Processing of Dyneon PTFEFine Powder

3M™ Dyneon™

PTFE Fine Powder

Processing of Dyneon PTFEFine Powder

3M™ Dyneon™

PTFE Fine Powder

Contents

1 Introduction 4

1.1 About the Company 4

1.2 About our Flouoropolymer Product Family 4

2 Physical Fundamentals of PTFE Fine Powder Processing 6

2.1 Production of 3M™ Dyneon™ PTFE Fine Powder 6

2.2 Phenomenology of Paste Extrusion 6

2.2.1 Morphology of the Fine Powder 6

2.2.2 Paste Mixing 6

2.2.3 Preform Fabrication 6

2.2.4 Paste Extrusion in Funnel-Flow Design 7

2.3 Morphological Changes during Paste Extrusion 7

2.3.1 Morphology of the Paste Extrudate 7

2.3.2 Crack-up of the Secondary Particles 8

2.3.3 Reversible Deformation of the Primary Particles 8

3 Properties and Handling of Dyneon PTFE Fine Powder 9

3.1 Reduction Ratio 9

3.2 Extrusion Pressure 10

3.3 Particle Size and Particle Size Distribution 10

3.4 Specific Weight 10

3.5 Density 10

4 Fundamentals of PTFE Fine Powder Processing 11

4.1 Packaging and Storage 11

4.2 Preparation of the Extrusion Mixture 11

4.3 Powder Screening 11

4.4 Mixing with Lubricants 12

4.5 Pigmentation 13

4.6 Maturing of the Extrusion Mix 13

4.7 Preform Compression 13

4.8 Extrusion 13

5 Fabrication of Films, Tapes and Sealing Cords 14

5.1 Profile Extrusion 14

5.2 Calendering 15

5.3 Film and Sheet Drying 15

5.4 Film Stretching 16

5.5 Fabrication of Unstretched and Stretched Sealing Cords 16

6 Fabrication of Tubing and Hoses 17

6.1 Lubricants for Hose Extrusion 17

6.2 Tube Extrusion 17

6.3 Drying and Sintering of Tubes 18

6.4 Tube Testing 19

6.5 Typical Applications of PTFE Tubes 19

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7 Fabrication of Thick-Walled Pipes 20

7.1 Specimen Preparation and Lubricant 20

7.2 Carbon Black Pigmentation and Antistatic Treatment 20

7.3 Liner Extrusion 20

7.4 Drying and Sintering of the Liner 21

7.5 Typical PTFE Liner Applications 21

8 Fabrication of Cable Insulations 22

8.1 Preparation of the Extrusions Mix 22

8.2 Cable Extruder 23

8.3 Cable Extrusion 23

8.4 Drying and Sintering of Cables 23

9 3M™ Dyneon™ PTFE Fine Powder Compounds 24

9.1 Property Modification by Means of Fillers 24

9.2 Manufacturing Methods of PTFE Fine Powder Compounds 24

9.3 Typical Applications of PTFE Compounds 24

10 Special Applications 25

11 Trouble Shooting Guide 25

12 Compliance and Safety 27

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This brochure provides information on how to process 3M™ Dyneon™ PTFE Fine Powder for the manufacture of a va-riety of products, such as wire and cable insulation, tubes, pipes, tapes and sealing cords.

1.1 About the CompanyDyneon is one of the world’s leading fluoropolymer suppliers with de-cades of experience in developing applications. The company develops userfocused application possibilities for the entire Dyneon product range in its own technical service laboratories and research facilities in Europe, USA and Asia.

1.2 About our Fluoropolymer Product FamilyDyneon offers a broad product family of high-performance plastics that is divided into three product groups:

Fluoroelastomers are cross-linkable, amorphous copolymers with a molecular weight of 5 x 103 - 5 x 104 kg/kmol. They are extruded at temperatures of <150 °C and then cross-linked in a heated mould. The resulting vulcanisates lose their formability and weldability.

Fluorothermoplastics are partially crystalline copolymers with a molec-ular weight of 5 x 104 - 8 x 105 kg/kmol. Due to a modification of their copolymer composition, they feature a broad melting-point range of 100 - 320 °C. Fluorothermoplastics can be processed at temperatures of up to 400 °C with conventional thermoplastic technologies.

Figure 1.1: Division of different fluoropolymers

Introduction

Fluoropolymers

Fluoroelastomers

Fluorothermoplastics

PTFE

4

PTFE is a partially crystalline homopolymer with an exceptionally high molecular weight of 107 - 108 kg/kmol. This high molecular weight makes melt-processing of PTFE impossible. The product leaves the polymeriza-tion process in a highly crystalline form with a crystallization degree of > 90% providing it with a melting point of 340 - 345 °C. After the first melt, the crystallization degree decreases to approx. 60% and the melting point to 327 °C.

The proven properties of PTFE are:

excellent all-round chemical resistance

widest service temperature range of -200 °C to +260 °C

superior dielectric properties

no embrittlement or ageing

very good non-stick properties

dimensional stability and stress cracking resistance.

The PTFE group is subdivided into suspension polymers and emulsion polymers (Figure 1.2). Three further product groups evolve from the latter: micro-powders, dispersions and fine powders. Suspension polymers are divided into pre-sintered ram extrusion powders as well as non-free-flow-ing and free-flowing powder grades. PTFE compounds, i.e. blends of PTFE and fillers, are mostly made of suspension polymers. In addition, Dyneon has a number of fine powder compounds available.

Suspension polymers leave the polymerization process in the form of up to 2000 μm polymer particles with irregular shapes, so-called reactor beads. These are then milled until they are just 10 μm fine and, if neces-sary, agglomerated in an additional process step, where the fineness can be adjusted to particle sizes in the range of 100 - 600 μm.

Agglomerated products are free-flowing. Milled products are non-free-flowing and are also used for the manufacture of PTFE compounds. PTFE powders are processed by specially developed technologies, such as pressing and sintering as well as ram extrusion. The products that are particularly well suited for the ram extrusion process are those that are non-milled, yet pre-sintered, whose dosing and free-flowing properties are achieved by screening the rough and fine parts. Suspension polymers are mainly used for the production of semi-finished products which are then drilled, turned, planed or milled into a variety of finished articles.

Emulsion polymers are converted into water-dispersed latex particles during polymerization. These so-called primary particles are of spherical shape with a diameter of 180 - 250 nm. The primary particles are coagu-lated into so-called secondary particles with a diameter of approx. 400 - 600 μm and agglomerated into a free-flowing fine powder.

Emulsion polymers are divided into three groups according to their in-tended use: fine powders, micro-powders and dispersions.

Fine powders cannot be processed from the melt due to their high mo-lecular weight in the 107 - 108 kg/kmol range. A special technology, known as paste extrusion, has therefore been developed to produce high-quality finished articles, such as tubes, wire and cable insulations, and liners. Specific fine powder types have been developed for the various fields of application, described in the “3M™ Dyneon™ Polytetrafluoroethylene Prod-uct Comparison Guide” brochure.

PTFE dispersions with a solid content of up to 65% are used for impreg-nation and coating of metal surfaces and fibreglass.

Micropowders are used as additives for a variety of applications to im-prove slip and non-stick characteristics. Their molecular weight, similar to that of fluorothermoplastics, is in the range 5 x 104 - 106 kg/kmol. In contrast to fine powders or suspension polymers, micro-powders cannot be used for the production of semi-finished products, as a result of their low molecular weight.

The next chapter describes the theoretical and physical background of the materials, which helps to explain the careful steps that must be taken during the paste extrusion process.

Suspension polymers

Ram extrusion powders

Free-flowing powders

Non-free-flowing powders

Micropowders

Emulsion polymers

Fine powders

Dispersions

PTFE

Fig. 1.2: Various product groups within the PTFE family

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2.1 Production of 3M™ Dyneon™ PTFE Fine Powders

PTFE production starts with the synthesis of the non-natural monomer (tetrafluoroethylene, TFE) and leads through emulsion polymerization of the monomer in water to the final polymer. The watery emulsions con-sist of approx. 180 - 250 nm sized particles that agglomerate during the precipitation process, leading to the formation of secondary particles of approx. 400 - 600 μm in size. Then the water is decanted and the still wet fine powder is dried. The fine powder is highly shear-sensitive because the agglomerated particle has a very low mechanical stress resistance. The following pages describe the processes that occur during paste extrusion with the aim of pointing out those that are rheologically1 relevant in order to get a better understanding of their impact on the final properties of the paste extrudates.

2.2 Phenomenology of Paste Extrusion2.2.1 Morphology of the Fine PowderAs illustrated in Figure 2.1, the fine powder consists of “potato-like” sec-ondary particles with a diameter of approx. 500 μm and a specific weight of 500 g/l. The space filling degree is 25 vol %, i.e. 1 l of fine powder has an air-filled pore space of 750 ml.

