FIELD OF THE INVENTION

The present invention is in the field of a hot melt adhesive composition, use of said hot melt composition and products comprising said composition.

BACKGROUND OF THE INVENTION

A hot melt adhesive (HMA) , also known as a hot glue, is a form of thermoplastic adhesive. It is commonly supplied in solid form, as blocks, pillows, powders, films or sticks designed to be melted e.g. in an dedicated hot melt melter and to be applied using a hot melt applicator. Such a glue is tacky when hot, and solidifies in a relatively short time, typically from a few seconds to a few minutes exceptionally. Hot melt adhesives can also be applied by dipping or spraying.

Hot melt adhesives provide several advantages. Volatile organic compounds are reduced or eliminated, a drying or curing step is eliminated, they have a long shelf life, they do not lose thickness during solidifying, and usually can be disposed of without special precautions. They also have some disadvantages, such as a thermal load (due to hot application) of a substrate, loss of bond strength at higher temperatures, and complete melting of the adhesive.

General properties of hot melt adhesives are open time (working time to make a bond, where the surface still retains sufficient tack, can range from seconds for fast-setting adhesives to infinity for pressure- sensitive adhesives) , set time (time to form a bond of acceptable strength) , tack (degree of surface stickiness of the adhesive, and surface energy (influences wetting of different kind of surfaces) .

Hot melt glues often consist of one base material with various additives. The composition is then formulated to have a glass transition temperature (onset of brittleness) below a lowest service temperature and a suitably high melt temperature as well. Hot melt adhesives can be tailored for a given application, e.g. the melt viscosity, tack, strength and the crystallization rate (and corresponding open time) and crystallization level. It is considered that a lower level of crystallization, such as can be reached by using amorphous polymers will lead to softer adhesives such as which can be used for bonding soft and flexible materials.

Some of the possible base materials, each having advantages and disadvantages, are ethylene-vinyl acetate (EVA) copolymers, ethylene-acrylate copolymers, polyamides and polyesters, polyurethanes (PUR), or reactive urethanes, styrene block copolymers (SBC) , also called styrene copolymer adhe- sives and various other copolymers.

Polyolefins, such as polyethylene, suffer from various disadvantages, e.g. from cohesive failure, and relative high cost price.

In physical and organic chemistry, the dispersity is a measure of the heterogeneity of sizes of molecules or particles in a mixture. A collection of objects is called monodis- perse or uniform if the objects have the same size, shape, or mass. A sample of objects that have an inconsistent size, shape and mass distribution is called polydisperse or nonuniform. The objects can be in any form of chemical dispersion, such as particles in a colloid, and polymer molecules in a solvent. Polymers can possess a distribution of molecular mass; and particles often possess a wide distribution of size, surface area and mass .

The term dispersity, represented by the symbol D, can refer to either molecular mass or degree of polymerization. It can be calculated using the equation DM = Mw/Mn, where Mw is the weight-average molar mass and Mn is the number-average molar mass .

Many of the prior art adhesives need to be bonded directly _/ typically within a few seconds, as an open time is relatively short. That characteristics makes such an adhesive not suited for larger surfaces to be adhered.

It is also preferred to have an adhesive that has a long transferability .

It is often also important to have a high initial

strength. For many prior art adhesive initial strength is not sufficient. One has to wait before making the bond thus reducing open time.

For many applications a strong initial bond is required. Inherently it is difficult to provide a strong bond with an adhesive, especially with hot melt adhesives.

Adhesives are preferably also temperature stable. Some documents recite hot melt adhesives, which suffer from one or more drawbacks .

US 3,635,861 recites a pressure sensitive adhesive composition which comprises a block co-polymer of styrene and butadiene or isoprene, a rubber extending petroleum oil and a modified or unmodified rosin, a coumarone- indene resin, a poly- terpene resin, a diene-olefin aliphatic hydrocarbon resin or a polystyrene resin.

US6767424 (Bl) recites a hot melt adhesive containing at least one thermoplastic elastomer, at least one hydrocarbon resin, at least one poly-alpha-olefin, and at least one polar wa .

US2007117907 (Al) recites hot melt compositions based on isotactic, low molecular mass, low viscosity homopolymer or copolymer waxes and atactic polyalpha-olefins (APAOs) . Hot melt compositions of this kind can be used with outstanding effect as hot melt adhesives.

