Lecture 12: Paints and pigments part 2 Flashcards
1
Q
Simplified analytical workflow for paints
A
- Gross examination, recovery and collection
- Preliminary evaluation of physical characteristics
- Physical fit assessment - most probative value
- All microscopic techniques
- Microspectrophotometry - colour determination
- Infrared spectroscopy - organic binder & pigments
- Raman spectroscopy - inorganic pigments
- SEM-EDX & XRF - elemental composition of layers
- XRD - Crystal structure & polymorphs
- Pyrolysis-GC/MS & Microchemical Testing
2
Q
Colour perception
A
- Colour is a major factor in comparing fibres, paints and pigments but The perception of colour by a human observer is subject to a variety of factors.
- The human visual system is complex and adaptive. The phenomenon called simultaneous contrast is the perception of colour based on context
- Humans interpret colours differently, we adapt to different colours differently
- Colour perception begins with retinal cells known as cone cells
- The cones are usually labeled according to their wavelength of peak spectral sensitivity i.e. Short (S); Medium (M); Long (L)
- However, RGB is standardised in the digital space to identify what colour something should be reproduced as.
3
Q
Microspectrophotometry
A
- Combine a microscope with a spectrometer
- Gives the precise characteristic colour described as a distribution of wavelengths. Therefore it is independent of colour perception
- Microscope locates minute traces or shows how light interacts with the material under analysis
- Allows capture of both a magnified visual and a spectroscopic pattern at the same time. Increasing the number of identifying characteristics of any given material
- Instrumentation combines UV-Visible-NIR digital imaging with colorimetry and many other microscopic techniques.
- Independent of human colour perception
- Can combine it with any other microscopic techniques
4
Q
Comparison Microscopy
Exposing the layers
A
Viewing all coating layers in automotive paint simultaneously provides a large number of comparable features - can indicate end use, highlight after-market refinish etc.
* Cross section
* Colour & texture
* Layer sequence
* Relative layer thickness
* Pigment size & distribution
* Defects
* Weathering
* Dirt
* Solvent traps
5
Q
Automotive paint
A
- Common in car accidents, hit and runs, etc
- Automotive paint is complex, they don’t have a single layer.
- Automotive has a minimum of 4 layers!
- Allows us to be more discriminative as we have more layers.
- Being able to identify each layers is difficult.
- After market modification could be another coat.
- Together the layers are about 100 microns
- Top layers will give indications about the environment it’s going to be used in.
- Brand of car, year of car, etc
- Layers can include: electro deposited coat which is your first layer, then prime layer and then a base coat then probably a clear coat sat on top.
6
Q
4 different ways to analyse automotive paint sample
A
4 different ways to extract information about it
* Cross-section - Cutting across the end and looking at the microscopic details (cross section)
* Thin peels - Peeling of the single layers indivudsally, hard to do depending on how the layers stick together
* Wedge cut - Wedge cut and stair cut are ways to look at it without a cross section
* Stair step exposure - stair step allows for more probative analysis
7
Q
IR
Paint analysis
A
- IR is quick, non destrucive, little sample prep, can be run in two forms
- It is also the second best method for analysis paint and pigments.
- IR Spectroscopy is arguably the second most essential tool in forensic paint analysis - Quick; Minimally Destructive; Relatively Little Sample Prep.
- Can be run in transmitted or attenuated total reflection (ATR) for surfaces
- Can produce significant discrimination power .
8
Q
Limitation of IR for paint analysis
A
Limitations of IR include pigments below the cut-ff and LoD
9
Q
Raman for paint analysis
A
- Raman is an emerging application with real discrimination power
- Raman is Complementary to IR e.g. with inorganic pigments
- AND can discriminate crystalline polymorphs e.g. in Titanium Dioxide
- You can look if two samples are equivalent and at direct links between a sample and it’s source.
10
Q
XRF fluorescence theory
A
- When excited by an external X-ray, an electron from the inner ‘K’ shell is ejected from the atom, this creates a vacancy.
- An electron from the L (or M) shell fills the vacancy (they drop down in ordet to stabilise the atom), which leads to the energy of the particle being lowered.
- In the process of the electrons dropping down, the energy of the particle has been lowered it is converted and emitted as an xray which is emitted as a photon which can be observed. The energy is based on the size of the step.
- Which leads to the emission of this difference in energy in the form of an X-ray which is unique to this element.
- This produces a vacancy in the L or M shell, which then needs to be filled releasing another X-ray etc so the process continues and is unique to the element.
11
Q
What does XRF allow for?
A
- It gives us a unique identity of the element, particularly if you look at the whole process together. If you look at the numeber of shells, the distance between the shells, the electrons populating the shells, etc is unique to the element.
- Allows to detect specific elements
- Allows us to get more detail.
- We have discrete energy levels with an electron sitting in them charactersitc to a particular atom.
- If we take the energy levels and excite them using x-rays, they excite the higher energy levels (closest to the nucleus), so the electrons get ejected out of the atom leaving a hole. This hole creates a problem
- Electrons will start moving down to fill the hole.
- Measuring these released X-rays give us a unique identity of the element.
- Wavelength dispersive gives the best resolution.
- The ratio of the height not only gives elemental compostion but we could potnetally determine the amount of different elments present.
12
Q
Anything that fluoreseses
A
- Anything that fluoreses is something where a matieral absorbs energy and emits radiation at a different energy, usually lower but can be higher in some cases.
13
Q
XRF Instrumentation
A
- Energy dispersive XRF (EDXRF or EDX) has a fast collection time & can be in a handheld device although has relatively low resolution
- Energy dispersive has a lower resolution and is more used in forensics.
- Often a non-destructive technique with no sample preparation required.
- Offers the flexibility to be combined with other microscopic techniques.
14
Q
Electron microscopy
Paint
A
- The resolution of light microscopy is limited by the illuminating wavelength. Higher resolutions are achievable using electrons instead of light
- Non-destructive analysis of very small quantities of material possible this way (although beam damage CAN occur for sensitive samples)
- Allows for the rapid accumulation of results in high resolution
- Combining electron microscopy with XRF elemental analysis to increase discrimination power even more!!
- Can even give elemental composition of a material!!
- Electrons are strongly interacting and have a wavelength that makes them useful for imaging nanomaterials.
15
Q
Scanning electron microscopy
Paints
A
- Samples can be large as long as there is space in the sample chamber
- Scanning approach builds up image of one point at a time
- Allows us to pick up Seconday electrons - knocked out of atom
- Allows us to look at our backscattered electrons Inelastic
- Allows us to scan across a surface
