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PVC - A Persistent,
Versatile Choice!
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Mr. Kamalnain Kurra
Director – Innovative Performance Plastics |
Polyvinyl Chloride also referred
to as Vinyl is an extensively used thermoplastic in
all aspects of our lives. “Our lifesaving medical
industry being no exception is thriving on the
benefits of PVC”. This is a much-known fact to
all the stakeholders of the field. This article on
PVC brings to highlight the various aspects such as
history, production, classification, developments,
compounding, regulations, and its application in the
healthcare industry. The objective of this article
is to convey a basic understanding of the process of
PVC compounding and its far-reaching pursuit in the
medical industry. |
History - The evolution of
PVC goes back to over a century when it was first
polymerized during the period 1838-1872. The commercial
production of PVC is estimated during the early 20th
century. In 1913, German inventor Friedrich Klatte
patented a polymerization process to manufacture PVC. In
1926, Waldo Semon invented plasticized PVC providing a
synthetic replacement for increasingly costly natural
rubber. Ever since then there has been no looking back
for PVC with its endless list of usage – be it
industrial, construction, household, packaging, and
healthcare. PVC is widely used in pipes & fittings,
films & sheets, wires & cables, flooring & profiles.
Production - Most of the
PVC in use today is manufactured by combining ethylene
and chlorine. The electrolysis of saltwater produces
chlorine, which is combined with ethylene (separated
from oil feedstock during thermal cracking) to form
vinyl chloride monomer (VCM). Molecules of VCM are
polymerized to form PVC resin, to which appropriate
additives are incorporated to make a customized PVC
compound. Since ethylene is a product of the oil & gas
industry, PVC is considered a petrochemical
product. PVC can also be produced from hydrocarbons such
as coal or plant derivatives such as sugar cane.
Carbon and hydrogen are the main elements of most
thermoplastic as they are entirely derived from oil.
What makes PVC unique is the presence of Chlorine. PVC
is made from 57% salt and 43% oil. The presence of
chlorine with hydrocarbons provides several technical
advantages to PVC and helps it to deliver unbeatable
value. It makes PVC compatible with a wide range of
materials, adds to its versatility, and makes it flame
retardant. Chlorine plays a significant role during the
recycling of plastics as its presence helps as a marker
to distinguish PVC from other materials.
Classification - PVC being
a thermoplastic offers exceptional clarity, is weldable,
bondable (with adhesive or chemical), and can be
extruded, molded, or calendared. Depending on the
polymerization technique used for manufacturing PVC
polymer, it can further be classified as mass or bulk
polymerization grade, suspension grade, emulsion grade,
or solution process grade. The suspension process
accounts for over 80% of the global production.
 Developments
- All along the previous
year's PVC recipes have gone through a lot of changes to
offer better performance and processibility, address
environmental issues, and comply with legislation. Some
developments include elimination, restricted use, or
modification of heavy metal stabilizers (such as lead,
tin, Zinc), non-use of specific plasticizers (for the
medical industry), wider choice options for
pigmentation, lubricants, and additives. Needless to
mention the process improvements and choices at each
step of compounding. The key steps during compound
manufacturing are blending, compounding, and pelletizing.
Various blending methods available are High-speed mixer,
ribbon blending, paddle mixers and drum tumblers.
Available methods for compounding include kneading,
single or twin screw (with co-rotating or
counter-rotating) extrusion, Farrel continuous Mixer and
Banbury. Pelletization is the final step during
compounding, it can be done using air (strand) or
underwater (die face) pelletization. Compounders for the
medical industry normally use a selection from
High-Speed mixing, ribbon blending, kneading, screw
extrusion, and die face pelletization.
Compounding - PVC in its
original form does not offer flexibility. The addition
of plasticizer &/or other minor components to make the
PVC amenable and suitable for flexible or rigid
applications is known as compounding. These minor
components are added to enhance the functionality of PVC
and the quality of the end product. These salt and
pepper ingredients are Stabilizers, Lubricants, Fillers,
Processing aids, Pigments, Impact Modifies, and other
additives. Depending on the end produce the formulation
mixture can be dry blended or melt compounded. By and
large the two major components for flexible PVC compound
is the choice of PVC resin (identified by its K value)
and Plasticizer. Resin & Plasticizer temperature,
blending speed, time and temperature, sequence of
addition of ingredients, screw design, L/D Ratio, the
temperature at various zones in the barrel are the other
key considerations during compounding.
 K-Value
- An important parameter that
influences the property of PVC compounds is the K-Value
of PVC resin. K-Value represents a function of the
average molecular weight, the degree of polymerization
and the intrinsic viscosity of the resin. As a thumb
rule, low K-value resins are used for rigid and higher
for extrusion. For high-performance and specialty
compounds (such as pump, kink-free and steam
sterilizable compounds) further higher K-value resins
are preferred. The increasing Kvalue
offers higher plasticizer absorbance during
compounding, better thermal stability and mechanical
properties of the finished product, and a decrease in
processability. |
