WO2012131110A2

WO2012131110A2 - System for form pressing with high production efficiency

Info

Publication number: WO2012131110A2
Application number: PCT/EP2012/056003
Authority: WO
Inventors: Astrid Pedersen; Dick PAUW
Original assignee: Dynea Oy
Current assignee: Dynea Oy
Priority date: 2011-04-01
Filing date: 2012-04-02
Publication date: 2012-10-04
Anticipated expiration: 2013-10-01
Also published as: GB201105583D0; WO2012131110A3

Abstract

The invention relates to process of manufacture of wood products such as plywood and form pressed panels with improved production efficiency where an adhesive system (adhesive and hardener) is used together with a separate application hardener to facilitate the high production efficiency or production at reduced temperature or the combination of both. The process has at least 3 and preferably less than 50 alternating veneer layers A and B and comprises the steps of; - providing a glue mixture of adhesive I and hardener II on both sides of veneer layer(s) A; - providing a water based hardener III on both sides of veneer layers B; - laying-up in a stack at least one of said veneer layers A alternating with at least two veneer layers B - pressing the stack at elevated temperature and pressure to form the form-pressed product; and wherein said veneer layers A and B are stacked such, that the hardener III on veneer layers B get in contact with the glue mixture on veneer layer(s) A during pressing; and wherein the first and the last veneer in the stack should only have glue mixture or hardener solution on the inner side of the veneers, to avoid sticking in the press.

Description

SYSTEM FOR FORM PRESSING WITH HIGH PRODUCTION EFFICIENCY

FIELD OF THE PATENT

[0001] The invention relates to a process of manufacture of wood products such as plywood and form pressed panels wherein at least 3 and preferably less than 50 alternating veneer layers A and B are alternating laid-up with a glue mixture of adhesive and hardener on one or more veneer layers A or B followed by pressing the layers at elevated temperature and pressure. The process of the invention provides improved production efficiency to facilitate a reduced pressing time or production at reduced temperature or the combination of both. The invention further relates to a form pressed products or plywood products obtainable by the process and to a three component adhesive system for use in the process according to the invention.

BACKGROUND

[0002] Plywood and form -pressed products (also called form -bent products or shaped/curved plywood products) typically consist of from 3 to 50 layers of veneer glued together with an adhesive. Plywood is produced in a flat press whereas for form pressed products the gluing process is performed in a mould to give the final product the desired shape. In plywood the wood layers (also referred to as veneer sheets) are placed perpendicular to each other with the wood grain of each sheet cross wise, whereas in form-pressed products the veneer sheets are most commonly placed with the grain of each veneer sheet in the same direction.

[0003] When using a roller spreader the glue mix are usually applied on both sides of every second veneer sheet, whereby every second sheet goes through the roller spreader and the stack is laid up at the other side of the roller spreader by alternating veneer sheets with and without glue. The most commonly used adhesive for interior plywood and form pressed panels is urea formaldehyde type adhesive (UF) as it gives excellent performance as well as being inexpensive. Over the years the formaldehyde content in the adhesives has been reduced so that the formaldehyde emission from the final product has been minimized. Alternative technologies with even lower emissions have also been developed. [0004] The urea formaldehyde adhesives normally used consists of two components, a urea formaldehyde adhesive and a hardener/catalyst. The hardener is mixed with the adhesive to facilitate the cross linking reaction of the adhesive. The reactivity of the adhesive systems used for form pressing is mainly adjusted by the composition of the hardener and can vary from very low reactivity to medium reactivity. All the form press manufacturers would like to have as high reactivity as possible to have as efficient production as possible, but the limiting factor is the pot life of the system.

[0005] Pot life is the time span from mixing of the adhesive and hardener until the glue mix no longer can be handled due to its high viscosity. It takes time to apply the glue on the veneer sheets, to lay up the stack of veneer with glue, to transport it to the press and to put into the press. A few highly automated plants can tolerate short pot lives (1 ½ h at 20°C), but less automated plants may require the adhesive system to have pot lives of up to 10 - 14 h. Glue systems with such low reactivity require long pressing times and the production is not very efficient. [0006] Two different press technologies are commonly used, hot pressing and heating by radio frequency. In hot pressing the stack is placed between two heated press platen and pressed. The press platen typically holds temperatures in the range 70 - 130°C. The veneer stack and the glue lines heats up and the temperature facilitates the cure of the glue lines. The higher the temperature is the faster the curing reactions will be. The necessary pressing time is mainly dependent on the press temperature and the thickness of the stack.

[0007] For a thin stack, with few veneer layers, the time it takes to bring the whole stack up to high enough temperature for the curing to occur is short giving a short pressing time. For a thick stack with many veneer layers it takes long time for the core to heat up sufficiently to facilitate cure, hence the pressing time is long. [0008] In radio frequency presses the stack is pressed between two press platen. Electrical current is passed through the stack and creates heat. The stronger the generator for the radio frequency press is, the faster the stack heats up. With this press technique the core heats up faster than the outer layers of the stack.

[0009] The highest production efficiency is obtained by radio frequency heating as the whole panel is rapidly brought up to >60°C at which temperature the cure will proceed rapidly. For hot pressing it takes time for the heat to transfer from the hot press platen into the glue lines in the core of the panel, thus the time to obtain high enough temperature to facilitate curing within the panel is considerable and the pressing times longer. A method to speed up the cure or to start the curing reaction at lower temperatures would be highly valuable to the plywood and form press producers using hot pressing and give considerable increase in production capacity.

[0010] Traditionally the form pressing plants are increasing the capacity by buying additional presses which then again requires additional people. The large plants may have up to 40 - 50 different presses in their production. One gluing station (roller spreader where glue mix is applied on the veneer) can supply up to 4 - 5 presses. Such plants usually use adhesive systems with very long pot lives and assembly times and have a majority of hot presses.

[0011] Other plants have invested in additional roller spreaders and are servicing only one or two presses with each roller spreader, in such plants the majority of the presses are radio frequency presses. These plants tolerate adhesive systems with shorter pot lives and have shorter pressing times due to higher production efficiency. Regardless of press technology and setup of the individual plants is a way to increase the production capacity, without huge investments either in number of presses and people, of interest.

[0012] The form pressed panels consist of wood, a natural material that will shrink and swell depending on temperature and moisture content. The veneer sheets have a certain moisture content before being glued together (normally between 2 and 10%) and as the urea formaldehyde adhesives contains water as solvent, the resulting panel will have even higher moisture content. When the form pressed panels is finished they have high temperature, from 60 to 130°C. During the next hours and days the panels cools down to room temperature and the moisture content of the panel reaches equilibrium with the moisture content in the surrounding air. [0013] Due to these processes, and the shrinking and swelling properties of the wood, the shape of the panels will change. This process is called spring back. The higher the pressing temperature are or the longer the pressing time is the higher the temperature of the panels will be, and as a result the larger the spring back. Hence a way to reduce the final temperature of the panels, either by reducing the pressing time or the pressing temperature, would be greatly appreciated by the industry. [0014] PCT/EP2009/060462 describes a two component adhesive system for production of wood based products such as parquet and form pressed panels. The adhesive system consists of two adhesive components, namely and MF and a dispersion based adhesive component (preferably PVAc), mixed in a ratio from 1 :0.5 to 1 : 1.5. The two adhesive components can be mixed together before application or applied separately on two different veneer sheets.

[0015] WO 2007/025623 refers to a method for producing veneered compression mouldings in which at least one support and an outer veneer are two-dimensionally connected to one another in a form press to form a blank form part. It is provided a continuous layer of a viscous polyester impregnating agent applied to a surface of the outer veneer, the layer of impregnating agent is subsequently physically dried while retaining a flexibility of the outer veneer and the layer of impregnating agent is cured at an elevated temperature during the form pressing operation utilizing a metal foil during form pressing.

[0016] In WO 2008/078003 wood panels consisting of a number of veneers sheets is joined together with a thermoplastic material. The thermoplastic material is heated above the melting temperature to facilitate the joining. Reheating of the panels above the melting temperature of the thermoplastic material allows form pressing of the panels. The thermoplastic material can be applied as polymer sheets, in a melt form or in a dispersion form.

[0017] JP 2003-320501 concerns use of a non-formaldehyde adhesive system for production of plywood at reduced press temperatures compared to when using formaldehyde based adhesives. The adhesive system consists of polyvinyl alcohol and/or an aqueous emulsion like Butadiene/Styrene Copolymerisate, a filler and an isocyanate based compound. The cited document uses a PVOH/Isocyanate MDI system to achieve a curing at a lower temperature between 60°C and 100 °C. [0018] WO 89/05221 relates to a process for the production of plywood. In the process a formaldehyde based curable adhesive with low reactivity is applied to a surface of a veneer layer and a separate application hardener is applied to a veneer layer opposite to the one coated with adhesive in a zone along at least two edges on the opposed surfaces. A number of veneers are coated in this manner and the veneers are then assembled to a package which is cold pre-pressed during which time the adhesive in contact with the separate application hardener cures. After the pre-pressing the stack is held together by the cured strips by which the need for adhesive systems with good cold tack is eliminated. The stack is later cured by pressure and heat as normal. The disadvantage of this process is that it requires two different hardener systems, two separate pressing steps and means to apply hardener along the edges. Further, the process can be applied only in plywood technology where the veneer stacks are flat and cannot be used for form pressing of shaped pieces with 3 to 50 layers of veneer

[0019] WO 02/092711 describes separate application of adhesive and hardener to opposite surfaces. However, this document describes using specific two component adhesives for the adhesive joining of rigid panels like Parquet elements when laying a parquet floor, which does not relate to production of form-pressed products. [0020] EP 1 368 169 and EP 1 368 168 describe the separate treatment of veneer or other wood components, before or after the gluing process with a water solution of ammonium salts and urea to reduce the formaldehyde emission from the glued product.

[0021] US 2009/0162680 describes a process with short pressing times for multilayered veneer panels using an emulsion based adhesive cured with and isocyanate based hardener. In this document both sides of the veneer layers are used to be glued, one of them or in between with the use of a fast setting adhesive, wherein the hardener is already mixed. The process uses as fast setting adhesives styrene-Butadiene-Copolymer Latex; PVAc, EVA and pMDI.

[0022] WO2010/018202 describes a two component adhesive system for production of interior wood products, in particular form-pressed products, parquet floor elements and solid wood panels with very low formaldehyde emission. The two-component adhesive system comprises adhesive component I comprising a MF type adhesive resin and adhesive component II comprising a water based dispersion adhesive and a formaldehyde scavenger which adhesives can be applied to one surface or on opposing surfaces.

[0023] WO2004/104124 describes process for joining surfaces for production of wood products like plywood etc which provides high quality joints at a short pressing time. This is achieved in essence by drying off the adhesives to a higher solids content. The process comprises in the following order: applying an adhesive composition (A), comprising a solvent, onto a first surface and applying a component (C) comprising a solvent and optional adhesive (B) comprising a solvent onto a second surface, wherein the solvent is dried off, partially or completely by forced drying whereby the solids content of (C) is lower than (A) and pressing the two surfaces against each other Component (C) is water or an adhesive composition comprising hardeners. The drying of the adhesive can be done outside of the press and can reduce pressing times, but it still is a disadvantage from process economic point of view to introduce a tike consuming drying step and there remains a desire to further reduce the pressing times without risking adequate bond strength. [0024] Therefore, an object of the invention is to provide a process for the manufacture of form-pressed products having at least 3 and preferably less than 50 layers of veneer, which has one or more of the advantages of increasing the reactivity and the speed of curing, which results in significantly reduced pressing times; and/or reduced pressing temperature, which results in a reduction of the final temperature of the material, which reduces spring back and is more economical whilst maintaining a good pot life of the adhesive system.

