Fluoropolymer Foams

14 Fluoropolymer Foams 14.1 Introduction Foamed plastics are also called cellular polymers and expanded plastics, and have played a great role in everyday life. Sponge is a well-known open cellular polymer (Figure 14.1), but wood is the oldest form of foam. It is a naturally occurring foam of cellulose. The first commercial foam was sponge rubber introduced in the 1910s [1]. Polymeric foams possess special characteristics that render them attractive for many industrial and household applications. General polymeric foam attributes and drawbacks are shown in Table 14.1, of which the lightweight is most widely known. Other advantages of foamed plastics over nonfoamed polymers include improved thermal insulation, relatively high strength per unit weight, ease of foam formation, ease of molding, and lower dielectric constant (Figure 14.2). In general, reduction in foam cell size is desired for thermoplastic resins. In particular, for thin-wall foams (e.g., 125e250 mm), small voids to maximize the number of bubbles are desired for mechanical and electrical properties. This is especially so for smaller wire constructions of interest in the electronics field, so that, for example, foam cell

dimensions will be small with respect to the radial dimension of the thin insulation. Void space of foamed polymers is also important with regard to the capacitance of the foamed insulation material. It is important that the conductor with foamed insulation (primary) has a capacitance with low standard deviation. If the voids in the foamed insulation are not uniform, then capacitance variation can arise along a coated wire. Accordingly, small uniform bubbled distribution within the insulation material, which arises by creation of sphericalshaped cells, will result in uniform capacitance. Thus, the nucleating agents are selected to obtain the uniform cell distribution within the foamed insulation material for stable capacitance. Foamed fluoropolymers are also useful in applications other than wire insulation. Fluoropolymer foams have played an important role in the plenum cable market. The air content of the foam reduces the dielectric constant, loss factor (tangent), and permittivity of the electric insulation. Consequently, the signal speed and clarity improves at microwave frequencies. These foams can be processed by a variety of techniques including molding methods, extrusion, and calendering. An attractive feature of fluoropolymer foams is a reduction in the required

Table 14.1 Advantages and Disadvantages of Polymeric Foams Advantages

Disadvantages

Light weight

Variable density

Good thermal insulation property

Loss of certain mechanical properties

High strength per unit weight Ease of molding Impact strength Figure 14.1 Example of open-cell foam structure. Courtesy: Massachusets Inst of Tech, http://web.mit.edu/ dmse/csg/Research.html, March 2015.

Lower dielectric constant

Fluoroplastics, Volume 2. http://dx.doi.org/10.1016/B978-1-4557-3197-8.00014-6 Copyright © 2015 Elsevier Inc. All rights reserved.

412

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413

Figure 14.2 Relationship between dielectric constant and void content of thermoplastics [29].

weight of fluoropolymer per unit length of cable, thus reducing material cost.

14.2 Background Cells are bubbles frozen in size and shape after solidification of a molten plastic. A plastic can contain two types of cellsdclosed structure and open structure. In a closed structure (Figure 14.3) foam system, each cell is an independent closed entity. The cells resemble small glass bubbles that have been dispersed in the plastic. The walls of a closed cell have no holes in them. The cell will contain gas if the plastic is impermeable to the blowing gas. The cells of an open structure are interconnected, thus unable to

hold gas. Liquids and gases can usually move through the open cell structure, like common sponges. Plastic foams are also classified according to their mechanical properties. If the walls of the cell are stiff under stress and relatively inflexible, the foam is called rigid. The foam is flexible, if its wall collapses under stress. Both open and closed structure foams can form rigid and flexible foam.

14.3 Foaming Technology Many resins can be foamed by a variety of processes. Each method of foam manufacturing involves the steps of cell initiation, growth, and stabilization. The usual method for classification of foaming methods is based on the cell growth and stabilization. There are three general methods for producing foam from plastics: mechanical, chemical, and physical (Figure 14.4).

Figure 14.3 An example of closed-cell structure foam. Courtesy: MDI Products, www.mdiproducts.com, March 2015.

Figure 14.4 Chronological evolution of foaming technology [29].

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Initiation or nucleation is the formation of cells in the plastic, which are small discontinuities in the melt continuum. The expansion condition of the foam allows the growth of the nuclei. The governing drive force for the growth of each cell is the difference between the pressure inside and outside that cell as shown in Eqn (14.1) [2]. Surface tension of the melt (g) and the cell radius (r) are the two factors determining the pressure difference (DP). Surface tension of the melt is a function of many factors including the type of plastic, temperature, pressure, and other additives present. The pressure outside the cell is the pressure that the melt is under, and the pressure inside the cell is the pressure that is generated by the blowing/foaming agent. A gas or a solid could be used to initiate foaming, around which the blowing agent accumulated. DP ¼

2g r

(14.1)

Cell growth is a very complex process because the properties of the melt change during the cell growth phase. Few quantitative models have been established which completely describe the cell growth. Viscosity and pressure continually change during the growth phase. Viscosity change affects the cell growth rate and polymer flow. Pressure drop in the blowing agent follows Eqn (14.1) that is inversely proportional to the radius in contrast to volume. It is important to note that pressure is higher in small cells than larger cells, leading to intercell gas diffusion or breaking of the cell walls. Thermodynamically speaking, generation and growth of cells in a molten plastic is destabilizing. A fluid tends to minimize its free energy by reducing its surface area. Cell formation significantly increases the surface area of the polymer melt. The unstable foam at the end of the growth phase requires stabilization to retain the cells in the foamed state. These stabilization methods have traditionally been used to classify the foaming technique as either physical or chemical. Also, limited use of mechanical foaming is made in the industry.

14.3.1 Mechanical Foaming The application of mechanical foaming is limited. This technique is similar to whipping cream during which air is mixed in with the bulk of melt. Mixing simply entrains air into the molten plastic where air

becomes the foaming gas. An example of an application of this method is the foaming of vinyl plastisols for producing thick vinyl flooring material.

14.3.2 Chemical Foaming In this method, chemistry controls the foam formation process [3], that is, the rate of formation of the polymer during which the viscous fluid is converted to a cross-linked (three-dimensional) structure. Chemistry also controls the rate of activation of the blowing agent, which is either by a drop in solubility in the monomer solution as the reaction proceeds, or by thermal degradation. The characteristics of the blowing agent determines the amount of gas generated, the rate of gas generation, the foaming pressure, and the net amount of gas retained in the cells. This technique is not generally applicable to fluoroplastics because these resins are not typically cross-linked, in addition to their strict polymerization regimes.

