Adhesives Magazine

Ask Dr.Dave

November 29, 2003
Do you have an adhesives-related question? Send it to Dr. David Dunn, a technical consultant. His broad knowledge of the industry has helped hundreds of people. He may just have the answer you're looking for to help your business.

Question:

We are an eastern European company that produces adhesives for the food industry. We are interested in the development of casein glues for sticking paper labels to glass and to PET substrates using high-speed machines. What can you tell us about these adhesives, and can you recommend any raw materials and formulas for manufacturing them?

Answer:

Casein adhesives are very old products that were originally developed by Borden Chemicals Inc. from the byproduct remaining after their invention of condensed milk manufacturing, which was produced to supply fresh milk to the troops of the American Civil War. This former waste byproduct was quickly developed and became an industry standard adhesive from the 1860s until about 1930. Casein adhesive is prepared from casein curd, which is precipitated from skim milk or buttermilk either by allowing the milk to sour naturally or by adding acid to it. To make the adhesive, the curds are dissolved in alkaline water (such as limewater) to which other chemicals like formaldehyde or copper chloride are added. Most casein adhesive is sold as a dry powder and must be mixed with water before it can be used. Once mixed with water, it has a relatively short pot life. Pot life can be increased, but only at the expense of moisture resistance. Until the introduction of adhesives based on synthetic resins, casein glues were very important in wood bonding because of their superior moisture resistance, but have since been replaced by phenolic and amino resins.

Casein adhesives have several characteristics that have limited their usefulness. They tend to stain those species of wood that are rich in tannic acid (such as oak), so that largely excluded them from the furniture industry. However, casein adhesives are still quite important in the labeling of beverage bottles, preserve jars and other glass containers. They can be applied to wet or dry, cold or hot surfaces and can easily be removed by cleaning machines.

Question:

I am confused by the large number of cyanoacrylate instant glues on the market; they all claim to stick to everything. How can I tell a good one from a bad one?

Answer:

This is an interesting question. Cyanoacrylates have transitioned from being specialty adhesives made by a handful of manufacturers to general purpose, almost commodity-type products, particularly in the consumer sector where a range of products is available from many sources worldwide. Most of the products are based on two or three basic monomers, but there is a lot of science and art in formulating these products to get maximum performance and shelf stability. The adhesives do vary widely in their prices and performance. The important factors to consider when choosing a particular brand are: 1.) What do you need to assemble or repair?; 2.) How long do you want the repair to last?; and 3.) Is shelf life of the adhesive important to you? If you need to do a quick one-shot repair on a non-critical household item, then buying the lowest cost adhesive from your local store will probably work for most applications. On the other hand, if you are doing a critical assembly operation on an industrial or medical product, then I would strongly recommend that you work with one of the very large companies with well-known brand names. You may pay a lot more for products from these companies, but what you will get is extensive technical support and testing. Modern developments in these adhesives have resulted in products that are particularly good on surfaces that were once considered "problem" surfaces, including wood, paper and polyethylene plastics.