discuss about the impact toughening mechanism of glassy polymers

1. Mary Mincy
CBPST

2.  Impact Strength
 Glassy polymers
 Toughening Mechanisms
 Properties ; before & after toughening

3.  Ability of a material to absorb applied energy
or ability of material to resist fracture under
stress applied at high speed
 It is expressed in j/m or kg cm/cm of notch
 It determines the toughness of material
 ASTM D 256-06: standard test method for
determining the izod pendulum impact
resistance of plastics

4.  Measures toughness
 Energy absorb by a specimen up to
fracture

5.  Hard, rigid, and transparent thermoplastic
 It have well defined and easily observed
craze structures
 Low water absorption
 Good dimensional stability
 Colourability
 Reasonable chemical resistance
 Good electrical insulation properties
 Eg.: PS, PMMA,PC

6.  Brittle
 Fails easily under impact

7.  The main aim of the rubber modification of
glassy polymers is to improve their toughness.
Methods to increase the toughness
 copolymerization
 Incorporation of a second phase like other
thermoplastics
 Inorganic materials
 Spherical rubber particles
The last mechanism is mostly used

8.  Rubber like materials have long chains
with higher flexibility and mobility
which are joined in network
 Due to higher mobility the chain alter
their configuration rather fast so able
to bear higher loads
 On removal of the external forces, it
goes back to the original dimensions

9.  Impact toughness of glassy polymers can
be improved by the dispersion of rubber
particle.
 Rubber content and rubber particle size
play an important role in Toughening
Mechanisms.
 The optimum rubber particle diameter is
known to be 1-2µm

10. Well dispersed rubber particle are able to
induce different mechanisms of
toughening :
1. Crazing
2. Shear yielding
3. Combined crazing and shear yielding

11.  Crazing is the
phenomenon that
produces a network
of fine cracks on
the surface of the
glassy polymers
 Craze can be
observed by
transition electron
microscope (TEM)

12.  Crazing is creation of small crack occur in rubber
particle in which impact energy is absorbed
 This energy will be used to make the cracks
 It will protect the matrix from sudden failure
TEM micrograph of crazing zone

13. Schematic. Model of the craze extension with
increase in applied strain

14.  In shear yielding
the rubber
particle get
yielded to the
matrix
perpendicular to
the applied force
which is holding
the matrix from
sudden failure

15.  Rubber content and the gel content are almost the same between
sample A and sample C, Impact toughness is higher for sample
C. This suggests that the particle size affects the lmpact
toughness even in samples where the rubber content is the
same.
Sample content,
wt %
Rubber
Wt % gell Ave
rubber
particle
diameter
um
Izod
impact
Strength
Kg.cm/c
m2
A 8.6 23.0 0.2 6.9
B 6.5 18.3 1.0 7.7
C 8.4 23.2 4.9 7.8

16.  Properties of PS & HIPS
PROPERTIES PS HIPS
Specific gravity 1.05 1.05
Tensile strength (MPa) 32.4-56.5 16.0-41.3
Tensile modulus (MPa) 3103-3276 1653-2549
Flexural modulus (MPa) 3103-3448 1791-2687
Elongation at break (%) 1.2-3.6 1.0-1.5
Impact strength (izod) (j/m) 13.3-24 48.1-219
Hardness 60-84 63-76
HDT(0c) 76-108 69-76
Glass transition temperature
(0c)
74-110 93-105
Dielectric strength 19.7 11.8-19.7