[0001] The present invention relates to a method for the preparation of a particle board
by building up the board in layers with a middle layer of coarse material, a surface
layer of fine material on each side of the board and, optionally, one or more intermediate
layers between the middle layer and each surface layer.
[0002] In the conventional preparation of multi-layer boards according to the principle
related above there is either no fractionation of the material within any of the different
layers or a stepless fractionation of primarily the material of the surface layer.
In both cases a surface of the finished board is obtained which comprises a considerable
part of comparatively coarse material. The reason for this in the case when the material
of the surface layer is fractionated is that this material on its application is distributed
with its coarser parts nearest to the previous layer in the board and with the finest
parts of the material at the surface so that the last-mentioned parts are ground off
on grinding the board after pressing and hence will be lost. In this way also a great
part of the binder which is used for the surface layer will be lost, because the finer
the particle material the more binder is absorbed by said material on application
of the binder thereto. The presence of coarser particles at the surface of the board
gives rise to irregularities on the influence of moisture or solvents owing to the
fact that the greater particles will rise more than the finer particles as a result
of said influence. This phenomenon appears for instance in the case of thin surface
finishings such as when gluing a thin film to the board or when applying lacquer to
the board.
[0003] An object of the present invention is to provide a method of the above mentioned
type, by means of which method a surface of throughout fine material can be obtained.
[0004] Another object of the invention is to provide a method of the above mentioned type,
by means of which method the consumption of binder for the surface layer can be cut
down and the loss of binder by material ground away can be reduced.
[0005] These objects are achieved in accordance with the invention by means of a method
which is characterized in that the material for the surface layer is fractionated
and applied with the finest part of the material of the surface layer next to the
previous layer which in the board is placed further in and with the coarsest part
of the material of the surface layer at the surface of the board and in that the surface
layer on grinding after pressing is ground down to the desired fineness of the material
at the surface of the board.
[0006] The solution of the problem in accordance with the present invention thus implies
a break with the conventional thinking according to which either non-fractionated
particle material or fractionated particle.material has been applied which latter
material without exception has been applied with increasing fineness out towards the
surface of the board.
[0007] In comparison with such prior art methods which are using non-fractionated material
for the surface layer, the method according to the invention provides a board with
considerably higher volumetric weight and bending strength at the same percentage
addition of binder and a considerably lower percentage addition of binder can be used
for the attainment of the same bending strength.
[0008] The fractionation of the material for the surface layer in the method according to
the invention is preferably carried out by methods known per se in connection with
the stepless fractionation of particle material but the application of the fractionated
material is carried out in reverse order in comparison with the conventional methods
for the application of fractionated material, i.e. according to the invention the
direction of the blowing in wind sifters is changed to be the reversed respectively
the direction of rotation is changed in case of throwing rolls to be the reversed
in comparison with the conventional methods. However, also a fractionation step-by-step
of the material for the surface layer into two or more fractions can be used, if desired.
EXAMPLE
[0009] Glue-coated particles for the middle and surface layers of a three-layer board were
taken from the production in the factory and were pressed to boards having a thickness
of 15mm in a laboratory press at 200
0C with a pressing time of 7 seconds/mm. The material of the surface layer amounted
to about 35 % by weight calculated on the weight of the total particle material.
[0010] The material for the surface layer was fractionated according to the invention into
2 fractions, one being in the range of 0 - 5 mm and the other above 5 mm. The two
fractions were strewed with the finer fraction next to the material of the middle
layer and on both sides thereof and with the coarser fraction farthest.out on the
two-sides of the boards. Two different percentages of glue (11.5 and 7.8 % by weight
calculated on the weight of the surface layer) were used, three boards being pressed
at each percentage of glue.
[0011] Two further boards were pressed in the same way but with a material for the surface
layer to which there had been added 5 % by weight of non-glued wood dust calculated
on the weight of the material for the .surface layers; the content of glue was 11.5
% by weight calculated on the weight of the material for the surface layers.
[0012] Three boards with non-fractionated material for the surface layers and with a content
of 11.5 % by weight of glue calculated on the weight of the material for the surface
layers were pressed as controls.
[0013] All boards were ground after the pressing.
[0014] Five samples were taken from each board and examined with respect to the volumetric
weight and the bending strength. The results thus obtained are given in the Table
below in the form of mean values obtained for the five samples.
Method for the preparation of a particle board by building up the board in layers
with a middle layer of coarse material, a surface layer of fine material on each side
of the board and, optionally, one or more intermediate layers between the middle layer
and each surface layer, characterized in that the material for the surface layer is
fractionated and applied with the finest part of the material of the surface layer
next to the previous layer which in the board is placed further in and with the coarsest
part of the material of the surface layer at the surface of the board and in that
the surface layer on grinding after pressing is ground down to the desired fineness
of the material at the surface of the board.
