Thermoplastic Vulcanizate Recycled Content Grade: Advanced Formulations And Sustainable Manufacturing Strategies

Molecular Composition And Structural Characteristics Of Thermoplastic Vulcanizate Recycled Content Grade

Thermoplastic vulcanizate recycled content grades are engineered composite materials featuring a biphasic morphology wherein crosslinked elastomer particles (typically 0.5–10 μm in diameter) are dispersed within a continuous thermoplastic matrix 2. The fundamental architecture consists of dynamically vulcanized rubber—commonly ethylene propylene diene monomer (EPDM) containing ≥40 wt% ethylene-derived units—embedded in an isotactic polypropylene (PP) or thermoplastic polyurethane (TPU) matrix 9,16. What distinguishes recycled content grades is the incorporation of devulcanized rubber derived from post-consumer sources such as end-of-life tires, which undergoes controlled depolymerization to restore processability while retaining sufficient molecular weight for subsequent re-crosslinking 1.

The recycled rubber component typically comprises crosslinkable devulcanized rubber blended with virgin polyolefins at ratios ranging from 40:60 to 70:30 (recycled:virgin) by weight 1. Critical to phase integrity is the inclusion of compatibilizers—often propylene-ethylene-diene terpolymers (PEDM) containing ≥60 wt% propylene-derived units with heat of fusion (Hf) values of 2–10 J/g—which facilitate interfacial adhesion between the polar recycled rubber domains and nonpolar thermoplastic matrix 9. The molecular architecture is further optimized through multimodal polymer compositions, wherein 45–75 wt% of a first high-molecular-weight elastomer fraction (Mooney viscosity >200 ML at 125°C) is combined with 25–55 wt% of a second lower-viscosity fraction to balance processability with mechanical performance 10.

Recent formulations incorporate re-refined oils—petroleum-derived extenders that have undergone secondary refining from used lubricants—at loadings of 30–250 parts per hundred rubber (phr), which simultaneously reduce apparent viscosity during processing and lower the carbon footprint by 15–30% compared to virgin paraffinic oils 3,4. The thermoplastic resin content typically ranges from 20–300 phr (or 5–50 wt% of total composition), with the weight ratio of cured elastomer to thermoplastic maintained below 1.25 to ensure adequate continuous-phase connectivity for thermoplastic processability 6,10.

Precursors And Synthesis Routes For Thermoplastic Vulcanizate Recycled Content Grade

Devulcanized Rubber Precursors From Post-Consumer Sources

The primary recycled precursor in these TPV grades is devulcanized rubber obtained through thermomechanical, chemical, or microwave-assisted depolymerization of vulcanized scrap rubber 1. End-of-life tire rubber undergoes size reduction to <5 mm crumb, followed by devulcanization at 180–250°C under shear forces of 50–150 MPa for 5–15 minutes, which selectively cleaves polysulfidic and disulfidic crosslinks (bond dissociation energy ~250–270 kJ/mol) while preserving the carbon-carbon backbone 1. The resulting devulcanized rubber exhibits Mooney viscosity of 40–80 ML (1+4 at 100°C) and sol fraction of 30–60 wt%, indicating partial network breakdown with retention of sufficient molecular entanglements for re-vulcanization 1.

Quality control parameters for devulcanized rubber include: residual crosslink density <0.5×10⁻⁴ mol/cm³ (measured by equilibrium swelling in toluene), gel content 40–70 wt%, and absence of metal contaminants (<50 ppm Fe, <20 ppm Zn) that could interfere with subsequent cure chemistry 1. The devulcanized material is typically supplied as free-flowing granules with particle size <8 mm and bulk density 0.45–0.55 g/cm³ to facilitate metered feeding into compounding equipment 15.

Re-Refined Oil Integration And Carbon Footprint Reduction

Re-refined oils represent a critical sustainability component, produced by vacuum distillation and hydrotreatment of used motor oils to remove oxidation products, metal wear particles, and additive residues 3,4. These recycled extenders meet Group II base oil specifications (viscosity index >80, sulfur content <0.03 wt%, saturates >90 wt%) and are incorporated at 50–150 phr to plasticize the elastomer phase and reduce melt viscosity during dynamic vulcanization 3. Comparative lifecycle analysis demonstrates that re-refined oil-based TPV formulations exhibit 18–25% lower cradle-to-gate CO₂ equivalent emissions (typically 2.8–3.2 kg CO₂e per kg TPV) versus virgin paraffinic oil formulations (3.5–4.0 kg CO₂e per kg TPV) 4.

