Plastic Component FAILURE ANALYSIS Several unique analytical techniques to comprehensively analyze plastic parts can provide the information needed to determine the nature and cause of plastic component failures. Jeffrey A. Jansen* Stork Technimet Inc. New Berlin, Wisconsin lthough procedures for failure analysis of metals are well documented, those for plastic parts are not. The final goal of both metallurgical and polymeric failure As plastic materials find further engineering applications, an understanding of the appropriate characterization tech- Ainvestigations remains the same, namely the de- niques will increase in importance. termination of the mode and cause of failure. There- fore, the general steps involved in evaluating a ally contain additives such as reinforcing fillers, failed plastic part parallel those applicable to a plasticizers, colorants, antidegradants, and process metallic component. The first step should include aids. It is this combination of molecular structure a thorough inspection, initially with an optical and complex formulation that requires specialized stereomicroscope, and subsequently with a scan- testing. This leads to the most significant variation ning electron microscope. in the failure investigations, which is the evalua- For both metals and plastics, the purpose of this tion of chemical composition. initial inspection is to characterize the fracture sur- Normally, the chemical composition of metals is face, and to determine the failure mode and crack analyzed by one of several elemental spectroscopic origin location. However, because of inherent phys- techniques, such as Inductively Coupled Plasma ical differences, the fracture surface morphology Spectroscopy (ICP). The analysis is relatively varies significantly between metals and plastics. straightforward, with the results detailing the ele- In subsequent steps, mechanical testing provides mental make-up of the material, thus facilitating evaluation of tensile, impact, and hardness prop- the identification of the alloy. Conversely, failure erties of both types of materials. While procedures analysis of a polymeric material requires several vary, the primary purpose of comparing the mea- different tests to derive comparable data regarding sured results to a specification, or to data generated the material composition. This article reviews the by a known good sample, remains constant. Cross test methods unique to polymeric materials, specif- sections of both types are prepared and inspected, ically those for evaluating composition and molec- but two slightly different aspects of the materials ular structure. are evaluated. In the investigation of a metal, the microstructure is analyzed. With plastics, the de- Fourier transform infrared spectroscopy gree of fusion and the orientation and dispersion Fourier transform infrared spectroscopy (FTIR) is of the filler materials are determined. a nondestructive micro-analytical spectroscopic While many aspects of the investigation are sim- technique that involves the study of molecular vi- ilar, it is important to recognize the distinct differ- brations. A continuous beam of infrared electro- ences that necessitate unique testing programs. The magnetic radiation is passed through a sample, principal difference is based on composition and causing individual molecular bonds and groups of structure. Unlike metals, polymers have a molec- bonds to vibrate at characteristic frequencies, and ular structure, which includes characteristics such as to absorb infrared energy at corresponding wave- molecular weight, crystallinity, and orientation, lengths. Because of this, organic materials having which has a significant impact on the properties of unique molecular structures generate distinct pat- the molded article. Additionally, plastic resins usu- terns of absorption. The resulting FTIR spectrum *Member of ASM International is considered the “fingerprint” of that particular 56 ADVANCED MATERIALS & PROCESSES/MAY2001 Polystyrene 0.1 0.0 Styrene: acrylonitrile copolymer 0.1 0.0 0.5 Polycarbonate material. Results can be manually interpreted or, 1.0 more commonly, searched against a library of ref- Poly (methyl methacrylate) erence spectra. FTIR can identify resins, contami- nants, chemical agents, and molecular degradation. 