1. Technical Field of the Invention
The present invention relates to a multilayered article including a layer of elastomer overmoulded onto a thermoplastic and vulcanized so as to adhere directly to the latter.
It also relates to a process for the preparation of a composite article including a vulcanized elastomer associated directly with a thermoplastic.
In the case where the thermoplastic has barrier properties in respect of fuels or heating fluids, a double layer may be sufficient for the application of the invention in the form of pipes for petrol or conditioned air. In the case where the thermoplastic does not have barrier properties per se, one (or more) layer(s) will be needed inside, for the same purpose, in order to ensure the imperviousness.
2. Description of the Related Art
The processes for assembling articles (tubular ones or sheets) made of vulcanized elastomer (synthetic or natural) that is associated with thermoplastics which are commonly employed are based on the extrusion-sheathing of a thermoplastic previously coated with an adhesive. The outer layer of elastomer is then vulcanized in an autoclave with hot air, with steam pressure or with radiation, which are usual in the rubber industry.
Nevertheless, a considerable saving where the process is concerned could be achieved by doing away with the application of the adhesive.
One of the objectives of the present invention is to propose a process for the preparation of a composite article as defined above, avoiding the application of adhesive.
Another objective of the present invention is to propose an article as defined above exhibiting a high peel strength of the vulcanized elastomer sheath when subjected to the separating stress, preferably higher than 2 daN/cm.
The problems which the invention intends to solve are the following:
the interfacial adhesion in the case of multilayered articles (especially, in the case of tubular articles, between the outer elastomeric layer and the inner thermoplastic layer),
the protection of thermoplastic pipes from hot spots by a vulcanized elastomer, an example of application in the motor vehicle sector.
For the conditioned air application the pipes are traditionally made of textile-reinforced rubber. With the new legislation, a barrier layer against fluorohydrocarbons must be incorporated, and this is done with a barrier thermoplastic. This barrier thermoplastic layer must be fine so as not to impair the acoustic damping properties of the rubber.
For the petrol pipe application these pipes are today made of textile-reinforced rubber. The problems related to the rubber are: the permeability, the swelling, the limited resistance to peroxidized fuel and the extraction of various products, resulting in the advantage of introducing a thermoplastic inner barrier layer.
The advantage of the interfacial adhesion is linked, among others, with the better properties of the pipes (flexibility, decrease in the crunching of the pipes and impact strength) and easier use during the fixing of connections than on a pipe consisting of two separate parts (a rubber jacket and a thermoplastic liner).
Other advantages linked with the use of the present invention will appear in the course of the description which follows.
Japanese Patent Application JP 5-44874 describes pipes with an outer layer of epichlorohydrin rubber and an inner layer of nylon or of a fluoro resin, but with the use of adhesive.
In Patent Applications DE 4232946 and GB 2023626 use has also been made of an adhesion promoter or of an adhesive for pipes made of polyamide (or fluoropolymer, in the former), covered with rubber.
French Patent FR 2660404 describes pipes produced by direct coextrusion of a vulcanizable elastomer on a polyolefin; the vulcanizable elastomer is chosen from nitrile-PVC or EPDM rubbers and nothing is said concerning the vulcanization or the mechanism which makes the adhesiveness possible.
German Application DE 3914011 describes a pipe including an outer layer of polyolefinic elastomer (devoid of carboxylic or other functional groups) and on inner layer of thermoplastic, for example polyamide, but it is accepted that it would be preferable to employ adhesive between the two layersxe2x80x94which is furthermore described explicitly in a subsequent application DE 4026161.
The vulcanized synthetic or natural elastomers which are suitable for making use of the present invention are well known to a person skilled in the art, the term xe2x80x9celastomerxe2x80x9d in the definition of the present invention meaning that it may consist of mixtures of several elastomers.
These elastomers or elastomer mixtures exhibit a residual compression set (RCS) at 100xc2x0 C. which is lower than 50%, generally between 5 and 40% and preferably lower than 30%. These vulcanized elastomers originate from the corresponding vulcanizable elastomers.
