Paints Midterm

Stain

Solution with a dye or suspension of pigment designed to impart color to wood No binder, no film

Chemical Classifications (Microchem)

Solvents: acetone, chloroform, methylethyl ketone, methylene chloride, toluene Acids: nitric, sulfuric, hydrochloric, acetic Reagents: diphenylamine, LeRosen

Condensation

Step-by-step polymerization Chains may combine directly Two monomers form a covalent bond w/ elimination of a small molecule Most common are polyesters and polyamides EACH combination produces waste

Tacticity

The substituent arrangement around the back bone -Isotactic = all on one side -Syndiotactic = alternating -Atactic = randomly oriented

Polymer types

Thermoplastic, thermosetting and elastomers

Binders

Thermoplastic- softens when heated and hardens when cooled. re-dissolve in suitable solveent Thermosetting- permanent, forms cross-linkages when heated or subjected to UV light, cannot redissolve

Common white pigments

Titanium Dioxide - Rutile TiO2 or Anatase TiO2 ZnO BaSO4 CaCO3 PbCO3

Metamerism

When colors match under one type of illumination, but not when under different kind of lighting

Continuum X-Rays

- "white noise" -not specific to any element -also called Brensstrahlung radiation

Homopolymer v. Copolymer

- Homopolymer = same monomer throughout entire molecule - Copolymer = two or more different monomers, combines the properties of the monomers Alternating: A-B-A-B-A-B Random: A-A-B-A-B-B (most common) Block: A-A-A-B-B-B

Color Perception is affected by..

- Illumination -Angle of observation -Surrounding color -Sample size -Observer

Paint usually consists of....

- Pigments= color and or to modify physical properties -Binder (resin)= binds pigments and the paint film to substrate -Solvent= dissolves binder, not always present -Additives= usually to improve properties

Why use FT-IR?

- To identify binder, pigment, oils, etc -Key for PDQ (Paint Data Query) -Non-destructive

Contents of PDQ

- about 20,000 vehicles with library of over 74,000 layers of paint -from 1973 to now - includes most domestic and foreign cars marketed in North America -samples: 25% from factories, 75% from field - main is FT-IR

PyGC Weaknesses

- can only analyze organics - can complicate sample intro into GC column - biased against larger fragments - destructive

PyGC Strengths

- does not require major additional instrumentation - fast with limited sample prep - can differentiate samples that have indistinguishable IR spec and elemental compositions (SEM-EDS) - small sample size required 5-100 ug

Observer variation

-1 in 12 males and 1 in 250 females are color deficient -most common color deficiency is partial green

Metallic Coats (for OEM)

-1st coat is sprayed in several coats (not in US) -2nd coat is pigmented base metallic coat, then clear coat Flakes vary in morphology from different manufacturers

Specific Reactions

-Acetone dissolves lacquers -LeRosen has a magenta reaction with aromatics -Diphenylamine has a deep ink blue reaction with nitrates

Common Binders

-Acrylic: all -Alkyd: oil based, auto OEM -Polyester: Auto OEM, spray paint -Polyurethane: Auto OEM and refinish, non-pigmented -Epoxy: Auto OEM, architectural, spray paint (withstands heat well) -Vinyl Acetate: architectural -Nitrocellulose: auto refinish, spray paints, tools, nail polish

Types of paint

-Automotice -Vehicular (motorcycles, bikes, boats) -Architectural/Structural -Special Purpose (tools, safes)

2. Electrocoat (EC) Primer (for OEM)

-Build up acts as a resistor to decrease deposition - Controls thickness, allows coating in hard to reach places, first paint coat, aint-corrosion pigments included -Submerge in a charged bath and the body has the opposite charge

FT-IR Microscope

-Cassegrainian objectives -Individual layers -Need very thin sections - 4000-650 cm-1

Color-shifting Pigment (for OEM)

-Chromaflair - aluminum sandwiched between magnesium oxide -metallic flakes are aligned, not randomly oriented -very pricy -looks like different colors @ different angles

Physical features of evidence to note

-Color -Approx amount/size -Texture -Layer sequence -Relative thickness of layers -presence of metallic vs non-metallic -distribution of pigment -weathering characteristics - substrate

Limitations of FT-IR

-Complex mixtures -Minor components are masked -Thin layers are difficult to pass the beam only through the one layer --> increases signal to noise ratio -detection of binder in non-gloss architectural paints and some automotive primers

Cross-linking & effects

-Covalent bonds between neighboring polymer chains -Effects: no or light cross-linking = flexible, softer heavily cross-linked = harder, more brittle

Paint transfer is dependent on...

