Patent Description:
Polymers are often tested in accordance with one of several ASTM methods. Examples include ASTM D6048, D5289, D6204, D6601, D7050 and D7605. ASTM <NUM> describes the use of a variable frequency test and also discloses the capability of performing a variable temperature test. ASTM D6601 describes the conditions for evaluating a specimen at more than one strain amplitude during a single test. Instruments operating in accordance with these ASTM tests are known.

Existing torsional rheometers include an upper die, an upper housing, and an upper seal plate (upper assembly), and a lower die, a lower housing and a lower seal plate (lower assembly). One example of an existing torsional rheometer is the RPA <NUM> sold by Alpha Technologies, Inc. The RPA <NUM> is a dynamic mechanical rheological tester (DMRT) designed to test raw elastomers or mixed rubber before, during and after cure in a single test. During a test, the upper assembly is lowered onto the lower assembly that contains an excess amount of a test specimen. A closing force, which may be in one example approximately <NUM> pounds force, squeezes the polymer sample and forms a sealed die cavity with the sample under pressure between the upper and lower dies. The closing force is distributed between the polymer sample in the die cavity and the seal plate housing. The faces of the upper and lower seal plates come into direct contact with each other which then holds the faces of the upper and lower dies a fixed distance apart. This typically produces a die cavity pressure near <NUM> psi. but the pressure can vary due to factors such as the test temperature and the viscosity of the material. The lower die is then oscillated by a drive system and the force from the lower die is transmitted through the sample to the upper die where the resulting reaction torque is measured by a transducer mounted to the back of the upper die.

<CIT> discloses a curing characteristics measuring apparatus in which a sample chamber having a fixed surface and a rotation-vibrating drive surface is charged with a sample having a viscoelasticity. A torsional vibration is applied to the sample through the drive surface. A shearing stress generated in the sample is observed as a torque through the fixed surface or the drive surface, wherein mechanism is provided for reducing volume of the sample chamber following volume contraction generated in curing process of the sample. However, the structure disclosed in <CIT> inter alia fails to disclose a load bearing compliant member that allows for self-correction of a drop in pressure caused by shrinkage of a sample.

<CIT> provides apparatus modifications to a sealed die material testing apparatus having dies with associated seal plates. These modifications enable the apparatus to be employed as a non-sealed die system and further include adapting the seal plates to carry spacers and adapting the apparatus to affect a gap between the seal plates. <CIT> also discloses a rheometer that confines a sample of an elastomer under pressure for measuring the curing characteristics of that sample.

There are some rheological tests where it is beneficial to first run a subtest on the sample at an elevated temperature and then run an additional subtest at a reduced temperature. As the dies cool, the sample also cools, causing it to shrink, reducing the pressure in the die cavity. If the first test is a cure test, the sample often shrinks even more during the transition and there is a further loss of cavity pressure. At some point the polymer shrinkage becomes so severe that the interface between the die surface and the polymer breaks down, allowing the sample to slip on the die face and reducing the strain imparted to the material. This produces a reduction in the signal and often a shift in the peak phase of the signal relative to the lower die movement. Once this slippage occurs, the test results become inaccurate and often not repeatable.

The claimed invention is defined in the subject matter of the independent claims, wherein the dependent claims specify preferred embodiments.

The invention relates to a torsional rheometer according to claim <NUM>.

Another aspect of the invention relates to a method for compensating for reduction of cavity pressure due to shrinkage of a test sample in a die cavity of a torsional according to claim <NUM>.

In the drawings, each identical and nearly-identical component that is illustrated in various figures is represented by a like numeral. Various embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which:.

