Waterborne Adhesives for Rigid Packaging Uses

While aqueous solutions, dispersions and emulsions have been the most commonly used adhesives in the packaging industry,¹ there are many settings in which hot-melts and other products are preferred. Thats in part down to one major weakness in waterborne formulations - long setting times, caused by the need for water to evaporate.² However, in recent years researchers have striven to overcome this, and have gone some way towards rectifying the deficiency.

Waterborne adhesives are often used in packaging in conjunction with paper and board materials. Due to their relatively low viscosity, water-based adhesives can be sprayed or extruded in a thin layer that forms a strong adhesive bond. Their low viscosity also means that they can penetrate porous substrates, rapidly giving a high degree of tack and stronger bonding than is achievable with hot-melt adhesives.

While hot-melt adhesives soften at temperatures over 75°C and may become brittle near 0°C waterborne adhesives remain bonded strongly over a greater temperature range. They can also make packaging, for example corrugated containers, that can be re-pulped and recycled without removing or separating the adhesive layers, another distinct advantage over hot-melts.

Yet, in applications like sealing corrugated fibreboard containers made from pre-cut flats, setting time remains the most critical property. Under the type of compression typically used on carton sealing lines, a hot-melt adhesive will set in about 1 to 3 seconds, as compared with 10 to 20 seconds for a water-based adhesive.

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Figure 1: Succinic anhydrides that can reduce the hydrophilicity of polymers emulsified in waterborne adhesives, increasing their setting speed.

While its possible to try and overcome this difficulty through adapting the adhesive dispensing process, novel adhesive formulations can also tackle it. For example, many waterborne packaging adhesives are based on starch or poly(vinyl acetate) (PVA) and poly(ethylene-vinyl acetate) (EVA) resin emulsions. In these curing speed can be accelerated by incorporating a starch modified with succinic anhydride functionality.³ Octenyl succinic anhydride (OSA) and dodecenyl succinic anhydride (DDSA) are particularly effective in improving the drying speed of waterborne EVA and PVA adhesives.

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Anhydride waterborne adhesives dry quicker

Anhydride molecules are attached to hydroxyl groups within the composition of the formulation. An appropriate cross-linker is then applied to tie up the hydrophilic groups and make them perform hydrophobically. This reaction serves to change the surface tension and cause the water molecules to disperse more rapidly, and therefore promote improved drying rates. For OSA groups the crosslinker would be aluminium sulfate, while in DDSA groups it would be calcium chloride.

While starch contains hydroxyl groups that can be modified with these anhydrides, EVA and PVA do not. Instead, formulations based on these polymers tend to exploit hydroxyl-containing chemicals, for example as stabilizing and dispersing agents. These can also be starch molecules, as well as dextrins or polyvinyl alcohol (PVOH). Some of the fastest curing waterborne adhesives are based on EVA stabilized by PVOH and containing starch which contains between 2-5 percent by weight of OSA groups bound to it.

These anhydride-modified corn starches are commercially available, and can simply be added to waterborne adhesives to modify them. For example, researchers have added an aluminium sulfate crosslinked corn starch carrier containing 3% OSA directly to EVA emulsions containing PVOH colloid groups and allowed it to mix for thirty minutes. They then assessed these different formulations setting speed on the wire side of unbleached Kraft base stock, and also gauged bond strength in a Kraft fibre tear test. Untreated EVA resins took eleven seconds to set, and scored 65 percent in the tear test. Adding five percent crosslinked starch to the formulation reduced setting time to five seconds, and increased the tear test performance to 100 percent, which is considered the ultimate bond strength.