-Degree of force -Condition of paint (dry, wet, etc) -Duration of contact -Nature of object struck

Solvent

-Dissolves binder, provides consistency -Organic or aqueous -Factors: odor, toxicity, volatility, cost -Not present in dry paint -> evaporates Vehicle = binder + solvent

PDQ Limitations

-Does not contain all vehicles produced, all paint systems on a vehicle, all top coat colors, or batch differences - Not a population database, a representative database -Limited by what is in the database

Backscattered Electrons

-Elastic collisions with specimen atoms -BS electrons have 80% or more of their original energy -generation of backscattered e- shooting e- at something with lots of other e- around it, the e- beam can bounce off an e- "primary e- beam has elastic collisions with e- cloud -have too much energy to be affected by any pull by detectors, so the backscatter detector is directly above the sample -visual indicator of higher and lower atomic numbers -"middle energy"

1. Phosphate process (for OEM)

-Electrogalvanizing with Zinc - corrosion protection and adhesion, sacrificial coat that will react with atmospheric O2 and water instead of the body of metal, very thin -steps: alkaline degreasing, water rinse, zinc phosphate electrogalvanizing, water rinsing, chromic acid rinsing, DI water rinsing, drying in an oven for 10 min @360-400 F

3. Top Coat (for OEM)

-Function is color -composition currently: polyester melamine, acrylic melamine, polyurethane -typically sprayed on, very thin -"flashed off" (curing and removing solvent) before the clear coat is applied

Attenuated Total Reflectance (ATR)

-IR beam enters at the critical angle of total internal reflection -Sample prep is almost non-existent -Small sample size -only good for top and bottom layers.. cannot reach other layer -ATR objective, sample sits on pressure sensor

Affects of Tacticity

-Increasing polymer chains density -> more crystalline -Regular arrangement (isotactic and syndiotactic) increase crystallization -No order (atactic) -> no chain packing, amorphous polymers

Pigments

-Insoluble, fine powder -Can be organic, inorganic, or metallic -Extender pigment= (inorganic) to modify gloss, texture, viscosity, and reduce cost (much cheaper!!)

How does PyGC work?

-Large molecules break at their weakest points producing smaller more volatile fragments capable of analysis by GC -Transfer heat to the sample as instantaneously and consistently as possible -Pyrolysis chamber should have a small internal volume and a rapid carrier flow -The sample size should be small and consistent

Thermoplastic Polymers

-Long, linear or lightly branched chains -No primary valence bonding between chains -Weak secondary valence bonding between chains in close proximity (H-bonding) -Has the ability to go back to amorphous shape and reform -> recyclables -Polyethylene, polypropylene, polystyrene, polyvinyl chloride (PVC), polyamide

Thermosetting Cross-Linking Agents

-Melamine (most common) -Urea -Epoxy -Styrene -Phenol-formaldehyde

Pearlescent paints (for OEM)

-Mica flakes sandwiched between TiO2 or iron oxide -reflecting light undergoes interference giving off sparkles of different colors

SEM Vacuum System

-Need a mean free path for the electron beam -Vacuum greater than 5x10-5 torr -Typically utilizes both a rough pump and a diffusion or turbo molecular pump -Reasons: Filament will oxidize, column must be clear of particles so that e beam doesn't interact with anything else, and moisture condensing of "cold finger" - add liquid N to keep detector cold

SEM Coating

-Non-conductive samples "charge", which leads to electric discharge, beam deflection, cracking, overheating, sample loss -coat with a thick layer of carbon = non conductive samples collect e and don't know where to go - Charging -to solve, coat with conductive material, carbon or gold

Factors that affect polymer configuration

-Order or arrangement of polymer chain -amorphous v. crystalline -Homopolymer v. copolymerization -Tg (glass transition temp - where glass softens) -Flexibility -Adhesion -Tacticity -Impact resistance, etc

