Open time

The open time of an adhesive is the time it can remain on an adherend before the joint needs to be closed. For epoxies this would typically be about 50% of the fixture time as any longer time could result in the adhesive not wetting and thus not adhering properly to the second substrate. 

UV adhesives have an almost infinite open time although exposure to workshop light and daylight can initiate slow polymerisation. 

Cyanoacrylates would normally have a relatively short open time as they are very sensitive to surface moisture and to a lesser extent atmospheric moisture. In most cyanoacrylate applications, the joint shonld be closed within a few seconds and certainly not more than a few minutes. 

Room-temperature-vulcanising silicone adhesives will skin over in about 10 minutes and so the joint should be closed within a five-minute period to ensure that full wetting and therefore adhesion takes place. 

For fully automatic dispensing systems, this means that some consideration must be made to ensure that the joint is closed within the specified time. 

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Case 1. The particles are statistically distributed around the ring. Then, the number of escaping particles will be proportional both to the time interval (opening time) dt and to the total number of particles in the container. The result is a first-order rate law. 

Solvents. Solvents in house paints serve several essential purposes. They keep the binder dispersed or dissolved and the pigments dispersed in an easy-to-use state. Solvents allow the paint to be appHed in the correct thickness and uniformity, and evaporate from the paint film after the paint is apphed. Solvent choice is limited mainly to a solvent that is compatible with the binder system and that has the desked evaporation rate and toxicity profile. The volatility or evaporation rate of a solvent determines to a large extent the open-time and dry-time properties of a paint (6). 

Mineral spirits, a type of petroleum distillate popular for use in solvent-based house paints, consist mainly of aUphatic hydrocarbons with a trace of aromatics. This type of solvent finds use in oil- and alkyd-based house paints because of its good solvency with typical house paint binders and its relatively slow evaporation rate which imparts good bmshabiUty, open-time, and leveling. Other properties include lower odor, relatively lower cost, as well as safety and health hazard characteristics comparable to most other organic solvents. 

Phenolic resin substantially increases open time and peel strength of the formulation (80). For example, higher methylol and methylene ether contents of the resin improves peel strength and elevated temperature resistance. Adhesive properties are also influenced by the molecular weight distribution of the phenoHc low molecular weight reduces adhesion (82). 

Compounding is quite different for the two systems. The solvent base system is dependent on magnesium oxide and a /-butylphenoHc resin in the formulation to provide specific adhesion, tack, and added strength. Neither of these materials have proven useful in latex adhesive formulations due to colloidal incompatibihty. In addition, 2inc oxide slowly reacts with carboxylated latexes and reduces their tack. Zinc oxide is an acceptable additive to anionic latex, however. Other tackifying resins, such as rosin acids and esters, must be used with anionic latexes to provide sufficient tack and open time. 

Volume of vessel (free volume V) Shape of vessel (area and aspect ratio) Type of dust cloud distribution (ISO method/pneumatic-loading method) Dust explosihility characteristics Maximum explosion overpressure P ax Maximum explosion constant K ax Minimum ignition temperature MIT Type of explosion suppressant and its suppression efficiency Type of HRD suppressors number and free volume of HRD suppressors and the outlet diameter and valve opening time Suppressant charge and propelling agent pressure Fittings elbow and/or stub pipe and type of nozzle Type of explosion detector(s) dynamic or threshold pressure, UV or IR radiation, effective system activation overpressure Hardware deployment location of HRD suppressor(s) on vessel... 

It is observed that there may be asymmetry in the system as long as the short-circuit condition lasts, as illustrated in Figure 13.20, i.e. up to the opening of the interrupting device. (For opening times of interrupters, refer to Table 19.1.) But the content of the asymmetry may be quite feeble after three or four cycles. Flowever. if the short-circuit condition still prevails, such as when... 

Neoprene 400. It contains a fast-crystallizing polymer with the highest chlorine content and the highest uncured cohesive strength among all anionic latexes. However, this latex has a relatively short open time and requires high lamination pressures to achieve coalescence. 

Tackifiers and modifiers are generally added to improve the adhesive performance of synthetic elastomers. All resins added to an adhesive formulation modify their properties (viscosity, open time, tack) and therefore these resins are also called... 

In contact adhesives, the so-called tack open time is important. This can be defined as the time available after the adhesive is applied during which the surface remains tacky enough for the application of the adherend. It can be easily measured by applying a thin layer of fresh adhesive on Kraft paper and making a bond at different times until no bond is obtained. 

The solvent plays an important role in the performance of rubber base adhesives. The solvent is the carrier for all components of the adhesive. Furthermore, the solvent controls the viscosity, open time, tack and adhesion of rubber base adhesives. 

Chlorinated rubber is also used to promote the adhesion of solvent-borne CR adhesives to metals and plasticized PVC. Addition of a low molecular weight chlorinated rubber (containing about 65 wt% chlorine) improves the shear strength and creep resistance of polychloroprene adhesives [75] but a reduction in open time is also produced. A heat reactivation (process in which the surface of the adhesive film is raised to 90-100°C to destroy the crystallinity of the film and allowing diffusion to produce polymer chain interlocking more rapidly) restores tack to the polychloroprene adhesives. 

