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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resins, casein glues were very important in wood bonding because of their superior moisture resistance, but have since been replaced by phenolic and amino resins.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.