One-third of the plastic produced in the world contains polyethylene. It is actually easier and cheaper to produce than to recycle; most plastics end up in nature and take centuries to decompose. It is thus important that material recycling is cost-effective to break out of the single-use plastic preference model.
The chemical resistance of plastics: an obstacle to recycling
The success of plastics is due in part to their chemical inertness. In fact, PVC and lead water pipes have long been used instead of metal because they are not corrosive. The same goes for plastic bottles that can contain highly toxic chemicals. Surprisingly, this chemical inertness also poses a difficulty, as it prevents natural decomposition within an acceptable time frame: it takes nature hundreds of years to break down an average plastic bag. For the same reason, manual chemical recycling is complex and energy-consuming. So, on paper, it is an interesting alternative to mechanical recycling, but the energy costs of returning the monomer are often too high, which is problematic from both an environmental and economic point of view. A boiler-making company can design plastic tubes from recycled plastics.
An innovative method
However, it is possible to produce high-value-added molecules by chemical recycling without going back to the monomer. Research reported in a scientific magazine shows how a university team has succeeded in synthesizing alkyl aromatic molecules. These are often found in lubricants, paints, solvents, pharmaceuticals, and detergents. They are obtained from polyethylene waste. The advantage of this method is that no solvent or hydrogen gas is used, but only an alumina and platinum (Pt/Al2O3) based catalyst, which can break the carbon-carbon chains and reorganize the molecular composition of the polymer to form the aromatic molecules. Recently, it has been reported in the media that aromatic molecules are notoriously complex to synthesize. During the formation of aromatics from polyolefins, hydrogen is simultaneously generated and reused to break the polymer chains. Finally, a long alkyl aromatic chain is obtained, which is a fascinating result. In this way, a boiler-making company can proceed to plastic boiler-making.
Recycling strategies
Recycling plastic waste from a given deposit requires pre-consideration of a number of fundamental elements: mechanical, chemical, or energetic before finalizing a recycling strategy. This analysis is a guarantee of success for the project. Many factors must be taken into account, and the most important ones are the only ones mentioned here. A tool for this decision has been developed for this purpose, taking into account a few basic criteria. First of all, the geographical properties of the deposit. This location, especially its geographical dispersion, generates enormous collection and transport costs. These represent a significant part of the cost of realization. Few recyclers carry out their own waste collection, preferring systems set up by collection specialists or by law. Also, the extent of the sediments, especially the available fraction, remains an essential element. This will determine the size of the industrial recycling unit, which has a major impact on the final product cost. Dispersed and low-tonnage deposits are unlikely to be recovered due to the high cost of recycling. The second factor is the quality of the deposit. The condition of the waste collected from the holder is a factor to consider. The degree of contamination, especially the nature of the contamination, must be carefully analyzed. This can hinder or even stop the recycling process.
Some issues for recyclers
Mixtures of plastic materials create the largest pools, and in most cases, these wastes are mixed at their production sites. Some manufacturers recycle these mixtures to produce low-value products that meet the specifications of their intended application. In addition to the problem of inorganic or organic contaminants that are not compatible with the polymer and that alter its final properties, there are several polymers with different chemical characteristics. In fact, in most cases, the polymers are totally immiscible with each other in the molten state, leading to a multiphase mixture without any interaction between the phases. The final properties are thus modified. Some polymer pairs still have some suitability. This is the case with PVDF and PMMA, which produce materials with suitable properties. Unfortunately, most polymers encountered are classified as incompatible and immiscible blends, so the recyclers have to find solutions where the poor properties of the materials do not pose an obstacle. Moreover, on an economic level, the high cost of pollution and sorting operations compromises the economy of this recycling sector. Selective collections are of great value here, as they help to reduce the complexity of these purification steps.