Sometimes, a sticky situation is exactly where you want to be. That’s definitely the case when adhesives are a part of your product or manufacturing process. And, borates have a long history as a go-to additive for improved adhesive properties in the formulation of starch-based adhesives.
Adhesives treated with borates
Traditionally, adhesives were created from naturally sticky substances: tree saps, starches, and animal extracts. Thousands of years ago, portions of Babylonian statues were glued with a tar-like material. Ancient Greeks and Romans made adhesives from animal bones and blood, egg whites, and other foodstuffs.
Adhesives & Sealants Industry (ASI) magazine notes that many of these traditional starch-based adhesive formulas included borax to improve durability, stability, and wet tack. The use of boron compounds in adhesives has been popular for centuries. ASI points out that, while only a theory, the idea that a borate-containing ore might have contributed to the bonding properties of the cloth used in Egyptian mummification ceremonies isn’t too far-fetched.
But the formulation of starch-based adhesives really took off during the Industrial Revolution, when they were developed for use in furniture manufacturing. In the 1930s, the Stein-Hall process—used to produce starch adhesives for the manufacture of corrugated cardboard boxes—featured the addition of borax. The process is still used in the packaging industry.
What makes borates such an important addition for adhesives? The key is the borate anion’s ability to induce chemical changes. As a natural polymeric product, starch is already a good adhesive. But for industrial purposes, it can have too slow a tack and too low a viscosity. Borate compounds create a more highly branched chain polymer with a higher molecular weight. As a result, starch-based adhesives gain improved tack, viscosity, and fluid properties.
Better bonding with boron
Today, borates continue to be a part of better adhesives.
By adding borates to starch-based and dextrin-based adhesives, manufacturers see increased viscosity, resulting in greater tack, faster bonding, and better fluid properties. These qualities are vital for several industries, including packaging and construction and home-decoration. Borate-treated adhesives are less likely to overrun bonded materials, help keep machinery running smoothly, and can provide additional benefits such as a fire-retardancy and wood protection.
Borax decahydrate, boric acid, sodium borate, and sodium metaborate are essential to the adhesives used in the manufacture of many paper-bonding and paper products, including:
- Paper grocery and multiwall bags, paper boxes, and corrugated box board
- Bookbinding and wallpaper pastes
- Carton and case sealing
- Laminated paper board
- Textile sizing
- Tube winding
Wood and cellulosic products
Borax makes an excellent addition to urea-formaldehyde (UF) and phenol formaldehyde (PF) adhesive formulations, used in the making of:
- Medium-density fiberboard (MDF)
Borates can significantly reduce formaldehyde emissions, without affecting mechanical strength. And boric acid, used in polyurethane-based (PU) wood glues, helps to protect wood without reducing bonding strength.
Soy protein-based adhesives are also popular for use in oriented strand board (OSB) and plywood, especially as a formaldehyde-free adhesive. Here, the addition of boric acid boosts the shear strength of bonded samples when treated with water—a sign of improved water resistance and adhesion.
Boric acid or borax is also used in tile-adhesive thickener. Thanks to these borates, manufacturers can see better thickening action than formulations without borates.