In this study, three types of polyethylene, low-density (LDPE), linear low-density (LLDPE),
and high-density (HDPE) polyethylene, were used as polymer matrices to prepare sisal fibre
reinforced polyethylene composites containing 10-30 wt% fibre. The untreated and the
dicumyl peroxide (DCP) treated composites were prepared by melt mixing, followed by hot
melt pressing. The influence of the DCP treatment, the polyethylene molecular
characteristics, and the sisal fibre loadings on the morphology and on the thermal,
mechanical, and dynamic mechanical properties of the composites was investigated. The gel
contents of the composites varied significantly depending on the polyethylene molecular
characteristics. The LLDPE composites had the highest gel content values followed by LDPE
and then HDPE, for which the gel content did not change significantly. These results strongly
suggested the presence of grafting of the polyethylene chains onto the sisal fibre surfaces
combined with crosslinking between the polymer chains. The morphologies of the
cryofractured surfaces and the xylene-extracted samples further confirmed the presence of the
grafting, particularly in the case of the treated LLDPE and LDPE composites. The SEM
micrographs of the treated LLDPE and LDPE composites showed better interfacial adhesion
between the polymers and the sisal fibres. For HDPE composites, however, such interfacial
bonding was not observed from the SEM micrographs. The SEM images of all the untreated
polyethylene composites showed poor interfacial interactions. TGA analyses showed that the
treatment did significantly affect the thermal stabilities of the composites, and all the
untreated and the treated samples were thermally less stable than the neat polymer matrices.
The DSC results demonstrated that the crystallization and melting behaviour of all the
untreated polyethylene composites remained unaffected. However, both the DCP treatment
and the sisal fibre loadings to some extent influenced the crystallization and melting
behaviour of the LLDPE composites, whereas those of the LDPE composites were only
slightly affected. The treated HDPE composites, however, did not show significant changes
in their crystallization and melting behaviour. The elongation at break for all the treated and
the untreated polyethylene composites showed similar trends and the treatment did not bring
about any differences. Compared to the untreated composites, the tensile strength and the
Youngâs modulus of the treated LLDPE and LDPE composites were remarkably higher,
whereas the Youngâs modulus of the treated HDPE composites was observably lower and no
significant effect on the tensile strength was noticed. The storage modulus of the LLDPE and LDPE composites showed good correlation with the tensile testing results. The tan δ curves
showed a slight increase in the glass transition temperatures for the treated composites. The
storage modulus of the treated HDPE composites remarkably decreased, and the tan δ curves
did not show the β-relaxation as in the case of the other two polymers.
The effect of the incorporation of sisal whickers on the properties of poly(lactic acid) was
also investigated in this study. Untreated and the MA/DCP and DCP treated PLA
nanocomposites, with sisal whiskers loadings of 2 and 6 wt%, were prepared by melt mixing
and hot melt pressing. The dispersion of the whiskers in the PLA matrix as well as the
thermal and viscoelastic properties of the nanocomposites were determined using TEM, DSC,
TGA, and DMA. The dispersion of the whiskers was found to be similar, whether the
samples were treated or not. The presence and the amount of whiskers in the untreated
nanocomposites slightly decreased the calculated percent crystallinity, but the Tm, Tc and Tg
remained fairly constant compared to neat PLA. The type of treatment was also found to
influence the crystallization and melting behaviour of the nanocomposites. The TGA results
showed that neither the sisal whiskers loading nor the treatment had a significant effect on the
thermal stabilities of the nanocomposites. The incorporation of the whiskers remarkably
reduced the intensity of the glass transition in the tan δ curve, and all the nanocomposites
showed higher storage modulus values compared to the neat PLA. The type of treatment did
not really influence the stiffness of the samples.
Entirely bio-based nanocomposites of PFA and sisal whiskers were prepared by an in situ
polymerization method. The effect of increased sisal whiskers loadings (1 and 2 wt%) on the
thermal and the dynamic mechanical properties of the nanocomposites were studied. No
significant changes in the thermal stabilities of the nanocomposites could be seen. The
storage moduli of the nanocomposites were significantly increased by the presence and the
amount of sisal whiskers, and the intensity of the glass transition relaxation in the tan δ curve
observably decreased and slightly shifted to lower temperatures.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ufs/oai:etd.uovs.ac.za:etd-09172013-091706 |
| Date | 17 September 2013 |
| Creators | Ahmad, Essa Esmail Mohammad |
| Contributors | Prof AS Luyt |
| Publisher | University of the Free State |
| Source Sets | South African National ETD Portal |
| Language | en-uk |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.uovs.ac.za//theses/available/etd-09172013-091706/restricted/ |
| Rights | unrestricted, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to University Free State or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
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