EP0254618B1 - Preformed inhibitor based on rubber gum for a composite propellant with a polyurethane binder - Google Patents

Description

The invention relates to a preformed inhibitor for composite propellant.

The invention relates more particularly to a preformed inhibitor having as matrix a rubber rubber to which a composite propellant with a polyurethane binder is directly adhered.

By inhibitor is meant any part which is placed on at least part of a propellant block to limit the combustion surface thereof. The inhibitors of the present invention also play a role in protecting the structure of the craft.

In the context of the present invention, the term inhibitor also designates parts of the inhibition of a block, such as for example the bottoms or the end faces, the other parts of the inhibition being able to be carried out with other inhibitory compositions.

• un premier groupe dans lequel la composition inhibitrice est déposée sur la surface du bloc de propergol déjà réticulé,
• un second groupe de procédés dans lequel la composition inhibitrice est coulée dans un moule et au moins partiellement réticulée pour former une enveloppe dans laquelle sera coulée le propergol.

The propellant block inhibition methods can be classified into two groups:

• a first group in which the inhibiting composition is deposited on the surface of the already crosslinked propellant block,
• a second group of processes in which the inhibitor composition is cast in a mold and at least partially crosslinked to form an envelope in which the propellant will be cast.

The methods of the second group make it possible to manufacture free blocks or glued molded blocks. In the latter case, the inhibitor is preformed directly in the structure of the self-propelled vehicle, the propellant then being poured into this structure.

The inhibitors of the present invention are used for carrying out the methods of the second group.

Inhibitors are already known having as matrix rubber rubbers. However, these inhibitors are used with a composite propellant based on polybutadiene with terminal carboxyl functions (PBCT), and do not adhere to the composite propellant based on polybutadiene with terminal hydroxyl functions (PBHT).

Furthermore, an inhibitor based on a gum rubber is known, the ethylene-propylene-diene-monomer elastomer, more commonly called EPDM for a composite propellant based on PBHT.

Patent FR-A-2 554 114 describes the use as an inhibitor of an EPDM gum rubber vulcanized with peroxides. This rubber comprises fillers, in particular carbonizable organic fibers to ensure good resistance to the effect of erosion by the flow of hot gases.

Patent FR-A-2 495 133 describes the use of a gasifiable organic filler, in particular of oxamide, in an inhibitor based on rubber EPDM gum; the role of this oxygenated charge being to reduce the formation of slag during the degradation of the inhibitor.

However, the adhesion of the inhibitor to the propellant is obtained by the presence of an adhesion layer also called "primary" deposited between the inhibitor and the propellant.

The need to provide this adhesion layer is a major drawback. In fact, the application of this adhesion layer to the inhibitor increases the duration and the manufacturing costs. In addition, it can generate defects in the load obtained, for example, by trapping air bubbles. To avoid any defect and to obtain a uniform adhesion on all the block this application requires great care.

The object of the present invention is in particular to remedy these drawbacks by proposing a preformed inhibitor based on gum rubber to which a propellant with a polyurethane binder is directly adhered.

To this end, the invention provides a preformed inhibitor for a composite propellant block based on a polyurethane binder obtained from a polyol and a polyisocyanate. This inhibitor comprises a rubber-based matrix, fillers, a rubber vulcanization system, and is characterized in that the rubber rubber is chosen from the group comprising ethylene-propylene-diene-monomer copolymers (called EPDM rubbers). and isoprene-isobutylene copolymers (called butyl rubbers), the vulcanization system of EPDM rubber being a system based on peroxide compounds, that of butyl rubber being a sulfur-based system comprising at least one vulcanization accelerator and devoid of vulcanization activator.

Thus, the composite propellants adhere directly to the inhibitors according to the invention, without the use of an adhesion or "primary" layer.

According to another characteristic of the invention, the inhibitor contains a bonding dopant.

When the matrix is based on EPDM rubber, the bonding dopant is oxamide, preferably present at a concentration of between 50 and 200 parts by weight per 100 parts of gum rubber.

