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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
what are organic pigments also called
lakes
26
what is the pigment used in vehicle tyres
carbon black
27
what are the most common yellow, orange and red organic pigments
Azo compounds - they have N=N bonds
28
what is paint? and what are other types of this
a type of surface coating others = varnishes, enamels, stains, lacquers
29
paints are - and - which is good news for forensic examiners
complex and variable
30
how are paints formulated
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
31
what is the purpose of binders
make paint brittle and stick, made of polymers e.g acrylic
32
what is the purpose of fillers
increases the thickness of the coating, bulk it out e.g talc
33
what is the purpose of drying agents
make a hardened surface - speed up polymerisation or cause some liquid to evaporate
34
what is the purpose of texturisers
create a smooth surface or desired surface
35
what is the purpose of emulsifiers
prevent separation - allow water and oil to mix - increase shelf life of paint
36
what is the purpose of fungicides
prevent mould growth
37
what is the purpose of plasticisers
increase flexibility of the paint, so if the surface bends to paint wont move or crack
38
where are the two most common sources of paint evidence seen
automotive crashes burglaries or break ins others graffiti art forgeries nail varnish (trace evidence left at a scene) road paint
39
what is interesting about the pain used on boats
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
why are glass beads added to road paint
to make the signs more reflective so drivers can see them
41
what must paint evidence not be recovered using and why therefore what should they be recovered using
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
should embedded flakes or paint transfers be removed at the scene
NO submit whole items if you can
43
should control samples be taken
yes if available - sample to substrate e.g with cars take samples down to the bare metal so you can compare layers
44
are paint samples refrigerated/frozen, why
No this can introduce moisture causing layers to come away keep at an ambient temperature
45
what is the general analytical workflow for paint analysis
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
what part of the analytical workflow has the most probative value
the physical fit assessment
47
what is MSP used for
colour determination
48
what is IR spectroscopy used for
analysis of organic binders/pigments
49
what is raman spectroscopy used for
inorganic pigments or carbon black
50
what is SEM-EDX or XRF used for
elemental analysis of the different layers
51
what is XRD used for
crystal structure and polymorph (different crystalline forms of the same substance) identification
52
53
what is the database used for analysis of automotive paint
Paint Data Query - has around 15,000 models of car
54
what can increase the distinctiveness when analysing paint sample from cars and walls
if they have been repainted without proper removal of the original
55
in a hit and run, what type of trasnfer of paint would be more useful that just a single transfer
cross transfer between the two vehicles
56
what are the layers in automotive paint generally from bottom to top
substrate material - iron or aluminium chemical treatment electro-deposition coating primer surfacer base coat clear coat
57
colour is a factor in the comparison of fibres, paints and pigments but what is an issue with this
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
what phenomenon describes the perception of colour based on context
simultaneous contrast
59
where does human perception of colour begin
in retina cells known as cone cells
60
what is MSP
microspectrophotometry - the combination of a microscope with a spectrometer the instrument combines UV-Visible-NIR digital imaging with colourimetry
61
how does the use of MSP overcome the subjectivity of human colour perception during forensic analysis
the colour is characterised based off a distribution of wavelengths
62
what information does MSP capture
a magnified visual image and a spectroscopic pattern = more characteristic than one or the other
63
what microscopic techniques can be useful in the analysis of paints and pigments
stereoscopic reflected light comparison fluorescence polarised thermal brightfield multispectral (NIR and UV)
64
when looking at automotive paint under a microscope what things are good to note (6)
colour texture layer sequence relative layer thickness pigment size and distribution defect, weathering, dirt, solvent traps
65
why are more layers of paint beneficial
more layers can be more discriminative due to more information and complexity
66
how many layers do 1 - automotive paint 2 - architectural paint tend to have
1 = 4 2 = 2
67
what are the 4 ways to look at the layers of paints
cross section thin peels - done using something sticky like Jlar wedge cut stair-step exposure
68
when are solvent traps likely to be seen in paint layers
when a spray has been used
69
name 5 methods of analysis that can give elemental composition information of materials
FTIR Raman SEM-EDX XRF XRD
70
what are the limitations of using IR spectroscopy for paint analysis
pigments present below the LOD
71
why is IR spectroscopy good for paint analysis
quick minimally destructive relatively little sample prep can have high discriminatory power
72
why is raman a good complimentary method with IR
good for inorganic pigments and looking at crystalline polymorphs such as Titanium dioxide
72
name two types of IR used in paint analysis
transmitted ATR (attenuated total reflection) = good for surface layers
73
when is XRF a good method to use
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
explain on a chemical level how XRF works
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
what makes something fluoresce
when energy is absorbed and re-emitted with differing energy (normally a lower energy but can be a higher)
76
how does XRF allow elemental identification
the x-rays emitted are measured and characteristic of each element as the energy levels are different for each element
77
what is EDXRF/EDX
energy dispersive XRF
78
what are the benefits and limitations of using energy dispersive XRF over wavelength dispersive
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
what other microscopic technique is XRF often combined with why is this technique good
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
what is the result of XRF analysis
a spectrum with peaks corresponding to the elements found within the sample
79
thin peels allow individual layers to be analysed but what can they sometimes result in
the loss of elemental data as tape is used
80
what is the risk of using cross sections or stair-step layer in XRF
the risk of electron beam penetration below the layer of interest and this interfering with the data obtained
81
what is XRD and what is it used for
X-ray diffraction used to see the arrangement of atoms within a crystal structure and how they stack together
82
what law is a model used to understand the conditions needed for diffraction
Bragg's law
83
what 4 things can XRD determine
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
which equation allows the average size of nanocrystals to be calculated
Scherrer equation

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