DISPERGATION OF VERMICULITES IN POLYMER MATRIX AND ELECTED PROPERTY CHARACTERIZATION OF PREPARED NANOCOMPOSITES

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1 DISPERGATION OF VERMICULITES IN POLYMER MATRIX AND ELECTED PROPERTY CHARACTERIZATION OF PREPARED NANOCOMPOSITES Dušan KIMMER a, Jan FENYK a, Martin ZATLOUKAL b, Petr SLOBODIAN b, Karla ČECH BARABASZOVÁ c a SPUR a.s., T. Bati 299, Zlín, Česká repulika b Centrum Polymerních Systémů, Polymerní Centrum, Univerzita Tomáše Bati ve Zlíně, nám. T.G.Masaryka 5555, Zlín, Česká republika c Centrum Nanotechnologií, VŠB Technická Univerzita Ostrava, 17. listopadu 15/2172, Ostrava Poruba, Česká republika Abstract Clay minerals in the form of nanofillers in polymer matrices are usually studied with the aim to improve mechanical properties, thermal and chemical stability and flame retardancy of nanocomposites. Clay mineral vermiculite was treated mechanically and chemically as a nanofiller and its exfoliation in polyolefins was studied. Effects of jet and ball milling and of chemical modification of vermiculite by means of intercalation with hexadecyl trimethyl ammonium bromide, modification with HCl and different homogenisation procedures including electrospinning of solution dispersions on exfoliation of vermiculite in a polymer matrix were studied in composites. Dispersion and content of nanoparticle in the matrix were characterized by X-ray diffraction analysis, scanning electron microscopy, light optical microscopy and thermogravimetric analysis. Effect of dispersion of vermiculite nanofiller particles in polyethylene on processing, mechanical and service properties of the composite was demonstrated by determining melt strength, by tensile and creep. Key words: Nanoclay, PE composites, vermiculite, melt strength, exfoliation 1. INTRODUCTION i) Treatment of the filler - intercalation in a polymer solution where silicate was dispersed and swelled in the solution of the polymer, which was gradually absorbed on thin layers of clay. By evaporating the solvent (in some samples by electrospinning) a sandwich structure with polymer chains closed between the clay layers was formed. As starting material Brazilian vermiculite was used. The vermiculite was ground in a ball mill and subsequently sieved to a fraction below 40 µm or the initial material treated in this way was ground in a jet mill to a fraction of approximately 5 µm. The above mentioned vermiculites were modified with hexadecyl trimethyl amonium bromide (HDTMAB) or HCl. In order to achieve the maximum exfoliation and subsequent uniform distribution of the filler in the polymer matrix also processing of the clay dispersion in a solution of ethylene-vinyl acetate (EVA) copolymer in an electrostatic field was tested when treating the filler. ii) Preparation of the composite - intercalation in melt with the treated clay being mixed into polymer melt aiming at preparing nanocomposites containing clay filler exfoliated (delaminated) to maximum.

2 2. EXPERIMENTAL 2.1 Raw materials used Polymers EVATANE EVA copolymer from Arkema Company, USA Bralen VA LD-PE both in granular and ground forms, manufactured by Istrochem Bratislava, Slovakia Bralen FB 2-17 LD-PE, ground. Manufactured by Istrochem Bratislava, Slovakia Intercalation additives HDTMAB - hexadecyl trimethyl amonium bromide supplied by Sigma-Aldrich HCl - hydrochloric acid manufactured by Vitrum, Czech Republic Vermiculites, treatment of vermiculites The starting material for preparation of the fillers was Superfind Brasilian vermiculite having an undersize of 40 µm manufactured by Grena a.s., Czech Republic. Tab. 1 A list of the filler samples prepared Sample No. Designation Characteristic 1 VER 40 Untreated VER (having an undersize of 40 μm) 2 VER J5 Untreated VER ground in a jet mill to approximately 5 μm 3 VER HDTMA 40 4 VER HDTMA J5 VER having an undersize of 40 μm treated with hexadecyl trimethyl amonium bromide VER ground in a jet mill treated with hexadecyl trimethyl amonium bromide (HDTMAB) 5 VER HCl 40 VER having an undersize of 40 μm treated with HCl 6 VER HCl J5 VER ground in a jet mill (particle size approximately 5 μm) treated with HCl Preparation of PE/vermiculites composites in melt Composites containing clay particles, EVA copolymer and a PE blend in a mass ratio of 3:9:88 and materials containing no EVA copolymer in a mass ratio of PE:clay = 97:3 were prepared. In order to determine precise proportions of separate components in the composite, presence of further substances (humidity, intercalation additives and, possibly, electrospinning additives) was checked, always gravimetrically, using thermogravimetric analysis (TGA). The PE matrix comprised a blend of three BRALEN LD-PE grades having a composition as follows: V20-60 (granulate), V20-60 (powder) and FB2-17 (powder) in a mass ratio of 4:5:3. The composites were mixed in a BRABENDER internal mixer at the temperature of 150 C and moulded in mechanical presses at the temperature of 160 C.

