Leathery state

Both kinetic and thermodynamic approaches have been used to measure and explain the abrupt change in properties as a polymer changes from a glassy to a leathery state. These involve the coefficient of expansion, the compressibility, the index of refraction, and the specific heat values. In the thermodynamic approach used by Gibbs and DiMarzio, the process is considered to be related to conformational entropy changes with temperature and is related to a second-order transition. There is also an abrupt change from the solid crystalline to the liquid state at the first-order transition or melting point Tm. 

Although the transition from a solid to a liquid state at the Tm is relatively precise and occurs over a short temperature range, the transition from a glassy solid to a leathery state occurs over somewhat broader temperature ranges around Tr The modulus, or stiffness, of the polymer decreases as the temperature is increased above the Tg, and the polymer changes from a leathery to a rubbery state. 

It is possible that there are in fact two secondary types of transitions one, the transition between the glassy and leathery states, called the Tg and an additional transition occurring between the leathery and rubbery regions. 

The most straightforward way to measure the effect of low temperatures on recovery is by means of a compression set or tension set test. Tests in compression are favoured and a method has been standardised internationally. The procedure is essentially the same as set measurements at normal or elevated temperatures and has been discussed in Chapter 10, Section 3.1. As the recovery of the rubber becomes more sluggish with reduction of temperature the dynamic loss tangent becomes larger and the resilience lower (see Chapter 9), and these parameters are sensitive measures of the effects of low temperatures. Procedures have not been standardized, but rebound resilience tests are inherently simple and quite commonly carried out as a function of temperature. It is found that resilience becomes a minimum when the rubber is in its most leathery state and rises again as the rubber becomes hard and brittle. 

On the basis of the shear strength in the glassy and leathery states (see Table 5.2) and the volume fraction of each state (similarly as for the modeling of E-modulus, only glassy and leathery states are considered), the modeling curves of the temperature-dependent shear strength were calculated according to the rule... 

The quasi-liquid state (ql) [50, 52,120] only exists in polymers and is localized at the transition between the glass and the melt, where low mobility and a retarded response to external forces are observed. The low mobility distinguishes this state from a low molar mass (ordinary) liquid which flows much more rapidly under comparable conditions. The retarded mechanical response compares with the leathery state of an elastomer (e.g. Chapter 1 in reference 92). In the present case the ql phase originates from PET. This is also the state in which cold crystallization occurs. The temperature of maximum cold crystallization is indicated in Figure 9.25. 

Most room-temperature cured polyester layups include a gel coat that is applied to the bare mold surface before addition of the fiberglass. After the gel coat reaches a near-cured or leathery state, it is coated with fresh catalyzed resin and covered with a specified number of fiberglass plies to complete the layup. 

At low temperatures all polymers are in the glassy state, by which it is meant that they are relatively hard and inflexible. At some temperature, which is characteristic for each polymer, the material becomes soft and flexible, and so enters the rubbery or leathery state. This change is the glass transition, and it occurs at the glass transition temperature Tg. 

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