Mechanical behaviour of low-density polyethylene modified with maleic anhydride in the solid state and composites based on it

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1 Plasticheskie Massy, No. 7, 2004, pp Mechanical behaviour of low-density polyethylene modified with maleic anhydride in the solid state and composites based on it A. N. Zelenetskii, V. P. Volkov, L. O. Bunina, A. S. Kechek yan, E. S. Obolonkova, and M. D. Sizova Institute for the Synthesis of Polymeric Materials (ISPM) of the Russian Academy of Sciences Selected from International Polymer Science and Technology, 31, No. 10, 2004, reference PM 04/07/24; transl. serial no Translation submitted by P. Curtis The modification of polyolefins (POs) makes it possible to broaden considerably the areas of their application. In particular, when carboxyl groups are introduced into the PO chain, there is a basic change in adhesion capacity, which enables the POs to be used as surface modifiers, adhesives, compatibilisers, and also binders in the manufacture of composite materials [1]. A promising method of modification is the solidphase mechanochemical graft copolymerisation of polyethylene (PE) and polypropylene (PP) with maleic anhydride (MA), making it possible to minimise the occurrence of secondary reactions of degradation, crosslinking, and oxidation during modification [2 4]. Here, in contrast to synthesis in the melt, the entire combination of mechanical properties of the initial polymers is retained. To date, no one has studied the mechanical characteristics of solid phase modified low-density polyethylene (LDPE). In the present work, an investigation has been made of the effect of solidphase modification on the mechanical properties of LDPE and its wood polymer composites, and its adhesion to aluminium foil. EXPERIMENTAL The comminution of granules of industrial LDPE of grade with a melt flow index of 8 g/10 min and a weight-average molecular weight of was carried out on a Bersstorf twin-screw extruder at 80 C. LDPE powder was mixed with maleic anhydride (MA) powder with a melting temperature of 54 C. Benzoyl peroxide (BP 0.5%) was used as the initiator. The mixture was fed to a Bersstorf twin-screw, continuous-action extruder consisting of a charging zone and four zones with a variable thermal regime. Treatment of the reactants was carried out in a solidphase regime on an extruder at temperatures in zones II to V of C, i.e. below the melting temperature of the polymer, for 7 70 min. The morphology of the particles was studied on a JSM LM scanning electron microscope of the Japanese company JEOL at a magnification of Sheets were moulded from powder materials on a closed compression mould at a temperature of 155 C and a pressure of 5 MPa. For tensile tests, specimens were cut out of the sheets in the form of dumb-bell testpieces with a working length of 22 mm and a width of 3.3 mm. Tests of the materials were carried out on an Autograph AGS-10 kng Shimadzu machine (Japan) at an elongation rate of 22 mm/min. Flex testing of composites with a wood filler was carried out on specimens moulded in the form of bars on a WPM machine (Germany). The ratio of polymer (control or modified) and wood filler was 50:50. Birch sawdust with a particle size of <0.25 mm was used as the wood filler. Specimens were moulded in a closed compression mould at T = 180 C and a pressure of 10 MPa. The cross-breaking strength was determined. T/40 International Polymer Science and Technology, Vol. 32, No. 1, 2005

2 The adhesion between the aluminium foil and the control or modified LDPE was measured on Al polymer Al laminates. Similar laminates were prepared from fabric and were tested by the same procedure [2]. RESULTS AND DISCUSSION Study of the Morphology of the Polymeric Materials Important characteristics of powder materials are the shape, size, and surface structure of the particles. During modification and processing, the probability of the polymer getting into the working part of the extruder and its residence time there depend on these parameters. The shape, size, and surface structure of particles were studied by means of electron