During the manufacture of virgin material, inline screening detects impurities, with computerised counting and measuring. Gels are classified by size and frequency of appearance and batches of virgin material are sorted according to these findings. However, not all impurities can be eliminated. At this point, gels are typically found, as shown in image 1.
Image 2 shows a polymer defect in virgin material that found its way into the outer layer of a multilayer film.
Failure in films – small cause, big damage
How dramatic the effect of such impurities may be is demonstrated in images 3 and 4. Image 3 shows a torn hole; image 4 shows the cause of high tension within the film that led to the damage (image 4 is an enlarged section of image 3). Here, a small particle of improperly dissolved additive rips the film. Starting point is a tiny (approx100µm) silicate grain originating from a poorly-homogenised molten mass – small cause, big damage.
Common causes of improper dissolving can include low melt temperature, weak homogenisation or poor quality additive.
Image 5 shows LLDPE film at the edge of a hole, created by a grain of starch. The starch grain was introduced into the polymer-stream by a contaminated silo transporter.
Film failures – typical and untypical
The accidental mixing of polymers with different melting points is likely to result in a gel, as demonstrated in images 6 and 7. Image 6 shows a gel from outside the film while image 7 provides a cross-section view. The gel consists of HDPE included in a PP matrix. Mixing different types of polymers generally occurs in plants with poorly-developed waste management.
Image 8 shows a classic ‘black spot’ – thermally-damaged polymer. Black spots occur during plastics conversion when there’s ‘dead space’ in the flow of the molten mass. Plastic accumulating in the dead space is further heated and begins to disintegrate, while the mass of plastic flows by and transports small parts of destroyed material into the finished film. The proper design of processing and recycling equipment can overcome such problems.
Image 9 shows a cellulose fibre embedded in a polymer matrix; such particles are usually brought into the polymer stream through poor cleaning and maintenance.
High quality resin from production waste
The goal of pelletising polyolefin industrial waste material is to provide resin with equivalent characteristics to virgin material for use in manufacturing high-quality products.
Industrial waste materials range from offcuts to waste film. It’s important to prevent material from being further contaminated and to preserve mechanical properties as far as possible.
Undisputed is the fact, that the lengths of the polymeric molecule chains have the biggest influence on the mechanical properties – highest attention therefore has to be directed towards treating the material with caution. The preferred choice are recycling systems that maintain thermal exposure (slow and uniform heating and stringent temperature control) and minimise mechanical stress due to shear.
Shredding of plastic waste should be done with slow rotating knives to avoid partial overheating of material. The designs of drum-like shredders have been proven appropriate. The feeding of shredded material to the extruder should provide for pressure build-up, but should not thermally stress the material. The feeding itself should be quick enough, as freshly cut plastics tend to oxidise and change colour (indicators of possible material degradation). Heat transfer into the material takes place in the temperature-controlled extruder by applying minimal shear and maintaining the lowest possible level of melt temperature and pressure.
Besides keeping temperature and pressure settings under control, other design aspects to be considered include avoiding dead space in which molten plastic can accumulate and ensuring easy accessibility for cleaning and maintenance of machine parts in contact with the material.
In order to minimise the shear applied to the material, it’s not recommended to employ directional changes to the molten mass.
Filtration of the molten mass
The performance of filtration systems depends on cleaning performance. In other words, filtration performance is equivalent to the level of contamination of waste material versus the desired purity of the recycled pellets. In all plastic filtration processes, a mesh gathers impurities and separates them from the molten mass. Depending on specific demands, combinations of different meshes are arranged to provide desired performance.
In the area of plastics recycling, continuous filter systems with back flushing provide excellent results. The choice of system takes into account filtration performance and desired level of automation.
[Ed’s note: Next Generation Recyclingmaschinen (NGR) builds plastics recycling machines for the pelletising of post-industrial and post-consumer waste. These high-quality pellets can be converted into high-quality products. NGR equipment operates in some 770 applications on all continents. The company, based in Austria, is represented in South Africa by Technimac.]