Kannadaguli & Ramaiah Kotra
For more than four decades, almost all nonwovens required a chemical
binder in order to provide any measure of structural integrity. In
addition, the binder was called upon to contribute and convey numerous
properties that were necessary for the effective performance of the
During this extended period, binders were essentially the weak element
in developing fully acceptable nonwoven fabrics. The fibers that were
available to the nonwoven industry were the same fibers that were
available to the textile and other fiber-based industries; hence, the
fibers were fully acceptable. It was generally the binder that limited the
performance of the nonwoven fabric.
The deficiencies cited against nonwovens generally were deficiencies
attributable to an inadequate binder. Common complaints are as follows:
- The fabric doesn't have enough strength.
- The fabric is too stiff.
- The fabric has inadequate absorbency.
- The fabric shows poor launderability.
- The fabric has inadequate dry cleanability.
- The fabric simply doesn't feel like a textile.
As a consequence, a great deal of effort has been put
into the development and continuous improvement of chemical binders. The
steady improvements in nonwovens performance that occurred over a period
of many years were, in no small measure, due to improvements in the
performance and utility of the binder.
In the very early stages of nonwovens development,
different types of natural resins and glues were used to bond nonwovens.
While they conveyed some integrity and strength to these webs, they also
had many glaring deficiencies. Consequently, synthetic binders were
developed to meet the structural and performance requirements of nonwoven
Polyvinyl acetate was the first successful synthetic
binder used in substantial volume. This material had distinctly superior
adhesive properties, strength, and performance compared to the early
natural adhesives. This binder had considerable flexibility in
formulation, and could easily be applied to fiber webs by a variety of
application processes, including print bonding.
The industry was faced with the inevitable compromise
in fabric properties of nonwovens bonded with synthetic materials. In
order to build strength in the fabric, increasing amounts of resin must be
applied, which results in more stiffness. If softness is necessary, it can
be achieved, but primarily by sacrificing strength.
A substantial improvement in this trade-off of strength
and softness was achieved with the introduction of acrylic-based latex
binders in the 1950s and 1960s. By proper selection of co-monomers, it is
possible to build improved softness properties with adequate strength.
Consequently, these binders became widely used by most of the nonwovens
industry, despite the somewhat higher cost.
As polymer technology for manufacturers of synthetic
binder systems improved, a greater variety of chemical building blocks
became available with much greater flexibility in terms of binder
strength, durability, and other properties. The introduction of
cross-linkable and self-crosslinking binder polymers turned out an
entirely new range of fabric properties. This was particularly noteworthy
in durable nonwovens where such durability features as washability and dry
cleanability were important.
The construction of a nonwoven with suitable binders is
to achieve improved characteristics such as strength, softness, adhesion,
firmness, durability, stiffness, fire retardence, hydrophilicity,
hydrophobicity, anti-microbial properties, organic compatibility, reduced
surface tension, improved dimensional stability and solvent, wash and acid
resistance. The following list illustrates some general considerations
required for an ideal binder. The required properties can be varied
depending on the end-uses.
Strength: The strength of a nonwoven fabric is more closely
related to the strength of the applied binder.
Adhesion to Fibers: Even though the mechanism of adhesion is not
completely understood, the adhesion strength of the binder-to-fiber bond
has to be considered.
Flexibility/handle: The some movements of fibers should be
allowed, especially when a soft hand is desired.
Elastic Recovery: To avoid the permanent deformation of fabric,
good elastic recovery is required under strain.
Resistance to washing/ Drying cleaning: Some nonwoven products
need durability in cleaning processes according to their end-uses.
Resistance to aging: The binder should be stable and not be
degraded in the fabric during storage and use.
Good color and color retention: Diverse ranges of colors are
required, and also the colorfastness and yellowing problems should be
Other: Flame resistance, resistance to chemicals, air, oxygen,
light, heat, etc.
Classification of Binders
Due to their diversity binders may be classified into
several categories based on polymer (binder) chemical structure,
functionality and the type of curing reactions.
1. Classification based on chemical structure.
There are three main kinds of binders: butadiene
copolymers, acrylates, and vinyl copolymers. The chemical compositions
influence Tg, hardness and softness, hydrophobicity and hydrophilicity,
elasticity, aging, and dry tensile strength of binders. The higher the Tg,
the stiffer the hand and higher the dry tensile strength of binders.
