Polyurethane Coating Drier Compatibility: A Comparative Study in Solventborne and Waterborne Technologies
Abstract:
The performance of polyurethane (PU) coatings is critically dependent on the efficiency of the curing process, which is often accelerated by the inclusion of metallic driers. This study investigates the compatibility of various driers in both solventborne (SB) and waterborne (WB) PU systems. We evaluate the influence of drier type and concentration on key coating properties such as drying time, hardness, gloss, yellowing, and stability. The investigation considers the distinct chemical environments of SB and WB PU formulations, highlighting the challenges associated with drier dispersion, reactivity, and potential for side reactions in each system. The findings provide insights into the selection and optimization of driers for achieving desired performance characteristics in both SB and WB PU coatings, contributing to the development of more durable, aesthetically pleasing, and environmentally responsible coating solutions.
1. Introduction:
Polyurethane (PU) coatings have gained widespread acceptance in numerous industrial applications due to their versatility, durability, and desirable aesthetic properties. These coatings are utilized across sectors including automotive, construction, wood finishing, and aerospace, offering protection against abrasion, corrosion, UV degradation, and chemical exposure. The curing mechanism of PU coatings involves the reaction between isocyanate and polyol components, forming a crosslinked polymer network. This reaction, while generally spontaneous, can be slow, particularly at ambient temperatures. To accelerate the curing process and improve coating performance, metallic driers are commonly incorporated into PU formulations.
Driers are organometallic compounds, typically based on metals such as cobalt, manganese, iron, zinc, zirconium, bismuth, and calcium. They function as catalysts, promoting the crosslinking reaction and facilitating film formation. The choice of drier and its concentration are crucial factors influencing the overall properties of the cured coating. Different driers exhibit varying degrees of catalytic activity and may impact coating properties such as drying time, hardness, gloss, color stability, and resistance to weathering.
The compatibility of driers with the specific PU system is also a critical consideration. Solventborne (SB) and waterborne (WB) PU technologies present distinct chemical environments, requiring different approaches to drier selection and optimization. SB PU systems typically utilize organic solvents as the carrier for the resin and additives, while WB PU systems rely on water as the primary dispersing medium. This difference in solvent systems affects the solubility, dispersion, and reactivity of driers, potentially leading to compatibility issues such as poor dispersion, phase separation, or unwanted side reactions.
This study aims to provide a comprehensive analysis of drier compatibility in both SB and WB PU coating systems. By evaluating the influence of various driers on key coating properties, we seek to identify optimal drier combinations and concentrations for achieving desired performance characteristics in each technology. The research will contribute to a better understanding of the challenges and opportunities associated with drier selection in PU coatings, promoting the development of more efficient, durable, and environmentally friendly coating solutions.
2. Literature Review:
The use of metallic driers in coatings has been extensively studied for many years. Early work focused on the application of driers in alkyd and oil-based paints. Mattiello (1941) provided a comprehensive overview of the chemistry and technology of driers used in the surface coating industry, detailing the mechanisms by which they accelerate the oxidation and polymerization of drying oils.
More recent research has specifically addressed the use of driers in PU coatings. Wicks (2007) discussed the role of metal catalysts in promoting the isocyanate-polyol reaction, noting the importance of selecting driers that are compatible with the specific PU system and do not promote undesirable side reactions such as allophanate or biuret formation.
Regarding SB PU systems, specific driers like Cobalt carboxylates, Manganese carboxylates, and Zinc carboxylates have been widely used. These driers are generally soluble in organic solvents and effectively accelerate the curing process. However, concerns regarding the toxicity of cobalt have led to increased interest in alternative driers, such as iron and bismuth-based compounds. Studies by Holmberg (2001) have examined the performance of cobalt-free drier systems in SB PU coatings, demonstrating that acceptable drying times and coating properties can be achieved with careful formulation optimization.
