Bonding to Silicone Rubber

Silicone rubber is difficult to bond with cyanoacrylates. The adhesive does not wet the surface properly without special surface treatment, due to the very low surface energy, and therefore will not bond. Polypropylene (PP), polyethylene (PE), polytetrafluoro ethylene (PTFE) and acetal plastics and Santoprene rubber also fall into this category and cannot be bonded without prior surface preparation. 

Surface primers are now available which can act as an adhesion promoter. Loctite 770 (Henkel Loctite Adhesives Ltd) is a heptane-based primer which can be brushed or sprayed onto the surface of the rubber and then used with an ethyl-based cyanoacrylate. 

This primer is recommended for PP, PE, Santoprene and other similar rubbers as well as PTFE. It frequently enables cyanoacrylates to successfully bond polyacetals as well. 

The primer should be applied once only to the surface over-application will nullify the effect of the primer. The on-part life of the primer is typically eight hours, but best results are achieved if the surfaces are bonded immediately. 

The Loctite 770 primer contains a fluorescent additive to reveal where the primer has been applied. Treated parts can be inspected simply by passing them under a low intensity, blue UV lamp. 

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The ideal surface for contact with human blood is the surface of blood vessels, and the immediate surface contains heparinoid complexes. Heparin, a negatively charged polysaccharide, has been bonded to silicon rubber and other polymers. In one procedure, a quaternary ammonium compound is first adsorbed on die polymer substrate and heparin is 111 turn adsorbed on the positively charged surface. Chemical bonding of heparin has also been achieved. Such surfaces do not cause clotting of contacted blood. 

Foly(dimethylsiloxane) (silicone rubber) membranes are fabricated by hydrolysis of alkox-ysilyl terminal groups of silicone-rubber precursors [oligo(dimethylsiloxane) derivatives and crosslinking agents], followed by condensation. Covalent bonding of neutral carriers carrying an alkoxysilyl group to silicone rubber is, therefore, feasible by simple reaction of the silicone-rubber precursor with alkoxysilylated neutral carriers, as schematically shown in Scheme 1 [44]. 

It must be remembered that although silicone elastomer is selfreleasing from epoxy resins and from clean metal surfaces, it will bond to silicone release agents. It is therefore essential to mould the rubber prior to treating any parts of the mould with release agent. 

The health and environmental aspects of poly(dimethylsiloxane) fluids have been reviewed. Sea water saturated with poly(dimethylsiloxane) fluid shows no toxic activity against phytoplanktons, molluscs, crustaceans, and fish. The toxicity of the pyrolysis gases from organosiloxane polymer fluids, rubbers, and resins has been shown to be the least toxic from those of 300 materials tested. Two excellent texts have been published on the analysis of silicones both in bulk and additive forms. > A comparative study of techniques for the trace analysis of Si—H and Si—Cl groups in poly(organosiloxane)s shows that levels of 0.01 p.p.m. H and 1.0 p.p.m. Cl bonded to silicon may be determined, ... 

The bonding of silicone rubbers to other materials or to themselves is a very wide field. Most commonly they are classified as ... 

It is technically possible to produce very strong bonding between silicone rubber and another material. However, the silicone-plastic, silicone-metal or silicone composite obtained usually... 

Finally, conventional sealants, based on silicone rubber formulations, are used to bond in silicone rubber sealant strips. 

Devine AT, et al. Bonding Lexan Polycarbonate to Silicone Rubber for Gas Masks. Picatinny Arsenal Technical Report... 

Silicone Rubber. These polymers are based on chains of siUcon rather than carbon atoms, and owe thek temperature properties to thek unique stmcture. The most common types of siUcone mbbers are specifically and almost exclusively the polysdoxanes. The Si—O—Si bonds can rotate much more freely than the C—C bond, or even the C—O bond, so the siUcone chain is much more flexible and less affected by temperature (see Silicon COMPOUNDS, silicones). 

Chen et al. utUized a direct chemical reaction with a given solution (wet treatment) to modify the surface of the silicone rubber. The presence of a layer of PEO on a biomaterial surface is accompanied by reductions in protein adsorption, and cell and bacterial adhesion. In order to obtain a PEO layer on top of the silicone rabber surface, the surface was firstly modihed by incorporating an Si-H bond using (MeHSiO) , and followed by PEO grafting to the surface using a platinum-catalyzed hydrosilylation reaction. These PEO-modified surfaces were demonstrated by fibrinogen adsorption both from buffer and plasma, as well as albumin adsorption from buffer. Reductions in protein adsorption of as much as 90% were noted on these surfaces. 

The design of bioeompatible (blood compatible) potentiometric ion sensors was described in this chapter. Sensing membranes fabricated by crosslinked poly(dimethylsiloxane) (silicone rubber) and sol gel-derived materials are excellent for potentiometric ion sensors. Their sensor membrane properties are comparable to conventional plasticized-PVC membranes, and their thrombogenic properties are superior to the PVC-based membranes. Specifically, membranes modified chemically by neutral carriers and anion excluders are very promising, because the toxicity is alleviated drastically. The sensor properties are still excellent in spite of the chemical bonding of neutral carriers on membranes. 

Silicone-based release agents, although extremely effective in their designed function, can be a real hazard if used incorrectly. Through-transfer to rubber surfaces during cavity loading will destroy any attempt at bonding to other substrates. The liquid silicones are often supplied as water emulsions. 

While unaffected by water, styrofoam is dissolved by many organic solvents and is unsuitable for high-temperature applications because its heat-distortion temperature is around 77°C. Molded styrofoam objects are produced commercially from expandable polystyrene beads, but this process does not appear attractive for laboratory applications because polyurethane foams are much easier to foam in place. However, extruded polystyrene foam is available in slabs and boards which may be sawed, carved, or sanded into desired shapes and may be cemented. It is generally undesirable to join expanded polystyrene parts with cements that contain solvents which will dissolve the plastic and thus cause collapse of the cellular structure. This excludes from use a large number of cements which contain volatile aromatic hydrocarbons, ketones, or esters. Some suitable cements are room-temperature-vulcanizing silicone rubber (see below) and solvent-free epoxy cements. When a strong bond is not necessary, polyvinyl-acetate emulsion (Elmer s Glue-All) will work. 

Hydrosilylation (the addition of R3SiH to a double bond) is an important reaction in the silicone polymer industry. It is used for curing silicone rubber, by cross-linking polymer chains. The reaction is catalyzed by Pt and Rh complexes, following the cycle shown in Figure 3.45. 

A silicone rubber adhesive layer is used in US-A-4081819 to bond an HgCdTe substrate, which includes an epitaxial layer, to a second substrate. The silicone rubber adhesive reduces the risk that the substrate cracks when cooled to cryogenic temperatures. 

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