Patent Description:
Curtain coating has been used for coating substrates, e.g., lignoceullulosic substrates such as sheets of wood, because it allows for an increase in production rates and the same time allows for a reduction of the amount of coating material per square meter of the substrate. Curtain coating provides a curtain of the coating material, and a substrate is driven through said curtain, whereby a controlled amount of coating material can be applied onto the surface of the substrate in order to obtain the desired properties of the final product using a minimal amount of coating material. This offers an advantage over other coating methods such as roller coating, wherein the substrate passes through rolls impregnated with the coating material and wherein the amount of coating material being applied onto the substrate is hard to control due to the pressure applied onto the substrate passing through the rolls.

However, not all coating formulations are suitable for curtain coating. In particular, some resin adhesive formulations fail to provide a homogeneous and continuous curtain due to inappropriate rheological properties and viscosity of the coating formulations, such as materials or formulations having pseudoplastic behavior. This can result in breaks in the curtain and defects in the coated product, e.g., resulting in portions of the substrate surface which are not provided with the coating. In order to solve such problems, the amount of coating material used for forming the curtain may be increased, which ultimately increases the amount of coating applied to the substrate, thereby eliminating one of the main advantages of curtain coating, which is the reduction of the amount of coating material that is applied to the substrate.

Phenol formaldehyde resins (also referred to as phenolic resins) are commonly used in adhesive formulations and are resins of choice for coating substrates such as lignocellulosic substrates, e.g., sheets of wood or wood veneers. In particular, phenol formaldehyde resins may be used in the preparation of coated materials such as plywood. They may also be used in the preparation of other coated materials such as veneered board or laminated veneer lumber (LVL).

Given the advantages of curtain coating there is an interest in applying phenolic resins by curtain coating.

Some documents describe phenolic resin formulations that may be applied using curtain coating.

For instance, <CIT> describes a process of bonding surfaces, such as a wood veneer to a sheet of hardboard, or to a number of wood veneers to form a plywood panel, comprising applying separately and successively to at least one of the surfaces to be bonded, first, a spray coating of a liquid hardenable resinous component, and second, a component comprising a powder hardener for the liquid resinous component and, optionally, a filler, bringing the surfaces together and allowing the composition to dry. The liquid resinous component may be a phenol formaldehyde resin and the powder component include both a filler and hardener, such as wood-flour, slate-flour, wheat-flour, starch, clay, calcium sulphate, chalk as the filler and, as the hardener, ammonium salt which may have added to it citric acid, oxalic acid, tartaric acid, aluminium sulphate, magnesium silicofluoride or the like. A curtain coater is used to apply the liquid resinous component and the powder component is applied by a vibrating screen. A slow acting hardener may be incorporated in the resinous component as well as a rapid acting hardener in the powder component. However, the methods described require applying different components to the substrate separately which adds complexity to the coating process.

<CIT> and <CIT> describe potassium-modified phenolic resins (also referred to as resol resins) with a significant improvement in cure speed without loss of flowability. These resins are described to act as though they were lower molecular weight condensation products. Reduced application rates are possible. The combination of faster cure and lower application rates is described to allow such resins to be used as effective adhesives for plywood, for example, with veneer and interior plies having a higher moisture content than was previously possible. Generally, resins according to the invention may contain from about <NUM>% to about <NUM>%, and preferably from about <NUM>% to about <NUM>% by weight, of potassium hydroxide, or more. The KOH-modified resole resins are described to be applied to the wood by conventional equipment including curtain coaters among others such as roll coaters.

<CIT> describes a liquid adhesive composition for use in adhesive applicators of the curtain-coating class, such as are used in coating plywood veneers in the manufacture of plywood, comprising: an aqueous, alkaline phenol-aldehyde resinous adhesive, a filler, and a minor proportion of <NUM>,<NUM>,<NUM>,<NUM>-tetramethyl-<NUM>-decyne-<NUM>,<NUM>-diol, or of the ethylene glycol-, propylene glycol-, and butylene glycol-ethers thereof having from <NUM> to <NUM> mols of glycol per mol of diol. The diol or diol ether serves the function of improving the curtain forming qualities of the adhesive composition. In particular, its inclusion in the composition insures curtain uniformity, i.e., the absence of curtain breaks and tears, and uniform spread on the substrate.

<CIT> describes adhesive compositions and methods of using the compositions in the production of laminated veneer lumber (LVL). The adhesive compositions comprise a thermosetting phenol-aldehyde resin having at least one of (A) a number average molecular weight (Mn) of at least about <NUM>, (B) a weight average molecular weight (Mw) of at least about <NUM>, and (C) a Z-average molecular weight (Mz) of at least about <NUM>, wherein said Mn, Mw, and Mz are measured using gel permeation chromatography (GPC), a ketone-aldehyde resin cure promoter, and optionally other components (e.g., a tack-promoter or a catalyst). The adhesive compositions minimize or eliminate the art-recognized problems of glue line dryout and steam blowout, associated with LVL manufacture from both low-moisture veneers and high-moisture veneers, respectively. Furthermore, the adhesive compositions provide fast tack-build and curing as well as ultimately good bonding characteristics. Curtain coating may be used.

<CIT> describes phenolic plywood adhesive containing lignosulfonates and a trialkyl phosphate having alkyl substituents of from <NUM> to <NUM> carbon atoms for use in a curtain coater.

Even though several phenol formaldehyde formulations have been described which may be applied by curtain coating, there is still need for a phenol formaldehyde resin formulation which allows the application of phenol formaldehyde resin adhesives by curtain coating without resulting in breaks on the curtain and without compromising the good adhesion properties of the final product.

A phenol formaldehyde resin formulation has been found which has appropriate rheological and viscosity properties which allow for the formulation of an adhesive which advantageously can be applied by curtain coating. The formulation allows for the adhesive to be applied by curtain coating in controlled amounts and to provide lower amounts of coating that can be achieved by, e.g., roller coating. Furthermore, such phenol formaldehyde resin formulations have been found to provide coated materials, such as plywood, which typically display good adhesion properties (e.g. Class III bonding quality rating according to EN <NUM>-<NUM>:<NUM> and to EN <NUM>-<NUM>: <NUM> (confirmed in <NUM>) European Standards. Such formulations have also been found to provide coated materials that have good formaldehyde emission properties (e.g. to have an E-<NUM> formaldehyde emissions rating according to EN <NUM>:<NUM>+A1:<NUM> European Standard, and/or to be Title VI compliant according to the Toxic Substances Control Act (TSCA) of the United States, and/or even to have a three star rating or four star rating according to the Japanese Industrial Standard JIS <NUM>:<NUM>).

