Bonded linings

In these simplified representations called bond line formulas or carbon skeleton dia grams, the only atoms specifically written m are those that are neither carbon nor hydro gen bound to carbon Hydrogens bound to these heteroatoms are shown however... 

Expand the following bond line representations to show all the... 

Unbranched alkanes are sometimes referred to as straight chain alkanes but as we 11 see m Chapter 3 their chains are not straight but instead tend to adopt the zigzag shape portrayed m the bond line formulas introduced m Section 1 7... 

Bond line formula of n pentane Bond line formula of n hexane... 

Much of the communication between insects involves chemical messengers called pheromones A species of cockroach secretes a substance from its mandibular glands that alerts other cockroaches to its presence and causes them to congregate One of the principal components of this aggregation pheromone is the alkane shown in the bond line formula that follows Give the molecular formula of this substance and represent it by a condensed formula... 

Write condensed and bond line formulas for the five isomeric... 

We can use bond line formulas to represent alkenes in much the same way that we use them to represent alkanes Consider the following alkene... 

Bond line formula (Section 1 7) Formula in which connec tions between carbons are shown but individual carbons and hydrogens are not The bond line formula... 

Emulsion Adhesives. The most widely used emulsion-based adhesive is that based upon poly(vinyl acetate)—poly(vinyl alcohol) copolymers formed by free-radical polymerization in an emulsion system. Poly(vinyl alcohol) is typically formed by hydrolysis of the poly(vinyl acetate). The properties of the emulsion are derived from the polymer employed in the polymerization as weU as from the system used to emulsify the polymer in water. The emulsion is stabilized by a combination of a surfactant plus a coUoid protection system. The protective coUoids are similar to those used paint (qv) to stabilize latex. For poly(vinyl acetate), the protective coUoids are isolated from natural gums and ceUulosic resins (carboxymethylceUulose or hydroxyethjdceUulose). The hydroHzed polymer may also be used. The physical properties of the poly(vinyl acetate) polymer can be modified by changing the co-monomer used in the polymerization. Any material which is free-radically active and participates in an emulsion polymerization can be employed. Plasticizers (qv), tackifiers, viscosity modifiers, solvents (added to coalesce the emulsion particles), fillers, humectants, and other materials are often added to the adhesive to meet specifications for the intended appHcation. Because the presence of foam in the bond line could decrease performance of the adhesion joint, agents that control the amount of air entrapped in an adhesive bond must be added. Biocides are also necessary many of the materials that are used to stabilize poly(vinyl acetate) emulsions are natural products. Poly(vinyl acetate) adhesives known as "white glue" or "carpenter s glue" are available under a number of different trade names. AppHcations are found mosdy in the area of adhesion to paper and wood (see Vinyl polymers). 

When loading or unloading a tank tmck or tank car, static bonding lines must be attached between the vehicle and the fixed piping system. Tank tmck drivers may be inexperienced in the handling of chemicals and, therefore, it is essential that the plant personnel be alert in checking the driver s operations and in correcting any possible deficiencies (see Transportation). 

Fitzpatrick et al. [41] used small-spot XPS to determine the failure mechanism of adhesively bonded, phosphated hot-dipped galvanized steel (HDGS) upon exposure to a humid environment. Substrates were prepared by applying a phosphate conversion coating and then a chromate rinse to HDGS. Lap joints were prepared from substrates having dimensions of 110 x 20 x 1.2 mm using a polybutadiene (PBD) adhesive with a bond line thickness of 250 p,m. The Joints were exposed to 95% RH at 35 C for 12 months and then pulled to failure. 

Another important factor is the corrosiveness of the adhesive. This may be especially important in those cases where the PSA has direct contact with the bare wire, the electronic component, or the silicon wafer in a dicing operation. In those cases where an electrical current is running through the device, electrolytic corrosion processes may occur, especially if moisture can penetrate into the adhesive or bond line. 

