PARTS GO

M any OEMs and contract manufac- turers have the facilities to test basic physical properties to assure the quality and performance of their

products. This works well for the vast majority of day-to-day business needs, but most companies have been faced at some point with the unexpected failure of aproven --- design, often in the abbsewd any conscious change in materials or production.

The consequences can be disastrous, depending largely on how early the problem is discovered and how efficiently it can be analyzed and cor- rected. The results can range from failures iden- tified in routine quality control, which allow the manufacturer to halt further production until the problem can be investigated; to failures in the field, which can be far more grave. Depending on the application, the potential safety hazards, product liability, recalls, and damaged relationships can all loom large.

This article discusses the many analytical tech- nologies available for determining the cause of fail- ures, in particular as they relate to failure of plastic parts. Technologies covered include optical mi- croscopy, transmission electron microscopy, scan- ning electron microscopy, atomic force microscopy, energy dispersive X-ray spectroscopy, thermal analysis, dynamic mechanical analysis, and chromatography.

First steps The initial step in analyzing a failure is to get a

complete history on the part and material, in- cluding the service environment, load, physical composition, and molding conditions. If the failure --

___- - - - - - invoTvesafrmeme mbahon area is typically examined with a low-power stereo microscope, as technicians look for patterns in the break and/or the wear surface that give clues to the mechanism and ultimate cause of failure.

Optical microscopy provides the opportunity to observe the part and the fractures at low enlarge- ment factors. The stereomicroscope is especially useful as an initial review to gain an overall sense of the failure, and it offers great flexibility in spec- imen size, orientation, and lighting choices. The compound light microscope is also valuable for examining crystalline materials in polarized light.

Transmission electron microscopy (TEM): Often, the imaging process advances to TEM to

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At times, a single technique can make great strides in determining cause offailure, but in many cases, multiple methods will be needed to make a confident diagnosis and find the underlying problem or combination of issues causing the failure.

Kevin Battjes Impact Analytical Midland, Michigan

zis photo is an SEM micrograph ofa fractured glass-reinforced nylon part. The image allows analysts to s t u 4 &tags ~$thglass-matrix interface m d d e

- Facrtir%%uTacTKagIn fiZtwT can be as high as 50,000X or more, although most polymer materials have a practical limit of10,OOOX to 20,000X.

view detailed morphological images that define m the failure mechanisms. Imaging: the fracture ~ o i n t

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and analyzing the elemental composition can help detect foreign particles, non-homogeneous mixing, cold welds, cyclic loading, poor adhesion to re- inforcing fibers, and void formation.

Scanning electron microscopy Scanning electron microscopy (SEM) is an ex-

cellent tool for examining surfaces at high magru- fications. It produces a pseudo three-dimensional image without many of the aberrations that are troublesome in optical microscopy. Magnification

ADVANCED MATERIALS & PROCESSES/IULY 2005

Polymer parts and adhesives - .quently at i__ . . ncidents are plastic components or

polymeric adhesives, which continue to replace traditional metal parts and fasteners at an increasing rate, as manufacturers seek to reduce weight, lower production costs and/or improve the corrosion resist- ance of their products. Compounding the problem for many engi- neers is a general lack of experience with plastics as opposed to metals, where modes of failure and their underlying causes are drastically different.

The motivators that drive the choice of polymeric materials can be either economic or performance-related. Weight reduction is often a key objective, particularly in automotive applications. Polymeric ma- terials offer features such as sound absorption, aesthetics and cost sav- ings, as well as the versatility to conform and/or mold into complex shapes. In some parts (such as valve covers, fans, fuel and emissions components, vacuum control systems and fluid reservoirs), the flex- ibility and/or corrosion resistance of a polymer can deliver superior performance over a metal counterpart.

Within the realm of polymeric materials, the selection of specific grades of plastics, adhesives, and elastomers can also have a signifi- cant impact on profitability. Even the difference of a few cents per pound from one resin or supplier to another can be enough of an impetus to make a change, at least on paper. But when a part fails, the anticipated savings can quickly evaporate from the ensuing testing, analysis, and corrections.

typically ranges from as low as 20X to as high as 50,000X or more. (Because of their low signal yield, good insulating properties and chemical compo- sition, most polymer materials have a practical magnification limit of 10,000X to 20,000X.)