3 2.2 Physical methods used to characterize the materials TGA analysis Thermogravimetric analysis was carried out to determine content of the clay filler in the samples prepared by electrospinning. Since some modified samples of the vermiculites contained additives, thermogravimetric analysis (TGA) was used to determine the clay content in each sample in order to be able to prepare composites containing a constant amount of the vermiculate. The analysis was done on a TA INSTRUMENTS TG Q500 analyser. Measuring conditions used to measure all samples were as follows: carrier gas - nitrogen (60 ml/min) mass of the samples: 0.02 ± g the samples were heated from 25 C to the final temperature of 600 C, the heating rate being 20 C per minute. Scanning electron microscopy (SEM) SEM images were made on a Vega 3 microscope supplied by the Tescan Company (Brno, Czech Republic). Creep test Dumbbel test specimens (according to EN ISO 3167 standard) were cut out using an eccentric cutting machine. Cross-section of the test specimens: approximately 7.0 mm2, test specimens gauge length: 100 mm. Conditions used in measurements of all test specimens were as follows: test temperature: 25 C. pre-stress of approximately 0.44 MPa for a time period of 2 minutes. The stress proper of approximately 2.3 MPa (the precise value is given by the real cross-section of the test piece) was applied during the tests (creep tests) for a time period of 7200 minutes. The stress stage was followed by a stress release (reverse creep) at a stress level of approximately 0.44 MPa for a time period of approximately minutes. Melt strength Tensile characteristics of polymer melts were determined employing ARES 2000 rotational rheometers and a SER-HV-A01 Sentmanat elongational rheometer. The properties measured were extentional stress as a function of extentional strain and tensile viscosity as a function of time at two rates of elongational extentional strain (0.1 s-1, 1 s-1) and a temperature of 150 C. Analysis of nanocomposite plates by optical microscopy In order to assess the level of mixing (exfoliation) of nanoclay fillers samples (as listed in Table 1) in PE/EVA polymer matrices the optical microscopy technique was used. Samples of the composite plates were studied with the aid of Olympus BX51 optical microscope equipped with Olympus UC30 camera in a light field of a polarized light using the passing-light mode. X-ray diffraction The analyses of nanocomposites were conducted on an INEL X-ray powder diffractometer incorporating a position-sensitive CPS120 curved detector. A radiation source was a copper anticathode and the X-ray radiation was monochromatized by a germanium monochromator. The diffraction on the nanocomposite plates proceeded for a period of 30 minutes at a working voltage of 35 kv and a current of 15 ma.

4 3. RESULTS AND DISCUSSION 3.1 Mechanical treatments of fillers The fillers used in the polymer composites were treated by methods of both chemical modification (intercalation with HDTMAB, HCl leaching) and mechanical destruction (grinding in a jet mill - Figs. 1 and 2). Fig. 1 SEM image of untreated vermiculite, undersizes 40 µm Fig. 2 SEM image of jet ground untreated vermiculite from Fig Dispersion of filler in polymer matrix Nanostructures containing modified vermiculites dispersed by electrospinning process Contents of clays in nanostructures comprising nanofibres from EVA copolymer and clay particles were determined by TGA and are summarized in Table 2. Efficiency of transition of the clay particles from the polymer solution to nanostructures was very good. Smaller particles and particles intercalated with the aid of HDTMAB are incorporated into the nanostructures with a higher efficiency. Tab. 2 Clay content in polymer solution prior to electrospinning and in nanostructure after the electrospinning process Vermiculite used Theoretical content of clay prior to electrospinning (%) Content of clay in samples after electrospinning (%) Efficiency of transition of clay from solution to nanostructure (%) VER VER J VER 40 HDTMA VER HDTMA J

