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Designing of epoxy resin systems for cryogenic use
Cryogenics 45 (2005) 141–148 www.elsevier.com/locate/cryogenics
Designing of epoxy resin systems for cryogenic use T. Ueki a
a,*
, S. Nishijima b, Y. Izumi
b
Arisawa Manufacturing Co., Ltd, 1-Nakadahara, Niigata, Joetsu 943-8610, Japan b Osaka University, 2-1 Yamada-oka, Osaka, Suita 565-0871, Japan
Received 16 January 2004; received in revised form 4 July 2004; accepted 28 July 2004
Abstract The mechanical and thermal properties of several types of epoxy systems were designed based on the chemical structure, network structure and morphology aiming at cryogenic application. In this research di-epoxies or multifunctional epoxies were cured by several kinds of hardeners such as anhydride, amine or phenol and were blended with polycarbonate, carboxyl-terminated butadiene acrylonitrile copolymer or phenoxy. The mechanical properties and thermal properties of these cured epoxies were measured at room and liquid nitrogen temperature. It was found that the two-dimensional network structured linear polymer shows high performance even at cryogenic temperature. It was concluded that the controls of the structures are very important to optimize epoxy systems for cryogenic application. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Epoxy resin; Phenoxy; Linear polymer
1. Introduction Epoxy resins have been widely used as the matrix of composites because of their good electric insulating, mechanical, and easy fabricating properties. Additionally the composites have also been used for cryogenic applications. When the temperature is decreased down to cryogenic temperatures, the internal stress due to the thermal contraction is generated in a matrix resin. Fracture of the matrix is induced when the thermal stress induced stress intensity factor exceeds the fracture toughness of the resin. It is, therefore, important that the fracture toughness of the epoxy resin is improved even at cryogenic temperature. To attain the high performances of polymers, it is necessary to control the molecular structure, e.g. chemical and network structure. To control the morphology of the polymers is also
*
Corresponding author. Fax: +81 25 524 5281. E-mail address: [email protected] (T. Ueki).
0011-2275/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.cryogenics.2004.07.002
needed when the different types of polymers are blended. In epoxy resins there are many kinds of chemical unit such as diphenylol propane, diphenylol methane, tetrabromo diphenylol propane, phenol novolac, diamines, diacids, diols and so on. This difference brings the change of the polymer rigidity. The controls of crosslinking density and the epoxide equivalent by increasing the unit number of main chain cause the different network structure. The functionality of epoxy groups also causes the change of physical properties of epoxy resins. The different types of epoxy hardeners such as acid anhydride, amine and phenol also induce the change of performances of epoxy resins. By choosing the different types of epoxy and hardener the chemical structure can be designed in epoxy resin. It is known that the different network structures cause the different properties even if the polymers have the same chemical structure. The network structure can also be controlled by the chemical reaction. The epoxy resins are usually rigid and brittle. To improve the fracture toughness of epoxy the plasticizer is usually added. The morphology such as
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T. Ueki et al. / Cryogenics 45 (2005) 141–148
the sea-island structure and interpenetrating network also determine the physical properties of epoxy. When the morphology is optimized, the modified toughened polymer could be obtained showing high crack-resistance without degradation of other properties. Changing the types and amount of modifier or the curing process can change the morphology. It is very important to clarify how to control the molecular structure to obtain high performance polymer effectively and economically at cryogenic temperatures [1–4]. In this research high performance polymer for cryogenic use has been developed and the results will be presented.
2. Experiment 2.1. Specimen Fig. 1 shows the structures of epoxy resin and hardener used in this work. The typical reactions are shown in Fig. 2.
three-dimensional polymer by cross-linking the multi functional epoxy resin of Phenol novolac with BPA. This reaction is shown in Fig. 2(5). 2.4. Control of morphology Table 3 shows these systems to examine the effects of morphology. The several types of modifier were chosen and added to the sample B. The blend ratio of the modifier was 10 phr, which means that 10 g of the modifier are added to 100 g of epoxy resin. First the modifiers were melted by heat homogeneously and were added to epoxy resin. The PC, phenoxy and CTBN added to epoxy resin are named B1, B2 and B3. The modifier phase is separated from epoxy phase due to the different solubility. Furthermore the two-dimensional phenoxy shown in Fig. 1(5) is added to the three-dimensional epoxy systems at the fraction of 10, 30, 50 and 70 wt %, which can be bought on commercial base, has melting point beyond 150 °C and is difficult to handle owing to very high viscosity even if molten state. The systems are shown in Table 4.
