Note: Descriptions are shown in the official language in which they were submitted.
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ADHESIVES CONTAINING EXOTHERMIC CROSSLINKER A~ND ALUM~A
~;'IELD OF l~E INV~I-ION
The invention relates to phenol-formaldehyde resin formulations, particularly those
5 used in adhesive compositions for structural wood products, having a shortened curing time
and a faster temperature rise rate compared to conventional adhesives.
BAcKGRouND OF T~E INvENnoN
Phenol-formaldehyde resins are conventionally used as adhesives in composite wood
products and particularly for laminated veneer lumber (LVL) and plywood. The LVL and
10 plywood products are typically prepared from an assembly of layers of 0.125 inch thick
(3.2 mm) sheets of wood (referred to in the art as "plies" or "veneersn) with a resinous
adhesive between each layer. A platen applies col~pressive forces and heat to the product
assembly as the resin cures in a ~hot press~ step. LVL products exhibit a thickness of
generally 1.5 - 3 inches (38-76 mm) and are made of 13-22 wood plies having parallel
lS grain patterns. LVL are widely used for beams and joist members. Plywood is made of
up to 7 wood plies with perpendicular grain patterns and thicknesses up to about I inch (25
mm).
The productivity of a manufacturing facility for LVL and plywood products and
other wood composite products using the same adhesives are directly tied to the curing rate
20 of the resin in the hot press. Shorter curing times are sought to improve the productivity
of the manufacturing process. Higher platen temperatures could undoubtedly increase the
rate of temperature increase across the glue lines of the product and reduce the required
curing time. At high temperatures, however, the wood plies are subject to excessive drying
which adversely affects the physical properties of the final product. Lower temperature
25 platen temperatures could be used, but the curing times required becomes uneconomical.
Curing conditions with a target glue line temperature of just about 212 F (100 C)
represent a generally useful balance point between curing rate and avoidance of excessive
drying of the wood.
It would be desirable to have an adhesive and manufacluring process that wou1d
30 exhibit reduced curing times with faster heat transfer through and between glue layers to
increase further manufacturing productivity. Reduced curing times ~ould translate directly
2 ~ 6
into increased plant productivity, Increased heat transfer rates would help to reduce
lo~alized heating in the adhesive and around the wood to generate a sufficient temperature
gradient to overcome the ins~ tin~ effects of the wood materials. The result would be an
increase in manufacturing productivity with the same or better product performance
5 characteristics.
SUMMARY OF TÆ INVEN~ON
It is an objective of the invention to provide a resole phenol-formaldehyde adhesive
composition suitable for composite wood products and a method for their manufacture that
exhibits a rapid curing rate. Plywood and LVL products are particularly well suited to
10 such improvements.
It is another objective of the invention to provide a resole phenol-formaldehydeadhesive composition suitable for composite wood products generally and a method for
their manufacture that exhibits higher rates of heat transfer through and between glue lines
compared to conventional resin forrnulations.
It is a further objective of the invention to provide composite wood products and
processes for their manufacture that exhibit an overall reduction in the time required for
curing the phenol-formaldehyde adhesion resin and improve the heat transfer characteristics
within the adhesive-containing areas.
In accordance with these and other objectives of the invention which will become20 apparent from the description of the invention herein, adhesive compositions, composite
wood products, and their process of manufacture all relate to phenol-formaldehyde (PF)
resin formulations containing a PF resin prepared under a~kaline conditions at aformaldehyde to phenol mole ratio within the range from about 2:1 to about 2.5:1; a
crosslinking agent that can exothermically form crosslinking bonds with reactive sites in
25 the phenolic resin; and finely divided aluminum oxide within the range from about 0.1 wt%
to about 5 wt% based on the formulation weight.
The present invention provides an adhesive composition, composite wood product,
and process of manufacture that is characterized by adhesives having a reduced curing time
and better heat transfer than adhesives currently in use. The productivity of the
30 manufacturing process is thus improved.
A~ty D~t. No 35~11 2
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BR~ DESC~ON QF ~E DRAWINGS
Figures 1 and 2 are graphs depicting the increased temperature levels and shortened
press times attained by examples of the invention for LVL samples.
