Borates in Adhesives
Uses of Borates in Starch & Dextrin Adhesive
An adhesive is any substance that, when applied, binds two surfaces together and resists separation. Other names for adhesive could include glue, mucilage and paste. There are many types of adhesives, but the primary purpose of this discussion surrounds natural polymeric ones such as starch and dextrin adhesives
In his book “The Technology of Adhesives,” John Delmonte advises that starch adhesives were used on postage stamps when they were first issued in 1840, and that the first U.S. Patent 61,991 on a dextrin adhesive was issued in 1867. Historically speaking, the Stein‑Hall* process developed a further breakthrough in starch chemistry in the late 1930's. The process replaced sodium silicate with a starch-based adhesive during the production of corrugated paper and paperboard. This industry continues to require the greatest demand for starch chemistry and the need for borates and borax in adhesives as well.
Dextrin is one of any number of carbohydrates having the same chemical composition but a smaller and less complex molecule. They are polysaccharides and are produced as intermediate products in the hydrolysis of starch by heat, acids and enzymes. Their nature and chemical behavior depend to a great extent on the kind of starch from which they are derived. For example, some react with iodine to give a reddish-brown color, others a blue, and still others yield no color at all. For commercial use dextrin is prepared by heating dry starch or starch treated with acids to produce a colorless or yellowish, tasteless, odorless powder which, when mixed with water, forms a strongly adhesive paste. It is used widely in adhesives, e.g., for postage stamps, envelopes, and wallpapers, and for sizing paper and textiles.
Dextrin/starch-based adhesives are made from natural polymers derived from roots, tubers and seeds of higher plants. These seeds include maize, potatoes, wheat, rice and tapioca. Starch-based adhesive is either cold or warm water-soluble depending on the application.
Benefits – Borates and borax in adhesives
- Increased viscosity through cross-linking of starch molecule
- Improved film forming of adhesive
- Improved water holding properties of the adhesive
- Improved wet tack in the presence of Sodium hydroxide
- To affect the gel point of the total starch adhesive
Major Paper Corrugation Starch Adhesive Components
- Carrier Starch – Cooked
- Raw or Slurry Starch
- Sodium hydroxide (Caustic Soda)
- Other Possible Additives
- Defoaming agents
- Water resistant agents
- Wetting agents
Borate Starch Chemistry
A heated aqueous starch solution of caustic soda causes the granules of starch to swell as water is absorbed into the starch matrix. The caustic soda solution is there to break the hydrogen bonds between long chain starch molecules. This reduces the crystalline matrices and a starch paste like material is formed.
Boric acid with a portion of the caustic soda to yield yet another borate in-situ called sodium metaborate which has a much higher solubility than Borax 10 Mol or Etibor 48 (Borax 5 Mol). Inter-chain linkages are typically formed through the borate anion structure resulting in desirable modifications of the physical properties of the system.
The result is a change in this polymeric structure of the starch molecule to a more highly branched chain polymer of higher molecular weight. As a result a cross-linking effect takes place between the starch’s hydroxyl groups and the borate affinity for these groups.
This in turn produces an adhesive with increased viscosity, quicker tack and better fluid properties as mentioned earlier. These are the qualities that paper corrugated boxes require where quick tack is important at a reasonably low temperature to maintain the high operating speeds of paper box corrugated operations.
Borate compounds are also used as a peptizing agent in the manufacture of casein-based and dextrin based adhesives. The role of borate compounds is in the production of adhesives is to control the viscosity.
Starch/Dextrin Adhesive Applications
- Carton sealing
- Case sealing
- Gummed tape & paper
- Laminated paperboard
- Paper bags
- Paper corrugated boxes
- Textile sizing
- Tube winding
Other Cross-linking Chemistries
Still other compounds, both man-made and natural can be cross-linked using borates with the hydroxyl-based chemicals found.
Such compounds include:
- Carboxymethylcellulose (CMC)
- Guar & Xanthium gum
- Polyvinyl acetate
- Polyvinyl alcohol
Typically by changing the borate species and concentration, the polymer can increase in viscosity. This modification produces an interesting experiment between the molecular structure of these aforementioned compounds and the bulk properties of macromolecules. Typically alkaline borates will offer a better cross-linking chemistry than non-alkaline products.
American Borate Company Products: