The failure of plastic components can be quite different from their metal counterparts. Both types of materials can fail through tensile overload, fatigue, or wear. However, plastic failure can also occur via thermal degradation, chemical degradation, and environmental degradation.
This article is the first in a two-part series about the failure of plastic components. Here, in Part I, we’ll take a closer look at plastic failure by means of various degradation mechanisms.
What Is Considered Plastic?
Plastic is a synthetic material consisting mainly of long-chain repeating units or monomers that are organic-based. Common examples include polyethylene, polystyrene, polyvinyl chloride, urethane, and nylon. There are two basic types of plastics:
- Thermoplastic materials soften at elevated temperatures and can be molded to a specific shape that is retained upon cooling. Polyethylene terephthalate and polycarbonate are examples. Although plastic materials may have similar repeating units, unlike metals, they can differ with respect to molecular weight, additives, plasticizers, UV stabilizers, flame retardants, and coloring pigments. Since thermoplastic materials consist of long hydrocarbon chains, at room temperature they can be crystalline, non-crystalline (amorphous), or semi-crystalline. How the plastic resins are formulated can significantly impact their properties and the failure mechanism.
- Thermoset plastics are irreversibly hardened when heated and subsequently cannot be reshaped. At that temperature, thermosets undergo cross-linking reactions that make them more heat-resistant than thermoplastics. Typical examples include epoxy resins and vulcanized rubber. Thus, how the plastic resins are formulated, such as the inclusion of additives, can significantly impact their properties and the failure mechanism.
Mechanical Degradation
Thermoplastics can fracture by rapid, brittle crack propagation or by a ductile mechanism with significant deformation. For a given plastic, the ductile failure mode can occur at a temperature above what is referred to as the glass transition temperature (Tg), where there is increased polymer chain flexibility. The Tg can be increased with the addition of a plasticizer, decreased with the incorporation of fillers, or changed by processing conditions.
The failure mode at a temperature below Tg will be brittle. Oriented polymer chains can increase the tensile strength and tear resistance of the plastic, with the failure mode varying depending on the orientation of the applied stress to the polymer chains. Depending on the Tg, cross-linked thermoset plastics tend to fail by brittle crack propagation because of a high Tg. On the other hand, cross-linked rubbers—having a very low Tg—experience extensive deformation before failure. Thus, the mechanical properties of plastics are determined by the viscoelasticity of the specific plastic, as well as all included additives.
Environmental Stress Cracking
One of the most common premature failure modes is environmental stress cracking (ESC). ESC is caused by the simultaneous and synergistic combination of chemical and stress exposure, and is a form of accelerated creep rupture.
Specific, small molecules that have a certain affinity for the plastic can preferentially absorb into the plastic and increase the free volume of the polymer network, which promotes polymer chain mobility. Polymer chains can then realign in the direction of the strain at the defect/crack front. Subsequently, planar micro-voids emerge between the highly oriented chains, forming a craze zone.
The voids can then coalesce to form crazes that initiate the crack. This continues until the crack propagation reaches a catastrophic size, causing the ultimate failure. Further absorption of the molecule into the plastic can produce swelling that further facilitates the crazing. Non-crystalline plastics are the most susceptible to ESC.
Chemical Degradation
Although plastics do not corrode like metals, aggressive environments can cause changes in the properties and structure of the polymer chains. For example, acetone can soften and even dissolve polyvinyl chloride and polystyrene plastics. To varying degrees, plastics are permeable to organic chemicals. This sorption can cause swelling, plasticization, dissolution, or leaching of additives. These changes may involve a decrease in molecular weight by chain scission or the incorporation of a new chemical group into the polymer chain.
Chain scission can cause a reduction in tensile strength and fracture toughness. Depending on the extent of the chain scission, a release of gaseous fragments can produce crazing, bubbles, and crack formation. New chemical groups incorporated into the polymer chain can increase Tg by restricting chain mobility and decreasing free volume, which can make the plastic more brittle.
A 3D rendering of a polymer chain. The chain is made up of individual monomers, which are linked together by chemical bonds. (Source: LE/iStock)
Thermal Degradation and Thermal Oxidation
Thermal degradation occurs when plastics are exposed to high temperatures that cause the polymer chain to break via chain scission. Embrittlement, loss of ductility, chalking, color change, or cracking will result.
Thermal oxidation is a type of ESC, but the aggressive agent is oxygen. Oxygen is absorbed into the plastic and reacts via free radicals to break the polymer chains into smaller fragments, which reduces the molecular weight of the polymer chain. This mechanism will accelerate with time since the reaction tends to be autocatalytic. Polyethylene tends to be the least resistant to thermal oxidation, while polytetrafluoroethylene is the most resistant to it. Antioxidants are added to minimize thermal oxidation.
Photo-Oxidation / UV Degradation
Photo-oxidation (commonly called UV degradation) of plastics is caused by exposure of intense sunlight in conjunction with air that then breaks down the polymer chains. This results in a loss of physical properties, such as loss of impact strength, with the plastic becoming brittle, changing color, chalking, and cracking. Photo-oxidation is similar to thermal oxidation because it is driven by free radical chain scission reactions that produce peroxy-radicals. This also reduces the molecular weight of the polymer chain.
Short-wavelength UV radiation is most severe. Degradation damage is usually limited to the surface layer that is exposed to the UV radiation, with the depth determined by the extent of oxygen diffusion. This degraded layer will then become extremely brittle.
Polypropylene and low-density polyethylene have greater susceptibility to UV degradation, while high-density polyethylene and polycarbonate have better resistance. Although nylon is susceptible to degradation, nylon 6 is more resistant than nylon 6/6.
Various additives are added to improve resistance to UV degradation. Fillers such as TiO2 and carbon black pigments can act as radiation blockers. Organic absorbers like benzophenones and benzotriazoles can absorb UV radiation and convert it into heat. Stabilizers such as hindered amine light stabilizers work by trapping and scavenging any free radicals that are formed. On the other hand, some additives—e.g., flame-retardant agents, biocides, and plasticizers—can act as a chromophore and make the plastic more susceptible to UV degradation.
Microbial Degradation
Since most plastics are derived from petroleum, they are inherently not biodegradable. However, many of the additives are biodegradable, and these are the components that can be attacked by microorganisms that can then negatively modify the properties of the plastic.
For microbial attack to initiate, bacteria and/or fungi must colonize the surface and then produce exoenzymes that metabolize the susceptible constituents. Some polymers (such as polylactic acid) are biodegradable, where the non-crystalline regions are attacked first. However, even with these plastics, the biodegradation process is quite slow.
Conclusion
Plastics can fail by mechanisms different to those observed for metals. Parameters that impact the degradation of plastics include the type of polymer, effectiveness of stabilizing additives, operating temperature, type and strength of aggressive service environment, and applied or residual component stresses. Identifying the cause of the degradation requires a complex materials analysis.
In the next article, we’ll take a look at the techniques and analytical instrumentation for determining the failure mechanism.
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