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A stress relaxation rig for use in γ-irradiated environments
Polymer Testing 6 (1986) 379-386
A Stress Relaxation Rig for Use in y-Irradiated Environments S. W. A d d y , t D. W. C l e g g t A . A. Collyer~t t Department of Metals and Materials Engineering and ~:Department of Applied Physics, Sheffield City Polytechnic, Pond Street, SheffieldS1 lWB, UK and P. C r u m CEGB, Bedminster Down, Bristol BS13 8AN, UK SUMMARY A rig has been constructed that enables stress relaxation tests to be carried out on elastomers under uniaxial compression. Testing may be undertaken during the ),-irradiation of the sample in an inert atmosphere over the temperature range 25-200 °C. Successful studies have been made on polydimethyl siloxane (PDMS) elastomer using this apparatus.
1.
INTRODUCTION
1.1. Background In previous studies on PDMS, the authors have shown that the physical stress relaxation takes place rapidly. 1 Any stress relaxation that occurs in this material over a period of time is, therefore, due to chemical changes rather than physical ones. These chemical changes are brought about by the change in crosslink density and the reduction in the load-bearing chains due to chain scission during the course of the experiment. Changes in the elastomers can, therefore, be followed by monitoring the stress relaxation. The effect of y-irradiation on the long-term mechanical properties of the elastomers and on the chemical changes occurring can be studied by this method. This requires a stress 379 Polymer Testing 0142-9418/86/$03-50 © Elsevier Applied Science Publishers Ltd, England, 1986. Printed in Northern Ireland
380
S. W. Addy, D. W. Clegg, A. A. CoUyer, P. Crum
relaxation rig that can function remotely, and for this reason the present apparatus was designed and built. 1.2. l~neory It has long been established that stress relaxation may be used as an investigative tool to reveal the kinetics of chemical reactions that break down the elastomer network structure. During degradation, the stress is relieved in the sample by scission of load-bearing network chains. Tobolsky2'3 proposed a simple theory of degrading polymer networks resulting in the following expression relating the
(O(t)
stress ratio \o(O)/ to the number of scissions of load-bearing chains
qm(t) in time t.
o(t) = [ --qm(t)] o(O) exp [ N¢(O) J
(1)
where N¢(O) is the initial number of load-bearing chains per unit volume. A simple re-arrangement allows the number of scissions to be estimated as follows.
[ °(oI
qm(t) = -Nc(0) In L o(0) J
(2)
In elastomers that undergo both chain scission and crosslinking reactions, the kinetics of the crosslinking reaction may be evaluated by using conventional stress relaxation data in conjunction with intermittent data. The following relationship can be developed relating the difference in intermittent and continuous stress ratio to the fraction of crosslinkages formed in time t. oi(/)
ac(t) = ANcR(t)
a,(o) at(o)
NoR(O)
where ANcR(t) is the number of crosslinks per unit volume occurring in a time t and NeR(O) is the initial number of crosslinks per unit volume. 4 2.
DESIGN OF THE RIG
The stress relaxation rig is shown in Fig. 1, schematically, and in Fig. 2. The whole assembly is enclosed between two circular flanges,
A stress relaxation rig for use in y-irradiated environments
Silicone Rubber Seal
Upper Flange
[-
Kaolin
Kaolin
Wool
Insulation
Insulation
Strip
Furnace ~" Sample J 1 "
UpPer Platen
Spring "~ ~ Lower
~
I- ~
LOaded Pins
Composite Platen
j~ Aluminium
Glass ~"
Cylinder
Lower Flange
Pneumatic
Housing
Cell
Fig. 1. A schematic diagram of the stress relaxation rig.
381
382
S. W. Addy, D. W. Clegg, A. A. Collyer, P. Crum
Fig. 2. The stress relaxation rig.
