EPDM Blends Having

19 Morphology of L o w Density Polyethylene/ EPDM

Blends Having Tensile Strength

Downloaded by MONASH UNIV on November 18, 2015 | http://pubs.acs.org Publication Date: June 1, 1979 | doi: 10.1021/ba-1979-0176.ch019

Synergism GEOFFREY A. LINDSAY, CHLOE J. SINGLETON, CHARLES J. CARMAN, and RONALD W. SMITH B. F. Goodrich Co., Chemical and Corporate Divisions, 9921 Brecksville Road, Brecksville, OH 44141

Blends of low density polyethylene (LDPE) with certain crystalline ethylene-propylene-diene monomer (EPDM) rubbers have tensile strengths greater than either component. Differential calorimetry scans of these blends and their components have been compared with those of low strength blends of LDPE with amorphous EPDM rubber. Upon cooling the high strength blend from the melt, LDPE crystallites appear to nucleate the crystallization of some high ethylene segments of the EPDM rubber. Nodules, believed to be crystallites, were observed on ion-etched surfaces by scanning electron microscopy. High strength blends had a 50% larger nodule size than other blends, which may also result from high ethylene segments of EPDM crystallized on the surfaces of LDPE crystallites.

"D atiuk, Herman, and Healy discovered that blends of certain ethylenepropylene-diene monomer ( E P D M ) rubbers with polyethylene have surprisingly high tensile strengths ( I ) . Several patents have been published on these and related blends (2,3,4). These blends are not crosslinked i n the normal sense, but can be melted and molded repeatedly, retaining their high strength upon cooling. W e have begun to study the morphology of these blends i n an attempt to learn what causes the tensile strength synergism. This chapter gives our initial results and interpretations. 0-8412-0457-8/79/33-176-367$05.00/0 © 1979 American Chemical Society In Multiphase Polymers; Cooper, S., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

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MULTIPHASE

Table I.

Physical Properties of Polymers

Polymer type

EPDM

Tradename Obtained from Crystallinity (%) Density, g/cm W t % ethylene Wt % E N B Mooney viscosity M e l t index, g/ 10 min* β

3

b


0

POLYMERS

EPDM

LDPE

Epcar 845 Epcar 847 Epolene C-14 B.F.Goodrich B.F.Goodrich Eastman 0 6 32 .87 .88 .918 56 71 100 4 4 0 56 55 — — 1.6

* From x-ray diffraction scans. Ethylidene norbornene. Four minute warm up/four minute run at 6

c d

A S T M D 1 2 3 8 2.16

120°C.

kg/190°C.

Experimental The polymers used and some of their physical properties are listed in Table I. Polymers were mixed and blended on a two-roll m i l l at 450 K . Samples were compression molded at 450 Κ for 7 m i n and cooled in the press with tap water for 5 min. A S T M D412 6.35-mm ( VA in. ) dumb bells were cut parallel to the mill grain from sheets having 1.9-mm (75 mils ) thickness. Instron tensile tests were carried out at least 48 hr after molding. P u l l rate was 50.8 cm/min (20 in./min). A Perkin-Elmer D S C - 2 differential scanning calorimeter equipped with an Automatic Scanning Zero was used. D S C conditions were 20 K / m i n on a 20.8-mW sensitivity with a 24-mg aluminum reference. A J E O L J S M 50A scanning electron microscope was used. Polymer surfaces were etched with a Hummer II sputtering device using argon ions and coated with a 20-nm thick sputtered film of gold/palladium alloy.

Results and Discussion The structure of the E P D M rubber is very important to the strength of these blends. Those E P D M rubbers which have crystallinity melting above room temperature give blends with the highest tensile strengths. E P D M rubbers having crystallinity levels (by x-ray diffraction measure ments ) i n the range 1-20% are of greatest interest. Above 20%, the E P D M behaves more like a plastic than a rubber. Blends of amorphous E P D M rubber with L D P E do not have tensile strength synergism. Commercial E P D M rubbers have monomer sequence distributions ranging between random and alternating i n the first-order Markovian sense. From our previous experience they must contain at least about 68 wt % ethylene to have runs of ethylene long enough to crystallize above room temperature at rest.

