Slips and falls on icy surfaces can cause serious injuries of people. The primary risk factor for slipping incidents is undoubtedly the decreased friction coefficient between the shoe sole and the ice or snow surface. Nowadays environmental protection has been gaining significance and becoming highly important for the various innovation strategies. In rubber industry the concept of environmental protection is more often associated with the maximum use of elastomers and ingredients from renewable sources in the manufacture of rubber products. The aim of this work is to investigate the possibilities of using elastomers and ingredients from renewable sources—epoxidized natural rubber, silica obtained by rice husks incineration and microcrystalline cellulose—as fillers and rapeseed oil as a process additive in compositions, intended for the manufacture of soles for winter footwear having an increased coefficient of friction to icy surfaces. The tribological tests based on the coefficient of friction evaluated the adhesion of the composites to the icy surfaces at different temperatures. The complex evaluation of developed composites revealed those containing microcrystalline cellulose and biogenic amorphous silica at a 1:1 ratio as the most suitable for making footwear soles because of the best combination of physicо-mechanical properties and coefficient of friction.
Slips and falls are quite common on icy surfaces and can cause serious injuries. It is believed that slipping accounts for 16% of all incidents and 43% of all fall incidents in the Scandinavian countries [
Derler et al. [
Patent literature describes a number of compositions of rubber compounds with enhanced traction on ice and snow which comprise: 5-norbornene at 5 ÷ 100 phr [
Nowadays the use of carbon black in rubber industry is restricted since it has been classified as a Group 2B carcinogenic agent, which is possibly carcinogenic to human beings [
For instance, a number of authors have reported improved mechanical properties, resistance to heat aging, processability, heat buildup and dynamic mechanical properties of the vulcanizates when using microcrystalline cellulose as a filler [
In the same time it is known that the chemical modification of natural rubber latex via epoxidation yields epoxidised natural rubber (ENR) that has improved properties depending on the degree of epoxidation [
Taking into account the above mentioned the aim of the work is to investigate the possibilities of using elastomers (epoxidized natural rubber) and ingredients from renewable sources―silica obtained by rice husks incineration and microcrystalline cellulose―as fillers, and rapeseed oil, as a process additive, to prepare compositions, intended for manufacturing soles for winter footwear having an increased coefficient of friction to icy surfaces.
Epoxyprene 25 (Made in Thailand) comprising 25% of epoxy groups; glass transition temperature (Tg) minus 47˚С, 0.97 g∙cm−3 density and Mooney viscosity ML (1 + 4), 100˚С 50 - 70 in amount of 100 phr was used for the investigations carried out.
Biogenic amorphous silica obtained by rice husk incineration (SRHI) was used as filler [
Standard silica Ultrasil 7000 GR with the following characteristics was from Evonik, Germany: 175 m2∙g−1 B.E.T. specific surface area; 0.55 g∙cm−3 bulk density; 5.5% weight loss at 105˚C and pH 6.5 of an aqueous solution.
Carbon black N550 characterized by: particle size 50 ÷ 65 nm; 45 ÷ 60 m2∙g−1 B.E.T. specific surface area; oil number 96 ÷ 100 ml/100g and 7 ÷ 9 pH, at an amount of 3 phr was used as a pigment.
Microcrystalline cellulose (Sigma-Aldrich, USA) with characteristics as follow: particle size ranging from 5 to 30 μm, pH 5 - 7.5; water-soluble substances ≤ 0.24% and weight loss at 105˚C ≤ 5%.
The compatibilizers used were:
-bis(triethoxysilylpropyl)tetrasulfide (TESPT)―silane Si 69®―a bifunctional sulfur-containing organosilane from Evonik Industries. The sulfur content of TESPT was about 22.5 wt.%; molecular weight of 532 g∙mol−1 and 1.1 g∙cm−3 density. The compound was used only in the silica-containing composites.
-3-thiocyanato-propyl triethoxysilane (Si - 264, also manufactured by Evonik Industries) with a sulfur content of 12.5 wt.% and average molecular weight of 263 g∙mol−1 was used in the compounds containing microcrystalline cellulose.
The other ingredients used for manufacturing the rubber composites (zinc oxide, stearic acid, anti-aging agents, vulcanization accelerators and activators and vulcanization agents) were standard for the rubber industry, at an equal quantity for each sample.
