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Electron beam irradiation conditions and foam seat properties in polypropylene-polyethylene blends
Pergamon
Radiat. Phys. Chem. Vol. 46, No. 4-6, pp. 905-908, 1995 Elsevier Science Ltd. Printed in Great Britain
0969-806X(95)00289-8
ELECTRON BEAM IRRADIATION CONDITIONS AND FOAM SEAT PROPERTIES IN POLYPROPYLENE-POLYETHYLENE BLENDS
S. TOKUDA and T. KEMMOTSU Hiratsuka Research Laboratory, The Furukawa Electric Co., LTD. 5 - 1 - 9 , Higashi-Yawata, Hiratsuka 254, Japan
ABSTRACT High expansion polypropylene foam (20-100kg/m 3) is industrially produced by blending polypropylene with thermal decomposition foaming agent, which is then irradiated with electron beam to induce crosslinking, and is finally heated, causing the foaming agent to decompose and generate foams. Here, crosslinking stage also serves to increase the melt viscosity of the mixture so that an appropriate value is obtained for foaming. In order to obtain desirable final product, the important factors in the above process are the material properties of the polypropylene and the appropriate control of crosslinking, which is governed by the selection of crosslinking-promoter and the irradiation conditions. We have used polypropylene random copolymer (R-PP) with low ethylene content and performed studies on the relation between the amount of electron beam irradiation and the degree of crosslinking and also on the effect of the multifunctional monomers as crosslinking-promoter. We have also produced foams using blends of this R - P P and linear low density polyethylene (L-LDPE) and evaluated their mechanical properties and their heat resistance.
KEYWORDS Crosslinking, Foam, Irradiation, Polypropylene, Multi functional monomers
INTRODUCTION Polyolefin foam is used in a wide range of fields as materials for heat insulation, cushioning, and automobile interiors because it is easy to form and has superior heat insulation, flexibility, and cushioning properties. Polypropylene foam, in particular, has better mechanical strength, heat resistance and heat formability than polyethylene foam and is expected to find wider applications in the future. In the foaming process, polypropylene blended with a thermal decomposition foaming agent is heated, causing the foaming agent to break down and yield foams. However when ordinary polypropylene is used, the melt viscosity suddenly decreases when it is heated above its melting point, in which case the gas generated from the foaming agent cannot be retained inside the resin. This makes it difficult to obtain foam with a high expansion ratio (20-100kg/m 3 ), making it impossible to control the size and the number of cells. Therefore on an industrial basis, methods are used in which the resin is first irradiated with an electron beam to produce the right degree of crosslinking, then heat is applied to create foaming.
905
S. Tokuda and T. Kemmotsu
906
Nonetheless, the electron beams irradiated onto polypropylene breakes the molecular chains, which becomes prevalent over the crosslinking reaction. There are two generally known methods to avoid this problem; one is to add multifunctional monomers as crosslinking-promoter and the other is to copolymerize polypropylene to enhance the crosslinking reaction. Also, polypropylene foams are brittle at low temperatures, sometime cracking at near 0 *C, and an effective way to solve this problem is to blend polypropylene with polyethylene that has low grass transition temperature. These considerations led us to add multifunctional monomers to various types of R - P P and LLDPE, investigate the relationship between the amount of electron beam irradiation and the degree of crosslinking, create foams of R-PP/LLDPE blends, and study their mechanical properties and heat resistance.
EXPERIMENTAL
Materials Three types of R - P P and one type of LLDPE were used. The physical properties of these various resins are listed in Table 1. Azodicarbonamide (decomposition temperature 205 *C ) was used as thermal decomposition foaming agent. Four types of multifunctional monomers were used: trimethylolpropane trimethacrylate (TMPT), trimethylolpropane tfiacrylate (TMPTA), tetramethylolmethane tetracerylate (TMMT), and triallyl isoeyanurate(TAIC). Table 1. Characteristics of R - P P and LLDPE Melt Flow Rate (210 *C ,2.16kg) R-PP 1 R-PP 2 R-PP 3
8.0 2.0 1.8
LLDPE
*4.0
Ethylene cont. (%) 4.0 3.8 5.8
density (g/cm a ) 0.90 0.90 0.90 0.92
"190 *C ,2.16kg
Sample preparation The samples used for evaluating the crosslinking characteristics were made by adding 5 x 10 - a tool of the multifunctional monomer to 100 g of resin, which was kneaded with two rollers having a surface temperature of 160 *C, and pressed into sheet 1.4 mm thick. The samples used for making foams were made by the same procedure with 13 g of thermal decomposition foaming agent and 5 x 10-a tool of multifunctional monomer added to 100 g of resin. These sheets were irradiated from both sides with electron beams in the atmosphere with an acceleration voltage of 450 keV using an electron beam irradiator made by Nisshin High Voltage(Ltd.).