In Multiphase Polymers; Cooper, S., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

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LINDSAY

E T

Tensile Strength Synergism

AL.

A low level of crystallinity ( 1-3% ) is sometimes difficult to detect by x-ray diffraction techniques. Differential scanning calorimetry ( D S C ) does detect these first-order thermal transitions. However, crystallinity is difficult to quantify by D S C because of uncertainties i n locating the baseline. This uncertainty is a result of an overlap of both the E P D M T and L D P E melting endotherm with the two ends of the E P D M melting endotherm. C N M R has proved to be a good method of characterizing the monomer sequence distribution of ethylene-propylene copolymers (3,4,5). Those E P D M rubbers with an appropriate fraction of long ethylene runs also give blends with polyethylene having unusually high tensile strengths. W e plan to make this the subject of a future paper. Figure 1 shows tensile strengths for the amorphous and crystalline E P D M blended with various levels of L D P E . The strength of the crystalline E P D M blend goes through a maximum between 20-40 wt % L D P E , where it is greater than either component. The strength of the amorphous E P D M blend is much lower. Tensile properties of these two E P D M rubbers are fairly representative of what can be obtained using commercial polymers having a 125°C Mooney viscosity near 55. Tensile strengths as high as 2900 psi have g

1 3


369

0

20

40

60

80

100

Wt. % LDPE

Figure 1.

Tensile data



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MULTIPHASE

POLYMERS

been obtained with blends of L D P E and an E P D M having a slightly higher ethylene content. Lower tensile strengths have been reported for blends of L D P E a n d E P copolymer having only 43 wt % ethylene (6). Tensile strength also decreases as the molecular weight of the E P ( D ) M decreases. Figure 2 shows tensile-yield strengths for blends of the crystalline E P D M with various levels of L D P E . The curve increases monotonically as expected if no phase inversion occurs. Since amorphous L D P E has a glass-transition temperature near that of E P D M (7) and since the L P D E has only 27% crystallinity, one should not expect a rubber-to-rigid phase transition. Our wide-angle x-ray diffraction ( W A X D ) measurements have shown that stretching pure Epcar 847 induces considerable orientation and crystallinity—just as is the case with natural rubber. The blend of Epcar 847 with 50 phr L D P E also undergoes stress-induced crystallization. However, quantitative or qualitative differences i n amounts of

20 40 60 80 WEIGHT % LDPE in BLEND

Figure 2.

Tensile yield strength vs. composition for blends of Epcar 847 and LDPE


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A L .

TEMPERATURE(Κ) 200

250

300


Π—ι—ι—ι—ι—ι—I—I—I—I ρ RI

Figure 3.

350

400

I—I—ι—ι—ι—I—I—I—I—Γ

DSC scans of Epcar 847

Figure 3. DSC SCANS of EPCAR 847.

stress-induced orientation between the pure crystalline rubber and its blend with L D P E are impossible to measure. The amorphous Epcar 845 and its blend with L D P E show no stress-induced orientation or crystal lization by W A X D measurements. The crystallinity as measured by W A X D agrees with that calculated from a weighted average of the crystallinity of the individual components. In our differential scanning calorimetry ( D S C ) measurements, a l l samples were quenched i n liquid nitrogen from room temperature to 170 K, heated at 20 K / m i n to 470 K , cooled at 20 K/min to 170 K , and reheated to 470 K . Figure 3 shows the first and second heating scans of the crystalline E P D M rubber. The first D S C heating scans give good representations of the morphology present i n tensile dumbbell specimens which were annealed at least two days at room temperature. The location and shape of the endotherm changes considerably i n the second heating scan. Shih and Cluff also have shown how sensitive the endothermic response of crystalline ethylene copolymers can be to the thermal history of the sample ( 8 ) . Figure 4 shows the first and second D S C heating scans of the blend of crystalline E P D M with 50 phr L D P E . T h e L D P E endotherm changes very little between the first and second heating scan. The rubbers endotherms i n the blend change similar to those of the pure rubber as shown in Figure 3.



