Hot melt adhesive

The foregoing and other objects and advantages of the present invention are accomplished by a hot melt adhesive comprising a blend of a low molecular weight polyolefin, olefin-carboxylic acid graft polymer or olefin-maleic anhydride graft polymer or blend thereof; a propylene-ethylene copolymer or propylene-ethylene-butene terpolymer; and a tackifying resin or blend of tackifying resins. The blend can include an antioxidant or blends of antioxidants, a high molecular weight or functionalized polyolefin or a copolymer or terpolymer of a vinyl monomer (e.g., ethylene/vinyl acetate).

FIELD AND BACKGROUND OF THE INVENTION 
The present invention relates to a hot melt adhesive. More particularly, 
the invention relates to an adhesive which has desirable adhesive 
properties and which facilitates recycling cellulosic (e.g., paper) 
products having the adhesive thereon. 
Hot melt and pressure sensitive adhesives for cellulosic products are 
typically water insoluble and thus are difficult to remove during 
recycling such as during a repulping process. The repulping process 
typically comprises mixing water and paper, and the fibers are separated 
via mechanical action. The slurry may then be passed through screens and 
centrifugal cleaners to remove non-fibrous contaminants such as glass, 
metal, plastic, dirt, and other unwanted solid materials. After waste 
paper is repulped, it is formed into paper or paper board on the moving 
wire of a fourdrinier machine or on the cylinders of a cylinder machine, 
pressed and dried to remove the remaining water. 
The failure to remove all of the water insoluble adhesive from the repulped 
paper results in papers of low quality, inconsistent composition, and 
non-uniform appearance and surfaces. Additionally, the sticky nature of 
the adhesive can create processing difficulties in commonly used automated 
machinery. Both of these problems are due to the fact that the adhesives 
tend to agglomerate and form globules commonly called "stickies." 
Various solutions to the problem associated with hot melt and pressure 
sensitive adhesives have been suggested. For example, U.S. Pat. No. 
3,891,584 to Ray-Chaudhuri et al. proposes a water dispersible hot melt 
adhesive comprising 75 to 95 parts of a graft copolymer of vinyl monomer 
and a water soluble polyalkylene oxide polymer, and 5 to 25 parts of a 
tackifying resin. U.S. Pat. No. 4,176,054 to Kelley proposes providing the 
adhesive with a magnetic or paramagnetic material integrated therein. 
These solutions have not, however, been widely embraced by the industry 
due to cost and unacceptable adhesive properties of the resulting hot melt 
adhesive. 
It is therefore an object of the present invention to provide a hot melt 
adhesive which facilitates the recycling of cellulosic products bearing 
the adhesive. 
Still another object of the present invention is to provide a hot melt 
adhesive which obviates the problems associated with the formation of 
stickies during recycling. 
Another object of the invention is to provide a hot melt adhesive which is 
characterized by aggressive tack and good machining properties. 
SUMMARY OF THE INVENTION 
The foregoing and other objects and advantages of the present invention are 
accomplished by a hot melt adhesive comprising a blend of a low molecular 
weight (i.e., less than about 50,000) polyolefin, olefin-carboxylic acid 
graft polymer or olefin-maleic anhydride graft polymer or blends thereof; 
a propylene-ethylene copolymer or propylene-ethylene-butene terpolymer; 
and a tackifying resin or blend of tackifying resins. The hot melt 
adhesive blend can include an antioxidant or blend of antioxidants, a high 
molecular weight polyolefin, a functionalized polyolefin (e.g., an 
ethylene vinyl silane copolymer), or a copolymer or terpolymer of a vinyl 
monomer (e.g., ethylene/vinyl acetate). 
The hot melt adhesive of the present invention can be used for example in 
packaging and bookbinding, and particularly in perfect bound books. 
Perfect binding is the term used to describe a method of book binding in 
which the sheets making up a book are stacked in order to form a so-called 
signature which is then held in a clamp while adhesive is applied to the 
edge or spine to be bound whereafter a cover sheet or strip is brought 
into contact with the spine before the adhesive sets. Perfect binding 
originally was used as a cheap and rapid binding method for paperback or 
pocket books. However, it is now used for binding magazines, catalogues, 
directories, etc., and even for binding signatures for case-bound books. 
The adhesive advantageously can be formed into a thin film in the molten 
state, wet easily and provides good tack on papers such as coated paper, 
enameled stock, cross and long grain, coated or uncoated sheets and ground 
wood. The adhesives of the present invention are quick setting and can be 
used in a wide range of viscosities. The adhesive of the present invention 
provides good mechanical properties such as high bonding strength, 
flexibility, high tensile strength, and resistance to aging, mold growth 
and cold crack. Cellulosic products with the adhesive thereon can be 
recycled by repulping inasmuch as the adhesive of the present invention 
does not form stickies at elevated temperatures. 
DETAILED DESCRIPTION OF THE INVENTION 
As stated above the hot melt adhesive of the present invention is suitable 
for use with cellulosic products which can be repulped to recycle. The hot 
melt adhesive comprises a low molecular weight polyolefin, 
olefin-carboxylic acid graft polymer or olefin-maleic anhydride graft 
polymer or blends thereof; a propylene-ethylene copolymer or 
propylene-ethylene-butene terpolymer; and a tackifying resin or blend of 
tackifying resins. 
The low molecular weight polyolefin is preferably polyethylene, 
polypropylene or polybutylene and has a molecular weight of less than 
about 50,000, and typically from about 2,000 to 50,000. The 
olefin-carboxylic acid graft polymer preferably is a graft polymer of 
ethylene or propylene and a carboxylic acid having 1 to 6 to carbon atoms 
such as acrylic acid or methyacrylic acid. The olefin-maleic anhydride 
graft polymer preferably is a graft polymer of ethylene or propylene and 
maleic anhydride. Selection of various graft polymers and polymerization 
techniques will be within the skill of one in the art. 
The propylene ethylene copolymers and the propylene-ethylene-butene 
terpolymers are formed using conventional copolymerization techniques. 
The tackifying resin is a low molecular weight (i.e., from about 500 to 
25,0000) polymer resin that is used to provide hot tack and adhesion, and 
can control compatibility with other polymeric components. Exemplary 
tackifying resins include aliphatic hydrocarbons, aromatic hydrocarbons, 
aromatic/aliphatic hydrocarbons, dimerized rosin, pentaerythritol esters 
of rosin, glycerol esters of rosin, hydrogenated pentaerythritol rosin 
esters, styrenated terpenes, rosin acids, styrene based resins, 
polyterpenes, phenolic modified terpenes, and styrene-modified 
hydrocarbons and blends thereof such as described in U.S. Pat. Nos. 
4,140,733 to Meyer, Jr. et al. and 4,722,650 to Allen et al., the 
disclosures of which are incorporated herein by reference in their 
entirety. 
The composition is formed batchwise (single stage) by blending the 
components under conditions sufficient to form the hot melt adhesive. 
Typically, the low molecular weight polyolefin, olefin-carboxylic acid 
graft polymer or olefin-maleic anhydride graft polymer and the tackifying 
resin or blend of tackify resin are mixed in a kettle jacket at a 
temperature of about 150.degree. C. to 200.degree. C. Any antioxidants, 
stabilizers, compatibilizers and catalysts are added and heated to 
150.degree. C. to 200.degree. C. until all the ingredients are molten. 
Mixing is commenced and heat is applied to maintain a temperature of about 
160.degree. C. to 180.degree. C. The propylene-ethylene copolymer or 
propylene-ethylene-butene terpolymer is added while maintaining the 
temperature at about 160.degree. C. to 180.degree. C. The entire mixture 
is then homogeneously blended at a temperature of about 160.degree. C. to 
180.degree. C. The resulting composition typically has a viscosity of 
about 500 to 10,000 cps at 175.degree. C., a softening point of about 
130.degree. C. to 165.degree. C. and a maximum cold crack of at least 
+5.degree. C. Other manufacturing techniques include continuous feed 
extrusion, sequential polymerization or by blending batch polymers, the 
selection and operation of these techniques will be within the skilled of 
one in the art. 
The hot melt adhesive can optionally include a high molecular weight 
polyolefin such as polyethylene or polypropylene having a molecular weight 
greater than about 50,000, and often greater than about 100,000. The hot 
melt adhesive could also include a copolymer or terpolymer of a vinyl 
monomer, e.g., an ethylene-vinyl acetate (EVA) copolymer such as described 
in U.S. Pat. Nos. 3,615,106 to Flanagan et al., 4,140,733 to Meyer, Jr. et 
al. and 4,960,295 to Bodouroglou, the disclosures of which are 
incorporated herein in their entirety. The hot melt adhesive could also 
include a functionalized polyolefin such as an ethylene vinyl silane 
copolymer. Such a copolymer can be crosslinked using moisture which 
contributes to the copolymer being non-disintegradable. 
The hot melt adhesive can include an antioxidant or blend of antioxidants. 
Suitable antioxidants are high molecular weight hindered phenols and 
multifunctional phenols, amines and phosphites such as sulfur and 
phosphorous-containing phenols. Hindered phenols are well known to those 
skilled in the art and are described in U.S. Pat. Nos. 4,660,858 to 
Flanagan et al. and 4,140,733 to Allen, Jr. et al., the disclosures of 
which are incorporated herein by reference in its entirety. The hindered 
phenols may be characterized as phenolic compounds which also contain 
sterically bulky radicals in close proximity to the phenolic hydroxyl 
group thereof. In particular, tertiary butyl groups generally are 
substituted onto the benzene ring in at least one of the other positions 
relative to the phenolic hydroxy group. The presence of these sterically 
bulky substituted radicals in the vicinity of the hydroxyl group serves to 
retard its stretching frequency and, correspondingly, its reactivity; this 
steric hindrance thus providing the phenolic compound with its stabilizing 
properties. Representative hindered phenols include: butylhydroxyphenol 
("BHT"); 4,4'-bis(.alpha..alpha.-dimethylbenzyl) diphenylamine; 
1,3,4-trimethyl 2,4,6-tris (3,5-di-tertbutyl-4-hydroxybenzyl)benzene; 
2,2'-thiodiethyl-bis (3,5-di-tertbutyl-4-hydroxyhydrocinnamate); 
pentaerythrityl tetrakis-3(3,5-di-tertbutyl-4-hydroxybenzyl) propionate; 
n-octadecyl-3(3,5-di-tertbutyl-4-hydroxphenyl)propionate; 
4,4'-methylenbis(2,6-tert-butylphenol); 4,4'-thiobis 
(6-tertbutyl-o-cresol); 2,6-di-tertbutylphenol; 
6-(4-hydroxyphenoxy)-2,4-bis(n-octylthio)-1,3,5-triazine; di-n-octadecyl 
3,5-di-tert-butyl-4-hydroxybenzoate; and sorbitol 
hexa[3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate]. 
The performance of these antioxidants may be further enhanced by utilizing, 
in conjunction therewith, known synergists such as, for example 
thiodipropionate esters and phosphites; particularly useful is 
distearylthiodipropionate. Antioxidants and blends of antioxidants, if 
used, are generally present in amounts of about 0.1 to 2.0 weight percent 
by weight, and preferably about 0.25 to 1.0 percent by weight. 
The adhesive can also include components such as diluents or modifiers in 
the form of various waxes. As will be understood by those skilled in the 
art, the inclusion of waxes in the formulations reduces viscosity, 
enhances hot melt processing, influences the open time of the particular 
adhesive formulation and regulates the heat resistance of the bound book. 
Waxes which are suitable for inclusion in hot melt formulations include 
oxidized waxes, oxidized polyethylene, high melting point microcrystalline 
waxes, 40.degree. C. to 70.degree. C. paraffins, synthetic waxes, and the 
like. Wax modifiers or low molecular weight polymers may also be added in 
small percentages. The waxes included are most preferably high melting 
point microcrystalline and/or synthetic waxes. The adhesive will 
preferably contain between about 10 and about 30 percent by weight and, 
most preferably, between about 15 and about 20 percent by waxes. 
The hot melt adhesive of the invention can be utilized with various 
cellulosic materials. The adhesive can be used in packaging and 
bookbinding. For example, in bookbinding, the hot melt adhesive is 
particularly adapted for use in perfect bound books. This technique is 
used for binding magazines, catalogues, directories (e.g., telephone 
books) and the like. Such books create a great source of waste and 
represent a major prospect for recycling via repulping techniques. 
Generally, a perfect bound book, such as described in U.S. Pat. No. 
4,960,295 to Bodouroglou, comprises a plurality of sheets forming a book 
block. These sheets are stacked in order to form a so-called "signature" 
which is then held in a clamp while adhesive is applied to the edge or 
spine to be bound whereafter a cover sheet or strip is brought into 
contact with the spine before the adhesive sets. The book block has a 
backbone of the hot melt adhesive. The book block can optionally include a 
primer layer based on allow viscosity water-based vinyl acetate-ethylene 
(VAE) emulsion such as described in the Bodouroglou patent. 
The process of binding the perfect bound book is typically a so-called 
"one-shot" operation although a so-called "two-shot" technique can be used 
if the optional primer layer is applied to the book block. In operation, 
the process comprises applying the hot melt adhesive of the present 
invention to the book block, and subjecting the hot melt adhesive to 
conditions sufficient to solidify the adhesive to bind the perfect bound 
book. Typically, the adhesive is applied at a temperature of about 
170.degree. C. and 190.degree. C., and is set by cooling to less than 
about 165.degree. C.

EXAMPLES 
Example 1 
A hot melt adhesive is prepared by following the procedures described 
below. A clean kettle, equipped with a mechanical mixer and heated to 
150.degree. C., is charged with approximately 20 parts of (A) hydrogenated 
C.sub.5 aliphatic hydrocarbon tackifying resin with 130.degree. C. ring 
and ball softening point (Brookfield viscosity of 1200 cps at 190.degree. 
C.); (B) 0.5 parts of butyl-hydroxyphenol; (C) 0.3 parts of 2,2' 
thiodiethyl-bis (3,5 di-tertbutyl-4-hydroxyhydrocinnamate), and (D) 2 
parts of 4,4'-bis(.alpha..alpha.-dimethylbenzyl) diphenylamine. It is 
mixed thoroughly, followed by the addition of 10 parts of (E) maleic 
anhydride grafted polyethylene (106.degree. C. softening point, 
m.w.=26,000/m.w.=5,600); 15 parts of (F) maleic anhydride grafted 
polypropylene (157.degree. C. softening point); and 55 parts of (G) 
propylene-ethylene-butene terpolymer (softening point 155.degree. C., 
m.w., 45,000). The mixture is continued to mix at 
175.degree..+-.10.degree. C. for about 2 hours until a homogeneous 
solution is obtained. The resulting hot melt adhesive has a viscosity of 
5,000 cps at 175.degree. C., and softening point of 155.degree. C. The hot 
melt adhesive is used as a one-shot bookbinding adhesive exhibiting a page 
pull value of &gt;35 lbs, page flex of &gt;75, and cold crack of &lt;0.degree. C. 
Example 2 
A telephone directory, 3/4 of an inch thick, bound using the hot melt 
adhesive in Example 1, is subjected to repulping using the following 
procedure. The spine of the telephone directory is cut to make the total 
width of the directory about 1/4" from the glue line. This is further cut 
into pieces 3/8" to 1/2 in width. About 10 pieces of such paper cuts 
containing adhesive film, each weighing about 1 gram, are mixed with 50 
grams of blotting paper, thus making 60 grams of total paper, and placed 
in a disintegrator. Two liters of tap water is added to the disintegrator 
and the temperature is maintained at 55.degree..+-.5.degree. C. The 
disintegrator is run for about 30,000 revolutions (about 10 minutes). At 
the end of the blending cycle, the sample is diluted with an additional 4 
liters of water (25.degree. C.). The slurry is filtered to remove the hot 
melt adhesive. The hot melt adhesive thereby screened is examined for 
breakage or disintegration of the adhesive film. It is found that all the 
adhesive pieces are intact within the same dimension (unchanged). The 100 
ml pulped slurry is used for hand sheet machine and subjected to high 
temperature evaluation (about 120.degree. C.) between two Kraft papers to 
determine the formation of any stickies. From this test, no glue is 
observed thus confirming that the adhesive does not disintegrate under the 
disintegration (repulping) process. 