US2004081795 (Al) recites a hot melt adhesive composition which is based on an isotactic polypropylene random copolymer (RCP) . The composition contains about 4%-50% by weight of the RCP copolymer, about 20%-65% by weight of a compatible tacki- fier, about 0%-40% by weight of a plasticizer, about 0%-3% by weight of a stabilizer, about 0%-40% by weight of a wax, and optionally about 0%-60% by weight of an atactic poly-alpha- olefin (APAO) . The adhesive composition may be used in a number of applications such as, for example, in disposable nonwo- ven hygienic articles, paper converting, flexible packaging, wood working, carton and case sealing, labeling and other assembly applications.

It is therefore an object of the present invention to provide a hot melt adhesive which overcomes one or more of the above disadvantages, without jeopardizing functionality and advantages .

SUMMARY OF THE INVENTION

The present invention relates to a hot melt adhesive according to claim 1, a use thereof according to claim 12 and a product according to claim 14. The present hot melt is considered to be a relatively slow hot melt.

The present adhesive surprisingly provides as improvements a high initial strength (> 15 sec), suitable vis- coelastic properties, strong bonding (< 15 sec.) , long transferability, good wetting, low-temperature flexibility, good anti-blocking, good stability, long shelf life (> 12 months) , stress release during formation of a bond, applicability over a broad temperature range (e.g. 120-180 °C, typically 150-160 °C) , application on a variety of substrates, such as porous flexible substrates, good cohesive strength, and long open time (> 3 min.), and which can be used in combination with a broad range of additives, organic compounds and elastomers, respectively, compared to other adhesives, especially hot melt adhesives. The present hot melt adhesive provides an open time of at least 10 seconds, more typically at least 3 minutes, and often at least 5 minutes . It is therefore especially suited for application on relatively large surfaces, such as in a matrass, in foam and in furniture. It is very suited for difficult-to-bond plastics. It provides a very good adhesion, such as to polypropylene and products comprising polypropylene, such as foam, as well as for wood and metal bonding. It also transfers the hot melt from a first surface to a second surface, which is typically established optically.

The present adhesive comprises a C ₅ -C ₅₀ organic compound, such as a 0 ₆ -0 ₂₅ organic compound. This organic compound can unexpectedly be added in high amounts to the present adhesive, without jeopardizing functionality thereof; in fact it has been found that the organic compound even improves the functionality. The organic compound is preferably a liquid compound .

The present adhesive comprises a synthetic thermoplastic polymer having weight average molecular weight of 80,000 to 200,000 and a dispersity of 1-4.2, preferably from 1-3.5, more preferably from 1-3, even more preferably from 1- 2.5, such as from 1.1-2, wherein the dispersity is defined as above (DM = Mw/Mn, where Mw is the weight-average molar mass and Mn is the number-average molar mass) . For prior art adhesive it is considered necessary to have a high dispersity polymer, typically a DM of about 5 or higher, in order to obtain required characteristics, such as good adhesive properties. It therefore came as a surprise that relatively low dispersity polymers could be used. In fact these low dispersity polymers, albeit available on the market, are put on the market for very different uses, such as non-woven fabrics, resin modifiers, elastics and pigment-dispersing agent.

The Mw is calculated as follows: wherein Ni is the number of molecules of molecular mass Mi. The mass average molecular mass is determined by static light scattering on a Brookhaven 90Plus.

The number average molar mass M _n is determined by measuring the molecular mass of n polymer molecules, summing the masses, and dividing by n .

The number average molecular mass of a polymer is determined by gel permeation chromatography, in particular by a Waters GPC using a Styragel column.

The present adhesive further comprises an elastomer and preferably a wax.

In view of the present invention the following is considered relevant.

Amorphous polyolefin (APO/APAO) polymers are compatible with many solvents, tackifiers, waxes, and polymers; they find wide use in many adhesive applications. APO hot melts have good fuel and acid resistance, moderate heat resistance, are tacky, soft and flexible, have good adhesion and longer open times than crystalline polyolefins . APOs tend to have lower melt viscosity, better adhesion, longer open times and slow set times than comparable EVAs . Some APOs can be used alone, but often they are compounded with tackifiers, waxes, and plasticizers (e.g., mineral oil, poly-butene oil). Examples of APOs include amorphous (atactic) propylene (APP) , amorphous propylene/ethylene (APE) , amorphous propylene/butene (APB) , amorphous propylene/hexene (APH) , amorphous propyl- ene/ethylene/butene . APOs show relatively low cohesion, the entangled polymer chains have fairly high degree of freedom of movement. Under mechanical load, most of the strain is dissipated by elongation and disentanglement of polymer chains.

Thereby the present invention provides a solution to one or more of the above mentioned problems.

