J. Pack. Set Technol Vol. 2 No. 2 (1993)

Apparatus and Permeability of CO 2 Dissolved in Water
through LDPE and Nylon 6 Films

Tsutomu NAKAGAWA*, Atsuko NARUSE*, Akon HIGUCHI*

The apparatus by which the measurement of the permeability coefficient of films ofcarbon dioxide dissolved in water through films was carried out was constructed. Theapparatus consists of a carbon dioxide electrode in a liquid and liquid diffusion cell.
Using this, permeability of low density polyethylene (LDPE) film and Nylon 6 film ofcarbon dioxide dissolved in water through films were measured. Polystyrene (PS) film,
which is used as a standard film for oxygen permeability coefficient was also measured.
Even for th e h yd rophob ic films, n amely LDPE film and PS film, th e ap parent
permeability coefficients were observed to be slightly higher than those in a gas phase
due to the effect of absorbed water in the films. The apparent permeability coefficientof Nylon 6 was about two orders of magnitude higher than that in gas phase due toplasticizing effect of water.
Keywords ; Gas permeability, Carbon dioxide, LDPE, Nylon 6

INTRODUCTION

hydrophilic and hydrophobic monomeric units
stru ctures 9) .

Permeabilities of films of oxygen dissolvedin water have been studied to clarify theeffect of water on the permeability 1)-7) . The

Few researchers 9) studied transport of dissolved gas except oxygen in the films usinga gas electrode. It seems that no interest havebeen paid for the transport of carbon dioxide
dissolved in water. The concentration of carb o n d io x i d e i n p a ck ag e s o f f o o d s i n s o r mpermeability of dissolved oxygen is mostly
determined by applying an oxygen electrodein water and water diffusion cell. One of theauthors also reported the effect of water on
the oxygen permeability for films having bothcases should be important to keep theirquality. Iezdorczyk et. al 9) . reported diffusion*Department of Industrial Chemistry, Meiji University, School of Science and Engineering, 1-1-1, Higashi- mita, Tama -ku, Kaw a sak i- sh i, K an ag awa, 214 Japan-96-Apparatus and Permeability of CO 2 Dissolved in Water

coefficients of carbon dioxide dissolved inwater through several films by the time lag
method, but they did not measure any ofpermeability. This work dealt with the apparatus for permeability measurement and thepermeability coefficients of carbon dioxide
dissolved in water through films, which arebeing used for food packaging, by applying

A diagram of the permeation apparatus isshown in Fig. 1. The permeation cell consistsof two glass chambers separated by a film.
The cell volume of the upstream or down
stream side was 200ml, respectively. The effective film area was 6.84cm2. The revolution
speed of magnetic spinbars in the cells wascontrolled (i.e., 430 rpm in this study) bya magnetic stirrer having a revolution metera carbon dioxide electrode in a liquid andliquid permeation cell.

(HS3D, Iuchi Seiseido Co., Ltd.). The pH-type
CO2 electrode made by TOA Elec tronics Ltd.
was used as a sensor to determine the concentration of carbon dioxide. A diagram of theelectrode is shown in Fig. 2.

EXPERIMENTAL
Samples used in this study are low density
polyethylene (LDPE) film. Nylon 6 film andpolystyrene (PS) film (Stylex, Mitsubishi
Plastics Ind. Ltd.), which is used as a standard
film. Carbon dioxide of more than 99.99%
purity was used as received. Ultrapure waterby Toraypure LV- 10T reverse osmosis system(Toray Co., Ltd.) was used throughout theexperiment.

-Fig. 1 Diagram of permeation apparatus : (A)CO2electrode ; (B)O-ring ; (C) film ; (D) magnetic
stirrer ; (E) water bath.

Fig. 2 Structure of CO2 electrode. A glass electrode ;B sheath; C reference electrode; D Diaphragm ;E spacer ; F high sensitive glass membrane ;H internal filling solution ; I micro hole.

- 9 7 -

J . P a ck . S et . Te c h n o l Vol . 2 N o . 2 ( 1 9 9 3 )After water in the cells was equilibrated
with CO 2 atomospheri-cally, the upstream
water was pumped out and saturated waterwith CO 2 at 1 atm was immediately injected
i n t o t h e u p s t r e a m s i d e c e l l a t t i me = 0 s .
Permeability coefficients were calculated fromthe concentration change of CO 2 monitoredby the CO 2 electrode.

CALCULATION OF THE CO 2
PERMEABILITY COEFFICIENT
The pH changes were monitored by differences of the electromotive forces with thereference electrode. This is expressed asfollows :Concentration of CO 2 / v*v - 1 %Fig. 3

Plot of electromotive force vs. concentrationin gas phase for carbon dioxide electrodeat 25°C in this system.