Furthermore, phenol formaldehyde resin formulations as described herein have been found to have relatively high molecular weights which provide for relatively short curing times which are advantageous for applications, such as plywood manufacture, wherein pressing times may be reduced.

In particular, a first aspect of the invention relates to a resin formulation comprising:.

wherein the adhesive has a viscosity from <NUM> to <NUM> Pa*s and a solids content from <NUM> to <NUM> wt. % based on the total amount of adhesive.

Several additional aspects of the invention relate to.

These and other aspects of the invention are further described in the detailed description of the invention, here below.

The instant invention relates to a resin formulation comprising:.

wherein the resin formulation has a solids content from <NUM> to <NUM> wt. %, based on the total weight of resin formulation; as determined according to ISO <NUM>:<NUM> and a) a viscosity from <NUM> Pa*s to <NUM> Pa*s measured at <NUM> using a viscometer.

Phenol formaldehyde resins are commonly used in adhesive formulations and are known to a skilled person. Phenol formaldehyde resins may be typically characterized by the proportion of the different monomers conforming the resins.

Phenol formaldehyde resins used in resin formulations as described herein have a formaldehyde to phenol molar ratio from <NUM>:<NUM> to <NUM>:<NUM>. The proportion of formaldehyde to phenol may be determined taking into account the total molar amount of formaldehyde and phenol used in the preparation of the resins. In several embodiments, the formaldehyde to phenol molar ratio may be from <NUM>:<NUM> to <NUM>:<NUM>.

It has been found that resins with such proportion of formaldehyde to phenol allows for the formulation of adhesives which typically result in products (e.g., coated materials) with (relatively) low emissions of free formaldehyde. In particular, may provide coated materials with an E-<NUM> formaldehyde emissions rating according to EN <NUM>:<NUM>+A1:<NUM> European Standard, and/or which are Title VI compliant according to the Toxic Substances Control Act (TSCA) of the United States, and/or even have a three star rating or four star rating according to the Japanese Industrial Standard JIS <NUM>:<NUM>).

Phenol formaldehyde resins used in resin formulations as described herein have an average molecular weight (Mw) from <NUM> to <NUM> Da and a molecular weight distribution from <NUM> to <NUM> Da, wherein from <NUM> to <NUM> wt. % of the phenol formaldehyde resin has a Mw above <NUM> Da, as determined by gel permeation chromatography (GPC). The presence of such high molecular weights advantageously provides resin formulations with relatively short curing times which are advantageous for applications, such as plywood manufacture, wherein pressing times may be reduced. Higher percentages of high molecular weights would adversely have a viscosity which would be too high for curtain coating and would adversely have a low stability which would difficult handling of the resin.

Phenol formaldehyde resins used in resin formulations as described herein further have from <NUM> to <NUM> wt. % of the phenol formaldehyde resin having molecular weights from <NUM> to <NUM> Da. The presence of such amounts of intermediate molecular wights may advantageously provide resin formulations which result in adhesives with desirable tack which allow for a relatively long assembly time.

The Mw and the molecular weight distribution as well as the wt. % of resin having a specific Mw are determined by GPC. GPC is a common method for the determination of the molecular weight of polymeric materials and needs no further elucidation here. For instance, a GPC Refractive Index (RI) detector may be used (such as a Waters RI detector for GPC). Specific conditions that may be mentioned as an example are, e.g., the use of DMF as a solvent at a flow rate of <NUM>/min. A double column for low (e.g., a range of <NUM>-<NUM> Da) and high molecular weights (e.g., a range of <NUM>-<NUM> Da) may be used. Suitable calibration standards are known in the art. For instance, a polyethylene glycol (PEG) calibration standard may be used. A GPC curve for a resin as described herein will first show the polymer chains of higher molecular weight (which have lower retention times) and will later show polymer chains with lower molecular weight (which have a higher retention time). As a mode of example GPC curves for resin formulations prepared according to the examples are shown in <FIG>.

An average molecular weight (Mw) from <NUM> to <NUM> Da, has a meaning as commonly understood in the art. In particular, the average molecular weight (Mw) is also known as the weight average molecular weight and is defined by the following formula:
<MAT> , where Mi is the molecular weight of a chain and Ni is the number of chains of that molecular weight.

The Mw can be determined by GPC as indicated above. Typically, a commercial GPC apparatus and software can automatically provide the Mw for an analyzed sample. An example of a suitable commercial software may be EMPOWER™ software from WATERS.

The average molecular weight (Mw) is distinct from the wt. % of resin having a specific molecular weight as defined above and below. The average molecular weight (Mw) is also distinct from the number average molecular weight (Mn). Compared to the Mn, the Mw takes into account the molecular weight of a chain in determining contributions to the molecular weight average. The more massive the chain, the more the chain contributes to Mw.

In particular, the weight average molecular weight may be from <NUM> to <NUM> Da, and more in particular from <NUM> to <NUM> Da.

A molecular weight distribution from <NUM> to <NUM> Da is used herein to indicate that the polymeric chains of the phenol formaldehyde resin have a minimum molecular weight of at least <NUM> Da and a maximum molecular weight of at most <NUM> Da, as evidenced by GPC. In particular, a GPC curve obtained for a phenol formaldehyde resin as described herein may start at an elution time for polymeric chains of at most <NUM> Da and end at an elution time for polymeric chains of at least <NUM> Da. As a mode of example molecular weight distribution curves from resins as described herein and obtained in the examples (e.g., resin formulation C and comparative resin formulation D of example <NUM>) are included in <FIG>. As it can be seen the curve starts at around <NUM> minutes of elution (which corresponds to the signal of polymeric chains of molecular weights of <NUM> Da) and ends at around <NUM> minutes of elution (which corresponds to polymeric chains of molecular weights of <NUM> Da).

From <NUM> to <NUM> wt. % of the phenol formaldehyde resin has a molecular weight above <NUM> Da, as determined by gel permeation chromatography (GPC).

From <NUM> to <NUM> wt. % of the phenol formaldehyde resin having molecular weights from <NUM> to <NUM> Da, as determined by gel permeation chromatography (GPC).

% of the resin having a molecular weight above <NUM> Da or molecular weights from <NUM> to <NUM> Da is understood as the weight amount of the resin having polymer chains of a specific molecular weight in Da within the defined range.

