It should be noted that PVC resin, 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 ingredient
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.
1.
PLASTICIZERS
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 calenering.
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) phthlate
(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 t tri-mellitates
-
Cost reduction Cereclor, chlorinated parafins
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:
⑴ efficiency- This is the level or
concentration needed to give a stated hardness, flexibility or modulus.
⑵ the effect on low temperature flexibility.
⑶ solvating power: This influence the
fluxing rate of the compound at a given temperature or at a minimum fluxing
temperature.
The fluxing rate relates directly
to processing time.
Performance: 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 ℉ 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 stabilziers have been
found to be:
⑴ Metal soaps: Barium-cadmium solids and liquids:
Mark 725, Mark 311
⑵ Organo tin compounds: octyl tin mercaptide: Mark
OTM
⑶ Epoxies: epoxidized soya oil (G-62)
The above are most likely most effectively
only when used in combination (synergism).
What are some of the criteria in choosing a
stabilzier system?
⑴ The ability to prevent discoloration
⑵ 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.
⑶ Plate-Out- a potential side-affect of processing
and has been linked to certain barium-cadium stabilizers.
⑷ Compatability with the resin system- for obvious
reasons.
⑸ Resistance to sunlight staining: atmospheric
discoloration.
