Paste adhesives formulations

Heat curing epoxy systems may consist of one or two components. The nomenclature is similar to that employed with the room temperature curing adhesives (e.g., resin component, curing agent component, 2Ksystem, etc.). Many heat curing epoxy adhesives systems are liquids or pastes. However, heat curing systems also can be processed into solid adhesive forms (e.g., powders and films). This chapter describes the paste and liquid formulations solid adhesive forms are considered separately in Chap. 13. 

Guidelines for formulating solid epoxy adhesives are similar to those employed for liquid or paste adhesives. Table 13.4 shows starting formulations for several epoxy adhesive tapes and films. 

The rubber and resin are coreacted by heating the mixture at around 100 °C in the presence of a Lewis base. Most commonly tertiary amines are used for this purpose. The morphology of the rubber dispersion is important to bond performance and is governed by the coreaction procedure. This material may be formulated into a useful aerospace paste adhesive by the following combination of materials (pbw) ... 

In general, specific processes for formulating adhesives are proprietary with the adhesives manufacturer. However, some basic methods and considerations for formulating paste, film, and frozen adhesives are given here. 

Figure 3.22 Manufacturing steps in formulating one-part and two-part paste adhesives.

A third approach to underfilling involves applying an insulative thermoplastic preform (film) prior to attaching the device. Under pressure and heat, the preform softens and flows around the solder bumps, then solidifies quickly on cooUng. The devices may then be solder reflowed to make the connections or the preform may be formulated to flow and encapsulate the solder during solder reflow. Table 5.6 lists some commercially available preforms used as underfills. Anisotropic film adhesives have also been used as underfill preforms, in which case z-direction conductive paths are formed beneath the solder bumps at the same time that the rest of the insulating film flows and cures around the bumps. Anisotropic paste adhesives can be similarly used (Figure 5.11). 

To assure optimum conductance, the adhesive must be applied as thinly and uniformly as possible. To control the thicknesses bond lines, thermally conductive paste adhesives have been formulated with collapsible spacers. The spacers are reported to control bond line thickness to 30 pm. Bond line thicknesses and uniformity may also be achieved by using film or preform tape adhesives and controlling the applied pressure and heat during cure. 

Paste adhesives. Paste adhesive formulations are produced by mixing the base resin with low-molecular weight diluents or solvents, if necessary, to reduce viscosity. Other prepolymer resins may be added and agitated until dissolved. Viscosity and temperature are continuously monitored and more diluents or resin added as required. Finally, one or more types of fillers are added and mixed and the viscosity of the mixture is adjusted. In incorporating the filler into the resin, best results are obtained when the filler is preheated... 

Bondline thickness. The control of bondline thickness is essential for high-power devices. A thin bondline is required since thickness is directly proportional to the junction-to-case thermal resistance, Qjq (see Ch. 2). Specially formulated solvent-based hybrid adhesives can produce uniform bondline thicknesses of one mil or less. Small hard spacers, acting as shims, added to paste adhesive formulations, have also been used to control thickness.The use ofpreforms instead of paste adhesives also assures reproducible thicknesses provided uniform pressure and temperature are applied during curing. 

The benefit of such an approach is that a dispersion of the precipitated polymer can be made in liquid epoxy resin, which can then be used as a formulating ingredient in its own right in, for instance, paste adhesives. 

Reactive liquid polymers (RLPs) The Uquid rubbers most often used in epoxy adhesives are those based on acrylonitrile-butadiene copolymers or on long-chain polyethers. To act as effective tougheners, however, they cannot be directly added to the formulation but have to be prereacted with some of the epoxy resin, or sometimes with the amino hardeners in two component paste adhesives (adduct fonnatiou). To achieve this, only RLPs with reactive eud groups are used by the formulator. 

One-part pastes and films are generally heat curing, although a few one-part paste adhesives are formulated to cure on exposure to UV. 

Three adhesives are used in paper bag manufacture side seam adhesives, bottom paste adhesives, and cross pastes. The side seam adhesive is used to form a cylinder from a flat sheet of paper. This adhesive must develop a strong bond quickly, so the tube can be cut for further operation. The viscosity should be about 3,000 cP. and solids about 25%. One formulation is water 68%, dextrin 28% heat to 160°F (71°C), add 3% borax, heat to 190°F (88° C), add 3% preservative. A water-resistant formulation is water 1700 pounds, white dextrin 700, soap 2, urea-formaldehyde 70, heat to 200°F (93°C), dilute with cold water to 260 gal volume, and add 14 pounds ammonium chloride. This formulation should be used immediately. Its pH is about 6. ... 

Bottom paste adhesives are applied to one end of the tube formed above to close that end to form a bag bottom. These pastes are usually made from unconverted starches. Soap and/or salt may be added to produce a thixotropic paste (flows under shear but sets up if left undisturbed). A water-resistant formulation is com starch 13%, poly(vinylalcohol) 4.5%, poly(vinylacetate) 1 %, soap 0.1%, water 81%, heat to 90°C (194°F), cool to room temperature. ... 

Redux 850 This two-component, essentially hot-curing, paste adhesive is a good example of the formulator co-operating with the end-user. 

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