Super durable Powder Coatings
Crosslinking technology is at the core of any successful powder coating. Without efficient crosslinking the properties associated with powder coatings such as chemical resistance, UV-outdoor resistance and mechanical properties such as impact and scratch resistance would not be possible (Orleans et al., 1998). For architectural coatings and super durable coatings, the so-called epoxy functional crosslinkers Triglycidyl Isocyanurate (TGIC) or Araldite and hydroxyalkyl-amide (HAA) or PRIMID are used (Foster, 2004).
Idealized base structure of a polyester resin
Schematic 1:
Schematic 1 illustrates the structure of an idealized polyester resin with both acidic and alcoholic functionality. The functionality can of course be varied by increasing or decreasing the ratios of organic acids and glycols. In Schematic 1, the use of iso-phthalic acid in the polyester structure is higher, leading to a so-called super-durable polyester. Due to the presence of isophthalic acid in the polymer backbone, properties such as flexibility, corrosion resistance and chemical resistance is enhanced. Isophthalic acid is also not as prone to hydrolysis as phthalic acid based polyesters (Dagher et al., 2004).
Examples of di-acids and glycols used in the synthesis of polyester resins
Schematic 2:
In Schematic 2, some of the acids and glycols used to synthesize polyesters in general are shown. The type of acid and the type of alcohol has a lot of influence on the flexibility, durability and chemical resistant properties and it is therefore of interest to use the correct type of crosslinker to gain maximum performance from the polyester, be it conventional or super-durable polyesters.
TGIC cured polyesters are generally linear carboxylic acid terminated polymers (Danick, 2004). They are cured for 10 to 15 minutes at 160 to 180 ºC. The chemical reaction involves a reaction between the carboxylic acid group and the oxirane functionality of the TGIC. TGIC cured polyesters provide robust polyesters which remains a sought-after property in any manufacturing environment. By this is meant:
- Film thickness is not critical. It works in both thin and thick films leaving cured, blemish free films.
- It has exterior durability and corrosion resistance.
- It shows color stability and offers resistance to over-bake conditions.
Super durable polyesters have become the benchmark in performance for all coatings, not just powder coatings (Ozsagiroglu et al., 2012). These polymers offer value for money, and they are designed to keep their color and gloss for periods of 5 to 10 years longer compared to conventional polyesters. These enhanced properties have created a demand for them in interior architectural coatings as well as outdoor applications.
The chemical differences between TGIC and HAA is best explained by looking at their crosslinking mechanisms.
Crosslinking reaction of TGIC with a polyester (PES) containing an alcohol group (OH)
Schematic 3
TGIC crosslinking can take place from 160 to 180 ºC and there is no splitting out of water molecules. Curing can also be accelerated by including a catalyst in the powder formulation. In contrast to this, HAA or Primid sees the splitting out of water and requires a slightly higher curing temperature compared to TGIC.
Crosslinking reaction of HAA with a polyester (PES) containing a carboxylic acid group (COOH)
Schematic 4
The presence of water above its boiling point in the polyester matrix can lead to surface defects in the final film such as pinholes and blistering. However, the addition of additives such as benzoin can help to eliminate surface defects and there is no reason why thin and thicker films can effectively be sprayed and cured.
Catalysts for HAA crosslinkers have not been identified as in the case of TGIC. In the light of this, an option to consider is to increase the crosslinking reactivity of HAA crosslinkers by using higher molecular weight polyesters with similar functionality.
Regional players and market drivers
The powder coatings market is estimated to be EUR 10 – 12 billion with a bullish compound annual growth rate (CAGR) of upwards of 6 % (Silva, 2021). China consumes about 50% of the global market in powder coatings in terms of local demand.
Estimated global demand for powder coatings in 2021
Figure 1:
The growth in powder coating demand globally has resulted in raw material shortages of TGIC and HAA. In addition, it is recognized that powder coatings offer greater environmental protection and is displacing older coating technologies at a rapid rate in multiple market segments such as architectural/extrusion, furniture, appliances, heavy duty and agricultural as well as construction equipment. Another factor benefitting the growth in powder coatings is the consolidation seen in the industry by Akzo Nobel and PPG acquisitions of Stahl Performance Powder Coatings and Alpha Coating Technologies respectively. In the USA, the estimated percentage of each market segment for powder coatings is given in Figure 2.
Size of market segments as percentages in the USA
Figure 2:
The availability and regional selectivity when it comes to crosslinkers depend very much on the countries or areas of the globe where they are being used. TGIC has been on the US Environmental Protection Agency’s list but is still used in that country. In Europe it is frowned upon as it is regarded as a category 2 mutagen. The raw materials to make TGIC may also be in short supply due to disrupted shipping supply lines from China to the USA, COVID 19 and severe weather experienced in 2021. In Europe, HAA is used widely.
