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|Title: ||Environmental stress cracking in thermoplastic polyurethanes|
|Authors: ||Patel, Prakesh I.|
|Issue Date: ||1982|
|Publisher: ||© P.I. Patel|
|Abstract: ||This research has been concerned with investigating stress
cracking phenomena which occur when thermoplastic polyurethanes
(TPU's) are immersed in seawater at low temperatures. The investigation
was concerned with testing the following hypothesis:
a) that cracking of TPU was due to environmental stress cracking
b) due to structure and morphology changes which occur during
the ageing period.
The combined effects of polyurethane chemical backbone, domain structure
and crystallinity on stress cracking in TPU were studied.
Further, the investigation concerned itself with studying, on stress
cracking, the effects of storage conditioning the TPU granules prior
to processing, the processing conditions and various postcuring
treatments of TPU. Characterisation and analytical techniques employed to study
the structure of TPU's investigated consisted of thermal analysis
and X-ray diffraction. Molecular weight distribution was studied
by gel permeation chromatography and solution viscosity techniques.
Processing'was evaluated by melt flow index (MFI), injection moulding
A new accelerated ageing test for environmental stress cracking
has been developed which uses high pressures. Also, a new type of
ESC chemical class of reagent which relates the hydrogen bonding parameter
of the ESC agent to that of the TPU's has been discovered and
a theory developed. A test method has been designed to measure, in
active environments, the existence of a critical strain for all the
TPU examined whether commercial or laboratory synthesised materials.
Results show that the stress cracking in TPU's is due to the
following events: (a) a large reduction in molecular weight due to the processing, and (b) the combined effects of applied stresses
and the hydrolysis of the polymer. Molecular weight reduction in
TPU's and their resistance to ESe was also found dependent on the
preconditioning history of unprocessed TPU granules, as well as
any postcuring operations applied to the processed materials.
It has been established that, the ESe resistance of TPU's
can be improved by (i) optimising processing conditions, (ii) by
insertion of slight crosslinking or by selecting certain types of
diisocyanates, specifically p~phenylene diisocyanate, and trans
A mechanism to explain ESe of TPU's inactive and mild environments
is also proposed.|
|Description: ||A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.|
|Appears in Collections:||PhD Theses (Materials)|
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