Doklady Physics, Vol. 47, No. 11, 2002, pp. 822–824. Translated from Doklady Akademii Nauk, Vol. 387, No. 3, 2002, pp. 329–332.Original Russian Text Copyright © 2002 by Serenko, Goncharuk, Bazhenov.

TECHNICAL PHYSICS

Deformability of Particle-Filled Compositesat Brittle Fracture

O. A. Serenko, G. P. Goncharuk, and S. L. BazhenovPresented by Academician N.F. Bakeev April 8, 2002

Received April 19, 2002

Strain at the brittle rupture of rubber-particle-filledcomposites based on polymeric matrices deformed viathe neck propagation was found to be equal to the valuefor the onset of the neck propagation in the initialmatrix polymer. This characteristic is important for par-ticle-filled composites, because it determines their ulti-mate elongation at brittle fracture.

Conventionally, caoutchouc particles are introducedinto brittle polymers, for example, polystyrene, toincrease their shock viscosity and deformability [1].However, the introduction of caoutchouc or rubber par-ticles into a plastic polymer reduces its ultimate strain[2, 3]. For a certain critical filler content, the compositebecomes brittle, more precisely, quasibrittle, which isaccompanied by a sharp decrease in the rupture strainby a factor of about 100 [3]. The translation from plas-tic to brittle fracture of the composite occurs becausethe formed neck loses its capability to propagate alongthe sample with a certain content of particles, and itruptures during the neck formation [4, 5].

Translation to brittle fracture occurs when the initialmatrix is deformed via the neck propagation [4, 5]. Oth-erwise, the introduction of a filler into the polymer onlymonotonically reduces rupture strain in a wide intervalof compositions [6]. Brittle fracture is observed in anarrow range of filling degree that is determined by thetype of the matrix and by the capability of the matrix forstrain hardening [3]. The typical interval of the brittlebehavior of a composite containing a hard mineral orelastic particles lies from 10 to 40 vol % of filler. Forhigher contents of rubber particles, the next translationoccurs from the brittle fracture to the homogeneousplastic deformation and is accompanied by an increasein the rupture strain of the filled polymer [7]. The pur-pose of this study is to investigate the deformability ofcomposites consisting of polyethylene matrices andrubber particles in the region of quasibrittle behavior.

Institute of Synthetic Polymeric Materials,Russian Academy of Sciences, ul. Profsoyuznaya 70, Moscow, 117393 Russia

1028-3358/02/4711- $22.00 © 20822

To prepare composite materials, we used low-den-sity polyethylene (LDPE), medium-density polyethyl-ene (MDPE), their mixtures, and mixtures of LDPE andhigh-density polyethylene (HDPE). The brands andproperties of the polymers, as well as the properties ofpolyethylene mixtures, are presented in the table. Weused a polydisperse rubber crumb with a particle sizeranging from 0.05 to 0.6 mm as a filler and varied thefilling degree from 0 to 36 vol %.

The composites were prepared by mixing in a sin-gle-screw laboratory extruder. Then, 2-mm plates wereformed by pressing the mixtures at a temperature of160°C and a pressure of 10 MPa and were cooled to20°C under pressure. From the plates, we cut samplesin the form of a two-sided paddle with a 5 × 35-mmworking area.

The mechanical tests of the composites were carriedout on a Shimadzu Autograph AGS-10 kNG universaltesting machine and on a 2038R-005 dynamometer atroom temperature and a tension rate of 20 mm/min. Thesample surface was studied by a computerized opticalmicroscope Q × 3.

All the polymeric matrices being investigated weredeformed with the formation of a neck. Figure 1 showstypical stress–strain curves for filled polymers with var-ious contents of rubber particles. For a low filler con-tent, the composite ruptures as the neck propagatesacross the working area of the sample (curve 2). As thefilling degree increases, the plastic deformation of thecomposite changes to brittle, more precisely to qua-sibrittle, fracture (curve 3). The material ruptures whenthe neck is formed. Further filling leads to the disap-pearance of the sharp yield point in the stress–straindiagram of the composite (curve 4).

Figure 2 shows the typical appearance of the rup-tured material containing 36 vol % of rubber particles.In the rupture plane of the sample, we observe a con-traction associated with neck formation.

Figure 3 shows the most characteristic dependencesof rupture strain εc for the composites being investi-gated on rubber-particle content Vf . The introduction ofa small amount of rubber particles (1.6 vol %) substan-tially reduces material deformability due to the large

002 MAIK “Nauka/Interperiodica”

DEFORMABILITY OF PARTICLE-FILLED COMPOSITES AT BRITTLE FRACTURE 823

size (up to 0.6 mm) of particles of the polydisperse elas-tic filler. The filling of the polymers under investigationwith a monodisperse elastic filler of smaller particlesincreases rupture strains in the composites for lowdegrees of filling but does not affect the deformationmechanism and the general form of εc as a function ofVf . The filled polymer is deformed via the formation

1

234

16

8

0 200 400ε, %

σ, MPa

εd

Fig. 1. Stress–strain curves of the composites based onmatrix no. 9 from the table. The content of an elastic filleris (1) 0, (2) 6, (3) 9, and (4) 26 vol %. The arrow indicatesthe neck-formation strain εd in the matrix polymer.