DhTA~ DESCRIFIION
The present invention relates to an adhesive formulation containing a phenol-
formaldehyde resole resin, a cros~linking agent for exothermically forming crosslink bonds
within said resin and for acting as an in situ source of heat, and a finely divided aluminum
oxide. These adhesive formulations are useful for bonding wood composites in general
with LVL and plywood products being particularly well suited to such adhesive
improvements.
The phenols employed in the formation of the phenolic resins generally include any
phenol which has heretofore been employed in the formation of phenolic resole resins and
which are not substituted at either the two ortho positions or at one ortho position and the
para position, such unsubstituted positions being necessary for the desired polymerization
reactions to occur. Phenols substituted in these positions may be used in lesser quantities
as is known in the art to control molecular weight by a chain capping reaction. Any one,
all, or none of the remaining carbon atoms of the phenol ring can be substituted in a
conventional fashion. The nature of these substituents can vary widely, and it is only
necessary that the substituent not interfere in the polymerization of the aldehyde with the
phenol at the ortho and/or para positions thereof (except for molecular weight control as
noted above).
Substituted phenols employed in the formation of the phenolic resins include alkyl
substituted phenols, aryl substituted phenols, cycloalkyl substituted phenols, alkenyl-
substituted phenols, alkoxy substituted phenols, aryloxy substituted phenols, and halogen
substituted phenols, the foregoing substituents possibly containing from 1-26, and
preferably from 1-9, carbon atoms.
Specific examples of suitable phenols for preparing the resole resin compositionuseful in the present invention include, inter alia: phenol; o-cresol; m-cresol; p-cresol; 3,5-
xylenol; 3,4-xylenol; 3,4,5-trimethylphenol; 3-ethyl phenol; 3,5-diethyl phenol; p-butyl
I phenol; 3,5-dibutyl phenol; p-amyl phenol; p-cyclohexyl phenol; p-octyl phenol; 3,5-
I I A~ Dln No. 35~1`1 3
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dicyclohexyl phenol; p-phenyl phenol; p-crotyl phenol; 3,5-dimethoxy phenol; 3,4,5-
trimethoxy phenol; p-ethoxy phenol; p-buto~y phenol; 3-methyl-4-methoxy phenol; and p-
phenoxy phenol. Ordinary phenol is preferred for most applications with up to about 5
wt% of a phenolic compound other than phenol, e.g., 3,5-xylenol and/or m-cresol, being
5 present as part of the phenolic component of the resin.
The aldehydes reacted with the phenol component similarly can include any of thealdehydes heretofore employed in the formation of the phenolic resole resins for the present
invention. In general, the aldehydes for reaction with the phenolic component have the
formula R'CHO wherein R' is a hydrogen or a hydrocarbon radical of 1-8 carbon atoms.
10 Suitable aldehydes include, inter alia: formaldehyde, acetaldehyde, propionaldehyde,
furfuraldehyde, and benzaldehyde. Ordinary formaldehyde is preferred for most
applications, and the formaldehyde can be supplied in any of its commonly available forms,
e.g., formalin solutions or paraforrnaldehyde.
For the present invention, the formaldehyde to phenol mole ratio of the phenolicresin is preferably adjusted to within the range from about 2:1 to about 2.5:1, preferably
a ratio within the range from about 2.2:1 to about 2.4:1.
An exothermic crosslinking agent for e~cothermic crosslinking of the phenolic resin
can be formed in-situ within the resin or added separately. Preferred crosslinking agents
include hexamethylenetetramine and paraformaldehyde. For generating
20 hexamethylenetetramine in situ when formaldehyde is used as the aldehyde reactant,
arnmonium hydro~ide can be added to the phenolic resin to scavenge free, unreacted
formaldehyde and thereby form hexamethylenetetramine from the reaction of ammonium
hydroxide and the formaldehyde. Hexamethylenetetramine as well as paraformaldehyde
can also be added as discrete components aRer formation of the phenolic resin from the
25 aldehyde and phenol. Regardless of how generated or added, the exothermic crosslinking
agent content should fall within the range from about 0.5-5 wt% based on total weight of
the adhesive to provide an exothermic crosslinking reaction with reactive phenol sites in
the phenolic resin.
The aluminum oxide particles added to the adhesive formulation can be in a dry or
30 hydrated form and should be evenly distributed throughout the final adhesive. Preferably,
A~v. Dh. .~lo. 35~1~ 4
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the aluminum oxide particles have a median particle size within the range from about 0.1
~rn to about 5 ~m with a median within the range of 0.5 to 3 ~m being particularly
preferred.
While not wishing to be bound by a particular theory, the aluminum oxide is
5 believed to serve two roles in adhesive formulations according to the present invention.
One role is for enh~ncing the heat transfer characteristics of the adhesive so the applied
heat from the pre-press and hot press as well as any heat generated by the crosslinking
agent within the adhesive is more evenly distributed throughout the glue layer. The
improved heat transfer can be attributed to the relatively higher heat transfer coefficient of
aluminum oxide (20 BTU/hr-ft2- F/ft) which is much higher than the coefficient of 0.8
BTU/hr-ft~- F/ft for wood. In addition, applied heat during the curing cycle will kansfer
throughout the glue layer more quickly. The other function of the alumina is that of a
catalyst for the exotherrnic reaction of the crosslinking agent with reactive phenol sites.
The exothermic heat from this crosslinking reaction is a source of in situ generated heat
15 that assists in curing the resin adhesive.
If desired, any of the conventional phenol-formaldehyde additives and extenders
norrnally used in phenolic resin-based adhesives can be included in adhesives according to
the present invention. Such additives include, for example, cellulosic flours (e.g., wheat
flour and corn flour~, clay, lignocellulosic flours (e.g., pecan shell flour, walnut shell flour,
20 and corn cob flour), and soda ash. Because the adhesive formulations according to the
invention are based on resole resins, sodium hydroxide and soda ash can be used to assist
in curing the resin as well as promoting heat transfer through the adhesive forrnulation.
Sodium hydroxide is a particu]arly useful additive for swelling lignocellulosic fibers and
for ~Csi~tin~ in forming a good adhesive bond. Preferably 1-2% by weight NaOH is used
25 in the adhesive formulation and is typically added as a 50% aqueous solution.Bondable wood that is useful for making composite wood products according to theinvention may be in the form of wood strips, veneer, wood chips, meal, sawdust, and
flour, as well as leached or chemically treated solid wood having substAntially unimpaired
wood cellulose structure characteristics. More specifically, the adhesive resin formulations
30 ~ of the invention are regarded as most useful in connection with the production of LVL and
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plywood. The resin adhesives are expected to be useful, however, for the production of
particleboard and other wood products.
In the preparation of LVL and plywood products, adhesive forrnulations accordingto the invention can be applied to the veneer layers by any of the conventional application
S methods. Curtain coating is particularly pl~fe.~
Assembly of the LVL and plywood products is straightforward, and conventional
procedures can be used. At least one side of a first planar sheet of wood is c~ated with
a phenol-formaldehyde resin adhesive made by a process which comprises contacting
formaldehyde with a phenol under alkaline conditions and a formaldehyde to phenol mole
ratio within the range from about 2:1 to about 2.5:1 to form the resin, mixing the resin
with: (a) a crosslinhn~ agent to forrn crosslinkin~ bonds among reactive sites in the
phenolic resin with the attendant generation of reaction heat, and (b) from about 0.1 wt%
to about 10 wt% finely divided aluminum oxide; at least one side of a second planar sheet
of wood is coated with the same phenol-formaldehyde resin adhesive composition coated
15 on at least one side of the first sheet; and at least the coated side of the first sheet is
pressed against at least the coated side of the second sheet while applying heat to the
adhesive on each sheet at conditions sufficient to cure the adhesive between the first and
second sheets. Sufficient plies of wood sheets are used in a quantity and with a quality
sufficient to make plywood or laminated veneer lumber products with each layer being
20 adhered to the adjacent layers with adhesives made according to the present invention.
