Chapter 8 Fiber Properties and Identification
Yiping Qiu
Donghua University
Fiber Properties and Identification
? Mechanical properties
– Tensile properties
? Tensile deformation
l 0
l f
? l
Fiber Properties and Identification
? Tensile deformation,some concepts
– Elongation
0lll f ???
– Strain
00
0
l
l
l
ll f ?
?
?
??
Fiber Properties and Identification
? Tensile deformation
– Load,
? Force applied to extend the fiber
? Unfair to compare force applied to fibers with
different sizes.
– Stress
–Engineering stress
A
Fσ ?
Unit,Pa = N/m2
or dyne/cm2
psi = pound/in2
Fiber Properties and Identification
? Tensile deformation
– For textile yarns and fibers,hard to
determine cross-section area
?use linear density,
?gf/denier or N/tex
Fiber Properties and Identification
? Tensile testing of fibers
– Testing conditions,
?ASTM standard for textile testing:
? 70° F (21° C) and 65% relative humidity (RH)
– Stress-strain curves
?
?
0
Fiber Properties and Identification
? Stress-strain curves
– Initial modulus
?The slope of the first section of the stress-strain
curve (ratio of the stress to strain)
? also called Young’s modulus
? closely follows Hook’s Law
? smaller modulus,easier to elongate under small
stresses.
? For different applications,different requirements,
? ordinary apparels,high initial modulus
? women’s sheer hosiery,low initial modulus
? composites,extremely high initial modulus
Fiber Properties and Identification
? Stress-strain curves
– Initial modulus
?
?
0
Fiber Properties and Identification
? Stress-strain curves
– Yield point
? The point at which the stress-strain curve
flattens or changes its slope significantly
?
?
0
Y i e l d po i n t
?
?
0
Y i e l d p o i n t
Fiber Properties and Identification
? Stress-strain curves
– Yield point
? The polymers,yield” or molecular chains start
to move and pass one another.
? The deformation is permanent.
? The slope of the stress-strain curve after yield
point relies on the strength of intermolecular
forces.
? Most fibers have yield point but not cotton and
flax due to strong intermolecular forces,
Fiber Properties and Identification
? Stress-strain curves
– Hardening point
? The polymer molecules
become more oriented as
strain increases and the
fiber reaches a
deformation limit,where
the molecules have the
highest orientation and
(stress) hardening occurs.
?
?
0
H a r d e n i n g
p o i n t
Fiber Properties and Identification
? Stress-strain curves
– Failure point
? Where the fiber fails.
? Sometimes not necessary the maximum stress
point.
? Depends on definition.
Fiber Properties and Identification
? Stress-strain curves
– Tenacity
? Stress at the point of rupture.
? Stress at the maximum load.
? Unit,N/tex,gf/denier
Fiber Properties and Identification
? Stress-strain
curves
– Strain or elongation at
break
? % of strain at the
point of rupture.
? Strain at the
maximum load.
?
?
0
S t r a in a t b r e a k
S t r e s s a t b r e a k
o r t e n a c it y
Fiber Properties and Identification
? Stress-strain curves
– Work of rupture
? Physics,Work = Force ′ Distance = F ′ s
? If F = f(s) is not a constant
?When change of distance ?s is very small,F is
almost a constant,thus
??w = F ? s or
?when ?s ? 0,dw = f(s)ds
?therefore,??
??
??
ll
dssfdwW
00
)(
Fiber Properties and Identification
? Stress-strain
curves
– Work of rupture
? Area under load-
displacement
curve.
– Specific work of
rupture
? Area under stress-
strain curve
?
?
0
S pe c i f i c W o r k
of R up t ur e
Fiber Properties and Identification
? Elastic recovery
?percent of strain being recovered when load is
released.
? Occurs at the initial part of the stress-strain curve
where deformation is small and almost no
relative movement of molecules.
? Energy absorbed can be recovered completely.
? Elastic recovery ? as cross-link ?
? ? as H-bond and ionic bond ?
? ? as strain or stress ?
Fiber Properties and Identification
? Resilience (Work of recovery)
?The ratio of energy recovered to energy absorbed
? May be extensional,flexural,compressional or
torsional.
? Is a function of deformation.
?? as crimp and cross-link ?
?? as H-bond and ionic bond ?
?? as strain ?
? Resilience ?? wrinkle ?
– e.g,PET vs Cotton and Rayon
Fiber Properties and Identification
? Flexural rigidity (stiffness)
?How easy a fiber can be bent
? Defined as EI where
E = Young’s modulus of the fiber
I = Moment of inertia of the fiber
Mathematically,
??
