Cohesive strength .

Neoprene—phenohc contact adhesives, known for thein high green strength and peel values, contain a resole-type resin prepared from 4-/-butylphenol. The alkyl group increases compatibiHty and reduces cross-linking. This resin reacts or complexes with the metal oxide, eg, MgO, contained in the formulation, and increases the cohesive strength of the adhesive. In fact, the reactivity with MgO is frequently measured to determine the effectiveness of heat-reactive phenoHcs in the formulation. 

A funnel flow bin typically exhibits a first-in/last-out type of flow sequence. If the material has sufficient cohesive strength, it may bridge over the outlet. Also, if the narrow flow channel empties out, a stable rathole may form. This stable rathole decreases the bin s five or usable capacity, causes materials to cake or spoil, and/or enhances segregation problems. Collapsing ratholes may impose loads on the stmcture that it was not designed to withstand. 

In order to characterize this bonding tendency, the flow function of a material must be deterrnined. Data on flow function can be generated in a testing laboratory by measuring the cohesive strength of the bulk soHd as a function of consoHdation pressure appHed to it. Such strength is directly related to the abihty of the material to form arches and ratholes in bins and hoppers. 

A material s flow function is usually measured on the same tester as the wall friction angle, although the cell arrangement is somewhat different (Fig. 6). ConsoHdation values are easily controUed, and the cohesive strength of the bulk soHd is determined by measuring interparticle shear stresses while some predeterrnined normal pressure is being appHed. 

There are two mechanisms by which arching can occur particle interlocking and cohesive strength. The minimum outlet size required to prevent mechanical interlocking of particles is directly related to the size of the particles. The diameter of a circular outlet must be at least six to eight times the particle size, and the width of a slotted outlet must be at least three to four times the particle size. These ratios normally only govern the outlet size of mass flow hoppers if the particles are at least 0.6 cm or larger. 

By employing additives to improve interfacial adhesion and the cohesive strength of the mbber phase, natural mbber can compete with ethylene—propylene mbbers as an impact modifier for polypropylene. These hard grades, containing between 15 and 25% natural mbber, have the potential for use in the automotive and domestic markets, eg, in bumpers, spoilers, grilles, electrical connectors, and floor tiles. 

Polyurethane adhesives are known for excellent adhesion, flexibihty, toughness, high cohesive strength, and fast cure rates. Polyurethane adhesives rely on the curing of multifunctional isocyanate-terrninated prepolymers with moisture or on the reaction with the substrate, eg, wood and ceUulosic fibers. Two-component adhesives consist of an isocyanate prepolymer, which is cured with low equivalent weight diols, polyols, diamines, or polyamines. Such systems can be used neat or as solution. The two components are kept separately before apphcation. Two-component polyurethane systems are also used as hot-melt adhesives. 

Adhesives. Poly(vinyl alcohol) is used as a component in a wide variety of general-purpose adhesives to bond ceUulosic materials, such as paper and paperboard, wood textiles, some metal foils, and porous ceramic surfaces, to each other. It is also an effective binder for pigments and other finely divided powders. Both fully and partially hydrolyzed grades are used. Sensitivity to water increases with decreasing degree of hydrolysis and the addition of plasticizer. Poly(vinyl alcohol) in many appHcations is employed as an additive to other polymer systems to improve the cohesive strength, film flexibiUty, moisture resistance, and other properties. It is incorporated into a wide variety of adhesives through its use as a protective coUoid in emulsion p olymerization. 

An example that shows that the cohesive strength of a material is less than that of the adhesional strength of the interface is that of the nominal 50,000 mile steel belted radial tire. It is a simple calculation to show that, on average, a tire leaves a monolayer of rubber particles on the road every time it makes a rotation. In essence, the strength of the adhesional bonding between the road and the tire is greater than that of the rubber within the tire. 

Despite these early successes in the commercialization of acrylic polymers, no acrylic PSAs were manufactured on a larger scale until many years later. One of the primary reasons for the initial commercial failure of the acrylic PSAs was their lack of cohesive strength. Unlike the higher Tg, plastic-like polymers obtained from monomers like methylmethacrylate, polymers synthesized from alkyl acrylates typically formed sticky, cold-flowing materials with little if any utility. 

The increased probability of maintaining high cohesive strength at elevated temperatures because the polymer is not in a diluted state. 

It is for this reason that the discovery by Ulrich was of significant importance to the successful development of acrylic PSAs. He found that by copolymerizing polar monomers, such as acrylic acid, one could greatly increase the cohesive strength of the polymer allowing PSA articles coated with this type of material to sustain a load without premature shear failure. These polar monomers commonly... 

These types of polar monomer provide sites for hydrogen bonding which increase the cohesive strength of the PSA because of strong inter-chain interaction, and they can also allow for hydrogen bonding or other polar interactions with some substrates. 

One of the other benefits of incorporating polar monomers in the PSA is the enhancement in cohesive strength. This can be observed in the form of higher shear holding in a static shear test and/or better creep resistance of the adhesive when subject to a constant load. 

As the amount of acrylic acid in the polymer increases, the degree of hydrogen bonding between polymer chains also increases causing the cohesive strength to improve without the need for crosslinking. Very similar observations can be made for other polar monomers, such as acrylamide. 

Introduction. Crosslinking of the acrylic polymer can be a very important factor in formulating a PSA. As can be expected, for a given PSA composition, it is typically observed that the cohesive strength of a non-crosslinked polymer decreases with the decreasing molecular weight. This drop in performance can be... 

The acid/base interaction between the two polymers significantly increases the cohesive strength of the polymer blend at normal use temperatures but at elevated temperature the interaction can be interrupted and the polymer can still be melt processed. Other examples of basic polymers use for crosslinking include polyethylenimines, vinyl pyridine copolymers, and the like. 

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