PROCESS FOR MAKING MICROCELLULAR POLYURETHANE ELASTOMERS WITH IMPROVED PROCESSABILITY

Abstract: An improved process for making pigmented, microcellular polyurethane elastomers by reacting a resin component and an isocyanate-terminated prepolymer is disclosed. The resin component includes a polyol and two chain extenders, one of which is 1,3-propanediol or a branched C3-C5 diol. The chain extender combination enables successful elastomer processing throughout a broad range (at least a 5 DEG C) of molding temperatures and surprisingly overcomes pigment-induced problems with processability and physical properties. The process is ideal for making polyurethane shoe soles and midsoles.

Description: PROCESS FOR MAKING MICROCELLULAR POLYURETHANE ELASTOMERS WITH IMPROVED PROCESSABILITY FIELD OF THE INVENTION The invention relates to a way to improve processability in the preparation of microceliular elastomers. In particular, the invention improves the processing characteristics and physical properties of microceliular elastomers that contain a pigment. BACKGROUND OF THE INVENTION Microceliular polyurethane elastomers are well known, as is illustrated, for example, by U.S. Pat. Nos. 5,728,745 and 5,250,582. These references and others teach the value of using polyether polyols having very low unsaturation levels to make the elastomers. In making shoe soles, these microceliular products are valuable because of their reduced shrinkage compared with elastomers made with conventional polyols that have higher unsaturation levels (see column 22 of U.S. Pat. No. 5,728,745). A typical microceliular polyurethane elastomer formulation reacts a polyol or mixture of polyols with water, an amine blowing catalyst, an organometallic urethane catalyst, one or more chain extenders, and a surfactant with an organic polyisocyanate (e.g., TDI, MDI, or the like). Various processes are known for combining the reactants. In the common "one-shot" approach, the "B side" components (polyols, surfactants, catalysts, chain extenders, and water) are pre-mixed before combining and reacting them with the polyisocyanate ("A side"). Another approach is called the "prepolymer method." In this method, a prepolymer is prepared, usually by reacting the polyisocyanate with a portion of the polyols, chain extenders, or both to give an NCO-terminated intermediate. The NCO-terminated prepolymer is then reacted with a resin component or "B side" (a mixture of water, catalysts, surfactant, and the remaining amounts of polyol and/or chain extender) to produce the microceliular elastomer. A pigment is often included in the formulation. Typical pigments are carbon black, titanium dioxide, iron oxides, or the like. While pigments are often a necessary part of the formulation, they can adversely affect both elastomer processability and physical/mechanical properties. We found that black pigments (e.g., carbon black) can interfere with processability of an otherwise acceptable microceliular elastomer formulation. Moreover, when products can be made, they often lack the required physical properties for high-performance applications. A possible solution is to use a mixture of chain extenders. While 1 ,4- butanediol, ethylene glycol, and diethylene glycol are commonly used as chain extenders, only one chain extender is normally used. We found that various mixtures of 1 ,4-butanediol, ethylene glycol, and diethylene glycol were only partially satisfactory in overcoming the processing problems created by the presence of a pigment, particularly carbon black, in the prepolymer approach to making microceliular elastomers (see Table 1 below). Moreover, the overall physical properties of elastomers made with chain extender mixtures also dropped dramatically compared with the properties of elastomers made using 1 ,4-butanediol or ethylene glycol alone (see Table 2 below). In sum, a better way to make microceliular polyurethane elastomers is needed. Preferably, microceliular elastomers containing pigments, especially black pigments, could be made using the prepolymer method while avoiding the processing difficulties introduced by the pigment. A preferred process would be easy to practice with conventional equipment and inexpensive, readily available starting materials. A valuable process would take advantage of the benefits of low-unsaturation polyols. Ideally, the resulting pigmented elastomers would maintain an excellent overall level of physical and mechanical properties, particularly good tensile, elongation, and tear properties, as well as high resilience and low compression sets. SUMMARY OF THE INVENTION The invention is an improved prepolymer process for making microceliular polyurethane elastomers that contain a pigment. The process comprises making a microceliular elastomer by reacting a resin component ("B side") with an isocyanate-terminated prepolymer ("A side") in the presence of the pigment to produce a microceliular polyurethane elastomer. The resin component comprises a first