Binders for nonwovens

Nonwoven fabrics characterized by a superior balance of strength and softness are formed utilizing an aqueous emulsion prepared by the emulsion polymerization of: 30 to 50% by weight of vinyl ester of an alkanoic acid; 10 to 30% by weight ethylene; 30 to 50% by weight of C.sub.4 -C.sub.8 alkyl acrylate; and 1 to 5% by weight of copolymerizable N-methylol containing monomer; wherein the polymerization is performed using batch or semi-batch techniques.

BACKGROUND OF THE INVENTION 
Nonwoven fabrics, or nonwovens, have gained great acceptance in the 
industry for a wide range of applications, particularly as replacements 
for woven fabrics in constructions such as for facings or topsheets in 
diapers, incontinent pads, bed pads, sanitary napkins, hospital gowns, and 
other single and multi-use nonwovens. For such uses it is desirable to 
produce a nonwoven which closely resembles the drape, flexibility and 
softness (hand) of a textile and yet is as strong as possible. 
When an adhesive binder is used to bond the loosely assembled webs of 
fibers in the nonwoven, the particular binder employed plays an important 
role in determining the final properties of the nonwoven since it 
contributes to the presence or absence of a wide range of properties 
including the wet and dry tensile, tear strength, softness, absorbency, 
and resilience as well as the visual aesthetics. Acrylic latices have 
generally been used as binders where softness is the most important 
criteria, however the resultant nonwovens have suffered in strength. 
Ethylene/vinyl acetate-based binders yield the necessary strength 
properties but are deficient in softness for some applications requiring 
extreme softness. Efforts have been made to soften the ethylene/vinyl 
acetate binders by interpolymerization with the appropriate acrylate 
functionalities; however, this has also only been accomplished with a 
consequent reduction in the strength of the binder. As a result of this 
loss in strength, no more than 25% by weight acrylate functional has been 
employed in ethylene/vinyl acetate based binders for non-wovens. 
SUMMARY OF THE INVENTION 
We have now found that latex binders for use in forming nonwovens can be 
prepared by the emulsion polymerization of: 
30 to 50% by weight of a vinyl ester of an alkanoic acid; 
10 to 30% by weight ethylene; 
30 to 50% by weight of a C.sub.4 -C.sub.8 alkyl acrylate; and 
1 to 5% by weight of copolymerizable N-methylol containing monomer; 
wherein the polymerization is performed using batch or semi-batch emulsion 
polymerization techniques. 
Surprisingly, nonwovens prepared with these binders possess the desirable 
softness characteristic of binders containing high acrylate content, with 
no reduction, indeed often with improvement, in the tensile strength 
properties. 
As used herein, the term "batch" refers to a process whereby all the major 
monomers are charged to the reactor initially with the N-methylol 
containing monomer added uniformly and concurrently with the initiators. 
The term "semi-batch" refers to a process whereby the vinyl ester and 
ethylene are charged initially and the N-methylol containing monomer and 
acrylate components are pre-emulsified and added uniformly and 
concurrently with the initiators. 
These processes are in contrast to conventional slow-addition processes 
used to prepare acrylate-containing binder emusions for nonwovens such as 
that disclosed in U.S. Pat. No. 4,044,197 wherein water, emulsifying 
agents and optionally a minor portion of the monomers are initially 
charged in the reactor and the monomers then added gradually with the 
initiators over the course of the reaction. 
In a preferred embodiment of the invention, a small amount of an N-methylol 
containing thermoset polymer such as melamine formaldehyde condensate is 
post-added to the emulsion in an amount of 0.5 to 5%. When utilizing these 
thermosets, smaller amounts of the N-methylol containing monomer are 
required to achieve comparable strength. As an example, conventional 
binders for use in specific applications where wet strength is important 
require 2-5% N-methylol containing monomers such as N-methylol acrylamide 
(NMA); when thermosets are used comparable results may be obtained with 
only about 0.5-2% NMA. Since NMA increases the stiffness of the nonwoven, 
these lower NMA levels are advantageous because they provide comparable 
strength with a softer product than could be obtained at the higher 
levels. 
