Where now for Lamination?
By Tom Kerchiss, RK Print Coat Instruments
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 in thickness (and usually much less). These thin gauge structures or assemblages are cost effective and provide many benefits including, gas/barrier resistance, durability, flexibility, scuff and chemical resistance and often so much more. Lamination often compares favourably with alternative process techniques such as co-extrusion.
Flexibility and cost containment are driving forces behind many of the successful processing developments of today. For instance, lamination is the only process that allows for aluminium foil, metallised papers and non-plastics to be used in combination, thereby extending retail shelf life minimising product waste and retail throwaway while enabling brand owners and others in the prototype/process development chain to develop innovative new products.
Many products have laminates in their make up; laminated materials are used in credit card and debit card production, identity and security documents, for the production of waterproof fabrics and for use in applications where the environmental conditions may be harsh. A good example with regard to the latter 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 are going to be subjected to extremes of temperature.
Of course not everything is straightforward a converter choosing to laminate must give some thought to application end user requirements and to the matter of 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. Equally it is 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.
There are a variety of methods for applying a laminate adhesive including wet and dry techniques. With wet laminating the adhesive is applied to one of the substrates either by 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 of course necessary to take into account factors surrounding the use of water-based and solvent-based adhesives. These must be dried in an oven after application; heat sensitive substrates would in this instance be inappropriate if these substrates are to be coated and which would then go on to be dried in an oven. Exposure to heat in an oven would if a heat sensitive substrate such as polyolefin was going to be used the result would be shrinkage and distortion.
When it comes to substrate, coating and the lamination process itself none of them 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 of paper may result in tearing or in creasing which can be due to the coating used and to factors such as whether or not the paper selected can withstand the stresses and strains of traveling through the laminator and the associated processes, the drying, the lamination in the nip and tension transients.
Even aluminium foil when used as a combination layer within a lamination structure can sometimes be tricky and needs watching. This is especially the case when the foil is ultra thin and is inclined to be subject to microscopic nicks as it travels at high speed through the laminator. Metallised films bring with it its own host of difficulties. For one thing they can be easily abraded or scratched during their travels through the laminator as the primary coated substrate.
Laminating demands as much 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. Coat weight, nip temperature, treatment level and tension control will also influence bond strength and affect final product quality.
Lets turn our attention to dry lamination and the opportunities that this type of lamination affords. Dry lamination differs from wet lamination in the liquid adhesive is dried prior to lamination. The adhesive can be applied to one substrate and dried; alternatively the adhesive can be applied as a hot melt filmic layer. In effect the adhesive becomes another filmic adhesive layer. Bonding is generally achieved at high temperature and high-pressure nip. In effect the high temperature and high nip pressure are sufficient to cause the adhesive to flow, which as it cools and gels creates a highly effective and instant bond. Dry bond is certainly worth considering. Unlike wet laminating dry bond can be used on a wider range of substrates including film with film and film with foil.
Until comparatively recent times the adhesive 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 have included 100 percent reactive solids, which of course overcomes the problems associated with VOC’s but also reduce energy costs.
Other options that we will touch on here but will not go into 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, and, as mentioned earlier the high temperature and nip achieve the 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 considered to be not commercially viable for short runs. If the quantity of material to be used is relatively short, then adhesive laminating is often a good option. There are plus and negative points to consider. The various extrusion possibilities do allow users to try out different polymer. In film laminating the converter is more often than not restricted to films provided by the customer.
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 to other films, to paper and foil substrates. In flexible packaging UV laminates have found favour as an alternative to many pressure sensitive applications. UV curable laminates provides for protection of printed graphics, colour and design particularly against abrasion, chemicals, moisture and tearing.
Technology that enables product developers, converters and others that are connected in some way with coating/laminating product quality/product development to determine matters such as correct product processes to use under real world conditions and prior to commercial commitment are generally well received. 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.
The Rotary Koater and its high tech bespoke counterpart, the VCM 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 extrusion head, heated hose and heated tank with pump and is ideal for high viscosity adhesives. Conversely for low viscosity adhesives below 5,000 centipoises with a gravure coating set up is the technique of choice.
Wet or dry laminating possibilities also exist. Considering the Rotary Koater for one moment, 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.
Another system, also developed by RK Print Coat Instruments is the VCM. This system is aimed at customers with a different profile; customers that 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.
Source: RK Print Coat Instruments Ltd