SUMMARY OF THE INVENTION

[0025] The present invention addresses these problems by providing a fast curing, efficient pressing process for the manufacture of form-pressed products having at least 3 and preferably less than 50 alternating veneer layers A and B, said process comprising the steps of;

- providing a glue mixture of adhesive I and hardener II on both sides of one or more veneer layers A;

- providing a water based hardener ΙΠ on both sides of two or more veneer layer(s) B;

- laying-up in a stack veneer layers A alternating with veneer layers B,

- pressing the stack at elevated temperature and pressure to form the form-pressed product; and wherein said veneer layers A and B are stacked such that the hardener ΙΠ on veneer layers B get in contact with the glue mixture on veneer layers A during pressing and wherein the first and the last veneer in the stack only have glue mixture or hardener ΙΠ on the inner side of the veneers. [0026] The hardener composition ΠΙ on veneer layer B can either be applied directly before assembly of the veneer layers in the stack, so that the veneer layer B is still wet at the time of laying up in the stack (also referred to as assembly); or alternatively the hardener composition ΙΠ on the veneer layer is dried sufficient time before the assembly; for hours, days or even months, so that the veneer layer B is dry at the time of assembly. As described in the examples it is preferred to dry the hardener ΙΠ as it allows longer assembly times. [0027] In the process according to the invention the hardener ΠΙ is more reactive than hardener II and preferably hardener ΙΠ has a lower pH than hardener II, in particular the pH of hardener Π is between 2 and 6, preferably between 2.5 and 5 and the pH of hardener ΠΙ is between 0 and 2.5, preferably 0 and 2 and more preferably between 0 and 1. Preferably the hardener salts (lib) in hardener II are chosen from the group of acid generating salts, preferably ammonium salts such as ammonium chloride, ammonium sulphate and ammonium phosphates, or acidic salts, preferably aluminium salts such as aluminium nitrate, chloride or sulphate and wherein the hardener ΠΙ comprises acids or salts thereof (in.b) in an amount of 5 to 30 wt%, preferably para toluene sulfonic acid, citric acid, oxalic acid, lactic acid, sulphuric acid, phosphoric acid, sulfamic acid, maleic acid and salts thereof.

[0028] In a preferred embodiments of the process of the invention there is provided a process for manufacture of a form pressed product using a three-component adhesive system comprising:

(I) an adhesive I comprising (in wt% relative to the total weight of adhesive I); - La) 50- 75 wt% of a urea formaldehyde (UF) type resin; and

- Lb) 0 - 20 wt% of organic or inorganic filler; and

- I.c) 0 - 10 wt% of further additives; and

- I.d) 25 - 40 wt% of water; and

(Π) a hardener II being a powder or water based mix-in hardener comprising (in wt% relative to the total weight of hardener Π) in case of a water based hardener

-II. a) 10 - 45 wt% of water; and

- n.b) 1 - 15 wt% of hardener salts; and

- n.c) 10 - 45 wt% of urea; and - Il.d) 0 - 10 wt% of further additives; and

- Lie) 10 - 40 wt% of organic or inorganic fillers; and

in case of a powder hardener

- LLf) 0 wt% of water; and - n.g) 1 - 15 wt% of hardener salts; and

- n.h) 10 - 65 wt% of urea; and

- Π.ί) 0 - 5 wt% of further additives; and

- n.j) 10 - 50 wt% of organic or inorganic fillers;

(ΙΠ) a water based separate application hardener ΙΠ comprising (in wt% relative to the total weight of hardener ΙΠ)

- m.a) 50 - 80 wt% of water; and

- m.b) 5 - 30 wt% of acids or salts thereof; and

- m.c) 0 - 10 wt% of urea; and - ΙΠ. d) 0 - 10 wt% of further additives

[0029] The invention also relates to the above three-component adhesive system and its use for the manufacture of plywood or form pressed products, in particular in the process according to the invention.

[0030] Preferably, the adhesive I and the hardener Π are mixed in a weight ratio of 100:5 to 100:40, preferably 100: 10 to 100:30 and the application rate of the separate application hardener ΠΙ on the wood is 5 -30 g/m², preferably 10 - 20 g/m². The ratio is chosen by the skilled person in view of optimising pressing time versus pot-life. Preferably, the adhesive I has a molar ratio of formaldehyde (F) to urea (U) between 1.00 and 1.70, preferably 1.15 and 1.60 and most preferably between 1.20 and 1.50. [0031] It was surprisingly found as exemplified by the examples that the process according to the invention reduces the pressing time leading to increased productivity while maintaining the bond quality of the glued product. The reduced pressing time is independent of the pot life; it can be obtained with adhesive systems with everything from short to long pot lives. It was found that with a normal good pot life of at least 8 - 10 hours at 20 °C, the pressing time could be reduced by at least 40%, sometimes at least 50% and even at least 60%.

DETAILED DESCRIPTION OF THE INVENTION

[0032] Plywood and form-pressed products (also called form-bent products or shaped/curved plywood products) typically consist of from 3 to 50 layers of veneer glued together with an adhesive system. Plywood is produced in a flat press whereas for form pressed products the gluing process is performed in a mould to give the final product the desired shape. In plywood, the wood layers (also referred to as veneer sheets) are placed perpendicular to each other with the wood grain of each sheet crosswise, whereas in form-pressed products the veneer sheets are most commonly placed with the grain of each veneer sheet in the same direction. The veneer sheets are typically from 0.5 to 3 mm thick. Most common wood species for form pressing is beech, but in principle all types of wood can be used both for production of plywood and for production of form pressed panels.

[0033] The present invention provides a fast curing, efficient pressing process for the manufacture of form- pressed products and plywood with at least 3 and preferably less than 50 layers of veneer; and said process comprises several steps.

[0034] The first step is providing a glue mixture of adhesive I and hardener II on both sides of veneer layers A. The application of the mixture of the adhesive I and the hardener Π onto the surface of the veneer layers can be done by any suitable method known in the art, such as spraying, brushing, extruding, roll-spreading, curtain-coating etc. forming shapes such as droplets, one or several strands, beads or a substantially continuous layer. In cases where the glue mix is applied on the first, third, fifth veneer etc. as well as the last veneer, the first and the last veneer is only applied glue mix on one side, the inner side of the veneer, to avoid sticking in the press. [0035] In the cases where the glue mix is applied on the second, fourth, sixth etc. veneer, the glue mix is applied on both sides of each veneer. The adhesive I and the hardener II are mixed in a weight ratio of 100:5 to 100:40, preferably 100: 10 to 100:30. The glue mix is applied on the wood layer in an amount between 100 and 250 g/m², preferably between 120 and 180 g/m², more preferably between 150 and 160 g/m². [0036] The second step is providing the water based hardener ΠΙ on both sides of veneer layers B. The separate application hardener ΙΠ is preferably applied in an amount of 5 and 30 g/m², more preferably between 10 and 20 g/m². The application of the hardener ΙΠ onto the surface of the veneer layers can be done by any suitable method known in the art, such as spraying, brushing, extruding, roll-spreading, curtain-coating etc. forming a continuous layer. The hardener ΠΙ composition on veneer layers B can either be applied directly before assembly of the veneers in the stack so that the veneers still are wet at the time of assembly or before (hours, days or months) the assembly so that the veneer layers B are dry at the time of assembly.

