Method for Making Coated Molded Articles

1. Field of the Invention:

The invention relates to thermoset fiber-reinforced molded articles having in-mold applied coatings.

2. Description of the Prior Art:

MATCHED METAL MOLDING--Fiber-reinforced, thermosetting resinous products are produced in elevated temperature, matched metal molds. One of the shortcomings of this process is that the molded article surface tends to be irregular and tends to have surface exposure of the reinforcing fibers, usually glass fibers.

IN-MOLD COATINGS--The in-mold coating technique has been developed to improve the quality of the surface of a molded article after the curing of the article in a matched metal mold has been substantially advanced, U.S. Pat. No. 4,081,578. In the matched metal molding process, two or more shaped mold elements cooperate to define a mold cavity. A molding composition (polymerizable thermosetting resin and fibrous reinforcing material and usually fillers, catalysts, mold release agents, and other reagents) is introduced into a mold cavity between the matched metal mold surfaces. The shaped mold elements are brought together to spread the molding composition throughout the mold cavity and to confine the molding composition until curing is essentially completed. The heated mold elements initiate polymerization. When the polymerization is well advanced, i.e., the article has assumed the shape of the mold cavity, the mold surfaces are separated slightly, usually a distance from 3 to 40 mils, and a liquid, in-mold, coating composition is injected under pressure into the mold cavity. Thereafter the mold elements are again brought together under pressure which causes the liquid coating composition to disperse over the surface of the partially cured article and to conform with the surface of the contiguous mold element. After a suitable time, the polymerization of the article is essentially completed and the polymerization of the coating composition is essentially completed. At that time the mold elements are separated and the coated, molded article is removed.

Two-component in-mold coating compositions are known, U.S. Pat. No. 4,081,578, which employ polyisocyanates as one ingredient and employ hydroxy-terminated high molecular weight unsaturated polyester resin as another ingredient. These compositions have limited pot life after being mixed and must be used promptly. These compositions have exhibited some inadequate adhesive properties.

There is a demonstrated need for a single-component coating composition which can be prepared and retained for extended periods of time by the molding operator.

According to the present invention, an in-mold coating composition is provided for thermoset fiber-reinforced plastic molded articles. The composition includes a polyurethane polyacrylate and copolymerizable ethylenically unsaturated monomer along with a polymerization initiator and fillers which may include inert particulate fillers, pigments, mold release agents. The polyurethane polyacrylate is a reaction product, substantially free of unreacted--NCO radicals of

1. an organic diisocyanate;

2. a hydroxy alkyl acrylate or methacrylate;

3. an organic diol.

The polyurethane polyacrylate constitutes from 0.075 to 5.0 parts by weight for each part by weight of the ethylenically unsaturated copolymerizable monomer.

A polyurethane polyacrylate or polymethacrylate is an oligomer which can be prepared by combining an organic diisocyanate, a hydroxy alkyl acrylate or methacrylate and an organic diol.

Organic Diisocyanate--A preferred organic diisocyanate is toluene diisocyanate (TDI). The readily available mixture of the 2,4- and 2,6- isomers of TDI is useful. Other organic diisocyanates include isophorone diisocyanate, polymethylene polyphenyl diisocyanate. In general a mixture of polyisocyanates and monoisocyanates having an--NCO equivalent of 1.8 to 2.2 is useful and can be categorized as a diisocyanate for the present purposes.

The Hydroxy Alkyl Acrylate or Methacrylate--In general these materials include hydroxy ethyl acrylate or methacrylate, hydroxy propyl acrylate or methacrylate, hydroxy butyl acrylate or methacrylate, a hydroxyl group is usually attached to the beta carbon of the alkyl radical. The alkyl radical may contain up to 8 carbon atoms.

The following structural formula describes useful hydroxy alkyl acrylates and methacrylates: ##STR1## wherein R=--H or --CH.sub.3 ; and

n=an integer from 2 to 8.

The Organic Diol--An organic diol is employed as the core of the oligomer which comprises the polyurethane polyacrylate or methacrylate. The proportion of organic diol in the polyurethane polyacrylate influences the size of each oligomer. Useful organic diols include alkylene diols such as ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol, polyethylene glycol, polypropylene glycol, hydroxy alkyl ethers of butane diol and the like; diester diols such as the diester reaction product of dicarboxylic acid or anhydride and an organic diol as already described, for example, a diester of adipic acid and diethylene glycol is a useful organic diol for the present purposes. Alkylene oxide adducts of diester diols may be employed.

Proportions--From 0.15 to 0.5 mols of the organic diol are supplied for each mol of the hydroxy alkyl acrylate or methacrylate. The organic diisocyanate is supplied such that the NCO/OH ratio in the system is 0.9 to 1.0.

The polyurethane polyacrylate or methacrylate is formed by combining the organic diol, the organic diisocyanate and the hydroxy alkyl acrylate or methacrylate in the presence of a suitable catalyst for the reaction of --NCO radicals with hydroxyl radicals, for example, a tin catalyst. A preferred procedure is to combine the organic diisocyanate with the hydroxy alkyl acrylate or methacrylate to produce a urethane product having an acrylic or methacrylic radical and an unreacted --NCO radical. That monoisocyanate is then combined with the organic diol at about 80.degree. C. until substantially all of the --NCO radicals are reacted. The resulting oligomer is dissolved in a suitable ethylenically unsaturated copolymerizable monomer to reduce the viscosity of the mixture to a useful level. In general the polyurethane polyacrylate will comprise from about 7 to 90 percent by weight of the resinous ingredients (of the coating composition) and the copolymerizable ethylenically unsaturated monomer will comprise from about 93 to 10 percent of the resinous ingredients (of the coating composition). Thus the polyurethane polyacrylate constitutes from about 0.075 to about 5.0 parts by weight for each part by weight of the ethylenically unsaturated copolymerizable monomer.

