Crystalline structure of polypropylene in blends with thermoplastic elastomers after electron beam irradiation

Crystalline structure of polypropylene in blends with thermoplastic elastomers after electron beam irradiation

ARTICLE IN PRESS Radiation Physics and Chemistry 75 (2006) 259–267 www.elsevier.com/locate/radphyschem Crystalline structure of polypropylene in ble...

304KB Sizes 0 Downloads 67 Views

ARTICLE IN PRESS

Radiation Physics and Chemistry 75 (2006) 259–267 www.elsevier.com/locate/radphyschem

Crystalline structure of polypropylene in blends with thermoplastic elastomers after electron beam irradiation Ryszard Stellera, Danuta Z˙ uchowskaa,, Wanda Meissnera, Dominik Pauksztab, Jo´zef Garbarczykb a

Department of Chemistry, Wroc!aw University of Technology, Wybrzez˙e Wyspian´skiego 27, Wroc!aw 50-370, Poland Institute of Chemical Technology and Engineering, Poznan´ University of Technology, pl. Sk!odowskiej-Curie 2, Poznan´ 60-965, Poland

b

Received 10 January 2005; accepted 15 May 2005

Abstract Isotactic polypropylene (PP) was blended in extruder with 0–50% addition of styrene–ethylene/butylene–styrene (SEBS) and styrene–butadiene–styrene (SBS) block copolymers. Granulated blends were irradiated with electron beam (60 kGy) and 1 week later processed with injection molding machine. Properties of samples molded from irradiated and non-irradiated granulates were investigated using DSC, WAXS, MFR, SEM and mechanical and solubility tests. It was found that the SEBS based systems are more resistant to irradiation in comparison to similar blends with SBS copolymer. Such behavior can be explained by the presence of double bonds in elastic SBS block. Irradiation of PP-SBS blends leads to considerable structure changes of crystalline and amorphous PP phases and elastic SBS phase. It indicates creation of new (inter)phase consisting of products of grafting and cross-linking reactions. Irradiated PP-SBS blends show significant improvement of impact strength at low temperatures. r 2005 Elsevier Ltd. All rights reserved. Keywords: Radiation modification; Polypropylene blends; SEBS and SBS copolymers

1. Introduction Radiation modification of polymers is one of the modern methods, which enable the receiving of new polymeric materials with specific properties. Unmodified polypropylene and elastomer toughened polypropylene have a large technical meaning. Among other things, they are used for production of medical equipment capable of radiation sterilization (Ishigaki and Yoshii, Corresponding author. Tel.: +48 71 320 36 33; fax: +48 71 320 36 78. E-mail address: [email protected] ˙ uchowska). (D. Z

1992; Singh and Silverman, 1992). The irradiation of polymers with electron beam (EB) or X-rays causes usually the chain cracking or atoms (e.g. hydrogen) split off resulting in the creation of macroradicals. The end effect of the transformation of macroradicals is a molecular weight decrease and/or the creation of crosslinked structures. Both the degradation and the crosslinking proceed simultaneously, and the domination one of them depends on the polymer chemical structure. The degradation prevails in polypropylene, while in polyethylene or in unsaturated elastomers, e.g. polybutadiene, the cross-linking process is more intense. Irradiation changes the chemical structure of polyolefins, especially in the amorphous phase, while the