Foaming Foam was obtained by heating the crosslinking sheets in an oven at 230 *(2 for 5 minutes, which caused the foaming agent to decompose.
9th International Meeting on Radiation Processing
907
Characterization The degree of crosslinking of the sheets was obtained by measuring the gel fraction after the samples were immersed in xylene at 120 *C for 24 hours. The mechanical strength of the foam was evaluated by its tensil repture strength and its 25% compression strength (JIS K-6767). The heat resistance was evaluated by using thermal mechanical analysis (TMA), to measure the temperature at which a thermal probe 2 mm in diameter begins to penetrate the sample under 5 - g load, when the sample is heated at a rate of 2 *C/min.
RESULT AND DISCUSSION
Crosslinking characteristics of R - P P Figure 1 shows the relationship between irradiation dose and gel fraction when various types of multifunctional monomers are added to R - P P 2. It is known that the type of monomer that is added greatly affects the crosslinking characcteristics of the polypropylene, and if we classify the four multifunctional monomers used in the evaluation, we have two general types: type 1, which shows high gel fraction at low irradiation dose, then goes down temporarily, and thereafter gradually rises (TMPTA, TMMT), and type 2, in which the gel fraction increases with increasing irradiation dose(TMPT, TAIC).
60 • TMPT 40
c TMPTA [] TMMT
20
TAIC O 0
0
20
40 60 80 I r r a d i a t i o n dose kGy
100
120
Fig 1. Relationships between irradiation dose and gel fraction of R - P P 2 Table 2 shows the results of measuring the number of electric discharge holes produced under electron beam irradiation. The monomers can be classified into two tying: one that produces relatively few electric discharge holes, and another that produces increasing number of holes as the irradiation dose is increased. These types correspond to the type 1 and type 2 for crosslinking characteristics. Table 2. Number of electric discharge holes when multifunctional monomers are added (10 a holea/m a ) Multifunctional monomer
TMPT TMPTA TMMT TMC
Irradiation dose (kGy) 5
i0
20
50
100
0 0 0 0
1.4 0.4 0.2 1.8
4.8 0.8 1.0 3.2
10.6 5.8 1.8 16.6
24.4 7.8 4.2 18.4
908
s. Tokuda and T. Kemmotsu
Thus it is found that in manufacturing polypropylene foam on an idustrial basis it is possible to have a high degree of crosslinking with a low irradiation dose, and that type 1 multifunetional monomers, which produce few electric discharge holes, which are a product defect, are very useful as crosslinking- promoter.
Foam characteristics Next, in a R - P P / LLDPE (50/50) blend, using type 1 multifunctional monomers the optimum irradiation dose was selected and foames were made. Table 3 lists the physical properties of the foam made from each R - P P and, for comparison, of ordinary polyethylene foam. In each case the density of the foam is 50 kg/m 3
Table 3. Physical properties of blend foams
Matrix
Tensile strength (MPa)
25% compressin strength (MPa)
Heat resistance
( *C )
R-PPI / LLDPE R-PP2 / LLDPE R-PP3 / LLDPE
0.60 0.75 0.70
0.09 0.11 0.09
130 135 130
Polyethylene(LDPE)
0.50
0.06
80
This table reveals that the blend foams have better mechanical strength and heat resistance than the polyethylene foam. Among the blends, R - P P 2, with low MFR and low ethylene content, has the best mechanical strength and heat resistance, and similar results are obtained even when the type of multifunctional monmer is changed. In order words, the presence of a multifimctional monomer greatly affects the crosslinking characteristics of the R - P P and is effective in reducing the irradiation dose and number of electric discharge holes when foam is manufactured, but the physical propreties of the foam are determined largely by the type of R - P P that forms the matrix.
CONCLUSIONS In adding various multifunctional monomers to R - P P and examing the resulting crosslinking characteristics, it was learned that a high gel fraction is shown from low irradiation dose, and that multifunctional monomers of the type that produces few electric discharge holes are very effective as crosslinking-promoter. We have also found that, in R-PP/LLDPE blends, the physical propertise of the foams are primarily affected by the physical properties of the R-PP, and little by the crosslinking-promoter. All the blend foams obtained have better mechanical strength and heat resistance than polyethylene foam and are expected to find wide application in the future.
