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The effect of radiation cross-linking on the mechanical properties of polyethylene sheets
P_,,~m. Phys. Chem. Vol. 29, No. 3, pp. 175-177, 1987 Int. J. Ra~t. Appl. instrum. Part C
0146-5724/87 $3.00+ 0.00 Copyright ~ 1987Petl~,tmonJournals Lid
Printed in Great Britain. All fights rmerved
THE EFFECT OF RADIATION CROSS-LINKING ON THE MECHANICAL PROPERTIES OF POLYETHYLENE SHEETS? LIU DONGYUAN,ZHANG LIANSHUI,WANO YAQ! and CHL~ WENYaU Department of Chemistry, Beijing Normal University, lkijin8, China (Received 29 September 1985)
AlmrEt--The high density polyethylene (HDPE) sheets thickness 3 mm were irradiated by S°Co y-ray. Its gel fraction increases with increasing dosage. The melt index of irradiated samples decreases sLt,nifimmtly with dosage from i kGy to 12 kGy and the number average molecular weight of samples incr~__~ linearly. The mechanical properties including yield strength, breaking strength and breaking elongation percentage of irradiated HDPE sheets increase at low doses and then decrease at high doses. All of them have a peak value at 180 kGy. The Young's modulus of samples also increases linearly with increasing dosage. The aging tests of samples in irradiation dines of 180 and 360 kGy were carried out respectively and compared with non-irradiated samples.
INTI~ODUCTION
gamma source setting in The Institute of Biophysics, Academia Sinica. The dose rate was 0.40 kGy/min measured by a ceric-cerour dosimeter. The aging tests were made at room temperature. Gel fraction m e a s ~ t s were refluxed with xylene for 24 h. (" Melt indexes were measured on a XRZ400-1 Melt Index Detector made in the Mechanical Factory in Jifin University, at a temperature of 190±0.5°C, load 21608. Crystallinitics were measured by a Perkin Elmer 180 Infrared Absorption Spectrometer. According to GBI040-79 ~s) mechanical properties measurements were carried out on WD-I Extension Tester, made by Changehung Non-metallic Tester Factory, Stretching speed 30 ram/rain.
The radiation cross-linking effects of polyethylene have been studied extensively by many investigators. Polyethylene products which were cross-linked, such as electric wire and cable, heat shrinkable connectors and packaginge, and foamed plastics, have had wide applications. A part of these products including low density polyethylene (LDPE) and high density polyethylene (HDPE) products which were manufactured by radiation processing have become commercialized.(I-3) Recently there has been much attention to reduce the density of HDPE and increase its molecular weight or flavability, thus improving its mechanical properties so that its practical application would expand. (*,5) Du Plessis et al. have irradiated HDPE sheets with gas additive and used them in orthopaedic operations. (') There is an improvement on the mechanical properties of HDPE sheets in this paper. The primary experimental result is reported. The relationship between the gel fraction, the melt index and molecular weight with dosage were investigated and the mechanical properties of HDPE as well as the aging tests have been done.
I E S U L T S AND DISCUSSION
1. Gel fraction--dosage relationship
The gel fraction of HDPE sheets increases with an increase in dosage. The relation between gel fraction of HDPE sheets and absorbed dose is shown in Fig. 1. Within 25-500 kGy, the gel fraction of irradiated HDPE sheets increases with an increase in dosage. Below 100kGy, gel fraction of irradiated samples increases quickly, then stays almost constant above 100 kGy.
EXPERIMENTAL
The HDPE sheets were produced by ikijing Ninth Plastic Factory, thickness 3 nun, density 0.942 g/cm 3, melt index 1.70 g/10 rain. The samples were irradiated in air atmosphere at room temperature with 6°Co ~'This paper was presented at the 2nd Japan--China Bilateral Symposium on Radiation Chemistry, Osaka, Japan (September 1985). 175 II.,p.C.29/~.--A
2. Melt index-dosage relationship
The melt index of irradiated HDPE sheets decreases significantly with increasing of dosage. The relation between melt index of irradiated HDPE sheets and absorbed dose is shown in Fig. 2. The melt index of irradiated HDPE sheets decreases significantly with dosage from 1 to 12 kGy. At
176
LIU DONOYUANet al. 100
--
240
80--
220
60
200
Eu
.,¢ v
¢.~ 40
240
~
220
~
oJ 200 E u ~.
"~ 180 .¢:
1 80 ,¢P ¢¢
,~
160
460
"~ 140 )"
140
120
120
L.
