Flexible PVC is primarily made pliable by the addition of substances called plasticizers during its manufacturing process.
Understanding PVC's Flexibility
Polyvinyl Chloride (PVC) is a versatile plastic available in several forms. As mentioned in the reference, it is widely available in two broad categories: Rigid and Flexible. While the basic polymer chain of PVC is inherently rigid due to strong intermolecular forces, its flexibility is achieved through specific modifications.
The reference states, "Flexible PVC is formed by the addition of compatible plasticizers to PVC which lower the crystallinity." This is the key process that transforms rigid PVC into its flexible counterpart.
The Role of Plasticizers
Plasticizers are typically low-molecular-weight organic compounds that are added to a polymer to increase its flexibility, decrease its hardness, and reduce its viscosity. When added to PVC:
- They insert themselves between the polymer chains.
- They weaken the intermolecular forces (like Van der Waals forces) holding the chains together.
- This allows the polymer chains to move more freely relative to each other.
How Plasticizers Affect Crystallinity
Polymers can have crystalline regions, where the polymer chains are highly ordered, and amorphous regions, where they are disordered. Rigid PVC has a degree of inherent crystallinity which contributes to its stiffness. Plasticizers disrupt these ordered (crystalline) regions.
By lowering the crystallinity and increasing the space between polymer chains, plasticizers reduce the material's brittleness and glass transition temperature (Tg), making it softer, more elastic, and significantly more flexible.
Common Plasticizers
A common type of plasticizer used for PVC are phthalates, although there is a trend towards using non-phthalate alternatives due to health and environmental concerns. Other types include adipates, trimellitates, and epoxidized vegetable oils.
Flexible vs. Rigid PVC
Here's a quick comparison based on the modification and properties:
| Feature | Rigid PVC | Flexible PVC |
|---|---|---|
| Key Additive | None (mainly PVC polymer + stabilizers) | Plasticizers (e.g., phthalates, adipates) |
| Crystallinity | Higher relative crystallinity | Lower crystallinity |
| Flexibility | Very rigid | Flexible, pliable, elastic |
| Density | Higher (e.g., ~1.4 g/cm³) | Lower (as per reference: 1.1-1.35 g/cm³) |
| Typical Uses | Pipes, window frames, flooring | Cables, hoses, synthetic leather, medical tubing |
Note: Density values can vary based on specific formulation.
Practical Applications
The addition of plasticizers makes flexible PVC suitable for a wide range of applications where rigidity would be impractical. Examples include:
- Electrical Cables: The flexible insulation around wires.
- Hoses: Garden hoses, industrial hoses.
- Medical Devices: IV bags, tubing, catheters.
- Footwear: Soles and uppers.
- Flooring: Resilient sheet flooring.
- Toys: Many common plastic toys.
- Textiles: Synthetic leather, coated fabrics.
In conclusion, the transformation of rigid PVC into its flexible form is a direct result of incorporating plasticizers, which effectively increase the mobility of polymer chains by reducing the material's crystallinity.
