CA2093026A1 - Adhesives containing exothermic crosslinker and alumina - Google Patents
Adhesives containing exothermic crosslinker and aluminaInfo
- Publication number
- CA2093026A1 CA2093026A1 CA 2093026 CA2093026A CA2093026A1 CA 2093026 A1 CA2093026 A1 CA 2093026A1 CA 2093026 CA2093026 CA 2093026 CA 2093026 A CA2093026 A CA 2093026A CA 2093026 A1 CA2093026 A1 CA 2093026A1
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- Canada
- Prior art keywords
- phenol
- formaldehyde
- wood
- adhesive
- resin
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
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Abstract
Resole resins having a formaldehyde to phenol mole ratio within the range from about 2 to about 2.5 are used in an adhesive formulation having a crosslinking agent and finely divided aluminum oxide. The crosslinking agent provides in-situ reaction heat to enhance the curing rate of the resin. The aluminum oxide appears to serve both as a catalyst for the crosslinking reactions of the crosslinking agent and as a heat transfer facilitator for distributing the exothermic heat as well as the applied heat during the hot press step.
Description
~2~3~2~
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 ,3Q~6 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 20~3~2~
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 ~3~6 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 ~ Dl~l No. 35~14 5 2~3~
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.
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 ~ ~9~ 6 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 ~3~6 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
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 ,3Q~6 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 20~3~2~
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 ~3~6 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 ~ Dl~l No. 35~14 5 2~3~
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.
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 ~ ~9~ 6 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 ~3~6 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
Claims (11)
1. An adhesive composition comprising: (a) a phenol-formaldehyde resin prepared under alkaline conditions at a formaldehyde to phenol mole ratio within the range from about 2 to about 2.5; (b) a crosslinking agent able to form crosslinking bonds with reactive sites in the phenolic resin and exothermic heat from the crosslinking reactions; and (c) finely divided aluminum oxide within the range from about 0.1 wt% to about 10 wt%.
2. A composition according to claim 1 wherein the formaldehyde to phenol mole ratio is within the range from about 2.2 to about 2.4.
3. A composition as in claim 1 wherein the crosslinking agent comprises hexamethylenetetramine.
4. A composition as in claim 3 wherein the hexamethylenetetramine is formed in situ by adding ammonium hydroxide to said resin and allowing the ammonium hydroxide to react with free formaldehyde.
5. A laminated wood product comprising:
at least a first planar sheet of wood;
at least a second planar sheet of wood; and a cured adhesive disposed between and acting to adhere the first sheet to the second sheet whereby said adhesive has been made by a process which comprises contacting formaldehyde with a phenol under alkaline conditions at a formaldehyde to phenol mole ratio within the range from about 2 to about 2.5 in the presence of a crosslinking agent able to exothermically form crosslinking bonds with reactive sites in the phenolic resin and from about 0.1 wt% to about 10 wt% finely divided aluminum oxide.
at least a first planar sheet of wood;
at least a second planar sheet of wood; and a cured adhesive disposed between and acting to adhere the first sheet to the second sheet whereby said adhesive has been made by a process which comprises contacting formaldehyde with a phenol under alkaline conditions at a formaldehyde to phenol mole ratio within the range from about 2 to about 2.5 in the presence of a crosslinking agent able to exothermically form crosslinking bonds with reactive sites in the phenolic resin and from about 0.1 wt% to about 10 wt% finely divided aluminum oxide.
6. A wood product according to claim 5 wherein the formaldehyde to phenol mole ratio is within the range from about 2.2 to about 2.4.
7. A wood product as in claim 5 wherein the crosslinking agent comprises hexamethylenetetramine.
8. A wood product as in claim 7 wherein the hexamethylenetetramine is formed in situ by adding ammonium hydroxide to said resin and allowing the ammonium hydroxide to react with free formaldehyde.
9. A wood product as in claim 5 further comprising plies of planar sheets of wood in a quantity and quality sufficient to make a plywood product.
10. A wood product as in claim 5 further comprising plies of planar sheets of wood in a quantity and quality sufficient to make a laminated veneer lumber product.
11. A process for making a laminated veneer lumber wood product, said process comprising:
coating at least one side of a first planar sheet of veneer wood 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 to about 2.5 in the presence of a crosslinking agent able to exothermically form crosslinking bonds with reactive sites on the phenol and from about 0.1 wt% to about 10 wt% finely divided aluminum oxide;
coating at least one side of a second planar sheet of veneer wood with the same phenol-formaldehyde resin adhesive composition coated on at least one side of the first veneer sheet; and pressing at least the coated side of the first veneer sheet against at least the coated side of the second veneer sheet while applying heat to the adhesive on each sheet of veneer at conditions sufficient to cure the adhesive between the first and second veneer sheets.
coating at least one side of a first planar sheet of veneer wood 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 to about 2.5 in the presence of a crosslinking agent able to exothermically form crosslinking bonds with reactive sites on the phenol and from about 0.1 wt% to about 10 wt% finely divided aluminum oxide;
coating at least one side of a second planar sheet of veneer wood with the same phenol-formaldehyde resin adhesive composition coated on at least one side of the first veneer sheet; and pressing at least the coated side of the first veneer sheet against at least the coated side of the second veneer sheet while applying heat to the adhesive on each sheet of veneer at conditions sufficient to cure the adhesive between the first and second veneer sheets.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US87548192A | 1992-04-29 | 1992-04-29 | |
| US875,481 | 1992-04-29 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2093026A1 true CA2093026A1 (en) | 1993-10-30 |
Family
ID=25365885
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2093026 Abandoned CA2093026A1 (en) | 1992-04-29 | 1993-03-31 | Adhesives containing exothermic crosslinker and alumina |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2093026A1 (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN112852361A (en) * | 2021-03-26 | 2021-05-28 | 广西南宁市完美木业有限公司 | Anticorrosive easy-demoulding building template surface adhesive and preparation method thereof |
-
1993
- 1993-03-31 CA CA 2093026 patent/CA2093026A1/en not_active Abandoned
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN112852361A (en) * | 2021-03-26 | 2021-05-28 | 广西南宁市完美木业有限公司 | Anticorrosive easy-demoulding building template surface adhesive and preparation method thereof |
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