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Gamma radiation effects on polydimethylsiloxane rubber foams under different radiation conditions
Nuclear Instruments and Methods in Physics Research B 307 (2013) 570–574
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Gamma radiation effects on polydimethylsiloxane rubber foams under different radiation conditions H.L. Sui a,b, X.Y. Liu b, F.C. Zhong b, X.Y. Li c, L. Wang b, X. Ju a,⇑ a
Department of Physics, University of Science and Technology Beijing, Beijing 100083, PR China Institute of Chemical Materials, CAEP, Mianyang 621900, PR China c Institute of Nuclear Physics and Chemistry, CAEP, Mianyang 621900, PR China b
a r t i c l e
i n f o
Article history: Received 21 September 2012 Received in revised form 30 March 2013 Accepted 31 March 2013 Available online 19 April 2013 Keywords: Polydimethylsiloxane Gamma radiation Orthogonal design
a b s t r a c t Polydimethylsiloxane rubber foams were irradiated by gamma ray under different radiation conditions designed by orthogonal design method. Compression set measurement, infrared attenuated total reflectance spectroscopy (ATR) and X-ray induced photoelectron spectroscopy (XPS) were used. Three aging factors’ influence effects on the mechanical property and chemical structure were studied. It was found that among the three factors and the chosen levels, both properties were affected most by radiation dose, while radiation dose rate had no obvious influence on both properties. The stiffening of the rubber foams was caused by cross-linking reactions in the Si–CH3. At the same radiation dose, the rigidity of the foams irradiated in air was lower than that in nitrogen. When polydimethylsiloxane was irradiated at a high dose in sealed nitrogen atmosphere, carbon element distribution would be changed. Hydrocarbons produced by gamma ray in the sealed tube would make the carbon content in the skin-deep higher than that in the middle, which indicated that polydimethylsiloxane rubber foams storing in a sealed atmosphere filled with enough hydrocarbons should be helpful to extend the service life. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Siloxane rubber and siloxane rubber foams have been widely used in the areas of aerospace and nucleus industries for their fine electric insulation, wide service temperature range, good thermostability and excellent resistance to oxidation and radiations [1]. In many applications, siloxane foam is exposed to radiations, and some of their performances will become disabled, which could affect the security of the related systems. In 1957, Robert [2] has been investigated several kinds of siloxane rubbers by gamma radiation. It was concluded that poly-methyl-vinyl siloxane rubber could endure 500–1000 kGy gamma radiation. Radiation effects of polysiloxane can be affected by many factors, such as chemical or physical structures, radiation dose, radiation dose rate, radiation temperature, stress, radiation atmosphere and so on. So, before you get start to investigate the polymer’s radiation effects, it is better to determine which factor has the most influence on the radiation effects, and which has the least influence. In the previous studies, most researchers have followed a traditional experimental method, i.e. varying one factor while keeping others constant [3–5]. If a traditional experimental method is employed to study the effects of four factors, each with three levels, ⇑ Corresponding author. Tel./fax: +86 10 62333921. E-mail address: [email protected] (X. Ju). 0168-583X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.03.059
there are a total of 64 experiments. Thus, this method is time consuming and calls for too many resources. Orthogonal design method is developed to optimize the experimental conditions and it can be used to study how much the selected factors affect the results [6]. If orthogonal design is used in the above experiment of four factors, each with three levels, it only needs 9 experiments. It is obvious that orthogonal design can significantly reduce many resources. In this study, the experiment was designed by orthogonal design methods. Compression set measurement was used to study the mechanical property. ATR and XPS were performed to study the structure property. In order to make sure how the aging factors (radiation dose, radiation dose rate and radiation environment) affect the performances of polydimethylsiloxane rubber foam. The effects of the three factors on mechanical properties and chemical structures were studied by range and variance analysis. Furthermore, radiation effects were also been studied. 2. Experimental 2.1. Materials Polydimethylsiloxane rubber foams were supplied by Institute of Chemistry Materials in China Academy of Engineering Physics. Polydimethylsiloxane (PDMS) with a few vinyl groups (<5%) has
H.L. Sui et al. / Nuclear Instruments and Methods in Physics Research B 307 (2013) 570–574
a number-average molecular weight Mn 350000. Precipitated silica with a size of 1–3 lm was used as the filler. Dicumyl peroxide (DCP) was used as curing agent. Blowing agents were carbamide and 3, 7-dinitroso-1.3.5.7-tetraazobicyclo-nonane. PDMS was mixed with silica filler. Then curing agent and blowing agents were added into the mixture in order. After purified and dried, the polydimethylsiloxane rubber foams were prepared. Then, the foams were cut into sheets with 1.5 mm in thickness for measurements. 2.2. Gamma radiation 60
Co source was used to irradiate the samples up to 800 kGy at two different dose rates (65 Gy/min and 130 Gy/min) at room temperature at Institute of Nuclear Physics and Chemistry. Samples irradiated in air were placed in polyethylene bags and samples irradiated in nitrogen were sealed in special glass tubes filled with 50 kPa inert gas (N2).