The secondary particle consists of some 1010 primary particles that are statistically packed in a spherical agglomeration. The packing density is 55 vol %. A statistical spherical packing of spheres with identical size can, regardless of the sphere diameter, achieve a maximum filling degree of 62 vol %. The spherical primary particles have an extremely tight particle size distribution. The PTFE they contain is in a highly crystalline form.

The potato-like form of the secondary particles ensures the free-flow abili-ty (Fig. 2.1 left). The grainy, island-like structure of the secondary particle, which can be easily seen, illustrates the statistically packed spherical ag-glomeration (Fig. 2.1 centre). The particles are “grape-like agglomerates” of 1010 primary particles that can only be seen at higher magnifications (Fig 2.1 right).

2.2.2 Paste MixingBy adding lubricants, the pore space of the secondary particle is filled. Organic PTFE-wetting fluids are used as lubricants, mostly higher boiling point hydrocarbons (benzenes). In practical use, 20 weight parts of ben-zene are mixed with 100 weight parts of PTFE. The air in the interior of the secondary particle is displaced by adding lubricants. The potato-like shape of the secondary particle is not modified by this. The paste with the additive still retains its free-flow ability, while the specific weight increases to about 700 g/l. The air-filled pore space between the secondary particles is in the range of 500 ml/l.

2.2.3 Preform FabricationThe air between the secondary par-ticles is removed by compressing them in a cylinder with a pressure of approx. 30 to 50 bar, which increases the density of the lubricant-containing material to 1650 g/l. The shape of the secondary particle and the primary particles is thereby preserved (Fig. 2.2). The cylindrical rod resulting from this is called the preform or billet. From the measured density, a filling degree of approx 63 vol %. From the rheolog-ical point of view, the fine powder has a paste-like state in the billet.

Rheologically, the paste can be defined as a heterogeneous 2-part system consisting of an immobilized fluid and a plastically deformable solid. This system flows when forces exceeding a certain minimum force are applied, and it is irreversibly deformed. The immobilized fluid thereby becomes a lubricant (matrix) and the deformed primary particles are a filling sub-stance (islands).

Physical Fundamentals

of PTFE Fine Powder Processing

1Rheology, the science of flowing materials and deformation. It was established because the linear elasticity theory with Hook’s law and hydrodynamics with Newton’s law of friction were not sufficient to describe the deformation behaviour of certain substances.

Fig. 2.1: Morphology of the fine powder; left: optical-microscope image of a secondary particle; centre and right: SEM micrograph of the surface of the secondary particle shown in different magnification

200 μm 100 μm 1 μm

Fig. 2.2: Morphology of the fracture surface of the compressed billet

200 μm

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2.2.4 Paste Extrusion in Funnel-Flow DesignThe irreversible deformation during extrusion is illustrated in Fig 2.3. The billet is transferred to a cylindrical metallic hopper and then pressed through a funnel, also made of metal, at a certain pressure, the so-called paste extrusion pressure. The narrowing of the cross-section in the funnel is characterized by the reduction ratio (RR), which is the ratio of the areas of the funnel inlet and the funnel outlet. Attached to the funnel outlet is a very short piece of pipe, the so-called guide in which the “flowing paste is calmed”.

In the hopper, the billet material shows plug flow behaviour, as it does not stick to the metallic wall. A real flow process, from the rheological point of view, only starts at the funnel inlet, shown as flow threads in Fig. 2.3. The flow velocity increases in direct proportion to the narrowing of the RR cross-section.

The crowding of the flow threads generates a shear gradient in the direction of the flow. This forces the paste material into an irreversible, plastic deformation. The extruded material gains mechanical stability from the deformation, the so-called “green stability”, both in its lubri-cant-containing, wet state and in the dry state. The wet extruded material has a density of approx. 1.8 g/cm3, while the dried extrudate has a density of 1.6 g/cm3 and a space filling degree of 70 vol %. This means that the the-oretical maximum packing density for spheres of identical size has nearly been reached. The next section provides a phenomenological ex-planation of the processes that are necessary to achieve a high filling degree.

Fig. 2.3: The irreversible plastic deformation of the paste during extrusion

Fig. 2.4: The fibrous structure of the extruded material. SEM micrograph of a split extrudate. The primary particles in the fibrils are arranged in the direction of flow like beads on a string

Fig. 2.5: Scanning electron-microscope image of a split extrudate

RV=1

RV=2

RV=4

RV=40

RV=400

2.3 Morphological Changes during Paste Extrusion2.3.1 Morphology of the Paste ExtrudateFig. 2.4 shows the fibrous structure of an externally smooth paste extrudate that has been split in the direction of flow. The secondary particles, still intact in the billet (Fig. 2.3), have been irreversibly deformed into fibrils. The fibrils consist of a “pearlstring-like” alignment of primary particles in the direction of flow.

The electron-microscope image in Fig. 2.5 of the split extruded material shows that individual primary particles (200 nm) have been preserved. They are clearly recognizable and survived the high shear gradient that is necessary for extrusion. The preservation of these primary particles is ensured by the lubricant.

Two processes during extrusion can be clearly distinguished: irreversible deformation, so-called crack-up, of the potato-like secondary particle in bead-string-like fibrils,

the kneading of the primary particles as a reversible deformation.

The specific energy necessary for the two forced processes in relation to the volume corresponds to the extrusion pressure. The two processes are explained in detail in the following section.

100 μm

1 μm

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2.3.2 Crack-up of the Secondary ParticlesThe crack-up of the secondary particle, also called paste fibrillation, is illustrated in Fig. 2.3. It shows the longitudinal section of the funnel cone. The fine powder is mixed with lubricant containing dyed secondary particles before paste preparation. The dye used is benzene insoluble. It is thus possible to see how the deformation of the dyed secondary particles into “longitudinal cylinders” increases with increasing reduction ratio RR.

Deformation is an adaptation to the flow threads (see Fig. 2.3). The crosssection of the cylinder becomes smaller the higher the reduction ratio (RR) gets. The decrease of the cross-section is inversely proportional to the RR. It is the result of packet-like regrouping of large primary particle clusters that are transported in the flow direction to the head of the fibrillating secondary particle. The transportation of the cluster is inevitably coupled with primary particles changing place. This induces the pearl-stringlike alignment of the primary particles. The secondary particle is cracked up.

The pearl-string-like alignment of the primary particles should ideally be homogeneous up to ranges of <10 μm, i.e. larger unaligned “grape-like” clusters should be avoided. Such clusters lead to irreg-ular, unsmooth surfaces of the sintered finished product (orange peel).

2.3.3 Reversible Deformation of Primary ParticlesPaste extrusion is accompanied by an enlargement of the extrudate, i.e. the extrudate has a larger diameter than the guide. This can be proof that an elastic deformation of the primary particles has taken place, as they are the only ones able to store elastic energy. The shear gradient in the flow direction deforms the spherical primary particles into ellipsoids. The primary particles are kneaded. After leaving the guide, the elastic tensions relax and the primary particle returns to its original spherical shape, as shown in Fig. 2.6. This means that the deformation is reversible. All that remains is a more compact structure.

The changed melting behaviour of the paste material after kneading is another major proof that a reversible deformation has taken place, as illustrated in Fig. 2.7. The DSC diagram shows a uniform melting peak for the original and a “bimodal” melting peak for the extruded paste material. The crystalline structure has obviously been changed through the kneading. The original orderly alignment has been partly destroyed.

Fig. 2.6: Reversible deformation of the primary particles. Above: before the area reduction in the extruder. Centre: maximum deformation at the tip of the extrusion die. Below: relaxation after leaving the extrusion die.

Fig. 2.7: DSC diagram: on the left, unkneaded (original from the drum) and on the right kneaded (extruded) PTFE paste; both samples are unsintered

290 300 310 320 330 340 350 360Temperature (°C)

Perkin-Elmer Thermal Analysis

300 310 320 330 340 350 360Temperature (°C)

Peak = 345 °C

Area = 714 mJDelta H = 66 J/g

Onset = 338 °C

Peak = 347 °C

Onset = 340 °C

Peak = 338 °COnset = 335 °C

Area = 8.6 mJDelta H = 0.8 J/g

Area = 662 mJDelta H = 64 J/g

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PTFE fine powder has a very high specific sur-face with a porous, spongelike structure that is characterized by a very high absorption capaci-ty for liquid hydrocarbons (lubricants).

The emulsion polymer powders are also often referred to as powder paste. This is because they undergo a processing step where the ma-terial has a paste-like feel. Its extremely high molecular weight, however, results in such a high melting viscosity that processing requires a special technology, the socalled paste extru-sion. The first process step, therefore, is to add a lubricant to enable processing of the powder paste. The powder paste with lubricant added is still powdery with good free-flowing properties. With special paste extruders (ram extruders), a variety of profiles, tubes, sealing cords, wire insulation and even large pipe liners can be fabricated at slightly elevated temperatures. The extruded profiles can then be calendered into tapes and films for use as thread sealing tapes and electrical insulation films for cables. If porous structures are required, a subsequent stretching process can be used. After the dry-ing process in which the lubricant is removed, a final sintering process with temperatures of 360 - 380 °C gives the products their high me-chanical stability and transparency.

