NBSIR 81-2418

Fabrication of Ultra-Drawn Thick

PVDF Transducers

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Materials Science

Polymer Science and Standards Division

Washington, DC 20234

December 1981

Final Report

Prepared for: Naval Ocean Systems Center

San Diego, California 92152

NBSIR 81-2418

FABRICATION OF ULTRA-DRAWN THICK

PVDF TRANSDUCERS

A. J. Bur and A. K. Tsao

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Materials Science

Polymer Science and Standards Division

Washington, DC 20234

December 1981

Final Report

Contract No. N66001 -81 -MP00024

Charles Hicks, Contract Officer

Reproduction in whole or in part is permittecffor any

purpose of the United States Government.

This document has been approved for public release

and sale; its distribution is unlimited.

Prepared for: Naval Ocean Systems Center

San Diego, California

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
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Fabrication of Ultra-Drawn Thick PVDF Transducers

A. J. Bur and A. K. Tsao

Polymer Science and Standards Division

Center for Materials Science

National Bureau of Standards

Washington, D.C. 20234

Abstract

Ultra-drawn PVDF (polyvinyl idene fluoride) transducers have been fabricated

from pellet resin material with thicknesses of 0.23, 0.30 and 0.50 mm. The

samples were mechanically ultra-drawn beyond their natural 4:1 draw ratio to

a 7:1 ratio in order to enhance the molecular orientation and thereby optimize

the piezoelectric activity. The samples were characterized using modulus and

x-ray measurements which showed that modulus increases with increasing draw

ratio and that the crystalline portion of the 7:1 drav/n material was mostly s

phase. Ten out of fourteen samples were successfully poled at room temperature

with fields ranging from 1.6 to 2.5 MV/cm. The pyroelectric activities ranged

nc from 2.0 to 4.1 —

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8.6 to 15.6 pC/N.
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Introduction

An accepted method of fabricating PVDF transducers is mechanical orientation of sheets or film followed by poling of the sample with fields of 2 MV/cm

or more at room temperature. One reason for mechanical orientation of the

sample is to achieve greater breakdown strength and thereby permit the application of high poling fields^

2 According to studies by Shuford and coworkers in 1976 and recently by

3 Nix et al , the piezoelectric activity of PVDF can be optimized by mechanically

ultra-drawing beyond the ^'natural" 4:1 draw ratio. In the above studies, different

techniques of mechanical drawing were used but the properties of the ultra-drawn

2 film were similar. In the first study , a thin film sheet was fed from a storage

drum through an oven at 80 °C to a take-up drum which rotated at a speed higher

than the feed drum. Draw ratios of 6:1 could be obtained by rotating the take-up

drum six times faster than the feed drum. Typical take-up rates were 30 m/min.

In the second case^, Nix et al , using an Instron^ mechanical testing machine worked

at much -slower draw rates and higher temperatures, 0.2 cm/min over a temperature

range from 80 to 140 *^0. Stretching was done in one step, that is, the sample was

extended through the yield point, necking and on to strain hardening in the range

of draw ratios from 4:1 to 8:1. In both cases, it was found that for a constant

set of poling conditions, the piezoelectric activity increased with increasing

draw ratio.

In this project our goal has been to develop a processing technique which can

be used to produce and characterize thick ultra-drawn transducers of PVDF without

special drawing equipment. The procedures which are described below include compression molding of PVDF samples from pellet resin material, mechanical orientation

of the material, characterization of the drawn material using modulus and x-ray measur

ments, poling the drawn material at high electrical fields and measuring the piezo

a) Identification of a commercial product is made only to facilitate experimental

reproducibil ity and does not imply endorsement by NBS.
- 2

-

and pyroelectric activity. The nominal sample thicknesses which we desired were |

I 0.23, 0.30, and 0.5 mm (9, 12 and 20 mils).

