What are the components of a typical disposable diaper?
Most disposable diapers are made
with the following basic components:
1)
Polyethylene or cloth-like film: This is used as the back sheet, that
prevents the liquids from leaking out of the diaper. The back-sheet can also be
given a cloth-like look, by adding a thin polypropylene non-woven sheet to the
film, using either the hot melt process or the heat and pressure method with
direct extrusion to the nonwoven. Contrary to popular belief, the cloth-like
back sheet is not cloth -it is made of plastics. Breathable cloth-like material
can also be used instead of the film. Not many know that even a breathable
diaper with 200 ml of urine loses less than 2.5% of its weight over a period of
24 hours and this evaporation is enough the cool the diaper, which may not be as
comfortable at night. For more information about "breathability" please use
this link: breathable diapers.
2)
Tissue: A special tissue paper that is different from the regular bathroom
tissue and has a higher elasticity and wet strength is another important
component of a diaper. The tissue essentially serves as a carrier for the pad (the
pad is the absorbent core of the diaper) and helps reduce the pin holes created
during the compression process carried out by continuous drum forming systems.
This tissue, typically at 16 grams/m2 (also abbreviated as GSM) or more,
protects the inner plastic from the superabsorbent particles. Instead of
tissue, it is possible to use a low gage SMS nonwoven material as the carrier (for
the pad), it can be placed right next to the back sheet or as a full wrap
material around the core. In order for the SMS carrier to be cost competitive
against tissue paper, it needs to be less than 12 GSM.
3) Hot
Melts: They are used to glue the different components of the diaper, such as
the pad and the elastics. They are made of a mixture of resins, oils and
tackifiers. The hot melt adhesive is applied in molten form and when it cools
down it provides the required bonding force to glue the materials. Most of the
times two types of adhesives are used: a construction adhesive, for the back
sheet and the nonwovens, and an elastomeric adhesive, for the leg and waist foam
elastics. The elastomeric adhesive has higher elasticity and bonding strength
and it is generally more expensive than construction adhesives. When the diaper
pad is very thin, another specialty adhesive known as "pad integrity adhesive"
is also used to add strength to the diaper core when it is wet. This integrity
adhesive is specially useful when SAP loadings exceed 25% of the total pad
weight -i.e. when the weight of the SAP is more than a quarter of the weight of
the pad. For a list of hot melt suppliers follow this link:
Hot Melts.
Click to see Drawing with Hot Melt Zones
4)
Hydrophobic Non-woven: It is used as a top sheet for the leg cuffs; it
prevents water from passing through. It is made of polypropylene resin without
any added surface surfactants. The hydrophobic nonwoven prevents leakage out
of diaper. By applying a surfactant to a restricted area, it is possible to
make a roll of hydrophobic nonwoven only partially philic. This is known as the
Zebra process and it is an important feature designed to avoid leakage during
leg cuff construction. For nonwoven suppliers use this link:
Nonwovens.
5)
Hydrophilic Non-woven: It is the main top sheet, the top surface that is in
contact with the baby's skin. It allows the liquids to flow into the diaper
core. The difference between the two non-wovens (philic and phobic) is the
surfactant treatment used in the process. The surfactant treatment reduces the
surface tension of the nonwoven, reduces the contact angle with the liquid and
allows it to pass. Flow dynamics within the diaper core prevent liquids from
returning to the surface. Most nonwovens used in diapers are made with the spun
bonding process, though it is possible to use thermal bonded nonwovens also,
which are softer but have lower resistance and strength. Trough Air Bonded
nonwovens which are more lofty, can also be used. For nonwoven suppliers use
this link: Nonwovens.
6)
Elastics: Used to improve the fit of the diaper, usually made of
polyurethane or polyester foam, synthetic rubber or Lycra (also known with the
generic name Spandex). They are used in cuffs, for the waist and the legs; they
can also be used as lateral side panels and in tape construction. Most
gasketing cuffs use spandex to provide a seal with the baby's legs. Spandex
can stretch as much as 400% of its original length before it breaks, however it
is typically used at less than 300% stretch. New generations of softer and
stronger elastic materials are reportedly in the pipeline. For a list of
suppliers use this link: Elastomerics.
7)
Lateral Tapes: In premium diapers, VelcroR type materials have
been used to provide mechanical grip, it is also known as the "hook tape". In
lower priced diapers, adhesive tapes made of polypropylene are used. Then there
are new versions of elasticized Nonwoven Velcro Tapes. In a few years baby
diapers may replace training pants with the help of these new stretchable
fastening systems that offer the same characteristics to the consumer but cost
less. Some adult diapers use what is called the "target tape" system, where the
tape has two adhesive tabs to avoid the need for a frontal tape. This is a
cheaper alternative for adult diapers but not as good as the one using a frontal
tape which does not require repositioning of the tape on top of the target.
For a list of suppliers use this link:
Tapes and
Clossure systems.
8)
Frontal Tapes:
This is used to facilitate
multiple repositioning of the lateral tape without tearing the back-sheet, it is
made of polypropylene film and attached to the front of the diaper with adhesive.
Its use has helped to reduce the thickness of the poly film without the risk of
potential tears associated with the opening of the lateral tapes from the
backsheet. In premium diapers, a special loop system has been developed in
order to use of Velcro type fasteners (also called the "hook and loop" system).
