Determination of Acid and Alkali Resistance Tensile Strength of Rubber

Acid and alkali resistance tensile strength testing evaluates the ability of rubber materials to retain their mechanical integrity after exposure to aggressive chemical media. This article presents a comprehensive technical overview of the testing standards, sample preparation, immersion procedures, and result calculation methods used by accredited third-party laboratories. The content is based on internationally recognized standards including ISO 37, ISO 1817, ASTM D412, ASTM D471, GB/T 528, and GB/T 1690.

Technical Overview

Rubber materials are widely used in chemical processing environments where they are continuously or intermittently exposed to acids, alkalis, salts, and other corrosive media. Prolonged contact with such chemicals can cause swelling, extraction of plasticizers, chain scission, or additional crosslinking, all of which degrade the tensile properties of the rubber. The determination of acid and alkali resistance tensile strength quantifies this degradation by comparing the tensile strength of rubber specimens before and after immersion in specified chemical reagents under controlled temperature and time conditions.

The test provides essential data for material selection, quality control, formulation development, and service-life prediction of rubber components intended for chemical service. The core reference standards for this determination include:

• ISO 37 / GB/T 528 — Rubber, vulcanized or thermoplastic — Determination of tensile stress-strain properties
• ISO 1817 / GB/T 1690 — Rubber, vulcanized or thermoplastic — Determination of the effect of liquids (chemical resistance immersion testing)
• ASTM D412 — Standard Test Methods for Vulcanized Rubber and Thermoplastic Elastomers — Tension
• ASTM D471 — Standard Test Method for Rubber Property — Effect of Liquids

Testing Samples

Standard Dumbbell Specimens

The primary test piece is the standard dumbbell (dog-bone) specimen die-cut from vulcanized rubber sheets with a preferred thickness of 2.0 mm ± 0.2 mm. Type 1, Type 2, or Type 4 dumbbells per ISO 37 / GB/T 528 are most commonly used. For each test condition (each reagent, temperature, and immersion duration), a minimum of five dumbbell specimens are required for the exposed group, and an additional set of at least five specimens is retained as the unexposed control group.

Finished Rubber Products

When the test object is a finished product, specimens may be taken directly from seals, gaskets, hoses, linings, diaphragms, or sheets, provided that the specimen geometry and thickness comply with the requirements of the tensile standard. Products such as O-rings may be tested as whole rings where applicable.

Sample Preparation and Conditioning

All specimens must be free from surface defects, bubbles, and mechanical damage. Prior to testing, specimens shall be conditioned at the standard laboratory atmosphere of 23 °C ± 2 °C and 50% ± 5% relative humidity for a minimum of 16 hours, unless otherwise specified. The time interval between vulcanization and testing should comply with the relevant standard requirements.

Testing Items

The acid and alkali resistance tensile strength determination typically includes the following evaluation parameters:

• Tensile strength before immersion (original value)
• Tensile strength after immersion in acid media (e.g., sulfuric acid, hydrochloric acid, nitric acid)
• Tensile strength after immersion in alkali media (e.g., sodium hydroxide solution)
• Percentage change in tensile strength after immersion
• Elongation at break and its percentage change after immersion
• 100% and 300% tensile stress (modulus) change after immersion, where applicable
• Supplementary indicators: mass change, volume change, and hardness change (Shore A) after immersion

Testing Methods

Immersion Conditions

Typical reagents and test conditions are selected according to the intended service environment and the requirements of the product specification. Common examples include:

• Sulfuric acid (H2SO4) at concentrations of 10%, 30%, or 50%
• Hydrochloric acid (HCl) at concentrations of 10% or 20%
• Sodium hydroxide (NaOH) at concentrations of 10%, 20%, or 40%
• Immersion temperatures: 23 °C (standard), or elevated temperatures such as 50 °C, 70 °C, or 100 °C
• Immersion durations: 24 h, 72 h, 168 h, or extended periods as specified

Test Procedure

The general test sequence is as follows:

