graphics have the huge advantage of being electronically distributable over telecommunications equipment. Thus, one skilled in the art can distribute an image to many physically remote locations using secure data transmission lines or the Internet for later inkjet printing at such remote locations. The means of communication coupled with the media capable of printing durable, precise image graphics changes the way companies or organizations warn, educate, entertain or advertise in brilliant multicolor image graphics.
Moreover, media of the present invention can be illuminated from their viewing side, a reflective lighting property, or can be illuminated from its non-viewing side, a transmissive lighting property. Thus, the brilliant multicolor image graphics are capable of being viewed in natural or artificial light without loss of its color qualities regardless of the location of the light source.
The base medium is a non-porous film suitable for either or both backlit (transmissive) and opaque (reflective) viewing applications. Preferably, the base medium is particularly suitable for rigid xe2x80x9cdrop-inxe2x80x9d type backlit signage materials for lightboxes. Therefore, another aspect of the invention is a combination of translucent sheets or transparent sheets and an inkjet receptor medium as above, which is also translucent, thereby producing a xe2x80x9cdrop-inxe2x80x9d backlit image graphic. When coated onto clear polyester film, the dried coating layer can act both as a diffuser with good light transmission and also act as the imaging layer described above.
After printing an image using an ink jet printer with either dye or pigment-based inks, the inkjet receptor medium of the present invention may give a translucent graphic viewable on a lightbox with both the light on and off.
Another aspect of the present invention is a method of making the inkjet receptor medium identified above, where a solvent-based coating formulation as described below is applied to the non-porous base medium on one major surface thereon, and then the solvent is evaporated to form an imaging layer.
Yet another aspect of the present invention is an inkjet receptor medium that has an imaging layer and also an image printed thereon, whereby the image after drying is fixed by hot-rolling. This finished article therefore comprises in order, a base film (such as a polyester), a hot-melt adhesive layer, and a porous coating which acts as an ink jet receptive layer giving good images.
In another embodiment, the article is printed, allowed to dry, and hot-roll laminated with a hot-melt adhesive overlaminate, and the image is thereby encapsulated between the two layers of the hot-melt adhesive. The porous coating transparentizes somewhat showing the ingress of the hot-melt material into the pores, and so the image is now protected from direct exposure to the elements such as water and direct exposure to air. After encapsulation, the rub resistance and strength of the coating improves because the layer is now more of a continuous film and not weakened by the frequent pores which have been at least partially filled by the hot-melt material.
An advantage of the invention is the solvent-based coating formulation minimizes manufacturing complexities of delivering a coating layer to a base medium.
Other features and advantages will be explained in relation to the following embodiments of the invention.
Non-Porous Base Medium
The base medium useful for the present invention can be any polymeric material that can be uniformly coated by a solvent-based coating formulation to generate an inkjet receptor medium of the present invention. The base medium can be transparent, clear, translucent, colored, non-colored, or opaque, or a combination thereof, as required by those creating the image graphic.
The base medium preferably has a thickness ranging from about 25 microns to about 750 microns and more preferably from about 50 microns to about 250 microns.
The base medium can be rigid, flexible, elastic, or otherwise, again as required by those creating the image graphic.
Nonlimiting examples of polymers useful in the creation of the base medium include polyolefins, polyurethanes, polyesters, acrylics, polycarbonates, polyvinyl chlorides and other vinyl polymers and copolymers, polystyrenes. Presently preferred is a polyester film in the range of thickness from about 110 to about 180 xcexcm thickness due to low cost and handling.
The size of the base medium is only limited by the capacity of the printer through which the medium can pass for printing. Printers directed to personal or business usage are usually small-format, i.e., less than about 56 cm printing width, whereas printers directed to commercial or industrial usage are usually large-format, i.e., greater than that printing width of 56 cm. As the digital revolution in image graphics continues to occur, many more uses of inkjet printers will be found, especially for those industries that distribute an image to many locations before printing it.
Imaging Layer
Solvent-soluble Multivalent Cationic Salts
Solvent-soluble multivalent cationic salts used in the present invention provide a critical element for precise, durable image graphics: inhibition of ink migration on an imaging layer in the presence of water, where the imaging layer is water-insoluble. These cationic salts interact with the pigment particles of the ink to fix such pigment particles within the porous imaging layer.
Nonlimiting examples of solvent-soluble multivalent cationic salts include those salts composed of cations selected from the group consisting of zinc, aluminum, calcium, magnesium, chromium, and manganese and anions selected from the group consisting of chloride, bromide, iodide, and nitrate.
Preferred examples of such salts include anhydrous zinc bromide, anhydrous calcium bromide, and anhydrous calcium chloride.
The amount of salts that can be used in the coating solution for coating the base medium range from about 0.1% to about 10% and preferably from about 0.75% to about 3% weight percent of the solids of the coating formulation.
Solvent
Organic solvents used in the present invention are capable of solvating the solvent-soluble multivalent cationic salts and other ingredients of the coating formulation preferably alone, or in a mixtures with another organic solvent. Nonlimiting examples of such organic solvents include ketones such as methyl ethyl ketone, acetone, isobutyl ketone, cyclohexanone and methyl isobutyl ketone; hydrocarbons such as cyclohexane, heptane, toluene, and xylenes; alcohols such as ethanol, butanol, isopropanol, pentanol; mineral oils; esters such as ethyl acetate, and butyl acetate; PM acetate; carbitol acetate; and glycol alkyl ethers and combinations thereof. Preferred organic solvents for the present invention have limited adverse environmental effects. Particularly preferred organic solvents have a boiling point between about 80xc2x0 C. to about 160xc2x0 C.
Binder
Preferred binders for retaining the solvent-soluble multivalent cationic salts in the imaging layer have low cost, easy manufacturing and processing features, and can form tough layers on base media described above, with or without the use of a priming layer between the imaging layer and the base medium. These are water-insoluble, and binders should be soluble in the solvent used for the coating formulation to assure even delivery of the coating to the base medium.
Nonlimiting examples of binders include acrylic acid copolymers, poly(meth)acrylates, polyvinyl acetals (such as polyvinyl butyra and polyvinyl formal) vinyl acetate copolymers, polyurethanes, vinyl chloride polymers and copolymers such as VYNS (a copolymer of vinyl chloride and vinyl acetate from Union Carbide of Danbury, Conn., USA), VAGH (a terpolymer of vinyl chloride, vinyl acetate and vinyl alcohol from Union Carbide of Danbury, Conn., USA) and the like known to those skilled in the art for producing high quality, low cost layers in laminate constructions. These binders are readily commercially available as resins from large and small manufacturers. Particularly preferred as binders for the present invention include Paraloid B82 brand methyl methyacrylate polymer from Rohm and Haas of Philadelphia, Pa., USA; and VYHH (a copolymer of vinyl chloride and vinyle acetate from Union Carbide of Danbury, Conn., USA).
The amount of binder that can be used in the coating solution for coating the base medium range from about 10% to about 50% and preferably from about 20% to about 40% weight percent of the total coating solids.
