Acrylic Fibers: Properties & Applications

Acrylic Fibres

• Acrylic fibres are the third largest class of synthetic fibre after polyester and nylons
• Commercial acrylic fibre was developed by Dupont in the US and named Orlon
• Modified acrylic fibre was developed by Union Carbide and named Dynel
• Acrylic fibre monomer is acrylonitrile.
• Modified acrylic fibre involves acrylonitrile co-polymerized with Vinylidiene chloride vinyl chloride.
• Halogenated monomers provide flame resistance, making them suitable for home furnishing, protective coatings, sleepwear, and hospital blankets

Properties of Acrylic Fibres

• Soft and lightweight
• Durable and strong
• High crease recovery
• Color fastness to washing and sunlight
• Easy to launder and low maintenance cost
• High abrasion resistance
• Good aesthetics and high luster
• Good wicking action enables quick moisture and sweat transfer, facilitating quick drying
• Non-allergenic and non-toxic
• Resistant to mildew, insects, oils, and chemicals
• Highly resistant to deterioration from sunlight exposure

Monomers and Characteristics of Major Synthetic Fibres

• Acrylic Fiber:
  • Monomer: Acrylonitrile
  • Basic Chemicals: Propylene, ammonia
  • Density: 1.17
  • Moisture regain: 1.5-2.5
  • Sticking point= 235 °C
  • Silk-like luster
  • Good resistance to weathering, alkalies, and acids
  • High bulking
  • Tensile strength 2-3 gm/denier
  • Elongations at break is 16-21%
• Modified Acrylics
  • Monomer: Acrylonitrile, vinyl chloride, vinylidene chloride
  • Basic Chemicals: Propylene, ammonia, ethylene
  • Moisture regain: 1.5-2.5
  • Sticking point = 235 °C
  • Good resistance to weathering, alkalies, and acids
  • High bulking
  • Good resistance to combustion
• Polypropylene:
  • Monomer: Propylene
  • Basic Chemicals: Propylene
  • Density: 0.85-0.94
  • Moisture regain: <0.1
  • Melting point: 168-171 °C
  • Good resistance to bacteria, chemicals, and water

Acrylonitrile

• Chemical formula: CH2=CH-CN
• An important monomer in manufacturing acrylic fibres.
• Earlier manufacturing routes using acetylene, ethylene oxide, or acetaldehyde have been replaced by the propylene route
• Now made using the propylene route due to the availability of cheaper propylene from steam cracker plants, involving ammono-oxidation of propylene.
• Also used in manufacturing nitrile rubber, ABS and SAN plastics, adiponitrile, and acrylamide
• Used in the manufacture of acrylates, intermediates for flocculants, pharmaceuticals, antioxidants, dyes and surface-active agents

Routes for the Manufacture of Acrylonitrile

• Acetylene Route:
  • CH=CH + HCN → CH2 = CHCN
• Ethylene Oxide Route:
  • H2C-CH2 + HCN → HOCH2CH2CN
  • HOCH2CH2CN → H2C = CHCN + H2O
• Acetaldehyde Route:
  • CH3CHO + HCN → CH3CHOHCN
  • CH3CHOHCN → CH2 = CHCN + H2O
• Propylene Route:
  • CH2=CH-CH3 + NH3 + 3/2 O2 → CH2 = CHCN

Acrylonitrile by Ammoxidation of Propylene

• A typical plant includes reactor, acrylonitrile recovery, acrylonitrile purification, and HCN purification sections.
• Propylene, ammonia, and air are fed to a fluidized bed catalytic reactor where ammoxidation of propylene occurs
• The reaction is highly exothermic

Key Steps

• Catalyst preparation: using bismuth and molybdenum.
• Mixing of propylene, ammonia, and oxygen in a 1:1:6 ratio.
• Macrylonitrile acetonitrile, hydrogen cyanide and unreacted elements are fed to a fludised bed reactor
• Removal of ammonia
• Absorption of components from ammonia-free gas in water to separate non-condensables
• Stripping of organic components to separate HCN.
• Separation of Acrylonitrile and acetonitrile via extractive distillation using water
• Purification of acetonitrile and acrylonitrile

Reactions during the Formation of Acrylonitrile

• CH2=CH-CH3+ O2 → CH2=CH-CHO + H2O
• CH2=CH-CHO + NH3 → CH2=CH-CH=NH + H2O
• CH2=CH-CH=NH+ ½ O2 → CH2=CH-CN + H2O
• Overall reaction: CH2=CH-CH3 + NH3 + 3/2 O2 → CH2=CH-CN + 3 H2O
• Side reactions also occur, producing byproducts such as CH3CN, HCN, CO, and CO2

Acrylic Fiber Manufacturing

• Manufacturing can be divided into two main parts: polymerization and spinning
• Polymerization:
  • Includes copolymer composition, catalyst system, polymerization reaction, and monomer recovery
  • Processes include bulk, suspension, emulsion, and solution polymerization
  • Suspension polymerization is often used for fibre-grade acrylic polymers
  • Emulsion polymerization is used for modified acrylic fibres
  • Inorganic compounds generate radical
• Spinning:
  • Includes solution/dope preparation, spinning techniques, and finishing.
• Dry Spinning:
  • Dimethyl formamide (DMF) is used.
  • The DMF spin dope contains the polymer, thermal stabilizers, and delustrant
  • The DMF is evaporated by circulating inert gas at 300-35 °C
• Wet Spinning:
  • Sodium thiocyanate is commonly used
  • Fibre is spun into a liquid bath with a solvent non-solvent mixture called coagulant

Polyurethane

• Discovered by Otto Bayer and co-workers in 1937
• Thermosetting polymers that combine the elasticity of rubber with the toughness and durability of metal
• Global polyurethane demand forecast to grow at a CAGR of 5.8% from 2010 to 2016, with the Asia-Pacific region accounting for over 60% of the total
• Flexible and rigid polyurethane foams made up the bulk of the total end-use segment in 2010, accounting for 60% of the full amount
• Polymer formed by combining two or more isocyanate functional groups and two or more hydroxyl groups
• Alcohol and isocyanate groups combine to form a urethane linkage
• Polyurethanes are made by addition of polyols and polyfunctional isocyanates
• Commonly used isocyanates: toluene di-isocyanate (TDI), diphenyl methane diisocyanate (MDI), Hexamethylene di-isocyanate (HDI)
• Polyols may be either polyether polyols or polyester polyols, with polyether polyols being more commonly used
• Catalysts include aliphatic and cyclo-aliphatic tertiary amines and organic tin compounds
• Typical reaction: NOC-R-NCO + NHO-R'-OH → (R-NH-COO-R'-OCO-NH)n

Polyurethane Foam

• Polyurethane rigid foam has good structural strength, adheres well to most substances, offers processing flexibility, and lasts long
• Rigid polyurethane foams are used for insulation in refrigerators, thermo-ware, cold sore panels, and refrigerated trucks
• Rigid polyurethane foam is a cellular polymer filled with a gaseous blowing agent, which lowers heat conductivity