Wood adhesives modified with surface active agents

The present invention provides an improved adhesive composition for wood gluing, comprising a formaldehyde condensation resin and a minor amount of a surface active agent. This invention further provides a method for manufacture of composite wood products comprising applying an adhesive composition, which includes a formaldehyde condensation resin and a small amount of a surface active agent, to the wood and curing the adhesive.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention is concerned with the wood gluing art. It provides an 
adhesive composition and a method for gluing wood veneers and particles 
together. 
2. Description of Related Art 
Many approaches to improving the bonding by wood adhesives have been tried. 
In particular, a number of workers have tried to improve the interaction 
between the adhesive and the wood surface, and to increase penetration of 
the adhesive into the wood. 
Penetration is important as shown by Rowell (1978), Int. J. Adhesion 
Adhesive. 7:183-8, who looked at surface interactions between wood and 
adhesive by changing the surface properties of the wood. Aspen wood was 
acetylated with acetic anhydride then glued with phenolic adhesives. The 
water-soluble phenolic resin could not penetrate into the acetylated 
wood's surface, and the reduction in penetration resulted in poor bonding. 
Lambuth (1967), U.S. Pat. No. 3,342,776, taught a way of increasing the 
penetration of phenolic resins to increase bond strength in resultant glue 
bonds. The initial condensation reaction between phenol and formaldehyde 
is controlled to maximize addition of formaldehyde to phenol while 
minimizing the dimerization reaction between substituted phenols. The 
resulting resin has low viscosity to encourage penetration but cures 
rapidly, and after curing, the glue joint has improved strength. 
Hse (1971), Forest Prod. J., 21:44 52; (1972), Holzfors., 26:82-85, studied 
the effects of variations in surface tension on glue bond strength in 
southern pine plywood made with phenolic resins. The surface tension was 
varied by changing the level of sodium hydroxide catalyst present during 
the condensation reaction of the resin to change the extent of methylol 
derivatization of the phenol. The glue bond strength, measured as shear 
resistance or as wood failure percent, increased as surface tension 
decreased. However, in these experiments the surface tension property was 
inextricably confounded with the state of condensation in the phenolic 
resin. Therefore, it is unclear whether the improved bond strength is 
really the result of changes in surface tension or whether the true 
effector is the change in conditions during the condensation reaction. 
Perhaps as a result, other components to modify surface tension such as 
surfactants have not been tried with phenolic resins for southern pine 
plywood. 
Eblez (1978), Holzfors., 32:82-92, reported studies with resorcinol 
adhesives for beech plywood, where the surface tension was modified by 
addition of low molecular weight aliphatic alcohols. Some improvement in 
strength was observed on the addition of methanol or isopropanol, although 
the concentration is unspecified. Addition of benzyl alcohol had no effect 
on the glue strength. These changes in glue strength seemed to correlate 
inversely with the wetting ability of the adhesive. In general, as wetting 
ability of the adhesive mixture increased, the strength of the resulting 
bond decreased, which argues against using surfactants to change adhesive 
wetting ability. 
Modification of urea-formaldehyde adhesives with surfactants has been 
attempted, but has not been uniformly successful. Horioka, et al. (1956) 
Jap. Forest Exp. Stn. Bull., 89:1-55, reported that a small amount of a 
surface active compound had little or no effect on the bond strength of 
beech veneer plywood made with urea-formaldehyde adhesive. When plywood 
panels were soaked before they were tested, presence of a surface active 
compound in the adhesive decreased the bond strength. When plywood was not 
pre-soaked before testing, a small amount of surfactant in the adhesive 
gave some increase in bond strength, while increasing amounts of 
surfactant lowered bond strength. 
Herczeg (1965), Forest Prod. J., 15:499-505, used polyoxyethylene sorbitan 
esters to modify the surface tension of urea adhesives. When these 
adhesives were used to make plywood, he found the highest bond strength 
for resins containing 0.025% polyoxyethylene sorbitan esters. 
The art of plywood manufacture has long recognized that manufacture of 
plywood from wet veneers results in poor bonding of the finished products, 
and similar effects have been observed in other composite wood products 
manufactured from wood furnish whose moisture content was too high. 
