FORM 2
THE PATENTS ACT, 1970 (39 of 1970)
&
The Patents Rules, 2003 COMPLETE SPECIFICATION
(See section 10 and rule 13)
1. TITLE OF THE INVENTION :
"IMPROVED HOT MELT ADHESIVES'

2. APPLICANT (S)
(a) NAME
(b) NATIONALITY
(c) ADDRESS

ICI INDIA LTD. INDIA
DLF Plaza Tower, 10th Floor, DLF Qutab Enclave Phase I, Gurgaon 122 022, Haryana, INDIA

3. PREAMBLE TO THE DESCRIPTION

PROVISIONAL
The following specification describes the invention

COMPLETE
The following specification particularly describes the invention and the manner in which it is to be performed.

4. DESCRIPTION (Description shall start from next page)
5. CLAIMS (not applicable for provisional specification. Claims should start with the
preamble - "l/we claim" on separate page)
6. DATE AND SIGNATURE (to be given at the end of last page of specification)
7. ABSTRACT OF THE INVENTION (to be given along with complete specification on separate page)
Note:-
'Repeat boxes in case of more than one entry.
*To be signed by the applicant(s) or by authorized registered patent agent.
'Name of the applicant should be given in full, family name in the beginning.
'Complete address of the applicant should be given stating the postal index no./code, state and
Country Strike out the column which is/are not applicable.

FIELD OF THE INVENTION:
The present invention relates to a synergistic adhesive composition. It also relates to .novel hot melt adhesives that retain good adhesion at elevated temperatures and a process for making such hot melt adhesives.
BACKGROUND TO THE INVENTION
Hot melt adhesives (HMAs) are thermoplastic materials that are solid at room temperature.. When heated to a liquid or molten form, the HMA can be applied to a substrate. If the substrate is then contacted with another substrate and the HMA brought to ambient temperature, it solidifies to give a bond.
A typical hot melt adhesive formulation comprises a base polymer, a tackifier and wax as the primary ingredient. Optional components include other polymer resins, fillers, plasticisers and anti-oxidants. The most common base polymer used in HMA has been an ethylene copolymer, in particular, an ethylene vinyl acetate (EVA) copolymer. Most EVA copolymer used in HMA applications have a vinyl acetate content of 28-33%.
The use of hot melt adhesives is growing because such adhesives do not use solvents and allow fast production speeds over conventional water or solvent based

adhesives. The hot melt adhesives need to combine contrasting characteristics like fluidity (i.e. low melt viscosity) and temperature resistance and strength at elevated temperatures- Additionally, the adhesive bond so formed should not fail at elevated temperatures. This remains a major technological challenge.
In addition, UV varnish coated substrates are being increasingly used in preference to laminates or aqueous varnish coatings for many applications. Conventional hot melt adhesives, however, fail to bond these substrates.
Various means have been employed for improving the strength and utility of hot melt adhesives at high temperatures.
Curing after application of the hot melt adhesive is one approach. An example of this is the hot melt adhesive based on a polyurethane system which cures irreversibly when exposed to the atmosphere.
Styrene containing polymers have been disclosed in the prior art for high temperature applications. Elastomeric block copolymers comprising of a 'hard' segment like polystyrene and a 'soft' segment like polybutadiene, can be independently used as adhesives for high temperature resistance. However, their use is limited due to their

nigh shear at processing temperatures and their high cost.
Adhesive blends of styrene block copolymers with EVA resins are reported in the prior art. However, some of the problems associated with the use of styrene block copolymers as adhesives remain even when they are incorporated in EVA resins.
PRIOR ART
Thermoplastic, styrene containing polymers are reported as tackifiers for hot melt adhesives. Tackifiers are resins that are added to the formulation to improve the stickiness and wetting properties. Thus,
US 4412030 discloses hot melt adhesives comprising styrene resins and a polymer from a group comprising copolymers of ethylene vinyl acetate, atactic polypropylene and rubbers. The styrene resin is based on styrene, alpha methyl styrene and phenol and is present in an amount between 2 5 and 7 5% with respect to the other polymer. The formulations are particularly useful as pressure sensitive adhesives.
WO 9318107 discloses alpha-methyl styrene/vinyl toluene copolymers as optional tackifiers for hot melt adhesives

comprising of EVA and a thermoplastic polyester. These are shown to be good adhesives for high temperature applications, in particular for PVC substrates. The tackifier is present in an amount from 15-40% in the adhesive.
US 4039733 discloses the use of petroleum resins as tackifiers in hot melt adhesives. The copolymer comprises the spent C4 or C5 petroleum fraction from which dienic constituents have been removed, styrene and divinylbenzene, with the limitation that the styrene content in the final copolymer does not exceed 80%. The resins so produced are claimed to be tackifiers for hot melt adhesives..
The applicants have now found that hot melt adhesives comprising an ethylene copolymer and a styrene polymer are easily processable and have improved strength and do not show adhesive failure at high temperatures. The said finding is surprising with regard to prior art. In addition, such HMAs show good bonding with UV varnish coated substrates.

SUMMARY
Thus, according to the present invention, there is provided a synergistic adhesive composition comprising a) an ethylene copolymer and b) a cross-hnked homo and/or copolymer of styrene or a styrene derivative, wherein the styrene content in the cross-hnked polymer is more than 80%. The compositions also contain a tackifier and wax.
The hot. melt adhesives of the invention show improved rigidity and strength and better adhesion properties and can help bind a variety of substrates. They are particularly useful for UV varnish coated substrates. The hot melt adhesives of the invention can be used for high temperature applications as they show improved performance at elevated temperatures.
STATEMENT OF INVENTION:
Accordingly, the invention provides a synergistic adhesive composition comprising an ethylene copolymer and a cross-hnked homo or copolymer of styrene and/or a styrene derivative, wherein the styrene content in the cross-linked polymer is more than 80%, the amount of ethylene co¬polymer is about 20 to 60%, the composition optionally comprising a tackifier, wax, filler, stabilizer and monomer.
The invention also provides a method of manufacturing an adhesive composition comprising the step of mixing by melt blending an ethylene copolymer and a cross-hnked homo or copolymer of styrene and/or a styrene derivative, at a temperature in the range of 100 to 180 C, wherein the styrene content in the cross-linked polymer is more than 80%, the amount of ethylene co-polymer is about 20 to 60%, and optionally mixing a tackifier, wax, filler, stabilizer and monomer.
DETAILED DESCRIPTION OF THE INVENTION
All percentages and ratios used herein are by weight unless otherwise specified.
The Adhesive Composition
The adhesive composition of the invention comprises a) an ethylene copolymer b) a cross-hnked
homo and/or copolymer of styrene or a styrene derivative (hereinafter

referred to as the cross-linked polymer), wherein the styrene content in the cross-linked polymer is more than 80% c) a tackifier and d) a wax.
The adhesive composition comprises from 2 0 to 60% ethylene copolymer (based on the blend weight). Preferably, the ethylene copolymer is present in an amount from 30 to 50% and more preferably, the ethylene copolymer is present in an amount from 35 to 45%.
The adhesive composition comprises 1 to 25% of the cross-linked polymer. Preferably it is present in an amount from 5 to 2 0% and more preferably in an amount from 5 to 10% in the hot melt adhesive.
Wax and tackifier are preferably present in an amount
from 10 to 40% and 10 to 50% respectively. Optional
ingredients that may be added include fillers,
antioxidants, stabilisers and other polymers.
METHOD OF MANUFACTURE
The adhesive compositions of the present invention may be prepared by melt blending using conventional mixing or blending methods as disclosed in the prior art. An example is melt blending using a hot melt mixer with a centrally mounted pitched blade turbine agitator. While