571
The changes in chemical structure were examined by infrared attenuated total reflection (ATR) spectroscopy using NICOLET 6700 FT-IR instrument. ATR spectra were measured with a resolution of 1 cm1 in the range of 400–4000 cm1. Every spectrum was the results of 32 co-added scans. X-ray induced photoelectron spectroscopy (XPS) was performed using ESCALAB 250 with Al Ka X-ray (1486.6 eV). Chemical elements content of the un-irradiated sample and the sample irradiated with 800 kGy were tested. The test locations of every sample were shown in Fig. 1. Radiation dose, radiation dose rate and radiation atmosphere are the usual factors which could affect the radiation effects. Among orthogonal tables, the appropriate one for arranging the three factors with four levels and two levels is L8(4 23). No interaction among the three factors was considered here. The selections of factors and the levels are shown in Table 1. 3. Results and discussion
2.3. Measurements 3.1. Mechanical property Compression property was carried out at a constant crosshead speed of 0.5 mm/min at room temperature. The nonstandard specimen size was 65 12 1.5 mm3. Every radiation condition has three same samples.
Fig. 1. The test locations of the vertical section for XPS.
Table 1 Factors and their levels. Factos levels
Dose (kGy) A
Dose rate (Gy/min) B
Atmosphere C
1 2 3 4
50 200 400 800
65 130
N2 Air
The non-linear compression curves of polydimethylsiloxane rubber foams irradiated in air and nitrogen are shown in Fig. 2. In order to make a quantitative analysis, e0.42 (The strain when the stress is 0.42 MPa) is chosen to describe mechanical property and the results are shown in the Table 2. Range and variance analysis results are shown in Table 3. Range analysis is aimed to investigate which aging factor affected the mechanical property most and which one affected least. Usually, the higher the mean range R is, the greater the influence effect on the results is. It is shown in Table 3 that radiation dose with the highest R affects the mechanical property most, while radiation dose rate least. F-test in variance analysis is usually used to determine whether the factor’s effect is significant or could be ignored. The method to calculate F value can be obtained from Yang’s study [7]. F value of each factor (for example A) is simply the ratio of the mean of squared deviations QA to the mean of squared error Qe. The mean squared deviations QA is equal to the sum of squared deviations SQA divided by the degree of freedom associated with the design factor. QA can be calculated from the following Eq. (1)
Q A ¼ p ½ðX 1A xÞ2 þ ðX 2A xÞ2 þ þ ðX mA xÞ2
ð1Þ
where, x is mean of all test values. p is the degree of each level used in the test. Generally, the higher the F is, the more obvious significance the effect is. The variance analysis shows that radiation dose
Fig. 2. The compression stress–strain curves of polydimethylsiloxane rubber foams irradiated in air and nitrogen at different radiation dose.
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Table 2 The results of mechanical and structure properties. Number
Factors
1 2 3 4 5 6 7 8
Results
A (kGy)
B (Gy/min)
C
e0.42
dSi–CH3/C–H
1(50) 1(50) 2(200) 2(200) 3(400) 3(400) 4(800) 4(800)
1(65) 2(130) 1(65) 2(130) 1(65) 2(130) 1(65) 2(130)
1(N2) 2(Air) 2(Air) 1(N2) 1(N2) 2(Air) 2(Air) 1(N2)
42.03 41.58 38.34 31.56 26.86 31.28 22.84 15.29
4.46 4.53 4.23 4.11 3.18 3.44 3.27 3.6
Table 3 Results of range and variance analysis of e0.42. Factors
X1a
X2
X3
X4
Rb
F
F-limitc
A B C
41.81 32.52 28.94
34.95 29.93 33.51
29.07
19.07
22.74 2.59 4.58
61.07 4.42 13.78
F0.05(3,2)=19.2 F0.1(1,2)=8.53 F0.1(1,2)=8.53
a The X value for each level is the average values of the samples with the same level. For example, X1 of factor A was calculated by X1A = (42.03 + 41.58)/2 = 41.805. b The symbol R is the difference between the highest and the lowest among X1, X2, X3, X4. For example, R of factor A was calculated by RA = 41.805 19.065 = 22.74. c F-limit value Fa(fA, fe) can be obtained from the F distribution table. a is a given significance level. fA and fe are the degrees of freedom of the design factor and error respectively. Usually, F > F0.05(fA, fe) means the factor has a very significant effect on the quality characteristic, and F0.1(fA, fe) < F < F0.05(fA, fe) means the factor has a significant effect.