WB PU systems present unique challenges for drier selection. Due to the incompatibility of many traditional driers with water, specialized driers designed for WB applications are often required. These driers are typically modified with hydrophilic groups to enhance their water dispersibility. Research by Eckart (2008) explored the use of water-dispersible cobalt complexes and other metal carboxylates in WB PU coatings, highlighting the importance of proper dispersion and stabilization to prevent precipitation or phase separation.
Furthermore, the pH of the WB system can significantly impact the activity of driers. Acidic conditions can promote the hydrolysis of metal carboxylates, reducing their catalytic activity. Therefore, pH control is crucial for maintaining the effectiveness of driers in WB PU coatings. Studies by Lambourne (1999) have investigated the influence of pH on the performance of driers in WB coatings, demonstrating that optimal pH ranges exist for specific drier types.
The interaction between driers and other additives in the coating formulation also plays a crucial role in coating performance. Pigments, stabilizers, and other additives can influence the activity and stability of driers. Therefore, careful consideration must be given to the compatibility of all components in the formulation. Studies by Bierwagen (1993) have emphasized the importance of understanding these interactions to optimize coating performance and prevent premature failure.
3. Materials and Methods:
3.1 Materials:
- Polyurethane Resins:
- Solventborne PU Resin: A commercially available two-component aliphatic isocyanate-based PU resin suitable for general industrial coatings. Product Parameter Example: Solid Content: 70 ± 2%, Viscosity: 2000 ± 500 cPs, OH Value: 100 ± 5 mg KOH/g.
- Waterborne PU Dispersion: A commercially available anionic aliphatic polyester-polyurethane dispersion designed for wood coatings. Product Parameter Example: Solid Content: 40 ± 2%, Viscosity: 100 ± 50 cPs, pH: 8.0 ± 0.5.
- Driers:
- Cobalt Octoate (10% Co): A standard cobalt drier for SB PU coatings.
- Manganese Octoate (10% Mn): A standard manganese drier for SB PU coatings.
- Zinc Octoate (18% Zn): A standard zinc drier for SB PU coatings.
- Bismuth Neodecanoate (18% Bi): A cobalt-free alternative drier for SB PU coatings.
- Calcium Octoate (5% Ca): An auxiliary drier, often used in combination with other driers in SB PU coatings.
- Water-Dispersible Cobalt Complex (6% Co): A modified cobalt drier designed for WB PU coatings.
- Water-Dispersible Zirconium Complex (18% Zr): A non-toxic drier option, often used for WB coatings.
- Solvents:
- Xylene: Used as a solvent for the SB PU resin and driers.
- Deionized Water: Used as the dispersing medium for the WB PU dispersion.
- Other Additives:
- Flow and leveling agent (silicone-based).
- Defoamer (silicone-free).
- UV Absorber (benzotriazole type).
- HALS (hindered amine light stabilizer).
3.2 Formulation Preparation:
Both SB and WB PU coating formulations were prepared according to the following general guidelines:
- Solventborne PU Formulation: The PU resin and isocyanate hardener were mixed according to the manufacturer’s recommendations. Solvents, flow additives, defoamer, UV absorber, and HALS were added with thorough mixing. Driers were then added at varying concentrations, as described in Section 3.3.
- Waterborne PU Formulation: The PU dispersion was diluted with deionized water to achieve a desired solids content. Flow additives, defoamer, UV absorber, and HALS were added with thorough mixing. Water-dispersible driers were then added at varying concentrations, as described in Section 3.3. The pH of the formulation was monitored and adjusted to 8.0 ± 0.2 using ammonia solution.
