Wednesday, October 19, 2011

It costs HOW MUCH?!?

We've been looking lately at polymers that have acidic products after hydrolysis. Polyesters are the natural choice, with aliphatic polyesters (such as polylactic acid (PLA)) being the preferred choice (as opposed to semi-aromatic polyesters like polyethylene terephthalate (PET), polyethylene naphthalate (PEN)…).

Polylactic acid (PLA) is often called polylactide, which is perhaps a more technically correct name. In the polymerization of lactic acid, water is produced as a byproduct and it is difficult to reach high molecular weights with the water hanging around the reactor. Instead, the lactic acid is first dimerized to form a lactide – six member ring (and water).
The water is now easily separated from the lactide, and the lactide can then undergo a ring opening polymerization to form polylactic acid or again, the more technically correct name of polylactide [1], without creating any water.

If you study the illustrations above, you can see that this two-step polymerization can be performed with all α-hydroxy acids. Glycolic acid is a shorter α-hydroxy acid that forms glycolides that then are polymerized to polyglycolic acid (PGA). I thought PGA would be another good candidate for our screening tests but was floored by the price. One source has 10 g for $450! [2] [3].

Now I am well aware that PGA, and various PLA-PGA copolymers are commonly used for medical fasteners (sutures, staples…) and that those materials will be horribly expensive, but this particular application is worlds away from any medical usage. I'm just looking for some cheap industrial grade and am not finding it.

[1] If we are going to insist that all (additive) polymers are going to be named on their starting materials, than we better get rid of polyvinyl alcohol – it will now be called hydrolyzed polyvinyl acetate – polyvinyl butyrate – it will now be called butyrated polyvinyl acetate – and

[2] You know you looking at an expensive material when the material prices start with 1 gram quantities.

[3] The same source has HDPE for $37 a kilo, about a 16x markup over bulk commodity pricing. That "initial" markup is not what I am complaining about. I expect to be paying that for small lab quantities.

1 comment:

Patrick said...

A quick scan over the UK suppliers reveals that (like lactic acid) it's usually supplied as a saturated aqueous solution - a 70% solution goes for about £10/litre. The neat stuff is about £200/kg! (sigma-aldrich).