NO156290B - PROCEDURE FOR THE MANUFACTURE OF HARD AND HALF-HARD FIBER PLATES. - Google Patents

Description

The invention relates to a method for producing hard and semi-hard fiber boards by impregnating lignocellulose fiber material with a urea/formaldehyde resin binder mixture, drying and pressing.

The currently common practice is for the manufacturer of this type of fiberboard to use one binder mixture obtained from a urea/formaldehyde resin which is in turn produced from a commercially available urea/formaldehyde precondensate.

Urea/formaldehyde precondensates are commercially available high concentrations of formaldehyde solutions that are stabilized with urea.

A resin is produced from the precondensate by reaction with urea (or urea and melamine) and this resin is used to produce a binder mixture with which the fibers of lignin/cellulose material are impregnated (usually soft woods are used, e.g. pine, eucalyptus, poplar etc., but other types of wood and products such as bagasse, straw etc. can be used), and the impregnated fiber is then transferred to a press, where the fiber boards are produced using heat (by hot fluids flowing through the press tables or in the case of electrical systems: elements, high frequency) and pressure. Nevertheless, this process is fraught with various disadvantages.

First, there is a high degree of instability, as the resin remains stable for a period of less than 48 hours.

Secondly, the fibers are not correctly impregnated, as the instability of the resin means that the degree of polymerization increases with time, which makes it impossible to achieve correct, stable conditions for fiber impregnation. This incorrect impregnation causes spots of binder mixture to form on the surface of the fibreboard, with the disadvantage (besides the irregular appearance of the board's surface) that problems arise when applying finishes such as paint, varnish, plastic-treated paper etc. . different degree of absorption in the areas showing spots of dry resin compared to areas without spots.

Thirdly, there are problems with the transfer of the impregnated fibres, as the incorrect fiber impregnation and the high stickiness of the resin used cause the formation of lumps of stuck together fibers (balls of stuck together fibres), which lead to serious problems in the systems ( air or mechanical) to transfer the impregnated fibers to the press.

Furthermore, it is impossible to impregnate before the fibers have dried, because as the resin is fast-hardening, only dry fibers can be impregnated (i.e. after passage through the drying apparatus) because the passage of the impregnated fibers through a drying apparatus causes pre-gelation before the material reaches the press , which gives a fiber board with poor mechanical properties.

In order to solve the above-mentioned problems, the invention provides a method of the aforementioned type which is mainly characterized by the use of a binder mixture made from a non-adhesive urea/formaldehyde resin which has a solids content of 60-70%, preferably approx. 65%, and which at a temperature of 25°C and a solids content of 65% has a viscosity of 150-300 mPa-s and a free formaldehyde content of less than 1%, preferably 0.45-0.65%, 40-55% water by weight calculated on the resin, 0.3-0.6% catalyst in the form of aluminum chloride or sulphate, and a retarding agent comprising a solution of ammonia or ammonia and urea, the resulting binder mixture having a resin solid - content of 40-45% and a viscosity at 25°C of less than 15 mPa*s, that the fiber material is ground in a measuring device at a pressure exceeding 300 kPa, that the wet fibers when they leave the measuring device are impregnated while relieving the pressure from the high pressure in the measuring device to approximately atmospheric pressure, taking advantage of the expansion of the fibers that then takes place, and that the impregnated fibers are then dried and pressed.

According to a further embodiment, 1-15% of the urea in the resin is replaced with melamine and/or dicyandiamide.

The fiberboard produced by the method according to the invention provides significant improvements compared to the previously known fiberboards, especially compared to particle boards.

The following advantages should be highlighted:

A homogeneous appearance of the surface and the inside of the plate. This homogeneity even allows the plate to be shaped.

Greater bending strength.

Greater resistance to the removal of screws from the plate.

As previously stated, the resin used to produce the binder mixture has a solids content of 60-70%, preferably 65%, and at 25°C and a solids content of 65% it has a viscosity of 150-300 mPa-s.

In some cases, it may offer an economic advantage to produce a similar resin, but with a lower solids content (e.g. when the resin is to be used in a place very close to where it is produced, the insignificant increase in transport costs due to the higher weight of the product is compensated for by the reduction of the energy required to concentrate the resin). Such a resin with a solids content of 60% has a viscosity at 25°C of 80-140 mPa-s.

Despite these lower viscosity values which would indicate (for resins with similar solids content and formaldehyde and urea content) that they are resins with a low degree of polymerization and high levels of free formaldehyde, surprisingly, free formaldehyde values are obtained of less than 1%, more precisely 0.45-0.65% (the determination carried out by the traditional "cold-sulphite" method).

When impregnating the fibers, the binder mixture behaves as if it had no stickiness, so that the formation of lumps or "balls" of stuck together fibers is avoided, which is the source of significant problems in the transfer (air or mechanical) of the glued fibers, and it is likewise avoided that the fiber board exhibits irregularities with regard to appearance and/or mechanical properties, which irregularities are caused by some of these lumps or "balls" of stuck together fibers reaching the press.

For impregnation of the wet fibers with resin, dosing devices are required to regulate the amount of fibers and resin. The weight ratio that is normally used is

weight of dry resin

> weight of dry fiber > °'<07>

As indicated above, the fibers are impregnated while wet.