Among the latter there may be mentioned natural rubber, polyisoprene which has a high double-bond content in a cis position, a polymerized emulsion based on styrene/butadiene copolymer, a polymerized solution based on styrene/butadiene copolymer, a polybutadiene which has a high double-bond content in a cis position, obtained by catalysis with nickel, cobalt, titanium or neodymium, a halogenated ethylene/propylene/diene terpolymer, a halogenated butyl rubber, a styrene/butadiene block copolymer, a styrene/isoprene block copolymer, the halogenated products of the above polymers, an acrylonitrile/butadiene copolymer, an acrylic elastomer, a fluoro elastomer, an epichlorohydrin elastomer and chloroprene.
Some elastomers mentioned above may be functionalized by means of carboxylic (or anhydride), epoxy or amino groups, by grafting, in a known manner, of these elastomers or, in the case of elastomer mixtures, for example with acrylic elastomers.
Among the abovementioned elastomers those included in the following group will be advantageously chosen: carboxylated nitrile elastomers, acrylic elastomers, carboxylated polybutadienes, grafted ethylene/propylene/diene terpolymers, epichlorohydrin elastomers or mixtures of these polymers with the same elastomers but ungrafted, such as nitrile rubbers, hydrogenated nitriles, polybutadienes and ethylene/propylene/diene terpolymers, by themselves or mixed.
The thermoplastic may be chosen from polyamides 6, 66, 11 and 12 and preferably polyamides 11 and 12 (plasticized or otherwise) or their copolymers or blends of these polyamides with polyolefins, from polyesters (for example polybutylene terephthalate), ethylene/tetrafluoroethylene (ETFE) copolymers, copolymers containing ethylene/vinyl alcohol units and polyvinylidene fluoride (PVDF) or mixtures containing it.
Included in the term xe2x80x9cpolyvinylidene fluoridexe2x80x9d is the homopolymer or the copolymers containing at least 70% by weight of vinylidene fluoride. The polyvinylidene fluoride may also be mixed with another thermoplastic polymer on condition that at least 50% by weight of polyvinylidene fluoride is present in the mixture.
An important example of a mixture containing polyvinylidene fluoride is the composition described in European Application EP 450994: a polymethacrylate plus an additive consisting of PVDF and of an acrylic or methacrylic elastomer.
The subject of the invention therefore includes a layer of rubber adherent to a thermoplastic, but it may also include one or a number of other layers, optionally secured by a binder; there is therefore: vulcanized rubber/thermoplastic/other layers with or without bond, for example: PA12/binder/PA6/EVOH/PA6 or PA12/binder/PVDF/PA12/PBT/PA12.
The invention is particularly useful for sheathing pipes which have one of the abovementioned thermoplastics as outer layer: polyamides and their blends, polyesters, and the like.
Another subject of the invention is a process for the preparation of the composite articles described above, characterized in that a thermoplastic is overmoulded, at an appropriate temperature, with an elastomeric composition comprising a synthetic or natural elastomer optionally containing carboxylic (or dicarboxylic acid anhydride), epoxy or amino functional groups, a crosslinking system and optionally various adjuvants and fillers, and in that the layer of elastomeric composition obtained is vulcanized.
The vulcanization temperature is preferably between xe2x88x925xc2x0 C. and +30xc2x0 C. in relation to the Vicat point of the said thermoplastic in contact with the elastomer.
The elastomeric composition and its vulcanization kinetics are such that the duration of the vulcanization cycle does not exceed 15 minutes and that the composite article exhibits a high peel strength of the vulcanized elastomer layer (preferably higher than 2 daN/cm).
The vulcanized elastomers and thermoplastics forming the composite material are normally associated sufficiently strongly to prevent any separation during a normal stress, bearing in mind the desired utilization. Thus, within the meaning of the present text, the term xe2x80x9cseparationxe2x80x9d implies the application to the material of a force which is considerably higher than that to which the said material should normally be subjected.
The separation resistance is assessed by a peeling test on a strip of pipe less than 5 mm in width, cut along a generatrix. The peel strength will preferably be advantageously higher than 2 daN/cm.
In the case of a carboxylated unsaturated elastomer, the vulcanization of the rubber having taken place by virtue of the double bonds and of the carboxylic groups, it is necessary to prevent all the latter, which are more reactive, from being consumed for the crosslinking, since they are needed for the adhesion with the thermoplastic. This is why the vulcanization temperature must be controlled well to have a sufficiently long scorch time and thus the carboxylic groups which are still free must be allowed time to react with the thermoplastic.
