No abstract

The values of the elastic modulus (E) and the yield stress (Y) and the shape of the strain-stress curves of different polyethylenes have been related to the microhardness (MH) of these materials, considering the roles that the crystalline, amorphous, and interfacial regions play in the deformation of semicrystalline polymers. Linear relations between microhardness and degree of crystallinity, (1 -n&, and between the logarithms of E and MH have been found. The variations of the parameters 100Y/E and s i n / Y (sin is the relative minimum value of the nominal stress for elongations above the yield point) as a function of (1 -A ) , follow different trends for linear and branched polyethylenes. Moreover, it has been found that the ratio MH/Y is smaller than three for these materials, approaching this theoretical value (Tabor relation) only for the highest crystallinity levels. ZUSAMMENFASSUNG:Die Werte des Elastizitatsmoduls (E) und der FlieBspannung (Y) und die Gestalt der Spannungs-Dehnungskurven wurden mit der Mikrohilrte (MH) dieser Materialien verkniipft, unter Beachtung der Rolle, die die kristallinen, amorphen und Phasengrenzbereiche bei der Verformung der kristallinen Polymeren spielen. Es wurden lineare Beziehungen zwischen der Mikroharte und dem Kristallinitatsgrad (1 -n ) , und zwischen den Logarithmen von E und MH gefunden. Die Anderungen der Parameter 100 Y/E und s i n / Y ist das relative Minimum der Nominalspannung fur Dehnungen oberhalb der Streckgrenze) als Funktion von (1 -h& folgen unterschiedlichen Trends fur lineare und verzweigte Polyethylene. AuBerdem zeigte sich, dal3 das Verhaltnis MH/Y fur diese Materialien kleiner als drei ist, wobei dieser theoretische Wert (Taborgleichung) nur bei den hochsten Kristallinitatsgraden angenahert wird.

No abstract

SYNOPSISHot-melt pressure-sensitive adhesives based on styrene-butadiene block copolymers with aliphatic and aromatic tackifying resins and plastifying oils have been analyzed. The importance of the resin structure in the compatibility with the block copolymer and the influence of the different paraffinic-naphthenic character of the oil in PSA performance have been shown. Ternary systems with a fixed polymer content (30%) and with variable resin and oil contents show a good miscibility over the whole range of compositions, and only one glass transition temperature was found in each composition. The relationship between chemical composition and bulk performance are expressed in terms of the viscoelastic behavior of the adhesives, measured by DMTA. It has been shown that at a given resin content there is a minimum on tan 6 peak vs. temperature, the melt viscosities present a plateau region and the tack strength shows a maximum. An important conclusion is that phase separation is not a requirement for maximum tack; some restricted miscibility is enough, present in a few microdomains of the blend. 0 1996 John Wiley & Sons, Inc. I NTRODU CTlO NHot-Melt pressure-sensitive adhesives are generally blends of block copolymers with modifying resins and other additives in order to improve the characteristics and performance.The viscoelastic properties of the bulk adhesive depend on the chemical composition of its components and are related to the final properties of these materials.'-" In a general sense, the bonding and debonding processes that take place, corresponding to the adhesion to a surface and to the resisce to separation, respectively, are viscoelastic processes. Bonding takes place a t low rates and a t low deformation. On the contrary, debonding is a high rate and high deformation process. Because of these general rheological characteristics, the properties of the adhesive are strongly dependent on their composition, qualitative and quantitatively considered and, therefore, on the compati-* To whom correspondence should be addressed Journal of Applied Polymer Science, Vol. 62, 1263-1275 (1996) 0 1996 John Wiley & Sons, Inc.CCC 0021-S995/96/OS1263-13 bility of the components in a system that it is multiphase. As a consequence of the varied composition in the blends of the main component, elastomer, with tackyfing resins and oils, the thermal transitions and the dynamic-mechanical relaxations may be very variable, conditioning the performance and final properties such as tack, peel strength, and shear strength. Thus, thermal transition measurements have been carried out by Differential Scanning Calorimetry (DSC) and the viscoelastic relaxations have been determined by Dynamic Mechanical Thermal Analysis (DMTA) , by observing the differences in mechanical properties as a function of Changes in the storage ( E ' ) and loss ( E " ) moduli or in the tan 6 = E " / E r have been used to characterize the behavior of both the pure components and the blends. Earlier studies on natural rubber-based adhesives and the influ...

No abstract