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Comparison of permeability determined by permeation cell and immersion methods for organic solvents through protective gloves
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POLYMER TESTING Polymer Testing 25 (2006) 975–984 www.elsevier.com/locate/polytest
Test Method
Comparison of permeability determined by permeation cell and immersion methods for organic solvents through protective gloves Keh-Ping Chao, Jim-Shoung Lai, Hsueh-Chien Lin, Ya-Ping Hsu Department of Occupational Safety and Health, China Medical University, Taiwan, ROC Received 18 April 2006; accepted 22 June 2006
Abstract The chemical resistance of protective gloves has been tested using the ASTM F739 standard permeation method, as well as the immersion gravimetric method. Diffusion and solubility coefficients of four organic solvents, benzene, toluene, ethyl benzene and p-xylene, in nitrile and neoprene gloves were investigated. The diffusion coefficients determined by the ASTM F739 method were consistent with the immersion test (r2 ¼ 0.714, p ¼ 0.008), and their permeation coefficients showed excellent correlation (r2 ¼ 0.979, p ¼ 0.011) for nitrile gloves. Using a one-dimensional diffusion equation based on Fick’s second law, the simulation results indicated that the immersion test could be inappropriate for determining the diffusion and solubility coefficients of organic solvent permeation through neoprene gloves. For the practical purpose of effective evaluation, a test should be conducted to obtain the permeability using ASTM F739 method which can closely simulate the exposure conditions of workers wearing protective gloves. r 2006 Elsevier Ltd. All rights reserved. Keywords: ASTM F739; Immersion method; Protective glove; Diffusion coefficient; Solubility coefficient; Permeability
1. Introduction Selection and use of the right type of protective gloves is important in preventing skin exposure to hazardous chemicals in the workplace. Chemical resistant gloves are commercially made of polymeric materials such as nitrile and neoprene. The methods used to evaluate the chemical resistance of protective gloves can be classified in two categories: immersion/sorption method and permeation methCorresponding author. Tel.:886 4 22053366 x3503; fax: 886 4 22070500. E-mail address: [email protected] (K.-P. Chao).
0142-9418/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2006.06.007
ods. For the permeation test, ASTM F739 [1] and European EN374 [2] standard methods have been widely applied to measure the resistance of protective gloves to permeation by liquid chemicals. These two standards define the same test cell and similar methods to obtain the permeation rate of the chemical through protective gloves. In ASTM F739 and European EN374 methods, a permeation cell is used where the glove sample divides the permeation cell into two chambers: the permeant chamber (challenge chamber) which is filled with the test chemical and in contact with the outer face of glove sample, and the collection chamber where the permeant is collected and
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analyzed. In general, these tests should be performed using analytical techniques in the laboratory under controlled conditions [3]. ISO 6179 and 6529 also specify permeation methods for rubber and for protective clothing, respectively. However, in both these methods the test chemical permeates vertically through polymer film and, therefore, the gravitational force of the liquid may have an effect on the permeation process. For the mostly practical cases, the liquid chemical permeates horizontally through protective gloves, which is similar to the ASTM F739 and EN 374 methods. Several researchers have studied the chemical resistance of protective gloves using the ASTM F739 standard method but the ISO 6179 and 6529 methods are not commonly used for this purpose. The immersion method is a simple and expedient gravimetric test. The glove sample is completely immersed in the chemical and weighed at specified time intervals. In contrast to the permeation cell methods, both surfaces of the glove sample are in contact with the test chemical in the immersion method. The immersion method can be employed as a rapid means to assess the chemical resistance of protective gloves, and there is a need to correlate the data obtained from it with the ASTM F739 method. In this study, ASTM F739 permeation experiments and immersion tests were conducted for the nitrile and neoprene gloves with four neat organic solvents (i.e. benzene, toluene, ethyl benzene and p-xylene). Solubility and Fick’s diffusion coefficients of these solvents in gloves were estimated for ASTM F739 and immersion tests, respectively. Comparisons were made between the permeation coefficients and simulations of the ASTM F739 experiment data using Fick’s second law. The present work will provide an approach to quantitatively interpret the immersion test data in relation to ASTM F739 permeation experiment results. 2. Theory 2.1. Diffusion through polymer films Permeation of organic solvents through protective gloves is dependent on the molecular interactions between the solvent and polymer and involves three mass transfer mechanisms. The permeant is first dissolved into the contact surface of glove. Subsequently, the permeant diffuses through the
glove and is desorbed from the opposite surface. Permeation is mainly a function of solubility and diffusivity of solvents through the polymer with desorption playing an insignificant role [4,5]. One-dimensional diffusion of organic solvent through a polymer glove can be described by Fick’s first law [4–15] J ¼ D
dC Z , dZ
(1)
where J is the permeation rate per unit area (M L2 T1); D is the diffusion coefficient of organic solvent in the polymer (L2T1); CZ is the solvent concentration in the polymer (M L3); and Z is the distance along the direction of diffusion (L). The diffusion coefficient is assumed to be, in principle, independent of the concentration CZ and time. The diffusion into the polymer glove can be expressed by Fick’s second law with a constant diffusivity as follows: qC Z q2 C Z ¼D qt qZ2
(2)
Several models have been proposed to estimate the diffusion coefficient for liquid chemical permeation through polymer membrane. Crank employed a diaphragm cell to investigate the permeation of volatile organic compounds (VOC) vapor through polymer membrane with thickness L and assumed that the boundary conditions for Eq. (2) were CZ ¼ L equal to zero, and CZ ¼ 0 equal to the solubility, SP (ML3), of the VOC in the polymer membrane [6]. According to the solutions of Eq. (2), the diffusion coefficient, Dp (L2T1), of VOC in the polymer membrane is given by: Dp ¼
L2 , 6 tl
(3)
where tl is the lag time (T) which is given by the time–axis intercept from the extrapolation of the steady state permeation portion of the cumulative permeation curve. Based on the assumptions of Crank, the steady state permeation rate, Js,p (ML2T1), for polymer membrane can be determined by Eq. (1) as follows [5,7–10]:
J s;p ¼ Dp
C Z¼L C Z¼0 Dp S p ¼ . L L
(4)
