Failure mechanisms for plastic components are quite different from those of metal components, but the overall approach remains the same. This includes gathering information; preliminary examination and documentation; and macroscopic examination of the fracture surface, secondary cracking, and surface condition. The primary difference in the analysis of plastic components is the analytical tools used to identify the material of construction and the required properties.
As noted in Part 1 of this series, plastics consist of long-chain organic repeating units. Plastic properties can be modified by various additives that provide increased ductility or strength, coloring, UV resistance, improved oxidation resistance, and enhanced processing. Even for the same polymer, the properties and characteristics may be different. (It depends on the molecular weight distribution, molecular orientation, and crystallinity.) This article will describe some of the useful techniques that can provide insight into the cause of the failure of the plastic component.
Identifying the Type of Plastic
Probably the most common first analytical approach is to identify the type of polymer using Fourier transform infrared spectroscopy (FTIR). When infrared radiation passes through the plastic, some radiation is absorbed, and some of it passes through. The spectrum of the absorbed light provides a fingerprint of the molecular structure. Since the molecular structure of polyethylene is different from nylon, for example, the basic spectrum is different.
Subsequently, the identification of the plastic can be obtained by comparing that spectrum to spectra of known plastics. This results in a qualitative identification of the base polymer. Note that this technique cannot easily differentiate between polymers having similar molecular structures. FTIR can provide some information as to the presence of various additives and filler materials if present above about 1%.
To determine whether the failed plastic was as originally specified, a sample of an unused plastic component must be analyzed and the spectra compared. Low-level additives, such as antioxidants, may not be detected. An advantage of this technique is the identification of absorbed chemicals, which can be useful in determining the cause of failure. Such causes include chemical attack and environmental stress cracking.
FTIR instrument with the IR spectrum of the sample that was analyzed, as shown on the monitor. (Source: Suriphon Singha / iStock)
Another vibrational spectroscopy technique is Raman spectroscopy. While FTIR can provide bulk composition and identify polar groups such as OH and C=O bonds, Raman is better for symmetric bonds such as C-C and C=C bonds, crystallinity, and inorganic fillers. The presence of absorbed water can cause interference with an FTIR spectrum, while fluorescence can impact the Raman spectrum. Due to these differences, both techniques are quite complementary and useful for identifying various aspects of the failed plastic. Both techniques, combined with a microscope, can also be used to analyze and compare small and unique locations to the bulk. This, in turn, can be useful for identifying the failure mechanism.
How to Determine Why the Plastic Is Brittle
Since polymers are often semi-crystalline, the amorphous phase softens at a temperature referred to as the glass transition temperature (Tg). The glass transition represents the reversible change from a hard or brittle state to a viscous or rubbery condition. This parameter can provide information regarding the aging of the plastic, effectiveness of the plasticizer, resistance of the plastic to oxidation, and whether the plastic component met the design requirements for the temperature of operation.
When comparing results to a used or different sample, the measured Tg can vary based various factors. These include the technique used, heating rate, anisotropic nature of the sample, and data reduction. Tg can also vary based on the crystallinity of the polymer, extent of cross-linking, and type and quantity of plasticizer used.
DMA, DSC, and TMA
Techniques for measuring Tg include dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), and thermomechanical analysis (TMA). The best technique is a function of the material, as to whether it is a thin film or bulk sample, and the required sensitivity of onset of stiffness versus peak viscous flow.
- DMA applies an oscillating stress/strain and measures the response as a function of temperature. It is highly sensitive to material stiffness loss but requires a thin film sample. Results can vary depending on the frequency of the oscillation.
- DSC measures the heat flow difference between the sample and reference as a function of temperature. It can also be used determine the melting point of the polymer. DSC requires a smaller sample and is easy to operate, but is less sensitive than DMA. The heating rate can impact the measurement for both instrumental methods.
- TMA measures the changes in sample dimensions of expansion or penetration as a function of temperature and is appropriate for larger samples. The dimensional changes are related to Tg.
As with infrared spectroscopy, an unused sample for comparison is the best approach when performing a plastic failure analysis to detect changes in Tg or other transitions caused during service. Measuring Tg by various techniques for polymethyl methacrylate is described in research,1 along with relevant ASTM methods. These can be used as guidelines for performing the measurement.
Other Useful Techniques
When the appropriate loadings of various additives—such as plasticizer, flame retardants, or UV stabilizers—are required as part of the plastic failure analysis, thermogravimetric analysis (TGA) is an excellent tool. TGA can also provide insight into thermal or oxidation degradation mechanisms. TGA measures the amount and rate of change in the sample weight as a function of temperature or time at a given temperature. The inclusion of a mass spectrometer can identify the evolved gases from the volatilization or decomposition of the plastic components.
Failure modes that might cause a loss of molecular weight or distribution of the polymer chain include thermal degradation, photo or UV oxidation, chemical attack (such as by hydrolysis or acid), and biodegradation.
Gel permeation chromatography (GPC) is a size exclusion chromatographic technique that uses a packed column to segregate the various components. Multiple columns are used to separate the polymeric and oligomeric constituents. The polymer is further separated by molecular weight, producing a molecular weight distribution of the plastic. Other techniques, such as melt flow index or solution viscosity, do not provide a distribution of molecular weights.
However, the melt flow index or melt flow rate, which measures the viscosity of the plastic in the molten state, is a fairly simple technique for measuring the viscosity molecular weight. The sample is heated through the melting or softening point, and then extruded through a standard-sized orifice. The viscosity molecular weight is inversely proportional to the melt flow rate. More details are provided in ASTM D1238.
Mechanical tests are often performed to evaluate whether the properties of the failed polymer have changed. These can include tensile, flexural, and impact resistance properties. There are various ASTM methods for such tests since sample size is a critical parameter. Durometer testing of elastomers and softer plastics is simple anf helpful for evaluating changes in the properties of the plastic. The depth of penetration by the spring-loaded probe is measured on a standardized scale. The penetration depth for the applied force is related to the elastic modulus and viscoelastic behavior of the material, but is not a fundamental property. Note: There are different scales depending on the “softness” of the material; see ASTM D2240 for more details.
Scanning electron microscopy of the fracture surface can provide insight into whether the failure mode was via a brittle or ductile mechanism, the crack origin, and possible defects such as voids or impurities. Examining the fracture surface of a fiber-reinforced plastic can assist in evaluating the particle distribution, porosity, and adhesion of the reinforcement with the polymer.
Conclusion
Conducting a plastic failure analysis of a broken component often requires analytical tools that are different to those for a metallurgical failure analysis. With the techniques discussed here, comparison to an unused plastic component is often necessary to conclusively evaluate whether the plastic, additives, molecular structure, molecular weight, or properties have changed during or because of service. Such analyses can assist in determining the cause of the failure.
***
1O.V. Startsev and M.P. Lebedev, Polymer Science Series A, 60, 2018, pp. 907–919 (2018)
Useful References: