A process for producing composite particle board from rice husk

Abstract: A process is disclosed for producing composite particle board from rice husk. The rice husk is cleaned and may be split prior to admixture with a substantially moisture-free resin to form a furnish. The furnish is spread to form a layer which is then subjected to the step of hot pressing and cooling. The resin comprises a condensation product of cashew nut shell liquid and/or cardanol with phenol paraformaldehyde and alkaline catalyst.

Description: FIELD OF INVENTION This invention relates to a process for producing composite particle boards from rice husk and composite boards produced therefrom. PRIOR ART It is known in the art to produce particle board from agro-wastes using phenolic resins. Such a known process envisages the step of blending rice husk with a phenolic resin containing a substantial quantity of water. Due to the presence of water, the blend is subjected to the step of drying for removal of the excess moisture content during or after the resin is coated on the rice husk or furnish. The furnish is then hot pressed to form particle boards. Several disadvantages are associated with the use of such a known phenolic resin. One such disadvantage is that a drying operation results in a precuring of the resin, thereby diminishing its adhesive strength and resulting in poor quality particle boards. Moreover, since rice husk is a material of low bulk density, handling of the resin coated husk or furnish for drying is not convenient. Another disadvantage is that of the control of the moisture content of the furnish. It is generally known that the furnish should have a moisture content, but which simultaneously should not be substantial in quantity as an excess of moisture could result in the formation of blisters or steam pockets in the formed boards. Thus, as rice husk itself contains moisture, the use of the known resin containing substantial quantities of water results in a furnish having a high moisture content, which is detrimental to the formation of the board. Furthermore, even if the furnish is subjected to the step of drying, it is difficult to control the correct content of moisture in the furnish. OBJECTS OF THE INVENTION An object of this invention is to propose an improved process for the production of composite particle boards from rice husk and composite boards produced therefrom. Another object of this invention is to propose a process for the production of composite particle boards from rice husk which no longer requires the step of drying of the furnish. Still another object of this invention is to propose a process for the production of composite particle boards from rice husk which no longer is attended with the disadvantage of formation of steam pockets or blisters. DESCRIPTION OF THE INVENTION According to this invention there is provided a process for producing composite particle boards from rice husk which comprises in blending cleaned rice husk with a substantially moisture-free resin to form a furnish, said resin comprising a condensation product of cashew nut shell liquid and/or cardanol with phenol, paraformaldehyde and an alkaline catalyst, spreading the furnish to form a layer, subjecting the furnish layer to the step of hot pressing and subsequent cooling to form the board. It is generally known that rice husk is of a boat shape. It has now been found that such a rice husk is not receptive to a uniform coating of resin. Thus, in accordance with this invention, rice husk may first be subjected to the step of splitting to provide a surface more receptive to resin. Such a step of splitting is important, as in the absence thereof, the bulk density of rice husk would be low and, consequentially, the thickness of furnished mat would be high, and that when such a furnished mat is subjected to the step of pressing, the edges of the mat would tend to spread. As the density in the spread portion would be lower than the main body portion of the board, the spread portion would need to be trimmed and, thereby, resulting in a loss of material. The split rice husk is subjected to the step of cleaning and grading by any known method for removal of dust and other extraneous matter. One of the concepts of the present invention resides in the use of a substantially moisture free resin so that, firstly, the moisture content of the furnish can be controlled and, secondly, the step of drying of the furnish is avoided. The step of control on the moisture content of the furnish is important, as the presence of a correct moisture content provides a board without blisters and steam pockets. The furnish should contain a moisture content of 6 to 15% based on dry weight, and preferably between 8 to 