PVC Additives (Part 2)

Lead Stabilisers

Lead compounds, usually based on a mixture of lead salts, e.g., tribasic lead sulfate or dibasic lead phosphite and lead soaps with some lubricating action, dibasic lead stearate or normal lead stearate have been very cost-effective heat stabilisers for many years. They primarily function as very efficient HCl absorbers.

Dibasic lead phosphite is also a very effective light stabiliser. They are particularly suitable for wire and cable electrical insulation because of their complete non-conductivity and the inert nature of the chlorides formed with HCl. Technical limitations to their use have been opacity and sulfur staining. Their cumulative toxicity has been mainly concerned with worker exposure and consequently they have been heavily regulated. Apart from wire and cable, their use has been restricted to unplasticised-PVC (PVC-U) applications in Europe and Asia. North America has never accepted lead compounds in PVC-U applications, preferring the use of organotin compounds.

Non- or low-dusting products, which include the lead components in a safe handling state, in combination with lubricants (one-pack), have been available for many years in different product forms: flake, granule, tablet, and dosage packs. These one-packs are tailored to suit the processing technique, primarily extrusion and injection moulding, and end-use specification.

However, the EU hazard and labeling classification of lead stabilisers as reprotoxic with possible risk of impaired fertility and dangerous for the environment, in addition to their harmful and dangerous cumulative effects has resulted in their agreed phase-out in Europe. Another complicating factor is their presence (heavy metal) causing issues for waste management strategies. A new UK standard reducing the lead level in drinking water between 2003 and 2013 was an additional factor.

Calcium Zinc-based Stabilisers (Ca Zn)

One of the most successful developments in recent years has been the progress made in solid Ca Zn stabiliser formulations to replace lead on a cost-effective basis. It has also been important that the components of these systems have a satisfactory low-toxicity profile.

Ca Zn formulations are a complex blend consisting primarily of:

? Calcium soap (stearate or laurate)

? Zinc soap (stearate or laurate)

? Acid scavengers such as a hydrotalcite, zeolite, and metal oxides/hydroxides

? Organic co-stabilisers such as the diketone stearoyl benzoylmethane

? Antioxidant

? Polyols, and so on.

Calcium stearate acts as an acid acceptor in addition to providing lubrication. Zinc stearate is used to improve initial and early colour, in combination with the co-stabiliser. This functions by the co-stabiliser complexing the zinc stearate and so preventing the early formation of zinc chloride, which is a prodegradant. It should also be noted that PVC resins may react differently in the presence of zinc (zinc sensitive) and this has an influence on stabiliser formulations. Synthetic hydrotalcites and zeolites (already used in other industries) have been developed for use in PVC stabilisation. These form addition complexes at degrading sites. Such sites tend to be deactivated and the catalytic and highly mobile HCl captured before elimination of further HCl. Hydrotalcites have a layer structure based on magnesium aluminium hydroxycarbonates containing exchangeable anions. Zeolites are crystalline sodium aluminosilicates with a cubic crystal structure of zeolite.

They are microporous with uniform pores and a high internal surface area. Antioxidants are included at very low levels to inhibit the oxidation of the polymer matrix arising not only from thermal processing but also from subsequent photochemical and environmental influences.

Antioxidants are included at very low levels to inhibit the oxidation of the polymer matrix arising not only from thermal processing but also from subsequent photochemical and environmental influences. Solid Ca Zn systems have also been developed for plasticised-PVC (PVC-P) applications as replacements for liquid Ba Zn stabilisers where volatility and emissions may be an issue at the processing and end-use stages. Specific Ca/Zn systems are also available for food contact and medical use meeting the strict regulations that these materials have to satisfy.

Calcium and zinc are essential to life at low doses. The recommended daily intake for zinc is approximately 15 mg for adults. At high doses, toxic effects are observable. Zinc stearate is under review, under the EU Existing Chemicals Directive.

Calcium Organic-based Stabilisers

Another major development has been the replacement of zinc with a specific organic co-stabiliser that does not rely on zinc to generate good initial and early colour. The performance of the patented uracil co-stabiliser is linked to the efficiency of conjugation and electron transfer to retard dehydrochlorination and shorten polyene sequences. Zinc-free stabilisers are claimed to have a better processing window (no influence from zinc sensitivity) than Ca Zn, although initially there were also some mistaken perceptions about zinc being a ‘heavy metal’. Organic-based systems have been developed as lead replacements for rigid pipe and pipe fittings.

Liquid Stabilisers

Organotin Compounds

Organotin compounds are primarily based on methyl, butyl, or octyl derivatives, usually mixtures of dialkyl and monoalkyl, bound to the tin atom through a covalent C–Sn bond. Taking up the other positions are high molecular weight, highly ionic organic groups linked through a sulfur atom (mercaptide) or oxygen atom (carboxylate). The general formula is RxSnL4–x .

