Laminating adhesives bond strength values

Two other variations are used to avoid the bending forces that occur with simple ASTM D 1002 specimens the laminated lap shear specimen (ASTM D 3165) shown in Fig. 20.6a and the double-lap specimen (ASTM D 3528) shown in Fig. 20.6b. These specimens minimize the joint eccentricity and provide higher strength values than does the singleoverlap specimen. For the specimen in Fig. 20.6a, the overlap joint can be made from saw cuts in the top and bottom substrates of a bonded laminate. This process negates the effects of extruded adhesive at the edges of the lap and the sheared edge of the standard type of lap shear specimen. As a result, the chances of deformation and uneven surface preparation are lessened. 

Peel values are recorded in pounds per inch of width of the bonded specimen. They tend to fluctuate more than any other adhesive test result because of the extremely small area at which the stress is localized during loading. Even during the test, the peel strength values tend to fluctuate depending on the type of adhesive, adherend, and condition of the test. In preparing the samples, care must be taken to produce void-free laminated bond lines. 

The high melt viscosity of Larc-TPI at 350°C (10 Pas) prohibits its use as an adhesive for bonding metallic or laminate skins on core honeycombs to make high strength sandwich structures. The work of Chow et al. discussed in Section 4.3.2.2 demonstrates that the melt viscosity of the poly(isoimide) form 21 of Fare TPI is of the order of 10 Pas at 240-250°C, compared to 2 x 10 Pas for polyimide 22 [25]. A semi-crystalline form of Larc-TPI has been obtained by chemical imidisation of the polyamic acid with acetic anhydride and triethylamine [63]. The differential scanning calorimetric curve exhibits an endotherm at 274°C due to melting of the crystalline sites. The value of the initial inherent viscosity (0.22 dl g ) indicates, however, that the lower melt viscosity also results from lower molecular weight polymer. 

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