Polyolefin Elastomer Low Hardness Grade: Comprehensive Analysis Of Formulation, Properties, And Industrial Applications

Molecular Composition And Structural Characteristics Of Polyolefin Elastomer Low Hardness Grade

Low hardness polyolefin elastomers are predominantly based on heterophasic compositions comprising a crystalline propylene homopolymer or copolymer matrix (component A) and a soft elastomeric copolymer phase (component B) 7916. The crystalline phase typically consists of propylene homopolymer or copolymers containing up to 15 wt% ethylene or other C3-C8 alpha-olefin comonomers, providing structural integrity and processability 716. The elastomeric phase is composed of ethylene copolymers with C3-C20 alpha-olefins, containing 15-40 wt% ethylene to ensure low-temperature flexibility and rubber-like elasticity 7916.

The achievement of low hardness (Shore A <70) requires careful optimization of the weight ratio between these phases. Patent literature demonstrates that compositions with 10-50 wt% crystalline propylene component (A) and 50-90 wt% elastomeric ethylene copolymer component (B) yield Shore A hardness values below 90, with compression set values ranging from 30-65% and elongation-to-compression-set ratios exceeding 8 1620. For ultra-low hardness grades (Shore A 10-40), the elastomeric phase content must be increased further, often exceeding 70 wt%, while maintaining flexural modulus below 150 MPa 7916.

The molecular architecture of the elastomeric phase critically influences final properties. Ethylene-alpha-olefin copolymers with 74-95 mol% ethylene content and high melt flow rate ratios (MLRA/ML ≥8) provide optimal low-temperature toughness and impact resistance 15. The degree of crystallinity in the elastomeric phase should remain below 40% to preserve flexibility 10. Advanced formulations incorporate cyclic olefins (0.5-20 mol%) alongside ethylene and C3-C14 alpha-olefins to fine-tune glass transition temperature (-50°C to 30°C) and molecular weight distribution (5,000-150,000 g/mol) 1314.

Crosslinking mechanisms further differentiate low hardness polyolefin elastomers. Dynamic vulcanization using alkenyl-substituted alkoxysilane grafting agents in the presence of water generates thermoplastic vulcanizate (TPV) elastomers with compression set values of 45-65%, elongation-at-break-to-compression-set ratios above 10, and Shore A hardness below 90 7916. Static curing of silane-grafted heterophasic compositions yields thermoset polyolefin elastomers with compression set 30-65%, maintaining Shore A hardness below 90 and elongation-to-compression-set ratios above 8 20. Silane-crosslinked polyolefin elastomers with density <0.90 g/cm³ achieve compression set values of 5.0-35.0% (ASTM D395, 22 hrs @ 70°C) through incorporation of silane crosslinkers, grafting initiators, and condensation catalysts 10.

Key Performance Properties And Hardness Ranges In Polyolefin Elastomer Low Hardness Grade

Hardness Specifications And Measurement Standards

Low hardness polyolefin elastomers span a broad hardness spectrum, categorized by Shore scale measurements:

• Ultra-Low Hardness (Shore OO 35-52): Achieved through high styrene content styrene-ethylene-ethylene-propylene-styrene (SEEPS) copolymers blended with conventional SEEPS and plasticizers, yielding gel-like tactile properties suitable for cushioning and vibration damping 6.
• Low Hardness (Shore A 10-40): Thermosetting polyurethane elastomers formulated from difunctional polyols (total unsaturation ≤0.01 meq/g) and 4,4'-diphenylmethane diisocyanate (NCO content 3-6%) exhibit JIS A hardness 10-40, with low compression set, minimal moisture absorption, excellent dimensional stability, and non-bleeding characteristics 23.
• Moderate Low Hardness (Shore A 40-70): Polyetherester elastomers with Shore D 25-50 (equivalent to Shore A ~70-85) combine low crystalline polyetherester (melting point 130-170°C) with polybutyleneterephthalate and epoxy-functionalized polyolefin or polyacryl copolymers (10-50 parts by weight per 100 parts polyetherester) 1.
• Soft Hardness (Shore A 70-90): Thermoplastic vulcanizate elastomers from dynamically crosslinked heterophasic polyolefin compositions achieve Shore A <90 with compression set 45-65% and elongation-to-compression-set ratios >10 7916.