Figure 5 compares the second D S C heating scans of pure L D P E , a blend of amorphous E P D M with 50 phr L D P E , and a blend of crystalline E P D M with 50 phr L D P E . The L D P E endotherm is decreased three or four kelvins i n the blends compared with pure L D P E . This could be caused by: (a) slightly smaller or less perfect crystallites i n the blend and/or (b) better heat transfer to the L P D E crystallites i n the blend attributable to intimate contact with molten E P D M chains . Figures 6 and 7 show D S C cooling scans of the pure rubbers, L D P E , and blends of rubber with 50 phr L D P E . The crystallization temperature ( T ) of L D P E (largest exothermic peak) is decreased six kelvins i n the presence of the amorphous E P D M and is decreased 12 kelvins i n the presence of the crystalline E P D M . This T decrease could be attributable to partial solubility of E P D M rubber and L D P E . The soluble E P D M chains would have to be expelled from the L D P E crystalcr

c r


LINDSAY

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A L .



19.


373

374

MULTIPHASE

POLYMERS

lization zones. This would retard the L D P E crystallization rate and lower T . Because the crystalline E P D M has a higher ethylene content than the amorphous E P D M , it should be more soluble with L D P E . The D S C cooling scan of L D P E shows a small exotherm at 325 Κ (see Figure 6 ) . This could be caused b y less perfect crystals or paracrystalline L D P E regions. Perhaps it represents L D P E segments containing the branch points (9) which must lie outside the normal chain-folded lamellae. Depending on crystallization conditions, some segments are normally trapped as loose loops or cilia (10). I n the amor phous r u b b e r / L D P E blend, this small exotherm at 325 Κ has not been shifted. However, i n the crystalline r u b b e r / L D P E blend (see Figure 7 ) ,


c r

Figure 6. DSC cooling scans of an Epcar 845/LDPE blend ( ) and its individual components ( ). Blend contains 50 phr LDPE.



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375


348

3601!

Figure 7.

DSC cooling scans of an Epcar 847/LDPE blend ( ) and its individual components ( ). Blend contains 50 phr LDPE.

this small exotherm is absent or shifted and overlapped. This may indicate an interaction of this L D P E structure and high ethylene E P D M segments. In Figure 7 one can see that the temperature of a portion of the E P D M exotherm is increased i n the blend compared with the pure E P D M . A possible explanation is that L D P E crystallites are nucleating the crystallization of high-ethylene-EPDM segments. I n this case, E P D M crystallites would form layers on L D P E crystallites. Figure 8 shows scanning electron photomicrographs of ion-etched surfaces of the three pure polymers. Presumably, argon-ion bombard-



376

MULTIPHASE

POLYMERS

Figure 8. SEM photomicrographs. 25500 X magnification. Ion-etched surfaces. Polymers and % crystallinity as follows: (a) (top) Epcar 847 (EPDM), 6; (b) (bottom left) Epcar 845 (EPDM), 0; (c) (bottom right) Epolene C-14(LDPE), 32. ment selectively etches a soft amorphous polymer to a greater extent than a hard crystalline polymer. Hence, the nodules seen i n these photomicrographs are probably crystallite residues. Pure amorphous Epcar 845 has no nodules visible by S E M as expected. Diameters of nodules i n pure L D P E and Epcar 847 are about 60 nm on the average. Figure 9 shows S E M photomicrographs of ion-etched surfaces of blends of LDPE/amorphous Epcar 845 and of LDPE/crystalline Epcar 847. Nodule diameters of the LDPE/amorphous rubber blend average



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Figure 9. SEM photomicrographs. 24000 X. Ion-etched surfaces, (a) (hit) Epcar 845 + 50 phr LDPE (amorphous EPDM). (h) (right) Epcar 847 + 50 phr LDPE (crystalline EPDM). about 60 nm—the same as for pure L D P E . Nodule diameters of the LDPE/crystalline rubber blend average about 90 nm. This larger size may be attributable to E P D M crystallized on surfaces of L D P E crystallites. This is in line with D S C data which indicates L D P E crystallites nucleate the crystallization of high ethylene E P D M . One might expect the nodule diameter of pure L D P E to be the same as that in the amorphous r u b b e r / L D P E blend. This could result if the same proportion of L D P E nucleated the crystals and if no amorphous E P D M lay inside the L D P E crystallites. However, the concentration of crystallites would be lower in the blend. It is impossible for us to measure the concentration of crystallites in this blend. The resolution is inadequate and the etching depth is not accurately known. W e w i l l have to look at blends containing less L D P E to see if the crystallite concentration decreases. N o spherulites are seen in these blends by polarized optical microscopy. However, these nodules are too small for optical resolution, and may indeed be spherulites or aggregates of lamellae. Figure 10 compares a transmission electron ( T E M ) photomicrograph, (a), with S E M photomicrographs, (b) and (c), of the same blend of crystalline E P D M and 50 phr L D P E . A thin section of the blend was stained with osmium tetroxide and examined with our Phillips E M 1 0 0 B T E M . In the T E M photomicrograph, 10a, the light spots are probably L D P E crystallites, and the gray regions are probably E P D M rubber (black spots are probably osmium residue). Figure 10b is a view of a solvent-etched surface which has been gently swabbed with xylene, a good solvent for amorphous E P D M . Figure 10c is the ion-etched surface.



