ELASTOMER BASED COMPOSITES FILLED WITH LAYERED FILLERS AND IONIC LIQUIDS

This thesis was focused on the preparation and the study of elastomer composites filled

with layered fillers (cationic clays, anionic clays and graphene-like platelets) with improved

mechanical properties, decreased swelling in solvents, increased UV stability and reduced gas

permeability. The use of layered minerals in rubber formulations has ecologic and economic

importance and offers the potential for environmentally friendly rubber products. Moreover,

the melt mixing method used in this work for the rubber/layered fillers composites preparation

is considered as a one of the most eco-friendly routes that excludes the use of organic solvents

and is the most suitable for an industrial scale production. In this work, the layered minerals

where investigated not only in terms of their use as reinforcing fillers for rubbers but also as

crosslinking agents, gas barrier and UV stability enhancers. The multifunctionality of these

fillers makes them interesting materials with broad potential applicability for the

manufacturing of high performance elastomers. The layered fillers tested belong to a class of

cationic clays (natural and synthetic hectorite), anionic clays (hydrotalcites or magnesium

aluminum layered double hydroxides MgAl-LDHs) and graphene-based materials. The first

part of thesis focused on the evaluation of morphological, structural, thermal and surface

characteristics of the layered minerals by following techniques: scanning electron microscopy

(SEM), X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR),

Thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS) and inverse gas

chromatography (IGC). Subsequently, the rubber composites were prepared either by 1-step

mixing method in a laboratory two-roll mill or by 2-step method involving (1) processing in

an internal mixer and (2) milling in the two-roll mill. The influences of layered fillers in the

amounts form 2.5 – 40 phr (depending on the types of particulate filler) on the curing

behavior, crosslink density, tensile properties, mechano-dynamical properties, thermal

behavior and morphology of elastomer composites were investigated. The elastomer

composites containing increasing concentrations of imidazolium ionic liquids ILs, were

tested for their rheometric, mechanical, ionic conductivity and morphological properties. The

following techniques were used: DMA (dynamic-mechanical analysis), differential scanning

calorimetry (DSC), broadband dielectric spectroscopy (BDS), Fourier-transmission infrared

spectral analysis (FTIR), thermogravimetric analysis (TGA), X-ray analysis (XRD), scanning

electron microscopy (SEM).

Different structural and surface properties of layered fillers such as particles aspect ratio,

specific surface area and surface activity were investigated as factors impacting the

reinforcement of acrylonitrile-butadiene rubber (NBR) and carboxylated acrylonitrile-

butadiene rubber (XNBR). The NBR was vulcanized with the use of conventional sulfur-base

cure system which leads to the formation sulfur bridges between rubber chains. The XNBR

was crosslinked by the metal ions-containing curatives (ZnO, MgAl-LDH) where ionic bonds

are generated through the reaction between carboxylic groups and metal ions. Special

attention has been devoted to the XNBR systems containing magnesium aluminum layered

double hydroxides (MgAl-LDHs) varying in Mg/Al ratios, layers aspect ratios and particles

morphologies. This thesis presented that the simultaneous application of MgAl-LDH as a

filler and as a crosslinking agent in XNBR provides not only environmentally friendly, zinc-

oxide free product but also ionic elastomer composite with improved mechanical, barrier and

transparent properties. It was observed that the extent of the reinforcement and cure degree of

XNBR is strongly related to the content of magnesium ions in LDH crystalline structure and

the aspect ratio of layers. By conducting rheometric, dynamic mechanical (DMA) and Fourier

transform infrared spectroscopy (FTIR) studies it was demonstrated that improvements in

mechanical properties arise from the existence of ionic crosslinks via metal-carboxylate

crosslinking provided by MgAl-LDH. This thesis considers also the potential application of

hydrophilic and hydrophobic imidazolium ionic liquids which are characterized by high ionic

conductivities as multifunctional additives in rubber formulations. Such organic compounds

could serve simultaneous functions as dispersing agents in rubber matrix, plasticizers and

ionic conductivity enhancers. The optimal concentration and type of ionic liquids were

selected for obtaining a good compromise between mechanical and conductivity properties of

rubber composites. The rubber composite with improved mechanical, morphological

properties and increased ionic conductivity up to 10-7

S/cm was realized by the use of

hydrophobic 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ionic and 1-

butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquids at low

concentrations from 5 to 10 parts per hundred rubber (phr). Increased length of alkyl chain to

1-hexyl-3-methylimidazolium or higher concentration of ionic liquid than 10 phr in rubber

matrix led to a decrease in mechanical properties due to the plasticizing effect of the excess of

ionic liquid in rubber composite material. Graphene fillers with various specific areas, surface

chemistry and particles morphology were also investigated as novel reinforcing fillers for

acrylonitrile-butadiene rubber (NBR) and carboxylated nitrile butadiene rubber (XNBR).

Both kinds of rubbers showed significantly improved UV stability in the presence of layered

carbon filler. A substantial improvement in mechanical properties of XNBR was observed

even at low filler concentration of 5 phr.