0969-806X/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2005.05.018

ARTICLE IN PRESS 260

R. Steller et al. / Radiation Physics and Chemistry 75 (2006) 259–267

crystalline phase is more resistant to its action. It follows from different experimental data reviewed by (Batheja et al., 1995; Mukherjee et al., 1986) that the relatively low doses of high-energy radiation either do not alter or somewhat decrease the crystallinity degree of polyolefins. For instance, the crystallinity variations of PP irradiated with the dose smaller than 150 Mrad are also small. On the other hand, (Sen and Kumar, 1995) have found that the X-ray irradiation of PP yarn in the time longer that 5 h causes partial destruction of crystallites due to different processes connected with radiation. One of them is the radiation oxidation, which proceeds in polyolefins at the boundaries of crystalline regions. Therefore, its intensity depends on dimensions and distribution of crystalline phase, crystal lattice defects and the character of transient regions between crystallites (Sirota et al., 1995). Processes occurring in semicrystalline polymers under the action of ionizing radiation are complex and they depend on many factors, e.g. the content of antioxidants and other additives. For this reason the experimental results quoted by various authors are sometimes contradictory. The use of different elastomers, such as EPM or EPDM copolymers, as polypropylene impact modifiers can disturb the PP crystallization after irradiation (Dongyuan et al., 1990; Geuskens and Nedelkos, 1996). It was found in our previous studies (Z˙ uchowska ˙ uchowska and Zago´rski, 1999) that in PP-SBS 1997; Z blends after electron beam irradiation (with the dose 30 or 60 kGy) no significant changes of the crystalline phase content determined by WAXS were observed. However, the mean size of crystallites undergoes visible variations. The temperature of the beginning of crystalline phase melting measured by DSC both for pure PP and for PP in blends with SBS (50/50) after irradiation is shifted towards lower temperatures in comparison with non-irradiated samples. This behavior can also testify to the changes in structure and size of crystallites. The study of irradiation effect on crystalline phase content in pure PP and in PP blended with block elastomers (SBS or SEBS) was carried out applying the dose 60 kGy. This dose is slightly larger than that recommended for medical equipment sterilization, which should not change the crystalline structure of PP. According to literature data (Yoshii et al., 1995; ˙ uchowska, 1988; Z ˙ uchowska et al., 2003) macroZ radicals in irradiated PP have a long life time reaching a few months. Hence, the degradation and/or crosslinking proceeds even if the radiation action has finished. It leads to some lowering of the tensile strength and to a violent decrease of the elongation at break, i.e. to the rise of PP brittleness. During the processing of irradiated PP the degradation and cross-linking acceleratad by the action of stress and temperature fields may take place.

2. Experimental 2.1. Polymers

 Isotactic polypropylene (PP)—Malen P (J-400) man

ufactured by PKN ORLEN S.A. Triblock copolymer styrene/butadiene/styrene (SBS)–Kraton D1102CU manufactured by Shell, styrene content 29% Triblock copolymer styrene/ethylene-co-buthylene/ styrene (SEBS)—Kraton G1652, manufactured by Shell, styrene content 30%

2.2. Preparation of blends Pure PP (as the reference material) and PP with 25% and 50% elastomer (SBS or SEBS) were processed (blended) twice at 458–483 K in a single-screw extruder and ground after cooling. A part of granulated PP and PP-SBS/SEBS blends was subjected to electron beam irradiation (13 MeV, high monochromatic) with the linear accelerator LAE 13/9 in the Institute of Nuclear Chemistry and Technology in Warsaw. The total dose 60 kGy (1 Gy ¼ 1 J/kg) was applied in two stages (2  30 kGy) in the time interval, which was indispensable for cooling of primary irradiated material to the room temperature. Test samples were prepared from irradiated (and non-irradiated) granulate by injection molding at 473 K about 1 week after irradiation. 2.3. Methods of measurements The crystalline structure of PP was determined by means of wide angle X-ray scattering method (WAXS). CuKa radiation monochromatized with a Ni filter was used. Measurements at standard parameters were performed within the angle range 2y ¼ 10–301. Heat of fusion and melting point of PP were measured using the differential scanning calorimeter (Polymer Laboratories). Samples (ca. 6 mg) were heated in the range 293–473 K with the rate 5 K/min. Glass temperatures were determined by with dynamic mechanical analyser (Polymer Laboratories). Test samples were subjected to sinusoidal bending with the frequency 10 Hz in the temperature range 173–423 K at the heating rate 5 K/min. The morphology of irradiated and non-irradiated blends was determined by SEM observations of fractures (after cooling in liquid nitrogen) of injected specimens. The flowability of PP-SBS/SEBS blends was characterized by melt flow rate (MFR) measurements with conventional plastometer (Zwick) at standard conditions (49 N, 463 K).

