TECHNICAL FIELD

The present invention relates to an aminoplast resin formulation, an adhesive comprising the same suitable for curtain coating and a process for manufacturing the aminoplast resin formulation. The present invention also relates to a process for manufacturing a coated material by applying said adhesive by curtain coating to a coated material, in particular plywood, veneered board or laminated veneer lumber (LVL), obtainable or obtained by said process and to a coated material, in particular plywood, veneered board or laminated veneer lumber (LVL), comprising the adhesive of the invention.

STATE OF THE ART

Curtain coating has been used for coating substrates, e.g., lignocellulosic substrates such as sheets of wood or wood veneers, 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.

Aminoplast 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, aminoplast 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). However, commercial or common aminoplast resins are typically applied by roller coating.

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

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

For instance, GB 1 ,075,893 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 urea-formaldehyde resin and the powder component include both a filler and hardener, such as woodflour, 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, aluminum 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.

US 8 257 554 describes a thermosetting ureaformaldehyde (UF) resin composition containing a rheological-enhancing amount of a thickening agent, the use of the resin composition for formulating an adhesive binder for preparing fiber mats and the fiber mats made using the adhesive binder, wherein improved mat properties and faster cure speeds can be obtained. By modifying a thermosetting UF resin composition with a small amount of a thickening agent, the rheological characteristic of an adhesive binder formulation made using the thermosetting UF resin composition is significantly improved, allowing the adhesive binder formulation to be applied evenly at high mat manufacturing speeds even using a curtain coater. However, this document is occupied with fiber substrates and the viscosities described even after addition of the thickening agent are low (e.g., of 201-2020 centipoise, i.e., about 0.2 Pa*s). Furthermore, as it is directed to forming mats embedding fiber in the adhesive fiber, this document is not occupied with reducing the amount of material being applied by curtain coating onto a substrate.

There is a need for an aminoplast resin formulation which allows the application of aminoplast resin adhesives by curtain coating allowing a reduction of the amount of adhesive per square meter of substrate, e.g., compared to that achieved by other methods such as roller coating, without breaks on the curtain and without compromising the good adhesion properties of the final product.

BRIEF DESCRIPTION OF THE INVENTION

An aminoplast 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 aminoplast resin formulations have been found to provide coated materials, such as plywood, which typically display good adhesion properties (e.g., Class I, Class II or even Class III bonding quality rating according to EN 314- 1 :2004 and to EN 314-2: 1993 (confirmed in 2013) European Standards. Such formulations have also been found to provide coated materials that have good formaldehyde emission properties (e.g., to have an E-1 formaldehyde emissions rating according to EN 636:2012+A1 :2015 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 1460:2015).

In particular, a first aspect of the invention relates to a resin formulation comprising: an aminoplast resin selected from ureaformaldehyde (UF) resin and a melamine ureaformaldehyde (MUF) resin; and a surfactant mixture comprising an anionic surfactant and a non-ionic surfactant, wherein said resin formulation has a viscosity from 0.05 to 3 Pa*s and a solids content from 40 to 70 wt.%.

Additional aspects of the invention relate to an adhesive comprising said resin formulation, a filler comprising 20-100 wt.% of an organic filler and 0-80 wt.% of an inorganic filler, a catalyst, and water, wherein the adhesive has a viscosity of 0.5-4 Pa*s and has a solids content amount of the resin formulation of 12-45 wt.%.

Several additional aspects of the invention relate to a process for manufacturing said resin formulation; a use of said adhesive in curtain coating; a process for manufacturing a coated material comprising applying said adhesive on to a substrate, wherein the adhesive is applied by curtain coating; a coated material, e.g., plywood veneered board or laminated veneer lumber (LVL), obtainable by or obtained by such a process; a coated material comprising said adhesive, in particular wherein the coated material is selected from plywood, veneered board or laminated veneer lumber (LVL).

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

DETAILED DESCRIPTION OF THE INVENTION

The instant invention relates to a resin formulation comprising: an aminoplast resin selected from ureaformaldehyde (UF) resin and a melamine ureaformaldehyde (MUF) resin having a molar ratio of aldehyde groups to urea groups from 4.5:1 to 0.5:1 ; and a surfactant mixture comprising an anionic surfactant and a non-ionic surfactant, wherein said anionic surfactant is present in an amount of 1-10 wt.% and said non-ionic surfactant is present in an amount from 1 to 40 wt.%, based on the total weight of the surfactant mixture; wherein the surfactant mixture is present in an amount from 0.1 to 5 wt.% based on the total weight of the aminoplast resin, and wherein said resin formulation has a viscosity from 0.05 to 3 Pa*s and a solids content from 40 to 70 wt.%.

