By Claudia Henkel and Michael Latinski, Oerlikon Manmade Fibers
Almost 500,000 tons of nonwovens are currently used annually in the production of filters, which corresponds to around 10% of current worldwide industrial nonwovens manufactured.
The filter market comprises air and liquid filtration. While more than 170,000 tons of nonwovens were manufactured for gas/air filters in 2015, the volume for liquid filters was almost twice that at around 295,000 tons.
Depending on the specific application, the demands placed on filters are extremely high. For example, high-efficiency particulate air filters for clean rooms must filter < 1 µm particles from the air with virtually 100% efficiency.
Filters must achieve their separation performance with the lowest possible pressure drop – i.e. filter resistance. Pressure drop is one of the most important quality features of filters: the lower the pressure drop, the more energy efficient the filter of the corresponding filter class.
A good example is vacuum cleaner filters: the class depends on the energy consumption, which is also influenced by the vacuum cleaner filter. If the filter is too dense, the vacuum cleaner draws more electricity and the energy consumption increases.
Highly efficient filters can either be manufactured from either synthetic fibers or glass fibers. Filters in the high efficiency particulate air (HEPA) and ultra-low particulate air (ULPA) classes – the classes with high filter separation performances – are still produced using glass fibers.
However, synthetic filters offer benefits: glass fiber filters achieve their separation performance only by means of the fineness of the pores, which is why the pressure drop is relatively high.
By contrast, filters manufactured from synthetic fibers can be electrostatically charged. As a result, their pores can be comparatively larger, which in turn considerably reduces the pressure drop.
Further, glass fiber filters harbor the risk that glass fiber particles can be released, which end up in the clean air, result in contamination and could be breathed in.
Meltblown technology is one of the most efficient methods for producing very fine and highly separating filter media made from synthetic fibers.
Depending on the application, the pore size of a meltblown nonwoven material ranges from 5-40 µm. Smaller pores increase the mechanical filtration performance, albeit at the expense of higher pressure losses.
The fineness of the meltblown fibers used for filter media lies in the 200-2,500 nm range. However, even fibers with nanoscale fineness are often not sufficient to filter the finest particles from air or liquid flows.
As a result, the nonwovens are treated online or offline; for example, they are electrostatically charged to increase their filter performance.
The filter efficiency can be considerably increased by means of so-called electro-charging, where the nonwovens are electrostatically charged.
There are two options for charging nonwovens for industrial applications: triboelectric charging and corona charging; at present, corona charging is the method predominantly used.
Oerlikon Nonwoven, one of the leading manufacturers of meltblown systems, is currently developing its own concept for electrostatic charging of meltblown nonwoven materials. This distinguishes itself from concepts currently available on the market owing to its high flexibility with regards to charging the most varied nonwovens.
Users can choose from a large number of variation possibilities and set the optimum charging method depending on the filter application.
Initial laboratory trials have shown that the Oerlikon Nonwoven charging unit can also be used to manufacture EPA- and HEPA-class filters. For example, a H14-class filter with an efficiency of 99.995% at a pressure drop of less than 100 Pa has been produced.
The polymers used for manufacture are just as diverse as the applications for the filters. The spectrum ranges from polyesters such as polyethylene terephthalate, polylactic acid and polybutylene terephthalate, polyamide and polyolefins such as polypropylene and polyethylene, through to special polymers such as polyphenylene sulfide and thermoplastic polyurethane.
All these and other raw materials can be spun using the Oerlikon Nonwoven meltblown process. This process is characterized by its constant melt pressure distribution and simultaneously consistent dwell time across the entire width of the spinning beam, which in turn ensures particularly homogeneous nonwoven properties and basis weights.
Further, the innovative guidance and distribution of the process air outside the coat-hanger distributor offered by the Oerlikon Nonwoven technology prevents so-called hotspots. As a result, even extremely sensitive raw materials can be reliably processed into high-quality nonwoven media.
In-house R&D Center
As a high-tech company with a clear commitment to research and development, Oerlikon Nonwoven is dedicated to continual innovation: at the comprehensively equipped meltblown R&D Center, the processes and machine technology – along with the manufacture of application-optimized nonwovens – are continually tested and further developed.
The pilot line has a spinning width of 550 mm and comes with additional equipment. A comprehensive range of testing devices and tools is available for analyzing nonwovens.
By contacting the Oerlikon Nonwoven sales department, customers can also utilize the laboratory for their own product development and optimization.
The trend in filtration applications is increasingly focusing on ever more efficient filter media with minimum pressure drop; the Oerlikon Nonwoven meltblown technology is particularly suitable for this.
This is also underlined by the orders placed with the Neumünster, Germany-based systems manufacturer by leading meltblown nonwoven material manufacturers over the past few months.
The overwhelming share of filter media still used today is produced from multilayer nonwoven laminates: manufacturers generally have to resort to nonwovens that are available on the market.
By using the Oerlikon Nonwoven meltblown technology, these filter media can be directly manufactured in a single-step process, whereby the producer has complete control over the raw materials deployed and lower manufacturing costs, as well as being able to reliably and consistently fulfill the high quality requirements of its customers.
For more information contact:
Oerlikon Manmade Fibers