PROCESS FOR MAKING BOARD

Abstract: In a process for making board according to the wet method, a stock is prepared from fibres produced from a fibrous raw material, and a binder is added prior to dewatering and pressing. The process is distinguished by adding the binder to the fibres before the preparation of the stock. The binder may be a water-soluble binder which is precipitated and fixed to the fibres by acidification. Acidification may take place in connection with the preparation of the stock or prior thereto as well as prior to the addition of the binder. The binder can be added to the fibres in the blow line from the defibrator for producing the fibres from the fibrous raw material, or it may be added to the fibres before the defibrator.

Description: PROCESS FOR MAKING BOARD The present invention relates to a process for making board according to what is commonly referred to as the wet method. More specifically, the invention concerns an improved process for making board, by adding the binder at an early stage of the process. Insulation board, building board and hardboard, for instance, can be produced by this process. When making board according to the wet method, a stock is prepared from fibres produced from a fibrous raw material, such as chips and sawdust, by mixing the fibres with water in several steps. A binder is added to bind the fibres in the board, whereupon the stock is dewatered and the fibre pulp is pressed. The binder is added to the stock and, if water-soluble, is precipitated and fixed to the fibres by acidification of the stock, e.g. by alum or acid. The binder may be added at a site where the stock concentration is about 5-15%, e.g. in the level box, whereupon precipitation and fixation by acidification take place where the stock concentration is about 2-3%, e.g. in the machine chest. Alternatively, both the addition of the binder and the precipitation thereof on to the fibres may take place in the machine chest. This process has the disadvantage that the binder, when precipitated, deposits not only on the fibres, but also on the remaining material in the stock. Apart from useful fibres, the stock contains fibre fragments, i.e. what is commonly referred to as fines, as well as dissolved carbohydrate material and, to a certain extent, ligneous substance in dissolved and colloidal form. Since the water in the process is at least partly recycled by using, in the preparation of the stock, the white water from the dewater¬ ing step, the contents of these by-products will be com¬ paratively high. As a portion of the binder deposits on these components, this portion will be of no avail to the fibre matrix. It has thus become necessary to increase the amount of binder added, which nevertheless has not given the desired result in certain cases. The dosage of resin is also restricted by the fact that dewatering of the stock is rendered more difficult if the amount of resin exceeds a certain level (about 6-8%). In addition, many board manu¬ facturers in Europe have made their processes more or less closed for environmental reasons (reduced discharge of COD-consuming materials to recipients) . As a result, the contents of fines and dissolved substances in the white water have increased even further. However, the present invention largely obviates these disadvantages. One object of the invention is, therefore, to utilize the adhesive on the fibre matrix more effectively in the making of board. Another object of the invention is to enable dosage of the desired amount of resin on the fibres without affecting the subsequent dewatering. As appears from the appended claims, the objects of the invention are achieved by a process for making board according to the wet method, in which a stock is prepared from fibres produced from a fibr.ous raw material, such as chips and sawdust, and a binder is added prior to dewater- ing and pressing, the binder being added to the fibres before the preparation of the stock. It has surprisingly been found that the binder can be utilized far more effec¬ tively if added to the fibres early in the process, before the supply of water for preparing the stock, resulting in improved properties of the board. Also, it has become pos¬ sible to add very large amounts of binder without adversely affecting the subsequent process. In the process according to the invention, conven¬ tional equipment is used for making board according to the wet method. Thus, the wood raw material is conducted, by preheating with steam, to a defibrator for disintegration. The resulting fibre pulp is conveyed through a blow line to chests where the stock is prepared. Fresh water and/or white water is added to the chests, resulting in a stock having a content of about 5-15%. This stock is conducted to a refiner for final adjustment of the beating degree. From the refiner, the stock is conveyed to a machine chest, where the final dilution with water/white water is per- formed. At this stage, the stock has a content of about 2- 3%. This stock is conducted to a wet machine to be dewater- ed, pressed and dried/press-dried (hot press). In the process according to the invention, the binder may be added to the fibres at . any time before the site where a substantial amount of water is added for preparing the stock. The binder is suitably added in the blow line from the defibrator. Here, the fibres have a dry solids content of about 40-60% by weight. This constitutes a preferred embodiment of the invention. The binder may also be added to the fibres before the defibrator. In the inventive process, use is made of such binders as are predominant in the art. These are curing organic or inorganic binders, such as phenolic resins, amino resins, silicates or mixtures thereof. Water-soluble binders are generally used. When water-soluble resins are employed, they have to be precipitated and fixed to the fibres, which is brought about by acidification, e.g. by means of alum or acid. Preferably, acidification is performed in connection with the preparation of the stock or prior thereto, and in that case also prior to the addition of the binder. When added in connection with the preparation of the stock, the precipitating reagent may be added to the first dilution chest, where the stock concentration is about 1-15% by weight, usually 5-15% by weight. If acidi¬ fication is allowed to take place at a late stage of the process, e.g. in the machine chest, there is a risk that some of the water-soluble binder applied to the fibre matrix is dissolved and instead precipitated on the other material in the stock, resulting in impaired performance. With the present invention, it is also possible to "control" the penetration of resin into the fibre. As a result, the properties of the board can be influenced in different directions, e.g. internal bond strength and modulus of rupture versus swelling and water absorption. If a sparingly soluble or concentrated resin is used, pene¬ tration is reduced and the resin is concentrated on the fibre surfaces. This enhances the possibility of obtaining active glue joints between the fibres, resulting in higher strength. In the opposite case, i.e. when diluted and low- viscosity resins are used, penetration increases, resulting in lower swelling and, above all, lower water absorption. Penetration may also be reduced by adding alum/acid before the defibrator or before the site where the binder is added. Thus, the binder will be precipitated directly on the surface of the acid fibre, and penetration is reduced to a minimum. Consequently, the present invention enables several ways of controlling in a given direction the properties of the end product. Also, the process according to the invention enables the addition of very large amounts of binder without ren¬ dering dewatering on the wet machine more difficult. The prior-art process restricts the dosage of the binder to about 6-8%, requiring at that the aid of certain floccu¬ lating chemicals. If the dosage exceeds this level, dewa¬ tering from the wire is impeded to such an extent that ' there is a risk of crushing the sheet in the press section. In the process according to the invention, there are no dosing problems with contents of from about 0.5% by weight to about 40% by weight (based on dry resin on a dry fibre matrix), suitably up to about 15-20% by weight. Generally, the binder is used in an amount of about 0.5-10% by weight, preferably 1-5% by weight. Making board according to the wet method by the pro¬ cess according to the invention does not influence the other process conditions. The binder can be added through a simple connecting tube on the blow line after the defi- brator. The amount of binder is adjusted by a dosing pump. The binder can be precipitated in e.g. a first dilution chest at a stock concentration of about 10% by means of alum. Then, the sized stock is allowed to continue in the process via a refiner, a dilution chest, a machine chest and a wet machine in conventional manner. Thus, the process according to the invention is fairly easy to incorporate in existing processes. The invention will now be described in more detail with the aid of the following Examples, in which the part and percentage figures concern parts by weight and per cent by weight, unless otherwise stated. Example 1 The making of insulation board. Phenolic resin having a concentration of 40% by weight was applied by so-called blow-line dosage immediately after the defibrator through an insert pipe centered in the blow line. Thus, the phenolic resin was applied to concentrated hot fibre having a temperature of about 150°C (a dry solids content of about 50% by weight). The resin was dosed by a suitable dosing pump, and was precipitated and fixed to the fibre at a high stock concentration (8-10%) immediately after the cyclone in the first dilution chest. Precipitation was brought about by the addition of an alum solution to the diluting water. The sized fibre (the stock) was then conducted further in the process via refiners, machine chests and a wet machine in conventional manner. The board thickness was 13 mm. Dosing Phenolic resin MOR IB Swelling Water abs process dosage % MPa MPa 2 h % 2 h % According to the invention 6.5 0.26 1.9 12.4 Added in ma¬ chine chest typical va¬ 3.0- 0.08- 2.3- 18-30 lues* 3.5 0.12 3.5 Standard board 2.7 0.11 7.0 173 *) An addition of wax forms part of this grade. Example 2 Insulation board 16 mm. The process according to Example 1 was repeated, the only difference being that use was here made of concentrated phenolic resin of 43% by weight, i.e. having higher vis¬ cosity than the resin used in Example 1. Dosing Phenolic resin MOR IB Swelling Water abs. process dosage % MPa MPa 2 h % 2 h % According to the invention 5 4.0 0.16 3.2 15 Added in ma¬ chine chest 2 2.2 0.07 3.5 22 *) An addition of wax (2%) forms part of this grade. It is apparent how the properties can be controlled towards increased strength by reducing resin penetration, ' although with lesser effect on swelling and water absorption.

