Abstract
As the majority of the people are becoming more aware of the adverse effects of petroleum-based products consumption and of the continued diminishing of natural fuel oil reserves all over the world, the search for more environmentally friendly and renewable substitute is becoming the focus of many studies. Such substitute products include the soybean-based adhesives, and soybean-based adhesive bonded wood composites and particle boards. In this study, a “literature review” was conducted in order to identify the many issues associated with the search for a replacement for petroleum-based adhesives. Accordingly, the issues identified are (1) environmental friendliness, such as the biodegradability and release of toxic gases like formaldehyde and phenols; (2) inferior strength and durability in terms water resistance and shear strength of soybean-based adhesives compared to petroleum based adhesives; (3) and the higher cost of soybean-based adhesive, soybean-based adhesive bonded wood composites and particle boards production. This study also reviewed information on the most current methods and techniques used to address these issues. The 100% biodegradability of soybean-base adhesives, soybean-based adhesives bonded wood composites and particle boards can be achieved by avoiding the use of petrochemical or synthetic plasticizers and crosslinkers such phenols and formaldehyde and replacing them with naturally occurring substances such as jute and silicon dioxide (SiO2). The emission of toxic gases such as formaldehyde can also be avoided using such technique. The strength and durability of the soybean soybean-base adhesives, soybean-based adhesives bonded wood composites and particle boards can improve by using different methods of dispersion and curing. Such methods include the use of denaturants like enzymes, co-resin systems, and variation of solid content. Water resistance can be improved by controlling or manipulating the pressing time, pressing temperature, pressing temperature, and adding naturally occurring additives such as jute and SiO2. The resistance to sheer stress can also be improved via the same approach. The cost of producing soybean-based adhesives, soybean-based adhesive bonded wood composites and particle boards can be further decreased to make the products more economical by different approaches. Such approaches include: the use of lower grade soybean meal such as soy flour over higher grade meals such as soy protein concentrate; and the use of recycled fiber sources such as recycled newspapers.. Lastly, it was concluded in this study that the soybean plant (Glycine ma) has high potential to become a source of protein for making protein-based adhesives, particularly, soybean-based adhesives. Moreover, it was also concluded that 100% biodegradable and strong and durable soybean-based wood composites and particle boards can be produced using soybean-based adhesives as binders.
Introduction
As the supply of fuel oil continues to diminish due to the rapid growth of industrialization and the limited sources of fossil fuels, researchers are continually finding ways to find the suitable substitutes to petroleum-based products, such as adhesives. Adhesives play a very important role in infrastructure and material science as they are indispensable in producing highly used materials such as wood composites and particle boards. These materials usually find their use on many household items and equipment. Candidate adhesives are preferably highly biodegradable, durable, and must be from renewable sources such as plants, which can be harvested and re-grown on different seasons. Note that it must be that the source is renewable as such improves the sustainability of the technology being developed from it. Bio-degradability is also important as such can help ensure that the cycling of substances that constitute the source continues. Moreover, the biodegradability of industrial products is now one of the major focuses of many researchers because there are decreasing numbers of areas which can serve as landfills. An ideal industrial product, which is adhesive in this case, is that it should be 100% biodegradable. In other words, the biodegradability and renewability of the product is an important aspect for it to be considered a green technology, aside from its physical and chemical properties (McGraw-Hill Yearbook of Science & Technology 355).
Why Soybean should be used as Source for Making Adhesives
While there are many plants that can be used to create biodegradable adhesives there are certain criteria that produces must accomplish in order to decide which one to use. Such criteria include the abundance of the plant source. Note that the making of renewable adhesives is in sync with the intention to fully replace the use of petroleum-based adhesives in the future; hence, it is only necessary that the plant source be highly abundant. Moreover, the price of producing the said source is indirectly proportional to its degree of abundance, that is, the more abundant the source is, the cheaper is its production. Another criterion is the composition of the plant. Accordingly, the chemical composition of the plant should be suited for making adhesives. Plants that are good candidates should have chemicals that have chemical properties for efficient bonding. Such chemical properties are determined by the shape of the chemical molecules as well as their functional groups.
