Effect of Mixing Method on Properties of Ethylene Vinyl Acetate Copolymer/Natural Rubber Thermoplastic Vulcanizates

Abstract

Thermoplastic vulcanizate (TPV) has excellent elastomeric properties and can be reprocessed multiple times. TPV is typically produced by using the dynamic vulcanization (DV) method in which rubber is crosslinked simultaneously with thermoplastics. Peroxide-crosslinked TPV can increase the compatibility between rubber and thermoplastics but loses its reprocessability due to excess crosslinking in the latter. In this work, we overcome this obstacle by using a two-step mixing method to prepare fully crosslinked elastomers of ethylene vinyl acetate copolymer (EVA) and natural rubber (NR). Each sample formulation was prepared with three different mixing methods for comparison: NR-DV, Split-DV, and All-DV. For NR-DV, NR was crosslinked prior to the addition of EVA together with the thermal stabilizer (TS). For Split-DV, a small amount of EVA and NR was crosslinked prior to the addition of EVA and TS. In the All-DV method, EVA and NR were crosslinked, and then TS was added. The appearance and processability of the samples were affected by the degree of crosslinking. NR-DV showed a non-homogeneous texture. Although the samples of the All-DV method appeared homogeneous, their mechanical and rheological properties were inferior to those of the Split-DV method. The mechanical properties of the Split-DV samples were not significantly changed after reprocessing 10 times. Therefore, Split-DV is the preferred method for TPV production.

Keywords: capillary rheometer; dynamic vulcanization; ethylene vinyl acetate copolymer; mixing method; natural rubber; thermoplastic vulcanizate.

Figure A1

Test of tension set.

Scheme 1

Schematic diagram of mixing methods: (a) NR-DV (b) Split-DV, (c) All-DV.

Scheme 2

Schematic diagram of reprocessed cycle of thermoplastic vulcanizate (TPV) in this research (n is the number of cycles).

Figure 1

Optical appearance of compression molded TPV samples.

Figure 2

Fractured surfaces of ethylene vinyl acetate copolymer (EVA)/natural rubber (NR) TPVs with magnification of 2,000 and 5,000 (a) sTPV 2000×, (b) sTPV, 5000×, (c) aTPV 2000× and (d) aTPV 5000×. sTPV: TPVs prepared by Split-DV, aTPV: TPVs prepared by All-DV.

Figure 3

Differential scanning calorimetry (DSC) thermograms of uncured and cured EVA, NR, as well as EVA/NR TPVs: the second heating scan (a) and cooling scan (b) at heating/cooling rate of 10 °C/min.

Figure 4

Tensile storage and loss moduli of uncured EVA, uncured NR, cured EVA, cured NR, and EVA/NR TPVs samples prepared with different methods.

Figure 5

Steady-state shear viscosity of uncured EVA and EVA/NR TPVs as a function of shear rate at 190 °C from a capillary rheometer.

Figure 6

Stress–strain curves of uncured EVA, cured EVA, cured NR, and EVA/NR TPVs at (a) overall (b) 0–200% strain.

Figure 6

Stress–strain curves of uncured EVA, cured EVA, cured NR, and EVA/NR TPVs at (a) overall (b) 0–200% strain.

Figure 7

Stress–strain curves of sTPV that was reprocessed 0, 5, and 10 times (a) overall (b) 0–200% strain.