A stress relaxation rig for use in ),-irradiated environments
383
which are supported by four pillars. The sample is loaded automatically by a pneumatic ram, which is driven by compressed air. When the ram is activated, the lower platen is raised and held at a constant displacement throughout the test by the four spring-loaded pins located in the recess of the platen. The pneumatic cell consists of a single-acting 25 mm stroke ram (Schrader Bellows Model No. 40-9010000). The ram is surrounded by an anodised aluminium housing, which contains the spring-loaded pins. This housing acts as a guide for the lower platen, ensuring that it is presented squarely to the sample. The lower platen is a composite which enables its length to be altered in order to accommodate samples of various sizes. The upper platen, which is connected to the load cell, is separated by a kaolin wool strip in order to prevent heat transfer to the load cell. The load cell (AJB Associates Model No. 462) consists of a simple rectangular casting containing a bridge network of four strain gauges, which are secured with epoxy resin. The load cell is connected to a combined bridge supply, amplifier and balance unit (FE-359-TA Fylde Instruments). The output from the unit is fed into a channel of a 5401 Honeywell chart recorder. The sample is heated using a sample tube furnace, working from a 12 V supply. Temperature control is achieved by using a Chromel/ Alumel thermocouple, which is placed in the furnace adjacent to the sample. This thermocouple is used in conjunction with a Honeywell temperature control device (Model No. C.L.40). The output from the thermocouple is also fed to the Honeywell recorder so that both stress and temperature can be constantly monitored. The assembly can be enclosed in a glass cylinder, which is located on the silicone rubber seals set in the recesses of the top and bottom flanges. These seals are made from a room-temperature vulcanising (RTV) elastomer known commercially as Silcaset. This enables tests to be undertaken under inert or any other specified environment. 3.
RESULTS
3.1. Performance of the rig The stress relaxation rig performed successfully at an absorbed dose rate of 1.05 x 105 R h -1 for a total test period of 36 weeks, giving a
384
S. W. Addy, D. W. Clegg, A. A. Collyer, P. Crum
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r, I
r, 0 t~ q3
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.=. e3
.~ o .~ o °.~ o .~o •
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,4 U
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A stress relaxation rig for use in y-irradiated environments
385
total absorbed dose of 6.35 x 108 R. The strain gauges secured by epoxy resin were still functioning, although the insulation on the bridge-connecting wires (silicone resin) had disintegrated. Upon dismantling the pneumatic ram, it was found that the nylon spacers had become hard and brittle, crumbling to a powder. However, the natural rubber seals were still intact, retaining their elastomeric properties, although a slight increase in hardness was observed. 3.2. Results from stress relaxation tests
Figure 3 shows results obtained for PDMS elastomer in a 3,-irradiated environment in the temperature range 40-200 °C. The curves result from the combination of the thermal and radiation scission reaction in the elastomer, and are discussed more fully in ref. 5.
4.
DISCUSSION
The present design enables only continuous stress relaxation tests to be performed. Hence only the kinetics of chain scission reactions may be studied. A simple modification to the rig consisting of removing the spring-loaded pins and replacing the single-acting ram with a double-acting ram would allow intermittent tests to be undertaken. The kinetics of any crosslinking reactions could then be studied using the theory outlined previously. In such a test, the sample is degraded in the unstressed condition. At regular time intervals the sample is rapidly loaded to some preset strain and then rapidly unloaded. It is assumed that the amount of crosslink formation taking place during the loading-unloading operation would be negligible.
ACKNOWLED GEMENTS The authors would like to thank SERC for the provision of a CASE Award for financing the work and CEGB for the provision of the ),-irradiation facilities.
386
S. w. Addy, D. W. Clegg, A. A. Collyer, P. Crum
REFERENCES 1. Clegg, D. W., Addy, S. W., Collyer, A. A., Jones, I. K. and Webb, S. W. (1984). Polym. Commun., 25(3), 73. 2. Tobolsky, A. V. (1953). J. Chem. Phys., 21, 614. 3. Tobolsky, A. V., Metz, D. J. and Mesrobian, R. B. (1950). J. Amer. Chem. Soc., 72, 1942. 4. Murakami, K. and Ono, K. (1979). Chemorheology of Polymers, Polymer Science Library 1. Amsterdam, Elsevier. 5. Addy, S. W., Clegg, D. W., Collyer, A. A., Cortield, G. C. and Crum, P. (1985). In: Advances in Rheology-3, (Eds B. Mena, A. Gar6ia-Rej6n and C. Rangel-Nafaille). Amsterdam, Elsevier, pp. 35-43.
