GB2032805A - A method for preventing or reducing acid-induced discolouration of wood - Google Patents

Abstract

Discolouration of wood involving reddening due to acid treatments of the wood is prevented or reduced by coating the surface of the wood with a layer of a resin which is impervious to the acid before the wood is treated with the acid-based wood finish, for example a lacquer.

Description

SPECIFICATION A method for preventing or reducing acid-induced discolouration of wood The present invention relates to a method for preventing or reducing acid-induced discolouration of wood, and the particular application of this method to use in the furniture industry.

Certain woods, when subjected to conventional acidic or acid-catalysed wood finishing treatment, are known to undergo a discolouration or reddening of the narural wood colour. This effect becomes more marked with time, and is undesirable in situations where colour-matched wood is required, for example in a suite of furniture. The discolouration is particularly apparent at a joint between two or more pieces of wood having different degrees of reddening.

The natural colour of wood is due to the presence of a group pf pigmented chamicals and their polymerisation products, The constituents of the group have, as their common characteristic, an aromatic and often polynuclear structure often carrying one or more -OH groups, on account of which they are often commonly referred to as polyphenols. One such type of constituent commonly found is the anthocyanins (glycosides of pyrylium salts which have, as their parent structure, flavylium chloride), which are formed by the partial conversion of their unpigmented precursors, the leuco-anthocyanins.

The presence of anthocyanin pigment precursors, and therefore a consequent liability to redden, has been confirmed in many of the hardwoods, including even such light-coloured species as European beech and sycamore.

Other constituents of the polyphenol group include tannic acids, hydroxy-flavones, flavonols, stilbenes, anthraquinones and napthaquinones. The carbonyl group is frequently present in the polyphenols, these carbonyl groups acting jointly with the aromatic structures as chromophores.

The conversion of leuco-anthocyanins to anthocyanins influenced by the ionic character of the molecules concerned, and is particularly favoured under acid conditions. An increased hydrogen ion concentration, such as is furnished by an acid-based wood finish, will therefore cause an increased rate of conversion into anthocyanin, with a concomitant increase in reddening of the natural wood colour.

Although the problems of acid-induced discolouration, and the consequent costs of replacement of furniture and other wooden installations which have suffered unacceptable colour changes in service, have been well known for a long time, there is little evidence of systematic research with an aim to prevent the risk of discoloration.

According to the present invention, there is provided a method for preventing or reducing acidinduced discoloration of wood, comprising coating the wood surface with an impervious layer of resin, followed by treatment of the thus-coated wood with an acid based wood finish.

The resin, for use as the impermeable layer in the process of the present invention, may be defined with reference to its preferred properties. The resin preferably has an initial solubility in approprate solvents for convenience of application to wood surfaces. When dry, the resin preferably is neutral or of low acidity with a pH not less than 3, most preferably not less than 4.0. It is substantially free from the risk of yielding significant quantities of acid on aging. The resin and the acid-catalysed wood finish are selected so that the resin is resistant to solvent in the acid-catalysed wood finish.

The resin layer may generally be of a thickness in the range of from 1 0or to 300u, for example 200u.

Preferably, the impervious layer is based on an acrylate-based resin, and more preferably consists of a polymethyl-methacrylate resin. Other resins based on the acrylate monoester, which if desired may be present as a copolymer with, for example, a polyethylene, may be used in the method of the invention.

Other resins which are envisaged as being potentially suitable for use in forming the impermeable layer are as follows:- organo-silico oxide polymers; epoxies; polyesters; alkyd resins; cellulose ethers; polystyrene and copolymers; polyvinyl acetate and copolymers, and polyvinyl chloride and copolymers.

Specific resins which have proved to give a very high degree of reduction or elimination of discolouration are polymethyl methacrylate; carboxymethyl cellulose (especially with 0.7-0.8 substitution) in the form for example of its sodium salt; polymethacrylic acid; and polyvinyl acetate. On the other hand, methyl cellulose of both high and low substitution appears to yield less favourable results.

Water-soluble resins (for example sodium carboxymethyl-cellulose or polymethacrylic acid) my be of considerable advantage in view of the currently growing interest in water-based finishes. Such finishes may well include some acid-catalysed cross-linking resins in their formulation. Polyvinyl acetate requires organic solvents for dissolution.

For a barrier to be reasonably effective, the solid (resin) content of the solutions can vary within a wide range, for example 0.1 %-99.9%, but in practice it is generally desirable that it be at least 5%, preferably in the range of 1 0%-30%.

Application may be by spraying, dipping or curtain-coating and in the event of the solid content of the solution being too low, for whatever reason, two or more consecutive applications, with adequate intervals for drying between them being allowed, must be made.

