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Trace Evidence > Lecture 11&12 - Paints and Pigments > Flashcards

Lecture 11&12 - Paints and Pigments Flashcards

1
Q

what is the main purpose of dyes and pigments

A

to enhance the appeal of a product

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2
Q

what is the main property separating dyes and pigments

A

dyes = soluble (a solid that then dissolves)
pigments - insoluble - remain as particles

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3
Q

how are dyes and pigments different in their composition

A

dyes = organic based
pigments = equally organic and inorganic based

the inorganic can be useful in forensics

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4
Q

what is the difference in the use of dyes and pigments

A

dyes = more used in textiles
pigments = paints, ceramic, glass

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5
Q

how are dyes and pigments classified

A

using the Colour Index Scheme (CI) which is now called the colour index international

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6
Q

what is the Colour Index International

A

a list of all commercially available dyes and pigments globally

regularly updated

each colourant is given a generic name

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7
Q

what does the generic name of a dye or pigment in the CI represent

A

the application, hue and CI number

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8
Q

what is hue

A

essentially the colour

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9
Q

what are the three elements that make up the colour of something

A

hue = colour dependent on wavelength
saturation = purity of the colour (intensity)
value = lightness or darkness

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10
Q

what are other words for saturation of colour

what are other words for the value of colour

A

strength, chroma

brightness, luminance

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11
Q

what is the colour of a pigment determined by

A

the light absorption/reflection -
we see the colour that is reflected others are absorbed

also crystal lattice and particle size/shape

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12
Q

what is the opacity of a pigment related to

A

the refractive index - determined by the degree of light scattering

opacity = degree of cover of pigment

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13
Q

how is a gloss achieved and what is the opposite of a gloss

A

using small particles at low concentrations finely dispersed in the formulation

opposite = matte

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14
Q

what does a higher refractive index result in terms of opacity

A

higher RI = higher opacity = higher coverage

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15
Q

what is a common pigment used in paint

A

titanium dioxide

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16
Q

what two things do pigments provide to a formulation

A

colour and opacity (how much light can get through)

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17
Q

how do organic and inorganic pigments differ

A

inorganic = cheaper, resistant to heat, light, weathering, solvents and chemicals, metal based

organic = better colour intensity and brightness, lower RI so more transparent (less opaque)

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18
Q

where are organic pigments used

A

in printing inks due to translucent characteristic allowing colours to work in harmony

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19
Q

where are inorganic pigments used

A

in something you want to remain the colour it is e.g car paint or any other paint

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20
Q

what are the purpose of extenders and what pigments are used as these

A

to bulk things out

non hiding white pigments

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21
Q

particle size of pigments can impact what?

A

the opacity

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22
Q

what size particles are used in material aiming for max opacity and give some examples of these materials

A

pigment grade = 0.25 micrometre

toothpaste, makeup, paper

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23
Q

what size particles are used in material aiming for a lower opacity and give some examples of these materials

A

nano particles = less than 0.1 micrometre

moisturisers, suncream

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24
Q

name two of the most used coloured inorganic pigments

A

iron oxides (can be red, yellow or black)
chromium oxide

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25
Q

what are organic pigments also called

A

lakes

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26
Q

what is the pigment used in vehicle tyres

A

carbon black

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27
Q

what are the most common yellow, orange and red organic pigments

A

Azo compounds - they have N=N bonds

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28
Q

what is paint? and what are other types of this

A

a type of surface coating

others = varnishes, enamels, stains, lacquers

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29
Q

paints are - and - which is good news for forensic examiners

A

complex and variable

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30
Q

how are paints formulated

A

pigments and additives are suspended in a binder

can also include:
fillers
drying agents
texturisers
emulsifiers
plasticisers
fungicides, biocides and insecticides
UV stabilisers
corrosion inhibitors
plasticisers

