screw conveyor directly to the particles in the blender. ...... mixing action in the blender after application of the resin on the wood .... Without mastering this inter-.
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 1 are reported the main wood adhesives in use today with their main applications.
III.
OVERVIEW ON REQUIREMENTS CONCERNING WOOD ADHESIVES
Table 2 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 3. 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 1
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 1 and 4, FPY, FPO), DIN 68763 (V20) and EN 312-2 to 4 and 312-6; V100, particleboard according to DIN 68763 and EN 312-5 and 312-7, option 2 (internal bond after boil test according to EN 1087-1); V313, particleboard according to EN 312-5 and 312-7, option 1 (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 1 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 2 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
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Table 3
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 4 for different panel products describe the level of formaldehyde emission which is low enough to prevent Copyright © 2003 by Taylor & Francis Group, LLC
Table 4 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 þ UF MUPF, PMUF
urea–formaldehyde 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 5 summarizes the various influences of the molar ratio F/U on various properties of woodbased panels. Table 6 summerizes the influence of the molar rations F/U and F/(NH2)2,
Table 5 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
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Table 6 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 do 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. 1.
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
in which it develops after application of the adhesive can be adjusted by the degree of condensation of the resin as well as by using special resin preparation procedures [33–35]. Also various additives can increase the cold tack of the adhesive resins, e.g., some thermoplastic polymers such as poly(vinyl alcohol). 2. Isocyanate (PMDI) as Accelerator and Fortifier for UF Resins Polymeric methylenediisocyanate (PMDI) can be used as an accelerator and as a special crosslinker for UF resins. UF resins and PMDI can be sprayed separately without prior mixing onto the particles [36,37] or for improved performance the two resins can be premixed and then applied [8,38,39]. In the usual mixing procedure PMDI is pumped under high pressure into the UF resin [40,41]. Usually 0.5 to 1.0% PMDI based on dry particles is used, whereas at the same time the UF gluing factor might be reduced slightly. The specific press time is said to be reduced by up to 1 s/mm. Addition of PMDI to UF resins with a very low molar ratio was also recommended to achieve low formaldehyde emission. The poor properties of the UF resin due to its very low molar ratio can then be improved by the addition of PMDI [42–45]. B.
Improvement of the Hygroscopic Behavior of Boards by Melamine Fortified UF Resins (MUF, MUPF and PMUF Resins)
The resin used has a crucial influence on the properties of wood-based panels. Depending on the requirements, different resin types are selected for use. Whereas UF resins are mainly used for interior boards (for use in dry conditions, e.g., in furniture manufacturing), a higher water resistance can be achieved by incoroporating melamine and also some phenol into the resin (melamine fortified UF resins, MUF, MUPF, PMUF). The level of melamine addition and especially the resin manufacturing sequence used in relation to how melamine is incorporated in the resin can be very different. The different types of these resins which exist today are given in Table 7. The different resistances of these resins against hydrolysis are based on their differences at the molecular level. The methylene bridge linking the nitrogens of amido groups can be split rather easily by water attack in UF resins. The same is not so easy in the case of M(U)F resins, mainly due to the much lower water solubility of melamine itself which is a consequence of the water repellency
Table 7 Molar Ratios F/(NH2)2 of MUF/MUPF Resins Currently in Use in the Wood-Based Panels Industry F/(NH2)2 molar ratio 1.20 to 1.35 0.98 to 1.15
1.00
Resin type Resins for water resistant plywood, in the case of the addition of a formaldehyde catcher E1 particleboard resin and E1 MDF resin for water resistant boards (PB: EN 312-5 and 312-7; MDF: EN 622-5). For particleboards according to option 1 (V313 cycle test) MUF resins can be used; for boards according to option 2 (V100 2 h boiling test, tested wet) MUPF or MUF with a special approval is necessary. In this case, especially for the MDF production, formaldehyde catchers are added Special resins for boards with very low formaldehyde emission during board service [81,82]