In the case of a Butyl rubber matrix, the doping agent is diethylene glycol (DEG), preferably present at a concentration of between 1 and 5 parts by weight per 100 parts of gum rubber.

As butyl rubbers suitable for the invention, mention may be made of the various known butyl rubbers, the percentage of unsaturation being indifferent, as well as halogenated butyl rubbers, such as chlorobutyl.

The sulfur vulcanization systems suitable for the invention are those containing sulfur, and vulcanization accelerators. Sulfur can be supplied by vulcanization accelerators.

By way of example, as a vulcanization accelerator suitable for the invention, mention may be made of zinc salts and dithiocarbamic acid such as zinc dibutyldithiocarbamate, zinc diethyl dithiocarbamate or zinc ethylphenyl dithiocarbamate , or thiurams such as, mercaptobenzothiazole (MTB), benzothiazyl disulfide (MBTS), or guanidines or amines, or the like.

The sulfur vulcanization systems suitable for the invention do not contain a vulcanization activator such as, for example, stearic acid, zinc oxide. These products have a detrimental effect on the propellant adhesion properties.

Vulcanization systems suitable for EPDM rubbers include one or more peroxide compounds such as, for example, dicumyl peroxide; 2,5 dimethyl 2,5 Bis peroxide; 2,5 dimethyl 2,5 dimethyl hexane peroxide; 2,5 dimethyl 2,5 bis (t-butyl) hexyne 3 peroxide; 1,3 bis (t-butylisopropyl) benzene peroxide; 1-1 bis (t-butyl) 3.3.5 trimethylcyclohexane peroxide and t-butylcumyl peroxide or the like.

The concentration of peroxide or sulfur vulcanization system is that indicated and recommended by the rubber manufacturers for their use.

Other fillers can be added to the rubber such as fillers which reinforce the mechanical properties.

Advantageously, at least part of these charges is an electrically conductive charge, such as, for example, carbon black. Preferably the concentration by weight of this conductive filler is between 5 and 50 parts per 100 parts of gum rubber.

The addition of such an electrically conductive charge makes it possible to obtain an inhibitor having an improved electrical conductivity which is very advantageous for ensuring the evacuation of the electrostatic current generated in the charges or machines, in particular during the storage of these charges.

Without departing from the scope of the invention, it is possible to add various additives known to those skilled in the art, such as, for example, antioxidants, etc.

Polyurethane binder propellants are composite propellants comprising an inert binder, an oxidant, a fuel, ballistic additives, catalysts. In addition, these compositions can contain an explosive such as tetramethylenetetranitramine, trimethylenetrinitramine, for example.

As is known to those skilled in the art, other compounds not mentioned above can be added to these propellant compositions.

Propellants suitable for the invention have as an inert binder, a polyurethane obtained by reaction of a polyol with a polyisocyanate.

As preferred polyols of the invention, mention may be made of polybutadienes containing terminal hydroxyl functions (PBHT), in particular those marketed by the company ARCO under the trade names R45M and R45HT, polyalkylene ether glycols such as polypropylene glycols or polyethylene glycols, polyalkylene ester glycols, or the like.

As polyisocyanates suitable for the invention, aromatic, aliphatic, arylaliphatic or alkylaromatic polyisocyanates can be used.

Difunctional isocyanates are preferred, such as, for example, 2,4-tolylene diisocyanate; 2,6-tolylene diisocyanate; isophorone diisocyanate, hexamethylene diisocyanate, dimeryl diisocyanate or the like.

Other objects, details and characteristics of the invention will appear more clearly in the light of the detailed description which follows, given with reference to the examples given solely by way of illustration and indication.

The inhibitors of the invention are prepared according to conventional methods of using rubbers.

Thus, by way of example, the compositions are prepared in a mixer by first plasticizing the rubber compound at ambient temperature, then by introducing fillers such as carbon black, oxamide. After homogenization of the assembly, the various additives such as adhesion doping agents and antioxidant agents are always introduced at ambient temperature. Finally, the vulcanizing agents are mixed with the composition.

The composition is calendered in the form of a sheet with an average thickness of 5 millimeters.