Fortunately a plant called, Glycine ma, which is commonly called soybean, passes both criteria. Accordingly, soybean is produced in different countries around the world and can be grown on both tropical and temperate regions. In the United States alone, 90% of the fuel stock for making biodiesel is derived from soybean. This percentage means that there is also a great abundance of soybean meal, which is the raw material needed for making soybean-based adhesives (Liang, Gao and Shi 1). Soybean meal has a high concentration of soy protein – studies show that soybean meal has 43% proteins (Gao et. al. 689). Other studies on the composition of soybean meal or soybean flour show that is has a protein content of 44% to 55% (wt%); its fiber, water, and ash content ranges from 20% to 25% (wt%), and its carbohydrate content is 30% (wt%). The protein content of the soybean meal flour can be concentrated up to 65% to 72% - many industries thrive in the purification of soy meal to soy protein concentrate (Schimitz 4). What is more interesting about the proteins contained in soybean meal is that they have high abundance of, hydroxyl (-OH), carboxylic (-COOH) and amino (-NH2) functional groups. Note that these groups are reactive under different reaction conditions. With the right acidity or basicity of the reaction mixture, the soybean meal proteins can easily react with different substances such as cellulose, lignin, or resins to form wood composites or particle boards (Liang, Gao and Shi 1). Note that in the usual use of soybean proteins for making protein-based adhesives, the soybean meal is first dispersed in alkaline solutions. The alkaline solution allows the de-protonation of the carboxylic groups making it ionic or highly polar. This change in the overall molecular charge of the protein allows it to bind to nearby molecules such as the crosslinkers in a wood composite (Schimitz 28). These two properties of soybean make it a good candidate as a renewable source for making adhesives that could replace petroleum-based adhesives in the future. In fact, soybean-based adhesives have already been in use in the 1920s (Liu 123).
Soybean-based adhesives vs. Petroleum-based Adhesives
Despite the fact that soybean-based adhesives were already in use several decades ago; it did not gain prominence in the market due to the production of petroleum-based adhesives. Accordingly, the use of soybean-based adhesives had its peak during the 1960s and eventually plummeted down to almost zero use in the 1970s due to the popularity of petroleum-based adhesives (Wescott, Frihart and Traska 860). The main reason the latter dominated the market is its ease of production, its relatively cheaper price, and many superior qualities over soybean-based adhesives such as durability and water resistance (Gao et. al 946). Moreover, soybean-based adhesives have their disadvantages compared to petroleum-based adhesives. Accordingly, the former have lower bond strength, lower pot life or stability, higher viscosity, and high water absorption capacity than the latter (Zhang and Sun 383). Nevertheless, researchers through many years of studies have found recently that the properties of soybean-base adhesive and soybean-based adhesive bonded materials can further improve.
The key reason for the advantages of petroleum-based adhesives lies primarily on the nature of the chemical species which compose it. Accordingly, petroleum-based products such as petroleum-based adhesives are usually polymers made up very few monomers. In other words, the chemical composition of the adhesive is quite simpler in terms of chemical structures and uniformity. The uniform functional groups of the monomers allow their equal dispersion and more uniform bonding across the wood composites which they bond together. Also, the simplicity of the functional groups that they have, and the number thereof makes them less reactive to different substances to which the finished product (wood composites and particle boards) may come in contact. The case is different, however, for protein-based adhesives such as soybean-based adhesives. They have more types of monomeric units which chemical structures have more branches than the monomers of petroleum-based adhesives. This property is far from ideality f uniformity and non-reactiveness is to be desired. It is this nature of the monomeric units in soybean proteins which is the main source of the problems and issues related to the inferior quality of adhesives derived from it (McGraw-Hill Yearbook of Science & Technology 355)
What are the Issues Associated with Petroleum-based and Soybean-based Adhesives
The main issue associated with soybean-based adhesive is its high cost of production and inferior quality compared to petroleum-based products. Accordingly, while it is a fact that soybean-based adhesives come renewable sources, their use become limited due to their strength and durability. That is when it comes to strength and durability, petroleum-based products dominate. Moreover, at present, the price of manufacturing soybean-based adhesives remains higher at the industrial scale. Nevertheless, there are also issues associated with petroleum-based products such as their rapidly increasing prices due to the increasing petroleum price all over the world. Another issue associated with petroleum-based products is their low biodegradability and the high concentration of formaldehyde and phenol which is needed to produce them. These chemicals (phenols and formaldehyde) usually become airborne during and after the production process of making the adhesives and the wood composites and particle boards. In other words, petroleum-based adhesives are potential health hazards and are not environmentally friendly. Hence, researchers are now met with the challenge of improving the properties of soybean-based adhesives, soybean-based adhesive bonded wood composites and particle boards (Schimitz 111; Gao et. al. 5624; Wescott and Frihart 201; and McGraw-Hill Yearbook of Science & Technology 354).