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characteristic of the triazine ring of melamine. The equivalent methylene bridge is instead very stable to hydrolytic attack in phenolic resins. The melamine fortified products, however, are much more expensive due to the much higher price of melamine compared to urea. Therefore, the content of melamine in these resins is as high as strictly necessary but always as low as possible. A MUF resin, at parity of all other conditions, yields a lower pH drop after addition of the hardener than a UF resin [46]. This lower drop of the pH due to the buffer capacity of the triazine ring of melamine, however, also causes a decrease of the hardening rate of the resin and, therefore, a lengthening of its gel time [1], hence a lengthening of the hot press time is necessary. This is also seen in the shifts of the exothermic differential scanning calorimetry (DSC) peak of hardening which are observed in thermal experiments [47]. The deterioration of a bondline and hence its durability under conditions of weathering is determined essentially by: The failure of the resin (low hydrolysis resistance, degradation of the hardened resin causing loss of bonding strength). The failure of the interface between the resin and the wood surface (replacement of physical bondings between resin and reactive wood surface sites by water or other nonresin chemicals). The adhesion of UF resins to cellulose is sensitive to water not only due to the already mentioned lability to hydrolysis of the methylene bridge and of its partial reversibility, but also because theoretical calculations have shown that on most cellulose sites the average adhesion of water to cellulose is stronger than that of UF oligomers [8,48]. Thus, water can displace hardened UF resins from the surface of a wood joint. The inverse effect is valid for PF resins [8,49]. The breaking of bondings due to mechanical forces and stresses: water causes swelling and, therefore, movement of the structural components of the wood-based panels (cyclic stresses due to swelling and shrinking, including stress rupture). The durability of a glue line can be enhanced by the incorporation of hydrophobic chains into the hardened network. This was done by introducing urea-capped di- and trifunctional amines containing aliphatic chains into the resin structure or by using the hydrochloride salts of some of these amines as a curing agent [50–54]. By this approach some flexibility is introduced into the hardened network, which should decrease internal stresses. In UF resins the aminomethylene link is susceptible to hydrolysis and, therefore, it is unstable at higher relative humidity, especially at elevated temperatures [55,56]. Water also causes degradation of the UF resin with greater devastating effect the higher is the temperature of the water in which the boards are immersed. This different behavior of boards at different temperatures also is the basis for standard tests on which is based the classification of bondlines, resins, and bonded wood products. These classes include the lowest requirements (interior use) for the normal production of UF-bonded boards up to water and weather resistant boards (V100 boiling test, V313 cycle test, water and boil proof (WBP), and others) according to various national and international standard specifications. Hardened UF resins can also be hydrolyzed by moisture or water, due to the relative weakness of the bond between the nitrogen of the urea and the carbon of the methylene bridge, and this is especially so at higher temperatures. During this reaction the methylene bridge is eliminated as formaldehyde [57,58]. The amount of liberated formaldehyde can be taken under certain circumstances as a measure of the resistance of the resin against Copyright © 2003 by Taylor & Francis Group, LLC
hydrolysis. The main parameters influencing the rate and extent of the hydrolysis are temperature, pH, and degree of hardening of the resin [59]. The acid which has induced the hardening of the resin can also and especially induce such a hydrolysis and hence loss of bonding strength. Another approach to increase the resistance of UF resins against hydrolysis is therefore, based on the fact that the resin acid hardening causes acid residues in the glue line. Myers [60] pointed out that in the case of such an acid hardening system the decrease in the durability of adhesive bonds could be initiated both by the hydrolysis of the wood cell wall polymers adjacent to the glue line as well as in the case of UF-bonded products by acidcatalyzed resin degradation. A neutral pH glue line, therefore, should show a distinctly higher hydrolysis resistance. The amount of hardener (acids, acidic substances, latent hardeners) therefore should always be adjusted to the desired hardening conditions (press temperature, press time, and other parameters) and never follow ‘‘the more the better.’’ Thus, too high an addition of hardener can cause brittleness of the cured resin and a very high acid residue in the glue line. However, glue-line neutralization must not take place as long as the hardening reaction is ongoing, otherwise this would delay or even prevent curing. This aspect is quite a challenge which in practice has not yet really been solved. Higuchi and Sakata [61] found that a complete removal of acidic substances by soaking plywood test specimens in an aqueous sodium bicarbonate solution resulted in considerable increase in water resistance of UF glue lines. Another attempt was made by these authors [62,63] using glass powder as an acid scavenger, which reacts only slowly with the remaining acid of the glue line and, therefore, does not interfere with acid hardening of the resin. Dutkiewicz [64] obtained some good results in the neutralization of the inherent acidity of a hardened UF-bonded glue line by the addition of polymers containing amino or amido groups. All these solutions, however, are not used as yet in broader industrial applications. Laminate floorings require a very low, long term (24 h) thickness swelling of the MDF/high density fiberboard (HDF) or particleboard cores of which they are composed. Requirements usually are a maximum value of 8 or 10%, sometimes a maximum value of 6% or even lower, all figures based on the original thickness of the board. Such low percentage thickness swelling results cannot usually be obtained by just using straight UF resins, whereas the incorporation of melamine in the resin is a suitable way to achieve the desired results. Other possibilities could be a pretreatment of the particles or the fibers (e.g., acetylation) or a special posttreatment of the board. The necessary melamine content in the resin depends on various parameters, e.g., the type of wood furnish, the pressing parameters (pressure profile, density profile), and on resin consumption which can vary between a few percent up to more than 30%, based on liquid adhesive resin. Due to the considerable cost of melamine itself the content of melamine must always be only as high as necessary but as low as possible. Other important parameters are the resin manufacturing procedure, which considerably influences the thickness swelling of the boards even at the same adhesive solids content and at the same content of melamine. Melamine fortified UF resins and MUF resins can be manufactured in a variety of ways, for example: (i)
(ii)
By cocondensation of melamine, urea, and formaldehyde in a multistep reaction [65–69]. In this regard a comprehensive study of the various reaction types was done by Mercer and Pizzi [70]. They especially compared the sequence of the additions of melamine and urea. By mixing of an MF resin with a UF resin according to the desired composition of the resin [71–73].
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(iii)
(iv)
By addition of melamine in various forms (pure melamine, MF/MUF powder resin) to a UF resin during the application of the glue mix. In the case of the addition of pure melamine, a UF resin of a higher molar ratio must be used, otherwise there is not enough formaldehyde available to react with the melamine in order to incorporate it into the resin. Melamine also can be added in the form of melamine salts such as acetates, formates, or oxalates [74–78], which decompose in the aqueous resin mix only at higher temperatures and enable some savings of melamine for the same degree of water resistance compared to original MUF resins. Additionally they act as a hardener. Some of the reasons why melamine salts yield a saving in melamine content have also been identified [74].
The higher the content of melamine, the higher is the stability of the hardened resin towards the influence of humidity and water (hydrolysis resistance) [79,80]. Resins containing melamine can be characterized by the molar ratio F/(NH2)2 (Table 7) or by the triple molar ratio F:U:M. The mass portion of melamine in the resin can be described based on (i) the liquid resin, (ii) the resin solids content, or (iii) the sum of urea and melamine in the resin. One of the most interesting tasks is to clarify if there is a real cocondensation within MUF resins or if two independent networks are formed, which only penetrate each other. The application of MUF resins is very similar to the UF resins, with the difference that the level of hardener addition is usually much higher. MUPF resins are mainly used for the production of so-called V100 exterior grade boards according to DIN 68763 and EN 312-5 and 312-7, option 2. They contain small amounts of phenol. Production procedures are described in patents and in the literature [83–87] and a coreaction has been demonstrated here, although often not contributing to resin effectiveness [83,84,88,89]. PMF/PMUF resins, in which the amount of phenol is much higher than in MUPF resins, usually contain only little or no urea at all. The analysis of the molecular structure of these resins has shown that either there is no cocondensation between the phenol and the melamine, but that there exist two distinct networks [90–93], or that cocondensation can indeed occur [88]. The reason for this is the different reactivities of the phenol methylols and the melamine methylols, depending under which pH conditions the reaction is carried out.
