Hot-melt adhesives based on vinyl polymer

A hot-melt adhesive especially useful in the construction of disposable diapers contains a vinyl polymer of 40-90% by weight of a C.sub.1 -C.sub.12 alkyl acrylate and 10-60% by weight of an alpha-olefin of C.sub.20 -C.sub.40, wherein the alkyl acrylate may be partially replaced with vinyl acetate or with a C.sub.6 -C.sub.12 olefin or with mixtures thereof, provided that the polymer contain at least 10% by weight of the alkyl acrylate. The adhesive may also contain a tackifying resin, a wax, an oil or a stabilizer depending on the specific end-use desired. In a preferred embodiment the polymer contains 20-45% by weight of the alpha-olefin.

BACKGROUND OF THE INVENTION 
This invention relates to a hot-melt adhesive based on vinyl polymer and to 
a process for adhering surfaces therewith. 
Hot-melt adhesives are bonding agents which achieve a solid state and 
resultant strength by cooling, as contrasted with other adhesives which 
set or harden by chemical reaction or loss of solvent or water vehicle. 
Prior to heating the hot-melt adhesive is a thermoplastic material in the 
form of a 100% by weight solid. Application of heat melts the adhesive so 
that it can be readily applied to the substrate desired. After removal of 
heat, the adhesive returns to the solid state by simple cooling. 
Combinations of polymers and copolymers containing polyethylene, rubbers 
and the like have been proposed in the past as the basic component of 
hot-melt adhesive compositions. Often such compositions include several 
ingredients additional to the base polymer to improve specific properties 
of the adhesive such as its viscosity in the molten state (melt 
viscosity), peel strength, flexibility, stability and the like. 
The base polymer currently of choice for use in hot-melt adhesives designed 
to give good adhesion between polar and nonpolar surfaces is a copolymer 
of ethylene and vinyl acetate. Such polymers are produced by polymerizing 
ethylene and vinyl acetate at very high pressures of 15,000 to 40,000 psi 
(1055-2812 kg/cm.sup.2) and at temperatures of from 150.degree. to 
250.degree. C. A hydrocarbon wax is generally added to the copolymer thus 
obtained in amounts of 25 to 50% by weight of the total mixture to improve 
the hot-melt performance of the copolymer. Such waxes as are employed, 
however, have become increasingly costly and difficult to obtain due to 
the uncertainty in supply of petroleum available. 
It is an object of the present invention to provide a novel hot-melt 
adhesive based on a polymer derived from higher alpha-olefins which 
requires no hydrocarbon wax yet performs comparably to commercially 
available hot-melt formulations. 
It is another object to provide a process for adhering surfaces with such 
an adhesive. 
SUMMARY OF THE INVENTION 
The above and related objects are achieved in a novel hot-melt adhesive 
comprising a vinyl polymer comprising: 
(a) 40-90% by weight of a C.sub.1 -C.sub.12 alkyl acrylate and 
(b) 10-60% by weight of an alpha-olefin of C.sub.20 -C.sub.40, wherein said 
alkyl acrylate may be partially replaced with up to 60% by weight, based 
on the polymer, of vinyl acetate or with up to 10% by weight, based on the 
polymer, of C.sub.6 -C.sub.12 olefin or with a mixture of vinyl acetate 
and C.sub.6 -C.sub.12 olefin, provided that the polymer contain at least 
10% by weight of said alkyl acrylate. 
The adhesive may optionally contain a tackifying resin, wax, oil and/or 
stabilizer if desired. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The hot-melt adhesive of this invention is a vinyl polymer which is derived 
from at least two basic monomers, each in an amount within a given range. 
The alkyl acrylate monomer required to prepare the polymer generally 
comprises 40 to 90% by weight of the polymer. The choice of particular 
alkyl acrylate and amount thereof determines the character of the polymer 
and, thus, of the resulting adhesive. For example, methyl acrylate lends 
hardness and brittleness to the adhesive, ethyl acrylate results in tough 
rubbery polymers, and butyl and octyl acrylates in large amounts yield 
pressure-sensitive adhesives. The alkyl acrylate useful herein contains 1 
to 12 carbon atoms in the alkyl portion, and preferably is ethyl or butyl 
acrylate. 
The alpha-olefin monomer required in the basic vinyl polymer of the 
adhesive is a solid at room temperature and contains from 20 to 40 carbon 
atoms, preferably 20-30 carbon atoms. The olefin portion of the polymer 
retains its crystallinity in the polymer by side-chain crystallization and 
thus imparts cohesive strength to the adhesive. Thus, olefins of less than 
20 carbon atoms are not useful herein since they tend to be liquids or 
cold-flowing solids under ambient conditions. The preferred olefins herein 
are those mixture of olefins such as the C.sub.20 -C.sub.24 olefin mixture 
supplied by Gulf Oil Chemicals Company under the trademark Gulftene 20-24 
prepared by Ziegler polymerization of ethylene and primarily composed of 
eicosene (49 weight percent), docosene (42 weight percent) and tetracosene 
(8 weight percent). Also preferred is the C.sub.24 -C.sub.28 olefin 
mixture composed of 30% C.sub.24 olefin, 39% C.sub.26 olefin, 20% C.sub.28 
olefin and 10% C.sub.30+ olefin sold under the trademark Gulftene 24-28. 
Another mixture which is useful herein is the C.sub.30+ olefin mixture of 
28% C.sub.28 olefin and 78% C.sub.30+ olefin sold under the trademark 
Gulftene 30+. 
The amount of alpha-olefin monomer useful herein is 10-60%, and preferably 
20-45% by weight of the polymer. If the amount of olefin is higher than 
about 60%, chain transfer occurs to a large degree with the result that 
the polymer is a wax-like material with an unsuitably low viscosity. If, 
on the other hand, the amount of olefin in the polymer is too low, the 
resulting adhesive will not have the requisite cohesive strength due to 
lack of crystallinity in the polymer. 
In addition to the two required monomers mentioned above, the polymer may 
optionally be derived from additional monomers if desired. Thus, the alkyl 
acrylate monomer can be partially replaced with up to about 60% by weight, 
based on the polymer, of vinyl acetate or with up to 10% by weight, based 
on the polymer, of a C.sub.6 -C.sub.12 olefin or mixtures thereof, 
provided that the polymer contain at least 10% by weight of alkyl 
acrylate. The amount of vinyl acetate to be added will depend on the 
degree of brittleness desired in the adhesive. The C.sub.6 -C.sub.12 
olefins, which are particularly preferred because they lend internal 
plasticity to the polymer, cannot be used in amounts of more than about 
10% of the polymer due to their chain transfer properties. An especially 
preferred polymer for use as an adhesive herein is a terpolymer of 10-30% 
by weight alkyl acrylate, 30-60% by weight of vinyl acetate and 10-60% by 
weight of the alpha-olefin. Other monomers such as allyl esters or 
alpha-olefins of C.sub.3 -C.sub. 19 may be employed in amounts of up to 
15% by weight to replace the alkyl acrylate monomer, if they do not 
adversely affect the desired properties of the hot-melt adhesive to a 
significant degree. Examples of unsuitable monomers herein are styrene and 
alkyl methacrylate, which tend to homopolymerize. 
The polymers of this invention are prepared by free-radical-initiated 
polymerizations which are well known and described in the art. In a 
typical method of preparing the polymers, all of the olefin, a portion of 
the other monomers and the free-radical initiator (such as benzoyl 
peroxide or tert-butyl perbenzoate) are mixed together in a vessel and the 
mixture is heated to reflux with stirring. Thereafter, a mixture of the 
remaining amounts of vinyl acetate and alkyl acrylate is slowly added over 
about three hours. Heating is continued until the internal temperature 
reaches 150.degree. C. (about three hours). A viscous polymer is formed 
and the antioxidant or stabilizer is added. The residual monomers if any 
are removed by vacuum distillation. 
It is to be noted that a non-uniform polymer may lead to a useless adhesive 
which separates into two or more phases in the melt. In preparing uniform 
polymers it is desirable to consider the relative reactivities of the 
monomers, especially when olefins of molecular weight greater than a 
C.sub.24 molecular weight are employed. 
The preferred polymer herein has a viscosity in the range of 500 to 20,000 
cps, but a viscosity outside this range is still within the scope of this 
invention if the hot-melt adhesive formulated from the polymer functions 
adequately. 
While the vinyl polymers of the present invention are useful by themselves 
as adhesives without being modified in any way, it may be advantageous in 
certain applications to blend other components therewith. For example, 
tackifying resins may be incorporated into the hot-melt adhesive to alter 
or optimize the properties desired in the adhesive such as its bond 
strength and melt viscosity. The tackifier may also produce a reinforcing 
or plasticizing (flexibility) effect or contribute to stickiness (better 
molten tack) and particular wetting ability. Representative of suitable 
tackifying resins are, for example, the rosin and rosin esters (such as 
those available from Hercules Chemical Co. under the trademark FORAL), 
aromatic hydrocarbon resins including styrene polymers and copolymers 
(such as the low-molecular-weight polystyrene resins available from 
Hercules Chemical Co. under the trademarks PICCOLASTIC and KRISTALEX 
RESINS) and alphamethylstyrene/vinyl toluene resins (available from 
Hercules Chemical Co. under the trademark PICCOTEX), the coumarone-indene 
resins, aromatic-aliphatic copolymer resins, aromatic petroleum oils, 
phenolic resins, and the like. The tackifying resin, if employed, is 
typically present in amount of 5-25% by weight of the total adhesive. 
Additionally, various other modifiers may be employed including stabilizers 
or antioxidants such as Irganox 1010, butylated hydroxy-toluene or alkyl 
aryl phosphite, plasticizers such as dioctyl phthlalate, pigments such as 
calcium carbonate, silicon dioxide, titanium dioxide and zinc oxide, oils 
such as white mineral oil and castor oil, and waxes such as paraffin, 
microcrystalline or synthetic waxes and equivalents thereof such a 
C.sub.20 -C.sub.40 olefins. The waxes, which are preferably employed in 
amounts no greater than 25% by weight of the adhesive, function so as to 
reduce the melt viscosity or alter the cohesive or dryness characteristics 
without appreciably decreasing bond strength. Furthermore, addition of oil 
or wax tends to decrease adhesion of the polymer to give an adhesive with 
a removable peel strength as discussed hereinafter. 
In the formation of the hot-melt adhesive, the various additives such as 
antioxidants, plasticizers and tackifying resins are generally added at 
the end of polymerization, either to the polymerization vessel (prior or 
subsequent to removing residual monomer) or to a separate mixing vessel; 
in some instances, however, one or more ingredients may be added in the 
beginning of the polymerization, providing, of course, that they do not 
detrimentally interfere with the polymerization. By way of illustration, 
plasticizer may be added at the beginning of polymerization and the 
antioxidants and tackifying resins added at the end thereof. In the 
typical procedure the formulations are prepared by remelting the vinyl 
polymer in a stainless steel beaker at about 150.degree. C. and blending 
therewith the optional components with heating and stirring at a rate and 
timing such that lumping is avoided. Mixing and heating are continued 
until a smooth, homogeneous mass is obtained, whereupon the resultant 
hot-melt adhesive is drawn off. The adhesive thus obtained may be used 
immediately in hot pots or may be molten-extruded or converted into 
cylinders, slugs or billets depending on the physical character of the 
solid adhesive. If desired, the adhesive may be placed in cooling pans or 
drums and held in bulk for later use. 
A particularly important area for utilization of the hot-melt adhesives of 
the present invention is in preparing disposable diapers. Thus, the 
adhesive is applied hot to one side of a polyethylene layer which is then 
joined to the cellulosic layer, and on cooling the layers adhere firmly. 
When the diaper is to be discarded, the layers are peeled apart, allowing 
separate disposal of the parts of the construction, a desirable feature 
when different means are advised for disposal of each part. The bond 
obtained in such application has high cohesive strength but is readily 
removable on demand. Depending on the application and the substrates 
employed, the adhesive may also be used to form a permanent bond between 
the layers which will not separate without tearing of the layers. 
Other areas for utilization include the construction of disposable sanitary 
products such as sanitary napkins and other such applications wherein 
hot-melt adhesives of the character described herein would be useful. 
The following examples illustrate the efficacy of the hot-melt adhesives 
herein. All parts and percentages are given by weight and all temperatures 
in degrees Centigrade unless otherwise noted. 
The following test procedures were used to evaluate the polymers and 
hot-melt adhesive compositions of this invention: 
MELT VISCOSITY TEST 
The molten viscosities of the polymer and of the hot-melt adhesive at a 
given temperature (usually 120.degree. to 150.degree. C.) are measured in 
centipoises (cps) utilizing a Thermosel Viscometer as supplied by 
Brookfield Engineering of Stoughton, Mass. Thermal stability of the 
adhesive is determined by measuring the viscosity of the adhesive after 
aging for one week at 150.degree. C. 
PEEL STRENGTH (REMOVABLE BOND) 
A molten film of the adhesive of 50 to 100 microns thickness is cast using 
a heated glass rod or Bird applicator on a Teflon (trademark) coated steel 
sheet and allowed to solidify. Strips of 0.65 cm width are cut from the 
film and inserted between pieces of polyethylene film and nonwoven cloth. 
A heat-sealing machine with a pressure of 1.4 kg/cm.sup.2 and a 
temperature of 135.degree. C. is applied to the lamination for three 
seconds to form the bond. Peel strength is tested by peeling the 
polyethylene film from the nonwoven using an Instron Tensile Tester with a 
separation rate of 5.1 cm per minute. The peel strength is measured in 
gram units and is in the range of 15-40 g for best results. 
PEEL STRENTH (PERMANENT BOND) 
The hot-melt adhesive is applied as a bead onto polyethylene film at 
93.degree. to 107.degree. C. in a stream of about 1.6 mm diameter flowing 
from a glass rod. The adhesive bead is immediately chilled by a water 
spray to avoid distortion of the polyethylene film. The bead on the 
polyethylene is bonded to nonwoven paper by rolling over the combined 
substrates with a 2 kg rubber-coated roller. The bond is tested by peeling 
the substrates apart either slowly by hand or on an Instron Tensile Tester 
with a separation rate of 5.1 cm per minute. A permanent bond produces a 
tear in the nonwoven paper when separated.

EXAMPLE I 
A one-liter round-bottom flask was charged with 150 g of C.sub.20 -C.sub.24 
olefin sold under the trademark Gulftene 20-24, 90 g of vinyl acetate, 1.5 
g of ethyl acrylate, 2.5 g of tert-butyl perbenzoate and 10 g of a 50% 
mixture of benzoyl peroxide and dicyclohexyl phthalate. The mixture was 
heated to reflux with stirring and a solution containing 210 g of vinyl 
acetate and 48.5 g of ethyl acrylate was added over a three-hour period. 
The boiling point of the mixture remained between 80.degree. and 
86.degree. C. during the addition. Heating was continued until the 
internal temperature reached 150.degree. C. (three hours). One gram of 
Irganox 1010 (antioxidant) was then added and the unreacted monomer (2.7%) 
removed by vacuum distillation. A terpolymer (Polymer I) was thereby 
produced. 
Polymer II was prepared in an indentical manner except that butyl acrylate 
was substitued for ethyl acrylate. 
Adhesives A-D in Table I were prepared by mixing either Polymer I or 
Polymer II with the indicated ingredients in the given amounts at 
150.degree. C. until a homogeneous mass was obtained. The properties of 
the polymers and the adhesives are given in Table I. 
TABLE I 
______________________________________ 
Adhesive 
Ingredients (parts): 
A B C D 
______________________________________ 
Polymer I (containing 30% 
75 75 75 -- 
olefin, 60% vinyl acetate 
and 10% ethyl acrylate) 
Polymer II (containing 30% 
-- -- -- 75 
olefin, 60% vinyl acetate 
and 10% butyl acrylate) 
Paraffin wax 25 20 -- 5 
White mineral oil 
-- 5 -- 20 
Polystyrene of softening 
-- -- 25 -- 
point 25 (sold by Hercules 
under the trademark Picco- 
lastic A 25) 
Properties: 
Melt Viscosity of Polymer 
3875 3875 3875 3500 
at 150.degree. C. (cps) 
Melt Viscosity of Adhesive 
2900 2875 2920 2690 
at 120.degree. C. (cps) 
Melt Viscosity of Adhesive 
* 3000 * 2875 
at 120.degree. C. after one week 
aging at 150.degree. C. (cps) 
Peel Strength (Removable) (g) 
20 25 30 35 
______________________________________ 
*Not determined. 
It is seen that the removable peel strengths for all four adhesives were 
comparable to the peel strength of 24-32 g obtained on testing a 
commercial diaper of Procter & Gamble Co. bonded with a commercially 
available hot-melt adhesive used as a comparison. Furthermore, the melt 
viscosity of each adhesive is desirably reduced as compared with the melt 
viscosity of the base polymer alone. 
EXAMPLE II 
The procedure of Example I was used to prepare Polymers III-V employing the 
ingredients given below in the indicated amounts: 
______________________________________ 
Ingredients for Polymerization 
Polymers 
(parts): III IV V 
______________________________________ 
Initial Charge: 
C.sub.24 -C.sub.28 olefin 
30 20 40 
1-decene 5 10 -- 
vinyl acetate 9 12 12 
ethyl acrylate 1 -- -- 
butyl acrylate -- 1 0.6 
50% mixture of benzoyl peroxide 
0.1 0.4 1.0 
and dicyclohexyl phthalate 
t-butyl perbenzoate 
0.5 0.5 0.5 
Slow Add: 
Vinyl acetate 21 28 28 
Ethyl acrylate 34 -- -- 
Butyl acrylate -- 29 19.4 
50% mixture of benzoyl peroxide 
0.5 0.4 1.0 
and dicyclohexyl phthalate 
Melt Viscosity of Polymer 
1440 1000 800 
at 150.degree. C. (cps): (at 120.degree. C.) 
