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Abstract:
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Investigations on the fracture behaviour of polymer blends is the topic of
this thesis. The blends selected are PP/HDPE and PS/HIPS. PP/HDPE blend is
chosen due to its commercial importance and PS/HIPS blend is selected to study
the transition from brittle fracture to ductile fracture.PP/HDPE blends were prepared at different compositions by melt blending
at 180°C and fracture failure process was investigated by conducting notch
sensitivity test and tensile test at different strain rates. The effects of two types of
modifiers (particulate and elastomer) on the fracture behaviour and notch
sensitivity of PP/HDPE blends were studied. The modifiers used are calcium
carbonate, a hard particulate filler commonly used in plastics and Ethylene
Propylene Diene Monomer (EPDM). They were added in 2%, 4% and 6% by
weight of the blends.The study shows that the mechanical properties of PP/HDPE blends can be
optimized by selecting proper blend compositions. The selected modifiers are
found to alter and improve the fracture behaviour and notch sensitivity of the
blends. Particulate fillers like calcium carbonate can be used for making the
mechanical behaviour more stable at the various blend compositions. The
resistance to notch sensitivity of the blends is found to be marginally lower in the
presence of calcium carbonate. The elastomeric modifier EPDM produces a better
stability of the mechanical behaviour. A low concentration of EPDM is sufficient
to effect such a change. EPDM significantly improves the resistance to notch
sensitivity of the blends. The study shows that judicious selection of modifiers can
improve the fracture behaviour and notch sensitivity of PP/HDPE blends and help
these materials to be used for critical applications.For investigating the transition in fracture behaviour and failure modes,
PS/HIPS blends were selected. The blends were prepared by melt mixing followed
by injection moulding to prepare the specimens for conducting tensile, impact and
flexure tests. These tests were used to simulate the various conditions which
promote failure.The tensile behaviour of unnotched and notched PS/HIPS blend samples
were evaluated at slow speeds. Tensile strengths and moduli were found to
increase at the higher testing speed for all the blend combinations whereas
maximum strain at break was found to decrease. For a particular speed of testing,
the tensile strength and modulus show only a very slight decrease as HIPS content
is increased up to about 40%. However, there is a drastic decrease on increasing
the HIPS content thereafter.The maximum strain at break shows only a very slight change up to about
40% HIPS content and thereafter shows a remarkable increase. The notched
specimens also follow a comparable trend even though the notch sensitivity is seen
high for PS rich blends containing up to 40% HIPS. The notch sensitivity
marginally decreases with increase in HIPS content. At the same time, it is found
to increase with the increase in strain rate. It is observed that blends containing
more than 40% HIPS fail in ductile mode.The impact characteristics of PSIHIPS blends studied were impact strength,
the energy absorbed by the test specimen and impact toughness. Remarkable
increase in impact strength is observed as HIPS content in the blend exceeds 40%.
The energy absorbed by the test specimens and the impact toughness also show a
comparable trend.Flexural testing which helps to characterize the load bearing capacity was
conducted on PS/HIPS blend samples at the two different testing speeds of
5mmlmin and 10 mm/min. The flexural strength increases with increase in testing
speed for all the blend compositions. At both the speeds, remarkable reduction in
flexural strength is observed as HIPS content in the blend exceeds 40%. The
flexural strain and flexural energy absorbed by the specimens are found to increase
with increase in HIPS content. At both the testing speeds, brittle fracture is
observed for PS rich blends whereas HIPS rich blends show ductile mode of
failure.Photoelastic investigations were conducted on PS/HIPS blend samples to
analyze their failure modes. A plane polariscope with a broad source of light was
utilized for the study. The coloured isochromatic fringes formed indicate the
presence of residual stress concentration in the blend samples. The coverage made
by the fringes on the test specimens varies with the blend composition and it
shows a reducing trend with the increase in HIPS content. This indicates that the
presence of residual stress is a contributing factor leading to brittle fracture in PS
rich blends and this tendency gradually falls with increase in HIPS content and
leads to their ductile mode of failure. |