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Understanding biaxially and monaxially oriented films

Solid-phase orientation processes create very significant changes in film properties that can be quite valuable in packaging structures.

These changes permit films that are lighter weight and thinner than standard blown or cast films to satisfy performance needs by generating significant increases in such properties as tensile strength, toughness, heat stability, tear initiation resistance, and barrier.

Virtually all solid-phase film orientation takes place in-line with polymer extrusion. This is due to cost and uniformity requirements for packaging applications. Solid-phase process choices include:

• Biaxial or monaxial orientation
• Flat or tubular format
• Simultaneous or sequential machine direction (MD) and cross-machine direction (CD)

Flat biaxial orientation
Flat biaxial orientation is the most prevalent film orientation process. Most of the biaxially oriented polypropylene (BOPP) and polyester (BOPET) films used in packaging are produced using the technique detailed below. Very large units, capable of producing finished film over 300 inches wide, are being installed around the world, bringing down processing costs per pound and creating plentiful supplies for the growing global flexible packaging market.

The Process: A unit commonly called a tenter frame line performs flat biaxial orientation. The term “tenter frame” refers to the CD orientation section. The process starts with a plasticating extruder feeding a first module that operates similarly to a cast film line. (For more on cast films, download the “Understanding film manufacturing techniques” PDF here.)

With flat biaxial orientation, there are multiple chill and surface finishing rolls to handle the much thicker sheet that is formed. Orientation (or “stretching”) thins the polymer web proportionally to the orientation or stretch ratios. For example, to get to a 1-mil finished film with 5:1 stretch ratios in both MD and CD, the process must start out with roughly a 25-mil web of quenched polymer.

Once the extruded sheet has been quenched and its temperature stabilized, MD stretching is accomplished by pulling the sheet using rolls rotating with progressively increasing surface speeds; stretching takes place in small gaps between successive rollers. The small gaps minimize the potential for “neck in,” which is a reduction of the width of the web during the drawing or thinning processes.

Following MD stretching, endless chains with clips grasp the edges of the moving web and carry it into a multistage hot-air oven. The first oven stage is used to equilibrate the web at the desired temperature; this is followed by CD stretching, as the two tracks carrying the chains and clips are gradually moved farther apart, making the sheet wider. Further temperature conditioning in the final oven sections provides annealing or stress relaxation for dimensional stability and cools the film prior to winding.

Applications: BOPP and BOPET are workhorse flexible packaging films, principally in laminations, where they can be used in as-oriented form, but often are vacuum-metallized, where the films themselves do not offer sufficient barrier protection. BOPP is widely used in salty, dry snack packaging, as it offers better moisture vapor barrier than BOPET. BOPP also has the lowest density of the commonly oriented packaging films, yielding more than 6% more area per pound of film at 0.70-mil thickness than 0.48-mil BOPET.

BOPET is a popular surface film in laminations where its superior stiffness, heat-resistance, and oxygen-barrier properties compared to BOPP make it a good choice for a wide variety of products. BOPET is more stable through printing and laminating processes than BOPP and is often preferred where high-quality graphics are required. It is the surface film of choice for retort pouches because of its dimensional stability through retort sterilization.

Simultaneous flat film biaxial orientation
Machines to simultaneously biaxially orient flat films had been sought for years, and workable systems are now available from machine suppliers. These machines are also equipped with clips that grasp the edges of the moving web, but in this case, prior to any solid-phase orientation. A variety of mechanical strategies are used to simultaneously move the clips farther from each other in both MD and CD.

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