Depolymerisation Depolymerised

If boiled with water an aqueous solution is obtained which, owing to depolymerisation, gives the reactions for formaldehyde. 

The acetaldehyde should be freshly distilled (b.p. 20-5-21°). It can be conveniently prepared by depolymerising pure dry paraldehyde (see Section 111,65). 

The reaction probably proceeds as follows. Crotonaldehyde is first formed by oondensation of the depolymerised acetaldehyde in the presence of acid ... 

The action of sulphuric acid alone upon acetone cyanohydrin affords a-methylacrylic acid. The methyl methacrylate polymers are the nearest approach to an organic glass so far developed, and are marketed as Perspex (sheet or rod) or Dialcon (powder) in Great Britain and as Plexiglass and Luciie in the U.S.A. They are readily depolymerised to the monomers upon distillation. The constitution of methyl methacrylate polymer has been given as ... 

Place 25 g. of methyl methacrylate polymer (G.B. Diakon (powder). Perspex (sheet) U.S.A. Lucite, Plexiglass) in a 100 ml. Claisen flask, attach an efficient condenser e.g., of the double smface type) and distil with a small luminous flame move the flame to and fro around the sides of the flask. At about 300° the polymer softens and undergoes rapid depolymerisation to the monomer, methyl methacrylate, which distils over into the receiver. Continue the distillation until only a small black residue (3-4 g.) remains. Redistil the hquid it passes over at 100-110°, mainly at 100-102°. The yield of methyl methacrylate (monomer) is 20 g. If the monomer is to be kept for any period, add 0 -1 g. of hydro quinone to act as a stabiUser or inhibitor of polymerisation. 

X,9. DEPOLYMERISATION OF A HEXAMETHYLENE-DIAMINE-ADIPIC ACID POLYMER (NYLON 66 )... 

The details of the commercial preparation of acetal homo- and copolymers are discussed later. One aspect of the polymerisation so pervades the chemistry of the resulting polymers that familiarity with it is a prerequisite for understanding the chemistry of the polymers, the often subde differences between homo- and copolymers, and the difficulties which had to be overcome to make the polymers commercially useful. The ionic polymerisations of formaldehyde and trioxane are equiUbrium reactions. Unless suitable measures are taken, polymer will begin to revert to monomeric formaldehyde at processing temperatures by depolymerisation (called unsipping) which begins at chain ends. 

Properly end-capped acetal resins, substantially free of ionic impurities, are relatively thermally stable. However, the methylene groups in the polymer backbone are sites for peroxidation or hydroperoxidation reactions which ultimately lead to scission and depolymerisation. Thus antioxidants (qv), especially hindered phenols, are included in most commercially available acetal resins for optimal thermal oxidative stabiUty. 

Weak links, particularly terminal weak links, can be the site of initiation of a chain unzipping reaction. A monomer or other simple molecule may be abstracted from the end of the chain in such a way that the new chain end is also unstable. The reaction repeats itself and the polymer depolymerises or otherwise degrades. This phenomenon occurs to a serious extent with polyacetals, polyfmethyl methacrylate) and, it is believed, with PVC. 

Some polymers such as the polyacetals (polyformaldehyde) and poly(methyl methacrylate) depolymerise to monomer on heating. At processing temperatures such monomers are in the gaseous phase and even where there is only a small amount of depolymerisation a large number of bubbles can be formed in the products. 

The esterification reaction may be carried out with a number of different anhydrides but the literature indicates that acetic anhydride is preferred. The reaction is catalysed by amines and the soluble salts of the alkali metals. The presence of free acid has an adverse effect on the esterification reaction, the presence of hydrogen ions causing depolymerisation by an unzipping mechanism. Reaction temperatures may be in the range of 130-200°C. Sodium acetate is a particularly effective catalyst. Esterification at 139°C, the boiling point of acetic anhydride, in the presence of 0.01% sodium acetate (based on the anhydride) is substantially complete within 5 minutes. In the absence of such a catalyst the percentage esterification is of the order of only 35% after 15 minutes. 

Stepwise thermal- or base-eatalysed hydrolytic depolymerisation initiated from the hemi-formal chain end with the evolution of formaldehyde. The main reasons for end-capping and copolymerisation mechanisms described above are carried out in order to minimise this reaction. 

