Adhesives thickness

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  

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Adhesive thickness (peel force increases with increasing thickness.)... 

Property Strength requirement (minimum) Adhesive thickness... 

Fig. 12. Adhesive thick adherend shear stress/strain. Reproduced by permission of the Boeing Company.

Sela N., Ishai O. and Banks-Sills L. (1989). The effect of adhesive thickness on interlaminar fracture toughness of interleaved CFRP specimens. Composites 20, 257-264. 

Another option is to design mechanical shims into the joint itself. The parts to be assembled are designed with bps or stops so that the adhesive cannot flow out of the joint area or so that a certain predetermined adhesive thickness is always maintained. 

Primer (source) Adhesive Thickness Application process Properties... 

The width of the lap shear specimen is generally 1 in. The recommended length of overlap, for metal substrates of 0.064-in thickness, is 0.5 + 0.05 in however, it is recommended that the overlap length be chosen so that the yield point of the substrate is not exceeded. In lap shear specimens, an optimum adhesive thickness exists. For maximum bond strengths, the optimum thickness varies with adhesives of different moduli (from about 2 mils for high-modulus adhesives to about 6 mils for low-modulus adhesives).5... 

The rate of peel loading is more important than in lap shear loading, and it should be known and controlled as closely as possible. The rate at which the load is applied is usually specified in the ASTM test procedure. Adhesive thickness also has a significant effect on peel strength values, as does the angle of peeling. 

Entry Adhesive Thickness Approximate Adhesive Uptake Ply Adhesion Average Coeff. Var. Set Time... 

Figure 10. Approximate stress distribution (oy) for L type specimen predicted for load P3 of lON/mm. Predictions based on 5 stepwise adhesive thicknesses from 0.8 mm to 0.08 mm at a = 4.8 mm, 5.6 mm, 6 mm and 6.4 mm. See text for details.

MAJOR PRODUCT APPLICATIONS conductive inks, pastes, coatings, adhesives, thick films, battery plates, electrical contacts, powder metallurgy, capacitor inks... 

FIG. 6—Effect of adhesive thickness on load at failure and location from which cracks grow for button tensile test. Specimens made of PMMA with a diameter of 1,5 in. The model adhesive was Thiokol Chemical Corp. Solithane polyurethane. Crack growth was observed and recorded with a video camcorder. 

In a similar manner, differences in adhesive thickness also complicate the comparison between adhesive joints. Say, for example, a person was comparing the lap joint strength of two adhesives in which the amount of overlap and the adherends were identical, but the thickness of the adhesives differed. If the first adhesive thickness was 1.3 mm and the second was 6.4 mm. the adhesive thickness effect would likely swamp any differences due to the adhesive type. Similar effects of thickness are noted for tensile tests. Not only does the relative thickness of the adhesive affect the load at failure, but it may also influence the point from which cracks are likely to grow. 

The adhesive thickness was about 0.1 mm. The dimensions of the bonded area were 6 in xl in (15 cmx2.5 cm). AU adherends (for shear and peel) were aluminum 2024-T3 alloy pretreated by unsealed chromic anodization. The substrates were cleaned with trichloroethylene prior to bonding. The shear and peel tests were conducted on an Instron 4481 tester. 

Before the application of the adhesive, the bonding surfaces have to be prepared. To clean the surfaces properly, the aluminum of the chassis is grit-blasted (corundum 20-30 pm grit) and the GRP of the cabin is ground with a very fine grinding pad. Then, any dirt and dust particles are removed with the Sika Cleaner-205 and brushed with a suitable Sika Primer. Then the cabin is set onto the chassis in the correct position using polyurethane distance holders to control the adhesive thickness, before Sikaflex -254 Booster is injected into the joint with the booster pump equipment, pressed, and skinned into the joint with a spatula. A small gap with a depth of 5 mm has to remain later, this gap is filled with Sikaflex -265 to achieve a durable seal. 

Sheet structure Three layers (with mold release film) Polyester film liner (50 pm), 25 pm adhesive thickness... 

As was the case for tensile specimens, the stress distribution in lap joints is intricately related to the details of the specimen geometry [2,6-16]. Such factors as amount of overlap, adhesive thickness, adherend thickness, relative stiffness (moduli) of the adhesive and adherend, and other factors critically influence the stress distribution. The maximum... 

Figure 14 Photographs showing the loci of crack growth for different double cantilever beam adhesive joint specimens pulled to failure, (a) The ERR calculated by FEM for this specimen was largest down the center of the adhesive, (b) the ERR calculated by FEM for this specimen was largest near the lower edge of the adhesive, and (c) the ERR calculated by FEM for this specimen was essentially independent of position across the adhesive thickness.

With increasing applied stress, the rectangular cross-section of the adhesive under stress will become increasingly trapezoidal until eventually disentanglement occurs (Fig. 11). The test is now terminated, having provided that force necessary to initiate disentanglement. The test also provides secondary information in that, if the adhesive thickness is known accurately and the distance moved before disentanglement is also known from the recorder chart, then the ratio of these two dimensions is a measure of the deformability of the adhesive, a key factor in pressure-sensitive adhesive behavior. 

Figure 6 Effect of adhesive thickness on anisotropically conductive adhesive interconnections.

Ji, G., Ouyang, Z., Li, G., Ibekwe, S.I., and Pang, S.S. (2010) Effects of adhesive thickness on global and local Mode-I interfacial fiacture of bonded joints. International Journal of Solids and Structures, 47, 2445-2458. 

It is noted that the residual tensile strength depends primarily on the strength of the thermoplastic particles and the thickness of the thermoplastic layer within the narrowed crack. The reason is that the molten thermoplastic forms a thin film in the narrowed crack under the recovery pressure by the SMPFs, similar to the adhesive layer in an adhesively bonded joint (here the fractured two half beams serve as the adherends). It has been well demonstrated that the tensile and shear resistance of adhesively bonded joints highly depends on the adhesive thickness. A thinner adhesive layer usually leads to higher peel and shear resistance [28-32]. This is one reason why a 100% pre-strained SMPF leads to a higher healing efficiency because... 

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