Paints and Polymers 1

What is paint?

A pigmented coating.

What is a coating?

A liquid that changes into a solid protective/decorative layer

What is paint composed of?

• Binders

• Pigments

• Additives (which will most likely be in concentrations too small to detect)

• Volatile components

What is a binder?

The portion of paint that holds the paint together and is responsible for adhering it to the surface the paint is covering

Pigment characteristics

• insoluble particles

• decorative pigments impart color and appearance to paint

  • done through absorption, scattering, and/or interference

• Some pigments add anticorrosion

Extender pigment characteristics

• Used to add bulk

• Modifies certain properties of the paint, reduces cost, and helps cover the substrate

• Sometimes referred to as “fillers”

• Refractive index will be close to binder

What is the main white pigment?

Titanium oxide

Titanium oxide comes in what two forms?

• Anatase

• Rutile

Anatase characteristics

Has a more vivid white color

Rutile characteristics

Has more covering power (hides the substrate better)

What is used to achieve metallic finishes?

Aluminum flakes are used as pigments

Aluminum is opaque to visible light, but will also hamper infrared analysis of paint when examined in transmission.

Nickel, stainless steel, and copper alloy pigments may also be used.

Zinc pigments are used and are helpful as anti-rust

Effect pigment characteristics

• alter appearance through reflection, diffraction, and the interference of light

  • includes pearlescence and iridescence

Interference Pigments

tend to be regularly shaped, with multiple layers, and the color that is seen will depend on the angle that it’s viewed at.

Pearlescent pigments

Is mica flakes coated in iron oxide or titanium dioxide.

Additives

• are substances added to paint in small quantities with the purpose of improving specific properties

  • They serve multiple purposes and are normally not reliably detectable; they therefore are of little value for comparison purposes.

Volatile components

• Refers to paint’s solvents

  • Not typically present in dried samples

Spraying

• Paint is atomized, directed towards the surface being painted.

  • overspray is common, and is useful in comparison

Dipping

• Item is dipped into paint

  • For automobile parts, an electrical current is applied to the paint and the body to assist in deposition

Electrodeposition

• Layers applied in this manner are known as “e-coats”

• The part is dipped in an aqueous solution with paint particles that are electrically charged.

• Charged particles and part are oppositely charged.

  • Commonly used in automobile parts

Automotive coatings

OEM = Original Equipment Manufacturing (Paint that is applied at the factory)

Automotive pretreatments

• used for both corrosion protection and to assist in paint adherence to the substrate

• “Conversion coat”

• Can be made of zinc phosphate or zirconium oxide

  • Applied by electroplating in a THIN layer

Automotive primer

• protects surface while also filling defects and preparing for adhesion to additional layers

• most cars have 2 primer layers

• first layer is electrodeposited (e-coat)

  • second layer MAY be applied- antichip

Automotive primer: 3rd Layer

• primer surfacer

• can be applied extra thick to act as antichip if antichip layer hasn’t been separately applied

Automotive Basecoat

• sometimes called the clearcoat, though typically clearcoat is a separate layer

• if there is only a basecoat (no clearcoat) the system is called a monocoat

Automotive Clearcoat

• Topmost layer of paint OEM

• Acts partially as a UV blocker

• Can add a tinted clearcoat to add color to the system

• If there are two clearcoats and the bottom one contains effect pigments, then the system is a “tri-coat” system

Monocoat System

Basecoat only

Tricoat System

• Clearcoat

• Clearcoat with effect pigments

• Basecoat

Quadcoat System

• Clearcoat

• Tinted transparent basecoat

• Clearcoat

• Basecoat with effect pigment

Architectural Paints

• Coarser textured than automotive paint.

• Tend to be applied in thicker, more uneven layers

Road Paint

• May have glass spheres acting as reflecting agents

• Contains lead

What is the strongest association that can be made?

A physical break match

Thin cross-section

can expose the true color of layers

Thin peels

individual layers that can be used for microscopic comparison

Wedge Cuts

quick way to expose all of the layers for comparison, but they can distort the apparent thickness of layers

Fourier Transform Infrared Spectrometer (FTIR)

This technique works by exposing the sample to a range of IR frequencies, some of which are absorbed by the sample, based on its molecular structure producing a spectra which can then be compared

• can provide a very high discrimination between samples

• precludes detection of many non-organic components

Michelson Interferometer

• A coherent beam of light enters the interferometer.

• The beam is split by the beam splitter.

• The first half of the beam travels to, and is reflected back, by a mirror at a fixed stationary distance.

• The second half of the beam travels to, and is reflected back, by a mirror that constantly moves closer and further back than the distance of the stationary mirror.

• The two halves recombine at the beam splitter and at different times experience constructive and destructive interference (varying by wavelength).

• The re-formed beam travels through the sample and on to the sensor.

Interferogram

When the moving mirror is at the same distance as the stationary mirror, the two halves form constructive interference at all wavelengths, this forms the “center burst” of the interferogram