Example 3 
This example demonstrates that the hot melt adhesive of the present 
invention can include a polyolefin such as polyethylene and atactic 
polypropylene. The procedure followed uses 10 parts of maleic hydride 
grafted polypropylene (F) from Example 1, 20 parts of hydrogenated C.sub.5 
aliphatic hydrocarbon tackifying resin (A), 30 parts of polyethylene 
polymer, and 40 parts of atactic polypropylene. In addition to this the 
same level of antioxidants package (i.e., components (B) to (D)) as 
reported in Example 1 is included. The resulting hot melt adhesive, with a 
softening point of 155.degree. C. and viscosity of 4,600 cps at 
175.degree. C., was found to be suitable as a bookbinding adhesive. The 
product is found to pass the pulping and stickies tests. 
Example 4 
The example demonstrates that one can use a small amount of wax as an 
additive in the hot melt adhesive of Example 1 without significantly 
affecting the properties. The procedure of Example 1 is produced using 15 
parts of tackifying resin (A), 10 parts of (F), 45 parts of (G), 25 parts 
of polyethylene polymer, and 5 parts of polyethylene wax (drop melting 
point=115.degree..+-.2.degree. C.; m.w.=700 to 1200). The antioxidant 
package of Example 1 is used. The resulting hot melt adhesive passed the 
repulping and stickies test, and had the adhesive performance satisfactory 
for bookbinding applications. 
Example 5 
This example demonstrates the properties of a hot melt adhesive which could 
be used in packaging applications (for example, case and carton sealing). 
The procedure of Example 1 was followed using about 15 parts of aliphatic 
hydrocarbon tackifying resin (A), 15 parts of pentaerythritol resin ester, 
13 parts of maleic anhydride grafted polyterpene (F), 49.5 parts of 
propylene-ethylene copolymer (141.degree. C. softening point), and 0.5 
parts of a hindered phenol antioxidant (C). The resulting product had a 
viscosity of 3,000 cps at 175.degree. C., softening point of 140.degree. 
C., and cold crack below 5.degree. C. The product was found to be suitable 
as a packaging adhesive in terms of open time (&lt;5 sec), and set speed (&lt;2 
sec). The adhesive was found to pass the repulping and stickies test as 
described in Example 2. 
Comparative Example 1 
This is a comparative example using a conventional hot melt adhesive for 
bookbinding application based on an EVA polymer. In a typical example, the 
hot melt is prepared by blending a mixture of 25.7 parts of paraffin wax 
(melting point=58.degree. C. to 60.degree. C.), 30.7 parts of 
.alpha.-methylstyrene hydrocarbon resin (melting point=80.degree. C. to 
90.degree. C.), 33.8 parts of EVA polymer (33 percent vinyl acetate; melt 
index=43), and 9.9 parts of EVA polymer (28 percent vinyl acetate; melt 
index=40). The resulting hot melt adhesive has a softening point of 
80.degree..+-.2.degree. C. and viscosity of 3599 .+-.500 cps at 
177.degree. C. This adhesive was used as a bookbinding adhesive and 
subjected to the repulping test of Example 2. The result was formation of 
stickies and the adhesive tended to disintegrate in the repulping test 
causing blockage of filters. 
Comparative Example 2 
This was a comparative example for packaging application using a 
polyethylene polymer along with tackifying resins without the use of 
maleic anhydride grafted polyolefin and copolymers or terpolymers. In a 
typical example, a hot melt adhesive is prepared using 66.3 parts of a low 
density polyethylene polymer (melting point =104.degree. C.; melt 
index=2250), 2 parts of low density polyethylene (melting point 
=110.degree. C.; melt index=200), 15 parts hydrocarbon tackifying resin 
(A) of Example 1, 16.2 parts of another hydrocarbon tackifying resin and 
softening point of 115.degree. C. and viscosity of 400 cps at 150.degree. 
C., 0.3 parts of antioxidant (C) of Example 1, and 0.2 parts of amine 
antioxidant (D) of Example 1. The resulting hot melt adhesive has a 
softening point of 97.degree..+-.3.degree. C. and a viscosity of 2000 
.+-.400 cps at 177.degree. C. Although the adhesive was suitable for 
packaging and is being used commercially, when subjected to the repulping 
conditions of Example 2, results were the break-up of the adhesive and 
formation of stickies. 
While various embodiments have been disclosed and described herein, it will 
be appreciated that various changes and modifications can be made by those 
skilled in the art without departing from the true spirit and scope of the 
invention, as defined in the following claims.