Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Patent
1986-10-03
1988-07-19
Welsh, Maurice J.
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
525127, 525131, 528 48, 528 75, C08G 1830
Patent
active
047586257
DESCRIPTION:
BRIEF SUMMARY
This invention relates to surface coatings particularly to urethane crosslinked acrylic coatings.
Australian Pat. No. 476,431 discloses the vapour catalysed curing process which enables a coating of an acrylic polymer-isocyanate mix to be cured at room temperature by the use of a vapourized catalyst.
U.K. Pat. No. 2093049 provides a number of resin/isocyanate mixtures for use in the vapour catalysed curing process. These compositions provide conventional coating formulations particularly suited for vapour catalysis. Polyester, polyether and acrylic polyols are disclosed as being useful backbone polymers for use with the isocyanate. European Pat. No. 0,029,598 discloses a coating composition having 50 to 95% of an acrylic polymer having a molecular weight from 500 to 10,000 and an hydroxyl content of at least 2% with 5 to 50% by weight of a polyisocyanate crosslinking agent.
It is an object of this invention to provide an improved composition for forming the cured acrylic-urethane coating so that the resulting coating exhibits improved toughness characteristics and has a quick drying time. Toughness is a combination of hardness and ductility or resilience which are desirable properties in coatings.
In particular it is an object of the invention to develop polymer formulations which exhibit the characteristic of early initial hardness so that the coatings are resistant to damage within half an hour of application.
To this end the present invention provides a coating composition curable with a vapourized catalyst and having high initial hardness characteristics formed by mixing a poly-isocyanate cross linking agent with an acrylic polymer in a water free solvent, having a good evaporation rate in an acrylic urethane polymer system, said acrylic polymer having a molecular weight below 20,000, preferably less than 10,000 and a content of 15 to 50% by weight of an hydroxy unsaturated monomer and the remaining monomers being selected to provide the final polymer with a glass transition temperature in the range of -40.degree. C. to 55.degree. C., preferably -10.degree. C. to 30.degree. C.
By maintaining a low molecular weight and having a relatively high number of hydroxyl groups the possibility exists for creating urethane crosslinks of length comparable to that of the acrylic backbone polymer. It is thought that it is this structure that results in a tougher coating film. Further, the predominance of these urethane linkages means that the crosslinked polymer is less soluble in the carrier solvent after curing and it is thought that this results in liberation of the solvent from the film which aids drying of the coating.
The hydroxyl containing monomer which is required to be present in the acrylic polymer comprises 15 to 50% by weight of total monomers and preferably comprises 25% to 35% by weight of total monomers. Preferably the hydroxyl containing monomer is an acrylic monomer and the most preferred monomers of this class are hydroxy ethyl acrylate (HEA), and hydroxy ethyl methacrylate (HEMA). With hydroxy ethyl acrylate the otimum initial hardness characteristics are obtained with polymers in which HEA content is within the range of 25 to 35% moles on total moles of monomer and preferably about 30 mole %. The pot life optimum of at least 12 hours can be achieved within the range of 15 to 45% HEA content. It has been found that surprisingly HEA tends to provide better initial hardness and also equal to better final hardness than HEMA when used as the principal hydroxy monomer. Hydroxypropyl acrylate (HPA) and hydroxy propyl methacrylate (HPMA) may also be used, but as they contain a high proportion of secondary alcohols, they cure more slowly. They may be used in conjunction with HEA and HEMA to extend the pot-life.
As well as the acrylic and methacrylic hydroxy monomers other hydroxy monomers may be used, preferably in admixture with acrylic or methacrylic hydroxy monomers. By using monomers which provide a longer hydroxyl terminated side chain preferably greater than 2 carbon atoms in length, the final cure
REFERENCES:
patent: 3836491 (1974-09-01), Taft et al.
patent: 3874898 (1975-04-01), McInnes et al.
patent: 4517222 (1985-05-01), Blegen
patent: 4659799 (1987-04-01), Nahas et al.
Boyack Robert M.
Rao Rama
Timms Raymond N.
Acquah S. A.
Albright & Wilson (Australia) Limited
Barron Alexis
Welsh Maurice J.
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