The special processing technologies required for paste extrusion place very high demands on the raw material. Contaminants such as dust, lint from garments, or hair would decompose at the high sintering temperatures, resulting in discoloration and deterioration of the material’s properties. The powder paste is soft, deforma-ble and shear-sensitive. Ensuring proper pack-aging, storage and transportation conditions is therefore of the utmost importance. The free-flowability of 3M Dyneon PTFE fine powder depends on the temperature and humidity.

To avoid contamination of the fine powder when opening the drums, we recommend that the lid and drum be thoroughly cleaned beforehand (e.g. wiped with a moist cloth and dried).

Tip:

Properties and Handling of 3M™ Dyneon™ PTFE Fine Powders

Fig. 3.1: Determination of PTFE phase transformation with the help of DSC

TemperatureThe higher the temperature, the worse the free-flow ability of the powder becomes. Good free-flow behaviour is a prerequisite for homo-geneous absorption and distribution of the lu-bricant. The two crystal modification phases at 19 °C and 29 °C (Fig. 3.1) are specific to PTFE. They are coupled with a significant deterioration of the PTFE powder’s free-flow ability. PTFE is prone to clumping. Above 30 °C, it is virtually impossible to achieve good quality paste pro-cessing.

In storage, maximum temperature limits must therefore be absolutely observed in order to ensure optimum free-flow ability. Temperatures below 19 °C (e.g. 15 °C) have proven to be very effective.

HumidityTo prevent flaws in the finished product, the residual humidity of the fine powders must be very low (max. 0.04%). This is taken into ac-count both during the manufacturing process and in the selection of packaging and storage. The converter of the raw materials has to keep these requirements in mind, too. For example, where the air humidity is very high, cooled drums should only be opened in a cooled sample preparation room to prevent condensation of the humidity from contaminating the raw material.

3.1 Reduction Ratio

The reduction ratio (RR) is a unit-less number calculated from the ratio of the cross-sec-tional area of the extrusion cylinder minus the cross-section area of the mandrel rod and the crosssection of the extrusion die minus the cross-section of the mandrel tip. When extrud-ing full profiles, the crosssections of mandrel rod and mandrel tip are not taken into account.

Reduction ratio =

EC: cross-section of the extrusion cylinderM: cross-section of the mandrel rodED: cross-section of the extrusion dieMT: cross-section of the mandrel tipExamples of different reduction ratios (RR):

Thick-walled liners: RR = 10 to 50Thin-walled liners: RR = 50 to 500Micro-tubes: RR = 500 to 2000Cable insulations: RR = 300 to 3000

EC - MED - MT

-10 0 10 20 30 40Temperature (°C)

Area = 66 mJDelta H = 6.2 J/g

Area = 3.3 mJDelta H = 0.3 J/g

Onset = 24°C

Peak = 29 °COnset = 11 °C

Peak = 19 °C

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3.2 Extrusion PressureAs per ISO 12086, 3M™ Dyneon™ PTFE Fine Powder types are evaluated according to their extrusion pressure with a reduction ratio of RR = 400 or RR = 1600, which allows a statement as to their processability with a specific RR. Extrusion pressure is defined as the pressure in bar or MPa under standardized conditions (RR = 400 or 1600) that builds up in the paste extruder when extruding a mixture of PTFE fine powder and lubricant, and which is exerted on the material.

The extrusion pressure is dependent on the extrusion conditions, e.g. type of fine powder, lubricant, extrusion velocity, RR, die geometry (die angle and land length) and temperature. The extrusion pressure increases with increas-ing reduction ratio, as illustrated in Fig. 3.2.

Fig. 3.2: Dependency of extrusion pressure on reduction ratio (RR) for different types of fine powder

140

120

100

80

60

40

20

0

Material for low RR, e.g. linersMaterial for medium RR, e.g. large tubesMaterial for high RR, e.g. small tubes

0 200 400 600 800 1000 1200 1400 1600 1800

3.3 Particle Size and Particle Size DistributionThe free-flow ability and thus the processability of the dry powder are determined by the surface structure of the agglomerate particle and the particle size distribution. The average particle size ranges from 500 - 600 μm. The particle size distribution is determined by means of fractional screening. When pigments are added to powder paste that is too coarsegrained, inho-mogeneous colour distribution can result.

3.4 Specific WeightThe specific weight of PTFE powder is ex-pressed as kg/m3 (mass per volume). Directly related to this are mass and volume of the pre-form (also commonly called billet; see Section 4.7 Compression of the Preform).

The specific weight is thus a value to be taken into account when dimensioning the preform press. Fine powders tend to compact during transportation and storage. This tendency is even greater when the transition point at 19 °C has been exceeded, so the powder must be loosened before processing by means of cooling and screening.

Due to this tendency to clump, even under light pressure, a special preparation of the specimen is necessary in order to measure the specific weight. It should be determined in accord-ance with ISO 12086 and should be approx. 500 kg/m3.

3.5 DensityThe density of the finished parts made from sin-tered paste PTFE is, according to product type and processing parameters, between 2.14 and 2.17 g/cm3 and enables evaluation and quality control of the finished product. Low molecular weight or slow cooling results in products with high crystallinity and thus a density that is too high. High molecular weight or too rapid cooling results in products with low crystallinity and thus a density that is too low.

The density of the crystalline portion is higher (2.288 g/cm3) than the density of the amor-phous portion (1.966 g/cm3).

The mechanical properties of the PTFE are es-sentially determined by the amorphous portion of the product and to a lesser extent by the crystalline portion. For the reasons given above, a precise interpretation of the density is only possible if specimen preparation and process-ing parameters are precisely defined.

The fabrication of plates, the sintering condi-tions and density measurement (Standard Spe-cific Gravity, SSG) are described in ISO 12086.

Reduction Ratio (RR)

Extru

sion

Pre

ssur

e in

MPa

10

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4.1 Packaging and Storage3M™ Dyneon™ PTFE Fine Powders are produced in electronically-con-trolled processes (process control system) and filled under clean-room conditions (clean room class 100). They are packed in tightly-closable plastic drums with a filling quantity of 25 kg.

The PTFE production facility, as well as the quality of the drum container with lid and seal, eliminates the need for dry bags, which also avoids pos-sible contamination due to damage to the dry bag. The material is filled and stored at temperatures below 19 °C. In the hotter months of the year, the product is usually shipped in refrigerated trucks in order to avoid clumping due to transportation and/or heat and to maintain the good free-flowability of the fine powder.

In order to preserve these powder properties, it is recommended that customers store the products in refrigerated rooms; where possible at temperatures below 19 °C, the point at which crystallite is transformed. A room temperature of 15 °C is recommended. Should the fine powder be found to be clumpy or contain agglomerates, despite these precautionary measures, then the latter can be sieved out (caution: do not apply pressure to the particles, do not contaminate the powder). The separated agglom-erates should be refrigerated for 2 to 3 days at a temperature of between 5 and 10 °C, and then shaken in order to break apart the agglomerates. They should then be screened at temperatures below 19 °C by which the agglomerates should fall apart into free-flowing powder.

Fig. 4.1 shows the cooling curve (determined by experiment) of a fine pow-der drum with 25 kg content with a temperature of 30 °C. The ambient temperature is 5 °C; the temperature sensor is in the middle of the powder in the fine powder drum. It takes more than 24 hours until the fine powder material is ready for further processing and approx. 3 days to cool the material down to 5 °C (also see Section 4.3). A more practical solution would be a cold room temperature of 15 °C where the cooling of the PTFE down to 15 °C extends over several days.

4.2 Preparation of the Extrusion MixtureIn order to avoid flaws in the finished product, care must be taken during processing of the fine powder to avoid all excess mechanical stress of the powder, as it is highly shear- sensitive. It is recommended to shake the powder carefully or scoop it out in order to avoid crushing the particles.

4.3 Powder Screening Before pouring it into the mixing container, the fine powder should be screened in order to break apart any agglomerates and to loosen it. The mesh size of the screen should be 3 to 4 mm. The use of riddle sifters is also possible, which allows harder agglomerates to be broken apart. Larger clumps that do not fall apart should be removed from the screen and collected in a separate container. The separated agglomerate parti-cles can be reprocessed through cooling and renewed screening (also see Section 4.1). Utmost cleanliness is important during the open screening process. Moisture absorption due to air condensation must be avoided by maintaining the drum at ambient temperature and reclosing it immediately after powder removal. PTFE is a good electrical insulator, so when dosing PTFE it is necessary to avoid high pouring speeds, as the material could otherwise become charged with static electricity and then explode in com-bination with the lubricant.