______________________________________ 
The composition and properties of the Adhesives E-G prepared from Polymers 
III-V, respectively, are given in Table II. 
__________________________________________________________________________ 
Adhesives 
E F G 
__________________________________________________________________________ 
Ingredients (parts): 
Polymer III (tetrapolymer containing 
90 -- -- 
30% olefin, 30% vinyl acetate, 35% 
ethyl acrylate and 5% decene) 
Polymer IV (tetrapolymer containing 
-- 90 -- 
20% olefin, 40% vinyl acetate, 
30% butyl acrylate and 10% decene) 
Polymer V (terpolymer containing 40% 
-- -- 100 
olefin, 40% vinyl acetate and 20% 
butyl acrylate) 
Alphamethyl styrene/styrene copolymer 
10 -- -- 
(sold by Hercules under the trademark 
KRISTALEX 85) 
Microcrystalline wax (m.p. 195.degree. C.) 
-- 10 -- 
Alkyl aryl phosphite (stabilizer) 
-- 0.25 
-- 
Properties: 
Melt Viscosity of Adhesive at 150.degree. C. (cps) 
940 1440 
800 
(at 120.degree. C.) 
Peel Strength (permanent) 
100% tear 
* * 
Peel Strength (removable) (g) 
* 20 30 
__________________________________________________________________________ 
*Not appropriate 
The results indicate that tetrapolymers are also effective in formulating a 
suitable hot-melt adhesive for permanent and removable bonds. In addition, 
it is seen that the polymer can be employed as an adhesive as 100% 
material. 
EXAMPLE III 
Polymer VI (copolymer containing 30% olefin and 70% ethyl acrylate) was 
prepared by charging a 500-ml round-bottom flask with 75 g of C.sub.20 
-C.sub.24 olefin of Example I, 9 g of ethyl acrylate and 1.5 g of t-butyl 
perbenzoate. The solution was heated to 115.degree. C. under a nitrogen 
atmosphere and 166 g of ethyl acrylate was added over a two-hour period 
while maintaining a temperature around 115.degree. C. The reaction mixture 
was heated for one hour at 130.degree. C. Then 0.5 g of Irganox 1010 
(trademark) was added and the volatile materials (0.3%) were removed by 
vacuum distillation. The viscosity of the polymer was 1810 cps at 
150.degree. C. 
An adhesive formulation prepared with 90 parts of Polymer VI and 10 parts 
of alpha-methyl styrene/styrene copolymer had the following properties: 
______________________________________ 
Melt Viscosity at 150.degree. C.: 
1575 cps 
Melt Viscosity of Adhesive 
1450 cps 
at 150.degree. C. after 3 days aging 
at 150.degree. C.: 
Peel Strength (Removable): 
34 g 
______________________________________ 
EXAMPLE IV (Comparative) 
Polymer VII was prepared by the method described in Example I employing the 
following ingredients: 
______________________________________ 
Initial charge: Parts 
______________________________________ 
1-hexadecene 25 
vinyl acetate 18 
ethyl acrylate 0.5 
50% mixture of benzoyl peroxide 
2.5 
and dicyclohexyl phthalate 
t-butyl perbenzoate 2.5 
Slow addition: 
vinyl acetate 42 
ethyl acrylate 14.5 
50% mixture of benzoyl peroxide 
2.5 
and dicyclohexyl phthalate 
______________________________________ 
The terpolymer thus obtained (containing 25% olefin, 60% vinyl acetate and 
15% ethyl acrylate) has a melt viscosity at 120.degree. C. of 6050 cps and 
was ineffective as an adhesive due to its relative weakness and 
cold-flowing properties. 
In summary, the present invention provides a hot-melt adhesive containing a 
vinyl polymer derived from higher alpha-olefins requiring no wax for its 
good performance. 
Now that the preferred embodiments of the invention have been described in 
detail, various modifications and improvements thereon will become readily 
apparent to the practitioner. Accordingly, the spirit and scope of the 
present invention are to be limited only by the appended claims, and not 
by the foregoing specification.