A two step sulfur removal for treatment of hydrocarbonaceous fuel intended for use in a fuel cell comprising a mild hydrotreating step followed by an extraction step reduces the sulfur content in fuel to 5 ppm total sulfur or less and a fuel processor suitable for carrying out the process.

The present invention is directed to a process for reducing the sulfur content in a fuel, such as gasoline, to a very low level in order to make it suitable for use in a fuel cell.
Fuel cells offer certain advantages over conventional internal combustion engines in certain applications. Fuel cells are usually more efficient and emit less pollutants into the environment when compared to heat engines. Accordingly, fuel cell systems are being proposed for both stationary and mobile applications which have traditionally been occupied by internal combustion engines. Fuel cells with which the present invention is concerned are referred to as proton exchange membrane (PEM) fuel cells. PEM fuel cells require hydrogen as a fuel and a source of oxygen for their operation. During operation the hydrogen is contacted with a catalyst, usually a platinum based catalyst, where it is ionized and gives up an electron. The resulting electrical charge provides a source of electricity which may be used to power an electric motor.
Demonstration PEM fuel cell systems typically have used pure hydrogen as a fuel. However, hydrogen has a number of significant disadvantages as a practical fuel for commercial applications. Therefore, other fuels such as natural gas and methanol which are easily converted to hydrogen at the site of the fuel cell have been proposed, but these fuels also have serious drawbacks. For example, methanol is expensive as a fuel, lacks an extensive distribution network, and presents groundwater pollution problems. Natural gas while useful for stationary applications is less practical for wide spread use as a transportation fuel due to its storage and handling problems. Gasoline has been suggested as a suitable alternative fuel for use in fuel cells and has the advantages over other fuels of being relatively inexpensive, of already being widely available through a commercial distribution network, and of lacking the storage problems associated with gases such as hydrogen and natural gas. However, a principal disadvantage of gasoline as a fuel for use in fuel cells is its sulfur content. The catalysts used to convert the hydrogen and oxygen in the fuel cell to electricity are very sensitive to even very low levels of sulfur and are rapidly deactivated at the sulfur levels normally present in conventional gasoline which typically falls within the range of from about 10 to 50 ppm. Gasoline intended for fuel cells would require a significantly lower sulfur content than 10 ppm, usually 5ppm or less, preferably less than 1 ppm and even more preferably below 0.5 ppm.
Gasoline is generally prepared from a number of blend streams. Typical examples include butanes, light straight run, isomerate, FCC cracked products, hydrocracked naphtha, coker gasoline, alkylate, reformate, added ethers, etc. Of these, gasoline blend stocks from the FCC, the reformer and the alkylation unit account for a major portion of the gasoline pool. FCC gasoline, and if present, coker naphtha and pyrolysis gasoline, generally contribute a substantial portion of the pool sulfur.
Sulfur present in the gasoline pool may be in one of several molecular forms, including thiophenes, mercaptans and disulfides. Typical thiophenes include thiophene ( less than (CH:CH)2 greater than S) and its alkylated derivatives, and benzothiophene (alternatively thianaphthene). Typical mercaptans occurring in the sulfur-containing gasoline streams include thiophenol (C6H5SH), and the alkylthiols from ethanethiol to nonanethiol, with potentially smaller amounts of the higher alkylthiols.
A number of methods have been proposed for removing sulfur from gasoline. In general, hydrotreating is the method of choice, on account of the cost and ease of processing using the catalytic method. However, sulfur removal by hydrotreating has several disadvantages for preparing fuels suitable for use in a fuel cell. Hydrotreating requires relatively severe operating conditions which make it impractical in most instances to include the hydrotreating unit as part of a fuel processor in association with the fuel cell itself. In addition, hydrotreating converts olefins present in the gasoline which leads to octane loss. A special advantage of the process of the present invention is that the process is capable of reducing the sulfur content of the gasoline to a level suitable for use in fuel cells while minimizing octane loss. Therefore, fuels prepared according to the present invention may serve either conventional internal combustion engines or fuel cells.
According to U.S. Pat. No. 3,957,625, the sulfur impurities tend to concentrate in the heavy fraction of the gasoline and a method for removing the sulfur includes hydrodesulfurization of the heavy fraction of the catalytically cracked gasoline so as to retain the octane contribution from the olefins which are found mainly in the lighter fraction. U.S. Pat. No. 5,290,427 teaches fractionating a sulfur containing gasoline feed, and introducing each fraction in turn into a hydrodesulfurization reactor at spaced locations along the length of the reactor according to boiling point. By this method, low boiling, olefin containing fractions are treated for a relatively shorter time, and higher boiling fractions, with lesser amounts of olefins and higher amounts of sulfur containing molecules, are treated for a relatively longer time. U.S. Pat. No. 5,290,427 further teaches contacting the intermediate product from the hydrodesulfurization reaction zone with an acidic catalyst, reportedly to produce a product having a higher octane number than that of the intermediate product.
In U.S. Pat. No. 4,049,542, Gibson et al. discloses a process in which a copper catalyst is used to desulfurize an olefinic hydrocarbon feed such as catalytically cracked light naphtha. This catalyst is stated to promote desulfurization while retaining the olefins and their contribution to product octane.
In U.S. Pat. 5,059,304 a desulfurization process is described in which a naphtha feed is subjected to a mild reforming step followed by treatment with sulfur sorbent to remove the hydrogen sulfide. The reforming step is conducted at a temperature above 575xc2x0 F., and preferably above 660xc2x0 F.
The process of the present invention is particularly advantageous because, due to the very mild process conditions employed, it may be used in direct association with the fuel cell, that is, the desulfurization process may be carried out as part of an integrated fuel cell system. This is of major importance since the presence of sulfur at the levels with which the present invention is concerned may readily result from contamination during handling, such as during tankage or passage through contaminated fuel lines. Therefore, fuels which as manufactured may meet the stringent sulfur standard required for use in fuel cells, may easily become contaminated during delivery to the site of the fuel cell. With the present invention it is possible to desulfurize the fuel at the site of the fuel cell, if stationary, or in the case of a vehicle by use of an on-board fuel processor. Thus the desulfurized fuel may be fed directly into the fuel cell with minimal opportunity for contamination.
The present invention is directed to a process for desulfurizing a hydrocarbonaceous fuel intended for use in a fuel cell which comprises (a) contacting a hydrocarbonaceous fuel containing sulfur with hydrogen in the presence of a hydrotreating catalyst at a temperature of between 300xc2x0 F. and 500xc2x0 F., a total pressure between 50 psig and 500 psig, and a feed rate between 0.5 hrxe2x88x921 and 10 hrxe2x88x921 for a time sufficient to convert up to 95% of the thiophenes present in the fuel, to produce a hydrotreated hydrocarbonaceous fuel; (b) contacting the hydrotreated hydrocarbonaceous fuel with a solid adsorbent or a liquid extractant selected to rereduce the sulfur compounds remaining in the hydrotreated hydrocarbonaceous to about 5.0 ppm or less; (c) recovering a hydrocarbonaceous fuel having a reduced sulfur content; and (d) supplying the hydrocarbonaceous fuel having reduced sulfur as fuel for a fuel cell. When referring to fuel cells in this disclosure, it should be understood that the reference is to PEM fuel cells.
The present process is a multi-step process for removing sulfur from a sulfur-containing hydrocarbonaceous fuel intended for use in a fuel cell. While other hydrocarbonaceous fuels suitable for use in fuel cells may be desulfurized by the present process, the process is particularly advantageous when the hydrocarbonaceous fuel is gasoline or gasoline mixed with other hydrocarbons, such as, for example, methanol. A hydrotreating step at very mild conditions converts a portion of the sulfur compounds present and produces a hydrotreated product from which at least a significant portion, preferably substantially all, of the remaining sulfur compounds may be more easily removed by adsorption or extraction. The process of this invention may be employed as part of a fuel processor associated with the fuel cell. Gasoline streams produced using the process of the present invention may be produced which contain less than about 5 ppm total sulfur, more preferably less than about 1 ppm total sulfur, and still more preferably less than about 0.5 ppm total sulfur. Ideally the amount of amount of sulfur should be as low as possible, therefore total sulfur levels of less than 0.1 ppm would be most preferred.
The present invention is also directed to a mobile fuel processor suitable for desulfurizing a hydrocarbonaceous fuel intended for use in a PEM fuel cell used to power a transportation platform , said fuel processor comprising (a) a hydrotreating zone suitable for contacting said hydrocarbonaceous fuel with hydrogen in the presence of a hydrotreating catalyst at a temperature of between 300xc2x0 F. and 500xc2x0 F., a total pressure of between 50 psig and 500 psig, a space velocity of between 0.5 hrxe2x88x921 and 10 hrxe2x88x921 for time sufficient to convert up to 95% of the thiophenes present in the fuel and (b) an adsoption zone containing a solid adsorbent or liquid extractant capable of reducing the total sulfur compounds present in the fuel leaving the hydrotreating zone to 5.0 ppm or less. Two possible configurations of the fuel processor are possible. In one embodiment the hydrotreating catalyst and adsorbant, most likely a solid adsorbant, are located in a single reactor chamber. In a second embodiment the hydrotreating zone and the adsorption zone are physically located in separate chambers within the fuel processor.
The term transportation platform refers to a mobile platform intended for civilian or military use for transporting passengers, livestock, goods, commodities, ores, munitions, and the like, including, for example, motor vehicles, such as cars and trucks, various construction vehicles, tanks, trains, airplanes, watercraft including ships, etc.
Sulfur compounds present in gasoline occur principally as mercaptans, aromatic heterocyclic compounds and disulfides. Relative amounts of each depend on a number of factors, many of which are refinery, process and feed specific. In general, heavier fractions contain a larger amount of sulfur compounds, and a larger fraction of these sulfur compounds are in the form of aromatic heterocyclic compounds. In addition, certain streams commonly blended for gasoline, e.g. FCC feedstocks, contain high amounts of the heterocyclic compounds. Gasoline streams containing significant amounts of these heterocyclic compounds are difficult to process. Very severe operating conditions have been conventionally specified for hydrotreating processes to desulfurize gasoline streams. Adsorption processes, used as an alternative to hydrogen processing, have very low removal efficiencies, since the aromatic heterocyclic sulfur compounds have adsorptive properties similar to the aromatic compounds in the hydrocarbon matrix.
Rather than operating a hydrotreating step at high severities in order to reduce the sulfur content of a sulfur-containing hydrocarbonaceous fuel such as gasoline, the hydrotreating step in the present process is operated at very mild conditions of temperature, pressure and feed rate. Among other factors, the present invention is based on the surprising discovery that, during the mild hydrotreating step, which converts up to 95% of the aromatic heterocyclic sulfur compounds, the amount of mercaptans present in the gasoline blend actually increases while the amount of the aromatic heterocyclic sulfur compounds is reduced. Rather than hydrotreating to remove the mercaptans remaining after a substantial portion of the heterocyclic sulfur compounds are converted, these remaining mercaptans in the process are then removed by contacting the hydrocarbonaceous fuel with a solid adsorbent or a liquid extractant having an affinity for the mercaptan sulfur compounds. This contacting step to sorb the remaining sulfur compounds can be achieved at very high hydrocarbonaceous fuel yields and in the case of gasoline with little or no reduction in octane. An additional advantage of the present process over conventional processes which use a single adsorption step to remove sulfur from fuel streams is that the two step process of this invention results in less total loss of the hydrocarbons making up the fuel. The reason is that the mercaptans formed in the mild hydrotreating step of the present process contain a significantly lower hydrocarbon to sulfur ratio than the thiophene forms that would otherwise be present. For example, the following table compares the molecular weight and hydrocarbon to sulfur ratios of several sulfur-containing hydrocarbons.
The table compares the higher ratios of hydrocarbons to sulfur of three common sulfur contaminants in gasoline to ethyl mercaptan which is one of the primary forms of sulfur remaining in the fuel after the mild hydrotreating step. Thus significantly less hydrocarbons would be expected to be lost in the soption step resulting a higher yield of fuel.
The present two-step sulfur removal process is equally suited to streams containing high amounts of mercaptans, such as a coker pentane/hexane stream. Such mercaptan-containing streams may be suitably treated by contacting the stream with a solid adsorbent or a liquid extractant in an extraction process. However, the use of a very mild hydrotreating step, according to the present process, followed by an adsorption or extraction step, provides good sulfur removal, while requiring a considerably smaller processing unit for conducting the adsorption or extraction step. Thus, the use of the mild hydrotreating step is particularly advantageous for use in preparing fuels for use in a fuel cell operating in a motor vehicle.
The present process is effective for reducing the sulfur content of a gasoline or mixture containing gasoline and other hydrocarbons. As used herein, the term xe2x80x9cgasolinexe2x80x9d is intended to include hydrocarbons from individual refinery streams suitable for use as a blend stock for gasoline, or a blended gasoline stream containing two or more streams, each of which are suitable for use as a gasoline blend stock. A suitable gasoline blend stock, when blended with other refinery streams, produces a combined stream which meets the requirements for gasoline, which requirements are well documented in Federal and State regulations.
The feed to the process comprises a sulfur-containing petroleum fraction which boils in the gasoline boiling range, including FCC gasoline, coker pentane/hexane, coker naphtha, FCC naphtha, straight run gasoline, and mixtures containing two or more of these streams. Such gasoline blending streams typically have a normal boiling point within the range of 0xc2x0 C. and 260xc2x0 C., as determined by an ASTM D86 distillation. Feeds of this type include light naphthas typically having a boiling range of about C6 to 165xc2x0 C. (330xc2x0 F.); full range naphthas, typically having a boiling range of about C5 to 215xc2x0 C. (420xc2x0 F.), heavier naphtha fractions boiling in the range of about 125xc2x0 C. to 210xc2x0 C. (260xc2x0 F.-412xc2x0 F.), or heavy gasoline fractions boiling at, or at least within, the range of about 165xc2x0 C. to 260xc2x0 C. (330xc2x0 F.-500xc2x0 F.), preferably about 165xc2x0 C. to 210xc2x0 C. (330xc2x0 F.-412xc2x0 F.). In general, a gasoline fuel will distill over the range of from about room temperature to 260xc2x0 C. (500xc2x0 F.).
In addition, suitable hydrocarbonaceous fuels may include mixtures containing other hydrocarbons in association with gasoline. For example, methanol may be mixed with the gasoline, preferably in an amount of less than 10 weight percent methanol. While gasoline or mixtures containing gasoline are the preferred hydrocarbonaceous fuel for use in carrying out the present invention, other sulfur-containing fuels may also be employed. For example, suitable fuels for use in fuel cells may contain other hydrocarbons in varying amounts, such as, for example,. alcohols or aromatics.
Aromatic heterocyclic compounds include alkyl substituted thiophene, thiophenol, alkylthiophene and benzothiophene. Among the aromatic heterocyclic compounds of particular interest in this application are thiophene, 2-methylthiophene, 3-methylthiophene, ethylthiophenes, dimethylthiophenes, trimethylthiophenes, benzothiophene and dimethylbenzothiophene. These aromatic heterocyclic compounds are collectively termed xe2x80x9cthiophenesxe2x80x9d.
Mercaptans which will be removed by the process of this invention often contain from 2-10 carbon atoms, and are illustrated by materials such as 1-ethanthiol, 2-propanethiol, 2-butanethiol, 2-methyl-2-propanethiol, pentanethiol, hexanethiol, heptanethiol, octanethiol, nonanethiol and thiophenol.