A Stress Relaxation Rig for Use in y-Irradiated Environments S. W. A d d y , t D. W. C l e g g t A . A. Collyer~t t Department of Metals and Materials Engineering and ~:Department of Applied Physics, Sheffield City Polytechnic, Pond Street, SheffieldS1 lWB, UK and P. C r u m CEGB, Bedminster Down, Bristol BS13 8AN, UK SUMMARY A rig has been constructed that enables stress relaxation tests to be carried out on elastomers under uniaxial compression. Testing may be undertaken during the ),-irradiation of the sample in an inert atmosphere over the temperature range 25-200 °C. Successful studies have been made on polydimethyl siloxane (PDMS) elastomer using this apparatus.
1.
INTRODUCTION
1.1. Background In previous studies on PDMS, the authors have shown that the physical stress relaxation takes place rapidly. 1 Any stress relaxation that occurs in this material over a period of time is, therefore, due to chemical changes rather than physical ones. These chemical changes are brought about by the change in crosslink density and the reduction in the load-bearing chains due to chain scission during the course of the experiment. Changes in the elastomers can, therefore, be followed by monitoring the stress relaxation. The effect of y-irradiation on the long-term mechanical properties of the elastomers and on the chemical changes occurring can be studied by this method. This requires a stress 379 Polymer Testing 0142-9418/86/$03-50 © Elsevier Applied Science Publishers Ltd, England, 1986. Printed in Northern Ireland
380
S. W. Addy, D. W. Clegg, A. A. CoUyer, P. Crum
relaxation rig that can function remotely, and for this reason the present apparatus was designed and built. 1.2. l~neory It has long been established that stress relaxation may be used as an investigative tool to reveal the kinetics of chemical reactions that break down the elastomer network structure. During degradation, the stress is relieved in the sample by scission of load-bearing network chains. Tobolsky2'3 proposed a simple theory of degrading polymer networks resulting in the following expression relating the
(O(t)
stress ratio \o(O)/ to the number of scissions of load-bearing chains
qm(t) in time t.
o(t) = [ --qm(t)] o(O) exp [ N¢(O) J
(1)
where N¢(O) is the initial number of load-bearing chains per unit volume. A simple re-arrangement allows the number of scissions to be estimated as follows.
[ °(oI
qm(t) = -Nc(0) In L o(0) J
(2)
In elastomers that undergo both chain scission and crosslinking reactions, the kinetics of the crosslinking reaction may be evaluated by using conventional stress relaxation data in conjunction with intermittent data. The following relationship can be developed relating the difference in intermittent and continuous stress ratio to the fraction of crosslinkages formed in time t. oi(/)
ac(t) = ANcR(t)
a,(o) at(o)
NoR(O)
where ANcR(t) is the number of crosslinks per unit volume occurring in a time t and NeR(O) is the initial number of crosslinks per unit volume. 4 2.
DESIGN OF THE RIG
The stress relaxation rig is shown in Fig. 1, schematically, and in Fig. 2. The whole assembly is enclosed between two circular flanges,
A stress relaxation rig for use in y-irradiated environments
Silicone Rubber Seal
Upper Flange
[-
Kaolin
Kaolin
Wool
Insulation
Insulation
Strip
Furnace ~" Sample J 1 "
UpPer Platen
Spring "~ ~ Lower
~
I- ~
LOaded Pins
Composite Platen
j~ Aluminium
Glass ~"
Cylinder
Lower Flange
Pneumatic
Housing
Cell
Fig. 1. A schematic diagram of the stress relaxation rig.
381
382
S. W. Addy, D. W. Clegg, A. A. Collyer, P. Crum
Fig. 2. The stress relaxation rig.