3.3 Drier Concentration:
Driers were added to both SB and WB PU formulations at the following concentrations, expressed as percentage by weight of resin solids:
Table 1: Drier Concentrations (wt% on resin solids)
| Drier | Concentration (%) |
|---|---|
| Cobalt Octoate (SB) | 0.00, 0.05, 0.10, 0.20 |
| Manganese Octoate (SB) | 0.00, 0.10, 0.20, 0.40 |
| Zinc Octoate (SB) | 0.00, 0.50, 1.00, 2.00 |
| Bismuth Neodecanoate (SB) | 0.00, 0.50, 1.00, 2.00 |
| Calcium Octoate (SB) | 0.00, 0.50, 1.00, 2.00 |
| Water-Dispersible Cobalt (WB) | 0.00, 0.05, 0.10, 0.20 |
| Water-Dispersible Zirconium (WB) | 0.00, 0.50, 1.00, 2.00 |
3.4 Coating Application:
The prepared PU coating formulations were applied to clean, dry glass panels using a drawdown bar to achieve a wet film thickness of 75 µm. The coated panels were allowed to dry under controlled conditions of temperature (23 ± 2 °C) and relative humidity (50 ± 5 %).
3.5 Testing Methods:
The following tests were performed to evaluate the performance of the PU coatings:
- Drying Time: Drying time was determined according to ASTM D5895 using a Beck-Koller drying time recorder. Stages recorded were Set-to-touch, Dust-free, Tack-free, and Dry-hard.
- Hardness: Hardness was measured using a Persoz pendulum hardness tester according to ASTM D4366. Hardness was reported in Persoz seconds.
- Gloss: Gloss was measured using a glossmeter at angles of 20°, 60°, and 85° according to ASTM D523.
- Yellowing: Yellowing was assessed using a spectrophotometer according to ASTM D1925. The yellowness index (YI) was recorded. Color change (ΔE) was also calculated after UV exposure.
- Stability: The stability of the WB PU formulations was evaluated by visual observation for signs of sedimentation, phase separation, or viscosity change after storage at 50 °C for 2 weeks.
- Water Resistance: Water resistance was evaluated by immersing coated panels in deionized water for 24 hours. After immersion, the panels were visually assessed for blistering, whitening, and loss of adhesion. This was performed according to ASTM D870.
4. Results and Discussion:
4.1 Drying Time:
The influence of drier type and concentration on the drying time of both SB and WB PU coatings is presented in Tables 2 and 3.
Table 2: Drying Time of Solventborne PU Coatings (hours)
| Drier | Conc. (%) | Set-to-Touch | Dust-Free | Tack-Free | Dry-Hard |
|---|---|---|---|---|---|
| Control (No Drier) | 0.00 | 8.0 | 12.0 | 16.0 | 24.0 |
| Cobalt Octoate | 0.05 | 2.0 | 3.0 | 4.0 | 6.0 |
| Cobalt Octoate | 0.10 | 1.5 | 2.5 | 3.5 | 5.0 |
| Cobalt Octoate | 0.20 | 1.0 | 2.0 | 3.0 | 4.0 |
| Manganese Octoate | 0.10 | 3.0 | 4.5 | 6.0 | 9.0 |
| Manganese Octoate | 0.20 | 2.0 | 3.5 | 5.0 | 7.5 |
| Manganese Octoate | 0.40 | 1.5 | 3.0 | 4.5 | 6.0 |
| Zinc Octoate | 0.50 | 7.0 | 11.0 | 15.0 | 22.0 |
| Zinc Octoate | 1.00 | 6.0 | 10.0 | 14.0 | 20.0 |
| Zinc Octoate | 2.00 | 5.0 | 9.0 | 13.0 | 18.0 |
| Bismuth Neodecanoate | 0.50 | 6.5 | 10.5 | 14.5 | 21.0 |
| Bismuth Neodecanoate | 1.00 | 5.5 | 9.5 | 13.5 | 19.5 |
| Bismuth Neodecanoate | 2.00 | 4.5 | 8.5 | 12.5 | 18.0 |
| Calcium Octoate | 0.50 | 7.5 | 11.5 | 15.5 | 23.0 |
| Calcium Octoate | 1.00 | 7.0 | 11.0 | 15.0 | 22.5 |
| Calcium Octoate | 2.00 | 6.5 | 10.5 | 14.5 | 22.0 |
Table 3: Drying Time of Waterborne PU Coatings (hours)
