1. A primer course on natural vs plastic and ways to distinguish
Fibers Analysis
A primer course on natural vs plastic and ways to distinguish

2. OBJECTIVES Be able to identify the types of fibers
Be able to define fibers
Be able to understand importance of Fiber Transfer
Be able to differentiate between types of Fiber Transfer
Be able to understand the evidential value of fibers.
Be able to define the methods of examination.
Be able to understand the Microscopy Technique
Be able to understand the TLC Technique
Be able to understand the Pyrolysis Gas Chromatography Technique

3. DEFINITION OF FIBER
A fiber is the smallest unit of a textile material.
Fibers can occur naturally as plant and animal fibers, but they can also be man-made.
A fiber can be spun with other fibers to form a yarn that can be woven or knitted to form a fabric.
Fibers are usually collected from clothing, carpeting, furniture, beds, and blankets.

4. Types of Fibers
Natural fibers are derived in whole from animal or plant sources.
- Examples: wool, mohair, cashmere, furs, and cotton.
Man-made fibers are manufactured.
- Regenerated fibers are manufactured from natural raw materials and include rayon, acetate, and triacetate
- Synthetic fibers are produced solely from synthetic chemicals and include nylons, polyesters, and acrylics.
Polymers, or macromolecules, are synthetic fibers composed of a large number of atoms arranged in repeating units known as monomers.

5. Types of Fibers
Many different natural fibers originating from plants and animals are used in the production of fabric.
Cotton fibers are the plant fibers most commonly used in textile materials
The animal fiber most frequently used in the production of textile materials is wool, and the most common wool fibers originate from sheep.

6. Cross-section of a man-made fiber
Types of Fibers
Man-made fibers can be defined as a fiber of a particular chemical composition that has been manufactured into a particular shape and size, contains a certain amount of various additives, and has been processed in a particular way
More than half of all fibers used in the production of textile materials are man-made.
Some man-made fibers originate from natural materials such as cotton or wood; others originate from synthetic materials.
Cross-section of a man-made fiber

7. FIBER TRANSFER
The type and length of fiber, the type of spinning method, and the type of fabric construction all affect the transfer of fibers and the significance of fiber associations.
This becomes very important when there is a possibility of fiber transfer between a suspect and a victim during the commission of a crime.
The discovery of cross transfers and multiple fiber transfers between the suspect's clothing and the victim's clothing dramatically increases the probability that these two individuals had physical contact.

8. FIBER TRANSFER
When fibers are transferred from the fabric of an individual's clothing to the clothing of another individual, it is called a primary transfer.
As these same fibers are transferred to other objects during subsequent contacts, secondary transfers are occurring.

9. Fiber Evidence
The quality of the fiber evidence depends on the ability of the analysts to identify the origin of the fiber or at least be able to narrow the possibilities to a limited number of sources.
Obviously, if the examiner is presented with fabrics that can be exactly fitted together at their torn edges, it is a virtual certainty that the fabrics were of common source.
Matching dyed synthetic fibers or dyed natural fibers on the clothing of a victim to fibers on a suspect’s clothing can be very helpful to an investigation, whereas the matching of common fibers such as white cotton or blue denim cotton would be less helpful.

10. Fiber Evidence
Microscopic comparisons between questioned and standard or reference fibers are initially undertaken for color and diameter characteristics, using a comparison microscope.
Other morphological features that could be important in comparing fibers are:
- Lengthwise striations on the surface of the fiber
- The presence of delustering particles that reduce shine
- The cross-sectional shape of the fiber
Compositional differences may exist in the dyes that were applied to the fibers during the manufacturing process.

11. Methods For Fiber Comparison
The visible light microspectrophotometer is a convenient way for analysts to compare the colors of fibers through spectral patterns.
A more detailed analysis of the fiber’s dye composition can be obtained through a chromatography separation.
Infrared spectrophotometry is a rapid and reliable method for identifying the generic class of fibers, as does the polarizing microscope.
Depending on the class of fiber, each polarized plane of light will have a characteristic index of refraction.

