Pre-Treatment Studies on Various Wood Types with High-Performance Light Stabilizers

Summary

Lignin (wood component) degrades readily when exposed to sunlight. To preserve wood appearance, water-based clear coatings containing UV absorbers and light stabilizers are commonly applied to surfaces for protection. However, the benefit of such application is limited. According to our studies, subjecting the wood surface to pretreatment before applying a clear topcoat can effectively enhance protection of lignin against UV degradation. Eversorb® AQ, a product combining UV absorber and HALS, is manufactured by Everlight Chemical Industrial Corporation (ECIC) specifically for water-based clear coatings. Eversorb® SB, produced for wood pretreatments, can protect lignin from the effects of UV light. The wood protection efficacy factor for Eversorb® AQ is 14.69%. The wood protection efficacy factor for Eversorb® SB was determined to be 13.35%. Eversorb® AQ and Eversorb® SB exhibit synergistic interaction. The efficacy factor is 10.09%. Both Eversorb® AQ and Eversorb® SB are products with good performance for wood protection. This article is a comparative study using accelerated weathering method to test different wood types treated with Eversorb® SB previously or differently in different solvents. Design of Experiments (DOE) was used in this study. The data confirm that treatment with Eversorb® SB demonstrates better performance in UV effect testing. Lignin is a complex organic polymer containing hydroxyl functional groups (-OH). Two types of auxiliary solvents, isopropyl alcohol and butyl carbitol, were tested in the system with solubility parameters of 11.5 and 9.9 respectively. After degradation testing, Eversorb® SB mixed with isopropyl alcohol yielded better results than for butyl carbitol. Because softwoods contain higher lignin content (25–35%) than hardwoods (18–25%), theoretically, Eversorb® SB pretreatment can provide better protection to softwoods. Our data support the hypothesis; pretreatment with Eversorb® SB provides much greater protection on softwoods (Southern Pine and Douglas Fir) than on hardwoods (Cedar, Southern Birch and African Teak).

Introduction

Water-based clear coatings were developed to address the high VOC issues typically associated with traditional solvent-based coatings [1]. Most customers use water-based clear wood coatings to protect wood materials. However, sunlight and UV radiation can penetrate coatings on wood surfaces causing yellowing. Yellowing results from both the coating and lignin. Wood begins to yellow due to photodegradation of lignin, which is a combination of lignin, cellulose and hemicellulose [2-6].

The photodegradation pathway of lignin [7-8] is shown in Figure 1.

Figure 1. Photodegradation pathway of lignin

Eversorb® AQ is a water-based light stabilizer. Eversorb® SB is a lignin stabilizer designed for wood pretreatments. Test data confirmed that increasing Eversorb® AQ concentration in the clear topcoat can effectively protect Southern Pine from UV light degradation. Other experiments show that pretreatment of Southern Pine with Eversorb® SB before clear topcoat application provides better weathering resistance than without Eversorb® SB pretreatment. Additionally, the data suggest that by increasing coating thickness, light fastness of Southern Pine can be improved to some extent, but increasing the dosage of light stabilizers in clear coating or pretreating southern pine before varnish application are more effective methods [9]. Lignin is a complex organic polymer containing hydroxyl functional groups (-OH). To check whether solvents affect the protection performance of Eversorb® SB, different auxiliary solvents were used in the experiment to mix with Eversorb® SB. Theoretically, Eversorb® SB pretreatment could have better performance on softwoods than hardwoods due to high lignin content. While softwoods contain 25–35% lignin, hardwoods contain 18–25% lignin [10]. The subject of this paper covers five different wood types including softwoods (Southern Pine and Douglas Fir) and hardwoods (Cedar, Southern Birch and African Teak).

Experiment

Three model formulations were used in this study: acrylic-based water-based clear coating, wood pretreatments and auxiliary solvents. Detailed compositions are shown in Tables 1, 2 and 3. Five different wood types were selected in the region as shown in Figure 2. Design of Experiments (DOE) was used to design test studies and analyze test data. The DOE technique enabled us to validate the data.