14.3.3 Physical Foaming In the physical foaming technique, a blowing (foaming) agent is added to the plastic, which volatilizes during the melting process. This agent can be either a liquid or a gas. A nucleating compound may have to be used to control the cell size. The chemical structure and composition of the plastic define the foaming process conditions. The key variables are temperature, type of foaming agent, and the cooling rate of the expanded structure in order to stabilize it dimensionally. The nature of the foaming agent and its concentration in the plastic determine [3] the rate of gas evolution, gas pressure, gas retention in the cells, and heat absorption/release due to the degradation/activation of the blowing agent. This is the technique by which fluoropolymer foams are produced. In a typical process, a solid nucleating agent that is stable at the processing temperature is added to the fluoroplastic. The nucleation agent plays a number of roles including uniform cell shape and dimensions, control of the cell number and size, and broad foaming window. Without a nucleating agent, a foam structure with irregularly shaped cells with limited void content is produced that has excessive compression recovery and poor physical properties. Boron nitride has been found to be the ideal nucleating agent for meltprocessible fluoropolymers [4]. Carbon dioxide and nitrogen have replaced FreonÒ as blowing agents in

14: F LUOROPOLYMER F OAMS

response to ozone layer environmental concerns with chlorofluorocarbons. Conventional nucleating agents include boron nitride, calcium carbonate, magnesium dioxide, lead oxide, barium oxide, antimony oxide, magnesium carbonate, zinc carbonate, barium carbonate, carbon black, graphite, alumina, calcium silicate, calcium metasilicate, and calcium sulfate. Polytetrafluoroethylene (PTFE) is a nonmeltprocessible fluoroplastic. It can be foamed by a somewhat different technology than those used for melt-processible fluoropolymers.

14.4 Foam Manufacturing Processes A variety of processes can be employed to produce “foamed” parts. The process has to accommodate the three stages of initiation, growth, and stabilization. These processes are classified by the manner of achieving pressure differences between the outside and inside of the foam cell. If the external pressure is lowered, the process is called decompression foaming. The process in which the internal cell pressure grows is called expandable foaming. Other methods to create cellular structure are sintering the resin particles in the presence of a gas or dispersing a gas or a solid in the molten polymer. Decompression and expandable foaming can be stabilized by both chemical and physical methods. Common plastic processes such as extrusion, injection molding, and compression molding are used to produce foamed articles.

415

Table 14.2 Dielectric Properties of Fluoroplastics at 1 MHz [21] Resin

Dielectric Constant

Dissipation Factor

PTFE

2.1

<0.0004

FEP

2.1

0.0002

PFA

2.1

0.0002

ETFE

2.6

0.007

ECTFE

2.6

0.014

PVDF

8.0

0.16

Abbreviations: PTFE, Polytetrafluoroethylene; FEP, TFE/HFP copolymer; PFA, perfluoroalkoxy; ETFE, ethylenetetrafluoroethylene; ECTFE, ethylene-chlorotrifluoroethylene; PVDF, polyvinylidene fluoride.

the insulation. The decrease in the dielectric constant is proportional; for example, FEP (copolymer of tetrafluoroethylene (TFE) and hexafluoropropylene (HFP)) insulation with 60% void content had a dielectric constant of 1.3 [5]. More uniform foam cell size and smaller cells yield foams with the best electrical properties. The low dielectric constant and dissipation factor reduce signal loss and cross talk. They also allow miniaturization of the circuitry because of the good insulation properties of fluoropolymers at very high voltages. These cables are suitable for transmission in microwave frequencies in excess of 10 GHz. The impact of void content on PFA and FEP properties can be seen in Figure 14.5.

14.6 Foaming Technology 14.5 Benefits of Fluoroplastic Foams The main driver for fluoroplastic foams has been the insulation for data transmission cables. An example is coaxial cables that have relatively thick insulation. Its low dielectric constant and dissipation factor are desirable electrical properties. Air has the ideal dielectric constant (1.0). The ideal dissipation factor for data-cable insulation is zero. Perfluoropolymers have low dielectric constant and dissipation factor values (Table 14.2). Foaming perfluorinated fluoropolymers further reduces the dielectric constants toward 1.0 and moves the dissipation factors closer to zero because the resin is replaced with air-filled cells in

There are various foamable fluoropolymer resin options available, each having unique capabilities and limitations. Selecting the correct resin for the application is important for cost, ease of processing, and desired electrical performance. Designing and processing cables within the materials capabilities will produce quality products with high yields. Processing equipment selection and process conditions are critical to insure a stable process, maintain minimum product variation, and achieve the lowest cost operation. Special techniques, such as the addition of solid skin layer(s) to foam constructions, can provide additional improvements to processing and performance. In this section, foaming techniques are covered based on the fluoropolymer type, beginning

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F LUOROPLASTICS , VOLUME 2

Figure 14.5 Effect of voids on FEP and PFA properties [30].

with perfluoroplastics. Next, foaming of partially fluorinated fluoropolymers are reviewed, combining ethylene-tetrafluoroethylene (ETFE) and ethylenechlorotrifluoroethylene (ECTFE) and separately covering polyvinylidene fluoride (PVDF). Technological reviews are based primarily on patents and publications that focus on potentially commercial methods of foaming fluoroplastics.

14.6.1 Foaming Melt-Processible Perfluoropolymers One of the earliest reports of a fluoroplastic foaming technique was issued by Randa in 1963 [4]. A physical foaming method was used in which a fluoromethane was the foaming agent along with boron nitride particles (preferred particle size <10 mm) to control the cell size. Examples of fluoromethanes included dichlorodifluoromethane, chlorodifluoromethane, dibromodifluoromethane, and chlorobromofluoromethane. Foams of meltprocessible FEP were produced by this technique (Figure 14.6). An example of this foaming technology was demonstrated on an FEP resin with a melt viscosity of 8.2  104 P. The resin was milled with 1% by weight of boron nitride in a Banbury mixer for 15 min before applying mechanical energy and raising the temperature of the mixture to 350  C. Boron nitride was well dispersed in the FEP resin. Afterward, the mixture was shredded into small

Figure 14.6 Elements of manufacturing foamed fluoropolymers [29].

cubes. These cubes were exposed to chlorodifluoromethane for 5 days at a pressure of 150 kPa at room temperature. The cubes were subsequently extruded onto a 19-gauge wire, using a 38-mm single extruder equipped with a 2.25-mm extrusion die. The melt temperature and pressure were 390  C and 2.2 MPa. The extrusion die near the orifice was heated to 500  C by means of an induction heater. The coated wire was quenched in a water bath that was located 5 cm from the die outlet. The thickness of the foam insulation on the wire was 1.12 mm. The foam cells had a diameter ranging from 25 to 75 mm and the total void content was 53%. The density of the foam was 1.02 g/cm3 and it had a dielectric constant of 1.47. Table 14.3 shows the effect of foaming agents, exposure time, and pressure on the properties of the foam. All of the tested FEPs

14: F LUOROPOLYMER F OAMS

417

Table 14.3 Effect of Foaming Process Variables on FEP Foam Properties [4] Exposure Time, Days

Pressure, kPa

Void Content, %

Foam, Density, g/cm3

Dielectric Constant

Cell Diameter, mm

5

150

53

1.02

1.47

25e75

Chlorodifluoromethane

4

445

57

0.93

e

375

Dichlorodifluoromethane

4

100

28

1.50

1.76

13e50

1,2-Dichloro1,1,2,2-tetrafluoro ethane

3

100

<1

e

e

e

Foaming Agent Chlorodifluoromethane a

a

Contains 1% by weight aluminum oxide.