By utilizing the emulsion polymerization procedures described herein, 
applicants have been able to obtain latex binders which, when used in the 
formation of nonwovens, give products characterized by a balance of 
softness and strength heretofore achievable only by use of thermal bonding 
techniques. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The vinyl esters utilized herein are the esters of alkanoic acids having 
from one to about 13 carbon atoms. Typical examples include: vinyl 
formate, vinyl acetate, vinyl propionate, vinyl butyrate, vinyl 
isobutyrate, vinyl valerate, vinyl 2-ethyl-hexanoate, vinyl isooctanoate, 
vinyl nonoate, vinyl decanoate, vinyl pivalate, vinyl versatate, etc. Of 
the foregoing, vinyl acetate is the preferred monomer because of its ready 
availability and low cost. 
The N-methylol component is generally N-methylol acrylamide although other 
mono-olefinically unsaturated compounds containing an N-methylol group and 
capable of copolymerizing with ethylene and the vinyl ester may also be 
employed. Such other compounds include, for example, N-methylol 
methacrylamide or lower alkanol ethers thereof, or mixtures thereof. 
The alkyl acrylates used herein are those containing 4 to 8 carbon atoms in 
the alkyl group and incude butyl, hexyl, 2-ethyl hexyl and octyl acrylate. 
The corresponding methacrylates may also be used herein. 
Optionally, mono-ethylenically or polyethylenically unsaturated 
copolymerizable monomers known for use in free-radical initiated 
polymerizations may also be present in small amounts. In addition, certain 
copolymerizable monomers which assist in the stability of the copolymer 
emulsion, e.g., acrylamide and vinyl sulfonic acid, are also useful herein 
as latex stabilizers. These optionally present monomers, if employed, are 
added in very low amounts of from 0.1 to about 2% by weight of the monomer 
mixture. 
In accordance with either the batch or semi-batch procedures utilized 
herein the vinyl acetate, ethylene, acrylate and the N-methylol containing 
monomer are polymerized in a aqueous medium under pressures not exceeding 
100 atmospheres in the presence of a catalyst and at least one emulsifying 
agent, the aqueous system being maintained by a suitable buffering agent 
at a pH of 2 to 6, the catalyst being added incrementally or continuously. 
If a batch process is used, the vinyl acetate and the acrylate components 
are suspended in water and are thoroughly agitated in the presence of 
ethylene under the working pressure to effect solution of the ethylene in 
the vinyl acetate and acrylate up to the substantial limit of its 
solubility under the condition existing in the reacton zone, while the 
vinyl acetate and acrylate are gradually heated to polymerization 
temperature. The homogenization period is followed by a polymerization 
period during which the catalyst, which consists of a main catalyst or 
initiator, and may include an activator, is added incrementally or 
continuously together with the N-methylol containing monomer, the pressure 
in the system being maintained substantially constant by application of a 
constant ethylene pressure if required. The semi-batch process is similar 
but some or all of the acrylate component is pre-emulsified with the 
N-methylol containing monomer and then added incrementally or continuously 
as the polymerization proceeds. 
Suitable as polymerization catalysts are the water-soluble free- 
radical-formers generally used in emulsion polymerization, such as 
hydrogen peroxide, sodium persulfate, potassium persulfate and ammonium 
persulfate, as well as tert-butyl hydroperoxide, in amounts of between 
0.01 and 3% by weight, preferably 0.01 and 1% by weight based on the total 
amount of the emulsion. They can be used alone or together with reducing 
agents such as sodium formaldehyde-sulfoxylate, iron-II-salts, sodium 
dithionite, sodium hydrogen sulfite, sodium sulfite, sodium thiosulfate, 
as redox catalysts in amounts of 0.01 to 3% by weight, preferably 0.01 to 
1% by weight, based on the total amount of the emulsion. The 
free-radical-formers can be charged in the aqueous emulsifier solution or 
be added during the polymerization in doses. 