Fig. 3. e0.42 of polydimethylsiloxane rubber foams exposed to gamma radiation with various radiation dose.
and atmosphere affect mechanical property significantly since F > F0.05(3,2), while radiation dose rate has no obvious influence. So, radiation dose rate could be ignored in our experiments, the curves for e0.42 with various radiation doses shown in Fig. 3 can be used to study the radiation effects on polydimethylsiloxane rubber foams. Generally, the higher the cross-linking density is, the harder and more brittle the polysiloxane rubber becomes [5]. Fig. 3 shows that the strain at the same stress becomes smaller and smaller, which indicates that polydimethylsiloxane rubber foams become harder and harder with the increasing of radiation dose either in air or in nitrogen. When exposed to 800 kGy radiation dose, the foams could be easily milled into powder. This result reveals that cross-
linking reactions happened during gamma radiation. One interesting phenomenon is that the mechanical property of the foams irradiated in nitrogen is more easily became bad than that irradiated in air. At almost any dose, the rigidity of foams irradiated in nitrogen is higher than the foams irradiated in air. 3.2. Changes in chemical structure ATR-FTIR spectra of polydimethylsiloxane rubber foams before and after gamma radiation are shown in Fig. 4. There are no new peaks appearing, but the position and the relative intensity of some peaks have been changed. The absorption double peaks at 1015 and 1086 cm1 are due to Si–O–Si stretching vibrations (m(Si–O– Si1), m(Si–O–Si2)) [8]. The peaks at around 800 and 1260 cm1 are mainly caused by Si–CH3 stretching vibrations (m(Si–CH3)) and C–H3 bending vibrations (d(C–H3)) [8]. The 2963 cm1 absorption peaks are ascribed to C–H stretching vibrations (m(C–H)) [8]. It is well known that the peak position is closely related to the chemical environment, and the peak intensity sometimes can reflect the content of the bond. In order to find which bond is influenced most of all the chemical bonds by gamma radiation, the foams irradiated at a dose of 800 kGy were studied. The changes in some character peak positions and absolute height are shown in the Table 4. The peak positions of m(C–H), d(C–H3) and m(Si– O–Si1) have not been changed obviously. But the peak positions of m(Si–O–Si2) and m(Si–CH3) have been decreased by almost 3 cm1 and 2 cm1 respectively at 800 kGy. Besides, the m(Si– CH3) peak height is decreased most of all, while m(C–H) least. The changes of m(Si–CH3) peak height are about twice as much as changes of m(C–H). So Si–CH3 bond in the chemical structures is affected most during gamma radiation, while radiation has least affect on C–H bond of all the bonds. In this case, C–H peak (around 2963 cm1) is chosen as the reference peak to calculate peak height ratio to study the changes of chemical structure. The peak height ratio (dSi–CH3/C–H) of m(Si–CH3) is shown in the Table 2. The results of range and variance analysis are shown in Table 5. Radiation dose with highest R is found to influence the chemical structure most. Furthermore, F-test indicates that the chemical structure is affected by radiation dose very significantly since F = 22.192 > F0.05(3,2), while radiation dose rate and radiation atmosphere have no obvious influence on the chemical structure during gamma radiation. Therefore, radiation dose rate can be ignored and the curves about dSi–CH3/C–H with various radiation doses are shown in Fig. 5. Si–CH3 as the most easily affected bond, its peak height ratio dSi–CH3/C–H decreases with the radiation dose increasing below 400 kGy for samples irradiated both in nitrogen and in air, which reveals that the content of Si–CH3 decreases. But dSi–CH3/C–H turns to increase when the radiation dose is over 400 kGy for the foams irradiated in nitrogen. Gamma ray as a high-energy photon has the ability to break any kinds of bonds, such as Si–O, Si–C, C–H in polydimethylsiloxane molecules. Then many free radicals can be produced. The crosslinking reaction would be happened between polydimethylsiloxane molecules in the positions of –CH2 or –O–Si–O–, and CH3 (methyl radical) could react with H (hydrogen radical) or CH3 forming hydrocarbons such as CH4, CH3CH3 and so on [4]. This is the reason why dSi–CH3/C–H decreases. The foams irradiated in nitrogen are sealed in glass tubes, so that the hydrocarbons which are produced by radical reactions would accumulate in the tubes during irradiation. Hydrocarbon radicals or H would be reproduced under radiation. So Hydrocarbon radicals could diffuse to the surface of foams and react with –O–Si either from the rubber molecules or from the SiO2 fillings. The higher the radiation dose is, the more the produced hydrocarbon radicals are, and this effect should be more obvious. This is why the content of Si–CH3 bond
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Fig. 4. ATR-FTIR spectra of polydimethylsiloxane rubber foams before and after irradiation in air and nitrogen.
