Preparation of Densified Wood Impregnated With Phenolic Resins

The present invention relates to a method of preparing densified wood impregnated with phenolic resin. More particularly, it relates to an improved method of preparing densified wood having a high phenolic resin content and a high mechanical strength by impregnating and reinforcing the wood with a phenolic resin solution obtained by blending about 0.6 to about 4 parts by weight of methanol, acetone, or a mixture of methanol and acetone per part by weight of the solid content of a resole-type phenolic resin under a high pressure of about 10-50 Kgs/cm.sup.2.

Densified wood impregnated with phenolic resins is known and is used for electric insulation boards, handles of knives, door knobs, etc. However, since the phenolic resins used for these purposes generally do not easily permeate into wood tissues, densified wood has been manufactured by slicing wood into veneers of 1-2 mm in thickness and putting the veneers into the impregnation tank of an impregnator, and after evacuating the air of the tank, feeding the phenolic resin into the tank and impregnating the veneers with the phenolic resin from the sliced faces under vacuum, or further impregnating the veneers with the phenolic resin for a short time under a pressure of 0-5 Kgs/cm.sup.2, then taking out the impregnated veneers from the tank and removing the excessive resin on the surfaces of the veneers, pre-drying the veneers at a temperature of 80.degree.-100.degree. C, and after piling up the veneers in 10-50 layers, hot-pressing the veneers at a temperature of 130.degree.-200.degree. C under a pressure of 20-100 Kgs/cm.sup.2 for 20-100 minutes, and followed by finishing.

The aforementioned conventional method for manufacturing such densified wood, is defective in that the yield of the densified wood product from the raw timber is low because of the necessity to slice the wood into veneers before impregnation. Wood resources are now gradually decreasing, and especially the domestic broadleaved tree resources such as birch, beech, maple, etc. which have dense textures and beautiful grains, and which are most suitable as raw materials for phenolic resin-impregnated densified wood preparation are being exhausted markedly. Accordingly, increasing the yield of the densified wood product from the raw timber is highly desirable. To achieve this purpose, it is desirable that blocks of raw timber be impregnated with phenolic resins without previous slicing of the wood into veneer. However with the conventional impregnating techniques mentioned above, phenolic resin cannot penetrate deeply into the unsliced wood blocks.

The purpose of the present invention is to provide an improved method for making densified wood. This is achieved according to the invention by impregnating wood e.g. wood blocks, with a resole-type phenolic resin pre-condensate solution mixed, i.e. diluted, with an organic solvent such as methanol or acetone. Wood impregnation by the present mixtures proceeds from the cross-sectional faces of the blocks deep into the center thereof, facilitated by the good permeability of the phenolic resin pre-condensate into the wood vessels.

The phenolic resins used in the present invention are phenol-formaldehyde condensates generally designated as resole-type phenolic resins, which include resins which, in the precondensate stage, are alcohol-soluble, i.e. soluble in the alcohol but insoluble in water, and water-soluble, i.e. soluble in both the alcohol and water. The impregnating of a block wood with phenolic resin is greatly affected by the properties and states of the resin.

More particularly, the important factors controlling the impregnation of wood with phenolic resins are:

(1) the molecular weight of the pre-condensate of the phenolic resins, (2) the affinity of the pre-condensate of the phenolic resin for the wood, (3) the viscosity of the solution of the phenolic resin pre-condensate, and (4) the type and amount of the solvent contained in phenolic resin solution and used for dilution.

First, the lower the molecular weight of the pre-condensate of the phenolic resin, the smaller will be the flow resistance in the fine vessels of the wood block during the impregnation and the better is the result of impregnation. The pre-condensate of the resole-type phenolic resin generally has a molecular weight of 100 to 500 and is superior in impregnating ability to novolac-type phenolic resin whose molecular weight is generally in the range of 100 to 1200.

Second, the higher the affinity of the phenolic resin pre-condensate towards the wood, the better will be the fixed state of the resin inside the wood tissue during the subsequent drying process. At this point, the pre-condensate of the resole-type phenolic resin contains more methylol groups than the pre-condensate of the novolak-type phenolic resin and, in consequence, exhibits a higher affinity to the cellulose that constitutes the wood tissue and thus gives a more outstanding product.

Next, with respect to viscosity, the lower the viscosity of the impregnating solution, the better will be the permeability of the resin solution into the wood. Therefore, when the viscosity of the resin solution is high, it is desirable to dilute the phenolic resin solution with a solvent, and thereby lower the viscosity of the solution. However, as the solvent use for this purpose must be evaporated when the impregnated wood is dried by heating, an optimal degree of dilution is determined by both the rate of permeation and the efficiency of evaporation. Therefore, the type and amount of the solvent contained in the phenolic resin pre-condensate solution or used as diluting agent are the most important factors of all when impregnation of the block wood with the solution is performed under a high pressure of 10-50 Kgs/cm.sup.2. It is particularly important that the permeation rates of both the phenolic resin pre-condensate and the solvent be as near to each other as possible in order to ensure deep impregnation of the pre-condensate of phenolic resin into the wood block. If the permeation rate of the phenolic resin pre-condensate differs to a great extent from that of the solvent, selective adsorption of them by the wood may occur so that complete impregnation of the interior of the wood blocks with the phenolic resin pre-condensate becomes impossible. However, as the molecular weight of the phenolic resin pre-condensate is generally higher than that of the solvent, the rate of permeation into the inside of wood block of the phenolic resin pre-condensate is generally lower than that of the solvent. So, in order to make the permeation rates of the phenolic resin pre-condensate approximate that of the solvent as closely as possible, a variety of solvents have been tested by the present inventors. As a result, it was found that wood blocks impregnated with a very high content of the phenolic resin could be obtained by the use of a phenolic resin solution prepared from about 100 parts by weight of the phenolic resin solids and about 60 to about 400 parts by weight of methanol, acetone, or a mixture of methanol and acetone.

After wood blocks have been impregnated with the pre-condensate of the phenolic resin in an impregnating tank under pressure, they may be directly transferred to a dryer, dried and heated to obtain a densified wood. However, when the wood blocks are of small size, the densified wood block is advantageously obtained by hot-pressing a mass of piled blocks which have been impregnated according to the invention.

In order to describe the present invention more concretely, several examples are presented below, although it should be understood that these examples do not restrict the scope of the present invention. Parts and percentages cited in the examples and claims are by weight and unless otherwise noted.

Twenty pieces of block sapwood of beech, 300 cm in length, 30 cm in width and 5 cm in thickness, were dried to a moisture content of less than 15% and charged to an impregnation tank of an impregnation apparatus. Meanwhile, an impregnating solution was prepared by mixing 100 parts by weight of water-soluble resole-type phenolic resin (formaldehyde/phenol = 1.5 (mole ratio); catalyst, caustic soda; solid content of the resin, 50%; water content, 50%) with 150 parts by weight of methanol. Then, the impregnation tank charged with the beech blocks was deaerated for 1 hour under vacuum of 70-76 cm Hg. The above impregnating solution was charged to the tank. The impregnation of the blocks with the impregnating solution was carried out for 12 hours under a pressure of 30 Kgs/cm.sup.2. Subsequently, the beech blocks were discharged from the tank, and excess impregnating solution on the wood surfaces was removed. The blocks were air-dried for 2 days, then charged to a dryer and heated for 12 hours at a temperature of 120.degree.-130.degree. C to evaporate the water and methanol and to fix and then cure the pre-condensate of the phenolic resin inside the wood. After cooling, the blocks were finished to obtain a densified wood. For comparison with respect to the phenolic resin content and bending strength of the densified wood, comparable blocks impregnated with water-soluble resole-type phenolic resin in absence of methanol under a pressure of either 5 Kgs/cm.sup.2 or 30 Kgs/cm.sup.2 were prepared. The results are shown in Table 1.

Ten blocks of cedar, 400 cm in length, 40 cm in width and 20 cm in thickness, which had been dried to a moisture content of less than 15% were charged to an impregnation tank of the impregnator. Meanwhile, an impregnating solution was prepared by mixing 100 parts by weight of water-soluble resole-type phenolic resin (formaldehyde/phenol = 1.7; catalyst, caustic soda; solid content of the resin 45%; water content 55%) with 50 parts by weight of methanol. The impregnation tank charged with cedar blocks was then deaerated for an hour at a vacuum of 70-76 cm Hg, the impregnating solution mentioned above was charged and the impregnation of the blocks with the impregnating solution was carried out for 10 hours under a pressure of 10 Kgs/cm.sup.2. Subsequently, the cedar blocks were discharged from the tank, and the excess impregnating solution on the wood surfaces was removed. The blocks were air-dried for 2 days, and charged to a dryer and heated for 15 hours at a temperature of 120.degree.-130.degree. C to evaporate the water and methanol and to fix and cure the pre-condensate of the phenolic resin inside the wood. After cooling, the blocks were finished to obtain a densified wood product. The phenolic resin content and bending strength of the densified wood impregnated according to the above process, were compared to those of comparable blocks prepared by impregnation with the water-soluble resole-type phenolic resins in absence of the methanol under a pressure of either 3 Kgs/cm.sup.2 or 10 Kgs/cm.sup.2. The results are shown in Table 2.

Thirty blocks of birch, 200 cm in length, 4 cm in thickness and 30 cm in width, which had been dried to a moisture content of less than 15%, were charged to an impregnation tank of the impregnator. Meanwhile, an impregnating solution was prepared by mixing 100 parts by weight of alcohol-soluble resole-type phenolic resin (formaldehyde/phenol = 1.1; catalyst, ammonia; solid content of the resin, 65%; water, 15%; methanol 10%; and acetone, 5%) with 20 parts by weight of methanol and 20 parts by weight of acetone. The impregnation tank charged with the birch blocks was then deaerated for about 1 hour at a vacuum of 70-76 cm Hg, the impregnating solution described above was charged and the impregnation of the wood blocks with the solution was carried out for 12 hours under a pressure of 50 Kgs/cm.sup.2. Subsequently, the birch blocks were discharged from the tank, and the excess impregnating solution on the wood surfaces was removed. The birch blocks were air-dried for 2 days, and charged to a dryer and heated for 12 hours at a temperature of 120.degree.-130.degree. C to evaporate the water, methanol and acetone and to fix and cure the pre-condensate of the phenolic resin inside the wood. After cooling, the blocks were finished to obtain a densified wood product.

The phenolic resin content and bending strength of densified wood impregnated according to the above process were compared to those of comparable blocks prepared by impregnation with the alcohol-soluble resole-type phenolic resin solution with methanol and acetone under a pressure of 5 Kgs/cm.sup.2, both in the absence and presence of the added methanol-acetone mixture. The results are shown in Table 3.

It is to be understood that the invention is not limited to the specific examples which have been offered merely as illustrative of the invention and that modifications can be made therein without departing from the spirit of the invention.