Heat stabilizers

As with UV light, heat tends to oxidize polymers. The symptoms are embrittlement, melt flow instability, loss of tensile properties and discolouration. The mechanism of stabilization is therefore to prevent oxidation or to mitigate its effects. Plastics, particularly thermoplastics, may also require stabilization protection against degradation from heat during processing or in use. This article reviews the systems currently available, recent developments in technology and the present market trends in heat stabilizers.

How they work

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.

For applications in contact with food, FDA and BGA regulations recommend liquid antioxidants 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 centered on technical improvement of the product, and easier handling and dispersion. The main technical objectives have been a 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.

Antioxidants interrupt the degradation process in two ways, depending on structure:

It is possible to use more than one type of antioxidant, producing a desired result by different routes. Synergism between two stabilizers can often produce better results than a single system at the same concentration and, in polymers such as polyethylene and polypropylene, the total effect of synergism can be to improve efficiency of the antioxidant by up to 200%. A considerable amount of recent development has also focused on harnessing this phenomenon.

Primary antioxidants

 

Ciba's Irganox 1141 is a new liquid antioxidant for applications including ABS, MBS and PVC

Primary antioxidants react rapidly and are termed 'radical scavengers'. The most important are sterically hindered phenolics and secondary aromatic amines.

Hindered phenolic is a high molecular weight antioxidant, for polymer systems that are sensitive to thermal and oxidative degradation, due to formation of free radicals and peroxides. It protects against degradation at high processing temperatures and is highly efficient, low in volatility and non-staining, with wide toxicological clearance. The efficiency can be enhanced by using hindered phenolic with other antioxidants such as phosphites and thioesters, producing synergistic effects for effective and economical formulations. It is normally available as a free-flowing powder, but 50% aqueous dispersions are also available. There is a trend towards non-dusting products, including pelletised solids and liquids.

The antioxidant is effective at very low dosages (0.01 - 0.1%) in low and high density polyethylene (especially carbon black-filled grades for pipe and copper cable insulation), polypropylene (especially hot water applications), high impact polystyrene, ABS and MBS. It can also be added to PVC plasticisers (in which it can be dissolved) to inhibit oxidative degradation and embrittlement of PVC wire and cable insulation. The material can be used in polyamides and hot melt adhesive formulations. It is losing ground, however, in PU foam because of volatility, as the use of non-CFC blowing agents requires higher processing temperatures. 
 

Secondary antioxidants

Secondary antioxidants react with hydroperoxides to produce non-radical products and are therefore often termed 'hydroperoxide decomposers'. They differ from primary phenols and amines in that they are decomposed by reaction with hydroperoxide, rather than containing it. They are particularly useful in synergistic combinations with primary antioxidants.

Systems that do not contain a phenolic can provide good colour stability and gas fade resistance, which are important properties in PP fibres and other applications. A breakthrough was claimed by Ciba with FS Systems, the first of which was based on a new hydroxyl-amine - a high molecular weight compound offering outstanding compatibility with polypropylene, which functions through several different stabilization mechanisms to give both processing and long-term thermal ageing stability. It also appears to improve the activity of the hindered amine stabilizer that is also a component of the system, as both a thermal and light stabilizer.

Phosphite/phosphonites are generally regarded as the most effective stabilizers during processing, protecting both the polymer and the primary antioxidant. Hydrolytically stable phosphites are the most frequently used processing stabilizer in high-performance additive systems. Among recent developments are systems with better colour fidelity and handling properties. Typical is Dover's Doverphos HiPure 4, which is a high-purity tris-nonylphenyl phosphite (TNPP) processing and heat stabilizer and is claimed to reduce overall costs. With 0.1% residual nonyl phenol, it is FDA-approved for food-contact applications and is also used in medical applications, colour-critical polyolefins and styrenic block copolymers. It is effective also in acrylics, elastomers, nylon, polycarbonate, poly-urethanes, polystyrene, PVC, ABS and PET.

A breakthrough in lactone chemistry for polymer stabilization (making the manufacturing process significantly more efficient) was recently claimed by Ciba. The technology is based on a combination of a high performance phosphite or phosphonite processing stabilizer and a hindered phenolic antioxidant, offering an improved cost-performance profile compared with traditional two-component systems based on a phosphite/phosphonite and a phenolic antioxidant.

Ciba recently commissioned its first plant to manufacture the phosphorus-based stablizer P-EPQ, and will market it under the Irgafos brand name in several forms, including the superior pastille form FD, which offers advantageous handling properties. It also forms a component in the lactone Irganox XP 400 stabilizers.

A solid phosphite antioxidant has been developed by GE Specialty Chemicals in its Ultranox range. Designed to meet the demand for a high activity stabilizer with superior hydrocarbon stability and improved handling characteristics, it is based on butyl ethyl propane diol chemistry, rather than the usual pentaerythritol. It has been granted FDA approval for food contact in certain applications and is expected to find applications in polyolefins, styrenics, PVC, engineering thermoplastics, elastomers and adhesives.

Irganox XP stabilizer gives good processing stability, efficient colour control and good additive compatibility for food packaging (Photo: Ciba)

Blends of phosphites with primary antioxidants are also a centre of active development. Blends (such as the Irganox LC range) of primary antioxidants and a high temperature hydrolytically-stable organophosphite secondary antioxidant have been developed for high temperature processing of polyolefins, polyamides and polycarbonates in colour-critical applications. Irganox LM blends of primary antioxidants and a new phosphite processing stabilizer offer melting at 90oC and can be applied to polymer reactor products, especially polyolefins, linear polyesters, polycarbonates, polyamides, HIPS, ABS, SAN and elastomers.

