The rheometer is located in a non-return branch of a molten polymer line and includes, in succession, a fixed-speed pump, a first capillary tube, and a second capillary tube in series with the first tube. The second tube has a diameter less than or equal to that of the first tube, and emerges into a fluid medium at regulated pressure. Pressure-sensors at the inlet of each tube determine the pressure loss between the inlet of each tube and the fluid medium at regulated pressure. The temperature is monitored by means that are at least party external to the body of the rheometer.

BACKGROUND OF THE INVENTION 
The invention relates to a method of characterizing the rheological 
properties of a plastic material, and more particularly to a rheometer 
intended for the determination of the working-up capacity of a molten 
visco-elastic polymeric material. 
It is known that information on the shear modulus and on the viscosity of a 
polymeric material is of extreme importance in characterizing commercial 
plastics, particularly thermo-plastics, since these properties determine 
the specific applications for which the plastics are intended. Thus, 
thermo-plastic resins intended for extrusion have visco-elastic properties 
which are very different from those intended for injection-moulding. 
These rheological phenomena have already given rise to the study and 
development of apparatuses capable of providing data, the interpretation 
of which makes it possible to calculate the desired properties. Thus U.S. 
Pat. No. 3,559,464 describes a rheometer for continuous monitoring, 
arranged in parallel with the main polymer flow line and comprising (a) a 
heat-controlled block provided with a first capillary tube and with a 
second capillary tube of the same diameter but of a greater length than 
the first tube, (b) a pump of variable speed connected to the block and 
inserted between the two capillaries, and (c) differential pressure gauges 
fitted on the capillaries. This device constitutes a pressure regulation 
system which is intricate and difficult to maintain, and suffers the 
disadvantage of allowing measurement of the rheological properties for 
only a single shear-level of the visco-elastic material. From this 
standpoint it therefore provides incomplete results and allows only an 
approximate determination of the working-up capacity of the polymers. 
A first object of the inventon is therefore to provide an apparatus which 
can, by measurements at different shear-levels, determine with great 
precision the working-up capacity of a molten visco-elastic polymeric 
material. A second object of the invention is, when a lesser precision in 
the determination of the rheological behavior of a polymer is acceptable, 
to provide an apparatus whose construction and maintenance are easier than 
those of known apparatuses and which can more easily be adapted to the 
direct and speedy monitoring of very large polymerization units. 
SUMMARY OF THE INVENTION 
To achieve the foregoing objects and in accordance with the purpose of the 
invention, as embodied and broadly described herein, the rheometer of this 
invention is arranged in a non-return branch of a molten polymer line and 
comprises (1) a body including a fixed-speed pump, a first capillary tube 
downstream of the pump, a second capillary tube in series with the first 
capillary tube and of a diameter less than or equal to that of the first 
tube, the second tube emerging in a fluid medium at regulated pressure, 
and a pressure-sensor fitted at the inlet of each tube in order to measure 
the pressure loss between the inlet of each tube and the fluid medium at 
regulated pressure, and (2) means for monitoring the temperature which are 
at least partly external to the body. 
If the process of preparation of the polymer involves the presence of 
molten polymer in the polymerization plant, the rheometer of the invention 
can be located in this part of the plant. In ethylene polymerization 
plants under high pressure, the rheometer will be connected to the outlet 
of the separator. 
This apparatus is useful for rheological information on very diverse 
polymers, e.g., polystyrene and all polyolefins, namely low and high 
density polyethylenes, polypropylene, and polybutene. As usual in this 
technique, the external part of the temperature monitoring means includes 
appropriate isothermic and adiabatic jackets, whose purpose is to keep the 
temperature of the molten visco-elastic material circulating in the body 
constant. The fluid medium at regulated pressure is situated at the outlet 
of the second capillary tube and is generally a gaseous medium such as air 
at atmospheric pressure; it can also be an inert gaseous medium, such as 
nitrogen or argon, where it is necessary to evaluate or measure other 
properties, for example chemical properties, of the polymer at the outlet 
of the rheometer. It should be noted that the components of the rheometer 
according to the invention, i.e., the fixed-speed pump and the 
pressure-sensors functioning with reference to a known pressure, are of 
simpler design and are easier to maintain than those incorporated in the 
apparatus of U.S. Pat. No. 3,559,464. 
According to a preferred embodiment of the invention, the temperature 
monitoring means of the rheometer comprise not only the external part 
discussed above, but also a part inside the body, this part being arranged 
between the fixed-speed pump and the first capillary tube. This internal 
part may comprise, for example, a tubular heat-exchange zone. The diameter 
of this zone should be as small as possible in order to promote the 
efficiency of this heat-exchange. However, the choice of this diameter is 
limited by the absolute necessity of avoiding any rheological modification 
of the polymer, such as the so-called melt-fracture phenomenon, during its 
flow through the tubular heat-exchange zone. 