The fillers used, silica and microcrystalline cellulose, were investigated by TEM/XEDS. The TEM observations were performed on a TEM JEOL 2100 instrument, at an accelerating voltage of 200 kV. The specimens were prepared by grinding and ultrasonic dispersing in ethanol for 6 min. The suspensions were dripped on standard holey carbon/Cu grids. The chemical elemental composition of the fillers was determined by use of OXFORD INSTRUMENTS, X-MAXN 80 T.
The surface compositions of the NR samples filled with silica were investigated by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS).
X-ray analysis was performed on a Bruker D8 Advance diffractometer at Cu-Kα radiation, in the range of 10˚ to 80˚ 2θ degrees.
The XPS measurements were carried out on an AXIS Supra electron spectrometer (Kratos Analitycal Ltd.) using AlKα radiation with photon energy of 1486.6 eV. The energy calibration was performed by normalizing the C1s line of adsorbed adventitious hydrocarbons to 284.6 eV. The binding energies (BE) were determined with an accuracy of ±0.1 eV. The chemical composition of the samples was determined monitoring the areas and binding energies of C1s, O1s and Si2p photoelectron peaks. Using the commercial data-processing software of Kratos Analytical Ltd. the concentrations of the different chemical elements in atomic percentages were calculated by normalizing the areas of the photoelectron peaks to their relative sensitivity factors.
The elastomers and ingredients used are described in the Experimental Part above (2.1. Elastomers and Ingredients). The differences in the formulations of the compounds used to manufacture soles for sporting winter boots are shown in
The compounds were prepared on an open two-roll laboratory mill (rolls Length/Diameter 320 × 160 mm, friction 1.17 and 25 min−1 speed of the slower rotating roll). The rubber compounds were vulcanized in the form of 400 × 400 mm plates on an electrically hydraulic vulcanization press at 150˚C, at 10 MPa and for a time determined by the vulcanization isotherms of the compounds taken on a MDR 2000, Alpha Technology vulcameter.
The compounds and vulcanizates were characterized as follows:
・ Vulcanization characteristics―determined according to ISO 3417: 2002;
・ Physico-mechanical properties (Modules at 100% and 300% elongation, tensile strength, relative elongation, residual elongation)―according to ISO 37: 2002 and EN 12 803;
・ Shore A hardness according to ISO 7619:2001
・ Dynamic characteristics―the tangent of the mechanical loss angle (tan δ) at a frequency of 5 Hz, 64 μm deformation, in the temperature range of minus 80 to +80˚C, in particular at minus 10˚C, minus 4˚C, and minus 1˚C, at a heating rate of 3˚C∙min−1. The size of the samples tested was as follows: 10 mm width, 25 mm length and 2 mm thickness. A dynamic mechanical thermal analyzer, manufactured by Rheometric Scientific was used.
・ Wear resistance: according to EN 12 770;
Component | ENR 0 | ЕMCC 1 | ЕMCC 2 | ЕMCC 3 |
---|---|---|---|---|
Microcrystalline cellulose | - | 60 | 30 | 30 |
Silica from rice husk | - | - | 30 | |
Silica Ultrasil 7000 GR | - | - | - | 30 |
Si-264 | - | 6 | 3 | 3 |
Si 69®) | - | - | 3 | 3 |
・ Bending resistance: according to BDS 5658-72;
・ Tribological characteristics: the static and dynamic coefficient of friction were determined using a laboratory apparatus and methods
A functional scheme of the device used is presented in
A small parallelepiped ice block was used as a contrabody. The icy surface was evened via treatment with liquid sandpaper STRUERSP180. The samples tested were evacuated into the camera for 30 min prior to the measurements. For each experiment the measurements were performed trice and the average value was taken into account. The standard deviations, SD for each measurement was within the accuracy of the method ± 0.01. The dynamic friction coefficient µ was calculated by the formula (Equation (1)):
μ = T / P (1)
The measurements were carried out at temperatures below zero: −10˚С (corresponding to the so called “dry” ice); −4˚С (corresponding to the so called “wet” ice) and −1˚С (corresponding to so called “melting” ice, i.e. ice covered by a layer of water).
