O. A. Serenko

The effect of cold rolling on crack propagation behavior in high-density polyethylene

Crack propagation behavior in HDPE was studied. The preliminary orientation of the polymer, which is deformed in its isotropic state via necking and breaks down at the neck propagation stage, improves the crack resistance and ductility of the material. The critical crack opening in preoriented HDPE samples dramatically increases at relatively low draw ratios of cold rolling while the speed of transverse crack propagation decreases.

Deformability of Particle-Filled Composites at Brittle Fracture

Strain at the brittle rupture of rubber-particle-filled composites based on polymeric matrices deformed via the neck propagation was found to be equal to the value for the onset of the neck propagation in the initial matrix polymer. This characteristic is important for particle-filled composites, because it determines their ultimate elongation at brittle fracture. Conventionally, caoutchouc particles are introduced into brittle polymers, for example, polystyrene, to increase their shock viscosity and deformability [1]. However, the introduction of caoutchouc or rubber particles into a plastic polymer reduces its ultimate strain [2, 3]. For a certain critical filler content, the composite becomes brittle, more precisely, quasibrittle, which is accompanied by a sharp decrease in the rupture strain by a factor of about 100 [3]. The translation from plastic to brittle fracture of the composite occurs because the formed neck loses its capability to propagate along the sample with a certain content of particles, and it ruptures during the neck formation [4, 5].

Criterion of the Appearance of Diamond-Shaped Pores in Dispersely Filled Polymers

Introduction of a small fraction of large particles often embrittles a polymer, and the rupture of the polymer occurs at small relative elongation. The sharp loss of the deformability of the composite is caused by the appearance of so-called diamond-shaped pores [1] observed previously in [2, 3]. It was shown that the size of particles responsible for the appearance of diamondshaped pores is determined by the critical crack opening and, therefore, by the breakdown viscosity of the matrix polymer. Rupture of particles or their separation from the matrix under tension gives rise to the formation of pores whose shape is determined by the size of particles [1]. Small and large particles form oval pores and diamondshaped pores, respectively. With further tension, the two types of pores that appear behave differently. An oval pore develops only along the material-elongation direction. A diamond-shaped pore grows in three directions, parallel and perpendicular to the sample tension axis, in particular, along the sample thickness, which leads to early failure. In polymers deformed by the propagation of a neck, the problem is compounded, because the growth of pores is often localized in the narrow formed neck. As a result, the material breaks down at small macroscopic strain. Although the fracture process (growth of diamond-shaped pores) is typically plastic at the mesoscale, the material behaves as a brittle material at the macroscale. This work aims to determine the dependence of the critical size of particles at which diamond-shaped cracks appear on the properties of the polymer matrix. Lukoten F 3802 medium-density polyethylene, Lipol A4-70 polypropylene, and 168030-070 low-density polyethylene are used for composites. Polymers were filled with powdered-rubber particles with sizes from 50 to 600 μm. A monodisperse filler was obtained by grading the polymers into grain sizes with a standard set of sieves. Each polymer was mixed with rubber particles in a single-screw laboratory extruder. The filler

Effect of Temperature on the Stress-Strain Behavior of a Polypropylene-Particulate Rubber Composite

Composites based on polypropylene and rubber particles were studied at different temperatures. It was found that, as the temperature is elevated, the type of defects that are formed near large filler particles changes from a crack to a diamond-shaped void and, next, to an elliptical or slit-type void. The change in the defect type predetermines the change of the composite failure mechanism at a constant particulate-filler content from brittle fracture before the yield point to fracture during neck formation or propagation and, finally, to non-uniform plastic drawing with a stable neck growth.

The effect of filler content on the lower yield stress of polymer composites

The effect of the concentration of the dispersed elastic filler on the lower yield stress of matrix composites based on plastic polymers is studied. As the matrix polymers, LDPE-HDPE and LDPE-(medium-density PE) are used. The elastic filler is rubber crumb prepared by roll grinding of worn tires or by deformation grinding of ethylene-propylene-diene rubber. Irrespective of the type of filler particles and their adhesion to the polymer matrix, the lower yield stress σd of the composite is described by the linear law σd = σdm(1 − Vf), where σdm is the lower yield stress of the polymer matrix and Vf is the volume content of the filler. Analysis of the published data shows that this relationship is quite general and describes the effect of rigid inorganic particles on the lower yield stress when adhesion between the filler particles and the matrix is poor.

The brittle-ductile transition in rubber-filled polymers

Composites based on various polymers and rubber particles as a filler were studied. As the filler concentration was increased, the transition from necking to brittle fracture and then to uniform ductile yielding was observed. The criterion for the brittle-ductile transition, which is accompanied by an increase in the elongation at break, is equality between the tensile strength and the upper yield stress of the filled composite. Upon the brittle-ductile transition, the critical concentration of rubber particles is determined by two parameters: the height of the yield drop (difference between the upper and lower yield stresses of matrix polymer) and adhesive strength at the interface between the matrix polymer and filler particles (in the case of good adhesion, tensile strength of rubber particles). The larger the yield drop, the broader the concentration range corresponding to the polymer brittle fracture. The enhancement of adhesion between the matrix and the particles makes it possible to displace the brittle-ductile transition to lower filler contents and, hence, to narrow the region of brittle fracture of the composite.

