Adhesives Used in the Production of Wood Composite Materials

Wood-based composite materials have traditionally been produced worldwide in various forms, with examples including particleboard, fiberboard, oriented strand board (OSB), plywood, blockboard, and glulam products. The production of these materials involves joining wood particles of different shapes and sizes (particles, fibers, veneer sheets, solid wood, etc.) with the aid of adhesives to create a complex structure. This structure is known as a matrix structure, in which wood particles serve as 'reinforcing elements'. Within this matrix structure composed of adhesive-wood mixture, wood provides strength and load-bearing properties, while the matrix material (adhesive) distributes loads across the entire material width and ensures high resistance to deformation forces that may occur during transition to plastic deformation (Anonymous 2010; Vasiliev and Morozov, 2001). Wood-based composite materials are generally engineering design materials. For this purpose, by modifying wood-adhesive combinations according to the intended use and expected performance characteristics, it is possible to produce a wide variety of materials.

Through these manufactured products, it is aimed to develop certain properties expected from wood-based materials, which are briefly listed below:

• Absence of some natural growth defects found in wood materials, • Resistance to biotic and abiotic pests, • Suitability for fast and economical production, • High strength (durability) properties, • Improved thermal and acoustic insulation properties, • Ability to acquire aesthetic properties. In achieving these briefly mentioned properties, in addition to wood type, the type of adhesive selected and its method of application are among the most important considerations (Anonymous, 2010; Youngquist, 1999). The presence of thousands of wood particles of varying sizes within a matrix structure is made possible through adhesives. Indeed, in the production of wood composite panel materials, which we describe as engineering design products, adhesives represent the second largest input after wood. In this regard, knowledge of the properties of the adhesives used is important for successful production processes. Various theories exist regarding the bonding mechanism created between wood particles and adhesive molecules, and although some issues remain unexplained, the most important factor is the homogeneous distribution of the adhesive solution over the wood surface and the formation of chemical bonds (Marra, 1992; Pizzi, 1993; Pizzi and Mittal, 2003; Şahin 2013). In this study, three of the most important formaldehyde-based thermoset adhesives utilized extensively in the forest products industry have been briefly explained. More detailed information on adhesives, wood-adhesive-composite combinations, and product properties can be obtained from various sources (Frihart, 2005; Marra, 1992; Pizzi, 1993; Pizzi and Mittal, 2003; Şahin 2013; Tank, 1993; Yılmaz, 2006). Figure 1 shows a summary of a typical particleboard production process flow diagram.

2. Historical Development of Adhesives

The earliest examples of the use of substances for joining materials date back to 4000 BCE. Archaeologists conducting research have noted that in prehistoric times, earthen vessels and ceramics buried alongside deceased individuals were repaired with various tree resins having adhesive properties after breaking. In the Knossos Palace in Crete, lime was used as a binder together with certain blue and red pigments in wall paints (Şahin, 2013; Tank, 1993). Although adhesives have been known and used by mankind for thousands of years, the vast majority of joining technology used today was developed during the 20th century. In particular, the discovery of plastic and flexible materials such as synthetic rubber brought rapid advances in adhesive recipes and formulations, providing chemists with the opportunity to improve the properties of adhesives. Thus, a new generation of adhesives emerged that were more flexible, hardened more easily, and were more resistant to the damaging effects of time and temperature. Development was not limited to adhesive types and recipes alone; new adhesive application techniques were also developed. Furthermore, as a result of technological advances, many new types of adhesives with high resistance to air, humidity, temperature, abiotic and biotic pests were developed (Pizzi, 1993; Şahin 2013; Yılmaz, 2006)

3. Thermoset Adhesives Used in Wood-Based Composite Material Production

Today, in the production of engineered wood composite panel materials produced extensively in the forest products industry, adhesives represent the second largest input after wood. Because wood particles (particles, fibers, or solid surfaces) can be effectively joined with the aid of liquid adhesives. During this joining process, wood surfaces and the adhesive together form a complex (matrix) system. For this purpose, formaldehyde-based thermoset adhesives with easily adjustable recipes and modifiable performance properties are used worldwide for different uses (interior or exterior spaces) (Gillespie, et al. 1978; Maloney, 1977; Pizzi, 1993; Şahin 2013; Tank, 1993; Yılmaz, 2006). Below, the properties and variables of urea-formaldehyde, melamine-formaldehyde, and phenol-formaldehyde adhesives, which are most commonly used in the production of wood-based composite panel materials, are briefly explained.

