Keith Berry, technical services manager at Chemique Adhesives (01922 459321) claims that it can match any traditional solvent-based adhesive with alternatives. Solvents are a group of chemicals that can be classified as volatile organic compounds, VOCs, and include dichloromethane, toluene, acetone and ethyl acetate, which form vapours that may be harmful to health, and which may also be flammable. One of the functions of solvents is to act as a carrier, reducing the adhesive’s viscosity to make application easier. In addition, solvents can be used to delay a chemical reaction within the adhesive until the solvent has evaporated. That means that the adhesive can be applied to a substrate before it cures.
Due to the VOCs within solvent-based adhesives, manufacturing operations using solvent-based adhesives are required to install extraction toremove the vapour so it cannot be inhaled by operators using the adhesive. Also, the extraction must not re-circulate the air. Such requirements can lead to expensive and wasteful systems, Berry says.
Chemique offers two principal adhesive alternatives. One is to replace the solvent with water. The water in water-based adhesives acts as a carrier for polymers such as acrylics, SBR (synthetic rubber) and polychloroprene (neoprene). If the adhesive is spray-applied, operators still require a face mask or extraction in the form of a spray booth (pictured above) to prevent workers from breathing in particles of adhesive. However, the extraction would be more cost-effective, as the air can be recirculated as long as the system can filter particulates.
Water-based adhesives are emulsions of adhesive and water. With brushable variants, the water needs to be removed either through evaporation or absorption into the substrate before they reach full strength (although they still have some degree of initial tack). In contrast, with sprayable water-based adhesives, the adhesive is forced out of the water. That can be done physically through shear, where molecules of the pressurised fluid are torn apart at the spray head nozzle. Or it can be done chemically, through for example changing the pH in a two-part formulation. Using a two-part water-based adhesive will provide a high level of initial tack, states Berry. And although water is the carrier in these adhesives, they are formulated to be water-resistant, so the adhesive will not re-emulsify in contact with water, he adds.
These different chemistries influence their behaviour in the factory, Berry cautions. “One of the differences that we often find with people using solvent-based is that they spray a lot of adhesive down, and think that if you add more it’s better. They can get away with that because the solvent evaporates quickly. But with water-based, if they want it to work in a process, they may need to put down less. In addition to adding more adhesive, they are also adding more water, which will take longer to evaporate than solvents. Also, some sprayable solvent-based adhesives work in one-way stick: you spray on one surface and the adhesive transfers to the other. Some sprayable water-based adhesives require two-way stick; like contact adhesives, you spray adhesive on both surfaces and stick them together. So it can seem like you are spraying more but in actual fact you aren’t.”
That’s not all, he adds. “One of the advantages of solvent-based adhesives is it can reach near full strength extremely quickly. What lots of people would when comparing water-based with solvent-based adhesives is to stick some foam together and then try to pull it apart straight away.”
Changing the adhesive might require a cultural shift in terms of production. Adds Berry: “The issue is to get people not to do what they normally would. Don’t look at the stuck foam straight away; put it through the entire process, and then check adhesion. Talking to customers, we say, ‘yes, this is a different method, you will see differences, but when you get to the end, it is the same. Once the item is out to customers, you won’t see any difference, and that is where it matters.’”
And, finally, while water-based adhesives can be slightly more expensive, they are likely to offer a higher ratio of adhesive to carrier, so a given amount goes a longer way.
Water-based adhesives are not the only alternative to solvents; users might also consider a hot-melt adhesive, solid at room temperature, in which heat functions as the carrier. Hot-melts come in pressure-sensitive (tacky) and non-pressure-sensitive types. The heat involved might range from 150-180°C. Because the chemical formulations use polymers with boiling points at far higher temperatures, there is no significant off-gassing, so face masks don’t need to be worn, although workers still need protection from burns. Hot-melt adhesives, pictured in solid form below and shown being dispensed at right and bottom, include EVAs and polyolefins.
Hot melts tend to be applied with a heated gun, which provides a bead or swirl pattern, or a spray arrangement fed by a heated hopper and hose. Most can be reheated multiple times; that means that workers need not clean the nozzle too carefully at the end of a shift, as the adhesive will re-melt next time. (Much more care needs to be taken in the case of solvent- or water-based spray glues, as once the carrier has gone, a plug of adhesive may form and block the head).
Although working with different types of adhesives, particularly anaerobics, Warren Wilkinson, joint MD of Bondloc, agrees that there is a general trend away from flammable products, and reducing or limiting hazardous solvents and monomers.
At Bondloc (01299 269 269), this drive has continued since the implementation of the CLP legislation (EC 1272:2008). Over the past three years, the company has developed a range of anaerobic adhesives that do not carry hazard labels and which have reduced hazard phrases.
While standard versions may be classed as a skin irritant or corrosive, its new ‘white label’ adhesives are not. The range of what he describes as core products include a low, medium and high-strength threadlocker, pipe seal thread sealant, a gasket product, a flexible gasket product and a high-strength retainer.
He says: “To develop the white label products, we review the base formulation: the resin structure, the monomer system, and look at what gives it strength, temperature resistance, and what is controlling viscosity. And then we look at substituting it with a less-hazardous alternative.”
Formulating an adhesive that is totally non-symbolled, totally green and 100% safe is a difficult challenge, he says. The ethos of the project he describes as, “What can you do with the current product range where you retain product performance, but you have something that reduces hazards or reduces exposure limits.”
For most applications, the white label formulations should offer similar performance to the standard product, although not necessarily at extremes. Wilkinson states: “From a visual point of view, a performance point of view, a curing process point of view, we do not expect it to work any differently to the current system. It is still an anaerobic curing mechanism; it still cures in the absence of air, in contact with a metal ion.”
The real benefit of the white label products is their acceptability from the point of view of a health and safety officer reviewing datasheets. “We’ve had products put forward before, and if anything were to flag up on the register, something that is a potential hazard to an operator, it’s not accepted as a raw material on site, and would instantly get rejected. This means that the introduction and approval of white-label products is much easier than it would be normally.”
He concludes: “From conception, it was something that we thought we should do, and look to add to the range. Ethically it’s where we need to focus, and something that customers want to know: that a manufacturer of core adhesive products can produce a reduced-hazard version without compromising performance.