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PTFE Recycling

Logo https://pageflow.neuman-esser.de/ptfe-recycling-en

Story

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Recycling has been one of the most important social and economic topics for many years. Good recycling management means that considerably fewer resources are needed. The resulting energy-saving protects the environment and thus the climate. However, recycled material frequently has a considerably lower quality and therefore a lower value than the source material.
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Polytetrafluorethylene (PTFE) is viewed as particularly difficult to recycle. In a special and extremely work-intense process, the recyclates must be crushed in order to achieve approximately the same particle size as for new goods. However, if some anomalies are taken into account, astounding results can be achieved, because the recyclates not only reach the same quality as new goods, they even exceed it!
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PTFE

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For what?

PTFE or polytetrafluorethylene is often used in industrial areas such as sealing technology, electrotechnology or in chemical plants.

Character profile

The polymer has a sensational profile: high temperature and weather resistance, very good resistance to chemicals and a low friction coefficient are just some of the properties to be emphasized. This makes PTFE a material with extremely versatile use, in particular when subjected to high temperatures or heavy friction.

Problematic properties

However, the material also has its drawbacks. As a result of low intermolecular interaction between the polymer chains, pure PTFE demonstrates low modulus of elasticity, a high cold flow as well as a high wear rate.

Tailor-made filling material mix

In order to compensate these properties and to improve the mechanical properties on the other hand, a tailor-made filling material mix is needed. In particular in the area of sealing materials as used in reciprocating compressors, this optimization is highly relevant. The thermal load and the dynamic pressure stress are particularly high there. Depending on the process gas to be compressed, individual filling material combinations are required here.

PTFE recycling – a great challenge

The special and individual processing make the recycling of PTFE a really demanding challenge. Finally, all filling material mixes consist of highly individual materials in accordance with the respective requirements.

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STASSKOL

The company STASSKOL is a solution provider for highly specialized recipes based on high-performance plastics, such as PTFE, for various areas of application. This also explains why the sealing experts from STASSKOL also occupy themselves intensely with the topic of PTFE recycling.
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Recycling

PTFE not only has positive properties: a low module of elasticity, high cold flow as well as a high wear rate are characteristic for the material. In order to compensate these properties, a filling material mix and also a dry lubricant are added to the PTFE. Its mechanical properties are improved considerably, simultaneously compensating the increased friction coefficient caused by the filling material.

A very work-intense process is necessary for the sustainable handling of PTFE because PTFE is extremely difficult to recycle. These work-intense process flows finally crush the plastic so that the particle size is as small as possible. The amazing result shows that the recycled material not only meets the quality of the new goods, but even exceeds the quality of the original material.
In order to ensure successful implementation of the process, the so-called Press Sinter Method is applied. This can either be implemented as Cold Compression Molding (CCM) or as Hot Compression Molding (HCM). When using the press and sinter method for PTFE, the material or the mix must again be crushed to the original particle size of some 20 to 40 µm. Primarily, small particle sizes are required.





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Mono-fraction collection

of chips and remnants of the corresponding material

Coarse crushing

of all the PTFE waste collected with a customary cutting mill

Fine grinding

the crushed particles with a special jet impact classifier mill

Mixing in

the ground material into the new goods in different proportions using a powder mixer
Manufacturing semi-finished products under standard processing conditions

Production of semi-finished products

of semi-finished products

Checking the properties of the materials

Using hardness and density measurement, tension tests and tribological characterization

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Process engineering

STASSKOL has used two materials to check whether and how PTFE can be recycled and which properties the processed material has: SK202 and SK801.

Whilst SK202 is a CCM-PTFE with glass fiber, carbon and graphite, SK801 is an HCM-PTFE with carbon fiber and thermoplastic filling material.

These two materials were selected in order to enable application of both the cold and hot compression processes. The cold pressing process is a greater challenge because the size of the pulverulent particles plays a special role. A mixture with a significant proportion of carbon and glass fibers was selected because here the danger of reduction of the fiber length ratio and consequently a direct effect on the material’s properties exists.

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Initially, the materials were collected by fraction and coarsely crushed with a customary cutting mill. However, as very fine particle sizes of a mere 50 µm must be achieved, considerably more technical know-how is necessary for the following demanding grinding process.