12. Microscopy Microscopic View:
Microscopic examination provides the quickest, most accurate, and least destructive means of determining the microscopic characteristics and polymer type of textile fibers.
Microscopic View:
Acetate
Dacron

13. Stereomicroscope Should be used first to examine fibers.
Physical features such as crimp, length, color, relative diameter, luster, apparent cross section, damage, and adhering debris should be noted.
Fibers are then tentatively classified into broad groups such as synthetic, natural, or inorganic.

14. Comparison Microscope
If all of the characteristics are the same under the stereoscope, then the comparison microscope is used.
A point-by-point and side-by-side comparison provides the most discriminating method of determining if two or more fibers are consistent with originating from the same source.
Comparisons should be made
under the same illumination
conditions at the same magnifications.
This requires color balancing
the light sources.
A balanced neutral background
color is optimal.
Side-by-side Comparison
Bright Field Adjustment

15. Polarized Light Microscope
Perhaps the most versatile of all microscopes; allows the analyst to actually see and manipulate the sample of interest.
Refractive indices, birefringence, and dispersion can all be quantitatively determined.

16. Microspectrophotometry
Using a grating spectrometer, light absorbed by or reflected from a sample is separated into its component wavelengths, and intensity at each wavelength plotted.
Microscope linked to a
Spectrophotometer
- IR Absorption spectrum
- UV/VIS Absorption Spectrum
Microscope - FTIR

17. Microspectrophotometry
IR spectography identifies generic subtypes indistinguishable by microscopic exam .
Use of IR microscopes coupled with Fourier transform infrared (FTIR) spectrometers has greatly simplified the IR analysis of single fibers.
Advantages:
Nondestructive
Not limited to sample size
Disadvantages
Reactive dyes
Chemical composition
Tentative identification

18. Thin-Layer Chromatography
An inexpensive, simple, well-documented technique that can be used (under certain conditions) to complement the use of visible spectroscopy in comparisons of fiber colorants.
Dye components are separated by their differential migration caused by a mobile phase flowing through a porous, adsorptive medium.
Should be considered for single-fiber comparisons only when it is not possible to discriminate between the fibers of interest using other techniques, such as comparison microscopy and microspectrophotometry in the visible range

19. Thin-Layer Chromatography
Technique
Extraction of dyes
Solid stationary phase
Liquid moving phase
Capillary action
Chromatogram
Interpretation
Rf (retention factor)
Color
Proportions
Scanning densitometer
Fluorescence
Analysis of Chromatograms
Positive association
Exclusion
Inconclusive

20. Pyrolysis Gas Chromatography
Pyrolysis is a destructive analytical method.
When the heat energy applied to the polymer chains is greater than the energy of specific bonds in that polymer chain, these bonds will fragment.
In PGC, the fragments generated by pyrolysis are introduced into a gas chromatograph for separation and characterization
PGC can be used to ID the generic type of an unknown fiber, and in some cases it can ID subclasses within a generic class

21. Analysis of Fibrous Materials
The analyst should perform a combination of methods that extract the greatest potential for discrimination between samples.
A minimum of 2 of the analytical techniques must be performed for each category.

22. Chemical analyses that can be done on fibers and plastics.
Melting temperature
Density - liquid gradients
Resistance to solvents
Refractive index
IR spectrum

23. Notable plastics - Polyethylene
Probably most common plastic – glad bags and packing material, children’s toys – thermoplastic
Simple formula:
Not quite amorphous!
Glass transition -130 to -80 C
Melting point 130 C
Tensile yield (strength) 25 MPa
Tensile modulus (stiffness) 1 GPa (soft)
Density 0.95

24. Notable plastics - Acrylic
Most common optical plastic - refractive index very close to glass (1.5), aka Plexiglas, Lucite
Full name polymethyl methacrylate (PMMA).
Also an important fiber, paint.
Glass transition 110 C
Melting point 130 C
Tensile yield 50 MPa
Tensile modulus 30 GPa
Density 1.15

25. High performance plastics
Epoxy resin is made from the 2-part kits.
It’s the basis of composites like fiberglass, carbon fiber composites etc.
Apart from an excellent glue, it is an important molding compound for rapid prototyping.
Tensile strength 60 MPa
Stiffness 2.6 GPa

26. High performance plastics
Kevlar is an aramid polymer:
Chains are stiff and straight.
Highly crystalline polymer, difficult to process.
Melting temperature 500 C
Tensile strength 3.6 GPa, about 4x steel!

