Plastic Films Primer

Adhesive tape substrates are predominantly made from three classes of materials: paper, specialty films such as polyimide, metals, PTFE etc. and plastic films. This primer will discuss the various plastics used to manufacture adhesive tapes:

The majority of plastic films are made from low density (LDPE), linear low density (LLDPE) polyethylene, and high-density polyethylene (HDPE) resins.

Recent advances in resin technology has allowed the production of additional grades of polyethylene, especially LLDPE, with enhanced performance characteristics.  New processes, such as the use of metallocene catalysts, are improving properties of linear polyethylene and polypropylene through precise control of molecular weight and composition.

Non polyethylene resins constitute the remainder of plastic film types found in the marketplace.  Polypropylene (PP), polyvinyl chloride (PVC), and nylon resins comprise the bulk of these other film types.  Increasingly, certain multi-ply or co-extruded films are used in specialty applications (specialty bags, some bubble wrap, etc.) that seek to combine performance attributes of two or more resins for a specific application.

Tips for Identifying Film and Resin Types

Techniques for identifying film resin types include evaluating clarity, stretch and strength properties, feel and flexibility, and even burning characteristics. Testing burning characteristics should be done outdoors with extreme caution, using a small sample, burning only one corner with a lighter or match. Each resin and its identifying characteristics are described below:

Low Density Polyethylene (LDPE)

LDPE was the first commercial polyethylene and remains widely used in the production of custom bags for durable goods and other products.  LDPE is increasingly yielding market share to LLDPE, an enhanced variation of the same resin, and specialty blends.

Common LDPE Film Identifiers:

  • Unpigmented films have high clarity
  • Burn test: smells like a candle
  • Will strand when pulled in molten state
  • Moderate stretch & strength characteristics

In addition to film applications, LDPE is found in film-related products, including packing foams and bubble wrap materials.

Medium Density Polyethylene (MDPE)

This is a variation on the production of low-density polyethylene, using a similar process. This resin is commonly used as a low-cost alternative to other resins in film applications where strength is not required.

Common MDPE Film Identifiers:

  • Unpigmented films have moderate clarity
  • Burn test: smells like a candle
  • Will strand when pulled in molten state
  • Poor stretch & strength characteristics

Linear Low-Density Polyethylene (LLDPE)

Linear low-density polyethylene was developed in 1978. It is an attractive alternative to LDPE because its production process is less costly than high-pressure processes used to produce standard low-density resins. Its improved stretch and strength characteristics, relative to LDPE, have led to increased use in things like stretch wrap and bags.

Common LLDPE Film Identifiers:

  • Unpigmented films have moderate clarity
  • Slightly tacky feel to the touch
  • Burn test: smells like a candle
  • Will strand when pulled in molten state
  • Very good stretch & strength characteristics

High Density Polyethylene (HDPE)

HDPE, a common resin used in the production of a variety of rigid container plastics, plays an important role in certain film applications. Produced predominantly by low-pressure processes, it is relatively cost-competitive with LLDPE, and offers improved strength, making it the preferred choice for grocery and merchandise sack manufacturing.

Common HDPE Film Identifiers:

  • Unpigmented films have some opacity
  • Crinkles to the touch
  • Burn test: smells like a candle
  • Will not strand when pulled in molten state
  • Moderate stretch - high strength Unpigmented LLDPE (Stretch Wrap)

Polypropylene (PP)

Polypropylene has the lowest density of any common thermoplastic but has good strength properties.  Commonly used in heavy gauge woven bags or tarps.

Common PP Film Identifiers:

  • Common resin in heavy gauge woven bags or tarps
  • Burn test: distinct sweet wood odor

A type of PP used increasingly in film production is oriented polypropylene (OPP). OPP is produced by stretching the film while it is hot, to improve strength by orienting the molecules. OPP can be used to produce low-cost films with good clarity.  These films are difficult to discern from PE films, except by texture and burn testing.

Common OPP Film Identifiers:

  • Unpigmented films have high clarity
  • Common resin in garment bags
  • Burn test: distinct rosewood odor
  • Fairly stiff, crinkles to touch, tends to return to original shape.

Co-extruded Recyclable Films

There are a variety of niche film products where two or more of the resins described above are combined to yield a film product with enhanced properties for a specific application.  Most often, this involves the combination of LDPE and HDPE.  In such applications, HDPE contributes strength, while LDPE provides a smooth, flexible surface with high printability.

Common applications include:

  • Woven Lumber Wraps — woven HDPE tapes, laminated with LDPE film: woven texture is apparent, but surface is smooth; often different color on opposite sides.
  • Mailing pouches, and bank bags — HDPE and LDPE layers laminated: typically different color on opposite sides, relatively stiff feel.

Identifying Limited and Non-Recyclable Films

Cross-linked Polyethylene

Cross-linked polyethylene films are films which have been altered on a molecular level to significantly improve their strength. The e-beam or chemical processes required to create cross-linked films are expensive, thereby limiting the number of applications where such films are found.  Unfortunately, cross-linked films are also non-recyclable because they cannot be re-melted. These can sometimes be replaced by other polyethylene films with strength-enhancing additives.

Common Cross-link Film Identifiers:

  • No stretch - very high strength - difficult to tear.
  • Crinkles to touch
  • Yellowish hue when crumpled.

Polyvinyl Chloride (PVC) / Polyvinylidene (PVDC)

Vinyl films are common in food contact applications, especially frozen foods, because of their low oxygen permeability and strong cold temperature properties.

Vinyl is sometimes laminated to lower cost LDPE films to achieve combined attributes. Vinyl films are recyclable, but due to contamination problems associated with common uses and its specialized recycling lines required, recycling is not cost-effective at this time. A common PVDC wrap is the Saran® brand food wrap.

 Nylon Web and Cast Nylon Films

Nylon films are also somewhat common in food contact applications requiring stronger packaging, such as soup pouches or seafood packaging. Nylon films may also be found laminated to LDPE films, and also offer low oxygen and odor permeability and strong cold temperature properties.

Common Nylon Film Identifiers:

  • Check with mfr. on all food contact films
  • Typically thick, high-strength films

Cast Polystyrene (PS) Films

These films are found in a small number of niche applications, and represent a very small fraction of the overall film stream, and are only recyclable in specialty programs. They are used in applications such as carrier sheets for other manufacturing materials. Because they are much crisper (less flexible) than most other plastic films, they may be classified as rigid sheet rather than films, and readily identifiable.

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