[0037] In cases where hardener ΠΙ is applied on the first, third, fifth veneer etc as well as the last veneer, the first and the last veneer is only applied hardener on one side, the inner side of the veneer, to avoid sticking in the press. In the cases where the hardener is applied on the second, fourth, sixth etc veneer, the hardener is applied on both sides of each veneer.

[0038] The third step involves laying-up in a stack at least one of said veneer layers A alternating with at least two veneer layers B, wherein said veneer layers A and B are stacked such that the hardener ΠΙ composition on veneer layers B contacts the glue mix composition on veneer layer(s) A during pressing. The first and the last veneer in the stack should only have glue mixture or hardener ΙΠ solution on the inner side of the veneers, to avoid sticking in the press. In cases where the stack only comprises 3 veneer sheets, only the internal veneer(s) has glue mix or hardener ΠΙ on both sides.

[0039] It is pressing machine depending whether the layer A or the layer B is the starting layer of the veneer, resulting in sequence A-B-A-B-A-B-A[ ] or B-A-B-A-B-A-

B[ ] respectively; it further depends on the most practical way and the preferences in different plants in regard to the available equipment.

[0040] The process can be used both with hot pressing and radio frequency heating. It can be used both to reduce the pressing time and/or to reduce the temperature of the pressing plates in the hot press and will give significant increase in production efficiency, reduced electricity cost for each produced unit, lower temperature of the final panel and less tendency of spring back of the glued panels.

[0041] The fourth step involves pressing the plywood in a flat press or, in the case of form pressed product, moulding the stack at elevated temperature and pressure. In both cases the stack is pressed in a heated press between 40 and 120°C, preferably 50 to 100°C or most preferably 60 to 100°C or radio frequency (RF) press where temperatures between 50 and 110°C is normally created. The pressure is normally 0.5 and 2 N/mm², preferably between 0.6 and 1.5 N/mm².

[0042] Adhesive I preferably comprises urea formaldehyde polymer to an amount of 50 to 75 wt%, more preferably 60 to 72 wt% or most preferably 65 to 71 wt% (La) to the total weight of the adhesive I, with a molar ratio of formaldehyde to urea in the range 1.00 to 1.70 or more preferably 1.15 to 1.60 or most preferably 1.20 to 1.50. Furthermore, adhesive I comprises 25 to 40 wt% of water (Id), preferably 27 to 35 wt% or most preferably 28 to 33 wt%. The adhesive I has high pH, preferably between 7 and 10, more preferably between 8 and 9. This is preferred in the view of the stability of the adhesive solution, because the correct pH of the adhesive prevents premature cross linking of the adhesive.

[0043] Filler may be included in adhesive I and/or hardener Π to adjust the glue mix viscosity and improve the bond quality by adjusting the penetration of the glue into the wood and allow the adhesive to better accommodate irregularities in the surface of the substrate. The filler may be both inorganic and organic in origin. Examples of suitable organic fillers are different types of flours and starches, wood flours or wood fiber, different types of nut shell flours. Examples of suitable inorganic fillers are clay, in particular china clay (kaolin), talc and calcium carbonate. The amount of organic and/or inorganic filler (Lb) in the adhesive I is between 0-20 wt%, preferably 0.1-20 wt% , more preferably 2 to 10 wt% or most preferably 3 to 7 wt%. Further, adhesive I may comprise 0-10 wt% of one or more further additives (I.c), preferably 1 to 8 wt% or most preferably 2 to 7 wt% chosen from the group of anti -foaming agents, thickeners, surfactants, pigments, colorants, rheology modifiers, and/or flexibilisers.

[0044] In the process of the invention the glue mixture is mixed from an adhesive I and a hardener Π before, preferably shortly before, applying the glue mixture to veneer A. The fillers in composition I may be same or different from the fillers of (Π). Which filler to use depends on achieving stable compositions in the different compositions. In the resulting glue mixture it is irrelevant where the fillers come from. The same applies to the water content in the resulting glue mixture.

[0045] The hardener II is either a powder hardener or water based hardener. The powder hardener will be free of water (Il.f) with exception to the water already present in the fillers (n.j) and hardener salts (Kg) at the time of blending. The water based hardener Π comprises between 20 and 45 wt% water (Ka) in which the hardener salts (Kb) are dissolved.

[0046] The hardener II may contain one or more of the acidic salts and/or acid generating salts in combination with each other. Acid generating salts are salts that react with formaldehyde to generate acid, examples of such salts are ammonium salts such as ammonium chloride, ammonium sulphate and ammonium phosphates. Acidic salts are salts that give acidic pH when they are dissolved in water. Examples of such salts are aluminium salts such as aluminium nitrate, chloride or sulphate. The salts are combined in the amounts necessary to give the desired reactivity of the hardener. The hardener Π may contain both acid generating salts and acidic salts.

[0047] The water based hardener Π may contain between 10 to 45 wt% urea, preferably 20 to 43 wt% or most preferably 25 to 40 wt% urea (He) to modify the molar ratio of the adhesive to prevent excess emission of formaldehyde from the glued panels. Further, water based hardener Π may comprise of 10-45%, preferably 20-45% water (Il.a), 1- 15wt% hardener salts (n.b), 0-10 wt% of one or more further additives (II. d), preferably 1-10 wt%, more preferably 1 to 8 wt% chosen from the group of anti-foaming agents, thickeners, surfactants, pigments, colorants, rheology modifiers, and/or flexibilisers. The amount of organic and/or inorganic fillers (He) in the water based hardener II is preferably 10-50 wt%, preferably 10- 40 wt% but more preferably 12 to 38 wt%. The water based hardener II has acidic pH, preferably the pH is between 2 and 6, more preferably between 2.5 and 6 and even more preferably between 2.5 and 5. The glue mix has a pH of about 7 or 6 which decreases by reaction of reaction of formaldehyde with acid generating salts.

