Field of the Invention

[001] The invention relates to resin binder compositions that may be used for a wide variety of bonded structures including cellulosic materials such as particle board and plywood, veneer sheets, and non-woven materials such as mineral wool and glass fibers. The resin compositions of the invention exhibit very low formaldehyde emissions when used in bonded structures in comparison to resin compositions of the prior art. The invention further relates to methods of making such resin compositions, methods of making bonded structures using such resin compositions, and bonded structures bound by such resin compositions.

Background

[002] Urea-formaldehyde (UF) resins and melamine-formaldehyde (MF) resins are known in the prior art. It was also known to combine melamine with UF resins in order to improve their mechanical properties.

[003] A known problem with both UF and MF resins is that during curing and particularly afterwards they tend to emit formaldehyde, a toxic irritant. U.S. Patent No. 5,017,641 attempts to address the formaldehyde emission problem in UF resins by adding ethanedial in an amount between 0.1 and 0.5 moles per mole of urea and a triazine. A 3- to 8-fold reduction in formaldehyde emission (17O ⁰ C for 7 minutes) was observed as compared to UF resins without ethanedial and triazine. The preferred fibrous web products according to that patent emitted between 0.1% and 0.2% formaldehyde by weight of the solid resin during curing. Patent application number SI 9800068 takes the same approach of adding ethanedial to UF resins. In both cases, the ethanedial was co-condensed with the formaldehyde and urea. [004] An approach for reduction of formaldehyde emissions is the use of formaldehyde scavengers, compounds which react with formaldehyde to form non-volatile compounds. Several patents refer to this technology. Patent application WO 2008026056 describes a resin composition deriving from proteinaceous material which has good reactivity and achieves lowered formaldehyde emission by fast cross-linking at a high degree into hydrolysis resistible network and providing an efficient formaldehyde scavenger at the right time in the application. This system can realize a similar low level of formaldehyde emission but a different technology than used in present invention.

[005] Another approach to formaldehyde reduction was to add dialdehydes such as ethanedial as a post-condensation reaction additive to UF resins. For example, in patent application number JP-07- 126596 ethanedial was added to UF resins after the urea and formaldehyde were fully condensed. It is believed that the post-condensation addition of ethanedial improves the curing conditions, thus binding more formaldehyde in the cured resin and allowing less free formaldehyde to be emitted.

[006] US4,039,496 describes a water soluble textile finishing resin composition comprising fully etherified substantially fully methylolated melamine resins which is reacted in a separate subsequent step with urea, formaldehyde and glyoxal. The resin composition has a relatively low free formaldehyde content and good storage stability.

Summary

[007] While significant advances have thus been made in reducing formaldehyde emissions from curing resins, there is a constant need for further reduction of formaldehyde emissions until it can be eliminated altogether without compromising the material properties of the resin. [008] According to the invention there is provided a resin composition obtainable by forming a reaction mixture by combining different components (Ia) an amino component having 2-6 amino groups; (Ib) an aldehyde component; (2) a trifunctional amino component; and (3) a crosslinking component for the amino components (Ia) and/or (2), and reacting said mixture at a temperature between 6O ⁰ C and 120 ⁰ C and at a pH lower than 7.

[009] The present inventors have surprisingly found a synergistic interaction between a crosslinking agent such as ethanedial and a tri-functional amino component, such as melamine in reducing formaldehyde in amino-formaldehyde resins to a much greater extent than would be expected based on using a crosslinking agent such as ethanedial and a tri-functional amino component, such as melamine, alone or than would be expected when using a resin comprising fully condensated tri-functional amino component such as melamine that can no longer react with the crosslinking component for the trifunctional amino component.

[0010] In a particular embodiment, the components of the invention the amino component (Ia) is an non-cyclic amino compound, i.e. not comprising cyclic groups, whereas component (2) is a cyclic trifunctional amino component. The resin composition may comprising an amino- formaldehyde resin reacted with an at least trifunctional amino component and a crosslinker for the trifunctional amino component, preferably a dialdehyde. Preferably, the amino resin is an UF resin, the trifunctional amino component is melamine, and the dialdehyde is ethanedial. The resin in the resin composition preferably does not comprise a substantial amount of components other than the mentioned components Ia, Ib, 2 and 3, in particular the resin is not etherified.

