Towards a greener plastic

Until now, by definition, PLASTICS have been a group of synthetic polymers derived primarily from petroleum, which, thanks to their properties and versatility, have been able to be extended to practically any area of our lives, our production systems and even our economy.

In fact, they are the second most used application of oil, accounting for about 4% of its production, consuming more than 265 million tons a year worldwide, with production that has an average annual growth of 5% over the last two decades.

We all use plastics, from the poorest countries, which are at levels of about 30 kg/year per inhabitant, to the richest countries, which can be as high as 150 kg/year per inhabitant. It's so common and widespread in our lives that wherever we are we always see something made of plastic.

The bad news is that plastic comes from petroleum, which is a non-renewable source of supply that is also increasingly scarce and expensive, and in addition, from the point of view of its end of useful life, it becomes waste generated in large volumes with a high persistence in the environment, since it has practically no biodegradability.

In Europe, of the 46.4 million tons consumed in 2010, some 24.7 million tons were destined for waste, representing approximately 53%. To this we must add an immature management system, in which 42% of waste ends up in landfill, while 33.8% goes to incineration, and only 24.1% to recycling.

On the other hand, their poor biodegradability means that those plastics that do not enter the waste management circuit can end up dispersing through the environment and causing damage to ecosystems, either through ingestion, drowning, or the release of any of their chemical additives (fillers, dyes, plasticizers, etc.) that can be very toxic.

One of the main lines of work that has emerged in recent years is the use of renewable raw materials for the manufacture of plastics, mainly of vegetable origin, giving rise to those known as bioplastics, plastics that thus have a more environmentally friendly origin and a greater sustainability character, even having a smaller ecological and carbon footprint.

Some bioplastics such as PLA (polylactic acid), synthesized from corn, are already on a good track record. Developed by Cargill in 1987, PLA entered industrial production five years later, and is currently being developed by several companies that have already achieved high molecular weights and continuous, low-cost production processes.

PLA is a polyester similar to PET (polyethylene terephthalate) but with lower thermo-mechanical stability and more biodegradable in nature. The advances made today in their formulation make it possible to obtain thermoplastic polymers that have different properties (crystallinity, elasticity, melting temperature, etc.), and therefore multiple applications.

In one way or another, a good number of the bioplastics that are available today in commercial-scale markets come from starch. Materials such as PSM (Plastarch Material) stand out, a resin produced from the direct enzymatic transformation of starch and cellulose to produce a thermoplastic polymer that is impermeable and resistant to changes in temperature and organic solvents, very similar to Polypropylene (PP), which is also fully biodegradable in its original composition.

In addition to those seen, other bioplastics are also having an interesting development in recent years. These are Polyhydroxyalkanoates (PHAs). Unlike the other bioplastics seen, where the natural origin is for the monomer (which then has to be chemically polymerized), PHAs are aliphatic polyesters produced in a single stage by the bacteria themselves, which in situations of environmental stress produce them as if it were a reservoir.

As PHAs, polymers such as Poly-3-Hydroxybutyrate (PHB), which is the shortest chain obtained so far and the one produced on a larger scale by the industry, are already being produced, although other variants are already reaching the industrial production scale.

PHAs have high polymerization and share extremely interesting properties, such as their insolubility in water, their biocompatibility or their piezoelectric properties, among others, properties that, once barriers in their production are overcome, will make these polymers a very interesting material in the market.

Such is the pull of bioplastics that many traditional petroleum-derived plastics, such as PE or PET, are moving to “bio” production, changing their original raw material for a vegetable origin, using monomers derived from the fermentation or chemical transformation of a natural base (for example the fermentation of sugars to obtain ethylene)

PSM glass — Photo by Aaron Bihari on Flickr.

Another workhorse today is biodegradability, a property that occurs in polymers, both by excess and by default, and that conditions their durability and compatibility both in the environment and in the face of certain uses that you want to give them.

To increase the degradability of some thermoplastics, and essentially of the well-known Polyethylene (PE) and Polypropylene (PP), the plastics industry launched oxo-biodegradable plastics on the market in the seventies, a solution that, however, did not find a definitive market and that, in fact, some countries such as Germany have banned.

The problem with these oxobiodegradable plastics is the current absence of their own treatment route and the incompatibility in many cases with existing ones, since they require oxygen and a certain amount of time for their degradation, being incompatible with the recycling chain.

Many of the polymers currently used and that are soluble in water are not biodegradable either. An example would be Polyvinyl Alcohol (PVOH), which can be found in soap capsules for clothes or dishwashers sold by some brands. Although these wrappers “disappear” in our washing machines, this does not mean that the product degrades, but only that it dissolves, requiring in any case the action of very specific bacteria for subsequent degradation.

The fact that a plastic is biodegradable should imply the possibility of decomposing due to its reaction with the natural environment, through existing natural degradation pathways, both aerobic and anaerobic, without the need for intermediate non-spontaneous chemical reactions, and generating compounds in its degradation that can be integrated into the natural cycle without deteriorating it.

To complement this section, we must also take into account the incursion into the market of “compostable” plastics, which are defined as those that are capable of completely degrading under the conditions that occur in a composting process of organic matter, thus being able to close their life cycle and be integrated into the treatment that should be given to all organic waste.

Coca-Cola bottle made from recycled, plant-based plastic

Compostable plastics go one step beyond biodegradability, and are intended to be returned to nature as nutrients, so it must be ensured that they comply, in any case, with the provisions of international standards (UNE) EN 13,432 and EN 14,995.

The culture of change towards bioplastics and the reduction of the environmental impact of polymers is, in any case, an unstoppable movement. The global production of bioplastics now reaches 1.5 million tons, representing a turnover of 4.4 billion tons, led by non-biodegradable plastics of natural origin, and the forecast for 2016 is an increase of 500% with an expected production volume of more than 5 million tons.
If you want to know more about bioplastics visit the full article at: Environmental Quality