- October 01, 2003, James D. Stobie, Macro Engineering & Technology
PEER-REVIEWED TECHNICAL PAPER:
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Application: Examining and comparing high barrier polymers and structures provides guidelines for matching film performance with application.
The manufacturing process for high barrier heat-shrinkable packaging films is the double bubble process. It is a two-stage process that includes extruding, quenching, and re-inflating a polymer tube. This technique allows the processor to produce films with balanced machine direction (MD) vs. trans-machine direction (TD) shrinkage with exceptional optical and mechanical properties. The barrier polymers used include polyvinylidene chloride (PVDC), ethylene vinyl alcohol (EVOH), and polyamide (PA). The structural resins are polyolefins. Tie layers are necessary when a barrier material does not adhere to a structural polymer.
In the coextruded double bubble process, a polymer extrudes downward as a molten tube. The tube undergoes rapid quenching in water followed by collapsing the tube and pulling it through a nip. The external water quench can be a cascade spray or immersion bath. Proprietary devices such as water-cooled mandrels can cool the internal film surface. The temperature variation around the tube circumference should not exceed ± 1°C, or the gauge distribution will be adversely affected.
The thick quenched tube-typical thickness of approximately 0.4-1.0 mm at a temperature of approximately 30°C has little or no orientation at this stage. It is then fed into a hot water bath or an infrared or air heated oven. The tube now at the appropriate temperature is then re-inflated and biaxially oriented. The amount of inflation or blow up ratio controls the TD orientation. The speed ratio between the reblowing nip and the collapsing nip controls the MD orientation. The bubble is then cooled and collapsed into a layflat form and transported to a winder.
When the reheating uses an infrared oven, the collapsing nip and winder are typically mounted on a rotating platform for optimum gauge randomization and finished roll geometry. For this process, the collapsing nip is normally on a motorized ball/screw assembly for height adjustment. A layflat width control system with a sensor mounted after the nip will monitor and maintain layflat width by raising or lowering the nip assembly as necessary. In the hot water bath reheating system, the re-inflated tube may be collapsed and directed into an oscillating hauloff/nip assembly for gauge randomization and then into a winder. The final wound product is a layflat tube with film thickness of approximately 50-100 µ range. The winder oscillates in the cross direction to improve finished roll geometry.
If an annealed or heat set film is required such as is typical of polyamide biaxially oriented blown films, a further reheating and re-inflation step is necessary. This is a “triple bubble.” This additional step will improve the dimensional stability and minimize the tendency of polyamide to shrink in a wound roll while still retaining requisite shrinkage for product packaging. As in all high quality film coextrusion applications, gravimetric feeding and control systems are best to ensure consistent structure and layer thickness uniformity.
Biaxially oriented multilayer PA shrink films have wide use as sausage casing and some cheese packaging applications. Typical structures are three layer PA/tie/polyethylene or PA/tie/PA, five layer PA/tie/PA/tie/polyethylene, or a combination with EVOH such as PA/tie/EVOH/tie/ionomer. The rapid cooling achieved with a water quenching process suppresses crystallization and produces films with excellent clarity and flexibility. This same rapid quench process produces films with poor dimensional stability necessitating an annealing step for the film before winding.
PA resins offer fairly good oxygen barrier, good aroma and flavor barrier, superior heat resistance, and good toughness and scuff resistance. Limitations are a requirement for an adhesive layer to bond with polyolefins and relatively moderate moisture barrier. In biaxially oriented applications, PA6 is typically blended with PA6/66 for improved stretching properties and processibility. Water quench temperatures should be as low as possible such as <10°C, and reheat film temperatures must be a minimum of 55°C for reblowing. High molecular weight PA resins provide better bubble stability.
Depending upon material blend, blowup ratio, and annealing conditions, PA coextrusion films will have shrinkage of 15%-20% in a hot water cooking bath temperature of 65°C-85°C. A wide range of PA materials are available that have FDA approval for food contact applications.
Coextruded biaxially oriented films containing EVOH have use in processed meat, cheese, and some dry food packaging applications. EVOH has excellent oxygen barrier in dry applications, but it does not have good moisture barrier. In addition, its oxygen barrier diminishes at elevated relative humidity levels. Due to these limitations, EVOH is normally coextruded with polyolefins. They can provide some protection from moisture. Similar to PA materials, EVOH requires an adhesive layer to bond with polyolefins. EVOH will adhere to PA resins without the use of an adhesive. EVOH has use with PA resins in some sausage applications and is also finding markets in CAP/MAP gas flushed packaging applications.
Shrinkage with EVOH films is similar to PA materials — typically in the 15%-20% range at temperatures below 85°C. Higher shrinkage rates are possible at higher temperatures, but these elevated temperatures can cause discoloration and other undesirable side effects to meat being packaged. The EVOH commonly used in double bubble films is a 44 mol percentage ethylene. Recent findings have shown that the oxygen barrier of EVOH can increase as much as 20% during the orientation and heating processes encountered in a biaxial orientation process.
PVDC is the only polymer that offers excellent barrier to both oxygen and moisture. In addition, moisture does not influence the oxygen transmission rate (OTR) of PVDC as it does with EVOH. PVDC also offers excellent barrier to aroma, flavor, gases, and chemicals. PVDC extrudes well but requires special consideration in machine design and proper training of operators due to its heat sensitive and corrosive nature. In addition to barrier properties, PVDC provides excellent shrink characteristics up to 40%-50% at 85°C depending on blow up and MD stretch ratios, clarity, cold temperature toughness, and sealability.
PVDC coextrudes well with other polymers but does require an adhesive layer — typically EVA or EMA — to adhere to polyolefins. Structures can be three layer such as polyethylene/PVDC/ethyelene vinyl acetate or five layer such as low density polyethylene/ethylene vinyl acetate/PVDC/ethylene vinyl acetate/sealant. The primary market for these films is primal and sub-primal meat packaging applications.
Shrink bags containing PVDC have had use for almost 50 years for packaging fresh red meat. The PVDC is particularly suitable for this application due to the following attributes:
High level of shrinkage to fully collapse around irregular shaped cuts of meat
Low activation temperature for shrinkage provides no discoloration to the meat
Softness and elasticity
Excellent oxygen, moisture, odor, and grease barrier characteristics.
The PVDC content of a typical shrink bag is approximately 10% with the balance being primarily polyethylene.
Turnkey equipment for the production of biaxially oriented barrier shrink films is now commercially available for film structures containing any of the three common high barrier polymers — nylon, EVOH, or PVDC. To achieve the desired combination of barrier, optical, and mechanical properties required for specific applications, a processor should consult with raw material and machinery manufacturers to optimize the process.