378

MULTIPHASE

POLYMERS

Figure 10. Epcar 847 -f 50 phr LDPE. (a) (top) TEM photomicrograph; 10560 X ; OsO -stained thin section, (b) (bottom left) SEM photomicrographs; 24000 X ; ion-etched surface; (c) (bottom right) xylene-etched surface. h

Crystallite diameters estimated from Figure 10a, 10b, and 10c, are 140, 120, and 90 nm, respectively. Hence, these three different sample preparation techniques give crystallite-diameter-size estimates of the same order of magnitude. Summary and Conclusions Blends of L D P E with certain partially crystalline E P D M rubbers exhibit tensile strengths greater than that of either pure component. Blends of L D P E with amorphous E P D M have tensile strengths less than expected from a weighted average of the pure components' tensile strengths.



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D S C measurements show that the crystallization temperature of L D P E is decreased 6 and 12 kelvins i n blends with E P D M rubber having 56 and 72 wt % ethylene respectively. W e believe this indicates partial miscibility between melts of L D P E and high ethylene E P D M rubber. D S C cooling scans also give evidence that upon cooling the high ethylene E P D M / L D P E blend from the melt, the L D P E crystallites nucleate the crystallization of high ethylene E P D M segments. S E M observations of ion-etched surfaces of these blends a d d more evidence that high ethylene E P D M rubber crystallizes on L D P E lamellae in the surface regions of L D P E crystallites. Hence, the morphology of these high tensile blends appears to consist of E P D M reinforced with many tiny L D P E spherulites—much like carbon-black-reinforced S B R . A fraction of high ethylene E P D M chains crystallizes i n surface regions of neighboring L D P E spherulites, tying them together i n a virtually crosslinked network. The crystalline E P D M rubber undergoes further crystallization upon stretching, just like natural rubber. T h e blend of crystalline E P D M with L D P E also stress crystallizes, and its special morphology must help distribute the tensile load more evenly. Acknowledgment The authors would like to thank the B . F . Goodrich C o . for granting permission to publish our findings.

Literature Cited

RECEIVED

1. Batiuk, M., Herman, R. M., Healy, J. C., U.S. Patent 3,919,358 (1975), assigned to BFGoodrich. 2. Batiuk, M., Herman, R. M., Healy, J. C., U.S. Patent 3,941,859 (1976), assigned to BFGoodrich. 3. Carman, C. J., Batiuk, M., Herman, R. M., U.S. Patent 4,046,840 (1977), assigned to BFGoodrich. 4. Stricharczuk, P. T., U.S. Patent 4,036,912 (1977), assigned to BFGoodrich. 5. Carman, C. J., Harrington, R. Α., Wilkes, C. Ε., Macromolecules (1977) 10, 536. 6. Robertson, R. E., Paul, D. R., J. Appl. Polym. Sci. (1973) 17, 2579. 7. Maurer, J. J., Rubber Chem.Technol.(1965) 38, 979. 8. Shih, C. K., Cluff, E. F., J. Appl. Polym. Sci. (1977) 21, 2885. 9. Bovey, F. Α., Schilling, F. C., McCrakin, F. L., Wagner, H . L . , Macromolecules (1976) 9, 76. 10. Boyer, R. F., J. Macromol. Sci., Phys. (1973) B8(3-4), 503. April 14, 1978.


EPDM Blends Having

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