Footwear Soles Research Articles

Exploring the Mechanical and Thermal Impact of Natural Fillers on Thermoplastic Polyurethane and Styrene–Butadiene Rubber Footwear Sole Materials

The increasing concern for sustainability in the footwear industry has spurred the exploration of eco-friendly alternatives for materials commonly used in sole manufacturing. This study examined the effect of incorporating rice straw and cellulose as fillers into soles made from either styrene–butadiene rubber (SBR) or thermoplastic polyurethane (TPU). Both fillers were used as a substitute in mass percentages ranging from 5 to 20% in the original SBR and TPU formulas, and their impact on mechanical properties such as abrasion and tear resistance, as well as thermal properties, was thoroughly evaluated. The results demonstrated that the inclusion of fillers affects the overall performance of the soles, with the optimal balance of mechanical and thermal properties observed at a 10% filler content. At this level, improvements in durability were achieved without significantly compromising flexibility or abrasion resistance. Thermal analysis revealed increased thermal stability at moderate filler contents. This research not only offers a sustainable alternative to traditional materials but also enhances sole performance by improving the composition. Furthermore, this study paves the way for future research on the feasibility of incorporating eco-friendly materials into other consumer product applications, highlighting a commitment to innovation and sustainability in product design.

Simultaneous crosslinking and foaming of ethylene‐propylene diene terpolymers (EPDM) organoclay composite foams

AbstractThis study used ethylene‐propylene diene monomers (EPDM), an elastomer, to develop foams incorporating clay as filler using simple compounding, chemical foaming, and peroxide‐mediated light crosslinking methods. The low‐temperature batch mixer was employed for compounding azodicarbonamide (ADC) as a foaming agent, dicumyl peroxide (DCP) as a crosslinker, followed by compression molding for sheet preparation and the high‐temperature foaming process. Fourier transform infrared spectroscopy (FTIR) equipped with a hot stage attenuated total reflection (ATR) was employed to elucidate the ADC decomposition and foaming. The DCP (0.25 phr) content was optimized based on the foaming expansion ratio (>4) and stability, which was further optimized in conjuncture with clay to generate high‐performance composite foams. The incorporation of 1% organoclay enhances the expansion ratio and specific tensile strength by >7 and ~ 270%, respectively, compared with EPDM foams without clay while displaying a density of 0.11 g/cm3. Overall, the developed foaming process can be extended to other elastomeric polymers for various applications such as protective gears (e.g., helmets and shin guards), thermal/sound insulation, packaging/containment, footwear soles, oil absorption, etc.

Upcycling of textile and footwear wastes for synergistical reinforcement of cement mortar

The utilization of waste polymeric materials from textile, clothing and footwear (TCF) industry as aggregates in the cement is a valuable approach to reduce plastic pollution. However, the low modulus of polymers and poor interfacial interactions with the cement deteriorate their applications. Herein, we report an upcycling strategy for the recovery of waste fabrics and footwear sole materials from TCF industry as cement mortar reinforcement. The exfoliation of fabrics and pulverization of footwear soles enable them to expose the reactive groups which can absorb Ca2+ dissolved in water, thus helping the formation of C–H–S gel at the organic–inorganic interfacial transition zones between polymers and cement. It was determined by laser confocal Raman spectra. Hence, the exfoliated polyester fibers can prevent the crack expansion of cement and the devulcanized sole elastomers can dissipate energy while subjected to the external force, synergistically enhancing the mechanical properties. The flexural and compressive strengths of cement containing 0.5 wt% PFW and 5 wt% EPW are 48.9% and 59.6% higher than the neat cement. This work suggests a new strategy to realize value-added utilization of waste polymers which are difficult to be classified and separated.

Strategic selection tool for thermoplastic materials in a renewable circular economy: Identifying future circular polymers

