Film adhesives application

TPEs find use in automotive, wire and cable, footwear, polymer modification, hose and tube, mechanical, bitumen modification, construction, adhesives and coatings, and film/sheet applications. 

Many different silane coupling agents are available for improving the adhesion between polymers and metals or fillers and fibers [1]. Although organofunctional silanes have been reported to be very effective in such adhesion applications, the exact mechanism by which they function has not been determined conclusively in certain cases. Several techniques have been used to characterize silane films formed on various substrates, such as glass, aluminum, and steel. Some of these methods are XPS [2, 3], FUR [4], IETS [5, 6], NMR [7], and Raman spectroscopy [8]. 

Sakamoto YI, Takeda N, Takeda T, Tokoh A, Tang DY (1992) New type film-adhesives for microelectronics application. Proc-Electron Compon Technol Conf 42nd 2315... 

Film Adhesive. A further type which is of importance commercially is the film adhesive which exists usually as pre-formed shapes on suitable substrate materials (often, siliconized release paper). When being applied, such adhesives are activated by heat or pressure and so converted into a semi-fluid condition, when they can wet the surface to be bonded. The pressure-sensitive adhesives are an example of this type—the application of finger pressure alone being sufficient to form the bond. 

In hot mixing or elevated-temperature curing of an epoxy system, vapor pressure could also be of concern relative to the quality of the adhesive bond. If the components in an epoxy system become too hot, boiling can occur, resulting in gas bubbles. If gas bubbles become trapped in the cured adhesive film, they can lead to reduction of cohesive strength and stress risers. For many adhesive applications, particularly those in the electrical and electronic industries (due to possible ionization of air voids), complete removal of any gas bubbles from the epoxy is essential. 

The final thickness of epoxy tape or film adhesives is on the order of 5 to 15 mils. These adhesives may be soft and tacky, or stiff and dry, depending on their formulation. The soft and tacky products are valuable in products requiring application to contoured or vertical surfaces. If the product is especially tacky, a release liner (e.g., polyethylene film, coated paper) is generally used to keep the film from blocking. The stiff and dry products are generally used for flat surfaces where speed and ease of application are required. 

Tape and film adhesives are most often used to bond large areas, such as for applications in the aerospace industry. For example, the joining of aluminum honeycomb structure to flat metal skins is often accomplished with thermosetting epoxy film adhesives. These films (Fig. 13.1) can easily be applied without the need to mix, meter, or apply a liquid coating. 

Structural adhesives that are commonly used for composites are supplied in two basic forms semisolid B-stage film and thixotropic pastes. The film adhesives are cast or extruded onto carrier fabrics or films and partially cured to a semisolid. They can easily be handled, cut, and applied to the joint area. There is no need for mixing, metering, or dispensing of liquid components. In use, these adhesive systems are activated by heat and pressure. The semisolid B-stage film liquefies briefly on application of heat and then cures to an insoluble state. Epoxy, polyimides, epoxy-nylons, epoxy-phenolic, and nitrile-phenolic adhesives are available as B-stage film. 

Poly(vinyl acetate) (PVAc), -[CH2-CH(OCOCH3) , is also prepared as an emulsion for adhesive applications, and is familiar to users as white glue. As mentioned already, the Tg of PVAc is above room temperature, which makes the polymer rigid and brittle at room temperature. For adhesive application, therefore, an external plasticizer, such as dibutyl phthalate, is added to lower the Tg below room temperature and to facilitate film formation from emulsions. 

The percentage of solids in the dispersion adhesives is 35-55%, in some cases as much as 70%, the rest is water, which after the adhesive application must either be completely evaporated (ventilation time) or diffuse into the joined parts (wood, textiles, concrete, foams, leather). The film formation temperature in types containing plasticizers and flexibilizing additives is lower than in types that are free of plasticizers. Additives, so-called high boilers (ethyl glycol, ethyl glycol acetate, ethylhexanol), are used to bring the film formation temperature down. 

Current interest in siloxane polyimides is triggered by opportunities for such materials in military, aerospace and electronic applications as coatings, films, adhesives, molding compounds and composite matrix materials which are subject to demanding operating conditions. These polymers offer advantages such as excellent interlevel adhesion, plasma resistance, low water absorption, and stability at high temperatures. 

Minimum drying time In the case of solvent-based adhesives, the time period between the adhesive application and the fixing of the adherends, to enable the evaporation of the major part of the solvent from the liquid adhesive film. 

Although they have been incorporated in paints for their durable, hard, bright, anticorrosive and adhesive films, industrial applications of lacs and of urushiol have been limited because of their high viscosity, low drying rate, requiring a regulated temperature and humidity, their suseptibility to an alkaline environment and the contact sensitivity of human skin. 

MAJOR USES Used in the manufacture of polyvinylidene chloride copolymers, flexible films for food packaging, flame retardant coatings and adhesive applications. 

ORIGIN/INDUSTRY SOURCES/USES not a natural product formed by the anaerobic biodegradation of trichloroethylene and by the hydrolysis of 1,1,1-trichloroethane manufacturer of polyvinylidene copolymers and methyl chloroform flexible films for food packing (Saran and Velon wraps) flame retardant coatings for fiber and carpet backing in pipes coating for steel pipes adhesive applications... 

---