Quality of Recycled Plastic
Plastic is a very complex material. It consists of different types of plastics which cannot be recycled together, and of many different types of products with individual product designs. It is a challenge for the recycling, particularly when it comes to quality and usability of the recycled plastic.
(a) Collection(b) Sorting(c) Size reduction and cleaning(d) Further separation(e) Current advances in plastic recycling
“Today, consumers only have a single container for plastic waste, but if the plastic from food packaging is to be reused for new foods, it should not be mixed with plastic that has been used for other purposes—empty shampoo bottles, for example,” Marie Kampmann Eriksen explains.
In addition, the different types of plastics such as PET and PP must be separated. It can be difficult, as packaging for meat, for example, may consist of a plastic tray made of one type of plastic—and plastic film made of a different type.
“If the plastics are not separated, the quality of the recycled material degrades. Similarly, paper residues from e.g., labels, which are often glued to the plastic packaging contribute to ‘contamination’ of the plastic,” says Marie Kampmann Eriksen.
The researchers stress that good sorting of plastic waste from our households is essential in order to achieve better recycling, but that it is necessary to supplement with other measures if we want to take the vision of circular economy in the plastic area to the next step.
“As early as when the plastic products are designed, we must consider recycling. It may, for example, be considered in the preparation of plastic packaging, where all parts, e.g., both container and lid are made of the same type of plastic,” says Professor Thomas Fruergaard Astrup. “In addition, the product design should in any case be aligned with the layout of the waste handling and plastic waste sorting.”
Plastics are inexpensive, lightweight and durable materials, which can readily be moulded into a variety of products that find use in a wide range of applications. As a consequence, the production of plastics has increased markedly over the last 60 years. However, current levels of their usage and disposal generate several environmental problems. Around 4 per cent of world oil and gas production, a non-renewable resource, is used as feedstock for plastics and a further 3–4% is expended to provide energy for their manufacture. A major portion of plastic produced each year is used to make disposable items of packaging or other short-lived products that are discarded within a year of manufacture. These two observations alone indicate that our current use of plastics is not sustainable. In addition, because of the durability of the polymers involved, substantial quantities of discarded end-of-life plastics are accumulating as debris in landfills and in natural habitats worldwide.
Recycling is one of the most important actions currently available to reduce these impacts and represents one of the most dynamic areas in the plastics industry today. Recycling provides opportunities to reduce oil usage, carbon dioxide emissions and the quantities of waste requiring disposal. Here, we briefly set recycling into context against other waste-reduction strategies, namely reduction in material use through downgauging or product reuse, the use of alternative biodegradable materials and energy recovery as fuel. Check this article about the future of plastic recycling https://www.naraloca.com/post/what-does-the-future-of-plastic-recycling-looks-like.
While plastics have been recycled since the 1970s, the quantities that are recycled vary geographically, according to plastic type and application. Recycling of packaging materials has seen rapid expansion over the last decades in a number of countries. Advances in technologies and systems for the collection, sorting and reprocessing of recyclable plastics are creating new opportunities for recycling, and with the combined actions of the public, industry and governments it may be possible to divert the majority of plastic waste from landfills to recycling over the next decades.
Plastic materials can be recycled in a variety of ways and the ease of recycling varies among polymer type, package design and product type. For example, rigid containers consisting of a single polymer are simpler and more economic to recycle than multi-layer and multi-component packages.
Thermoplastics, including PET, PE and PP all have high potential to be mechanically recycled. Thermosetting polymers such as unsaturated polyester or epoxy resin cannot be mechanically recycled, except to be potentially re-used as filler materials once they have been size-reduced or pulverized to fine particles or powders (Rebeiz & Craft 1995). This is because thermoset plastics are permanently cross-linked in manufacture, and therefore cannot be re-melted and re-formed. Recycling of cross-linked rubber from car tyres back to rubber crumb for re-manufacture into other products does occur and this is expected to grow owing to the EU Directive on Landfill of Waste (1999/31/EC), which bans the landfill of tyres and tyre waste.
A major challenge for producing recycled resins from plastic wastes is that most different plastic types are not compatible with each other because of inherent immiscibility at the molecular level, and differences in processing requirements at a macro-scale. For example, a small amount of PVC contaminant present in a PET recycle stream will degrade the recycled PET resin owing to evolution of hydrochloric acid gas from the PVC at a higher temperature required to melt and reprocess PET. Conversely, PET in a PVC recycle stream will form solid lumps of undispersed crystalline PET, which significantly reduces the value of the recycled material.
Hence, it is often not technically feasible to add recovered plastic to virgin polymer without decreasing at least some quality attributes of the virgin plastic such as colour, clarity or mechanical properties such as impact strength. Most uses of recycled resin either blend the recycled resin with virgin resin—often done with polyolefin films for non-critical applications such as refuse bags, and non-pressure-rated irrigation or drainage pipes, or for use in multi-layer applications, where the recycled resin is sandwiched between surface layers of virgin resin.
The ability to substitute recycled plastic for virgin polymer generally depends on the purity of the recovered plastic feed and the property requirements of the plastic product to be made. This has led to current recycling schemes for post-consumer waste that concentrate on the most easily separated packages, such as PET soft-drink and water bottles and HDPE milk bottles, which can be positively identified and sorted out of a co-mingled waste stream. Conversely, there is limited recycling of multi-layer/multi-component articles because these result in contamination between polymer types. Post-consumer recycling therefore comprises of several key steps: collection, sorting, cleaning, size reduction and separation, and/or compatibilization to reduce contamination by incompatible polymers.
