Rubber Tension Set Testing: Determination of Tensile Permanent Deformation of Rubber

Tension set testing quantifies the permanent deformation a rubber material retains after being stretched to a specified elongation, held for a defined period, and released, providing a direct measure of elastic recovery and long-term reliability. This article explains the principle of rubber tensile permanent deformation measurement, the relevant ISO, ASTM, and IEC standards, specimen requirements, step-by-step test procedures, calculation formulas, and typical industrial applications. It also answers frequently asked questions to help engineers and quality professionals select suitable test conditions and interpret results correctly.

Technical Overview

Tension set, also referred to as tensile permanent deformation or tensile permanent set, is the residual extension that remains in a rubber test piece after it has been elongated by a specified amount, held in that position for a defined time, and then allowed to recover freely. The result is expressed as a percentage of the original gauge length. An ideal elastomer would return completely to its original dimensions, while real crosslinked rubber exhibits a finite set due to molecular chain slippage, rearrangement of the filler network, incomplete crosslinking, or degradation of the polymer network.

The principal standard governing this measurement is ISO 2285, which specifies methods for determining tension set under constant elongation, as well as tension set, elongation, and creep under constant tensile load for vulcanized and thermoplastic rubbers. In tensile property testing, ASTM D412 includes procedures for set after break and set after a specified elongation. Specimen geometry follows ISO 37, GB/T 528, and ASTM D412 dumbbell or ring dies, while conditioning atmospheres follow ISO 291. For crosslinked cable compounds, the hot set test in IEC 60811-507 is a specialized elevated-temperature tension set method. JIS K 6273 is the widely used Japanese equivalent of ISO 2285.

Lower tension set values indicate better elastic recovery and dimensional stability. The test is routinely used alongside tensile strength, elongation at break, and compression set to qualify compounds, evaluate cure state, and assess aging resistance in both in-house laboratories and accredited third-party laboratories.

Testing Samples

Test pieces are normally cut from standard cured sheets with a preferred thickness of 2.0 mm plus or minus 0.2 mm. Specimens taken from finished products may also be tested when their shape and thickness permit meaningful results.

Standard Vulcanized Rubber Sheets

Dumbbell specimens are stamped from compression-molded or calendar-cured sheets using a certified cutting die. The gauge section must be free of voids, nicks, and visible defects, and the direction of milling or grain should be recorded because anisotropy can influence recovery behavior.

Thermoplastic Elastomers

TPE, TPV, and SEBS-based materials are prepared as injection-molded or compression-molded plaques and cut into the same dumbbell geometry. Because thermoplastic phases can flow under sustained strain, dwell time and test temperature must be tightly controlled to ensure reproducible results.

Finished Rubber Products

Strips or ring specimens can be die-cut or sectioned from products such as O-rings, tubing, belts, membranes, and gloves. Results on product samples may deviate from standard sheet data due to processing history, geometry, and non-uniform crosslinking, so the sampling location should always be documented.

Aged and Conditioned Specimens

Tension set is a sensitive indicator of degradation. Specimens exposed to heat aging per ISO 188, ozone, or fluid immersion can be retested to quantify the loss of elastic recovery caused by chain scission, additional crosslinking, or extractable plasticizer loss.

Testing Items

Typical tension set test programs include the following measurement items:

• Tension set under constant elongation at standard laboratory temperature of 23 °C and 50 % relative humidity
• Tension set at elevated temperatures, such as 70 °C, 100 °C, or 125 °C, in a circulating air oven
• Tensile set after rupture, determined immediately following a tensile strength test to break
• Elongation and creep under a constant tensile load, with residual set measured after unloading
• Hot set and hot elongation verification for crosslinked cable insulation and sheathing compounds
• Change in tension set after heat aging or chemical immersion, comparing aged and unaged specimens

Commonly specified parameter combinations include elongations of 50 %, 100 %, or 200 %, dwell periods of 24 h or 72 h, and a free recovery period of 30 minutes at standard laboratory temperature before the final length measurement.

Testing Methods

Specimen Preparation and Conditioning

Dumbbell specimens, for example ISO 37 Type 2 or ASTM D412 Die C, are marked with a fine ink line or optical marks at the gauge length, typically 20 mm or 25 mm, using an accurate template. Thickness is measured at three points across the gauge section with a dead-weight thickness gauge. Specimens are conditioned at 23 plus or minus 2 °C and 50 plus or minus 5 % relative humidity for at least 16 hours in accordance with ISO 291. A minimum of three specimens is tested and the median value is reported.

