Thermal degradation of poly(methyl methacrylate) polymers: Kinetics and recovery of monomers using a fluidized bed reactor

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Abstract

Poly(methyl methacrylate) (PMMA) is a versatile transparent thermoplastic that is used in a wide range of fields and applications, such as in the lenses of exterior lights of automobiles, in electronics, in food containers and in artificial marble. When PMMA is subject to thermal degradation conditions, it degrades mainly into its monomer, methyl methacrylate (MMA). In this study, a virgin PMMA copolymer composed of 97.5% methyl methacrylate, 2.5% methyl acrylate (MA) and PMMA waste plastics were investigated. Kinetic measurements of the thermal degradation of the virgin copolymer were carried out with a TGA. The Chatterjee–Conrad (CC) and Freeman–Caroll (FC) models were applied to determine the pyrolysis kinetic parameters. Both models simulated the experimental DTG curves fairly well. In addition, pyrolysis of the virgin copolymer and the PMMA wastes was carried out using a fluidized bed reactor to recover the monomer. The reaction temperatures ranged from 450 to 500 °C. In each case, the rate of oil recovery was over 97%, and the content of the MMA in the oils was up to 98 wt%. The MMA recovered from the experiments can be used as feedstock for the polymerization of PMMA.

Introduction

Waste plastics are usually disposed of in landfill sites or utilized energetically, as for instance, by incineration. Modern society, however, requires reasonable alternatives to cope effectively with needs for energy and environmental conservation. Researchers consider the thermal degradation of waste plastics an alternative to traditional waste plastic management. In thermal degradation, plastic wastes can be converted into chemical feedstock, which can be used to produce valuable new products such as oils and gases of high caloric value. Thermal degradation of various plastics has been extensively investigated recently, for which various reactors have been used, such as vessels, autoclaves, rotary kilns and fluidized beds [1], [2], [3]. Most researches have focused on the thermal degradation of widely used plastic wastes, such as polyolefins. There are few commercial applications because of the low heat transfer rates of the melting polymer and the low value of the product oils. In contrast, thermal degradation of PMMA can lead to economic feedstock recycling because of its specific degradation behavior to yield the monomer, methyl methacrylate (MMA). Above 400 °C, it will be degraded almost into the monomer [4], [5]. The recovery of MMA from PMMA has been drawing much attention recently due to its high current cost of above 2000 $/ton. Typical reactors for the degradation of PMMA are molten metal baths, extruders and fluidized bed reactors [6], [7]. The molten bath is composed essentially of tin and lead metal, but it can release hazardous organic metal compounds [8].

The annual production of PMMA plastics in Korea amounts to 100,000 tons. The PMMA that is mainly produced in Korea is a copolymer composed of MMA and methyl acrylate (MA) or ethyl acrylate (EA), to facilitate the processing of PMMA products. In this study, thermal degradation of a virgin PMMA and waste PMMA plastics is conducted in a fluidized bed pyrolysis plant to obtain the optimal reaction temperature for a high yield of monomer. In addition, the kinetic characteristics of the thermal degradation of a virgin PMMA copolymer were also investigated using a TGA. Some researchers have already carried out kinetic experiments on the degradation of PMMA [9], [10]. In this study, the Chatterjee–Conrad (CC) and Freeman–Caroll (FC) models were applied to determine the kinetic parameters.

Section snippets

Materials

The virgin PMMA copolymer was supplied by the company LG MMA. It was the bead type and was composed of 97.5% methyl methacrylate (MMA) and 2.5% methyl acrylate (MA). It was radically polymerized in emulsion and its weight-average molecular weight was about 100,000 g/mol. The waste PMMA plastics used in the experiments were automobile taillight lenses and light guide plates. They were collected from scrapped cars and an electronic waste separation facility, respectively. They were then pretreated

Thermal degradation of the virgin PMMA and waste PMMA plastics in a TGA

The TGA curve for the thermal degradation of the virgin PMMA is shown in Fig. 2; of the automobile taillight lens, in Fig. 3; and of the waste light guide plate, in Fig. 4. The fractional conversion is as follows: x = (m0  m)/(m0  m), in which m0 is the initial sample weight, m is the residual weight at infinite time, and m is the residual sample weight at time t. As the heating rate increased, the temperature range at which weight loss took place moved to a higher-temperature region. The main

Conclusion

The thermogravimetric analysis shows that the main thermal degradation took place at the temperature range of 350–400 °C. At a higher heating rate, the temperature range at which weight loss took place moved to a higher-temperature region. The Freeman–Carroll method was used to compare the experimental DTG curve and the kinetic-model-predicted DTG curve. Lyon's approximation was chosen to calculate the fractional conversion. There was fairly good agreement between the experimental results and

Acknowledgment

This study is supported by the Ministry of Environment of Korea as “The Eco-technopia 21 Project.”

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