Boron Compounds in Adhesives and Coatings
Boron compounds are used as a peptizing agent in the manufacture of casein- and dextrin-based adhesives. The main role of boron compounds in the production of adhesives is to control the viscosity. In the presence of small amounts of sodium hydroxide, borax increases branching of the polymer chain that comprises the starch. The resulting higher molecular weight improves the wet tack of the adhesive.
The higher viscosity further helps with water retention so that excessive water doesn’t penetrate the substrate being glued. Borate’s effect on viscosity is also critical to the ability to process the adhesive on manufacturing equipment at the needed flow rates. Additionally, borates enhance flame retardant and fungicidal properties. Borates also help an adhesive's initial film formation. The major use of starch based adhesives are in the manufacture of corrugated cardboard and cardboard tubes for products such as toilet paper and kitchen foils and papers. They are also used for glass bottle labels and wall papers.
Adhesive formulations based on polyvinyl alcohol (PVA) also use borax as a gelling agent. This saves on the cost of synthetic PVA raw materials, preservatives and biocides. Adhesive grouts based on magnesium phosphates use borates so that the product can be sold in liquid form, which is preferred in certain regional markets.