I Preparation of Samples _

'

The raw material used for fabricating our thick specimens was Solef 1012 PVDF ,

! in pellet form^. Solef 5008 powder was also used but this material was found to be j

unsatisfactory because it tended to stick tenaciously to the mold surface. The !

pellet material was formed into disks, 14 cm (5% in.) in diameter and of appro- j

priate thickness, by compression molding under vacuum at 190 °C. The mold, which |

is shown schematically in Figure 1, has two cylindrical stainless steel plates whic

| are the mold surfaces. For convenient handling the stainless steel plates are I

attached to aluminum backing plates. A vacuum seal is provided by o-ring seals beI tween the steel plates and an aluminum collar which surrounds the steel plates. A f

vacuum port, situated at the side of the collar, provides a connection to the vacuu

'

I pump.

During molding, the two steel plates' are held apart by a stainless steel

separation ring, the thickness of which determines the thickness of the molded

disk. Three sizes of separation rings were used. The rings, each with an ID of

| 14 cm (5% in.) and an OD of 16 cm (6% in.), had thicknesses of 0.96 mm (38 mils),

1.27 mm (50 mils), and 1.88 mm (74 mils). These rings respectively yielded final

j

i sample thicknesses after ul tra-drawing of 0.23 mm (9 mils), 0.30 mm (12 mils),

|

i and 0. 50 mm (20 mi 1 s) . !

I Prior to compression of a sample, the mold, o-rings, o-ring seats and collar !

were cleaned with ethanol. After the o-rings and collar were treated with sili- [ cone vacuum grease, the o-rings were positioned in their seats and the collar was |

set in place around the bottom plate.

The amount of PVDF material placed in the mold was in excess of the mold , volume in order to ensure that no voids remained unfilled. This resulted in a |

sample disk which was approximately 10% larger than the separation ring. The mass

pellet resin used with each separation ring was: 38 gm for the 38 mil ring; 50 gm '
-3-

for the 50 mil ring and 71 gm for the 74 mil ring. After the pellets were placed in

the mold, the mold was connected to the vacuum pump and placed in a hydraulic press

with top and bottom in contact with the heated platens of the press but not under

pressure.

The hydraulic press has platens with surface dimensions of 30.5 cm x 30.5 cm

(12 in. square). Temperature controlled heaters and cooling coils are embedded in

the platens so that the temperature can be set in the range from 50 to 400 °C

with ± 5 °C control. The cooling coils, which are connected to a water line, were

used to quickly cool the platens from the high molding temperature to room temperature.

If the platens of the press were preheated to 190 °C, then it would take

approximately 30 minutes for the sample to reach its melting temperature. The

pressure which we usually achieved was 50-60 ym of Hg (6.65-8.0 Pa); higher pressures,

up to 200 urn (26.6 Pa), were also found to yield satisfactory results.

The temperature of the interface between platen and mold was monitored by a

copper-constantan thermocouple probe which fit into a groove cut into the aluminum

facing of the mold. When this temperature reached 175 °C, melting of the sample

soon followed and this was noted by a slight decrease (2 mm) in the separation

between the top and bottom plates of the mold. When the temperature of the interface reached 190 °C, the mold was placed under a pressure of 1.9 x 10^ N/m^

(265 lbs. /in. ^). Under constant pressure, the temperature was held at 190 °C for

10 minutes after which time the heaters were turned off and the water line was

opened to the cooling coils. Care was taken to dissipate safely the steam which

was generated by the hot platens. Using the cooling coils it was possible to

change the temperature of the interface from 190 °C to 50 °C in 8 minutes.

It was found that by quickly cooling the mold under pressure, we were able to

produce samples with uniform surfaces. By contrast, samples slowly cooled under

pressure, usually had surface pock marks and streaks which we attributed to non-uniform

temperature distribution of the stainless steel plates as the mold was cooling through
the freezing point of the polymer.

