Adhesives in wood industry

47 Adhesives in the Wood Industry Manfred Dunky Dynea Austria GmbH, Krems, Austria I INTRODUCTION Progress in research and development within the wood-based industry and within the adhesive industry has shown many successes during the past few decades Notwithstanding this, the industrial requirements of the wood industry still induce technical improvement in the adhesives and their application in this area What drives this technical development is the search for ‘‘cheaper,’’ ‘‘faster-curing,’’ and ‘‘more complex’’ adhesives The first two requirements are caused by the heightened competition within the wood industry and efforts to minimize costs at a certain level of product quality and performance The requirement ‘‘more complex’’ stands for new and specialized products and process Adhesives play a central role within wood-based panels production The quality of bonding and hence the properties of the wood-based panels are determined mainly by the type and quality of the adhesives Development in wood-based panels, therefore, is always linked to development in adhesives and resins Both the wood-based panels industry and the adhesive industry shown a high commitment to and great capability towards innovation The best evidence for this is the considerable diversity of types of adhesives used for the production of wood-based panels Well known basic chemicals have been used for a long time for the production of adhesives and their resins, the most important ones being formaldehyde, urea, melamine, phenol, resorcinol, and isocyanate The greater part of the adhesive resins and adhesives currently used for wood-based panels is produced with these few raw materials The ‘‘how to cook the resins’’ and the ‘‘how to formulate the adhesive’’ therefore become more and more complicated and sophisticated and are key factors to meet today’s requirements of the wood-based panels industry The quality of bonding and hence the properties and performance of the wood-based panels and beams are determined by three main parameters: the wood, especially the wood surface, including the interface between the wood surface and the bondline the applied adhesive the working conditions and process parameters Copyright © 2003 by Taylor & Francis Group, LLC Good quality bonding and adequate properties of the wood-based panels can be attained only if each of these three parameters contributes to the necessary extent to the bonding and production process In this chapter are then covered the types of adhesives used in the wood industry and their characteristics The influences on their performance of the adhesives’ physicochemical characteristics, of their application parameters, of the wood itself, and of the wood composite process parameters are also described In wood adhesives the application parameters other than the characteristics of the adhesive itself account for around 50% of performance II TYPES OF WOOD ADHESIVES In the wood-based panels industry a great variety of adhesives are currently is use Condensation resins based on formaldehyde represent the biggest volume within the wood adhesives field They are prepared by the reaction of formaldehyde with various chemicals such as urea, melamine, phenol, resorcinol, or combinations thereof At delivery these adhesive resins are mainly liquid and consist of linear or branched oligomers and polymers in aqueous solution or dispersion During hardening and gelling they convert to three-dimensionally crosslinked and, therefore, insoluble and nonmeltable networks The hardening conditions used can be acidic (for aminoplastic resins), highly alkaline (for phenolic resins), or neutral to lightly alkaline (for resorcinol resins) Isocyanates [especially polymeric 4,40 -diphenyl methane diisocyanate (PMDI)] are another important chemical group used for various applications in the wood industry, especially for water resistant bonds In Table are reported the main wood adhesives in use today with their main applications III OVERVIEW ON REQUIREMENTS CONCERNING WOOD ADHESIVES Table summarizes the general parameters of importance for wood adhesives Research and development in adhesives and resins are mainly driven by the requirements of the bonding and production processes and by the intended properties of the wood-based panels These requirements are summarized in Table The necessity to achieve shorter press times is omnipresent within the woodworking industry, to keep production costs low An increased production rate gives the chance to reduce production costs This is only valid when the market is able to absorb such a high level of production Shorter press times within a given production line and for certain types of wood-based panels can be achieved by, among others: highly reactive adhesive resins possessing rapid gelling and hardening and steep increase in bonding strength even at a low degree of chemical curing highly reactive adhesive glue mixes obtained by the addition of accelerators, special hardeners, crosslinkers, and others the optimization of the pressing process, e.g., by increasing the effect of the steam shock by (i) increased press temperatures, (ii) a more marked difference in the moisture content between the surface and the core layer of the panel before hot pressing, or (iii) an additional steam injection step constancy of as many parameters of the production process as possible Copyright © 2003 by Taylor & Francis Group, LLC Table Fields of