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FSCI6030 - Trace Evidence > Paint and Pigments > Flashcards

Paint and Pigments Flashcards

1
Q

What is the collective term for dyes and pigments?

what are their main intention?

A
  • colourants
  • mainly designed to enhance a product’s appeal
  • aesthetic purposes - visual appeal
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2
Q

What is the fundamental differences between dyes and pigments? (solubility, organic/inorganic components and uses)

A
  • dyes:
  • soluble (start of as solid but they dissolve into solvent)
  • primarily organic based
  • greater use in textiles
  • pigments:
  • insoluble particles dispersed in a matrix (remain as particles dispersed/suspended in matrix)
  • have equal representation of organic and inorganic components
  • used in paints, inks, plastics, cement, ceramics, glass
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3
Q

How are dyes and pigments classified?

Describe this?

How does it work to classify a colourant?

What extra information does it provide?

A
  • using the colour index international (CI)
  • it is a comprehensive list of all commercially available dyes and pigments globally
  • it is web based and regularly updated
  • people subscribe to it and pay a fee to access it
  • each colourant is assigned a generic name, which incorporates its application, its hue, and its CI number
  • it also gives:
  • application methods and properties (how to use the pigment/dye)
  • companies that manufacture (tie analysis from lab back to origin)
  • any trade names
  • chemical composition
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4
Q

What are the three indicators of colour?

What are two other terms that can also be used?

A
  • hue
  • this is colour
  • dependent on wavelength
  • saturation (intensity, strength, chroma)
  • purity of colour: how pure/vibrant it is or how close to true colour it is or not
  • brightness (luminance, value)
  • how light or dark it is
  • tint = addition of white
  • shade = addition of black
  • these affect the brightness but not hue
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5
Q

How is the colour of a pigment determined?

What else is colour determined by?

A
  • determined by light absorption/reflection
  • light we see is what is reflected, other bits are absorbed
  • colour is also determined by crystal lattice (how atoms packed within crystal structure) and particle size/shape
  • can tweak these synthetically
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6
Q

What is opacity of pigment?

How is opacity of pigment determined?

What is it directly related to?

A
  • opacity is the degree of cover (how much light is allowed through)
  • by degree of light scattering
  • it is directly related to the refractive index
  • higher RI = higher opacity = higher coverage
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7
Q

What two things do pigments provide?

What are the two pigment options and describe the properties of each?

What type of pigment is used in printer inks?

A
  • colour and opacity
  • cheapest option: inorganic pigments (mineral derived or metal based)
  • resistant to heat, light, weathering, solvents and chemicals
  • good form of pigment to go in things you want to stay that colour
  • organic pigments (lakes)
  • offer better colour intensity and brightness but have a lower RI = lower opacity = transparent
  • organic pigments used in printer inks rather than inorganic
  • if they were inorganic pigments would splat on page and wouldn’t be able to see colours – all be totally opaque
  • need translucency in printer ink so pigments can work together in harmony to produce different colours we see on a print out
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8
Q

How can gloss paint be achieved?

How about matte?

A
  • gloss can be achieved by using smaller particles at low concentrations finely dispersed in the formulation
  • matte - do the opposite
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9
Q

The size of what determines properties?

Use titanium dioxide as an example to explain this

A
  • particle size determines properties
  • TiO2 is a hiding white pigment
  • TiO2 pigment grade for maximum opacity = 0.25 um
  • used in toothpaste, paper, paint, makeup
  • nano TiO2 particles < 0.1 um
  • used for sunscreens
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10
Q

What is the two types of pigments in terms of hiding white pigments?

What are the most important coloured inorganic pigments?

What is carbon black used in?

What are most important yellow, orange and red pigments composed of?

What is an important metallic inorganic pigment?

What are other types of pigments?

A
  • hiding white pigment - TiO2
  • non-hiding white pigment used as extenders (bulk stuff out) - calcium carbonate, talc, china clay and silica
  • iron oxides and chromium oxides (pigment green 17)
  • 2 types used in vehicle tyres
  • azo compounds (N=N) e.g. pigment yellow 1
  • aluminium flake (pigment metal 1)
  • many speciality pigments - pearlescent and optically-variable pigments
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11
Q

What is paint the most common thing of?

