I n d . Eng. Chem. Res. 1993,32, 279-287
279
Wettability of Nonwoven Fabrics. 1. Effect of Fluorochemical Finishes on Water Repellency A. Majid Sarmadi,* Young Ah Kwon, and Raymond A. Young Department of Environment, Textiles and Design, School of Family Resources and Consumer Sciences, University of Wisconsin-Madison, 1300 Linden Drive, Madison, Wisconsin 53706
The effect of fluorochemical finishes on the liquid penetration phenomena of nonwoven fabrics has been analyzed in this study. The contact angle, obtained from the Wilhelmy technique, combined with the fabric pore radius was used for an estimation of the capillary flow rates based on the Washburn equation. The expected capillary flow rates were compared with the experimental values of the spontaneous water uptake rate obtained from the demand wettability and static absorption tests. Results of this investigation indicate that the expected capillary flow rates were proportional to the experimental water uptake rates. The lowest contact angle of the wood pulp fibers and the largest pore radius resulted in the highest rate of water penetration from an unfinished Sontara fabric; however, a fluorochemical finish effectively reduced the surface energy of the Sontara fabric and provided excellent water repellency. Polypropylene nonwoven fabrics, containing fibers with a micrometer size radius, had the highest contact angle, providing the least water penetration and, thus, the greatest water repellency compared with the other nonwovens. 1. Introduction
As we have learned of the hazardous effects of pesticides on human health, the need to protect persons from harmful exposure has become urgent. For workers who handle harmful chemicals, exposure can be reduced by wearing the appropriate clothing. However, none of the conventional garment materials are known to be appropriate for use in all possible conditions of pesticide use. Nonwovens have gained wide acceptance as protective garments in medical, industrial, and agricultural areas because of cost, comfort, and low permeability. A recent study of Hobbs et al. (1986) has shown that fluorochemical (FC) treated nonwoven fabrics enhance barrier properties against liquid penetration. These nonwovens, due to their permeable structure, are expected to have better comfort properties than less permeable nonwovens such as Tyvek. However, a review of the literature revealed relatively little work on the evaluation of these fabrics. Thus, the first objective of this research was to determine the effect of a fluorochemical repellent finish on the liquid penetration of various nonwoven fabrics used as protective garments. A second objective of the study was to investigate the principles which govern resistance against the penetration of liquids into porous nonwoven fabrics. 2. Background In general, most studies on liquid penetration through porous materials have used theoretical models to draw conclusions from experimental resdts. Although it has not yet been totally clarified whether the absorption of liquids into textiles can be used in all cases as a measure of the resistance of textiles to the penetration of liquids, the measurement of the absorption of liquids by textiles has been the most widely applied method for evaluating resistance to liquid flow. It has been commonly considered that an increase in the absorption of liquids, which is the first step in the penetration mechanism, results in increased penetration. The interactions of liquids with fiber surfaces can be evaluated using the concepts of contact angle, critical surface energy, and fiber pore structure. Measurement of
* Author to whom correspondence should be addressed. 0888-5885/93/2632-0279$04.00/0
these parameters has been used to attain a better understanding of fiber wetting and resistance to liquid flow in fabrics. 2.1. Wetting Measurements for Single Fibers. Wilhelmy balance techniques can be used to determine the surface energetics of single fibers by measuring the attractive force across the interface between a liquid surface and a partially immersed solid (Miller and Young, 1975). The pull exerted on the fiber inserted into a liquid is expressed by the following equation: (1) F, = pYlvCOS e where F, = measured force, ylv= surface tension of the liquid, p = perimeter of the solid, and 0 = equilibrium contact angle of the solid-liquid interface a t the threephase contact line. To obtain wettability parameters such as contact angle and work of adhesion, it is necessary to know the perimeter of the solid at the three-phase interface and the surface tension of the liquid. The work of adhesion due to polar and dispersion forces can also be determined by this technique using the Fowkes approach. Fowkes (1962) reported that the forces present at a liquid.-& interface can be separated into contributions arising from dispersion forces and polar interactions, such as hydrogen bonding: Ys = Td + Y P
(2)
where yd is the contribution to surface tension arising from dispersion forces and y p is the force arising from polar interactions. The interfacial tensions can be calculated from the dispersion force components, yd, and the polar force component, y p , of the surface tension using eq 3 (Owens and Wendt, 1969). By measuring contact angles for a fiber ySl = yS + YI - 2(7,dyld)”*
- ~ ( Y , P Y ~ P ) ’ / ~ (3)
in two different liquids, one polar and the other nonpolar, two simultaneous equations are obtained, which can be solved for and ysP. Fox et al. (1953) demonstrated that there is a linear relationship between cos e for a homologous series of liquids and the surface tension of these liquids. A plot of the cosine of the contact angle versus the surface tension for a number of organic liquids gave a straight line, which, 0 1993 American Chemical Society
280 Ind. Eng. Chem. Res., Vol. 32,No. 2,1993
when extrapolated to cos 0 = 1 (Oo of contact angle), provided the critical surface tension, yc. 2.2. Effect of Roughness on Wetting. Wenzel(1936) expressed the effect of roughness on the contact angle by the roughness factor, fBl, given by fSl = A * / A = cos e*/cos e (4) or COS
e* = fsl COS e
(5)