In order to remove the sample from the mold, the plates were separated using a

set of three 0.6 cm (% in.) diameter rods which were inserted through holes in the

overlapping edge of the top plate and reached to the overlapping bottom edge (Figure ])

With the mold set up on blocks in the press, pressure was applied to the rods forcing

I the plates to separate.

| Mechanical Drawing _ |

i The sample to be mechanically oriented was sliced from the center of the

molded disk in the shape of a rectangle 7 cm wide by 14 cm long. Gauge marks were

penned on the sample for the purpose of measuring subsequent draw ratios. Mechanical

drawing was carried out using an Instron testing machine which was adapted with a

temperature control jacket^.

Secure and tight grips on the sample are essential for uniform drawing. The ;

grips which we used were hardened steel flats with surface dimensions of 7.6 cm i

wide X 5.1 cm high (3 in. x 2 in.). The flats were connected to a 3.8 cm (1% in.)

[

I diameter bolt which was threaded through the yoke of the grip assembly. The sample i;

|i

i was placed in the grips with sheets of fine grit sand paper, no. 400, between the i,

sample and grip surfaces with the abrasive side against the sample. A sample surface

| area as large as possible was placed between the grips, i.e. 5.1 cm (2 in.) from each

|

1

! end of the sample was kept in the grips. The grip bolts were tightened manually with |

as large a torque as possible by using a ratchet wrench with a 30 cm handle. As the ’

i

ti, temperature of the oven was raised, the sample grips were retightened at 120 °C. |

Because extension of the sample was limited by the size of the oven, drawing j'

was carried out in two steps: first, a 15 cm length of 4:1 drawn material was obj| tained by drawing at a rate of 0.25 cm/min. at 140 °C; second, this 15 cm sample was'

reclamped in the Instron and extended at a rate of 0.12 cm/min. at 140 "“C. The

I initial grip separation for the 4:1 draw was 2.54 cm and the final separation was

15.2 cm. For the 7:1 draw, the initial grip and final grip separations were 3.81 cm

and 10.2 cm. These grip separations do not correspond exactly to 4:1 and 7:1 ratios
-5-

because there was normally some sample slippage out of the grips. Routinely, we

were able to obtain draw ratios of 6.5:1 to 7:1, but, drawing beyond 7:1 usually resulted in transverse fracture or a split in the direction of orientation.

Characteristic load vs extension curves are shown in Figure 2. In Figure 2(a)

are shown the data for the natural 4:1 draw ratio. The sample begins to neck at

the maximum load and then proceeds to yield over the horizontal portion of the

curve where the load is constant with extension. By contrast, in Figure 2(b), which

is for ultra-drawing beyond the 4:1 ratio, the load is continually increasing with

extension. Since the cross sectional area of the sample decreases with extension

during ultra-draw, a continually increasing load indicates that the Young's modulus

of the sample is increasing also.

It was found that ultra-drawing unoriented PVDF to a draw ratio of 7:1 caused

a thickness reduction by a factor of 4.5. That is, in order to obtain an ultradrawn sample of 0.30 mm (12 mils) thickness, the unoriented starting material had

to be 1.37 mm (54 mils) thick. The reduction in the width of the sample was by a

factor of 1.6.

Poling Procedures

In order to avoid flashover and corona problems at the high poling voltages

(45 kV and greater), all poling was carried out under oil in a cell which is

sketched in Figure 3. The maximum sample size was limited to the inside diameter

of the cell which is 4.4 cm (1.75 in.). The two ports on the cell are used to introduce silicone oil into the cell under vacuum. Oil is pulled through a 20 urn

pore filter connected to one port by a vacuum line connected to the other. Voltage

is applied to the sample via the two 6.3 mm (% in.) diameter rods between which

the sample is squeezed. During poling, the chamber itself was immersed in a larger

transformer oil bath so that corona did not become a problem.

Electrodes were attached to all samples by using silver paint. In order to

avoid breakdown due to high fields at the edge of the electrodes, the edges were

raised. This was done by encircling the central area of the electrode with a
raised ring of epoxy. The electrode was then painted onto the epoxy ring. Flashover problems were minimized by having an apron of material extending beyond the

electrodes of the sample.

