Shear strength adhesives

The interphase provided by the adhesion promoter may be hard or soft and could affect the mechanical properties. A soft interphase, for example, can significantly improve fatigue and other properties. A soft interphase will reduce stress concentrations. A rigid interphase improves stress transfer of resin to the filler or adherend and improves interfacial shear strength. Adhesion promoters generally increase adhesion between the resin matrix and substrate, thus raising the fracture energy required to initiate a crack. 

Tensile shear strength (adhesive strength), in the sense of this standard, is defined as the maximum force Fmax at the break of the bonded joint in relation to adherend surface A. The adherend surface A results from the test piece width h (25 mm) and the overlap length lu (12.5 mm) ... 

Formulation of speeific SA for special applications, e.g. high shear strength adhesives for applications involving pre-stressing with improved ductile behaviour in order to avoid the concentration of shear stresses over a short anchorage length... 

R. A. Peters and T. J. Logan, "Microvoid Epoxy Adhesives For High Peel and Shear Strength", Adhesives Age, 18, 17, April, 1975. 

The routine compositional and functional testing done on the adhesives includes gas chromatographic testing for purity, potentiometric titrations for acid stabilizer concentrations, accelerated thermal stabiUty tests for shelf life, fixture time cure speed tests, and assorted ASTM tests for tensile shear strengths, peel and impact strengths, and hot strengths. 

The physical properties of polyurethane adhesives result from a special form of phase separation which occurs in the cross-linked polyurethane stmcture. The urethane portions of polyurethanes tend to separate from the polyol portion of the resin, providing good shear strength, good low temperature flexibiUty, and high peel strength. Catalysts such as dibutyltin dilaurate [77-58-7], stannous octoate [1912-83-0], l,4-diazabicyclo[2.2.2]octane... 

Polyimides of 6FDA and aUphatic diamines with good low temperature processkig and low moisture swelling are known to be useful as hot-melt adhesives (109). Aluminum strips bonded by this polymer (177°C/172 kPa (25 psi) for 15 min) exhibited a lap-shear strength of 53 MPa (7690 psi) at room temperature and 35 MPa (5090 psi) at 100°C. The heat- and moisture-resistant 6F-containing Pis useful ki electronic devices are prepared from... 

Adhesives are used to transfer loads and are typically designed with much higher tensile and shear strengths than sealants. The most important rating of an adhesive ia many appHcatioas is the determiaatioa of how much load it can handle. Some sealants are used as adhesives and some adhesives as sealants and thus arises the occasional blurring of their roles. If the material s primary function is the exclusion of wiad, water, dirt, etc, it is a sealant. 

Wear. Ceramics generally exhibit excellent wear properties. Wear is deterrnined by a ceramic s friction and adhesion behavior, and occurs by two mechanisms adhesive wear and abrasive wear (43). Adhesive wear occurs when interfacial adhesion produces a localized Kj when the body on one side of the interface is moved relative to the other. If the strength of either of the materials is lower than the interfacial shear strength, fracture occurs. Lubricants (see Lubricants and lubrication) minimize adhesion between adj acent surfaces by providing an interlayer that shears easily. Abrasive wear occurs when one material is softer than the other. Particles originating in the harder material are introduced into the interface between the two materials and plow into and remove material from the softer material (52). Hard particles from extrinsic sources can also cause abrasive wear, and wear may occur in both of the materials depending on the hardness of the particle. 

Assuming the work of adhesion to be measurable, one must next ask if it can be related to practical adhesion. If so, it may be a useful predictor of adhesion. The prospect at first looks bleak. The perfect disjoining of phases contemplated by Eq. 1 almost never occurs, and it takes no account of the existence of an interphase , as discussed earlier. Nonetheless, modeling the complex real interphase as a true mathematical interface has led to quantitative relationships between mechanical quantities and the work of adhesion. For example, Cox [22] suggested a linear relationship between Wa and the interfacial shear strength, r, in a fiber-matrix composite as follows ... 

Fig. 19. Inlerfacial shear strengths of various fiber/matrix composites as a function of the work of adhesion as determined by IGC. 1, glass fiber-poly (ethylene) 2, carbon fiber-epoxy B 3, carbon fiber-epoxy A and 4, carbon fiber-PEEK. Redrawn from ref. [102].

Primer (source) Adhesive Tensile lap shear strength (psi) ... 

BRs were found to have a rate-sensitive mechanical response with very low tensile and shear strengths [63]. The stress-strain curves of the adhesives were characterized by an initial elastic response followed by a region of large plastic flow. 

Flexibilized epoxy resins are important structural adhesives [69]. Liquid functionally terminated nitrile rubbers are excellent flexibilizing agents for epoxy resins. This liquid nitrile rubber can be reacted into the epoxy matrix if it contains carboxylated terminated functionalities or by adding an amine terminated rubber. The main effects produced by addition of liquid nitrile rubber in epoxy formulations is the increase in T-peel strength and in low-temperature lap shear strength, without reducing the elevated temperature lap shear. 

Chlorinated rubber is also used to promote the adhesion of solvent-borne CR adhesives to metals and plasticized PVC. Addition of a low molecular weight chlorinated rubber (containing about 65 wt% chlorine) improves the shear strength and creep resistance of polychloroprene adhesives [75] but a reduction in open time is also produced. A heat reactivation (process in which the surface of the adhesive film is raised to 90-100°C to destroy the crystallinity of the film and allowing diffusion to produce polymer chain interlocking more rapidly) restores tack to the polychloroprene adhesives. 

St. Clair et. al. investigated a series of maleimide and nadimide terminated polyimides and developed LARC-13 [8,9]. Changing the terminal group from maleimide to nadimide, the value of the lap shear strength of a titanium lap shear joint increased from 7 to 19 MPa [9]. They also added an elastomeric component to the adhesive formulation. The introduction of 15 wt% of a rubbery component, ATBN (amine terminated butadiene nitrile polymer) and ADMS (aniline terminated polydimethyl siloxane) enhanced the adhesive properties as follows 19 MPa to 25 MPa (ATBN) titanium T-peel strength 0.2 kN/m to 1.4... 

The lapshear tensile shear strength of formulation A without any rubber, declines to ca. 50% of its original strength after 24 h at 12I C. In contrast, the lapshear ten.sile shear strength of formulation C exhibits no loss of adhesion under the same conditions. 

In other studies [115], the bond strengths of joints made from steel substrates coated with a variety of oils and waxes ( 6 mg/cm ) and Joined with acrylic adhesive were investigated. Lap-shear strengths up to 15 MPa were obtained with room temperature curing. Very little degradation was seen after 1000 h of... 

Lap shear strengths for EB-cured acrylic adhesives on graphite composites... 

Adhesive Curing dose (kGy) Man. specs. Lap shear strength (MPa) ... 

There has been very little work done on the development of EB-curable epoxy adhesives. When undertaking this development work the authors had two objectives. The first objective was to develop a series of adhesives for bonding aluminum-to-aluminum (Al-Al) and composite-to-composite (C-C) with lap shear strengths of 30 MPa or greater at room temperature. The second objective was that the... 

We have also looked at the lap shear strength of selected EB-ciirable epoxy adhesives. Because the adhesives being developed were being used for both aluminum-to-aluminum and composite-to-composite applications the lap shear strengths for both adherends was measured. Aluminum adherends were T2024 phosphoric acid anodized according to ASTM 3933. The composite adherends... 

Lap shear strengths (MPa) of selected EB-cured epoxy adhesives at various temperatures... 

Effects on lap shear strength of EB-cured epoxy adhesives from different surface preparations on aluminum and composite adherends... 

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