Social scientists have opined that social behavior is influenced both by environment and genetic makeup, the latter relating to our body chemistry. Interestingly, many polymers' resistance to cracking follows these same two influences. Whereas some cracking results from excessive loads, or from thermal or UV degradation, a large percentage of plastic part failure results from a phenomenon termed environmental stress cracking (ESC), which is the result of an external chemical acting on a plastic part that contains internal stresses. The stress at which ESC cracking occurs is often well below the nominal fracture stress of the material.
ENVIRONMENTAL STRESS CRACKING EXAMPLES
We often see examples of ESC in components that are subjected to regular exposure to cleaning agents. Hospital equipment, which will often see alcohols, surfactants, and disinfectants during reprocessing, often experience ESC after multiple cleaning cycles. Whereas ESC may not lead to parts failure or loss of functionality, they do result in a marred appearance, and may also lead to disinfection issues due to the generation of internal surfaces that may harbor microorganisms.
Polycarbonate is another thermoplastic that is susceptible to environmental stress cracking. Windows made from polycarbonate often show evidence of crazing (fine, silvery cracks) over time, which is the result of ESC (look at an airplane window next time you fly; you will likely see some crazing). Similar to the description above, crazing does not necessarily mean that the window’s integrity is compromised. Rather, the crazes have relieved localized stresses and may have stopped growing.
ESC occurs when a polymer component is exposed to a chemical when under stress. The stress may be residual stress resulting from a molding or machining process, or can be an externally applied stress during its application use. The chemical, almost always a liquid, does not result in bulk solvation or chemical degradation of the polymer. Rather, typical ESC chemicals are weak solvents for the polymer in question, and result in partial chain disentanglement in the regions of high stress. This chain disentanglement leads to localized plastic flow at stresses below the normal yield or fracture stresses, resulting in craze formation followed by cracking. The kinetics of ESC depend on the rates of the ESC chemical absorption and the relative rates of craze and crack formation, which are also influenced by the mechanical stresses in the samples.
ESC is characterized by brittle fracture surfaces, even occurring in polymers that normally exhibit ductile behavior. The crack morphology usually shows a smooth fracture plane, indicating a slow growing crack, as opposed to a striated surface indicating fatigue behavior. Since ESC often results in craze formation ahead of the crack, residual craze fibrils are sometimes evident on the fracture surface. ESC cracks often initiate from the surface, which is the source of the chemical cracking agent.
NATURE VS. NURTURE
So what materials (nature) and what environmental conditions (nurture) lead to situations where ESC is more likely? On the nature side, lower molecular weight polymers are more susceptible, since chain disentanglement is more likely. Additionally, polymers that have a lower amount of crystallinity, or that are amorphous, are more likely to exhibit ESC behavior, since crystalline domains have less free volume available for an ESC chemical to occupy. For this reason, polystyrene and polymethyl methacrylate (amorphous) are more susceptible to ESC than polyethylene (semicrystalline).
On the environmental side, the chemical makeup of the polymer is the primary determinant of which chemicals may act as ESC agents for a given polymer. Potential ESC agents include chlorinated hydrocarbons, aromatics, carbonyls, fatty acid esters, alcohols and aliphatic hydrocarbons, to name a few. Additionally, lower molecular weight ESC agents are more active than higher molecular weight due to improved ability to diffuse into the polymer structure.
Also on the environmental side, the nature of the loading is important. Tensile stresses are required to induce ESC; compression will not cause ESC, unless they lead to a component of tensile orientation in another plane. Additionally, residual stresses resulting from material orientation during injection molding can result in ESC.
There are multiple approaches to testing materials and components for their propensity to exhibit ESC. ASTM F484 “Standard Test Method for Stress Crazing of Acrylic Plastics in Contact with Liquid or Semi-Liquid Compounds" describes a method where plaques of acrylic materials are flexed to specific levels of strain, and hence stress, while exposed to potential ESC agents for periods of time, while examining the plaques for evidence of craze or crack formation. Strain (stress) can be applied to samples in many ways, including a circular strain jig (detailed in ASTM D543 “Evaluating the Resistance of Plastics to Chemical Reagents”) or an elliptical Bergen jig, which applies a range of strains to a single sample bar. Chemical exposures can also be performed in several ways including immersion, wet patch, wipe, or spray.
CPG has many years of experience investigating cracks in polymeric devices, and can assist in determining the root cause and preventing their appearance.