Two-Component Reactive Adhesives

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  

Cold-curing reactive adhesives are adhesives that enable strong and functional bonded joints due to the monomers reactivity already at room temperature. Additional heat supply may contribute to a reduction of the curing time, especially in the case of epoxy resin adhesives, but need not necessarily to be done. 

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Reactive adhesives are usually available in two different tubes or tins (mostly called resin and hardener), and have to be mixed according to the weight and volume units defined by the manufacturer. Such adhesives are typically two-component reactive adhesives because of the two components A and B that have to be mixed. 

Adhesives that have to be mixed with a second component for curing (two-component reactive adhesives). 

Film adhesives have to be strictly delimitated from adhesive tapes and adhesive strips (Sections 5.6 and 5.7). Mainly blocked two-component reactive adhesives (Section 3.1.4) are used raw materials. For transport and storage (at low temperatures) they are applied to a - nonadhesive - substrate. Prior to processing they are removed and then applied between the adherends (compounding) and cured under pressure and heat (Sections 3.1.4, 3.2.2 and 4.1.2). Special film adhesives (e.g., phenolic resin nitrile rubber) are also activated by suitable solvents. 

To avoid inner tensions through thermal stress, it is recommended to use only adhesives that cure at room temperature. The adhesive selection is limited by the fact that many applications require an invisible glueline. In such cases, adhesives with fillers are excluded, cyanoacrylates and in particular radiationcuring products (Section 9.3.3) are the suitable choice. If the visual appearance of the bonded joint is not important, two-component reactive adhesives based on expoxides, polyurethanes, methacrylates, contact adhesives and, if required, adhesive tapes are recommended. 

In many cases, glass and even porcelain bondings are carried out as repairs in households. In such cases, it has to be pointed out that, compared to the two-component reactive adhesives, in particular epoxides, cyanoacrylates show only a limited bond strength in the face of high temperature and humidity stress in connection with rinsing agents in the cleaning equipment. 

Adhesive films Consisting of two-component reactive adhesives applied to a nonadhesive substrate for transport and storage purposes that is removed prior to processing. Curing occurs via chemical reaction under application of heat and pressure. Adhesive films are also commercially available as physically setting films, see heat-sealing adhesive. 

Due to the existence of different adhesive systems and the potential hazards associated with each system, there are different types of pack ing as well as stor e conditions and shelf fives. Adhesives can be differentiated based on specific criterion and properties. In the literature, one can find various classifications based on the assembly process, delivery form, adhesion mechanism, or application. Within the same adhesive group, the adhesives can be further differentiated based on their physical states or characteristics liquid, paste, or solid (throi viscosity measurements and rheological characterization), and solvent based, water based, or those without volatile content (through solid content measurements). Another classification can be made based on the adhesion mechanism physically dried adhesives from solution, solidified hot-melt adhesive, or chemically cross-linked single- and two-component reactive adhesives. 

Storage and Shelf Life of Two-Component Reactive Adhesives... 

Highly reactive two-component epoxy adhesives have several advantages. These adhesives can come to a practical handling strength in minutes at room temperatures and can develop useful bond strengths at ambient temperatures as low as —20°C. The various uses for these adhesives include... 

A Plexus two-component reactive MMA adhesive system was able to resolve the problem for the OEM. The nature of the MMA adhesive means it is compatible with many types of thermoplastic regrind materials, and the adhesive does not need to be cut from the bonded part. Test data clearly showed that there were minimal effects on the injection molding parameters of the plastic and that there was minimal loss of properties when compared to the virgin material (see Figures 1 and 2). 

In the mid-seventies, spurred by developments from the DuPont Corporation, a new kind of two-component acrylic adhesive system was introduced to the joining industry. The DuPont technology has been widely licensed and is now referred to by numerous terms such as "second-generation acrylics", "reactive adhesives", "modified acrylics", "toughened acrylics", etc. These adhesives seemed to answer the need for improvements over some of the major disadvantages found in anaerobic structural adhesives, namely those of cost, the need for scrupulous surface preparation, and the ability to bond plastic surfaces. 

