Page 129 - CW E-Magazine (7-11-2023)

Point of View

Change is coming in heat stabiliser options for PVC

processing

The processing of polymers into the myriad forms and shapes that find industrial and consumer use is aided by a plethora of
chemicals bandied together as polymer additives. These serve to ensure the desired rheological properties when a solid polymer is
melted before it is given the shape and form needed; aesthetics (colour, feel, etc.); resistance to the environment into which it is put
to use (heat, light, UV radiation, moisture, etc.); barrier properties (permeability to oxygen, water vapour); and mechanical properties
(rigidity, strength, etc.).

The chemistry of the additives vary and their choice is determined by several factors including the polymer being processed; the
performance needed; costs; and increasingly their safety to human & animal health and their fate in the environment at the end of life.

Processing PVC
Polyvinyl chloride (PVC) is one of the largest thermoplastics in use. It is a versatile polymer that can be extruded into pipes and
profiles for doors & windows; calendered into sheets; blown into film; or injection moulded into complex shapes. The applications to
which it can be put to can be broadly classified as rigid and flexible – reflecting the physical condition of the article. Notable in the
former are pipes and profiles, and in the latter, films and sheets. Though PVC is intrinsically a rigid polymer, it can be made flexible
by incorporating additives broadly known as plasticisers. Their usage levels (more than one, if needed) vary, as do their chemistries.
The most common are the phthalates, but some items belonging to this class have fallen out of favour as they have been proven to
be harmful to human health if released from the polymer matrix and ingested (for examples, in food contact applications and toys).

Consequent to these concerns, safer alternatives have been brought to market, and their adoption is partly driven by governmental
regulation that prohibit or restrict usage. In several instances, the choice is also determined by demands posed by customers – be
they industrial ones or consumer-facing FMCG companies. The latter, in particular, prefer to stay ahead of regulation and at times
make ingredient switches on mere suspicion of safety concerns.

Heat stabilisers for PVC
Heat stabilisers are an important category of PVC additives that do exactly what the name suggests. i.e., stabilise the polymer
backbone from thermal degradation during processing. PVC is particularly prone to this due to the chlorine atom present in the
polymer chain, which results in the liberation of hydrogen chloride gas, that can contribute to further destruction of the polymer chain.

Historically, the most important class of products used for heat stabilisation of PVC have been lead compounds, (e.g., lead
stearates). They are highly effective over a wide range of processing conditions; have a long history of use and familiarity in the
industry; and are cheap. But the harmful effects of lead on the human body and on the environment are well-proven and have led to
the phase-out of the metal from many applications, including PVC processing. Tetraethyllead, for example, is no longer used as an
anti-knock in gasoline; and in the paint industry no longer uses lead compounds as drying aids, even in India.

The phase-out of lead in PVC processing is more or less complete in much of the developed world, but there are remnants of it in
some developing countries. Globally, as a result of the growing use of non-lead stabiliser options, the quantum of PVC stabilised with
lead has fallen from ~20-mt in 2016 to under ~6-mt in 2022. The bad news is that India alone accounted for ~2-mt of this, giving
it the dubious distinction of being the largest user of this unsafe stabiliser option in the world.

But this will change in coming years, as the PVC processing industry here has only just embarked on a mandatory phase-out plan,
which has begun with elimination of lead from pipes carrying drinking water, and will extend over the next four years to cover other
pipes (agricultural, sewage and rainwater) and non-pipe uses.

The importance of PVC pipes
The importance of pipes in the Indian PVC market cannot be over-emphasised. Unlike the rest of the world, pipes (and fittings)
account for ~80% of Indian demand for PVC, which surpassed 3.7-mt in FY23. Other important end-uses include wire & cable (7%),

Chemical Weekly November 7, 2023 129

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