Figure 2: Free Radical ProcessJason Ott
Curran 2
Materials Science
3-25-99
A Report on Polyvinyl Chloride (PVC)
History
Regnault discovered vinyl chloride in the early 1800's. PVC was first discovered in 1912 by Fritz Klatte, a German chemist. Having obtained some spare acetylene, he decided to react it with hydrochloric acid, on a whim. This produced vinyl chloride. Thinking he'd failed, he put it on a shelf, where it proceeded to polymerize into a polymer. When he found it, he had no idea what to do with it, but patented it anyway, in Germany. His company never did anything with it, failing to realize the potential it had for commercial use.
Ten years later, his patent expired. The next year, 1926, Waldo Semon, an American Chemist with B.F. Goodrich, discovered the same PVC, without any previous knowledge of Klatte's discovery. He showed it to his boss, who patented it in America. They thought it would make an excellent material for shower curtains. The waterproof nature of the material soon brought on more creative uses, and Goodrich made a fortune on the material. Klatte never saw a penny.
Uses
PVC is known for many uses. It is a thermoplastic, which is both water and fire resistant, making it excellent in many situations. Its most common uses include plumbing, linoleum, sports equipment, house siding, packaging material, and more. It is regarded as one of the best sterile plastics, and is used exclusively in some medical institutions of Europe. In the 70's, it was popular as the vinyl roofing for automobiles, and is currently used as interior paneling in vehicles. Its versatility has made it one of the highest produced plastics in the world.
Characteristics
PVC is impermeable, water and flame resistant, recyclable, durable, cheap, and easy to make. Many of these characteristics are due to its unique structure (Fig.1).
The substitution of hydrogen with chlorine gives PVC several special properties. For one, when burned, the chlorine atoms are released, inhibiting combustion, effectively rendering PVC resistant to fire. PVC is also resistant to alkalis, nonoxidising acids, hypochlorite solutions, and aliphatic hydrocarbons (PVC Properties).
PVC degrades when exposed to heat and light-- HCL is eliminated and causes oxidation.
Synthesis
PVC is created through free radical polymerization, which is also used for other polymers such as polystyrene, and polyvinyl acetate.
Figure 2: Free Radical Process
The free radical process is carried out using initiators, such as 2-2'-azo-bis-isobutyrylnitrile (AIBN). These molecules split in a unique fashion (Fig.2) which makes them very useful in the synthesis of polymers. They split in such a manner that two fragments, initiator fragments or free radicals, remain. Each free radical has one unpaired electron, such as the one attached to the C atom in figure 2. The nitrogen atoms break the bond with the carbon atoms and bond with each other, leaving the free electron. This electron will bond with most any unpaired electron.
In a vinyl chloride monomer, a double bond exists between the two carbon atoms (Fig.3). This bond is not strong, and when in the presence of free radicals, is easily broken. The resulting unpaired electrons are then paired with the AIBN free radicals.
These radicals only take one electron--there are two when the monomer is broken. This new compound--AIBN free radical and vinyl chloride monomer--in essence form another free radical, with the carbon atom's extra electron forming a bond with something else, usually another vinyl chloride monomer. This process, as is apparent, is a chain reaction, a self-perpetuating reaction that feeds upon itself.
The process can cause impurities. Due to the unpredictable nature of the process, bonds can be broken between the carbon and hydrogen atoms as well. This can lead to branching; an occurrence meaning the single strand polymer has developed a strand branching from what was formerly the carbon-hydrogen bond (Fig.4).
The process is completed when one of the detached hydrogen atoms attaches to the unpaired electron at the end of the PVC chain, or when the carbon atoms form a double bond through a process called disproportionation, which results in the free hydrogen atom.
Even with the unpredictable nature of free radical polymerization, and the possibility of branching and fragments of initiators, the process is by far the easiest and most economical method to date.
PVC can be made in two forms (Fig.5), depending on the formulation and the type/proportion of plasticizer. The forms are Rigid Polyvinyl Chloride (RPVC) and Chlorinated Polyvinyl Chloride (CPVC).
|
RPVC |
CPVC |
|
|
Structure |
Amorphous |
Amorphous |
|
Specific Density |
1.35 |
1.56 |
|
Water Absorption Rate |
0.20% |
0.15% |
|
Elongation |
20% |
40% |
|
Tensile Strength |
6500psi |
8700psi |
|
Compression Strength |
11000psi |
14500psi |
|
Flexural Strength |
12100psi |
15500psi |
|
Flexural Modulus |
400000psi |
435000psi |
|
Impact (IZOD ft. lbs/in) |
5 |
12 |
|
Hardness |
R105 |
R110 |
|
Utilization Temperature:min/max |
14/140 |
?/178 |
|
Melting Point(F) |
176 |
175 |
|
Coefficient of Expansion |
0.000045 |
0.00004 |
|
Specific Gravity |
1.39 |
1.1-1.7 |
|
Specific Heat |
0.25 |
.3-.5 |
|
Transparency |
Clear |
Clear |
|
UV Resistance |
Good |
Poor |
|
Chemical Resistance |
||
|
Acids |
Poor |
Excellent |
|
Alkalis |
Excellent |
Excellent |
|
Solvents |
Good |
Good |
|
Fabrication |
||
|
Bonding |
Good |
Good |
|
Ultrasonic Welding |
Good |
Poor |
|
Machining |
Good |
Poor |
Figure 5: Physical Properties of RPVC and CPVC
Bibliography