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, contaminants, chemical agents, and molecular degradation. 0.5 • Resin identifcation: FTIR is the principal analytical 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 technique, 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 polycarbonate, 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 consistent 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 oxidation 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 in a Fig. 5 — DSC thermogram showing a clear difference in molded part with diminished tensile strength and the melting points of PBT and PET. plastic lower modulus. Additionally, if contaminants are from a semi-crystalline resin. Generally, higher resin are detected by FTIR within a material, TGA can quan- levels of crystallinity result in improved tensile responsible tify their respective loading. strength, increased stiffness, and superior chemical resistance. Reduced crystallinity is often the result for specific Differential scanning calorimetry of molding the part in a relatively cold tool, pro- end Differential scanning calorimetry (DSC) is a ducing frozen-in amorphous regions within the properties. thermoanalytical technique in which temperature preferentially crystalline structure. and heat flow are measured as a function of time To evaluate the condition of a failed part, the ma- and temperature. The measurements provide quan- terial is first analyzed as-received. Then the spec- titative and qualitative information about physical imen is heated through melting and allowed to cool and chemical changes involving endothermic or slowly through recrystallization, which produces exothermic processes, or changes in heat capacity. optimal crystallinity. Some materials, such as poly- DSC can determine melting point, heat of fusion, acetal (POM) resins, simply show a significantly glass transition temperature, and degree of cure. reduced heat of fusion in the initial DSC heating • Melting point: The most fundamental property run, as compared to the second run after slow measured by DSC is the melting point (Tm) of a cooling. Other materials, including polyphthala- semi-crystalline plastic resin. The melting point of a mide, exhibit a low temperature exothermic tran- material, which is observed as an endothermic transition, as shown in Fig. 6. sition, is a secondary means of resin identification, • Glass transition temperature: DSC can also yield particularly when different materials within a resin other information regarding failed plastic parts. As family cannot be spectrally distinguished by FTIR. part of a standard DSC analysis, the glass transi- This is the case for thermoplastic polyesters, such tion temperature (Tg) and recrystallization temper- as polyethylene terephthalate (PET) and poly- ature can be determined. These parameters can pro- butylene terephthalate (PBT). These materials provide information on the material regarding duce consistent FTIR spectra because of the simi- degradation, and the presence of nucleating agents larities in their structures, but have distinct melting within the resin. Specialized testing can also deter- points, as illustrated in Fig. 5. mine oxidative stability. This is applicable to a • Heat of fusion: In conjunction with determining failure analysis in determining if the failed mate- the melting point of a material, DSC measures the rial contained the proper level of antioxidants, be- corresponding heat of fusion. This is simply the en- cause deficiency in the loading level of such stabi- ergy required to melt the sample, and it is signifi- lizers can lead to premature failures. cant because it indicates the degree of crystallinity. • Degree of cure: DSC can also evaluate thermoset The crystalline state has a great effect on the phys- plastics for degree of cure. If a molded component ical properties of a molded plastic article produced is under-cured, the part exhibits a significant re-

58 ADVANCED MATERIALS & PROCESSES/MAY2001 0.2 80 Part tested o after slow cooling 287C, Grille, as received o 28J/g 60 Grille, annealed at 150C 0.0 Thermo- mechanical 142oC 40 -0.2 304oC analysis o 281C, 20 25J/g (TMA) is a Part tested o -0.4 139C, as received 0 thermal 17J/g analysis Low temperature crystallization 300oC 40 80 120 160 method in 50 150 250 350 o Temperature, oC Temperature, C which linear Fig. 6 — An automotive component produced from a PPS Fig. 7 — TMA thermogram shows a comparison of the co- or volumetric resin failed under relatively low stress levels. DSC analysis efficients of thermal expansion (CTE) of the base polysulfone showed that the material underwent low temperature crystal- material and the steel insert material. The difference in CTE dimensional lization. This was indicative that the material was under- in conjunction with repeated temperature changes led to pre- changes are crystallized as molded. mature failure. measured as duction in mechanical properties and may be likely residual stress. Additionally, molded-in stress is to fail. often sufficient to facilitate environmental stress a function of cracking. Molded-in stress is evident in the ther- temperature, Thermomechanical analysis mogram as a sharp expansion at the glass transi- time, or force. Thermomechanical analysis (TMA) is a thermal tion temperature. Annealing the sample just above analysis method in which linear or volumetric di- the glass transition temperature relieves the mensional changes are measured as a function of molded-in stress and results in the expected temperature, time, or force. TMA data can be ac- contraction. quired in several compression modes, including expansion, penetration, dilatometry, rheometry, Melt flow rate and flexure; or in tension mode. TMA is often over- Melt flow rate is an indication of the rate of ex- looked as a failure analysis tool, but can provide trusion of thermoplastic materials, and is a mea- insight into critical aspects of material properties sure of the viscosity of the plastic resin. A sample as part of such investigations. These aspects include is placed in the molten state, as it would be during coefficient of thermal expansion, glass transition injection molding, and forced through a standard- temperature, and residual molded-in stress. ized die under prescribed conditions of tempera- • Coefficient of thermal expansion: The most ture and load. The quantity of material that is ex- common application of TMA in failure analysis is truded in a standard time period is measured and the measurement of the coefficient of thermal ex- normalized to the final test units, grams per ten pansion (CTE). This can be a very important prop- minutes. Melt flow rate increases with reduced melt erty, particularly when a plastic component has viscosity caused by declining average molecular molded-in metal inserts. A typical analysis result weight. As such, the test is a practical, efficient illustrating this is presented in Fig. 7. Repeated, method to compare the average molecular weights cyclic movement across a wide temperature range of similar materials. can result in fatigue failure, initiating at the inter- Resin suppliers and plastic molders commonly face between two materials having considerably determine melt flow rate as a quality control tool, different coefficients of thermal expansion. and nominal data is often readily available on pub- • Glass transition temperature: Another important lished material data sheets. Melt flow rate is mea- parameter measured by TMA is the glass transition sured as part of a failure analysis to evaluate the temperature (Tg) of semi-crystalline and amorphous extent of molecular degradation caused by service thermoplastic resins. While DSCcan also measure conditions, or more commonly, the molding Tg, TMA offers a more sensitive technique, particu- process. The test is most effective when the results larly when the transition is subtle. The glass transition from a failed part are compared with the results temperature is an important parameter of a mate- from the same lot of virgin molding resin in the rial, because plastic resins exhibit significantly dif- part. ferent properties below and above this point. For Alternatively, data from a different lot of molding polypropylene homopolymers, this point is usually resin, or published literature values, can also be the around 25°C (77°F), and the material is relatively basis for comparison. However, this renders the in- brittle at lower temperatures. For amorphous resins, terpretation of the data somewhat ambiguous, the material usually loses all of its load-bearing ca- making the assessment of molecular degradation pabilities at the glass transition temperature. more difficult. ■ • Residual stress: Perhaps the most important parameter measured by TMA is molded-in residual stress. Such stress is often produced during molding For more information: Jeffrey A. Jansen, Stork Tech- processes, and is a significant factor in many plastic nimet, Inc., 2345 South 170thStreet, New Berlin, WI 53151; failure investigations. It must be remembered that tel: 262/782-6344; e-mail: [email protected]; the total stress is the sum of external stresses and Web site: www.technimet.com.

ADVANCED MATERIALS & PROCESSES/MAY2001 59