Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Thls invention relates to adhesive formulations particularly
suitable for forming hot-pressed wood composites, such as particleboard
and waferboard. A fast-curing adheæive with non-sticking properties i8
provided fro~ a llquid polyisocyanate, a hydrocarbon oil and a higher
fatty acid salt.
Description of the Prior Art
Phenolformaldehyde (PF) resins have been used exclusively for
some time for outdoor type particleboard and waferboard making. The use
of these reslns presents certain disadvantages. The density of boards
made from PF resln ls-5-10~ higher than that of aminoplast bonded boards.
The rate of thickness swelling i9 high because of the high alkalinity of
the re~in and some discoloration may occur due to diffuslon of unbound
phenolic residues. With thick bosrds, press times are long. ~arious
polyurethane type adhesives also have been used in these context~.
The use of i60cyanates as an adhesive offers some advantages
over PF resins. The denfiity of 4,4'-diphe~ylmethane diisocyanate (MDI)
bonded boards may be reduced by 10 to 15% without reducing their strength
properties; press times may be reduced since isocyanates do not contain
water which would need to be evaporated. ~ith 3-layer particleboard6, a
higher proportion of fine particles may be incorporated in the core. The
weather resistance of isocyanate-bonded particleboards or waferboards is
equal to that of PF-bonded boards and the bond quality is not affected
within a broad moisture range, i.e. 0 to ~5%. The disadvantages of iso-
cyanate bonding are the high cost of the isocyanate (offset by the above
mentioned advantages) and sticking to metal parts which necessitates the
use of anti-sticking agents on the metal surfaces.
Polyisocyanate-formaldehyde binder systems are known in forming
wood composites with release agent coatings on caul sheets normally being
necessary on hot-pressing (~ee United States Patents 3,919,017 and
3,930,110, Shoemaker et al). In United States Patent 3,870,665, 11 ~arch
1975, Diehr et al, the use of polyisocyanates in con~unction with com-
pounds which catalyze the formation of isocyanurates from isocyanates as
mold release agents, is described. Many types of such catalytic com-
pounds are mentioned with one type being certain metal salts of carbox-
ylic acids.
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Polyisocyanate adhesive systems used in con~unction wLth
selected phosphates as internal mold release agents, are described in
Canadian Patent 1,123,817, May 18, 19~2, McLaughlin et al~
It would be desirable to provide alternatives to the release
agents mentioned which function internally, are inexpensive, and are
readily available.
Description of the Invention
According to this invention, a heat-curable adhesive
co~position is provided, which does not stick significantly to hot mold
surfaces, comprising:
a) a liquid polyisocyanate,
b) a hydrocarbon oil, and
c) a salt of a higher fatty acid having at least 14 carbon atoms.
The adheslve composition may be in the form of an emulsion in the oil.
The liquid polyisocyanate may be selected from a wide variety
~nown to form adhesive systems. Such polyisocyanates include 4,4'-di
phenylmethane diisocyanate; 1,4-butylene diisocyanate; 2,4-toluylene di-
isocyanate; 4,4'-biphenylene diisocyanate; hexamethylene dlisocyanate;
and triphenylmethane triisocyanate. Liquid dimers, trimer~, tetramers,
etc. (oligomers) thereof may be used.
The hydrocarbon oil is selected from petroleum fractions of
light to heavy viscosity, particularly those similar to a ~ineral oil or
paraEfin oil. A suitable viscosity range is from about 125 to 350
Saybolt viscosity units at 100F. These oils are non-reactive with the
polyisocyanate.
The fatty acid moiety should be selected from those having at
least 14 carbon atoms. Suitable fatty acids include myristic, palmitic,
stearic, oleic, linoleic, behenic and mixtures thereof. The fatty acid
is used ln the form of a salt with a polyvalent cation, for instance,
zinc, copper, iron, cobalt, calcium, magnesium, and aluminum. We have
found the cations of zinc or calcium very suitable.
The relative proportions of the three components can vary
within the approximate range by wt. 100:40-200:5-30. Preferably the
proportions are about 100:100:16.
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The hydrocarbon oil and the fatty acld salt have been found to
bestow very effective anti-sticking and mold release properties on the
polyisocyanate without interfering significantly with the wood adhesi~e
properties.
The emulsion formation may be facilitated using small amounts
of emulsifiers of the nonionic type, for example, esters or modified
esters of fatty acids, such as sorbitan mo~olaurate (Span ~ 20) and
polyoxyethylene sorbitan monostearate ~Tween ~ 60), other equivalent
emulsifiers and mixtures thereof. The emulsion is very stable and can be
sprayed onto the wood with standard spray means. Where any water is
present in the emulsion, or where there is a slow reaction between the
polyisocyanate and emulsifier used, the emulsion should be used as soon
as posslble after preparation.