Liquid blended systems (such as GE's Weston blends of trisnonylphenyl phosphite and octadecyl 3.5 di-tert-butyl-4-hydrozyhydrocinnamate) reduce the number of feeders required and eliminate on-site mixing, reduce loss from dusting and give more accurate feeding ratios.

Anox PAM blends from Great Lakes is a blend of Anox 20, a hindered phenol antioxidant, and Alkanox P-24, an organo-phosphite antioxidant. The blend provides good protection against thermo-oxidative degradation during long term ageing, while providing process stability to polymer systems such as polypropylene, polycarbonate, HDEP, ABS and other engineering thermoplastics.
Great Lakes has also introduced a product specifically for the wire and cable industry, where the presence of metals in plastics can lead to the introduction and accelerated production of degradation products. Lowinox MD24 combines both an antioxidant and a metal deactivator for the protection of polyolefins and elastomers used in wire and cable insulation. Protection is provided from the auto-oxidation process that can occur through the presence of trace metals from catalyst residues, for example.

Thioethers increase long-term stability in conjunction with phenolic antioxidants. Their use is limited to applications where possible effect on odour or taste and negative interaction with HALS (hindered amine light stabilizers) is not important.

Replacement of heavy metals

Environmental concern has promoted the development of systems to replace lead (especially from cables and cellular products). Cadmium-based stabilizers are also being phased out. To achieve the same effect, however, the replacements have to be a complicated mixtures of salts. Organotin and calcium/zinc systems are favoured at present, but much will depend on the shape of any legislation in the future. In general, alternatives based on calcium and zinc are less effective, but are cheaper than those based on aluminium or magnesium. Water absorption can be a problem with systems not using heavy metals.

Ciba has developed both tin and calcium zinc stabilizers for cellular PVC. Liquid systems, for easy processing, are generally featured in new developments. Akcros has a barium/zinc liquid stabilizer for flexible PVC, Lankromark LZ 1727, which gives good clarity and is self-lubricating. Bärlocher has patented a stabilizer based on calcium/aluminium hydroxyphosphite, claimed to be an environmentally-friendly raw material superior to all other known systems in external applications.

Performance comparable with lead, together with greater thermal stability, better weathering than CaZn and improved processability is claimed by Atofina for a new organotin, Thermolite PA 2400. With the addition of a new methyltin product line to its US butyl and octyl tin facilities, the company has become the only US producer to have all three tin stabilizers for the PVC industry. Methyltin stabilizers are used to improve the performance of colour development during processing, and to add strength and stability to PVC products. They can be used in the production of rigid PVC exterior building products such as window profiles, fencing and siding, and also in bottles, calendered sheet, pipe and injection moulded fittings.

Commercial trends

With a world consumption of around 280 000 tonnes, heat stabilizers are considerably larger than UV stabilizers. PVC is overwhelmingly the largest user - which may put some check on growth - but, as thermoplastics are processed faster and hotter, there are many niche opportunities opening up for other materials such as polyolefins and engineering plastics.

Within the total there will be significant changes in materials, in response to environmental concerns. There is a decline in use of lead compounds (the most popular type) and the threatened phase-out of cadmium will have a major impact, but will signal opportunities for other stabilizers.

The most significant of the heat stabilizers are lead compounds, which accounted for nearly 68% of volume in 1989, but by 1994 had declined to 64%. Barium/cadmium compounds are the next most important, followed by organotin compounds (which actually come second in value terms). Barium/zinc and calcium/zinc compounds have a high growth rate due to substitition in some cases of barium/zinc for barium/cadmium because of fears for the effects of cadmium on the environment and health, after it had been found that soluble cadmium products could have an adverse effect on the environment when used at above certain critical levels.

Irganox HP is a high performance melt processing stabilizer system from Ciba

Unfortunately, in the absence of any specific data on cadmium in insoluble form, the restrictions were also applied to these materials as a precautionary measure. This has led to confusion, with the result that many legislative bodies have taken their own decisions.

In the USA, OSHA issued a Notice of Proposed Rulemaking, significantly reducing the permissible exposure limits for cadmium, while a new risk assessment study on lead toxicity is expected. In the meantime, individual states have passed their own laws restricting the use of cadmium and lead.

In Europe, after introducing increasing restrictions on the use of cadmium over the last 20 years, legislators are suddenly embarrassed by their own latest study which concludes that cadmium pigments (at least) do not present any significant threat to human health or to the environment. It is unlikely that there will be any dramatic reversal of the legislation.

Lead stabilizers hold about 60% of the European PVC stabilizer market and organotins 10-15%, while the remainder is liquid or paste combinations of calcium and barium salts with zinc (according to estimates by Akcros). Stabilizers for UPVC window profiles are: lead-based 68%, barium, cadmium and lead, and barium cadmium are 29%, calcium- and zinc-based are 3%.

In the $200m US market, the main heat stabilizers include lead compounds, organotin compounds and mixed metal/salt blends based on chemicals such as barium, cadmium and zinc, and significant changes are expected. Over $75m of the business is in lead and cadmium-containing stabilizers, but these are under increasing scrutiny on health and safety grounds.