Thermo-oxidative dehydrochlorination of rigid and plasticised poly(vinyl chloride)/poly(methyl methacrylate) blends

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Abstract

The aim of this work was to study the thermo-oxidative dehydrochlorination of rigid and plasticised poly(vinyl chloride)/poly(methyl methacrylate) blends. For that purpose, blends of variable compositions from 0 to 100 wt% were prepared in the presence (15, 30 and 50 wt%) and in the absence of diethyl-2-hexyl phthalate as plasticiser. Their miscibility was investigated by using differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR). Their thermo-oxidative degradation at 180 ± 1 °C was studied and the amount of HCl released from PVC was measured by a continuous potentiometric method. Degraded samples were characterised, after purification, by FTIR spectroscopy and UV–visible spectroscopy. The results showed that the two polymers are miscible up to 60 wt% of poly(methyl methacrylate) (PMMA). This miscibility is due to a specific interaction of hydrogen bonding type between carbonyl groups (Cdouble bondO) of PMMA and hydrogen (CHCl) groups of PVC as shown by FTIR analysis. On the other hand, PMMA exerted a stabilizing effect on the thermal degradation of PVC by reducing the zip dehydrochlorination, leading to the formation of shorter polyenes.

Introduction

Polymer blending is one of the most contemporary ways in the development of new polymeric materials [1]. The polymer blends often exhibit properties that are superior to any individual component polymer. However, the manifestation of superior properties depends upon the miscibility of homo polymers on the molecular scale. The main method to find out the number of amorphous phases in polymer blends is the determination of the number of glass transition temperatures (Tg) because each Tg corresponds to one amorphous phase [2]. The most often used experimental method for Tg determination is differential scanning calorimetry (DSC).

Poly(vinyl chloride) (PVC) is one of the most important and widely used thermoplastics due to its many valuable properties like low price, good processability, chemical resistance and low flammability. Its principal drawback, however, is low thermal stability at processing temperatures. In practice, this problem is overcome by the use of heat stabilizers [3]. Several polymers are mixed with PVC as polymer plasticisers or processing aids. Polymethacrylates, especially poly(methyl methacrylate) (PMMA), are used as processing aids for PVC [4]. Blends of PVC and PMMA were first studied by Schurer et al. [5] who concluded that PVC was partially miscible with atactic and syndiotactic PMMA but almost completely immiscible with isotactic PMMA. The PMMA/PVC blend is a well-known system in which a hydrogen bonding type of specific interaction involving the α-hydrogen of PVC and the carbonyl group of PMMA is expected [6]. In previous studies, this specific interaction was shown by using Fourier transform infrared spectroscopy [7], [8], [9].

Polymers and their blends are often processed in the melt, which makes the thermal stability of these materials of primary importance. McNeill et al. [10], [11] studied the thermal degradation of PVC and PMMA mixtures and found that the monomer formed is methyl methacrylate from early depolymerisation of PMMA initiated by Cl radicals propagating dehydrochlorination (DHC) of PVC at a much lower temperature than that of PMMA depolymerisation when heated alone. In previous studies [8], [12], we have found that PMMA exerted a stabilizing effect on the thermal degradation of PVC under nitrogen atmosphere.

In the present work, miscibility and thermo-oxidative dehydrochlorination of rigid and plasticised PVC/PMMA blends have been investigated.

Section snippets

Materials

Commercial grades of resins and additives listed in Table 1 were used as received. The K value of PVC is 65–67 according to DIN 53-726; ρ(PVC) = 0.54 g/cm3; ρ(PMMA) = 1.18 g/cm3.

Sample preparation

Blends of variable compositions from 0 to 100 wt% were prepared in the presence of 0, 15, 30 and 50 wt% of di(ethyl-2-hexyl)phthalate (DEHP), 1 wt% of lubricant and 3 wt% of heat stabilizer. The amounts of the three additives were added according to the amount of PVC in the blend. Melt mixing was performed at 175 °C on a two-roll

DSC analysis

The glass transition temperatures (Tg) of rigid and plasticised PVC/PMMA blends are reported in Table 2, Table 3, respectively. For miscible polymers, a single Tg is observed [2]. From the obtained results and according to this Tg criterion of miscibility, this polymer pair appears to be miscible up to about 50 wt% of PMMA. It is known that the presence of a plasticiser decreases the Tg value. In our case and for a same blend composition, all the plasticised blends showed lower Tg in comparison

Conclusions

The DSC analysis of the rigid and plasticised PVC/PMMA blends showed polymer miscibility up to about 50 wt% of PMMA. This miscibility is due to a specific hydrogen bonding interaction between carbonyl groups (Cdouble bondO) of PMMA and hydrogen from (CHCl) groups of PVC as evidenced by FTIR analysis. The deconvolution of the carbonyl band of PMMA in the miscibility range showed two contributions due to bonded and non-bonded carbonyl groups while the deconvolution outside the miscibility range showed only

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