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
