A Comprehensive Kinetic Model for Polymerization of Styrene with Ground Tire Rubber

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Polyethylene (PE)/GTR blends

Sonnier et al. (2007) reported that GTR showed poor adhesion to a high density PE and worsened the tensile properties of the latter. The size of the GTR was 0.6-0.7mm and the PE/GTR ratio was from 100/0 to 30/70. The GTR was treated by oxidation method (KMnO4) [31].
Scaffaro et al. (2005) used the GTR of 0.4-0.7mm in size and the PE/GTR ratio was from 75/25 to 25/75. The blends were processed in a twin screw extruder. The injection molded samples showed better performance than compression molded ones [32].
Shojaei et al. (2007) blended the PE and GTR in a twin screw extruder. They found that with increasing GTR content, the yield stress and impact strength decreased. The size of the GTR was below 0.4mm and the PE/GTR ratio was from 75/25 to 25/75 [33].

Polypropylene (PP)/GTR blends

PP has good performance. However its toughness is poor, limiting its applications. Therefore, numerous methods have been proposed to overcome this shortcoming. One of them is to use the GTR as an impact modifier. Fuhrmann et al. (1999) studied PP/GTR blends in a twin screw extruder. The size of the GTR was 0.4-0.7mm and the PP/GTR ratio was from 100/0 to 40/60.The specimen was produced by injection molding. They showed that the notched charpy impact strength at room temperature increased with increasing GTR content [34].
Kuznetsova et al. (2004) studied rubber devulcanization by a themomechenial method in a twin screw extruder. The sizes of the GTR were below 0.4mm and 0.4- 3 0.7mm, respectively. The PP/GTR was from 100/0 to 50/50.They found that smaller GTR particles resulted in slightly better mechanical properties [35].
Awang et al. showed that tensile properties of PP/modified-GTR blends were better than those of PP/unmodified-GTR ones [2]. An improved dispersion and a size reduction of particles were observed in the micrographs of tensile fracture surfaces. It indicated that the modifications of GTR were capable of offering an improved interfacial adhesion which should lead to improved properties.

Polyvinyl chloride (PVC)/GTR blends

Naskar et al. (2002) used chlorinated ground rubber tire (Cl-GRT) particles as fillers for a plasticized PVC. The physical properties of the Cl-GRT-filled PVC compound were improved compared with those of the non-chlorinated counterpart. Moreover, the Cl-GRT-filled composite was found to be re-processable like the unfilled PVC compound [5]. Stelescu et al. (2013) focused on the polymer composites based on plasticized PVC and rubber powder from vulcanized nitrile rubber waste. Lower hardness, higher elongation at break, a better tensile strength, and better ozone resistance were the good properties of the new polymer composites. Moreover, the polymer composites had good fluidity that could be processed by injection, extrusion, and compression molding [36].

High Impact-Modified Polystyrene (HIPS)

The incorporation of polybutadiene is an earliest way to overcome the brittleness of GPPS. Ostromislensky [38] invented the high impact-modified HIPS, which has been commercialized since the 1950s. He used the cellular rubber particles to toughen PS during the manufacturing process and the rubber particles were embedded into the PS matrix. However, the transparency of PS was lost by using the rubber toughening. As the research continued, core/shell particles were used to toughen PS, such as styrene—butadiene block copolymers. Due to the small domains of this copolymer in the PS, the resulting material is translucent impact polystyrene [37].

Free radical polymerization mechanism of styrene

Before a kinetic model is established, the mechanism of free radical polymerization of styrene should be described first. There are four significant reactions that take place in free radical polymerization of styrene: initiation, propagation , termination and transfer.