ARTICLE IN PRESS R. Steller et al. / Radiation Physics and Chemistry 75 (2006) 259–267

The notched impact strength at 298 and 225 K was measured by means of a Charpy hammer (Ceast) with liquid nitrogen cooled measuring chamber. Tensile properties were determined with a conventional testing machine (Instron) at standard conditions (tensile rate 50 mm/min).

3. Results and discussion SBS and SEBS copolymers and their blends with PP dissolve only partly in organic solvents after electron beam irradiation (dose 60 kGy). An increase of the content of hot xylene insoluble parts (gel) in both irradiated copolymers is connected with a violent viscosity rise. The viscosity is so high that both copolymers practically do not flow during MFR measurements at standard conditions (Table 1). They begin to flow only during compression or injection molding, i.e. under much larger pressure than that reigning in plastometer during MFR determination. The gel content in SBS copolymer is about two times larger in comparison with SEBS copolymer. It is probably due to different chemical structures of elastic blocks in these copolymers. SBS polybutadiene block containing the double bonds is more susceptible to the processes of radiation cross-linking than the elastic SEBS block, which consists of ethylene and butylene mers. The differences in reactivity of elastic blocks in both copolymers cause that they behave completely different in blends with PP when subjected to irradiation. Assuming that the gel is created mainly by the elastomer phase, it can be seen (Table 1) that PP does not effect the radiation cross-linking of SEBS. The gel content with respect to the elastomer phase is almost the same (ca. 14%) in the pure SEBS and in the (50/50) blend. In the case of PP-SBS blends the PP addition increases the gel fraction with respect to SBS more than two times (ca. 70%). It means that a part of PP macroradicals created by irradiation acts either as cross-linking agent or it is grafted onto polybutadiene block of SBS. This conclusion is confirmed by the fact that PP-SBS blend (50/50)

261

in contrast to similar PP-SEBS blend practically does not flow during MFR measurement, although the flowability of SBS before irradiation is ca. 20 times higher than that of SEBS. The flowability of PP after electron beam irradiation is considerably larger in comparison with the virgin polymer. The corresponding MFR values differ almost 15 times. Such a sudden viscosity diminution is an evidence of the PP chain cracking during irradiation without formation of cross-linked structures and gel creation. This phenomenon is also reported in the literature (Yoshii et al., 1995). The irradiation does not influence apparently PP glass temperature in contrast to both copolymers, which after irradiation and injection molding preserve their domain structures confirmed by the existence of two glass transitions (Table 2). The low temperature transition corresponds to the vitrification of the elastic block, while the high temperature one is connected with the polystyrene block. The last transition (ca. 368 K) is practically independent of the copolymer type, blend composition and irradiation. Significant changes of the glass temperature are connected with the elastic blocks and the PP matrix. For the pure SBS and SEBS copolymers only small shifts of elastic block glass transitions towards higher temperatures are observed. They can be treated as a result of decreased mobility of elastic segments after irradiation due to the cross-linking reactions. This result is consistent with the gel content measurements as well as with the literature reports (Singh and Silverman, 1992). In PP-SBS/SEBS blends the glass temperatures of elastic SBS/SEBS blocks and PP matrix are changed in comparison with initial polymers (Table 2). In nonirradiated blends containing 25% of any copolymer all glass transitions are shifted towards considerably lower temperatures in comparison with pure components. This behavior can be treated as an evidence of a strong affinity between elastic blocks and PP matrix resulting in their increased mobility. Somewhat different behavior can be observed for non-irradiated blends with 50% elastomer. In this case the glass temperatures of elastic