The re-refined oil must exhibit kinematic viscosity of 95–110 cSt at 40°C, pour point <−15°C, and flash point >200°C to ensure adequate low-temperature flexibility and processing safety 3. Compatibility with EPDM rubber is verified through Hansen solubility parameter matching (δ_oil ≈ 16.5–17.2 MPa^0.5 vs. δ_EPDM ≈ 16.8 MPa^0.5) and absence of phase separation after 168 hours at 100°C 4.

Dynamic Vulcanization Process Parameters

The synthesis of recycled content TPV grades employs continuous dynamic vulcanization in twin-screw extruders or batch internal mixers operating at 180–220°C with screw speeds of 200–400 rpm 1,15. The process sequence involves:

• Stage 1 (Melting & Dispersion, 0–2 min): Thermoplastic resin and devulcanized rubber are co-fed and melt-mixed at 180–190°C to achieve initial phase blending, with specific mechanical energy input of 0.15–0.25 kWh/kg 15.

• Stage 2 (Oil Addition & Homogenization, 2–4 min): Re-refined oil is injected at 190–200°C under vacuum (50–100 mbar) to minimize oxidative degradation, reducing blend viscosity from ~50,000 Pa·s to 8,000–12,000 Pa·s at 100 s⁻¹ shear rate 3,10.

• Stage 3 (Cure System Addition, 4–5 min): Phenolic resin curatives (2–4 phr) with stannous chloride activator (0.5–1.5 phr) or peroxide systems (0.5–2 phr dicumyl peroxide) are introduced, initiating crosslinking reactions with t₉₀ (90% cure time) of 3–6 minutes at 200°C 1,10.

• Stage 4 (Dynamic Vulcanization, 5–8 min): Intensive mixing at 200–220°C and shear rates >1000 s⁻¹ fragments the curing rubber into discrete particles (0.5–5 μm) while crosslink density increases to 1.5–3.0×10⁻⁴ mol/cm³, as confirmed by rheometric torque plateau 1,15.

Critical process controls include maintaining melt temperature below 230°C to prevent thermoplastic degradation (onset of PP chain scission), residence time of 6–10 minutes to achieve >85% cure conversion, and specific energy input of 0.35–0.50 kWh/kg total 15. The extrudate is pelletized underwater at 15–25°C and dried to <0.05 wt% moisture before packaging 1.

Physical And Mechanical Properties Of Recycled Content Thermoplastic Vulcanizate Grades

Tensile And Elastic Performance Characteristics

Optimized recycled content TPV formulations achieve tensile strength at break of 8–14 MPa (ASTM D412, dumbbell specimens, 500 mm/min crosshead speed), representing 75–90% of virgin-material TPV performance 5. Ultimate elongation ranges from 200–450%, with permanent set after 100% extension typically <15%, indicating effective elastic recovery 6,5. The elastic modulus at 100% strain (M100) falls within 3.5–6.5 MPa, while Shore A hardness spans 55–85 depending on oil loading and cure density 2,16.

Tear strength measured by ASTM D624 Die C method yields values of 25–40 kN/m (140–225 lb-f/in) at 23°C, which is critical for applications involving sharp-edge contact or cyclic flexing 5. Compression set after 22 hours at 70°C under 25% deflection ranges from 25–45%, with lower values (<30%) achieved through optimized cure systems and higher crosslink density 1. Dynamic mechanical analysis (DMA) reveals storage modulus (E') of 15–35 MPa at 23°C (1 Hz), with tan δ peak (glass transition) occurring at −45 to −35°C for EPDM-based grades, confirming retention of low-temperature flexibility 10.

Comparative testing demonstrates that TPVs containing 50 wt% devulcanized tire rubber exhibit tensile strength of 9.2 MPa and elongation of 320%, versus 11.5 MPa and 380% for virgin EPDM controls—a performance retention of 80% and 84% respectively 1. The slight reduction is attributed to residual crosslink heterogeneity in devulcanized rubber and presence of inert fillers (carbon black, silica) from the original tire formulation 1.

Thermal Stability And Processing Window

Thermogravimetric analysis (TGA) under nitrogen atmosphere shows onset of decomposition (5% mass loss) at 320–360°C for recycled content TPV grades, with maximum decomposition rate occurring at 420–460°C 1. The processing window—defined as the temperature range between melting point of the thermoplastic phase (Tm = 160–168°C for PP) and onset of thermal degradation—spans 165–230°C, providing adequate latitude for injection molding (190–220°C) and extrusion (180–210°C) operations 15.