For instance, the wt. % of resin having a specific molecular weight (above <NUM> Da or from <NUM> to <NUM> Da) may be defined by <MAT> <MAT> where Mi is the molecular weight of a chain and Ni is the number of chains of that molecular weight, and ><NUM> indicates the molecular weights and chains which are above <NUM> Da, total indicates all the molecular weights and chains of the sample and <NUM>-<NUM> indicate the molecular weights and chains which are within the range of from <NUM> to <NUM> Da.

Typically, a commercial GPC apparatus and software can automatically provide, for an analyzed sample, the wt. % of a sample having a specific molecular weight. As a mode of example, a suitable commercial software may be EMPOWER™ software from WATERS, which can provide for a given a molecular weight threshold or molecular weight range the wt. % of analyzed sample within that threshold (e.g., above <NUM> Da) or range (e.g., from <NUM> to <NUM> Da).

Phenol formaldehyde resins used in resin formulations as described herein, may have a number average molecular weight (Mn) from <NUM> to <NUM> Da, in particular from <NUM> to <NUM> Da.

The Mn has a meaning as commonly understood in the art and is defined by the following formula: <MAT>.

The Mn can also be determined by GPC as indicated above. Typically, a commercial GPC apparatus and software, such as EMPOWER™ software from WATERS, can provide the Mn for an analyzed sample.

Resin formulations as described herein may have a polydispersity index (PDI) defined as Mw/Mn, may typically be from <NUM> to <NUM> in particular from <NUM> to <NUM>, more in particular from <NUM> to <NUM>, and yet more in particular from <NUM> to <NUM>. Such PDIs may be regarded as a consequence of the wt. % of the resin having a molecular weight above <NUM> Da and molecular weights from <NUM> to <NUM> Da, as described above. However, not all resin formulations having such PDI will have a wt. % of the resin within the molecular weight ranges as described herein (i.e., from <NUM> to <NUM> wt. % above <NUM> Da and from <NUM> to <NUM> wt. % from <NUM> to <NUM> Da).

As described above, the combination of high and medium molecular weights in a resin formulation as described herein, has advantageous properties for adhesive formulations comprising the same both with respect to the application of the formulations, e.g., by curtain coating, and with respect to the final properties of the products obtained thereby, as described in more detail below.

A resin formulation as described herein has a solids content from <NUM> to <NUM> wt. %, based on the total weight of resin formulation. In particular the solids content may be from <NUM> to <NUM> wt. %, more in particular of about <NUM> wt.

The solids content is defined as the amount of non-volatile matter contained in a formulation are evaporated under specified conditions of temperature and time, e.g., in a resin formulation. The solids content may be known by knowing the amounts of the different components present in the resin formulation. The solids content may also be determined by methods known in the art, e.g., according to ISO <NUM>:<NUM>.

Resin formulations as described herein may comprise a surfactant mixture comprising an anionic surfactant and a non-ionic surfactant.

It has been found that such surfactant mixtures allow minimizing the flow rate at which the curtain is stable, thereby ultimately achieving low amounts of phenol formaldehyde resin being applied by curtain coting. This has not been achieved by other commonly used surfactants used on their own.

The surfactant mixture may be particularly present in a resin formulation as described herein in an amount from <NUM> to <NUM> wt. % based on the total amount of phenol formaldehyde resin.

The anionic surfactant of a surfactant mixture as described herein may be a salified alkyl phosphate, such as an organic ester of ortho-phosphoric acid which is at least partially salified. In particular, the alkyl phosphate may a mixture of be fully esterified phosphate, e.g., a phosphate comprising three alkylations, and fully salified phosphate or partially salified alkyl phosphate, i.e., a phosphate comprising a one to two alkylations and one to two alkali metal cations (e.g., potassium and/or sodium cations). The salified alkyl phosphate may also be partially salified or a mixture of partially salified alkyl phosphates, i.e., comprising one or more phosphates comprising one to two alkylations and one to two alkali metal cations (e.g. potassium and/or sodium cations). Specific examples of suitable salified alkyl phosphate anionic surfactants include, e.g., potassium cetyl phosphate, potassium C9-<NUM> alkyl phosphate, potassium C11-<NUM> alkyl phosphate, potassium C12-<NUM> alkyl phosphate, potassium C12-<NUM> alkyl phosphate, potassium lauryl phosphate, disodium lauryl phosphate, disodium oleyl phosphate, sodium lauryl phosphate.

The non-ionic surfactant of a surfactant mixture as described herein may be a glycol ether. In particular, the glycol ether may be a glycol ether of formula R(OCH<NUM>CH<NUM>)nOH wherein n = <NUM>, <NUM> or <NUM> and R is an alkyl group. In particular, the R alkyl group may be selected from methyl, ethyl, propyl and butyl. More in particular, the glycol ether may be selected from <NUM>-(<NUM>-butoxy-ethoxy)-ethanol, diethylene glycol monobutyl ether, diethylene glycol monoethyl ether, diethylene glycol monohexyl ether, diethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monohexyl ether, ethylene glycol monomethyl ether, ethylene glycol monooctyl ether, ethylene glycol monopropyl ether, triethylene glycol monoethyl ether, and triethylene glycol monomethyl ether. In particular embodiments, the glycol ether may be <NUM>-(<NUM>-butoxy-ethoxy)-ethanol, also referred to as diethylene glycol monobutyl ether (DEGBE).

Said anionic surfactant may be typically present in the surfactant mixture in an amount of <NUM>-<NUM> wt. %, in particular from <NUM> to <NUM> wt. %, more in particular from1. <NUM> to <NUM> wt.

Said non-ionic surfactant may be typically present in the surfactant mixture in an amount from <NUM> to <NUM> wt. %, in particular from <NUM> to <NUM> wt. % more in particular from <NUM> to <NUM> wt. %, based on the total weight of the surfactant mixture. The remaining of the surfactant mixture may be an appropriate solvent, e.g., water.

A resin formulation as described herein may have from <NUM> to <NUM> wt. % of an aqueous solvent based on the total amount of phenol formaldehyde resin, said aqueous solvent comprising <NUM> to <NUM> wt. % of water and <NUM> to <NUM> wt. % of a C1-C3 linear or branched alcohol, based on the total weight of aqueous solvent, in particular <NUM> to <NUM> wt. % of water and <NUM> to <NUM> wt. % of the a C1-C3 linear or branched alcohol.

