Improving cut/chip/chunk resistance in truck tyres by the use of para-amid chopped fibres

Transcription

1 Gummi Fasern Kunststoffe, No., 00, pp Improving cut/chip/chunk resistance in truck tyres by the use of para-amid chopped fibres R.N. Datta Teijin Twaron BV, Arnhem, Netherlands Selected from International Polymer Science and Technology, 1, No. 10, 00, reference GK 0/0/109; transl. serial no. 16 Translation submitted by C. Hinchliffe SUMMARY The tyre industry is continually striving to improve the cut/chip/chunk resistance of tyre treads while simultaneously maintaining a high level of product quality and product performance. Several approaches have been explored in which the fillers are varied, the cure package is optimised or polymers and their blend ratios are changed, but although each approach provides improvements with regard to cut/chip/chunk resistance, this is at the expense of hysteresis, heat buildup and fatigue properties. With its complete portfolio of aramid fibres, p-aramid (Twaron and Technora) and m-aramid (Teijin Conex), the company Teijin Twaron is the world leader in the field of chopped fibre rubber reinforcement. These chopped fibres provide rubber compounders with a potent tool for expanding the performance of rubber products beyond their current horizons 1. INTRODUCTION Although cut/chip/chunk damage to tyre treads is mostly associated with off-road vehicles, this phenomenon has also been observed in tyres for trucks which predominantly travel on country roads and in all-season tyres for cars. Cut/chip/chunk damage to the tread rubber caused by repeated contact with sharp stones, kerbs etc shortens the lifetime of a tyre. There have been numerous attempts to improve cut/ chip/chunk resistance all of them produced positive results as far as cut/chip/chunk resistance is concerned, but this was always at the expense of other important tyre properties such as hysteresis, heat buildup etc. Elastomers reinforced with continuous fibres are well known [1-], but composites of this kind are mainly restricted to special areas of tyres (for example, the casing, the belt or the cap ply) and drive belts and tubes. More complicated structures such as the tread or subtread cannot be produced from an elastomer reinforced with continuous fibres. This paper deals with the use of chopped aramid fibres in typical compounds for truck tyres with the objective of improving cut/chip/chunk resistance together with hysteresis, heat buildup and other dynamic properties. It will present the results of tyre tests in order to confirm the advantages observed during laboratory tests.. EXPERIMENTAL The mixtures were cured with TBBS and sulfur. Zinc oxide, stearic acid, carbon black, aromatic oil and antioxidants were added during the first mixing stage in a Banbury kneader. Poly-(paraphenylene terephthalamide) (PPTA), poly-(metaphenylene isophthalamide) (MPIA) and co-poly- (paraphenylene/, -oxydiphenylene terephthalamide) (PP/ODPTA) (Figure 1) were also introduced into the Banbury kneader during the mixing. Table 1 is a comparison of the physical properties of the fibres used. Tables and show the formulations investigated..1 Compounding and Curing Premixes of the different formulations were produced in a Banbury kneader using a standard procedure. The required quantity of fibres was added to the premix, again in the Banbury, and sufficient time allowed for the dispersion of the fibres in the matrix. The curing T/10 International Polymer Science and Technology, Vol., No. 6, 00

2 Figure 1. Structures of poly-(paraphenylene terephthalamide) (PPTA, Twaron), poly-(metaphenylene isophthalamide) (MPIA, Conex) co-poly-(paraphenylene/, -oxydiphenylene terephthalamide) (PP/ODPTA, Technora) Table 1. Comparison of the physical properties of the aramid fibres Fibres Modulus of elasticity, GPa Tensile strength Elangation at break, % PPTA, Twaron MPIA, Conex PP/ODPTA, Technora Table. Formulations Mix components 1 NR SMR CV BR Buna CB TOR Vestenamer Carbon black N Carbon black N-660 Aromatic oil Zinc oxide Stearic acid Antioxidant 6PPD mm mm PP ODPTA CF mm PP/ODPTA DCF mm Accelerator, TBBS Sulfur..... Note: CF stands for chopped fi bres, DCF for dipped chopped fi bres International Polymer Science and Technology, Vol., No. 6, 00 T/11

