Note: Descriptions are shown in the official language in which they were submitted.
21.~~93f
The present invention relates to bonding agents that can be
hardened when heated, that are compatible with wood and
other products that contain cellulose, and which axe
suitable, in particular, for manufacturing wood materials,
s such as, for example, chipboard.
In the course of the search for natural and, in particular,
for renewable resources for manufacturing bonding agents for
wood materials, it has also been found that tannins can be
io used (J. Macromol. SCI.-Chem. A 16 (7), 1243-1250 (1981).
Despite good availability, they have not been widely used
because the tensile strength of wood materials that are
bonded with tannin is not satisfactory, particularly after
i5 such products have been stored in water.
For this reason, it is an object of the present invention to
describe a bonding agent that is based on tannin, with which
wood materials having significantly improved mechanical
2o properties can be manufactured.
It has been found that tannins that contain units of the
general structure
R
- O
O
O-
O-
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21.29936
wherein R represents hydrogen or a hydroxy group and the
free bonds at the oxygen radicals are either saturated with
hydrogen or are bonds to other molecular groups such as
sugar radicals or radicals of other or identical polyvalent
compounds with phenolic hydroxyl groups, display a bonding
capability that is superior in comparison to other tannins
if they are used together with a material that liberates
formaldehyde when heated and are used as a bonding agent for
products that contain cellulose. Tensile strength increases
1o and achieves values as are found, for example, in the case
of corresponding materials that are bonded with phenol
resin. The reduction of transverse tensile strength aftex
storage in water is clearly reduced, and swelling values in
boiling water are similarly reduced.
In this regard, it is an added advantage that these tannins
can be used without prior breakdown treatment as is known in
the case of other tannins.
2o Suitable tannins are those from the group of procyanidines
and prodelphinidines, in particular tannin from pine bark or
from the pecan nut. Although mixtures of these tannins
with, for example, minosa or quebracho tannin in the ratio
of 2:5 to 5:2 have a longer hardening time, they cannot,
2s however, be used. These tannins are commercially available
products and up to now have been used mainly as tanning
agents. Commercial quality is adequate for use as foreseen
in the present invention.
- 2 -
Substances that liberate formaldehyde when heated are, for
example, paraformaldehyde, trioxane and, in particular,
hexamethylenetetramine (hexa).
In the simplest case, the thermosetting bonding agents
according to the present invention are mixtures of tannin
that contains structural units of the formula
O-
- O
O v
O -
io and a substance that releases formaldehyde when heated, the
proportion of material that releases formaldehyde, in
particular hexamethylenetetramine, amounting to 3.0 to 9%-
wt, relative to the tannin.
i5 However, it is also within the scope of the present
invention if, in addition to the special types of tannin and
a substance that releases formaldehyde when heated, the
bonding agent also contains up to 50%-wt of a novolak or up
to 20%-wt of an isocyanate compound that is at least
2o difunctional, such as, for example, diisocyanatonaphthaline,
4,4,°,4 " -triisocyanato-triphenylmethane, isophoron-,
toluol- or hexamethylene- diisocyanate or diisocyanato-
diphenylmethane (MDI), when, in practical use, the di- or
polyisocyanates are applied to the products that contain
2s cellulose in parallel with the dry mixture of the tannin
- 3
;.
;:
that is used according to the present invention and the
substance that releases formaldehyde when heated, before
these are pressed to form the appropriate materials.
Suitable products that contain cellulose are, for example,
wood chips, fibres based on cellulose, such as, for example,
cotton or flax, or straw, from which chipboards, textile
fleeces, or accoustic and thermal insulating panels are
produced.
io
The materials are manufactured in such a way that the
bonding agents according to the present invention are mixed
with products that contain cellulose, the mixture is placed
in a mould, and then hardened under pressure at a
is temperature that is above the decomposition temperature of
the substance that releases the formaldehyde and below the
decomposition temperature of the tannin, preferably at 150
to 210°C. Depending on the working material that is used
and the thickness that is desixed, the pressure will be in
2o the range from 0.1 to 4 MPa/mm2. Because it is possible,
for example, to manufacture three layered chipboards, with a
pressure of 2 to 3.5 MPa/mm2.
Depending on the desired material and the desired strength,
25 the quantity of bonding agent will in the range of 4 to 20%-
wt, relative to the product that contains the cellulose.
The bonding agent can be in the form of a solid mixture or
a~ a solution of the components in water, alcohol, or water-
alcohol mixtures.
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'",:>:' . ,-; ,.'
2~.~9~36
Suitable solutions can be stored at room temperature for
several weeks without any major increase in viscosity. In
the case of aqueous solutions, it is recommended that a
fungicide be added in order to prevent the formation of
s fungus.
