Detection of cross-linking in pre-cure stage polymeric materials by measuring their resistance

A method for detecting extent of cross-linking of a high impedance polymer material during a pre-cure state, the method including: placing an insulated ground plane adjacent to the polymer material and substantially parallel to a sensor; applying a test signal through a sensor to the material and through a reference resistance; determining a voltage difference between the test signal applied to the material and the reference resistance as the reference voltage; and correlating the voltage difference as a relative indicator of the extent of cross-linking which has occurred within the polymer material. Also disclosed is a device for implementing the subject method.

BACKGROUND OF THE INVENTION 
The present invention relates to a DETECTION OF CROSS-LINKING IN PRE-CURE 
STAGE POLYMERIC MATERIALS BY MEASURING THEIR RESISTANCE. 
1. Field of Invention 
This invention relates to a process for determining the extent of 
cross-linking which has occurred in a polymeric material such as paint, 
dental resin, B-staged resin, etc. More particularly, the present 
invention pertains to the detection of extent of curing of such materials 
in their pre-cure stage. 
2. Prior Art 
Thermosetting resins form a class of very useful plastics which have been 
applied throughout the aerospace industry, construction industry, 
automotive manufacturing, medical applications, adhesives, and in 
virtually every area where permanent characteristics of weatherability, 
structural stiffness, strength and ease of manufacture through molding 
process provides an advantage over competing metals, ceramics and other 
compositions. Dental applications include filling and facia materials 
which are applied to the tooth in liquid form and then polymerized by UV 
radiation or other known techniques. Many paint compositions are a form of 
thermosetting resin whose application depends on having a uniform liquid 
state which can be readily applied by brush or air gun. Matched die, 
filament winding, transfer molding, lay up molding and pultrusion 
techniques for fabricating structural and component parts, housings, etc., 
depend on maintenance of a flowable condition which can wet fibers or 
quickly fill mold cavities in a liquid state. 
These resin materials are typically manufactured in a low viscous liquid 
state wherein the polymer material has incurred minimal cross-linking 
prior to the curing stage. It is, of course, this cross-linking that 
solidifies the thermosetting composition into a permanent, rigid structure 
characterizing this group of plastics. The shelf life of such products is 
significant, because premature curing results in a permanent, irreversible 
condition which makes the material useless for further processing. Indeed, 
the extent of waste arising because of premature curing of thermosetting 
materials is substantial. In industries where partially cured materials 
must be discarded for safety reasons, the losses are even more 
significant. For example, the manufacture of high performance aircraft 
components from resins that have already partly cured could result in 
weakened structures that put life in jeopardy. Therefore, it is very 
likely that a substantial amount of good resin is discarded because of 
suspicion of excessive pre-cure. 
Because most resins will inherently begin cross-linking upon manufacture 
and will continue such cross-linking until finally cured, measures are 
taken to reduce and control this process. The primary control measure is 
to maintain the resins at low temperatures to reduce reaction rates to a 
minimum. This low temperature environment needs to be maintained until the 
material is ready for final curing. Unfortunately, the resin material 
appearance does not always reflect the degree of curing which has occurred 
during this pre-cure stage. If variations in temperature occur during 
storage, their impact may be substantially unknown. Therefore, the extent 
of cure is often a risk factor that must be considered with the choice of 
any particular resin. 
With paints and adhesives, viscosity provides a useful measure of 
acceptability of pre-cure. In general, their shelf life is determined by 
the time required for the material to set up or become too viscous to flow 
well. There are, however, no current tests to determine the actual state 
of cross-linking in paints and adhesives. Current practice is to examine 
the viscosity of the materials qualitatively as noted, or perform sample 
tests to determine the performance of these resins in a particular 
application. 
With respect to polymers used in a matrix material for fiber reinforced 
composites, there are two distinct time periods during which cross linking 
takes place. The first period can be called the shelf life of the material 
and the second is the curing cycle. Typically, thermosetting resins for 
composites are stored at very low temperatures such as -0 degrees F. The 
curing cycle occurs when the resins are subjected to heat/radiation and/or 
pressure during molding processes. For example, elevated temperatures in 
the range of 200 to 400 degrees F., and occasionally as high as 700 
degrees F., are common for curing these polymers and can enable the 
completion of cross linking in a short time interval. 
Users of fiber reinforced thermosetting composites have created several 
mechanical tests to evaluate the state of cumulative cross linking in the 
storage and pre-cure stages. For example, tack and drape properties give 
an indication of the extent of cure. These tests are acknowledged to be 
highly subjective and unreliable, and are at best general qualitative 
indicators having little quantitative value. 
A more specific application of thermosetting resins for composite materials 
is to impregnate a layer of fiber reinforcement with resin, and then store 
this "pre-preg" or "B-staged" material for later use. Obviously, this 
B-staged material will have a limited shelf life, depending upon the rate 
of continued cross linking, which is affected mainly by temperature. It is 
presently difficult, subjective, and consumptive of material to test the 
B-staged material for the extent of cross linking. If the B-staged 
material has reached a particular stage of cross linkage, it is no longer 
usable material and must be discarded on the basis of storage time, rather 
than on the actual amount of cross linking. 
There is increasing interest in the composites industry to monitor, adjust 
and optimize the cure cycle of thermoset polymers. Accordingly, it is 
known to evaluate cross linking during actual cure using viscometers, 
infrared meters, and microdielectrometers. This period of evaluation is 
characterized by the resins being subjected to high temperatures used to 
fully complete the curing of the materials. The primary interest is to 
identify the gelation point and then to confirm final stage at which the 
curing process is complete, so that the final product can be removed 
without extending cure time and conditions beyond that which is necessary. 
This enables efficient use of expensive equipment and also insures that 
the manufactured part is not removed from the mold prior to complete cross 
linking. 
The present inventors are unaware of any activity designed to determine the 
extent of cross linking in polymers prior to the actual curing process 
within a high temperature environment. Specifically, adhesives and paints 
represent a broad class of polymers which do not require devoted 
temperatures to cure to final stage. Because curing in such polymers is an 
ongoing process at a somewhat continuous rate, neither intermediate nor 
final cure status is generally measured. Where polymers are cured in two 
stages representing pre-cure and elevated final process, the only point of 
measurement of cross linking in polymers has occurred only during the 
elevated conditions, with little regard for cross linking during the 
pre-cure stage or shelf-life period. 
This practice may have arisen in part from an assumption that the 
electrical response of any polymer at low temperatures applied during 
storage would not provide enough signal to show a measurable change as the 
pre-cure cross linking continues. 
What is needed is an effective method for detecting the extent of cure in 
polymer materials during the shelf-life period, to enable more effective 
determination of whether specific batches or lots of polymer have exceeded 
safe limits in the pre-cure stage. Such procedures could provide 
quantitative determination of which resins must be discarded and which can 
be safely used, and yield substantial savings in cost and natural 
resources. 
OBJECTS AND SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a device and method for 
enabling the determination of the extent of cross linking in any resistive 
polymer. 
It is yet another object of this invention to provide a device and method 
as stated above which can be effectively applied during the pre-cure stage 
of a polymer, wherein the polymer is maintained at low temperatures for 
minimizing cross linking. 
A further object of the present invention is to provide a method and system 
for providing ongoing or continuous detection of cross linking within a 
pre-cure polymer, which can be applied to paints, adhesives, caulking, 
dental resins, resins for composites and molding systems. 
Yet another object of this invention is to provide the objects noted above 
within a low cost system which is convenient to use under virtually all 
situations. 
These and other objects are realized in a method for detecting extent of 
cross-linking of a high impedance polymer material during a pre-cure stage 
at low temperature. This method includes the steps of: 
a) placing a ground plane adjacent to the polymer material to be tested 
such that a parallel shunting path to ground is created thereby removing a 
reactive portion of an applied electric field and leaving a resistive 
portion; 
b) applying a test signal through a sensor by creating an electric field 
through the polymer material in the pre-cure stage to determine a level of 
resistance and corresponding sample voltage representative of a degree of 
cross-linking within the material; 
c) applying the same test signal through the sensor to a reference material 
having a fixed resistance to determine a reference voltage; 
d) determining a voltage difference between the test signal applied to the 
polymer at pre-cure stage and the test signal applied to the reference 
material as the reference voltage; and 
e) correlating the voltage difference as a relative indicator of the extent 
of cross-linking which has occurred within the polymer material, based on 
comparison of magnitude of the voltage difference with respect to a 
comparable potential range of resistance for the polymer material from its 
lower impedance stage at minimal cross-linking to its high impedance stage 
at maximum impedance for total cross-linking. 