The structures of two main butadiene copolymers are shown as follows:
|Styrene/Butadiene rubber (SBR)
where f denotes a phenyl group.
|Butadiene/Acylonitrile Rubber (NBR)
The butadiene polymers are cross-linked by
polysulphides, and their properties are modified by different copolymers.
The butadiene monomers provide elasticity while styrene and acrylonitrile
monomers give tensile strength, and oil and solvent resistance,
respectively. Their disadvantages are oxidation and discoloration due to
residual double bonds in their polymer chains.
Acrylic acid derivatives
Acrylic binders are the most widely used and versatile
binders available with various modifications. The properties of acrylic
binders differ according to their derivatives and copolymers. The
structures of the common acrylic polymers are as follows:
|Acrylic Acid Derivatives
Acrylic Acid: -CH2-CH-
Metholyated Acrylaminde: -CH2-CH-
They are frequently copolymerized with styrene,
acrylonitrile, vinyl chloride or vinyl acetate, depending on the desirable
properties. Some of these properties are hardness from styrene, solvent
resistance from acrylonitrile, flame retardancy from vinyl chloride, and
cost benefits from vinyl acetate.
There are two main binders for vinyl copolymers: vinyl
chloride and vinyl acetate. Since the vinyl binders are stiff, they are
plasticized externally or internally. As internal plasticizers, ethylene
and acrylate monomers are used, and external plasticizers consist of vinyl
chloride. Due to its low Tg, vinyl acetate is not that stiff, and its
advantage is low cost. The chlorides cause yellowing problems. The
chemical structures are closely related Tg and stiffness of binders. The
Tg's of many homopolymers are listed in
Click to enlarge image...
2. Classification based on functionality
The functionality of binders is in the functional
groups attached to polymer chains, which influences wet and solvent
properties. To modify binder properties, copolymerization with a small
amount of monomers with special functionality is performed. The main
functionalities in binders are carboxyl and amide side chains.
This functionality is related to binders which contain
acrylic acid or methacrylic acid by copolymerization. The binders are
crosslinkable since the functional group, carboxylic acid, provides sites
for crosslinking reactions. Two probable cross-linking reactions of
carboxylated lattices are shown as follows:
COOH Heat C=O
COOH +ZnO Zn + H20
CH2 NH CH2-NH
R -------------------> R +H20
CH2 NH CH2-NH
This functionality is related to binders containing
acrylamide by copolymerization. The amide functionality provides
crosslinking sites, and even the binders are self-crosslinkable .
N-metylol amide functionality
This functionality is obtained after acrylamide is
reacted with formaldehyde. The binders containing the substituted
acrylamide groups have self-crosslinking properties, and the possible
reaction as follows:
O=C=NH2 Heat O=C
O=C-NH2 -NH3 O=C
CH2OH ------------------> CH2
3.Classification based on type of curing reactions.
The classification of reactivity refers to
crosslinkability of binders, which is related to reaction with curing
resins, crosslinking agents. The most common curing resin is melamine
formaldehyde condensate resin which involves reaction of n-methylol
The polymers do not contain any of the functional
groups. They can not crosslink , even with external curing resins.
The polymers contain acid or amide functional groups.
They can react with added curing resins, but the degree of crosslinking is
The polymers contain n-methylol functional groups. They
can react with themselves, and a high crosslink density can be obtained by
adding curing resins.
Recent trends in chemical bonding: Although
nonwoven manufacturers are seeking alternative technologies such as
thermal bonding, chemical bonding still has its advantages and a promising
market. Chemical bonding allows more room for fabric designs and fiber
selections. Both disposable and durable products are supplied to roll
goods producers and fiber manufacturers. On the environmental front,
increasingly strict regulations and guidelines are driving a trend towards
alternative products and technologies. Manufacturers and end-product
suppliers alike are seeking ultra-low or formaldehyde-free binders. The
growing consideration of the environmental impact of chemical binder and
additives has become a focus of debate on the national and international
Latex binder chemical types
A latex polymer consists of an aqueous medium with
extremely fine liquid or solid polymer particles dispersed therein. The
latex polymer generally is produced via free radical emulsion
polymerization in water, whereby a vinyl monomer is combined with a small
amount of other monomer (co-monomer) to create a high molecular weight
polymer. The latex dispersion also will carry surfactants, stabilizers,
and other additives to convey realistic properties to the latex itself.