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2.2. Estimation of diffusion coefficient from immersion test In an immersion test, the polymer membrane is exposed to the organic solvent for a definite time and the mass uptake in the polymer is measured. The change of polymer membrane weight, Ct, is determined as Wt Wo Ct ¼ , (5) W1 Wo where Wo is the initial weight of polymer (M); Wt is the weight of polymer at time t (M); WN is the maximum weight of polymer (M). Furthermore, the sorption curve is determined by plotting Ct against the square root of time. Under the conditions of the immersion test, CZ ¼ 0 and CZ ¼ L in Eq. (2) are assumed to be the solubility, Si (ML3), of organic solvent in polymer membrane. The diffusion coefficient, Di (L2T1), of organic solvent in polymer membrane with thickness L can be estimated using the sorption curve as follows [9,11–15]: 2 Ly Di ¼ p (6) 4 where y is the slope of the initial linear portion of the sorption curve (i.e. before 50–55% of equilibrium). Several researchers have conducted immersion tests to obtain the solubilities for chemical permeation through polymer membranes [4,10–15]. The solubility Si is experimentally determined as follows: W1 Wo Si ¼ , (7) Vo where Vo is the initial volume of polymer membrane (L3). 3. Experimental 3.1. Chemicals and glove samples Organic solvents used for this study were benzene (Merck, Germany), toluene (Merck, Germany), ethyl benzene (Fluka, Switzerland), and p-xylene (Merck, Germany). These organic solvents were selected because of their broad use in industry and the resistance given by nitrile and neoprene gloves. Some properties of these solvents are summarized in Table 1. The tested solvents were of reagent grade
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Table 1 Physical and chemical properties of organic solvents Chemical
Benzene
Toluene
Ethyl benzene
p-Xylene
Formula Grade Purity (%) SG DHv d Log Kow LDL
C6H6 GR 99 0.88 8089 9.2 2.12 0.06
C7H8 GR 99 0.87 9080 8.9 2.73 0.75
C8H10 GC 98 0.87 10,098 8.8 3.15 0.31
C8H10 GR 99.8 0.86 10,128 8.8 3.15 0.04
Properties are at 251C and abbreviations are as follows. SG, specific gravity in kg/L; DHv, molar heat of vaporization (cal/ mole) [17]; d, solubility parameter ((cal/cm3)1/2) [17]; Kow, octanol-water partition coefficient [18]; LDL, Limit Detection Level (mg/L).
or higher purity (498%) and were used without further purification. As indicated in Table 2, the glove materials tested were unsupported/unlined nitrile and neoprene rubber. The test samples with a 51 mm (200 ) diameter were cut from the palms of the gloves. The thickness of each sample was taken as the average of four randomly located measurements made using a dial thickness gauge (Teclock Co., Japan) to an accuracy of 0.001 cm. The glove samples were rinsed with water and conditioned for at least 24 h at 2571 1C and relative humidity of 5075%. 3.2. Permeation experiment Permeation experiments were conducted using the ASTM F739 liquid chemical permeation test method with an open loop system [1]. The sketch of the experimental system and associated equipment is shown in Fig. 1. The 51-mm glass permeation cell (PTC-200, Pesce Lab Sales, Kennett Square, PA) was assembled and immersed into the water bath at a temperature of 2571 1C. Nitrogen was used as the collection medium at a flow rate of 125 mL/min (50–150 mL/min suggested in ASTM F739 method). The temperature of the nitrogen was equilibrated with the water by passing the gas through a glass chamber placed in the water bath. All equipment was connected with Teflon tubing and the downstream line was kept at room temperature of 2572 1C. During the experimental run, 100 mL gas samples were collected from the downstream sampling point at 15-minute intervals using a gas-tight syringe. The
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978 Table 2 Experimental glove samples Material
Brand
Thickness (mm)
Solubility parametera (cal/cm3)1/2
Nitrile Neoprene
Best 737-11 (Menlo, Ga.) MAPA-414 (Brunswick, OH)
0.5770.02 0.9570.04
11.2 10.1
a
Data obtained from Perkins and Tippit (1985) [19].
f low meter
Valve
water bath @25+ −1ºC glove collection chamber
cylinder
glass chamber
challenge chamber
permeation cell
sampling point Fig. 1. Experimental setup of open-loop permeation system.
gas samples were immediately injected into the gas chromatograph (GC) equipped with a flame ionization detector (Auto-System XL, Perkin Elmer, Norwalk, CT.). The temperature of the capillary GC column (DB-5, J and W, Folsom, CA.) was kept at an initial temperature of 60 1C for one minute, then ramped to 150 1C at a rate of 8 1C/min and held for one minute. The temperatures of the injection port and detector were maintained at 200 and 250 1C, respectively. Each permeation experiment was conducted over 3 h to obtain the steady state concentrations. 3.3. Immersion test Circularly cut glove samples were completely immersed in screw-tight Teflon bottles containing the experimental solvents. The Teflon bottles were placed inside the oven at a constant temperature of 2571 1C. After being taken from the sealed bottle at suitably selected time intervals, the glove sample was gently blotted using filter papers to remove the surface liquid. The sample was weighed immediately
using a highly sensitive electronic balance (AG245, Mettler Toledo, Switzerland) with an accuracy of 0.0001 g. After that, the glove sample was placed back in the experimental solvent and efforts were made to minimize the time needed to weigh the sample. The glove samples were weighed until the maximum value was reached (i.e. equilibrium sorption). The immersion tests and permeation experiments were conducted in triplicate for each solvent. 4. Results and discussion 4.1. Permeability of ASTM F739 experiments The permeation of organic solvents through nitrile and neoprene gloves is shown in Fig. 2. According to ASTM F739 method [1], the experimental steady state permeation rate Js,p for an open loop system is calculated as follows: J s;p ¼
CsQ , A
(8)
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where Cs is the steady state concentration of permeant in the collection medium (M L3); Q is the flow rate of the collection medium (125 mL/min); A is the surface area of the glove sample exposed to the permeant (20.26 cm2). For the ASTM F739 experiment, the diffusion coefficients Dp were estimated using Eq. (3), and the solubilities Sp were obtained by substituting Dp and Js,p into Eq. (4) as follows:
30
S p ¼ J s;p
180 Benzene
Nitrile
Toluene
Conc. (mg/ L)
150
Ethylbenzene p-Xylene
120 90
0
50
100
(a)
150 200 Time (min)
250
L . Dp
(9)
Furthermore, the solubility coefficient, Kp (dimensionless), of organic solvent in the polymer glove can be determined using the Nerst distribution function as follows [4]:
0 300
200 Neoprene
180
Kp ¼
160
Sp , Cf
(10)
where Cf is the equilibrium concentration of permeant (M L3). In this study, Cf is the density of the test solvent. The permeation coefficient or permeability, Pp (L2T1), is defined as a function of diffusion and solubility coefficients, and is calculated as follows [4,13,14]:
140 Conc. (mg/L)
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120 100 80 60 40 20
Pp ¼ K p Dp .