Improving the Strength and Durability of Soybean-Based Adhesives, wood composites, and particle boards
There are many factors that a researcher needs to focus upon in order to ensure the improvement of the strength and durability of soybean-based adhesives, and soybean-based adhesive bonded wood composites and particle boards. One of this is the adhesive’s viscosity. Fortunately, researchers have found ways to manipulate the viscosity. The viscosity of the soybean-based adhesive, for example, can be controlled by varying the solid content of the adhesive. The concentration of proteins in the reaction mixture controls the viscosity of the adhesive as they control the intermolecular interaction resulting from their unfolded protein molecules. Among the many parts of soybean proteins, it is the disulfide bonds which form between different peptide chains as well as the electrostatic interactions along the protein molecules which contribute significantly to the viscosity of the adhesive. These two factors are called the “viscosity forming forces” in soybean-based adhesives; hence, they are the ones that are monitored and manipulated to vary the viscosity of the adhesive (14).Note that viscosity is highly important in controlling the adhesive behavior and the durability of the adhesive – not necessarily the bond strength which will form between the adhesive and other composite components. Schimitz explained that the operating limit for viscosity for adhesives used in wood composites is diverse which usually ranges from 500 cps to 75, 000 cps. What is promising about soybean-based adhesives is that its viscosity can be within the said range; hence, it can be used for different wood fiber materials for different wood composites and particle boards. Schimitz also explained in his study that, it is the viscosity of the adhesive which will determine the hardness or softness of the wood composite or particle board created from it, as well as its water absorption capacity. In other words, the viscosity of the soybean-based adhesive also determines the water resistance of wood composite and particle board derived from it. For particle boards and wood composites, which are relatively soft and have low water resistance, the operating viscosity of the soybean-adhesive was found to be 500 cps to 5,000 cps; for harder more water resistant boards and wood such as those wood composites used for lamination the working viscosity is from 5, 000 cps to 25, 000 cps; and those which are highly resistant to water have viscosities ranging from 8, 000 cps and 20, 000 cps (Schimitz 111).
Aside from increasing or decreasing the solid content of the reaction mixture to manipulate the viscosity, there are also other ways to accomplish such. In a study conducted by James M. Wescott and Charles R. Frihart, entitled, “Competitive Soybean Flour/Phenol-Formaldehyde Adhesives for Oriented Strand board,” it was proven that the viscosity of soybean-based adhesives can be improved by adding small portions of phenols and formaldehyde. They have also shown that the adhesive becomes water-resistant when it copolymerizes with crosslinking agents – such technique also increases the pot life or stability of the adhesive because formaldehyde, which use is to preserve biological specimen (Wescott and Frihart 201). Other studies have also shown that the physical properties – particularly its binding efficiency and reactivity to water (water absorbing capacity) – of soybean-based adhesives can be varied by using it on different wood fibers (Liang, Gao and Shi 2; McGraw-Hill Yearbook of Science & Technology 356). Wescott, Frihart, and Traska have shown in their study entitled, “High-soy-containing water-durable adhesives,” that the water-resistance of the composite wood or particle board can be improved by using a three-step process. The first step involved the denaturation of the soy proteins via alkali hydrolysis using sodium hydroxide (NaOH) and ethylene glycol as phase transfer agent. The mixture was then heated to 70oC with stirring and ultimately to 90oC for a few minutes. The second step was the modification of the denatured proteins with formaldehyde. The third step was cross polymerization with phenol-formaldehyde system. The water resistance of the resulting product was then compared to that of commercially available petroleum-based resins (pure phenol-formaldehyde system). Results of the comparison showed that the soybean-based adhesive was more water resistant by 3.3% compared to the commercially available resin or adhesive. It is also important to note that in the said experiment the internal bond strength of commercially available petroleum based adhesives – phenol/formaldehyde system – was also compared to that of the soybean-based adhesive. Results showed that the strength of the latter was higher by 17.9% to that of the former (Wescott, Frihart, and Traska 871).