The presence of a C1-C3 linear or branched alcohol may advantageously confer an adequate viscosity for a phenol formaldehyde resin with a molecular weight as described herein in combination of a relatively high solids content, which in turn may result in adhesives with properties particularly adequate for curtain coating, e.g., particularly adequate fluidity. For instance, the alcohol may be selected from methanol, ethanol, propanol or isopropanol and may preferably be methanol.

A resin formulation as described herein may particularly have at least one of.

As indicated resin formulation as described herein may have a viscosity from <NUM> Pa*s to <NUM> Pa*s. The viscosity may be determined by methods known in the art. In particular, the viscosity may be measured at <NUM> using a viscometer, e.g., a Brookfield DV2T viscometer. Such viscosities advantageously may further facilitate handling of the resin formulation (e.g., its pumping) and the formulation of an adhesive for its application by curtain coating. Furthermore, such viscosities may further advantageously prevent the phenol formaldehyde resin itself from having a low penetration into the substrate onto which it is applied.

As indicated, a resin formulation as described herein may have a pH from <NUM> to <NUM>. Such pH may advantageously provide formulations with a reactivity and water compatibility, which may further facilitate handling of the resin. A pH of the formulation outside of this range may adversely lower the reactivity of the resins.

A resin formulation as described herein may also advantageously have a free-formaldehyde content of less than <NUM> wt. %, as determined according to ISO <NUM>:<NUM>, and a residual phenol content of less than <NUM> wt. % as determined according to ISO <NUM>:<NUM>, in particular of less than <NUM> wt. % and more in particular of at most <NUM> wt. Surprisingly, such low formaldehyde and phenol contents allow the application of a resin formulation as described herein by curtain coating in adhesives as described herein, and at the same time achieve good adhesion properties of the final products to which it is applied.

A resin formulation as described herein may also advantageously have a curing time <NUM> of <NUM>-<NUM> minutes. The curing time is determined by methods known in the art. In particular, it may be determined according to ISO <NUM>:<NUM>.

Such curing times may be generally obtained thanks to the combination of properties of the resin formulation and are optimal for the typical applications for phenol formaldehyde resins. For instance, in the manufacture of plywood, such curing times allow the application of the resin to a wood veneer and subsequent building of the stack of wood veneers followed by its compression to provide the final plywood products in a timeframe which is suitable for industrial application. Such advantages also apply to the manufacture of related coated materials such as veneered board or laminated veneer lumber (LVL).

A resin formulation as described herein typically has a) a viscosity from <NUM> Pa*s to <NUM> Pa*s, measured at <NUM> using a viscometer.

A resin formulation as described herein in addition to a) a viscosity from <NUM> Pa*s to <NUM> Pa*s may further have at least one of b) a pH from <NUM> to <NUM>; c) a free-formaldehyde content of less than <NUM> wt. %, as determined according to ISO <NUM>:<NUM>; d) a residual phenol content of less than <NUM> wt. % as determined according to ISO <NUM>:<NUM>; and e) a curing time at <NUM> of <NUM>-<NUM> minutes as determined according to ISO <NUM>:<NUM>.

In several embodiments, a resin formulation as described herein has a) a viscosity from <NUM> Pa*s to <NUM> Pa*s and may additionally have two or three of b), c), d) and e), as defined above.

In several embodiments, a resin formulation as described herein may particularly have all of a), b), c), d) and e).

A resin formulation as described herein may comprise from <NUM> to <NUM> wt. % of urea, based on the total amount of phenol formaldehyde resin, in particular, it may comprise from <NUM> to <NUM> wt. The presence of a small amount of urea in phenol formaldehyde resins may be typically used in the art to, e.g., increase the solids content of the final formulation, to reduce formaldehyde emissions and/or to increase the plasticity of the resin formulation. However, the presence of urea in resin formulations as described herein is not necessary and it may be preferred that they have no urea. For instance, a urea content of <NUM> wt. % may be preferred. Nonetheless, since the presence of urea has been found not to detrimentally affect the properties of phenol formaldehyde resins as described herein, if desired, urea may also be used.

It has been surprisingly found that a phenol formaldehyde resin as described herein, in particular in combination with a surfactant mixture and optionally also a C1-C3 linear or branched alcohol as described herein, allows for a resin formulation with relatively high molecular weights in combination with a solids content and viscosity which provide an adhesive which can be advantageously applied by curtain coating without compromising the bonding properties of the final products. In particular, a resin formulation is provided adequate wettability fluidity and physical properties for curtain coating applications. More in particular, such resin formulations may be used in adhesives which result in coated products which have excellent bonding properties.

Accordingly, the instant invention further relates to an adhesive comprising from <NUM> to <NUM> wt. % (in particular from <NUM> to <NUM> wt. %) of a resin formulation as described herein; from <NUM> to <NUM> wt. % of a filler, said filler comprising from <NUM> to <NUM> wt. % of an organic filler and from <NUM> to <NUM> wt. % of an inorganic filler; and from <NUM> to <NUM> wt. % of water, wherein the adhesive has a viscosity from <NUM> to <NUM> Pa*s and a solids content from <NUM> to <NUM> wt. %, and wherein the wt. % is based on the total amount of adhesive.

A filler is present in an adhesive as described herein in an amount of from <NUM> to <NUM> wt. %, based on the total amount of adhesive. In particular, the filler may be present in an amount from <NUM> to <NUM> wt.

A filler as described herein comprises <NUM> to <NUM> wt. % of an organic filler and <NUM> to <NUM> wt. % of an inorganic filler, in particular <NUM> to <NUM> wt. % of an organic filler and <NUM> to <NUM> wt. % of an inorganic filler, more in particular <NUM> to <NUM> wt. % of an organic filler and <NUM> to <NUM> wt. % of an inorganic filler.

An organic filler may be selected from wheat flour, rye flour, olive stone flour, corn flour, coconut husk flour, wood flour, carbohydroxymethyllcellulose (CMC), xanthan gum and starch. An organic filler may preferably be selected from wheat flour, olive stone flour and wood flour.

An inorganic filler may be selected from calcium carbonate, sodium carbonate, potassium carbonate, chalk, barium sulfate, talcum powder, calcium sulfate and gypsum. An inorganic filler may preferably be calcium carbonate.

