Polysiloxanes adhesive

Since the locus of failure can clearly distinguish between adhesive and cohesive failures, the following discussion separates loss of adherence into loss of adhesion and loss of cohesion. In the loss of cohesion it is the polysiloxane network that degrades, which can be dealt with independently of the substrate. The loss of adhesion, however, is dependent on the cure chemistry of the silicone, the chemical and physical properties of the substrates, and the specific mechanisms of adhesion involved. 

The latter thicker adhesion layers are most likely polysiloxane condensation polymer layers. Photoresist adhesion to these layers may be electrostatic in nature, while the adhesion of the promoter layers to the gold substrate may be the result of chemisorption forces between the chelating moieties of the promoter layer and the gold surface atoms. 

Because the mass sensitivity of the QCM is a function of radial distance, being highest in the center (Fig. 4.5b), the lateral particle distribution also affects the magnitude of the response. The stickiness of the crystal can be enhanced either chemically or electrostatically. Figure 4.7 shows the schematic of a QCM-based instrument for detection of aerosols in which the crystal is one of the electrodes to which a DC voltage is applied. In order to chemically increase adhesion, coatings of polysiloxanes or silicone grease have been used. 

Polysiloxane-imides were prepared from BPDA, aromatic diamines and ami-nopropylsiloxane and heat imidized to form heat-resistant coatings with excellent adhesion... 

Titanates have been instrumental in the bonding of fluorinated resins to packaging films, poly(hydantoin)—polyester to polyester wire enamel, polysulfide sealant to polyurethane (a phosphated titanate is recommended), polyethylene to cellophane using a titanated polyethylenimine, and silicone mbber sealant to metal or plastic support using polysilane (Si—H) plus polysiloxane (Si—OR) and titanate as the adhesive ingredients (450—454). Polyester film coated first with a titanium alkoxide, then with a poly(vinyl alcohol)—polyethylenimine blend, becomes impermeable to gases (455). 

Three primary mechanisms have been suggested for enhanced adhesion via silane coupling agents.5 The classical explanation is that the functional group on the silane molecule reacts with the adhesive resin. Another possibility is that the polysiloxane surface layer has an open porous structure. The liquid adhesive penetrates the porosity and then hardens to form an interpenetrating interphase region. The third mechanism applies only to polymeric adherends. It is possible that the solvent used to dilute and apply the silane adhesion promoter opens the molecular structure on the substrate surface, allowing the silane to penetrate and diffuse into the adherend. 

The only high-temperature resin family that retains a moderate amount of flexibility is the polysiloxanes. A significant amount of research has been devoted to trying to marry the properties of siloxanes with epoxy resins to obtain less brittle, high-temperature adhesives. However, these efforts have yet to result in commercial adhesives systems. 

Polysiloxanes (silicones) are highly hydrophobic, inert polymers. As aresult, many silicone-based materials display good water repellency and weather resistance. Furthermore, the polysiloxane chain s flexibility gives rise to the macroscopic properties of smoothness and Inbricity. This makes silicones attractive candidates for the modification of snrfaces, snch as textiles, paper and leather. Polysiloxanes, however, have poor adhesive properties, dne to their limited interaction with other materials. Functional gronps have to be introdnced for this purpose. They are inserted at the end of the silicone polymer chain or into the side chains. 

The values are within the limits for electronic application. The seal is thermally stable up to 180 °C. The investigations show that by tailoring chemical structures of sol-gel derived organically modified hetero polysiloxanes, mechanical and adhesive structures can be tailored for needs of application. 

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