Sulfur in gasoline originating from these gasoline streams may be in one of several molecular forms, including thiophenes, mercaptans and disulfides. For a given gasoline stream, the sulfur compounds tend to be concentrated in the higher boiling portions of the stream. Such a stream may be fractionated, and the heavier fraction treated using the present process. Alternatively, the entire stream may be treated. For example, light gasoline streams which are particularly rich in sulfur compounds, such as coker pentane/hexane, may be suitably treated as a blend stream which also contains a higher boiling, lower sulfur containing component. In general, gasoline streams suited for treating in the present process contain greater than about 10 ppm thiophenic compounds. Typically, streams containing more than 40 ppm thiophenic compounds, up to 2000 ppm thiophenic compounds and higher may be treated as described herein. After treatment according to the invention, the sulfur content is desirably less than about 5 ppm, preferably less than 1.0 ppm and most preferably less than 0.5 ppm.
Sulfur content in commercial gasolines are mandated for reduction. The total sulfur content of commercial gasoline to be desulfurized in the present process can exceed 50 ppm by weight, and typically can range from about 10 ppm to as much as 400 ppm sulfur. For fractions containing at least 5 vol. % boiling over about 380xc2x0 F. (193xc2x0 C.), the sulfur content may exceed about 100 ppm by weight and may be as high as 400 ppm by weight or even higher.
Standard analytical procedures are available for determining the total sulfur content of a gasoline stream. Gas chromatographic procedures are also readily available for analyzing specifically for thiophenic, mercaptan, and disulfide compounds in the stream.
In the present process a sulfur-containing gasoline stream is contacted with hydrogen in the presence of a hydrotreating catalyst at hydrotreating conditions and for a time sufficient to convert up to 95%, and preferably between about 70% and about 90% of the thiophenes present in the gasoline. Hydrotreating conditions suitable in the present process are particularly mild conditions, and include a reaction temperature between 300xc2x0 F. and 500xc2x0 F., preferably between 350xc2x0 F. and 475xc2x0 F., e.g. 400xc2x0 F.; a total reaction pressure between 50 psig and 500 psig, preferably between 100 psig and 400 psig, e.g. between 100 psig and 200 psig, with a hydrogen partial pressure between 70 psig and 140 psig; and a space velocity between 0.5 hrxe2x88x921 and 10 hrxe2x88x921, preferably between 1 hrxe2x88x921 and 4 hrxe2x88x921, e.g. 1.2 hrxe2x88x921.
The hydrogen feed to the hydrotreating process is desirably of high purity, particularly with respect to sulfur content (including H2S content). Hydrogen containing less than 10 ppm H2S is preferred, with hydrogen containing less than 500 ppm H2S being more preferred and hydrogen containing less than 0.25 ppm H2S being still more preferred. Hydrogen purity with respect to H2S (and other sulfur species) content is most preferably as low as the economics of the process permits. Using an H2S scavenger in a hydrogen recycle loop helps maintain high hydrogen purity at low hydrogen costs. In mobile applications of the process of the invention, hydrogen may be obtained from the same source of the hydrogen used to operate the fuel cell. This is usually from the on-board fuel processor.
The catalyst for use in the hydrotreating reaction zone is a conventional hydrotreating catalyst of the type used to carry out hydrodenitrification and hydrodesulfurization reactions, and having substantially no cracking activity. Those familiar with the art recognize that such catalysts generally are constituted by a metal from Group VI and a metal from Group VIII placed on a non-acidic oxide such as alumina. The commercial catalysts generally fall into one or more of the numerous nickel-molybdenum or cobalt-molybdenum, or nickel-tungsten or cobalt-tungsten families. The catalytic metals are supported by alumina or other low acidic support material. Such catalysts to be useful in the present invention do not have cracking activity, that is they are non-zeolitic non-acidic catalysts which function to promote hydrodentrification and hydrodesulfurization reactions. Such catalysts are well known in the art. The amount(s) of hydrogenation component(s) in the catalyst suitably range from about 0.5% to about 10% by weight of Group VIII metal component(s) and from about 5% to about 25% by weight of Group VI metal component(s), calculated as metal(s) per 100 parts by weight of total catalyst. The hydrogenation components in the catalyst may be in the oxidic and/or the sulphidic form. If a combination of at least a Group VI and a Group VIII metal component is present as (mixed) oxides, it will be subjected to a sulphiding treatment prior to proper use in hydrotreating. Suitably, the catalyst comprises one or more components of nickel and/or cobalt and one or more components of molybdenum and/or tungsten and/or one or more components of platinum and/or palladium.
In the process, the hydrotreated gasoline stream recovered from the hydrotreating step is contacted with a solid adsorbent or a liquid extractant to remove some or all of the sulfur compounds remaining in the hydrotreated gasoline stream. Both the solid adsorbent and the liquid extractant are characterized by an affinity for the sulfur compounds in the gasoline stream. The process for extracting the sulfur compounds into a liquid extractant involves a liquid phase which is substantially immiscible in the gasoline stream, and which preferentially absorbs or dissolves the sulfur compounds for removal from the gasoline stream. Either an aqueous or substantially organic liquid extractant may be used. For example, n-formylmorpholine may be used to extract thiophenes from the gasoline stream.
The Merox process is particularly suited for extracting light (e.g. C5xe2x88x92) mercaptans. A description of the Merox process may be found, for example, in D. L. Holbrook, xe2x80x9cUOP Merox Processxe2x80x9d, Handbook of Petroleum Refining Processes, (Second Edition) ed. Robert A. Meyers, McGraw-Hill, 1997, p. 11.29. In such mercaptan-extraction units, fresh feed is charged to an extraction column, where mercaptans are extracted by a countercurrent caustic stream. The mercaptans are converted over a metal chelate catalyst in the presence of air to a disulphide. An oily disulphide layer is recovered from the caustic. The disulfide oil can be injected into the charge to a hydrotreating unit or sold as a specialty product. A similar process is disclosed in U.S. Pat. No. 4,290,913.
The sulfur compounds remaining in the gasoline stream may also be removed by passing the gasoline stream over a solid adsorbent. Example solid adsorbents comprise materials including alumina, a zeolitic material, a clay material, included acid activated clay, activated carbon, copper on activated carbon, cobalt disulfophthalocyanine on activated carbon, metal oxides such as ZnO and magnesium aluminum oxide and UOP""s Mixed Oxide Solid Solution (MOSS) material. U.S. Pat. No. 5,807,475 discloses a process for removing sulfur containing compounds from a liquid hydrocarbon stream using an adsorbent selected from the group consisting of NiX zeolite, MoX zeolite, NiY zeolite, a smectite layered clay and mixtures thereof. U.S. Pat. No. 5,843,300 discloses a similar process using an impregnated potassium-exchanged Y zeolite for adsorbing organic compounds from a petroleum feedstock. U.S. Pat. No. 5,360,536 discloses a similar process using a clay adsorbent. These disclosures are incorporated herein by reference for all purposes. Reactants for converting mercaptans, such as DuPont""s Oxone process, are also suitable.
The entire disclosure of U.S. Pat. No. 5,582,714, which is incorporated herein by reference for all purposes, teaches an extraction process for removing sulfur from gasoline. Both the adsorption and the extraction process may be operated over a wide range of temperatures and pressures. However, pressures and temperatures near ambient (i.e. less than about 50xc2x0 C. and less than 2 atmospheres pressure) are preferred.
Preferably, no more than 5 ppm total sulfur remains in the fuel following the sorption step using either the solid adsorbent or the liquid extractant to remove the remaining sulfur. More preferred fuel product recovered from the process contain less than about 1.0 ppm total sulfur and still more preferably less than about 0.5 ppm total sulfur. Most preferably the total sulfur content of the fuel will be less than 0.1 ppm.