the mixing temperature is dependent on the components of the HMA, temperatures in the range of 100-180°C are suitable, with a preferred temperature range being 120-150°C. The ingredients can be added in any order but a preferred method of addition would be to add the cross-linked polymer and the ethylene copolymer at the end. Stabilisers can be added during or after the blending. The mixing time is dependent on the components of the HMA, however, mixing for 30 minutes to two hours gives good mixing. The adhesive is then allowed to cool. The solid adhesive can be any form known in the conventional literature, examples of which are blocks, chiclets, noodles, sheets and slats.
A preferred industrial method would be to drop the temperature of the mixer to 110°C and pump the HMA through a filter to a cooling belt using a dispenser. The strands of the hot melt adhesive the solidify on the cooling belt and can be cut into pellets using a pelletiser.
THE ETHYLENE COPOLYMER
The ethylene copolymer of the invention is such that the ethylene content is at least 55% and preferably does not exceed 81%. More preferably, the ethylene content is from 70 to 80% and most preferably from 70-75%.

The other monomer may include vinyl acetate, acrylic esters and vinyl esters or their mixtures thereof. When mixtures of the monomers are used, the proportions in which they are not used is not critical. The acrylic esters are preferably C2-C6 monomers and more preferably methyl acrylate and butyl acrylate.
Other ethylenically unsaturated monomers known in the art may also be present in the copolymer, but it is preferred that the amount in the copolymer does not exceed 3%. Examples of such monomers, without being limited by the same, are methyl methacrylate, maleic anhydride, C2-C4 alkenes, styrene, vinyl chloride, vinylidene chloride, vinyl pyridine, vinyl pyrrolidone, vinyl acetals, acrylamide, acrylic acid and methacrylic acid.
Preferably, an ethylene vinyl acetate (EVA) copolymer is used as the ethylene copolymer. More preferred are the copolymers wherein the vinyl acetate is present in an amount from 20 to 30%. Preferably, such a copolymer has a melt index of 150-400, more preferably from 300-400 and most preferably from 350-400. The commercially available EVAFLEX210™ and DUR-O-SET E200™ are examples of EVA copolymers that can be used in the hot melt adhesives of the invention.

THE CROSS-LINKED POLYMER
The cross-linked polymer is such that more than 80% of the polymer is styrene and/or a styrene derivative. When referring to the styrene content of the polymer, the term includes styrene and its derivatives. Preferably, the styrene content is 90-99.5% and more preferably from 95 to 99.5%.
Examples of styrene derivatives include alpha methyl styrene, 2-methyl styrene, 3-methyl styrene, 4-methyl styrene, dimethyl styrene, trimethyl styrene, 4-tert butyl styrene, 4-phenyl styrene, chlorostyrene, 5-tert butyl-2-methyl styrene, 4-acetyl styrene, 4-benzoyl styrene and 4-butoxy styrene.
Other monomers or their mixtures may be present in the cross-linked polymer but it is preferred that the amount in the polymer is not greater than 20%. The monomers include C2-C6 acrylic polymers (acids and esters), acrylamide, vinyl chloride, vinylidene chloride, vinyl alcohol, vinyl pyridine, vinyl pyrrolidone, vinyl sulfonic acid, maleic anhydride, C2-C6 alkene polymers, vinyl acetals and epicholorhydrin.

Ethylenically unsaturated monomers are preferred. Especially preferred are the acrylate monomers and most preferred are methyl acrylate and methyl methacrylate.
Cross-linking can be obtained by any of known cross-linking reagents like divinylbenzene and N,N' methylenebisacrylamide.
Divinylbenzene is especially preferred to obtain the cross-linked polymer. Preferably, the cross-linking reagent is present in an amount from 0.5 to 2 0% in the final polymer, more preferably from 0.5 to 10% and most preferably from 0.5 to 2% in the final polymer.
The cross-linked polymer is preferably synthesised using any of conventional addition polymerisation methods. Examples of known methods include emulsion polymerisation, suspension polymerisation and free radical polymerisation. Especially preferred is aqueous suspension polymerisation.
In aqueous suspension polymerisation, the monomer is suspended in water in the presence of a surfactant. Benzoyl peroxide is used as an initiator and the polymerisation is initiated by heating the mixture above 60°C. The reaction is continued till fine beads settle down at the bottom of the reactor and the monomer

