Induction cured powder coatings for temperature sensitive substrates
According to the present invention, powder coating compositions comprise one or more than one thermoplastic or thermosetting polymer or resin and one or more than one finely divided magnetic material, such as a ferromagnetic material. Preferred magnetic materials include Ni 1−x Zn x Fe 2 O 4 compounds, wherein 0.4≦X≦0.75, piezoelectric compounds, ferrimagnetic δFeOOH, Fe—Ni—B, Cu 2 MnIn, transparent polymer-Cobalt oxide nanocomposites and soft ferrites. In addition, the present invention provides methods of making powder coatings on a substrate comprising applying to a substrate a resinous powder coating composition comprising one or more than one magnetic material to form a powder coating layer, followed by induction heating to melt the applied powder coating to form a coating film and, optionally, to cure the powder coating. The powder coatings remain at a pre-selected temperature equal to or less than the Curie temperature (T C ) of the one or more than one finely divided magnetic material during induction heating of the powder coating without directly heating the substrate. Powder coatings on metal parts may also be induction heat cured.
1. A powder coating composition comprising one or more than one curable or thermoplastic resin or polymer and one or more than one finely divided magnetic material having a Curie temperature (T C ) of from 50° C. to 250° C., wherein the said one or more than one finely divided magnetic material is chosen from Ni 1−x Zn x Fe 2 O 4 compounds, wherein 0.4<X<0.75, ferrimagnetic δFeOOH, Cu 2 MnIn, soft ferrites, Fe—Ni—B, transparent polymer-Cobalt oxide nanocomposites, mixtures thereof; ferromagnetic metals, ferromagnetic alloys, ferromagnetic compounds, ferrimagnetic compounds, and paramagnetic Ni-rich austenite,
wherein the ferromagnetic compounds are chosen from Ni 1−x Zn x Fe 2 O 4 , MnO—Fe 2 O 3 , Ni—Zn—Fe 2 O 3 and Ni—Zn alloys having a T C of less than 250° C.; densified and heat treated ferromagnetic compounds, vitrified compounds from ferromagnetic metals and multilayer particles, and
further wherein, when the said one or more than one finely divided magnetic material is chosen from ferromagnetic metals, ferromagnetic alloys, and ferromagnetic compounds, the said composition comprises the said ferromagnetic metals, ferromagnetic alloys, ferromagnetic compounds in the amount of from 0.1 parts per hundred parts resin (phr) to 10 phr.
2. A powder coating composition as claimed in claim 1 , wherein the said finely divided magnetic material is chosen from Ni 1−x Zn x Fe 2 O 4 compounds, wherein 0.4<X<0.75, ferrimagnetic δFeOOH, Cu 2 MnIn, soft ferrites, Fe—Ni—B, transparent polymer-Cobalt oxide nanocomposites, mixtures thereof, ferrimagnetic compounds, and paramagnetic Ni-rich austenite and the said composition comprises the said one or more than one finely divided magnetic materia in the amount of from 0.1 parts per hundred parts resin (phr) to 60 phr.
3. A powder coating composition as claimed in claim 1 , wherein the said one or more than one finely divided magnetic material is chosen from Ni 1−x Zn x Fe 2 O 4 compounds, wherein 0.4<X<0.75, ferrimagnetic δFeOOH, Cu 2 MnIn, soft ferrites, Fe—Ni—B, transparent polymer-Cobalt oxide nanocomposites, and mixtures thereof.
4. A powder coating composition as claimed in claim 1 , wherein the said polymer or resin is chosen from epoxy resins, polyesters, urethanes, acrylics, unsaturated polyesters, silicones, vinyl ether resins, acrylic prepolymers, acrylic resins, and mixtures, hybrids and combinations thereof.
5. A powder coating composition as claimed in claim 1 , wherein the said polymer or resin is thermoplastic and is chosen from polyamides, polyolefins, polyvinylidene fluoride (PVDF) resins, polyacetals, ethylene-vinyl acetate copolymers, polystyrenes, polyacrylates, ketone resins, polyvinyl butyrals, mixtures, hybrids and combinations thereof.
6. A powder coating composition as claimed in claim 4 , wherein the said polymer or resin is an unsaturated polyester comprising active hydrogens, and the composition further comprises one or more radical polymerizable cross-linker, polymer or prepolymer, one or more thermal initiator, and one or more mold release agent.
7. A powder coating composition as claimed in claim 1 , wherein the said one or more than one finely divided magnetic material has an average particle size of from 15 nm to 25 μm.
8. A method of making a powder coating from a powder coating composition as claimed in any one claims 1 to 7 comprising:
applying the said powder coating composition to a substrate to form a powder coating layer; and
induction heating the said powder coating layer to melt it to form a coating film and, optionally, to cure the said film.