[0048] The powder hardener Π may contain no water (Hf), between 10 to 65 wt% urea, preferably 10 to 50 wt% or most preferably 20 to 40 wt% urea (Il.h) to modify the molar ratio of the adhesive to prevent excess emission of formaldehyde from the glued panels. Further, powder hardener II may comprise of 1- 15wt% hardener salts (II. g), 0-5 wt% of one or more further additives (Hi) chosen from the group of thickeners, pigments, colorants, and/or rheology modifiers. The amount of organic and/or inorganic fillers (Il.j) in the powder hardener II is preferably 10-50 wt% but more preferably 12 to 40 wt%. The powder hardener II has acidic pH, preferably between pH 2 and pH 6, more preferably pH 2.5 and pH 5 if it is solved in water.

[0049] The separate application hardener HI is water based, containing 50 to 80 wt% or preferably 60 to 70 w% water (in.a). The water based separate application hardener ΙΠ contains acids (ffi.b) and/or salts thereof in an amount of 5 to 30 wt%, preferably 10 to 25%. Generally all organic and inorganic acids and/or salts thereof will work; examples of suitable acids are para toluene sulfonic acid, citric acid, oxalic acid, lactic acid, sulphuric acid, phosphoric acid, sulfamic acid, maleic acid and salts thereof. When producing water based separate application hardener ΠΙ containing maleic acid, this is often produced by use of maleic anhydride, maleic acid is formed by dissolving the maleic anhydride in water. Urea (Ox) may be optionally added into water based separate application hardener ΠΙ to scavenge formaldehyde in the cured glue and amounts to 0 - 10wt%, preferably 1 to 8 wt%. Further, water based separate application hardener ΠΙ may comprise 0-10 wt% of one or more further additives (Hid) or preferably 1 to 8 wt% chosen from the group of anti -foaming agents, thickeners, surfactants, pigments, colorants, rheology modifiers, and/or flexibilisers and preferably no adhesive. Preferably the hardener ΠΙ essentially consists of the above described components ffi.a to ffi.d. The water based separate application hardener ΠΙ has acidic pH, between 0 and 2.5, preferably 0 and 2, more preferably between 0 and 1. The hardener ΙΠ is more reactive than hardener Π, preferably hardener ΠΙ has a lower pH than hardener Π.

[0050] Thus, the invention has been described by reference to certain embodiments discussed above. It will be recognized that these embodiments are susceptible to various modifications and alternative forms well known to those of skill in the art. Further modifications in addition to those described above may be made to the structures and techniques described herein without departing from the spirit and scope of the invention. Accordingly, although specific embodiments have been described, these are examples only and are not limiting upon the scope of the invention.

EXAMPLES AND TEST METHODS

Test methods Viscosity measurements

[0051] Viscosity was measured according ISO 2555: 1999 at 20°C or 25°C, on adhesive alone, the hardener alone or mixtures of adhesives and hardeners. The measurement was done on 200 g samples in metal beakers. The samples were adjusted to the desired temperature 20°C or 25°C before the measurement. The measurements were done by use of a Brookfield instrument, model DV-1 +. For viscosities below 10 000 mPa.s spindle 4 were used with a speed of 20 rpm (rotations per minute). For viscosities between 10 000 and 20 000 mPa.s spindle 5 were used with a speed of 20 rpm. Pot life measurements

[0052] Pot lives are measured on mixtures of adhesive and hardener. The pot life is defined as the time it takes from the two components is mixed until the viscosity has increased to 12000 mPa.s. At this viscosity the glue mix is too thick to handle it properly. The pot life is dependent on the temperature of the glue mix; the higher the temperature the shorter the pot life. The standard temperature for measurement of pot life is 20°C, but for glue mixtures with very low reactivity, where the pot life would be very long at 20°C for about 6-8 hours the measurements are done at higher temperatures (25 or 30°C), this has to be reported with the measured value. [0053] The measurement of the pot life is started at the time of mixing of adhesive and hardener. After the mixing the glue mix is adjusted to correct temperature (usually 20°C) and the viscosity is measured as described in the section for viscosity measurement. After the measurement the metal beaker, in which the glue mix is stored, is placed in water with the desired temperature. The viscosity of the glue mix is measured at regular intervals, for example every 15^th minute for glue mixes with short pot life and every 30^th or 60^th minute for glue mixes with long pot life.

Breaking test for screening of layer bond quality (quick manual test)

[0054] Two sheets of 1.5 mm thick beech veneer ( 20 by 40 cm) are glued together. The bond quality is assessed by cutting strips of 4 cm from the panel and laying the strips onto the edge of e.g. a table, so that the majority of the strips looking over the edge. The strips are then broken by pulling them quickly over the edge so that the breaking pattern can be observed.

This test is done at specific times after pressing (immediately, after 1 min, after 2 min etc.).

Inspection of the breaking pattern shows how well the glue line has cured. Insufficient cure causes the veneer sheets to separate (delaminate) or break with a very ragged edge. With sufficient cure (fully cured glue lines) the break is clean or nearly clean with no signs of delamination.

Knife test according to British Standard 1455

[0055] Gluing tests were done according to British Standard 1455 to evaluate bond quality. Pieces (15 x 15 cm) were cut from panels consisting of sheets of veneer glued together with parallel wood grain. Half of the samples were kept as they are dry and at room temperature for test of dry strength (dry test, DRY test) whereas the rest of the samples are soaked in water (15 ± 5°C) for 16 - 24 h (interior test, INT test). After the time all the samples (dry and wet) are tested. The aim of the test is to open the glue line (this is the glue matter in between two veneers. If the glue line is able to be opened, the bonding is weak and gets a low number (e.g.O or 2), if the glue line is strong, the opening of the glue line is likely only possible by damaging of the veneers. In this case, the sample glue line gets a high number (e.g. 8 or 10)

[0056] The gluing quality of the pieces was assessed by inserting a chisel into the glue lines to split the wood pieces apart with a hammer. The resulting fracture surface was visually inspected and fracture of the wood vs. fracture in the glue line as such was assessed. The results are evaluated by comparing the wood failure with examples given in the standard. Results are graded in a scale from 0 to 10 based on evaluation of the amount of wood fracture. Pieces with 100 % wood fracture indicates a glue line of excellent quality as there are no fractures in the glue line as such and is given a grade of 10. Pieces with no wood fracture were graded as 0. Grades above 6 are acceptable and grades above 8 were considered very good. The grading of each glue line is reported separately both for the dry test and the INT test.