[0011] It was surprisingly found that a relatively low amount of the tri-functional amino component (2) (in particular melamine) is sufficient to produce a significant synergistic effect with dialdehyde in reducing formaldehyde emissions. The amount of component (2) is preferably less than 30%, more preferably less than 25, 15 wt% or even less than 10 or 7% and most preferably less than 5 % (wt to total resin composition in aqueous solution having total resin solids content between 40 and 70wt%). Preferably, the amount of amino-formaldehyde (Ia + Ib) (relative to total dry resin weight in the resin composition) is at least 70 wt%, more preferably at least 80wt%, even more preferably at least 90wt%.

[0012] Preferably, for the preferred resin composition based on UF, triazine and dialdehyde, the ranges for the various components (relative to one weight unit of formaldehyde) are F:U = 1 :0.25 to 1 :1.5, more preferably 1 :0.4 to 1: 1.5, most preferably 1:0.6 - 1 :1.3, F:Triazine = 1:0.05 to 1:40, more preferably 1 :0.05 - 1 : 15, most preferably 1 :0.05 - 1 :5, F:Dialdehyde = 1 :0 to 1 :40, more preferably 1 :0.05 - 1 :15, most preferably 1 :0.05 - 1 :5.

[0013] The methods of the invention include mixing of certain components and submitting them to changed pH and/or temperature conditions, which favor covalent reactions, such as condensation reactions such as between an amino resin with an at least trifunctional amino component and a crosslinker, preferably a dialdehyde.

[0014] A method for making the resin composition according to the invention comprises the following steps in order: providing in a reaction mixture components (Ia) an amino component having 2-6 amino groups, (Ib) an aldehyde component, (2) a trifunctional amino component, and (3) a crosslinking component for the trifunctional amino component; wherein the components Ia, Ib, 2 and 3 are provided in uncondensed form or in pre-condensed form, reacting the reaction mixture at a temperature between 6O ⁰ C and 120 ⁰ C and at a pH of below 7, stopping the reaction by raising the pH of the reaction product to above 7 and cooling to a temperature of below 60 ⁰ C and optionally adding additional amounts of the amino component (of type Ia) after the temperature of the mixture has been adjusted to less than 60 ⁰ C. [0015] In some embodiments, the reaction is stopped when the mixture has reached a viscosity of at least about 100 cP, measured at 30 ⁰ C using the Brookfield spindle 18.

[0016] It is preferred that the components Ia, Ib, 2 and 3 are provided in the reaction mixture in uncondensed form. However, in some embodiments of the resins and methods of the invention, at least two of the components amino component, aldehyde component, trifunctional amino component, and crosslinking component for the trifunctional amino component may be co- condensed before addition of the remaining components, preferably component (Ia) and/or (2) may be co-condensed with component (Ib) or component (2) may be co-condensed with component (3). Preferably component (Ia) may be co-condensed with component (Ib). It is noted that the co-condensed means condensed to some partial degree but not fully condensed such that the trifunctional amino component (2) and the crosslinking component for the trifunctional amino component (3) can still react.

[0017] The invention also relates to a bonded structure comprising a particulate material, preferably a cellulosic material, and a resin according to the invention wherein the resin is cured and to the use of such a resin composition as a binder for bonded structures, especially cellulosic products including particle board and plywood, veneer sheets, and non-woven materials such as mineral wool and glass fibers. The bonded structures are preferably particle board, preferably medium density fiberboard or paper bonded finish foil. The bonded structures according to the invention have very low formaldehyde emission levels of less than 0.3 mg/1, preferably less than 0.2 mg/1, most preferably less than 0.1 mg/1 as measured by the Japanese desiccator method.

