Recycling of Polyolefins
- Recycling of Polyolefins -
Moving Towards a Closed Loop Economy with Polyolefins
We come in contact on a daily basis, with items like bread and garbage bags, bottles, containers, hoses, carpets, bumpers and many other products. At a constituent level these are all united by something called polyolefins.
As new materials, polyolefins have introduced new possibilities and benefits at attractive price points. Their annual growth rate remains exponential. Polyolefins are used in every sector of life and are processed by every significant polymer processing technology. Whereas they are commonly perceived as commodity polymers, and their vast production and consumption volumes are testimony to this, polyolefins can also be speciality and outstanding niche materials, with unique properties and features.
The popularity of polyolefins is due to their excellent combination of chemical and physical properties along with low-cost, superior process-ability, and good recyclability. By controlling crystallinity and molecular weight, polyolefins have been produced for a wide range of commercial applications.
Polyolefins are polymers which can be described as a large molecule made up of numerous repeated subunits. The word itself derives from Greek with “polus” meaning many and “mero” meaning parts. Polymers are both man made and are naturally occurring. Rubber is an example of a natural polymeric material that is extremely useful due to its molecular polymer chain with its excellent elastic properties.
While both man-made and natural polymers can exhibit elastic properties, polymers can exhibit a wide range of additional useful properties. Depending on the desired use, polymers can be finely tuned to leverage advantageous properties such as being reflective, impact resistant, tough, brittle, translucent, malleable, soft, elastic, inelastic or insulative.
Examples of Polymers
- Polypropylene (PP) – Tubs, bins, containers
- Polyethylene low density (LDPE) - Grocery bags
- Polyethylene high density (HDPE) – Crates, bottles, drums
- Poly(vinyl chloride) (PVC) - Piping, decking
- Polystyrene (PS) - Toys, foam
- Polytetrafluoroethylene (PTFE, Teflon) - non-stick pans, electrical insulation
- Poly(methyl methacrylate) (PMMA, Lucite, Plexiglas) - Face shields, skylights
- Poly(vinyl acetate) (PVAc) - Paints, adhesives
- Polychloroprene (cis + trans) (Neoprene) – Wetsuits
- Polyethylene terephthalate (PET) – Beverage bottles
Most polymers, commonly referred to as plastics or thermoplastics, are not cross linked polymers which means that they can be melted by applying heat, moulded, then cooled to become solid again. This process can be repeated countless times, making thermoplastics recyclable. Plastic cool-drink bottles are melted down and can be reused to make everything from carpet to fleece jackets, or made into new water bottles. This is all done simply with the addition of heat. Cross linked polymers, also called thermoset resins, on the other hand cannot re-bond after the cross linked bond between molecules is broken. Cross linked polymers often exhibit desired properties such as higher strength, rigidity, thermal properties and hardness. However, they cannot be reformed, reshaped, or recycled.
Polymerization is the method of creating a synthetic polymer by combining many small monomer molecules into a chain held together by covalent bonds. There are two major forms of polymerization, step growth polymerization and chain growth polymerization. The main difference between the two types of polymerization is that in chain growth polymerization, monomer molecules are added to the chain one at a time. In the case of step growth polymerization, monomer molecules can bond directly with one another.
If one were to look at a polymer chain close up, you would see that the visual structure and physical properties of the molecule chain would mimic the actual physical properties of the polymer. For example, if the polymer chain is comprised of tightly twisted bonds between monomers and are difficult to break, chances are this polymer will be strong and tough. Or, if a polymer chain on a molecular level exhibits stretchy characteristics, chances are this polymer will have flexible properties as well.
How are Polyolefins made?
As mentioned previously, polyolefins are the largest class of organic thermoplastic polymers. Polyolefin materials are made up of only carbon and hydrogen atoms with a carbon-carbon bond forming the core of the molecule.
Uncomplicated and inexpensive to dye and mould, polyolefins are preferred plastic resins in industry and for consumer goods, usually processed by extrusion, injection moulding, blow moulding, and rotational moulding methods. An inherent characteristic common to all polyolefins is a non-polar, non-porous, lowenergy surface that is not receptive to inks, and lacquers without special oxidative pre-treatment.
Most polyolefin resins for injection-moulding are used in pellet form. The pellets are about one eighth of an inch in diameter and usually somewhat translucent to white in colour. Many polyolefin resins contain additives, such as thermal and UV stabilisers and pigments. Polyolefin resins are classified as thermoplastics, which means that they can be melted, solidified and melted again. This contrasts with thermoset resins, such as phenolics, which, once solidified, cannot be reprocessed.
Polyolefins - the largest group of thermoplastics.
