Intro to Forensic science-lecture notes- week 16- paint evidence PDF

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FS-week 16- lecture notes- paint evidence Definitions    

Chemistry of paint is very complex Forensic role is to compare paint evidence from scene to known source Individual evidence only in cases where large enough paint samples exists to physically match. Paint o A suspension of pigment in a film former also called vehicle). o Usually not a solution. Pigment is suspended in film former. o Pigment is usually a mixture of inorganic metal salts. o Purpose is to impart colour. o Film former is usually organic polymers o Purposes are to protect surface and hold pigments

How does it help?  

Providing investigative leads for a case. E.g identification of a vehicle type in a hit and run through automotive paint and glass analysis.

Where is it found? 

Paint and paint products o On various surfaces that may have transferred from one object to another, such as in a car crash. o Clothing of pedestrians struck by a vehicle are routinely examined for this type of paint fragment transfer.

What is it? 

Trace evidence o Physical evidence that results from the transfer of small quantities of materials in the same category as;  Hair  Textile fibres  Glass fragment  Gunshot residue pigments

Paint as evidence 

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Paint chips o Pieces of paint which have come off surfaces. o They usually contain all the layers of the paint Paint smears o The top layer of paint which has loosened owing to being wet or air oxidation. o Smears onto another surface after brushing contact.

o

Layer structure not present

Paint-components        

Binders Natural or synthetic resins to which pigments are added. Extenders Add bulk to increase the covering capacity of paint Modifiers Change the gloss, flexibility, hardness and duration. E.G lead oxide toughens the paint and makes it resistant to weather Pigments Organic or inorganic

Paint evidence 

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How o From damaged doors, window frames o Between motor vehicles, from vehicle to person o From a weapon o Wet applied o From a spray can Comparison o Colour o Layer structure o Chemical composition pigments/resin Evidential value o Limited to conclusive; often good o Identify usage of paint

How does paint dry  

When paint dries, a thin polymer film is formed that adheres to the surface and suspends the pigments. Paint dries by a number of different mechanisms; o Solvent evaporation  Rust proof paints o Heating (thermosetting)  Automotive o Oxidation  Drying oils  Artistic paints  Linseed oil

Other coatings 

Varnish o A film former, commonly polyurethane, dissolved in a solvent, which normally doesn’t contain a stain o Purpose is to protect wood

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Stain o A mixture of organic dyes dissolved in solvent made to penetrate mood and stain it Enamel Originally a glossy, thermosetting paint Now nay paint which dries glossy

Paint layering

Paint-physical fits

Recovery

How is paint analysed? 

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By several o Micro-chemical tests o Instrumental techniques Examples o GC-MS o IR o X-ray diffraction o Fluorescence o Spectrophotofluorometry o SEM 

Colour comparison is central to forensic pain analyses

Paint analysis

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Electron microscopy o Inorganic pigments o Excellent for single, top layer analysis and paint smears UV-visible spectrophotometry o Must be able to separate pigment from film former and dissolve it. Solubility o Use of solvents  Acetone  Dicholomethane  Pyridine  Acrylic lacquares are solutble in acetone o Pyrolysis GC  Analysis of film formers

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Bulk technique All layers analysed together

Class and individualisation   

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Noting the number of paint colours and shades that are known to exist Colour, more than any other property, imparts paint with its most distinctive forensic characteristics Questioned and known specimens are best compared side by side under stereoscopic microscope for o Colour o Surface texture o Colour layer sequence

The same components of class characteristic statements and individual linkage statements are the central features of this important forensic science discipline.

Pyrolyzing unit 

Burning paint and analysing it

Analysis of paint 

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FTIR o Will determine type of film former o Does not show pigments unless they are organic o Is a bulk method; if more than one layer is present, IR will be a composite. Colour layer analysis o Requires cross section of paint chip o Cannot be done on paint smears o Same number and order of layers o Relative thickness must be same o Is a class test

Case interpretation   

Hit and run RTC Paint chips left at crime scene on victim Chips had 11 distinct layers of various colours

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Samples taken from suspect auto showed 11 layers with matching colour-layer sequencing Paint examiner testified that paint found at scene came from the suspect car (Individualised chip to car) Defence lawyer asked examiner the following hypothetical questions “what would your conclusion have been if there has been only 10 matching layers?” Where is the defence going with this line of questioning? Why does this question present a problem for the examiner? One possible answer: I cannot give a definitive answer. I take each case on an individual basis and reach a conclusion based on the evidence in that case. It would not be possible to generalise about an optimal number of layers of paint that are present....