Another aspect of this invention is represented by a device for testing 
extent of cross-linking of a polymer material in a pre-cure stage, wherein 
the device comprises a signal generator capable of generating a low 
frequency, low amplitude signal with an attached sensor adapted for 
receiving a coating of the polymer material to be tested, wherein the 
sensor has a known impedance. A reference material is provided which has a 
resistance approximately equal to the geometric mean of (i) the impedance 
of the sensor with polymer material in its lower-resistivity state, and 
(ii) the expected impedance of the polymer material when the polymer 
material has reached its high resistivity state upon full curing. The 
device includes voltage means for determining voltage difference between a 
signal detected through the sensor with polymer material and a signal 
detected through the sensor at the reference material. Means are provided 
and coupled to the voltage means for converting the voltage difference to 
a factor representing the extent of cross-linking which has occurred 
within the polymer material. 
Other objects and features of the present invention will become apparent to 
those skilled in the art, based upon the following detailed description of 
a preferred embodiment, taken in combination with the accompanying 
drawings.

DETAILED DESCRIPTION OF THE INVENTION 
The present inventors have discovered that it is possible to determine the 
extent of cross-linking in any resistive polymer, even during the pre-cure 
stage. 
Specifically, the invention comprises a method for detecting extent of 
cross-linking of a high impedance polymer material during a pre-cure 
stage. The first step of this method involves placing a ground plane 
adjacent to the polymer material to be tested such that a parallel 
shunting path to ground is created, but which is electrically isolated 
from the polymer material. The next step is to then apply a test signal 
through a sensor to the polymer material in the pre-cure stage to 
determine a level of resistance and corresponding sample voltage 
representative of a degree of cross-linking within the material. Normally, 
the electric field is comprised of a resistive and a reactive component, 
and both are measured when measuring impedance of the polymer material. 
However, the introduction of a parallel shunting ground plane removes the 
reactive portion of the electric field. This leaves only the resistive 
portion of the electric field which is unaffected by the insulated ground 
plane. 
Typically, the test signal will be an electric current whose amplitude is 
inversely proportional to the resistance of the polymer in accordance with 
Ohms law I=E/R. Other techniques of measuring the resistance of the 
material may likewise be employed. 
The test signal is conducted directly into the polymer by means of an 
interdigitated electrode sensor which may or may not be placed in contact 
with the polymer. The specific geometry of the probe is only important 
insofar as the electrodes have an interdigitated relationship and have 
rounded edges to minimize the strength of an applied electric field at any 
particular location, thus preventing the chemical or molecular breakdown 
of the material being sensed. Any conductive material coupled at one end 
to a voltage source may be used as a probe. Where the sensor is used with 
high resistivity resins, the probe should be shielded by a shielding means 
coupled around the sensor to shield from static electricity. 
The next step of this methodology is applying the same test signal as 
applied in the previous step through the sensor to a reference material, 
such as a fixed value resistor. This provides the quantitative character 
of the procedure. The reference material should have a fixed resistance to 
determine a reference voltage. A voltage difference between the test 
signal applied to the polymer at pre-cure stage and the test signal 
applied to the reference material as the reference voltage is then 
determined. This voltage difference serves as a relative indicator of the 
extent of cross-linking which has occurred within the polymer material, 
based on comparison of magnitude of the voltage difference with respect to 
a comparable potential range of resistance for the polymer material from 
its lower impedance stage at minimal cross-linking to its high impedance 
stage at maximum impedance for total cross-linking. 
The mechanics of processing the voltage difference to get an indication of 
the actual extent of cross-linking may vary. The preferred technique 
represented in the disclosed figures involves converting the alternating 
voltage to direct current and inputting this direct current to a display 
device which gives direct readout of a value which can be correlated with 
the extent of cross-linking of the polymer. This direct readout comprises 
a DC voltage ranging from approximately 0.5 volts at low resistance to 0.0 
volts at high resistance, representing a range in magnitude of resistance 
of at least approximately 1.times.10.sup.4 ohms. This may extend as high 
as 10.sup.8 ohms. 