The improvement of wood composites and particle boards is also made possible by varying the composition of the entire composite or by varying the temperature and pressing time. The improvement in physical properties of plywood bonded with soybean-based adhesive was best illustrated in an experiment conducted by Qiang Gao, Jianzhang Li, Shedon Shi, Kaiwen Liang and Xiumai Zhang, entitled, “Soybean meal-based adhesive reinforced with cellulose nanowhiskers.” Accordingly, they have compared the water resistances of 5 different plywood which are bonded with different soybean-based adhesive systems. Each of the plywood produced from each system underwent a test for its water resistance property in three cycles. There were ten trials for each system. The systems were: (1) pure soybean meal adhesive, (2) soybean meal/ cellulose nanowhiskers adhesive, (3) soybean meal/ cellulose nanowhiskers adhesive/ sodium hydroxide adhesive, (4) soybean meal/ cellulose nanowhiskers adhesive/ sodium hydroxide/ PEG adhesive, and (5) soybean meal/ sodium hydroxide/ PEG adhesive. The results of their experiment showed that the fourth system was most water resistant among the five, and the pure soybean meal adhesive system (system 1) was the least water resistant. The hierarchy of the systems in terms of increasing water resistance is as follows: 1 > 2 > 3 > 5 > 4. The water resistance advantage of the fourth system is about 20% of that of the fifth (Gao et. al 5624 -5627)
The role of pressing temperature was also studied by Qiang Gao, Jianzhang Li, Shedon Shi, Xiumai Zhang and Kaiwen Liang, entitled, “Improved plywood strength and lowered emission from soybean meal/melamine urea-formaldehyde adhesives.” Accordingly, their experiment tested whether pressing temperature could have a significant effect in the durability in terms of strength of the soybean-based wood composites. They have obtained the following results in figure 1. Accordingly, the best fit curve which describes the relationship between pressing temperature and durability in terms of wet sheer strength is a polynomial curve described by a second degree equation. Note that a linear curve has a Pearson Product Moment of Correlation value (R2) close to that in the figure; hence, a linear curve would also suffice to describe the relationship between the two variables. It is evident in the curve that as the pressing temperature increases, so is the durability of the soybean-based wood composite and soybean-based adhesive. It is important to note, however, that based from the behaviour of the curve, the increase is not linear; that is, as the temperature further increases from T = 150oC, the durability increases more rapidly. Nevertheless, it is also to be noted that proteins degrade on extremely high temperatures, and, therefore, the researchers who did the experiment stopped at 150oC. They have also studied if pressing time will have an effect on the durability of soybean-based adhesives and their corresponding bonded wood composites. Their experiment results showed that there is no significant correlation between the two variables.
Figure 1: Effect of Pressing Temperature on the Durability of Soybean-based adhesives (Gao et al 692)
Other studies have also shown that by using co-resin systems, such as soybean-phenol-formaldehyde systems (SPF), the viscosity and stability, of soybean-based adhesives can improve. In a study conducted by James M. Wescott, Amy Traska, Charles R. Frihart, and Linda Lorenz, entitled, “Durable Soy-Based Adhesive Dispersions” three experimental setups were made. The set-ups were to test how pressing time and composition of wood composites or particle boards can be manipulated in order to affect the water resistance of the end product. Accordingly, the three experimental set-ups or the three types of wood composites were kenaf fiber/soy protein/DS 3530, kenaf fiber/DS 3530, and kenaf fiber/soy protein. Note that two of these set-ups contained soybean meal flour, which is the source of soybean protein. The experimenters then subjected each set-up to different pressing time and then measured the degree of swelling of each of the set-ups due to water absorption for each pressing time. The experiment showed promising results because it proves that pressing time can increase the water-resistance of two of the set-ups, hence proving that composition is an essential factor in determining water resistance. The have also shown that one the set-ups behaved in reverse to that of the two. Note that water-resistance is inversely proportional to the swelling of the wood composites. Also, the use of co-resin system to improve soybean-adhesive properties is an example of the technique called dispersion (Wescott and Traska 263 – 264).
