forensic analysis of paints and coatings Flashcards
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
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.
reasons for paint
For aesthetic reasons – Pretty colours!
For protection of the coated material such as ironwork
To instil new properties to the coated material – Anti radar paint for example
For security purposes (Anti-climb)
A variety of different types of paint exist, but all are composed of four rudimentary elements
Vehicle or binder
Pigments
Extenders and Additives
Solvent
binder
The portion of the paint that forms the film over a surface
Binds all of the paint components into a single mass or film coating
Generally composed of resins or polymers of varying type and complexity
Characteristics can be readily altered by variation of chemical composition
Gloss or lack thereof – Often for aesthetics or practicality
Toughness – According to intended use
Durability – According to environment
Flexibility – According to nature of substrate
various types
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
acrylic 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
name 5 acrylic polymers
vinyl acetate
acrylic acid
methacrylic acid
styrene
2-hydroxyethyl acetate
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
alkyd resins/polyesters - composted 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
formulation of aklyds/polyesters
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
fatty acid - alkyd/polyesters
Often Linoleic Acid
Greater degree of unsaturation tends to promote more rapid curing
polyol - alkyd/polyester
Glycerol (3 OH)
Pentaerthritol (4 OH)
Sorbitol (6 OH)
dibasic acid - alkyd/polyester
Terepthalic acid
Pthalic Anhydride
oxidative phosphorylation
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
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
positives of inorganic pigments
- Opaque
- Light-fast
- Generally inexpensive
negatives of inorganic pigments
- Dull colouration
- Low colouring strength
- Insoluble in many cases
- Some safety issues – Lead or arsenic (i.e. Napoleonic Green) Paints!
see pp for
pigments and their colours
positives of organic pigments
- Bright
- Vivid colours
- Excellent colouring strength
negatives of organic pigments
- Transparent
- Many not light fast
- Expensive
name 2 oddities
VANTA Black
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
- similar technology to the standard material but can be sprayed
- Whilst not quite as black, it still only reflects 0.23% of incident light
what are additives used for
to extend or lower the cost of the coating
give two paint additives
- Calcium Carbonate – Reduces gloss and may lower fade
- Calcium Sulphate – Improve whiteness and improve blending
name 8 thing additives often 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
name 7 analytical techniques for pain analysis
- 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
- 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
what is 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 using microphotospectrometry
- 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
- Differences of >14.5 CIE are commonly seen on older weathered cars
infrared 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
infrared spec techniques available
- 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
sample prep in infrared spec
- 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
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
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
SEM-EDX 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
SEM-EDX visual examination
• Visual examination may be a useful discriminator
- Visual examination may aid in differentiation
- DP further increased by use of elemental mapping
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
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…
• 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
chromatography - pyrolysis
- Multiple pathways for polymer degradation
chromatography - random chain cleavage
- Olefin and vinyl polymers with polymethylene backbone
- Produce a series of random oligomer fragments
chromatography - 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
chromatography - chain depropagation
- The reverse of polymerisation!
- Thermally stable monomer units yielded
- Polymethyl and polybutyl methacrylates
chromatography - directed chain cleavage
- Cleavage occurs at weak points (Condensation polymers)
- Polyamides and polyesters
- Produce dibasic acids and diamines
chromatography - 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
see pp for
Pyr-GC and Pyr-GC/MS
