Condensation polymers depolymerization

Because commercial synthetic thermoplastic polymers are either addition polymers or condensation polymers, depolymerization occurs by different routes. Addition polymers, for which the synthesis reactions are essentially not reversible, depolymerize by pyrolysis or such severe chemical attack that few useful monomers can be practically recovered. With pyrolysis, a wide spectrum of species are created, which offers little in the way of valuable reaction products without costly separation processes. The overall yield to desired products can be unattractively low. 

Equilibrium between Monomer and Polymer. A monomer-with-polymer equilibrium is quite different from the polymer-with-condensation-product equilibrium discussed in Section 13.1.1. If the condensation product is removed from the reaction mixture, a condensation polymer increases in molecular weight. If the monomer is removed when it is in equilibrium with the polymer, the polymer depolymerizes to re-form the monomer. At temperatures suitable for long-term use, the equihbrium will be shifted toward stable polymer. However, at fabrication temperatures and at the high temperatures common in devolatilization, the production of monomer and low-molecular-weight ohgomers can be significant. 

Representative condensation polymers are listed in Table I. The list is by no means exhaustive, but it serves to indicate the variety of condensation reactions which may be employed in the synthesis of polymers. Cellulose and proteins, although their syntheses have not been accomplished by condensation polymerization in the laboratory, nevertheless are included within the definition of condensation polymers on the ground that they can be degraded, hydrolytically, to monomers differing from the structural units by the addition of the elements of a molecule of water. This is denoted by the direction of the arrows in the table, indicating depolymerization. 

Cyclic oligomers of condensation polymers such as polycarbonates and polyesters have been known for quite some time. Early work by Carothers in the 1930s showed that preparation of aliphatic cyclic oligomers was possible via distillative depolymerization [1, 2], However, little interest in the all-aliphatics was generated, due to the low glass transition temperatures of these materials. Other small-ring, all-aliphatic cyclic ester systems, such as caprolactone, lactide... 

Proteins are nature s polyamide condensation polymers. A protein is formed by polymerization of o-artiino acids, with the amino group on the carbon atom next to the carboxylic acid. Biologists call the bond formed a peptide rather than an amide. In the food chain these amino acids are continuously hydrolyzed and polymerized back into polymers, which the host can use in its tissues. These polymerization and depolymerization reactions in biological systems are all controlled by enzyme catalysts that produce extreme selectivity to the desired proteins. 

Scheme 5 suggests that every step of the ADMET polymerization cycle is in equilibrium and that, by shifting the relative concentrations of the condensate and polymer, depolymerization would result. In fact it has been shown that various unsaturated polymers can be depolymerized with excess ethylene, as well as substituted ethylenes. These depolymerizations can be done either with the tungsten or the molybdenum versions of Schrock s catalyst. 

Pyrolysis treatments are interesting regarding the aforementioned plastic refuse makeup. Other successful treatments for feedstock recycling of condensation polymers (PET, ABS, etc.), that allows for the depolymerization and recovery of their constituent monomers (e.g. hydrolysis, alcoholysis, methanolysis, etc.), cannot be applied for polyolefin plastics recycling. In contrast, pyrolysis of polyolefins yields valuable hydrocarbon mixtures of... 

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. 

Plastics with a carbonyl group can be converted to monomers by hydrolysis or glycolysis. Condensation polymers such as polyesters and nylons can be depolymerized to form monomers. For Polyurethanes (PURs), what is obtained is not the initial monomer, but a reaction product of the monomer diamine, which can be converted to diisocyanate. For PURs. hydrolysis is attractive as they can be easily broken down to polyols and diamines. The only issue is to separate them later. Steam-assisted hydrolysis has been shown to yield 60 to 80 percent recovery of polyols from PUR foam products. A twin screw extruder can be used as a reactor for hydrolysis. Glycolysis of PURS, yields mixture of polyols that can be reused directly. 

The major disadvantage of chemical depolymerization is that it is almost completely restricted to the recycling of condensation polymers, and is of no use for the decomposition of most addition polymers, which are the main components of the plastic waste stream. Condensation polymers are obtained by the random reaction of two molecules, which may be monomers, oligomers or higher molecular weight intermediates, which proceeds with the liberation of a small molecule as the chain bonds are formed. Chemical depolymerization takes place by promoting the reverse reaction of the polymer formation, usually through the reaction of those small molecules with the polymeric chains. Several resins widely used on a commercial scale are based on condensation polymers, such as polyesters, polyamides, polyacetals, polycarbonates, etc. However, these polymers account for less than 15% of the total plastic wastes (see Chapter 1). 

Depending on the chemical agent used to break down the polymer, different depolymerization routes can be envisaged glycolysis, methanolysis, hydrolysis, ammonolysis, etc. In the following sections of this chapter, these alternatives are reviewed for those condensation polymers having the most significant commercial applications. It must be pointed out that a majority of the studies on chemical depolymerization of plastic wastes is reported in patents works published in the scientific literature are relatively scarce. 