Dispersion takes into consideration the chemistry of proteins as components of adhesives. In order to understand how such technique works, it first important to understand why petroleum-based adhesives have better properties than protein-based adhesives. Accordingly, as aforementioned, petroleum-based adhesives are usually polymers that have few monomers that have similar functional groups. Protein-based adhesives such as soybean-based adhesives, on the other hand, are usually made up of approximately 20 monomers of amino acids. These monomers have side chains that contain different functional groups (McGraw-Hill Yearbook of Science & Technology 355). Soybean proteins have the following amino acids: aspartic acid, glutamic acid, alanine, leucine, valine, arginine, glycine, and lysine. All these amino acids have different functional groups (Kim and Natravali 5400). Among the many amino acids, the following have the highest abundance in soybean meal lysine (6.8%), histidine (3.4%), arginine (7.7%), tyrosine (4.2%), tryptophan (1.3%) and Serine (5.4%). These amino acids have either highly polar or ionizable side chains which make them ideal for binding cellulosic fibers in a wood composite of particle boards.
These amino acids also play an important role in solubilizing the soybean meal in H2O causing them to disperse uniformly. Moreover, with the simple manipulation of the pH of the reaction mixture the solubility of the amino acids varies. For example, an acidic pH will render the majority of the aforementioned amino acids insoluble. On the bonding properties of the amino acids with crosslinkers such as phenols, the lysine side chains are the ones that usually responsible. The reaction between the two is known as “condensation reaction” and is also dependent on the reaction mixtures’ pH. Note also, that not only the protein fractions of the soybean meal are responsible for the integrity of the adhesive properties of the resulting adhesive but also to the carbohydrates present on the meal. Carbohydrates can co-polymerize in two ways, namely: through the Maillard reaction, and or through its reaction with phenols.
Note, however, that the presence of such side chains allows the soy protein molecules to have secondary, tertiary, and quaternary structures which are far more complex than those of the petroleum polymers. Dispersion alters the tertiary structures of soy protein allowing them to bond uniformly the fibers which they come in contact. As the uniformity increases its adhesive properties also increases.
The said uniformity is the result of dispersion which is in turn is the result of manipulating the degree of solubility of soybean proteins. Note that the majority of soy proteins are globulins, which are essentially insoluble in water at their respective isoelectric point or pI, but are soluble below or above it. The pI for these globulins is 4.5; hence soy proteins are essentially insoluble within the pH range of 3.75 to 5.25. Their maximum solubility, however lies between the pH ranges of 1.5 to 2.5 and 6.3 to 14. Researchers, therefore usually manipulate the pH of the reaction mixture. However, aside from pH manipulation there are also other methods that can control soy protein solubility which include chemical and heat treatment. An example of such is the hydrolysis of soy protein into much smaller peptides near its pI. The ultimate result of such hydrolysis is the change in the net charge distribution of the soy protein which can relax its tertiary and quaternary structures. In other words, the proteins themselves become more dispersed into the reaction mixture resulting to a more uniform distribution and uniform bonding of other wood composite components (Liu 59 – 63).
In using pH manipulation as a way to disperse proteins by denaturing them, researchers have found out that strongly alkaline pH indeed increase the level of dispersion or the solubility of soy proteins. Nevertheless, when the pH is too alkaline or basic, the color of the soybean-based adhesive bonded wood composite or particle board becomes darker. Therefore, researchers are also finding ways to increase dispersion and solubility without affecting the physical properties of the end product. Fortunately, there is heating. Another method that is currently being used and continuously improved is called protein curing. In this particular technique, the tertiary structure of soybean protein is destroyed using either pressure and or increased temperature (McGraw-Hill Yearbook of Science & Technology 355). Researchers have shown that heating above 60oC results to the unfolding of the proteins and dissociation of protein subunits. The dissociation is the result of the disruption of the hydrophobic linkages. Such increase the surface area available to make bonds with other component put into the reaction mixture for making wood composites and particle boards. Nevertheless, this technique also has its drawback. Accordingly, when too much or too long heating is done to the fresh soy proteins, their solubility in water diminishes because there is an over exposure of the hydrophobic parts of the soy proteins.
