Degradation of Gonometa postica silk, which occurs during bleaching, is not only a
problem for the consumer but also for the producer. The purpose of this study was
to determine the effect of hydrogen peroxide bleaching on Gonometa postica silk.
The specific goal was to determine the influence that temperature, pH and time
duration has on the releasing of hydrogen peroxide, colour change, breaking
strength, stiffness and shrinkage.
A literature study was undertaken to describe the physical properties, chemical and
molecular structure of Gonometa postica silk. Research on the degradation effect of
hydrogen peroxide on silk was investigated and the factors that influenced the
bleaching effect, were identified.
An experimental study was carried out to determine the releasing of hydrogen
peroxide at 50°C, 60°C, 70°C and 90°C at a pH 8 and pH 10. The Gonometa
postica silk was bleached with hydrogen peroxide in a 5 l water bath and dried
afterwards. The samples were kept in standard atmosphere conditions of 20 ± 1°C
and 65 ± 2% relative humidity before and during testing. The colour change,
breaking strength, stiffness and shrinkage of Gonometa postica silk were
determined after hydrogen peroxide bleaching.
The release of hydrogen peroxide was determined by potassium permanganate
titrations during the bleach action, while the colour changes (delta E) of the
samples were determined by a colorimeter. The instron tensile tester was used to
determine the breaking strength of bleached and unbleached samples according to
the SABS method 13934 â 1:1999. The breaking strength was measured in Newton
and elongation in mm. The stiffness of the silk was measured by a Shirley stiffness
tester. Shrinkage caused during bleaching was determined in grain difference and
expressed in percentages. To illustrate the degradation caused by bleaching,
scanning electron microscope photos were taken. The results indicated that temperature, pH and time duration plays an important
role in the bleaching of Gonometa postica silk. At a pH 10, the colour change
(bleaching effect) increased rapidly as the temperature increased. More bleaching
(colour change) could be seen at low temperatures at a pH value of 8. High
temperatures damaged the fibre, which lowered the breaking strength, and caused
an increase in the stiffness of the fibre as the temperature incresed. Hydrogen
peroxide released quicker at high temperatures. Different temperatures did not
influence shrinkage.
The pH-value was the factor with the highest influence on the release of hydrogen
peroxide. More bleaching and release of hydrogen peroxide could be seen at a pH
value of 8 and better results were obtained in colour change at a pH value of 10 at
low temperatures. The pH-value had no significant influence on breaking strength,
stiffness and shrinkage. Better results could be seen with an increase in time of
bleaching.
The results showed that 70°C for 3 hours at a pH value of 8 are the best conditions
for bleaching Gonometa postica silk. Althought some damage to the fibres was
evident under these treatment conditions, an effective white colour was obtained.
The conclusion that can thus be made is that factors, such as temperature and pHvalue,
have the biggest influence on colour change, tensile strength and stiffness of
Gonometa postica silk.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ufs/oai:etd.uovs.ac.za:etd-09232008-120555 |
| Date | 23 September 2008 |
| Creators | Olivier, Yolande |
| Contributors | Prof HJH Steyn |
| 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-09232008-120555/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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