where A* = surface area of a regicin of the rough surface, A = surface area which would be given if the region were smooth, B* = measured (or apparent) contact angle on a rough surface, and 8 = intrinsic contact angle on a smooth surface. Equation 5 indicates that there would be a decrease in an apparent contact angle, 8*, with increasing surface roughness when 0 < 90° and an increase in an apparent contact angle with increasing surface roughness when 8 > 90° (Cassi, 1948). 2.3. The Interaction of Liquids with Porous Materials. When the liquid movement is perpendicular to the plane of the fabric and penetrates through the face of the fabric, the measurement is referred to as demand wettability (Miller and Tyomkin, 1984). The development of methods for evaluation of such spontaneous uptake of liquids by fibrous materials has been reported by several workers (Buras et al., 1950; Lichstein, 1974;Miller and Tyomkin, 1984). The demand wettability test described by Miller and Tyomkin (1984)is the most recently improved approach. With a zero gravity head ( P h = 01,the spontaneous uptake rate of water by fabrics may be written as the following equation (Bryant, 1984):
where R = effective radius of the pore, q = liquid viscosity, and L = distance within the pore. From eq 6, the rate at which the liquid front advances is derived in ita integrated form. This equation was applied by Washburn to penetration of liquids into any porous body consisting of assemblies of cylindrical capillary tubes. Because of the complexity of the pore geometry in textile systems, this simple cylindrical tube model has also been used in the studies by Palmer (1953),Gillespie (1959),Swanson and Cordingly (1969),Laughlin and Davis (1960),Burgeni and Kapur (19671,Dyba and Miller (1969),Chwastiak (19731, Haynea and Dixon (1977),Madan et al. (1978),Minor and Schwartz (1980),Miller and Tyomkin (1984),Yang (1985), Kawase et al. (19861,and Hodgson and Berg (1988). Recently, Gupta (1988)showed that the estimated capillary flow rates of nonwoven fabrics based on the Washburn equation are well correlated to measured water uptake rates. Since the volume of penetrated liquid is directly proportional to penetration length, the volume at which liquid penetrates through porous channels can be written as
V" = KILn = Kzt
(7)
where V = volume of penetrated liquid, L = penetration length of liquid, t = time, n = 2, and K1 and Kz= constants; or
V = Kmt* (8) where m = l / n = 0.5. However, V = Kzt does not hold for liquid flow into textile systems (Carli and Simioni, 1979). An approximate flow law can be formulated by taking the logarithm of both sides of eq 8:
log V = m log K
+ m log t
(9) The plot of the penetration data (volume or length) versus time on a doublelogarithmicplot gives m,the slope of the h e a r portion of the curve. It is expected that the increase in water uptake as a function of time will follow a power law having the same exponent as that found in the height versus time relation. 2.4. Modification of the Surface Energy by FC Finishes. Water repellency is a condition of limited wettability. The repellency of textile materials depends upon their own chemical and physical properties as well as those of the contaminated or finished substances. To improve the water repellency of textile materials, the following conditions are required: (1)fabric treatment that gives as high a contact angle as possible (e > 90°) and as large a negative value of LV(defined below) as possible, (2)a fabric with low critical surface tension, and (3)fabric construction with minimum channel size. A porous substrate can be considered as a bundle of capillaries; the following equation describes capillary pressure, AZ? AP = (27,"COS e,)/R (10) or
AP = W
- %l)l/R
(11) To impart water repellency, the effective pressure gradient (AP)must be negative. According to eq 11, a negative PP is possible only if the valuea of ysvis smaller than ya. This can be achieved by adding a water repellent finish to the fabric. 2.5. Surface Modification by Corona Discharge Treatment. Little is known about surface modification of textiles by corona discharge treatments except that they increase the wettability of such hydrophobic polymers as polyethylene and polypropylene (Thorsen, 1971). Thorsen found that a corona discharge increases the wettability of wool by water and that this increase could be reversed by adsorption of a cationic surfactant. It is expected that anionic polar sites formed on a fiber surface as a result of corona discharge treatment would enhance the wettability of the fabric. S
"
3. Materials and Methods 3.1. Fabrics. Three types of nonwoven fabrics, m e k , Sontara, and SMS (spun-bondedlmelt-blown/spunbonded), were used in this study. Ten yards of each fabric was treated with a fluorochemical fiiish (Tyvek finished by the University of Tennessee-Knoxville, Sontara by Du Pont, and SMS by Kimberly-Clark). Two weights of 100% polypropylene SMS fabrics (61and 81 g/m2) were analyzed to determine the effect of fabric weight on wettability. Tyvek is Du Pont's registered trademark for a group of durable nonwoven produds made from 100% high density polyethylene fibers by an integrated spinning and bonding process. Tyvek has a very smooth surface and a light weight. The corona treatment of Tyvek is performed to improve ita ability to absorb finishes. Sontara is Du Pont's trade name for a spun-laced nonwoven fabric which is a composite of a dry laid web of polyester staple fibers and a layer of wood pulp fibers. The randomly oriented short wood pulp fibers on the face side of the fabric readily absorb water and chemicals such as repellent finishes. Polyester fibers on the back side of Sontara make the fabric more hydrophobic. Due to proprietary considerations it was not possible to obtain the complete specifications for these fabrics in terms of their processing conditions. However, it was possible
Ind. Eng. Chem. Res., Vol. 32, No. 2, 1993 281 Table 1. Formulation for Fluorochemical Finish chemical % deionized water 98.040 lithium nitrate 0.030 octanol 0.025 fluorochemical (3M FC-808) 1.207 hexanol 0.699
