Bond line thickness

The gap between the parts and therefore the thickness of the adhesive film has an important bearing on the characteristics of the joint [4], A thick bond line ( 0.25 mm) will generally be a weakening feature for cyanoacrylates as the mechanical strength of the cured cyanoacrylate film is likely to be less than the plastic or other substrate 

Epoxies, poljmrethanes, two-part acrylics and the adhesive/sealant products (silicones, and modified silanes) all have excellent gap-filling capabilities and some of these products will offer excellent resistance to impact loading and peel loads. 

Assemblies with varying joint gaps should be avoided as much as possible. This practice will avoid uneven stress distributions, irregular pressures in the assembly and internal curing stresses. 

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Fitzpatrick et al. [41] used small-spot XPS to determine the failure mechanism of adhesively bonded, phosphated hot-dipped galvanized steel (HDGS) upon exposure to a humid environment. Substrates were prepared by applying a phosphate conversion coating and then a chromate rinse to HDGS. Lap joints were prepared from substrates having dimensions of 110 x 20 x 1.2 mm using a polybutadiene (PBD) adhesive with a bond line thickness of 250 p,m. The Joints were exposed to 95% RH at 35 C for 12 months and then pulled to failure. 

Flow properties (viscosity, thixotropy) Bond line thickness Coefficient of thermal expansion Shrinkage... 

Thickeners, fillers, and thixotropic agents are also commonly used to control flow and the bond line s thickness within a joint. Scrims, carriers, and woven reinforcements are other methods commonly used to control bond line thickness. 

Viscosity Ease of compounding Dispensing and flow Bond line thickness control... 

The viscosity of epoxy resins and curing agents can be used to control the bond line thickness within the adhesive joint. But the bond line can also be regulated by the incorporation of fillers, by the use of scrim cloth or woven tapes as internal shims within the adhesive itself, or by the careful regulation of the cure cycle. 

It can provide for a practical and reproducible bond line thickness in the final joint. 

The first three factors are generally controlled by the rheological properties of the liquid adhesive through the application of fillers in the formulation. The final factor can be controlled through the viscosity however, other methods are also possible to control the bond line thickness. 

The type and amount of fillers are chosen so that a practical bond line thickness will result after application of the necessary pressure (usually only contact pressure, approximately 5 psi) during cure. Ordinarily, the objective is a bond line thickness of 2 to 10 mils. Consideration, of course, must be given to the curing temperature. Viscosity of the formulation could drastically be reduced at elevated temperatures, and unless there is a furrow designed into the joint to contain the adhesive, much of the adhesive could flow out of the joint area before the adhesive is completely cured. 

Supporting fibers in epoxy adhesive tapes are useful in that they provide for a positive stop under bonding pressure. This can be used to control bond line thickness and to help distribute stresses evenly during service. The supporting fibers that are used in these adhesives are primarily for the purposes of carrying the adhesive and convenient application to the substrate. Their reinforcing function within the epoxy matrix is generally considered to be of secondary importance. 

Many film adhesives have a supporting carrier or reinforcement fabric incorporated into the adhesive to improve handling of the film and provide control of bond line thickness. The carriers are usually glass, polyester, or nylon fabrics of knitted, woven, or nonwoven construction. The difficulty with such carriers is that they can provide an effective way of moisture entering the bulk of the adhesive. Moisture can wick along the fiber-adhesive interface. Nylon carriers should especially be reviewed since they have a strong tendency to absorb moisture. 

Liquids. Liquids are the most common form of adhesive, and they can be applied by a variety of methods. Liquids have an advantage in that they are relatively easy to transfer, meter, and mix. They also tend to wet the substrate easily and provide uniform bond line thickness. However, they have the disadvantages of sometimes being messy, requiring cleanup, and having a relatively high degree of waste. 

Application requires a relatively high degree of care to ensure nonwrinkling and removal of separator sheets. Films are often supported on scrim that distributes stress in the cured joint and ensures a uniform bond line thickness throughout the bonded area. 

The amount of the applied adhesive and the final bond line thickness must be monitored because they can have a significant effect on joint strength. Curing conditions should be monitored for pressure, heat-up rate, maximum and minimum temperatures during cure, time at the required temperature, and cool-down rate. The primary concerns are to ensure the following ... 

The objectives of this test pattern is to analytically resolve these problems into three manageable segments. The first task will be to define the viscoelastic kinetic properties of a material as a function of various reaction temperatures. These properties (viscosity, viscous modulus, elastic modulus, tan delta) define the rate of change in the polymers overall reaction "character" as it will relate to article flow consolidation, phase separation particle distribution, bond line thickness and gas-liquid transport mechanics. These are the properties primarily responsible for consistent production behavior and structural properties. This test is also utilized as a quality assurance technique for incoming materials. The reaction rates are an excellent screening criteria to ensure the polymer system is "behaviorally" identical to its predecessor. The second objective is to allow modeling for effects of process variables. This will allow the material to undergo environmental... 

Storage and shipping stability is poor over 20 C. Many of the current products on the market are handled in refrigerated containers, under h°C. Bond line thickness is limited, e.g. 3-10 mils is... 

Bond line thickness and flow out shape often fluids are compressed between two surfaces to create a mechanical bond, electrical interface, and/or thermal interface. When this is the case, the shape of the fluid as dispensed can significantly affect the bond line thickness, flow out, and coverage of the interface. Any part geometries that cause variation in the bond line thickness will likewise influence the volumetric tolerances of the material to be dispensed. 

AIC materials contain a relatively low percentage of conductive particles suspended in an adhesive. They make electrical contact when compressed between two electrical surfaces and conduct electricity in one axis. The particles must be size matched to the bond line thickness or they will create shorts between lines. 

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. 

Yet, a third equation defines maximum stress for rectangular devices taking into account the size of the device, bond line thickness, and cure and exposure temperatures, in addition to modulus and CTEs. ... 

Based on these equations, stresses that can result in failures are a function of many parameters including modulus of elasticity, expansion coefficients of the adhesive and adherends, glass-transition temperature, cure temperature, operating or exposure temperatures, and bond line thickness. 

The control of bond line thickness is essential for high-power devices. A thin bond line is required since thickness is directly proportional to the junction-to-case thermal resistance, jc (see Chapter 2). Specially formulated solvent-based hybrid adhesives... 

The handling and reliability advantages of tape and film adhesive include ready to use, no need for mixing, no degassing, and no possibility for error in adding catalyst. Tapes permit a variety of lay-up techniques, which facilitate the production of virtually defect-free structures. The use of a mesh support helps to control the bond-line thickness with tape adhesives. 

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 ... 

Employ a film adhesive that becomes highly viscous during the cure cycle, preventing excessive adhesive flow-out. With supported films, the adhesive carrier itself can act as the shim. Generally, the cured bond line thickness will be determined by the original thickness of the adhesive film. 

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