12%. Simultaneously, rice husk itself contains a moisture content, which normally is in the proximity of 8 to 12%. Thus, prior to the step of blending with resin, the moisture content of rice husk is determined and only, if required, water is added to the blend so that the furnish contains the required moisture content of 6 to 15%, and preferably 8 to 12%. The upper and lower limits of the moisture content contained in the furnish is dependant on the thickness of the board. If the board is thin, for example 6mm, the moisture content can then be upto 15%. However, if the board has a thickness of, for example, 25 mm a moisture content of the lower value of 6% is preferred. A moisture content of 6 to 15% in the furnish is required because when the furnish is subjected to the step of hot pressing, the resin melts but does not flow adequately to wet adjacent particles. Further, to permit a bonding of rice husk to form the board, the surfaces of rice husk must get together, and for which a flexibility of rice husk is required. Such a flexibility is imparted through the moisture contained in the furnish. If the moisture content contained in the furnish is greater than 15%, and as temperatures as high as 170 to 180°C are employed in the step of hot pressing, only a negligible amount of moisture is allowed to escape from the edges. Thus, water is entrapped at high temperature during such a step of pressing, which is converted to steam on release or removal of pressure, and which results in the possible formation of blisters. However, if the furnish contains less than 6% of moisture, then the flexibility property is not present and, whereby, the wetting of the rice husk with resin is reduced. In accordance with this invention, rice husk is blended with 5 to 20% of resin to obtain a furnish. If more than 20% of resin is added to rice husk, the process is uneconomical. If the furnish contains less than 5% of resin, the board does not contain enough strength. Preferably, 10 to 12% of resin is added to rice husk. The furnish layer is subjected to the step of hot pressing at a temperature of 150 to 200°C, and preferably at a temperature of 170 to 180°C. The pressure applied to the furnish layer is 5 to 15 kg/cm 2 , and preferably 10 to 12 kg/cm 2 . The density of the particle board may vary from 400 to 1200 kg/m 3 . The board prepared by the process of the present invention may advantageously be used for overlaid and/or reinforced composite particle boards. In the instance of a reinforced board, one or more fibrous materials such as coconut fibre, bamboo slivers and wood strips are incorporated in the board. Coconut fibre, bamboo and wood used as reinforcement increases the strength of the boards and incorporation of short length or chopped fibre in the rice husk increases the internal bond strength of the boards prepared therewith. Coconut fibre, bamboo and wood strips may also be used as surface reinforcements for the rice husk particle board. Such surface reinfrocements increases the bonding strength and the decorative value of the resulting board. Rice husk and the reinforcement such as chopped fibre is blended with a coconut shell liquid and/or cardanol-phenol-formaldehyde adhesive resin which is substantially moisture free. This admixture is then spread out evenly to form a layer and then hot pressed to obtain reinforced boards. Alternatively, and in the instance where the reinforcement is in the form of strips and slivers, the reinforcement is coated with coconut shell liquid and/or cardanol-phenol-formaldehyde resin and then embedded in an even layer of resin coated rice husk furnish and hot pressed to form the reinforced board. Overlaying is effected on one side or on both sides to provide a compact overlaid finished board. Known phenolic resin adhesives and resin adhesives compatible with coconut shell liquid and/or cardanol-phenol-formaldehyde resin may be used for bonding the overlays to the rice husk. Overlays such as bamboo mats, coconut fibre felt, wood veneer and the like are coated with a coconut shell liquid cardanol-phenol-formaldehyde resin or a phenol-formaldehyde adhesive resin or any other resin compatible with coconut shell liquid and/or cardanol-phenol-formaldehyde resin. Resin coated rice husk furnish is then spread evenly thereon. Another resin coated overlaying material may be placed over the rice husk furnish layer if overlaying is required on both sides. In each of the overlaying, the layers should have a total moisture content of 6 to 15%. This assembly may then be subjected to hot pressing. This invention further relates to a process for preparing an improved adhesive resin suitable for bonding rice husk which comprises in the steps of admixing cashewnut shell liquid