3 Table. Optimisation of formulations by reducing the amount of carbon black Mix components NR SMR CV BR Buna CB TOR Vestenamer Carbon black N Carbon black N-660 Aromatic oil Zinc oxide Stearic acid Antioxidant 6PPD mm mm PP ODPTA CF mm PP/ODPTA DCF mm Accelerator, TBBS Sulfur..... agents were added on a roll mill. The direction of rolling and the rotational speed were kept constant in order to achieve a maximum degree of orientation of the fibres. Finally, test plates were cut and cured in a compression moulding process at 160 C for the time t90.. Processing Properties The processing and vulcanisation properties of the compounds were determined using a Mooney viscosimeter (MV 000EA) or an oscillating-chamber rheometer.. Tests The tensile stress properties of the vulcanisates were determined using a Zwick 1 universal testing machine according to ISO 7. Other physical test methods used were: Hardness ISO 8 Tear propagation resistance (crescent test pieces with a 1 mm incision) ISO :199 DIN abrasion DIN 16 The fatigue properties were determined using a Monsanto testing device under deformation with a constant energy of. x 10 kjm -. Cut/chip/ chunk resistance was determined using a homemade cut, chip and chunk tester. The experiments were performed with a tyre running temperature of 70 C with an ageing duration of two days at 100 C. Dynamic and mechanical tests were performed on a Metrovib viscoanalyser which was operated at 60 C and 1 Hz with a DSA value of 1%. The evaluation of the vulcanisate structure was performed by means of equilibrium swelling in toluene using the procedure described by Ellis and Welding []. The Vr value determined in this was converted into the elastic Mooney-Rivlin constant (C1) and finally into the concentration of chemical crosslinks using the equations quoted in literature [, 6]. The content of mono-, di- and polysulfidic crosslinks were determined using the method described elsewhere [7, 8].. Test Bench and Road Tests The test bench tests were performed on tyres which had been retreaded with the test formulations containing PP/ODPTA from Technora DCF. The formulation corresponded to the formulation with. phr PP/ODPTA described in the previous section. We also produced a second series of retreaded tyres containing a tread formulation for use as a control. The dimensions of the new tyres were 9/R., Michelin XZA. The measurements of heat buildup, kilometrage and rolling resistance were performed in the factory of a renowned tyre manufacturer. The tests to determine rolling resistance and heat buildup were performed on a drum test bench with a diameter of 1.71 m. Rolling resistance T/1 International Polymer Science and Technology, Vol., No. 6, 00

4 was determined from the consumption of energy of the tyres exposed to 0% of the permissible load at 0, 60, 80 and 100 km/h. The investigations into heat buildup were performed under exposure to 80% of the permissible load at 110 km/h. The temperature was measured using a pyrometer with a needle sensor that was inserted at six pre-specified points into the shoulder (four points) and base of the subtread in the central grooves (crown). The endurance tests were performed at 100 km/h and at 80% of the permissible load until the tyres burst immediately after the last measurements of heat buildup at 110 km/h. For the road tests, ten heavy trucks were fitted with the tyres to be tested. Each truck was given two reference tyres and two test tyres in the drive position. The road tests were performed with vehicles which were primarily used for long journeys on trunk roads and main roads. The road tests were performed from February to November. Measurements of the depth of the tread profile were performed in the internal, two middle and external profile grooves using a vernier slide gauge. The abrasion losses from all tyres were recorded.. RESULTS AND DISCUSSION.1 Impact of Poly-(metaphenylene isophthalamide) (MPIA, Conex) and copoly-(paraphenylene/, -oxydiphenylene terephthalamide) (PP/ODPTA, Technora) As expected, the addition of MPIA or PP/ODPTA (in the form of both chopped fibres and dipped chopped fibres) increased the maximum rheometer torque without impairing the scorch time, vulcanisation time or Mooney viscosity. Table shows the values for the processing characteristics and the vulcanising characteristics. Figure is a typical rheogram depicting the impact of MPIA and Technora chopped fibres on the vulcanisation characteristics. The use of chopped fibres in a typical formulation for a truck tread (Table ) increases the hardness and modulus without any clear impacts on the values for tensile strength, elongation at break and tear propagation resistance. The corresponding numerical values are shown in Table and Table. Figure demonstrates that the addition of these chopped fibres has a slightly positive impact on abrasion loss. Figure. Vulcanisation characteristics determined at 160 C for compounds 1 to (lowest curve: control, rest: + chopped fibres) Table. Processing and vulcanisation properties Properties 1 Processing properties Mooney viscosities ML(1+) at 100 C Rheometer vulcanisation at 160 C Minimum torque, Nm Maximum torque, Nm Ts, min T90, min International Polymer Science and Technology, Vol., No. 6, 00 T/1

