Polyvinyl chloride (PVC) is a widely used polymer. The range of additives used with this polymer can alter its physical properties to create a tough rigid polymer used for water pipes through to a pliable material used for fabric applications.
Properties of Polyvinyl chloride
PVC is a thermoplastic polymer. Its properties are usually categorized based on rigid and flexible PVCs.
Properties
| Property | Unit of measurement | Rigid PVC | Flexible PVC |
|---|---|---|---|
| Density | g/cm3 | 1.3–1.45 | 1.1–1.35 |
| Thermal conductivity | W/(m·K) | 0.14–0.28 | 0.14–0.17 |
| Yield strength | psi | 4,500–8,700 | 31–60 |
| Yield strength | MPa | 1,450–3,600 | 10.0–24.8 |
| Young’s modulus | psi | 490,000 | – |
| Young’s modulus | GPa | 3.4 | – |
| Flexural strength (yield) | psi | 10,500 | – |
| Flexural strength (yield) | MPa | 72 | – |
| Compression strength | psi | 9,500 | – |
| Compression strength | MPa | 66 | – |
| Coefficient of thermal expansion (linear) | mm/(mm °C) | 5×10 ^?5 | – |
| °C | 65–100 | Not recommended | |
| Resistivity | Ω m | 10^16 | 10^12 ~ 10^15 |
| Surface resistivity | Ω | 10^13 ~ 10^14 | 10^11 ~ 10^12 |
Typical Applications
– water distribution piping
– home plumbing
– house siding
– business machine housings
– electronics packaging
– medical apparatus
– packaging for foodstuffs
PVC Injection Molding Processing Conditions
| Conditions Name | Value |
|---|---|
| Drying | Not usually necessary as PVC absorbs very little water. |
| Melt Temperature | 160°C–220°C [320°F–428°F] |
| Mold Temperature | 20°C–70°C [68°F–158°F] |
| Material Injection Pressure | Up to 150 MPa |
| Packing Pressure | Up to 100 MPa |
| Injection Speed | Relatively slow to avoid material degradation |
Runners and Gates for PVC injection molds
All conventional gate types can be used. Pin-point and circular tapered gates are used for molding small components and fan gates are typically used for thick sections. The minimum diameter of pin-point or circular tapered gates should be 1 mm and the thickness of fan gates should not be less than 1 mm.
Sprues should be as short as possible; typical runner sizes are 6–10 mm and should have a full round cross-section. Insulated hot runners and certain types of hot sprue bushings can be used with PVC.
Chemical and Physical Properties
Rigid (unplasticized) PVC, which is produced from sodium chloride and natural gas, is one of the most widely used plastic materials. The repeat chemical structure is vinyl chloride. Additives are mixed with PVC to make it processible. PVC grades produced by suspension or mass polymerization techniques are the major types used for melt processing. PVC is substantially an amorphous material.
The additives used include stabilizers, lubricants, processing aids, pigments, impact modifiers, and fillers. The features of PVC include low combustibility, toughness (designed to be virtually unbreakable), good weatherability (color and impact retention and no loss in stiffness), and excellent dimensional stability. PVC is highly resistant to oxidizing and reducing agents, and strong acids. However, unplasticized PVC is not recommended for environmental and continuous use above 60°C [140°F]. It is not resistant to concentrated oxidizing acids such as sulfuric or nitric acid, and is unsuitable for use with aromatic and chlorinated hydrocarbons.
It is very important to process the material at the correct melt temperature; otherwise, severe problems from decomposition (which produces hydrochloric acid which in turn accelerates decomposition) could result.
PVC is a relatively stiff flow material and has a narrow processing range. The molecular weight determines the flow characteristics. Materials with a higher molecular weight are more difficult to process. This can be modified by the addition of lubricants. Typically, however, relatively low molecular weight grades are used in molding.
Shrinkage is fairly low (0.002–0.006 mm/mm [0.2–0.6%]).
