Paints and Pigments Flashcards
Dyes and Pigments
- Collectively termed “colourants”
- Dyes are soluble
- Pigments are insoluble particels dispersed in a matrix
- Mainly designed to enhance a product’ss appeal
What are dyes and pigments made of and where are they used?
- Dyes are primarily organic based, used in textiles
- Pigments are equally represented by organic and inorganic components, used in paints, inks, plastics, cement, ceramics, glass
What are dyes and pigments classified by?
Colour Index Scheme (CI)
What does the Colour Index Scheme tell you about a colourant?
- Generic name
- Its hue
- Its CI number
- Application methods
- Properties
- Companies that manufacture it
- Trade names
- Chemical composition
What are the three colour characteristics?
- Hue
- Saturation
- Value
Define Hue
Colour that is dependent upon wavelength
Define saturation and other names that mean the same
Purity of colour
(Also called intenisty, stength or chroma)
Define value and give the other names for it
How light or dark the colour is
(Brightness and luminance)
What effects the value but not the hue of a colour?
Tint and shade
What is tint?
In terms of colour
Addition of white
What is shade?
In terms of colour
Addition of black
What is colour determined by?
- Light absorption/reflection
- Crystal lattice
- Particle size/shape
What determines the opacity of pigments?
- Determined by the degree of light scattering
- Higher refractive index (RI), higher the opacity, higher coverage
Pigment size
How do you get a glossy or matt pigment?
Glossy: Use small particles at low concentrations, finely dispersed in the formulation
Matt: Opposite of gloss
What are the benefits of inorganic pigments?
- Resistant to heat
- Resistant to light
- Resistant to weathering
- Resistant to solvents
- Resistant to chemicals
- Cheaper (mineral derived or metal based are cheapest)
What are the benefits of using organic pigments?
- Better colour intenisty and brightness
- Lower RI = lower opacity = more transparent
- Need to have translucency to work in harmony to produce the colours on a print out
What does different particle size mean in pigments with examples?
Larger particle size for maximum opacity
* TiO2 pigment grade (0.25 μm)
Smaller size for sunscreens
* Nano TiO2 particles (<0.1 μm)
What are non-hiding white pigments used for, with examples?
Extenders (bulk stuff out)
* Calcium carbonate
* Talc
* China clay
* Silica
What are organic pigments sometimes called?
Lakes
Why are paints good for forensic analysis?
Huge variety and complexity so there is good discrimination between samples
What are the main components of paint?
- Binders
- Pigments
- Fillers
- Drying agents
- Texturisers
- Emulsifiers
- Fungicides, biocides, insecticides
- UV stabilisers and corrosion inhibitors
- Plasticisers
What are binders and give examples?
It is what makes paint, paint like (sticky and brittle)
* Polymers
* Acrylic
What do pigments do in paint with examples?
Gives paint its colour and opacity
* TiO2
* ZnO
* BaSO4
* Fe2O3
What are fillers and examples?
Increase the thickness of coatings
* Non-hiding white powders
* Talc
What are drying agents?
Speed up polymerisation or evaporate off the liquid content
What are emulsifiers?
Prevent separation and increase shelf life - force the mixing of oil and water
What are texturisers with examples?
They smooth the surface
* Solvents
What do UV stabilisers do?
Stop UV degradation
What do plasticisers do?
Increase flexibility
Where are paints found?
- Automotive and clear coats
- Architectural
- Other vehicle coatings (bike, marine, aircraft)
- Tool coating
- Spray paints
- Road paitns
- Works of art (forgeries)
- Nail varnish
What are some recovery considerations of paint?
- Paint chips are very delicate
- Must not be lifted with J-lar or acetate
- Glass or plastic vial then bag
- Embedded flakes must not be removed at scene
- Paint transfers should not be lifted (submit whole item)
- Take controls if available
- Dont refrigerate or freeze (introduces water which can peel apart the paint)
What is the analytical workflow for paint and what are each technique used for?
- Gross examination, recovery and collection
- Preliminary evaluation of physical characteristics
- Physical fit assessment (most probative value)
- All microscopic evaluation
- Microspectrophotometry - colour determination
- Infrared spectroscopy - organic binders/pigments
- Raman spectroscopy - inorganic pigments
- SEM-EDX, XRF - elemental composition of layer
- XRD - crystal structure, polymorphs
What is the global automotive paint database called?
Paint Data Query
What increases the value of paint in forensic science?
- Car resprays/wall repainting - increase distinctiveness
- Cross transfer - increased significance and value
What effects colour perception from person to person?
- Different lighting
- Cones respond to colour in different wats from person to person
What is microspectrophotometry and how is it used to analyse colour?
- Microscope combined with spectrometer
- Gives the precise characteristic colour described as a distribution of wavelengths - independent of colour perception
- Can look at the UV to NIR range of wavelengths - can go further if combined with other microscopic techniques
- It locates minute traces or shows us how light interacts with the material under analysis
How is comparision microscopy used to analyse paint?
- Can view all coating layers simultaneously
- Colour and texture
- Layer sequence
- Relative layer thickness
- Pigment size and distribution
- Defects (weathers, dirt, solvent traps)
What are the four ways that you can look at paint under microscopy?
- Cross-section
- Thin peels - each individual layer is peeled off (hard to do but allows the identification of each layer)
- Wedge cut - dont have to take paint off the sample
- Stair-step exposure - dont have to take paint off the sample
What do solvent traps tell you in paint?
Bubbles from spraying - tell you if the car might have been resprayed and the type of solvent used
Why is IR and Raman used to analyse paint (benefits + limitations)?
- Quick
- Minimally destructive
- Relatively little sample prep
- Can be run in transmitted or attenuated total reflection (ATR)
- Can discriminate crystalline polymorphs
- Limitation of IR - pigments below the cut-off (in terms of wavelength numbers) and LoD - complex paint or a small sample wont be able to be detected
Theory of it
How does X-Ray Fluorescence (XRF) work?
- Anything that fluoresces is a material that absorbs energy and it emits radiation at a different energy
- After excitation, the atoms were left with a vacancy so to respond to this, electrons start to move down from other shells to the lower shells, this allows us to observe it and stabilises the atom
- Energy of the particle is lowered and is converted and emitted as a photo to allow us to observe it
- The amount of energy depends on how far the electron has gone
- Energy is unique to the atoms so we can identify it
Benefits of using XRF
- Fast collection time
- Can be in a handheld device - although realtively low resolution
- Often non-destructive
- No sample prep required
- Tells us the makeup of the sample
Benefits of electron microscopy with paints
- Higher resolutions are achievable using electrons instead of light
- Non-destructive analysis of small quanities - beam damage can occur for sensitive sample
- Rapid accumlation of results in high resolution
- Give elemental composition
- Combine electron microscopy with XRF to increase discrimination power
Sample prep for SEM-EDX & XRF for paints and pigments
- Thicker cross-sections or a stair-step layer exposure - at risk of electron beam penetration beyond the layer of interest
- Thin peels for data for a particular layer - might lose elemental dator of some minor components
Benefit of SEM-EDX with paints
- Fast
- Generally non-destructive
- sensitive to low concs
XRF vs SEM-EDX
XRF limit of detection is even lower than SEM-EDX although larger sample sizes are necessary
What can XRD determine in paint?
- Lattice parameters
- Phase composition of the sample
- Crystal structure
- Crystallite size - smaller than ~120 nm create broadening in diffraction peaks
How many iron oxide varients are there and what are they?
3
* red
* yellow
* black
