Note: Descriptions are shown in the official language in which they were submitted.
1~:358499
This in~ention relates to the bonding together of
solid lignocellulosic materials, particularly of wood.
Bonding of lignocellulosic materials, such as
wood, is widely used commercially, such as i~ the
; manufacture of plywood or particl~ boards. I~ pr~sent
commercial bonding pxocedures, adhesives, such as urea-
or phenol formaldehyde are employed, which are spread or
- otherwise applied to the surface of the material, and
penetrate the wood structure whereby bonding is effected
by the aahesive. Procedures have been proposed to effect
such bonding by chemical reactions between reagents and
the wood itself, but have not met commercial accepta~ce.
Accordin~ to one aspect of the present invention, ;~
there is provided a method of bonding solid lignocellulosic
materials, which comprises applying a bonding composition
- comprising at le~st one carbohydrate and an acid capable
of catalysing hydrolysis of the carbohydrate to a surface
of the material and pressing surfaces of the material
together at an elevated temperature.
~he method of the invention may be used for
manufacture of laminates, plywood and particle boards
without the use of traditional adhesives.
It is believed that bonding is created by chemical
transformation of the carbohydrates which are applied to
the wood surface and carbohydrates present in a layer of
wood surface.
It is well known that wood is a high polymeric
substance of a complicated structure composed of three
classes of compounds, namely carbohydrates, ligni~ and
~0 extractives. The main carbohydrate, cellulose, is a
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10584~9
polysaccharide build up of glucose units. Lignin is
a complicated phe~olic compou~d the structure of which
has ~ot bee~ entirely determined. Not much is k~owu
about the character of the bond betwee~ the carbohydrates
and lignin, although, generally speaking, lignin seems to
function as a binder of cellulose microfibrils. The
function of extractives appears to be manifold, their
disease protective function probably is the most important.
-~ The chemical reactions involved in the bonding
system in the prese~t invention have not been full~
elucidated. Theoretically, several reactio~ systems may
be i~volved at the same time. The most important,
however, appears to be hydrolytic degradation of
polysaccharide~s and lignin by the action of the acid and
repolymerization of degradation products thereof.
The action of acids on polysaccharides results in
hydrolysis into the ccmponent monosaccharides, followed
by further transformation of the resulting mono-
saccharides: thus x~loses are dehydrated to furfural,
i 20 glucose a~d mannose to hydroxymethyl furfuralO ~hese
furanes are extremely reactive and further transform
into a large variety of che~ical species, in the ca~e of
glucose, chiefly to levulinic acid, as well as to ill
defined polymeric humins. Various mechanis~s for
polymerization reactions of hydrolysis products have been
proposed to explain their formation.
~he action of strong acids on lignin results in
condensation of various types. In mild acidic media
partial hydrolytic degradation of lignin into a variety
of products takes place. Yields, however, are small
1C~5~3499
because of concurrent polymerization of degradation
products. In acidic media a coupling of degradation
products of carbohydrate and lignin appears to be a
theoretical possibility.
It is believed that all or some of the above-
mentioned reactions ma~ be involved in the bonding
process of the present invention.
It has been found that the strength of bonds
formed between pieces of wood using the present
inve~tion is at least comparable to the stre~gth of bonds
achieved by hitherto used adhesives.
In the manufacture of plywood it is only necessary
to cover a surface of a wood veneer with a liquid carrier
containing the carbohydrate and acid, bring such surface
into contact with the surface of another veneer which may
or may not have a coating of the bonding composition, and
press them at an elevated temperature in a conventional
pres~.
Alternatively, the liquid composition containing
the carbohydrate and acid may be cooked before application
to the material to be bonded to transform carbohydrates
into furane type intermediate products. Preferred
cooking conditions are lO0 to 130C for from 30 to 90
minutes. The cooking results in colour change to brown
or black indicating the level of chemical transformation
Or carbohydrates. ~he cooked composition can be applied
to a wood surface in the same way as an uncobked one,
followed by pressing under heat and pressure Cooking
may be advantageous in cases where shorter pressing times
are required.
