The present invention relates to a process for the microencapsulation of a water-insoluble adhesive in envelopes of polymeric material. As is known, when two surfaces have to be joined, these latter are spread with suitable adhesives which immediately fulfil their function which is to join the two surfaces.

The adhesives are dissolved in solvents or dispersed in water in order to make it easier to apply them in a thin layer.

After evaporation of the solvent, the adhesive is then able to perform its function, which is to exert a joining force between the two surfaces. In the particular field of devising self-sealing forms, that is to say printed forms which carry par¬ ticular adhesives in certain zones, some problems arise which derive from the fact that the use of fast laser- type printers such as, for example, IBM 3800, IBM 3835, SIEMENS 2500 or SIEMENS 2900, may prove to be incom¬ patible since the zone where the adhesive is applied may be compromised by the printing apparatus used.

This is due to the fact that, after the text has been printed, this form undergoes a rise in temperature such that undue softening of the adhesive is caused and, during the hot fixing of the text, partial detachment of the adhesive by adhering to the pressure rolls can occur.

This kind of disadvantage leads quite quickly to a stoppage of the press with the obvious adverse conse- quences and, in some cases it becomes downright impos¬ sible to use self-sealing systems with the hot laser-type printers.

To overcome these disadvantages, a process of microencapsulation of one or more polymers, which con- stitute the adhesive material, and one or more polymer solvents, also separately microencapsulated, has already been introduced.

The two microcapsules, appropriately spread on the two surfaces to be joined, cause rupture of the envelope when subjected to a defined pressure, with the adhesive substance and the solvent consequently flowing out and then forming an adhesive solution which can be used directly for sticking the two surfaces together.

In US Patent 2,907,682, a tape spread with two types of microcapsules is illustrated, one type including the solvent and the other type enclosing an adhesive material, which is substantially solid but soluble in the solvent enclosed in the first type of microcapsule.

This type of solution has process disadvantages due, for example, to the fact that any undue action of pressure causes rupture of the microcapsules before the end of the operation to which the treated surfaces must be subjected, with inevitable spillage of the adhesive substance.

Another type of adhesive used on self-sealing forms is the thermoplastic type which, in the final stage of closure of the envelopes, presupposes heating to a temperature which is about 20° higher than the melting point of the adhesive itself.

This type of heat-sealing adhesive too, however, cannot be used for laser printers, where the temperature of the form can be as much as 200°C and is then far higher than the melting point of the adhesive and can therefore cause detachment thereof, with deposition on the pressure rolls.

It is theoretically possible to have recourse to adhesives having melting points above 200°C, but this leads to difficulties in the choice of polymers which may- undergo degradation at this temperature, and the difficulties in spreading at an excessive temperature.

Moreover, the thermoplastic polymers are not crystalline, so that they presuppose a period of softening at a temperature remote from the melting point, or a. transformation of state.

In US Patent 4,132,322, the formation of conven¬ tional envelopes is illustrated, which have edges for sealing the envelope, which have been treated with thermoplastic adhesives microencapsulated and then dried. When the side edges of the envelope are folded and brought into contact with the lower edge, a localised pressure causes rupture of the microcapsules, outflow of the adhesive and the consequent closure of the envelope. Examples of thermoplastic microencapsulated adhesives are the vinyl polymers and copolymers, vinyl/ acrylic copolymers, cellulose esters, colophony esters, polyamide polymers and others.

In such cases,, however, it turns out not to be possible to obtain a laigh concentration of adhesive or water-insoluble material and, moreover, the thickness of the envelope enclosing the adhesive is of a certain consiste; y, so that a high pressure is required for the rupture of the microcapsules and the consequent outflow of the adhesive. Other types of known adhesives based, for example, on emulsions of styrene/butadiene copolymers, neoprene or natural rubbers, which have a considerable adhesive strength, show the peculiar property of being able to adhere only to layers treated beforehand with the same polymers.