Adhesives formulated to provide good bonding for veneers with low moisture 
content (from 0-6%), give poor results when applied to intermediate or 
high moisture veneer (7-25%), and an adhesive formulated for use with 
intermediate moisture veneer will not run properly on plywood forming 
machines set to accommodate low moisture veneers. Wet furnish or veneer 
tends to cause over-penetration during hot-pressing, so that too little 
active bonding component is retained at the interface after curing is 
complete. The excessive flow is caused both by dilution of the adhesive 
and by retardation of cure, which produces a longer flow period. If 
particulate extenders are present, they cannot perform their usual 
function of holding active bonding component such as resin at the 
interface, and instead become detrimental by further weakening the cured 
interfacial layer. Many different solutions to the problem of 
over-penetration have been proposed including both adding modifiers to the 
adhesive and changing the way the adhesive resins are prepared in order to 
change the penetration properties of the adhesive. 
Kuhr (1967), Canadian Patent No. 754,646, noted when gluing plywood with 
phenolic resins that the efficacy of bonding is sensitive to the moisture 
content of the wood and also that using too high temperature during hot 
press curing of plywood made with high-moisture-content veneer often 
resulted in steam blisters, damaging the panels. Kuhr taught that the 
addition of between 0.75% and 4.5% polyvinyl alcohol resin to 
phenol-formaldehyde adhesive when manufacturing either plywood or 
chipboard improves stability of the adhesive before pressing, and permits 
lower press temperatures as well as reduced drying time between applying 
the adhesive and pressing. Inclusion of polyvinyl alcohol is taught to 
reduce the sensitivity of the adhesive to variation in initial wood 
moisture content and to reduce the frequency of steam blisters as a 
defect. 
Most of the techniques available for minimizing over-penetration of 
adhesive merely shift the emphasis to another manufacturing problem-type. 
For instance, using higher molecular weight resin will reduce 
over-penetration but at the expense of reducing the amount of time 
available between application of the resin and final curing of the glue 
bond. Addition of a limited amount of vinyl resin to phenolic adhesives, 
as reported by Kuhr, has relatively few detrimental side effects, so long 
as the amount of vinyl resin added is small, but as a result the benefit 
is also limited. There exists a need for adhesive modifiers which reduce 
over-penetration and which may serve as an alternative to vinyl resins or 
may be used in an additive fashion with the existing modifiers. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide a wood adhesive with improved 
bonding properties. It is a further object of this invention to provide a 
method for the manufacture of plywood from high moisture veneers and to 
provide an adhesive for composite wood products that is effective with 
high moisture furnishes. It is still another object of this invention to 
provide a modified adhesive composition for wood that reduces 
over-penetration. Still another object of this invention is to provide an 
adhesive modifier for wood adhesive whose effects are additive with 
modifiers such as vinyl resins. Still another object of this invention is 
to provide a modified adhesive composition for wood that permits extension 
of the time between application and curing of the adhesive without drying 
out, but which is not susceptible to over-penetration that would reduce 
the strength of final cured glue bonds. These and other objects can be 
achieved by the invention disclosed herein. 
This invention contemplates an adhesive composition comprising a 
formaldehyde condensation resin and a minor amount of a surface active 
compound. This invention further contemplates a method for the production 
of composite wood products which is effective using high moisture furnish. 
The method comprises application of an adhesive comprising a formaldehyde 
condensation resin and a minor amount of a surface active compound to the 
furnish and then curing the adhesive. 
In gluing wood laminates or particles, it is sometimes necessary to use 
high moisture content veneers or furnish, but this tends to cause poor 
bonding due to over-penetration of the adhesive. The inventor has observed 
that the incorporation of small amounts of certain surface tension 
modifiers (surfactants) decreases over-penetration without causing the 
dryout problems produced by most other modifiers or resin modifications. 
Experiments have shown that the beneficial effect of the surface active 
agent is additive to that of, for instance, vinyl resin, so that it is 
possible by using both modifiers to obtain greater reduction of 
over-penetration than that which could be achieved with either one 
separately. Stated another way, the surface active agent seems to broaden 
the tolerance of a formaldehyde condensation adhesive to moist veneer, 
whether or not other modifiers are present. It could alternatively also 
replace more expensive modifiers such as vinyl resin. 