Aminoplast resins are commonly used in adhesive formulations and are known to a skilled person. Ureaformaldehyde (UF) and Melamine ureaformaldehyde (MUF) resins may be typically characterized by the proportion of the different monomers conforming the resins. Ureaformaldehyde (UF) resins are constituted by urea and formaldehyde monomers and Melamine ureaformaldehyde (MUF) are constituted by urea, formaldehyde and melamine monomers.

Ureaformaldehyde (UF) and Melamine ureaformaldehyde (MUF) resins used in resin formulations as described herein have a molar ratio of aldehyde groups to urea groups from 4.5:1 to 0.5:1.

The proportion of aldehyde groups to urea groups may be determined taking into account the total molar amount of aldehyde groups and urea groups used in the preparation of the resins.

In several embodiments the aminoplast resin may be a UF resin having a molar ratio of aldehyde groups to urea groups 1.75:1 to 0.75:1, in particular from 1.6:1 to 0.8:1.

In several embodiments the aminoplast resin may be a MUF resin having a molar ratio of aldehyde groups to urea groups from 4.5:1 to 0.75:1 , in particular from 4.3:1 to 0.8:1.

It has been found that resins with such proportion of aldehyde groups to urea groups allows for the formulation of adhesives which result on products (e.g., coated materials) with (relatively) low emissions of free formaldehyde. In particular, provide coated materials with an E-1 formaldehyde emissions rating according to EN 636:2012+A1:2015 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 1460:2015).

Furthermore, resins with a proportion of aldehyde groups to urea groups outside of these ranges may be detrimental to the bonding properties of the final products.

MUF resins as described herein may have an amount of melamine from 0.3 to 40 wt.% based on the total amount of aminoplast resin, depending on the desired properties of the MUF resins.

Depending on the final application of the resin formulation, a UF or a MUF may be preferred, e.g., depending on the bonding quality desired. In particular, UF resins may typically provide products with a class I bonding quality rating according to EN 314-1:2004 and to EN 314-2: 1993 (confirmed in 2013) European Standards and MUF resins may typically provide products with class II or class III according to the same standards.

The aminoplast resin typically is the only resin component of the resin formulation. Accordingly, the proportion of aminoplast resin based on the total resin solids of the resin formulation typically is of 100 wt.%. Also, the only monomers typically conforming the resin component of the resin formulation are urea, formaldehyde and optionally melamine, as no other monomers are typically reacted with these. Resin formulations as described herein 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 aminoplast resin being applied by curtain coating. This has not been achieved by other commonly used surfactants (e.g. a polyether modified polydimethylsiloxane) used on their own.

In contrast with other resin formulations, surfactant mixtures as described herein are not used for the stabilization of emulsions, as resin formulations as described herein are not emulsions, as described in more detail below.

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 C9-15 alkyl phosphate, potassium C11-15 alkyl phosphate, potassium C12-13 alkyl phosphate, potassium C12-14 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(OCH2CH2)nOH wherein n = 1, 2 or 3 and R is an alkyl group. The R alkyl group may be selected from methyl, ethyl, propyl and butyl. In particular, the glycol ether is selected from 2-(2-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 2-(2-butoxy-ethoxy)-ethanol, also referred to as diethylene glycol monobutyl ether (DEGBE).

Said anionic surfactant is typically present in the surfactant mixture in an amount of 1- 10 wt.%, in particular from 1.0 to 7.5 wt.%, more in particular froml.O to 5.0 wt.%.

Said non-ionic surfactant is typically present in the surfactant mixture in an amount from 1 to 40 wt.%, in particular from 5 to 30 wt.% more in particular from 10 to 20 wt.%, based on the total weight of the surfactant mixture. The remaining of the surfactant mixture may be an appropriate solvent, e.g., water.

The surfactant mixture is present in a resin formulation as described herein in an amount from 0.1 to 5 wt.% based on the total weight of the aminoplast resin, in particular from 0.5 to 4 wt.% more in particular from 1 to 3 wt.%. Lower amounts do not provide the advantageous effects of the surfactant mixture and higher amounts, even though they may not be disadvantageous, make the product more expensive without providing any additional benefits.

The surfactants of the surfactant mixture are present in admixture with the aminoplast resin, i.e., without copolymerizing with the aminoplast resin component of the resin formulation. As indicated above the surfactants of the surfactant mixture render the resin formulation particularly suitable for curtain coating applications, and are not used for stabilizing an emulsion, as the resin formulation typically is not in the form of an emulsion.

A resin formulation as described herein has a viscosity from 0.05 to 3 Pa*s, in particular from 0.1 to 2.5 Pa*s, more in particular from 0.2 to 2 Pa*s. The viscosity may be determined by methods known in the art. In particular, the viscosity may be measured at 25 °C using, e.g., a Brookfield DV2T viscometer.

Such viscosities advantageously 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 prevent the aminoplast resin itself from having a low penetration into the substrate onto which it is applied.

A resin formulation as described herein has a solids content from 40 to 70 wt.%, in particular the solids content may be from 55 to 65 wt.%.

The solids content is defined as the amount of non-volatile matter contained in a formulation, e.g., in a resin formulation. The solids content may be known by knowing the amounts of the different components present in, e.g., the resin formulation.

Accordingly, the solids content of a resin formulation as described corresponds to the dry weight of the non-volatile matter present in the formulation and includes the dry weight of the aminoplast resin and of the surfactant mixture. The resin solids content refers to the solids content of the resin components of the resin formulation, e.g., the aminoplast resin.

The solids content may also be determined by methods known in the art, e.g., by drying a known weight amount of resin formulation until a constant weight is obtained and the solids content is the proportion of dry weight with respect to the weight of resin formulation prior to drying. Drying may be performed in an oven at a temperature of, e.g., 105 °C.

Solids contents in ranges as indicated herein advantageously provide a good stability to the aminoplast resin formulation.

A resin formulation as described herein typically is a waterborne resin formulation, given the presence of water as carrying medium. Typically, the water content of the resin formulation may be from 30 to 60 wt.%, in particular from 35 to 45 wt.% of water based on the total weight of the formulation. Typically, resin formulations as described herein are not emulsions.

A resin formulation as described herein may comprise components other than an aminoplast resin, a surfactant mixture as described herein, but may typically consist essentially of an aminoplast resin, a surfactant mixture as described herein, and water.

It has been surprisingly found that the combination of an aminoplast resins and a surfactant mixture as described allows for a resin formulation with a solids amount and viscosity which can satisfactorily provide an adhesive which can be advantageously applied by curtain coating. In particular, provide a resin formulation with adequate wettability fluidity and physical properties for curtain coating applications.

Accordingly, the instant invention further relates to an adhesive in comprising a resin formulation as described herein and further comprising a filler in an amount from 10 to 80 pph of resin formulation, said filler comprising 20-100 wt.% of an organic filler and 0-80 wt.% of an inorganic filler; a catalyst in an amount from 5 to 10 wt.% based on the solids content amount of the resin formulation; and water in an amount from 5 to 80 pph of resin formulation, wherein the adhesive has a viscosity of 0.5-4 Pa*s and has a solids content amount of the resin formulation of 12-45 wt.%.

A filler is present in an adhesive as described herein in an amount of from 10 to 80 pph of resin formulation, “pph” being “parts per hundred”, which with respect to the filler amount this means that for hundred weight parts of resin formulation the adhesive comprises from 10 to 80 weight parts of filler. In particular, the filler may be present in an amount from 25 to 75 pph, more in particular from 30 to 60 pph. Lower filler amounts do not provide the desired rheological behavior for the adhesive and higher filler amounts adversely affect the physical properties of the final product.

A filler as described herein comprises 20 to 100 wt.% of an organic filler and 0 to 80 wt.% of an inorganic filler, in particular 40 to 95 wt.% of an organic filler and 5 to 60 wt.% of an inorganic filler, more in particular 50 to 90 wt.% of an organic filler and 10 to 50 wt.% of an inorganic filler. Generally, if an inorganic filler is present an organic filler will be present in equal or greater amounts than the 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 and wood flour.

If present, an inorganic filler may be selected from sodium carbonate, calcium carbonate, calcium sulfate, mica powder, barite powder, kaolin and gypsum. An inorganic filler may preferably be calcium carbonate.

The presence of organic fillers on their own or 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 aminoplast resin is evenly distributed on the surface of the coated substrate. This ultimately ensures good adhesion properties with a reduced amount of aminoplast resin, e.g., when compared to products obtained by other types of coating such as roller coating.

A catalyst is present in an adhesive as described herein in order to speed up the curing of the aminoplast resin. A catalyst is present in an amount from 5 to 10 wt.% based on the solids content amount of the resin formulation. In particular; the catalyst may be present in an amount from 6 to 8 wt.%. Such amounts achieve the desired reactivity without compromising other properties of the formulation and without increasing the costs of the formulation unnecessarily. A catalyst in an adhesive as described herein may preferably be an ammonium salt catalyst or a weak organic acid catalyst. An ammonium salt catalyst as may particularly be selected from ammonium nitrate, ammonium sulfate, ammonium chloride, ammonium iodide, ammonium carbonate, and ammonium phosphate, and may be preferably selected form ammonium nitrate, ammonium sulfate and ammonium chloride. A weak organic acid catalyst may particularly be selected from lactic acid, citric acid, formic acid and acetic acid. A catalyst may preferably be an ammonium salt catalyst.

Water is present in an adhesive as described herein in order to provide a viscosity and solids content to the adhesive which is suitable for curtain coating. Water is present in an amount from 5 to 80 pph of resin formulation, in particular water may be present in an amount from 10 to 75 pph, and more in particular from 20 to 60 pph. 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. requiring longer pressing 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 0.5 to 4.0 Pa*s, in particular may have a viscosity from 0.6 to 3.0 Pa*s, in particular from 0.8 to 2.0 Pa*s. The viscosity may be measured as indicated above for the resin formulation.

