Vinyl Chemistry and Basic Compounding
PVC is the common abbreviation for polyvinylchloride, one member of a large class of polymers, called vinyl. Most versatile of the thermoplastics, vinyl polymers are also among the oldest. They - when suitably compounded - range in form from soft and flexible to hard and rigid, either of which may be solid or cellular.
Polyvinyl chloride polymer is, of course, produced from vinyl chloride monomer. The classical method of VCl manufacture is from the reaction of H Cl and acetylene:
H Cl + C2H2 CH2CHCl
Acetylene vinyl chloride monomer
This is a somewhat inefficient and expensive process. The method presently used involves the oxychlorination of ethylene to make ethylene dechloride which is subsequently cracked to vinyl chloride.
2 H Cl + ½ O2 + Cl2 + 2 C2H4 2 C2H4Cl2 + H2O
(ethylene) (ethylene dichloride)
C2H4Cl2 CH2 CHCl + HCl
The derivations and reactions involved are shown schematically below:
Vinyl chloride monomer, then, is the basic repeating unit of a polyvinyl chloride chain. This monomer is an easily liquefiable gas with a pleasant odor (B.P. - 20° C). The polymerization of this material is then carried out to produce high molecular weight polymer.
Polymerization processes available are as follows:
- Suspension: Monomer is dispersed in water to form a suspension where the reaction occurs.
- Particle shape is like popcorn and particle size of the order of 50 micrometers.
- Emulsion: Monomer is emulsified in water. Particle sizes are usually less than 1 micrometer.
- Chain lengths and hence molecular weight can be controlled by polymerization temperatures.
RESIN TYPES AND CHARACTERISTICS
PVC resins can be classified as either general purpose or dispersion. General purpose resins are usually produced by suspension polymerization and the calendering resins used at C.G.T. fall into this category. Dispersion resins which are used in plastisols and organosols are produced primarily by emulsion polymerization where the fine particles are obtained. (A)Characteristics of general-purpose suspension PVC:
The most important is molecular weight due to its great effect on processing and end product properties. Further, processing may also be affected by molecular weight distribution and by the degree of branching.
Particle size and particle-size-distribution will affect compounding, processing and bulk handling. Large, fairly uniform particles are easier to handle and process. Fine particles will absorb plasticizer less evenly during dry blending. For most commercial suspension resins particles range from 50 - 150 micrometers.
Gels are large resin particles that failed to fuse completely during processing and appear as small spots or lumps on finished film.
There are inherent variations in heat stability amongst vinyl resins. These are attributed to differences in initiators, residual catalysts and impurities.
(B) Characteristics of Emulsion Resins or Dispersion Resins:
These resins fuse most rapidly because of their fine particle structure. Particle sizes range from 0.5 to 2.0 micrometers. Particle size and particle-size distribution affect the viscosity and stability of plastisols. Complete fusion of dispersion resins is generally considered to indicate complete solvation which is the solution formation of a resin by a solvent or plasticizer.
It should be noted that PVC resins, of themselves, are of no practical use. When fused they are hard, brittle compounds. Their inherent limited heat stability make any type of processing difficult if not impossible. Therefore, in order to produce a useful product other ingredients are added to the PVC resin for the purpose of:
- increasing flexibility
- providing adequate heat stability
- improving processability
- imparting aesthetic appeal
Let's consider these ingredients in some detail:
Plasticizers are low boiling liquids or low molecular weight solids that are added to resins to alter processing and physical properties. They increase resin flexibility, softness and elongation. They increase low temperature flexibility but decrease hardness. They also reduce processing, temperatures and melt viscosity in the case of calendering.
Plasticizers fall into two categories based on their solvating power and compatibility with resins:
A. Primary Plasticizers: are able to solvate resins and retain compatibility on aging. Samples of these would be:
- DOP Dioctyl phthalate
- S-711 Di (n-hexyl; n-octyl; n-decyl) phthalate (linear)
- DIDP Di-iso decyl phthate
B. Secondary Plasticizers: are so defined because of their limited solubility and compatibility and are, therefore, used only in conjunction with primary plasticizers. The ratio of primary to secondary depends on the type and quantity of the particular plasticizers.