2.2. Control of chemical structure Table 1 shows the employed systems to control the chemical structure of epoxy resin. The DGEBA shown in Fig. 1(1) is the most frequently used epoxy resin, and the DGEBF shown in Fig. 1(2) has lower viscosity than DGEBA. Sample A was cured with anhydride MNA shown in Fig. 1(6), which composes the threedimension network with ester bond. This reaction is shown in Fig. 2(1). Sample B was cured with aromatic amine DETDA shown in Fig. 1(7), which composes three-dimensional network with hydrogen groups and epoxy groups. The reaction shown Fig. 2(2) yields the secondary alcohol and shows low viscosity and hence this system is used for vacuum impregnation. Sample C was cured with the amine DICY shown in Fig. 1(9), which is the most common hardener. These were used to study the effects of chemical structures at cryogenic temperature. 2.3. Control of network structure Table 2 shows the systems to control the network structures. The epoxy molecules cross-linked to form the various networks having various structures, which can be controlled by selecting the main chain and hardener. Samples D and E have the two-dimensional network showing both thermal resistance and high fracture toughness even at cryogenic temperatures. They consist of the high molecular weight liner polymers having more than 100-repeated unit. They are named generically phenoxy resin as same as the polymer shown in Fig. 1(5). These reactions are shown in Fig. 2(3,4). Sample F was cured not to form the two-dimensional but the
3. Measurement Thermal contraction was measured by the strain gauge attached on the sample block [5]. In Table 3 the thermal contraction from 100 °C to 23 °C of room temperature (RT), is also shown as noted. The thermal contraction down to liquid nitrogen temperature (LNT) from room temperature was measured. Glass transition temperature (Tg) was measured by the differential scanning calorimeter (DSC). Specific gravity was measured by the ArchimedesÕ method. Fracture toughness (Kq), elastic modulus (Er) and crack index were evaluated as the mechanical properties. The Kq and Er were measured by the three-point-bend method. Pre-cracks were introduced with a knife-edge. Crack index defined in this work corresponds to the crack density in the matrix resin of 3D-FRP at RT when the epoxy impregnated the three-dimensional glass fabric was cured at 100 °C for 10 h. The crack density was evaluated and was classified into 10 classes. For example Index 10 shows that there is no crack in the resin [6,7].
4. Result and discussion 4.1. Effect of chemical structure The relationship between Er and Kq of epoxy resins with different chemical structure is shown in Fig. 3. It shows that Er increases by approximately twice with decreasing temperature from room temperature to liquid nitrogen temperature while Kq increases slightly.
T. Ueki et al. / Cryogenics 45 (2005) 141–148
143
CH3 CH2 CH
C
CH2 O
O
CH2 CH
O
CH2
O
CH3
(1)
CH2
CH2 CH CH2 O
CH2 CH CH2
O
O
O
(2) CH3 CH2 CH
CH3
CH2 O
O
CH2 CH
O
CH2
O CH3
CH3
(3) CH2
CH2
CH2
CH2
O
O
CH2
CH2
CH2
O
O
CH2
CH2
CH2
O
O
CH2 n
(4)
CH3 R
O
CH3
C
O
C H2 C
CH3
CH2 O
C
O
R
CH3
OH n
(5)
O
CH3
C
NH2
O
NH2
C
CH3
C2H5
C2H5
O (6)
(7)
CH3 OH
C
OH
CH3 BPA
(8)
NH NH2 C
NH CN (9)
Fig. 1. Structures of the test epoxy resins and the hardeners: (1) DGEBA: diglycidylether of bisphenol A, (2) DGEBF: diglycidylether of bisphenol F, (3) DGEBiphenyl: diglycidylether of biphenyl, (4) Phenol novolac type epoxy, (5) Phenoxy resin, (6) MNA: methylnadic anhydride, (7) DETDA: diethyltoluene diamine, (8) BPA: bisphenol A, (9) DICY: dicyandiamide.
The relationship between specific gravity and Kq is shown in Fig. 4. The relationship between Kq and thermal contraction from room to liquid nitrogen temperature is shown in Fig. 5. The relationship between Tg and Kq is shown in Fig. 6. Samples A and C have almost
same values of specific gravity and thermal contraction. B has the lowest specific gravity but it shows the lowest thermal contraction. Usually it has been thought that the epoxy having higher specific gravity shows the lowest thermal contraction. It is not always correct
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T. Ueki et al. / Cryogenics 45 (2005) 141–148
O
C
C O
(1)
R
C
H3C
H3C
C
OR O
O
MNA
C
C
OR
O
O
C
OH
OH
C
CH3
OH
O
O O
OH O CH2 CH
CH2 O
+
R
(2)
R
NH2
CH2 O
CH2 CH
CH2 O
N
amine
epoxy
CH2 CH
OH DGEBiphenyl
CH3
(3) R
+
BPA
CH3
O
CH3
O
C H2 C
CH3
CH2 O
O
R
OH CH3
CH3
DGEBA
+
BPA
CH3
(4) R
O
CH3
CH3
n
CH3
C
O
CH2 C
CH3
CH2 O
C
O
R
CH3
OH n
Phenol novolac
+
BPA
OH CH2
CH2
CH3 C
CH2 O
O
R
CH2
CH2
(5)
O
CH3
n O CH2
CH3 CH2 OH
CH2 O
C
O O
R OH
CH3 CH2
CH2
CH3 CH2
O
C
O
R
CH3
Fig. 2. Chemical reactions of epoxy resins with the hardeners: (1) Chemical reaction of epoxy with anhydride. (2) Chemical reaction of epoxy with amine. (3) Two-dimensional linear polymer of DGEBiphenyl with BPA. (4) Two-dimensional linear polymer of DGEBA with BPA. (5) Cross-linked three-dimensional polymer of multifunctional epoxy with BPA.
