modeling the vulcanization reaction of silicone rubber.

by:Keyuan     2020-06-04
Silicone rubber has become more and more popular in the past 50 years.
This thermosolid rubber maintains its mechanical and electrical properties over a wide temperature range and is therefore a natural choice for aerospace applications and medical devices [1]1, 2].
Seals for the automotive and aerospace industries, connectors and cables for applications and telecom, implants and equipment for medical purposes, and the production of packaging and baking trays for the food industry [3].
Silicone rubber is a thermosolid elastic material with a backbone for exchanging silicone and oxygen atoms as well as methyl or vinyl side bases.
All hot processes
The active curing thermostat, including silicone rubber, is mainly controlled by a heating and irreversible chemical reaction called vulcanization or curing.
The vulcanization process to improve the performance of the final product forms a three dimensional network [4-6].
Two types of reactions can lead to the cross-reaction of sulfide silica soil: peroxide and platinum catalysislinking [7, 8].
Silicone rubber is mainly composed of silicone polymers and fillers.
Depending on the polymer and vulcanization process used, they can be classified as low temperature vulcanization rubber (LTV orRTV)
And high temperature vulcanization rubber (HTV).
The RTV silicon fiber is a soft paste or sticky liquid.
HTV silicone rubber has two different physical states: liquid and solid.
Because they have high viscosity, solid silicone rubber (
Commonly referred to as high consistency silicone rubber)
The same way as ordinary organic rubber.
Solid silicone rubber is formed using a linear polymer with a molecular weight between 400,000-000g [mol. sup. -1].
These polymers, which contain an average of 6000 siloxyunits, are water-clear Newton liquids with viscosity between 15, 000 and 30,000 Pa s.
Solid silicone rubber is usually cured using two peroxide catalysts: aroyl-
Peroxide and hydrocarbonperoxides [7, 8]. Aroyl-
The peroxide is used when the vulcanization process can be completed without pressure and high reaction rate is allowed. Alkyl-
Peroxide is only used for low pressure vulcanization because they do not form carbonated acid as a decomposition product [1, 8].
The scheme of the reaction is shown in the figure. 1.
Reactive evinyl double base of polymer (Fig. 1a)
Reaction with no oxygen
Free radicals already produced in the peroxide group (Fig. 1b)
In the first phase of the reaction (Fig. 1c).
Vulcanization is the free radical of achievedwhen (Fig. 1c)
Connect yourself to another polymer chain to form a bridge.
The peroxide catalyst has a high reaction rate.
Solid silicone rubber is a pre-treated material, which is cured to a certain extent and then stored in the form of foil
Packed bar and carton.
In order to process solid rubber, a piece of paper is then cut or stamped to the desired size and placed in a compression or transfer mold.
After vulcanization, the part is removed from the mold, deflated, and the peroxide decomposition product is released in post-processingcuring process.
Liquid silicone rubber (LSR)
Has the same structure as solid silicone rubber.
However, the chain length of polydisulfur silicone for LSR is lower, about 6 times.
Therefore, the viscosity of the polymer is reduced by approximately 1000 times [1-3, 7].
The vulcanization of liquid silicone rubber is almost entirely done with Platinum
Catalytic Silicon hydrogen addition reaction shown in Figure 1
2. in a reaction that does not produce-products.
Similar to solid silicone rubber, the vinyl double bond on the polymer (Fig. 2a)
Interact with the Platinum Center, which has a free coordination website (Fig. 2b)
And activate the double bond.
Vinyl group crossover
By transforming the double bond to link, a single bond to the polymer chain is created;
In this case, for the inclusion of Si-H groups (Fig. 3c).
The catalyst becomes free and can be used again for further crossoverlinking.
Liquid silicone rubber is supplied in the form of a barrel or barrel.
Due to the low viscosity of these rubber, they can be pumped to the vulcanization equipment through pipes and pipes.