Tin Mercaptides

R can be methyl, butyl, or octyl (mono- or dialkylation). L is 2-ethyhexyl thioglycolate (as used in rigid bottles and films) or 2-mercaptoethyloleate (reverse ester). The performance of organotin mercaptides is not only based on the tin metal content, but on the organotin species, mercaptide ligand chemistry, and organic co-stabiliser. In North America, the use of lower cost, reverse ester thiotins is common for PVC-U applications covering pipe, profile and sidings, and foam. Tin, acting as a base, reacts with the HCl initially released during PVC processing (the base strength being such so as not to extract HCl from the PVC). The monoalkyltin mercaptide acts quickly to react with the labile chlorine to generate the corresponding trichloride which can further catalyse decomposition. The dialkyltin mercaptide neutralises this compound and the resulting dichloride does not catalyse any further decomposition, and also reactivates the mono-stabiliser. In addition the organotin mercaptide stabilisers are very effective antioxidants. Addition levels are 0.5–1.5 phr.

The main features of tin mercaptide stabilisers are that they provide good initial and long-term colour hold coupled with excellent clarity, linked to their good compatibility (due to high molecular weights and ester function in the mercaptan ligands).

The methyl and octyl versions are approved (up to a maximum level) for use in rigid food contact and medical applications. Limitations to their use include their relatively high cost and unpleasant odour and their use is now almost exclusively in PVC-U applications.

Tin Carboxylates

R is predominantly butyl (dialkylation). L is alkyl maleate or laurate. Organotin maleates are relatively less efficient than the tin mercaptides but have superior light stability due to the presence of the maleic acid structure which is able to react with conjugated double bonds (Diels–Alder reaction). They usually also contain antioxidants at a low level. Addition levels are 1.5–3.0 phr. They require particular lubrication systems due to their anti-lubricating effect.

In Europe, the annual production of tin mercaptides and carboxylates has remained fairly static around the 14 kt level (www.vinyl2010.org). This primarily reflects an effective decrease of PVC use in packaging.

Mixed Metal Compounds

Liquid mixed metal stabilisers are used almost exclusively in PVC-P applications. The stabilisers are formulated to suit specific requirements, such as clarity, good initial colour, long-term stability, compatibility with filled, pigmented systems, and suitability for post-processing techniques. Usually a good compromise of all the necessary requirements is achieved, but specific stabiliser formulations are designed to meet one or two specific needs.

Liquid mixed metal heat stabilisers are blends of metal soaps or salts, in combination with organophosphite esters and co-stabilisers in a liquid medium. The soaps or salts can be ethylhexanoate, nonylphenate, para-tert-butylbenzoate, oleate, and so on. Aryl-alkyl or alkyl organophosphites are liquid esters, which replace the labile chlorine (particularly in the presence of zinc), scavenge HCl, decompose peroxides, and act as complex Lewis acids (by accepting electron pairs). They also assist in the solubilisation of the stabiliser components.

Manufacture is based on in situ reaction of the metal oxides or hydroxides with the appropriate acid components. In addition to cost performance criteria, important aspects are storage stability, easy handling, and low odor.

Strongly basic carboxylates, derived from barium or calcium, are mostly HCl scavengers. Zinc and cadmium carboxylates are also able to scavenge HCl, but also substitute the allylic chlorine atoms. The synergism between the two types is attributed to a fast exchange reaction between zinc or cadmium chloride and barium or calcium carboxylates. These reactions regenerate the active zinc (or cadmium) carboxylate and also avoid the catalytic effect in PVC degradation of zinc (or cadmium) chlorides.

Epoxidised soyabean oil (ESBO) or octyl epoxy stearate at 2.0–5.0 phr is normally also included in PVC-P formulations. This is a secondary stabiliser/plasticiser which acts as an HCl scavenger.

One issue with liquid stabilisers is the emission of volatile components (phenol from the organophosphite, solvent from the liquid dispersion medium, and so on) during processing and from the end-use application, e.g., floor and wall coverings. Analytical techniques are now available to detect volatile organic compounds (VOC) and so influence stabiliser developments to improve this aspect.

Barium Cadmium (Ba Cd)

Ba Cd-based systems (which may also include zinc) have been available for many years due to their excellent cost effectiveness in combination with good initial colour and long-term stability. However, in the European area, their use was voluntarily phased out by the PVC industry in 2001, due to severe restrictions relating to environmental and toxicity (carcinogenic) concerns over cadmium (which accumulates in the body). Ba Zn stabilisers were the preferred option. Cadmium-based stabilisers are still used in the USA and Asia Pacific areas, but are coming under increasing health and environmental scrutiny. Ba Zn systems have been and continue to be developed.

Barium Zinc (Ba Zn)

Ba Zn-based systems now have the majority market share for stabilisation of PVC-P in Europe due to extensive formulation development based on increasing the barium content and the important role of organic co-stabilisers. Examples of organic co-stabilisers are the diketone dibenzoylmethane, dehydroacetic acid and pyrrolidine-2,4-diones.

Calcium Zinc (Ca Zn)

Traditionally Ca Zn has been less effective as a heat stabiliser, but much work has been carried out to develop more sophisticated and higher efficiency heat stabilisers, primarily to replace Ba Zn.

This is based on the fact that barium is a heavy metal. However, there is no evidence of health or environmental concerns. Barium does not build up in the body and in the USA, the Environmental Protection Agency has concluded that barium is unlikely to cause cancer in humans by ingestion. It is also not regulated in air.

Components of Ca Zn stabiliser systems are similar to those present in Ba Zn but vary in concentration. In Europe, the annual production fi gure of European Stabiliser Producers Association (ESPA) members for liquid Ba Zn and Ca Zn is approximately 14 kt (www.vinyl2010.org).