Bhukan Parbhoo

The dependence of the rate of crosslinking in poly(dimethyl siloxane) on the thickness of coatings

Abstract We consider reasons why the crosslinking reaction rate in poly(dimethyl siloxane) (PDMS) network coatings might differ from the rate found in the bulk and specifically examine the influence of coating thickness. Infrared spectroscopic ellipsometry (IRSE) is employed as an in situ probe of the reactions between vinyl (–CHCH2) end groups on PDMS and SiH groups in a crosslinker and between unreacted SiH groups and hydroxyl/silanol groups within PDMS coatings, all on silicon substrates. Measurements of the concentrations of SiH groups (using the characteristic vibration at 2160 cm−1) were obtained from coatings between 1 and 27 μm in thickness, over temperatures ranging from 25 to 120 °C. First-order kinetics are exhibited in the consumption of SiH groups. The reaction rate constant is found to decrease with increasing coating thickness. Although there is evidence that the Pt catalyst segregates to the interface with the substrate, this phenomenon does not appear to have an impact on the thickness dependence. The diffusion of water into the silicone might be the rate-limiting step in the reactions, however, and therefore lead to the observed thickness dependence of the reaction rate.

Chapter 14 – Fundamental aspects of adhesion technology in silicones

Publisher Summary This chapter reviews the general structures and properties of silicone polymers. It describes the cross linking chemistry and the properties of the cross linked networks. It then discusses the promotion of adhesive and cohesive strength. The buildup of adhesion and the loss of adhesive strength are explained in the light of the fundamental theories of adhesion. The chapter illustrates the use of silicones in various adhesion applications and leads to the design of specific adhesive and sealant products. The surface of the substrate, the silicone/substrate interface, and the bulk properties of silicones play significant and influential roles that affect practical adhesion and the performance of the silicone. The design of silicone adhesives, sealants, coatings, encapsulants, or any products where adhesion property is needed requires the development chemist to have a thorough understanding of both silicone chemistry and adhesion phenomena. The chapter presents a detailed review of the theories that are proposed for the mechanisms of adhesion. The various theories underlying each mechanism is briefly outlined and qualitatively illustrated in the chapter with specific examples.

Polymerization of Cyclosiloxanes

The cyclodiorganopolysiloxanes (1) are one of the two principal classes of siloxane intermediate used in the manufacture of siloxane polymers. The other is short chain linear α,ω-polydisiloxanols (2). Both are formed simultaneously during the hydrolysis of the appropriate diorganodichlorosilane. The cyclosiloxane fraction can be separated from the linear polysiloxanols by simple flash distillation. Ring-opening polymerization of the cyclosiloxanes, which can be effected by both basic and acidic catalysts, produces high molecular weight linear polymers. Molecular weight control is obtained by the use of hexaorganodisiloxanes (3), which act as chain transfer agents. The polymerization with both catalyst systems is a complex equilibrium polymerization and comprises a series of competing reactions involving both cyclic and linear species as represented below (equations 1–5) for octamethylcyclotetrasiloxane (D4) and hexamethyldisiloxane (M2).