The Wool-Like Comfort of Acrylic Fiber: Textile Fiber Lecture #19 (in a Series on Textile Fibers)

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We’re shifting gears to explore Acrylic Fiber, a synthetic fiber often lauded for its ability to mimic the warmth and bulk of wool. Developed in the mid-20th century, acrylic has become a popular choice for a wide range of textile applications due to its desirable properties and relatively low cost. In this lecture, we will delve into the history of acrylic fiber, its preparation from chemical building blocks, and its fundamental chemical structure.

A Synthetic Alternative to Wool:

The development of acrylic fibers emerged from the pursuit of synthetic alternatives to natural fibers, particularly wool. Research into acrylic polymers gained momentum in the early to mid-1900s. H.W. Coover at Eastman Kodak is credited with some early significant work in the field of cyanoacrylate polymers, though not directly textile acrylics. However, it was Carbide and Carbon Chemicals Corporation (later part of Union Carbide) in the United States that first produced acrylic fibers commercially in the 1950s under the trade name Orlon.

The introduction of acrylic fibers provided a more affordable, lightweight, and often easier-care alternative to wool. It quickly found applications in sweaters, blankets, and other textile products where a wool-like feel was desired. Over time, various modifications and copolymers of acrylic have been developed to enhance its properties and expand its uses.

Building Blocks of Softness: Preparation and Chemical Structure

Acrylic fibers are synthetic fibers made from a polymer (polyacrylonitrile, PAN) containing at least 85% by weight of acrylonitrile monomer. Copolymers are often used to improve dyeability and other properties. Common comonomers include vinyl acetate, methyl methacrylate, or vinyl chloride.

Preparation of Acrylic Fiber:

The production of acrylic fiber involves the addition polymerization of acrylonitrile:

  1. Monomer Production: Acrylonitrile (CH₂=CHCN) is typically derived from propylene and ammonia through a process called ammoxidation.
  2. Polymerization: Acrylonitrile monomers are linked together in long chains through addition polymerization. This process involves a chain reaction initiated by a catalyst, where the double bonds in the acrylonitrile molecules break and form single bonds with neighboring monomers, creating the polyacrylonitrile polymer. The reaction can be simplified as: n CH₂=CHCN (Acrylonitrile) → [-CH₂-CH(CN)-]n (Polyacrylonitrile) Copolymers are produced by including other monomers along with acrylonitrile in the polymerization process.
  3. Dissolving the Polymer: Polyacrylonitrile is not easily melt-spun due to its high melting point and decomposition before melting. Therefore, the polymer is dissolved in a suitable solvent, such as dimethylformamide (DMF), dimethylacetamide (DMAc), or sodium thiocyanate.
  4. Spinning: The polymer solution is then forced through spinnerets into a coagulation bath (wet spinning) or evaporated in a warm air chamber (dry spinning) to solidify the fibers. Gel spinning is another method used for high-strength acrylic fibers.
  5. Drawing, Washing, and Drying: The newly formed fibers are stretched (drawn) to align the polymer molecules and increase their strength. They are then washed to remove residual solvents and dried.
  6. Crimping and Cutting: To enhance their bulk, warmth, and wool-like handle, acrylic fibers are often crimped (waved or curled). Finally, the continuous filaments are cut into staple fibers of desired lengths for spinning into yarns.

Chemical Structure of Acrylic Fiber (Polyacrylonitrile):

The repeating unit of polyacrylonitrile has a relatively simple structure: a carbon backbone with a pendant nitrile group (-CN) on every other carbon atom.

The repeating unit’s structure can be visualized as:

     H   H
     |   |
    -C - C -
     |   |
     H   CN

Key Structural Features:

  • Linear Polymer Chains: Polyacrylonitrile consists of long linear chains of repeating acrylonitrile units.
  • Nitrile Groups (-CN): The presence of the polar nitrile groups contributes to intermolecular forces between the polymer chains, influencing the fiber’s properties. These forces are not as strong as the hydrogen bonding in nylon or the ester linkages in polyester, which accounts for acrylic’s lower strength compared to those fibers.
  • Limited Crystallinity: Acrylic fibers generally have a lower degree of crystallinity compared to fibers like polyester or nylon, which contributes to their softer, more bulky handle. The copolymers used can further disrupt crystallinity.

In our next lecture, we will explore the key properties of acrylic fibers, examining how their chemical structure and the manufacturing processes influence their characteristics, such as warmth, bulk, dyeability, and resistance to sunlight.

Thank you for your attention.