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

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

Wood-plastic composite (WPC) materials have found extensive use in the construction of residential decks. Although recognized as a low-maintenance product, WPC lumber is not immune to the effects of weathering. Sunlight and water exposure can lead to color change and physical degradation. (1), (2)

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Previous WPC preservation studies have evaluated the effectiveness of light stabilizers incorporated during the wood fiber/thermoplastic compounding process.(3), (4) This study presents the results of accelerated exposure tests of WPC materials preserved by the application of a light-stabilized clearcoat.

DISCUSSION

Four light stabilizer (LS) options (including a no light stabilizer case and three different combinations of UV absorber and hindered amine free radical scavenger) were tested in each of three waterborne acrylic topcoats on each of two wood-plastic composite (WPC) substrates (one made with polyethylene and a second made with polypropylene).

Corresponding samples, as well as samples of each substrate without any topcoat, were prepared for both accelerated weathering in a xenon arc weatherometer (1000 hr, CAM 7) and for exterior exposure on a test rack.

The unprotected WPC samples exposed to accelerated weathering lightened significantly. The corresponding CIELAB [DELTA]L* values were in the range of 13 to 14. The samples protected with topcoats that lacked light-stabilizer additives fared better. The samples protected with light-stabilized topcoats showed the best color stability. The [DELTA]L* values corresponding to the best-performing topcoat resin/light stabilizer combination were in the range of 4 to 6.

The exterior exposure test is ongoing. One of the coatings failed within the first three months of exposure (although this same coating performed reasonably well under accelerated weathering test conditions). Through six months exposure the exterior weathering test results showed that across both WPG substrates, and 2 of 3 resin types, the application of a coating provides significant protection against color fade. Light stabilization yielded a further incremental benefit in one of the two successful coatings, but made little apparent difference in the other.

MATERIALS AND METHODS

Two types of commercial WPC decking material, each made with wood flour and each compounded with a different plastic, were procured for use in this study: (a) WPC-1: made with polyethylene plastic; and (b) WPC-2: made with polypropylene plastic.

Three different waterborne resin systems were used to prepare the topcoats:

(a) Resin A: a 100% acrylic emulsion polymer developed for low- to zero-VOC exterior semitransparent wood stains

(b) Resin B: an all-acrylic emulsion polymer that features extensive crosslinking for sufficient hardness to withstand heavy foot traffic

(c) Resin C: a 100% acrylic binder with good adhesion and water-blocking properties.

Coatings were prepared from each of these binders by mixing the following constituents in order, with continuous stirring:

(a) Specified quantities of resin and water

(b) A pre-mixed solution of Dowanol DPnB and water

(c) Propylene glycol (where applicable)

(d) Defoamer and fungicide/mildewcide

(e) A pre-mixed solution of Acrysol[TM] RM-825 thickener and water (to adjust viscosity, target=70 KU)

Four different light stabilization options were tested in each of the three waterborne acrylic topcoats:

(a) No light stabilizer (LS) additives

(b) LS Blend

(c) UV absorber 1 (UVA-1) plus hindered amine light stabilizer 1 (HALS-1)

(d) UV absorber 2 (UVA-2) plus HALS-1

The "LS Blend" is a liquid product consisting of 100% active material. It contains a benzotriazole UV absorber with a broad UV absorption spectrum, high thermal stability, and good photopermanence. It also contains a hindered amine free radical scavenger based on a substituted tetramethyl piperidine with an N-[CH.sub.3] (methyl) head group. This product can be used in waterborne coatings, but typically requires the use of a cosolvent for incorporation.

The UVA-1, UVA-2, and HALS-1 are products based on a newly developed encapsulation technology to render organic, water-insoluble UVA compatible with waterborne coating systems. (5), (6) These novel dispersions exhibit long-term storage stability even after being added to the liquid paint without any sedimentation, and are designed for easy, stir-in incorporation into aqueous paints without the use of cosolvents or high-energy dispersion equipment.

The concentration of active ingredient in UVA-1 is 20% w/w. This is a red-shifted hydroxy-phenyl-s-tri-azine chromophore that provides strong protection in the UV-A range of the UV spectrum. It has high heat stability and excellent photopermanence.

The concentration of active ingredient in UVA-2 is also 20% w/w. Again, this is a hydroxy-phenyl-s-tri-azine chromophore with superior thermal stability and photopermanence. However, its peak absorbance occurs in the shorter wavelength range of the UV spectrum.

The concentration of active ingredient in HALS-1 is 30% w/w. This HALS is based on a substituted tetramethyl piperidine with an N-OR (aminoether) head group. Relative to the HALS contained in the LS Blend product, this variant has low basicity to minimize the potential for interaction with acidic media or catalyst residues in the paint.