Studies on the hydro-thermal and viscoelastic properties of leather
The temperature dependent viscoelastic behaviour of synthetic polymers has been shown to be of high importance for their properties and applications. However for leather this aspect of viscoelasticity has not been studied. In order to further the understanding of structure-property relationships, the viscoelastic transitions of leather were investigated in this research work. This thesis reports the dynamic mechanical behaviour of leather within the temperature region of —100 to 300°C, where three major viscoelastic transitions were identified, termed a., [ and y. The 3 transition peak represents the glass transition temperature of the amorphous region of collagen molecules. It is also shown that tanning agents act as plasticisers and depress the glass transition temperature to a lower temperature. Thus the tanning process itself may be viewed as a plasticisation of the collagen molecule. In this event, tanning molecules interpose themselves between the collagen chains, thus reducing the forces holding the chain together. Different tanning agents show differing degrees of plasticisation. The effect of water on the viscoelastic transitions of leather was also investigated. It was shown that leather remains in a transitional viscoelastic region between -50 and 70°C regardless of the moisture content of a sample. This imparts unique properties to leather. Initially, the absorbed water molecules act as a plasticiser and depress viscoelastic transitions to a lower temperature region. Depending on the leather type, above a certain percentage of absorbed water, splitting of the glass transition peak is observed. This may be due to a preferential hydration of certain hydrophilic amino acid residues, leading to the separation of the transitions due to hydrophobic and hydrophilic amino acid residues. It is demonstrated that the rate of stress relaxation is temperature dependent and the stress relaxation property of leather above and below the glass transition differs greatly. Two critical temperatures related to heat setting were identified, which may be termed the critical and the optimum temperature. The critical temperature is the temperature above which the set increases markedly and has been positively identified as the glass transition temperature. Finally, changes in the dynamic modulus during the drying of leather revealed information concerning the nature of the moisture-leather relationship at the critical stages accompanying drying. It is concluded that leather undergoes three different phases during drying, when only the final phase is related to the hnal stillness of the leather
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