0 0
50
100
(b)
150 200 Time (min)
250
(11)
Table 3 shows the average diffusion, solubility and permeation coefficients of organic solvents for the ASTM F739 experiments. From the solubility parameter theory [4], if the solubility parameters, d (M0.5 L T1), of a polymer and a permeant are close
300
Fig. 2. Concentrations of organic solvents in collection medium for ASTM F739 experiments.
Table 3 Diffusion, solubility and permeation coefficients of organic solvents Chemical
Permeation experiment Js,p (mg/cm2/ min)
Immersion test
Dp (107 cm2/s)
Kp
Pp ¼ DpKp (107 cm2/s)
Di (107 cm2/s)
Ki
Pi ¼ DiKi (107 cm2/s)
n
Nitrile glove Benzene Toluene Ethyl benzene p-Xylene
818.60 551.13 232.80 166.63
2.63 1.95 1.48 1.22
3.11 3.08 1.69 1.52
8.18 6.01 2.50 1.85
4.23 3.81 2.29 1.95
1.86 1.77 1.39 1.11
7.87 6.74 3.18 2.16
0.65 0.77 0.71 0.73
Neoprene glove Benzene Toluene Ethyl benzene p-Xylene
1285.34 711.56 248.05 263.57
4.61 5.33 3.78 5.09
4.02 1.74 0.83 0.93
18.53 9.27 3.14 4.73
4.02 5.17 5.46 6.47
2.83 2.89 2.58 2.65
11.38 14.94 14.09 17.15
0.58 0.61 0.72 0.71
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980 20
1.2 Nitrile
2
(1) Nitrile, R = -0.841, p = 0.083 2
(2) Neoprene, R = -0.976, p = 0.012
1.0 0.8 Ct (w/w)
Pp (10-7cm2/s)
15
(2)
10
0.6 0.4
5
Benzene
(1)
Toluene
0.2
Ethyl benzene p-Xylene
0.0
0 0
1
2
3
4
∆δ ((cal/cm3)1/2)
0
5
10 √ t (min)
15
20
5
10 √ t (min)
15
20
(a) 1.2
Fig. 3. Correlations of Pp to the solubility parameter theory.
Neoprene
1.0
4.2. Sorption kinetics of immersion tests pffiffi Fig. 4 shows the Ct t sorption curves of test solvents for nitrile and neoprene gloves. In general, the weight of glove sample increased linearly in the initial half-hour, and reached sorption equilibrium
0.8 Ct (w/w)
to each other, the likelihood of permeation is increased. Therefore, the less the difference in the solubility parameter (i.e. Dd), the more permeation there is through the glove. The solubility parameters for the tested solvents and gloves are presented in Tables 1 and 2. Fig. 3 shows that the permeation coefficients Pp for nitrile and neoprene gloves were inversely correlated to Dd with r2 ¼ 0.841 (p ¼ 0.083) and r2 ¼ 0.976 (p ¼ 0.012), respectively. As a result, the solubility parameter theory can be used to describe the permeability of organic solvents through nitrile and neoprene gloves. The potential solvent solubility is related to solvent polarity. In general, the lower the octanol– water partition coefficient (Kow) of a chemical, the higher the polarity. As the nitrile and neoprene gloves are polar materials, the chemical with lower Kow will have a stronger attraction to the nitrile and neoprene gloves; this is the so-called ‘‘like dissolves like’’. The permeation coefficients Pp were found to be significantly correlated to log Kow of organic solvents for nitrile (r2 ¼ 0.939, p ¼ 0.031) and neoprene (r2 ¼ 0.989, p ¼ 0.005) gloves. This result may imply that the higher the polarity (i.e. lower value of log Kow), the more the permeation of organic solvent through the protective glove.
0.6 0.4 0.2 0.0
0 (b)
Fig. 4. Sorption curves for immersion tests.
after approximately one hour. Using the initial sorption results (i.e. before 50–55% equilibrium), the diffusion mechanism can be classified by the value of n from the following relation [13–15]: C t ¼ Ktn ,
(12)
where K and n are empirical parameters and can be obtained by the least-squares analysis of log Ct and log t. The Fickian diffusion is characterized by n ¼ 0.5, and non-Fickian or anomalous diffusion by n varying from 0.5 to 1. Table 3 shows that the values of empirical parameter n for the tested solvents ranged between 0.58 and 0.77. Therefore, the diffusion mechanism of tested solvents in the nitrile and neoprene gloves was non-Fickian type. This was the reason that the sorption curves were slightly sigmoidal during the
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40
981
20 Nitrile
Benzene Ethlybenzene
30
15
p-Xylene
Pi (10-7 cm2/s)
Thickness increase (%)
Toluene
20
(2)
10 (1)
5 10
2
(1) Nitrile, R = 0.979, p = 0.011 2
(2) Neoprene, R = -0.607, p = 0.221
0 0
0 30
0 (a)
60 Time (min)
90
120
5
10 Pp
(10-7
15
20
cm2/s)
Fig. 6. Comparison of permeation coefficients determined by ASTM F739 and immersion experiments.