20
I 10
0
I 20 Dole
I 30
I 40
I 50
OB
(10 kGy)
Fig. 1. Relation between the gel fraction o f H D P E sheets
and absorbed dose. 1oo
12 kGy, the value of the melt index is one-tenth that of unirradiated samples. These experimental results indicate that the molecular weight of HDPE sheets increases with increasing dosage. On polyethylene, the approximate equation of the relation between number average molecular weight and melt index is as follows~9) (~1~/2 = 188 -
30 log (M.I.)
(1)
where ~14~,is number average molecular weight, and M.I. is the melt index of polyethylene. The values of number average molecular weight of HDPE sheets were obtained from the melt index using the empirical equation (1). The JtT, values were shown in Fig. 2, too. Dosage ranges from 1 to 29 kGy, and the relation between bT, and absorbed dose have a linearity. An /~. value with dose of 29 kGy is twice that of unirradiated samples. 3. The effect of radiation cross-linking on the mechanical properties of H D P E sheets The influences of irradiation on the mechanical properties of polymers were different depending on the polymer cross-links or degradations. Radiation induced degradation always makes mechanical 2.0
7O,000
1.6
60,000
I 10
I 20
I 40
1 oo 50
Dose ( 1 0 k G y )
Fig. 3. Influence of absorbed dose on both the yield strength and the breaking strength of HDPE sheets. properties in polymers worse. On the other hand, radiation cross-linking sometimes may improve the mechanical properties of polymers. After high dose irradiation, different kinds of polymers change and become brittle. °°) Influences of irradiation on both yield strength and breaking strength of HDPE sheets are shown in Fig. 3. Figure 3 indicated that yield strength and breaking strength increase at low doses and then decrease at high doses. Each of them has a peak value at 180 kGy. Influences of irradiation on both breaking elongation percentage and Young's modulus of HDPE sheets are shown in Fig. 4. The shape of the curve of breaking elongation percentage is similar to Fig. 3, and also similar to work of Takeda. (') Young's modulus increases linearly with dosage.
.500
60
N
70
5 ~
~/~//
60
g
c
I 30
1400i
m
o"r= 1.2
50,000
ex t-i 0.8 :E
40,0001~=
0.4
0
3O,OO0
05
t,0 Dose
, 1.5 2.0 (10 kGy)
2.5
".,o,OOo 3.0
Fig. 2. Influence of absorbed dose on both M.I. and ~ . of HDPE sheets.
0 50 E le) OD C = 40 0 >30
I
0
10
I
I
20 30 Dose (10 kGyl
I
4.0
50
,.5o
Fig. 4. Influence of absorbed dose on both the breaking elongation and Young's modulus of HDPE sheets.
Radiation effect on HDPE sheets 700 -
Table I. The crystallinityof HDPEsheets in variousradiation dines Dose (kGy) 0 100 180 250 400 490 Crystailinity (%) 82.9 82.3 80.7 79.4 76.3 74.4 In general, molecular weight and crystallinity of polyethylene have an influence on its mechanical properties. Higher molecular weight polyethylene has more resistance to stretching, i.e. yield strength and breaking strength will he greater. The crystal phase and the amorphous region in polyethylene are alternative and there are spherulites in the crystal phase. HDPE sheets have the properties of rigidity and softness both due to two phases in existence. The rigidity of HDPE sheets increases with crystallinity. Figure 1 showed that gel fraction of samples increased with dosage, so that the molecular weight also increased, but the crystallinity of samples decreases with increasing dosage at higher doses. The detailed data are shown in Table 1. The curves of the yield strength, breaking strength in Fig. 3, and breaking elongation percentage in Fig. 4 are explained as follows: below doses of 180 kGy, the increasing molecular weight of samples is the dominant effect, then the decreasing crystallinity is the effective factor above 180 kGy. The crystallinity of HDPE sheets decreases with dosage so that Young's modulus increases linearly as shown in Fig. 4 and are softened.
177
600
500 C
O
_o
4 0 0 _o---o---.o..~
n
W
300 ~
200
.
.
I
50
~
I
100
Aging
I
I
150
time
200
I
250
I 300
(days)
Fig. 6. Changes of the breaking elongation of HDPE sheets with aging time. O, Unirradiation; I-1, irradiation to 180kGy; A, irradiation to 360kGy. tal points, but when irradiated at a dose of 360 kGy, it is lower than that of the unirradiated sample. The breaking elongation percentage of HDPE sheets is shown in Fig. 6. That of irradiated 180, 360kGy and unirradiated was changed insignificantly after aging. In an aging period of 270 days, the effect of dose on the breaking elongation percentage is similar to the breaking strength in Fig. 5. The values of both are raised in due order of 180 kGy > unirradiated > 360 kGy.