Particulate
The coating formulation includes particulates in an amount and size sufficient to assist in providing a porous structure in the ultimate imaging layer. Additionally, the particles may provide surface variation and protection of the pigment-based particles delivered in the inkjet inks for the final product. Nonlimiting examples of particulates include those disclosed in the prior art such as starch, silica, zeolites, clay articles, insoluble silicates such as calcium silicate, alumina, talc, titanium dioxide and the like. Because the coating formulation is solvent-based, the particulates need to be insoluble in the solvents used in the coating formulations. Moreover, it has been found in this invention that a crosslinked polyvinylpyrrolidone particle is particularly useful for providing a good image when printed with both pigment or dye-based aqueous ink jet inks. It is also an advantage that a receptor medium such as described, while primarily of use in receiving pigment-based ink jets to give a water-fast fade-resistant image, can also optionally be used to print with dye-based inks. Such crosslinked polyvinylpyrrolidone particles are commercially available from a number of sources in a number of particle size distributions, including BASF of Wyandotte, Mich., USA under the Luvicross(copyright) M brand.
Mean particle size for the particulates can range from about 1 xcexcm to about 25 xcexcm and preferably from about 4 xcexcm to about 15 xcexcm.
When a crosslinked polyvinylpyrrolidone particulate is used with a binder and a solvent-soluble multivalent cationic salt in the coating formulation, the amount of particulate to be used is determined by its weight/weight ratio with the binder. The particulate:binder W/W (weight/weight) ratio can range from about 1:1 to about 9:1 and preferably from about 1.7:1 to about 2.0:1 and most preferably about 1.8:1. Other particulates may require a different W/W ratio with the binder because it is really the V/V (volume/volume) ratio that concerns the imaging layer after the solvent has evaporated for the binder to hold the particulates in place adequately.
Because the base medium is a solid film without any discernable porosity, the present imaging layer comprising particulates with the binder and the solvent-soluble multivalent cationic salts in the coating formulation inherently provides a porosity for the imaging layer. While not being bound by theory, it is believed that a porous coating layer is formed from the evaporation of solvent from the coating formulation, leaving a disorganized collection of particulates bound by the binder within which the solvent-soluble multivalent cationic salts reside. The pores are able to quickly imbibe the ink providing a quick drying medium. This porous structure may be facilitated by the use of particulates that are irregular in shape (e.g. non-spherical). The imaging layer is not unlike the popular confection of xe2x80x9cpeanut brittlexe2x80x9d with the binder holding together the particulate xe2x80x9cpeanutsxe2x80x9d and enormous porosity in the binder xe2x80x9cbrittlexe2x80x9d formed by solvent evaporation.
Optional Priming Layer
Depending on the type of base medium, to provide an excellent surface for the imaging layer, a priming layer can be provided between the base medium and the imaging layer delivered by the solvent-based system. Nonlimiting examples of such priming layers include poly(vinylidene chloride) or solvent-adhesion primers such as found on Mitsubishi Diafoil 4507 brand polyester (available from Mitsubishi Polyester Film, 2001 Hood Road, P.O. Box 1400, Greer, S.C. 29652).
Alternatively or in addition to priming the base medium, surface alteration treatments can be used to enhance adhesion to the base film such as corona treatment, surface ablation, surface abrasion, and the like known to those skilled in the art.
Optional Adhesive Layer and Optional Release Liner
The receptor medium optionally has an adhesive layer on the opposite major surface of the base medium that is optionally but preferably protected by a release liner. After imaging, the image receptor medium can be adhered to a horizontal or vertical, interior or exterior surface to warn, educate, entertain, advertise, etc.
The choice of adhesive and release liner depends on usage desired for the image graphic.
Pressure sensitive adhesives can be any conventional pressure sensitive adhesive that adheres to both membrane and to the surface of the item upon which the inkjet receptor medium having the permanent, precise image is destined to be placed. Pressure sensitive adhesives are generally described in Satas, Ed., Handbook of Pressure Sensitive Adhesives 2nd Ed. (Von Nostrand Reinhold 1989), the disclosure of which is incorporated by reference. Pressure sensitive adhesives are commercially available from a number of sources. Particularly preferred are acrylate pressure sensitive adhesives commercially available from Minnesota Mining and and Manufacturing Company of St. Paul, Minn. and generally described in U.S. Pat. Nos. 5,141,790, 4,605,592, 5,045,386, and 5,229,207 and EPO Patent Publication EP 0 570 515 B1 (Steelman et al.). Another suitable adhesive is disclosed in copending, coassigned, U.S. patent application Ser. No. 08/775,844, now U.S. Pat. No. 6,197,397, the disclosure of which is incorporated by reference.
Release liners are also well known and commercially available from a number of sources. Nonlimiting examples of release liners include silicone coated kraft paper, silicone coated polyethylene coated paper, silicone coated or non-coated polymeric materials such as polyethylene or polypropylene, as well as the aforementioned base materials coated with polymeric release agents such as silicone urea, urethanes, and long chain alkyl acrylates, such as defined in U.S. Pat. Nos. 3,957,724; 4,567,073; 4,313,988; 3,997,702; 4,614,667; 5,202,190; and 5,290,615; the disclosures of which are incorporated by reference herein and those liners commercially available as Polyslik brand liners from Rexam Release of Oakbrook, Ill., USA and EXHERE brand liners from P.H. Glatfelter Company of Spring Grove, Pa., USA.
Alternatively, one can provide mechanical fasteners on the opposing surface as disclosed in copending, coassigned, U.S. patent application Ser. No. 08/930,957, the disclosure of which is incorporated by reference.
When used in a xe2x80x9cdrop-inxe2x80x9d backlit condition, the inkjet receptor medium has no adhesive or mechanical fasteners on the opposing major surface of the medium, although adhesives and fasteners can be limited to perimeter regions of the medium to secure the imaged medium to supporting rigid sheets. The translucent coating applied to a transparent or translucent receptor medium can also be used in second surface applications, for example by affixing the imaged graphic on the inside of a transparent viewing surface such as a window or the plastic front of a lightbox, vending machine etc. using a transparent double-sided sheet adhesive such as 8560 application adhesive (available from 3M Commercial Graphics Division, 3M Center, Maplewood, Minn. 55144-1000).
Optional Additives
Optional additives to the imaging layer could include coparticulates such as silica or titanium dioxide to increase optical opacity. Such coparticulates may optionally be less than 1 xcexcm, and preferably between about 10 and about 100 nanometers in size. Also optionally added are UV and/or heat stabilizers such as hindered amine light stabilizers (HALS), UV absorbers, antioxidants and heat-stabilizers. Such additives are well known in the art and are available from companies such as Ciba Geigy Additives (7 Skyline Drive, Hawthorne, N.Y. 10532-2188), Cytec Industries Inc. (P.O. Box 426, Westmont, Ill. 60559-0426), Sandoz (4000 Monroe Road, Charlotte, N.C. 28205) or BASF (BASF Aktiengesellschaft Farbmittel und ProzeBchemikalien, 67056 Ludwigshafen, Germany). Other additives could include cobinders, plasticizers for the binders present, and surfactants.