What is paint?

Why is it good in forensics?

A
  • most common surface coating (other include varnishes, enamels, stains, lacquers
  • paint is a suspension of pigment and additives in a binder
  • there is a huge amount of complexity and variability (good chance at good discrimination between samples)
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12
Q

What are the nine main components of paint?

A

1 - binders
- polymer
- film former (what gives you that film that lays on top that makes it a surface coating) e.g. acrylic
- this is what makes paint paint - what makes it go hard once dried

2 - pigments
- by composition these are the most common ingredient
- what gives it it’s colour and opacity
- e.g. hiding white pigments designed to cover

3 - fillers
- increase thickness of coating e.g. talc

4 - drying agents
- speed up polymerisation (not just evaporating off liquid content)

5 - texturisers
- gives smooth surface e.g. solvents

6 - emulsifiers
- compounds that cause mixing of oil and water
- prevents the separation to increase shelf life
- if didn’t have it, when get paint tin, the two layers won’t mix

7 - fungicides, biocides, insecticides
- stop mould, mushrooms, bugs and bacteria

8 - UV stabilisers (stops UV degradation) and corrosion inhibitors (stops corrosion)

9 - plasticisers
- increase flexibility
- if car panel bends in wind or open car door onto another car door and flex it, paint would move and wouldn’t split and crack

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13
Q

What eight places are paints found in forensic contexts?

A

1 - automotive and clear coats (hit-and-run)
2 - architectural (applied to building) - burglaries and forced entries
3 - other vehicular coatings e.g. bike, marine, aircraft
4 - tool coatings (burglaries)
5 - spray paints (graffiti)
6 - road paints (RTC, road markings have distinctive glass beads added to paint to increase reflection so that it will be easier to see lines
7 - works of art (forgeries)
8 - nail varnish (although nitrocellulose based (don’t tend to be acrylic based))

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14
Q

What are recovery considerations of paint? (what not to lift them with and why, what is best option, embedded flakes, paint transfers, control samples, 2 caveats of control samples, temperature control)

A
  • paint chips, flakes, fragments are very delicate so need to be careful
  • must not be lifted with J-Lar or acetate:
  • hard to get off after and risks splitting layers apart or breaking up further once get back to lab
  • if going to analyse in situ, adhesive tapes are made of polymers - will interfere with analysis
  • glass/plastic vial is the best option then bag
  • if use druggist fold then would have to put in box instead of evidence bag as paper can bend
  • embedded flakes must not be removed at scene
  • if possible whole item should be recovered to lab so controlled removal can take place
  • where got paint contact transfer and smears don’t try and lift it submit whole item
  • always take control samples if available (sample down to substrate)
  • in case of hit and run if don’t have vehicle that hit car can’t control sample
  • 2 caveats:
    1 - have to sample right down to metal substrate but paint layers go on quite a way right down to bare metal but even the bare metal itself is coated before primer is put on top so important to recover all layers

2 - important to sample as close to area of damage as is reasonably possible
- if car has been resprayed on any one panel, if don’t take paint control from the area of impact but from another panel that has had a respray - it will not be like for like comparison
- tricky because don’t want to take control from actual area of impact - don’t want - don’t want to play around with impact site and possible interfering with evidence
- as close to impact site as possible but not on it

  • do not refrigerate or freeze paint samples
  • this introduces water and water can peel apart layers of paint
  • must keep at ambient temperature
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15
Q

What is ten step analytical workflow for the analysis of paint?

A

1 - gross examination, recovery and collection
2 - preliminary evaluation of physical characteristics
3 - physical fit assessment (most probative value)
4 - all microscopic techniques
5 - microspectrophotometry (colour determination)
6 - infrared spectroscopy (organic binders/pigments)
7 - raman spectroscopy (inorganic pigments)
SPECIFIC TO INORGANICS:
8 - SEM-EDX, XRF (elemental composition of layers)
9 - XRD (crystal structure - really useful if got polymorphs e.g. titanium dioxide
10 - Pyrolysis-GC-MS and microchemical tests (destructive so do these as a last resort when done everything else - burn, disintegrate, dissolve sample and will be gone)

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16
Q

What can be interpreted in the interpretation of paint?

