Assignment: DIAPERS
Submitted to: Sir TOUCIF sb
DISPOSIBLE DIAPERS
A disposable diaper consists of an absorbent pad sandwiched between two sheets of nonwoven fabric. The pad is specially
designed to absorb and retain body fluids, and the nonwoven fabric gives the diaper a comfortable shape and helps prevent
leakage.
The earliest disposables used wood pulp fluff, cellulose wadding, fluff cellulose, or cotton fibers as the absorbent material. These
materials did not absorb very much moisture for their weight, however. Consequently, diapers made from these materials were
extremely bulky. More efficient absorbent polymers were developed to address this issue.
Raw material
Absorbent Pad:
The single most important property of a diaper, cloth or disposable, is its ability to absorb and retain
moisture. Cotton material used in cloth diapers is reasonably absorbent, but synthetic polymers far exceed the capacity of
natural fibers. Today's state-of-the-art disposable diaper will absorb 15 times its weight in water. This phenomenal absorption
capacity is due to the absorbent pad found in the core of the diaper. This pad is composed of two essential elements, a
hydrophilic, or water-loving, polymer and a fibrous material such as wood pulp. The polymer is made of fine particles of an
acrylic acid derivative, such as sodium acrylate, potassium acrylate, or an alkyl acrylate. These polymeric particles act as tiny
sponges that retain many times their weight in water. The absorbent pad is formed on a movable conveyer belt that passes
through a long "forming chamber." At various points in the chamber, pressurized nozzles spray either polymer particles or
fibrous material onto the conveyor surface. The bottom of the conveyor is perforated, and as the pad material is sprayed onto
the belt, a vacuum is applied from below so that the fibers are pulled down to form a flat pad.
Nonwoven:
The absorbent pad is at the core of the diaper. It is held in place by nonwoven fabric sheets that form the body of the diaper. A
dry laid process, such as the "meltblown" method, is typically used to make nonwoven diaper fabrics. Polypropylene is typically
the material used for the permeable top sheet, while polyethylene is the resin of choice for the non-permeable back sheet.
Meltblown Method:
A typical MB process consists of the following elements: extruder, metering pumps, die assembly, web formation, and winding.
Submitted by: Engr. M.Hassan Rafe
013-NTU-6002
Assignment: DIAPERS
Submitted to: Sir TOUCIF sb
Extruder:
It consists of a heated barrel with a rotating screw inside. Its main function is to melt the polymer pellets or granules and feed
them to the next step/element. The forward movement of the pellets in the extruder is along the hot walls of the barrel
between the flights of the screw. The melting of the pellets in the extruder is due to the heat and friction of the viscous flow and
the mechanical action between the screw and the walls of the barrel. The extruder is divided in to three different zones,
i)
Feed Zone: In the feed zone the polymer pellets are preheated and pushed to the next zone.
ii)
Transition Zone: The transition zone has a decreasing depth channel in order to compress and homogenize the
melted polymer.
iii)
Metering Zone: This is the last zone in the extruder whose main purpose is to generate maximum pressure in order
to pump the molten polymer in the forward direction. At this point the breaker plate controls the pressure generated with a screen
pack placed near to the screw discharge. The breaker plate also filters out any impurities such as dirt, foreign particle metal
particles and melted polymer lumps.
Schematic of a screw extruder
METERING PUMP:
The metering pump is a positive-displacement and constant-volume device for uniform melt delivery to the die assembly. It
ensures consistent flow of clean polymer mix under process variations in viscosity, pressure, and temperature.
DIE ASSEMBLY (Fiber forming assembly):
The die assembly is the most important element of the melt blown process. It has three distinct components: polymer-feed
distribution, die nosepiece, and air manifolds.
(a) Feed distribution:
Submitted by: Engr. M.Hassan Rafe
013-NTU-6002
Assignment: DIAPERS
Submitted to: Sir TOUCIF sb
The feed distribution balances the flow and the residence time across the width of the die. Coat
hanger type feed distribution is widely used in the manufacturing of nonwoven for diapers because it gives both even
polymer flow and even residence time across the full width of the die.
(b) Die Nosepiece:
From the feed distribution channel the polymer melt goes directly to the die nosepiece. The die
nosepiece is a wide, hollow, and tapered piece of metal having several hundred orifices or holes across the width. The
polymer melt is extruded from these holes to form filament strands which are subsequently attenuated by hot air to
form fine fibers.