The most common catalyst used in urethane adhesives is a tin(lV) salt, dibutyltin dilaurate. Tin(IV) salts are known to catalyze degradation reactions at high temperatures [30J. Tin(II) salts, such as stannous octoate, are excellent urethane catalysts but can hydrolyze easily in the presence of water and deactivate. More recently, bismuth carboxylates, such as bismuth neodecanoate, have been found to be active urethane catalysts with good selectivity toward the hydroxyl/isocyanate reaction, as opposed to catalyzing the water/isocyanate reaction, which, in turn, could cause foaming in an adhesive bond line [31]. 

As a result of the reduction in viscosity, the adhesive is now fully activated. The heating of the bond line ceases. The viscosity of the adhesive is now low enough so that the adhesive can be mated with a second substrate, and proper wetting of the second substrate can occur. 

Phase III in Fig. 5 is marked by a cooling of the bond line, which causes the bulk viscosity of the adhesive to rise. During phase IV, another sharp increase in viscosity is observed. This is caused by the re-crystallization of the polymer... 

The crystallization kinetics defines the open time of the bond. For automated industrial processes, a fast crystallizing backbone, such as hexamethylene adipate, is often highly desirable. Once the bond line cools, crystallization can occur in less than 2 min. Thus, minimal time is needed to hold or clamp the substrates until fixturing strength is achieved. For specialty or non-automated processes, the PUD backbone might be based on a polyester polyol with slow crystallization kinetics. This gives the adhesive end user additional open time, after the adhesive has been activated, in which to make the bond. The crystallization kinetics for various waterborne dispersions were determined by Dormish and Witowski by following the Shore hardness. Open times of up to 40 min were measured [60]. 

If blocking agents come off too quickly, foaming in the bond line can result, especially if the substrates are non-porous. One exception to this rule is E-caprolactam. It remains in the adhesive and can act as a plasticizer, which can aid adhesion at elevated temperatures. 

Activation by a metal surface also takes place in the commercially important anaerobic adhesives. These one-part adhesives are stable in the package, but cure quickly in an oxygen-free environment such as a tightly controlled bond line. Important applications include thread-locking, sealing, retaining, and some structural bonding [111]. A representative model formulation has recently been described [112] (Fig. 3). 

Blooming is the phenomenon of the formation of a white residue of cured adhesive near a bond line. It is the result of the evaporation of a small amount of the adhesive monomer from where it was applied onto the adjoining surfaee and its subsequent polymerization. The use of higher molecular weight monomers, such as the alkoxy esters, eliminates this problem. 

The quality of bonding and hence the properties and the performanee of the wood-based panels and beams are determined by three main parameters (1) the wood, espeeially the wood surface, including the interface between the wood surface and the bond line (2) the adhesive (3) the working conditions and process parameters. 

The deterioration of a bond line can occur due to (1) the failure of the resin (low hydrolysis resistance, degradation of the hardened resin causing loss of bonding strength) (2) the failure of the interface between resin and wood surface (replacement of secondary forces between resin and reactive wood surface sites by water or other non-resin chemicals) (3) the breaking of bonds due to mechanical forces and stresses (the influence of water will cause swelling and therefore movement of the structural components of the wood-based panels). 

One-component systems are starting to be used for structural glulam and fingerjointing applications. The bond line reaches the necessary green strength... 

Two-component systems consist of (1) polyol or polyamine, and (2) isocyanate. The hardening starts with the mixing of the two components. Due to the low viscosities of the two components, they can be used without addition of solvents. The mass ratio between the two components determines the properties of the bond line. Linear polyols and a lower surplus of isocyanates give flexible bond lines, whereas branched polyols and higher amounts of isocyanates lead to hard and brittle bond lines. The pot life of the two-component systems is determined by the reactivity of the two components, the temperature and the addition of catalysts. The pot life can vary between 0.5 and 24 h. The cure at room temperature is completed within 3 to 20 h. 

Thermoplastic, linear and saturated polyester give, depending on their chemical composition, hard or elastic and tacky bond lines they have relatively high melting viscosities bond lines are resistant against moisture, water and UV. 

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