Properties and Selection

Powder Coating Properties and Selection

Akzo Nobel Group produces both thermoplastic and thermosetting powder coatings, the majority of demand in the market being for thermosetting products.

Thermoplastic coatings do not chemically react upon temperature increase, but they melt and flow out onto the substrate. Application is usually in the industrial market coating wire, pipes and accessories. The thickness of these coating films is typically around 250 microns or more. 

Thermosetting coatings also melt upon temperature increase, but under go a simultaneous chemical reaction and polymerise through cross-linking into a resistant film. Once this chemical reaction has occurred the powder coating film cannot melt again. These coatings are used in both the decorative and the industrial markets. 

For decorative thermosetting coatings film thickness ranges from 20-80 microns. Functional thermosetting coatings require film thickness’ from microns up to several millimetres (depending on application). 

Properties of thermosetting decorative powder coatings based upon the formulation 
Chapter 1 discusses the raw materials that are required for the production of powder coatings, these being resin, curing agents, pigments, extenders and additives. The selection of the individual components and their composition (formulation) will be influenced by: 
1.    The ultimate film properties required including gloss, hardness, flexibility, adhesion, chemical and corrosion          resistance 
2.    Application technique 
3.    Curing conditions including type of oven, curing time and temperature 
4.    Production procedures and conditions. 

Below we detail how component properties influence the quality of the ultimate powder coating material. 

Resin: 
The selection of the correct grade of resin or blend of resins is very important, as these form the basic properties of the powder coating material and also control the film properties such as melting point, flow and levelling. Relatively low molecular weight resin, as solid grades, have a softening point between 60ºC & 110ºC. 
Low melting points can result in a tendency for powder to ‘cake’ during storage. They also have an extreme degree of flow on curing where a low degree of ‘sharp edge coverage’ is obtained due to the coating flowing away from the edges.

Usual resins include epoxies, polyurethane, polyester and acrylics. 

Curing Agent (also known as hardener)
The hardener is used to cross-link the resin at a given temperature. The degree of cross-linking can also be used to determine the gloss level, degree of surface ‘orange peel’ and other aspects including structure and texture effects. The curing agent should be unreactive at room temperature, remaining latent up to 100ºC and should react fully between 100ºC & 180ºC. This reaction should not be so rapid as to prevent complete flow out of the fused resin and not so slow that it creates commercial implications. 
Usual crosslinkers are amines, anhydrides and blocked isocyanates. Catalysts are used to accelerate the curing speed. 

Pigments and Extenders: 
Pigments must be inert, fast to light and heat resistant. As with most coatings they are used to create a decorative effect. 
§    Titanium dioxide creates white, pastel and light tints
§    Carbon black creates blacks and greys
§    Phthalocyanine creates blues and greens
§    Aluminium and bronze creates metallic effects. 
Organic pigments have to be handled with care as some of them can react during processing and curing. This can result in loss of brightness and cleanliness and in these cases alternative pigmentation have to be used. 

Certain inorganic extenders can be incorporated into the formulations without reducing the film quality. Usually extenders are of high specific gravity and although they reduce the raw material cost they can adversely affect the area covered by the powder. 

Additives: 
even after the optimum resin, hardener and pigments have been selected, adjustments to the formulation may still be required to modify flow and film properties to suit the application and curing conditions (i.e. Thixotropic agents to slow down the flow and UV stabilisers). Other functions of additives are: 
§    Increase/decrease electrostatic attraction
§    Increase/decrease surface levelling
§    Creation of decorative effects
§    Decrease stoving temperature requirement
§    Changing conductivity
§    Increase re-coatability
§    Increase surface hardness. 

Below are five types of thermosetting powder coatings and a comparison of their main properties (also see Table 1).

Epoxy powders: can be formulated to give high gloss and smooth coatings with excellent adhesion, flexibility and hardness as well as solvent and chemical resistance. The main deficiencies are their poor tolerance to heat and light as well as their pronounced tendency to yellow at elevated temperatures and exposure to diffused day light. 

