properties of polytetrafluoroethylene (ptfe) paste extrudates

Properties of Polytetrafluoroethylene (PTFE) Paste Extrudates

ALFONSIUS B. ARIAWAN', SINA EBNESAJJAD2 and SAWB G. HATZIKIRIAKOS'* *

Deparbnent of Chemical and Biological Engineering The University of British Columbia

2216 Main Mall, Vancouver, BC, V6T 124, Can&

Dupont Fluoroproducts Chestnut RUII Plaza, P.O. B0~80713, Wilmingto~ DE 19880-0713

In this work, we have studied the effects of extrusion die design, resin molecular structure, and lubricant concentration on the properties of FTFE paste extrudates by performing macroscopic extrusion pressure measurements, Raman spectrwcopy, differential scanning calorimetry and mechanical testing on the exhudates. Five resins of Werent molecular structures were tested. We have found that a balance between fibril quantity and quality (in terms of fibril orientation and continuousness) is necessary to ensure acceptable products, as illustrated through the effects of the operating variables on the extrudate tensile strength. The number of fibrils formed during extrusion can be increased by extrudmg the paste through a die of larger reduction ratio or by decreasing the lubricant content in the paste, thereby increasing the extrusion pressure. However, excessive pressure will cause fibril breakage. By using a die of larger entmnce angle, the extent of fibrillation is also increased, a l t h o m the quality of the fibrils is somewhat compromised. Increasing the die aspect (L/D) ratio does not increase the extent of fibrillation. However, it increases the degree of fibril orientation and ensures smoother extrudate. Finally, we have found that extrudates obtained using a paste of higher molecular weight are mechanically superior.

INTRODUCTION

olytetrafluoroethylene (FTFE) is a highly crys- P talline polymer with a high melting temperature of approximately 342°C. It forms a stable melt even at 380°C. with melt viscosity equaling about 10 GPa.s (1). It is not possible to fabricate FTFE using conventional polymer melt processes because of this high melting temperature and melt viscosity. Instead, tech- niques involving cold pressing and sintering, such as paste extrusion, must be employed (2-5). These tech- niques resemble those used in metallurgy, but are un- conventional as far as thermoplastic processing is concerned.

In FTFE paste extrusion, fine powder resin of individual particle.diameter of approximately 0.2 pm is first mixed with a lubricating liquid (lube) to form a paste. A typical lube concentration varies from 16 to

.Corresponding author. E-mail address: [email protected] IS. G. HatAkwakw).

25 wt?! (4, 6). The paste is then compacted at a typical pressure of 2 MPa to produce a cylindrical billet (preform) that is h e of air void (5-7). The next step in- volves the extrusion of the preform using a ram ex- truder at a temperature slightly higher than 30°C (4). This is usually followed by the evaporation of the lube through an oven and then sintering, for processes such as wire coating and tube fabrication. In the pro- duction of FTFE tapes, the extrudate is calendered before passing through the oven, with no sintering to follow. More detailed descriptions of the processes can be found elsewhere (4.6,8).

The fabrication of extrudates of considerable mechanical strength, even without sintering, from a free flowing fine powder resin is possible with PTFE because of its unique phase transition properties. PTFE has two transition temperatures of approximately 19°C and 30°C which are particularly important because of their proximity to ambient temperature (2-5). Below 19°C. shearing will cause PTFE crystals to slide past each other, retaining their identity. Above 19°C. FTFE molecules are packed more loosely, and shearing

POLYMER ENGINEERING AND SCIENCE, JUNE 2002, Vol. 42, No. 6 1247

Aljonsius B. Ariawan, Sina Ebnesajad, Savvas G. Hatzikiriak 0s

will cause the unwinding of crystallites, creating fibrils (4, 5). At temperatures greater than 30°C, a hgher degree of fibrillation can be achieved. This property has made it possible for FTFE paste extrusion to be performed near ambient temperature, producing a mechanically strong extrudate as these fibrils are formed and oriented in the flow direction (5, 9, 10). This is shown in Flg. 1 in two SEM micrographs obtained before extrusion (Rg. 14 and after extrusion (Rg. 1 b). The creation of fibrils is clear. In our previous studies, we have shown that fibrilla-

tion occurs in the contraction zone of the extrusion die (9). We have also proposed a mechanism for fibrillation shown schematically in Fig. 2 (4, 9). The compacted resin particles entering the die contraction

zone are highly compressed because of the reduction in the flow cross-sectional area. This results in the mechanical interlocking of particles. At the contraction zone and exit of the die, these connected particles experience an accelerated flow, which causes the rne- chanically locked crystallites to be consequently unwound, creating fibrils of submicron sizes. Differential scanning calorimetry (DSC) experiments have indicated that the first heat of melting of FTFE resins, which is proportional to the degree of crystallinity of the resins, is consistently lower after extrusion, pre- sumably due to the unwinding of crystallites during fibrillation (9). scannmg electron microscopy (SEW have also shown that the resin particles retain their original spherical shape, even after extrusion, indicating no permanent deformation (see Rg. 1). This essentially excludes the possibility of fibrils as being deformed particles.