Two components (
Components A and B)
If the vulcanization process may begin to occur, it is sent to the static mixer through the luffing machine pump;
Using a cold runner can avoid material loss in the feed line [8]. [
Figure 1 slightly][
Figure 2:
Each step of the silicone rubber vulcanization process can be found in the literature [1-4, 7]
, But there is not a model of motion that describes crossoverLink process.
The vulcanization process can be described as A reaction between the two chemical groups represented by A and B, which connect two segments of the polymer chain.
Then the concentration of the unresponsive Asor Bs was tracked ,[C. sub. A]or [C. sub. B].
The degree of cure can be defined [4,5], [
Figure 3 slightly]
They are simple mathematical expressions that match the experimental data and can be used directly for numerical simulation.
In this paper, Kissinger model and Kamal-
In order to get a deeper understanding of the reaction and find the value that really makes sense for the activation energy, acid.
The use of the combined model results in a more realistic and robust response model.
The activation energy of silicone rubber was determined from the experimental data using the Kissinger model.
Determination of other parameters of the reaction with Kamal-
Sourour model, proposed Hernandez-with the same method-
Ortiz and Oswald [22, 23].
The kinetic parameters are used to track the degree of vulcanization as time and temperature change.
Experimental procedures for liquid silicone rubber (LSR)is a two-
Component System.
Components containing platinum catalyst and component B contain methyl hydrogen Silicon for cross-reaction
Link and alcohol inhibitor [8,23].
Components A and B of liquid silicone rubber are stored in A two-component box.
In a dark cabinet, place the cartridge at room temperature with a room temperature and humidity of 35%.
Using a static mixer with 13 mixing elements, mix the two components in a 1:1 ratio.
Five types of liquid silicone rubber, expressed as LSR-a, LSR-b,LSR-c, LSR-d, and LSR-
E and a solid silicone rubber expressed in hours were analyzed by thermal analysis to determine the progress of the vulcanization process.
The specific properties studied are peak temperature, reaction heat, and degree of reaction.
[Differential scanning Heat Meter]24]
Equipment manufactured by Netzsch (Phox DSC 200 PC)
Used to measure the reaction heat of the sample.
Analyze all reactions with a sealed aluminum pan.
The quality of the sample ranges from 10 mg to 30 mg.
A sealed empty pot is referenced.
The total heat of the reaction was measured by dynamic scanning of 20-150 [degrees]
C. use heating rate 1. 0, 2. 5, 5. 0, and 10. 0 K/min.
Using multiple scanning rates, the effect of time and temperature on the vulcanization reaction was deeply understood.
Repeatability was obtained at each heating rate.
All the experiments were carried out under nitrogen purification.
The curing kinetic model relates the reaction rate to temperature and degree of reaction.
To study the cross.
Two different methods are used: Image-only and mechanical [5, 6, 9].
The phenomenal method studies the reaction from a macro level and considers the reaction as a whole process.
Observation of mechanical methods-
The process is connected from a micro level and the reaction is studied as a series of separate steps. [
Figure 4 slightly]
The mechanism model is based on the chemometric equilibrium of the reaction substance involved in the basic reaction and the mechanism of each reaction.
As a result, these models are more difficult to obtain, but they represent vulcanization dynamics better than the only model.
The image-only model is semi-empirical and does not provide a clear description of the vulcanization process and the mechanism behind it.
These models are simple to calculate and can be widely used to describe cross-cutting
The link process for different resource systems.
They are based on the reaction rate as a function of the amount and rate constant of the reaction resin, as described in [4-6,9]
, Dc/defined [12, 13]
, Dc/However, in the later period, it is excessive
Predict the degree of cure [20, 21].
This deviation is caused by the diffusion effect.
Therefore, it is necessary for Kamal-
Sourour model, including diffusion
Limited part of the reaction
However, for elastic materials, it is not necessary to modify the constant of the reaction due to diffusion, because the material is more than vulcanization at its glass transition temperature.
The model accurately predicts the whole process, and these materials are cured before the glass transition point.
This condition gives sufficient free volume between molecules to allow free movement during molecular crossing
The link process, so diffusion does not play a big role in [4, 5].
