US3690922A - Conditioning of wood to stabilise its colour - Google Patents

Abstract

THE INVENTION RELATES TO A PROCESS FOR IMPARTING STABILIZED COLOUR TO WOOD. IN THE PROCESS, THE WOOD IS TREATED WITH A PERSULFATE OXIDISING AGENT SO AS TO PRODUCE THEREIN LIGHT-STABLE PRODUCTS WHICH STABILISE THE COLOUR OF THE WOOD.

Description

United States Patent 3,690,922 CONDITIONING OF WOOD TO STABILISE ITS COLOUR Stephen Dombay, Stevenage, England, assignor to Furniture Industry Research Association, Stevenage, England No Drawing. Continuation-impart of application Ser. No. 687,464, Dec. 4, 1967. This application Sept. 1, 1970, Ser. No. 68,781 Claims priority, application Great Britain, Dec. 8, 1966, 55,026/66 The portion of the term of the patent subsequent to Jan. 12, 1988, has been disclaimed Int. Cl. B27k 5/02 US. Cl. 117-57 19 Claims ABSTRACT OF THE DISCLOSURE The invention relates to a process for imparting stabilized colour to wood. In the process, the wood is treated with a persulfate oxidising agent so as to produce therein light-stable products which stabilise the colour of the wood.

This is a continuation-in-part application of my copending application Ser. No. 687,464 filed on Dec. 4,

1967, now Pat. No. 3,554,785.

The invention relates to a method of treating wood so as to produce a stable colour resistant to changes due to photochemical action.

It has been found that many woods change colour on exposure to light and that this is due to photochemical activity. The change in colour is particularly rapid with freshly-exposed surfaces.

Attempts have been made in the past to overcome this problem by applying reducing agents to the woods or by reducing the effect of light by incorporating ultra-violet absorbers therein. It has now surprisingly been found that it is possible to achieve a more satisfactory result than in the past by a particular oxidation of ligneous ingredients. By means of the invention a stabilised colour may be imparted to the material, so that for example only slight or imperceptible changes take place in that colour even after quite long exposure to light. The invention is particularly applicable to solid wood or wood in the form of a veneer.

The invention provides a method for producing stable colour in wood material which comprises treating the material with a persulfate oxidising agent so as to oxidise natural substances present in the material in a particular way whereby there are produced oxidation products which are substantially stable to light, so that the colour of the material containing such oxidation products is stabilized.

The material to which the invention is especially applicable is mahogany, sapele, walnut, afrormosia, munin= ga, utile, iroko, tola, oak, cherry and maple woods. The term walnut" covers all walnuts, including European, American and African walnuts and pecan, and the term mahogany covers all mahoganies, including American and African mahoganies.

The oxidising agent may of course be a mixture of oxidising agents. As indicated above, the persulphate oxidising agent produces relatively light-stable oxidation prodnets in the material. The use of a persulphate is crucial in yielding the improved stable colour. Persulphates (for example ammonium or alkali metal persulphate) either alone or in combination with other agents such as hypochlorites or peroxides (added with the persulphate's or separately therefrom) have been found particularly satis= factory in producing a good stable colour. Hypochlorous acid (for example with a pH of 2 to 7) is in some cases very suitable as the other agent, or the other agent may be obtained by adding free acid to an alkaline solution of 3,690,922 Patented Sept. 12, 1972 hypochlorite. If a hypochlorite is used, it may be an alkaline solution containing at least 0.1% of available chlorine.

The persulphate is used normally as a solution containing from 10 g. per litre up to the solubility limit thereof.

Chlorates and perchlorates have been found unsuitable because the colour produced is less stable; nitrates may give an inadequately stable colour for some purposes. Other contemplated oxidising agents include iodates, periodates, bromates and perbromates, but these are rather expensive. None of these gives the remarkable and sur prising effects given by persulphates.