These sheets are then cut to the desired dimensions and placed in a mold where they are shaped and vulcanized under pressure, at a temperature of the order of 160 ° C. and a duration of 30 to 60 minutes. The number of sheets deposited depends on the thickness of the part to be produced.

Of course, the vulcanization temperature and duration are given for information only and are determined, in a manner well known to those skilled in the art, by measuring a mechanical property to determine the optimum degree of vulcanization suitable for rubber application. For example, shear strength can be monitored using a cone rheometer.

The part thus formed is placed either in a mold when it is desired to make free blocks, or in the structure of a machine when the propellant charge is of the "molded-glued" type.

The propellant is then poured into the mold or structure, and crosslinked by heat treatment according to a process well known to those skilled in the art.

Advantageously, the surface of the rubber part is sanded, dusted and dried before the propellant is poured.

The propellant used in Examples 1 to 6 has the following composition:

Sm :

contrainte maximale en MPa

ε :

allongement élastique en %

E :

module d'Young en MPa

er :

allongement relatif à la rupture en %

ainsi que les caractéristiques de l'adhésion au propergol :

C :

résistance au cisaillement en Mpa

T :

résistance à la traction au MPa

P :

résistance au pelage en daN/cm

For each inhibitor, its mechanical properties are determined:

Sm:

maximum stress in MPa

ε:

elastic elongation in%

E:

Young's modulus in MPa

er:

elongation relative to rupture in%

as well as the characteristics of the propellant adhesion:

VS :

shear strength in Mpa

T:

tensile strength at MPa

P:

peel strength in daN / cm

Example 1

The inhibitor is based on butyl gum rubber and has the following composition expressed in parts by weight per 100 parts of gum rubber (phr).

• Sm = 8,7 MPa
• ε = 163 %
• E = 5,3 MPa
• er = 960 %

The mechanical properties measured at 20 ° C after vulcanization at 160 ° C for 30 minutes are:

• Sm = 8.7 MPa
• ε = 163%
• E = 5.3 MPa
• er = 960%

• C = 0,47 MPa
• T = 0,80 MPa
• P = 1,3 daN/cm

The characteristics of the bonding are:

• C = 0.47 MPa
• T = 0.80 MPa
• P = 1.3 daN / cm

Example 2

A composition identical to that of Example 1 is produced, but by replacing the isoprene-isobutylene copolymer (Butyl 065) with an isoprene-isobutylene copolymer also marketed by ESSO under the name Butyl 365 containing 2.25 moles of isoprene per 100 moles of isobutylene.

• Sm = 13,5 MPa
• ε = 219 %
• E = 6,2 MPa
• er = 990 %

This inhibitor, after vulcanization at 160 ° C for 30 minutes has the following mechanical properties:

• Sm = 13.5 MPa
• ε = 219%
• E = 6.2 MPa
• er = 990%

• C = 0,53 MPa
• T = 0,87 MPa
• P = 1,2 daN/cm

The characteristics of the bonding are:

• C = 0.53 MPa
• T = 0.87 MPa
• P = 1.2 daN / cm

Example 3

The inhibitor has the composition of that of Example 2 in which a bonding dopant, diethylene glycol, has been added in a proportion of 2 phr.

• Sm = 7 MPa
• ε = 122 %
• E = 5,8 MPa
• er = 1020 %

et les caractéristiques de l'adhésion ont les valeurs suivantes :

• C = 0,56 MPa
• T = 0,90 MPa
• P = 1,3 daN/cm

The mechanical properties are:

• Sm = 7 MPa
• ε = 122%
• E = 5.8 MPa
• er = 1020%

and the characteristics of the membership have the following values:

• C = 0.56 MPa
• T = 0.90 MPa
• P = 1.3 daN / cm

Example 4

The composition of the inhibitor is as follows:

The mechanical and characteristic properties of the adhesion are:

Example 5

The composition of the inhibitor is as follows:

The mechanical properties and characteristics of the bonding are:

Example 6

A test is carried out with the same composition as for Example 5 but without adding oxamide, and replacing the dicumyl peroxide with 2,5-dimethyl 2,5 Bis peroxide.

