Resins makeup the bulk of most powder coatings anywhere from 50 to 90% by mass. THe use of the the proper resin can make the difference between a super-durable AAMA2604 grade coating and a mediocre coating. In super-durable coatings resins should constitute atleast 65% of the total mass to extend their respective durable properties to the finished product. Using large amounts of filler in SD coatings can have a negative influence on the lifetime and quality of the finished product.The use of the correct hardening agent is of paramount importance to obtain good mechanical properties such as impact resistance in the finished coating product.
Since resins make up the bulk of the finished powder coating by mass their quality should not be compromised. Selecting a resin with an appropriate glass transition temperature, cure profile, reactivity, gel time and melt viscosity are all important factors to consider. The resin choice can affect the mechanical properties, chemical properties and how well a coating can withstand corrosion and UV exposure. Resins however should be considered in tandem with the appropriate cross-linking agent or hardener since the improper or inadequate amounts can lead to coating failures and disbondment. Popular suppliers of quality resins include Allnex, Uralac (DSM), and Huntsman.
Epoxy powder coatings are suitable for use in interior applications but their use should be limited in any application where there will be significant UV exposure. Epoxy coatings exhibit poor UV resistance and as such color and gloss will fade quickly when used in an exterior application and chalking becomes apparent. Epoxies do however have excellent chemical resistance a feature of the strong ether linkage formed when the epoxide ring is opened during the cure process. These resins are also quite strong and tough. Epoxy resins can also be mixed with polyester resin to create a hybrid type resin (see below). Epoxy coatings find use in indoor applications, coatings for structural rebar and pipelines and have recently seen a resurgence of interest as “fusion bonded epoxy” and their use on pipelines in adverse conditions.
Epoxy resins are based on the 3 member C-O-C epoxide ring system which is under considerable strain at the atomic level. This ring strain means that epoxies can react with a variety of nucleophilic agents including amides, anhydrides, phenols or carboxylates. Although epoxy resins typically require a dicyanamide or phenolic curing agent the use of some catalysts can cause self-polymerisation to occur. The use of bisphenol A and epichlorohydrin are quite common to make epoxy resins. The repeating structure of the generic epoxy resin shown below can be altered to effect different properties in the resin. For values of n lower than 2 the resulting resin is not solid enough to be used as a powder coating resin. Values of n greater than 2 result in stable solids at room temperature, and epoxy resins of type 3.0 (n = 3) are most common in powder coating epoxy resins. As n increases so does the molecular weight of the epoxy resin resulting in increased viscosity and texture.
The use of o-cresol Novolac in epoxy resins leads to increased functionality which will produce a harder, more chemical and heat resistant epoxy coating. Replacement of some or all of the bisphenol A with o-cresol Novolac results in an Epoxy Cresol Novolac (ECN).
Epoxy resins utilize a variety of nucleophilic hardening agents including thiols, amines, alcohols, phenols, and anhydrides. One of the most common amine hardening agents is dicyandiamide which is commonly used with an imidazole catalyst acting as a Lewis base. Dicyandiamide has poor solubility and as such requires higher than stoichiometric amounts oftentimes to be truly effective. Insolubility of the hardening agent also makes achieving high gloss epoxies based on dicyandiamide difficult to achieve, a problem that has been circumvented by using a toluene substituted derivative of dicyandiamide.
Polyesters are some of the most commonly used resins in powder coating today. There exists hundreds of different types of polyester resin with varying degrees of functionality. Polyester resins can be either hydroxyl terminated or carboxyl terminated. Carboxyl terminated resins react with hardeners such as TGIC, Primid, and any epoxide including glycidyl methylacrylacte (GMA) and epoxy resins in hybrid systems. Hydroxyl terminated polyesters react with isocyanate or amine hardeners.
Popular monomer choices are terephtallic acid and neopentyl glycol which can be combined in varying amounts to form the ester shown below (4-((3-hydroxy-2,2-dimethylpropoxy)carbonyl)benzoic acid). Esters are a common functional group encountered in organic chemistry and often have pleasant aromas. In the case of the reaction below the reaction product has one ester bond (bolded in red). The reaction scheme below depicts only two of these monomers reacting to form the ester, the reaction can be repeated to form longer chain polyester derivatives with either hydroxyl or carboxyl termination. The choice of these monomers is largely in part to their ease of procurement from large scale processes and the physical characteristics they impart to the resultant polymers (room temperature stability) and low cost. Using an excess of either the acid or glycol will in large part determine the nature of the resulting polymer’s terminus (hydroxyl or carboxyl). The reaction product below has both a hydroxyl terminal (left hand side of ester molecule) and a carboxyl terminal (right hand side of ester molecule).
