Silicone networks formation

Silicone Network Formation. Silicone rubber has a three-dimensional network structure consisting of cross-linked polydiorganosiloxane chains. Three reaction types are generally employed for the formation of silicone networks (207) peroxide-induced free-radical processes, hydrosilylation addition reactions, and... 

After the components are mixed and heated, the catalyst initiates the cross-linking reaction by addition of the Si—H group to the double bond (eq. 37). Latent cure catalysts have been developed that allow the formulation of one-component products (499). These systems work by incorporation of platinum ligands that deactivate the platinum hydrosilylation catalysts at room temperature. When heated to temperatures above 100°C, these catalysts become active (see also Silicone Network Formation). 

Silicone adhesives are generally applied in a liquid and uncured state. It is therefore the physical and chemical properties of the polymers, or more precisely of the polymer formulation, that guide the various processes leading to the formation of the cured silicone network. The choice of the cure system can be guided by a variety of parameters that includes cure time and temperature, rheological properties in relation with the application process, substrates, the environment the adhesive joints will be subjected to and its subsequent durability, and of course, cost. 

The epoxy/siloxane/PACM-20 mixture was poured into a hot (120 °C) RTV-silicone mold of the precise shapes to be used for solid-state testing. The mixture was cured at 160 °C for 2.5 hours. The curing time and temperature chosen were considered to provide enough mobility for network formation. This conclusion was partially based on earlier studies which found a glass transition temperature of 150 °C for Epon 828/PACM-20 3S). 

A physical model for the formation energies is more easily addressed for undoped a-Si H because the dopants do not have to be included. The origin of the distribution is the disorder of the silicon network. Defect generation is expected to occur when a silicon-silicon bond... 

In the case of PSCSs when x > 0.1, most of the products were found soluble and were converted into the corresponding PCS at 450 °C, under atmospheric pressure. These spinnable PCS were transformed into SiC-based materials with ceramic yields very close to those of Yajima ( 60 %). PSCSs of formula (MeRSi),.j-(HR SiCH2SiR H)j also were prepared in order to appreciate the influence of R and R on the carbon content of the ceramic. Otherwise, multinuclear solid state NMR studies indicated that the network was first built around silicon atoms (formation of SiC4) then, at higher temperature, aroimd carbon atoms (formation of CSi4). 

At the same time, in a number of cases a discrepancy is observed between the position of the maximum of G" and the point where viscosity goes to infinity, for example, during network-formation of silicone rubbers [22]. In this example, the difference between these two points reaches 7% in conversion which suggests that the gel point cannot be identified with the help of dynamical mechanical measurements [12]. 

Mixed metal alkoxide systems are also of interest as a means of creating additional hybrid systems. However, recognition of the large differences in their hydrolysis and condensation rates is crucial. For example, if titanium isopropoxide is made to react under the same conditions as might be used for TEOS, hydrolysis and condensation rapidly occur and lead to particulate rather than network formation of Ti02- Cocondensation with TEOS under these conditions does not occur because of the fast precipitation of the titanium dioxide species. Indeed, of the general metal alkoxides, those based on silicon tend to be more easily controlled because of their slower hydrolysis... 

Many thermoset polymers of major commercial importance are synthesized by step-growth polymerization, as the case of unsaturated polyester, polyurethanes, melamines, phenolic and urea formaldehyde resins, epoxy resins, silicons, etc. In these systems, the crosslinking process, which leads to a polymer network formation, is usually referred to as curing. 

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