Fig. 4.1: Cooling time of a 25 kg powder drum at an ambient temperature of 5 °C and of 15 °C, curves determined by experiment

It is recommended that you earth all containers coming in contact with PTFE and PTFE lubricants and use metallic containers.

Tip:30

25

20

15

10

5

00 10 20 30 40 50 60 70 80 90

Cooling cure at 15 °C ambient temperatureCooling cure at 5 °C ambient temperature

Pow

der t

empe

ratu

re in

°C

Cooling time in hours

Fundamentals of

PTFE Fine Powder Processing

11

4.4 Mixing with LubricantsAliphatic hydrocarbons with different boiling ranges have proven useful as lubricants for paste extru-sion. The choice of the lubricant is dependent on the type of extrusion material.

Lubricants with a higher boiling range are usually used for thin-wall applications requiring a calender-ing process, such as films. Lubricants with a lower boiling range are used for thick-walled extrusion materials such as liners.

The selected lubricant should be well absorbed by the fine powder and equally well removed after the extrusion. It should also not cause discolorations during sintering. Depending on the application and the lubricant type, the lubricant content amounts to 17 to 25 parts by weight related to 100 parts by weight of 3M™ Dyneon™ PTFE Fine Powder. The quantity of the lubricant is stated in parts by weight for simplicity’s sake. However, it would be more correct to say that the optimum volume of lubricant is added to the PTFE fine powder, as the void volumes between the primary particles that has to be filled. Here, the density of the lubricant, that may vary by around 10 - 15 %, plays a role.

The lubricant is added to the powder in the centre of the mixing container, not at its edge. Table 4.1 shows a selection of usable lubricants. The mixing procedure should be carried out at a temperature below 19 °C as the fine powder has a better free-flow behaviour at these temperatures. Depending on the type of mixer (dolly or tumbling mixer), the mixing time is between 20 and 30 minutes with a speed set at 20 to 30 revolutions per minute. The powder mix should flow and not splash in the mixing container. The lubricant is absorbed uniformly by the powder. The mixing containers must be tightly sealed in order to avoid evaporation losses. The mixing container should be filled to a maximum of 2/3 of its volume in order to attain a good mix.

Earthing is important when mixing the fine powder with the flammable lubricant due to the ignition risk of the lubricant vapours, e.g. ignition caused by electrostatic charge (also see Section 4.3). The benzene concentration in the working rooms must be monitored with the help of suitable room air monitoring devices. Good ventilation should also be provided.

Table 4.1: Selection of lubricants for use in paste extrusion

Lubricant Manufacturer Boiling range of the lubricant

Density Applications

Shell Sol 100/140 Shell 105 - 137 °C 0.730 g/cm3 Pipes, cables, tubes

Isopar E Exxon 118 - 143 °C 0.724 g/cm3

Shell Sol T Shell 187 - 215 °C 0.761 g/cm3 Tapes

Shell Sol D70 Shell 190 - 250 °C 0.798 g/cm3

Isopar K Exxon 181 - 204 °C 0.766 g/cm3

Isopar L Exxon 189 - 210 °C 0.773 g/cm3

Isopar M Exxon 206 - 245 °C 0.790 g/cm3

Ignitable lubricant/air mixes can be avoided by using a good suction ventilation system with a high air-flow rate. The lower explo-sion limit is not attained.

Tip:

Lubricant mixing may cause the formation of ignitable lubricant air mixes in the range of 0.8 vol % to 6.5 vol % lubricant (also see the material safety data sheet of the respective lubricant as well as safety regulations concerning the handling of flammable solvents or vapours).

Warning!

12

4.5 PigmentationThe following procedures are recommended for pigmenting or colouring the powder paste: When using liquid colour suspensions, add these to the lubricant before mixing with the powder paste. If the pigment is to be mixed with the powder paste in a dry state (e.g. for anti-static applications, carbon black dyeing), the pigment is screened directly onto the powder and the mix is then homogenized in dry state through rolling. After that, the lubricant is added and handled as described in Section 4.4. This avoids formation of agglomerates to a large extent. If any agglomerates persist, cooling as per Section 4.1 or screening as per Section 4.3 should be carried out.

4.6 Maturing of the Extrusion MixA homogeneous distribution of the lubricant in the PTFE can be obtained by letting the mix “ma-ture”. This ripening process should take over-night or at best over 24 hours in tightly sealed containers. Longer times are not necessary.

4.7 Preform CompressionIn this processing step, the mix of 3M™ Dyneon™ PTFE Fine Powder and lubricant is fed into a preform press where it is compacted into a cy-lindrical preform.

The aim of the compression is to eliminate the air contained in the mix of powder paste and lubricant and to bring the mix into a form that can be fed into the extrusion cylinder without any problems. The cylinder of the preform press should be three times the length of the preform, as the powder is compressed to 1/3 of its vol-ume. The mix of powder and lubricant should be compacted slowly so as to allow the air to com-pletely escape from the mix in the preform cyl-inder. This process can be supported by a vacu-um placed at the ventilation bores. Pre-pressing takes several minutes at a pressure of approx. 30 - 50 bar. The quality of the finished products is, among other things, dependent on a preform without cracks. The compression pressure is therefore only slowly decreased and care must be taken when removing the preform from the preform cylinder. The compacted part must then be immediately processed to reduce evap-oration of the lubricant from the surfaces to a minimum.

Inhomogeneous distribution of the lubricant results in quality and dimensional variations of the finished product. The preform is fed into the paste extruder – the cylinder of which should have a diameter that is 1 mm larger than the outer diameter of the preform.

4.8 ExtrusionBecause of the different handling procedures, the topic of “extrusion” is dealt with in specific sections titled “Fabrication of Films and Tapes”, “Fabrication of Tubes”, “Fabrication of Thick-Walled Pipes (Liners)” and “Fabrication of Ca-ble and Wire Insulations”. For paste extrusion, ram extruders with a relatively simple design are used where the preform is extruded through the die of a ram extruder.

Higher temperatures (e.g. 30 °C) during the maturing process ensure improved distribution of the lubricant and provide more homogeneous extrusion quality.

Tip:

Preforms containing lubricant can be stored in air-tight, sealed containers. In order to ensure a uniform lubricant atmosphere in the storage drum, a cloth impregnated with lubricant is enclosed in the container. This prevents the surface of the preform from drying out. In doing so, the precautionary measures stated in Section 4.3 must be observed.

Tip:

13

5

Films, tapes and sealing cords can be fabricat-ed from 3M™ Dyneon™ PTFE extrusion paste in stretched, unstretched, sintered or unsintered forms. The following applications are known: Thread sealing tapes Flat or plate seals Electrical insulation tapes for wound insulations

Tape cable Varns

The key processing steps for the fabrication of films and tapes are shown in Fig. 5.1: Preparation of the extrusion mix (Section 4.2 - 4.6)

Preform compression (Section 4.7) Profile extrusion Film calendering Film drying Mono- or biaxial film stretching Film sintering, cutting and packaging, if required

The content of lubricant is 21 - 25 parts by weight related to 100 parts by weight of PTFE. In contrast to tube extrusion, higherboiling lu-bricants (boiling range of 180 - 250 °C, also see Tab. 4.1) are used in order to avoid lubricant loss during calendering. This also has a favour-able impact on the calendering behaviour (even edges, splice tendency).

Fig. 5.2: Extrusion profiles for tape fabrication: a dog-bone, b rectangular and c round profile

Fig. 5.1: Diagram of a tape facility: a extrusion, b calendering, c drying, d stretching, e cutting and f wind-up

Place a lint-free cloth impregnated with lubricant into the container to ensure a uniform lubricant at-mosphere and to prevent the extrudate surface from drying out. In doing so, the precautionary measures described in Section 4.3 must be observed.

Tip:

a b c d e f

Extruders for profile fabrication can be of relatively simple design. A cylinder with a nozzle or a ram with a mechanical or hydraulic drive that runs pres-sureindependently at a constant velocity are basically all that is needed. The latter is necessary as the pressure can change during extrusion. A small-dimensioned extruder is normally enough to attain the reduction ratio of 100:1 that is required for profile extrusion.

It is possible to set preform on preform, in order to minimize loss of mate-rial that stays in the cone of the extruder due to the design. The touching surfaces of the two preforms can be roughened with a fork or similar device in order to improve the mixing of the two materials and thus avoid a prede-termined breaking point in the profile. The forming tool consists of a conical reducer and a die with parallel guide.

The enclosed angle of the cone is between 20° and 40° (also refer to Section 6.2, Fig. 6.2, but without mandrel rod). In practical use, the round extrudate has proven to be a useful solution for extrusion profiles.