Application for Various Wood Adhesives Adhesive type UF MUF MF/MUF MUPF PF/PUF RF PMDI PVAc old nat.adhesives nat.adhesives inorg.adhesives activation V20 V100 V313 FP MDF PLW HLB MH ven furn x x x xa xa x x x x x x x x x x x x x x x xb x x x x x x x x x x x x x x x x xc x UF, urea–formaldehyde resin; MUF, melamine fortified UF resin; MF/MUF, melamine and melamine–urea resins (MF resins are only used mixed/coreacted with UF resins; MUPF, melamine–urea–phenol–formaldehyde resin; PF/PUF, phenol and phenol–urea–formaldehyde resin; (P)RF, resorcinol–(phenol–)formaldehyde resin; PMDI, polymeric methylenediisocyanate; PVAc, polyvinylacetate adhesive; old nat.adhesives, old (historic) natural adhesives (e.g., starch, glutin, casein adhesives); nat.adhesives, natural adhesives (e.g., tannins, lignins, carbohydrates); inorg.adhesives, inorganic adhesives (e.g., cement, gypsum); activation: activation constituents of wood to function as adhesives (i.e., lignin) V20, particleboard according to DIN 68761 (parts and 4, FPY, FPO), DIN 68763 (V20) and EN 312-2 to and 312-6; V100, particleboard according to DIN 68763 and EN 312-5 and 312-7, option (internal bond after boil test according to EN 1087-1); V313, particleboard according to EN 312-5 and 312-7, option (cycle test according to EN 321); FP, hardboard (wet process) according to EN 622-2; MDF, medium density fiberboard according to EN 622-5; PLW, plywood according to EN 636 with various resistance against influence of moisture and water; HLB, laminated beams; MH, solid wood panels according to OeNORM B 3021 to B 3023 (prEN 12775, prEN 13353 part to 3, prEN 13017-1 and 2, prEN 13354); ven., veneering and covering with foils; furn., production of furniture a Partly powder resins b Boards with reduced thickness swelling, e.g., for laminate flooring c Special production method Table General Requirements for Wood Adhesives Composition, solids content, viscosity, purity Color and smell Sufficient storage stability for given transport and storage conditions Easy application Low transport and application risks Proper gluing quality Climate resistance Hardening characteristic: reactivity, hardening, crosslinking Compatibility for additives Cold tack behavior Ecological behavior: Life cycle analysis (LCA), waste water, disposal, etc Emission of monomers, Volatile organic compounds (VOC), formaldehyde during production of the wood-based panels and during their use Copyright © 2003 by Taylor & Francis Group, LLC Table Actual Requirements in the Production and in the Development of Wood Adhesives Shorter press times, shorter cycle times Better hygroscopic behavior of boards (e.g., lower thickness swelling, higher resistance against the influence of humidity and water, better outdoor performance) Cheaper raw materials and alternative products Modification of the wood surface Life cycle assessment, energy and raw material balances, recycling and reuse Reduction of emissions during the production and the use of wood-based panels Cheaper raw materials are another way to reduce production costs This includes, for example, the minimization of the melamine content in a MUF resin, to produce boards with reduced thickness swelling or increased resistance against the influence of water and high humidity of the surrounding air Impeding factors (often temporary) can be the shortage of raw materials for the adhesives, as was the case with methanol and melamine during the 1990s Life cycle analysis and recycling of bonded wood boards also concerns the adhesive resins used, since adhesives and resins are one of the major raw materials in the production of wood-based panels This includes, for example, the impact of the adhesives on various environmental issues such as waste water and effluent management, noxious gas emission during panel production and from the finished boards, or the reuse of panels to burn for energy generation Furthermore, for certain recycling processes the type of resin has also a crucial influence on their feasibility and efficiency Gas emission from wood-based panels during their production can be caused by chemicals inherent to wood itself, such as terpenes or free acids, as well as by volatile compounds and residual monomers coming from the adhesive The emission of formaldehyde especially is a matter of concern, but so are possible emissions and discharges of free phenols or other materials The formaldehyde emission noted only after panel manufacture and adhesive resin hardening is due, on the one hand, to the residual, unreacted formaldehyde present in urea–formaldehyde (UF)-bonded boards, or as gas trapped in the wood or dissolved in the moisture still present in the panel On the other hand, in aminoplastic resins the hydrolysis of weakly bonded formaldehyde from Nmethylol groups, acetals, and hemiacetals as well as in more severe cases of hydrolysis (e.g., at high relative humidity) from methylene ether bridges, increases again the content of emittable formaldehyde after resin hardening In contrast to phenolic resins, a permanent reservoir of potentially emittable formaldehyde is the consequence of the presence of these weakly bonded structures This explains the continuous, yet low, release of formaldehyde from UF-bonded wood-based panels even over long periods However, the level of emission depends on the environmental conditions, a fact which may be described by the resin hydrolysis rate which indicates if this formaldehyde reservoir will or will