GB2138008A - Ethylene Copolymers for Low Viscosity Hot Melt Systems - Google Patents

Abstract

Ethylene-vinyl acetate copolymers having a melt index of 1000 to 10000 and containing from 1 to 11% vinyl acetate are useful in low viscosity hot melt compositions, preferably in wax-free binary compositions with natural or synthetic resin tackifiers. The adhesives show long open times with short setting times, and low viscosity making them particularly suitable for high speed sealing operations such as packaging and labelling. In certain sealing applications the melt index of the copolymer may be as low as 600. The EVAs can be prepared by a high pressure polymerization, preferably by using chain transfer agent such as isobutylene to achieve high melt indices.

Description

SPECIFICATION Ethylene Copolymers for Low Viscosity Hot Melt Systems This invention concerns ethylene vinyl acetate copolymers for hot melt systems where low viscosities are required such as in labelling or packaging applications.

Hot melt adhesives (HMA) are compositions applied to a substrate in a molten state, the composition developing the specific final properties for which it was formulated on cooling. The compositions are normally solid at ambient temperatures, and become tacky in their molten state. In packaging applications the hot melt adhesives are used to seal wrappers, cartons and similar packages.

The components of the package to be bonded are joined with the molten adhesive which sets when it cools to form a strong adhesive bond. In labelling the adhesive is used to apply a label to a bottle, carton or similar package. In such applications high speed operations require low viscosity hot melt systems.

One conventional form of HMA is based on a ternary composition comprising: (a) high molecular weight ethylene-vinyl acetate copolymer; (b) a resin; and (c) a wax.

The ethylene-vinyl acetate copolymer (which term will be abbreviated herein to "EVA") is the "backbone" polymer for HMA and other hot melt compositions, and contributes to the mechanical and adhesive properties of such compositions. Conventional EVA used in such applications is characterized by a melt index of 550 or less. Melt index is expressed herein in units of gms/1 0 mins as measured by ASTM D1238 and all references to melt indices of EVAs in connection with the invention are to be construed accordingly. The resin component b), which is normally chosen from a wide range of natural and synthetic resins, functions as a tackifier and also assists wetting. The wax component c) is present to reduce viscosity since high viscosities mean that application temperatures are raised to get an adhesive to flow, and this may result in unwanted thermal degradation.The wax also acts as an extender and a wetting agent and in some compositions as a set time improver. For many applications it is necessary to employ a relatively expensive synthetic wax to achieve a desired combination of properties. "Encyclopaedia of Polymer Science and Technology", Interscience, 1967, 6, p41 416, discusses the use of EVA in wax-based hot melt systems and summarises the effect on adhesive properties of using EVAs of differing vinyl acetate content and melt index, showing trends for commercially available EVAs with a melt index in the range of 3 to 450, with a single EVA having a melt index over 1000, Allied Chemical product AC-400.The article states that lower melt index indicates improved toughness, better barrier properties in creased sections, higher viscosity and better hot tack properties.

Other hot melt compositions containing waxes are known. GB 1 462 628 describes the high pressure production of ethylene-vinyl acetate-isobutylene terpolymers containing 1 8 to 28% by weight vinyl acetate and 0.5 to 3% by weight isobutylene and having a melt index of at least 150, preferably from 1 50 to 550, and their use in hot melt coating compositions with a wax. US 2 877 1 96 discloses EVA/petroleum wax blends in which the EVA has a melt index of from 0.5 to 1 000 as compositions for surface coating.CA 742287 describes similar EVA/wax blends containing a high molecular weight EVA component with a melt index below 80 and a low molecular weight component having a vinyl acetate content between 10% and 20%, an average molecular weight between 1000 and 5000, melt viscosity at 3000F (1 490C) between 50 and 600 centipoises (50--600 mPa.s), inherent viscosity (0.25% toluene solution at 300 C) between 0.1 and 0.3 and softening point (ring and ball) between 1900F and 210 F (880C-990C).

Some wax-free compositions have been described containing an EVA and a tackifier resin, but these compositions are based on known, low melt index EVA which results in a viscous composition only suitable for high temperature operation when there is a danger of degradation.

