Ageing Behavior of Rubber Compounds Prepared with Different ZnO Types

Zinc oxide is considered as the most widely used activator that influences curing reaction kinetics and promotes short sulphide crosslinks to achieve higher crosslink density in rubber compounds. Besides its effect on curing process, it has beneficial effects on the physical and mechanical properties of rubber as well. However, its level should be minimized in rubber compounds because of its toxicity for human health and environment, especially on aquatic wildlife. One of the potential routes for decreasing ZnO level is to use composite ZnO materials where ZnO particles are coated. In this study, composite ZnO materials having ZnO:CaCO3 ratio as 40:60, 60:40 and 90:10 are used in SBR/BR compounds and their effects on mechanical properties under ageing conditions are investigated and compared with the conventional white seal ZnO and active ZnO types. The findings have shown that all composite materials have no negative effect on mechanical properties under ageing conditions, thus they can be used as alternative materials to conventional activators.


Introduction
Rubber is a linear polymer and has many widespread applications such as tires, gloves, footwears etc. due to its good elasticity, low hardness, high elongation at break and good resistance to corrosive chemicals [1,2].Although tires and other rubber products are technologically advanced products, the sustainability of their production, use, disposal, and reuse has become main issues in the frame of circular economy model [3].Rubber undergoes a curing process which is necessary for the majority of uses to transform sticky polymer into an elastic material [4].The tire industry has a standardized procedure for curing rubber with sulfur at high temperatures, namely vulcanization process, in order to improve its mechanical properties and retain its molded shape [5].One environmental concern in the vulcanization process is the use of zinc oxide, a crucial and widely used curing activator.It increases process efficiency by shortening reaction time, providing energy conservation, and cost reduction [6][7][8].Several studies reveal that zinc leaching occurs during the whole tire life and it leads to a potential environmental impact with toxicity and cytotoxicity for the aquatic organisms [9].Variety of aquatic organisms are affected by high zinc concentration which lead to poisonous reactions causing reproductive, developmental, and behavioral disorders [10,11].The United States Environmental Protection Agency (EPA) has set a limit for the maximum allowable concentration of dissolved zinc in water, based on the metal's toxicity to aquatic life [12].Thus, the release of zinc to nature should be controlled and it is an important issue to keep the amount of zinc in rubber compounds at low levels.
CaCO3 is a non-reinforcing filler that is widely used in rubber compounds to reduce cost.Composite ZnO materials are designed by coating ZnO on CaCO3 core material.Despite their lower ZnO content, they have high activity owing to their large surface areas and better dispersion provided by CaCO3 component.The main goal of this study is to explore the possibility of reducing the amount of ZnO used in rubber matrix by replacing conventional activators with composite ZnO, thus the usage of ZnO is reduced by 10-60 %.Trial compounds are prepared using these materials, their curing properties and mechanical performance upon ageing are studied.The results are compared with those of control compounds prepared by conventional activators to determine whether composite ZnO materials can be good alternatives to provide environmental and economic gains.

Materials
All mixing ingredients are used as received.Raw materials and their properties used in compound preparation are given in Table 1.The properties of the ZnO materials used in this study are given in Table 2. White seal zinc oxide (WS-ZnO), active zinc oxide (Ac-ZnO), and composite ZnO materials produced via wet process by coating ZnO particles (20-40 nm) on precipitated CaCO3 particles (50-200 nm) as core with ratios of 40:60 (C-ZnO40:60), 60:40 (C-ZnO60:40), and 90:10 (C-ZnO90:10) are used as activators.

