Lecture 11&12 - Paints and Pigments Flashcards
what is the main purpose of dyes and pigments
to enhance the appeal of a product
what is the main property separating dyes and pigments
dyes = soluble (a solid that then dissolves)
pigments - insoluble - remain as particles
how are dyes and pigments different in their composition
dyes = organic based
pigments = equally organic and inorganic based
the inorganic can be useful in forensics
what is the difference in the use of dyes and pigments
dyes = more used in textiles
pigments = paints, ceramic, glass
how are dyes and pigments classified
using the Colour Index Scheme (CI) which is now called the colour index international
what is the Colour Index International
a list of all commercially available dyes and pigments globally
regularly updated
each colourant is given a generic name
what does the generic name of a dye or pigment in the CI represent
the application, hue and CI number
what is hue
essentially the colour
what are the three elements that make up the colour of something
hue = colour dependent on wavelength
saturation = purity of the colour (intensity)
value = lightness or darkness
what are other words for saturation of colour
what are other words for the value of colour
strength, chroma
brightness, luminance
what is the colour of a pigment determined by
the light absorption/reflection -
we see the colour that is reflected others are absorbed
also crystal lattice and particle size/shape
what is the opacity of a pigment related to
the refractive index - determined by the degree of light scattering
opacity = degree of cover of pigment
how is a gloss achieved and what is the opposite of a gloss
using small particles at low concentrations finely dispersed in the formulation
opposite = matte
what does a higher refractive index result in terms of opacity
higher RI = higher opacity = higher coverage
what is a common pigment used in paint
titanium dioxide
what two things do pigments provide to a formulation
colour and opacity (how much light can get through)
how do organic and inorganic pigments differ
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)
where are organic pigments used
in printing inks due to translucent characteristic allowing colours to work in harmony
where are inorganic pigments used
in something you want to remain the colour it is e.g car paint or any other paint
what are the purpose of extenders and what pigments are used as these
to bulk things out
non hiding white pigments
particle size of pigments can impact what?
the opacity
what size particles are used in material aiming for max opacity and give some examples of these materials
pigment grade = 0.25 micrometre
toothpaste, makeup, paper
what size particles are used in material aiming for a lower opacity and give some examples of these materials
nano particles = less than 0.1 micrometre
moisturisers, suncream
name two of the most used coloured inorganic pigments
iron oxides (can be red, yellow or black)
chromium oxide
what are organic pigments also called
lakes
what is the pigment used in vehicle tyres
carbon black
what are the most common yellow, orange and red organic pigments
Azo compounds - they have N=N bonds
what is paint? and what are other types of this
a type of surface coating
others = varnishes, enamels, stains, lacquers
paints are - and - which is good news for forensic examiners
complex and variable
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
what is the purpose of binders
make paint brittle and stick, made of polymers e.g acrylic
what is the purpose of fillers
increases the thickness of the coating, bulk it out e.g talc
what is the purpose of drying agents
make a hardened surface - speed up polymerisation or cause some liquid to evaporate
what is the purpose of texturisers
create a smooth surface or desired surface
what is the purpose of emulsifiers
prevent separation - allow water and oil to mix - increase shelf life of paint
what is the purpose of fungicides
prevent mould growth
what is the purpose of plasticisers
increase flexibility of the paint, so if the surface bends to paint wont move or crack
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
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
why are glass beads added to road paint
to make the signs more reflective so drivers can see them
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
should embedded flakes or paint transfers be removed at the scene
NO
submit whole items if you can
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
are paint samples refrigerated/frozen, why
No this can introduce moisture causing layers to come away
keep at an ambient temperature
what is the general analytical workflow for paint analysis
- gross examination, recovery, collection
- preliminary evaluation of physical characteristics
- physical fit assessment
- microscopic techniques
- MSP - microspectrophotometry
- IR spectroscopy
- Raman spectroscopy
- SEM-EDX and XRF
- XRD
- GC-MS and other chemical tests
what part of the analytical workflow has the most probative value
the physical fit assessment
what is MSP used for
colour determination
what is IR spectroscopy used for
analysis of organic binders/pigments
what is raman spectroscopy used for
inorganic pigments or carbon black
what is SEM-EDX or XRF used for
elemental analysis of the different layers
what is XRD used for
crystal structure and polymorph (different crystalline forms of the same substance) identification
what is the database used for analysis of automotive paint
Paint Data Query - has around 15,000 models of car
what can increase the distinctiveness when analysing paint sample from cars and walls
if they have been repainted without proper removal of the original
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
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
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
what phenomenon describes the perception of colour based on context
simultaneous contrast
where does human perception of colour begin
in retina cells known as cone cells
what is MSP
microspectrophotometry - the combination of a microscope with a spectrometer
the instrument combines UV-Visible-NIR digital imaging with colourimetry
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
what information does MSP capture
a magnified visual image and a spectroscopic pattern = more characteristic than one or the other
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)
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
why are more layers of paint beneficial
more layers can be more discriminative due to more information and complexity
how many layers do
1 - automotive paint
2 - architectural paint
tend to have
1 = 4
2 = 2
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
when are solvent traps likely to be seen in paint layers
when a spray has been used
name 5 methods of analysis that can give elemental composition information of materials
FTIR
Raman
SEM-EDX
XRF
XRD
what are the limitations of using IR spectroscopy for paint analysis
pigments present below the LOD
why is IR spectroscopy good for paint analysis
quick
minimally destructive
relatively little sample prep
can have high discriminatory power
why is raman a good complimentary method with IR
good for inorganic pigments and looking at crystalline polymorphs such as Titanium dioxide
name two types of IR used in paint analysis
transmitted
ATR (attenuated total reflection) = good for surface layers
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
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
what makes something fluoresce
when energy is absorbed and re-emitted with differing energy
(normally a lower energy but can be a higher)
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
what is EDXRF/EDX
energy dispersive XRF
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)
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
what is the result of XRF analysis
a spectrum with peaks corresponding to the elements found within the sample
thin peels allow individual layers to be analysed but what can they sometimes result in
the loss of elemental data as tape is used
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
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
what law is a model used to understand the conditions needed for diffraction
Bragg’s law
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
which equation allows the average size of nanocrystals to be calculated
Scherrer equation
