Moisture-curable, single-component sealants are used to provide liquid and gaseous barriers in various applications. Such applications include bonding of dissimilar materials, sealing of expansion joints, assembling curtain walls and side walls, weatherproofing, constructing roofing systems, and sealing the perimeters around doors, windows and other building components (i.e., perimeter sealing). Weather proofing applications might include truck trailers, buses, recreational vehicles, and utility trailers. Dissimilar materials that may, for example, be sealed and bonded with the inventive sealants include cement-containing products, metals, plastics, glass, and composites of any of the foregoing. The inventive sealants may also be used, for example, for the maintenance and repair of trailers, recreational vehicles, and rail units. Commercially viable sealants strike an acceptable balance among mechanical and rheological properties, such as cure speed, shelf life, extrusion rate, sag resistance, elongation, modulus, tensile strength, adhesion to various surfaces, and thermal and ultraviolet light stability.
Two major moisture-curable, single-component sealant technologies have been found to be useful in such applications. These are silicone-based and urethane-based sealants. Silicone-based and urethane-based sealants each have many beneficial characteristics, yet each have different, but equally undesirable characteristics. The silicone-based sealants generally exhibit superior weather resistance, mechanical and rheological properties (e.g., elastic recovery), heat resistance and adhesion to a variety of substrates, and tend to be nonfoaming in high humidity. Even so, silicone-based sealants tend to be very difficult to compound due to the incompatibility of silicones with many sealant additives, to be unpaintable, to accumulate dirt and dust, and to contain fluids that stain porous substrates. In addition, silicone-based sealants containing alkoxy groups tend to cure slowly as they age in the package, resulting in short sealant shelf lives.
Urethane-based sealants generally exhibit superior mechanical and rheological properties, and adhere well to a variety of substrates, as do silicone-based sealants. Unlike silicone-base sealants, however, urethane-based sealants tend not to appreciably accumulate dust, tend to be relatively easy to compound as compared to silicone-based sealants, and tend not to stain substrates. Even so, urethane-based sealants generally discolor upon exposure to ultraviolet light, foam when cured in hot humid environments, and cannot accommodate large joint movements. In addition, while urethane-based sealants made using aliphatic isocyanates tend not to discolor, they tend to have relatively slow cure rates. Thus, sealants that maximize the beneficial characteristics of each technology, yet which minimize the undesirable characteristics are needed. These sealants are commonly referred to as "hybrids".
Hybrid sealants based on moisture-curable hydrolyzable alkoxysilane functional polyether urethane prepolymers have been proposed in an attempt to combine many of the beneficial properties of each of the urethane-based and silicone-based technologies, while avoiding or minimizing the undesirable properties of each technology. Such prepolymers contain both hydrolyzable silyl groups which cross-link by a silane polycondensation reaction in the presence of moisture, and other functional groups. Since moisture is typically present in the atmosphere, sealants containing these prepolymers may be referred to as "atmospheric-curable sealants." Hybrid sealants are typically made by compounding the moisture-curable alkoxysilane functional polyether urethane prepolymers with rheological modifiers, adhesion promoters, oxidative stabilizers, plasticizers, and cure catalysts.
The moisture-curable alkoxysilane functional polyether urethane prepolymers may be prepared by a variety of methods, including the reaction or addition of isocyanate functional polymers (i.e., polyether urethane polymers) with amino alkylalkoxysilanes. This reaction results in the termination of some or all pendant isocyanate groups of the polyether urethane prepolymer with the amino group of the amino alkylalkoxysilane. By way of definition, the termination of pendant isocyanate groups with the amino group of the amino alkylalkoxysilane is termed "end-capping," the resulting polymer is termed an "endcapped" or "silylated" polymer, and the molecule used to terminate the isocyanate groups is termed the "endcap". The polyether urethane prepolymer is generally prepared by reacting a polyol in the presence of a catalyst with an isocyanate to form the polyether urethane prepolymer.
Endcaps useful to terminate the isocyanate groups of polyurethane prepolymers containing the adduct of amino alkylalkoxysilanes and maleic acid esters having an alkyl group with less than four carbon atoms have been proposed. The amino groups utilized are primary amines, such as gamma-aminopropyltrimethoxysilane (hereinafter "APTMS"). APTMS must be used as a precursor when used as an endcap because APTMS has active hydrogen atoms that tend to associate with polar groups, such as urethane linkages in the prepolymer. These associations result in compounding difficulties like building of viscosity and slower cure times.
