Solvent-free adhesives: “The customer must take the journey with the product!”

Solvent-free adhesives for shoes have been on the market since the end of the 1980s. Meanwhile, the industry has further improved on the bonding strengths of adhesives. Despite advances, however, there are several stumbling blocks in handling adhesives that users should be aware of. How do users work with solvent-free adhesives and what have their product journeys been like?
By Julia Knaut

For years, Master Pedorthist Siegfried Horsch had worked with solvent-based adhesives in his workshop in the Swabian town of Nattheim. He was always coughing; eventually, the cough became chronic and Horsch started getting asthma ­attacks. An examination by his doctor showed that Horsch was allergic to phthalic acid, a component present in virtually every solvent-based adhesive. This meant that if our master ­pedorthist wanted to continue working in his profession, he would have to go out and find a solvent-free ­adhesive free of phthalic acid. After some research, Horsch finally found a material supplier who recommended he try the dispersion adhesive Aquilim manufactured by Renia in Cologne.
“I switched over in 2013 and was initially very impressed with the bonding properties of the adhesive. For about a year, I was continually satisfied with the results I got working with Aquilim; then the problems began,” Siegfried Horsch remembers. The adhesive sprays started liquefying and he received many complaints from his customers because their foot orthotics kept coming apart. Siegfried Horsch was working with the adhesives Aquilim 250 and Aquilim 315 (note: According to Renia, Aquilim 250 they no longer produce because the raw material supply has been stopped). At the beginning, he was using Aquilim 315 for footbeds, foot orthoses and their covers. After multiple tests, the Master Pedorthist still also works with the solvent-free adhesive Aquilim today, but has adapted a special solution to meet his needs.The handling of solvent-free adhesives confronts users with special challenges, but also entails changes in work processes. “The feedback we received on Aquilim varied greatly: some of it was very positive, some also negative,” explains Dr. Martin Buchholz, chemist at Renia in Cologne. This experience can be confirmed by Michael Tiffinger, General Manager of the Kessler leather trading company in Munich: “Several of our customers like working with these adhesives, others not so much. Overall, we definitely need to rethink the processing,” says Tiffinger, voicing his concern.
The chemist Martin Buchholz distinguishes between three problem areas that, he believes, arise in processing.
In his estimation, the adhesive result is significantly influenced by the brushes used, as well as the application thickness. Other impact factors to be considered, however, include the pedorthist's work style, drying times and material combinations. When processing Aquilim dispersion adhesives, Buchholz recommends using brushes made of the finest synthetic bristles available. Such brushes allow a very fine and thin adhesive coating to be applied and, in contrast to natural bristles, do not soak up fluid as heavily. After use, the brush should always be immediately placed in water or back in the adhesive solution, as it will otherwise harden and the adhesive no longer dissolves. A look at the basic chemical properties of solvent-free adhesives gives us the reasons for this. Adhesives like Aquilim are based on adhesive systems that do not dissolve, but rather disperse. In other words, the adhesive consists of water-binding solids. The strength of the adhesives is exerted by evaporation of the water, in that the previously bound solids “move” closer together in the adhesive film until, finally, there is no more dispersion and the final adhesive strength is reached. Once this reaction has taken place, the solids can no longer be dissolved in water; the evaporation of the dispersion is an irreversible process. “Solid is solid, therefore the brush should go back in water right away,” Martin Buchholz stresses. He urges users to pay more attention to work materials. Brushes should always be cleaned at the end of the workday.
In addition, the adhesive coating also has an effect on the adhesive result. Because of the significantly higher solid fraction in solvent-free adhesives, users can coat a larger surface with a thinner adhesive layer, which also allows them to reduce the overall amount of adhesive required. For comparison: Solvent-based adhesives contain between 10 and 20 percent solids, whereas this proportion accounts for between 45 and 55 percent of the solvent-free alternative Aquilim.