The polymerization is carried out at a pH of between 2 and 7, preferably 
between 3 and 5. In order to maintain the pH range, it may be useful to 
work in the presence of customary buffer systems, for example, in the 
presence of alkali metal acetates, alkali metal carbonates, alkai metal 
phosphates. Polymerization regulators, like mercaptans, aldehydes, 
chloroform, methylene chloride and trichloroethylene, can also be added in 
some cases. 
The dispersing agents are all the emulsifiers generally used in emulsion 
polymerization, as well as optionally present protective colloids. It is 
also possible to use emulsifiers alone or in mixtures with protective 
colloids. 
The emulsifiers can be anionic, cationic or non-ionic surface-active 
compounds. Suitable anionic emulsifiers are, for example, alkyl 
sulfonates, alkylaryl sulfonates, alkyl sulfates, sulfates of 
hydroxyalkanols, alkyl and alkylaryl disulfonates, sulfonated fatty acids, 
sulfates and phosphates of polyethoxylated alkanols and alkylphenols, as 
well as esters of sulfosuccinic acid. Suitable cationic emulsifiers are, 
for example, alkyl quaternary ammonium salts, and alkyl quaternary 
phosphonium salts. Examples of suitable non-ionic emulsifiers are the 
addition products of 5 to 50 mols of ethylene oxide adducted to 
straight-chained and branch-chained alkanols with 6 to 22 carbon atoms, or 
alkylphenols, or higher fatty acids, or higher fatty acid amides, or 
primary and secondary higher alkyl amines; as well as block copolymers of 
propylene oxide with ethylene oxide and mixtures thereof. Preferably 
nonionic and/or anionic emulsifiers are used as emulsifying agents in 
amounts of 1 to 6% by weight of the polymerisate. 
Suitable protective colloids optionally employed are partially or 
completely saponified polyvinyl alcohol with degrees of hydrolysis between 
75 and 100% and viscosities of between 3 and 48 cps, measured as a 4% 
aqueous solution at 20.degree. C.; water-soluble cellulose ether 
derivatives, like hydroxyethyl cellulose; hydroxypropyl cellulose, 
methylcellulose or carboxymethyl cellulose; water-soluble starch ethers; 
polyacrylic acid or water-soluble polyacrylic acid copolymers with 
acrylamide and/or alkyl acrylates; poly-N-vinyl compounds of open-chained 
or cyclic carboxylic acid amides; and mixtures thereof. 
The copolymers according to the invention have a glass transition 
temperture of between -45.degree. to -20.degree. C. and dry to form soft 
flexible films. They are generally crosslinked in a weakly acid pH range 
or in the presence of latent acid catalysts at elevated temperature. The 
optimum crosslinking temperatures are between 100.degree. and 200.degree. 
C., preferably between 130.degree. and 160.degree. C. Acid catalysts 
accelerate the crosslinking. Such acid catalysts are mineral acids or 
organic acids, such as phosphoric acid, tartaric acid, citric acid, or 
acid salts, such as chromium -III salts, aluminum chloride, ammonium 
chloride, zinc nitrate or magnesium chloride. 
The process of making the vinyl acetate-ethylene-acrylate-N-methylol 
containing interpolymer latices generally comprises the preparation of an 
aqueous solution containing at least some of the emulsifying agent and 
stabilizer, and the pH buffering system. This aqueous solution and the 
initial charge of vinyl acetate are added to the polymerization vessel and 
ethylene pressure is applied to the desired value. The quantity of 
ethylene entering into the copolymer is influenced by the pressure, the 
agitation, and the viscosity of the polymerization medium. Thus, to 
increase the ethylene content of the copolymer, higher pressures are 
employed. A pressure of at least about 10 atmospheres is most suitably 
employed. As previously mentioned, the mixture is thoroughly agitated to 
dissolve the ethylene, agitation being continued until substantial 
equilbrium is achieved. This generally requires about 15 minutes. However, 
less time may be required depending upon the vessel, the efficiency of 
agitation, the specific system, and the like. When high ethylene contents 
are desired, a higher degree of agitation should be employed. In any case, 
by measuring the pressure drop of the ethylene in conventional manner, the 
realization of substantial equilibrium can be easily determined. 
