How to reduce powder coating cure temperature
Powder Coating Temperature & Improving Energy Efficiency
Improving the energy efficiency of industrial coatings, has along with environmental concerns, become a crucial challenge facing the world today. In the past, concerns were raised by energy and environmental watch groups about the impacts on the world economy, the quality of people’s personal lives and the negative impact on fauna and flora.
To introduce energy efficiency and lower the powder coating temperature, aspects of the process, management and the chemistry of powder coatings must be assessed.
Powder Coating Process and Management
The management of the powder coating process to reduce the temperature and improve energy efficiency is of cardinal importance. It is also important to understand the various processes involved with powder coating thoroughly. Only by having an in-depth understanding can interventions to help lower energy consumption be realized. Some key areas where management and process intervention can lower energy consumption are (Aquino, 2007):
Minimizing high temperature powder coating operations
An immediate example is the use of lower curing temperature powder coating resins. It is estimated that lowering the cure oven by 24 ºC could save up to 600 000Btu/h. A low powder coating temperature releases significant energy savings in dollar or Euro terms.
Containment of heated air
Installing air shields at appropriate places can prevent hot air from escaping and carrying with it massive amounts of heat that is being wasted. Introduction of air seals in the ovens could for example enable shortened curing times and faster conveyor speeds.
Investing in automation and control.
More advanced process operation equipment can unlock the potential of monitoring each area of intense energy use and optimizing it for a specific product. Real-time tracking of heating profiles of specific parts in the process can identify which of the many parts of the powder coating process are working efficiently or not.
Powder Coating Chemistry: Low Energy Consumption
To achieve further energy savings, lowering the powder coating temperature involves modifying the resins. Achieving lower curing temperature resins involves an understanding of the complex interaction of the molecular weight, molecular structure and functionality of the resin polymers. Polymer molecular weights, for example in polyethers contribute to changes in the melting points and crystallization temperatures (Johansson et al., 2020).
Figure 1. Poly (ethylene glycol) melting temperature and crystallization temperature as a function of molecular weight.
Figure 1 illustrates the non-linear dependance of the melting point and the crystallization temperature of selected poly (ethylene glycols). A similar relationship exists for other aliphatic polymers including aliphatic polyesters (Shen et al., 2017), Polymer molecular structure has a significant influence on the properties of polymers such as the glass transition temperatures for industrial polyesters. Structures of polyesters in Figure 2 illustrate that the rigidity of the polymer increases through the incorporation of groups that contain benzene rings with functionality (terephthalic acid and iso-phthalic acid). These groups disrupt the ability of the polymer chain to rotate and in doing so increase the glass transition and melting point temperatures.
Figure 2. The influence of molecular structure on the glass transition temperature and melting points of some selected polyesters and a polycarbonate.
The reason why polymers such as poly (ethylene glycol) and poly (oxymethylene) have low melting points and crystallinity has to do with their structure. The structure of these polymers allows for rotation around the oxygen atoms as well as the carbon atoms without hinderance leading to less energy required to melt and flow.
Figure 3. An example of a polyethylene glycol. The red arrow indicates a swivel point
In Figure 3 the principle that allows the low melting- and crystallinity temperatures of polyether is explained. The oxygen and carbon atoms allow for movement and act as swivel points to give the polymer chain great mobility. The principles illustrated in the examples above are used to design polymers that have defined micro-structures such as Spandex, a polyurethane whose molecular structure and its contribution to certain properties at a microscopic scale is depicted below.
Figure 4. The molecular structure of Spandex. The concepts explained previously are combined to give hard and soft segments in the same molecule.
The principles that govern the physical properties of soft rubbery polymers such as poly (ethylene glycol) and the segments that resist rotation and contribute to higher glass transitions are all found in the same molecular repeat structure. It is also the basis on which resins are designed to allow for lower curing temperatures.
A further aspect of the molecular manipulation of resin structures is the functionality. Allnex has introduced a series of polyesters with low curing temperatures and various functionalities.
Figure 7. A theoretical polyester structure that allows for lower melting temperatures and greater functionality.
An example of a possible polyester with enhanced COOH functionality as well as a molecular design that allows it to melt and flow at a lower temperature (essential for low temperature crosslinking), is shown in Figure 5. In Figure 5, the blue arrows indicate molecular swivel points in the molecular structure due to the introduction of monomers such as propylene glycol and enhanced functionality due to trimellitic acid. Hybrid polyester/epoxy coatings are not new but companies such as Allnex have been able to produce COOH functional polyesters and epoxy resins with epoxy equivalent weights (EEW) of 680 to 800 g.eq-1 with much lower curing requirements. An example of a high molecular weight epoxy resin is given below in Figure 6.
