WO1997039076A1

WO1997039076A1 - Styrene-isoprene-styrene tri block copolymers with adapted diblock copolymers

Info

Publication number: WO1997039076A1
Application number: PCT/EP1997/001906
Authority: WO
Inventors: Eric-Jack Gerard; Noël Raymond Maurice DE KEYZER; Jacqueline Margaretha Mensen-Veurink; Birgitte Maria Ludovica Christine Van De Vliet
Original assignee: Shell Internationale Research Maatschappij B.V.
Priority date: 1996-04-16
Filing date: 1997-04-15
Publication date: 1997-10-23

Abstract

Hot melt adhesive compositions comprising: (a,1) from 75-90 parts by weight of a triblock copolymer A1B1A2 where A1 and A2 represent predominantly poly(monovinyl aromatic) blocks of the same or different peak molecular weight in the range of from 5,000 to 20,000 g/mol, wherein B1 represents a predominantly poly(isoprene) block while the average apparent molecular weight of the total triblock copolymer is in the range of from 160,000 to 250,000 g/mol, and the monovinyl aromatic monomer content is in the range of from 15 to 25 wt.%; (a,2) from 10 to 25 parts by weight of a diblock copolymer A3B2, wherein A3 represents a predominantly poly(monovinyl aromatic) block having a peak molecular weight in the range of from 5,000 to 20,000 g/mol, and wherein B2 represents a poly(isoprene) block having an average apparent molecular weight calculated from GPC measurements which is from 80 to 110 % of the average molecular weight calculated from GPC measurements of block B1, and wherein the monovinyl aromatic monomer content is in the range of from 5 to 17 %; (b) from 50 to 400 parts by weight of a tackifying resin; (c) from 0 to 100 parts by weight of a plasticiser and/or softening agent.

Description

STY ENE-ISOPRENE-STYRENE TRI BLOCK COPOLYMERS WITH ADAPTED DIBLOCK COPOLYMERS

The invention relates to styrene-isoprene-styrene triblock copolymers with adapted diblock copolymers and to adhesive, sealant and coating compositions comprising them. More particular the invention relates to tailor made triblock copolymers-diblock copolymers, containing different molecular weight blocks from predominantly isoprene, and to hot melt adhesives, especially for use in packaging tape formulations.

Block copolymer blends of triblock copolymers and diblock copolymers comprising different molecular weight blocks are known from e.g. US patent no. 5,089,550. Said patent discloses a block copolymer composition, comprising a block copolymer composition for use in adhesives, comprising 50-85° by weight of a block copolymer A]_-B_]_ from an aromatic vinyl compound and preferably styrene, and a conjugated diene and preferably isoprene, respectively, A^ being <20% by weight of (Aj+B^) , A_]_ and B_ having a molecular weight >12,000 and >150,000, respectively; and 15-50?, by weight of a block copolymer A2-B2-A3 from an aromat:c vinyl compound, a con ugated diene and aromatic vinyl compound, respectively, A2+A3 being >20f> by weight of A2+B2+ 3, 2 and A3 having a molecular weight >12,000 and (molecular weight of B2)£ (molecular weight of BT ) /I .5.

In addition, from e.g. US patent no. 5,405,903 a process is known for preparing block copolymer blend compositions which process comprises the subsequent steps of polymerising vinyl aromatic monomer (e.g. styrene) adding conjugated diene monomer (e.g. isoprene) to the polymerisation mixture, adding additional initiator, adding a second portion of conjugated diene monomer, adding a second portion of vinyl aromatic monomer and adding a terminating agent. Said process provides a block copolymer composition, consisting of a blend of a triblock copolymer A-B-B'-A' and a diblock copolymer B'-A', wherein the molar we. "ht ratio B'/B'B depends on the moment of the introduction of additional initiator. The molar weight ratio B'/B'B is specified in the range of from 0.3 to 0.7, more preferably from 0.4 to 0.6 and even more preferably from 0.45 to 0.55.

European patent application no. 0 636 654 discloses a colour stable block copolymer composition for hot melt adhesives, comprising linear styrene/isoprene/styrene triblock copolymer and a specific stabiliser set in an amount of from 0.1 to 5 parts by weight per 100 parts by weight of linear block copolymer.