In a study conducted by Gao et. al., entitled, “Improved plywood strength and lowered emission from soybean meal/melamine urea-formaldehyde adhesives,” it was shown that by increasing the pressing temperature and or pressing pressure will result to the improvement of the stability and water-resistance of the protein-based adhesive. Such technique also reduces the formaldehyde emission, which is an important advantage of soybean-based adhesive production over petroleum-based adhesive production (Gao et. al. 689).
As aforementioned the environmental friendliness of the soybean-based adhesives is one of the reasons why it is becoming more favored over petroleum-based adhesives at present. Note that the possibility of decreasing formaldehyde emissions in soybean-based adhesive production makes it more environmentally friendly than petroleum-based adhesives. Formaldehyde is toxic. It can cause the neurons to degrade faster than normal, and it can also cause cancer. The environmental friendliness of soybean –based adhesives and soybean –based adhesive bonded wood composites can be improved by varying the pressing temperature of the reactive mixture; as such can have an effect on its formaldehyde emissions. Accordingly, on the aforementioned experiment by Gao et. al., entitled, “Improved plywood strength and lowered emission from soybean meal/melamine urea-formaldehyde adhesives,” the effect of pressing temperature on formaldehyde emissions of soybean-based adhesives was studied. Their experimental results have shown that there is an inverse relationship between the two variables. That is, as the pressing temperature increases, the formaldehyde emission also increases. Figure 2 summarizes these results.
Figure 2: Effect of Pressing Temperature to Formaldehyde emission on soybean-based adhesives (Gao et. al. 692)
Moreover, highly biodegradable adhesives are perceived to be more environmentally friendly than those which are less biodegradable. It is important to note that one of the advantages of soybean-based adhesives over petroleum-based adhesives is that the former is more biodegradable than the latter. In fact, its biodegradability can be increased to 100%. In a study conducted by Narendra Reddy and Yiqi Yang, entitled, “Completely biodegradable soy protein-jute biocomposites developed using water without any chemical as plasticizer,” it was shown that 100% biodegradable soybean-adhesives can be manufactured. It was explained in the study that one of the major reasons of the decreased biodegradability of soybean-based adhesives is the used o chemical or synthetic plasticizers. Hence, in the study jute fiber was used as the plasticizer. The flexural strength, tensile modulus, and tensile strength of the soybean-based adhesive were also compared to that of polypropylene adhesive. The result of the comparison proved that the soybean-based adhesive performed better on all three aspects (Reddy and Yang 35). In a study conducted by Hua-Neng Xu, Shufeng Ma, Wenping Lv, Zhouping Wang, entitled, “Soy protein adhesives improved by silicon dioxide SiO2 nanoparticles (nano-SiO2) for plywoods,” they have tried to investigate the feasibility of using nano-SiO2 as a component for wood composites that make use of soybean proteins as adhesives. Results from the study revealed increased dry strength of the wood composites and particle boards, increased water resistance, and increase biodegradability. The experimenters explained that the said improvement in the properties of the soybean-based adhesive bonded wood composite was due to the increased interaction of the adhesive to the wood fibers and the filling of the voids by the nano-SiO2 particles. Note that since the voids are all covered with nano-SiO2 particles, then water molecules will be less susceptible to fill them. The biodegradability of the wood composites was attributed to the fact that no synthetic plasticizer was used and the SiO2 is a naturally occurring substance that need not be degraded (Xu, Ma, Lv, and Wang 191).