The oxidising agent is normally applied to the material, but it may sometimes be found convenient to produce it in situ in the material, for example by applying to the material substances which will react to form the oxidising agent. a

The oxidising agent may be applied to the material in the form of a solution thereof. The material is preferably completely immersed in the solution, or treated therewith by swabbing or the like.

The oxidation must, of course, be a fairly gentle one so as not to destroy the nature of the material, but it may be sufiiciently strong so that the oxidation products have a colour somewhat different from the colour of the ma-= terial before treatment.

This confers a further advantage on the invention in that it can be used for the additional purpose of providing the treated material with a greater uniformity of colour than it had before treatment. By this means different samples of wood within the same species may be given more similar colours and this facilitates matching of materials of the same species from different sources.

This advantage is of particular value in the furniture industry. A furniture factory frequently has to get its supplies of wood of one species from a number of different sources. It would often happen that pieces of wood from different sources would be brought together in a single piece of furniture, but hitherto this has often had to be avoided because the different pieces of wood, although of the same species, have often had different colours, so that a piece of furniture containing them would be commercially unacceptable. By means of the present invention, this source of inefficiency in furniture manufacture can be avoided, as the wood from the various sources can be treated by the present method so as to be more uniform in colour.

A further advantage is that the method of the invention can be used to confer greater uniformity of colour in a single piece of wood. Thus, it sometimes occurs that exposure of part of the surface of a piece of wood to light causes localised discolouration or darkening, leaving a patchy appearance. Treatment of the surface by the present method can bring the variously coloured portions into greater colour uniformity.

It is, of course, not necessary to treat the full thickness of a piece of the material of which only the surface is to be seen in use. It has been found generally adequate in practice to treat the wood to a depth of up to 1 mm. and a stabilised layer of this thickness is frequently more than adequate.

It is frequently found desirable to use an oxidation catalyst (for example one containing permanganate or cupric ions) so as to accelerate the oxidation, and thus save time in carrying out the process. The oxidation catalyst may fog xample be applied as a single solution with the oxidising agent, or as a separate solution.

The most suitable auxiliary oxidising agent (if any), solvent, pH concentration, time of application and the like may vary from wood to wood and may partly depend on the particular final colour and the thickness of the stabilised layer which is desired. The most suitable parameters can in any particular case be found by tests involving variation of the parameters indicated above.

An important preferred feature of the invention is the control of the pH during the oxidation. It has been found that the colour and its stability may partly depend on the pH. Frequently it is very desirable to apply pH adjusting agents to the material so as to impart a suitable pH during treatment; generally, an alkaline pH is advantageous.

Of course, the treatment according to the invention may be followed by a neutralising step so as to give the treated material a final pH which is substantially neutral. Thus, if the only agent used is an alkaline persulphate solution, this may be followed by an acid wash to neutralise the alkali remaining in the material.

The pH adjusting agent may for example be as a single solution with the oxidising agent, or as a separate solution. Many oxidising agents (such as persulphates) have been found to form acid reaction products when applied in the present process; if it is found that these acid products, particularly if added to the natural acidity of many woods, cause too low a pH, then a suitable alkaline agent may be applied to the material. Sometimes the required pH may be maintained by means of a pH buffer, for example a phosphate or acetate buffer.

It is generally convenient to use the oxidising agent in the form of a solution containing a pH adjusting agent to impart a predetermined pH (optionally with at least 0.1 g. of an oxidation catalyst per litre), the remainder of the solution consisting essentially of water with or without a lower alcohol and optionally containing a surface-active agent.

Where the treatment imparts an acid or alkaline pH to the material, it may be desirable to bring the pH to a moderate value, say -8, by after treatment of the material with an alkaline or acid material, as indicated above.

The single oxidising step may leave a surface layer of wood which is not only stabilised in colour but which has an acceptable colour. This has been found to be the case with, for example, African mahogany and European walnut.

