Sanjeev S Koni Assistant Professor Centurion University of Technology and Management INTRODUCTION The analysis of paint samples is often required in the Forensic Science Laboratories These samples come from variety of sources such as motor vehicles in HitRun cases and door and windows in ID: 1027792
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1. THE USE OF MICROSCOPY, SPECTROSCOPY AND CHROMATOGARAPHY TECHNIQUES FOR PAINT EXAMINATIONSanjeev S KoniAssistant ProfessorCenturion University of Technology and Management.
2. INTRODUCTIONThe analysis of paint samples is often required in the Forensic Science Laboratories. These samples come from variety of sources such as motor vehicles in Hit-Run cases and door and windows in breaking and entry cases. The analytical problems facing the forensic scientist is the comparison of paint samples located during the course of an examination, with samples obtained from the possible sources to establish the common source of origin.
3. COMPONENTS OF PAINTPaint is commonly known as pigmented coating. The main components of Paint are: 1. Pigment 2. Binder 3.Solvent 4. Additives.
4. ROLE OF COMPONENTSPigment: A finely ground, inorganic or organic, insoluble, and dispersed particle. Besides color, a pigment may provide many of the essential properties of paint such as opacity, hardness, durability, and corrosion resistance.
5. ROLE OF COMPONENTS The pigments can be classified as natural or synthetic type. Natural pigments include clays, calcium carbonate, mica, silica and talcs while synthetic pigments include engineered molecules such as precipitated calcium carbonate and synthetic pyrogenic silica. Hiding pigments such as Titanium dioxide, Phthalo blue and Red iron oxide protects substrate from effects of UV radiation and make paint opaque.
6. ROLE OF COMPONENTSBinders: A nonvolatile portion of the liquid vehicle of a coating, which serves to bind or cement the pigment particles together .It includes synthetic or natural resins such as cement alkyds, vinyl-acrylics, vinyl acetate ethylene (VAE), polyurethanes, polyesters, melamine, resins or oils.
7. ROLE OF COMPONENTSSolvent: The main function of solvent is to adjust the curing property of the viscosity of the paint.It controls flow and application properties, and affects the stability of the paint while in liquid state. Its main function is as the carrier for non volatile components. Petroleum distillate, Esters , Glycol ethers are used as solvents.
8. ROLE OF COMPONENTSAdditives[Modifier]: Any substance that added in small quantities to improve the properties of paint are called additives.Additives may include substances such as driers, corrosion inhibitors, catalysts, ultraviolet absorbers, and plasticizers.
9. AUTOMOBILE PAINTSCoating : A generic term for paint, lacquer, enamel, or other liquid or liquefiable material that is converted to a solid, protective, or decorative film or a combination of these types of films after application. Automobile finishing has at least four coatings: 1. Electro coat primer 2. Primer Surfacer 3. Base coat 4. Clear coat.
10. LAYERS OF COATING
11. ELECTROCOAT PRIMERThey are epoxy-based resins electroplated on to surface to prevent corrosion They are uniform in color. They are first layer. They are usually black or grey in color.
12. PRIMER SURFACEROriginally intended for corrosion control before base coat.It smoothes out e or hides imperfections or seams.Color pigments are used to prime for color. Ex. Gray used for pastelsRed oxide used under dark colors
13. BASE COATBase coat is the pigment that provides appearance of the finish. Integrity depends on its ability to resist weather, UV radiation, and acid rainAcrylic based polymer comprises binder.Heavy metals like lead and chrome no longer used. In favor now is organic based pigments.
14. CLEAR COATThey are unpigmented coat used to improve gloss, durability, and appearance.Most of them are acrylic basedPolyurethane clear coats are becoming more popular. It offers great resistance and gives good appearance.
15. PHYSICAL AND CHEMICAL FEATURES OF PAINTPaint films are characterized by number of physical and chemical features. The physical characteristics may include color, layer sequence, thickness, surface and layer features, contaminants and weathering. Chemical components may include pigments, polymers and additives. These features can be evaluated by various chemicals and instrumental methods.
16. SAMPLE PREPARATION AND LAYER ANALYSISThe layers in a paint film are identified by viewing the sample edges at magnifications ranging between 5× and 100×. Definitive paint layer identification usually requires sample preparation techniques such as manual or microtome sectioning, edge mounting and polishing, or both. A combination of techniques may be required to fully characterize the layer structure. The extent of sample manipulation and preparation will depend on the amount of paint available and its characteristics.
17. SAMPLE PREPARATION AND LAYER ANALYSISPaint layer structure can be observed by using a scalpel blade to make an oblique cut through a sample. The larger surface area created by this angled cut may enhance layer visualization and assist in the detection of layer in homogeneities. The preparation of thin-sections and the separation of paint layers can be accomplished with a scalpel blade. Preliminary solvent tests can be conducted on the manually prepared sections and layer fractions. Subtle differences in color, pigment appearance, surface details, inclusions, metallic and pearlescent flake size and distribution, and layer defects may require microscopical comparisons of the edge, oblique cut, and surface views of known and questioned paint samples. These comparisons must be carried out with both samples positioned side by side and in the same field of view.