When used as a binder, the latex typically is combined
with other components to provide the formulated binder ready for
application to the fiber web. The formulated binder conveys many
characteristics that are not possessed by the straight binder.
Consequently, there is a substantial chemistry involved in combining the
latex with the other components in order to prepare the formulated binder.
Binders are quite dependent upon the glass transition
temperature (Tg) of the monomer unit selected to form the polymer. The
lower the (Tg) of the monomer units, the softer is the resulting polymer.
A sampling of the most common monomers used in the manufacture of
latex polymer for nonwoven binders include the following materials:
Butyl Acrylate -52
Ethyl Acrylate -22
Vinyl Acetate +30
Vinyl Chloride +80
Methyl Methacrylate +105
The monomers selected for forming the polymeric latex
also have considerable influence on the hydrophilic or hydrophobic nature
of the binder. This can affect the wet strength of the nonwoven fabric as
well as a host of absorbency characteristics.
With the current capabilities of polymerization
chemistry, there is considerable versatility for each chemical type.
Despite this range of properties, the commonly employed nonwovens binders
generally are characterized by a fairly well defined set of properties.
These properties can be modified to some degree by incorporation of other
agents, but they provide a useful guide in classifying the kind of
performance to be expected from each type of binder.
Types of nonwoven binders
The following comparison of latex binder chemical types
provides an indication of the relative performance, as well as the
advantages and disadvantages of each type of binder. As indicated, the
binder properties can be modified considerably by the presence of
Acrylic: These binders offer the greatest
durability, color stability, and dry/wet performance. Acrylic binders have
the widest range of fabric hand properties. They can be formulated to vary
from very soft (Tg = -40oC) to extremely hard (Tg = 105oC).
These binders can be used in virtually all nonwovens applications,
although they tend to be more costly. These polymers can be made to
cross-link, with substantial improvement in durability.
Styrenated Acrylics: These are tough, hydrophobic
binders. The resulting textile hand ranges from soft-to-firm (Tg varies
20oC to +105oC ).These binders can be used in
applications where there is a need for some wet strength without
crosslinking. The use of this type of latex binder does involve some
sacrifice in UV and solvent resistance.
Vinyl Acetate (VAC): The vinyl acetate binders are
firm (Tg = +30oC to +40oC); however, they are
relatively low cost and find extensive use. They offer good dry strength
and toughness, but are somewhat hydrophilic and have a tendency to yellow
when subjected to heat.
Vinyl Acrylics: These binders are more hydrophobic
than the straight VAC binders. They provide excellent toughness,
flexibility, and better color stability. They are the compromise between
VAC and acrylic, and can compete on a cost/performance basis. The hand
range is limited to intermediate softness (Tg = -10oC) to a
firm hand (Tg = +30oC).
Ethylene Vinyl Acetate (EVA): These latex binders
have a (Tg range of
20oC to +115oC, which is
equivalent to soft ranging to an intermediate textile hand. They exhibit
high wet strength, coupled with excellent absorbency. In general, they are
less costly than acrylics. They do have a tendency to have more of an odor
compared to other binders. They are used primarily in wipes, air-laid pulp
fabrics and similar applications.
Styrene-Butadiene (S/B, SB, or styrene butadiene
rubber): These binders have an excellent combination of flexibility
and toughness. They range in hardness from very soft (Tg = -30oC)
to very firm (Tg = +80oC). However, the (Tg of an SB binder is
not strictly comparable to other classes of nonwoven binders. The
styrene-to-butadiene ratio (S/B ratio) is the most common method for
describing the relative hand resulting from the use of these binders. The
higher the styrene content, the firmer the hand. When cross-linked, this
class of binder is very hydrophobic and durable. They are affected
somewhat by heat and light because of their tendency to oxidize.