In many cases, it has been found desirable to effect the method in stages, the first stage being as described above and the second stage being a second treatment with a second oxidising agent so as to modify the stabilised colour (which may reinforce stabilisation), this has been found desirable in, for example, the cases of afrormosia and African walnut. Hypochlorite and hydrogen peroxide are examples of second oxidising agents which have been found beneficial in certain cases. For example, medium dark stabilised shades of colour have successfully been produced in African walnut and in afrormosia by using a persulphate as first oxidising agent and a hypochlorite as a second oxidising agent.

A final stage of washing the material with a liquid of pH other than 7 is a desirable optional feature of the method of the invention for the purpose of neutralising any excess acidity or alkalinity of the surface of the material and giving it a pH of 5-8. This liquid can also act as a wash to remove any unwanted solid substance on the surface of the material.

After treatment by the method of the invention, the material may be dried in air or force-dried, for example by a current of heated air. Some time is desirably allowed to elapse between the treatment and the force-drying; the minimum time varies from wood to wood, but a time of at least 5 minutes is generally required for best results.

In addition to the above general teaching, the invention provides specific guidance to cover most practical case's. In particular, the following ranges of compositions are an important feature of the invention.

While a treatment with the persulphate only is very advantageous in giving a stable colour, it is possible to help in controlling the actual shade of colour and in giving optimum results in some cases by using additional steps and carrying out the process in stages.

For the first stage of the method of the invention, it is preferred to use as oxidising agent, particularly for dark or medium coloured woods such as 'sapele (Entandrophragma cylindricum African mahogany (Khaya ivorensis), afrormosia (Afrormosia elata), muninga (Pter0 carpus angolensis), utile (Entandrophragma utile), iroko (Chlorophora excelsa), tola (Oxystimga oxyphyllum), African walnut (Lovoa klaineana), and European walnut (Juglans regia), a solution containing from 10 g. per litre up to the solubility limit of potassium, sodium or ammonium persulphate, the remainder of the solution consisting essentially of water and a lower alcohol and optionally a surface-active agent (such as 5-15 ml. of Teepol, a surfactant based on the sodium salts of secondary alkyl sulfates, cf. Kingzetts Chemical Encyclopedia, 9th edition, p. 959 or Lissapol, a surfactant, either sodium oleylsulfate (Lissapol-C) or an octylcresol condensation product with ethylene oxide (Lissapol NX), cf. Hackhs Chemical Dictionary, 4th edition, p. 392). Such a solution is herein called solution A.

For light-coloured woods (such as oak or maple) it is sometimes advantageous to precede the persulphate treatment by a bleaching step, for example with an alkaline hypochlorite solution containing 1-5% of available chlorine, or a hydrogen peroxide solution.

For the second oxidising stage (if one is used), it is preferred either to use a solution containing, per litre, sodium hypochlorite in an amount equivalent to 500 ml. of a sodium hypochlorite solution of 10-12% available chlorine, and a pH adjusting agent in an amount to give the desired pH, the remainder consisting essentially of water, or to use a 10-100 volumes hydrogen peroxide solution which desirably contains 1-5% ammonium acetate by weight. Such a solution is herein called solution B. The sodium hypochlorite solution is generally used when a persulphate has been used as the first or main oxidising agent that is, in many cases where solution A has been used). The hydrogen peroxide solution is generally used after solution A in the case of teak, and after solution B in the case of light-coloured woods.

In its application to the material, the selected solution is in each case preferably swabbed on to the surface of the material for approximately 5 minutes. Between successive stages a period of time is preferably allowed for drying; this time may be reduced by force-drying, for example in a current of warm air. Desirably the material is allowed to remain at least five minutes at room temperature.

If the material to be treated is sapele, African mahogany, afrormosia, muninga, utile, iroko or tola, and the final colour is to be light, then it may be treated in a first stage with a solution A (having an alkaline pH), in a second stage with a solution B (having an alkaline pH), and in a final stage with a solution of a weak acid (for example containing 20-100 ml. of glacial acetic acid per litre) to restore the pH to 5-8. In this case preferred solutions A and B contain respectively the following ingredients in approximately the following amounts per litre- A: 30 g. of potassium persulphate, 30 g. of sodium carbonate, 0.5 g. of potassium permanganate, ml. of methylated spirit, 10 ml. of of Teepol or Lissapol, and water to make up a litre; B: 200 ml. of sodium hypochlorite solution containing 10-12% available chlorine, 10 g. of sodium carbonate, and water to make up a litre.