70 Neoprene
Thickness increase (%)
60 50 40 30 20 10 0
0 (b)
30
60 Time (min)
90
120
Fig. 5. Thickness increase of glove samples for immersion tests.
initial period shown in Fig. 4. The non-Fickian diffusion is due to the effect of swelling of the glove samples in the immersion test [6,15]. Fig. 5 shows that the thickness of glove samples was increased by approximately 15–50% for the immersion test. Most of the swelling occurred during the initial half hour of the test. Swelling can be because the nitrile and neoprene elastomers were softened and the molecular chains were subjected to large forces [16]. 4.3. Comparison between permeation and immersion For the immersion test, the diffusion coefficient Di and solubility Si were determined using Eqs. (6) and (7), respectively. The solubility coefficient Ki was calculated by substituting Si into Eq. (10). Also,
the permeation coefficient, Pi (L2T1), was obtained by substituting Di and Ki into Eq. (11). From the equilibrium data of immersion tests, the maximum solvent uptake (i.e. WNWo) was significantly correlated to Pi for nitrile (r2 ¼ 0.998, p ¼ 0.001), however, an inverse correlation (r2 ¼ 0.349, p ¼ 0.409) was obtained for neoprene gloves. Observation of Pp and Pi presented in Table 3 suggests that the nitrile glove is more resistant to the tested solvents than neoprene. As shown in Table 3, Di obtained from the immersion test was generally greater than Dp for the four solvents, but they were of the same order of magnitude. Di was found to be significantly correlated to Dp with a regression coefficient r2 ¼ 0.714 (p ¼ 0.008). Fig. 6 presents a comparison of the permeation coefficients determined from ASTM F739 and immersion tests. For nitrile gloves, Fig. 6 shows that Pp and Pi were significantly correlated (r2 ¼ 0.979, p ¼ 0.011), and the results were very close to 1:1 line indicating an excellent agreement between immersion and ASTM F739 methods. However, it is unreasonable that Pp was inversely correlated to Pi (r2 ¼ 0.607, p ¼ 0.221) for neoprene gloves. As shown in Table 4, the thickness of nitrile and neoprene samples for the permeation experiment was increased by 11.3–13.9% and 14.8–24.4%, respectively. For the immersion test, the swelling of neoprene sample was as high as approximately 50%, and much greater than in the permeation experiments. For the immersion tests, the polymer relaxation can have a significant effect on the permeant mobility of neoprene samples.
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982 Table 4 Swelling of glove samples Chemical
Permeation (%)
Immersion (%)
Nitrile glove Benzene Toluene Ethyl benzene p-Xylene
11.3 13.9 12.2 11.4
22.9 19.6 15.3 15.2
Neoprene glove Benzene Toluene Ethyl benzene p-Xylene
14.8 20.2 24.4 21.9
49.1 50.2 48.3 49.9
4.4. Simulation of permeation concentration in collection medium Assuming the boundary conditions, CZ(0,t) ¼ Sp ¼ KpCf and CZ(L,t) ¼ 0, and the initial condition, CZ(Z,0) ¼ 0, the concentration profile CZ in the glove sample is obtained by solving Eq. (2) Z C Z ðZ; tÞ ¼ K p C f 1 L ! 1 np X 2 Dp ðnp=LÞ2 t e Z . ð13Þ sin np L n¼1 By taking the mass balance for the collection medium, the permeation concentration C (M L3) in the open loop system is determined as follows: ADp C¼ V
"Z
t
eðQt=V Þ ,
# ! 1 K pCf X K p C f Dp ðnp=LÞ2 t ðQt=V Þ þ e 2 dt e L L n¼1
ð14Þ
where V is the volume of collection chamber (100 mL). Fig. 7 shows the simulation results of Eq. (14) using diffusion coefficient Dp and solubility coefficient Kp determined from the permeation experiments. For the four organic solvents, Dp and Kp were able to appropriately simulate the permeation results, especially for the steady state conditions. As shown in Fig. 7, the simulated solvent concentrations were slightly greater than the experimental results during the initial period. This deviation may be as a result of the non-Fickian diffusion and neglecting the increase of sample thickness in simulation equations. The other possible reason can be the effects of sorption of the organic solvents
Fig. 7. Simulated concentrations of organic solvents in collection medium using Dp and Kp.
on the permeation process [7,8]. The higher concentrations of modeling results implied that the glove samples may be not in equilibrium with the organic solvents during the initial period of permeation. Fig. 8 shows the simulated results of Eq. (14) for the organic solvents permeation through nitrile and neoprene gloves using diffusion coefficient Di and solubility coefficient Ki. For the reason that a significant correlation (r2 ¼ 0.979) was obtained between Pp and Pi for the nitrile glove, Fig. 8(a) shows that Di and Ki were able to approximately
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obtained using the steady state permeation and sorption data from ASTM F739 and immersion methods, respectively. As expected, the larger differences in solubility parameter between solvents and glove samples resulted in a lower permeation through gloves. Although the glove samples were exposed to solvents under different conditions, the diffusion and permeation coefficients were found to be correlated well for these two methods. In the immersion test, it is noted that the swelling of neoprene samples may have a significant effect on the permeation process. A one-dimenionsal Fick’s diffusion equation was able to simulate well the ASTM F739 experimental results using diffusion coefficient Dp and solubility coefficient Kp as the boundary condition. On the other hand, the diffusion coefficient Di and solubility coefficient Ki obtained by the immersion test were inappropriate in simulating the results of ASTM F739 experiments. To assess the exposure of workers to solvents, the diffusion and solubility coefficients should be obtained using the ASTM F739 method which is more consistent with the conditions of using protective gloves. Acknowledgments The study was financially supported by the National Science Council, Taiwan, ROC (NSC932211-E-039-002). The authors are also grateful to China Medical University (CMU94-080) for support.
References Fig. 8. Simulations of permeation concentrations using Di and Ki.
simulate the steady state permeation. On the contrary, the simulations were much more scattered for neoprene gloves shown in Fig. 8(b). The simulation results indicated that the immersion test could be inappropriate for determining the diffusion and solubility coefficients of organic solvent permeation through neoprene gloves. 5. Conclusion Fick’s diffusion coefficients and solubility coefficients of benzene, toluene, ethyl benzene and p-xylene in nitrile and neoprene gloves were
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[7] K.P. Chao, V.S. Wang, P.H. Lee, J. Occup. Environ. Hyg. 1 (2) (2004) 57–61. [8] K.P. Chao, P.H. Lee, M.J. Wu, J. Hazard. Mater. 99 (2) (2003) 191–201. [9] N. Vahdat, V.D. Sullivan, J. Appl. Polym. Sci. 79 (7) (2001) 1265–1272. [10] N. Vahdat, J. Appl. Polym. Sci. 42 (12) (1991) 3165–3171. [11] X. Lu, S.S. Que Hee, J. Hazard. Mater. 59 (2–3) (1998) 279–285. [12] S.S. Que Hee, Appl. Occup. Environ. Hyg. 11 (2) (1996) 117–124. [13] Siddaramaiah, S. Roopa, U. Premakumar, A. Varadarajulu, J. Appl. Polymer Sci. 67 (1) (1998) 101–112.