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lOS8499
In some cases it may be advantageous to expose
wood veneers or particles covered by the liquid bonding
composition to an elevated temperature for a short
period of time prior to pressing Such preheating
causes partial chemical transformation of carbohydrates
which may shorten the pressing time or lower the pressing
temperature required. ~emperatures of preheating up to
140C m~v be employed for a period of time up to about 15
- minutes.
The bonding composition may include a liquid
carrier which is non-reactant with the lignocellulosic
material, such as water, ethyl alcohol and other solvents.
In general the vapors from the carrier may readîly escape
from the pres~ during the pressing. The composition may
also contain other chemical reagents capable of affecting
the bonding reaction, i.e. accelerating or reducing the
`extent of the reaction in which the carbohydrate
participates depending upon the reaction conditions which
may vary widely. Such reagents may be incorporated in
the desired amount in the carrier liquid together with the
carbohydrate and the acids. Mixtures of various
carbohydrates as well as mixtures of various acids in a
carrier may be employed.
Excess amounts of carbohydrates and acid catalyst
applied to the surface of wood do not affect the
efficiency of bonding but are uneconomical. It is
merely necessary to have sufficient carbohydrate and acid
to effect the bonding reaction in a press under heat and
pressure. The minimum amount of carbohydrate and acid
required is variable depending on the pH of the wood and
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lC~S84~9
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carbohydrates, kind a~d reactivity of carbohydrate,
temperature, moisture content sf wood, desired reaction
speed and other factors.
It is known that hydrolysis of polymeric
carbohydrates, such as cellulose, hemicelluloses and
starch into monomeric units is an unhomogenous process,
because of differe~t reaction rates of different
carbohydrates. It can be assumed that also rates of
transformation of simple sugars, such as glucose, xylose,
sucrose into furane type compounds and their
f condensation are variable Therefore, the rate of
reaction may be varied between quite wide limitsO
~he optimum amount of carbohydrate used will ~ary
depending upon the character of the wood, reacti~ity or
other properties of the carbohydrate used, surface
roughness of the wood and the pressing conditions
desired. Only a film of the composition need be applied
which can be con~eniently done by brushing, spraying or
roller spreading. Typically an amount of the bonding
~omposition may be employed which will provide from 2 to
32 grams of carbohydrate per lOOO cm2 of area.
~`or example when wood laminates Pre prepared wi~h
Douglas fir veneers and a mixture of sucrose and starch
in the ratio l:l as a carbohydrate, sulfuric acid as a
catalyst in water as the carrier and pressing conditions
of 12 ~g/c~2 and temperature of l60C for 7 ~inutes, an
amou~t of carbohydrate of about 5 grams per square foot
will suffice. The minimum amount of carbohydrate under
these conditions may be 2.5 grams per square foot. ~he
preferred amount of sulfuric acid as a catalyst in this
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las~4~s
case would be about l to 2% of the amount of carbo-
hydrate.
From the preceding, it will be seen that a wide
variety of carbohydrates may be employed, including
cellulose, hemicelluloses, sugars such as glucose and
; sucrose, starch, wheat or corn flour, molasses of
various origin and mixtures of carbohydrates.
Inexpensive molasses represent an attractive possibility.
The acids which may be used include sulphuric, hydro-
chloric~ phosphoric and acetic acid.
The amount of acid present is generally from l
to 20/o by weight on the carbohydrate; the preferred ratio
- ~ depsnds on the identity of carbohydrate and acid. It is
preferred to keep the amount of acid used to the minimum
level necessary to catalyze the carbohydrate transformation
into the furane type compoun~s and their polymerization.
A large excess of acid may be harmful to wood strength
over a long period of time. The amount of hydrochloric
acid, which i~ a strong acid, necessary to catalyze the
transformation of the mixture of sucrose and starch in
the ratio of l:l is about 1-2% of acid to carbohydrate by
weight. The amount of weak acid, suck as acetic of
phosphoric required would be higher than that of
hydrochloric acid. ~he transformation of polymeric
carbohydrates, such as cellulose, into furane type
compounds requires a higher proportion of acid than
; simpler carbohydratest such as glucose or sucrose~ for a
given reaction rate.