The presence of organic fillers in combination with inorganic fillers has been found to advantageously ensure that the adhesive has appropriate rheological properties to form and stabilize the curtain of adhesive during curtain coating and at the same time ensures that the phenol formaldehyde resin is evenly distributed on the surface of the coated substrate. This ultimately ensures good adhesion properties with a reduced amount of phenol formaldehyde resin, e.g., when compared to products obtained by other types of coating such as roller coating.

Water is present in an adhesive as described herein in order to provide a viscosity and a solids content to the adhesive which is suitable for curtain coating. Water is present in an amount from <NUM> to <NUM> wt. % is based on the total amount of adhesive, in particular water may be present in an amount from <NUM> to <NUM> wt. Lower or higher amounts of water generally result in curtains which are not stable or products with an excessive or a deficient amount of adhesive. An excessive amount of water would also be detrimental to the desired viscosity properties and on the properties of the final product, e.g., longer compression times or could cause bleeding of the surface boards in, e.g., a plywood, and/or blisters at different points of the board.

An adhesive as described herein has a viscosity from <NUM> to <NUM> Pa*s, in particular may have a viscosity from <NUM> to <NUM> Pa*s, in particular from <NUM> to <NUM> Pa*s. The viscosity may be measured as indicated above for the resin formulation.

An adhesive as described herein has a solids content is based on the total amount of adhesive from <NUM> to <NUM> wt. %, in particular from <NUM> to <NUM> wt. The solids content amount of the adhesive can be established as described above for the resin formulation.

An adhesive having a viscosity and a solids content amount of the resin formulation within these ranges has been found to advantageously provide a continuous curtain and to provide desired low amounts of a continuous coating on a substrate.

The instant invention further relates to a process for manufacturing a resin formulation as described herein comprising:.

In a method as described herein, alkali metal hydroxide is added in three or more (e.g. <NUM> or <NUM>) separate fractions in order to gradually increase the viscosity the phenol formaldehyde resin. The total amount of alkali metal hydroxide added is from <NUM> to <NUM> wt. % based on the total amount of a phenol formaldehyde resin.

Such sequential addition of alkali metal hydroxide has been found to advantageously allow obtaining resins with a relatively high molecular weight as defined herein and at the same time resin formulations with a relatively high solids content as also defined herein.

A process as described herein may further comprise ii) adding to the phenol formaldehyde resin from <NUM> to <NUM> wt. % of a surfactant mixture comprising an anionic surfactant and a non-ionic surfactant, the wt. % being based on the total amount of resin formulation. The addition of the surfactant mixture may be typically performed at the end of condensation or may also be performed during condensation. It may be preferred to add the surfactant mixture at the end of condensation or after condensation, e.g., during the cooling of the resin. For instance, addition may be performed at a temperature from <NUM> to <NUM> or from <NUM> to <NUM> or may be performed after cooling <NUM> to <NUM>.

In several embodiments, condensing (i) may be performed in the presence of from <NUM> to <NUM> wt. %, in particular from <NUM> to <NUM> wt. %, of a C1-C3 linear or branched alcohol and from <NUM> to <NUM> wt. % of water, based on the total weight amount of the resin formulation.

It has been surprisingly found that the addition of a C1-C3 linear or branched alcohol in the condensation step (i) may advantageously provide a resin formulation with a viscosity and solids content particularly suitable for use in adhesives comprising the same for curtain coating applications, without compromising the condensation of the resin or the bonding properties of the final products.

In several embodiments, a process as described herein may comprise adding to the phenol formaldehyde resin from <NUM> to <NUM> wt. % of urea, in particular from <NUM> to <NUM> wt. % of urea, based on the total weight amount of the resin formulation.

In several embodiments, in a process as described herein condensing (i) comprises:.

wherein the weight percentages (wt. %) are based on the total weight amount of the resin formulation.

In processes as described herein the total amount of alkali metal hydroxide may be added sequentially as detailed. The gradual addition of sodium hydroxide allows for the gradual increase of viscosity, e.g., as also detailed. This has been found to allow reaching higher viscosities and higher molecular weights whilst at the same time reaching relatively high solids content.

Heating and cooling steps are typically used in phenol formaldehyde manufacture and need no further elucidation herein.

An alkali metal hydroxide used in processes as described herein may be selected from sodium hydroxide, potassium hydroxide, barium hydroxide, magnesium hydroxide, and zinc hydroxide and may preferably be selected from sodium hydroxide and potassium hydroxide.

A method as described herein provides resin formulations with a combination of properties that render them particularly suitable for curtain coating applications. As evidenced in the examples, formulations with the same monomer constituents obtained by other methods do not typically have such adequate properties. Accordingly, the instant invention further relates to resin formulations obtainable by or obtained by a method as described herein.

As indicated, resin formulations as described herein are advantageously used in adhesives, as also described above, for curtain coating applications.

Adhesives as described herein may be prepared with resin formulations as also described herein by methods known in the art.

The instant invention further relates to the use of an adhesive as described herein in curtain coating or for coating a substrate by curtain coating.

Accordingly, the instant invention also relates a process for manufacturing a coated material comprising applying an adhesive as described herein on to a substrate, wherein the adhesive is applied by curtain coating to an amount from <NUM> to <NUM>/m<NUM> based on the amount of adhesive per square meter of substrate, to provide a curtain coated substrate.

Any substrate which may be desired to be coated with a phenol formaldehyde resin-based adhesive and may be suitably coated by curtain coating may be used. Examples of suitable substrates include, e.g., lignocellulosic substrates, metallic substrates or polymeric substrates. Lignocellulosic substrates, and in particular wood panels or wood veneers may be preferred. For instance, wood veneers, e.g., for plywood, may have a thickness from <NUM> to <NUM>, in particular from <NUM> to <NUM>, more in particular from <NUM> to <NUM>. Wood composites such as medium density fiberboards (MDF) may also be used as lignocellulosic substrates.

Curtain coating may be applied as commonly performed in the art. Curtain coating is achieved by providing a curtain of adhesive and allowing a substrate to be coated to pass through the curtain, e.g., by placing the substrate on a conveyor belt and letting it advance under the curtain of adhesive.

Advantageously a phenol formaldehyde resin formulation and an adhesive as described herein allow for the adhesive to be used in curtain coating systems or apparatus already being used in the art without the need of adapting them for a particular phenol formaldehyde resin formulation or adhesive as described herein. For instance, commercial curtain coaters may be used.

In particular, an adhesive as described herein may be applied by curtain coating to an amount from <NUM> to <NUM>/m<NUM> based on the amount of adhesive per square meter of substrate, to provide a curtain coated substrate. The adhesive amount on the curtain coated substrate may be determined by weighing a substrate of a known surface area before and after coating.