As may be expected, the extent of fibrillation and the quality of the fibrils formed during paste extrusion are signifhntly affected by the resin properties, extrusion conditions and the design of the extrusion die. These variables consequently affect the final product properties, such as the mechanical strength of unsin- tered calendered tapes, the dielectric breakdown property of wires and the stretch void index of tubes and hoses (4, 11).

In this work, we study the effects of die design, resin molecular structure and extrusion conditions on the mechanical properties of FTFE paste extrudates, and relate them to the quantity and quality of the fibrils formed during the extrusion. The organiza- tion of this paper is as follows. the next section, the materials studied in this work are described, followed by a general description of the experimental equipment and procedures. The results are presented in section the Results and Discussion and discussed sequentially in relation to the issue of fibril quantity and quality, and the effects of die design (die reduction ratio, entrance angle and aspect (L/D) ratio), resin molecular structure and lubricant concentration on the mechanical properties of extrudates. Fi- nally, the conclusions and a brief summary of the work are discussed.

EXPERIMENTAL

Materials

Experiments were performed using five grades of FIFE the powder resins. The resins were supplied by DuPont Fluoroproducts, with properties listed in Table 1. Three of the resins have a homopolymer structure with different molecular weights, and two have a slight degree of branching due to the incorpo- ration of another perfluorinated monomer. Since it is not possible to measure the molecular weight of PTFE using conventional GPC analysis (because of the in- solubility of PTFE in many solvents), the relative magnitude of the resin molecular weights is determined proportionally from their melt creep viscosities. The

@) FYg. 1 . SEM micrographs of resin 2: (a) before processing (virgin) and (b) aBer extrusion (extrusion direction is Mi- cated by the arrow]. ZRe same mrphobgies are observed for other pastes.

1248 POLYMER ENGINEERING AND SCIENCE, JUNE 2002, Vol. 42, No. 6

properties of @TiW Paste Extrudates

m. 2. Schematic d@rm illustrating the proposed mechanism forjibriUation. (a) compacted resin particles enter the die conical zone, [b) resin particles are highlg compressed and in antact with one another in the die conical wne, resulting in the mechanical locking of crystallites, (c) upon exiting the die, particles retum to their original spherical shape, and entangled crystallites are unwound, creatingjibrils that connect the particles (9).

term “creep viscosity” invented by DuPont to charac- terize FIFE resins (12). A small rectangular (sintered) FIFE sample is produced, and at one end, a weight is hung. The temperature is then raised to 380°C. From the elongation data versus time, the viscosity is calculated by an empirical formula given by Holmes (12). For resins of the same molecular structure, it is also possible to infer molecular weights from the standard specitic gravity (SSG) data, with the highest molecular weight resin having the smallest SSG (2).

A n isoparaffinic compound under the trade name of ISOPAR G@ (supplied by ExxonMobil Chemicals) was used as the lubricant.

PmCedures

Pastes were prepared by mixing FTFE fine powder resins with the lubricant at a temperature lower than 19°C. using a horizontal roll mixer at 15 rpm for approximately twenty minutes. The low temperature mixing that is below the FTFE transition temperature is to ensure that the resin is not damaged prior

to extrusion (4, 6). The resulting mixture (paste) was then aged at room temperature for 24 hours to allow more uniform wetting of the resin particles by the lubricant. Following this, the paste was preformed into a cylindrical billet by subjecting it to a pressure of 2 MPa using an Instron capillary rheometer, equipped with a blank die. This is schematically shown in Fcg. 3. Upon the completion of preforming, the blank die was replaced by a tapered die and extrusion then pro- ceeded at a constant piston speed. Tapered dies of various diameter (DJ, entrance angle (a), length to diameter ratio (L/D), and reduction or contraction ratio ((D,/DJ2) were used. The extrudates obtained were then dried in an oven at approximately 60°C until their weights remained approximately unchanged with further heating.

