Antistatic compositions based on polyamide

Details Antistatic compositions based on polyamide Antistatic compositions based on polyamide

2000-10-06

2003-01-14

Medley, Margaret (Department: 1714)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

At least one aryl ring which is part of a fused or bridged...

C524S496000, C524S252000, C524S511000

Reexamination Certificate

active

06506830

ABSTRACT:

This invention relates to compositions based on polyamide that can be used in particular for the production of single-layer or multilayer tubes and/or ducts in the field of transport and/or storage of hydrocarbons.
In automobiles, under the action of the injection pump, the gasoline circulates at high speed in the tubes that connect the engine to the tank, whereby these tubes are obtained from compositions that are based primarily on polyamide 11 or 12 (RILSAN). In some cases, the rubbing of gasoline/inside wall of the tube can produce electrostatic charges, whose accumulation can lead to an electric discharge (spark) that can ignite the gasoline with catastrophic consequences (explosion). Also, it is necessary to limit the surface resistivity of the inside face of the tube to a value that is generally less than 10
6
ohms (&OHgr;).
Furthermore, these polyamide-based compositions should meet the other criteria of the specifications of the gasoline line and in particular cold shock resistance. In addition, the polyamide composition, made antistatic, should be extrudable: it is therefore sought to limit as much as possible its viscosity in the molten state. It should also be chemically resistant to peroxidized gasolines.
It is known to lower the surface resistivity of resins or polymeric materials by incorporating in them conductive and/or semiconductive materials, such as carbon black, steel fibers, carbon fibers, particles (fibers, strips, spheres, etc.) that are gold-, silver- or nickel-plated or covered by a fine layer of polymer that is inherently conductive or semi-conductive.
Among these materials, the carbon black is used more particularly because of its great commercial availability and its good performances.
When the level of carbon black is increased in a polymeric composition, the resistivity first changes little. Then, when a critical level of carbon black, called percolation threshold, is reached, the resistivity drops very abruptly until a relatively stable level (plateau zone) is reached, where increasing the carbon black level brings about very little change in resistivity.
The technical report “Ketjenblack EC—BLACK 94/01” of the AKZO NOBEL Company indicates that a conductive and/or semi-conductive carbon black is all the more effective—i.e., it is necessary to add little of it to the polymer to give it antistatic properties—as its structure is developed. The structure of a carbon black reflects the manner in which the basic carbon-containing particles that constitute the carbon black are arranged in aggregates, even in agglomerates. The structure of a carbon black can be expressed by its specific surface area (measured by the nitrogen adsorption method—BET method—according to ASTM Standard D 3037-89), as well as by its DBP (di-butyl-phthalate) absorption (measured according to ASTM Standard D 2414-90). The carbon blacks that are marketed by the AKZO NOBEL Company are very structured and characterized by a large BET surface area and high DBP absorption. They are often designated as extra-conductive carbon blacks. Thanks to their developed structure, the percolation threshold is reached at a low addition rate.
Beyond its electro conductive and/or semi-conductive properties, the carbon black acts like a filler, such as, for example, talc, chalk, kaolin, and therefore affects many other physical and chemical properties.
Thus, one skilled in the art knows that when the proportion of filler increases, the viscosity of the polymer/feedstock mixture increases, as well as the modulus of elasticity of the composition. The increase in viscosity is observed by, for example, a measure of the fluidity index (MWI=melt flow index). Also, when the capacity factor increases, the durability or resistance to impact of the charged polymer, expressed by, for example, measurement of elongation at break or impact strength, decreases. The increase of the viscosity and the reduction of the impact resistance are all the larger the higher the proportion of filler.
Thus, one skilled in the art naturally prefers to reduce the proportion of filler to impart the desired property to the polymer/feedstock mixture while affecting the other properties, such as viscosity or impact resistance, as little as possible. Thus, if the task at hand is to obtain a low surface resistivity, one skilled in the art will rather use extra-conductive carbon blacks.
It was thus noted that for polyamide 12, with inherent viscosity 1.65 (measured at 20° C. for a sample of 0.5 g in 100 g of meta-cresol), plasticized by 11.4% by mass of n-butyl benzene sulfonamide (BBSA) and containing at least 6% by mass of Ketjenblack EC 600 jD carbon black of the AKZO NOBEL Company (characterized by DBP absorption that is greater than 400 ml/g and by a BET surface area that is greater than 1000 m
2
/g), the surface resistivity on the tube is less than 10
6
ohms. It was noted, moreover, for this same polyamide, that the plateau zone 10
2
-10
3
ohms) is reached starting at 10% by mass of Ketjenblack EC 600 JD carbon black.
It seems, however, that this carbon black, that can be designated as “structured” or “more structured,” disperses poorly in the polyamide in the molten state, which leads to the presence of agglomerates. These agglomerates have a negative effect on the impact strength.
It has now been discovered, surprisingly enough, that by going against the teaching of the prior art that relates to, on the one hand, the selection of the type of carbon black, and, on the other hand, its amount used, namely by using a conductive carbon black and/or a “less structured” semi-conductive carbon black than the extra-conductive carbon black above and in addition by using such a carbon black in a larger amount than the preceding extra-conductive carbon black, polyamide compositions that have better impact strength as well as better Theological properties (with equivalent resistivity levels) are obtained.
The fact of using a less structured carbon black requires increasing the content to obtain the same antistatism level—generally the goal is to produce a surface resistivity of less than 10
6
ohms. Despite this higher addition rate of carbon black, better rheological properties (a lower viscosity in the molten state, which is demonstrated by a higher fluidity index (MW-I)) and impact strength (impact resistance) are obtained. This is all the more surprising since in general—and as emphasized above—the more the proportion of filler is increased, the more it is precisely these properties that are degraded.
Thus, this invention, residing in the selection of this “less structured” carbon black, does not produce a better compromise of antistatism/other properties, but leads to a polyamide-based antistatic composition that has inherently better rheological properties and impact strength.
This invention therefore first has as its object a composition of antistatic polyamide, comprising at least one polyamide and a sufficient amount of carbon black to make it antistatic, characterized by the fact that the carbon black is at least a carbon black that is selected from among those that have a specific BET surface area, measured according to ASTM Standard D 3037-89, from 5 to 200 m
2
/g, in particular from 20 to 100 m
2
/g, and DBP absorption, measured according to ASTM Standard D 2414-90, from 50 to 300 ml/100 g, in particular from 125 to 250 ml/100 g. (The measurement of the DBP absorption is that of a pore volume that is expressed in ml of di-butyl-phthalate per 100 g of carbon black.)
The carbon blacks according to the invention can be designated as conductive or semi-conductive contrary to extra-conductive carbon blacks that are used according to the prior art, which generally have a BET surface area that is greater than 500 m
2
/g and DBP absorption that is greater than 300 ml/100 g.
Furthermore, the polyamide-based antistatic compositions of the invention preferably contain 16 to 30% by mass of these “less structured” conductive or semi-conductive carbon black(s) and more particularly 17.5 to 23% by mass, relative to the total compos

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