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POLYMER TESTING Polymer Testing 25 (2006) 975–984 www.elsevier.com/locate/polytest
Test Method
Comparison of permeability determined by permeation cell and immersion methods for organic solvents through protective gloves Keh-Ping Chao, Jim-Shoung Lai, Hsueh-Chien Lin, Ya-Ping Hsu Department of Occupational Safety and Health, China Medical University, Taiwan, ROC Received 18 April 2006; accepted 22 June 2006
Abstract The chemical resistance of protective gloves has been tested using the ASTM F739 standard permeation method, as well as the immersion gravimetric method. Diffusion and solubility coefficients of four organic solvents, benzene, toluene, ethyl benzene and p-xylene, in nitrile and neoprene gloves were investigated. The diffusion coefficients determined by the ASTM F739 method were consistent with the immersion test (r2 ¼ 0.714, p ¼ 0.008), and their permeation coefficients showed excellent correlation (r2 ¼ 0.979, p ¼ 0.011) for nitrile gloves. Using a one-dimensional diffusion equation based on Fick’s second law, the simulation results indicated that the immersion test could be inappropriate for determining the diffusion and solubility coefficients of organic solvent permeation through neoprene gloves. For the practical purpose of effective evaluation, a test should be conducted to obtain the permeability using ASTM F739 method which can closely simulate the exposure conditions of workers wearing protective gloves. r 2006 Elsevier Ltd. All rights reserved. Keywords: ASTM F739; Immersion method; Protective glove; Diffusion coefficient; Solubility coefficient; Permeability
1. Introduction Selection and use of the right type of protective gloves is important in preventing skin exposure to hazardous chemicals in the workplace. Chemical resistant gloves are commercially made of polymeric materials such as nitrile and neoprene. The methods used to evaluate the chemical resistance of protective gloves can be classified in two categories: immersion/sorption method and permeation methCorresponding author. Tel.:886 4 22053366 x3503; fax: 886 4 22070500. E-mail address: [email protected] (K.-P. Chao).
0142-9418/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2006.06.007
ods. For the permeation test, ASTM F739 [1] and European EN374 [2] standard methods have been widely applied to measure the resistance of protective gloves to permeation by liquid chemicals. These two standards define the same test cell and similar methods to obtain the permeation rate of the chemical through protective gloves. In ASTM F739 and European EN374 methods, a permeation cell is used where the glove sample divides the permeation cell into two chambers: the permeant chamber (challenge chamber) which is filled with the test chemical and in contact with the outer face of glove sample, and the collection chamber where the permeant is collected and
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analyzed. In general, these tests should be performed using analytical techniques in the laboratory under controlled conditions [3]. ISO 6179 and 6529 also specify permeation methods for rubber and for protective clothing, respectively. However, in both these methods the test chemical permeates vertically through polymer film and, therefore, the gravitational force of the liquid may have an effect on the permeation process. For the mostly practical cases, the liquid chemical permeates horizontally through protective gloves, which is similar to the ASTM F739 and EN 374 methods. Several researchers have studied the chemical resistance of protective gloves using the ASTM F739 standard method but the ISO 6179 and 6529 methods are not commonly used for this purpose. The immersion method is a simple and expedient gravimetric test. The glove sample is completely immersed in the chemical and weighed at specified time intervals. In contrast to the permeation cell methods, both surfaces of the glove sample are in contact with the test chemical in the immersion method. The immersion method can be employed as a rapid means to assess the chemical resistance of protective gloves, and there is a need to correlate the data obtained from it with the ASTM F739 method. In this study, ASTM F739 permeation experiments and immersion tests were conducted for the nitrile and neoprene gloves with four neat organic solvents (i.e. benzene, toluene, ethyl benzene and p-xylene). Solubility and Fick’s diffusion coefficients of these solvents in gloves were estimated for ASTM F739 and immersion tests, respectively. Comparisons were made between the permeation coefficients and simulations of the ASTM F739 experiment data using Fick’s second law. The present work will provide an approach to quantitatively interpret the immersion test data in relation to ASTM F739 permeation experiment results. 2. Theory 2.1. Diffusion through polymer films Permeation of organic solvents through protective gloves is dependent on the molecular interactions between the solvent and polymer and involves three mass transfer mechanisms. The permeant is first dissolved into the contact surface of glove. Subsequently, the permeant diffuses through the
glove and is desorbed from the opposite surface. Permeation is mainly a function of solubility and diffusivity of solvents through the polymer with desorption playing an insignificant role [4,5]. One-dimensional diffusion of organic solvent through a polymer glove can be described by Fick’s first law [4–15] J ¼ D
dC Z , dZ
(1)
where J is the permeation rate per unit area (M L2 T1); D is the diffusion coefficient of organic solvent in the polymer (L2T1); CZ is the solvent concentration in the polymer (M L3); and Z is the distance along the direction of diffusion (L). The diffusion coefficient is assumed to be, in principle, independent of the concentration CZ and time. The diffusion into the polymer glove can be expressed by Fick’s second law with a constant diffusivity as follows: qC Z q2 C Z ¼D qt qZ2
(2)
Several models have been proposed to estimate the diffusion coefficient for liquid chemical permeation through polymer membrane. Crank employed a diaphragm cell to investigate the permeation of volatile organic compounds (VOC) vapor through polymer membrane with thickness L and assumed that the boundary conditions for Eq. (2) were CZ ¼ L equal to zero, and CZ ¼ 0 equal to the solubility, SP (ML3), of the VOC in the polymer membrane [6]. According to the solutions of Eq. (2), the diffusion coefficient, Dp (L2T1), of VOC in the polymer membrane is given by: Dp ¼
L2 , 6 tl
(3)
where tl is the lag time (T) which is given by the time–axis intercept from the extrapolation of the steady state permeation portion of the cumulative permeation curve. Based on the assumptions of Crank, the steady state permeation rate, Js,p (ML2T1), for polymer membrane can be determined by Eq. (1) as follows [5,7–10]:
J s;p ¼ Dp
C Z¼L C Z¼0 Dp S p ¼ . L L
(4)
