Wednesday, September 08, 2010

Raising the Tg of PLA

Polylactic acid (PLA) has always struggled with a low heat resistance. A popular example of its weakness is that a cup made from it would not be able to hold hot coffee. My fellow blogger "Th' Gaussling" has stated that this is due to the "low" Tg, previously noted in this blog as being about 60 oC [1] and that raising it would help. While there certainly is value in attempting to raise it [2], to correlate the weakness to Tg alone is incorrect. Polypropylene has a significantly lower Tg of about -17 oC and yet is able to handle plenty high temperatures. Why? Because of the crystallinity and its occurrence at such high levels that you can completely overlook that it is above its Tg in most circumstances.

If you are working with a purely amorphous material, yes, Tg is all important, but for semi-crystalline materials, it is only part of the description and it can be a very small part at that.

[1] Let's all play nicey-nice here and not get into any of the well-established arguments about measuring Tg, o.k.?
[2]Here is latest effort but there are others going back at least 5 years.


孙尉翔 said...

PLA also suffers low thermal stability during processing (much worse than PET). As a polyester it also crystallizes slowly. Otherwise, it is indeed more favorable to extend the annealing time in order to obtain a high crystalline PLA product than to raise its Tg.

John said...

Issues with increasing the crystallinity would/could include a decrease in opacity and longer processing times.

Th' Gaussling said...

My dear fellow. I certainly defer to your superior knowledge of rheology. I am an admitted and serial small-molecule chemist. The goal of raising the Tg was given to me upon my arrival to the PLA project. But you are certainly right about the weak correlation of Tg wih the suitability for holding hot liquids. Soon, we became more interested in other moving targets like comonomers that would lower the Tg and alter other properties in order to remove the brittle character of PLA films for packaging. Sharp corners were prone to fracture in PLA packaging. Rattle was viewed as undesirable by the marketing folks as well for film packaging applications. Actually, I spent most of my time working on identifying comonomers for lowering the crystallinity.

Anonymous said...

A simple way to lower the crystallinity of PLA is NOT to purify the starting cyclic lactide monomer from meso-lactide, its natural 'contaminant'. Unlike the D- and L-lactide, meso-lactide has both chiral tertiary hydrogens sticking out on the opposite sides of the ring. 10-15 % of meso-lactide in the lactide cyclic monomer is enough to produce a completely amorphous polymer.