Master curves for the sulphur assisted crosslinking reaction of natural rubber in the presence of nano- and nano-structured sp2 carbon allotropes

In this paper, master curves are reported for the crosslinking of a diene rubber with a sulphur based system in the presence of either nano- or nano-structured carbon allotropes, such as carbon nanotubes (CNT), a nanosized graphite with high surface area (HSAG) and carbon black (CB). Poly(1,4-cis-isoprene) from Hevea Brasiliensis was the diene rubber and crosslinking was performed in temperatures ranging from 151 to 180°C, with carbon allotropes below and above their percolation threshold. Such carbon allotropes were characterized by different aspect ratio, surface area and pH. However, in the crosslinking reaction, they revealed common behaviour. In fact, the specific interfacial area could be used to correlate crosslinking parameters, such as induction time (ts1) and activation energy (Ea) calculated by applying the autocatalytic model. Monotonous decrease of ts1 and increase of Ea were observed, with points lying on master curves, regardless of the nature of the carbon allotropes. Remarkable differences were however observed in the structure of the crosslinking network: when the carbon allotrope was above the percolation threshold much larger crosslinking density was obtained in the presence of CNT whereas composites based on HSAG became soluble in hydrocarbon solvent, after the reaction with a thiol. Proposed explanation of these results is based on the reactivity of carbon allotropes with sulphur and sulphur based compounds, demonstrated through the reaction of 1-dodecanethiol and sulphur with CNT and HSAG and with a model substrate such as anthracene

Layered double hydroxides with low Al content and new intercalate structures

Layered double hydroxides (LDHs) containing Mg2+ and Al3+ as divalent and trivalent cations, respectively, have been synthesized by a procedure based on hydrolysis of a mixture of metal alcoholates, and have been characterized mainly by Wide Angle X-ray diffraction (WAXD) and Fourier Transform Infrared (FTIR) measurements. The used procedure is suitable to prepare LDHs with Al molar fractions, x(Al)=Al/(Al+Mg), as low as 0.1. The same LDH preparation procedure, for x(Al)<0.1, produces thermally stable intercalates with alkoxy anions, exhibiting layers similar to those of brucite, which maintain a high degree of order only in the hexagonal packing of the hydroxide groups. In particular, the basal periodicity increases from nearly 0.8 nm up to nearly 1.3 nm, due to the parallel staking of hexanolate groups in the clay galleries. (C) 2012 Elsevier B.V. All rights reserved

Reduction of filler networking in silica based elastomeric nanocomposites with exfoliated organo-montmorillonite

Montmorillonite with dimethyl di(hydrogenatedtalloyl) ammonium as the compensating cation was added to a silica based elastomeric composite and the hybrid filler system led to reduction of filler networking phenomenon, better stability of elastic modulus with temperature, enhancement of stresses at all elongations, improvement of ultimate properties. This composite was based on a blend of natural rubber, poly(1,4-cis-isoprene) and poly(styrene-co-butadiene) from anionic polymerization and contained 70 parts per hundred rubber (phr) of silica. The organically modified clay (OC) was below the threshold required to establish an hybrid OC-silica filler network. Such threshold (about 7 phr) was estimated by preparing silica based nanocomposites containing various amounts of OC and determining shear storage and loss moduli as a function of strain amplitude. This work demonstrates that exfoliated OC favour lower dissipation of energy of silica based elastomeric composites under dynamic mechanical stresses and paves the way for further large scale applications

Serinol: a biosourced building block for better mechanical reinforcement and sustainable vulcanization of rubber compounds