Conveniently the charge is brought to polymerization temperature during 
this agitation period. Agitation can be effected by shaking, by means of 
an agitator, or other known mechanism. The polymerization is then 
initiated by introducing initial amouts of the catalyst, and of the 
activator when used. After polymerization has started, the catalyst and 
the activator are incrementally added as required to continue 
polymerization, and the N-methylol containing monomer and in the case of 
the semi-batch process, the acrylates are similarly added. 
As mentioned, the reaction is generally continued until the residual vinyl 
acetate, acrylate and N-methylol monomer content is below about 1%. The 
completed reaction product is then allowed to cool to about room 
temperature, while sealed from the atmosphere. 
By following the procedure described above, particularly the initial 
saturation of the polymerization mixture with ethylene before 
polymerization is initiated, there can be produced the stable vinyl 
acetate-ethylene-acrylate-N-methylol containing interpolymer latex 
characterized above, with the copolymer having an ethylene content of 10 
to 30%, an intrinsic viscosity of 1 to 2.5 dl./g. (measured in dimethyl 
formamide) and an average particle size of 0.1 to 2 microns, with the 
latex having a high solids content of up to 60% or more. 
The vinyl acetate-ethylene-acrylate-N-methylol containing binder described 
above is suitably used to prepare nonwoven fabrics by a variety of methods 
known to the art which in general, involve the impregnation of a loosely 
assembled web of fibers with the binder latex, followed by moderate 
heating to dry the web. In the case of the present invention this moderate 
heating also serves to cure the binder, that is, by forming a crosslinked 
interpolymer. Before the binder is applied it is optionally mixed with a 
suitable catalyst for the N-methylol groups present as comonomer and 
thermoset. Thus, acid catalysts such as mineral acids, e.g. HCl, or 
organic acids, e.g., oxalic acid, or acid salts such as ammonium chloride, 
are suitably used, as known in the art. The amount of catalyst is 
generally about 0.5 to 2% of the total resin. 
As discussed previously, it may also be desirable to improve the strength 
of the monomer using such lower levels of the N-methylol containing 
monomers as will provide for extremely soft materials. This may be 
accomplished by replacing 0.5 to 5% by weight of the latex binder solids 
with an N-methylol containing thermoset polymer. Suitable polymers are 
represented by the following formula 
##STR1## 
wherein 
(a) X is &gt;CH.sub.2 or &gt;CHOH; 
(b) X--X can be 
##STR2## 
(c) Y is &gt;CH.sub.2 or RN&lt; wherein R is lower alkyl or hydroxy lower lower 
alkyl: 
(d) M.sub.1 is--CH.sub.2 OH; 
(e) each of M.sub.2 and M.sub.3 is H or a --CH.sub.2 OR.sup.1 group wherein 
R.sup.1 is a lower alkyl group and n is 1 or 2. 
Typical examples of these thermoset polymers are monoethylolmelamine, 
dimethylolmelamine, trimethylolmelamine, tetramethylolmelamine, 
pentamethylolmelamine, hexamethylolmelamine, N-methoxymethyl 
N'-methylolmelamine, dimethylolethylene urea, monomethylol urea, 
dimethylol urea, dimethylolethyltriazone, dimethylolhydroxyethyltriazone, 
tetramethylolacetylene diurea, dimethylolpropylene urea, 
dimethyloldihydroxyethylene urea, N-butoxymethyl N-methylol urea and 
N-methymethyl N-methylol urea. 
Additionally there may also be present in the latex binders other additives 
conventionally employed in similar binders including defoamers, pigments, 
catalysts, wetting agents, thickeners, external plasticizers, etc. The 
choice of materials as well as the amounts employed are well known to 
those skilled in the art. These materials may be added just before 
application, if their stability in the dispersion or solution is low, or 
they may be formulated into the aqueous dispersion of the binder and 
stored if the stability in aqueous dispersion is high. 
The starting fibrous web can be formed by any one of the conventional 
techniques for depositing or arranging fibers in a web or layer. These 
techniques incude carding, garnetting, air-laying, and the like. 
Individual webs or thin layers formed by one or more of these techniques 
can also be lapped or laminated to provide a thicker layer for conversion 
into a heavier fabric. In general, the fibers extend in a plurality of 
diverse directions in general alignment with the major plane of the 
fabric, overlapping, intersecting and supporting one another to form an 
open, porous structure. When reference is made to "cellulose" fibers, 
those fibers containing predominately C.sub.6 H.sub.10 O.sub.5 groupings 
are meant. Thus, examples of the fibers to be used in the starting web are 
the natural cellulose fibers such as wood pulp, and chemically modified 
celluloses such as regenerated cellulose. Often the fibrous starting web 
contains at least 50% cellulose fibers, whether they be natural or 
synthetic, or a combination thereof. Other fibers in the starting web may 
comprise natural fibers such as wool; artificial fibers such as cellulose 
acetate; synethetic fibers such as polyamides, i.e., nylon, polyesters, 
i.e., "Dacron", acrylics, i.e., "Dynel," "Acrilan," "Orlon," polyolefins, 
i.e., polyethylene, polyvinyl chloride, polyurethane, etc., alone or in 
combination with one another. 
The fibrous starting layer or web suitably weighs from about 5 to 65 grams 
per square yard and generally weighs about 10 to 40 grams per square yard. 
This fibrous starting layer, regardless of its method of preparation, is 
then subjected to at least one of the several types of latex bonding 
operations to anchor the individual fibers together to form a 
self-sustaining web. Some of the better-known methods of bonding are 
overall impregnation, spraying or printing the web with intermittent or 
continuous straight or wavy lines or areas of binder extending generally 
transversely or diagonally across the web additionally, if desired, along 
the web. 
The amount of binder, calculated on a dry basis, applied to the fibrous 
starting web suitably ranges from about 10 to about 100 parts or more per 
100 parts of the starting web, and preferably from about 20 to about 45 
per 100 parts of the starting web. The impregnated web is then dried and 
cured. Thus, the fabrics are suitably dried by passing them through an air 
oven or over a series of heated cans or the like and then through a curing 
oven or sections of hot cans. Ordinarily, convection air drying is 
effected at 65.degree.-95.degree. C. for 2-6 min., followed by curing at 
145.degree.-155.degree. C. for 1-5 min. or more. However, other 
time-temperature relationships can be employed, as is well known in the 
art, shorter times at higher temperatures or longer times at lower 
temperatures being used. For example, the curing step can be carried out 
at about 135.degree. C. for about 15 minutes or more in a laboratory or 
pilot line but may require only 2 to 20 seconds on high pressure high 
efficiency steam cans used in high speed production. If desired, the 
drying and curing can be effected in a single exposure or step. 
Nonwoven fabrics prepared in accordance with this invention have greater 
strength than other resin bonded nonwovens of comparable softness levels 
and, as such, are competitive with woven fabrics and thermally bonded 
polyolefins. 
The following examples are given to illustrate the present invention, but 
it will be understood that they are intended to be illustrative only and 
not limitative of the invention. In the examples, all parts are by weight 
unless otherwise indicated.

The procedures utilized to prepare the binders produced in the examples are 
as follows: 
EXAMPLE 1 
A 10 liter stainless steel autoclave equipped with heating/cooling means, 
variable rate stirrer and means of metering monomers and initiators was 
employed. To the 10 liter autoclave was charged 450 g (of a 20% w/w 
solution) sodium alkyl aryl polyethylene oxide sulphate (3 moles ethylene 
oxide), 40 g (of a 70% w/w solution in water) alkyl aryl polyethylene 
oxide (30 mole ethylene oxide), 90 g (of a 25% w/w solution in water) 
sodium vinyl sulphonate, 2 g sodium formaldehyde sulphoxylate, 0.5 g 
sodium acetate, 5 g (of a 1% solution in water) ferrous sulphate solution 
and 2500 g water. After purging with nitrogen all the vinyl acetate (2000 
g) was added and the reactor was pressurized to 750 psi with ethylene and 
equilibrated at 50.degree. C. for 15 minutes. 