ADHESIVE SYSTEMS

[0057] The composition of adhesives used in the examples is given in Table 1. The composition of the hardeners used in the examples is given in Table 2.

Table 1 Adhesive systems

* wt% relative to the total weight of adhesive system Sample Nr. 2-1 2-2 2-3 2-4 2-5

Hardener Number Π in ΠΙ Π Π

Water* 29.5 68 83.4 34.8 ammonium chloride * (solid) 4 2 6

Monoammonium phosphate* 4

Urea (solid) * 40 7.5 3.8 40 60

Aluminium nitrate with 9 crystal water (solid) * 4 9

Sulphuric acid (100%)* 3.3 1.7

Sulfamic acid (solid) *

Maleic anhydride (solid) * 21 10.5

Thickener (solids) * 0.2 0.1 0.4

Filler(s) (solids) * 22 13 25

Additives (liquids) * 0.5 0.8 5

Table 2 Hardeners

* wt% relative to the total weight of hardener II and ΙΠ Example 1 Effect of use of separate application hardener III

[0058] Two 1.5 mm thick veneer sheets of beech were glued together with an adhesive system consisting of a mixture of a adhesive I and hardener II at a pressing temperature of 95°C with a pressure of 1.2 N/mm². The pressing were done for varying duration between 0,5 and 3 minutes.

[0059] The glue mix was applied on the surface of only one of the veneers (160 g/m²). In the comparative examples (Al . l, A1.2 and A1.3) the second veneer was untreated. In the tests according to the invention (B 1.4, B 1.5, C 1.6 and C 1.7) a separate application hardener ΠΙ was applied on the second surface (ca 20 g/m²) and allowed to dry. The veneer surface with the separate application hardener ΙΠ was assembled with the veneer surface on which the glue mix was applied and then pressed in a hot press under the conditions described above. The bond quality was evaluated as described in the test method "Breaking test". Adhesive Evaluation of bond quality after gluing system

Separate Pressing

Sample (mixed

application time

Nr. 100/20

hardener (min) After 1 min After 2 min After 3 min parts by

weight)

Full Full Full

Al . l (1-1/2-1)* none 1

delamination delamination delamination

Partly Partly Partly

A1.2 1-1/2-1 none 2.5

delaminated delaminated delaminated

Some Signs of Good

A1.3 1-1/2-1 none 3.0

delamination delamination bonding

Very good Very good Very good

B1.4 1-1/2-1 2-2 1

bonding bonding bonding

Good Good Very good

B1.5 1-1/2-1 2-2 0.5

bonding bonding bonding

Very good Very good Very good

CI .6 1-1/2-1 2-3 1

bonding bonding bonding

Good Very good Very good

CI .7 1-1/2-1 2-3 0.5

bonding bonding bonding

Table 3 Breaking test to compare necessary pressing times with and without separate application hardeners. * The pot life of this adhesive system is 8 - 10 h at 20°C.

[0060] The glue system 1-1/2-1 used in the comparative example Al . l has relatively low reactivity (pot life 8 - 10 h at 20°C) and requires a pressing time of 3 min or longer to cure the glue line between two 1.5 mm thick veneer sheets at a pressing temperature of 95°C (A1.3). In the examples according to the invention one of the surfaces is treated with separate application hardener. This reduces the pressing time to 30 seconds for both the separate application hardeners that were tested (B1.5, CI .7). As the adhesive system is only brought in contact with the separate application hardener after assembly of the veneers the reduction in pressing time does not affect the pot life of the adhesive system.

Example 2 Breaking test and knife test of 7 layer veneer panels

[0061] In this example seven 1.5 mm thick veneer sheets were glued together at a pressing temperature of 95 °C with a pressure of 1.2 N/mm². [0062] In the comparative examples D2.1 and D2.2, only untreated veneer sheets were used and the adhesive system (160g/m²) was applied on the inner sides of veneer nr 1 and 7 as well as of both sides of the third and the fifth veneer sheets. All the seven veneer sheets were then assembled and pressed.

[0063] In the examples according to the invention, E2.3 and E2.4, the second, fourth and sixth veneer sheet were treated with separate application hardener (20 g/m²) on both sides and allowed to dry. The adhesive system (160g/m²) was applied on the inner sides of veneer nr 1 and 7 as well as of both sides of the third and the fifth veneer sheets. All the seven veneer sheets were then assembled and pressed. The bond quality was evaluated by cutting strips of the panels and performing the breaking test as described above.

Table 4 Comparison of necessary pressing times with and without separate application hardener.

[0064] The invention gives at least 75% reduction of the pressing time: Panels according to the inventive example E2.4 require only 1 ½ min pressing time whereas the panels in the comparative example D2.2 requires 6 min pressing time.

[0065] The aforementioned glued panels were tested according to British Standard 14455 (the "Knife test"). In addition were panels according to the invention produced with lower application rate of separate application hardener (10 g/m²) to allow evaluatiol7n of the effect of the dosage (F2.5 and F2.6). The results are given in Table 5. The test properties are given in [0057]. The line of numbers in Table 5 (e.g. 2-2-5-6-5-6) indicate the evaluation given by the examiner to the glue lines of each sample veneer stack by opening the glue lines with chisel and hammer according to British Standard 1455.

Table 5 DRY test and INT test of the panels glued in Table 4 plus additional panels glued with reduced amount of separate application hardener.

[0066] For the comparative examples, test panel D2.1 is inferior to test panel D2.2 in both the dry test and the INT test. This confirms that 5 min pressing time as used for panel D2.1 is too short for this glue system. The bond quality for the panels glued according to the invention, E2.3, E2.4, F2.5 and F2.6 show acceptable bonding for both pressing times and both application rates of separate application hardener. Hence, it can be concluded, that 1 ½ min pressing time is sufficient both with 10 and 20 g/m² application of separate application hardener (E2.4 and F2.6). This shows that the invention is able to achieve acceptable bonding with a significant reduces pressing time.