Preparation of the Coating Formulation and Delivery to the Base Medium
The coating formulation is solvent-based and uncomplicated to prepare because the various ingredients except the particulate are preferably soluble in the solvent chosen. For purposes of the present invention, a xe2x80x9csolvent based coating formulationxe2x80x9d is a formulation wherein the majority of the materials present in the formulation that are liquid at room temperature are organic materials. Such formulations may additionally comprise water in smaller proportions. Preferably, the solvent based coating formulation comprises less than 30% water, more preferably less than 20% water, and most preferably less than 10% water. The coating formulation should be thoroughly mixed and the resulting dispersion screened to assure an appropriate size of particulate for the wet coating weight desired for the formation of the imaging layer. The coating formulation is preferably shelf stable, so that it does not form a non-reversible agglomeration during the expected duration between preparation of the coating formulation and application to an intended non-porous base medium.
The coating formulation can be applied in a thickness to the base medium depending on the amount of ink likely to be printed on the inkjet receptor medium. Preferably, the solvent based coating formulation has a wet coating thickness from about 50 xcexcm to about 500 xcexcm, and preferably from about 152 xcexcm (6 mils) to about 200 xcexcm (8 mils) when the solution is approximately 32.5% solids (weight solids to weight of solution) and the particulate is Luvicross M and the binder is Paraloid B82 and the weight ratio of particulate to the binder is 1.8.
The imaging layer preferably has a dry coating weight ranging from about 20 g/m2 to about 80 g/m2 and preferably from about 25 g/m2 to about 60 g/m2.
Preferably, the void volume of the pores is 20% to 80% of the dried imaging layer volume. More preferably, the void volume of the pores is 30% to 60% of the dried imaging layer volume. Void volume is evaluated by any appropriate means in the art, such as imbibing the image layer with a liquid material to determine the volume available for such liquid, estimation using photomicrographs or other visual techniques, or calculation by determining overall volume and subtracting actual image layer volume by density determination. An Evaluation technique is mercury pore symmetry.
Optional Encapsulation of Image Graphics
While the image graphic created by pigment particles inhibited from migration alone can avoid the use of clear coats or overlaminates yet retain durable, precise image graphics, an optional additional step in the formation of the final image graphic is desired. This step could also protect images made by printing with dye-based inks. When the particulates are present in the imaging layer and the solvent has evaporated, an inherent, porosity has been formed. The image can be fixed through the use of heat and pressure in the location where it was printed when an adjacent hot-melt layer is present or if the particulates were to melt into the binder. Thus, the solvent-soluble multivalent cationic salts provide a rapid ink fixing, whereas this optional hot melt processing step provides an additional and highly durable ink fixing.
The coating formulation gives fine quality ink jet images when printed on an HP Designjet 2500CP or HP Designjet 3500CP printer. When coated onto a clear polyester film as the base medium, the imaging layer produces good graphics (when printed) for lightbox applications, both with and without a diffuser. The use of solvent-soluble multivalent cationic salts in the imaging layer of the present invention imparts a good degree of water-resistance to the images after printing with pigment-based aqueous inkjet inks from an inkjet printer, and also some improvement in the water-resistance to dye images printed by ink jet.
The coatings can also be applied to opaque base films giving good inkjet receptor media which dry very quickly to the touch. Because solvent-soluble multivalent cationic salts are used in these opaque imaging applications, both an increase in water-resistance and improvement in reflected viewing density are seen.
The coatings with ink fixing abilities therefore show utility as an imaging layer for an inkjet receptor medium that can be applied to a base film (or other sheet material, e.g. paper, synthetic paper etc.) and be printed using an inkjet printer to give a poster, banner, or other type of image graphic which is substantially water-resistant without the need for a clear coat or overlaminate, and can be put outside for at least a short period of time without the ink running in rain.
The hot-melt encapsulation articles and processes are useful because they provide a method by which a fabricator can print a graphic using ink jet printing, and then pass the material through a hot laminator (potentially with or without the use of a hot-melt overlaminate) and encapsulate the image. The resultin imaged graphic is water-fast and protected from the elements and could be put outside even under harsh conditions. The encapsulation of the coating, which involves filling the pores, makes the coating and therefore the resultant image much tougher, more water resistant, and potentially more UV-resistant.

Image receptor medium and method of making and using same


A hydrocracking catalyst is provided that includes a crystalline molecular sieve material component having a faujasite structure and an alpha acidity of less than 1, preferably 0.3 or less, and a dispersed Group VIII noble metal component. The extremely low acidity allows selective hydrocracking of the aromatic and naphthenic species in a feedstock, while limiting the cracking of paraffins. The catalyst produces improved yields of products, such as diesel fuel, at high conversion rates and with high cetane values.

The present invention relates to novel hydrocracking catalysts. More particularly, the invention relates to noble metal containing low acidic hydrocracking catalysts.
Hydrocracking utilizing a zeolite catalyst is widely used in petroleum refining for converting various petroleum fractions to lighter and more valuable products, especially gasoline and distillates such as jet fuels, diesel fuels and heating oils.
Known hydrocracking catalysts comprise an acid cracking component and a hydrogenation component. The acid component can be an amorphous material such as an acidic clay or amorphous silica-alumina or, alternatively, a zeolite. Large pore zeolites such as zeolites X and Y possessing relatively low silica:alumina ratios, e.g., less than about 40:1, have been conventionally used for this purpose because the principal components of the feedstocks (gas oils, coker bottoms, reduced crudes, recycle oils, FCC bottoms) are higher molecular weight hydrocarbons which will not enter the internal pore structure of the smaller pore zeolites and therefore will not undergo conversion. The hydrogenation component may be a noble metal such as platinum or palladium or a non-noble metal such as nickel, molybdenum or tungsten or a combination of these metals.
A measurement of acidity provides an indication of the amount of framework aluminum in a zeolite. The reduction of acidity indicates that a portion of framework alumina is being lost. Thus, a higher silica to alumina ratio will correspond to a lower acidity. Although the silica to alumina ratio is most often used to refer to the cracking activity of a catalyst, alpha acidity is a more accurate measurement of such activity.
Distillate selective hydrocracking catalysts which rely upon the acidic components of the zeolite for hydrocracking activity cause a significant amount of unwanted secondary cracking of paraffins along with the desired cracking of aromatic and naphthenic components. This secondary cracking results in distillate yield loss.
There is, therefore, a need for a distillate selective hydrocracking catalyst with very low acidity, i.e. a hydrocracking catalyst which does not rely upon acidity of the zeolite for its hydrocracking activity. It has been found that such a catalyst is useful in various hydrocracking processes, such as a process for increasing diesel fuel cetane levels described in co-pending application 09/330,386, filed concurrently herewith.
The present invention is a hydrocracking catalyst composition that includes a large pore crystalline molecular sieve material component having a faujasite structure and an alpha acidity of less than 1, and a Group VIII noble metal component. Preferred crystalline sieve materials are zeolite Y and zeolite USY. In the preferred embodiment, the crystalline sieve material has an alpha acidity of about 0.3 or less. The Group VIII noble metal component can be platinum, palladium, iridium, rhodium, or a combination of one or more of these noble metals. Platinum is preferred. The content of the Group VIII noble metal component can vary between about 0.01 and about 5 wt % of the catalyst.
