Getting on in plastic films
By Tom Kerchiss, RK Print Coat Instruments
Paperboard, corrugated material, glass bottles, wooden packing crates and to some extent hessian, jute and woven fabrics of one type or another accounted for a major proportion of packaging materials up until the end of the 1960s and the beginning of the 1970s. Although a limited range of plastic filmic materials was around during this time the cost was prohibitive and application potential was considered to be too small and specialised. However, nothing remains the same and resin formulators and film manufacturers began to unveil new products that were more price competitive and which could be used either as a replacement for existing materials or could serve as a material foundation for an exciting new category of products– termed flexible packaging.
But first, mention must be made of polyolefin. These films provided niche opportunities for the converter and for packaging printer and for end users. As with everything else there needed to be a market for these products. Fortuitously this coincided with a time when consumers were looking for low cost products that were brightly coloured, attractive and that were more in keeping with a growing optimism about life in general. Plastics were among a whole host of developments and innovations that were seen as finally ending the era of austerity that had previously dominated so much of life after the Second World War.
Lets look at how the polyolefin films first made inroads into the packaging market, paving the way for later products. Polyolefin is a collective term for polyethylene and polypropylene. Polyethylene was the simplest and most inexpensive plastic film to develop and manufacture at the time and initially was available as either high density or low density. Polypropylene, a harder, denser and more transparent material became commercially available much later.
Polyolefin films to begin with were used as a replacement for cellophane. Not only did the new materials provide better physical properties and moisture resistance they did so at a much lower cost. As cellophane lost market share to polyolefin manufacturers and investors gained confidence and further investment in film product development took place, notably in areas such as polypropylene and for items such as snack food packaging. PP offered good tensile strength and modulus values. Most importantly from a converter’s perspective PP is readily convertible, even when variants such as bi-axially orientated polypropylene (BOPP) are taken into account. BOPP itself provides the stiffness and stand up properties that are needed for modern day heavy- duty plastic bag and sack applications.
Another evolutionary step in the manufacture and converting of filmic materials was the use of coatings, first for the heat sealing of bags and as a way of enhancing the look of a product and most importantly for extending product shelf life and minimising spoilage caused by gases, contaminants, moisture and light.
Package food scientists and others also turned their attention to developing the means to improve oxygen transmission resistance. Polyvinyl Dichloride and Ethylene Vinyl Alcohol (EVA) material compounds proved effective. The former, known as PvDC was often applied using in-line systems on packaging presses by the converter.
Bringing us up to date both commercial and industrial demands place greater pressure on film producers, technologists of one kind or another, converters and everyone else to process new and often unfamiliar materials, often for products that are themselves at the cutting edge of technology. Convertibility is a critical consideration; many of the products, materials and consumables involved are too all intent and purpose new. Failure in any area, whether processing a multi-web structure, lamination technique, ink or a coating material that is contaminated or out of spec may jeopardise looks and performance.
The same is also true of press and converting machine performance, colour must be right but so too must rub and chemical resistance, durability and flexibility and so forth. Product development tools and quality control systems have an increasingly prominent role in processing both in terms of commercial and product viability – but more about that later.
Many of the materials now on the processing scene are specialist in that they are engineered for specific purposes. More often than not they have a thin profile, frequently less than 0.25mm in thickness.
Made in small quantities for applications that include electronics, automotive/aerospace, medical, security, packaging, civil engineering and thermal/light regulatory devices, these specialist films tend to be at the high end of the price spectrum, consequently converting is generally conducted with care in order to minimise waste.
Coating films to improve anti-static resistance and abrasion resistance as well optimise adhesion and to provide for barrier resistance continues. Films are often coated to provide be-spoke properties. Films can and are sometimes metallised to alter their electrical properties or reduce moisture permeability. Films can be mono-layered, multi-layered, laminated or co-extruded, etc.
Lets briefly consider some of the materials that the converter may come across. Polyester associated films and materials such as PET or polyethylene terephthalate is employed in a variety of applications including flexible packaging. Good printability and machinability and properties such as a resistance to extremes of temperature, puncture resistance, strength and barrier resistance make materials like polyester films a popular choice. It’s not uncommon now to find that for package applications, one may find that the pack is constructed with several layers of substrate and coatings. For instance, PET may be in the top most outer layer of a beverage pack, below this is the print and primer coating layer followed perhaps by an LDPE (Low Density Polyethylene)/foil layer and in our hypothetical structure a final layer which is a LDPE/heat seal layer.
Packaging technologists and converters will take a different approach to the one outlined above depending upon circumstances. If the product is a retort pack the layering may look something like the following: PET/print and an adhesive coating layer/foil/primer coating and LDPE.
Although PET made its debut as long ago as the 1950s further development is on going. Certain grades of PET are being used in areas such as cosmetics and personal care products. In some instances the film is embossed with holographic design.
Returning to the polyamide films, further refinements has resulted in them becoming well know and accepted for high temperature applications. They are noted for their ability to maintain the integrity of di-electric properties at temperatures as high as 240 C and they are used in coil winding insulation for industrial applications such as the manufacture of motors. For still higher applications films made from (PEEK) Polyaryletheretherketone are employed. These offer good resistance not only to temperatures in excess of 260 C but are also resistant to solvent, chemical and even fire and smoke.
Mention must be made of the transparent fluropolymer films and the nylons. The former is widely used in the chemical and in the manufacture of electronics, for instance: circuit boards. It also features increasingly in the packaging of medical and pharmaceutical products. With regard to nylons, ordinary bi-axially orientated nylon (BONN) is well known. A more exotic version known at Nylon6 is used as a matrix for nano film structures.
As mentioned earlier films as with other materials convertibility and printability are an issue. Converters and consumable providers need to minimise process inconsistencies as much as possible. Films destined for packaging applications must print well and must be manufactured with minimal gauge band variation. Time is also of the essence, process variables must be bought under control and problems rectified as soon as possible. After all the printer, the converter and other producers and suppliers are only earning revenue when a job can be mounted on a machine, run and completed, ready for the next job to take its place.
Colour communication or modern day proofing devices such as the FlexiProof and variants for users and producers of flexo inks and the K Printing Proofer for proofing flexo ink, gravure and gravure-offset have been developed by RK Print Coat Instruments. Another system that minimises process problems, speeds product development and which can be used to trial materials, formulas and undertake small scale production is the Rotary Koater, a pilot coating, printing and laminating system.
The Rotary Koater enables users to select from almost two-dozen print head and coating technologies; wet or dry lamination can be undertaken and a variety of drying technologies including hot air, infrared and UV curing are available. The Rotary Koater is particularly suited for laboratories, research centres and converters faced with frequently changing product requirements, where different coating and other technologies may need to be assessed.
For clients with known print/coat or laminating requirements, companies or organisations that require a bespoke built to order machine the VCM may be the answer. The VCM is built to high specifications and is designed for seamless integration into product development, trialling, workflow and investigative workflows of one type or another.
Source: RK Print Coat Instruments Ltd