to obtain the formulation of the fluorochemical finish on the SMS fabrics (Table I). Ten samples of each fabric were utilized for each wettability test. All fabrics were extracted with benzene for 2 h in a Soxhlet extractor. Control samples included both extracted and unextracted samples. 3.2. Chemicals. The liquids used for the contact angle measurements were triple-distilled water, glycerol, formamide, ethylene glycol, methylene iodide, and pyridine. These liquids were selected because they provide a wide range of surface tensions and boiling points and because studies have provided information about their spreading behavior on solid surfaces (Fox and Zisman, 1952). All liquids were of spectrophotometric grade. The surface tensions, viscosities, and densities at 20 OC were assumed to be equal to those listed in the literature (Handbook of Chemistry and Physics, 1983) for the test liquids. 3.3. Wetting Force Measurements for Single Fiber Wettability. Using the Wilhelmy wetting method (Miller and Young, 1975),the advancing and receding forces were obtained through immersion and emersion of the fiber in and out of the liquid. The force increased as the fiber detached from the water surface; this point is called maximum wetting force. Assuming that the mean maximum wetting force corresponds to zero degrees, the advancing contact angle is calculated as described by Miller and Tyomkin (1984). cos 8, = Fw(a)/Fw(m) (12) where Fw(a) = wetting force for advancing, Fw(m) = maximum wetting force generated during pull out, and 8, = effective contact angles during movement of the liquid down the solid. All measurements were made in a conditioned room at 50% relative humidity and 70 OF. (This is the Tappi standard for paper products.) The results of this test are reported for the average 10 fibers pulled out from each fabric. 3.4. Demand Wettability. The demand wettability test as modified by Miller and Tyomkin (1984)was used in this investigation. The amount of weight loss indicated by the recording balance was recorded every 5 s for 1 min and then every 15 s up to 20 min. Experimental quantities measured were the uptake rate (AWIAt), the slope of linear portion of the weight-loss m e , the induction period before the flow of liquid started t(i), the time to reach 50% saturation t(50),and the total amount of liquid taken up AW(m). 3.6. Water Repellency Test: Static Absorption. AATCC Test Method 21-1983was applied for measuring the resistance of fabrics to wetting by water. This method is especially suitable for measuring the effectivenessof the water-repellent finishes applied to fabrics. 3.6. Application of Washburn Equation. The expected capillary flow rate based on eq 6 can be calculated using valuea of 72.8 X lo+' N/pm and 9.826 X lo4 N / b m e) for q, the advancing contact angles of single fibers determined from the Wilhelmy balance technique, and the pore size radius, R,described by Gupta (1988). The experimental values of the spontaneous uptake rate of the demand wettability test and of the static absorption test
Table 11. Contact Angle (degrees) of Individual Fibers from Nonwoven Fabrics sample fibe? polyethylene in regular Tyvek polyethylene in corona-treatedTyvek wood pulp fiber in regular Sontara wood pulp fiber in FC-finished Sontara polyester fiber in regular Sontara polyester fiber in FC-finished Sontara polypropylene in unfinished SMS (L) polypropylene in FC-finished SMS (L) polypropylene in unfinished SMS (H) polypropylene in FC-finished SMS (H)
before extraction after extraction face side face side back side 72 f 4 82+a 76+4 79 f 7
74f6
73f7
25 i 12
67 f 13
6
65 f 7
82 f 10
b
75 f 5
C
70 f 5
70 f 5
C
66 f 3
*4
103 f 10
106 f 5
91 f 9
105 f 13
99 f 15
102 f 14
80 f 12
96 f 17
104i 11
96f 8
94 f 3
90
"L, 61 g/m2; H,81 g/m2; FC, fluorochemical. *There were no wood pulp fibers on the back side. cThere were no polyester fibers on the face side.
were plotted against the expected capillary flow rates calculated by the Washburn equation. 3.7. Scanning Electron Microscope (SEM). Scanning electron photomicrographs were obtained for individual fibers and fabrics on a scanning electron microscope (Hitachi Model 5-570). 4. Results and Discussion 4.1. Wettability of Single Fibers. Table 11 shows the average advancing contact anglea for 10 fibers from the face and back side of each fabric subjected to extractions. It also contains data for the fiber samples from the face side of fabrics which were not subjected to extractions. A stick-alip wetting profile was noted for all the fibers as the liquid moved along the fiber surface. These scanning variations are most likely due to the presence of localized surface variations which make the wetting force either slightly larger or smaller than its average value. Therefore, the mean value of the trace was taken as the advancing force value for these scans,(Milleret al., 1983). Effect of Extraction. Analysis of variance showed that the effect of extractions was significant for the wettability of the following fibers: polyethylene from regular Tyvek, wood pulp from both unfinished and F C - f d e d Sontara, and polypropylene from unfinished heavy weight SMS. In contrast to the contact angle of the polypropylene fibers from heavyweight unfiihed SMS, which decreaaed in magnitude, the water contact angles for polyethylene fibers from regular Tyvek and for wood pulp fibers from both unfinished and FC-fiiished Sontara increased after extraction. Scanning electron micrographs (Figures 1-4) demonstrated that the extractions with benzene removed undesirable materials. The extracted surface of the fibers appeared to be cleaner and smoother. Thus the removal of surface contaminants and some of the repellent finish increased the wettability. Effect of FC Finish. The effect of the fluorochemical finish on the wettability of individual fibers was statistically significant for all nonwovens except lightweight SMS (61g/m2). The FC finish did not significantly influence the water contact angle of the polypropylene fibers from the lightweight SMS. However, the effect of the FC finieh on the water contact angle for polypropylene fibers from
282 Ind. Eng. Chem. Res., Vol. 32, No. 2, 1993 Table 111. Average cos 8, of Test Liquids from t h e Fabrics" liauid _____~ fiber no. WA FA 0.14 0.41 1 avg 0.12 0.13 std 0.30 0.44 3 avg 0.07 0.14 std 0.75 4w avg 0.49 0.10 std 0.10 0.57 4P avg 0.27 0.16 std 0.11 0.07 0.16 5w avg 0.05 0.12 std 0.39 0.45 5P avg 0.06 0.11 std -0.02 0.30 7 avg 0.09 0.14 std -0.19 0.22 8 avg 0.10 std 0.10 0.18 10 avg -0.11 0.10 std 0.11 -0.12 0.14 11 avg 0.09 0.05 std
for the 10 Fibers
~
EG 0.52 0.16 0.65 0.17 0.78 0.06 0.63 0.17 0.27 0.17 0.54 0.12 0.58 0.11 0.32 0.09 0.41 0.18 0.41 0.07