4. The aging tests
It is shown in Fig. 5 that the breaking strength of HDPE sheets irradiated at doses 180 and 360 kGy and unirradiated, were changed after aging. In an aging period of 270 days, the breaking strength of samples decreased slightly. The breaking strength in a sample irradiated at a dose of 180kGy is higher than that of an unirradiated sample at all experimen-
3°°I
CONCLUSION The HDPE sheets of thickness 3 mm were irradiated by e°Co ),-rays at room temperature. The mechanical properties of the sheets were improved when they were irradiated at an optimum dose of 180 kGy. On the other hand, above 180 kGy, the mechanical properties of sheets changed for the worse. The effect of irradiation dose on the mechanical properties of HDPE sheets was retained significantly in an aging period of 270 days.
250 REFERENCES
~' I o o t g} m
50
0
I
I ~)
I 100 Aging
I I 150 200 time (days)
I 250
I 3oo
Fig. 5. Changes of the breaking strength of HDPE sheets with aging time. O, Unirradiation; r'l, Irradiation to 180 kGy; A, Irradiation to 360 kGy.
1. E. Butta and A. Charlesby, J. Poiym. Sci. 1958, 33, 119. 2. Chen Wenxiu, Bao Huaying, Jia Haishun, Liu Dongyuan and Lu Xiangdi, J. Nat. Sci. of Beijing Normal Un~rsity, 1979, 2, 58. 3. W. J. Chappms, M. A. Mier and J. Siverman, Radiat. Phys. Chem. 1982, 20, 323. 4. Plastics Tec/mo/. 1984, 30, 61. 5. Plastics Industry (Chinese), 1985, 1, 2. 6. T. A. Du Plessis, C. J. Grobbelaar and F. Marois, Radiat. Phys. Chem. 1977, 9, 647. 7. Lu Xiang Di, Bao Huaying, Lfi Gongxu, Jia Haishun and Chert Wenxiu, J. Nat. Sci. of Beijing Normal University, 1982, 4, 57. 8. GBI040-79 The Elongation Test of Plastics (Chinese). 9. The Writing Group of "The Polyethylene". The Polyethylene (Chinese), p. 92. SPPH (China), 1972. 10. A. Chapiro, Radlat. Chem. Polymeric Systems, p. 344. John Wiley, New York, 1962. !1. Y. Takeda, R,,d~t. Phys. Chem. 1981, Ig, 853.
0146-5724/87 $3.00+ 0.00 Copyright ~ 1987Petl~,tmonJournals Lid
Printed in Great Britain. All fights rmerved
THE EFFECT OF RADIATION CROSS-LINKING ON THE MECHANICAL PROPERTIES OF POLYETHYLENE SHEETS? LIU DONGYUAN,ZHANG LIANSHUI,WANO YAQ! and CHL~ WENYaU Department of Chemistry, Beijing Normal University, lkijin8, China (Received 29 September 1985)
AlmrEt--The high density polyethylene (HDPE) sheets thickness 3 mm were irradiated by S°Co y-ray. Its gel fraction increases with increasing dosage. The melt index of irradiated samples decreases sLt,nifimmtly with dosage from i kGy to 12 kGy and the number average molecular weight of samples incr~__~ linearly. The mechanical properties including yield strength, breaking strength and breaking elongation percentage of irradiated HDPE sheets increase at low doses and then decrease at high doses. All of them have a peak value at 180 kGy. The Young's modulus of samples also increases linearly with increasing dosage. The aging tests of samples in irradiation dines of 180 and 360 kGy were carried out respectively and compared with non-irradiated samples.
INTI~ODUCTION
gamma source setting in The Institute of Biophysics, Academia Sinica. The dose rate was 0.40 kGy/min measured by a ceric-cerour dosimeter. The aging tests were made at room temperature. Gel fraction m e a s ~ t s were refluxed with xylene for 24 h. (" Melt indexes were measured on a XRZ400-1 Melt Index Detector made in the Mechanical Factory in Jifin University, at a temperature of 190±0.5°C, load 21608. Crystallinitics were measured by a Perkin Elmer 180 Infrared Absorption Spectrometer. According to GBI040-79 ~s) mechanical properties measurements were carried out on WD-I Extension Tester, made by Changehung Non-metallic Tester Factory, Stretching speed 30 ram/rain.