The following poling conditions were used: for the 0.23 mm (9 mil) specimens,

45 kV (2 MV/cm) for 1 minute at room temperature; for the 0.30 mm (12 mil) specimens,

75 kV (2.5 MV/cm) for 1 minute at room temperature; and for the 0.5 mm (20 mil)

specimens, 82 kV (1.6 MV/cm) for 1 minute at room temperature.

Characterization of Samples

The samples were selectively characterized by modulus and x-ray measurements

and all successfully poled samples were characterized by their piezo and pyroelectric

coefficients.

a. Modulus Measurements

The Young's modulus was obtained from a stress measurement at 10 sec in a stress

relaxation experiment. In this experiment, the sample is held at a constant deformation while a load is applied in the direction of orientation and the stress

is monitored as a function time. It was found that the stress-strain behavior

was linear up to 3% strain for oriented specimens but only up to 1% for the one unoriented sample. The samples for the modulus measurements were die cut strips which

were 0.2 cm in width by 5 cm long with thickness in the range 0.1 to 0.5 mm. The

accuracy of the modulus measurement is estimated to be ± 5%.

The results of the modulus measurements are shown in Table I and Figure 4.

The ultra-drawn sample, 24, with a draw ratio of 7:1, has a Young's modulus of

3

3.1 GPa. This is in good agreement with the measurements of Nix et al and is a

factor of 1.8 greater than the modulus for the unoriented material and 20% higher

than the modulus of sample 27 for which the draw ratio is 4:1.

b. X-ray Measurements

Wide angle x-ray measurements were made using CuKa radiation and a graphite

monochromator. X-ray patterns were recorded in reflection with the x-ray beams

perpendicular to axis of draw and in transmission with the axis of draw tilted at
-7-

an angle with respect to the beam. The sample was tilted in order to obtain

(001) reflections of the 3 phase and (002) of the a phase. The ratio of the

4

intensity of the a and 3 reflections along with structure factors of these

reflections were considered in making an estimate of the relative amounts of

a and 3 phase present.

In Figure 5, plots of x-ray intensity vs. 2e for 4:1 and 7:1 drawn samples

are shown. These plots are for the transmission experiment in which (001) planes

are detected. It is well-known that stretching PVDF to 4:1 draw ratio at temperatures above 100 °C results in a phase as shown in Figure 5(a). However, as shown

2 3 previously, * ultra-drawing to ratios higher than 4:1 produces 3 phase material

as seen in Figure 5(b). Considering the relative magnitudes of the 002 a and 001 3

4

peaks in Figure 5(b) and the relative structure factors , it is estimated that 3

phase is 90% of the crystalline phase.

c. Pyro and Piezoelectric Measurements

The pyroelectric and piezoelectric activities of these samples were measured

5

in a temperature pressure cell which has been previously described. In this

measurement, current is generated by the transducer in proportion to the change

in pressure or temperature with time. The estimated accuracy of pyroelectric

and piezoelectric coefficients is estimated to be ± 5 %.

In Table II we have listed the hydrostatic piezoelectric coefficient d^

and the pyroelectric coefficient Py along with other information for all samples

we have measured thus far. The prime mark on the sample designation indicates

that the data for these samples has been corrected for the fringing contribution

by punching out the electroded portion and measuring it separately from the rest

of the sample. The data for the other samples has not been corrected and the

apparent high numbers should be considered with caution.

Discussion

Our two-step drawing procedure, as it is traced out in Figure 2, contrasts

2 3 3 • with the methods previously published. * Although Nix et al took the precaution
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to cut dumbbell shaped samples for drawing experiments, we have found that thick

rectangular samples could be drawn without transverse fracture or tearing at the

grip. While some slippage from the grip occurred and probably neutralized the

high stress there, the final product was not affected by the initial rectangular

shape.