MI 0.70 0.15 0.77 0.05 0.60 0.09 1.00 0.00 -0.23 0.31 0.69 0.19 0.82 0.07 0.70 0.08 0.66 0.13 0.85 0.08
WA,water; FA, formamide; MI, methylene iodide; EG,ethylene glycol. 1, polyethylene from regular Tyvek; 2, polyethylene from FC-finished Tyvek; 3,polyethylene from corona-treated Tyvek; 4W, wood pulp from unfinished Sontara; 4P,polyester from unfinished Sontara; 5W,wood pulp from FC-finished Sontara; 5P, polyester from FC-finished Sontara; 7,polypropylene from unfinished S M S 8, polypropylene from FC-finished SMS; 10, polypropylene from unfinished SMS; 11, polypropylene from FC-finished SMS. (I
the heavyweight SMS (81 g/m2) was significant: it was significantly larger than that for polypropylene from the corresponding unfinished SMS. For unfinished SMS fabrics of both weights the difference between the water contact angle of the fibers from the face side and that of the fibers from the back side was not significant. Effect of Corona Discharge. Corona discharge significantly reduced the water contact angle of the polyethylene fibers from w e k and increased their wettability. There are two possible explanations for the above results: (1) after corona treatment, the increase in the roughness of the surface of polyethylene fibers decreased the water contact angle (Wenzel, 1936)and/or (2)corona treatment of polyethylene fibers produced polar sites on the outer surface of the fibers resulting in lower contact angles. 4.2. Surface Energy of Fibers. Critical Surface Tension of Fibers. For a fabric to be resistant to liquid penetration, the free energy of the fibers should be minimized. The critical surface tension for the fibers was obtained from Zisman plots, in which cos 0 was plotted against the surface tension of selected polar liquids. The average cos 8, of selected test liquids on fibers is shown in Table III. The advancing contact angle was determined by the Wilhelmy balance technique. Although yc is empirical in nature, it has been extremely useful in examining the wetting properties of many types of polymers, both finished and unfinished fibers. As can be seen by the extrapolated values for y c in Table IV, fluorochemical finishing resulta in a decrease in the critical surface tension of wood pulp fibers from 35.6 X lo4 to 16.7 X N/gm. This suggests that the FC-finished wood pulp fibers are much more difficult to wet, when compared with the unfinished wood pulp fibers, than would be expected. It is believed that the fluorochemical tails oriented themselves away from the wood pulp fibers to produce a very low surface energy barrier. Table IV also shows that the yc of the polypropylene fibers was not reduced by the fluorochemical finish. It is
Table IV. Components of the Surface Energy of Fibers" fiber no. ysdb yad/ys ySpb Y ~ ~ / Y ~ ~s b ~ c b 8.80 0.24 36.89 29.5 0.76 1 28.09 5.00 0.13 39.38 33.9 3 34.30 0.87 15.09 0.36 26.33 0.74 41.43 35.6 4W 1.99 0.04 53.29 29.5 4P 51.30 0.96 0.01 35.30 5w 0.25 0.99 35.55 16.7 5P 34.14 0.79 8.83 0.21 42.97 20.3 2.53 0.04 7 55.83 0.96 58.37 30.2 52.05 0.94 3.57 0.06 55.61 31.8 8 44.14 0.98 0.96 0.02 45.10 23.8 10 0.08 71.01 31.4 5.36 11 65.64 0.92 1, polyethylene from regular Tyvek; 3,polyethylene from corona-treated Tyvek; 4 W , wood pulp from unfinished Sontara; 4P, polyester from unfinished Sontara; 5W, wood pulp from FC-finished Sontara; 5P, polyester from FC-finished Sontara; 7, polypropylene from unfinished SMS; 8, polypropylene from FC-finished SMS; 10, polypropylene from unfinished SMS;11, polypropylene from FC-finished SMS. yap, polar force component of fibers;,:y dispersion force component of fibers; yo critical surface tension of fibers; ys, surface energy of fibers. bReported as Nlrm.
possible that close packing of the fluorine in the finish was not possible when applied to these fibers; therefore, underlying hydrophilic atoms in the fluorochemical formulation were exposed, preventing a reduction in the y c of polypropylene (Wadsworth et al., 1985). Although the unfinished polypropylene exhibited critical surface tension values similar to that of FC-finished polypropylene, it is apparent from Table 111that the wetting behavior is different. The unfinished polypropylene fiber has more affinity for the high surface energy liquids compared to the FC-finished polypropylene fiber. According to eq 3,the maximum interaction between the two phases occurs when the proportions of dispersion, polar, and hydrogen-bonding forces in each of the two phases are the same. It is likely that the changes in these parameters changed the value of yc. Dispersion and polar forces of the fibers were calculated and are discussed in the following section. It is also important to point out that Zisman plots were originally devised with nonpolar liquids, numerous other investigators have also noted deviations when using polar liquids for this type of analysis (Fox et al., 1953;Ellison and Zisman, 1954;Fowkes, 1962;Dettre and Johnson, 1964,1965;Johnson and Dettra, 1969;Owens and Wendt, 1969). Separation of Surface Energy into Components. The dispersion force component, yt, and polar force component, yIP,were obtained from eq 3 (Fox et al., 1953;Wu, 1971)and the contact angles of the above liquids on fibers. Table V lists the surface tension components for the fibers calculated from data using different pairs of liquids. The pairing is to show polar versus nonpolar contribution to surface free energy. Table IV summarizes the average total surface free energy, average dispersion force component, and average polar force component. The use of polar liquids to determine y c on nonpolar fibers led to a value less than ye. The interfacial tension of ys - y c had a positive value. Treatment with fluorochemical finishes decreased the dispersion force component ratio, y$/ya, and increased the polar force component ratio, ysP/ys. This finding supports the conclusions of Ellison and Zisman (19541,who have noted an increase in wettability by water or surfaces in the range of low atom percent fluorine substitution (0-25%). They explained the increase in the polar component on the basis of hydrogen bonding between water and the relatively high electronegative fluorine atom. They concluded that the electronegativity of the fluorine substituents will decrease with additional fluorine substitution.
