TGIC and HAA crosslinkers for 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 standard – and superdurable architectural coating, the so-called epoxy functional crosslinker triglycidyl isocyanurate (TGIC) and the hydroxy functional β-hydroxyalkyl-amide (HAA) are used (Foster, 2004).
Schematic 1 illustrates the structure of an idealized TGIC polyester resin with both acidic and hydroxyl 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 isophthalic acid in the TGIC polyester structure is higher, leading to a so-called superdurable polyester. Due to the presence of isophthalic acid in the polymer backbone, properties such as corrosion resistance and chemical resistance are enhanced. Isophthalic acid is also not as prone to hydrolysis as terephthalic acid based polyesters (Dagher et al., 2004).
In Schematic 2, some of the acids and glycols used to synthesize a TGIC polyester (and HAA polyester) in general are shown. The type of acid and the type of alcohol has a lot of influence on the flexibility, durability and chemical resistance properties. It is therefore critical to get the raw material selection correct to ensure the desired coating properties. (Foster, 2004), (Orleans et al., 1998), (Coating et al., 2021).
Schematic 2. Examples of di-acids and glycols used in the synthesis of polyester resins.
TGIC cured polyesters are generally linear carboxylic acid terminated polymers (Danick, 2004). Generally, they are cured for 10 to 15 minutes at 160 to 200ºC. The chemical reaction involves a reaction between the carboxylic acid group and the oxirane functionality of the TGIC. TGIC cured polyesters provide robust coatings which remains a sought-after property in any manufacturing environment. By this it 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.
Superdurable TGIC 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.
TGIC powder coatings have become the de facto standard for exterior durability. Mechanical (impact resistance) and chemical resistance have already been mentioned but it is especially their outstanding UV resistance that give them exterior durability that is head and shoulders above any standard solvent-, or water borne coating or indeed, any urethane based powder coating (Environmentally & Choice, 2017), (Liberto, n.d.). This UV resistance is directly related to the chemical structure of the polyester, and in particular for superdurable TGIC polyester resins.
Chemical crosslinking mechanisms of TGIC and HAA polyesters
The chemical differences between TGIC and HAA polyester is best explained by looking at their crosslinking mechanisms.
TGIC crosslinking can take place from 160 to 200 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 sees the splitting out of water and requires a slightly higher curing temperature compared to TGIC.
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, it is possible to increase the functionality of polyester to increase reactivity.
Powder coating transfer efficiencies
Given the advantages of TGIC powder coatings discussed to this point, it is crucial to remember that all the advances and technology to produce the best coatings in the world can be undone if the proper application procedures are not followed. Here some tips are offered to improve first-pass transfer efficiency (Guides & Mohar, 2015).
If powder particles are too small or too big, transfer efficiency cannot be achieved. Generally, it is found that particles between 25 to 75 microns achieve 70% transfer efficiency. Below or above this leads to inefficiencies.
Powder coating particles require an electric charge to adhere to the metal frame. The whole interplay between force, voltage and microamps needs to be optimized to get the most efficient coating. A crucial part of the process is the corona gun that provides a high voltage of 100 kV or higher through an optimally placed electrode inside the gun. Other factors that have to be taken into consideration include distance of the gun from the substrate, using the voltage of the gun to push the powder particles to the edges, avoiding back ionization by employing a current limiter.
Venturi air pumps used before 2008, used compressed air to drive powder to the gun but the high velocity at which this occurred had a detrimental effect on transfer efficiency. Newer venturi pump systems have overcome these flaws. In these designs, atomizing air maintains the correct velocity. A separate airflow operates to regulate the amount of powder fed to the gun.
Digitally controlled dense phase technology
Recently, digitally controlled dense-phase technology has been commercialized. The amount of airflow is controlled to ensure reproducible coating efficiencies. It is reported that the dense-phase technology can deliver a denser cloud of powder at a reduced velocity, and this largely overcomes aerodynamic forces that works against powder application.
Powder booth design
Booths with metal sides affect first pass efficiency by acting as a magnet for the powder (steel, stainless steel). It is important to install to use the right design and in recent years, propriety engineered plastics have become the materials of choice in this regard. For example, a non-conductive plastic such as polypropylene will attract a lot less powder compared to a stainless-steel booth.
To obtain more powder on parts require more parts in front of the gun. If it is possible, electronic monitoring of the lines can help to optimize the most efficient number of parts in front of the booth. In a really integrated environment the electronic monitoring and powder control systems to the guns could be integrated. However, the initial cost has to be recovered quickly for this to make sense financially. In addition, the racks and the hooks have to be free of powder to increase conductivity.
Arguably the biggest success factor are the technicians that are part of the operations. If preventative maintenance is not a top priority, productivity will quickly fall and with it, profitability.
Powder coating market dynamics
The powder coatings market is estimated to be EUR 10 – 12 billion with a bullish compound annual growth rate in the region of 6% (Silva,2021). China consumes about 50% of the global market in powder coatings in terms of local demand.
The growth in powder coating demand globally has resulted in raw material shortages of TGIC and HAA (PF Online, 2021). In addition, it is recognized that powder coatings offer greater environmental protection and can displace older coating technologies in multiple market segments such as architectural extrusions, furniture, appliances, heavy duty and agricultural as well as construction equipment (Us & Subscription, 2022).
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. The estimated percentage of each market segment for global powder coatings is given in 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 the European Union 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 the European Union, HAA is used widely.
Expected market demand for TGIC and HAA
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). 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 European Union since the 1990’s, the following drivers may shape and even the playing field even further in favor of TGIC free crosslinkers (Biller & Powder, 2021):
Sustainability addresses the economic, environmental and social impact of technology. Although alternative feedstocks such as Allnex’s C5 and C6 sugar based polyester TGIC resins are an example, lower curing products use less energy while coatings with greater durability saves energy, materials and labor (Challener, 2018).
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 polyester and binder development. Today, a curing time of 10 to 15 minutes at 140 to 160 ºC is an achievable goal.
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 superdurable TGIC polyester resins increase longevity by enhanced UV durability and corrosion resistance. These “superdurable” polyester powder coatings are based on isophthalic acid and has been described previously.
Standard durable and superdurable polyester resins can both use TGIC and HAA as crosslinkers. In the case of TGIC, possible mutagenic effects preclude its use in the European Union 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 free).
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.
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