Note: Descriptions are shown in the official language in which they were submitted.
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A method of manufacturing bonded products
of cellulose or cellulose derivatives
It is well known that so-called ionising radiation, both
mechanically produced radiation and that obtained from na-
tural sources, can cause chemical and physical changes in poly-
meric material. Thus, some polymeric materials obtain an
increase in their molecular weight and in their melting
point subsequent to being exposed to such radiation, owing
to the fact that new chemical bonds are formed in the material.
Other polymers are influenced to a lesser extent or are de- ~;
10 composed into products of lower molecular weight, the me-
chanical properties of these polymers often being impaired
at the same time.
The effect caused by the radiation can be influenced in some
way, by exposing the polymer to said radiation in the presence
15 of various additives, for example in a manner such that a
material which has been decomposed can also be re-linked and
the properties thereof improved thereby. Further, it has been
found that the presence of air when irradiating the material
also has a certain significance, since in many cases the oxygen
20 accelerates the decomposition of the material.
It is also known that cellulose, cellulose derivatives and
various natural cellulose composites, such as wood and
vegetable fibres, belong to those polymeric materials most
25 sensitive to radiation. If one excepts an insignificant
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positive effect at very small doses, i.e. ~ 10 krad, the
mechanical properties of these materials are greatly
impaired with increasing doses of radiation. It is also
known that the properties of a plurality of condensation
polymers between formaldehyde and urea, melamine or
different phenol types, are impaired by radiation.
The present invention is based on the surprising
discovery that chipboard or particle board bonded with
formaldehyde-urea-resin exhibits highly improved
mechanical properties subsequent to exposing the board to
radiation.
In accordance with one aspect of the invention there ~ - -
is provided a method of manufacturing bonded products
comprising cellulose or a cellulose derivative, said
method comprising: bonding the cellulose or cellulose
derivative with an adhesive comprising a condensation
resin between an aldehyde and a reagent selected from the
group consisting of urea, melamine and a phenol type; and
irradiating the bonded product to at most 5 Mrad with
ionizing radiation so that the material undergoes
cross-linking, residual quantities of uncured adhesive
substances being reduced simultaneously therewith.
In accordance with another aspect of the invention
there is provided a method of improving the mechanical
properties of manufactured bonded products comprising
cellulose or a cellulose derivative, bonded with an
adhesive comprising a condensation resin between and a
reagent selected from the group consisting of urea,
melamine and a phenol type; said method comprising
irradiating the bonded product to at most 5 Mrad with
ionizing radiation so that the material undergoes
cross-linking, residual quantities of uncured adhesive
substances being reduced simultaneously therewith.
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One Mrad corresponds to an amount of energy taken up
in the material of 10 Joule/g or 10 Ws/g.
The cellulose or cellulose derivative may be, for
example, saw dust, wood chips, paper or vegetable fibers.
s In order to improve the properties of the afore-
mentioned products, it is also desirable to be able to
increase the content of aldehyde in the adhesive. This
cannot be done, however, without the surplus being removed
in some way. The present invention constitutes a
significant step forward in this respect, owing to the
fact that, in addition to improving the mechanical
properties of the products, the radiation also contributes
to reducing the residual content of aldehyde in the
material.
The invention can also be applied together with
various additives containing those functional groups which
experience has shown will improve the extent to which -
cross-linking takes place when the material is subjected
to radiation. An example
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of such an additive is hydrocarbon with olefinic, acrylic
or other comparable groups. It also lies within the scope
of the invention to modify the adhesive substance itself,
in a manner such that these groups are chemically bound to
one or more of the components of the resin.
Example 1
Five groups of four test plates of chipboard or particle
board bonded with urea- formaldehyde-resin were irradiated
with 60Co ar- radiation in different dosages, and the sur-
10 face hardness of the plates was determined at differentlocations by forcing a ball into the surface of the plates.
Dose (Mrad) Surface hardness (working units)
Blind-test 2.17 + 0.13
0.10 2.41 i 0.03
0.33 2.35 + 0.06
1.0 2.31 + 0.09
4.0 2.28 + 0.06
2.12 + 0.12
Example 2
20 Four groups of four test plates of the type described above
were irradiated with 60Co ~r-radiation in different dosages,
whereafter test rods measuring 60 x 13.9 x 16 mm were manu-
factured from the plates and impact tested at a distance o.f
46.8 mm between the points of impact.
Dose (Mrad) Energy absorbed (kpcm)
Blind-test 10.4 + 0.1
0.10 12.1 + 0.6
0.33 10.6 + 0.5
1.0 11.0 + 0.3
9.7 + 0.5
The plates were irradiated at room temperature and exposed
to the effect of oxygen in the air. The dosing rate was
approximately 0.04 krad/s.
The material may also be irradiated at elevated temperatures
and at other dosing rates than those mentioned here. Slightly
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improved effects can be expected, in actual fact, when the
dosing rate is increased,owing to the fact that the extent to
which linking takes place is then more pronounced.
The above examples show that the radiation gives similar
changes in the properties of the selected materials. The most
noticeable improvement was already obtained at 0.10 Mrad. It
will also be understood that the material can be irradiated
to dosages of several Mrad, without lowering the mechanical
strength of the material, compared with non-irradiated samples.
10 This is of great significance when the irradiating step is
primarily intended to eliminate residual quantities of formal-
dehyde.
The material may be irradiated during the manufacture of
the bonded product or in immediate conjunction therewith. Alter-
15 natively the material may be irradiated at a later stage, forexample when the product is substantially finished and ready
for use, or even shortly after the material has been put into
use. When the said product has the form of a semi-manufacture,
the irradiating operation - if it has not previously been
20 carried out - may be undertaken when the semi-manufacture is
converted to the final product.
The irradiating operation can be carried out either with
the intention of influencing the surface layer of the material
or in a manner such as to cause the radiation to pass comple-
25 tely through the material. In the former case, it is an ad-
vantage to use a less expensive and simpler type of accelera-
tor.
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