The radiation cross-linking effects of polyethylene have been studied extensively by many investigators. Polyethylene products which were cross-linked, such as electric wire and cable, heat shrinkable connectors and packaginge, and foamed plastics, have had wide applications. A part of these products including low density polyethylene (LDPE) and high density polyethylene (HDPE) products which were manufactured by radiation processing have become commercialized.(I-3) Recently there has been much attention to reduce the density of HDPE and increase its molecular weight or flavability, thus improving its mechanical properties so that its practical application would expand. (*,5) Du Plessis et al. have irradiated HDPE sheets with gas additive and used them in orthopaedic operations. (') There is an improvement on the mechanical properties of HDPE sheets in this paper. The primary experimental result is reported. The relationship between the gel fraction, the melt index and molecular weight with dosage were investigated and the mechanical properties of HDPE as well as the aging tests have been done.
I E S U L T S AND DISCUSSION
1. Gel fraction--dosage relationship
The gel fraction of HDPE sheets increases with an increase in dosage. The relation between gel fraction of HDPE sheets and absorbed dose is shown in Fig. 1. Within 25-500 kGy, the gel fraction of irradiated HDPE sheets increases with an increase in dosage. Below 100kGy, gel fraction of irradiated samples increases quickly, then stays almost constant above 100 kGy.
EXPERIMENTAL
The HDPE sheets were produced by ikijing Ninth Plastic Factory, thickness 3 nun, density 0.942 g/cm 3, melt index 1.70 g/10 rain. The samples were irradiated in air atmosphere at room temperature with 6°Co ~'This paper was presented at the 2nd Japan--China Bilateral Symposium on Radiation Chemistry, Osaka, Japan (September 1985). 175 II.,p.C.29/~.--A
2. Melt index-dosage relationship
The melt index of irradiated HDPE sheets decreases significantly with increasing of dosage. The relation between melt index of irradiated HDPE sheets and absorbed dose is shown in Fig. 2. The melt index of irradiated HDPE sheets decreases significantly with dosage from 1 to 12 kGy. At
176
LIU DONOYUANet al. 100
--
240
80--
220
60
200
Eu
.,¢ v
¢.~ 40
240
~
220
~
oJ 200 E u ~.
"~ 180 .¢:
1 80 ,¢P ¢¢
,~
160
460
"~ 140 )"
140
120
120
L.
20
I 10
0
I 20 Dole
I 30
I 40
I 50
OB
(10 kGy)
Fig. 1. Relation between the gel fraction o f H D P E sheets
and absorbed dose. 1oo
12 kGy, the value of the melt index is one-tenth that of unirradiated samples. These experimental results indicate that the molecular weight of HDPE sheets increases with increasing dosage. On polyethylene, the approximate equation of the relation between number average molecular weight and melt index is as follows~9) (~1~/2 = 188 -
30 log (M.I.)
(1)
where ~14~,is number average molecular weight, and M.I. is the melt index of polyethylene. The values of number average molecular weight of HDPE sheets were obtained from the melt index using the empirical equation (1). The JtT, values were shown in Fig. 2, too. Dosage ranges from 1 to 29 kGy, and the relation between bT, and absorbed dose have a linearity. An /~. value with dose of 29 kGy is twice that of unirradiated samples. 3. The effect of radiation cross-linking on the mechanical properties of H D P E sheets The influences of irradiation on the mechanical properties of polymers were different depending on the polymer cross-links or degradations. Radiation induced degradation always makes mechanical 2.0
7O,000
1.6
60,000
I 10
I 20
I 40
1 oo 50
Dose ( 1 0 k G y )
Fig. 3. Influence of absorbed dose on both the yield strength and the breaking strength of HDPE sheets. properties in polymers worse. On the other hand, radiation cross-linking sometimes may improve the mechanical properties of polymers. After high dose irradiation, different kinds of polymers change and become brittle. °°) Influences of irradiation on both yield strength and breaking strength of HDPE sheets are shown in Fig. 3. Figure 3 indicated that yield strength and breaking strength increase at low doses and then decrease at high doses. Each of them has a peak value at 180 kGy. Influences of irradiation on both breaking elongation percentage and Young's modulus of HDPE sheets are shown in Fig. 4. The shape of the curve of breaking elongation percentage is similar to Fig. 3, and also similar to work of Takeda. (') Young's modulus increases linearly with dosage.
.500
60
N
70
5 ~
~/~//
60
g
c
I 30
1400i
m
o"r= 1.2
50,000
ex t-i 0.8 :E
40,0001~=
0.4
0
3O,OO0
05
t,0 Dose
, 1.5 2.0 (10 kGy)
2.5
".,o,OOo 3.0
Fig. 2. Influence of absorbed dose on both M.I. and ~ . of HDPE sheets.