T. Ueki et al. / Cryogenics 45 (2005) 141–148 Table 1 Test epoxy samples of three-dimensional polymers
145
Table 4 Test epoxy samples of modified with phenoxy
Sample
Epoxy resin/hardener
Blend ratio (g)
Phenoxy/C epoxy resin Blend ratio (g)
A B C
DGEBF/MNA DGEBF/DETDA DGEBF/DICY
100/85 100/25 100/4
10/90 30/70 50/50 70/30
The test samples were cured at 150 °C for 10 h.
The test samples were cured at 150 °C for 10 h.
Table 2 Test epoxy samples of two and three-dimensional polymers Epoxy resin/hardener
Blend ratio (g)
D E F
DGEBA/BPA DGEBiphenyl/BPA Phenol novolac/BPA
100/60 100/60 100/60
15 295K LNT
Kq(MPam1/2)
Sample
The test samples were cured at 150 °C for 10 h.
especially at low temperatures. C has the lowest Tg and Kq. It can be concluded that the hardener changes the physical properties, even if the chemical structure of the main chain is identical. It suggests that the local dissipation affects the physical property of epoxy at cryogenic temperatures.
10
A B B A
5
0
C
C
0
5
10
15
Er(GPa)
4.2. Effect of network structure Fig. 3. Relationship between Kq and YoungÕs modulus Er.
The relationship between Er and Kq of the epoxies with different network structure is shown in Fig. 7. Samples D and E have two-dimensional and F has three-dimensional networks. The Er of two-dimensional network polymers is slightly smaller than that of threedimensional network polymer at room temperature. The Kq of D and E is almost equal to those at room temperature. But at liquid nitrogen temperature D and E show much higher Kq than that of F. D and E have phenoxy structures as shown in Fig. 2(3,4). The relationships between Kq and specific gravity, between Kq and Tg, and between Kq and thermal contraction are shown in Figs. 8–10 respectively. The specific gravity of twodimensional network polymer D and E is slightly smaller than that of three-dimensional network polymer F. The Tg and thermal contraction of D and E are almost same as those of F. The two-dimensional polymer sample D and E showed very high value of Kq at cryogenic
15
Kq(MPam1/2)
295K 10
B
A
5 C
0 1.10
1.15
1.20
1.25
1.30
1.35
Specific gravity Fig. 4. Relationship between Kq and specific gravity.
Table 3 Test epoxy samples of modified polymers Sample
Modifier
Flexural modulus Er (GPa)
Thermal contraction a (%) (100 °C ! 23 °C)
Thermal residual stress rr (MPa) (100 °C ! 23 °C)
B B1 B2 B3
Non PC/10 phr Phenoxy/10 phr CTBN/10 phr
5.29 5.98 5.83 5.60
0.48 0.47 0.49 0.57
2.5 2.8 2.9 3.2
The test samples were cured at 100 °C for 10 h.
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T. Ueki et al. / Cryogenics 45 (2005) 141–148
15
15
295K
Kq(MPam1/2)
Kq(MPam1/2)
LNT 10 B A 5
C
0 0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
10
D 5
0 1.10
1.3
F
E
1.15
Thermal contraction (RT-LNT) (%)
1.20 1.25 Specific gravity
1.30
1.35
Fig. 8. Relationship between Kq and specific gravity.
Fig. 5. Relationship between Kq and thermal contraction from RT to LNT.
15 15 D 295K LNT
Kq(MPam1/2)
Kq(MPam1/2)
295K LNT 10
B
5
A
E
10
D
5
F E
C
0 250
0 250 300
350
400
450
300
350
500
400
450
500
Tg(K)
Tg(K) Fig. 9. Relationship between Kq and Tg. Fig. 6. Relationship between Kq and Tg.
15
15
D
LNT
D
1/2
Kq(MPam )
Kq(MPam1/2)
295K LNT E
10
D 5
E
F
F
0
5
10
15
F
5
0 0.5
0
E
10
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
Thermal contraction (RT-LNT) (%)
Er(GPa) Fig. 7. Relationship between Kq and YoungÕs modulus Er.