Because of the factors discussed above it is to be expected that the market demand for TGIC is expected to grow only modestly at 2 – 3% (Ncn & Click, 2022). The estimated market size of TGIC is USD 110 – 115 billion. In 2019 it was estimated that the market size for TGIC in the USA was 43.5% while HAA was only 3 – 4% while it is expected to grow to 5 – 7% at the expense of TGIC in coming years. Although TGIC has held a dominant position in the USA since the 1970’s and HAA held dominance in Europe since the 1990’s, the following drivers may shape and even the playing field even further in favor of non-TGIC crosslinkers (Biller & Powder, 2021):
SUSTAINABILITY
Sustainability addresses the economic, environmental and social impact of technology. Although alternative feedstocks such as Allnex’s C5 and C6 sugar based polyester resins are an example, lower curing products use less energy while coatings with greater durability saves energy, materials and labor(Challener, 2018). Recycling is embraced by Allnex by incorporating 25% recycled polyethylene terephthalate (PET) from recycled bottles into their polyester resins. Similarly, Sherwin Williams has the Powdura ECO Hybrid coating line in which each pound of powder is equivalent to 10 PET bottles. Batelle Memorial Institute has designed a powder resin based on renewable plant-based feedstock and low temperature curing technology.
LOW TEMPERATURE CURING
Although a sub-genre of sustainability, low temperature curing as exhibited by both TGIC and HAA, remains an active field of investigation. Shorter curing times remains the goal to drive crosslinker development. Ideally, a curing time of 10 to 15 minutes at 150 to 160 ºC is an achievable goal.
HYPER-DURABILITY
To apply fresh coats of paint is costly, can involve coatings with high volatile organic component (VOC) levels, and is labor intensive. Powder coatings, especially the super-durable polyester resins increase longevity by enhanced UV durability and corrosion resistance. These “super-durable” polyester powder coatings are based on isophthalic acid and has been described previously.
Conclusion
Conventional and super durable polyester resins can both use TGIC and HAA as crosslinkers. In the case of TGIC, possible mutagenic effects preclude its use in Europe while it is still widely used in the USA and China. Stricter environmental requirements and the drive to source sustainable answers to tomorrows social and economic challenges, may prompt a move towards the use of HAA. TGIC may see a corresponding loss in market share in the USA and possibly worldwide because of this. Furthermore, COVID 19, supply chain issues between the USA and China, has negatively influenced the availability of TGIC. It is possible that global events such as the conflict in the Ukraine will only add to a distressed TGIC market. Its saving grace is the entrenched use in the USA and the robustness of the powder application process associated with TGIC.
References
Biller, B. K., & Powder, T. (2021). New Advances in Powder Coating Technology. Focus on Powder Coatings, 2021(8), 4–5. https://doi.org/10.1016/j.fopow.2021.07.019
Challener, C. (2018). Market Update: Advances in Powder Coatings – American Coatings Association. Coating Tech, Vol. 15, No. 8. https://www.paint.org/coatingstech-magazine/articles/market-update-advances-in-powder-coatings/
Dagher, H. J., Iqbal, A., & Bogner, B. (2004). Durability of Isophthalic Polyester Composites Used in Civil Engineering Applications. 12(3), 169–182.
Danick, C. (2004). Resin and Cross-linker Chemistry for Powder Coatings By :
Foster, D. (2004). The effect of crosslinking chemistry on superdurable powder coatings. Industrial Paint and Powder, 80(2), 17–20.
Ncn, W., & Click, L. (2022). Triglycidyl Isocyanurate ( TGIC ) Market Size In 2022 is estimated to grow at a modest CAGR of more than 2 . 3 % During the forecast period 2022-2026 with Top Countries Data | In-depth 131 Pages Report. 8–11.
Orleans, N., Danick, A. C., & Technologies, M. (1998). Low Temperature Crosslinking for Powder Coatings Presented at “ Crosslinking for the Coatings Chemist ” at the 1998 Federation of Societies for Coatings Technology 1998 FSCT International Coatings Conference.
Ozsagiroglu, E., Iyisan, B., & Guvenilir, Y. A. (2012). Biodegradation and characterization studies of different kinds of polyurethanes with several enzyme solutions. Polish Journal of Environmental Studies, 21(6), 1777–1782.
Silva, S. (2021). Market Report : Driving growth in powder. 9–11.