Fixing, adherends

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

Since the chemical reaction of the two components A and B begins in a pot immediately after mixing, this ready-made adhesive mixture requires speedy application. Otherwise the reaction for the formation of the AB polymer (the adhesive layer) will have developed to such an extent already prior to the application to the adherends that the expected strength of the bonded joint is impaired. Between the mixing of the adhesive mix and its application to the adherends and their fixing only a certain time span is allowed - which may vary for the individual reactive adhesives. This time is called the pot life. Depending on the reactivity of the A and B monomers, pot life can lie in the range of minutes or even hours. 

So, one-component systems, for example for aircraft construction, are preferably produced as adhesive films, which have to be stored at low temperatures (up to approximately -20 °C). When they are cut to the dimensions of the adherends (compounding) and the adherends are fixed, they will be cured at high temperatures (approximately 140-160 °C). 

After the application of the adhesive, the adherends have to be fixed immediately, since otherwise the beginning polymerization will lead to a reduction of the bonding strength. 

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. 

This variation implies that the peroxide hardener dissolved in an organic solvent will be applied to one of the two adherends. After the evaporation of the solvent, the hardener remains on the surface in a very thin layer where it can remain for a sufficiently long time without changing. The resin component provided with the accelerator, which is not subjected to pot-life limitation, will be applied to the other adherend. Only when both adherends are fixed does the contact between hardener and resin/accelerator system lead to the chemical reaction of the adhesive layer formation. This procedure is common in industrial use, but also advantageous for trade and semi-industrial use, since there are no pot-life limitations. Care has to be taken that the adhesive layer is not too thick since otherwise the amount of hardener applied in a thin layer to one of the adherends is not sufficient for complete curing. Since the hardener is applied... 

The adherends have to be fixed immediately after the adhesive is applied, since otherwise early curing (skin formation on the adhesive surface) will set in causing a deterioration of the adhesive properties. 

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. 

Solvent-based adhesives are adhesives with polymers dissolved or pasted in organic solvents. The solvents or solvent mixtures are only processing aids and have to be removed, either partly or completely, from the applied liquid adhesive layer through evaporation or penetration prior to the fixing of the adherends. The first case is necessary for solvent-impermeable materials (metals, glass, thermosetting plastics), the second case concerns porous and solvent-permeable materials (paper, cardboard, wood, leather). This process can be accelerated by heat supply. Solvents are mainly esters, ketones, if applicable, portions of different alcohols. The total solvent portion ranges between 75-85%. 

In this context, the term wet adhesive requires a special explanation. In processing, it serves the differentiation between contact adhesives and solvent-based adhesives and describes the still (partially) liquid state of the adhesive prior to the fixing of the adherends. This term is not a common designation of adhesives in the broader sense. 

Fixing of adherends under pressure and final setting of adhesive layer... 

Application of surface pressure When processing solvent-based adhesives, it is in any case recommendable to apply equally distributed pressure to the adhesive layer after the fixing of the adherends. This causes the polymer molecules existing on both adherends to get jammed or entangled", which increases the cohesion strength of the adhesive layer. 

In the case of permeable, respectively, porous materials (papers, cardboards, wood, etc.) the fixing of the adherends is already possible when the minimum... 

When fixing the adherends, it has to be considered that a later adjustment is not possible. It is recommendable to use a fixing device (edge, or the like, to align the adherends with). 

Bond the adherends as long as the adhesive is wet, then fix the bonded joint under pressure. 

Adhesive tapes are also suitable as fixing aids for the manufacturing of bonded joints to secure adherends against moving (in the case of heat-curing adhesives, however, applicable only to a limited extent). 

Mix the adhesive only after the adherends and their surfaces have been adequately prepared and the devices for the ensuing fixing of the adherends after the adhesive application are available. 

For individual bonds, screw clamps and weights are suitable for pressure application, however, these technical aids are only reasonable for small numbers of items. For series bonds it is necessary to manufacture special fixing devices according to the geometry of the adherends. 

The fixing of adherends is also possible in a simple way with adhesive tapes that, in case of hot curing, have to be heat resistant. 

The strength values mentioned above depend on the respective construction conditions as well as on the stress duration. An example A plastic hook fixed to a tile by means of a pressure-sensitive adhesive and exposed to stress can come off in the course of time due to a failure of the adhesive layer (creeping). In the case of dynamic stress, the hook can break inside, the bonded joint remains unaffected. In this case, the adherend is the weaker link in the strength chain . 

Adhesives with shorter pot life are only processible in serial bonding by means of automatic mixing and dosing systems and fixing of the adherends at the same time. 

Efficient serial bondings with high production speed are only possible with fast-curing, respectively, setting adhesives. Slowly curing adhesives require costly devices for the fixing of the adherends. 

Assembly time (closed) Period of time during which a bonded joint has to be fixed until it is strong enough to resist a displacement of the adherends by external force effects. 

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

Fixing The fixation of the adherends, with or without pressure, in the desired position during the curing process. 

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. 

Minimum drying time In the case of solvent-based adhesives, the time period between the adhesive application and the fixing of the adherends, to enable the evaporation of the major part of the solvent from the liquid adhesive film. 

Solvent-based adhesives Adhesives with adhesive-layer forming substances (polymers) being solved or pasted. Depending on the character of the adherends, the solvents must evaporate completely or partly prior to fixing. 

---