conversion reaches near 100%. The polymer is separated by filtration, steam sparged to remove the volatiles and dried. The cross-linked polymer is obtained as a white, free flowing fine powder comprising of substantially spherical beads.
The cross-linked polymer is compatible with the other components of the HMA.
TACKIFIERS AND WAXES
The adhesive compositions of the invention also contain tackifiers and waxes.
Examples of tackifiers are polyaromatics, cumarone-indene and other resins"from the coal tar or petroleum industry such that the softening point is > 85°C, wood gum or tall oil rosin acids, the esters of these rosin acids wherein the alcohol is chosen from glycols, polyols, glycerol, petroleum resins, pentaerythritol and related products, polyterpenes, terpene phenolics and non cross-linked low molecular weight styrene resins.
Waxes that can be used in the formulation include both polar and non polar waxes or their mixtures thereof. Examples of these are paraffin wax, microcrystalline wax,

animal and vegetable wax, polyethylene based waxes, Fischer-Tropsch waxes amongst others.
OPTIONAL INGREDIENTS
Optional ingredients that can be used in the hot melt adhesives of the invention include fillers, anti¬oxidants, plasticisers, stabilisers, colorants and other polymers.
Fillers that can be used in the formulation include calcium carbonate, silica, titanium oxide, talc, clays, barium sulphate or their mixtures thereof. The amount of filler in the formulation can be between 2-30%.
Antioxidants include alkylphenols, hydroxyalkyl propionates, hydroxybenzyl compounds, alkylidene bisphenols, secondary aromatic amines, thiobisphenols, thioethers, phosphites and phosphonites and sterically hindered amines.
Where required other polymers may also be added to the formulation. Without being limited by the same, examples of such polymers are styrene block copolymers, epoxidised block copolymers, polyesters, nylon polymers and olefin polymers.

Further examples of such optional additives may be as given in Handbook of Adhesive Raw Materials, E.W. Flick, Noyes Publication, 2nd Ed, 1989.
METHOD OF APPLICATION AND PERFORMANCE OF THE ADHESIVE
The hot melt adhesives of the invention are fluid during application.
The hot melt adhesive of the invention can be applied by any of the methods known in the packaging industry. These include the dauber, wheel and jet. A description of each of these methods is provided in Industrial Packaging Adhesives, ed K. Booth, Blackie, Glasgow and London, 1990.
Particularly preferred methods of application are the single orifice and multiple orifice nozzles.
Industrial applications of such adhesives include flap sealing and carton sealing, wood working, book binding amongst others. A variety of substrates can be bonded using the resins of the invention, examples of such substrates are paper to paper, paper to aluminium foil, paper to laminates like BOPP, PP, polyester, paper to UV varnish coat coated substrates, paper to aqueous varnish substrates, paper to wood and wood to wood, amongst

others. The adhesives of the present invention are useful for surfaces like UV varnish coated substrates wherein conventional EVA adhesives fail. An example of such a substrate is UV varnish coated paper.
High temperature performance is indicated by adhesive failure. Adhesive failure refers to the spontaneous opening of the adhesive bond when subjected to temperatures above 45°C.
An example of sealing requiring high temperature
resistance, to prevent adhesive failure, are cartons
which are exposed to temperatures up to 80°C during
transport and storage.
The invention is further illustrated by the following non-limiting Examples, in which parts and percentages are by weight unless otherwise specified.
Reference is made in the Examples to Figure 1 of the accompanying drawings.

EXAMPLES
CROSS-LINKED POLYSTYRENE
Cross-linked polystyrene samples were procured from Delmar Trading Corporation, India. The samples were free flowing powders comprising of spherical beads. The particle size of the polystyrene was less than 106A. The specific gravity of all samples was between 1.05 and 1.06.
Three samples were used for evaluation. The compositions were as indicated in Table 1. An increase in the amount of divihyl benzene (DVB) indicates increased cross-linking.
Table 1

