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Radiation effects on aging behaviour of oligobutadiene-base urethane polymer
Radiat.
Pergamoo O!m-m6x(95)oo44s3
Phys. Chem. Vol. 48, No. 3, pp. 333-335, 1996 Copyright Q 1596 Elsevier Science Ltd Prinkd in Great Britain. All rights reserved 0969-806X/96 $15.00 + 0.00
RADIATION EFFECTS ON AGING BEHAVIOUR OF OLIGOBUTADIENE-BASE URETHANE POLYMER V. G. DEDGAONKAR,’ ‘Department of Chemistry, ‘High Energy Materials
P. B. NAVLE’ and P. G. SHROTRI’ Pune 41 I 007, India and Pune 41 I 021, India
University of Pune, Research Laboratory,
(Received
3 October
1995,)
Abstract-The effect of radiation curing (5 kCi @‘Co)on the aging behaviour of hydroxyl terminated polybutadiene-base propellant-binder system is studied. The elastomeric products were investigated for their mechanical properties, such as tensile strength, elongation, etc. After 2 years of aging the values of these properties did not alter significantly and remained better than for the aged samples which were cured thermally (65°C). Binders prepared using HTPB synthesized by different suppliers indicated distinct differences between them after aging. Copyright c 1996 Elsevier Science Ltd.
INTRODUCI’ION
eventually transforming it into cracked grains. The present work deals with the effect on the aging process when radiation treatment is given to the samples during the preparation of the hydroxyl terminated polybutadiene-based propellant-binder (HTPB).
During the aging period, and particularly while being exposed to a varying environment, the mechanical and ballistic properties and the bonding between propellant and liner are expected to remain within narrowly specified limits. Hence information about the behaviour of a propellant as it undergoes aging (Christiansen ef al., 1981) is vital. Accelerated aging tests (Martinez-Pardo and Vera-Graziano, 1995) are therefore conducted, usually by means of elevating the temperature, and the data are extrapolated following different empirical methods to evaluate life
FXPERMENTAL
Samples of HTPB pre-polymer (S,, S, and S,) supplied by three different manufacturers were used. The general formula of HTPB is
H~2~~:xJj~x~H2~H I ,2-vinyl
1,4-cis
expectancy. An inherent disadvantage of such tests is that degradative reactions occurring (So and Chen, 1992) at higher temperatures are not necessarily the same that may occur under the conditions the propellant is likely to encounter. In the case of polybutadiene-base propellant binders the carbonarbon unsaturation is reported (Oberth, 1987) to cause elimination of hydrogen atom adjacent to carbon thus yielding the allylic radical. This allylic radical may undergo further oxidation and/or may crosslink to an adjacent chain during the aging process. Even at ordinary temperatures, the chemical reactions that proceed slowly cause the propellant to harden (Myers, 1975),
1,4-trans
Typical characteristics of HTPB are summarized (Deuskar et al., 1992) in Table 1. Its curing was accomplished by means of gamma irradiation (Dedgaonkar et al., 1993a, 1994a,b) and in the presence of toluene diisocyanate. This TDI was a mixture of 80% of 2,4- and 20% of 2,6-isomers: CH3 I
333
NC0
OCN
CH3 I
NC0
V. G. Dedgaonkar ef al.
334 Tabk
1. Characteristics (Deuskar et al., 1992) and radiation dose for HTPB obtained from different sources
curing
HTPB samples Characteristics Hydroxyl value (mg KOH) Number of average molecular weight by VP0 method Microsrruclure Content (%) 1,4-trans I ,4-L+ I ,4-vinyl Dose (kGy) required for complete consumption of NC0
s.4
s,
SC
39 3060
40 3000
39 3030
63 17 21 360
64 16 20 240
55 16 29 240
Crosslinking was introduced into the polymer matrix by irradiating the HT’F’B-TDI mixture (NCO/OH ratio = 1) at the curing stage itself. For this a @‘Cosource of strength 5 kCi was employed. The details of the irradiation procedure are reported elsewhere (Dedgaonkar et al., 1993b). The cured samples were stored at ambient temperatures over a period of 2 years and evaluated for aging. Their mechanical properties such as, ultimate tensile stress, tensile strain, cb, initial Young’s 6 b, ultimate modulus, Ei, and energy at break, U,, were investigated.
-&_ 0
I
I
I
240
480
720
Radiation dose (kGy) Fig. 2. Modulus and strain data after aging (2 years) for :ontainine HTPB obtained from different sources S,, S, and S,.
1 0 t
I 240
I 480
I 720
Radiation dose (kGy)
Thermal treatment (6s’C)
Fig. I. Influence of radiation/thermal treatment on the HTPB-l-D1 system mechanical properties of (NCO/OH = 1). V, v fre$sFles; A, A aged (2 years)
0
480
720
Radiation dose (kGy) Fig. 3. Energy and stress data after aging (2 years) for binders containing HTPB obtained from different sources S,, S, and SC.
Radiation effects on aging behaviour of polyurethanes
335
Table 2. Mechanical properties after aging (2 years) of the HTPETDI system cured by radiation (120 kGy),!themxal treatment (65°C). Binder contained HTPB (S,, S, and S,) obtained from different sources System TEatmeIlt E, (MPa) fh (%) fY,‘&iPa) u* (J)
RESULTS
SA Radiation 0.33 120 0.20
0.11
SB Hut
SC
Radiation
Heat
0.56 136 0.35 0.21
0.70 83 0.32 0.13
0.40 9x 0.17 0.12
AND DISCUSSION
Samples (S,) of elastomers containing HTPETDI with NCOjOH ratio at unity, which werecured by both
radiation and thermal treatments and their mechanical properties determined after storage (2 y) at room temperature, are presented in Fig. 1. This figure also includes the data obtained for the non-aged samples for comparison, One notices that at any radiation dose all the properties namely, a,, Q,, E, and U,, remain practically unaltered even after storing. These results indicate that aging reactions (Trong-Mind et al., 199 l), if occurring in the irradiated samples, are slow and insignificant in producing any physico-chemical change responsible for influencing the mechanical behaviour of the system. The insignificance of the time parameter may be attributed to radiation-induced crosslinking of easily approachable allyhc radical sites. Absence of such sites results in the disappearance of further crosslinking reactions that normally occur after keeping. Irradiation at moderate levels (c 180 kGy) of high-density polyethylene (Dongyuan et al., 1987) also did not show any significant change in the mechanical properties within the investigated period of 270 days. In fact at lower doses (< 120 kGy) the present samples after aging showed mechanical properties slightly higher compared to those samples cured thermally at 65 f 5°C (Table 2). The properties of aged samples of radiation-crosslinked silicone rubber compounds were also reported (Vokal et al., 1986) to be comparable or slightly better than those of the chemically cured samples. When the HTFS-TDI systems containing S,, S, and Sc were exposed to radiation, the curing doses required for complete consumption of isocyanate were 360, 240 and 240 kGy, respectively. In spite of the curing dose variation, the mechanical properties of these aged samples exhibited identical trends (Figs 2 and 3), namely as irradiation progresses: (i) a6 and E, increasing and (ii) cb and U, decreasing. Further, systems containing SC showed higher values of EI at all doses and an abrupt rise above 600 kGy. Here, as expected, the trends in Lowere exactly opposite. The higher E, values indicate the occurrence of higher order crosslinking among polybutadiene segments. The excessive crosslinkages are in all probability due to the higher content of 1,Zvinyl double bonds (see Table 1) which are more reactive (O’Donnell and Whittaker, 1992). The abrupt rise in Ei is, however, difficult to explain.
Radiation 0.73 71 0.33 0.14
Heat 0.67 76 0.33 0.1 I
Acknowledgement-P.B.N. is grateful to Dr Haridwar Singh, Director, H.E.M.R.L., Pune, for his timely help and encourdgement.
REFERENCES
Christiansen A. G., Layton L. H. and Carpenter R. L. (1981) HTPB nronellant a&a. J. S~acecrofi 18, 3. Dedgaonkar V. G., ‘Navle P.-B.-and Shrotri P. G..(1993a) Diol-diisocyanate polymerisation by gamma irradiation. J. Radioanal. Nucl. Chem. Lett. 176, 77.