Ind. Eng. Chem. Res., Vol. 32, No. 2,1993 283 Table V. Polar (7,') and Dispersion Force Components (7:) of Fibers in Various Combinations of Liquids" combination of liquids N/fim) fiberno. componenta WA/MI GL/MI FA/MI AVG 20.10 32.59 28.09 1 Ysd 31.58 18.27 2.82 8.80 Y2 5.30 48.38 58.66 43.91 2 Ysd 24.69 1.76 7.97 6.70 Yap 10.60 36.87 36.84 34.38 3 Ysd 29.43 2.18 2.14 5.00 Yap 10.70 20.16 8.31 15.09 4w Ysd 16.81 11.66 45.92 26.33 YSP 21.41 51.87 51.30 46.72 55.29 4P Ysd Y sp 4.90 0.21 0.86 1.99 0.21 0.25 0.28 0.24 5w Ysd 26.67 51.39 35.30 Y SP 27.85 52.32 28.79 34.14 5P Ysd 21.31 2.50 5.31 8.80 YSP 18.60 67.10 53.40 55.03 7 Ysd 47.01 6.60 0.53 2.53 YSP 0.47 64.37 45.72 52.05 8 Ysd 46.05 10.33 0.34 3.57 Y 0.04 43.87 44.14 49.47 39.09 10 YS 2.15 0.38 0.96 YSP 0.34 70.67 71.26 65.64 11 Ysd 54.99 7.76 8.28 5.36 YSP 0.06 WA, water; GL, glycerol; FA, formamide; MI, methylene iodide. 1, polyethylene from regular Tyvek; 2, polyethylene from FC-finished Tyvek; 3,polyethylene from corona-treatedTyvek; 4 W ,wood pulp from unfinished Sontara; 4P,polyester from unfinished Sontara; 5W, wood pulp from FC-finished Sontara; 5P,polyester from FC-finished Sontara; 7, polypropylene from unfinished SMS; 8, polypropylene from FC-finished SMS; 10,polypropylene from unfinished SMS; 11, polypropylene from FC-finished SMS. Table VI. Demand Wettability Test on the Face Side of Nonwoven Fabrics"
1 f 0.5 regular T y e k corona-treated Tyvek 1 0.5 1 f 0.5 regular Sontara FC-finished Sontara 5 f 2 unfiniehed SMS (L) 5 * 2 FC-finished SMS (L) unfinished SMS (H) 5 f 2 FC-finished SMS (H) 3 f l
10 f 5 63.0 f 12.0 5f2 105.6 f 14.1 15 f 5 599.1 f 75.7 330 45 37.4 f 3.0 330 60 9.4 f 6.3
-
* *
330 f 60 210 f 45
-
9.8 24.7
* 5.0 * 3.0
3.2 f 1.2 10.6 f 2.9 20.0 f 0.2 0.6 & 0.1 0.1 0.0
-
*
0.1 f 0.0 0.6 f 0.1
t(i), induction period before flow of liquid starts, t(60), time to reach 60% saturation; AW(m), total amount of liquid taken up; AW/At, spontaneOu rate of water uptake; L, 61 g/m2; H,81 g/mZ;FC, fluorochemical; -, no spontaneou water uptake observed.
Owens and Wendt (1969) supported this conclusion, but they suggested that the hydrogen-bonding ability of fluorinated polymers does not decrease until the fluorine content exceeds 50 atom %. 4.3. Demand Wettability of Fabrics. Effect of Finish. Table VI contains the averages and standard deviations for the demand wettability test on the nonwoven fabrics. The corona-treated Tyvek fabric exhibited greater total water uptake than regular Tyvek. This result is consistent with the finding that the water contact angle of the corona-treated fibers was smaller than that of the regular Tyvek. The water uptake for the unfinished Sontara fabric differed from the other fabrics in that the initial uptake with the Sontara fabric could not be clearly determined because of the very rapid penetration of water. This is probably due to the combination of hydrophilic wood pulp fibers on the face side of the Sontara fabric and the continuous capillary structure formed from the smooth polyester fibers on the back side of the fabric. After the very rapid initial absorption rate, a nearly constant, steady rate was observed. Most of the liquid flow phenomena for the
Sontara fabrics occurred in less than 15 s. The unfinished Sontara was the most absorbent material among the nonwoven evaluated in this study. Therefore, this fabric would be the least appropriate as a protective material against liquid penetration. The effect of the FC finish on the total water uptake was significant for the Sontara fabric. For spontaneous liquid movement, the conventional understanding is that the contact angle of the fiber should be less than 90°. For fabrics which are composed of fibers having large advancing contact angles (0 > 90'1, the level of water penetration into pores is very small although these microamounts are detectable by the demand wettability test. However, as Miller and Tyomkin (1984) pointed out, the water penetration need not be restricted to contact angles less than 90' when the pore wells are curved. Both FC-finished and unfinished SMS fabrics exhibited the longest induction period and lowest uptake rates under all conditions. However, the demand wettability test showed that all SMS fabrics continued to increase in water uptake over time. This is possibly due to the continuous water migration through the finer capillaries in the melt-blown layer of the SMS fabrics. As the scanning electron micrograph shows, the open capillary spaces of the melt-blown layer can be directly contacted by water. The average total water uptake of the unfinished SMS of 61 g/m2 was higher than that of FC-finished SMS of the same weight. The data obtained from the demand wettability test showed that the water repellency of the control unfinished SMS (81 g/m2) was greater than that of the FC-finished SMS of the same weight; however, the of individual polypropylene fibers from the SMS fabric (81 g/m2) was not significantly reduced by the FC finish. There are two possible reasons for the above results. 1. The mechanical forces imposed during finishing may have increased surface nonuniformity of the fibers (Wadsworth et al., 1985), thereby resulting in a greater tendency to take up water. 