THE USE OF MICROSCOPICAL METHODS FOR CHARACTERIZATION AND FAILURE/CONTAMINATION ANALYSIS OF ADHESIVE TAPES

Transcription

1 THE USE OF MICROSCOPICAL METHODS FOR CHARACTERIZATION AND FAILURE/CONTAMINATION ANALYSIS OF ADHESIVE TAPES.< Sctt Steffier, Senir Research Micrscpist, McCrne Assciates Inc., Westmnt, IL Intrductin A variety f techniques invlving light micrscpy and small particle islatin and identificatin are available fr the analysis f adhesive tapes. These techniques can be applied t bth film and adhesive layers f tapes themselves, as well as t any defects r cntaminants which may be present and which may result in an unacceptable appearance and/r perfrmance f the prduct. Defects in different layers f the plymer film, as well as adhesive distributin can be visualized using blique, transmitted, episcpie, darkfield r flurescent illuminatin, as well as cntrast enhancement techniques such as phase cntrast r Nmarski differential interference cntrast micrscpy. Additinal characterizatin f layer thickness and adhesin failure, pigment distributin, and distributin f cntaminant particles is pssible using micrtmed crss sectins, cut either by hand r mechanically. Particulate cntaminatin dwn t.micrn-sized particles can be readily islated and identified using plarized light micrscpy (PLM), infrared micrspectrscpy (IR), Raman spectrscpy, andscanning electrn micrscpy with energy dispersive x-ray spectrscpy (SEM/EDS). '. Characterizatin f Tape Cmpnents : In sme cases, a prject will require as cmplete a characterizatin as pssible f the nrmal cmpnents f a tape sample, including film layers, fabric, adhesive and pigments/fillers. This is mst ften needed in cases f reverse engineering f a empetitr's prduct r in frensic science when, fr evidentiary purpses, tw r mre tape samples must be cmpared t determine their similarity. Identificatin f the varius cmpnents f a tape sample can be readily accmplished even n small amunts f material, using a variety f micranalytical techniques. < Characterizatin f Plymer Film If the plymer film backing is knwn t be a single layer, characterizatin f the rganic prtin is easily accmplished by shaving ff a thin sectin with a micrscalpel and pressing the sample nt a salt plate fr analysis by infrared micrspectrsepy. On bulk samples, infrared micrspectrsepy by attentuated ttal reflectance (ATR) can als be useful. If multiple layers are present, they can smetimes be separated by freezing r immersin in a suitable slvent, then analyzed separately. Mre cmmnly, a crss-sectin f the plymer film is cut, either by hand r with a micrtme. Figure 1 illustrates a typical sectin f a multilayer plymer film. In sme cases, acceptable hand sectins can be cut using a Tefln cated duble-edged razr blade under a lw pwer steremicrscpe, hwever mechanically micrtmed sectins are generally preferable due t their greater unifrmity. The thickness and unifrmity f the varius layers can readily be determined using a micrscpe equipped with a calibrated eyepiece reticule. In sme cases, a cntrast enhancement technique such as phase micrscpy must be emplyed t clearly visualize the brders between varius layers. Infrared micrspectrscpy can als be perfrmed n the individual layers in a crss-sectin, prvided they are nt t thin. If a sample cntains layers t thin t be reslved by a single IR analysis, an infrared line map acrss the entire film thickness can prvide chemical infrmatin abut the thinner layers. Alternatively, a sectin can be cut at an blique angle, s that each layer presents a wider aspect. In sme case, the analysis f very thin layers can als be accmplished by delaminatin f the sample, fllwed by ATR spectrscpy f the apprpriate surface. In samples when the plymer film is t heavily filled r 29

