Adhesive Technology
Adhesive materials
Adhesives are composed of materials that must do two things: (1) adhere strongly to the surfaces being bonded and (2) possess a substantial cohesive strength that prevents this material from easily being pulled or sheared apart after it sets. Polymers are materials which can do both jobs well. Polymers are molecules possessing long, entangled or inter-connected ("crosslinked") chains. Many varieties of adhesives are produced from synthetic, petrochemical-based polymers and each is suited for different applications depending on the surfaces being bonded, the use conditions such as temperature and moisture exposure, and bond strength and cost requirements. Polymers are also produced in nature. Examples are proteins, polysaccharides, polyesters, and rubber. Petrochemical-based adhesives are frequently favored in industry due to their higher performance, the ease in producing materials with modified or tuned properties, and lower cost.
Bio-Based Adhesives
However, this is not always the case and in some applications, biologically produced (bio-based) adhesives possess superior adhesive performance and other attributes at a competitive price. Bio-based polymers for adhesives are obtained from plants (starch), animals (casein, milk protein), and microorganisms (BioPolyBond). Although bio-based polymers have been found useful as adhesives, they are not necessarily used by their producing organisms for that purpose - they are just polymers with properties that happen to make them good adhesives in the industrial world.
However, there are polymers that are produced and utilized by organisms in the natural world primarily for their adhesive properties and some have very unique performance, for example the ability to set and retain strength in wet environments (see "Biological Adhesives," Smith and Callow, eds.). Currently, there is much research focussed on understanding these materials with the ultimate goal of development of new, commercially useful adhesives.
When comparing adhesives produced from biological sources with those from petrochemical sources, it is also important to consider the big picture. What are the costs due to effects on human health and our environment? When analyzing the overall lifecycle of the two types of products, it is apparent that bio-based products are often the best option and this provides the driving force for continued development.
Microbial polymers
In nature, microorganisms produce polymers for many purposes, including as adhesives to help them stick to surfaces and to each other in masses called biofilms. In most cases, such as industrial water systems and medical devices, this behavior can cause big problems. However, it also presents us with a good place to look for new adhesive materials. Figure 1 shows a thin-section electron micrograph of the adhesive matrix anchoring bacterial cells to a submerged surface. The bacteria excrete an adhesive polysaccharide to form a biofilm in aqueous environments.
Production of polysaccharide polymers from microorganisms can be performed on an industrial scale. Some microbial polysaccharides which are produced commercially are used as thickening agents for water solutions and have been developed for food and industrial applications. For example, xanthan gum is produced at an annual volume of 50 million pounds and sells for about $4 per pound. A unique feature of the polysaccharides produced by bacteria is that a single bacterial strain can be cultivated under different conditions to produce polysaccharides with different chemical structures. In addition, biotechnology research continues to provide process and product improvements so that it is likely that these materials will become more cost effective.
Specialty Biopolymers adhesives
Specialty Biopolymers LLC uses bacterial polysaccharides as the primary polymer for adhesives. The polysaccharide polymers have relatively polar structures and thus adhere well to other polar, high surface-energy materials and have a high cohesive strength after setting. The adhesives are water-based and require evaporation or absorption of the water to set. Thus, they are best suited for porous substrates like paper products and wood. These adhesives are entirely bio-based, derived from renewable raw materials, and after their successful use, they are readily biodegradable.
An example of how bacterial polysaccharide adhesives are tested for their value as wood adhesives is shown in Figure 2 which illustrates the standard "shear block" method of ASTM D905. Two pieces of wood are first bonded together using adhesive formulations. After setting, the wood blocks are subjected to a compression force along the plane of the bond line at a defined load rate using an Instron testing machine. Shear strength is defined as the force required to break a specimen divided by the area of the adhesive bond. The surfaces can them be examined to determine the type of failure (Figure 3).
Figure 2. Testing shear strength of bonded
wood specimens.
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Figure 3. Wood specimens after shearing.