is polymer heat of fusion relevant to quench cooling?

[anthonym]

I am setting up an ANSYS heat transfer model for the rapid quenching of an extruded polyester (PET) sheet, such that this otherwise semi-crystalline thermoplastic remains amorphous. In view of this, and the fact that differential scanning calorimetry (DSC) quench cooling curves don't show an exotherm in the vicinity of the melting point, would it be correct to ignore the heat of fusion and consider only the heat capacity?

Thanks for your help!

Anthony Mrse

 

[25362]

Anthonym, thinking aloud:
Although the DSC draws an enthalpy cooling curve w/o any sharp transition, PET shows to have a T_M = 265^oC. As in most linear polyesters, below T_M and down to the glass point (for PET, T_G = 70^oC) PET chains have yet sufficient mobility -when in high viscosity melts- to disentangle and show some degree of crystallinity in the presence of nucleating agents, even for packaging or bottle grade PET.
The heat capacity as published: 1200-1350 J/(K.kg).

Although probably of a low proportional value, it remains a question of whether to totally discard enthalpy of melting, even on rapid quenching of extruded PET. I wouldn't. By the way, PETP is the IUPAC name for PET.

Good luck!

 

[anthonym]

Thanks for your thoughtful reply to my original post!

We are interested in the time required to drop the sheet temperature to about 220C, at which point we should be able redirect the sheet around a bar in the quench tank without causing "ripples" to be frozen in. Therefore, the sensible heat loss corresponding to the decrease in temperature from 265C to 220C is of the same order as the heat of fusion cited for a typical 30-40% crystalline PET. Am I correct in assuming that the heat of fusion of completely amorphous PET would be zero, so that your caution to me is based on the likelyhood that our "amorphous" sheet likely has some residual (e.g., 1-2%) crystallinity?

As I understand it, the crystalline fraction of a sample is calculated from the heat of melting (as calculated from a DSC heat-up trace) by dividing by the heat of melting of a 100% crystalline reference polymer (100-140 J/g published for PET). This implies that the heat of melting of the amorphous component is zero, correct?

Thanks again, Anthony.

 

[25362]

My personal experience is with waxy materials. High crystallization rates, even when cooling or quenching with liquid nitrogen (!), make it almost impossible for P/E melts to give amorphous polymers; quenching of PET melts, on the other hand, may lead to amorphous polymers. If there are no internal nuclei, or external nucleating agents (2-10 nm in size) such as additives, dust, or even the container walls, crystals may not form resulting in a totally amorphous solid. After the first nucleation is there, a second solidifying step (crystal growth) may warp or shrink the polymer sheet.
Since melting is defined as the thermal transition from a crystal to an isotropic melt, you are right: no crystals, no enthalpy of fusion.