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2.2. Estimation of diffusion coefficient from immersion test In an immersion test, the polymer membrane is exposed to the organic solvent for a definite time and the mass uptake in the polymer is measured. The change of polymer membrane weight, Ct, is determined as Wt Wo Ct ¼ , (5) W1 Wo where Wo is the initial weight of polymer (M); Wt is the weight of polymer at time t (M); WN is the maximum weight of polymer (M). Furthermore, the sorption curve is determined by plotting Ct against the square root of time. Under the conditions of the immersion test, CZ ¼ 0 and CZ ¼ L in Eq. (2) are assumed to be the solubility, Si (ML3), of organic solvent in polymer membrane. The diffusion coefficient, Di (L2T1), of organic solvent in polymer membrane with thickness L can be estimated using the sorption curve as follows [9,11–15]: 2 Ly Di ¼ p (6) 4 where y is the slope of the initial linear portion of the sorption curve (i.e. before 50–55% of equilibrium). Several researchers have conducted immersion tests to obtain the solubilities for chemical permeation through polymer membranes [4,10–15]. The solubility Si is experimentally determined as follows: W1 Wo Si ¼ , (7) Vo where Vo is the initial volume of polymer membrane (L3). 3. Experimental 3.1. Chemicals and glove samples Organic solvents used for this study were benzene (Merck, Germany), toluene (Merck, Germany), ethyl benzene (Fluka, Switzerland), and p-xylene (Merck, Germany). These organic solvents were selected because of their broad use in industry and the resistance given by nitrile and neoprene gloves. Some properties of these solvents are summarized in Table 1. The tested solvents were of reagent grade
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Table 1 Physical and chemical properties of organic solvents Chemical
Benzene
Toluene
Ethyl benzene
p-Xylene
Formula Grade Purity (%) SG DHv d Log Kow LDL
C6H6 GR 99 0.88 8089 9.2 2.12 0.06
C7H8 GR 99 0.87 9080 8.9 2.73 0.75
C8H10 GC 98 0.87 10,098 8.8 3.15 0.31
C8H10 GR 99.8 0.86 10,128 8.8 3.15 0.04
Properties are at 251C and abbreviations are as follows. SG, specific gravity in kg/L; DHv, molar heat of vaporization (cal/ mole) [17]; d, solubility parameter ((cal/cm3)1/2) [17]; Kow, octanol-water partition coefficient [18]; LDL, Limit Detection Level (mg/L).
or higher purity (498%) and were used without further purification. As indicated in Table 2, the glove materials tested were unsupported/unlined nitrile and neoprene rubber. The test samples with a 51 mm (200 ) diameter were cut from the palms of the gloves. The thickness of each sample was taken as the average of four randomly located measurements made using a dial thickness gauge (Teclock Co., Japan) to an accuracy of 0.001 cm. The glove samples were rinsed with water and conditioned for at least 24 h at 2571 1C and relative humidity of 5075%. 3.2. Permeation experiment Permeation experiments were conducted using the ASTM F739 liquid chemical permeation test method with an open loop system [1]. The sketch of the experimental system and associated equipment is shown in Fig. 1. The 51-mm glass permeation cell (PTC-200, Pesce Lab Sales, Kennett Square, PA) was assembled and immersed into the water bath at a temperature of 2571 1C. Nitrogen was used as the collection medium at a flow rate of 125 mL/min (50–150 mL/min suggested in ASTM F739 method). The temperature of the nitrogen was equilibrated with the water by passing the gas through a glass chamber placed in the water bath. All equipment was connected with Teflon tubing and the downstream line was kept at room temperature of 2572 1C. During the experimental run, 100 mL gas samples were collected from the downstream sampling point at 15-minute intervals using a gas-tight syringe. The
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978 Table 2 Experimental glove samples Material
Brand
Thickness (mm)
Solubility parametera (cal/cm3)1/2
Nitrile Neoprene
Best 737-11 (Menlo, Ga.) MAPA-414 (Brunswick, OH)
0.5770.02 0.9570.04
11.2 10.1
a
Data obtained from Perkins and Tippit (1985) [19].
f low meter
Valve
water bath @25+ −1ºC glove collection chamber
cylinder
glass chamber
challenge chamber
permeation cell
sampling point Fig. 1. Experimental setup of open-loop permeation system.
gas samples were immediately injected into the gas chromatograph (GC) equipped with a flame ionization detector (Auto-System XL, Perkin Elmer, Norwalk, CT.). The temperature of the capillary GC column (DB-5, J and W, Folsom, CA.) was kept at an initial temperature of 60 1C for one minute, then ramped to 150 1C at a rate of 8 1C/min and held for one minute. The temperatures of the injection port and detector were maintained at 200 and 250 1C, respectively. Each permeation experiment was conducted over 3 h to obtain the steady state concentrations. 3.3. Immersion test Circularly cut glove samples were completely immersed in screw-tight Teflon bottles containing the experimental solvents. The Teflon bottles were placed inside the oven at a constant temperature of 2571 1C. After being taken from the sealed bottle at suitably selected time intervals, the glove sample was gently blotted using filter papers to remove the surface liquid. The sample was weighed immediately
using a highly sensitive electronic balance (AG245, Mettler Toledo, Switzerland) with an accuracy of 0.0001 g. After that, the glove sample was placed back in the experimental solvent and efforts were made to minimize the time needed to weigh the sample. The glove samples were weighed until the maximum value was reached (i.e. equilibrium sorption). The immersion tests and permeation experiments were conducted in triplicate for each solvent. 4. Results and discussion 4.1. Permeability of ASTM F739 experiments The permeation of organic solvents through nitrile and neoprene gloves is shown in Fig. 2. According to ASTM F739 method [1], the experimental steady state permeation rate Js,p for an open loop system is calculated as follows: J s;p ¼
CsQ , A
(8)
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where Cs is the steady state concentration of permeant in the collection medium (M L3); Q is the flow rate of the collection medium (125 mL/min); A is the surface area of the glove sample exposed to the permeant (20.26 cm2). For the ASTM F739 experiment, the diffusion coefficients Dp were estimated using Eq. (3), and the solubilities Sp were obtained by substituting Dp and Js,p into Eq. (4) as follows:
30
S p ¼ J s;p
180 Benzene
Nitrile
Toluene
Conc. (mg/ L)
150
Ethylbenzene p-Xylene
120 90
0
50
100
(a)
150 200 Time (min)
250
L . Dp
(9)
Furthermore, the solubility coefficient, Kp (dimensionless), of organic solvent in the polymer glove can be determined using the Nerst distribution function as follows [4]:
0 300
200 Neoprene
180
Kp ¼
160
Sp , Cf
(10)
where Cf is the equilibrium concentration of permeant (M L3). In this study, Cf is the density of the test solvent. The permeation coefficient or permeability, Pp (L2T1), is defined as a function of diffusion and solubility coefficients, and is calculated as follows [4,13,14]:
140 Conc. (mg/L)
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120 100 80 60 40 20
Pp ¼ K p Dp .