Oxidative attack at random along the chain leading to chain scission and subsequent depolymerisation. Initial chain scission is reduced by the use of antioxidants (see Chapter 7) and in recent formulations hindered phenols seemed to be preferred. It is reported that 2,2 -methylenebis-(4-methyl-6-t-butylphenol) is present in Celcon and 4,4 -butylidene bis-(3-methyl-6-t-butylphenol) in Derlin. The copolymerisation helps to reduce the rate of depolymerisation where initiation of depolymerisation is not completely prevented. 

Thermal depolymerisation through scission of C—O bonds can occur catastrophically above 270°C and care must be taken not to exceed this temperature during processing. 

At room temperature there is only a small decrease in free energy on conversion of monomer to polymer. At higher temperatures the magnitude of the free energy change decreases and becomes zero at 127°C above this temperature the thermodynamics indicate that depolymerisation will take place. Thus it is absolutely vital to stabilise the polyacetal resin both internally and externally to form a polymer which is sufficiently stable for processing at the desired elevated temperatures. 

Care has to be taken in the polymerisation of aldehyde polymers in order to achieve reproducible results. It is also difficult to stabilise most of the products since thermodynamics frequently favour depolymerisation at temperatures a little above or at room temperature. 

The polymer is liable to depolymerisation at temperatures just above T. In the case of pure polymer there is a tendency for the few spherulites to grow to sizes up to 1mm diameter. Spherulite size may be reduced by the use of nucleating agents and by fast cooling. 

Clarson, S.J., Depolymerisation, degradation and thermal properties of siloxane polymers. In Clarson, S.J. and Semiyen, J.A. (Eds.), Siloxane Polymers, Polymer Science and Technology Series. PTR Prentice Hall, Englewood Cliffs, NJ, 1993, pp. 216-244. 

In other instances the reactions appear to occur in sequence down the chain, for example in the depolymerisation reaction of polyformaldehyde (polyacetal) and polymethyl methacrylate which are referred to as zippering or sometimes unzippering reactions. In other cases cyclisation reactions can occur such as on heating polyacrylonitrile ... 

Thermal stability is largely concerned with chemical reactivity which may involve oxygen, u.v. radiation or depolymerisation reactions. The presence of weak links and the possibility of chain reactions involving polymer chains may lead to polymers having lower thermal stability than predicted from studies of low molecular weight analogues. 

A number of thermoplastics undergo depolymerisation on heating. These include poly(styrene), poly(methyl methacrylate), and poly(oxymethylene). Such depolymerisation will occur regardless of the prevailing oxygen concentration and under well aerated conditions will provide a ready source of fuel for sustained combustion. 

Recycling can include chemical recycling. This involves plastics waste being processed chemically, either by cracking or depolymerisation, to... 

A number of cracking/depolymerisation processes are currently operating commercially. These include the Texaco gasification process and the BP Chemicals polymer cracking process. Both have been operating since the mid 1990s. 

In the Texaco process, there are two steps, an initial liquefaction step followed by treatment in an entrained bed gasifier. The liquefaction step involves heating the plastic scrap in such a way that partial depolymerisation occurs, generating a heavy oil and some gas fractions. Some of the gas is recycled as fuel for the process. 

Texaco gasification is based on a combination of two process steps, a liquefaction step and an entrained bed gasifier. In the liquefaction step the plastic waste is cracked under relatively mild thermal conditions. This depolymerisation results in a synthetic heavy oil and a gas fraction, which in part is condensable. The noncondensable fraction is used as a fuel in the process. The process is very comparable to the cracking of vacuum residues that originate from oil recycling processes. 

As indicated above, the plant consists of a VCC part and a depolymerisation part. Depolymerisation allows for further processing of the residues in the VCC section. The depolymerisation takes place between 350-400 °C. Here, at the same time chlorine is released. Over 80% of the chlorine input will become available as HCl in the light fraction and washed out in a purification process yielding technical HCl. 

The gaseous product of the depolymerisation is partially condensed. The condensate, containing 18% of the chlorine input, is fed into a hydrotreater. The HCl is eliminated with the formation of water. The resulting Cl-free condensate and gas are mixed with the depolymerisate for treatment in the VCC section. 

The depolymerisate is hydrogenated in the VCC section at 400-450 °C. This takes place under high pressure... 

When the VEBA plant was operational, it had the following input specifications for the depolymerisation section ... 

---