Leather and Footwear Journal

Volume 25, no 4

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EFFECT OF STRUCTURAL CHARACTERISTICS ON HARDNESS AND COMPRESSION SET OF 3D PRINTED TPU INSOLE
- Pages 193-202
  Van-Huan BUI^*
  - Department of Textile-Leather and Fashion, School of Materials Science and Engineering, Hanoi University of Science, and Technology, No. 1, Dai Co Viet, Bach Mai ward, Hanoi, Vietnam, huan.buivan@hust.edu.vn
  ABSTRACT. Hardness and compression set are important characteristics of shoe insole materials, especially for insoles intended for diabetic patients. In this study, we investigated the effects of several structural factors including the hardness of thermoplastic polyurethane (TPU), infill density, and internal structural pattern on the hardness and compression set of 3D-printed insoles. TPUs with hardness levels of 60A, 70A, and 95A were used to fabricate test specimens with different infill patterns and infill densities using FDM technology. The specimens were then subjected to hardness and compression set testing. We found that the specimens exhibited good compression set performance, meeting the requirements for shoe insole materials due to the inherent elasticity of TPU. The hardness of the 3D-printed specimens primarily depended on the hardness of the TPU material and the infill density, while the influence of the infill pattern was less significant. TPU95A is suitable for producing rigid insole components, whereas TPU60A and TPU70A are appropriate for fabricating soft cushioning layers. However, using TPU70A combined with the Grid pattern in BambuLab software is more efficient for 3D printing custom insoles in terms of material consumption and printing time. The mathematical models established between infill density and the hardness of 3D-printed specimens demonstrated very strong correlation coefficients. These models can be used to determine the required infill density to achieve specific hardness levels in different regions of the insole, thereby helping to minimize peak plantar pressure in diabetic patients. The results of this study provide a foundation for the design and manufacture of cost-effective custom insoles for diabetic patients in Vietnam.
  
  KEY WORDS: 3D printed insoles, TPU materials, FDM technology, infill pattern, infill density
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MULTI-LAYER FIREFIGHTING FOOTWEAR UPPER WITH ENHANCED THERMAL AND CHEMICAL PROTECTION: MATERIALS INNOVATION, FUNCTIONAL VALIDATION, AND WEARER COMFORT
- Pages 203-224
  Arun Kumar GAIKWAD^*, Adity SAXENA
  - Woxsen University, Telangana, India, arunkumar.gaikwad@woxsen.edu.in, Dean.SD@woxsen.edu.in
  ABSTRACT. Current firefighting footwear faces significant limitations in providing simultaneous thermal protection, chemical resistance, and wearer comfort during prolonged emergency operations. Existing single-layer or dual-layer designs often compromise protection for breathability or vice versa, leading to heat stress, chemical exposure risks, and reduced operational effectiveness. The objective was to develop and validate a novel multi-layer composite upper system that integrates advanced materials to achieve superior thermal protection, chemical barrier properties, toxic gas filtration, and enhanced wearer comfort while maintaining structural integrity and durability. A nine-layer composite system was designed incorporating: high-grade leather base, PVC-coated Kevlar outer shell, aluminium trihydrate (ATH) nanoparticle flame retardant layer, thermoplastic polyurethane (TPU) chemical barrier membrane, activated carbon filtration layer, vacuum-insulated metallic foil thermal insulation, aramid Fiber structural reinforcement, hydrophilic polyurethane moisture management layer, and memory foam with bamboo charcoal comfort interface. Validation protocols included thermal testing (Heat Transfer Index, Radiant Heat Transfer Index), chemical resistance evaluation (permeation testing), mechanical property assessment (puncture resistance, tear strength), and comfort metrics (water vapor resistance, thermal load). The multi-layer system demonstrated Heat Transfer Index values of 18.2 ± 1.4 s (>17 s requirement), Radiant Heat Transfer Index of 21.8 ± 2.1 s (>18 s requirement), and chemical breakthrough times >480 minutes for common hazardous substances. Puncture resistance increased by 340% compared to conventional designs while maintaining water vapor resistance below 15 m²·Pa/W. Flame spread index was reduced to <25, with Limiting Oxygen Index (LOI) values exceeding 28%. Field trials showed 23% reduction in heat stress indicators and 89% user satisfaction rating for comfort. The innovative multi-layer firefighting footwear upper successfully addresses critical limitations of existing designs by providing enhanced protection without compromising wearer comfort. The integration of advanced materials through systematic layer optimization offers a promising approach for next-generation firefighter personal protective equipment.
  