To progress towards a renewable circular economy for thermoplastic materials it is imperative to decouple from fossil feedstocks, to maximise looping strategies and to manufacture occasionally littered articles from readily biodegradable materials. This transition is complex due to the combination of stringent technical specifications that are required for ordinary plastic products and the demands that all end-of-life scenarios foist on these products. The presented strategic material selection tool for fast moving consumer goods in a renewable circular economy prioritises their suitability for the expected end-of-life fates and the contrived technical performance. This framework is tested for 17 common consumer articles and 21 biobased plastics. The strategic selection tool shows that consumer articles that are made from foamed and fibrous plastics, such as matrasses and textiles, can potentially be produced from biobased alternatives, such as biobased poly(ethylene terephthalate) (PET), poly(trimethylene terephthalate) (PTT), poly(butylene succinate) (PBS), poly(butylene succinate-co-adipate) (PBSA) and poly(butylene adipate-co-terephthalate) (PBAT). On the other hand, the tool also reveals that there are currently no adequate alternatives in barrier (food) packages and in elastomeric products such as tyres, soles of footwear and gloves. Biobased PET is a good polymer for beverage bottles provided that the leakage to the natural environment is minimised with an effective collection, reuse and recycling system. Although there are no viable single-biobased-polymeric alternatives for flexible packages to pack for instance dried foods, solutions could be developed in the form of multi-layered films of various biobased and biodegradable materials. But it would also imply that a dedicated new recycling technology needs to be developed for such multilayer films. The presented tool demonstrates that the technology is ready to start the transition towards a renewable circular economy for consumer articles such as matrasses, cushions, beverage bottles. Simultaneously, new biobased polymeric solutions need to be developed for multiple other applications such as tyres, footwear, gloves and flexible barrier packaging.

A study on the occurrence of glass and paint across various cities in the United States—Part I: Background presence of glass in the general population

The plausibility of finding glass fragments by chance in the general population becomes a relevant question when assessing glass recovered from someone suspected of being involved in an illegal activity implicating glass breaking. This research addresses this question in United States regions by searching the apparel of individuals assumed to be unconnected with crimes involving glass breaking. A large-scale study was conducted in the two metropolitan areas of Houston, TX, and Pittsburgh, PA, and the two suburban areas of Huntsville, TX, and Morgantown, WV. A total of 1,883 items of clothing and footwear were searched from 410 subjects, using tape liftings for garment areas and scrapings from the footwear soles. A total of eight glass fragments were recovered, five from the Houston set and one from each of the three other locations. No more than one glass fragment was recovered on a given item. The findings confirm that it is highly unlikely to observe glass fragments on the garments of individuals from the general population, and it is even less likely to find many fragments. Examination of glass by refractive index and micro-x-ray fluorescence spectrometry enabled their differentiation and classification by end-use. A zeta distribution and the R package fitPS developed for this study were used to estimate two parameters typically used for activity level likelihood ratio formulas: the probabilities of observing a given number of groups of fragments on target garments (P) and a particular number of fragments within an observed group (S).

Trajectory Optimization of Glue Spraying on Athletic Footwear Sole Based on Multivariable Spray Torch Model

Trajectory Optimization of Glue Spraying on Athletic Footwear Sole Based on Multivariable Spray Torch Model

Cleaner technologies to minimise the environmental footprint of the footwear bonding process

Advances in cleaner technologies have a critical role in reaching Europe’s climate and environmental goals. More precisely, in the case of the footwear bonding process its environmental impact is mainly related to the VOCs emissions resulting from the use of organic solvents in the most common adhesives used, as well as during the application of some surface treatments such as halogenation. Moreover, the use of raw materials of fossil origin also contributes to its carbon footprint. The aim of this work is to deploy cleaner technologies to minimise the environmental footprint of the footwear bonding process through several approaches at different steps of the process life cycle, also to contribute to footwear circularity. This work focuses on the synthesis and application of reactive hot melt polyurethanes from CO2-based polyols as biobased adhesives, and the study of plasma-based surface treatments (low-pressure plasma and atmospheric pressure plasma) to improve adhesion properties, using styrene-butadiene vulcanised (SBR) rubber as a representative footwear soling material. The influence of CO2-based polyols on the polyurethane structure and the physicochemical effects of the plasma treatments on the rubber surface, as well as the final adhesion properties have been evaluated by means of different experimental techniques. The results showed that the values obtained for the T-peel strength of the adhesive joints studied, with the combination of the application of the plasma surface treatments and the biobased reactive hotmelt, exceed the minimum quality requirements for footwear according to the standardised tests. As a result, the deployment of such cleaner technologies regarding adhesives and surface treatments in the footwear bonding process will contribute to improving footwear carbon footprint and its circularity.

