Polylactic Acid (PLA)- A polydisperse trail of tears.
Many hearts have been broken in the attempt to get PLA on the market. In my case, I bailed from a tenure track asst prof slot to join a startup planning to scale up PLA production. It was quite exciting for a year and then it went belly up. These days, I’m a bit more cautious. I’m not bitter about it. It was a good introduction to polymer science and the marvels of chemical engineering.
In response to a question about PLA, I thought I’d elaborate on it a bit.
I’d be curious to find out more about the PLA experience, particularly the timing. PLA is certainly a big hit right now. Natureworks is sold out, and it has also found some niche applications – surgical staples for instance.
The problem seems to be that if you develop the polymer first and the application second, then you will have a difficult sell. If you go the other direction, it is an easy sell but you are left with lots of little applications.
Whereas we failed with PLA, Dow Cargill LLC has apparently turned it into an ongoing product called NatureWorks.
In case you haven’t heard, PLA is polylactic acid. In its most common manifestation, it is the homopolymer of what is designated as the L enantiomer, which is produced from fermentation. Out of respect for my colleagues I won’t name the now defunct startup company.
Most everyone agrees that the marketing appeal of PLA is that it will biodegrade in the environment all the way to carbon dioxide and water, at least in principle. I qualify this assertion because it has been found that this biodegradation requires a fair amount of moisture to progress in a reasonable time. Landfills can be dry, fetid heaps that are not automatically conducive to rapid breakdown of organic materials. At least on the timescale of a few decades.
In the microbial world, many microorganisms have the enzymatic machinery to biodegrade PLA to lactic acid (LA) and beyond. LA is a natural compound that is judged to have a benign fate in the environment because it is such a common metabolite. In principle LA could be fully metabolized to CO2 and water once it is depolymerized from the PLA. So went the sales pitch.
In the 1990′s, people were concerned that landfills were rapidly filling to the brim with smelly disposable diapers and plastic junk. There seems to be less public debate on this today, but I assume that the landfill issue remains largely unresolved.
PLA is made by an esterification reaction called ROP- ring opening polymerization. PLA is not made directly from LA. It is made from the ring opening polymerization of lactide, the cyclodimer of LA. This way there is no evolved water to add reversability to the polymerization. And lactide is quite reactive. Initiation of this highly strained monomer can be started with an initiator like an alcohol or an HO-terminated polyether in the presence of a Lewis acid catalyst (tin (II) octoate) in the lactide melt phase.
Lactide can be made by the direct cyclodimerization of lactic acid or by a back-biting reaction of oligomeric PLA made by heating LA. I don’t know for sure, but I think that the back-biting reaction may be the major route to lactide today.
There is a lot of IP out there covering specialized applications of PLA. Medical and dental implants, sutures, timed released chemotherapy, etc. PLA will slowly come apart in vivo over time, so it can serve as a kind of scaffold for bone or tissue regrowth or for metered drug release. But this is a small and specialized market.
The big money is in packaging materials- blown films in particular. However, there are technical challenges here owing to a few of the properties of PLA homopolymer. PLA has a relatively high Tg, so films will rattle and sharp package corners will crack. PLA’s crystallinity can be good or bad depending on the application. PLA also has a tendency to have an amber color and it’s films can block.
Commodity polymer films have to be dirt cheap. The premium films are colorless and low haze, have a high gloss, and have a low Tg. There is a whole industry already producing such premium material from inexpensive feedstocks- the polyolefin industry. Sometimes people parse polystyrene and polyvinyl chloride as industries distinct from polyethylene and polypropylene. Polyethylene, polypropylene, polystyrene, and polyvinyl chloride are the predominant synthetic polymer feedstocks used by the packaging industry. They are well dug into the market with established feedstock supply lines and a global presence.
Enter PLA. PLA is ultimately a fermentation product. To get the right tacticity, you need enantiomerically pure LA. The best way to get it is to ferment sugars. LA must be fermented from a carbohydrate source, isolated from the broth (!!), converted to lactide, and polymerized. Fermentation is a low space yield process. The microbes must be kept alive- excessive LA will kill them owing to low pH. You’ll need a cheap source of carbohydrates.
One of the best sources is corn starch, so a big corn wet mill will be required to produce it. The economics of PLA requires that a producer be vertically integrated from starch to fermentation to monomer production to polymerization. Energy and corn prices will have a large impact on your economics.
I’ll spare you the details going forward. Suffice it to say that PLA can’t compete with polyolefins on a price per pound basis at the present time. PLA is boutique polymer at best for the forseeable future. My former company, the defunct PLA startup, felt that the best market segment for PLA was the market occupied by nylon films, due to the comparable cost and food contact and barrier properties. I have no idea what the economics look like now, 10 years later.
I wish all of the players well in the PLA business. It is a worthwhile endeavor and I wish that my experience had turned out differently. So it goes.
For an updated post on PLA, follow this link.