The polymerization was started by metering in a solution of 25 g. tertiary 
butyl hydroperoxide in 250 g of water and 25 g sodium formaldehyde 
sulphoxylate in 250 g of water. The initiators were added at a uniform 
rate over a period of 51/4 hours. 
Concurrently added with the initiators over a period of 4 hrs was a 
pre-emulsified blend of 2000 g butyl acrylate and 150 g N-methylol 
acrylamide (48% w/w solution in water) in a solution of 450 g (of a 20% 
w/w solution in water) sodium alkyl aryl polyethylene oxide sulphate (3 
mole ethylene oxide), 25 g (of a 70% w/w solution in water) alkyl aryl 
polyethylene oxide (30 mole ethylene oxide) and 1 g sodium acetate in 400 
g water. 
During the polymerization, the temperature of the reaction was maintained 
at 55.degree.-60.degree. C. by means of cooling and at the end of the 
reaction, the emulsion was transferred to an evacuated vessel (30 liter) 
to remove residual ethylene from the system. Composition and analysis of 
the latex is given in Table 1. 
In Example 1a, the same procedure was repeated using a higher level (about 
500 g) of N-methylolacrylamide. 
EXAMPLE 2 
The procedure was as in Example 1, except that the vinyl acetate charge was 
2400 g instead of 2000 g and the butyl acrylate was 1600 g. 
EXAMPLE 3 
The procedure was as in Example 1, except that 2800 g of vinyl acetate and 
1200 g of butyl acrylate were used. 
EXAMPLE 4 
The procedure was as in Example 1, except that 2800 g. of vinyl acetate and 
1200 g of 2-ethylhexyl acrylate were used. 
COMISON EXAMPLE 5 
The following three examples utilize the slow addition technique typically 
used to prepare the vinyl acetate, ethylene, acrylate nonwoven binders of 
the prior art. 
To the 10 liter autoclave was charged 90 g (of a 20% w/w solution in water 
sodium alkyl aryl polyethylene oxide sulphate (3 moles ethylene oxide), 6 
g (of a 70% w/w solution in water) alkyl aryl polyethylene oxide (30 mole 
ethylene oxide), 20 g (of a 25% w/w solution sodium vinyl sulphonate, 2 g 
sodium formaldehyde sulphoxylate 0.5 g sodium acetate, 5 g (of a 1% w/w 
solution in water) ferrous sulphate solution and 2000 g water. After 
purging with nitrogen, 300 g vinyl acetate and 100 g butyl acrylate were 
charged to the reactor. The reactor was then pressurized to 750 psi with 
ethylene and equilibrated at 50.degree. C. for 15 minutes. The 
polymerization was started by metering in a solution of 35 g tertiary 
butyl hydroperoxide in 250 g water and 35 g sodium formaldehyde 
sulphoxylate in 250 g water over a period of 61/2 hours. 
Concurrently added with the initiators over a period of 4 hrs was a 
pre-emulsified blend of 1900 g butyl acrylate, 1700 g. vinyl acetate, 150 
g (48% w/w solution in water) N-methylol acrylamide, 810 g (of a 20% w/w 
solution in water) sodium alkyl aryl polyethylene oxide sulphate (3 mole 
ethylene oxide), 60 g (of a 70% w/w solution in water) alkyl aryl 
polyethylene oxide (30 mole ethylene oxide), 1 g sodium acetate, 60 g (of 
a 25% w/w solution in water) sodium vinyl sulphonate in 600 g water. 
During the polymerization, the temperature of the reaction was maintained 
at 55-60.degree. C. by means of cooling and the pressure at 750 psi of 
ethylene by adding it when necessary. At the end of the additions of 
monomers and catalysts, the emulsion was transferred to an evacuated 
vessel following the procedure in Ex 1. 
COMISON EXAMPLE 6 
The procedure was as in Example 5, except that ethylene was omitted from 
the polymerization and the initial charge was 40 g butyl acrylate and 160 
g vinyl acetate. The pre-emulsified monomer charge was also changed with 
the vinyl acetate being 860 g and the butyl acrylate being 2960 g. 
