Environmental Stress Cracking - What It Is and How to Avoid It
Why do transparent plastics like acrylic and polycarbonate craze following contact with cleaning chemicals? Why do intricately machined Ultem® PEI and Noryl® modified PPO parts sometimes craze and crack after a short time on the shelf? It turns out that these are two examples of the same failure mode known as environmental stress cracking (ESC).

It has been estimated that ESC is one of the most common failure modes for plastic parts. This is almost certainly the case for amorphous thermoplastics where up to 40% of all failures are related tA ESC. The phenomenon occurs as stress in the material interacts synergistically with an environmental factor, resulting in crazing and eventual cracking over time.

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Contact with dilute ethyl alcohol hand sanitizer causes crazing in a thin strip of polycarbonate in a bending apparatus within 2 hours. Note that crazes form perpendicular to the primary tensile stress direction (left). Severe cracks form over the course of a few months as glass cleaner is used to frequently clean an acrylic display case (right).

In the case of a highly stressed fabricated part cracking on the shelf, the only environmental factor facilitating the onset of crazing is the air in the room. More commonly, a chemical environment is responsible for rapid craze development. Examples include the repeated use of glass cleaner resulting in crazes emanating from the bottom edges of an acrylic window, or an incompatible cutting fluid used during machining causing crazes near part features. The former is the prototypical example of ESC.

Creep Rupture and the Ductile to Brittle Transition
At high stress levels, plastics tend to exhibit high levels of deformation before failing ductility in a relatively short period of time. At low stress levels, plastics tend to fail in a brittle fashion over longer periods of time as a function of their fatigue characteristics.

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Figure 1. At low levels of stress, long-term brittle failure is favored over ductile yielding for typical amorphous plastics. Source: Wright, D.C., Environmental stress cracking of plastics. 1996, Shrewsbury: Rapra Technology.

Environmental stress cracking is essentially creep rupture that is accelerated by an adverse environment. In other words, there is a time-to-failure in air and a much shorter one in an adverse environment (e.g., chemical, vibration, cyclic stress loading, or UV-irradiation).

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Figure 2. Typical tensile creep curves for a polymer in air (solid lines) and in contact with an ESC fluid (dashed lines) subject to various tensile stress levels. Contact with the ESC fluid accelerates rupture. Source: Adapted from McKeen, Laurence W. (2015). Effect of Creep and other Time Related Factors on Plastics and Elastomers (3rd Edition). Elsevier.

Which Plastics are Most at Risk?
It turns out that strong resistance to ESC is imparted by a semi-crystalline molecular structure. This is why crazes are rarely observed in UHMW-PE, Delrin®, Nylon and PEEK parts. Crystalline regions resist solvent absorption and block craze propagation. On the other hand, amorphous thermoplastics like polycarbonate and polystyrene allow for the ready absorption of fluids and vapors which are compatible with the resin from a solubility perspective. Moreover, they have no crystalline regions to prevent craze propagation. For these reasons, plastics with completely disordered molecular structures (i.e., amorphous) are substantially more susceptible to stress-cracking.

ESC Does Not Necessarily Involve Chemical Attack
Note that ESC involves the physical interaction of a chemical, essentially acting as a plasticizer, with a polymer matrix. Solvents preferentially absorb into areas of high stress which increases local molecular mobility in the polymer matrix, and micro-voids form between orienting chains. This is the synergy between stress and environmental conditions which facilitates ESC. Next, the voids coalesce to form crazes which rupture to form catastrophic cracks.

Chemical attack is a separate (but somewhat related) phenomenon which involves interactions at the chemical level, such as chain scission reactions which reduce the polymer molecular weight. Chemical attack typically results in embrittlement, warping, color change, generation of leachable components, etc.

Tips to Avoid ESC
Minimize Stress
The principal factor contributing to ESC is the amount of stress in the part. For example, threading a polycarbonate part can create an extreme level of localized stress. Cracks often form over time via creep rupture, without chemical contact. If an aggressive chemical does contact the area, crazes form rapidly. It may be helpful to consider the following best practices of plastic part design:

• Apply generous radii to sharp corners wherever possible.
• Position thru-holes as far as possible from part edges or other features.
• Avoid countersinking flat head screws and instead use round head or pan head screws with flat washers on both sides.
• Oversize through holes to allow for thermal expansion (CTE) mismatch between the plastic part and the mating material (usually a metal).
• Only use inserts specially designed for plastics. Heat or ultrasonic installation minimizes stress.
• Apply the minimum amount of torque on fasteners as allowable by the application.
• Stress-relieving/annealing cycles may be required for certain machined parts.

Minimize Contact with Aggressive Environments
ESC is most commonly facilitated by unanticipated contact with chemicals, usually in the vapor state. Various materials used in assembly or maintenance operations including adhesives, lubricants, cleaning agents, anti-rust agents, plasticizers, and oils may be ESC agents for certain plastics. It is good practice to evaluate all potentially aggressive chemicals that the plastic part may contact. Assembly hardware is often coated with anti-rust agents which tend to be ESC agents for transparent plastics. Cleaning fasteners with dish soap greatly reduces the risk of these solutions/coatings causing ESC. Additionally, elastomeric washers tend to outgas ESC agents which can be aggressive enough to cause crazing at fastening points. Increasing the temperature or chemical concentration increases the capacity of a chemical to interact with a polymer in both physical and chemical facets. In terms of contributing to ESC, these factors are secondary to the level of stress in the part.

Chemical Resistance Data
It is important to mention chemical resistance reference materials which are ubiquitous in the commercial literature. Chemical resistance data for plastic materials is often generated with unstressed test specimens. The absence of stress (which is one of the two “ingredients” for ESC) may result in “compatible” ratings being assigned to chemicals which may in fact induce ESC when parts are in stressed conditions.

The criteria for resistance ratings are also somewhat vague in common reference materials. Typically, ratings are based on observations such as weight-gain, dimensional change, color change, warping, and possibly ESC. Despite these considerations, handbooks and online data entries can be quite helpful as initial points of reference.

For more information, contact Timothy Buchanan, Technical Service Engineer, Curbell Plastics, Inc., one of the nation’s top suppliers of plastic sheet, rod, tube, tapes, and fabricated parts. For more information contact Mr. Buchanan at 716-667-3377 x7412, E-mail: tbuchanan@curbellplastics.com. Web: www.curbellplastics.com.