Advantages of the present description are detailed throughout the description. DETAILED DESCRIPTION OF THE INVENTION The present invention relates in a first aspect to a hot melt adhesive composition according to claim 1.

The elastomer is present in a relative low amount. It preferably has a molecular weight of 20-150 kg/mole, more preferably 30-100 kg/mole, such as 50-70 kg/mole. It has been found that especially this molecular weight range provides good characteristics to the present adhesive. The molecular weight is determined by size exclusion chromatography, such as a Viscotek SEC-MALS 20 of Malvern Instruments.

In an example of the present adhesive the organic compound is selected from liquid alkanes, alkenes, aromatic hydrocarbons having 1-5 aromatic groups, such as 3-4 aromatic groups, such as naphthalene, oils, such as light and middle weight oils, mineral oil, naphthenic oils, poly-butene oil, and combinations thereof. Naphthenic oils may relate to hydro- treated heavy naphthenic distillate. It may have less than 50% non-aromatic hydrocarbons. A broad range of organic compounds may be used in the present adhesive.

In an example of the present adhesive the organic compounds is present in an amount of 5-60 wt.%, preferably 10- 50 wt. %, more preferably 20-45 wt . %, such as 30-40 wt.%. Such is considered a very high amount. As noted addition of an organic compound is thereby hardly limited; further the characteristics of the present adhesive remain good.

In an example of the present adhesive the synthetic polymer is one or more atactic polymers, preferably a metallo- cene catalyst based atactic polymer, more preferably a

poly (alfa) alkene polymer, such as poly (alfa) propylene ,

poly (alfa) butadiene , poly (alfa) methylpentene , most preferably more than 80 wt.% polypropylene. It is even more preferred to use 85 wt.%-99.99 wt.% polypropylene, such as 90 wt.%-99 wt.% polypropylene. Small amounts of other polymers may be present.

In atactic polymers the substituents are placed randomly along the chain. Typically the percentage of meso diads is between 1 and 99%. For the present hot melt 10-90 % meso- diads are prefably used, more preferably 25-75%, e.g. 40%, 50% and 60%. For the present hot melt 10-90 % racemo-diads are prefably used, more preferably 25-75%, e.g. 40%, 50% and 60%. With the aid of spectroscopic techniques such as NMR it is possible to pinpoint the composition of a polymer in terms of the percentages for each triad. The irregular structure of an atactic polymer makes it impossible or at least very difficult for the polymer chains to stack in a regular fashion. As a consequence an atactic polymer cannot crystallize and forms a glass instead. Polymers that are formed by free-radical mechanisms such as polyvinyl chloride are usually atactic. Due to their random nature atactic polymers are usually amorphous.

In an example of the present adhesive the synthetic polymer is present in an amount of 10-40 wt.%, preferably 12- 30 wt. %, more preferably 15-25 wt . %, such as 18-22 wt.%. Best performances in terms as described above are found for these adhesives.

In an example of the present adhesive the synthetic polymer is 70-100% atactic, preferably 90-99.99% atactic, more preferably 95-99.9% atactic, such as 98-99% atactic. Surprisingly highly atactic polymers with low polydispersity may be used in adhesives, especially in relative high amounts.

In an example of the present adhesive the elastomer is one or more of a copolymer, a block polymer, and a homopol- ymer, preferably a styrene block polymer.

These elastomers preferably have a tri-block structure, such as an A-B-A or A-B-C structure. A central block B is preferably chemically more flexible whereas block A and C are chemically more inflexible. The A- and/or C-block preferably have a glass transition temperature that is above ambient temperature, preferably a glass transition temperature somewhat lower than a temperature of application, whereas B-blocks preferably have a glass transition temperature that is around or below ambient temperature. The present elastomer preferably has one or more of the characteristics: being macromolecular; a glass transition temperature (of at least a part thereof) Tg is below a temperature of application; being amorphous; having a moderate degree of crosslinking; and having low secondary forces between molecules. The glass-liquid transition is a reversible transition in amorphous materials or in amorphous regions from a hard and relatively brittle state into a molten or rubber- like state. It is determined by differential scanning calorimetry (DSC) using ASTM D3418-03 and an Intertek.

In an example of the present adhesive the elastomer is present in an amount of 1-15 wt.%, preferably 2-10 wt. %, more preferably 3-8 wt . %, such as 4-6 wt.%.