For broader tapes, a rectangular or “dog-bone” profile is preferable. Fig. 5.2 shows possible extrusion profiles. The tools or areas coming into con-tact with the preform should be smoothly polished and made of stainless steel. Steps or edges at the transition from cylinder to cone should also be avoided. Profiles of the highest quality are attained when the extrusion cylinder and die have a temperature of approx. 30 to 40 °C. After extrusion, the extrudate is wound up and calendered. If the extrudate is to be stored, it should be placed in a tightly sealed container to avoid lubricant escape.

5.1 Profile Extrusion

a b c

Fabrication of Films, Tapes and Sealing Cords

14

5.2 CalenderingTwo-roll calenders equipped with an appropriate feeding system are used to calender profiles. The shape of the feeding system is similar to a fishtail or coat hanger profile, as shown in Fig. 5.3.

The calender rolls usually have a diameter of 300 to 400mm and widths of approx. 400 mm.

A temperature of around 40 °C is recommended for the tool surface and a roll speed of approx. 30 revolutions per minute (depending on the roll diameter). The adjustment of the sheet thickness requires a highly precise fine adjustment of the roll gap across the width of the film.

Calendering from profile to sheet is usually done in one step. For thick-walled profiles, this process can be performed in several steps until the required width and thickness are obtained. The calendered width of the sheets is dependent on both the PTFE type and the following parameters:

shape of the profile

type and shape of the fishtail guide and its distance to the roll gap

sheet thickness

lubricant content

roll surface

In general, sheet widths of 240 to 270 mm at a tape thickness of 100 μm can be achieved by using a dog-bone profile and a calender of the type described above.

5.3 Film and Sheet DryingThe film or sheeting is dried at a temperature of 160 to 200 °C. The line speed or the length of the oven must be selected so as to enable complete removal of the lubricant. Calendering and drying should be performed independently of each other as both steps achieve their optimal results at different speeds.

Ignitable lubricant/air mixes can be avoided by ensuring a high air throughput in the drying oven. The lower explosion limit is not attained.

Tip:

The surface of the roll should not have a high-gloss polish but have a slight coarseness or a coarse polish of the crosssection in order to ensure enhanced creep of the sheet. This can be achieved by treating the surface with abrasive with a grit of 200, for example.

Tip:

Fig. 5.3: Feeding of the extrudate through the fishtail profile of a two-roll calender

Fishtail profile

Extrudate

The drying process of the sheet may cause the formation of ignitable lubricant air mixes in the range of 0.8 vol % to 6.5 vol % lubricant (also see the material safety data sheet of the respective lubricant as well as safety regulations concerning the handling of flammable solvents or vapours).

Warning!

15

5.4 Film StretchingThe dried films or tapes are stretched for cer-tain applications. In practical use, stretching of the free-running PTFE sheets at temperatures of between 280 °C and 300 °C has proven effective (used for thread sealing tapes). The sheet is stretched with two different roll sys-tems that are running at different speeds in the running direction. The sheet is clamped with roll systems that can be made of steel or combina-tions of steel and rubber rolls.

The film or sheet can be stretched up to a ratio of between 1:10 and 1:15 without formation of irregularities, with minimal reduction of thick-ness and width. The material becomes highly porous and stretching results in a substantial reduction of the specific weight of the sheet/film. Fig. 5.4 shows the development of the film or sheet’s density in relation to the stretch ratio.

The films and sheets are distinguished as ei-ther monoaxially and biaxially stretched. Fig. 5.5 shows SEM micrograms of monoaxially stretched (left) and biaxially stretched (right) PTFE. The fibres between the PTFE particles can be seen. Monoaxially stretched PTFE is used for thread sealing tape. The magnified image part in Fig. 5.6 shows the transition from an island to a fibre. Biaxially stretched PTFE is widely used for breathable, impermeable membranes in the clothing industry. The space between fibres is large enough to let water vapour pass through the membrane (breathable), yet small enough to keep water drops out (impermeable). Films, sheets or tapes can also be sintered and then stretched after the sintering process in order to produce high tensile-strength films, sheets, tapes or yarns.

Fig. 5.4: Dependence of film density to the stretching ratio at a stretching temperature of 300 °C and a stretching speed of 1.7 m/min

5.5 Fabrication of Unstretched and Stretched Sealing CordsSealing cords generally refer to round or rectangular extrudates that can be used in stretched or unstretched form, depending on their application. The technology used for fabricating sealing cords is very similar to that used for tape fabrication. As with tape production, the profiles are fabricated by means of extrusion with appropriate dies being used to correspond to the profile dimension. The ex-trudate is subsequently dried. For stretched sealing cords, the extrudates are stretched using similar equipment and conditions as for sheet stretching. The final product should generally have a density of approx. 0.65 g/cm3, which corresponds to a stretch ratio of around 1:3 to 1:4.

Uniform cord dimensions can be achieved by feeding the extrudates through special calibrating rolls. During the same process step, adhesive tape can be applied to provide the sealing cords with a fixation aid for installation.

Fig. 5.5: SEM micrographs of monoaxially and biaxially stretched PTFE sheeting Fig. 5.6: SEM micrographs of monoaxially stretched PTFE sheeting. The image shows the transition from an island to a fibre

2.0

1.5

1.0

0.5

0

Tape

thic

knes

s in

g/c

m3

0 10 20 30 40 50 60 70 80 90

Stretch ratio 1 : x

16

6

Table 6.1: Lubricants suitable for use in tube extrusion

Fig. 6.1: Hydraulic paste extruder for tube fabrication

Paste extrusion is used to fabricate extremely thin-walled micro- and spaghetti hoses as well as thin-walled industrial tubes from 3M™ Dyneon™ PTFE Fine Powder. Dimensions range from approx. 0.1 mm to approx. 25 mm interior diameter with wall thicknesses of around 0.1 to 2 mm.The extruded tubes can be further processed to fabricate: shrink tubes corrugated tubes and braided tubes

6.1 Lubricants for Hose ExtrusionThe lubricants used for hose extrusion have a lower boiling range (100 to 150 °C) than those used for tape fabrication to enable easier evaporation of the lubricant during the short time spent in the drying oven. The following lubricants listed in Table 6.1 can be used:

Lubricant ManufacturerBoiling rangeof the lubricant

Specific gravity Application

Shell Sol 100/140 Shell 105 - 137 °C 0.730 g/cm3 Pipes,cables,hosesIsopar E Exxon 118 - 143 °C 0.724 g/cm3

6.2 Tube ExtrusionTubes made from paste are fabricated using special ram extruders (paste extruder) with an inner mandrel that determines the interior diameter of the tube, and the extruders can be set up either horizontally or vertically. The extrudates are then dried and sintered. Specimen preparation and fab-rication of the preforms is described in Sections 4.2 to 4.7.

The extrusion process is discontinuous. Extrusion is stopped after each extruded preform to return the ram and insert a new preform. Large paste extruders are able to process several preforms with a total weight of up to 100 kg in one step. Arbitrary enlargement of the preform’s diameter is not possi-ble as this might lead to excess reduction ratios and high local shear resulting in flaws of the extrusion materials. Limits are imposed by the material here, as high reduction ratios result in high extrusion pressures. This means that you must have a cylinder with suitable diameter depending on the tube dimension. Cylinder diameter can vary in a range of 25 mm to 250 mm as the various tube types have different maximum reduction ratios. The extruder drive has to ensure that the set extrusion speed stays constant regardless of the changing extrusion pressure. This is necessary to achieve constant drying and sintering conditions in the downstream drying oven – a precondition for consistent tube dimensions and quality. The extruder can have either a hydraulic or mechanical drive. Experience has shown that extruders with hydraulic drives now provide precise extrusion conditions and can be more easily produced. Fig. 6.1 shows the set-up of an extruder used for processing 3M Dyneon PTFE.

To keep investment costs and the number of ex-truders low, it has proven useful to apply different sleeves and rams, thus making it possible to achieve the optimum reduction ratio of a material.

Tip:

Suction

Cross-bar

Ram

Hydraulic cylinder

Extrusion cylinder

PTFE filling

Mandrel rod with mandrel tipExtrusion die

Chain mechanism

Drying zone (2-3 m)

Sintering zone (2-3 m)

Cooling zone

Wind-up (optional)

Oven Suction

Fabrication of

Tubing and Hoses

17

In contrast to profile extrusion, tube extrusion requires an additional mandrel in the extruder. This mandrel is fixed to a cross-bar or a frame plate and runs freely without any fixation in the die area. During extrusion the ram presses the material over the mandrel. Precise centring of the mandrel tip in the die is required to achieve

a uniform wall thickness of the tube. There have been good results with flexible mandrels made of polyacetale or polyimide. The tool consists of a conic reducer and a die with parallel guide (land length). The land length L is determined by the die diameter D. For thick-walled and large-dimensional extrudates, the ratio is

and for thin-walled, small-dimensioned tubes the maximum ratio is

The enclosed angle of the cone is between 20° and 40°. The higher the reduction ratio, the smaller this angle must become in order to keep the extrusion pressure in limits. Fig. 6.2 illustrates the dies of a paste extruder for tube fabrication.