The dried extrudates were tested for their tensile strength according to ASTM D1710-96 (FTFE rods) using an Instron mechanical testing machine ( 13). In an attempt to describe quantitatively the extent of fibrillation and the degree of fibril orientation in the

Table 1. Properties of PTFE Fine Powder Resins Studied in This Work. Relative Magnitude of the Resin Molecular Weight Can Be Inferred Proportionally From the Melt Creep Viscosity Data.

Resin Relative M Avg. Dia. (pm) SSG Melt Creep Viscosity (Pas)

Copolymer series: 1 Low 2 High

3 Low 4 Medium 5 High

Homopolymer series:

0.209 2.1 57 0.204 2.1 53

0.177 2.220 0.21 6 2.185 0.263 2.154

1.6 x 109 2.1 x 109

1.2 x 10’0 2.8 X 10’O 3.2 X 10’O

~~~ ~~

POLYMER ENGINEERING AND SCIENCE, JUNE 2002, Vol. 42, No. 6

~~

1249

Arfonsius B. Ariawan, Sina Ebnesaijad, Sauvas G. Hatzikiriak 0s

4 Electrical Heaters I

Die entrance

Die capillary zon

Tapered Capillary Die

Fig. 3. Schematic diagram of Instron capillary rheometer. BlankdiewasusedinplaceofcaplUarydieduring&orming.

extrudates, measurements of latent heat of melting using standard DSC procedures and Raman spectroscopy were performed, respectively. The procedure for Raman spectroscopy involved the projection of po- larized excited €?aman spectra onto the -dates in the direction parallel and perpendicular to the extrusion direction. The ratios of appropriate Raman intensity peah in the two directions were then calculated to provide a quantitative indication on the preferred fibril orientation. The procedure has been described in detail and reported previously in the literature by sev- eral authors in their work to determine the degree of molecular orientation in drawn polymer samples (14 - 16).

RESULTS AND DISCUSSION

Qmmtitat4ve Descriptions of Fibrillation

In order to completely describe the state of fibrillation in an extrudate, it is necessary to consider both the extent of fibrillation in the extrudate and the quality of the fibrils that are formed. During paste extrusion, a significant portion of the extrusion pressure is dissipated to overcome the strain hardening and vis- cous resistance contributions of the paste flow (9). The frictional forces between individual solid particles, and between the bulk paste and the die wall also give rise to additional pressure drop. These factors consequently translate into the creation of fibrils, in accord with the proposed mechanism for fibrillation discussed

above. Therefore, the extrusion pressure can be used as a quantitative measure of the extent of fibrillation that occurs during extrusion. For example, the extrusion of paste through a die of larger reduction ratio re- quires a higher extrusion pressure (9) and consequently, this will result in an extrudate that is more fibrillated, as will be discussed later. However, it is noted that since fibrillation occurs only in the die entrance (contraction) zone, the additional pressure drop across the capillary part of the die should not be taken into account when describing the extent of fibrillation during extrusion.

Since fibrillation effectively results in the decrease in resin crystallinity due to the unwinding of crystallites (Q), it is also theoretically possible to q u a n q the extent of fibrillation in an extrudate using DSC. It is known that the first heat of melting of a polymer is di- rectly proportional to the degree of resin crystallinity (2). Therefore, the difference in the first heat of melting of the paste, AH,,,, before and after extrusion should be indicative of the number of fibrils formed during extrusion, and correlates with the extrusion pressure. In Rg. 4, the relative difference in the first heat of melting of the paste before and after extrusion under various processing conditions is plotted as function of steady state extrusion pressure. Amid the scatter, it is still possible to extract a positive correla- tion between the extrusion pressure and the difference in AH,,,. The scatter can be attributed to the fact that the samples tested in the calorimetry represent only minute parts of the extrudates, making the results highly localized, and hence, sensitive to any local variations in the amount of fibrils. Nevertheless, from the presented results, we are able to validate the proposed mechanism for fibrillation, in which fibrils are considered as unwound crystallites. and con- firmed the significance of extrusion pressure in fibril formation. From FSg. 4, one can also see that different correlations are obtained for different resins, which implies that different levels of extrusion pressure are required to fibrillate resins of different molecular structure under identical conditions.