C. Reactivity and Hardening Reactions During the curing process a three-dimensional network is built up. This leads to an insoluble resin which is no longer thermoformable. The hardening reaction is the continuation of the acid condensation process during resin production. The acid hardening conditions can be adjusted (i) by the addition of a hardener (usually ammonium salts such as ammonium sulfate or ammonium nitrate) or (ii) by the direct addition of acids (maleic acid, formic acid, phosphoric acid, and others) or of acidic substances, which dissociate in water (e.g., aluminum sulfate). Ammonium chloride has not been in use in the particleboard and MDF industry for several years because of the generation of hydrochloric acid during combustion of wood-based panels causing corrosion problems and because of the suspected formation of dioxins [94]. Ammonium sulfate reacts with the free formaldehyde in the resin to generate sulfuric acid, which decreases the pH; this low pH and hence the acid conditions enable the Copyright © 2003 by Taylor & Francis Group, LLC
condensation reaction to restart and finally the gelling and hardening of the resin takes place. The pH decrease takes place with a rate depending on the relative amounts of available free formaldehyde and hardener and is greatly accelerated by heat [46,61]. UF resins differ from other formaldehyde resins (e.g., MF, MUF, and PF) due to their high reactivity, and hence the short hot-press times which are achievable. Hot press times shorter than 4 s/mm board thickness are possible in the production of particleboards with modern, long continuous press lines. This requires highly reactive UF resins, an adequate amount of hardener, as high a press temperature as possible, and a marked difference in moisture content of the glued wood particles in the surface and the core of the mat before hot pressing. This moisture gradient induces the so-called steam shock effect even without the additional steam injection often used in North American plants. The optimal moisture content of the glued particles is 6 to 7% in the core and 11 to 13% in the surface. The lower the moisture content in the core, the higher the surface moisture content can be. However, a critical total moisture content in the mat must not be exceeded as this might cause problems with steam ventilation and even steam blisters in the panel. For this it is necessary to have low moisture content of the glued core particles and it is necessary to be thrifty with any extra addition of water in the mat core. The lower the resin solids content on the wood, the lower is the amount of water applied to the wood furnish and hence the lower is the moisture content of the glued core particles. For the surface layers, on the other hand, additional water is necessary in the glue mix to increase the moisture content of the glued particles. This additional water, however, cannot be replaced by a higher moisture content of the dried particles themselves before blending, because this water must be available quickly for a strong steam shock effect. This would not be the case if the water would still be present in the wood furnish as the internal wood cell wall moisture content. The mechanism of the hardening reaction of a MUPF/PMUF resin is not really clear. MUF resins harden is the acid range, whereas phenolic resins have their minimum of reactivity under these conditions. There is then the possibility that the phenolic portion of the resin might not really be incorporated into the aminoplastic portion of the resin during hardening. Different opinions and confusing reports have been advanced as regards PMF resin hardening. During the hardening of PMF resins either no cocondensation occurs [95] and in the hardened state two independent interpenetrating networks exist, or some cocondensation is reported to occur [88]. Only in model reactions between phenolmethylols and melamine have indications for a cocondensation via methylene bridges between the phenolic nucleus and the amino group of the melamine been found by 1H nuclear magnetic resonance (NMR). In order to increase the capacity of a production line, especially by shortening the panel hot press times, adhesive resins with a reactivity as high as possible should be used. This includes two parameters: a short gel time and a rapid and instantaneous bond strength development, even at a low degree of chemical curing. The reactivity of a resin at a certain molar ratio F/U or F/(NH2)2 is determined mainly by its preparation procedure and the quality of the raw materials used. Figure 1 shows the comparison of two straight low formaldehyde emission (E1) UF resins with the same molar ratio, but prepared according to different manufacturing procedures. The differences between the two resins are clearly evident by their different rates of strength increase obtained in the so-called ABES (Automatic Bonding Evaluation System) test [96]. Resin A shows a distinctly quicker increase in bond strength than resin B, a fact which also has been verified in the industrial scale production of boards. Copyright © 2003 by Taylor & Francis Group, LLC
Figure 1 Comparison of two UF resins with the same molar ratio F/U, but with different reactivities, due to different preparation procedures, tested by means of the Automatic Bonding Evaluation System (ABES) according to Humphrey [96,97]. UF-resin A, UF resin with F/U ¼ 1.08 and special preparation procedure for higher reactivity; UF-resin B, traditional UF resin with F/U ¼ 1.08. Table 8 Acceleration of Aminoplastic Resins by Addition of an Accelerator [98] Standard glue mix (parts by weight) Component liquid UF resin (F/U ¼ 1.05) accelerator hardener solution (ammonium sulfate 20%) formaldehyde catcher (urea) Property calculated molar ratio F/U of the glue mix gelation time at 100 C (s)
100 — 10 — 1.05 44
Glue mix with accelerator (parts by weight) 100 2.5 10 2 1.05 36
1. Glue Mixes with Enhanced Reactivity Table 8 describes an example of the use of an accelerator which distinctly increases the gelling rate of a core layer glue mix, hence enabling a significant shortening of the necessary press time. The quick reaction of the accelerator with the hardener salt generates the acid for the acid-induced hardening reaction of the resin. The accelerator is mixed with the resin just prior to use. Since it does not contain any hardener or acid, there is no limiting pot life of this premix. To compensate for the additional formaldehyde, small amounts of formaldehyde catchers are recommended for addition to the glue mix. 2.