polyol, a first chain extender, and an additive amount of a second chain extender selected from 1 ,3-propanediol and branched C 3 -C 5 diols. The key component is the second chain extender, which is used in an amount effective to allow successful elastomer processing throughout at least a 5°C range of molding temperatures. We surprisingly found that using 1 ,3-propanediol or a branched C 3 -C 5 diol in addition to the usual ethylene glycol or 1 ,4-butanediol chain extender solves the problems of poor processability and poor physical properties that occur when a pigment is used in a prepolymer process to make microceliular polyurethane elastomers. The elastomers exhibit an excellent overall balance of physical and mechanical properties, including good tensile, elongation, and tear properties, high resilience, and low compression sets. The process is ideal for making elastomers useful as midsoles or shoe soles because it enables their fabrication over a broad range of molding temperatures. DETAILED DESCRIPTION OF THE INVENTION In the process of the invention, the resin component ("B side") comprises a first polyol, a first chain extender, and an additive amount of a second chain extender selected from 1 ,3-propanediol and branched C 3 -C 5 diols. Polyols useful in the process of the invention are prepared by ring- opening polymerization of cyclic ethers, and include epoxide polymers, oxetane polymers, tetrahydrofuran polymers, and the like. Preferred are polyether polyols made by polymerizing epoxides, especially propylene oxide and/or ethylene oxide. Particularly preferred are propylene oxide- based polyols that are capped or tipped with oxyethylene groups. The polyols can be made by any desired method, including conventional base catalysis. Preferred first polyols have a high content of primary hydroxyl end groups. Such polyols are normally made by tipping or capping the ends of an polyoxypropylene polyol with oxyethylene units. By "high-primary," we mean polyols having at least about 50% primary hydroxyl groups. More preferably, the polyols have at least about 65% primary hydroxyl groups; most preferred are polyols having at least about 75% primary hydroxyl groups. Preferred first polyols also have low unsaturation. By "low- unsaturation," we mean less than about 0.02 meq/g of unsaturation as measured by standard methods, such as ASTM D-2849-69, "Testing of Urethane Foam Polyol Raw Materials." More preferred polyols have unsaturations less than about 0.01 meq/g; most preferred are polyols having unsaturations less than about 0.007 meq/g. Polyols with very low unsaturation levels are conveniently made via double metal cyanide catalysis as described, for example, in U.S. Pat. Nos. 5,470,813 and 5,482,908, the teachings of which are incorporated herein by reference. Suitable first polyols have a nominal hydroxyl functionality within the range of 2 to 6. Preferably, the first polyol has an average hydroxyl functionality less than about 3. A more preferred range is from about 1.8 to about 3.0. In addition, the polyol preferably has a number average molecular weight within the range of about 500 to about 50,000. A more preferred range is from about 1000 to about 6000; most preferred is the range from about 2000 to about 6000. The first polyol preferably has an oxyethylene content of at least about 5 wt.%, more preferably from about 10 to about 20 wt.%, which can be present internally, as a tip, or as an endcap. Preferably, most of the oxyethylene content is located toward the end of the polyol to provide for the desirable high content of primary hydroxyl groups. The first polyol is typically the major component of the "B side." Generally, it comprises at least about 40 wt.% of the resin component. A preferred range is from about 45 to about 90 wt.%, more preferably from about 50 to about 70 wt.% of the resin component. The resin component also includes a first chain extender. Useful first chain extenders have at least two active hydrogens, and include low molecular weight diols, diamines, aminoalcohols, dithiols, or the like. Preferably, the chain extenders have number average molecular weights less than about 400, more preferably less than about 300. Diols are preferred first chain extenders, especially ones with an even number of carbons between the hydroxyl groups. Suitable chain extenders include, for example, ethylene glycol, 1,4-butanediol, diethylene glycol, dipropylene glycol, trimethylene glycol, tripropylene glycol, cyclohexanedimethanol, 1 ,6- hexanediol, ethylene diamine, ethanedithiol, and the like, and mixtures thereof. Particularly preferred are dipropylene glycol, ethylene glycol, and 1 ,4-butanediol. A very minor proportion of chain extender having 3 or more active hydrogens (e.g., glycerine) can be included if desired. The first chain extender is a minor "B" side component. Typically, it comprises less than about 30 wt.% of the resin component. A preferred range is from about 1 to about 20 wt.%, more preferably from about 3 to about 10 wt.% of the resin component. A minor yet key piece of the resin component is the second chain extender, which is selected from 1 ,3-propanediol and branched C 3 -C 5 diols and is used in an additive amount. By "additive amount," we mean that the second chain extender is used in addition to the normal amount of first chain extender. Generally, the second chain extender is used in an amount effective to allow successful elastomer processing throughout at least a 5°C range of molding temperatures. Preferred second chain extenders are even more forgiving, allowing successful elastomer processing throughout at least a 10°C range of molding temperatures. The amount of second chain extender used is preferably up to about 50 wt.% based on the total amount of chain extenders. More preferably, the amount used is within the range of about 5 to about 25 wt.%, most preferably from about 6 to about 20 wt.%, based on the total amount of chain extenders. The second chain extender is 1 ,3-propanediol (a linear C 3 diol) or a branched C 3 -C 5 diol. Suitable branched C 3 -C 5 diols include, for example, propylene glycol, 2-methyl-1 ,3-propanediol, 1 ,3-butanediol, neopentyl glycol, 1 ,3-dimethyl-1 ,3-propanediol, 1 ,2-dimethylpropanediol, and the like, and mixtures thereof. 2-Methyl-1,3-propanediol is particularly preferred. The resin component optionally includes additional polyols. Preferably, the resin component includes a polymer polyol. Suitable polymer polyols include the well-known variety prepared by in situ polymerization of vinyl monomers in a base polyol to give a stable dispersion of polymer particles in the base polyol, such as styrene-acrylonitrile (SAN) polymer polyols. Other suitable polymer polyols include PIPA and PHD polyols, which are-like the SAN polymer polyols-commercially available. These polymer polyols have polymer solids contents generally in the range of about 5 to about 50 wt.%. When a polymer polyol is included, it is preferred to use an amount within the range of 5 to about 45 wt.% based on the total amount of resin component. An isocyanate-terminated prepolymer ("A side") reacts with the resin component ("B side") in the process of the invention. The prepolymer is the reaction product of a polyisocyanate and a second polyol. The polyisocyanate is an aromatic, aliphatic, or cycloaliphatic isocyanate that contains at least two free NCO groups. Suitable polyisocyanates include diphenylmethane diisocyanates (MDIs), polymeric MDIs, MDI variants, toluene diisocyanates, hexamethylene diisocyanate, isophorone diisocyanate, and the like, and mixtures thereof. Preferred polyisocyanates are 4,4'-MDI, other MDI blends that contain a substantial proportion of the 4,4-MDI isomer, and modified MDIs made by reacting MDI with itself of another component to introduce carbodiimide, allophanate, urea, urethane, biuret, or other linkages into the structure (MDI variants). Particularly preferred are 4,4'-MDI, carbodiimide-modified MDIs, and mixtures thereof. The amount of polyisocyanate used is preferably the amount needed to give an NCO-terminated prepolymer or quasiprepolymer having a free NCO content within the range of about 15 to about 30 wt.%, more preferably from about 18 to about 28 wt.%. The prepolymer includes a second polyol, which may be the same as or different from the first ("B side") polyol. The second polyol preferably has the same general characteristics as the first, however, i.e., preferably low unsaturation (less than about 0.02 meq/g) and preferably a high content (at least about 50%) of primary hydroxyl groups. The second polyol is a minor "A side" component, preferably comprising from about 1 to less than about 50 wt.% of the prepolymer component; a more preferred range is from about 2 to about 20 wt.%. The prepolymer may include a minor proportion of one or both of the chain extender components. The prepolymer is generally made by combining the second polyol and polyisocyanate and heating the mixture at a temperature and for a time effective to produce an isocyanate-terminated prepolymer. Heating continues until the prepolymer reaches the desired content of free NCO groups. After the prepolymer has been made, it is combined with the resin component using conventional techniques to make the microceliular elastomer. The resin component is a well-blended mixture of the first polyol, the first chain extender, the second chain extender, the pigment, and other optional components such as blowing agents, surfactant, catalysts, and the like. The elastomers can be made by hand casting or machine. The "A side" and "B side" components are combined, rapidly mixed, and injected or poured into open or closed molds. The formulations described herein are well suited for use with commercial molding equipment (such as the "Green Machine") for making midsoles and shoe soles by closed molding techniques. Preferably, the process of the invention is performed in the presence of a blowing agent. Suitable