The triblock copolymer is prepared by anionic sequential polymerisation of styrene and isoprene, optionally in combination with a second initiation, to provide additional diblock copolymer m an amount of less than 40 wt%, relative to the total weight of block copolymer . It will be appreciated that for economic and environmental reasons it would be desirable to be able to provide hot melt adhesive compositions having a low melt viscosity at the processing temperatures of the most relevant applications, that is m the range from 100 to 200 °C.

A low melt viscosity not only reduces the need for organic solvent-based adhesive formulations (compositions) m a number of applications, but also allows for formulation freedom in other applications. For example, in a number of adhesive formulations a pJasticiser, typically oil, is added to reduce the viscosity thereof. Thus, m such formulations it would be possible to lower the amount of plasticiser whilst maintaining the same melt viscosity.

A reduction in the amount of plasticiser, in particular oil, in adhesive formulations would provide a better formulation stability (less oil migration) , and better shear properties.

It would further be desirable to be able to provide hot melt adhesive compositions which have acceptable adhesive properties on difficult substrates, that is substrates which normally cannot be easily wetted by the said compositions, such as foams and kraft, including recycled kraft. In particular it would be desirable to be able to provide hot melt adhesive compositions for use in packaging tapes, having a low hot melt viscosity and acceptable or even improved adhesive properties.

Surprisingly, block copolymer compositions have now been found which impart improvements n one or more respects in hot melt adhesive compositions. In particular, block copolymer compositions have been found which impart a lower melt viscosity to such adhesive compositions as compared with compositions where only the block copolymer composition is different, whilst having acceptable, preferably improved, adhesive properties on difficult substrates.

Accordingly, the present invention relates to hot melt adhesive compositions, comprising

(a,l) from 75-90 parts by weight of a triblock copolymer A B]_A2 where A and A2 represent predominantly poly (monovinyl aromatic) blocks of the same or different peak molecular weight in the range of from 5,000 to 20,000 g/mol, wherein B_ represents a predominantly poly (isoprene) block while the average apparent molecular weight of the total triblock copolymer is m the range of from 160,000 to 250,000 g/mol, and the monovinyl aromatic monomer content is in the range of from 10 to 30 wto, preferably from 15 to 25?>wt. (a, 2) from 10 to 25 parts by weight of a diblock copolymer A3B2, wherein A3 represents a predominantly pol (monovinyl aromatic) block having a peak molecular weight in the range of from 5,000 to 20,000 g/mol, and wherein B represents a poly (isoprene) block having an average apparent molecular weight calculated from

GPC measurements which is from 80 to 110 o, preferably from 80 to 100". more preferably from 80 to 95o, of the average apparent molecular weight, calculated from GPC measurements, of the block Bi, and wherein the monovinyl aromatic monomer content is in the range of from 5 to 17%, preferably from 8 to 13%. (b) from 50 to 400 parts by weight of a tackifymg resm, preferably from 100 to 200 parts by weight. (c) from 0 to 100 parts by weight of a plasticiser and/or softening agent, preferably from 0 to 50 parts by weight. Preferably, blocks A2 and A3 have substantially the same molecular weight, more preferably, blocks Aη , A2 and A3 have substantially the same molecular weight. It will be appreciated that another aspect of the present invention is formed by block copolymer compositions, comprising the hereinbefore specified components (a,l) and (a, 2), optionally mixed with a stabiliser.

The block copolymer composition, comprising the components (a,l) and (a, 2) may be stabilised by means of an amount of from 0.1 to 5 parts by weight and preferably from 0.2 to 1 parts by weight of a stabiliser per 100 parts by weight of (a,l) and (a, 2) . The stabiliser (antioxidant) , used m accordance with the practice of the present invention includes high molecular weight hindered phenol derivatives, high molecular weight hindered amme or triazine derivatives or phosphorous and/or sulphur-containing hindered compounds, or mixtures thereof.