not lead to unpleasantly high emission values [1–4] The higher this hydrolysis rate is, the higher is the potential reservoir of formaldehyde which contributes to subsequent formaldehyde emission The problem of formaldehyde emission after adhesive hardening in panel manufacture can fortunately be regarded today as solved, due to clear and stringent emission regulations in many European and other countries and to successful long term R&D investement by the chemical industry and the wood working industry The so-called E1-emission class regulations shown in Table for different panel products describe the level of formaldehyde emission which is low enough to prevent Copyright © 2003 by Taylor & Francis Group, LLC Table Actual Regulations Concerning Formaldehyde Emission from Wood-Based Panels According to the German Regulation of Prohibition of Chemicals (formerly Regulation of Hazardous Substances) for E1 Emission Class (the Lowest Emission Types panels) (a) Maximum steady state concentration in a climate chamber: 0.1 ppm (prEN 717-1; 1995) (b) Laboratory test methods (based on experimental correlation experiences): Particleboard: 6.5 mg/100 g dry board as perforator value (EN 120; 1992) MDF: 7.0 mg/100 g dry board as perforator value (EN 120; 1992) Plywood: 2.5 mg/h-m2 with gas analysis method (EN 717-2) Particleboard and MDF: correction of the perforator value to 6.5% board moisture content any danger, irritation, or inflammation of the mucous membranes in the eyes, nose, and mouth However, it is important that not only the boards themselves, but also veneering and carpenters’ adhesives, lacquers, varnishes, and other sources of formaldehyde be controlled, since they also might contribute to a close environment formaldehyde steady-state concentration [1–4] IV AMINOPLASTIC ADHESIVE RESINS (UREA RESINS, MELAMINE RESINS) The various aminoplastic resins are the most important class of adhesives in the woodbased panels industry, especially for the production of particleboards and medium density fibreboard (MDF), and partly also for oriented strandboard (OSB), plywood, blockboards, and some other types of wood panels They are also used in the furniture industry as well as in carpenters’ shops Aminoplastic adhesive resins are formed by the reaction of urea and/or melamine with formaldehyde Based on the raw materials that are used various types of resins can be prepared, namely: UF MF MUF mUF MF ỵ UF MUPF, PMUF ureaformaldehyde resin melamine–formaldehyde resin melamine–urea–formaldehyde cocondensation resin melamine fortified UF resins mixture of an MF and a UF resin melamine–urea–phenol–formaldehyde cocondensation resin The most important parameters for the aminoplastic resins are: (a) The type of monomers used (b) The relative molar ratio of the various monomers in the resin: F/U molar ratio of formaldedhyde to urea F/M molar ratio of formaldehyde to melamine F/(NH2)2 molar ratio of formaldehyde to amide or amine groups, whereby urea counts for two NH2 groups, and melamine for three NH2 groups (c) The purity of the different raw materials, e.g., the level of residual methanol or formic acid in formaldehyde, biuret in urea, or ammeline and ammelide in melamine Copyright © 2003 by Taylor & Francis Group, LLC (d) The reaction procedures used, e.g the the the the the pH variation sequence temperature variation sequence types and amount of alkaline and acidic catalysts sequence of addition of the different raw materials duration of the different reaction steps in the cooking procedures The production of aminoplastic adhesive resins is usually a multistep procedure where both alkaline and acidic steps occur Aminoplastic resins can be prepared in a variety of different types for all the different needs in wood bonding This can be achieved by just using the three main monomers mentioned above and varying the preparation procedure A UF Resins Urea–formaldehyde resins [1–9] are based on a series of consecutive reactions of urea and formaldehyde Using different conditions of reaction and preparation a practically endless variety of condensed UF chemical structures is possible UF resins are thermosetting resins and consist of linear or branched oligomers and polymers always admixed with some amounts of monomers The presence of some unreacted urea is often helpful to achieve specific effects, e.g., a better storage stability of the resin The presence of free formaldehyde has, however, both positive and negative effects On the one hand, it is necessary to induce the subsequent hardening reaction while, on the other hand, it causes a certain level of formaldehyde emission during the hot press, resin hardening cycle Even in the hardened state, low levels of residual formaldehyde can lead to the displeasing odor of formaldehyde emission from the boards while in service This fact has changed significantly the composition and formulation of UF resins during the past 20 years After hardening, UF resins consist of insoluble, three-dimensional networks which cannot be melted or thermoformed again In their application stage UF resins are used as water solutions or dispersions or even in the form of still soluble spray dried powders These, however, in most cases have to be redissolved and redispersed in water for application Despite the fact that UF resins consist of only the two main components, namely urea and formaldehyde, a broad variety of possible reactions and resin structures can be achieved The basic