Table 1 Gel content and melt flow rate in PP-SBS/SEBS systems before and after electron beam (EB) irradiation System

Copolymer content (%)

Gel fraction (%)

MFR, g/10 min

Before EB

After EB

Before EB

After EB

PP-SBS

0 50 100

0 0 0

0 34.3 31.0

4.1 1.9 6.3

63.8 0.1 0

PP-SEBS

0 50 100

0 0 0

0 7.1 14.0

4.1 1.3 0.3

63.8 7.2 0.1

ARTICLE IN PRESS R. Steller et al. / Radiation Physics and Chemistry 75 (2006) 259–267

262

Table 2 Glass temperatures of elastic blocks and PP matrix in PP-SBS/SEBS systems before and after electron beam (EB) irradiation System

Copolymer content (%)

Glass temperature (K) Elastic block

PP matrix

Before EB

After EB

Before EB

After EB

PP-SBS

0 25 50 100

– 191 199 201

– 184 198 202

290 288 286 –

290 287 301

PP-SEBS

0 25 50 100

– 234 239 241

– 236 240 242

290 288 287 –

290 287 289 –

0.25 0.20

tan δ

0.15 0.10 0.05 0.00 -100

-50

(a)

0 50 Temperature (°C)

100

150

0.25 0.20 0.15 tan δ

blocks are higher (but still lower than those of pure components), while for PP the opposite tendency occurs. Taking into account these observations it can be generally stated that an increase in the content of any blend component (elastomer or PP) lowers the glass temperature of the second component, while the glass temperature of the first component becomes closer to that, which is characteristic of the pure polymer. This is probably due to the fact that with decreasing concentration of a component it becomes more finely dispersed, and hence, more strongly influenced by the second (dominating) component. For this reason the property changes of the dispersed phase are more intense in comparison with the continuous phase. The irradiation leads to considerable changes of the behavior of blends containing SBS and SEBS copolymers. For blends with SBS a further decrease of the glass temperature of elastic block takes place, while for SEBS blends the opposite tendency can be seen. The most glaring differences between PP-SEBS and PP-SBS systems concerning the Tg changes of both the elastic block and PP matrix appear for blends with 50% copolymer. The DMA spectra (representing tg d as a function of temperature) of PP-SEBS blend before and after irradiation are qualitatively similar (Fig. 1a and b). The glass temperatures of elastic SEBS block and PP matrix increase slightly after irradiation. This is probably due to the improved miscibility of elastic SEBS block with amorphous part of PP matrix evoked by the products of cross-linking and grafting reactions. For PP-SBS blends (50/50) significant qualitative and quantitative differences in DMA spectra before and after irradiation can be observed (Fig. 2a and b). The main qualitative difference is the existence in the irradiated blend spectrum of a very sharp peak at ca. 338 K, which can be treated as an evidence of the creation of a new (inter)phase. It is accompanied by a very strong rise of the glass temperature of PP matrix

0.10 0.05 0.00 -100

(b)

-50

0 50 Temperature (°C)

100

150

Fig. 1. DMA spectra of PP-SEBS blend with 50% elastomer at 10 Hz (—) and at 1 Hz (- - - -): (a) before irradiation, (b) after irradiation.

(ca. 15 K) and some Tg lowering of the elastic block (ca. 2 K). Simultaneously, the intensity of the low temperature peak decreases. The new phase consists probably of a part of the elastic SBS phase, which was cross-linked and grafted by the PP radiolysis products. The large amount of gel in PP-SBS (50/50) blend confirms also indirectly the creation of such phase. It is quite possible that during the melt blending of irradiated PP-SBS system a specific phase separation within SBS elastomer takes place, i.e. the ‘‘modified’’ and ‘‘unmodified’’ parts of elastic phase create separate regions. The new phase