An adhesive as described herein has a solids content amount of the resin formulation from 12 to 45 wt.%, in particular from 20 to 40 wt.%, more in particular from 25 to 35 wt.%. The solids content amount of resin formulation can be established by the solids content of the resin formulation (as described above for the resin formulation) and by the amount of resin formulation present in the adhesive. For instance, when using a resin formulation having a solids content of 50 wt.%, an adhesive comprising 50 wt.% of a weight amount of resin formulation based on the total weight amount of adhesive, will have a solids content amount of the resin formulation of 25 wt.%.

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 an aminoplast resin formulation as described herein comprising mixing an aminoplast resin and a surfactant mixture as described herein. In particular, a process for manufacturing a resin formulation comprises mixing an aminoplast resin selected from ureaformaldehyde (UF) resin and a melamine ureaformaldehyde (MUF) resin having a molar ratio of aldehyde groups to urea groups from 4.5:1 to 0.5:1; and from 0.1 to 5 wt.% based on the total weight of the aminoplast resin of a surfactant mixture comprising an anionic surfactant and a non-ionic surfactant, wherein said anionic surfactant is present in an amount from 1 to 10 wt.% and said non-ionic surfactant is present in an amount from 1 to 40 wt.%, based on the total weight of the surfactant mixture.

In several embodiments, an aminoplast resin may be prepared from the monomers constituting the aminoplast resin and/or from previously prepared polymeric components (e.g., ureaformaldehyde concentrate (UFc) and/or melamine formaldehyde concentrate (MFc)) as starting material, wherein the molar ratio of aldehyde groups to urea groups of the aminoplast resin obtained is from 4.5:1 to 0.5:1.

A suitable previously made UFc may preferably have a F:U molar ratio of 6.0:1 to 3.0:1 and/or may typically comprise hydroxymethylolated urea and free formaldehyde.

A suitable previously made MFc may preferably have a F:M molar ratio of 40:1-20:1

An aminoplast resin as described herein may also be prepared from a mixture of UF and MUF resins, wherein the molar ratio of aldehyde groups to urea groups of the mixture is from 4.5:1 to 0.5:1.

Ureaformaldehyde (UF) resins may be prepared from formaldehyde (F) and urea monomers (U), or from a previously made polymeric ureaformaldehyde concentrate (UFc) which may be commercially available or may also be prepared from its constituting monomers. A previously made UFc may typically be reacted with further urea (and optionally formaldehyde) to obtain a UF resin with a molar ratio of aldehyde groups to urea groups from 4.5:1 to 0.5:1.

Melamine ureaformaldehyde (MUF) resins may be prepared from formaldehyde (F), urea (U) and melamine (M) monomers, or may be prepared from previously made polymeric melamine formaldehyde concentrate (MFc) and/or a previously made polymeric ureaformaldehyde concentrate (UFc), which may be commercially available or may also be prepared from its constituting monomers. For instance, in several embodiments MFc may be further reacted with urea (and optionally melamine and/or formaldheyde) to provide the MUF resin with a molar ratio of aldehyde groups to urea groups from 4.5:1 to 0.5:1. In other embodiments UFc may be further reacted with melamine (and optionally urea and/or formaldehyde) to provide the MUF resin with a molar ratio of aldehyde groups to urea groups from 4.5:1 to 0.5:1. In yet other embodiments MFc may be reacted with UFc (and optionally urea, melamine and/or formaldehyde) to obtain the MUF resin with a molar ratio of aldehyde groups to urea groups from 4.5:1 to 0.5:1. The instant invention further relates to a process for manufacturing an aminoplast resin formulation as described herein comprising: a) mixing formaldehyde (F) and urea (II) at a F:U molar ratio of 6.0:1 to 3.0:1 to provide a first reaction mixture and adjusting the pH to a pH of 4.5 to 10.0; b) heating the first reaction mixture to a temperature of at least 70°C, in particular at least 85°C, more in particular from 90 to 100°C, yet more in particular from 92 to 98°C, for 30 to 90 minutes, in particular for 30 to 60 minutes or for 30 to 45 minutes, to provide a second reaction mixture comprising hydroxymethylolated urea and free formaldehyde; c) adjusting the pH to a pH of 4.5 to 10.0 and adding urea to the second reaction mixture to a F: U molar ratio from 4.5:1 to 1.5:1 to provide a third reaction mixture; d) heating the third reaction mixture to a temperature from 70 to 98°C, in particular from 80 to 95 °C, to provide a fourth reaction mixture having a free formaldehyde content of less than 2.5 wt.%, in particular less than 2 wt.%; e) cooling the fourth reaction mixture to a temperature from 40 to 65°C and adjusting the pH to a pH of 7.0 to 10.0 to slow down the reaction and adding urea to provide a final reaction mixture comprising an aminoplast resin with a F:ll molar ratio from 4.5:1 to 0.5:1 , in particular from 4.3:1 to 0.75:1 and having a free formaldehyde content of less than 1 wt.%, in particular less than 0.2 wt.% and yet more in particular of less than 0.1 wt.%; f) cooling the final reaction mixture to a temperature from 20 °C to 35 °C. wherein a surfactant mixture comprising an anionic surfactant and a non-ionic surfactant, wherein said anionic surfactant is present in an amount from 1 to 10 wt.% and said non-ionic surfactant is present in an amount from 1 to 40 wt.%, based on the total weight of the surfactant mixture, is added to the second reaction mixture after step b), to the fourth reaction mixture after step d) and/or to the final reaction mixture after step e) (e.g. prior to, during and/or after cooling of step f)) to a total amount from 0.1 to 5 wt.% based on the total amount of the aminoplast resin of the final reaction mixture to provide a resin formulation having a viscosity from 0.05 to 3 Pa*s and a solids content from 40 to 70 %.

