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EVERYTHING YOU WANTED TO KNOW ABOUT PTFE

HISTORY OF PTFE

The story of PTFE began on April 6, 1938 at Du Pont's Jackson Laboratory in New Jersey, USA. Dr. Roy J. Plunkett, who was working with gases related to FREON fluorinated refrigerants, discovered that one sample left overnight in a cylinder had polymerised spontaneously to a white waxy solid.

Testing showed this solid to be a remarkable material. It was resistant to practically every known chemical or solvent; its surface was extremely slippery; almost no substance would stick to it; moisture did not cause it to swell and it did not degrade after long-term exposure to sunlight. It has melting point of 342C but, as opposed to conventional thermoplastics, the resin would not flow above that melting point. It could therefore not be processed by the usual methods used with thermoplastic materials. Du Pont then developed processing techniques which were not unlike those used for shaping powered metals. Using this technology, Du Pont engineers were able to compress and sinter PTFE resins into blocks that could then be machined in to the d desired shapes. Later, Du Pont developed ways to make dispersions of the resin in water; the dispersions could be used to coat glass cloth and make enamels. Next a powder was developed that could be blended with a lubricant and extruded as a paste to the desired shape and then sintered to coat wire and manufacture tubing.

COMPOUNDS

In spite of its remarkable properties, pure or 'unfilled' PTFE was inadequate for a number of more demanding engineering uses. In particular, its cold flow or creep kept PTFE out of mechanical applications such as those involving very heavy loads. In the sixties, Du Pont discovered that the physical properties of PTFE could be improved by the addition of fillers. A range of filled granular compounds were found to be highly suitable for gaskets, valve seats, shaft seals piston rings, high-voltage switches, bearings, pipe linings etc. It turned out that through a proper combination of base resin and one or more fillers, compounds could be tailor-made for many end-uses.

In general, the fillers were found to give:

  • Improved resistance to cold flow or creep
  • Reduced wear and friction
  • Increased stiffness
  • Increased thermal conductivity
  • Increased thermal dimensional stability
  • Increased surface hardness
  • Increased electrical conductivity

CHOICE OF FILLERS

Over time, the fillers which gained popularity amongst processors and end-users alike were glass, bronze, carbon, graphite and molybdenum disulphide. In addition, compounds with other fillers, or combinations of fillers, or compounds with standard fillers in non-standard percentages, have also been developed. The percentage of filler is usually between 5 and 40% by volume. Below 5%, the impact of the filler on most properties is insignificant and above 40 volume percent most physical properties drop sharply.

Table 1 below shows a number of PTFE compounds currently used.

Type

Filler Type

%  Filler by Wt.

% Filler by Vol.

Standard Specific Gravity

Glass 15%

Glass

15

13.2

2.20

Glass 20%

Glass

20

17.8

2.21

Glass 25%

Glass

25

22.4

2.22

Glass 40%

Glass

40

36.5

2.22

Moly 5%

MoS2

5

2.3

2.22

Graphite 15%

Graphite

15

14.5

2.12

Bronze 40%

Bronze

40

14.0

3.05

Bronze 60%

Bronze

60

26.7

3.90

Glass 20%/ Graphite 5%

Glass/graphite

20/5

22.7

2.20

Glass 10%/ Graphite 10%

Glass/graphite

10/10

18.5

2.17

Glass 15%/ Moly 5%

Glass/ MoS2

15/5

15.9

2.26

Carbon 23%/ Graphite 2%

Carbon/graphite

23/2

28.4

2.05

Carbon 29%/ Graphite 3%

Carbon/graphite

29/3

35.2

2.00

Unfilled

Unfilled

-

-

2.16

ABOUT FILLERS

INTRODUCTION

Practically any material that can withstand the sintering temperature of PTFE can be used as a filler. Characteristics such as particle shape and size and the chemical composition of the filler greatly affect the prerties of the compound. Also crucial is the blending process. PTFE must be blended with the appropriate fillers (and any pigment additives which the end-user may want) using specialised equipment to ensure that the blend is fully uniform. Improper blending will result in a final product with poor physical properties and can also be visually displeasing.

Listed below are some of the key fillers and their contribution to the properties of PTFE:

GLASS

Glass fibre is the most widely used filler. It improves the creep resistance of PTFE both at low and high temperature. It is chemically stable (except to strong alkalis and hydrofluoric acid-HF). It has little effect on the electrical properties of PTFE and improves its wear and friction behaviour. An often cited problem with glass-filled PTFE is discoloration of the finished parts, in particular on the inside of large billets.