In one embodiment of the fuel processor, the hydrotreating zone and the adsoption zone are incorporated into a single reaction chamber. In this configuration the hydrotreating catalyst and solid adsorbant are mixed or otherwise contained in the same reactor. From a process perspective this represents the most efficient design, since the mercaptans formed in the mild hydrotreating step are immediately scavenged by the adsorbant. However, despite its efficiency this configuration has a couple of significant disadvantages. One disadvantage is that most conventional hydrotreating catalysts must be pre-sulfided in order to be active. When the hydrotreating catalyst and the adsorbant are physically located in the proximity to one another the adsorbant may desulfide the hydrotreating catalyst shortening its effective life by deactivating it. Another disadvantage of this configuration is that it is more difficult to replace the spent adsorbant without also replacing the hydrotreating catalyst. Since the hydrotreating catalyst is relatively expensive and is expected to have a significantly longer life than the adsorbant, it is not desirable to dispose of the hydrotreating catalyst at the same time as the adsorbant is replaced. In the second configuration, the hydrotreating zone and the adsorption zone are physically separated into two separate chambers within the fuel processor. The advantage of this configuration is that it is easier to replace the adsorbant when it has been spent. Since the hydrotreating catalyst has an almost indefinite life, it is advantageous to be able to replace the spent adsorbant without also having to replace the hydrotreating catalyst at the same time. A disadvantage of this configuration over the first mentioned configuration, is that a greater quantity of adsorbent would be required to scavenge the sulfur compounds present in the feed. However, in this latter configuration, the adsorbant may be contained in disposable canister which may be replaced periodically. In this configuration the fuel to be processed is first pasted to a hydrotreating reactor where the sulfur compounds are converted and is then passed to the second adsorption chamber where the converted sulfur compounds are scavenged. The following examples are presented as illustrating various embodiments of this invention and are not intended as an undue limitation on the generally broad scope of the invention as set out in the rest of this disclosure and the appended claims.

Mild hydrotreating/extraction process for low sulfur fuel for use in fuel cells


A description is given of a switching device (1) comprising a transparent substrate (3), a switching film (5) comprising a hydride of scandium and magnesium, covered with a palladium layer (7). By exchange of hydrogen, the switching film can be reversibly switched from a transparent state to a mirror-like state with zero transmission via an intermediate black absorbing state. The conversion between both states is reversible, and this phenomenon can be used, for example, in an optical switching element or sun roof.

The invention relates to an optical switching device comprising a substrate and a switching film which comprises a hydride of a trivalent metal and magnesium, which hydride can be reversibly switched from a low-hydrogen, mirror-like composition to a high-hydrogen, transparent composition by an exchange of hydrogen. The invention further relates to a switching film, applicable in an optical switching device. The invention also relates to an electrochemical switching device comprising such a switching film. The invention additionally relates to applications of such a switching device.
In the relevant switching devices, the optical properties are governed by external influences, such as gas pressure, an electric potential or current.
A switching device according to the preamble is known from the European patent application EP-A-0 871 926 filed by the applicant. The switching film which is disclosed in said application comprises hydrides of a trivalent metal, such as gadolinium, lutetium, yttrium and lanthanum, and magnesium, preferably covered with a palladium layer. Said switching films provide excellent results in terms of the large contrasts between their low-hydrogen and high-hydrogen state. To this end reference is made to xe2x80x9cContrast enhancement of rare-earth switchable mirrors through microscopic shutter effectxe2x80x9d, by Nagengast et al., Applied physics letters, volume 75, nr. 14. However, the phase separation into magnesium and rare-earth hydride as discussed in the latter publication, after repetitive cycling also results in a relatively short life-time of the switchable mirror.
It is an object of the present invention to provide an optical switching device with an improved life-time.
In accordance with the invention, this object is achieved by an optical switching device according to the preamble, which switching device is characterized in that the trivalent metal comprises scandium.
It has been found that alloys of scandium and magnesium are very good hydrogen storage materials. As the scandium and magnesium atoms are of about the same size, a so called solid solution is formed. Phase separation into magnesium and scandium hydride induced by repetitive cycling of the materials between mirror-like and transparent or black states does substantially not occur, or at least to a far lesser extent than with magnesium-rare earth alloys. Accordingly, life-time problems due to the formation of large magnesium particles in the switching layer can be avoided or at least substantially delayed.
The scandium magnesium switching film according to the present invention has a very high transmission in the transparent state and a very low transmission in the non-transparent state to almost zero. This means a high contrast, which is the ratio between both transmission values. The scandium magnesium switching film can provide for three stable states which are governed by the hydrogen content of the switching film, namely, as the hydrogen content increases: a mirror-like state, a black absorbing state, and a transparent state.
Advantageously, the switching film comprises 1-50 at. % scandium and 50-99 at. % magnesium, more advantageously 15-40 at. % scandium and 60-85 at. % magnesium and preferably 35 at. % scandium and 65 at. % magnesium.
The specific amounts of the different components in the alloy are determined by weighting the kinetics and the hydrogen uptake against each other.
Instead of an alloy of scandium and magnesium, a stack of alternating thin layers of scandium and Mg may be used, e.g. a multilayer stack of 50 Mg|Sc pairs. Such a multilayer has the additional advantage of increasing the switching speed between the optical states.
In a particular embodiment, the scandium-magnesium switching film additionally comprises another trivalent metal.
The switching film may, for example, comprise a hydride of more than one trivalent metal and magnesium, such as scandium-gadolinium-magnesium hydride, ScGdMgHx. Of course, combinations of scandium with another trivalent metal may also be used.
In an advantageous embodiment the scandium-magnesium switching film additionally comprises an element selected from the group of nickel, aluminum, chromium, silicon, iron, cobalt, silver, zirconium, niobium, tantalum and hafnium.
The addition of 1% Ni in Sc30Mg70 prolongs the life by a factor 4, while the addition of 2% Ni results in a life prolonging by a factor 11.
Switching of the switching film takes place with hydrogen. The transmission of the switching film is governed by the hydrogen content: the transmission increases as the hydrogen content increases. If molecular hydrogen gas is supplied to the switching film, the transmission increases as the hydrogen pressure increases. The hydrogen must be dissociated to atomic H. The rate of dissociation can be increased by providing the surface of the switching film with a catalytically active layer, such as a thin layer of palladium having a thickness, for example, of 5 nm. At said thickness, the palladium layer is discontinuous. The layer thickness is not critical and is chosen to be in the range between 2 and 25 nm. Thin layers of 2 to 10 nm are preferred, however, because the thickness of the palladium layer determines the maximum transmission of the switching device. In addition, the palladium layer protects the underlying switching film against oxidation.
Apart from palladium, other catalytically active metals which promote hydrogen dissociation, such as platinum, nickel and cobalt, or alloys with these metals, can be provided on the switching film.
In an advantageous embodiment the catalytically active layer comprises a layer of Ag(x)Pd(1xe2x88x92x), where x is about 0.25.
Such catalyst layer shows a significant improved behaviour in terms of cycling durability.
In order to improve the cycling durability in an alternative manner a protective layer is placed between the catalytically active layer and the switching film.
Said protective layer preferably comprises a NiZr or a ZrOH compound and has a thickness of 10 to 100 mn.
The molecular hydrogen can be passed from a gas cylinder filled with H2 to the switching film at room temperature in a simple manner. A low-hydrogen-content mirror-like switching film then changes to a transparent hydrogen-rich state. This conversion is reversible: the transparent film is converted to a mirror-like state by heating and/or evacuation of hydrogen. Said reversible conversion can take place around room temperature, or higher temperatures.