Swedish Patent 346553 discloses an adhesive melt comprising an ethylene vinyl acetate and either a styrene-diene copolymer resin or a thermoplastic resin; Japanese Patent Publication 51 - 1 49327 discloses a binary composition of an EVA having melt index of 1 5 to 450 in conjunction with a resin derived from C5 to C9 naphtha fractions and having a low cyclopentadiene and dicyclopentadiene content. Japanese Patent Publication 53-147733 describes a HMA intended for extrusion coating based on an EVA plus a tackifier, but teaches the addition of a second EVA to produce the appropriate properties in the composition. Only low melt index EVAs are disclosed.US 3448178 discloses HMA compositions containing EVA with a high molecular weight of 200,000 to 500,000 in conjunction with a phenolic, modified terpene resin. US 391 4489 discloses an ethylene/vinyl ester copolymer having a melt index of 2 to 100 in conjunction with a resin having specific physical characteristics to give the composition high tensile strength. US 3926878 concerns hot melt compositions containing a specific hydrogenated aromatic petroleum resin in conjunction with an EVA having a melt index of 1 to 400.An article by Duncan and Bergerhouse, "Adhesives Age" March 1980, pp 37-41, describes hot melt pressure sensitive adhesive compositions containing a tackifier and an ethylene/vinyl acetate copolymer having 1 9-60% vinyl acetate; the highest melt index disclosed is 1 50.

To reduce viscosity of the EVA/resin mixtures based on conventional, low melt index EVAs, it is usual to add a wax as described in the prior art mentioned above. Other modifiers have been employed in hot melt compositions.

US 4 283 317 describes wax-free hot melt adhesive compositions based on an -olefin copolymer and a tall oil rosin and containing a hydrocarbon oil. GB 1 485 253 describes hot melt compositions comprising an EVA having 10 to 45 wt% vinyl acetate and 1 to 1000 g/l 0 min melt index and/or a rubber together with a low molecular weight propylene-ethylene copolymer.

Binary compositions based on relatively low molecular weight polyethylene homopolymers have been proposed, and are described, for example, in GB 1 345 620, but such polyethylene-based HMA compositions have been found to have poor adhesion.

A new class of high melt index EVA copolymers with low vinyl acetate content has now been found to be useful in low viscosity hot melt systems.

EVA as used in conventional hot melt compositions generally has a melt index below 1 000, and most usually below 550. Very high melt index EVA is also known. "Encyclopaedia of Polymer Science and Technology" discloses AC-400 as an EVA with melt index of "over 1000" suggesting that this parameter had not been precisely measured. The vinyl acetate content of this commercially available EVA is 14-1 6% and the melt index has been measured as 23500, and it is thus effectively a synthetic wax, differing from normal synthetic waxes in that it is a copolymer rather than a polyethylene homopolymer.

GB 1 260 486 discloses AC-400 as an auxiliary polymer in at coating composition in addition to the film-forming copolymer and AC-400 is described therein as a "polyethylene wax containing vinyl acetate". GB 1 233 797 discloses a hot melt adhesive composition containing AC-400 with a particular combination of rosin esters, the composition displays cold tack-that is, adhesive properties at room temperatures-which makes the composition highly unsuitable for hot melt adhesives for packaging as it would lead to problems in handling the composition. US 3 368 991 describes transfer inks containing waxy EVA containing 10 to 30% vinyl acetate and having an average molecular weight between 1 000 and 5000.These waxy copolymers have a kinematic viscosity at 1 400C of from 50 to 2000 centipoises (50--2000 mPa.s) and a melt index very greatly in excess of 1 000, the correlation being given as 1000 melt index roughiy equivalent to 11000 centipoises (11000 in mPa.s), indicating that the waxy copolymers have a melt index of many thousands. A further class of EVA having very low molecular weight and thus very high melt index is used as additives for petroleum fractions, for example as described in GB 1 374 051. Typically such polymers are of such low molecular weight that the melt index is virtually immeasurably high. EP 3489 describes an EVA additive having 22 to 36% vinyl acetate and a melt index of 50 to 1000.

