Peeling strength

The cured polymers are hard, clear, and glassy thermoplastic resins with high tensile strengths. The polymers, because of their highly polar stmcture, exhibit excellent adhesion to a wide variety of substrate combinations. They tend to be somewhat britde and have only low to moderate impact and peel strengths. The addition of fillers such as poly (methyl methacrylate) (PMMA) reduces the brittleness somewhat. Newer formulations are now available that contain dissolved elastomeric materials of various types. These mbber-modifted products have been found to offer adhesive bonds of considerably improved toughness (3,4). 

The physical properties of polyurethane adhesives result from a special form of phase separation which occurs in the cross-linked polyurethane stmcture. The urethane portions of polyurethanes tend to separate from the polyol portion of the resin, providing good shear strength, good low temperature flexibiUty, and high peel strength. Catalysts such as dibutyltin dilaurate [77-58-7], stannous octoate [1912-83-0], l,4-diazabicyclo[2.2.2]octane... 

Ethylene—tetrafluoroethylene copolymers respond weU to melt bonding to untreated aluminum, steel, and copper with peel strengths above 3.5 kN/m (20 Ibf/in.). Eor melt bonding to itself, hot-plate welding is used. The material is heated to 271—276°C, and the parts are pressed together during cooling. 

Phenolic resin substantially increases open time and peel strength of the formulation (80). For example, higher methylol and methylene ether contents of the resin improves peel strength and elevated temperature resistance. Adhesive properties are also influenced by the molecular weight distribution of the phenoHc low molecular weight reduces adhesion (82). 

Waterborne contact adhesives contain an elastomer in latex form, usually an acryflc or neoprene-based latex, and a heat-reactive, cross-linkable phenohc resin in the form of an aqueous dispersion. The phenoHc resin improves metal adhesion, green strength, and peel strength at elevated temperature. A typical formulation contains three parts latex and one part phenohc dispersion (dry weight bases). Although metal oxides may be added, reaction of the oxide with the phenohc resin does not occur readily. 

Speciali2ed copolymer latices, which are inherently and permanently tacky, are available as pressure-sensitive emulsions. They are mechanically stable and have excellent machinabiUty. They are compatible with many other PVAc latices and, therefore, can be easily blended with other resins for modification of surface tack, peel strength, and creep. 

Fig. 15. Adhesion expressed as peel strength of poly(vinyl alcohol) film on polyester film (8). To convert N/m to ppi, divide by 175.

Low viscosity cellulose acetate is used in lacquers and protective coatings for paper, metal, glass, and other substrates and as an adhesive for cellulose photographic film because of its quick bonding rate and excellent bond peel strength (135) (see Coatings). Heat-sensitive adhesives for textiles have also been prepared from cellulose acetate (136). Extmded cellulose acetate film makes an excellent base for transparent pressure-sensitive tape (137) (see Adhesives). 

In addition to transport properties, the adhesive properties are characterized by tensile measurements. For instance, the peel strength is deterrnined by measuring the force required to pull the adhesive from a substrate at a constant speed in a controUed temperature and humidity environment. 

B533 peel strength measurement of plated plastics... 

Fig. 21. Peel strengths of various adhesives against poly(ethylene terephthalate) adherends vs. the solubility parameter difference between adhesive and adherend. Drawn using data from ref. [72].

Amauroux, N, Petit, J. and Leger, L., Role of interfacial resistance to shear stress on adhesive peel strength. Langmuir, 17(21), 6510-6517 (2001). 

Influxes when 7 = 0, no influxes, the peel strength goes to zero. We also have G 7, such that G increases linearly with 7. 

True vs. apparent strength the visco-plastic energy dissipation dominated the magnitude of the peel strength. However, little dissipation occurs if the interface becomes very weak. In other words, some influxes are necessary to produce sufficient stress to activate the viscoelastic deformation in the body of the A. 

Let us examine this relation for typical values of the A/B interface = 1 (max energy dissipation in the A layer) I = 1 (max strength of interface with influxes) E = 12,000psi (Tq = 4000 psi /t = 30 mils (10 in) Lc = 4 x lO" in. We obtain for both terms G = 20 pli (energy dissipated) + 0.08 pli (true interface strength with max influxes), or G 20 pli, which says that the measured peel strength is dominated by visco-plastic deformation processes. 

Type Load bearing/holding power (kPa) Peel strength (N/m)... 

Rubber base adhesives develop strength faster than most other polymeric types. Fig. 1 [3J shows the differences in the development of peel strength for several rubber polymers (without additional additives, except an antioxidant). Natural... 

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. 

Fig. 25. Evolution of the tack of polychloroprene-aromatic hydrocarbon resin blends as a function of the resin content. Tack was obtained as the immediate T-peel strength of joints produced with 0.6 mm thick styrene-butadiene rubber strips placed in contact without application of pressure. Peeling rate = 10 cm/min.

The chemical nature of the tackifier also affects the compatibility of resin-elastomer blends. For polychloroprene (a polar elastomer) higher tack is obtained with a polar resin (PF blend in Fig. 27) than with a non-polar resin (PA blend in Fig. 27). Further, the adhesion of resin-elastomer blends also decreases by increasing the aromatic content of the resin [29]. Fig. 28 shows a decrease in T-peel strength of styrene-butadiene rubber/polychloroprene-hydrocarbon resin blends by increasing the MMAP cloud point. Because the higher the MMAP... 

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