Multilayer bamer f i i s on polyethylene polymerized from CF3H/C2H4 plasmas

M. Walker*, K. M. Baumgartner and E. Rauchle

Institut fiir Plasmaforschung der Universitat Stuttgart, Pfaffenwaldring 3 1, D-70569 Stuttgart, FRG

The barrier properties of thin fluorocarbon films toward toluene are studied. Mixtures of trifluoromethane and ethylene are used as monomers in an electron cyclotron resonance plasma discharge. The measured permeation fluxes of toluene through polyethylene show that thin fluorocarbon layers strongly reduce the permeation, e. g. by a factor of about 3000 when an appro- priate tapered layer is deposited on polyethylene foils. Different tapering of the barrier layers was investigated with respect to low permeation and good adhesion of the films. The amount of permeated toluene through the untreated and treated poly- ethylene foils are measured as a function of time using a gas chromatograph with a flame ionization detector.

1. Introduction

Polyethylene, due to its low price, low weight and ease of processing has acquired considerable importance in the packaging industry. The trend in the past ten years in the packaging industry is to replace metal containers by poly- ethylene. Although polyethylene (PE) is the preferred ma- terial, it shows a relatively large permeation for various or- ganic liquids [ 1-31, e. g. gasoline, and can often not be used as a container material without further modification. It is well known that surface modification by chemical fluorina- tion or sulfonation improves the barrier properties of PE considerably. In the case of fluorination, the reaction of el- emental fluorine and polyethylene yields a surface film re- sembling that of polytetrafluorethylene [4, 51. Surface flu- orinated PE-HD containers are used, for example, as fuel tanks in automobiles. In comparison with the chemical flu- orination, a fluorination by plasma processes has the advan- tage that only a small amount of chemically aggressive sub- stances is involved.

The permeation process by which a molecule permeates through a polymer involves the following stages [6]: 1. The absorption of the permeating species onto the sur-

face of the polymer and the solution of the molecules into the polymer matrix.

2. The diffusion through the polymer. 3. The desorption from the opposite polymer surface.

It is well known that PE as a nonpolar polymer is more permeable to nonpolar liquids, e. g. hydrocarbons, than to polar liquids. When the liquid dissolves in the polymer, the permeation rate depends on the solubility of the liquid in the surface coating. In a simplified description the fluorination of PE causes a decrease in solubility and a corresponding decrease in permeation. This means that a surface treatment of PE affects only the first step of the permeation process, the absorption and the solution of the liquid in the polymer.

In this paper, we apply plasma processing technique to deposit thin barrier layers on PE to improve the barrier properties toward toluene. The polymer films were pre- pared in a low pressure, 2.45 GHz electron cyclotron reson- ance plasma. Trifluoromethane and ethylene were used as monomers in an amount which is only a small fraction in comparison with the usual chemical treatment.

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2. Theory

Fick’s first law is the fundamental law of diffusion and states that the flux in the x-directionj, is proportional to the concentration gradient dcldx [2, 71

dc j x = -D(c) -

dx Here c(x, r ) is the local concentration of the permeant (in the units particles per cm’), j , is the particle flux diffusing across unit area in unit time and D(c) is the diffusion coeffi- cient. Equation (1) describes a one-dimensional diffusion process, where x is perpendicular to the film (see Fig. 1).

Polymer Film ‘t

Fig. 1. Boundary and initial conditions in a permeation experiment and a typical steady state concentration distribution.

In an ideal gas-polymer system, the diffusion coefficient is independent of the concentration, whereas for organic permeants the coefficient is usually a function of concentra- tion [ 1,2,8]. For the investigations discussed here a difk- sion coefficient of the form

D = DO exp(yc) (2)

can be applied [9-111. Here y is a characteristic parameter of the polymer-permeant system and Do is the diffusion coefficient in the limit c + 0.

The diffusion coefficient of Eq. (2) is an approximate form in the limit of small concentration of a more general expression for D(c) given in the paper of Huang and co- workers [12]. In the investigations discussed here the low concentration approximation is valid because the diffusion is strongly diminished by the applied barrier films.

Fick’s second law of diffusion describes the non-steady state and can be written as

dr dw (3)

466 Acta Polymer., 47.466-469 (1996) 0 VCH Verlagsgesellschaft mbH, D-6945 1 Weinheim, 1996 0323-7648/96/1010-466$10.00 + .2510

In the steady state (dcldt = 0), the concentration distribution c,(x) is given by the solution of the equation

( D ( C ) $) = 0 dx (4)

For the boundary and initial conditions (see Fig. 1)

c(x = 0, t ) = co

c(x = L, t) = 0

( 5 )

(6)

the stationary concentration distribution is obtained [7] as a solution of Eq. (4):

Here co is the concentration for x = 0 and L is the thickness of the polymer. A substitution of Eq. (7) into Fick’s first law yields the steady state flux:

As a result of Eq. (8), the steady state flux increases ex- ponentially with an increase in the concentration co. Thus, a “small” reduction of co leads to a “large” reduction in the steady state flux. In the case of fluorinated PE, the surface film causes a decrease in concentration co and according to Eq. (8) a corresponding decrease in the permeated fluxj,. The fluorinated surface of PE acts as a solvent barrier layer 141.

