Adhesion-promoting Fillers

Formulation. Polysulftde-based sealants are formulated with appropriate ingredients to obtain the desired properties for a particular appHcation. A typical formulation contains Hquid polysulftde polymer, curing agent, cure accelerators (bases) or retarders (acids), fillers, plasticizers, thixotropes, and adhesion promoters. 

Silicone Heat-Cured Rubber. Sihcone elastomers are made by vulcanising high molecular weight (>5 x 10 mol wt) linear polydimethylsiloxane polymer, often called gum. Fillers are used in these formulations to increase strength through reinforcement. Extending fillers and various additives, eg, antioxidants, adhesion promoters, and pigments, can be used to obtain certain properties (59,357,364). 

Various additives and fillers may be employed. Calcium carbonate, talc, carbon black, titanium dioxide, and wollastonite are commonly used as fillers. Plasticizers are often utilized also. Plasticizers may reduce viscosity and may help adhesion to certain substrates. Thixotropes such as fumed silica, structured clays, precipitated silica, PVC powder, etc. can be added. Adhesion promoters, such as silane coupling agents, may also be used in the formulation [69]. 

Acrylics. There are two principal classes of acrylic sealants latex acrylics and solvent-release actylics. High molecular weight latex acrylic polymers are prepared by emulsion polymerization of alkyl esters of acrylic acid, The emulsion polymers are compounded inlo sealants by adding fillers, plasticizers, freeze-thaw stabilizers, thickeners, and adhesion promoters. As is true of the silicone lalex sealants, die acrylic latex sealants are easy to apply and clean with water. 

Polyurethane sealant formulations use TDI or MDI prepolymers made from polyether polyols. The sealants contain 30—50% of the prepolymer the remainder consists of pigments, fillers, plasticizers, adhesion promoters, and other additives. The curing of the sealant is conducted with atmospheric moisture. One-component windshield sealants utilize diethyl malonate-blocked MDI prepolymers (46). Several polyurethane hybrid systems, containing epoxies, silicones, or polysulfide, are also used. 

Future chapters describe the other raw materials that contribute to the epoxy adhesive formulation (curing agents and catalysts, Chap. 5 solvents and diluents, Chap. 6 hybrid resins, Chap. 7 flexibilizers and tougheners, Chap. 8 fillers, Chap. 9 and adhesion promoters, Chap. 10). Complete adhesive formulations are then discussed in subsequent chapters. 

Adhesion promoters are a group of specialty bifunctional compounds that can react chemically with both the substrate and the adhesive. The adhesion promoter forms covalent bonds across the interface that are both strong and durable. Adhesion promoters can be applied directly to the substrate, similar to primers, or they can be mixed with the adhesive itself. When mixed into the adhesive formulation, adhesion promoters are sometimes referred to as coupling agents because they can interact with both the substrate and the other fillers in the formulation. 

Adhesion promoters can also be applied directly to mineral fillers or fibers prior to their addition to a formulation. The adhesion promoter provides for a formulation that has several unique benefits ... 

The interphase provided by the adhesion promoter may be hard or soft and could affect the mechanical properties. A soft interphase, for example, can significantly improve fatigue and other properties. A soft interphase will reduce stress concentrations. A rigid interphase improves stress transfer of resin to the filler or adherend and improves interfacial shear strength. Adhesion promoters generally increase adhesion between the resin matrix and substrate, thus raising the fracture energy required to initiate a crack. 

The moisture resistance properties of filled molding compounds are enhanced by the treatment of the fillers with silane adhesion promoters prior to compounding. Silane promoters on wollastonite fillers in thermoplastic polyester molding compounds (50 percent filled) will improve the flexural strength after 16 h in 50°C water by as much as 40 percent. Silane-treated silica fillers have been found to significantly increase the moisture resistance of epoxy adhesives used in the electronics industry for chip, surface-mounted, and printed-circuit processes.8... 

Other additives used to improve the performance of radiant cured adhesives are similar to those that might be found in more conventional adhesives. These include adhesion promoters, fillers, light stabilizers, antioxidants, and plasticizers. 

Chapters 4 through 10 describe the basic raw materials that are commonly employed in formulating epoxy adhesives. These include the epoxy resins, curing agents and catalysts, solvents and diluents, resinous modifiers, flexibilizers and tougheners, fillers, and adhesion promoters. 

The chemicals used for coating and laminating are polymeric materials, either naturally occurring or produced synthetically. These include natural and synthetic rubbers, polyvinyl chloride, polyvinyl alcohol, acrylic, phenohc resins, polyurethanes, silicones, fluorochemicals, epoxy resins and polyesters." Coating formulations typically include auxiliaries such as plasticizers, adhesion promoters, viscosity regulators, pigments, fillers, flame retardants, catalysts and the like. ... 

Both one-component and two-component silicone sealants contain polydimethyl siloxane as the base polymer along with fillers such as calcium carbonate and/or fumed silica fillers, plasticisers (silicone oil) and a variety of cross-linking agents and adhesion promoters. Two-component sealants utilise a catalyst such as dibutyl tin dilaurate, alkyl silicate esters and metallic salts (Maslow, 1982). 

Silylated urethane polymers allow formulators to produce fast-cure construction joint sealants with superior performance properties. The sealant is composed of silylated urethane polymer, plasticiser (e.g. diisodecyl phthalate), fillers (calcium carbonates), fumed silica, adhesion promoter, dehydrating agent, and catalyst (dibutyltin dilaurate). 

In PET films containing CaCOs, the adhesion of the film was less than that of unfilled film. This is due to the replacement of the adhesion promoting surface by filler which has no adhesive properties.In UV cured adhesives, quartz filler contributed to a faster development of adhesion due to its transparency to the UV radiation used for curing.In polymer blends filler was accumulated at the interphase between polymers. Adhesion depended on the interaction of filler with both polymers and on the particle size of filler. ... 

A similar mechanism acts in the case of reactive adhesion promoters such as polyurethanes. Here water is a factor, as well as surface functional groups of fillers. The adhesion promoter is exhausted within the bulk of organic material (coating, adhesive, etc.) and unable to perform the task. 

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