contained 1% by weight boron nitride except for one composition that contained 1% by weight aluminum oxide. Further work has been done to address the issue of boron nitride cost, which is quite expensive. It was discovered [6] that, in addition to boron nitride, a small amount of an inorganic salt could be included which enhanced the foam nucleation by reducing the foam cell size. The composition incorporated 0.05e1.0% by weight of boron nitride and 25e1000 part per million of the inorganic salt (preferred particle size <5 mm). Suitable salts had a specific relationship between the radius of the cation, the valence of the cation, and the acid strength of the protonated anion (see Eqn (14.2)). Examples of salts that were effective included bicarbonates and tetraborates of Li, Ca, Sr, and Na, while bicarbonates of Ba, K, Rb, Mg, and Zn were ineffective. Table 14.4 shows the required concentration of various salts in combination with 0.25% boron nitride that yields 50% void in FEP. 0:36ð14  pKa Þ  0:52  ðr  0:2qÞ2  0:11cð14  pKa Þ  0:28

Table 14.4 Effective Concentration of Various Salts to Obtain FEP with 50% Void Volume [6] Salt Concentration, ppm

Average Cell Size, mm

Lithium carbonate

88

165

Sodium carbonate

67 134

200 150

Sodium tetraborate

130 1000

165 610

Potassium tetraborate

125

150

Calcium tetraborate

125

150

Sodium fluorosilicate

125

180

Barium nitrate

50

675

100 200

175 225

400

250

50

325

100

200

200

200

100

450

200

125

400

250

Salt

(14.2) Aluminum phosphate

˚ r ¼ crystal ionic radius of the cation, A q ¼ valence of the cation pKa ¼ logKa Here, Ka is the equilibrium constant for the dissociation in the following reaction, where H is hydrogen and A is the salt ion. H A1n 4H þ þ An

Sodium sulfite

To generate foam, a gas (e.g., nitrogen, carbon dioxide, and chlorodifluoromethane) is continuously injected into the extruder (Figure 14.7), processing a fluoropolymer that has been compounded with

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F LUOROPLASTICS , VOLUME 2

Table 14.5 Extruder Design and Processing Conditions for Foaming FEP and PFA [6] Polymer Parameter

FEP

PFA

27.8

27.8

Melt draw

Melt draw

Extruder: Figure 14.7 Schematic of a melt extruder for foaming fluoropolymers [5].

L/D ratio Extrusion regime

Extruder screw design:(mm depth/number of turns)

a nucleating agent such as boron nitride. The extruder is specially designed for foaming process. A blowing agent such as chlorodifluoromethane was dissolved in the compounded resin. This agent was suitable because it easily dissolved in the molten fluoroplastics and had adequate thermal stability to withstand temperatures up to 380  C. In this system, foam cell formation starts shortly after the molten resin containing the blowing agent exits the extrusion die. Melt draw contributes to foam nucleation. Cell growth continues while the melt travels between the die exit and the water bath. Cell growth is stopped when the melt enters the cooling water trough and the polymer begins to solidify. Typical examples of extruder design and processing conditions for foaming FEP and PFA are given in Table 14.5. Figure 14.8 shows the effect of the concentration of calcium tetraborate (second salt) on the average size of the FEP foam cells. It appears that foam cell size reaches a minimum between 100 and 200 ppm of calcium tetraborate independent of boron nitride concentration in the range of 0.25e0.50% by weight. Each salt has a specific concentration at which minimum cell size is achieved. Other important variables affecting the foam cell size include wire speed and extrusion cone length (viscoelastic effect) in wire insulation. Tables 14.6 and 14.7 show the effect of these parameters for FEP containing 0.5% by weight boron nitride and foamed by injection of chlorodifluoromethane. The void content of the foam was 50%  5%. Additional improvements have been made to the nucleation agent in the foaming process using a blowing agent or injected gas. A number of patents [7e9] have described the compositions of more effective nucleating agents containing at least one of a special class of sulfonic and phosphonic acids, and salts of the acids. These agents enhanced foam nucleation as evidenced by smaller cells, higher foam void contents, and/or greater uniformity of cell size.

Feed zone

6.35/10

6.35/10

Metering zone

1.39/4

1.39/4

Gas zone

3.05/4

3.05/4

2.15/3.5

2.15/3.5

6.35/4

6.35/4

Rear

340

370

Center rear

380

390

Center front

370

380

Front

375

380

Adaptor

365

380

Crosshead

300

340

Die

325

315

Melt

385

395

Screw speed, rpm

30

25

Wire size, AWG

22

24

Wire speed, m/min

21

43

Wire preheat, C

65

120

Crosshead melt pressure, MPa

10.3

9.0

Gas pressure, kPa

825

825

Vacuum, mm H2O

380

250

Die size, mm

4.57

4.57

Guide tip size (outside diameter), mm

1.90

2.54

Foamed core size, mm

3.68

2.54

Pumping zone Mixing zone Temperatures,  C:



The compositions contained 250e3000 ppm by weight of one of the acids and/or salts with the following chemical formula: h i ZðCF2 Þx ðCF2 CFXÞp ðR0 Þy ðCH2 Þz RO3 M (14.3) n

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419

Table 14.8 Relative Efficiency of Bivalent Nucleating Agents [7]

Figure 14.8 Effect of a second salt (calcium tetraborate) on the foam cell size of FEP [6]. Table 14.6 Effect of Wire Speed on the FEP Foam Cell Size [6] Wire Speed, m/min

Average Cell Size, mm

Number of Cells per cm3

4.6

500

7.6  104

9.1

380

1.7  105

21

180

1.6  106

37

75

2.3  107

Table 14.7 Effect of Cone Length on the FEP Foam Cell Size [6] Cone Length, cm

Average Cell Size, mm

Number of Cells per cm3

1.9

150

2.8  106

7.6

200

1.2  106

12.7

560

5.4  104

in which Z is CCl3, CCl2H, H, F, Cl, or Br; X is selected from H, F, or CCl; R is sulfur or phosphorous; M is H or metallic, ammonium, substituted ammonium, or quaternary ammonium cation; x is an integer between 0 and 10; p is an integer between 0 and 6; y is 0 or 1; z is an integer between 0 and 10; Z is CCl3 or CCl2H; and n is the valence of M. Preferably, M is a metallic ion that is stable at the foaming

Metal in the Salt (14.3)

Total Void Content, %

Average Cell Size, mm

Number of Cells per cm3

Barium

55

75

2,300,000

Strontium

59

100

763,000

Calcium

41

220

72,200

Zinc

20

508

24,800

temperature. R0 is a C1 to C16 straight or branchedchain perfluoroalkylene diradical containing a perfluorinated alicyclic ring or a C1 to C6 perfluorinated aliphatic polyether diradical with repeat units selected from CF2O, CF2CF2O, and CF2CF(CF3)O. Different salts have great differences [7] in their nucleating (foaming) efficiency as shown in Table 14.8 for bivalent salt telomer B sulfonic acid (TBSA) generally described by the chemical formula in Eqn (14.3). TBSA has a chemical formula of F(CF2)nCH2CH2SO3H in which n is a mixture of 6, 8, 10, and 12 but predominantly 8. The number of cells per unit volume and the average foam cell diameter have defined nucleating efficiency. Barium was by far the most effective metal among the group that was tested. Examples of the effectiveness of specific compositions of these nucleating agents have been presented in Table 14.9. An FEP polymer was dry blended with 5% by weight of boron nitride concentrate in a 28-mm twinscrew extruder at the appropriate ratio to yield a compound containing 0.25% by weight boron nitride. This composition was called “A.” Compound “B” was composed of 0.25% by weight boron nitride