Such polymers spread on a sheet of paper, then, do not give rise to adhesion to another sheet of the same paper, but only to a. sheet of paper which has been appropriately treated with the same polymers. The adhesion of such polyners to hot surfaces, i.e. about 200°C, is inhibited by the presence of non-stick com¬ pounds such as tetrafluoroethylene, silicone oils, polyethylene and others.

The presence of cellulose esters further reduces the adhesive strength ^' thereof.

This embodiment also has the disadvantage of being relatively stable in time, because the nature of the adhesive is such that it changes in air, with its

initial properties deteriorating after a few months.

The object underlying the invention is exactly that of solving the problems explained above, by provid¬ ing a process which makes it possible to produce a microencapsulated adhesive at a very high concentration of adhesive material and with the possibility of obtain¬ ing rupture of the microcapsules without having to exert particularly high forces.

Within the scope of the object explained above, a particular object of the invention is that of providing microcapsules which, even if they happen to be present on the surface for any reason, before the final phase of adhesion, do not show the properties of adhesive material, so that they do not cause problems during the normal treatment phases.

A further purpose of the present invention is that of providing a process which allows a very wide field of uses and thus becomes suitable for all those fields where the presence of surfaces provided with adhesives is necessary only at the end of a certain cycle of production or treatment.

The object explained above and also the purposes mentioned and others which will become clear below, are achieved by a process for the microencapsulation of a water-insoluble adhesive in envelopes of polymeric material, which is characterised in that it comprises preparing an aqueous solution with a polymeric emulsify¬ ing agent having hydroxyl groups and containing a second component of the envelope, emulsifying the aqueous solution with a water-immiscible adhesive containing a non-polymeric or prepolymeric crosslinking agent which constitutes a first component of the envelope, and effecting a polycondensation reaction to obtain a polymeric wall which constitutes the envelope encapsulat- ing the adhesive.

In short, the process allows adhesives to be microencapsulated in a concentration greater than 50% by weight in very soft and fragile microenvelopes of a

polymeric ther osetting character and then, having brought the surfaces treated with the microencapsulated adhesives together, to subject them to pressure in order to rupture the microenvelopes to obtain outflow of the adhesive and the consequent gluing of the two surfaces. To carry out the microencapsulation process according to the invention it is therefore envisaged to use a water-insoluble adhesive which contains a crosslinking agent belonging to the group comprising polyfunctional isocyanates such as diphenylmethane 4,4- diisocyanate, toluene diisocyanate, hexamethylene diisocyanate, triphenylmethane triisocyanate and mixtures and adducts of such isocyanates with trimethylolpropane. The aqueous solution in which the adhesive is emulsified contains a polymeric emulsifying agent with hydroxyl groups in the molecule.

The crosslinking reaction between the poly¬ functional isoσyanate and the polymeric emulsifying agent is a polycondensation which is carried out at a tempera- ture between 40 and 80°C for a period of time between 1 and 3 hours on average.

The ratio between the polymeric emulsifying agent and the crosslinking agent is preferably between 1 and 10 parts by weight of emulsifier per part of crosslinking agent.

As such water-soluble adhesives, a solution of natural rubber in the following formulation can be used: light crepe rubber 100, colophony ester (adhesion promoter) 50-100 plasticiser (mineral oil) 10-20 antioxidant 2246 (hindered phenol) 1-2 solvent (toluene, xylene, trichloroethylene) 300-600 The adhesion promoter can also be a hydrocarbon resin or a chetonic, terpenic, maleic or polyammidic resin.

Moreover, the adhesion promoter can be a solution of a 30/70 styrene/butadiene copolymer in dichloropropane or a solution of a polychloroprene/methacrylic acid

copolymer .

The following are indicated as polymeric emulsifying agents containing hydroxyl groups: polyvinyl alcohols, methylcellulose, isopropylcellulose, hydroxy- ethylcellulose, esterified starch and the like.

