| Summary: | 博士 === 國立交通大學 === 應用化學系所 === 92 === Laminar polymeric films are usually fabricated by coextrusion and blending
processes. In a coextrusion process, a laminar polymeric film is formed into
multilayer and parallel structures. In a blending process, on the other hand,
the laminar polymeric film formed has laminar morphology of dispersed phases
in its blend film. Because of the wide range of applications of laminar
polymeric films as packaging materials, studies of the processes for forming,
and the characteristics of, laminar polymeric films have become increasingly
important. In this study, the aims are to predict the properties of the
multilayer film and investigate the effect of adhesive on the laminar
polymeric film.
In Chapters 2 and 3, we successfully fabricated three-layer (A/B/C) films,
comprising high-density polyethylene (HDPE), tie layer [high-density
polyethylene-grafted maleic anhydride (HDPE-g-MAH]], and polyamide-6 (PA-6),
by a coextrusion blown-film process. The tensile behavior of the three-layer
film can also be predicted from its component layers by using an additive rule
and an empirical constitutive equation — , where σT, εT, and are the
true stress, the true strain, and the true strain rate, respectively, K and γ
ε are constants, and m is the strain rate sensitivity — and a simplified
constitutive equation — , where εT, σ0, and γ are the true strain, true
yield stress and the strain hardening parameter, respectively — over the
range of plastic deformation. There exists a good agreement between the
experimental and predicted data at low crosshead speeds, but there is a
relatively large discrepancy between them at high speeds, for both constitutive
equations, because of the heat generated during deformation. The valid strain
range for the latter is smaller, however, than that for the former. On the
other hand, the series model was examined to predict permeability of HDPE/tie/
PA-6 three-layer film; there exists a good agreement between experimental data
and this model for predicting both gas and water vapor permeabilities of three-
layer films containing various volume fractions of PA-6.
Conventionally, one or more tie layers are used in coextrusion processes, e.
g., in the preparation of HDPE/tie/PA-6 mentioned above, but having additional
tie layers in a coextruded film makes the fabrication process more complex and
expensive. To eliminate the need for tie layer(s) and to reduce the number of
layers, we have also successfully fabricated three-layer (A/B/A) films,
comprising a varying content of ethylene–vinyl alcohol copolymer (EVOH) as
the internal layer and blends of low-density polyethylene (LDPE) and adhesive [
low-density ethylene grafted with maleic anhydride (LDPE-g-MAH]] as the
external layers, by a coextrusion blown-film process. In Chapter 4 ,we
describe our investigation of the mechanical properties and compare their
oxygen and water vapor permeabilities to a series model reflecting the content
of adhesive. The peel strength increased sharply at LDPE-g-MAH content > 12.5
wt%; we associate this increase with a promotion of adhesion between layers
that arises from the formation of ester bonds, as determined by FTIR
spectroscopy, between EVOH and LDPE-g-MAH. The tensile strength did not change
significantly upon increasing the LDPE-g-MAH content, but it had a small
effect on elongation and modulus in both the machine and transverse directions.
Tear strength decreased continuously, in both directions, upon increasing the
LDPE-g-MAH content. The oxygen permeabilities of the three-layer films
remained almost constant upon varying the amount of LDPE-g-MAH and all
followed the series model. The water vapor permeabilities of the three-layer
films, however, were affected by the degree of hydrogen bonding, which we
analyzed by FTIR spectroscopy, to result in a discrepancy between the
experimental findings and the series model, especially when the EVOH content
was increased.
An alternative process to fabricate laminar polymeric film is the blending
process. In Chapter 5, we investigated the morphological, thermal, barrier,
and mechanical properties of low-density polyethylene/ethylene–vinyl alcohol
blend (LDPE/EVOH; 85/15 wt%) in highly and biaxially oriented blown films. We
used linear low-density polyethylene-grafted maleic anhydride (LDPE-g-MAH) in
various amounts as the compatibilizer for this immiscible system. Thermal
analyses of the blend films indicated that their melting temperatures,
crystallization temperatures, and heats of fusion remain almost constant upon
varying the amount of compatibilizer. The addition of the compatibilizer did
not adversely affect the inherent properties of the blends, especially their
barrier properties, through constraint effects of the grafted EVOH (EVOH-g-
LD). The heat of fusion of EVOH obtained during the first heating was much
higher than that of the second as a result of the stress-induced
crystallization that occurs during the blown-film process. Oxygen permeation
measurements demonstated that the oxygen barrier properties of both highly and
biaxially oriented blown films decrease upon increasing the amount of
compatibilizer, although morphological analysis indicated that the blends
exhibit better laminar dispersion of the EVOH phase in the LDPE. The increase
in oxygen permeability results from the formation of microvoids at the
interface between the two phases during the blown-film process. Mechanical
measurements indicated that there exists an optimal amount of LDPE-g-MAH at
which both the tensile and tear properties are maximized in both the machine
and transverse directions.
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