Melt flow rate (MFR) measured at 230°C under 2.16 kg load ranges from 5–25 g/10 min depending on oil content and molecular weight distribution, with higher MFR grades (15–25 g/10 min) preferred for thin-wall injection molding and lower MFR grades (5–12 g/10 min) suited for extrusion profiles 2,10. Capillary rheometry at 200°C reveals shear-thinning behavior with apparent viscosity decreasing from 8,000 Pa·s at 10 s⁻¹ to 800 Pa·s at 1000 s⁻¹, facilitating mold filling in complex geometries 3.

Heat aging resistance is evaluated by exposing specimens to 100°C air circulation for 168 hours, after which tensile strength retention should exceed 75% and elongation retention >60% to meet automotive weatherseal specifications 8. Oxidative induction time (OIT) measured by differential scanning calorimetry (DSC) under oxygen atmosphere at 200°C typically ranges from 8–15 minutes for stabilized formulations containing 0.3–0.8 wt% hindered phenolic antioxidants 1.

Chemical Resistance And Environmental Durability

Recycled content TPV grades demonstrate volume swell of <15% after 168 hours immersion in ASTM Oil No. 3 at 100°C, and <8% in distilled water at 70°C, indicating adequate resistance to automotive fluids and aqueous environments 8. Resistance to polar solvents is moderate, with methanol and ethanol causing 20–35% volume swell due to plasticizer extraction, while nonpolar hydrocarbons (hexane, toluene) induce 10–18% swelling primarily through rubber phase interaction 1.

Ozone resistance testing per ASTM D1149 (50 pphm ozone, 40°C, 20% strain, 168 hours) shows no visible cracking for properly cured grades, attributing to the saturated backbone of EPDM rubber and protective effect of the thermoplastic continuous phase 16. UV weathering in QUV-A chamber (340 nm, 0.89 W/m²·nm, 8 hours UV at 60°C / 4 hours condensation at 50°C, 2000 hours total) results in <20% tensile strength loss and ΔE color change <5 units for carbon black-pigmented grades 1.

Low-temperature impact resistance measured by Izod method (ASTM D256, notched specimens) at −40°C yields values of 8–15 kJ/m², sufficient for automotive exterior applications in cold climates 6. Brittle point determined by ASTM D746 occurs below −50°C for optimized formulations, ensuring flexibility across the typical automotive service temperature range of −40 to +120°C 9.

Applications Of Thermoplastic Vulcanizate Recycled Content Grade Across Industries

Automotive Weathersealing And Glazing Systems

Recycled content TPV grades are increasingly specified for automotive door seals, window encapsulation, and trunk seals where combination of elastic recovery, low compression set, and environmental durability is required 8,17. These applications demand materials that maintain sealing force over −40 to +90°C service range while withstanding 10+ years outdoor exposure to UV, ozone, and temperature cycling 8. Typical formulations for weatherseals contain 55–65 wt% EPDM (30–40% from devulcanized sources), 25–35 wt% PP, and 40–60 phr re-refined paraffinic oil, achieving Shore A hardness of 60–70 and compression set <35% 1,3.

A critical performance metric is coefficient of friction (COF) against glass and painted metal surfaces, which must remain <0.6 (static) and <0.4 (dynamic) to prevent squeak and stick-slip noise during window operation 8. This is achieved through incorporation of 2–5 wt% migratory silicone additives (polydimethylsiloxane with viscosity 10,000–50,000 cSt) that bloom to the surface, creating a lubricious boundary layer 8. Co-extrusion processes combine dense TPV skin layers (for sealing) with microcellular foam cores (density 0.3–0.5 g/cm³, cell size 50–200 μm) to reduce weight by 20–30% while maintaining sealing performance 17.

Injection-molded corner pieces and end-caps utilize higher-flow TPV grades (MFR 18–25 g/10 min at 230°C) with 40–50 wt% recycled content, achieving cycle times of 25–40 seconds for parts weighing 50–150 g 8. Adhesion to extruded profiles is accomplished through overmolding at 200–215°C, where the molten TPV partially melts and interdiffuses with the substrate surface, creating bond strengths of 8–12 N/mm (180° peel test) without primers or adhesives 8.

Under-Hood Automotive Components

The thermal stability and fluid resistance of recycled content TPVs enable their use in under-hood applications including air intake ducts, turbocharger hoses, and engine mount isolators 17. These components must withstand continuous exposure to 120–150°C with intermittent peaks to 180°C, plus contact with engine oils, coolants, and fuel vapors 6. Formulations for under-hood use typically employ thermoplastic copolyester elastomer (TPCE) matrices instead of PP, combined with highly saturated EPDM containing <3% diene content to maximize thermal oxidation resistance 6.

A representative under-hood TPV grade contains 35–45