Besides the number of fibrils present in the extrudate, the quality of the fibrils is an equally important issue to consider when describing the mechanical properties of extrudates. The quality of fibrils can be described in terms of their degree of orientation and continuousness. A preliminary attempt to quanw the degree of fibril orientation in an extrudate using Raman spectroscopy shows promising results. Figures 5a and b show typical Raman spectra of unprocessed and processed powder, respectively. In the case of unprocessed powder (Rg. 54, no difference in the scattering intensity at all Raman shifts is observed between the two polarization geometries (i.e. parallel or perpendicular to the extrusion direction), indicating no preferred orientation (isotropicity). In the case of processed powder (Rg. 5b), however, there is a definite difference in the scattering intensity at major Raman shifts, particularly at 734 cm-1 and 1383 cm-l between


Properties of PlFE) Paste Exb-udates

0.09 1 0.08 - - .- e '.

1' 0.07 Q \ h

0.06 1

I 0.05 -

.= E

'P .- 2 0.04 - Y 3

0.03

0.02 -

-

4l 0 0

A 0 8

v v

v

0 20 40 60 80 1 00 120

Steady State Extrusion Pressure (MPa)

Fig. 4. Correlations between the relatiue dismences in AH,,, of pastes before and after exbusion and the steady state exbusion pressure (under various exbusion conditions). TRe fact that the di@rences in AH,,, are always positiw indicates that the resin crys- taLlinity is consistently lower after exbusion.

the two polarization geometies. The assignment of Raman scattering bands for PTFE is difFicult and con- troversial, especially those at 734 cm-' and 1383 cm-' (17, 18). There is a general agreement that these bands are due to C-F and C-C stretching (17). How- ever, as to which band is due to which stretching, the issue is yet to be resolved. Nevertheless, we have found that the Raman scattering intensity at 1383 cm-' is consistently enhanced in the polarization geometries parallel to the extrusion direction, while that at 734 cm-' is enhanced in the polarization geometry perpendicular to the extrusion direction. From SEM, it is evident that for the fibrils to become oriented, the PTFE molecules are straightened with their backbones parallel to the extrusion direction (see r;fg. 1). Therefore, we can definitely conclude that the Raman scattering band at 1383 cm-l corresponds to C-C (backbone) stretching. The ratio of the Raman scattering intensities at 1383 cm-' between the two polarization geometries (parallel to perpendicular) will then provide a quantitative measure of the preferred fibril orientation. A ratio of unity indicates isotropicity or no preferred orientation, while a ratio greater than unity indicates preferred orientation in the direction parallel to the extrusion direction.

We have also used Raman spectroscopy to track the development of fibril orientation in the extrudate during the course of an extrusion experiment. This is

shown in Fig. 6 for resin 4. The transient extrusion pressure response is also plotted in the same figure. The degree of fibril orientation, as shown with refer- ence to the right axis, initially rises with the extrusion pressure. However, when the extrusion pressure is at its peak, a sudden drop in the degree of fibril orientation is observed. When the process has reached steady state, the extrusion pressure and the degree of fibril orientation in the extrudate level off to their steady state values. The low degree of fibril orientation at the peak pressure explains the commonly observed dielectric spark test failures in the wire coating process at approximately the same point during the extrusion (11). When the extrusion pressure is at its peak, the extrudate is highly accelerated out of the extrusion die. This causes the extension and breakage of fibrils. Consequently, the fibrils become less continuous and are more chaotic, and hence, exhibit lower degree of preferred orientation. This results in a nonuniform sintering and the manifestation of voids in the final product, causing the commonly observed spark test failures. An industrially acceptable paste extrudate should,

therefore, be sufficiently fibrillated, with the fibrils being mostly unbroken (continuous) and exhibit high degree of orientation in the extrusion direction. It is also important that the extent of fibrillation and the quality of the fibrils are uniform along the extrudate.


0s . . . Alfonsius B. Ariawan, Sina EbnesaJad, Savvas G. Hatdanak

1 ~ " ' 1 " ' ~ 1 " " 1 " ' 1 ~ " "

. . . . . . . . . . . Parallel to extrusion direction - Perpendicular to extrusion direction

734 cm-'

250 500 750 1000 1250 1500 1750

Rarnan Shift (crn-')

(4

. . . . . . . . . . . Parallel to extrusion direction I- Perpendicular to extrusion direction

250 500 750 I000 1250 1500 1750