microscopy. Electron micrographs of particles of the control (i.e. deformed by comminution) LDPE are presented in Figure 1a d. It can be seen that the particles, both coarse and fine, chiefly have an anisometric extended shape, while individual particles are flattened. Also, fibre-like particles are observed (Figure 1a c). The length of the particles ranges from a few to hundreds of micrometres, the transverse dimension of the particles ranges from fractions of a micrometre to 10 μm, and the surface of the particles (Figure 1c and d) is smoothed. Electron micrographs of particles of modified LDPE are presented in Figure 2a d. Both coarse particles (up to 500 μm, Figure 2a and b) and fine particles (1 10 μm, Figure 2c and d) chiefly have an oval shape, in contrast to unmodified LDPE. The coarse particles are loose agglomerates of finer particles. On the surface of the coarse particles there are pores (up to 40 μm) between individual finer agglomerates of particles (Figure 2d). Change in the particle shape is connected with the presence of graft carboxyl groups. Interaction of carboxyl groups emerging at the surface promotes the folding of the initial lengthened LDPE particles into oval particles and their agglomeration. Figure 1. Electron micrographs of the control LDPE: a 10 µm; b, c 50 µm; d 20 µm Figure 2. Electron micrographs of modified extracted LDPE: a 1 µm; b 200 µm; c 100 µm; d 10 µm Deformation and Strength Properties of the Polymeric Materials The main deformation and strength characteristics of the materials were determined from stress strain diagrams. Stress strain diagrams of the control and modified LDPE before and after extraction of the modifier (MA) from it are given in Figure 3. All the materials were obtained from powder with a particle size of less than μm. It can be seen that the nature of deformation of the two materials is similar, and the spread of experimental data is small and amounts to 10% (Figure 3, Table 1). Stress strain diagrams of the control and modified polyethylene are given in Figure 4. The yield point of Figure 3. Stress strain diagrams of LDPE and modified LDPE (<0.315 µm fraction) International Polymer Science and Technology, Vol. 32, No. 1, 2005 T/41

3 Figure 4. Average stress strain diagrams of LDPE and modified LDPE after extraction of unreacted modifier (<0.315 µm fraction) Figure 5. Effect of extraction on stress strain diagrams of modified extracted LDPE of <0.315 µm fraction: 1 extracted; 2 not extracted Table 1. Effect of modification on properties of LDPE, fine fraction (<0.315 µm) Modified LDPE extracted LDPE E E, MPa ΔE, MPa V, % σ T σ T, MPa Δσ T, MPa V, % ε T ε T, % Δε T, % V, % σ f σ f,mpa Δσ f, MPa V, % ε b ε b, % Δε b, % V, % Table 2. Effect of extraction on mechanical properties of specimens of modified LDPE, fine fraction (<0.315 µm) Modified unextracted LDPE Modified extracted LDPE E E, MPa ΔE, MPa V, % σ T σ T, MPa Δσ T, MPa V, % ε T ε T, % Δε T, % V, % σ f σ f, MPa Δσ f, MPa V, % ε b ε b, % Δε b, % V, % the control polyethylene is less pronounced than that of the modified polyethylene. In the strain range from 75 to 400%, the entire LDPE specimen passes into a neck. The section of strain hardening of the material is complex in nature: after a fairly rapid increase in stress, a rapid reduction is observed, in some cases up to the formation of a plateau. Then the rate of increase in stress again increases, but without reaching the rate on the first section. The observed effect comprises a so-called second-order neck [5]. With the formation of the primary neck, the first stage of orientation occurs, i.e. shear strain of the amorphous and poorly ordered phase in the direction of the maximum shear stress. Then, coarse supermolecular formations begin to be divided into simpler composite elements and orientation of the crystallites begins, which is characterised by the appearance of the second-order neck. In the case of modified LDPE the yield point is pronounced, and the specimens passes into a neck earlier than in the case of the control polyethylene (from 30 to 350%). In the T/42 International Polymer Science and Technology, Vol. 32, No. 1, 2005

