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pvc products
2011-07-11 10:58:25
Preparation
The repeating unit of polyvinyl chloride
Space-filling model of a part of a PVC chain
Polyvinyl chloride is produced by polymerization of the vinyl chloride monomer (VCM), as shown.[5] About 57% of its mass is chlorine.
By far the most widely used production process is suspension polymerization. In this process, VCM and water are introduced into the polymerization reactor and a polymerization initiator, along with other chemical additives, are added to initiate the polymerization reaction. The contents of the reaction vessel are continually mixed to maintain the suspension and ensure a uniform particle size of the PVC resin. The reaction is exothermic, and thus requires a cooling mechanism to maintain the reactor contents at the appropriate temperature. As the volumes also contract during the reaction (PVC is denser than VCM), water is continually added to the mixture to maintain the suspension.
Once the reaction has run its course, the resulting PVC slurry is degassed and stripped to remove excess VCM (which is recycled) then passed though a centrifuge to remove water. The slurry is further dried in a hot air bed, and the resulting powder sieved before storage or pelletization. Normally, the resulting PVC has a VCM content of less than 1 part per million.
Other production processes, such as micro-suspension polymerization and emulsion polymerization, produce PVC with smaller particle sizes (10 μm vs. 120-150 μm for suspension PVC) with slightly different properties and with somewhat different sets of applications.
The product of the polymerization process is unmodified PVC. Before PVC can be made into finished products, it almost always requires conversion into a compound by the incorporation of additives such as heat stabilizers, UV stabilizers, lubricants, plasticizers, processing aids, impact modifiers, thermal modifiers, fillers, flame retardants, biocides, blowing agents and smoke suppressors, and, optionally pigments.[6]
[edit] Properties
The properties for PVC are usually categorized based on rigid and flexible PVCs.
| Property | Rigid PVC | Flexible PVC |
|---|---|---|
| 1.3–1.45 | 1.1–1.35 | |
| Thermal conductivity [W/(m·K)][8] | 0.14–0.28 | 0.14–0.17 |
| Yield strength [MPa][7] | 31–60 | 10–25 |
| Young's modulus [psi] | 490,000[9] | |
| Flexural strength (yield) [psi] | 10,500[9] | |
| Compression strength [psi] | 9500[9] | |
| Coefficient of thermal expansion (linear) [mm/(mm °C)] | 5×10−5[9] | |
| Vicat B [°C][8] | 65–100 | Not recommended |
| Resistivity [Ω m][10][11] | 1016 | 1012–1015 |
| Surface resistivity [Ω][10][11] | 1013–1014 | 1011–1012 |
[edit] History
PVC was accidentally discovered at least twice in the 19th century, first in 1835 by Henri Victor Regnault and in 1872 by Eugen Baumann. On both occasions the polymer appeared as a white solid inside flasks of vinyl chloride that had been left exposed to sunlight. In the early 20th century the Russian chemist Ivan Ostromislensky and Fritz Klatte of the German chemical company Griesheim-Elektron both attempted to use PVC (polyvinyl chloride) in commercial products, but difficulties in processing the rigid, sometimes brittle polymer blocked their efforts. Waldo Semon and the B.F. Goodrich Company developed a method in 1926 to plasticize PVC by blending it with various additives. The result was a more flexible and more easily processed material that soon achieved widespread commercial use.
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