The characteristics of water-based adhesivesHow water-based adhesives can be successfully used in sole attaching processes.
Sole bond failure still represents a major cause of footwear consumer complaints. The growth in the variety of materials used for both upper and solings means that now there is even greater demand for better knowledge and improved control of adhesives during the shoemaking process.
With an increasing awareness of the health and safety issues arising from manufacturing processes, it is becoming ever more important to reduce the use of solvents in shoemaking operations. This is particularly so with many countries adopting legislation to reduce exposure of operatives to volatile organic compounds (VOCs) and reduce emissions to the environment. One result is an increased demand for alternatives to solvent-based adhesives (SBAs) and primers, which are the main source of VOCs in any shoemaking factory. There is also a rise in consumer awareness of such issues, increasing the pressure on brands to address their impact on the environment.
A wide range of organic solvent-free formulations is available, the most common type being water-based adhesive (WBA). A water-based alternative has the advantage that, in the vast majority of applications, WBA can be used on current production lines with existing equipment. However, there is still a lack of confidence in these formulations and switching to a WBA is often approached with some reserve. If done correctly and with a few simple rules being followed water-based formulations can produce better bond strengths which can be achieved at a neutral cost – at the same price as if an SBA were being used.
Before the switch, it is important to ensure that the current process is being correctly carried out so that bad habits are not simply transferred to the water-based lines.
Why even consider WBAs? There is a risk in changing any process, especially where current methods are trusted and quality is good. When switching to a water-based formulation, it is not to remedy a flawed process. The change is done for other benefits and must be achieved with cost neutrality. To this end, it is important to understand the cost of the solvent-based operation, which is not as big a challenge as it may seem. An adhesive and primer usage audit can be carried out – this will involve measuring the mass of adhesive and primers over short runs to determine usage in terms of grams per pair (of liquid adhesive). Sole bond tests should be carried out to set a benchmark and determine the coating rates (the actual amount of dry adhesive that remains in the product). Once the switch is made, it is important not to revert to the old system. The goal should be to achieve or improve on the benchmarks set.
This article focuses on the normal Asian methods of production of cemented constructions with adhesive applied by hand, although many of the principles apply to other production methods involving adhesives.
A typical stock fitting/assembly of a modern construction may consist of a sole unit of several different materials, rubber out sole, EVA or PE die cut sheet or re moulded midsole / inserts with a polymer shank or flexure plate. The sole stock is assembled on a traditional line, after which the sole is cemented to a lasted leather upper.
Typical consumption figures of the primer would be 25g (liquid mass) per pair for stock and 28g for assembly. The contents of the primer will be mostly solvent (99 per cent), so a conservative estimate expects 50g of evaporated volatile organic compounds (VOCs) per pair from the priming operation.
For the SBAs used, 28g (liquid mass) for the stock and 53g for the assembly would not be untypical. This means an additional 67g of solvent have evaporated. These figures also include other losses, such as evaporation from the pots (measured as high as 10 per cent mass per hour). So for each pair produced of even a relatively simple design, 117g of solvent – mostly MEK – will evaporate into the atmosphere. To put this into context, for the production of a million pairs of shoes a year, 117 metric tonnes of MEK will be put into the atmosphere, which is a considerable amount. In the example previously used, switching to WBAs could reduce this by 67 tonnes a year – on just a million pairs of one style.
WBAs can also improve working conditions, although they are not a complete solution. Primers look set to remain solvent-based for at least the near future, for varying reasons that will be discussed later.
Considering the cost
The cost of WBA outwardly appears to be considerably more than traditional solvent-based formulations – double or even three times the cost not being unusual. This is clearly a barrier against change and is the most obvious hurdle to confront.
SBAs contain in the region of 8-17 per cent of solids, most typically around 12 per cent. This is dry mass that will ultimately end up in the bond. Solids content will vary, depending on both the quality of the supplier and the formulation. WBA will contain near to 50 per cent of solids, which means three or more times as much adhesive will be present than in solvent-based formulations.
This is important, as it means that an application of only one coat of WBA is likely to be required to achieve the correct bond, rather than two or three of a solvent-based formulation. Because of this, the cost per bond remains neutral. In our field trials, switching to WBA proved to be cost neutral and, in several examples, was a real cost reduction. There are several other factors that contribute to this result. Firstly, SBAs will typically evaporate at rates of 8-10 per cent by mass per hour. This is the solvent evaporating, which has the effect of increasing the solids content of the adhesive. Typically, this will be counteracted by topping up the pots with additional adhesive or adding solvent before the pots are filled. The overall effect is that the actual solid content varies enormously during the shift, and is mostly judged by eye.
WBAs do not suffer this problem, as the solids content remains largely stable (although pots that have stood for a time should be agitated at intervals to prevent the adhesive settling out). In addition, water-based formulations are generally slow to dry and milky white in colour. This has two advantages for manual adhesive application. It is very easy to see where the adhesive has been applied also it does not skin over quickly, and it remains the milky colour until force dried. SBAs tend to be almost transparent, especially when dry (which can happen very quickly in hot conditions).
Humid air affects the drying of WBAs but not formulations which are solvent-based, which just dry more quickly the warmer it is. Quick drying can mean that areas of the product already coated are often unnecessarily re-coated. It is also more difficult to see if the adhesive is applied to the edge, and frequently the worker will run over the edge to make sure. Being far more visible, WBAs allow workers to apply them more evenly and with less waste. Looking once again at our example, 28g of SBA were needed for the stock compared to only 11g of a water-based formulation to introduce the same amount of adhesive in the bond. There is no reason why costs should increase if changing to using WBA. In fact, because of the factors outlined above, some cost savings can be made.
