Adhesive assembly time

Assembly time Refers to the elapsed time after an adhesive is applied... 

In those cases where water is absorbed very rapidly it would be expected that adhesion values would be very high. They are. That is, if the open assembly time is low. But, if the film is applied to a very absorptive surface and then held for 30-60 minutes before closing with an unspread surface, the solvent will have been removed prematurely, and the bond subsequently obtained will be poor due to low penetration. Thus, all of the factors listed in Table I can potentially cause poor curing. 

Alkaline-dispersed amylaceous materials in phenolic plywood adhesives for improved assembly time tolerance and prepress tack. 

Straight casein lumber laminating adhesives for their superior open and closed beam assembly time tolerance and gap-filling properties. 

Subsequent work was, therefore, directed to substitution of resorcinol, a much more expensive phenol, in cold-setting, waterproof adhesives (38). Formulations based on 30 to 60% of extract mixed with a resorcinol-formaldehyde condensate and additional formaldehyde met pot-life and assembly time requirements for timber lamination. Test bonds passed requirements of the major performance standards in the United States, but competition from lower cost, phenol-modified resorcinol resins and the lack of longterm commercial perfor-... 

The total assembly time, from the applieation of the adhesive to entry into the hot press, ranges from 20 to 40 minutes. For part of that time the glue is exposed and can lose moisture rapidly. The viseosity of the glue and other adhesive characteristics change over time and the bonding strength after curing in the hot press is influenced by sueh factors. Automated layup minimizes open assembly and total assembly times. 

Such adhesives are particularly suitable for smaller adherend surfaces, since for large-surface applications the open assembly time is not sufficient, thus curing may occur before the adherends are fixed. 

Hot-melt adhesives, which belong to thermoplastics (Section 3.3.1), are liquefied by heat supply, for example, in electrically heated nozzles of the application device, and then applied to the adherends. Since the hot-melt cools down very quickly the adherends have to be affixed immediately. The open assembly time of these adhesives, that is, the period of time between the application of the adhesive and the fixing of the adherends, is very short and must not be exceeded. The open assembly time strongly depends on the heat conductivity properties of the adherends the faster they dissipate the heat from the melt, the shorter is the open assembly time. Hot-melt adhesives are available in the form of blocks, rods, films, granulate or even in the form of powder. 

Due to the limited open assembly time, large area bondings are only possible on a restricted basis. Special hot-melt adhesives are available for the bonding of larger surfaces for industrial purposes, and especially adjusted applicators are utilized (spray application). 

Amount of adhesive applied The thicker the adhesive layer, the more solvents must evaporate. This may result in an extension of the open assembly time. 

Due to the long open assembly time inherent in the contact adhesives (Section 5.2) they are also applied to bonding of large area surfaces, for example, in wood processing for the application of veneers. 

Since the latter possibility is only given in case of porous surfaces, dispersion adhesives are mainly used for materials that are capable of binding the water of the liquid adhesive layer by absorption . The remaining polymer particles with their inherent tack fuse to an adhesive layer (film formation). The open assembly time (Section 5.2) in the processing of dispersion adhesives is strongly influenced by the moisture content of the adherends and the relative humidity. The open assembly time increases with higher humidity. 

Choose adhesives with longer open assembly times... 

Very short open assembly time, fix adherends immediately after adhesive application. 

Hot-melt adhesives for small-size bonded joints (owing to short open assembly time). 

Assembly time (open) Period of time between the adhesive application and the fixing of the adherends. 

Hot-melt adhesives Hot-melt adhesives are applied to the adherends in a fused form and set by cooling. Their open assembly time is very short, thus adherends have to be fixed immediately after adhesive application. 

However, good penetration into the wood is a very important aspect of wood bonding. Standards such as ASTM D 2559 require bond formation within the minimum and maximum of the recommended open and closed assembly times [9], Sufficient penetration into the wood is considered important for good bond formation, but overpenetration produces a starved bondline that is the weak link. Overpenetration does not occur with non-porous substrates thus, different factors need to be considered in formulating and using wood adhesives. A lower viscosity adhesive is normally better for the wetting and adhesion, but for wood the adhesive can be so thin as to overpenetrate into the wood. 

PVA-based adhesives exhibit excellent adhesion properties and adhere to a host of different substrates such as metal, porcelain, wood, paper and various textiles. The adhesives are quick-setting, a property which shortens press and assembly times in manufacturing processes. 

Table 4 Delamination of Phenolic FRP-Wood Bondline under Different Assembly Times (with RF Adhesive and without HMR Primer)...

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