characteristics of UF resins can be ascribed at a molecular level to: their high reactivity their waterborne state, which renders these resins ideal for use in the woodworking industry the reversibility of their aminomethylene bridge, which also explains the low resistance of UF resins to water and moisture attack, especially at higher temperatures; this is also one of the reasons for the hydrolysis leading to subsequent formaldehyde emission The reaction of urea and formaldehyde is basically a two-step process, usually consisting of an alkaline methylolation (hydroxymethylation) step and an acid condensation step The methylolation reaction, which usually is performed at a high molar ratio (F/U ¼ 1.8 to 2.5), is the addition of up to three (four in theory) molecules of bifunctional formaldehyde to one molecule of urea to give methylolureas; the types and the proportions Copyright © 2003 by Taylor & Francis Group, LLC of the formed methylol groups depend on the molar ratio F/U Each methylolation step has its own rate constant ki, with different values for the forward and the backward reactions The formation of these methylol groups mostly depends on the molar ratio F/U The higher the molar ratio used, the higher the molecular weight the methylolated species formed tends to be The UF resin itself is formed in the acid condensation step, where still the same high molar ratios as in the alkaline methylolation step is used (F/ U ¼ 1.8 to 2.5): the methylol groups, urea and the free formaldehyde react with linear and partly branched molecules with medium and even higher molar masses, forming the polydisperse molar mass distribution pattern characteristic of UF resins Molar ratios lower than approximately 1.8 during this acid condensation step tend to cause resin precipitation The final UF resin has a low F/U molar ratio obtained by the addition of the so-called second urea, which might also be added in several steps [8,9] The second urea process step needs particular care It is important for the production of resins with good performance, especially at the very low molar ratios usually in use now in the production of particleboards and MDFs This last step also includes the distillation of the resin solution to usually 66% resin solids content, which is performed by vacuum distillation in the reactor itself or in a thin layer evaporator Industrial manufacturing procedures usually are proprietary and are described in depth in the literature only in rare cases [7–11] The type of bonding between the urea molecules depends on the conditions used: low temperatures and slightly acid pHs favor the formation of methylene ether bridges (–CH2– O–CH2–) and higher temperatures and lower pHs lead preferentially to the formation of more stable methylene bridges (–CH2–) Ether bridges can be rearranged to methylene bridges by splitting off formaldehyde One ether bridge needs two formaldehyde molecules and additionally it is not as stable as a methylene bridge, hence it is highly recommended to follow procedures that minimize the formation of such ether groups in UF resins In the literature other types of resin preparation procedures are also described Some of these yield uron structures in high proportion [12–15] or triazinone rings in the resins [15–17] The latter are formed by the reaction of ammonia or an amine, respectively, with urea and an excess of formaldehyde under alkaline conditions These resins are used, e.g., to enhance the wet strength of paper The following chemical species are present in UF resins: free formaldehyde, which is in steady state with the remaining methylol groups and the post-added urea monomeric methylol groups, which have been formed mainly by the reaction of the post-added urea with the high content of free formaldehyde at the still high molar ratio of the acid condensation step oligomeric methylol groups, which have not reacted further in the acid condensation reaction or which have been formed by the above-mentioned reaction of postadded urea molecules with higher molar masses, which constitute the real polymer portion of the resin The condensation reaction as well as the increase in the molar mass can also be monitored by gel permeation chromatography (GPC) [18,19] At longer acid condensation steps, molecules with higher molar mass form and the GPC peaks shift to lower elution volumes Because of the necessity to limit the subsequent formaldehyde emission, the molar ratio F/U has been decreased constantly over the years [20] The main differences between Copyright © 2003 by Taylor & Francis Group, LLC the UF resins with high and low formaldehyde content are the reactivity of the resin due to the different contents of free formaldehyde and the degree of crosslinking in the cured network The main challenge has been to reduce the content of formaldehyde in the UF resins and to achieve this without any major changes in the performance of the resins In theory this is not possible, because formaldehyde is the reactive partner in the reaction of urea and formaldehyde during the condensation reaction as well as curing Decreasing the molar ratio F/U means lowering the degree of branching and crosslinking in the hardened network, which unavoidably leads to a lower cohesive bonding strength The degree of crosslinking is directly related to the molar ratio of the two components The UF resin formulators have revolutionized UF resin chemistry in the past 30 years For example, in a straight UF resin