Detailed Description

The amino (Ia ^* ) -Aldehyde (Ib) component [0018] The basic component of the resin compositions of the invention is an amino (Ia)- aldehyde (Ib) resin. Preferably, in the resin composition the amino component (Ia) is an non- cyclic amino compound, i.e. not comprising cyclic groups, whereas component (2) is a cyclic trifunctional amino component.

[0019] Component (Ia) is preferably chosen form groups of Urea, Biuret, Triuret, Tetrauret, Pentauret, Hexauret, Methoxycarbonylurea, Ethoxycarbonylurea, tert.-Butylurea, Bis- 1,4- butyldiureid, guanidine and Acetylene-diurea Methoxycarbonylbiuret, Ethoxycarbonylbiuret, 1- Methylbiuret, 1-Ethylbiuret, 1-Propylbiuret, Acetylbiuret, Diacetylbiuret, 1,1-Dimethylbiuret, Ethylendibiuret and/or 1 ,2-Diethylbiuret, 3-Methyltriuret, 3-Ethyltriuret, 3-Methoxytriuret and/or 1,1 -Dimethyltriuret and salts and derivatives thereof.

[0020] The Amino-Aldehyde component is preferably a urea-formaldehyde (UF) resin. In a preferred embodiment, the UF resin used as the major component of the resin composition of the invention can be prepared from urea and formaldehyde monomers in any manner known to those skilled in the art.

[0021] UF reactants are commercially available in many forms. Any form that can react with the other reactants and which does not introduce extraneous moieties deleterious to the desired reaction and reaction product can be used in the preparation of the UF resins of the invention.

[0022] The aldehyde component (Ib) of the invention is preferably formaldehyde. Formaldehyde for use in UF resins is commercially available in many forms. Paraformaldehyde (solid, polymerized formaldehyde) and formalin solutions (aqueous solutions of formaldehyde, sometimes with methanol, in various concentrations) are commonly used forms as well as UFC (Urea-Formaldehyde-Concentrate) solutions. Formaldehyde is also available as a gas. Any of these forms is suitable for use in preparing UF resins. Formalin solutions are preferred. [0023] The amino component (Ia), preferably urea, is also available in many forms. Solid urea, such as prill, and urea solutions, typically aqueous solutions, are commonly available. Urea may be combined with another moiety, most typically formaldehyde and urea-formaldehyde adducts, often in aqueous solution. Any form of urea or urea in combination with formaldehyde is suitable for use in the practice of the invention.

[0024] Any of a wide variety of procedures used for reacting the principal urea and formaldehyde components to form a UF thermosetting resin composition also can be used, such as staged monomer addition, staged catalyst addition, pH control, amine modification and the like. Generally, the urea and formaldehyde are reacted at a mole ratio of formaldehyde to urea in the range of about 1.1 :1 to 4:1, and more often at an F:U mole ratio of about 2.1 :1 to 3.2:1. Generally, the UF resin is highly water dilutable, if not water soluble.

[0025] Many suitable thermosetting UF resins are commercially available including those sold by Georgia Pacific Resins, Inc. for glass fiber mat applications, and those sold by Hexion and Dynea. These resins contain reactive methylol groups that upon curing form methylene or ether linkages. Such methylol-containing urea adducts may include N,N'-dimethylol, dihydroxy- methylolethylene. N,N'-bis(methoxymethyl), N,N'-dimethylolpropylene, 5,5-dimethyl-N,N'- dimethylolethylene, N,N'-dimelhylolethylene, and the like.

[0026] UF resins useful in the practice of the invention generally contain 45 to 70 weight percent non-volatiles, preferably 55 to 65 weight percent non-volatiles. They generally have a viscosity of 50 to 600 centipoise, preferably 150 to 400 centipoise. They normally exhibit a pH of 7.0 to 9.0, preferably 7.5 to 8.5. And they often have a free formaldehyde level of not more than about 3.0%, and a water dilutability of 1:1 to 100:1, preferably 5: 1 and above. [0027] US2004/0082697 describes certain UF and/or MF resin modified inorganic particles wherein lamellar inorganic particles like clay are swollen with a very dilute urea or melamine solution and then reacted with formaldehyde and/or glyoxal to form modified inorganic particles having resin interlaminarly inside the particles. It is noted that the resin of the resin composition according to the invention is formed in the absence of such inorganic particles and is much more concentrated (45 - 70 wt%). The resin composition according to the invention when it is formed does not comprise inorganic particles, but inorganic particles may be added after formation of the resin. These particles are however not interlaminar resin modified.

