Open assembly time

Figure 3. The absorption of alcohol-water solvent by wood near glue line (0 min open assembly time and 15 min closed time)...

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. 

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). 

Adherend or ambient temperature The higher the temperature, the faster the evaporation of the solvents this again leads to a reduction of the open assembly time. 

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. 

Particularly suitable for small (mm2, cm2) adherend surfaces. Less suitable for larger (> approximately DIN A6) adherend surfaces due to the short open assembly time. 

Consider open assembly time, minimum drying time, maximum drying time (Figure 5.2). 

Long open assembly time, thus advantageous for large area bonded joints. 

For the extension of the open assembly time of thermally well conductive adherends (metals), preheating up to the approximately melt temperature is required. 

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

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. 

Open assembly time Maximum period of time between the application of the adhesive and the fixing of the adherends. 

Bonded assemblies composed of 6 lamellae are prepared from straight-grained spruce with a density of (425 25) kg/m at(12 l)% moisture content. They must be bonded with the shortest and the longest closed assembly times given in the technical data sheet of the product to be tested. The open assembly time is set to be shorter than 5 min and the pressure applied for bonding is set to 0.6 N/mm. ... 

The formulations given in Table 7.8 are representative of hot melt adhesive compositions with open assembly times of greater than 2 s, which makes them suitable for the application on rigid supports such as standard cardboards. The hot melt compositions can be prepared by simple mixing its components at a temperature of 100-200°C, until a homogeneous mixture is obtained. 

Open time—See open assembly time under time, assembly. 

Time, assembly n. The time interval between the spreading of the adhesive on the adher-end and the application of pressure or heat, or both, to the assembly. Note—For assemblies involving multiple layers or parts, the assembly time begins with the spreading of the adhesive on the first adherend. (1) Open assembly time is the time interval between the spreading of the adhesive on the adherend and the completion of assembly of the parts for bonding. (2) Closed assembly time is the time interval between... 

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