4 strain range from 175 to 350%, a neck develops with slow strain hardening, and then a rapid increase in hardening is observed. The specimen fails before the formation of the second-order neck. Modification leads (Table 1) to a strong increase in the elastic modulus and tensile strength, which seems to be due to the formation of a physical network of intermolecular hydrogen bonds of graft carboxyl groups. The elongation corresponding to the yield point and the breaking elongation decrease (Table 1). Stress strain diagrams for extracted and non-extracted modified polyethylene from powders with a particle size of <0.315 μm (Figure 5) differ little. However, for the non-extracted material the section of strain hardening begins earlier (220%) than for the extracted material (350%), and the elastic modulus and yield point are lowered (Table 2). Probably, in the non-extracted modified LDPE, modifier molecules are associated with graft groups and prevent the formation of a physical network of intermolecular hydrogen bonds, and therefore the elastic modulus remains as in the initial LDPE. Figure 6 gives average stress strain diagrams for extracted modified LDPE of two fractions: material from particles of <1 μm size and <0.315 μm size. The change in the fractional composition of the initial particles of the material has little effect on the nature of the stress strain diagrams, but the curve for the fine fraction of the material is positioned higher, i.e. the strength properties are higher. Figure 6. Effect of initial particle size on stress strain diagrams of modified extracted LDPE: 1 <0.315 µm fraction; 2 <1.0 µm fraction From Table 3 it follows that, for material from powder of smaller particle size, the elastic modulus, the yield point, and the stress causing failure are greater, which may be connected with the greater homogeneity of the material from the finer powder. The deformation properties of the materials of different particle size differ little. Table 3. Influence of initial particle size on mechanical properties of specimens of modified extracted LDPE Modified extracted LDPE <1 µm <0.315 µm E E, MPa ΔE, MPa V, % σ T σ T, MPa Δσ T, MPa V, % ε T ε T, % Δε T, %% V, % σ f σ f,mpa Δσ f, MPa V, % ε b ε b, % Δε b, % V, % The results of investigations of modified PE by means of gel permeation chromatography are presented in Table 4. It can be seen that the weight-average molecular weight of the modified PE decreases in all experiments. Experiments 2 to 4 (Table 4) differ in treatment time. The longer the time of the reaction, the greater is the fall in the MW. The initial initiator concentration affects the fall in the MW (Table 4, experiments 3 and 5). In experiment 3, with double the initial initiator content, the fall in the MW is more significant than in experiment 5. Under identical modification conditions, the different type of monomer has little effect on the fall in the MW (Table 4, experiments 6 and 7). Thus, with the solid-phase modification of PE with different comonomers in the extruder, the molecular weight decreases. Furthermore, the degradation of PE is sensitive to the amount of initiator in the initial mixture. However, all the above relates to the weight-average indices. Not only do the number-average indices not decrease, in a number of cases they increase, and significantly. In this case the MWD of the specimens is narrowed. The compensation effect present is caused, strange though this may seem, by the formation of a more homogeneously distributed molecular structure of the PE during its mechanochemical modification. The LDPE modified in the solid state possesses considerably higher adhesion to aluminium (by an order of magnitude) and cotton fabric than the control polyethylene International Polymer Science and Technology, Vol. 32, No. 1, 2005 T/43