Our copending European Application 82-305382.2 describes EVAs having a vinyl acetate content of from 11 to 40% by weight (based on the copolymer) and a melt index of from 700 to 10000, and their use in wax-free hot melt compositions and other hot melt systems. It further describes hot melt roadmarking compositions comprising an EVA having a vinyl acetate content of from 10 to 22% by weight vinyl acetate and a melt index of from 2000 to 7500.

We have found that certain EVAs with low vinyl acetate content and high melt index may be useful in hot melt systems where low viscosity is required.

Accordingly, in one aspect this invention provides a hot melt adhesive composition with a viscosity with a viscosity (measured at 1 800C) not exceeding 3000 mPas comprising- a base polymer and a tackifier, the base polymer comprises at least one ethylene vinyl acetate copolymer having a vinyl acetate content of from 1 to 11% by weight (based on the copolymer) and a melt index of from 1 000 to 10000.

The invention also provides for the use of the EVA defined above in low viscosity hot melt systems, and to a method of sealing in which a hot melt adhesive composition as defined above is applied in a molten state to at least one of two surfaces to be joined, the surfaces are brought into contact and thereafter cooled so that a bond is formed between the surfaces.

The composition of the invention may include one dr more waxes such as paraffin waxes, microwaxes and synthetic waxes such as low molecular weight ethylene homopolymers and copolymers including high melt index (greater than 10,000 melt index) EVAs or hydrocarbon waxes produced by the Fischer-Tropsch process. However, in a preferred aspect the hot melt compositions and systems of the invention are wax-free.

By the term "wax-free" as used herein it is meant that the HMA or composition contains substantially no viscosity-modifying, natural or synthetic wax in addition to the defined EVA and tackifier component. The system or composition is thus substantially free, inter alia, from the waxes mentioned above.

The compositions and systems of the invention may also contain other conventional base polymers, such as conventional EVAs, and thus the low vinyl acetate, high melt index EVAs used in the invention may be used as partial replacement for such conventional polymers. In a preferred aspect, however, the base polymer component consists essentially of the EVA having a vinyl acetate content of from 1 to 11% by weight and a melt index of from 1 000 to 10000, hereinafter referred to as the "EVA of the invention".

The vinyl acetate content of the EVA of the invention affects the performance of the HMA in that lower VA content results in faster setting but a tendency for the adhesive strength to be reduced. Thus, it is possible to tailor a HMA composition to a particular application by suitable choice of the VA content of the EVA to achieve a particular balance of setting time and adhesive strength. A preferred range of VA content is from 1 to 10% by weight, more preferably from 1 to 7.5% by weight and most preferably from 4.5 to 7.5% by weight.

The EVAs of the invention have relatively higher melt indices than is conventional for hot melt EVAs yet lower melt index than EVAs used as fuel additives. It is preferred for the melt index of the EVAs to be in the range of 1 500 to 7500, more preferably 2000 to 6500, but again within these ranges the optimum melt index may be chosen with regard to the desired viscosity of the HMA.

The invention may be employed in low viscosity hot melt compositions particularly for packaging and for labelling in a variety of compositions.

The tackifier employed in such wax-free hot melt compositions may be a natural or a synthetic resin. Among the natural resins which may be employed are the polyterpenes, rosins, rosin esters and derivatives, as well as their hydrogenated forms. Various synthetic petroleum resins may be employed, and suitable resins include products obtained by thermal or catalytic polymerization of aliphatic (including cyclic olefins) and/or aromatic hydrocarbons and hydrogenated derivatives thereof, as well as mixtures of such resins. By way of example, the so-called aliphatic resins may be obtained by polymerization of C5 naphtha fractions containing C5 conjugated diolefins such as isoprene, 1,3- pentadiene or mixtures thereof, or these fractions with other components such as cyclopentadiene, methyl cyclopentadiene or their dimers or codimers.Optionally these resins may be hydrogenated. The so-called aromatic resins may be obtained by polymerizing a petroleum fraction containing polymerizable aromatic hydrocarbons such as styrene, -methylstyrene, vinyl toluene, vinyl xylene, propenyl benzene, indene, methyl indene and ethyl indene, and optionally hydrogenating the resultant resin. Resins may also be prepared from pure monomers such as a-methyl styrene and/or vinyl toluene.