for wood particleboard the above mentioned molar ratio F/U was approximately 1.6 at the end of the 1970s It is now 1.02–1.08, but the requirements for the boards (e.g., internal bond strength or percent thickness swelling in water) as given in the quality standards are still unaltered Also the reactivity of the resin during hardening, besides the degree of crosslinking of the cured resins, depends on the availability of free formaldehyde in the system It has, however, to be considered that it is neither the content of free formaldehyde itself nor the molar ratio which should be taken as the decisive and only criterion for the classification of a resin concerning its subsequent level of formaldehyde emission In reality the composition of the glue mix as well as the various process parameters during board production also determine the level of formaldehyde emission Depending on the type of board and the process of application, it is sometimes recommended to use a UF resin with a low molar ration F/U (e.g., F/U ¼ 1.03), hence presenting a low content of free formaldehyde; while sometimes the use of a resin with higher molar ratio (e.g., F/U ¼ 1.10) to which a formaldehyde catcher has been added in the glue mix will give better results Which of these two possible ways is the better one in practice can only be decided by trial and error in each case The higher the molar ratio F/U, the higher is the content of free formaldehyde in the resin Assuming stable conditions in the resins, which means that, e.g., post-added urea has had enough time to react with the resin, the content of free formaldehyde is very similar even for different manufacturing procedures The content of formaldehyde in a straight UF resin is approximately 0.1% at F/U ¼ 1.1 and 1% at F/U ¼ 1.8 [19–21] It also decreases with time due to aging reactions where this formaldehyde reacts further Table summarizes the various influences of the molar ratio F/U on various properties of woodbased panels Table summerizes the influence of the molar rations F/U and F/(NH2)2, Table Influence of the Molar Ratio on Various Properties of UF-Bonded Wood-Based Panels Decreasing the molar ratio leads to a decrease of an increase of the formaldehyde emission during the production of the wood-based panels the subsequent formaldehyde emission the mechanical properties the degree of hardening the thickness swelling and the water absorption the susceptibility of hydrolysis Copyright © 2003 by Taylor & Francis Group, LLC Table Molar Ratios F/U and F/(NH2)2, Respectively, of Pure and Melamine Fortified UF Resins Currently in Use in the Wood-Based Panels Industry F/U or F/(NH2)2 molar ratio 1.55 to 1.85 1.30 to 1.60 1.20 to 1.30 1.00 to 1.10 below 1.00 Resin type Classical plywood UF resin, also cold setting; use is only possible with special hardeners and additives, e.g., melamine containing glue mixes for an enhanced water resistance UF plywood resin; use for interior boards without special requirements concerning water resistance; to produce panels with low subsequent formaldehyde emission, the addition of formaldehyde catchers is necessary Plywood or furniture resin with low content of formaldehyde; also without addition of catchers, products with a low subsequent formaldehyde emission can be produced E1 particleboard and E1 MDF resins; especially in MDF production further addition of catchers is necessary Modification or fortification with melamine can be done MDF resins and special glue resins for boards with a very low formaldehyde emission; in most cases modified or fortified with melamine respectively, of pure and melamine fortified UF resins currently in use in the wood-based panels industry The molar mass distribution of UF resins is determined by the degree of condensation and by the addition of urea (and sometimes also other components) after the condensation step; this again shifts the resin mass distribution towards lower average molar masses For this reason the molar mass distribution is much broader than for other polymers: it starts at the low molar mass monomers (the molecular weight of formaldehyde is 30, for urea it is 60) and goes up to more polymerized structures It is not clearly known, however, what are really the highest molar masses in a UF resin Molar masses of up to 500,000, determined by light scattering, have been reported [18,22] The conditions of molecular level shear within the chromatographic columns [23] should guarantee that all physically bonded clusters, caused by the interaction of the polar groups present in the resins and which might simulate too high a molar mass, are separated and that these high numbers between 100,000 and 500,000, measured using low angle laser light scattering (LALLS) coupled to GPC, really describe the macromolecular structure of a UF resin in the right manner A second important argument for this statement is the fact that up to such a high molar mass the on-line calibration curve determined in the GPC–LALLS run is stable and more or less linear It does not show any sudden transition as would be the case of a too sharp increase in apparent molar mass if molecular clustering occurred again after the material has passed through the column The molar mass distribution (and the degree of condensation) is one of the most important characteristics of the resin and it determines several properties of the resin Consequence of highly condensed resin structures (high molar masses) are: the viscosity