Unit 2 - Lecture 10 Topic 2 Flashcards
(38 cards)
What forms does rayon come in?
Regenerated or precipitated cellulose is “rayon”, but where that fibre has been made by a viscose process, this would be called “viscose” or “viscose rayon”. In a modified viscose process for high wet modulus, this creates “modal” or “modal rayon”. In a solvent extrusion process, where no chemical intermediates are formed, “lyocell” or “lyocell rayon” is created.
How is cellulose involved in rayon?
Rayon fibres are 100% regenerated cellulose. Cellulose (wood pulp) is dissolved, possibly chemically altered and regenerated (or precipitated) into cellulose again.
How does rayon react to water?
Rayon is hydrophilic (water-loving). Generally, the weak fibres tend to be weaker when wet (unlike natural cellulosic fibres), however modal rayon can be similar in strength as cotton. Water is attracted to the –OH groups in viscose and it has high amorphous regions.
Does viscose rayon have high or low degrees of polymerization?
Short, between ~250-700.
Does viscose rayon have high or low crystalline properties?
Low.
Is viscose rayon orientated?
It is not well oriented with the fibre axis.
What are the surface features of viscose rayon?
It has lengthwise striations and a serrated cross-sectional shape.
How does the cross-sectional shape of viscose rayon form?
Serrated cross-sectional shape occurs as the solvent evaporates after wet-spinning and the fibre partially collapses.
Is rayon used in filament or staple lengths?
Both!
What is the tensile strength of viscose and why?
It has low tensile strength that is even lower when wet. This is because there are relatively few polymers to support the load due to it’s low orientation and low proportion of crystalline areas therefore less H-bonding capability.
What is the elongation property of viscose like?
It has high elongation at break. Polymers can easily slip past each other so the fibre elongates with a small pulling force.
What is the elastic recovery of viscose?
It has poor elastic recovery. Viscose fabrics tend to wrinkle and stretch easily because H-bonds will break and reform in new positions and there is easy slippage of the H- bonds which contributes to stretching.
How does viscose bode with static electricity?
Viscose does not build up static electricity due to the good moisture absorption.
Why are viscose fibres soft and easily bent?
Due to amorphous regions and the polymers are not aligned well enough to form many strong H-bonds.
What is the resiliency of viscose like?
Viscose easily wrinkles as H-bonds are broken when bent, compressed and formed in new positions as with cotton and linen.
What are the lustre and drape properties of viscose?
Viscose can have high lustre as it is a manufactured fibre with a manufactured fibre shape. It can also be made duller by adding finishes and drapes well.
Modal rayon:
This is a modification of normal viscose rayon. It goes by other names like ‘high wet modulus (HWM) rayon’, and ‘polynosic’. The fibrils are manufactured into the structure and it has a rounder cross section than viscose rayon.
What is the durability of modal rayon?
It has higher strength than viscose because of it’s higher DP and more crystalline structure.
What are the absorption and stiffness properties of modal?
It is slightly less absorbent than viscose rayon and stiffer than viscose, but softer than cotton.
What is the dimensional stability and fibre blend use of modal?
Modal has a better dimensional stability than viscose and is often blended with other fibres as it has a higher modulus than viscose (eg, cotton, nylon, polyester).
Why is lyocell environmentally friendly?
The creation of lyocell is an environmentally friendly process because only one solvent is used which is recycled. There are managed forests for producing lyocell. It is a wet-spun regenerated cellulosic fibre.
What are the microscopic physical features of lyocell?
Lyocell has smooth surfaces and round cross sections.
How crystalline is lyocell?
More crystalline than viscose & modal.
What is Lyocell’s trademark name?
Tencel.