Preliminary end-of-service-life study of the soles of protective footwear resistant to selected mechanical factors and mineral oil

This work aimed to study the end of service life of soles of protective footwear resistant to selected mechanical factors and mineral oil. Three sole variants were examined; made from poly(ethylene-co-vinyl acetate) (EVA), poly(vinyl chloride) (PVC) and polyurethane (PU), currently widely used in all-rubber protective footwear. The preliminary study focused on the abrasion resistance and bending strength of the three sole materials after different times of exposure to mineral oil. Changes in density and hardness of the examined materials were evaluated following exposure to cyclical factors. Statistical analysis was performed to identify significant differences between the three types of polymers in terms of abrasion resistance, density and hardness following exposure to mineral oil for three different periods. Surface morphology of the sole materials was examined by means of scanning electron microscopy. The presented studies elucidate the effects of mineral oil on the basic mechanical parameters of all-rubber footwear soles.

Robotised atmospheric plasma treatment to improve the adhesion of vulcanised and thermoplastic rubber materials for a more sustainable footwear

Surface treatment is essential to ensure the resistance and durability of adhesive upper-to-sole joints for footwear. This study investigates robotised atmospheric pressure plasma technology to increase adhesion on styrene-butadiene vulcanised (SBR) and styrene-butadiene-styrene thermoplastic (SBS) rubbers, which constitutes a more sustainable surface treatment as an alternative to solvent-based chemical treatments (i.e., halogenation). In this work, the different parameters such as application speed and distance of the nozzle of the atmospheric plasma system have been optimised and the effects of the plasma treatment on the physicochemical properties in the materials’ surface, roughness, wettability, as well as the adhesion properties have been evaluated by means of different experimental techniques: Fourier Transform Infrared Spectroscopy (FTIR), X-Ray Photoelectron Spectroscopy (XPS), Field Emission Scanning Electron (FESEM), Contact Angles (CA) and Surface Free Energy (SFE) measurements and T-peel strength tests. The results show that the values obtained for the T-peel strength of the reactive hot melt polyurethane adhesive (HMPUR) joints studied exceed the minimum quality requirements for footwear according to standardised tests. In consequence, the optimisation and validation of operating plasma conditions and adhesion properties of rubbers could consolidate atmospheric plasma as a solvent-free alternative for the preparation of footwear soling materials. The great advantage of this technology is its introduction into the production line by fully automating the bonding process with the use of robots.

Carbon nanomaterials as additives to ethylene vinyl acetate copolymer foams for sport footwear applications

AbstractEthylene vinyl acetate (EVA) copolymer foams with addition of 1.0, 2.5, and 5 wt% of carbon nanomaterials (graphene, graphene oxide, carbon nanotubes, and multiwalled carbon nanotubes) were fabricated. Properties of these foams were compared with reference sample (no additives) and samples containing carbon black. The following properties of these samples were examined: density, compressive stress (50% compression), impact behavior, thermal conductivity, and hydrophobicity. The morphology was studied using scanning electron microscopy. The samples were additionally characterized using thermal gravimetric analysis and differential scanning calorimetry. Increasing weight percentage of carbon‐based additives led to foam structure with lower porosity and higher density. Samples with a higher degree of cross‐linking were less dense (~10%) as compared to the reference sample and also displayed better mechanical and thermal properties. Increasing weight percentages of additives led to higher compressive strength and higher energy absorption. Samples with 5 wt% of additives and increased quantity of cross‐linking agent exhibited great mechanical properties. Despite their lower density, these samples had higher values of compressive stress, 10%–15% higher than those of the reference sample, and higher energy return properties. All samples follow the trend of increasing values of compressive strength with the increase in density, for some samples, this correlation has quasi linear character. The obtained results show that EVA foams with the addition of carbon black, graphene, carbon nanotubes, and multiwalled carbon nanotubes have properties desirable in high‐performance materials for sport footwear soles. Multiwalled carbon nanotube additives could be incorporated at low weight percentages and low degree of crosslinking into EVA matrix to achieve lower density and enhanced energy repulsion and cushioning effects.

Poly(hydroxyurethane) Adhesives and Coatings: State-of-the-Art and Future Directions.

