Ultimate FAQ Guide for Footwear Materials, Shoe Chemicals, Shoe-making, Auxiliaries

Ultimate FAQ Guide for Footwear Materials, Shoe Chemicals & Auxiliaries

Consolidated all high-frequency industry FAQs from Google, GISMA technical documents, H.B. Fuller chemical manuals and global environmental compliance standards. Plain text without tables/charts, easy to copy, search and edit.

Part 1: Shoe Adhesives & Surface Primers

Q1: How to select PU adhesive, water-based PU adhesive and CR neoprene contact adhesive for shoemaking?

Solvent-based PU Adhesive

Features: Solvent carrier, ultra-high bonding strength, fast curing, good crystallinity, excellent resistance to water, heat and repeated flexing.

Applications: Mass upper-outsole bonding for sneakers and leather dress shoes, compatible with EVA, PU, rubber, TPR and other common outsole materials.

Water-based PU Adhesive

Features: Diluted with water, extremely low VOCs, no harsh odor. Mandated for European & American eco-friendly orders.

Operation challenge: Water has high heat of vaporization. The drying tunnel must run strictly at 60–65°C until the adhesive film turns fully transparent before heat activation and lamination.

CR Neoprene Contact Adhesive

Features: Outstanding initial tack, elastic film, long open time, great wettability on leather and fabrics.

Applications: Moccasin lasting, Goodyear welt construction, lining & midsole cloth lamination, edge folding positioning.

Limitation: Poor long-term creep resistance. Not allowed for heavy-load upper-outsole main bonding, which may cause delamination after long wear.

Q2: What is shoe primer? Why will delamination occur without primer coating on substrates?

Reaction Mechanism

Substrates like EVA, PP, TPR and vulcanized rubber are non-polar high-crystallinity polymers with low surface energy, naturally repelling adhesives.

Primers act as surface activators: their solvents swell the substrate surface and graft polar groups (chlorinated grafts) onto the material, providing chemical bonding sites for polar PU adhesives.

Primer Matching for Different Outsoles

EVA outsoles: Special EVA primer to remove silicone wax and mold release residues from molding.

Vulcanized rubber (RB): Chlorinated primer to micro-etch and acidify rubber surfaces.

Production Consequence

Missing coating, insufficient coating or wrong primer grade prevents adhesive wetting, leading to large-area upper-outsole separation during flexing endurance tests.

Q3: Functions, mixing ratio and pot life of hardener / crosslinker

Core Function

Most hardeners are polyisocyanates. Mixed with PU adhesive, they crosslink polyurethane chains to form a 3D network, drastically improving heat resistance (anti-delamination under high temperature inside cars), hydrolysis resistance and peel strength.

Standard Mix Ratio

Mass production ratio: 3%–5% hardener by weight of base adhesive.

Too little hardener: insufficient crosslinking, weak heat & hydrolysis resistance.

Too much hardener: brittle adhesive film prone to cracking under stress.

Pot Life / Working Time

Crosslinking starts immediately after mixing. The blended adhesive must be fully consumed within 4–8 hours. Expired adhesive loses reactivity permanently even after reheating and must be discarded.

Part 2: Upper Materials & Reinforcement Auxiliaries

Q1: What is TPU hot melt film required for mesh & Flyknit uppers? Applications & Process

Modern sneakers widely use engineered mesh and 3D Flyknit fabric with high porosity and elasticity. These fabrics fray easily during cutting & stitching and fail to retain toe shape.

Process: Laminated onto fabric backside via high-frequency heat press with customized-thickness TPU hot melt film.

Core Uses: Local upper reinforcement to prevent tearing; base material for seamless hot pressing, KPU embossing, eyelet reinforcement and heel counter molding.

Q2: Differences between fiber insole board, regenerated leather (Salpa) and steel shank

Fiberboard / paper insole

Made of wood & cotton fiber hot-pressed with latex. Low cost, stable sweat absorption.

Applications: Heel reinforcement and full-length midsole for casual shoes and entry-level sneakers.

Regenerated leather (Salpa)

Produced by crushing genuine leather scraps and repolymerizing with natural latex. Offers genuine leather-level flex resistance, resilience and toughness, superior water resistance to fiberboard.

Applications: High-end leather dress shoes and mid-to-high women's footwear.