Mechanical And Elastic Performance

Low hardness polyolefin elastomers exhibit exceptional elongation at break, often exceeding 500%, with some formulations reaching >800% 79. Compression set—a critical indicator of elastic recovery—ranges from 5% to 65% depending on crosslinking method and composition 101620. Silane-crosslinked polyolefin elastomers demonstrate compression set as low as 5.0-35.0% (ASTM D395, 22 hrs @ 70°C), significantly outperforming non-crosslinked analogs 10.

Flexural modulus for low hardness grades remains ≤150 MPa, ensuring flexibility without sacrificing structural integrity 791620. Tensile strength typically ranges from 2-10 MPa, with tear strength values of 10-50 kN/m, depending on filler content and crosslink density 4. High-strength variants incorporating hydrogenated triblock copolymers, non-aromatic oils, polyolefin resins, inorganic fillers, polyphenylene ether resins, and silicone resins achieve enhanced abrasion resistance while maintaining low hardness and high strength 4.

Thermal Stability And Temperature Performance

Low hardness polyolefin elastomers maintain performance across wide temperature ranges. Glass transition temperatures (Tg) span -50°C to 30°C, enabling flexibility in sub-zero environments 1314. Service temperature ranges extend from -40°C to 120°C for automotive interior applications, with some formulations stable up to 150°C for under-the-hood components 7916. Thermogravimetric analysis (TGA) confirms thermal decomposition onset temperatures >300°C for crosslinked grades, ensuring long-term stability in elevated-temperature environments 4.

Chemical Resistance And Environmental Durability

Polyolefin elastomers exhibit excellent resistance to polar solvents, acids, bases, and aqueous media due to their non-polar hydrocarbon backbone 716. Low moisture absorption (<0.5 wt% after 24 hrs immersion) prevents dimensional changes and maintains mechanical properties in humid conditions 23. UV stabilization packages and antioxidant additives extend outdoor weathering resistance, with <10% property degradation after 2000 hrs accelerated aging (ASTM G154) 4.

Formulation Strategies And Compounding Techniques For Polyolefin Elastomer Low Hardness Grade

Base Polymer Selection And Blending Ratios

Achieving target hardness requires precise selection of base polymers and their ratios. For Shore A <70, formulations typically employ 50-90 wt% elastomeric ethylene-alpha-olefin copolymer (15-40 wt% ethylene) blended with 10-50 wt% crystalline propylene homopolymer or copolymer 7916. Ultra-low hardness grades (Shore OO 35-52) utilize high styrene content SEEPS (styrene content >40 wt%) combined with conventional SEEPS and plasticizers at ratios of 1:1 to 3:1 6.

Polyetherester elastomers for Shore D 25-50 hardness blend low crystalline polyetherester (melting point 130-170°C) with higher-hardness polyetherester (Shore D 51-80) at ratios of 100:80 to 100:230 parts by weight, incorporating polybutyleneterephthalate (4-200 parts) and epoxy-functionalized polyolefin or polyacryl copolymers (10-50 parts) 1.

Crosslinking And Vulcanization Methods

Dynamic vulcanization represents the preferred method for producing thermoplastic vulcanizate (TPV) elastomers with low hardness. The process involves melt-blending heterophasic polyolefin compositions in twin-screw extruders at 180-220°C, followed by addition of alkenyl-substituted alkoxysilane grafting agents (0.5-3 wt%) and water (0.1-1 wt%) to initiate in-situ crosslinking 7916. Crosslinking occurs within 2-5 minutes residence time, generating finely dispersed crosslinked elastomer domains (0.5-5 μm) within the thermoplastic matrix 16.

Static curing of silane-grafted compositions provides an alternative route. Silane-functionalized polyolefin elastomers (grafting degree 0.5-2 wt%) are compounded with condensation catalysts (dibutyltin dilaurate, 0.01-0.1 wt%) and moisture-cured at 23°C/50% RH for 3-7 days, achieving compression set 30-65% and Shore A <90 20. Peroxide-based crosslinking using organic peroxides (0.1-1 part per 100 parts polymer) combined with acrylic acid metallic salt mixtures (0.1-5 parts) generates foamed elastomers with high rebound resilience and low compression set 12.

Plasticizers And Processing Aids

Non-aromatic process oils (paraffinic or naphthenic, 10-50 phr) reduce hardness and improve processability without compromising mechanical properties 46. Polyamide wax (5-10 parts per 100 parts polymer) enhances low-temperature activation in hot-melt adhesive applications while maintaining ultra-low hardness 17. Compatibilizers such as maleic anhydride-grafted polyolefins (3-10 parts) improve interfacial adhesion between crystalline and elastomeric phases, reducing phase separation and enhancing mechanical integrity 117.

