Mixing 2 components adhesives

Commercial epoxy adhesives are composed primarily of an epoxy resin and a curing agent. Various additives and modifiers are added to the formulation to provide specific properties. Example trade names and suppliers of these ingredients are included in App. A. The curing agent may be incorporated into the resin to provide a single-component adhesive, or else it may be provided in a separate container to be mixed into the resin immediately prior to application. 

The primary advantage of solid epoxy adhesives is that they avoid the disadvantages of working with liquids. Waste and cleanup are minimized, and health problems are reduced because the end user handles only a solid substance. Since they are essentially one-component adhesives, they also eliminate the need to meter and mix individual components. 

Paste adhesives are supplied as either one- or two-component adhesive systems. They can be used in applications where pressure cannot be applied. Some two-part pastes cure at room temperature after the appropriate proportions are mixed. Epoxy, urethane, and acrylic adhesives are all available as paste adhesives. 

Heating the adhesive before atomization enables heavier adhesive buildup, reduces overspray losses, and minimizes contamination from atmospheric water vapor. Spray methods can be used on both small and large production runs. The liquid to be sprayed is generally in solvent solution. Sizable amounts of product may be lost from overspray. Two-component adhesives are usually mixed prior to placement in the spray gun reservoir. Application systems are available, however, that meter and mix the adhesive within the spray gun barrel. This is ideal for fast-reacting systems, but guns must be thoroughly cleaned to avoid buildup of polymerized product. 

Complete procedures for mixing the adhesive components, if more than one. 

Small (much less than stoichiometric) amounts of diamines sometimes are added to the hydroxyl portion of the adhesive to provide a rapid but limited molecular weight increase as soon as the components are mixed.61,62 In this way a two-package adhesive can be made that will flow easily before mixing but will not readily flow after mixing. Such adhesives can be applied to vertical surfaces or overhead, and will remain in place until the bonds are closed and cured. 

ReACt 784 [Hernon], TM for a two component, no mix, structural adhesive compound. 

To eliminate errors resulting from mixing or due to the exothermic reaction heat, so-called 1C adhesives were developed. These EP resins are only single-component adhesives in a procedural sense Chemically they are stiU two-component or multiple-component adhesives. These resins frequently contain additional catalysts that influence the course and kinetics of the reaction. 

In the case of two-component reactive adhesives, a second hardener component B is added to the resin component A in the mixing ratio prescribed by the adhesive manufacturer. Both components are then mixed evenly according to the processes described in Section 7.2.2 and applied to the adherends. The following adhesive types are characteristic for such two-component adhesives ... 

The mixing of adhesives is conducive to the compounding of the components of a reactive adhesive in the prescribed ratio to trigger off the chemical reaction of curing. 

It is advisable to use a stainless steel, glass or wood spatula and a clean working area (glass, aluminum foil) or a disposable plastic cup for the mixing of adhesives (preferably polythylene or polypropylene since plastics like polystyrene, polycarbonate or polyvinyl chloride may swell due to components of the adhesive). Tip For frequent bonding processes with low adhesive consumption, even so-called pill cups (content approximately 20 ml) of polyethylene or polypropylene are suitable, which are available in drug stores. 

One-component adhesive Adhesive that does not require mixing with another adhesive component prior to application. 

Static mixing tube Mixing device for two-component adhesives, mainly used for adhesives with identical mixing portions and processing viscosities. Mixing occurs by layer formation via staggered mixing impellers. 

Two-component adhesives typically consist of low-equivalent-weight isocyanate or prepolymer that is cured with a low-equivalent-weight polyol or polyamine. They may be 100% solids or solvent borne. Since the two components will cure rapidly when mixed, they must be kept separate until just before application. Application is followed quickly by mating of the two substrates to be bonded. 

This company s new 2600-093 Micro-Kiss series mix-dispense valves are designed for the low-flow mixing and dispensing of two-component adhesives and sealants such as silicones, epoxies, urethanes, and acrylics. The valves can be used to apply two-part adhesives and sealants for bonding, gasketing, potting and filling processes in aerospace, appliance, contractor, defense, electronics, filters, marine. 

The 2-component adhesives comprise of one part that is 100% polyol and a second part that is a partial reaction of polyol with isocyanate. This second part typically contains approximately 30% free isocyanate. When the two components are mixed, the free isocyanate in the second part reacts with the hydroxyl groups in the first part and these two components then polymerize to form an adhesive film. With this type of product, it can be appreciated that the cure rate is not therefore moisture dependent. 

Adhesives may also be classified by the way they are applied or cured. Hence, anaerobic adhesives are adhesives that set only in the absence of air, for instance, when confined between plates or sheets. A contact adhesive is one that is apparently dry to the touch but will adhere to itself instantaneously on contact (also called contact bond adhesive and dry bond adhesive). A heat-activated adhesive is a dry adhesive film that is made tacky or fluid by application of heat or heat and pressure to the assembly. A pressure-sensitive adhesive is a viscoelastic material that in solvent-free form remains permanently tacky. Such materials will adhere instantaneously to most solid surfaces with the application of very slight pressure. Room-temperature setting adhesives are those that set in the temperature range of 20-30°C. These are usually two-component adhesives that must be mixed before application. A solvent adhesive is an adhesive that has a volatile organic liquid as a vehicle and sets or becomes tacky after the solvent has evaporated. A solvent-activated adhesive is a dry adhesive film that is rendered tacky just before use by applicafion of a solvent. 

This rapidly developing group of materials, based on a variety of acrylic monomers, has viscosities which are fairly readily modified giving thin liquids, syrupy resins and thixotropic semi-solids. Some are truly single-component adhesives - the anaerobic versions (see Sections 5.1.2 and 5.1.12.1) - while others require some form of hardener. The hardener may be a surface primer or mixed directly into the adhesive. 

The chemical reactions that form the solid adhesive have to be blocked for a sufficient period to allow the adhesive to get to its final destination, i.e. the bonding joint. Some adhesives, which after mixing with their reaction partners spontaneously react at room temperature, are sold as two-component adhesives. These are present as resin and hardener in separate containers and are hence physically apart. They are only mixed together a short time before application. 

With single-component adhesives, the adhesive components are pre-mixed in their final proportions. They are, however, chemically blocked. As long as they are not subjected to the specific conditions which activate the hardener, they will not bond. They require either high temperature or substances or media (light and humidity) from their surroundings to initiate the curing mechanism. The containers in which this type of adhesive is stored are carefully chosen to prevent any undesirable reaction. 

Figure 82 Mixing of two-component adhesives, mechanical proportioner, fixed ratio (Hydra Cat from Graco).

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