5 magnified 150 magnified 500 magnified 1500 Fig. 3 Nanostructure prepared from EVA copolymer and HDTMAB-intercalated VER 40 by electrospinning (filler 3 from Table 1 was used) magnified 150 magnified 500 magnified 1500 Fig. 4 Nanostructure prepared from EVA copolymer and VER HDTMA J5 by electrospinning (filler 4 from Table 1 was used) Fig. 5 TGA analysis of nanostructure comprising EVA and VER HDTMA J5 Determination of filler dispersion level by optical microscopy The photographs in Figs. 4 to 11 show the surface area of the composite plate mm in size, which is representative for the given sample. Particle edges, well visible at this magnification, were made clearer by framing thus making the distribution in the polymer matrix more pronounced. Based on measuring the

6 particles size the particles can be counted and included into separate categories according to their size. Though the ascertained numbers and size of the particles are approximate only they do make possible to assess intensity of mixing (exfoliation) of the nanoclay in the polymer. Fig. 6 PE/EVA and VER 40 composite Fig.7 PE/EVA and VER 40 composite - processing of the filler by electrospinning Untreated samples of VER 40 and VER J5 homogenized poorly with the polymer matrix when common plastics processing methods were used. A decrease in the number of larger agglomerates and hence an improvement in exfoliation of the filler in the polymer can be achieved by using the HDTMAB intercalation additive (Figs. 10 to 13) and by jet grinding of the clay (Figs. 12 and 13). An enhancement of the filler dispersion in the matrix can always be observed when the filler has been treated by electrospinning (Figs.7, 9, 11 and 13). Fig. 8 PE/EVA and VER J5 composite Fig. 9 PE/EVA and VER J5 composite - processing of the filler by electrospinning

7 Fig. 10 Composite comprising PE/EVA and VER HDTMA 40 Fig. 11 Composite comprising PE/EVA and VER HDTMA 40 - processing of the filler by electrospinning Fig. 12 Composite comprising PE/EVA and VER HDTMA J5 Fig. 13 Composite comprising PE/EVA and VER HDTMA J5 - processing of the filler by electrospinning X-ray diffraction analysis Diffraction analysis is one of the common methods carried out to monitor structure changes of the materials studied. A change in diffraction positions and diffraction intensities of vermiculite clay nanofiller was studied at diffraction angles of 2θ in the range from 1 to 10 degrees. Diffractions of PE and PE/EVA polymers occur at the diffraction angles of 2θ in the area ranging from 15 to 25 degrees. The diffraction records of each clay nanofiller were compared with those of nanocomposites containing these nanofiller. Fig. 18 shows diffraction records of VER 40 powder nanofiller and composites into which this filler has been mixed, i.e. PE/EVA containing VER 40, and PE/EVA containing VER 40 dispersed by electrospinning. X-ray records made on plates from PE and PE/EVA polymers were inserted into this figure in order to be able to compare them with diffraction records of nanocomposites. A comparison of diffraction angle areas shows that positions of basal diffractions of vermiculite have not changed; hence no change in the vermiculate layered structure and its exfoliation in the polymer matrix of the composite occurred. An identical situation exists also in case of the composites containing VER J5 jet ground vermiculite (Fig. 15). Interesting results were provided by X-ray diffraction records of the HDTMAB intercalated composites (Figs. 16 and 17). Here, interactions of VER HDTMA 40 and also of VER HDTMA J5 nanoclays with polymer matrices are evident. Changes in diffraction positions and formation of new diffractions, particularly in the