Table 4 The changes of the peak position and intensity for samples irradiated at a dose of 800 kGy compared with the un-irradiated results.
m(C–H)
Peaks
a
In N2 In air a b
m(Si–O–Si2)
d(C–H3) b
m(Si–O–Si1)
m(Si–CH3)
D
E
D
E
D
E
D
E
D
E
0.35 0.05
14.15% 25.47%
0.3 0.34
21.67% 37.77%
2.58 2.67
27.82% 44.73%
0.43 0.15
21.65% 37.50%
1.57 1.75
34.00% 48.09%
D represents the change of peak position (cm1). E represents the change of peak intensity.
Table 5 Results of range and variance analysis of dSi–C/C–H. Factors
Y1
Y2
Y3
Y4
R
F
F-limit
A B C
4.5 3.79 3.84
4.17 3.92 3.87
3.31
3.44
1.19 0.14 0.03
22.19 1.22 0.07
F0.05(3,2) = 19.2 F0.05(1,2) = 18.5 F0.05(1,2) = 18.5
Table 6 The carbon contents (mole fraction) in different locations for the un-irradiated and irradiated (800 kGy) samples. Number
1 2 3 4 5 Mean
Un-irradiated results
Irradiated results
Inside (%)
Skin-deep (%)
Inside (%)
Skin-deep (%)
46.81 46.83 46.89 46.5 46.77 46.76
46.64 46.58 46.85 46.69 46.66 46.68
44.02 45.41 45.14 45.34 45.81 45.14
45.4 45.95 45.72 46.61 46.38 46.01
diated in nitrogen by 800 kGy, the carbon element content in the skin-deep is a little more than that inside. This result shows that when polydimethylsiloxane is irradiated in sealed nitrogen atmosphere, carbon element distribution would be changed, and this also proves the above explanation why the content of Si–CH3 in the skin-deep of the foams irradiated in a sealed tube increases a little over 400 kGy. 4. Conclusions
Fig. 5. dSi–C/C–H of polydimethylsiloxane rubber foams exposed to gamma radiation with various radiation dose.
in the skin-deep of the foams increases over 400 kGy for foams irradiated in a sealed tube. If this is right, the carbon element content in the skin-deep of the foams would be higher than that inside. XPS is the right method to prove it. The chemical elements content of different locations of un-irradiated and irradiated by 800 kGy were shown in Table 6. For the un-irradiated sample, the carbon element content in the skin-deep is no difference with that inside. However, when the sample is irra-
Among the three factors and the chosen levels, radiation dose affected the mechanical properties and chemical structures most, and its influence effects on mechanical property and chemical structures were very significant, while radiation dose rate had no obvious influence on both properties. The foams became harder and harder with the radiation dose increasing due to the crosslinking reactions, and the mechanical property of the foams irradiated in nitrogen was more easily became bad than irradiated in air. Gamma radiation had the most influence on Si–CH3, while least influence on C–H bonds in the siloxane chemical structures. The content of Si–CH3 decreased with the radiation dose increasing due to the cross-linking reactions in the Si–CH3. When polydimethylsiloxane is irradiated in sealed nitrogen atmosphere, carbon element distribution would be changed. The carbon content in the skin-deep would be higher than that in the middle due to the
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diffuseness of hydrocarbon radicals in the atmosphere. So, polydimethylsiloxane rubber foams storing in a sealed atmosphere filled with hydrocarbons should be helpful to extend the service life. Acknowledgements This work was supported financially by National Defense PreResearch Foundation of China 426020404. The authors would like to thank Mr. Jiabin Liu and Mr. Yaogang Shi for their great help in sample preparation. References [1] M. Patel, A.R. Skinner, Thermal ageing studies on room-temperature vulcanized polysiloxane rubbers, Polym. Degrad. Stab. 73 (2001) 399–402.
[2] H. Robert, Rubber Age 82 (1957) 461–470. [3] R.S. Maxwell, R. Cohenour, W. Sung, D. Solyom, M. Patel, The effects of cradiation on the thermal, mechanical and segmental dynamics of a silica filled, room temperature vulcanized polysiloxane rubber, Polym. Degrad. Stab. 80 (2003) 443–450. [4] W. Huang, F. Yibei, W. Chaoyang, X. Yunshu, B. Zhishang, A study on radiation resistance of siloxane foam containing phenyl, Radiat. Phys. Chem. 64 (2002) 229–233. [5] A. Chien, R. Maxwell, D. Chambers, B. Balazs, J. LeMay, Characterization of radiation-induced aging in silica-reinforced polysiloxane composites, Radiat. Phys. Chem. 59 (2000) 493–500. [6] J.L. Chen, K.C. Au, Y.S. Wong, N.F.Y. Tam, Using orthogonal design to determine optimal conditions for biodegradation of phenanthrene in mangrove sediment slurry, J. Hazard. Mater. 176 (2010) 666–676. [7] W.H. Yang, Y.S. Tarng, Design optimization of cutting parameters for turning operations based on the Taguchi method, J. Mater. Process. Technol. 84 (1998) 122–129. [8] A. Grill, Low and Ultralow Dielectric Constant Films Prepared by Plasmaenhanced Chemical Vapor Deposition, John Wiley & Sons, Ltd., USA, 2007.