The Group VIII noble metal component is located within the catalyst in dispersed clusters. In the preferred embodiment, the particle size of the Group VIII metal on the catalyst is less than about 10 xc3x85. Dispersion of the metal can also be measured by hydrogen chemisorption technique in terms of the H/metal ratio. In the preferred embodiment, when platinum is used as the noble metal component, the H/Pt ratio between about 1.1 and 1.5.
In a preferred embodiment, the catalyst is prepared by a method which includes: (a) replacing hydrogen on hydroxyl nest groups located within said crystalline molecular sieve material component with said Group VIII noble metal component by impregnation or cation exchange in a basic solution; (b) drying said catalyst to remove said basic solution; and (c) calcining said catalyst. Preferably, the basic solution has a pH of from about 7.5 to about 10.0.
The catalysts of the present invention have a low acidity that provides selective hydrocracking of the aromatics and naphthenes in a feedstock, while limiting the cracking of paraffins. The catalysts produce improved yields of distillate products, such as diesel fuel, at high conversion rates.
The catalysts of the present invention include a large pore crystalline molecular sieve material component with a faujasite structure having an alpha acidity of less than 1 and, preferably, about 0.3 or less. The catalysts also contain a Group VIII noble metal component.
Unlike catalysts known in the art, the catalysts of the present invention do not rely on acidity to drive the hydrocracking reactions. Hydrocracking is driven by the Group VIII noble metal component, which acts as a hydrogenation and hydrocracking component. The crystalline molecular sieve material acts as a host for the Group VIII noble metal component. The low acidity permits the hydrocracking of the aromatics and naphthenic species, but minimizes secondary cracking of paraffins. Also, the faujasitic crystalline molecular sieve material provides reactant selectivity for adsorbing aromatic and naphthenic hydrocarbon structures, as opposed to paraffins. The preference of the catalyst for ringed structures allows the paraffins to pass through with minimal hydrocracking or hydroisomerization, thereby retaining a high distillate yield.
Constraint Index is a convenient measure of the extent to which a crystalline sieve material allows molecules of varying sizes access to its internal structure. Materials which provide highly restricted access to their internal structures have a high Constraint Index value and small pore size, e.g. less than 5 angstroms. On the other hand, materials which provide relatively free access to their internal porous crystalline sieve structure have a low Constraint Index value, and usually pores of large size, e.g. greater than 7 angstroms. The method by which Constraint Index is determined is described fully in U.S. Pat. No. 4,016,218, incorporated herein by reference.
Large pore crystalline sieve materials are typically defined as having a Constraint Index of 2 or less. Crystalline sieve materials having a Constraint Index of 2-12 are generally regarded to be medium pore materials.
The Constraint Index (CI) is calculated as follows:                               Constraint                                xe2x80x83                              Index                =                                            log              10                                                  xe2x80x83                                                  (                                    fraction                                      xe2x80x83                                    of                                      xe2x80x83                                    n                                      -                                    hexane                                      xe2x80x83                                    remaining                                      )                                                          log              10                                                  xe2x80x83                                                  (                                    fraction                                      xe2x80x83                                    of                                      xe2x80x83                                    3                                      -                                    methylpentane                                      xe2x80x83                                    remaining                                      )                                                          (        1        )
The Constraint Index approximates the ratio of the cracking rate constants for the two hydrocarbons.
The catalysts of the invention contain a large pore crystalline sieve material component with a Constraint Index of less than 2. The materials have a pore size sufficiently large to admit most of the components normally found in a feedstock and generally have a pore size greater than 7 Angstroms. These materials can include zeolites, such as Zeolite beta, Zeolite Y, Ultrastable Y (USY), Dealuminized Y (DEALY), Mordenite, ZSM-3, ZSM-4, ZSM-18 and ZSM-20.
The large pore crystalline sieve materials useful for the catalysts of the invention are of the faujasite family. Within the ranges specified above, preferred crystalline sieve materials useful for the catalysts of the invention are zeolite Y and zeolite USY. Zeolite USY is most preferred.
The Constraint Index provides a means for identifying the crystalline molecular sieve materials which are particularly useful in the present invention. However, the CI is the cumulative result of several variables that are used in the determination and calculation thereof. Thus, other variables, such as the cracking temperature, the conversion rate, the crystal size of the zeolite, the presence of possibly occluded contaminants and binders intimately combined with the zeolite may affect the CI. Therefore, it is possible to select test conditions (e.g., temperature) that establish more than one value for the Constraint Index of a particular crystalline sieve material. This explains the range of Constraint Indices for some materials. Accordingly, it is understood to those skilled in the art that the CI, as utilized herein, while affording a highly useful means for characterizing the zeolites of interest, is an approximation. However, in all instances, at a temperature within the above-specified range of 290xc2x0 C. to about 538xc2x0 C., the CI will have a value for any given crystalline molecular sieve material of particular interest herein of 2 or less.
It is possible to judge from a known crystalline structure whether a sufficient pore size exists. Pore windows are formed by rings of silicon and aluminum atoms. 12-membered rings are preferred in the catalyst of the invention in order to be sufficiently large to admit the components normally found in a feedstock. Such a pore size is also sufficiently large to allow paraffinic materials to pass through.
The crystalline sieve materials that can be utilized in the hydrocracking catalyst have a hydrocarbon sorption capacity for n-hexane of at least about 5 percent. The hydrocarbon sorption capacity of a zeolite is determined by measuring its sorption at 25xc2x0 C. and at 40 mm Hg (5333 Pa) hydrocarbon pressure in an inert carrier such as helium. The sorption test is conveniently carried out in a thermogravimetric analysis (TGA) with helium as a carrier gas flowing over the zeolite at 25xc2x0 C. The hydrocarbon of interest, e.g., n-hexane, is introduced into the gas stream adjusted to 40 mm Hg hydrocarbon pressure and the hydrocarbon uptake, measured as an increase in zeolite weight, is recorded. The sorption capacity may then be calculated as a percentage in accordance with the relationship:                               Hydrocarbon                                xe2x80x83                              Sorption                                xe2x80x83                              Capacity                                xe2x80x83                                          (            %            )                          =                                                            Wt                .                                  xe2x80x83                                                of                                                          xe2x80x83                                          Hydrocarbon                                            xe2x80x83                                          Sorbed                                                      Wt                .                                  xe2x80x83                                                of                                                          xe2x80x83                                          zeolite                                xc3x97          100                                    (        2        )
The catalysts of the invention contain a Group VIII noble metal component. This metal component acts to catalyze both hydrogenation and hydrocracking of the aromatic and naphthenic species within the feedstock. Suitable noble metal components include platinum, palladium, iridium and rhodium, or a combination thereof Platinum is preferred. The hydrocracking process is driven by the affinity of the aromatic and naphthenic hydrocarbon molecules to the noble metal supported within the highly siliceous faujasite crystalline molecular sieve material.
The amount of the Group VIII noble metal component can range from about 0.01 to about 5% by weight and is normally from about 0.1 to about 3% by weight, preferably about 0.3 to about 2 wt %. The precise amount will, of course, vary with the nature of the component. Less of the highly active noble metals, particularly platinum, is required than of less active metals. Because the hydrocracking activity of the catalyst is metal catalyzed, it is preferred that a larger volume of the metal be incorporated into the catalyst.
