Thermoplastic Vulcanizate Low Hardness Grade: Comprehensive Analysis Of Formulation, Processing, And Applications

Molecular Composition And Structural Characteristics Of Thermoplastic Vulcanizate Low Hardness Grade

Low hardness thermoplastic vulcanizates are heterogeneous polymer blends comprising a continuous thermoplastic phase and a dispersed, dynamically crosslinked rubber phase, with the rubber content typically exceeding 60 wt% to achieve the desired softness 1. The thermoplastic component most commonly consists of polypropylene (PP), particularly random propylene copolymers with melting points below 105°C, which facilitate processing at lower temperatures and contribute to the overall flexibility of the compound 1. In advanced formulations, propylene-based elastomers formed from at least three alpha-olefins are incorporated to further reduce hardness, enabling Shore A values of 45 or below while maintaining processability 2.

The rubber phase predominantly comprises ethylene-propylene-diene monomer (EPDM) rubber or ethylene-propylene copolymers, selected for their excellent weatherability, ozone resistance, and compatibility with polyolefin thermoplastics 67. The molecular weight of the rubber component significantly influences final properties; EPDM rubbers with weight-average molecular weights (Mw) ranging from 500,000 to 3,000,000 g/mol and narrow molecular weight distributions (Mw/Mn ≤ 4.0) are preferred for achieving optimal dispersion morphology and mechanical performance 9. The branching architecture of the rubber, quantified by the g'vis parameter (≥0.90), ensures adequate entanglement density for stress transfer while permitting the fine particle dispersion necessary for thermoplastic processing 9.

Dynamic vulcanization is effected using phenolic resin curatives or silicon-containing crosslinking systems, which selectively crosslink the rubber phase during high-shear melt mixing 1014. The degree of cure is critical: achieving greater than 94 wt% gel content (insoluble fraction in cyclohexane at 23°C) ensures dimensional stability and elastic recovery, while avoiding excessive crosslinking that would compromise processability 1014. The crosslinked rubber particles, typically 0.5–10 μm in diameter, are uniformly dispersed within the thermoplastic matrix, creating a co-continuous or near-co-continuous morphology at low hardness grades 16.

Process oils, predominantly paraffinic or naphthenic types, are incorporated at high loadings (often exceeding 200 phr relative to rubber content) to plasticize both phases and reduce hardness 6. The oil-to-elastomer ratio is a primary determinant of softness; ratios of 2:1 or higher are common in ultra-soft grades, though this must be balanced against potential oil migration and surface exudation 6. The thermoplastic-to-rubber weight ratio typically ranges from 15:85 to 30:70 in low hardness formulations, with the lower thermoplastic content providing maximum softness while maintaining just sufficient continuous phase for melt processing 16.

Formulation Strategies For Achieving Shore A Hardness Below 50

Achieving Shore A hardness values below 50 in thermoplastic vulcanizates requires systematic optimization of multiple formulation variables, each contributing synergistically to the final softness profile.

Thermoplastic Phase Selection And Modification

The choice of thermoplastic resin profoundly impacts both hardness and processing characteristics. Random propylene copolymers containing 1–20 wt% ethylene or higher alpha-olefins exhibit lower crystallinity and melting points (80–105°C) compared to isotactic polypropylene homopolymer, directly reducing the hardness contribution of the continuous phase 118. These copolymers are characterized by g' ratios less than 1.0 (measured at number-average molecular weight), indicating long-chain branching that disrupts crystalline packing and enhances chain mobility 13. Melt flow rates (MFR) of 0.01–50 g/10 min (2.16 kg at 230°C) provide adequate processability while maintaining molecular weight sufficient for mechanical integrity 13.

In specialized formulations, thermoplastic polyurethanes (TPU) with Shore A hardness ≥70A are blended with softer rubbers at weight ratios of 30:70 to 70:30, where the TPU hardness must exceed the rubber hardness by at least 19 Shore A points to establish a stable continuous phase 35. This approach is particularly effective for applications requiring enhanced abrasion resistance and grip, such as footwear outsoles, while maintaining overall softness through high rubber loading 35.

Butene-1-based polymers, either as homopolymers or copolymers, are increasingly employed in the thermoplastic phase at concentrations of 15–50 wt% (relative to total thermoplastic content) to reduce hardness and improve compression set resistance 1014. Isotactic poly(butene-1) exhibits a lower modulus than polypropylene and undergoes a crystal phase transition that contributes to long-term dimensional stability 10. The combination of 50–85 wt% propylene-based polymer with 15–50 wt% butene-1-based polymer in the thermoplastic phase enables hardness reduction while maintaining tensile strength above 8 MPa and tear strength exceeding 190 lb-f/in 14.