Type: E-glass
Milled fibres, nominal diameter 13 um
Nominal length 0.8 mm
Aspect ratio: min. 10
Density: 2.5 

CARBON

Amorphous carbon is one of the most inert fillers, except in oxidizing environments where glass performs better. Carbon adds to the creep resistance, increases the hardness and raises the thermal conductivity of PTFE. Carbon-filled compounds have excellent wear properties, in particular when combined with graphite. The combination of the above properties makes carbon/graphite compounds the preferred material for non-lubricated piston rings. The use of a softer carbon has the additional advantage that it lowers tool wear during machining, thus allowing machining to very close tolerances. Carbon-containing compounds have some electrical conductivity and are therefore antistatic

Base: Amorphous petroleum coke
Purity: > 99% C
Particle Size: < 75 um
Density: 4.8

CARBON FIBRE

Addition of carbon fibre to PTFE changes its physical properties in the same way as glass fibre does: Lower deformation under load, higher compressive and flex modulus and increased hardness.

In general, it requires less carbon fibre than glass fibre to achieve the same effect on PTFE. Carbon fibre is chemically inert and can be used in strong alkali and in HF, where glass-filled compounds fail. Compounds of PTFE with carbon fibre have the advantage of higher thermal conductivity and lower thermal expansion coefficients than glass-filled ones with the same filler percentages, and they are lighter.

They wear less in contract with most metals, and are also less abrasive on the mating surface. The wear in water is particularly low. This makes carbon-fibre-filled PTFE an excellent bearing material, especially when lubricated with water. It is widely used in the automotive industry for bearings and seal rings, for example in water pumps and in shock absorbers.

GRAPHITE

Graphite is a crystalline modification of high purity carbon. Graphite-filled PTFE has one of the lowest coefficients of friction. It has excellent wear properties, in particular against soft metals, displays high load-carrying capability in high-speed contact applications and is chemically inert. It is often used in combination with other fillers.

Source: synthetic
Purity: > 99%C
Irregular shaped
Particle size: < 75 um
Density: 2.26

BRONZE

Bronze is an alloy of copper and tin. Addition of high percentages of bronze powder to PTFE results in a compound having high thermal conductivity and better creep resistance than most other compounds. Bronze-filled PTFE is often used for components in hydraulic systems, but is not suited for electrical applications and is attacked by certain chemicals. Bronze has tendency to oxidise: bronze-filled compounds should therefore be used fresh and containers should always be kept closed. Some discoloration of the finished part during the sintering cycle is normal and has no impact on its quality.

Cu/Sn: 9/1
Low in phosphorus
Particle size: < 60um
Particle shape: Spherical (Bronze 60%)
                            Irregular (2146-N)
Density: 8.95

MOLYBDENUM DISULFIDE (MoS2)

Molybdenum disulfide adds to the hardness and stiffness of PTFE and reduces friction. It has little effect on its electrical properties. It is quite inert chemically and dissolves only in strongly oxidising acids. It is normally used in low percentages and together with other fillers. Compounds containing molybdenum disulfide need special attention during performing and sintering.

Source: mineral
Purity: > 98%
Particle size:  < 65um
Density: 4.9

ALUMINA (Al2O3)

Alumina or aluminium oxide is an excellent electrical insulator and is used to improve mechanical properties of compounds used in high voltage applications. As it is very hard, machining of the sintered part should be avoided whenever possible.

CALCIUM FLUORIDE (CaF2)

Calcium fluoride is a suitable filler for PTFE in uses where it comes in contact with chemicals that attack glass, such as hydrofluoric acid and strong alkalis. High purity grades of CaF2 are also used in electrical applications.

PIGMENTS

It is possible to pigment PTFE; using inorganic pigments that withstand the sintering temperature of PTFE Pigments do not significantly charge the properties of PTFE. Combinations of pigments and other fillers can be used.

POLYMERS

In recent years, polymeric fillers with sufficient heat stability to be used in PTFE have become available. Some remarkable properties have been obtained with polymer-filled compounds, particularly with respect to friction against soft metals.

MICA

Mica is mineral with a plate-like structure. During processing, the particles orient themselves perpendicular to the pressing direction. This results in very low shrinkage and low thermal expansion in the cross direction. Tensile properties are poor, so that mica-filled compounds are only suitable for parts under compressive stress.

PROPERTIES OF PTFE COMPOUNDS

INTRODUCTION

In reviewing the properties of PTFE compounds it is important to keep in mind that the way a part is prepared effects its final properties. Perform pressures, sintering cycles, shape and thickness of the specimen and direction of moulding all play a part. Thus, the values that a processor measures on his finished part may deviate from values quoted in the literature. For a number of tests, a standard method for preparing the test sample has been described. Most tests generally refer to ASTM methods.