Epoxy Polyester/ Hybrid Coatings
Hybrid coatings seek to exploit the benefits of two or more different resin classes. By far the most common are PE-epoxy based resin systems which typically consist of 60-70% polyester resin and 40-30% epoxy resin. These hybrid systems are a good low cost alternative but again suffer from reduced UV resistance and should be used in interior applications only.
Polyester is the most commonly used resin in powder coatings today. In north American markets the polyester resin is most commonly crosslinked with the tri functional epoxide hardener triglycidyl isocyanurate (TGIC). TGIC can bond with up to three resin molecules to form a 3D network of polymer chains in the cured coating. Each of the three-membered epoxide rings will react with the carboxyl terminus of a PE resin in a similar fashion to polyester/epoxy resin bonding. TGIC based powder coatings display excellent weatherability, mechanical flexibility, chemical resistance and hardness and can be used in both interior and exterior applications including architectural. The use of TGIC is banned in Europe and many other countries. Polyester resins commonly have an acid value of about 33 which would require a 93:7 ratio of polyester resin to TGIC. Although 33 is a common acid value for polyester resins there is variance among these values so it is not uncommon to see 95:5 and even as high as 90:10 ratios in some polyester:TGIC systems. Increased amounts of TGIC will lead to higher overall costs (TGIC is more expensive than PE resin) but will result in a coating that has better impact/mechanical properties and better chemical resistance.
Due to toxicity concerns about TGIC formulators have been trying to develop alternative hardening agents. Chemistries of the suitable replacement hardeners have to date been either glycidyl containing agents or hydroxyalkylamide agents (HAA) such as the popular Primid®.
Polyester TGIC-free (β-hydroxyalkylamide)
TGIC free polyester resins find large market share in Europe and in Quebec. These TGIC free PE coatings employ a β-hydroxyalkylamide (HAA) as a hardening agent. The most common HAA agent is Primid® and has four sites for which crosslinking can occur. Primid based powder coatings have very similar characteristics to that of TGIC-PE coatings, and even though corrosion resistance has traditionally been less than that of TGIC systems, these new PE-HAA powder systems are now capable of achieving first class corrosion resistance. TGIC free PE coatings that use Primid are typically formulated with a binder ratio of 95:5 resin to Primid hardener although resin choice can alter this.
Polyester Urethane (Polyurethane)
Polyester-Urethane based hybrid coatings are a mixture of polyester and urethane chemistry and are commonly referred to as urethane. These hybrids show remarkably improved UV resistance and are suitable for both interior and exterior applications. This type of powder coating can achieve excellent appearance at relatively low film build, and given the large variety of polyester resins available it is possible to formulate coatings of different chemical and mechanical strengths. PE-urethane coatings can be hard tough and chemically resistant.
A hydroxyl functional polyester resin is crosslinked with an isocyanate reagent in a vigorous reaction. Because of the speed of reaction it is necessary that the isocyanate group be chemically blocked to prevent polymerization during manufacture.
Typical blocking agents are of the Ɛ-caprolactam type which can be unmasked at temperatures in excess of 180°C. This blocking group means that sufficient curing is of paramount importance in these polyurethane coatings. An unwanted side effect of curing polyurethane coatings is the evolution of the caprolactam upon demasking.
As with any resin that contains polyester there exists a large variety of polyesters to choose from. Stoichiometry ranges broadly with hydroxyl numbers ranging from 20 to 350. Utilizing a polyester with a low hydroxyl value results in an affordable product with good impact resistance, good weatherability, but reduced chemical resistance to that of a higher valued hydroxyl polyester. It is typical for formulators to use a polyester with a hydroxyl value between 30 and 50 which results in a product that has excellent mechanical properties and weatherability. Using a polyester resin with a higher OH value will result in increased chemical resistance at the expense of mechanical properties and so the needs of the consumer should be given careful consideration when doing so.
Although not a recommended practice polyurethanes can often be made cheaper by using less than the stoichiometric amount of urethane (isocyanate) resin. Since these urethane crosslinkers are often many times more expensive than the polyester resin using less can amount to significant cost savings. More interesting is that oftentimes using 80 to 90% of the stoichiometric amount of urethane cross linker results in no loss of performance!