3. Experimental

Thin polymer films were deposited in an electron cyclo- tron resonance (ECR) plasma. The experimental set-up is shown in Fig. 2. The magnetic induction of 0.0875 T re- quired for the ECR-heating atf= 2.45 GHz is produced by CoSm permanent magnets. The magnetic field configura- tion allows the production of a linearly extended plasma. The plasma generation mechanism is described in detail in

For the deposition of fluorocarbon polymer films, mix- tures of trifluoromethane (CF3H) and ethylene (C2H4) are used as monomers. The monomer inlet system consists of several mass flow controllers and mass flow meters. The partial flow rates of each component was measured in the

~ 3 1 .

pressure g a w

monomer inlet system

i KWY movable maanet configuration

vacuum system

Fig. 2. Plasma reactor.

Acta Polymer., 47, 46-69 (1996)

units of sccm. For example, a CF3H : C2H4 mixture ratio of 30 : 2.5 means a partial flow rate of 30 sccm CF3H and 2.5 sccm CzH4 during the deposition process. The plasma pro- cess runs at a pressure of 3 Pa typically, and an average microwave power of 300 W.

To determine the bamer properties of the plasma poly- merized layers, permeability measurements of plasma treated and untreated polyethylene foils were performed. Commercial 1 10 pm thick PE-HD foils from BP Chemicals with a density of 0.955 g/cm3 were used. Toluene from Merck Chemicals was of analytical grade.

The permeation experiments were carried out in a device shown in Fig. 3 [14]. The permeation cell consists of two compartments seperated by the PE foil. The permeant is in- troduced in the upper compartment of this cell. A carrier gas (N2) flows at a constant rate through the other compart- ment and sweeps the permeated toluene molecules to a flame ionisation detector (FID). The resulting signal of the FID is continuously recorded. The permeation measure- ments were performed with a nitrogen flow rate of 20 sccm, adjusted by a mass flow controller (MFC). The flow rate was controlled during the measurements by a mass flow meter (MFM). Both compartments of the permeation cell are at atmospheric pressure, thus no support of the PE foils is required. The permeation cell was placed in a constant temperature bath which was maintained within 0.5 “C at the desired temperature. Both compartments of the permeation cell were sealed by O-rings. The exposed circular area of the foils was 43 cm2. The upper compartment contains a further O-ring for sealing the PE foil against the water of the temperature bath.

eatlon cell

I MFM I

,Br+5 gas chromatograph

Fig. 3. Schematic diagram of the device for liquid permeation studies (MFC = mass flow controller, MFM = mass flow meter).

4. Results and discussion

The plasma polymerized films obtained from different CF3H : C2H4 mixtures were deposited on PE foils, then the permeation rate of toluene through these foils was measured. The permeation measurements were performed at a temperature of 40 “C.

Fig. 4 shows the permeation curves versus time for (a) an uncoated PE foil, (b) a coated PE foil with a plasma polymer from pure CF3H (CF3H: C2H4 mixture ratio of 30 : 0) and (c) a coated PE foil from a CF3H : C2H4 mixture ratio of 30 : 2.5. Additionally Fig. 4 shows the permeation curves of a multilayer plasma film consisting of six single

Multilayer bamer films on polyethylene 467

’01’ I I

CGH : C& mixture ratio thickness

lime [sec]

Fig. 4. Toluene flux density as a function of time for (a) an uncoated PE foil and for (b)-(e) different plasma polymerized films on the surface of PE.

films (graph (d)) and seven single films (graph (e)) de- posited from different CF3H : C2H4 mixture ratios. The multilayer were obtained by varying the partial flow rates of each component during the deposition process. Table 1 summarizes the steady state fluxesjs, the film thickness and the retention factors for the different barrier films studied.

The steady state fluxj, decreases from 2.34 x 10l6 to- luene molecules/cm2 s (TM/cm2 s) for the uncoated PE foil to 1.90 x 1015 TM/cm2 s for the plasma polymer from pure CF3H. A further reduction ofjs can be obtained by a small addition of C2H4 in a CF3H plasma. A plasma polymer ob- tained from a CF3H : C2H4 mixture ratio of 30 : 2.5 reduces

j , to 2.25 x lOI4 TWcm s. This means, that a 0.1 pm thick plasma polymer layer reduces the toluene permeability of PE by a factor of approximately 100.

