Paints and Coatings

Question 1

What sources of paint might forensics run into?

Answer 1

Automotive, architectural, specialty

Question 2

What scenes would paint become relevant evidence?

Answer 2

Break and enter
Road incidents
Vandalism
Art fraud

Question 3

Forensic analysis of coatings

Answer 3

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

Question 4

What are the four main elements of paint?

Answer 4

Solvent - suspension; uniform application
Binder - holds paint together
Pigment - impart colour, opacity, or other
Additives - corrosion, UV, plasticizers, catalysts

Question 5

Paint - binders

Answer 5

Create cohesive solid paint - polymerization/pre-polymer species
Non-convertible: single molecule; extensive cross linking
Convertible: evaporation, entanglement of polymer chains

Question 6

What are the common polymer groups used in paint binders?

Answer 6

Acrylics
Alkyds
Polyesters
Urethanes
Epoxies
Cellulose
Silicones

Question 7

Automotive binders - paint layers

Answer 7

1. Electrocoat primer
2. Primer surfaces
3. Basecoat
4. Clearcoat

Question 8

Electrocoat primer

Answer 8

Corrosion resistance

Epoxy based

Question 9

Primer surface

Answer 9

Hide imperfections
Maybe pigments
Epoxy-modified polyester or polyurethane

Question 10

Basecoat

Answer 10

Colour and aesthetic
Acrylic based polymer
Al - metallic

Question 11

Clearcoat

Answer 11

Unpigmented resin
Gloss
Durability
Appearance

Question 12

Lacquers

Answer 12

Pre-polymer dissolved in solvent
Evaporation
Weak forces - London, polar -> Redissolve
Eg. nail polish, nitrocellulose

Question 13

Latex paints

Answer 13

Small micelles of polymer suspected in solvent
Evaporation -> micelles coalesce, further polymerization
Solubility depends on polymer and cross linking

Question 14

Pigments

Answer 14

Coloured, white, or black
Crystalline in paint
Inorganic, organic, organometallic
Electron transitions

Question 15

Inorganic pigments

Answer 15

Titanium dioxide - high RI
Aluminium and alkali earth metals
D-orbital splitting

Question 16

What do inorganic pigments usually contains

Answer 16

Iron oxides
Chromates
Ferrocyanides
Bismuth vanadate/molybdate
Chromium oxides
Cadmium sulphides

Question 17

Organometallic pigments

Answer 17

Metal-ligand bonding
D-orbital splitting
Inorganic-organic hybrids - “lake” - organic bound to inorganic alumina

Question 18

Polymorphism - titanium dioxide

Answer 18

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

Question 19

Organic pigments

Answer 19

Pi orbitals
Wide variety of colours
Flat planar systems, extensive conjugation
Better colour, more vivid, smaller particles, less durable, expensive

Question 20

Extender pigments

Answer 20

Do not have optical effects - similar RI to binder
Viscosity
Texture
Strength
Chemical resistance
Abrasion resistance

Ex: Talc, Kaolin, Calcite, Baryte

Question 21

Effect pigments

Answer 21

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

Question 22

Paint additives

Answer 22

UV protection - home, deck, car
UV stabilizers quench free radicals
Both can migrate

Question 23

Forensic analysis of paint samples

Answer 23

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

Question 24

ASTM E1610 STANDARD GUIDE FOR PAINT ANALYSIS AND COMPARISON

Answer 24

• 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

Question 25

Sample preparation of paint

Answer 25

Oblique cut

Cross-sections - layer thickness, colour, pigment distribution

Question 26

Stereomicroscopy of paint samples

Answer 26

Exclude based on layer structure
Colour and shape across sample
Surface
# of layers
Sequence
Homogeneity 
Texture
Thickness
Gloss

Question 27

PLM & fluorescence

Answer 27

Differentiate specimens based on mineral composition
Differentiate by light absorbance and degree of refraction
Sample that can transmit light
Effect pigments differentiated

Question 28

ASTM E2937 STANDARD GUIDE FOR USING IR SPECTROSCOPY IN FORENSIC PAINT EXAMINATIONS

Answer 28

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

Question 29

Limitations of IR for paint analysis

Answer 29

Melamine migrates
Weathering and other substances
Analyze the middle

Question 30

ASTM E2808 STANDARD GUIDE FOR USING MSP AND COLOUR MEASURMENT IN FORENSIC PAINT ANALYSIS

Answer 30

Crystal field effects of transition metals
Electron transitions conjugated compounds
Electron transitions in compounds with conductive energy
Wave optics - scattering, interference, diffraction

Question 31

ASTM E2809 SEM/EDS ANALYSIS OF PAINT

Answer 31

Define and compare layer structure

Bulk elemental composition - layers and particles

Question 32

Limitations of SEM/EDS for paint analysis

Answer 32

• no trace concentrations
• need conductive coating
• cant remove sample from embedding
• quantitative analysis not appropriate - heterogeneity