According to an alternative form of the process the thermoplastic layer is overmoulded with the elastomer composition which is extruded on an elastomeric extruder at a temperature of between 50xc2x0 C. and 120xc2x0 C., in a sheathing die (that is to say a crosshead die). The unvulcanized article is placed, possibly after cutting out, in a conventional rubber vulcanization autoclave (especially with hot air, steam, infrared and the like) the temperature of which is between xe2x88x925xc2x0 C. and +30xc2x0 C. in relation to the Vicat point of the said thermoplastic in contact with the elastomer.
According to another alternative form of the process the article is obtained by coextrusion of the elastomer at the same time as the thermoplastic(s). In this case the adhesion produced during the coextrusion process and the vulcanization operation are performed in line or by reprocessing.
The vulcanization systems employed for producing these composites are well known to a person skilled in the art and, consequently, the invention is not limited to systems of any one particular type. It suffices for the latter to meet the criterion relating to the vulcanization kinetics, defined in the definition of the invention indicated above.
When the elastomer is based on unsaturated monomer (butadiene, isoprene, vinylidenenorbornene, etc), four types of vulcanization systems may be mentioned:
sulphur systems consisting of sulphur used in combination with the usual accelerators such as dithiocarbamate metal salts (zinc or tellurium dimethyl-dithiocarbamate and the like), thiuram disulphides (thiuram tetramethyldisulphide, and the like), sulpheramides, and the like.
These systems may also contain zinc oxide used in combination with stearic acid.
Sulphur-donor systems, in which most of the sulphur employed for the bridging originates from sulphur-containing molecules such as the organosulophur compounds referred to above.
Systems containing phenolic resins, consisting of difunctional phenol-formaldehyde resins which may be halogenated, used in combination with accelerators such as stannous chloride or zinc oxide.
Peroxide systems. They make it possible to have a product which is more stable to heat, white, as in the case of the sulphur-donor systems. Any free radical donors may be employed (dicumyl peroxides and the like) in association with zinc oxide and stearic acid.
When the elastomer is acrylic (polybutyl acrylate with acidic or expoxy functional groups or any other reactive functional group permitting the crosslinking), the usual crose l inking agents are employed, which are based on diamines (ortho-toluidylguanidine, diphenylguanidine and the like) or on blocked diamines (hexamethylenediamine carbamate and the like).
When the elastomer is the epichlorohydrin elastomer (homopolymer, copolyme r or terpolymer), crosslinking agents based on amine (2-mercaptoimidazoline, triazines, and the like) are employed.
The elastomeric compositions may be modified for some special properties (for example improvement in the mechanical properties) by the addition of fillers such as carbon black, silica, kaolin, clay, talc, chalk and the like. These fillers may be surface-treated with silanes, polyethylene glycols or any other coupling molecule. In general the proportion of fillers in parts by weight is between 5 and 100 per 100 parts of elastomers.
In addition, the compositions may be made flexible using plasticizers such as petroleum-derived mineral oils, esters of phthalic acid or of sebacic acid, liquid polymer plasticizers such as optionally carboxylated polybutadiene of low mass and other plasticizers which are well known to a person skilled in the art.
The combinations of vulcanizing agent which are employed for making use of the process are such that they should allow a complete crosslinking of the elastomer according to kinetics which result in good properties of resistance to separation, as mentioned in the definition of the invention and, in general, in good rubber properties (measured as a residual compression set at 100xc2x0 C., tensile mechanical properties and the like).
The vulcanization temperature in the autoclave will be advantageously between 130 and 190xc2x0 C.
The kinetics measured with the aid of an oscillating rheometer will be advantageously such that the characteristic time for 90% vulcanization, t90, does not exceed 15 minutes at 160xc2x0 C. and advantageously will be between 5 and 10 minutes.
Furthermore, it has been found that the time of the beginning of vulcanization (or setting time) corresponding to an increase in torque of 0.2 Nm is an important factor for obtaining materials exhibiting good performance. Thus, it is advantageous that the abovementioned increase in torque should be reached in a time longer than or equal to 4 minutes at the moulding temperature, and preferably between 4 and 6 minutes.