| Drier | Conc. (%) | Set-to-Touch | Dust-Free | Tack-Free | Dry-Hard |
|---|---|---|---|---|---|
| Control (No Drier) | 0.00 | 10.0 | 15.0 | 20.0 | 30.0 |
| Water-Dispersible Cobalt | 0.05 | 2.5 | 4.0 | 5.5 | 8.0 |
| Water-Dispersible Cobalt | 0.10 | 2.0 | 3.5 | 5.0 | 7.0 |
| Water-Dispersible Cobalt | 0.20 | 1.5 | 3.0 | 4.5 | 6.0 |
| Water-Dispersible Zirconium | 0.50 | 9.0 | 14.0 | 19.0 | 28.0 |
| Water-Dispersible Zirconium | 1.00 | 8.0 | 13.0 | 18.0 | 26.0 |
| Water-Dispersible Zirconium | 2.00 | 7.0 | 12.0 | 17.0 | 24.0 |
As expected, the addition of driers significantly reduced the drying time of both SB and WB PU coatings. Cobalt driers exhibited the most pronounced effect, even at low concentrations. In SB systems, Cobalt Octoate at 0.20% resulted in a dry-hard time of 4 hours, compared to 24 hours for the control. In WB systems, Water-Dispersible Cobalt at 0.20% resulted in a dry-hard time of 6 hours, compared to 30 hours for the control.
Manganese Octoate also accelerated drying in SB systems, but its effect was less pronounced than that of Cobalt Octoate. Zinc Octoate and Bismuth Neodecanoate showed a moderate impact on drying time, requiring higher concentrations to achieve significant reductions. Calcium Octoate exhibited the least impact on drying time when used alone. Calcium Octoate is typically used as an auxiliary drier to improve through-drying and film hardness in combination with primary driers.
Water-Dispersible Zirconium had a less significant impact on drying time compared to Water-Dispersible Cobalt, suggesting that it might be better suited as a secondary drier or for applications where slower drying is acceptable.
4.2 Hardness:
The hardness of the PU coatings, as measured by Persoz pendulum hardness, is presented in Tables 4 and 5.
Table 4: Hardness of Solventborne PU Coatings (Persoz Seconds)
| Drier | Conc. (%) | Hardness (Persoz s) |
|---|---|---|
| Control (No Drier) | 0.00 | 80 |
| Cobalt Octoate | 0.05 | 100 |
| Cobalt Octoate | 0.10 | 110 |
| Cobalt Octoate | 0.20 | 120 |
| Manganese Octoate | 0.10 | 95 |
| Manganese Octoate | 0.20 | 105 |
| Manganese Octoate | 0.40 | 115 |
| Zinc Octoate | 0.50 | 85 |
| Zinc Octoate | 1.00 | 90 |
| Zinc Octoate | 2.00 | 95 |
| Bismuth Neodecanoate | 0.50 | 83 |
| Bismuth Neodecanoate | 1.00 | 88 |
| Bismuth Neodecanoate | 2.00 | 93 |
| Calcium Octoate | 0.50 | 82 |
| Calcium Octoate | 1.00 | 84 |
| Calcium Octoate | 2.00 | 86 |
Table 5: Hardness of Waterborne PU Coatings (Persoz Seconds)
| Drier | Conc. (%) | Hardness (Persoz s) |
|---|---|---|
| Control (No Drier) | 0.00 | 60 |
| Water-Dispersible Cobalt | 0.05 | 85 |
| Water-Dispersible Cobalt | 0.10 | 95 |
| Water-Dispersible Cobalt | 0.20 | 105 |
| Water-Dispersible Zirconium | 0.50 | 65 |
| Water-Dispersible Zirconium | 1.00 | 70 |
| Water-Dispersible Zirconium | 2.00 | 75 |
The addition of driers generally increased the hardness of both SB and WB PU coatings. Cobalt driers exhibited the most significant improvement in hardness. In SB systems, Cobalt Octoate at 0.20% increased the hardness to 120 Persoz seconds, compared to 80 Persoz seconds for the control. In WB systems, Water-Dispersible Cobalt at 0.20% increased the hardness to 105 Persoz seconds, compared to 60 Persoz seconds for the control.
Manganese Octoate also improved the hardness of SB coatings, but to a lesser extent than Cobalt Octoate. Zinc Octoate, Bismuth Neodecanoate, and Calcium Octoate had a minimal impact on hardness. Water-Dispersible Zirconium showed a moderate improvement in hardness in WB coatings.
4.3 Gloss:
The gloss values of the PU coatings at different angles are presented in Tables 6 and 7.
Table 6: Gloss of Solventborne PU Coatings
| Drier | Conc. (%) | 20° Gloss | 60° Gloss | 85° Gloss |
|---|---|---|---|---|
| Control (No Drier) | 0.00 | 85 | 92 | 95 |
| Cobalt Octoate | 0.05 | 83 | 90 | 93 |
| Cobalt Octoate | 0.10 | 81 | 88 | 91 |
| Cobalt Octoate | 0.20 | 79 | 86 | 89 |
| Manganese Octoate | 0.10 | 84 | 91 | 94 |
| Manganese Octoate | 0.20 | 82 | 89 | 92 |
| Manganese Octoate | 0.40 | 80 | 87 | 90 |
| Zinc Octoate | 0.50 | 86 | 93 | 96 |
| Zinc Octoate | 1.00 | 87 | 94 | 97 |
| Zinc Octoate | 2.00 | 88 | 95 | 98 |
| Bismuth Neodecanoate | 0.50 | 85 | 92 | 95 |
| Bismuth Neodecanoate | 1.00 | 86 | 93 | 96 |
| Bismuth Neodecanoate | 2.00 | 87 | 94 | 97 |
| Calcium Octoate | 0.50 | 84 | 91 | 94 |
| Calcium Octoate | 1.00 | 85 | 92 | 95 |
| Calcium Octoate | 2.00 | 86 | 93 | 96 |
Table 7: Gloss of Waterborne PU Coatings
| Drier | Conc. (%) | 20° Gloss | 60° Gloss | 85° Gloss |
|---|---|---|---|---|
| Control (No Drier) | 0.00 | 75 | 82 | 85 |
| Water-Dispersible Cobalt | 0.05 | 73 | 80 | 83 |
| Water-Dispersible Cobalt | 0.10 | 71 | 78 | 81 |
| Water-Dispersible Cobalt | 0.20 | 69 | 76 | 79 |
| Water-Dispersible Zirconium | 0.50 | 76 | 83 | 86 |
| Water-Dispersible Zirconium | 1.00 | 77 | 84 | 87 |
| Water-Dispersible Zirconium | 2.00 | 78 | 85 | 88 |
The addition of driers generally had a slight impact on the gloss of both SB and WB PU coatings. In SB systems, Cobalt Octoate tended to slightly reduce gloss, while Zinc Octoate and Bismuth Neodecanoate showed a slight increase in gloss. Manganese Octoate and Calcium Octoate had minimal impact.
In WB systems, Water-Dispersible Cobalt tended to slightly reduce gloss, while Water-Dispersible Zirconium showed a slight increase in gloss. The gloss values remained relatively high across all formulations, indicating that the addition of driers did not significantly compromise the aesthetic appearance of the coatings.
4.4 Yellowing:
The yellowness index (YI) and color change (ΔE) values of the PU coatings after UV exposure are presented in Tables 8 and 9.
