What Is Polyvinylidene Chloride? Composition, Features & Applications
Polyvinylidene chloride (PVDC) is defined as a synthetic resin, which is produced by the polymerization of vinylidene chloride. It can be used principally inflexible, clear, and impermeable plastic food wrap.
Properties of Polyvinylidene Chloride
Vinylidene chloride (with the bonding CH2=CCl2) is a colourless, clear, and toxic liquid. It is obtained from trichloroethane (CH2=CHCl3) via dehydrochlorination (the removal of hydrogen chloride [HCl]) of that compound by alkali treatment. In water, the liquid can be suspended either as fine droplets for PVDC processing, or else, it is treated with soaplike surfactants and also dispersed as an emulsion of smaller particles in water.
The vinylidene-chloride monomers (small and single-unit molecules) can be joined together to form large and multiple-unit polymers using the free-radical initiator action. The polymer is obtained from the water phase either as beads or dry powder, which may be melted for extrusion into the plastic film.
The outstanding property of PVDC is given as its low permeability to the water vapour and gases - making it ideal in food packaging. Copolymers of vinylidene chloride, including the other monomers, are also marketed. The well-known is Saran, which is a copolymer consisting of around 87% vinylidene chloride and 13% vinyl chloride. The Dow Chemical Company first introduced Saran in 1939, and it is still a common transparent food wrap today.
Polyvinylidene Chloride Structure
Let us look at the polyvinylidene chloride structure below.
The chemical structure of Polyvinylidene chloride can be represented as follows:
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Fiber Types
Saran fibre is available in monofilament, multifilament-twist, and staple types. Thermochromic (colour changing) and luminescent (glow in the dark) fibres are also available.
Characteristics
Let us look at a few of the characteristics of Polyvinylidene chloride:
Compatibility With Additives
The chemical structure of PVC's Chlorine polar groups, as well as its amorphous nature, allows it to easily mix with a variety of substances. Several qualities can be imbued in products based on the additives used in PVC manufacturing, including anti-mist, different colours, fire inhibiting, elasticity, impact resistance, durability, and microbe prevention.
Durability
The factor which most strongly affects the product’s durability under conditions of typical use is the resistance to oxidation by atmospheric oxygen. PVC has excellent longevity due to its high resistance to oxidative reactions. In a test performed by the Japan PVC Pipe and Fittings Association, for example, 35-year-old underground pipes showed no signs of deterioration. This specific durability applies even through the process of recycling because the re-converted product’s physical properties are virtually similar to those made from virgin PVC resin.
Fire Resistance
Fire resistance is one of the major qualities of PVC that make it popular in several industries like building products. Also, PVC is a thermoplastic, which is made of 57% chlorine derived from common salt, and when ignited, its chlorine content will extinguish the flames. PVC has a high ignition temperature of 455°C. Because the heat released by the PVC when ignited is much lower compared to the temperatures released by other plastics such as PP and PE, it is less likely to spread the fire to other materials that increase the desirability of building products.
Electrical Insulation
PVC holds a good dielectric strength. It means it can withstand a considerable amount of electric field strength without breaking down its insulation properties. When combined with the fire-retardant properties of PVC, this dialectic strength makes it ideal for usage in insulation tape, communication cables, switch boxes, residential electrical cables, and wire covering.
Oil and Chemical Resistance
Although PVC swells or dissolves in aromatic hydrocarbons, cyclic, and ketone ethers, it is difficult, not easy, to dissolve in other organic chemicals. Also, it is almost resistant to all the inorganic chemicals. This makes it more ideal for the usage in gas exhaust pipes or tubes and ducts of all kinds, including medical applications.
Forms
PVC is available in two basic forms: Rigid (at the time, abbreviated as PVC) and versatile. Polyvinyl Chloride (Vinyl or PVC) is given as a versatile and cheap thermoplastic polymer, which is widely utilized in the housing and building industry to supply window and door profiles, pipes (waste-water and drinking water), cable and wire insulation, medical devices, and more.