Atomic hydrogen can also be obtained in other ways, such as by electrolytic reduction of water at the switching film in accordance with the following reaction:
H2O+exe2x88x92xe2x86x92H+OHxe2x88x92
Atomic hydrogen can additionally be generated from a hydrogen plasma. In this case, a catalytically active layer, for example, of palladium is not necessary. Atomic hydrogen can also originate from another metal hydride, such as metal alloys for hydrogen storage, which are known per se.
The switching film in accordance with the invention is thin, i.e. its film thickness is less than 2 xcexcm. The film thickness of the switching film preferably ranges between 100 and 1,000 nm. As hydrogen must diffuse in the switching film, the film thickness determines the rate of fall conversion from the mirror-like to the transparent state, and conversely.
The object of the invention, i.e. to provide an optical device which can be switched electrochemically, is achieved by a device comprising a first and a second electrode separated by an ion-conducting electrolyte, the first electrode comprising a switching film of hydrides of scandium and magnesium, which switching film is provided with an electrocatalytic metal layer in contact with the electrolyte, so that by applying a potential or current between the electrodes a change in optical transmission of the switching layer is detectable when the hydride is electrochemically converted from a low-hydrogen-content mirror-like state to a hydrogen-rich transparent state by an exchange of hydrogen, and vice versa.
The optical switching device according to this embodiment of the invention is an electrochemical cell, in which one of the electrodes comprises a switching film of hydrides of scandium and magnesium, which film is in contact with a ion-conducting electrolyte via a thin catalytic metal layer. The switching film is loaded with hydrogen by electrolytic reduction of protons, or a hydrogen-containing species, such as water, or by oxidation of Hxe2x88x92-ions, by applying a potential between the electrodes. At this electrode/electrolyte interface e.g. water is reduced to atomic hydrogen. The generated atomic hydrogen (H) will cause a conversion from the mirror-like state to the transparent state. Changing the potential leads to oxidation of the transparent state into the mirror-like state. In this way a reversible electro-optical switch is obtained.
In order to increase the rate of hydriding and dehydriding, and thus the switching speed, the metal-hydride-containing switching film is provided with a thin layer of an electrocatalytic metal or alloy, such as palladium, platinum or nickel. These metals catalyze inter alia the reduction of protons to hydrogen. Other suitable catalytic metals are the alloys of the so called AB2 and AB5 types, such as TiNi2 and LaNi5. In addition, this metal layer protects the underlying switching film against corrosion by the electrolyte. As explained herein above, this layer has a thickness in the range between 2 and 25 nm. Thin layers of 2 to 10 nm are preferred, however, because the thickness of the film determines the maximum transmission of the switching device.
The electrolyte must be a good ion conductor, but it must be an isolator for electrons in order to prevent self-discharge of the device. As an electrolyte liquid, use can be made of electrolytes, such as an aqueous solution of KOH. Such a solution is a good ion conductor, and the metal hydrides are stable in it. The electrolyte may also be present in the gel or solid state.
Use is most preferably made of transparent solid-state electrolytes, because of the simplicity of the device; they prevent sealing problems, and the device is easier to handle. Both solid inorganic and organic compounds can be used. Examples of inorganic electrolytes, which are good proton (H+) conductors, are hydrated oxides such as Ta2O5.nH2O, Nb2O5.nH2O, CeO2.nH2O, Sb2O5.nH2O, Zr(HPO4)2.nH2O, ZrO(2+x)Hy and V2O5.nH2O, H3PO4(WO3)12.29H2O, H3PO4(MoO3)12.29H2O, [Mg2Gd(OH)6]OH.2H2O and anhydrous compounds such as KH2PO4, KH2AsO4, CeHSO4, CeHSeO4, Mg(OH)2 and compounds of the type MCeO3 (M=Mg, BA, Ca, Sr), in which a part of CE has been substituted by Yb, Gd or Nb. Also glasses may be used, such as alkali-free zirconium phosphate glass. Examples of good ion (H3+) conductors are HUO2PO4.4H2O and oxonium xcex2-alumina. Examples of good Hxe2x88x92-ion conductors are CaCl2/CaH2, Ba2NH and SrLiH3. An example of a solid organic electrolyte is poly(2-acrylamido-2-methyl-propane-sulphonic acid).
Various transparent materials can be used for the counter or second electrode. Examples are hydrogenated oxidic materials such as TiO2, WO3, NiOz, Rh2O3 and V2O5. Said materials can be loaded with hydrogen, by sputtering in a hydrogen atmosphere, or electrochemically in a separate step. Also thin layers of hydride-forming intermetallic AB2 and AB5 compounds, such as TiNi2 and LaNi5, can be used. Another possibility is the use of the same material as that used for the switching film, which results in the formation of a symmetrical switching device. Said materials are provided in the form of a layer with a thickness comparable to that of the switching film. The thickness is chosen in such a way that the hydrogen capacity in the second electrode is sufficient to convert the switching film from the mirror-like state to the transparent state, and vice versa.
Substrates onto which the layers of the switching device may be provided are transparent materials, such as glass, quartz, diamond, aluminium oxide or (flexible) synthetic resin. The substrate may be plane or curved.
The switching film is applied as a thin layer onto the substrate by means of conventional methods such as vacuum evaporation, sputtering, laser ablation, chemical vapor deposition or electroplating. In this respect, it is important that during and after application of the switching layer, the metal of the switching film is not subject to oxidation. In a vacuum-evaporation process, this is achieved by maintaining the pressure, in particular, of the residual gases water and oxygen, at a low level below 10xe2x88x926 to 10xe2x88x927 mbar. By subjecting the trivalent metal and magnesium to hydrogen at a pressure of 10xe2x88x922 mbar, the metals in the switching film can be converted to the hydride state with a low hydrogen content, which film has a mirror-like appearance and is non-transparent.
The catalytically active layer, for example, of Pd, and the layer of the second electrode, can likewise be applied by means of one of the above-mentioned methods.
The inorganic solid-state electrolyte can also be applied as a thin layer by one of the above-mentioned methods. Inorganic oxidic electrolytes may also be manufactured by a sol-gel process, starting from a suitable alkoxy compound. Organic electrolytes may be applied e.g. by spin coating.
As thin films of metal hydrides have a sufficient electrical conductance, a thin transparent layer of indium-tin oxide (ITO) between the substrate and the switching layer, which is usual in conventional electrochromic devices, may be omitted. In this way, the switching device according to the invention is simpler than a conventional electrochromic display.
An example of a possible layer sequence in an electrochemical optical switching device according to the invention is:
substrate|ScMgHx|Pd|KOH|TiOOHy|ITO
In this configuration, ScMgHx is initially in the low-hydrogen, mirror-like state, whereas the tungsten oxide is loaded with hydrogen: TiOOHy. The TiOOHy layer, which serves as the second electrode, is transparent. When a negative potential is applied to the ScMgHx film, OHxe2x88x92 ions and H2O are transferred through the electrolyte, and H2O is reduced at the ScMgx film. By this electrochemical reaction ScMgHx is converted to ScMgHx+xcex4, which is transparent and neutral grey, whereas TiOOHyxe2x88x92xcex4 remains transparent: the device has become transparent in this state, or is switched from the mirror-like state to the transparent state, provided the Pd layer is transparent. The switching process is reversible. When a positive potential is applied to the ScMgHx+xcex4 film, this film loses hydrogen and the device becomes mirror-like and non-transparent again. This switching process can be repeated many times and takes place at a low voltage below 10 V.