5 Table. Physical properties (vulcanisation corresponding to t90 at 160 C) Properties 1 Hardness IRHD Modulus 0%, MPa Modulus 100%, MPa Tensile strength, MPa Elongation at break, % Figure. Impacts on tear propagation resistance and abrasion loss Table 6. Heat buildup on the Goodrich flexometer at 100 C (load: 11 kg, amplitude:. mm, duration: h) Properties 1 Temperature increase, C Compression set, % A clear impact is observed with the values obtained for heat buildup. Both MPIA (Conex) and PP/ODPTA (Technora) improve heat buildup on exposure to a dynamic load. The numerical values are shown in Table 6.. Optimisation of Modulus/Hardness by the Partial Replacement of Carbon Black with Aramid Chopped Fibres Generally, reducing the carbon black content results in a reduction in both the hardness and modulus values and, below a certain critical level, could have a negative impact on abrasion resistance and elongation properties. As discussed above, the addition of aramid fibres results in an increase in both the modulus and hardness values and hence has a positive impact on the abrasion properties. If the values for hardness and modulus are different, this results in complications when comparing systems and for this reason it was our objective to replace some of the carbon black by chopped fibres to make it possible to retain the hardness and the modulus. This will facilitate an effective comparison of the dynamic properties. The formulation used in this optimisation is shown in Table (compounds 6-10). The quantity of chopped fibres is selected to be equal to 1 phr based on the model for our optimisation study. As expected, the replacement of phr carbon black by 1 phr of either chopped fibres (MPIA) or dipped chopped fibres (PP/ODPTA) essentially results in the retention of torque, scorch time and vulcanisation time. The corresponding values are shown in Table 7. The processing characteristics shown in Table 7 (Mooney viscosity) are virtually unchanged. The physical properties of the vulcanisates are shown in Table 8. This table reveals that there is no change in the hardness or elongation properties as the result of the optimisation involving the replacement of carbon black by chopped fibres. Chopped fibres have an only scarcely perceivable impact on tear propagation resistance and abrasion characteristics. These effects are shown separately in Figure. T/1 International Polymer Science and Technology, Vol., No. 6, 00

6 Table 7. Processing and vulcanisation properties Properties/compounds Processing properties Mooney viscosity ML(1+) at 100 C Rheometer vulcanisation at 160 Minimum torque, Nm Maximum torque, Nm Ts, min T90, min Table 8. Physical properties (vulcanisation corresponding to t90 at 160 C) Properties Hardness IRHD Modulus 0%, MPa Modulus 100%, MPa Tensile strength, MPa Elongation at break, % Figure. Impacts on tear propagation resistance and abrasion resistance The most obvious impacts of the use of aramid-based chopped fibres are in the hysteresis as manifested both in the heat buildup and the viscoelastic properties. The decisive measurements are shown in Tables 9 and 10. In this context, it is important to stress that the reduction in the delta tangent as a result of the use of dipped PP/ODPTA (Technora) chopped fibres is up to > 0. It is to be expected that this should significantly decrease the rolling resistance of truck tyres. The use of PP/ODPTA (Technora DCF, Compound No 10) in a modified tread formulation increases both the dynamic fatigue resistance in a test to determine the crack growth and the fatigue to rupture measured with a constant energy. Figures and 6 clearly demonstrate the benefit of chopped fibres obtained from Technora. In order to test cut growth and fatigue to rupture, samples of the control material and compound with PP/ODPTA (Technora DCF, Compound 10) were tested on a home-made apparatus to determine their cut/chip/ chunk resistance. The impact (of the added chopped fibres) shown in Figure 7 is remarkable. The addition of 1 phr PP/ODPTA (Technora DCF) extended the fatigue to rupture from 1 to 18 days and increased the rating from to. Finally, we performed a structural evaluation of the vulcanisates to enable us to establish a correlation between the vulcanisate properties and the improvement in the fine structure of the network. The results may be seen in Table 11. This table clearly shows that aramidbased chopped fibres are able to improve the network. International Polymer Science and Technology, Vol., No. 6, 00 T/1

7 Table 9. Heat buildup on the Goodrich flexometer at 100 C (load: 11 kg, amplitude:. mm, duration: h) Properties Temperature increase, C Compression set, % Table 10. Viscoelastic properties (temperature: 60 C; deformation amplitude: 1%, frequency: 1 Hz) Properties/compounds Storage modulus E, MPa Loss modulus E, MPa Delta tangent (-%) Loss of compliance MPa Figure. De mattia cut growth Figure 6. Fatigue to rupture at constant energy TRANSLATION Figure 7. Cut/chip/chunk resistance (test temperature: 70 C, ageing temperature: 100 C/d, 0 stress cycles) As a result, with the same vulcanisation times, some of the polysulfide bonds are converted into monosulfide bonds. This leads one to suspect that aramid chopped fibres such as (poly-(paraphenylene terephthalamide) poly- (metaphenylene isophthalamide) (MPIA) and co-poly- (paraphenylene/, -oxydiphenylene terephthalamide) (PP/ODPTA) are able to participate in the vulcanisation mechanism and hence help to improve the vulcanisate properties. T/16 International Polymer Science and Technology, Vol., No. 6, 00

