Relating the shear stress at the cone surface to the measured torque and the shear rate to the angular velocity of the plate, the expression for the viscosity h is obtained as h Z3Kq sin
Trang 1which is measured by means of a transducer The polymer sample is placed in the space between the cone and plate and the torque experienced by the stationary cone is measured for different rotational speeds of the plate
Relating the shear stress at the cone surface to the measured torque and the shear rate to the angular velocity of the plate, the expression for the viscosity (h) is obtained as
h Z3Kq sin a 2pR3
where K is the torsional constant and q is the deflection of the spring; Rpis the radius and u is the angular velocity of the plate; and a is the angle of the cone While q and u are experimentally determined quantities, K and a are obtained by calibration on other materials
The cone and plate viscometer gives reliable experimental data over an extensive range of shear rates (10–4–104secK1) Not only can it be used to measure viscosities in simple shear, but it can also be used to determine the dynamic properties of viscoelastic materials The unit is also set up to measure the normal stresses exhibited by viscoelastics, i.e., those perpendicular to the plane of shear
3.2.17.2 Capillary Rheometers
These rheometers are widely used to study the rheological behavior of molten polymers As shown in Figure 3.35 the fluid is forced from a reservoir into and through a fine-bore tube, or capillary, by either mechanical or pneumatic means The fluid is maintained at isothermal conditions by electrical temperature control methods Either the extrusion pressure or volumetric flow rate can be controlled
as the independent variable with the other being the measured dependent variable
Under steady flow and isothermal conditions for an incompressible fluid (assuming only axial flow and no slip at the wall), the viscous force resisting the motion of a column of fluid in the capillary is equal
to the applied force tending to move the column in the direction of flow Thus,
t ZRDP
where R and L are the radius and length of the column and DP is the pressure drop across the capillary The shear stress t is therefore zero at the center of the capillary and increases to a maximum value at the capillary wall This maximum value is the one generally used for the shear stress in capillary flow
Air bearing Transducer
Transducer
Amplifiers Recorders
Torsional spring
Constant
speed
motor
Gear assembly Bearing
FIGURE 3.34 Scheme of a Weissenberg Rheogoniometer.
Trang 2which is measured by means of a transducer The polymer sample is placed in the space between the cone and plate and the torque experienced by the stationary cone is measured for different rotational speeds of the plate
Relating the shear stress at the cone surface to the measured torque and the shear rate to the angular velocity of the plate, the expression for the viscosity (h) is obtained as
h Z3Kq sin a 2pR3
where K is the torsional constant and q is the deflection of the spring; Rpis the radius and u is the angular velocity of the plate; and a is the angle of the cone While q and u are experimentally determined quantities, K and a are obtained by calibration on other materials
The cone and plate viscometer gives reliable experimental data over an extensive range of shear rates (10–4–104secK1) Not only can it be used to measure viscosities in simple shear, but it can also be used to determine the dynamic properties of viscoelastic materials The unit is also set up to measure the normal stresses exhibited by viscoelastics, i.e., those perpendicular to the plane of shear
3.2.17.2 Capillary Rheometers
These rheometers are widely used to study the rheological behavior of molten polymers As shown in Figure 3.35 the fluid is forced from a reservoir into and through a fine-bore tube, or capillary, by either mechanical or pneumatic means The fluid is maintained at isothermal conditions by electrical temperature control methods Either the extrusion pressure or volumetric flow rate can be controlled
as the independent variable with the other being the measured dependent variable
Under steady flow and isothermal conditions for an incompressible fluid (assuming only axial flow and no slip at the wall), the viscous force resisting the motion of a column of fluid in the capillary is equal
to the applied force tending to move the column in the direction of flow Thus,
t ZRDP
where R and L are the radius and length of the column and DP is the pressure drop across the capillary The shear stress t is therefore zero at the center of the capillary and increases to a maximum value at the capillary wall This maximum value is the one generally used for the shear stress in capillary flow
Air bearing Transducer
Transducer
Amplifiers Recorders
Torsional spring
Constant
speed
motor
Gear assembly Bearing
FIGURE 3.34 Scheme of a Weissenberg Rheogoniometer.