Fig. 10. Relationship between Kq and thermal contraction from RT to LNT.
temperature. This can be explained by the difference of intermolecular forces. In two-dimensional polymer the intermolecular forces are van der Waals binding or
hydrogen bond and hence even at the cryogenic temperature the stress can be relaxed by special rearrangement of molecules or in other words the local dissipation
T. Ueki et al. / Cryogenics 45 (2005) 141–148
15
295K LNT 1/2
Kq(MPam )
mode. On the other hand the molecules of the threedimensional cross-linked polymer are bonded hardly by covalent bond and hence the stress relaxation is not easy at cryogenic temperature. The molecules of twodimensional polymer are folded to decrease the unoccupied space of molecular level. This may be the reason why the two-dimensional polymer does not show high thermal contraction. It is found that the network structure should be given priority than the chemical structure in improving the fracture toughness at cryogenic temperature.
147
10
5
0
4.3. Effect of morphology
0
20
40
60
80
100
Phenoxy(%) Fig. 12. Change of Kq with the amount two-dimensional polymer phenoxy.
1.26 1.25 1.24
Specific gravity
Fig. 11 shows the cracking index of 3D-GFRP having various epoxy matrixes. There are almost no cracks in the matrix of phenoxy resin such as sample D or the matrix modified by phenoxy as B2. It is thought that B2 has the semi-IPN structure. On the other hand it seems that B1 and B3, which were added thermoplastics such as PC and CTBN in B system, have the sea-island structure. In this research this structure does not improve but degrades the crack resistance. It was concluded that phenoxy resin has an excellent performance to improve the crack resistance as a modifier. The phenoxy resin is found to be promising to improve the fracture toughness at cryogenic temperature not only by the matrix but also by the modifier. Fig. 12 shows the change of Kq with the amount of phenoxy in DGEBF epoxy shown in Table 4. The Kq shows slight decrease at small amount of adding it and then gradually increases at room temperature. At liquid nitrogen temperature there is a great improvement of Kq. The samples, blend ratios of phenoxy/epoxy were 10/90 and 30/70, could not be measured Kq because they were broken by cooling down to LNT. Fig. 13 shows the change of specific gravity with the amount of phenoxy addition. The specific gravity decreases linearly with amount of phenoxy. Fig. 14 shows the change
1.23 1.22 1.21 1.20 1.19 1.18
0
20
40 60 Phenoxy(%)
80
100
Fig. 13. Change of specific gravity with the amount of phenoxy.
370 360 350
Tg(K)
10
330
8
Crack index
340
320 6
310 4
300
0
20
40
60
80
100
Phenoxy(%)
2
Fig. 14. Change of Tg with amount of phenoxy.
0
A
B
B1
B2
B3
D
Sample Fig. 11. Relationship between the crack index and the resin formulation of 3D-GFRP.
of Tg with amount of phenoxy. It seems that there is a change of morphology at the blend ratios of phenoxy/ epoxy from 10/90 to 30/70 because Kq and Tg shows decrease.
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5. Conclusion Several kinds of epoxy have been cured and were subjected to the evaluation test of physical properties, both at room and at cryogenic temperatures to establish the design methodology of epoxy resin, which can be used even at cryogenic temperatures. The chemical structure, network structure and the morphology were paid attention. It was found that the two-dimensional network structured epoxies (phenoxy) were promising because they showed high fracture toughness even at cryogenic temperatures. The phenoxy could be also applied as the modifier of the epoxy, which is to be used at cryogenic temperatures [8,9]. References [1] Sawa F, Nishijima S, Okada T. Molecular design of an epoxy for cryogenic temperatures. Cryogenics 1995;35:767–9.
[2] Moriyama H, Inoue Y, Mitsui H, Sanada Y, Kobayashi Y. Several properties of impregnating epoxy resin used for superconducting coils. Adv Cryog Eng 1992;38:339–46. [3] Sawa F, Nishijima S, Ohtani Y, Matsushita K, Okada T. Fracture toughness relaxation of epoxy resins at cryogenic temperature. Adv Cryog Eng (Mater) 1995;40:1113–9. [4] Yamamoto A, Ueki T, Mukai H, Hosaka S, Toda Y, Makida Y, Mine S, Makishima K. A composite electrical insulation in superconducting magnets. Adv Cryog Eng (Mater) 1998;44:239–44. [5] Nojima K, Ueki T, Asano K, Nishijima S, Okada T. Composite matrix for low thermal contraction down to cryogenic temperature. 2035–38. [6] Sekiya H, Moriyama H, Mitsui H, Nishijima S, Okada T. Effect of epoxy cracking resistance on the stability of impregnated superconducting solenoids. Cryogenics 1995;35:809–11. [7] Nishijima S, Takahata K, Okada T. Local temperature rise after quench due to epoxy cracking in impregnated windings. Adv Cryog Eng 1988;3:135–42. [8] Ueki T, Nojima K, Asano K, Nishijima S, Okada T. Improvement of fracture toughness of epoxy resin for cryogenic. 2061–64. [9] Ueki T, Nojima K, Asano K, Nishijima S, Okada T. Toughening of epoxy resin systems for cryogenic use. Adv Cryog Eng (Mater) 1998;44:277–83.
Designing of epoxy resin systems for cryogenic use T. Ueki a
a,*
, S. Nishijima b, Y. Izumi
b
Arisawa Manufacturing Co., Ltd, 1-Nakadahara, Niigata, Joetsu 943-8610, Japan b Osaka University, 2-1 Yamada-oka, Osaka, Suita 565-0871, Japan
Received 16 January 2004; received in revised form 4 July 2004; accepted 28 July 2004
Abstract The mechanical and thermal properties of several types of epoxy systems were designed based on the chemical structure, network structure and morphology aiming at cryogenic application. In this research di-epoxies or multifunctional epoxies were cured by several kinds of hardeners such as anhydride, amine or phenol and were blended with polycarbonate, carboxyl-terminated butadiene acrylonitrile copolymer or phenoxy. The mechanical properties and thermal properties of these cured epoxies were measured at room and liquid nitrogen temperature. It was found that the two-dimensional network structured linear polymer shows high performance even at cryogenic temperature. It was concluded that the controls of the structures are very important to optimize epoxy systems for cryogenic application. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Epoxy resin; Phenoxy; Linear polymer
1. Introduction Epoxy resins have been widely used as the matrix of composites because of their good electric insulating, mechanical, and easy fabricating properties. Additionally the composites have also been used for cryogenic applications. When the temperature is decreased down to cryogenic temperatures, the internal stress due to the thermal contraction is generated in a matrix resin. Fracture of the matrix is induced when the thermal stress induced stress intensity factor exceeds the fracture toughness of the resin. It is, therefore, important that the fracture toughness of the epoxy resin is improved even at cryogenic temperature. To attain the high performances of polymers, it is necessary to control the molecular structure, e.g. chemical and network structure. To control the morphology of the polymers is also
*
Corresponding author. Fax: +81 25 524 5281. E-mail address: [email protected] (T. Ueki).
0011-2275/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.cryogenics.2004.07.002
needed when the different types of polymers are blended. In epoxy resins there are many kinds of chemical unit such as diphenylol propane, diphenylol methane, tetrabromo diphenylol propane, phenol novolac, diamines, diacids, diols and so on. This difference brings the change of the polymer rigidity. The controls of crosslinking density and the epoxide equivalent by increasing the unit number of main chain cause the different network structure. The functionality of epoxy groups also causes the change of physical properties of epoxy resins. The different types of epoxy hardeners such as acid anhydride, amine and phenol also induce the change of performances of epoxy resins. By choosing the different types of epoxy and hardener the chemical structure can be designed in epoxy resin. It is known that the different network structures cause the different properties even if the polymers have the same chemical structure. The network structure can also be controlled by the chemical reaction. The epoxy resins are usually rigid and brittle. To improve the fracture toughness of epoxy the plasticizer is usually added. The morphology such as
142
T. Ueki et al. / Cryogenics 45 (2005) 141–148
the sea-island structure and interpenetrating network also determine the physical properties of epoxy. When the morphology is optimized, the modified toughened polymer could be obtained showing high crack-resistance without degradation of other properties. Changing the types and amount of modifier or the curing process can change the morphology. It is very important to clarify how to control the molecular structure to obtain high performance polymer effectively and economically at cryogenic temperatures [1–4]. In this research high performance polymer for cryogenic use has been developed and the results will be presented.
2. Experiment 2.1. Specimen Fig. 1 shows the structures of epoxy resin and hardener used in this work. The typical reactions are shown in Fig. 2.
three-dimensional polymer by cross-linking the multi functional epoxy resin of Phenol novolac with BPA. This reaction is shown in Fig. 2(5). 2.4. Control of morphology Table 3 shows these systems to examine the effects of morphology. The several types of modifier were chosen and added to the sample B. The blend ratio of the modifier was 10 phr, which means that 10 g of the modifier are added to 100 g of epoxy resin. First the modifiers were melted by heat homogeneously and were added to epoxy resin. The PC, phenoxy and CTBN added to epoxy resin are named B1, B2 and B3. The modifier phase is separated from epoxy phase due to the different solubility. Furthermore the two-dimensional phenoxy shown in Fig. 1(5) is added to the three-dimensional epoxy systems at the fraction of 10, 30, 50 and 70 wt %, which can be bought on commercial base, has melting point beyond 150 °C and is difficult to handle owing to very high viscosity even if molten state. The systems are shown in Table 4.