Spray adhesives always tend to score points thanks to their lower material consumption. However, Martin Buchholz recommends that pedorthists spray larger surfaces with Aquilim; in other words, parts measuring from 20 to 30 cm in length. In particular, the user should test the adhesives on materials they have worked with previously. This is really the only way to find out how much adhesive they require for each type of material, as well as where the possibilities and limitations lie.
But Martin Buchholz also points to “problem materials” that do not bond with solvent-free adhesives. Working with strongly absorbent PUR materials, for example, generally proves to be difficult - the user must sometimes apply the adhesive as many as three or four times, and then allow the water to evaporate. “Unlike with solvent-based adhesives, five to ten minutes is not sufficient, so the pedorthist has to adapt his or her time accordingly,” Buchholz notes. However, this process can be accelerated by oven-drying prior to bonding the adhesive parts in the deep-drawing machine. Another option, according to Buchholz, is to apply a barrier layer: The user should pre-coat the PUR with one thin layer of adhesive, allow this to dry and then apply a second thin adhesive layer. The drying process can be accelerated using a hot-air gun or in a hot air circulating oven.
In general, industry has successfully further developed solvent-free adhesives in terms of strength, spectrum of use and processing properties. Currently, however, the solvent-based adhesives still have broader multi-functional usability, particularly when it comes to special materials. Buchholz recommends pedorthists start by choosing a subsector in which to work with adhesives and gather experience first. And don’t forget: Aquilim has to be stored. The water-based dispersed adhesives should not be stored under 4 °C, optimally at 20 °C.
Based on feedback from his customers, Michael Tiffinger – from the leather trading company Kessler – confirmed that solvent-free adhesive is sensitive to processing. The user has to roughen up the materials to be bonded “fresh”, blow off the dust particles and adhere exactly to the drying times. According to his customers, vegetable-tanned leathers in particular caused problems when used as top covers for foot orthoses. Michael Tiffinger also gets a range of completely different reactions to Aquilim from his customers: Whereas one customer from the pedorthic sector (with three employees) has been exclusively working with Aquilim for five years now — in other words, has completely converted to solvent-free adhesives — there are other customers who have gone back to solvent-based adhesives. “Many take a two-pronged approach and use both types of adhesives,” explains Tiffinger. Overall, he sees great potential for this type of adhesive and believes there will be an increasing demand on the part of customers. At the moment, his customers are still exercising caution, as the necessary awareness for less health-harmful adhesives has not ­taken hold in their minds yet. Michael Tiffinger sums up: “The customers themselves have to go on the journey with the product. Otherwise, they’re just not going to accept it.”
Sven Berneis of the Dresden-based company “berneis natürlich-aktiv GmbH” has been working with Aquilim for over two years now. He currently uses Aquilim as a spray adhesive for foot orthosis fabrication and for milled foot orthoses. At Berneis’s workshop, foot orthosis fabrication takes place in a separate room where adhesive workplaces have been set up with a special exhaust unit and filter system. The pedorthist rates the adhesive results as “positive across the board”. But he also points out: “Before introducing new covering materials or other substances, you’re forced to test the adhesive capability – not everything sticks.” Moreover, employees’ learning curve and the time it took to convert work processes were very short. One particularly important aspect, however, is the effort it takes to persuade employees of the benefits.
Next on his agenda, Sven Berneis wants to test solvent-free coating adhesives for upper construction. He can't use spray adhesive in this area because a corresponding exhaust suction unit is lacking. Therefore, the work processes for the upper construction are currently being revamped. He does not expect the same time savings with coating adhesives as can be achieved when using spray adhesives.This raises the question: How should one-man workshops and smaller pedorthic companies with small-scale shoe and foot orthoses production use solvent-free adhesive alternatives successfully?
Pedorthist Siegfried Horsch has found his own combination solution to meet his needs for bonding tasks involved with foot orthoses: First, he coats the foot orthosis with the neoprene adhesive Europren 2618: one of the few products on the market without any allergy-triggering phthalic acid. He then sprays the coating material with Aquilim 315. For outer soles, Horsch uses the solvent-free universal adhesive Aquilim 250, which he brushes on. He rates the adhesive results as more positive than with Aquilim 315, although he finds that the individual work steps take more time (halogenating, drying etc.). He can accelerate the processes in a heating oven, but nevertheless still needs a place to cool and dry the soles, i.e. free space — a rare commodity in his workshop, also given that he cannot do any further work on the sole during this time.
Whether the workshop is a smaller or larger one – factors like ambient temperature, humidity, evaporation time and available space required in the handling of solvent-free adhesives means that work processes have to be adapted accordingly. In particular smaller operations are forced to organize and streamline their workflows, especially if, like Siegfried Horsch for example, they only manufacture foot orthoses on specific workdays.