COMISON EXAMPLE 7 
The procedure was as in Example 5, except that ethylene was omitted from 
the polymerization and the initial charge was 40 g butyl acrylate and 160 
g vinyl acetate. The pre-emulsified monomer charge was also changed with 
the vinyl acetate being 1240 g and the butyl acrylate being 2560 g. 
EXAMPLE 8 
This example illustrates the use of the batch polymerization process in 
preparing nonwoven binders of the present invention. 
To the 10 liter autoclave was charged 675 g (of a 20% w/w solution in 
water) sodium alkyl aryl polyethylene oxide sulphate (3 moles ethylene 
oxide), 50 g (of a 70% w/w solution in water) alkyl aryl polyethylene 
oxide (30 moles ethylene oxide), 60 g (of a 25% w/w solution in water) 
sodium vinyl suphonate, 0.5 g sodium acetate, 2 g sodium formaldehyde 
sulphoxylate, 5 g (of a 1% w/w solution in water) ferrous sulphate 
solution and 2000 g water. After purging with nitrogen, 1500 g vinyl 
acetate and 1500 g butyl acrylate were charged to the reactor. The reactor 
was then pressurized to 650 psi with ethylene and equilibrated at 
50.degree. C. for 15 minutes. The polymerization was then started by 
metering in a solution of 12 g tertiary butyl hydroperoxide in 225 g water 
and 10 g sodium formaldehyde sulphoxylate in 225 g water over a period of 
6 hrs. uniformly. 
Concurrently added with the initiators over a period of 4 hrs. was 110 g 
N-methylol-acrylamide (48% w/w solution in water) in 370 g water. 
During the polymerization, the temperature of the reaction was maintained 
at 55.degree.-60.degree. C. by means of cooling. At the end of the 
initiator slow additions, the product was transferred to an evacuated 
vessel (30 liter) to remove residual ethylene from the system. 
The results obtained by testing the binders of Examples 1-8 are shown in 
Table 1 and are compared with a commercially employed vinyl 
acetate/ethylene/N-methylol acrylamide polymer (designated CONTROL). In 
the Table, the abbreviations SB, SA and B are used to represent 
semi-batch, slow addition and batch polymerization techniques 
respectively. The results are also graphed and provided as FIGS. I and II 
where FIG. I shows the relation between dry tensile strength and softness, 
and FIG. II a similar relationship using wet tensile strength values. In 
the graphs, the points designated by a circle indicate those nonwovens 
falling within the scope of the claims, while the points designated by a 
square represent control or comparative compositions. 
In preparing samples for testing, lengths of 15 gram per square yard 
polyester were saturated using a Butterworth Padder and a bath of 100 
parts dry binder, 2 parts surfactant, 1 part catalyst, 2 parts melamine 
formaldehyde thermoset and sufficient water to give a 25% solids dilution, 
with a dry pick up of approximately 40 to 45 parts binder per 100 parts 
polyester web. The saturated web was dried for 2 minutes at 145.degree. C. 
in a laboratory contact drier. 
The tensile tests were run on a standard Instron tester set at 3 inch gauge 
length and 5 inch crosshead speed. The wet tensile was run after soaking 
specimens one minute in a 0.5% solution of Aerosol OT wetting agent. 
Results shown reflects the average of 10 tests. 
The softness or hand of a nonwoven is difficult to test using quantitative 
techniques. There is a correlation between softness of the nonwoven and Tg 
of the binder system, however since Tg is the temperature at which the 
polymer changes from a glassy to a rubbery state (which for soft nonwoven 
binder is generally in the range of -20.degree. C. to -35.degree. C. or 
lower), neither measured Tg nor calculated Tg is a completely adequate 
measure of the perceived softness of a binder at ambient conditions. 
Nonetheless, for binders using the same class of comonomers for example, 
vinyl acrylic binders, ethylene-vinyl acetate binders, etc, the lower the 
Tg of the copolymer, the greater the softness of the nonwoven made 
therewith. 