Marine/Boat Paint

-Resist salt water and atmospheric conditions -Can vary on the boat depending on location/purpose of boat (above deck/below water) -bottom paint - usually a primer and barrier coat, controlled depletion polymer (slowly dissolved and released) prevents growth on the boat -gel coats: pigment filled resins with additives, all polyester based barrier coat -> gel coat ->fiberglass with polyester resin

Thermosetting Polymers

-Shorter polymer chains react with other short polymers -3D structure with cross links (primary bonds) -Can't be broken without permanently destroying the structure -Most automotive paints -Irreversibly hanged with heat or UV light -Excellent thermal stability and rigidity -Melamine and urea act as cross linkers in these polymers -Phenolic resins, epoxy resins, polyurethane, melamine-formaldehyde resins

Xirallic (for OEM)

-Similar to pearlescent -Produces interference colors like pearlescent pigments do -Less satiny, more flashy - aluminum oxide sandwiched between a metal oxide

Specialty Finishes

-Spray paints - typical binders include alkyd enamels, acrylic lacquers, urethanes, epoxies -Wrinkle finishes - safes, appliances, filing cabients, often nitrocellulose lacquers -reflective paint - street signs, license plates, glass beads and foil -roof coatings - aluminum, acrylic, bitumen

Pearlescent Pigments

-TiO2 or Fe2O3 coating on mica or bismuth oxychloride -High refractive index coating on transparent particles -Mimic the luster of pearls -Interference colors formed when light waves reflected off different and multiple surfaces

2nd Primer Surfacer (for OEM)

-Typically sprayed -Solvent based, polyester/melamine or acrylic/melamine -Powder coat, epoxy or epoxy isocyanate -Function is to full surface imperfections and increase adhesion to color coat -Thickness is 35-50 um -Anti chip coating, applied over e-coat to lower areas of the car, typically a urethane resin

Collection of Known (K) Paint

-Very close to but outside of the damaged area on the SAME PANEL (door, bumper, etc) - Sample size ~ nickel - All the way down to the substrate

Cycle Paints

-Very similar to automotive paints -motorcycle paints use same binder systems as automotive -bikes: alkyd enamels, acrylic lacquers, alkyd-urea enamels (not in auto)

Electron Gun Assembly - SEM scope

-Wehnelt cylinder/tungsten hairpin filament - cylinder has neg potential, e gits the bend and pops off and wont be attracted to the cap -thermionic emission, anode plate, accelerating voltage -accelerating voltage controls how fast the e go, everything else controlling the beam. scanning coil moves back and forth to move the beam back and forth ALL INSIDE OF COLUMN

Redundant apeturing

-beam goes through the aperture (you pick size) through sample then through another aperture -good because scattering can go through another layer and you don't want that, second aperture ensures you only get that one layer

To exclude, look for difference in...

-color -texture -layer sequence -presence of metallic flakes -size and distribution of pigments

MSP Limitations

-cross sections must be very thin and flat to achieve transmission -some paint pigments can only be analyzed by reflectance -sample conditions like UV bleaching, damage, environmental conditions -featureless spectra with very wide and weak abs. bands

Powder Coating (for OEM)

-finely ground solid consisting of a mixture of ingredients in a resin base. solids melt, flow and coalesce into a film with heat -environmentally good - no solvent -thermoplastic resin - charged, has pigment -epoxy or epoxy isocyanate -strong finish but not always pretty

FT-IR Reflectance

-first surface mirror -cross section cut and rolled flat -resolution and sensitivity not as good in as transition mode -want sample on top a silver slide

Diamond compression cell

-for FT-IR -creating a thin pathway for the beam -certain frequency is absorbed, others will just pass through detector -set compensator to 1 -KBr background - won't absorb in this region -destructive and constructive interferences

How does SEM magnification work?