Table 14.9 Efficiency of Compositions of Nucleating Agents [7] Total Void Content, %

Average Cell Size, mm

Number of Cells per cm3

A

55

381

18,000

B

60

152

312,000

C

55

100

2,800,000

D

58

76

12,000,000

Compound

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F LUOROPLASTICS , VOLUME 2

weight and 110 ppm of calcium tetraborate. The resin was extruded as wire insulation and foamed by continuous gas injection of nitrogen into the extruder. The foamed insulation had an outer diameter of 2.3e2.5 mm. The process took place in a 45-mm single screw extruder by using a temperature profile similar to those shown for FEP in Table 14.5. Unique methods have been offered for foaming perfluoropolymers. For example, a self-foaming composition has been developed [12] by utilizing the end groups of the fluoropolymers. PFA and FEP that contain a sufficient number of end groups consisting of carboxylic acid or salts, eCOOH or eCOOM, were found to be foamable; M was one or more of the alkali metal or alkaline earth metal ions. These end groups are produced during the emulsion polymerization of TFE and comonomers. Other end groups such as eCOF are converted to eCOOH by hydrolysis. The desirable minimum concentration of end groups was 50 end groups per 106 carbon atoms, which yielded a void fraction of 20% or more. To foam resin with unstable end groups, compositions containing nucleating agents were prepared for extrusion according to the following procedure. The nucleating agent was dry blended with FEP to prepare a concentrate containing 10 times the desired final concentration. The concentrate was diluted in the extruder during the extrusion of the wire insulation. The extrusion foaming process was similar to that used for ordinary extrusion of full-density fluoropolymer resin. No gas was injected into the extruder, nor was a chemical blowing agent added to the resin. The void level was controlled by temperature regulation and the hold-up time of the resin in the extruder. An example of processing self-foaming FEP into wire insulation is shown [12] in Table 14.11.

and 330 ppm of the potassium salt of TBSA (defined in the previous paragraph). Compound “C” consisted of 0.25% by weight boron nitride and 125 ppm calcium tetraborate. Compound “D” had a composition of 0.25% by weight boron nitride, 125 ppm calcium tetraborate, and 330 ppm of the potassium salt of TBSA. It can be seen from the results in Table 14.9 that the compound “B” containing the potassium salt of TBSA gave enhanced foam nucleation as compared to “A.” The best efficiency was obtained with a threecomponent nucleating agent consisting of boron nitride, potassium salt of TBSA, and calcium tetraborate. Subsequent work by Randa and Buckmaster [8,9] has shown that other compounds, in addition to aliphatic free acids and salts of sulfonic and phosphonic acids (Eqn (14.3), form suitable nucleating agents; these compounds may contain either oxygen or cycloalkyl groups. Aromatic sulfonic and phosphonic acids in which the aromatic ring is optionally substituted with alkyl, fluorine-containing alkyl, and/ or hydroxyl groups. Additional improvements have been made to the type and the composition of nucleating agents for melt-processible fluoropolymers [10,11]. For example, the effectiveness of boron nitride was enhanced by controlling the shape and size of its particles [10]. The improved boron nitride produced smaller foam cells. It was obtained as crystal platelets, generally rounded, that have been grown to final size and have been deagglomerated without milling. Table 14.10 gives an example of the effect of boron nitride particle morphology and size on foaming efficiency when it is incorporated in an FEP resin (melt flow rate (MFR)d7 g/10 min) as a nucleating agent. It was composed of 0.25% boron nitride by

Table 14.10 Effect of Boron Nitride Particles on the Foaming Efficiency FEP [10] Total Void Content, %

Average Cell Size, mm

Dielectric Constant

Exterior Surface Condition

Compound

Boron Nitride Characteristics

A

Unmilled, average particle size 4 mm and surface area of 6.1 m2/g

63

36

1.32

Smooth

B

Unmilled, average particle size 2 mm and surface area of 9.7 m2/g

60

36

1.35

Smooth

C

Milled, average particle size 6.3 mm and surface area of 7.1 m2/g

54

89

Rough

14: F LUOROPOLYMER F OAMS

421

Table 14.11 Extruder Design and Processing Conditions for Self-Foaming FEP [12]

Table 14.11 Extruder Design and Processing Conditions for Self-Foaming FEP [12] (Continued )

FEP Composition No. Parameter

1

2

3

Extruder:

FEP Composition No. Parameter

1

2

3

Extrudate properties:

Barrel diameter, mm

45

60

45

Foam diameter, mm

2.36

2.33

2.35

Die diameter, mm

5.58

4.32

5.58

Capacitance, pF/m

61.9

60.2

62.3

Guide diameter, mm

1.90

1.91

1.91

Dielectric constant

1.44

1.40

1.46

Foam cell diameter, mm

90

150

150

Voids, %

50

55

50

Screw flights/depth: (number of flights/mm depth) Feed zone

10/8.3

12/11.4

12/11.4

Transition

3/e

3/e

1/e

Metering zone

4/3.1

none

4/3.1

Mixing zone

4/5.1

none

4/5.1

Pumping zone

6/2.8

5/3.8

6/2.8

Mixing zone

4/5.1

10/7.1

4/5.1

Rear

338

357

338

Center rear

338

360

338

Center

338

360

338

Center front

341

362

341

Front

341

362

341

Clamp

357

357

357

Adaptor

316

343

316

Crosshead

282

341

282

Die

274

357

271

Melt

338

362

339

Wire preheat

135

82

115

Screw speed, rpm

20

14

20

Crosshead pressure, MPa

7.0

17.2

8.9

Crosshead vacuum, mm Hg

e

100

80

Wire speed, m/min

76

56

42

Core length, mm

51

19

51

Air gap, m

12

10

10

Temperatures,  C:

(Continued )

A composition was prepared by blending two grades of FEP. The FEP resins respectively had MFR of 19.2 g/10 min (60% by weight of the blend) and 7.2 g/10 min. The resins contained 400 and 340 carboxyl groups per million carbon atoms, plus 55 and 46 ppm of potassium, respectively. The FEP resin blend was combined with 0.25% (by weight) of boron nitride and 110 ppm of calcium tetraborate. Carbon dioxide can be used as a blowing agent for foaming melt-processible fluoropolymers [13]. Carbon dioxide foaming takes place at temperatures below those required for melt extrusion of fluoropolymers. Perfluoropolymers and partially fluorinated fluoropolymers are respectively processed (extruded) at about 60  C and 35  C above their melting points. Homopolymers and copolymers of chlorotrifluoroethylene (CTFE) and TFE can be foamed by supercritical carbon dioxide. The optimal means of the introduction of carbon dioxide was injection into the extruder at a pressure equal to or greater than its critical pressure to insure that it was in a supercritical state. The preferred range of MFR of fluoropolymers was 5e25 g/10 min. For example, a single screw extruder with a diameter of 32 mm and a length-to-diameter ratio of 30/1 was used to foam the polymer by continuous injection of carbon dioxide. The extruder was equipped with a decompression screw having a lowpressure zone. Carbon dioxide was injected into the low-pressure zone; pressure increased as the mixture of polymer and CO2 was pumped along the extruder