3.1. Urea-Formaldehyde Adhesive (UF)

Urea-formaldehyde adhesive, produced through the condensation of urea and formaldehyde, is among the most widely used adhesives in wood-based composite panel sheet production today. Moreover, for special purposes, it can also be used in the form of mixed adhesive recipes by adding to casein, blood albumin, PVA adhesive, and melamine resin (Anonymous, 2010; Pizzi, 1993; Youngquist, 1999; Şahin 2013). In a water-soluble type UF adhesive, 1.5-2.5 mol of formaldehyde is used per 1 mol of urea. The condensation reaction initiated in an alkaline environment transforms first to monomethylol urea and then to dimethylol urea according to the purpose. Although it proceeds slowly, the condensation process also results in resin formation in an acidic environment. By adjusting the molecular ratio of urea to formaldehyde between 1:1.0-2.0, it is possible to obtain resins of varying degrees of effectiveness. In particular, resin with low formaldehyde content is normally very active, but has a short shelf life due to free amid (NH2-C=O) group condensation. In contrast, resin with high formaldehyde content can be used after longer storage periods. However, during the hardening process and afterwards, the bonded products release large amounts of formaldehyde (emission), resulting in toxic gas emissions (formaldehyde) (Pizzi, 1993; Şahin, 2005). Detailed information on formaldehyde emissions and their effects can be obtained from other sources (Pizzi, 1993; Şahin, et al. 2011). A hardener must be added to harden the resin. By adding an ammonium chloride (NH4Cl)/ammonia (NH3) buffer system as a hardener to the UF adhesive formulation, ammonia and formaldehyde use the hydrogen of ammonium chloride, and the free formaldehyde in the adhesive solution or, more slowly, the methyl (-CH3) groups of the resin react to form a hexamethylene tetramine structure. As increasing the proportion of ammonium chloride in the mixture has a slowing effect on hardening, it is generally desired that the ammonium chloride content in the hardener does not exceed 15% (Şahin, 2005 and 2013; Pizzi, 1993; Tank, 1993). To improve the properties and usage characteristics of UF adhesive, certain additives and fillers are introduced. Examples of additive materials added to UF adhesive include: • Grains or raw flour (wheat, rye, corn, rice), • Raw or natural starch (corn, rice, sorghum), • Legume flour (peas, beans), • Water-soluble cellulose ethers (methyl cellulose, carboxymethyl cellulose). However, excessive increase in the proportion of additives in the solution causes a reduction in UF adhesive's resistance to water. For this reason, the additive proportion should not exceed 10%. UF adhesive is the primary choice in the wood sector for general use purposes (interior spaces) in panel sheet production due to being more economical than other synthetic resins (phenol-, melamine-formaldehyde), easier to use, and requiring lower pressing temperatures. Currently, approximately 90% of wood-based composite panel materials produced worldwide are manufactured with UF adhesive. It is not highly resistant to water and temperature. To improve hot water resistance, it can be used as a mixture with melamine or resorcinol adhesives (Pizzi and Mittal, 2003; Şahin, 2013).

