What is the Difference Between Polyvinyl Chloride (PVC) and Polyvinylidene Chloride (PVDC)?
The main differences between polyvinyl chloride (PVC) and polyvinylidene chloride (PVDC) are as follows:
1. Chemical Structure
PVC: Polyvinyl chloride is polymerized from vinyl chloride monomers. Each repeating unit in the molecular chain contains one chlorine atom, resulting in a linear, amorphous polymer.
PVDC: Polyvinylidene chloride is polymerized from vinylidene chloride monomers. Each repeating unit contains two chlorine atoms. It exhibits higher molecular chain symmetry and a crystallinity of 50%-60%.
2. Physical Properties
Heat Resistance: PVC softens at approximately 80℃, with a long-term operating temperature not exceeding 60℃. It easily decomposes at high temperatures, producing HCl gas.
PVDC has a melting point of 198-205℃ and can withstand temperatures up to 100℃. Some copolymers can be used for short periods at 140℃, making it suitable for microwave heating.
Mechanical Properties: PVC has higher hardness but is more brittle, requiring the addition of plasticizers to improve flexibility.
PVDC has higher crystallinity, resulting in superior tensile strength and toughness compared to PVC, and also exhibits stronger oil resistance and chemical corrosion resistance.
3. Barrier Properties
PVC has an oxygen permeability of approximately 15-30 cm³·mm/(m²·24h·MPa), exhibiting moderate barrier properties.
PVDC has an oxygen permeability as low as 1.2 cm³·mm/(m²·24h·MPa), which is 1/10 to 1/20 that of PVC, and possesses excellent moisture barrier properties, earning it the title of "golden barrier."
4. Processing Difficulty
PVC has a wide processing temperature range and can be molded through extrusion, injection molding, calendering, etc., resulting in lower costs.
PVDC homopolymers, due to their close melting and decomposition temperatures, require copolymerization (e.g., with vinyl chloride or acrylonitrile) to reduce processing difficulty, leading to complex processes and high equipment requirements.
5. Environmental Friendliness and Safety
PVC production discharges chlorinated wastewater, and incineration releases highly toxic substances such as dioxins. The EU has restricted its use in food packaging. Recycling is difficult, requiring separation from other plastics.
PVDC incineration produces a relatively small amount of HCl, but specialized treatment equipment is still required. Some copolymers can be recycled using enzymatic hydrolysis, meet FDA certification, and are safe for direct food contact.
6. Application Areas
PVC is widely used in the construction industry (e.g., pipes, window and door profiles), industrial applications (e.g., wire and cable insulation), and daily necessities (e.g., raincoats, cling film).
PVDC is mainly used in food packaging (e.g., sausage casings, cheese packaging), pharmaceutical packaging (e.g., blister packs), and specialized fields (e.g., military moisture-proof packaging) where high barrier properties are required.
7. Cost
PVC has abundant raw materials, mature production processes, and low prices, making it the world's third most widely used plastic.
PVDC, due to its complex copolymerization process and high monomer costs, is approximately 3-5 times more expensive than PVC and is mainly used in the high-end barrier plastic market.
