Durability Testing of Rubber

Table of Contents

Introduction
Influencing Factors
Accelerated Aging
The Philosophy of Aging Processes
What to Remember about the Aging Process
Aging
Weather Simulation ISO 4665
Outdoor Aging
Ozone Tests ISO 1431
Abrasion Testing ISO 4649
Dynamic Fatigue ISO 132, ISO 4666, ISO 6943

Introduction

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.

Influencing Factors

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.

Accelerated Aging

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.

Aging

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:

  1. Oxidative degradation, brought about by oxygen, which causes breaks in the polymer chain.
  2. Thermal degradation, caused by thermal movements in the polymer chains, which cause breaks in the polymer chain.
  3. 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

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.

Weather Simulation ISO 4665

Weather simulation is testing that is performed indoors and is conducted in special weather aging cabinets. In a weather cabinet, temperature, sun and rain are simulated. The normal testing cycle has a light intensity of 1000 W/m2, 55 °C black panel temperature and a rain cycle of 18 minutes of rain and 102 minutes dry. Only light is used in certain simpler devices.

The properties most frequently examined are color changes and variations in tensile strength and elongation at break. The testing is fairly accelerated and a six weeks test corresponds to just about 2–3 years outdoors.

Outdoor Aging

Outdoor aging of rubber is mostly performed with elongated test pieces in order to form an opinion of the effect of ozone, in addition to variations in the tensile strength and appearance. The testing is performed for at least a year and is frequently performed as a comparison between varied materials.

Ozone cabinet

Ozone cabinet (Photo: SATRA)

Ozone Tests ISO 1431

Testing a rubber material’s ozone resistance is carried out in special ozone chambers, generally at an ozone concentration of 50 pphm (parts per hundred million) and at a temperature of +40 °C. Since rubber is more effortlessly attacked by ozone when it is elongated, the testing is performed in a rig with changing degrees of elongation, from 5 to 80%.

The samples are checked at specific time intervals, 2, 4, 8, 24, 48, 72 and 96 hours, and the time taken to form the first crack in each elongation is then noted.

Abrasion Testing ISO 4649

Abrasive resistance is one of the most important properties of rubber, but it is at the same time complicated to test in the laboratory.

In laboratory tests, results must be attained within a short time period and therefore the testing is accelerated, but this explains the fact that the abrasion mechanism is not the same as in practice and the results do not correlate to what actually takes place in reality.

Abrasion tester

Abrasion tester (Photo: Bareiss)

Generally, there are four different abrasion mechanisms for rubber materials:

Type Abrasion mechanism Abrasion patterns Example
Pyrolysis wear Frictional heat breaks down the rubber in liquid and gaseous substances Sticky surface Braking tracks on asphalt
Abrasive wear Sharp particles tear parts of the rubber surface Scores in the longitudinal direction Sandpaper wear
Wear by chip The friction exceeds the rubber’s shear formation strength, whereby the material is removed as small chips Wave shape patterns, with wave patterns in the direction of glide Eraser wear
Fatigue wear The forces in the surface are too small to break at the first cycle, but repeated cycles lead to the material being removed as very small particles Smooth, polished surface Fatigue wear, polishing wear

The most common method of testing abrasion for a rubber material is to allow a loaded sample slip against a roller covered in emery cloth. The weight loss is determined after a wear distance of 40 m and is then calculated over to a volume loss. Eventual variations in the emery cloth are corrected with the help of a reference piece of rubber, and the results are then corrected. The downside of this method is the fact that the wear intensity is greater than what is allowed in practice.

Dynamic Fatigue ISO 132, ISO 4666, ISO 6943

Dynamic fatigue testing can be performed in a number of ways. Products are frequently tested in ways similar to their actual usage in servo hydraulic machines or eccentric driven test rigs. The time taken until break is determined.

Dynamic fatigue test on materials is frequently performed as bending or elongation fatigue in a so-called De Mattia machine. Test pieces are stretched or bent using a frequency of 5 Hz, until a break happens.

A remarkable fact to be noted is that a rubber which is tested for elongation lasts much longer if it is preloaded than if it has zero tension during part of the elongation cycle.

Fatigue testing in a De Mattia machine

Fatigue testing in a De Mattia machine (Photo: MonTech)

Elastocon AB

This information has been sourced, reviewed and adapted from materials provided by Elastocon AB.

For more information on this source, please visit Elastocon AB.

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