Necking Behavior of Low-Density Polyethylene-lsotactic Polypropylene Blends: A Morphological Investigation

GIOVANNI RIZZO and GIUSEPPE SPADARO

Zstituto di Zngegneria Chimica, University of Palerrno, Italy

The tensile behavior of low-density polyethylene-isotactic polypropylene blends was investigated at room temperature. Neck formation and propagation along the whole length of the samples were observed for the whole range of composition. This behavior, which is not indicated by most data available in the literature, was examined in relation to sample morphology by scanning electron microscopy. The results of this investiga- tion indicated some differences between the morphology of these materials and the morphology of blends which do not undergo necking propagation.

INTRODUCTION

n the past several years, blends of various poly- I mers have been extensively studied (1-4), in view of either recycling them from wastes or ob- taining new materials. Unfortunately, as most of these polymers are incompatible in the blend, their mechanical properties are rather poor,

Recently, as a part of a study on the modifications induced in polymeric systems by gamma radiation, physico-chemical and mechanical properties of both irradiated and nonirradiated low-density-poly- ethylene (LDPE) -isotactic polypropylene (iPP) blends have been investigated (5, 6). As for the mechanical properties of nonirradiated blends, an anomalous behavior, with respect to most of data reported in the literature, has been observed. In particular many authors report that polyethylene- polypropylene blends do not undergo any necking propagation and consequently fail at very low elon- gations. This is interpreted in the light of their two- phase structure (7-1 1).

The above mentioned LDPE-iPP blends, on the contrary, showed the formation of a neck which propagated along the whole length of the sample, allowing elongation at break of the same order of magnitude as the two pure components.

In order to establish any relation with the differ- ent tensile behavior, the morphology of our blends was compared with that of blends which did not undergo necking propagation. The results indicate that the ability of the 1,lend to propagate the neck may be related to the structure of the material.

EXPERIMENTAL The two resins were a low-density polyethylene,

Fertene AF.5 1800 (MFI = 0.14; p = 0.919) and an isotactic polypropylene, Moplen D60P (MFI = 0.39; p = 0.906) manufactured by Montedison.

Blends containing 2.5, Fj0, and '7.5 percent LDPE were prepared. In the following discussion the blend composition will be indicated by the weight

percentage of LDPE. Melt blending of weighed quantities of granules was accomplished in a Bra- bender plasticorder at 180 "C and 20 rpm for 15 minutes. The pure components were subjected to the same procedure, in order to make the data comparable to those of the blends.

In order to obtain a material having a tensile behavior similar to that described in (9), a blend containing 50 percent LDPE was prepared with a different mixing time, which was lowered from 15 minutes to 2 minutes. In the following, this will be referred to as "Sobis" blend.

The samples for tensile tests, about 10 mm wide and 0.5 mm thick, were cut from sheets obtained by compression molding in a laboratory Carver press. The polymer was kept at 200 "C and 2.5 x 1 0-' GPa for five minutes; a subsequent rapid cool- ing was obtained by running cold water through the press platens. A thermal treatment for five days at 80 "C was performed. As in another work (5), the tensile behavior of these samples was compared with that of the irradiated materials, for which the thermal treatment was necessary in order to com- pletely remove the trapped free radicals.

Mechanical tests were performed at room tem- perature (-25 "C) by an Instron testing machine, model 111Fj. The deformation rate was 0.5 min-'. The reported data are averaged values of at least five tests.

The morphological investigation was accom- plished by a Philips scanning electron microscope at several magnifications up to 5000. The samples were fractured at liquid nitrogen temperature and the fracture surfaces were made conductive by the deposition of a layer of gold and palladium in a vacuum chamber.

RESULTS AND DISCUSSION

The presence of two individual phases in these blends was revealed by thermal analysis (5). Two melting temperatures were observed, indicating that 110 co-crystallization phenomena occurred.

264 POLYMER ENGINEERING AND SCIENCE, MARCH, 1984, Vol. 24, No. 4

Necking Behavior of Low-Density Polyethylene-lsotactic Polypropylene Blends

I 1

1.

E

Fig. I Typical stress-strain curves from tensile tests. a is the nominal stress. Blend composition is defined by LDPE weight percentage.

Typical tensile curves are shown in Fig. 1 as nominal stress versus strain.

As for blends mixed for 1.5 minutes, the Young moduli and the yield properties monotonically vary between the values relative to the pure components (6). It can be observed that all blends undergo necking propagation, so that all samples fail at high elongation. As for the ultimate nominal strength, a slight minimum is found for the 75 percent blend, which, on the other hand, has the lowest elongation at break.

The tensile behavior of 5Obis blend, on the con- trary, is characterized by neck formation, which does not propagate along the sample.

The ultimate elongation, Eb, is reported in Fig. 2 versus LQPE percentage in the blend. For compar- ison, t b values from Re5 9 are dso reported. It is evident that all our blends but 50bis are character- ized by elongation at break of the same order of magnitude as the pure components, whereas the other blends fail at elongation markedly lower than their homopolymers, according to most of data available in the literature (7, 8, 10, 11).

