Starch- and Dextrin-Based Adhesives

Starch- and dextrin-based adhesives play a large part in industrial production, especially the packaging industry. Starch and dextrin are principally used for bonding paper products. Most corrugated boxboard for making cartons is bonded with starchbased adhesives, and other porous substrates can be easily joined with these versatile adhesives.

Starch and dextrin adhesives are readily available, low in cost, and easy to apply from water dispersion. They are considered to be the least expensive class of paper-packaging adhesive. Formulated starch and dextrin adhesives can be applied hot or cold. These adhesives are generally supplied as powder and mixed with water prior to use to form a relatively thick paste. Starch and dextrin cure by the loss of moisture. Since these adhesives cure to a thermosetting structure, they have excellent heat resistance. Another significant advantage is their very slow curing rate, allowing ample assembly time. Disadvantages include poor moisture resistance and mold growth.

Although starch and dextrin have been used as adhesives for many decades, there are several important reasons why these natural adhesives will not be entirely replaced by synthetic products. The following advantages ensure that they will continue to fill particular niches in the marketplace.1

* Good availability

* Relatively low cost

* Stable quality

* Good adhesion to cellulose and many porous substrates

* Insoluble in oils and fats

* Non-toxic and biodegradable

* Heat resistant

This article will review the raw materials that go into starch and dextrin adhesives, the methods of adhesive manufacture, and the additives that are commonly used in formulations for improvement of specific properties. Finally, the common applications and formulations of starch- and dextrin-based adhesives will be reviewed.

RAW MATERIALS

Both starch and dextrin (dextrin is starch that is processed further) are considered carbohydrate polymers, and they come from basically the same sources. Starch is a natural polymer (a polysaccharide) derived from the seeds, roots and leaves of plants. Only a few plants yield starch in sufficient quantity to be economical: corn, wheat, potato, rice, tapioca and sago. The quality of the starch must be high to produce high-quality adhesives.

Starch is made up of two molecules: amylose and amylopectin. Amylose consists of long helical chains, and amylopectin has a branched structure. Starch does not have a uniform composition....