Paints and Coatings Flashcards
What sources of paint might forensics run into?
Automotive, architectural, specialty
What scenes would paint become relevant evidence?
Break and enter
Road incidents
Vandalism
Art fraud
Forensic analysis of coatings
Is the material paint?
How is it relevant to the case?
Comparison of two or more samples for common source
Identification of vehicle make/model
Identification or art forgeries based on paint type/colour
What are the four main elements of paint?
Solvent - suspension; uniform application
Binder - holds paint together
Pigment - impart colour, opacity, or other
Additives - corrosion, UV, plasticizers, catalysts
Paint - binders
Create cohesive solid paint - polymerization/pre-polymer species
Non-convertible: single molecule; extensive cross linking
Convertible: evaporation, entanglement of polymer chains
What are the common polymer groups used in paint binders?
Acrylics Alkyds Polyesters Urethanes Epoxies Cellulose Silicones
Automotive binders - paint layers
- Electrocoat primer
- Primer surfaces
- Basecoat
- Clearcoat
Electrocoat primer
Corrosion resistance
Epoxy based
Primer surface
Hide imperfections
Maybe pigments
Epoxy-modified polyester or polyurethane
Basecoat
Colour and aesthetic
Acrylic based polymer
Al - metallic
Clearcoat
Unpigmented resin
Gloss
Durability
Appearance
Lacquers
Pre-polymer dissolved in solvent
Evaporation
Weak forces - London, polar -> Redissolve
Eg. nail polish, nitrocellulose
Latex paints
Small micelles of polymer suspected in solvent
Evaporation -> micelles coalesce, further polymerization
Solubility depends on polymer and cross linking
Pigments
Coloured, white, or black
Crystalline in paint
Inorganic, organic, organometallic
Electron transitions
Inorganic pigments
Titanium dioxide - high RI
Aluminium and alkali earth metals
D-orbital splitting
What do inorganic pigments usually contains
Iron oxides Chromates Ferrocyanides Bismuth vanadate/molybdate Chromium oxides Cadmium sulphides
Organometallic pigments
Metal-ligand bonding
D-orbital splitting
Inorganic-organic hybrids - “lake” - organic bound to inorganic alumina
Polymorphism - titanium dioxide
More than one crystal structure Polymorphs may have different colours Stability Titanium dioxide: Anatase, rutile, Brookite Brookite not in paint Rutile more common - larger RI
Organic pigments
Pi orbitals
Wide variety of colours
Flat planar systems, extensive conjugation
Better colour, more vivid, smaller particles, less durable, expensive
Extender pigments
Do not have optical effects - similar RI to binder Viscosity Texture Strength Chemical resistance Abrasion resistance
Ex: Talc, Kaolin, Calcite, Baryte
Effect pigments
Sparkling effects - aluminum flakes
Silver: saturated fatty lubricants to force the flakes to the paint surface
Pearlescence: constructive/destructive interference; layers of titanium dioxide and mica; layer thickness
Paint additives
UV protection - home, deck, car
UV stabilizers quench free radicals
Both can migrate
Forensic analysis of paint samples
Complexity of paint Organic and inorganic components Rarely present as single layer Can’t predict layer sequence Heterogeneity Degradation Migration, continual cross linking, additional substances applied
ASTM E1610 STANDARD GUIDE FOR PAINT ANALYSIS AND COMPARISON
- consistent approach, analytical science
- sample size, condition, layer structure complexity
- More specific and least destructive tests first
Description -> additional samples (if needed) -> physical characteristics -> yes/no -> physical match? -> yes/no -> evaluate analytical approach -> sample prep -> binder/pigment/additive analysis
Sample preparation of paint
Oblique cut
Cross-sections - layer thickness, colour, pigment distribution
Stereomicroscopy of paint samples
Exclude based on layer structure Colour and shape across sample Surface # of layers Sequence Homogeneity Texture Thickness Gloss
PLM & fluorescence
Differentiate specimens based on mineral composition
Differentiate by light absorbance and degree of refraction
Sample that can transmit light
Effect pigments differentiated
ASTM E2937 STANDARD GUIDE FOR USING IR SPECTROSCOPY IN FORENSIC PAINT EXAMINATIONS
Classify binders and pigments
Probable paint types: auto, architectural, maintenance
Many common paints - based on esters and urethanes
CARBONYL BAND 1650 - 1750
C-O BOND STRETCH 1000-1300
MELAMINE MODIFICATION 1550, 815
Limitations of IR for paint analysis
Melamine migrates
Weathering and other substances
Analyze the middle
ASTM E2808 STANDARD GUIDE FOR USING MSP AND COLOUR MEASURMENT IN FORENSIC PAINT ANALYSIS
Crystal field effects of transition metals
Electron transitions conjugated compounds
Electron transitions in compounds with conductive energy
Wave optics - scattering, interference, diffraction
ASTM E2809 SEM/EDS ANALYSIS OF PAINT
Define and compare layer structure
Bulk elemental composition - layers and particles
Limitations of SEM/EDS for paint analysis
- no trace concentrations
- need conductive coating
- cant remove sample from embedding
- quantitative analysis not appropriate - heterogeneity