Fillers And Reinforcing Agents

Inorganic fillers (calcium carbonate, talc, silica) at loadings of 5-30 phr increase modulus and tear strength without significantly raising hardness 4. Silicone resins (1-5 phr) impart surface lubricity and antimicrobial activity, beneficial for medical and consumer applications 4. Polyphenylene ether resins (5-15 phr) enhance heat resistance and dimensional stability at elevated temperatures 4.

Additives For Stability And Performance

Antioxidants (hindered phenols, phosphites, 0.1-0.5 wt%) prevent thermal and oxidative degradation during processing and service 417. UV stabilizers (hindered amine light stabilizers, benzotriazoles, 0.2-1.0 wt%) extend outdoor durability 4. Anti-yellowing agents (0.5-2 wt%) maintain aesthetic appearance in transparent or light-colored grades 17. Antimicrobial agents (silver-based compounds, quaternary ammonium salts, 0.1-1.0 wt%) provide hygiene benefits for medical and food-contact applications 4.

Processing Technologies And Manufacturing Considerations For Polyolefin Elastomer Low Hardness Grade

Extrusion And Compounding

Twin-screw extrusion at 160-220°C with screw speeds of 200-400 rpm ensures homogeneous dispersion of elastomeric and crystalline phases 7916. Temperature profiles typically range from 170°C (feed zone) to 200°C (die zone) for polyolefin TPVs, with residence times of 60-120 seconds 16. For silane-grafted systems, moisture must be rigorously excluded during compounding (moisture content <0.05%) to prevent premature crosslinking 1020.

Underwater pelletizing immediately after extrusion minimizes thermal degradation and produces uniform pellets (2-4 mm diameter) suitable for subsequent processing 17. Dehydration and drying to moisture content ≤0.05% are mandatory before injection molding or film casting 17.

Injection Molding

Low hardness polyolefin elastomers are injection-molded at barrel temperatures of 180-220°C, mold temperatures of 30-60°C, and injection pressures of 50-100 MPa 616. Cycle times range from 20-60 seconds depending on part geometry and wall thickness 6. Gate design must accommodate high melt viscosity (typically 10,000-50,000 Pa·s at 200°C, 100 s⁻¹ shear rate) to prevent flow marks and weld lines 6.

For ultra-low hardness grades (Shore OO 35-52), injection molding requires specialized equipment with low-compression-ratio screws (2.0-2.5:1) and enlarged gates to prevent shear-induced degradation 6. Mold release agents (silicone-based, 0.1-0.5 wt%) facilitate demolding of soft, tacky surfaces 6.

Film And Sheet Extrusion

Cast film extrusion at 180-210°C with chill roll temperatures of 20-40°C produces films of 10-500 μm thickness for adhesive, packaging, and artificial leather applications 1117. Multi-layer coextrusion enables production of composite structures with graded hardness: a top layer of Shore A 70-90 (thickness 5-150 μm) provides abrasion resistance, while a middle layer of Shore A <70 (thickness 100-600 μm) delivers softness and cushioning 11.

Blown film extrusion at 170-200°C with blow-up ratios of 2.0-3.5 yields films with balanced mechanical properties and excellent clarity for medical and hygiene applications 11. Film thickness uniformity (±5%) is critical for consistent bonding performance in adhesive applications 17.

Foaming Processes

Chemical foaming agents (azodicarbonamide, 0.5-3 wt%) or physical blowing agents (supercritical CO₂, N₂) generate foamed elastomers with densities of 0.2-0.6 g/cm³ 1220. Foaming temperatures of 160-180°C and pressures of 10-20 MPa produce uniform cell structures (cell size 50-500 μm) with closed-cell contents >80% 12. Peroxide-crosslinked formulations exhibit superior cell stability and rebound resilience compared to non-crosslinked analogs 12.

Quality Control And Testing Protocols

Hardness measurement per ASTM D2240 (Shore A, Shore OO) or ISO 868 at 23°C/50% RH after 24 hrs conditioning ensures specification compliance 236. Compression set testing per ASTM D395 (22 hrs @ 70°C, 25% deflection) quantifies elastic recovery 101620. Tensile properties (tensile strength, elongation at break, modulus) are measured per ASTM D412 or ISO 37 47. Tear strength per ASTM D624 (Die C) assesses resistance to crack propagation 4.

Thermal analysis via differential scanning calorimetry (DSC) determines melting point, crystallinity, and glass transition temperature 1314. Thermogravimetric analysis (TGA