Applicants have discovered that highly dispersed Group VIII noble metal particles acting as the hydrogenation/hydrocracking component reside on severely dealuminated crystalline sieve material. The dispersion of the noble metal, such as Pt (platinum), can be measured by the cluster size of the noble metal component. The cluster of noble metal particles within the catalyst should be less than about 10 xc3x85. For platinum, a cluster size of about 10 xc3x85 would be about 30-40 atoms. This smaller particle size and greater dispersion provides a greater surface area for the hydrocarbon to contact the hydrogenating/hydrocracking Group VIII noble metal component.
The dispersion of the noble metal can also be measured by the hydrogen chemisorption technique. This technique is well known in the art and is described in J. R. Anderson, Structure of Metallic Catalysts, Academic Press, London, pp. 289-394 (1975), which is incorporated herein by reference. In the hydrogen chemisorption technique, the amount of dispersion of the noble metal, such as Pt (platinum), is expressed in terms of the H/Pt ratio.
An increase in the amount of hydrogen absorbed by a catalyst will correspond to an increase in the H/Pt ratio. A higher H/Pt ratio corresponds to a higher platinum dispersion. Typically, an H/Pt value of greater than 1 indicates the average platinum particle size of a given catalyst is less than 1 nm. For example, an H/Pt value of 1.1 indicates the platinum particles within the catalyst form cluster sizes of less than about 10 xc3x85. In the process of the invention, the H/Pt ratio can be greater than about 0.8, preferably between about 1.1 and 1.5. The H/noble metal ratio will vary based upon the hydrogen chemisorption stoichiometry of the metal. For example, if rhodium is used as the Group VIII noble metal component, the H/Rh ratio will be almost twice as high as the H/Pt ratio, i.e. greater than about 1.6 and, preferably between about 2.2 and 3.0. Regardless of which Group VIII noble metal is used, the noble metal cluster particle size should be less than 10 xc3x85.
The acidity of the catalyst can be measured by its Alpha Value, also called alpha acidity. The catalyst of the invention has an alpha acidity of less than 1, preferably about 0.3 or less. The Alpha Value is an approximate indication of the catalytic cracking activity of the catalyst compared to a standard catalyst and it gives the relative rate constant (rate of normal hexane conversion per volume of catalyst per unit time). It is based on the activity of the highly active silica-alumina cracking catalyst which has an Alpha of 1 (Rate Constant=0.016 secxe2x88x921). The test for alpha acidity is described in U.S. Pat. No. 3,354,078; in the Journal of Catalysis, 4, 527 (1965); 6, 278 (1966); 61, 395 (1980), each incorporated by reference as to that description. The experimental conditions of the test used therein include a constant temperature of 538xc2x0 C. and a variable flow rate as described in the Journal of Catalysis, 61, 395 (1980).
Alpha acidity provides a measure of framework alumina. The reduction of alpha indicates that a portion of the framework aluminum is being lost. It should be understood that the silica to alumina ratio referred to in this specification is the structural or framework ratio, that is, the ratio of the SiO4 to the Al2O4 tetrahedra which, together, constitute the structure of the crystalline sieve material. This ratio can vary according to the analytical procedure used for its determination. For example, a gross chemical analysis may include aluminum which is present in the form of cations associated with the acidic sites on the zeolite thereby giving a low silica:alumina ratio. Similarly, if the ratio is determined by thermogravimetric analysis (TGA) of ammonia desorption, a low ammonia titration may be obtained if cationic aluminum prevents exchange of the ammonium ions onto the acidic sites. These disparities are particularly troublesome when certain dealuminization treatments are employed which result in the presence of ionic aluminum free of the zeolite structure. Therefore, the alpha acidity should be determined in hydrogen form.
A number of different methods are known for increasing the structural silica:alumina ratios of various zeolites. Many of these methods rely upon the removal of aluminum from the structural framework of the zeolite employing suitable chemical agents. Specific methods for preparing dealuminized zeolites are described in the following to which reference may be made for specific details: xe2x80x9cCatalysis by Zeolitesxe2x80x9d (International Symposium on Zeolites, Lyon, Sep. 9-11, 1980), Elsevier Scientific Publishing Co., Amsterdam, 1980 (dealuminization of zeolite Y with silicon tetrachloride); U.S. Pat. No. 3,442,795 and U.K. Pat. No. 1,058,188 (hydrolysis and removal of aluminum by chelation); U.K. Pat. No. 1,061,847 (acid extraction of aluminum); U.S. Pat. No 3,493,519 (aluminum removal by steaming and chelation); U.S. Pat. No. 3,591,488 (aluminum removal by steaming); U.S. Pat. No. 4,273,753 (dealuminization by silicon halide and oxyhalides); U.S. Pat. No. 3,691,099 (aluminum extraction with acid); U.S. Pat. No. 4,093,560 (dealuminization by treatment with salts); U.S. Pat. No. 3,937,791 (aluminum removal with Cr(Ill) solutions); U.S. Pat. No. 3,506,400 (steaming followed by chelation); U.S. Pat. No. 3,640,681 (extraction of aluminum with acetylacetonate followed by dehydroxylation); U.S. Pat. No. 3,836,561 (removal of aluminum with acid); German Offenleg. No. 2,510,740 (treatment of zeolite with chlorine or chlorine-containing gases at high temperatures), Dutch Pat. No. 7,604,264 (acid extraction), Japanese Pat. No. 53/101,003 (treatment with EDTA or other materials to remove aluminum) and J. Catalysis, 54, 295 (1978) (hydrothermal treatment followed by acid extraction).
The preferred dealuminization method for preparing the catalyst of the invention is steaming dealuminization, due to its convenience and low cost. More specifically, the preferred method of obtaining the low acidic crystalline molecular sieve material utilized in the catalyst of the invention is through steaming an already low acidic USY zeolite (e.g., alpha acidity of about 10 or less) to the level required, i.e. an alpha acidity of less than 1.
The steaming dealuminization method includes contacting the USY zeolite with steam at an elevated temperature of about 550 to about 815xc2x0 C. for a period of time, e.g., about 0.5 to about 24 hours, sufficient for structural alumina to be displaced, thereby lowering the alpha acidity to the desired level of less than 1, preferably less than 0.3. In contrast, the alkaline cation exchange method of dealuminization is less useful because it tends introduce residual protons upon H2 reduction during hydroprocessing, which may contribute unwanted acidity to the catalyst and also reduce the noble metal catalyzed hydrocracking activity.
The Group VIII noble metal component can be incorporated by any means known in the art. However, it should be noted that a hydrogenation/hydrocracking component, such as noble metal, would not be incorporated into such a dealuminated crystalline molecular sieve material under conventional exchange conditions because very few exchange sites exist for the noble metal cationic precursors.
The preferred methods of incorporating the Group VIII noble metal component onto the interior sites of the crystalline molecular sieve material are either impregnation or cation exchange. The metal can be incorporated in the form of a cationic or neutral complex. Pt(NH3)42+and cationic complexes of this type are convenient for exchanging metals onto the crystalline molecular sieve component. Anionic complexes are not preferred.
