FIELD OF THE INVENTION

The present invention relates to starch-based adhesives, and more particularly to a method of making dispersible adhesives in a dry form by forming into a melt a particulate synthetic or natural polymer having adhesive properties and starch and/or a starch derivative in the presence of an emulsifying and/or dispersing agent and cooling said melt into a solidified particulate form. In a further embodiment of the present invention said particulate form is a foam.

BACKGROUND TO THE INVENTION

It is known that starch may be heated under pressure to form a melt suitable for the production of shaped articles. Such a method is described in US-patent 4 673 438 the contents of which are incorporated herein by reference. It is further known from US- patent 5 095 054 that starch may be blended with thermoplastic polymers to form a melt which is useful for forming ^' shaped articles. These starch-based blends are of particular relevance to the present invention and therefore the content of US-patent 5 095 054 is incorporated herein by reference. Many of the starch based blends described in US-patent 5 095 054 have adhesive properties depending on the polymer(s) used in the blend. For example many of these blends are useful as hot melt adhesives for intermediate tie layers in composite materials. However, the use of the solidified material as obtained according to US-patent 5 095 054 as a dispersible adhesive has several drawbacks, especially because the dispersibility of the blend even when obtained in a powdery form is too low. The same is true whether the starch is partially or wholly replaced by a starch derivative.

The present invention refers to an improvement in the production of

dispersible adhesives based on starch and/or a starch derivative. The present invention also refers to the products obtained by said processes.

Products of the present invention are easy to make and have excellent dispersibility characteristics. The method according to this invention of making such dispersible adhesives brings many important advantages to the method of producing dispersible adhesives, flexibility in production means and reduction of costs.

SUMMARY OF THE INVENTION

Specifically the present invention refers to a method of making a dispersible adhesive in dry form (i) by forming into a melt a particulate synthetic or natural polymer having adhesive properties and starch and/or a starch derivative in the presence of an emulsifying and/or dispersing agent, whereby the mixture is heated above the melting point of the starch to form a uniform melt and for a time long enough substantially to destroy the crystalline and granular structure of the starch and (ii) subsequently cooling said melt into a solidified particulate form. In a further embodiment of the present invention said melt is extruded and cooled in such a way that a particulate foamed form is obtained.

The composition may further comprise one or more plasticisers.

The starch and/or the starch derivatives as well as the polymers with adhesive properties present in the adhesives made according to the present invention are known in the art.

The compositions made according to this invention may further comprise foaming agents, nucleating agents; lubricants; stabilisers; anti-microbial agents, colouring agents; flame retardants; boron-containing compounds; alkali and alkaline earth metal salts; thermal stabilisers; and melt flow accelerators; and

mixtures thereof.

It is particularly preferred that the solidified dispersible adhesive is in the form of a foam, which preferably has a bulk density in the range of from about 8.0 to about 60kg/m ³ . The present invention will be further apparent from the following description taken in conjunction with the accompanying examples and appended claims.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a method of making a dispersible adhesive in dry form comprising (i) forming into a melt a particulate synthetic or natural polymer having adhesive properties and starch and/or a starch derivative in the presence of an emulsifying and/or dispersing agent whereby the mixture is heated above the melting point of the starch to form a uniform melt and for a time long enough to destroy the crystalline and granular structure of the starch and (ii) subsequently cooling said melt into a solidified particulate form.

In a further embodiment of the present invention said melt is extruded in such a way that a particulate foamed form is obtained.

Redispersible adhesives are known in the art.

Within the scope of this invention, any type of commercially available starch may be used such as native starch selected from potatoes, rice, tapioca, corn, pea, rye, oats, and wheat. The starch may be physically modified. The starch may also have an increased amylose content, for example higher than 50%. The use of high amylose starches, however, is not critical for the present invention.

The starch derivative may be a starch ester or starch ether whic may be selected from the group consisting of C ₂ to C ₂₂ starch ester or ethers.

In a first embodiment the starch derivative may be a starch ether, preferably selected from the group consisting of methylstarch, hydroxymethylstarch, hydroxyethylstarch, hydroxypropylstarch, hydroxyethyl-methylstarch, hydroxypropyl-methylstarch, hydroxybutyl-methylstarch, carboxyalkyl starches and carboxyalkyl alkylstarches. Preferred from these are hydroxyalkylstarches suc as hydroxyethylstarch and hydroxypropylstarch. Most preferred i hydroxypropy1starch. The degree of substitution for the mentione substituted starches is preferably about 0.01 to about 1.0, mor preferably from about 0.05 to about 0.2. Most preferred is a degre of substitution of about 0.08 to about 0.15.