Steel shank

High-carbon steel sheet embedded inside midsoles of high heels and curved casual shoes to bear body weight and resist shoe torsion. Insufficient rigidity leads to heel collapse and breakage.

Q3: Roles of toe puff & heel counter; selection between solvent-based counter and low-temperature hot melt counter

Counter materials are hard sheets sandwiched between upper and lining to lock shoe silhouette.

Solvent-impregnated chemical counter

Manufactured by soaking non-woven fabric in chemical liquid. Requires toluene / thinner to soften during lasting, hardens irreversibly at room temperature.

Drawback: Severe solvent pollution, phased out by mainstream factories. Only retained for heavy safety boots, military boots and rigid Martin boots.

Low-temperature hot melt counter

Knitted / thin non-woven fabric coated with TPU or PCL hot melt adhesive on both sides. Softens at 80–110°C via heel molding machines and sets after cooling.

Advantages: Repairable with reheating, zero solvent residue, eco-friendly. Standard mass-production material for all sneakers and branded leather shoes.

Part 3: Outsole Materials & Rubber Compounding Chemicals

Q1: Pros, cons and applications of five mainstream outsole materials: RB, EVA, TPR, PU, TPU

RB Vulcanized Rubber Outsole

Advantages: Superior abrasion resistance, strong anti-slip traction, good tear resistance and elasticity.

Disadvantages: High density adds shoe weight; vulcanization causes heavy pollution; non-recyclable.

Applications: Basketball outsoles, hiking footwear, industrial safety work shoes.

EVA Foam Midsole

Advantages: Ultra-lightweight, soft, excellent shock absorption, easy color tinting.

Disadvantages: Permanent compression set after long-term flexing (easy to flatten and lose cushioning).

Applications: Running shoe midsoles, lightweight casual shoes, beach slippers.

TPR / TR Thermoplastic Rubber

Advantages: SBS-based thermoplastic elastomer, direct injection molding, short cycle time, low cost, 100% recyclable scraps.

Disadvantages: Average abrasion & tear resistance; stiffens and cracks under low temperature.

Applications: Fast-fashion footwear outsoles, general casual shoe full soles.

PU Polyurethane Elastomer

Advantages: Ultra-low foam density, premium soft feel, long-lasting rebound and fatigue resistance via microporous structure.

Disadvantages: High hydrophilicity; prone to hydrolysis and disintegration after 2–3 years of storage.

Applications: Premium leather dress shoes, comfort elderly shoes, medical work footwear.

TPU Thermoplastic Polyurethane

Advantages: Ultra-high mechanical strength, superior abrasion, puncture & tear resistance, wide adjustable hardness range.

Disadvantages: Relatively heavy; susceptible to oxidative yellowing over storage.

Applications: Soccer shoe studs, stability support shanks for high-performance running shoes.

Q2: Technical breakthrough of supercritical foaming (E-TPU popcorn) vs traditional chemical foaming

Traditional Chemical Foaming

AC azodicarbonamide chemical foaming agent is added during compounding. High temperature decomposes the agent to release nitrogen & CO₂ for expansion.

Defects: Uneven cell size; residual toxic ammonia and formamide in finished soles.

Supercritical Physical Foaming

Supercritical CO₂ or nitrogen penetrates TPU / PEBAX pellets under controlled high pressure & temperature. Instant pressure reduction triggers internal gas expansion for one-step uniform micropore formation.

Advantages: No chemical foaming agents; rebound rate lifted from 50% to 70%–80%; nearly 50% weight reduction; zero toxic residues, fully eco-compliant.

Q3: Mechanism of abrasion additives and anti-slip agents in rubber compounding

Abrasion Resistance Additives

Raw materials: Ultra-high molecular weight silicone powder, polysiloxane modified polymers, high-activity silicates.

Mechanism: Blended during rubber mixing to reduce micro friction coefficient of cured outsoles, drastically lowering Akron & DIN abrasion loss without sacrificing rubber hardness.

Anti-Slip Agents

Raw materials: Modified silica gel, porous nano inorganic water-absorbent fillers.

Mechanism: Raise outsole surface tension to break water/oil film on slippery floors instantly via micropore absorption, expanding actual contact area between sole and ground to pass EU SRC highest anti-slip certification.

