Rubber Testing & control Other testing
Other testing
Below you will find a selection of other testing that is not covered in the sections on mechanical, thermal, and durability testing.
Discoloration
Discoloration from rubber is caused by the migration of substances from the rubber material to adjacent materials. The substances that migrate are primarily antioxidants and plasticizers. Discoloration manifests as a slightly light brown to black surface on the material in contact with the rubber and is often particularly noticeable if the surface that was in contact with the rubber is exposed to sunlight.
Discoloration is usually divided into the following types:
Contact Discoloration Discoloration in the contact surface between rubber and substrate.
Migration staining Discoloration that spreads beyond the contact area.
Penetration staining Discoloration that penetrates a material in contact with rubber.
Extraction discoloration Discoloration that occurs when water flows over a rubber material and then flows over another surface, transferring discoloring substances.
The standard for staining tests of rubber is ISO 3865.
Dispersion
It is important that the fillers and especially carbon black are well dispersed in the rubber compound; otherwise, the mechanical properties such as tensile strength and abrasion resistance will deteriorate. The finer the grain of a filler, the better its reinforcing properties, but the more difficult it is to disperse.
The most common way to determine carbon black dispersion is to slice a vulcanized rubber piece with a sharp knife. Afterwards, the cut surface is examined under a microscope. Poor dispersion appears as unevenness, and it is compared to a reference scale to rate the dispersion.
The standard for dispersion testing of rubber is ISO 11345.
Permeability
Liquids and gases diffuse through rubber. The rate of diffusion depends on the type of rubber, the formulation, and the medium.
Diffusion is usually divided into three phases:
1. The medium dissolves into the rubber on one side.
2. diffusion (migration) through rubber.
3. evaporation from the opposite surface.
Different rubber materials have different gas permeability, and there are several methods available to determine permeability.
To determine the liquid permeability, rubber can be used as a barrier in a vessel containing the liquid to be tested. The container is weighed at regular intervals to determine the weight loss due to diffusion.
For determining the gas permeability, a rubber sheet can be placed between two chambers. One chamber is pressurized with the gas to be used, e.g., air, to a pressure of 0.3-1.5 MPa, and the other chamber is kept at ambient pressure.
Standards for testing the permeability of rubber are ISO 6179 and ISO 2782.
Fire test
All rubber materials are more or less flammable. To improve the fire resistance of a rubber material, chlorinated substances, such as chlorinated paraffin, can be added.
Rubber's fire resistance can be tested using vertical or horizontal methods. The most common method for testing rubber is to ignite a strip at one end with a Bunsen burner and study how long the material burns after the flame is removed.
The standard for fire testing of rubber is ISO 16 22 22.
Electrical tests
To determine electrical conductivity or resistivity, several methods can be employed. The principle, however, involves applying a voltage across a rubber sample and measuring the current passing through the rubber. This may sound simple, but in practice, it's very difficult as it often deals with very small currents, and the rubber's resistivity changes with factors like temperature, time, and load.
Standards for electrical testing of rubber are ISO 1853, ISO 1278, and ISO 2951.
Thermogravimetric analysis
ISO 9924
Thermogravimetric analysis, TGA, is used to determine the content of plasticizers, rubber polymer, carbon black, and ash.
A sample is weighed and heated from 70°C to 300°C in a nitrogen atmosphere, which yields the volatile matter content. Subsequently, the heating continues to 550°C. During this stage, other organic substances, primarily consisting of the polymer, are released. Finally, the atmosphere is switched to air and the temperature is raised to 650°C, burning off the carbon black. What remains is ash, consisting of inorganic fillers.
The standard for thermogravimetric analysis of rubber is ISO 9924.
Weather simulation
Weather simulation (ISO 4665) is the name of the test when it takes place indoors and is performed in special weather aging apparatuses. Here, sun, temperature, and rain are simulated. A common test cycle has a light intensity of 1,000 W/m², a 55°C black panel temperature, and a rain cycle of 18 minutes of rain and 102 minutes dry.
Properties often examined are color changes and changes in fracture strength and elongation at break.
The standard for weather simulation of rubber is ISO 4665.
Abrasion test
The most common method for testing abrasion on a rubber material is to slide a loaded test piece against an emery cloth-covered roller. After an abrasion distance of 40 m, the test piece's weight loss is determined and converted into a volume loss.
Standard for abrasion testing of rubber is ISO 4649.
Ozone resistance
Testing of rubber materials for ozone resistance is performed in special ozone chambers. Since rubber is more susceptible to ozone attack when stretched, the test is conducted in a testing rig with the test specimens stretched from 5 to 80% of their original length.
The specimen is checked at certain time intervals, and the time to the first crack at each elongation is recorded.
The standard for ozone testing of rubber is ISO 1431.