Paints Flashcards
Chemical composition of glass types:
- Soda Lime
- Borosilicate
- Lead Silicate
Soda Lime glass:
Made by melting silica from sand, soda ash, limestone, magnesium oxide, aluminum oxide, potassium oxide, barium oxide
Borosilicate glass:
Contains boric oxide and silica; excellent thermal and shock resistant; made into headlamps and cookware
Lead Silicate glass:
Contains lead oxide and silica; used in a range of optical, crystal and electrical glass products.
Glass manufacture process:
- Storage , weighing and mixing raw materials.
- Melting of raw materials; refining and homogenizing
- Forming the melt into required shape
- Annealing of the glass
- Warehousing and secondary processing
Transfer and persistence of breaking glass:
- Minute fragments transferred to breaker
- Called backscatter or “backward fragmentation”
- Secondary transfer occur in small percentages
Factors that effect transfer of glass fragments :
- Manner glass was broken
- Type and amount of glass broken
- Distance between glass and breaking glass
Factors that effect persistence of glass transfer:
- Type of clothing worn
- Length of time from break and seizure of clothing
- Activity of wearer
- Whether clothing was damp
The Becke Line:
is a “halo” that can be seen on the inside of the glass on the left, indicating the glass has a higher refractive index than the liquid medium. The Becke Line as seen on the right on the outside of the glass, indicating just the opposite.
Quality of measurements:
Edge- edge tracings and edge count
Edge- Edge count ranges 1-99
“Contrast”- contrast of Becke Line depends on edge morphology of glass fragment and instrumentation & debris contamination.
“Control”- control samples usually display large edge counts and less variation then recovered fragments.
Elemental analysis of glass serve two purposes:
-Classification according to end use (from window or container)
Based on composition of major elements
- Discrimination; differentiate glass with same end use, example: glass
from 2 different windows; relies on minor or trace elements
Elemental Techniques:
- X ray florescence spectrometry
- Micro ZRF
- Energy dispersive X-ray spectroscopy
- Inductively coupled plasma mass spectrometry (dissolution of sample)
- Laser ablation ICP-MS (solid sample)
Reasons why substrates are painted:
For protection and decoration
Resin or Binder System (paint):
“Resin portion” of paint system is part of holds everything together and imparts most of the physical characteristics as well as curing mechanism and durability qualities to final coating.
Curing mechanism (paint system):
Way the wet paint or powder particle becomes a solid continuous film (done by polymerization or evaporation)
Resin Solution Coating Types:
Solvent borne - made fluid with use of solvent
Waterborne - some or all of solvent is water
Emulsion Coating (latex)- resin in the form of particles dispersed in immiscible liquid
Powder Coating- consist of epoxy, polyester or acrylic resin systems;
can be cured by heat, exposure to UV
Solvents:
- adjust paint for environmental conditions to be used in
- referred to as thinner or reducer
- ability of paint to remain on vertical surface without dripping (evaporating)
- solubilize resin and adjust viscosity of paint
- permits ease of application to service w/no protection qualities
Additives:
- Enhance performance of paint
- improve durability of dried paint
- contain chemicals to prevent mold, mildew, fungus and algae growth
- can be seen as treatment for issues encountered with wet paint or
treatment for environmental issues encountered in dry film
Solvents in architectural paints:
“Solvent thinned coatings” -based on resins soluble in organic solvents
“Water thinned coatings” - latex
Interior paints come in:
Gloss, semi-gloss and flat
Gloss finish:
Hard, easily cleaned and reflects light
Binder to pigment ratio is higher than in semi-gloss
Adhesion of subsequent coating minimal
Semi Gloss:
Binder to pigment ratio higher than flat paint
More pigment than gloss
Silky appearance
Flat finish:
No gloss
Adhesion is good between flat film and successive coating, no need for primer
Auto paint layers:
“Primer”- closest to substrate
“Base coat”- layer that contain color and effect pigments
“Clear coat”- applied over wet or uncured coat
Assume re-coating done factory when:
Layer system consists of series of base coats and clear coats with no additional primer
Auto is painted aftermarket when:
There is “primer” over base coat or clear coat.
Optical examination (auto paint):
Stereo-microscopy and microscopy - morphology of paint *surface characteristics, number of layers, size and appearance to
specific type of paint; use of episcopic illumination allows observation of
effect pigments and of surface defects space as striae or scratches.
Analysis of auto paint:
Bright field- enhances contrast of red layers
Dark field- enhances color contrast between opaque layers
Crossed polarizers- allow observation of birefringent material
UV- allows good observation of clear coat layers based on flourescence
IR Spectrometry:
Technique of choice for organic and inorganic components of paint system; fast, repeatable, “semi- destructive”, high discriminating power and existence of databases
FTIR:
Provides molecular structure information on organic and inorganic components of “coating”
Raman Spectroscopy:
Disadvantage: Possible florescence of sample that will mask Raman spectrum
Advantage: no sample prep, high spatial resolution
Elementary Analysis: SEM/EDS X-ray
Most commonly used in forensic labs
Provides indirect ID of inorganic content of paint
Technique will characterize extenders and “inorganic” pigments
Rapid analysis and sensitive, complimentary as it analyzes inorganic component
Comparison purposes in qualitative or semi quantitative modes
Level of Association:
Level 1- physical match of sample (paint came from source).
Level 2- high degree of association (unusual characteristics)
Level 3- conventional association (nothing unusual in population)
Level 4- limited association (only certain aspects of paint could be examined and information from such a sample would be limited).
Level 5- inconclusive (similarities but can not be associated)
Level 6- Elimination/exclusion (fundamental difference)
