Pressure-sensitive viscoelastic properties

Chu, S.G., Viscoelastic properties of pressure sensitive adhesives. In Satas, D. (Ed.), Handbook of Pressure Sensitive Adhesive Technology. Van Nostrand Reinhold, New York, 1989, p. 191. 

Tackifying resins enhance the adhesion of non-polar elastomers by improving wettability, increasing polarity and altering the viscoelastic properties. Dahlquist [31 ] established the first evidence of the modification of the viscoelastic properties of an elastomer by adding resins, and demonstrated that the performance of pressure-sensitive adhesives was related to the creep compliance. Later, Aubrey and Sherriff [32] demonstrated that a relationship between peel strength and viscoelasticity in natural rubber-low molecular resins blends existed. Class and Chu [33] used the dynamic mechanical measurements to demonstrate that compatible resins with an elastomer produced a decrease in the elastic modulus at room temperature and an increase in the tan <5 peak (which indicated the glass transition temperature of the resin-elastomer blend). Resins which are incompatible with an elastomer caused an increase in the elastic modulus at room temperature and showed two distinct maxima in the tan <5 curve. 

Acoustic-wave devices are sensitive to a large number of physical and chemical measurands. These include such parameters as temperature, pressure, acceleration, stress, and the adjacent medium s density, viscoelastic properties, and electrical conductivity. Indeed, it is this wide range of measurand sensitivities that makes AW devices attractive for a wide variety of sensor applications. However, since one is interested in exploiting only one of these sensitivities for a particular application, all other responses become undesirable interferences. Thus, it is essential that the sensor environment be carefully controlled to eliminate the effects of sensor cross-sensitivities. 

Polymeric materials composed of flexible molecular chains respond very sensitively to changes in vanous surrounding conditions such as temperature, stress, pressure, and so on For example, temperature variation induces in polymeric materials drastic changes m mechamcal and electrical properties as well as the phase transitions of melting and crystallization. Since these phenomena are accompanied by considerable vanation in viscoelastic characteristics and thus in density (p) and elastic modulus (E) of the materials, the ultrasonic method which can detect quite sensitively these viscoelastic properties becomes one of the most valuable and important methods for investigating the macro- as well as microscopic structural changes occurring in the materials. 

J. B. Class and S. G. Chu, "Viscoelastic Properties of Rubber-Resin Pressure Sensitive Adhesive Formulations," in Ref. 14. 

In a complementary paper by Class and Chu, model resins - polystyrene and poly(vinylcyclohexane) - in combination with natural and styrene-butadiene rubbers, were used to study effects of resin structure, molecular weight and concentration on viscoelastic properties of pressure-sensitive adhesives resulting from these combinations. 

There have been very few studies reported on the viscoelastic properties of rubber-resin pressure sensitive adhesive systems. In 1973, M. Sherriff and co-workers (1) reported on the effect of adding poly (j3-pinene) resin to natural rubber. Based on a G master curve, they showed that the resin shifted the entry to the transition zone to a lower frequency and reduced the modulus in the rubbery plateau. G. Kraus and K.W. Rollman (2) reported in 1977 on their study of resins blended with styrene-isoprene-styrene block copolymers. They showed that the addition of a resin increased the glass transition temperature of the rubbery mid-block and decreased the plateau modulus. Accordingly, a satisfactory tackifying resin should produce these changes. 

RELATIONSHIP BETWEEN VISCOELASTIC PROPERTIES AND PRESSURE SENSITIVE ADHESIVE PERFORMANCE... 

Plateau modulus and tan 6 peak temperature may not be the most relevant viscoelastic properties to relate to pressure sensitive adhesive performance. Other viscoelastic parameters may be better predictors. This will be determined in continuing work to relate viscoelastic properties to pressure sensitive adhesive performance. 

