Polyethylene, depolymerization

Upon thermal destruction of polyethylene the chain transfer reactions are predominant, but depolymerization proceeds to a much lesser extent. As a result, the products of destruction represent the polymeric chain fragments of different length, and monomeric ethylene is formed to the extent of 1-3% by mass of polyethylene. C—C bonds in polypropylene are less strong than in polyethylene because of the fact that each second carbon atom in the main chain is the tertiary one. 

Boric acid esters provide for thermal stabilization of low-pressure polyethylene to a variable degree (Table 7). The difference in efficiency derives from the nature of polyester. Boric acid esters of aliphatic diols and triols are less efficient than the aromatic ones. Among polyesters of aromatic diols and triols, polyesters of boric acid and pyrocatechol exhibit the highest efficiency. Boric acid polyesters provide inhibition of polyethylene thermal destruction following the radical-chain mechanism, are unsuitable for inhibition of polystyrene depolymerization following the molecular pattern and have little effect as inhibitors of polypropylene thermal destruction following the hydrogen-transfer mechanism. 

Depolymerization, e.g., polyethylene terephthalate and cellulose hydrolysis Hydrothermal oxidation of organic wastes in water Crystallization, particle formation, and coatings Antisolvent crystallization, rapid expansion from supercritical fluid solution (RESS)... 

Polyethylene terephthalate also has the tendency, because it is produced by a condensation polymerization process, to depolymerize under high pressure and temperatures in the presence of water. Although this is usually a negative attribute, it can be utilized to regenerate pure monomers which can be repolymerized to make fresh polymer. This avoids the issues experienced by reprocessing resins, as the new resin has not experienced a previous heat history. A major drawback to this process is the requirement that the monomers used in polymerization processes must be highly pure, Unfortunately, this process is extremely costly and not performed on a commercial scale. 

Claims of perpetual motion create moments of mirth and consternation for those knowledgeable in the laws of thermodynamics. Yet, is it only hyperbole when a responsible journal such as the European Plastics News [1] proclaims that depolymerization of polyethylene terephthalate (PET) can be repeated indefinitely The second law of thermodynamics brings us back to reality. The depolymerization of PET does not operate at 100% yields, but does offer the opportunity for near-stoichiometric recovery of the monomers used to make the polyester. With high yields of potentially valuable monomers, the commercial potential for polyester depolymerization to regain feedstocks must be considered. 

One of the earliest published studies on extraction in twin-screw extruders was conducted by Todd (1974). In this work devolatilization was conducted under vacuum using two different polymeric systems, polystyrene in one and polyethylene in the other. In the case of polystyrene, styrene was not used as the volatUe component so as to avoid problems associated with further polymerization or depolymerization instead, use was made of mixtures of thiophene and toluene or ethylbenzene. Todd found good agreement between the measured exit concentrations of the volatile component and the predicted values using Pe = 40 in the solution to Eq. (38) (see Fig. 15). The value of 5 in Eq. (39) was not reported and it is not known whether a value was chosen to provide a fit with the data or whether it was known a priori. In any event, what is clear is that the exit concentration varies with IVwhich suggests that mass transfer is occur-... 

Earlier transition metals, as zirconium and hafnium, are still more active in hydrogenolysis, which allows zirconium hydrides to be used in depolymerization reactions (hydrogenolysis of polyethylene and polypropylene) [89], In this case, the zirconium hydride was supported on silica-alumina. Aluminum hydrides close to [(=SiO)3ZrH] sites would increase their electrophilicity and, thus, their catalytic activity. A catalyst prepared in this way was able to convert low-density polyethylene (MW 125000) into saturated oligomers (after 5h) or lower alkanes at 150°C (100% conversion). It was also able to cleave commercial isotactic polypropylene (MW 250000) under hydrogen at about 190 °C (40% of the starting polypropylene was converted into lower alkanes after 15 h of reaction). 

Manufacture. The manufacture of 1,4-cyclohexanedimethanol can be accomplished by the catalytic reduction under pressure of dimethyl terephthalate in a methanol solution (47,65). This glycol also may be prepared by the depolymerization and catalytic reduction of linear polyesters that have alkylene terephthalates as primary constituents. Polyethylene terephthalate) may be hydrogenated in the presence of methanol under pressure and heat to give good yields of the glycol (see Polyesters) (66,67). 

AIST A process for converting PET (polyethylene tererephthalate) into virgin plastic. The scrap PET is first depolymerized by heating with ethylene glycol and a metal salt catalyst. 

Shabtai [27] has foimd that SO4 ions attached to AI2O3 surfaces act as a super-acid catalysts for the depolymerization of polyethylene. 

While condensation polymers such as PET and polyamides can be broken down into their monomer nnits by thermal depolymerization processes, vinyl (addition) polymers snch as polyethylene and polypropylene are very difficnlt to decompose to monomers. This is becanse of random scission of the carbon-carbon bonds of the polymer chains during thermal degradation, which prodnces a broad prodnct range. 

Serrano et al. [11] studied the use of a laboratory-scale screw kiln reactor to transform low-density polyethylene (LDPE) into petrochemical feedstock. In this process, pyrolysis was carried out at reaction temperatures of 400-550°C and screw speeds of 3-20 rpm (Figure 19.6). In this process the plastic feed is initially heated in a feed hopper until the feed is melted. The melted plastic is then fed into the screw conveyor where it is depolymerized into gas, liquid and solid. The hopper is equipped with a stirrer to mix the feed plastic. Nitrogen is also used to provide an inert medium for pyrolysis. 

Propane as a degradation product of ohgomerized (polymerized) ethylene was ruled out because ethylene alone under similar conditions does not give any propane. Under similar conditions, but under hydrogen pressure, polyethylene is quantitatively depolymerized to form C3 to Ce alkanes, 85% of which are isobutane and isopentane. These results further substantiate the direct alkane-alkylation reaction and the intermediacy of the pentacoordinate carbonium ion. 

The model chosen to describe the degradation of polyethylene was random chain scission. Lenz (3,) in his section on degradation reactions of polymers cites work which supports the contention that polyethylene does thermally degrade in a random chain scission manner as opposed to depolymerization. For this model a statistical treatment has been developed by Montroll and Simha (). The extent of reaction may be related to the number average molecular weight by ... 

It can be obtained by depolymerization and hydrogenation of polyethylene terepb-thalate in the presence of methanol, but it is produced industrially by Tenttessee Eastman from dimethyl terephthalate. 

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