In many cases, the fracture surfaces of a failed part indicates the failure mechanism by character- istic features along the fracture. Microscopy can discover many clues to the failure, from identi- fying the point of origin, to the propagation direc- tion, to the mechanical properties.

Specimen preparation for SEM is relatively simple and quick, generally consisting of excising the sample area of interest and applying a vacuum- deposited conductive layer to avoid charging in the electron beam. In blends of incompatible poly- mers or block copolymers, phase separation has a significant impact on the end use properties of the material, so it's important to know the morphology of the components. Additive amounts and pro- cessing conditions also affect the phase domain type and size.

Energy, keV

Used for analyzingfillers, additives and contaminants i n polymer materials, Energy Dispersive X-Ray Spectroscopy evaluates the elemental composition ofde- posits and residues found in problematic materials. This method is especially ef- fective i n identifying unknozun particulates and foreign bodies in failed parts, and helps identify potential sources.

To generate SEM images of the separated phases, specimens are frozen and fractured in a reproducible manner. Preparation of the sample usually involves carbon coating by evaporation and/or sputter coating with a heavy atom. (Gold or gold/palla- dium are most common.) The component phases are often readily identified, and micrographs are recorded for measurement and reference.

Transmission electron microscopy Transmission electron microscopy (TEM) pro-

vides high magnification imaging and electron dif- fraction capability for the ultra-structural analysis of polymer systems. Magrufncation typically ranges from 500X to 500,000X. Specimen formats may be whole mounts, thin films, replicas, or ultra-micro- tomed thin sections. Staining techniques enhance image contrast, based on functional group consti- tution. TEM is the tool of choice when character- izing the phase morphology of blended or copoly- merized materials. Polymeric materials such as high impact polystyrene, ABS, TPO, and filled plas- tics are also readily characterized by TEM methods.

Atomic force microscopy Atomic force microscopy (AFM) is an important

new analytical technique for study of surface fea- tures with a resolution down to the atomic level. In contrast to traditional microscopes, in which electron or photon beams create images, an atomic force microscope is based on a mechanical probe that moves across the surface to reveal various sur- face properties such as topography, friction, and hardness.

AFM combines ease of use with an unparalleled ability to image surface features under ambient conditions, without extensive sample preparation. Atomic force microscopes can magrufy surface fea- tures by as much as 100,000,000X, and as a result, single atoms and surface molecules are often di- rectly observable.

Energy dispersive X-ray spectroscopy Energy dispersive X-ray spectroscopy (EDS) is

based on an electron beam that strikes the spec- imen, whch then emits X-rays characteristic of the material. Thus, EDS enables identification of the elemental composition of the material in question. This technique is especially effective in deter- mining the chemical composition of unknown par- ticulates and foreign bodies in materials or parts that are causing failure issues.

EDS is appropriate for analyzing fillers, addi- tives and contaminants in polymer materials. It also helps answer questions regarding the compo- sition of deposits and residues found in problem- atic systems and materials.

Fourier transform infrared spectroscopy Fourier transform infrared spectroscopy (IT-IR)

is a popular tool for identifying contaminants and determining the presence of functional groups in chemical systems. Because IR absorption depends upon a dipole moment change during each molec- ular vibration and/or molecular rotation, it's pos-

ADVANCED MATERIALS & PROCESSES/JULY 2005

sible to conduct functional group analysis, as well as molecular identification and characterization.

Some FT-IR systems include a polymers and ad- ditives digital library, which provides an excellent tool for identifying an unknown polymer. A small amount of the material can be excised from the molded article and dissolved in a solvent. A thin film of the polymer is cast and its infrared spec- trum is measured. By searching the library, a match can often be quickly found for the spectrum, thereby identifying the material. An experienced analyst is invaluable in interpreting the spectra and their implications for the material in question.