3.2. Melamine-Formaldehyde Adhesive (MF)

This adhesive, also called melamine adhesive in short, is produced as a result of the condensation reaction of melamine resin with formaldehyde, just like urea-formaldehyde adhesives. This adhesive can harden at temperatures between 90-160ºC without the addition of any hardening agent. MF is very similar to urea-formaldehyde adhesive in appearance and chemical properties. The difference between UF and MF is that MF condensation and hardening can occur not only under acidic conditions but also under neutral and even alkaline conditions (Pizzi, 1993; Şahin, 2013; Tank, 1993). Up to 50% fillers can be used for melamine formaldehyde resin. For this purpose, organic fillers such as kaolin, gypsum, starch, and coconut shell flour are used. Increase in solid matter content affects resistance to water. When 100% filler is added, the adhesive shows resistance to boiling water, when 150-200% is added it shows resistance to water at ambient temperature, and even when over 200% filler is added, bonding is possible with the adhesive, but water resistance decreases significantly (Şahin, 2013; Pizzi and Mittal, 2003). MF adhesive, which is colorless, hard, and capable of creating durable bonds, shows similarity to phenol-formaldehyde due to its ability to create very strong bonds with panel surfaces, and to urea-formaldehyde adhesive in terms of chemical reaction capability and general properties. By mixing urea with melamine, an adhesive called melamine-urea-formaldehyde (MUF) can be obtained, which has properties between MF-UF adhesive characteristics (Şahin, 2013). Since boiling water resistance is not required in many wood material adhesives, especially in furniture manufacturing, high proportions of additives can be added to melamine adhesive. The additives mentioned for urea adhesives and their usage rates are also applicable here. As briefly explained above, in terms of chemical properties it is very similar to urea-formaldehyde resin adhesive, but has some different properties explained below (Pizzi, 1993; Pizzi and Mittal, 2003; Şahin, 2013). These are: • Melamine resins can harden between 90-160°C without any hardening agent added, • The hardened adhesive film is more resistant to water and atmospheric conditions than urea-formaldehyde resin, • Its resistance to light and heat is high. Melamine resins can be used mixed with urea resins, blood albumin, and PVA dispersion resins. This situation allows for more economical use of the expensive melamine resin while maintaining effectiveness. Notably, melamine-urea-formaldehyde (MUF) resin obtained by adding 50% urea-formaldehyde shows adhesive strength close to that obtained using only melamine resin, and the adhesive prepared in this mixture can be used for longer periods without deterioration. For wood material that requires limited durability, melamine-formaldehyde adhesive reinforced with urea-formaldehyde (MUF) provides quality and adhesive strength between UF and MF. Table 1 presents the properties of a urea-formaldehyde (UF) and melamine-urea-formaldehyde (MUF) adhesive solution typically used in the particleboard industry (Şahin, 2013). When the MF/UF mixture is adjusted between 25-75%, it is possible to produce products that are sufficiently water-resistant. Panel sheets prepared from MF adhesive are more resistant to moisture, heat, and external effects than PF, and more durable than MUF and UF. In general, hardened melamine adhesive is insoluble in water and highly resistant to boiling water. Due to this property, it can be used effectively in marine vessels that require high water resistance properties and in panel sheets to be used in exterior spaces. In some applications (for example, plywood, OSB) it is preferred because its color is lighter than phenolic adhesive. Furthermore, it is possible to produce panel products suitable for use in exterior atmospheric conditions, moisture and heat-resistant products, such as those used in construction applications for concrete work, on exterior building facades, and on roofs. In Europe, MF adhesive is widely used in place of PF adhesive at bonding locations where exterior atmospheric conditions and water resistance are required (Şahin, 2013). One of the major disadvantages of this adhesive is its high cost. By adding UF or MF adhesive to paper pulp, papers with high wet strength can be produced. Although melamine-formaldehyde adhesive is expensive, this property balances its high cost. In the production of water-resistant papers, the adhesive solution is added to the paper pulp at the stage between a beater (such as a hollander or refiner) and the paper machine. For this purpose, 12% hydrochloric acid is also added to the MF adhesive to be used. The adhesive hardens as the paper strips pass between the hot cylinders of the calender (Şahin, 2013). Furthermore, decorative papers treated with melamine resin as a surface coating/laminate material on panel sheets are preferred because of their resistance to temperature, water, alcohol, and alkalies. It is used as an adhesive for shoe sole and heel lining. Due to its resistance to high temperatures and bonding strength, it also finds use in the adhesion of brake linings (Şahin, 2013; Tank, 1993).