Rarnan Shift (cm-')

(b)

Fig. 5. 15pica2 Raman spectroscopy results for (4 unprocessed powder and lb) paste e&udate.


Properties of PEW) Paste Esmudates

100-3

80 - a" z g 60

2? a .4 40

-E

Y

=I v) fn

c rn

w

20

I 1 1 , I I I I I I I 1 I , I I , 1 , , 1.75 Resin 4 with 18wt% Lubricant

Vol. Flowrate = 125 mrn3/s, T = 35'C a = 30'. Reduction Ratio = 336:1, UD = 20

- 1.70 -

- 1.65 2 2 - .- E

- 1.60 9 fn C

C C - (0

- 0

2 - 1.55 6 Extrusion Pressure 1.50 -

The consequence of these conditions is a mechanically strong extrudate. Considering this, we may use the tensile strength of an extrudate as a quality indi- cator, since it takes into account the combined effects of fibril quantity and quality. Furthermore, mechanical testing results are generally more reproducible with less variability, making extrudate tensile strength a suitable parameter for this purpose.

EHect of Die Reduction Ratio

The effect of die reduction ratio on the extrudate tensile strength is shown in Fig. 7 for resins 2 and 4. it can be seen that increasing the die reduction ratio ini- increases the tensile strength of an extrudate. At larger reduction ratios, the effect diminishes. In fact, the results for resin 2 indicate a small decrease in the extrudate tensile strength at a reduction ratio of 352: 1.

In a die of larger reduction ratio, resin particles are squeezed against one another to a greater extent over a longer flow path, resulting in a higher extrusion pressure. This allows for a greater extent of mechanical interlocking to occur between adjacent particles. A greater number of fibrils are consequently created as these interlocked crystallites are unwound while exiting the die. Provided that the quality of the fibrils is unaffected by the increase in the die reduction ratio, this translates into a mechanically stronger extrudate.

However, this is only partially observed in FYg. 7. Beyond a certain reduction ratio, the extrudate tensile strength is approximately unchanged or, as in the case of resin 2. decreases with the further increase in the die reduction ratio. The higher extrusion pressure

associated with a die of larger reduction ratio (see FIg. 7) causes the exhvdate to be accelerated (spurted) out of the die at a greater velocity. This results in the breakage of some fibrils in the extrudate (as discussed above) and an overall reduction in the fibril quality. The consequence is a mechanical weakening of the extrudate. These competing effects between fibril quantity and quality affect the extrudate tensile strength in the opposing directions and become more important at larger reduction ratios, accounting for the observed trend in Fig. 7.

It is noted that the maximum die reduction ratio that is suitable for a particular resin is partly depen- dent on the molecular properties of the resin. For example, as can be seen kom Flg. 7, the extrudate obtained using resin 2 with a die of reduction ratio of 352: 1 has a lower tensile strength than that obtained with a die of reduction ratio of 156: 1. However, this is not observed with resin 4. It is also worthwhile to note that with resin 5, it was not possible to extrude the paste through a die of reduction ratio of 3521. An extrusion attempt resulted in a high velocity spurting of periodically broken extrudates, due to the excessively high extrusion pressure, as will be discussed later.

Effect of Die Entrance Angle

m e 8 depicts the effect of die entrance angle at constant throughput on the extrudate tensile strength for resin 3. The steady state extrusion pressure corresponding to each experimental run is also shown in the figure for comparison. The extrusion pressure generally increases with the increase in the die entrance angle, although at very smdl entrance angles

POLYMER ENGINEERING AND SCIENCE, JUNE 2002, Vol. 42, No. 6 1 253

Alfonsius B. Ariawan, Sina EbnesaJad, Savvas G. Hatzikiriak 0s

m 23.5 MPa m -

48.6 MPa -

-

- 0 Resin 2 with 18wt.% Lube

UD = 20, a = 45', T = 5OoC Vol. Flowrate = 30.2 rnrn3/s Resin 4 with 18wt.% Lube

- 1

0 a, UD = 0, a = 45', T = 35'C

Vol. Flowrate = 75.4 rnrn3/s m c.

1 1.5 MPa X Lu

8) 25.6 MPa ' 1 q0.3MPa

62.4 -I MP

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 50 100 150 200 250 300 350 400

Die Reduction Ratio

m. 7. The e f f i of die reduction ratio on evbudate tensile strength Also shown are the steady state extrusion pressures corresponding to each m t a l run

0.9 v - 1

Resin 3 with 18wt.% Lubricant Reduction Ratio = 352:1, UD = 0, T = 35OC

Vol. Flowrate = 75.4 mm3/s 0.6 1

0 10 20 30 40 50

Die Entrance Angle (a)

Flg. 8. The effect of die entrance angle on exbudate tensile strength. Also shown are the steadg state extrusion pressures corre- spading to each eqwhe&d run.