In an example of the present adhesive the elastomer is one or more of a natural rubber, an artificial rubber, such as a styrene-isoprene-styrene block copolymer, a styrene- butadiene-rubber, a styrene-butadiene- styrene block copolymer, a styrene-ethylene-butadiene-styrene block copolymer, a styrene-isoprene- butadiene-styrene block copolymer, a nitril- isoprene rubber, and isoprene- isoprene rubber.

Examples are styrene-butadiene-styrene (SBS) , providing high-strength, styrene-isoprene-styrene (SIS) providing low-viscosity and high initial strength, styrene- ethylene/butylene- styrene (SEBS) providing low self-adhering, and styrene-ethylene/propylene (SEP) .

In an example the present adhesive comprises one or more additives .

Additives may relate to a wax, such as microcrystal- line waxes, fatty amide waxes, PE-waxes, and PP-waxes; to a plasticizer, such as a paraffin oil, polyisobutylene , naph- thenic oil, and a chlorinated paraffin; to an antioxidant and stabilizer; to a pigment and dye; and to a tackifying resin, such as a terpene-phenol resin, a rosins, a terpene and a modified terpene, an aliphatic, cycloaliphatic and aromatic resin, a hydrogenated hydrocarbon resin, tackifier resin emulsion, modified rosin resin, polymerized rosin resin, a-pinene resins, β-pinene resins, terpene resins, terpene phenol resins, alkylphenol resins, styrene resins, xylene resins, couma- rone resins, indene resins, hydrocarbon resins and combinations thereof, and their derivates, and their mixtures.

In an example the present adhesive comprises 0.01-30 wt . % wax, preferably 1-2 wt . %, such as paraffin. The wax is preferably high crystalline short chain was, such as a C ₂ -C ₅ polymer wax, wherein a melting point of the wax is preferably lower than 100 °C, more preferably lower than 90 °C. The melting point is measured using a so-called slip melting point (SMP) or "slip point" measurement. It is determined by casting a 10 mm column of the solid wax in a glass tube with an internal diameter of 1 mm and a length of 80 mm, The tube is subsequently immersed in a temperature-controlled water bath. The slip point is considered to be the temperature at which the column of the solid begins to rise in the tube due to buoyancy, and because the outside surface of the solid is molten. The accuracy is ± 0.5 °C per individual measurement, using the ring and ball method EN 1427.

In an example of the present adhesive comprises 0.01- 50 wt. % resin, preferably 1-30 wt . %.

In an example of the present adhesive comprises 0.01- 5 wt. % plasticizer, preferably 1-2 wt . %.

In an example of the present adhesive comprises 0.01- 5 wt . % stabilizer, preferably 0.1-1 wt . %.

In a second aspect the present invention relates to a use of the present adhesive composition for one or more of obtaining a high initial strength, low quantity (< 2 g/m) , long transferability, and long open time.

The present hot melt adhesives may be used for corrugated fiberboard boxes, disposable diaper, paperboard cartons, assembly of parts in manufacturing, and in electronic devices .

In an example the present adhesive composition is used in one-sided or two-sided bonding, especially of bonding of large surfaces of 0.5-10 m ² , such as 1-5 m ² , such as in a matrass, in foam and in furniture.

In a third aspect the present invention relates to a product comprising an adhesive composition according to the invention .

The invention is further detailed by the accompanying figures and examples, which are exemplary and explanatory of nature and are not limiting the scope of the invention. To the person skilled in the art it may be clear that many variants, being obvious or not, may be conceivable falling within the scope of protection, defined by the present claims .

EXAMPLES/EXPERIMENTS

The invention although described in detailed explanatory context may be best understood in conjunction with the accompanying examples and figures.

Example 1: 4910 comparison with commercial hot melt based on APAO with high (poly) dispersity

The performance of the present adhesive was compared with a standard APAO based hot melt, i.e. Sabamelt 4910. The test involved using a spray gun to glue together foam under tension at different open times. The present adhesive is bonded securely within 15 seconds and typically less, and was substantially faster than the commercial hot melt which required 30 seconds till 1 minute. The open time of the present adhesive averaged 3 minutes, substantially longer than the commercial hot melt (2 minutes) . Thus showing that the use of a APAO of low (poly) dispersity in combination with a elastomer and an organic compound creates at the customer a much broader application window, which is less dependent on mass (e.g. beads of 1,5 - 5 g/m) and settings, generating a robust process.

Example 2 :

Example comparing different organic compounds:

Examples 3 and 4 demonstrate the importance of the organic compound and the chemical nature of this compound.

It should be appreciated that for commercial application it may be preferable to use one or more variations of the present system, which would similar be to the ones disclosed in the present application and are within the spirit of the invention .