The following examples illustrate the binding agent
according to the present invention, as used to manufacture
chipboards.
io
Example
Woodchips are sprayed with a water-alcohol solution that
contains bonding agent at the rate of 11%-wt of the wood,
is and these are then dried. Subsequently, they are formed in
the usual manner (2.5 N/mm2; 195°C) to form panels measuring
400 x 350 x 12 mm, pressed, and hardened.
Density, transverse tensile strength of the panels, a. dry
2o and b. after storage in water (2 hours in boiling water,
subsequently dried for 16 hours at 105°C) as well as the
swell thickness are determined after 2 hour storage in
boiling water, with various mixture proportions of the
bonding agent components, the quantity of bonding agent, and
2s the press time.
2129936
proportion of pecan nut tannin varies accordingly, from 92.5
to 94.5%.
In Example 2, the proportion of hexamethylenetetramine
relative to the tannin, amounts to 6.4%, i.e., the
proportion of pecan nut tannin is 93.6%. The press time is
varied.
Example 3 shows values for a three-layer chipboard at 6.4%
io hexamethylenetetramine in the bonding agent, and with a
bonding agent content of 10% in the inner layers, and 12% in
the outer layers.
Examples 4 and 5 show values that axe analogous to Examples
is 1 and 2 when mimosa (wattle) tannin is used..
The following tables show the values that were obtained:
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t:v
~1~~~~~
TABLES
Example 1 Example 4
s Hexamethylenetetramine (comparison)
7.50 6.40 5.5% 7.50 6.50
Press time
io [sec/mm] 17.5 17.5 17.5 17.5 17.5
Density
[kg/m3] 729 730 736 731 711
15 Transverse
tensile strength,
dry
[MPa] 0.82 0.81 0.79 0.45 0.43
2o Transverse
tensile strength
after storage
in water
[MPa] 0.25 0.27 0.26 0.09 0.07
25
Swelling
[%] 25.0 23.9 22.1 32.1 31.5
HCHO emission
30 [mg/100 g; WKI] 1.71 1.45 0.84 2.01 1.87
23.2~~~6
Example 2 Example 5
(6.4o (comparison)
hexa)
(6.5% hexa)
Press time
[sec/mm] 17.5 12.5 8.5 30.0 17.5
12.5
Density
io[kg/m3] 730 738 733 714 731
722
Transverse
tensile strength,
i dry
s
[MPa] 0.81 0.75 0.67 0.5 0.45
0.41
Transverse
2otensile strength
after storage
in water
[MPa] 0.27 0.19 0.17 0.13 0.09
0.03
25
Swelling
[%] 23.9 22.4 20.9 29.7 32.1
36.7
3oHCHQ emission
[mg/100 g; WKI] 1.45 1.32 1.1 1.64 2.01
1.28
_ g _
y
21 :;336
Example 3
Press time
[sec/mm] 12 . 5
Bonding agent
[o]
middle-/cover
layers 10/I2
ioMoisture - chips
after coating
[%]
middle-/cover
layer 28/26
i5Density
[kg/m3] 723
Transverse
tensile strength,
2odry
[MPa] 0 . 03
Transverse
tensile strength
25after storage
in water
[MPa] 0 . 2 7
Swelling
30[%] 22.1
HCHO emission
[mg/100 g; WKI] 1.4
- 21.~9~36
Examx>le 6
Single layer chipboards were produced and tested according
to the general description for the examples (press time 15
s/mm at 190°C, 2.5 N/mm2) .
Bonding agent:
Tannin from pine bark
io (37% aqueous solution) 337 parts by weight .
Hexamethylenetetramine
(40% aqueous solution) 19.3 parts by weight
Zinc acetate
(25% aqueous solution) 19.3 parts by weight
1s Water 10 parts by weight
Results:
Density (kg/m3) : 708
2o Transverse tensile strength, dry (MPa): 0.23
Transverse tensile strength after storage
in water (MPa) : 0.1
HCHO emission (mg/100 g): 0.1
25 Examt~le 7
Three-layer chipboards were produced and tested according to
the general description of the examples (press time 15 s/mm
at 190°C; 2.5 N/mm2).
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,. : . : . . , , ; ~ .. , .. : :; ;.
-. v , ; . . ,
. ::
, ~ '
~,
~
~ .
. .,', v , !'; . ,,. ~
- ' . ; ; . '
'. i,~:
, . ~,,,.:.,~ . f
~~..i~~'~~~
Bonding agent: Core layers Outer layers
Tannin from pine bark
s (37% aqueous solution) 327 327 parts by weight
Hexamethylenetetramine
(40% aqueous solution) 19.5 19.5 parts by weight
Zinc acetate
(25% aqueous solution) 24.0 19.5 parts by weight
ao Water 12 -- parts by weight
Results:
Density (kg/m3) : 723
15 Transverse tensile strength, dry (MPa): 0.80
Transverse tensile strength after storage
in water (MPa): 0.29
HCHO emission (mg/100 g): 0.16
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