The effect of temperature on the stress-strain behavior of composites based on high-density polyethylene and rubber particles

The failure behavior of composites based on HDPE, which breaks down at the necking stage, and dispersed rubber particles is studied. In was shown that the materials containing at most 8 vol % filler experience the brittle-to-ductile transition with increasing temperature. It was assumed that the ductility retained at elevated temperatures by the composites based on a polymer with unstable neck propagation is due to the interplay of two factors, the decrease in the upper yield point of the matrix polymer and the increase in the polymer draw ratio in the neck. These factors markedly reduce the sensitivity of the materials to the presence of defects and facilitate neck formation and propagation, as well as change the form of the defects from cracks to slitlike pores.

Properties of Ultrahighly Filled Composites Based on Polymers and Ground Rubber 1

The stress-strain and strength properties of ultrahighly filled composites based on thermoplastic polymers and ground rubber wastes are studied. The content of the elastic filler is higher than 70 wt %. As is shown, introduction of minor amounts of the plastic polymer, which serves as the binder for the filler particles, makes it possible to improve the strength properties of ultrahighly filled composites and to prepare materials of a desired thickness. A correlation between the stress-strain properties of the plastic polymer-rubber systems and the effective viscosity of the matrix polymer is established. When a polymer with homogeneous deforma- tion and good adhesion to the elastic filler is used as the matrix, the resultant composites are characterized by properties close to those of vulcanized rubbers. A new method is proposed for processing of ground rubber wastes and preparation of materials that are similar to hard rubbers.

Ductile-to-ductile transition in dispersely filled composites based on thermoplastic polymers

The fracture mechanism for rubber-filled composites based on gutta-percha, LDPE, medium-density PE, and rubber particles has been studied. An increase in the concentration of filler particles leads to a change in the stress-strain behavior of the composites from neck propagation to homogeneous plastic deformation. For the filled composites, the criterion for the ductile-to-ductile transition is the equality of yield and draw stresses. The critical concentration of rubber particles at the ductile-to-ductile transition is controlled by the ratio between the yield stress of matrix polymer and the neck propagation stress. Transition from neck propagation to homogeneous plastic flow of the material is accomplished under two conditions: the breaking strength of the polymer matrix should be higher than the yield stress, and stretching of the composite should not be accompanied by the formation of diamond cracks. The latter condition is fulfilled when the dimensions of rubber particles are below a certain critical value, which is determined by the ductility of the matrix.

Ductile-to-Ductile Transition in Particle-Filled Polymer Composites

One of the main disadvantages of particle-filledcomposites based on thermoplastic matrices is a sharpdrop in the fracture strain in compositions with a fillerconcentration of about 10 vol % and higher. This isexplained by the transition from ductile to brittle frac-ture of the composite. In this paper, it is shown thatembrittlement of a particle-filled composite based on aductile polymer deforming with neck propagation canbe avoided if the following conditions are fulfilled:(i) the ultimate strength of the matrix exceeds its upperyield stress and (ii) extension of the composite is notaccompanied by the formation of diamond pores(cracks). The latter condition is satisfied when the aver-age particle size is below a certain critical value deter-mined by the fracture toughness of the polymer matrix[1].It is known that, when the filler particles are intro-duced into a thermoplastic polymer matrix, the com-posite fracture deformation decreases. If a uniformlyyielding plastic polymer is used as a matrix [2], then thefracture deformation monotonically decreases withincreasing filler content. Composites based on thematrices exhibiting neck formation upon extensionshow a substantially different behavior and, at a certaincritical concentration of the filler, they become brittle.As a result, the fracture deformation sharply decreases,by approximately two orders of magnitude [3]. How-ever, along with the materials in which the transitionfrom ductile to brittle fracture takes place, there arecomposites capable of retaining ductility in a widerange of filler concentrations [4, 5]. The aim of thisstudy was to find the conditions making it possible toavoid the embrittlement of materials based on ductilematrices deforming with the neck propagation.The experiments were performed with low-densitypolyethylene (LDPE) (15803-020 grade), medium-density polyethylene (MDPE) (F 3802 B grade), andtrans-polyisoprene (PI). As a filler, we used polydis-perse particles of vulcanized rubber based on ethylene-propylene-diene terpolymer (EPT) and isoprene rubber(IR) with an average particle size in the range from 10to 600 µm. The conditions under which the materialsbased on LDPE and MDPE matrices were prepared aredescribed elsewhere [6, 7]. Rubber particles and EPTwere blended in a press at a temperature of 100 °C. Theobtained blends were used to make 1-mm-thick platesat a temperature of 150°C and a pressure of 10 MPawith subsequent cooling under pressure. The filler con-tent was varied from 0.02 to 0.36 volume fractions(2−40 wt %).The standard dog-bone specimens for mechanicaltesting were cut from the plates and had a width of5 mm and a length of 35 mm. The tests were performedunder uniaxial tension conditions on a 203R-005 test-ing machine at an extension rate of 20 mm/min.Mechanical tests with the samples of LDPE–IR andPI–EPT composites were performed at room tempera-ture. The samples of MDPE–EPT composites weretested in a thermal chamber at a temperature of 80°C.In this case, a sample was held at the given temperaturefor 5 min prior to test.The mechanical behavior of particle-filled compos-ites was analyzed using an approach developed previ-ously [3, 8, 9], according to which three competingmechanisms of deformation in a filled composite wereconsidered: (i) neck propagation, (ii) brittle fracture,and (iii) uniform ductile yielding. Each of these mech-anisms corresponds to a certain formal parameterdepending on the filler particle concentration. The neckpropagation is characterized by the draw stress σ