Properties of PlFE) Paste Esctnrdates

0.6

0.5

(less than a ~7.5~1, frictional force becomes important and this trend reverses (9). As discussed above, this increase in the extrusion pressure translates into extrudates that are more fibrillated. However, while this is true, it appears that the overall quality of the fibrils is increasingly compromised with the increase in the die entrance angle. This is indicated by the decrease in the extrudate tensile strength shown in Rg. 8, con- trary to an expected increase.

The increase in the steady state extrusion pressure with the increase in the die entrance angle is due to the paste flow path that becomes correspondingly less streamlined. Previous visualization experiments showed that the paste flow pattern in the die contraction zone becomes more "deformed" as the die entrance angle is increased (9). Because of this, the resulting extrudate exhibits a greater extent of fibrillation. However, the degree of fibril orientation and possibly, continuousness, decrease. A greater number of fibrils exiting the die will be oriented in directions other than the flow directions, owing to the lesser streamking of the flow. This can be expected to result in a larger swelling of the extrudate. Indeed, diameter measurements of -dates obtained using dies of various entrance angles indicate this effect, and this is shown in FTg. 9. It can be seen from Rg. 9

Resin 3 with 18wt.% Lubricant Reduction Ratio = 352:1, UD = 0, T = 35OC -

Vol. Flowrate = 75.4 mm3/s -

i " " " " " " " " " " " "

that extrudate obtained using a die of smaller entrance angle exhibits less swell, due to the greater streamlining of the flow. On the other hand, extrudate obtained using a die of larger entrance angle exhibits greater swell. The overall effect is a reduction in the fibril quality as the die entrance angle is increased, and consequently a mechanically weaker extrudate is obtained (Rg. 8).

(LID) Ratio of Die

Figure 10 shows the effects of die L/D ratio on the tensile strength and diameter of paste extrudate. It can be seen that increasing the die L / D ratio increases the mechanical strength of the extrudate. This is due to the improvement in the overall fibril quality (orientation). The creation of fibrils occurs only in the die entrance zone (see Fig. 3), where the interlocking of particles is possible. In the die capillary zone, the fibrillated paste flows in a plug flow manner, and the pressure drop across the capillary length is due mainly to the frictional losses at the die wall (9). Therefore, -dates that are produced using dies of the same reduction ratio and entrance angle will exhibit the same extent of fibrillation, regardless of the length of the die capillary zone.

Effect of --to-Diameter


Alfonsius B. Ariawan, Sinu EbnesaJad, Savvas G. Hatzikirfak 0s

p--------------- # /

/ /

,,/ Resin 4 with 18wt.% Lubricant

T = 35OC, Vol. Fiowrate = 75.4 mm3/s Reduction Ratio = 352:1, a = 4 5 O , h

-0- Extrudate Tensile Strength 1p- Extrudate Diameter

4 0.8 1

h

I E

{ 0.5

0.4 0 10 20 30 40 50

Die UD Ratio

Q. 10. TXe effect of die L/D ratio on exbudak tensile strength and diameter.

The die capillary zone, however, is important in im- proving the quality of fibrils in the extrudate. This is somewhat evident from Rg. 10, where one can see that increasing the die L/D ratio suppresses the extent of exbudate swell. This implies that by increasing the capillary length, spurting of fibrils near the exit of the die entrance (contraction) zone is contained within the capillary channel. Consequently, the randomness of fibril orientation is reduced and the overall fibril quality in the exbudate is improved. Visually, this is also apparent in Rg. 11. An extrudate obtained using a die of L / D = 0 (Rg. 1 la) often shows a fibrous appearance on the surface, which is visible even by naked eye. This is due to the presence of broken fibrils that are chaotically oriented, as they are spurted out of the die at a high extrusion pressure. The extrudate appearance is vastly improved when a die having an L/D > 0 is used (Rg. l lb ) . This explains the increase in the -date tensile strength as the die L/D ratio is increased (Rg. 10). However, it is noted that since this effect occurs near the exit of the die contraction zone, beyond a certain capillary length, there will be no further improvement in the overall fibril quality, as shown by the plateau in the curves in Rg. 10. Apparently, using a die having L/D 3 20 will opti- mize the process in terms of product quality.

Effect of Resin Molecular Structure (b)

Rg. 11. pictures of ewbudates (resin 5) obtained using dies of ( d L / D = O a n d ( b ) L / D = 1OunderthesameewperimenM conditions. Note the visual dixerence in the extrudate SLU- faces. ?he same effect was observed with other exbudates.

