Adhesives comparative properties

Most of the polymer s characteristics stem from its molecular stmcture, which like POE, promotes solubiUty in a variety of solvents in addition to water. It exhibits Newtonian rheology and is mechanically stable relative to other thermoplastics. It also forms miscible blends with a variety of other polymers. The water solubiUty and hot meltable characteristics promote adhesion in a number of appHcations. PEOX has been observed to promote adhesion comparable with PVP and PVA on aluminum foil, cellophane, nylon, poly(methyl methacrylate), and poly(ethylene terephthalate), and in composite systems improved tensile strength and Izod impact properties have been noted. 

Epoxy-polysulfide systems can be formulated either as a liquid DGEBA epoxy mixed with liquid poly sulfide polymer or as an epoxy-terminated polysulfide polymer either may be cured with a tertiary amine such as DMP-30. Table 11.19 describes the formulation and shows the physical properties of these epoxy-polysulfide adhesives compared to an unmodified epoxy adhesive. 

Cycloaliphatic and polycyloaliphatic polyamines are finding increasing use as non-toxic replacements. Polycycloaliphatics have shown outstanding mechanical properties, high acid and solvent resistance and similar adhesion compared with standard plasticised aromatic amines (Hunt and Ashcroft, 1994). 

The tunable metallocene catalyst with a well-defined polymerization mechanism provides distinctive advantages in the preparation of new polymers with well-controlled molecular structures, especially functional polyolefins that are very difficult to prepare by other methods. Since the discovery of HDPE and i-PP about half a century ago, functionalization of polyolefin has been a scientifically challenging and industrially important area. The constant interest, despite lack of effective functionalization chemistry, is due to the strong desire to improve polyolefin s poor interactive properties. The hydrophobicity and low surface energy of polyolefin has limited its applications, especially in the areas of coating, blends, and composites, in which adhesion, comparability, dispersion, and paintability are paramount. 

In the early 1950 s, B.F. Goodrich introduced the first commercial elastomer based on ionic interactions, a poly(butadiene-co-acry-lon1trile-co-acrylic acid). Typically less than 6% of carboxylic monomer 1s employed in order to preserve the elastomeric properties inherent in these systems. When neutralized to the zinc salt, these elastomers display enhanced tensile properties and improved adhesion compared to conventional copolymers. This enhancement of properties can be directly attributed to ionic associations between the metal carboxylate groups. 

The (physically wetting) polyaniline primer CORRPASSIV was compared with a fast-drying two-component epoxy resin primer (Messrs. Finalin, Type 144) and a metal-reactive two-component primer with adhesion-promoting properties (Type 918). All three primers were coated with a two-component polyurethane paint (B412). CORRPASSIV, both with and without top coat, exhibited no rust formation beneath the primer. The slight undermining receded after the test, and even within the cross and scratch injuries, it was virtually impossible to detect any corrosion. 

This current state has to do with the discovery that the action of fundamental adhesion forces is not restricted to the interface. They not only fix some layer of adhesive molecules on the surface of the adherend but they can exert strong influence on the formation of chemical and morphological structure as well as on molecular mobility in the adjacent region of the adhesive during solidification. Hence an interphase is formed. These interphases depend on the combination of adhesive and adherend surface and on the process of contact formation as well. Due to its distinct structure, the interphase possesses properties that can be much different from the behavior of the bulk adhesive. Compared to the complexity of the problem, we just start understanding what is behind interphases. 

The hkelihood that the specialty and commercial polymers can be from the same polymer class also increases the chances for comparable properties and thereby improves adhesion between the composite layers. Finally, co-spinning involves little additional processing facilities save the additional dope supply system. 

David et al have tested these new diblock copolymers as additives for adhesion/anticorrosive properties of metals. In order to protect against corrosion, a coating system was prepared consisting of a blend of poly(VDF) (inhibiting water penetration) with poly(MMA)-Z)-poly(monophosphonic acrylate) diblock copolymer with 10 wt% [compared to poly(VDF)]. This diblock copolymer was obtained by atom transfer radical polymerization using post-functionalization and hydrolysis to introduce the phosphonic acid function onto the polymeric backbone. The anticorrosive properties of virgin poly(VDF) were also determined without any additive to be compared... 

George et al. (1996) treated pineapple leaf fiber with polymethylene-polyphe-nyl-isocyanate (C15H10N2O2) solution at 50°C for 30 min to improve the fiber-matrix interfacial adhesion. Comparing silane and isocyanate-treated wood fiber-PS composites, it was reported that isocyanate treatment was more effective than silane treatment in enhancing the mechanical properties of cellulose fiber-PS composites (Maldas et al. 1989). 

Aspects of this subject are also dealt with in other articles, notably Acrylic adhesives. Durability - fundamentals. Joint design general. Joint design cylindrical joints. Joint design strength and fracture perspectives. In the article on Toughened acrylic adhesives, some properties are compared with those of Epoxide adhesives and anaerobic adhesives. 

When all the conditions are optimum, particularly the polyurethane to robber ratio, then the chemistry and nature of the precipitated phase is altered and a degree of blend synergy takes place, which leads to an improvement in the adhesive shear properties, compared with a standard CTBN-tonghened system, coupled with a marked increase in peel strength, particularly under dynamic loading conditions. 

Table V. Comparative Properties of High-Temperature Adhesives... 

Other studies on the properties of block copolymers with more complicated architectures exist, although not all of them consider tack. Some studies comparing properties of radial versus simple block architecture allow to compare the effect of chemical versus physical crosslinks in the case of structured systems. The data shows that there is a decrease of melt viscosity together with better adhesive properties for star copolymers [44]. This is related to a point that was discussed earlier the different architectures of the molecules lead to different physical crosslinking densities. In the same spirit, block copolymers with four different arms, two polyisoprene, and two polystyrene-fc-poly(ethylene/butylene) arms were studied, and led to a better combination of shear strength and melt viscosity for adhesive applications [45], compared to the conventional linear SIS and SEBS triblocks. 

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