Drying of the barrier coat applied from solutions may be speeded up by heating, e.g. as in ovens, though an initial 'flash-off" period that can vary from 5 minutes to 30 minutes according to solvent balance and composition, will normally be desirable. In any event it is of great importance to allow the barrier coat to dry thoroughly then to cool it thoroughly before applying the acid-catalysed finish to minimise any risk of the barrier being penetrated or softened by solvents in the finish. In this context the mixing of aqueous systems with organic solvent-based systems, e.g. sodium carboxymethylcellulose for barrier coat, to be followed by application of a conventional acid-catalysed finish would be advantageous, the former having a good resistance to attack to organic solvents.

Barrier materials may, if necessary, be blended with non-acidic and otherwise compatible additives to improve or modify their technical properties, e.g. plasticity, hardness, colour (by pigmentation) and in other respects, as may be desired.

Wood finishes which cause discolouration include, for example, lacquers such as those comprising urea and melamine-formaldehyde resins which required acid conditions for curing. Many flameretardant coatings are based on resins which also require acid curing conditions, as do certain adhesives used in woodworking. An increase in recent years in the use of radiant heat for the drying of acidcatalysed wood finishes has been found to lead to a corresponding increase in the risk of reddening of the wood caused by these finishes.

Commonly occurring strong acids of practical importance to wood finishing and veneering include hydrochloric, sulphuric, aryl sulphonic and other, less known, sometimes synthetic acids. Mineral acids occur frequently in two-pack acid-catalysed glues and other finishes.

Many, though not all, species of wood are subject to acid-induced colouration. Common species of wood which have an observed strong tendency to redden include beech (Fagus spp.), for example European beech (Fagus sylvatica), elm (Ulmus spp.), the mahoganies (Swietenia spp., Khaya spp., etc.), cherry (Prunus spp.), maple (Acer spp.), many eucalypts (Eucalpytus spp.), makere (Mimusops spp.), sapele, utile, etc. (Entandrophragma spp.). Certain softwood are also subject to a tendency to acidinduced discolouration; they include Pseudotsuga spp., Abies spp., Pinus spp., Taxus spp., Picea spp.

The heartwood is normally more susceptible than the sapwood.

For acid-sensitive veneers to redden owing to anthocyanin pigment formation, it is essential that the reactants, i.e. the acid and the pigment precursors, are brought into a common physical phase to allow the reaction to proceed. This condition is usually provided by a hydroxylic solvent, containing the acid and some water, diffusing into the wood fibre, where solubilisation of the leuco-anthocyanin takes place. Thus, for a barrier coat to be effective, it must be able to prevent diffusion and consequently interaction of the potential reactants. In this sense, the barrier coat must be an impervious layer.

An indication of the effectiveness of the method of the invention (and an indication of a way in which to discriminate between such a method and methods not in accordance with the invention) is shown in the following Examples by a comparison of the changes in surface reflectances of veneers, following exposure to radiant heat, which have been provided with varying combinations of barrier coat and top coat.

Although reddening due to the action of acids on susceptible woods can also proceed in the cold, the preference of radiant heat to convectional (oven) heat for a laboratory tests is made on the grounds of better provision being possible for solvent retention by lacquered veneer, which is a desirable condition for the conversion of leuco-anthocyanin to the red anthocyanin.

250W-275W reflectors lamps for short to medium wave length infra-red radiation were used in the Examples. The colour changes, i.e. the extent to which reddening of the veneers occurred, were assessed instrumentally with an abridged spectrophotometerfor coloured surfaces.

Calibration was made against a white ceramic tile as an arbitrary standard white, which was acceptable for the essential purpose of this study. The reflectance measurements were carried out with four colour filters of approximate peak transmissions values of 470 nm(blue), 520 nm (green), 580 nm (yellow) and 700 nm (red).

Thus, a polymethyl-methacryiate resin-based lacquer having a 24% solids content was applied to cherry wood-veneered panels with a hand spreader, laying down a wet film having a thickness of 200,u.

The two sets of test panels had been previously conditioned at 35% and 85% relative humidity, respectively, for about 10 days each.

For the top coat, a commercial two-pack sulphuric acid-catalysed urea-formaldehyde finish with a total solids content of 31% was used, likewise applied with a 200,u spreader. The proportion of catalyst to lacquer was, however, varied to include the recommended mixing ratios and also mixes in which the catalyst was present in 50% excess of the normal proportion.

Exposure of the test panels to infra-red radiation at a surface temperature of about 70 C was begun one hour following coating and maintained for a total period of two hours. (In such a test the surface temperature and the period in which it is maintained may for example be varied as follows: 800C with an exposure period of 1 hour; or 900C with an exposure period of 2 hour).