Highly Reactive Adhesive Resins in Plywood, Parquet Flooring, and Door Production Plywood, parquet flooring, and doors are usually produced using aminoplastic adhesives. The press time necessary for these applications depends on the press temperature, the total thickness of the wood layers which have to be heated through, and the reactivity of the resin glue mix. Traditional adhesive resin systems need rather long press times due to their Copyright © 2003 by Taylor & Francis Group, LLC
low reactivity, causing a low capacity of the production line. In these systems the hardener is premixed in greater proportion with the adhesive resin. The limitation of these is the too short pot life obtained after hardener addition, causing early gelling of the glue mix in the storage vessel. Using smaller glue mixes increases the chances of improving the resin reactivity. With this presupposition very reactive hardeners can be used. They decrease the gel time of the glue mix and hence the necessary press times. Special aminoplastic resin systems with distinctly higher reactivity have therefore been developed to fulfill the requirements of each customer in terms of saving time, energy, and costs. The higher the reactivity, the higher is the capacity of the production line, or the lower is the necessary press temperature at a given press time. Lower temperatures are beneficial for the quality of the wood itself as well as for saving energy costs. It has been shown that such very reactive hardeners perform favorably also when in liquid form. This enables the use of various acids or acidic substances in the formulations of these hardeners. Such reactive adhesive systems usually consist of two liquid components, one being a high viscosity resin and the other a high viscosity liquid hardener. The hardener contains some inorganic fillers or organic thickeners. The mixing of these two components is performed just prior to the application of the resin mix to the roll coater. The liquid–liquid two-component mixer is installed preferably above the roll coater, in order to reduce considerably the amount of each batch of prepared glue mix. If a long stop of the production occurs the lost amount of the ready-to-use glue mix is rather small. Another advantage of this system is that both components can be pumped directly from the storage vessels to the mixer, without the use of any powder. The disadvantage of these two-component systems is the fixed ratio between the extender and the hardener. If the amount of extender should be changed, the amount of the hardener itself is also changed and hence the glue mix reactivity and pot life are changed too. Because of the well known marked influence of the temperature on the pot life, cooling of the whole system is necessary using a chilled water cooler. The raw adhesive resin should have a temperature not higher than 15 C prior to use, which is especially important in summer due to the higher room temperature. Cooling of the adhesive resin can be performed in a small vessel with cooling coils, which is installed between the storage tank and the mixer. Additionally the roll coater itself also needs cooled cylinders in order to stabilize the temperature at approximately 15 C. Figure 2 shows the scheme of a liquid– liquid two-component mixing station.