ANISOTROPY

In virgin, unfilled PTFE and in PTFE compounds with spherical fillers, properties are usually isotropic, i.e. the same in all directions. Irregular or longitudinal fillers orientates themselves during compression moulding, so that properties in the mould direction (MD) differ from these in the cross direction (CD). In the following paragraphs both MD and CD properties are given where appropriate.

DENSITY

See table 1 (above)

PROPERTIES AT LOW TEMPERATURES

Unlike most conventional plastics, which become brittle at cryogenic temperatures, PTFE still has some ductility at-269C, the boiling point of helium. As a result PTFE can be used at extremely low temperatures, such as those in outer space. Some properties have been summarised in table 2. Like most other plastics, PTFE expands and contracts under the influence of heat and cold more than metals, but the addition of fillers reduces its coefficient of thermal expansion.

PROPERTIES AT HIGH TEMPERATURES/ THERMAL STABILITY

PTFE has excellent resistance of heat. It is capable of continuous service at 260C and can withstand temperatures up to 360C for limited periods. At these conditions, the extent of degradation still remains small. For example: at 390C, the weight loss of PTFE is still less than 0.1 percent per hour. The physical properties however, fall off with increasing temperature, and above 260C mechanical properties become a limiting factor. Thus, the addition of fillers to raise compressive strength and stiffness can be even more beneficial than at room temperature. Fillers do not affect the heat stability of the PTFE itself. Most fillers are stable up to 400C. Molybdenum-disulfide-filled PTFE should be sintered under special precautions, as the filler starts to oxidise at normal sintering temperature.

PROPERTIES IN VACUUM

PTFE has an extremely low vapour pressure (<10-5mbar at 1200C) and can be used safely in vacuum. The same is true for most filled compounds. The exception is compounds that contain graphite, as this material disintegrates under vacuum conditions in the absence of traces of water.

THERMAL EXPANSION

The coefficient of linear thermal expansion of unfilled PTFE is not constant over its useful temperature range. A marked change in volume of 1.0 to 1.8% is evident for PTFE resins in the transaction zone from 10 to 250C. A part which has been machined on either side of this zone will obviously change dimensions if permitted to go through the zone. Thus, final operating temperature of a precision part must be accurately determined. Measurement on a production basis must allow for this volume change if the transition zone is traversed in either manufacture or operation of the part. The expansion of PTFE compounds is generally lower than that of unfilled PTFE (Tables 3 and 4). With some fillers it is higher in the moulding direction (MD) than in the cross direction (CD). To obtain compounds with the lowest thermal expansion, the use of mica or carbon fibre filler is recommended.  

TABLE 2: TYPICAL PROPERTIES OF PTFE FILLED AND UNFILLED RESINS AT LOW TEMPERATURES

Property

Test method

Units

Temperature0C

Unfilled (1)

Compound

65% Bronze

15% Carbon

25% Glass Fibre

Tensile yield strength

ASTM D 1708

N/mm2

-253

-196

-183

-129

-79

+23

123

92

(84)

53

36

18

-

47

(44)

(32)

(26)

12

-

44

(41)

(32)

(24)

8

30

24

(23)

(18)

(14)

10

Ultimate tensile strength

ASTM D 1708

N/mm2

-253

-196

-183

-129

-79

+23

124

103

(95)

63

42

36

-

47

(45)

(35)

(26)

15

-

47

(45)

(35)

(27)

16

31

25

(24)

21

20

19

Tensile modulus

ASTM D1708

N/mm2

-253

-196

-183

-129

-79

+23

4300

3200

3100

2100

1400

750

-

-

-

-

-

-

-

-

-

-

-

-

3200

2600

(2500)

(2000)

(1600)

800

Elongation

ASTM D 1708

%

-253

-196

-183

-129

-79

+23

3

10

(12)

70

130

400

0

4

(5)

(34)

(70)

200

0

2

(4)

(23)

(50)

140

1

4

(6)

(35)

(80)

240

Flexural strength

ASTM D790

N/mm2

-253

-196

-183

-129

-79

+23

Did not break

Did not break

Did not break

Did not break

Flexural modulus

ASTM D790

N/mm2

-253

-196

-183

-129

-79

+23

5100

4700

4600

3200

1600

700

-

-

-

-

-

-

-

-

-

-

-

-

2800

2500

2300

1900

1700

1000

Compressive strength

ASTM D 625

N/mm2

-253

-196

-183

-129

-79

+23

220

171

(168)