Compo~ite wood products may be prepared by steps comprising:
i) applying to the wood components, as a mlxture,
(a) a liquid polyisocyanate
(b) a hydrocarbon oil, and
(c) a salt of a higher fatty acid having at least 14
carbon atoms;
ii) sub~ecting the coated components to heat and pressure by hot-press-
ing surfaces sufficient to consolidate the componen~s and cure the
adhesive; and
iii) separating the composite wood product from the hot-pressing surfaces
without significant sticking thereto.
The amount of the composition applied to the wood surf2ces normally is
within ~he range of about 0.5 to about 5% by wt. of the dry wood compon-
ents.
The wood should not be abnormally wet or too dry: a moisture
content within the range from about 3 to about 10% by wt, i8 preferred.
Hot-pressing may be carried out at temperatures within the
approximate range from 150 to 300C for from about 0.75 to about 15 min.,
preferably about 1-4 min. at about 200C. Pressures applied suitably are
within about 400-1200 psi but these are not critical.
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The following examples are illustrative.
The aspen wafers used in this work were prepared with a labora-
tory waferizer~ After sifting, flake thickness varied from 23-30 thou-
sandthæ of an inch ~0.58-0.78 mm), the length from 1.0-1.8 inches (25.6-
46 mm), and width from 0.25-1.25 inches (6.4-31 mm), the maJority being
in the 0.75 in. range (19.2 mm). The wafers were airdried on trays for a
few days, then in a forced air oven to a molsture content of 4-6% by wt.
The following will illustrate the preparation and use of a
polyisocyanate emulsion in paraffin oil.
Emulsion preparation: An emulsion was prepared having the
following composltion, (given in parts by weight per part of polyisocyan-
ate) paraffln oil: 1, mixed emulsifier: 0~094, water: 0,085, zlnc stear-
ate: 0.162. The polyisocyanate was a mixture of monomer, dimer, trimer
and tetramer of 4,4'-diphenylmethane diisocyanate (MDI-E 441 Mobay). The
paraffin oil was added to the MDI with vigorous stirring~ then the mixed
emulsifler was added, followed by the water and the ~inc stearate. The
mixed emulsifler had the following composition: Span 20 [trademark]:
85%; Tween 60 [trademarkl: 15% by weight. These two emulsifying agents
were mixed separately, then added to the MDI-paraffin oil mixture.
This procedure could be simpliEied by simply adding the stea-
rate powder to the MDI-paraffin oll mixture with strong stirring. An
emulsion i8 also obtained which is stable for 2 hours or so, and could be
used in this condition.
Spraying and Boardmaking: The emulsion was sprayed shortly
after its preparation onto aspen wafers of 4-6% moisture content in a
rotary drum equipped with a spraying system. The amount of MDI in the
emulsion sprayed was 1.5% oE the weight of oven dried wafers.
The coated wafers were placed in a wooden frame set over a car-
bon steel caul plate. This plate was given a light coat of a commercial
anti-sticking compound before pressing the first board only. About 1500
gms. of coated wafers were used to give board deDsities varying from
39-41 pounds per cu. ft. The mat was then pressed to stops at 500 psi
and 410F (210C) for 3 min. Board thickness was 7/16 inch ~ 2 mm).
Five boards were prepared in this manner without one board sticking to
the caul plate~ In a control experiment, the caul plate was similarly
coated with an anti-sticking compound, but the wafers were sprayed with
the same emulsion containing no zinc stearate. The second board made
stuck very badly to the caul plate.
The boards were conditioned at 65% R.H. for a week before
determinlng their modulus of rupture (MOR), modulus of elasticity (MOE)
and the internal bond strength (IB). The wet MOR was determined after
boiling in water for 2 hours. Results are shown in Table I.
TABLE I
Strength Properties of Aspen
Poplar Waferboards
Adhesive Components MOR MOE IB Densit~
(~ of o.d. wafers) dry wet(psi) psi(10-3) psi lbs/ft
Paraffln Zinc
MDI Oil Stearate
1.5% 1.5% 0.25~ 4484 2143 8~6 47 39.1
All MOR and IB values exceed CSA Standard 0188, namely, 2000
psi ln the dry state, 1000 psi after 2 hours of boiling in water and 40
psi for IB.
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