[0028] The reactants for making the UF resin can, optionally, further include a small amount of a resin modifier such as ammonia, an alkanolamine, or a polyamine such as an alkyl primary diamine (e.g., ethylenediamine (EDA)). Additional modifiers, such as melamine, ethylene ureas, and primary and secondary and tertiary amines, for example, dicyanodiamide, can also be incorporated during manufacturing into the resin composition. Concentrations of these modifiers, when present in the reaction mixture, can be 0.05 to 20 weight percent, based on the total resin solids. These types of modifiers promote hydrolysis resistance, polymer flexibility, and lower formaldehyde emissions in the cured resin.

The tri-functional amino component (2)

[0029] Component (2) of resin composition of the invention is an at least trifunctional amine, preferably a cyclic urea, preferably having an aromatic structure, like triazines. A particularly suitable component is Melamine (m) (also referred to as (2,4,6-Triamino-l,3,5-triazine; Cyanuro-triamide; Cyanurotriamine; Cyanuramide). Melamine is a broadly available chemical, and may be use in the present context in any form, as long as the water solubility and water dispersibility is not compromised. [0030] Other suitable tri-functional amino components include melamine derivatives such as 4,6-Diamino-l,3,5-triazin-2(lH)-one; 6-Amino-l,3,5-triazine-2,4(lH,3H)-dione; N2-(4,6- diamino-l,3,5-triazin-2-yl)-l,3,5-triazine-2,4,6-triamine and l,3,4,6,7,9,9b-Heptaazaphenalene- 2,5,8-triamine. If such materials are used, the molar ratios to other ingredients as will be discussed herein below, might need to be adjusted, and the weight ratios may change. Those of ordinary skill in the art would readily appreciate how to adjust these parameters.

Crosslinking components (3) for the tri-functional amine

[0031] Component (3) in the resin composition of the invention is a component which can undergo cross-linking reactions with the component (2) in the reaction mixture, the tri-functional amino component. Suitable components include components comprising at least one reactive aldehyde group and at least a further reactive groups such as an acidic group, such as glyoxylic acid. Other suitable components comprise at least two aldehyde groups, such as tri-aldehydes or di-aldehydes

[0032] Preferred aldehydes are aliphatic. More preferred are short chain C2 to C 12 dialdehydes, so as to not negatively impact the solubility properties, for example, Ethanedial, Butanedial, 2-

Butene-l,4-dial, Hexanedial, Pentanedial, 2-Hydroxy-hexanedial, 3-Hydroxymethyl-5-methoxy-

2-methyl-hexanedial, Heptanedial, Octanedial, Propanedial, Propanedial-2-on, 2-

Methylpropanedial, Propanedial-2-on, Butanedial, Hexanedial, Decanedial, 2,4-

Hexadienedialdehyde, 1,4-Benzenedialdehyde, 1 ,2-Benzenedialdehyde, and 1,3- Benzenedialdehyde. Even more preferred are unbranched C2 to C5 di-aldehydes, and most preferred is Ethanedial. Preferably, the crosslinking component is in liquid form, such as in aqueous solution. Other ingredients

[0033] Other ingredients may be added to the resin compositions of the invention that will beneficially affect the material properties of the resins without detrimentally affecting the formaldehyde reduction. For example, plasticizers or catalysts known in the art to be useful in resin formulations, but also organic and inorganic fillers may be used with the present invention

[0034] Essentially any conventional hardener as useful for UF resins and not contradicting the process conditions as described herein below may be applied in the present invention.

