Study of the Distribution of Rubber Particles in Ground Tire Rubber/Polypropylene Blends

Goal number 12 of the UN 2030 Agenda for Sustainable Development is sustainable consumption and productions. Recycling has a central role in assuring to meet the target in 2030. Indeed, only in EU, the annual total waste of spent tire is 3.7 million tons [1], this is why it is impelling to find a sustainable way to recycle this material. This article investigates the feasibility of the use of ground tire rubber (GTR) as feedstock for injection molding, by blending it with polypropylene. One of the applications is to create functional anisotropic surfaces due to the roughness introduced by rubber particles.
In order to study the possibility to use GTR rubber as feedstock for injection molding, different blends between ground tire rubber and polypropylene were investigated. In literature is possible to find examples of studies where GTR is used as filler in a thermoplastic matrix [2,3,4,5], in this work, instead, the content of GTR has been increased until 80wt%. In addition the GTR powder used for the blending has a particle size lower than 74 μm (200 mesh), in [2,4,5] the dimension of GTR powder is larger than 0,2 mm. By using a smaller particle size of rubber powder is possible to increase the surface energy of the single rubber particle, improving the overall dispersion of rubber in the matrix, as explained by Xun et al. in [3]. Moreover the use of GTR with small particle size is crucial in order to achieve desired optical properties by micro features on the surface.
The focus of this work is on the investigation of the distribution of GTR particles in a blend between ground tire rubber and polypropylene, because the mechanical property, the surface finishing and the optical functionality of GTR/PP blend are highly related to the distribution.
The GTR powder used was produced in a cryogenic grinding implant and subsequently mixed with polypropylene. Three different content of GTR were used:60wt%, 70wt% and 80wt%. The blending between PP and GTR was done with a tween screw extruder, the extruded material was than shredded and finally injection molded in dogbone specimens and in samples with specific micro-structured surfaces. The distribution of the GTR powders was visualized by SEM images at three crosssections of one sample. The cross sections were taken near the injection gate, in the center and far from the injection gate. The cross sections near the gate and far from the gate showed a uniform distribution between PP and GTR. This is not true for the cross section in the center of the specimen (see Figure 1) where it was possible to observe an accumulation of PP near the surface. EDS and sulfur combustion analyses verified the distribution.The roughness was measured, and correlated with GTR content and particle distribution, in specific positions for each sample: near the injection gate, in the center and far from the injection gate. From the roughness investigation it was possible to observe an increase in roughness when also the content of GTR in the blends was raised, meaning that the rubber powder contributes for the roughness variation of the blends. In addition, a remarkable variation of the roughness was notable throughout the samples, in particular the roughness is higher in the position far from the gate and lower in the center.The uneven distribution between PP and GTR, showed by the abovementioned analysis, influences the mechanical properties of the blends. Hardness test showed higher hardness in the center of the sample, where the content of GTR is lower, and lower hardness in the local position far from the gate, where GTR is more concentrated.