How adhesives are made

Neanderthals made glue from birch bark about 200,000 years ago, and every glue since works by sticking or by reacting.

stations: 8
things upstream: 26
raw sources: 7
steps from the ground: 7

The short version

Hot-melt adhesive is made in big jacketed kettles or sigma-blade mixers. Tackifiers and waxes melt first, then thick polymers go in over several additions, along with additives. A sample goes to the lab, and approved batches are pumped out to be filled. Water-based glues disperse the polymer in water instead, while reactive epoxies and polyurethanes are sold as separate parts.

Choose the base polymer

Makers start from polymers such as polyethylene, polyacrylates, polyurethanes and epoxies, picked to suit the job. Reactive types stay as separate parts that harden only when combined.

Goes in
Polyethylene Acrylic Epoxy Polyurethane
Melt in a heated kettle

The heavy steel kettle warms to mixing temperature, and the thin ingredients, tackifiers and waxes or oils, melt first. One patent preheats its kettle to about 350 degrees Fahrenheit.

Goes in
Paraffin wax
Feed in thick polymers

Polymers and elastomers go in over several additions. A vertical kettle has a mixer emulsifier, a high-speed blade and an anchor agitator that scrapes the wall to keep the adhesive from degrading in the heat.
Shear in sigma blades

Some plants use a horizontal mixer instead, with two large sigma-shaped blades running toward each other at different speeds in a curved trough. The tight gap at the wall shears the mix.
Extrude it continuously

Pressure-sensitive hot-melts are often made continuously in an extruder rather than in batches.
Thin with water

For water-based and solvent-based glues, the mix is emulsified in water or dissolved in a solvent. When the liquid evaporates, a sticky polymer film stays behind.

Goes in
Water
Sample for quality control

A sample is drawn from the kettle for testing. Only a batch that meets the specification gets pumped onward.
Mold, chill and pack

One patent pumps molten hot-melt into molds, chills them, empties out the solid blocks, dusts them and sends them to packaging.

End of the lineAdhesive Next it goes into adhesive bandage, credit card, duct tape and 81 more.

Worth repeating

About 6,000 years ago, people in one burial site repaired broken clay pots with tree resin.
Adhesives are classed as reactive or non-reactive, depending on whether they harden by a chemical reaction.