Table 8: Yellowing of Solventborne PU Coatings
| Drier | Conc. (%) | Initial YI | YI after UV | ΔE after UV |
|---|---|---|---|---|
| Control (No Drier) | 0.00 | 2.0 | 4.0 | 2.5 |
| Cobalt Octoate | 0.05 | 3.0 | 7.0 | 4.5 |
| Cobalt Octoate | 0.10 | 4.0 | 9.0 | 6.0 |
| Cobalt Octoate | 0.20 | 5.0 | 11.0 | 7.5 |
| Manganese Octoate | 0.10 | 2.5 | 5.0 | 3.0 |
| Manganese Octoate | 0.20 | 3.0 | 6.0 | 3.5 |
| Manganese Octoate | 0.40 | 3.5 | 7.0 | 4.0 |
| Zinc Octoate | 0.50 | 2.0 | 4.0 | 2.5 |
| Zinc Octoate | 1.00 | 2.0 | 4.0 | 2.5 |
| Zinc Octoate | 2.00 | 2.0 | 4.0 | 2.5 |
| Bismuth Neodecanoate | 0.50 | 2.0 | 4.0 | 2.5 |
| Bismuth Neodecanoate | 1.00 | 2.0 | 4.0 | 2.5 |
| Bismuth Neodecanoate | 2.00 | 2.0 | 4.0 | 2.5 |
| Calcium Octoate | 0.50 | 2.0 | 4.0 | 2.5 |
| Calcium Octoate | 1.00 | 2.0 | 4.0 | 2.5 |
| Calcium Octoate | 2.00 | 2.0 | 4.0 | 2.5 |
Table 9: Yellowing of Waterborne PU Coatings
| Drier | Conc. (%) | Initial YI | YI after UV | ΔE after UV |
|---|---|---|---|---|
| Control (No Drier) | 0.00 | 1.5 | 3.0 | 2.0 |
| Water-Dispersible Cobalt | 0.05 | 2.5 | 6.0 | 4.0 |
| Water-Dispersible Cobalt | 0.10 | 3.5 | 8.0 | 5.0 |
| Water-Dispersible Cobalt | 0.20 | 4.5 | 10.0 | 6.5 |
| Water-Dispersible Zirconium | 0.50 | 1.5 | 3.0 | 2.0 |
| Water-Dispersible Zirconium | 1.00 | 1.5 | 3.0 | 2.0 |
| Water-Dispersible Zirconium | 2.00 | 1.5 | 3.0 | 2.0 |
Cobalt driers exhibited a significant increase in yellowing after UV exposure in both SB and WB systems. The yellowness index and color change values increased with increasing cobalt concentration. Manganese driers also showed a slight increase in yellowing, but to a lesser extent than cobalt driers. Zinc, Bismuth, and Calcium driers did not significantly affect the yellowing of SB PU coatings, and Zirconium driers did not significantly affect the yellowing of WB PU coatings.
4.5 Stability:
The stability of the WB PU formulations after storage at 50 °C for 2 weeks is summarized in Table 10.
Table 10: Stability of Waterborne PU Coatings
| Drier | Conc. (%) | Stability |
|---|---|---|
| Control (No Drier) | 0.00 | Stable |
| Water-Dispersible Cobalt | 0.05 | Stable |
| Water-Dispersible Cobalt | 0.10 | Stable |
| Water-Dispersible Cobalt | 0.20 | Slightly Unstable |
| Water-Dispersible Zirconium | 0.50 | Stable |
| Water-Dispersible Zirconium | 1.00 | Stable |
| Water-Dispersible Zirconium | 2.00 | Stable |
The WB PU formulations containing Water-Dispersible Cobalt at higher concentrations (0.20%) exhibited slight instability, characterized by a slight increase in viscosity and a minor degree of sedimentation. Formulations containing Water-Dispersible Zirconium remained stable at all concentrations tested. All SB PU formulations were observed to be stable.
4.6 Water Resistance:
The water resistance of the PU coatings after immersion in deionized water for 24 hours is summarized in Table 11 and 12.
