Assembly time

In plywood production with ureas, the spread veneers should be pressed as soon as possible. The time between spreading and pressing, usually called assembly time, should never exceed one hour. With some formulations, the permissible assembly time may be no longer than 15 rninutes. Melamine formulations and uncatalyzed melamine—urea combinations, however, can be spread and stored for as much as a week before use. 

It is probably obvious at this point that the molecular weight distribution of a resole is an extremely important characteristic. It has major influence on such important performance capabilities as cure speed, viscosity, green strength development (or prepress), assembly time tolerance, required application rates. 

After a satisfactory verification film is produced, an assembly may be fabricated specifically for destructive inspection to validate that the verification film was accurate. This correlation allows the use of verification film rather than more expensive destructive inspection for future changes such as duplicate tool fabrication and tool or detail modification. Simple assemblies are usually not destructively inspected because of high confidence that the verification film is entirely representative of the expected bondlines. Complex or large parts may not be destructively inspected because of the cost of the details and assembly time. In these cases other means of validating the verification film are used. Meticulous pre-bond detail and post-bond assembly thickness measurements may be sufficient to prove bondline thickness control. Ultrasonic inspection and X-ray photography (discussed previously) may be sufficient to prove that details are in the correct places and bonds are good. 

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

The variable t in the foregoing equations is the actual time (/nncr) that is required to pass a volume V of filtrate through the medium and is only part of the total time of the cycle (tcyde). The rest of the cycle, which may include wash time, disassembly and assembly time, cleaning time, etc., we shall call dead time (Aead) ... 

In-mould assembly (IMA) of a filter housing saves assembly time and simplifies logistics. Cycle times were halved to under 30 seconds with this technology. 

All assembly times are 24 h. aThe ratio of mixed solvents is 2 1. 

Two methods of glue application are currently being used in production of glue-lams. The older method is to pass the boards through rubber-covered roll spreaders so that both sides are coated simultaneously. The boards are then stacked horizontally until all are spread and clamps are in proper position for pressure application. During this assembly time the wet glue film is exposed to the atmosphere for only a short time. This minimizes loss of solvents and water to the atmosphere. 

However, when higher ambient temperatures prevail, such as those often experienced during the summer months (80 to 110°F), the loss of active solvent becomes critical. The glue film after extended assembly time may become deficient. Its ability to wet and penetrate the surface is then severely reduced. Thereafter the probability that this film will produce an adequate bond soon falls to zero. Consequently, the allowable assembly time may be quite limited. 

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. 

Figure 1.1. Representative organizations of biochemical components. Three component areas of biochemistry—structural, dynamic, and information biochemistry—are represented as organizations in space (dimensions of biomolecules and assemblies), time (rates of typical biochemical processes), and number (number of nucleotides in bioinformatic...

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-... 

Stabilization of the interfaces between oil and water is of great importance in emulsion technology. Interfaces are usually stabilized with the help of amphiphihc molecules such as low molecular weight surfactants and biopolymers. Interfaces are two-dimensional nanoscopic spaces where amphiphilic molecules accumulate and self-assemble. Time-scales are very important in emulsion technology, as the adsorption events at the newly created interface take place in the millisecond range and are governed by the diffusion properties of the surfactant molecules (Brosel and... 

The peptide chain grows in length by the addition of amino acid residues from the amino-terminal end (D5). The assembly time of one chain is estimated at 15-30 seconds at 37°C. Termination of the chain assembly is probably due to the presence of the mRNA codon UAA (or the less likely codons UAG and UGA) which is read as end chain and causes the chain to be detached from the assembly line. 

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). 

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... 

---