Here, the professionals for fine and finest grinding are involved in the project: the NEUMAN & ESSER Process Technology experts.

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To achieve the desired particle size, the grinding experiments were carried out using an ICM 15 from NEUMAN & ESSER Process Technology.

For impact grinding, the grinding material is filled into the grinding zone via a constant air flow. During grinding, the grinding tools throw the feed material into the liner and crush it. The constantly present purge air flow carries the ground material out of the grinding zone. This material discharge is blocked by a fast-rotating classifier wheel. The grinding fineness can be set flexibly using the jet impact classifier’s rotary speed. The classifier wheel can only be passed by particles which are small enough to find a route through a gap in the classifier wheel at a high speed. The variation of the parameters grinding speed (m/sec), mass flow (kg/h) and the classifier wheel speed (m/sec), the process can be regulated to achieve the desired particle size. The extent of filling of the grinding chamber also influences the result decisively.

The advantages of the ICM are in its narrow grain distribution with precise upper grain restrictions of 20 μm to 2,000 μm and the low particulate matter shares.

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Grinding

At a constant rotational speed of the liner of 128 m/sec and a constant rotational speed of the classifier wheel of 10 m/sec, mass flow was varied in order to determine the effect on the particle size. The particle size (D50 value) of the ground goods was determined after the process using a laser diffraction spectrometer (type: Mastersizer 2000, manufacturer: Malvern).
At a throughput of 6.9 kg/h for the SK202, a particle size of 47.7 µm resulted and at a throughput of 2.5 kg/h a particle size of 44.6 µm.
For the SK801, the particle size was 56.7 µm at 9.7 kg/h throughput and 47.5 µm at 2.7 kg/h throughput.

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Marc Giersemehl

Technical Managing Director NEUMAN & ESSER Process Technology

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As the results show, reduction of the mass flow for both materials caused a slight reduction of the particle size. For all grinding experiments, particle sizes of approx. 50 µm could be achieved. For economy’s sake, the grist at high throughput was selected for the further experiments. The ground recyclate was integrated into the new goods with a powder mixer at various concentrations for this purpose.
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Analysis procedure

The mixtures included by this means were processed as test semi-finished products, solid rods with 50 mm diameter under normal processing conditions (CCM for SK202 and HCM for SK801). Both tensile specimens and test pins with a diameter of 8 mm are manufactured from these semi-finished products for mechanical and tribological characterization. Moreover, the hardness (shore D) and density of the samples are determined. Mechanical characterization took place using Z005 tensile test equipment from the company Zwick as type “SPI standard FD-105” micro tension bars. The hardness test was carried out using a manual measurement device from the company BAQ, the density was determined gravimetrically.


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The determination of the wear resistance was made using an reciprocating tribometer - self-made by STASSKOL. In it, four samples can be simultaneously quantified with regard to friction and wear using different process gases on various counterfaces. Characterization took place under reciprocating motion with 20 bar contact pressure, a temperature of 120 °C and at an average speed of 2.7 m/sec. The gas type selected was the respective process gas for which the corresponding sealing material is used. This is nitrogen for SK202 and hydrogen for SK801.
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SK202 properties: Compare original to 100% recyclate share
SK202 properties: Compare original to 100% recyclate share
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It is conspicuous with regard to the test results that for cold processed SK202 the semi-finished product consisting of 100% recyclate (sample A2) shows considerably reduced mechanical properties and a lower density and hardness than new goods (sample A1) (table). This is because the increased particle size of the recyclate is approx. 48 µm. The PTFE and the filling materials for the new goods, except the glass fiber, have an average particle size of some 25 - 30 µm. Therefore, the powder can be compressed less densely when cold pressing the recyclate to produce the preform. The air contained in the preform remains in the material during the following sintering process because it runs depressurized.
SK202 properties: Compare original to 100% recyclate share
SK202 properties: Compare original to 100% recyclate share
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SK202 properties dependent on the recyclate share
SK202 properties dependent on the recyclate share
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In order to determine the maximum possible recyclate share in the SK202, mixtures with 10, 20, 30 and 40 percent by weight of recycled material were prepared (samples A3 to A6) and processed as test semi-finished products. Here, we can observe that the properties of the pure new goods were even exceeded with 10, 20, and 30 percent recyclate by weight. Both the modulus of elasticity and the elongation at break are above the new goods values and the low wear rate and lower friction coefficient show that also the wear properties have improved due to the introduction of the recyclate. With a share of 40 percent by weight, the wear properties remain at a very good level, however the density and mechanical properties worsen.
SK202 properties dependent on the recyclate share
SK202 properties dependent on the recyclate share
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SK801 properties: comparison of the original and 100% recyclate share
SK801 properties: comparison of the original and 100% recyclate share
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In the SK801, manufactured with hot pressing, only a slight reduction of the hardness, the modulus of elasticity and the tensile strength can be determined at 100% recyclate share (specimen B2) compared to new goods (specimen B1). In particular, the wear properties within a hydrogen atmosphere profit from the recycling process. It reduces the wear factor and the friction coefficient is more than halved. Therefore, further experiments with staggered recyclate concentrations were not necessary.
SK801 properties: comparison of the original and 100% recyclate share
SK801 properties: comparison of the original and 100% recyclate share
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Knowledge

