Because of its structure, PVC is pa

Because of its structure, PVC is particularly sensitive to heat and is by far the
largest user of heat stabilizers. Other vulnerable polymers are chlorinated
polyethylene and PVC/ABS blends. The increasing use of engineering plastics in
applications involving prolonged exposure to heat also calls for special stabilizer
systems.
Another important growth area for heat stabilizers is recycled materials,
where they will be used increasingly in inhibiting degradation and secondly in
re-stabilizing post-use plastics waste.
There are many different stabilizer systems for plastics, depending on the type
and products of oxidation. Metallic salts were originally used to stabilize PVC, the
most common being based on barium, cadmium, lead, or zinc, often mixed
together to obtain a synergistic effect. Organometallic compounds are also used,
mainly based on tin. A third group is non-metallic organic stabilizers, in which
phosphites play an important role, improving transparency, initial colour, and
light fastness. Epoxies (particularly derivatives of soya bean oil) are also used,
acting also as plasticizers, for non-toxic products.
Traditional stabilizer systems for polyolefins are based on a combination of a
phenolic anti-oxidant and a phosphorus-based melt processing stabilizer, the
phenolic providing melt processing stability as a donor of hydrogen atoms and a
scavenger of free radicals, and a level of thermal stability. The phosphorus-based
additive functions as a hydroperoxide decomposer during the melt compounding
stage.
For applications in contact with food, FDA and EGA regulations recommend
liquid anti-oxidants based on vitamin E. These have been developed as patented
systems and also open up new areas of application in polyolefins and
polyurethane foam systems.
Development in recent years has centred on technical improvement of the
product, and easier handling and dispersion. The main technical objectives have
been more durable effect at lower dosage levels, with good retention of colour
and transparency when required. Improvement of toxicological properties, for
food-contact and medical applications, has also been a continuing aim of
developers. For improved handling, pelletized and liquid systems have been
introduced, and there is a general trend towards greater use of masterbatch. The
most expensive stabilizers are organotin stabilizers. Lead compounds are the
cheapest.
Liquid stabilizers can be lubricating or non-lubricating and can contribute to
colour and/or volatility, firstly throughout the process and subsequently in the
finished product. They include barium/cadmium (zinc), barium/zinc, and
calcium/zinc in technical and low-toxicity grades and stabilizer/activator
combinations. Liquid barium/cadmium/zinc has been used for many years for
the most demanding outdoor applications, such as coil coating.
Antioxidants are frequently recommended as part of a multi-component
package to help achieve the best possible protection against photodegradation.
Chemistries include: phenolics - highly efficient; ABS - good FDA approval;
phenolic/phosphite blends - exceptional resistance to discoloration; thioesters -
exceptional heat stabilization in blend/synergy with phenolics.