ARTICLE IN PRESS R. Steller et al. / Radiation Physics and Chemistry 75 (2006) 259–267

interacts very strongly with PP matrix. It leads to the large increase of the glass temperature of amorphous PP phase. On the other hand, the existence of the ‘‘unmodified’’ part of elastic phase consisting of a smaller amount of polybutadiene chains with increased mobility can explain the negative shift and intensity decrease of the low temperature peak in DMA spectrum of PP-SBS (50/50) blend after irradiation. The electron beam irradiation influences not only the structure and properties of elastic SEBS/SBS blocks and amorphous PP phase but also the crystalline PP phase. It follows from the results of DSC and WAXS measurements. The DSC method made possible to determine the melting point and heat of fusion of pure 0.45 0.40 0.35

tan δ

0.30 0.25 0.20 0.15 0.10 0.05 0.00 -100

-50

0 50 Temperature (°C)

(a)

100

150

100

150

0.45 0.40 0.35

tan δ

0.30 0.25 0.20 0.15 0.10 0.05 0.00 -100

-50

0

(b)

50

Temperature (°C)

Fig. 2. DMA spectra of PP-SBS blend with 50% elastomer at 10 Hz (—) and at 1 Hz (- - - -) (a) before irradiation, (b) after irradiation.

263

PP and its blends with both copolymers (Table 3), while with the use of WAXS the content of crystalline PP phase in all systems was determined (Table 4). It follows from the data presented in Table 3 that the irradiation leads to a decrease of the PP melting point for all systems differing by the elastomer and composition. This phenomenon can be attributed to an increase of a number of structural defects in crystalline PP phase caused directly by irradiation and/or by more intense interactions between PP matrix and elastic phase as an indirect result of irradiation. The last statement is confirmed by the fact that the melting point depression in SBS containing systems is on average larger than in similar systems with SEBS. Moreover, the irradiated SBS based systems have considerably lower values of the heat of fusion in comparison with other systems (irradiated or non-irradiated), which are in this respect similar . This is especially visible for the blend with 50% SBS copolymer discussed above and it can also testify to some PP crystallization hindrance after irradiation and blending resulting in a decrease of crystalline phase content. Such possibility is confirmed by the results of WAXS studies. The diffraction spectra were subjected to separation of maximums originating from the presence of different crystalline phases. The background lines and the areas connected with the amorphous phase were also determined. The individual maximums were separated numerically according to the Hindeleh–Johnson method (Rabiej, 1991). Experimental diffractograms and diffractograms with separated diffraction lines and smoothed experimental lines obtained for PP and PPSBS/SEBS blends with 50% copolymer before and after irradiation are shown in Figs. 3–5. The degree of crystallinity and the contents of polymorphous forms were determined from the areas under separated maximums. The hexagonal b phase content in PP was calculated according to the Turner–Jones formula (Rabiej, 1991) from the areas under maximums, which result from the presence of a and b forms. The degree of PP crystallinity in PP-SBS/SEBS blends was calculated taking into account the areas connected with amorphous

Table 3 Melting point end heat of fusion of PP matrix in PP-SBS/SEBS systems before and after electron beam (EB) irradiation System

Copolymer content (%)

Melting point (K)

Heat of fusion (J/g)

Before EB

After EB

Before EB

After EB

PP-SBS

0 25 50

440 439 439

437 435 433

72 73 76

72 67 48

PP-SEBS

0 25 50

440 440 439

437 437 436

72 70 70

72 75 76

ARTICLE IN PRESS R. Steller et al. / Radiation Physics and Chemistry 75 (2006) 259–267

264

Table 4 Degree of crystallinity of PP matrix in PP-SBS/SEBS systems before and after electron beam (EB) irradiation System

Copolymer content (%)

Degree of crystallinity of PP (%)

Content of the b form in crystalline phase of PP (%)

