Moving to water-based adhesives

How SATRA’s new service is assisting factories on the ground with implementing the change to WBA, giving positive results for the environment and profit line.

by Tom Bayes

Brand owners are under ever-increasing pressure from consumers to display their green credentials. In the world of fast fashion and seasonal styles, this is especially difficult – particularly if margins are limited and products are very cost sensitive. Small steps are required, which should be cost-neutral if possible. One of the easy-win scenarios is to use water-based products during manufacture and reduce the use of solvents.

While we often hear about the need for a reduction in CO₂ production, it is rare to be told about the issues associated with volatile solvents. These are scientifically grouped as ‘Volatile Organic Compounds’ (VOCs) and cover a wide range of products. They are mainly found as organic solvents and are used as carriers for dissolved products such as paints and adhesives. Of course, these solvents must evaporate for the product to cure and there lies the issue – they pass into the environment. The majority are quite toxic (acetone being one of the exceptions) and workplace limits should be strictly adhered to. However, when they reach the atmosphere, these solvents combine with oxides of nitrogen and form the brown photochemical smog seen in some cities. Some researchers have produced convincing evidence that the presence of these chemicals now rivals smoking as a major cause of respiratory diseases and subsequent mortality rates.

To gain some perspective, the tops of spectacular thunder clouds reach to around 52,000 feet above sea level. The air temperature at this height is -55°C, and a human would need a pressure suit (such as an astronaut wears) to survive.

In contrast, most humans live in a zone from sea level to an altitude of around 15,000 feet, above which it is normally a struggle to breath. Unfortunately, photochemical smog sits in this area up to around 15,000 feet, which is a very thin ‘veneer’ in which most life on earth exists. On the scale of a football, this is just the thickness of a sheet of paper. Therefore, it is extremely important to look after it.

Why VOCs are still used

VOCs are emitted from many sources. The ‘new smell’ in a car – or, indeed, from a freshly opened shoe box – is caused by VOCs. Certain industry sectors have gone to great lengths to reduce VOCs, especially where indoor air quality is concerned. Many paint products are now water-based formulations and legislation exists for floorings and furnishings to ensure that the release of these solvents is kept to a minimum. There are still large quantities of VOCs released by adhesives, despite water-based alternatives being available for many years. Why is this?

There are actually some very good and understandable reasons. Specialist adhesives exist, of course, but also solvent-based adhesives (‘SBAs’) are quite tolerant of bad practice and poor priming. They tend to be better at wetting hard-to-adhere materials and have an inbuilt cleaning ability if, for example, there is excess mould release remaining. In addition, solvent-based adhesives tend to dry faster and they are not affected by humidity levels. As a result, water-based adhesives (‘WBAs’) are still treated with a certain amount of suspicion in the industry, probably because there is an ongoing attempt to adhere difficult materials and get high bond strengths. However, it is important to remember that the solvent is just a carrier for the adhesive, which can just as well be carried by water.

Water-based adhesives differ from their solvent-based alternatives in certain respects, and these differences must be respected. Moving to water-based adhesive is not something that is done for an easy life – this step is taken to help reduce the impact solvents have on the environment and improve ‘green credentials’. The main difference is that in a solvent-based adhesive, the adhesive component (whether polyurethane or chloroprene) is dissolved in the solvent in the same way sugar dissolves in a mug of hot coffee.

In a similar manner, the solvent has a capacity – a limit on the amount of solute dissolved. This means that solvent-based adhesives actually contain far less actual adhesive than an equivalent water-based adhesive (usually 12 to 18 per cent), and this is called the ‘solids content’. Water-based adhesives are a suspension of small but solid particles of adhesives carried rather like silt in river water, which can settle out and need frequent agitation. However, the solids content of typical water-based adhesives is around 50 per cent by mass. They contain considerably more adhesive per kilogram, which is why they tend to be more viscous. Hence, while they take longer to dry than solvent-based formulations (approximately twice as long), only one application is required to achieve the same coating rate.