Polyolefins are plastic resins polymerised from petroleum-based gases of which the two main gases are ethylene and propylene. Ethylene is the principal raw material for making polyethylene (and ethylene copolymer resins) while propylene is the main ingredient for making polypropylene (and propylene copolymer resins), both of which are very popular due to their low cost and wide range of applications. Consequently, the growth of the polyolefin industry in the last 60 years is the story primarily (in terms of volume and impact) of the growth of polyethylene and polypropylene. In South Africa, the two main producers of polyolefins are Sasol Polymers and Safripol.
It is really polyethylene that started it all. The first record of this word appears in the work of the French chemist Pierre Eugène Marcellin Berthelot, who reported in 1869 on his studies of ethylene exposed to boiling alkali, in which he described the olefin fraction boiling at 280–300 °C as “polyethylene”. It is believed that these may have been ethylene polymers, or perhaps oligomers, but they were not solids. Polyethylene was discovered as an unexpected experimental result in 1931 at the research laboratory of Imperial Chemical Industries. Since polyethylene was seen to have many suitable properties, the research was continued and the first plant went online in 1939 just before World War II.
The three major segments of polyethylene (LDPE and its copolymers; HDPE; LLDPE) are now an industry of almost 100 million tons with a value of $183bn. At over 31 % of the global plastic market, polyethylene has indeed become the world’s leading synthetic macromolecule.
The outstanding growth of the polyethylene industry over more than 80 years is shown clearly in the figure below.
Low density polyethylene (LDPE)
In order to manufacture LDPE resins you need high pressure, high temperature and autoclave polymerization reactors. Ethylene is pumped into the reactors and combined with a catalyst or initiator to make LDPE. The LDPE melt formed flows to a separator where unused gas is removed, recovered, and recycled back into the process. The LDPE is then fed to an extruder for pelletisation. Additives, if required for specific applications, are incorporated at this point.
High density polyethylene (HDPE)
There are a number of basic processes used for making HDPE for injection moulding applications, of which solution and slurry process are the most common. In contrast to LDPE production, relatively low pressures and temperatures are used to produce HDPE. The granular polymer leaves the reactor system in a liquid slurry and is separated and dried. It is then conveyed to an extruder where additives are incorporated prior to pelletizing.
Linear low density polyethylene (LLDPE)
A gas phase process is used for making LLDPE. This process is quite different from the LDPE process, but similar to the HDPE process. The major difference compared the LDPE process are that relatively low pressure and low temperature polymerization reactors are used. Unlike HDPE, the polymer exits the reactor in a dry granular form, which is subsequently compounded with additives in an extruder. With changes in catalysts and operating conditions, HDPE resins also can be produced in some of these LLDPE reactors.
The production of polyethylene in the mid-1950s was followed shortly by the discovery and rapid exploration of polypropylene.
Even though it is comparatively a late-comer to the polyolefin market, it can be argued that polypropylene (PP) is the world’s largest polymer. From its invention in 1953 and commercialization in 1957, it has become an industry of (2018 estimate) 86 million tons (27 % of the worldwide plastic market) with a value of over $135 billion. On the same scale as the PE growth curve shown above, the figure below shows the PP production since invention.
The past fifty years or more, the development of polyolefin technology has been centred around the discovery of new catalyst systems. As a consequence, the successful catalyst systems were also accompanied with new production processes, such as gas phase, suspension, solution, etc., for industrial-scale production. However, despite the great scientific and commercial success, there are shortfalls of polyolefin materials that prevent their even wider usage in other areas currently occupied by other polymers that are much more expensive and less environmentally friendly materials.
Polyolefin Injection Moulding
The most widely used plastics for injection moulding are polyolefins. Polyolefins that can be injection moulded include:
- Low density polyethylene (LDPE)
- Linear low density polyethylene (LLDPE)
- High density polyethylene (HDPE)
- Ethylene copolymers, such as ethylene vinyl acetate (EVA)
- Polypropylene and propylene copolymers (PP)
- Thermoplastic olefins (TPO)
In general, the advantage of injection moulded polyolefins compared with other plastics is that they are, lightweight, outstanding chemical resistance, good toughness at lower temperatures, excellent dielectric properties and are non-hygroscopic. In addition, the basic properties of polyolefins can be modified with a broad range of fillers, reinforcements and chemical modifiers. Furthermore, polyolefins are considered to be relatively easy to injection mould.