Dedgaonkar V. G., Navle P. B. and Shrotri P. G. (1993b) Properties of hydroxyl-terminated polybutadiene-base elastomers under the influence of y-radiation. J. Radioanal. Nucl. Chem. L.etr. 176, 153. Dedgaonkar V. G., Navle P. B. and Shrotri P. G. (1994a) Influence of gamma radiation on the crosslinking properties of oligobutadiene-base polyurethane elastomers. In Symposium on Radiation and Photochemistry, Vol. 2, p. 17. BARC, Bombay. Dedgaonkar V. G., Navle P. B. and Shrotri P. G. (l994b) Polymerisation of HTPB-base binder by gamma radiation. In 7th National Seminar on High Energy Marerials, p. 53. VSSC, Thiruvananthapuram --_ Deuskar V. D.. Nazare A. N.. Ghorpade V. G., Deshmukh S. M. and Murthy G. D. (1992) influence of characteristics of HTPB on mechanical properties and ballistic behaviour of composite propellant. In Colloquium on HTPB, p. 205. VSSC, Thiruvananthapuram. Dongyudn L., Lianshui Z., Yagi W. and Wenxiu C. (1987) The effect of radiation crosslinking on the mechanical properties of polyethylene sheets. Radio?. Phys. Chem. 29, 175.
Martinez-Pardo M. E. and Vera-Graziano R. (1995) Gamma radiation induced crosslinking of polyethylene/ethylene-vinylacetate blends. Radiar. Phys. Chem. 45, 93.
Myers G. E. (1975) Chemical structural aging effects. Gout Rep. Announce (U.S.) 75, 162. Oberth A. E. (1987) Principles of solid propellant develop ment. CPIA Publication 469. O’Donnell J. H. and Whittaker A. K. (1992) A solid-state “C-NMR study of crosslinking in polybutadiene by y-radiation: effect of microstructure and dose. J. Polymer bci. Part A, Polymer Chem. 30, 185. So H. and Chen U. D. (1992) The effects of temuerature aging and moisture’ on some mechanical behaviours of particulate-filled rubber. J. Polymer Engng 11, 245. Trong-Mind D., Wen-Yen C. and Kuo-Huang H. (1991) The thermal aging of filled polyurethane. J. Appl. Polymer. Sci. 43, 2193.
Vokal A., Kourim P., Sussmilchova J., Heidingsfeldova M. and Kopecky B. (1986) Comparison of thermal and radiation curing of silicone rubber. Radial. Phys. Chem. 28, 497.
Pergamoo O!m-m6x(95)oo44s3
Phys. Chem. Vol. 48, No. 3, pp. 333-335, 1996 Copyright Q 1596 Elsevier Science Ltd Prinkd in Great Britain. All rights reserved 0969-806X/96 $15.00 + 0.00
RADIATION EFFECTS ON AGING BEHAVIOUR OF OLIGOBUTADIENE-BASE URETHANE POLYMER V. G. DEDGAONKAR,’ ‘Department of Chemistry, ‘High Energy Materials
P. B. NAVLE’ and P. G. SHROTRI’ Pune 41 I 007, India and Pune 41 I 021, India
University of Pune, Research Laboratory,
(Received
3 October
1995,)
Abstract-The effect of radiation curing (5 kCi @‘Co)on the aging behaviour of hydroxyl terminated polybutadiene-base propellant-binder system is studied. The elastomeric products were investigated for their mechanical properties, such as tensile strength, elongation, etc. After 2 years of aging the values of these properties did not alter significantly and remained better than for the aged samples which were cured thermally (65°C). Binders prepared using HTPB synthesized by different suppliers indicated distinct differences between them after aging. Copyright c 1996 Elsevier Science Ltd.
INTRODUCI’ION
eventually transforming it into cracked grains. The present work deals with the effect on the aging process when radiation treatment is given to the samples during the preparation of the hydroxyl terminated polybutadiene-based propellant-binder (HTPB).