U.S. Pat. No. 5,364,955 (Zweiner et al.), U.S. Pat. No. 5,866,651 (Moren et al.), U.S. Pat. No. 3,033,815 (Pike et al.), and European Patent Application Nos. EP 0 831 108 A1 (Waldman et al.) and EP 0 864 575 A2 (Roesler et al.) disclose endcaps, including aspartic acid ester endcaps formed from maleic acid esters, and primary amino alkylalkoxysilanes. Specifically, these patents and applications disclose urethane prepolymers reacted with endcaps formed from dimethyl, diethyl, and/or dibutyl maleic acid esters and amino alkylalkoxysilanes. These patents and applications disclose that these endcaps may be prepared by an addition reaction, known as Michael addition, of the amino alkylalkoxysilane with the beta-olefinic carbon atom of the dimethyl, diethyl and/or dibutyl maleic acid ester.
Sag resistance and elongation tend to be low, and extrusion rates and tensile strength tend to be high for conventional dialkyl maleate endcaps. None of the above patents and patent applications discloses dialkyl maleic acid ester endcap precursors having alkyl groups containing more than four carbon atoms.
In addition, conventional endcapped polyurethane prepolymers may utilize polyether diols or triols. Polyether diols conventionally used typically have relatively low molecular weights. This is believed to be because conventional polyether diols having higher molecular weights, for example, molecular weights of 6000 or higher, tend to have undesirably high monol contents. While the presence of at least some monol is typically unavoidable in polyether diols, a relatively high monol content is highly undesirable because monols react with isocyanates thereby reducing crosslinking and curing of the prepolymer. Thus, conventional polyether diols are generally commercially available having only relatively low molecular weights.
Conventional polyether diols typically have, for example, monol contents of about 6% by weight for a polyether diol of about 2000 molecular weight, and of about 31% by weight for a polyether diol of about 4000 molecular weight. Lawry, B. D. et al., "High Performance Moisture-Cured Systems Based in Acclaim Polyether Polyols", presented at The Adhesives and Sealant Council's 1996 Int'l Conference, San Francisco, Calif., Nov. 5, 1996. For example, typically polyether diols having molecular weights of only about 2000 are used. Conventional polyether diols having higher molecular weights of about 4000 tend to have poor cure rates or do not cure. Even conventional diols with lower molecular weights unavoidably have some amounts of undesirable monols. Using lower molecular weight diols, while having the advantage of lowering the monol content and improving the cure rate, necessitates using more isocyanates and accordingly more amino alkylalkoxysilanes which are expensive.
U.S. Pat. No. 5,866,651 (Moren et al.) and European Patent Application Nos. EP 0 831 108 Al (Waldman et al.) disclose endcapped polyether urethane prepolymers made from conventional polyether diols having relatively low molecular weights. In particular, U.S. Pat. No. 5,866,651 teaches using ethylene oxide and propylene oxide having average molecular weights ranging from about 2000 to about 8000, and more preferably from about 3000 to about 6000. European Patent Application No. EP 0 831 108 A1 teaches using polypropylene glycols with average molecular weights ranging from 500 to 6000, and more narrowly from 1000 to 4000. None of these teaches using polyether diols having average molecular weights of from about 6000 to about 20000, and more preferably from about 8000 to about 12000. In addition, none of these teaches a monol content of any kind. Even further, none of U.S. Pat. No. 5,364,955 (Zweiner et al.), U.S. Pat. No. 5,866,651 (Moren et al.), U.S. Pat. No. 3,033,815 (Pike et al.), and European Patent Application Nos. EP 0 831 108 A1 (Waldman et al.) and EP 0 864 575 A2 (Roesler et al.) teach a sealant having alkyl groups with more than four carbon atoms and high molecular weight, low monol content polyols.
What is desired therefore is a single-component, moisture-curable prepolymer and sealant containing silylated polyether urethane prepolymers prepared from the adduct of at least one dialkyl maleic acid ester having alkyl groups containing more than four carbon atoms and a primary amino alkylalkoxysilane to form a single-component, moisture-curable sealant. What is further desired is a single-component, moisture-curable prepolymer and sealant containing silylated polyether urethane prepolymers prepared from such adducts and reacted with polyether diols having relatively high molecular weights and relatively low monol contents. What is even further desired is a prepolymer and sealant as described above having improved mechanical and rheological properties.