forensic analysis of paint and coatings Flashcards
Paints as evidence
Automotive paint
- Hit and run – ~28,000 in 2017, of which more than 100 result in a fatality
- Chips or smears of paint material may be transferred from the offending vehicle to the victim’s body or vehicle
- May be the only forensically relevant exhibit in a case
Break and enter
- Structural paints are often found on tools recovered from suspects of burglary – Typically contact between the working end and a painted surface
- Crowbars, screwdrivers etc.
Stolen items
- Iron/metal railings and gates!
What is paint?
- Substance used to coat another material
- For aesthetic reasons
- For protection of the material
- To instil new properties to the coated material
- For security purposes
Paint is composed of four rudimentary elements
- Vehicle or binder
- Pigments
- Extenders and additives
- Solvent
Vehicle
- The portion of the paint that forms the film over a surface
- Binds all the pain components into a single mass or film coating
- Composed of resins or polymers of varying type and complexity
Characteristics of paint on a vehicle can be readily altered by variation of chemical composition
- Gloss or lack thereof
- Toughness
- Durability
- Flexibility
Various types of pain according to application
- Acrylic – Commonly emulsions used for everyday internal application
- Alkyd/Polyester – Gloss paints/White goods/Occasionally vehicle paints
- Epoxy – Metal paints, can coatings and vehicle paints
- Urethane – Often used in vehicle paints
- Vinyl – Commonly emulsions
- Phenolic – Often car paints
- Amino resin – Melamine paints
- Cellulose – Older/Vintage vehicle paints
Acrylic resins
- Most acrylic paints consist of long polymer chains
- These comprise of monomer units which may act as a link in the chain (non-functional monomers) or allows chains to bind together (functional monomers)
- Functional monomers always contain functional groups where cross-linking can take place between chains to allow formation of an acrylic resin
Formulation
- Acrylic paints commonly consist of an emulsion of acrylic polymer, pigment and water, which acts as the solvent
- Acrylic paints are often therefore described as water-based emulsion paints
Acrylic polymers
Monomers Units - Vinyl Acetate - Vinyl Paint – Very Common Acrylic acid* - Levels control brittleness, flexibility Methacrylic acid* - Controls flexibility but can cause brittleness - Plexiglas Styrene - Improved chemical resistance 2-Hydroxyethyl acetate* - Improved durability
Acrylic film formation- coalescence formation
- Acrylic resins curing process
- As the paint is applied, it forms a film which when left to dry hardens
- This is due to the evaporation of water or absorption by the substrate
- As water is lost, the acrylic polymers are drawn into close contact as capillary forces act to bring them together
- The forces eventually pack the polymer spheres against each other forming a continuous cohesive film held together by the deformation and combination of the polymer chains
- This type of film formation is known as coalescence
Alkyds/polyesters
Composed of three components
- Fatty acids such as linoleic acid
- Polyol such as glycerol
- These form ester bonds
- A dibasic acid or anhydride is also
- added to the mix to allow cross-linking
alkyds/polyesters
formulation
- Molecular weight of resin varied by acid and polyol content
- Drying time varied by choice of oil/fatty acid
- Oils often composed of 2 or 3 fatty acids
- Dissolved in an appropriate solvent such as white spirit or xylene
Alkyd/Polyester Polymers
Monomers Units • Fatty Acid • Often Linoleic Acid • Greater degree of unsaturation tends to promote more rapid curing Polyol • Glycerol (3 OH) • Pentaerthritol (4 OH) • Sorbitol (6 OH) Dibasic Acid • Terepthalic acid • Pthalic Anhydride
Heated until correct viscosity is reached and dissolved
Alkyd/Polyester Film formation
Oxidative Formation
- Polyester/Alkyd resins curing process
- Curing takes place as a result of an oxidative process – Often catalysed
- The double bonds present in the fatty acid are attacked by atmospheric oxygen forming an hydroperoxide group
- This reacts with other fatty acids forming a bridge effectively crosslinking the individual molecules forming a film
Various other vehicle types are commonly used
Epoxy resins
- Often 2-Chloro-1,2-epoxypropane and
- bisphenol-A
- Sometimes cured using a hardener such
- as Triethylenetetramine which brings about polymerisation
Urethanes
- Another 2 component polymerisation mixture
Nitrocellulose
- Solvent evaporation curing
- Single component dissolved in solvent
- Evaporation of solvent leads to deposition of lacquer
Hiding Pigments
- Materials added to paint producing a coating to hide medium
- Hiding pigments have the ability to hide the surface of the painted substrate using the lowest possible film thickness
- Hiding pigments generally have an RI of 1.5 or above
- Most common is rutile or TiO2 with an RI of 2.72
Semi-hiding/coloured pigments
- Generally Pigments with a poorer hiding power than rutile
- Various types with numerous colours
Colouring pigments
- Materials added to paint producing a coating of specified colour Inorganic pigments - Chromates - Oxides - Ferrocyanides Organic Pigments - Azo pigments (N=N) - Dioxazines - Anthraquinones - Phtalocyanines
Inorganic pigments
- Mineral sources Positives • Opaque • Light-fast • Generally inexpensive Negatives • Dull colouration • Low colouring strength • Insoluble in many cases • Some safety issues – Lead or arsenic (i.e. Napoleonic Green) Paints!