Polyurethane (PU) adhesives and coatings are widely used to fabricate high-quality materials due to their excellent properties and their versatile nature, which stems from the wide range of commercially available polyisocyanate and polyol precursors. This polymer family has traditionally been used in a wide range of adhesive applications including the bonding of footwear soles, bonding of wood (flooring) to concrete (subflooring), in the automotive industry for adhering different car parts, and in rotor blades, in which large surfaces are required to be adhered. Moreover, PUs are also frequently applied as coatings/paints for automotive finishes and can be applied over a wide range of substrates such as wood, metal, plastic, and textiles. One of the major drawbacks of this polymer family lies in the use of toxic isocyanate-based starting materials. In the context of the REACH regulation, which places restrictions on the use of substances containing free isocyanates, it is now urgent to find greener routes to PUs. While non-isocyanate polyurethanes (NIPUs) based on the polyaddition of poly(cyclic carbonate)s to polyamines have emerged in the past decade as greener alternatives to conventional PUs, their industrial implementation is at an early stage of development. In this review article, recent advances in the application of NIPUs in the field of adhesives and coatings are summarized. The article also draws attention to the opportunities and challenges of implementing NIPUs at the industrial scale.

Cellular materials used in athletic footwear applications: Experimental, analytical and numerical investigations

In this paper, the mechanical behavior of hyperelastic foams used in athletic footwear soles is investigated for compression and tensile loadings. Specimens were cut using profiled tools from aged footwear manufactured by five different companies. Based on the experimental results, the modified Ogden hyperelastic strain-energy density function was calibrated and evaluated analytically and numerically, yielding good results.

Antimycotic Activity of Sumac Extract in Composites Based on Epoxidized Natural Rubber for Application in Footwear Soles Production

Antimycotic Activity of Sumac Extract in Composites Based on Epoxidized Natural Rubber for Application in Footwear Soles Production

Characterisation of Shoe Soling Material Prepared by using Rubber and Leather Solid Waste

The ultimate aim of this research is to develop footwear soling materials from solid waste generated from leather industry. The Chrome shaving from the leather sector is used as a solid waste in this study. Styrene butadiene, Nitryl, Ethylene propylene monomer (EPDM) and Isoprene elastomers were used for this research. The rubber and chrome shavings mixes were prepared by using industrial two roll mill. Various propositions of rubbers, chrome shavings and nano fillers were characterised in this research. The developed soling materials were tested for physical testing like hardness, abrasion resistance, tensile strength, density and elongation at break and compared with compared with commercially available soling material. In this research four different experiments has been conducted among the four experiments the soling material prepared using a isoprene rubber-70 Phr and EPDM rubber - 30Phr with chrome shavings -50 Phr and KLN(Silica based) nano fillers- 7 Phr meets the required parameters of commercial soling material. Hence, it is concluded that chrome shaving wastes from tannery in soling material preparation is one of the best remedy for the environmental issues.

Biodegradable Polymer Composite based on Recycled Polyurethane and Finished Leather Waste

The issue of polluting waste has been addressed over the years by means of several methods: depollution, recovery by reuse and/or energy recycling and/or mechanical and/or chemical recycling. In the integrated plastic waste management concept, along with these methods are included the possibilities of reducing the quantities by using biodegradable polymeric materials. The goal paper is to recover footwear waste after use: polymer and finished leather by making new biodegradable polymeric composites. The waste is ground to 1-5 mm and compounded by melt extrusion-granulation, processable by injection into new products (footwear soles) with pre-established advanced features. Biodegradable polymer composite based on post-consumption thermoplastic polyurethane (WTPU) waste compounded with post-consumption finished leather waste, ground and functionalized. This composite will be made into a low-density product, with low cost, recovery and reuse of waste, containing both polymer and protein, and last but not least, biodegradable. The methodology for making the new materials involves the following steps: sorting waste, grinding, functionalization and compounding. These operations are easy to manage and do not involve new equipment. Compounding, the most important operation, will be carried out on a corotating twin-screw extruder-granulator with the possibility of adapting the length of the screw depending on the material (L/D-35). Waste transformation (ground and functionalized) in new value-added products will lead to remarkable improvements in the life cycle of raw materials and the sustainable use of this waste, contributing to sustainability, improving eco-efficiency and economic efficiency and reducing the “pressure” of waste on the environment.

Innowacyjna metoda otrzymywania polioli poliestrowych przeznaczonych do produkcji spodow obuwniczych

A new technology for the production of polyester polyols using a newly separated side stream from the cyclohexane oxidation process as a raw material was developed. The innovation consisted in the introduction of the stage of separation of the waste product stream, which allowed to obtain a fraction with composition suitable to ensure optimal properties of the synthesized polyol intended for polyurethane systems used in the production of footwear soles. The obtained prepolyester fraction was characterized by a high content of bifunctional aliphatic compounds, with a minor share of low molecular weight monoacids, contributing to the deterioration of final product quality. The fraction resulting from the separation is an alternative raw material to the commonly used adipic acid. The parameters of footwear soles manufactured from microporous polyurethanes based on the obtained polyesters were evaluated. It was found that polyesters produced from new raw material are characterized by similar or higher physical and mechanical properties in comparison to the existing products of Purinova Company.