0 50 E le) OD C = 40 0 >30
I
0
10
I
I
20 30 Dose (10 kGyl
I
4.0
50
,.5o
Fig. 4. Influence of absorbed dose on both the breaking elongation and Young's modulus of HDPE sheets.
Radiation effect on HDPE sheets 700 -
Table I. The crystallinityof HDPEsheets in variousradiation dines Dose (kGy) 0 100 180 250 400 490 Crystailinity (%) 82.9 82.3 80.7 79.4 76.3 74.4 In general, molecular weight and crystallinity of polyethylene have an influence on its mechanical properties. Higher molecular weight polyethylene has more resistance to stretching, i.e. yield strength and breaking strength will he greater. The crystal phase and the amorphous region in polyethylene are alternative and there are spherulites in the crystal phase. HDPE sheets have the properties of rigidity and softness both due to two phases in existence. The rigidity of HDPE sheets increases with crystallinity. Figure 1 showed that gel fraction of samples increased with dosage, so that the molecular weight also increased, but the crystallinity of samples decreases with increasing dosage at higher doses. The detailed data are shown in Table 1. The curves of the yield strength, breaking strength in Fig. 3, and breaking elongation percentage in Fig. 4 are explained as follows: below doses of 180 kGy, the increasing molecular weight of samples is the dominant effect, then the decreasing crystallinity is the effective factor above 180 kGy. The crystallinity of HDPE sheets decreases with dosage so that Young's modulus increases linearly as shown in Fig. 4 and are softened.
177
600
500 C
O
_o
4 0 0 _o---o---.o..~
n
W
300 ~
200
.
.
I
50
~
I
100
Aging
I
I
150
time
200
I
250
I 300
(days)
Fig. 6. Changes of the breaking elongation of HDPE sheets with aging time. O, Unirradiation; I-1, irradiation to 180kGy; A, irradiation to 360kGy. tal points, but when irradiated at a dose of 360 kGy, it is lower than that of the unirradiated sample. The breaking elongation percentage of HDPE sheets is shown in Fig. 6. That of irradiated 180, 360kGy and unirradiated was changed insignificantly after aging. In an aging period of 270 days, the effect of dose on the breaking elongation percentage is similar to the breaking strength in Fig. 5. The values of both are raised in due order of 180 kGy > unirradiated > 360 kGy.
4. The aging tests
It is shown in Fig. 5 that the breaking strength of HDPE sheets irradiated at doses 180 and 360 kGy and unirradiated, were changed after aging. In an aging period of 270 days, the breaking strength of samples decreased slightly. The breaking strength in a sample irradiated at a dose of 180kGy is higher than that of an unirradiated sample at all experimen-
3°°I
CONCLUSION The HDPE sheets of thickness 3 mm were irradiated by e°Co ),-rays at room temperature. The mechanical properties of the sheets were improved when they were irradiated at an optimum dose of 180 kGy. On the other hand, above 180 kGy, the mechanical properties of sheets changed for the worse. The effect of irradiation dose on the mechanical properties of HDPE sheets was retained significantly in an aging period of 270 days.
250 REFERENCES
~' I o o t g} m
50
0
I
I ~)
I 100 Aging
I I 150 200 time (days)
I 250
I 3oo
Fig. 5. Changes of the breaking strength of HDPE sheets with aging time. O, Unirradiation; r'l, Irradiation to 180 kGy; A, Irradiation to 360 kGy.
1. E. Butta and A. Charlesby, J. Poiym. Sci. 1958, 33, 119. 2. Chen Wenxiu, Bao Huaying, Jia Haishun, Liu Dongyuan and Lu Xiangdi, J. Nat. Sci. of Beijing Normal Un~rsity, 1979, 2, 58. 3. W. J. Chappms, M. A. Mier and J. Siverman, Radiat. Phys. Chem. 1982, 20, 323. 4. Plastics Tec/mo/. 1984, 30, 61. 5. Plastics Industry (Chinese), 1985, 1, 2. 6. T. A. Du Plessis, C. J. Grobbelaar and F. Marois, Radiat. Phys. Chem. 1977, 9, 647. 7. Lu Xiang Di, Bao Huaying, Lfi Gongxu, Jia Haishun and Chert Wenxiu, J. Nat. Sci. of Beijing Normal University, 1982, 4, 57. 8. GBI040-79 The Elongation Test of Plastics (Chinese). 9. The Writing Group of "The Polyethylene". The Polyethylene (Chinese), p. 92. SPPH (China), 1972. 10. A. Chapiro, Radlat. Chem. Polymeric Systems, p. 344. John Wiley, New York, 1962. !1. Y. Takeda, R,,d~t. Phys. Chem. 1981, Ig, 853.