Polyvinyl Chloride (PVC): The homopolymer of
polyvinyl chloride is a very hard, rigid polymer (Tg = +80oC).
This polymer must be plasticized to provide flexibility and film-forming
properties. Normally, the (PVC) binders used in nonwovens are softened
internally by co-polymerizing the vinyl chloride or with softer acrylic
monomers. The hand range of most of these polymers is still relatively
firm (Tg is greater than the +30oC). Because this type of
polymer is quite thermoplastic, it performs well in heat and dielectric
sealing applications. This can be an advantage in some uses. Also, the
chlorine content of the polymer promotes flame retardancy. This feature is
one of the primary benefits of utilizing this type of binder. However, the
chlorine also conveys the tendency to yellow upon heat aging, due to
elimination of hydrogen chloride from the polymer.
Ethylene/Vinyl Chloride (EVCI): Binders in this
class have a slightly broader hand range (Tg = OoC to +30oC)
without the external plasticization required of (PVC) binders. The
presence of the chlorine again conveys some flame retardancy. These
binders exhibit good acid resistance, fair water resistance, and excellent
adhesion to synthetic fibers. There is some tendency to yellow upon aging.
In essence, this is an internally plasticized (PVC) binder, considering
the ethylene monomer to be the softener.
The formulation of binding solution is an art since
many ingredients are involved and many different possibilities exist for
different end-uses. Some of the characteristics, and the types of
formulation agents utilized to obtain them include the following.
Surfactants : offer improvement in binder adhesion, stability, and
ability to be converted into a foam
External cross-linkers: provide cross-links with binder polymer to
provide improved performance
- Defoamers: utilized to minimize foam in processing
- Repellent agents : convey water or oil repellency
- Salts: added to impart low flame response properties and to convey
- Thickeners: added to control the rheology of the binder liquid
- Catalysts: added to facilitate curing and to promote cross-linking
- Acids and bases: added to control pH of the latex
- Dyes and pigments: provide color to the binder and fabric
- Fillers: added to reduce binder tack and to lower cost
- Optical brighteners: added to increase whiteness
- Sewing aids: added to provide lubrication during fabrication
The purposes of wetting agents, mainly nonionic or
anionic surfactants, are to enhance binder penetration through webs,
improve the affinity between binder and fibers. The crosslinker, which has
multi-functional groups, is generally added to increase crosslink density
and to improve durability and resistance to deformation. The typical
reaction of the common crosslinking agent, melamine formaldehyde, is
illustrated as follows:
H OH----CH2 H2C----OH H
Order of Formulation
In terms of adding ingredients into a binding bath, the
compatibility of ingredients should be confirmed because the orders are
extremely important . The milky white color of most binders impedes a
check on the white-color indication of non-compatible ingredients, so most
ingredients are first added to the dilution water. After the compatibility
is assured, binders are added and then thickeners added to adjust
viscosity. For the stability of the binding solution, catalysts are added
just before application. Some water may be added to reach a desirable
solid level. The summarized order is as follows:
Some water, and the others, such as dyes and
pigments, fillers, clays, optical brighteners, sewing aids, etc.
Bonding nonwoven webs
Web consolidation or nonwoven bonding processes
interlock preferentially arranged fiber or film assemblies by mechanical,
chemical, solvent, and/or thermal means. The degree of bonding is a
primary factor in determining fabric integrity (strength), porosity
flexibility, softness, and density (loft, thickness).
Bonding may be carried out as a separate and distinct
operation, but generally is carried out as a sequential operation in
tandem with web formation. In some fabric constructions, more than one
bonding process may be used to enhance physical or chemical properties.
Mechanical consolidation methods include needlefelting,
stitchbonding, and hydroentangling. Chemical consolidation methods involve
applying adhesive binders to webs by saturating, spraying, printing, or
foaming techniques. Solvent bonding involves softening or partially
dissolving fibers with a solvent to provide self-bonding surfaces. Thermal
bonding involves the use of heat and often pressure to fuse or weld fibers
together at points of intersection or in patterned bond sites.
