Handling strength

Unlike clay-based systems, modem ceramics require additives, termed binders, to provide the plasticity required for ductile-forming methods to be used. These organic additives serve to modify the rheological behavior of the ceramic suspensions and impart handling strength to the green, as-formed ceramic bodies. Their... 

Epoxy adhesives are often given a postcure. Room temperature curing and elevated-temperature curing epoxy adhesives are cured to a state in which the resin is hard and the bond is capable of handling (i.e., handling strength). This provides the opportunity of... 

Anhydrides are sometimes used in epoxy adhesives to provide specific properties or to provide improvements in handling strengths. The most important anhydride in epoxy adhesive formulations is pyromellitic dianhydride (PMDA), which provides very high temperature properties. 

Polymercaptans, which cure at 0° to — 20°C, are attracting attention in low-temperature curing adhesive formulation. At normal room temperature, polymercaptan has a pot life of 2 to 10 min and reaches handling strength in 10 to 30 min. An example of a typical mercaptan and its reaction sequence with an epoxy group is shown in Fig. 5.12. 

Can be cured at room temperature to a handling strength and then exposed to elevated temperatures for final cure... 

The pot life of polyamide or amidoamine cured epoxy adhesives is generally on the order of hours at room temperature. Full cure is achieved in 5 to 7 days at room temperature, and handling strength is achieved in about 16 to 24 h. A faster cure can be achieved in 20 min to 4 h by heating to 60 to 150°C. When room temperature cures are required, an accelerator such as an amine is often added to the formulation. 

Highly reactive two-component epoxy adhesives have several advantages. These adhesives can come to a practical handling strength in minutes at room temperatures and can develop useful bond strengths at ambient temperatures as low as —20°C. The various uses for these adhesives include... 

Table 11.12 shows two rapid-setting, room temperature cure epoxy adhesives based on epoxy acrylate resins with aliphatic amine curing agents. These adhesives have gel times of less than 5 min for a 100-g mass. The bond strength development is rapid with handling strength occurring in about 1 h at room temperature. 

The cationic photoinitiator has relatively limited absorbance spectra, and the cure is more sluggish than a uv cured epoxy-acrylate. As a result, the uv may be used to develop handling strength in the epoxy adhesive, and a room temperature or elevated-temperature postcure may be necessary after the initial uv exposure for the adhesive to achieve ultimate properties. 

Typically a full cure will take less than 1 min and can occur with certain adhesives in 15 to 30 s. The partial cure process is usually employed on larger, more difficult to fixture parts. Heat times for sufficient handling strength can be as low as 3 to 10 s. A secondary heat source such as a final paint cure oven will then fully cure the adhesive. 

Structural adhesives usually require curing by the application of heat, the addition of a catalyst, the addition of pressure, or a combination of the three. The strength developed in the adhesive joint at various times during the curing process may be determined by lap shear tensile specimens. This test is commonly used to determine when an adhesive or sealant is fully cured or when the system reaches a handling strength so that the assembled product can be moved with moderate care. 

General-purpose, flexible epoxy 8—12 h handling strength... 

Long working life 8—12 h handling strength rigid... 

Applications for these foamed products are many. An example is meat trays that are normally produced from PS foamed sheet with a thickness of approximately 0.095 in. and a density of about 5 lb/ft3. These trays are manufactured with little skin. The meat should not stick to the plastic when frozen, nor should the meat juice penetrate into the tray, and the tray should not break when handled. Strength is achieved by correcdy orienting the sheet resulting in cells that are round from a plan view. The cell size, which is determined by the amount and type of nucleating agent, is kept fairly fine to give the trays a soft, glossy appearance. 

Immediate handling strength, no displacement possible after fixation of adherends. 

Handling strength Strength of a bonded joint that allows for further processing in the work flow. 

Initial strength Strength, also handling strength, an adhesive develops shortly after the application and fixation of the adherends adhesive property important for further processing. 

Some adhesives will bond cured elastomers. Typically, a thin layer (0.0025 mm thickness) of the activator is applied to one of the adherends and a layer of the adhesive (0.026-0.26 mm) is placed on the other adher-end. The two substrates are then pressed together and secured until adequate handling strength develops. Most acrylic adhesives cure to this point in 2-20 minutes, but some cure in as little as 10 seconds. In all cases, cure is completed within 24 hours. These adhesives provide excellent shear, peel, and impact strengths at temperatures ranging from -107°C to -i-121 °C. These adhesives can withstand short exposures up to 177°C. ... 

For jigging purposes, and to provide handling strength prior to adhesive cure, supplementary mechanical fasteners are desirable. A further consideration is the unproved peel resistance and impact performance, which may be realized by strategic mechanical fastening. In this respect, spot welding remains the favoured method. 

To achieve ease of manufacture with total continuous automation, a single component, cyanoacrylate, (see Cyanoacrylate adhesives) is now used. It is applied automatically to one of the joint faces and the faces mated immediately. The adhesive develops initial handling strength in 4 s. 

Is it absolutely essential that handling strength be Yes rejectallbutbekmnr 19... 

Must handling strength be developed within 60 Yes act ijlosuyz 19... 

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