Resilient tires and wheels – Spring wheels – Spring encircling rigid annulus
Reexamination Certificate
1999-12-06
2003-04-22
Cain, Edward J. (Department: 1714)
Resilient tires and wheels
Spring wheels
Spring encircling rigid annulus
C152S556000, C057S236000, C057S238000, C057S243000
Reexamination Certificate
active
06550507
ABSTRACT:
BACKGROUND OF INVENTION
The present invention relates to thermotropic aromatic polyester(amide) fibers, more particularly to the monofilaments of such polymers, and to processes for obtaining such monofilaments.
The production from thermotropic aromatic polyester(amide) of conventional multifilament fibers formed of a large number of filaments of low elementary diameter (typically from about 20 to 30 &mgr;m) or of unit monofilaments of large diameter (at least 40 &mgr;m) by melt-spinning the polymer, generally followed by heat treatment referred to as post-polycondensation, is a known technique.
International Application WO 92/12018 (equivalent patents EP-B-517 870 and U.S. Pat. No. 5,427,165) describes in particular reinforcement assemblies intended to replace steel cables in tires, these assemblies being formed of monofilaments of liquid-crystal organic polymers having very high mechanical properties, in particular of aromatic polyester. To obtain these aromatic polyester monofilaments, the molten polymer is extruded at 340° C. through the capillary of a spinneret, the diameter of which is 800 &mgr;m and the temperature of which is 270° C.; the liquid jet emerging from the spinneret is stretched in air (stretching ratio about 20), then solidified by passing into a thermal quenching zone. The as-spun monofilament thus obtained is taken from a winding device at a speed of 590 m/min, then subjected to the post-polycondensation heat treatment on the takeup reel: this post-polycondensation phase, which is particularly long for this type of polymer (several hours) in fact involves the treatment being carried out on a reel, generally in an oven, and not on a single-thread passing continuously through this oven. After heat treatment, the monofilaments, for a diameter of about 180 &mgr;m, have the following mechanical properties: initial modulus of 4300 cN/tex, elongation at break of 2.5% and tenacity of 130 cN/tex. Owing to the liquid-crystal nature of the initial polymer, the single-threads, already in the as-spun state, have a very high initial modulus, greater than 4000 cN/tex, the post-polycondensation heat treatment being essentially intended to increase the tenacity of the spun products.
However, one major disadvantage of the as-spun monofilaments described above is that they have the special characteristic of contracting in the hot state. This property, which is probably linked to release from constraints “frozen” during spinning, makes it difficult to perform the subsequent post-polycondensation phase, and it is detrimental to the quality of the heat-treated monofilaments which derive therefrom, as is explained hereafter.
It happens that unless the monofilaments are allowed the possibility of contracting freely during their heat treatment, on their support reel, the latter will develop very major tensions which may result in partial damaging thereof, or even in self-breaking. Furthermore, there is the risk of interfilament “marriage” between adjacent or superposed turns, this risk being due to the combined action of the tension of contraction and of the temperature; such marriage, if it takes place, may prevent satisfactory later unwinding of the treated monofilaments.
In order to limit the above risks, although it has been attempted, before the treatment of the as-spun single-threads, to wind them up again at very low speed (several tens of meters per minute) to obtain as low as possible a tension on the support reel, this operation is costly from an industrial point of view and difficult to carry out when large lengths of monofilaments have to be treated. Attempts have also been made to utilise geometries of crossed overlapping winding limiting the contact between the threads, but the contraction then induces flexion-compression damage at the contact points.
To allow the monofilaments, on the contrary, the possibility of contracting freely during their treatment, experiments have been made using specific flexible reels which contract to a greater or lesser extent under the effect of the tension (variable diameter), this avoiding prior rewinding operations under very low tension. The use of such reels, although admittedly rather impractical and more costly, in particular reveals another major drawback of these as-spun monofilaments: their self-compression upon the thermal contraction in the majority of cases involves irreversible structural damage, revealed on the treated products by the presence of compression defects well-known under the name of “kink-bands”, once a critical compression threshold, which is relatively low for this type of polymer, is exceeded.
Thus, whatever the solution adopted, none has hitherto proved completely satisfactory with respect to the various problems posed by as-spun single-threads which contract in the hot state, in particular during their thermal post-polycondensation treatment.
Some of the disadvantages described previously are moreover not specific to monofilaments of high diameter, and have already been described for conventional multifilament fibers of thermotropic aromatic polyester(amide).
Nevertheless, all these drawbacks are generally exacerbated on monofilaments owing to their greater diameter: the damage to a filament after heat treatment may for example pass unnoticed on a multifilament fiber formed of several hundreds of filaments, whereas it most frequently becomes crippling on a unit monofilament of large diameter.
SUMMARY OF THE INVENTION
The first object of the invention is to overcome the above drawbacks by proposing a new monofilament of thermotropic aromatic polyester(amide) which, in the as-spun stage, has the characteristic of not contracting when hot.
This as-spun monofilament satisfies the following conditions: D≧40; Te>45; &Dgr;L≧0, D being its diameter (in &mgr;m) or its thickness in the case of an oblong or flattened shape, Te its tenacity (in cN/tex) and &Dgr;L its variation in length (in %) after 2 minutes at 235±5° C. at an initial tension of 0.2 cN/tex.
When the monofilament of the invention is intended to reinforce articles of plastics material and/or of rubber, in particular tires, D preferably lies within a range from 80 to 230 &mgr;m, more preferably from 100 to 200 &mgr;m.
Compared with the monofilaments of thermotropic aromatic polyester(amide) of the prior art, in particular within the range of 80 to 230 &mgr;m above, the as-spun monofilament of the invention has the advantage of having, for a given polymer and a given diameter D, a lower tensile modulus combined with an elongation at break which is generally higher, which constitutes an advantageous compromise. For it is known that generally, for fibers of liquid-crystal origin having very high mechanical properties, such a combination favours better flexion-compression properties, which are particularly desired when articles of plastics material and/or of rubber, in particular tires, have to be reinforced; this better compromise observed on the as-spun monofilaments is retained on the heat-treated monofilaments which derive therefrom.
Thus, preferably, the as-spun monofilament of the invention satisfies the conditions:
Mi<4000; Ar>2, Mi being its initial modulus (in cN/tex) and Ar its elongation at break (in %).
The monofilament of the invention is obtained by means of a novel, specific spinning process which constitutes another subject of the invention, this process being characterized in that it comprises the following stages:
a) melting the polymer;
b) extruding the molten polymer through a spinneret comprising at least one extrusion capillary;
c) on emerging from the capillary, structuring the flow of the polymer by stretching in a layer of gaseous fluid, preferably air, for a predetermined time ts;
d) at the end of the time ts, thermally quenching the flow of polymer thus structured by passing it through a quenching liquid, preferably water, so as to solidify it;
e) after possibly drying it, winding the monofilament thus obtained, the time ts (in seconds) being linked to the diameter or thickness D (in &mgr;m) of the as-spun
Aubry Jean-Claude
Esnault Philippe
Baker & Botts L.L.P.
Cain Edward J.
Michelin & Recherche et Technique S.A.
Wyrozebski Katarzyna
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