REFERENCE N.Shiina et al., Japan Plastics Age., December, 37-48 (1972) A.Osakada et al., Japan chemical quarterly V - l , 55-59 (1971) A.Charlesby and P.J.Fydelor, Int. f. Radiat. Phy. Chem., 4_, 107 (1972) N.Sagane et al., Radiat. phys. Chem., 18. 99-108 (1981) A.Nojiri et al., Furukawa, Review 1982, 2_, 36. A.Nojiri et al., Radiat. Phys. Chem. 3_, 339-346 (1985)
Radiat. Phys. Chem. Vol. 46, No. 4-6, pp. 905-908, 1995 Elsevier Science Ltd. Printed in Great Britain
0969-806X(95)00289-8
ELECTRON BEAM IRRADIATION CONDITIONS AND FOAM SEAT PROPERTIES IN POLYPROPYLENE-POLYETHYLENE BLENDS
S. TOKUDA and T. KEMMOTSU Hiratsuka Research Laboratory, The Furukawa Electric Co., LTD. 5 - 1 - 9 , Higashi-Yawata, Hiratsuka 254, Japan
ABSTRACT High expansion polypropylene foam (20-100kg/m 3) is industrially produced by blending polypropylene with thermal decomposition foaming agent, which is then irradiated with electron beam to induce crosslinking, and is finally heated, causing the foaming agent to decompose and generate foams. Here, crosslinking stage also serves to increase the melt viscosity of the mixture so that an appropriate value is obtained for foaming. In order to obtain desirable final product, the important factors in the above process are the material properties of the polypropylene and the appropriate control of crosslinking, which is governed by the selection of crosslinking-promoter and the irradiation conditions. We have used polypropylene random copolymer (R-PP) with low ethylene content and performed studies on the relation between the amount of electron beam irradiation and the degree of crosslinking and also on the effect of the multifunctional monomers as crosslinking-promoter. We have also produced foams using blends of this R - P P and linear low density polyethylene (L-LDPE) and evaluated their mechanical properties and their heat resistance.
KEYWORDS Crosslinking, Foam, Irradiation, Polypropylene, Multi functional monomers
INTRODUCTION Polyolefin foam is used in a wide range of fields as materials for heat insulation, cushioning, and automobile interiors because it is easy to form and has superior heat insulation, flexibility, and cushioning properties. Polypropylene foam, in particular, has better mechanical strength, heat resistance and heat formability than polyethylene foam and is expected to find wider applications in the future. In the foaming process, polypropylene blended with a thermal decomposition foaming agent is heated, causing the foaming agent to break down and yield foams. However when ordinary polypropylene is used, the melt viscosity suddenly decreases when it is heated above its melting point, in which case the gas generated from the foaming agent cannot be retained inside the resin. This makes it difficult to obtain foam with a high expansion ratio (20-100kg/m 3 ), making it impossible to control the size and the number of cells. Therefore on an industrial basis, methods are used in which the resin is first irradiated with an electron beam to produce the right degree of crosslinking, then heat is applied to create foaming.
905
S. Tokuda and T. Kemmotsu
906
Nonetheless, the electron beams irradiated onto polypropylene breakes the molecular chains, which becomes prevalent over the crosslinking reaction. There are two generally known methods to avoid this problem; one is to add multifunctional monomers as crosslinking-promoter and the other is to copolymerize polypropylene to enhance the crosslinking reaction. Also, polypropylene foams are brittle at low temperatures, sometime cracking at near 0 *C, and an effective way to solve this problem is to blend polypropylene with polyethylene that has low grass transition temperature. These considerations led us to add multifunctional monomers to various types of R - P P and LLDPE, investigate the relationship between the amount of electron beam irradiation and the degree of crosslinking, create foams of R-PP/LLDPE blends, and study their mechanical properties and heat resistance.
EXPERIMENTAL
Materials Three types of R - P P and one type of LLDPE were used. The physical properties of these various resins are listed in Table 1. Azodicarbonamide (decomposition temperature 205 *C ) was used as thermal decomposition foaming agent. Four types of multifunctional monomers were used: trimethylolpropane trimethacrylate (TMPT), trimethylolpropane tfiacrylate (TMPTA), tetramethylolmethane tetracerylate (TMMT), and triallyl isoeyanurate(TAIC). Table 1. Characteristics of R - P P and LLDPE Melt Flow Rate (210 *C ,2.16kg) R-PP 1 R-PP 2 R-PP 3
8.0 2.0 1.8
LLDPE
*4.0
Ethylene cont. (%) 4.0 3.8 5.8
density (g/cm a ) 0.90 0.90 0.90 0.92
"190 *C ,2.16kg
Sample preparation The samples used for evaluating the crosslinking characteristics were made by adding 5 x 10 - a tool of the multifunctional monomer to 100 g of resin, which was kneaded with two rollers having a surface temperature of 160 *C, and pressed into sheet 1.4 mm thick. The samples used for making foams were made by the same procedure with 13 g of thermal decomposition foaming agent and 5 x 10-a tool of multifunctional monomer added to 100 g of resin. These sheets were irradiated from both sides with electron beams in the atmosphere with an acceleration voltage of 450 keV using an electron beam irradiator made by Nisshin High Voltage(Ltd.).