Other chemically modified resins may also be used-for example, modification with a,P-unsaturated acid or anhydride. Examples of other resins include mixed aliphatic/aromatic resins and modified terpene resins such as phenolic modified terpene resins.

A great advantage of the invention, however, lies in the use of the EVAs of the invention to replace both the conventional polymer component and the viscosity modifier component of hot melt compositions; the resulting wax-free compositions not only have the advantage of avoiding the need for wax or other viscosity modifiers, but also show improved performance over compositions using conventional EVA or other polymers.

The wax-free hot melt adhesive compositions of the invention may desirably contain the EVA and tackifier in a weight ratio of from 95:5 to 5:95. A high proportion of EVA tends to result in high viscosity, better cohesive strength, better tensile strength and better cold flexibility, whereas high resin contents tend to enhance adhesion and wetting.

Preferably the weight ratio of EVA to tackifier is in the range of from 95:5 to 50:50, and most preferably the binary compositions contain from 50 to 70% by weight of the composition of EVA.

However, it is a feature of the wax-free compositions that the properties do not vary as significantly as in conventional ternary compositions with varying ratios of EVA and tackifier. Thus, while ternary compositions are frequently formulated for particular applications within relatively narrow ranges of EVA, wax and tackifier content, the wax-free compositions of the invention show much greater flexibility, as well as greater predictability in terms of their behaviour and performance.

As aiready indicated the wax-free compositions have low viscosity without having to resort to the addition of waxes. The compositions of the invention are formulated to give a viscosity of not more than 3000 mPa.s (at 1 800C), more preferably from 500 to 2000 mPa.s and most preferably from 1000 to 2000 mPa.s. It is a feature of the compositions of the present invention that the low overail viscosity is obtained without an undesirable reduction in adhesive properties. It is a further feature that the compositions have good viscosity stability.

In many applications of HMA it is highly desirable for the HMA when molten to remain capable of sticking for a relatively long period to enable the materials to be bonded to be correctly positioned. that is to say, the "open-time"-the time available from applying molten HMA to the substrates to be bonded until the substrates are brought together-should be long. At the same time, the "setting time"-the time from bringing the substrates together until the bond is set-should be short.

Previously, formulations with long open time tended to have long setting times and vice versa. It is a particular feature for the present invention that it is possible to formulate compositions which combine a long open time and a short setting time even without wax. Packaging lines generally operate at relatively high speeds so that setting times of less than 1 second are desirable. When sealing larger boxes and corrugated cardboard cases a setting time of 0.5 to 1 second is particularly preferred, whereas smaller cartons such as cigarette packets use high speed lines whelp setting times of 0.1 to 0.5 seconds are preferred.

For particular applications, it may be appropriate fcr the hot melt compositions of the invention to contain other components common in HMA formulation. Thus, for example, the compositions may also contain other polymers, antioxidants, plasticizers, fillers and flow improvers.

HMAs prepared in accordance with this invention may be used to bond a variety of substrates including metals such as aluminium, polyolefins such as polyethylene and polypropylene, other polymers such as polyvinylchloride, polyamides and polyesters, paper, textiles, treated and untreated cardboard and wood.

The EVAs described above and certain EVAs with lower melt index may also be useful in other sealing applications such as providing layers on caps for bottles and on the ends of cans. Metal caps for bottles are provided with a polymer sealant layer to promote a good seal. In addition cans, particularly for beverages, employ a sealant between the separate parts of the can, again to give a good seal. It has been found that a hot melt system comprising an EVA having a vinyl acetate content of from 1 to 11% by weight and a melt index of from 600 to 3000, preferably 1000 to 2500, may be used as sealant, particularly in cap sealing and can sealing.