In a process as described herein the total amount of formaldehyde is typically added to the process in step a) whereas urea is added sequentially, in different steps, achieving the desired final molar ratio of aldehyde groups to urea groups of the aminoplast resin in step e).

In a process as described herein the surfactant mixture of the resin formulation may be added to the second reaction mixture after step b), to the fourth reaction mixture after step d) and/or to the final reaction mixture after step e) (e.g. prior to, during and/or after cooling of step f)). It has been surprisingly found that the presence of the surfactant mixture during the preparation of the aminoplast resin does not adversely affect the properties of the resin but to the contrary it may favor the blending of the surfactant components within the resin. For instance, in several embodiments the surfactant mixture may be added to the second reaction mixture after step b). In other embodiments, the surfactant mixture may be added to the fourth reaction mixture after step d). In yet other embodiments the surfactant mixture may be added to the final reaction mixture after step e). In yet other embodiments the surfactant mixture may be added to the final reaction mixture prior to, during and/or after cooling of step f), in particular after step f). In yet other embodiments the surfactant mixture may be added sequentially after two or more of step b), step d), step e) and step f).

In a process as described herein the free formaldehyde content may be determined by the European Standard EN 1243:2011.

In several embodiments the aminoplast resin may be ureaformaldehyde (UF) resin. If the aminoplast resin is UF the process for manufacturing a resin formulation may preferably be a process as described herein wherein in step a) the F:U molar ratio is from 6.0:1 to 3.75:1 and the pH is adjusted to an acidic pH of 4.5 to 6.5, in particular of 5.0 to 6.0 or to a basic pH from 7.0 to 10.0, in particular from 8.0 to 9.5; in step c), if necessary, the pH is adjusted to an acidic pH from 4.5 to 6.5, in particular from 5.0 to 6.0 and urea is added to a F:U molar ratio from 2.2:1 to 1.5:1 in step d) heating is performed to a temperature from 85 to 98°C, in particular from 90 to 95°C; in step e) the pH is adjusted to a pH from 7.0 to 9.0 and urea is added to a F:U molar ratio from 1.75: 1 to 0.75: 1 , in particular from 1.6: 1 to 0.8: 1.

In several embodiments the aminoplast resin may be melamine ureaformaldehyde (MUF) resin. If the aminoplast resin is MUF the process for manufacturing a resin formulation may preferably be a process as described herein wherein in step a) the F:U molar ratio is from 6.0:1 to 3.0:1 and the pH is adjusted to a basic pH of 7.0 to 10.0, in particular 8.0 to 9.5; in step c), if necessary, the pH is adjusted to a basic pH of 8.0 to 10.0, and urea is added to a F:U molar ratio from 4.5:1-1.5:1 and melamine is added to an amount from 0.3 to 40 wt.% based on the total amount of aminoplast resin of the final reaction mixture in step d) heating is performed to a temperature from 70 to 98°C, in particular from 80 to 90 °C; in step e) the pH is adjusted to a pH of 8.5 to 10.0, urea is added to a F:U molar ratio from 4.5: 1 to 0.75: 1 , in particular from 4.3: 1 to 0.8: 1.

Such specific conditions for UF or MUF resins may be useful to optimize the method for specific aminoplast resins.

Wherein the aminoplast resin is a mixture of UF and MUF, the mixture may be obtained by obtaining each of the resin formulations as described above and then mixing the MUF and UF resin formulations.

As indicated, resin formulations as described herein are advantageously used in adhesives 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.

A substrate may be coated with adhesives as described herein to provide a coated material wherein the substrate is adhered onto another substrate, or to provide a coated material with a protective coating or a coating with a specific finish.

