Adherence of Viscoelastic Solids

As discussed in Section 2.3, a crack propagating under a constant crack extension force G — w undergoes a drag proportional to w and is a function of its velocity v one can 

Thus in Eq. (54) surface properties (w) and viscoelastic properties cp(v) are completely decoupled from elastic properties, geometry, and loading conditions included in G. The dimensionless function cp(6(t v) is a characteristic of crack propagation in Mode I in the material. Once (p( jv) is known, Eq. (54) allows one to predict any feature such as kinetics of detachment at fixed load, fixed grips, or fixed cross-head velocity. 

FIGURE 6. Kinetics of crack propagation for the unloading of a glass ball on polyurethane from / = 50 mN to P — -30 mN, for two temperatures. The dashed line is the theoretical value of the inflection point. (From Reference 10.) 

Water adsorption decreases the Dupre energy of adhesion, and hence the viscoelastic losses according to Eq. (54). This point was verified by measuring the rolling resistance 91 of a glass cylinder rolling on an inclined sample (G — 91//) as a function of velocity for various 

FIGURE 7. Reduced crack extension force vs. crack velocity for glass-polyurethane systems. (From Reference 10.) 

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D. Maugis and M. Barquins, Fracture mechanics and adherence of viscoelastic solids, in Adhesion and Adsorption of Polymers (L. H. Lee, ed.). Part A, pp. 203-277, Plenum Press, New York (1980). 

Contact mechanics, in the classical sense, describes the behavior of solids in contact under the action of an external load. The first studies in the area of contact mechanics date back to the seminal publication "On the contact of elastic solids of Heinrich Hertz in 1882 [ 1 ]. The original Hertz theory was applied to frictionless non-adhering surfaces of perfectly elastic solids. Lee and Radok [2], Graham [3], and Yang [4] developed the theories of contact mechanics of viscoelastic solids. None of these treatments, however, accounted for the role of interfacial adhesive interactions. 

In adherence of solids, surface effects, rheological effects and fracture mechanics are mixed in an intricate manner, and no experiment can be performed involving only the chemical aspects. The adherence of elastic solids is now clear that of viscoelastics is in progress for other materials, the constitutive equation must be known and taken into account. 

Surface and Bulk Properties in Adherence of Elastic-Viscoelastic Solids... 

Measurement of C requires more sophisticated and expensive rheometers and more involved experimental procedures. It must be remembered that experiments have to he carried out below the critical strain value (see Sec II), or in [he region of linear viscoelastic behavior. This region is determined by measuring the complex modulus G as a function of the applied strain at a constant oscillation frequency (usually 1 Hz). Up to 7, G does not vary with the strain above Yr, G tends to drop. The evaluation of oscillatory parameters is more often restricted to product formulation studies and research. However, a controlled-fall penetrometer may be used to compare the degree of elasticity between different samples. Creep compliance and creep relaxation experiments may be obtained by means of this type of device. In fact, a penetrometer may be the only way to assess viscoeIa.sticity when the sample does not adhere to solid surfaces, or adheres too well, or cures to become a solid or semisolid. This is the case of many dental products such as fillings, impression putties, sealants, and cements. 

There are three general classes of adhesives. These are hot melt adhesives (HMA), pressure sensitive adhesives (PSA), and hot melt pressure sensitive adhesives (HMPSA). A HMA is a 100 % solids, thermoplastic composition which is compounded and applied molten at elevated temperatures and whose strength is obtained solely by the removal of heat. A PSA is a viscoelastic material which is permanently tacky at room temperature, such that a low force contact to a surface will cause the material to adhere instantaneously. A HMPSA is a viscoelastic material which is permanently tacky at room temperature and which is applied as a 100 % solids, thermoplastic composition in the molten state. Solvent based systems are the oldest technology. Water based systems have the advantage that they avoid flammability and generally do not give off obnoxious odors. Basic adhesive components within each of the three classes are ... 

Adhesives may also be classified by the way they are applied or cured. Hence, anaerobic adhesives are adhesives that set only in the absence of air, for instance, when confined between plates or sheets. A contact adhesive is one that is apparently dry to the touch but will adhere to itself instantaneously on contact (also called contact bond adhesive and dry bond adhesive). A heat-activated adhesive is a dry adhesive film that is made tacky or fluid by application of heat or heat and pressure to the assembly. A pressure-sensitive adhesive is a viscoelastic material that in solvent-free form remains permanently tacky. Such materials will adhere instantaneously to most solid surfaces with the application of very slight pressure. Room-temperature setting adhesives are those that set in the temperature range of 20-30°C. These are usually two-component adhesives that must be mixed before application. A solvent adhesive is an adhesive that has a volatile organic liquid as a vehicle and sets or becomes tacky after the solvent has evaporated. A solvent-activated adhesive is a dry adhesive film that is rendered tacky just before use by applicafion of a solvent. 

The recorded force first increases, then decreases. The maximum value, termed the tack force, is a measure of the adherence under this particular experimental condition and has no clear physical significance. The area under the curve, termed the tack energy, is equal to the work jGda of the cohesive stress at the crack tip. Tackiness refers to the ability of an elastomer to adhere instantaneously to a solid surface, or to itself, after a brief time of contact under low pressure. Probe tack testing can be analyzed by Eq. (54), and tack curves obtained by computer integration almost coincide with experimental ones. S) Figure 8 pertains to a spherical probe and shows that even at a very low cross-head velocity the viscoelastic effects considerably increase the adherence force compared to the elastic (or quasi-static) adherence force at fixed displacement (point D). 

Adhesive, pressure-sensitive A viscoelastic material that, in solvent-free form, reiitains permanently tacky at room temperature. Such material will adhere instantaneously and tenaciously to most solid surfaces with the application of very slight manual pressure. These adhesives are frequently used on tapes. An adhesive that, in the dry state, is aggressively and permanently tacky at room temperature and firmly adheres to a variety of dissimilar surfaces on contact without the need for more than finger or hand pressure. 

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