Differential analysis of very small particles (VSP) from the contact surfaces and recessed areas of footwear

Prior work has shown that the contact surfaces of footwear rapidly lose very small particles (VSP) when walking on dry soil. Other research, with more general sampling from shoe soles, has shown that particles can persist much longer. Given rapid losses from contact surfaces, we hypothesize that non-contact, recessed areas of footwear retain particles from prior exposures. Here we conduct differential analysis of VSP recovered separately from contact and recessed surfaces, following a controlled series of environmental exposures.Work boots and tennis shoes were exposed by walking distances of 250 m, sequentially, in three environmental sites. VSP were harvested separately from contact and recessed surfaces using a moist swabbing procedure. VSP were analyzed by microscopy and the proportions attributable to each site were determined using a newly developed statistical model.The principal findings are (1) contact surfaces of footwear are dominated by VSP attributable to the most recent site of exposure, (2) recessed surfaces of footwear retain VSP from prior exposures, (3) sole type appears to be a source of major differences in the amount of VSP from prior exposures remaining in recessed areas, and (4) when VSP attributable to prior exposures are found, there is no clear trend for dominance between earlier exposures.The significance of these findings is considerable. In cases where the last site visited is of interest, VSP from contact surfaces of footwear will give a nearly pure sample of that site. In cases where prior sites visited are of interest, the VSP from the contact surfaces can be used as a background signal, providing a means for differential analysis of VSP mixtures found in the recessed areas of the sole. The strong influence of shoe sole type on the retention of VSP from prior exposures indicates that efforts to explore the rates of VSP loss from recessed areas will need to take characteristics of the sole type into account. The absence of a clear trend for dominance between earlier exposures suggests that once traces are lodged within the recessed areas of footwear soles they can be retained for long periods of time. These findings support prior research on the retention of particles on footwear that shows periods of long retention following rapid initial losses of trace materials.The differential analysis of VSP is a significant contribution to the methods available for the deconvolution of particle mixtures into fractions that are characteristic of their contributing sources.

Journey history reconstruction from the soils and sediments on footwear: An empirical approach

The value of environmental evidence for reconstructing journey histories has significant potential given the high transferability of sediments and the interaction of footwear with the ground. The importance of empirical evidence bases to underpin the collection, analysis, interpretation and presentation of forensic trace materials is increasingly acknowledged. This paper presents two experimental studies designed to address the transfer and persistence of sediments on the soles of footwear in forensically relevant scenarios, by means of quartz grain surface texture analysis, a technique which has been demonstrated to be able to distinguish between samples of mixed provenance.It was identified that there is a consistent trend of transfer and persistence of sediments from hypothetical pre-, syn- and post-crime event locations across the sole of the shoe, with sediments from ‘older’ locations likely to be retained in small proportions. Furthermore, the arch of the shoe (the area of lowest foot pressure distribution) typically (but not exclusively) retained the highest proportion of grain types from previous locations including the crime scene. A lack of chronological layering of the retained sediments was observed indicating that techniques that can identify the components of mixed provenance samples are important for analysing footwear sediment samples. It was also identified that the type of footwear appeared to have an influence on what particles were retained, with high relief soles that incorporate recessed areas being more likely to retain sediments transferred from ‘older’ locations from the journey history. In addition, the inners of footwear were found to retain sediments from multiple locations from the journey history that are less susceptible to differential loss in comparison to the outer sole. These findings provide important data that can form the basis for the effective collection, analysis and interpretation of sediments recovered from both the outer soles and inners of footwear, building on the findings of previously published studies. These data offer insights that enable inferences to be made about mixed source sediments that are identified on footwear in casework, and provide the beginnings of an empirical basis for assessing the significance of such sediment particles for a specific forensic reconstruction.

Elastomer Composites with Enhanced Ice Grip Based on Renewable Resources

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.

Nanotechnology can provide a real breakthrough in the anti-slip properties of safety footwear soles

Slips, trips and falls are the most common causes of major injuries at work (Leamon & Murphy, 1995); the majority of them occur in the presence of floor contaminants as water or oil.Soles and footw...