Phenol-formaldehyde-resin treatment of Scots pine sapwood for the reduction of resin exudation through coatings

ABSTRACT

Brown discolouration caused by resin exudation from knots is a problem for a range of light-coloured painted pines, and will negatively affect the appearance of the finish. To solve this problem, a hot-and-cold bath impregnation process of wood prior to painting was tested. Sawn timber, 18 × 120 mm in cross-section dimension, were heated in an oven and then immediately immersed in a cold liquid containing a phenol-formaldehyde-based solution, filling a 1–5 mm thick layer beneath the surface with phenol-formaldehyde. After curing, the timbers were painted with a white coloured coating system intended for exterior use, and tested in artificial weathering test (QUV). The phenol-formaldehyde treatment greatly reduced the discolouration of the coating compared to non-impregnated wood.

KEYWORDS:

• Wood
• resin exudation
• phenol-formaldedye resin
• coatings

Introduction

A common problem in surface-coated wood products is that extractives migrate from inside the wood, especially through the knots, and resurface under or penetrate through the paint layer, and thereby change the adhesion and appearance of the painted surface. This problem increased when water-borne paints were introduced on the market (Ekstedt 2000). Methods to resolve or reduce this problem such as sealing the knot-surface with shellac before the coating system have been applied, but the most common industrial method has been to increase the amount of paint (increasing the total thickness of the coating), which increases overall cost of the product and also increases the environmental impact. It is well known that pine species, and in particular their knots, contain considerably high contents of extractives (Willför et al. 2005, Holmbom et al. 2007). Such polyphenolic compounds can be mobilised by moisture and heat and can migrate to the surface of wood coatings and through degradation by UV radiation, resulting in coloured breakdown products appearing as a yellow or brown stain on the surface of the wood coating (Suttie and Ekstedt 2004).

Impregnation of wood with phenol-formaldehyde (PF) resin is a well-known industrial procedure to improve biological durability, moisture resistance and dimensional stability of sawn timber (Stamm and Seborg 1936, Sandberg et al. 2021). On an industrial scale, the PF-resin impregnation usually reaches a full-cell impregnation level under increased pressure in a closed system (autoclave), but for sealing only the wood surface a full-cell impregnation is unnecessary. A hot-and-cold bath process with a resin will normally result in a penetration of a few millimetres beneath the wood surface, which should give a protective layer that the extractives may not migrate through, as well as possibly reducing the roughness of the surface to be coated. The hot-and-cold bath process is an open process were sawn timber is first heated in a kiln or in a hot bath containing water or a preservative, after which the heated material is transferred to a cold preservative solution for impregnation (Hunt and Garratt 1967). The heating stage causes air in the lumen of the outer layers of the wood to expand and during cooling, the warm air in the wood cells contracts and creates a “partial vacuum” which sucks the preservative solution into the cells. Kollmann and Côté (1984) and Ormrod and Van Dalfsen (1993) claim that the hot-and-cold-bath method is the most cost-effective non-pressure method.

The objective was to study the effect on resin penetration through the coating of wood samples that has undergone PF resin impregnated in a hot-and-cold bath process prior to coating. The focus was on resin penetration from knots in Scots pine through an applied white-coloured coating, as this combination has been shown to be especially problematic in practice.

Materials and method

Scots pine (Pinus sylvestris L.) sapwood sideboards with cross-sectional dimensions of 18 × 120 mm and a moisture content of 18% were selected from a sawmill production in Northern Sweden. Two sections, each containing at least one large knot on the flat-side surfaces, were cut from each board giving 9 specimens with a length of 95 mm for further treatment, and 9 matched specimens as reference.

The cross-sections of the specimen were sealed with silicone gel and aluminium foil to prevent any penetration of impregnation resin in the longitudinal direction.

The phenol formaldehyde (PF) resin (Prefere Resins Finland Oy) was prepared in two types of solutions for the treatment: pure PF resin and PF resin mixed with methanol in a ratio of 1:1. Both solutions contained small amounts of phenol, formaldehyde, and sodium toluene-4-sulfonate, according to the manufacturer’s specification.

A combination of primer (Tinova primer exterior, white, No. 5207655) and white top-coating (Super tech. base, white, No. 5207722) for exterior use, supplied by AkzoNobel® Decorative Coatings AB, was used for the coating.

The following procedure was used:

1. The specimens were preheated for 2 h at 130°C for treatments A and B and at 160°C for treatment C. For treatment A and C pure PF was used, for treatment B PF diluted with methanol (1:1) was used (Table 1).

2. While still hot, the specimens were completely submerged for 2 h in a cold PF resin solution, and then cured at 20°C for 12 h.

3. The gain in weight due to the PF-impregnation was determined (prior to impregnation, the specimens were dried at 103°C for approximately 12 h to determine the dry weight), and the cross-section seals were cut away (ca. 10 mm), and the depth of PF penetration determined.