This invention relates to an improved torsional rheometer system for testing polymers. One aspect of the invention relates to an apparatus that compensates for reduction in the pressure of the die cavity caused by shrinkage of the polymer sample in the die cavity during testing. This aspect includes a compliant member disposed in series with a load-bearing component in which the compliant member is configured to deflect as the pressure in the die cavity drops to cause the die cavity to close more tightly on the sample to maintain cavity pressure. This reduction in pressure may be caused by factors such as cooling of the test sample, curing of the test sample, or a lowering of the temperature used in a test. Another aspect relates to a method for compensating for reduction of cavity pressure in the die cavity of a torsional rheometer. This aspect includes placing a compliant member in series with the load-bearing components of the die cavity so that the compliant member deflects as cavity pressure drops to maintain cavity pressure during cooling.

With reference now to the drawings, and more particularly to <FIG> thereof, an example of a prior art torsional rheometer will now be described. <FIG> is a schematic representation of such a torsional rheometer, such as a RPA <NUM> rheometer. <FIG> does not disclose all components thereof, but only the components necessary to gain an understanding of the invention. Furthermore, <FIG> is not drawn to scale or in a way that accurately represents the size and shape of each component.

Torsional rheometer <NUM> includes a first or upper assembly <NUM> and a second or lower assembly <NUM>. Upper assembly <NUM> includes an upper cross-head <NUM>, an upper die assembly <NUM>, an upper seal plate housing <NUM>, an upper insulator ring <NUM> and an upper seal plate <NUM>. Upper cross-head <NUM> is driven upwardly and downwardly relative to lower assembly <NUM> by an air cylinder <NUM> which is shown schematically in <FIG>. Air cylinder <NUM> is connected to upper cross-head <NUM> by a mount <NUM>. Air cylinder <NUM> moves upper cross-head <NUM>, and thus upper die assembly <NUM> toward and away from lower assembly <NUM> in a manner well-known to those of skill in the art. An air regulator <NUM> regulates the pressure applied by air cylinder <NUM>. Torque and pressure transducers <NUM> are coupled to upper die assembly <NUM> to measure torque applied to upper die assembly <NUM> and pressure within the die cavity respectively.

Lower assembly <NUM> includes a fixed table plate <NUM> which supports and stabilizes rheometer <NUM>. Lower assembly <NUM> also includes a lower die assembly <NUM>. Lower die assembly <NUM> may be oscillated back and forth about its central axis by a drive motor <NUM>. Surrounding lower die assembly <NUM> is a housing <NUM>. Housing <NUM> includes an upper portion <NUM> and a lower portion <NUM>. Disposed between upper portion <NUM> and lower portion <NUM> is a shim <NUM>. Housing <NUM> rests on table plate <NUM>. Sitting atop housing <NUM> is an insulator ring <NUM>. Disposed on top of insulator ring <NUM> is a lower seal plate <NUM>.

A die cavity <NUM> is disposed between lower surface <NUM> of upper die assembly <NUM>, and upper surface <NUM> of lower die assembly <NUM>. A test sample <NUM> may be placed in die cavity <NUM> and clamped between upper seal plate <NUM>, and lower seal plate <NUM>. Air cylinder <NUM> applies pressure to upper die assembly <NUM> to capture sample <NUM> between upper seal plate <NUM> and lower seal plate <NUM>, and between surfaces <NUM> and <NUM>. During a test, typically upper die assembly <NUM>, and lower die assembly <NUM> are heated. To initiate a test, after insertion of test sample <NUM> into die cavity <NUM>, the upper assembly <NUM> is lowered onto lower assembly <NUM>. Initially, die cavity <NUM> contains an excess amount of a test sample. A closing force of approximately <NUM> pounds force squeezes test sample <NUM> and forms a sealed die cavity <NUM> in which the sample is under pressure between both lower surface <NUM> of upper die assembly <NUM> and upper surface <NUM> of lower die assembly <NUM>. Initially, this closing force is distributed between the force applied to sample <NUM> in die cavity <NUM> and the force applied by upper seal plate <NUM> to lower seal plate <NUM>. This force typically produces a die cavity pressure near <NUM> psi. However, this pressure can vary due to factors such as test temperature and the viscosity of the material. Lower die assembly <NUM> is then oscillated by drive motor <NUM>, and the force from the lower die assembly <NUM> is transmitted through sample <NUM> to upper die assembly <NUM> where the resulting reaction torque is measured by transducer <NUM>.

