IE920406A1 - Process for the manufacture of wood-fibre boards - Google Patents

Abstract

Process for the production of wood fibre boards, in which the raw material is subjected to defibration, is subsequently glue-treated with an aqueous bonding agent liquor in a blow pipe, and the resultant glue-treated fibre material, after drying to the required residual moisture, is spread into mat courses and is moulded under pressure and temperature to form boards, characterised in that the glue treatment takes place using a liquor having a solid resin content of 20 to 35% by mass and, in so doing, a quantity of bonding agent is used which amounts to less than 10% by mass of solid resin, relative to the total calculated absolutely dry fibre material.

Description

The invention relates to a process for the manufacture of wood-fibre boards, in which the raw material undergoes fibre separation and is then glue-coated with an aqueous binding agent bath in a blowpipe, and the glue-coated fibre obtained, after drying to the required residual moisture content, is spread into webs of matting and, at raised pressure and temperature, is pressed into boards. The invention relates especially to such a process for the manufacture of medium density fibreboards (MDF boards) and high density fibreboards (HDF boards) which contain a reduced amount of binding agent.

The mechanical properties of fibreboards are determined not only by the quality of the fibre separation process but also to a considerable degree by the binding agent used for the binding of the fibre material. Under otherwise similar conditions an increase in the mechanical strength of the boards accompanies an increase in the amount of binding agent used for the binding of the fibre materials.

Because, however, the binding agent material represents a considerable cost factor in the production of fibreboards, there is an endeavour, with regard to the efficienty of fibre board production, to use the smallest possible amount of binding agent.

Furthermore, there is the endeavour to reduce the amount of binding agent in order to lower environmental pollution from the formaldehyde separation which occurs particularly in aminoplastic binding agents. However, using the usual process for the manufacture of woodfibre boards, a reduction in the amount of binding agent added, as previously mentioned, leads to a drop in the strength of the boards so manufactured.

There have been previous attempts to reduce the amount of binding agent without detrimental effect on the strength, by various methods using apparatus to obtain the best possible distribution of the glue when adding the binding agent in the blowpipe; that is to achieve as large a number of glue points as possible, which are important for the bond strength. In these attempts, moreover, binding agent baths with a solid resin content of around 50 % by weight were used.

In the area of chipboard production, a process using reduced addition of binding agent has been described, wherein diluted binding agent with a solid resin content of preferably 22 to 30 % by weight is used (EPB-0 010 537).

By means of this process a more uniform distribution of the binding agent on chips of widely varying sizes is achieved, and thereby also a reduction in the amount of binding agent. The raw material used in this process undergoes comminution and subsequent desiccation down to a humidity of preferably less than 2 % before it is glue-coated with an aqueous bath of an aminoplast. The glue-coating of the chips takes place in a glueing mixer. In addition thickening agents such as cellulose derivatives are added to the binding agent bath, in order to avoid penetration of the binding agent into the chip material. The humidity of the glue-coated chip material, which is held below 12 %, is determined by the amount of water added to the chip material in the binding agent bath. This publication thus refers to a conventional process for the manufacture of chipboard, wherein the raw material is first dried and then glue-coated, as opposed to the invention which relates to a dry process for the production of fibrebords, whereby the initial material is first gluecoated and then dried.

With such dry processes it has so far not been possible to manufacture fibreboards which manifest adequate strength despite a reduced amount of binding agent. In order to achieve an adequate strength in accordance with existing standards, to date it has always been necessary to use more than 12 % by weight solid resin, by reference to absolutely dry fibre material, in fibreboards (including MDF and HDF boards) produced by the dry process.

It is an objection of the present invention to devise a process for the manufacture of fibreboards as mentioned in the introduction, whereby a further improved use of the binding agent used for the glue-coating of the fibre material can be achieved, the objective being that a certain board strength can be obtained using a given fibre material with a lesser amount of solid resin than has been necessary up to the present. The invention aims also to devise a process which shall be practicable without fundamental change in the equipment already available for the above-mentioned known processes, or without substantial change in the time required for the process. This is achieved according to the invention, in that the glue-coating is done with a bath containing 20 to 35 % by weight, preferably 25 to 35 % by weight of solid resin, and in that an amount of binding agent is used which is less than 10 % by weight, preferably 7 to 9 % by weight solid resin, based on the total fibre material reckoned at absolute dryness. It is possible to manufacture wood-fibre boards in a dry process, using highly diluted aqueous binding agent according to the invention, whereby the mechanical properties of the fibreboards well meet the standard requirements and at the same time a considerable saving in binding agent is made possible when compared with boards manufactured by the procedures of conventional technology. By means of the process according to the invention it is possible to manufacture wood-fibre boards with a solid resin content of less than 10 % by weight which are of adequate strength in accordance with existing standards.

With the use of highly diluted binding agent baths as provided in the process according to the invention, the obvious result is the most extensive possible covering of the fibre material with binding agent; and a large increase in the number of the adhesion points, which are essential for the strengthening of the boards, results.

A further advantage of this process is that, by means of the high dilution of the binding agent used, a cooling effect on the glue-coated fibres ensues, as a part of the water contained in the binding agent mixture immediately evaporates, thus extracting heat from the entire fibre/binding agent system. In this way a premature setting of the binding agent present on the glue-coated fibre is prevented. Such pre-setting can likewise result in a decrease in the board strength.

The present process according to the invention can be carried out without substantial change in the processing technique. Shortly after the fibre material makes its exit from the fibre separator (refiner) the glue-coating thereof takes place in the blowpipe in conventional manner.