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
Figure 2 Scheme of a liquid–liquid two-component glue mixing station. Copyright © 2003 by Taylor & Francis Group, LLC
glue resin tank hardener tank twin pumps filter unit flow sensor mixer switchbox distributor level sensor roll coater ventilation valve
D. Correlations Between the Composition of Aminoplastic Resins and the Properties of the Wood-Based Panels Not much work has been done up to now concerning the prediction of bond strengths and other board properties based on the results of the analysis of the adhesive resin in its liquid state. What has been investigated and derived up to now are correlation equations that correlate the chemical structures in various UF resins having different molar ratios F/U and different types of preparations with the achievable internal bond strengths of the boards as well as the formaldehyde emission measured after resin hardening. The basic aim of such experiments is the prediction of the properties of the woodbased panels, hence of the adhesive resin in its hardened state, based on the composition and the properties of the liquid resins used before their hardening. For this purpose various structural components were determined by means of NMR spectroscopy and the ratios of the amounts of the various structural components were calculated, for example: (i)
(ii)
(iii)
for UF resins: free urea related to total urea methylene bridges with crosslinking related to total sum of methylene bridges sum of methylene bridges in relation to sum of methylols for MF resins: unreacted melamine to monosubstituted melamine unreacted melamine to total melamine number of methylene bridges in relation to the number of methylol groups degree of branching: number of branching sites at methylene bridges in relation to total number of methylene bridges for MUF resins: sum of unreacted melamine and urea to sum of substituted melamine and urea number of methylene bridges in relation to number of methylol groups or to the sum of methylene bridges and methylol groups
These ratios then are correlated to various properties of the wood-based panels, e.g., internal bond strength or subsequent formaldehyde emission. Various papers in the literature describe examples of such correlations and present workable predictive equations. For UF resins: Ferg [30], Ferg et al. [99,100]; for MF resins: Mercer and Pizzi [101]; for MUF resins: Mercer and Pizzi [102], Panamgama and Pizzi [103]. For certain boards, some good correlations exist. Even these equations, however, cannot predict all properties for all types of UF resins. This is because it must be assumed that a general correlation for various resins and various panels cannot exist. Other correlation equations might have to be used sometime. However, the types of equations that have already been published describe how a universal equation for this task might look. Only the coefficient need to be changed from case to case. These results are of some importance, because they show that at least for a certain combination of resin type and board type, correlations do indeed exist. It will be the task for chemists and technologists to evaluate in further detail all possible parameters as well as their influence on the performance of the resins and the wood-based panels. It can also be assumed that the various parameters mentioned above will also be decisive for other combinations, even if the numerical values of the coefficients within individual equations might differ. The range of the molar ratio under investigation in the papers mentioned above was rather broad. Copyright © 2003 by Taylor & Francis Group, LLC
Table 9 UF Resin Glue Mixes for the Production of Particleboard and MDF, Parts by Weight Components/resin mixes
PB-CLa
PB-FLa
PB-CLb
PB-FLb
MDFa
PB–UF resina,c MDF–UF resind MUF resine Water Hardener solutionf Urea solutiong
100 — — — 8 up to 5
100 — — 10–20 2 up to 5
— — 100 — 15 up to 5
— — 100 10–20 6 up to 5
— 100 — 30–80 2 15
PB, particleboard; CL, core layer; FL, face layer. a For use in dry conditions. b For use in moist conditions. c UF resin with molar F/U ¼ 1.03 to 1.08. d UF resin with molar F/U 0.98 to 1.02. e MUF resin with molar F/(NH2)2 1.03 to 1.08. f Ammonium sulfate solution (20%). g Urea solution (40%).
It would not appear to be possible to use these equations for predictions within narrow ranges of molar ratio, e.g., the usual range of an E1 UF resin with approximate F/U ¼ 1.03 to 1.10. A method showing how different resin preparation procedures, for equal molar ratio resins, can be included in these correlation equations also needs to be developed. E.
Glue Resin Mixes
Table 9 summarizes some resin glue mixes for different applications in the production of particleboard and MDF. Table 10 summarizes various resin glue mixes for different applications in the production of plywood, parquet flooring, and furniture.