Thermal analysis Thermal analysis includes a number of tech-

niques that characterize the thermal and physical properties of a polymeric material. Specific deter- minations include glass transition temperature, enthalpy of phase transition, kinetic constant, crys- talline melting point, specific heat, crystallinity, degradation temperature, viscoelastic properties, percent weight loss, material softening point, and coefficient of thermal expansion. The thermal and crystalline properties of polymeric materials play a significant role in understanding part perform- ance in the intended applications.

Thermogravimetric analysis (TGA) is a thermal analysis technique in which changes in the weight (mass) of a sample are measured as a function of temperature and/or time. TGAis fre- quently chosen to determine polymer degradation temperatures, residual solvent levels, and absorbed moisture content. The inorganic glass fiber con- tent of polymer and composite samples can also be readily analyzed by TGA.

In operation, the polymer resin is degraded and burned off in heating these specimens to high tem- peratures in an air atmosphere. The noncom- bustible glass fiber is left behind as a residue, so the weight fraction of glass fiber in the composite can be determined by a TGA residue analysis rou- tine. This test is widely used for analysis of filled systems to evaluate the material against the man- ufacturer's or supplier's specifications. It can also serve to examine moisture content in resins or molded articles.

Dynamic mechanical analysis ( D M ) is an- other thermal technique for measuring changes in the viscoelastic response of a material as a func- tion of temperature, time, or deformation fre- quency. DMA can determine quantitative flexural storage and loss moduli, shear storage and loss moduli, and tan delta (damping coefficient or loss factor), as well as the dynamic and complex vis- cosity of materials. The technique is also particu- larly useful for qualitatively characterizing the glass transition temperature and other sub-Tg tran- sitions of polymer and composite materials.

Block copolymer samples generally exhibit at least two distinct material phases. Because the me-

4000 3000 2000 1500 1000 500 Wave numbers, cm-1

Fourier Transfom lnfrared Spectroscopy is a toolfor ident$jing unknown con- taminants by extracting and analyzing non-volatile additives. The peaks in this graphic correspond tofunctional groups and chemical bond types. Peak positions and shapes help analysts identify specific chemistries that might contribute to part failure.

on block copolymers generally show two distinct Tg's, which are related to the Tg's of the respective homopolymer components in the block copolymer.

Chromatography Gas chromatography (GC) is the most common

analytical technique for determining the number and concentration of components in a volatile mixture, or the presence of volatile imvurities. It can also aid in the iositive identification ofla polymer compound. Additives extracted from the polymer can be detected and identified by GC, as well.

In operation, a sample, usually a liquid or a solid dissolved in solvent, is injected into the gas chro- matograph. The sample is flash-vaporized and brought into the column by the carrier gas. As the sample passes through the column, it is separated into its individual components, and as each one passes through the detector, it appears as a deflec- tion on a chart. The area under this deflection is ~rovortional to the concentration of the comvo- I I I

nent in the original sample. When combined with mass spectrometry (MS),

the sample mixture is first separated by gas chro- matography. Upon entering the mass spectro- graph, the components in each peak are identified by their molecular ion and fragmentation pattern.

High-performance liquid chromatography (HPLC) is the method of choice for separating non- volatile, thermally unstable and/or polar compo- nents. Solid or liquid samples are dissolved in an appropriate solvent and injected into a liquid chro- matograph, where the components are separated bv selective retention within the stationarv phase. i s the analytes flow through the detector: &ere is a deflection on the chart. As with gas chromatog- raphy the area underneath the curve is proportional to the concentration of the analyte in solution. e

chanical loss characteris6cs of a material are di- For m o ~ infomation: Kevin Battjes is a Research spe rectly related to molecular-level motions, the D M cialist at Imvact Analvtical. 1910 W. St. Andrews Road. damping signal is particularly sensitive to a ma- Midland, MI 48640; ;el: 989/832-5555; fax: 989/ 832: terial's glass transition process. DMA loss profiles 5560; Web site: www.impactanalytical.com.

ADVANCED MATERIALS & PROCESSES/JULY 2005