Light-stabilized coatings for preservation of wood-plastic composite decking

JCT CoatingsTech, March, 2009 by Robert Waldron, Brett Moyer

The 6-in. panels prepared for accelerated exposure testing were weathered for 1000 hr in a xenon arc weatherometer using the Xe-WOM CAM 7 cycle according to DIN EN ISO 11341 A. This cycle consists of:

(a) 102 min of light (0.35 W/[m.sup.2] @ 340 nm) with a black panel temperature of (60[ or -]2)[degrees]C and a relative humidity of (50[ or -]5)% at the end of the dry period

(b) 18 min of light and spray with a black panel temperature of (40[ or -]2) [degrees]C and a relative humidity of (95[ or -]5)%

RESULTS

The color stability results for the WPC panels exposed to accelerated weathering are shown in Table 5. The uncoated WPC-1 and WPC-2 samples lightened (or faded) considerably. The measured CIELAB [DELTA]L* values were 14.3 and 13.4, respectively. The samples protected with topcoats that lacked light-stabilizer additives fared better. The samples protected with Resin C topcoats containing 2% w/w UVA-1 and 1% w/w HALS-1 showed the best color stability, with [DELTA]L* = 4.6 for the WPC-1 sample and [DELTA]L*=5.8 for the WPC-2 sample.

The [DELTA]L* data for the WPC-1 and WPC-2 panels exposed in the xenon arc weatherometer are plotted in Figures 1 and 2, respectively. Figure 3 is a photograph of two weathered WPC-1 panels. The one on the left is the unprotected sample. The one on the right was protected with the Resin C topcoat containing UVA-1 and HALS-1. Likewise, Figure 4 is a photograph of two weathered WPC-2 panels. Again, the one on the left is the unprotected sample and the one on the right was protected with the Resin C topcoat containing UVA-1 and HALS-1.

Color stability data for the WPC panels exposed outdoors are shown in Tables 6 and 7. The data in Table 6 were measured after three months' exposure, while the data in Table 7 were measured after six months' exposure. Resin B coating failure was observed on both WPC substrates within the first three months of exterior exposure. This problem involved extensive cracking, and was somewhat worse on WPC-1 than on WPC-2. The Resin B coating continued to deteriorate during the second three-month exposure interval. Much better durability was obtained with the Resin A and Resin C coatings.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

Table 5--Color Measurement Results for Samples Exposed to
Accelerated Weathering

               WPC-1 [DELTA] E*              WPC-2 [DELTA]E*

           No     Resin  Resin  Resin     No    Resin  Resin  Resin C
         Coating    A      B      C    Coating    A      B

No        14.3     --            --     13.4     --     --       --
coating

No LS      --      8.8    9.2    7.3     --     11.3   11.3     9.9
3% LS      --      7.9    7.2    7.2     --      9.1    8.1     7.7
blend

2%         --      6.9    6.5    5.4     --      8.3    6.4     6.8
UVA-2,
1%
HALS-1

2%         --      6.3    6.3    6.0     --      7.1    7.3     6.5
UVA-1,
1%
HALS-1

               WPC-1 [DELTA] L*              WPC-2[DELTA] L*

           No     Resin  Resin  Resin    No     Resin  Resin  Resin C
         Coating    A      B      C    Coating    A      B

No        14.3     --     --     --     13.4     --     --       --
coating

No LS      --      8.6    9.1    6.7     --     11.1   11.2     9.6
3% LS      --      7.6    6.8    6.5     --      8.8    7.9     7.1
blend

2%         --      6.1    5.8    4.6     --      8.0    6.0     6.1
UVA-2,
1%

HALS-1
2%         --      5.8    5.4    4.6     --       6.0   7.1     5.8
UVA-1,
1%
HALS-1

All LS concentrations stated as weigh percent actives on tinder solids.

Table 6--Color Measurement Results for Samples Exposed to Three Month's
Exterior Weathering

                 WPC-1, [DELTA] E*           WPC-2, [DELTA] E*

Exterior     Resin A  Resin B  Resin C  Resin A  Resin B  Resin C
Weathering
3-Months
Exposure

No coating     4.6      7.2      6.3      9.1      6.1      7.7

No LC          2.1     11.6      1.3      2.5      7.7      1.8

3% LS          2.1     13.3      3.4      1.5      4.2      1.3
Blend

2% UVA-1,1%    3.1     10.6      3.4      1.8      8.3      1.9
HALS-1

2% UVA-2,      1.5     11.8      1.5      3.4      8.1      2.4
1% HALS-1

No coating     6.5      6.6      7.0      8.5      9.4      7.6

                 WPC-1, [DELTA] L*           WPC-2, [DELTA] L*

Exterior     Resin A  Resin B  Resin C  Resin A  Resin B  Resin C
Weathering
3-moths
Exposure

No coating     4.2      7.0      6.1      9.1      5.9      7.6

No LS          2.0     10.3      1.0      2.4      6.3      0.6

3% LS          1.8     11.5      3.2      0.3      3.2      1.3
blend

2% UVA-1,      2.6     10.1      3.1      1.6      5.6      1.8
1% HALS-1

2% UVA-2,      1.0     10.8      1.4      3.3      6.8      1.9
1% HALS-1

No coating     6.4      6.4      6.9      8.5      9.2      7.5

All LS concentrations stated in terms of weight percent actives on
binder solids.

Table 7--Color Measurement Results for Samples Exposed to Three Months'
Exterior Weathering

                   WPC-1, [DELTA]E*          WPC-2, [DELTA]E*

Exterior      Resin A  Resin B  Resin C  Resin A  Resin B  Resin C
Weathering 3
Months
Exposure

No coating      5.3      9.3      7.6      9.3      8.7      6.6

No LC           7.3     15.2      3.8      6.4     10.8      5.4

3% LS blend     4.5     15.1      5.6      4.4     11.7      5.8

2% UVA-1, 1%    4.6     13.4      4.5      4.0     11.6      4.9
HALS-1

2% UVA-2, 1%    4.4     13.8      3.8      7.3     10.7      5.4
HALS-1

No coating     10.0      8.5      9.7      7.7      8.7      7.5

                   WPC-1, [DELTA]L*          WPC-2, [DELTA]L*

Exterior      Resin A  Resin B  Resin C  Resin A  Resin B  Resin C
Wealthering
3 Months
Exposure

No coating      4.0      9.0      7.0      9.2      8.6      6.4

No LS           7.2     14.4      3.7      6.4     10.1      5.3

3% LS blend     4.3     14.1      5.3      4.1     10.5      5.6

2% UVA-1, 1%    4.4     13.0      4.3      3.6      9.5      4.7
HALS-1

2% UVA-2, 1%    4.3     13.0      3.6      7.2      9.7      5.2
HALS-1

No coating      9.9      8.1      9.6      7.6      8.5      7.2

All LS concentrations stated in terms of weight percent actives on
binder solids.