Boron in Adhesives, Caulks, and Sealants


Starch—a component of almost every plant—is a natural polymeric product and has long been used as an adhesive. (According to literature, the ancient Egyptians used a starch adhesive as a bonding agent in the making of papyrus.) Borate additives can significantly improve its adhesive qualities.

For many industrial applications, the natural tack of starch is too slow, and its viscosity too low. But when treated with a hot aqueous solution of soda ash or caustic soda plus a borate compound, extensive chemical changes occur. Interchain linkages can be formed through borester anions, creating a more highly branched chain polymer with a higher molecular weight. The result is improved viscosity, tack, and fluid properties. The starch and adhesive industry depends on these unique characteristics in the production of numerous products, including:


Borates in starch adhesives

In adhesives, borates impart characteristics that the starch and starch adhesive industry depends on.
  • Corrugated box board
  • Paper bags (grocery and multiwall)
  • Paper boxes
  • Carton and case sealing
  • Paper and board tube winding
  • Laminated paper board
  • Gummed paper and tape
  • Textile sizing
Large amounts of dextrin, mostly borated, are used in making paper boxes.


In tube winding, the correct tack speed in the adhesive is critical.

Faster tack with borates—perfect for corrugated cardboard and cardboard tubes

The patented Stein-Hall process exploits starch’s natural gelatinizing properties to produce a fluid, concentrated suspension of starch granules that is applied to corrugated cardboard and other products, such as convolute or spiral paper or cardboard tubes. When heated, the granules swell and burst, creating adhesion.

Among other ingredients that create this suspension, borates are used to increase the rate of gelatinization and achieve optimum viscosity. By speeding tack, borates help to keep machinery running smoothly and quickly. In tube winding, borates’ ability to improve the fluidity of the adhesive is also prized, as the correct flow and tack for the speed of the tube run and length of cutoff are essential.

Using borates for quicker tack in corrugated cardboard

When starch is treated with 20 Mule Team® borax decahydrate, Neobor®, Optibor®, or 20 Mule Team sodium metaborate, extensive chemical changes—such as interchain linkages formed through borester anions—create desirable modifications of the starch adhesive’s physical properties. These qualities are essential for use in corrugated paperboard machines and other forming operations.
Borax Products

Borates increase viscosity for better box sealing

Borated dextrin is often used to make paper boxes. Here, as in the creation of corrugated cardboard, borates’ ability to improve the viscosity, tack speed, and fluid properties of the adhesive—even at fairly low temperatures—is highly valuable.

Box-sealing adhesives must be thin without being gummy or stringy and must wet the board without soaking it under pressure. Starch-based adhesives used during the ending process are usually high-solubility/low-viscosity types blended with borax (10-15%) and high in solids (40-55%). They must have adequate tack, be low-foaming, and impart minimum warping (dimensional changes). Similarly, carton sealing, and gummed paper (such as stamps and envelopes) require adhesives with controlled viscosity and viscous stability.

Increased viscosity

Better fluidity for making paper bags

The paper bag industry is a large consumer of starch-based adhesives, with grocery and multiwall bags consuming the most adhesive. In grocery bags, seam adhesive must be fluid, tacky, non-foaming, and fairly stable in viscosity. Bottom pastes should be sufficiently thixotropic, to keep the paste roll covered without breaking down appreciably under the roll action. They should also adhere sufficiently and release well from the stencil applying the paste; have enough wet tack to hold the bottoms closed until the bags are bundled, wrapped, or weighted; and form a good dry bond.

Borax Products

More uses: Caulks, sealants, and epoxy

20 Mule Team borax borates also provide crosslinking to help tackify many caulks and sealants. But they also provide another benefit: Fire retardancy. Firebrake® ZB is used in certain sealants that prevent the spread of fire between adjacent rooms, through expansion of the sealant into openings associated with doors, cables, and pipes when heated. In both halogen-containing and halogen-free epoxy adhesives, Firebrake ZB is used as flame retardant, smoke suppressant, and afterglow suppressant. In halogen-containing epoxy, Firebrake ZB can replace 40-100% of antimony trioxide in epoxy formulations without affecting flame performance, and Firebrake ZB is shown to suppress smoke formation even in the presence of antimony oxide. Plus, Firebrake ZB is an effective char promoter, helping with intumescent applications of epoxy resins. In halogen-free epoxy, Firebrake ZB displays synergy in fire test performance when used in conjunction with ammonium polyphosphate, alumina trihydrate, and silica. It is an effective smoke suppressant in many formulations and promotes char formation.

20 Mule Team Borax Products

These 20 Mule Team Borax products are developed for use in the manufacture of adhesives, caulks, and sealants.

Borax Decahydrate

From detergents to dyes to adhesives, this mild alkaline salt does it all, particularly excelling as a buffering and fluxing agent. Learn More


These products are unique zinc borates combining the best of zinc and boron oxides that can be used to develop fire-retardant formulations. Learn More


With lower transportation, handling, and storage costs, this concentrated sodium borate is used in glass, fiberglass, cleaning products, and flame retardants. Learn More

Optibor boric acid

From reducing melting temperatures in fiberglass production to inhibiting corrosion in fuel additives, Optibor has a multitude of uses in numerous industries. Learn More

Sodium Metaborate

Provides increased viscosity, quicker tack, and better fluidity in starch and dextrin adhesives. Helps stabilize hydrogen peroxide solutions and neutralize acidic oxidation by-products in textile processing. Learn More

More About Borates in Adhesives, Caulks, and Sealants

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Mummies to Modern Industry: Borates in Adhesives
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U.S. Borax, part of Rio Tinto, is a global leader in the supply and science of borates—naturally-occurring minerals containing boron and other elements. We are 1,000 people serving 500 customers with more than 1,700 delivery locations globally. We supply 30% of the world’s need for refined borates from our world-class mine in Boron, California, about 100 miles northeast of Los Angeles.  Learn more about Rio Tinto.

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