and/or cardanol with phenol, heating the said mixture in presence of an alkaline catalyst, adding para-formaldehyde to the said reaction mixture while the heating is continued to complete the condensation reaction and thereafter cooling the reaction product. In the preparation of the improved resin according to the invention, the reactants are taken according to the proportion set forth below :- CNSL/Cardanol 20% to 90% by weight Phenol 10% to 80% by weight Paraformaldehyde equivalent to 1.4 to 2 moles of formaldehyde based on the CNSL and phenol used. Alkaline catalyst in the range of 1 to 5% by weight of the CNSL/Cardanol and phenol used. The catalyst is dissolved in 2 to 10 times its weight of water before adding to the condensation mixture. The condensation takes place at the temperature range of 50 to 95°C. The heating is generally continued for a period of 25 to 45 minutes initially. The second stage of condensation after the addition of a further quantity of phenol is carried out at the temperature range of 55 to 95°C for a period of 30 to 45 minutes to complete the condensation reaction. The following examples are only illustrative and not restrictive on the scope of the invention. EXAMPLE 1 50 Kg cleaned rice husk is blended with 5 Kg of the adhesive resin of the present invention in a muller for 15 mts. 15 Kg of the furnish so formed is taken out and is spread out in an even layer. This is subjected to hot pressing at 170 °C for 15 minutes to a nominal thickness of 1.2 cm. The board produced is then cooled and the edges trimmed to a nominal size 1.2 m x 1.2 m x 1.2 cm. The board prepared accordingly has a specific gravity of 0.70. EXAMPLE 2 38 Kg of the furnish prepared according to example 1 is laid out as an even mat and hot pressed at 170°C for 25 minutes, to a nominal thickness of 1.9 cm. The board is cooled and trimmed to a nominal size of 1.8 M x 1.2M x 1.9 CM and has a specific gravity of 0.82. EXAMPLE 3 Bamboo slivers of about 1 mm thick, about 15 mm wide and about 1.4 m to 2 m length are coated with CNSL and/or phenol-formaldehyde resin which is substantially moisture free. 50 Kg. cleaned rice husk is blended with 6 Kg. of CNSL and/or cardanol-phenol-formaldehyde resin substantially free of water. 24 Kg.of the furnish thus produced is spread out as an even layer and the resin coated bamboo slivers are placed over the layer at approximately 10 cm intervals both along and across the length of the furnish layer. Another 24 Kg. of the said furnish is spread over the bamboo slivers in a uniform layer and the assembly pressed at 175-185°C for 30 minutes to a nominal thickness of 2.0 cm. The bamboo reinforced composite rice husk board that is formed is cooled and trimmed to a nominal size of 1.8 m x 1.2 m x 2.0 cm. It has a specific gravity of 0.85. EXAMPLE 4 Two bamboo mats of the size 1.5 m x 1.5 m are evenly coated with a CNSL and/or cardanol-phenol-formaldehyde resin which is substantially moisture free. 50 Kg. rice husk is blended with 5.5 Kg. of the said resin and 18 Kg. of the furnish so obtained is spread as an even layer on one of the said resin coated bamboo mats. The other mat is then placed on top of the furnish layer. This assembly is then pressed in a hot press at 160 to 170°C for 20 mts.to obtain a bamboo mat overlaid composite particle board having a nominal thickness of 1.2 cm. The board is cooled and trimmed to a nominal size of 1.2 m x 1.2 m x 1.2 cm. This board has a specific gravity of 0.78. EXAMPLE 5 5.45 Kg. phenol, 0.31 kg sodium hydroxide dissolved in 2.35 Kg water, 26.5 Kg CNSL and 1.56 Kg paraformaldehyde are mixed together and heated to 50 to 55°C. In about 25 minutes, 8.15 Kg of paraformaldehyde are added to the above reaction mixtue in 12 approximatley equal quantities, at about 4 mins. intervals each. The heating is continued during the addition of paraformadehyde, 5.9 Kg phenol and 0.15 Kg of sodium hydroxide dissolved in 0.75 Kg water are added to the reaction of mixture, while continuing the heating. The temperature is then raised to around 65 to 70°C, and the mixture is heated for around 40 minutes to complete the condensation. The resinous product is then cooled. EXAMPLE 6 7.5 Kg phenol, 0.37 Kg sodium hydroxide dissolved in 2.7 Kg water, 13.4 Kg cardonol and 1.5 Kg paraformaldehyde are mixed together and heated to 50 to 55°C. To this 11.0 Kg of paraformaldehyde are added in approximately 12 equal quantities, at 4 mins. intervals while continuing the heating. Subsequently a further quantity of 12.6 Kg phenol and 0.22 Kg sodium hydroxide dissolved in 1.23 Kg water is added to the mixture, and the temperature raised to 65 to 70°C. The heating is continued for 30 minutes to complete the condensation reaction. The product is thereafter cooled.

Claims: A process for producing composite particle boards from rice husk which comprises in blending cleaned and split rice husk with a substantially moisture-free resin to form a furnish, said resin comprising a condensation product of cashew nut shell liquid and/or cardanol with phenol, paraformaldehyde and an alkaline catalyst, spreading the furnish to form a layer, subjecting the furnish layer to the step of hot pressing and subsequent cooling to form the board. A process as claimed in claim 1 wherein the furnish has a moisture content of 6 to 15%, and preferably 8 to 12%. A process as claimed in claim 1 wherein the rice husk is subjected to the step of splitting and then blended with said resin. A process as claimed in claim 1 wherein rice husk is blended with 5 to 20% of resin to obtain a furnish. A process as claimed in claim 4 wherein rice husk is blended with 10 to 12% of resin to obtain a furnish. A process as claimed in claim 1 wherein the furnish layer is subjected to the step of hot pressing at a temperature of 150 to 200°C, and preferably at 170 to 180°C. A process as claimed in claim 1 wherein the furnish layer is subjected to the step of hot pressing at a pressure of 5 to 15 Kg/cm 2 , and preferably at 10 to 12 Kg/cm 2 . A process as claimed in claim 1 wherein reinforcements such as coconut, bamboo and wood in the form of fibers are blended with said husk and resin. A process as claimed in claim 1 wherein reinforcements such as coconut, bamboo and wood in the form of slivers and strips are coated with a resin and formed into a layer or layers provided on one or both sides of said furnish and that the total moisture content being between 6 to 15%. A process as claimed in claim 1 wherein the resin is prepared from a reaction mixture comprising in the steps of admixing cashewnut shell liquid and/or cardanol with part of phenol, heating the mixture in presence of an alkaline catalyst, such as an aqueous solution of sodium hydroxide, adding paraformaldehyde to the said reaction mixture while the heating is continued to complete the condensation reaction and thereafter cooling the reaction product. A process as claimed in claim 10 wherein the remainder of phenol is added to the reaction mixture subsequent to the addition of paraformaldehyde. A process as claimed in claim 10 where in the reaction mixture is initially heated to a temperature range of 50 to 55°C for 25 minutes. A process as claimed in claim 11 wherein the reaction mixture is heated to 55 to 95°C for a period of 30 minutes after the addition of the further quantity of phenol. A process as claimed in claim 10 wherein 20 to 90% parts of cashewnut shell liquid is admixed with 10 to 80% by wt. of phenol for resin preparation by condensation with paraformaldehyde. A process as claimed in claim 10 wherein para-formaldehyde equivalent to 1.4 to 2 moles of formaldehyde based on cashwnut shell liquid and/or cardanol and phenol is added to the reaction mixture. A process as claimed in claim 10 wherein the alkaline catalyst added in equivalent to 1 to 5% by weight of cashewnut shell liquid cardanol and phenol.

Inventors: JOSEPH GEORGE, JOSEPH, GEORGE, Joseph, George, 226/64, 243, 18th Cross (Behind
Applicants: NAT RES DEV, NATIONAL RESEARCH DEVELOPMENT CORPORATION
Application year: 1995
Main classification: B27N3
IPC codes: B27N3