8 area of smaller angles 2θ, took place. Therefore, an intercalation of HDTMA into the interlayer (space between the layers) and exfoliation of the layered (stratified) structure of the vermiculite are assumed here. PE-EVA+VER 40 el. PE-EVA+VER 40 PE-EVA Fig. 14 Comparison of X-ray diffractions of PE homopolymer, PE/EVA blend, VER 40 filler, of a composite comprising PE/EVA and VER 40 and of a composite comprising PE/EVA and VER 40 el. treated by electrospinning process PE-EVA+VER J5 PE-EVA+VER J5 el. Fig. 15 Comparison of X-ray diffractions of PE homopolymer, PE/EVA blend, jet-ground VER J5 filler, of a composite comprising PE/EVA and VER J5 and of a composite comprising PE/EVA and VER J5 el. prepared by electrospinning process

9 PE-EVA+VER HDTMA 40 PE-EVA+VER HDTMA 40 el. Fig. 16 Comparison of X-ray diffractions of HDTMA intercalated VER 40 filler, of a composite comprising PE/EVA and VER 40 HDTMA and of a composite comprising PE/EVA and VER HDTMA 40 el. treated by electrospinning process PE-EVA+VER HDTMA J5 PE-EVA+VER HDTMA J5 el. Fig. 17 X-ray diffraction records of intercalated and jet ground VER HDTMA J5 nanofiller and of plates of nanocomposite comprising PE/EVA and HDTMA J5 and of a composite comprising PE/EVA and HDTMA J5 el. where the filler was dispersed by electrospinning 3.3 Preparation of composites by mixing in melt The fillers were treated by jet grinding, modified with intercalation additives or hydrochloric acid and possibly also pre-treated by electrospinning. All the fillers were then mixed with PE or also with EVA copolymer in a Brabender plasticorder using classical plastics processing procedures and the composites thus prepared were moulded to form plates for tests aimed at determining their properties. Selected properties of the composites comprising PE/EVA polymer matrix are presented in Table 3.

10 Tab. 3 Mechanical properties of composites comprising PE/EVA and VER Filler incorporation method Composite composition Initial creep modulus (MPa) Elasticity modulus from tensile test (MPa) Vermiculite used PE EVA VER Humidit y and (%) (%) (%) additive s (%) Electrospinning and mixing in melt Mixing in melt 3.4 Creep compliance tests VER VER J VER HDTMA VER HDTMA J VER VER J VER HDTMA VER HDTMA J VER HCl VER HCl J Effect of homogenization by electrospinning on creep modulus of composites Electrospinning is used to achieve a thorough incorporation of small filler particles into polymer matrix and a maximum suppression of agglomeration. In case of comparable materials it is apparent that a certain improvement of creep modulus occurs (Figs. 18 and 19). However, in no case the improvement is great enough to counterbalance the complexity and high costs of masterbatch preparation. Fig. 18 Creep modulus of VER 40 treated by electrospinning and mixed in melt, VER 40 mixed in melt only and a reference blend of PE and EVA copolymer Fig. 19 Creep modulus of jet ground VER J5 treated by electrospinning and mixed in melt, mixed in melt only and a reference blend of PE and EVA copolymer Effect of jet grinding and intercalation additives on modulus of composites in creep and tensile tests Good results with respect to polymer reinforcement were achieved in case of VER HDTMA 40. The intercalation agent affects here positively moduli at the start of straining. However, after a certain time of loading, values of the creep moduli of the polymer matrix alone are higher than those of the composites (Fig. 20).

11 Treatment of clays with HCl results in poorer creep properties of composites in comparison with those of the PE/EVA polymer blend alone (Fig. 21). Fig. 18 Creep modulus of VER 40 treated by electrospinning and mixed in melt, VER 40 mixed in melt only and a reference blend of PE and EVA copolymer Fig. 19 Creep modulus of jet ground VER J5 treated by electrospinning and mixed in melt, mixed in melt only and a reference blend of PE and EVA copolymer Log t (s) HDTMA 40 HDTMA J5 Fig. 20 Creep moduli of composites comprising PE/EVA and HDTMAB- intercalated VER Fig. 21 Creep moduli of composites comprising PE and HCl-treated VER mixed in melt only 3.5 Melt strength A very important processing property in extrusion of plastics is strength of the melt. PE composites containing small amounts of nucleation additives, no EVA copolymer and an appropriate quantity of the clay filler are compared.