Examinations show that PTFE can be reused by fraction via an efficient grinding process. In-part, this can even improve its properties vis-à-vis pure new goods. For materials which are processed with cold compression and following pressure-free sintering, the limit for the fraction of recyclate is some 20 to 30 percent by weight. This has to do with the pressure-free sintering process during which the air trapped during sintering remains in the material when the green pellets are manufactured.

That is different for hot compressed PTFE materials. Here, despite the increased particle size, a semi-finished product can be won from pure recyclate whose properties can compete with those of new goods. That also has to do with the molding process during which the material is impinged with pressure whilst it is in a thermoelastic state above the glass transition temperature. As a result, air trapped in the green pellet can escape, providing a considerably denser material structure as opposed to cold compression materials.

The results show that waste from well-filled sealing materials can be recycled based on PTFE and then reintroduced into the production process. This is not only interesting with regard to strengthened environmental protection, but can also provide processing companies with a financial advantage. Moreover, properties such as the resistance to wear are influenced positively due to use of the recycled material.

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Process integration

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Conclusion

Marc Langela

Central Division of Technology STASSKOL

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STASSKOL

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STASSKOL has been active in sealing technology for over 100 years. With the foundation of the company in 1920 as Deventer Werke, the combination of tradition and progress is the key to success. STASSKOL has test equipment and in-house material production that is unique in the world and is therefore able to develop individual plastics tailored specifically to the customer's application in a very short time. However, the company not only offers semi-finished products, but also professional processing of the semi-finished products into machine components. State-of-the-art machinery is available for this purpose. The experts intensively develop their own materials, which are successfully used in many areas such as the food industry, aerospace, vacuum technology, mechanical engineering or the petrochemical industry. Thanks to the application of a special manufacturing process, STASSKOL high-performance plastics have outstanding properties. In addition to high-performance plastics, the product portfolio includes seals for compressors and shaft seals.

Translated with www.DeepL.com/Translator (free version)
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Founded in 1920

as “Deventer Werke”[works]

Locations

in Germany, USA, China.

114 employees

worldwide

Sealing experts

for compressors and rotating equipment

High-performance plastics

up to 1,200 mm diameter

In-house material production

Own research & development

State-of-the-art test bench

Patent forge

Tribometer

Endurance test for sealing elements under realistic conditions and functional testing and wear behavior

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The family-managed and modern family business has its headquarters in Staßfurt, Germany. Additional sales locations can be found in China, India and Japan and a further manufacturing site in the USA. A total of some 120 employees work for the NEUMAN & ESSER GROUP company.
STASSKOL products must meet the most stringent requirements and are constantly developed further. Experience and manufacturing know-how are passed on from generation to generation and are very valuable for the company.
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In addition to the high-performance plastics, STASSKOL’s product portfolio includes seals for compressors and shaft seals. Digital solutions such as the Purge Panel for Rotating Systems (PPRS) and service (also for consultancy, seminars and engineering) round off the product range.


