Understanding Polar vs. Non-Polar Rubber Material
Understanding polar and non-polar rubber material could support us to choose the right rubber material during design phrase.
1. Definition and Core Differences
The fundamental distinction between polar and non-polar rubbers lies in the presence and density of polar groups within their molecular chains. Polar groups (e.g., -CN, -Cl, -F, -COO-) alter charge distribution via inductive or conjugative effects, significantly impacting material polarity, intermolecular forces, and macroscopic properties.
Polar Rubbers:
Contain strong polar groups (e.g., nitrile in NBR, fluorine in FKM).
Exhibit strong intermolecular forces, excellent oil/solvent resistance, but poor low-temperature performance.
Non-Polar Rubbers:
Dominated by C-H structures (e.g., NR, EPDM).
Rely on van der Waals forces, offering superior elasticity and low-temperature flexibility but weak oil resistance.
2. Classification and Mechanisms of Typical Rubber Material
|
Rubber Type |
Chemical Structure |
Polarity Classification |
Key Characteristics |
|
NR |
Polyisoprene (C₅H₈) |
Non-polar |
Elasticity, low Tg (~-60°C) |
|
NBR |
Acrylonitrile-butadiene copolymer |
Strong polar |
High Tg (~-40°C), oil-resistant |
|
EPDM |
Ethylene-propylene-diene terpolymer |
Non-polar |
Thermal stability, ozone resistance |
|
CR |
Chloroprene homopolymer |
Weak-to-moderate polar |
Balanced mechanical and chemical properties |
|
FKM |
Vinylidene fluoride-hexafluoropropylene copolymer |
Strong polar |
Extreme heat/chemical resistance (Td >400°C) |
|
VMQ |
Polydimethylsiloxane (-Si-O-) |
Non-polar |
Low surface energy, high thermal stability |
3. Experimental Characterization Techniques
We can measure the relevant properties of materials through the following experimental advices, and these characterization parameters can help us verify the characteristics of polar and non-polar materials.
3.1 Differential Scanning Calorimetry (DSC)
Polarity Indicator: Glass transition temperature (Tg)
Insight: Rigid chains in polar rubbers (e.g., FKM) may decouple Tg from polarity; cross-validate with FTIR.
3.2 Thermogravimetric Analysis (TGA)
Polarity Indicator: Thermal decomposition temperature (Td)
Polar rubbers (e.g., FKM: >400°Td>400°C) outperform non-polar types (e.g., NR: ≈300° Td≈300°C)
Controversy: Non-polar VMQ shows high Td (~450°C) due to Si-O bond strength, necessitating FTIR validation.
3.3 Fourier Transform Infrared Spectroscopy (FTIR)
Core Method: Detects polar group signatures (e.g., NBR’s -CN at 2240 cm⁻¹, FKM’s C-F at 1100–1200 cm⁻¹).
Limitation: Weak polar groups (e.g., -Cl in CR) may be masked; use ATR-FTIR for surface sensitivity.
3.4 Dynamic Mechanical Analysis (DMA)
Key Metrics: Temperature-dependent storage modulus (′E′) and loss factor (tan�tanδ).
Polar rubbers (e.g., HNBR) show broad tanδ peaks (-30–100°C), reflecting heterogeneous structures.
Non-polar EPDM displays sharp tanδ peaks (homogeneous structure).
3.5 Mooney Viscometer and Curemetry
Polarity Impact: Polar rubbers (e.g., FKM: ML >80) exhibit high viscosity and slow curing due to hindered chain mobility.
Anomaly: CR (moderate polarity) shows lower viscosity (ML ≈40) than NBR, attributed to chlorine’s steric effects.
4. Challenging Traditional Perspectives
4.1 Reclassifying Chloroprene Rubber (CR)
Contradiction: CR’s oil resistance exceeds EPDM but lags behind NBR.
Revised View: Based on solubility parameters (�≈19.2 MPa1/2δ≈19.2MPa1/2), CR aligns closer to weakly polar rubbers.
4.2 Silicone Rubber’s Polarity Paradox
VMQ: Non-polar due to symmetric methyl groups offsetting Si-O polarity.
FVMQ: Fluorination (-CF₃) increases surface energy (24 →32 mN/m), enabling polarity tuning.
4.3 EPDM’s Polarity Clarification
Debunked: Despite minor polar dienes (e.g., ENB), EPDM remains non-polar unless polar monomer content exceeds 5% (per FTIR/DSC data).