A comparison of EDX spectroscopy maps shows that the chemical elemental composition of the three fillers is quite different. The data are presented as Figures 1(a)-(c) in Supporting Information. Noteworthy is also that the elemental composition of standard silica is quite different from that of silica obtained by rice husk incineration. The atomic percentage of the elements is also quite different (
The data in
Chemical element | Ultrasil 7000 GR | SRHI | MCC |
---|---|---|---|
O | 47.94 | 49.58 | 5.68 |
Na | 0.34 | 0.17 | 0.98 |
Al | 38.77 | 2.04 | - |
Si | 12.79 | 29.02 | 0.51 |
Ca | 0.16 | 7.42 | - |
Mg | - | 0.31 | - |
P | - | 0.23 | - |
K | - | 9.31 | - |
Mn | - | 1.15 | - |
Fe | - | 0.62 | - |
C | - | - | 92.71 |
Cl | - | - | 0.12 |
to the fact that in the first case we have a synthetic product and in the other a natural one. There is a large difference in the size and shape of the elementary particles (
The XRD data presented in
The XRD patterns for elastomers EMCC 2 and EMCC 3 (
The surface composition of the samples was investigated by XPS,
The vulcanization characteristics of the investigated compounds are summarized in
As seen from the results summarized in
The physicochemical characteristics of vulcanizates containing elastomers and ingredients from renewable resources are summarized in
The analysis of the results in
Characteristic | ENR0 | ЕMCC1 | ЕMCC2 | ЕMCC3 |
---|---|---|---|---|
ML, dNm | 0.00 | 0.03 | 0.43 | 0.84 |
MH, dNm | 4.53 | 9.61 | 20.26 | 25.71 |
ΔM = MH − ML | 4.53 | 9.58 | 19.83 | 24.87 |
Ts1, min:sec | 4:19 | 3:57 | 4:29 | 5:11 |
Ts2, min:sec | 4:34 | 4:10 | 4:51 | 5:31 |
T50, min:sec | 4:39 | 4:50 | 5:46 | 7:21 |
T90, min:sec | 5:35 | 7:32 | 9:38 | 12:08 |
Parameter | ENR 0 | EMCC 1 | EMCC 2 | EMCC 3 | Recommended values |
---|---|---|---|---|---|
Modulus at 100% of elongation, M100, MPa | 0.3 | 0.7 | 1.4 | 2.1 | - |
Modulus at 300% of elongation, M300, MPa | 0.7 | 2.1 | 4.8 | 5.7 | - |
Tensile strength, σ, MPa | 9.0 | 3.5 | 10.7 | 12.0 | Min 8 [ |
Relative elongation, ε1, % | 750 | 450 | 570 | 590 | Min 300 [ |
Residual elongation, ε2, % | 10 | 15 | 30 | 30 | - |
Shore A Hardness, relative units | 45 | 60 | 70 | 71 | 60 - 88 [ |
Abrasion resistance, mm3 | 410 | 400 | 195 | 175 | Max 150 [ |
Density,g∙cm−3 | 0.95 | 1.05 | 1.14 | 1.16 | Max 1.25 [ |
Fatigue failure resistance, cycles | >60,000 | >60,000 | >60,000 | >60,000 | Min 40,000 [ |
Tear strength, N∙mm−1 | 2.0 | 2.5 | 8.1 | 8.9 | Min 8 [ |
TEM studies, described further on. The low reinforcement potency of microcrystalline cellulose, in addition to its low specific surface area and large particle size [
All composites tested were subjected to experiments to determine their “fatigue failure resistance” index as described in [
Dynamic mechanical thermal analysis (DMTA) allows to define the complex dynamic modulus E*, the storage modulus E' and the losses modulus E" of a rubber material by the reaction of the material against the impact of an oscillating force causing a sinusoidal load. The relationship between them can be represented by the equation E* = E'+ iE", i.e. the ability of the composite to store energy (the storage modulus expresses the elastic component of the complex dynamic modulus) and its ability to lose energy (the loss modulus expresses the plastic component of the complex dynamic modulus) [
As the results presented show, the composites containing only microcrystalline cellulose as a filler are outstanding with the highest tangent of mechanical losses angle at all the studied temperatures, with predicted highest adhesion to icy surfaces, respectively. However, it should be kept in mind that those composites have the poorest physico-mechanical performance. Of the composites containing silica, those with SHRI have higher values of the tangent of mechanical losses angle. The composites with standard silica have the lowest values. A number of authors [
microcrystalline cellulose have the lowest Shore A hardness value (60 μm), while the composites containing SRHI and standard silica have significantly higher hardness values (70 and 71 equivalents, respectively). Shore A hardness is directly related to traction with icy surfaces―the lower the hardness, the higher the traction is, and vice versa. On the other hand, the chemical nature and structure of the filler determine its effect on the material hardness.
