Note: Descriptions are shown in the official language in which they were submitted.
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Wood/plastic Composites, their Production and Use
FIELD OF THE INVENTION
This invention relates to formaldehyde-free plastic/wood composites
having improved resistance to water, to their production and to their use.
BACKGROUND OF THE INVENTION
According to Ullmann, Enzyklopadie der technischen Chemie, 4th
Edition, Vol. 12, pages 709 et seq, wood-based materials may be divided
into the following classes of semi-finished products:
Wood chip boards or particle boards are generally understood to be
boards of mechanically produced chips of wood or wood-containing parts
which are made by gluing under pressure with a binder. The synthetic resins
or binders used are selected from urea resins or aminoplastics, phenolic
resins or mixed resins of urea, melamine, phenol and formaldehyde.
Isocyanates, particularly those based on diphenylmethane diisocyanate, and
crosslinkable polymers are also used. The properties of chipboards can be
varied through the size, shape and arrangement of the chips and the amount
of synthetic resin or binder used (ca. 5 - 10%). High-quality boards comprise
several layers with a surface layer of particularly fine particles. For use in
furniture making, chipboards can be coated with decorative films, priming
films and veneers. Here, a density-based distinction is drawn between flat-
pressed boards with a medium density of 500 to 800 kg/m3 and light flat-
pressed boards with a density of about 300 kg/m3.
Japanese Publication No. 58-185670, published October 29, 1983,
describes binders for chipboards based on a 4,4'-diphenylmethane
diisocyanate fraction. According to this document, the chipboards are
moistened with water so that the diisocyanate mentioned can be reacted
during hot pressing at 150 C/25 kp/cm2. The polyurethane-containing
chipboards obtained have improved flexural strength.
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Wood-fiber chipboards are made from wood fibers or lignocellulose-
containing material. Under the effect of heat, moisture and mechanical
pressure in fiberizing machines, the lignin-, cellulose and hemicellulose-
containing raw material is broken down into its fiber-like, anatomical basic
elements in the form of individual fibers and fiber bundles. In the course of
the manufacturing process, the fibrous material is shaped, compacted and
pressed. The matting of the fibers and the natural binding forces are
primarily
used for this purpose. The binding forces can be increased by adding binding
and hydrophobicizing agents and by thermal and other aftertreatments. The
physical and strength properties can thus be adapted to the intended
application.
According to German Institute for Standardization Document No. DIN
68 753 published January, 1976, wood-fiber chipboards are divided into hard
boards with a density of more than 800 kg/m3, medium-hard boards with a
density of more than 350 kg/m3 to 800 kg/m3 and porous boards with a density
of 230 to 350 kg/m3. Both in the wet process and in the dry process, up to 25
kg of resin and 1.5 to 20 kg of paraffin - per tonne of wood-fiber chipboard
produced - are required for binding and hydrophobicizing. In the wet process
predominantly in use today, the process water has to be circulated with a
content of soluble material of up to 2.0-2.5% which is highly energy-intensive
at a water temperature of up to 65 C. In addition, formaldehyde has to be
added in a quantity of 0.02 to 0.2% to avoid troublesome staining of the wood-
fiber chipboards by the highly concentrated circuit water.
At the present time, medium-hard wood-fiber chipboards are mainly
marketed in semifinished form as medium-density fiberboards (MDFs) which
are made with formaldehyde-containing condensation resins. However,
through the continuous emission of carcinogenic formaldehyde vapors, in
some cases for several years, products such as these are no longer wanted
on ecological grounds. In the furniture industry, the situation is remedied by
giving MDFs an additional coating to bring the emission of formaldehyde
below the legally specified limits. In addition, although MDFs have better
dimensional stability than natural wood at typical air humidity levels of 35
to
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85%, it is still not good enough for certain applications. In addition, MDFs
made with synthetic urea-formaldehyde binders are not suitable for use in
high-humidity environments, particularly in water.
Wood/plastic composites are understood to be wood/plastic
combinations which are obtained by treating wood with monomers or
prepolymers. They are composite materials where the wood is impregnated
with the liquid starting material and the monomer deposited in the wood is
subsequently polymerized. Liquid monomers and solutions, for example
methacrylate prepolymers or unsaturated polyesters dissolved in styrene, are
preferably used in practice. The polymers primarily increase the strength of
the wood, above all its hardness and its compressive strength. Finally, the
aesthetic effect of natural wood not only is not impaired, it is actually
enhanced in many cases. Despite these advantages, wood/plastic
composites have hitherto been used to only a very limited extent for special
applications, for example for parquet floors, sports equipment, kitchen
utensils
and tool handles.
In contrast to the pure impregnating process for making wood/plastic
composites, the skinpreg process comprises surface impregnation with
plastics which penetrate into the wood to different depths under light
pressure
without completely impregnating it.