Table 1. Composition of water-based model formulation

Table 2. Wood pretreatment model formulations

Table 3. Solubility parameters of different auxiliary solvents (Unit: (cal/cm³)1/2)

Screening Design of Different Wood Types and Auxiliary Solvents Using EV-AQ and EV-SB

A screening design (Table 4) was used to identify factors providing minimum color change after degradation testing. The mathematical model derived from the data shows all factors – X1: 5 different wood types (Cedar, Douglas Fir, Southern Birch, African Teak and Southern Pine). X2: 3 types of application (X2-1: Topcoat: Water-Based Acrylic Resin, X2-2: Wood Pretreatment: Butyl Carbitol mixed with EV-SB, Topcoat: Acrylic Resin mixed with EV-AQ and X2-3: Wood Pretreatment: IPA mixed with EV-SB, Topcoat: Acrylic Resin mixed with EV-AQ).

Figure 2. Different wood types

Table 4. Screening design details and test results

Optimal Prediction Profiler

The prediction profile (for example, Southern Pine) shows the effect of 3 types of treatments after 300 hours of weathering test. For the sample using only water-based acrylic resin, yellowing is significant after degradation testing. (Color difference ΔE=31.85) (as shown in Figure 3). 10% EV-SB in pretreatment mixed with Butyl Carbitol and 2% EV-AQ in topcoat. Performance was better than untreated with UV. (Color difference ΔE=10.25) (Figure 4). 10% EV-SB mixed with IPA in pretreatment and 2% EV-AQ in topcoat. Southern Pine showed the best light fastness. (Color difference ΔE=6.55) (Figure 5). Photographs of different wood types after degradation testing are shown in Figures 6 through 10.

Figure 3. Effect of Southern Pine coated with acrylic resin after 300 hours of weathering

Figure 4. Effect of Southern Pine and pretreatments with butyl carbitol after 300 hours of weathering (Prediction Profiler)

Figure 5. Effect of Southern Pine and pretreatments with IPA after 300 hours of weathering (Prediction Profiler)

Figure 6. Photo of Douglas Fir after testing for 300 hours

Figure 7. Photo of African Teak after degradation testing for 300 hours

Figure 8. Photo of Southern Birch after degradation testing for 300 hours

Figure 9. Photo of Southern Pine after weathering test for 300hrs

Figure 10. Photo of Cedar after degradation testing for 300 hours

Conclusion

After aging testing, the results show that the combination of Eversorb® SB and Eversorb® AQ can significantly protect wood surfaces against yellowing. Additionally, Eversorb® SB mixed with isopropyl alcohol can provide better performance than Eversorb® SB mixed with butyl carbitol.

Discussion

Environmentally friendly coatings are the future development trend. We have developed many unique products specifically for coatings. Eversorb® AQ, a combination of UV absorber and HALS, has been developed for water-based clear coatings. Eversorb® SB, designed for wood pretreatments, can protect lignin from UV light degradation. Lignin is a complex organic polymer containing hydroxyl functional groups (-OH). The solubility parameter of isopropyl alcohol and butyl carbitol is 11.5 and 9.9 respectively. After aging testing, the results show that Eversorb® SB in isopropyl alcohol can provide better performance than butyl carbitol. In other words, solvents with higher solubility parameters can provide better protection. Because softwoods contain higher lignin content (25–35%) than hardwoods (18–25%), theoretically Eversorb® SB pretreatment can provide better protection to softwoods. Our data show that pretreatments with Eversorb® SB provide much greater protection on softwoods (Southern Pine and Douglas Fir) compared to hardwoods (Cedar, Southern Birch and African Teak). Coating manufacturers may need to conduct experiments to confirm the compatibility of coating systems. Additionally, a UV absorber and light stabilizer package specifically formulated to meet your specific requirements can also be recommended. Yung-Chi Yang Technical Manager, Specialty Chemicals Everlight Chemical Industrial Corp       Samir Günaştı General Manager ELİTE A.Ş.    

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