The mechanical properties and characteristics of the bonding are:

Examples 5 and 6 show the effect of oxamide on the characteristics of the bonding.

Example 7 was carried out with a propellant of the following composition:

The composition of the inhibitor tested, its mechanical properties and its bonding characteristics are as follows:

Examples 8 and 9 are comparative examples not falling within the scope of the present invention.

Example 8

The composition of the inhibitor, which is based on butyl gum rubber, differs from that of Example 1 in that a vulcanization activator is added in the following weight proportions:

• C = 0,48 MPa
• T = 0,51 MPa
• P = 0,7 daN/cm

The bonding properties on a propellant identical to that used in Example 1 are as follows:

• C = 0.48 MPa
• T = 0.51 MPa
• P = 0.7 daN / cm

These results clearly show the harmful effect of the presence of a vulcanization activator on the bonding, in particular on the tensile and peel strength.

Example 9

The bonding properties of an inhibitor based on an EPDM gum rubber identical to that of Example 4 were also evaluated, but using a sulfur vulcanization system in the following proportions:

• C = 0,44 MPa
• T = 0,48 MPa
• P = 0,3 daN/cm

The bonding properties obtained are:

• C = 0.44 MPa
• T = 0.48 MPa
• P = 0.3 daN / cm

These results compared to those of Example 4 clearly show the technical effect (direct bonding between the propellant and the inhibitor) obtained with a vulcanization system with peroxide compounds.

Claims (9)

1. Preformed inhibitor for a propellant with a polyurethane binder, comprising

   - a matrix based on gum rubber,

   - at least one filler,

   - a system for vulcanising the gum rubber, characterised in that :

   a) the gum rubber is chosen from the group comprising ethylene-propylene-diene monomer (EPDM) copolymers and isoprene-isobutylene (Butyl) copolymers,

   b) the system for vulcanising the gum rubber is :

   - a system based on peroxide compounds when the gum rubber is an EPDM copolymer,

   - a sulphur-based system comprising at least one vulcanisation accelerator and devoid of vulcanisation activators when the gum rubber is a Butyl copolymer

   c) the inhibitor also comprises :

   - oxamide when the gum rubber is an EPDM copolymer,

   - diethylene glycol when the gum rubber is a Butyl copolymer.
2. Inhibitor according to Claim 1, characterised in that the oxamide concentration is between 50 and 200 parts by weight per 100 parts of EPDM gum rubber.
3. Inhibitor according to Claim 1, characterised in that the diethylene glycol concentration is between 1 and 5 parts by weight per 100 parts of Butyl gum rubber.
4. Inhibitor according to one of the preceding claims, characterised in that at least one filler is an electrically conductive filler.
5. Inhibitor according to Claim 4, characterised in that the concentration of the electrically conductive filler is between 5 and 50 parts by weight per 100 parts of gum rubber.
6. Inhibitor according to one of Claims 1, 2, 4 and 5, characterised in that the system for vulcanising the EPDM rubber contains at least one peroxide compound chosen from the group containing dicumyl peroxide, 2,5-dimethyl-2,5-bisperoxide, 2,5-dimethyl-2,5-dimethylhexane peroxide, 2,5-dimethyl-2,5-bis(t-butyl)-3-hexyne peroxide, 1,3-bis(t-butylisopropyl)benzene peroxide, t-butylcumyl peroxide and 1,1-bis(t-butyl)-3,3,5-trimethylcyclohexane peroxide.
7. Inhibitor according to one of claims 1, 3, 4 and 5, characterised in that the system for vulcanising the butyl gum rubber contains sulphur and a vulcanisation accelerator.
8. Inhibitor according to any one of the preceding claims, to which a propellant with a polyurethane binder adheres directly without the insertion of a layer of adhesive.
9. Inhibitor to which a propellant with a polyurethane binder according to Claim 8 adheres directly, where the polyurethane binder is obtained by crosslinking a polybutadiene containing end hydroxyl functional groups (PBHT) with a polyisocyanate.