at a given solids content increases [19,24] the flowing ability is reduced Copyright © 2003 by Taylor & Francis Group, LLC the the the the the wetting behavior of a wood surface becomes worse [24] penetration into the wood surface is reduced [25,26] distribution of the resin on the furnish (particles, fibers) worsens water dilutability of the resin becomes lower portion of the resin that remains soluble in water decreases [22] Diluting the resin with a surplus of water causes precipitation of parts of the resin These parts preferably contain the higher molar mass molecules of the resin and their relative proportion increases at higher degrees of condensation [22] Information on correlations between the molar mass distribution (degree of condensation) and mechanical and hygroscopic properties of the boards produced, however, is rather rare and often equivocal [7,19,27–29] The influence of the degree of condensation is mostly felt during the application and the hardening reaction (wetting behavior and penetration into the wood surface which depend on the degree of condensation) At higher temperatures, during the curing hot press cycle, the viscosity of the resin drops, before the onset of hardening again leads to an increase of viscosity With this temporary lowering of the viscosity the adhesive wetting behavior improves significantly, but its substrate penetration behavior also changes The reactivity of an aminoplastic resin seems to be independent of its viscosity (degrees of condensation), at parity of molar ratio Ferg [30] mentioned that the bonding strength increased with the degree of condensation of the applied UF resin The higher molar masses (higher viscosity resin fractions) give a more stable glue line and determine the cohesive properties of the hardened resin [7] Also Rice [29] and Narkarai and Wantanabe [28] reported that the resistance of a bondline against water attack and redrying increased with the viscosity of the resin The reason again might be that resins with an advanced degree of condensation remain to a greater extent in the glue line, avoiding resin overabsorption by the substrate and hence avoiding starving of the bondline Rice [29] found an increase of the thickness of the glue line with an increased viscosity of the resin, obviously due to its lower penetration into the wood substrate However, it must be taken into consideration that the strength and stability of a glue line decrease with increased glue-line thickness [31] According to the findings of Sodhi [32] the bonding strength decreases the longer is the waiting time before application of the glue mix Once the hardening reaction has started and, therefore, the average molar mass has started to increase, the worse the resin wetting behavior and its penetration in the wood surface appears to be Cold Tack Properties of UF Resins Cold tack means that the particle mat has attained some strength already after the prepress at ambient temperature, without any hardening reaction having occurred This ‘‘green’’ strength is necessary for better handling of the particle mat during transfer on the production line This can well be the case in multiopening presses, in special forming presses, or in plywood mills, where the glued veneer layers are prepressed to fit into the openings of the presses At least a low level of cold tack is also necessary to avoid blowing out and loss of the fine wood particles from the surface when panels enter a continuous press at high belt speeds On the other hand, cold tack can lead to agglomeration of fine wood particles and fibers in the forming station Cold tack is generated during the dry out of glue line, and reaches a maximum after a certain period of time After this point the cold tack decreases again, when the glue line starts to dry out Both the intensity of the cold tack as well as the optimum length of time Copyright © 2003 by Taylor & Francis Group, LLC Figure 10 Particle size distribution and mass gluing factor of the individual particle size fractions for the separate gluing example CLỵFL The resin consumption was assumed to be 6.5% resin solids content/dry wood in the core layer (CL) and 11.0% in the face layer (FL) The mass proportions are CL:FL ¼ 60:40 (After ref 429.) Figure 11 Fractionated mass gluing factors of industrially glued core layer particles The mass gluing factor during blending was 9.5% resin solids/dry particles (After ref 429.) Assuming that the gluing of particles of different sizes is performed randomly with their surface area as the decisive parameter, for various homogeneous particle size fractions and for different particle size mixtures the theoretical mass gluing factors and the distribution of the resin solids content can be calculated and correlated with the same values obtained experimentally, by analysis There are some indications [431–433], however, that glue distribution is not exclusively influenced by the surface area of the particles, but has a certain preference for coarser particles This may be due to the effectiveness of the adhesive application, thus to the separation and distribution of resin droplets, or to the mixing action in the blender after application of the resin on the wood particles (wiping effect) The concept that the particle surface area exclusively influences gluing is quite clearly invalid, if glue droplets and the surface to be glued have similar size Meinecke and Klauditz [431] mentioned diameters of glue droplets of to 