This loop tape can use a "locked loop" or a "brushed loop" in order to provide a
softer texture or a stronger grip. A new generation of nonwoven materials
expected to be commercialized in a few years, may eliminate the need for frontal
tapes - the whole backsheet will be used to reposition mechanical tapes. The
frontal tape can have a printed design which can be random or synchronized, some
patents may protect the use of synchronized printing in some markets. Tapes
and Clossure systems.
9) Cellulose:
Used in the
construction of the pad, it gives integrity and absorbing capacity to the diaper.
The capacity of normal cellulose pulp is around 10 cc of water per gram of pulp
when the diaper is in "free swell" but less than 2 cc when subjected to 5 KPa
of pressure; that is why a superabsorbent material is also needed to hold the
liquids under pressure. Cellulose comes from pine trees, generally obtained
from well managed forests. Liquids are absorbed by the capillaries in the void
spaces between the fibers and the surface tension angle between the fibers and
the water. Typical fiber length used in diapers is about 2.6 mm. An
alternative to pulp is to use air laid synthetic fibers. However, it is still
difficult for air laid synthetics to compete with pulp, unless it is a niche
market product and thickness is more important for the consumer (as in case of
some sanitary napkins and the adult diapers used by active people) than the cost.
Cellulose acetate, the material used to make cigarette filters, has been used in
some absorbent products. PP synthetic fibers has also been attempted for
absorbent core formation. For a list of pulp suppliers use this link:
Fluff Pulp.
10)
Acquisition and Distribution Layer:
Also known with its
abbreviation ADL, it is a sub layer used between the top sheet and the absorbent
core. Sometimes used in full length but mostly preferred as a patch near the
target zone where urine is most likely to be deposited. This sub layer is
specially needed when the absorbent core is very thin -the sub layer quickly
moves liquids into the absorbent core and reduces potential leakage. The ADL is
very important to provide a sense of dryness to the skin, providing additional
separation between the wet pad and the skin. ADL's should be used whenever the
mix of SAP in the absorbent core exceeds about 15% by weight or when the liquid
penetration time requires a boost in order to avoid diaper leakage due to liquid
accumulation inside the diaper. ADL's are made either of through air bond (TAB)
nonwovens, "curly" fibers such as in P&G's pampers and some Ontex diapers, or
some kind of "high loft" nonwoven. An aperture film, made of perforated plastic
film, has also been used successfully in some markets. Lower priced diapers
sometimes use resin bonded nonwovens, but they do not work as well. For
acquisition nonwoven suppliers use this link:
Nonwovens.
11)
Sodium Polyacrylate: Also known as super-absorbent or "SAP"
(super absorbent polymer), Kimberly Clark used to call it SAM (super absorbent
material). It is typically used in fine granular form (like table salt).
It helps improve capacity for better retention in a disposable diaper, allowing
the product to be thinner with improved performance and less usage of pine fluff
pulp. The molecular structure of the polyacrylate has sodium carboxylate groups
hanging off the main chain. When it comes in contact with water, the
sodium detaches itself, leaving only carboxyl ions. Being negatively charged,
these ions repel one another so that the polymer unwinds and absorbs water,
which is attracted by the sodium atoms. The polymer also has cross-links, which
effectively leads to a three-dimensional structure. It has high molecular
weight of more than a million; thus, instead of getting dissolved, it solidifies
into a gel. The Hydrogen in the water (H-O-H) is trapped by the acrylate due to
the atomic bonds associated with the polarity forces between the atoms.
Electrolytes in the liquid, such as salt minerals (urine contains 0.9% of
minerals), reduce polarity, thereby affecting superabsorbent properties,
specially with regard to the superabsorbent capacity for liquid retention.
This is the main reason why diapers containing SAP should never be tested with
plain water. Linear molecular configurations have less total capacity than
non-linear molecules but, on the other hand, retention of liquid in a linear
molecule is higher than in a non-linear molecule, due to improved polarity. For
a list of SAP suppliers, please use this link:
SAP
Example of a linear
molecule
The superabsorbent can be designed to
absorb higher amounts of liquids (with less retention) or very high retentions (but
lower capacity). In addition, a surface cross linker can be added to the
superabsorbent particle to help it move liquids while it is saturated. This
helps avoid formation of "gel blocks", the phenomenon that describes the
impossibility of moving liquids once a SAP particle gets saturated. Please read
the history section for more information. To get more answers about this
superabsorbent material, please use the link: Frequent
questions about SAP
12) Top Sheet surface add-on lotions:
In order to create novelties for product differentiation, several topical
lotions are added to the nonwoven top sheet, among others: Aloe Vera, Vitamin E,
Petrolatum, Almond Oil, Vitamin D, Oat Extract, Jojoba, etc. There is another
trend to use antibacterial lotions (such as tertiary ammonia or silver salt
compounds); however, many pediatricians are against its use for obvious reasons.
13) Decorated Films and wetness
indicators: For even greater product differentiation, some diapers use
decorated films underneath the cloth-like backsheet. Some use as many as nine
inks with all kinds of well known characters such as Disney, Sesame Street,
Soccer teams, etc. Another gimmick they use is a wetness indicator. This is
typically used for adult products but some baby diapers also use it.
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