2. The pressure applied during the fmishing of the fabric may have increased the exposed area of the melt-blown layer, which is primarily responsible for water uptake in the fabric structure. Thus, FC-finished SMS (81 g/m2) absorbed more water than the corresponding unfinished SMS even though the water contact angle for the individual fibers from the finished SMS (81 g/m2) was significantly lower than for those from the corresponding unfinished SMS. Minimizing the tension on the fabrics and the pressure applied to the pad during the FC-finishing process could produce a more desirable level of water repellency. The total water uptake by FC-finished SMS of 81 g/m2 was significantly larger than that of FC-finished SMS of 61 g/m2. Corresponding results were obtained for unfinished SMS: the water uptake for unfinished SMS of 81 g/m2 was also larger than that of 61 g/m2 fabric. This is probably the result of a greater pore volume in the heavier weight fabric. However, further studies are recommended before making definite conclusions concerning the effect of the weight of the fabrics on water repellency. It was also noted that the thickness of the SMS fabrics of both weights was decreased by FC finishing. The demand wettability test indicated a higher degree of wettability on the back side of both Tyvek and SMS fabrics compared to the face side of the same fabrics. The ease of water penetration into the back side compared to the face of the same fabric is explained by the fact that repellent finishes were only applied to the face side. Differences in the geometries of the back and face sides are another factor
284 Ind. Eng. Chem. Res., Vol. 32,No. 2, 1993 Table VII. Static Absorption Test Data (70Water AbsorDtion) of Nonwoven Fabricsa % water absorption extracted unextracted fabric 6.1 avg 17.5 regular Tyvek std 0.2 4.3 8.4 avg 22.7 corona-treated Tyvek 2.6 std 5.7 avg 48.8 59.2 regular Sontara 3.3 std 3.2 20.3 avg 33.6 FC-finished Sontara 2.6 std 0.8 2.5 avg 11.3 unfinished light SMS 1.1 std 2.9 avg 14.8 8.2 FC-finished light SMS 1.6 std 3.9 2.0 avg 10.7 unfinished heavy SMS std 1.2 0.3 15.4 0.0 FC-finished heavy SMS avg 0.0 std 3.8 OAvg, average of 10 samples; std, standard deviation of 10 samples.
which could influence demand wettability. This geometry difference depends on the fabric structure, and it is responsible for the differences in pore volume and size distribution for each side of the Tyvek and SMS fabrics. Under the scanning electron microscope the back sides of all the fabrics tested exhibited relatively more continuous capillaries than the face sides. Therefore, the back sides of fabrics probably hold more water and exhibit faster spontaneous water uptake rate than the face sides. Effect of Fiber Radius. The Wilhelmy balance technique was used for determination of all single-fiber radii. The radius of the polypropylene fibers in the melt-blown layer and thme in the spun-bonded layer of the SMS fabric ranged from 0.5 to 2.0 pm and 6.0 to 11.0 pm, respectively. The radius of the wood pulp fibers in the Sontara fabric ranges from 6.0 to 20.0 pm. The radii of the polyester fibers in the Sontara fabric ranged from 6.0 to 10.0 pm while the radius of polyethylene fibers in the W e k fabric ranged from 4.0 to 16.0 pm. The fiber radii observed by the SEM study agreed well with the values obtained by the Wilhelmy balance technique. SMS,having fine fibers and closely packed construction, exhibited a very slow water uptake rate in the demand wettability test. On the other hand, unfinished Sontara, made from fibers with a larger radius and hence a larger interfiber capillary radius, exhibited the highest rate of water uptake. These results are consistent with the Washburn equation, which predicts that the larger the fiber and corresponding pore size, the larger the water uptake of the fabrics (assuming the other factors are equal). 4.4. Static Absorption Test. Table VI1 shows the results of the static absorption test for the nonwoven fabrics. Since the FC finishes were applied to only one side of the fabrics, the static absorption test offered only a crude estimation of the effect of FC finishes on the wettability of the test fabrics. However, the general trends for the wettabilities agreed well with the results for the maximum amount of water uptake in the demand wettability test. The average percentages of water uptake for the extracted control fabrics increased in the following order: unfinished Sontara > FC-finished Sontara > coronatreated Tyvek > regular Tyvek > FC-finished Tyvek > FC-finished SMS of 81 g/m2 > FC-finished SMS of 61 g/m2 > unfinished SMS of 61 g/m2 > unfinished SMS of 81 g/m2. On the basis of the results of the static absorption
Table VIII. Pore Radius and Expected Rate of Capillary Flow Based on the Washburn Equation’ pore radius rete, L 2 / t (rate)l’*,L / t 1 / 2 extracted fabric (urn) (um/s) (.X I 0 .um/e1/2) .. . .. , . , I regular Tyvek 2.24 f 1.30 1.10f 0.64 1.05 f 0.80 corona-treated Tyvek 1.52f 0.61 1.59 f 0.64 1.26 f 0.80 regular Sontara 2.25 f 1.24 3.43 f 1.05 5.06 f 1.55 FC-finished Sontara 1.90f 0.62 0.98f 0.32 0.99 f 0.56 unfinished SMS (L) 1.50f 0.43 [-1.221 f 0.35 [-1.111 0.59 FC-finished SMS (L) 0.95 f 0.24 [-1.801 f 0.61 [-1.341f 0.78 unfinished SMS (H) 1.51 f 0.65 0.60f 0.15 0.77f 0.39 FC-finished SMS (H) 1.52 f 0.65 [-0.61]f 0.26 [-0).78] f 0.51
*
a
The negative values in [ ] indicate that the liquid goes down instead
of moving up.