Typically, the adhesive will be evenly distributed over the substrate and will cover <NUM>% of the surface of the substrate. The distribution of the coating over the substrate can be determined visually.

The amount of adhesive may be regulated to provide a desired amount per surface of the substrate. In particular, the amount of adhesive may be regulated in a curtain coater by varying the flow of adhesive provided by the curtain and the speed at which the substrate goes through the curtain.

As a mode of example, a substrate may be passed through a curtain of adhesive at a speed, e.g., from <NUM> to <NUM>/min, in particular from <NUM> to <NUM>/min. For a specific flow of adhesive, higher speeds could reduce the amount of adhesive but may detrimentally result in areas of the substrate surface free of adhesive, lower speeds increased the amount of adhesive placed onto the surface substrate without providing additional benefits on, e.g., bonding properties and resulting in higher formaldehyde emissions.

It has been found that such adhesive amounts provide coated substrates which may be used to provide products with satisfactory adhesion properties whilst keeping the adhesive amounts low.

More in particular, the adhesive may be applied to an amount from <NUM> to <NUM>/m<NUM>, <NUM> to <NUM>/m<NUM> or from <NUM> to <NUM>/m<NUM>.

The amount of adhesive applied may be chosen to meet specific needs of the final products wherein the coated substrate will be used. For instance, higher adhesive amounts within said ranges may be desired when a higher adhesion may be required.

The curtain coated substrate as such may be the coated material or the curtain coated substrate may be further processed to provide the coated material.

For instance, a process for manufacturing a coated material as described herein may further comprise pressing the curtain coated substrate at a temperature from <NUM> to <NUM>, in particular from <NUM> to <NUM> and more in particular from <NUM> to <NUM>, and at a pressure from <NUM> to <NUM> MPa for a length of time from <NUM> to <NUM> minutes per mm of total thickness of the curtain coated substrate being pressed. Resin formulations as described herein have been found to advantageously provide adhesives which upon application by curtain coating allow relatively short pressing times which is advantageous in, e.g., plywood manufacture.

Pressing the curtain coated surface is performed to cure the phenol formaldehyde resin and to provide the coated material with desired bonding properties. The coated substrate may be compressed together with other coated substrates.

In a process for manufacturing a coated material as described herein the coated material may be selected from plywood, veneered board or laminated veneer lumber (LVL), and it may particularly be plywood.

Accordingly, the present invention further relates to coated material, obtainable by or obtained by a process for manufacturing a coated material as described herein. In particular, the coated material may be selected from plywood, veneered board or laminated veneer lumber (LVL), and more in particular the coated material may be plywood,.

The process for manufacturing plywood, veneered board or laminated veneer lumber (LVL) as a coated material corresponds to processes typically used in the art with the exception that an adhesive comprising a phenol formaldehyde resin as described herein is applied by curtain coating.

For instance, a plywood may be obtained from an uneven number of wood veneers and the number of wood veneers and the nature of the wood will vary depending on the wishes for the final products. As a mode of example, the number of wood veneers may vary from <NUM> to <NUM>, in particular from <NUM> to <NUM> and more in particular from <NUM> to <NUM> and the thickness of the wood veneers may be from <NUM> to <NUM>, in particular from <NUM> to <NUM>, more in particular from <NUM> to <NUM>.

A process for manufacturing plywood may generally comprise placing a first wood veneer in a conveyor belt, with the wood grain on a first direction, to allow the wood veneer to pass through a curtain of an adhesive, placing a subsequent wood veneer with the wood grain of the wood on the opposite direction to that first direction. Subsequent wood veneers are placed alternatively with the wood grain on the first direction and the wood grain on the opposite direction. The wood veneers are then placed on top of each other to provide a stack of wood veneers wherein adjacent wood veneers have the wood grain in opposite directions and with the surface of the wood veneer which is free of adhesive being placed on top of the surface containing adhesive of a previous wood veneer, until the last wood veneer is to be placed to obtain a stack of wood veneers of desired thickness. The last wood veneer to be placed on the stack is free of any adhesive.

The stack of wood veneers may be pressed at a temperature from <NUM> to <NUM>, in particular from <NUM> to <NUM> and more in particular from <NUM> to <NUM>, and at a pressure from <NUM> to <NUM> MPa for a length of time from <NUM> to <NUM> minutes per mm of total thickness of the curtain coated substrate being pressed.

The stack of wood veneers may be directly submitted to such conditions or may have been submitted to a previous compression step wherein the stack of veneers is subjected to a pressure from <NUM> to <NUM> MPa, for <NUM> to <NUM> minutes at room temperature (e.g., at a temperature from <NUM> to <NUM>). Such previous compression step may advantageously prevent drying out of the resin and improve adhesion, particularly if compression cannot be performed immediately after stacking.

A plywood obtained by processes as described herein advantageously displays good bonding quality and acceptable or low formaldehyde emissions.

In particular, it has been found that methods as described herein can produce plywood with phenol formaldehyde resin-based adhesive which has good bonding properties with a low amount of adhesive being used.

For instance, a plywood may be obtained wherein wood veneers are adhered by an adhesive as described herein, wherein said plywood has a class III bonding quality rating according to EN <NUM>-<NUM>:<NUM> and to EN <NUM>-<NUM>: <NUM> (confirmed in <NUM>) European Standards and has an E-<NUM> formaldehyde emissions rating according to EN <NUM>:<NUM>+A1:<NUM> European Standard and/or is Title VI compliant according to the Toxic Substances Control Act (TSCA) of the United States, and/or has a three star rating or four star rating according to the Japanese Industrial Standard JIS <NUM>:<NUM>).

In several further embodiments, the adhesive may be applied to one of the surfaces of a wood veneer to an amount from <NUM> to <NUM>/m<NUM> as described above.

Other coated materials, such as veneered board or laminated veneer lumber (LVL), may also typically have such an amount of adhesive applied.

Accordingly, the instant invention also relates to a coated material, comprising an adhesive as described herein, in particular the coated material may be selected from plywood, veneered board or laminated veneer lumber (LVL), and more in particular the coated material may be plywood, yet more in particular a plywood with properties as described herein.

As indicated, the adhesive adhering wood veneers may be typically applied to one of the surfaces of the wood veneers. Even though methods applying adhesive to both surfaces of the wood veneers may be envisaged, in an industrial production of plywood, adhesive would typically only be applied to one surface.