• Measure the initial tensile strength of the control specimens in accordance with ISO 37 / ASTM D412 / GB/T 528 using a universal tensile testing machine, and record the original values.
• Immerse the test-group specimens completely in the specified chemical reagent, ensuring at least 10 mL of liquid per square centimeter of specimen surface and keeping specimens separated and fully submerged.
• Maintain the specified immersion temperature within a tolerance of ±2 °C using a temperature-controlled bath, and keep the vessel closed to prevent evaporation and concentration drift.
• At the end of the immersion period, remove the specimens, quickly rinse them with cold running water (for acid or alkali media) and blot them dry with filter cloth or paper without rubbing.
• Allow the specimens to recover for the specified rest period (typically 30 minutes for mechanical testing after immersion, or longer where reabsorption of volatiles is involved) before tensile testing.
• Measure the tensile strength of the exposed specimens under identical conditions as the control group.

Result Calculation

The percentage change in tensile strength is calculated as follows: change in tensile strength (%) = (T2 − T1) / T1 × 100, where T1 is the median tensile strength of the unexposed control specimens and T2 is the median tensile strength of the specimens after immersion. A negative value indicates a loss of tensile strength caused by chemical attack. The same calculation principle applies to elongation at break, modulus, mass, volume, and hardness. Acceptance criteria are defined by the relevant product standard or the agreement between the purchaser and supplier; for example, many chemical-resistant rubber specifications require that the retained tensile strength after immersion be no less than 70% to 80% of the original value.

Applications

Acid and alkali resistance tensile strength data are essential for rubber products operating in chemically aggressive environments. Typical application areas include:

• Rubber linings for chemical reactors, storage tanks, and pipelines carrying corrosive media
• Seals, gaskets, and O-rings used in chemical processing equipment, pumps, and valves
• Chemical hoses, expansion joints, and flexible connectors for acid and alkali transfer
• Rubber diaphragms for diaphragm pumps and metering devices in chemical dosing systems
• Acid-resistant rubber rollers, sheets, and conveyor belting in metallurgical and electroplating plants
• Battery components and encapsulation requiring long-term acid resistance
• Protective articles such as chemical-resistant gloves and aprons
• Material research and formulation comparison for fluoroelastomers (FKM), EPDM, butyl rubber (IIR), nitrile rubber (NBR), neoprene (CR), and natural rubber (NR) compounds

FAQ

What is the difference between this test and a standard tensile test?

A standard tensile test measures the tensile properties of rubber in its original state, whereas this test measures the tensile strength after the specimens have been aged by immersion in acid or alkali solutions. The key deliverable is the percentage change in tensile strength, which reflects the chemical resistance of the material.

How many specimens are required for one test condition?

At least five dumbbell specimens are needed for the immersion group and another five for the control group for each reagent, temperature, and duration combination. Additional specimens are recommended if supplementary mass, volume, or hardness measurements are required.

How are immersion reagent, temperature, and duration selected?

These parameters are determined by the relevant product standard or by the customer based on the actual service environment. Common practice uses 20% sulfuric acid or 20% sodium hydroxide at 23 °C or elevated temperature for 24 h to 168 h, with harsher conditions applied for accelerated evaluation.

Why must specimens be washed and rested after immersion?

Washing removes residual reagent from the surface that could continue attacking the rubber or corrode the testing fixtures, and the rest period allows surface liquid to evaporate and the specimen condition to stabilize so that the tensile results are comparable and reproducible.

What does a negative change in tensile strength indicate?

A negative value indicates that the chemical medium has degraded the rubber network through swelling, plasticizer extraction, hydrolysis, chain scission, or oxidation, resulting in a loss of tensile strength. A severe negative change means the material is unsuitable for the tested chemical environment.

Can hardness and mass change be tested together with tensile strength?

Yes. Under the same immersion conditions, parallel measurements of hardness change per ISO 48 / GB/T 531.1, mass change, and volume change per ISO 1817 / ASTM D471 / GB/T 1690 are frequently performed to provide a complete picture of chemical resistance.