Polyesters pyrolysis

Several commercial polyester fabrics are flame retarded using low levels of phosphoms additives that cause them to melt and drip more readily than fabrics without the flame retardant. This mechanism can be completely defeated by the presence of nonthermoplastic component such as infusible fibers, pigments, or by siUcone oils which can form pyrolysis products capable of impeding melt flow (27,28). 

Xylenes. The main appHcation of xylene isomers, primarily p- and 0-xylenes, is in the manufacture of plasticizers and polyester fibers and resins. Demands for xylene isomers and other aromatics such as benzene have steadily been increasing over the last two decades. The major source of xylenes is the catalytic reforming of naphtha and the pyrolysis of naphtha and gas oils. A significant amount of toluene and Cg aromatics, which have lower petrochemical value, is also produced by these processes. More valuable p- or 0-xylene isomers can be manufactured from these low value aromatics in a process complex consisting of transalkylation, eg, the Tatoray process and Mobil s toluene disproportionation (M lDP) and selective toluene disproportionation (MSTDP) processes isomerization, eg, the UOP Isomar process (88) and Mobil s high temperature isomerization (MHTI), low pressure isomerization (MLPI), and vapor-phase isomerization (MVPI) processes (89) and xylene isomer separation, eg, the UOP Parex process (90). 

An excess of crotonaldehyde or aUphatic, ahcyhc, and aromatic hydrocarbons and their derivatives is used as a solvent to produce compounds of molecular weights of 1000—5000 (25—28). After removal of unreacted components and solvent, the adduct referred to as polyester is decomposed in acidic media or by pyrolysis (29—36). Proper operation of acidic decomposition can give high yields of pure /n j ,/n7 j -2,4-hexadienoic acid, whereas the pyrolysis gives a mixture of isomers that must be converted to the pure trans,trans form. The thermal decomposition is carried out in the presence of alkaU or amine catalysts. A simultaneous codistillation of the sorbic acid as it forms and the component used as the solvent can simplify the process scheme. The catalyst remains in the reaction batch. Suitable solvents and entraining agents include most inert Hquids that bod at 200—300°C, eg, aUphatic hydrocarbons. When the polyester is spHt thermally at 170—180°C and the sorbic acid is distilled direcdy with the solvent, production and purification can be combined in a single step. The solvent can be reused after removal of the sorbic acid (34). The isomeric mixture can be converted to the thermodynamically more stable trans,trans form in the presence of iodine, alkaU, or sulfuric or hydrochloric acid (37,38). 

In addition to the pathways depicted above, a 4-center concerted mechanism yielding ketenes has been reported during die vacuum pyrolysis of aliphatic polyesters (Scheme 2.4).89,90... 

A pyrolysis technique was investigated as a method for the chemical recycling of glass fibre-reinforced unsaturated polyester SMC composites. The proeess yielded liquid products and gases and also a solid residue formed in the pyrolysis of glass fibres and fillers. The solid residue was used as a reinforeement/filler in unsaturated polyester BMC composites, and the influenee on mechanical properties was studied in comparison with BMC prepared entirely from virgin materials. 

Y. Ishida, H. Othani and S. Tsuge, Effects of solvents and inorganic salts on the reactive pyrolysis of aromatic polyester in the presence of tetramethylammonium hydroxide studied hy pyrolysis gas chromatography/mass spectrometry, J. Anal. Appl. Pyrol., 33, 167 180 (1995). 

Levin, B.C., A summary of the NBS litterature Reviews on the chemical nature and toxicity of the pyrolysis and combustion from seven plastics acrylonitrite-butadien-styrenes (ABS), nylons, polyesters, polyetylenes, polysterenes, poly(vinyl-chlorides) and rigid polyurethane foams, KB SIR 85-3267, 1986... 

The /3-lactone dimer of dimethylketene can be prepared by pyrolysis of its polyester, which is formed by the base-catalyzed polymerization of dimethylketene.3"6 In addition to the rearrangement of the normal dimer described above,6 the direct dimerization of dimethylketene in the presence of aluminum chloride3 or trialkyl phosphites7 leads to the /3-lactone dimer. 

The color of the polymer can also be affected by inappropriate reaction conditions in the polymerization process, such as temperature, residence time, deposits of degraded polymer or the presence of oxygen. Degradation of polyesters and the generation of chromophores are thermally effected [29b, 29c, 39], The mechanism of thermal decomposition is based on the pyrolysis of esters and the formation of unsaturated compounds, which can then polymerize into colored products. It can be assumed that the discoloration takes place via polymerization of the vinyl ester end groups or by further reaction of AA to polyene aldehydes. 

Aromatic polyesters, commercially important molding resin materials, show a low degree of flammability and produce high percentages of char on exposure to a flame or on heating to pyrolysis conditions (9). 