While the majority of the previously discussed studies still used small portions of formaldehyde or phenols in their experiments, there some studied that prove that formaldehyde and phenols can be totally eliminated in soybean-based adhesives and soybean-based adhesive bonded wood composite wood and particleboards. One of such studies was performed by Kaichang Li, Svetlana Peshkova, and Xinglian Geng. The said study was entitled, “Investigation of soy protein-kymene adhesive systems for wood composites.” The new adhesive system was a simple combination of soy protein isolate (SPI) and Kymene 557H. Note that Kymene is used in paper production as a wet strength agent. It will be the first time that such agent will be used in wood composite. The resulting wood composite was then tested for its shear strength and compared with that of formaldehyde-based wood composites. Results of the test showed that the SPI-Kymene system produce wood composites that have comparable or higher shear strength than wood composites derived from the formaldehyde-based system. Li, Peshkova and Geng proposed in the said study that Kymene served as curing agent in the SPI-Kymene system.
Aside from pH manipulation, heating, and variation of pressing pressure, researchers have found a way to disperse and cure soy proteins without adversely changing their solubility and physical properties, such as color of the end product, which is the wood composite or particle board. This technique is the use of enzymes to cure the soy proteins. Enzymes themselves are proteins and hence they usually highly soluble on water as much as soy proteins are. The enzymes used are called protease enzymes, and they usually catalyze the hydrolysis of proteins. In a study conducted by Schimitz, it was shown that some of the enzymes which can be used effectively in improving the quality of soybean-based adhesive bonded wood composites and particle boards are alcalase, pepsin, papain, and trypsin. Note that the major advantages of using enzymes to cure soybean proteins are: high reaction rates, specificity on which bonds to cleave; and milder reaction conditions (low temperature needed for curing) using low-cost equipment for the processing. It is important that in selecting which enzyme to use that a good knowledge of the nature of the protein source be known. Different enzymes catalyze the curing or denaturation of different sets of peptides or even up to the amino acid level. For example, it was aforementioned that lysine is the amino acid which is highly responsible for forming cross links with phenols crosslinkers. Lysine moieties need to be cleaved first before they can bind or react with phenols and it is on this stage of the reaction that the enzymes are of great use. Note further that two much denaturation is not good for in making adhesives because such will eventually adversely affect the water solubility of the adhesive. Specificity of the enzymatic action is the key towards achieving efficient protein denaturation. That is, if the enzyme is specific enough to target the lysine amino acids while avoiding the rest then it is less likely that unwanted breaking or excess cleavage of bonds will take place while also making sure that all necessary cross-linking could take place between the crosslinkers. However, it should be pointed out that for adhesives having high solid contents, enzyme treatment may not be sufficient to decrease viscosity at the workable range. Also, keen monitoring of reaction parameters such as concentration of reactants, reaction temperature, or pH should be closely monitored if enzymatic denaturation is to be used over other methods of soy-protein denaturation or curing (Schimitz 17).
As discussed previously, one of the disadvantages of soybean-based adhesives compared to petroleum-based adhesives is that the former is more expensive than the latter. Nevertheless, researchers have found ways to decrease the cost of producing high quality soybean-based adhesive and soybean-based adhesive bonded wood composites and particle boards. According to a study conducted by Wescott and Traska the cost of soybean protein-based wood composites and particle boards can be reduced so that they are 20% to 40% cheaper compared to their counterparts that used different adhesives such as petroleum-based adhesives such as those resins that are composed by formaldehyde and phenols. Wescott and Frihart reported an even bigger savings in their cost analysis study between petroleum-based adhesives and soybean-based adhesives. Results of their cost analysis showed that the soybean-based adhesive is cheaper by 37.8% to 58.5% compared to petroleum-based adhesive. Note that the petroleum based-adhesive used in their cost analysis was the phenol-formaldehyde-based adhesive, which is the most common petroleum-based adhesive in the market. Table 1, summarizes the results of the cost analysis.