In Figure 2(b) the change in slope at 10% extension was observed for all

specimens. It is a consequence of the two stage drawing procedure. After a

sample has been drawn to 4:1 ratio and it is released from the clamps, its length

retracts by approximately 10%. When this sample is reclamped in the Instron, the

initial extension retraces the 10% retraction until the load reaches a level corresponding to the yield load of Figure 2(a). At this point, drawing beyond the

4:1 ratio commences as stress on the sample continues to increase with extension.

As mentioned above, the increasing stress with decreasing width and thickness

dimensions means that the modulus of the sample is also increasing. We interpret

this to mean that energy is being used to orient and extend the molecular structure

and that yield or plastic flow is not occurring.

2 3

As shown by Shuford et al and Nix et al , a stress induced crystalline phase

transition from a to g phase occurs as the draw ratio increases from 4:1 to 7:1.

Their data indicates that the 3 phase grows at the expense of the a phase and there

appears to be no increase in crystal 1 inity. While the x-ray patterns show that

orientation occurs in the crystalline regions, the increase in modulus during ultra

drawing indicates an increase in the number of tie molecules in the amorphous regici

2 I and corresponding increase in orientation of the amorphous phase ’

. This is supported by birefringence measurements which show a saturation in the crystal oriental'

9

tion at b:l draw ratio .

I When the piezo and pyroelectric activities of the punched out samples, the ( first five listed in Table II, are compared with the data of Nix et al , significant

'

I. differences can be seen. For the same draw ratio and poling field our pyroelectrii ;

coefficients are 25% lower than theirs while for d, there is no significant differ , h ^ I
-9-

ence. We do not know the source of this difference other than to point out that

the time scale of the two measurements was different by a factor of 20. Our experiment employed a temperature change from 23 °C to 25 at the rate of 0.6 °C/min

whereas Nix et al used a large temperature change from 10 °C to 30 °C at a rate of

0.2 °C/sec. We have assessed our experiment considering these factors and can find

no source of error to account for the 25% difference.

Conclusions

The fabrication of thick PVDF ultra-drawn transducers can be achieved by

using straight forward drawing and poling procedures. Drawing was done in two steps:

the sample was first drawn to its natural 4:1 draw ratio and then reclamped and

drawn to a ratio of 7:1. It was not necessary to use a dumbbell shaped sample; a

rectangular shape was found to be adequate and did not lead to tearing or fracture

at the grip.

Poling, which was done in oil at room temperature for 1 minute, was carried

out successfully as long as edges of the electrodes were raised so that high

edge fields were avoided. Ten out of fourteen samples were poled at fields in the

range 1.6 to 2.5 MV/cm.
Acknowledgements

The authors wish to acknowledge the assistance of Mr. Chester A. Harding

and Drs. John M. Crissman and Martin Misakian for their help with x-ray,

modulus and poling experiments. The support of the Naval Ocean System

Center is also gratefully acknowledged.
References

1. A. J. Bur, Polymer, 1288 (1981 ).

2. R. J. Shuford, A. F. Wilde, J. J. Ricca, and G. R. Thomas, Poly. Eng.

and Sci. 16, 25 (1976).

3. E. L. Nix, L. Holt, J. C. McGrath and I. M. Ward, Ferroelectrics 3^, 103

(1981).

4. R. Hasegawa, Y. Takahashi , Y. Chatani and H. Tadokora, Polymer J. 3, 600

(1972).

