49
This correlation seems to be a promising approach to tailor and improve the tensile
properties of PU/waste rubber composites.
Recently, Strakšys et al. (2018) studied the impact of the shape and size of the
waste rubber particles, the adhesive content and the curing conditions on the PU/
GTR composites. The results showed that the tensile and compressive properties of
the composites were more dependent on the adhesive content than on the curing
conditions. The characteristics of rubber particles were important for the composites studied because they affect the mechanism of void formation, thus wetting of
rubber particles with the adhesive. As a result, structure of these particles has a
significant impact on the mechanical properties of the material.
Some research groups, for a better understanding of the mechanisms of formation of holes during sintering, have investigated the morphology, shape parameters
and compression behavior of GTR (Ferrer-Giménez et al. 2009; Nadal et al. 2016).
Due to the high elasticity of rubber, the main technical problem for the industrial
application of reactive sintering has been related to the high pressure necessary for
efficient compression of crosslinked rubber particles. This problem has been solved
in the Sustainable Moulding of Articles from Recycled Tires (SMART) project
financed by the European Commission. The aim of the SMART project was to
develop a new GTR moulding process without additives or binders (Quadrini
et al. 2013).
This technology allows to produce large-size tiles (1 m
2
) with a thickness of up
to 50 mm, characterized by a tensile strength of ~0.6 MPa, an elongation at break of
~31% and a hardness of ~66 °ShA (Quadrini et al. 2019). Quadrini et al. (2019) also
presented calculations that have clearly shown that the direct reactive sintering of
GTR is ~50% cheaper compared to the conventional process with the application of
PU adhesive as a binder.
Another solution to eliminate the high pressure problems necessary for GTR
compression is the application of smaller particles (Gugliemotti et al. 2012). In this
sense, Shen et al. (2019) studied the effects of grinding conditions (ambient/cryogenic) and GTR particle size (in the range of 75–350 μm) for reactive sintering in
the presence of sulfur curing system (N-tert-butyl 2-benzothiazole sulfenamide
(TBBS) - 0.8 phr and sulfur 1.2 phr). It was observed that the tensile strength of the
rubber sheets increased for GTR with a smaller size and a more developed particle
surface. On the other hand, the result showed that the moulding pressure had no
effect on density, tensile strength or elongation at break of the materials obtained.
However, it should be noted that reducing the particle size increased the energy
consumption of the grounding process, which obviously affects the final
costs of GTR.
Prut et al. (2015) indicated that the addition of up to 2 phr of sulfur had a beneficial impact on the tensile properties of the revulcanized ethylene-propylene-diene
monomer, while a higher content of sulfur resulted in deterioration of the mechanical properties.
For a better understanding of the current progress in reactive sintering of rubber
waste. A summary and a comparison of the performance properties of the products
prepared by different research groups is shown in Table 4.1. The data presented
4 Reactive Processing and Functionalization of Ground Tire Rubber
This correlation seems to be a promising approach to tailor and improve the tensile
properties of PU/waste rubber composites.
Recently, Strakšys et al. (2018) studied the impact of the shape and size of the
waste rubber particles, the adhesive content and the curing conditions on the PU/
GTR composites. The results showed that the tensile and compressive properties of
the composites were more dependent on the adhesive content than on the curing
conditions. The characteristics of rubber particles were important for the composites studied because they affect the mechanism of void formation, thus wetting of
rubber particles with the adhesive. As a result, structure of these particles has a
significant impact on the mechanical properties of the material.
Some research groups, for a better understanding of the mechanisms of formation of holes during sintering, have investigated the morphology, shape parameters
and compression behavior of GTR (Ferrer-Giménez et al. 2009; Nadal et al. 2016).
Due to the high elasticity of rubber, the main technical problem for the industrial
application of reactive sintering has been related to the high pressure necessary for
efficient compression of crosslinked rubber particles. This problem has been solved
in the Sustainable Moulding of Articles from Recycled Tires (SMART) project
financed by the European Commission. The aim of the SMART project was to
develop a new GTR moulding process without additives or binders (Quadrini
et al. 2013).
This technology allows to produce large-size tiles (1 m
2
) with a thickness of up
to 50 mm, characterized by a tensile strength of ~0.6 MPa, an elongation at break of
~31% and a hardness of ~66 °ShA (Quadrini et al. 2019). Quadrini et al. (2019) also
presented calculations that have clearly shown that the direct reactive sintering of
GTR is ~50% cheaper compared to the conventional process with the application of
PU adhesive as a binder.
Another solution to eliminate the high pressure problems necessary for GTR
compression is the application of smaller particles (Gugliemotti et al. 2012). In this
sense, Shen et al. (2019) studied the effects of grinding conditions (ambient/cryogenic) and GTR particle size (in the range of 75–350 μm) for reactive sintering in
the presence of sulfur curing system (N-tert-butyl 2-benzothiazole sulfenamide
(TBBS) - 0.8 phr and sulfur 1.2 phr). It was observed that the tensile strength of the
rubber sheets increased for GTR with a smaller size and a more developed particle
surface. On the other hand, the result showed that the moulding pressure had no
effect on density, tensile strength or elongation at break of the materials obtained.
However, it should be noted that reducing the particle size increased the energy
consumption of the grounding process, which obviously affects the final
costs of GTR.
Prut et al. (2015) indicated that the addition of up to 2 phr of sulfur had a beneficial impact on the tensile properties of the revulcanized ethylene-propylene-diene
monomer, while a higher content of sulfur resulted in deterioration of the mechanical properties.
For a better understanding of the current progress in reactive sintering of rubber
waste. A summary and a comparison of the performance properties of the products
prepared by different research groups is shown in Table 4.1. The data presented
4 Reactive Processing and Functionalization of Ground Tire Rubber