The test signal is applied by generating a low frequency signal of less 
than 10 Hz, having a low amplitude of less than 20 volts peak to peak, and 
by applying this signal to the polymer in uncured stage and to the 
reference resistance. In a more preferred embodiment, the low frequency 
signal is approximately 0.1 Hz to 5 Hz, and consists of a low amplitude of 
less than 1 volt peak to peak. 
FIGS. 1 and 2 illustrate implementation of this invention by structuring 
the polymer material and the reference material within the circuit as a 
voltage divider wherein the voltage output is proportional to the ratio of 
the resistance of the reference material to total resistance of the 
polymer material plus the reference material. This circuit can sense a 
resistance change in the order of 10.sup.4 ohms from the fresh stage of 
the material to the cured stage. Such a range is typical for resins, 
plastics, paints, adhesives and caulks. In addition the circuit can be 
adjusted to begin sensing in the fresh stage at anywhere from 10.sup.3 
ohms to 10.sup.8 ohms, finishing up in the cured stage at anywhere from 
10.sup.7 ohms to 10.sup.13 ohms. 
FIG. 1 shows a block diagram in which the signal generator 20 is a 
sinusoidal generator which provides a 1 Hz, 1 Volt p-p signal (Vappl) 
which is applied to the sensor 21 with a coated polymer to be tested and 
an insulated ground plane. This signal is then applied to the reference 
resistor 22. The voltage between these function blocks (Vd) is then 
buffered 24 and filtered 25, after which the resulting signal (Vf) is 
converted to DC with a peak detector 26. V(out) 27 is then a DC voltage 
ranging from approximately 0.5 volts (for lower resistivity) up to 0.0 
volts (for high-resistivity). The illustrated circuit can sense a range of 
about 1.times.10.sup.4 ohms. 
An important part of this sensor is the resin-covered sensor combined with 
the reference. One embodiment of the sensor is shown in FIG. 3. This 
device comprises an upper 30 and lower 31 casement, with the 
interdigitated electrode sensor component 32 enclosed therein. Contact 
pins 34 are electrically coupled to contacts 35, 36, and 37 of the sensor 
component. Contacts 35 are at ground potential, while contacts 36 and 37 
provide the voltage differential V(subscript D) for indicating the extent 
of polymerization. These contacts 36 and 37 are coupled to the respective 
interdigitated terminal electrode 38 and 39. The actual measurement of 
resistance is made by placing the polymer 40 to be tested on two or more 
of the adjacent terminal electrodes 38 and 39 to provide a conductive path 
for measuring resistance through the material. The measurement with this 
hardware has always produced a measure of impedance, including a resistive 
and a reactive component. However, the present invention modifies the 
structure of the sensor 21 by adding two additional components, a layer of 
insulating material 50 and a ground plane 51. The purpose of the 
insulating material 50 is to isolate the ground plane 51 from the sensor 
21. With the ground plane 51 isolated from the circuit, the resistive 
portion of the measurement is unaffected, but the reactive portion of the 
electric field passing through the polymer is substantially shunted to 
ground through the ground plane 51. Contacts 35 are at ground potential 
and are connected to the ground plane 51. Thus, the measurement becomes 
substantially a measure of resistance instead of impedance. 
The advantage of a resistance measurement is being able to avoid the 
complications introduced by a vector measurement such as impedance. This 
is accomplished while still applying an AC test signal. Thus, the 
additional benefit of using AC and avoiding a polarity buildup in the high 
impedance polymer material is obtained while overcoming the difficulty of 
separating resistive and reactive elements in the measurement. 
This grid of interdigitated electrodes may be etched or plated on a 
substrate in accordance with standard technology. The pins 34 are coupled 
by wires to appropriate contacts of the circuitry described in FIG. 2. The 
important consideration when etching the grid is avoiding sharp corners 
such as might be produced at the ends of electrodes. FIG. 4 provides 
close-up detail of an interdigitated electrode which illustrates this 
feature. 