Foaming Foam was obtained by heating the crosslinking sheets in an oven at 230 *(2 for 5 minutes, which caused the foaming agent to decompose.
9th International Meeting on Radiation Processing
907
Characterization The degree of crosslinking of the sheets was obtained by measuring the gel fraction after the samples were immersed in xylene at 120 *C for 24 hours. The mechanical strength of the foam was evaluated by its tensil repture strength and its 25% compression strength (JIS K-6767). The heat resistance was evaluated by using thermal mechanical analysis (TMA), to measure the temperature at which a thermal probe 2 mm in diameter begins to penetrate the sample under 5 - g load, when the sample is heated at a rate of 2 *C/min.
RESULT AND DISCUSSION
Crosslinking characteristics of R - P P Figure 1 shows the relationship between irradiation dose and gel fraction when various types of multifunctional monomers are added to R - P P 2. It is known that the type of monomer that is added greatly affects the crosslinking characcteristics of the polypropylene, and if we classify the four multifunctional monomers used in the evaluation, we have two general types: type 1, which shows high gel fraction at low irradiation dose, then goes down temporarily, and thereafter gradually rises (TMPTA, TMMT), and type 2, in which the gel fraction increases with increasing irradiation dose(TMPT, TAIC).
60 • TMPT 40
c TMPTA [] TMMT
20
TAIC O 0
0
20
40 60 80 I r r a d i a t i o n dose kGy
100
120
Fig 1. Relationships between irradiation dose and gel fraction of R - P P 2 Table 2 shows the results of measuring the number of electric discharge holes produced under electron beam irradiation. The monomers can be classified into two tying: one that produces relatively few electric discharge holes, and another that produces increasing number of holes as the irradiation dose is increased. These types correspond to the type 1 and type 2 for crosslinking characteristics. Table 2. Number of electric discharge holes when multifunctional monomers are added (10 a holea/m a ) Multifunctional monomer
TMPT TMPTA TMMT TMC
Irradiation dose (kGy) 5
i0
20
50
100
0 0 0 0
1.4 0.4 0.2 1.8
4.8 0.8 1.0 3.2
10.6 5.8 1.8 16.6
24.4 7.8 4.2 18.4
908
s. Tokuda and T. Kemmotsu
Thus it is found that in manufacturing polypropylene foam on an idustrial basis it is possible to have a high degree of crosslinking with a low irradiation dose, and that type 1 multifunetional monomers, which produce few electric discharge holes, which are a product defect, are very useful as crosslinking- promoter.
Foam characteristics Next, in a R - P P / LLDPE (50/50) blend, using type 1 multifunctional monomers the optimum irradiation dose was selected and foames were made. Table 3 lists the physical properties of the foam made from each R - P P and, for comparison, of ordinary polyethylene foam. In each case the density of the foam is 50 kg/m 3
Table 3. Physical properties of blend foams
Matrix
Tensile strength (MPa)
25% compressin strength (MPa)
Heat resistance
( *C )
R-PPI / LLDPE R-PP2 / LLDPE R-PP3 / LLDPE
0.60 0.75 0.70
0.09 0.11 0.09
130 135 130
Polyethylene(LDPE)
0.50
0.06
80
This table reveals that the blend foams have better mechanical strength and heat resistance than the polyethylene foam. Among the blends, R - P P 2, with low MFR and low ethylene content, has the best mechanical strength and heat resistance, and similar results are obtained even when the type of multifunctional monmer is changed. In order words, the presence of a multifimctional monomer greatly affects the crosslinking characteristics of the R - P P and is effective in reducing the irradiation dose and number of electric discharge holes when foam is manufactured, but the physical propreties of the foam are determined largely by the type of R - P P that forms the matrix.
CONCLUSIONS In adding various multifunctional monomers to R - P P and examing the resulting crosslinking characteristics, it was learned that a high gel fraction is shown from low irradiation dose, and that multifunctional monomers of the type that produces few electric discharge holes are very effective as crosslinking-promoter. We have also found that, in R-PP/LLDPE blends, the physical propertise of the foams are primarily affected by the physical properties of the R-PP, and little by the crosslinking-promoter. All the blend foams obtained have better mechanical strength and heat resistance than polyethylene foam and are expected to find wide application in the future.
REFERENCE N.Shiina et al., Japan Plastics Age., December, 37-48 (1972) A.Osakada et al., Japan chemical quarterly V - l , 55-59 (1971) A.Charlesby and P.J.Fydelor, Int. f. Radiat. Phy. Chem., 4_, 107 (1972) N.Sagane et al., Radiat. phys. Chem., 18. 99-108 (1981) A.Nojiri et al., Furukawa, Review 1982, 2_, 36. A.Nojiri et al., Radiat. Phys. Chem. 3_, 339-346 (1985)