Molding compounds for use in induction heating applications and heating elements molded from these compounds
View Patent ↗The invention provides molding compounds that are particularly suitable to be molded into an article such as a heating element that will conduct heat and not burn when an electric current is passed through the article. These compounds are generally liquid thermosetting molding resins which comprise a thermoset resin matrix such as a terephthalate polyester which can include blends of polyester and/or vinyl ester with a significant loading of conductive inorganic filler, typically graphite. The compositions also include flame and sound retardant additives, and glass fibers. They are further formulated to meet the desired molding characteristics; to withstand the operating temperatures to which they will be exposed; and to have a predetermined strength and a desirable user interface including appearance, and odor. Typically, the compounds will have a glass transition temperature from about 160° C. (320° F.) to about 195° C. (383° F.).
1. A method of making a heating element comprising molding the element from a composition comprising:
a) an unsaturated prepolymer resin which comprises a blend of an unsaturated polyester and a vinyl ester resin;
b) an unsaturated material copolymerizable with said resin and including a terminal ethylene group;
c) from about 10 to about 200 parts by weight of graphite based upon 100 parts of resin, which is the weight of a) and b);
d) an initiator to initiate said copolymerization; and
e) a flame retardant.
2. A method of making a heating element as set forth in claim 1 wherein the graphite is present in from about 25 to about 100 parts by weight based upon 100 parts by weight of resin.
3. A method of making a heating element as set forth in claim 2 further comprising glass fiber.
4. A method of making a heating element as set forth in claim 3 wherein the amount of glass fiber is from about 10 to about 70 parts by weight based upon 100 parts by weight of resin.
5. A method of making a heating element as set forth in claim 4 wherein the amount of glass fiber is from about 20 to about 35 parts by weight based upon 100 parts by weight of resin.
6. A method of making a heating element as set forth in claim 5 wherein the prepolymer comprises a polyester terephthlate blended with an unsaturated polyester.
7. A method of making a heating element as set forth in claim 6 wherein the ratio of polyester terephthlate to unsaturated polyester is from about 5:95 to about 25:75.
8. A method of making a heating element as set forth in claim 7 wherein the flame retardant is antimony trioxide.
9. A method of making a heating element as set forth in claim 8 wherein the antimony trioxide is present in an amount of from about 5 parts to about 20 parts by weight based upon 100 parts by weight of resin.
10. A method of making a heating element as set forth in claim 1 wherein the heating element is a blower housing.