MATERIALS | MEDICAL TUBING


Right: Ineos says that its Styroflex 4G80 is tailored for tubing applications


“Extrusion residence times and residence time distribution must be taken into consideration: proper equipment sizing is critical,” he said. Gels can be a significant source of TPU tubing


scrap – though for very small gels there is no evidence of having any impact on performance, he said. However, these cosmetic and visual defects are generally unacceptable. Gels could be particles of unmelted polymer, agglomerates of hard block structures, or contami- nants. They can be divided into two types – P-gels (caused during polymerisation, and present before extrusion begins); and E-gels (which are formed during the extrusion process). “Gels caused by extrusion can be minimised by proper equipment design,” he said.


“should be the standard” for a number of reasons – including higher heat stability, increased tensile and flexural properties and predictable performance.


PU alternative While PVC is the traditional material of choice for medical tubes, alternatives are available – including thermoplastic polyurethane (TPU). Ajay Padsalgikay, senior principal scientist at DSM Biomedical, said that TPUs are a useful material for medical tubing – and that their surface properties play a key role in biocompatibility. “Surface chemistry, roughness and surface


energy play an important role in cell– material interactions,” he said. “These characteristics determine protein adsorption and dictate the subsequent inflammatory response.” In order to control these properties, he said that


careful processing of the material is needed – in- cluding correct drying, melt temperature, rheology, screw design, gels and downstream operations. For instance, he said that insufficiently dried


material can lead to reduced molecular weight and mechanical properties after extrusion. “Moisture during extrusion can also cause


streaks on the product and foaming of the melt,” he added.


Flow properties are also critical to the process-


ing of TPUs, as viscosity affects how the material moves through the melt processing equipment. Many methods can be used to assess melt flow properties, which is vital in order to assess their likely behaviour, he said. Screw design is also important: TPUs are sensitive to high shear, so low channel depths in the metering section, and aggressive mixing zones should be avoided, he said.


14 PIPE & PROFILE EXTRUSION | May 2020


Styrene benefits Another alternative material – used widely else- where – is polystyrene (PS). Ineos, a leading producer of styrenics, says that its Styroflex 4G80 is tailored for tubing applications. The material is a TPE based on styrene and butadiene (a hard-soft-hard block sequence SBS). This structure, says the company, gives it a range of key characteristics, including: kink resistance; high roller clamp performance; good bonding; and high clarity.


Ineos says that it also has good processability: it has a wide processing window (170-250°C) and is also around 20% less dense than competing materials – leading to lighter end products. In physical tests, the material showed superior dynamic kink resistance, compared with another Styroflex grade.


Insty R&D also performed a test of its stability in


an IV pump. Here, it was found to work successfully over the course of a 24-hour test. It also performed relatively well in a bonding study.


Water facts As well as using cutting edge materials, extruders must keep close control of the production methods they use.


Christian Herrild, director of growth strategies at US-based extruder Teel Plastics, explained the importance of water sensitivity during tube production. Water is typically used to cool extruded pipe. Its


high heat capacity means it is a very efficient coolant. It can also be used as lubrication in sizing devices “There are three factors in controlling water sensitivity,” said Herrild. “Time, temperature and interaction with water.”


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