The steaming dealuminzation process described above creates defect sites, also called hydroxyl nests, where the structural alumina has been removed. The formation of hydroxyl nests is described in Gao, Z. et. al., xe2x80x9cEffect of Dealumination Defects on the Properties of Zeolite Yxe2x80x9d, J. Applied Catalysis, 56:1 pp. 83-94 (1989); Thakur, D., et. al., xe2x80x9cExistence of Hydroxyl Nests in Acid-Extracted Mordenites,xe2x80x9d J. Catal., 24:1 pp. 543-6 (1972), which are incorporated herein by reference as to those descriptions. Hydroxyl nests can also be created by other dealumination processes listed above, such as acid leaching (see, Thakur et. al.), or can be created during synthesis of the crystalline molecular sieve material component.
In the preferred method of preparing the catalyst of the invention, the Group VIII noble metal component is introduced onto the interior sites of the crystalline molecular sieve material component via impregnation or cation exchange with the hydroxyl nest sites in a basic solution, preferably pH of from about 7.5 to 10, more preferably pH 8-9. The solution can be inorganic, such a H2O, or organic such as alcohol. In this basic solution, the hydrogen on the hydroxyl nest sites can be replaced with the Group VIII noble metal containing cations, such as at Pt (NH3)42+.
After the Group VIII noble metal component is incorporated onto the interior sites of the crystalline molecular sieve material, the basic solution is removed by drying at about 130-140xc2x0 C. for several hours. The catalyst is then dry air calcined for several hours, preferably 3-4 hours, at a temperature of about 350xc2x0 C.
To be useful in a reactor, the catalyst will need to be formed either into an extrudate, beads, pellets, or the like. To form the catalyst, an inert support can be used that will not induce acidity in the catalyst, such as self- and/or silica binding of the catalyst. A binder that is not inert, such as alumina, should not be used since aluminum could migrate from the binder and become re-inserted into the crystalline molecular sieve material. This re-insertion can lead to creation of the undesirable acidity sites during the post steaming treatment.
The preferred low acidic hydrocracking catalyst is a dealuminated Pt/USY catalyst.

Noble metal containing low acidic hydrocracking catalysts


One embodiment of the present invention provides a system that ensures that progress is made in an environment that supports execution of obstruction-free operations. During execution, when a process pi invokes an operation, the system checks a panic flag, which indicates whether a progress-ensuring mechanism is to be activated. If the panic flag is set, the progress-ensuring mechanism is activated, which causes the system to attempt to perform the operation by coordinating actions between processes to ensure that progress is made in spite of contention between the processes. On the other hand, if the panic flag is not set, the system attempts to perform the operation essentially as if the progress-ensuring mechanism were not present. In this case, if there is an indication that contention between processes is impeding progress, the system sets the panic flag, which causes the progress-ensuring mechanism to be activated so that processes will coordinate their actions to ensure that progress is made.

Before presenting our transformation, we introduce background on nonblocking shared data structures, nonblocking progress conditions, and asynchronous and semisynchronous models of computation, and briefly describe some previous results that use semisynchronous models to analyze implementations.
Nonblocking Shared Data Structures
Today, almost all concurrent programs rely on blocking constructs such as mutual exclusion locks for synchronizing access to shared data structures. The use of locks introduces numerous problems, including deadlock, performance bottlenecks, and priority inversion. Researchers have investigated nonblocking implementations in the hope of eliminating these problems.
An implementation of a shared data structure in a shared memory system provides a representation of the data structure using base objects in the system and provides techniques for the processes of the system to perform operations on the data structure.
Most nonblocking applications are based on an optimistic approach to synchronization, in which an operation is attempted but may fail to take effect if another concurrent operation interferes. In this case, the operation is retried. A significant source of difficulty is guaranteeing that an operation is not retried repeatedly without ever completing. Generally, stronger nonblocking progress guarantees are more difficult to achieve, and require techniques that are more complicated and more expensive.
Nonblocking Progress Conditions
A wait-free implementation guarantees that when a process performs an operation, it completes the operation in a finite number of its own steps, regardless of how fast or slowly other processes execute, and even if they stop executing permanently. (See M. Herlihy. Wait-free synchronization. ACM Transactions on Programming Languages and Systems, 13(1):124-149, January 1991.) Such strong progress guarantees are attractive, but often very difficult to achieve. Most wait-free techniques in the literature are too complicated and too expensive to be useful in practice.
A lock-free implementation guarantees that, starting from any state in which one or more processes are executing operations, some process will complete its operation within a finite number of steps. This weaker progress condition usually makes lock-free implementations easier to design than wait-free ones. Simple and practical lock-free implementations have been achieved for a small number of important data structures, such as stacks, queues, and work-stealing deques. Lock-freedom has generally been regarded as acceptable because well known contention management techniques such as backoff are effective at reducing contention when it arises, thereby achieving progress in practice, despite the lack of the strong theoretical guarantee of wait-freedom. (See A. Agarwal and M. Cherian. Adaptive backoff synchronization techniques. In Proceedings of the 16th International Symposium on Computer Architecture, pages 396-406, May 1989.)
Herlihy, Luchangco, and Moir [Herlihy2003A] recently proposed the obstruction-free approach to implementing nonblocking operations for shared data structures. An obstruction-free implementation simply guarantees that a process will complete its operation if it eventually executes enough steps without interference from other processes. Thus, if two or more processes repeatedly interfere with each other, it is possible that none of them completes its operation. The view is that, because contention management techniques are required to achieve acceptable performance when contention arises anyway, it is unnecessary to make any progress guarantees in the case of contention between concurrent operations.
Several examples in the literature suggest that by providing only obstruction-free progress guarantees, significantly simpler implementations can be achieved that are faster in the uncontended case. Furthermore, although an implementation that is obstruction-free but not lock-free will exhibit livelock if contention is ignored, experience shows that livelock can be effectively avoided by using simple contention-management strategies.
Asynchronous and Semisynchronous Models of Computation
Concurrent techniques are usually required to ensure safety properties regardless of how the steps of concurrent processes are interleaved, and (therefore) regardless of how fast or slowly any process executes. In other words, these techniques should be proved safe in an asynchronous model of computation, in which the steps of processes are scheduled by an adversarial scheduler that can perform many steps of a process consecutively or perform them arbitrarily far apart. In such a model, it is impossible for a process to determine whether another process has crashed (i.e., stopped executing) or is just running very slowly.
Of course, in reality, there are limits to how fast or slowly processes can run. Some techniques exploit assumptions about these limits to improve in various ways on techniques designed for an asynchronous model. Such techniques are analyzed in synchronous or semisynchronous models of computation that embody timing assumptions made about the target execution environment.
In a synchronous model, all processes execute steps at the same rate (until they crash). This means that if a process does not perform a step when it should, other processes can detect that it has crashed. However, if the correctness of a particular technique depends on all (non-crashed) processes performing steps precisely at a given rate, then tiny variations in execution rate, for example due to one processor becoming warmer than another, can cause incorrect behavior. Consequently, such techniques are not generally practical.