Compound processing
An SBR/BR based tread compound recipe (Table 3) is adapted from literature [13] and prepared using an internal mixer with 1.2 L capacity.The produced compound is milled several times at 70 °C to achieve a thickness of 2.2 mm using a Farrel/6" laboratory mill and allowed to cool at room temperature.(1)

Mechanical properties
Heat generation of the compounds is measured with a flexometer according to ASTM D3182 standard by stretching the sample at constant load and constant compression.The change in sample length and the temperature increase at the base of the sample are recorded.The permanent deformation values of the samples are calculated by using equation (2) where to is the original thickness and tf is the final thickness after testing: Tensile test specimens are prepared and tested by Tensile Tester according to ASTM D412.Thermal ageing studies are conducted by keeping the samples in oven for 24, 48, 72, and 96 h at 100 ℃.Tensile strength, elongation at break, and modulus values (stress values at 100 % and 300 % strain) are reported for unaged and aged samples.Hardness tests are carried out by Shore A hardness instrument using the tensile sheets as stacked to proper thickness.

Evaluation of mechanical properties
The resistance to heat generation and resistance to permanent deformation of the compounds are determined by heat build-up and compression set data, respectively by flexometer tests and are given in Table 5.Although flexometer data appear to be comparable for the studied compounds, it can be noted that the trial compound TT4 shows similar performance to control compound TC1 whereas trial compounds TT6 and TT9 behave similar to control compound TC2.This can be attributed to the surface area values of the ZnO materials given in Table 2. Tensile strength (Figure 1), elongation at break (Figure 2), and modulus values at low and high strain (Figure 3) are reported for unaged and aged compounds as determined by tensile tests.Tensile data indicate that composite ZnO materials have no adverse effect on these properties; however, trial compound TT4 and control compound TC1 demonstrate slightly poorer ageing performance compared to the other compounds in tensile strength data.This trend is similar with the flexometer data and is also attributed to the lower surface area values of the ZnO materials used in TT4 and TC1.On the other hand, elongation at break data exhibit similar ageing characteristics for all studied compounds.Upon ageing, all compounds reveal increasing tendency in moduli and hardness (Figure 4) which can be explained by continuing crosslinking process and causing restriction of polymer chain mobility.All compounds have similar moduli and hardness values for all ageing times.

Conclusion
In this study, composite ZnO materials having ZnO:CaCO3 ratio as 40:60, 60:40 and 90:10 are used in SBR/BR compounds and their effects on curing behavior and mechanical properties (tensile strength, elongation at break, modulus, and hardness) under ageing conditions are investigated by comparing with white seal ZnO and active ZnO materials.It is observed that trial compounds have similar cure behavior as the control ones.Although all control and trial compounds have similar ageing behavior in terms of mechanical properties, those utilizing ZnO materials with lower surface area have slightly worse performance in terms of heat build-up, compression set, and tensile strength.Thus, it can be concluded that composite ZnO materials can be expected to be used as alternatives to conventional activators to provide economic and environmental gains.

Table 3 .
SBR/BR compound recipe given in parts per hundred of rubber (phr)

Figure 1 .
Figure 1.Change of tensile strength upon ageing at room temperature

Figure 2 .Figure 4 .
Figure 2. Change of elongation at break upon ageing at room temperature

Table 1 .
Properties of the raw materials used in the compounds

Table 2 .
Properties of the ZnO types used in the compounds a Total surface area of both coat and core material for composite ZnO types Cure characteristics of control and trial compounds are determined by Mooney viscometer instrument and Moving Die Rheometer (MDR, operating conditions: 25-200 °C and 1.667 Hz).The viscosity, scorch characteristics, and processability of the green compounds are tested at 130 °C according to ASTM D1646 by Mooney test.MDR is operated under isothermal test conditions with constant strain and frequency according to ASTM D5289, the tests are carried out at 160 °C, and 15 min.

Table 4
shows the results of Mooney and MDR tests.ML (1+4) data indicating the Mooney viscosity at 4 min test time after 1 min pre-heating and scorch times determined by both Mooney (t5) and MDR (ts2) tests are similar for all compounds.Cure times (t90), torque difference (MH-ML), and cure rate index (CRI) as determined by MDR data indicate that all control and trial compounds have similar cure properties.

Table 4 .
Cure properties of the compounds