DOKLADY PHYSICS Vol. 47 No. 11 2002

and propagation of a neck. For a certain filler content,the material loses deformability and ruptures as theneck is formed. With the further increase in Vf , the ulti-mate elongation of the composite remains almost con-stant for the amounts of filling being investigated.

Deformability decreases sharply for a certain criti-cal filler content, because the plastic behavior of the

10 mm

Fig. 2. Appearance of the fractured sample of the compositebased on matrix no. 9 from the table and containing36 vol % of an elastic filler.

Composition and properties of polymeric matrices (engineering magnitudes of stresses are given)

No. Composition Polymer brandUpper

yield pointσy, MPa

Neck-propa-gation stress

σd, MPa

Neck-for-mation

strain εd, %

Strengthσc, MPa

Ultimate strain εc, %

1 LDPE 16803-070 8.0 7.8 70 12.7 550

2 LDPE 15803-020 10.2 10.0 86 16.7 600

3 95 wt % LDPE + 5 wt % HDPE

15803-020 + 277-73 8.9 8.7 90 11.8 380

4 90 wt % LDPE + 10 wt % HDPE

" 9.3 9.0 77 11.5 360

5 85 wt % LDPE + 15 wt % HDPE

" 9.3 8.8 76 10.2 340

6 75 wt % LDPE + 25 wt % HDPE

" 10.1 9.5 60 10.9 320

7 70 wt % LDPE + 30 wt % HDPE

" 11.4 9.8 53 11.8 340

8 HDPE F 3802 B 21.3 15.5 27 26.5 720

9 70 wt % LDPE + 30 wt % HDPE

15803-020 + F 3802B 14.7 12.1 60 14.3 450

10 LDPE 16204-020 11.0 10.6 100 16.8 600

11 80 wt % LDPE +20 wt % HDPE

16204-020 + 277-73 13.7 11.6 70 15.6 520

12 70 wt % LDPE + 30 wt % HDPE

" 16.4 12.5 35 15.8 510

824

SERENKO

et al

.

composite (interval I, Fig. 3) changes to brittle fracture(interval II, Fig. 3) [3]. The critical filling for thischange in the deformation mechanism depends on boththe reinforcement degree [3, 4] and the elongationdegree of the matrix-polymer neck.

The ultimate elongation at brittle rupture (interval II,Fig. 3) is virtually independent of the filler content but

12

34, 5

800

0 40Vf, %

400

20

ε, %

III

Fig. 3. Ultimate strain in composites based on matrices(1) no. 1, (2) no. 9, (3) no. 11, (4) no. 12, and (5) no. 8 (num-bers according to table). Intervals I and II correspond to theplastic deformation and brittle rupture of the materials,respectively.

120

0 40 εd, %

(8)

(12)

(7)(6)(1, 11)

(5) (10)

(3)

(2)(4)

(9)80

40

80 120

εc, %

Fig. 4. Ultimate strain in composites at brittle fracture vs.the neck-formation strain in the matrix polymers, whosenumbers according to the table are given in the parentheses.

varies for various matrix polymers. All the compositesin this region of compositions rupture in the process ofneck formation before its propagation along the sam-ple. Figure 4 shows the correlation between the rupturestrain εc of the composite at brittle fracture and thestrain εd at the onset of neck propagation in the initialmatrix polymer. The latter value is denoted by thearrow in Fig. 1. The dependence shown in Fig. 4 is lin-ear and has a slope close to 45°. Consequently, the rup-ture strain εc of the composite is equal to the strain εd atthe onset of neck propagation in the initial matrixpolymer:

εc = εd .

This correlation explains why the brittle rupture ofvarious composites occurs at different ultimate elonga-tions. The brittle rupture of a composite based on amatrix polymer where the neck is formed for low (high)strains as, for example, in MDPE (LDPE) occurs at low(high) ultimate elongation.

Thus, the strain at the onset of neck propagation isan important characteristic of the material and deter-mines the deformability of filled composite systems atbrittle fracture.

REFERENCES

1. C. B. Bucknall, Toughened Plastics (Applied SciencePublishers Ltd., London, 1977; Khimiya, Leningrad,1981).

2. N. A. Erina, S. G. Karpova, O. A. Ledneva, et al.,Vysokomol. Soedin., Ser. B 37 (8), 1392 (1995).

3. O. A. Serenko, V. S. Avinkin, and S. L. Bazhenov,Vysokomol. Soedin., Ser. A 44 (8), 457 (2002).

4. S. Bazhenov, Polym. Eng. Sci. 35 (10), 813 (1995).5. Al. Al. Berlin, V. A. Topolkaraev, and S. L. Bazhenov,

Physical Aspects of Production of Failure and Deforma-tion, Collection of Scientific Works (Fiziko-Tekh-nicheskiœ Inst., Leningrad, 1987).

6. S. L. Bazhenov, V. S. Avinkin, and O. A. Serenko, Dokl.Akad. Nauk 382 (3), 341 (2002) [Dokl. Phys. 47, 72(2002)].

7. S. L. Bazhenov, G. P. Goncharuk, and O. A. Serenko,Dokl. Akad. Nauk 379 (5), 620 (2001) [Dokl. Phys. 46,562 (2001)].

Translated by V. Bukhanov

DOKLADY PHYSICS Vol. 47 No. 11 2002