To interpret these differences, the morphology of our samples was examined. The results of scan- ning electron microscopy of the undrawn speci- mens are illustrated in Fig. 3 for blends mixed for 15 minutes and in Fig. 4 for the 50bis blend.

In Fig. 3, typical features of the morphology of pure components are clearly evidenced by micro- graphs (u ) and (e): LDPE has a moon-like structure where bundles of short lamellae (lp or less) are easy to recognize, whereas iPP lamellae are many micrometers in length. Micrograph (b) shows that 7.4 percent blend keeps the texture of short lamel- lae observed in pure LDPE, though slightly modi- fied by the presence of iPP. As for S O and 25 percent blends, micrographs (c) and (d), respec- tively, it is possible to observe a matrix in which polypropylene lamellae are distinguishable. It is worth noting that iPP lamellae are smaller than those observed in the pure polymer (micrograph e ) . This can be attributed to the presence of poly- ethylene, which acts as a nucleating agent for iPP

A

P + @

. 4 l P

A A

A A

‘ A

0 26 60 76 $00

LOPE %

Fig. 2. Elongation at break, Cb, versus blend composition. 0 averaged values measured for mixing time of 15 min

averaged ualue measured for mixing time of 2 min A averaged values from ReJ 9.

(9). Moreover the morphology of these blends is characterized by the presence of very small spher- ical domains of LDPE (less than 1p diameter), some of which are visible on the fracture surface, while others are evident from the holes left in the matrix. As a general consideration, in no case it was possible to observe larger LDPE “islands” as reported by Lovinger and Williams (9), who found the presence of LDPE islands up to l op diameter in their blends prepared by a two roll mixer.

On the contrary, morphological features as in (9) are evident in micrograph shown in Fig. 4 , which is relative to 50bis blend; islands of polyethylene many micrometers in length can be observed.

Micrographs reported in Figs. 3 and 4 and in (9) indicate that the mixing procedure affects the mor- phology of blends. This may explain the different tensile behavior, i.e., high elongation at break after neck propagation along the whole length of the sample in our blends mixed for 15 minutes versus low elongation at break without any neck propa- gation both in our blend mixed for 2 minutes and in (9). In fact the neck formation and propagation imply interlamellar and intralamellar slip which produce the transformation of the initial spherulitic structure into a fibrillar structure (12-14). It is reasonable that in a material which is made by two incompatible phases, their mutual boundaries play an important role on its properties; in the blends characterized by a good mixing of the two compo- nents (Fig. 3 ) the strong interactions between the crystallites and their tie molecules of the two LDPE and iPP distinct phases make possible structure rearrangements, such that the local stresses may be distributed over the sample, which, rather than fail, can deform into a fibrillar structure. Figure 5 is a micrograph of the necked zone of a blend after necking propagation along the whole length of the sample.

The consideration that the mixing procedure strongly affects the blend properties is confirmed by the presence in the literature of a wide variety

POLYMER ENGINEERING AND SCIENCE, MARCH, 1984, Yo/. 24, No. 4 265

G. Rizzo and G. Spadaro

Fig. 3. A4icrogru~iii~s of rmtlrutoi scriiip1e.s fi-uctirrcvl i n liqriirl iiitro- gc’ri. a , b, c, tl uiid e ref.. to 100, 7.5, 50, 2.5, atid 0 percetit LDPE i n the l h i t l , rrspectioely. The riglit .side of’microgruph r is u 4 . 4

fo ld rnugnijcution of ihe window drawn on tile left side, the same mugiiifi:cution ratio as tlw other micrographs, i.e. 5000.

266 POLYMER ENGINEERING AND SCIENCE, MARCH, 1984, Vol. 24, No. 4

Necking Behavior of Low-Density Polyethylene-lsotactic Polypropylene Blends

Finally, it is worth reporting that the thermal treatment under vacuum at 80 “C for five days (see Experimental) did not affect the necking behavior of our blends, as confirmed by many tests per- formed on samples just molded in the Carver press, without any thermal treatment.

CONCLUSIONS The ability of LDPE-iPP blends to undergo neck-

ing formation and propagation in tensile tests, which is unusual behavior with respect to most of data available in the literature, may be related to the morphology of the blends, as evidenced by scanning electron microscopy. It may be reasonably interpreted on the basis of the neck propagation mechanism. This is only, however, a partial view of the necking phenomenon, which is affected by many other factors not taken into account in this work.

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Fig. 4. Micrograph of an undrawn sample of 50bk blend, The magngcation ratio is 5000.

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Fig. 5. Micrograph of a drawn blend sample in the necked zone. The sample is relatiue to the blend containing 50 percent LDPE mixed for 15 minutes. The right side is a 6.1 fold magnifzation of the window drawn on the left side, i.e. 6700.

of behavior in relation to different blending proce- dures (8-10, 15).

However, it has to be pointed out that the pres- ence of such a complex phenomenon as necking cannot be satisfactorily explained just by the mor- phology of the blend, as it is affected by many factors, such as the molecular weight of the two polymers, their crystallinity and type of crystalline structure (1 6, 17), the testing temperature (18), etc.

POLYMER EMWEERlW AND SCIENCE, MARCH, 1984, Yd. 24, No. 4 267