Thermoplastic natural rubber based on linear-low-density polyethylene

Thermoplastic natural rubber (TPNR) is one of the alternative natural rubber (NR) products obtained by blending NR with any compatible thermoplastics, e.g., high-density polyethylene (HDPE), low-density polyethylene (LDPE), linear-low-density polyethylene (LLDPE), polystyrene (PS), polylactic acid (PLA) at the temperature above the melting point of plastics, which is always above the softening point of NR. Unlike vulcanized NR, TPNR is recyclable and convertible to the final articles using the existing plastic technologies, including extrusion and injection molding. TPNR materials based on NR/LLDPE blends filled with two types of inorganic fillers, i.e., titanium dioxide and silica, were prepared and their properties were discussed in the current work. Silica provided a better reinforcing effect to the NR/LLDPE blends, whereas the blends filled with titanium dioxide exhibited excellent UVA and weather resistance. The resulting NR/LLDPE blends filled with inorganic fillers can be injection molded to form the products with desired shapes.


Introduction
Natural rubber (NR) is a biobased polymeric material from the Para rubber tree (Hevea brasiliensis). NR has high molecular weight and exhibits excellent water resistance, but low grease resistance. Furthermore, NR is sensitive to oxidation because of unsaturated double bonds; metal, light, and heat can accelerate the oxidation of NR as well. Consequently, NR properties may be changed over time during outdoor uses.
Thermoplastic natural rubber (TPNR) is a kind of thermoplastic elastomer, which is one of the alternative NR products obtained by blending NR with any compatible thermoplastics, e.g., highdensity polyethylene (HDPE) [1], low-density polyethylene (LDPE) [2], linear-low-density polyethylene (LLDPE) [3], polystyrene (PS) [4], polylactic acid (PLA) [5] at the temperature above the melting point of plastics, which is always above the softening point of NR. Unlike vulcanized NR, TPNR is recyclable and convertible to the final articles using the existing plastic technologies, including extrusion and injection molding.
Herein, TPNR materials based on NR/LLDPE blends filled with two types of inorganic fillers, i.e., titanium dioxide and silica were prepared and their properties, including mechanical, morphological, and QUV and weather-resistant ones were discussed.

Characterization and property testing of NR/LLDPE blends filled with TiO2 and/or SiO2
The tensile property was tested according to ISO37:2011 using an Instron universal testing machine (Instron, USA) with a test speed of 500 mm/min and a gauge length of 30 mm. Tensile strength, Young's modulus, and elongation at break of each sample were reported as averages from the measurement of three specimens. Shore A hardness of the sample was tested according to the ASTM D2240-05 using a GS-719N Durometer (Teclock, Japan).
Morphological characteristics at tensile fractured surfaces of the samples were examined using a JSM-6610 series scanning electron microscope (SEM, JEOL, USA) at an acceleration voltage of 10 kV. The SEM micrographs were taken at a magnification of 500. The sample surfaces were coated with a thin layer of gold to prevent charging before observation.
Ozone resistance test was performed according to ISO1431-1:2004. Samples were stretched to 120% of their original length and then exposed to the ozone with a concentration of 50 pphm at 40°C for 7 h. The crack of the samples was then observed.
QUV and weathering resistance determination was carried out according to ASTM G154-16. The sample specimens were exposed to UVA at 60°C for 8 h and punctuated with 50°C of water vapor for 4 h for the overall test period of 168 h using an Accelerated Weathering Tester (QUV/spray, Q-LAB, USA). The color of each specimen was then examined using a spectrophotometer (Miniscan EZ, HunterLab, USA) and the change in color as compared with the unexposed specimen was observed.
The results were statistically analyzed using IBM SPSS Statistics Version 20 (Statsoft, Oklahoma), with analysis of variance (ANOVA) and Duncan's test at significant differences at 5% level.

Results and discussion
The tensile properties and hardness of all samples were shown in Figure 1. Although tensile properties and hardness of NR/LLDPE blend and NR/LLDPE/inorganic filler composites were insignificantly different, tensile strength, Young's modulus, and hardness of the composites tended to increase with increasing inorganic filler concentration. It should be pointed that elongation at break of the composites was lower than that of the blend and hardly changed as a function of inorganic fillers concentrations. The results implied that the composites became more rigid due to the reinforcing effect.
SEM images at tensile fracture surfaces also supported the above mechanical property results as demonstrated in Figure 2. NR/LLDPE blend shows rough tensile fractured surface, reflecting extensible IOP Publishing doi:10.1088/1757-899X/1234/1/012008 3 material. However, the roughness of tensile fractured surfaces slightly decreased when inorganic fillers were added, corresponding to stiffer material. The reduction of surface roughness was more outstanding in the composite filled with SiO2, particularly with increasing SiO2 concentration.  QUV and weathering resistance properties of the blend and composites were evaluated by the difference in CIELAB color space value before and after exposure to the UVA, water vapor, and heat. The smaller difference indicates the better resistances. Figure 3A-C shows that the difference in CIELAB color space (L * , a * , and b * ) values of the composites filled with TiO2 was lower than those of the blend and the composites filled with SiO2, suggesting that TiO2 provided the best resistance against UVA and weather to the NR/LLDPE blend. The result in Figure 3D was also in good agreement with the CIELAB color space as the composites filled with TiO2 possessed a higher Grey Scale value than others, reflecting less change in color after UVA and water vapor exposure.

Conclusions
TPNR materials based on LLDPE were successfully prepared. Two types of inorganic fillers, i.e., TiO2 and SiO2, were incorporated. The obtained NR/LLDPE composites filled with inorganic additives showed slightly increased tensile strength, Young's modulus, and hardness, which were in good agreement with the morphology observed by SEM. SiO2 provided the composites with better mechanical properties, whereas TiO2 improved their QUV and weather resistance.