Semisynchronous models relax these timing requirements, allowing processes to execute steps at different rates, and even allowing the rate at which a particular process executes to vary over time. However, it is assumed that there is an upper bound on the relative execution rates of any pair of processes. To be more precise, let us define the maximum step time of an execution as the longest time between the completion times of consecutive steps of any process. We define minimum step time analogously. Semisynchronous models assume that there exists a finite R such that in all executions, the ratio of the maximum and minimum step times is at most R. The evaluation of techniques in semisynchronous models has value for the practitioner because real-world systems satisfy the assumptions of such models, and for the theoretician in understanding the limitations of assumptions on timing.
In the known-bound model, R is known by all processes. (See [Alur1997] R. Alur, H. Attiya, and G. Taubenfeld. Time-adaptive algorithms for synchronization. SIAM Journal of Computing, 26(2):539-556, 1997.) This implies that a process can wait long enough to guarantee that every other process has taken another step, or has crashed. Some techniques that depend on knowledge of R can violate safety requirements in systems that do not satisfy the assumed bound. Conservative estimates of the bound for a particular system generally translate into worse performance, so designers are faced with a dangerous tradeoff in using such techniques. Thus, such techniques are not easily portable and indeed may violate safety properties in a given system if the system stops satisfying the timing assumptions, for example due to increased temperature.
In the unknown-bound model [Alur1997], R is not known to processes. Thus, in contrast to the synchronous and known-bound models, a process does not know how long to wait to ensure that every other process that has not crashed takes a step. Therefore, it is not possible for a process to detect that another process has crashed. Nonetheless, it is possible for applications to wait for increasingly longer periods, and to exploit the knowledge that eventually all non-crashed processes have taken a step during one of these periods. It has been shown that an application that is correct in this model does not violate any of its safety properties even in an asynchronous model, although progress properties proved in the unknown-bound model may not hold in an asynchronous model.
Applications that are correct in an asynchronous model are nonetheless sometimes analyzed in a synchronous or semisynchronous model, thus allowing the analysis to depend on various timing assumptions. Because contention management techniques such as backoff fundamentally rely on operations waiting for some time before retrying, they cannot be meaningfully analyzed in an asynchronous model of computation, which has no notion of time whatsoever.
In this description, we show how to transform any obstruction-free implementation into one that guarantees that every process performing an operation eventually completes the operation, when analyzed in the unknown-bound model. Thus, the resulting application is safe to use in any non-real-time application, and guarantees that every operation eventually completes in any practical system.
Our Transformation
We begin by explaining some simple ways of ensuring progress for each operation under various different assumptions and models. These ideas motivate the techniques used in our technique, and explain why they are needed under the weak assumptions of the unknown-bound model.
First, if we assume that processes never crash, then it is easy to ensure progress, even in an asynchronous model. This is achieved by ordering operations using timestamps, and having each process wait until all earlier operations in this order have completed before performing the steps of its own operation. This ensures that operations do not encounter contention with concurrent operations while executing the original obstruction-free technique, so every operation eventually completes. However, if a process does crash while it has a pending operation, no operations with later timestamps can be executed.
In a synchronous model, if all processes know an upper bound B on the number of consecutive steps that must be taken by a process to ensure that its operation completes, then it is easy to guarantee that each operation completes, even if processes can crash. The idea is, again, to order operations using timestamps and to have processes refrain from executing their operations while operations with earlier timestamps are pending. However, unlike in the asynchronous model, a process can detect if another process crashed while executing its operation: if the operation is not completed within B steps, then the process executing it must have crashed. In this case, a process can execute its operation when every operation with an earlier timestamp has either completed, or will not interfere further because the process executing it has crashed.
A similar approach works in the known-bound model. In this case, a process that is waiting for an earlier operation than its own to complete must conservatively assume that it is executing its steps at the maximum speed allowed by the model relative to the speed of the process executing the earlier operation. Thus, in this model, a process must wait for RB steps in order to be sure that another process has had time to execute B steps, where R is the ratio of the maximum and minimum step times.
However, this technique does not work in the unknown-bound model because the bound R is not known to processes. In fact, in this model, it is impossible for one process to determine that another process has crashed. Nonetheless, ideas similar to those described above can be used to guarantee that each operation executed by a process that does not crash will complete even in the unknown-bound model. The key idea is that, rather than delaying for an amount of time that is known to be long enough to allow another process to take B steps, a process can delay for increasingly long periods of time while an earlier operation has not completed.
Each time a process performs b steps of its operation, for some constant b, it increases a counter. This serves the dual purposes of demonstrating that it has not crashed, and therefore must be deferred to by later operations, as well as increasing the number of steps for which later operations must defer. After a process has waited the required number of steps for an earlier operation that has not been completed and whose counter has not been increased, it assumes that the process performing the earlier operation has crashed. Consequently, it removes the timestamp of that operation from the order of operations under consideration and proceeds.
In case the process executing the earlier operation has, in fact, not crashed, it reinstates its operation into the order (using its original timestamp). With this arrangement, if a process does crash while executing an operation, then it is removed from consideration and does not prevent progress by other operations. On the other hand, if an operation fails to complete because others did not wait long enough, then they will wait longer next time, so the bound provided by the model ensures that eventually they will wait long enough and the operation will complete.
It is important to note that the worst-case bound R for a particular system might be very high, because a process might occasionally take a very long time between two steps. However, the application has no knowledge of the bound, so the bound does not affect the performance of the application; only the particular execution behavior does. Furthermore, even if an unlikely sequence of events causes progress to take a long time, this has no bearing on how the application behaves in the future. In practice, processes run at approximately the same speed most of the time. Therefore, the effective bound will generally be small, even if, in theory, the actual bound is very large.
This description captures the key idea about how we transform implementations to provide progress guarantees in the unknown-bound model. However, because this strategy essentially amounts to eliminating concurrency, it would not be practical if simply used as described. Therefore, our transformation does not employ this strategy until some process determines that it has executed the original operation too long without making progress.
The application produced by applying our transformation to an obstruction-free application OFApp (which may include a contention manager) is shown in Table 1. We now describe the transformed application in more detail. The PANIC flag is used to regulate when the strategy to ensure progress should be used. Referring to both Table 1 and the flow chart illustrated in FIG. 2, when a process invokes an operation, it first checks this flag (N1; step 204) and, if it is not set (i.e., has the value FALSE), executes up to B steps of its original application (N2; step 208), where B is a parameter of the transformation. If these steps are sufficient to complete its operation, the process simply returns (N3-N4). Observe that if every operation completes within B steps, then the PANIC flag remains unset (FALSE), so the transformed application behaves exactly like the original one, except that it must read one variable, which is likely to be cached. Thus, by choosing B appropriately, we ensure that our transformation introduces very little overhead, if the original contention manager is effective.
TABLE 1invoke(op)N1:if   PANICN2:  execute up to B steps of OFAppN3:  if op is completeN4:    return responseN5:  PANIC   TRUE//panic modeP1:t   fetch-and-increment(C)P2:A[i]   1repeatP3:  T[i]   t  //find minimum time stamp; reset all othersP4:  m   t  k   iP5:  for each j ≠ iP6:    s T[j]P7:    if s < mP8:      T[k]   ∞P9:      m   s      k   j    elseP10:      if(s<∞) T[j]   ∞P11:  if k = i    repeatP12:      execute up to b steps of OFAlgP13:      if (op is complete)P14:        T[i]   ∞P15:        PANIC   FALSEP16:        return responseP17:      A[i]   A[i]+ 1P18:      PANIC   TRUEP19:    until (T[i] = ∞)  else    repeatP20:      a   A[k]P21:      wait a stepsP22:      s   T[k]P23:    until a = A[k] or s ≠ mP24:    if (s = m) T[k]   ∞P25:until (op is complete)
If its operation fails to complete within B steps, the process sets the PANIC flag (to have the value TRUE) (N5; step 206). Thereafter, until the flag is reset, all new operations see that the PANIC flag is set (to have the value TRUE) and begin to participate in the strategy to ensure progress (P1-P25).