[0035] Hardener or curing agents may be added just after the condensation reaction between the components la/b, 2 and 3 (step f and g) but imminently before the application to a bondable material. Alternatively, latent hardener may be added, which will not induce the curing until they are activated, e.g. after shipment of the resin composition with the latent hardener to the place, where this is combined with the bondable material, and the curing is initiated. Alternatively, the hardener or curing agent may be shipped separately to the place of application, and be mixed with the curable resin composition just prior to the application to the bondable material.

[0036] The hardener may be any inorganic salt, for example, ammonium nitrate or ammonium sulfate. Preferably, the hardener is ammonium nitrate present in a quantity of about 0.5% by weight.

[0037] In one aspect, the invention includes a method for manufacturing a resin composition according to the invention comprising the following steps in order: (a) providing at least four components comprising: (Ia) an amino component having 2-6 amino groups, (Ib) an aldehyde component, (2) a trifunctional amino component, and (3) a crosslinking component for the trifunctional amino component. The components form a reaction mixture wherein the components Ia, Ib, 2 and 3 can be provided in uncondensed form or in part in pre-condensed form. The pre-condensed form can be prepared according to steps (b) to (e); (b) combining at least two of the at least four components having amino and aldehyde reactive groups to form a first mixture, (c) adjusting the pH of the first mixture to between 2-10 to allow formation of covalent bonds between the components in the first mixture;(d) heating the first mixture to a temperature between 60 ⁰ C and 100 ⁰ C; (e) adding the remaining of the at least four components that were not added in step (b) to form the second mixture. The second mixture is uncondensed or in part pre-condensed but does not comprise fully condensed amino formaldehyde resin, The full condensation reaction is done by (f) adjusting the temperature of the second mixture to between 60 ⁰ C and 120 ⁰ C; (g) adjusting the pH of the second mixture to below 7 to start the reaction. When the desired viscosity of preferably at least lOOcP at 3O ⁰ C is achieved (Brookfield spindle 18); (h) stopping the reaction by raising the pH of the mixture to above 7 and by (i) adjusting the temperature of the mixture to less than about 60 ⁰ C, (j) optionally adding additional amounts of the amino component (of type Ia) after the temperature of the mixture has been adjusted to less than 60°C.

[0038] A first particular execution of this method comprises adding a crosslinking component such as a dialdehyde or a mixture of a crosslinking component such as a dialdehyde and an aldehyde component such as formaldehyde to a solution of an aldehyde such as formaldehyde. The pH is adjusted to 4-10 before or after subsequent addition of the amino component and a trifunctional amino component such as a triazine.

[0039] In another embodiment step (b) comprises combining the aldehyde and amino components, and step (e) comprises adding the crosslinking component and the trifunctional amino component. [0040] In both executions the condensation reaction in the mixture is done by heating to 60- 120°C. The pH is lowered below 7.0 into acidic/slight acidic range for condensation. The condensation reaction is allowed to proceed until a predetermined viscosity is reached, after which pH is adjusted back above 7.0 to neutral to slightly alkaline condition in order to halt the condensation reaction. A second or third or greater quantity of urea may be added in stages, depending on the desired properties of the final resin product. Finally, the mixture is cooled and the pH is adjusted to basic, for example, 7-1 1. Those who are skilled in the art will readily appreciate that there are alternative orders to add the components.

Chemical properties of the resin

[0041] The resin compositions of the invention are particularly useful resin compositions, and in particular as resin compositions allowing the manufacturing of bonded structures exhibiting very low formaldehyde emissions, when the four key components as described above (such as urea, formaldehyde, dialdehyde and triazine) are combined and optionally covalently reacted such as co-condensed.

[0042] As with conventional resins, the condensation and addition reactions before the curing upon the application to the bondable material (as described herein below) should not be complete or excessive, but just to a degree that the resin is still water soluble and/or dilutable or capable of forming a stable dispersion.