The EVA of the present invention is conveniently prepared by a high pressure polymerization process in which ethylene and an appropriate amount of vinyl acetate are poiymerized at a temperature not greater than 320 C and at a pressure of 300 to 3000 kg/cm2, optionally in the presence of an appropriate amount of a chain transfer agent, to give the desired melt index to the EVA product.

The polymerization may be effected in a conventional autoclave or tubular reactor. The reaction temperature is normally between 130 and 2500C and is preferably from 145 to 2300 C. The lower the reaction pressure the higher the output of the polymerization reaction but at the expense of higher consumption of chain transfer agent and initiator. Thus, the preferred pressure range is from 450 to 2100 kg/cm2, more preferably from 600 to 1850 kg/cm2.The particular conditions are chosen having regard to the product required.

The chain transfer agent or modifier may be introduced into the reaction vessel to adjust the melt index of the product. The amount of chain transfer agent used therefore depends on the agent chosen and the final melt index sought, as well as the reaction conditions employed and vinyl acetate content since the latter acts as a chain transfer agent itself. Typically the amount of chain transfer agent will be from Oto 25% of the reactor feed. The use of chain transfer agents is well known, and given the requirements specified herein for the melt index of the EVA it is believed to be within the competence of one skilled in the art to determine a suitable agent and rate of addition empirically.By way of illustration it may be said that suitable agents include those with C5 values (measured at 1 360 atmosphere and 130 C for homopolymerization of ethylene) of up to 1.0 and include hydrogen, acetone, butyraldehyde, cyclohexanone, butene-1, propylene, butane and isobutylene. C5 values of up to 0.1 are preferred. It has been found that isobutylene is a convenient chain transfer agent.

It is to be noted that in some instances there will be incorporation of the chain transfer agent into the EVA copolymer. For example, when isobutylene is employed the product will be effectively an ethylene/vinyl acetate/isobutylene terpolymer. As used herein, the terms "EVA", "EVA copolymer" and "ethylene vinyl acetate copolymer" include polymers containing such additional comonomers.

As an alternative to using a chain transfer agent, it has been found possible to prepare certain EVAs of the invention by operating at lower pressures of preferably 300 to 1000 kg/cm2.

The polymerization will usually be initiated by a free radical catalyst such as a peroxide. Also conventional additives such as antioxidants and carriers for the catalyst may be present according to conventional practice.

The invention will now be described in more detail, though only by way of illustration, with reference to the accompanying drawing which is a schematic block diagram of an apparatus for preparing an EVA of the invention.

Ethylene is introduced at 45 kg/cm2 into compressor 1 where it is compressed to about 1 50 kg/cm2, then vinyl acetate and chain transfer agent are added and the combined feed is further compressed in compressor 2 to approximately 1 550 kg/cm2. The high pressure feed is then introduced into autoclave 3 fitted with stirrer 4. A free radical catalyst is introduced via line 5 and at separate points on the autoclave (not shown). The autoclave 3 may be cooled or heated as appropriate to maintain the desired reaction temperature.

The formed polymer, together with unreacted material and inpurities is taken via line 6 to a high pressure separator 7. Unreacted monomer separated off are recycled via heat exchanger 11 to compressor 2. The remainder of the output is fed to low pressure separator 8, from which EVA is collected and fed to extruder 9 where it is formed into pellets. The unpolymerized material separated at 8 is fed via heat exchanger 12 to a small compressor 10. Impurities are separated off in a purge 13, and unreacted monomers are recycled to compressor 1.

The following Examples and Tests are now given, though again only by way of illustration, to show certain aspects of the invention and its evaluation in more detail.

EXAMPLE 1: Preparation of EVA An apparatus as described hereinbefore in relation to the drawing was operated under the following conditions: Reactor pressure=1740 kg/cm2 Reactor temperature=145 C Feed temperature=480C Catalyst: 20% solution of t-butyl perpivalate in isododecane Antioxidant: butylhydroxytoluene Chain transfer agent: 4% (by weight) isobutylene.

The product obtained is an ethylene/vinyl acetate copolymer containing 5.4% by weight vinyl acetate, and incorporated isobutylene and having a melt index of 2590.