To further increase the quantity of micro¬ encapsulated adhesive, other emulsifiers belonging to the group of the naphthalenesulphonate/formaldehyde conden¬ sates, such as Tamol SN from Rohm and Has or Humifen NBL85 from GAF, which are the sodium salt of the con¬ densed naphthalenesulphonic acid, are added to the emulsifiers which take part in the crosslinking action.

Surprisingly, it is found that, in fact, the sodium salt of sulphonated lignin and the sodium salt of the naphthalenesulphonate/formaldehyde condensate, known as Tamol N 8906 or Tamol NN9401 from BASF or a Humifen 85 from GAF, of the formula:

make it possible to increase the dry solids content of adhesive substance to more than 50% in the finished emulsion and to minimise the thickness of the envelope of the microcapsule, in order to promote in this way the rupture by relatively small pressures, that is to say 0.2-2 atmospheres.

The thin and fragile thickness of the envelope, the high concentration of the adhesive to be microencap¬ sulated and the minimisation of the materials constitut¬ ing the envelope of the microcapsules to from 2 to 10% on adhesive allow outstanding functional results to be obtained for the adhesive produced by the process which is the subject of the invention.

The invention is applicable to a wide variety of polycondensation reactions between at least two reagents capable of reacting at the interface between the internal hydrophobic phase and the continuous or external hydrophilic phase to form microcapsules.

A certain number of basic types of polyconden¬ sation reactions are known and can be used in the present process.

The envelope of the microcapsules can, for example, be constituted by: polyamide, polysulphonamide, polyester, polycarbonate, polyurethane or polyurea or mixtures of these same polymers.

In other words, the walls of the microcapsules can consist of a polyamide/polyester or polyamide/ polyurea mixture.

Those substances are indicated below as first component of the envelope or wall of the microcapsules which are soluble in the material which is to be microencapsulated, that is to say the adhesive, while the substances soluble in the continuous or external phase and capable of reacting with the first component and giving rise to the wall or envelope of the microcapsule are indicated as second component.

The second component of the wall of the micro- capsules is soluble in water and insoluble in the organic and discontinuous phase.

The first component of the wall of the micro¬ capsules is insoluble in water or in the continuous or external phase, but is soluble in the internal or organic phase.

The two components react with one another to give rise to a polymeric wall all around the material which is to be microencapsulated, that is to say the adhesive.

The types of polymer constituting the finished wall of the microcapsules and functioning as the first and second components, which are used in the microencap¬ sulation process described above, are illustrated in the table which follows.

As specific examples of polycondensation reactions, to which the encapsulation process is applicable, the following are indicated according to the invention:

- Diacids or polyacids in the organic or discon¬ tinuous phase and diamines or polyamines in the con¬ tinuous aqueous phase, giving an envelope of the finished microcapsule, consisting of a polyamide polymer.

- Diisocyanates or polyisocyanates in the organic phase with diamines or polyamines in the aqueous phase, giving a reaction product at the interface, which consists of a urea.

- Diisocyanates or polyisocyanates in the organic phase with diols or polyols in the aqueous phase, giving a polyurethane as the reaction product at the interface.

- Chlorinated diacids or polyacids in the organic phase with diols or polyols in the aqueous phase, giving a polyester as the reaction product at the interface.

First reaction Second reaction Polymer con- component component constituting the wall

Chlorinated Diamines or Polyamide diacids or polyamines polyacids Diisocyanates Diamines or Polyurea or polyiso¬ polyamines cyanates Diisocyanates Diols or Polyurethane or polyiso¬ polyols cyanates Chlorinated Diols or Polyester diacids or polyols polyacids

Examples of chlorinated diacids: sebacoyl chloride, adipoyl chloride.

Examples of chlorinated polyacids: benzene-1,3,5- trisulphonyl chloride.

Examples of amines and polyamines: ethylene- diamine, hexamethylenediamine, polyalkylene-polyamines. Examples of diols or glycols: ethylene glycol, tri ethylolpropane, pentanediol, pyrogallol. Examples of isocyanates: toluene diisocyanate, hexamethylene diisosyanate.