A process pi participating in this strategy first acquires a timestamp (P1; step 214), initializes its activity counter A[i] (P2; step 216), and then repeats loop P3-P25 until its operation is complete. In each iteration of this loop, the system first determines which process pk has the minimum time stamp and sets all other time stamps to ∞ (step 218). To accomplish this, each process pi announces its timestamp in T[i] (P3) and then searches for the minimum (i.e., oldest) timestamp announced by any process. All timestamps that are not ∞, but are larger than the minimum timestamp it observes, are replaced by ∞ (P4-P10).
If pi determines that it has the minimum timestamp (P11), then it repeatedly takes up to b steps of the original application (P12; step 222) (where the constant b is a parameter of the application), increases its activity counter (P17; step 225), and sets the PANIC flag (to have the value TRUE) (P18; step 226). Note that the PANIC flag may have been unset (to have the value FALSE) because some other process completed its operation (P15). Setting the PANIC flag (to have the value TRUE) ensures that new operations continue to participate in the strategy to ensure progress. Process pi repeats these steps until either its operation finishes (P13-P16) or some other process overwrites its timestamp with ∞ (P19). The latter case indicates that this other process has read an older timestamp (P8, P10) or thinks that pi may have crashed (P24). On the other hand, if pi finishes its operation, it sets T[i] to ∞ (P14; step 227), unsets the PANIC flag (to have the value FALSE) (P15; step 228), and returns from the invocation.
If process pi determines that some other process pk has the minimum timestamp (P11), then pi enters loop P20-P23. During each iteration of this loop, pi reads pk's activity counter A[k] (P20) and waits for the indicated number of steps (P21; step 230). If pk's timestamp is overwritten during this period of time, then either pk has completed its operation, another process thought that pk had crashed, or another process saw an operation with a smaller timestamp. In this case, pi exits the loop (P23). If pk's timestamp is not overwritten by another value and pk does not increase its activity counter during the period of time while pi was waiting at line P21, then pk may have crashed, so pi exits the loop (P23) and overwrites pk's timestamp with ∞ (P24; step 232). The system then returns to step 218. In doing so, the process pi sets T[i] again to have its time stamp. Note that each process remembers its time stamp, and this memory is not wiped out when other processes set T[i] to infinity. Process pi uses this "remembered" time stamp to restore its timestamp T[i] when process pi transitions from step 232 to step 218.
We now informally explain why our strategy ensures a guarantee of progress. Eventually, in every iteration of loop P3-P25, process pi enters loop P12-P19. Meanwhile, other processes determine that pi's timestamp is the minimum and wait for a number of steps indicated by pi's activity counter A[i]. If pi doesn't complete its operation within b steps, then it increases its activity counter A[i]. Eventually, no process resets T[i] to ∞, and A[i] becomes large enough so that each process executing loop P20-P23 waits long enough at P21 so that pi increases A[i] during this period. Thus, eventually, all other active processes remain in loop P20-P23, so no process except pi executes steps of the original application. Hence, obstruction freedom guarantees that pi eventually completes its operation.
On the other hand, if pi crashes, then the other processes will no longer see A[i] change, will stop waiting for pi, and will overwrite its timestamp with ∞. Then the way is clear for the next operation in timestamp order (if any) to make progress.
An important feature of the transformed implementation is that, if the original contention manager is occasionally ineffective, causing the PANIC flag to be set, the PANIC flag will be reset and normal execution will resume, provided the original contention manager does not remain ineffective. To see this, recall that every operation by a non-crashed process eventually completes, and note that each operation either sees that PANIC is unset (has the value FALSE) and does not set it (N1), or unsets PANIC (to have the value FALSE) before returning (P15). Furthermore, PANIC is set (to have the value TRUE) only by an operation that has executed either B or b steps of the original application (including the original contention manager) without completing. Thus, with appropriate choices for B and b, we ensure that our mechanism continues to be invoked only if the original contention manager continues to be ineffective.
Note that a number of aspects of the exemplary code that appears in Table 1 can be modified. For example, the choice of specific values for B and b can depend on the particular operation being executed (and the process executing it). Hence, the parameters B and b do not have to be a constant parameter of the transformation. The only requirement is that there is some upper bound for B over all operations. Note that a process can raise the PANIC flag earlier if it finds that it is encountering too much contention, so B is just a worst-case bound, and should be chosen conservatively. Furthermore, it is desirable for the value of b to be constant for each operation. That is, once b is chosen for a particular operation, it should use the same value of b throughout. Though again, b is an upper bound; the process may choose to execute fewer steps of OFAlg, as long as it executes at least one step.
We can also allow A[i] to be initialized (on line P2) with any value, and the choice of what value to use may depend on the operation. Also, at line P17, the process may increase its activity counter by any amount, as long as it increases it. (In the case where the activity counter is an integer, the process has to increase by at least 1.)
Concluding Remarks
We have shown that any obstruction-free application can be transformed into a new application that is wait-free when analyzed in the unknown-bound semisynchronous model of computation. Our transformation can be applied to an obstruction-free implementation, together with any valid contention manager, and the transformed implementation behaves like the original as long as the chosen contention manager is effective. Because real-world systems satisfy the assumptions of the model we consider, our result shows that obstruction-free applications and ad hoc contention managers can be used in practice without sacrificing the strong progress guarantees of wait-freedom.
Our result can easily be made stronger from both practical and theoretical points of view. First, as presented, our transformation introduces the need to know of the maximum number of processes that use the implementation. However, this disadvantage can easily be eliminated using results of Herlihy, Luchangco and Moir. (See M. Herlihy, V. Luchangco, and M. Moir. Space- and time-adaptive nonblocking algorithms. In Proceedings of Computing: The Australasian Theory Symposium (CATS), 2003.) From a theoretical point of view, our use of the fetch-and-increment can be eliminated by using standard time stamping techniques based on an array of single-writer-multiple-reader registers. Thus, our transformation is applicable in a wide range of shared memory systems, as it does not depend on any special support for synchronization.
Furthermore, because we can replace the fetch-and-increment with time-stamping techniques based on registers, it is possible to apply our transformation to truly distributed systems without shared memory. Note that existing techniques make it possible to implement shared memory on a distributed, message-passing system as long as not more than half the processes are faulty.
The foregoing descriptions of embodiments of the present invention have been presented only for purposes of illustration and description. They are not intended to be exhaustive or to limit the present invention to the forms disclosed. Accordingly, many modifications and variations will be apparent to practitioners skilled in the art. Additionally, the above disclosure is not intended to limit the present invention. The scope of the present invention is defined by the appended claims.

Ensuring progress in a system that supports execution of obstruction-free operations