An old idea may help solve the problem of plastic waste

PLASTICS were once regarded as wonder-materials. They are still ubiquitous, but find less favour than they used to because of the very stability and persistence that won them plaudits in the first place. Persistence is not a quality to be desired in something that gets thrown away, and so much plastic is used in packaging, and in articles that are disposable, that many people now see conventional petrochemical plastics as a nuisance and a threat.

The search is on, then, for biodegradable alternatives. One possibility has recently been explored by David Schiraldi of Case Western Reserve University, in Ohio, and his colleagues. They propose to reach back into history and revive the use of a feedstock that was used to make some of the first plastics invented: milk.

What they actually suggest is using casein, the principal protein found in milk. The curds (of "curds and whey") are mostly made of this protein. In 1889 a French chemist called Jean-Jacques Trillat discovered that if casein is treated with formaldehyde the result is a hard, shiny substance that does well as a substitute for materials like ivory and tortoiseshell. So widespread was the enthusiasm for the new material that Queen Mary herself ordered several pieces of jewellery made from it. However, casein-based plastic of this sort is too brittle for general use. It was eventually superseded by the modern, petrochemical variety, and manufacture stopped altogether by the 1970s.

The thought of reviving it, though, has never quite gone away, and these days the fact that it is made mostly of protein, and could thus be chewed up by bacteria, is regarded as a virtue—if only the structural weakness could be overcome. Dr Schiraldi's approach does this by using a silicate clay called sodium montmorillonite as a skeleton that holds the plastic together.

Sodium montmorillonite can be freeze-dried into a spongelike material known as an aerogel. Aerogels are famously fragile. But that is because they are mostly empty space. Indeed, they are sometimes nicknamed "solid smoke". This fragility disguises an underlying stiffness. Filling the pores in the aerogel with plastic should remove its fragility and, conversely, the network of clay molecules in the aerogel will stop the plastic cracking. So the researchers reckoned that if they mixed casein with the clay and added glyceraldehyde (which substitutes for the poisonous formaldehyde used in the original plastic), they might be able to make something really rather useful.

To test their ideas the team mixed a solution of casein with glyceraldehyde and sodium montmorillonite and vigorously stirred the result to get rid of the bubbles before freezing it at 80°C below zero. Once frozen, the material was placed in a freeze-dryer that removed all the water from it over the course of four days. It was then cured for 24 hours in an oven at 80°C above zero.

The researchers report in Biomacromolecules that their new material matches the stiffness, strength and compressibility of expanded polystyrene, a common packaging material that is the bane of many a rubbish dump. However, unlike polystyrene, it goes away once it has been dumped. An initial experiment suggested that 20% of it vanished within 18 days in a dump-like environment. A comedown, perhaps, from being a queen's brooch. But far more useful.