**4. Irradiation stability of PTFE**

**Figure 4.** A blue pigment (3%) mixed non free flow PTFE powder.

172 Sintering Techniques of Materials

**3. Recycling PTFE scrap material**

**Figure 5.** Sintered rods made from blue pigmented PTFE, glass mixed PTFE and carbon mixed PTFE

Use of plastics has the capability to cause danger to both environment and human life. PTFE scrap can be neither incinerated nor dumped! In the waste incineration processes which have been usual up to now, highly-corrosive vapors are released which also damage the incineration From Table 1 it is seen that among all plastics, PTFE has the least stability against ionizing radiation. Heavy gamma or electron irradiation (several kilo Gray) has been found to break down carbon-carbon bonds in the polymer chain in the PTFE scrap and reduce its molecular weight which makes it very brittle and the end product is a white, free-flowing PTFE powder which was found to be useful as additives in other materials or systems (see Fig.6). While the turnings of PTFE scrap before irradiation are tough and elastic, those after irradiation in air crumbles into a powdery material. The molecular weight of irradiated PTFE is in the range of a few tens of thousands to a few hundreds of thousands, compared to several million for the unirradiated resins. When irradiated in vacuum or inert atmosphere, the cleavage of the bonds produces highly stable radicals. The recombination of these stable radicals prevents rapid degradation of PTFE, as the molecular weight rebuilds. When irradiation is conducted in air, as is the case in the present experiment, the radicals react with oxygen leading to smaller molecular weight PTFE chains fairly quickly.

**Figure 6.** PTFE scrap before (left) and after gamma irradiation (right)


**Table 1.** Radiation Stability of thermoplastic polymers [3]

#### **4.1. PTFE Micropowder**

The irradiated and pulverized PTFE scrap differs from PTFE granular resins and fine powders because of the very small particle size, typically in the range of 2 to 20 μm (and hence the word *micropowder*), low molecular weight and the way they are handled and processed. Micro powders were first developed as an outlet for the disposal of scrap resin. The recycled PTFE powder cannot be used for the same applications as PTFE since its properties are quite different; it cannot be molded in the same way and it does not possesses the same plastic properties of virgin PTFE. However, it can be used as an additive/lubricant in other materials (printing ink, thermoplastics, elastomers, coatings and lubricants) and can be used over a wide range of temperatures from -190 to 250°C and depending on the application, may provide nonstick properties, improved lubricity, better wear resistance and reinforcing properties [4]. More of these applications can be seen in the pamphlets published by Dupont. Micro powders down to four micron range used as additives for Inks, Oils, Lubricants, Paints and Coatings, Cosmetics and Thermoplastics show enhanced lubrication and wear resistant properties.


**Table 2.** Effect of electron irradiation dose on PTFE Micropowder Particle Size

**Plastic Ionizing Radiation Stability**

Fair

The irradiated and pulverized PTFE scrap differs from PTFE granular resins and fine powders because of the very small particle size, typically in the range of 2 to 20 μm (and hence the word *micropowder*), low molecular weight and the way they are handled and processed. Micro powders were first developed as an outlet for the disposal of scrap resin. The recycled PTFE powder cannot be used for the same applications as PTFE since its properties are quite different; it cannot be molded in the same way and it does not possesses the same plastic properties of virgin PTFE. However, it can be used as an additive/lubricant in other materials (printing ink, thermoplastics, elastomers, coatings and lubricants) and can be used over a wide range of temperatures from -190 to 250°C and depending on the application, may provide nonstick properties, improved lubricity, better wear resistance and reinforcing properties [4]. More of these applications can be seen in the pamphlets published by Dupont. Micro powders down

ABS Fair

– Aliphatic Fair – Aromatic Excellent Cellulosics Fair

– PTFE Poor

– PVF Good – Polyvinylidene fluoride Good – Copolymers of ethylene & TFE Good – Polycarbonate Good Polyesters – aromatic Good

– PE Good – PP Fair – Polymethylpentene Good – Copolymers Good Polystyrene Excellent Polystyrene acrylonitrile Good Polysulfones Excellent