Constant-Elongation Tension Set Procedure

The initial distance between the gauge marks, L0, is measured to within 1 % using a calibrated rule or optical comparator. The specimen is then mounted in a stretching fixture and elongated to the specified extension, for example doubling the mark spacing for 100 % elongation. The fixture holds the specimen at the chosen elongation for the specified dwell time, either at room temperature or inside a temperature-controlled air oven complying with ISO 188 requirements for elevated-temperature testing. At the end of the dwell period, the specimen is released quickly and uniformly without subjecting it to additional strain, then allowed to recover freely for 30 minutes at standard laboratory temperature. The recovered distance between the marks, L1, is measured, and the tension set is calculated as (L1 minus L0) divided by L0, multiplied by 100 percent.

Tensile Set After Rupture per ASTM D412

In a conventional tensile test, the specimen is pulled to break at the specified separation speed. After a defined rest interval, commonly 10 minutes, the two fractured ends are carefully butted together and the distance between the original bench marks is measured. The set after break is calculated from the difference between this distance and the initial gauge length. A variant of the method elongates the specimen to a specified extension, holds it briefly, allows retraction, and records the residual set at a given retraction time.

Constant-Load Creep and Recovery Method

ISO 2285 also defines a procedure in which a constant tensile load, applied as a dead weight, produces a defined stress in the gauge section. The increase in gauge length is recorded continuously as creep, and after removal of the load the residual extension is measured to obtain the tension set under constant tensile load. This method is particularly useful for materials subjected to sustained static loading in service.

Data Expression and Reporting

The test report should identify the standard followed, specimen type and thickness, test temperature, applied elongation or load, dwell time, recovery time, and the individual and median set values in percent. Any deviation from the reference procedure, such as product-derived specimens or modified recovery periods, must be clearly stated so results remain comparable between laboratories.

Applications

Tension set data support design, qualification, and quality control decisions across many rubber-intensive industries:

• Seals and gaskets: O-rings and lip seals are stretched during installation and must recover fully to maintain sealing force over time
• Wire and cable: hot set testing verifies the degree of crosslinking in XLPE and EPR insulation, with typical acceptance limits such as maximum elongation under load and maximum permanent set after release per IEC 60811-507
• Elastic consumer products: rubber bands, elastic straps, and waistbands require low permanent set after repeated stretching
• Automotive components: hoses, bellows, diaphragms, and vibration isolators where sustained strain occurs under heat
• Medical gloves and elastomeric closures: dimensional recovery after donning or compression is critical to fit and function
• Material development: comparing formulations to optimize crosslink density, cure system, filler loading, and plasticizer content
• Aging and lifetime assessment: tracking the increase in tension set after thermal or environmental exposure as an indicator of network degradation

FAQ

What is the difference between tension set and compression set?

Both measure permanent deformation and the loss of elastic recovery, but under opposite modes of strain. Tension set stretches the specimen, while compression set, measured per ISO 815, compresses a cylindrical specimen between plates. Seals loaded in compression are usually evaluated by compression set, whereas parts stretched in service are better represented by tension set.

What elongation level should be selected for testing?

The elongation should reflect the actual strain the part experiences in service. Common choices are 100 % or 200 % for general compounds, while product specifications, particularly in the cable industry, define their own levels. Higher elongation, longer dwell time, and higher test temperature all increase the measured set.

What does a high tension set value indicate?

A high value signals poor elastic recovery, which may result from under-cure, insufficient crosslink density, excessive filler or plasticizer loading, molecular flow in thermoplastic phases, or oxidative degradation of the polymer network. In service, this can translate into leakage, loose fit, or loss of sealing and clamping force.

Are there universal pass or fail criteria?

No single limit applies to all products. Acceptance criteria are set by product standards or customer specifications; for example, cable standards typically limit permanent elongation after the hot set test, while seal specifications frequently require tension set below roughly 10 to 20 percent. The applicable limit must always be agreed in the technical specification.

Which standard should be referenced for a test request?

Specify ISO 2285 for constant-elongation or constant-load tension set, ASTM D412 for set determined during tensile testing, and IEC 60811-507 for hot set testing of crosslinked cable materials. Equivalent national standards such as JIS K 6273 may be referenced where regional compliance is required, and accredited third-party laboratories can advise on the most appropriate method for a given application.