Coating & Laminating
Controlling and containing filmic process variables
By Tom Kerchiss - RK Print Coat Instruments M
aterials today are oſten more than they seem at first glance; filmic and composite, coextruded or laminate
structures are oſten manufactured with non- conventional properties. For example, certain co-polyester films and coatings are made that are permeable to moisture and gases such as carbon dioxide and oxygen; at the same time though they are engineered to remain non-porous and impermeable to liquids such as water, blood and bacteria. When used as a mono-layer film, TPE co-
polyester resins (sometimes referred to as thermoplastic rubbers) have a non-stick tendency that makes them useful for packaging many food items; when used as a multi-layer structure they have an affinity with EVA (ethylene-vinyl-acetate) and EMA (ethyl methyl acrylate) co-polymers; they also enhance the performance properties of other polyesters and of nylon. Staying with filmic materials and subsequent conversion, films such as high density polyethylene (HDPE) are available in multi-layer cross laminated construction as well as in single ply, and can be clay coated as a speciality product offering good printability. Furthermore, if a film needs to be reinforced for strength but needs to remain flexible, and, if the film selected for the application is Polypropylene (PP) – calcium carbonate can be added. Over a third of all film used in applications such as flexible packaging is tried and tested polyethylene (PE), nevertheless with more than one hundred types of filmic materials commercially available, the creation of more complex films with enhanced performance features and often unique printing and converting idiosyncrasies continues. Depending upon selection these filmic structures can include layers that provide stiffness, strength, aesthetic, sealant and layers that exclude the ingress of gases and moisture. Materials commonly used in the exterior layer of a multi-web structure, which provides for dimensional stability, heat resistance, low co- efficient of friction (COF), clarity and printability include BOPP (bi-axially oriented polypropylene), OPET (oriented polyester) and BON (bi-axially oriented nylon). A barrier layer added provides a level of resistance to odour, light (UV) and so on but also provides chemical resistance and bond strength. Typically silicone dioxide (SiOx), ethylene vinyl alcohol (EVOH) and polyvinyledene dichloride (PvDc) are used. An integrated sealant layer located on the inside of a multi layer structure prevents gases from penetrating through the seals and into the package. In this instance the most common materials used are low density polyethylene (LDPE), ultra low density polyethylene (ULLDPE), linear low density polyethylene (LLDPE) and EVA (ethyl methyl ccrylate. Ease of printability and converting is critical but that does not always mean that
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processing goes according to plan. For example, when winding the non- porous nature of films may allow for easy entrapment of air between wrap layers and between web and rollers. The seemingly smooth nature of the filmic surface can further complicate matters in that it increases the incidence of surface scratches and abrasions. Variation in material calliper can also produce difficulties in processes ranging from laminating to slitting areas of relatively high calliper can have a detrimental affect on nip pressure, damaging delicate materials; diametrically variation may also force a web to steer towards the larger area. Material properties including those associated with adhesives used can be critical, influencing good coating quality and product performance. A defective substrate will cause yield loss and cannot be corrected by any additional processes in the coating line. Coating solution is of course important, meaning that mixing time and temperature profiles must be right. The correct raw materials must be added at the proper level and at the right points, and solution/dispersion properties – percentage solids, pH, particle size and purity must be within defined parameters. Other elements to consider include the selection of the correct coating applicator – which is not always immediately obvious, especially when engaged in laminating, undertaking research and development or when contract coating. Or indeed when asked to print or coat on an unfamiliar substrate or one known to be temperamental, an example being the TPE co-polyesters mentioned at the beginning of this article. Coating uniformity, the desired coat weight and the methods to achieve this are vital. All coating methods provide an inherent uniformity that they can achieve. If the wrong coating method is used desired uniformity might not be achievable. Defects in the coating that can render the coated product worthless include voids, pinholes, lack of coating uniformity (thick or thin) in the machine direction; orange peel effect and contamination due to gels or foreign matter – including airborne and static induced contaminants in the press room or converting plant environment. Voids in the coating are generally caused by poor adhesion between substrate and the adhesive or other coating flow. Pinholes can arise as a result of too high a coating speed. Each coating method offers advantages and disadvantages and sometimes defects are more or less associated with a particular coating technology. Extrusion coating is used for a variety of
purposes, one of which is hot melt coating, one of the many processes that can be carried out on the Rotary Koater, a pilot coating/printing and laminating system developed by RK Print Coat
Instruments. Extrusion coating is also possible with RK’s customer bespoke high end system the VCM. Depending upon the requirements of the application extrusion coating can be used for higher viscosity hot melt applications; for low viscosity products gravure coating is available. The Rotary Koater extrusion unit is comprised of
the extrusion head, heater hose and heated tank with pump. Adjustments can be made to the temperature of the system, the head setting and the pump speed to regulate coating weights down as low as 10gsm. For those adhesives with a viscosity of below 5,000 centipoises and with a maximum temperature of approximately 1,500C, products can be coated on the Rotary Koater with gravure cylinders using a reverse angle doctor blade setting. As a point of interest it should be noted that with hot melt adhesives the bond s formed by solidification of molten adhesive by the loss of heat. Resins, waxes and thermoplastics that melt in the temperature range of 100-2,500°C are used. Hot melts must flow and wet both the substrates and then cool rapidly to form a strong adhesive bond. This final bond strength is attained when the temperature falls to ambient conditions. The viscosity must be low at the application temperature, but not so low that excessive spreading and penetration occurs, otherwise blocking could occur. One of the big plusses for quality control,
product development and problem resolving devices, such as the Rotary Koater, is the range of interchangeable technologies available so that if, for example, reverse roll coating is not quite right, two or three roll reverse can be tried or perhaps slot die, air knife, gravure or another technology from a line up of coating, print and laminating specifically suited to the Rotary Koater. Test rigs and quality control devices and systems of one type or another play a very important role in manufacturing and processing and are of course not limited to plastic filmic materials but encompass a wide range of materials and consumables. The Rotary Koater is geared more towards users engaged in frequent product changes whereas the VCM is for customers with known and often high tech requirements. Both the Rotary Koater and VCM are able if required to undertake small-scale production runs.
www.rkprint.com
www.convertermag.co.uk
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