Patent Description:
Plywood is a sheet material manufactured from thin layers or "plies" of wood veneer that are glued together. It is an engineered wood from the family of wood-based panel which includes fiberboard and particle board (chipboard). Currently, artificial plywood is an important composite material with much wider application and much better performance than natural wood.

Adhesives are used to glue wood layers together to manufacture plywood. The majority of adhesives used for plywood are formaldehyde-based curing formulations, such as urea formaldehyde (UF) resin, and phenol formaldehyde (PF) resin. Urea formaldehyde (UF) and phenol formaldehyde (PF) take up over <NUM> % total market share. Although several formaldehyde-free curing formulations (such as polyisocyanate based on diphenylmethane-diisocyanate (pMDI)) exist, they suffer from significant drawbacks as follows:.

Recently some biomaterial-based adhesive systems such as flour-based or soybean-based adhesives were reported to be used for plywood, but they usually show weakness in workability and water resistance.

Governmental regulations are pushing towards even lower allowable formaldehyde emission levels for the wood-based panel industry (E0 <=<NUM>/L vs. E1 <=<NUM>/L, in which E0 and E1 both refer to the formaldehyde emission standard in China), and the public is becoming more and more aware of health hazards posed by formaldehyde such as irritation, allergy and even cancer and malformation. As such environmental concerns keep bringing pressure to upgrade of adhesives for wood. As an even worse scenario, under floor heating is a growing market in China, and the best option for under floor heating is plywood floor, but to heat the plywood floor will accelerate the formaldehyde release speed and boost the risk.

Currently in US, there is a certification for no-added formaldehyde (NAF) product manufacturers by the California Air Resources Board (CARB). In China, an industry association standard of biomaterial-based composite panels and final products of no-added formaldehyde has been recently launched (T/CNFPIA <NUM>-<NUM>).

<CIT>, <CIT>, <CIT> and <CIT> relate to further adhesive compositions based on acrylic polymers and eventually isocyanate cross-linking agents.

Therefore, there is a strong need in the wood-based panel industry for alternative formaldehyde free adhesive compositions for plywood that have desirable performances such as adhesion performance and good workability.

The present disclosure provides a novel formaldehyde free adhesive composition for plywood that has desirable performances, such as long open time, good adhesion performance and good workability.

In a first aspect, the present disclosure provides a formaldehyde free adhesive composition comprising:.

wherein said acrylic polymer has at least one structural unit of one or more ethylenically unsaturated monomers carrying at least one functional group, and said acrylic polymer has a glass transition temperature of -<NUM> to <NUM>.

In a second aspect, the present disclosure provides a method for producing a plywood, comprising:.

In a third aspect, the present disclosure provides a plywood obtained from a formaldehyde free adhesive composition according to the present disclosure.

As disclosed herein, "and/or" means "and, or as an alternative". All ranges include endpoints unless otherwise indicated.

As disclosed herein, the term "composition", "formulation" or "mixture" refers to a physical blend of different components, which is obtained by mixing simply different components by a physical means.

As disclosed herein, the term "formaldehyde free" means that the composition has no added formaldehyde and/or no added formaldehyde generators.

As disclosed herein, the term "glass transition temperature" or "Tg" is determined by differential scanning calorimetry (DSC).

As disclosed herein, the term "alkyl" or "alkoxy" refers to an alkyl or alkoxy having <NUM> to <NUM> carbon atoms, preferably <NUM>-<NUM> carbon atoms, more preferably, <NUM>-<NUM> carbon atoms.

The use of the term "(meth)" followed by another term such as acrylate in the present invention, refers to acrylate, methacrylate and mixtures thereof.

"Acrylic" means (meth)acrylic acid, (meth)alkyl acrylate, (meth)acrylamide, (meth)acrylonitrile and modified forms thereof, such as (meth)hydroxyalkyl acrylate.

Adhesion strength of a multilayer structure refers to interlayer adhesion strength between any two adjacent layers of the multilayer structure.