Rubber Phase Optimization And Crosslinking Control

The rubber component must be selected and crosslinked to provide maximum elasticity without compromising the softness objective. EPDM terpolymers containing 40–70 wt% ethylene, 25–55 wt% propylene, and 2–12 wt% non-conjugated diene (typically ethylidene norbornene or dicyclopentadiene) offer an optimal balance of cure rate, mechanical properties, and oil compatibility 711. The diene content provides crosslinking sites while maintaining the saturated backbone that confers excellent thermal and oxidative stability 7.

Phenolic resin curatives, particularly alkylphenol-formaldehyde resins activated with stannous chloride or zinc oxide, are preferred for EPDM vulcanization in TPV systems due to their ability to generate high crosslink densities without excessive scorch during dynamic vulcanization 1014. Curative loadings of 2–8 phr (relative to rubber content) are typical, with higher loadings increasing hardness through enhanced crosslink density 6. Silicon-containing curatives, such as bis(triethoxysilylpropyl)tetrasulfide, offer alternative crosslinking mechanisms with improved heat aging resistance, though they generally require longer cure times 10.

The degree of cure must be carefully controlled: gel contents below 90 wt% result in insufficient elastic recovery and poor compression set, while gel contents approaching 100 wt% increase hardness and reduce processability 1014. Dynamic vulcanization conditions—mixing temperature (180–230°C), rotor speed (50–150 rpm), and residence time (3–10 minutes)—are optimized to achieve the target gel content while maintaining fine rubber particle dispersion 6.

Plasticizer Selection And Loading Optimization

Process oils are essential for achieving low hardness grades, acting as both processing aids and permanent plasticizers. Paraffinic oils with kinematic viscosities of 100–400 cSt at 40°C are most commonly employed due to their excellent compatibility with polyolefins and EPDM, minimal volatility, and regulatory acceptance 6. Naphthenic oils offer enhanced low-temperature flexibility but may exhibit greater migration tendencies 6.

Oil loadings in ultra-soft TPV formulations typically range from 200 to 400 phr relative to rubber content, corresponding to 50–70 wt% of the total composition 16. At these high loadings, the oil plasticizes both the rubber particles and the thermoplastic matrix, dramatically reducing hardness while maintaining a processable melt viscosity 6. However, excessive oil content (>350 phr) can lead to surface exudation, dimensional instability, and reduced mechanical properties 6. The oil-to-elastomer ratio is a more critical parameter than absolute oil content; ratios of 2:1 to 3:1 are typical for Shore A hardness values of 20–35 6.

Alternative plasticizers, including bio-based oils (e.g., epoxidized soybean oil, castor oil derivatives) and low-molecular-weight polyolefin oligomers, are being explored to address sustainability concerns and improve specific performance attributes such as low-temperature flexibility or reduced migration 1. These alternatives must be carefully evaluated for compatibility, processing effects, and long-term stability.

Additive Systems For Property Enhancement

Beyond the primary polymer and oil components, various additives are incorporated to optimize processing, stability, and end-use performance:

• Nucleating agents (0.1–1.0 wt%) such as sodium benzoate or sorbitol derivatives accelerate crystallization of the thermoplastic phase during cooling, reducing cycle times in molding operations and improving dimensional stability 8. However, nucleating agents must be used judiciously in low hardness grades, as excessive crystallinity can increase hardness 8.

• Antioxidants and stabilizers (0.5–2.0 wt%), including hindered phenols, phosphites, and hindered amine light stabilizers (HALS), protect against thermal and oxidative degradation during processing and service 7. EPDM-based TPVs are inherently resistant to ozone attack due to the saturated backbone, but stabilization is still required for long-term heat aging performance 7.

• Fillers such as calcium carbonate, talc, or silica (0–30 wt%) can be added to reduce cost, improve stiffness, or modify surface properties, though filler addition generally increases hardness and must be minimized in ultra-soft grades 12. When fillers are employed, surface-treated grades with improved polymer-filler interactions are preferred to maintain mechanical properties 12.

• Compatibilizers (1–5 wt%), including maleic anhydride-grafted polyolefins or styrenic block copolymers, enhance interfacial adhesion between the thermoplastic and rubber phases, improving mechanical properties and reducing phase separation during processing 1617. In TPU-based low hardness TPVs, interfacial compatible resins at 5–15 phr are critical for achieving stable morphology 35.

• Foaming agents (0.5–5.0 wt%), such as azodicarbonamide or expandable microspheres, enable production of foamed TPV structures with densities reduced by 20–50%, further decreasing effective hardness and improving cushioning performance 2. Chemical foaming agents are activated during processing, generating gas that expands the melt; physical foaming agents (e.g., supercritical CO₂) offer cleaner processing but require specialized equipment 2.