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Gamma radiation effects on polydimethylsiloxane rubber foams under different radiation conditions H.L. Sui a,b, X.Y. Liu b, F.C. Zhong b, X.Y. Li c, L. Wang b, X. Ju a,⇑ a
Department of Physics, University of Science and Technology Beijing, Beijing 100083, PR China Institute of Chemical Materials, CAEP, Mianyang 621900, PR China c Institute of Nuclear Physics and Chemistry, CAEP, Mianyang 621900, PR China b
a r t i c l e
i n f o
Article history: Received 21 September 2012 Received in revised form 30 March 2013 Accepted 31 March 2013 Available online 19 April 2013 Keywords: Polydimethylsiloxane Gamma radiation Orthogonal design
a b s t r a c t Polydimethylsiloxane rubber foams were irradiated by gamma ray under different radiation conditions designed by orthogonal design method. Compression set measurement, infrared attenuated total reflectance spectroscopy (ATR) and X-ray induced photoelectron spectroscopy (XPS) were used. Three aging factors’ influence effects on the mechanical property and chemical structure were studied. It was found that among the three factors and the chosen levels, both properties were affected most by radiation dose, while radiation dose rate had no obvious influence on both properties. The stiffening of the rubber foams was caused by cross-linking reactions in the Si–CH3. At the same radiation dose, the rigidity of the foams irradiated in air was lower than that in nitrogen. When polydimethylsiloxane was irradiated at a high dose in sealed nitrogen atmosphere, carbon element distribution would be changed. Hydrocarbons produced by gamma ray in the sealed tube would make the carbon content in the skin-deep higher than that in the middle, which indicated that polydimethylsiloxane rubber foams storing in a sealed atmosphere filled with enough hydrocarbons should be helpful to extend the service life. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Siloxane rubber and siloxane rubber foams have been widely used in the areas of aerospace and nucleus industries for their fine electric insulation, wide service temperature range, good thermostability and excellent resistance to oxidation and radiations [1]. In many applications, siloxane foam is exposed to radiations, and some of their performances will become disabled, which could affect the security of the related systems. In 1957, Robert [2] has been investigated several kinds of siloxane rubbers by gamma radiation. It was concluded that poly-methyl-vinyl siloxane rubber could endure 500–1000 kGy gamma radiation. Radiation effects of polysiloxane can be affected by many factors, such as chemical or physical structures, radiation dose, radiation dose rate, radiation temperature, stress, radiation atmosphere and so on. So, before you get start to investigate the polymer’s radiation effects, it is better to determine which factor has the most influence on the radiation effects, and which has the least influence. In the previous studies, most researchers have followed a traditional experimental method, i.e. varying one factor while keeping others constant [3–5]. If a traditional experimental method is employed to study the effects of four factors, each with three levels, ⇑ Corresponding author. Tel./fax: +86 10 62333921. E-mail address: [email protected] (X. Ju). 0168-583X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.03.059
there are a total of 64 experiments. Thus, this method is time consuming and calls for too many resources. Orthogonal design method is developed to optimize the experimental conditions and it can be used to study how much the selected factors affect the results [6]. If orthogonal design is used in the above experiment of four factors, each with three levels, it only needs 9 experiments. It is obvious that orthogonal design can significantly reduce many resources. In this study, the experiment was designed by orthogonal design methods. Compression set measurement was used to study the mechanical property. ATR and XPS were performed to study the structure property. In order to make sure how the aging factors (radiation dose, radiation dose rate and radiation environment) affect the performances of polydimethylsiloxane rubber foam. The effects of the three factors on mechanical properties and chemical structures were studied by range and variance analysis. Furthermore, radiation effects were also been studied. 2. Experimental 2.1. Materials Polydimethylsiloxane rubber foams were supplied by Institute of Chemistry Materials in China Academy of Engineering Physics. Polydimethylsiloxane (PDMS) with a few vinyl groups (<5%) has
H.L. Sui et al. / Nuclear Instruments and Methods in Physics Research B 307 (2013) 570–574
a number-average molecular weight Mn 350000. Precipitated silica with a size of 1–3 lm was used as the filler. Dicumyl peroxide (DCP) was used as curing agent. Blowing agents were carbamide and 3, 7-dinitroso-1.3.5.7-tetraazobicyclo-nonane. PDMS was mixed with silica filler. Then curing agent and blowing agents were added into the mixture in order. After purified and dried, the polydimethylsiloxane rubber foams were prepared. Then, the foams were cut into sheets with 1.5 mm in thickness for measurements. 2.2. Gamma radiation 60
Co source was used to irradiate the samples up to 800 kGy at two different dose rates (65 Gy/min and 130 Gy/min) at room temperature at Institute of Nuclear Physics and Chemistry. Samples irradiated in air were placed in polyethylene bags and samples irradiated in nitrogen were sealed in special glass tubes filled with 50 kPa inert gas (N2).