Figure 6. Allnex has 50/50 polyester/epoxy blends with curing temperatures of 140 °C for 10 minutes and at 180 °C for 1 minute.
Crylcoat® Powder Coating Resins
Allnex polyesters under the brand name of Crylcoat® designed for the 50/50 epoxy hybrid systems typically have acid values in the region of 70 mg KOH/g. The carboxylic acid functionality reacts with the epoxy functionality to provide a thermoset as illustrated in Schematic 1 below.
Schematic 1. An example of an epoxy-functional resin reacting with a polyester having carboxylic acid functionality.
There systems are able to lower the curing temperature or powder coatings down to 140°C-130°C depending from the chosen resin and the required properties that the final coating has to achieve; moving to different polyester/epoxy ratio, the curing temperature is slightly increased to 160°C for the new developed resins, such as CRYLCOAT® 1757-6, an highly reactive TMA-free polyester used in the production of 70/30 hybrid powder coating.
Powder coating with hydroxy alkyl amides (HAA) or Primid
Allnex also provides polyesters that can react with hydroxy alkyl amides (HAA) such as Primid XL 552 at much reduced temperatures. Typical curing temperatures using low temperature curing polyesters and HAA are for example 160 °C for 10 minutes (Schematic 2). Depending on the area of application (industrial or architectural), the acid value of the polyester resins can vary from as low as 16 mg KOH/g to 90 mg KOH/g.
Schematic 2. The curing reaction involving hydroxyalkyl amide (HAA) and a carboxylic acid-functional polyester.
Powder coating with tryglycidylisocyanurate (TGIC)
Low temperature curing resins based on tryglycidylisocyanurate (TGIC) are also available. The TGIC is known as ARALDITE PT 810. Curing regimes for TGIC-based low curing polyesters include 12 minutes at 140 °C and 4 minutes at 200 °C (Schematic 3). This curing regime is also operative for the glycidyl ester curing agent ARALDITE PT 910 and 912.
Schematic 3. Curing process involving TGIC and acid-0functional polyester resin.
Additional improvements in low temperature crosslinking involves the use of polymers that are easily hydrolyzed or attacked by species that can react with the functional groups in the polymer backbone. A system such as this is described in EP 3478774A1 (Weaver et al., 2017). The synthesis of polyanhydrides are covered by Ghosh (Ghosh et al., 2022). The reaction is a steady build-up of molecular weight by splitting out water. A typical acid that can be considered for this type of reaction is sebacic acid.
Figure 7. molecular weight increase of a difunctional organic acid to yield a poly anhydride. In this case m < n < x.
Polyanhydrides are used in slow-release applications (Carbone & Uhrich, 2009). Depending on the structure and molecular weight of the polyanhydride, the rate at which hydrolysis can occur may be controlled.
Figure 8. Hydrolysis of polyanhydride based on salicylic acid and sebacic acid.
In Figure 8, water is shown as the nucleophile that attacks the polyanhydride copolymer. In the case of powder coatings, polymers with hydroxy -, carboxylic acid – and especially amine functionality can be successfully combined to provide low temperature curing powder coatings.
Revolutionize Your MDF Powder Coating with Low Temperature Curing
Aquino, R. (2007). Three steps to lower energy use. Products Finishing. https://www.pfonline.com/articles/three-steps-to-lower-energy-use
Carbone, A. L., & Uhrich, K. E. (2009). Design and synthesis of fast-degrading poly(anhydride-esters). Macromolecular Rapid Communications, 30(12), 1021–1026. https://doi.org/10.1002/marc.200900029
Ghosh, R., Arun, Y., Siman, P., & Domb, A. J. (2022). Synthesis of Aliphatic Polyanhydrides with Controllable and Reproducible Molecular Weight. Pharmaceutics, 14(7). https://doi.org/10.3390/pharmaceutics14071403
Johansson, P., Paberit, R., Rilby, E., Göhl, J., Swenson, J., Refaa, Z., & Jansson, H. (2020). Cycling stability of poly(ethylene glycol) of six molecular weights: Influence of thermal conditions for energy applications. ACS Applied Energy Materials, 3(11), 10578–10589. https://doi.org/10.1021/acsaem.0c01621
Shen, J., Caydamli, Y., Gurarslan, A., Li, S., & Tonelli, A. E. (2017). The glass transition temperatures of amorphous linear aliphatic polyesters. Polymer, 124(March 2018), 235–245. https://doi.org/10.1016/j.polymer.2017.07.054
Weaver, I. W., Watson, R., Hendrikus, R., Brinkhuis, G., Bosma, M., Jones, P., White, S., Baxevanis, D., & Cavalieri, R. (2017). EP3478774A1 Low temperature cure powder coating compositions.