– PVC Good – Copolymers Fair

**Table 1.** Radiation Stability of thermoplastic polymers [3]

**4.1. PTFE Micropowder**

Amides

Fluoroplastics

lene

Polyolefins

Polyvinyls

– Polychlorotrifluoroethy

174 Sintering Techniques of Materials

**Figure 7.** Particle size analysis of micro powder using laser light scattering.

Table 2 shows that with increasing electron irradiation dose in the range 5 – 25 Mrad, the average particle size of the PTFE micropowder decreases from 11.1 to 0.9 μm. The melt flow index goes up as the molecular weight of the powder goes down with increasing dose of irradiation. Electron irradiation is reported to cause cleavage of bonds and generation of gases such as HF acid vapor, which must be removed by means of adequate ventilation from the processing areas. Electron irradiation also increases the temperature of the sample which is held below 121°C by fractionating the irradiation. Both these problems are much less severe in gamma irradiation as the irradiation rate is several orders of magnitude less than that of electron irradiator.

**Figure 8.** Scanning electron micrograph of PTFE micro powder - irradiated and pulverized

Particle size distribution of the micropowder was carried out using a laser light scattering instrument (Aerosol Dust Monitor Model 1.108 of M/S GRIMM Aerosoltechnik, GmbH, Germany). The scattering angle by a single particle is inversely proportional to the size of the particle. By measuring the forward angle of scattering and intensity of the scattering light, both size distribution and number concentration could be obtained. Fig.7 shows that an average particle size of 0.26 μm was obtained on electron irradiation of the PTFE scrap followed by milling which was achieved by a jet mill or a hammer mill. In the jet mil particles strike against each other, causing them to fracture into smaller particles. The flaky morphology of imported scrap powder seen in Fig.8 offers better lubricant property of the micro powder.

## **5. Reprocessing of unirradiated PTFE scrap**

Unirradiated PTFE scrap can be recycled into many other products and used for rods, tubing and sheets etc by pulverization followed by suitable heat treatments similar to that of virgin PTFE. The recycled PTFE is known technically as "Reprocessed" or "Repro" PTFE or mechan‐ ical grade PTFE. Though off-white in color reprocessed PTFE has certain advantages over virgin PTFE namely low creep and better mechanical strength.

Unlike other thermosetting plastics, on heating to high temperatures PTFE products do not melt but only soften above 327°C. Therefore techniques other than those used for recycling conventional thermosetting plastics such as polyethylene have to be employed for recycling PTFE. One way is to grind PTFE scrap into fine powder and sinter it under pressure after suitable heat treatment and cleaning process. Another way is to decompose PTFE into its raw gaseous components (tetrafluoroethene and hexafluoropropene), cleaned and fed back into the production of new PTFE. Dyneon GmbH is building a pilot plant in Burgkirchen, Germany based on the latter technique, to recycle PTFE scrap. It will have capacity to recycle 500 metric tons of PTFE waste annually.

The most common way is to blend the pulverized scrap fine powder with pure PTFE to be used either in compression molding or ram extrusion. Before grinding, the scrap is usually shredded (Figs. 9 and 10) and heated to above its melting point to remove any volatile contaminants. Once ground, it is treated with acid to dissolve inorganics after which it is washed

**Figure 9.** PTFE scrap being shredded

in gamma irradiation as the irradiation rate is several orders of magnitude less than that of

Particle size distribution of the micropowder was carried out using a laser light scattering instrument (Aerosol Dust Monitor Model 1.108 of M/S GRIMM Aerosoltechnik, GmbH, Germany). The scattering angle by a single particle is inversely proportional to the size of the particle. By measuring the forward angle of scattering and intensity of the scattering light, both size distribution and number concentration could be obtained. Fig.7 shows that an average particle size of 0.26 μm was obtained on electron irradiation of the PTFE scrap followed by milling which was achieved by a jet mill or a hammer mill. In the jet mil particles strike against each other, causing them to fracture into smaller particles. The flaky morphology of imported