The reference sensor 22 is simply a fixed-value resistor chosen to be 
approximately equal to the geometric mean of the resistance of the sensor 
with material in its lower resistivity state, and the same resistance as 
expected when the material has reached its high resistivity state. In this 
manner, the sensor and the reference form a simple voltage divider. The 
output voltage from this divider is proportional to the ratio of the 
reference resistor to the total resistance of the sensor plus the 
reference resistor, as shown in Equation 1: 
##EQU1## 
Specific considerations are relevant to FIG. 2. For example, the purpose of 
the voltage reference stage is simply to allow the remaining op amp stages 
to operate in a pseudo-dual-supply mode. This is necessary because the 
circuit is to be battery powered, yet generate an AC signal with no DC 
offset as applied to the sensor. Resistor R5 is a multi-turn trim 
potentiometer. It is necessary to adjust the gain of the oscillator to the 
point where a steady amplitude signal is produced. 
The op amp chosen must have an input impedance in the area of 10.sup.12 
ohms and must operate from +3 volts. The op amp chosen for the test 
implementation of this invention was the Texas Instruments TSC27M4AIN. The 
buffer stage is necessary to prevent loading the voltage divider output 
voltage, Vd. The buffer stage raises the load impedance to about 10.sup.15 
ohms. The filter stage is an attempt to limit the bandwidth of the entire 
circuit and thereby reduce noise sensitivity to most stray voltages and 
all static electricity. For this reason, the enclosure should be carefully 
shielded. 
When energized, the oscillator stage may not automatically start 
oscillating and may require a jump-start. This is accomplished by simply 
disconnecting R8 from the reference ground voltage, and reconnecting it. 
It should also be noted that V(out) will not change quickly. Therefore, 
when testing a new or different sensor, C4 should be momentarily shorted 
out, then returned to normal. This will allow V(out) to settle more 
quickly to its final value. 
The above described structure is representative of a device for testing the 
extent of cross-linking of a polymer material in a pre-cure stage which is 
generally described to include (i) a signal generator capable of 
generating a low frequency, low amplitude signal; (ii) a sensor coupled to 
the signal generator and adapted for receiving a coating of the polymer 
material to be tested, the sensor having a known impedance; (iii) a ground 
plane parallel to the sensor but insulated from it by an insulating 
substrate; (iv) a reference material which has a resistivity approximately 
equal to the geometric mean of the resistance of the sensor with polymer 
material in its lower-resistivity state, and the expected resistance of 
the polymer material when the polymer material has reached its high 
resistivity state upon full curing; and (v) voltage means for determining 
the voltage difference between a signal detected through the sensor with 
polymer material and a signal detected through the sensor at the reference 
material. Converting means is coupled to the voltage means for converting 
the voltage difference to a factor representing the extent of 
cross-linking which has occurred within the polymer material. A display 
means may be coupled to the converting means to provide a visual readout 
of the extent of cross-linking in real time mode. 
The subject device can be correlated to the monitored polymer sample by 
numerous techniques. For example, a sample 40 of the polymer may be placed 
directly on the electrodes of the sensor as described in FIG. 3. This 
sensor can be permanently attached to the monitored polymer material so 
that the extent of polymerization can be checked at any time by merely 
inserting the pins 34 into a monitoring device 41 such as the hand held 
reader shown in FIG. 5. This reader 41 would contain the circuitry shown 
in FIG. 2, including a power supply for the signal generator. The reading 
is then displayed on the LCD 42, giving an accurate statement of condition 
for the batch of polymer to which the sample relates. This system could be 
readily applied with respect to batch shipments of adhesives, paints, 
caulks, and similar products which are stored and shipped in quantity. 
Once the reading is taken, the sensor 21 is returned to the material, to 
which it remains attached for future monitoring. 