27. Notable plastics – contd.
ABS – popular construction thermoplastic, used in FDM machines.
PVC – plumbing pipes, electrical insulation.
Nylon – most important fiber.
Polyester – 70s disco clothing – plastic bottles.
Polystyrene – computer housings, toys, also made into foam (Styrofoam).
Polycarbonate – strong, refractive index > glass, eyeglass material. A thermoset plastic.
Cellulose – natural wood fiber.

28. High performance plastics
PTFE – Polytetrafluoroethylene – aka Teflon long name, simple structure:
Exceptional resistance to solvents, great lubricant, nothing sticks to it!
The fluorine-carbon bonds are very strong, fluorines protect carbon backbone.
High melting point 330 C
High electrical breakdown – artificial muscle.
Technically a thermoplastic, but hard to process.

29. Determining the index of refraction of a plastic or fiber The Becke Line method
is a technique in optical mineralogy that helps determine the relative refractive index of two materials. It is done by lowering the stage (increasing the focal distance) of the petrographic microscope and observing which direction the light appears to 'move' toward. This movement will always go into the material of higher refractive index

30. Becke Lines
Becke Lines are a band or rim of light visible along the grain boundary in plane light when the grain mount is slightly out of focus.
The Becke line may lie inside or outside the material depending on how the microscope is focused
Becke lines are interpreted to be produced as a result of:
the lens effect and/or
internal reflection effect.

31. Becke Lines

32. fibers can act either as diverging lenses or converging lenses.
Lens Effect
Most fibers are thinner at their edges than in the middle, i.e. they have a lens shape and as such they act as a lens.
fibers can act either as diverging lenses or converging lenses.
Light is refracted away from the normal on passing into a material with a higher index of refraction and away from the mineral when passing into a material with a lower index of refraction as predicted by Snell’s Law

33. Diverging Lens Effect
If nmin < noil , the grain acts as a diverging lens, and light is concentrated in the immersion oil.

34. Converging Lens Effect
If nmin > noil the grain acts as a converging lens, concentrating light towards the centre of the grain.

35. Internal Reflection Effect
This hypothesis to explain why Becke Lines form, requires that grain edges be vertical, which in a normal thin section most grain edges are believed to be more or less vertical.
With the converging light hitting the vertical grain boundary, the light is either refracted or internally reflected, depending on angles of incidence and indices of refraction.
The combined result of refraction and internal reflection concentrates light into a thin band in the material of higher refractive index.

36. nmineral > noil
Rays 1 and 4 are refracted into the mineral. Rays 2 and 3 strike the vertical mineral-oil boundary at greater than the critical angle and are internally reflected back into the mineral.
The Becke Line is formed by the concentration of light inside the mineral grain

37. nmineral < noil
Rays 1 and 4 strike the vertical mineral-oil boundary at greater than the critical angle and are reflected back into the oil. Rays 2 and 3 are refracted into the oil.
The Becke Line is formed by the concentration of light outside the mineral grain

38. Measuring Becke Line Movement
To observe the Becke line:
use medium or high power,
The direction of movement of the Becke Line is determined by lowering the stage with the Becke Line always moving into the material with the higher refractive index.
The Becke Line can be considered to form from a cone of light that extends upwards from the edge of the mineral grain.

39. Direction of Becke Line Movement
Movement of the Becke line as the stage is lowered. The becke line may be considered to consist of a cone of light that extends upwards from the mineral grain.
If nmin < noil, the cone diverges upwards and if nmin > noil the cone converges upwards. If the stage is lowered, the plane of focus goes from F1 to F2 and the Becke Line appears to move towards the material of the higher refractive index.

40. Becke line in fibers

41. Compare between a series of different refractive index liquids