To ensure a smooth tube surface, the die is heated to a temperature of approx. 50 - 60 °C. The tools or areas coming into contact with the product should be smoothly polished and made of stainless steel. The necessary extrusion pressure is mainly dependent on the reduc-tion ratio and the product type. It is also it is

determined by lubricant content, temperature, die angle, land length and extrusion speed. Excess extrusion pressure may not only lead to high extruder stress but also to excess shear in the tube (cracks, deformations, etc.). If the extrusion pressure is too low, the extruded material may have irregularities in the form of surface coarseness. The relation between ex-trusion pressure and lubricant content during tube extrusion with different reduction ratios is illustrated in Fig. 6.3. The impacts of the lubricant quantity on the extrusion pressure, i.e. shrinkage of a selected tube dimension, are summarized in Table 6.2. The mechanical prop-erties are not affected by the lubricant quantity. Fig. 6.4 shows the possibility of variations of the final tube dimensions in relation to the removal speed of different lubricant contents in parts by weight (PW). It also provides information on the relationship between the shrinking proper-ties of the tube and the lubricant content. The extrusion velocity is dependent on the tube di-mension or wall thickness and the length of the drying and sintering oven. It can be in the range of 1 to 20 m/min.

6.3 Drying and Sintering of TubesThe extruded tube is dried and sintered in a continuous oven. In the drying zone, the lubri-cant is evaporated and removed above the boil-ing point of the lubricant and below the sintering temperature of the PTFE (150 to 250 °C).

The evaporation speed is dependent on the oven temperature, the throughput speed, the wall thickness and the tube dimensions. Com-plete evaporation of the lubricant at high extru-sion pressures requires higher temperatures or longer ovens. After drying, the tube is sintered at a temperature of 360 to 380 °C. Sintering conditions are dependent on

tube dimensions,

extrusion velocity and

temperature and length of the oven.

The drying and sintering process changes the dimensions of the tube. Crosswise shrinkage of 0 to 15 % and lengthwise shrinkage of 15 to 25 % are expected. This dimensional change must be taken into account when selecting the extrusion tool. It is influenced by the lubricant content, the temperature management and the tube weight drawing on the extrusion die.

Fig. 6.2: Extrusion die of a paste extruder for tube fabrication

Cylinder

Mandrel rod

Closing jaws20°- 40°

Die tool,exchangeable

Mandrel tip,exchangeable

Die heating

Land length

L = land lengthD = die diameter

L D

>1

L D

=10

Table 6.2: The impact of the lubricant quantity on extrusion pressure, shrinkage and mechanical properties of a selected tube dimension

Processingparameter Unit

Tool mm 13.3 x 11.4

Reduction ratio 210 : 1

Extrusions speed m/min 2.0

Lubricant Boiling range 100 bis 140 °C

Lubricant quantity Parts by wgt.Wgt. %

18 15.3

19 16

20 16.7

21 17.4

22 18

Extrusion pressure bar 205 161 135 101 91

Final tube dimensions

Outer diameter mm 12.0 11.7 11.6 11.5 11.3

Interior diameter mm 10.2 9.9 9.8 9.7 9.5

Shrinkage of outer diameter % 9.8 12.0 12.8 13.5 15.0

Tensile strength, lengswise N/mm2 29 30 31 30 31

Tensile strength, crosswise N/mm2 26 28 26 28 26

Elongation at break, lengthwise % 315 280 340 300 365

Elongation at break, crosswise % 625 605 620 615 585

18

6.4 Tube TestingTo monitor tube fabrication it is advisable to carry out regular checks of the specific gravity and the mechanical properties such as tensile strength and elongation at break.

Determination of the specific gravityThe “buoyancy method” has proven useful to determine the specific gravi-ty of the tube specimen. The test method is described in DIN 53479 under “Method A”.

Determination of Tensile Strength and Elongation at BreakSpecimen preparation and test conditions must be precisely defined as they have an influence on the measurement results. Testing of tensile strength and elongation at break is described in ISO 12086.

6.5 Typical Applications of PTFE Tubes Chemical industry: for storage of aggressive media, e.g. use in tank vehi-cles, sniffer probe lines for operation monitoring, lab devices.

Pharmaceutical and food industries Biotechnology Engineering Steam lines for laminating and vulcanization presses, extruders, calen-ders, purification equipment, plastic foaming equipment, spraying and painting equipment, glue lines for wood processing, hydraulics, air condi-tioning and cooling equipment

Engine and vehicle construction Exhaust pipes, fuel lines Bowden pulleys Electrical industry and electronics Insulations for electronic components

Fig. 6.3: Influence of extrusion pressure and lubricant content in tube extrusion at different reduction ratios

Fig. 6.4: Dependency of the tube diameter on the removal speed and the lubri-cant in parts by weight (PG)

The converter may fine-tune the final tube dimensions with removal speed and lubricant content.

Tip:

Formation of explosive mixes of lubricant and air can be avoided by using a proper ventilation system (also see Section 4.3).

17 18 19 20 21 22 23 24 25 26

RV =1900

RV =950

RV =460

Extru

sion

pre

ssur

e in

MPa

Lubricant quantity in parts by weight per 100 weight parts PTFE

120

100

80

60

40

20

0

3.5

3.0

2.5

2.0

1.5

1.0

4 6 8 10 12

Outer diameter at 19 GT

Interior diameter at 19 GT

Diameter die

Diameter mandrel

at 22 GT

at 22 GT

Tube

dia

met

er in

mm

Removal speed in m/min

Warning!

19

7

Liners are thick-walled pipes with wall thick-nesses of approx. 2 to 15 mm used for corro-sion-resistant lining of steel pipes in chemical plants. This brochure is limited to the descrip-tion of how seamless liner pipes are fabricated by means of paste extrusion. It does not deal with alternative linings implemented with 3M™ Dyneon™ PTFE suspension polymers (isostatic pressing, ram extrusion, peel films).

7.1 Specimen Preparation and LubricantSpecimen preparation does not differ from tube fabrication. The lubricants described in Section 6.1 can also be used for liner fabrication. Liner extrusion usually has lower reduction ratios in the range of 20 to 100. They cause low extru-sion pressure and low green strength of the liner pipes. The green strength of a liner refers to the stability of the extruded PTFE tube right after it leaves the extrusion die. At this point, the PTFE still contains lubricants and is very sensitive to mechanical stress. In order to increase the extrusion pressure and thus the green strength, 3M Dyneon PTFE liner types have a very high pressure level of their own. In addition, lubricant quantities of 17 to 20 parts by weight relating to 100 parts of PTFE by weight are used.

7.2 Carbon Black Pigmentation and Antistatic TreatmentThe highly conductive, fine carbon black pow-der must be added in its dry state. The carbon black is screened onto the PTFE powder. The mixture is homogenized in mixing vessels with the help of rolls or tumble mixers. 0.1 to 0.3 % weight of carbon black have proven efficient for black coloration, and around 1 to 3 % weight for antistatic treatment. The lubricant is added af-terwards. More detailed information is provided in Chapter 9, PTFE Fine Powder Compounds.

7.3 Liner ExtrusionFor liner extrusion the same ram extruders as described for tube extrusion (see Section 6.2) are used. Due to the heavy weight of the liner pipes, the extruders are generally set up horizontally and require substantially larger ex-trusion cylinders as a result of the large pipe dimensions. Fig. 7.1 shows how liners are fab-ricated.

The extruded liner is drawn over an interior supporting pipe, if required, and put into a sup-porting half pipe, taking the low green strength into account. Pipe and half pipe must be cor-

rosion-resistant in order to avoid liner discolor-ation. Unlike with tube extrusion, the mandrel diameter may exceed the size of the mandrel rod in order to enable the large liner dimension. The mechanical stress may be very high, which requires the use of large dimensioned mandrel rods made of high-strength steel. The marked areas at the mandrel of the tool in Fig. 7.2 show the spots exposed to the highest stress.

When fabricating thick-walled liners, the phe-nomenon of orange peel sometimes occurs. Due to the low reduction ratios, the shear gra-dient is sometimes reduced to such an extent that a sufficiently homogeneous crackup of the secondary particle is no longer ensured. This problem is obviously not caused by “over-shear” as is often assumed, and can therefore be solved by dramatically increasing the shear gradient, e.g. through a substantial increase of the extrusion speed.

Fig. 7.1: Horizontal extrusion of a liner along an inner supporting pipe into a supporting half pipe followed by drying and sintering in the oven

Horizontal extruderDrying andsintering oven

Liner

1

2

3

Supported by holed supporting half pipe

Inner supporting pipe

1 Supporting half pipe (stainles steel)2 Liner3 Inner supporting pipe (stainles steel)

Fabrication of

Thick-Walled Pipes

The inner supporting pipe and the supportinghalf pipe have deburred holes to enable easier lubricant evaporation during drying as well asreduced drying times. Alloys with high nickel content or aluminium are considered as dis-colorationfree materials.