Serinol and serinol derivatives were used to promote the mechanical reinforcement and the sulphur based crosslinking of rubber compounds. Reaction of serinol was performed with aldehydes and ketones, such as cinnamaldehyde, acetone, 2,5-hexanedione and camphor, obtaining the regiospecific synthesis of either imines or oxazolidines. Paal-Knorr reaction was as well carried out between serinol and 2,5-hexanedione, obtaining a serinol derivative containing a pyrrole ring, 2-(2,5-dimethyl-1H-pyrrol-1-yl)-1,3-propanediol, named as serinolpyrrole. All the reactions were performed in the absence of solvents and catalysts and were characterized by high and also quantitative yields. Thanks to the reactions’ specificity, pure substances were obtained, with controlled atoms hybridization. The aromatic pyrrole ring was exploited to form adducts of serinolpyrrole with carbon allotropes. Adducts of SP with carbon black promoted the reduction of filler networking in silica based compounds. Serinol and immine as well oxazolidine derivatives were efficient secondary accelerators in silica based compounds and are thus suitable substances for the replacement of DPG. Tuning of vulcanization kinetics was performed

Filler networking of a nanographite with a high shape anisotropy and synergism with carbon black in poly(1,4-cis-isoprene) based nanocomposites

ABSTRACTA nanoGraphite (nanoG) having a high surface area and a high shape anisotropy, defined as the ratio between the crystallite dimensions in a direction orthogonal and parallel to structural layers, was used to prepare nanocomposites based on poly(1,4-cis-isoprene) (IR), in the neat polymer matrix and in the presence of carbon black (CB). Tensile and dynamic-mechanical measurements showed that nanoG forms a filler network at a relatively low concentration in neat IR and a hybrid filler network at a lower nanoG concentration in the presence of CB. A synergistic effect between the two carbon allotropes was found: composites containing both fillers present initial modulus values much higher than those calculated through the simple addition of the initial moduli of the composites containing only CB or nanoG.</jats:p

Organoclays with hexagonal rotator order for the paraffinic chains of the compensating cation. Implications on the structure of clay polymer nanocomposites

Organically modified clays (OC) were prepared by reacting montmorillonite (Mt) and dimethyl ditallow ammonium chloride (2HTCl), optionally adding either stearic acid (SA) or 2-stearamidoethyl stearate (SAES). Reactions were performed either at the solid state, in the absence of any other chemical substance, or with the help of solvents such as water and alcohol, or in poly(1,4-cis-isoprene) as the reaction medium. Characterization was performed by means of TGA, XRD, DSC, IR analyses. Two types of OC were identified, with 3.6-4.0nm and about 6nm as d001 basal spacing, independently of the synthetic route. The 3.6-4nm value could be explained with the bilayer placement of the tilted paraffinic chains of the ammonium cation. The 6nm value could arise from the perpendicular placement of the paraffinic chains, associated with the presence in the interlayer space of either the acid or the amide. For the first time, it is documented the occurrence of hexagonal rotator order in the packing of the long hydrocarbon tails of the 2HT compensating cation. SA and, in particular, SAES were found to enhance this type of order. A third type of OC, with 2.5nm as d001 basal spacing was obtained through solvent extraction of the above mentioned OC. Results reported in this work show that the crystalline structure of an OC depends on type and amount of low molecular mass substances present in the interlayer space: compensating cations and guests such as an acid and/or an amide. For a given compensating cation, the interlayer distance can be varied by changing the amount of the cation and by adding a guest. The Mt/2HT organoclay studied in this work reveals 2.5, 3.6-4.0 and 6.0nm as the interlayer distances of the minima of energy. Polymer chains are not involved in the OC crystalline structure, even if the OC synthesis is performed in the polymer matrix. This work strongly suggests that the best explanation for most interlayer spacing variation observed for clays in polymer nanocomposites is the different placement of intercalated low molecular mass substances, rather than the generally assumed polymer chain intercalation. Moreover, it presents the synthesis of OC at the solid state as a versatile method for preparing a large variety of clay polymer nanocomposites. © 2013 Elsevier B.V

Hybrid filler systems for the mechanical reinforcement of isoprene rubber

In this paper, hybrid filler systems, based on either sp2 carbon allotropes such as carbon black and carbon nanotubes, or on inorganic oxides and hydroxides, such as silica and organoclay, are for the first time comparatively investigated. It is shown that, in isoprene rubber as the matrix, nano- and nano-structured fillers form hybrid filler networks. The effect of such hybrid filler systems on the mechanical reinforcement strongly depends on the nanofiller content