Modern society is extremely dependent on a steady supply of adhesives. Adhesives are used in many non-food consumer products, including books, fabricated building materials, apparel and house wares. With such widespread distribution and usage, the health effects of consumers' exposure to these adhesive formulations should be considered. Many currently available adhesive products use petroleum-based starting materials and require organic solvents. As these adhesives cure, the organic solvents are released in the form of potentially harmful or irritating volatile organic compounds (VOCs). VOCs can be harmful to human and animal health, and are a significant cause of indoor air and outdoor water supply pollution.
Many commonly used adhesives are of petroleum-derived feedstock molecules such formaldehyde, phenol, and methylene diisocyanate. These chemicals are both toxic to humans and animals and are volatile, easily making their way into the air, thus contributing to indoor air pollution. Formaldehyde, for example is classified as a carcinogen. Furthermore, petroleum-based adhesives often require solvents for distribution. Examples of these solvents include methylene chloride, toluene, and trichloroethane. Water-based (water soluble) adhesives represent an attractive alternative to petroleum-based adhesives, as water is inherently nontoxic, non-flammable, and safe to handle. Moreover, preparation of such an adhesive can be derived almost entirely from natural/renewable sources that do not produce VOCs upon curing.
One such class of water-based adhesives is dextrin-based adhesives. Dextrins are low molecular weight carbohydrates that are readily produced via hydrolysis of plant starch. This is achieved by dry roasting starch in the presence of an acid catalyst. Corn starch is the most commonly used starch due to its abundance and low cost. Potato, tapioca, and sago starches are other substrates that can be easily converted to dextrin. More specifically, dextrins are oligomers of D-glucose linked by either α-(1,4) or α-(1,6) glycosidic bonds. Given that these adhesives are water soluble they can therefore be distributed as water-based solutions. The majority of starch-based adhesives are used in the paper and textile industries as binders and sizing materials as well as glues and pastes.
Dextrins fall into three classes: white dextrins, yellow dextrins, and British gums. These classes are differentiated by their respective dry roasting times, temperatures, and amounts of catalyst used. British gums are typically dry roasted for 10 to 24 hours at temperatures between 150° C. and 180° C. in the presence of small amounts of acid catalyst. British gums are the highest molecular weight dextrin fragments, and as such they typically form the strongest adhesives. British gums contain pendant hydroxyl groups which form an extended network of inter- and intramolecular hydrogen bonds, thus producing a strong adhesive force. However, the extensive hydrogen bonding network makes these longer fragments of British gum dextrins less soluble in water because the crystalline hydrogen-bonded domains are difficult to separate and dissolve. Because of this, the maximum solids concentration of British gum dextrin fragments in the water solvent carrier is only about 25% (w/v), thus limiting the utility of these starch-based dextrin adhesives. In addition, these types of adhesives are susceptible to colonization by a variety of microbes including molds and fungi which can decrease the effective lifetime of the adhesive and the product into which it is incorporated.
Accordingly, there is a need for improved environmentally friendly adhesives and adhesive additives that improve the properties of existing starch-based adhesives.