Everything you need to know about absorbency for diapers.

Absorbency is probably the most talked about and most misunderstood part of cloth diaper making, hopefully this article simplifies it a little.

For those of us in the cloth diaper business, the holy grail is to find reusable materials that can offer the trimness and capacity of the gel+wood fluff material used in disposable diapers.  With new materials and combinations of fabrics we can now approach the disposible diaper in these 2 critical areas.  Before we get to explaining how, let's explore absorbency a little 

First, there are 2 critical factors to consider and evaluate:

1) Absorbency rate - how fast a material mops up moisture
2) Absorbent reserve - the amount of moisture a material will hold when saturated under normal working conditions

Absorbency Rate.

A full assault on a diaper can be 70 to 150ml of liquid, so for a soaker to work perfectly that amount of moisture must be absorbed within a few seconds otherwise moisture will flow off the soaker area either to the cuff of the diaper or straight through to the barrier layer.

Fabrics that have a lot of loft and space between fibers are the best at absorbing quickly because they trap moisture between the fibers before the fibers themselves are become saturated.  To illustrate this point let's compare a sheet of paper towel and a sheet of laser printer paper.  Both are made of the same fiber, both have the same amount of fiber, both will have absorbed the same amount of material when fully saturated.  But we all know the paper towel will blot the moisture up fast -- that's simply because it has space to trap the moisture between the fibers until the fibers wet out. Same goes for quilting cotton and flannel, cotton jersey and cotton fleece.

Fabrics with a high loft such as Zorb and microfiber terry toweling absorb very fast because they maintain their loft and provide lots of space between fibers to trap moisture.  Fleeces, Sherpa and terries also maintain their loft well when wet and absorb quickly.  Velours and flannels lose their loft instantly when wetted making them the lowest performers

Absorbent reserve.

Another important factor is the rate at which a fiber wets out.  Wetted fibers offer the best moisture holding power because the moisture is trapped inside the fiber as well as in the spaces between , WTo simplify the concept let's pretend a cotton fiber is a strand of spaghetti.   When you make spaghetti  you're simply wetting out the noodle -- it's done when it's absorbed water through to the core.  Like spaghetti, absorbent fibers take a little time to completely wet out.  They also wet out at different rates, making some better than others when used for absorbency. Of the popular fibers used in diaper making, viscose of Bamboo, cotton and hemp are absorbent --  microfiber is not so it

 

So, the rule of thumb here is the more lofty the fabric, the faster it will absorb. 

 

Absorbent reserve is the amount of moisture retained under normal working conditions. 

 

 

the amount of moisture a material will hold when saturated under normal working conditions

Reserve

 

 

of a sheet of printer paper and a  out.   be made to absorb quickly

There are 2 components to absorbency

that of fluddabsorbency and trimness Speed of Absorption

Distribution

Holding Capacity

One of the major applications of  Zorb is in absorbent products ranging from baby diapers, personal hygiene and adult incontinent products, and for industrial applications like air filtering, spill absorbers, and a variety of sanitary cleaning products.  The key technical benefits of Zorb are:

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. The absorbent capacity is mostly determined by the space between the fibers, and the resiliency of the fabric web in a wetted state. The absorbency rate is determined by the fabric's capillary action offset by the frictional drag of the fiber surfaces. Non-swelling fibers like polyester offer exceptional capillary action and low drag, so these fibers are great for distributing moisture through the fabric.  Swellign fibers, like cotton, bamboo and hemp, slightly increase the absorbing capacity and greatly improve the holding capacity when a fabric is under the pressure.   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.

 

The type of the fibers in the fabrics, hydrophilic (absorbs and swells: cotton, bamboo, hemp) or hydrophobic (does not absorb or swell: polyester, nylon, influences the inherent absorbent properties of the fabrics.  A hydrophilic fiber increases the capacity when it swells, capturing  moisture inside the fiber.  It also attracts and holds liquid on the outside the fiber, in capillaries, and the structural voids of the fabric. On the other hand, these properties reduce moisture distribution through the fabric and these fibers generally collapse the fabric web, so the ability to hold moisture in the fabric web is greatly diminished once these fabrics are saturated. 

 Hydrophobic fibers do not absorb moisture into their fibers,  They do however  he 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].