EXAMPLES 2 to 4: Preparation of Binary HMA Compositions Binary hot melt adhesive compositions were prepared using the EVA prepared in Example 1, in conjunction with Escorez 2203, a petroleum resin available from Exxon Chemical Company which comprises the product of a Friedel-Craft catalysed polymerization of a C5 naphtha fraction to which are added C5 branched olefins and styrene. Table 1 below gives the proportions of the EVA and resin in the binary compositions. In addition, to each composition was added 0.5% (based on the weight of the total composition) of Irganox 1076, a phenolic antioxidant available from Ciba-Geigy.

Formulation The Irganox 1076 inhibitor, half the EVA and half the resin were added under a nitrogen blanket to a 1 litre sigma-blade mixer at 1 750C and mixing was commenced. The remaining EVA and resin was then added in increment over 5 to 10 minutes. After 90 minutes mixing the formulation was removed and cooled under a nitrogen blanket.

TABLE 3 Example EVA (% by wt.) Resin (% by wt.) 2 90 10 3 70 30 4 60 40 EXAMPLES 5 and 6: Binary HMA Compositions Compositions were prepared by the procedure of Examples 2 to 4 as follows: TABLE 4 Example EVA:Resin Resin EVA VA(%) Melt Index 5 70:30 Escorez 2203 9.2 7.6 6 70:30 Escorez 2203 6530 8800 Comparative Example: Preparation of EVA To provide a comparison, EVA having a melt index outside the range specified for the invention were prepared. The properties of this product and the conditions for preparing it are set out in Table 5 below.

TABLE 5 Reactor Reactor Product Comparative Pressure Temp Chain Transfer Example (kg/cm2) (OC) Agent (wt%) VA(%) MI J 1740 145 Isobutylene 2% 5.5 735 Test Results-Performance of Examples 2 to 6, Comparative Example and Conventional Wax Containing Compositions The products of Examples 2 to 6 and the Comparative Example was tested in wax-free hot melt compositions and compared to conventional packaging ternary (wax-containing) compositions with the following compositions:: (parts by weight) Reference I II EVA-28%VA MI 400 33 33 Escorez 2203 33 33 Paraffin(650CMP) 33 Hard microwax* - 33 Irganox 1076 < -0.5- > *Muítiwax 180 MH from Witco Chemical Comparison of Reference I with Reference II shows the effect of using expensive hard microwax, which is unnecessary according to the invention.

The wax-free compositions containing the product of the Comparative Example were formulated with Escorez 2203 and 0.5% Irganox 1076, in weight ratios (EVA:resin) of 60:40, 70:30 and 90:10 using the procedure described in Examples 2 to 4.

Test Results The compositions were compared in the following series of tests.

Test Measurement Cloud Point (AMS 360-22) C Viscosity at 1 800C (ASTM D3236) mPa.s Viscosity stability-viscosity after 50 hours at 1 800C (ASTM D3236) mPa.s Colour in molten state (ASTM D1544) Gardner No.

Colour stability after 25 and 50 hours at 1 8O0C Gardner No.

(molten state) Skinning after 50 hours at 1 8O0C % of surface skinned Tensile strength at break point (ASTM D638) g/mm2 Elongation at break (ASTM D638) Open time at 1 8O0C (measured on Olinger machine secs.

at 1.5 kg/cm2 pressure Setting time at 1 800C (measured on Olinger machine secs.

at 1.5 kg/cm2 pressure) T-peel on polyethylene (PE) aluminium (Al) -g/cm polypropylene (PP) (ASTM D 1876) Hot shear at 40/60/800C (see below for procedure) hours Low temperature performance (see below @ C for procedure Loss of weight at 1800Cfor3 hours

Appearance} Visual check Stringing The results obtained are shown in Table 2.

Hot Shear Test Test strips were prepared by laminating the hot melt adhesive between sheet of polyethylene.

The test strips were cut to 1/2 inch (12.7 mm) width and had a shear contact area of 1/2 inchx 1/2 inch (12.7 mmx12.7 mm).

The test strips were then placed in an oven at the test temperature and a shear load of 0.5 kg was applied. The time for the HMA bond to fail was measured.