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 provide a curtain coated substrate. Any substrate which may be desired to be coated with an aminoplast resin-based adhesive and may be suitably coated by curtain coating may be used. Examples of suitable substrates include, e.g., lignocellulosic substrates, metal 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 0.3 to 3 mm, in particular from 0.5 to 2.5 mm, more in particular from 1 to 2 mm. 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 an aminoplast 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 aminoplast resin formulation or adhesive as described herein. For instance, commercial curtain coaters may be used.

In a process as described herein the adhesive may be applied to an amount from 130 to 250 g/m ² , based on the amount of adhesive per square meter of substrate to provide the 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 100% 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 the desired amount per surface of the substrate. In particular the amount of adhesive may be regulated by varying the flow of adhesive provided by the curtain and the speed on 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 90 to 140 m/min, in particular from 100 to 130 m/min.

Higher and lower speeds may also be used. Nonetheless, 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. At the same time such adhesive amounts typically provide products with acceptable formaldehyde emissions.

More in particular, the adhesive may be applied to an amount from 130 to 220 g/m ² or from 130 to 190 g/m ² .

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 it 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 80 to 160°C, in particular from 90 to 150°C and more in particular from 100 to 140°C, and at a pressure from 0.4 to 2.0 MPa for a length of time from 0.4 to 3 minutes per mm of total thickness of the curtain coated substrate being pressed.

Pressing the curtain coated surface is performed to cure the aminoplast resin and to provide the coated material with desired bonding properties or a coating with a desired finish. The coated substrate may be compressed alone or 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 a 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 may correspond to a process typically used in the art with the exception that an adhesive comprising an aminoplast resin as described herein is applied to wood veneers 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 3 to 29, in particular from 5 to 25 and more in particular from 7 to 23 and the thickness of the wood veneers may be from 0.3 to 3 mm, in particular from 0.5 to 2.5 mm, more in particular from 1 to 2 mm.

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 is typically pressed at a temperature from 80 to 160°C, in particular from 90 to 150°C and more in particular from 100 to 140°C, and at a pressure from 0.4 to 2.0 MPa for a length of time from 0.4 to 3 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 previously submitted to a previous compression step wherein the stack of veneers is subjected to a pressure from 0.3 to 1 MPa, for 2 to 30 minutes at room temperature (e.g., at a temperature from 16 to 26 °C). 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 obtainable or 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 aminoplast resin-based adhesive which have 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 I, class II or class III bonding quality rating according to EN 314-1 :2004 and to EN 314-2: 1993 (confirmed in 2013) European Standards and has an E-1 formaldehyde emissions rating according to EN 636:2012+A1:2015 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 1460:2015). Accordingly, the present invention also relates to a coated material which is such a plywood.

In several further embodiments, the adhesive may be applied to a wood veneer to an amount from 130 to 250 g/m ² .

Other coated materials, such as veneered board or laminated veneer lumber (LVL), may also typically have such properties.

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 such a 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.

The adhesive adhering wood veneers may be applied to at least one surface of the wood veneers. Accordingly, the adhesive adhering wood veneers may be applied to one surface of the wood veneers or to both 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. Accordingly, it may be preferred for the adhesive adhering wood veneers to be applied to one surface of the wood veneers. Methods of configuring and applying adhesive to wood veneers for the manufacture of plywood, veneered board, laminated veneer lumber (LVL), or other materials using wood veneers are known to the skilled person. For instance, wood veneers can be adhered to each other in such a way that a surface of a first wood veneer onto which the adhesive has been applied is contacted with a surface of a second wood veneer which is free of adhesive. If the adhesive adhering wood veneers is applied to both surfaces of a first wood veneer, the two contacting surfaces of the two wood veneers adjacent to said first wood veneer will typically be free of adhesive. Of course, a wood veneer which has a surface free of adhesive may have adhesive applied to the other surface, or if, e.g., it is the last wood veneer, may have the other surface also be free of adhesive.

As also indicated, the adhesive adhering wood veneers may be applied to an amount from 130 to 250 g/m ² , and it may be particularly applied to an amount from 130-220 g/m ² , or 130-190 g/m ² . When applied to both surfaces of a wood veneer, the amount of adhesive applied to each surface may typically be from 130 to 250 g/m ² , in particular from 130-220 g/m ² and more in particular from 130-190 g/m ² .

Amounts of aminoplast resin adhesive within the ranges described herein are 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 aminoplast resin adhesive as described herein display a good bonding quality. In particular, plywood products can be achieved displaying class I, class II or class III bonding quality rating according to EN 314-1:2004 and to EN 314-2: 1993 (confirmed in 2013) European Standards. The standards define the bond quality requirements for different applications and the testing conditions:

Bond Class 1 : Dry conditions - interior applications with no risk of wetting.

This is equivalent to a moisture content in the plywood corresponding to a temperature of 20°C and a relative humidity of the surrounding air only exceeding 65% for a few weeks of the year. As a guide, the average moisture content of most softwoods will not exceed 12%.