Table IX. Correlation Coefficients for Wettability Data conditiona correln coeff pore radius vs SAT absorption 0.832 pore radius vs DWT total uptake 0.896 pore radius vs DWT uptake rate 0.788 theor rate vs SAT % water absorption 0.855 theor rate vs DWT total uptake 0.890 theor rate vs DWT uptake rate 0.889 SAT vs DWT total uptake 0.862 SAT vs DWT uptake rate 0.760 a
SAT, static absorption test; DWT, demand wettability test.
test, neither FC finishing nor fabric weight improved the water repellency of the SMS fabric. 4.5. Application of Washburn’sEquation. The data obtained from the combined teats were plotted against the predicted capillary flow rate calculated by Washburn’s equation. Table VI11 shows the average and standard deviation of the pore radius and the theoretical rate of capillary flow from the Washburn equation. Table IX contains the correlation coefficients for the linear relationships between the expected capillary data and the experimental wetting data. As seen in Table IX, an increase in the fabric pore radius corresponded to an increase in the fabric wettability. Although the other parameters were not constant, it appears that the pore radius, proportional to the fiber radius as determined from the equation developed by Gupta (1988), may be used as a relative index of wettability of nonwoven fabrics when other parameters are known. The experimental wettabiliw data for the control fabrics also correlated with the theoretical rates of capillary flow derived from Washburn’s equation. 4.6. Scanning Electron Microscopy (SEM). The SEM micrographs (Figure 1) show that the polyethylene fibers from the Tyvek fabric exhibited many ruffles on their surface and the frequency increased with the corona treatment. It is possible that the temperature of the electron beam used for examination by the scanning electron microscope was high enough to cause the ruffing of the polyethylene fiber walls. A review of the literature indicates that the effect of heat from an electron beam on the appearance of polyethylene fibers has not received much attention. However, this observation on polyethylene fibers is in general agreement with Tripp et al. (1954),who found that heating cotton fibers at high temperatures for long periods caused a ruffled appearance on the primary wall. Another possible reason is that the Tyvek fabrics had been over-dried during manufacturing. The ruffles on the fiber surface may affect the wettability of fibers as well as the fabrics. Figure 2 contains scanning electron micrographs of the wall of the pulp fiber. Pita and scales are exhibited on the fibers of both unfinished and FC-finished Sontara fabrics.
Ind. Eng. Chem. Res., Vol. 32. No. 2,1993 286
d Figure 1. SEM micrograph of the wall of a polyethylene fiber in regular (a, top) and corona-treated (b, hattom) Tyvek fabric.
However, Fctinished wccd pulp fibers exhibited relatively smoother surfacea than the unfinished pulp fibers. Costing with the FC finish on the rough surface of these fibers concealed the fibrils, which were detected on the surface of the unfinished fiber. Under the scanning electron microscope both unfinished and FC-finished wood pulp fibers exhibited cracks in their periphery. In addition to numerous ridges and pita, these cracks may affect the wettability characteristics of the wood pulp fibers. Figure 2b shows the primary wall from a FC-finished wood pulp fiber after extraction with benzene at 70 'C for 2 h. It is obvious that noncellulosic materials were removed by the extraction procedures. The rougher surface of the primary wall was exposed by the extraction with benzene. This cellulosic structure has been described hy numerous other investigators (Tripp et al., 1954,1957; Goynes and Rollins, 1971). Parta a and b of Figure 3 are scanning electron micrographs of polyester fibers obtained from the back side of unfiished and FC-finished Sontara fabrics, respectively. The appearance of the polyester fiber on the back side of FC-fmished Sontara is similar to that of the polyester on the back side of the unfinished Sontara. Although the scanning electron micrographs did not show a significant difference in the surface appearance of the polyester fibers, this is not necessarily an indication of the absence of FC finishes on the fiber surfaces. As can be seen in Figure 4b, which illustrates a typical surface of a polypropylene fiber from a FC-fmished SMS fabric, the FC finish also did not change the surface ap-
Figure 2. SEM micrograph of the wall of B wood pulp fiber in (a. top) and FC-finished (b, bottom) Sontara fabric.
unfinished
pearance of polypropylene fibers. Obviously the FC finishes change the surface energy of the fibers without a visible effect on the topography of fiber. Polypropylene fibers from unfmished (Figure 4a) and those from FCfinished (Figure 4b) fabrics appear identical in the scanning electron micrographs. These fibers have smooth surfaces and closely resemble the surfaces of polyester fibers in the Sontara fabric. 5. Conclusions
1. The corona treatment applied to the Tyvek fabric resulted in an increase of the water absorption by a p proximately 30% over the untreated fabric. 2. The face sides of all fabrics which received a fluomehemid f d exhibited more water repellency thanthe back sides of the same fabrics. 3. Both fluorwhemical-finished and unfinished SMS fabrics had superior resistance to water penetration among the tested nonwovens. 4. Both lightweight (61 g/m2) and heavyweight (81 g/m2) SMS fabrics provided good resistance to water penetration. While the FC fmish was effective in reducing the water absorption of the lightweight SMS fabric, it was not effective in reducing the water absorption of the heavyweight SMS fabrics. These resulta indicate that it is not beneficial to finish heavyweight SMS fabrics because liquid penetration was not decreased by the FC f ~ For h the heavyweight SMS fabric an interaction of variables
286 Ind. Eng. Chem. Res., Val. 32,No. 2, 1993
I Figure 3. SEM of the surface of polyester fibers in unfinished (a, top) and FC-finished (b, bottom) Sontara fabric.