Reflectance measurements to asses the changes were made following a 48-hour dwell period and with reference to a white tile standard.

The results are shown in the following Table 1. Reddening is shown by a greater proportionate decrease in the reflectance of blue and green lights.

Table 1. Treatment combinations and surface reflectance changes after 48 h. dwell time following exposure to radiant heat.

Coating combinations Peak transmission of filters (nm) Lacquer/ 470 | 520 | 580 | 700 Conditioning Barrier Top catalyst Example RH % coat coat ratio. /C reflectance of standard white 16.9 36 1* 85 + - - 12.0 12.5 16.5 36 2 85 + + 9:1 12:0 12.5 17:0 38 3 85 + + 6:1 13.0 13.0 18.5 41 4 85 - + 9:1 10.5 10.0 15.0 38 5 85 - + 6:1 9.0 8.5 12.0 36.5 6 35 ± - - 12.0 13.0 17.5 39 7 35 + + 9::1 11.5 12.0 15.5 38 8 35 t + 9:1 10.5 10.5 15.0 37.5 9 35 - + 6:1 10.0 9.5 14.0 41.0 * Not subjected to radiant heat.

For a comparative graphic illustration of the results in Table 1, colour plots, based on the yellow and red tristimulus functions, calculated from the reflectance values of the blue, yellow and red lights, from a tungsten source, were constructed as illustrated in the graph on page 9.

As these tristimulus functions were obtained with simple equipment and under conditions not entirely corresponding to those specified in the standard C.l.E. system, the chromaticity values assigned to the veneered panels in the chart should be used only to assess the relative differences in colour due ta differential treatments of the samples used in the exercise. Used in this way, the benefits of the methacrylate barrier coat become apparent and so does the detrimental effect of the excess acid (Examples 4, 5, 8 and 9) in the absence of a barrier coat.

The further to the right the red function lies in the attached chart, the stronger the hue and to obviate the inevitable interference from the brownish ground colour present in all samples naturally, a "luminance" scale, based on the reflectance values of the yellow component alone, was included as an additional dimension. Thus when ail the dimensions are examined simultaneously, it is apparent that, in general, the brown element of the natural colour decreases as the red component increases.

It is uncertain whether the seemingly slight increase in the red component of the colour of the methacrylate-coated controls, as suggested by, e.g. Example No. 6 is real.or, which is more likely, this slight difference in the red component of the colour from those of the test pieces in Examples Nos. 1, 2 and 3 is mainly due to natural variation associated with the markedly varied figure of the species.

The enhancing influence of the higher environmental humidity in conditioning the samples before finishing is suggested by the difference in the results of the corresponding pairs of Examples Nos. 8 and 4, and Nos. 9 and 5. Examples 2, 3 and 7 are illustrative of the method of the invention, the remaining Examples are for the purposes of comparison only.

Claims (14)

1. A method for preventing or reducing acid-induced discolouration of wood comprising coating the wood surface with an impervious layer of a resin and treating the thus-coated wood with an acidbased wood finish.

2. A method according to claim 1 in which the resin layer has a thickness of 100--300FL.

3. A method according to claim 1 or 2 in which the wood is of beech (Fagus spp.), elm (Ulmus spp.), mahogany, (Swietenia spp.), African mahogany (Khaya spp.), sapele, utile (Entandrophragura spp.), cherry (Prunus spp.), maple (Acer spp.), eucalypts (Eucalyptus spp.) or makone (Mimusops spp.).

4. A method according to claim 1 or 2 in which the wood is of Pseudotsuga spp., Abies spp., Pinus spp., Taxus spp., or Picea spp.

5. A method according to any one of claims 1 to 4 in which the resin when dry has a pH not less than 3.

6. A method according to claim 5 in which the pH is not less than 4.0.

7. A method according to any of claims 1 to 6 in which the formation of a coating involves applying to the wood surface a solution of the resin containing 530% of the resin solids.

8. A method according to any of claims 1 to 7 in which the resin is an acrylate-based resin.

9. A method according to claim 8 in which the resin is a polymethyl methacrylate resin.

10. A method according to claim 8 in which the resin is polymethacrylic acid.

11. A method according to any of claims 1 to 7 in which the resin is polyvinyl acetate.

12. A method according to any of claims 1 to 7 in which the resin is carboxymethyl cellulose.

13. A method according to claim 1 2 in which the carboxymethyl cellulose is in the form of its sodium salt.

14. A method according to claim 1 substantially as hereinbefore described.

1 5. Wood protected from acid-induced discolouration by a method according to any of claims 1 to 14.