Bond Class 2: Humid conditions - protected exterior conditions, e.g., behind cladding or under roof coverings.

This is equivalent to a moisture content in the plywood corresponding to a temperature of 20°C and a relative humidity of the surrounding air only exceeding 85% for a few weeks per year. As a guide, the average moisture content of most softwoods will not exceed 20%.

Bond Class 3: Exterior conditions - unprotected exterior conditions over sustained periods.

This is equivalent to climatic conditions leading to a higher moisture content than would be suitable for bond class 2 environments.

The desired class rating may be achieved varying the amount of adhesive applied to the wood veneers conforming the plywood and/or by varying the type of aminoplast resins used in the adhesive. The amount of adhesive and type of aminoplast resins may be selected to achieve a desired rating with a desired amount of adhesive.

Adhesive amounts of 130-150 g/m ² may typically display Class I bonding quality ratings and adhesive amounts of 180-250 g/m ² may typically display Class II or Class III bonding quality ratings.

Typically, UF resins as described herein may provide plywood with Class I bonding quality rating and MUF resins may provide plywood with Class II or Class III bonding quality rating. For obtaining Class III bonding quality an adhesive as described herein may additionally comprise a reinforcer, such as resorcinol, in an amount of, e.g., 3 to 10 pph of resin formulation, in particular 5 to 8 pph.

It has also been found that aminoplast resin adhesives as described herein provide plywood products with acceptable or even low formaldehyde emissions. In particular, a plywood as described herein displays E-1 formaldehyde emissions rating according to EN 636:2012+A1:2015 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 1460:2015), a higher star rating meaning a lower formaldehyde emission.

Coated materials comprising aminoplast 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.

EXAMPLES

Example 1 : Preparation of UF resins and application of adhesives comprising the same in plywood manufacture by curtain coating.

Preparation of UF resins:

Aminoplast resin formulations comprising a UF resin and 1.0 wt.% and 2.0 wt% of a surfactant mixture based on the total weight of UF resin (referred to respectively as UF 1% and UF 2%) were prepared.

The surfactant mixture used comprised 1-5 wt.% of a salified alkyl phosphate as anionic surfactant and 10-20 wt.% of diethylene glycol monobutyl ether (DEGBE) as nonionic surfactant.

Formaldehyde was provided by FORESA and had a purity of 55%.

Urea was provided by FORESA and had a purity of 99%. Urea and formaldehyde at a F:U molar ratio of 5.0:1 were mixed in a stirred glass reactor provided with a temperature controller.

The pH was adjusted to a pH of 7.5-8.5 by adding sodium hydroxide.

The mixture was heated to a temperature of 90 °C for about 40 minutes, at which point the reaction mixture comprised hydroxymethylolated urea and free formaldehyde.

The pH was adjusted to 5.5 with formic acid and urea was added to the reaction mixture to obtain a F:U molar ratio of 2.2:1 and the reaction mixture was heated at 93 °C for about 90 minutes, at which point the reaction mixture had a free formaldehyde content of less than 2 wt.%. The reaction mixture was then cooled to a temperature of 55 °C and the pH was adjusted to 8.0 and urea was added to achieve an aminoplast resin with a final F:U molar ratio of 1.2:1.

To two batches of aminoplast resin 1.0 g and 2.0 g of surfactant mixture were added respectively to 100 g of aminoplast resin and mixed to provide resin formulations UF 1% and UF 2% respectively.

An aminoplast resin formulation comprising a UF resin and 1.5 wt.% of the surfactant mixture based on the total weight of UF resin (referred to as UF 1.5%) was prepared as described above for UF 1% and UF 1.5%, with the difference that the final F:U molar ratio was 0.95:1.

An aminoplast resin formulation comprising a UF resin and 1 wt.% of polyether modified polydimethylsiloxane as non-ionic surfactant based on the total weight of UF resin (referred to as UF 1% (nonionic)) was prepared as described above for UF 1%, with the same the final F:U molar ratio of 1.2:1.

The properties of the aminoplast resin formulations obtained are detailed in the following table:

*The surfactant amount is based on the total weight amount of aminoplast resin * Determined according to the European Standard EN 1243:2011

The solids content was determined by drying a sample of aminoplast resin formulation of a known weight amount in an oven at 105°C until the weight of the sample remained constant.

The viscosity was determined at 25 °C using a Brookfield DV2T viscometer.

Preparation of adhesives

UF resin formulation 1 and UF formulation 2 were used to prepare adhesives with the compositions as defined in the following table:

* The wt.% of the filler composition refers to the amount of filler component (i.e. calcium carbonate or wheat flour) over the total amount of filler. ^f The solids content refers to the solids content of aminoplast resin formulation

The different components were mixed in stirred tank.

The viscosity was determined at 25 °C using a Brookfield DV2T viscometer.

