10-23 ANNEALING

10-23 ANNEALING

In all forming processes polymers are subjected to many conditions that cause internal stresses and frozen strains. This adversely affects the physical properties, heat resistance, and thermal stability of polymers. To eliminate or reduce these defects, polymeric materials are subjected to annealing. At present amorphous thermoplastic polymers undergo commercial annealing only if high quality, such as dimensional stability, optimum strength, and heat resistance, is required. Thermosetting polymers are usually not annealed.

Annealing is carried out by heating the plastic object to a temperature just below its glass transition temperature, soaking it for a suitable time, and subsequently slowly cooling to room temperature. During annealing, the chain mobility increases so much that the highly strained macromolecules can relax almost completely and the relaxation of internal stresses and frozen strains occurs according to the Maxwell relation (see Equation 7-55)

Figure 10-11 illustrates the changes in the specific volume of an amorphous polymer with temperature for different cooling rates.

We can see that for a polymer the T is lower and the specific volume is greater after rapid quenching than after equilibrium cooling. Thus the quenched polymer will show heat distortion at lower temperatures and considerable changes in dimensions that are highly undesirable for the product. Annealing will eliminate these discrepancies and, as path DEC of Fig. 10-11 indicates, the annealed material will be much denser and will show higher distortion temperatures. Quantitatively, the time for annealing at a particular temperature can be evaluated, provided that the curves relating specific volume versus temperature for different cooling rates and the rate constant k for isothermal volume changes are given

FIGURE 10-11 Specific volume-temperature curves for an amorphous polymer. Curve Q results from

quenching, curve S from slow (equilibrium) cooling. T g and T g ′′′′ are the glass transition temperatures

of the polymer on quenching and slow cooling, respectively. DE represents the volume change on

annealing the quenched polymer at the annealing temperature T a , which is only slightly below the glass transition temperature T g . Points B and C represent the specific volumes at room temperature,

T r , or quenching and slow cooling, respectively. (From Annealing by Z. D. Jastrzebski, Encyclopedia of Polymer Science and Technology, Vol. 2, p. 140, edited by H. Mark, John Wiley &

Sons, Inc., New York, 1966.)

Crystalline polymers are frequently annealed for research purposes only. Generally, annealing slightly improves the impact strength of crystalline polymers by lowering their brittleness temperature. This is due to relaxation of internal stresses instead of to the morphological changes of the polymer structure. On the other hand, annealing may increase the size of spherulites so that the crystalline polymer may become susceptible to environmental stress cracking. Perhaps the most evident practical benefit of annealing crystalline polymers is the reduction of the possible future shrinkage. The annealing of semicrystalline polymers is a useful method of increasing their strengths. This method is applied frequently in the manufacture of fibers and films. Annealing has three effects: it increases the initial modulus, increases the ultimate tensile stress, and reduces the ultimate elongation.