A stress relaxation rig for use in ),-irradiated environments
383
which are supported by four pillars. The sample is loaded automatically by a pneumatic ram, which is driven by compressed air. When the ram is activated, the lower platen is raised and held at a constant displacement throughout the test by the four spring-loaded pins located in the recess of the platen. The pneumatic cell consists of a single-acting 25 mm stroke ram (Schrader Bellows Model No. 40-9010000). The ram is surrounded by an anodised aluminium housing, which contains the spring-loaded pins. This housing acts as a guide for the lower platen, ensuring that it is presented squarely to the sample. The lower platen is a composite which enables its length to be altered in order to accommodate samples of various sizes. The upper platen, which is connected to the load cell, is separated by a kaolin wool strip in order to prevent heat transfer to the load cell. The load cell (AJB Associates Model No. 462) consists of a simple rectangular casting containing a bridge network of four strain gauges, which are secured with epoxy resin. The load cell is connected to a combined bridge supply, amplifier and balance unit (FE-359-TA Fylde Instruments). The output from the unit is fed into a channel of a 5401 Honeywell chart recorder. The sample is heated using a sample tube furnace, working from a 12 V supply. Temperature control is achieved by using a Chromel/ Alumel thermocouple, which is placed in the furnace adjacent to the sample. This thermocouple is used in conjunction with a Honeywell temperature control device (Model No. C.L.40). The output from the thermocouple is also fed to the Honeywell recorder so that both stress and temperature can be constantly monitored. The assembly can be enclosed in a glass cylinder, which is located on the silicone rubber seals set in the recesses of the top and bottom flanges. These seals are made from a room-temperature vulcanising (RTV) elastomer known commercially as Silcaset. This enables tests to be undertaken under inert or any other specified environment. 3.
RESULTS
3.1. Performance of the rig The stress relaxation rig performed successfully at an absorbed dose rate of 1.05 x 105 R h -1 for a total test period of 36 weeks, giving a
384
S. W. Addy, D. W. Clegg, A. A. Collyer, P. Crum
I
l
I
i
l
I
I
I
0
r, I
r, 0 t~ q3
03
1l+
.=. e3
.~ o .~ o °.~ o .~o •
n
•
0 o "r~ t~ e3
,4 U
@
@
op
l
I
I
I
I
I
I
I
0
I
0
o
o
o
0
6
o
o
0
~
A stress relaxation rig for use in y-irradiated environments
385
total absorbed dose of 6.35 x 108 R. The strain gauges secured by epoxy resin were still functioning, although the insulation on the bridge-connecting wires (silicone resin) had disintegrated. Upon dismantling the pneumatic ram, it was found that the nylon spacers had become hard and brittle, crumbling to a powder. However, the natural rubber seals were still intact, retaining their elastomeric properties, although a slight increase in hardness was observed. 3.2. Results from stress relaxation tests
Figure 3 shows results obtained for PDMS elastomer in a 3,-irradiated environment in the temperature range 40-200 °C. The curves result from the combination of the thermal and radiation scission reaction in the elastomer, and are discussed more fully in ref. 5.
4.
DISCUSSION
The present design enables only continuous stress relaxation tests to be performed. Hence only the kinetics of chain scission reactions may be studied. A simple modification to the rig consisting of removing the spring-loaded pins and replacing the single-acting ram with a double-acting ram would allow intermittent tests to be undertaken. The kinetics of any crosslinking reactions could then be studied using the theory outlined previously. In such a test, the sample is degraded in the unstressed condition. At regular time intervals the sample is rapidly loaded to some preset strain and then rapidly unloaded. It is assumed that the amount of crosslink formation taking place during the loading-unloading operation would be negligible.
ACKNOWLED GEMENTS The authors would like to thank SERC for the provision of a CASE Award for financing the work and CEGB for the provision of the ),-irradiation facilities.
386
S. w. Addy, D. W. Clegg, A. A. Collyer, P. Crum
REFERENCES 1. Clegg, D. W., Addy, S. W., Collyer, A. A., Jones, I. K. and Webb, S. W. (1984). Polym. Commun., 25(3), 73. 2. Tobolsky, A. V. (1953). J. Chem. Phys., 21, 614. 3. Tobolsky, A. V., Metz, D. J. and Mesrobian, R. B. (1950). J. Amer. Chem. Soc., 72, 1942. 4. Murakami, K. and Ono, K. (1979). Chemorheology of Polymers, Polymer Science Library 1. Amsterdam, Elsevier. 5. Addy, S. W., Clegg, D. W., Collyer, A. A., Cortield, G. C. and Crum, P. (1985). In: Advances in Rheology-3, (Eds B. Mena, A. Gar6ia-Rej6n and C. Rangel-Nafaille). Amsterdam, Elsevier, pp. 35-43.