Japanese Publication No. 64-045440, published February 17, 1989,
describes isocyanate- or formaldehyde-based wood/foam compositions which
contain sawdust as filler. The foam obtained, with a density of 0.35 g/cm3,
possesses very high strength. The sawdust or wood powder is normally very
finely size-reduced wood which is used as a filter aid, as a filler, as an
additive
for rough fiber coatings, etc. However, there is nothing in the literature
reference in question to suggest that the foam is produced under high
pressure. Solvents are used.
Japanese Publication No. 63-303703, published December 12, 1988,
describes composites of fine vegetable fibers or vegetable particles, more
particularly wood powder, and a urethane prepolymer which are contacted
with water or steam before or after molding. A composite of this type has a
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density of 0.29 g/cm3, a compressive strength of 5.3 kg/cm2 and a tensile
strength of 3.4 kg/cm2. However, there is nothing in this literature reference
which directly or indirectly suggests that the composites are produced at
above-
atmospheric pressure. Solvents are used.
SUMMARY OF THE INVENTION
Accordingly, the problem addressed by the present invention was to
provide a new wood/plastic composite which would avoid the use of the
formaldehyde-containing binders still absolutely essential in the MDFs mainly
in
use today and which would also have advantageous performance properties.
Accordingly, the present invention relates to a wood/plastic composite
based on wood particles and/or cellulose-containing material and at least one
binder, the binder being a carbon-dioxide-eliminating two-component
polyurethane binder of a polyol, water and a polyisocyanate, characterized in
that
the binder is present in a quantity of 10 to 200 parts by weight, based on 100
parts by weight of the wood particles and/or the cellulose-containing
material, the
composite being obtainable by reaction of the wood particles and/or the
cellulose-containing material and the binder under a pressure of at least I
kp/cm2
and, more particularly, in the range from 50 to 100 kp/cm2.
In one aspect of the invention, the polyisocyanate is selected from the
group consisting of: aliphatic, alicyclic, and aromatic di- and
triisocyanates, and
reaction products of monomeric diisocyanates with low molecular weight diols.
In view of the marked increase in the hardness of the wood, even in the
interior of the composite, it is assumed that - depending on the ratio by
weight of
wood to binder, the size of the wood particles and the pressure applied - the
wood is strengthened by the polyurethane at its surface or throughout, i.e.
the
wood is present as a wood/plastic composite.
The wood/plastic composite has the following advantages over the prior
art:
- In contrast to the known wood-based materials mentioned above, it can be
made in any form, i.e. made-to-measure, for example in the form of
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boards, strips, cubes, squares, etc.
- It is suitable as a lightweight building material because it normally has
a density of 0.40 to 0.65 g/cm3. It is therefore a substitute for light and
medium flat-pressed boards or medium-hard wood-fiber chipboards, but
without the attendant formaldehyde problems.
- It does not swell in water at room temperature, i.e. its increase in
thickness after 24 hours in water at 20 C is <4 or 1% for thicknesses of 6
to 12 or >35 mm.
- In contrast to many of the wood-based materials and MDF boards still in
use today, it is formaldehyde-free and flame-retardant.
- The composites are so elastic that 5 mm diameter timber screws can be
screwed in without any splintering.
- The composites are also so dimensionally stable that threads can be cut
for SpaxTM screws, i.e. screws with a broad thread.
- By virtue of the polyurethane present, the composites may readily be
painted.
- Finally, the composites are characterized by their homogeneity, i.e. there
is none of the otherwise usual layer formation; in particular, there is no
inner layer and outer layer.
In one preferred embodiment of the wood/plastic composite according to
the invention, soft woods, for example woods of the spruce, pine, fir, larch,
birch,
alder, horse chestnut, aspen, willow, poplar and lime, are used as the wood
starting material. However, hard woods, for example beech, hawthorn,
blackthorn, ash, maple, walnut, apple, pear, yew or oak, may also be used.
Mixtures of soft wood and hard wood may also be used.
In another preferred embodiment, vegetable fibers, for example cotton,
jute, flax, hemp, bast, sisal, ramie, coconut fibers, yucca fibers or manila,
or
chemically modified fibers, such as the viscose fibers rayon and rayon staple,
cuoxam fibers, acetate fibers, and paper and cellulose yarns, may be used as
the
cellulose-containing material in the composite according to the invention.
In another aspect of the present invention, a process is provided
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comprising the steps of:
a) mixing the particles with the polyol component;
b) adding the polyisocyanate in excess and water to yield a mixture and
homogenizing said mixture;
c) introducing the mixture a closable pressure-tight mold optionally coated
with release agents and the mixture is reacted under a pressure of at
least 1 kg/cm3 and
d) removing the molding from the mold.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
The wood particles are present in the composite according to the
invention in the form of wood chips and/or wood particles or as cellulose-
containing material in particle sizes of at most 5 mm (thickness) x 20 mm
(width) x 50 mm (length). A thickness range of 0.5 to 3 mm, a width range of
1 to 15 mm and a length range of 3 to 40 mm are preferred.
