Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

Materials identification > forensic analysis of paint and coatings > Flashcards

forensic analysis of paint and coatings Flashcards

1
Q

Paints as evidence

Automotive paint

A
  • 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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

Break and enter

A
  • 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.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Stolen items

A
  • Iron/metal railings and gates!
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What is paint?

A
  • 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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

Paint is composed of four rudimentary elements

A
  • Vehicle or binder
  • Pigments
  • Extenders and additives
  • Solvent
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

Vehicle

A
  • 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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

Characteristics of paint on a vehicle can be readily altered by variation of chemical composition

A
  • Gloss or lack thereof
  • Toughness
  • Durability
  • Flexibility
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

Various types of pain according to application

A
  • 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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

Acrylic resins

A
  • 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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

Formulation

A
  • 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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

Acrylic polymers

A 
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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

Acrylic film formation- coalescence formation

A
  • 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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

Alkyds/polyesters

Composed of three components

A
  • 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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

alkyds/polyesters

formulation

A
  • 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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

Alkyd/Polyester Polymers

A 
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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

Alkyd/Polyester Film formation

Oxidative Formation

A
  • 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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

Various other vehicle types are commonly used

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

Hiding Pigments

A
  • 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
19
Q

Semi-hiding/coloured pigments

A
  • Generally Pigments with a poorer hiding power than rutile

- Various types with numerous colours

20
Q

Colouring pigments

A 
-	Materials added to paint producing a coating of specified colour
Inorganic pigments	
-	Chromates
-	Oxides
-	Ferrocyanides
Organic Pigments
-	Azo pigments (N=N)
-	Dioxazines
-	Anthraquinones
-	Phtalocyanines
21
Q

Inorganic pigments

A 
-	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!
22
Q

Organic pigments

A 
-	Oil source
Positives
•	Bright
•	Vivid colours
•	Excellent colouring strength
Negatives
•	Transparent
•	Many not light fast
•	Expensive
23
Q

Odditites

A

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
24
Q

paint additives

A

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

25
Q

Analysis of paint

A
  • 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
26
Q

Initial Physical examination

A

• 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

27
Q

Microspectrophotometry

A
  • 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
28
Q

Analysis of microspectrophotmetry

A
  • 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
29
Q

Analysis of paint- infra red spectroscopy

A
  • 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
30
Q

Several techniques of infra red spectroscopy

A
  • 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
31
Q

May require sample preparation

A
  • 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
32
Q

Analysis of paint SEM

A
  • 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
33
Q

Analysis of paint SEM

techniques

A
  • 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
34
Q

Analysis of paint SEM- EDX

A

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

35
Q

EDX

A

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…
36
Q

SEM-EDX qualitative analysis

A
  • 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
37
Q

SEM-EDX quantitative analysis

A
  • 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
38
Q

More about EDX

A
  • 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
39
Q

Chromatography

A
  • 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
40
Q

Pyr-gc

A
  • Typical chromatogram

- Paint type should be obvious

41
Q

Pyr-gc/ms

A
  • Identity of products typically confirmed using MS
42
Q

Chromatography

A

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

43
Q

Issues in pyr-gc

A
  • 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
44
Q

paint analysis process

A

ppt

Materials identification (21 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions