This invention relates to the manufacture of board of lignocellulose-containing material, such as fiber board.

Board of this kind is manufactured by disintegrating the material to particles containing fiber bundles and fibers. Glue is added to the disintegrated material and the material is dried and a web is formed which finally is pressed at increased temperature for curing the glue so that a board is formed. Fiber board is made of material which is disintegrated by defibering, i.e. of more or less free fibers.

The binding agent normally used is urea formaldehyde glue, which is admixed to the disintegrated material. The amount of glue can be 6% or more. The glue is relatively expensive, and it is therefore desired to minimize the glue addition. A smaller amount of glue, however, deteriorates the strength of the completed board. In view thereof, the glue addition must be optimized.

At the drying of the glued material, a great or small amount of water soluble formaldehyde is given off from the glue. After the separation of the drying gas from the fiber material, for example in a cyclone, the liquid content is condensed out of the gas. In conventional cleaning processes the water soluble formaldehyde then causes problems. The formaldehyde following along with the material to the final pressing also can cause problems when it is successively given off from the completed board. When glue is added to the material after its drying, the problem with formaldehyde separation arises at the pressing as well as from the completed board.

Fiber board manufactured according to the dry method normally is called MDF (Medium Density Fiberboard) and is a board product made of wood fiber. The yield exceeds 95%. The starting material preferably is wood chips, which are defibered with relatively low energy input, 100-500 kWh/ton fibers, for freeing the fibers. The defibered fiber material is glued, dried, formed to a web and pressed in a hot press. Alternatively, the material can be dried prior to the gluing. The material normally is preheated prior to the defibering to above 150°C whereafter the defibering is carried out under pressure at this high temperature

in order to facilitate the freeing of the fibers. At such a high temperature, however, the material can be degradated and iscoloured due to occurring hydrolysis. A short heating time reduces this shortcoming, but miscolouring yet cannot be avoided, and at the same time the energy consumption increases.

The urea formaldehyde glue normally used can be mixed with the fiber material on the conveying line (blow line) from the defibering apparatus or in a separate mixing device. The amount of admixed glue can be 6% or more.

When the fiber material subsequently is dried in a tube drier by means of gas at high temperature, a great or small amount of water soluble formaldehyde is given off from the glue. As stated above, the formaldehyde following along with the fiber material to the final pressing also can cause problems when it is given off successively from the completed board. This applies especially to gluing after the drying of the material, which can be carried out at the manufacture of fiber board.

The aforesaid problems can be reduced by the present invention, which simultaneously offers further advantages. According to the invention, the fiber material is treated with an impregnation liquid containing fiber surface activating chemicals and preheated before it is finally disintegrated.

Examples of such chemicals are such which affect the lignin, such as Na ₂ S0 ₃ and NaHS0 ₃ . These chemicals can be added in an amount of 1-30 kg/ton material expressed as Na ₂ S0 ₃ . According to the invention, it is possible to increase the ISO-brightness of the completed product, to reduce the energy consumption at the defibering, to reduce the glue amount at maintained strength or to increase the strength at maintained glue amount. Furthermore, also the formaldehyde separation can be reduced substantially.

The invention is described in greater detail in the following with reference to some embodiments thereof.

Fiberboard is manufactured by impregnating the raw material in the form of chips of, for example, softwood with lignin affecting chemicals, such as Na ₂ S0 ₃ and NaHS0 ₃ , in an amount of 1-30, preferably 10-20 kg/ton fiber material expressed as Na ₂ S0 ₃ .

The impregnation, which has to be carried out rapidly, can last some minute. Thereafter the temperature is increased to 150-200°C, preferably 160-180°C, for 2-3 minutes. However, the preheating can of course be performed prior to or at the same time as the impregnation. The pH value is kept within 2-12. This implies, that an impregnation solution with Na ₂ S0 ₃ and NaHS0 ₃ will contain sulphite in the form of HS0 ₃ ^" and/or S0 ₃ ^2" . The pH value suitably can be controlled by NaOH. Due to this chemical treatment (sulphonation) , the subsequent defibering can be carried out to the desired defibering degree with an energy input lower than normal. The energy consumption is in the interval 50- 400 kWh/ton fiber material. The energy saving can amount to about 10% compared with conventional defibering.

Owing to the impregnation, also the brightness of the completed fiber board increases.

As regards the fiber disintegration in the defibered material, it was found that both the fine material content and the shives content were reduced, i.e. the proportion of pri a fiber has increased.

It was also found at the impregnation according to the invention that the strength properties of the completed board have improved substantially. The tensile strength as well as the elasticity modules and the fiber bond have increased by at least 10%. Alternatively, this effect can be utilized for reducing the amount of glue addition at maintained strength properties. A 10% increase in the fiber bond corresponds to a 1% decrease in glue addition. The sulphonation according to the invention also implies, that the separation of formaldehyde can be reduced, due to the fact, that the sulphite ions in the impregnation solution react with the formaldehyde so that it is bound in the fiber material instead of being given off from the completed board.

EXAMPLE

Raw material in the form of chips was exposed to different impregnation treatment. One batch was treated with 10 kg Na ₂ S0 ₃ per ton chips, and one batch with 10 kg NaHS0 ₃ per ton chips. A reference batch was not impregnated.

The different batches were preheated to about 170°C and thereafter defibered. In the blow line after the defibering, urea formaldehyde glue of the type E2 was added in an amount of 10% dry glue calculated on dry fiber. The material was thereafter dried to a dry matter content of about 90%. The fibers were dry formed to a mat, which was prepressed with a pressure of about 1.5 MPa and hot pressed for 5-6 minutes at 170°C. The board thus manufactured was tested on strength, ISO-brightness and formaldehyde content (perforator value) . The relative energy consumption for the different batches was also measured. The results were as follows:

Property Reference Na ₂ S0 ₃ NaHS0 ₃ batch batch batch

Bending strength

MPa 38 . 4 43.8 40.0

Internal bond strength

MPa 1 . 8 2.1 2.0

ISO-brightness

% 24 26 26

Formaldehyde content mg CH ₂ O/ 100 g 27 20 18

Relative energy consumption 1.0 0.95 0.95

The results confirm the technical effects stated above.

The invention, of course, is not restricted to the embodiments described above, but can be varied within the scope of the invention idea.