Initiation reaction

Initiation reaction of styrene polymerization often includes the chemical initiation by the initiator and thermal initiation of styrene, the latter becoming important at temperatures above 100℃. For the chemical initiation of the free radical polymerization of styrene, Villalobos et al. (1993) [64] and Kotoulas (2003) [65] studied the polymerization of styrene using the benzoyl peroxide (BPO) and dicumyl peroxide (DCP), respectively. They measured the conversion, number and weight average molecular weights at different polymerization temperature (60-200℃), the temperature control was achieved using an oil bath. Yamazoe et al. (2001) used NearInfrared Spectroscopic to study the entire conversion range of free radical bulk polymerization of styrene [66]. The polymerization was maintained at 70℃for a initiator, dimethyl 2,2’azobis (isobutyrate).
For the thermal initiation of styrene, there are two kinds of initiation mechanisms. One involves two styrene molecules, first put forward by Flory et al. (1937) [67]. Bengough et al. (1978) supported Flory’s theory by thermal initiation of the polymerization of styrene from 60–140℃. Mayo et al. (1943-1953) [68]–[70] and Russell et al. (1953) [71] proposed a thermal initiation mechanisms involving three styrene molecules. They deduced this mechanism from the fact that the thermal initiation rate is proportional to the monomer concentration to the power of 2.5. Pryor et al. (1970) continued to investigate the thermal free radical polymerization of styrene by computer simulations [72].
A generalized thermal initiation mechanism is described below [65]. A reversible Diels-Alder dimerization of styrene leads to 1-phenyl-1, 2, 3, 9-tetrahydronaphtalene (AH). AH with one styrene forms one styryl (MR) and one 1-phenyltetralyl radical (AR). MR and AR could further produce polymer chains. The reaction of AH with styrene makes a ‘dead’ trimmer (D3).
2M − AH (1.1).
AH+M AR+MR (1.2).
AR+M kA R3 (1.3).

The development of kinetics model of polymerization of styrene

Prior to the 1970s, some of the basic works of modeling of radical polymerization of styrene were done by Hamielec (1967) [71] and Hui (1972) [72]. They found that a model of third order of the initiation rate respect to the monomer concentration was better than that of second order. Kotoulas (2003) [65] and Kiparissides et al. (1992) [78] showed that a mathematical model accounting for both chemical and thermal radical initiation could be used for temperatures ranging from 60 to 200℃. When the system becomes viscous, the diffusion-controlled phenomena may come into play in the polymerization process. The diffusion-controlled phenomena of the cage, glass and gel effects have been related to the initiation, propagation and termination reactions [65]. Diffusion controlled reactions of termination, propagation and initiation were described. However, they did show the critical points of different diffusion effects (gel effect, glass effect). O’Neil et al. (1998) [79] established a model with focus on the description of the onset of the gel effect. Huang et al. (1990) [80] developed a kinetic model which took into account the gel and glass effects via DSC measurements. Cavin et al. (2000) [81] proposed a model to describe the conversion, gel effect and the number average molecular weight.
More recently, Woloszyn et al. (2013) [82], [83] developed a mathematical model of thermal radical polymerization of styrene. They studied the chemically initiated free radical polymerization of styrene using DCP, at the temperatures ranging from 100 to 150℃ as well as using BPO, at temperatures ranging from 70 to 90℃. A generation and consumption of styrene adduct could be calculated by their model.
This section deals specifically with kinetic models for the graft polymerization of styrene onto rubber. Manaresi et al. (1975) [73] showed the graft polymerization of styrene solutions of polybutadiene (2-9 wt%) containing dicumyl peroxide (0.1-0.3 wt%) as initiator at 100℃. Graft efficiency was found to be independent of the peroxide concentration, but increased with the rubber content. The graft efficiency was given by Eq 1.8. = 4 (1.8).
where P was the PS (wt%) in the grafted fraction. R was the polybutadiene (wt%) in the initial solution.

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Thermogravimetric analysis (TGA)

The thermogravimetric analysis (TGA) can track the degradation of a product according to the variation of temperature in a given atmosphere. A nacelle containing a sample is placed in an oven that can be maintained under vacuum or be swept away by a carrier gas and control the total flow rate. The experiment conditions for TG (TGA Mettler Toledo STAR System) were as follows: temperature range room temperature to 750℃ [14]; heating rate, 15℃ min-1. The thermal degradation of the powder was under nitrogen flow, the flow rate was 50 ml min-1 of nitrogen [20-21].

Particle Size Distribution (PSD) by Malvern Mastersizer

The size of the GTR or GTR-g-PS particles obtained after polymerization and extrusion was measured directly using the light diffraction with a Mastersizer 2000 from Malvern Instruments. This device was responsive to the volume of the particles and could determine the size distribution of particles between 0.02 microns to 2000 microns.

Mechanical Testing

A mini-injection molding machine DSM (Figure 2.5) was used to mold specimens for impact testing. A polymer in the molten state is pushed into a mold where it is cooled to the desired shape. The sleeve of the injection molding machine can be fed directly from the microcompounder DSM to inject specimens from molten polymer.