158

143

118

-

-

-

-

-

-

-

-

-

-

-

-

188

152

(140)

(105)

(79)

24

Compressive modulus

ASTM D 625

N/mm2

-253

-196

-183

-129

-79

+23

6200

5500

(5400)

(4100)

(3000)

700

6200

5700

(55400)

(4300)

(3200)

550

6100

6000

(5500)

(4300)

(3200)

900

6800

5900

(5600)

(4400)

(3200)

860

Torsional modulus of rigidity

ASTM D 4043

N/mm2

-253

-196

-183

-129

-79

+23

2200

1500

(1400)

1000

500

160

-

-

-

-

-

-

-

-

-

-

-

-

1700

720

(690)

(520)

(390)

110

Izod impact strength (notched)

ASTM D 256

J/m

-253

-196

-183

-129

-79

+23

75

80

(85)

(97)

133

160

67

70

(73)

(86)

(121)

161

51

60

(64)

(103)

(139)

169

53

59

(63)

(98)

(143)

173

TABLE 3 : LINEAR THERMAL CONTRACTION

Material

Linear thermal contraction for 150C to: (% or um/m x 104)

-790C

-1290C

-1830C

-1970C

-2530C

SAE 1020 Steel

0.1

0.1

0.1

0.2

0.2

Copper

0.2

0.2

0.3

0.3

0.3

Aluminium

0.2

0.3

0.4

0.4

0.4

PTFE, unfilled

1.5

1.9

2.0

2.1

2.1

PTFE, 60% Bronze

0.8

1.0

1.2

1.2

1.4

PTFE, 25% Glass fibre (MD)

1.0

1.2

1.5

1.5

1.7

PTFE, 25% Glass fibre (CD)

0.6

0.8

0.9

0.9

0.9

PTFE, 15% Graphite 

0.9

1.2

1.4

1.4

1.5

TABLE 4: LINEAR COEFFICIENT OF THERMAL EXPANSION (x10-6/0C)

Temp Range (0C)

703-N

Glass 15%

Glass 25%

1146N

Carbon 23%/ Graphite 2%

Carbon 29%/ Graphite 3%

MD

CD

MD

CD

MD

CD

MD

CD

MD

CD

MD

CD

- 150 to + 15

103

96

88

74

83

61

70

66

79

57

67

50

-100 to + 15

119

109

102

86

96

69

80

77

90

64

77

57

-50 to + 15

131

117

111

93

1036

74

87

84

95

67

84

61

+15 to + 23

472

286

332

278

284

180

201

207

315

158

222

133

+23 to + 100

125

129

135

123

109

66

117

110

114

70

108

80

+23 to + 200

142

152

156

153

136

84

134

132

136

88

132

99

+23 to + 250

159

176

181

179

159

102

155

152

158

107

152

115

Moulded at Perform Pressure N/mm2

21

45

55

45

70

90

THERMAL CONDUCTIVITY

Unfilled PTFE has low thermal conductivity; adding fillers can increase it significantly. This is particularly important in bearing applications, where the maximum load a bearing can carry is determined by the rate at which heat developed through friction can be dissipated. Graphite is most effective for raising thermal conductivity of a number of PTFE compounds

UV RESISTANCE

Unfilled PTFE has excellent UV resistance and weather-ability, and the same is true for most of its compounds.

RADIATION RESISTANCE

PTFE has a relatively poor resistance to electron and gamma radiation and is not recommended for use where resistance to nuclear radiation is needed. Addition of fillers does not protect the base polymer sufficiently to allow compounds to be safely used in such environments.

TENSILE STRENGTH AND ELONGATION

Compounds of PTFE and filler always have lower tensile properties than virgin PTFE. This is due to the lack of adhesion between the filler and polymer particles. The bond between filler and PTFE is strictly mechanical: this is why irregular or fibrous fillers give the best tensile properties. Tensile strength and elongation for compounds are more a function of the percentage by volume of filler and not so much of the type of filler; the exceptions are carbon and graphite, where lower tensile properties result than might be expected on the basis of percentage of filler by volume.

As the temperature increases tensile strength falls rapidly. 

DEFORMATION UNDER LOAD

Addition of a small percentage of filler reduces the deformation under load substantially. Carbon is one of the most effective fillers in this respect.

TABLE 5: TENSILE PROPERTIES (1)

Temperature 230C

Unfilled

Glass 15%

Glass 25%

Bronze 60%

Carbon 23%/ Graphite 2%

Carbon 29%/ Graphite 3%