DETAILED DESCRIPTION OF THE INVENTION 
The adhesive composition of the invention comprises at least two 
components: a formaldehyde condensation resin and a surface active agent 
as described below. 
The surface active agent is a compound which effects polarity and surface 
tension of the adhesive composition. The most preferred agent is cetyl 
alcohol. However, other compounds with related properties are within the 
contemplation of this invention. These compounds include, but are not 
limited to, the aliphatic, monofunctional alcohols having between 10 and 
25 carbon atoms, especially between 14 and 20 carbon atoms, and the 
olefinic, monohydroxy alcohols having between 10 and 25 carbon atoms, 
especially between 16 and 22 carbon atoms. Other compounds which have HLB 
values similar to these alcohols are also contemplated as surface active 
agents of this invention, in particular, compounds with HLB in the range 
from about 1.0 to about 2.0. Examples of such compounds are shown in Table 
1. 
TABLE 1 
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Compound HLB 
______________________________________ 
cetyl alcohol 1.0 
oleic acid 1.0 
acetylated sucrose diester 
1.0 
ethylene glycol distearate 
1.3 
acetylated monoglycerides 
1.5 
sorbitan trioleate 1.8 
glycol dioleate 1.8 
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(Kirk-Othmer Encyclopedia of Chemical Technology, 3rd Ed., Vol. 8, p. 913 
 
Formaldehyde condensation resins contemplated by this invention are one or 
more of the condensation resins that are commonly used in the art for 
gluing wood products. This group includes phenolic novolac and resole 
resins, urea-formaldehyde resins, melamine-formaldehyde resins, 
resorcinol-formaldehyde resins, or mixtures of these resins. The resins 
may be single-stage resins or two-stage resins. Preferred resins are of 
the phenolic-type, in particular, phenol-formaldehyde resins. The 
particular resin or resin mixture chosen will be selected in view of the 
type of wood product in which the adhesive will be used, where the 
selection criteria are those commonly used to choose resins in the 
wood-gluing art (including cure speed, water solubility, cost, and 
durability). 
In addition to formaldehyde condensation resins and a minor amount of 
surface active compound, the adhesive compositions contemplated by this 
invention may include other additives of the sort used routinely in the 
art of wood-gluing to modify adhesive properties. Examples of some of the 
additives contemplated by this invention and the properties they modify 
are shown in Table 2. In particular, adhesives which contain both the 
surface active agent of this invention and one or more of the penetration 
control agents listed in Table 2 will enhance the performance of both 
modifiers, as shown in the Examples below. 
TABLE 2 
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Other Modifiers 
______________________________________ 
Viscosity control agents 
*extenders (absorb water) 
sodium hydroxide (swells the extenders) 
gums (natural gums and modified cellulose) 
Penetration control agents 
polyvinyl alcohol (limits depth of penetration) 
polyvinyl acetate (limits depth of penetration) 
*extenders (hold resin fractions at interface) 
gums (raise viscosity) 
borates, chromates, etc. (form complexes with resin) 
Cheapeners 
*extenders 
alkali metal silicate salts 
lignin 
lignosulfonate 
Strengtherners (may make cured film less brittle) 
*extender 
latex 
isocyanate resin 
Tackifiers 
gums 
latexes 
borates, chromates, and other salts 
Curing Agents (catalysts to accelerate curing) 
polyhydroxy benzenes 
aromatic amines 
aminophenols 
ketone-aldehyde resins 
alkylene carbonates 
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*argricultual residues, wood flour, nut shell flour, cereal flour, clays 
etc. 
The surface active agents of this invention, in an amount sufficient to 
enhance bond strength after curing, are combined with aldehyde 
condensation resins, and optionally other modifiers, to make up the 
adhesives of this invention. The surface active agents of this invention 
are usually added to the adhesive composition in an amount whereby the 
agent will be sufficient to coat the surface of the glue layer after it is 
applied to the wood material. The surface active agent should preferably 
make up from 0.01%-0.2%, and especially from 0.02%-0.08%, of the final 
adhesive weight. The formaldehyde condensation resin solids are present in 
the adhesive composition at from about 25% to about 60% of the total 
weight of the adhesive. In plywood adhesives, the resin is present at 
preferably from about 25% to about 30%. The resultant adhesives are used 
by directly substituting them for the adhesives routinely used to glue 
composite wood products in production process well-known to those skilled 
in this art. 