Table 1. Film thickness, toluene permeation rate and retention coef- ficient of different CF3H : CzH4 films plasma-polymerized on PE.

Monomer composition Film Steady state flux Retention of the barrier layer thickness [TWcm’ s] coefficient

“4 PE, uncoated - 2.34 x 10l6 -

CF3H : C2H4 mixture 100 1.90 x loL5 12.3

CF3H : CzH4 mixture 100 2.2s x 1014 104

“Multilayer” consisting 275 2.99 x 1013 780

“Multilayer” consisting 420 8.08 x 10” 2900

ratio of 30 : 0

ratio of 30 : 2.5

of six single films

of seven single films

The deposition of an appropriate tapered plasma polymer film leads to a further strong reduction of the toluene per- meabilip As shown in Fig. 4 j , decreases from 2.99 x 1013 TWcm s for a multilayer consisting of six single films to 8.08 x 10l2 TM/cm2 s for a multilayer consisting of seven single films. Figure 5 shows the structure of the multilayer on the surface of PE [ 151. The first film in the multilayer of Fig. 5 is plasma polymerized from a pure C2H4 plasma

30:O 115 nm

30: 1 35 nm 30 : 2.5 40 nm 30:4 50 nm

30:8 50 nm

30:m 55 nm

0 : 3 0 75 nm

polymer (PE-HD)

Fig. 5. Schematic diagram of a multilayer barrier structure consist- ing of seven single films.

(CF3H : C2H4 mixture ratio of 0 : 30). The following six films are plasma polymerized from different CF3H : C2H4 mixture ratios. The outer film of the barrier layer is de- posited from a pure CF3H plasma (CF3H : C2H4 mixture ratio of 30 : 0). The chemical composition of such multi- layer films were investigated by IR and XPS [14, 16, 171. It was shown, that the first film of the barrier layer consists mainly of CH2- and CH3-groups. The chemical and mech- anical properties of the C2H4 plasma polymer film is simi- lar to PE and can be described as a PE like film. The chemi- cally similarity with the substrate improves the adhesion of the film to the substrate. The following six films of the bar- rier layer contain CH-groups as well as CF-groups. The outer film shows absorption bands due to CF2- and CF3- groups and can be described as a polytetrafluorethylene (PTFE)-like film. The increase of the flow rate of CF3H during the deposition process leads to an increase of fluoro- carbon chains and a decrease of hydrocarbon chains. This means that the chemical composition changes from PE to a surface similar to PTFE.

As shown in Fig. 4 a single film, plasma-polymerized from a pure CF3H plasma, leads only to a small reduction of the toluene permeation rate. While such a plasma-poly- merized film is highly fluorinated, the adhesion between the plasma polymer and the PE substrate is poor [ 181. Chemical bonding between the deposited layer and the substrate is weak and results in the measured poor toluene barrier of the CF3H plasma polymer film. In the case of the multilayer structure, the C2H4 plasma polymer film acts like an ad- hesive layer between the polymer substrate and the follow- ing barrier layers. As shown in Fig. 4 the permeation is drastically reduced by a multilayer consisting of six single layers. A further reduction is obtained for a multilayer con- sisting of seven single layers. This means that for a good barrier property a smooth tapering from PE to the PTFE- like film is advantageous.

5. Conclusion

It is shown that a multilayer structure as a barrier sheath deposited from different CF3H : C2H4 plasmas is an effec- tive means for improving the barrier properties of poly-

468 Walker, Baumgartner, Rauchle Acta Polymer., 47, 46-69 (1996)

ethylene toward toluene, e. g. the steady state flux decreases by a factor fo about 3000 from 2.34 x 10l6 TM/cm2 s for the uncoated polyethylene to 8.08 x 10l2 TM/cm2 s for a multi- layer consisting of seven single layers.

The advantages of a multilayer structure are: (1) a good adhesion between the polymer substrate and the following plasma polymerized layers and (2) a low permeation flux especially for a smooth tapering of the barrier film struc- ture.

Acknowledgements

The authors would like to acknowledge the support of the Institut f ir Kunststoffpriifung und Kunststoffkunde der Universitit Stuttgart (IKP) for help in the permeation measurements. The authors thank Prof. Dr. U. Schumacher from the Institut f ir Plasmaforschung der Universitit Stutt- gart, Prof. Dr. G. Busse and Dipl. Ing. G. Taigel from IKP for useful discussions.

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Received June 10, I996

Final version July 31, 1996

Acta Polymer., 47, 466-469 (1996) Multilayer barrier films on polyethylene 469