Currently, Europe consumes more than 350,000 metric tons of powder coating per year. The annual rate worldwide is 1,000,000 metric tons - and rising (Figure 1). By , industry experts anticipate global demand to reach over 1.2 million metric tons, of which Europe will account for 500,000 metric tons.
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Polyurethane (PU) coatings are known for their excellent coating properties, good weather stability and versatility of formulation. In particular, powder coating technology - besides water-based dispersions - has made it possible to apply PU formulations in a solvent-free and, hence, environmentally friendly manner. However, conventional PU powder coatings release blocking agents during the hardening process, which are undesired both ecologically and economically.2 Internally blocked and, hence, emission-free PU powder coatings do not have this drawback.3 Unfortunately, up to now these blocking agent-free PU powder coatings required hardening temperatures of 180 °C and higher, excluding the possibility of applying these coatings on temperature-sensible substrates like wood, plastics and pre-assembled parts. In this article novel catalysts are described, which allow decreasing the hardening temperature to 120 °C and even lower.
b. Starting material: Isophorone diisocyanate (IPDI) uretdione were made at Degussa AG, Coatings & Colorants. Ethylhexanol and catalysts were purchased from Aldrich. The uretdione content was measured by reaction with butyl amine followed by back titration of excess amine with hydrochloric acid.
c. Powder coatings: Crosslinker: VESTAGON EP BF , Degussa AG, latent NCO-content 12.2 %, Tg 41- 44 °C; polyester resin: Crylcoat , Cytec, OH-value 58 mg KOH/g, Tg 57 °C; pigment: titanium dioxide, KRONOS , Kronos; acid scavenger: ARALDITE PT 912, Vantico; auxiliary agents: benzoin (degassing agent, Aldrich), RESIFLOW PV88 (levelling agent, Worlee.)
If we look at the thermodynamic equilibrium of the uretdione formation of IPDI we can observe that at low temperatures (around room temperature) the dimer is favored, whereas at 120 °C the monomer dominates (Figure 3). Thus, it is surprising that conventional curing of uretdione-based formulations takes place at 180 °C and higher. The reason for this discrepancy lies in a kinetic hindrance of the cleavage reaction. To overcome this obstacle, suitable catalysts have been searched for over the past 20 years and found only recently.
Although IPDI uretdione is a mixture of 10 different isomers,4 which vary in selectivity,5 reactivity and concentration, in this study we focused just on an average composition.
IPDI-uretdione was blocked irreversibly with 2-ethyl hexanol at the outer NCO-groups, leaving the uretdione ring intact (Figure 4). In a second step, the reaction of the excess 2-ethyl hexanol with the isocyanate units of the uretdione ring at 140 °C in the time curve then enabled the reactivity of selected catalysts to be determined. The content of the remaining uretdione groups was used as a measure of conversion.
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In addition, differential scanning calorimetry (DSC) was used as a hint for reactivity. In this way about 200 different catalysts were tested, and surprisingly, at least four different classes of useful catalysts were found. In Figure 5, the DSC curves can be seen for the uncatalyzed system, for dibutyltin dilaureate (DBTL) and for tetraalkyl ammonium carboxylate (TAAC) catalyzed formulations, respectively.
The activity of TAAC is strongly inhibited by acidic components, e.g., the remaining acid groups in OH-functional polyesters. In this case, compounds - so called acid scavengers - have to be added to intercept the acid groups and eventually to restore the activity of the catalyst. These acid scavengers are beneficial to adjust reactivity and in addition to improve the flow of the powder coating.
13C-NMR analysis revealed that besides the mainly formed urethane, isocyanurate groups are generated. The quantity of these structures depends on parameters like temperature, catalyst concentration and NCO/OH ratio. In the first few minutes of reaction even small amounts of allophanates could be detected, although after 30 minutes at 140 °C no allophanates are left, only urethanes (80%) and isocyanurates (20%). These side reactions lead to an increased consumption of isocyanate groups indicating a higher need of NCO compared with conventional polyurethane systems.
Table 1 shows powder formulation data.6 The NCO/OH ratio is 1.7:1. Obviously we were able to achieve good mechanical data and chemical resistance at 30 minutes at 130 °C and even lower. The reactivity can be adjusted either by the concentration of the catalyst or of the acid scavenger.
Highly reactive formulations tend to show pre-reaction during the processing of the powder. Even more detrimental to a good smoothness of the film is the start of the crosslink reaction before a sufficient flow can occur. Following the Arrhenius equation it is almost impossible to have a stable system at 120 °C for processing and good flow and at the same time show full conversion at 130 °C. However, due to the inhibiting effect of acids we gain some time for flowing as long as the reaction of acids and epoxides is not completed. Thus, we are able to achieve a fair smoothness (6), though not an excellent one (10). As a matter of fact, it is even possible to improve reactivity and smoothness significantly with Tg decreasing formulation ingredients, but usually those tend to influence the storage stability unfavorably.7Accelerated weathering tests (QUV-A, QUV-B, weather-o-meter) revealed a weather resistance of this novel low-temperature curing system, which is similar to conventional polyurethane formulation (cured at 200 °C) based on the same composition (Figure 7).6 If superdurable polyester resins are used, however, the weatherability is enhanced significantly. This stresses that the resin, not the hardener nor the catalyst, is the fragile link in this chain.
This paper was presented at the 8th Nürnberg Congress, Creative Advances in Coatings Technology, April in Nürnberg, Germany. The Congress is sponsored by FPL, PRA and the Vincentz Network.
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