Trang 3Wide-angle light scatter
Light source
Light source
Light trap
Light trap absorbs all light scattered by film less than 2½ ° After T1 is determined the sphere is rotated to measure T2
Photocell collecting all transmitted light reflected
Photocell collecting all wide-angle transmitted light scattered by film more than 2½° (T 2 )
(T1) Film sample
Film sample
Reflecting sphere
Reflecting sphere
Film Object
FIGURE 3.86 Test for haze of transparent plastics Haze, %Z100!T 2 /T 1 A low haze value is important for good short distance vision Standard test method: ASTM D1003.
Narrow-angle light scatter
Film
Object
Light
Annular photocell collecting light greater than that
at ½° to normal = T 1
Less than ½ °
to normal = T2
½ °
FIGURE 3.87 Measurement of narrow-angle light-scattering property of plastic film Clarity, %Z100!T 1 /(T 1 CT 2 ).
Trang 4TABLE 3.8 Heating Tests of Some Common Polymers
The material burns but self-extinguishes on removal from flame
Poly(vinyl chloride) Yellow–orange, green
bordered
Resembles hydrochloric acid and plasticizer (usually ester like)
Strongly acidic fumes (HCl), black residue Poly(vinylidene chloride) As above Resembles hydrochloric
acid
As above
Phenol-formaldehyde resin Yellow, smoky Phenol, formaldehyde Very difficult to ignite,
vapor reaction neutral Melamine- formaldehyde
resin
Pale yellow, light Ammonia, amines
(typically fish like), formaldehyde
Very difficult to ignite, vapor reaction alkaline
Nylons Yellow–orange, blue edge Resembling burnt hair Melts sharply to clear,
flowing liquid; melt can
be drawn into a fiber; vapor reaction alkaline
Chlorinated rubber Yellow, green bordered Acrid Strongly acidic fumes,
liberation of HCl; swollen, black residue The material burns and continues burning on removal from flame
Polybutadiene (BR) Yellow, blue base, smoky Disagreeable, sweet Chars readily; vapor
reaction neutral Polyisoprene (NR, gutta
percha, synthetic)
Yellow, sooty Pungent, disagreeable, like
burnt rubber
As above Styrene-butadiene
rubber (SBR)
Yellow, sooty Pungent, fruity smell
of styrene
As above Nitrile rubber (NBR) Yellow, sooty Like burnt rubber/
burnt hair
As above Butyl rubber (IIR) Practically smoke free
candle like
Slightly like burnt paper Melt does not char readily Polysulfide rubber (polymer
itself emits unpleasant,
mercaptan like odor)
Smoke-free, bluish Pungent; smell of H2S Yellow, acidic (SO2) fumes
Cellulose (cotton,
cellophane, viscose
rayon, etc.)
melting Cellulose acetate Yellow–green, sparks Acetic acid, burnt paper Melts, drips, burns rapidly,
chars, acidic fumes Cellulose acetate butyrate Dark yellow (edges slightly
blue), somewhat sooty, sparks
Acetic acid/butyric acid, burnt paper
Melts and forms drops which continue burning Cellulose nitrate
(plasticized with
camphor)
explosion
Ehtyl cellulose Pale yellow with blue–green
base
Slightly sweet, burnt paper Melts and chars Polyacrylonitrile Yellow Resembling burnt hair Dark residue; vapor
reaction alkaline Poly(vinyl acetate) Yellow, luminous, sooty Acetic acid Sticky residue, acidic vapor Poly(vinyl alcohol) Luminous, limited smoky Unpleasant, charry smell Burns in flame, self
extinguishing slowly on removal; black residue
(continued)