Examples of suitable stabilizers are those disclosed in European patent specification No. 0659787. Preferred representatives of stabilizers are pentaerythrityl-tetrakis (3, 5-dι-tert-butyl-4-hydroxy- hydrocmnamate) (IRGANOX 1010) ; octadecyl ester of 3,5- bis (1, 1-d -methylethyl) -4-hydroxy benzene propanoic acid (IRGANOX 1076) ; 2,4-bιs (n-octylthio) -6- (4- hydroxy-3, 5-dι-tert-butylanιlmo) -1, 3, 5-trιazme (IRGANOX 565) ; 2-tert-butyl-6- (3-tert-butyl-2 ' -hydroxy- 5-methylbenzyl) -4-methylphenyl acrylate (SUMILIZER GM) ; tris (nonylphenyl)phosphite; tris (mixed mono- and di- phenyl) -phosphite; bis (2, 4-dι-tert-butylphenyl) - pentaerythπtol diphosphite (ULTRANOX 626) ; distearyl pentaerythπtol diphosphite (WESTON 618) ; styrenated diphenylamme (NAUGARD 445) ; N-l, 3-dιmethylbutyl-N' - phenyl-paraphenylenediamme (SUMILIZER 116 PPD) ; tris (2, 4-dι-tert-butylphenyl) phosphite (IRGAFOS 168); 4,4- butylidene-bis- (3-methyl-6-tert-butylphenol) (SUMILIZER BBMS) ; or combinations thereof.

Most preferably are used combinations of tris (nonylphenyl) phosphite (TNPP) and IRGANOX 1076, IRGANOX 565, IRGANOX 1010 or SUMILIZER G . (IRGANOX, SUMILIZER, ULTRANOX, NAUGARD, WESTON and IRGAFOS are trade marks . )

The content of monovinyl aromatic monomer m the triblock copolymer is preferably from 15 to 25^" by weight, more preferably from 15 to 20,' by weight. In the triblock and diblock copolymers, the polymerised monovinyl aromatic blocks are preferably polymerised styrene blocks. The predominantly poly (isoprene) blocks of the triblock copolymers to be used in the adhesive compositions according to the present invention typically have average apparent molecular weights, calculated from GPC measurement of the initial prepared poly (monovinyl aromatic) blocks, the final diblock copolymer and the final complete triblock copolymer, in the range of from 80,000 to 250,000, preferably from 100,000 to 240,000, and more preferably in the range of from 100,000 to 200,000 g/mol.

It has now been found that the length of the predominantly, poly (monovinyl-aromatic) blocks has a significant influence on the Hot Melt Viscosity of the hot melt adhesive compositions according to the invention. Therefore, the predominantly poly

( onovinylaromatic) blocks preferably have a peak molecular weight ranging from 9,000 to 15,000 g/mol, more preferably from 9,000 to 11,000 g/mol.

Also, the average apparent molecular weight of the total triblock copolymer has an influence on the Hot Melt Viscosity of the hot melt adhesive compositions according to the invention. Preferably, the average apparent molecular weight of the triblock copolymer is less than 230,000 g/mol, more preferably from 170,000 to 220,000 g/mol, oven more preferably from 180,000 to 205,000 g/mol.

Thus, if one wants to improve existing commercially available block copolymer compositions for use n hot melt adhesive compositions, the said block copolymer compositions having a polyisoprene block in the diblock copolymer having an apparent average molecular weight which is less tnan 80' of the average apparent molecular weiqht of the polyisoprene block in the triblock copolymer, whilst retaining the same total polystyrene content, it is preferred to reduce the length of the polyisoprene block in the triblock copolymer .

With the term "predominantly poly (monovinyl aromatic", as used throughout the specification, is meant poly (monovinyl aromatic) homopolymer blocks e.g. pure poly (styrene) ; copolymer blocks derived from a monovinyl aromatic monomer such as styrene, and structurally related monomers e.g. a mixture of a major part of styrene and a minor part (<20 wt%) of an alkyl- substituted styrene, the alkyl substituent (s) containing from 1 to 4 carbon atoms, e.g. α- methylstyrene, o-methylstyrene, p-methylstyrene, p- tert.butyl styrene; 1, 3-dimethylstyrene or mixtures thereof) ; or copolymer blocks derived from a monovinyl aromatic monomer and structurally not related comonomers, such as styrene in a major amount and isoprene in a minor amount (<15 wt%) .

