The study of textile materials at different temperatures in the internal structure and properties of the law of change, for its reasonable processing and correct use has important significance. Fiber properties in the temperature transition point before and after the performance is significantly different, with different temperature transition point to characterize. From the content of the study, there are mainly thermodynamic properties, thermal shaping, thermal damage.
Thermodynamic properties, refers to the temperature change process, the mechanical properties of textile materials with the change in the characteristics. The internal structure of the vast majority of fiber materials is a two-phase structure, that is, the coexistence of the crystalline phase (crystalline zone) and the amorphous phase (amorphous zone). For the crystalline phase of the crystalline zone, under the action of heat its thermodynamic state has two kinds: one is the crystalline state after melting, its mechanical characteristics manifested as a rigid body, and has the characteristics of high strength, elongation is small, modulus is large; the other is the molten state after melting, its mechanical characteristics manifested as a viscous flow body. The two can be distinguished by the melting point. For the amorphous phase of the amorphous zone, under the action of heat its thermodynamic state has a brittle folding state, glassy state, high elasticity and viscous flow state, respectively, according to the size of the deformation ability to use the brittle folding transition temperature, glass transition temperature, viscous flow transition temperature to divide.
1.the thermodynamics of fiber materials, three states
For linear polymers, the viscous flow transition temperature of the amorphous phase of the material and the melting point of the crystalline often overlap with each other, it is difficult to distinguish, so the measurement of the thermodynamic properties of the fiber is the first change in the amorphous phase of the changes that are manifested in the typical curves of the hair shown in Figure 1.
Figure 1 Typical thermodynamic curve of fiber material
Figure 1 is under constant stress conditions fiber deformation capacity (solid line) and tensile modulus (dotted line) with the process of temperature change, the turning point for the glass transition temperature Tg, and viscous flow transition temperature Tf, and the transition temperature have a zone, which is amorphous polymer has mechanical three-state characteristics. Among them, most of the synthetic fibers mechanical three-state characteristics are more obvious, while natural fibers (cotton, linen, wool, silk) and regenerated cellulosic fibers, etc. in certain temperature rate (high temperature) does not show more obvious and viscous flow state characteristics, but direct decomposition, carbonization.
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1,Glass state
In the low temperature state, the energy of thermal movement of macromolecules within the fiber by the lower, the movement of units in the base, links, short branched chain and other short units, the chain is in the "frozen" state, the movement of the local vibration carry forward the bond length, bond angle changes. Therefore, the fiber pull Shen modulus is very high, high strength, deformation capacity is very small, and in addition to the external force, the deformation quickly disappeared, the fiber is hard and brittle, similar to the mechanical properties of the glass, so it is called glazed state (or hard glass state). When the temperature is further increased, the size of the motor unit increases, the fiber macromolecule chain segments have a certain ability to turn back, the fiber shows a certain flexibility, toughness, the force can be seen in the case of plastic deformation, this state is often referred to as the soft glass state (or known as forced high elasticity), the vast majority of fibers in the room temperature conditions in this state.
When the chain links, chain segments, main chain rotation and side base are frozen is called brittle folding state.
The glass transition temperature of textile fibers are mostly higher than room temperature, so under room temperature conditions, the clothes can maintain a certain degree of tensile strength and stiffness, such as spandex glass temperature below -40 ° C (polyether-type -70 ° C ~ -50 ° C) in the ambient environment with excellent elasticity.
2, high elastic state
When the temperature continues to rise above a certain temperature (glass transition temperature Tg), the tensile modulus of the fiber suddenly decreased, the fiber by the action of a small force on the occurrence of a large deformation, and when the external force is lifted, the deformation of rapid recovery. In the "temperature - deformation" or "temperature - modulus" curve appears a platform area, the mechanical behavior of this interval is similar to the mechanical characteristics of the rubber, the mechanical state of the fiber is called high elasticity state or rubber state. From the molecular movement mechanism, at this temperature within the fiber macromolecular chain has been "thawed", the chain can be rotated around the main chain axis, so that the macromolecules are easier to curl, straighten the deformation, and the deformation is also easy to generate through the thermal movement of the chain to restore the original form. This is a unique mechanical state of the polymer, the essence of elastic deformation is the chain movement of the macromolecule stretching a curling movement of the macro-performance.
3,Viscous flow state
When the temperature continues to rise to a certain temperature (viscous flow transition temperature Tf), the thermal movement of the macromolecules to overcome the intermolecular forces, the movement unit from the chain segments to expand to the macromolecular chain, the macromolecules which can be seen between the relative slip, the deformation ability to significantly increase and irreversible. Textile fibers present a viscous, flowable liquid state, this mechanical state of the fiber is called viscous flow state. When the degree of polymerization of macromolecules is very high, the intermolecular use of force is very great, the entanglement between the macromolecules is serious, the relative slip between the molecules is very difficult, there will be no viscous flow state.
The above from the molecular kinematics point of view describes the thermodynamic three states, from the phase point of view, the glassy state, high elasticity and viscous flow state are non-crystalline phase, that is, the arrangement of the macromolecules between the state of the random (disordered, amorphous) state.