Depolymerization, of PMMA

Methyl methacrylate (MMA) has the formula CH2 = ClCHa) (COOCH3). The vapor pressure of MMA due to its vaporization is estimated by [3] In P(Atm) = 11.79 — 4410/T(K), assuming a constant heat of vaporization. For the depolymerization reaction, the temperature which gives 1 atm of MMA (the product) is 164 C [26]. This temperature establishes the pressure equilibrium constant and therefore the Gibbs fiiee energy for the depolymerization reaction. Answer The depolymerization of PMMA rate constant given by [26] rate constant = 3.87 x 10 exp(—175 kJ/mole/RgT) gm/cm sec. 

For the depolymerization of PMMA, molten metal bath, dry distillation, extruder processes and fiuidized-bed processes are used [19], The depolymerization reactor of a molten metal bath consists essentially of a gas- or oil-heated metal bath. The metals used are those which have a low melting point such as tin and lead. The PMMA regrind is fed from the storage silos onto the stirred metal bath. Bath temperature and a residence time of some minutes are important for good yield and quality of the MMA. 

Products of degradation - depolymerization of PMMA chain in presence of PVC Chen, Q Wang, J Shen, J, Polym. Deg. Stab., 87, 527-533, 2005. 

It is now established that free radicals are involved in the decomposition. The most convincing evidence comes from (a) transfer in the degradation of PVC in the presence of labelled toluene, (b) initiation by PVC of the free radical depolymerization of PMMA in the temperature range of dehydrochlorination and well below the normal temperature for PMMA breakdown (see Section 15.11.1.1), (c) ESR evidence for the presence of radicals in degrading PVC, " and (d) initiation of dehydrochlorination by azodiisobutyronitrile. ... 

Scheme 34 Depolymerization of PMMA resulting from Cl radical attack...

The number average molecular weight of the PMMA sample was 24,000, and the amount of butyl lsopropenyl ketone unit in this polymer could be calculated to be 0.42mol% of MMA unit if a polymer molecule was assumed to contain one ketone unit. The amount of the ketone formed in the depolymerization reaction was 0.3mol% of MMA formed and corresponded to about 70% of the theoretical value. The depolymerization of the oligomer also gave low-boiling products in 53% yield, which contained about 9% of butyl lsopropenyl ketone. 

Solution A 10 cc green body will contain 0.01 moles of PMMA, which decomposes into 1.0 mole of methyl metacrylate (MMA = 100 gm/mole) that has a gaseous volume of 24,451 cc at 298 K and 1 atm pressure. The mass transfer of such huge amounts of gas volatiles is somtimes the rate determining step for depolymerization. 

Calculate the maximum time for all the rate determining steps for the binder burnout of PMMA from a spherical green body of radius i = 1.0 cm filled to a volume fraction of 0.8 with spherical 0.5 fim BaTiOs particles and 0.001 gms PMMA per cc. PMMA(Af , = 10,000) imdergoes depolymerization forming MMA (M - 100) at 350 C... 

N. Brand, Depolymerization of polymethylmethacrylate (PMMA), In Recycling and Recovery of Plastics, J. Brandrup, M. Bitmer, W. Michael and G. Menges (eds), Hanser Press, Munich 1996, pp. 488-493. 

At temperatures up to approximately 150°C u.v. irradiation of PMMA in the form of thin film results in random cleavage of the polymer molecules without depolymerization to monomer (2.) The accepted mechanism may be represented as follows ... 

In Fig. 14.3, the decomposition pathway of PMMA upon irradiation with UV laser light is shown [78] as it has been defined from products detected with time-of-flight mass spectroscopy (TOF-MS). The first three steps can be associated with incubation, whereas in step 4 depolymerization takes place. Incubation, as observed for PMMA, is often present in polymers that show weak absorption at the irradiation wavelength... 

Poly (methyl methacrylate) is characterized by crystal-clear hght transparency, unexcelled weatherability, and good chemical resistance and electrical and thermal properties. It has a useful combination of stiffness, density, and moderate toughness. PMMA has a moderate Tg of 105°C, a heat deflection temperature in the range of 74 to 100°C, and a service temperature of about 93°C. However, on pyrolysis, it is almost completely depolymerized to its monomer. The outstanding optical properties of PMMA combined with its excellent environmental resistance recommend it for applications requiring light transmission and outdoor exposure. Poly (methyl methacrylate) is used for specialized apphcations such as hard contact lenses. The hydroxyethyl ester of methaciyhc acid is the monomer of choice for the manufacture of soft contact lenses. Typical applications of poly(methyl methacrylate are shown in Table 15.6. 

Most polymers that are positive resists tend to depolymerize via a monomer-unzipping action when degraded, and poly(methyl methacrylate) (PMMA) is typical of this type. Unfortunately, the sensitivity of PMMA to electron beam radiation is low, and in an attempt to improve this feature, PMMA derivatives have been prepared by replacing the a-methyl group with more polar electron-withdrawing substituents, e.g.. Cl, CN, and CF3, to assist electron capture (Figure 16.5). Modification of the... 

These recovery values are fairly consistent regardless of pyrolysis temperature and heating rates. It seems that in the case of PMMA, as long as there is sufficient energy delivered to the sample in order to break a C-C bond, depolymerization occurs readily. 

Controlled depolymerization (as in pyrolysis) renders selectively pure monomers. In the case of PMMA (acrylics) it is successfully commercialized, and the yield is quite high. The pure monomer may replace virgin raw material. PET is also prone to depolymerization (via hydrolysis) as are other polycondensation resins, such as polyamides (Nylon) and even polyurethanes. 

Becanse PMMA has a low ceiling temperature, the formation of propagating radicals on radiolysis at 453 K, with G(S) = 10, Charlesby and Moore (40) found spontaneons depolymerization of the polymer to monomer at this temperature. In a later article, David and co-workers (239) showed that depol5mierization also occnrred when PMMA was irradiated at ambient temperatnre and then heated to 433 K, and that the jdeld of monomer was independent of the absorbed dose and the initial molecular weight of the PMMA. 

In Figure 10.16, we show the TGA data for PMMA and PMMA with 10% Cloisite 20A nanocomposite. From the figure we can see that pure PMMA has two main decomposition temperatures, the first around 250-325 °C for degradation of the unsaturated end groups, and the second at 350-450 °C for the depolymerization of random scission [21, 22]. In the insert, the derivatives of the spectra are plotted. The first peak in the derivative is significantly reduced when the clays are added, which may indicate that the end groups are... 

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