The aqueous emulsion of acrylic polymer can be prepared through free radical emulsion or suspension addition polymerization or by dispersion of a pre-formed polymer under shear into an aqueous medium. Monomers suitable for the preparation of the acrylic polymer include, but are not limited to, (meth)acrylic acids and (meth)acrylates, such as alkyl (meth)acrylates. Examples of alkyl (meth)acrylates are, but not limited to, methyl acrylate, ethyl acrylate, butyl acrylate, glycidyl methacrylate, <NUM>-ethylhexyl acrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate, and <NUM>-ethylhexyl methacrylate, and combinations thereof.

The acrylic polymer in the present disclosure may comprise structural units of one or more ethylenically unsaturated monomers carrying at least one hetero functional group. The hetero functional group may be selected from the group consisting of ureido, nitrile, alkoxysilane (preferably hydrolyzable alkoxysilane), or phosphorous group. Preferably, the hetero functional group may be selected from the group consisting of ureido, and nitrile. Suitable ureido functional monomer includes, for example, ureido group containing (meth)acrylic acid alkyl esters. Examples of suitable ureido monomers are illustrated below:
<CHM>
<CHM>
or mixtures thereof. Representative functional monomer like Norsocryl <NUM> can be obtained from Arkema. Suitable alkoxysilane functional monomer includes, for example, vinyltrialkoxysilanes such as vinyltrimethoxysilane; alkylvinyldialkoxysilanes; (meth)acryloxyalkyltrialkoxysilanes such as (meth)acryloxyethyltrimethoxysilane and (meth)acryloxypropyltrimethoxysilane; derivatives thereof, and combinations thereof. Preferred alkoxysilane functional monomer is Silquest A-<NUM> available from Momentive. Suitable nitrile functional monomer includes, for example, (alkyl)acrylonitrile, such as (meth)acrylonitrile. Suitable phosphorous functional monomer includes, for example, phosphorous-containing (meth)acrylates, such as phosphoethyl (meth)acrylate, phosphopropyl (meth)acrylate, phosphobutyl (meth)acrylate, salts thereof, and mixtures thereof; CH<NUM>=C(R)-C(O)-O-(R<NUM>O)n-P(O)(OH)<NUM>, wherein R=H or CH<NUM>, R<NUM>=alkyl, and n=<NUM>-<NUM>, such as SIPOMER PAM-<NUM>, SIPOMER PAM-<NUM>, and SIPOMER PAM-<NUM> all available from Solvay; phosphoalkoxy (meth)acrylates such as phospho ethylene glycol (meth)acrylate, phospho di-ethylene glycol (meth)acrylate, phospho tri-ethylene glycol (meth)acrylate, phospho propylene glycol (meth)acrylate, phospho di-propylene glycol (meth)acrylate, phospho tri-propylene glycol (meth)acrylate, salts thereof, and mixtures thereof.

The acrylic polymer may comprise, based on the weight of the polymer, from <NUM>% to <NUM>% by weight, from <NUM>% to <NUM>% by weight, from <NUM>% to <NUM>% by weight, or from <NUM>% to <NUM>% by weight, of structural units of one or more ethylenically unsaturated monomers carrying at least one hetero functional group.

The acrylic polymer may further comprise structural units of one or more styrene monomers. The styrene monomers may include, for example, styrene, substituted styrene, or mixtures thereof. The substituted styrene may include, for example, benzyl acrylate, <NUM>-phenoxyethyl acrylate, butylstyrene, methylstyrene, p-methoxystyrene, or mixtures thereof. Preferred styrene monomer is styrene. The polymer may comprise, by weight of the polymer, <NUM>% or more, <NUM>% or more, <NUM>% or more, <NUM>% or more, <NUM>% or more, <NUM>% or more, or even <NUM>% or more, and at the same time, <NUM>% or less, <NUM>% or less, <NUM>% or less, <NUM>% or less, or even <NUM>% or less, of structural unit(s) of the styrene monomer(s).