A
dAyI 2
Fiber Properties and Identification
? Flexural rigidity (stiffness)
? Thus,for a fiber with a round cross-section
4rI ?
Fiber Properties and Identification
? Abrasion resistance
? Very difficult to measure
? Determined largely by structure of yarn and
fabric.
? Affected by temperature and moisture content
e.g,Nylon has the highest and acetate lowest
Fiber Properties and Identification
? Flexibility
? Defined as number of times a fiber can be bent
180° before it fails,
? Crystallinity ?,flexibility ?
? Fiber diameter ?,flexibility ?
Fiber Properties and Identification
? Time related mechanical properties
– Creep,under a constant load or stress,
deformation or strain increases with time.
Fiber Properties and Identification
Force
Elongation
Time
Time
elastic
elongation
creep
deformation
instant strain recovery
creep recovery
Permanent deformation
Fiber Properties and Identification
? Time related mechanical properties
– Creep
? A = elastic elongation (initial)
? B = creep deformation
? C = instant strain recovery
? D = creep recovery
? E = permanent deformation
Fiber Properties and Identification
? Time related mechanical properties
– Stress relaxation
? A fiber is loaded at a constant deformation,
observe the stress change with time.
elongation
Time
Time
stress
Fiber Properties and Identification
? Time related mechanical properties
– Stress relaxation
? At a constant deformation,stress decreases
with time.
? Decrease of stress is due to the molecular
movement in the stressed fiber such as
unentanglement of molecules.
? The rate of relaxation is dependent on factors
like temperature,intermolecular forces,
stiffness of molecules.
Fiber Properties and Identification
? Sorption properties
– Terms
? Hydrophilic,water loving or likes water
? Hydrophobic,water avoiding
? Hygroscopic,hydrophilic fibers that absorbs
significant amount of water or moisture
without feeling wet to touch.
? Oleophilic,how tenaciously fibers hold
absorbed or adsorbed oil,not amount of oil
absorbed.
? Oleophobic,does not have a strong affinity
for oil,but readily releases oil in a detergent
solution.
Fiber Properties and Identification
? Sorption properties
– Vaporous water absorption
? % Moisture regain
M M
M
W e t d r y
d r y
?
? 100%
Fiber Properties and Identification
? Sorption properties
– Vaporous water absorption
? % Moisture content
M M
M
W e t d r y
w e t
? ? 100%
Fiber Properties and Identification
? Sorption properties
– Vaporous water absorption
? depends on temperature and relative
humidity as well as atmospheric pressure.
Fiber Properties and Identification
? Sorption properties
– relative humidity
? the amount of moisture (water vapor) being
held in air divided by the maximum amount of
water vapor can be held in air.
let h = mass of water/unit volume of air
and hs = mass of water when air is saturated at
the same temperature and pressure
RH hh
s
% ? ? 100%
Fiber Properties and Identification
? Sorption properties
– Importance of moisture regain
? weight of textile materials
? change of mechanical properties
? comfort considerations
– Why a fibrous material absorbs more
moisture?
? more polar groups or H-bonds in amorphous
regions.
Fiber Properties and Identification
? Sorption isotherm
RH
Moisture regain
0
A B C
Fiber Properties and Identification
? Sorption isotherm
– Segment A,H2O attaches to polar groups in
amorphous phase,Fast absorption due to
availability of large number of polar groups,
– Segment B,H2O still attaches to polar groups only
but less of them available ? slower absorption.
– Segment C,H2O attaches to polar groups as well
as H2O being absorbed previously in the capillary
pores of the fiber,also breaking up of H-bonds
opens the space unavailable before ? faster
absorption.
Fiber Properties and Identification
? Sorption isotherm
Note:
– Water absorption in amorphous regions only
– Significant amount of moisture absorption with
the existence of H-bond only
– Properties of a fiber change with its moisture
content.
– Absorption-desorption curves are different,
sorption hysteresis exists
Fiber Properties and Identification
? Sorption isotherm
Relative Humidity
Moisture regain
0
desorption
absorption
Fiber Properties and Identification
? Absorption of liquid water
– Swelling
Upon absorption of water,fibers swell
? larger diameter and longer
? which direction swells more depends on
molecular orientation
Fiber Properties and Identification
? Absorption of liquid water
– Mechanical property change due to water
absorption
? Tenacity
? in general,with H-bonds,tenacity?as moisture
regain?