In some rheological tests, it is beneficial to first run a subtest on test sample <NUM> at an elevated temperature, and then run an additional subtest on sample <NUM> at a reduced temperature. As upper die assembly <NUM> and lower die assembly <NUM> are cooled, sample <NUM> also cools, causing it to shrink. If the first test is a cure test, sample <NUM> often shrinks even more during the transition. Both of these effects reduce the pressure in die cavity <NUM>. At some point, the shrinkage of sample <NUM> may become so severe that the interfaces between sample <NUM> and surfaces <NUM> and <NUM> break down, allowing the sample to slip on surfaces <NUM> and <NUM>, reducing the strain imparted to the material. This produces a reduction in the signal and often a shift in the peak phase of the signal relative to the movement of lower die assembly <NUM>. Once the slippage occurs, the test results become inaccurate and often not repeatable.

These problems caused by reduction in die cavity pressure resulting from cooling and shrinkage of the sample in prior art torsional rheometers may be overcome by the improved torsional rheometer disclosed herein in <FIG>. <FIG> does not disclose all components of the torsional rheometer, but only the components material to an understanding of the invention. <FIG> also is not drawn to scale or in a way that accurately represents the size and shape of each component.

Torsional rheometer <NUM> of the present invention includes a first or upper assembly <NUM>, and a second or lower assembly <NUM>. Assembly <NUM> is substantially identical to assembly <NUM> of <FIG>. Assembly <NUM> includes a cross-head <NUM>, die assembly <NUM>, seal plate housing <NUM>, insulator ring <NUM>, and seal plate <NUM>. A force applying apparatus, such as air cylinder <NUM>, is coupled to cross-head <NUM> by mount <NUM>, in a manner similar to that described with respect to <FIG>, to move cross-head <NUM> and die assembly <NUM> toward and away from assembly <NUM>. Torque transducer <NUM> and pressure transducer <NUM> are coupled to die assembly <NUM>, and measure the reaction torque applied to die assembly <NUM> and pressure within die cavity <NUM>, respectively.

In one embodiment, air cylinder <NUM> may be an <NUM>-inch air cylinder, although other suitable air cylinders may be used. Air cylinder <NUM>, in one embodiment, may apply <NUM> pounds of force to the die assembly <NUM>. In other embodiments, air cylinder <NUM> could be replaced with other drive apparatuses, such as an electric motor or a hydraulic drive system or the like. An air regulator <NUM> may control the pressure in air cylinder <NUM>. In some embodiments, air regulator <NUM> may be programmed to maintain a desired pressure in cavity <NUM>. In this embodiment, measurements of cavity pressure by transducers <NUM> may be provided to air regulator <NUM> in a feedback loop <NUM> to assist in maintaining the cavity pressure without human interaction.

The second or lower assembly <NUM> will now be described with respect to <FIG>. As in <FIG>, assembly <NUM> includes a fixed table plate <NUM> upon which rheometer <NUM> rests and which supports and stabilizes rheometer <NUM>. Assembly <NUM> also includes a die assembly <NUM>, insulator ring <NUM> and a housing assembly <NUM>. A drive motor <NUM> may be provided to produce oscillatory motion of die assembly <NUM> in the same manner as described with respect to <FIG>. A seal plate <NUM> rests on insulator ring <NUM>. Die cavity <NUM> is disposed between surface <NUM> of die assembly <NUM> and surface <NUM> of die assembly <NUM>. Test sample <NUM> may be placed in die cavity <NUM> and clamped between seal plate <NUM> and seal plate <NUM>.