(c) Air Manifold:
The air manifolds supply the high velocity hot air through the slots on the top and bottom sides of the die
nosepiece. The processing temperature is 280 C. The exits from the top and bottom sides of the die through narrow air
gaps are shown in Figure.
WEB FORMATION:
As soon as the molten polymer is extruded from the die holes, high velocity hot air streams attenuate the
polymer streams to form microfibers. As the hot air stream containing the microfibers progresses toward the collector screen, it
draws in a large amount of surrounding air (also called secondary air) that cools and solidifies the fibers. The solidified fibers
subsequently get laid randomly because of the turbulence in the air stream, onto the collecting screen, forming a self-bonded
nonwoven web, but there is a small bias in the machine direction due to some directionality imparted by the moving collector.
Usually, a vacuum is applied to the inside of the collector screen to withdraw the hot air and enhance the fiber laying process.
WINDING & BONDING:
The melt-blown web is usually wound onto a cardboard core. The combination of fiber
entanglement and fiber-to-fiber bonding generally produce enough web cohesion so that the web can be readily used without
further bonding. However, additional bonding and finishing processes may further be applied to these melt-blown webs.
Additional bonding, over the fiber adhesion and fiber entanglement that occurs at lay down, is employed to alter web
characteristics. Thermal bonding is the most commonly used technique. The bonding can be overall (area bonding). Heated
rollers are then used to flatten the fibers and bond them together. Two, three or four roll calendars can be used, depending on
the weight of the web to be bonded and the degree of bonding desired.
CONVERSION:
Submitted by: Engr. M.Hassan Rafe
013-NTU-6002
Assignment: DIAPERS
Submitted to: Sir TOUCIF sb
3 Sheets of nonwoven fabric are formed from plastic resin. There is a web for the top sheet of POLYPROPALENE and another for
the bottom sheet of POLYETHYLENE. The absorbent pad of superabsorbent polymer is placed between these two sheets. These
three components are in long strips and must be joined together and cut into diaper-sized units. This is accomplished by feeding
the absorbent pad onto a conveyor with the polyethylene bottom sheet. The polypropylene top sheet is then fed into place, and
the compiled sheets are joined by gluing, heating, or ultrasonic welding. At some point, stretched elastic bands are attached
with adhesive. These elastic bands contract and gather the diaper together to ensure a snug fit and limit leakage.
FINISHING:
Skin-friendly finish is applied to the fiber web babies feel more comfy in their diapers. This finish is called
MELTCARE, which are largely based on derivatives of natural oils and fats which are solid at room temperature, but are selected
in such a way that they partially melt at temperatures around 35°C. Triglycerides, partial glycerides, fatty alcohols, fatty esters,
ethers, or carbonates and fatty acids are some of the preferred building blocks for care melts.
CHARACTERIZATION:
Absorbency: The key working part of an diaper is its absorbent core, where the fluid diffuses, is absorbed and is retained.
Testing with variable pressures at increasing levels of saturation reveals the strengths and weaknesses of the core in handling
the fluids.
Strength: The core of diaper is also tested for strength to ensure that it will reach its maximum absorption potential before core
failure occurs.
SAP Indicator: Special combinations of chemicals and indicators are applied to the diapers to show High SAP density (green
square) and low SAP density (blue square) areas can be clearly identified.
Retention: Retention tests indicate the fluid retention under stepped pressure. These provide a clear indication of the amount
of fluid that will be released at each pressure equivalent to the baby lying down, sitting or moving.
Rewet: Rewet tests are carried out in front, central and back zones, with and without the ADL layer, to show the intrinsic ability
of the absorbent core to prevent fluids from resurfacing, and the effect that the ADL has on the rewet of the diaper within the
critical load range.
Pad integrity: The diaper is tested using the Hardy Integrity Tester (HIT) apparatus. This indicates the overall ability of the
diaper core to remain unbroken when subjected to impact.
Wicking test: The diaper is tested for wicking with saline solution. The results of this test are combined with those of the pad
integrity to assist in determining the optimum density of the core. Increased strength and wicking must be balanced against the
speed of absorption and the soft feel of the diaper.
Submitted by: Engr. M.Hassan Rafe
013-NTU-6002