Before EB

After EB

Before EB

After EB

PP-SBS

0 25 50

58 56 49

59 54 46

9.1 0 0

4.5 0 0

PP-SEBS

0 25 50

58 57 52

59 55 49

9.1 0 Traces (ca. 2.0)

4.5 Traces 0

4000

2000

3500

1800

Intensity

Intensity

a

1600

3000 2500 a 2000

1200 c

1000

b

1500

b

1400

800 600

1000

c

400

500

200

0 10

12

14

16

18

(a)

20 2Θ

22

24

26

28

30

(a)

0 10

12

14

16

18

20 2Θ

22

24

26

30

2500

4000

a

3500

2000

3000 2500

a

Intensity

Intensity

28

2000 b 1500 1000

b

1500

c

1000

c

500

500

0

0 10

(b)

12

14

16

18

20 2Θ

22

24

26

28

30

10

(b)

12

14

16

18

20 2Θ

22

24

26

28

30

Fig. 3. X-ray patterns of polypropylene before (a) and after (a) irradiation: (a) experimental line before separation of maximums, (b) separated lines for crystalline phase, (c) separated line for amorphous phase.

Fig. 4. X-ray patterns of PP-SBS (50/50) blend before (a) and after (a) irradiation: (a) experimental line before separation of maximums, (b) separated lines for crystalline phase, (c) separated line for amorphous SBS.

regions of both copolymers. The results of measurements are summarized in Table 4. It follows from the data in Table 4 that the addition of SBS/SEBS elastomer hinders the PP crystallization resulting in a significant decrease of the content of crystalline phase. This process is intensified by irradiation especially for blends containing SBS elastomer. The blend with 50% SBS elastomer has the lowest content of the crystalline PP phase. This observation directly confirms the conclusions (presented previously), which can be drown from the results of other measurements.

Another interesting conclusion resulting from the WAXS studies is that the irradiation and especially the elastomer addition change not only the crystalline phase content but also its general structure. The irradiation lowers ca. two times the amount of the b form in pure PP, which almost completely vanishes if PP is blended with elastomers. In PP-SEBS systems traces of the b form can be detected. It can also be treated as a sign of generally lower interaction intensity between PP matrix and SEBS in comparison with SBS.

ARTICLE IN PRESS R. Steller et al. / Radiation Physics and Chemistry 75 (2006) 259–267

The different behavior of irradiated PP-SEBS and PPSBS blends is also reflected by their mechanical properties. The tensile strength (static property) measured at standard conditions varies generally within the range 15–35 MPA for different blends. It decreases ca. two times with increasing content of a weaker component, i.e. the elastomer, from 0 to 50%. The irradiation has no very significant effect on this property in contrast to impact strength (dynamic property), especially at low temperatures. The results of impact strength measure-

2500 a 2000 Intensity

b 1500 c 1000 500 0 10

12

14

16

18

(a)

20 2Θ

22

24

26

28

30

2000 a b

Intensity

1500

c 1000

500

(b)

0 10

12

14

16

18

20 2Θ

22

24

26

28

30

Fig. 5. X-ray patterns of PP-SEBS (50/50) blend before (a) and after (a) irradiation: (a) experimental line before separation of maximums, (b) separated lines for crystalline phase, (c) separated line for amorphous SEBS.