As also indicated, the adhesive adhering wood veneers may be applied to an amount from <NUM> to <NUM>/m<NUM>, and it may be particularly applied to an amount from <NUM> to <NUM>/m<NUM>, from <NUM> to <NUM>/m<NUM>, or from <NUM> to <NUM>/m<NUM>. Amounts of phenol formaldehyde resin adhesive within the ranges described herein may be lower than those typically achieved with other coating methods such as roller coating.

Surprisingly, it has been found that coated materials such as plywood, comprising an adhesive as described herein display a good bonding quality. In particular, plywood products can be achieved displaying class III bonding quality rating according to EN <NUM>-<NUM>:<NUM> and to EN <NUM>-<NUM>: <NUM> (confirmed in <NUM>) European Standards. The standards define the bond quality requirements for different applications and the testing conditions:.

This is equivalent to climatic conditions leading to a higher moisture content than would be suitable for bond class <NUM> environments.

It has also been found that phenol formaldehyde resin adhesives as described herein provide plywood products with acceptable or even low formaldehyde emissions. In particular, a plywood as described herein displays E-<NUM> formaldehyde emissions rating according to EN <NUM>:<NUM>+A1:<NUM> European Standard and/or is Title VI compliant according to the Toxic Substances Control Act (TSCA) of the United States, and/or has a three star rating or four star rating according to the Japanese Industrial Standard JIS <NUM>:<NUM>), a higher star rating meaning a lower formaldehyde emission.

Coated materials comprising phenol formaldehyde resin adhesives with such bonding and formaldehyde emission properties as described herein, have now been found possible by applying an adhesive as described herein by curtain coating as also described herein.

The instant invention is further illustrated with the following examples without being limited thereto or thereby.

<NUM> of a <NUM>% solution of formaldehyde were added into a reactor followed by <NUM> of phenol, <NUM> of water and <NUM> of methanol as the alcohol. The components were mixed and cooled to <NUM>.

Alkali metal hydroxide (selected from sodium hydroxide and potassium hydroxide, using a <NUM>% solution in both cases) was added sequentially in three portions, as follows.

A first portion of <NUM> of sodium hydroxide (Alkali <NUM>) was added to the reactor and the mixture was heated up to <NUM>. The reaction mixture was held at that temperature until the viscosity measured at <NUM> was <NUM> Pa*s. When the viscosity was reached the mixture was cooled to <NUM> and a second portion of <NUM> of potassium hydroxide (Alkali <NUM>) was added to the reactor. The mixture was heated again to <NUM> and this temperature was held until the viscosity measured at <NUM> was <NUM> Pa*s. When the viscosity was reached the mixture was cooled to <NUM> and a third portion of <NUM> of sodium hydroxide (Alkali <NUM>) was added to the reactor. The reaction mixture was heated again to <NUM> until the viscosity at <NUM> was <NUM> Pa*s. Then the reaction mixture was cooled to <NUM> and <NUM> of urea and <NUM> of the surfactant mixture were added to the reactor. The mixture was further cooled to <NUM>.

The resulting resin formulation comprised a combination of properties as indicated in table <NUM>, including a phenol formaldehyde resin with combination of high and medium molecular weights, a solids content of <NUM> wt. %, a viscosity <NUM> Pa*s at <NUM>° C and good fluidity for its formulation in an adhesive for curtain coating.

<NUM> of a <NUM>% solution of formaldehyde were added into a reactor followed by <NUM> of phenol and <NUM> of ethanol as the alcohol. The components were mixed and cooled to <NUM>.

Sodium hydroxide was added sequentially in three portions, using a <NUM>% solution of sodium hydroxide as follows.

A first portion of <NUM> of sodium hydroxide (Alkali <NUM>) was added to the reactor together with <NUM> of water and the mixture was heated up to <NUM>. The reaction mixture was held at that temperature until the viscosity measured at <NUM> was <NUM> Pa*s. When the viscosity was reached the mixture was cooled to <NUM> and a second portion of <NUM> of sodium hydroxide (Alkali <NUM>) was added to the reactor. The mixture was kept at <NUM> until the viscosity measured at <NUM> was <NUM> Pa*s. When the viscosity was reached the mixture was cooled to <NUM> and a third portion of <NUM> of sodium hydroxide (Alkali <NUM>) was added to the reactor. The reaction mixture was kept at <NUM> until the viscosity at <NUM> was <NUM> Pa*s. Then the reaction mixture was cooled to <NUM> and <NUM> of the surfactant mixture were added to the reactor. The mixture was further cooled to <NUM>.

The resulting resin formulation comprised a combination of properties as indicated in table <NUM>, including a phenol formaldehyde resin with combination of high and medium molecular weights, a solids content of <NUM> wt. %, a viscosity of <NUM> Pa*s at <NUM>° C and good fluidity for its formulation in an adhesive for curtain coating.

<NUM> of phenol were added into a reactor followed by <NUM> of a <NUM>% solution of formaldehyde and <NUM> of methanol as the alcohol. The components were mixed and cooled to <NUM>.

Sodium hydroxide was added sequentially in three portions, using a <NUM>% solution of sodium hydroxide, as follows:
A first portion of <NUM> of potassium hydroxide (Alkali <NUM>) was added to the reactor together with <NUM> of water and the mixture was heated up to <NUM>. The reaction mixture was held at that temperature until the viscosity measured at <NUM> was <NUM> Pa*s. When the viscosity was reached the mixture was cooled to <NUM> and a second portion of <NUM> of potassium hydroxide (Alkali <NUM>) was added to the reactor. The mixture was kept at <NUM> until the viscosity measured at <NUM> was <NUM> Pa*s. When the viscosity was reached the mixture was cooled to <NUM> and a third portion of <NUM> of potassium hydroxide (Alkali <NUM>) was added to the reactor. The reaction mixture was kept at <NUM> until the viscosity at <NUM> was <NUM> Pa*s. Then the reaction mixture was cooled to <NUM> and <NUM> of urea was added to the reactor. The reaction mixture was further cooled to <NUM> and <NUM> of the surfactant mixture were added to the reactor. The mixture was further cooled to <NUM>.

The resulting resin formulation comprised a combination of properties as indicated in table <NUM>, including a phenol formaldehyde resin with a combination of high and medium molecular weights, a solids content of <NUM> wt. %, a viscosity of <NUM> Pa*s at <NUM>° C and good fluidity for its formulation in an adhesive for curtain coating.