2.2. Control of chemical structure Table 1 shows the employed systems to control the chemical structure of epoxy resin. The DGEBA shown in Fig. 1(1) is the most frequently used epoxy resin, and the DGEBF shown in Fig. 1(2) has lower viscosity than DGEBA. Sample A was cured with anhydride MNA shown in Fig. 1(6), which composes the threedimension network with ester bond. This reaction is shown in Fig. 2(1). Sample B was cured with aromatic amine DETDA shown in Fig. 1(7), which composes three-dimensional network with hydrogen groups and epoxy groups. The reaction shown Fig. 2(2) yields the secondary alcohol and shows low viscosity and hence this system is used for vacuum impregnation. Sample C was cured with the amine DICY shown in Fig. 1(9), which is the most common hardener. These were used to study the effects of chemical structures at cryogenic temperature. 2.3. Control of network structure Table 2 shows the systems to control the network structures. The epoxy molecules cross-linked to form the various networks having various structures, which can be controlled by selecting the main chain and hardener. Samples D and E have the two-dimensional network showing both thermal resistance and high fracture toughness even at cryogenic temperatures. They consist of the high molecular weight liner polymers having more than 100-repeated unit. They are named generically phenoxy resin as same as the polymer shown in Fig. 1(5). These reactions are shown in Fig. 2(3,4). Sample F was cured not to form the two-dimensional but the
3. Measurement Thermal contraction was measured by the strain gauge attached on the sample block [5]. In Table 3 the thermal contraction from 100 °C to 23 °C of room temperature (RT), is also shown as noted. The thermal contraction down to liquid nitrogen temperature (LNT) from room temperature was measured. Glass transition temperature (Tg) was measured by the differential scanning calorimeter (DSC). Specific gravity was measured by the ArchimedesÕ method. Fracture toughness (Kq), elastic modulus (Er) and crack index were evaluated as the mechanical properties. The Kq and Er were measured by the three-point-bend method. Pre-cracks were introduced with a knife-edge. Crack index defined in this work corresponds to the crack density in the matrix resin of 3D-FRP at RT when the epoxy impregnated the three-dimensional glass fabric was cured at 100 °C for 10 h. The crack density was evaluated and was classified into 10 classes. For example Index 10 shows that there is no crack in the resin [6,7].
4. Result and discussion 4.1. Effect of chemical structure The relationship between Er and Kq of epoxy resins with different chemical structure is shown in Fig. 3. It shows that Er increases by approximately twice with decreasing temperature from room temperature to liquid nitrogen temperature while Kq increases slightly.
T. Ueki et al. / Cryogenics 45 (2005) 141–148
143
CH3 CH2 CH
C
CH2 O
O
CH2 CH
O
CH2
O
CH3
(1)
CH2
CH2 CH CH2 O
CH2 CH CH2
O
O
O
(2) CH3 CH2 CH
CH3
CH2 O
O
CH2 CH
O
CH2
O CH3
CH3
(3) CH2
CH2
CH2
CH2
O
O
CH2
CH2
CH2
O
O
CH2
CH2
CH2
O
O
CH2 n
(4)
CH3 R
O
CH3
C
O
C H2 C
CH3
CH2 O
C
O
R
CH3
OH n
(5)
O
CH3
C
NH2
O
NH2
C
CH3
C2H5
C2H5
O (6)
(7)
CH3 OH
C
OH
CH3 BPA
(8)
NH NH2 C
NH CN (9)
Fig. 1. Structures of the test epoxy resins and the hardeners: (1) DGEBA: diglycidylether of bisphenol A, (2) DGEBF: diglycidylether of bisphenol F, (3) DGEBiphenyl: diglycidylether of biphenyl, (4) Phenol novolac type epoxy, (5) Phenoxy resin, (6) MNA: methylnadic anhydride, (7) DETDA: diethyltoluene diamine, (8) BPA: bisphenol A, (9) DICY: dicyandiamide.
The relationship between specific gravity and Kq is shown in Fig. 4. The relationship between Kq and thermal contraction from room to liquid nitrogen temperature is shown in Fig. 5. The relationship between Tg and Kq is shown in Fig. 6. Samples A and C have almost
same values of specific gravity and thermal contraction. B has the lowest specific gravity but it shows the lowest thermal contraction. Usually it has been thought that the epoxy having higher specific gravity shows the lowest thermal contraction. It is not always correct
144
T. Ueki et al. / Cryogenics 45 (2005) 141–148
O
C
C O
(1)
R
C
H3C
H3C
C
OR O
O
MNA
C
C
OR
O
O
C
OH
OH
C
CH3
OH
O
O O
OH O CH2 CH
CH2 O
+
R
(2)
R
NH2
CH2 O
CH2 CH
CH2 O
N
amine
epoxy
CH2 CH
OH DGEBiphenyl
CH3
(3) R
+
BPA
CH3
O
CH3
O
C H2 C
CH3
CH2 O
O
R
OH CH3
CH3
DGEBA
+
BPA
CH3
(4) R
O
CH3
CH3
n
CH3
C
O
CH2 C
CH3
CH2 O
C
O
R
CH3
OH n
Phenol novolac
+
BPA
OH CH2
CH2
CH3 C
CH2 O
O
R
CH2
CH2
(5)
O
CH3
n O CH2
CH3 CH2 OH
CH2 O
C
O O
R OH
CH3 CH2
CH2
CH3 CH2
O
C
O
R
CH3
Fig. 2. Chemical reactions of epoxy resins with the hardeners: (1) Chemical reaction of epoxy with anhydride. (2) Chemical reaction of epoxy with amine. (3) Two-dimensional linear polymer of DGEBiphenyl with BPA. (4) Two-dimensional linear polymer of DGEBA with BPA. (5) Cross-linked three-dimensional polymer of multifunctional epoxy with BPA.
