Dry film adhesives

Converting Palladium to Palladium Sulfide. Crimson, invented by Shipley, employs a conversion step after the activator where palladium is changed to palladium sulfide, which is claimed to be more conductive for subsequent electrolytic copper plating. The enhancer stabilizes the conductive film so that it is chemically resistant to imaging steps. The stabilizer neutralizes residues from the enhancer, thereby preventing contamination of subsequent steps. The microetch selectively removes activator from copper surfaces to achieve optimum copper-to-copper bond and reliable dry film adhesion. The process works best in conveyorized horizontal equipment and can be followed by pattern or panel electroplating. (See Fig. 30.3.)... 

Cost per gallon Spr ability Brightness Sag resistance Storage stability Dry film hardness Dry film durability Chalking properties Detergent resistance Lightfastness Scrubbability Dry film adhesion Salt resistance Blister resistance Odor... 

AppHcation of an adhesion-promoting paint before metal spraying improves the coating. Color-coded paints, which indicate compatibiHty with specific plastics, can be appHed at 20 times the rate of grit blasting, typically at 0.025-mm dry film thickness. The main test and control method is cross-hatch adhesion. Among the most common plastics coated with such paints are polycarbonate, poly(phenylene ether), polystyrene, ABS, poly(vinyl chloride), polyethylene, polyester, and polyetherimide. 

One of the most common rubber adhesives are the contact adhesives. These adhesives are bonded by a diffusion process in which the adhesive is applied to both surfaces to be joined. To achieve optimum diffusion of polymer chains, two requirements are necessary (1) a high wettability of the adhesive by the smooth or rough substrate surfaces (2) adequate viscosity (in general rheological properties) of the adhesive to penetrate into the voids and roughness of the substrate surfaces. Both requirements can be easily achieved in liquid adhesives. Once the adhesive solution is applied on the surface of the substrate, spontaneous or forced evaporation of the solvent or water must be produced to obtain a dry adhesive film. In most cases, the dry-contact adhesive film contains residual solvent (about 5-10 wt%), which usually acts as a plasticizer. The time necessary... 

Water-borne adhesives. These contain between 40 and 80 wt% solids. They are completely untacky in the dried film and auto-adhere to itself under pressure. 

The paints were spray applied to a number of substrates. Two coatings gave a final dry film thickness between 85 to 130 microns. For ac impedance studies the coatings were applied to shot (steel) blasted cold rolled steel plates. For interface metal-loss studies by electrical resistance the coatings were applied to 10 micron Fe foils (99.85 purity), adhesively attached by Araldite 2003 epoxy paste on one side to grit blasted glass plates. 

Rev. impact (in-lb) Acetone rub X-Hatch adhesion 1/8" Mandrel bend Dry film thinkness ( im)... 

The science behind bonding technology is very extensive. A brief description along with some examples is given in the following text. It is important to prime the surfaces of the layers to be bonded, that is, cover the surfaces with a dilute solution of the adhesive mixed with an organic solvent to obtain a dried film of thickness between 0.0015 and 0.005 mm. 

Figure 4 shows the insulative ability of the above formulation. Three coats were applied over a primer to a total dry film thickness of 20 mils, allowing 24 hours dry time between coats. This graph indicates complete failure in 8 minutes. When subjected to other tests the coating lacked adhesion to metal, moisture resistance, and flexibility. After, puffing, the coating disintegrated under vibration. 

Pigments (if required)—to impart colour to the eventual dry film of adhesive. The nature of the solvent plays an important part in establishing the properties of the adhesive as examples, the volatility of the solvent influences the speed of drying and the time during which adhesive-coated surfaces are capable of forming a bond when brought into contact. 

The solids contents of these dispersions generally are of the order of 50% to 60%, occasionally even higher, so that one may deposit from a certain thickness of wet adhesive quite a thick dry film of polymer. Solvent-based adhesives generally have lower solids contents (20% to 30%), and since the shrinkage after drying of water-based dispersions also is lower such adhesives can be formulated to fill gaps between materials as well as to bond them. 

The lower reactivity of the aromatic amines in adhesive formulations is an advantage in that epoxy resin mixtures can be B-staged at room temperature (react to a glassy but fusible and thermoplastic intermediate structure) and will not fully cure for months. In this way, dry films and solid powders can be formulated as elevated-temperature curing, one-component adhesives with long shelf life. 

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. 

Epoxy-nylon film adhesive can be manufactured by solution casting processes. However, a more efficient and environmentally acceptable method is to calender dry blends of powdered nylon with a bquid epoxy resin with accelerators and other modifying resins directly... 

However, waterborne epoxy systems are not without certain disadvantages, which have limited their application as adhesives. These disadvantages include increased use of energy to evaporate the water and dry the adhesive, lower resistance of the cured film to high-humidity environments, and storage and application limitations due to potential freezing at low temperatures. 

Films. Both structural and nonstructural adhesives are commonly available in film form. Adhesives applied in the form of dry films offer a clean, hazard-free operation with minimum waste and excellent control of film thickness. However, the method is generally limited to parts with flat surfaces or simple curves. Optimum bond strength requires curing under heat and pressure, which may involve considerable equipment and floor space, particularly for large parts. Film material cost is high in comparison to liquids, but waste or material loss is the lowest of any application method. 

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