Low Temperature Performance Sheets of the HMA under test were prepared with a thickness of 0.9 mm. Test strips 1/2 inch5 inches (12.7 mm x127 mm) were cut from the sheets. The test strips were then arranged in a flex tester (a device in which the test strips may be bent around a metal cylinder 3 cms in diameter) and the tester with the test strips was then cooled to the test temperature in a freezing compartment.

After 1 6 hours the flex tester was operated to bend the strips through 1 800 around the cylinder in 1 5 seconds.

The temperature of any broken test strips was measured.

The test was started at a temperature of-90C and repeated at temperatures reduced in 3"C intervals. For each HMA the result recorded is the temperature of which the test strip failed.

T-Peel The laminated samples for carrying out the test according to ASTM Dl 876 were prepared by sandwiching a layer of adhesive between sheets of the substrate to be tested in a hot press for 45 seconds. Spacers were used between the sheets of substrate under test to ensure a uniform 1 50 m sample thickness. The hot press was used to melt the adhesive composition for 2 minutes, then light pressure was applied for 4 minutes to press the sheets of substrate together to give the sample thickness set by the spacers. The press temperatures were: PE 900C Aluminium 1 500C The results are given in Table 5 below.

TABLE 5 Composition 2 3 4 5 6 EVA:Resin 90:10 70:30 60:40 70:30 70:30 Cloud Point, C 92 87 82 79 77 Colour Gardner 6 9 10 Appearance Clear Clear Clear Clear Clear Viscosity mPa.s at 180 C 2620 1500 1087 670 485 Heat Stability Colour (50 hrs at 1800C) 12 15 17 Viscosity (50 hrs at 1 800C) 2590 1510 1095 Viscosity change, % -1 +0.6 +0.8 Skinning No 25 50 Tensile at break, g/mm2 380 289 309 ' 189 176 Elongation at break, % 63 138 1 67 88 Cold flexibility, C -29.5 -17.5 -10 -19.5 -18 Open time sec. at 180 C 9 15 18 14 16 Setting time sec. at 1 800C 0.8 0.6 0.5 0.5 0.8 T-Peel g/cm: PE/PE 75 90 25 225 175 AL/AL 100 100 100 600 250 Weight loss 3 hrs. at 1 800C 0.3 0.8 1.1 Stringing, % No No No TABLE 5 (Cont'd) Composition Comparative EVA:Resin 60:40 70:30 90:10 Cloud Point, CC 90 86 91 Colour Gardner 9 9 6 Appearance Clear Clear Clear Viscosity mPa.s at 1 800C 3200 4800 9700 Heat Stability Colour (50 hrs at 180 C) 16 14 11 Viscosity (50 hrs at 1800C) 3210 4880 9680 Viscosity change, % +0.3 +2 0 Skinning 90 75 No Tensile at break, g/mm2 392 404 487 Elongation at break, % 155 227 194 Cold flexibility, C -10 -15 -29.5 Opentimesec.at1800C 13 11 6 Setting time sec. at 1 800C 0.3 0.4 0.5 T-Peel g/cm:PE/PE 25 40 40 AL/AL 100 400 170 Weight loss 3 hrs. at 1 800C 1.2 1.0 0.4 Stringing, % No No No brittle slightly brittle TABLE 5 (Cont'd) Composition Reference I II Cloud Point, OC Colour Gardner 10 11 Appearance Viscosity mPa.s at 1 800C 275 406 Heat Stability Colour (50 hrs at 1800C) 17 > 17 Viscosity (50 hrs at 1 800 C) 336 402 Viscosity change, % +22 -1 Skinning No 100 Tensile at break, g/mm2 317 238 Elongation at break, % 36 72 Cold flexibility, OC -17 17 Open time sec. at 180"C 10 12 Setting time sec. at 1 800C 1.3 0.8 T-Peel g/cm:PE/PE 35 1500 AL/AL 400 600 Weight loss 3 hrs. at 1 800C - - These results of the Test demonstrate the surprising versatility of the compositions of the invention. The compositions of the invention show a useful combination of properties for hot melt adhesives in packaging applications, and have acceptable viscosities without the need for a wax component. They show low stringing,-good viscosity stability and good adhesion, together with short setting times and low viscosity. By contrast the compositions based on the Comparative Example have higher viscosity and the compositions of Reference I and II have longer setting time, and require the addition- of wax.