Model description and numerical algorithm of Kamal-
The Sourour diffusion model has six parameters to fit.
Assuming that x is an unknown vector parameter as defined, the fit must be done in a manner with a smaller second-order term accompanying coefficient.
If this condition does not satisfy a particular set of data, the extension does not guarantee that the higher order terms are small;
Therefore, higher order terms must be included in the expansion (i. e. third, fourth).
Parameters in the model are fitted by least squares
Square estimation algorithm developed by Marquardt [26, 27](
See also Press and others. [29]
Oddensen and Schnabel [30]).
Details of the numerical method for finding Amatrix can be found in Hernandez-
Ortiz and Oswald [22]. [
Figure 10 slightly][
Figure 11 omitted]
Results for each liquid silicone rubber, the repeatability and heating rate of vulcanization were obtained, as shown in Table 1. 5.
The data of the heating rate and the temperature at which the maximum reaction rate occurs ,[T. sub. peak]
, Draw and fit into a linear model.
Activation energy [E. sub. 1]
, Each silicone rubber was calculated based on the data of the four dynamic scanning rates tested.
The slope of this line corresponds to the negative ratio of the activation energy and the general gas constant R (8. 3145 J g[mol. sup. -1][K. sup. -1])
As shown in the figure. 3 and Table 6.
Kelvintemeratures are used to avoid negative temperatures in the fittedmodel.
The data obtained by dynamic DSC scan is used to determine Kamal-Sourour model.
This process is done by fitting the instantaneous vulcanization rate and vulcanization percentage at each specific temperature to the model describing the reaction.
The technology uses one or more dynamic DSC scans to determine a set of kinetic parameters that simulate the vulcanization process for all tested heating rates.
Experimental data and fitting Kamal-
The acid model of liquid silicone rubber is put together in Fig. 4-8.
The fitted model fits well with the dsc data.
Fitting parameters are listed in Table 711.
Order of reaction m and n and frequency factor [a. sub. 1]and[a. sub. 2]
Fit according to the temperature as a variable. [
Figure 12:
Kamal-parameters found
The Sourour model is a mathematical expression that fits the experimental data well.
However, as can be seen from Table 7 --
They have no physical meaning.
These parameters are highly correlated with temperature.
When the initial temperature conditions used for modeling were the same as the experimental data, they simulated the experimental results well.
However, when the fitting curve is generated under the initial temperature condition of the experimental data, the fitting model moves from the experimental curve.
This effect can be seen in the picture. 9.
In addition to the sensitivity to temperature changes, the model is very sensitive to the accuracy of the parameters.
Small changes in some parameters will have a great impact on the model.
This effect can be seen in figs. 10 and 11.
The repeatability of solid silicone rubber is lower than that of liquid silicone rubber because the material is pre-
A certain degree of vulcanization before storage.
Rubber continues to cross during storage
Link in different proportions.
The results of solid silicone and a liquid silicone rubber were also compared.
Take samples of various materials from one HSR batch and test them [23].
Due to the change of initial curing degree between samples, only one was selected to fit the model.
Figure 12 shows the fitting data and compares it with LSR for similar applications.
The fitting parameters of the HSR are taken from the previous work [22]
And summarized in Table 12.
It should be noted that for this fit, the Kissinger equation is not used to calculate the internal energy.
It can be seen that solid silicone rubber is vulcanization at a temperature higher than liquid silicone rubber, resulting in greater cycle time and energy costs.
In addition, the vulcanization rate of liquid silicone rubber is higher than that of hard silicone rubber.
A higher reaction rate significantly shortens the cycle time.
The author thanked Julian Salguero for his help in the experimental work.
The authors also thank SIMTEC silicone components for providing the materials and ComProTec, Inc.
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Osswald Polymer Engineering Center, Department of Mechanical Engineering, University of Wisconsin-
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Email: ossward @ engrwise.
Sponsor: Innovation Partnership (PFI)
National Science Foundation
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