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Trace evidence > Lecture 11: Paints and Pigments > Flashcards

Lecture 11: Paints and Pigments Flashcards

1
Q

Dyes and pigments

A
  • Collectively termed “colourants”
  • Mainly designed to enhance a product’s appeal
  • Fundamentally, dyes are soluble; pigments are insoluble particles dispersed in a matrix
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2
Q

Dyes

A
  • Start off as solids but they dissolve.
  • Dyes are primarily organic based; pigments equally represented by organic and inorganic components
  • Dyes are of greater use in textiles; pigments used in paints, inks, plastics, cement, ceramics, glass…
  • Classified using the Colour Index scheme (CI)
  • Inorganic components are the useful parts
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3
Q

Pigments

A
  • Remain as particles dispersed or suspended within a matrix.
  • Can be found anywhere, they have a broaden application so they’re more applicable for trace evidence.
  • Inks tend to be organic
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4
Q

Colour characteristics

Hue

A

Hue – colour, dependent upon wavelength

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4
Q

Colour index (international)

A
  • Produced by society of dyers and colourists
  • Comprehensive list of all commercially available dyes and pigments globally (regularly updated as people subscribe to it)
  • Each colourant is assigned a generic name, which incorporates its application, its hue, and its CI number (colour index number)
  • Also gives application methods and properties, companies that manufacture which allows you to trach it back to its origin, any trade names, and most importantly, the chemical composition!
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5
Q

Colour characteristics

Saturation

A

Saturation, intensity, strength, chroma – purity of colour
How close is it to the true colour

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6
Q

Colour characteristic

Tint

A

Tint = addition of white; shade = addition of black
Not one of the main 3 ways to characterise colour but still exists

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7
Q

Colour characteristics

Brightness

A

Brightness - luminance, value of lightness or darkness

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8
Q

How is gloss achieved?

A

Gloss can be achieved by using small particles, at low concentrations, finely dispersed in the formulation, opposite for matte

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9
Q

What determines the opacity of a pigment?

A

Opacity determined by degree of light scattering – directly related to refractive index (RI)

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10
Q

What determines the colour of a pigment?

A
  • Colour is determined by light absorption/reflection and, crystal lattice and particle size/shape.
  • We can synthetically change particle size and shape
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11
Q

What is opacity

A
  • It is the degree of cover / how good the coverage of a pigment is
  • How much light is allowed through
  • The higher the RI the higher the opacity and higher the coverage
  • TiO2 has a high RI
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12
Q

What do pigments provide?

A

Pigments provide both colour and opacity

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13
Q

Inorganic pigments

A
  • Inorganic pigments are resistant to heat, light, weathering, solvents and chemicals, and cheaper!
  • Inorganic pignmetns (mineral derived or metal based) are the cheapest option and are highly resistant.
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14
Q

Organic pigments

A
  • Organic pigments offer better colour intensity and brightness (degree of tint or shade), because have a lower RI → transparency.
  • Organic pigments are more expensive and have a lower opacity which is why they’re used for printing inks. If they were inorganic you wouldn’t be able to see the colours as they would be totally opaque, we need some degree of transluceney .
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15
Q

What does transparency allow?

A

Transparency allows colours to work together and be printed.

16
Q

What does particle size determine?

A

Particle size determines properties

17
Q

TiO2 pigment grade

A

TiO2 pigment grade for maximum opacity = 0.25 μm
This is a hiding white pigment.

18
Q

Nano TiO2 particles

A

Nano TiO2 particles <0.1 μm used for sunscreens

19
Q

Non hiding white pigments

A

Non-hiding white pigments used as extenders (they bulk stuff out)
Examples = calcium carbonate, talc, china clay, silica

20
Q

Most important coloured inorganic pigments

A

Most important coloured inorganic pigments = iron oxides and chromium oxide (Pigment Green 17)

21
Q

Carbon blacks

A

Pigments used in vehicle tyres

22
Q

What are organic pigments often called?

A

Lakes

23
Q

What is paint?

A
  • Most common surface coating
  • Others include varnishes, enamels, stains, lacquers
  • Like fibres, there is a huge amount of complexity and variability → great news for a forensic examiner!
  • Suspension of pigments and additives in a binder (See Trace Evidence Handbook for binder types)
  • Complexity of the paints allows for discrimination of the samples
  • Binder is the basis of paints, makes paint brittle and stick and what makes it go hard when it dries
24
Q

Binders

A

Binders – polymers – film former, e.g. acrylic
Most common ingredient in paint.
Makes it a surface coating.
Binder gives the basis, it’s a polymer.

25
Q

Pigments in paint

A

Pigments – TiO2, ZnO, BaSO4, Fe2O3, CoAl2O4
Next most commont ingredient, gives it its colour and opacity
These examples are all hiding white pigments, designed to cover.

