Question

Compared to grey fabric, heat set polyester fabric show

• A. High shear rigidity
• B. High density
• C. High bending rigidity
• D. Low density

Hint:

• Heat Setting of Polyester Thermal treatment to impart dimensional stability and modify properties.
• It involves heating above glass transition temperature ($T_g$) allowing molecular chains to rearrange, followed by cooling to "set" the structure.
• Key effects:
  • Improved dimensional stability (reduced shrinkage).
  • Increased crystallinity} and molecular order.
  • Increased density} due to more compact packing.
  • Often increased stiffness (bending and shear rigidity).
  • Modified handle and drape.
  • Changes in dyeability (usually reduced).
• Compared to grey (un-set) fabric, heat-set polyester fabric shows higher density.

Answer 1

The Correct Option is B

Heat setting is a thermal process applied to synthetic fabrics (like polyester, nylon) or blends containing them. The fabric is held under controlled dimensions (length and width) and exposed to high temperatures (below melting point but above glass transition temperature) for a specific time, followed by cooling. Effects of heat setting on polyester fabric:

• Dimensional Stability: This is a primary goal. Heat setting stabilizes the fabric structure by relaxing internal stresses and "setting" the dimensions. This reduces subsequent shrinkage or distortion during washing, ironing, or use.
• Crystallinity and Molecular Structure: Heat setting allows polymer chains in amorphous regions to rearrange and form more, or more perfect, crystalline structures. This generally leads to an increase in the degree of crystallinity and a more compact, ordered molecular structure.
• Physical Properties:
  • Density: Increased crystallinity and more compact packing of polymer chains typically lead to an increase in fabric density.
  • Stiffness (Bending and Shear Rigidity): The effect on stiffness can be complex. Increased crystallinity can make individual fibers stiffer. However, relaxation of stresses and changes in yarn mobility within the fabric structure can also occur. Often, heat-set fabrics might feel somewhat stiffer or firmer, or sometimes a specific handle (e.g., softer if combined with certain finishes or starting with highly stressed yarn) is targeted. Generally, an increase in rigidity (both bending and shear) is expected due to increased crystallinity, but it's not always a dramatic "high" value unless specifically engineered.
  • Strength and Extension: Can be affected, usually a slight increase in strength and decrease in extensibility might be observed.
  • Dyeability: Can be reduced due to increased crystallinity making dye penetration harder.
  • Handle and Drape: Can be significantly modified.
  • Crease Recovery: Often improved.

Comparing heat-set polyester fabric to grey (un-heat-set) polyester fabric: The grey fabric has not undergone this thermal stabilization and structural ordering. After heat setting, the polyester fabric will have:

• Better dimensional stability.
• Increased crystallinity.
• Increased density due to more ordered and compact molecular packing in crystalline regions.

Regarding rigidity options:

• (a) High shear rigidity & (c) High bending rigidity: Heat setting generally increases these due to increased crystallinity. So, the heat-set fabric would likely have higher rigidity than the grey fabric. The term "High" is relative.
• (b) High density: Correct. Density increases due to increased crystallinity and molecular packing.
• (d) Low density: Incorrect.

The most definitive and direct structural change leading to a clear property change is the increase in order and packing, resulting in higher density. While rigidities also tend to increase, "High density" is a very direct consequence of the increased crystallinity induced by heat setting. The options use "High" as a descriptor. Compared to the grey state, the heat-set state has higher density. \[ \boxed{\text{High density}} \]