FLUIDIZED BED PARTICLES HAVING ALKALINE RESISTANCE FOR COATING METAL SUBSTRATES
Polymer particles containing a polyoxymethylene polymer for coating metallic substrates are described. The polymeric particles contain a polyoxymethylene polymer in combination with a thermoplastic elastomer. The thermoplastic elastomer can include hard segments and soft segments. The soft segments may contain carbonate groups, ether groups, or caprolactone groups. Also disclosed is a process where the polymeric particles are used to form a fluidized bed for coating metallic substrates. In one embodiment, after being coated with the polymer particles, the coated metal substrate is gas cooled followed by cooling the coated substrate in a bath containing an aqueous medium.
1 . A powder well suited for powder coating metallic substrates comprising:
polymeric particles having a particle size distribution such that at least about 80% of the particles have a particle size of from about 30 microns to about 800 microns, the polymeric particles comprising:
(a) a polyoxymethylene polymer that contains terminal hydroxyl groups in an amount of from about 15 mmol/kg to 200 mmol/kg,
(b) an impact modifier comprising a thermoplastic elastomer having hard segments and soft segments, the soft segments comprising carbonate groups, ether groups or caprolactone groups; and
optionally a coupling agent for coupling the polyoxymethylene polymer to the impact modifier.
2 . A powder as defined in claim 1 , wherein the thermoplastic elastomer, comprises a thermoplastic polyurethane elastomer, the thermoplastic polyurethane elastomer being present in the polymeric particles in an amount from about 10% to about 30% by weight.
3 . A powder as defined in claim 2 , wherein the polymeric particles contain the coupling agent, the coupling agent comprising an isocyanate, the coupling agent being present in the polymeric particles in an amount from about 0.1% to about 3% by weight.
4 . A powder as defined in claim 1 , wherein at least 70% of the terminal groups of the polyoxymethylene polymer are hydroxyl groups.
5 . A powder as defined in claim 1 , wherein the polyoxymethylene polymer contains terminal hydroxyl groups in an amount greater than about 20 mmol/kg.
6 . A powder as defined in claim 1 , wherein the polyoxymethylene polymer is present in the polymeric particles in an amount from about 40% to about 90% by weight.
7 . A powder as defined in claim 1 , wherein the polymeric particles have been cryogenically ground.
8 . A powder as defined in claim 1 , wherein the soft segments of the thermoplastic elastomer comprise carbonate groups.
9 . A powder as defined in claim 1 , wherein the impact modifier has a Shore A hardness of from about 85 to about 95 according to ISO Test 868.
10 . A powder as defined in claim 1 , wherein the polymeric particles further comprise an acid scavenger.
11 . A powder as defined in claim 10 , wherein the acid scavenger comprises zinc oxide.
12 . A powder as defined in claim 10 , wherein the polymeric particles contain a plurality of acid scavengers.
13 . A powder as defined in claim 12 , wherein the acid scavengers comprise zinc oxide, tricalcium citrate, a copolyamide, or mixtures thereof.
14 . A powder as defined in claim 1 , wherein the polymeric particles further contain a chlorine scavenger.
15 . A powder as defined in claim 1 , wherein the polymeric particles further contain a nucleating agent.
16 . A metallic substrate coated with the powder defined in claim 1 .
17 . A rack for a dishwasher comprising a plurality of tines for holding dishes, the rack comprising a metal substrate coated with the polymeric particles defined in claim 1 .
18 . An article coated with a polymer composition as defined in claim 1 , the article comprising a railing, a fence, a rebar, a medical device, an ambulant transport part, a refrigerator part, a washing machine part, a clip, a fastener, a suspension spring, or an automotive part.