With the term "predominantly poly (isoprene) ", as used throughout the specification, is meant poly (isoprene) homopolymer blocks; blocks of isoprene and structurally related comonomers having from 2 to 6 carbon atoms, such as butadiene; blocks of isoprene and structurally net related comonomers, typically monovinyl aromatic monomers; or combinations thereof. Structurally related co-monomers typically may be present in an amount of up to 20?, by weight of the polymer block, preferably not more than 10V, by weight. Structurally not related comonomers typically may be present in an amount of up to 15\' by weight of the polymer block, preferably not more than 10- by weight, more preferably not more than 5 by weight.

The predominantly poly(isoprene) blocks can be derived from substantially pure isoprene feedstock as well as from lower concentrate isoprene feedstocks (>50 wt^'' isoprene) containing inert C5 paraffins and mono-olefins . The term inert means that in the isoprene containing mixture, polymerisation killers have been inactivated e.g. by treatment with finely divided sodium in oil.

Preferably block copolymers, containing substantially pure poly (styrene) and substantially pure poly (isoprene) are used m the hereinbefore defined hot melt adhesive compositions.

With the term "apparent molecular weight" as used throughout this specification, the molecular weight of a polymer is meant, except polystyrene itself, as measured with gel permeation chromatography (GPC) using polystyrene calibration standards as described m ASTM D 3016.

With the term "peak molecular weight" is meant the molecular weight corresponding to the peak m the Gel Permeation Chromatogram. For monodisperse polystyrene blocks, the peak molecular weight corresponds, substantially to the true molecular weight.

The average apparent molecular weight of the respective pol (isoprene) blocks can be calculated from the apparent molecular weights measured for the respective diblock and triblock copolymers and the initially prepared living poly (styrene) blocks.

It will be appreciated that the finally used block copolymers are consisting of mixtures of triblock copolymers A^B^A2 and a predetermined well adjusted amount of corresponding diblock copolymers A3B2. Such mixtures of block copolymers are obtainable by a polymerisation process as specified in US patent no. 5,405,903, or by mixing in such a predetermined well adjusted amount of diblock copolymer (A3B2) into the triblock copolymer before or after termination and/or recovery.

Examples of tackifymg resins useful in the compositions of this invention include hydrocarbon resins, synthetic polyterpenes, rosin esters and natural terpenes which are semi-solid or solid at ambient temperatures, and soften or become liquid, that is, have a ring and ball softening point (ASTM E 28- 58T) , at temperatures typically ranging from 70°C to 150°C, preferably from 85°C to 120°C. Such tackifying resins are often referred to as primary tackifying resins .

Exemplary of the primary tackifying resins are compatible resins such as (1) natural and modified rosins such as gum rosin, wood rosin, tall oil rosin, distilled rosin, hydrogenated rosin, dimerized rosin, and polymerised rosin; (2) glycerol and pentaerythritol esters of natural and modified rosins such as, for example, the glycerol ester of pale, wood rosin, the glycerol ester of hydrogenated rosin, the glycerol ester of polymerised rosin, the pentaerythritol ester of hydrogenated rosin, and the phenolic-modified pentaerythritol ester of rosin; (3) copolymer and terpolymers of natured terpenes, e.g., styrene/terpene and alpha methyl styrene/terpene; (4) polyterpene resins the latter polyterpene resins generally resulting from the polymerisation of terpene hydrocarbons, such as the bicyclic monoterpene known as pinene, in the presence of Fπedel-Crafts catalysts at moderately low temperatures; also included are the hydrogenated polyterpene resins; (5) phenolic modified terpene resins and hydrogenated derivatives thereof such as, for example, the resin product resulting from the condensation, in an acidic medium, of a bicyclic terpene and a phenol; (6) aliphatic petroleum hydrocarbon resins; the latter resins resulting from the polymerisation of monomers consisting primarily of olefins and diolefins; also included are the hydrogenated aliphatic petroleum hydrocarbon resins (7) . Mixed aromatic and aliphatic paraffin hydrocarbon resins, and the hydrogenated derivatives thereof; (8) .romatic modified alicyclic petroleum resins and the ydrogenated derivatives thereof; and (9) alicyclic .etroleum hydrocarbon resins and the hydrogenated lerivatives thereof. The preferred primary tackifying ⁵ esms for use in the practice ol this invention are epresented by sub-paragraphs (1), (6), (7) , (8) and 9) , supra.