The control system <NUM> for torsional rheometer <NUM> will now be described with respect to <FIG>. Control system <NUM> includes controller <NUM>, which may be any suitable controller. In one embodiment, the control functions are implemented in firmware executing on a circuit or processor. Controller <NUM> is connected to torque transducer <NUM> to receive and process measurements of the reaction torque applied to die assembly <NUM>. Controller <NUM> is also coupled to pressure transducer <NUM> to receive and process measurements of the pressure within die cavity <NUM>. Controller <NUM> is also coupled to air regulator <NUM> to control operation thereof. Cross-head <NUM> includes a valve <NUM> to which controller <NUM> is coupled. Valve <NUM>, which in one embodiment, is a solenoid valve, is turned on and off by controller <NUM> to control the air supply to air cylinder <NUM>. Both die assembly <NUM> and die assembly <NUM> are heated by an associated heater <NUM> and <NUM>, respectively. Accordingly, controller <NUM> is coupled to heater <NUM> and-to heater <NUM> to operate and control the heaters. Finally, a cooling system is provided for both die assembly <NUM> and die assembly <NUM> to allow performing tests at lower temperatures. This cooling system typically is forced air cooling, although other cooling systems may be used. Valve <NUM> turns the cooling system on and off, and is controlled by controller <NUM>. Valve <NUM> may be a solenoid valve.

As shown in <FIG>, there may be a small gap <NUM> between a lower surface <NUM> of seal plate <NUM> and an upper, facing surface <NUM> of die assembly <NUM>. Gap <NUM> permits movement of die assembly <NUM> toward and away from die assembly <NUM>, as will be described. For this same reason, end faces <NUM> of seal plate <NUM> may be spaced from opposed faces <NUM> of die assembly <NUM>. <FIG> also illustrate a small space <NUM> that may appear in die cavity <NUM> as a result of shrinkage of sample <NUM>.

Housing assembly <NUM> will now be described with respect to <FIG> and <FIG>. Housing assembly <NUM> may be disposed just below insulator ring <NUM>, and may include upper plate <NUM>, lower plate <NUM>, and middle plate <NUM> disposed between upper plate <NUM> and lower plate <NUM>. Disposed in housing assembly <NUM> are compliant members <NUM>. Members <NUM> are placed in series with other load-bearing components of assembly <NUM>, such as insulator ring <NUM>, and/or the components of housing assembly <NUM>. In the example shown in <FIG> and <FIG>, members <NUM> are disposed between upper plate <NUM> and middle plate <NUM>, although members <NUM> could also be positioned between middle plate <NUM> and lower plate <NUM> or between upper plate <NUM> and insulator ring <NUM> or between lower plate <NUM> and table <NUM>. Members <NUM> are configured to deflect with increased load, as will be described more fully hereinafter.

Members <NUM> may be any suitable and known devices which will deflect a small amount when a large load is applied. In the example of <FIG>, members <NUM> are shown to be spring washers <NUM>, which are mounted on alignment pins <NUM>. Spring washers <NUM> may be Belleville washers. In one embodiment, the Belleville washers may have a nominal spring rate of <NUM>,<NUM> pounds per inch, a working load of about <NUM> pounds, and a flat load of about <NUM> pounds. Deflection of these Belleville washers at a working load may be about <NUM> inches, in one example. Typically, in one embodiment, four spring washers <NUM> are used, however, in alternative embodiments, more or fewer washers <NUM> may be used. In an alternative embodiment, as shown in <FIG>, coiled compression springs <NUM> mounted on pins <NUM> could be used. In other embodiments, members <NUM> may be machine springs, leaf springs, a polymer material, or a composite material. Members <NUM> may be comprised of any material that can be compressed as long as the material would support a working load of approximately <NUM> pounds force. In other embodiments (not shown), a hydraulic system may be provided to raise housing assembly <NUM>, to provide the function of members <NUM>.

Positioned between middle plate <NUM> and lower plate <NUM>, in some embodiments, may be a shim <NUM>. Seal plate <NUM> may be attached to insulator ring <NUM> and to upper plate <NUM> by any known, suitable affixation devices. In one example, seal plate <NUM>, insulator ring <NUM> and upper plate <NUM> are bonded together using screws <NUM> although it is to be understood that other affixation devices may be used. Typically, although not necessarily, screws <NUM> are used to attach middle plate <NUM> to upper plate <NUM>. Finally, in one embodiment, screws <NUM> may attach lower plate <NUM> to middle plate <NUM>. It should be understood, however, that other known means may be used to affix the plates together, such as glue, bolts, clips, or rivets.