Alternately, the circuitry and sensor could be housed in a small, 
disposable unit such as that illustrated in FIG. 6. In this embodiment, 
the device 45 is a disposable unit which is coupled directly to the 
monitored polymer 46. Where the polymer 46 is prepreg material, the 
monitoring device 45 is loaded with a sample of representative polymer 
associated with the prepreg material 46. This device 45 is then 
permanently attached to the cardboard core 47 in visual position. When a 
reading is to be taken, the circuit may be activated by pressing a switch 
48 which energizes the circuit and gives a reading on the LCD 49. In this 
manner, wherever the roll of prepreg material is shipped, its extent of 
polymerization can be immediately read from the attached device 45. It 
will be apparent that numerous methods of permanent or temporary 
attachment may be envisioned. These may include sensors which have a 
sample of material embedded at the time of manufacture, as in FIG. 3, or 
may be sensors which are inserted directly into the monitored polymer. 
These features also suggest the use of the present invention as part of a 
more general method for monitoring extent of cross linking of polymer 
material which comprises the steps of (i) identifying polymer material in 
pre-cure stage; (ii) attaching a sensor in contact with the identified 
polymer as part of the material, which sensor enables intermittent or 
continuous reading of cure state of the polymer; and (iii) maintaining the 
sensor in contact with the polymer throughout the pre-cure stage of the 
polymer as a means for determining extent of cure of the material to which 
the sensor is attached. The same steps can be applied toward a batch of 
polymer material in pre-cure stage, wherein a sample of the polymer 
material is separated from the batch and the sensor is attached in contact 
with the sample of the identified polymer material. In this latter case, 
the material may be visually inaccessible, such as being in a closed 
container, but the sample which is attached to the outside of the 
container will be indicative of the contents. For this reason it is 
important that the sample being measured is fixed to the container so that 
the sample polymer experiences the same temperature and environmental 
conditions of the primary batch of polymer. The circuit described above 
could be in the form of a hand-held meter, which when attached to a 
sensor, would give a voltage proportional to the parameter of the material 
being measured. 
Several other alternative embodiments illustrate useful modifications to 
the preferred embodiment shown in FIG. 3. For example, the shape of the 
sensor and ground plane is quite flexible, depending upon the desired 
application. FIG. 7 shows that a hollow cylinder 52 can serve as the 
insulating substrate 90 between a ground plane 53 surrounding the outside 
of the cylinder 52, and the interdigitated electrode sensor 54 disposed on 
the inside surface of the cylinder 52. The benefit of this embodiment is 
that the surrounding ground plane 53 serves the dual function of shielding 
the sensor 54 from outside environmental noise interference while 
preventing the release of stray emissions. 
Another embodiment is illustrated in FIG. 8. It is sometimes the case that 
the polymer material 55 being tested will have conductive particles 56 
floating within it. These particles 56 can lead to a false reading of low 
resistance of the polymer material 55 if the particles should settle 
between interdigitated electrodes 57 and create a short or highly 
conductive path between electrodes 57. The polymer material 55 might even 
be substantially beyond a pre-cure state, but a false low resistance 
reading would indicate otherwise. Therefore, to obtain a resistance 
measurement of only the polymer material 55, a porous barrier 58 is placed 
over the sensor 57 on the insulating substrate 92 and the substantially 
parallel ground plane 93. The porous barrier 58 would pass the polymer 
material 55 while preventing the conductive particles 56 from touching the 
electrodes or coming near enough to cause a false resistance reading. It 
is therefore necessary that the polymer material 55 not be interfered with 
by the porous barrier 58. Some polymer materials 55 might require that the 
porous barrier 58 form a cavity 91 which is filled by polymer material 55 
for the resistance measurement. 
Another embodiment abandons the interdigitated electrode sensor design and 
is illustrated in FIG. 9. The plates 60 are the sensors of this 
embodiment, creating an electrical field which is generated from one plate 
60 to the other. Between the plates 60 is the polymer material 61 to be 
tested. To shunt the reactive component of the electric field, a ground 
plane 62 is placed "inside" the polymer 61. The ground plane 62 is any 
conductive, electrically isolated material parallel to the sensor plates 
60 which will still allow the resistive portion of the electric field to 
pass through. This is accomplished by making the ground plane 62 porous. 
For example, a plurality of holes 63 could be cut through the ground plane 
62. 
It will be apparent to those skilled in the art that the foregoing 
disclosure is merely representative of preferred embodiments of the 
invention and is not to be considered limiting, except as set forth in the 
following claims.