Organic pigments
- Oil source Positives • Bright • Vivid colours • Excellent colouring strength Negatives • Transparent • Many not light fast • Expensive
Odditites
VANTA Black S-VIS
- VANTA black
- Is the blackest material on the planet reflecting only 0.036% of incident light
- It is composed a vertically aligned (carbon) nanotube array which act a bit like a forest trapping light between the trunks of the trees
- Its does unfortunately have to be applied using extremely complex techniques
- VANTA black S-VIS
- VANTA black S-VIS is somewhat different and uses
- similar technology to the standard material but can
- be sprayed
- Whilst not quite as black, it still only reflects 0.23% of
- incident light
paint additives
Additives
• Additives often used to extend or lower the cost of the coating
• Calcium Carbonate – Reduces gloss and may lower fade
• Calcium Sulphate – Improve whiteness and improve blending
Other additives – May commonly include
• Antifreeze – Glycols
• Dispersing aids
• Wetting agents
• Thickeners and anti-drip
• Biocides – Long term and wide range
• Low temperature drying aids
• Antifoam agent
• Thixotropic Agents – Gives a thicker consistency until stirring
Analysis of paint
- Physical Examination – Triage process (Asking sensible questions)
- Microscopic Examination – Paint layer structures
- Microspectrophotometry – Colour/Spectral Analysis
- Scanning electron microscopy – Structure and composition
- Elemental analysis – XRD, XRF, EDX – Elemental composition (Pigments)
- FTIR – Vehicle composition
- PyrGC-MS – Vehicle composition
Initial Physical examination
• Triage process is vital to avoid unnecessary analysis
• Initial analysis may include simple physical
examination and visual colour comparison
• Examination of paint layer structure using
stereomicroscopy – Low magnification
• Sample attributes such as number, order,
thickness and texture of any layers present
• Considered within case context
• Once triage is complete
• The sample can be taken for further examination or excluded
• Colour analysis would seem a logical next step
Microspectrophotometry
- A method used to determine spectral response of a sample
- A microscopic technique used to produce a definitive spectrum or colour definition for fragments of paint
- Can be used in reflectance or transmission modes
- Comparison of spectrum or CIE colour values can be used for elimination or inclusion
- Accurate result requires dirt and blemish free surfaces correctly mounted – 90⁰ plane
- Samples may require polishing to improve accuracy
- DP may be >0.97 even with ‘white’ paints
Analysis of microspectrophotmetry
- Direct visual comparison or overlay of spectra
- Conversion of results into CIELAB colour units which can be compared to paint databases or scene samples
- Average colour difference for same sample analysis is often >1.86 units
- Difference greater than 8 CIE units might be eliminated where other issues such as weathering are not a factor but spectral comparison a wiser approach
- N.b Differences of >14.5 CIE are commonly seen on older weathered cars
Analysis of paint- infra red spectroscopy
- Provides molecular information
- This can be used to determine the vehicle or resin type used in the paint or coating
- Specific functional groups are characteristic of specific vehicle type
- Presence or absence of characteristic peaks
- Can be used to define or eliminate vehicle type
- Coupled with CIE colour can allow definition of paint manufacturer
Several techniques of infra red spectroscopy
- FTIR-ATR – Standard technique
- FTIR (Micro KBr Disc) – Formation of a KBr micro disc
- FTIR (Thin Film) – Direct FTIR of thin film
- FTIR- Microscopy – Use of FTIR microscope
May require sample preparation
- ATR, thin film and KBr techniques will require separation of individual paint layers
- FTIR microscopy may allow mounting of sample such that layer separation is unnecessary