Suitable hardening substance, for example ammonium chloride, ammonium phosphate, phosphoric acid, formic acid, are added in amounts customary in the art and depending on conditions in the respective installation, to harden the binding agents used during the hotpressing procedure. The addition of a special glue fortifier based on a lignin sulphonate has proved to be particularly advantageous. Furthermore extra formaldehyde scavengers, for example urea, can be added to the binding agent bath. The addition of paraffin emulsion or melted paraffin to make the fibreboards water-repellent is appropriately effected before their entry into the refiner.

The fibre material which is treated with binding agent in the manner described above is then dried in manner known per se to a maximum residual moisture content of 12 %, preferably a maximum of 10 %, before it is compressed into boards in conventional hot-presses after being formed into webs of matting.

The board compressing can be carried out in conventional stationary presses at temperatures of approximately 160°C to 190°C and pressures up to 35 bar or in Conti-presses at hot board temperatures up to 235°C and following the customary pressure programme. The pressing times in the invented process correspond to the pressing times also used to date.

A preferred embodiment of the process in accordance with the present invention, is that the aminoplastic binding agent being used is first diluted with water to a solid resin concentration of approximately 30 to 40 % by weight, and the necessary additives such as hardening substances, glue fortifiers, formaldehyde scavengers, are added in a suitable aqueous solution to the diluted binding agent via a mixing zone, immediately before said agent enters the blowpipe.

The invention will now be further explained with the help of examples, without being limited to them.

Example 1 For the manufacture of medium density fibre boards (MDF boards) fibre material was used which had been produced in manner known per se from pinewood chippings in a fibre separating machine (refiner). A mixture containing binding agent was added into the blowpipe through which the fibre material is carried from the refiner, shortly after its exit therefrom. This mixture containing binding agent is made up as follows: An aminoplastic El-glue for fibreboard production (HIACOLL MDF 15) was diluted in a mixing tank with water to a solid resin concentration of 33 % by weight.

In a second mixing tank a solution of 30 % by weight urea, 10 % by weight ammonium chloride and 2 % by weight phosphoric acid was prepared. With the help of metering pumps the two solutions were mixed together using a static mixer in such proportions that the final glue bath had a solid resin concentration of 28.3 % by weight. This glue bath was added in such an amount to the fibre material that a glueing factor of 8.2 % by weight of solid resin, referred to absolutely dry fibre, was obtained. The glue-coated fibre mixture was then dried to a residual moisture content of 10 % by weight in a known manner, was formed into a fibre mat and pressed to an MDF board at elevated pressure and temperature. The board manufactured in this way met industrial standards for MDF boards, (Standard Type MDF, formaldehyde content class A).

Example 2 A fibre material in accordance with Example 1 was gluecoated in manner known per se with the following mixture containing binding agent: 1,000 kg of HACOLL MDF 15, 100 kg urea solution (40 % by weight) and 1,000 1 water were mixed in a mixing tank, whereby the hard resin concentration in the mixture amounted to 31.4 % by weight. 100 kg of glue fortifier HIACOLL K1215 and 165 1 water were mixed in a second mixing tank. The two solutions were mixed together using metering pumps and a static mixer in such a ratio that the final concentration of solid resin in the mixture amounted to 27.0 % by weight.

This mixture was applied in such an amount to the fibre material that a glueing factor of 7.5 % by weight was obtained, referred to absolutely dry fibre material. After drying to approximately 10 % residual moisture, forming of a fibre mat and hot-pressing, an MDF board was obtained all of whose test values easily met the requirements for Standard Type MDF, formaldehyde content class A, according to the Industry Standard for MDF boards.

Example 3 Example 3 is a comparative example.

For the carried bath of 1,000 kg 150 kg 40 kg 50 kg glue-coating of the fibre material, which was out as described in Example 1 and 2, a glue the following composition was used: HIACOLL MDF-12 urea solution (40 % by weight) ammonium chloride solution (15 % by weight) water.

The composition of this glue bath corresponds to the state of the art. It has a solid resin concentration of 53.2 % by weight. In order that an MDF board produced using this bath attains the mechanical values required by the Industry Standard for MDF boards, 235 kg of glue bath has to be used for every 1,000 kg of absolutely dry fibre material, which corresponds to a glueing factor of 12.5 % by weight referred to absolutely dry fibre material.

Claims (6)

1. Process for the manufacture of wood-fibre boards in which the raw material fibres are separated, then glue-coated in an aqueous bath of binding agent in a blowpipe, and the glue-coated fibre material thus obtained is spread into webs of matting after drying to the required residual moisture content, and is pressed into boards at elevated pressure and temperature, characterized in that the glue-coating is done with a bath having a solid resin content of 20 to 35 % by weight, and that at this stage a binding agent is used which contains less than 10 % by weight solid resin based on the total absolutely dry fibre material.

2. Process according to claim 1 characterized in that the bath has a solid resin content of 25 to 35 % by weight and the amount of binding agent used is 7 to 9 % by weight solid resin based on the total absolutely dry fibre material.

3. Process according to claim 1 or 2 characterized in that the binding agent is an aminoplastic binding agent which is diluted with water to a solid resin concentration of 30 to 40 % by weight, to which additives such as hardening substances, glue fortifiers and formaldehyde scavengers are added in a suitable aqueous solution.

4. Process according to claim 3, characterized in that the additives are added to the aminoplastic binding agent immediately before its entry into the blowpipe, by way of a mixing zone.

5. A process according to claim 1 for the manufacture of woodfibre boards, substantially as hereinbefore described with particular reference to the accompanying Examples.

6. Wood-fibre boards whenever manufactured by a process claimed in a preceding claim.