12 From tensile characteristics of polymer melts employing a Sentmanat elongational rheometer it is apparent that the maximum strength at an elongational strain of 0.1 s-1 and T = 150 C is higher in composites containing the HDTMAB intercalated filler (Fig. 22), as well in those containing a jet ground and, at the same time, HCl-modified filler (Fig. 23) and increases with a decreasing content of the filler in the row 9, 6 and 3 mass percent of the filler studied. The positive effect of jet grinding and treatment of filler with 1M hydrochloric acid is demonstrated in Fig. 24 also by stress-strain dependences for composites containing 3 mass percent of filler. Fig. 22 Increase in melt strength with a decreasing content of VER HDTMA 40 nanoclay filler in PE matrix Fig. 23 Increase in melt strength with a decreasing content of VER HCl J5 nanoclay filler in PE matrix

13 Fig. 24 Melt strength values of composites containing 3 mass percent of VER 40, VER J5, VER HCl 40 and VER HCl J5 fillers in PE matrix The composites comprising PE matrix and nanoclays discussed in this paper are superior in strength to usually much more expensive highly branched LD-PE grades having melt of great strength. Dependences of extentional stress as a function of extentional strain, and tensile viscosity as a function of time are shown in Figs. 31 to 33 for PE Bralen FB a composite matrix, a reference highly branched PE (much more expensive than Bralen FB 2-17 under the investigation) and the composite based on Bralen PE containing 6 mass percent of HCl-modified vermiculite (VER HCl 40). Behaviour of the composite in regions of low strains or shorter times is rather similar to that of PE matrix alone but in regions of higher strains or longer time periods the properties of the composite are similar to those of highly branched PE. Fig. 25 A comparison of extentional stressstrain dependences at elongational strain of 0.1 s -1 for: PE Bralen FB 2-17 highly branched PE composite of Bralen PE and 6 mass % of VER HCl 40 filler

14 Fig. 26 A comparison of extentional stressstrain dependences at elongational strain of 1 s -1 for: PE Bralen FB 2-17 highly branched PE composite of Bralen PE and 6 mass % of VER HCl 40 filler Fig. 27 A comparison of dependences of tensile viscosities for PE Bralen FB 2-17 highly branched PE composite of Bralen PE and 6 mass % of VER HCl 40 filler 4. CONCLUSIONS A fibre-forming process in electrostatic field including a highly efficient incorporation of filler into nanostructures of EVA copolymer was developed. Though a favourable effect of electrospinning on dispersion of filler in composite compounds was demonstrated, in case of exfoliation of the filler in a polymer matrix usual plastics mixing procedures are adequate to achieve the required properties of the composites. Treatments of the filler do not result in a marked improvement of mechanical properties of PE/nanoclay composites.

15 The most interesting behaviour was exhibited by PE/VER composites in processing properties where a marked increase in strength of melt was achieved in composites containing HDTMAB additive or those modified with HCl. From the processing point of view, of a great interest is also the course of dependences of tensile strength on strain, or tensile viscosities on time. LITERATURE [1] TEIMOURI Y., NAZOCKDAST H. The effect of process parameters on physical and mechanical properties of commercial low density polyethylene/org-mmt nanocomposites. Journal of Material Science 2011, vol. 46, issue 20, p [2] FURLAN L. G., FERREIRA C. I., DAL CASTEL D., SANTOS K. S., MELLO A. C. E., LIBERMAN S. A., OVIEDO M. A. S., MAULER R. S. Effect of processing conditions on the mechanical and thermal properties of high-impact polypropylene nanocomposites. Materials Science and Engineering A 2011, vol. 528, issue 22-23, p [3] HUANG G., ZHU B., SHI H. Combination effect of organics-modified montmorillonite with intumescent flame retardants on thermal stability and fire behavior of polyethylene nanocomposites. Journal of Applied Polymer Science 2011, vol. 121, issue 3, p