Why do we laminate and why has it, over the years, become such an increasingly important production process? The short answer, perhaps, is that lamination allows for an almost unlimited range of often very dissimilar materials and structures to be economically combined in order that unique performance benefits can be obtained that would be difficult or near enough impossible to obtain using a single material.
Laminates when married together often have an overall thickness of no more than 400 to 500 microns (and usually much less). These thin gauge structures provide many benefits including, gas/barrier resistance, durability, flexibility, scuff and chemical resistance. Lamination often compares favourably with alternative process techniques such as co-extrusion.
Flexibility and cost containment are the driving forces behind many of the successful processing developments. For instance, lamination is the only process that allows aluminium foil, metallised papers and non-plastics to be used in a broad variety of combinations.
A whole raft of applications
Many products have laminates in their make up as they are used in environmental harsh conditions. A good example would be the personal care product sector. Items such as shampoo and shower gels, for instance, are taken into the shower where they are not only going to get wet but they’re going to be roughly handled and be subjected to extremes of temperature.
Some thoughts about applications
A converter choosing to laminate must give some thought to application end user requirements and to what type of adhesives and substrate to use. Obviously an aqueous adhesive would be out of the question if the chosen substrate were one that is sensitive to water such as paper. Adhesives that have a solvent component would not be appropriate for use with a substrate where a solvent sensitive coating is already being used. For example, it is also important that an adhesive that requires an elevated temperature to be effective is not used in conjunction with a heat sensitive coating.
It may come as a surprise to many that the adhesive used in a laminate is often chosen for more than just its bonding capability. Apart from sealing the various substrate layers together the properties of the adhesive contribute to final product performance. For instance, gas permeability may be important, but flame resistance, optical clarity, chemical and heat resistance, conductivity and even something that may seem totally unrelated such as thermoforming capability may be up for consideration.
Various methods of adhesive application
With wet laminating the adhesive is applied to one of the substrates either by an air knife or by a roller coating method. The coated substrate is conjoined with another substrate and the resulting laminate is air-dried or oven dried to remove solvents and complete the bond.
One of the necessary requirements for wet lamination is that one of the substrates must be porous such as paper or cardboard. It is also necessary to take into account factors surrounding the use of water-based and solvent-based adhesives which must be dried in an oven after application. Heat sensitive substrates such as polyolefin would in this instance be inappropriate if these substrates are to be coated and would then be dried in an oven as the result would be shrinkage and distortion.
When it comes to substrate, coating and the lamination process itself none of these items can be considered in isolation. Take a seemingly straightforward substrate such as paper. Much depends upon the grade of paper to be used and the thickness. The wrong choice may result in tearing or creasing which can be due to the coating being used and other factors such as whether or not the paper selected can withstand the stresses and strains of traveling through the laminator and the associated processes.
Laminating demands as much on forethought as any other process that a converter is likely to be engaged with. The chosen laminating adhesive must have sufficient cohesive strength and the necessary long life adhesive strength to bond sufficiently to each of the different substrates. Coating weight, nip temperature, treatment level and tension control will also influence bond strength and affect the final product quality.
Until comparatively recent times the adhesives used in dry lamination have for the most part been solvent based. Quality control devices and product development systems have overcome the environmental drawbacks to solvents and now water based adhesives such as acrylic emulsions have become available. Other developments include 100% reactive solids, which overcomes the problems associated with VOC’s but also reduces energy costs.
Other options that we will touch on here but not go into in too great a detail are the hot melts that are applied via extrusion or die coating. With a hot melt a pre-made film of hot melt material may be interleaved between two substrate layers, so that, as mentioned earlier, the high temperature and nip achieve successful lamination. Extrusion laminating again is the product of several substrates but involves a molten polymer. In this case the extrudate enters a nip between two rollers and acts as a sandwich between the differing substrates as they travel over the rollers.
Extruded operations are generally not considered to be commercially viable for short runs. If the quantity of material to be used is relatively short, then adhesive laminating is often a good option.
UV or energy curable adhesives
Lamination and attendant processes are at an exciting stage with regard to development, much has happened in recent years. For instance one might cite the progress being made with UV or energy curable limiting adhesives. Initially regarded as being too expensive and not advanced enough for modern commercial and industrial application, they have confounded their critics and are now used for a wide range of general and specialised application.
UV curable laminating adhesives can bond clear plastic films or to other films, and paper and foil substrates. In flexible packaging UV laminates have found favour as an alternative to many pressure sensitive applications as they provide protection for printed graphics, colour and design particularly against abrasion, chemicals, moisture and tearing.
The VCM and Rotary Koater
The VCM and Rotary Koater enables users to select the technology most suitable for the application, whether it is engaging in quality control evaluation, undertaking pilot runs or even carrying out production on a small scale.
Both systems are versatile and are able to undertake hot melt extrusion coating using two different methods depending upon the viscosity of the hot melt. The extrusion system contains a hot melt coating head that consists of an extrusion head, heated hose and heated tank with a pump and is ideal for high viscosity adhesives. Conversely for low viscosity adhesives below 5,000 centipoises a gravure coating set up is the technique of choice.
Wet or dry laminating possibilities also exist. Considering the Rotary Koater, this system is ideal for users faced with frequent product changeovers. It offers different web path possibilities for processes including lamination. For instance, gravure/hot air drying/lamination; corona treating/direct gravure coating/hot air frying/lamination; knife over roll coating/hot air drying/ lamination, and so on may be possible combination set-ups.
The VCM system is aimed at customers who have a specific task in mind and know exactly what coating/print/laminating/curing/drying technology is needed and furthermore need a purpose built machine in order to achieve their goals.