There are many products that use recycled materials instead of pure materials to create economic and process technology meanings. They are commonly used fibers, thermoformed sheets, nonwoven fibers, packaging tapes, and even masterbatches and bottles.
Each of these processes imposes requirements on the properties of the recycled PET material, close to the requirements imposed on the pure material. In addition to the molecular weight (often referred to as the intrinsic viscosity), foreign particles or contaminants in the melt (such as paper, sand, glue, PP, and PVC, and other plastics) will affect the quality of the product.
The highest quality requirements appear in the so-called "bottle-bottle" recycling process. In this case, the process is practical and economically viable only if it is determined and controlled by the source of the bottle, such as the backlog of recovered bottles.
If the bottle used comes from household plastic waste, there is a danger of unknown contamination. And it will also require highly sophisticated sorting and cleaning plants, increasing the cost of bottle blanks.
At present, the collection, packaging, sorting, grinding, cleaning, drying, extruding, filtering, chipping, and solid state polymerization processes of PET bottle flakes all contribute to the increase in efficiency and the resulting cost savings. An important trend is to reduce the production cost by directly processing the cleaned and dried bottle blanks into the above products, without the intermediate step of extruding the PET bottle blanks into pellets or flakes.
Direct processing of PET flakes into one of the above products makes the production process susceptible to interference. Filtration is particularly important. The filtration system must be able to handle the ever-changing pollution load, must operate automatically, use the filter media effectively, and of course it must have a high level of process stability.
Next, we will discuss some of the most important issues and influencing factors related to the processing of PET bottles and sheets, and propose possible solutions for how to deal with various issues. The emphasis is placed on this aspect of melt filtration.
The main influencing factors of the processing of PET bottles and flakes
When processing PET flakes, many interference factors may arise, causing further problems. The following table provides a concise overview of the main interference factors in the processing of PET bottles and flakes.
Example of direct recycling of PET flakes
a) PET bottle resin
Processing PET bottle flakes further into bottle resins requires the best material quality. Assuming that the scrap cleaning and dressing processes are operating efficiently, there are still many process technology needs to be met, and the goal is to obtain scraps that can compete with pure materials in quality. The following are some examples: optional pre-drying, extrusion with or without vacuum, melt filtration, particle cutting, crystallization, and SSP.
Because the melting time of PET must be absolute minimum, the filtration system is crucial.
In order to remove visible contaminants, the fineness of the filter must be between 25 and 50 mm. The scorched particles must not be present in the final product at all times, including the replacement of filters and the self-cleaning of filters. The filtration system, of course, must also be able to automatically adjust itself to handle the changing pollution load, an important feature of the fully automatic melt filtration system RSFgenius.
b) A-PET sheet
The use of PET bottle flakes is a very attractive method for thermoformed sheet manufacturers to reduce their raw material costs. The filtration system is also critical in this case: the sheet must be free of macroscopic defects (dark spots, fish eyes, etc.). In many cases, melt filtration occurs after the gear pump, and pressure-stabilized filtration systems are important. Because of the heavy cost pressures, the melt filtration system must be fully automated, and the use of filter elements is highly efficient, with little material loss due to self-cleaning (back flushing).
The RSFgenius melt filtration system meets this requirement. Filtration fineness varies with the application, but it is usually between 20 and 200 mm.
C) Locally oriented spun yarns, fixed length and short fibers
When spinning recycled pellets or using bottle fragments as feed, different processing principles work: The high-speed spinning process for producing locally oriented spun yarn requires an intrinsic viscosity of 0.62-0.64 dl/g. With bottle fragments, intrinsic viscosity can be controlled by pre-drying or controlled addition of an inherent viscosity modifier. To produce a completely dark or semi-gray appearance, Ti02 additives are also needed.
In order for the spinneret not to plug prematurely, efficient melt filtration is essential. The required filtration fineness is generally 20 mm or less.
The staple fibers are drawn under a slightly low intrinsic viscosity of 0.58-0.62 dl/g. In this case, the intrinsic viscosity is also determined by the degree of pre-drying and the addition of an intrinsic viscosity modifier. Effective filtration is critical to the economical efficiency of the operation by preventing premature clogging of the filter. The typical filter fineness is 36mm.
d) Packaging tape material, high tensile monofilament
High stretch packaging tape is extruded and uniaxially stretched. Monofilaments are melt-spun and then stretched out. Because of the high inherent viscosity of this product, the cross-sectional area is relatively thick, and the filtration fineness is usually between 70mm-120mmp. Larger foreign particles reduce the tensile strength of the packaging tape. Moreover, the regulation mode of the filter system operation is a decisive aspect, because the necessary dimensional stability is very important for deep processing and tensile strength.
The use of PET bottle flakes has an important influence on the production of strapping and tear-resistant monofilament, in order to keep raw material costs as low as possible, especially since recycled materials can be used without problems, as long as the production line is correspondingly design. Until now, PET pure material with an intrinsic viscosity of about 0.9-1.3 will be mixed. The mixing rate is related to the required strength of the straps and the quality and humidity of the scraps. Moreover, the design of the production line plays an important role. The possibility of using PET bottles for scrap is particularly determined by the consistency and fineness of the filtration system used. It is also possible to increase the intrinsic viscosity value of the scrap to 1.0 and higher so that the recycled material can be used 100%.
in conclusion
When processing PET flakes, melt filtration is very important. Many problems related to materials or production processes can only be solved by correct melt filtration. Gneuss' rotary filtering system RSFgenius has some perfect features. It can be fully automated, stable and efficient. Therefore, it is in full compliance with the requirements for the direct recycling of scrap PET bottles, as evidenced by its popularity around the world.
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