How To Systematically Enhance The Low-Temperature Resistance Of Rubber Products?

Against the backdrop of increasingly frequent extreme weather events and ever-more complex product operating environments, the performance of rubber materials under low-temperature conditions has become a crucial indicator for assessing their overall application value. Whether in automotive seals, cryogenic oil seals, polar exploration equipment, aerospace piping, cable sheathing, or military components, low-temperature resistance directly impacts service life and reliability.

This paper systematically summarises the low-temperature behaviour of current mainstream rubber materials, material selection strategies, plasticisation and crosslinking control, formulation optimisation approaches, alongside typical recommendations and reference data. It assists practitioners in comprehensively mastering modification methods for rubber materials operating at temperatures of -40°C and below.

I. Failure Mechanisms of Rubber Materials in Cryogenic Environments

Rubber faces two primary performance challenges at low temperatures:

1. Glass Transition (Tg): When ambient temperatures fall below a rubber's glass transition temperature (Tg), the material shifts from a highly elastic state to a glassy state. This manifests as a significant increase in rigidity and a marked decrease in flexibility, potentially culminating in brittle fracture.

2. Crystallisation behaviour: Certain rubbers (e.g., NR, CR) exhibit crystallisation at low temperatures, causing material hardening and even micro-cracking.

Consequently, the key to designing low-temperature-resistant formulations lies in lowering the Tg and suppressing low-temperature crystallisation behaviour.

II. Material Selection Priority Principle: Tg is the Primary Consideration

The following table presents typical glass transition temperatures (Tg) for common rubbers, sourced from literature compilations such as the Rubber Technology Handbook:

Recommended low-Tg material combinations: MVQ, BR, FVMQ, IIR, low-ACN NBR

III. Suppressing crystallisation behaviour: Combining polymer architecture and blending strategies

1. Risks and countermeasures for crystalline rubbers

Without appropriate compounding or plasticisation at low temperatures, materials such as NR and CR readily crystallise, leading to performance degradation.

Recommended countermeasures:

NR/BR blending: BR's non-crystalline nature effectively disrupts NR's crystallisation sequence, enhancing overall flexibility;

Controlling crosslink density: Moderately reducing crosslinking degree increases segment mobility, slowing crystallisation rate.

2. Silicone Rubber Crystallisation Countermeasures

Standard VMQ crystallises below -45°C. Introducing 5-7 mol% phenyl groups during polymerisation forms a PVMQ structure, effectively inhibiting crystallisation and extending the application lower limit to -90°C.

IV. Plasticiser Selection: The Determinant of Flexibility

An effective low-temperature plasticiser must satisfy: low viscosity, low volatility, high compatibility, low Tg, and no migration. Recommended types are as follows:

Special note: Low-molecular-weight ester monomers generally outperform high-viscosity polymeric plasticisers, with minimal quantities sufficient to enhance NR low-temperature performance.

V. Low-Temperature Design for Thermoplastic Elastomers

TPV (e.g., EPDM/PP): Switching to low-ethylene, low-crystallinity EPDM matrices extends their low-temperature limits;

TPU: Select ether-type structures with MDI as the prepolymer, which exhibit lower Tg and are suitable for cables and sealing products in cold environments.

VI. Key Points for Low-Temperature Optimisation of Various Speciality Rubbers

VII. Processing Control Recommendations

1. Control Crosslink Density

Whilst high crosslink density enhances mechanical properties, it compromises low-temperature flexibility. Sulphur content and accelerator ratios within the vulcanisation system must be appropriately regulated.

2. Co-agent Optimisation

For instance, introducing Ricon®-type liquid high-vinyl polybutadiene co-agents into EPDM peroxide vulcanisation systems outperforms traditional TMPTMA co-agents, delivering superior low-temperature performance.

VIII. Practical Case References

IX. Concluding Remarks: Systemic Thinking for Building Stable Low-Temperature Performance

Low-temperature resistance cannot be achieved through reliance on a single material or additive alone. It is the synergistic outcome of foundational rubber selection + filler matching + plasticising system + crosslinking structure + process window. Practitioners are advised to follow these principles during actual formulation design:

Define the operating temperature range, with Tg estimation as the starting point;

Integrate with actual measurements, such as TR-10, DSC, and DMA analysis for validation;

Systematically evaluate low-temperature behaviour across crosslinking, processing, ageing, and oil-medium conditions;

Conduct linked formulation-to-product validation, setting phased low-temperature resistance targets.