Absorption PROPERTIES
Updated:
April, 2004 - M. G. Kamath, Atul Dahiya, Raghavendra R.
Hegde
(Haoming Rong)
1. INTRODUCTION
One of the major applications of disposable nonwovens is in absorbent
materials, which constitute a broad range of products, ranging from baby
diapers, personal hygiene and adult incontinent pads to tampons, paper towels,
tissues and sponges. Fig. 1 shows the anatomy of a diaper where the
key requirement for absorbent materials at the cover sheet is its ability to
imbibe rapidly and hold large amount of fluid under pressure. Absorbency
rate and absorbent capacity are the two most important performance parameters
to be considered for absorbent applications of nonwovens. The absorbent
capacity is mainly determined by the interstitial space between the fibers, the
absorbing and swelling characteristics of the material and the resiliency of
the web in the wet state. The absorbency rate is governed by the balance
between the forces exerted by the capillaries and the frictional drag offered
by the fiber surfaces. For non-swelling materials, these properties are largely
controlled by the capillary sorption of fluid into the structure until
saturation is reached [1]. The
absorbency rate and absorbent capacity are affected by fiber mechanical and
surface properties, structure of the fabric (i.e., the size and the orientation
of flow channels), the nature of fluids imbibed, and the manner in which the
web or the product is tested or used [2-7]. Among those factors, the surface
wetting characteristics (contact angle) of the fibers in the web and the
structure of the web, such as the size, shape, orientation of
capillaries, and the extent of bonding, are most important.
Fig.
1: Anatomy of Diaper [10]
The polymer type of the fibers in the fabrics, hydrophilic or
hydrophobic, influences the inherent absorbent properties of the fabrics. A
hydrophilic fiber provides the capacity to absorb liquid via fiber imbibitions,
giving rise to fiber swelling. It also attracts and holds liquid external to
the fiber, in the capillaries, and structure voids. On the other hand, a
hydrophobic fiber has only the latter mechanism available to it normally [7].
The effect of the small amount of fiber finish (generally 0.1 to 0.5% by
weight) is also important since it is on the fiber surface. The particular
finish applied on the fiber can significantly change surface wetting property
of the fiber.
Fiber linear density and its cross-section area affect void volume,
capillary dimensions and the total number of capillaries per unit mass in the
fabrics. Fiber surface morphology, surface ruggedness, and core uniformity can
influence the absorbency performance to some extent. Fiber crimps influence the
packing density of the fabrics and further affect the thickness per unit mass
that affects the absorbency of the nonwoven fabrics. The nature of the crimps,
whether it is two-dimensional or three-dimensional, also has some effect.
The size of capillaries is affected by the thickness per unit mass and
the resiliency of the web, and the size, shape and the mechanical properties of
the fibers. The resiliency of the web is influenced by the nature and level of
bonding of the fabrics as well as the size, shape, and mechanical properties of
the constituent fibers [6].
2. MODELS & EQUATIONS
Models have been built to characterize the two parameters, absorbent capacity (C) and absorbency rate (Q). C (cc/g fluid/g) is given by the volume/mass of fluid absorbed at equilibrium divided by the dry mass of the specimen, while Q is given by the slope of the absorbency curve divided by the dry mass of the specimen. The model to calculate C is based on determining the total interstitial space available for holding fluid per unit dry mass of fiber. The equation is shown as follow [5,6]:
.........(1)
Where, A is the area of the web
T is the thickness of the web
Wf is the mass of the dry web
rf is the density of the dry fiber
Vd is the amount of fluid diffused into the structure of the fibers
a is the ratio of increase in volume of a fiber upon wetting to the volume of fluid diffused into the fiber.
In the above equation, "the second term is negligible compared to the first term, and the third term is nearly zero if a fiber is assumed to swell strictly by replacement of fiber volume with fluid volume" [6]. Thus, the dominant factor that controls the fabric absorbent capacity is the web thickness per unit mass on dry basis (T/Wf).
For
absorbency rate, the Washburn-Lucas's equation [8,9]
is applied.
........(2)
Where, S is the distance through which the fluid penetrated in time t
r is the mean pore radius of the capillary
gl is the surface tension of the fluid
q is the contact angle of the fiber
h is the viscosity of the fluid
t is the fluid penetrated time
Modifications are given to Washburn-Lucas's equation when applied to
the nonwoven webs in which the fluid spreads radially outward from a point in
the center. The modified equation is shown as follow:
...... (3)
Where, r is the mean pore radius of the capillary
gl is the surface tension of the fluid
q is the contact angle of the fiber
h is the viscosity of the fluid
T is the thickness of the web
Wf is the mass of the dry web
A is the area of the web
rf is the density of the dry fiber
In a given web and fluid system, only mean pore radius r and thickness per unit mass (T/Wf) in above equation are not constant. Predicted the value of r by the following equation based on the assumption that a capillary was bound by three fibers, oriented parallel or randomly, and the specific volume of the capillary unit cell equaled that of the parent web [3].
............(4)
for
,
Where the subscripts 1 and 2 represent different fiber types and
x is a constant with a value of 9x105
d is fiber denier
r is fiber density (g/cc)
f is mass fraction of a fiber in blend (f1 + f2 = 1)
REFFERENCES
- L. F. Fryer, B. S. Gupta, Determination of Pore Size Distribution in Fibrous Webs and Its Impact on Absorbency, "Proceedings of 1996 Nonwovens Conference," 1996, pp. 321-327.
- Chatterjee, P. K., "Absorbency,"
- Gupta, B. S., The Effect of Structural Factors on Absorbent Characteristics of Nonwovens, Tappi J. 71, 147-152 (1988).
- Gupta, B. S., and Crews, A. L., Nonwoven: An Advanced
Tutorial, "The Effect of Fluid Characteristics in Nonwovens," TAPPI Press,
- Gupta, B. S., and Hong, C. J., Changes in Dimensions of Web During Fluid Uptake and its Impact on Absorbency, Tappi J. 77, 181-188 (1994).
- Gupta, B. S., Whang, H. S., Capillary Absorption
Behaviors of Hydroentangled and Needlepunched Webs of Cellulosic Fibers,
"Proceedings of INDA-TEC 96: International nonwovens conference,"
- Gupta, B.S., and Smith, D. K., Nonwovens in Absorbent Materials, Textile Sci. and Technol. 13, 349-388 (2002).
- Lucas, R., Kolloid Z., "Ueber das Zeitgesetz des Kapillaren Aufstiegs von Flussigkeiten," 23, 15 (1918).
- Washburn, E.W., The Dynamics of Capillary Flow, Phys. Rev. 17(3), 273 (1921).
- Gupta, B. S. and L. C. Wadsworth, "Differentially Absorbent Cotton-Surfaced Spunbond Copoplyester and Spunbond PP with Wetting Agent," Proceedings , Seventh Nonwovens Conference at 2004 Beltwide Cotton Conferences, San Antonio , TX , January 5-9, 2004 .