DESCRIPTION

The present invention relates to an adhesive composition to be applied to a layer of already vulcanized rubber.

As it is well known to those skilled in the art, it is very easy to couple effectively a layer of not yet vulcanized rubber, whilst it is very difficult to couple a layer of already vulcanized rubber. In fact, in the case of not yet vulcanized rubber, an adhesive composition is applied thereto, wherein the adhesive effect is obtained above all during the vulcanization step.

However, above all in the aerospace field, constructive applications exist necessarily involving the coupling of already vulcanized rubber. The coupling can be done between vulcanized rubber and vulcanized rubber or vulcanized rubber and virgin rubber.

One of the processes where it is necessary to couple an already vulcanized rubber to a virgin rubber is, for example, the production of solid-propellant rocket engines.

The solid-propellant rocket engines generally comprise an axial-symmetrical outer casing, which houses a charge of solid propellant, an ignition system provided at an end of the casing and an end nozzle provided at the opposite end. The casing is internally coated with a thermal protection made of ablative material, typically constituted by elastomers charged with additives and/or fibers, to protect the inner surface of the casing against high temperatures and erosion.

Generally, the thermal protection consists of an intermediate cylindrical part formed by winding a web of reinforced ablative material on the surface of a mandrel, and of two end dome-shaped end parts formed out of the site of assemblage. The term "out of the site of assemblage" means that the two end portions are formed separately from the mandrel, and then mounted thereon.

In particular, the two end portions are mounted on the mandrel once they have already been vulcanized. According to this solution, on the mandrel the end portions of already vulcanized rubber are necessarily coupled to the intermediate portion being formed, i.e. still made of virgin (not vulcanized) rubber.

In other words, the thermal portion is made by coupling an intermediate portion of virgin rubber with two end portions of already vulcanized rubber.

As it is clearly apparent to those skilled in the art, the coupling between the intermediate portion and the end portions is a very crucial aspect. In fact, in order for the solid rocket engine to operate correctly, it is important that the coupling of the various parts of the thermal portion is really effective.

There was thus the need to have available an adhesive composition able to ensure the effective coupling of the already vulcanized rubber with either a virgin rubber or an already vulcanized rubber.

The inventors of the present invention have surprisingly found that the use of a compound currently used for making virgin rubber adhere to metal, when properly mixed with a rubber solubilized in an organic solvent, provides an adhesive composition capable of ensuring the effective adhesion of already vulcanized rubber both to already vulcanized rubber and virgin rubber, as well as to metal.

An object of the invention is an organic solvent-based adhesive composition comprising a crosslinking unsaturated-chain polymer base and a vulcanization system, said adhesive composition being characterized by comprising 50-200 phr, preferably 100-150 phr of a product with adhesive action comprising: xylene 30-80% wt, ethylbenzene 10-30% wt, zinc oxide 1-10% wt, and 1-5% wt of a compound selected from the group consisting of l,l-(l,3-phenylene)bis-lH-pirrole-2,5-dione, N,N’- (4,4’diphenylmethane) bismaleimide, diphenylmethanediisocyanate, 4,4'-methylenbis (N, N'-bis (2,3-epoxypropyl) aniline or mixtures thereof.

Said product with adhesive action is preferably marketed under the name Chemosil® 411 or Chemosil® 256 or Chemosil® 425 or Chemosil® 231 o Thixon® 547-EF or Thixon® 6110F. “Crosslinkable, unsaturated-chain polymer base” refers, hereinafter, to any natural or synthetic non-crosslinked polymer, adapted to gain all the chemical-physical and mechanical features typically gained by elastomers following cross-linking (vulcanization).

“Vulcanization system” refers, hereinafter, to a complex of vulcanizing agents that promote vulcanization of the polymer base when the mixture is brought to the vulcanization temperature.

Preferably, the cross -linkable unsaturated-chain polymer base is comprised in group consisting of EPDM, NBR, SBR, HTPB.

Preferably, the vulcanization system comprises a peroxide crosslinking agent.

Preferably, the adhesive composition of the present invention comprises xylene as a solvent.