The polymers constituting the wall of the micro- capsule represent from 5% up to 10% by weight of the organic content to be encapsulated, adhesive in the specific case.

The first reaction component, as is known, must dissolve in the organic phase (adhesive) without reacting with the solvents present in the adhesive, which are in general: xylene, monochlorobenzene or toluene. The optimum quantity of emulsifier can vary from

1 to 5% on the adhesive or internal water-insoluble phase. The microcapsules obtained according to the present invention do not require further treatments, such as separation of the liquid phase, but are used directly as such with previous addition of antifoam and ingredients which can vary the viscosity of the vehicle. For the antifoam, those of the silicone type are preferred, because they act as parting agents on the toner-fixing rollers in a laser printer. The agitation for stabilising the emulsion consisting of droplets of adhesive and aqueous phase is effected by means of special high-speed emulsifying apparatus capable of producing miσrovortices at high shear stress. The optimum dimension of the microcapsules is between 1 and 10 microns.

To give a more detailed illustration of the processes described above, some embodiment examples are given which obviously are not intended in a limiting sense.

The adhesive or internal phase will contain the first component of the envelope dissolved therein, while the aqueous phase will contain the emulsifier and the

second component of the envelope. The second component is added when the emulsion has been obtained. Example 1

0.56 g of polyvinyl alcohol (Mowiol 4/88 Hoechst) 33.20 " of deionised water

1.32 " of Humifen NBL 85 (GAF Italy) 56.29 " of adhesive (natural rubber, colophony ester, toluene) 5.63 " of toluene 2.80 " of TDI-trimethylolpropane (Desmodur L75

Bayer) 0.20 " of2-hydroxy-4-methoxybenzophenone (Uvasorb Met SIGMA)

100.00%

5.63 g of toluene, 2.80 g of Desmodur L75 and 0.20 g of Uvasorb Met are added to 56.29 g of adhesive. The mixture is mixed until the Desmodur L75 is dissolved in the adhesive.

In a special high-speed emulsifying apparatus, a solution of 0.56 g of Mowiol 4/88 and 1.32 g of Humifen

NBL 85 in 33.20 g of water is emulsified at about

10,000 rpm with 64.92 g of "adhesive phase" at a tempera- ture of 20°C for 1 minute.

The speed of the emulsifying apparatus is then reduced to 3,000 rpm and the temperature is gradually increased up to 50°C, mixing being continued for 2 hours with liberation of C0 ₂ and toluene. The dimension of the microcapsules is 1-2 microns, and the envelope of these consists of polyurethane.

The ready-to-use adhesive is obtained by the addition of 1% of silicone antifoam. Spreading on paper in appropriate stripes and evaporation of the water gives a non-tacky film and, after folding the paper in such a way that the two treated surfaces coincide and applying a pressure of

0.2-2 atmospheres, the two edges join to form an envelope.

With respect to the second example, it is stated that the operating phases remain unchanged from the first example, with the sole variation that the second element which will constitute the envelope is added to an emul¬ sion already prepared. Example 2

38.20 g of water

2.50 of sodium ligninsulphonate 1.20 of ethylenediamine 56.10 of adhesive (para, ketonic resin, solvent) 4.00 of xylene 2.80 of TDI 0.20 of Uvasorb Met

100.00

4 g of xylene, 2.80 g of TDI and 0.20 g of Uvasorb Met are added to 56.10 g of adhesive with stir- ring until a solution is obtained.

