Coating processes – With post-treatment of coating or coating material – Heating or drying
Patent
1995-01-13
1997-01-14
Short, Patricia A.
Coating processes
With post-treatment of coating or coating material
Heating or drying
4273855, 525 10, 525119, 525131, 525176, 525183, 525199, 525276, 525426, 525445, 525454, 525455, 525530, 525531, C08L 3302, C08L 3700, C08L 2902, C08L 4304
Patent
active
055937306
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a certain powder coating composition, and to a process for its preparation. More particularly, the invention relates to a powder coating polymer composition comprising a certain hybrid polymer system prepared by the insitu aqueous suspension polymerisation to form a granular polymer, especially an acrylic polymer, in the presence of another, preformed polymer.
It is well known to employ various polymer types in thermoset powder coating applications. Examples of such polymers include epoxy resins, polyester polymers, acrylic polymers and fluoropolymers. It is desirable for a number of applications to use combinations of different polymers so that e.g. the desirable properties of each polymer provide their contribution to the resulting coating or a desired new effect or appearance is achieved. For example, an acrylic polymer used in combination with a polyester polymer which when crosslinked up can give the enhanced durability of the acrylic polymer in combination with the improved mechanical properties of the polyester, or alternatively can provide a different but desired appearance (e.g. textured rather than glossy or matt).
A number of approaches have been described in the patent literature in order to achieve the degree of admixture necessary to give acceptable or required performance. JA88-182382 describes a polyester-acrylic hybrid prepared under conditions of melt transesterification to induce limited grafting. However the reaction conditions are such that control of melt viscosity due to premature reaction can be problematic. JA77-126433 describes the preparation of a carboxyl functional polyester in solvent and the subsequent insitu solution polymerisation to form an acrylic polymer followed by solvent stripping and solid polymer recovery. More typically, blending is achieved by coextrusion (EP 0314447, U.S. Pat. No. 4,499,239) although problems in achieving completely homogeneous mixing may be encountered.
An additional limitation is that these approaches may not allow certain ratios of either type of polymer to be prepared.
For example, binder compositions for use in powder coatings are disclosed in U.S. Pat. No. 4,499,239, which describes a resin composition consisting substantially of 60-97 wt. % of a polyester with an acid number between 15 and 200 and 3-40 wt. % of a glycidyl acrylate polymer. A disadvantage of the compositions of U.S. Pat. No. 4,499,239 is thought to be that the described mixtures have a maximum polyacrylate resin content of only 40 wt. % since at higher amounts by weight this resin is insufficiently miscible with the polyester resin also employed. As a result, the resins cannot be mixed in all the necessary ratios for the desired applications.
We have now discovered certain polymer compositions which are particularly useful for powder coating applications and do not suffer from the drawbacks discussed above.
According to the present invention there is provided a powder coating composition which comprises a hybrid polymer system in granular form of a polymer A formable from monomer(s) by aqueous suspension polymerisation and a polymer B, different to A, which polymer system has been obtained using a process which comprises dissolving polymer B in monomeric material to be used in the formation of polymer A, polymerising the monomer(s) to form polymer A using an aqueous suspension polymerisation process, and removing the aqueous phase to form a hybrid of the polymers A and B in granular form, and wherein polymer A and polymer B bear functional groups for imparting curability to the hybrid polymer system, and wherein further the hybrid polymer system has a number average molecular weight within the range of from 1500 to 15,000.
There is also provided according to the invention a process for the production of a powder coating composition, said composition comprising a hybrid polymer system in granular form of a polymer A formable from monomer(s) by aqueous suspension polymerisation and a polymer B, different to A, wherein said process comprises di
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Patent Abstracts of Japan, vol. 9, No. 324, C-320, abstract of JP, A, 60-155279, Aug. 15, 1985.
Hinde David C.
Padget John C.
Satgurunathan Rajasingham
Yeates Stephen G.
Short Patricia A.
Zeneca Limited
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