  KEYWORDS: firefighting footwear, thermal protection, chemical resistance, multi-layer composite, personal protective equipment, flame retardant materials, aramid reinforcement, comfort engineering
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RELATIONSHIP BETWEEN STRETCH AND PRESSURE OF KNITTED FABRIC FOR SHOE UPPERS FOR FEMALE DIABETIC PATIENTS
- Pages 225-242
  Thi-Kien-Chung CAO^1*, Van-Huan BUI², Vu-Luc TA¹, Hai-Kien LE¹
  - ¹Faculty of Garment Technology and Fashion Design, Hung Yen University of Technology and Education, Viet Tien, Hung Yen, Vietnam
  - ²School of Materials Science and Engineering, Hanoi University of Science and Technology, No. 1, Dai Co Viet, Hai Ba Trung, Hanoi, Vietnam
  ABSTRACT. Materials for producing shoes for diabetic patients must provide high comfort and protect the feet, preventing foot damage and ensuring aesthetics. The objective of this study was to analyze the impact of a knitted fabric on the pressure on the instep of female diabetic patients. In this study, three shoe samples with similar designs were explored, the shoe uppers were made from two type of space knit fabrics and one type of three layers fabric sample with polyester composition. The knit fabric structure has a top layer (single jersey, raschel) and a bottom layer (interlock, single jersey, and atlas). A Flexiforce A301 sensor was used to measure the pressure. Research was conducted on 45 female diabetics with the same foot length and were categorized into three groups based on the toe joint circumference size, with each group having a difference in the circumference size of 8 mm. Pressure was measured at two positions on the foot. Results show that when the shoe upper is elongated, the compression level increases, leading to an increase in the pressure on the foot. Furthermore, in different walking positions, the pressure value of the shoe upper on the foot varies. M2 presented the smallest pressure value among all three experimental groups, where the largest pressure of 98.66 ± 2.03 mmHg was observed at Posture 2 in Group 3. M1 had the largest pressure values at all stretch levels, where the pressure reached 120.65 ± 2.50 mmHg at Posture 2 in Group 3. With the shoe samples tested here, peak pressure measured on the different areas of the foot reached a maximum of 182 kPa, which is within the recommended limit of 200 kPa. In addition, pressure was determined on three types of knitted fabrics, when used in shoe uppers, which revealed Groups N1 and N2 to meet the pressure criteria. Knitted fabric with a stretchability of ≤ 10.74% is suitable for making shoe caps for female diabetic patients. This finding helps improve the process of choosing suitable materials to make shoe caps that can ensure comfortable pressure for female diabetic patients. These results provide a guideline for selecting suitable materials for fabricating shoe uppers with good comfort and pressure relief for female diabetic patients.
  
  KEY WORDS: pressure, knitted, shoe uppers, shoes for diabetic patients
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RESEARCH ON TECHNOLOGICAL FACTORS AFFECTING THE PEEL STRENGTH OF ZIPPERS
- Pages 243-254
  Nhat-Huy PHAM, Van-Huan BUI^*, Thanh-Thao PHAN^*
  - Department of Textile – Leather and Fashion, School of Materials Science and Engineering, Hanoi University of Science and Technology, No. 1, Dai Co Viet, Bach Mai ward, Hanoi, Vietnam, huan.buivan@hust.edu.vn, Thao.phanthanh@hust.edu.vn
  ABSTRACT. Thermoplastic adhesive bonding technology is widely used to attach zippers in the production of waterproof clothing, footwear, and leather goods, particularly waterproof sportswear. The durability of zipper bonds during use is critical to the overall quality of waterproof products. Numerous factors influence the durability of zipper bonds with waterproof materials, including the properties of the zipper tape material, the type of waterproof material, the type of adhesive, surface preparation, and bonding process parameters. In this study, an orthogonal experimental design was employed to investigate the effects of technological parameters, including temperature, bonding time, and pressure on the peel strength of two zipper samples bonded to waterproof-coated fabrics using thermoplastic polyurethane adhesive films. The peel strength of the zipper–fabric bonds was measured both after bonding and after 20 washing cycles. Using Design Expert statistical software, we analyzed the experimental data and developed mathematical models describing the relationships between the three process parameters and the peel strength of each zipper sample before and after washing. Based on these models, the optimal temperature, time, and pressure conditions were determined to ensure high peel strength of zippers bonded to waterproof-coated fabrics. The results of this study provide a foundation for further research on zipper adhesion technologies with different materials to improve the quality of waterproof products.
  
  KEY WORDS: zipper, zipper bonding technology, zipper peel strength
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3D PRINTING FOR PEDIATRIC FOOT ORTHOSES: CURRENT APPLICATIONS, CHALLENGES, AND FUTURE PERSPECTIVES
- Pages 255-272
  Shixuan CHEN^1,2, Han XU^1,2, Shiyang YAN^1,2*, Luming YANG^1,2*
  - ¹National Engineering Laboratory for Clean Technology of Leather Manufacture, Sichuan University, Chengdu 610065, Sichuan, P. R. China, cklwhy61@gmail.com, xuhanzft@163.com, yanshiyangscu@126.com, ylmll1982@126.com
  - ²College of Biomass Science and Engineering, Sichuan University, Chengdu 610065, Sichuan, P. R. China, cklwhy61@gmail.com, xuhanzft@163.com, yanshiyangscu@126.com, ylmll1982@126.com
  ABSTRACT. Pediatric foot deformities such as flexible flatfoot, clubfoot, and neuromuscular-related deformities can alter plantar loading, gait, and physical activity levels. Orthoses are widely used, but pediatric care requires frequent remakes during growth, and comfort strongly affects adherence. Additive manufacturing enables a digital workflow in which foot geometry is captured by three-dimensional scanning and translated into computer-aided design. Insoles, footwear components, or ankle-foot orthoses can then be fabricated with controlled geometry and regional stiffness. This review presents current applications of 3D-printed pediatric foot orthoses, synthesizing reported biomechanical outcomes and patient-reported experience across major indications. Available studies suggest that 3D-printed devices can achieve outcomes comparable to traditional orthoses in selected pediatric groups, with potential practical benefits such as lighter structures and better perceived fit in some reports. However, evidence is limited by small samples, short follow-up, and inconsistent reporting of design parameters and outcome measures. Future studies should report designs in a reproducible way and confirm durability, adherence, and clinical benefit through longer follow-up.
  
  KEY WORDS: 3D printing, pediatric orthoses, foot deformities, insoles, ankle-foot orthoses
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