Determination And Application Of The Glass Transitions Of Wool, Rayon And Cotton In Dyeing And Finishing Process

The glass transition, which is a transition of polymer material between glassy state and rubber state, is related to the textile processing and applicable properties of fibers directly. Many processing conditions may depend on fiber's glass transition temperature during the chemical fiber production, dyeing and finishing processes. The glass transitions of synthetic fibers such as polyester, nylon, etc., have been studied a lot, while the glass transitions of natural and regenerated fibers have not been tested by reliable methods. Thus from the point of view of the glass transition, the relevant principles of dyeing and finishing natural fibers have been reported in very few papers, and the relevant discussion and analysis remain qualitatively in the macro-level, so that the research about he processing principles has been limited considerably.Generally, dynamic mechanical analysis (DMA) is a kind of technologies, which measures the relationship between the dynamic modulus or mechanical loss and temperature under the alternating load and programmed temperature, in order to determine the viscoelastic performance of materials. DMA is an effective means about polymer structure, molecular motion and performance. Because the structural changes of polymers such as the glass transition and cross-linking are closely associated with changes in the state of molecular motion which can sensitively reflect in the dynamic mechanical performances. DMA has been widely used in composite materials, rubber, resin and other fields, but has been rarely seen in natural textile fiber application. Therefore, it is significant to explore the glass transition point of natural fibers by DMA. It is expected to determine glass transition points of wool, rayon arid cotton, and study the effect of dyeing and finishing processes on their dynamic mechanical properties by DMA, so as to provide some possible theoretical guidance to the processes.In this paper, the basic experimental conditions for textile fibers were firstly determined through many DMA trials, and then in dry/wet temperature scan modes and the humidity scan mode, wool, rayon and cotton were tested many times.Though the shape-like glass transition of wool was obtained by DMA temperature scan in the dry state, the peak value of tan 8 was very close to the melting point. So its authenticity might be suspicious. The transition temperature of the storage modulus curve was about 214℃, which might be the glass transition temperature. This could be used to analyze changes in wool properties, which had yet to be confirmed. In the test temperature range, the glass transition points of rayon and cotton were not detected by DMA temperature scans in the dry states.The obvious glass transition points of wool and rayon didn't appear through DMA temperature scans in different wet states. The glass transition point of cotton appeared at high humidity but not at low humidity, indicating sufficient water might affect its glass transition.Through DMA humidity scans, it was firstly found that both wool and rayon had a "glassy transition" as a function of humidity. When the temperature was 35℃, near body temperature, their glass transitions appeared in the relative humidity as 84.94% and 86.56% respectively. The transition humidity decreases with the increasing temperature, which may be some theoretical significant to study the wet processing mechanism and results of treatment through changes of the viscoelastic properties.In addition, it is also explored that the effects of dyeing and cross-linking processes on the dynamic mechanical properties of wool, rayon and cotton. The experimental results showed that in the DMA humidity spectra, dyeing had a little effect on fibers'dynamic mechanical properties and cross-linking processing affected the mechanical properties greatly. The introduction of dyestuffs to rayon made the peak of tanδweaker and increased the transition humidity slightly, because the hydrophobic part of dyestuff inside fibers hindered the water absorption of fiber segments. The peak value of tanδof cross-linked rayon was higher than the value of uncross-linked rayon, indicating that the cross-linked rayon consumed more energy at the glass transition point. The reason should be that the cross-linking between molecules has a certain restricted role on the movement of molecular chains.Dyeing process had no effect on the temperature/humidity spectrums of cotton thread, while cross-linking process affected it. The storage modulus of cross-linked cotton thread decreased and the loss factor increased relatively, which is due to the limitation of molecular cross-linking to the movement of molecular groups and chain segments. However, the application significance of these findings on dyeing and finishing processes still needs further exploration.