In a further embodiment of the invention, the starch derivative ma be a starch ester, preferably an acetate, a propionate, butyrate, pentanoate, or hexanoate. Most preferred are the acetates an propionates. The degree of substitution is preferably about 0.01 t about 0.80, and more preferably from about 0.02 to about 0.1.

It will be appreciated that the substituents and amount of suc substitution being dictated by the desired characteristics of th derivative in the dispersible adhesive which is a matter of routin optimization for the person skilled in the art.

The polymer having adhesive properties may be selected from know adhesives and as such are selected from the group consisting o polyvinylesters such as polyvinylacetates or polyvinylpropionates; polyacrylic acids and poly(acrylic acidJesters; polymethacryli acids and poly(methacrylic acid)esters; polyalkylene oxides; polycarbonates; polyarylethers; polyalkylethers; polyurethanes; cationically modified acrylates and methacrylates possessing tertiary or quaternary amino group; alkenol homopolymer

(polyvinylalcohols) ; alkenol copolymers which are obtained by copolymerization of vinyl acetate with a member selected from ethylene, propylene, isobutylene and/or styrene with subsequent hydrolysis of the vinyl ester group; alkylcelluloses; hydroxy- alkylcelluloses; hydroxy-alkylalkylcelluloses; cellulose esters; hydroxy-alkylcellulose esters; carboxy-alkylalkylcelluloses; carboxy-alkylcellulose; silylated polysaccharides, and mixtures thereof.

In one embodiment of the invention, said polymer having adhesive properties is selected from ethylene/vinyl acetate-copolymers; ethylene/vinyl alcohol-copolymers; ethylene/acrylic acid-co- polymers; ethylene/ ethyl acrylate-copolymers; ethylene/meth- acrylate-copolymers; polyurethane-copolymers; styrene/acrylo- nitrile-copolymers; polyacetals; methylcellulose; hydroxymethyl- cellulose; hydroxyethylcellulose; hydroxypropylcellulose; hydroxy- ethyl-methylcellulose; hydroxypropylmethylcellulose; hydroxybutyl- methylcellulose; cellulose acetylphthalate; hydroxy- propylmethylcellulose; carboxyalkylcelluloses; carboxy- alkylalkylcelluloses, and mixtures thereof.

Preferred are those on the basis of polyvinylacetates and co¬ polymers thereof; polyvinylpropionates and copolymers thereof; polymethacrylates and copolymers thereof, alkenol homopolymers and alkenol copolymers as mentioned above. Further preferred are those with a melt temperature between 90°C and 190°C, preferably those with a melt temperature between 90°C and 180°C.

The composition further contains an emulsifying and/or dispersing agent. Such emulsifying and dispersing agents are known. The dispersants can be anionic, cationic, amphoteric or nonionic surface-active compounds or mixtures thereof. Suitable anionic emulsifiers are, for example, alkyl sulfonates, alkylaryl sulfonates, alkyl sulfates, sulfates of hydroxylakanols, alkyl and alkylaryl disulfonates, sulfonated fatty acids, sulfates and

phosphates of polyethyoxylated alkanols and alkylphenols, as wel as esters of sulfosuccinic acid. Suitable cationic emulsifier are, for example, alkyl quaternary ammonium salts, and alky quaternary phosphonium salts. Examples of suitable non-ioni emulsifiers are the addition products of 5 to 50 mols of ethylen oxide adducted to straight-chain and branch-chain alkanols with to 22 carbon atoms, or alkylphenols, or higher fatty acids, o higher fatty acid amines, or primary and secondary higher alky amines; as well as block copolymers of propylene oxide wit ethylene oxide and mixtures thereof. Preferred are for exampl highly viscous aqueous solutions of polyvinylalcohols which may b fed into an extruder during processing or added to the initia mixture to be compounded. However, polyoxyethylene ethers and thei known derivatives which are known in the art as emulsifying and/o dispersing agents may also be used.

The emulsifying and/or dispersing agent (calculated as the dr substance) is present in a concentration of about 3% to about 25 by weight, preferably about 5% to about 10% by weight calculated t the weight of the polymer present in the composition.

It is particularly preferred that the adhesive produced accordin to the present method is in the form of a foam, which typically ma have a bulk density in the range of about 8.0 to about 60kg/m ³ whereby the surface area of the dispersible adhesive as well as it speed of dispersion are greatly increased.

Accordingly, the melt may comprise further known additives whic aid foam formation, particularly nucleating agents and foamin agents.

A suitable nucleating agent for use in the present method has particle size of from about 0.01 to about 5 microns, and is presen in the melt in an amount of up to about 10% by weight with respec to that of the melt. Suitable such nucleating agents includ

silica, titania, alumina, barium oxide, magnesium oxide, sodium chloride, potassium bromide, magnesium phosphate, barium sulphate, aluminum sulphate, boron nitrate and magnesium silicate, or mixtures thereof.

