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Prevent particle board from swelling when encountering water


Particle board is compressed from wood chips, so the thickness expansion rate after water absorption is much greater than that of wood itself. The German once made an experiment: put particle board in the air with 30% relative humidity for balance, and then put it in the air with 80% relative humidity for balance. The moisture content of particle board changed from 6% to 18%, and the thickness expansion rate was 4-9%, while the thickness expansion rate of wood itself in the corresponding experiment was only 2%. Among the ways to reduce the water absorption thickness expansion of particle board, there are several clear methods as follows:

1. Add waterproof agent (usually paraffin lotion)

For the paraffin lotion added to urea formaldehyde particle board, it is generally about 0.5% in foreign countries according to the calculation of solid paraffin to ultradry particle board. This dosage was accepted by everyone after research and extensive practice. Adding 0.25% paraffin can significantly reduce the thickness expansion value; When the dosage increased to 0.5% and 0.75%, the thickness expansion value did not show a significant decrease again. However, in special circumstances, if a certain tree species is not treated properly or the process is not appropriate, adding 0.5% paraffin cannot meet the requirements specified in the "standard". How can paraffin lotion give full play to its role? Understanding the wax application mechanism is of great help in analyzing the problems that arise in production. For example, when urea formaldehyde adhesive is used as adhesive, only a small amount of paraffin lotion (generally 0.5%) can be added. If such a small amount of paraffin lotion is not sprayed properly, it will not play a waterproof role. Therefore, paraffin lotion is usually applied together with glue to expand the spraying area of paraffin lotion. However, the specific gravity of paraffin lotion (0.86~0.92) is quite different from that of glue solution (1.20~1.30), so it is difficult to mix evenly. If the ratio of the size of glue drops sprayed to the size of paraffin lotion drops is not appropriate, it is difficult for glue drops to be evenly distributed on shavings with paraffin lotion drops. Generally, the diameter of glue drops sprayed is 0.1mm, sometimes to 0.2mm, and the paraffin lotion particles are about 10 microns, so each glue drop contains about 150 paraffin emulsion drops on average. If the diameter of the glue droplet is less than 0.06mm, it is difficult to fully encapsulate the paraffin emulsion droplets.

During the hot pressing process, the glue droplets are squeezed, and the paraffin droplets move towards the edge of the glue droplets. A portion of the paraffin droplets are close to each other, and as the pressure increases, the glue droplets are squeezed, and the paraffin droplets inside the glue droplets are also squeezed. At this point, the surrounding temperature precisely causes the paraffin to melt, and due to the different surface tension between the melted paraffin and the water-soluble adhesive, it is extruded by the adhesive to the surface of the particle to form a very thin wax film, which plays a waterproof role. Therefore, the reasonable wax application conditions should be: the concentration of paraffin lotion is 33~60%, and the diameter of paraffin droplets is 4~14 microns; The diameter of the paraffin droplet that ruptures after being squeezed should be 1/8-1/10 of the diameter at the time of spraying; The wax lotion drops are crushed just before the wax melting temperature and the glue curing. If the adhesive particles solidify before being crushed due to improper pressure and temperature coordination during hot pressing, the paraffin will be wrapped in the adhesive droplets, sometimes losing 20-30% of the paraffin. This also suggests why lightweight particle board with low compression ratio or particle board made of thick particleboard always have high thickness water absorption expansion rate. The glue is not squeezed to a certain extent, and it often fails to get the due waterproof performance, which is also the reason why MDF uses solid paraffin instead of paraffin lotion.

2. Different wood materials have significant differences in water absorption, thickness, and expansion rate

When particle board plants use certain materials to make boards, special attention should be paid to some specific links. For example, poplar, although the normal amount of paraffin lotion is also added to the board, the thickness expansion rate of water absorption is often high. This is due to the large gap between the tube hole and the catheter, and the tendency to fuzz, cut, and curl during the planing process, which increases the gap and capillary volume of the board. The difference in material quality between early and late poplar wood results in a significant deviation in the thickness of the wood particles during slicing, leading to an increase in the water absorption thickness expansion rate of the board. If processing such raw materials, it is necessary to pay attention to the operation of the material preparation section, such as sharp blades and appropriate moisture content of the raw materials. If it is still difficult to achieve the water absorption thickness expansion rate specified in the "standard", it is necessary to correspondingly increase the amount of adhesive used.

3. Increase the amount of glue used

As mentioned above, increasing the amount of adhesive used will have a greater inhibitory effect on the rebound of wood particles, and the water absorption thickness expansion value will inevitably decrease accordingly. This has a significant effect on both pine and poplar trees. In addition, some studies have shown that high temperature heat treatment (1550C-1800C) or unnecessary extension of hot pressing time for particle board, too long placing time of particleboard after mixing glue, too low moisture content, too slow rising speed of the press, too high hot pressing temperature, etc. will increase the water absorption thickness expansion rate of particle board. Therefore, these are all things that need to be noted in production. After the hot pressing of the board, hot stacking (800C) is not only beneficial for the release of internal stress, but also for the improvement of water absorption thickness expansion properties (plane tensile strength may decrease to some extent). Therefore, proper hot stacking after hot pressing is also one of the feasible methods to improve the water absorption thickness expansion rate of particle board.

The above content is all from the internet!