It is obvious that the differences in the structure, granulometry and crystallinity of the two types of silicon dioxide influences the tangent of mechanical loss angle. The predominant cristobalite phase in SRHI (
The coefficients of kinetic friction of the composites at different temperatures are shown on
As seen from the results obtained, the complex estimation (physico-mechanical characteristics and kinetic coefficient of friction to various icy surfaces) reveals that the samples containing microcrystalline cellulose and SRHI possess the best parameters (EMCC-2).
The samples containing only microcrystalline cellulose (EMCC-1) have the highest values of the friction coefficients but their physico-mechanical properties are quite poor. Samples containing microcrystalline cellulose and standard silica (EMCC-3) have values of the physico-mechanical parameters higher than those of composites with biogenic silica but lower friction coefficients to icy surfaces. The differences observed again could be explained by the different structure of the two silica fillers.
The following classification of the coefficients of kinetic friction and the class of adhesion (to ice)/slip is given in the specialized literature [
The above classification confirms that composites EMCC-1 and EMCC-2 with
Kinetic coefficient of friction | Degree of slip resistance |
---|---|
>0.30 | highly stable |
0.20 - 0.29 | stable |
0.15 - 0.19 | unstable |
0.05 - 0.14 | slipping |
<0.05 | highly slipping |
respect to slip resistance fall into the category “highly stable” and “stable”, whereas composite EMCC-3 in one of the categories falls into the “slipping” class with a coefficient value of 0.1. The correlation between the tangent of mechanical loss angle and the kinetic coefficient of friction with respect to the adhesion to icy surfaces is also confirmed. Since the tested composites differ only in the nature of the filler used (microcrystalline cellulose, standard silica and SRHI), it is evident that the observed effects can be explained by the significant differences in the particle size, composition, structure and properties of the fillers used. The results obtained implementing STEM-EDX, XRD and XPS methods reveal best the state of art.
It is obvious, however, that the higher hardness of the composites has a negative effect on their adhesion to icy surfaces, as other authors have established [
The case of microcrystalline cellulose (
Various variants of rubber compounds and vulcanizates based on them have
been developed for the production of soles with increased adhesion to icy surfaces. Those contain elastomers and ingredients from renewable natural sources―epoxidized natural rubber, microcrystalline cellulose, silica produced by rice husks incineration, rapeseed oil. The compounds and vulcanizates were classified with respect to the performance standards laid down for footwear soles based on elastomers and it was demonstrated that the materials meet those standards. The adhesion of the materials as well as tribological tests based on the coefficient of friction was evaluated by dynamic mechanical thermal analysis, at different temperatures.
The complex evaluation of the composites developed revealed those containing microcrystalline cellulose and biogenic amorphous silica at a 1:1 ratio to be the most suitable for making footwear soles. The reason for the higher coefficient of friction of the composite, containing SRHI compared to those of composites filled with standard silica is due to different crystallinity―a predominant cristobalite phase in the first case and a completely amorphous phase in the second.
The amount of elastomers and ingredients from renewable sources (elastomers, fillers, process additives) predominate in the developed composites reaching 89%. That indicates good capacity of those products to improve the friction coefficients to icy surfaces.
The implementation of this study was funded by the Operational Program 2014BG16RFOP002―Innovation and Competitiveness, Procedure: BG16RFOP002-1.002―Support for the development of innovations by start-ups, Grant Agreement BG16RFOP002-1.002-0028-C01 Eco-footwear soles on elastomeric basis with improved ice and snow grip.
Dishovski, N., Mitkova, F., Kandeva, M., Angelov, Y., Uzunov, I., Ivanov, M. and Klissurski, D. (2018) Elastomer Composites with Enhanced Ice Grip Based on Renewable Resources. Materials Sciences and Applications, 9, 412-429. https://doi.org/10.4236/msa.2018.94028