E=E0 ＋ k In a+H ・・・・・・・・・・・(1)
An apparent permeability coefficient for filmin such a system is calculated as follows :The volume permeated is expressed by thewhere E is the electromotive force obtained
experimentally, Eo is an intrinsic electromotive force in this system, k is RT In 10

following equation :/ (ZF) (Z=1 in this case), and a +H is anactivity of H+.F is the Faraday con－stant.
Equation (1) is also expressed as follows :where Q is the volume permeated throughthe film, P is the permeability coefficient, pt

E=E0 + k In (r C)・・・・・・・(2)=E0 + k ln r + k ln C・・・・・(3)and p 2 are partial pressure of gas at theupstream side and downstream side of thefilm, respectively, A is the film area and twhere r is an activity coefficient.
C=10(E－EO )/k・・・・・・・(4)is the time.

is the thickness of the film.

where Eo' =Eo + k In r. Fig. 3 shows therelationship between the concentration of CO2
in the outer solution of the electrode andE. The experiments were performed on thecondition of pH= 7.0 at the down stream side ;the self-dissociation of water was neglected.

Then permeability coefficient is expressed by
the following equation :- 9 8 -A p p a r at u s a n d P e r me a b i l i t y o f C O 2 D i s so lv e d in W a t e r

where Δp = p1－p2 and p2 is C/α. α is theBunsen absorption constant of CO2 (0.759cm
Hg-1 at 25°C.)10). The schematic representationof a transient transport run is shown in Fig.
4. As C in the eq. 4 is the concentrationof the CO 2 in the water, it should betransformed into volume of gaseous state, byusing eq. 9. The permeability coefficient wasdetermined from the mean values of threeto 3 five experiments. All measurements wereperformed at 25 0.1°C.
APA(9)t » -t ,where V is the cell volume at the downstream side. The permeation experiment wasstopped when the concentration of CO2 of thedownstream side reached to 3 % of that of
the upstream side. The average partial pressure of CO2 at the downstream side between
t 1 and t 2 was used.
RESULTS AND DISCUSSION
Fig. 5 shows the dependence of apparent
permeability coefficients on C for LDPE andPS films. The term "apparent permeability

273+t

Fig. 4 Schematic representation of a transient transport run.

- 9 9 -J. Pack. Set. Technol. Vol. 2 No. 2 (1993)

When C increases at the concentration C =0.001, response rate becomes faster to catch
up with the increasing rate of CO2 at thedownstream side, and this contributes to theincrease of the apparent permeability coefficient. When C comes near 0.001, the response
rate can follow the increasing of CO 2 concentration at the down-stream side. This contributes to the higher apparent permeability
coefficient than the permeability coefficient,P, which is calculated by eq. 9 indicates theresponse rate rather than the permeating rateof CO 2 . Therefore, each curve in Fig. 5 has

the permeability coefficient.
At C > 0.001, the permeability coefficient isindependent of C at because the measured
Fig. 5 Dependence of permeability coefficients of
CO2 on C, for LDPE(O)and PS (●) films
at 25℃ and po=1atm.

potential coincides with the actual potential
without delay due to the fast response timeof the electrode. The permeability coefficients
which were obtained at C > 0.001 are consider-coefficient" means the permeability coefficient

ed to be independent on concentration ofwhich composed of the intrinsic permeability

carbon dioxide. Such permeability coefficientsof polymer film and the permeability of water

are expressed as P in liquid phase. The per-l a y e r o n th e f il m. B o t h f i l ms s h o w th epermeability coefficient increase with theincreasing C at C < 0.001 (cm3 (STP) /cm3 )meability coefficients of CO2 through the films

and give a maximum near C=0.001. Thisbehavior probably arises from the slow response time o f the electrode at th e lo w
concentration of CO 2 . It was shown that it
took 12 min.from atmospherical concentration of CO2 (0.0003cm3 (STP) /cm3) to C = 0.001
and 4 min. from the atmospherical concentration to C =0.01 to show the actual potential
by the electrode11) . The apparent permeability
coefficient shows lower values due to the limitof response time at the beginning (C< 0.001).

-measured in gas phase (high vacuum method)
are summarized in Table 1 and are comparedwith those in the gas phase.
In the liquid phase a boundary layer resistance was supposed to exist on the film
interface and this made to detect lower permeability values than those in the gas
phase 1 ) 4 ) 6 ) and this resistance may contributeto the lower permeability than value measured
in the gas phase. Table 1 shows, however,that the apparent permeability coefficients inthe liquid phase are higher than those in theg a s p h a s e , f o r n ot o n ly o n t h e hy dr o p h il i c

100-Apparatus and Permeability of CO2 Dissolved in Waterfilm, nylon 6, but on hydrophobic films suchas PS and LDPE.
Since the measurable lower limit of permeability coefficient capable of being measured
in the liquid phase is estimated to be 1 x 10""cm3 CSTP) cm/ cnV'S'cm Hg from the blanklest* the higher permeability coef-ficient
observed in the liquid phase is not due tothe leak of COa at the downstream side. The
permeability coefficient of the nylon 6 filmin the liquid phase especially observed to betwoorders of magnitude higher than the permeability coefficient in the gas phase. Thiscan be explained by the plasticizing effect
12 of the water molecule in the film. The
plasticizing effect is predominant over thereduction of the permeability due to theboundary layer's resistance and this resultsin the higher permeability in the liquid phase.
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101 -(Received 6 January 1993)(Accepted 22 April 1993)