Processing Technologies And Manufacturing Considerations For Low Hardness Thermoplastic Vulcanizates

The production of low hardness TPV grades presents unique processing challenges due to the high rubber and oil content, requiring careful optimization of equipment, process parameters, and quality control procedures.

Dynamic Vulcanization Process Parameters

Dynamic vulcanization is typically conducted in continuous twin-screw extruders with co-rotating, intermeshing screw designs that provide high shear and efficient mixing 718. The process sequence involves:

1. Feeding and melting: The thermoplastic resin is fed into the first barrel zones (150–180°C) and melted to form a continuous phase. Rubber, in the form of bales or pellets, is introduced downstream and dispersed into the molten thermoplastic 18.

2. Oil incorporation: Process oil is injected through liquid feeders into the mixing zones, where it is absorbed by both polymer phases. High oil loadings require multiple injection points to prevent flooding and ensure uniform distribution 6.

3. Curative addition and crosslinking: The crosslinking system (curative, activators, accelerators) is added in dedicated mixing zones maintained at 180–220°C. The high shear environment (screw speeds of 200–400 rpm) breaks up the rubber phase into fine particles while simultaneously crosslinking them 618. Residence time in the vulcanization zone is typically 1–3 minutes, sufficient for achieving >94% gel content 10.

4. Additive incorporation: Stabilizers, fillers, and other additives are introduced in downstream zones after the primary vulcanization is complete, minimizing their exposure to high-temperature, high-shear conditions 18.

5. Devolatilization and pelletization: Volatile byproducts from the crosslinking reaction are removed through vacuum vents. The extrudate is cooled, pelletized, and packaged for subsequent processing 18.

Screw design is critical for low hardness grades: high-shear mixing elements (kneading blocks, turbine mixers) are required in the vulcanization zone to achieve fine rubber particle dispersion, while downstream zones employ conveying elements to minimize additional shear that could cause oil separation 18. Barrel temperature profiles are carefully controlled, with peak temperatures of 200–230°C in the vulcanization zone and cooling to 160–180°C in the die zone to prevent premature solidification 6.

Secondary Processing: Injection Molding, Extrusion, And Blow Molding

Low hardness TPV pellets are processed into finished parts using conventional thermoplastic equipment, though process parameters must be adjusted to accommodate the soft, highly filled nature of the material.

Injection molding is widely used for complex geometries such as seals, grips, and soft-touch components. Melt temperatures of 180–220°C and injection pressures of 50–120 MPa are typical, with lower values preferred to minimize oil separation and flash 1. Mold temperatures of 20–50°C promote rapid solidification and dimensional stability. Cycle times are generally longer than for rigid thermoplastics due to the lower thermal conductivity of the oil-extended compound 8. Gate design must minimize shear heating; fan gates or film gates are preferred over pin gates 1.

Extrusion is employed for profiles, tubing, and sheet applications. Single-screw extruders with L/D ratios of 24:1 to 30:1 and compression ratios of 2.5:1 to 3.5:1 provide adequate melting and conveying 8. Melt temperatures are maintained at 180–210°C, with die temperatures of 170–190°C. Die swell is typically 10–30% higher than for rigid polyolefins due to the elastic rubber phase, requiring die design adjustments 8. Cooling is achieved through water baths or air rings, with careful control to prevent surface defects from rapid oil migration 8.

Blow molding (extrusion blow molding and injection stretch blow molding) is used for hollow articles such as bellows and flexible containers. Parison programming is critical to achieve uniform wall thickness distribution, as the low melt strength of oil-extended TPVs can lead to excessive sag 1. Mold temperatures of 30–60°C and blow pressures of 0.4–0.8 MPa are typical 1.

Thermoforming of extruded TPV sheet is feasible for low hardness grades, though the process window is narrow due to the limited melt strength. Forming temperatures of 150–180°C and vacuum pressures of 0.6–0.9 bar are employed, with rapid cooling to set the shape 1.

Quality Control And Testing Protocols

Rigorous quality control is essential to ensure consistent performance of low hardness TPV grades. Key test methods include:

• Hardness measurement: Shore A hardness (ASTM D2240) is the primary specification parameter, measured at 15 seconds after indentation on unaged specimens. For ultra-soft grades (Shore A <30), Shore 00 scales may be more appropriate 27. Hardness is temperature-dependent; measurements at 23°C ± 2°C are standard, but testing at service temperatures (e.g., -40°C, 70°C) is recommended for automotive applications 11.

• Tensile properties: Tensile strength at break, elongation at break, and 100% modulus are measured per AST