Important issues to consider when choosing the web
consolidation method are economy; versatility; and product properties,
primarily absorbency, strength, softness, loft, and purity. A recurring
issue involves environmental requirements of both the process and the
product. Many techniques are done for specific properties of unique
fabrics; therefore, it is difficult to measure differences in cost. In
some instances, two or more bonding techniques compete. The system that is
most energy-efficient; environmentally sound or provides the preferred
fabric properties generally dominates.
CHEMICAL BONDING PROCESSES
Chemical or resin bonding is a generic term for
interlocking fibers by the application of a chemical binder. The chemical
binder most frequently used to consolidate fiber webs today is a
water-borne latex. Most latex binders are made from vinyl materials, such
as polyvinylacetate, polyvinylchloride, styrene/butadiene resin,
butadiene, and polyacrylic, or their combinations.
Latexes are extensively used as nonwoven binders,
because they are economical, versatile, easily applied, and effective
adhesives. The versatility of a chemical binder system can be indicated by
enumerating a few factors which are considered when such a system is
The chemical composition of the monomer or backbone
material determines stiffness/softness properties, strength, water
affinity (hydrophilic/hydrophobic balance), elasticity, durability, and
aging. The type and nature of functional side groups determines solvent
resistance, adhesive characteristics, and cross-linking nature. The type
and quantity of surfactant used influences the polymerization process,
polymer stability, and the application method.
Chemical binders are applied to webs in amounts ranging
from about 5% to as much as 60% by weight. In some instances, when clays
or other weighty additives are included, add-on levels can approach or
even exceed the weight of the web. Waterborne binders are applied by
spray, saturation, print, and foam methods. A general objective of each
method is to apply the binder material in a manner sufficient to interlock
the fibers and provide fabric properties required of the intended fabric
The common methods of bonding include saturation, foam,
spray, print and powder bonding. They are briefly introduced in the
Saturation bonding is used in conjunction with
processes which require rapid binder addition, such as card-bond systems,
and for fabric applications which require strength, stiffness, and maximum
fiber encapsulation, such as carrier fabrics. Fiber encapsulation is
achieved by totally immersing the web in a binder bath or by flooding the
web as it enters the nip point of a set of pressure rolls. Excess binder
is removed by vacuum or roll pressure.
Three variations of saturation bonding exist: screen,
dip/squeeze, and size-press. Screen saturation is used for medium-weight
nonwovens, such as interlinings. Dip/squeeze saturation is used for web
structures with strength sufficient to withstand immersion without
support, such as spunbonds . Size-press saturation is used in highspeed
processes, such as wet-laid nonwovens. Drying and curing may be carried
out on steam-heated drying cans or in thru-air ovens or perforated-drum
dryers. Binder addition levels range from 20% to 60%. Two
techniques, single screen saturator and applicator roll technique, are
illustrated in Figure 2
and Figure 3. Advantages of this method are simplicity,
controllable tensile strength and softness by choice and amount of
binders. The disadvantages are the great influence of binders on softness,
and the limitation in loftiness.
Foam bonding is a means to apply binder at low water
and high binder-solids concentration levels. The basic concept employed
involves using air as well as water as the binder diluent and carrier
medium. Foam-bonded nonwovens require less energy in drying, since less
water is used. The foam is generated by introducing air into the
formulated latex while mechanically agitating the binder solution.
Air/latex dilutions or blow ratios in the order of 5:25
are practiced for various products. With the addition of a stabilizing
agent to the binder solution, the foam can resist collapsing during
application and curing, and the bonded fabric will exhibit enhanced loft,
hand, and resilience. Non-stabilized foams are referred to as froths;
froth-bonded fabrics are similar in properties to some saturation-bonded
nonwovens. One example of this bonding is illustrated in
Figure 4 The advantages include less energy required to dry
the web, less binder migration and controllable softness by choices and
amount of binders. The disadvantages are difficulties in controlling
process and adequate foaming.