0 0
50
100
(b)
150 200 Time (min)
250
(11)
Table 3 shows the average diffusion, solubility and permeation coefficients of organic solvents for the ASTM F739 experiments. From the solubility parameter theory [4], if the solubility parameters, d (M0.5 L T1), of a polymer and a permeant are close
300
Fig. 2. Concentrations of organic solvents in collection medium for ASTM F739 experiments.
Table 3 Diffusion, solubility and permeation coefficients of organic solvents Chemical
Permeation experiment Js,p (mg/cm2/ min)
Immersion test
Dp (107 cm2/s)
Kp
Pp ¼ DpKp (107 cm2/s)
Di (107 cm2/s)
Ki
Pi ¼ DiKi (107 cm2/s)
n
Nitrile glove Benzene Toluene Ethyl benzene p-Xylene
818.60 551.13 232.80 166.63
2.63 1.95 1.48 1.22
3.11 3.08 1.69 1.52
8.18 6.01 2.50 1.85
4.23 3.81 2.29 1.95
1.86 1.77 1.39 1.11
7.87 6.74 3.18 2.16
0.65 0.77 0.71 0.73
Neoprene glove Benzene Toluene Ethyl benzene p-Xylene
1285.34 711.56 248.05 263.57
4.61 5.33 3.78 5.09
4.02 1.74 0.83 0.93
18.53 9.27 3.14 4.73
4.02 5.17 5.46 6.47
2.83 2.89 2.58 2.65
11.38 14.94 14.09 17.15
0.58 0.61 0.72 0.71
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980 20
1.2 Nitrile
2
(1) Nitrile, R = -0.841, p = 0.083 2
(2) Neoprene, R = -0.976, p = 0.012
1.0 0.8 Ct (w/w)
Pp (10-7cm2/s)
15
(2)
10
0.6 0.4
5
Benzene
(1)
Toluene
0.2
Ethyl benzene p-Xylene
0.0
0 0
1
2
3
4
∆δ ((cal/cm3)1/2)
0
5
10 √ t (min)
15
20
5
10 √ t (min)
15
20
(a) 1.2
Fig. 3. Correlations of Pp to the solubility parameter theory.
Neoprene
1.0
4.2. Sorption kinetics of immersion tests pffiffi Fig. 4 shows the Ct t sorption curves of test solvents for nitrile and neoprene gloves. In general, the weight of glove sample increased linearly in the initial half-hour, and reached sorption equilibrium
0.8 Ct (w/w)
to each other, the likelihood of permeation is increased. Therefore, the less the difference in the solubility parameter (i.e. Dd), the more permeation there is through the glove. The solubility parameters for the tested solvents and gloves are presented in Tables 1 and 2. Fig. 3 shows that the permeation coefficients Pp for nitrile and neoprene gloves were inversely correlated to Dd with r2 ¼ 0.841 (p ¼ 0.083) and r2 ¼ 0.976 (p ¼ 0.012), respectively. As a result, the solubility parameter theory can be used to describe the permeability of organic solvents through nitrile and neoprene gloves. The potential solvent solubility is related to solvent polarity. In general, the lower the octanol– water partition coefficient (Kow) of a chemical, the higher the polarity. As the nitrile and neoprene gloves are polar materials, the chemical with lower Kow will have a stronger attraction to the nitrile and neoprene gloves; this is the so-called ‘‘like dissolves like’’. The permeation coefficients Pp were found to be significantly correlated to log Kow of organic solvents for nitrile (r2 ¼ 0.939, p ¼ 0.031) and neoprene (r2 ¼ 0.989, p ¼ 0.005) gloves. This result may imply that the higher the polarity (i.e. lower value of log Kow), the more the permeation of organic solvent through the protective glove.
0.6 0.4 0.2 0.0
0 (b)
Fig. 4. Sorption curves for immersion tests.
after approximately one hour. Using the initial sorption results (i.e. before 50–55% equilibrium), the diffusion mechanism can be classified by the value of n from the following relation [13–15]: C t ¼ Ktn ,
(12)
where K and n are empirical parameters and can be obtained by the least-squares analysis of log Ct and log t. The Fickian diffusion is characterized by n ¼ 0.5, and non-Fickian or anomalous diffusion by n varying from 0.5 to 1. Table 3 shows that the values of empirical parameter n for the tested solvents ranged between 0.58 and 0.77. Therefore, the diffusion mechanism of tested solvents in the nitrile and neoprene gloves was non-Fickian type. This was the reason that the sorption curves were slightly sigmoidal during the
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40
981
20 Nitrile
Benzene Ethlybenzene
30
15
p-Xylene
Pi (10-7 cm2/s)
Thickness increase (%)
Toluene
20
(2)
10 (1)
5 10
2
(1) Nitrile, R = 0.979, p = 0.011 2
(2) Neoprene, R = -0.607, p = 0.221
0 0
0 30
0 (a)
60 Time (min)
90
120
5
10 Pp
(10-7
15
20
cm2/s)
Fig. 6. Comparison of permeation coefficients determined by ASTM F739 and immersion experiments.