Solvents. Solvents affect adhesive viscosity, bond strength development, open time, cost, and ultimate strength. Blends of three solvents (aromatic, aliphatic, oxygenates, e.g. ketones, esters) are generally added, and in their selection environmental and safety regulations must be considered. A graphical method has been proposed to predict the most adequate solvent blends for solvent-borne CR... 

The open tack time of the CR adhesives partially depends on the evaporation rate of the solvent blend. If a solvent evaporates slowly, the CR adhesive will retain tack longer, whereas if the solvent evaporates quickly, the cohesive strength will develop more rapidly. According to Table 13, addition of small amounts of xylene (generally lower than 5%) will increase the open time of CR adhesives. 

Bond strength can vary from a temporary bond (non-curing compound) to a substrate tearing bond (using phenolic-modified curing products). Solvent-borne CR adhesives can be formulated to have very short open times for fast production operations or to retain contact bond characteristics for up to 24 h. Heat and solvent reactivation can be used to re-impart tack to dried surfaces. 

In most adhesives, tackifier is the ingredient present in the highest proportion. Tackifying resins are primarily used to reduce adhesive viscosity and adjust the 7g of the adhesive s amorphous matrix phase. Through their effects on the other ingredients and the overall system they can also dramatically affect wet out, hot tack, open time, set speed, and heat resistance. 

Rather, microcrystalline waxes are used to provide adhesives with longer open times and better low temperature adhesion, due to better wet out, and toughness. 

A typical formula would consist of 18-28 parts of SBS rubber, 50-60 parts of an aliphatic/aromatic tackifier with about 15-30% aromaticity (for long open time), and 15-30 parts of a white oil or a very clean process oil. Formulations are designed to maximize open time, while maintaining adequate heat resi.stance (maintenance of bond strength upon aging at 40-55 C — warehouse conditions). 

Freezer grades normally rely on lower melting paraffin waxes and/or the slower setting and tougher microerystalline waxes. In direct contrast to the above, polymer content is higher, and resin Tg is lower. The resin system is chosen to maximize the open time of the polymer and also speeific adhesion. 

To maximize thermal stability at conventional running temperatures (350°F) EnBA-based systems are chosen [20]. This polymer is much more thermally stable than EVA. Open time is longer than EVA, so special resins are not required for this purpose, even in freezer-grade systems. 

Rubber-based adhesives provide softness and good low temperature flexibility (see Table 8). These properties make them the primary choice for the hinge application, which are two thin glue beads applied to the sides of the book block adjacent to the spine. These adhesive beads allow the book to open with the cover and help to protect the spine glue from stresses. Hinge glues have low if any wax, and are pressure sensitive. When used for the spine application, rubber-based adhesives require a water-based emulsion primer due to their short open time and thus low penetration of paper substrates. 

The crystallization kinetics defines the open time of the bond. For automated industrial processes, a fast crystallizing backbone, such as hexamethylene adipate, is often highly desirable. Once the bond line cools, crystallization can occur in less than 2 min. Thus, minimal time is needed to hold or clamp the substrates until fixturing strength is achieved. For specialty or non-automated processes, the PUD backbone might be based on a polyester polyol with slow crystallization kinetics. This gives the adhesive end user additional open time, after the adhesive has been activated, in which to make the bond. The crystallization kinetics for various waterborne dispersions were determined by Dormish and Witowski by following the Shore hardness. Open times of up to 40 min were measured [60]. 

The thermoplastic polyurethane (TPU) adhesives must, of necessity, contain low gel content because they must be processable in an extruder. Most adhesives are relatively linear, with a functionality of 2.0, although small amounts of branching may be introduced, usually at the expense of a lower melt flow. Good physical properties of TPU s are obtained when the thermoplastic urethanes have molecular weights of 100,000 or higher (see p. 56 in [63]). Most TPU adhesives are based on symmetrical polyesters with a fast crystallizing backbone or a backbone slightly modified to increase the open time. 

Open times of two-component urethanes can vary widely, depending on the level of catalyst. Reaction times can vary from 90 s to over 8 h. Dibutyltin dilaurate is the most common catalyst employed to catalyze the urethane reaction. This is normally added to the polyol side. A tertiary amine may also be added in small amounts. Tin catalysts do not catalyze the amine/isocyanate reaction very well. Acids, such as 2-ethyl hexanoic acid, may be employed to catalyze the amine/isocyanate reaction where needed. 

Tackifiers. The tackifiers usually are hydrocarbon resins (aliphatic C5, aromatic C9) or natural resins (polyterpenes, rosin and rosin derivates, tall oil rosin ester). They improve hot tack, wetting characteristics and open time and enhance adhesion. The content on tackifiers in a hot melt can be in the region of 10-25%. 

Clamp Opening Time Clamp Closing Time Clamp Open Time Hold Timer Setting... 

Neurosteroids prolong the mean open time of recombinant GABAa receptor channels. Whereas, at least in recombinant systems, the identity of the a and (3 subunits has little or no effect on neurosteroid action, substitution of the y subunit by a 8 subunit suppresses the GABA-modulatory activity of the neurosteroids. 

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