0.5 • Resin identifcation: FTIR is the principal analyt- ical technique for identification of polymeric ma- 3000 2000 1000 500 terials. Similar to verifying the alloy type in a met- Wave numbers, cm-1 allurgical investigation, FTIR allows a confirmation that the failed plastic part has been produced from Fig. 1 — FTIR spectral comparison showing clear differences between the results obtained on materials which exhibit a similar physical appearance. The absorption the specified resin type. Given the uniqueness of bands arise from distinct functionalities within the polymer structure. polymeric structures and the power of the tech- nique, it is readily apparent if a component has been Cracked component molded from the wrong material. For example, 0.05 several clear thermoplastic resins, such as poly- carbonate, poly(methyl methacrylate), polystyrene, and poly(styrene:acrylonitrile), have similar ap- 1.0 pearances, yet their infrared spectra are distinctly Poly (styrene: acrylonitrile: butadiene) different and identifiable, as illustrated in Fig. 1. 0.5 FTIR is usually the first analytical evaluation in a polymeric failure investigation. 1.0 • Contaminants: FTIR can provide much more Poly (2,6-dimethyl-1, 4-phenylene oxide) + polystyrene insight as a failure analysis tool. If a plastic resin 0.5 has been contaminated, particularly by another resin, the molded part will likely exhibit relatively Addition result brittle properties. FTIR analysis of the failed part 1.0 will reveal the presence of such contaminants. Spec- tral subtraction techniques can then be used to iso- 0.5 late the absorption bands that are attributed to the contaminant material, thus allowing identification. 3000 2000 1000 500 -1 An example of this is illustrated in Fig. 2. Often this Wave numbers, cm identification allows a determination of the source Fig. 2 — A molded part produced from an acrylonitrile:butadiene:styrene (ABS) of the contaminant material. resin produced results indicating the presence of contaminant poly(phenylene oxide) • Chemical agents: FTIR can be a powerful tool (PPO) resin. The presence of the contamination, which originated at the molding fa- for identifying the chemical agents responsible for cility, caused the component to exhibit brittle properties. chemical attack and environmental stress cracking. By analyzing the fracture surface, it is often pos- Thermogravimetric analysis sible to find trace residues of the chemicals that Thermogravimetric analysis (TGA) is a thermal were responsible for the failure. Once character- analysis technique that measures the amount and ized, the source of the chemical agents is frequently rate of change in the weight of a material as a func- apparent. tion of temperature or time in a controlled atmos- • Molecular degradation: Another piece of infor- phere. Weight can decrease due to volatilization mation detected by FTIR analysis is the indication of and decomposition, or increase through gas ab- molecular degradation. During degradation, such sorption or chemical reaction. as oxidation and hydrolysis, the molecular struc- TGA is a valuable analytical method for charac- ture of the polymer is altered. In the case of oxida- terizing the composition of polymeric-based mate- tion, carbonyl bonds are formed as part of the re- rials. TGA provides quantitative details that com- action, creating ketone, aldehyde, ester, and plement the qualitative data from FTIR testing. The carboxylic acid functionalities. Such degradation is relative loading levels of various constituents within most discernible in polyolefins such as polyeth- a plastic compound can be evaluated, as shown in ylene and polypropylene, as shown in Fig. 3. Hy- Fig. 4. These formulation components include poly- drolysis also results in molecular weight reduction, mers, plasticizers, additives, carbon black, mineral and introduces terminal hydroxyl groups that can fillers, and glass reinforcement. Such quantitative be observed within the FTIR spectrum. information is essential in failure analysis to verify ADVANCED MATERIALS & PROCESSES/MAY2001 57 100 0.3 Housing - discolored surface 477oC 67.5% 2.0 0.2 80 (14.7 mg) PP Oxidation products 0.1 1.5 60 1.4% (0.3 mg) carbon 0.2 1.0 Housing - core material 40 black Residue: 0.5 0.1 20 31% (6.7 mg) o glass 674C 0.0 200 400 600 800 1.0 Polypropylene Temperature, oC Fig. 4 — TGA thermogram showing the composition of a 0.5 polypropylene component. The material produced results con- sistent with the material description. -0.1 216oC, 26 J/g 3000 2000 1000 500 -1 Wave numbers, cm -0.3 o Fig. 3 — A polypropylene housing exhibited cracking and localized discoloration. 237C, 32 J/g Analysis of the darkened region showed absorption bands associated with severe ox- idation in addition to the base polypropylene. -0.5 PET that the part was fabricated from the proper mate- PBT o rial. Individual constituents within a formulated -0.7 250C plastic resin are responsible for specific end prop- Individual 224oC constituents erties, and deviations from the designated concen- trations can result in significant changes in me- 50 100 150 200 250 300 o within a chanical, physical, or chemical properties. For Temperature, C formulated example, reduced glass fiber content will result