Effectively and non-destructively clean polymer adhesions from woven screen belts
THE SITUATION
In spunbond systems, polyester-based woven screen belts are used for the filament deposit. During plant startup, as well as during production, it is inevitable that filaments will break and stick in the form of drops on the wire belt. During maintenance and downtime these polymer droplets must be removed from the screen belt in order to not to permanently impair the quality of the nonwoven fabric.
Presently, this is done with a highly manual and timely effort using tools, such as spatulas and Scotch-Brite. The polymer is placed in a soft state by means of hot air and removed with a special pliers. Despite caution, damage to the band structure occurs. Often the screen belts, which can be up to 5.40 m wide, get removed, which increases the risk of additional damage and contamination.
THE TASK
The Sächsische Textilforschungsinstitute e.V. (Saxon Textile Research Institute – short: STFI) is a non-profit research institute in the Free State of Saxony, Germany, committed to the long-standing traditions of Saxon textile research. Its process and product related R & D work reflects classic textile technologies as well as innovative, unconventional solutions for the widest range of applications. In doing so, a lively exchange with companies and research institutes in Germany and abroad is cultivated. With the existing know-how and through an intensive transfer of results, companies of the textile industry are supported.
The objective was to find a cleaning technology that makes it possible to effectively and non-destructively clean polymer adhesions from the screen belts. The screen belts should be able to be cleaned during system operation or maintenance cycles, but without removal from the system.
In addition to the search for an improved cleaning method compared to the current process, new screen belts had to be developed, which cause the polymer drops that inevitably occur in the spunbonding process to penetrate less deeply into the fabric structure. With the right cleaning technology and the new fabrics, a longer service life of the sieve belts should be achieved.
THE COLD JET SOLUTION
The Saxon Textile Research Institute has a Reicofil®4 spunbonding line on a technical scale, which is suitable for testing the screen belts and the cleaning system.
In a research project, new tissue structures were developed by the company Voith Paper Fabric & Roll Systems GmbH & Co. KG, Heidenheim, Germany, screen belts were produced and tested in the STFI. Various cleaning technologies were also investigated.
In the preliminary investigations on the screen belt cleaning, three different systems were tested: high-pressure water blasting, CO2 snow blasting and various CO2 dry ice pellet blasting.
High pressure water blasting is suitable for preventing clogging of the fabric structure during production caused by adhesion of fibers or monomers. However, it is not suitable for polymer attachment.
CO2 snow blasting is able to remove smaller polymer drops without damaging the screen belt structure. However, it lacks the kinetic energy to remove larger polymer drops. Another disadvantage is the high demand for compressed air. Snow blasting requires the installation of a mobile compressor. Snow blasting also requires a facility to obtain CO2 in gas cylinders.
Many systems based on uncut CO2 dry ice pellets do not apply enough energy to remove the droplets from the screen belt with little compressed air. At a pressure of 2 bar, the cleaning performance was insufficient, which meant that for all screen belt samples only very small polymer drops (≤ 3 – 5 mm) could be removed. At pressures of ≥ 3 bar, the fabric structure of all sieve belts was changed at exposure times of more than 3 seconds and visibly destroyed after a few more seconds.
One method that was different from all others was the i³ MicroClean DX® system with patented technology from Cold Jet GmbH in Weinsheim, Germany.
“The advantage was that in this case the compressed air system of the technical center of the STFI e. V. could be used, since the Cold Jet system’s maximum compressed air requirement is 1.4 m³ / min and also the existing system with the ½” compressed air connections and the pressure up to 7 bar”, says Sven Schuffenhauer, the responsible technician from the nonwovens / recycling department.
The dry ice pellets are shaved into sugar-sized particles (MicroParticles), then added to a special dosing system and then accelerated by a compressed air stream through nozzles with different outlet cross sections (round nozzle with 4.7 mm or wide nozzle 11.9 x 2.2 mm) and finally transported to the surface. Since ice pellets are used directly and the ice rate can be varied between 9 and 18 kg/h, only a small amount of CO2 escapes into the environment. Compared to CO2 snow blasting that is only 10 to 15%.
THE RESULTS
Initially, the newly developed screening belts from Voith were tested for their suitability for spunbond production on the spunbonded nonwoven line at STFI. The fabric structure had a lower open volume on top compared to conventional screen belts. Thereafter, the plant setting was deliberately chosen so that the screen belt surface was contaminated in a relatively short time with polymer droplets, in this case made of polypropylene.
According to the evaluation criteria defined in Table 1, when using the CO2 snow blasting system with optimal positioning (contamination with polypropylene or polylactide), the results for two of the developed screen bands can be assigned to grade 2 and for the other two bands to grade 3.
With the Mobile Cleaning System i³ MicroClean DX® from Cold Jet, all existing polymer drops could be removed individually without damaging the screen belt structure.
The MicroClean is a compact, single-hose, electric low-pressure blasting system with patented microparticle shaving technology. Small, odorless microparticles, which form a finer cleaning stream compared to conventional dry ice pellets, are accelerated to supersonic speed with compressed air. This is an effective, but gentle cleaning – even for sensitive surfaces. Upon impact, there is a combination of kinetic, thermal shock and thermodynamic effect. The latter causes the dry ice to sublimate under ambient conditions – that is, the dry ice converts from a solid directly into the gaseous phase. The dry ice expands within a few milliseconds to a volume of up to 700 times. This results in a kind of “mini-explosion” at the point of contacts that spreads over the surface. This “explosion wave” has a very efficient lifting force, which reliably removes the polymer drops from the screen belt. There were no changes in the dry ice treated surfaces.
“The results of the cleaning and abrasiveness tests were very convincing,” recalls Sven Schuffenhauer. “The belt moved at 6 m / min, which is already fast for a maintenance operation.”
For this, only a quantity of 9 to 12 kg/h of CO2 pellets was needed. The compressed air volume required was 0.7 to 1.0 m³/min, which corresponds to one-third to one-fourth of the consumption compared to other systems. The suitability of this system for screen belt cleaning in spunbonded systems could thus be proven.
According to the criteria defined in Table 1, the result in both cases was assigned to Grade 1.
EXPRESSION OF THANKS
We thank the Federal Ministry of Economics and Technology for the promotion of the funding project (Reg. No. MF 150119) within the funding program “R & D funding of non-profit external industrial research institutions in East Germany – Innovation Competence East (INNO-KOM-Ost) – Module: Market-Oriented Research and Development (MF) “.