Notes on Textile Fibers

Introduction to Textile Fibers

• Fibers from textiles are continuously shed and transferred, serving as common forensic evidence.

• Often treated as undervalued but hold significant qualitative and quantitative comparisons.

Definition of Textile Fibers

• Textile fiber: Basic element of fabrics, either natural or manufactured.

  • Natural Fibers: Exist in their natural state (e.g., cotton, wool).

  • Manufactured Fibers: Produced through manufacturing from non-fiber substances.

Classification of Textile Fibers

• By Chemical Makeup:

  • Protein Fibers: Composed of amino acids (e.g., wool).

  • Cellulosic Fibers: Derived from carbohydrates (e.g., cotton).

  • Mineral Fibers: Inorganic, composed of silica (e.g., asbestos).

  • Synthetic Fibers: Made from polymers from organic molecules (e.g., nylon, polyester).

Textile Fiber Identification Act (TFIA)

• Generic names of fibers established by the TFIA (1954) include:

  • Acetate: Fiber-forming substance is cellulose acetate.

  • Acrylic: Composed of at least 85% acrylonitrile.

  • Nylon: Composed of synthetic polyamide.

  • Others include Lyocell, Modal, Spandex, etc.

Physical Properties of Fibers

• Diameter: Generally between 0.0004–0.002 inch (10-50 µm).

• Fiber length ranges from 7/8 inch to several miles:

  • Filaments: Indefinite lengths (e.g., silk).

  • Staple Fibers: Typically cut lengths (7/8 to 8 inches).

Measurement Units

• Natural fibers measured in micrometers.

• Silk/manufactured fibers measured in denier (grams of 9000 m) or tex (grams of 1000 m).

Yarn Formation

• Yarn: Continuous strands suitable for textiles, twisted (S or Z twist).

• Plied Yarn: Made from smaller single yarns twisted together.

Fabric Construction

• Fabric Types: Defined by assembly methods:

  • Woven Fabrics: Interlace yarns in warp (lengthwise) and weft (crosswise).

  • Knitted Fabrics: Interlocking loops forming courses (width) and wales (length).

  • Nonwoven Fabrics: Mechanical interlocking or bonding methods (e.g., felt).

Fiber Characteristics

• Identification correlated with shape, crimp, color, cross-section:

  • Crimp: Waviness measured per unit length.

  • Color: Influenced by dyes (manufactured) or naturally occurring (natural fibers).

  • Cross-sectional Shape: Critical for fiber analysis; variability over 500 shapes in synthetic fibers.

• All fibers are polymers, large chain-like macromolecules.

Types of Natural Fibers

• Comprise over half of fiber production; most common is cotton.

  • Sourced from animals (wool, fur), plants (bast fibers), etc.

  • Animal Fibers: Require comprehensive reference collections for identification (e.g., hair from mammals).

  • Plant Fibers: Sourced from seed (e.g., cotton), stem (e.g., flax), leaf (e.g., sisal).

Manufacture of Fibers

• Synthetic Fibers: Formed by extruding a polymer solution (spinning dope) through spinneret holes.

• Optical properties and color help identify generic polymer classes.

• Dyes and Treatments: Affect appearance and identification.

Microscopy in Fiber Analysis

• Methods: Polarized light microscopy critical for fiber identification.

• Cross-Sectional Measurements: Important for understanding fiber structure; diameters range significantly between natural and synthetic fibers.

Optical Properties and Color Assessment

• Refractive Index: Fibers act as lenses; assessed using Becke line.

• Fluorescence: Some fibers emit light when excited by specific wavelengths; can be degraded by treatment.

• Color Analysis: Crucial in fiber comparison; methods include visual, chemical, and instrumental analyses.

• Instrumental Analysis (e.g., MSP): Provides precise, repeatable measurements for colored fibers.

Summary on Fiber Evidence

• Fibers are a prevalent form of physical evidence.

• Color and other inherent characteristics offer powerful means for establishing associations.

• Through careful analysis, fibers can yield informative evidence in forensic investigations.