Question

Describe the best process to analyze and attempt to identify the chemical components of fiber and paint evidence.

Answer 1

Dear student

Fiber Identification Methods

There are different types of textile fibers used in the apparels and also the types and numbering of yarns.Therefore it is necessary to learn the different methods to identify different textile fibers.

The knowledge of identifying the textile fibers helps a producer of garments to identify the type of fiber and the care to be taken in maintaining the fabrics made of a particular type of fiber. This is an important factor for labeling of the garments, which includes specifying the fiber content in the garment.

There are different tests which could be used for the identification of the textile fibers such as burning test, microscopic test, solubility test, optical test, density test etc. We would be discussing the basic tests like burning, feeling, microscopic and chemical tests of identifying the textile fibers.

Nontechnical testing does not require any special equipment or setting for identification of fiber. The different nontechnical methods of identifying the textile fibers are:

Feeling Test

Burning Test

These tests are useful and simple to perform and also provide ready identification under certain circumstances.

Microscopic Identification: Positive identification of many natural fibers is possible using the microscope

Solubility: The chemical structure of polymers in a fiber determines the fiber’s basic solubility characteristics

Heating and Burning Characteristics: The reaction of fibers to heat from an open flame is a useful guide in the identification of fibers.

Density or Specific Gravity: Fiber density may be used as an aid in fiber identification

Staining: Fibers have differing dyeing characteristics and affinities dependent on the chemical and morphological structure of the fiber.

Structural, Physical and Chemical Characterization

A number of methods are available for characterization of the structural, physical, and chemical properties of fibers.

Optical and Electron Microscopy: Optical microscopy (OM) has been used for many years as a reliable method to determine the gross morphology of a fiber in longitudinal as well as cross-sectional views.

Elemental and End-Group Analysis: The qualitative and quantitative analysis of the chemical elements and groups in a fiber may aid in identification and characterization of a fiber

Infrared Spectroscopy: Infrared spectroscopy is a valuable tool in the determination of functional groups within a fiber

Ultraviolet-Visible Spectroscopy: The ultraviolet-visible spectra of fibers, dyes, and finishes can provide clues concerning the structure of these materials, as well as show the nature of electronic transitions that occur within the material as light is absorbed in various wavelengths by unsaturated groups giving an
electronically excited molecule.

Nuclear Magnetic Resonance Spectroscopy:Nuclear magnetic resonance (NMR) spectroscopy measures the relative magnitude and direction (moment) of spin orientation of the nucleus of the individual atoms within a polymer from a fiber in solution in a high-intensity magnetic field

X-Ray Diffraction: X-rays diffracted from or reflected off of crystalline or semicrystalline polymeric materials will give patterns related to the crystalline and amorphous areas within a fiber

Thermal Analysis: Physical and chemical changes in fibers may be investigated by measuring changes in selected properties as small samples of fiber are heated at a steady rate over a given temperature range in an inert atmosphere such as nitrogen.

Molecular Weight Determination: Molecular weight determination methods provide information concerning the average size and distribution of individual polymer molecules making up a fiber

Mechanical and Tensile Property Measurements:Mechanical and tensile measurements for fibers include tenacity or tensile strength, elongation at break, recovery from limited elongation, stiffness (relative force required to bend the fiber), and recovery from bending.

Specific Gravity: The specific gravity of a fiber is a measure of its density in relation to the density of the same volume of water, and provides a method to
relate the mass per unit volume of a given fiber to that of other fibers

Environmental Properties: Environmental properties include those physical properties which relate to the environment in which a fiber is found. Moisture regain, solvent solubility, heat conductivity, the physical effect of heat, and the electrical properties depend on the environmental conditions surrounding
the fiber

Chemical Properties: The chemical properties of fibers include the effects of chemical agents including acids, bases, oxidizing agents, reducing agents, and biological agents such as molds and mildews on the fiber and light- and heat-induced chemical changes within the fiber

Paint Analysis

Sometimes forensic scientists need to analyze a paint sample -- for example, if a paint chip is found on the body of a hit-and-run victim and investigators are trying to match it to a make and model of car.

First, the scientists look at the appearance of the sample -- its color, thickness and texture. They examine the sample under a polarized light microscope to view its different layers. Then they can use one of several tests to analyze the sample:

Fourier transform infrared (FTIR) spectrometry determines the type of paint (chemicals, pigments, etc.) by analyzing the way in which its various components absorb infrared light.

Solvent tests expose the paint sample to various chemicals to look for reactions such as swelling, softening, curling and color changes.

Pyrolysis gas chromatography/mass spectrometry helps distinguish paints that have the same color, but a different chemical composition. The paint sample is heated until it breaks into fragments, and then is separated into its various components.

Regards

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