Acrylic powders: are widely used in surface coatings. They have good gloss and colour retention on exterior exposure as well as heat and alkali resistance. 

Polyester powders: general performance can be categorised between epoxy and acrylic powders. They have excellent durability and a high resistance to yellowing under ultra-violet light. Most coatings used on buildings today are based on linear polyesters cross- linked with TGIC. Today modern polyester powders are TGIC-free 

Epoxy polyester hybrid powders: are epoxy powders by origin containing a high percentage of special polyester resin (sometimes exceeding 50%). These hybrids have properties similar to those of epoxy powders, however, their additional advantage is that they have improved resistance to overbake yellowing and improved weatherability. Hybrid powders are now regarded as the main backbone of the powder coatings industry. 

Polyurethane powders: provide good all-round physical and chemical properties as well as giving good exterior durability.

 Table 1
  
Influence of the particle size distribution upon the properties of thermosetting decorative powder during the coating application process -Approximately 12 stages of the powder application process are sensitive to the substrate to be coated and also to a large extent to the particle size of the powder. In its original state this can only be influenced by the powder producing company. 

The following steps of the application process represent the most sensitive stages: 
1.    Transportation of the powder 
2.    Electrostatic charging of the powder particles and faraday cage penetration 
3.    Formation of a uniform powder cloud in the surrounding air 
4.    Deposition and build up rate of the powder onto the substrate 
5.    Transfer efficiency. 

Thermosetting powders are applied electrostatically, therefore the powder particles must be able to charge in an electrostatic field or through Tribo static (friction) charging. This charge must be sufficient to attract adequate particles to the surface and edges of the substrate. However, the deposition of the powder should not be so insulating that it prevents adequate film build up.

The fluidity of the powder must meet all internal and external transportation demands. The particle size determines to a great extent the tendency of the powder to be free flowing. This fluidity depends not only upon the powder material but also upon the shape and size of the particles. 

Most commercial powders have a particle size between 10 and 100 microns. Graph 1 represents this typical psd for a pigmented epoxy powder. If the range was too wide it could result in an increased mass of powder to be recovered and recycled, which would effect transfer efficiency.
 
Graph

Relationship between the particle size distribution of the powder and the powder coating film formation during the application process to enable the formation of smooth, non-porous films the powder deposited during the application process should be as densely packed as possible onto the substrate surface prior to fusion and cross-linking in the oven. This will ensure that shrinkage and the formation of voids, pinholes and orange peel should be minimised. 

The importance of the relationship between the powder particle size and the coating film thickness after curing is also shown in Figure 2. It illustrates the difficulty in achieving a uniform 25 micron coating layer with particles larger than 50-75 microns (unless the polymer has exceptionally good flow out).
 
Figure 2
Packed, uniform spheres result in void spaces of up to 48%. Thermosetting powders however consist of irregular multi-sided particles of varying size and psd. When these powder particles are deposited in a layer the void volume will be greater. Therefore, before melting and cross-linking the powder film can be between three and seven times the actual thickness of the final coating film after curing. 

The technology is not yet available to reduce the void volume in the deposited layer using existing methods. However, it is generally easier to obtain a smoother coating film with a lower particle size and a smaller psd. Unfortunately, it is extremely difficult to apply these smaller particle sizes. 

The melt-flow index rate of cure of the powder and the particle size are critical properties in achieving a smooth coating film while maintaining adequate edge cover.

The selection of the type and grade of raw materials and their use in the formulation is of great significance to the quality of the powder coating material. Together with the powder particle shape, size and psd they will influence the quality, appearance and performance of the powder coating film on the substrate. 

Akzo Nobel Powder Coatings have the facilities to assist both powder coating applicators and equipment manufacturers in their selection of the most suitable powder coating materials. 
Document courtesy of AkzoNobel Group

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