Various plasticismg and/or softening agents are ..seful n the practice of this invention. The ° jlasticiser and/or softening agent can be used to 'educe viscosity and/or improve tack properties espectively.

Softening agents which have been found useful nclude olefin oligomers and secondary tackifying 5 "esms. The oligomers may be polypropylenes, oolybutenes, hydrogenated polyisoprene, hydrogenated olybutadiene, polypiperylene and copolymers of oiperylene and isoprene, or the like having average ^•πolecular weights between 350 and 35,000. 0 Secondary tackifying resins are liquid at ambient emperature, that is 50°C or less. Suitable secondary ackifymg resins are typically selected from the same lasses (1) to (9) of primary tack fyung resins, ,upra. Preferably, the secondary tackifying resin is "^« elected from the class (1), (b), ( /) , (8) and (9) , upra.

Examples of suitable piasticisers include petroleum-derived oils, synthetic oils such as oils prepared by a Fischer-Tropsch synthesis, as well as 0 oqetable and animal oil and their derivatives.

The petroleum derived oils which may be employed ire relatively high boiling materials containing only a minor proportion of aromatic hydrocarbons (preferably 'ess than 30 and, more particularly, less than 15 by ^s weight of the oil) . Alternately, the oil may be i ot ily non-aromatic. Vegetable and animal oils include glyceryl esters of fatty acids and polymerisation products thereof.

If desired, a filler may also be present in the hot melt adhesive composition an amount from 0 to 50 parts by weight. Examples of suitable fillers include refractory oxides and carbonates.

According to a preferred embodiment of the present invention, the block copolymer constituents are prepared by sequential polymerisation, combination with reinitiation, using sec-butyllithium or n- butyllithium as initiator for both initiations in a solvent comprising cyclopentane or cyclohexane, termination of the living polymer (s) with water, alcohol, hydrogen, mono epoxy compounds and preferably alcohols and more preferably methanol, followed by recovery of the block copolymer (s) by hot water and/or steam coagulation and dewatenng and/or drying.

The stabiliser can be added after termination of living polymer or at any later stage of the manufacturing process, but preferably just after termination.

The solvent to be used for the preparation of the block copolymer (s) , is preferably substantially pure cyclohexane or substantially pure cyclopentane, or mixtures of one of these solvents as mam solvent with a minor amount (<50 t', of linear or branched alkanes having 5 to 7 carbon atoms such as n-hexane, isopentane or n-pentane . Very suitable are cyclohexane, cyclohexane/n-hexane mixtures, cyclohexane/isopentane- /n-hexane mixtures, cyclopentane and cyclopentane/- lsopentane mixtures. Cyclohexane/ sopentane mixtures, substanti lly pure cyclohexane or cyclopentane are particularly preferred and cyclopentane s the most preferred . It will be appreciated that an unexpected advantage of the hot melt adhesive compositions is a combination of a low melt viscosity at the processing temperatures, enabling said compositions to penetrate sufficiently into the pores of e.g. a foam substrate or kraft, including recycled kraft and acceptable adhesive properties meeting the modern requirements.

A low hot melt viscosity allows industrial processing at higher production speeds. Moreover, a low hot melt viscosity creates formulation freedom as set out hereinbefore, and allows industrial processing at lower temperatures.

The invention is further illustrated by the following examples without, however, restricting the scope of the invention to these examples.

