Dynamic adhesion tests

Rheometric Scientific markets several devices designed for characterizing viscoelastic fluids. These instmments measure the response of a Hquid to sinusoidal oscillatory motion to determine dynamic viscosity as well as storage and loss moduH. The Rheometric Scientific line includes a fluids spectrometer (RFS-II), a dynamic spectrometer (RDS-7700 series II), and a mechanical spectrometer (RMS-800). The fluids spectrometer is designed for fairly low viscosity materials. The dynamic spectrometer can be used to test soHds, melts, and Hquids at frequencies from 10 to 500 rad/s and as a function of strain ampHtude and temperature. It is a stripped down version of the extremely versatile mechanical spectrometer, which is both a dynamic viscometer and a dynamic mechanical testing device. The RMS-800 can carry out measurements under rotational shear, oscillatory shear, torsional motion, and tension compression, as well as normal stress measurements. Step strain, creep, and creep recovery modes are also available. It is used on a wide range of materials, including adhesives, pastes, mbber, and plastics. 

Both static and dynamic tests are employed to evaluate the adhesion strength of cord-mbber composites. The major static tests used in tire industry are H-adhesion, 90/180° peel test, tire cord adhesion test (TCAT) and co-axial shear pull-out test (CSPT). Although these methods are... 

However, use of the loss tangent in Eq. (13) is an oversimplification. Instead, we shall make use of an empirical dissipation law obtained in the study of rate-dependent adhesion of elastomers (19). Maugis and Barquins [19], in order to explain the dynamic adhesion of elastomers in a variety of test configurations (peel, flat punch, etc), proposed the following relation ... 

Pressure-sensitive adhesives are used in a great variety of applications, most commonly for adhesive tapes. In that case, they have to be tested by static shear test or dynamic shear test The difference between these two methods is that in static shear test a standard force is being applied to the test specimen and the adhesive failure is reported as the time it takes for failure to occur. The dynamic shear test involves a force being applied to the PSA tape at a specific rate of speed (typically 0.25 mm or 0.1 in. per minute). The value reported is as the peak force per unit area (Ib/in, also abbreviated as psi, and in SI units MPa) required to cause adhesive failure. The standards for adhesion shear tests are ASTM D3654, ISO EN 1943, and PSTC-107.i i ... 

Fundamental mechanisms of adhesion. All classical adhesion tests involve a rheological component, in the deformation of the near-interface material, and a surface chemical component. With the recent availability of microscopic techniques to study surface forces, one can possibly go after the surface chemical component, separately from the rheological component. More generally, the configurational and dynamic behavior of macromolecular interfacial regions remains a very rich area. 

Both the published literature and previously unpublished information obtained by the structural panel industry indicate that formaldehyde levels associated with panel products glued with phenol formaldehyde adhesives are extremely low. Large dynamic chamber tests which simulate conditions that might be found in tightly sealed residences indicate consistently that formaldehyde levels associated with freshly manufactured phenolic panel products are less than 0.1 parts per million. The data, as well as theoretical considerations, also indicate that the amount of formaldehyde contributed to the environment by phenolic panel products should rapidly approach zero as the small quantity of formaldehyde initially present in the products is released. 

The principles of time-temperature superposition can be used with equal success for dielectric measurements as well as dynamic mechanical tests. Analysis of the frequency dependence of the glass transition of the adhesive in the system described above shows that it follows a WLF type dependence whereas the transition of PET obeys Arrhenius behaviour. This type of study can be used to distinguish between different types of relaxation phenomena in materials. 

As mentioned earlier, the DMTA technique measures molecular motion in adhesives, and not heat changes as with DSC. Many adhesives exhibit time-dependent, reversible viscoelastic properties in deformation. Hence a viscoelactic material can be characterized by measuring its elastic modulus as a function of temperature. The modulus depends both on the method and the time of measurement. Dynamic mechanical tests are characterized by application of a small stress in a time-varying periodic or sinusoidal fashion. For viscoelastic materials when a sinusoidal deformation is applied, the stress is not in phase with displacement. A complex tensile modulus E ) or shear modulus (G ) can be obtained ... 

Viscoelastic characteristics of polymers may be measured by either static or dynamic mechanical tests. The most common static methods are by measurement of creep, the time-dependent deformation of a polymer sample under constant load, or stress relaxation, the time-dependent load required to maintain a polymer sample at a constant extent of deformation. The results of such tests are expressed as the time-dependent parameters, creep compliance J t) (instantaneous strain/stress) and stress relaxation modulus Git) (instantaneous stress/strain) respectively. The more important of these, from the point of view of adhesive joints, is creep compliance (see also Pressure-sensitive adhesives - adhesion properties). Typical curves of creep and creep recovery for an uncross-Unked rubber (approximated by a three-parameter model) and a cross-linked rubber (approximated by a Voigt element) are shown in Fig. 2. 

In extrusion of PP/PET blends, although PET had lower t than the blends, the blends extruded at higher volumetric rates, indicating that the output was controlled either by melting or the interlayer slip. It was shown that even if the sheath-and-core monofilaments of PP/PET blends could not be oriented (because of poor adhesion between the two polymers), melt blending still produced useful oriented product. The maximum draw ratio for PET was 7, whereas for PP/PET blends it was 11. The dynamic mechanical testing of blend monofilaments, containing le(PET) = 50 and 70 wt.%, indicated that Tg(PET) remained constant and the dynamic moduli were approximately additive. 

Finally, a case study describing the application of the hybrid interference fitted-adhesive bonded joint technique on the automotive steel wheel system is presented. The study was carried out in collaboration with the MW company, a division of the CLN group. The business of MW concerns the steel wheel market for passenger cars, light and heavy commercial vehicles and motorcycles. Bonded wheel prototypes were submitted to static and dynamic mechanical tests adopted for components validation. The results obtained on bonded wheel prototypes were compared with the traditional welded wheels and with the samples tested at laboratory level. 

Perhaps it is more understandable if we correlate adhesive performance to viscoelastic properties determined at the adhesive testing temperature. The bond is formed and destroyed during testing using conditions which differ in stress level, deformation rate, and extent of deformation at room temperature. Therefore, the measurement of viscoelastic properties at room temperature may be pertinent. Viscoelastic properties at constant temperature are determined by dynamic mechanical measurement over a range of frequencies, for example, from 0.1 to 100 rad/sec. We see that the frequency scan curves are approximately a reciprocal... 

This history of investigation of the mechanism of gekkotan adhesion has been paralleled by a series of studies that have attempted to understand the structure and function of the entire adhesive apparams. Other studies have endeavored to investigate the performance capabihties of the adhesive system of geckos in both static clinging and dynamic running tests. 

Some of the work they did to evaluate low stress/slow rate PSA deformation was completed using a probe material analyzer (Chemlnstruments PMA-IOOO). The unit was helpful in performing tests on a variety of PSAs due to the ability to program both low forces and very slow speeds into the test protocols. In addition, they performed static peel adhesion and dynamic shear test methods on other Chemlnstruments test units. 

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