110 mm, depending on the type of spraying and Lehmann [434] mentioned up to 200 mm The latter values are of the same order of magnitude as the size of the finest particles used for the calculations above Besides the surface area of the particles several other parameters also have some influence on the necessary resin consumption, e.g., type of boards, thickness of the sanding Copyright © 2003 by Taylor & Francis Group, LLC zone, type and capacity of the blenders, separation and spraying of the resin (depending on if only the wiping/spreading effect occurs during blending or if instead spraying of the resin is used), shape of the particles for the same particle sizes, dependence of the slenderness ratio on particle length, concentration and viscosity of the glue resin, or a partial size degradation of the coarser particles in the blender New strategies in blending take into account the reality of the higher resin consumption by the finer particles, e.g., by removing the dust and the finest particles from the particle mix before blending Also an exact screening and classifying of the particles before blending can improve the distribution of the resin on the particle surfaces and can help to spare some resin A lower consumption of resin not only means lower costs for the raw materials, but also helps to avoid various technological disadvantages With the resin, water is also applied to the particles; as long as this amount of water is low enough, especially in the core layer, no problem should occur with a too high vapor pressure during hot pressing Often, however, the moisture content of the glued core particles is too high, due to an excessive gluing factor The high vapor pressure in the board at the end of the press cycle tends to expand the fresh board; if venting is not done very carefully, blistering of the boards at the end of the continuous press or after the opening of the press might occur Additionally, the heat transfer by the steam shock can be delayed if the vapor pressure difference betweeen the face layer and the core layer is small If the moisture content of the glued core layer particles is high, the moisture in the glued face layer particles must be reduced Also spraying water onto the belt before the forming station and onto the surface of the formed mat cannot be done due to the problems with the too high moisture content in the mat and hence with the too high vapor pressure Gluing of particles is usually done in quickly rotating blenders by spraying the resin mix into the blender Due to the rotation of the blender a partial degradation in the size of the particles can occur While blending OSB strands this degradation must be avoided; this is done by using slowly rotating big blender drums with a diameter of approximately m The liquid adhesive is distributed by several atomizers in this blender drum Gluing of fibers in MDF production is usually done in the so-called blowline between the refiner and the dryer The advantage of this method is that it avoids resin spots at the surface of the board The disadvantage, however, is the fact that the resin passes the dryer and can suffer part precuring This causes some loss of usable resin (approximately 0.5 to 2% in absolute figures); therefore the glue consumption in blowline blending is higher than in the mechanical blending Due to this fact mechanical blenders have lately been installed again in a few factories The theory of turbulent flow blowlinegluing is not yet clearly defined [435,436] However, some equations attempting to describe it have been recently presented [436] B Wood Moisture Content The wood moisture content influences several important processes such as wetting, flow of the adhesive, penetration into the wood surface, and hardening of the adhesive in the gluing and production of wood-based panels In bonding solid wood usually a wood moisture content of to 14% is seen as optimal Lower wood moisture contents can cause a quick dryout of the glue spread due to a strong absorption of the water into the wood surface as well as wetting problems High moisture contents can lead to a high flow and an enhanced penetration into the wood, causing starved glue lines Copyright © 2003 by Taylor & Francis Group, LLC Additionally a high steam pressure can be generated which might give problems of blistering when the press opens or at the end of the continuous press Also the hardening of a condensation resin might be retarded or even hindered During the hot press cycle of the particleboard or MDF production, quick changes of temperature, moisture content, and steam pressure occur The gradients of temperature and moisture content determine significantly the hardening rate of the resin and hence the board properties These gradients together with the mechanical pressure applied to densify the mat are decisive for generating the density profile and hence for the application properties and performance of the boards The higher the moisture content of the glued face layer particles, the steeper the moisture gradient between the surface and the core of the mat and the quicker the heating up of the mat occurs In the fiber mat in MDF production no differences are seen in the moisture content of the outer layer and the inner layer due to the temperature applied to the mat, nevertheless a vapor pressure gradient occurs The moisture content of the glued particles is the sum of the wood moisture content and the water that is part of the applied