Another example of a possible layer sequence is the following layer sequence of a solid state device in which zirconium oxide is used as a proton conductor:
substrate|ScMgHw|Pd|ZrO(x+2)Hy|WO3Hz|ITO
By virtue of switching from a mirror-like, non-transparent state to a transparent state, optionally via an absorbing black state, and conversely, the switching device in accordance with the invention can be used in many applications. By virtue of this optical effect, the switching device can be used as an optical switching element, for example as a variable beam splitter, optical shutter, and for controlling the illuminance or the shape of light beams in luminaires. Dependent upon the film thickness of the switching film, this film can exhibit almost zero transmission in the mirror-like state. This enables a switching device having a great contrast to be manufactured. The switching device can also be used for data storage and in optical computing, and in applications such as architectural glass, vision control glass, sun roofs and rear-view mirrors. In the mirror-like state not only visible light is reflected, but also radiant heat or infrared light, so that such a switching device can also be used for climate control.
The switching device in accordance with the invention can also be used as a variable transmission filter on or in front of a display screen to improve the contrast of the picture.
By making a pattern in the metal-hydride layer, a thin display can be manufactured. The construction of such a display is much simpler than that of an LCD (liquid crystal display) due to the absence of an LC layer, orientation layer, retardation layer and polarization filter. By using three different trivalent metals, a three-color dot pattern can be obtained.


Oriented polyolefin composite films are disclosed comprised of core including an olefin heteropolymer core consisting essentially of an ethylene/propylene copolymer having less than 10%, by weight, ethylene and having a skin layer on at least one of its surfaces comprised of a high modulus polypropylene. These films are characterized by an oxygen and moisture vapor transmission rate greater than that of conventional polypropylene films and a tensile modulus great enough to permit their use as packaging films. This combination of properties makes these films suitable for packaging fresh produce items which continue to respire after they are harvested.

This invention relates to polyolefin films having certain unique properties. Specifically, it relates to clear polyolefin films that exhibit an increased oxygen transmission property as compared to conventional polypropylene films known to the art.
It is known that, for many applications, particularly packaging applications, polypropylene is less than totally satisfactory due to a relatively high permeability to oxygen. A great amount of effort has been expended in recent years to decrease the oxygen transmission characteristics of polypropylene. The best results to date have been obtained by metallization of the films.
There are, however, some applications in which a greater and controllable oxygen transmission rate is desired. This is particularly the case when fresh fruits, vegetables and flowers are to be packaged as these products continue to respire after they are packaged. Absence or insufficient levels of oxygen, which occurs as the oxygen originally present in the package is consumed, leads to premature senescence and spoilage of the products. At the same time, respiration leads to a build-up of moisture in the package, which can also lead to spoilage of the product if the moisture cannot escape from the package. It is thus desirable to provide polypropylene packaging wherein the oxygen content can be replenished as necessary and from which the moisture can escape.
Conventional polypropylene films of a thickness required for most applications, whether monolayer or composite, have oxygen and moisture vapor permeability values that are not sufficient to allow the optimum oxygen and moisture levels to be maintained in a sealed package containing products of the type mentioned.
It is known to prepare microporous films based on an opaque polymer mixture comprised of about 45% to 55% homopolypropylene and 55% to 45% of a copolymer of propylene and ethylene containing about 2% to 5% ethylene by weight, which films have greater oxygen permeability than films made of plain polypropylene. This polymer mixture is blended into a mixture comprised of about 40 to 60% of the polymer mixture and 60 to 40% of certain inorganic filler materials and is subjected to biaxial orientation. The filler material selected is one that causes voiding of the polymer matrix during the drawing operation. Exemplary of such voiding pigments are barium sulfate, calcium carbonate, silica, diatomaceous earth and titania. Calcium carbonate is said to be the preferred filler material. Anderson, U.S. Pat. No. 4,842,875, teaches the use of such films in the preparation of controlled atmosphere containers for use with fresh vegetables, fruits and flowers.
Microporous films known to the art as described above function reasonably well for preparing films having increased and, to a degree, controllable oxygen and moisture vapor permeability. However, the voiding pigments employed in the prior art are of a particle size large enough and are employed in concentrations great enough to result in formation of voids of such a size that the resultant films are almost totally opaque.
Polyethylene is also known to have sufficiently great oxygen transmission qualities to permit produce items to continue respiring after being packaged, but polyethylene films do not have sufficient tensile modulus to permit their use in most packaging applications.
U.S. Pat. No. 4,410,582, issued to Tsunashima et al., is directed principally to finger-tearable multilayer structures effectively usable as an adhesive tape substrate and in other applications. Although this patent discloses the use of a nucleated polypropylene in the skin, the disclosed purpose of adding the nucleating agent is to improve the transparency of the film. There is no disclosure of providing a nucleated polypropylene having any required properties for improving the OTR of the film, let alone providing the high modulus properties required in the instant invention. In fact, there is no disclosure that controlling the modulus properties of the polypropylene in the skin has any effect on OTR. It is stated in column 8, lines 15-20 that the substrate may be usable as a packaging film if a paper or metal foil is bonded to one or both surfaces. However, employing a metallized layer tends to reduce the OTR; making the films undesirable for packaging applications in which a high OTR is required.
European publication No. 0 243 965, describing an invention by Tsutomu, et al., discloses a multilayer packaging film for packaging fresh vegetables and fruits and including an antifogging additive in at least one surface layer that also has heat sealing properties. Although the surface layer is described generally as being composed of a homopolymer or copolymer of alpha-olefins, there is no suggestion that the alpha olefin should, or could by a high modulus polypropylene required for use in the instant invention. Moreover, the film disclosed in the ""965 publication teaches away from forming the core of the multilayer film from a polyolefin copolymer, by itself (see page 17).
In accordance with this invention, there are provided transparent biaxially oriented polyolefin films based on high OTR olefin heteropolymers, which films have a stiffness (i.e. tensile modulus) sufficient to have self-supporting, stand-alone characteristics such that they can be employed as packaging films. These films have increased oxygen and moisture vapor permeability, as compared with conventional, unmodified polypropylene film, and are substantially clearer than are films possessing these permeability characteristics that have heretofore been known to the art.
In accordance with this invention, there are provided transparent composite, oriented polyolefin films comprised of a core layer and at least one skin layer, said core layer being a heteropolymer of at least two xcex1-monoolefins, and having a thickness sufficient to give the composite structure stand-alone characteristics; said at least one skin layer being a high modulus polypropylene of a thickness such that its oxygen transmission rate (OTR) is higher than that of the core layer and the total composite structure having an oxygen transmission rate (OTR) of at least about 200 cc/100 sq. in/day/atmosphere, a moisture vapor transmission rate (MVTR) of at least about 0.5 gram/sq. in./day, and a tensile modulus of at least 185,000 psi in both the machine direction and cross-machine direction of the film. Most preferably the heteropolymer employed in this invention is an ethylene/propylene copolymer having less than 10% ethylene, by weight, and most preferably having about 4.5-6% ethylene, by weight.
The core layer in the composite films of this invention has a sufficient thickness to be xe2x80x9cself-supportingxe2x80x9d (hereinafter defined), independent of the surface layers.
The composite films of the invention are characterized by having oxygen and moisture vapor permeability substantially greater than those of conventional films based on polypropylene wherein the thickest layer of the film, i.e., the core layer, is polypropylene. The films of the invention are also clear and transparent so that a product packaged therein can be visually inspected for its quality without opening the package. At the same time, they have tensile properties that make them suitable for use as packaging films.
When used in this disclosure, the term xe2x80x9cxcex1-monoolefinxe2x80x9d refers to a linear unsaturated hydrocarbon monomer having one carbon-carbon double bond, which double bond is located at the end of the linear chain. The term is intended to include any such monomer having 6 carbon atoms or less, including ethylene and propylene.