In a preferred embodiment of the invention, a
~0 mixture of carbohydrates comprising sugars and starches,
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lOS8499
such as sucrose and wheat flour is used. ~or reasonswhich are not clear such mixtures produce a higher
degree of bonding than simple carbohydrates, i.e. onl~
sugars or only starches. It is advantageouQ to use
sugars and starches of approximately the same
decomposition rate. Simple carbohydrates are preferred
to polymeric ones because of their higher decompositio~
rate. Price and availability, however, is probably
the most important factor in deciding which raw material
to use. ~ow boiling point acids are preferred to high
boiling point acids because they are removed from wood
during pressing operation, which substantially eliminates
the possibilit~ of the acid attacking the bonded
lignocellulosic material over a long storage period.
Pressing conditions in the presQ will vary widely
depending upon variables, such as kind of carbohydrate,
ki~d of wood, kind and amount of catalyst and
requirements on the product. As usual for any given
-~ system, the lower the temperature, the longer the
pressing time and vice versa. The pressing temperature
should not exceed the temperature at which charring of
the lignocellulosic material will occur nor should the
pressure exceed that at which the lignocellulosic
material will undesirably crush. The preferred
temperature range is 140 to 200C and the preferred
pressure range 5 to 25 kg/cm2. The pressing time
required under these conditions is generally 0.3 to 2
min. per ~n of thickness.
The invention may be applied to any kind of
` 30 wood bonding such as in plywood or particle board
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production, wood lamination and other composite board
production. In the production of composite products
such as particle boards, the same procedure is
followed as for plywood manufacture except that the
particles are thoroughly covered by the carrier
containing carbohydrate and acid, which can be achieved
by spraying and mixing followed by board formation and
pressing in the press.
~ Embodiments of the invention will be described,
- 10 by way of illustration, in the following Examples.
Percentages are give~ by weight.
EXAMPIE I
Douglas fir veneers 130 cm square and 2.5 mm thick
with moisture co~tent of about 4% were brushed on one
surface with water solution containing 2~h of sucrose,
- 25~ starch and 1% of sulphuri~ acid in the amount of
about lO grams of the solution per area of 30 cm square.
After application of the solution a 3 ply plywood was
made by hot pressing in a conventional press at the
pressure of 12 kg/cm2 and temperature of 170C for 7
minutes. Shear strength tests on 20 specimens showed
shear strength of 16 kg/cm2 which was comparable to
similar products used phenol formaldehyde adhesive.
Shear strength in wet conditions after 4 hours boiling
water followed ~ 20 hours drying at 53C and again 4
hours boiling was about 8 k~/cm2.
EXAMPIE II
The same procedure wa~ used as in Example I,
except that prior to pressing veneers covered with the
same carrier solution were put into an oven at the
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1~5849g
temperature of about 140C for 5 minutes, whereupon the
veneer surfaees turned to a dark colour and the pressing
time was reduced to 5 minutes. The strength properties
were about the same as in ~xample I.
EXA~L~ III
.
A water solution containing 25% of sucrose, 2~/o
of wheat flour and 1% of sùlphuric acid was boiled until
the colour turned dark, which took about 30 minutes~
After cooling the solution was used for making Douglas
fir plywood panels under the same conditions as in
Example II. Strength properties were about the same as
in the previous examples.
EXAMPIE IV
:~
Douglas fir shavings having a moisture content of
about 4% were sprayed with water solution containing 25%
sucrose, 2~/~ starch and 1% of sulphuric acid. The
~- amount used was about l~/o of the solution to the wood
weight. A particle mat was formed from such sprayed
shavings which was transferred into a hot press with press
platen at temperature of 170C and particle board
pressed for 7 minutes. After cooling to room
temperature internal bond tests were performed which
showed internal bond value of about 5 kg/cm2 which is
above the requirements of the standards. The bond was
resistant to 4 hours boiling.
From the review of these examples, it is apparent
that the addition of small amount of carbohydrate and
acid to the surfaces to be bonded leads to bond formation
under heat and pressure. ~he fitrength of 'he bond is
comparable to the strength achieved b~ traditional
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1~351~4<3g
adhesives and the bonds are resistant to water. ~he
economical advantage of such system is great because
the amount of carbohydrate used represents only about
of the amount of adhesive for the same conditions;
and the price of some carbohydrate applicable in this
bonding system, such as molasses, represent only about
to 1/5 of the price of the most extensively used
sdhesives such as uree or phenol formeldehyde adhesives.
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