Polyvinylidene chloride (PVDC)

Description

Polyvinylidene Chloride (PVDC): The Unsung Hero and Environmental Quandary of Barrier Packaging

In the vast world of polymers, few materials can boast the unparalleled barrier properties of Polyvinylidene Chloride, more commonly known as PVDC. For decades, this unique plastic was the gold standard for packaging that demanded exceptional protection against gases, moisture, and odors, playing a pivotal role in extending the shelf life of food, pharmaceuticals, and sensitive industrial products. Yet, its story is also one of evolving environmental consciousness, leading to its gradual decline in many mainstream applications.

What is PVDC?

Polyvinylidene Chloride (PVDC) is a synthetic thermoplastic polymer derived from the polymerization of vinylidene chloride (VDC) monomers. While often confused with Polyvinyl Chloride (PVC) due to the similar “vinyl chloride” designation, PVDC is a distinct polymer with very different properties. It is often created as a copolymer with other monomers, most commonly vinyl chloride (creating PVDC-PVC copolymers), to enhance its processability and flexibility while maintaining its core barrier characteristics.

Perhaps its most famous incarnation was Saran Wrap, introduced by Dow Chemical in the 1940s. Its remarkable cling and freshness-preserving capabilities made it a staple in kitchens worldwide, solidifying PVDC’s reputation as a premium packaging material.

Unrivaled Barrier Properties

The defining characteristic of PVDC is its exceptional barrier performance. This property stems from its highly crystalline and dense molecular structure, which effectively creates a formidable wall against the permeation of:

1. Oxygen: Crucial for preventing oxidation, spoilage, and maintaining the freshness of oxygen-sensitive foods like meats, cheeses, and coffee.
2. Moisture/Water Vapor: Prevents dehydration of products and protects moisture-sensitive items from humidity.
3. Aromas and Odors: Inhibits the transfer of strong odors in or out of packaging, keeping products fresh and preventing cross-contamination of smells.

Beyond its barrier capabilities, PVDC also offers:

• Chemical Resistance: Good resistance to many chemicals, oils, and greases.
• Transparency: Can be produced as a clear film, allowing product visibility.
• Good Adhesion: Often used as a coating or an inner layer in multi-layer films due to its ability to bond well with other polymers.
• Shrinkability: Certain formulations can be used as shrink films for tight, protective packaging.

Key Applications

Due to its superior properties, PVDC found widespread use across various industries:

• Food Packaging:
  • Meat and Poultry: Vacuum packaging to extend shelf life and prevent freezer burn.
  • Cheese: Prevents drying out and maintains flavor.
  • Baked Goods: Keeps items fresh and prevents staleness.
  • Coffee: Protects against oxygen degradation, preserving aroma and flavor.
  • Snacks and Confectionery: Maintains crispness and prevents moisture absorption.
  • Aseptic Packaging: Used as an ultra-barrier layer in multi-layer cartons (e.g., Tetra Pak) for long-shelf-life beverages.
• Medical and Pharmaceutical Packaging:
  • Blister Packs: Provides a strong barrier for pills and medical devices, protecting against moisture and gases.
  • Sterile Packaging: Ensures product integrity during storage.
• Industrial Applications:
  • Moisture barriers in construction materials.
  • Corrosion protection for metals.
  • Coatings for fishing lures (Saran fishing lures were once popular).

The Environmental Imperative: Challenges and Decline

Despite its technical prowess, PVDC faces significant environmental challenges that have led to its decreased use, particularly in consumer-facing products:

1. Chlorine Content: Like PVC, PVDC contains chlorine. While generally stable, concerns arise during incineration, where the high temperatures can potentially release harmful byproducts such as dioxins and furans.
2. Recycling Difficulties: PVDC is rarely recycled in practice. When used in multi-layer packaging (which is most common), separating it from other plastics is extremely difficult, rendering these composites generally unrecyclable through conventional methods.
3. Persistence in Landfills: As a highly stable polymer, PVDC does not biodegrade, meaning it persists in landfills for very long periods.

In response to these environmental pressures and changing consumer preferences, many manufacturers have shifted away from PVDC, especially in single-use consumer products. For instance, Dow Chemical’s Saran Wrap brand in North America transitioned from PVDC to polyethylene (PE) in the early 2000s, a testament to this industry-wide shift. However, PVDC is still used in specific high-performance applications where its unique barrier properties are indispensable and no suitable, cost-effective alternative exists.

The Search for Alternatives

The decline of PVDC has spurred the development and adoption of alternative barrier materials, each with its own advantages and limitations:

• Ethylene Vinyl Alcohol (EVOH): Offers excellent oxygen barrier properties, especially under dry conditions. However, its barrier performance degrades significantly in the presence of moisture.
• Nylon (Polyamide – PA): Provides a good barrier to gases and aromas, and excellent puncture resistance.
• Polyethylene Terephthalate (PET): While not as strong a barrier as PVDC, specialized grades or coatings can offer improved barrier properties.
• Metallized Films and Aluminum Foil: Provide excellent, almost absolute, barriers to gases and moisture, but lack transparency and can be prone to pinholes.
• Silica (SiOx) or Alumina (AlOx) Coatings: Ultra-thin ceramic coatings applied to other plastic films, offering a transparent, high-performance barrier.

Conclusion

Polyvinylidene Chloride stands as a fascinating case study in material science and its intersection with environmental responsibility. For decades, its unparalleled barrier properties made it an indispensable material, extending the freshness and safety of countless products globally. While environmental concerns regarding its chlorine content and recyclability have led to its reduced prominence in everyday items, PVDC continues to serve critical, high-performance roles where its unique characteristics remain unmatched by alternatives. Its legacy is a reminder of the constant balance between technological innovation and sustainable practice in the pursuit of better materials.