Polyvinylidene Chloride Uses
Various polyvinylidene chloride uses are given below.
Packaging
Polyvinylidene chloride can be applied as a water-based coating to the other plastic films such as polyethylene terephthalate (PET) and biaxially-oriented polypropylene (BOPP). This coating will increase the film’s barrier properties, reducing the permeability of the film to the oxygen and flavours and hence extending the shelf life of the food, which is inside the package. It may also impart a high-gloss finish, which can be aesthetically pleasing and also provides a higher degree of scuff resistance if it is applied overprint.
Household
Polyvinylidene chloride can be used for household purposes such as cleaning cloths, screens, filters, shower curtains, tape, and garden furniture.
Industry
Polyvinylidene chloride is useful for industrial uses such as artificial turf, screens, underground materials, and waste-water treatment materials.
Miscellaneous
PVC is also much useful for various uses such as stuffed animals, doll hair, fishnet, fabrics, pyrotechnics, shoe insoles.
FAQs on Polyvinylidene Chloride: Properties, Structure, and Uses
1. What exactly is Polyvinylidene Chloride (PVDC)?
Polyvinylidene Chloride, commonly known as PVDC, is a synthetic polymer produced from the monomer vinylidene chloride. It is highly regarded for its exceptional barrier properties, making it resistant to oxygen, moisture, and odours, which is why it's a popular choice in packaging.
2. What are the most common real-world uses of PVDC?
PVDC is primarily used in applications where protection from environmental elements is crucial. Some common examples include:
- Food Packaging: It is famous for its use in household cling wrap (like Saran wrap) and as a coating on other plastic films to keep food fresh.
- Pharmaceuticals: Used in blister packaging to protect medicines from moisture and air.
- Coated Papers: Provides a barrier layer on paper products to make them resistant to grease and water.
- Fibres and Filaments: Used in making flame-resistant textiles, filters, and even doll hair.
3. How does PVDC compare to the more common PVC plastic?
While both are chlorine-containing polymers, PVDC and PVC have key differences. The main distinction is that PVDC offers a significantly better barrier against oxygen and water vapour than PVC. This makes PVDC ideal for preserving food freshness. PVC, on the other hand, is generally cheaper, more rigid, and more versatile, making it a staple in construction for pipes and window frames.
4. What is the basic chemical structure of Polyvinylidene Chloride?
The structure of PVDC comes from the polymerisation of vinylidene chloride monomers (CH₂=CCl₂). During this process, the double bond in each monomer breaks, and they link together to form a long chain. The repeating unit in the polymer is (-CH₂-CCl₂-)n. The presence of two chlorine atoms on the same carbon atom is key to its properties.
5. Why is PVDC so effective as a barrier in food packaging?
The effectiveness of PVDC as a barrier comes from its molecular structure. The high concentration of heavy chlorine atoms causes the polymer chains to pack together very tightly and in an orderly, crystalline fashion. This dense, crystalline structure leaves very little empty space for gas molecules like oxygen or water vapour to pass through, effectively sealing the food from the outside environment and preventing spoilage.
6. Are there any significant disadvantages to using PVDC?
Yes, a primary disadvantage of PVDC is its thermal instability. When heated to processing temperatures, it can undergo thermally induced dehydrochlorination, which means it starts to release hydrogen chloride gas. This degradation can cause the material to change colour from clear to an undesirable brown, leading to product loss during manufacturing.
7. How is Polyvinylidene Chloride (PVDC) different from Polyvinylidene Fluoride (PVDF)?
The main difference lies in the halogen atom attached to the polymer chain. PVDC contains chlorine atoms, while PVDF contains fluorine atoms. This change significantly alters their properties. PVDC is known for its superior barrier qualities, especially for food packaging. PVDF, on the other hand, is valued for its high thermal stability, chemical inertness, and piezoelectric properties, making it useful in high-performance electronics, chemical pipes, and architectural coatings.
