Chemical Resistance of Plastic and Rubber Materials
There are a number of processes which affect the resistance of polymers to chemical environments. Whilst numerous chemical resistance charts are available online and from material suppliers these tend to be based on very limited exposure conditions and tend to be relevant onto to the base polymer. Whilst a "desktop" review can highlight the most severe interaction with the polymers and is often a good starting point for assessing chemical resistance, chemical interactions are specific to the grade of polymer used (molecular weight, presence of fillers and additives etc) and the specific exposure conditions. Therefore, we always advise assessing the resistance of candidate materials for an application to specific chemical environments under service conditions and can develop test programmes to achieve this.
The effect of chemical envirnments on plastic and rubber materials can be characterised by three main processes:
Chemical Attack (Degradation)
There are many degradation processes that lead to a deterioration in material properties that can be grouped under the term “chemical attack”. These include oxidation, hydrolysis, acidolysis, halogenation and other processes involving irreversible changes to a polymer’s molecular structure. The chemical reacts with the polymer leading to “breaking” of molecules (chain scission) and degradation of performance, change in appearance etc. In addition, it is possible for a chemical to react with additives within the polymer compound subsequently facilitating attack of the polymer by other degradation mechanisms e.g. the loss of a stabiliser leading to reduced resistance to photodegradation in sunlight. The additive package may also be degraded without changing the polymer base material but still have an effect on the properties of the compound. Hence, the effect of a chemical can vary dependent upon the particular grade of a material being used.
Where polymers are likely to come into contact with chemical environments during service it is recommended that the resistance of the proposed material grade is assessed either under under service conditions or accelerated to predict long term service performance. Such assessments are based generally based on the ISO 175 or ASTM D543 for plastics and ISO 1817 or D471 for rubbers. Test programmes can be developed to incorporate elevates temperatures, pressures and chemical combinations as applicable to the end application.
Plasticisation, Solvation and Swelling
When a polymer with limited molecular mobility is immersed in a fluid, the fluid diffuses into the polymer until an equilibrium state is reached and no further weight change occurs. Solubility increases with temperature but is primarily dependent upon the intermolecular cohesive forces that bind the material together. The solubility is maximised where the cohesive forces of the polymer are similar to those of the fluid. This is summed up in the adage that “like dissolves like”. Where the solubility parameter of the fluid is similar to that of the polymer, significant absorption can occur, although as with any rule there are a number of notable exceptions such as the effect of water on many polymers.
In some cases, where a chemical is absorbed into the material this leads to swelling, softening, a reduction in strength, increase in surface tackiness etc. due to the increased mobility of the polymer chains caused by a reduction in interchain bonding. In the extreme case, the polymer will completely dissolve into the liquid. There is no chemical attack of the polymer and no chain scission.
Some materials, such as some polyamides (nylons) rely on absorption of moisture to improve the toughness of the material. The as moulded components exhibiting low strain acceptance and brittleness. The presence of moisture acts as a plasticiser increasing chain mobility producing an increase in toughness and strain acceptance.
Sometimes, the interaction of a solvent with a polymer is mild, but the additives in the polymer are extracted into the fluid resulting in a loss of stabilisers and a reduction in the life of the polymer. Elastomers which often contain plasticisers can be particularly susceptible to extraction of the plasticiser into the fluid environment resulting in stiffening of the material and other changes in physical properties.
Environmental Stress Cracking
Environmental stress cracking (ESC) is the premature initiation of failure by the simultaneous action of an agent (liquid, gas, grease, wax etc) and stress /strain. Amorphous thermoplastics such as polycarbonate, polystyrene, polysulphone, ABS etc tend to be more susceptible to ESC by a wider range of fluids than semi-crystalline polymers (e.g. polyamide, acetal etc).
In contrast with bulk plasticisation & solvation, ESC occurs by the environment /agent in question absorbing at localised regions typically at defects or areas under increased dilational stress. The absorbed fluid plasticises only the surrounding material, reducing the yield properties of the polymer in that location. Applied stress acts on these points and if the stress is sufficient a crack like defect called a craze will form. For a given grade of polymer, the level of stress / strain applied and the service temperature will be the main factors determining the time taken to initiate cracking with a particular environment.
Below a certain critical stress, cracking may not initiate for that particular polymer /environment pair. ESC has typically been variously assigned as a prime factor in between 15% to 25% of polymer failures. Much of the published chemical resistance data for polymers is performed on materials with no stress applied. Hence the assessment of potential for failure by ESC is reliant upon the designer carrying out physical tests.
In some instances, chemicals that are solvents for a particular polymer, or cause bulk plasticisation, can also act as ESC agents when present in only small amounts. An example of this is where solvent cement is used to join pipes and fittings. Excessive use of the cement or insufficient time between formation of the joint and subsequent pressurising of the system can result in traces of solvent initiating ESC failure in the pipe or fitting. Over time under dynamic stress, the crazes develop into fractures until at a critical crack size, sudden catastrophic failure occurs.
Using appropriate test procedures we are able to assess the ESC resistance of polymers to a wide range of chemical environments. Such test procedures can be either rapid screen techniques to assess the resistance to a wide range of environments or more specific assessments to fully characterise the ESC effects of specific interactions.
Click for additional information on Compatibility of Sealants with CPVC Pipes and Sprinkler Systems
Environmental Stress-Cracking of Ethylene - ASTM D1693
Whilst ESC is most prevalent in amorphous materials. Some semi-crystalline materials such a polyethylene also show the effect with specific environments. The environmental stress crack resistance of polyethylene materials is determined using the “Bell Telephone Test" described in ASTM D1693 - Standard Test Method for Environmental Stress-Cracking of Ethylene Plastics.
In this test a notch cut to a specified depth in a standard specimen. The sample flexed perpendicularly to the notch and held in a test jig. The jig is then placed in a surfactant solution (Igepal CO-630) at either 50°C and 100°C and the time for crack initiation across the samples determined.
Three test conditions (A-C) are defined in the standard which determine the test sample thickness, notch depth, surfactant concentration and exposure temperature. In general polyethylene with density between 0.910 and 0.925 are tested under condition A. If density is >0.925 then condition B is used. Condition C uses concentrated surfactant at 100°C and is used for materials with extremely high stress crack resistance.
The test result is defined by the F50 value which determined to time to 50% of the test samples failing. The F50 value is a useful comparator of the ESC performance of different polyethylene grades.