-generating e beam fired @ sample and will scan across the sample feeding signals back to the detector as it goes -pick a section to view and change the magnification by zooming in

X-Ray Emission

-goes hand in hand with secondary electrons -will shift e around to fill the vacancy, and e with more energy moves, and releases energy difference as x-ray emission -K shell is 1, L =2, M =3, size of orbital jumps are 1 =alpha, 2=beta, 3=gamma - highest energy

SEM

-high magnification (over 300,000X) -high res (~10nm) -large depth of field with 3D image -with energy dispersive spectrometer (EDS) provides elemental info

Secondary Electrons

-inelastic collision -energy from the primary beam is transferred to an electron which is then ejected -the closer an e is to the nucleus, the less energy it has -primary e beam ejects an e from the e cloud this is called the secondary e -shows surface texture and topography -lowest energy -detector --> has positive bias and attracts low energy electrons to it

Basic Components of PyGC

-inert carrier gas -sample holder: often a quartz tube or ceramic cup -pyrolysis heating device: 3 types -GC inlet

Microspectrophotometry (MSP)

-microscope with a device to measure the intensity of each wavelength of light (UV, vis, and NIR) -light source for visible - halogen or tungsten -light source for UV- xenon or mercury -for UV, must have mirrored or quartz objectives, quartz slides and coverslips, mounting media cannot be UV absorbing -absorbance v. wavelength

PDQ Participation Requirements

-must submit 60 samples a year -PDQ and KnowItAll software -VINAssist Software -munsell color books

MSP Advantages

-non-destructive -UV vis NIR can see beyond the naked eye -more objective comparison -little sample prep -can distinguish metameric samples -can analyze very small samples

Microchemical Testing

-observe and record observations immediately and for duration of test -K and Q samples should be side by side -Most reactions are non specific -> dissolving, swelling, color changes, effervescence (bubbling), bleeding, layer splitting

FT-IR sample prep

-sample is ground up with KBr -single layer -Bulk samples -Advantage of DTGS detector is 4000-400 cm-1

SEM Sample Prep

-smooth cross sections -Legible and well diagrammed drawings of samples -mount towards center of stub area - aluminum stub with sticky carbon tape on it -provide background spectra -mount paint cross sections parallel to each other, ideally oriented the same way -detectors have beryllium windows, we ignore C and Be

Submissions of evidence

-submit the entire tool or piece of possible -collect at least the size of a nickel -collect down to substrate with all layers

Pyrolysis

-the thermally driven decomposition of large organic macromolecules into characteristic fragments -must be in the absence of oxygen to avoid combustion

Observer based factors that effect color vision...

-tiredness, hunger, age, stress, medication, disease, UV exposure/damage

Tool Paint

-typical binders: melamine, epoxy, nitrocellulose -application is typically a powder coat -challenge is that usually alot of paint samples from alot of things the tool was used for

Plastic Body Parts (for OEM)

-usually color matched to the body -no baking bc it will melt plastic -paint must be more flexible over plastic

Latex Paints

-water based paints -traditionally poorer application properties -additives: thickeners, anti-foam agents -more susceptible to mold b/c it contains water

Bike Paint

-wet look, very shiny and glossy, may contain solid silver or gold layers, can have up to 20 layers that are tinted or colorless, clear layers -sprayed or powder coat finishes

4. Clear Coat (for OEM)

-wet on wet in two stages, usually pretty thick -functions are for shine, abrasion protection and good weather durability -baked to crosslink one layer --> flash off --> second layer -Comp: us is alkyd or acrylic, Europe is urethane, BMW is epoxy or acrylic -application is usually HVLP electrostatic spray

Diana Wright Study

-white paint as an exception to being able to distinguish? -FTIR discrimination -94.5 % -entire analytical scheme - 99.35%

MSP data collection process

1. Autoset optimize - sets integration time 2. Collect dark scan - noise of detector and stray light 3. Reference scan - "background" from slide, light source, etc 4. Sample scan - background is subtracted out

SEM Electron Interactions

1. Backscattered electrons 2. Secondary electrons 3. X-ray Emission

PyGC Heating Methods

1. Isothermal Furnace (microfurnace) 2. Inductive Heating Coil - Curie Point Pyrolysis 3. Resistive Heating using Platinum Filaments (most common for forensics) - needs to be in contact with the samples

Performing a PDQ Search

1. Paint Sample Analysis - physical analysis, OEM vs. repaint 2. Spectral coding 3. Enter parameters search 4. Search layer system query 5. Search refinish books 6. Refer to automotive new and report publications

Tippett, Emerson, Fereday study...