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F LUOROPLASTICS , VOLUME 2

Table 14.12 Extruder Design and Processing Conditions for Carbon Dioxide Foaming [13]

Table 14.12 Extruder Design and Processing Conditions for Carbon Dioxide Foaming [13] (Continued )

Extrusion Number Extrusion Number Parameter Polymer Type

1

2

3

4

PFA

PFA

FEP

FEP

Polymer Type

Nucleating agent: Boron nitride, %

0.25

Calcium tetraborate, ppm

110

Barium sulfide, ppm

180

0.25 110

180

0.25 120

e

0.25 120

e

0.06

e

e

e

25

40 e60

75 e100

e

39

48

e

e

e

e

e

Voids, %

Guide tip diameter, mm

1.90

e

e

e

334

307

Center rear

343

363

348

324

Center

344

363

347

329

Center front

344

363

347

323

Front

318

352

332

308

Clamp

316

329

305

283

Adaptor

307

316

295

272

Crosshead

277

316

309

279

Die

278

e

e

e

Melt (in extrusion die)

338

323

304

278

Screw speed, rpm

21

80

80

80

Crosshead pressure, MPa

7.69

3.21

5.9

8.93

CO2 injection pressure, MPa

2.86

2.17

2.17

2.17

Air Gap, m

0.6

e

e

e

(Continued )

FEP

Wall thickness, mm

3.26

343

FEP

e

Die diameter, mm

343

PFA

e

Slab

Rear

PFA

e

Slab

Temperatures, C:

4

2.36

Slab



3

Outside diameter, mm

Tubing

Die type

2

Extrudate properties:

Foam cell diameter, mm

Extruder:

1

Parameter

barrel toward the die. Extrusion conditions and foam properties have been listed in Table 14.12 for FEP and PFA, which are conventionally extruded at a melt temperature of 370e400  C. Fabricated articles of fluoropolymers have also been foamed by supercritical CO2 under heat [14]. The article was heated to a foamable state inside a pressure vessel where it could be pressurized and held with carbon dioxide. Carbon dioxide may be introduced prior to heating or until the article has been heated. The supercritical CO2 permeates the article, thus resulting in foaming. The temperature to which the vessel was heated depended on the particular fluoropolymer and on the viscosity in the case of melt-processible fluoropolymer. The minimum pressure was 7.4 MPa (critical pressure of CO2), while the temperature ranged from 220 to 250  C for FEP, and from 280 to 325  C for PFA. At the end, the pressure vessel was depressurized, while the article was in the foamable state followed by cooling. A PFA film was foamed at 300  C by initially pressurizing the vessel at 8.3 MPa with carbon dioxide, reaching a final pressure of 30 MPa at this temperature. A PFA film containing 0.25% boron nitride, 110 ppm calcium tetraborate, and 180 ppm barium sulfide had a void content of 72% and an

14: F LUOROPOLYMER F OAMS

average size of 10 mm. Raising the foaming temperature to 325  C altered the foam walls into a Swiss cheese-like appearance and raised the average foam cell diameter to 100 mm. At 280  C, it had a fibrillar foam structure. Foaming did not take place when carbon dioxide was replaced by nitrogen even when the initial pressure had been raised to 13.8 MPa. A composite structure comprised a fluoropolymer and a substrate is combined, and can also be foamed using the supercritical carbon dioxide technique [15]. The exterior of foamed wire insulation can be coated with a solid layer (jacket) of ETFE or ECTFE for wires used in telecommunications and fiber optics. This arrangement allows the foamed perfluoropolymer to provide good electrical properties, while the skin layer supplies sufficient mechanical properties. A foamed FEP wire insulation (core), with a thickness of 0.5 mm, was coated [16] with 51 mm of an ETFE jacket; it had a dielectric constant of 1.64 and dielectric breakdown strength of 26.6 kV/mm.

14.6.2 Foaming PTFE PTFE has been described in US Patent No. 4,304,713 [17] as a nucleating agent for making dielectric compositions for use in coaxial cables, and US Patent No. 5,314,925 [18] discusses fluoropolyolefin nucleating agents for molded thermoplastics. US Patent No. 5,716,665 [19] addresses the use of PTFE, as well as boron nitride, silicon nitride, alumina, talc, and zinc sulfide, as nucleating agents for foamable solid compositions based on thermoprocessible perfluoropolymers. PTFE has also been foamed [14,20] by heating it in supercritical CO2. The procedure was generally similar to those with a sintered film of granular [21] PTFE. The polymer was modified with 0.05e0.1% by weight of perfluoropropylvinylether. The foaming temperature was 310  C at a CO2 pressure of 24.2 MPa. The foamed PTFE had a void content of 40%. In another experiment, an unsintered cylindrical shape (billet) of PTFE was foamed by the supercritical carbon monoxide at 325  C at a final pressure of 29 MPa. The billet was cooled to 250  C over 25 min before venting the CO2. Skived tape from the billet had a void content of 33%. Paste-extruded [21] PTFE was foamed at 330  C and 24.2 MPa yielded 15.7% voids. Table 14.13

423

Table 14.13 Effect of Sintering and Temperature on Foaming of Polytetrafluoroethylene (PTFE) [14,20] Foaming Temperature,  C

Void Content, %

Unsintered

310

23.0

Sintered

310

12.9

Unsintered

330

19.4

Sintered

330

15.7

Unsintered

350

9.7

Sintered

350

14.7

State of PTFE

shows the effect of sintering and temperature on the void content of the paste-extruded PTFE. Expanded structures can be formed from PTFE by a number of other means [21]. For example, preforming and sintering cycles can be controlled to leave voids in the molded part. Another approach is to stretch tapes of PTFE uniaxially or biaxially to create expanded structures that contain pore/voids with controlled size. PTFE compounded with thermally unstable compounds that produce gases such as carbonates can also produce foamed structures. Carbonates decompose and generate carbon dioxide gas when PTFE is heated during the sintering cycle. Carbon dioxide expands in the void space and expands the molten PTFE.