Very importantly, waste must be minimised. If bad practice already exists prior to switching from solvent-based to water-based products, it is likely to continue. The cost of any wasted water-based formulation is far higher than the same amount of wasted SBA.
How often are the adhesives included in the costing of the production? The cost of adhesives and primers can easily be £0.20-0.25 ($0.30-0.40) per pair, and sometimes much more. A small saving per pair can become a very significant total.
However, the reality is that WBAs are harder to dry than their solvent-based equivalents. All the water must be driven off until the coating is clear or the bond will be compromised. Typically, the drying line is longer for water-based formulations but, as already noted, only one coat needs to be dried. When switching a typical line, removing the second coating station and effectively doubling the length of the drying alone is normally sufficient.
The nature of water-based adhesives
One common question asked is: “is the water not a solvent?” Not in this case. SBAs are so named because the adhesive is dissolved in a base, usually MEK or similar. There is a limit to how much of the adhesive (for example, polyurethane or chloroprene) can be dissolved in the solvent while keeping the viscosity suitable for the application. This is why the solids content of SBAs is relatively low. WBAs are not solutions, as the adhesive is not dissolved in the water. The water acts as a carrier and the adhesive is in the form of fine particles suspended in the water (a dispersion). This allows for the much higher solids contents achieved but requires different handling. Frequently, SBAs will need stirring to prevent skinning, WBAs should also be stirred, as they can start to settle out over long periods. Freshly delivered containers or those that have been stored must be thoroughly mixed before distribution and pots at workstations should be regularly agitated.
As with SBAs, water-based formulations often have additional additives in small proportions in order to slow hydrolysis and cross-linking. The manufacturer’s recommendations should be followed with regard to mixing.
General rules on bonding
In general, the preparation of the surfaces to be bonded is the same for both WBAs and SBAs. Usually, however, SBAs are more tolerant of poor preparation than water-based products. There is a chance that this may be used to cover up bad practice, so it is essential to ensure that processes are actually being done correctly prior to any switch to WBAs. It is extremely difficult to solve problems in the switch if poor practice has also been carried over.
Often the same primers will be used which, as was stated earlier, will probably remain solvent-based for some time. Surfaces must be well prepared because adhesives will not bond to greasy or dusty surfaces, and awareness of potential problems is important. For example, when halogenating thermoplastic rubber (TR), the solvent can attack the material and weaken it. In this case, extra care must be taken and drying times extended twofold. The component must also be allowed to dry free from stress and ideally on a flat surface, possibly upside down to allow draining, and not stacked. Failure to follow these procedures can cause cracks in the material. Many EVA primers have the characteristic of only working if the material to which they are applied is above a certain temperature. It is important to become familiar with the manufacturer’s recommendations and follow them accurately before blaming the adhesive for apparent problems.
Coating rates will be the same. Typically, a coating rate that leaves 3-5mg/cm2 dry adhesive on non-absorbent material should be considered optimum. Absorbent materials can be as high as 15mg/cm2, but it should be remembered that this is the dry mass of adhesive. It may take two or three coats of SBA, but just one coat of a water-based formulation. There is no point in over-application, as there is little evidence to suggest that this makes any difference to the bond and just serves to increase cost. In order to determine the correct coating rates, laboratory adhesive tests can be carried out on specially prepared bands (SATRA test method TM401/402 can be used).
When testing bonds, it is to be expected that green strengths for WBAs will be lower than with SBAs, and that water-based formulations take longer to cure (with full strength sometimes not achieved for five days). The ultimate cured strength of WBA is often higher than that of its solvent-based counterpart if care has been taken to ensure the correct application.
Drying times for WBAs are slower. This can be advantageous, as it allows for easier coverage and easier application. However, applicators containing metal will need to be stainless steel or non-ferrous if rusting is to be avoided.
As with all sole bonding, it is the 12mm or so at the edge of the unit which contributes to the bond strength. Adhesive applied elsewhere contributes little to the sole bond and can actually compromise drying. Units with areas prone to pooling (and especially cored units) are potentially very wasteful. Pools in voids should be avoided, as they will be very difficult to dry. Unless water-based formulations are properly dry, which is indicated by the milky colour disappearing, they give very poor results. This is solved by reducing the amount of adhesive in these areas rather than attempting to dry the excessive application.
Similar attention needs to be applied to Strobel or half Strobel constructions, and where lasting strips are added to reduce upper materials costs. High walled units are often used to help reduce costs and lasting strips of cheaper material may be added for lasting purposes. Centre areas and the lasting strip may have adhesive applied, but only enough to prevent components from moving during wear. These areas can absorb or pool large amounts of adhesive and this should be avoided if costs are to be controlled.
Hardeners are compounds that are added to adhesives to enhance their performance. Hardeners increase the glass transition temperature of the adhesive by crosslinking the adhesive molecules, thus increasing rigidity and the melting point. Although this reduces the pot life of the adhesive in comparison to single part adhesive, the enhanced performance of the bond is particularly relevant to footwear. Aside from an increase in the overall bond strength, the addition of a hardener greatly increases the longevity of the bond by reducing the amount of strength lost after being exposed to hydrolysis conditions. This is very important if the bond is expected to get wet such as in water resistant footwear, or footwear for use in hot and humid conditions.
Switching to WBAs can be cost-effective, improve working conditions and assist with greener credentials. SATRA can help you with this changeover, in addition to auditing current usage and practices to ensure successful implementation.
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