2000 structural paints sampled, 98% distinguished using only microscopy and microchemical tests

Latex

A suspension of pigment in a water-based emulsion of any of several resins

Metallic Pigments

Aluminum flakes (usually 0.1-2 um think and 5-200 um in diameter) leafing vs. nonleafing (random) Copper flakes Zinc Nickel

Black Pigments

Carbon black Petroleum black Black iron oxide Bone black (from charred bone)

Addition

Chain-growth polymerization 100% of monomer is used in reaction, monomer usually has a double bond Relatively fast, high heat formed Three Steps- initiation, propagation, termination Ex. polypropylene, polystyrene

Varnish

Clear solution of drying oils and resin in organic solvents, usually used to protect wood

Oil-Based Paints

Common binders are alkyd enamels, urethane enamel, acrylic lacquers, nitrocellulose lacquers typically contains pigments, drier solvents (mineral spirits or other org solvent)

The two ways to form polymers

Condensation and addition

Addition Termination Step

Coupling - most common, two growing chains find each other and can no longer grow once combined OR add a free radical to cap off the end of the chain

Crystalline v. Amorphous

Crystalline - regions make the material rigid, strong, and brittle upon impact Amorphous - regions provide flexibility

MSP QA/QC Procedures

Day of use: wavelength check with holmium oxide and didymium filters -photometric accuracy check, neutral density filters at 0.1, 0.5 and 1 (blocks all wavelengths equally) -filters have NIST certified values

Oil Classifications

Drying - unsaturated fatty acids, metallic driers added to speed up curing time (film formation) Semi-drying - mixed fatty acids, safflower, soya (not as rigid) Non-drying - saturated fatty acids, never cures completely in the air. heat and catalyst to fully cure (no cross-linking), flexible

Dye vs. Pigment

Dye - soluble in the binder and light can go through it Pigment - insoluble, powder disperses through binder, solid particles that light cannot pass through

Lacquer

Fast drying, clear or pigmented, forms a film by solvent evaporation and not by oxidation or polymerization (thermoplastic)

Millbase Step 3 - Filtering

Filter out impurities, paint is poured into drums or cans for distribution

Addition Propagation Step

Free radical causes monomer to radicalize, which attacks another monomer to lengthen the chain

Addition Initiation Step

Free radicals are most common initiators for what we do Done by heat or UV light

Millbase Step 1

High Speed Dispenser - serrated disk blender revolves at high speeds (100-2000 rpm) Ball Mill - horizontal, cylindrical vessel containing steel or porcelain balls, rotates for 6-48 hours Sand Mill/Bead Mill- vibrating vessel containing sand or hard beads

Column of SEM Scope

Includes: -Condenser lens -Stigmators (makes the beam have a round cross sectional shape for resolution) -Scanning coils -Objective lens

Polymer structures

Linear, branched, or cross-linked

Millbase Step 2 - Blending

Mixture transferred to another vessel More resin, additives, solvents, and tinters are added

Automotive Finishes (OEM)

Original equipment manufacturing 1. pretreatment 2. primer (atleast one) 3. basecoat 4. clearcoat

Latex Binders

PVA Acrylics (Polyvinyl acetate) - most common, medium priced PVA Ethylenes - more expensive Styrene Butadiene - lowest cost Acrylics - methyl methacrylate, thermoset or thermoplastic, more expensive than PVA-acrylics, durable/weather resistant (can be in oil based or latex paints)

Fracture match

Paint fragments of sufficient size are evaluated, the only CONCLUSIVE paint association

Millbase Manufacture

Physical process of pigment dispersion, objective is to produce a stable, fine dispersion, pigment in minimum of solvent and resin called millbase, liquid with very concentrated pigment in binder and solvent

Enamel

Pigmented coating, dries to a hard gloss (thermosetting)

Paint definition

Pigmented liquid converted to an opaque solid film after application "Surface coating" (forensic definition) - liquid that forms a protective and/or decorative coating when dried

Additives

Plasticizers - increase flexibility, weathering and impact resistance, usually phthalates Driers- accelerates hardening/curing, metallic driers like Co, Mn, Fe, Ca -Anti-foaming, anti-mildew, anti-skinning, freeze/thaw stabilizers, anti-sag, thickening

Common Extender pigments

Quartz (SiO2) Diatomaceous earth Mica Limestone (mainly CaCO3) Clay Talc BaSO4

Gonioappearance (Geometric Metamerism)

Samples that match at one angle of illumination but not when that angle of illumination or viewing angle is changed