The term xe2x80x9cheteropolymerxe2x80x9d is intended to mean an olefin polymer containing at least two xcex1-monoolefins. Typical of such heteropolymers are ethylene/propylene copolymers having less than 10% by weight ethylene and more preferably about 4.5 to 6% by weight of ethylene. Although other heteropolymers, i.e., butene--propylene copolymers containing about 5 to 34% by weight of butene-1 and ethylene--propylene--butene-1 terpolymers may be usable, they are less preferred than the aforementioned ethylene/propylene copolymers. Such heteropolymers can be converted to films and can be oriented within a temperature range comparable to the temperature range within which polypropylene is satisfactorily drawn to effect orientation.
The term xe2x80x9cself supportingxe2x80x9d in reference to a film or core layer means that the film or core layer can support itself without the aid of any additional supporting layers to provide the necessary strength and body and it can readily be handled without being adhered to another substrate. Such a film or core may also be referred to as a xe2x80x9cstand-alonexe2x80x9d film or core, as the case may be.
Heteropolymers such as those described are well known in the polyolefin art and are widely used as heat seal layers on commercial polypropylene composite films. In those applications, the heteropolymers comprise very thin layers on the surface of a relatively thick polypropylene core. The layers thus employed are so thin that, unless they are adhered to a core layer, they cannot be wound into a roll, nor can they be used for any purpose that requires them to be handled except when they are adhered to a supporting substrate., i.e. they are not self supporting and do not have stand-alone properties. As a general rule, when the heteropolymers are intended to serve as sealing layers, they are applied to the polypropylene core in layers of no more than about 0.1 mil in thickness, whereas the thicker polypropylene layer is usually about 0.70 to about 0.80 mils.
The heteropolymers are inherently more permeable to oxygen and moisture than is polypropylene per se. For example, a one mil layer of a typical oriented polypropylene exhibits OTR, measured according to ASTM D3985-81, on the order of about 125 to 150 cc/100 sq. in./atmosphere/day. A one mil layer of a copolymer of propylene containing 6% ethylene exhibits OTR on the order of about 300 to 350 cc/100 sq. in./atmosphere/day. However, when used as sealing layers, these latter materials are in such thin layers that they have virtually no effect on the OTR of a film to which they may be applied since the lower OTR of the polypropylene core does not permit all the oxygen that can pass through the sealant layer to pass through the polypropylene layer.
It has not heretofore been possible, however, to take advantage of the greater OTR of the heteropolymers in packaging of fresh produce items since the heteropolymers have not heretofore been shown to be amenable to the production of packaging films. These materials lack the higher degree of stereoregularity characteristic of polypropylene and thus they do not possess sufficient tensile strength and stiffness to form stand-alone films that can be employed as packaging films.
In accordance with this invention, it has been found that, by combining the heteropolymer with a high modulus polypropylene species, a clear, self supporting film can be prepared that has stand-alone properties and a sufficiently high OTR that it can be used for packaging fresh fruits, flowers and vegetables that require a continuous refreshing of the oxygen supply in the closed package. The high modulus polypropylene species can be combined with the heteropolymer by layering a core of the heteropolymer on either or both of its surfaces with a skin of the high modulus polypropylene species. Such composite films are clear, self-supporting films having a sufficiently high OTR that they can be used for packaging fresh fruits, flowers and vegetables that require a continuous refreshing of the oxygen supply in the closed package. Most preferably the sole polymer in the core is the heteropolymer and most preferably the heteropolymer consists essentially of an ethylene/propylene copolymer having less than 10%, by weight, ethylene, and more preferably about 4.5-6% by weight ethylene.
As stated above, when the heteropolymers employed in this invention are applied as heat seal layers on a polypropylene core, their greater OTR has virtually no effect on the OTR of the composite, i.e., the composite OTR is unchanged from that of a polypropylene core of the same thickness without the skins. This appears to be a rather straight-forward observation, readily explainable by the substantially greater thickness of the core and its substantially lower OTR, which does not permit all of the oxygen that can pass through the skin in a specified time period to continue on through the core in that same time period.
It is known that OTR is, to a degree, dependent on the thickness of a polyolefin film and increases in almost a straight line relationship to the thickness as the thickness decreases. Thus, by decreasing the thickness of a polypropylene layer and increasing that of a heteropolymer layer, a point will be reached at which the OTR of the polypropylene layer will be greater than that of the heteropolymer layer. At this point, based on the observation of the OTR effect of heteropolymer skins on a polypropylene core, one would anticipate that the OTR of the composite would be that of the heteropolymer core.
However, upon examining the results of the OTR testing of the films prepared in carrying out the work that led to this application, it was found that, when a thin polypropylene layer (or layers) is (are) applied to a high OTR core, the OTR of the composite is not the same as would have been expected of the core layer without the polypropylene skin(s). The OTR of the composite is less than that of the core layer alone even though the polypropylene skin layers are so thin as to have an OTR greater than that of the core.
This effect is explainable by the fact that, even though the OTR of the polypropylene skin layers is now greater than that of the heteropolymer core, the difference between the OTR values of the core and the skins is not nearly so great as that between the polypropylene core and the heteropolymer skins of conventional films. In films according to the invention, the time required for the oxygen to pass through the skin layer becomes a factor with respect to the composite OTR of the film. Since the OTR of a heteropolymer skin on prior art films is so high, the time required for oxygen to pass through is so small that it has a negligible effect on the composite OTR.
Since OTR is a rate, it can be represented by distance (i.e., film thickness) divided by time required for oxygen to pass through the film, i.e   OTR  =      thickness    time
Accordingly, the time required is equal to the thickness divided by the OTR, i.e.   Time  =      thickness    OTR
For a three layer film, the composite OTR is the composite film thickness divided by the sum of the times required for oxygen to pass through the individual layers, i.e.   compositeOTR  =                    thickness                  T          1                            OTR        1              +                  T        2                    OTR        2              +                  T        3                    OTR        3
Where T1 and T3 represent the thicknesses of the skin layers and T2 represents the thickness of the core layer of the composite while OTR1 and OTR3 and represent the OTR values of the corresponding skin layers and OTR2 represents the OTR value of the core layer. Since the OTR and the thickness of the individual layers can be easily measured, it is not necessary to know the absolute time required for the oxygen to pass through each layer.
With the above knowledge, it is now possible to tailor films to have a predetermined OTR for packaging specific items of produce. If one has sufficient information concerning the respiration rate of a product, one can vary the layer thicknesses to approximate that respiration rate sufficiently accurately for successful commercial use.
As noted earlier, the preferred heteropolymers usable in this invention are ethylene/propylene copolymers having less than 10%, by weight, ethylene and most preferably about 4.5-6% by weight ethylene. The use of such ethylene/propylene copolymers allows for the attainment of higher film OTR values than are possible with, for example, butene/propylene copolymers employing a percentage, by weight, of butene that is the same as the percentage, by weight, of ethylene employed in the ethylene/propylene copolymers. Thus, with the same amount of ethylene as butene in the copolymer, the OTR values are much higher in the films employing the ethylene/propylene copolymer. It is possible to achieve the same or similar OTR value by using either a butene/propylene copolymer or an ethylene/propylene copolymer; however, the percentage by weight of butene employed in the butene/propylene copolymer needs to be greater that the percentage by weight of ethylene employed in the ethylene/propylene copolymer. The use of a greater percentage of butene in the butene/propylene copolymer causes an undesired loss in the stiffness of the film.
In order for the films to have a stiffness sufficient to permit their use as packaging films, a high modulus polypropylene or polypropylene composition is employed in the skin layer. Reference to xe2x80x9chigh modulus polypropylenexe2x80x9d means a polypropylene or a polypropylene composition, which, when formed into a 70 gauge (0.7 mil thick) film drawn 7xc3x97 by 7xc3x97 on simultaneous draw equipment provides the film with a modulus value of at least about 450,000 psi in both the machine direction and cross-machine direction of the film. The most preferred high modulus polypropylene in this invention is a polypropylene homopolymer having a melt flow rate (MFR) in the range of about 2-4 dg/minute. Such a homopolymer is extremely well suited for the production of biaxially oriented film in accordance with this invention.
