Thermoplastic hollow molded body

Details Thermoplastic hollow molded body Thermoplastic hollow molded body

2001-07-12

2003-07-15

Nolan, Sandra M. (Department: 1772)

Stock material or miscellaneous articles

Hollow or container type article

Polymer or resin containing

C428S036700, C428S426000, C428S516000, C428S903300

Reexamination Certificate

active

06592955

ABSTRACT:

The present invention relates to a thermoplastic multilayer hollow body.
Thermoplastic multilayer hollow bodies have been developed to meet usage needs which require properties that cannot be conferred by a single thermoplastic. In particular, this technique is used when it is a question of manufacturing thermoplastic hollow bodies which have both high rigidity at ordinary temperature and good impermeability to certain liquids that they are intended to contain. In this case, the impermeabilizing function is generally conferred by a thin internal layer of low mechanical strength which is made of a material behaving as a barrier to the liquid and vapours of this liquid which are contained in the hollow body.
In certain situations, when for example the hollow body is in contact with a liquid which is electrically non-conducting and when this liquid rubs against a surface of this hollow body, static electricity can be generated at this surface because of the low electrical conductivity that standard plastics usually have. When the electrostatic potentials generated are sufficiently high, sparks may occur, particularly in dry atmospheric conditions. If the liquid in contact with the charged surface of the hollow body is inflammable, there is a risk of explosion and destruction of the hollow body and its environment.
Solutions have consequently been developed to counter the build-up of static electricity on the surface of hollow bodies liable to come into contact with poorly conducting inflammable fluids. Among these solutions are encountered devices for earthing the hollow bodies and their supports by means of metal components which are good electrical conductors or else the incorporation into the plastic of the hollow bodies of additives so as to make them good conductors.
However, in certain cases the presence of these additives at the surface of the hollow body means that the conventional impermeabilizing treatments carried out by making the surface to be treated come into contact with gaseous fluorine can no longer be carried out correctly, especially when the fluorination treatment is carried out during the blow moulding used to manufacture the hollow body.
Moreover, the incorporation of conductive additives throughout the thickness of the walls of a hollow body unfavourably alters the mechanical properties of the end product and frequently increases the cost to the extent of making it economically unacceptable.
Attempts have therefore been made to manufacture hollow bodies having a multilayer structure in which the conductive thermoplastic is at the surface and the impermeabilization is provided by the presence of an internal layer comprising an impermeable material (“barrier” material).
International Patent Application WO 95/05940 describes a composite thermoplastic material formed from five layers comprising a core of an ethylene/vinyl alcohol copolymer surrounded on each side by an adhesive layer followed by a base layer which may be a polyolefin, for example high-density polyolefin. A conductive material, such as a metal powder or carbon powder, may be incorporated into one of the base layers or applied to it in the form of an additional layer at the outer surface.
However, known multilayer structures comprising a conductive surface and an internal barrier layer require base layers which comprise a high-performance material and are relatively thick in order to stiffen the assembly and achieve acceptable mechanical properties. When they have to withstand stresses compatible with intensive use, for example when they are associated with fuel tanks for motor vehicles, this results in the need to process a large amount of high-performance material and consequently this results in a high cost both of the raw material and of the finished multilayer structure.
It is an object of the invention to provide a conductive multilayer hollow body which is able to withstand high mechanical stresses, is highly impermeable to the products that it is intended to contain and has a reasonable cost.
For this purpose, the invention relates to a thermoplastic multilayer hollow body which comprises an inner layer comprising a conductive compound based on a virgin polymer selected from polyolefins and polyamides, an outer layer comprising a virgin polymer selected from polyolefins and polyamides, and at least one barrier layer and at least one adhesive, according to which it also includes at least one layer comprising recycled plastic.
The term “hollow body” is understood to mean any structure whose surface has at least an empty or concave part. In particular, the hollow body may have the shape of a tube or a pipe.
According to the invention, the hollow body is a thermoplastic multilayer, that is to say it is formed from a stack of several layers comprising thermoplastic.
The term “thermoplastic” is understood to mean any thermoplastic polymer, including thermoplastic elastomers, and blends thereof. The term “polymer” is understood to mean both homopolymers and copolymers (especially binary or ternary copolymers). Non-limiting examples of such copolymers are: random copolymers, alternating copolymers, block copolymers and graft copolymers.
The thermoplastic of one particular layer may be the only one in the multilayer structure of the hollow body. It may also be found in at least two separate layers of this structure. When it is in only a single layer or, on the contrary, in more than one layer of the structure, a particular thermoplastic may be by itself or in the presence of other compounds chosen from additives of a wide variety of types, these being intended to modify the intrinsic properties of the thermoplastic and are well known to those skilled in the art. Examples of such additives are, non-limitingly: fillers, stabilizers, plasticizers, lubricants, UV screens, etc. It is also possible to combine at least two different thermoplastics within a single layer of the structure. In this case, the layer may also possibly contain at least one of the abovementioned additives.
According to the invention, the hollow body comprises an inner layer comprising a conductive compound based on a polymer selected from polyolefins and polyamides.
The term “inner layer” is understood to mean the innermost layer of the hollow body on the same side as a concave part of this body. Often, the inner layer is that in direct contact with the fluid.
The conductive compound is a compound comprising at least one thermoplastic selected from polyolefins and polyamides and at least one additive which gives the compound good antistatic properties. The intended objective is to give the inner layer a low electrical surface resistivity. In general, a surface resistivity is considered to be low when it is at most 10
6
ohms/□, as measured according to the procedure described in Standard CEI/IEC 93: 1980 (2nd edition). In practice, the conductive thermoplastic layers which comprise a conductive compound, the surface resistivity of which is at most 0.5×10
6
ohms/□ are preferred. Excellent results have been obtained with conductive layers having a surface resistivity of at least 0.1×10
6
ohms/□ and of at most 0.5×10
6
ohms/□.
Such conductive compounds comprise a polyolefin or polyamide base to which a certain amount of electrically conductive additive is added. As conductive additives, it is possible to use, non-limitingly, one or more of the following additives: carbon black, carbon powder or carbon fibres, and conductive polymers. The term “conductive polymer” is understood to mean polymers that are intrinsically electrically conductive, such as, for example, those belonging to the class of polythiophenes, polypyrroles and polyindoles. Such conductive polymers are described, for example, in European Patents 0240063, 0323656, 0375005, 0408105, 0525856, 0416672, 0413382, and 0469667 and in U.S. Pat. No. 5,290,891.
Additives based on elemental carbon are preferred. Carbon black is the conductive additive most often preferred.
The amount of conductive additive used is generally at le

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