Table of contents :

Chapter 1 Introduction
1.1 Introduction to Ground Tire Rubber (GTR)
1.1.1 Waste tire and its reutilization
1.1.2 Surface modification of GTR
1.1.3 Devulcanization of GTR
1.1.4 Plastics/GTR blends
1.2 Introduction to polystyrene (PS)
1.2.1 Polystyrene structure
1.2.2 General Purpose Polystyrene (GPPS)
1.2.3 High Impact-Modified Polystyrene (HIPS)
1.2.4 PS/plastics blends
1.2.5 Rubber modified PS
1.2.6 GTR toughened PS
1.3 Introduction to GTR/PS blends using a twin screw extruder
1.3.1 Screw profile
1.3.2 Screw speed
1.3.3 Barrel temperature
1.4 Introduction to the free radical polymerization kinetics of styrene
1.4.1 Free radical polymerization mechanism of styrene
1.4.2 Model of the polymerization kinetics of styrene
1.4.3 Literature on the graft polymerization of styrene onto rubber
1.5 Introduction Artificial Neural Networks (ANNs)
1.5.1 Artificial Neural Networks (ANNs)
1.5.2 Back Propagation network
Chapter 2 Experimental
2.1 Experimental Procedure
2.1.1 Materials
2.1.2 Experimental steps
2.1.3 Soxhlet extraction procedure
2.1.4 Solvent selection for GTR particle swelling
2.1.5 GTR particle size measurement procedure
2.1.6 Conversion measurement procedure
2.1.7 Grafting extent measurement procedure
2.2 Characterization Methods
2.2.1 Scanning Electron Microscopy (SEM)
2.2.2 Fourier Transform Infrared Spectroscopy (FTIR)
2.2.3 Gel Permeation Chromatography (GPC)
2.2.4 Differential Scanning Calorimetry analysis (DSC)
2.2.5 Thermogravimetric analysis (TGA)
2.2.6 Particle Size Distribution (PSD) by Malvern Mastersizer
2.2.7 Mechanical Testing
2.3 Extrusion of PS/GTR blends
2.3.1 Screw profile
2.3.2 Barrel temperature and screw speed
Chapter 3 A Comprehensive Kinetic Model for Polymerization of Styrene with Ground Tire Rubber
3.1 Overall kinetic scheme
3.1.1 Initiation Reactions
3.1.2 Propagation Reactions
3.1.3 Chain Transfer Reaction
3.1.4 Termination Reactions
3.2 Polymerization rate functions
3.3 Reactor design equations
3.3.1 Initiator, I
3.3.2 Monomer, M
3.3.3 Primary radical, PR
3.3.4 Rubber primary radical, GPR
3.3.5 Diels-Alder adduct, AH
3.3.6 Styryl radical, MR
3.3.7 1-Phenyl tetraryl radical, AR
3.3.8 ‘live’ and ‘dead’ polymer chain moments
3.3.λ ‘live’ and ‘dead’ graft polymer chain moments
3.3.10 Reactor volume, V
3.4 Polymer Properties
3.4.1 Monomer conversion, X
3.4.2 Graft efficiency, GE
3.4.3 Number and weight – average molecular weight of free polymer
3.4.4 Number and weight – average molecular weight of graft polymer
3.5 Diffusion controlled reactions
Chapter 4 Results and Discussion
4.1 Polymerization of styrene inside cross-linked GTR particles
4.1.1 Characterization of GTR-g-PS particles
4.1.2 Conversion and GE results
4.1.3 Mn and Mw of free PS
4.1.4 Effect of GTR-g-PS particles on the mechanical properties, compatibility and morphology of GTR/PS blends
4.1.5 Effects of the number of extrusion passage on the mechanical properties, size and shape of the GTR-g-PS particles
4.2 Polymerization of styrene onto GTR particles
4.2.1 SEM micrographs of GTR-g-PS particles
4.2.2 Effect of GTR content on the polymerization of styrene onto GTR particles
4.2.3 Effect of initiator concentration on the polymerization of styrene onto GTR particles
4.2.4 Effect of BPO/DCP on the polymerization of styrene onto GTR particles .
4.2.5 Effect of reaction conditions on the glass transition temperature of GTR-g- PS particles
4.2.6 Effect of reaction conditions on thermal stability of GTR-g-PS particles
4.2.7 Effects of number of extrusion passage on the mechanical properties and size of the GTR-g PS particles
4.3 ANNs model and kinetic model
4.3.1 Experimental design
4.3.2 Results of ANNs model
4.3.3 Results of kinetic model
Chapter 5 Conclusions


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