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Mono- and biaxial film: Technologies and markets

January 1, 2006

6 Min Read
Mono- and biaxial film: Technologies and markets

Practically everybody uses them to protect sensitive goods-high-quality stretched plastic films are a mass-produced article.

They are undoubtedly number one within the packaging material market and are progressively replacing paper, cardboard, aluminum foil, and other materials. Upon applying a particular process-mono- or biaxial orientation-the films attain a wealth of advantageous properties due to a change in the morphology of the film''s molecular structure:

Excellent mechanical properties, e.g., stiffness, tear, shock, or puncture resistanceImpermeability to moisture and steam.Good resistance to oils, fats, solvents, and heat and coldDimensional stability and scratch resistanceAttractive glossy appearance due to brilliant surface quality and high transparencyExcellent convertibility, printability, and sealability.Orientation methodsOrientation methods applied to produce such films are the double-bubble process and the tenter frame process. The double-bubble process is based on the principle of extruding a tube 40 to 50 times thicker than that of the film to be produced, and cooling this tube in a water bath. Actual biaxial stretching occurs through formation of a second bubble under regulated excess pressure between two nip rollers, orientation in machine direction being achieved by the second pair of nip rolls being driven at a faster speed (Figure 1) . With the tenter-frame process (Figure 1), the cast film derived from plastic granulate by means of extrusion is stretched in longitudinal and transverse direction to attain the required film dimensions. This is processed to become a very thin but rigid film; it can be processed either sequentially or simultaneously.Sequential lines first stretch the cast film in machine direction through a system of rollers; stretching is achieved by varying speeds between groups of rolls. Then the film enters the tenter, an oven-like device that uses two endless chains to grip and stretch the web in the transverse direction on diverting rails. Simultaneous systems stretch the film in both directions at the same time. This may be achieved mechanically, using either a pantographic chain extension design or by use of spindles combined with chain divergence angle.The limited yield and inflexibility of mechanical solutions led to the development of the LISIM technology, using linear motors driving clips without chain connections. This drive principle, also used on the Transrapid high-speed magnetic levitation train, allows a new level of freedom when manufacturing high-quality film in fast and extremely flexible production.Film raw materialsDifferent film types for a wide variety of packaging solutions come from a variety of polymers. In the last 25 years, biaxially oriented PP (BOPP) has flourished and become one of the leading flexible plastics packaging materials and is by far the most-used polymer for producing biaxially oriented films. Traditional applications are packaging for potato chips, snacks, confectionery, pasta and noodles, or tobacco. New applications have been developed in areas such as labels or bakery wraps. Besides this, film producers all over the world are continuously developing added-value products, such as specialty films for more demanding applications: wrap-around and shrinkable labels, high-barrier films, synthetic paper, lidding films, and more.Today, an increasing interest in other substrates like PET, PA, and PS for flexible packaging is to be taken into consideration. Biaxially oriented PET''s (BOPET) main benefits include its stiffness, as well as its excellent temperature and puncture resistance. Biaxially oriented PA (BOPA) offers the best aroma and gas barrier, good tearing properties, and an applicable temperature range of 40°C to 140°C. Biaxially or monoaxially oriented high-quality PS film is attractive because of its reduced brittleness, increased strength, high shrink values, and good stretching ratios. Influences on film production and equipment: Trends and developmentsThe film and packaging markets are undergoing major changes: The specialties of today will be the commodities of tomorrow. For the manufacturing of commodity films, it is vital to reduce production costs and increase production efficiency. Thus, line widths up to 10m, speeds of more than 500 m/min, and outputs of up to 6000 kg/hr are more and more state-of-the art, in combination with increased raw material performance. Composite packaging (aluminum, paper, cardboard, tin, film) is increasingly substituted with pure film packaging. This means the film property requirements are rising. Packaging has also become a key marketing tool at the point of sale. As a result, customers in supermarkets and stores are placing greater emphasis on attractive and/or appealing packaging.This leads to a greater demand for a variety of specialty films:Coextruded multilayer structures, up to seven layers for ultrahigh barrierShrink film and sleeves for trendy full-body shape labelsUltrahigh barrier film for laminationBiodegradable films for environmental protection and sustainabilityBOPA film highly suitable for frozen and cooked (microwavable) packagingMonoaxial shrink films for bundles and labelsFor the suppliers of film stretching lines, their offerings must include:Cost-effective lines for the economical production of commodity filmsManufacturing management systems to achieve utmost productivityA transparent overview of the entire operation to perform an optimized and efficient production flow.Closing the gap between today''s factory level automation and the producer''s ERP systemsHigh yield and flexible simultaneous lines for the production of value-added specialtiesMultifunction lines for the flexible pro-cessing of different raw materials, e.g., PET, PETG, and PVC for shrink application.Film producers'' benefits from new technologiesStretching line manufacturers'' research and development activities'' have to be carried out in close cooperation with raw material suppliers, film producers, and converters. The clear goals are to achieve cost efficiency, flexibility, reliability, and safety. Cost efficiencyHighly precise gravimetric dosing systems for optimal raw material mixture.Energy-saving design, e.g., with flexible twin-screw extrusion and advanced heat recovery systems.Multilayer T-die technology for enhanced layer distribution, and thus a constant film quality over the width.Active film tension control system at the pull roll to reduce film breaks and enhance film quality.Optimized film-cutting systems for reduced slit waste.Reduced maintenance costs by, e.g., direct-drive systems without gears, belts, or universal joints. FlexibilityAdvanced line layouts ensuring fast product changes thanks to increased automation.Precise direct-drive technology for consistent film quality.Fully automated resin handling and production control systems for accurate reproducibility of various products and resins.Proper thickness measurement and thickness control for optimal film and mill roll conformity.Automated mill roll/slit roll handling.Advanced simultaneous technologies for high-speed stretching in combination with a wide range of online, freely adjustable stretching patternsSafetyFurther automation to minimize all manual operations, especially during start and service phases.Lines with fully integrated safety management.Integration of ergonomic systems for user-friendly line operation.Automatic film threading for initial transport from the transverse direction orientation outlet, through the pull roll section, and to the winder as these equipment areas are the riskiest positions of film stretching lines.ConclusionThe market for oriented films, and therefore the market for film stretching equipment as well, is in the midst of consolidation worldwide, mainly due to saturation after a period of robust growth. But besides the constant drive for cost optimization, film producers are trying to develop added-value products, such as specialty films, to meet more demanding requirements and applications. Moreover, new markets such as Eastern Europe, Asia, and Russia are emerging, demanding high-quality film to satisfy their backlog demands.Several evolutionary forces are at work in the flexible packaging field, and innovation is the key to sustainable growth. New developments in raw materials, stretching equipment, film processing, and converting technologies need to be aligned to achieve maximum benefits throughout the value chain. Christian Aigner, marketing manager, Brückner Maschinenbau GmbH [email protected]

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