Collection of plastic wastes can be done by ‘bring-schemes’ or through kerbside collection. Bring-schemes tend to result in low collection rates in the absence of either highly committed public behaviour or deposit-refund schemes that impose a direct economic incentive to participate. Hence, the general trend is for collection of recyclable materials through kerbside collection alongside MSW. To maximize the cost efficiency of these programmes, most kerbside collections are of co-mingled recyclables (paper/board, glass, aluminium, steel and plastic containers). While kerbside collection schemes have been very successful at recovering plastic bottle packaging from homes, in terms of the overall consumption typically only 30–40% of post-consumer plastic bottles are recovered, as a lot of this sort of packaging comes from food and beverage consumed away from home. For this reason, it is important to develop effective ‘on-the-go’ and ‘office recycling’ collection schemes if overall collection rates for plastic packaging are to increase.
Sorting of co-mingled rigid recyclables occurs by both automatic and manual methods. Automated pre-sorting is usually sufficient to result in a plastics stream separate from glass, metals and paper (other than when attached, e.g. as labels and closures). Generally, clear PET and unpigmented HDPE milk bottles are positively identified and separated out of the stream. Automatic sorting of containers is now widely used by material recovery facility operators and also by many plastic recycling facilities. These systems generally use Fourier-transform near-infrared (FT-NIR) spectroscopy for polymer type analysis and also use optical colour recognition camera systems to sort the streams into clear and coloured fractions. Optical sorters can be used to differentiate between clear, light blue, dark blue, green and other coloured PET containers. Sorting performance can be maximized using multiple detectors, and sorting in series. Other sorting technologies include X-ray detection, which is used for separation of PVC containers, which are 59 per cent chlorine by weight and so can be easily distinguished (Arvanitoyannis & Bosnea 2001; Fisher 2003).
Most local authorities or material recovery facilities do not actively collect post-consumer flexible packaging as there are current deficiencies in the equipment that can easily separate flexibles. Many plastic recycling facilities use trommels and density-based air-classification systems to remove small amounts of flexibles such as some films and labels. There are, however, developments in this area and new technologies such as ballistic separators, sophisticated hydrocyclones and air-classifiers that will increase the ability to recover post-consumer flexible packaging (Fisher 2003).
Rigid plastics are typically ground into flakes and cleaned to remove food residues, pulp fibres and adhesives. The latest generation of wash plants use only 2–3 m3 of water per tonne of material, about one-half of that of previous equipment. Innovative technologies for the removal of organics and surface contaminants from flakes include ‘dry-cleaning’, which cleans surfaces through friction without using water.
After size reduction, a range of separation techniques can be applied. Sink/float separation in water can effectively separate polyolefins (PP, HDPE, L/LLDPE) from PVC, PET and PS. Use of different media can allow separation of PS from PET, but PVC cannot be removed from PET in this manner as their density ranges overlap. Other separation techniques such as air elutriation can also be used for removing low-density films from denser ground plastics (Chandra & Roy 2007), e.g. in removing labels from PET flakes.
Technologies for reducing PVC contaminants in PET flake include froth flotation (Drelich et al. 1998; Marques & Tenorio 2000)[JH1], FT-NIR or Raman emission spectroscopic detectors to enable flake ejection and using differing electrostatic properties (Park et al. 2007). For PET flake, thermal kilns can be used to selectively degrade minor amounts of PVC impurities, as PVC turns black on heating, enabling colour-sorting.
Various methods exist for flake-sorting, but traditional PET-sorting systems are predominantly restricted to separating; (i) coloured flakes from clear PET flakes and (ii) materials with different physical properties such as density from PET. New approaches such as laser-sorting systems can be used to remove other impurities such as silicones and nylon. ‘Laser-sorting’ uses emission spectroscopy to differentiate polymer types. These systems are likely to significantly improve the ability to separate complex mixtures as they can perform up to 860 000 spectra s−1 and can scan each individual flake. They have the advantage that they can be used to sort different plastics that are black—a problem with traditional automatic systems. The application of laser-sorting systems is likely to increase separation of WEEE and automotive plastics. These systems also have the capability to separate polymer by type or grade and can also separate polyolefinic materials such as PP from HDPE.
However, this is still a very novel approach and currently is only used in a small number of European recycling facilities.
Innovations in recycling technologies over the last decade include increasingly reliable detectors and sophisticated decision and recognition software that collectively increase the accuracy and productivity of automatic sorting—for example current FT-NIR detectors can operate for up to 8000 h between faults in the detectors.
Another area of innovation has been in finding higher value applications for recycled polymers in closed-loop processes, which can directly replace virgin polymer (see table 3). As an example, in the UK, since 2005 most PET sheet for thermoforming contains 50–70% recycled PET (rPET) through use of A/B/A layer sheet where the outer layers (A) are food-contact-approved virgin resin, and the inner layer (B) is rPET. Food-grade rPET is also now widely available in the market for direct food contact because of the development of ‘super-clean’ grades. These only have slight deterioration in clarity from virgin PET, and are being used at 30–50% replacement of virgin PET in many applications and at 100 per cent of the material in some bottles. Nara Loca Abadi is a recycled plastic specialist that promotes the use of recycled PET flakes, recycled PET chips, recycled PP & HDPE granules to various plastic and polyester manufacturers.