If the material to be treated is European walnut, or if it is sapele, African mahogany or utile, and the final colour is to be dark, then it may be treated in a first stage with a solution A (having an alkaline pH) and in a second, final stage with a solution of a weak acid (for example containing 20-100 ml. of glacial acetic acid per litre) to restore the pH to 6-8. In this case a preferred solution A contains the following ingredients in approxi tion B (having an alkaline pH) and in a third stage with 10 a solution of a weak acid (for example containing 20-100 ml. of glacial acetic acid per litre) to restore the pH to 6-8. In this case preferred solutions A and B contain respectively the following ingredients in approximately the following amounts per litre--A: 30 g. of ammonium persulphate, 100 ml. of 0.88 ammonia, 0.5 g. of potassium permanganate, 150 ml. of methylated spirit, 10 ml. of Teepol or Lissapol, and water to make up a litre; B: 200 ml. of sodium hypochlorite solution containing 10-12% available chlorine, 10 g. of sodium carbonate and water to make up a litre.

It has been found that, in general, pink or light red woods may be treated with the same solutions as sapele and African mahogany, while beige or pale brown woods may be treated with the same solutions as utile, iroko and tola.

After treatment by the method of the invention, the material may be subjected to a conventional finishing operation. For example, wood may be finished by de-nibbing or sanding after filling or sealing. Heavy sanding is desirably avoided as it may remove the layer stabilised by the method of the invention, if such layer (as is usual) is not more than one millimetre thick.

When using the method of the invention to bring differently coloured samples of the same species of wood into greater colour uniformity, it may be advantageous to apply to the lighter coloured samples a solution which has a darkening effect as well as a stablising effect, and to the darker samples a solution which has a lightening effect on colour as well as a stabilising effect. A solution B generally has a lightening affect and a solution A generally has a darkening efiect. These solutions are suitably used at half the strengths indicated herein, and may be followed by the treatment with acetic acid solution, the

latter being used at full strength. For example, in order to bring different samples of afrormosia into greater colour uniformity, the darker samples may be treated with a half-strength solution B and the lighter samples with a half-strength solution A, followed in both cases by treatment with acetic acid solution of the strength hereinbefore indicated.

The invention also provides wood materials, either solid woods or veneers, treated by the method of the invention, as well as furniture and other artifacts made of or containing such treated material.

It is important to note that the process of the invention not only produces stabilised colours: it also enables different colours to be obtained by variation of the parameters mentioned above, such as the pH, the nature and strength of the oxidising agents, and the use of more than one treatment with different agents.

The invention is illustrated by the following examples.

EXAMPLE 1SAPELE A solution A was used containing 50 g. of potassium persulphate, 50 g. of sodium carbonate, 0.5 g. of potassium permanganate, 150 ml. of methylated spirit, 10 ml. of Teepol and enough water to make up one litre. The solution was applied to a sample of sapele by swabbing for about 2 minutes. The sapele was then allowed to dry for about an hour. It was then swabbed for about 2 minutes with a solution B containing 200 ml. of a sodium hypochlorite solution containing 10-12% of available chlorine, 10 g. of sodium carbonate and water to make up a litre. The sapele was then allowed to dry for an hour. It was then swabbed for about 2 minutes with a solution of acetic acid containing 50 ml. of glacial acetic acid and water to make up a litre. It was then completely dried and coated with one layer of a clear nitrocellulose lacquer.

The treated sapele panel and a control (untreated) panel more exposed to a north light for eight weeks, their reflectance being measured before and after such exposure. The results are given in Table 1. The treated panel had a much more stable colour than the control panel.

EXAMPLE 2--AFRICAN WALNUT A panel of African walnut was treated as described in Example 1 except that solution A contained 50 g. of ammonium persulphate and 100 ml. of 0.88 ammonia in stead of the potassium persulphate and sodium carbonate.

