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Replacing Volatile Organic Solvents in Shoemaking Adhesives

While water-based adhesives permeated through footwear manufacturing during the 1990s,¹ fear of tearing the sole and upper apart long kept solvent-borne products exclusively dominant in that area. But in recent years, water-based formulations suitable even for this demanding application have emerged. With the help of this kind of adhesive, a multinational material producer this year unveiled an entirely solvent-free "green shoe concept".²

Figure 1: Waterborne polyurethane adhesives help deliver the "Green Shoe" concept. Credit: Bayer MaterialScience

To do this, water-based adhesive development has had to overcome the last exclusive bastion of solvent-borne products - heat-activated attachment of soles and uppers. That process traditionally applies one pre-coat solvent-borne polyurethane (PU) adhesive to the shoe upper, then lets the solvent evaporate, before the process is repeated with a second solvent-borne PU layer. Heating the upper then re-activates the adhesive, and pressing the sole and upper together while hot delivers a secure bond.

Meanwhile, solvent contributions to industrial volatile organic compound (VOC) emissions have become an increasing issue. VOCs react with nitrogen oxide gases in the presence of light to produce ozone, which can damage human health. Consequently, some countries have introduced legislation to reduce VOC emissions, for example in Europe³ and Japan.⁴ Another concern is that the presence of solvents in adhesives can have an adverse effect on workers making shoes. For example, researchers have found that increased genetic damage levels can be detected in shoe manufacturing workers exposed to toluene-borne adhesives compared to those using water-borne products.⁵

Though these issues have helped increase the use of water-based adhesives elsewhere in shoe production, the upper-to-sole bond's particular demands made the shift away from solvents harder. Nevertheless, during the 2000s researchers showed it was possible to bond soles to shoes without using solvents. For example, a British team used physical or chemical pre-treatments on the sole, but no pre-coat, before applying a water-borne PU.⁶ They found that PU and PVC shoe-soling materials could be difficult to bond thanks to cohesively weak surface layers, from release agents in the case of PU and plasticisers in PVC. Cryoblasting removed the release agent, while sodium hydroxide treatment could eliminate residual plasticiser, greatly increasing peel strength. As well as cohesively weak layers, SBR and SBS also suffer from difficult-to bond surface functionality. Pretreatment with water-soluble organic chlorine donors or electrochemical methods could again improve bond strengths.

At this stage aqueous PU adhesives in general hadn't been optimised for use in heat-reactivation bonding, with some products requiring excessive temperatures, and therefore too much energy, for activation. Others can attain the appropriate activation temperature, but only using expensive polyesters containing metal sulphonate groups.⁷ Inadequate initial heat resistance was typical, meaning that the bonded substrates could be parted too easily.⁸

Resolving this required a number of departures from traditional aqueous polyurethane adhesive manufacture. Difunctional polyols containing sulphonate groups, which had previously used to aid dispersion, were employed alongside emulsifiers with a lipophilic/hydrophilic balance of 12 to 18. Thanks to the sulphonate-functionalised polyols the amount of emulsifier needed can be minimised, which avoids making the adhesive excessively sensitive to water post application. Arguably the most important change is including polyethers that add to the isocyanate groups and that contain at least 50 per cent ethylene oxide functionality. These incorporate terminal hydrophilic chains and help ensure the desired high molecular weight structure of the polyurethanes.

The resulting crystalline polymer backbone can create a one-component aqueous adhesive that forms bonds capable of supporting a 0.5 kg weight at temperatures above 100°C. After conditioning the bond at 80°C, without a weight for 3 minutes, and then loading it with 2.5 kg at 80°C for 5 minutes, the adhesive parted at speeds of less than 1 mm per minute, compared with 14.5 mm/min for traditional aqueous formulations. Properties like these make such adhesives especially well suited for use in heat reactivation bonding with plasticised PVC, as well as polyurethane and EVA, soles and uppers made of real or synthetic leather.

Thanks to surface preparation and chemical technologies like these, solvent-free adhesives can now be reliably used thoughout all traditional footwear production processes. But even though adhesive companies can now help shoemakers eliminate solvent completely, should they, and will they?

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References

SpecialChem4Adhesives Members Reactions

shoeadhesives,updated technolgy - Oct 22, 2011

posted by emil schmid, R&D - Applied/ Formulation/ Product development at tack-service-international

i read you articel about shoeadsives.i was very clear and detailed.i want to give you for information and updated news about this.
- for eva and pyloncleaning also semiwaterbased cleaner can be used.but still a uv-curing primer solventbased or semiwaterbased must be used.
- for rubberprimer are since years ttca ,2-3 % in solvent.some changes are to usa na-dica,in water or tcca in semiwaterbased solution are used.however this still based on chloride and also can provoke yellowing or hyrdolisis.
- with our tacktreat as technolgy based on UVC-treatment all the primer use is eliminated,the process is lean and voc-free.at of course the results with PU-D 1 or 2 comp.are meeting all the requirements in sport shoes or other heavy dutyused shoes.it can be used on all sole materials,even with high contens of tafmer or engage.
this reduce materials,labour cost isleand ....

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