The acrylic polymer in the present disclosure is free of structural units of hydroxy-containing monomers.

In one embodiment, the aqueous emulsion of acrylic polymer is PRIMAL™ EC4642, PRIMAL™ EC4811, PRIMAL™ EC2848ER, PRIMAL™ AC 261P, available from the Dow Chemical Company.

The acrylic polymer in the present disclosure may have a weight average molecular weight of from <NUM>,<NUM> to <NUM>,<NUM>,<NUM>, from <NUM>,<NUM> to <NUM>,<NUM>, or from <NUM>,<NUM> to <NUM>,<NUM>. The weight average molecular weight may be measured by gel permeation chromatography (GPC) calibrated by the polystyrene standard.

The acrylic polymer in the present disclosure has a glass transition temperature of -<NUM> to <NUM>, preferably, -<NUM> to -<NUM>, more preferably -<NUM> to -<NUM>. Tg higher than <NUM> will not show enough initial adhesion while Tg lower than -<NUM> will not pass adhesion strength test. Nevertheless, other acrylic polymer emulsions outside this Tg range can also work if mixed with acrylic polymer emulsion within this Tg range.

The pH of the acrylic polymer in the present disclosure has a pH no higher than <NUM>.

In the adhesive composition of the present disclosure, the aqueous emulsion of acrylic polymer comprises <NUM>-<NUM>%, preferably, <NUM>-<NUM>%, more preferably, <NUM>-<NUM>% by weight of the composition, based on the solids of the composition.

The_aliphatic isocyanate cross-linker(s) is aliphatic hydrophilically modified cross-linker(s) with at least two isocyanate functionality. Preferably, the aliphatic hydrophilically modified cross-linker(s) with at least two isocyanate functionality is a hydrophilically modified HDI prepolymer, more preferably, the aliphatic isocyanate cross-linker(s) is hydrophilically modified hexamethylene diisocyanate trimer. Suitable examples of the aliphatic isocyanate cross-linker(s) include Bayhydur XP2487/<NUM> from Covestro and Aquolin <NUM> from Wanhua Chemicals Co.

In the adhesive composition of the present disclosure, the isocyanate crosslinker(s) comprises <NUM>-<NUM>%, preferably, <NUM>-<NUM>%, more preferably, <NUM>-<NUM>%, more preferably, <NUM>-<NUM>% by weight of the composition, based on the solids of the composition.

The adhesive composition according to the present disclosure may further comprise a rheology modifier. The rheology modifier may include, not limited to, a non-ionic urethane polymer, cellulose, cellulose ether, polyethylene glycol, starch ether, polyvinyl alcohol, polyimide, gum, flour and mixtures thereof. The rheology modifier was preferably chosen from non-associate thickener, such as cellulose ether.

The rheology modifier may be present, based on the total solid weight of the adhesive composition, in an amount of generally from <NUM> to <NUM>% by weight, from <NUM>% to <NUM>% by weight, or from <NUM>% to <NUM>% by weight, or from <NUM>% to <NUM>% by weight.

The adhesive composition according to the present disclosure may further comprise one or more defoamers. "Defoamers" herein refer to chemical additives that reduce and hinder the formation of foam. Defoamers may be silicone-based defoamers, mineral oil-based defoamers, ethylene oxide/propylene oxide-based defoamers, or mixtures thereof. Suitable commercially available defoamers include, for example, TEGO Airex <NUM> W and TEGO Foamex <NUM> polyether siloxane copolymer emulsions both available from TEGO, BYK-<NUM> silicone deformer available from BYK, NOPCO NXZ deformer available from NOPCO or mixtures thereof. The defoamer may be present, by weight of the total solid of the adhesive composition, in an amount of generally from <NUM> to <NUM>%, from <NUM>% to <NUM>%, or from <NUM>% to <NUM>%, or from <NUM>% to <NUM>%.