Unirradiated PTFE scrap can be recycled into many other products and used for rods, tubing and sheets etc by pulverization followed by suitable heat treatments similar to that of virgin PTFE. The recycled PTFE is known technically as "Reprocessed" or "Repro" PTFE or mechan‐ ical grade PTFE. Though off-white in color reprocessed PTFE has certain advantages over

Unlike other thermosetting plastics, on heating to high temperatures PTFE products do not melt but only soften above 327°C. Therefore techniques other than those used for recycling conventional thermosetting plastics such as polyethylene have to be employed for recycling PTFE. One way is to grind PTFE scrap into fine powder and sinter it under pressure after suitable heat treatment and cleaning process. Another way is to decompose PTFE into its raw

**Figure 8.** Scanning electron micrograph of PTFE micro powder - irradiated and pulverized

scrap powder seen in Fig.8 offers better lubricant property of the micro powder.

**5. Reprocessing of unirradiated PTFE scrap**

virgin PTFE namely low creep and better mechanical strength.

electron irradiator.

176 Sintering Techniques of Materials

**Figure 10.** Shredded scrap

Reprocessed PTFE grade powder is manufactured from pre-sintered PTFE shavings, scrap, etc. It exhibits most of the properties that the virgin grade does but is subject to occasional contamination within the material. This is the grade of choice when cost is a major concern and cleanliness is not an issue. When repro grade is mixed with PTFE the cost comes down. Such mixed grades are used when high purity is not required such as non critical chemical, electrical and mechanical applications [5-8]. However, virgin grade PTFE is the material of choice for use in pharmaceutical, food and beverage, and cosmetics industries or for medical/ electrical applications. Virgin PTFE has better friction characteristics, which may be important in some applications. Reprocessed grade PTFE is produced for thin sheets with a maximum thickness of 0.250", For thicker sheets virgin PTFE is used. However, virgin PTFE is known to undergo creep – deformation under load whereas the compressive strength and deformation under load for reprocessed PTFE are superior to virgin PTFE. Reprocessed grade PTFE also has superior wear resistance than virgin PTFE. Reprocessed grade PTFE rods are available in diameters ranging from 1/8 to 4 inch and lengths of 6 to 12 ft. Reprocessed PTFE is frequently specified for high performance bearings and bushings, particularly in applications that require resistance to corrosive chemicals.

#### **5.1. PTFE has different grades**

Grade A: 100% virgin material.

Grade B: 70% virgin material, with 30% recycled material.

Grade C: 50% - 50%

Grade D: 30% virgin, 70% recycled.

Grade E: 100% recycled

High purity reprocessed PTFE is white in color similar to virgin PTFE and is used for appli‐ cations ranging from extruded billets or molding into tubes, gaskets and ball-valve seats. Lower grade off-white reprocessed PTFE is blended with pre PTFE and is used for packing materials for valve stems and other applications (Table 3).


**Table 3.** Properties of different grades of reprocessed PTFE powder.

#### **5.2. Issues to be tackled in manufacturing reprocessed PTFE powder are the following**

**i.** Difficulties in grinding the scrap into a fine powder

Reprocessed PTFE grade powder is manufactured from pre-sintered PTFE shavings, scrap, etc. It exhibits most of the properties that the virgin grade does but is subject to occasional contamination within the material. This is the grade of choice when cost is a major concern and cleanliness is not an issue. When repro grade is mixed with PTFE the cost comes down. Such mixed grades are used when high purity is not required such as non critical chemical, electrical and mechanical applications [5-8]. However, virgin grade PTFE is the material of choice for use in pharmaceutical, food and beverage, and cosmetics industries or for medical/ electrical applications. Virgin PTFE has better friction characteristics, which may be important in some applications. Reprocessed grade PTFE is produced for thin sheets with a maximum thickness of 0.250", For thicker sheets virgin PTFE is used. However, virgin PTFE is known to undergo creep – deformation under load whereas the compressive strength and deformation under load for reprocessed PTFE are superior to virgin PTFE. Reprocessed grade PTFE also has superior wear resistance than virgin PTFE. Reprocessed grade PTFE rods are available in diameters ranging from 1/8 to 4 inch and lengths of 6 to 12 ft. Reprocessed PTFE is frequently specified for high performance bearings and bushings, particularly in applications that require