During the aging period, and particularly while being exposed to a varying environment, the mechanical and ballistic properties and the bonding between propellant and liner are expected to remain within narrowly specified limits. Hence information about the behaviour of a propellant as it undergoes aging (Christiansen ef al., 1981) is vital. Accelerated aging tests (Martinez-Pardo and Vera-Graziano, 1995) are therefore conducted, usually by means of elevating the temperature, and the data are extrapolated following different empirical methods to evaluate life
FXPERMENTAL
Samples of HTPB pre-polymer (S,, S, and S,) supplied by three different manufacturers were used. The general formula of HTPB is
H~2~~:xJj~x~H2~H I ,2-vinyl
1,4-cis
expectancy. An inherent disadvantage of such tests is that degradative reactions occurring (So and Chen, 1992) at higher temperatures are not necessarily the same that may occur under the conditions the propellant is likely to encounter. In the case of polybutadiene-base propellant binders the carbonarbon unsaturation is reported (Oberth, 1987) to cause elimination of hydrogen atom adjacent to carbon thus yielding the allylic radical. This allylic radical may undergo further oxidation and/or may crosslink to an adjacent chain during the aging process. Even at ordinary temperatures, the chemical reactions that proceed slowly cause the propellant to harden (Myers, 1975),
1,4-trans
Typical characteristics of HTPB are summarized (Deuskar et al., 1992) in Table 1. Its curing was accomplished by means of gamma irradiation (Dedgaonkar et al., 1993a, 1994a,b) and in the presence of toluene diisocyanate. This TDI was a mixture of 80% of 2,4- and 20% of 2,6-isomers: CH3 I
333
NC0
OCN
CH3 I
NC0
V. G. Dedgaonkar ef al.
334 Tabk
1. Characteristics (Deuskar et al., 1992) and radiation dose for HTPB obtained from different sources
curing
HTPB samples Characteristics Hydroxyl value (mg KOH) Number of average molecular weight by VP0 method Microsrruclure Content (%) 1,4-trans I ,4-L+ I ,4-vinyl Dose (kGy) required for complete consumption of NC0
s.4
s,
SC
39 3060
40 3000
39 3030
63 17 21 360
64 16 20 240
55 16 29 240
Crosslinking was introduced into the polymer matrix by irradiating the HT’F’B-TDI mixture (NCO/OH ratio = 1) at the curing stage itself. For this a @‘Cosource of strength 5 kCi was employed. The details of the irradiation procedure are reported elsewhere (Dedgaonkar et al., 1993b). The cured samples were stored at ambient temperatures over a period of 2 years and evaluated for aging. Their mechanical properties such as, ultimate tensile stress, tensile strain, cb, initial Young’s 6 b, ultimate modulus, Ei, and energy at break, U,, were investigated.
-&_ 0
I
I
I
240
480
720
Radiation dose (kGy) Fig. 2. Modulus and strain data after aging (2 years) for :ontainine HTPB obtained from different sources S,, S, and S,.
1 0 t
I 240
I 480
I 720
Radiation dose (kGy)
Thermal treatment (6s’C)
Fig. I. Influence of radiation/thermal treatment on the HTPB-l-D1 system mechanical properties of (NCO/OH = 1). V, v fre$sFles; A, A aged (2 years)
0
480
720
Radiation dose (kGy) Fig. 3. Energy and stress data after aging (2 years) for binders containing HTPB obtained from different sources S,, S, and SC.
Radiation effects on aging behaviour of polyurethanes
335
Table 2. Mechanical properties after aging (2 years) of the HTPETDI system cured by radiation (120 kGy),!themxal treatment (65°C). Binder contained HTPB (S,, S, and S,) obtained from different sources System TEatmeIlt E, (MPa) fh (%) fY,‘&iPa) u* (J)
RESULTS
SA Radiation 0.33 120 0.20
0.11
SB Hut
SC
Radiation
Heat
0.56 136 0.35 0.21
0.70 83 0.32 0.13
0.40 9x 0.17 0.12
AND DISCUSSION
Samples (S,) of elastomers containing HTPETDI with NCOjOH ratio at unity, which werecured by both
radiation and thermal treatments and their mechanical properties determined after storage (2 y) at room temperature, are presented in Fig. 1. This figure also includes the data obtained for the non-aged samples for comparison, One notices that at any radiation dose all the properties namely, a,, Q,, E, and U,, remain practically unaltered even after storing. These results indicate that aging reactions (Trong-Mind et al., 199 l), if occurring in the irradiated samples, are slow and insignificant in producing any physico-chemical change responsible for influencing the mechanical behaviour of the system. The insignificance of the time parameter may be attributed to radiation-induced crosslinking of easily approachable allyhc radical sites. Absence of such sites results in the disappearance of further crosslinking reactions that normally occur after keeping. Irradiation at moderate levels (c 180 kGy) of high-density polyethylene (Dongyuan et al., 1987) also did not show any significant change in the mechanical properties within the investigated period of 270 days. In fact at lower doses (< 120 kGy) the present samples after aging showed mechanical properties slightly higher compared to those samples cured thermally at 65 f 5°C (Table 2). The properties of aged samples of radiation-crosslinked silicone rubber compounds were also reported (Vokal et al., 1986) to be comparable or slightly better than those of the chemically cured samples. When the HTFS-TDI systems containing S,, S, and Sc were exposed to radiation, the curing doses required for complete consumption of isocyanate were 360, 240 and 240 kGy, respectively. In spite of the curing dose variation, the mechanical properties of these aged samples exhibited identical trends (Figs 2 and 3), namely as irradiation progresses: (i) a6 and E, increasing and (ii) cb and U, decreasing. Further, systems containing SC showed higher values of EI at all doses and an abrupt rise above 600 kGy. Here, as expected, the trends in Lowere exactly opposite. The higher E, values indicate the occurrence of higher order crosslinking among polybutadiene segments. The excessive crosslinkages are in all probability due to the higher content of 1,Zvinyl double bonds (see Table 1) which are more reactive (O’Donnell and Whittaker, 1992). The abrupt rise in Ei is, however, difficult to explain.
