The application of polyimide to ultrahigh vacuum seals

The application vacuum seals received

27 January

of polyimide

1967; accepted

Paul W Hait, Varian Associates,

14 August Pa/o A/to,

to ultrahigh

1967 California

In order to achieve ultrahigh vacuum levels, vacuum systems must be baked out to temperatures as high as 300” C. Until the present fime, the only satisfactory seals that have been demonstrated to withstand such bakeout temperatures are those with metal gasket material, usually OFHC copper. When system bakeout temperatures need not exceed 150” C, Viton A has been commonly used. This article describes measurements made recently on a new ultrahigh vacuum material called polyimide’. If withstands intermediate temperature bakeout up to 300” C and has a lower outgassing rate than Viton A. To examine the usefulness of polyimide as a sealing material compared to Viton A, the following criteria were chosen for experimental evaluation : (a) Outgassing rate (b) Pumpdown characteristics (c) Mass spectrographic analysis of outgassing products (d) Mechanical properties

Experimental apparatus for measuring the outgassing rate

A novel experimental technique was designed for this test. Figure 1 shows a diagram of the system used to analyze and compare the outgassing characteristics of polyimide and Viton A. It consists of a conductance-limited sputter-ion pump that provides a known pumping speed out of a central gauging chamber. The sample to be analyzed is placed in the cavity

formed between one of the uppermost bakeable valves VI and Va and its blanked-off port. After pumpdown and bakeout, a sample can be valved into the gauging system for measurement by opening the adjacent valve V, or Vz. By periodically opening and closing these valves, a modulated pumpdown curve can be obtained which indicates system pressure with and without sample material present. Since the pumping speed is known, pressure changes that occur on modulation can be used to calculate the instantaneous outgassing rate. This method of operation provides a check that the sample under test is indeed the source of the gas. Valve To Polyimlde

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100

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Figure 2. Modulated pumpdown curve with polyimide sample

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A typical modulated pumpdown curve is shown in Figure 2. This shows a system pumpdown at room temperature for an air-exposed polyimide gasket. When the valve to the test

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Figure 1. Experimental arrangement

Vacuumlvolume

17/number

10. Pergamon

Press LtdlPrinted

in Great Britain

547

Paul W Hait: The application

of polyimide

to ultrahigh

vacuum seals

sample is open, system pressure approaches asymptotically the curve for system pumpdown with polyimide present, after an initial overshoot due to pressure build-up in the sample cavity. When the valve to the test sample is closed, the pressure drops to approach the curve for system pressure in the absence of sample material. Outgassing characteristics function of temperature

of polyimide

and Viton

A as a

To determine the outgassing characteristics of polyimide and Viton A, sample gaskets having a cross-section & inch square and an outside diameter of 1% inches were used. The gaskets were first vacuum-processed at 100°C for twelve hours. Sample gaskets were then placed with pliers in the test-sample cavities under valves V, and Ve. The cavities were purged with dry nitrogen gas during this operation. With either valve VI or V, open to the sample gasket, the system was baked for twelve hours. Bakeout of polyimide was carried out at 300°C. With Viton A, however, because excessive outgassing had previously been observed above 225”C, bakeout was limited to 200%. At the end of the bakeout period the modulation technique was used to measure outgassing rates at fixed temperatures 25°C apart. From the results of this test, the outgassing rates of polyimide and Viton A were determined. The resulting curves showing outgassing rate as a function of temperature are given in Figure 3. Note that the outgassing rate of Viton A increases sharply at temperatures close to 200°C. 10-T-

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Figure 4. Outgassing of polyimide and Viton A after air exposure

without bakeout when the system was not baked. Gas analysis indicated that the polyimide was evolving the water it had adsorbed on exposure to air. The fact that polyimide is hygroscopic can be deduced from its chemical composition, which is similar to that of nylonz. Viton A, however, belongs to the teflon fluorocarbon family, which has very little affinity for water. The results of these tests show that Viton A is preferable for use in non-baked systems that will be exposed to atmosphere repeatedly. Polyimide is a better choice if system bakeout at temperatures of 250°C to 300°C will be required.

4’

Pumpdown characteristics 2-

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Gaskets of polyimide, Viton A, and OFHC copper were compared in the normal pumpdown of a system. The system

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Figure 3. Outgassing rates of Viton A and polyimide

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Other tests were run comparing the outgassing characteristics of polyimide and Viton A after the two materials had been subjected to a thirty-minute exposure to air, as shown in Figure 4. Polyimide was found to outgas more than Viton A

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Figure 5. Comparative pumpdown characteristics of Viton A, poly-

imide, and OFHC copper

Paul W Hait: The application

of polyimide

to ultrahigh

vacuum seals

consisted of a mechanical pump with a liquid-nitrogen-cooled trap, a 500 litre/sec sputter-ion pump, and a chamber 100 litres in volume. Air exposures, pumpdowns, and bakeouts were identical for all gasket materials tested. The test results, shown in Figure 5, show that although polyimide gaskets do not have the low outgassing rate that OFHC copper does after bakeout, they do allow the attainment of lower ultimate pressures than do Viton A gaskets under the same conditions. Mass spectrographic analysis Figure 6 shows the results obtained by mass spectrographic gas analysis performed at 300°C during the outgassing rate measurements with polyimide. The principal gases evolved were water vapour, carbon monoxide, carbon dioxide, and hydrogen. When the system was allowed to return to room temperature, the water vapour and hydrogen peaks were no longer detectable. The primary residual gases were then found I00

Table 1. ____ Ionized Gas

Relative

~_

Outgassing

Rate

Polyimide

Viton A

water vapour

3

4.5

carbon

monoxide

2

3

carbon

dioxide

1 1

2 -

1.5

.I5

carbon

1.5

Mechanical design considerations

In designing a valve, the mechanical characteristics of the sealing material must be taken into account. Polyimide is a hard, resilient substance (85 to 95 Rockwell H). Its coefficient of thermal expansion is approximately three times as large as that of Type 304 stainless steel. Its compressive stress/strain characteristic is such that permanent distortion occurs if it is compressed more than twenty per cent. In valves with polyimide gaskets, design of the mechanism must therefore restrict the amount of seal compression to less than 20 per cent under all sealing and bakeout conditions. Various shapes of polyimide gaskets have been tested. The most successful results were obtained with a gasket having a modified diamond-shaped cross-section, as shown in Figure 7.

co

Valve To Polyimide Sample Closed

Tota I System Preseurf 6.4xW torr

monoxide and carbon dioxide. Total pressure was approximately 2 X lO-s torr. Mass spectrographic analysis of the outgassing products of Viton A at 200°C showed that essentially the same gases were evolved as those from polyimide but that their outgassing rates were higher. Table I gives relative outgassing rates for the most common gases evolved, using as standards the hydrogen and carbon dioxide evolved from polyimide. Note that 50 per cent more water vapour and carbon monoxide, and twice as much carbon dioxide, were outgassed from Viton A as from polyimide.

hydrogen

Valve To Polyimide Sample Open

Total System Pressure 1.2xltitorr

to be carbon

$’AND I _:‘IVAL PISTON

FACE

POLYIMIDE GASKET / (NOTE DIAMOND SHAPE) GASKET

Figure 7. Cross-section

I2

I6

26

MASS Figure 6. Gas analysis of polyimide at 300°C

44

RETAINER

PLATE

design of polyimide gasket

This shape makes seals with high surface pressure feasible, while allowing maximum freedom for thermal expansion. Experiments have shown that the sealing surfaces of a polyimide gasket must be machined to a finish of 32 rms or better, and must have a circular lay, that is, no cross-scratches. This finish is required because plastic flow does not occur on the surface of polyimide nearly as readily as it does on other elastomers used for vacuum seals. 549

Paul W Hait: The application

of polyimide

to ultrahigh

vacuum seals

The torque needed to close polyimide seals on experimental valves was found to be about three times as large as that needed to close Viton A seals in the same configuration. However, this closing torque is still low enough for a knob to be comfortably turned by hand. The torque required to close a la-inch polyimide-sealed valve, for example, ranged from 24 to 8 foot-pounds, depending on the width and surface finish of the seal.

any leaks in the seat or bonnet seals that were detectable with a helium mass spectrometer. Cyclic bakeouts were also carried out between room temperature and 300°C. After 200 cycles, no deterioration was observed in the functioning of the valves. Because it has a lower outgassing rate at 250” to 300°C and withstands repeated baking to 3OOC, polyimide is superior to Viton A for use as a seal in hand-operated valves.

Effects of bakeout

References

Repeated bakeout tests were performed on both I-inch and 1t-inch polyimide-sealed valves. Baking to temperatures as high as 400°C proved to be satisfactory only if terminated within fifteen minutes. Polyimide distorts and begins to carbonize at these temperatures. This structural change results in brittleness and loss of elastomeric characteristics. Baking to 300°C while the valves were either open or closed did not cause

550

1 N W Todd et al, “Polyimides”, S G C Pimental Freeman

and

A McClellan,

Machine

Design, Juur

The Hydrogen

Bond,

1966, page 8 1. Ch 6,

W H

and Co, 1960.

3 S D Bruck,

“Thermal Degradation of an Aromatic Polypyromellitimide in Air and Vacuum, 1. Rates and Activation Energies”, Polymer, 5 (9). September

1964.

4 B R F Kendall

Adhesives

and M F Zabielski, “High Temperature-Insulating J Vat Sci and Tech&, 3 (3). 1966. for Vacuum Applications”,