265

ments are summarized in Table 5. It can be seen that at the room temperature (298 K) PP-SEBS and PP-SBS systems behave similarly, i.e. at the presence of elastomer before and after irradiation no fracture is observed. At a low temperature (225 K) the notched impact strength of non-irradiated blends containing SBS is considerably higher in comparison with similar systems with SEBS copolymer. This behavior is partly due to the fact that the glass temperature of elastic SEBS block lies above the measurement temperature (Table 2). For this reason it is in the glassy state during measurement contrary to the elastic SBS block with a much lower glass temperature. The irradiation lowers further the impact strength of PP-SEBS blends. It can be also explained by the rise of elastic block glass temperature caused by radiation cross-linking discussed previously. The PP-SBS blends behave at similar conditions quite different. Their impact strength increases after irradiation ca. two times. Such considerable toughening results partly from the fact that the glass temperature of elastic SBS block in blends becomes lower after irradiation (Table 2). On the other hand, the interfacial adhesion in these systems is probably much higher after irradiation in comparison with PP-SEBS blends. Such conclusion follows also from the results of other measurements, which were presented earlier. It is also confirmed directly by the results of SEM observations. Figs. 6a–c present as an example the fracture micrographs of injected specimens from PP-SBS (75/ 25) blend, which were obtained at different irradiation conditions. Fig. 6a represents the non-irradiated samples. In the case Fig. 6b the samples were injected using irradiated granulate after blending, while the samples from Fig. 6c were irradiated after molding. The comparison of Fig. 6a and c leads to the conclusion that the irradiation of specimens after molding does not change significantly the blend structure. In both cases numerous ‘‘reprints’’ of spherical elastomer particles along with loose individual particles are visible. It can be

Table 5 Notched impact strength in PP-SBS/SEBS systems before and after electron beam (EB) irradiation System

Copolymer content %

Notched impact strength (kJ/m2) 298 K

225 K

Before EB

After EB

Before EB

After EB

PP-SBS

0 25 50

2.16 No fracture No fracture

2.33 No fracture No fracture

1.6 2.7 5.6

1.4 5.3 11.8

PP-SEBS

0 25 50

2.16 No fracture No fracture

2.33 No fracture No fracture

1.6 2.0 2.4

1.4 1.8 1.9

ARTICLE IN PRESS 266

R. Steller et al. / Radiation Physics and Chemistry 75 (2006) 259–267

Another interesting conclusion, which can be drawn from SEM observations, is that the main structural changes caused by irradiation appear during processing in the molten state (van Gisbergen and Overbergh, 1992). They were also visible as flowability changes in MFR measurements described earlier. In our opinion the changes are mainly due to the presence of macroradicals generated by irradiation and trapped (a long time) within solid blend as a result of low mobility of macromolecules. It follows not only from the fact that in this case all blends were processed (or measured) at least 1 week after irradiation but also from independent EPR measurements (the details are not presented), which confirm the presence of macroradicals within such period of time.

4. Conclusion

Fig. 6. SEM micrographs of PP-SBS (75/25) blend fractures irradiated at different conditions: (a) non-irradiated, (b) irradiated as granulate before molding, (c) irradiated after molding.

treated as a sign of relatively poor adhesion between components. Quite different picture arises from Fig. 6b. The fracture surface is smoother, and the phase boundaries between components are blurred (covered). It can testify not only to an increase of interfacial adhesion but also to the structural changes of continuous PP phase.

Basing on the results of various measurements it was shown that irradiation with electron beam leads to considerable changes in the behavior and properties of PP-SEBS and PP-SBS blends. The elastomer type, and especially the chemical structure of its elastic block connected with the presence of double bonds, plays a dominant role with respect to the susceptibility of the system to radiation modification. It was shown that SEBS copolymer and its blends with PP are generally more resistant towards the action of electron beam, i.e. the property changes in this system are generally smaller in comparison with the SBS based blends. However, the irradiation improves commonly the ultimate properties of blends. It was already mentioned that the specimens used for property determinations were molded from irradiated (or non-irradiated) granulate ca. 1 week after irradiation. Significant structure and property changes of irradiated blends, which probably take place during melting and homogenization in the molding process 1 week after irradiation, testify to the large persistence of macroradicals created in irradiated granulate. This observation is also consistent with the literature data. It was found (Zago´rski, 1997, 1999, 2002) that various low molecular radiolysis products are trapped in polymers, and they can be activated during processing in molten state. Concluding, it seems that the radiation modification of PP-SEBS and PP-SBS blends can be a useful tool for improving their properties.