3.3. Phenol-Formaldehyde Adhesive (PF)

Phenol-formaldehyde adhesive, produced as a result of the reaction of phenol with formaldehyde, falls within the duroplast group and is sold on the market mostly in liquid form. It was commercially manufactured for the first time in 1910 in Germany by the Bakelite GmbH company. Phenol-formaldehyde adhesives are divided into four types based on their formation and use conditions: Novolak, Resol, Resit, and Resitol (Pizzi, 1993; Pizzi and Mittal, 2003; Şahin 2013; Tank, 1993). a) Novolak: This type of phenolic resin is obtained by condensing phenol and formaldehyde with the aid of acidic catalysts. b) Resol: Resol refers to the unconsolidated liquid form of phenolic resin. An adhesive with low condensation degree can be dissolved in organic solvents, but unlike Novolak, it hardens into an insoluble/undissolvable resin when concentrated acids are added in a thermal process and more often in the cold. c) Resit: Resin that hardens when heat or acid is applied while in liquid form is called resit. The three-dimensional structure of this macromolecule forming a network structure does not soften or melt as temperature increases, becoming a non-fusible state. d) Resitol: This type of adhesive, which does not dissolve completely in organic solvents but rather swells, becomes a state that does not melt with temperature but softens. Due to this property, resitol falls between resol and resit. The molecular weight of phenol-formaldehyde adhesive is important for determining the wood bonding capability. During hot pressing in panel sheet production, certain hardeners and fillers are added to ensure proper hardening. In the PF solution, NaOH is generally used in 5-7% proportion as a catalyst and to protect against reaction with wood surface extractive substances. Additionally, fillers such as grain flour, wood flour, and clay can be added up to 50% proportion. In the adhesive preparation section, the phenol-formaldehyde adhesive solution prepared according to recipe and ready for use lasts only 4-5 hours at room temperature and must be used within this period. Sheets produced with PF adhesive are much more resistant to exterior atmospheric conditions, moisture, and heat compared to sheets produced with MF, UF, or MUF adhesives. For this reason, sheets produced with PF are used in underwater construction and in the construction of marine vessels. The only disadvantage of this adhesive is its high cost and dark color. The high durability of phenolic resin hardened by heat results from the bridges created between phenolic groups being in the form of dimethylene ether (CH2-O-O-CH2), providing strength in the molecule (Pizzi, 1993; Şahin 2005 and 2013). Phenolic resins in hardened form are indefinitely resistant to boiling water. Phenolic adhesives, resistant to boiling water, dilute acids, dilute alkalies, oils, and organic solvents, can also resist animal and fungal effects. When used in joining wood treated with oil-soluble impregnating substances, the hardening time is prolonged (Pizzi, 1993; Şahin 2005 and 2013; Tank, 1993).

4. Conclusion and Recommendations

In joining two different materials (for example, wood-adhesive), many variables are involved. These variables can be classified as variables dependent on the adhesive, variables dependent on the adhesion process, and variables dependent on the wood material. Assuming that variables dependent on the adhesion process and wood material are constant, the variables in the production of wood composite materials with synthetic adhesives are briefly: • Adhesive type, • Viscosity of the adhesive, • Active matter content in the adhesive recipe, • Type and proportion of filler material in the adhesive recipe, • Total solid matter content in the adhesive recipe, • pH of the adhesive recipe, • Buffering properties of the adhesive, • Spreadability/mixability of the adhesive on the wood surface are closely related. By knowing and controlling these properties, appropriate bonding can be performed and various wood-based composite panel materials can be produced. Prof. Dr. H. Turgut Şahin / Department of Forest Industry Engineering - Faculty of Forestry - Süleyman Demirel University

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