8 Table 11. Distribution of different types of crosslink Properties 1 Total crosslinks Polysulfi de crosslinks Disulfi de crosslinks Monosulfi de crosslinks The number of crosslinks is expressed in gram-mol per gram of hydrocarbon rubber x 10. Results of the Tyre Tests The abrasion behaviour of the treads tested on the drive axles is shown in Figure 8. This clearly shows that the addition of PP/ODPTA (Technora DCF) significantly improves the lifetime of the tread as manifested in the tread abrasion. In addition to the improvement to the tread abrasion, a comparison of the abrasion pattern of the worn treads identified significant differences. Cuts, chips and chunks were identified in 90% of the reference tyres, while the experimental tyres had a smooth tread surface. This is clearly depicted in Figure 9. Figure 8. Results of tread abrasion tests Figure 9. Results of the tread abrasion tests Measurements of the heat buildup, the endurance and the rolling resistance were performed in the factory of a famous tyre manufacturer. The results are listed in Tables 1, 1 and 1. The heat buildup was measured in both the shoulder and the crown part of the tyres after a period of 1 hours during which the test speed was increased gradually from 0 to 110 km/h. The tyres were run at 80% of the permissible load. A clear reduction in the heat buildup of up to > 1 was identified in the tyres with the tread containing PP/ODPTA (Technora DCF). This reduction in the operating temperature resulted in both increased endurance and reduced rolling resistance. International Polymer Science and Technology, Vol., No. 6, 00 T/17

9 Table 1. Tyre running temperature (test bench) +PP/ODPTA, Technora, DCF Shoulder temperature, C Temperature in the crown, C 19 1 Table 1. Endurance test on the tyres +PP/ODPTA, Technora, DCF Increase % Endurance time achieved, h Distance achieved, km Table 1. Tyre rolling resistance +PP/ODPTA, Technora, DCF Decrease % 0 km/h KW km/h KW km/h KW km/h KW 7..6 The endurance was determined from continuous operation at 110 km/h at 80% of the permissible load. The tyres containing PP/ODPTA (Technora DCF) were identified as undergoing a reduction in rolling resistance of 1 to % throughout the entire range of test speeds. A reduction in the rolling resistance of this degree can lead to fuel savings of about -%.. CONCLUSIONS Small additions of chopped fi bres obtained from poly-(paraphenylene terephthalamide) (PPTA), poly- (metaphenylene isophthalamide) (MPIA) and co-poly- (paraphenylene/, -oxydiphenylene terephthalamide) (PP/ODPTA) enable material modifi cations to be obtained which result in better values for cut/chip/ chunk resistance, dynamic fatigue resistance, abrasion resistance, hysteresis and heat buildup in tyres. These material modifications consist in the use of lower carbon black quantities with the incorporation of chopped fibres into the formulation. In addition to improving the cut/chip/chunk resistance, the changes also resulted in a significant reduction in the rolling resistance and hence made an important contribution to savings in fuel consumption. REFERENCES 1. T.S. Solomen, Rubber Chem. Technol. 8, 61 (198). B.C. Begnoche, R.L. Keefe and A.G. Causa, Rubber Chem. Technol., 60, 689 (1987). R.L. Keefe, Jr., Rubber Chem. Technol., 8, 8 (198). E. Ellis and G.N. Welding, Rubber Chem. Technol. 7, 71 (196). P.J. Flory and R.J. Rehner, J. Chem. Phy. 11, 1 (19) 6. L. Mullins, J. Appl. Poly. Sci..1 (199) 7. B. Saville and A.A. Watson, Rubber Chem. Technol., 0, 100 (1967) 8. A.H.M Schotman, P.J.C. van Haeren, A.J.M. Weber, F.G.H. van Wijk, J.W. Hofstraat, A.G. Talma, A. Steenbergen and R.N. Datta, Rubber Chem. Technol. 69, 77 (1996) 6. ACKNOWLEDGEMENT The authors wish to thank the management of Teijin Twaron BV for permitting the publication of this work. (No date given) T/18 International Polymer Science and Technology, Vol., No. 6, 00