Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Patent
1998-02-02
2000-02-01
Seidleck, James J.
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
525222, 525238, 525218, C08J 302, C08K 320, C08L 3300
Patent
active
060204168
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention pertains to aqueous blends of colloidally dispersed polymers for use in making organic coatings which are hard and ductile at ambient temperature, and which remain stiff and elastic at temperatures well above their film-formation or drying temperature. In particular, the invention relates to blends of high-molecular-weight, thermoplastic polymers which are capable of developing these mechanical properties without conventional amounts of volatile organic coalescing aids and without the need for chemical cure.
BACKGROUND OF THE INVENTION
The performance of many coatings such as paints are governed by the mechanical properties of one or more organic polymers which serve either as the coating per se or as a binder for other components of the coating, such as pigments and fillers. For use in automotive paints, it is desirable that such polymers be hard at ambient temperature, as illustrated, for example, by a Knoop hardness number (KHN) greater than about 5 MPa (L. Dillinger, "Hardness Testing", LECO Corp., 3000 Lakeview Ave., St. Joseph, Mich.; and ASTM D 1474-68). It is also desirable that such polymers retain a certain degree of stiffness and elasticity at use temperatures of 60.degree. C. or higher, for example, exhibit a Young's modulus (E) greater than 10 MPa Amorphous polymers exhibit such properties only when their glass transition temperature T.sub.g is equal to or greater than the uppermost use temperature. The present invention relates to a new class of coating formulations to achieve these objectives.
Coatings and films are commonly characterized as either brittle or ductile depending upon the manner in which they fail under tensile loads (I. M. Ward, Mechanical Properties of Solid Polymers, Chap. 12, John Wiley & Sons, London, 1971). Brittle failure occurs at relatively small strains, for example.ltoreq.20%, following a monotonic increase in the load. By contrast, ductile failure occurs at greater elongations, following a peak in the load/extension curve which is indicative of necking. Highly cross-linked, thermosetting polymer resins are generally brittle, often with elongations<10%, whereas high-molecular-weight, linear thermoplastic polymers typically display a change in failure mode with temperature. At temperatures much below T.sub.g most thermoplastics are brittle, but they undergo a transition from brittle to ductile failure as the temperature is increased, and the temperature of this transition generally increases with increasing strain rate.
Ductility (especially elongations>10%) is a desirable property for coatings on flexible substrates and also for coatings, such as automotive paints, on metals, because ductility contributes to the ability of the coating to survive impacts which dent or bend the substrate without causing cracking or peeling. The present invention provides a means to prepare coatings with a good balance of hardness, ductility, and stiffness, from very high-molecular-weight polymers, without the need for conventionally high amounts of volatile organic components and without the need for cure chemistry.
Aqueous colloidal dispersions of polymers are increasingly important in the paint industry because the coating constituents can be obtained in relatively concentrated form (>20%), at moderate viscosities, and with little or no need for volatile organic solvents which constitute undesirable side-products in paint applications. However, the drying of such dispersions to form uniform, crack-free coatings are subject to certain well-known limitations. Such dispersions have been characterized, in each case, by a minimum film formation temperature, MFT, which is typically a few degrees below the glass transition temperature T.sub.g of the colloidal-polymer particles (See, for example, G. Allyn, Film Forming Compositions, R. R. Myers & J. S. Long, Ed., Marcel Dekker, N.Y., 1967). To whatever extent a polymer in a dispersion may be plasticized by other components of the dispersion, the MFT may be reduced accordingly. If a dispersion
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Andrews Gerald Donald
Mazur Stephen
E. I. Du Pont de Nemours and Company
Rajguru U. K
Seidleck James J.
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