The foll~wing e~amples are intended to facilitate an understanding of the present
invention without limitation on the scope of the attached claims.
EXAMPLES 1-6
Several 16-ply laminated veneer lumber (LVL) billets were constructed using a
25 commercial adhesive as a control in comparison to adhesives according to the invention.
The plies used to prepare the LVL billets were a nominal 1/8 inch thick Southern Yellow
Pine at 12 inches X 12 inches in size with a 7% average moisture content. The adhesives
were applied at 32-34 gtft2 over a 10 minute layup period. The prepress was at lS0 psi
for 4 minutes. The hot press was at 175-200 psi for 21 minutes at 330 F (166 C), one
30 billet at a time. Three billets were tested for each adhesive formulation.
D~ o. 35414 6
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The billets were monitored for the time under hot pressing conditions required to
reach a glueline temperature of 212 F (100 C) as measured by a probe located in the glue
layer between the 8th and 9th plies of each billet. This target temperature is required to
thermally cure the resin and form a durable bond.
S Each of the tested adhesives was formulated to have 44% total dry solids, 33 æ resin
solids, 1.4% NaOH, 0.5% soda ash, and a viscosity within the range of 1200-1800 cps.
Alumina was added to adhesives of the invention in an amount sufficient to replace a
co,lts~onding amount of extender and filler from the control adhesive formulation.
Hexamethylenetetramine (HMTA) in an amount of 1-2% by weight of the total adhesive
weight was added into the resin after the reaction between the formaldehyde and the phenol
before formulation into the adhesive. The commercial (control) adhesive had the
composition shown in Table 1.
Table 1
Ingredient Example 1
Wt. %
Water 13.9
Wheat flour 6.0
Minugel clay 1.0
Pe~an shell flour 6.5
50% NaOH 2.34
Soda ash 0.5
Resin (43~ solids) remainder
Aluminum oxide ----
A series of adhesive formulations were prepared to compare the effects of varied25 levels of hexamethylenetetramine as a crosslinking agent and metal-based materials as heat
transfer agent. The composition differences among the examples are shown in Table 2.
A~. Dl~t. No. 35414 7
Table 2
Ex. F/P Metal Hexamethylene- Avg. Press
molarModifier tetrarnine Time to 212 F
Ratio (wt%) (wt%) (min.)
1 2.0 ~ l 8.5
(control)
2 2.4 1% Al2O3 (2 wt% NH,OH 16.6
added to resin)
3 2.0 1% Al2O3 0.7% 18.3
4 2.2 1% Al2O3 1% 17.5
2.2 1% Al2O3 296 17.3
6 2.41% Al(0H)3 (2 wt% NH~OH 18.7
added to resin)
7 2.0 196 Al203 ~~~ 18.3
The rate of curing as measured by the rise in glueline temperature versus time is
15 presented in the attached figures 1 and 2 for 16-ply LVL. Temperature readings were
measured by a probe located between the 8th and 9th plies. Figure 1 is a comparison of
the curing rates for examples 1, 4, and 5. The adhesive containing hexamethylenetetramine
started out at a lower temperature rate than the control adhesive, but began to exceed the
glueline ~,~peldture within about 6 minutes. Overall reduction in the press time is in the
20 range of 5~6.
Figure 2 compares examples 1, 2, 6, and a control adhesive with a resole resin
having a formaldehyde to phenol mole ratio of 2.4. Example 6 containing 1% aluminum
hydroxide and a crosslinking agent generated in situ by the reaction between the ammonium
hydroxide and free formaldehyde did not increase the curing rate or reduce the overall
25 press time. Increasing the F/P ratio of the control resin did reduce the press time.
Example 2 with a 2.4 F/P ratio, crosslinking agent generated in situ by the reaction
A~y. D~t. No 3S~114 8
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between the ammonium hydroxide and free formaldehyde, and 1~ alumina reduced thepress time about 10%.
Although the present examples are intended for illustration purposes only, the results
of the examples shows that the present invention does result in an improvement in the art.
A~. Dh. No. 35414 9