Radiation 0.73 71 0.33 0.14
Heat 0.67 76 0.33 0.1 I
Acknowledgement-P.B.N. is grateful to Dr Haridwar Singh, Director, H.E.M.R.L., Pune, for his timely help and encourdgement.
REFERENCES
Christiansen A. G., Layton L. H. and Carpenter R. L. (1981) HTPB nronellant a&a. J. S~acecrofi 18, 3. Dedgaonkar V. G., ‘Navle P.-B.-and Shrotri P. G..(1993a) Diol-diisocyanate polymerisation by gamma irradiation. J. Radioanal. Nucl. Chem. Lett. 176, 77.
Dedgaonkar V. G., Navle P. B. and Shrotri P. G. (1993b) Properties of hydroxyl-terminated polybutadiene-base elastomers under the influence of y-radiation. J. Radioanal. Nucl. Chem. L.etr. 176, 153. Dedgaonkar V. G., Navle P. B. and Shrotri P. G. (1994a) Influence of gamma radiation on the crosslinking properties of oligobutadiene-base polyurethane elastomers. In Symposium on Radiation and Photochemistry, Vol. 2, p. 17. BARC, Bombay. Dedgaonkar V. G., Navle P. B. and Shrotri P. G. (l994b) Polymerisation of HTPB-base binder by gamma radiation. In 7th National Seminar on High Energy Marerials, p. 53. VSSC, Thiruvananthapuram --_ Deuskar V. D.. Nazare A. N.. Ghorpade V. G., Deshmukh S. M. and Murthy G. D. (1992) influence of characteristics of HTPB on mechanical properties and ballistic behaviour of composite propellant. In Colloquium on HTPB, p. 205. VSSC, Thiruvananthapuram. Dongyudn L., Lianshui Z., Yagi W. and Wenxiu C. (1987) The effect of radiation crosslinking on the mechanical properties of polyethylene sheets. Radio?. Phys. Chem. 29, 175.
Martinez-Pardo M. E. and Vera-Graziano R. (1995) Gamma radiation induced crosslinking of polyethylene/ethylene-vinylacetate blends. Radiar. Phys. Chem. 45, 93.
Myers G. E. (1975) Chemical structural aging effects. Gout Rep. Announce (U.S.) 75, 162. Oberth A. E. (1987) Principles of solid propellant develop ment. CPIA Publication 469. O’Donnell J. H. and Whittaker A. K. (1992) A solid-state “C-NMR study of crosslinking in polybutadiene by y-radiation: effect of microstructure and dose. J. Polymer bci. Part A, Polymer Chem. 30, 185. So H. and Chen U. D. (1992) The effects of temuerature aging and moisture’ on some mechanical behaviours of particulate-filled rubber. J. Polymer Engng 11, 245. Trong-Mind D., Wen-Yen C. and Kuo-Huang H. (1991) The thermal aging of filled polyurethane. J. Appl. Polymer. Sci. 43, 2193.
Vokal A., Kourim P., Sussmilchova J., Heidingsfeldova M. and Kopecky B. (1986) Comparison of thermal and radiation curing of silicone rubber. Radial. Phys. Chem. 28, 497.