2 pigmented t allw an adequate infrared analysis (as in the case f black electrical tape r cnductive carbn tape, fr example), r when absrptins due t pigments and fillers verlap imprtant bands in the IR spectrum f the plymer, the rganic prtin f the film can be islated by using a micrpyrlysis technique. Fr this analysis, a small prtin f the layer f interest is placed in a glass capillary tube with a drawn ut tip. Figure 2 shws a capillary pipette laded with a black plymer particle a few hundred micrmeters in size. The sample is heated briefly ver an alchl flame t pyrlyze the rganic cmpnent. The pyrlysis prduct is then rinsed frm the tube with a suitable slvent, such as methylene chlride, and sptted nt a salt plate. After evapratin f the slvent, the pyrlysis prduct can be easily analyzed by IR. Characterizatin f Adhesive The rganic prtin f the adhesive layer is als best analyzed by infrared micrspectrscpy, either by bringing an ATR crystal int cntact with the adhesive layer, r by remving a small amunt f the adhesive with a needle and smearing it ut n a salt plate fr direct IR analysis in transmissin mde. When using the latter technique, it is smetimes helpful t partially disslve the adhesive in a micrdrp f a slvent such as xylene r amyl acetate, and then allw it t dry as a thinner, mre unifrm film n the salt plate. This is als a useful technique when the adhesive is filled (as with a duct tape, fr example), as a micrscale extractin can be perfrmed directly n the salt plate t separate the sluble rganic prtin f the adhesive frm the insluble inrganic prtin. Inrganic pigments and fillers such as titanium dixide, calcium carbnate r aluminsilicate clay can be identified by a cmbinatin f light micrscpy, infrared micrspectrscpy and SEM/EDS. In sme cases, it may be f value t characterize the distributin f the adhesive. In the case f an paque adhesive r backing, this is easiest with blique lighting and a steremicrscpe. In cases where neither the plymer film nr the adhesive layer are paque, transmitted illuminatin r episcpic illuminatin (light directed at a 9 angle t the surface) are typically mre useful fr visualizing the cnturs f the adhesive surface. Figures 3 and 4 depict the adhesive layers f tw samples f duct tape, shwing nticeably different adhesive distributin.,j, : Characterizatin f Fabric/Fibers The fiber r fabric reinfrcement present in sme tapes can als be easily characterized, using steremicrscpy and PLM. The number f warp (and if present, weft) yarns per inch can be measured using a calibrated eyepiece scale. Individual yarns and fibers can be islated frm the adhesive with an excess f a suitable slvent and munted in an il f knwn refractive index fr PLM analysis. Mst synthetic and natural fibers can be quickly identified by PLM alne, based n their mrphlgy and ptical prperties. The mst cmmn types f fibers used in adhesive tapes, (cttn, plyester, glass and rayn) can all be distinguished at a glance by PLM. Phtmicrgraphs f these fur fibers are included in Figures 5 thrugh 8. Cttn fibers are immediately identifiable by their mrphlgy, appearing as twisted ribbns, while rayn fibers are distinguished by a series f striated lines running parallel t their length. Plyester appears as straight, nrmally rund fibers that have fairly high refractive indices and which are birefringent, appearing bright between crssed plars. Glass fibers als appear as straight rds, but are nt birefringent and will appear dark between crssed plars. Analysis f Defects and Cntaminants In additin t being useful fr characterizing the nrmal cmpnents f a tape sample, micrscpical and micrchemical techniques are als valuable fr lcating, islating and identifying cntaminant particles r defects in the tape itself. In mst cases, these methds are quicker and mre 21

3 i readily applicable t cntaminant analysis than the macrscale methds used t analyze bulk tape cmpnents. Lcatin f Defects and Cntaminants Defects in the tape itself will mst cmmnly be present in the plymer backing. Depending n the nature and lcatin f the defect, transmitted illuminatin (brightfield r between crssed plars), blique illuminatin r episcpic illuminatin may be mst useful fr visualizatin and phtmicrgraphy. In mst cases, a defect will be visible t sme degree with mre than ne illuminatin methd. Figures 9 and 1 illustrate typical defects in the plymer film backing f tape samples, viewed with transmitted light. Figures 11 thrugh 13 shw defects in the same brwn wrapping tape under threedifferent illuminatin cnditins. While the defects are visible in all three cases, episcpic illuminatin prvides by far the best image. If a defect is lcated within ne f the layers f a transparent multilayer film backing, its depth, and the layer it resides in, can be determined by fcusing dwn frm the surface f the film until the defect is in clear fcus and measuring the change in distance n the fine fcus knb f the micrscpe. This methd (called ptical sectining) is faster and easier than manual crss sectining, and allws all f a defect t be imaged, rather than just what is visible at the edge f a sectin. The Use f crssed plars (with a plarized light micrscpe)wili h~hlight areas where a plymer has been placed under stress due t either manufacturing flaws r 'damage, Stressed areas will be mre highly rientedthan the surrunding plymer and will display different brighmess and clrs due t differing birefringence. Figure 14 shws a sectin f a transparent plymer film with numerus stress-related defects, viewed between crssed plars. In the case f mre highly riented plymer films, crssed plars can help t highlight regins f lwer rientatin, which ~may be weaker. Occasinally, specialized illuminatin and cntrast enhancement techniques may be necessary t adequately visualize defects. Phase cntrast and Nmarski differential interference cntrast can be used t highlight features which are very similar in clr and refractive index t thesurrunding plymer and which cannt be easily distinguished with rdinary brightfield illuminatin. Defects t small t be reslved with rdinary transmitted light can be visualized using transmitted darkfield illuminatin, which will cause the defects t appear as bright spts against a dark backgrund. Cntaminant particles can usually be lcated using nly a lw pwer steremicrscpe, with magnificatins f abut 5X-6X, and blique lighting, althugh extremely small particles may require the higher magnificatin f a cmpund micrscpe. Rarely, flurescence micrscpy may be useful in highlighting cntaminants that fluresce differently than the surrunding matrix, thugh such particles are typically als visible with nrmal illuminatin as well. Islatin f Cntaminants Cntaminant particles in the adhesive layer can typically be remved using fine frceps and needles, with the aid f a steremicrscpe. Larger particles such as fibers r metal flakes can be pulled ut f the adhesive layer directly, then rinsed with slvent n a micrscpe slide t remve residual adhesive. Fr the remval f smaller particles, it is ften helpful t apply a small drp f slvent t the area in rder t partially disslve the adhesive, after which a small blb f sftened adhesive cntaining the particle(s) f interest can be remved with a needle. This sample can then be pressed ut n a glass slide under a cver slip fr micrscpical examinatin f the particles, with additinal slvent being added t further disperse the adhesive, if necessary. If it is necessary t cmpletely islate small particles frm the adhesive fr chemical analysis, a small smear f adhesive cntaining the particles f -:

