PETpla.net Insider 06 / 2016

MATERIAL / RECYCLING 22 PET planet insider Vol. 17 No. 06/16 www.petpla.net Narrow straps for secure transport PET strapping Based on an article by Thomas Schwachulla, freelance journalist For end users they will probably just be consumables for packages; packaging experts however like them for their mechanical advantages and easy handling: PET straps. a stamping unit to an oven where the stressed material is relaxed. This is followed by a cooling station, the printing unit for individual labelling and the winders for packaging. The extru- sion lines including the upstream and downstream peripheral equipment are almost completely redundant. This has the advantage that in case of failure of one of the plant components, e.g. a mixer, the switch to another mixer can be effected without stop- ping the extrusion line. A throughput of the extrusion lines of 450 to 600kg/h provides an idea of the required level of machine performance. PET is a difficult material At the start of its career as a strapping material, polyester was still considered an engineering plastic and therefore was expensive. It was not until the beginning of the 90s when PET beverage bottles conquered the European market that PET became a mass plastic. “The disposers and recy- clers suddenly provided us with enough recycling material to be able to counter the high cost of virgin material,” recalls Bernd Causemann. “In those days we mostly used regranulate because the flakes usually were too much contami- nated. In the mid-90s we then suc- ceeded to use bottle regranulate for polyester strap production,” reports Causemann. Today, Cyklop hardly processes virgin material any more, but primarily regranulate and regrind (flakes) from DSD material, which comes washed from the recyclers. Virgin material is used only if transpar- ent or white straps or special designs are ordered. PET belongs to the sensi- tive, highly hygroscopic polymers; it is very susceptible to hydrolysis and hence degrades under heat. This prop- erty is detrimental to the strength of the straps. The influence of humidity or the quality of PET can be assessed on the basis of IV value which is indicative of the mean molecular weight. “Regranulation of PET triggers some sort of “downcycling”, i.e. melt viscosity decreases due to the repeated thermal load. In the case of flakes, the bottles become thinner every time, leading to even more rapid degradation of the material. In 2004, we had IV values of 0.75-0.76dl/g for the flakes, today we are down to 0.69- 0.70dl/g”, summarises Causemann. In other words a conflict of interest: Bottle manufacturers want to produce thin bottles and therefore need low IV values. Cyklop, on the other hand, needs high IV values for stretching. According to Causemann, the ideal IV value is 0.83-0.87dl/g. The material is stored in twin- chamber silos from Zeppelin, nine for regranulate and three for regrind. Inside the silos, the chambers are separated by a centre cone and each chamber accommodates the load of one lorry. Batch separation has a practical background: All incoming material is subjected to an in-house IV test which lasts approximately 70-80min – no unloading permitted until after this test. If problems should still arise in spite of the test, the mate- rial can be removed from the cham- ber before it has mixed with the good material. Vacuum conveying is used to transport the material from the silos to the crystallisers. Before PET is processed, it must be dried at temperatures of 160-170°C to a residual moisture of approx. 0.2%. However, at about 80°C, the material reaches a glass transition range where the molecular structure changes from amorphous to partly crystalline. In this transition phase, the material becomes sticky and tends to clump together. For this reason, PET is first crystallised at 150-160°C, keeping it in constant motion to prevent clumping. After crystallisation – separately for the granules and flakes – the pre-dried material must be left to dry to its pro- cessing moisture level for a further five to six hours at a dry-air tempera- ture of 160-170°C before it can be conveyed to the extrusion lines. The era of PET straps actually started in 1979 when the European company Cyklop GmbH in Cologne was the first to take up the issue and made it a success together with the Dutch raw materials manufacturer Akzo. Since then, Cyklop has been manufacturing over 60,000,000km of polyester straps which is equivalent to 1,500 times the circumference of the earth. The success of the polyester straps is based on their numerous advantages over steel straps. PET may not have the tensile strength of steel, but this is not what really matters in load securement. Much more important is the so-called shock reserve when sudden impact is to be absorbed. In addition, elasticity – up to 14% with Cyklop straps, depending on design is crucial to compensate for loss of tension in the case of shrinking packages. By contrast, steel straps only have an elasticity of 1%. What is more, PET does not corrode, the risk of injury is much lower, it can be used for sensitive packages, and it is much lighter than steel. Compared to PP strapping, the glass transition temperature of PET (60-70°C) has the advantage that the packages are held together firmly even under heat. Producing straps only appears to be easy In Cyklop´s Cologne factory, the extrusion lines plus downstream peripherals manufacture about 50 different PET straps with tensile strengths of between 2.000 and 13.000N depending on cross-section. The extrusion lines run with up to ten single strands of a length of 90-100m each, including upstream and downstream peripherals. After the straps have left the nozzle head, they are passed through a water bath to cool down and subsequently are stretched twice. Then they run through RECYCLING S P E C I A L