glue mix Therefore, the moisture content of the glued particles mainly depends on the gluing factor Usual moisture contents of glued particles are: (a) for UF, 6.5–8.5% in the core layer and 10–13% in the face layer; (b) for PF, 11–14% in the core layer and 14–18% in the face layer The optimal moisture content of the glued and dried MDF fibers in the mat before the press is in the region of 9–11% The higher the moisture content of particles, the easier the face layer can be densified at the start of the press cycle; this leads to a lower density in the core layer Blistering at the end of the press cycle or at the end of the continuous press occurs if the steam pressure within the fresh, and still hot, board exceeds the internal bond strength of the board It should be noted that the bond strengths at higher temperatures are always lower than after cooling the board If blistering occurs using resins with low formaldehyde content, press time should be shortened instead of prolonged, because a longer press time would not increase the bond strength but certainly would increase the steam pressure in the board Careful venting as well as decreasing the moisture content of the glued particles and reducing the press temperature will help C Press Cycle During the hot press cycle the hardening of the resin and possible reactions of the adhesive with the wood substance take place The influential parameters are especially the press temperature and the moisture content in the mat Additional parameters are the wood density, porosity, swelling and shrinking behavior of the wood, structure at the surface, and wetting behavior During the press cycle several processes take place: transport of heat and moisture densification, increasing internal stresses, followed by relaxation processes adhesion between the particles or fibers increase of the bond strength in the glue line (cohesion) Models describing what occurs in a panel during hot pressing have been published [437–443] These take into consideration various conditions occurring during the hot press cycle such as heat transfer, temperature gradients, moisture content, steam pressure, bond strengths, and presence or absence of postcuring [437–443] Copyright © 2003 by Taylor & Francis Group, LLC Table 21 Press Strategy for Production of Particleboards Different particle structures: coarser in the core, finer in the face layer Press temperature: As high as possible, to enable a quick heating up of the core layer due to an optimal steam shock effect In continuous lines press temperatures decrease from the entrance to the outlet of the press In the last zone of the press even active cooling in a few cases is possible (decreasing steam pressure in the core layer) Moisture content of the glued particles: Core layer as dry as possible (ca 6–7% in the case of UF resins), face layer as high as possible (11–14%, depending on the proportion of the face layer in the board) Too high a moisture content can cause blistering Spraying of water onto both surfaces in order to enhance the steam shock, amount ca 20–40 g/m2 Press pressure profile: The variation of pressure during hot pressing can follow different sequences Quick densification with pressure maximum to enable a high density of the face layer and hence high modulus of elasticity (MOE) Sometimes a second densification step is used Table 22 Press Strategy for Production of MDF Despite the uniform fiber material, a certain density profile is created due to the action of heat and compression Two-step pressure profile with quick densification at the start of the hot press cycle and a second densification step for the inner layer Uniform moisture content of the glued and dried fibers across the thickness of the mat Higher moisture content in the outer layer would require a three-layer mat or spraying of water Tables 21 and 22 summarize the usual press strategies for the production of particleboards and MDF The warming up of the mat is performed by the so-called steam shock effect [442–447] The precondition for this is the high permeability to steam and gases of the particle or fiber mat [442,443,448,449] High moisture contents of the face layers and spraying of water on the surface layers sustain this effect The press temperature influences the possible press time and by this the capacity of the production line The minimum press time has to guarantee that the bond strength of the still hot board can withstand the internal steam pressure as well as the elastic springback in board thickness at press opening XIII CONCLUSIONS Wood is a very complex material Wood adhesives technology is an advanced science which blends the technology of adhesive preparation and formulation with a multitude of advanced application technologies to different wood products In many fields other than wood, good bonding depends mainly on the use of a good adhesive The situation is not as straightforward in wood gluing: in general one can obtain excellent wood panels when using a decidedly poor adhesive if the parameters governing the technology of manufacture of the wood product are well mastered This indicates the extent to which a high level application technology can play a predominant role in this field This is not Copyright © 2003 by Taylor & Francis Group, LLC valid for all wood products Of course, good results are better or easier to obtain if one uses an excellent adhesive However, just 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