An example of the possibility, indicated above, of carrying out a 
supplementary characterization of the working-up capacity of the polymer 
at the outlet of the rheometer is means rigidly fastened to the body of 
the rheometer for measuring the extensibility of the polymer in the molten 
state. As embodied herein, this means comprises, for example, a strip 
extensometer. This embodiment is described below and illustrated in the 
drawing. 
If great precision is required in the determination of the rheological 
behavior of the visco-elastic material, the rheometer of the invention 
must be able to provide measurements at various shear levels. For this 
purpose, its second capillary tube is arranged immediately in series with 
the first capillary tube and has a diameter which must be less than that 
of the latter. Preferably, the shear speed gradient in the first capillary 
tube is between 0.1 and 50 seconds.sup.-1 and the shear speed gradient in 
the second capillary tube is between 100 and 5,000 seconds.sup.-1. 
Furthermore, since the pressures measured by the pressure-sensors of the 
rheometer body are related to the length and to the diameter of the 
capillary tubes, and since, for reasons of convenience and reliability, it 
is desirable to measure pressures of the same order of magnitude, the 
length of the first capillary tube should preferably be much greater than 
that of the second tube. The second capillary tube may be removable and 
replaceable with a tube of another diameter and the first capillary tube 
may be linable to vary its diameter, in order to adapt the apparatus to 
the flow of resins having very different rheological characteristics 
(especially the melt index), as frequently occurs in very large capacity 
polymerization units. 
The accompanying drawing, which is incorporated in and constitutes a part 
of this Specification, illustrates one embodiment of the invention and, 
together with the description, serves to explain the principles of the 
invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Reference will now be made in detail to the present preferred embodiment of 
the invention, an example of which is illustrated in the accompanying 
drawing. 
The invention will be better understood by reference to the single figure, 
which represents schematically the rheometer with a double shear level, 
including, as embodied herein, a device for measuring the extensibility of 
the polymer in the molten state. In this figure, 1 denotes the main flow 
line of a polymer, for example the outlet line of a separator of a 
manufacturing unit of the polymer, from which a branch 2 conducts the 
visco-elastic material to the body 3 of a rheometer. As embodied herein, 
the body comprises successively a fixed-speed pump 4, a tubular 
heat-exchange zone 5, and a first capillary tube 6. In accordance with the 
invention, this tube is provided with pressure and temperature sensors 7 
and 7a at its inlet and at its outlet, which sensors are arranged in 
housings 8 and 8a and connected firstly to pressure recorders 9 and 9a and 
secondly to an electronic temperature-regulating system 10. The second 
capillary tube 11, like the zone 5 and the first tube 6, is located in a 
metal part 12 which is itself surrounded by a heating jacket 13, the 
operation of which is controlled by the system 10, the assembly being 
surrounded by an insulating jacket 14 made, for example, of glass-wool 
and/or asbestos. As embodied herein, after flowing through the capillary 
tubes, the strip of molten polymer arrives tangentially to the detection 
pulley 15 of the device for measuring the extensibility, also called an 
extensometer. This pulley is firmly fixed, via a rod 16, to a force sensor 
17, which is fixed to a carrier-plate 18 and connected electrically to a 
recorder 19 by an adjustable free-rotating pulley. The carrier 18, while 
being thermally insulated from the body 3 of the rheometer, is linked 
mechanically to it by any connecting means 23, such as a rod or plate, 
allowing easy dispersal of the heat given off in this area by the polymer 
strip. If desired to increase the precision of the measurement of the 
drawing force, the strip is passed over a second pulley 20 which is firmly 
fixed, via a rod 21, to the carrier-plate 18. The strip, held by these 
pulleys, is drawn by a fixed speed drawing-bench 22, for example of the 
compact type consisting of a synchronous motor and a gear box equipped 
with rubberized drawing pulleys. 
Reconsidering the various uses of the invention, if ordinary precision is 
sufficient for the determination of the rheological behavior of the 
visco-elastic material, the rheometer of the invention can carry out 
measurements at a single shear level. Its second capillary tube is then 
provided with the same diameter as the first tube and is not arranged 
immediately in series with the latter, but is separated therefrom by a 
perforated distance-piece having a diameter greater than the diameter of 
the capillary tubes, in order to allow relaxation of the visco-elastic 
material between said tubes. Furthermore, in this embodiment, the length 
of the first capillary tube is preferably at least twice that of the 
second tube. 
Although the present invention has been described by means of preferred 
embodiments, it will be apparent to those skilled in the art that various 
modifications and variations can be made in the rheometer of the invention 
without departing from the scope or spirit of the invention. For example, 
the rheometer may carry out measurements at n different shear levels, n 
being greater than 2, by including, in series, n capillary tubes with 
decreasing diameters and provided with n pressure-sensors.