4 L. interest can be placed n a micrscpe slide, IR windw r SEM stub, after which small drplets f slvent are successively drawn ver the sample with a needle until the adhesive matrix has been cleared. >. :T i Cntaminant particles at r near the surface f the plymer backing can als be easily remved by direct picking with micrtls. Particles that are imbedded within the film can ften be excised frm the plymer by using a sharpened needle and micrscalpel. Alternatively, a small sectin f the plymer cntaining the particle f interest can be cut ut and crss-sectined (usually by hand) t expse a prtin f the particle fr IR r SEM analysis. Figures 15 and 16 depict this technique. Fr particles that are particularly difficult t islate frm the plymer matrix by physical means, disslutin r melting f the plymer r lw temperature ashing may als be useful, depending n the nature f the particles and f the matrix. < Identificatin f Cntaminants Once cntaminant particles have been islated, a variety f micranalytical techniques can be used fr their identificatin. Light micrscpy, using a steremicrscpe and plarized light micrscpe shuld be the initial techniques in any scheme f analysis and will in many cases be sufficient t identify unknwn particles quickly and unambiguusly. Fibers, hairs, pigments, fd prducts, pllen grains, spres, glass, plant tissues, cmbustin prducts and minerals are just sme f the particle types that are readily identifiable with light micrscpy alne. A valuable advantage f PLM is that it allws different types f particles with essentially the same chemical makeup t be easily distinguished. Cttn and rayn fibers, fr example, are bth cmpsed f cellulse and will give very similar spectra if analyzed by IR r SEM, but are distinguishable at a glance under the micrscpe. Light micrscpy als has the advantage that it can smetimes be used t identify particles withut the need t islate them frm a matrix. If additinal chemical infrmatin abut a sample is desired (in the case f plymeric materials r metals, fr example), IR and SEM/EDS are typically emplyed as the next steps in a system f analysis. SEM/EDS can prvide infrmatin n the elemental cmpsitin f metal allys, minerals, pigments, glasses and ther inrganic materials, while IR prvides mlecular infrmatin abut rganic materials, and can als be used fr limited identificatin f inrganics. Raman spectrscpy is anther useful technique that prvides chemical infrmatin cmplementary t IR and which can be used t identify bth rganic and inrganic materials. Raman spectrscpy has the advantages f being effective n smaller particles than IR, as well as smetimes being able t analyze particles in situ. Raman is als useful fr the analysis f materials that are paque (such as graphite) and fr distinguishing different crystalline frms f the same cmpund (such as the rutile and anatase frms f titanium dixide). Cnclusin Light micrscpy is an extremely pwerful tl that can be applied t many types f particle identificatin and material characterizatin prblems and shuld be part f the repertire f any labratry invlved in addressing industrial issues f prduct failure r cntaminatin. Its speed and versatility in sample evaluatin and identificatin make it an invaluable tl that, in cmbinatin with ther micranalytical techniques, can lead t slutins f prblems that wuld therwise be intractable. 212

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