When producing the adhesive of the present invention in a foamed form, the polymer with adhesive properties may also be used in the form of a dispersion. In this sense the present invention also includes a method of making a dispersible adhesive in dry form comprising (i) forming into a melt an aqueous dispersion of a synthetic or natural polymer having adhesive properties and starch and/or a starch derivative, whereby the mixture is heated above the melting point of the starch to form a uniform melt and for a time long enough to destroy the crystalline and granular structure of the starch and (ii) subsequently extruding said melt into a solidified particulate foamed form. For this procedure all the additives as mentioned herein are the same with the exception that no emulsifying and/or dispersing agent need to be additionally added.

The melt may still further comprise at least one member selected from the group consisting of tackifiers, extenders; lubricants; stabilisers; anti-microbial > agents, colouring agents; flame retardants; boron-containing compounds; alkali and alkaline earth metal salts; thermal stabilisers; and melt flow accelerators; and mixtures thereof.

The boron containing compounds may function as reversible cross linking agents for at least some of the polymers having adhesive properties. Suitable compounds include boric acid, metaboric acid, alkali and alkaline earth metal salts, borax and derivatives thereof.

The ratio of polymer having adhesive properties to said starch or

8 starch derivative is not narrowly critical and the precise figu depends, for example, on the degree of adhesiveness of the polyme the required dispersion characteristics, and the desired degree adhesiveness of the final product. It is however desirable that t polymer having adhesive properties (calculated to the dry polyme is present in a concentration of about 10% to about 45% by weig calculated to the dry final composition and preferably in concentration of about 10% to 30% by weight calculated to the d final composition.

In order to melt the composition to a uniform melt according to t invention, whereby a thermoplastic melt is obtained, it is suitab heated in a screw and barrel of an extruder for a time sufficie to enable uniform melt formation. The temperature is preferab within the range of 100°C to 220°C, and more preferably within t range of from 160 to 200°C, the precise temperature being depende upon the type and nature of the composition used. The compositi preferably is heated in a vented extruder for achieving the corre moisture content at the end of the melt formation, especially f subsequent foaming purposes, when drying of the composition may required.

Pressures created in such a volume correspond to the vapo pressure of the most volatile material present (which in t present case is mostly water) at the used temperature. It will appreciated that pressures may be applied or generated as is kno in the use of a screw and barrel.

The preferred applied and/or generated pressures are in the ran of pressures which occur in extrusion and are known per se, bei up to about 150 x 10 ⁵ N/m ² , more preferably up to about 75 x 10 ⁵ N and most preferably up to about 50 x 10 ⁵ N/m ² .

Within the screw the granular mixture is heated to a temperatu which is generally within the range of about 120 to 220°C, mo

preferably within the range of about 120 to 190°C and mo preferably within the range of about 130 to 190°C.

The components of the composition can be fed each one separately the extruder or mixed previously. It is also possible to mix t starch and/or the starch derivative with the particulate polym and feed these to the extruder whilst the water together with t emulsifying and/or the dispersing agent are fed separately direct into the barrel.

The dispersible adhesive made according to this invention may re-dispersed preferably in water in a manner known per se.

The present invention will be further apparent from the followi examples.

Example 1

Native potato starch with a water content of about 22% by weight intensively mixed with 0.75% by weight of a mono fattyaci glyceride and 0.75% by weight lecithin and then blended wi particulate polyvinylacetate in a ratio of 3 : 1. Polyvinylalcoh (8% by weight with respect to the polymer) is added. The mixtu is fed to an extruder (Leistritz, 34 mm, twin-screw, vented) . T mixture is heated to a maximum temperature of 175°C. The ex temperature is about 115°C. An aqueous solution polyvinylalcohol is fed into the barrel of the extruder into t first section, so that the water content rises to over 40% weight of the composition. The water content is reduced to 24% weight with respect to the total composition and the melt extruded and immediately cut into fine pellets. These pellets m be added to water preferably in a high speed mixer at somewh elevated temperature (about 45 to 50°C and mixed until a unifo dispersion is obtained) .

Example 2

Example 1 is repeated whereby the exit temperature is 130°C and th extrudate is extruded to a foamed particles. A quicker formation o the dispersion is obtained.

Example 3

Example 1 is repeated and the potato starch is replaced b hydroxypropyl starch prepared from corn starch with a degree o substitution of about 0.1. Analoguous results are obtained.

It will be appreciated that it is not intended to limit th invention to the above examples only, many variations thereto an modifications thereof being possible to one skilled in the ar without departing from scope of the invention, which is defined b the appended claims.