The reflectance results obtained with the treated panel and a control panel are given in Table 1. The treated panel had much more stable colour than the control panel.

EXAMPLE 3-AFRORMOSIA A panel of afrormosia was treated as described in Example 1.

The reflectance results obtained with the treated panel and a control panel are given in Table 1. The treated panel had a much more stable colour than the control panel.

EXAMPLE 4EUROPEAN OAK A panel of European oak (Quercus robur) was pretreated being immersed for about 5 minutes in a solution contaiuing 30 g. of potassium persulphate, 30 g. of sodium carbonate, 0.5 g. of potassium permanganate, 150 ml. of methylated spirit, 10 ml. of surface-active agent and water to make up a litre. It was allowed to dry. It was then immersed for about 5 minutes in a solution A1 of sodium hypochlorite containing 3% of available chloride and 2% of sodium carbonate. It was then dipped, while still wet, into a solution of hydrogen peroxide of 40 volumes strength, with agitation. The wood was finally rinsed with water and dried.

TABLE 1 Reflectance values obtained with the FIRA Spectrophotometer using nine Iltord colour filters Nos. 601-600. Readings were taken of test panels and controls before and after an eight-week period of exposure to the north light. The values in the table represent percent reflectance oi a chrome yellow surface, used as standard (reflectance=100%). The values for controls are given in parentheses.

Filter No.

P 8 transmission my. 426 470 490 520 650 580 600 660 685 T C T O T C T O T O T O T O T O T C Wood:

Entgtdrophragma cylmdricum (sapele):

A 71 (74) Afrormosz'a elata (afrcrmosia)' NOTE.B=before exposure, A=elter exposure, T=treated specimens, 0 =control.

3% acetic acid in water and dried completely at ambient temperature.

EXAMPLE 9 Species of wood and solutions used were as in Example 8, but instead of leaving the samples at ambient temperature after application of solution A, they were exposed to a stream of warm air at 60 C, for 5 minutes, and instead of the final drying at ambient temperature they were dried completely at 60 Ca TABLE 2.REFLECT ANCE CHANGES FOR WHITE OAK Filters 1 2 3 4 5 6 8 8 9 Before exposure 76(97) 77(97) 5902 64(63) 55(66) 59(67) 66(71) 80(86) 93(96) After exposure 79(87) 79(87) 60(64) 55(61) 57(60) 60(61) saws 82(80) 93 90 EAMPLE 5 EXAMPLE 10 Samples of the followmlg Woods welie used: Samples of the following woods were used: giant (European, American or African) Cherry Oak Sapele and African walnut Cherry Maple Equal volumes of a solution A and a solution B, the Mahogany compositions of which were as in Example 9, were mixed Iroko together and the mixture was applied by swabbing for Sapele and about 3 minutes, then the samples were left at ambient Makore temperature for 15 minutes and washed with 3% acetic A solution A was used containing 40 grammes of ammonium persulphate, 100 millilitres of 0.88 ammonia, 150 ml. of methylated spirit, 10 ml. Teepol and enough water to make upon one litre. The solution was applied to samples of the above species by swabbing for about 2 minutes, then the samples were left to dry completely at ambient temperature and coated with a clear nitrocellulose lacquer,

EXAMPLE 6 Samples of all the woods listed in Example 5 were used, A solution A the composition of which was as that in Example 5 was applied to the samples by swabbing for about 5 minutes, then the samples were exposed to a stream of warm air of about 60 C. until completely dry and were finally coated with a clear nitrocellulose lacquer,

EXAMPLE 7 Samples of all the woods listed in Example 5 were used, A solution A the composition of which was as that in Example 5 was applied by swabbing for about 2 minutes, then the samples were left to partly dry at ambient temperature for 20 minutes, then washed with 3% acetic acid in water and dried completely.