26
Q

Fillers in paint

A

Fillers – increase thickness of coating, e.g. talc
bulk it out, non hiding white pigments

27
Q

Drying agents in paint

A

Drying agents – speed up polymerisation
Polymerise reactions

28
Q

Texturises in paint

A

Texturisers – smooth surface, e.g. solvents

29
Q

Eumsifiers in paint

A

Emulsifiers – prevent separation, increase shelf life
Causing mixing of oil and water, oil and water doesnt mix so it forces them to mix, prevents separation which increases shelf life

30
Q

PLasticisers in paint

A

Plasticisers – increase flexibility

31
Q

Other common ingredients in paint

A

Fungicides, biocides, insecticides
UV stabilisers and corrosion inhibitors

32
Q

Where are paints found?

A
  • (1st) Automotive and clear coats – hit-and-run
  • (2nd) Architectural – burglaries, forced entries
  • Other vehicular coatings, e.g. bike, marine, aircraft
  • Tool coatings – burglaries
  • Spray paints – graffiti
  • Road paints – RTC, distinctive glass beads
  • Works of art – forgeries
  • Nail varnish – associative trace evidence!
  • Cars didn’t use to have a sealant but they do now which adds a layer of complexity
  • Architectural - applied to a building
33
Q

Recovery considerations for paint

Part 1

A
  • Paint chips, flakes, fragments are very delicate
  • Must not be lifted with J-Lar, cellotape or acetate!
  • Glass or plastic vial is the best option, then bag
  • Embedded flakes must be not be removed at scene
  • Paint transfers should not be lifted – submit item
  • Controls if available – sample down to substrate
  • Don’t refrigerate or freeze!
  • Its hard to get them off once lifted and you risk breaking it up or splitting the layers further and if you’re going to analyse it, adhesive tape is made up of polymers which will interfere with analysis.
  • You can use the druggies fold but youll have to put it in a box not evidence bag as evidence bag is paper and can fold which will damage the sample
  • Whole item should be removed if possible when fragments are embedded
  • Contact transfers should not be lifted
34
Q

Recovery considerations for paint

art 2

A
  • At a scene don’t separate paint from sample, it should be collected as a while
  • You need to sample right down to metal substrate if taking car samples
  • Even the bare metal itself is coated before the primer is added
  • Its important to recover all of the layers
  • Its important to sample as cloe to the area of damage that is reasonably possible, bc if car has been resprayed on a panel and you take it from a different area it wont be a like-for-like comparison
  • You dont want to take sample from impact site itself!
  • Dont freeze them bc it introduces moisture which can peel apart layers of paint
  • Keep them at ambient temp
35
Q

Analytical workflow for painr

A
  • Gross examination, recovery and collection
  • Preliminary evaluation of physical characteristics
  • Physical fit assessment – most probative value as it is conclusive and trumps everything else.
  • All microscopic techniques
  • Microspectrophotometry – colour determination
  • Infrared spectroscopy – organic binders/pigments
  • Raman spectroscopy – inorganic pigments/C.Black
  • SEM-EDX, XRF – elemental composition of layers
  • XRD – crystal structure, polymorphs
  • Good for titanium dioxide
  • (Pyrolysis-GC/MS and microchemical tests) Destructive so last resort
  • High concentration of inorganic materials so its slightly different to the previous one
  • The methods below raman are inorganic specifics
  • XRD - gives crystal structure which is important if you have polymorphs
  • Trace evidence handbook states if you find a physical match you don’t need to do anything else
36
Q

Paint interpretation

Part 1

A
  • Number of fragments and their physical dimensions
  • Number, sequence, thickness and colour of layers
  • Application method → brush strokes? Air bubbles?
  • Was it applied with a roller, paint brush? Was it applied as a spray?
  • Surface topography → wrinkle finish, hammered finish
  • Size and distribution of pigments (presence or absence of speciality pigments?)
  • Defects, weathering, delamination, corrosion
  • Assess number of layers
  • Need msp colour values applied as a spray or using a roller?
  • Hammered finish tend to be with metal paint and adds a discriminating factor
  • Has it faded in sunlight?
  • Delaminate is when the layers start to peel apart.
37
Q

Paint interpretation

Part 2

A
  • Paint Data Query founded by the Royal Canadian Mounted Police 1975 → global database automotive finishes
  • PDQ ~50,000 paint components with sequences, it tells you the order of the layers
  • It will tell you what components you should expect to see and the order so it is a really good discriminating database.
  • Car resprays/wall repainting → increase distinctiveness
  • Cross-transfer when one car hits another → increased significance and value then single way transfer
  • Limited research on spray paint variability and transferred paint persistence.
  • Consider background levels

Trace evidence (22 decks)

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