18. GUIDE TO MOTOR VEHICLE IDENTIFICATION
19. PAINT SAMPLE GUIDE
20. TECHNIQUES EMPLOYED.For the examination of paint, the following instrumental techniques are applied. 1. Microscopy. 2. Spectroscopy. 3. Chromatography.
21. MICROSCOPYFollowing microscopic techniques are used to analyze the paint samples. 1. Polarized Light Microscopy[PLM] 2. Scanning Electron Microscopy[SEM] 3. Transmission Electron Microscopy[TEM] 4. Optical Light Microscopy.
22. POLARIZED LIGHT MICROSCOPY [PLM] Polarized Light Microscopy is appropriate for the examination of Layer structure as well as for the comparison or identification of particles present in a paint film including pigments, extenders, additives, and contaminants. Extenders and other components of a paint film are generally of sufficient size to be identified by their morphology and optical properties using this technique. Particles to be examined are generally 1-20 μm – smaller than one thousandth of a millimeter. Each type of pigment reacts differently to these polarized light rays so that every single particle can be identified.
23. POLARIZED LIGHT MICROSCOPE
24. POLARIZED LIGHT MICROSCOPY [PLM] Pigments under Polarized LightMineral Debris in paint under Polarized Light
25. SCANNING ELECTRON MICROSCOPE
26. SCANNING ELECTRON MICROSCOPYScanning electron microscopy-energy dispersive X-ray analysis (SEM-EDS) can be used to characterize the morphology and elemental composition of paint samples. The SEM raster an electron beam over a selected area of a sample, producing emission of signals including X-rays, back scattered electrons, and secondary electrons. Emitted X-rays produce information regarding the presence of specific elements, and the electron signals produce compositional and topographical visualization of a sample. The depth from which X-rays are produced (the analytical volume) is dependent upon beam energy, composition and density of the sample, and energy of the X-rays.
27. SCANNING ELECTRON MICROSCOPYAfter a hit and run accident, often small car paint flakes can be found at the crime scene. Car paint consists out of different layers of paint. Comparisons are made between flakes from a suspect vehicle and the specimen in order to find a match. The picture shows a paint flake revealing various paint layers.
28. TRANSMISSION ELECTRON MICROSCOPE
29. TRANSMISSION ELECTRON MICROSCOPYWith an electron microscope, featuring electromagnetic lenses and special detectors, it is possible to examine very small particles.In a Transmission Electron Microscope, electrons are speeded up in a vacuum until their wavelength is extremely short. Beams of these fast-moving electrons are focused on an extremely thin paint sample which absorbs or scatter them, there by forming an image of particles in the sample.
30. TRANSMISSION ELECTRON MICROSCOPYA TEM can achieve the magnification factor of approximately one million making possibly to view an object as small as atom.This sample is approximately 12 microns. This reveals that the paint chiefly consists of evenly dispersed extremely fine white pigment particles in binding medium.
31. OPTICAL LIGHT MICROSCOPYTo study the uppermost layer of a painting, researchers use an optical microscope. There are various kinds of microscope available, ranging from small models giving 3x to 10x magnification to large instruments with magnification factors of up to 50x. The microscope lenses enable the condition and certain physical aspects of the painting to be assessed, revealing details of the brushwork, the kinds of pigments used, the texture of the paint layer, restorations and damage.
32. OPTICAL LIGHT MICROSCOPY
33. SPECTROSCOPYFollowing microscopic techniques are used to analyze the paint samples. 1. Vibrational Spectroscopy [IR Spectroscopy] 2. X-Ray Fluorescence Spectroscopy 3. Microspectrophotometry 4. Raman Spectroscopy
34. VIBRATIONAL SPECTROSCOPYInfrared spectroscopy (IR) may be used to obtain information about binders, pigments, and additives used in various types of coating materials. Because the paint fragments to be analyzed are often quite small, a beam condensing or focusing device is normally required, and a Fourier transform infrared (FTIR) spectrometer is recommended.Both transmittance and reflectance techniques may be used for the analysis of coatings, but in most cases, transmittance methods are preferred because all the sampling wavelengths are subjected to the same path lengths and most of the reference data of coatings, binders, pigments, and additives consist of transmittance spectra.
35. VIBRATIONAL SPECTROSCOPYIn addition, transmittance data are not significantly affected by collection parameters such as type of refractive element used, angle of incidence chosen for analysis, or the degree to which the sample makes contact with the refractive element. These factors affect spectra obtained using internal reflectance methods.If a multiple-layer coating system is to be subjected to an infrared examination, optimal results can be obtained if each layer is isolated and analyzed separately. Methods that use solvents to assist in the sample preparation should be used with caution because they might alter the sample or result in the production of residual solvent spectral absorptions.