The moisture content of the wood particles or cellulose-containing
material in the composite according to the invention is normally from 5 to 20%
by weight. If desired, it may be increased by moistening with water or steam
or reduced by drying at elevated temperature. However, the moisture content
preferably corresponds to the equilibrium moisture content of the material at
ambient temperature.
The composites according to the invention may contain wires, cables,
wire nets, rods or the like, for example for stabilization.
The two-component polyurethane binder used in the composite
according to the invention consists of a reaction product of at least one
polyol
with at least one polyisocyanate.
The quantity in which the two reactants are used is always selected so
that the polyisocyanate is present in excess, i.e. the equivalent ratio of NCO
groups to OH groups is 5:1 and preferably 2:1 to 1.2:1.
The polyisocyanate used is normally an aliphatic, alicyclic or aromatic
diisocyanate or triisocyanate.
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The polyisocyanates preferably contain on average 2 to at most 4 NCO
groups. Examples of suitable isocyanates are 1,5-naphthylene diisocyanate,
4,4'-diphenylmethane diisocyanate (MDI), hydrogenated MDI (H12MDI),
xylylene diisocyanate (XDI), tetramethyl xylylene diisocyanate (TMXDI), 4,4'-
diphenyl dimethyl methane diisocyanate, di- and tetraalkyl diphenylmethane
diisocyanate, 4,4'-dibenzyl diisocyanate, 1,3-phenylene diisocyanate, 1,4-
phenylene diisocyanate, the isomers of tolylene diisocyanate (TDI), optionally
in admixture, 1-methyl-2,4-diisocyanatocyclohexane, 1,6-diisocyanato-2,2,4-
trimethyl hexane, 1,6-diisocyanato-2,4,4-trimethyl hexane, 1-isocyanato-
methyl-3-isocyanato-1,5,5-trimethyl cyclohexane (IPDI), chlorinated and
brominated diisocyanates, phosphorus-containing diisocyanates, 4,4'-
diisocyanatophenyl perfluoroethane, tetramethoxybutane-1,4-diisocyanate,
butane-1,4-diisocyanate, hexane-1,6-diisocyanate (HDI), dicyclohexyl-
methane diisocyanate, cyclohexane-1,4-diisocyanate, ethylene diisocyanate,
phthalic acid-bis-isocyanatoethyl ester. Other important diisocyanates are
trimethyl hexamethylene diisocyanate, 1,4-diisocyanatobutane, 1,12-
diisocyanatododecane and dimer fatty acid diisocyanate. Also of interest are
partly masked polyisocyanates which enable self-crosslinking polyurethanes
to be formed, for example dimeric tolylene diisocyanate. Finally, prepolymers,
i.e. oligomers containing several isocyanate groups, may also be used.
Prepolymers are obtained using a large excess of monomeric polyisocyanate,
for example in the presence of diols. Isocyanuratization products of HDI and
biuretization products of HDI may also be used.
The diisocyanates or polyisocyanates preferably used are aromatic
isocyanates, for example diphenylmethane diisocyanate, either in the form of
the pure isomers or in the form of a mixture of the 2,4'- and 4,4'-isomers, or
even carbodiimide-liquefied diphenylmethane diisocyanate (MDI) which is
commercially available, for example, as Isonate 143 L. The so-called "crude
MDI", i.e. the isomer/oligomer mixture of MDI commercially available, for
example, as PAPI or Desmodur VK may also be used. In addition, so-called
"quasi prepolymers", i.e. reaction products of MDI or tolylene diisocyanate
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(TDI) with low molecular weight diols, for example ethylene glycol, diethylene
glycol, propylene glycol, dipropylene glycol or triethylene glycol, are also
suitable.
Preferred diols and/or polyols for the binder are the liquid polyhydroxy
compounds containing two or three hydroxyl groups per molecule, for
example difunctional and/or trifunctional polypropylene glycols with molecular
weights in the range from 200 to 6,000 and preferably in the range from 400
to 3,000. Statistical and/or block copolymers of ethylene oxide and propylene
oxide may also be used. Another group of preferred polyether polyols are the
polytetramethylene glycols which are obtained, for example, by acidic
polymerization of tetrahydrofuran. The molecular weight of the
polytetramethylene glycols is in the range from 200 to 6,000 and preferably
in the range from 40 to 4,000.