A
  • number of fragments and their physical dimensions (measure them)
  • number, sequence, thickness, and colour of layers (distinction between tri and quad coat, visual is subjective, MSP is objective)
  • application method (brush strokes, roller, air bubbles?)
  • surface topography (is it specialist paint with wrinkle finish, does it have hammered finish (metal paint))
  • size and distribution of pigments (presence/absence of speciality pigments)
  • defects, weathering (faded in sunlight), delamination (layers start to peel), corrosion
17
Q

When talking about automotive paint in particular, what can be used?

When was this set up?

What is it and what info does it contain?

How does it work?

A
  • paint data query database
  • set up in 1975 and by 2000, Japan, EU and US were using it
  • it is a global database of all automotive finishes
  • it contains around 50,000 paint components with sequences for around 15,000 models of car
  • if put paint flake into database, will say what components you are likely to see and in what order they should appear in
18
Q

What other databases are there and what are these for?

A
  • e-VISART (artists pigments and paints)
  • RRUFF (for minerals - IR/XRD diffraction patterns of naturally occurring minerals e.g. ones found in naturally occurring mineral based inorganic pigments such as hematite, gertite, magnetite
  • IR and Raman users group - has some info on cultural heritage
19
Q

What are the considerations when interpreting paint evidence (respraying, cross transfer)

A
  • if car resprayed/wall repainted this increases distinctiveness as have a unique pattern
  • cross transfer when one car hits another car - paint is transferred which increases significance and value
20
Q

What is there limited research on in paint interpretation?

A

limited research on spray paint variability and transferred paint persistence

21
Q

What did NAS report say about paint interpretation

A
  • got to consider background levels of paint
22
Q

What are the schematic layers constructed in automotive paint?

A

bottom
substrate material (iron/aluminium)
chemical treatment
electro-deposition coating
primer surfacer
base coat
clear coat
top

23
Q

What is a major factor in comparing fibres, paints and pigments but is subjective for humans?

What can be said about human visual system?

A
  • colour
  • perception of colour by human is subject to variety of factors
  • phenomenon called simultaneous contrast is perception of colour based on context (human visual system is complex and adaptive)
  • depending on what it is around, colours can appear different e.g. blue and yellow
24
Q

How does colour perception occur for humans?

A
  • beings with retinal cells known as cone cells
  • cones are usually labelled according to their wavelength of peak spectral sensitivity i.e. short (S), medium (M), long (L)
  • even though this is often described is RGB - it isn’t the case in human perception
  • RGB is standardised in digital space to identify what colour something should be reproduced as
25
Q

What is microspectrophotometry?

What information does it provide?

How does it work?

What does it allow for?

A
  • combination of microscope (allows focus onto rays) and spectrometer (looks at absorption bands in visible range)
  • gives the precise characteristic colour described as a distribution of wavelengths
  • this is independent of human colour perception
  • sample sits on stage with illumination from below
  • comes into focussing lens
  • got binocular focusing
  • microscope locates minute traces or shows how light interacts with material under analysis
  • spectrometer allows visualisation of sample
  • gives number in terms of absorption of what wavelength of light exists
  • allows capture of both a magnified visual and spectroscopic pattern at the same time
  • this increases the number of identifying characteristics of any given material we’re looking at e.g. paint/pigment
26
Q

What techniques can microspectrophotometry be combined with?

A
  • instrumentation combines UV-visible-NIR digital imaging with colorimetry and many other microscopic techniques (doesn’t just stick to visible range)
  • can also combine with all other microscopic techniques we have spoken about before e.g.
  • reflected light - instead of transmission
  • comparison - to have equal conditions
  • thermal - compare heat on stage
  • multispectral - already talking about this as NIR and UV
  • fluorescence - exists alongside - can classify particular fluorescent colours
  • polarised - can do everything on sample at same time
27
Q

Why is comparison microscopy important in the analysis of automotive paint/architectural paint with multiple layers

A

automotive - minimum 4
architectural - 2 or 3

  • can view all coating layers simultaneously
  • allows for large number of comparable features
  • can indicate end use, top layer will give us environment it is exposed to, will highlight aftermarket refinish
28
Q

What are the four ways to extract information from automotive paint samples?