571
The changes in chemical structure were examined by infrared attenuated total reflection (ATR) spectroscopy using NICOLET 6700 FT-IR instrument. ATR spectra were measured with a resolution of 1 cm1 in the range of 400–4000 cm1. Every spectrum was the results of 32 co-added scans. X-ray induced photoelectron spectroscopy (XPS) was performed using ESCALAB 250 with Al Ka X-ray (1486.6 eV). Chemical elements content of the un-irradiated sample and the sample irradiated with 800 kGy were tested. The test locations of every sample were shown in Fig. 1. Radiation dose, radiation dose rate and radiation atmosphere are the usual factors which could affect the radiation effects. Among orthogonal tables, the appropriate one for arranging the three factors with four levels and two levels is L8(4 23). No interaction among the three factors was considered here. The selections of factors and the levels are shown in Table 1. 3. Results and discussion
2.3. Measurements 3.1. Mechanical property Compression property was carried out at a constant crosshead speed of 0.5 mm/min at room temperature. The nonstandard specimen size was 65 12 1.5 mm3. Every radiation condition has three same samples.
Fig. 1. The test locations of the vertical section for XPS.
Table 1 Factors and their levels. Factos levels
Dose (kGy) A
Dose rate (Gy/min) B
Atmosphere C
1 2 3 4
50 200 400 800
65 130
N2 Air
The non-linear compression curves of polydimethylsiloxane rubber foams irradiated in air and nitrogen are shown in Fig. 2. In order to make a quantitative analysis, e0.42 (The strain when the stress is 0.42 MPa) is chosen to describe mechanical property and the results are shown in the Table 2. Range and variance analysis results are shown in Table 3. Range analysis is aimed to investigate which aging factor affected the mechanical property most and which one affected least. Usually, the higher the mean range R is, the greater the influence effect on the results is. It is shown in Table 3 that radiation dose with the highest R affects the mechanical property most, while radiation dose rate least. F-test in variance analysis is usually used to determine whether the factor’s effect is significant or could be ignored. The method to calculate F value can be obtained from Yang’s study [7]. F value of each factor (for example A) is simply the ratio of the mean of squared deviations QA to the mean of squared error Qe. The mean squared deviations QA is equal to the sum of squared deviations SQA divided by the degree of freedom associated with the design factor. QA can be calculated from the following Eq. (1)
Q A ¼ p ½ðX 1A xÞ2 þ ðX 2A xÞ2 þ þ ðX mA xÞ2
ð1Þ
where, x is mean of all test values. p is the degree of each level used in the test. Generally, the higher the F is, the more obvious significance the effect is. The variance analysis shows that radiation dose
Fig. 2. The compression stress–strain curves of polydimethylsiloxane rubber foams irradiated in air and nitrogen at different radiation dose.
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Table 2 The results of mechanical and structure properties. Number
Factors
1 2 3 4 5 6 7 8
Results
A (kGy)
B (Gy/min)
C
e0.42
dSi–CH3/C–H
1(50) 1(50) 2(200) 2(200) 3(400) 3(400) 4(800) 4(800)
1(65) 2(130) 1(65) 2(130) 1(65) 2(130) 1(65) 2(130)
1(N2) 2(Air) 2(Air) 1(N2) 1(N2) 2(Air) 2(Air) 1(N2)
42.03 41.58 38.34 31.56 26.86 31.28 22.84 15.29
4.46 4.53 4.23 4.11 3.18 3.44 3.27 3.6
Table 3 Results of range and variance analysis of e0.42. Factors
X1a
X2
X3
X4
Rb
F
F-limitc
A B C
41.81 32.52 28.94
34.95 29.93 33.51
29.07
19.07
22.74 2.59 4.58
61.07 4.42 13.78
F0.05(3,2)=19.2 F0.1(1,2)=8.53 F0.1(1,2)=8.53
a The X value for each level is the average values of the samples with the same level. For example, X1 of factor A was calculated by X1A = (42.03 + 41.58)/2 = 41.805. b The symbol R is the difference between the highest and the lowest among X1, X2, X3, X4. For example, R of factor A was calculated by RA = 41.805 19.065 = 22.74. c F-limit value Fa(fA, fe) can be obtained from the F distribution table. a is a given significance level. fA and fe are the degrees of freedom of the design factor and error respectively. Usually, F > F0.05(fA, fe) means the factor has a very significant effect on the quality characteristic, and F0.1(fA, fe) < F < F0.05(fA, fe) means the factor has a significant effect.
Fig. 3. e0.42 of polydimethylsiloxane rubber foams exposed to gamma radiation with various radiation dose.
and atmosphere affect mechanical property significantly since F > F0.05(3,2), while radiation dose rate has no obvious influence. So, radiation dose rate could be ignored in our experiments, the curves for e0.42 with various radiation doses shown in Fig. 3 can be used to study the radiation effects on polydimethylsiloxane rubber foams. Generally, the higher the cross-linking density is, the harder and more brittle the polysiloxane rubber becomes [5]. Fig. 3 shows that the strain at the same stress becomes smaller and smaller, which indicates that polydimethylsiloxane rubber foams become harder and harder with the increasing of radiation dose either in air or in nitrogen. When exposed to 800 kGy radiation dose, the foams could be easily milled into powder. This result reveals that cross-
linking reactions happened during gamma radiation. One interesting phenomenon is that the mechanical property of the foams irradiated in nitrogen is more easily became bad than that irradiated in air. At almost any dose, the rigidity of foams irradiated in nitrogen is higher than the foams irradiated in air. 3.2. Changes in chemical structure ATR-FTIR spectra of polydimethylsiloxane rubber foams before and after gamma radiation are shown in Fig. 4. There are no new peaks appearing, but the position and the relative intensity of some peaks have been changed. The absorption double peaks at 1015 and 1086 cm1 are due to Si–O–Si stretching vibrations (m(Si–O– Si1), m(Si–O–Si2)) [8]. The peaks at around 800 and 1260 cm1 are mainly caused by Si–CH3 stretching vibrations (m(Si–CH3)) and C–H3 bending vibrations (d(C–H3)) [8]. The 2963 cm1 absorption peaks are ascribed to C–H stretching vibrations (m(C–H)) [8]. It is well known that the peak position is closely related to the chemical environment, and the peak intensity sometimes can reflect the content of the bond. In order to find which bond is influenced most of all the chemical bonds by gamma radiation, the foams irradiated at a dose of 800 kGy were studied. The changes in some character peak positions and absolute height are shown in the Table 4. The peak positions of m(C–H), d(C–H3) and m(Si– O–Si1) have not been changed obviously. But the peak positions of m(Si–O–Si2) and m(Si–CH3) have been decreased by almost 3 cm1 and 2 cm1 respectively at 800 kGy. Besides, the m(Si– CH3) peak height is decreased most of all, while m(C–H) least. The changes of m(Si–CH3) peak height are about twice as much as changes of m(C–H). So Si–CH3 bond in the chemical structures is affected most during gamma radiation, while radiation has least affect on C–H bond of all the bonds. In this case, C–H peak (around 2963 cm1) is chosen as the reference peak to calculate peak height ratio to study the changes of chemical structure. The peak height ratio (dSi–CH3/C–H) of m(Si–CH3) is shown in the Table 2. The results of range and variance analysis are shown in Table 5. Radiation dose with highest R is found to influence the chemical structure most. Furthermore, F-test indicates that the chemical structure is affected by radiation dose very significantly since F = 22.192 > F0.05(3,2), while radiation dose rate and radiation atmosphere have no obvious influence on the chemical structure during gamma radiation. Therefore, radiation dose rate can be ignored and the curves about dSi–CH3/C–H with various radiation doses are shown in Fig. 5. Si–CH3 as the most easily affected bond, its peak height ratio dSi–CH3/C–H decreases with the radiation dose increasing below 400 kGy for samples irradiated both in nitrogen and in air, which reveals that the content of Si–CH3 decreases. But dSi–CH3/C–H turns to increase when the radiation dose is over 400 kGy for the foams irradiated in nitrogen. Gamma ray as a high-energy photon has the ability to break any kinds of bonds, such as Si–O, Si–C, C–H in polydimethylsiloxane molecules. Then many free radicals can be produced. The crosslinking reaction would be happened between polydimethylsiloxane molecules in the positions of –CH2 or –O–Si–O–, and CH3 (methyl radical) could react with H (hydrogen radical) or CH3 forming hydrocarbons such as CH4, CH3CH3 and so on [4]. This is the reason why dSi–CH3/C–H decreases. The foams irradiated in nitrogen are sealed in glass tubes, so that the hydrocarbons which are produced by radical reactions would accumulate in the tubes during irradiation. Hydrocarbon radicals or H would be reproduced under radiation. So Hydrocarbon radicals could diffuse to the surface of foams and react with –O–Si either from the rubber molecules or from the SiO2 fillings. The higher the radiation dose is, the more the produced hydrocarbon radicals are, and this effect should be more obvious. This is why the content of Si–CH3 bond
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Fig. 4. ATR-FTIR spectra of polydimethylsiloxane rubber foams before and after irradiation in air and nitrogen.
Table 4 The changes of the peak position and intensity for samples irradiated at a dose of 800 kGy compared with the un-irradiated results.
m(C–H)
Peaks
a
In N2 In air a b
m(Si–O–Si2)
d(C–H3) b
m(Si–O–Si1)
m(Si–CH3)
D
E
D
E
D
E
D
E
D
E
0.35 0.05
14.15% 25.47%
0.3 0.34
21.67% 37.77%
2.58 2.67
27.82% 44.73%
0.43 0.15
21.65% 37.50%
1.57 1.75
34.00% 48.09%
D represents the change of peak position (cm1). E represents the change of peak intensity.
