Powder Coatings:

   Powder coatings are exactly what they say they are, powders.  What is environmentally friendly about powders, there are absolutely no solvents.  The coating is 100% solid when it is applied and 100% solid when the coating is cured.  Therefore there are no VOC emitted into the atmosphere.  The efficiency of powder coating is another advantage, this can be seen in the picture below.

        As you can see from the above figure, powder coatings not only are solvent free, but more of the coating is applied and less is wasted or lost.  This is a distinct advantage over solvent based coatings.  The traditional polymers of solvent based coatings are also used for powder coatings.  The polymer types are usually separated into 2 categories:  thermosets and thermoplastics.
 
    Thermosetting powder coatings are excellent coatings and very durable.  Some of the applications for these types of coatings are:  appliances, automotive, archetectural, lawn and garden, and general metal finishing.  To achieve the properties desired from thermoset powder coatings one must deal with the problem of curing these coatings.  Powder coatings are applied to the substrate as a powder.  The powder must then be heated so that the polymer can flow and level to form a thin continuous film.  However during this heating process the thermoset powder coating begins to crosslink.  If crosslinking happens to fast then the film with not be continuous, but if the coating is not heated enough you won't crosslink your polymer enough.  This compromise is very important to obtain thermoset powder coatings with the desired properties.  The heating required to flow and level the powders gives rise to the limitations of powder coatings.  The subtrates that powder coatings can be applied to are limited because of this heating process.  Low melting plastics and wood are just a couple of examples of subtrates in which heating for flow and leveling would be a major problem.
 
 
Thermoset Resins
Resin Functionality Structure Characteristics
Acrylic resins hydroxyl, carboxyl, epoxy, random functional group distribution hydrocarbon backbone
Epoxy resins epoxy groups, terminal functional groups ether linkages
Polyester resins hydroxyl, carboxyl, terminal functional groups ester linkages, heteroatom and hydrocarbon backbone

 
Common Resin/Hardener Combinations Employed in Powder Coatings
Epoxy System Resin Epoxy
Hardener Dicyandiamide and variations
Anhydrides, e.g. BTDA
Amines, e.g. DDS, MDA
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Polyester / Epoxy (hybrid) Resin Carboxyl Polyester
Hardener Epoxy
Polyester / TGIC Resin Carboxyl Polyester
Hardener Triglycidyl isocyanurate
Polyester / Polyurethane Resin Hydroxyl polyester
Hardener Blocked isocyanate, e.g. IPDI
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Acrylic Resin Carboxyl, hydroxyl, or glycidyl acrylics
Hardener  TGIC, blocked isocyanate or self curing acrylics

    Thermoplastic powders do not have the problem with crosslinking taking place while the polymers are flowing and leveling.  Thermoplastic powders already high molecular weight powders so flow and leveling is the major concern.
 
 
Thermoplastic Resins
Vinyls  PVC              
PVDF         		 PVC has low cost with good mechanical and protective properties.  PVDF has outstanding exterior durability.
Polyamides Nylon 11   
Nylon 12   		 Nylons 11 and 12 provide the necessary melt temperatures for powder application.  Nylons are abrasive, impact and wear resistant and self lubricating.
Polyesters
Polyesters require a molecular weight of 15,000 to achieve good mechanical properties.  Exhibit poor outdoor weathering.
Polyethylene
Polyethylene are usually applied by fluidized bed.  They provide chemical resistance, toughness, and outstanding electrical insulating properties.
Polypropylene
Polypropylene offers medium-high gloss, outstanding chemical and solvent resistance, good surface hardness, and abrasion resistance.  They are between 75-100% isotactic.

    Whether the powders are thermosetting or thermoplastic, Powder Coatings provide an efficient and environmentally friendly way to protect a substrate.

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