From the polystyrene content and the molecular weight of the block copolymers as determined by Gel

Permeation Chromatography (GPC) other data such as the molar weight ratio of the respective polyisoprene blocks in the triblock and diblock copolymers and the styrene weight ratio triblock/diblock can be derived. Examples

Hot melt adhesive formulations were prepared from two styrene-isoprene-styrene block copolymer compositions according to the present invention (A and B) and from a prior art block copolymer, earlier proposed for similar purposes, (KRATON D 1161 identified by C) , as listed in the following tables. All formulations were compounded in a WERNER and PFLEIDERER Z blade mixer (trademark) at 140°C under nitrogen blanket. Two lines of hot melt were applied with a MELTEX

(trademark) hot melt gun at 160°C onto 2.54 cm (one inch) wide substrates. The coated part was laminated with a second substrate layer by means of a standardised roller. The lamination took place immediately after applying the hot melt (warm) . A T-peel test was used to asses the adhesion/cohesion of the samples. All adhesive compositions were tested on polyether foam and Kraft (warm and cold) . The hot melt adhesive composition comprising C was compared with that one comprising block copolymer B on polyethylene security envelopes. In this application, the adhesive composition should not peel off easily when opening the envelope. The plastic must be damaged which will indicate that the envelope has been opened.

An acryl modified polyurethane foam was used to compare adhesive compositions comprising the block copolymers B and C. These kind of foams are difficult to adhere on because the acryl modifier is a release agent.

For this test, a 6 cm sample (2.54 cm wide) was coated over its first 3 cm with two lines of the hot melt adhesive, the sample was folded over 180°C and the coated part was stuck to the non-coated part. A two kilogram weight was put on the folded sample for 1 hour. The two parts were separated by hand order to assess the difference between the respective samples.

As is apparent from Table II, the Hot Melt Viscosity of composition A is significantly lower than the Ho*- Melt Viscosity of composition C. The Hot Melt Viscosity of composition B was 266 Pa.s, which was attributed to the high Mw of the styrene blocks in the block copolymer composition as compared with compositions A and C. Table I

Table II

Table III

a: difficult to peel off; b: easy to peel off

TABLE IV SPECIFICATION OF AUXILIARIES

TABLE V ADHESIVE TEST METHODS

* The Shear Adhesion Failure Temperature (SAFT) test is designed to give an indication of the short-term heat resistance under defined conditions, such as loads and temperature increase. The SAFT test is carried out in an air circulating oven equipped with an electro-mechanical device monitoring a stepwise increase of the temperature. Inside the oven, supports are attached to the side walls on which six aluminium racks can be placed under an angle of 2°. These racks support 6 metal or glass plates onto which the tape samples will be applied. These plates rest against a narrow ridge in which 6 slots are cut; the free part of the coated film on which the weights are hung, passes along these slots. One end of the strip is centred and stuck partially onto a glass plate; the other end of the strip is held so that it does not make contact with the plate but is positioned below. The strip is pressed firmly against the glass plate avoiding entrapment of air bubbles. The adhered part of the strip is cut at 1" of the glass edge. The surface is then 25.4 mm x 25.4 mm (1 inch²) . Finally, the test specimens are stored for 24 hours.

The racks with the samples are installed in the oven and weights of 500 or 1000 grams are hung to the free end of the strip and clamped with "Hofmann" hose clamps .

These weights are initially supported by a base plate which can move up and downwards by means of a "lab jack". Just before the start of the test, the base plate is moved downwards so that the weights now hang freely above the micro or photo-electrical switches. These switches are connected to a counter. The temperature in the oven is displayed on a digital thermometer . When a strip falls, the weight falls down on the micro switch and the timer is stopped. When the time to failure is known, the temperature at which failure occurred can be read off from the calculated time/temperature diagram. The SAFT value is the average of two temperatures recorded with two samples. Further hot melt adhesive formulations for use in packaging tapes were prepared as described herein before, from one styrene-isoprene-styrene block copolymer composition according to the invention (D) and, for comparison, two styrene-isoprene-styrene block copolymer compositions not according to the invention (E and F) , as listed m the following tables. The polyisoprene block of the diblock copolymer of composition F had an average apparent molecular weight of less than 80%, whereas m composition E the diblock content was greater than 25 parts by weight. Block copolymer composition F is a sample of the same grade as composition C.

As can be seen from Table VII, packaging tape formulations prepared from block copolymer compositions according to the invention not only have a low Hot Melt Viscosity (HMV) , but also sufficient adhesive properties .