For some applications, an adhesive composition having a high viscosity (e. > <NUM>, <NUM> cP) and high solid content (e. > <NUM>%) is required. In these cases, to offer enough pot-life, loading of NCO cross-linker will be restricted, and silane was used as additive to compensate for NCO cross-linker, thus delivering both sufficient pot-life and adhesion performance. The silane according to the present disclosure may be epoxy-functional silane.

The epoxy functional silane compound useful in the present invention is typically a saturated alkoxylated silane having an epoxy group. The epoxy functional silane compound may have at least one hydrolysable silane group. A preferred epoxy functional silane compound has the general formula (I):
<CHM>
where each R<NUM> independently represents an alkyl group having one to <NUM> carbon atoms; each OR<NUM> group independently represents an alkoxy group having one to <NUM> carbon atoms including, for example, methoxy, ethoxy, or a combination thereof; R<NUM> represents a bivalent organic group having a molecular weight of <NUM> or less, preferably, R<NUM> is a C<NUM>-C<NUM>, C<NUM>-C<NUM>, or C<NUM>-C<NUM> alkylene group; R<NUM> represents a hydrogen atom or an alkyl, aryl, or aralkyl group having one to <NUM> carbon atoms; and q is one, <NUM> or <NUM>. Examples of suitable epoxy functional silane compounds include <NUM>-glycidyloxypropyl trimethoxysilane, <NUM>-glycidyloxypropyl triethoxysilane, <NUM>-glycidyloxypropyl methyldiethoxysilane, <NUM>-glycidyloxypropyl methyldimethoxysilane, or mixtures thereof. Commercially available epoxy functional silane compounds may include Silquest A-<NUM> gamma-glycidoxypropyltrimethoxysilane from Momentive Performance Materials Inc.

The epoxy functional silanes useful in the present disclosure may be present in a combined amount of zero or more, <NUM>% or more, <NUM>% or more, <NUM>% or more, <NUM>% or more, <NUM>% or more, <NUM>% or more, or even <NUM>% or more, and at the same time, <NUM>% or less, <NUM>% or less, <NUM>% or less, <NUM>% or less, <NUM>% or less, <NUM>% or less, <NUM>% or less, <NUM>% or less, <NUM>% or less, or even <NUM>% or less, by weight of the total solid of the adhesive composition.

The adhesive composition according to the present disclosure may further comprise one or more fillers. The fillers may include, but not limited to, calcium carbonate, silica, silicate, gypsum, pulp, wood powder, flour powder and mixtures thereof, preferably CaCO<NUM>. The filler may be present, by weight of the total solid of the adhesive composition, in an amount of generally from <NUM> to <NUM>%, from <NUM>% to <NUM>%, or from <NUM>% to <NUM>%, or from <NUM> to <NUM>%.

In addition to the components described above, the adhesive composition of the present invention may further comprise any one or combination of the following additives: dispersants, buffers, neutralizers, humectants, mildewcides, biocides, colorants, flowing agents, anti-oxidants, plasticizers, leveling agents, thixotropic agents, adhesion promoters, and grind vehicles. In one embodiment, the additive is selected from the group consisting of silane and dispersants. When present, these additives may be present in a combined amount of from <NUM> to <NUM>% by weight or from <NUM>% to <NUM>% by weight, or from <NUM> to <NUM>% by weight, based on the total solid weight of the adhesive composition.

The adhesive composition in the present disclosure is free of metal oxide cross-linkers, such as ZnO cross-linker.

The adhesive composition is aqueous, and is preferably organic solvent free, that is, the adhesive composition comprises less than <NUM>%, preferably less than <NUM>%, and more preferably less than <NUM>%, or more preferably <NUM>%, by dry weight based on total dry weight of the adhesive composition, an organic solvent.

The organic solvent is a compound that is a liquid at <NUM>, and has a boiling point of below <NUM>.

<NUM> grams of emulsion were placed in a plastic dish and allowed to dry for <NUM> hours at room temperature. The resultant film was then subjected to <NUM> for <NUM> hours to further dry the film. A small piece of the film was cut off and placed in a TA Instruments standard aluminum hermetic pan which was sealed. The sample was subjected to two cycles of -<NUM> to <NUM> at a heating ramp rate of <NUM>/min. The glass transition temperature for the polymer is measured at the midpoint of the inflection using the half-height method.