A further object of the present invention is a method for preparing the adhesive composition of the invention. The method comprises a first mixing step, wherein a crosslinking unsaturated-chain polymer base, a plasticizer oil and a vulcanization system are mixed together; a solubilization step, wherein the mixture formed during the first mixing step is solubilized in an organic solvent thus forming a liquid rubber; and a second mixing step, wherein to the liquid rubber a product is added with adhesive action, comprising: methyl isobutyl ketone 60-93% wt, xylene 5-10% wt, phenol- formaldehyde polymer 1-5% wt, ethylbenzene 1-5% wt.

Preferably, said product with adhesive action is marketed under the name Chemosil® 411.

Below, embodiments are described just by way of non-limiting examples.

The adhesive composition of the invention has been made.

Firstly, a rubber mixture has been prepared by mixing the compounds shown in Table I (amounts in phr) at room temperature. TABLE I

NOREX HI has been used as plasticizer oil.

AEROSIL R 972 has been used as rheology modifier.

TRIGONOX 101-50D-PD has been used as peroxide crosslinking agent.

TRIALLYL CYANETRATE has been used as accelerator.

Then, the above rubber mixture has been solubilized in xylene with a weight ratio of 1 part of rubber mixture and 10 parts of xylene.

The rubber mixture has been solubilized in xylene, resulting in the formation of a liquid rubber.

Chemosil® 411 has been added to the liquid rubber with a 1:1 weight ratio (Chemosil® 41 Erubber mixture).

The adhesive composition obtained looks like a low-viscosity liquid. In particular, the viscosity of the adhesive composition has been found equal to 600 mPa.s at 25°C.

The method described above is significant for the effectiveness of the adhesive composition.

In this regard, an attempt has been made to prepare the adhesive composition by adding all the ingredients, including Chemosil ® 411, directly into xylene without previous preparation of a rubber mixture as defined above. The adhesive composition obtained has been found less effective than that obtained with the method of the invention.

The adhesive composition obtained can be applied by means of a brush or by spraying.

The adhesive composition prepared as described above has been tested for the coupling between: a layer of vulcanized rubber and a layer of virgin rubber; a layer of vulcanized rubber and a further layer of vulcanized rubber; a layer of vulcanized rubber and a metal. In order to verify the advantages of the adhesive composition of the invention, the adhesion tests have been repeated using both the only rubber mixture and the only Chemosil® 411 as adhesive agent. In order to have a comparison as significant as possible, the rubber mixture and Chemosil® 411, when used alone, have been used in the same amount as in the adhesive composition of the invention.

The tests have been carried out under the same conditions, in order to ensure the significance of the comparison. Specifically, the various adhesive agents have been applied by brush in three subsequent coats, every subsequent coat having been applied once the solvent was evaporated. After the last coat, the two parts to be coupled have been kept in contact with each other and vulcanized in autoclave at l65°C under a pressure higher than 2 bar.

After the time indicated above, on each coupling a traction measurement has been performed.

Table II shows the results of the traction measurements.

In Table II the term "INV." refers to the use of the adhesive composition of the invention (rubber mixture added with Chemosil® 411); the term "CONF.A" refers to the use of only the rubber mixture; the term "CONF.B" refers to the use of only Chemosil® 411. The values in Table II are in MPa. TABLE II

From the values of Table II it is clearly apparent that the adhesive composition of the invention endures greater adhesion with respect to the comparative examples.

The effectiveness of the adhesive composition of the invention has been extremely surprising, as to date Chemosil® 411 has been used only for making virgin rubber adhere to metal. Differently from what described for the adhesive composition of the invention, Chemosil® 411 was applied directly (without being first added to a liquid rubber) on the virgin rubber layer, and the adhesive action thereof was performed during the vulcanization phase at l30°C-l80°C. In other words, according to the prior art, i.e. before the present invention, a necessary condition for Chemosil® 411 to be effective was that it would be subjected to a vulcanization phase at approximately l50°C.

In conclusion, the inventors of the present invention have provided an extremely effective solvent-based adhesive composition for coupling already vulcanized rubber by using a compound that was known for the adhesive effectiveness for coupling virgin rubber and only if used in a process including vulcanization.