Major application areas for polyolefin injection moulding are:
- Automotive Parts
- Consumer Products
- Industrial Containers
- Materials Handling Equipment
- Sporting Goods
- Toys and Novelties
The injection moulding process begins with the gravity feeding of polyolefin pellets from a hopper into the plasticating / injection unit of the moulding machine. Heat and pressure are applied to the polyolefin resin, causing it to melt and flow. The melt is injected under high pressure into the mould. Pressure is maintained on the material in the cavity until it cools and solidifies. When the part temperatures have been reduced sufficiently below the material's distortion temperature, the mould opens and the part is ejected. The complete process is called a moulding cycle. The period between the start of the injection of the melt into the mould cavity and the opening of the mould is called the clamp close time. The total injection cycle time consists of the clamp close time plus the time required to open the mould, eject the part, and close the mould again.
There are four basic components to an injection moulding machine:
1. Injection unit
2. Clamp unit
3. Injection mould
4. Control system
The Moulding Cycle
Regardless of part size, weight, or mould complexity, nearly half of every injection cycle is spent cooling the part(s) to a temperature sufficient to allow ejection without post-mould distortion. There are several steps in the production of injection moulded parts. In most cases, the injection moulding cycle begins with the mould open, ejector pins/slides retracted, and the screw/ ram ready with the next shot of material.
The cycle then proceeds as follows:
1. Mould Close: Mould closes and clamp develops full closing pressure
2. Injection: Material is injected into the mould cavity
3. Packing: Material is packed into the mould to fill out the part
4. Plastication: Screw begins to rotate (or ram retracts) to develop the next shot of material
5. Cooling: Coinciding with the start of plastication, the cooling cycle begins (Note: Since coolant continuously circulates through the mould, cooling technically starts as soon as the melt contacts the cavity during injection)
6. Mould Open: Mould opens, slides retract, ejector pins activate, part(s) are ejected. The length of each of these steps will depend on the complexity of the mould, the size of the machine, and the geometry and end-use requirements of the part.
Post-consumer Plastic Recycling
Post-consumer Plastic Recycling is the re-manufacturing of plastic products like bags, wrappers, plastic tubing, pipes, crates, etc. that have been used for its original intended purpose and can no longer be used or re-used and that would have landed up on landfill to a raw material for the manufacturing of plastic products. In South Africa, polyolefins are the largest plastic commodity recycled by Volume / Mass / Percentage. The figure below shows the instances of recycling of these materials in South Africa over the last decade.
Watch a very informative video about Polyolefin Recycling by clicking here.
Recycled plastics, be they high-density or low-density polyethylene, are able to be recycled into a variety of products ranging from crates, bins, pallets, refuse bags and outdoor furniture through to dustbins, irrigation pipes, construction films and furniture covers. In addition, shopping trolleys, coat hangers, paint trays and toys can be manufactured from recycled polypropylene.
Polyco is a South African-based Polyolefin Recycling Company and estimates that plastic packaging consumes 508 000 tonnes per year of polymer while the estimated plastics recycling rate in South Africa in 2013 was 159 000 tonnes or just over 31%. The polyolefin market is set to grow in order to meet the Industry Waste Management Plan target for plastic which is 35% (238 000 tonnes) by 2019.
POLYCO’s 10 Focus Areas:
1. Collecting voluntary recycling levies from all polyolefin converters and importers in SA and employing the funds towards the organisations objectives.
2. Pursue these objectives within a manner consistent with government’s policies on waste management and with a commitment to sustainable development and recycling.
3. Working towards stipulated recycling targets specific to polyolefin recycling.
4. Ensuring the participation by all entities within the polyolefin value chain, in POLYCO activities.
5. Identifying, measuring and reaching company specific key performance indicators in order to monitor their accomplishments and to safeguard accountability to stakeholders.
6. Ensuring that collection and drop-off points are located strategically in all major urban areas.
7. Continually promoting recycling efficiency in the production, design, conversion, collection and recycling of polyolefins.
8. Initiating and participating in environmental education and awareness campaigns.
9. Striving to become the authoritative voice championing post-consumer polyolefin recycling in South Africa.
10. Engaging with research activities to develop alternative production input materials, based on renewable resources for a sustainable and viable future polyolefin industry.
For every ton of polyolefin that is recycled, a ton of plastic product is diverted from landfill and means that South Africa should be able to enter into a closed loop economy where we continuously extract value from existing materials rather than consumer new resources.
MPACT Plastic Containers Mpact Plastic Containers forms part of Mpact Limited and is the leading supplier of plastic containers in the southern African market. The company is committed to creating and implementing sustainable solutions from product innovation through to product recycling. To this end, the City of Cape Town and Mpact Plastic Containers were awarded the SAPRO Trophy for the Recycled Product of the Year for their Fifty/50 Wheelie Bin, in September 2015.
Mpact is also a founding member of Polyco.
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