Figure 4. SEM of the surface of palypropylene fibers in unfinished
may have occurred, reducing the effect of the finish. The heavyweight fabric had a different thickness and pore size distribution compared to the lightweight SMS, which certainly influenced water penetration. 5. Unfinished Sontara fabric showed the greatest wettability and the least water repellency. However, the FC finish proved more effective for promoting water repellency with Sontara than with SMS and Tyvek fabrics. The FC finish reduced penetration of water through the Sontara fabric up to 25 times. 6. The total water absorption, the rate of spontaneous water uptake, and the percentage of water absorption of the nonwoven fabrics were proportional to the calculated pore radius. The pore radius is proportional to the fiber radius assuming maximum packing of fibers in fabrics. 7. Data obtained in the demand wettabfity test and the static absorption water repellency test correlated well with the Washburn model, making the relative wettability predictable. 8. The large fiber radius and low water contact angle of the wood pulp fiber provided the unfinished Sontara fabric with a high absorbency and therefore less water repellency than any of the other nonwovens tested; conversely, the low micrometer size of the fiber radius and the high water contact angle of the polypropylene fibers in the SMS fabrics provided less water penetration and neater water repellency than any of the other nonwovens tested. 9. On the basis of the total resulta of the unfuished and finished fabrica, it ean be concluded that the initial ability of a hydrophobic fabric to prevent liquid penetration is influenced more by the fabric structure than by the critical
surface energy of the individual fibem.
(a, top) and FC-finished (b, bottom) fabric.
Acknowledgment The authors wish to express sincere gratitude to the members of the CSRS Southern F&gional Project (S 208) Committee for their help and stimulating discussions. Funding for reaearch was provided by the USDA, Southern Regional Hatch Project S 208. This contribution is indeed appreciated. Literature Cited Bryant, G. M. Dynamic Sorption of Semistable Foams by Fabrics, Part I: Implications for Textile Foam Application Proeeasea. Text. Res. J . 1984.54, 211-226. Bursa. Jr., Edmund. M.; Golethwait, C. F.; Kraemer, R. M. Messurement and Theory of Absorbency of Cotton Fabrim. Text. Res. J. 1950. 20, 239-248. Burgeni, A. A.; Kaput, C. Capillary Sorption Equilibria in Fiber Masses. Text. Res. J. 1967,37,356366. Carli, F.; Simioni, L. Limitations of the Washbwn Equation in Quantifying Penetration Rates. Phannaeeutid TeehnolCgy Laboratory, Carlo ERBA Research Institute: Via Imbonati, 24. Milan, Italy, 1979. Cassi, A. B. D. Contact Angles. Discrura. Faraday Sac. 1948, No.3, 11-16.
ChwaatiaL, S. A W i c N Method for Measuring Wetting Properties of Carbon Y m . J. Colloid Interface Sei. 1973,42 (2), 298-309. Dettre. R. H.;Johnson, Jr., R. E. Contact Angle Hysteresis 11. Contact Angle Measurements on Rough Surfaces. In Contact Angle, Wetrobility, and Adhesion;AdvanQe in Chemistry Series 43; Could, R. F.. Ed.; American Chemical Society: Washington, DC. 1964: pp 112-135.
Ind. Eng. Chem. Res. 1993,32, 281-293 Dettre, R. H.; Johnson, Jr., R. E. Contact Angle Hysteresis. IV. Contact Angle Measurements on Heterogeneous Surfaces. J. Phys. Chem. 1965,69, 1507-1515. Dyba, R. V.; Miller, B. Evaluation of Wettability from Capillary Rise Between Filaments. Text. Res. J. 1969,39,962-970. Ellison, A. G.; Zisman, W. A. Wettability Studies of Nylon, Polyethylene Terephthalate and Polystyrene. J. Phys. Chern. 1954, 58,503-506.
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Fox, H. W.; Zisman, W. A. The Spreading of Liquids on Low-Energy Surfaces. 11. Modified Tetrafluoroethylene Polymers. J.Colloid Interface Sci. 1952, 7, 109-121. Fox, H. W.; Hare, E. F.; Zisman, W. A. The Spreading of Liquids on Low-Energy Surfaces. VI. Branched Chain Monomers, Aromatic Surfaces, and Thin Liquid Films. J. Colloid Interface Sci. 1953, 8, 194-203.
Gillespie, T. The Capillary Rise of a Liquid in a Vertical Strip of Filter Paper. J. Colloid Sci. 1959, 14, 123-130. Goynes, W. R.; Rollins, M. L. A Scanning Electron-Microscope Study of Washer-Dryer Abrasion in Cotton Fibers. Text. Res. J. 1971, 41, 226-230.
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Wettability of Nonwoven Fabrics. 2. Effect of Cationic Surfactant Treatment A. Majid Sarmadi,* Young Ah Kwon,and Raymond A. Young Department of Environment, Textiles a n d Design, School of Family Resources a n d Consumer Sciences, University Of Wisconsin-Madison, 1300 Linden Drive, Madison, Wisconsin 53706
This study evaluated the effect of cationic surfactant treatments on the wettability of fluorochemicaland corona-treated nonwoven fabrics. Three types of nonwovens (Tyvek, Sontara, spun-bonded/melt-blown/spun-bonded)and two types of cationic surfactants (hexadecyltrimethylammonium bromide and octadecyltrimethylammoniumbromide) were evaluated in this work. On the basis of measurements of contact angle, demand wettability, and static absorption, surfactant build-up on the fiber surface reduced the water repellency of fabrics. 1. Introduction
Agricultural workers require protection from harmful therefore,proper be used during since these garment materials must primarily and physical nature of the fibrs, water, the fabric construction, finishing, and laundering should be
* Author to whom correspondence should be addressed.
evaluated for their effect on water uptake. In part 1 of this series (Sarmadi et al., 1993) the influence of fluorochemical finishes on liquid penetration of nonwoven fabrics was examined. In this study, the effect of cationic surfactant treatments on the wettability of nonwoven fabrics is evaluated. Althounh cationic softeners are used after washing for several p&poses, their effect on the wettability of fabics has not yet been totally clarified. Evans (1969) haa shown
088S-5885/93/2632-0281$04.00/00 1993 American Chemical Society