A

1 - cross section:
- cut across them all quite flat

2 - thin peels:
- put something on that will stick to a single layer then peel it off and do same with next layer
- fiddly and hard to do as depends on how well layers have adhered to each other
- if successful, can do more science to identify individual layers

3 - wedge cut:
- simpler to do

4 - stair step exposure:
- harder to do
- ways of being able to do it without having to take full cross section
- block edges on stair-step exposure allows us to do more probative elemental analysis

29
Q

What are three sets of elemental analysis for paint analysis?

A
  • IR and raman
  • SEM-EDX
  • XRF and XRD (crystallography)
30
Q

How can IR and raman be used in analysis of automotive paint samples?

IR (3 pros, 2 forms, why it is good in this analysis, 2 limitations)

Raman (what can it do, pros)

A
  • IR spectroscopy:
  • essential tool in forensic paint analysis
  • quick, minimally destructive, little sample prep
  • 2 forms:
  • transmission
  • attenuated total reflection (ATR) for surfaces - IR of surface layer (even less sample prep)
  • can produce significant discrimination power (77-99 % for spray paints/automotive paints, 94 % for architectural paint)
  • when IR and microscopical examination combined - 99.99 % discrimination power
  • limitations:
  • pigments below cut off (in terms of wave numbers) - won’t be able to pick up wavelengths
  • LoD - certain colours outside range
  • raman:
  • complementary to IR e.g. with inorganic pigments and can discriminate crystalline polymorphs (e.g. titanium dioxide)
  • less limited by colour
31
Q

Define fluorescence

Describe the theory of XRF

A
  • fluorescence - absorption of radiation at one energy and re-emission at a different energy
  • a material that does this is one that fluoresces
  • consider an atom with electrons associated with orbitals (or shells) with characteristic energy levels
  • when excited by an external X-ray, an electron from the inner ‘K’ shell is ejected from the atom, creating a vacancy (a hole) (electron is displaced because its been given enough energy from absorption)
  • problem now is we have a hole and atoms always want to go to lowest energy level
  • electron from L (or M) shell fills the vacancy
  • this leads to energy of particle being lowered
  • this leads to emission of this different in energy in form of X-ray (photon) which is unique to this element
  • measurement of these X-rays gives us a unique identity of element
  • k alpha - single step
  • k beta - multiple step
32
Q

Describe EDXRF instrumentation (collection time, handheld device, what form is better, destructivity and sample prep, combination with other techniques)

A
  • EDXRF has fast collection time
  • it can be in a handheld device although has low resolution
  • wavelength dispersive gives more information than energy dispersive
  • non-destructive and no sample prep required
  • offers flexibility to be combined with other microscopic techniques e.g. combined with electron microscopy, XRF elemental analysis has even more discrimination power
33
Q

How can SEM-EDX and XRF be combined for analysis of paints and pigments? (sample prep, thicker cross sections/stair step exposure pros and cons, thin peels pros and cons, SEM-EDX pros, XRF pro and con)

A
  • sample prep is critical for data evaluation
  • generally thicker cross sections or stair step layer exposure are employed
  • but this risks the electron beam penetrating beyond the specific layer of interest
  • thin peels ensure that data originates from particular layer but may result in loss of elemental data of some minor components
  • SEM-EDX is fast, non-destructive and sensitive to low concentrations (theoretical minimum of ~0.1% by weight)
  • XRF limit of detection is even lower than SEM-EDX although larger sample sizes are necessary
34
Q

What does elemental analysis in combination with other techniques provide?

A
  • elemental analysis in combination with other techniques provides significant discriminatory power and allows comparison with high confidence

FSCI6030 - Trace Evidence (13 decks)

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