Example 3 Assessment of the effect of use of dry and wet separate application hardener on the assembly time

[0067] The "assembly time" is the time from the glue mix is applied on the veneer until the stack is in the press and pressure is applied. When the assembly time is too long the bonding will be poor due to dry-out or pre-cure of the glue line. When using separate application hardener in addition to the glue mix, the risk is that the assembly time will be reduced due to pre-cure in the areas where the glue film comes in contact with the separate application hardener. Tests according to the invention were done to investigate the effect drying the veneer sheet when separate application hardener is applied (H3.5-H3.8), in comparison, when the separate application hardener ΠΙ was still wet (G3.1 to G3.4) compared to drying the hardener before use (H3.4 to H3.8). The gluing was performed as described in Example 1 : Two 1.5 mm thick veneer sheets of beech were glued together with an adhesive system consisting of a mixture of a adhesive I and hardener Π at a pressing temperature of 95°C with a pressure of 1.2 N/mm². The pressing were done for 2,5 minutes.

[0068] The glue mix was applied on the surface of only one of the veneers (160 g/m²). In the tests according to the invention (H3.5-H3.8) a separate application hardener ΙΠ was applied on the second surface (ca 20 g/m²) and allowed to dry. In the other test samples the surface was wet (G3.1-G3.4). The veneer surface with the separate application hardener ΠΙ was assembled with the veneer surface on which the glue mix was applied and then pressed in a hot press under the conditions described above. The bond quality was evaluated as described in the British Standard 1455 knife test. The results are given in Table 6.

Table 6 Investigation of the effect of using dry or wet separate application hardener on assembly time

[0069] The test results show that the maximum assembly times is longer when the separate application hardener is dried before gluing compared to gluing without drying the hardener. The test also shows that the best bonding results are achieved, when the veneer with the separate application hardener ΠΙ are dried.

Example 4 Use of separate application hardener in combination with a reactive adhesive system [0070] Example 1 shows that the pressing time can be reduced with 75% by using separate application hardener with an adhesive system with low reactivity (pot life 8 - 10 h at 20°C). In the current example the combination of a reactive adhesive system (pot life of ca lh 20 min at 20°C) and separate application hardener is investigated. The gluing was performed like in Example 1, but except the pressing temperature, which was in this example at 70°C, followed by a "breaking test". The results are given in Table 7.

Table 7 Comparison of necessary pressing times with and without separate application hardener.

[0071] For the comparative example (14.1) 3 min pressing time is the minimum to give good bonding. The tests according to the invention (J4.2, J4.3) show that the invention reduces the pressing time with 50%.

Example 5 Use of separate application hardener to reduce pressing time in panel of 9 veneer layers

[0072] Panels consisting of nine layers of veneers with parallel wood grain were glued with and without separate application hardener with a adhesive system with long pot life (4h at 25°C). The pressing time to achieve adequate bonding was investigated. Pressing temperature was 90°C at 1.0 N/mm². Evaluation of the bond quality was done by cutting 4 cm strips of the glued panels and performing the "breaking test" on these strips.

[0073] In the comparative examples, K5.1 and K5.2, only untreated veneer sheets were used and the adhesive system (160g/m²) was applied on the inner sides of veneer nr 1 and 9 as well as of both sides of the third, fifth and the seventh veneer sheets. All the nine veneer sheets were then assembled and pressed.

[0074] For the panels glued according to the invention, L5.3 and L5.4, the separate application hardener (20 g/m²) was applied on veneer number 2, 4, 6 and 8, whereas the adhesive system (160g/m²) was applied on the inner side of veneer 1 and 9 as well as on both sides of veneer 3, 5 and 7. All the nine veneer sheets were then assembled and pressed. The results are given in Table 8.

Table 8 Breaking test to compare necessary pressing times with and without separate application hardener, hot pressing

[0075] Use of separate application hardener according to the invention reduces the pressing time with ca 50% compared to the comparative example (K5.2 vs. L5.3).

[0076] The results above are obtained with hot pressing. The test was repeated with radio frequency pressing to investigate if the same effect is obtained also with this pressing technique. The results of the comparative tests, M6.1, M6.2 and M.6.3, are compared with a system according to the invention, N6.4, are given in Table 9. Adhesive

Separate Radio

Test system (mixed Evaluation of bond quality 2 min application frequency

nr. 100/10 parts by after gluing

hardener time (sec)

weight)

M6.1 1-3/2-5 None 45 Very good bonding

M6.2 1-3/2-5 None 25 Delamination

M6.3 1-3/2-5 None 20 Delamination

N6.4 1-3/2-5 2-2 20 Very good bonding

Table 9 Necessary pressing times with and without separate application hardener, radio frequency* pressing

*Techwood lab press from Techwood Denmark. The press heats the substrate to be pressed with radio frequency current (Voltage 3 x 230/400, Frequency 13,56 MHz). The current can be directed both horizontally and vertically through the substrate.

[0077] Use of separate hardener according to the invention reduces the pressing time with ca 50 % (K6.1 vs K.6.4). Thus, the same result is obtained regardless of pressing technique.

Example 6 Pot life measurements

It has been shown that use of separate application hardener according to the invention for gluing of form pressed constructions reduces the pressing time with 50% or more regardless of pressing technique. The current example illustrates why the separate application hardener cannot be mixed into the glue mix but have to be applied separately. The glue mix in the comparative example 07.1 is the same as for the comparative examples in Example 5 (K5.1 and K5.2 (Table 8) and M6.1, M6.2 and M6.3 (Table 9). The mixing ratio between glue mix and separate application hardener according to the invention in P7.2 are the same as the total composition of glue mix during pressing of the glue lines in L5.3, L5.4 and N6.4 in Table 8 and 9 respectively.

Table 10 Pot life measurements of glue mixes with and without separate application hardener

[0078] The results in Table 10 show that mixing the separate application hardener into the glue mix gives a glue mix with no pot life. It gels immediately and is not usable for any bonding operations in the wood working industry as it would gel in the application equipment immediately after mixing. Hence, to use the invention the separate application hardener has to be applied separately it cannot be mixed with the rest of the components.

Example 7 Comparison of the combination mix in system/separate application hardener vs adhesive/separate application hardener

[0079] Use of separate application hardener, according to the invention, has been shown to reduce the necessary pressing time with 50% when gluing panels consisting of several layers of veneer. An experiment was performed to see if the same good result can be obtained by separate application of the adhesive alone and a combination of the two hardeners. The example was designed so that the glue line contains exactly the same components in all tests; only how they are applied is varied. The gluing test was performed as described in Example 1. The results are given in Table 11.

