The effect of low temperature air plasma treatment on physico-chemical properties of kaolinite/polyethylene composites
Introduction
Characterization and modification of polymers are perhaps the most important aspects of polymer research, production and applications. Polyolefin polymers, especially thermoplastics have become an essential part of our everyday lives in the last few decades [1], [2]. Thermoplastics, such as polyethylene (PE) can offer useful mechanical, chemical, electrical properties, with low density, high formability and the ability to be recycled. Due to its low price per unit volume and its unique physico-chemical properties it is therefore, the world’s number one per volume most used thermoplastic [3], [4]. This semi-crystalline polymer can be classified according its density and divided into four groups: high density polyethylene (HDPE), low density polyethylene (LDPE), linear low density polyethylene (LLDPE), and very low density polyethylene (VLDPE) [5].
In the last few years, clay minerals were used as a cost reducer and as an additive triggering an improvement of the mechanical properties of polymer composites. Their surface modifications were used in catalysis, as an adsorbent, in sensors and as filler in polymer–clay composite systems [6]. The effect of incorporating inorganic fillers into the thermoplastic polymer network, results in the improvement of the physico-chemical and mechanical characteristics such as low air permeability, improved mechanical strength, modulus of elasticity and stiffness [7], [8], [9], [10], [11]. For this reason the research and development activities focused on composites containing inorganic filler is of expanding importance. Matrix modification, degree of crystallinity, type of reinforcement, quality of adhesion between filler and matrix, size, shape, size distribution of filler particles, addition of coupling agents affects the physico-chemical and thermo-mechanical properties, as well as internal structure and strength of aggregates [12], [13], [14], [15]. In most cases, silane coupling agents are added to react with inorganic substrates in order to form stable covalent bonds, thus altering the physical interactions of treated polymer/filler substrates. As is very well known, polymer/kaolin interface quality performance is essential for excellent overall composite system material/mechanical properties, where exact adjustment of the polymer matrix modulus and adhesive bond strength is vital for final synergistic increase of mechanical strength of the composite system [16]. An effect of coupling agents on mechanical properties improvement of polymer/inorganic filler composites was studied and referenced in early works of Arkles [17], [18] and Leyden [19].
Kaolin is a clay mineral also known as China clay or paper clay. The chief constituent of kaolin (Al2O3 2SiO2 H2O), is kaolinite, which consists of successive layers of octahedral alumina and tetrahedral silica, which alternate to form plate like hexagonal particles. The particles are flat disks or plate like in shape, with the disk radius of the order 5–10 times larger than the thickness. Thus a typical crystal is a platelet of 0.5–1 μm in diameter and 0.1 μm thick. The flat surface is negatively charged over the entire pH range, whilst the edges are positive at low pH, but negative at high pH, with an iso electric point at about pH 7. These edge effects arise due to the fracture of the lattice network and contain silica and alumina like sites [20], [21]. The electrokinetic behaviour of kaolinite should show an average of the surface and edge properties, but initial experiments revealed that the zeta potentials were negative over the entire pH range and were very similar to the silica zeta potential pH curve. This was explained by the large face to edge surface area ratio [20], [21]. Particle characterization of this clay mineral has previously been studied in our another paper by Lapcik et al. [20] and Greenwood et al. [21].
The mechanical properties of particulate-polymer composites depend strongly on the particle size, particle matrix interface adhesion and particle loading [22], [23]. Polymer composites are noted to show mechanical properties which depend on time, deformation rate and temperature. However, the filler introduces a high amount of interface in the matrix that affects the polymer crystallization process and modifies the structure of the polymer in the neighbourhood of the particle surface [22], [24].
The main aim of this paper was focused on studying the effect of low temperature air plasma treatment and silane coupling agent modification of nano/micro kaolinite filler particles on the physico-chemical, mechanical and thermal properties of polyethylene/kaolinite composite system with filler content ranging from 0 to 25 wt.%.
Section snippets
Materials
Two commercial polyethylenes were used in this study: HDPE TIPELIN 6300B and LLDPE LITEN, Unipetrol PND 33-300 (Czech Republic). As a virgin filler material kaolin (Imerys Minerals Ltd., Cornwall, UK) was used. For chemical modification of kaolin a silane coupling agent (3-aminopropyl)triethoxysilane (Sigma Aldrich, USA) was used.
Sample preparation
Air plasma treatment of the filler powder was performed in a Diener Femto (Diener Electronic, Germany) plasma reactor operating at 13.56 MHz frequency for 10 min, with generator power 100 W, air flow rate 5 cm3/min and processing pressure 35 Pa. The polymers were filled with virgin and plasma treated kaolinite powder with the following content: 0, 2.5, 5, 7.5, 10, 15 and 25 wt.%.
From a processing point of view, the important attitude was favourable dispersion of filler particles and minimizing their
Results and discussion
Fig. 1 shows results of the tensile strength measurements for virgin and chemically modified plasma treated filler particles composites with HDPE. It is evident, that the highest increase in ultimate tensile strength was found for the chemically modified filler at 7.5 wt.% concentration. Here the absolute increase of Fmax was found to be from 30.50 MPa to 32 MPa for deformation rate of 50 mm/s. In the filler concentration range of 2.5–7.5 wt.% a plateau region of Fmax was found, indicating highest
Conclusions
The effect of low temperature air plasma treatment and silane coupling agent modification of the surface properties of kaolin based nano filler used for preparation of polyethylene composites (both HDPE as well as LLDPE) was studied. The effect on the final composite physico-chemical, mechanical and thermal properties was studied over a wide range of degrees of filling. It was found that the silane coupling agent surface modification had the strongest effect resulting in improved mechanical
Acknowledgements
Financial support from the Operational Program Research and Development for Innovations – European Regional Development Fund (Grants CZ.1.05/2.1.00/03.0058 and CZ.1.05/2.1.00/03.0111) and of Tomas Bata University in Zlin Internal Grant Agency (Project No. IGA/FT/2013/001) are gratefully acknowledged. Authors would like to express their gratitude also to Erasmus EU teaching staff and student exchange program for partial financing of the project. Special thanks also to Assoc. Prof. D. Maňas and
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