As discussed herein before, the low Hot Melt Viscosity leaves scope for even improving upon the adhesive properties of the formulation. It belongs to the skill of the skilled adhesive for ulator to select appropriate formulations for improving adhesive properties whilst somewhat increasing the Hot Melt Viscosity. Preferably, the Hot Melt Viscosity in adhesive formulations is not more than 80 Pa.s at

190 °C, as most existing industrial facilities will have difficulty dealing with higher Hot Melt Viscosities . Table VI

Table VII

Claims

C L A I M S

1. Hot melt adhesive compositions comprising:

(a,1) from 75-90 parts by weight of a triblock copolymer A₁B₁A₂ where A₁ and A₂ represent predominantly poly (monovinyl aromatic) blocks of the same or different peak molecular weight in the range of from 5,000 to 20,000 g/mol, wherein B₁ represents a predominantly poly (isoprene) block while the average apparent molecular weight of the total triblock copolymer is in the range of from 160,000 to 250,000 g/mol, and the monovinyl aromatic monomer content is in the range of from 10 to 30 wt%,

(a, 2) from 10 to 25 parts by weight of a diblock

copolymer A₃B₂, wherein A₃ represents a

predominantly poly (monovinyl aromatic) block having a peak molecular weight in the range of from 5,000 to 20,000 g/mol, and wherein B₂

represents a poly (isoprene) block having an average apparent molecular weight calculated from GPC measurements which is from 80 to 110% of the apparent average molecular weight calculated from GPC measurements of block B₁ , and wherein the monovinyl aromatic monomer content is in the range of from 5 to 17%.

(b) from 50 to 400 parts by weight of a tackifying

resin.

(c) from 0 to 100 parts by weight of a plasticiser

and/or softening agent.

2. Hot melt adhesive composition according to claim 1, characterised in that the components (a,1) and (a,2) have been stabilised by means of a stabiliser in an amount from 0.2 to 1 parts by weight per 100 parts by weight of (a,1) and (a, 2).

3. Hot melt adhesive composition according to claims 1 or 2, characterised in that the predominantly

poly (isoprene) blocks of the triblock copolymers have average apparent molecular weights in the range of from 100,000 to 240,000 g/mol, calculated from GPC

measurements of the initial prepared

homopoly(monovinylaromatic) block, the final diblock copolymer and the triblock copolymer.

4. Hot melt adhesive composition according to claims 1-3, characterised in that the peak molecular wen ght of the predominantly poly (monovinylaromatic) blocks is in the range of from 9,000 to 15,000 g/mol.

5. Hot melt adhesive composition according to claims 1-4, characterised in that the tackifying resins

include hydrocarbon resins, synthetic polyterpenes, rosin esters and natural terpenes, which are semi solid or solid at ambient temperatures, and have a ring and ball softening point (ASTM E28 58T) at a temperature ranging from 70°C to 150°C.

6. Block copolymer composition, comprising:

(a,l) from 75-90 parts by weight of a triblock copolymer A₁B₁A₂ where A₁ and A₂ represent predominantly poly (monovinyl aromatic) blocks of the same or different peak molecular weight in the range of from 5,000 to 20,000 g/mol, wherein B₁ represents a predominantly poly ( isoprene) block while the average apparent molecular weight of the total triblock copolymer is in the range of from 160,000 to 250,000 g/mol, and the monovinyl aromatic monomer content is in the range of from 10 to 30 wt%,

(a, 2) from 10 to 25 parts by weight of a diblock

copolymer A₃B₂, wherein A₃ represents a

represents a poly (isoprene) block having an average molecular weight calculated from GPC measurements which is from 80 to 110% of the average molecular weight calculated from GPC measurements of block B₁, and wherein the

monovinyl aromatic monomer content is in the range of from 5 to 17%.

7. Block copolymer composition according to claim 6 characterised in that the components (a,1) and (a, 2) have been stabilised by means of a stabiliser in an amount from 0.2 to 1 parts by weight per 100 parts by weight of (a,1) and (a, 2).

8. Block copolymer composition according to claims 6 or 7 characterised in that the predominantly

measurements of the initial prepared

homopoly(monovinylaiomatic) block, the final diblock copolymer and the triblock copolymer.

9. Block copolymer composition according to claims 6- 8, characterised in that the peak molecular weight of the predominantly poly (monovinylaromatic) blocks is in the range of from 9,000 to 15,000 g/mol.