Crosslinking filled rubbers

Most of the room temperature crosslinking silicone rubbers are utilized for filling holes in buildings e.g. so-called expanding fillers, in the sanitary sector and for the sealing of windows. 

The mechanical and thermal properties of a range of poly(ethylene)/po-ly(ethylene propylene) (PE/PEP) copolymers with different architectures have been compared [2]. The tensile stress-strain properties of PE-PEP-PE and PEP-PE-PEP triblocks and a PE-PEP diblock are similar to each other at high PE content. This is because the mechanical properties are determined predominantly by the behaviour of the more continuous PE phase. For lower PE contents there are major differences in the mechanical properties of polymers with different architectures, that form a cubic-packed sphere phase. PE-PEP-PE triblocks were found to be thermoplastic elastomers, whereas PEP-PE-PEP triblocks behaved like particulate filled rubber. The difference was proposed to result from bridging of PE domains across spheres in PE-PEP-PE triblocks, which acted as physical crosslinks due to anchorage of the PE blocks in the semicrystalline domains. No such arrangement is possible for the PEP-PE-PEP or PE-PEP copolymers [2]. 

Several performance characteristics of rubber such as abrasion resistance, pendulum rebound, Mooney viscosity, modulus, Taber die swell, and rheological properties can be modeled by Eq 7.34. " A complex mathematical model, called links-nodes-blobs was also developed and experimentally tested to express the properties of a filled rubber network system. Blobs are the filler aggregates, nodes are crosslinks and links are interconnecting chains. The model not only allows for... 

It is observed that Slapp of nano-CaCOs is less than the commercial CaCOs filled SBR which is attributed to greater crosslinking of rubber, as the uniform dispersion of nano-CaCOs brings the chains closer and keeps them intact with nanoparticles. Swelling depends on elastomer crosslinking density and solvent used. Solvent penetration is more in commercial micron size CaCOa than the nano-CaCOs rubber composites. 

The tensile strength of nano-CaCOs filled SBR (2.58 MPa for 9 nm CaCOs) are higher than the commercial CaCOs (1.63 MPa) and the fly ash filled SBR (1.37 MPa) which means nano-CaCOs provides higher tensile strength as compared to commercial CaCOs and fly ash filled SBR which is attributed to uniform dispersion of nanofiller into the rubber matrix that intercalates the rubber matrix, and so the degree of crosslinking of rubber chain increases. 

The solvent resistance characteristics of non-crosslinked and crosslinked NR composites reinforced by multi-walled carbon nanotubes (MWCNTs). All the MWCNTs were individually dispersed within the rubber matrix where a three-dimensional cellulation structure was created when the added amount of MWCNTs exceeded 16 wt%. The non-crosslinked MWCNTs-filled rubber composite was not soluble, but swelled in toluene. They found that a continuous three-dimensional structure at the interface between the MWCNTs and the NR was extremely tough and thus contributed to the improvement in the elastic modulus and thermal stability of NR composites. 

The mechanical properties of the control NR composite, untreated HNT-filled NR nanocomposite, treated HNT-filled NR nanocomposite, and silica-filled NR composite are tabulated in Table 19.6. The addition of 10 phr HNT loading increased the tensile modulus and tensile strength compared to the neat NR vulcanizate. However, the values of elongation at break of the HNT filled rubber nanocomposite were decreased in comparison to the unfilled NR vulcanizate. The silica filled NR vulcanizate showed inferior properties as comparison to HNT filled nanocomposites. It is well known that the modulus of rubber vulcanizates is proportional to the degree of crosslink density. Therefore, modulus of rubber vulcanizates increased with the increased of crosslink density. In contrast, the elongation at break decreased with increasing crosslink density. 

A. Hasse, A. Wehmeier and H. D. Luginsland, Crosslinking and Reinforcement of Silica Silane-Filled Rubber Compounds, Rubber World Magazine s Electronic Publishing Division, Akron, Ohio, 2004. 

The use of stearic acid as a modifier for silica and other fillers like CaCOs and Mg(OH)2 has been reported. The authors found that the presence of adsorbed stearic acid on the filler surface reduces the hydrophilicity of the silica surface and enhances the compatibility between filler and matrix, which may lead to an improvement in filler dispersion and the related mechanical performance of composites. Kosmalska et al also investigated the adsorption of DPG, ZnO and sulfur on the silica surface and reported that the bonding of DPG/ZnO and ZnO to silica causes a reduction in the surface energy of silica from 66 mN/m to 28.75 mN/m and 35.49 mN/m, respectively. A similar effect of ZnO on the surface tension of silica was also found by Laning et alP and Reuvekamp et al. The adsorption of that additive and its impact on the scorch time and reduction of the crosslink density in silica-filled rubber compounds have been frequently characterized. ... 

For resistance to acid conditions alone, traditional filled and unfilled bituminous solutions (which have economic advantages), chlorinated rubber and shellac have been used. Crosslinking coatings, e.g. amine-cured epoxy resins, often blended with coal-tar which develops resistance to oils and solvents, have obvious advantages on chemical plant. 

Other additives such as silanes, titanates and zirconates are also used to overcome the processing characteristics of silica fillers. Silanes not only give improved processability of silica-filled compounds, but also provide improved crosslinks between the silica particle surface and the rubber molecular chains giving increased physical properties. The use of silane coupling agents at a... 

The advanced applications for nitrocellulose plastisol propellants require that they be integrally bonded to the motor case. Successful case bonding for the multiyear storage life of a rocket calls for special adhesives and liners which are completely compatible with these highly plasticized propellants. Best results have been obtained with a combination of an impervious rubber liner and a crosslinked adhesive system with a limited affinity for the plasticizers used in the propellants. Examples of effective liners are silica-filled butyl rubber and chlorinated synthetic rubber. Epoxy polyamides, isocyanate-crosslinked cellulose esters, and combinations of crosslinked phenol-formaldehyde and polyvinyl formal varnishes have proved to be effective adhesives between propellant and impervious liners. Pressure curing of the propellants helps... 

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