Paint Flashcards
What is paint and what is its composition?
A liquid mixture that hardens into a solid coating after its application to a surface.
It is made of pigments and a carrier which is comprised of a binder, solvent and additives.
Pigment - any material that is black, white or coloured, organic or inorganic, that retains a crystalline form
Dyes - colourants which dissolve in the paint
Explain the properties of pigments
- Provide colour
- Contribute to characteristics such as opacity, gloss , toughness and flow
- Mostly inorganic
- Insoluble in water
- Often less expensive than dyes of a similar colour
- Finely group (0.01 - 100 micrometres)
Coloured pigments
- Absorb certain wavelengths of visible light and transmit/scatter other wavelengths
- Many are natural in origin
- Some are synthetic
White Pigments
- Scatter all wavelengths of light due to their high Refractive Index
- Perceived as white by human eye
- Make paint opaque
- Protects the substrate from some uv light
- TiO2 - rutile - is widely used
- Often known as ‘hiding pigments’
Explain the properties of binders
- Substances in a paint that hold the pigment particles in suspension
- Consists of resins - polymers
- Transparent or with a slight hue
- Binders enable pigments to be distributed across and attached to a substrate or surface
- Nature and amount of binder determines many of the paint’s performance properties
Explain the properties of solvents
- Dissolve the binder polymer
- Adjust the viscosity of the paint
- Solvents evaporate and do not form part of the final film
- Can help in the application and flow of the paint
- Some paints do not include any solvents/diluents
Explain the two types of paints based on the contribution of the solvent and/or binder
Non-convertible paints (aka Thermoplastic paints)
- Films form after the evaporation of the solvent
- Polymers in the binders do not undergo any chemical changes on drying
- Film can be re-dissolved in the original solvent
- Films are hard-wearing and resistant to chemicals
Convertible Paints
- Coatings undergo a chemical change following or during application, which involves the cross-linking of binder polymers
- Coatings cannot be re-dissolved in the original solvent
- Domestic emulsion - polymer distributed in tiny droplets that form beads on the surface - the film is formed when an additive causes polymers in the beads to merge into one another to give a continuous film
- May be a one or two component system
One Component Systems
- Chemical reactants are already present in the paint and only start to react when the paint is applied in a thin film
Two Component Systems
- Chemical reaction occurs only when two components are mixed
- Before mixing, each component is stable and non-convertible on its own
- Tough, durable and resistant to chemicals, water and physical damage
What are additives?
Included to enhance or modify certain paint qualities
- Surface tension
- Viscosity (thickeners)
- Final appearance (flatteners)
- Anti-rust properties
- UV resistance
- Mildew/microbial resistance (biocides)
- Anti-freeze qualities
May also be contaminants from impurities in other materials
Explain different paint types
Primer - Provides better adhesion for further painting
Undercoat - Thick, viscous paint with high pigment levels
Applied over primer or previously painted surfaces, before the final coat
Sealer - Stops paint being absorbed by walls
Finishing/top coat - Increases resistance to damage and may be of certain appearance/glossiness
Explain different categories of paint
Architectural paints - Found in residential and business properties
Product coatings - Applied during the process of manufacturing products
Special purpose coatings - skid resistance, waterproofing
Art paints - used for general art activities
Vehicular paint
- Clearcoat - transparent protection, gloss
- Basecoat - Colour, metallic effect
- Filler - surface, stone chip resistance
- Electrocoat - corrosion protection
- Pre-treatment
- Substrate - steel, aluminium
What is the method of recovery of paint?
Entire item bearing the paint marks should be recovered
Loose paint flakes should be recovered by fingers or with forceps
Do not use adhesive tape
Packaging:
Sample storage:
- Beecham’s wrap
- Resealable plastic bags
- Petri-dishes
- Glass vials
Sample labelling
- Sample description
- Where and when it was found and recovered
- Case details
- Exhibit number
- Your details
Methods of Paint analysis
Physical fit
Layers
- Low powered microscopy
- Colour, thickness and order of paint layers can be used to include/exclude samples
- If the number, colour and sequence of layers match, then examine the texture, gloss and other visible characteristics
Colour and other physical characteristics
- Microspectrophotometry (MSP)
- A combination of an optical microscope and a spectrophotometer
- Measures the absorption of visible (and ultra-violet) radiation over a range of wavelengths
- A graph is produced showing the wavelengths of light absorbed and the extent of the absorption
- Non destructive technique
- Infrared Spectroscopy
- Absorption of IR energy is measured over a range of wavelengths, different bonds vibrate at different wavelengths, giving a characteristic spectrum for a given substance
- Identifies binders, pigments and additives used in paints and coatings
- Useful for smears
- Non-destructive
Chemical composition
- Microchemical tests
- Carried out on each layer to distinguish between visually similar paints
- Sample is subjected to a variety of oxidising, dehydrating or reducing agents that will cause it to curl, soften, wrinkle, swell, disintegrate, bubble, fizz, change colour
- Subjective and destructive
- Good for initial inclusion/exclusion
- Diphenylamine: indicates presence of oxidisig ions
- Chloroform: indicates presence of acrylic lacquers
- Acetone: indicates presence of acrylic and nitrocellulose lacquers
- Possible reactions
- Colour change
- Bleeding of sample colour
- Bubbling/effervescence
- Dissolving
- No visible reaction
Explain paint comparisons
Positive
- No significant differences observed between the samples
- Paint recovered from Exhibit A was found to be physically and chemically consistent with the paint from Exhibit B
- Therefore, there is extremely strong/very strong/ strong support for the proposition that the paint from Exhibit A originated from the same source as the paint in Exhibit B
Inconclusive
- Results in which the differences may or may not be significant
- Examinations of the paints from Exhibit A and Exhibit B found some physical and chemical similarities, as well as a number of differences
- Therefore there is no support for either proposition - whether the paints in Exhibit A and Exhibit B did or did not originate from the same source
Negative
- One, or more, significant differences between samples
- Paint from Exhibit A is not consistent with the paint from Exhibit B
- Therefore there is extremely strong/very strong/ strong support for the proposition that the paint from Exhibit A originated from a different source to the paint in Exhibit B
