Natural Cellulosic Fibers

Chapter 4: Natural Cellulosic Fibers

Chapter Objectives

• To identify cellulosic fibers.

• To explain the characteristics common to all cellulosic fibers.

• To describe processing, characteristics, and performance unique to each of the commonly used cellulosic fibers.

• To articulate the basic steps in processing natural cellulosic fibers.

• To integrate the properties of natural cellulosic fibers with target market needs.

Textile Fiber Classification

• Natural Fibers:

  • Protein (e.g., silk, alpaca, camel, cashmere, llama, mohair, vicuna, wool)

  • Cellulose:

    • Bast (e.g., flax, hemp, jute, ramie, kenaf, nettle, hibiscus, bamboo*)

    • Leaf (e.g., pina, abaca, sisal, manila, henequen)

    • Seed Hair (e.g., cotton, kapok, milkweed, coir)

  • Mineral (e.g., asbestos, glass)

  • Natural Polymer (e.g., azlon, PLA, chitosan)

• Manufactured Fibers (Generic classification based on chemical composition as defined by the Textile Fiber Products Identification Act):

  • Filament or Staple forms.

  • Examples include Rayon, Lyocell, Acetate, Triacetate, Acrylic, Aramid, Elastomer, Nylon, Polyester, Spandex, etc.

Note on Bamboo: While bamboo has been used in agriculture, papermaking, furniture, and architecture for thousands of years, most bamboo found in textiles receives extensive chemical processing in the same manner as rayon. As such, the Federal Trade Commission requires that companies describe this as rayon from bamboo. However, some bamboo is processed directly from the plant, justifying its inclusion as a natural cellulosic fiber.

Natural Cellulosic Fibers Overview

• Source: Derived from plants.

• Sub-divisions based on the part of the plant:

  • Seed Hair Fibers: Attached to the seeds in the pod.

    • Examples: Cotton, Kapok, Milkweed, Coir.

  • Bast Fibers: Fibrous part of the plant stem.

    • Examples: Flax, Ramie, Hemp, Jute, Kenaf, Hibiscus, Nettle, Bamboo.

  • Leaf Fibers: Come from the fibrous part of the leaf.

    • Examples: Pina, Abaca, Sisal, Seagrass, Raffia, Henequen.

  • Fruit Husk: Cover the hard shell of the fruit.

    • Example: Coir (from coconuts).

General Cellulosic Fiber Properties

• Absorbency: High

• Heat Conductivity: High (comfortable in hot weather)

• Resistance to High Temperatures: Good for care & use

• Low Resiliency: Wrinkle easily & require ironing

• Lacks Loft

• Electrical Conductivity: High (no static buildup)

• Heavy Fiber: Density of $1.5$

• Attacked by Mildew

• Flammable: Ignite quickly, smells like burning paper

• Chemical Reactivity:

  • Alkali (base): High resistance

  • Acids: Destroy cellulosic fibers

  • Oxidizing Agents: Bleachable, but must be controlled

Cellulosic Seed Fibers

Cotton

• Known as the 'King of fibers'.

• A staple fiber.

• Characteristics depend on seed variety, growing conditions, and processing.

History & Economic Significance

• Ancient origins in China, Egypt, and India, where weaving and spinning began.

• Eli Whitney mechanized cotton production in $1793$.

• Cotton became the leading U.S. export.

• During the $1980$s-$90$s, most U.S. cotton mills closed due to globalization and competition.

Cotton Production

• World Production: Primarily grown in tropical and sub-tropical regions.

• Almost $2/3$ of the world's cotton is grown in China, the United States, and India.

• Pakistan and Brazil also grow significant amounts.

• Main U.S. Varieties:

  • Upland ($95 \%$ of production)

  • Pima

Cotton Production Terms

• 'Boll': The seedpod of the cotton plant.

• Gin: A machine used to separate fiber from seed.

• Bale: A compressed package of cotton, weighing approximately $480$ lbs each.

• Lint: Usable cotton fibers removed during the ginning process.

• Linters: Short fibers, about $1/8$ inch long, that remain after ginning.

Physical Structure and Grading of Cotton

• Cotton is graded based on:

  • Length: Longer fibers are of higher grade.

  • Color: Whiter cotton is graded higher, except for naturally colored cotton.

  • Cleanliness: Brown flecks of “trash” are undesirable.

  • Fineness: Finer fibers are generally preferred.

  • Strength: Stronger fibers are better.

Cotton Fiber Length

• A staple fiber ranging from $1/2$ to $2$ inches, depending on the variety.

• Upland: Staple length of $7/8 - 1 5/16$ inches.

• Long Staple: Fibers between $1 5/16 - 1 9/16$ inches (e.g., Sea Island).

• Extra Long Staple (ELS): Examples include Pima and Supima® (a registered trademark for pima cotton meeting brand criteria), Egyptian*, and American Egyptian.

  • Note: Not all cotton grown in Egypt is Egyptian quality.

• Short Staple: Fibers less than $3/4$ inches.

• Pima and Egyptian cottons are notable examples of extra long staple fibers.

• Fiber length is included on fiber content and product labels because it indicates higher quality.

Distinctive Parts of Cotton Fiber

• Cuticle: A wax-like film covering the primary, or outer, wall.

• Secondary Wall: Composed of layers of cellulose.

• Lumen: The central canal through which nourishment travels during fiber development.

• Convolutions: Characteristic ribbon-like twists along the cotton fiber.

  • The central canal collapses after drying.

  • Reverse spirals in the secondary wall cause the fibers to twist.

  • Convolutions make yarn spinning easy because they cling to one another.

Cotton Fiber Fineness

• Ranges from $16-20$ micrometers.

• Cross-section shape varies with fiber maturity:

  • Immature fibers are U-shaped with a thin cell wall.

  • Mature fibers are nearly circular with a thick cell wall and a small central lumen.

• Every boll contains some immature fibers, which can cause problems during spinning and dyeing.

Contour & Shape of Cotton

• Surface Contour (longitudinal view): Has a natural twist, appearing ribbon-like under a microscope.

• Shape (cross-section): Kidney-shaped with a hollow central core known as a lumen.

Fiber Color of Cotton

• Typically off-white, except for naturally colored cotton.

• Off-white cotton requires bleaching before it can be dyed.

• Naturally colored cotton (available in limited quantities) does not require bleaching or dyeing, making it eco-friendly.

Cotton Fiber Class or Grade

• Depends on length, quality, and fineness.

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