Thixotropic adhesive

Thixotropy can be obtained at fairly low loading concentrations with colloidal silica, bentonite, metallic leafing powders, and hydrated magnesium aluminum silicates. If required, thixotropic adhesive pastes may be formulated which will not flow during cure even at elevated temperatures and which are useful for bonding loose-fitting joints. 

The addition of asbestos fibers at one time provided excellent thixotropic adhesive formulations, especially at elevated temperatures. However, health and environmental regulations have severely limited the use of this material. Today, fumed silica, precipitated calcium carbonate, certain clays, and cellulose and other fibers offer thixotropic properties at relatively low levels of loading. 

Highly viscous materials are not suitable. Only low viscosity, highly thixotropic adhesives can he used. 

Aerobic acrylics also show enhanced cure speed, tensile strength, moisture resistance, and the ability to resist automotive bake cycles, which can be as high as 450°F. The operation also requires fixturing through a variable 10 to 20 mil gap in less than 30 s. Thixotropic adhesive is applied to the area to be bonded and activator sprayed directly on top of the adhesive. The metal is immediately folded to form a hem joint. 

Lithopone Polyamide Zirconium oxychloride Zirconium potassium carbonate thixotrope, adhesives Aramid... 

In thixotropic adhesive gels based on polyvinylalcohol, plasticizers are selected to maintain optical clarity of gel. " In some processes, plasticizers are nsed to process the... 

Figure 1. Application of a thixotropic adhesive to be used in conjunction with GFRP rods to strengthen a bridge stringer (left) and an epoxy grout to repair a carrier beam in a roof structure (right) [8] (reprinted with permission from Rotafix Ltd.).

Normally a higher viscosity or thixotropic adhesive, a sealant, or a thin wood fascia is first applied to the surface to seal the fissures. Small diameter plastic tubes are left in place at regular intervals to be used later to inject in a lower viscosity adhesive, that will hopefully penetrate the whole depth of the fissure, and let the air out. Injection starts from the lower injection tubes until the adhesive starts coining out from the next lower tube, and moves up. Plastic tubes will be cut and disguised after cure. This is nevertheless a tricky operation as the right balance between gap filling and penetration ability of the adhesive must be found. 

For an adhesive to be used, a joint must allow the easy application of the adhesive, must allow for the adhesive to cure fully and must be designed to give uniform stress. Even in a simple face-to-face joint, it must be possible to apply adhesive to one surface and for it to remain there until the two parts are brought together and after that until curing takes place. These requirements highlight the need for a choice of thin, thick or thixotropic adhesives. 

Figure 2.11 Correct placement of a component pre-coated with dispenser-applied adhesive, a Thixotropic adhesive is placed on the plug by means of a spinning mandrel coupled with a shaped application head and doctor blade, b Air will be displaced on plug insertion - given the illustrative dimensions of Figure 2.12. This will ensure that the adhesive will be uniformly distributed and that there are no leakage paths.

Thixotropic adhesives, which are not subject to capillary flow, are needed to bond laminated rotors to their shafts because the laminated rotor will remove other (Newtonian) adhesives from the joint area by capillary action, preventing a satisfactory bond. 

Low-strength thixotropic adhesives were used successfully to seal the halves of a gearbox which had to be pressure tested immediately after assembly. The weak grade chosen allowed ready removal of the lid during maintenance and its high viscosity prevented leakage even before completely cured. 

A glue line of minimum 0.5 mm is used and the thixotropic adhesive is required to fill gaps up to 3 mm in the vertical or horizontal plane. Attachment of the heavy steel stiffening plate requires a few bolts to carry the load whilst the adhesive is setting. The bolts are set back from the edge of the plate in a low stress area so that the hole and the bolt do not interfere with the function of the stiffener (Fig. 41). 

Manufacturers Comments Solvent-free thixotropic adhesive sealant Bonding is insensitive to oils and over-bake. 99% solids. ... 

Manufecturers Comments RT cure. Non-corrosive, non-slump, thixotropic adhesive-sealant for glazing systems, boats, electrical uses. Good resistance to weathering. Good adhesion to masonry (no primer). 

Thixotropic - adhesives whose viscosity becomes lower as they are disturbed (i.e. mixed) but thicken at rest to avoid slump or flow out of the joint region. 

Figure 21 Rheological behaviour of (a) Newtonian solution of oligoimide 99 (IP 605) in NMP and (b) thixotropic adhesive paste IP 680 prepared by loading this solution with silver flakes. Insert (c) represents the cone and plate viscosimeter principle.

Caulk formulation typically consist of 30 to 60% emulsion, 30 to 60% filler, and 0 to 10% plasticizer. Small amounts of additives, such as dispersants, surfactants, preservatives, defoamers, thixotropes, adhesion promoters and solvents for tooling are added to optimize performance. 

For the majority of liquids the viscosity is independent of the velocity gradient and so they exhibit Newtonian behaviour. Many engineering adhesives, however, have nonlinear behaviour and are therefore termed non-Newtonian. A thixotropic adhesive is one which will reduce in viscosity as it is sheared (or moved). When the shearing... 

---