In one embodiment, an indicator, such as dial indicator <NUM>, may be provided to measure the deflection between plates <NUM> and <NUM>.

In operation, initially die assembly <NUM> is separated from die assembly <NUM> by air cylinder <NUM>. A test sample <NUM> may be placed in the cavity <NUM> so that it is substantially centered on die assembly <NUM>. A closing force is applied to die assembly <NUM> by air cylinder <NUM>. This closing force, in one embodiment, may be about <NUM> pounds to <NUM> pounds, and may cause test sample <NUM> to flow to fill die cavity <NUM> and then extend outwardly from die cavity <NUM> to lie between seal plate <NUM> and seal plate <NUM>. A first test may be performed at a first temperature by actuating drive motor <NUM> and heating die assemblies <NUM> and <NUM> to a desired temperature. Thereafter, drive motor <NUM> oscillates die assembly <NUM> back and forth about a zero position along a central, vertical axis.

If there were no sample in die cavity <NUM>, the load applied by air cylinder <NUM> would be transferred directly only to seal plate <NUM>. As a result of this applied load, members <NUM> would be compressed, or deflected. In one embodiment where the closing force is <NUM> pounds, this compression results in a gap of approximately <NUM> millimeters in die cavity <NUM>. If a sample <NUM> is placed in die cavity <NUM>, the same <NUM> pound closing force is now divided between the force applied to sample <NUM> in die cavity <NUM>, and seal plate <NUM> and ultimately members <NUM>. In this example, <NUM> pounds force may be applied to sample <NUM>, which is then transferred to die assembly <NUM>, and approximately <NUM> pounds is applied to housing assembly <NUM>. For this example, the force applied to housing assembly <NUM> is transferred to members <NUM> which causes a compression or deflection of members <NUM>. If spring washers <NUM> are utilized, the resulting die gap is about <NUM> millimeters, in this example.

As sample <NUM> cools and shrinks, or if the sample <NUM> is cured and shrinks, the pressure in die cavity <NUM> drops. As a result, a greater portion of the closing force, applied by air cylinder <NUM>, is transferred to housing assembly <NUM>, and a lesser force is applied to sample <NUM>. Thus, an additional force is applied to members <NUM> via seal plate <NUM> through housing assembly <NUM>. Further deflection of members <NUM> occurs, causing die assembly <NUM> and thus lower surface <NUM> and seal plates <NUM> and <NUM> to move downwardly with respect to upper surface <NUM> of die assembly <NUM>, which is stationary. This relative movement effectively reduces the size of die cavity <NUM>, thus increasing the pressure in die cavity <NUM>. This relative movement is permitted because of gaps <NUM> described above. In the example described above, in which <NUM> pounds of force is applied by air cylinder <NUM>, and in which spring washers <NUM> were used, the gap in die cavity becomes about <NUM> millimeters. Thus, members <NUM> may allow self-correction of the drop in die cavity pressure caused by shrinkage of sample <NUM>.

In another embodiment, as illustrated in <FIG>, the pressure within die cavity <NUM> may be measured by pressure transducer <NUM>, and this information may be provided to air regulator <NUM> and controller <NUM> in a feedback loop <NUM>. Air regulator <NUM> may be programmed to maintain the pressure within die cavity <NUM> at a desired value. Thus, as sample <NUM> shrinks due to changes in temperature or otherwise, and as the resulting pressure within die cavity <NUM> drops, a signal may be sent to air regulator <NUM> to increase the air pressure to air cylinder <NUM> which increases the pressure within die cavity <NUM>. In one example, in which the initial applied force by air cylinder <NUM> is <NUM> pounds, for a particular polymer, and for a significant temperature drop, the force applied by air cylinder <NUM> may be increased to about <NUM> pounds. This sort of controlled feedback loop may assure that the optimum pressure is maintained at all times. This embodiment may be employed instead of or in addition to the self-correction embodiment described above. This embodiment may be required for higher torques and/or lower testing temperatures.