Copolymerizable monomers include styrene, alpha methyl styrene, divinyl benzene, vinyl toluene, diallyl phthalate, acrylate or methacrylate esters such as ethyl acrylate, methyl acrylate, propyl acrylate, butyl acrylate, hexyl acrylate and the corresponding methacrylates; ethylene glycol diacrylate, ethylene glycol dimethacrylate, acrylic and methacrylic esters of diepoxides such as the acrylic or methacrylic diester of the diglycidyl ether of bisphenol-A.

The in-mold coating composition may contain inhibitors and stabilizers to retard premature polymerization of the composition in the manner well known in the molding composition arts. A catalyst for the reaction of --NCO radicals and --OH radicals also is included, preferably a tin catalyst such as dibutyl tin dilaurate or stannous octoate. Mold release agents such as zinc stearate are included in the composition. Inert particulate fillers and opacifiers may be added along with pigments if desired. An initiator for addition polymerization of the acrylates and copolymerizable monomers is included, e.g., organic peroxide initiators.

The molded articles customarily are fabricated from unsaturated polyester resin compositions which include

A. an unsaturated polyester resin usually formed by polyesterification of organic polyols and organic polycarboxylic acids, at least a portion of which includes ethylenically unsaturated polycarboxylic acid;

B. copolymerizable monomer for the unsaturated polyester resin, usually styrene;

C. fillers, including particulate inert fillers, catalysts for the condensation polymerization of the unsaturated polyester resin with the copolymerizable monomer;

D. mold release agents;

E. glass fiber reinforcing materials.

The molding composition is introduced into a matched metal mold at temperatures usually in the range of 300.degree. F. The mold is closed and the molding composition is allowed to polymerize and assume the shape of the interior of the mold.

After the molded article has at least partially polymerized and assumed the shape of the interior of the mold, the mold is opened and an in-mold coating composition is introduced. Thereafter the mold is closed once more causing the coating composition to spread over at least one surface of the molded article. The mold is retained in a closed position for a sufficient period to allow the molded article to complete further curing and to allow the coating composition to be cured as an adherent coating over at least one surface of the molded article. Thereafter the mold is opened and the thermoset glass fiber-reinforced plastic molded article is recovered having a cured, adherent coating of the present coating composition over at least one surface thereof.

The following ingredients were combined in a reactor:

1 mol toluene diisocyanate, a mixture of the 2,4- and 2,6- isomers;

1 mol hydroxy ethyl methacrylate;

1/2 mol of a difunctional polyester formed by reaction of adipic acid and diethylene glycol, said polyester having a low acid number and having a hydroxyl number from 200 to 210;

dibutyl tin dilaurate catalyst, 1/3 percent by weight based upon the weight of toluene diisocyanate;

500 p.p.m. a stabilizer;

200 p.p.m. an inhibitor.

Initially the toluene diisocyanate was combined with the hydroxy methyl methacrylate in the presence of the tin catalyst at about 65.degree. C. Thereafter the polyester was added slowly to the reaction kettle at about 80.degree. C. until substantially all of the --NCO radicals had been reacted. An in-mold coating composition was prepared from 70 parts by weight of the described polyurethane polyacrylate resin and 30 parts by weight styrene. This resinous mixture was combined with zinc stearate as a mold release agent, calcium carbonate filler, pigment pastes and t-butyl perbenzoate catalyst. The resulting mixture exhibited good shelf life in the absence of a catalyst and exhibited good catalyzed pot life after the catalyst was added.

An in-mold coated article was prepared in a mold comprising a 14".times.14.times. flat sheet of glass fiber-reinforced polyester, 1/8" thick. The coating composition was introduced into the open mold in a sufficient quantity to provide a coating 3 mils thick. The mold was retained at 300.degree. F. for 2 minutes.

The resulting product had a coating exhibiting:

pencil hardness=H;

Gardner gloss=35 (60.degree. angle);

adhesion--good;

color--uniform.

A polyurethane polyacrylate was prepared by combining, as described in Example 1, the following ingredients:

1 mol toluene diisocyanate, a mixture of 2,4- and 2,6- isomers;

1 mol hydroxy ethyl acrylate;

0.5 mol polypropylene glycol, 1000 molecular weight.

70 parts by weight of the resulting polyurethane polyacrylate were mixed with 30 parts by weight styrene. The resulting styrene solution of polyurethane polyacrylate was combined with a dimethacrylate ester of the diglycidyl ether of bisphenol-A in a styrene solution. The resulting mixture contained 1 part by weight of the styrene solution and 1 part by weight of the dimethacrylate ester of the diglycidyl ether of bisphenol-A in 30 percent styrene. This resinous mixture was combined with catalysts and fillers and applied as a coating to the same 14".times.14" molded article described in Example 1 at 300.degree. for 2 minutes. The resulting coated article had:

hardness=F;

a gloss of 42 (60.degree. angle);

adhesion--excellent.