T. Ueki et al. / Cryogenics 45 (2005) 141–148 Table 1 Test epoxy samples of three-dimensional polymers
145
Table 4 Test epoxy samples of modified with phenoxy
Sample
Epoxy resin/hardener
Blend ratio (g)
Phenoxy/C epoxy resin Blend ratio (g)
A B C
DGEBF/MNA DGEBF/DETDA DGEBF/DICY
100/85 100/25 100/4
10/90 30/70 50/50 70/30
The test samples were cured at 150 °C for 10 h.
The test samples were cured at 150 °C for 10 h.
Table 2 Test epoxy samples of two and three-dimensional polymers Epoxy resin/hardener
Blend ratio (g)
D E F
DGEBA/BPA DGEBiphenyl/BPA Phenol novolac/BPA
100/60 100/60 100/60
15 295K LNT
Kq(MPam1/2)
Sample
The test samples were cured at 150 °C for 10 h.
especially at low temperatures. C has the lowest Tg and Kq. It can be concluded that the hardener changes the physical properties, even if the chemical structure of the main chain is identical. It suggests that the local dissipation affects the physical property of epoxy at cryogenic temperatures.
10
A B B A
5
0
C
C
0
5
10
15
Er(GPa)
4.2. Effect of network structure Fig. 3. Relationship between Kq and YoungÕs modulus Er.
The relationship between Er and Kq of the epoxies with different network structure is shown in Fig. 7. Samples D and E have two-dimensional and F has three-dimensional networks. The Er of two-dimensional network polymers is slightly smaller than that of threedimensional network polymer at room temperature. The Kq of D and E is almost equal to those at room temperature. But at liquid nitrogen temperature D and E show much higher Kq than that of F. D and E have phenoxy structures as shown in Fig. 2(3,4). The relationships between Kq and specific gravity, between Kq and Tg, and between Kq and thermal contraction are shown in Figs. 8–10 respectively. The specific gravity of twodimensional network polymer D and E is slightly smaller than that of three-dimensional network polymer F. The Tg and thermal contraction of D and E are almost same as those of F. The two-dimensional polymer sample D and E showed very high value of Kq at cryogenic
15
Kq(MPam1/2)
295K 10
B
A
5 C
0 1.10
1.15
1.20
1.25
1.30
1.35
Specific gravity Fig. 4. Relationship between Kq and specific gravity.
Table 3 Test epoxy samples of modified polymers Sample
Modifier
Flexural modulus Er (GPa)
Thermal contraction a (%) (100 °C ! 23 °C)
Thermal residual stress rr (MPa) (100 °C ! 23 °C)
B B1 B2 B3
Non PC/10 phr Phenoxy/10 phr CTBN/10 phr
5.29 5.98 5.83 5.60
0.48 0.47 0.49 0.57
2.5 2.8 2.9 3.2
The test samples were cured at 100 °C for 10 h.
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T. Ueki et al. / Cryogenics 45 (2005) 141–148
15
15
295K
Kq(MPam1/2)
Kq(MPam1/2)
LNT 10 B A 5
C
0 0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
10
D 5
0 1.10
1.3
F
E
1.15
Thermal contraction (RT-LNT) (%)
1.20 1.25 Specific gravity
1.30
1.35
Fig. 8. Relationship between Kq and specific gravity.
Fig. 5. Relationship between Kq and thermal contraction from RT to LNT.
15 15 D 295K LNT
Kq(MPam1/2)
Kq(MPam1/2)
295K LNT 10
B
5
A
E
10
D
5
F E
C
0 250
0 250 300
350
400
450
300
350
500
400
450
500
Tg(K)
Tg(K) Fig. 9. Relationship between Kq and Tg. Fig. 6. Relationship between Kq and Tg.
15
15
D
LNT
D
1/2
Kq(MPam )
Kq(MPam1/2)
295K LNT E
10
D 5
E
F
F
0
5
10
15
F
5
0 0.5
0
E
10
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
Thermal contraction (RT-LNT) (%)
Er(GPa) Fig. 7. Relationship between Kq and YoungÕs modulus Er.