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31
Q

what is the purpose of binders

A

make paint brittle and stick, made of polymers e.g acrylic

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32
Q

what is the purpose of fillers

A

increases the thickness of the coating, bulk it out e.g talc

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33
Q

what is the purpose of drying agents

A

make a hardened surface - speed up polymerisation or cause some liquid to evaporate

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34
Q

what is the purpose of texturisers

A

create a smooth surface or desired surface

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35
Q

what is the purpose of emulsifiers

A

prevent separation - allow water and oil to mix - increase shelf life of paint

36
Q

what is the purpose of fungicides

A

prevent mould growth

37
Q

what is the purpose of plasticisers

A

increase flexibility of the paint, so if the surface bends to paint wont move or crack

38
Q

where are the two most common sources of paint evidence seen

A

automotive crashes
burglaries or break ins

others
graffiti
art forgeries
nail varnish (trace evidence left at a scene)
road paint

39
Q

what is interesting about the pain used on boats

A

there are different kinds of paint needed for parts of the boat that are in and out of the water - higher copper and zinc content when in water to prevent salt corrosion

40
Q

why are glass beads added to road paint

A

to make the signs more reflective so drivers can see them

41
Q

what must paint evidence not be recovered using and why

therefore what should they be recovered using

A

J-Lar or acetate

because paint chips, flakes and fragments are very delicate, risk of disrupting layers when removing, tape can interfere with analysis e.g raman as it is a polymer

glass or plastic vials then in evidence bag

42
Q

should embedded flakes or paint transfers be removed at the scene

A

NO

submit whole items if you can

43
Q

should control samples be taken

A

yes if available - sample to substrate

e.g with cars take samples down to the bare metal so you can compare layers

44
Q

are paint samples refrigerated/frozen, why

A

No this can introduce moisture causing layers to come away

keep at an ambient temperature

45
Q

what is the general analytical workflow for paint analysis

A
  1. gross examination, recovery, collection
  2. preliminary evaluation of physical characteristics
  3. physical fit assessment
  4. microscopic techniques
  5. MSP - microspectrophotometry
  6. IR spectroscopy
  7. Raman spectroscopy
  8. SEM-EDX and XRF
  9. XRD
  10. GC-MS and other chemical tests
46
Q

what part of the analytical workflow has the most probative value

A

the physical fit assessment

47
Q

what is MSP used for

A

colour determination

48
Q

what is IR spectroscopy used for

A

analysis of organic binders/pigments

49
Q

what is raman spectroscopy used for

A

inorganic pigments or carbon black

50
Q

what is SEM-EDX or XRF used for

A

elemental analysis of the different layers

51
Q

what is XRD used for

A

crystal structure and polymorph (different crystalline forms of the same substance) identification

52
Q
A
53
Q

what is the database used for analysis of automotive paint

A

Paint Data Query - has around 15,000 models of car

54
Q

what can increase the distinctiveness when analysing paint sample from cars and walls

A

if they have been repainted without proper removal of the original

55
Q

in a hit and run, what type of trasnfer of paint would be more useful that just a single transfer

A

cross transfer between the two vehicles

56
Q

what are the layers in automotive paint generally from bottom to top

A

substrate material - iron or aluminium
chemical treatment
electro-deposition coating
primer surfacer
base coat
clear coat

57
Q

colour is a factor in the comparison of fibres, paints and pigments but what is an issue with this

A

everyone’s perception of colour is different = subjective

how we judge colour is different depending on the surrounding conditions during observation - hence why lighting and mounting media is controlled in comparisons

58
Q

what phenomenon describes the perception of colour based on context

A

simultaneous contrast

59
Q

where does human perception of colour begin

A

in retina cells known as cone cells

60
Q

what is MSP

A

microspectrophotometry - the combination of a microscope with a spectrometer

the instrument combines UV-Visible-NIR digital imaging with colourimetry

61
Q

how does the use of MSP overcome the subjectivity of human colour perception during forensic analysis

A

the colour is characterised based off a distribution of wavelengths

62
Q

what information does MSP capture

A

a magnified visual image and a spectroscopic pattern = more characteristic than one or the other