Table 5 Results of range and variance analysis of dSi–C/C–H. Factors
Y1
Y2
Y3
Y4
R
F
F-limit
A B C
4.5 3.79 3.84
4.17 3.92 3.87
3.31
3.44
1.19 0.14 0.03
22.19 1.22 0.07
F0.05(3,2) = 19.2 F0.05(1,2) = 18.5 F0.05(1,2) = 18.5
Table 6 The carbon contents (mole fraction) in different locations for the un-irradiated and irradiated (800 kGy) samples. Number
1 2 3 4 5 Mean
Un-irradiated results
Irradiated results
Inside (%)
Skin-deep (%)
Inside (%)
Skin-deep (%)
46.81 46.83 46.89 46.5 46.77 46.76
46.64 46.58 46.85 46.69 46.66 46.68
44.02 45.41 45.14 45.34 45.81 45.14
45.4 45.95 45.72 46.61 46.38 46.01
diated in nitrogen by 800 kGy, the carbon element content in the skin-deep is a little more than that inside. This result shows that when polydimethylsiloxane is irradiated in sealed nitrogen atmosphere, carbon element distribution would be changed, and this also proves the above explanation why the content of Si–CH3 in the skin-deep of the foams irradiated in a sealed tube increases a little over 400 kGy. 4. Conclusions
Fig. 5. dSi–C/C–H of polydimethylsiloxane rubber foams exposed to gamma radiation with various radiation dose.
in the skin-deep of the foams increases over 400 kGy for foams irradiated in a sealed tube. If this is right, the carbon element content in the skin-deep of the foams would be higher than that inside. XPS is the right method to prove it. The chemical elements content of different locations of un-irradiated and irradiated by 800 kGy were shown in Table 6. For the un-irradiated sample, the carbon element content in the skin-deep is no difference with that inside. However, when the sample is irra-
Among the three factors and the chosen levels, radiation dose affected the mechanical properties and chemical structures most, and its influence effects on mechanical property and chemical structures were very significant, while radiation dose rate had no obvious influence on both properties. The foams became harder and harder with the radiation dose increasing due to the crosslinking reactions, and the mechanical property of the foams irradiated in nitrogen was more easily became bad than irradiated in air. Gamma radiation had the most influence on Si–CH3, while least influence on C–H bonds in the siloxane chemical structures. The content of Si–CH3 decreased with the radiation dose increasing due to the cross-linking reactions in the Si–CH3. When polydimethylsiloxane is irradiated in sealed nitrogen atmosphere, carbon element distribution would be changed. The carbon content in the skin-deep would be higher than that in the middle due to the
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diffuseness of hydrocarbon radicals in the atmosphere. So, polydimethylsiloxane rubber foams storing in a sealed atmosphere filled with hydrocarbons should be helpful to extend the service life. Acknowledgements This work was supported financially by National Defense PreResearch Foundation of China 426020404. The authors would like to thank Mr. Jiabin Liu and Mr. Yaogang Shi for their great help in sample preparation. References [1] M. Patel, A.R. Skinner, Thermal ageing studies on room-temperature vulcanized polysiloxane rubbers, Polym. Degrad. Stab. 73 (2001) 399–402.
[2] H. Robert, Rubber Age 82 (1957) 461–470. [3] R.S. Maxwell, R. Cohenour, W. Sung, D. Solyom, M. Patel, The effects of cradiation on the thermal, mechanical and segmental dynamics of a silica filled, room temperature vulcanized polysiloxane rubber, Polym. Degrad. Stab. 80 (2003) 443–450. [4] W. Huang, F. Yibei, W. Chaoyang, X. Yunshu, B. Zhishang, A study on radiation resistance of siloxane foam containing phenyl, Radiat. Phys. Chem. 64 (2002) 229–233. [5] A. Chien, R. Maxwell, D. Chambers, B. Balazs, J. LeMay, Characterization of radiation-induced aging in silica-reinforced polysiloxane composites, Radiat. Phys. Chem. 59 (2000) 493–500. [6] J.L. Chen, K.C. Au, Y.S. Wong, N.F.Y. Tam, Using orthogonal design to determine optimal conditions for biodegradation of phenanthrene in mangrove sediment slurry, J. Hazard. Mater. 176 (2010) 666–676. [7] W.H. Yang, Y.S. Tarng, Design optimization of cutting parameters for turning operations based on the Taguchi method, J. Mater. Process. Technol. 84 (1998) 122–129. [8] A. Grill, Low and Ultralow Dielectric Constant Films Prepared by Plasmaenhanced Chemical Vapor Deposition, John Wiley & Sons, Ltd., USA, 2007.