Silicone rubber is a thermoset elastomer having a backbone made up of alternating silicon and oxygen atoms and methyl or vinyl side groups (See Fig. 1).
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Figure 1. Molecular Structure of Silicone Rubber
Properties
The key properties of silicone rubber are given below:
- Good temperature stable
- Excellent electrical conductivity
- Available in medical quality grades
- Easy to color
Applications
Major application areas of silicone rubber are listed below:
- Aerospace
- Electronics
- Medicine
- Consumer goods
Types
Silicone rubber comes in solid and liquid types. Both have a similar basic structure, but need different cure and processing methods.
Processing Methods
High Consistency Rubber
High consistency rubber (HCR) or solid silicone rubber is manufactured in large batches. The components are mixed up at high temperatures followed by the addition of a peroxide catalyst. As cross-linking starts the reaction is interrupted before the completion of vulcanization. The mildly cross-linked silicone rubber is then rolled out in the form of sheets for storage and shipping.
Liquid Silicone Rubber
Liquid silicone rubber (LSR) is a two-component system containing a platinum catalyst (A), a cross-linker, methylhydrogensiloxane (B), and an alcohol inhibitor. The two components are mixed only at the time of processing. LSR is normally processed using cold runner injection molding equipment. The elastomer’s long chains cross link during vulcanization, releasing a quanta of energy to make it an exothermic reaction. The catalyst creates bonds in between the long chains, thus giving rise to a three dimensional matrix. This enhances the mechanical properties of the rubber.
The following sections discuss the vulcanization process of silicone rubber, which helps to optimize the conditions for processing of silicone rubber and also to choose the correct material for every application.
DSC Measurement of Vulcanization
Vulcanization can be analyzed with a Differential Scanning Calorimeter (DSC). Two small pans, one having the specimen and the other empty for reference, are placed in the heating chamber of the DSC. During a thermal sweep, the sample pan will absorb more heat as it has a higher heat capacity. The heat absorbed will be proportional to the difference between the heat capacities of the specimen and the reference. The energy released by the exothermic reaction is proportional to the number of cross-linked bonds formed. The sample vulcanization is measured by assessing the energy released. The heat emitted during vulcanization can be determined by the following equation:
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where Q is the heat released up to time t and •Q is the change in heat of the sample. The total heat of the reaction, QT, is given by
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where τ-final is the time when the reaction is complete. The reaction rate, dc/dt, is then determined as,
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The calculated data is then applied to the theoretical model. The silicone vulcanization reaction can be illustrated using an autocatalytic model. For an autocatalytic curing reaction, the Kamal-Sourour reaction model shown in Equation 4 is used.
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where m and n are the reaction orders, c is the extent of the vulcanization reaction, defined by c = Q/QT , and k1 and k2 are the Arhenius overall constants defined by,
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where
a1 is the fitted rate coefficient
E1 is the activation energy
R is the universal gas law constant
T is the vulcanization temperature.
The parameters from the Kamal-Sourour model that need to be applied to the experimental data can be represented as an unknown vector quantity, through a least-squares estimation algorithm put forth by Marquardt.
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The parameters can then be expanded into a power series with temperature (T) as the independent variable
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where i = 1,…,6 and aij are the new goal of the fitting.
Experimental Procedures
A Netzsch (DSC 200 PC) DSC was used to determine reaction heat for the samples. Sealed aluminum pans were utilized. The sample mass was in the range 10 to 30 mg and reference was an empty pan. The total reaction heat was calculated using a dynamic scan with heating rates of 1, 2.5, 5, and 10 K/min. Multiple scanning rates were used to determine the effect of temperature and time on vulcanization. Repeatability was determined for all heating rates and scans were performed under a nitrogen purge. Both the components of LSR were mixed with a Mixpac (DMA 50) 1:1 static mixer in a Kenics mixing chamber. For PDMS, the ratio of Sylgard 184 curing agent to elastomer used was a 1:10.
Results
Repeatability was calculated for the DSC data. At every vulcanization rate, model fitting was performed for the LSR and HCR. Figs. 2 to 5 show that the DSC data agrees with the theory. Figure 5 illustrates the relationship between the LSR and PDMS.
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Figure 2. Dynamic Scans of HCR
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Figure 3. Dynamic Scans of LSR
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Figure 4. Dynamic Scans of PDMS
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Figure 5. Comparison of Dynamic Scans of LSR and PDMS
Table 1 provides the total heat of reaction and the peak temperatures for all materials.
Table 1. Table 1. LSR. Fitted values using a 5 K/min scan.
|
Parameter
|
O(1) |
O(T) |
O(T2) |
| m |
7.4140E+01
|
-7.0540E-02
|
-3.5051E-04
|
| n |
-1.5130E+00
|
-3.3828E-03
|
2.7015E-05
|
| a1 |
7.8546E+01
|
-4.6229E-01
|
6.8078E-04
|
| E1 |
5.3011E+04
|
6.3376E+00
|
-2.8889E-01
|
| a2 |
3.4989E+01
|
2.7318E-01
|
2.2352E-04
|
| E2 |
-1.3530E+05
|
1.1428E+02
|
8.7977E-01
|
Graphs having the experimental data and fitted models for LSR and HCR are shown in Figs. 6 and 7.
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Figure 6. Fitted Model and Experimental Data for HCR
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Figure 7. Fitted Model and Experimental Data for LSR
Tables 1 and 2 summarise the corresponding values of m, n, a1, E1, a2, and E2 for both LSR and HCR, respectively.
Table 2.HCR. Fitted values using a 5 K/min scan.
|
Parameter
|
O(1) |
O(T) |
O(T2) |
| m |
8.4455E+01
|
-5.5926E-01
|
9.0122E-04
|
| n |
2.0456E+01
|
-1.0317E-01
|
1.2983E-04
|
| a1 |
1.6682E-22
|
-7.1679E-21
|
1.8524E-23
|
| E1 |
1.1447E+05
|
-6.5515E+02
|
2.1787E-01
|
| a2 |
-1.5320E+03
|
6.6209E+00
|
-7.1566E-03
|
| E2 |
-2.2411E+05
|
1.4717E+01
|
1.0666E+00
|
Table 3 provides the peak temperature for the three materials at all scan rates
Table 3. Total Heat of Reaction and Peak Temperature
|
Material
|
Heating Rate (K/min) |
Peak Temperature (°C) |
| HCR |
1
|
157.7
|
|
2.5
|
170.6
|
|
5
|
177.6
|
|
10
|
183.3
|
| LSR |
1
|
73.4
|
|
2.5
|
81.7
|
|
5
|
91.2
|
|
10
|
95.9
|
| PDMS |
2.5
|
82.0
|
|
5
|
90.5
|
|
10
|
100.8
|
Conclusion
Vulcanization of LSR and HCR, as well as PDMS, was measured using the DSC. The modified Kamal-Sourour model was used to distinguish the vulcanization reaction of liquid and solid silicone rubber from dynamic DSC experimental data. A group of vulcanization kinetics parameters was determined using a non-linear least squares Levenberg-Marquardt algorithm. The model led to good agreement between the predictions and the experimental data.
About Simtec
SIMTEC is a research and technology driven company. Since their establishment in 2002 in Madison, Wisconsin, USA, they have been continuously developing Extraordinary Solutions™ for leading industries worldwide with services ranging from prototyping to serial production of high precision Liquid Silicone Rubber (LSR), overmolded and Two Shot (LSR/Thermoplastics) components.
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This information has been sourced, reviewed and adapted from materials provided by Simtec.
For more information on this source, please visit Simtec.