Gas permeation of fibre reinforced plastics

PII: S0011-2275(97)00125-2

Cryogenics 38 (1998) 143–147  1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0011-2275/98/$19.00

Gas permeation of fibre reinforced plastics ¨ Jens Humpenoder ¨ Forschungszentrum Karlsruhe, Institut fur Materialforschung II, D-76021 Karlsruhe, Germany

Received 3 September 1996; revised 7 July 1997 Propulsion with liquid hydrogen or methane is under development. Storage and transportation of these fuels is part of these activities. Lightweight vessels out of fibre composites are promising candidates. Their disadvantage might be gas permeation, especially when exposed to mechanical and thermal cycling. For several applications of superconducting magnets (pulsed magnets, SQUID detectors), non-metallic and non-magnetic cryostats are required. Fibre glass composites are promising candidate materials, but again a sufficient low helium gas permeation is required. The permeation of helium, hydrogen and methane gas has been investigated. The principal arrangement for measuring gas permeation through polymers, fibre glass and carbon reinforced epoxy resins will be shown. The behaviour of coating materials or organic and metallic liners is a further topic of these measurements. Special emphasis has been put on the influence of thermal and mechanical cycling on the permeation rate.  1998 Elsevier Science Ltd. All rights reserved Keywords: gas permeation; low temperatures; polymers; fibre composites

There is increasing interest in using fibre reinforced plastics as materials for storage and transportation vessels for liquefied gases1. These composite materials offer several advantages, such as high specific strength, relatively low thermal conductivity and non-magnetic behaviour. But there are also some disadvantages which reduce their applicability. Especially the larger gas permeation compared to metals and the surface outgassing characteristics of the fibre reinforced plastics may be troublesome. Therefore knowledge of the gas permeability of these materials, in particular at cryogenic temperatures, is very important. Gas permeation is frozen at sufficiently low temperatures, but polymers become brittle and microcracks might increase permeability. Thus, thermal and mechanical preloading is an important topic of these investigations. The influence of liners or surface layers is of further interest.

Materials The gas permeability of different composites and polymers has been investigated. Fibre reinforced plastics: 쐌 glass fibre/epoxy crossplies (GFEP) 쐌 carbon fibre/epoxy crossplies (CFEP) Thermoplastics: 쐌 polyamide (PA) 쐌 high-density polyethylene (PE-HD) 쐌 polyvinylchloride (PVC) Thermosetting plastics: 쐌 epoxy resin

All samples are flat discs, 54 mm in diameter and approximately 2 mm thick (Table 1).

Apparatus and measuring method Permeation measurements in a temperature range between 325 K and 77 K have been performed with a permeation cell (Figure 1), which is surrounded by an evaporator cryostat and attached to a mass spectrometer (Figure 2). The peripheral devices are shown in Figure 3. The permeation detection efficiency of the mass spectrometer is about 10−15 m2 s−1. Good vacuum-tight sealing at low temperatures is obtained with indium wires. The permeation measurements have been made with helium, hydrogen and methane as penetrant gases. A permeation cell for gas permeation measurements is shown schematically in Figure 1. A flat disc sample with a thickness d and a surface area A separates a volume V1 filled with gas under pressure p1 from another volume V0 which is continuously evacuated. The gas leakage rate Q, driven by the pressure difference ⌬p, is detected with a mass spectrometer or leak detector as a function of time t until a steady state is established and the leak rate has attained a saturation value Q0. The permeability P can be calculated from: Q0 = P

A ⌬p d

(1)

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¨ Gas permeation of fibre reinforced plastics: Jens Humpenoder Table 1 Fibre reinforced epoxy crossplies Material

GFEP

Resin system Fibre material Fibre content

GFEP

LY556/HY917 Ciba Geigy E-glass 56 vol%

CFEP

LY556/HY917 Ciba Geigy E-glass 67 vol%

LY556/HY917 Ciba Geigy Toho HTA7 60 vol%

冉 冊

S = S0 exp −

ES RT

(3b)

where D0 and S0 are pre-exponential factors, ED is the activation energy of diffusion, ES is the energy of solution, R is the gas constant, and T is the absolute temperature (Figure 4). From the definition P = D·S it follows that:

