The durability of rubber materials is influenced by several environmental factors along with the mechanical stresses that are brought about by the use of the product itself.
It was well known from an early stage in the development of rubber materials that factors such as sunlight, heat, oxygen in air and humidity in general speed up the degradation of rubber. Mechanical loads, impurities, microorganisms, erosion and other special influences occur based on the application of the rubber.
Generally, there is no time to wait for a test under real conditions. It could in fact take decades to obtain the natural results. Accelerated aging is thus used. This means the factors that lead to natural aging are reinforced. This could take place both outdoors – in a tropical rain forest or desert – and indoors in climate chambers, ovens or weather simulators. Unfortunately, this is usually done without proper critical analysis. The aging process is expedited far too much. The material is literally grilled. The accelerating aging process then becomes totally different from the natural process. The result is incorrect predictions of the actual durability.
The Philosophy of Aging Processes
First, the functional environment must be carefully analyzed, so that the most critical degradation factors in each application are identified. Using the available knowledge, it is then determined how far the acceleration can be taken. The available facts and knowledge are seldom sufficient in order to establish the maximum permissible acceleration or to translate the results into an exact number of years under real conditions. The acceleration thus has to be performed in moderation and by using rules of thumb.
If durability testing is to be performed seriously, long testing times – a year is not uncommon – must be expected. Indeed, it is always better to wait a long time to obtain a more correct result than to obtain an incorrect one rapidly.
What to Remember about the Aging Process
In all aging processes, it is particularly important to maintain a constant temperature and in specific cases a constant relative humidity in air. This is because the speed of a chemical reaction is more or less doubled for a temperature increase of 10 °C – and aging is in most cases a chemical reaction. Generally, the highest deviation of ±1 °C and ±5% RH is permitted. In all aging, and especially for prolonged testing times (up to a year and more can take place), it is very important to be certain that the temperature has been maintained within the permitted tolerance during the entire testing procedure.
Another significant factor refers to the flow of air. During the aging process, the oxygen in the air is used up and degradation products are produced. The oxygen concentration must be kept at a constant level in order to make the testing reproducible, and the degradation products must be ventilated. The air must be changed between 3 and 10 times per hour to meet these requirements.
The device must thus be equipped with an air supply and flow meters. The air speed must also be low, or otherwise the oxidation rate can increase and antioxidants and plasticizers be ventilated off.
When rubber material ages with time, this generally manifests it in reduced extensibility and increased stiffness. Effortlessly oxidized materials, as for example natural rubber, become softer for prolonged aging times.
When a rubber material ages, among other things, the following reactions occur:
- Oxidative degradation, brought about by oxygen, which causes breaks in the polymer chain.
- Thermal degradation, caused by thermal movements in the polymer chains, which cause breaks in the polymer chain.
- Additional cross-linking caused by the remains of curing agents. In curing systems with high sulfur content, disulfide and polysulfide links can break up and produce new crosslinks of the di- and mono-sulfide type.
The variations in a rubber material during aging can be analyzed by testing for several properties. The most common way to test the effect of aging on a rubber material is to conduct a tensile test and then measure the change in hardness. The total aging effects are considered to be most apparent in the decrease in elongation at break. The additional crosslinking is most apparent in the increase in tensile strength and increase in hardness.
For testing tension set, the oxidative reaction is generally dominant, caused by the thin cross section of the test piece, which allows the penetration of oxygen.
Cell aging oven (Photo: Elastocon).
Thermal degradation can be dominant for compression set testing, since the test pieces are fairly thick and protected on two sides by the compression plates, which leave just a small surface area exposed to the air.
For compression set testing, additional crosslinking is a key reaction during the first hours and days, while degradation reactions dominate in long-term tests. A 24 hours compression set test is thus frequently used as a control test of the degree of vulcanization of the rubber.
In relaxation testing, additional crosslinking does not influence the result, because the newly developed crosslinks do not contribute to any variation in the counter forces measured.
This means that a relaxation test in elongation reveals both thermal and oxidative degradation, while a relaxation test for compression mainly shows thermal degradation. If a relaxation test is performed in an inert atmosphere, such as nitrogen, the thermal degradation can be separated from the oxidative degradation. Stress relaxation is generally divided into two types of relaxation – chemical and physical relaxation. Physical relaxation takes place mostly during the first minutes and hours of a relaxation test and are brought about by the relocation of filler particles, in which these together with the chains of polymers detect new states of rest. Chemical stress relaxation comprises mainly of breaks in the polymer chain and is brought about by thermal and oxidative degradation.
Aging effects increase at increasing temperature and as a rule, a temperature rise of 10 °C decreases the life cycle by half.
The service life of rubber materials is affected by chemical reactions and also by mechanical influences, such as dynamic fatigue and abrasion.
A more serious approach for estimating the life time for a material is to use an Arrhenius diagram. In order to draw an Arrhenius diagram, it is necessary to first determine the life time for a material for at least three temperatures. The properties used in order to determine the life time are mostly elongation at break, compression set, tensile strength and relaxation. The properties chosen and the level, at which the function of the material stops, relies on what the material is to be used for. Often the time used is that which the particular property deteriorates to 50% of the initial level.
To be able to decide the time when the criteria is reached, it is necessary to perform the testing using logarithmic time intervals, for example, 1, 2, 4, 8, 16 and 32 etc. days. The testing times can become long, particularly at the lowest temperature, up to a year is not uncommon.
When the life time from at least three temperatures have been attained, these are entered on the Arrhenius diagram, with the Y-axis as Ln time and X-axis as temperature, expressed in 1/T, where T is the temperature in degrees Kelvin. Then a straight line is drawn through the time/temperature points and which can then be extrapolated for the life length at lower temperatures.
What may sometimes occur is that a straight line cannot be drawn between the points and which then demonstrates that the aging reaction has not been the same temperatures at all. Usually, it is the highest temperature that varies and which means that a lower temperature may have to be used. It is also necessary to note that the further one extrapolates from the lowest tested temperature, the uncertainty becomes larger.
Rain is generally not a problem for most types of rubber, except for some, which are sensitive to hydrolysis, meaning that damp environments increase the degradation. The rubber type that is most sensitive to hydrolysis is polyurethane rubber, particularly the ester types. Even silicone rubber is affected by hydrolysis, but only at high temperature in steam. Rain, in combination with sunlight, can indeed cause chalking on light rubber materials. Chalking arises when the rain washes away the outer layer of oxidized and degraded rubber polymers so that the light filler granules come out. Weather testing can be conducted outdoors or indoors.
This information has been sourced, reviewed and adapted from materials provided by Elastocon AB.
For more information on this source, please visit Elastocon AB.