(78c) Studying the Pulverization Mechanism of Low Density Cross-Linked Natural Rubber

Pulverization is an important process in industry to reduce the size of solid material. Small particle size improves the homogeneity of the feed, temperature distribution among materials during the solid process or molding, the efficient active sites for physical and chemical phenomena, dissolving rate and mixing quality. Because of the high molecular weight and low thermal conductivity of polymeric material, using small particles rather than polymeric granules provides more flexibility in polymer processes. Three basic technologies are used to produce the polymer powders:

Suspension or emulsion polymerization Precipitation of powder from dilute polymer solutions Mechanical grinding of solid polymers [1].

The first and second technologies are used to obtain virgin polymer powder [1], although removing media, emulsifier and organic solvents are costly processes and usually are not entirely successful (especially emulsifier removal). Mechanical grinding can be used for both virgin and waste polymers [2]. Mechanical grinding makes it the most common pulverization practice because of its large-scale capacity [1]. The fundamental idea in the mechanical size reduction technique is applying stresses to the solid material sufficient to break it into small pieces. The energy dissipation in pulverization processes is large. It is believed that most of the lost energy is due to the equipment rather than to pulverizing the material [2,3,4,5]. The proper grinding method for material is chosen based on the material mechanical properties, their desired size, and the energy source convenience for the process. In general, the various techniques fall into these categories: crushing, impacting, cutting, and exploding [3].

One of the mechanical methods for pulverizing material simultaneously uses high pressure and shear force. This idea has been inspired by Bridgman, who established an apparatus with two disks that could pulverize metals by applying hydrostatic pressure to the sample placed between the disks [6]. Enikolopian extended the idea of the high pressure and shear force to pulverize polymers by using an extrusion process [4]. Further on, the Bridgman anvil was used in the Center of Excellence in Polymer Science and Engineering (CEPSE) at Illinois Institute of Technology (IIT) to study the quality of the produced cross-linked polymer particles [7]. Although the mechanism of pulverization by Solid State Shear Extrusion (SSSE) has not been completely understood, it has been used in the last few years for blending non-cross-linked polymers [8-10], recycling both non-cross-linked and cross-linked polymers [11] and pulverizing them as a feed for a second process [12-13]. This process for blending polymers is called Solid State Shear Pulverization, \textrm{$S^{3}P$} [14]. The output particles in a pulverization process must be reproducible and the change of the particle average size should be less than $3 \%$ [15].

The output particle can be used as filler in polymer composites, or in a mix with virgin polymers in asphalt [16]. Recently a method for producing high value material such as Interpenetrating Polymer Network (IPN) from the recycled rubber powder has been proposed [13].

In this work, Solid State Shear Extrusion (SSSE) as a mechanical size reduction technique is applied to waste low-cross-link-density natural rubber. The Particle Size Distribution (PSD) of the produced particles is analyzed. The first goal of this analysis is to optimize the process conditions to obtain desirable output PSD (desired average particle size, or a narrow PSD), and the second goal is to find a relation between the process conditions and the output PSD. The output PSD shows a non-monotonic behavior. Based on these results and previous work on the SSSE process using polyethylene and polystyrene [3,7,17], it can be concluded that the pulverization mechanism depends on the material structure, the distribution of the dispersed phase in matrix (in case of the filled polymer or blend polymers) and the nature of the interactions between components. Because rubber is the material that is subjected to the SSSE process in this work, it is focused on the structure of rubber, its filler, particularly carbon black (CB). The PSD of the produced particles from two more rubber samples with the same compositions as waste-rubber: one with CB and the other one without have been studied.

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