- Comparison of spectra may show small differences related to each technique
Analysis of paint SEM
- An excellent method for analysis of paint chips
- Provides visual and elemental analysis
- Individual paint layers must be exposed to prevent penetration of analysis into lower layers
Analysis of paint SEM
techniques
- Embedding and polish – Paint is embedded in resin and each layer is individually analysed
- Cross-section – Paint is presented as a cross-section
- Stair Step – Each individual layer is exposed as a stair cross section
- Thin Peels – Each layer is individually peeled from the whole
- These are then carbon coated
Analysis of paint SEM- EDX
Each paint preparatory technique has its disadvantages
- Stair step may provide false information if EDX over penetration occurs
- Thin Peels may give weak elemental data due to the thin cross section presented and may require lengthy analysis
Visual examination may be a useful discriminator
- Visual examination may aid in differentiation
- DP further increased by use of elemental mapping
EDX
EDX provides a powerful tool in elemental analysis
- This can be used comparatively or as a tool for determining extender and pigment types
- The presence of lack of certain elementals can provide strong evidence to support the use of specific pigment or extender and thus increase discriminatory power
- Location of element potentially of use…
SEM-EDX qualitative analysis
- Direct comparison of elemental composition can eliminate a sample if an element is demonstrated in only one of the samples
- Distribution mapping can be used qualitatively to eliminate some samples
SEM-EDX quantitative analysis
- Should only be used for flat, clean, smooth samples as topographical differences may cause problems
- Elements often characterised verbally rather than numerically:
- <1% weight – Trace
- 1-10% weight – Minor
- > 10% weight – Major
More about EDX
- Beware of elemental differences caused by topography or differences in homogeneity
- Contamination may be problematic and endemic of certain sample preparation techniques
- It may be possible to use spot analysis to view individual pigment particles and use this for comparison against other techniques
- Use of XRF may be appropriate in some cases
- Greater sensitivity
- Poor resolution and requires greater sample bulk
Chromatography
- A useful technique in the examination of paint fragments
- Pyrolysis GC is one of the most commonly used techniques
- Sample thermally degraded in an inert atmosphere at 750⁰+
- Pyrolysis causes the material to fragment producing pyrolysis products characteristic of the composition of the original material
- The mixture of products is then fed onto a GC column and separated into its constituents
- Detection is by FID, MS or FTIR
Pyr-gc
- Typical chromatogram
- Paint type should be obvious
Pyr-gc/ms
- Identity of products typically confirmed using MS
Chromatography
Pyrolysis
- Multiple pathways for polymer degradation
Random chain cleavage
- Olefin and vinyl polymers with polymethylene backbone
- Produce a series of random oligomer fragments
Side-Chain scission
- Side chain groups may be expelled
- Backbone may fragment and produce cyclic or aromatic compounds
- Common for PVC and polyvinylacetates
- HCl, MeCOOH and aromatics may be produced
Chain depropagation
- The reverse of polymerisation!
- Thermally stable monomer units yielded
- Polymethyl and polybutyl methacrylates
Directed chain cleavage
- Cleavage occurs at weak points (Condensation polymers)
- Polyamides and polyesters
- Produce dibasic acids and diamines
The products produced are often quite characteristic
Issues in pyr-gc
- Class type often easily ascertained
- A destructive technique
- However low sample requirements offset this issue
- Sensitive – Low microgram level
- Mass spec further increases sensitivity and specificity
paint analysis process
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