In such a case, the wet fibers impregnated with a binder mixture are dried using heat, and if the binder mixture in such a case does not have the right properties, it is partially or completely gelled when passing through the drying apparatus, which reduces or eliminates its bond strength when gelled in the hot crucible press to form the plate. This problem is solved by producing a binder mixture where a catalyst, usually ammonium chloride or sulphate, has been added to the resin in an amount of 0.3-0.6%, which is greater than the amount used for dry fibers . Ammonia cup solution is also added (if the ammonia used is the usual commercially available 25 percent solution, the added amount is 0.5-1.5%) or ammonia and urea, which serve as retarders. Similarly, the amount of water that is added, greater than during the impregnation of dry fibres, as a mixture is obtained which has a resin solids content of 40-45%.

Thus, the mixture that impregnates the fibers is not exposed to embossing when passing through the dryer, and it gels correctly during the hot pressing operation in which the fiberboard is formed.

When this binder mixture is available, the impregnation can be carried out by taking advantage of the expansion that takes place when the fibers leave the grinding apparatus, which generally operates at high pressure (above 300 kPa), and is transferred to the inlet of the drying apparatus, which operates practically at atmospheric pressure. The measured amount of resin can be added here without the need for any stirring mechanism.

The impregnation that is achieved is generally of high quality. This can be observed i.a. in that there are no spots on the plate's surface which are formed by drops of poorly absorbed mixture in the fibres.

Example 1

Production of binder mixture

The binder mixture was prepared with a urea/formaldehyde resin having a solids content of 65%, a viscosity at 25°C of 225 mPa-s and a free-formaldehyde value of 0.50%, in a mixing vessel equipped with agitators and in such manner and in such conditions as set forth below.

100 parts by weight of the above resin was poured into the preparation vessel. Then 40 parts by weight were water and 4

parts by weight of a catalyst added. The catalyst was prepared as follows: To 100 parts of water were added 27 parts of 25 per cent ammonia which served as retarder and 13 parts of ammonium chloride. The stirred binder mixture of resin and catalyst had a gelation time at 100°C of 3 min and very good storage stability. The viscosity of the thus prepared mixture was 15 mPa-s. The resin solids content was 45%.

Example 2

Impregnation of wet fibers with the mixture prepared according to example 1

The fiber used was a mixture of pine and poplar chips in quantities of 90 and 10% respectively.

The fibers were pre-treated with a paraffin emulsion. The amount of paraffin added over the dry fibers was 1% paraffin solids.

The amount of binder corresponding to 11,320 g of wet fibers with a moisture content of 50% was 1,107 g, which gave 12,427 g of glued fibers.

When the bonded fibers had been dried to the proper moisture level for fiberboard production, they weighed 7,000 g. The material thus obtained contained 5,660 g dry fibers, 498 g resin solids and 840 g water.

Example 3

Production of fiberboard using the impregnated fibers according to example 2

Fibers which had been impregnated according to example 2 (fibres dried after the impregnation) were used for the production of fiberboard according to the following process: The impregnated fibers were formed in a 500 x 500 mm laboratory mould. They were pre-pressed cold in a 500 x 500 mm Siempelkamp laboratory press for 60 s at a pressure of 1.47 MPa. The edges of the block were trimmed to 450 x 450 mm before hot pressing.

The amount of impregnated fiber of 7000 g was sufficient to produce a finished 17 mm thick plate of 450 x 450 mm with a density of 720 kg/m^.

The pressing of the cold pre-pressed fiber blocks was carried out in a Siempelkamp laboratory press with both press tables electrically heated to a temperature of 180°C under a pressure of 2.26 MPa and automatic venting device.

The press time was 6 min.

After sanding the plates with sandpaper, the following was achieved

results achieved:

The above-mentioned experiments were carried out with fiber boards which

had been produced with densities adjusted to a particular value.

They can also be manufactured with other densities than those chosen, and in that case there will be a variation mainly in the value for tensile strength and, to a lesser extent, bending strength.

These values decrease at lower densities and increase at higher ones

densities.

Claims (5)

1. Process for the production of hard and semi-hard fiberboards by impregnation of lignocellulosic fiber material with a urea/formaldehyde resin binder mixture, drying and pressing, characterized in that a binder mixture is used made from a tack-free urea/formaldehyde resin which has a solids content of 60-70%, preferably approx. 65%, and which at a temperature of 25°C and a solids content of 65% has a viscosity of 150-300 mPa-s and a free formaldehyde content of less than 1%, preferably 0.45- 0.65%, 40-55% water by weight calculated on the resin, 0.3-0.6% catalyst in the form of aluminum chloride or -sulphate, and a retarder comprising a solution of ammonia or ammonia and urea, the resulting binder mixture having a resin solids content of 40-45% and a certain cosity at 25°C of less than 15l:mPa-s, that the fiber material is milled in a measuring device at a pressure exceeding 300 kPa, that the wet fibers as they leave the measuring device are impregnated while relieving the pressure from the high pressure in the measuring device to approximately atmospheric pressure, making use of the expansion of the fibers that then takes place, and that the impregnated fibers are then dried and pressed. 2. Method as stated in claim 1, characterized in that the impregnated fiber material is first pre-pressed in a cold state and then hot-pressed. 3. Method as stated in claim 1 or 2, characterized in that the material is treated before impregnation with a paraffin emulsion which provides paraffin solids in an amount of approx. 1% by weight calculated on the dry fibres. 4. Method as stated in one of the preceding claims, characterized in that the binder mixture is used in such an amount that the weight ratio between dry resin and dry fibers is between 0.07 and 0.11. 5. Method as stated in one of the preceding claims, characterized in that a binder mixture is used where 1-15% of the urea in the resin has been replaced with melamine and/or dicyandiamide.