Yet another embodiment will now be described with respect to <FIG>. Where significant cooling is expected or occurs during subsequent testing, or significant shrinkage of the sample <NUM> occurs, as noted above, the self-correcting technique may not be sufficient to maintain the pressure of die cavity <NUM> at the desired level. Moreover, for various reasons, it may not be desirable to include a feedback loop in the device to control a pre-programmed air regulator <NUM>. In this embodiment, the amount of shrinkage to be expected in a particular polymer sample for a particular change in temperature may be empirically determined. In this way, air regulator <NUM> may be pre-programmed to apply greater or lesser pressure via air cylinder <NUM> on die assembly <NUM> at specified times in a testing cycle based upon the testing to be done and the sample being tested to maintain the cavity pressure.

It should be appreciated that while compliant member <NUM> has been shown as being provided on a portion of rheometer <NUM> which is on a side of die cavity <NUM> opposite from the side on which the force is applied by air cylinder <NUM>, in other embodiments, compliant member <NUM> could be provided on the housing on the same side of die cavity <NUM> as the force is being applied by a cylinder <NUM>. In other words, housing <NUM> could be provided with a structure similar to that of housing assembly <NUM>, so that deflection of compliant members <NUM> would permit die assembly <NUM> and surface <NUM> thereof to move downwardly toward die assembly <NUM> to close the gap in die cavity <NUM>. In this embodiment, a gap would need to be provided between an upper surface of seal plate <NUM>, and a lower surface directly above seal plate <NUM> on die assembly <NUM> to accommodate movement of die assembly <NUM> with respect to seal plate <NUM>. In most other respects, this embodiment would operate in substantially the same way as the embodiments described herein.

Use of ordinal terms such as "first," "second," "third," etc. in the claims to modify a claim element does not by itself connote any priority, precedence or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements.

The use of "including," "comprising," "having," "containing," "involving," and variations thereof herein, is meant to encompass the items listed thereafter and equivalents thereof as well as additional terms.

The word "exemplary" is used herein to mean serving as an example, instance, or illustration. Any embodiment, implementation, process, feature, etc. described herein as exemplary should therefore be understood to be an illustrative example and should not be understood to be a preferred or advantageous example, unless otherwise indicated.

Claim 1:
A torsional rheometer (<NUM>) comprising:
a first die assembly (<NUM>) having a first bearing surface (<NUM>);
a second die assembly (<NUM>) having a second bearing surface (<NUM>) facing the first bearing surface on the first die assembly to form a die cavity (<NUM>) therebetween for placement of a test sample (<NUM>);
a first seal plate (<NUM>) associated with the first die assembly (<NUM>);
a second seal plate (<NUM>) associated with the second die assembly (<NUM>), the first and second seal plates being configured to capture and hold the test sample therebetween;
a force applicator (<NUM>) configured to apply a load to urge the first die assembly against the second die assembly and to urge the first seal plate against the second seal plate to capture and hold the test sample in the die cavity;
at least one compliant member (<NUM>) coupled to the second seal plate, the at least one compliant member placed in series with other load-bearing components (<NUM>, <NUM>) associated with the second die assembly, the at least one compliant member configured to deflect or be compressed in response to a load being applied by the force applicator to the second seal plate; and
a housing assembly (<NUM>) supporting the second seal plate, the housing assembly configured to bear at least part of the load applied by the force applicator to the second seal plate through the first seal plate, the housing assembly including the at least one compliant member,
wherein a gap (<NUM>) is located between the second seal plate (<NUM>) and the second die assembly (<NUM>) to permit movement of the second die assembly toward and away from the first die assembly when the test sample is captured and held in the die cavity (<NUM>).