Adhesive starved joint

Alkyl cyanoacrylate monomers typically have viscosities in the range of 1-3 cP, which are much too low to permit convenient use in many industrial applications. Water thin adhesives will flow from the bondlines, resulting in adhesive-starved joints and low bond strengths. Migrating adhesive could also bond adjacent... 

Control of Viscosity. To ensure that adhesives and sealants function well during their application and end use, the formulator must be able to control the flow properties of the product. The challenge that faces the formulator is that the adhesive or sealant may need different flow characteristics at different times. For example, adhesives must flow readily so that they can be evenly applied to a substrate and wet out the surface. Yet, there should not be an excess of penetration into porous substrates, nor should the adhesive run or bleed to create a starved joint. Certain adhesives and sealants must be capable of convenient flow application by trowel or extrusion, but they must also exhibit sag and slump resistance, once applied. Therefore, the flow properties, or rheology, of the material must fit the desired method of application. 

Viscosity of adhesive decrease at elevated temperature could result in starved joint... 

Another important factor with regard to adherends is the uniformity and tolerances allowed in the joint fit-up. Improper tolerances on the geometry of the joint can lead to adhesive-starved areas or areas where the bond is so thick that unwanted peel and cleavage forces can easily develop. 

As with other acrylics, the monomers are liquids of low viscosity that polymerize very easily in the presence of a slightly basic surface containing adsorbed water. Polymerization is ionic. The resulting polymers have different properties, depending on the alkyl group. The methyl ester (methyl-2-cyanoacrylate) is the most commonly used compound. This material is formulated with a thickener (to prevent starved joints from being formed) and a plasticizer to make it more resistant to shock loading. The thickener can be a polymer of the same monomer. An essential feature is a stabilizer to prevent polymerization in the adhesive container, which is usually made of polyethylene. 

No appearance of adhesive around edges of joint or adhesive bond line too thick Clamping pressure too low Starved joint Curing temperature too low Increase pressure. Check that clamps are seating properly Apply more adhesive Use higher curing temperature. Check that temperature is above the minimum specified... 

Adhesive bond line too thin Clsunping pressure too high Curing temperature too high Starved joint Lessen pressure Use lower curing temperature Apply more adhesive... 

Thickeners added to an adhesive will raise viscosity and permit dilution with water. This reduces the total solids of the adhesive and thus lowers its (wet) cost. Thickeners release water slowly, and when combined with lower solids, they also slow the setting (prolong open time) of the adhesive. Thickeners such as polyvinyl alcohol, starch, and hydroxy ethyl cellulose will improve adhesion to porous substrates by reducing penetration of an emulsion into the substrate and decreasing the likelihood of a starved joint. ... 

Adherends Improperly Positioned, Clamped, or Braced During Drying. Positioning is very important. In the case of contact cements there is no second chance to position the substrates, since bond is instantaneous. In other types of installations the correction of improper positioning can result in too much adjustment, thereby wiping adhesive from one of the substrates, resulting in a starved joint, a bond with inadequate adhesive. 

Form of adhesive. Solid, liquid or paste, if solid in film form is it supported on a carrier Pastes sometimes contain balotini to ensure correct spacing of the adherends and thus void a starved joint. Primers are always low viscosity solutions from which solvent must be allowed to evaporate they may need a separate cure or partial cure. The bare items of the above list can now be filled out with more detail without the danger of obscuring an overall view of those items about which decisions must be made. This now follows using substantially the same headings as were taken in the list. 

Of less importance for structural adhesives is item (iii) except where concrete is one of the adherends. Even in this case, however, penetration of the adhesive into the substrate may be a very desirable and, as epoxy adhesives will be those normally involved, viscosity can be adjusted to avoid any possibility of a starved joint. 

No appearance of adhesive around edges of joint or adhesive bond line too thick Clamping pressure too low Starved joint Curing temperature too low... 

Controlling flow or viscosity is an important part of the adhesive formulation process. If the adhesive has a propensity to flow easily before and during cure, then one risks the possibility of a final joint that is starved of adhesive material. If the adhesive flows only with the application of a great amount of external pressure, then one risks the possibility of entrapping air at the interface and getting too thick of a bond line. These factors could result in localized high-stress areas within the joint and reduction of the ultimate joint strength. 

Regardless of the heating source, elevated-temperature curing epoxy adhesives necessitate that the parts be held with clamps, pressure pads, or fixturing methods to apply constant and uniform pressure throughout the curing cycle. Reduction in viscosity due to elevated temperatures could result in excessive adhesive flow during cure. This could result in joints that are starved of adhesive or movement of the parts, if not firmly fixtured. 

It is highly desirable to have a uniformly thin (0.05-0.25 mm) adhesive bond line. Starved adhesive joints (where some areas have no adhesive), however, will result in poor bonds. Three basic methods are used to control adhesive (bond line) thickness ... 

Joint, starved—A joint that has an insufficient amount of adhesive to produce a satisfactory bond. This condition may result from too thin a spread to fill the gap between the adherends, excessive penetration of the adhesive into the adherend (when porous), too short an assembly time, or the use of excessive pressure. 

Joint, starved n. A joint that has an insuflfi-cient amount of adhesive to produce a satisfactory bond. Note - This condition may... 

But porous adherends require the application of sufficient adhesive to avoid a starved glue line. If the joint is to be exposed to considerable shear stress, the glue line should be somewhat thicker. 

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