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Materials Identification > forensic analysis of paints and coatings > Flashcards

forensic analysis of paints and coatings Flashcards

1
Q

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

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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.

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3
Q

reasons for paint

A

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)

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4
Q

A variety of different types of paint exist, but all are composed of four rudimentary elements

A

Vehicle or binder

Pigments

Extenders and Additives

Solvent

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5
Q

binder

A

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

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6
Q

Characteristics can be readily altered by variation of chemical composition

A

Gloss or lack thereof – Often for aesthetics or practicality

Toughness – According to intended use

Durability – According to environment

Flexibility – According to nature of substrate

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7
Q

various types

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

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8
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

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9
Q

acrylic 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

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10
Q

name 5 acrylic polymers

A

vinyl acetate

acrylic acid

methacrylic acid

styrene

2-hydroxyethyl acetate

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11
Q

vinyl acetate

A

vinyl paint

very common

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12
Q

acrylic acid

A

levels control brittleness

flexibility

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13
Q

methacrylic acid

A

controls flexibility but can cause brittleness - plexiglas

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14
Q

styrene

A

improved chemical resistance

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15
Q

2-hydroxyethyl acetate

A

improved durability

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16
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

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17
Q

alkyd resins/polyesters - composted 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

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18
Q

formulation of aklyds/polyesters

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

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19
Q

fatty acid - alkyd/polyesters

A

Often Linoleic Acid

Greater degree of unsaturation tends to promote more rapid curing

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20
Q

polyol - alkyd/polyester

A

Glycerol (3 OH)

Pentaerthritol (4 OH)

Sorbitol (6 OH)

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21
Q

dibasic acid - alkyd/polyester

A

Terepthalic acid

Pthalic Anhydride

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22
Q

oxidative phosphorylation

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

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23
Q

epoxy resins

A

Often 2-Chloro-1,2-epoxypropane and bisphenol-A

Sometimes cured using a hardener such as Triethylenetetramine which brings about polymerisation

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24
Q

urethanes

A

Another 2 component polymerisation mixture

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25
Q

nitrocellulose

A

Solvent evaporation curing

Single component dissolved in solvent

Evaporation of solvent leads to deposition of lacquer

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26
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

27
Q

semi-hiding/coloured pigments

A

Generally, Pigments with a poorer hiding power than rutile

Various types with numerous colours

28
Q

colouring pigments

A

• Materials added to paint producing a coating of specified colour

29
Q

inorganic pigments

A

Chromates

Oxides

Ferrocyanides

30
Q

organic pigments

A

Azo pigments (N=N)

Dioxazines

Anthraquinones

Phtalocyanines

31
Q

positives of inorganic pigments

A
  • Opaque
  • Light-fast
  • Generally inexpensive
32
Q

negatives of inorganic pigments

A
  • Dull colouration
  • Low colouring strength
  • Insoluble in many cases
  • Some safety issues – Lead or arsenic (i.e. Napoleonic Green) Paints!
33
Q

see pp for

A

pigments and their colours

34
Q

positives of organic pigments

A
  • Bright
  • Vivid colours
  • Excellent colouring strength
35
Q

negatives of organic pigments

A
  • Transparent
  • Many not light fast
  • Expensive
36
Q

name 2 oddities

A

VANTA Black

VANTA black S-VIS

37
Q

VANTA Black

A
  • 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
38
Q

VANTA Black S-VIS

A
  • similar technology to the standard material but can be sprayed
  • Whilst not quite as black, it still only reflects 0.23% of incident light
39
Q

what are additives used for

A

to extend or lower the cost of the coating

40
Q

give two paint additives

A
  • Calcium Carbonate – Reduces gloss and may lower fade

- Calcium Sulphate – Improve whiteness and improve blending

41
Q

name 8 thing additives often include

A
  • 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
42
Q

name 7 analytical techniques for pain analysis

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
43
Q

initial physical examination

A
  • 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
44
Q

once triage is complete

A
  • The sample can be taken for further examination or excluded
  • Colour analysis would seem a logical next step
45
Q

what is 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
46
Q

analysis using microphotospectrometry

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
  • Differences of >14.5 CIE are commonly seen on older weathered cars
47
Q

infrared 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

48
Q

infrared spec techniques available

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
49
Q

sample prep in infrared spec

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
50
Q

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
51
Q

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
52
Q

SEM-EDX disadvantages

A
  • 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
53
Q

SEM-EDX visual examination

A

• Visual examination may be a useful discriminator

  • Visual examination may aid in differentiation
  • DP further increased by use of elemental mapping
54
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
55
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
56
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…
• 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

57
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
58
Q

chromatography - pyrolysis

A
  • Multiple pathways for polymer degradation
59
Q

chromatography - random chain cleavage

A
  • Olefin and vinyl polymers with polymethylene backbone

- Produce a series of random oligomer fragments

60
Q

chromatography - side chain scission

A
  • 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
61
Q

chromatography - chain depropagation

A
  • The reverse of polymerisation!
  • Thermally stable monomer units yielded
  • Polymethyl and polybutyl methacrylates
62
Q

chromatography - directed chain cleavage

A
  • Cleavage occurs at weak points (Condensation polymers)
  • Polyamides and polyesters
  • Produce dibasic acids and diamines
63
Q

chromatography - 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
64
Q

see pp for

A

Pyr-GC and Pyr-GC/MS

Materials Identification (17 decks)

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