冉

P = D0S0 exp − Figure 1 Permeation cell

Temperature dependence of gas permeation For homogeneous materials, the mechanisms of gas transport through the sample can be expressed by a process of solution on the high pressure side of the sample, subsequent diffusion of the gas molecules through it, followed by gas evaporation on the low pressure side2. Solution and evaporation are much faster than the diffusion, thus diffusion controls the time dependence of the permeation process [see Equation (6)]. Permeation P is the product of diffusion D and solubility S: P = S·D

冉 冊

(4)

where EP is the activation energy of permeation and P0 is again a pre-exponential factor. Generally4: ED ⬎ 0 and 兩ES兩 ⬍ ED

(5)

Results Examples of the temperature dependence of the permeability are shown in Figures 5–7 for polymers and composites. The results are compiled in Table 2.

(2)

The temperature dependence of diffusion and solubility obeys an Arrhenius equation3:

冉 冊

ED D = D0 exp − RT

Figure 2 Evaporator cryostat

144

冊

ED + ES EP ⬅ P0 exp − RT RT

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(3a)

Discussion • The results of the temperature dependence show that the gas permeation obeys an Arrhenius type [Equation (4)]. This is true for polymers and fibre composites.

¨ Gas permeation of fibre reinforced plastics: Jens Humpenoder

Figure 3 Evaporator cryostat and peripheral devices

Figure 4 Schematic presentation of the temperature dependence of the permeability

Figure 6 Helium, hydrogen and methane permeability of PVC

Figure 5 Helium and hydrogen permeability of PVC, PE-HD and PA

Figure 7 Helium permeability of different fibre reinforced composites and an epoxy resin

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¨ Gas permeation of fibre reinforced plastics: Jens Humpenoder Table 2 Permeation P and the permeation barrier EP for helium, hydrogen and methane Helium

EP (kJ mol−1 )

Materials GFEP (67 vol%) GFEP (56 vol%) CFEP (60 vol%) PVC PE-HD PA Epoxy resin

20 20 20 16 22 23 22

Hydrogen

P at 20°C (m2 s−1 ) 2.1 2.9 4.0 1.2 1.6 2.8 1.9

× × × × × × ×

10−13 10−13 10−13 10−11 10−12 10−13 10−12

EP (kJ mol−1 )

Time dependence For homogeneous materials it is possible to determine the diffusion D separately by the time dependence of the leak rate Q5. At a non-steady state the leakage flux Q(t) can be expressed as the solution of Fick’s law: Q冑t ⬵ Q0

冪 ␲ exp冉 − 4t冊 4␶

␶

(6)

A plot of ln(Q√t) versus 1/t yields a straight line with slope − ␶/4 and axis intercept ln(Q0√4␶/␲ ) on the ordinate which yields the saturation value Q0. So it is possible to determine for homogeneous materials the permeability P [Equation (1)], the diffusion D, and the solubility S after a relatively short time by extrapolation. This method is not valid for inhomogeneous materials, such as composites. A plot of ln(Q√t) versus 1/t yields no straight line as shown in Figure 8. For heterogeneous materials, such as fibre composites, the theory is more complicated; models are not well developed. The time constant ␶ is a measure for the time needed for approaching saturation of the flux Q:

␶=

d2 D

3.1 4.5 1.7 2.0 3.4 1.4

× × × × × ×

10−13 10−13 10−11 10−12 10−13 10−12

EP (kJ mol−1 )

P at 20°C (m2 s−1 )

28 42

1.2 × 10−13 2.8 × 10−13

Results The diffusion D has been determined from the time dependence of the leak rate at different temperatures. The results are given for different gases (Figure 9). The results are compiled in Table 3. The diffusion D of the small helium atoms is higher than that of the larger hydrogen and methane molecules. The barrier ED is determined not only by the size of the penetrating gas molecules, but also by their dipole moments.

Influence of sample treatments Especially at low temperatures the polymer matrices become more brittle and more sensitive to mechanical and thermal loading.

Mechanical cycling Fibre composites have been mechanically cycled by four point bending at 77 K and 293 K until the onset of a decrease of the bending modulus. As seen from Figure 10, the permeation parameter P increased by only less than 30%. Thus, it can be concluded that mechanical cycling is of less importance for the permeability even at 77 K.