14.6.3 Foaming ETFE and ECTFE Some of the fluorinated copolymers of ethylene such as ETFE and ECTFE polymers can be fabricated into foams that are useful for wire insulation applications. These compositions are usually composed of 40e60 mol% of ethylene and have an ethylene to CTFE or TFE mole ratio of 1:1. Small amounts of other fluorinated comonomers are polymerized along with the two main monomers to improve the mechanical properties of the interpolymers. In one technique, ECTFE was blended [22,23] with a nucleating agent consisting of talc or carbonate or an oxide (or mixtures) of a metal selected from the group including magnesium, calcium, barium, zinc, and lead. The desired range of the nucleating agent was from 0.1 to 5 parts per 100 parts of fluoropolymer resin. Up to 30 parts of blowing agent could be incorporated, although 0.1e5 parts per 100 parts were normally sufficient. Calcium

424

F LUOROPLASTICS , VOLUME 2

carbonate and hydrazodicarboxylates were the preferred nucleating and blowing agents. Hydrazodicarboxylates are known to thermally decompose to produce olefins, carbon dioxide, carbon monoxide, alcohols, and a small amount of nitrogen. The blend was heated to a temperature above the melting point of the polymer under pressure (to prevent foaming) followed by depressurization. In a typical foaming trial, 1 part by weight of magnesium carbonate and 1 part by weight of diisopropyl hydrazodicarboxylate were added to 100 parts of ECTFE. The ECTFE resin had a density of 1.68 g/cm3 and an MFR of 15 g/10 min (at a temperature of 275  C and a load of 2160 g). The mixture was dispersed uniformly in a ribbon blender and extruded through a 38-mm extruder. The extruder had a length-to-diameter ratio of 20 to 1 and was equipped with a crosshead wire-coating die. Temperatures ranged from 232 to 277  C in the extruder when coating a 22-gauge wire. The coating had a density of 0.92 g/cm3 (or 45% void) and average foam cell diameter of 100 mm. Table 14.14 shows the effect of the nucleating agent (2 parts by weight per 100 parts) on the ECTFE foam properties of foamed strands about 3.1 mm in diameter. Wires have been insulated [24] and jacketed with ETFE and ECTFE resulting in excellent electrical and mechanical properties. A 24-gauge wire was insulated with an ETFE jacket and core. The jacket was solid (25-mm thick) and covered a foam insulation core (0.127 mm). This insulation had 45% void content and a dielectric breakdown voltage of 20 kV/mm. Similar to perfluoropolymers, ETFE and ECTFE can be foamed [14] using carbon dioxide. ECTFE resin was formed into a film and placed in the

pressure vessel. The vessel was pressurized with carbon dioxide to 6.9 MPa, then heated to 220  C and held at this temperature for 1 h. Heating the vessel raised the gas pressure to 18 MPa, which was released by venting. The vessel was cooled to room temperature and the film was removed. The appearance of the film turned opaque from transparent due to foaming; it had a void content of 56%.

14.6.4 Foaming PVDF PVDF has been converted [25e28] to expanded structures for applications such as electrical wire insulation (with a dielectric constant below 5.0) and ultrafiltration membranes. In this method, PVDF and its copolymers or terpolymers were blended with 0.05e5.0% by weight of the polymer of a nucleating agent, 0.05e5.0% by weight of the polymer of a blowing agent, and 0.05e5.0% by weight of the polymer of a dispersing aid. Ideally, the mixture should contain at least 70% by weight of PVDF. A homogeneous powder was formed by melt compounding this mixture using high shear melt blending at below the activation temperature of the blowing agent. Finally, the homogenous blend was extruded onto wire at an appropriate temperature to form foam and subsequently quenched. The nucleating agent should have an average particle size of less than 2 mm. Examples of nucleating agents include calcium carbonate, magnesium oxide, titanium oxide, carbon black, calcium metasilicate, magnesium hydroxide, antimony oxide, lead carbonate, barium oxide, zinc carbonate, molybdenum disulfide, and others. The optimal concentration and activation temperature of the blowing

Table 14.14 Effect of Nucleating Agent on the Ethylene-Chlorotrifluoroethylene Foam Properties [22,23] Density, g/cm3

Void Content, %

Average Cell Diameter, mm

Surface Quality

Talcum

0.68

59

100

Smooth

Calcium oxide

0.72

57

125

Smooth

Magnesium oxide

0.95

43

125

Smooth

Titanium oxide

0.7

58

300

Slightly Rough

Carbon black

1.0

40

200

Slightly Rough

None

0.67

60

400

Slightly Rough

Nucleating Agent

14: F LUOROPOLYMER F OAMS

425

agent is <3% by weight and 220  C, respectively. The most desirable blowing agent was diisopropyl hydrazodicarboxylate and the preferred dispersing aid was dibutyl phthalate or dioctyl phthalate. The nucleating agent and the dispersing aid could be preblended and added to the polymer. The blowing agent was defined as a compound with the formula ROOCeHNeNHeCOOR0 . R and R0 were selected from the group consisting of secondary and tertiary alkyl groups [25,26]. These alkyl groups may have 3e5 carbon atoms. R and R0 could also be selected from the group the included straight-chain and branched-chain alkyl groups having from 1 to 8 carbon atoms, cycloalkyl groups having from 5 to 8 carbon atoms, aryl radicals having from 6 to 10 carbon atoms, and alkaryl and aralkyl radicals having from 7 to 19 carbon atoms. The best method to produce a homogenous mixture from the components of the PVDF foam mixture was to extrude them through a twin-screw extruder into small pellets. A suitable extruder had a length-to-diameter ratio of 24:1 and a compression ratio of 3e5:1, with sharp or gradual transition zones. A high temperature was employed to activate the foaming agent. The most effective place for the activation of the blowing agent was the metering zone because the heat transfer to melt was uniform which resulted in a more homogeneous foam activation and cell structure in the end product. Table 14.15 shows the general effect of process variables on a PVDF foam structure. Table 14.16 shows the composition of a PVDF blend for foam fabrication. This recipe was mixed in an intensive power blender to produce a homogeneous powder blend which was then melt blended in a high shear twin-screw extruder at 200 rpm and a temperature of 200  C. Pellets were produced by extruding the melt into water and chopping the

quenched extrudate. The foamable blend was extruded, through a 25-mm extruder, according to the conditions given in Table 14.17. A comparison of the properties of foamed and solid PVDF wire insulation is presented in Table 14.18. Table 14.16 Composition of a Polyvinylidene Fluoride (PVDF) Foam Blend [25,26] Content, wt.%

Material PVDF (KynarÒ 461 sold by Ato fina)

95

Nucleating agent (CaCO3)

1.0

Dispersing aid (dibutyl phthalate)

3.0

Blowing agent (diisopropyl hydrazodicarboxylate)

1.0

Table 14.17 Wire Extrusion Conditions for Polyvinylidene Fluoride Foaming Blend [25] Extrusion Parameter Conductor

Value 24 AWG

Screw speed, rpm

50

Line speed, m/min

150

Die to quench bath distance, mm

50

Extruder temperature,  C: Zone 1

210

Zone 2

230

Zone 3

285

Gate

240

Crosshead

240

Forming die

230

Table 14.15 General Effects of Process Variables on the Polyvinylidene Fluoride Foam Structure [25,26] Process Variables

Change

Foam Insulation Density

Cell Size

Cell Wall Thickness

Line speed

þ

þ





Die to quench bath distance

þ



þ

þ

Extruder speed

þ

0

 (Slight)

