Process stabilisers

The key theme of this three day international conference is the role the PVC industry can play in creating a sustainable future. Papers are divided into nine sessions Strategic direction Challenges and markets PVC profiles Flexibles Polymerisation Sustainability Processing Stabilisers Additives... 

PD antioxidants may destroy hydroperoxides either stoichiometrically or catalytically. The commonly used processing stabilisers, the alkyl or aryl phosphite esters are representative of the first class ... 

Compounds (Kosolapoff Maiereds) Wiley Vol6 pp 211-577 1973. It is used as a processing stabiliser for polymers. 

Fleenor [47, 50] investigated bis(2,4-di- -butylphenyl) pentaerythritoldiphosphite (Ultranox 626 Borg-Warner) as a process stabiliser in PET. The author stated that the stabilising effect was mainly due to trapping of catalyst residues, and that the additive could also deactivate hydroperoxides. One problem to be overcome with the use of any additive in PET is the high processing temperature. 

In an article in 1998 examining new trends in stabilising additives. Solera [55] noted that, while phosphites are effective process stabilisers in PET, they can themselves be subject to hydrolytic degradation. A new phosphorus-based stabiliser with increased hydrolytic stability was suggested in the form of 2,2, 2 -nitrilo[triethyl-tris(3,3, 5,5 -tetra- -butyl-l,T-biphenyl-2,2 -diyl)]phosphite (Irgafos 12 Ciba). 

Ashton and co-workers [57] assessed a wide range of phosphites for their process-stabilising abilities in PET. Additives used were ... 

While b is well known as an effective process stabiliser, the authors found that a was best, possibly due to its having similar high activity, but significantly improved hydrolysis resistance. It is also said to be more soluble in the polymer than any of the other additives. 

Tock [13] suggested that the process stabiliser bis(2,4-di-f-butylphenyl)pentaerythritoldiphosphite (Ultranox 626 GE) was also a moderately effective antioxidant. 

Eew radical scavengers besides the HALS have been suggested as UV stabilisers for aromatic polyesters. Some time ago, it was noted that certain nitro compounds could be used as UV stabilisers for polyesters, but closer reading of the documents showed this to be limited to certain sulfonated copolyesters [202]. Benzofuran-2-ones have been suggested as UV stabilisers with a good chain breaking -acceptor action, i.e., capable of scavenging alkyl radicals [203, 204, 156], but these are more efficacious as thermal antioxidants and process stabilisers, especially when used in conjunction with hindered phenolic antioxidants. 

It will be recognised that in-process stabilisation of peroxygens is related to safety of operations, discussed in the following section. 

Fabric finishing processes, stabilisation and surface treatments, are carried out to ensure fabric stability and to modify its surface characteristics to regulate the fabric permeability. 

In most polymers (e.g. polyolefins) process stabilisation for first use can be achieved with relatively low concentrations (0.05-0.1%) of hindered phenol or phosphite antioxidant. The stabihsation of PVC requires much higher concentrations (generally 2-4% on the PVC content) in order to combat the mechanically initiated HCl elimination referred to in Section 3. Some of them may also stabihse the polymer to the environment, but their abihty to do this depends entirely on the amormt of stabihser that remains after the reprocessing operation and how much chemical damage has been done to the pol3rmer dru ing the primary processing operation and in subsequent service. 

By themselves, chain-breaking donor antioxidants such as the hindered phenols, rV, are ineffective processing stabilisers for PVC since alone they are not able to inhibit the HCl unzipping reactions discussed above. However, they do inhibit hydroperoxide formation and are widely used as synergists with the basic stabilisation systems discussed above and are always included in commercial PVC stabiliser packages for plasticised formulations (see above, Section 4.1). 

Thermoplastics are usually processed in the molten state, at temperatures in the range 150-350 °C depending on the melting point and viscosity of the polymer. There are many standard stabilisation packages on the market, often containing a process stabiliser and a long-term stabiliser. Most of the stabilisers are synthetic, although recently, a natural hindered phenol, a-tocopherol (vitamin E) was found to be effective in polyolefins [141-143] and has been commercialised. For further information, the reader can consult books explicitly dedicated to stabilisation of polymers [132, 144]. 

When an adequate yield of this intermediate has been produced, excess EG is removed by distillation at atmospheric pressure and at a temperature of between 235 and 250 °C. A phosphoms compound, e.g. polyphosphoric acid is added at this stage as a processing stabiliser. Prior to further polymerisation, the manganese catalyst has to be deactivated. Otherwise, the manganese will not only continue to catalyse the polymerisation of PET, but also cause the formation of undesirable side reactions which will increase the yellowness of the final PET and result in poor thermal stability. 

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