Secondary plasticizers are used to impart special properties such as:
- low temperature flexibility: DMODA (di-normal octyl decyl adipate)
DOZ (di-octyl azelate)
DOA (di-octyl adipate)
- flame retardance: Reofas 65 (tri-iso propyl phenyl phosphate)
- electrical properties: tri-mellitates
- cost reduction: Cereclor, chlorinated paraffins
In a separate category are the polymeric plasticizers. These are long chain molecules and are made from adipic, azelaic, sebacic acids and propylene and butylene glycols. The efficiency of polymerics is poor but volatility and migration are superior. An example of a polymeric plasticizer is Paraplex G-54.
The characteristics sought in plasticizers can be summarized as follows:
- efficiency - This is the level or concentration needed to give a stated hardness, flexibility or modulus.
- the effect on low temperature flexibility.
- solvating power: This influences the fluxing rate of the compound at a given temperature or at a minimum fluxing temperature.
The fluxing rate relates directly to processing time.
Permanence: This relates to volatility, extraction resistance, compatibility.
2. HEAT STABILIZERS:
The chief purpose of a heat stabilizer is to prevent discoloration during processing of the resin compound. Degradation begins with the evolution of Hydrogen Chloride, at about 200° F Increasing sharply with time and temperature. Color changes parallel the amount of degradation running from white to yellow to brown to black. Therefore, the need for heat stabilizers.
The most effective stabilizers have been found to be:
- Metal soaps: Barium -cadmium solids and liquids : Mark 725, Mark 311
- Organo tin compounds: octyl tin mercaptide: Mark OTM
- Epoxies: epoxidized soya oil (G-62)
The above are most likely most effective only when used in combination (synergism).
What are some of the criteria in choosing a stabilizer system?
- The ability to prevent discoloration.
- The amount of lubrication involved. In calandering this can be of
- critical importance. Mark 725 has low lubricating effect while Mark 311 contributes high lubrication effect.
- Plate-Out - a potential side-affect of processing and has been linked to certain barium-cadium stabilizers.
- Compatability with the resin systems - for obvious reasons.
- Resistance to sulpher staining: atmospheric discoloration.
Essentially fillers are added to formulations to reduce costs, although they may offer other advantages - such as opacity, resistance to blocking, reduced plate-out, improved dry blending. On the other side, fillers can reduce tensile and tear strength, reduce elongation, cause stress whitening, reduce low temperature performance.
The most common fillers used with PVC are calcined clays, and water-ground and precipitated calcium carbonates of particle size around 3 micrometers. Other fillers are silicas and talcs.
Examples of fillers used at C.G.T. are:
- water ground calcium carbonate :
Hi-Fine # 80
These materials are of prime importance in PVC processing. They:
- Improve the internal flow characteristics of the compound.
- Reduce the tendency for the compound to stick to the process machinery.
- Improve the surface smoothness of the finished product.
- Improve heat stability by lowering internal and/or external friction.
Examples of lubricants, with which you may be familiar, are stearic acid, calcium stearate, Wax E, polyethylene AC 617
5. PROCESSING AIDS:
These may be regarded as low-melt viscosity, compatable solid plasticizers. They are added to lower processing temperature, improve roll release on calendars, reduce plate-out, promote fusion.
They are usually added at concentrations of 5.0%. The most widely used processing aids are acrylic resins of which acryloid K 120N is an example.
7. OTHER ADDITIVES
There are several other additives which we will list and comment on briefly:
- Impact Modifiers: These are used in rigid vinyls to improve impact resistance. These are usually acrylic or ABS polymers used at 10 - 15 phr levels. Examples are: Kureha BTA 111, Blendex 301.
- Light Stabilizers: for resistance to ultraviolet radiation. They are used in low concentrations 0.5 - 1.5 phr. An example is Tinuvin P which is produced by Ciba-Geigy.
- Flame Retardants: PVC is inherently self-extinguishing. However, the plasticizers and additives are not. Therefore, flame retardants are added. The most widely known one is antimony tri-oxide.
- Anti-Static Agents
- Fungicides: Vinyzene BP-5
- Foaming Agents:Chemicals that decompose at predetermined temperatures to produce a certain volume of gas within the molten vinyl and thereby create a foam.
- Colorants: Both pigments and dyes can be used. However, dyes, which are soluble organic substances, are used sparingly due to their tendency toward migration and extract ability. Heat resistance of colorants must be carefully evaluate.
In summary, we have seen that a vinyl compound consists of the following components:
- PVC resin - plasticizer - heat stabilizer - lubricant - special additive - colorants.
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