[0043] The resin compositions of the invention preferably have a free formaldehyde content of less than 0.01- 0.8% by weight of solid content, preferably less than 0.5%, more preferably less than 0.2% and most preferably less than 0.1% as determined by titration on the uncured resin composition. However, for certain hardener or curing compounds such as ammonium salts, it may be desirable to have a certain amount of residual formaldehyde in the resin. [0044] Without wishing to be bound by a theory, it is believed, that the degree of reaction such as of the condensation is such that essentially all of the resin is reacted into oligomers but not into the completely cured polymeric network as can be found in the final bonded structure. The degree of reaction such as of the condensation may be measured by the viscosity of the resin during the reaction or condensation. For the resins of the invention, the viscosity at which it is preferable to stop the condensation reaction is at least 100 cP, measured by means of a Brookefield® viscosimeter at 30°C with spindle No 18.

[0045] The physical properties of the resin can typically be readily adjusted to be comparable to the ones of conventional resins. Preferably, the resulting resins exhibit a good water dilutability, and their viscosity may thus be adjusted by adding or evaporating water. Preferably, the resulting resins are storage stable. Other properties of the resin may vary according to the application and the bondable material.

[0046] The present invention includes application of the resin to a bondable material and the forming of the bonded structure. Resin compositions as described above which may be produced according to the processes as described in the above may be applied in a broad range of applications to create bonded structures. To this end, the condensed resin compositions are added in the uncured state to bondable material. Upon curing of the resin, the bonded structure is created.

[0047] In general terms, the resin composition as described may replace conventional UF resins in any useful application. They may even broaden the application field to areas in which conventional UF resins are not considered for due to their high formaldehyde emissions, even if such emissions are reduced as compared to pure or unmodified UF resins. [0048] The present invention is particularly useful in the context of bonding cellulosic based materials. Such materials may be wood fibres, chips, particles, sheets or the like, which then may be formed into boards, such as fiber boards, MDF boards, HDF, OSB boards, glue-lam, form-bent, parquet and the like, as well known to a person skilled in the art, and the resin compositions according to the present invention may be the binder in such structures. Such materials may also be essentially solid wood pieces, such as to form beams or form pressing structures or slices, such as veneers, and the resin composition may be the adhesive in such structures.

[0049] The resins of the invention may also be used for resin impregnated webs, such as paper webs and overlays. The resins of the present invention are further particularly useful in the context of bonding non-woven structures. Such structures may be based on discrete or essentially endless fibres of thermoplastic or thermoset materials, but may also be based on mineral, glass, or similar materials. Yet a further application is to use the resins according to the present invention with sand or sand-like materials such as in foundry applications. The present invention may further be applied to the making of abrasive articles, whereby abrasive particles are bonded to a carrier. The present invention can further be applied to the making of foamed structures, where by the resin may brought into a foam structure by conventional means and be cured.

[0050] The present invention includes any combination of such materials, such as when veneers are bonded to a fiber board, and a composition according to the present invention may be the binder of the fiber board and the same or a different composition may be the adhesive.

[0051] In order to create a bonded structure, the properties of the resin composition may be suitably modified to allow good application. Such modification may relate to adjusting viscosity or concentration depending on the application technology, such as coating, spraying, or soaking.

For certain applications, it may be highly preferred that the resin penetrates into the structure or components of the structure, such as into solid wood, into wood pieces, chips, strands or fibers, etc. The bonded material is typically brought into a form corresponding to the desired form of the bonded structure, and to resin composition is added prior to or after this forming. Thus, when forming e.g. fiber boards, the wooden fibres are mixed with the resin composition, which might already comprise a hardener, and the mixture is brought into a board form and - e.g. by temperature - curing is initiated.

[0052] The curing step accomplishes the forming of the final polymeric network structure and in the resins of the invention causes efficient and relatively complete formaldehyde bonding within the polymeric network structure.

EXAMPLE 1 (Particle board)

[0053] A fully condensed melamine-urea-formaldehyde control resin (Prefere 10L023 El resin available from Dynea Austria) was used to show the effect of addition of ethanedial (see Table

1).

Table 1 [0054] In this table, F:(NH ₂ ) ₂ indicates the Formaldehyde to amino (urea) ratio, not including the amino groups added with Melamine. Mel% indicates the percentage of melamine by weight of the resin composition ED% indicates the weight percent added to the liquid resin of ethanedial.

[0055] The control resins were used to make samples of particle board using a rubberwood and hardwood particle blend. Formaldehyde emissions from the particle board samples were measured using the Japanese standard dessicator method for determining formaldehyde emission

(JIS A 1460), and the European perforator value method (EN 120) was used to measure the free formaldehyde in the samples at 6.5% moisture. Internal bond strength was also measured using European standard EN 319 to determine the perpendicular tensile strength with a Zwick ® testing unit. The results of these tests are presented in Table 2.

Table 2 [0056] These tests demonstrate that formaldehyde emission both in desiccator and perforator method was reduced in the particle board samples prepared with a resin of control example IA to control example 1. The internal bond strength remained largely unchanged.

EXAMPLES 2-5 (Medium Density Fiberboard - Rubber-wood)

[0057] The resins were prepared as follows. The 50% w/w mixture of ethanedial and formaldehyde was added to a solution of formaldehyde. The pH was adjusted to be slightly basic. Urea and melamine were added to the solution. The mixture was heated to 80°C, and the pH was adjusted to 5.5-5.6. The condensation reaction was allowed to proceed until a viscosity of between 200 and 300 cP (measured at 30 ⁰ C) was reached, after which pH was adjusted back above 7.0 to neutral to slightly alkaline condition in order to halt the condensation reaction. Second and third quantities of urea were added in stages, and the mixture was cooled and the pH was adjusted to basic. Resins were prepared according to the formulations in Table 3.

Table 3

[0058] Each resin was used to prepare a sample of medium density fiberboard (MDF) using rubberwood fibers and 0.5% Ammonium Nitrate was added as a hardener. The sample density and resin loads for each of examples 2-5 are as indicated in Table 3. Resin load is weight of resin per total weight of the MDF sample.

[0059] Each MDF sample was tested using the Japanese dessicator method to measure formaldehyde emissions and the internal bond strengths were measured. The results of these measurements are presented in Table 4.

Table 4

I Inventive Example 5 I 0.03 I I 0.83 I

[0060] In each case, the addition of the formaldehyde/ethanedial mixture (the "b" control examples) reduced the formaldehyde emission by about half. But the compositions of the invention, which add the formaldehyde/ethanedial mixture and 2% by weight of melamine led to a surprisingly drastic reduction in formaldehyde emission, leading to MDF samples with almost no formaldehyde emission. Such a synergistic reaction of melamine and ethanedial was a completely unexpected result.

Example 6

[0061] Other dialdehydes may be used to make the resin compositions of the invention. In this example, pentanedial was used. Resins were prepared according to the formulations in Table 5.

Table 5

[0062] The resins were prepared according to the method described in Example 1. Each resin was used to prepare a sample of medium density fiberboard (MDF) using mixed tropical hardwood fibers, and 0.5% Ammonium Nitrate was added as a hardener. Formaldehyde emissions and internal bond strength were measured, and the results are presented in Table 6.

Table 6

[0063] Thus the combination of pentanedial and melamine are effective to reduce formaldehyde emissions by more than half in this example.

Example 7

[0064] Several tests were conducted to determine the most beneficial amount of ethanedial in the Melamine-Urea-Formaldehyde resin formulations of the invention. Further tests were conducted to determine the most beneficial time to add ethanedial, pre- or post-condensation. To conduct these tests, resins were prepared according to the formulations in Table 7.

Table 7

[0065] The resins were prepared according to the method described in Example 1. Each resin was used to prepare a sample of medium density fiberboard (MDF) using mixed tropical hardwood fibers, and 0.5% Ammonium Nitrate was added as a hardener. Formaldehyde emissions and internal bond strength were measured, and the results are presented in Table 8.

Table 8

[0066] These tests show that this example (inventive example 6a) resulted in very low formaldehyde emissions.