36. VIBRATIONAL SPECTROSCOPYSpectrograph of 3 different layers.Layer-1 : Protective coating.Layer-2: Base coat and poly propylene polymerLayer-3: Binder layer.
37. X-Ray Fluorescence Spectrometer
38. X-Ray Fluorescence SpectroscopyXRF is an elemental analysis technique based upon the emission of characteristic X-rays following excitation of the sample by an X-ray source. XRF analysis is less spatially discriminating than SEM-EDS because of its larger analytical beam size and the greater penetration depth of X-rays compared to electrons. However, the limits of detection for most elements are generally better than for SEM-EDS, and the higher energy X-ray lines produced by higher energy excitation typical of XRF can be useful during qualitative analysis.
39. X-Ray Fluorescence SpectroscopyBecause of the significant penetration depth of the primary X-rays, XRF analysis will generally yield elemental data from several, if not all, layers of a typical multilayer paint fragment simultaneously. Because variations in layer thickness may cause variations in the X-ray ratios of elements present, this technique can be used only comparatively or qualitatively.
40. X-Ray Fluorescence SpectroscopyX-Ray Spectroscopy Sheds Light on Leonardo da Vinci’s Faces
41. MICROSPECTROPHOTOMETER
42. MICROSPECTROPHOTOMETRYMicrospectrophotometry may be required to provide objective color data for paint comparison because of the typically small size of samples. The technique can be applied to the outer surfaces of paint films by diffuse reflectance (DR) measurements with visible spectrum illumination. Diffuse reflectance measurements of paint surfaces are affected profoundly by surface conditions such as weathering, abrasion, contamination, and texture.
43. MICROSPECTROPHOTOMETRYThis fact can provide useful discriminating information when an examiner is faced with distinguishing different surfaces that were originally painted with the same paint formulation.Careful reference sampling is essential to the success of color comparisons of such surfaces. Diffuse reflectance techniques can also be used on the edges of thin paint layers much as it is on outer paint surfaces.
44. MICROSPECTROPHOTOMETRYBefore analysis, questioned and known samples can be mounted side by side on edge and polished to a smooth surface using a polish of 3-micron grit size or less. The requirement for consistent surface finish characteristics for all samples is achieved easily if the known and questioned samples are mounted and prepared in a single mount. When required for the discrimination of similarly colored paint layers, the surface finish of a polished sample must approach the size of the smallest pigment particles present.
45. RAMAN SPECTROMETER
46. RAMAN SPECTROSCOPYRaman spectroscopy is not yet used for routine paint examination, but it has many potential benefits, particularly the speed in which it can be applied and the fact that it is a non-invasive method that can be used to screen paints of all types.This focuses on discriminating single layer white architectural paint. The white paint samples varied in manufacturer and finish (gloss, matt, satin), but were all nominally “pure white”. The samples were prepared according to the manufacturers’ instructions, painted onto fresh aluminum surfaces (1 cm2).
47. RAMAN SPECTROSCOPYThe Raman Spectrograph of White paint samples .Three Raman Paint Spectrograph.
48. RAMAN SPECTROSCOPYThe major advantage of Raman approach is the high reproducibility in spectra which allows even a small difference in relative intensity [rather than simple absence or presence of features ] to be used in discriminating different samples.Raman spectroscopy is rapid technique that gives information regarding both binder and inorganic constituents.
49. PYROLYSIS GAS CHROMATOGRAPHY
50. CHROMATOGRAPHYThe most common chromatographic techniques employed for analysis of paints is Pyrolysis Gas Chromatography.Automobile paints are analyzed by this techniques.Automotive finishes are highly complex polymer systems made of multiple layers of copolymers, each formulated to perform a specific function.
51. CHROMATOGRAPHYThe inclusion of inorganic compounds such as oxides of metals and aluminum particles for opacity and visual effect make the finished product difficult to analyze using standard laboratory instruments.They may, however, be analyzed by gas chromatography mass spectrometry with the addition of a pyrolyzer at the GC-inlet.Pyrolysis reduces the polymeric content of the paint to volatiles compatible with gas chromatography and mass spectrometry, leaving the inorganic constituents behind.
52. CHROMATOGRAPHYThe paints used for automobiles usually include familiar polymer monomers such as methyl methacrylate, styrene, butyl acrylate and butyl methacrylate.The presence or absence of specific monomers helps distinguish one paint from others, as does the relative amounts of the monomers common to two paints. It also helps in determining approximate age of paints.
53. CHROMATOGRAPHYTotal Ion Chromatogram resulting from the pyrolysis of apaint sample at 750 °C for 15 seconds.Mass spectrum of peak number 7 from the pyrogram shown inFigure 1, identified as hydroxypropyl methacrylate.
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