Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Reexamination Certificate
2000-01-19
2002-07-16
Sergent, Rabon (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Cellular products or processes of preparing a cellular...
C521S130000, C521S160000, C521S176000, C521S174000, C528S059000, C528S061000, C528S064000, C528S065000, C528S063000, C528S067000, C528S076000, C528S060000
Reexamination Certificate
active
06420445
ABSTRACT:
TECHNICAL FIELD
The present invention pertains to polyurethene and polyurethane/urea heat-cured elastomers prepared by the chain extension or moisture cure of isocyanate-terminated prepolymers. More particularly, the present invention pertains to prepolymers prepared by reacting a stoichiometric excess of a di- or polyisocyanate (other than toluene diisocyanate) with a polyol component comprising a high molecular weight, low unsaturation polyoxypropylene polyol and a very low molecular weight polyol. The subject elastomers have improved hardness, resilience, tear strength, and compression set compared with otherwise similar elastomers having the same hard segment content. Films prepared from the compositions have exceptional tensile and tear strengths.
BACKGROUND ART
Polyurethane heat-cured elastomers may, in general, be subdivided into two major classes distinguished by the type of monomeric isocyanate used to prepare the elastomer precursor prepolymer. When methylene diphenylene diisocyanate (MDI) or MDI variants are used to form the prepolymers, the isocyanate group to polyol hydroxyl equivalent ratio may be quite high. The range of isocyanate group content possible provides flexibility in formulating. However, the high reactivity of the MDI isocyanate groups generally requires use of diol chain extenders in cast elastomer systems, as amines with suitable reactivity are not commercially available. To maintain a desirable balance of properties, elastomers prepared from MDI are thus polyurethane elastomers and not polyurethane/urea elastomers which have a distinct status in the art.
Heat-cured polyurethane/urea elastomers should not be confused with reaction injection molded polyurethane/urea (RIM) systems. In the latter, MDI and modified MDIs are generally used in conjunction with a reactive diamine such as diethyltoluene diamine and injected into highly rigid molds at high pressures. Prepolymers are generally not used in such systems except in minor amount, since in the very short period prior to gelation, the rapidly reacting mixture must traverse the entire, often complex mold. Thus, low viscosity systems are desired, in conjunction with very high pressure, short duration injection. RIM processes have acquired a separate status in the art. RIM system are not heat-cured, but rather rapidly cure without heat.
Toluene diisocyanate (TDI) based elastomers are the second major heat-cured elastomer class, and the largest class in terms of elastomer produced. Approximately 65 percent of heat-cured polyurethane elastomers are TDI-based systems. In practice, TDI-based prepolymers having high isocyanate group content are seldom used, and amine rather than diol chain extenders are employed to provide hard segments having urea linkages. The resulting elastomers are thus polyurethane/urea elastomers. High isocyanate group contents are generally avoided with TDI-based prepolymers due to the volatility of TDI.
Due to the limited NCO/OH ratio, the % NCO content of the isocyanate-terminated prepolymers derived from TDI is limited to a maximum of about 10% by weight. At such limited isocyanate contents, formulation flexibility is reduced. Moreover, as the urea hard segment content is related to the isocyanate group content in amine cured systems, preparation of elastomers with high tensile strength and other desirable physical properties is rendered more difficult.
Many attempts have been made to increase polyurethane elastomer physical properties, and many have been successful in elevating certain physical properties, but often at the expense of others. For example, in U.S. Pat. No. 4,934,425 the preparation of polyurethane/urea elastomers having improved dynamic properties is exemplified. The elastomers were prepared from TDI-based prepolymers containing a high molecular weight polytetramethylene ether glycol (PTMEG) of 2000 Da molecular weight and a moderate molecular weight PTMEG of 1000 Da molecular weight. Elastomers prepared from the blended prepolymers showed considerable im provement in resistance to heat build-up in rubber tire applications, indicative of lower hysteresis. However, while dynamic properties improved, no improvement was noted with respect to tensile strength, elongation, or hardness.
In U.S. Pat. No. 3,963,681 the preparation of polyurethane/urea elastomers with improved cut-growth and flex-crack resistance for tire applications is disclosed. The TDI-based prepolymers were prepared from a blend of a very high molecular weight PTMEG (molecular weight above 4500 Da) and a moderate molecular weight PTMEG, the blend having an average molecular weight of between 1000 Da and 4500 Da. The molecular weight of the higher molecular weight PTMEG is required to be higher than the “critical molecular weight” of PTMEG polymers, while the molecular weight of the lower molecular weight component must be less than this value. Tensile strengths as well as cut-growth were improved. However, not all polyether polyols are known to have a “critical molecular weight.” Furthermore, the molecular weights are very critical; blends of 800 Da PTMEG and 3800 Da PTMEG are disclosed as being unsuitable, for example.
U.S. Pat. No. 5,077,371 addressed hardness build in polyurethane/urea cast elastomers. The rate of hardness build is important as cast elastomer parts cannot be demolded prior to developing adequate green strength without risk of damage to the parts. Rapid development of hardness allows more rapid demold. Cure can be completed in an oven outside the mold, and production rates increased correspondingly. The addition of the dimer of toluene diisocyanate (TDI dimer) in amounts of 0.3 to 6 weight percent of total isocyanate to the isocyanate component used to prepare bimodally distributed PTMEG prepolymers was found to increase the rate of hardness build. Unfortunately, of the remaining physical properties, hardness, elongation, and rebound remained virtually unchanged, tensile strength and particularly elongation actually decreased, and only tear strength showed significant improvement. The necessity of preparing the TDI dimer adds additional time and expense to elastomer formulation.
PTMEG has been traditionally used in preparing high performance polyurethane/urea elastomers, as illustrated by the three foregoing U.S. Pat. Nos. 4,934,425, 3,963,681, and 5,077,371. PTMEG is a premium priced polyol. However, PTMEG continues to be used today despite its much greater cost as compared to polyoxyalkylene polyols such as polyoxyethylene glycols and polyoxypropylene glycols, due to the desirable physical properties of the polyurethane which may be obtained through its use.
Moisture-cured polyurethane elastomers are often used as caulks and sealants. Rather than incorporating a diamine to react with isocyanate to form the linking urea hard segments, moisture-cured elastomers rely on the reaction of free isocyanate groups with moisture to form urea linkages. Many moisture-cured films and sealants exhibit relatively low physical properties, particularly tensile strength and/or tear strength, and therefore improvement in these and other properties is desired.
The majority of polyoxyalkylene polyether polyols are polymerized through base catalysis. For example, polyoxypropylene diols are prepared by the base catalyzed oxypropylation of a difunctional initiator such as propylene glycol. During base catalyzed oxypropylation, a competing rearrangement of propylene oxide to allyl alcohol continually introduces an unsaturated, monofunctional, oxyalkylatable species into the reactor. The oxyalkylation of this monofunctional species yields allyl-terminated polyoxypropylene monols. The rearrangement is discussed in Block and Graft Polymerization, Vol. 2, Ceresa, Ed., John Wiley & Sons, pp. 17-21. Unsaturation is measured in accordance with ASTM D-2849-69 “Testing Urethane Foam Polyol Raw Materials,” and expressed as milliequivalents of unsaturation per gram of polyol (meq/g).
Due to the continual creation of allyl alcohol and its subsequent oxypropylation, the average functionality of the polyol mixture de
Barksby Nigel
Clift Susan M.
Lawrey Bruce D.
Bayer Antwerp N.V.
Brooks & Kushman P.C.
Sergent Rabon
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