Silica/resin systems

A special study has been dedicated to the nature of bonds in the system, in particular in the silica-resin system. The interaction energies of the different kinds of H-bonds decrease in the order C-O-H- 0(H)-Si > C=0- H-O-Si > C-(H)0" H-O-Si, which is summarized in Table 1. 

The Role of Resins and Silica/Resin Systems (see also Chapter 9.2.3)... 

Comparative sag resistance properties of various commercial types of fumed silica are shown in Table 9.7. In most liquid epoxy resin systems, fumed silica is only employed at a 1 to 3 wt percent basis to provide thixotropic characteristics. 

Covers a two-part epoxy-resin system in the form of a bisphenol "A" epoxy resin filled with fumed silica and carbon microspheres and a separate aromatic diamine curing agent. 

Immobilization of a sulfonated chiral manganese-salen catalyst on a fimctio-nalized Merrifield resin yielded a remarkably active epoxidation catalyst [42]. Its activity and enantioselectivity was examined by epoxidation of 6-cyanochromene, indene, styrene, 4-methylstyrene, and trans-stilbene using m-CPBA/NMO and quantitative yields were obtained in less than 5 min. Enantioselectivities were between 33% (4-methylstyrene) and 96% ee (6-cyanochromene). The same complex was also supported on silica and a layered double hydroxide (LDEI) and the catalytic performances of the systems were compared. Recycling experiments were carried out and the silica-based system showed metal leaching combined with a significant decrease in yield and ee. The layered double hydroxide- and resin-catalysts exhibited a slight decrease in activity and constant ee values in five consecutive reactions. 

The filler effects on the chemoviscosity of thermosetting resins have not been studied extensively, but are vital to understanding the rheology of filled thermosets. For example, the effects of filler concentration on viscosity can be used in process control to monitor batch-to-batch variations or to provide essential information for research into alternative filler/resin batches. Ng and Manas-Zloczower (1993) examined an epoxy-resin system with silica filler and established that the elastic modulus of the resin can be expressed in terms of... 

Kogan et al. (1988) examined the chemorheology of silica- and carbon-fibre-filled epoxyresin systems. They found unusual effects of carbon fibre on the uncured rheology and chemorheology of filled epoxy-resin systems, and related these to the anisotropic namre of the filler shape and the effect of filler surfaces on the kinetics. 

De Miranda et al. (1997) showed the effects of silica flour on the cure of DGEBA/DDM epoxy-resin systems. They noted that there were appreciable decreases in reaction rates for systems with higher filler concentrations than 10 wt.% at high temperatures and conversions above 50%. 

Calculation procedures for adsorption and ion exchange differ only in detail from liquid-liquid extraction since an ion-exchange resin or adsorbent is analogous to the solvent in extraction. All coordinate systems used to represent solvent-solute or liquid-vapor equilibria may be used to display three-component solid-liquid, or solid-gas phase equilibria states. For the case of gas adsorption, equilibria are usually a function of pressure and temperature, and so isobaric and isothermal displays such as Fig. 3.21, which represents the propane-propylene-silica gel system, are convenient. 

Throughout the years, efforts have been made to develop resin systems that are by nature low in plateout properties, and also to develop additive packages that would additionally aid in reducing plateout." It has become a relatively common practice with fluorescent color concentrate suppliers to add a small percentage (i.e., 3-8%) of silica to the masterbatch during compounding. The addition of silica materials has shown to substantially reduce the amount of plateout from fluorescent color concentrates. Due to the improvements that have occurred with different silica products, a study was undertaken to determine if any differences were apparent between fumed and precipitated silica. 

Silica (silicon dioxide) particles, synthetic or natural based, have commonly been used in various polymer systems. Silica has a low coefficient of thermal expansion and high stiffness, translating into increased modulus of the compounded polymer. However, silica filler particles are not flake or plate like as talc or mica particles are, and typically have low aspect ratios. This means that unless the particles are very small, a silica filler addition provides a relatively low surface area for contacting the polymer, and thus it reinforces the resin system less than platy fillers. Researchers have also noted that mica-filled PP, for instance, contains fewer voids than silica-filled PP, which helps explain the higher strengths of mica compounds at 20% filler loadings. This tendency to form voids or cavities increases as silica filler content increases [7-20, 7-21). 

Fillers. In practice most epoxy resin systems have fillers incorporated, often simply to reduce cost although they may also assist in gap filling, reduction of creep, reduction of exotherm, corrosion inhibition and fire retardation. Their incorporation will also alter the physical and mechanical properties of the adhesive. Construction resins in particular often include a large volume fraction of sand or silica. 

Fig. 2. Relative viscosity in a shear rate step profile at D =1 s, 10 s and 100 s, each 120 s, respectively, of four resin systems Wacker HDK N20 and HDK HI 8 in Palatal P4 (UP resin) and in Atlac 590 (VE resin) 35 wt% styrene 3 wt% fumed silica.

UPE resins can be used as clear castings or in combination with particulate fillers or fibres. The resin was developed to meet the demand of lightweight materials in military application. The first functional use of UPE was in radome. Because of the obvious advantages of easy processability and low cost, it was used in a wide range of applications in civil sectors such as tanks, pipes, and electronic gears. Some of the important products based on cast UPE resins are encapsulation of electronic assembly, buttons, door handles, knives, umbrellas, industrial wood and furniture finishing. A filled resin system using limestone, silica, and china clay are used for floor tiles. The major use of UPE is as a matrix for fibre-reinforced composites. Such composites have wide applications in automobile and construction industries such as boats, water-skis and television antennae. Examples of applications of UPE resins are presented in Table 2.7. 

Chemically resistant mortars are formulated using an inorganic binder or liquid resin system, fillers such as silica, carbon, or combinations thereof, and a hardener or catalyst system. 

Carbon, silica, or combinations of the two are the most popular fillers used. Carbon is the most inert of the fillers and therefore is the one most often selected. The most popular resin systems, both organic and inorganic, will be discussed, starting with the organic systems, many of which are also used as monolithic surfacings. 

As stated previously, precipitated silica is being used to replace the carbon black filler in so-called green tyres where it has been shown to reduce rolling resistance [43-46], and so help to reduce fuel consumption. Since the late 1960s, silica has been used in combination with resin systems and at that time PPG introduced a system that consisted of HI-SIL hydrated silica, resorcinol and hexamethylene tetramine (HRH). Initially it was used chiefly for bonding various rubbers to textiles, but such systems have found increasing use in wire coat and belt compounds. 

In general, fillers reduce the specific adhesion and cohesive strength of adhesive films. For these reasons, they are rarely used in low solids contact adhesives. In some formulations, e.g., Neoprene-terpene phenolics, the inclusion of a fine particle size reinforcing silica such as HiSil 233 (PPG Industries) will increase film strength. This effect is lost in formulations using reacted magnesium resinate systems. 

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