Other suitable polyols are the liquid polyesters which may be obtained
by condensation of di- and tricarboxylic acids, for example adipic acid,
sebacic acid and glutaric acid, with low molecular weight diols and triols,
for
example ethylene glycol, propylene glycol, diethylene glycol, triethylene
glycol, dipropylene glycol, butane-1,4-diol, hexane-1,6-diol, glycerol or
trimethylol propane.
Another group of polyols suitable for use in accordance with the
invention are the polyesters based on s-caprolactone which are also known
as "polycaprolactones".
However, polyester polyols of oleochemical origin may also be used.
Oleochemical polyester polyols may be obtained, for example, by complete
ring opening of epoxidized triglycerides of an at least partly olefinically
unsaturated fatty-acid-containing fatty mixture with one or more alcohols
containing 1 to 12 carbon atoms and subsequent partial transesterification of
the triglyceride derivatives to form alkyl ester polyols containing 1 to 12
carbon atoms in the alkyl group. Other suitable polyols are polycarbonate
polyols and dimer diols (Henkel KGaA) and, in particular, castor oil and
derivatives thereof. The hydroxyfunctional polybutadienes commercially
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obtainable, for example as "Poly-bd" may also be used as polyols for the
composites according to the invention.
The present invention also relates to a process for producing the
wood/plastic composites in which the wood particles and/or the cellulose-
containing material are/is first mixed with the polyol component, the other
component(s), more particularly the polyisocyanate in excess, is/are added
to the resulting mixture, the mixture is homogenized and then introduced into
a closable, pressure-tight mold optionally coated with release agents, the
reaction mixture is reacted under a pressure of at least 1 kp/cm2 and the
composite is removed or freed from the mold after cooling.
The mixing and reaction steps mentioned above are carried out at
temperatures of 10 to 30 C and more particularly at room temperature (18 to
25 C). The pressure treatment in the process according to the invention is
sourced by the reaction of the reaction mixture under the natural reaction
pressure. If necessary, however, pressure may also be supplied from outside
in known manner in the form of an inert gas or even steam.
In the process according to the invention, the reaction in the mold and
hence the formation of the composite takes 5 to 30 minutes and preferably 10
to 20 minutes.
Closable pressure-tight molds are used in the process according to the
invention.
There is normally no need to provide a release agent, more particularly
in the form of a Teflon coating, between the pressure reactor and the
composite. In certain cases, however, type 39-5001, 39-4487, 37-3200 and
36-3182 Acmos release agents for PUR are preferably used.
Finally, the present invention relates to the use of composites of the
type mentioned above or produced by the process described above in the
form of boards, strips, cubes, squares etc., more particularly in humid
environments or outdoors. The present invention also relates to the use of
the composites obtainable by the process described above as semi-finished
products or for cladding purposes in the building industry. The composites
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according to the invention may also be used as a packaging material, as a
floor covering, as stairs or as ornamental beams. These uses of the
composites preferably involve the interior fitting-out of vehicles, more
particularly motor vehicles, such as automobiles and camping vehicles, but
also caravans, ships and aircraft. Altematively, the composites according to
the invention may be used for decorative purposes outdoors or in the
domestic and institutional sectors, more particularly in kitchens and
bathrooms.
The invention is illustrated by the following non-limiting Examples.
EXAMPLES
Example 1:
A) Starting products
a) Polyol component:
trifunctional polyether polyol based on glycerol,
ethylene oxide and propylene oxide 83.8
glycerol 6.0
soya polyol modified with ethylene oxide 6.0
water 2.2
Tegostab B 8404 (Goldschmidt) 1.3
dibutyl tin dilaurate 0.7
b) Isocyanate component:
diphenylmethane-4,4'-diisocyanate 100
(crude MDI with a viscosity of 200 to 220 mPas)
B) Production
1500 g of wood chips (pine) up to 4 cm in length are intensively mixed
with 1,000 g of the polyol component of the foam system. After addition of
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1,000 g of the isocyanate and remixing, the mixture was quickly introduced
into a metal mold around 6.5 dm3 in size. The mold was immediately closed
with a cover. After 30 minutes the foam-containing wood/plastic composite
is removed from the mold.
The composite obtained has a density of 0.6 g/cm3 and a smooth
surface and can be mechanically treated like wood, for example sawn,
planed, sanded and drilled. Threads can also be cut into the material.
C) Application
The composite obtained in accordance with the Production Example
was compared for quality with a medium-density fiberboard (MDF board)
which had been produced with formaldehyde-containing condensation resins
and had exactly the same thickness. It was found above all that the
composite according to the invention has significantly lower water absorption
than the MDF board.
Table :
Water absorption and swelling of the composites according to the invention
compared with MDF boards
Density Water absorption Increase in thickness
[g/cm] [%] [%]
After storage in water for 24 h
Board thickness 6-12 >35 6-12 >35
[mm]
MDF board 0.72 20 16 8 5
Composite 0.60 14 7.5 4 1
according to the
invention