The modified resin adhesives of this invention are particularly useful in 
the manufacture of plywood from southern pine veneers. Especially 
preferred for this application are resole resins. A typical resole for 
plywood is in the range of 40-45% total solids, 5-8% NaOH, and 1.9-2.3 F/P 
mole ratio. The NaOH is added in 2 or 3 portions at various stages of the 
resin synthesis. 
This invention also contemplates a method for the manufacture of gluded 
wood products comprising using the adhesives of this invention to bond 
wood veneers or wood particles together. Composite wood products which may 
be manufactured using the adhesive composition contemplated by this 
invention include softwood plywood, hardwood plywood, oriented strand 
board, chipboard, hardboard, particle board, fiberboard and LVL (laminated 
veneer lumber). Also within the contemplation of this invention are 
phenolic or aminoplast molding compositions comprising resin, wood flour 
filler and a minor amount of a surface active compound. 
The processes of manufacture of softwood plywood, hardwood plywood, 
oriented strandboard, chip board, hardboard, particle board, fiberboard, 
and LVL are well-known to those of ordinary skill in the art and fully 
discussed in the literature. In the method contemplated by this invention, 
the adhesive of this invention, comprising a formaldehyde condensation 
resin and a minor amount of surface-active agent, is substituted for the 
adhesive of the prior art. Other changes in the process comprise only 
minor adjustments of process variables to optimize the process, and are 
routine matters within the skill of the ordinary worker. 
In a particular embodiment, this invention contemplates a manufacturing 
process for plywood comprising southern pine veneers in which the veneer 
may have a moisture content from 0-25%, preferably from 5-15%. The 
adhesive of this invention permits use of veneers with a range of moisture 
content in contrast with the prior art adhesives which are restricted to 
moisture levels of either less than 7%, or alternatively between 7 and 
12%. A modified plywood adhesive composition comprising a formaldehyde 
condensation resin and a surface active agent (surfactant) is applied to 
the veneers, which are formed together and pressed at a temperature of 
from about 285.degree. to 350.degree. F. for a pressed-cycle time of from 
about 2.5 to 4.5 minutes for 3-ply 1/2-inch construction. Press times will 
vary for other constructions.

The adhesive compositions, and methods for using them, provided by this 
invention are further described in the subsequent Examples which are 
provided only to examplify the practice of the invention. They are not 
intended to limit the scope of the invention as disclosed herein. 
EXAMPLE 1: 
This example shows the effects of polyvinyl acetate and cetyl alcohol in 
wet veneer gluing. 
The base adhesive described in Table 3a was modified by the addition of 
polyvinyl acetate, cetyl alcohol, or both, as described in Table 3b. The 
adhesive was applied to three-ply, 1/2" southern pine plywood panels at 
glue spreads of 75-81 lbs/MDGL, on 15.3%-15.8% average moisture content 
veneers. The plywood panels then stood for 15-17 minutes before four 
minutes pre-press at 150 psi. The total assembly time was 60-62 minutes. 
The hot press cycle included one minute pre-cure and four minutes hot 
press at 200 psi 300.degree. F. and was followed by 15 hours in hot stack. 
The wood failure data in Table 3b indicates poor bonding of the high 
moisture veneers with control adhesive. Addition of either polyvinyl 
acetate or cetyl alcohol results in substantial improvement, and most 
significantly, a combination of both adhesives results in further 
improvement over either additive alone. 
TABLE 3 
______________________________________ 
Effects of Polyvinyl Acetate and Cetyl Alcohol 
in Wet Veneer Gluing 
______________________________________ 
(a) Adhesive Composition 
______________________________________ 
9.33% water 
8.77% 5% borax 
64.36% resin* 
4.81% Phenofil 
4.67% Plybond wheat flour 
2.12% 50% NaOH 
5.94% curing agent 
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% Wood 
(b) Results Failure by V-P Test 
______________________________________ 
1. Control. 54.3 
2. PVA. 1% Polyvac 16 emulsion 
67.5 
is added to resin. 