The effect of resin melt creep viscosity (molecular weight) on the extrudate tensile strength is shown in


Properties of @?WE) Paste Esmudates

2 -

Fig. 12. The results for both the homopolymer and copolymer series are plotted in the figure. It can be seen that increasing the resin molecular weight increases the mechanical strength of the extrudate for resins in the same series. However, one must be cau- tious about extending this conclusion across different polymer series. Further studies involving more samples are necessary before a general conclusion can be made.

It is worthwhile to note that the steady state extrusion pressure generally increases with the increase in the resin molecular weight. In some cases, it is not even possible to extrude a paste, owing to its high molecular weight. For example, as discussed above, an extrusion attempt using resin 5 at a reduction ratio of 352:l resulted in the spurting of broken extrudate pieces out of the die in a discontinuous fash- ion, owing to the excessively high pressure that broke the fibrils. The increase in the extrusion pressure with resin molecular weight, however, is caused by the 'hardness' of the higher molecular weight resin particles (making them less squeezable), rather than to the creation of more fibrils (7). In fact, the generally larger particle size of higher molecular weight resin implies that there will be a reduction in the contact area between adjacent particles, and consequently, it is more reasonable to expect a smaller extent of fibrillation. The fact that these particles are also less squeezable

:. -. -

further contributes to this. Furthermore, a higher molecular weight resin particle is made up of a lesser number of molecules, although they are longer. Hence, there will be fewer molecules to be unwound to create fibrils. Prelimimry analysis using DSC has also indicated this. The difference in the first heat of melting for a higher molecular weight paste tends to be smaller than that for a lower molecular weight paste (this is also apparent from Q. 4). This implies that the state of crystallinity in the higher molecular weight case is only slightly disturbed (i.e. less fibrillation).

Extrudates obtained from a lower molecular weght resin should, therefore, exhibit a better mechanical (tensile) property, sin-ce they are more readily fibrillated, as discussed above. However, this is not observed in F'ig. 12. It appears that, although extrudates obtained from a lower molecular weight paste are composed of more fibrils, these fibrils are weaker compared to those obtained using a higher molecular weght paste. The difference in the strength of these fibrils may be due to the fact that, in the higher molecular weight case, molecules are longer and the entanglement density is lugher. The fibrils created are, therefore, more continuous, as compared with the shorter fibrils associated with the lower molecular weght paste. Hence, although a lower molecular weight paste extrudate is more fibrillated, it is relatively easy

1 wi 18wt o/ Lubricant m 1 ~ 5 5 % = Ja, a = d ~ , Reduction Ratio = 56:l. Vol. Flowrate = 75.4 mm3/s

Reduction Ratio = 336: 1 Vol. Flowrate = 124.9 mm3/s

UD = 20, a = 30°, m

Melt Creep Viscosity * 1 O-'' (Pas)

Ffg. 12. 'Ihe effect of resin melt creep viscosity (molecular &ht) on extmdak tensile strength Lines are drawn to guide the eye. Fiued symbols represent copolymer series and Unmd symbols represent hornpolymer series.


Alfonsius B. Ariawan, Sina EbnesuJad, Savvas G. Hatzikiriak 0s

Resin 3, a = 45' Reduction Ratio = 352:1, UD = 0, T = 35OC

Vol. Flowrate = 75.4 rnrn3/s -

-

to break. On the other hand, extrudate obtained using a lugh molecular welght paste is less fibrillated. How- ever, the fibrils formed are parts of longer molecules with increased entanglement density that makes them remain intact during the flow. This evidently results in a greater extrudate tensile strength, as can be seen in Fig. 12.

Effect of Lubricant Concentration

The effect of lubricant concentration on the extrudate tensile strength is depicted in Fig. 13 for resin 3. It can be seen that there is an optimum lubricant concentration at which the extrudate tensile strength is maximized. An extrusion with a lubricant level less than the optimum results in an extrusion pressure that is too high and breaks the fibrils, although more fibrils are formed because of the increased friction between the resin particles. However, when the lubricant concentration is unnecessarily high, the extrudate becomes excessively wet and weak, and some- times not able to hold its shape, owing to the excessive local slippage between flowing particles in the die, which results in a lesser number of fibrils being formed. Although the optimum lubricant concentration is expected to depend on the average particle size of the resin (molecular structure), it is not expected to be a major practical issue, since the average particle sizes of paste extrusion grade resins are typically very similar (4, 6).

