Thermal and mechanical properties of polyolefins/wax PCM blends prepared with and without expanded graphite
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Date
2014-12
Authors
Mochane, Mokgaotsa Jonas
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State (Qwaqwa Campus)
Abstract
The study deals with the preparation of polyolefin/wax blends as form-stable, solid-liquid phase
change materials (PCM) with the aim of enhancing both the thermal conductivity and flame
resistance properties of the shape-stabilized PCMs. In such a composite, the wax serves as a
latent heat storage material and the polyolefins (EVA and PP) act as supporting materials,
preventing leakage of the molten wax and providing structural strength. To improve the thermal
conductivity and flammability resistance of these blends, expanded graphite at loadings of 3, 6
and 9 wt% was added into the samples, whereas the polymer/wax blend was kept at a 1:1 weight
ratio. To further improve the flammability resistance, combinations of EG with Cloisite 15A clay
and diammonium phosphate (DAP) in EVA and an EVA/wax blend were investigated. Both the
blends and composites were subjected to different characterization techniques in order to
establish their morphology and thermal and mechanical properties. The techniques used were
scanning electron microscopy (SEM), differential scanning calorimetry (DSC), thermal
conductivity, thermogravimetric analysis (TGA), cone calorimetry, dynamic mechanical analysis
(DMA), X-ray diffraction (XRD), tensile testing, and impact testing. It was generally observed
that the EG particles agglomerated in the absence of wax, but dispersed much better in the blends
when wax was present, probably because the wax penetrated in between the EG layers as a result
of the better interaction between wax and EG, and separated the layers into smaller and better
dispersed EG particles. This gave rise to better thermal conductivity and flame resistance. In the
presence of EG+Cloisite 15A the material formed a dense and stable char layer (carbonized
ceramic) which significantly improved the flame resistance of the materials. It was observed that
the thermal degradation mechanisms of the polymers and blends did not change in the presence
of EG, although the EG particles retarded the evolution of the volatile degradation products.
There were no significant changes in the melting temperature of EVA in the EVA/EG
composites, while the crystallinities of EVA were observably lower in the presence of EG. The
EVA composites showed a decrease in impact strength with increasing EG and wax contents.
The impact strength of the PP/wax/EG composites increased with increasing EG content in all
the samples, but decreased with increasing wax content.
Description
Keywords
Thesis (Ph.D. (Chemistry))--University of the Free State (Qwaqwa Campus), 2014, Graphite, Polyfins, Waxes, Polymers -- Thermal properties