References Bhateja, S.K., Duerst, R.W., Martens, J.A., Andrews, E.H., 1995. Radiation-induced enhancement of crystallinity in polymers. J.M.S.–Rev. Macromol. Chem. Phys C35 (4), 581–659.

ARTICLE IN PRESS R. Steller et al. / Radiation Physics and Chemistry 75 (2006) 259–267 Dongyuan, L., Czvikovszky, T., Dobo, J., Somogyi, A., 1990. The effect of impact modifier and of nucleating agent on the radiation tolerance of polypropylene. Radiat. Phys. Chem. 35 (1–4), 199–203. Geuskens, G., Nedelkos, G., 1996. The post-irradation oxidation of polypropylene. II: Influence on the mechanical properties. Polym. Degrad. Stabil. 51, 223–225. Ishigaki, I., Yoshii, F., 1992. Radiation effects on polymer materials in radiation sterilization of medical sypplies. Radiat. Phys. Chem. 39 (6), 527–533. Mukherjee, A.K., Gupta, B.D., Sharma, P.K., 1986. Radiationinduced changes in polyolefins. J. M. S.-Rev. Macromol. Chem. Phys. C26 (3), 415–439. Rabiej, S., 1991. A comparision of two X-ray diffraction procedures for crystallinity determination. Eur. Polym. J. 27 (9), 947–954. Sen, K., Kumar, P.J., 1995. Influence of gamma-irradiation on structural and mechanical properties of polypropylene Yarn. Appl. Polym. Sci. 55, 857–863. Singh, A., Silverman, J., 1992. Radiation processing: an overviews. In: Singh, A., Silverman, J. (Eds.), Radiation Processing of Polymers. Hanser Publishers, Munich, Vienna, New York, 1992, pp. 1–14. Sirota, A.G., Verkhovets, A.P., Auslender, V.L., 1995. Strength characteristic properties of polyethylene crosslinked by radiational-chemical method. Radiat. Phys. Chem. 46 (4-6), 999–1005. van Gisbergen, J., Overbergh, N., 1992. Radiation effect on polymer blends. In: Singh, A., Silverman, J. (Eds.),

267

Radiation Processing of Polymers. Hanser Publishers, New York, pp. 51–69. Yoshii, F., Meligi, G., Sasaki, T., Makuuchi, K., Rabie, A.M., Nishimoto, S., 1995. Effect of irradiation on the degradability of polypropylene in the natural environment. Polym. Degrad. Stabil. 49, 315–321. Zago´rski, Z.P., 1997. Selected topics in the radiation chemistry of polymers. Polimery 42 (3), 141–147 (in Polish). Zago´rski, Z.P., 1999. Solid state radiation chemistry-features important in basic research and application. Radiat. Phys. Chem. 56 (5–6), 559–565. Zago´rski, Z.P., 2002. Modification, degradation and stabilization of polymers in view of the classification of radiation spurs. Radiat. Phys. Chem. 63 (1), 9–19. ˙ uchowska, D., 1997. The effect of fast electron irradiation on the Z properties and structure of isotactic polyproipylene (iPP)-thermoplastic elastomer blends. Polimery (Polish) 42 (3), 182–188. ˙ uchowska, D., Zago´rski, Z.P., 1999. Modification of polymer Z blends by irradiation. Polimery (Polish) 44 (7–8), 516–521 and International Polymer Science Technology, RAPRA Technology LTD 1999; 26 (10), T/87–92. ˙ uchowska, D., 1988. Elastomers modified by electron beam Z irradiation. In: New Trends in Rubber Industry. Seventh International Rubber Symposium. Zlin, 10-11. 11 1998; pp. 78–84. ˙ uchowska, D., Zago´rski, Z.P., Przybytniak, G.K., Rafalski, Z A., 2003. Influence of butadiene/styrene copolymers on the modification of polypropylene in electron beam irradiation. Intern. J. Polymeric Mater. 52 (4), 335–344.