Adhesively bonded linings

The most common catalyst used in urethane adhesives is a tin(lV) salt, dibutyltin dilaurate. Tin(IV) salts are known to catalyze degradation reactions at high temperatures [30J. Tin(II) salts, such as stannous octoate, are excellent urethane catalysts but can hydrolyze easily in the presence of water and deactivate. More recently, bismuth carboxylates, such as bismuth neodecanoate, have been found to be active urethane catalysts with good selectivity toward the hydroxyl/isocyanate reaction, as opposed to catalyzing the water/isocyanate reaction, which, in turn, could cause foaming in an adhesive bond line [31]. 

As an aside, we note that the FDEMS sensor input information can also be used to detect the onset of phase separation in toughened thermoset systems and to monitor cure in thin film coatings and adhesive bond lines. It is particularly important that the FDEMS sensor is also very sensitive to changes in the mechanical properties of the resin due to degradation. As such, it can be used for accelerated aging studies and as a dosimeter to monitoring the composite part during use to determine the knockdown in the required performance properties with time. 

Appropriate fillers have been used to produce adhesives with high electrical conductivity. Note that, regardless of the adhesive system itself, electrical conductivity is improved by minimizing the adhesive bond line and by minimizing the organic or nonconductive part of the adhesive. 

If both substrates to be bonded are nonconducting, then the adhesive formulation must contain a susceptor material. Susceptors can have a small percentage of magnetic iron oxide, iron filings, or carbon additives. A susceptor can also be a steel screen or perforated steel foil that is embedded in the adhesive bond line. It has been found that graphite fiber composites used in the automotive and aerospace industries are sufficiently conductive that they can be successfully heated with induction. Design considerations must be taken into account in placement of the graphite reinforcement, so that the material heats uniformly. 

The adhesive bond line is usually very thin and well protected from the chemical itself. This is especially true if the adherends are nonporous and nonpermeable to the chemical environments in question. 

Plasticizer migration from the vinyl part into the adhesive bond line can degrade the strength of the joint. Adhesives must be tested for their ability to resist the plasticizer. PVC can be made with a variety of plasticizers. An adhesive suitable for a certain flexible PVC formulation may not be compatible with a PVC from another supplier. Nitrile rubber adhesives have been found to be very resistant to plasticizers and are often the preferred adhesive for flexible PVC films. However, certain epoxy adhesive formulations have also been found to provide excellent adhesion to flexible PVC substrates. Several such starting formulations are presented in Table 16.7. A comparison of the performance of several classes of adhesive when bonding PVC to itself and to various other materials is given in Table 16.15. 

If a cure of 60 min at 150°C is recommended, this does not mean that the assembly should be simply placed in a 150°C over for 60 min. The temperature is to be measured at the adhesive bond line. A large part will act as a heat sink and may require substantial time for the adhesive in the bond line to reach the necessary temperature. In this example, total oven time would be 60 min in addition to whatever time is required to bring the adhesive up to 150°C. Bond line temperatures are best measured by thermocouples placed very close to the adhesive. In some cases, it may be desirable to place the thermocouple in the adhesive joint for the first few assemblies being cured. 

Adhesive bond line too thin Clamping pressure too high Lessen pressure... 

Polyvinyl Chloride (PVC) With PVC plasticizer migration to the adhesive bond line can cause difficulties, especially in the softer, highly plasticized materials. Adhesives must be tested for their ability to resist the plasticizer. Nitrile-rubber adhesives are resistant to plasticizers. Polyurethanes and neoprenes are also used. Even rigid PVC contains up to 5% plasticizer. Most vinyl materials are fairly easy to... 

Figure 8.2 Adhesively bonded lining ("fop thermoforming of sheets, and (middle and bottom) finished lined head. (Courtesy Electrochemical Engineering and Manufacturing Company.)...

Alternative removal methods have been reported, but none is widely used. Of these, one involves sliding a hot wire, through which current is passed, between the substrate and package. This process simultaneously softens and slices through the adhesive bond line. Another method consists of inserting a resistance heater on the underside of the substrate to generate additional localized heating at the adhesive interface. 

After RGA data are collected and analyzed, the package is delidded to examine the inside components, interconnections, materials, and surfaces. Because removing the lid is a destructive process, it should be done in a lid-down position with special equipment to vacuum off any debris. After delidding, the components, surfaces, and connections are optically examined followed by analytical testing such surface analysis for contaminants, especially chloride and sodium ions. Scanning acoustical microscopy (SAM) is very useful in detecting voids, cracks or delamination within the adhesive or at the adhesive bond line. An excellent overview of SAM and its variations may be found in Ardebili and Pecht. ... 

Acoustic microscopy is a widely used method for analyzing the integrity of adhesive bond lines. Acoustic microscopy is based on the fact that each material has an acoustic impedance designated as ... 

When voids are present in a bond line or when delamination occurs, the air that fills the voids has an acoustic impedance of 0 kg/m second and a large amplitude and total negative reflectance occurs. Most polymeric materials have acoustic impedances of 2.0-4.5 X 10 kg/m second compared to copper (42 x 10 kg/m second) and silicon (20 x 10 kg/m second). Examples of acoustic micrographs showing voids and delamination in an adhesive bond line are pictured in Figs. 6.6 and 6.7. 

Bonding, Fig. 5 General installation of an adhesive bonding line (Cognard 2006)... 

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 ... 

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... 

Induction and dielectric heating are the fastest heating methods because they focus heat at or near the adhesive bond line. Workpiece heating rates greater than 100° F/s are possible with induction heating. For induction heating to work, the adhesive must be filled with... 

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