5 Table 4. GPC data of modified and initial PE in o-dichlorobenzene (140 C, c = 0.05 wt.%) Experiment No. Modifying monomer V, wt.% DAA, wt.% Content of graft monomer c 10 3, mol/unit Reaction time, min M w 10 3 M n 10 3 M w /M n Maleic acid Disodium salt of Table 5. Adhesion properties of control and modified LDPE No. Polymer Tear strength at separation angle of 180, σ tear 10 3, N/m Al Cotton fabric Reference 1 Control LDPE [2] 2 Modifi ed LDPE [2] 3 Modifi ed PE 0.25 [6] (Table 4). It must be emphasised that PE modified in the melt [6] has lower adhesion (Table 5, No. 3). Increase in the adhesion of modified polyethylene to aluminium may be due to the emergence of hydrogen bonds between the graft carboxyl groups of the LDPE and the hydroxyl groups of the surface layer of the aluminium [7] and to the formation of salt bonds. Adhesion is the main factor affecting the mechanical properties of the composite. The degree of strengthening of the polymer composites depends on the adhesion of the filler to the matrix [8]. In the case of the absence of adhesion, there is no strengthening. Owing to chemical modification of the matrix, the adhesion of PE to the filler increases. The cross-breaking strength of composites based on wood and modified LDPE is considerably higher than that of composites with the same wood content but with the control polyethylene (Table 6, Nos. 1 to 5). For example, with a low polymer content (10%), the strength of the composite with modified LDPE is almost twice that of a composite with the same amount of wood but the control LDPE and amounts to 27.0 MPa. With a ratio of components (wood polymer) of 50:50 and 70:30, the cross-breaking strength of the composite is 40% higher than that of the composite with the control polyethylene and amounts to 32.5 MPa. Interest in composites based on wood and polyethylene modified with MA is increasing at present. Thus, Balasuria et al. [9] used a stronger matrix (HDPE modified with MA in the melt) and obtained a composite with a strength of 42.0 MPa (50% wood) and a composite with a strength of 34.0 MPa (60% wood). The highest strength (about 70 MPa) was possessed by composites based on solid-phase modified PP and wood (50 70%) [2]. Increase in the adhesion of MA-modified polyethylene to cotton fabric and increase in the cross-breaking strength of composites containing modified polyethylene and sawdust are probably due to the interaction of graft carboxyl groups with hydroxyl groups of the cellulose with the formation of hydrogen bonds. CONCLUSIONS Thus, when a small amount ( mol/ethylene unit) of comonomer groups are introduced into the T/44 International Polymer Science and Technology, Vol. 32, No. 1, 2005

6 Table 6. Effect of modification of LDPE on cross-breaking strength of composites based on polymer and comminuted wood No. Polymer Composition, % Cross breaking Reference Polymer Sawdust strength σ cb, MPa 1 Control LDPE [1] 2 Control LDPE [1] 3 Modifi ed LDPE [1] 4 Modifi ed LDPE [1] 5 Modifi ed LDPE [1] 6 Modifi ed HDPE [9] 7 Modifi ed HDPE [9] chain of LDPE by means of solid-phase extrusion, there is a considerable increase in the elastic modulus of the polymer with a certain reduction in the tensile strain. Solid phase modified POs possess considerably higher adhesion properties compared with POs modified in the melt. This was particularly clear from the very high strength indices of wood polymer composites based on them. This seems to be due to the specific nature of solid-phase modification, which makes it possible, firstly, to produce a special (differing sharply from the structure of polymers modified in the melt) structure of copolymers containing graft polar groups in the chain that are capable of forming high-strength hydrogen bonds with hydroxyl and carboxyl groups, and secondly to retain the deformation and strength characteristics of the initial polymers. REFERENCES 1. E. V. Prut and A. N. Zelenetskii, Uspekhi Khimii, 70, No. 1, 2001, pp N. S. Enikolopov et al., Vys. Soed., A36, No. 4, 1994, pp V. P. Volkov et al., Plast. Massy, No. 3, 1997, pp A. N. Zelenetskii et al., Vys. Soed., A41, No. 5, 1999, pp A. S. Naumenko, Mekhanika Polimerov. Kratkie Soobshcheniya, 5, 1974, p N. Gaylord, US Patent , S. Hindin and S. Weller, J. Phys. Chem., 60, 1956, p E. S. Zelenetskii et al., Vys. Soed., A36, No. 4, 1994, pp P. Balasuria et al., Appl. Polym. Sci., 83, 2002, pp (No date given) International Polymer Science and Technology, Vol. 32, No. 1, 2005 T/45