Sample Styrene {%) DVB {%)
CPS 1 99 1
CPS 2 98 2
CPS 3 96 4

PREPARATION OF THE HOT MELT ADHESIVE
Hot melt adhesives were prepared as follows.
Wax (Paraflint HI™ ex Schuman Sasol) was heated to 140°C on a constant temperature bath. An antioxidant, butylated hydroxy toluene (BHT ex lonol), and an ester gum was charged into the vessel. Cross-linked polystyrene was added and dispersed thoroughly followed by the addition of ethylene vinyl acetate copolymer (EVAFLEX 210™ ex Dupont Mitsui, amount of vinyl acetate 28%, amount of ethylene 72%). The components were mixed at 140°C till a uniform melt was obtained. The hot melt adhesive composition was poured into a mould and allowed to cool.
EXAMPLES 1 TO 3, COMPARATIVE EXAMPLES A TO C
The hot melt adhesive compositions are given in Table 2, in parts by weight.

Table 2

Ingredient A B C 1 2 3
Wax 14 14 14 14 14 14
Antioxidant 0.5 0.5 0.5 0.5 0.5 0.5
Ester gum 38 38 38 38 38 . 38
EVA copolymer 38 - 38 38 38 38
Uncrosslinked Polystyrene .9.5
Polystyrene CPS1 - 9.5 - 9.5 - -
Polystyrene CPS2 - - - - 9.5 -
Polystyrene CPS3 - - - - - 9.5
Weight ratio EVA copolymer to polystyrene 1:0.25
Examples 1 to 3 are within the invention, Examples A to C are comparative.
Comparative Example A contained no styrene copolymer.
Comparative Example B which did not contain the EVA copolymer did not show satisfactory adhesive properties.
Comparative Example C comprising a styrene copolymer which is not cross-linked could not be formulated.

The viscosity, tensile strength and hardness of the hot melt adhesives of Examples 1 to 3 and Comparative Example A were determined.
Viscosity
The HMA was melted and the melt temperature was maintained at 180°C. The viscosity of the HMA melt was determined on a Brookfield Viscometer Model DV2. Measurements were made at an RPM of 2 0 using spindle number 27.
Tensile Strength and Percent Elongation
Tensile strength and percent elongation were determined on a Instron Universal Testing Machine. The HMA was compression moulded at 90 °C into a sheet. Dumbbell shaped specimens were then cut. Measurements were made at 50 mm/minute cross-head speed.
Shore Hardness
Shore A hardness was determined on a Shore Hardness Measurement Tester. Measurements were made on a compression moulded sheet.

Property A 1 2 3
Viscosity (cps) 1375 1962 1987 1900
Tensile strength (Kg/Sq.cm) 33 41 40 39
Percent Elongation 850 '200 180 200
Shore A Hardness 65 75 75 74
Table 3 shows that incorporation of the cross-linked polymer results in an increase in rigidity and strength of the composition. The increase in viscosity does not compromise high temperature fluidity.
EXAMPLES 4 TO 6
In these examples, the effect of using increased amounts of cross-linked polymer was examined. The formulation details and properties are presented in Table 4, together with the corresponding details and properties of Comparative Example A.

Ingredients A 4 5 6
Wax 14 14 '14 14
Antioxidant 0.5 0.5 ■0.5 0.5
Ester gum 38 38 38 38
EVA copolymer 38 38 38 38
Polystyrene (CPS3) 0 2.5 5 7.5
Viscosity (cps) 1375 1425 1537 1812
Tensile strength (Kg/Sq.cm) 33 37 38 39
Percent Elongation 850 420 200 250
Shore Hardness 65 74 73 76

The data on percent elongation shows that an increase in the amount of the cross-linked polymer results in a further increase in rigidity of the composition.