? but for cotton and flax or some other natural cellulose
fibers,the opposite is true due to structural
characteristics.
? No effect on fibers that do not absorb water.
Fiber Properties and Identification
? Absorption of liquid water
– Mechanical property change due to water
absorption
? Modulus
? in general,with H-bonds,modulus? as moisture
regain?because water breaks H-bonds (works like a
plasticizer).
?Nissan’s equation,E = E0exp(a-bM).
? Wet initial modulus,important for laundry,e.g,rayon
? No effect on hydrophobic fibers.
Heat of wetting
? Wetting of a fiber is an exothermal reaction,
heat released when fibers absorb water.
? Expressed as calories per gram (cal/g) or
Joules per gram of dry fiber.
? Amount of heat is proportional to # of H-
bonds and nature of H-bonds.
? High for hygroscopic fibers; low for non-
hygroscopic fibers
Heat of wetting
? Influences comfort
? RH increases,absorb more water; increase
temperature; feel warmer.
? RH decreases; release water; decrease
temperature; feel cooler.
Oil absorption or release (Oleophilicity)
? Fibers absorb the most oil are not necessary
those that hold it most tenaciously,In general,
fibers absorb more oil also tend to release it
easily:
? e.g,Fibers with H-bonds absorb more oil but release it
easily
? Most synthetic fibers do not absorb as much oil but do
hold it tenaciously due to their low surface energy such
that water cannot wet their surface easily.
Thermal Properties
? Specific heat and thermal conductivity
– Specific heat,a measurement of the amount of
heat required to change the temperature 1 ° C of
a unit mass of the fiber.
? A characteristic of the material
? Not as important as heat of wetting that
involves a lot more heat.
– Thermal conductivity,a measure of rate of heat
flow through fibers (not a single fiber but a mass of
fibers)
? Unit,Cal of heat/cm2 of fibrous loft
Heat Resistance
? Heat resistant temperature,at which a fiber
starts to degrade,evidenced as decrease of
mechanical properties (weaker and brittle)
Softening and melting
? Glass transition temperature,(Tg)
polymeric fibers all have a temperature above that a
fiber changes from a glassy state to a rubbery
state (amorphous phase start to,melt”).
? Melting temperature,(Tm)
at which the crystalline phase melts or molecules in
crystalline phase start to move around.
? Decomposition temperature,
at which polymer molecules decompose into small
molecules.
Softening and melting
? Note
– Tg is extremely important for performance of a
fiber.
– Tm and Tg depend on intermolecular forces,
structure,configuration and stiffness of the
molecule.
– Melting temperature exists only when the polymer
has crystalline region and a decomposition
temperature higher than Tm.
– Fibers with Tm or a softening temperature are
thermoplastic fibers.
Combustibility
? Conditions for combustion,
– availability of fuel (combustible volatiles)
– availability of O2
– heat provided,
? ignition temperature for most of the fibers,780-
1000F,415° C–538° C
Combustibility
? Noncombustible fibers
– no fuel
– or completely oxidized
– or extremely stable structures
? Combustible fibers
– normal
– flame resistant
? either inherently or modified,containing stable structures
or Cl,P,Br or F that changes decomposition processes
Combustibility
? Measurement of flammability
– Limiting oxygen index
? % of oxygen in air when a fibrous material combusts
? 78% N2 and 21% O2 in air
LOI < 21 ?flammable
LOI > 21 ?flame retardant
Chemical properties
? Chemical reactivity:
– Reactivity of a fiber to chemicals
– Related to modifications that a fiber can undergo
(mainly surface treatments)
e.g,fiber dyeability depends on the chemical
structure of the fiber.
Chemical properties
? Chemical reactivity:
– More importantly,Chemical resistance
? expose to certain chemicals breaks down polymer chains
and leads to degradation of the fiber.
? Chemical resistance is proportional to
– the type and strength of the covalent bonds along a
polymer backbone,
– presence of reactive groups in the polymer
– crystallinity
Resistance to biological organisms
? Cellulose and protein fibers can be food for
– insects,e.g moths,beetles,and silver fish
– Micro organisms,e.g,mildew,fungi,bacterial that
emit a substance (enzyme) breaks down polymers.
Electrical resistivity
? Definition,the voltage across the fiber divided
by the current through it.
? Mostly insulators,high resistivity
? Electrical resistivity largely depends on
moisture content
? Static charge,
– depends on conductivity and
– how tightly the electrons are held by the polymer
chain,e.g for olefins,no static charge built up due
to the tight holding of electrons by H.