Tip:

20

Fig. 7.2: Head of a liner extruder with extrusion tool

For safety and health reasons it is important to ensure good suction ventilation of the vapours generated during drying and sintering (also see Section 4.3).

Closing clamps

152.4(6“)

60°

30

50

70°

117

Mand-rel rod

Die

Mandrel

Cylinder

127 (5“)

The tube sections of up to 10 m length are both dried and sintered horizontally in the oven. The pipes are put in metal supporting half pipes with the interior supporting pipe so as to avoid defor-mation of the liner pipes during drying and sin-tering (Fig. 7.1). The drying conditions must be adjusted to the dimensions of the semifinished products and the boiling range of the lubricant. This is necessary to allow complete removal of the lubricant thus avoiding the sintering process being impacted by the lubricant.

Lubricant residues may lead to discoloration, cracks and bubble formation. The following dry-ing and sintering times and temperatures are recommended in accordance with wall-thick-ness and diameter:

Drying: 2 to 3 hours at 150 to 200 °CSintering: 1 to 3 hours at 360 to 380 °C

Cooling can be performed quickly or slowly, depending on the desired crystallinity level. The speed when passing through the gelling point at 310 to 320 °C defines the crystallinity. Fast cooling reduces the crystallinity and enhances the flexibility, while slow cooling increases the crystallinity and the specific gravity while reduc-ing permeability.

7.4 Drying and Sintering of the Liner

PTFE liners are generally used in chemical plant construction and in the pharmaceutical industry for pipes, columns, compensators and fittings to provide protection against aggressive media.

7.5 Typical PTFE Liner Applications

Cylinder

Warning!

21

8

The superior dielectric properties of 3M™ Dyneon™ PTFE coupled with high temperature resistance, all-round chemical resistance and inflammability under normal conditions (limiting oxygen index, LOI >95), are the key features that define 3M Dyneon PTFE as the material of choice for wire and cable insulations.

Similar to tube extrusion, paste extrusion with special wire cable extruders has established itself as a suitable processing method. The individual steps are described in Sections 8.2 to 8.4.

8.1 Preparation of the Extrusion Mix

Especially for cable fabrication, a prepared paste mix already containing lubricant is screened again into the preform press through a sifter of 3 to 5 mm mesh sizes. Cable insu-lations mostly have a low wall strength, which means that larger agglomerates in the insula-tion may not lead to flaws. Care must be taken that the compacting process in the prepress is slow so that the air can completely escape from the lubricant-containing powder paste. In addition, the pressure used to compress the preform should not exceed 30 to 50 bars and should be maintained for approx. 5 minutes.The quality of the cable insulation is highly de-pendent on the flawless fabrication of the pre-form which should therefore be given closest attention.

Fig. 8.2: PTFE cable extruder consisting of an unwind roll (a1) and dancer roll (a2), extruder (b), a drying- (c) and sinter-ing oven (d), deflector roll (e), a wire puller (f), electrical breakdown test device (g) and wire take-up roll (h)

Fig 8.1: Qualitative representation of the optimal lubri-cant amount for cable extrusion related to the number of electrical breakdowns

The preform is then slowly relaxed in order to avoid cracks. It should either be immediately processed or stored in an airtight container in order to avoid lubricant loss caused by evaporation (also see Section 5.1). Lubricants with a low boiling range are preferred as the dwell time in the drying oven is very short due to the high extrusion speed. The lubricant quantity is variable over a wide range in order to lower the extrusion pressure when reduction ratios are very high. The range for an optimum lubricant quantity is, however, very small in order to minimize the number of electrical breakdowns (Fig. 8.1, also see 6.2 and Fig. 6.3).

vapour extraction system

drying zone

electrical control cabinet

sintering zone

nozzle heating

barrel heating

wire take-up roll

electrical dreakdown test devicewire pullerdancer roll screw driveunwind roll

Num

ber o

f ele

ctric

albr

eakd

owns

Lubricant amount

Fabrication of

Cable Insulations

22

Fig. 8.3: Extrusion die of the wire cable extruder

8.2 Cable ExtruderFig. 8.2 provides an overview of a cable insulation facility. The extrusion system consists of a wire unwind roll (a

1), a dancer roll (a

2), the extruder (b), a drying- (c) and sintering oven (d), a deflector

roll (e), a wire puller (f), the electrical breakdown test device (g) and the wire take-up roll (h).The cable extruder can be set up either vertically or horizontally. A wire guiding pipe is used instead of an extrusion dome in the centre of the extrusion cylinder in order to ensure uniform thickness of the cable insulation. Due to the high extrusion speed, the production of cable insulations requires long drying and sintering ovens, which are usually arranged parallel to each other to ensure better space utilization and also require a deflector roll, as shown in Fig. 8.2. The alignment of drying and sintering ovens as shown here is a compromise solution as the ovens are in vertical position on top of each other. For construction reasons it is often necessary to deflect the wire by 180 degrees after leaving the drying oven in order to go through the sintering oven afterwards. Alternatively, the wire can also be deflected several times in the oven in order to increase the dwell time. After leaving the sintering route the coated wire passes through a thickness meter and its dielectric strength is tested.

8.3 Cable ExtrusionThe preform with inner bore is inserted into the extrusion cylinder of a paste extruder and then pressed through a die with the help of a ram. The extruded paste material coats the wire that is guided through the extruder head at the same time. As the extrusion pressure changes during pro-cessing, the machine design has to ensure that ram speed and therefore extrusion speed are kept at a constant level. This is particularly important for high reduction ratios during extrusion (compare Fig. 6.3 of tube fabrication). The diameter of the extrudate after leaving the die is higher than the inner diameter of the die. This is referred to as the PTFE “swelling rate” explained by the release of the elastic deformation energy of the particles.

It is also advisable to heat both the extrusion cylinder and the extrusion die (40 to 60 °C) in order to ensure that the surface of the extrudate is as smooth as possible. For thin wires, a dancer device is used after unwinding that compensates for pull variations and prevents the wire from breaking. Apart from the die angle, the die diameter and the land length, extrusion is decisively influenced by the clearance (“a” in Fig. 8.3) between the upper edge of the wire guide tip and the lower edge of the cylindrical guiding system (land length).

The clearance “a” is correctly set when the material speeds of the PTFE and the wire are identical at the exit from the wire guiding pipe. When clearance “a” gets bigger, product quality deteriorates which is reflected in low insulation strengths. When clearance “a” gets smaller the ring gap immedi-ately in front of the land length can be narrowed to such an extent that it leads to material over-shear and increase of the extrusion pressure. The adhesion of PTFE to the wire is equally reduced. In practical use, the optimum clearance “a” should be determined for each type of material and wire and for each tool. Depending on the reduction ratio, the optimum die angle is between 20° and 30° (Fig. 8.3). In addition, all edges coming into contact with the paste material should be rounded. The ex-trusion speed is determined by the material’s shear sensitivity and the conditions in the downstream drying and sintering ovens.

8.4 Drying and Sintering of CablesAfter the extrusion, the insulation must be dried at a temperature of around 150 to 200 °C. Any remaining lubricant in the extrudate may lead to brownish discolorations, cracks and electrical flaws during sintering. Sintering takes place at temperatures of above 345 °C, preferably at 360 to 380 °C. The coated wire must be run at decreasing speed the thicker the insulation material gets. PTFE is a good thermal insulator and prevents complete drying or sintering of the cable insulation if drying and sintering times are too short or temperatures are too low.

It may be advantageous to have a wire speed that is slightly above the speed of the extrudate as it allows better coating of the wire and therefore better insulation properties.

Tip:

Sufficient ventilation must be ensured (also see Section 5.3).

Land length

Distance (a)

Wire guiding pipe

Die

Mandrel rod

Wire

Warning!

23

9

PTFE offers a variety of excellent properties including:

broadest service temperature range of all plastic materials lowest coefficient of friction of all known solids excellent chemical resistance non-stick properties superior electrical and dielectric properties non-flammable under normal conditions (limiting oxygen index, LOI >95)

However, PTFE does have some properties that restrict its use in applica-tions like seals, self-lubricating bearings, etc: deformation under load (cold flow) poor thermal conductivity high thermal expansion coefficient limited wear resistance

By incorporating selected fillers into the fine powder, these properties can be offset. It is therefore possible to create property profiles that are tai-lored to specific applications.