CONCLUSIONS

In this work, we have studied the effects of die design, resin molecular structure and lubricant concentration on the mechanical properties of paste extrudates and their relations to the quantity and quality of the fibrils formed during the extrusion process. We have also attempted to q u a n w the extent fibrillation in an extrudate using DSC and Raman spectroscopy, in order to illustrate the issue of fibril quantity and quality. It was found that a balance between fibril quantity and quality is necessary to ensure industrially acceptable products. The extent of fibrillation in an extrudate can be increased by performing extrusion using a die of larger reduction ratio or entrance angle, or by decreasing the lubricant content in the paste matrix, which essentially increases the extrusion pressure level. However, too high an extrusion pressure tends to break the fibrils and is, therefore, detrimental to the quality of the extrudate. By increasing the die entrance angle, the quality of the fibrils formed is also somewhat compromised, owing to the less s t r e g of the flow. Increasing the length of the die capillary zone (or L / D ratio) was found to increase the exhudate tensile strength, owing to the increased degree of fibril orientation in the extrudate. The extrudate was also found to be visually more at- tractive (less fibrous) and exhibit less swelling. It was also noted that the pressure drop across the capillary length of the die does not contribute to the formation

h

a" 2 Y

5

F Q) C

Q) ([I 0 3

X w

Y

L CI

15 16 17 18 19 20 21 22 23 Lubricant Concentration (wt.%)

Flg. 13. The eflect of lubricant concentration on exbudate tensile strength.

1258 POLYMER ENGINEERING AND SCIENCE, JUNE 2002, Vol. 42, NO. 6

Properties of PiTE) Paste Esmudates

of more fibrils. Finally, it was found that a higher molecular weight resin produces stronger fibrils, which accounts for the mechanical superiority of the extrudate.

ACKNOWLEDGMENT

This work has been supported financially by a grant from DuPont Fluoroproducts, Wilmington, Delaware.

REFERENCES 1. C. A. Sperati, in J. Brandrup and E. H. Immergut, eds.,

Polymer Handbook, 3rd edition, pp.v35-v56, John Wiley and Sons. New York (1989).

2. S. V. Gangal, in J . I. Kroschwitz (ed.), Kirk-OthmerEncy- Clopedia Of Chemical Technology, 4th Ed., 621-44 John W i l y and Sons, New York (1999).

3. T. A. Blanchet, in 0. Olabisi, ed., Handbook of Thermo- plastics, 981-1000, Marcel Dekker, New York (1997).

4. S. EbnesaBad, Fluoroplastics VoL 1: Non-MeliProcessible Fluoroplastics, Plastics Design Library, William Andrew Corp., New York (2000).

5. S. Mazur, in M. Narkis ed., Polymer Powder Technology, 44 1- 8 1, John Wiley and Sons, New York ( 1995).

6. DuPont Fluoroproducts, Teflon@ PTFE Fluoropolymer Resin - Processing Guide for Fine Powder Resins, TecM- cal Bulletin (1994).

7. A. B. Ariawan, S. Ebnesaljad, and S. G. Hatzikiriakos, Preforming Behavior of Polytetrafluoroethylene Paste, Powder Technology, 121,249-258 (2001).

8. J. Lontz, J. A. Jaffee, L. E. Robb, and W. B. Happoldt, Jr., I d Erg. Chem. 44, 1805 11952).

9. A. B. Ariawan, S. Ebnesaljad, and S. G. Hatzikirakos, Paste Extrusion of Polytetrafhoroethylene (FTFE) Fine Powder Resins, Can. J. Chem Eng., manuscript submit- ted for publication (200 1).

10. E. E. I.&& and C. M. Winchester, Z n d Eng. C h , 45, 1123 (1953).

11. J. McAdams, m a t e communication, April 2000. 12. D. A. Holmes and E. W. Fasig, JL, U.S. Patent

3,819,594 (1974). 13 ASTM D1710-96, Standard Specification for Ewtruded

and Compression Molded Polgtetramroettylene (PTFE) Rod and Heavy-Walled Tubing (1996).

14. G. A. Voyiatzis, G. Petekidis, D. Vlassopoulos, E. I. Kamitsos, and A. Bruggeman, Macromolecules. 29, 2244 (1996).

15. K. S. Andrikopoullos, D. Vlassopoulos, G. A. Voyiatzis, Y. D. Yiannapoulos, and E. I. Kamitsos, M m m W s , 31,

16. V. Deimede, K. S. Andrikopodos, G. A. Voyiatzis, F. Kon- standakopoulou, and J. K Kallitsis, Macromolecules, 32, 8848 (1999).

17. D. I. Bower and W. F. Maddams, lRe Vibrational Spec- troscopy OfPoh~mers, Cambridge University Press, New York (1989).

18. R J. Lehnert, P. J. Hendra, N. Everall, and N. J. Clay- den, Polymer, 38, 1521 (1997).

5465-73 (1998).


properties of polytetrafluoroethylene (ptfe) paste extrudates

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