Reactive— liquid components chemically react with each other to form a solid (two-component epoxy adhesives, cyanoacrylates or super glues react with water, some adhesives react on exposure to light or radiation)... 

Reactive liquid polymers (RLPs) The Uquid rubbers most often used in epoxy adhesives are those based on acrylonitrile-butadiene copolymers or on long-chain polyethers. To act as effective tougheners, however, they cannot be directly added to the formulation but have to be prereacted with some of the epoxy resin, or sometimes with the amino hardeners in two component paste adhesives (adduct fonnatiou). To achieve this, only RLPs with reactive eud groups are used by the formulator. 

Two-component reactive quasi-prepolymer liquid adhesives where one component forms the polyol prepolymer stream and the other the diisocyanate prepolymer stream. 

Prefabricated Construction. Sandwich elements are widely used in prefabricated construction. In this case, bonding must meet stringent requirements with regard to stability and durability, especially in the case of exterior wall elements. Deflections attributable to difierential temperature or moisture-induced expansion or contraction can amount to as much as 5% of the length. The adhesive layer must be permeable to water vapor or act as a water barrier, depending on the structure and function of a given element. The adhesives used are two-component reactive resins, based mainly on epoxy resins, but polyurethane resins are used as well, as are elastomer-based contact adhesives that are applied on both sides. 

On the other hand, components in two-package systems are typically much more reactive at ambient conditions and are mixed just prior to application. For instance, highly cross-linked systems until relative high solid contents that cure at low temperature can only be achieved by the two-package application mode [33]. A review of processing requirements and physical properties of one and two-component urethane adhesives is presented elsewhere [34]. 

The most important types of two-component adhesives are moisture-sensitive polyurethane, comprising polyol resin and isocyanate catalyst, and heat-sensitive epoxy, comprising bisphenol A or F resins and polyamine catalyst. Both single- and two-component adhesive systems are available in the market. Single-component reactive adhesives, which include cyanoacrylate and silicone, are also widely known, with the latter often being used as a sealant. 

These subgel prepolymers have been manufactured by industry for some time and used in two-component adhesives or coating materials. For one-component materials, storage below the actual Tg, whenever applicable, is the most efficient method for blocking the reactivities. This kind of blocking is used in powder coatings. 

Eor adhesive applications, it is more convenient to have the two components of the epoxy resin system react at a reasonable rate at room temperature. Replacing the anhydride component by a suitable, more reactive polyfunctional amine enables this to occur at room temperature. Several aliphatic and aromatic polyfunctional amines are used for this purpose (Eq. 21.28). 

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. 

The basic principle of curing of a reactive adhesive to form an adhesive layer is a chemical reaction, as described in Section 3.1. The given resin component A requires a partner for reaction. There are two different possibilities of partner selection ... 

The (physically wetting) polyaniline primer CORRPASSIV was compared with a fast-drying two-component epoxy resin primer (Messrs. Finalin, Type 144) and a metal-reactive two-component primer with adhesion-promoting properties (Type 918). All three primers were coated with a two-component polyurethane paint (B412). CORRPASSIV, both with and without top coat, exhibited no rust formation beneath the primer. The slight undermining receded after the test, and even within the cross and scratch injuries, it was virtually impossible to detect any corrosion. 

An unusual type of two component system has recently been introduced in the adhesive industry (1,). These are called reactive adhesives, second generation acrylics, toughened acrylics, modified acrylics (2) or "honeymoon adhesives" (in Europe). 

Adhesion properties of epoxies, complete reactivity with no volatiles during cure, and minimal shrinkage make the materials outstanding for adhesives, particularly in structural applications. The most commonly known adhesive applications involve the two-component liquids or pastes, which cure at... 

Polyurethane pressure-sensitive adhesives began appearing in the early 1970s [15]. By 1978 advanced two-component automotive structural adhesives (Goodyear) were commercially available. Waterborne polyurethane adhesives received additional attention during this period [16]. In 1984, Bostik developed reactive hot-melt adhesives [17]. Polyurethane adhesives are sold into an ever-widening array of markets and products, where they are known for their excellent adhesion, flexibility, low-temperature... 

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