Help is available

To assist manufacturers, SATRA has developed a water-based adhesive consultation service to offer manufacturers and brand owners of all magnitudes an in-country service to move production lines to water-based adhesives. The service involves three phases. Initially, a factory visit is made in order to assess the current situation – in effect, an audit. As already explained, water-based adhesives are less tolerant of bad practice and any example needs to be identified and rectified. The initial audit covers the following areas: i) pre-production testing of prototypes and new materials, ii) adhesive and primer storage – to include a check on adhesive shelf life, iii) adhesive mixing, iv) primer mixing, v) preparation of uppers, vi) preparation of solings, vii) adhesive application, viii) drying, ix) activation, x) pressing, xi) moulded-on soles, if applicable, xii) construction – considering the suitability of bonding margins and fit, xiii) assessment of sole bonds, xiv) health and safety, xv) environmental aspects, and xvi) staff training.

The results of this audit provide a list of corrective actions to be addressed in phase two. This list can be quite comprehensive and include, for example, calibration for the equipment used in the sole bonding operation.

In phase two, a second visit is made to implement the corrective actions and make the move to water-based adhesives. This will include such details as application training, use of the correct types of brushes, priming, drying times and pressing pressures, and dwell times. As already mentioned, only one coat is generally required due to the higher solids content, and so appropriate minor line reorganisation is carried out.

Phase three involves a third visit to ascertain that all the corrective actions remain in place and to resolve any outstanding issues, such as insufficient roughing. During this visit, random samples are collected and returned to a SATRA laboratory for sole bond testing. In addition, measurements are taken in order to estimate the solvent-based adhesive usage and coating rates. The usage rate should be at least 30 to 40 per cent less than for solvent-based adhesives (and often more), remembering that one of the priorities is to achieve improved environmental credentials without an increase in cost.

The solvent-based adhesive coating rates are used in conjunction with the area of the bond and the solids content of the adhesive to estimate the actual mass of VOCs that would normally evaporate. Wastage estimates from SATRA’s historical data (such as normal evaporation from the pots) is applied and an overall figure is calculated for the reduction in VOCs for normal production quantities. It is perfectly possible to carry out a much more comprehensive audit of adhesive usage at additional cost if required. However, in SATRA’s experience, as long as the coating rates are reasonable, this more simple method of assessment produces values that are extremely accurate.

Saving money and the environment

In a recent project using the service outlined above on children’s footwear with just one cemented bond (a one-piece moulded unit), the reduction in VOCs released to the atmosphere was between 5.4 and 5.9 tonnes per year, based on a very moderate yearly production of 500,000 pairs. A sole unit consisting of a cemented midsole and outsole can easily be twice this number, and adult sizes with more complex cemented units can approach 30 tonnes per year. Very significant reductions in atmospheric pollution can be made by moving to water-based adhesives.

It is hard to visualise these quantities, but 5.9 tonnes of solvent equate to approximately 7,400 litres of liquid solvent. When fully evaporated, this is sufficient to fill 670,000 cubic metres – the volume of 268 Olympic-sized swimming pools – with a lethal level of vapour, or no less than three-quarters of a cubic kilometre of air which has a clearly detectable smell.

Moving to the use of water-based adhesives can be challenging, and is often first treated with suspicion accompanied with a mountain of negative reasons. The approach taken by SATRA is to fix the assembly line first, ensuring that it is operating correctly and efficiently, and thus already making savings and improving quality. A typical adult’s style manufactured in Asia with a complex multipart cemented sole unit can easily require $0.50 to $0.60 of adhesive and primers per pair. A saving of just 6 cents per pair through increased efficiency and care can add up. On a typical Asian line producing around 5,000 pairs a day, 6 cents a pair represents a saving of around $60,000 per year.

SATRA’s approach is to involve all the stakeholders at every level – the brand owner, the sourcing company and the factory staff. The move to water-based adhesive is then made with the assistance of SATRA experts, following a few simple rules while keeping a watching eye on quality. The subsequent reduction in pollution is demonstrable and scientifically quantifiable – with hard, undeniable evidence that can be used for marketing. As previously mentioned, the move to water-based adhesives is at least cost-neutral and ensures quality is improved rather than compromised.

Publishing Data

This article was originally published on page 28 of the October 2021 issue of SATRA Bulletin.

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