KAAMPLE 7 AND COMPARATIVE EXAMPLE D
Stiffness Modulus
The change in stiffness modulus with temperature was determined for
a) a control formulation (Comparative Example D) containing Paraflint HI wax ex Schuman Sasol (14%), BHT ex Ionol (0.5%), a mixture of ethylene vinyl acetate copolymers ex Dupont Mitsui (42.5%) and ester gum (42.7 5%) and
b) formulation (Example 7) containing Paraflint Hi wax ex Schuman Sasol (14%), BHT ex Ionol (0.5%), a mixture of ethylene vinyl acetate copolymers ex Dupont Mitsui (42.5%), ester gum (42.75%) and CPS 1 (9.5%).
Measurements were carried out on a DMA at a frequency of 2 Hz and a static stress of 8X10-3 sans.
Figure 1 shows the plot of stiffness modulus vs. temperature. The HMA of Example 7 failed at a higher temperature (52°C) than the HMA of Comparative Example D (45°C) , indicating greater high temperature stability.

Adhesive failure
The adhesive failure of sealed cartons was determined for the adhesives of Example 7 and Comparative Example D.
The two compositions were applied using a conventional HMA nozzle applicator at 180°C to seal filled tea cartons. The tea cartons were UV-varnish coated cartons.
The tea cartons were put in an oven maintained at 7 0°C for 96 hours. Adhesive failure of boxes was determined at the end of this period.
Boxes sealed using the formulation of Example 7 did not open even after 96 hours. 40% of the boxes sealed using the composition of Comparative Example D showed adhesive failure within.16 hours.
Thus, the hot melt adhesives of the present invention show superior strength and rigidity and show improved high temperature stability.

We claim:
1. A synergistic adhesive composition comprising an ethylene copolymer and a cross-linked homo or copolymer of styrene and/or a styrene derivative, wherein the styrene content in the cross-linked polymer is more than 80%, the amount of ethylene co¬polymer is about 2 0 to 60%, the composition optionally comprising a tackifier, wax, filler, stabilizer and monomer.
2. A composition as claimed in claim 1, wherein the tackifier and wax is present in an amount of 10 to 30% by wt.
3. A composition as claimed in claim 2, wherein the wax and tackifier are present in an amount 20 to 50%:
4. A composition as claimed in claim 1 wherein the ethylene copolymer is present in an amount from 30 to 50%.
5. A composition as claimed in any one of the preceding claims, wherein cross-linked polymer is present in an amount of from 2 to 2 5%.
6. A composition as claimed in any one of the preceding claims, wherein the cross-linked polymer is more than 80% of the polymer is styrene and/or a styrene derivative.

A composition as claimed in claim 6 wherein the styrene content is from 90-99.5%.
A composition as claimed in claim 7, wherein the styrene derivative is selected from alpha methyl styrene, 2-methyl styrene, 3-methyl styrene, 4-methyl styrene, dimethyl styrene, trimethyl styrene, 4-tert butyl styrene, 4-phenyl styrene, chlorostyrene, 5-tert butyl-2-methyl styrene, 4-acetyl styrene, 4-benzoyl styrene and 4-butoxy styrene.
A composition as claimed in claim 6, wherein cross-linked polymer is present in an amount of from 5 to 20%.
A composition as claimed in any one of claims 1 to 10 wherein the ethylene copolymer is such that the ethylene content is at least 60% and does not exceed 81%.
A composition as claimed in claim 11, wherein the ethylene content is from 70 to 80%.
A composition as claimed in any one of claims 1 to 12 wherein the monomer is selected from vinyl acetate, acrylic esters and vinyl esters or mixtures thereof. A composition as claimed in claim 13, wherein the monomers or their mixtures is present in the cross-linked polymer in an amount not greater than 2 0%. A method of manufacturing an adhesive composition as claimed in claim 1 comprising the step of mixing by melt blending an ethylene copolymer and a cross-linked homo or copolymer of styrene and/or a styrene derivative, at a temperature in the range of 100 to 180°C, wherein the styrene content in the cross-linked polymer is more than 80%, the amount of ethylene co-

polymer is about 20 to 60%, and optionally mixing a tackifier, wax, filler, stabilizer and monomer.
15. A method as claimed in claim 14 wherein the mixing 5temperature is in the range 120-150°C.
16. A method as claimed in anyone of claim 14 or 15, wherein the cross-linked polymer and the ethylene copolymer are added after other ingredients have been added.
Dated this 21st day of September 2 000
RajeshwariH
Of K & S Partners
Attorney for applicant