9.1 Property Modification by Means of Fillers3M™ Dyneon™ PTFE Compounds are able to broaden the application scope of PTFE fine powders even further. Fillers make it possible to optimize properties thus enabling use of PTFE in applications where pressure- and wear resistance are of utmost importance. The addition of fillers changes the properties of fine powders, as shown in Table 9.1. The following fillers are generally used to reduce abrasion and deformation under load or to increase thermal or electrical conductivity:

Tab. 9.1: Property changes of fine powders through filler compounds

Property Increased* Reduced*Deformation under load

Wear resistance

Hardness

Thermal expansion coefficient

Thermal conductivity

Electrical conductivity

Tensile strength

Elongation at break

Service temperature range

Chemical resistance

Coefficient of friction

*depending on filler type and quantity

3M™ Dyneon™

PTFE Fine Powder Compounds

Glass Carbon Graphite Carbon black

Bronze High-performance polymers, such as PEEK, PPSO

2 or PI.

Combinations of these and other suitable fillers make it possible to tailor custom compounds for specific applications.

There are two different methods for producing Dyneon PTFE Fine Powder Compounds. In the first, the filler is added to the fine powder in the form of powder or as a suspension together with the lubricant. This process is often called the dry-mix method. In the second, since Dyneon has the ability to produce compounds directly from the dispersion, the filler can be added to the PTFE dispersion and the powder paste is then mixed together with the filler (wet mix). Which production method is the most appropriate is mainly determined by the requirements placed on the compound and the specific application. Dyneon will be glad to help you find the appropriate compound for your needs.

Bowden pulleys, for example, are applications where low wear and low friction are important. Here, it is not only important that the PTFE part has a lower wear but also that the opposing surface is not prematurely worn by the filler. Ideal fillers here would be carbon, graphite or high-performance polymers, such as PPSO

2. Often, increased pressure resistance is required

in addition to chemical and thermal resistance. This is particularly impor-tant for seals. Glass would be an ideal filler for such an application. Carbon black is used for antistatic treatment of liners and tubes. Here you need a surface resistivity of ≤ 109 Ω.

9.3 Typical Applications of PTFE Fine Powder Compounds

9.2 Manufacturing Methods of PTFE Fine Powder Compounds

24

10

11

3M™ Dyneon™ PTFE Fine Powders are also used as anti-drip agents for thermoplastic plastics. In this special field of use, the fibrillation properties of the PTFE powder paste are used, in order to prevent melting of the thermoplastics in the case of fire.

Production processes do not always run trouble-free. The reasons for this are manifold. This chapter therefore provides a number of suggestions on how to find possible causes and solutions for such problems. As causes can be very complex the following table 11.1 does not claim to be exhaustive.

Table 11.1: Problems occurring during paste extrusion, their possible causes and remedy suggestions.

Problem Possible cause Suggested remedy

Contamination of the semi-finished product

Contaminated lubricant has been added Filter lubricant

Change the lubricant batch

During opening of the powder drum Before opening, remove dirt particles from the outside

of the drum to avoid contamination

Earth the drum to avoid electrostatic charges

Clean the preparation room

Prior extrusion contained fillers Clean extruder

Brown coloration of the semi-finished product

Lubricant has not been fully removed Increase drying period

Increase drying temperature

Use lubricant with lower boiling point

Improve suction in the oven

Repeat sintering, brown colour will disappear

in most cases

Extrudate is brittle Extrusion pressure is too low, green strength is too low Increase reduction ratio

Reduce lubricant quantity

Use material with higher extrusion pressure

Increase extrusion speed

Semi-finished product is torn in extrusion direction

Mechanical damage in green state Treat extrudate more carefully

Use lubricant with higher boiling point

Check die for mechanical damage

Sintered semi-finished product has low tear resistance but high density and elongation at break

Sintering of semi-finished product too long or too hot Check temperature profile of the sintering oven

Choose lower sintering temperature (360 to 380 °C)

Check for oven malfunctions

Liner is torn lengthwise and crosswise to extrusion direction

Inner supporting pipe was too big Use smaller inner supporting pipe

Irregular cooling after sintering Ensure uniform cold air distribution

Check for malfunctions of oven or cooling unit

Inner tensions or irregular shrinkage because cooling

was too fast

Slow down cooling process

Check for malfunctions of oven or cooling unit

Semi-finished product stuck to contact surface during

sintering

Check contact surface for roughness or flaws

Special Applications

Trouble Shooting Guide

25

Table 11.1: Problems occurring during paste extrusion, their possible causes and remedy suggestions (continued).

Problem Possible causes Suggested remedy

Semi-finished product burst open Drying temperature is too high Reduce drying temperature to the range between boiling

point of the lubricant and sintering temperature

Check for oven malfunctions

Moisture Dry lubricant

Water condensation when opening the powder drum,

bring drum to room temperature

Trapped air during preform fabrication Check machine parameters (pressure, time, closing

speed, etc.)

Drill ventilation bores

Partial tapering of the tube diameter or wavy extrudate, “snake-effect”

Too much lubricant Reduce lubricant content

White dots in the semi-finished product Contamination or PTFE residues from prior extrusions Clean extruder

Squeezed powder paste Treat powder more carefully

Check lubricant level

Screen agglomerates

Partial occurrence of streaks Excess lubricant Reduce lubricant level

Squeezed powder paste Treat powder more carefully

Screen agglomerates

Irregular lubricant distribution Extend mixing time

Let lubricant-powder mix rest overnight at 30 °C

Scaly surface of the semi-finished pro-duct (orange peel)

Shearing in extrusion die too low Increase reduction ratio

Increase extrusion speed

Rough tool finish Polish

If lateral polish is applied, polish longitudinal

Lack of lubricant Increase lubricant level

Irregular surface Lack of lubricant

Irregular lubricant distribution

Increase lubricant level

Let lubricant/powder mix rest overnight at 30 °C

Inconsistent drying and sintering conditions Check for oven malfunctions

Filler agglomerates in dry-mixed fine powder compound Reduce filler-particle size

Increase dimensions of semi-finished product

Grind, crush or screen filler

Filler or filler additives not temperature-resistant enough

Pressed billet does not fit into extruder Trapped air during preform pressing; preform has

expanded

Increase compression pressure suddenly to let air

escape

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If you have any questions regarding compliance with national and inter-national regulations of legislators or associations, please contact Dyneon GmbH (contacts see last page).

Due to lack of experience, we cannot recommend applications in the medical field (implants). Applications in this field are therefore the sole responsibility of the manufacturer.

General recommendations on health and safety in processing, on work hygiene and on measures to be taken in the event of accident are detailed in our material safety data sheets.

You will find further notes on the safe handling of fluoropolymers in the brochure “Guide for the safe handling of Fluoropolymers Resins” by PlasticsEurope, Box 3, 1160 Brussels, Belgium, Tel. +32 (2) 676 17 32.

Compliance and Safety

27

Dyneon is a 3M company. Dyneon is a trademark of 3M. 03/2015 All rights reserved.© Dyneon 2015 | PTFEFP201503EN

Dyneon GmbH3M Advanced Materials DivisionCarl-Schurz-Straße 141453 Neuss, GermanyPhone +49 (0) 2131 14 2265Fax +49 (0) 2131 14 3857www.dyneon.eu

Technical Information and Test DataTechnical information and guidance provided by Dyneon personnel is based upon data and testing which is believed to be reliable. Such advice is intended for use by persons with appropriate technical understanding, knowledge and skills relating to PTFE compounds. No licence under any Dyneon or third party intellectual rights is granted or implied by virtue of this information.

General recommendations on health and safety in processing, on work hygiene and on measures to be taken in the event of accident are detailed in our material safety data sheets.

You will find further notes on the safe handling of fluoropolymers in the brochure “Guide for the safe handling of Fluoropolymers Resins” by PlasticsEurope, Box 3, B-1160 Brussels, Tel. +32 (2) 676 17 32.

The present edition replaces all previous versions. Please make sure and inquire if in doubt whether you have the latest edition.

Important NoticeAll information set forth herein is based on our present state of knowledge and is intended to provide general notes regarding products and their uses. It should not therefore be construed as a guarantee of specific properties of the products described or their suitability for a particular application. Because conditions of product use are outside Dyneon’s control and vary widely, user must evaluate and determine whether a Dyneon product will be suitable for user’s intended application before using it. The quality of our products is warranted under our General Terms and Conditions of Sale as now are or hereafter may be in force.

Where to go for more information

Dyneon Customer ServiceEurope Phone: 00 800 396 366 27 Fax: 00 800 396 366 39 Italy Phone: 800 791 018 Fax: 800 781 019

Dyneon GmbHCarl-Schurz-Str. 1 41453 Neuss Germany Phone: +49 (0) 2131 14 2265 Fax : +49 (0) 2131 14 3857

Dyneon B.V.Tunnelweg 95 6468 EJ KerkradeThe NetherlandsPhone: +31 45 567 9600 Fax: +31 45 567 9619

3M Advanced Materials Division6744 33rd Street North Oakdale, MN 55128 USA Phone: +1 800 810 8499 Fax: +1 800 635 8061

www.dyneon.eu