Thermal cycling Thermal cycling to 77 K also did not influence the permeability, not even after 100 cycles. This is shown in Figure 11 for fibre glass composites and helium gas.

(7)

Figure 8 Schematic presentation of the time dependence of the leak rate for homogeneous materials and composites

146

P at 20°C (m2 s−1 )

26 26 20 27 26 28

• At room temperature the permeations P for helium and hydrogen are quite similar; that for methane is significantly lower and EP (methane) ⬎ EP (hydrogen) ⬎ EP (helium). • The permeability of fibre glass is lower than that of the carbon fibres.

Methane

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Figure 9 Diffusion of helium, hydrogen and methane through PE-HD

¨ Gas permeation of fibre reinforced plastics: Jens Humpenoder Table 3 Diffusion D and the diffusion barrier ED for helium, hydrogen and methane Helium Materials PVC PE-HD

ED (kJ mol−1 ) 16 23

Hydrogen

D at 20°C (m2 s−1 ) 9.8 × 10−10 4.6 × 10−10

ED (kJ mol−1 ) 22 27

D at 20°C (m2 s−1 )

Methane

ED (kJ mol−1 )

3.2 × 10−10 1.9 × 10−10

D at 20°C (m2 s−1 ) 6.6 × 10−12

and did not improve their barrier protection against permeation: Thick surface coatings→mechanically unstable Thin surface coatings→low or no effect on permeability Thicker metal foils can be applied when embedded within a composite. The following foils have been used: • aluminium foil (thickness 240 ␮m); • tin foil (thickness 250 or 100 ␮m). Figure 10 Helium permeability with and without mechanical cycling

Composite samples with metal foils embedded did not show any gas permeability at all (detection efficiency 10−15 m2 s−1 ). However, with aluminium foils delamination occurred under thermal and mechanical cycling. Excellent results, by contrast, have been observed for composites with tin foils. There is an excellent adhesion between tin and epoxy matrix, which is resistant to fatigue loading. Thus, this material seems to be a promising candidate for building vessels for liquefied gases.

Conclusions

Figure 11 Helium permeability with and without thermal cycling

Influence of surface layers and liners The influence has been examined on the permeability of different coating materials. GFEP and CFEP samples have been applied to surface layers or liners. Different surface layers 쐌 polyimide (Kapton) have been used: 쐌 amorphous hydrogenous carbon (aCH) 쐌 titanium nitride 쐌 metals (copper, gold, silver) For these surface coatings only thicknesses of the order of several micrometres have been used. Otherwise the adhesive bond between composite and coating materials failed. Therefore, it was not possible to apply thicker films. Thin surface layers and liners, however, did not decrease the permeability of the composite materials appreciably,

At room temperature, the gas permeability of fibre glass and carbon fibre reinforced epoxy resins cannot be neglected; however, at cryogenic temperatures it nearly disappears completely. In a temperature range down to 77 K, the tested composite samples did not show formation of cracks inside the fibre/matrix bond, which would lead to an increase of permeability. Thermal and mechanical cycling did not make a large effect. Fibre reinforced epoxy resins with tin foil embedded are materials suitable for building storage and transportation vessels for liquefied gases; it is resistant to thermal and mechanical fatigue, and fully impermeable to gases (at room temperature P ⬍ 10−15 m2 s−1 ).

References 1. Evans, D. and Morgan, J.T., Gas permeability through composite materials. Cryogenics, 1988, 28, 283–284. 2. Evans, D. and Morgan, J.T., Cryogen containment in composite vessels. Adv. Cryo. Eng., 1985, 32, 127–133. 3. Barrer, R.M., Permeation, diffusion and solution of gases in organic polymers. Trans. Faraday Soc., 1939, 35, 628–643. 4. Schmidt, W., Permeation von Gasen durch Kunststoffe im Temperaturbereich 150 K–300 K. Siemens Laborbericht, Erlangen, Germany, 1988. 5. Rogers, W.A., Buritz, R.S. and Alpert, D., Diffusion coefficient, solubility, and permeability for helium in glass. J. Appl. Phys., 1954, 25, 868–875.

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