In a high-speed emulsifying apparatus, a solution of sodium ligninsulphonate in 33.20 g of water is emul¬ sified with 63.10 g of adhesive phase at 10,000 rpm and T - 20 ^β C for 1 minute. After reducing the revolutions to 3,000/minute,

1.2 g of ethylenediamine (second component of the envelope) is added to the emulsifier and mixing is continued for about one half hour. An envelope of polyurea forms at the interface. With the addition of 1% of silicone antifoam, the adhesive is ready for use. Example 3

31.00 g of water 2.50 " of Tamol N 8906 (BASF) 3.00 " of ethylene glycol

56.00 " of adhesive (polychloroprene, hydrocarbon resin, solvent)

2.80 " of Uradur p49 (aromatic isocyanate

Stabilital) 4.50 " of xylene 0.20 " of Alvinox 100 (hindered phenol - SIGMA)

100.00

The ethylene glycol is added when the emulsion has been obtained.

The envelope at the interface consists of polyurethane.

Example 4 is carried out using the same procedure as in Examples 2 and 3. Example 4

The envelope obtained is of the nature of a polyester and is formed by the reaction of adipic acid (adipoyl chloride) and the polyols pentanediol and pyrogallol.

31.10 g of water 2.50 " of pentanediol 0.50 " of pyrogallol

2.50 " of Tamol NN 9401 (BASF) 56.00 " of adhesive (styrene/butadiene in solution) 3.00 " of adipoyl chloride 4.00 " of white spirit 0.20 " of Uvasorb Met

100.00

Example 5

30.00 g of water 2.00 " of 50% NaOH solution

2.50 " of Orotan SN (Rohm and Haas) emulsifier 3.00 " of hexamethylenediamine

55.30 " of adhesive (para, esterified colophony, solvent) 3.00 " of adipoyl chloride 4.00 " of xylene 0.20 " of Uvasorb Met

100.00

In this example again, the adipoyl chloride is the first component of the envelope and is dissolved in the adhesive which is the material to be micro¬ encapsulated, and the hexamethylenediamine is the second component of the envelope and is added when the emulsion has been obtained.

Example 6 33.00 g of water

0.60 " of polyvinyl alcohol of low viscosity (Airvol 203 - Air Products) of Tamol NN9401 BASF of adhesive (acrylic polymers in heptane) of heptane of Desmodur L75 of Uvasorb Met

In this example again, the first component is

Desmodur L75 and the second component of the envelope is the polyvinyl alcohol; the polycondensation reaction takes place at the interface between the two phases.

From the above .it is clear that the particular subject of the invention is the microencapsulation of an

adhesive having diverse characteristics in a polymeric envelope obtained by polycondensation in an aqueous emulsion.

The adhesive can be of the reversible or irrever- sible type, in the sense that the two edges treated with adhesive and joined under a localised pressure can still be detached and reattached under pressure in the case of the reversible type, or remain permanently joined in the case of the irreversible type. In particular, the microencapsulated adhesive described is suitable for self-sealing forms, for which it is required not to adhere to the untreated paper, to withstand the temperature of a laser printer, not to contaminate the pressure rollers, to join the two treated edges, after folding, by means of a pressure in order to form an envelope which is at the same time the content and the container.

One of the special features of the invention is the provision of a process of applying adhesive to surfaces which display their adhesive property only at the end of the production cycle as soon as appropriate pressures are applied and the treated surfaces are facing one another.

Moreover, another application of the micro- encapsulated adhesive described concerns the self- adhesive labels sector.

The notable advantage is the fact that the self- adhesive labels would no longer require to be accompanied by a siliconated paper which protects the adhesive properties thereof, since the envelope containing the adhesive is already sufficient for this purpose.

A pressure exerted on the label, which can be done simply by the thumb, effects rupture of the micro¬ capsules with outflow of the adhesive, which causes the label to adhere to the desired support.

An analogous application relates to the handling of postage stamps, at present effected by wettable adhesives.

The treatment of postage stamps with the adhesive described in the invention would not require wetting on the back, but simply the pressure exerted by the thumb would be sufficient to cause rupture of the capsules and consequent adhesion to letters, postcards and envelopes in general.

The invention thus conceived is susceptible to numerous modifications and variants which are all covered by the scope of the inventive concept. Moreover, all the details can be substituted by other technically equivalent elements.

In practice, the materials used can be modified in any way provided they are compatible with the specific application.