One way of limiting the cost of soybean-based adhesives is by using cheaper types of soybean meal. In a study conducted by John Schimitz, entitled, “Enzyme modified soy flour adhesives,” he compared the quality of the end products (soybean-based adhesives bonded composite woods and particle boards) and the cost of two different types of soybean meal flour in making soybean-based adhesives bonded wood composites. The two types of soybean meal flour used were raw soy flour (SF) and Soy Protein Isolate (SPI). Note that the latter is relatively more expensive than the former and is usually the type used by numerous researchers and soybean-based adhesive producers due to its higher or better quality. Schimitz have used enzymatic hydrolysis for both types and used formaldehyde as crosslinkers. The two systems produced from the two sources showed that both of them can be used to produce wood composites with the same or similar properties even if they undergo similar processes. Schimitz explained this finding by pointing out that the properties of the wood composite, such elasticity or flexibility is dependent on the extent of cross-linking efficiency of the formaldehyde additive and the structure of the soy protein. Since, enzymatic hydrolysis hydrolyzed both types of protein sources equally or the same extent, then the same degree of cross-linking was observed. It should be pointed out that the type of soybean protein source does not necessarily affect the strength of bonding between the wood fibers in a wood composite of particle board. Accordingly, in a study conducted by Charles R. Frihart and Holly Satori, entitled, “Soy flour dispersibility and performance as wood adhesive,” it was studied whether the different sources of commercial and non-commercial soybean flour would have different effects on the bonding strength established between the other components of the wood composite and the soy proteins. Results from their experiment showed that there is no significant difference among the different soy flour sources when it comes to the strength of bonds their proteins form with other wood composite components. It should be noted here that the difference between the proteins among the flour sources is their degree of denaturation. Commercially available proteins are less denatured than the raw sources. Another way of reducing the cost of soybean-based adhesive bonded wood composites and particle boards are through recycling. In a publication made by the United States Department of Agriculture Economic Research Service, entitled, “Soybean Meal and Oil Make Inroads in New Industrial Applications,” it was noted that old newspapers can be used to make particles boards and wood composites. The composite produced from such material has comparable properties, such as strength, durability and water resistance to those which were made from raw or fresh materials. What is economical about soybean-based adhesive bonded wood composites and particle boards is that any source of cellulosic fibers will do, even rice husks or from cogon grass (United States Department of Agriculture Economic Research Service 13, 15).
Conclusion
While it is true that at present, despite the plummeting increase in the price of fuel oil and their associated petroleum-based derivatives such as petroleum-based adhesives are still more prominent in the market, products that come from renewable sources will eventually become more prominent in the future. Such product includes the soybean-based adhesives and the soybean-based adhesive bonded wood composites or particle boards. These products are still being improved by many researchers, especially from the field of material science. The overwhelming number of evidences derived from the literatures – some of them were discussed above – proves that the quality of the said products can be significantly improved. That is the challenges associated with low water resistance, durability, and flexural strength can be addressed by the diverse methods now available. Some of these methods include dispersion through the use of co-resin systems, denaturation, curing, and incorporation of nanotechnology (cellulose nanowhiskerss, and nano-SiO2 particles), enzymatic curing and protein denaturation, manipulation of the reaction mixtures’ pH level, manipulation pressing time, and pressing temperature. The challenge in producing soybean-based adhesives and soybean-based adhesive bonded wood composites and particle boards at lower cost can also be addressed by using cheaper source of soy protein – such as raw soybean meal flour over soy protein concentrate. Such can also be achieved by using cheaper sources of fibers which can be bonded together as wood composites, such as fibers from grasses and from recycled newspapers. The challenge to produce highly biodegradable soybean-based adhesive bonded wood composites or particle boards can be efficiently addressed by using naturally occurring substances such as jute, kenaf fibers, and SiO2; and the avoidance of using synthetic plasticizers. Such strategy can help assure the production of 100% biodegradable soybean-based adhesive bonded wood composites or particle boards. Moreover, since the main advantage of producing soybean-based adhesives over petroleum based adhesives is the former’s environmental friendliness, then it should be also concluded that the possibility of producing soybean-based adhesives without using even minute amounts of phenol or formaldehyde would make it popular in the adhesive market. Fortunately, such product can be produced using other curing and crosslinking agents which are accessible already in the market such as the Kaymene. Lastly, it is concluded in this research that the abundance of soybean meal flour and the existence of numerous methods of using it for making wood composites and particle boards make it an ideal candidate for making 100% biodegradable, high quality protein-based adhesive.
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