blowing agents are those well known in the art of formulating microceliular polyurethane elastomers. They include "physical" blowing agents, such as low-boiling halocarbons (e.g., CFCs, HCFCs, methylene chloride) or hydrocarbons (e.g., butane, pentane), inert gases (e.g., nitrogen, argon, carbon dioxide), or the like, and "reactive" blowing agents, such as water and other active-hydrogen compounds that react with NCO groups to liberate gases. Mixtures of blowing agents can be used. Water is a particularly preferred blowing agent. The blowing agent is used in an amount needed to produce a microceliular elastomer having a density of less than 0.6 g/cm 3 . Preferably, the resulting elastomer has a density within the range of about 0.02 to about 0.5 g/cm 3 ; most preferred is the range from about 0.1 to about 0.4 g/cm 3 . The process optionally includes other conventional urethane foam components, such as surfactants, blowing catalysts, urethane catalysts, UV stabilizers, crosslinkers, antioxidants, other polyols, and/or other additives. These optional ingredients are preferably mixed thoroughly with the resin component before reacting it with the "A side" to make the elastomer. A pigment is included in the process of the invention. Suitable pigments include, for example, carbon black, titanium dioxide, transition or main group metal oxides or hydroxides (e.g., iron oxides), or the like. While any desired pigment can be included, the process is particularly valuable for use with black pigments such as carbon black. The pigment is preferably present in an amount within the range of about 0.1 to about 10 wt.%, more preferably from about 0.5 to about 5 wt.% based on the total amount of the formulation. The process of the invention enables successful elastomer processing over a wide temperature range. At minimum, elastomers can be processed throughout at least a 5°C range, more preferably at least a 10°C range, of molding temperatures. While any suitable molding temperature can be used, molding is generally performed at a temperature within the range of about 30°C to about 100°C; a more preferred range is from about 40°C to about 60°C. As Table 3 below shows, the use of 1 ,3-propanediol or a branched C 3 -C 5 diol as a second chain extender according to the process of the invention allows defect-free pigmented products to be made over a broad temperature range (see Examples 1-4). Without the second chain extender, however, the temperature window for successful processing becomes too narrow (Comparative Example 12). Interestingly, the pigment causes the processing problem. This is evident from Comparative Examples 11 and 13, which reveal good elastomer processing (with or without a second chain extender) in the absence of the pigment. The process of the invention also offers physical property advantages. As Table 3 shows, low-density microceliular elastomers made by the process have an excellent balance of properties, including good Asker C hardness, resilience, tensile, tear, and compression set properties. This is particularly evident when Examples 1-4 are compared with Comparative Examples 1-10 (see Tables 1 and 2). As Table 1 shows, the usual chain extenders (1 ,4-butanediol, ethylene glycol, diethylene glycol, or mixtures of these) rarely give a product with high enough hardness (i.e., greater than about 65 on the Asker C scale). Moreover, as Tables 2 and 3 show, the tensile strength, split tear strength, resilience, and compression set properties generally improve versus the comparative samples when an additive amount of a 1 ,3-propanediol or a branched C 3 -C 5 diol chain extender is included in the formulation. In sum, the process of the invention makes it possible to formulate-with ease-pigmented, low-density elastomers with excellent physical properties. The following examples merely illustrate the invention. Those skilled in the art will recognize many variations that are within the spirit of the invention and scope of the claims. EXAMPLES 1-4 Preparation of Microceliular Polyurethane Elastomers The "Green Machine" is used to mold 10-mm microceliular elastomer plaques by reaction injection molding, at 40 g/s, mixtures of the prepolymer ("A side") and resin component ("B side") as detailed in the Formulation below. The resin component and prepolymer stream temperatures are about 35°C. Mold temperatures are in the 40-60°C range. The products are demolded in 6-7 minutes. The pigmented samples are molded at a density of 0.40 g/cm 3 , and the clear samples (no pigment) are molded at a density of 0.36 g/cm 3 . Physical properties appear in Table 3. COMPARATIVE EXAMPLES 1-10 The procedure of Examples 1-4 is followed, except that the chain extender consists of 1 ,4-butanediol, ethylene glycol, diethylene glycol, or mixtures of these, in the relative proportions specified in Table 1. In these examples, the processing window for making defect-free parts is practically nil; the parts have splits, voids, shrinkage problems, inadequate