4. The specimens were dried at 130°C for 5 h, and cooled to 20°C before the specimens were painted with one layer of primer and two layers of top-coating by brush application, and dried at 20°C for 24 h.

5. The painted specimens were thereafter exposed to an artificial weathering test involving exposure in a QUV-spray during 12 weeks (Q-Lab Europe, Ltd., Express Trading Estate, Stone Hill Rd, Farnworth, Bolton, England) according to the European standard EN 927-6 (CEN 2006) and the colour of the coated surface was measured.

Table 1. Weight gain (± standard deviation) and resin penetration of PF-treated wood.

Treatment		Weight gain (%)	Resin penetration (mm)
A	Preheating at 130°C and soaking in pure PF	6.0 ± 1.4	1–3
B	Preheating at 130°C and soaking in PF diluted with methanol 1:1	11.5 ± 4.0	2–5
C	Preheating at 160°C and soaking in pure PF	6.0 ± 0.8	1–3

The colour was measured according to ISO 7724-2 (ISO 1984) with a Konica Minolta spectrophotometer CM 2600d (Konica Minolta Inc., Osaka, Japan), and the result was repeated in terms of the CIELAB scale with the three-dimensional L*, a*, b* parameters. The parameter L* represents the luminosity, a* and b* indicate respectively the red-green and yellow–blue contributions. The change in colour (ΔE) between two different measurements is calculated as: (1) $\mathrm{\Delta }E=\sqrt{{\left(L_2^{\ast }-L_1^{\ast }\right)}^2+{\left(a_2^{\ast }-a_1^{\ast }\right)}^2+{\left(b_2^{\ast }-b_1^{\ast }\right)}^2}$(1) where the suffixes 1 and 2 indicate the two different occasions. The colour of the coated wood before and after accelerated aging was measured separately for areas containing knots and areas which had no knots.

Results

Table 1 shows that the weight increase after impregnation with PF resin was not greatly affected by the preheating temperature. A greater weight gain was observed in the case of the PF-solution diluted with methanol, which is expected as the specimen diluted with methanol (boiling point 64.7°C) was cured at 20°C which means the solvent remain trapped inside the wood. The visual observation of penetration depth essentially confirmed the results of weight gain. The deeper impregnation with the diluted solution was probably due to the lower viscosity of the applied solution.

The results of the colour measurements after aging of both treated and untreated specimens are summarised in Table 2. Coated surfaces without knots of both PF-treated and untreated specimens had a similar lightness after exposure to UV, but the average colour change (ΔE) for areas with knots was considerably lower than for untreated specimens. This indicated that the treatment effectively reduced the staining of knots with respect to the untreated samples. The reduction in ΔE was approximately 70%. Figure 1 shows a comparison between the different surfaces after accelerated aging.

Figure 1. Treated (left) and untreated reference (right) specimens after accelerated aging: (a) treatment A, (b) treatment B, and (c) treatment C.

Table 2. Colour measurements after accelerated weathering test of painted specimens treated with PF resin (treatments A, B and C) and untreated specimens (reference).

Treatment	Area without knots			Areas with knots			ΔE*
	L*	a*	b*	L*	a*	b*
A	95.2 ± 0.6	0.0 ± 0.7	2.4 ± 0.4	91.9 ± 1.8	0.1 ± 0.6	7.2 ± 1.9	5.9 ± 2.6
Reference to A	96.1 ± 0.3	−0.2 ± 0.7	2.4 ± 0.2	84.0 ± 2.8	3.8 ± 1.0	16.7 ± 1.5	19.2 ± 3.2
B	94.7 ± 0.9	−0.7 ± 0.1	2.2 ± 0.2	90.7 ± 0.8	0.1 ± 0.4	7.3 ± 1.2	6.5 ± 1.8
Reference to B	95.8 ± 0.1	−0.6 ± 0.0	2.2 ± 0.2	84.0 ± 2.1	3.7 ± 0,5	16.5 ± 0.3	19.1 ± 1.7
C	94.9 ± 0.5	−0.8 ± 0.0	1.8 ± 0.2	93.5 ± 0.5	−0.5 ± 0.2	5.1 ± 0.6	3.6 ± 0.8
Reference to C	96.4 ± 0.1	−0.6 ± 0.0	2.3 ± 0.3	83.7 ± 3.6	3.9 ± 1.6	16.5 ± 2.4	19.6 ± 4.6

Conclusion

The phenol formaldehyde treatment using the studied variation of the hot-and-cold bath process was tested as a means of preventing resin exudation from knots. The studied treatment greatly reduced the migration of resin and extractives through knots in coated wood. This method can be considered as a pre-treatment prior to painting in order to reduce migration of extractives from knots through coatings on wood panels.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

Support from CT WOOD – a centre of excellence at Luleå University of Technology supported by the Swedish wood industry – is also acknowledged, along with additional support through the project “Advanced research supporting the forestry and wood-processing sector’s adaptation to global change and the 4th industrial revolution”, OP RDE [grant number CZ.02.1.01/0.0/0.0/16_019/0000803].