Suitable and preferred polypropylenes for use in the skin layer, which exhibit the required high modulus characteristics include the newly developed high crystallinity polypropylene. This material, available under several trade names, is defined as having an isotactic index of at least 93%, i.e., the polymer contains no more than about 7%, and preferably no more than about 2 to 6% xylene soluble materials. Typical high crystallinity polypropylene is further characterized by higher stiffness, greater surface hardness, lower heat deflection at high temperatures, lower heat shrinkage and better creep properties than conventional isotactic polypropylenes, which have isotactic index generally less than 93%. Typical high crystallinity polypropylenes that can be employed include ACCPRO 9117, ACCPRO 9119 and ACCPRO 9218 (all available from Amoco Polymers, Alpharetta, Ga.); Chisso HF 5010 and Chisso XF 2805 (available from Chisso Chemical Co. Ltd., Tokyo, Japan) and FF035C, a nucleated isotactic polypropylene (available from Aristech Chemical Co., Pittsburgh, Pa.).
Reference to the specified isotactic index throughout this application refers to the value determined by NMR spectroscopy.
High modulus polypropylenes also include blends of conventional isotactic polypropylene with low molecular weight hydrocarbon resins. Inclusion of about 5% or more of the low molecular weight hydrocarbon resin in a conventional polypropylene increases the modulus to the desired high modulus range. The upper limit of low molecular weight resin is about 25% by weight.
The low molecular weight hydrocarbon resins are hydrogenated or unhydrogenated resins derived from olefin monomers, such as the resins derived from terpene monomers, coal tar fractions and petroleum feedstocks. In general, the low molecular weight resins are characterized by a molecular weight less than about 5000, a Tg of about 50 to 100xc2x0 C. and a softening point less than about 140xc2x0 C. Suitable resins include those prepared from terpene monomers (e.g., limonene, alpha and beta pinene, such as Piccolyte resins from Hercules Incorporated, Wilmington, Del., and Zonatac resins from Arizona Chemical Company, Panama City, Fla.). Other low molecular weight resins are prepared from hydrocarbon monomers and mixtures thereof, such as C5 monomers (e.g., piperylene, cyclopentene, cyclopentadiene, and isoprene), oligomerized C5 monomers, particularly the thermally oligomerized C5 monomers such as the hydrogenated thermally oligomerized cyclopentadiene resins sold under the trade name Escorez (for example Escorez 5300) by Exxon Chemical Co. of Baytown, Tex. Others are prepared from C9 monomers, particularly the monomers derived from C9 petroleum fractions which are mixtures of aromatics, including styrene, methyl styrene, alpha methyl styrene, vinyl naphthalene, the indenes and methyl indenes and, additionally, pure aromatic monomers, including styrene, xcex1-methyl-styrene and vinyltoluene. Examples of these resins include hydrogenated xcex1-methyl styrene--vinyl toluene resins sold under the trade name Regalrez by Hercules Incorporated of Wilmington, Del. The hydrogenated C9 and pure monomer resins are preferred. Particularly preferred are the hydrogenated cyclopentadiene resins and the hydrogenated aromatic resins derived from pure aromatic monomers, e.g., the hydrogenated xcex1-methyl styrene--vinyltoluene copolymers.
Another polypropylene having the high modulus required of the skin layers of the films of this invention can be prepared by blending conventional commercial isotactic polypropylene prepared via Ziegler-Natta catalysis with a polypropylene prepared by use of a metallocene catalyst. Such blends and films prepared therewith are described in copending and commonly assigned U.S. patent application Ser. No. 09/055,389, filed Apr. 6, 1998. In the referenced application, it is shown that very significant increases in tensile modulus are realized when these polymer types are blended. Such modulus improvements are realized with blends containing 10 to 90% of the Ziegler-Natta polymer.
Films according to the invention are generally of a total thickness from about 0.5 to about 2 mil. A high modulus polypropylene skin layer can be applied to one or both surfaces of the heteropolymer core. Preferably, it will be applied to both surfaces and, most preferably, the skin thickness will be the same on both sides.
Although the results vary with different heteropolymer cores, the combination of increased OTR and satisfactory tensile modulus sought in the films of the invention is generally realized when the polypropylene skin layer or layers constitute about 14 to 56% of the thickness of the composite structure. Preferably a skin layer will comprise about 30 to 56% of the thickness of the composite structure. It is surprising to find that a skin layer of high modulus polypropylene as thin as that employed in the films of this invention can impart to the entire film the modulus required of a packaging film. When a skin layer is applied to both surfaces, the two layers should each be of a thickness such that its OTR is greater than that of the core layer.
As suggested hereinabove, the films of this invention have several properties that make them unique and desirable for the packaging of fresh produce products. First, they are of a stiffness required of packaging materials generally. Thus, they exhibit a tensile modulus greater than about 185,000 psi in both the machine-direction and cross-machine direction. While this is not as high as the modulus of conventional polypropylene films in which the polypropylene is the primary component, it is sufficient for many packaging applications. In addition, the films exhibit an OTR greater than 200 cc/100 sq. in./day/atmosphere; a level at which the packaged product can continue to respire for a reasonable time after it has been harvested, thus assuring a reasonable shelf life for the product. Thirdly, the films are found to have a moisture vapor transmission rate greater than that of conventional polypropylene films, whereby they permit the escape of moisture generated by the produce product""s continued respiration after it is packaged. Finally, the films retain a high degree of clarity and transparency, so that the packaged product can be inspected for its appearance by the potential purchaser
The multilayer films of the invention can be prepared by coextrusion, extrusion coating or lamination. The preferred method is coextrusion wherein the various layers are extruded simultaneously through a multi-layer die and immediately brought together in the molten state so that they are permanently bonded upon solidifying.
Film forming and drawing to effect orientation can be carried out by conventional techniques, i.e. the well known tubular (bubble) process or the equally well known tenter process can be employed. When the films are prepared by the bubble process, the draw is effected simultaneously and uniformly in the machine and cross directions to about 3xc3x97 to 7xc3x97 and preferably 5xc3x97 to 7xc3x97. Using the tenter process, drawing is carried out sequentially to about 3xc3x97 to 7xc3x97 in the machine direction and to about 7xc3x97 to 11xc3x97 in the cross direction.
The heteropolymer core of the composite film can have the high modulus skin on one or both of its surfaces. In the embodiment of the invention wherein the high modulus polypropylene skin is present on only one surface of the heteropolymer core, the other surface can have no skin at all or it can have a different functional layer. Functional layers that can be employed as the other layer include such layers as, e.g, a heat seal layer. Such a layer will be of a material of lower melting point than the core so that when heat is applied to effect the seal, the orientation of the core layer will not be disturbed. A commonly used heat seal layer is comprised of a terpolymer of propylene, ethylene and butene-1, unless, of course, that is the core heteropolymer. Other polymers that can be employed as a heat seal layer include polyvinyl or polyvinylidene chloride.
Another frequently used functional layer is a cold seal layer. In this layer, the polymer component will frequently be the same as that of the core layer, though, here again, it need not be the same. To prepare a cold seal film, a cold seal adhesive, such as, e.g., a rubber adhesive is applied to the surface of the oriented film.
Another commonly used functional layer is a slip layer to facilitate handling of the film during later converting operations. Such a layer is comprised of a polymer containing a slip agent such as a high molecular weight fatty acid amide. A functional layer may also contain an antiblock additive to facilitate unwinding of the film after it has been wound at the terminus of the film manufacturing process. These types of layers can be comprised of polypropylene if the layer is more oxygen permeable than the heteropolymer core and accordingly will not decrease the OTR of the total film structure. Preferably, such layers will be made of the same heteropolymer as is employed in the core layer.