Environmental stress cracking
Environmental Stress Cracking (ESC) is one of the most common causes of unexpected brittle failure of thermoplastic (especially amorphous) polymers known at present.
These corrosive environmental liquids are called 'secondary chemical agents', are often organic, and are defined as solvents not anticipated to come into contact with the plastic during its lifetime of use.
Failure is rarely associated with primary chemical agents, as these materials are anticipated to come into contact with the polymer during its lifetime, and thus compatibility is ensured prior to use.
These are broken when the mechanical stresses cause minute cracks in the polymer and they propagate rapidly under the harsh environmental conditions.
The rate of ESC is dependent on many factors including the polymer's chemical makeup, bonding, crystallinity, surface roughness, molecular weight and residual stress.
[2] ESC may occur continuously, or a piece-wise start and stop mechanism There is an array of experimentally derived evidence to support the above theories: ESC generally occurs at the surface of a plastic and doesn't require the secondary chemical agent to penetrate the material significantly, which leaves the bulk properties unmodified.
This theory provides an explanation for the decrease in the stress needed to propagate the craze in the presence of surface-active reagents such as detergents and high temperature.
Stress cracking agents, such as detergents, act to lower the cohesive forces which maintain the tie molecules in the crystallites, thus facilitating their "pull-out" and disentanglement from the lamellae.
The number of tie molecules and the strength of the crystals that anchor them are considered the controlling factors in determining the polymer resistance to ESC.
A common method in the polymer industry is use of the Bergen jig, which subjects the sample to variable strain during a single test.
[12] Further, new tests have been developed where the time for crack initiation under transverse loading and an aggressive solvent (10% Igepal CO-630 solution) is evaluated.
[14][15] In summary, though, there is not a singular descriptor that is applicable to ESC—rather, the specific fracture is dependent on the material, conditions, and secondary chemical agents present .
Scanning electron microscopy and fractographic methods have historically been used to analyze the failure mechanism, particularly in high density polyethylene (HDPE).
However, the long testing time and high costs associated with these methods slow down the R&D activities for designing materials with higher resistance to stress cracking.
To overcome these challenges, a new simpler and faster method was developed by SABIC to assess ESCR for high density polyethylene (HDPE) materials.
Because of the key role of tie-molecules and entanglements in resisting environmental stress cracking in polyethylene, it follows that ESCR and strain hardening behaviors can very well be correlated.
The strain hardening region of the stress-strain curve is considered to be the homogeneously deforming part well above the natural draw ratio, which is determined by presence of the neck propagation, and below the maximum elongation.
[17] To determine the cause of the fracture, the SAN piano key was heated above its glass transition temperature for a short time.