Part 4: Closed-Loop Troubleshooting: Fully Resolve EVA Outsole Delamination

Problem: Frequent upper-outsole separation of molded EVA foam soles on mass-production lines

EVA accumulates silicone wax release agent during molding and features ultra-low surface energy. Mandatory physical + chemical full-process control as below:

Physical Sanding & Chemical Degreasing

Sand all bonding surfaces with coarse grinding wheel to remove glossy foam outer layer and wax, exposing inner micropores for mechanical interlock.

For irregular hard-to-sand grooves: Wipe repeatedly in one direction with lint-free cloth soaked in mineral spirits / cyclohexanone to fully dissolve silicone residues.

Special EVA Primer Coating & Energy Monitoring

Apply uniform UV-curable EVA primer after cleaning. Monitor UV curing line energy meter constantly to stabilize output at 800–1200 mJ/cm².

Insufficient energy: primer fails crosslink activation.

Excessive energy: over-oxidation, brittleness and bubbling on EVA surface.

For heat-activated primers: Lock oven temperature at 60–65°C with minimum 3–5 minutes baking to fully evaporate all solvents.

Standardized PU Adhesive & Hardener Mixing

Use high-viscosity PU outsole adhesive only. Add 3%–5% polyisocyanate hardener on-site, stir electrically for over 5 minutes and label mixing time clearly. Discard any adhesive exceeding 4-hour pot life.

High-Pressure Press Lamination

Send bonded shoe immediately to full-auto sole press after upper-outsole closing. Calibrate dual pump pressure to 3.5–4.5 kg/cm² with minimum 5-second holding time to enable full adhesive penetration and interlocking.

Part 5: Global Eco Restrictions, RSL & Factory Compliance Standards

Q1: Mandatory chemical limits under EU REACH Regulation and Brand RSL for footwear materials

REACH & SVHC Substances of Very High Concern

Covers all finished shoes and auxiliary materials exported to EU. Strictly restricts phthalate plasticizers (DEHP, DBP etc.), carcinogenic AZO colorants, allergen DMFu (dimethyl fumarate) from improper anti-mold storage.

Any single SVHC substance exceeding 0.1% by weight triggers mandatory notification, customs detention, product recall and heavy fines.

RSL vs MRSL

RSL (Restricted Substances List): Governs chemical residues inside finished footwear (AFIRM standards of Nike, Adidas and other major brands).

MRSL (Manufacturing Restricted Substances List): Restricts chemical raw material formulas at factory production source.

Critical red line: Hexavalent Chromium Cr(VI). Leather material limit is usually <3 mg/kg or non-detectable. Improper trivalent chrome tanning oxidizes into highly carcinogenic & allergenic Cr(VI).

Q2: Traceability requirements of GRS Global Recycled Standard for footwear materials

Recycled Content Threshold

To attach official GRS eco-label on single components (recycled polyester mesh, recycled laces, recycled rubber soles): minimum 20% post-consumer / pre-consumer recycled material share.

To make explicit recycled content marketing claims: minimum 50% recycled ratio required.

Full-Chain TC Transaction Certificate Traceability

GRS requires closed-loop supply chain tracking beyond lab testing. Every transaction link – plastic chip factory, spinning mill, lamination plant, footwear manufacturer – must apply for unique TC certificates from certification bodies. Broken TC chain at any stage disqualifies finished shoes from carrying recycled eco-labels.

Q3: LWG Leather Working Group Certification: Audit Core Modules & Zero-Pass Failure Clauses

Certification Definition

World's most authoritative sustainability audit for leather supply chains, conducted by third parties SGS / Eurofins. No single-leather lab testing; 2–3 days full on-site inspection of tannery environmental operations.

17 Core Audit Modules

Per-square-foot leather power & water consumption quantification, 100% MRSL-compliant chemical management, legal hazardous solid waste disposal, independent wastewater treatment plant (WTP) operation standards, etc.

Rating Rules & Zero-Tolerance Failure Items

Ratings: Gold ≥85 points, Silver ≥75 points, Bronze ≥65 points. Not average total score based – single severe non-compliance downgrades overall rating directly.

Zero-pass fatal violations leading to full audit failure: uncontrolled Cr(VI) generation, missing real-time WTP online monitoring, illegal sewage discharge.