Wood adhesive performance evaluations are carried out according to China national standard (GB <NUM>-<NUM>):.

Pot-life reflects the viscosity stability of wood binder, which is very important for the application for wood adhesive. Due to the highly reactive nature of isocyanate, the pot-life will strongly impact the feasibility of binder formulations. The evaluation procedures are listed as follows:.

Adhesive compositions of CE1-<NUM> and IE1-<NUM> were prepared according to the followings preparation procedures:.

II) Plywood board sample preparation procedures:.

The percentages within the parentheses are the solid content of the component based on the total solid weight of the adhesive composition.

The results in the following Table <NUM> gives the detailed adhesion strength data of four samples for example formulations.

The result in the following Table <NUM> gives the pot-life data for CE6-<NUM> and IE1-<NUM>.

Compared with traditional formaldehyde based adhesives, our disclosure is a formaldehyde-free package with comparable adhesion performance proved by experimental data. Furthermore, compared with existing formaldehyde-free techniques, our disclosure is a systematic package that deliver good adaptability to common manufacturing line and application methods (referred to as "workability" in the following content), which was originally designed for formaldehyde adhesives. These competencies were not covered in existing patents, including good water resistance and high adhesion strength, sufficient pot-life, good workability and the initial adhesion after room temperature pressing that was required in manufacturing cycles.

As demonstrated in Table <NUM>, pure acrylic emulsion did not pass adhesion strength test (CE-<NUM>), and acrylic polymer with relatively higher Tg did not pass either (CE-<NUM> which contains TIANBA™ <NUM>). If industrial flour was largely adopted as filler, the pot-life would be short and the performance would also be poor (CE-<NUM>).

Acrylic polymer with fit Tg can contribute to good performance in adhesion strength (IE-<NUM> to IE-<NUM>). However, in IE-<NUM> to IE-<NUM>, the acrylic polymer (EC4811and EC2848ER) possess hetero-functional group on the polymer backbone. If they were replaced by acrylic polymer without hetero-functional groups (TIANBA™ <NUM>, ELASTENE™ <NUM> and ROBOND™ PS-<NUM>) as in CE-<NUM> to CE-<NUM>, although the Tg were similar, CE-<NUM> to CE-<NUM> could not pass adhesion strength test. IE-<NUM> verified that a simple mixture of an aqueous acrylic emulsion (PRIMAL™ EC2848ER) and an isocyanate cross-linker could also work for a plywood.

Pot-life control is important for isocyanate to be adopted in wood adhesives. The pot-life will be influenced by pH of acrylic polymer emulsion, as higher pH will accelerate the reaction of isocyanate. As shown in Table <NUM>, the adoption of PRIMAL™ EC2540 with pH > <NUM> resulted in poor pot-life (CE-<NUM>). The pH of PRIMAL™ EC4811 and PRIMAL™ EC2848ER were both lower than <NUM>, which contributed to a good pot-life performance (IE-<NUM> to IE-<NUM>). Due to the highly reactive nature, aromatic isocyanates were not suitable for wood adhesive application. Therefore, PAPI <NUM> (pMDI, polymeric diphenylmethane diisocyanate) will also result in quick failure in pot-life (CE-<NUM>), while aliphatic isocyanate with milder reaction speed will help to achieve good pot-life performance (IE-1to IE-<NUM>).

Claim 1:
A formaldehyde free adhesive composition comprising:
(a) an aqueous emulsion of acrylic polymer; and
(b) an aliphatic isocyanate crosslinker(s);
wherein said acrylic polymer has at least one structural unit of one or more ethylenically unsaturated monomers carrying at least one hetero functional group, and said acrylic polymer has a glass transition temperature of -<NUM> to <NUM> as measured by differential scanning calorimetry.