In the case of the nonwoven samples tested herein, a panel test was also 
run to determine the relative softness by rating the samples in order of 
softest to firmest by feeling the drape and pliability of the samples. The 
softest sample was rated as 1, the next a 2, etc., for the total numbers 
tested. The results reported show the average of five panelist ratings for 
each sample. 
As shown in the Table, binders produced utilizing the batch process 
(Example 8) as well as the semi-batch process (Examples 1-a, 2 and 4) 
exhibit a good balance of strength vs. hand (softness) as opposed to the 
slow addition processes of Examples 5, 6 and 7. 
More specifically, the benefits of the present invention with with respect 
to maximizing the balance of the contradictory properties of softness and 
strength will be recognized from an analysis of Table I in conjunction 
with the graphs of FIGS. I and II. Comparisons may be made along either 
axis with the understanding that at equal strengths, the preferred binder 
is that which gives the softest nonwoven and at equal softness levels, 
preference is given to the strongest binder. 
Thus Examples 1 and 8 show binder compositions having an optimum level of 
softness and strength achieved using the batch or semi-batch process 
required by the invention. When these properties are compared with those 
obtained from the same polymer composition prepared in Example 5, it is 
seen that the increased level of acrylate when incorporated using the slow 
addition techniques used in prior art nonwoven binder preparations, while 
softening the hand, substantially reduces the wet and dry tensile 
strengths. 
A comparison of the results of Example 4 of the invention with Comparative 
Example 6, shows that an equal level of softness can be achieved using 
very high levels of butyl acrylate with the slow addition process and 
using substantially less 2-ethyl hexyl acrylate with the semi-batch 
process. Note however, that the slow addition process, while producing the 
"softest" product also produces they weakest binder. In contrast, the 
binder of Example 4 gives high wet and dry tensile strength values. 
A comparison of Examples 1, 2,and 4 as opposed to Example 3 show that at 
least about 30% of the acrylate monomer is required to obtain adequate 
softness for use as a binder in very soft most nonwoven applications. 
Example 2 and 4 illustrate the differences in softness achieved using the 
same amount of different acrylate monomers. 
The control represents the "softest" product that can be obtained using the 
ethylene, vinYl acetate, NMA binders of the prior art. This composition 
contains 35 parts ethylene (the highest amount of ethylene that can be 
generally be incorporated using standard techniques of emulsion 
polymerization). The binder, while providing adequate strength is too 
stiff for many nonwoven applications such as for disposable diapers made 
by thermal bonding. On the other hand, the vinyl acrylic binders of 
Examples 6 and 7, while being soft enough for these applications, are 
unacceptably deficient in wet and dry strength properties. 
It will be apparent that various changes and modifications may be made in 
the embodiments of the invention described above, without departing from 
the scope of the invention, as defined in the appended claims, and it is 
intended therefore, that all matter contained in the foregoing description 
shall be interpreted as illustrative only and not as limitative of the 
invention. 
TABLE 1 
__________________________________________________________________________ 
HAND 
MONOMER COMPOSITION (%) 
PROCESS 
TENSILE STRENGTH (1 = SOFT 
EXAMPLE 
BA VA 2EHA 
E NMA TYPE DRY (lbs./inch) 
WET (lbs./inch) 
7 = HARD) 
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1 40 40 -- 20 1.5 SB 1.71 1.13 5 
1a 40 40 -- 20 5.0* 
SB 1.46 1.09 5 
2 32 48 -- 20 1.5 SB 1.30 0.93 6 
3 24 56 -- 20 1.5 SB 1.63 1.04 7 
4 -- 48 32 20 1.5 SB 1.12 0.85 2 
5 40 40 -- 20 1.5 SA 0.82 0.66 5 
6 75 25 -- -- 1.5 SA 0.38 0.37 2 
7 65 35 -- -- 1.5 SA 0.90 0.68 4 
8 40 40 -- 20 1.5 B 1.53 0.83 5 
Control 
-- 65 -- 35 4.5 SA 1.31 0.83 7 
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*No melamine formaldehyde thermoset was utilized in this formulation.