High purity reprocessed PTFE is white in color similar to virgin PTFE and is used for appli‐ cations ranging from extruded billets or molding into tubes, gaskets and ball-valve seats. Lower grade off-white reprocessed PTFE is blended with pre PTFE and is used for packing

**Repro (%) < 5% 10-20% >20%**

Slight off white No notable change Off white

Water absorption < 1% > 1% > 2%

Tensile strength Slightly reduced Reduced by 10% Reduced by 20% Dielectric strength Slightly reduced Reduced by 10% Not suitable Wear resistance Slightly reduced Slightly reduced Not suitable

Component finish Smooth Rough Rough – with powdery

burrs

resistance to corrosive chemicals.

**5.1. PTFE has different grades**

Grade A: 100% virgin material.

Grade D: 30% virgin, 70% recycled.

Chemical resistance No notable change

**Table 3.** Properties of different grades of reprocessed PTFE powder.

Grade C: 50% - 50%

178 Sintering Techniques of Materials

Grade E: 100% recycled

Visual

Grade B: 70% virgin material, with 30% recycled material.

materials for valve stems and other applications (Table 3).


Efforts needed to solve the above problems are described below.

**Figure 11.** Charred organic impurities on pre-heating the shredded scrap

**Figure 12.** Major charred impurities are removed by handpicking

**Figure 13.** Shredded PTFE scrap after pre-heating treatment

#### **5.3. Grinding PTFE scrap - Kirk-othemer encyclopedia of chemicals**

A technique known as Shear Extrusion Pulverisation based on Bridgeman – Anvil was used for this purpose. This technique is also known as


It is a physico-chemical process in which cohesive forces within the polymer are broken by means of mechanically induced stress.

The process is based on "Bridgeman phenomenon" and is realized inside a specially deigned pulverizer

In PSP, the polymer is subjected to simultaneous action of axial compression and shear stress between two mirror-like smooth working surfaces in the pulverizer and the pulverized

The process is realized below the melting point of polymers so the technical properties of recycled polymers remain unchanged after polymerization

**Figure 14.** Pre-sintered scrap powder

**Figure 13.** Shredded PTFE scrap after pre-heating treatment

for this purpose. This technique is also known as

recycled polymers remain unchanged after polymerization

**•** Double disc mill

180 Sintering Techniques of Materials

pulverizer

**•** Solid state pulverization (SSP)

**•** Pressure shear pulverization (PSP).

means of mechanically induced stress.

**5.3. Grinding PTFE scrap - Kirk-othemer encyclopedia of chemicals**

A technique known as Shear Extrusion Pulverisation based on Bridgeman – Anvil was used

It is a physico-chemical process in which cohesive forces within the polymer are broken by

The process is based on "Bridgeman phenomenon" and is realized inside a specially deigned

In PSP, the polymer is subjected to simultaneous action of axial compression and shear stress between two mirror-like smooth working surfaces in the pulverizer and the pulverized

The process is realized below the melting point of polymers so the technical properties of

**Figure 15.** Ground scrap powder after pre-sintering

**Figure 16.** Ground PTFE powder being mixed with virgin PTFE powder.

**Figure 17.** Mixture being cold pressed into pellet.

**Figure 18.** Fillers like graphite or pigments normally do not melt or agglomerate nor interact with PTFE during sinter‐ ing at atmospheric pressure but this is not the case with reprocessed PTFE when used alone or used as a filler with virgin PTFE

**Figure 19.** Reprocessed PTFE filled (40%) PTFE pellet before (left) and after (right) sintering at atmospheric pressure. The surface became rough on sintering due to agglomeration of reprocessed particles and their migration to surface of the pellet. Reprocessed PTFE is amenable to sintering only under pressure or under ram extrusion.

**Figure 20.** Sintered rods at atmospheric pressure – note pellets from 100% virgin scrap (second from left) crack and pop out perhaps due to the release of volatile gases whereas the pellet from 100% virgin PTFE (extreme right) are mil‐ ky white in color and exhibit good integrity on sintering at atmospheric pressure.