The structure of the low molecular weight resin is very important to its compatibility with elastomers and, consequently, to its effect on viscoelastic properties and performance as a pressure sensitive adhesive. A completely aromatic resin such as polystyrene has poor compatibility with natural rubber, but is compatible with styrene-butadiene rubber. A cycloaliphatic resin such as poly(vinyl cyclohexane) is compatible with natural rubber and is incompatible with styrene-butadiene rubber. An alkyl aromatic resin such as poly-(tert-butyl styrene) is compatible with both elastomers. 

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. 

The viscoelastic contribution to the fractnre energy in an adhesion test is, in principle, rate and temperatnre-dependent. Where this contribution is significant, it has been possible to nationalise adhesion measurements using the WLF transform-see Adhesion - fundamental and practical. Pressure-sensitive adhesives - adhesion properties and Tensile... 

A pressure-sensitive adhesive (PSA) is a viscoelastic material that adheres without the need of more than light pressure and requires no activation by water, solvent, or heat. The bond that is formed should be a permanent bond in the sense that the PSA remains bonded imless removal is desired and activated by the user. The Pressure Sensitive Tape Council suggests a number of other desirable features of many PSAs, including aggressive tack, the ability to adhere to a variety of surfaces, and to be removed cleanly without leaving residue. A common element to all PSAs is a polymeric network. This network may be uncross-linked or cross-linked, and it may also contain nonpolymeric additives that affect adhesive properties. 

In Figure 8, the viscoelastic properties of natural rubber latex (Hartex 103 from Firestone Co.) and milled smoked sheet natural rubber were examined. Both natural rubbers have tan 8 peak maximum temperature at -58°C. However, the latex has higher room-temperature modulus than the milled natural rubber. Natural rubber latex based pressure-sensitive adhesives offer an advantage over solvent-based systems (milled smoked sheet) because of the molecular-weight difference between the two systems. The high-molecular-weight portion of natural rubber is insoluble in solvent and therefore cannot be used in solvent-based adhesives. Natural... 

We have characterized the viscoelastic properties of commercial pressure-sensitive tapes and labels during our collaboration with customers and co-suppliers. In Figure 16, we plot... 

In 1966, Dahlquist(i7) defined the requirement for a good pressure-sensitive adhesive as an adhesive with one-second shear creep compliance greater than J(t) = 1 x 10cm /dyn (Figure 17). Recently, we characterized the viscoelastic properties of many conmiercial tapes and label adhesives and found that the glass transition temperature and modulus (G ) at the application temperature are the most important requirements for good performances (Figures 18, 43, and 52). The requirements for tape and label adhesives are somewhat different for... 

A good PSA can be made if we understand the rubber-resin compatibility. The compatibility of natural rubber with various resins is explained in papers published by Class and Chu.GO-13) The compatibility of resin and rubber can be determined by measuring viscoelastic properties of the blend. Compatibility is identified (in the G vs. temperature plot) by a pronounced shift of the tan 8 peak maximum temperature (Tg), associated with a decrease in the storage modulus in the plateau. An incompatible system is confirmed by a minimal shift of the tan 8 peak maximum temperature along with an increase in the storage modulus in the plateau (Figure 21). A second peak in tan 8 may be apparent in the incompatible system. Compatibility of rubber-resin systems depends on the structure, molecular weight, and concentration of the resin in the blends. The compatible systems exhibit pressure-sensitive adhesive performance at some ratio of rubber to resin. The incompatible systems, on the other... 

The performance of various pressure-sensitive adhesives can be related to the viscoelastic properties of the bulk adhesives at various temperatures. By focusing on certain temperature and modulus ranges, we can address needs such as cold-temperature performance, room-temperature applications, shear performance, and melt processing requirements. It is obvious develop a satisfactory adhesive system (Figure 52). The empirical windows required for the various pressure-sensitive adhesives were obtained by Carpei<22) as shown in Figure 53. Also seen are the viscoelastic properties of Piccotac HM2162L/Kraton 1107/oil blends. 

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