What’s in electrostatic thermosetting powders?

The ultimate success of any powder coating depends on both the equipment parameters and the nature, composition and condition of the powder.

There is a huge variety of powders to choose from and the final selection will depend on the end use requirement of the coated substrate.

In thermosetting systems, the polymer is usually a low molecular weight species which melts and flows during fusion and simultaneously undergoes a chemical conversion to a thermoset (cured or cross-linked) condition.  Once this condition has been achieved it can no longer be re-melted to a plastic condition as is the case with thermoplastic compositions.

The cross-linking reaction occurs in thermosetting powders between the functional (reactive) groups of the resin and those of the curing agent.  Obviously to achieve a smooth film the polymer must flow out to a continuous, even film before cure is initiated.

A powder is a compromise, balancing the softening temperature with the melt viscosity or fusion characteristics and the speed of cross-linking.  As with solvent-based paints thermosetting powders can be formulated to produce high and low gloss decorative coatings, aluminium, bronze & metallics and textures & hammered finishes.

Powder composition

The basic components required for the manufacture of thermosetting powders are:

  • Polymers
  • Hardener, catalyst, cross-linking or curing agent(s)
  • Flow control additive

The choice of components will be influenced by:

  • The film properties, such as gloss, colour,   hardness, flexibility, adhesion and chemical resistance.
  • Application technique.
  • The curing time and temperature.

Polymers

Solid grades of resin are used for powder coatings.  The selection of suitable grades is extremely important, as these control properties such as melting point, flow, levelling and film properties.  AkzoNobel Powder coatings boasts its very own Polymer Innovation facility in the UK which is dedicated to research and innovation solely in the field of polymers.

Resins having a softening point between 70-110oC are usually employed.  Resins of lower melting point may have a pronounced tendency to ‘cake’ in powder form on storage and have such a marked degree of flow on curing, that only a low degree of ‘sharp edge coverage’ is obtained.  By contrast, those with a higher softening point may have insufficient flow on curing and tend to give an orange peel effect, but coverage on sharp edges is good.  In addition there is a danger that at the higher processing temperatures required during manufacture the hardener may start to react with the polymer.

Hardener, catalyst, cross-linking or curing agent

The method of manufacture, application, curing and properties required in the coating all influence the choice of curing agent to be used with the particular type of polymers to be used in the formulation.  The hardener should be unreactive at room temperature, remaining latent up to 100oC and should react fully between this temperature and 180oC.  The reaction should not be so rapid as to prevent complete flow out of the fused resin but too long a curing time must be avoided because of commercial considerations.

Pigment and extender

Pigments which are currently used in solvent-borne conventional surface coatings may be used for powder coatings, provided they are chemically inert, fast to light and heat resistant.

Titanium dioxide is used virtually exclusively in the manufacture of white, pastel and light tints.  Carbon black is used for blacks and greys.  For more highly coloured coatings a wide variety of both organic and inorganic pigments are used, although emphasis is now moving onto the organic types as reduction in heavy metals such as lead is made.  However, some organic reds have a tendency to react during processing, losing their brightness and cleanliness so careful choice is important to ensure stability.  Aluminium and bronze powders are used to give metallic effects.

Certain inorganic extenders can be incorporated in formulations without reducing the gloss, flow or mechanical film properties.  The extenders are usually of high specific gravity and although reducing the raw material cost they also adversely affect the area covered by the powder.  The true economics of these can only be calculated from assessing the unit area covered per unit of powder.  Other extender types can be used to purposefully adjust the gloss level and appearance of the coatings.

Flow control additive

Having selected the appropriate resin/hardener/pigment system, adjustments to the formulation are usually required to modify flow and film properties to suit application and curing conditions.

It is normally essential to incorporate flow control agents, as without them powder coatings tend to pinhole, ciss or crater during curing and orange peel effects may appear.  The flow control agents reduce the tendency of the resin to ciss by decreasing the surface tension of the system and by promoting flow-out they give smoother films.  Acrylic polymers and other resinous materials have been used successfully in this context.

Occasionally coatings may tend to flow out too rapidly giving poor ‘sharp edge coverage’.  To give adequate cover on these parts and to prevent sagging, thixotropic agents such as the organic derivatives of montmorillonite, eg. ‘Bentones’ finely divided silicas and other extenders may be used.  However they have to be judiciously chosen otherwise film properties, such as gloss and colour, may be affected.

Epoxy powders

These can be formulated to give very high gloss and smooth coatings with excellent adhesion, flexibility and chemical resistance.  The main deficiency is a pronounced tendency to yellow at elevated temperatures or when exposed to daylight.  In addition, on exterior exposure they chalk rapidly.  However film integrity on exterior exposure is excellent.  The curing of epoxy powders is an additions mechanism and no volatiles are released during stoving.

Polyester powders

These powsers have excellent outdoor durability coupled with high resistance to yellowing under ultra-violet light and have to be stoved at extended times and elevated temperatures compared with epoxies.  Their general chemical resistance is somewhat lower than epoxies.

Epoxy-Polyester hybrid powders

These powders are a widely used class of thermosetting powders in which the epoxy resin acts as a curing agent for the polyester resin.  Depending on the choice of resins and their relative proportions, the properties of the hybrid will lie between those of the pure epoxy and the pure polyester.  The presence of the epoxy resin leaves this type of resin susceptible to chalking on exterior exposure.

Polyurethane powders

Polyurethane powders are based on hydroxy-containing polyesters reacted with masked (or blocked) isocyanates.  They provide good all-round physical and chemical properties as well as giving good exterior durability.

Table 1 summarises the main difference between the principal thermosetting polymers used in powder coatings.