The effect of kaolin modification of silane coupling agents on the properties of the polyethylene composites

The effect of kaolin modification of silane coupling agents on the properties of the polyethylene composites This paper shows the results of using the modified kaolin by silane coupling agents in HDPE composite and the effect of surface modifications of fillers on the properties of polyethylene composites. In the first stage pure and modified kaolin was subjected to a number of tests in order to determine the backfill density, water and paraffin oil absorbability, the surface area and pore volume, the morphology of their grains, thermal analysis and the FT-IR spectroscopy. In the second stage the composites, which were moulded into the samples that could be subjected to further tests, were produced. The samples were characterised by the determination of the hardness according to Shore, the elasticity modulus, the tensile strength and tearing strength. The modification of the kaolin surface has resulted in a substantial improvement of the strength parameters of the obtained polyethylene composites.


INTRODUCTION
Kaolin is a mineral filler of natural origin most often used in the production of plastic and elastomer materials. The composites obtained after their introduction into the polymer matrix are characterised by the improved strengthening physical properties 1 . One of the main components of kaolin is the mineral kaolinite, being hydrated aluminium silicate of the formula: Al 2 O 3 . 2SiO 2 . 2H 2 O. Kaolinite is 1:1 dioctahedral clay mineral composed of structurally asymmetric layers. One side of the layer is gibbsite-like with aluminium atoms coordinated octahedrially with applied oxygen atoms and hydroxyls. The other side of the layer is constituted by the silicate layer structure, where the silicon atoms are coordinated thetrahedrially to oxygen 2 . At the interface of each group of the layers there are hydroxyl groups. Three fourth of the OHgroups being the external hydroxide groups are at the surface of a kaolinite packet and their protons take part in hydrogen bonds of the lengths of 3.13, 3.00 and 2.29 . The other OHgroups are inside the packet; they are called the internal hydroxyl groups, and are involved in the shortest hydrogen bonds of 2.96 . Hydrogen bond formation changes the polarization of the OH bonds so that the positive charge is shifted towards the protons 3 . The lamellar structure of the kaolinite is responsible for their excellent cleavage and easy separability into thin sheets. The lamellar packets are electrically neutral. The packets are linked through the hydrogen bonds made with the involvement of the hydroxyl groups coordinated about a single packet and about the silica atoms in the neighbouring packet 4 . The paper reports on the production of kaolin filled polymers, their characterisation and the effects of the kaolin modifications with silane coupling agents significantly improving the strength of the composites.

Substrates
The KOG kaolin from the Surmin-Kaolin mine in Nowogrodziec near Boles³awiec was used. The kaolin surface was modified with the following silane -coupling agents made by Unisil, Tarnów:

Modification of fillers
The surface modification was performed at room temperature in a 1:1 water-ethanol solution with a 1 -3% vol addition of the silane coupling agent, in the MPW-309 mixer until the plastification of the plastic solid. The airdried sample was ground in the mortar and sieved through a mesh size 0.063mm.

Filler testing
Pure and modified kaolin was subjected to a number of tests in order to determine their bulk density PN-80/C-04404/03, water 5 and paraffin oil absorbability PN-87/C-04404/1. The surface area and pore volume distribution were determined on the ASAP 2010 Sorptometer and the SEM photographs of the samples were taken to assess the morphology of their grains by the PHILIPS SEM 515. The gravimetric measurements (TG, DTG, DTA methods) were determined on a thermogravimeter Setaram TGA 6 . The IR spectra were taken on the Bruker FT-IR IFS 66/s spectrometer.

Composite preparation
Polyethylene Hostallen ACP 5831 D made by Basell Orlen Polyolefins was selected because of a wide range of applications for the production of objects that have to show high mechanical strength. The composites with the modified fillers were produced at the Institute of Plastics Processing "Metalchem" in Toruñ, using a screw extruder BTSK 20/40D made by Bûhler . There were 4% of fillers in the composites. The composites were moulded into the samples that could be subjected to further tests.

Composite testing
The samples were characterised by the hardness according to the Shore scale PN-80C/04238, the PN-EN ISO 527/1998 elasticity modulus, the tensile strength and tearing strength at the TIRA test 27025 stand PN-EN ISO 527/1998.   The modification of kaolin with silane coupling agents significantly improves the physicochemical parameters of the mineral fillers (Tables 1 -3).

RESULTS AND DISCUSSION
The surface area increased, the pore sizes are reduced, the bulk density increases and the hydrophilous-hydrophobic properties were improved. The obtained fillers are characterised by significant thermal stability (Figs. 1 -4).
On heating the samples release the water that can originate from the following:   In the range of 1150 -1225 o C the spinel phase is transformed into mullite and the silica becomes cristobalite 6 .
The results of the FT-IR study confirm the presence of kaolinite by revealing four characteristic absorption bands, at about 3600 cm -1 assigned to the OH groups vibrations. The absorption bands permit the identification of the types of lamellar silicates as the spectrum of halloysite showed two bands, while those of nakrite and dyckit show three  .1 -7), of this filler shows the uniform size of the grains and a considerable increase in the surface area as a result of the formation of agglutinations and additional layers on the surface of the pseudo-hexagonal sheets. On the basis of the results obtained the most effective one is the modification with 3methacryloxypropyltrimethoxysilane and vinyltriethoxysilane as the use of such modified fillers has significantly improved the properties of polyethylene composites (greater hardness and elasticity modulus) with respect to those of the unfilled polymer and the polymer filled with the unmodified filler. The kaolin modification with the above two compounds also brings a correction in the physico-or mechanical parameters: the tearing strength and mean tensile stress increase, while the unit elongation on stretching and on tearing decreases. The SEM photographs of the composite (Phot. 8 -15), reveal a homogeneous distribution of the modified filler in the polymer matrix, achieved thanks to the modification improved character of the filler-polymer and filler-filler interactions. The composites filled with the modified kaolin are more stable than the unfilled polymer or the polymer filled with the unmodified kaolin.

CONCLUSIONS
The modification of kaolin fillers with silane coupling agents significantly improves their physicochemical properties and hence the properties of the obtained composites. As follows from our results, the modification of the fillers with silane coupling agents substantially improves their dispersion in the polymer matrix and hence reduces the tendency towards the agglomeration of particles, leading to a decreased strength of the filler-filler interactions and the increased strength of the filler-polymer interactions. The modified fillers are stronger bonded to the polymer which improves the physico-or mechanical parameters of the composite and permits the extension of the range of their applications.  Table 4. The elasticity modulus and the hardness of the polyethylene composites Table 5. The mechanical parameters of the polyethylene composites Table 6. The mechanical parameters of the polyethylene composites