hardness, or a combination of these defects (see Table 1). Physical properties of these elastomers appear in Table 2. COMPARATIVE EXAMPLES 11-13 In Comparative Example 11 , the standard Formulation is used, except that no second chain extender (1 ,3-propanediol or branched C 3 -C 5 diol) is included, and no pigment is included. A satisfactory (though pigment-free) product is made. In Comparative Example 12, the standard Formulation is used, except that no second chain extender is included. The sample cannot be processed successfully using mold temperatures in the usual 40°C to 60°C range. The products have poor skin quality and subsurface bubbles. In Comparative Example 13, the standard Formulation is used, including a second chain extender, except that no pigment is included. A satisfactory (though pigment-free) product is made. Table 3 summarizes the processing and physical properties of these elastomers. The preceding examples are meant only as illustrations; the following claims define the scope of the invention. Formulation Resin component ("B" side) pbw Polyol A 1 50-60 Polyol B 2 5-10 Polyol C 3 30-40 Water 0.3-0.8 Ethylene glycol 4-8 Second chain extender 0.5-2 Silicone surfactant 0.3 Black pigment 2 Amine catalyst 0.9-1.1 Organotin catalyst 0.01-0.04 Prepolvmer ("A" side: 24 wt.% NCO) 4,4 -MDI 70-80 Carbodiimide-modified MDI 4-12 Polyol A 2-8 NCO/OH index 100 1 Polyoxypropyiene diol having Mn=4000, about 20 wt.% oxyethylene content (5% internal, 15% cap), and a primary OH group content of about 85%. 2 Polyoxypropyiene triol having Mn=6000, about 20 wt.% oxyethylene content (5% internal, 15% cap), and a primary OH group content of about 85%. 3 SAN-type polymer polyol, 43 wt.% solids content, hydroxyl number 20 mg KOH/g.

Claims: CLAIMS 1. A process for the production of a microceliular polyurethane elastomer having a density of less than 0.6 g/cm 3 , which comprises reacting a resin component ("B side") with an isocyanate-terminated prepolymer ("A side") in the presence of a pigment and optionally in the presence of a blowing agent, a surfactant, a blowing catalyst, and a urethane catalyst; wherein the resin component comprises a first polyol, a first chain extender, and a second chain extender selected from the group consisting of 1 ,3-propanediol and branched C 3 -C 5 diols; wherein the prepolymer comprises the reaction product of a polyisocyanate and a second polyol, which may be the same as, or different from, the first polyol. 2. A process according to Claim 1 wherein at least one of the polyols is a low- unsatu ration polyol. 3. A process according to Claim 1 or Claim 2 wherein at least one of the polyols is a high-primary, low-unsatu ration polyol. 4. A process according to any preceding claim wherein the resin component includes a polymer polyol. 5. A process according to any preceding claim wherein the first polyol is a high- primary, low-unsaturation polyol, and the resin component includes a polymer polyol; and wherein the second polyol is a high-primary, low-unsaturation polyol, which may be the same as, or different from, the first polyol. 6. A process according to any preceding claim wherein the second chain extender is selected from the group consisting of 1 ,3-propanediol, 2-methyl- 1 ,3-propanediol, neopentyl glycol, and 1 ,3-butanediol. 7. A process according to Claim 6 wherein the second chain extender is 2-methyl-1 ,3-propanediol. 8. A process according to any preceding claim wherein the second chain extender is used in an amount effective to allow successful elastomer processing throughout at least a 10°C range of molding temperatures, said range of molding temperatures being within the range of 40°C to 60°C. 9. A process according to any preceding claim wherein the second chain extender is used in an amount up to 50 wt.% based on the total amount of chain extenders. 10. A process according to Claim 9 wherein the second chain extender is used in an amount within the range of 5 to 25 wt.% based on the total amount of chain extenders. 11. A process according to any preceding claim wherein the polyisocyanate is a mixture which comprises a major proportion of 4,4'-MDI and a minor proportion of carbodiimide-modified MDI. 12. A process according to any preceding claim wherein the pigment comprises carbon black. 13. A process according to Claim 12 wherein the carbon black is present in an amount within the range of 0.5 to 5 wt.% based on the total amount of formulation. 14. A process according to any preceding claim wherein the elastomer has a density within the range of 0.02 to 0.5 g/cm 3 . 15. A microceliular polyurethane elastomer having a density of less than 0.6 g/cm 3 produced by the process of any preceding claim. 16. A shoe sole or midsole made by the process of any preceding claim.

Inventors: DAI GIAC DZUNG, LAM JORDAN, LI XIAOHU, DAI, GIAC, DZUNG, LAM, JORDAN, LI, XIAOHU
Applicants: BAYER ANTWERPEN NV, BAYER ANTWERPEN N.V.
Application year: 2000
Main classification: C08G18
IPC codes: C08G18