3. Cetyl Alcohol. 0.45% cetyl 
66.5 
alcohol is added to curing agent 
4. PVA and Cetyl Alcohol. Resin 
76.3 
is same as in 2; curing agent is 
same as in 3. 
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*Resin is a 2.2 F/P mole ratio phenolic resin with 43.0% solids and a 
viscosity of 628 cps. 
EXAMPLE 2: 
This example shows the results of varying dose levels of cetyl alcohol. 
Cetyl alcohol is added, at the dose levels indicated in Table 4b, to the 
phenolic plywood adhesive described in Table 4a. Three-ply, 1/2" southern 
pine plywood panels were prepared using these adhesives at 81-88 
pounds/MDGL on veneers of the moisture content indicated in Table 4. 
Plywood from the low moisture veneers stood for five minutes before 
pre-press while plywood from 7.1% and 10% moisture content veneers stood 
for ten minutes before pre-press. The press cycle included four minutes 
pre-press at 150 psi, and 3.2 minutes hot press at 200 psi, 330.degree. F. 
The hot stack hold was for 16 hours. Total assembly times (TAT) for each 
veneer are also shown in Table 4. 
Cetyl alcohol provided a benefit in increased percent wood failure levels 
for each type of plywood prepared. However, the benefits of cetyl alcohol 
are much more apparent in the high moisture veneer plywoods. It is also 
apparent from Table 4 that the benefits of cetyl alcohol addition are 
achieved by a small addition of the compound, since the highest wood 
failure levels occurred where the adhesive contained from 0.03-0.06% cetyl 
alcohol. Further addition of cetyl alcohol above this level results in 
less benefit. 
TABLE 4 
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Gluing Results with Several Levels of Cetyl Alcohol 
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(a) Adhesive Composition 
Parts by Weight 
______________________________________ 
GP-553 resin* 67.00 
Water 14.68 
Co-Cob (Dehyco Co. extender) 
9.16 
Plybond wheat flour 5.50 
50% NaOH 3.66 
100.00 
______________________________________ 
(b) Results % Wood Failure by V-P Test 
% Cetyl Alcohol: 0 0.03 0.06 0.09 
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Production Conditions 
20 min. TAT, 4.8% 83.0 89.5 92.3 94.0 
veneer m.c. 
20 min. TAT, 10.0% 
63.8 98.8 98.0 63.3 
veneer m.c. 
60 min. TAT, 4.8% 93.8 97.3 96.3 91.3 
veneer m.c. 
60 min. TAT, 10.0% 
85.0 99.0 97.8 87.0 
veneer m.c. 
120 min. TAT, 7.1% 
97.0 99.3 99.0 97.5 
veneer m.c. 
Average 84.5 96.8 96.7 86.6 
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*Cetyl alcohol, when used, was predissolved in methanol and added to the 
GP553 resin using a high shear mixer. GP553 is a southern pine plywood 
resin marketed by GeorgiaPacific Resins, Inc. 
EXAMPLE 3: 
This example shows that cetyl alcohol addition in beneficial for adhesives 
prepared from a mixture of phenolic and melamine condensation resins. 
Melamine-formaldehyde and phenol-formaldehyde condensation resins were 
combined with extenders, water, and modifiers to produce adhesive 
compositions that differed only in the Adhesive A contained no cetyl 
alcohol and Adhesive B contained 0.017% cetyl alcohol. These adhesives 
were used in the preparation of three-ply, 3/8" southern pine plywood with 
4-6% veneer moisture content, 78-86 #/MDGL spread, a ten-minute stand time 
before prepress, a four-minute prepress at 150 psi, a one-minute precure, 
a 21/2-minute hot-press at 33.degree. F., and a 16-hour hot stack prior to 
testing. The APA vacuum-pressure test results are given in Table 5. 
TABLE 5 
______________________________________ 
% Wood Failure by V-P Test 
Adhesive A 
Adhesive B 
(control) 
(+ cetyl alcohol) 
______________________________________ 
30-minute total assembly 
88.3 96.8 
60-minute total assembly 
86.5 94.5 
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