Table 11 Necessary pressing times with and without separate application hardener, radio frequency pressing [0080] The tests in Table 11 show how the bonding properties were under two separate conditions: First (Q8.1.) the hardeners II and II were combined and applied in one veneer side, while the adhesive I was applied in the side of another veneer. Stacking and pressing the veneers together show, that at least 3 minutes pressing time are necessary to achieve a good bonding. The second test (R8.2) was made according to the invention by combining adhesive I and hardener Π (glue mix) on one side of the veneer and the separate hardener ΙΠ on the side of another veneer. Stacking and pressing the veneers together show, that at least 1 minute pressing time results in good bonding. Overall, the bond quality in the inventive example R8.2 is much better to the bond quality obtained in the comparative example R8.1.

Claims

WHAT IS CLAIMED IS:

1. A process for the manufacture of form-pressed or plywood products having at least 3 and preferably less than 50 alternating veneer layers A and B, said process comprising the steps of;

- laying-up in a stack veneer layers A alternating with veneer layers B,

- pressing the stack at elevated temperature and pressure to form the form-pressed product; and wherein said veneer layers A and B are stacked such that the hardener ΠΙ on veneer layers B get in contact with the glue mixture on veneer layers A during pressing and wherein the first and the last veneer in the stack only have glue mixture or hardener ΠΙ on the inner side of the veneers.

2. The process according to claim 1, wherein the hardener ΙΠ on veneer layer B is applied directly before laying-up of the veneer layers in the stack, so that the veneer layer B is still wet at the time of laying-up or alternatively wherein the hardener ΙΠ on the veneer layer B is dried before laying-up.

3. The process according to claims 1 - 2 wherein the stack is pressed in a flat press or in a form press by heat between 40 and 120°C, preferably 50 to 100°C or most preferably 60 to 100°C, at a pressure between 0.5 and 2 N/mm², preferably between 0.6 and 1.5 N/mm², preferably the heating being achieved by radio frequency (RF) to a temperature between 50 and l lO°C.

4. The process according to claims 1 - 3 wherein hardener ΙΠ is more reactive than hardener Π, preferably hardener ΠΙ has a lower pH than hardener II.

5. The process according to claims 1- 4 wherein the pH of hardener Π is between 2 and

6. preferably between 2.5 and 5 and the pH of hardener ΠΙ is between 0 and 2.5, preferably 0 and 2 and more preferably between 0 and 1.

6. The process according to claims 1 - 5 wherein the hardener salts (lib) in hardener II are chosen from the group of acid generating salts, preferably ammonium salts such as ammonium chloride, ammonium sulphate and ammonium phosphates, or acidic salts, preferably aluminium salts such as aluminium nitrate, chloride or sulphate and wherein the hardener ΠΙ comprises acids or salts thereof (in.b) in an amount of 5 to 30 wt%, preferably para toluene sulfonic acid, citric acid, oxalic acid, lactic acid, sulphuric acid, phosphoric acid, sulfamic acid, maleic acid and salts thereof.

7. The process according to claims 1 - 6 wherein:

(I) the adhesive I comprises (in wt% relative to the total weight of adhesive I); - La) 50- 75 wt% of a urea formaldehyde (UF) type resin; and

- Lb) 0 - 20 wt% of organic or inorganic filler; and

- I.c) 0 - 10 wt% of further additives; and

- I.d) 25 - 40 wt% of water; and

(Π) the hardener Π is a powder or water based mix-in hardener comprising (in wt% relative to the total weight of hardener II) in case of a water based hardener

-II. a) 10 - 45 wt% of water; and

- n.b) 1 - 15 wt% of hardener salts; and

- n.c) 10 - 45 wt% of urea; and - n.d) 0 - 10 wt% of further additives; and

- Lie) 10 - 40 wt% of organic or inorganic fillers; and

in case of a powder hardener - Π.-f) 0 wt% of water; and

- n.g) 1 - 15 wt% of hardener salts; and

- n.h) 10 - 65 wt% of urea; and

- Π.ί) 0 - 5 wt% of further additives; and

- n.j) 10 - 50 wt% of organic or inorganic fillers;

(ΙΠ) the water based separate application hardener ΙΠ comprising (in wt% relative to the total weight of hardener ΙΠ)

- m.a) 50 - 80 wt% of water; and

- m.b) 5 - 30 wt% of acids or salts thereof; and - ΙΠ. c) 0 - 10 wt% of urea; and

- m.d) 0 - 10 wt% of further additives.

8. The process according to claims 1- 7, wherein the adhesive I is an urea formaldehyde adhesive comprising 50 to 75 wt% or more preferably 60 to 72 wt% urea formaldehyde polymer (La) to the total weight of the adhesive I and has a molar ratio of formaldehyde (F) to urea (U) between 1.00 and 1.70, preferably 1.15 and 1.60 and most preferably between 1.20 and 1.50.

9. The process according to claims 1 -8 wherein the glue mixture comprises adhesive I and the hardener II mixed in a weight ratio of 100:5 to 100:40, preferably 100: 10 to 100:30

10. Form-pressed or plywood products obtainable by the process according to claims 1 - 9.

11. A three-component adhesive system for use in the process according to anyone of claims 1 - 9 comprising:

(I) an adhesive I comprising (in wt% relative to the total weight of adhesive I);

- La) 50- 75 wt% of a urea formaldehyde (UF) type resin; and - Lb) 0-20 wt% of organic or inorganic filler; and

- I.c) 0-10 wt o of further additives; and

- Id) 25 - 40 wt% of water; and

-II. a) 10 -45 wt% of water; and

- n.b) 1-15 wt% of hardener salts; and

-n.c) 10-45 wt% of urea; and

-n.d) 0- 10 wt% of further additives; and

- lie) 10-40 wt% of organic or inorganic fillers; and in case of a powder hardener

- Hf) 0 wt% of water; and

- n.g) 1-15 wt% of hardener salts; and

-n.h) 10-65 wt% of urea; and

- Π.ΐ) 0-5 wt% of further additives; and

- n.j) 10 - 50 wt% of organic or inorganic fillers;

-in.a) 50- 80 wt% of water; and

- m.b) 5-30 wt% of acids or salts thereof; and

- m.c) 0-10 wt% of urea; and

- m.d) 0-10 wt% of further additives 12. The use of the three-component adhesive system according to claim 11, for the manufacture of form -pressed or plywood products.