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Thomas Borchardt

Managing Director STASSKOL GmbH

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Exkurs

Sintering is a manufacturing process for molded parts, with which semi-finished products and finished parts can be manufactured. Here, powder masses are initially molded to provide minimal cohesion of the powder particles. Then, the pressed preform is compressed and cured via heat treatment. The preform is either manufactured by the injection of powder masses (for technical products) or molding and drying (e.g. for clay).
When sintering, three stations are passed through, where the volume and porosity of the preform are reduced. Initially, the preform is compressed, before its porosity is reduced during the second phase. During the third step, powder particle sinter necks are formed via surface diffusion between the powder particles, providing the sintered body its rigidity.

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Due to their high molecular weight, some high-performance materials, also including PTFE, cannot be processed using common thermoplastic production methods such as extrusion and injection molding. Here, compression molding and sintering processes must be applied in order to create a uniform and stable composite material from the pulverulent recipe. In principle, we distinguish Cold Compression Molding, CCM and Hot Compression Molding, HCM here.

Cold compression
In particular, cold compression is used for processing thermoplastics which do not soften when heated, e.g. fluoroplastics. Here, the material is pressed into cold molds and subjected to heat treatment after extrusion from the mold by sintering.

Hot compression
For hot compression, pulverulent recipes are pressed under high pressure (≥ 700 bar) to preforms in a mold and the tool then heated to a temperature higher than the melting point. After the softening temperature is exceeded, molding pressure is again exerted on the material structure. Therefore, also in case of very well-filled recipes, it is also possible to achieve an excellent bond between the filler components and the plastic matrix.

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Exkurs

Sintering is a manufacturing process for molded parts, with which semi-finished products and finished parts can be manufactured. Here, powder masses are initially molded to provide minimal cohesion of the powder particles. Then, the pressed preform is compressed and cured via heat treatment. The preform is either manufactured by the injection of powder masses (for technical products) or molding and drying (e.g. for clay).
When sintering, three stations are passed through, where the volume and porosity of the preform are reduced. Initially, the preform is compressed, before its porosity is reduced during the second phase. During the third step, powder particle sinter necks are formed via surface diffusion between the powder particles, providing the sintered body its rigidity.

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Vollbild
Due to their high molecular weight, some high-performance materials, also including PTFE, cannot be processed using common thermoplastic production methods such as extrusion and injection molding. Here, compression molding and sintering processes must be applied in order to create a uniform and stable composite material from the pulverulent recipe. In principle, we distinguish Cold Compression Molding, CCM and Hot Compression Molding, HCM here.

Cold compression
In particular, cold compression is used for processing thermoplastics which do not soften when heated, e.g. fluoroplastics. Here, the material is pressed into cold molds and subjected to heat treatment after extrusion from the mold by sintering.

Hot compression
For hot compression, pulverulent recipes are pressed under high pressure (≥ 700 bar) to preforms in a mold and the tool then heated to a temperature higher than the melting point. After the softening temperature is exceeded, molding pressure is again exerted on the material structure. Therefore, also in case of very well-filled recipes, it is also possible to achieve an excellent bond between the filler components and the plastic matrix.

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PT

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NEUMAN & ESSER Process Technology has had its own top equipped Test Center for carrying out really individual practical experiments since 1982. At the beginning of an experiment, the most important feed material properties are determined, then the grinding experiments are conducted and evaluation of the results follows.
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Manfred Salgert

Managing Director NEUMAN & ESSER Process Technology

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Leading provider

for sustainable process solutions for solids

71 employees

worldwide

Headquarters in Germany

Further locations in the USA and Brazil as well as offices in Egypt, China and Malaysia

Almost 100 years’ experience

in mechanical manufacturing

Generously dimensioned Test Center

for scaling-up and dimensioning tailored production and crushing systems for the customer

Solution provider for grinding and classifying systems

including downstream and upstream system components

Portfolio

with mills, rounding machines, air classifiers, cyclone separators and filters for dry particle processing

Traditional applications

in the ceramics, pigment, preparation and fertilizer industries and also for powder coating production

Sustainable solutions

for electromobility and energy storage batteries, recovered carbon black as well as protein enrichment

After Sales Service

for modernization of and revamping existing grinding systems, also third-party devices

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