5. M. G. Broadhurst, G. Malmberg, F. I. Mopsik and W. P. Harris in "Electret,

Charge Storage and Transport in Dielectrics," ed. by M. M. Perlman, The

Electrochemical Soc., Princeton, NO, p. 492, (1973).
Table I

Sample

26

27

24

Modulus^

Condition

draw ratio

unoriented

4:1

7:1

Young's Modulus

GPa

1.7

2.6

3.1

obtained from a stress relaxation experiment at 10 seconds
Table II

Sample Thickness Draw Ratio Polinq Field Py

, .. ^h

mm MV/cm nC/cm^K pC/N

15d" 0.24 6.5:1 1.9 2.62 15.5

15e" 0.24 6.5:1 1.9 2.82 14.5

23b 0.22 7:1 2.1 2,86 13.3

24b" 0.21 7:1 2.1 2.75 15.6

18a" 0.25 6.5:1 1.8 2.50 12.2

23a 0.22 7:1 2.1 2,2 11.2

29 0.33 7:1 2.4 3.5 15

30b 0.30 6.5:1 2.5 4.1 18

16 0.52 6:1 1.6 2

’. 4 11

31b 0.53 6.5:1 1.6 2.0 8.6
Figure Captions

1. A side view of the vacuum compression mold is shown.

2. (a) The trace of a load vs. extension curve for the primary draw to

4:1 draw ratio is shown.

(b) The trace of a load vs. extension curve for the secondary draw from

4:1 to 7:1 draw ratio is shown.

3. A cross-section view of the poling cell is shown.

4. Stress-strain curves which were obtained from various stress relaxation

experiments are shown. A, unoriented; o, 4:1 draw ratio; •, 7:1 draw

ratio.

5. X-ray intensity vs. 2e for transmission observations are shown, e is

the angle of reflection, (a) 4:1 draw ratio; (b) 7:1 draw ratio.

o oCsl ooao O CO CO

^ oo^

S1\I01M3N QVOl

EXTENSION

cm
SN01M3N aVOl
Poling Chamber

TO VACUUM TO FILTER

Scale: 1 cm chamber = 1 cm drawing

Fi'ijure 2
STRESS

Pa

STRAIN %
-RAY

INTENSITY

(ARBITRARY

UNITS)

cjare

44

20 degrees
X-RAY

INTENSITY

(ARBITRARY

UNITS)

F/'j ure 5 ( k)

NBS»n4A (REV. 2-80

U.S. DEPT. OF COMM. 1. PUBLICATION OR 2. Performing Organ. Report No. 3. Publication Date

BIBLIOGRAPHIC DATA

REPORT NO.

SHEET (See instructions) NBSIR 81-2418 December 1981

4. TITLE AND SUBTITLE

"Fabrication of Ultra-Drawn Thick PVDF Transducers"

5. AUTHOR(S)

A.J, Bur and A.K. Tsao

So PERFORMING ORGANIZATION (If joint or other than NBS, see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Coverec

SrSPWSO'RTfqGTUR'G'ANlZATlON NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

10.

SUPPLEMENTARY NOTES

I

~

; Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

abstract (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

Ultra-drawn PVDF (polyvinyl i dene fluoride) transducers have been fabricated

from pellet resin material with thicknesses of 0.23, 0.30 and 0.50 mm. The

samples were* mechanical ly ultra-drawn beyond their natural 4:1 draw ratio to

a 7:1 ratio in order to enhance the molecular orientation and thereby optimize

the piezoelectric activity. The samples were characterized using modulus and

x-ray measurements which showed that modulus increases with increasing draw

ratio and that the crystalline portion of the 7:1 drawn material was mostly 3 phase. Ten out of fourteen samples were successfully poled at room temperature

with fields ranging from 1.6 to 2.5 MV/cm. The pyroelectric activities ranged

from 2.0 to 4.1 and the hydrostatic piezoelectric coefficients ranged from

8.6 to 15.6 pC/N?"’

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s)

Draw ratio; modulus; orientation; piezoelectricity; poly(vinyl idene fluoride;

pyroelectricity; ultra-drawn; x-ray

13. availability 14. NO. OF

PRINTED PAGES

1 Xi Unlimited

1 For Official Distribution. Do Not Release to NTIS 24

: 1 Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 1 . 20402. 15. Price

Order From National Technical Information Service (NTIS), Springfield, VA. 22161

$5.00

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