PHOTO: Fig 1: Diagram of the fluorination process flow
BUSCH Vacuum Pumps, a German company established over 53 years ago is one of the world’s largest suppliers of vacuum pumps, liquid ring pumps and over-pressure blowers. Its Africa head office, Busch Vacuum Pumps and Systems Africa, is based in Johannesburg and supported by sales and service branches in Cape Town and Durban.
A recent case study of the use of Busch Vacuum Pumps used in the fluorination of plastic parts reveals how vacuum technology is a core component.
The fluorination of plastic parts to achieve different surface properties is an important pre-treatment step in the processing of plastic parts. German company, Fluor Technik System GmbH offers fluorination of plastic parts to change the wetting behaviour, the adhesion properties, the friction or stickiness and the barrier properties of a variety of plastics.
“The core component of a fluorination system is the vacuum technology,” says Bernd Moeller, MD of Fluor Technik System. “This is because the quality of the treated parts depends on the reliability of this technology.”
Fluor Technik System also manufactures customer-specific fluorination systems and sells these worldwide. To generate the necessary process vacuum, dry-compressing vacuum systems from Busch are installed, each with a Cobra NS screw vacuum pump as a backing pump.
Local Busch service technician for start-up and training
Busch Africa is able to supply most pumps, blowers and spare part kits ex-stock and is equipped to service and overhaul pumps in its workshop, or to carry out service work and commissioning at a customer’s site.
“We have engineers in house in SA that can assist with projects involving vacuum pumps and centralised vacuum systems. They are supported by Busch centres of excellence in Germany, England and the Netherlands for detailed system design and construction. All designs, drawings and documentation are completed to the highest standards,” said Sean Pieterse, MD of Busch Africa.
“The Busch Africa office is a 100% owned subsidiary of Busch International, meaning customers work directly with the OEM company, which is unique in Africa and benefits our customers with flexibility, expert technical advice and very competitive pricing” he added.
In Moeller’s fluorination systems, he only uses vacuum systems from Busch that can be run completely ‘dry’; that is, without any operating fluids in the compression chambers.
The fluorination process
The fluorination process (Fig 1) can be carried out within different systems. These either consist of a process chamber, loaded with a batch each time (offline systems), or web systems, with which plastic web material is continuously fluorinated as it passes through (inline systems). A gas mixture comprising 10%fluorine and 90%oxygen is used as the fluorinating agent. The change in the surface properties is caused by the highly reactive fluorinated gas. This replaces a portion of the hydrogen atoms in the surface material with fluorine atoms. The fluorination process is therefore a chemical reaction and not a type of coating. The depth of penetration of the fluorine atoms into the substrate is confined to the molecular level. The properties of the base material and its dimensions remain completely unchanged. Only the surface energy and the polarity are influenced by the fluorine, and these effects are lasting and stable.
Through targeted process control, the fluorination results can be precisely adjusted to the base polymer and the intended application. The basic process parameters are the dwell time in the process chamber, the temperature, and the fluorine concentration, which is determined by the pressure. These parameters enter into the fully automatic process control as a ‘prescription’. All the parts to be fluorinated therefore receive an individual ‘prescription’ that is tailored to the base material and the intended surface property as part of a process that is always replicable.The gears of the Cobra NS screw vacuum pumps and the gears of the Puma vacuum boosters are lubricated with prefluorinated polyethers. This ensures that the reactive fluorine cannot bond with the operating fluid. In the case of vacuum reactors that are rectangular in shape, gas circulation is required within the process chamber in order to ensure that it fills all areas completely and evenly and to guarantee homogeneous surface processing. Water-cooled Puma vacuum boosters (Fig 3) are also used in this process and are operated independently from the vacuum system.
A Cobra NS (Fig. 2) screw vacuum pump is used as a backing pump and a Panda vacuum booster is installed upstream of this. Cobra NS screw vacuum pumps were developed by Busch for the semiconductor industry. They offer a high level of process reliability and thanks to their optimized screw profile, they achieve short evacuation times. Cobra NS screw vacuum pumps are hermetically sealed and therefore no fluorinated gas can escape into the surrounding area.
Both vacuum pumps, including the drives, are water cooled, which means that the operating temperature can be precisely adjusted. Various monitoring devices report any faults that might occur as soon as they are present. The temperatures of the vacuum pumps themselves, the operating fluid in the gears and the coolant temperature are constantly measured as part of this. The quantities of coolant and oxygen used to rinse the Cobra NS once the process has finished is also monitored.
Moeller has thoroughly tested the vacuum system from Busch and is utterly convinced that it represents a robust and reliable solution. The vacuum systems are in operation between 8 and 24 hours at a time, depending on the applicable working hours.
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