þ

Wire preheat temperature

þ



þ

þ

Guide tube position in the die

þ

þ





426

F LUOROPLASTICS , VOLUME 2

Table 14.18 Typical Properties of Foamed and Solid Polyvinylidene Fluoride (PVDF) Wire Insulation [25,26]

Table 14.19 RG-62 Coaxial Cable Requirements according to the Military Specifications MIL C-17/30B [5]

Foamed PVDF

Solid PVDF

Insulation wall thickness, mm

175

175

Density, g/cm3

0.80

1.76

Void content, %

55

0

Diameter of core insulation, mm

Average cell size, mm

15e25

0

Maximum capacitance, pf/cm

Tensile modulus, MPa

20

41.5

Velocity of propagation

Tensile break strength, MPa

0.25

0.9

Impedance, U

50e80

100e400

Flexural modulus, MPa

19

41.5

Dielectric constant at 100 Hz

3.6

8.2

Dielectric strength, kV/mm

22.4

81

200e300

850

Variable

Tensile break elongation, %

Insulation resistance, Megaohm/300 m

Cable Parameter

Value

Inner diameter of conductor, mm

0.6426  0.0025

Core insulation material

14.7 Extrusion Foaming of MeltProcessible Perfluoropolymers Techniques for continuous foaming processes for FEP and PFA have been described in this section based on technical bulletins and other publications by various companies [29e31]. In one example, a coaxial cable (RG-62) was coated [5] and jacketed to meet the requirements of the military

FEP or PFA 3.708  0.127

83% 93  5

specifications MIL C-17/30B. The blowing agent was F-22 (CF2HCl) which has now been replaced by ozone-safe gases such as nitrogen. Table 14.19 lists the requirements of the RG-62 cable according to specifications. Winding of a polyethylene filament on the conductor and under the jacket creates a spiral void space that reduces the dielectric constant. Extruder screws for FEP and PFA melt extrusion have two important zones; a feed and a shallow metering zone. An extruder screw has to provide a uniform mixture of the nucleating and the blowing agents with the resin. In this example, boron nitride and F-22 were the nucleating and blowing agents. F22 was dissolved in the polymer within a special zone that was designed in the extruder. This zone was sealed on the two ends by molten fluoropolymer and contained approximately 25% molten polymer and 75% empty space. The blowing agent was injected into this zone of the extruder at an elevated pressure. A schematic of the five key zones of the extruder is shown in Figure 14.7. Figure 14.9 shows a schematic

Figure 14.9 Overall process for foam core insulation of a conductor [5].

14: F LUOROPOLYMER F OAMS

427

Table 14.20 Details of Extruder Screw Design [5] Extruder Screw Zone

Number of Flights

Channel Depth, mm

10.0

6.35

Transition

0.5

e

Metering

4.0

1.27

Transition

0.5

e

Blowing agent injection

4.0

10.16

Transition

0.5

e

Pumping

3.5

1.91

Mixing head and tips

4.8

6.35

27.8

e

Feed

Total

of the overall process for producing a foam core insulated wire. A long barrel extruder with a lengthto-diameter ratio of 28 to 1 was required to fit all these zones. The details of the screw design have been shown in Table 14.20. Figure 14.10 presents a schematic diagram of the gas injection system for the introduction of the blowing agent into the extruder. It is composed of a gas cylinder (1), a regulator (2), a shutoff valve (3), a check valve (4), a bleed valve (5), a relief valve (6), a shutoff valve at the extruder (7), and an injection probe through the barrel wall of the extruder (8). A pressure about 414 kPa was required for the injection of the blowing agent into the extruder. A fixed center or an adjustable crosshead could be used for the extrusion of foam onto the conductor (Figure 14.11). Operating conditions for FEP and PFA foaming have been given in Figure 14.12(A). A boron

Figure 14.10 Schematic diagram of gas injection system for foaming [5].

Figure 14.11 Schematic diagram of crosshead design [5].

428

F LUOROPLASTICS , VOLUME 2

Figure 14.12 Operating conditions and foam formation scheme in wire insulation [5].

nitride concentrate was blended at a 1:9 ratio with virgin resin. The concentration of boron nitride depends on the desired void content and foam cell size. For example, when the concentrate contained 2% boron nitride, FEP wire insulation had a void content of 60% and a dielectric constant of 1.3. The vacuum in the crosshead die pulled the insulation tightly on the conductor. The extrudate underwent a slight draw (Figure 14.12(B)) before foaming occurred. In summary, conductors can be coated with a foamed insulation by a continuous wire process to produce coaxial cables. Even though only the production process for RG-62 was described, processing conditions can be altered to make other foam-insulated coaxial cables. One invention [32] pertains to foaming a perfluoroalkoxy resin such as copolymers of TFE and perfluoromethyl vinyl ether, and ECTFE and FEP. The chosen nucleating agent consisted of synergistic amounts of TiO2, selected inorganic salts and, optionally, sulfonic acid salts and/or phosphonic acid salts. The composition of the nucleating agent system depended on the specific resin, combination of components, and the desired foam structure. Typically, the nucleating agents included TiO2 in concentration of 100e1500 ppm of the weight of the composition. Useful inorganic salt was present in the range of 50e500 ppm by weight of the composition. Optionally, a sulfonic acid salt and/or phosphonic acid salt or combinations of them are used in

the concentration range of 200e1500 ppm by weight of the total composition. A wide variety of inorganic salts can be used. Sodium tetraborate (Na2B4O7) and calcium tetraborate (CaB4O7) are effective. The sulfonic acid salts that are effective in this development include salts of CF3CF2 (CF2CF2) nCH2CH2SO3X in which X is either H or NH4 and n ¼ 2e4. Barium salts of sulfonic acid salt such as ZONYLÒ BAS Supplied by DuPont works well. TiO2, inorganic salt, and sulfonic acid salt and/or phosphonic acid salt if desired, can be dry blended in desired proportions and melt extruded to obtain a compounded composition. Alternatively, individual concentrates of the TiO2, inorganic salt, and, optionally, sulfonic acid salt and/ or phosphonic acid salt in thermoplastic resins can be blended with the same or a different compatible thermoplastic resin and then extruded to the desired composition [32]. A fluorinated polymer composition was made including a TFE/HFP-based copolymer and 0.01 to 3 parts by mass per 100 parts the copolymer of a PTFE with a standard specific gravity of 2.15e2.30 [33]. A mixture was obtained by blending an aqueous dispersion of the TFE/HFP-based copolymer with an aqueous dispersion of the PTFE resulting in coagulation followed by drying the polymer mixture and finally melt extrusion of the resin. This composition increases the speed of wire insulation, at the same time, reduces the proportion of insulation defects so