In spray bonding, binders are sprayed onto moving webs. Spray bonding
is used for fabric applications which require the maintenance of highloft
or bulk, such as fiberfill and air-laid pulp wipes. The binder is atomized
by air pressure, hydraulic pressure, or centrifugal force and is applied
to the upper surfaces of the web in fine droplet form through a system of
Lower-web-surface binder addition is accomplished by
reversing web direction on a second conveyor and passing the web under a
second spray station. After each spraying, the web is passed through a
heating zone to remove water, and the binder is cured (set/cross-linked)
in a third heating zone. For uniform binder distribution, spray
nozzles are carefully engineered. Typical spray bonding is illustrated in
Print bonding applies binder in predetermined areas only and is used
for fabric applications that require some areas of the fabric to be
binder-free, such as wipes and coverstocks. Many lightweight nonwovens are
print bonded. Printing patterns are designed to enhance strength, fluid
transport, softness, hand, absorbency, and drape. Print bonding is most
often carded out with gravure rolls. Binder addition levels are dependent
on engraved area and depth as well as binder-solids level. Increased
pattern versatility can be achieved with the use of rotary screen rolls.
Drying and curing are carried out on heated drums or steam-heated cans.
In print bonding, high viscose binders are applied to limited,
patterned areas. A prewet/prebond step is required for enough strength of
webs, and typical steps in this bonding are in
Figure 6 There are two types of printers: rotary screen and
rotogravure printers. Binders are applied through a hollow applicator roll
in rotary screen printer, while in rotogravure printer they are applied by
an engraved applicator roll as shown in
Figure 7 The main advantage is that outstanding softness of
nonwoven fabrics with adequate strength can be achieved.
In powder bonding, the adhesive powder of thermoplastic polymers is
applied onto webs by heat and pressure. Polyesters and polyolefins with
low Tg's and molecular weight can be used as powder binders. A typical
bonding line is illustrated in
Figure 8 The advantages are the bulky structure of dense
nonwovens and the applicability of polyester or polypropylene webs. The
disadvantage lies in difficulties of suitable particle sizes and ranges,
and their distribution.
The following list provides an indication of those nonwoven end-use
applications in which binder-bonded nonwovens are utilized:
1. Typical nonwoven applications:
- Wipes and towels
- Medical nonwovens
- Roofing products
- Apparel interlinings
- Filter media
- Coating substrates
- Automotive trim
- Carrier fabrics
2. Typical highloft nonwovens:
- Bedding products
- Furniture applications
- Filtration media
- Automotive trim
In the latter part of the 1970s and 1980s, thermal bonding technology
grew rapidly, providing the industry with a realistic method to produce
strong and soft nonwoven fabrics without the use of a chemical binder.
This development provided substantial advances in performance and
properties of many types of nonwovens. One quality of this new bonding
technique was that these nonwovens contained no formaldehyde and no
chemical additives to cause consumer concern. Naturally, this movement
depressed the interest of chemical binders within the industry and has
resulted in a decline in binder usage.
Despite this setback, significant improvements and advances have
continued to be made by the synthetic polymer industry, to the benefit of
the range of nonwoven products that continue to utilize chemical bonding
methods. These improvements have involved such developments as
formaldehyde-free binders, low-cure temperature binders, complex
copolymers with unique characteristics, moldable binders, and others.. In
the future, new types of binders may be combined with the present choices,
for example, by copolymerization. Also, new bonding technology may occur.
In addition, new ideas such as reactive binders which can be covalently
bonded with fibers, will be continually investigated.
W.E. Devry, "Latex bonding Chemistry and Processes."
A. Derelich, "Nonwoven Textile Fabrics", Kirk-Othmer:
Encyclopedia of Chemical Technology, Vol. 16, 3rd Ed, p104-124,
B.M. Lichstein, The Nonwovens Handbook, Inda Association of the
Nonwoven Fabric Industry, New York, 1988.
J. Lunenschloss and W. Albrecht, Non-woven Bonded Fabrics, John
Wiley & Sons Inc., New York, 1985.
A.E. Meazey, " Binders used in Bonded Fiber Fabric Production",
Nonwovens '71, The textile Trade Press, England, 1971.
M.F. Meyer and W. A.; Haili, " Nonwovens and Laminates Made with
Polyester Adhesive Powders, "Eastman Kodak Company.
J.M. Oelkers and E.J. Sweeney," Latex Binders and Bonding
Techniques of Disposables", 1988.
Ellen Lees Wuagneux, " And how would you like your nonwovens?"
Nonwoven Industry Oct. 64-72 (1997).
INDA, Book of Paper, 1997
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