Preparation of plywood products:

Plywood products using the adhesives A, B, C, D and E were prepared by curtain coating as follows.

For obtaining a plywood of about 19 mm thickness starting from wood veneers of about 2.5 mm thickness a total of 9 plywood veneers were necessary.

8 of said 9 wood veneers having a surface of 500x500 mm were subjected to curtain coating with adhesives A, B, C, D or E in 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 (A, B, C, D or E), they were placed on top of each other to provide a stack of 8 wood veneers wherein adjacent wood veneers have the wood grain in opposite directions.

For adhesive E a higher flow was necessary in order to obtain a stable curtain. This ultimately resulted in a higher amount of adhesive E (having the non-ionic surfactant polyether modified polydimethylsiloxane) being applied onto the wood veneers compared to adhesives A, B, C or D (having a surfactant mixture of non-ionic and anionic surfactants).

A 9 ^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 110°C, and at a pressure of 0.49 MPa for a length of time of 1 min per mm of the total thickness of the final plywood, i.e. 19 minutes.

The plywood products prepared with the different adhesives were tested according to EN 314-1 :2004 and to EN 314-2: 1993 (confirmed in 2013) European Standards and EN 636:2012+A1 :2015 European Standard, as indicated in the table below. The formaldehyde emissions of Plywood D were tested according to the Toxic Substances Control Act (TSCA) of the United States.

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

*EN 314-1 :2004 and to EN 314-2: 1993 (confirmed in 2013) European Standards ^f EN 636:2012+A1 :2015 European Standard

*Toxic Substances Control Act (TSCA) of the United States

Example 2: Preparation of MUF resins and application of adhesives comprising the same in plywood manufacture by curtain coating.

Preparation of MUF resins:

Aminoplast resin formulations comprising a MUF resin and 1.0 wt.% and 1.5 wt.% of a surfactant mixture based on the total weight of MUF resin (referred to respectively as MUF 1% and MUF 1.5%) were prepared.

The surfactant mixture, formaldehyde and urea were the same as those used in Example 1.

Melamine was provided by FORESA and had a purity of 99%

Urea and formaldehyde at a F:U molar ratio of 5.0:1 were mixed in a stirred glass tank reactor with temperature controller.

The pH was adjusted to 9.0 by adding sodium hydroxide.

The mixture was heated to a temperature of 92 °C for about 40 minutes, at which point the reaction mixture comprised hydroxymethylolated urea and free formaldehyde.

The pH was maintained at 9.0-9.5, melamine was added to an amount of 25 wt.% based on the total amount of aminoplast resin of the final reaction mixture and urea was added to the reaction mixture to obtain a F:U molar ratio of 3.5. The reaction mixture was heated at 90 °C, then cooled at 80°C for about 90 minutes, at which point the reaction mixture had a free formaldehyde content of less than 2 wt.% The pH was maintained above 8.5 during the reaction time. The reaction mixture was then cooled to a temperature of 55 °C and the pH was adjusted to 9.0 and urea was added to achieve an aminoplast resin with a final F:U molar ratio of 2.5:1. Another resin was prepared following the procedure above metioned with 30% of melamine F:U ratio during condensation of 3.5 and final F:U ratio of 3.0:1

Two batches of aminoplast resin 1.0 g and 1.5 g of surfactant mixture were added respectively to 100 g of aminoplast resin A and B respectively and mixed to provide resin formulations MUF 1 % and MUF 1.5%

The properties of the aminoplast resin formulations obtained are detailed in the following table:

Properties of the aminoplast resin formulations obtained: *The surfactant amount is based on the total weight amount of aminoplast resin

* Determined according to the European Standard EN 1243:2011

Preparation of adhesives

MUF resin formulation 1 and MUF formulation 2 were used to prepare adhesives with the compositions as defined in the following table:

* The wt.% of the filler composition refers to the amount of filler component (i.e. , calcium carbonate or wheat flour) over the total amount of filler.

* The solids content refers to the solids content of aminoplast resin formulation Preparation of plywood products:

Plywood products using the adhesives F and G were prepared by curtain coating as described above for plywood products A, B, C, D and E.

The plywood products prepared with the different adhesives were tested according to EN 314-1 :2004 and to EN 314-2: 1993 (confirmed in 2013) European Standards for bonding quality and EN 636:2012+A1:2015 European Standard for formaldehyde emissions.

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

*EN 314-1 :2004 and to EN 314-2: 1993 (confirmed in 2013) European Standards

In this text, the term “comprises” and its derivations (such as “comprising”, etc.) should not be understood in an excluding sense, that is, these terms should not be interpreted as excluding the possibility that what is described and defined may include further elements, steps, etc.

On the other hand, the invention is obviously not limited to the specific embodiment(s) described herein, but also encompasses any variations that may be considered by any person skilled in the art (for example, as regards the choice of materials, dimensions, components, configuration, etc.), within the general scope of the invention as defined in the claims.