14: F LUOROPOLYMER F OAMS

that the productivity may be improved and the cost may be reduced. Venkataraman and Young developed a process for making an extruded foamable composition [34]. The foamable composition included a partially crystalline melt-processible perfluoropolymer and a nucleating package. The process makes a foamed product having uniform foam cell size at high speeds without loss of product quality. A triple nucleating package of the extrusion process of the present invention comprised boron nitride (91.1  0.5 wt%), calcium tetraborate (2.5  0.2 wt%), and ZonylÒ BAS (6.4  0.2 wt%) was used. This foam-nucleating package was compounded into TeflonÒ FEP TE9494 (manufactured fluoropolymer, a TFE/HFP/PEVE perfluoropolymer with a MFR of 30 g/10 min). A master batch was formed with a boron nitride content of approximately 4 wt% of the final composition. TeflonÒ FEP TE9494 fluoropolymer was fluorinated in the melt and is substantially free of metal ions. The concentration of unstable end groups eCF2CH2OH, eCONH2, eCOF, and eCOOH was less than 20 per million carbon atoms. Pellets were formed by compounding operations performed on a Kombi-plast (by Coperion Corp) extruder consisting of a 28-mm twin-screw extruder and a 38-mm single screw extruder. The master batch pellets and pellets of the base fluoropolymer (TeflonÒ FEP TE9494) were dry blended at a ratio of 1:9 to form a foamed thermoplastic composition which was subsequently fed to a Nokia-Maillefer 45-mm extrusion wireline to extrude insulation onto AWG 23 solid copper conductor (0.57 mm). The extruder had a length/diameter ratio of 30:1 and was equipped with a mixing screw in order to provide uniform temperature and dispersion of nitrogen into the melt. The foamed fluoropolymer composition material was extruded onto wire at a speed of 300 m/min to produce an insulated wire about 0.20 mm in thickness with void contents ranging from 15 to 35 wt%. Die and guider tip combinations that typically yielded drawdown ratios of 30e40 were utilized. The density of the foamed insulation is determined by cutting a length of insulated conductor, removing the insulation, measuring the volume in cubic centimeters of the insulation, and dividing that value into the weight in grams of the insulation. The density is the average of measurements of at least five samples, each being 30 cm in length. The density of the unfoamed insulation is 2.15 [34].

429

Burch, Venkataraman, and Young [35,36] reported a novel approach for foaming by mixing fluoropolymers characterized by dissimilarities. They discovered those dissimilarities contribute to improved foamed insulation on the conductor of signal transmission cable. One example was a foamable composition consisted of a TFE/HFP copolymer, a TFE/PAVE copolymer (with a pendent alkyl group with 1 to 4 carbon atoms) and a foam-nucleating agent. The melting temperature of TFE/PAVE copolymer had to be 35  C greater than the melting temperature of TFE/HFP copolymer. For example, common FEP (TFE/HFP copolymer) has a melting temperature of 250e260  C and PFA (TFE/PAVE copolymer) has a melting temperature of 300e310  C. An example is described next. Two TFE/PAVE copolymers are used in the example: PFA (A) is a copolymer of TFE and 3.7 wt % PPVE with an MFR of 5.4 g/10 min, which had been fluorinated to convert the end groups to CF3 leaving no more than a total of six residual unstable end groups per 106 carbon atoms. This copolymer had a melting temperature of 305  C. PFA (B) was a copolymer of TFE and 7.3 wt% PEVE with an MFR of 6.6 g/10 min, which had also been fluorinated with no more than a total of six residual unstable end groups. This copolymer had a melting temperature of 288  C. The FEP polymer used in the examples had 10 to 11 wt% HFP and 1e1.5 wt% PEVE and the rest was TFE [35]. MFR of FEP was 30 g/10 min and it had 50 wire affinity end groups per 106 carbon atoms. These wire affinity end groups, primarily COOH, form during the polymerization process. The remaining end groups are stable CF3 generated about by fluorination of the FEP. The foam-nucleating agent is a mixture of 91.1 wt% boron nitride, 2.5 wt% calcium tetraborate, and 6.4 wt% of the barium salt of TBSA [7]. This agent is prepared as a 4 wt% concentrate in the FEP. To form a foamable PFA/FEP composition, extruded pellets of the foam cell nucleating agent concentrate are dry blended with pellets of the PFA and FEP and then subjected to the extrusion wire-coating/foaming process. A dry blend of 56 parts by weight of PFA (B) and 44 parts by weight of the FEP, together with foamnucleating agent concentrate is formed wherein the wt% of the nucleating agent is 0.30 wt%, based on the total weight of the composition. The MFR of the blend is 14.9 g/10 min. The extrusion foaming

430

conditions are conventional. The extruder is injected with nitrogen gas at high pressure. The drawdown ratio (e.g., DDR, in a tubular die, is defined as the ratio of the cross-sectional area of the annular die opening to the cross-sectional area of the finished insulation) of the extruded fluoropolymer composition is about 7 and the temperature of the copper conductor is ambient temperature, that is, no preheat is applied. The extrusion conditions are such that the foaming is delayed until the extruded polymer is in contact with the copper conductor [35]. The length of the molten copolymer cone is 5.08 cm. The line speed of the extrusion foaming process is 44.2 m/min. There is no formation of flakes on the die face during the extrusion foaming run. The foam-insulated wire is then formed into a coaxial cable by conventional procedure, including the braiding of strips of conductive metal over the foamed insulation to form the outer conductor and the application of a polymer jacket over the outer conductor. The dimensions of the coaxial cable are 0.58-mm diameter central conductor and outer foam diameter of 2.6 mm, whereby the thickness of the foamed insulation is about 1.0 mm. The void content of the foamed insulation is 47%. The composition of the foamed insulation exhibits a dissipation factor of 0.0003 at 10 GHz. This coaxial cable exhibits a return loss of 30 dB at 1 GHz, and the strip force required to break the adhesion between the foamed insulation and the central conductor is 25.5 N. Similar results are obtained when the foam cell nucleating agent is mixed directly with the PFA (B)/FEP composition to be extrusion foamed, rather than using a polymer concentrate of the foam cell nucleating agent [35]. The composition exhibits a dissipation factor at 10 GHz of 0.0003 and the cable made from the foamed composition as its insulation exhibits attenuation at 3 GHz of 22.5 dB/30.5 m. The foamed insulation exhibits a capacitance of 55.1 pf/m indicating a highly foamed structure. Another patent describes foamable compositions that include a partially crystalline melt-processible perfluoropolymer and a foam-nucleating package [37]. The foamable composition has a uniform foam cell size where the foam cell size of at least 90% of the foamed cells is 50 mm or less. The foamnucleating package ranged from 0.1 to 10 wt% of the combined weight of the perfluoropolymer and the foam-nucleating package.

F LUOROPLASTICS , VOLUME 2

14.8 Summary Partially fluorinated fluoroplastics are also foamed for essentially the same reason as those of the perfluoropolymers [30]. ECTFE and ETFE both have somewhat higher dielectric constants and significantly larger dissipation factors than FEP and PFA. Foamed ECTFE and ETFE have the advantage of stronger mechanical properties than perfluoroplastics, although their dielectric properties are inferior to those of FEP and PFA. The inherently high dielectric constant for PVDF (8e10) renders it unsuitable for data transmission applications. PVDF foam compounds have significantly lower dielectric constants. For example, PVDF at 55% void has a dielectric constant of 3.5 similar to that of polyvinyl chloride compounds currently used in applications such as long-length telephone primaries.

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