EXAMPLE 8 Samples of the following woods were used:

Iroko Pecan Sapele Cherry and Makore Maple Walnut acid in water, The samples were finally dried at ambient temperature.

EXAMPLE 11 Species, solutions and method as in Example 10, except that the final drying was carried out in a stream of warm air, approximately 60 C,

EXAMPLE 12 Samples of oak were used.

A bleaching solution containing 15% (50 volumes) hydrogen peroxide in water was first applied as a single swab and after an interval of 5 minutes a second solution identical in composition with solution B in Example 8 was applied by swabbing for 2 minutes, then the samples were left to partly dry at ambient temperature for about 20 minutes and swabbed with a third solution containing 30 grammes of potassium persulphate, 20 grammes of so dium carbonate, ml, methylated spirit, 10 ml. Teepol and enough water to make one litre and after an interval of 15 minutes, the samples were washed with 3% acetic acid in water and dried completely,

In all of Examples 5 to 12, the resulting treated samples were found to be much more stable in colour than central (untreated) samples of the same woods,

What is claimed is:

1. A process for producing stable colour in a wood comprising subjecting wood selected from the group consisting of sapele, mahogany, walnut, oak, cherry, maple, afrormosia, muninga, utile, iroko and tola to a treatment with a persulfate oxidizing agent under alkaline conditions whereby a stabilized colour is imparted to the wood.

2. A process according to claim 1 wherein the persulfate is selected from the group consisting of potassium and ammonium persulfate.

3. A process according to claim 1 wherein, after the treatment with the persulfate oxidizing agent, the pH of the wood surface is adjusted to from about 5 to about 8 with a weak acid,

4. A process according to claim 1 wherein the wood being treated is a mahogany 5. A process according to claim 1 wherein the wood being treated is a walnut.

6. A process according to claim 1 wherein the wood being treated is oak.

7. A process according to claim 1 wherein the wood being treated is cherry.

8. A process according to claim 1 wherein the wood being treated is maple.

9. A process for producing stable colour in a wood comprising subjecting wood selected from the group consisting of mahogany, walnut, oak, cherry and maple to a first treatment with a persulfate oxidizing agent under alkaline conditions, subjecting the wood to a second treat-= ment with a hypochlorite oxidizing agent under alkaline conditions and then adjusting the pH to from about 5 to about 8 with a weak acid whereby a stabilized colour is imparted to the wood.

10. A process according to claim 9 wherein the persulfate is selected from the group consisting of potassium and ammonium persulfate.

11. A process according to claim 9 wherein the pH adjustment is elfected with acetic acid.

12. A process according to claim 9 wherein the per= sulfate oxidizing agent is catalyzed with a catalyst se-= lected from the group consisting of permanganate and cupric ions.

13. A process according to claim 9 wherein the wood being treated is a mahogany.

14. A process according to claim 9 wherein the wood being treated is a walnut.

15. A process according to claim 9 wherein an aqueous potassium persulfate solution is employed containing per liter about 30 gms. of potassium sulfate, about 30 gms. of sodium carbonate, about 0.5 gm. of potassium permanganate, about 150 ml. of methylated spirit, about 10 ml. of a surface-active agent, and water to make up a liter, and the hypochlorite is added as an aqueous solution containing per liter about 200 ml. of sodium hypochlorite solution containing about 10-12% available chlorine, and about 10 gms. of sodium carbonate.

16. A process according to claim 9 wherein the pH adjustment is elfected with an aqueous solution that contains from about 20 to about ml. of glacial acetic acid per liter of solution.

17. A process according to claim 9 wherein the wood being treated is cherry.

18. A process according to claim 9 wherein the wood being treated is oak.

19. A process according to claim 9 wherein the wood being treated is maple.

References Cited UNITED STATES PATENTS 2,096,400 10/ 1937 Kritchevsky 81 11 3,348,903 10/1967 Baier 8-111 3,429,772 2/1969 Harvey et al. 8-111 XR 3,554,785 1/1971 Dombay 117-57 MAYER WEINBLATI, Primary Examiner US. Cl. X.R.