70 Neoprene
Thickness increase (%)
60 50 40 30 20 10 0
0 (b)
30
60 Time (min)
90
120
Fig. 5. Thickness increase of glove samples for immersion tests.
initial period shown in Fig. 4. The non-Fickian diffusion is due to the effect of swelling of the glove samples in the immersion test [6,15]. Fig. 5 shows that the thickness of glove samples was increased by approximately 15–50% for the immersion test. Most of the swelling occurred during the initial half hour of the test. Swelling can be because the nitrile and neoprene elastomers were softened and the molecular chains were subjected to large forces [16]. 4.3. Comparison between permeation and immersion For the immersion test, the diffusion coefficient Di and solubility Si were determined using Eqs. (6) and (7), respectively. The solubility coefficient Ki was calculated by substituting Si into Eq. (10). Also,
the permeation coefficient, Pi (L2T1), was obtained by substituting Di and Ki into Eq. (11). From the equilibrium data of immersion tests, the maximum solvent uptake (i.e. WNWo) was significantly correlated to Pi for nitrile (r2 ¼ 0.998, p ¼ 0.001), however, an inverse correlation (r2 ¼ 0.349, p ¼ 0.409) was obtained for neoprene gloves. Observation of Pp and Pi presented in Table 3 suggests that the nitrile glove is more resistant to the tested solvents than neoprene. As shown in Table 3, Di obtained from the immersion test was generally greater than Dp for the four solvents, but they were of the same order of magnitude. Di was found to be significantly correlated to Dp with a regression coefficient r2 ¼ 0.714 (p ¼ 0.008). Fig. 6 presents a comparison of the permeation coefficients determined from ASTM F739 and immersion tests. For nitrile gloves, Fig. 6 shows that Pp and Pi were significantly correlated (r2 ¼ 0.979, p ¼ 0.011), and the results were very close to 1:1 line indicating an excellent agreement between immersion and ASTM F739 methods. However, it is unreasonable that Pp was inversely correlated to Pi (r2 ¼ 0.607, p ¼ 0.221) for neoprene gloves. As shown in Table 4, the thickness of nitrile and neoprene samples for the permeation experiment was increased by 11.3–13.9% and 14.8–24.4%, respectively. For the immersion test, the swelling of neoprene sample was as high as approximately 50%, and much greater than in the permeation experiments. For the immersion tests, the polymer relaxation can have a significant effect on the permeant mobility of neoprene samples.
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982 Table 4 Swelling of glove samples Chemical
Permeation (%)
Immersion (%)
Nitrile glove Benzene Toluene Ethyl benzene p-Xylene
11.3 13.9 12.2 11.4
22.9 19.6 15.3 15.2
Neoprene glove Benzene Toluene Ethyl benzene p-Xylene
14.8 20.2 24.4 21.9
49.1 50.2 48.3 49.9
4.4. Simulation of permeation concentration in collection medium Assuming the boundary conditions, CZ(0,t) ¼ Sp ¼ KpCf and CZ(L,t) ¼ 0, and the initial condition, CZ(Z,0) ¼ 0, the concentration profile CZ in the glove sample is obtained by solving Eq. (2) Z C Z ðZ; tÞ ¼ K p C f 1 L ! 1 np X 2 Dp ðnp=LÞ2 t e Z . ð13Þ sin np L n¼1 By taking the mass balance for the collection medium, the permeation concentration C (M L3) in the open loop system is determined as follows: ADp C¼ V
"Z
t
eðQt=V Þ ,
# ! 1 K pCf X K p C f Dp ðnp=LÞ2 t ðQt=V Þ þ e 2 dt e L L n¼1
ð14Þ
where V is the volume of collection chamber (100 mL). Fig. 7 shows the simulation results of Eq. (14) using diffusion coefficient Dp and solubility coefficient Kp determined from the permeation experiments. For the four organic solvents, Dp and Kp were able to appropriately simulate the permeation results, especially for the steady state conditions. As shown in Fig. 7, the simulated solvent concentrations were slightly greater than the experimental results during the initial period. This deviation may be as a result of the non-Fickian diffusion and neglecting the increase of sample thickness in simulation equations. The other possible reason can be the effects of sorption of the organic solvents
Fig. 7. Simulated concentrations of organic solvents in collection medium using Dp and Kp.
on the permeation process [7,8]. The higher concentrations of modeling results implied that the glove samples may be not in equilibrium with the organic solvents during the initial period of permeation. Fig. 8 shows the simulated results of Eq. (14) for the organic solvents permeation through nitrile and neoprene gloves using diffusion coefficient Di and solubility coefficient Ki. For the reason that a significant correlation (r2 ¼ 0.979) was obtained between Pp and Pi for the nitrile glove, Fig. 8(a) shows that Di and Ki were able to approximately
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obtained using the steady state permeation and sorption data from ASTM F739 and immersion methods, respectively. As expected, the larger differences in solubility parameter between solvents and glove samples resulted in a lower permeation through gloves. Although the glove samples were exposed to solvents under different conditions, the diffusion and permeation coefficients were found to be correlated well for these two methods. In the immersion test, it is noted that the swelling of neoprene samples may have a significant effect on the permeation process. A one-dimenionsal Fick’s diffusion equation was able to simulate well the ASTM F739 experimental results using diffusion coefficient Dp and solubility coefficient Kp as the boundary condition. On the other hand, the diffusion coefficient Di and solubility coefficient Ki obtained by the immersion test were inappropriate in simulating the results of ASTM F739 experiments. To assess the exposure of workers to solvents, the diffusion and solubility coefficients should be obtained using the ASTM F739 method which is more consistent with the conditions of using protective gloves. Acknowledgments The study was financially supported by the National Science Council, Taiwan, ROC (NSC932211-E-039-002). The authors are also grateful to China Medical University (CMU94-080) for support.
References Fig. 8. Simulations of permeation concentrations using Di and Ki.
simulate the steady state permeation. On the contrary, the simulations were much more scattered for neoprene gloves shown in Fig. 8(b). The simulation results indicated that the immersion test could be inappropriate for determining the diffusion and solubility coefficients of organic solvent permeation through neoprene gloves. 5. Conclusion Fick’s diffusion coefficients and solubility coefficients of benzene, toluene, ethyl benzene and p-xylene in nitrile and neoprene gloves were
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