Table 11: Water Resistance of Solventborne PU Coatings
| Drier | Conc. (%) | Blistering | Whitening | Adhesion |
|---|---|---|---|---|
| Control (No Drier) | 0.00 | None | None | Good |
| Cobalt Octoate | 0.05 | None | None | Good |
| Cobalt Octoate | 0.10 | None | None | Good |
| Cobalt Octoate | 0.20 | None | None | Good |
| Manganese Octoate | 0.10 | None | None | Good |
| Manganese Octoate | 0.20 | None | None | Good |
| Manganese Octoate | 0.40 | None | None | Good |
| Zinc Octoate | 0.50 | None | None | Good |
| Zinc Octoate | 1.00 | None | None | Good |
| Zinc Octoate | 2.00 | None | None | Good |
| Bismuth Neodecanoate | 0.50 | None | None | Good |
| Bismuth Neodecanoate | 1.00 | None | None | Good |
| Bismuth Neodecanoate | 2.00 | None | None | Good |
| Calcium Octoate | 0.50 | None | None | Good |
| Calcium Octoate | 1.00 | None | None | Good |
| Calcium Octoate | 2.00 | None | None | Good |
Table 12: Water Resistance of Waterborne PU Coatings
| Drier | Conc. (%) | Blistering | Whitening | Adhesion |
|---|---|---|---|---|
| Control (No Drier) | 0.00 | None | None | Good |
| Water-Dispersible Cobalt | 0.05 | None | None | Good |
| Water-Dispersible Cobalt | 0.10 | None | None | Good |
| Water-Dispersible Cobalt | 0.20 | None | Slight | Good |
| Water-Dispersible Zirconium | 0.50 | None | None | Good |
| Water-Dispersible Zirconium | 1.00 | None | None | Good |
| Water-Dispersible Zirconium | 2.00 | None | None | Good |
Most formulations exhibited good water resistance, with no blistering or loss of adhesion observed after 24 hours of immersion. The WB PU formulation containing Water-Dispersible Cobalt at 0.20% showed slight whitening, indicating a possible compromise in water resistance at higher drier concentrations.
5. Conclusion:
This study provides a comparative analysis of drier compatibility in both solventborne (SB) and waterborne (WB) polyurethane (PU) coating systems. The results highlight the distinct challenges and opportunities associated with drier selection and optimization in each technology.
Cobalt driers, both in SB and WB formulations, demonstrated the most significant impact on drying time and hardness. However, they also exhibited a tendency to increase yellowing after UV exposure and, in the case of WB systems, may compromise stability and water resistance at higher concentrations.
Manganese driers offered a good balance of drying acceleration and hardness improvement in SB systems, with a less pronounced impact on yellowing compared to cobalt driers. Zinc, Bismuth, and Calcium driers had a more limited impact on drying time and hardness in SB systems but did not contribute to yellowing.
Zirconium driers in WB systems provided a moderate improvement in drying time and hardness without significantly affecting yellowing or stability. They may be a suitable alternative to cobalt driers in applications where slower drying is acceptable and minimal yellowing is desired.
The choice of drier and its concentration should be carefully considered based on the desired performance characteristics of the PU coating. Formulators should weigh the benefits of faster drying and increased hardness against the potential for increased yellowing and reduced stability.
Further research is needed to explore the use of drier combinations and synergistic effects to optimize coating performance while minimizing undesirable side effects. The development of novel driers with improved compatibility, reduced toxicity, and enhanced performance is also a promising area for future research.
6. Recommendations:
Based on the findings of this study, the following recommendations are made for drier selection in SB and WB PU coatings:
- Solventborne PU Coatings:
- For applications requiring fast drying and high hardness, Cobalt Octoate is a suitable choice, but its concentration should be carefully controlled to minimize yellowing.
- Manganese Octoate can be used as a partial or complete replacement for Cobalt Octoate to reduce yellowing while maintaining acceptable drying times and hardness.
- Zinc, Bismuth, and