Comparative resin formulations A-C were respectively prepared in the same way as resin formulations A-C, wherein the alcohol (methanol or ethanol as the case may be) is not added at the beginning of the preparation of the resin but is added at the end after cooling to <NUM>. The properties of the resin formulations obtained are detailed in Table <NUM>.

Sodium hydroxide was added in a single portion, using a <NUM>% solution of sodium hydroxide, as follows.

A first and only portion of <NUM> of sodium hydroxide (Alkali <NUM>) was added to the reactor together with <NUM> of water and the mixture was heated up to <NUM>. The reaction mixture was held at that temperature until the viscosity measured at <NUM> was <NUM> Pa*s. When the viscosity was reached the mixture was cooled to <NUM> and <NUM> of urea were added to the reactor. The reaction mixture was further cooled to <NUM> and <NUM> of the surfactant mixture were added to the reactor. The mixture was further cooled to <NUM>.

The resulting resin formulation comprised a combination of properties as indicated in table <NUM>, including a phenol formaldehyde resin with a proportion of high and medium molecular weights outside of the ranges of the present invention, a solids content of <NUM> wt. %, a viscosity of <NUM> Pa*s at <NUM>° C and high curing times.

<FIG> shows the GPC curve for resin formulation C (solid line) and for the resin formulation D (dotted line).

As it can be seen the curve of formulation C starts at around <NUM> minutes of elution (which corresponds to the signal of polymeric chains of molecular weights of around <NUM> Da) and ends at around <NUM> minutes of elution (which corresponds to of polymeric chains of molecular weights of around <NUM> Da). In contrast the curve of formulation D starts at <NUM> minutes of elution (which corresponds to the signal of polymeric chains of molecular weights of around 20000Da) and ends at the same elution time of around <NUM> minutes.

From comparing the two curves it can also be clearly seen that the population of high molecular chains for resin formulation C (i.e., above <NUM> Da and therefore with retention times below <NUM> minutes) is considerably higher than for formulation D as also reflected in Table <NUM> above (with a calculated <NUM> wt. % above <NUM> Da for formulation C vs a calculated <NUM> wt. % above <NUM> Da for formulation D).

Further it can also be observed that the population of intermediate molecular weights, from <NUM> to <NUM> Da (i.e., from a retention time of <NUM> to <NUM> minutes) is also maintained high for formulation C (with a calculated value of <NUM> wt.

The resin formulations A-C and comparative resin formulations A-D of example <NUM> were used in the preparation of adhesives A-C and comparative adhesives A-D to be used in curtain coating.

For each adhesive the ingredients of Table <NUM> were mixed in the indicated proportions. The properties of the adhesives obtained are also indicated.

The curing of the resin formulation C and comparative resin formulation D, was monitored by rheology using a Discovery HR-<NUM> Hybrid Rheometer (from TA Instruments) using the software Trios v5.

The rheological profiles obtained for the two resin formulations are shown in <FIG>. It was observed that adhesive comprising resin formulation C (with a higher wt. % of the resin having a molecular weight above <NUM> than adhesives comprising comparative resin formulation D) showed a higher modulus at low temperatures (< <NUM>), which is advantageous because it improves the adhesive properties during the process of preparation of the plywood, as the veneers have a more consistent behavior as they come out of the press. On the other hand, at the press temperature (<NUM>-<NUM>) the adhesive with resin formulation C is found to reach higher modulus values which is associated to good mechanical properties.

The adhesives A-C and comparative adhesives A-D were used to manufacture plywood from wood veneers using a curtain coater as described below.

Comparative adhesives A-C were unsuited for application by curtain coating. When curtain coating was attempted for comparative adhesives A-C, due to the low viscosity and unsuited fluidity of the adhesives, the curtain of adhesive displayed breaks resulting in areas of the wood veneer surface lacking adhesive.

For obtaining a plywood of about <NUM> thickness starting from wood veneers of about <NUM> thickness a total of <NUM> plywood veneers were necessary.

<NUM> of said <NUM> wood veneers having a surface of 500x500 mm were subjected to curtain coating with adhesives in A-C and comparative adhesive D in a curtain coater.

A first wood veneer was placed in the conveyor belt of the curtain coater, with the wood grain on a first direction, to allow the wood veneer to pass through the curtain of adhesive. The subsequent wood veneer was placed on the conveyor belt with the wood grain of the wood on the opposite direction to that first direction. Subsequent wood veneers were placed alternatively with the wood grain on the first direction and the wood grain on the opposite direction.

As the wood veneers were provided with the adhesive, they were placed on top of each other to provide a stack of <NUM> wood veneers wherein adjacent wood veneers have the wood grain in opposite directions.

A <NUM>th wood veneer which was free of adhesive was placed on top of the surface containing adhesive of the last wood veneer of the stack, with the wood grain in the same direction as the first wood veneer of the stack.

The stack of wood veneers was then pressed at a temperature from <NUM>, and at a pressure of <NUM> MPa for a length of time of <NUM> per mm of the total thickness of the final plywood, i.e. <NUM> minutes.

For comparative adhesive D a higher compressing time was necessary due to a longer curing time required.

Plywood products prepared with the different adhesives were tested according to EN <NUM>-<NUM>:<NUM> and to EN <NUM>-<NUM>: <NUM> (confirmed in <NUM>) European Standards for bonding properties and EN <NUM>:<NUM>+A1:<NUM> European Standard or according to the Toxic Substances Control Act (TSCA) of the United States for formaldehyde emissions, as indicated in Table <NUM> below.

The properties of the plywood obtained with the different adhesives are detailed in the following table:.

Claim 1:
A resin formulation comprising:
- a phenol formaldehyde resin
∘ having a formaldehyde to phenol molar ratio from <NUM>:<NUM> to <NUM>:<NUM>, and
∘ having an average molecular weight (Mw) from <NUM> to <NUM> Da and a molecular weight distribution from <NUM> to <NUM> Da,
wherein from <NUM> to <NUM> wt.% of the phenol formaldehyde resin has a Mw above <NUM> Da and from <NUM> to <NUM> wt.% of the phenol formaldehyde resin has a Mw from <NUM> to <NUM> Da, as determined by gel permeation chromatography (GPC);
wherein the resin formulation has a solids content from <NUM> to <NUM> wt.%, based on the total weight of resin formulation and a) a viscosity from <NUM> Pa*s to <NUM> Pa*s measured at <NUM> using a viscometer.