V. PHENOLIC RESINS Phenolic resins [phenol–formaldehyde (PF) resins] show complete resistance to hydrolysis of the C–C bond between the aromatic nucleus and the methylene bridge and, therefore, are used for water and weather resistant glue lines and boards such as water and weather proof particleboards, OSB, MDF, or plywood for use under exterior weather conditions. Another advantage of phenolic resins is the very low formaldehyde emission in service, after hardening, also due to the stability of the methylene bridges between aromatic nuclei. The disadvantages of phenolic resins are the distinctly longer press times necessary for hardening when compared to UF resins, the dark color of the glue line and of the board surface as well as a higher equilibrium moisture content of the boards due to the hygroscopicity of the high alkali content of the board. The preparation procedure of a phenolic resin is a multistage process, characterized by the time, sequence, and amount (in the case of several steps) of the additions of phenol, formaldehyde, and alkali as the most important raw materials. Similarly to all other formaldehyde condensation resins two main reactions predominate: Methylolation: there is no special preference for ortho or para substitution, preference which, however, could be achieved using special catalysts [104–106]. Copyright © 2003 by Taylor & Francis Group, LLC
Table 10 UF Resin Glue Mixes for the Production of Plywood, Parquet Flooring, and Furniture, Parts by Weight Components/resin mixes UF resina UF resinb UF resinc Extenderd Water Hardener solutione Hardener solutionf Powder hardenerg Powder hardenerh Liquid hardeneri
A
B
C
D
E
100 — — 20 — 10 — — — —
100 — — 40 10–20 — 20 — — —
100 — — 10 — — — 3 — —
— 100 — — — — — — 25 —
— — 100 — — — — — — 10–20
Glue mix A: standard glue mix. Glue mix B: containing higher proportion of fillers than in A. Glue mix C: high solids content, gives an enhanced water resistance to the glue line. Glue mix D: two-component glue mix: liquid resin þ ready-to-use hardener in powder form, no addition of other components necessary. Glue mix E: twocomponent glue mix: high viscosity liquid resin þ high viscosity liquid hardener. a UF resin with molar F/U 1.3. b UF resin with molar F/U 1.5 to 1.6. c High viscosity UF resin with molar F/U 1.3 to 1.4. d Extender: rye- or wheat-flour. In some cases some inorganic fillers are also included. e For example, ammonium sulfate solution (20%). f For example, ammonium sulfate–urea solution (20%/20%). g For example, ammonium sulfate in powder form. h Ready-to-use powder hardener, containing powdered hardener, formaldehyde catcher, extenders, and other additives. i High viscosity filled hardener, containing inorganic fillers or organic thickeners, hardener, and sometimes some formaldehyde catcher and other additives.
Methylolation is strongly exothermic and includes the risk of an uncontrolled reaction [107]. Condensation methylene and methylene ether linkages are formed; the latter do not exist at high alkaline conditions. During this stage chains are formed, still carrying free methylol groups. The reaction is stopped just by cooling down the preparation reactor thus preventing resin gelling. Phenolic resins contain oligomeric and polymeric chains as well as monomeric methylolphenols, free formaldehyde, and unreacted phenol. The contents of both monomers have to be minimized by the proper preparation procedure. Various preparation procedures are described in the literature and in patents [108–117]. Special PF resins consisting of a two-phase system of a highly condensed and insoluble PF resin and a lower condensation standard PF resin have also been prepared [118] and used industrially. Another two-phase resin consists of a highly condensed PF resin still in an aqueous solution and a PF dispersion [119]. The purpose of such special resins is the gluing of panel products of higher moisture content wood, where the danger of overpenetration of the resin into the wood surface would cause a starved glue line and other serious problems. The properties of the resins are determined mainly by the F/P molar ratio, the concentration of phenol and formaldehyde in the resin, the type and amount of the preparation catalyst (in most cases alkaline), and the reaction conditions. The reaction Copyright © 2003 by Taylor & Francis Group, LLC
itself is performed in an aqueous system without addition of organic solvents. The higher the F/P molar ratio, the higher is the reactivity of the resin hence the higher its hardening rate [120], the degree of branching, and the three-dimensional crosslinking. At lower F/P molar ratios linear molecules are formed preferably. Chow et al. [121] found an increase in the bonding strength of plywood with increasing F/P molar ratio; however, the bonding strength remained constant for molar ratios higher than 1.4. This value is still distinctly lower than the common industrial molar ratios of PF resins for wood adhesives. Usually sodium hydroxide is used as a catalyst, in an amount up to one mole per mole phenol (molar ratio NaOH/P), which corresponds to a portion of alkali in the liquid resin of approximately 10% by weight. The pH of a phenolic resin is in the range of 10 to 13. The preponderant part of the alkali is free NaOH, and a smaller part is present as sodium phenate. The alkali is necessary to keep the resin water soluble via the phenate ion formation in order to achieve a degree of condensation as high as possible at a viscosity that still can be used in practice. Additionally the alkali content significantly lowers the viscosity of the reaction mixture. Thus, the higher the alkali content of the resin, the higher is its possible degree of condensation, hence the greater is the reactivity of the resin and the higher its hardening rate and, therefore, the shorter is the necessary press time. High alkali contents have also some disadvantages. The equilibrium moisture content in humid climates increases with the alkaline content as do some hygroscopic-dependent properties (longitudinal stability, thickness swelling, water absorption), and some mechanical properties (creep behavior) become worse. The alkali content also causes a cleavage of the acetyl groups of the hemicelluloses. This leads to an enhanced emission of acetic acid compared to UF-bonded boards. The higher the alkali content, the higher is the emission of acetic acid. In European Norms EN 312-5 and 312-7 the content of alkali is limited to 2.0% for the whole board and 1.7% for the face layer, both figures being based on oven-dried mass of the board. Besides NaOH, other basic catalysts can be and are used, such as Ba(OH)2, LiOH, Na2CO3, ammonia, or hexamine. However, with some notable exceptions, these are not used in practice. The type of catalyst significantly determines the properties of the resins [122–124]. Replacing alkali in PF-bonded boards could give some advantages. Ammonia being a gas evaporates during the hot press process and does not therefore contribute to the alkali content and the hygroscopicity of the boards. It is important to hold a fairly high pH as long as possible during hot pressing in order to guarantee a high reactivity and hence a short press time [125,126]. The condensation process of PF resins can be followed by monitoring the increase in viscosity and by gel permeation chromatography (GPC) to measure the molar mass distribution. Chromatograms have been obtained by Duval et al. [122], Ellis and Steiner [127], Gobec et al. [128], Kim et al. [129], and Nieh and Sellers [130]. The penetration behavior strongly depends on the molar masses present in the resin: the higher the molar masses (approximately equivalent to the viscosity of the resin at the same solid content), the worse is the wettability and the lower is the penetration into the wood surface [131,132]. The lower molar masses are responsible for the good wettability, however, too low a molar mass can cause overpenetration and hence starved glue lines. Contact angles of phenolic resins on wood increase strongly with the viscosity of the resin, which increases with the molar masses [133]. The higher molar masses remain at the wood surface and form the glue line, but they will not anchor as well in the wood surface. Depending on the porosity of the wood surface, a certain portion with higher molar masses must be present to avoid an overpenetration into the wood, causing a starved glue line; this means a certain ratio between low and high molar masses is necessary Copyright © 2003 by Taylor & Francis Group, LLC
Table 11
Properties of PF Adhesive Resins
Solids content (%) Total alkali (%) Free alkali (%) Viscosity (mPa s) Density (g/ml)
Particleboard CL
Particleboard FL
AW100-Plywood
46–48 7–9 6–8 300–700 ca. 1.23
ca. 45 3–4 2–3 300–500 ca. 1.18
46–48 7–10 6–9 500–800 ca. 1.23
CL, core layer; FL, face layer; AW100-plywood according to DIN 68705.
[127,130,134–141]. Gollob and co-workers [109,142] found a decrease in the wood failure with increased molar mass averages of PF resins. The penetration behavior of resins into the wood surface also is influenced by various other parameters, such as wood species, amount of glue spread, press temperature, and pressure and hardening time. The temperature of the wood surface and of the glue line and hence the viscosity of the resin (which itself also depends on the degree of advancement of the resin at the time of measuring) influences the penetration behavior of the resin [143]. Table 11 summarizes the properties of various PF resins. The contents of free monomers (formaldehyde, phenol) depend on the type of the resin and the preparation procedure. Usual values are