63
Q

what microscopic techniques can be useful in the analysis of paints and pigments

A

stereoscopic
reflected light
comparison
fluorescence
polarised
thermal
brightfield
multispectral (NIR and UV)

64
Q

when looking at automotive paint under a microscope what things are good to note (6)

A

colour
texture
layer sequence
relative layer thickness
pigment size and distribution
defect, weathering, dirt, solvent traps

65
Q

why are more layers of paint beneficial

A

more layers can be more discriminative due to more information and complexity

66
Q

how many layers do
1 - automotive paint
2 - architectural paint
tend to have

A

1 = 4
2 = 2

67
Q

what are the 4 ways to look at the layers of paints

A

cross section
thin peels - done using something sticky like Jlar
wedge cut
stair-step exposure

68
Q

when are solvent traps likely to be seen in paint layers

A

when a spray has been used

69
Q

name 5 methods of analysis that can give elemental composition information of materials

A

FTIR
Raman
SEM-EDX
XRF
XRD

70
Q

what are the limitations of using IR spectroscopy for paint analysis

A

pigments present below the LOD

71
Q

why is IR spectroscopy good for paint analysis

A

quick
minimally destructive
relatively little sample prep
can have high discriminatory power

72
Q

why is raman a good complimentary method with IR

A

good for inorganic pigments and looking at crystalline polymorphs such as Titanium dioxide

72
Q

name two types of IR used in paint analysis

A

transmitted
ATR (attenuated total reflection) = good for surface layers

73
Q

when is XRF a good method to use

A

when the sample may be too small or not have any discriminative features to be identified using microscopy, IR, raman or MSP

good for anything that fluoresces

74
Q

explain on a chemical level how XRF works

A

an atom has electrons in orbitals with specific energy levels

when electrons are excited by a external X-Ray, one from the inner K shell (closest to nucleus) is ejected from the atom creating a vacancy

an electron from a shell further out fills this vacancy so the energy of the particle is lowered

this leads to the emission energy in form of an x-ray that is different to the one that originally came in

75
Q

what makes something fluoresce

A

when energy is absorbed and re-emitted with differing energy

(normally a lower energy but can be a higher)

76
Q

how does XRF allow elemental identification

A

the x-rays emitted are measured and characteristic of each element as the energy levels are different for each element

77
Q

what is EDXRF/EDX

A

energy dispersive XRF

78
Q

what are the benefits and limitations of using energy dispersive XRF over wavelength dispersive

A

fast collection time
can be a handheld device and benchtop
non destructive
no sample prep

lower resolution
doesn’t give as much info (but too much info isn’t always needed)

78
Q

what other microscopic technique is XRF often combined with

why is this technique good

A

SEM to give SEM-EDX

a point can be selected on the SEM image and the XRF spectrum for that point can be found

good because combines elemental analysis with visual analysis giving higher discriminatory power

XRF has lower LOD but SEM-EDX allows for larger samples

78
Q

what is the result of XRF analysis

A

a spectrum with peaks corresponding to the elements found within the sample

79
Q

thin peels allow individual layers to be analysed but what can they sometimes result in

A

the loss of elemental data as tape is used

80
Q

what is the risk of using cross sections or stair-step layer in XRF

A

the risk of electron beam penetration below the layer of interest and this interfering with the data obtained

81
Q

what is XRD and what is it used for

A

X-ray diffraction
used to see the arrangement of atoms within a crystal structure and how they stack together

82
Q

what law is a model used to understand the conditions needed for diffraction

A

Bragg’s law

83
Q

what 4 things can XRD determine

A

lattice parameters - gives info on alloying or doping

phase composition of sample - compositional info

crystal structure - gives texture and orientation in the bulk

crystallite size - bulk structural info

84
Q

which equation allows the average size of nanocrystals to be calculated

A

Scherrer equation

Trace Evidence (16 decks)

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