In conclusion, we would like to mention that, in addition to this new direction, a large consumer of metal alkoxides (initially aluminium and titanium) is by tradition the technology of materials, where the alkoxides are used for hy-drophobization and for cross-linking of the polyhydroxocompounds, epoxides and polyester resins, and organosilicon polymers. The products of the partial hydrolysis and pyrolysis of alkoxides — polyorganometalloxanes — are applied as components of the thermally stable coatings [48J. 

Pyrolysis is the process of thermal degradation of a substance into smaller, less complex molecules. Many processes exist to thermally depolymerize tires to salable products. Almost any organic substance can be decomposed this way, including rice hulls, polyester fabric, nut shells, coal and heavy crude oil. Pyrolysis is also known as destructive distillation, thermal depolymerization, thermal cracking, coking, and carbonization. 

The pyrolysis unit in Centralis converts 100 tires per hour (about one ton, assuming each tire weighs 20 pounds) to 600 pounds of carbon black, 90 gallons of oil, and 30 therms (8000 ft3) of vapor gas. In addition to tire rubber, Conrad s unit has been used to pyrolyze substances as diverse as rice hulls, nut shells, biomass (including wood, paper, and compost), and plastics (including polyester, polyethylene, and propylene).1... 

Because of the ease of formation of these flammable pyrolysis products, polyesters have LOI values of 20-22 vol% (see Table 2.4), and hence, burn readily and because of the styrene content, give heavy soot formation. As these resins are cured at room temperature, bromine-containing flame retardants, which would decompose in melt-processed, thermoplastic polymers, may be effectively used. 

Providing flame retardancy for fibre blends has proved to be a difficult task. Fibre blends, especially blends of natural fibres with synthetic fibres, usually exhibit a flammability that is worse than that of either component alone. Natural fibres develop a great deal of char during pyrolysis, whereas synthetic fibres often melt and drip when heated. This combination of thermal properties in a fabric made from a fibre blend results in a situation where the melted synthetic material is held in the contact with the heat source by the charred natural fibre. The natural fibre char acts as a candle wick for the molten synthetic material, allowing it to bum readily. This can be demonstrated by the LOl values of cotton (18-19), polyester (20-21) and a 50/50 blend of both (LOl 18), indicating ahigher flammability of the blend as described later (Section 8.11). But a rare case of the opposite behaviour is also known (modacrylic fibres with LOl 33 and cotton in blends from 40-60 % can raise the LOl to 35). 

Research on the pyrolysis of thermoset plastics is less common than thermoplastic pyrolysis research. Thermosets are most often used in composite materials which contain many different components, mainly fibre reinforcement, fillers and the thermoset or polymer, which is the matrix or continuous phase. There has been interest in the application of the technology of pyrolysis to recycle composite plastics [25, 26]. Product yields of gas, oil/wax and char are complicated and misleading because of the wide variety of formulations used in the production of the composite. For example, a high amount of filler and fibre reinforcement results in a high solid residue and inevitably a reduced gas and oiFwax yield. Similarly, in many cases, the polymeric resin is a mixture of different thermosets and thermoplastics and for real-world samples, the formulation is proprietary information. Table 11.4 shows the product yield for the pyrolysis of polyurethane, polyester, polyamide and polycarbonate in a fluidized-bed pyrolysis reactor [9]. 

Figure 11.7 Fourier transform infrared spectra of the oils/waxes derived from the pyrolysis of polyester resin (a), phenolic resin (b) and epoxy resin (c)...

Thermoset plastics have also been pyrolysed with a view to obtain chemicals for recycling into the petrochemical industry. Pyrolysis of a polyester/styrene copolymer resin composite produced a wax which consisted of 96 wt% of phthalic anhydride and an oil composed of 26 wt% styrene. The phthalic anhydride is used as a modifying agent in polyester resin manufacture and can also be used as a cross-linking agent for epoxy resins. Phthalic anhydride is a characteristic early degradation product of unsaturated thermoset polyesters derived from orf/io-phthalic acid [56, 57]. Kaminsky et al. [9] investigated the pyrolysis of polyester at 768°C in a fiuidized-bed reactor and reported 18.1 wt% conversion to benzene. 

S. J. Evans, P. J. Haines and G. A. Skinner, Pyrolysis-gas-chromatographic study of a series of polyester thermosets. J. Anal. Appl. Pyrolysis, 55, 13-58, (2000). 

A. M. CunUffe and P. T. Williams, Characterisation of products from the recycling of glass fibre reinforced polyester waste by pyrolysis. Fuel, 82, 2223-2230, (2003). J. H. Harker and J. R. Backhurst, Fuel and Energy, Academic Press London, 1981. A. C. Albertson and S. Karlsson, Polyethylene degradation products, In Agricultural and Synthetic Polymers, ACS Symposium Series 433, J. E. Glass and G. Swift (eds), American Chemical Society, Washington DC, 60-64, 1990. 

J. A. Hiltz, Pyrolysis-gas chromatography mass-spectrometry identification of styrene cross-linked polyester and vinyl ester resins, J. Anal. Appl. Pyrolysis, 22, 113-128, (1991). 

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