Claims: CLAIMS 1. A process for making board according to the wet method, in which a stock is prepared. from fibres produced from a fibrous raw material, and a binder is added prior to dewatering and pressing, c h a r a c t e r i s e d by adding the binder to the fibres before the preparation of the stock. 2. A process as claimed in claim 1, c h a r a c ¬ t e r i s e d in that use is made of a water-soluble binder which is precipitated and fixed to the fibres by acidification. 3. A process as claimed in claim 2, c h a r a c ¬ t e r i s e d in that acidification takes place in con¬ nection with the preparation of the stock. 4. A process as claimed in claim 2, c h a r a c ¬ t e r i s e d in that acidification takes place before the preparation of the stock. 5. A process as claimed in claim 2, c h a r a c ¬ t e r i s e d in that acidification takes place before the addition of the binder. 6. A process as claimed in any one of claims 2-5, c h a r a c t e r i s e d in that acidification takes place with alum or acid. 7. A process as claimed in claim 1, c h a r a c - t e r i s e d in that the binder is added to the fibres in the blow line from the defibrator for producing the fibres from the fibrous raw material. 8. A process as claimed in claim 1, c h a r a c ¬ t e r i s e d in that the binder is added to the fibres before the defibrator for producing the fibres from the fibrous raw material. 9. A process as claimed in claim 1, c h a r a c ¬ t e r i s e d in that the binder is a sparingly soluble or concentrated resin. 10. A process as claimed in claim 1, c h a r a c ¬ t e r i s e d in that the binder is a diluted, low-vis¬ cosity resin. 11. A process as claimed in claim 1, c h a r a c - t e r i s e d in that the binder is added in an amount of from about 0.5% by weight to about 40% by weight (based on dry resin on a dry fibre matrix).

Inventors: FLODMAN LEIF, NORDQVIST JAN-ERIK, FLODMAN, LEIF, NORDQVIST, JAN-ERIK
Applicants: CASCO NOBEL AB
Application year: 1993
Main classification: D21H23
IPC codes: D21B1, D21H17, D21H23, D21J1