Viscosity Measurement A number of small samples of EVA were taken during changing polymerjzarion conditions in the preparations described hereinbefo.re. These samples were analyzed to determine-VA content, the melt index was measured by ASTM D-l 238 and the viscosity was measured using a Brookfield viscometer at several temperatures. The results are given in Table 6 below.

This shows the EVA for use in the invention all have viscosity at 1 500C in excess of 2000 mPa.s and thus a viscosity at 1 400C considerably in excess of 2000 mPa.s.

TABLE 6 Ml Viscosity in mPa.s (g/10 min) VA (%) 1500C 1900C 1640 5 13280 5300 2590 5 6930 2850 3500 5 4860 2050 6050 2 2700 1150 6870 10 2390 1004 *calculated from viscosity measured at 1 300C and 1 500 C.

Claims (14)

1. A hot melt composition with a viscosity (measured at 1800 C) not exceeding 3000 mPas, comprising a base polymer component and a tackifier, in which the base polymer component comprises at least one ethylene-vinyl acetate copolymer having a vinyl acetate content of from 1 to 11% by weight (based on the copolymer) and a melt index of from 1000 to 10000.

2. A hot melt composition in claim 1, in which the EVA has a vinyl acetate content of from 1 to 10% by weight.

3. A composition as claimed in claim 1 or 2, in which the copolymer has a vinyl acetate content of from 1 to 7.5% by weight.

4. A composition as claimed in claim 3, in which the copolymer has a vinyl acetate content of from 4.5 to 7.5% by weight.

5. A composition as claimed in any of claims 1 to 4, in which the copolymer has a melt index of 1500 to 7500.

6. A composition as claimed in claim 5, in which the copolymer has a melt index of 2000 to 6500.

7. A composition as claimed in any of claims 1 to 6, which also contains a wax.

8. A wax-free composition with a viscosity (measured at 1 800C) not exceeding 3000 mPas, comprising a base polymer component and a tackifier, in which the base polymer component comprises at least one ethylene-vinyl acetate copolymer having a vinyl acetate content of from 1 to 11 % by weight (based on the copolymer) and a melt index of from 1000 to 1 0000.

9. A wax-free composition as claimed in claim 8, in which the base polymer consists of at least one EVA having a vinyl acetate content of from 1 to 11% by weight and a melt index of from 1 000 to 10000.

10. A composition as claimed in claim 8 or claim 9, which contains the EVA copolymer and tackifier in a weight ratio of from 95:5 to 5:95.

11. A composition as claimed in any of claims 1 to 10, which contains from 50 to 70% (by weight of the composition) of the EVA.

12. A composition as claimed in any of claims 1 to 8, which has a setting time (measured at 1 800 C) of not more than 1 second.

1 3. A method of sealing, in which a composition as claimed in any of claims 1 to 10 is applied in a molten state to at least one of two surfaces to be joined, the surfaces are brought into contact and cooled so that a bond is formed therebetween.

14. The use in a hot melt system with a viscosity (measured at 180 C) not exceeding 3000 mPas of an EVA copolymer having a vinyl acetate content of from 1 to 11% by weight and a melt index of 1000 to 10000.

1 5. The use as sealant of a hot melt system comprising an EVA having a vinyl acetate content of from 1 to 11% by weight and a melt index of from 600 to 3000.

1 6. A process for the preparation of a composition as claimed in any of claims 1 to 1 5, in which an ethylene vinyl acetate copolymer is prepared by polymerizing ethylene and an appropriate amount of vinyl acetate at a temperature not greater than 3200C and at a pressure of 300 to 3000 kg/cm2, optionally in the presence of an appropriate amount of a chain transfer agent, to give an EVA product with the desired melt index, and the EVA product is thereafter mixed with a tackifier, and optionally additional components, to form the desired composition.