Fig. 10. Relationship between Kq and thermal contraction from RT to LNT.
temperature. This can be explained by the difference of intermolecular forces. In two-dimensional polymer the intermolecular forces are van der Waals binding or
hydrogen bond and hence even at the cryogenic temperature the stress can be relaxed by special rearrangement of molecules or in other words the local dissipation
T. Ueki et al. / Cryogenics 45 (2005) 141–148
15
295K LNT 1/2
Kq(MPam )
mode. On the other hand the molecules of the threedimensional cross-linked polymer are bonded hardly by covalent bond and hence the stress relaxation is not easy at cryogenic temperature. The molecules of twodimensional polymer are folded to decrease the unoccupied space of molecular level. This may be the reason why the two-dimensional polymer does not show high thermal contraction. It is found that the network structure should be given priority than the chemical structure in improving the fracture toughness at cryogenic temperature.
147
10
5
0
4.3. Effect of morphology
0
20
40
60
80
100
Phenoxy(%) Fig. 12. Change of Kq with the amount two-dimensional polymer phenoxy.
1.26 1.25 1.24
Specific gravity
Fig. 11 shows the cracking index of 3D-GFRP having various epoxy matrixes. There are almost no cracks in the matrix of phenoxy resin such as sample D or the matrix modified by phenoxy as B2. It is thought that B2 has the semi-IPN structure. On the other hand it seems that B1 and B3, which were added thermoplastics such as PC and CTBN in B system, have the sea-island structure. In this research this structure does not improve but degrades the crack resistance. It was concluded that phenoxy resin has an excellent performance to improve the crack resistance as a modifier. The phenoxy resin is found to be promising to improve the fracture toughness at cryogenic temperature not only by the matrix but also by the modifier. Fig. 12 shows the change of Kq with the amount of phenoxy in DGEBF epoxy shown in Table 4. The Kq shows slight decrease at small amount of adding it and then gradually increases at room temperature. At liquid nitrogen temperature there is a great improvement of Kq. The samples, blend ratios of phenoxy/epoxy were 10/90 and 30/70, could not be measured Kq because they were broken by cooling down to LNT. Fig. 13 shows the change of specific gravity with the amount of phenoxy addition. The specific gravity decreases linearly with amount of phenoxy. Fig. 14 shows the change
1.23 1.22 1.21 1.20 1.19 1.18
0
20
40 60 Phenoxy(%)
80
100
Fig. 13. Change of specific gravity with the amount of phenoxy.
370 360 350
Tg(K)
10
330
8
Crack index
340
320 6
310 4
300
0
20
40
60
80
100
Phenoxy(%)
2
Fig. 14. Change of Tg with amount of phenoxy.
0
A
B
B1
B2
B3
D
Sample Fig. 11. Relationship between the crack index and the resin formulation of 3D-GFRP.
of Tg with amount of phenoxy. It seems that there is a change of morphology at the blend ratios of phenoxy/ epoxy from 10/90 to 30/70 because Kq and Tg shows decrease.
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T. Ueki et al. / Cryogenics 45 (2005) 141–148
5. Conclusion Several kinds of epoxy have been cured and were subjected to the evaluation test of physical properties, both at room and at cryogenic temperatures to establish the design methodology of epoxy resin, which can be used even at cryogenic temperatures. The chemical structure, network structure and the morphology were paid attention. It was found that the two-dimensional network structured epoxies (phenoxy) were promising because they showed high fracture toughness even at cryogenic temperatures. The phenoxy could be also applied as the modifier of the epoxy, which is to be used at cryogenic temperatures [8,9]. References [1] Sawa F, Nishijima S, Okada T. Molecular design of an epoxy for cryogenic temperatures. Cryogenics 1995;35:767–9.
[2] Moriyama H, Inoue Y, Mitsui H, Sanada Y, Kobayashi Y. Several properties of impregnating epoxy resin used for superconducting coils. Adv Cryog Eng 1992;38:339–46. [3] Sawa F, Nishijima S, Ohtani Y, Matsushita K, Okada T. Fracture toughness relaxation of epoxy resins at cryogenic temperature. Adv Cryog Eng (Mater) 1995;40:1113–9. [4] Yamamoto A, Ueki T, Mukai H, Hosaka S, Toda Y, Makida Y, Mine S, Makishima K. A composite electrical insulation in superconducting magnets. Adv Cryog Eng (Mater) 1998;44:239–44. [5] Nojima K, Ueki T, Asano K, Nishijima S, Okada T. Composite matrix for low thermal contraction down to cryogenic temperature. 2035–38. [6] Sekiya H, Moriyama H, Mitsui H, Nishijima S, Okada T. Effect of epoxy cracking resistance on the stability of impregnated superconducting solenoids. Cryogenics 1995;35:809–11. [7] Nishijima S, Takahata K, Okada T. Local temperature rise after quench due to epoxy cracking in impregnated windings. Adv Cryog Eng 1988;3:135–42. [8] Ueki T, Nojima K, Asano K, Nishijima S, Okada T. Improvement of fracture toughness of epoxy resin for cryogenic. 2061–64. [9] Ueki T, Nojima K, Asano K, Nishijima S, Okada T. Toughening of epoxy resin systems for cryogenic use. Adv Cryog Eng (Mater) 1998;44:277–83.