Thermal processes are mainly used for the feedstock recycling of addition polymers whereas, as stated in Chapter 2, condensation polymers are preferably depolymerized by reaction with certain chemical agents. The present chapter will deal with the thermal decomposition of polyethylene, polypropylene, polystyrene and polyvinyl chloride, which are the main components of the plastic waste stream (see Chapter 1). Nevertheless, the thermal degradation of some condensation polymers will also be mentioned, because they can appear mixed with polyolefins and other addition polymers in the plastic waste stream. Both the thermal decomposition of individual plastics and of plastic mixtures will be discussed. Likewise, the thermal coprocessing of plastic wastes with other materials (e.g. coal and biomass) will be considered in this chapter. Finally, the thermal degradation of rubber wastes will also be reviewed because in recent years much research effort has been devoted to the recovery of valuable products by the pyrolysis of used tyres. 

This section briefly describes the thermal behaviour and conversion of other plastics, including materials such as PET, that are condensation polymers. As described in Chapter 2, condensation polymers are best depolymerized by chemolysis. However, the knowledge of both their thermal stability and the products derived from their thermal decomposition is of interest because in many cases they are present as contaminants in wastes containing addition polymers. 

Chapter 2 discusses depolymerization processes based on the chemical cleavage of polymer molecules to convert them back into the raw monomers. The latter can be reused in the manufacture of new polymers, with properties similar to those of the virgin resins. However, this alternative is mainly used for condensation polymers, and is not successful for the degradation of most addition polymers. Glycolysis, methanolysis, hydrolysis and ammonolysis are the main treatments considered. Chemical depolymerization of polyesters, polyurethanes and polyamides is reviewed. 

Hydrolysis is the principal degradation mechanism for the condensation polymers. From the point of view of chemistry, the equilibrium molecular weight of these polymers is determined by the H O concentration at given temperature, T. However, owing to the moisture absorption from the air, the reaction equilibrium is shifted toward depolymerization. The rate of hydrolytic depolymerization depends on the moisture content, T and the presence of catalyst. Since these polymers are also subject to free-radical and oxidative processes (that lead to formation of unsaturations, hence the... 

Acrylic adhesives are essentially acrylic monomers which achieve excellent bonding upon polymerization. Typical examples are cyanoacrylates and ethylene glycol dimethacrylates. Cyanoacrylates [28] are obtained by depolymerization of a condensation polymer derived from a malonic acid derivative and formaldehyde. 

Generalized recycling routes applicable to all thermoplastic polymers (both addition and condensation) are illustrated schematically in Figure 0.8. This may be described as a crude, rough depolymerization as opposed to the precise surgical depolymerization achieved by the hydrolysis of a condensation polymer. Taking the procedure from the start ... 

There is also uncertainty in the regulatory status of tertiary recycling when it does not result in the direct production of monomers suitable for polymerization into new plastic. The European Commission has at times supported the chemical recycling (depolymerization) of condensation polymers such as polyethylene terephthalate back to monomer (e.g., dimethyl terephthalate) as recycling for the purpose of government-mandated plastics recycling rate calculations, but not the liquefaction of polyolefin plastics back to petrochemical feedstocks for reprocessing in a refinery. Discussions around these types of definitional issues, and their environmental and economic implications, are likely to continue for many years to come. 

Nylon recycling has increased substantially in the last several years. Most recycling efforts have focused on recovery of carpet. According to the U.S. Department of Energy, about 3.5 billion lb of waste carpet are discarded each year in the United States, with about 30% of them made from nylon 6. (For more on carpet recycling, see Sec. 12.4.15.) Recycling systems for condensation polymers, such as nylon and PET, can more effectively use chemical depolymerization techniques than can systems for addition polymers such as polyolefins and PVC. Most of the efforts directed at nylon recycling have taken this route. 

Hydrolysis reactions have been widely reported in NCW for low-molecular-weight molecules as well as for polymeric materials. Mandoki reported a process for depoly-merizing condensation polymers using NCW without addition of bases or acids. More particularly, polyethylene terephthalate, polybutene terephthalate, nylon 6, and nylon 66 were hydrolytically depolymerized (Fig. 9.25). 

Where depolymerization is least likeily to occur so that the condensation is irreversible and siloxane bonds cannot be hydrolyzed once they are formed, the condensation process may resemble classical polycondensation of poly functional organic monomer resulting in a three dimensional molecular network. Owing to the insolubility of silica under these conditions the condensation polymer of siloxane chains cannot undergo rearrangement into particles. 

Fourth, one may think of chemical recycling recycling constituents or conversion products of the nanocomposite. In the case of condensation polymers with ether, ester, or amide linkages, depolymerization is an option that can give rise to relatively high yields of the original constituents of the polymer [69,71-73,99]. The constituents obtained by depolymerization in turn can be used for repolymerization [69]. 

Condensation occurs most readily at a pH value equal to the piC of the participating silanol group. This representation becomes less vaUd at pH values above 10, where the rate constant of the depolymerization reaction k 2 ) becomes significant and at very low pH values where acids exert a catalytic influence on polymerization. The piC of monosilicic acid is 9.91 0.04 (51). The piC value of Si—OH decreases to 6.5 in higher order sihcate polymers (52), which is consistent with piC values of 6.8 0.2 reported for the surface silanol groups of sihca gel (53). Thus, the acidity of silanol functionahties increases as the degree of polymerization of the anion increases. However, the exact relationship between the connectivity of the silanol sihcon and SiOH acidity is not known. 

---