Liquid adhesives reactive liquids

As mentioned at the outset, hot melt adhesive s primary advantage is process speed. Fleat resistance and substrate penetration are typically inferior to liquid adhesives (neat reactive systems, solvent, or water-based). Current research and development is therefore focused on maximizing the process advantages of hot melts and minimizing their performance deficiencies. Optimizing hot melt... 

These applications are a domain that is shared by UV and EB processes, because both can directly convert reactive liquids into solids almost instantly. There are specific areas where EB irradiation is more suitable than the UV curing process. In general, these include applications where thick layers of coatings or adhesives are applied. Other instances are coatings with high levels of inorganic pigments or fillers, which usually cannot be cured by UV radiation because of their opacity. As pointed... 

Ihe polymers of the 2-cyanoacrylic esters, more commonly known as the alkyl 2-cyanoacrylates, are hard glassy resins that exhibit excellent adhesion to a wide variety of materials. The polymers are spontaneously formed when their liquid precursors or monomers are placed between two closely fitting surfaces. Tile spontaneous polymerization of these very reactive liquids and the excellent adhesion properties of the cured resins combine to make these compounds a unique class of single-component, ambient-temperature-curing adhesive of great versatility (Table 3). The materials that can be bonded run the gamut from metals, plastics, most elastomers, fabrics, and woods to many ceramics. 

In principle, an equality between the thermodynamic work of adhesion of liquid-solid systems and the work needed to separate an interface might be expected for simple systems and this has been observed for failure of adhesive-polymer interfaces bonded by van der Waals forces, (Kinloch 1987). Similarly, empirical correlations of interfacial strengths and work of adhesion values of solidified interfaces have been reported for some nominally non-reactive pure metal/ceramic systems. However, mechanical separation of such interfaces is a complex process that usually involves plastic deformation of the lattices, and hence their works of fracture are often at least ten and sometimes one hundred times larger than the works of adhesion, (Howe 1993). Nevertheless, for non-reactive metal/ceramic couples, it is now widely recognised that the energy dissipated by plasticity (and as a result the fracture energy of the interface) scales with the thermodynamic work of adhesion (Reimanis et al. 1991, Howe 1993, Tomsiaet al. 1995). 

The majority of adhesive laminations in multilayer flexible packaging are manufactured using the dry-bond process. In this technique, a liquid adhesive is applied to one substrate. The adhesive is then dried using hot air. This dried surface can be adhered to a second substrate using heat and pressure at a nip point. The adhesive formulations themselves represent a reactive chemistry (typically urethanes or acrylics) that is chosen to withstand the processing and storage/distribution environment of the filled product. The adhesives polymerise and/or cross-link during production of the laminated product. 

Adhesives are nonmetaUic substances used to join two surfaces by means of surface adherence (adhesion) and inherent strength (cohesion), DIN 16920. This definition of adhesives does not cover water glass adhesives, adhesive ceramics, or adhesive mortars. The substances used as adhesives are polymers that go through a liquid phase at least once (reactive adhesives) or more than once (hotmelts, thermally activated adhesives). The liquid phase can also be achieved by dissolution in suitable solvents (nonreactive adhesives). In dispersion adhesives, the polymer molecules are dispersed (finely distributed) in a liquid - usually water - whereby the polymer molecules themselves are not dissolved. Fig. 6. These adhesives are also known as water-based or aqueous adhesives. It must be remembered that solvents are contained in these adhesives in addition to the water. Genuine aqueous adhesives contain less than 5% solvents in the liquid phase. The dispersions crnitain, in contrast to the solute adhesive molecules, additional substances, disposal of which requires specific additional measures. Since the dispersions represent stable systems in water, the water-resistance of such adhesives is reduced. Their thermal and water resistance can be increased by additional crosslinking (usually with isocyanates). 

In this case, the liquid adhesive applied to the adherends consists of the monomer molecules ready for a chemical reaction (Sections 2.1.2 and 2.1.3). Due to their small size they are mostly liquid. After the application of the adhesive and the joining of the adherends to be bonded, a chemical reaction occurs in the glueline. From the (liquid) monomers the solid ( hard ) adhesive layer develops. This time-dependent process is called curing or setting. Since it is triggered by a chemical reaction, one talks of chemically reacting adhesives or of reactive adhesives. 

Microencapsulated adhesive Reactive adhesive mixture, with the (liquid) components encapsulated by a protective skin in the form of finest drops, preventing a reaction during storage. Only after the destruction of the capsule wall, for example, by screwing a nut onto a suchlike coated screw, does a chemical reaction set in and an adhesive layer develop. 

The acrylic core-shell polymers are considered to offer superior ultraviolet-light and thermal-oxidative aging properties than does the more conventional reactive liquid polymeric toughener, CTBN. Hence, there is current interest in the use of acrylic core-shell polymers as tougheners for adhesives and composite matrices that possess a relatively high glass-transition temperature. 

In this paper/ we shall describe the development of adhesives during the last several years and the trends for further advances. In general/ traditionally used / solvent-borne systems (4) will be gradually replaced by waterborne systems/ hot-meltS/ nonvolatile solid (or liquid) systems/ two-part adhesiveS/ radiation-curable adhesiveS/ and powder and reactive liquid systems (Table 1). 

The use of elastomeric or flexibilizing modifiers occurred and grew with epoxy resins first. Various aspects of toughened epoxy adhesives have been covered in reviews by the present authors (2,3), where the elastomeric modifiers have essentially been carboxylic, liquid and solid butadiene/acrylonitrile polymers. There has not been a systematic review, however, of these and other reactive liquid polybutadiene/acrylonitriles in the burgeoning areas of acrylic, anaerobic and radiation-curable systems. Thus, this paper s intent. 

Ralph Drake has degrees from Case-Western Research University. He has been with B. F. Goodrich Chemical Group for 26 years and spent much of that time associated with new products, many of them adhesive raw materials. He is currently Marketing Manager for Reactive Liquid Polymers. He has over 40 patents and technical publications. 

These are liquid ingredients added to an adhesive to reduce the concentration of the binder component. Diluents are added principally to lower the viscosity and to modify the processing conditions of some adhesives. Reactive diluents do not evaporate, as would solvents. They react with the binder during the cure cycle and are incorporated in the cured adhesive. 

One-part epoxy adhesives include solvent-free liquid resins, solutions in solvent, liquid resin pastes, fusible powders, sticks, pellets and paste, supported and unsupported films, and preformed shapes to fit a particular joint. Two-part epoxy adhesives are usually comprised of the resin and the curing agent, which are mixed just prior to use. The components may be liquids, putties, or liquid and hardener powder. They may also contain plasticizers, reactive diluents, fillers, and resinous modifiers. The processing conditions are determined by the curing agent employed. In general, two-part systems are mixed, applied within the recommended pot life (a few minutes to several hours), and cured at room temperature for up to 24 hours, or at elevated temperatures to reduce the cure time. Typical cure conditions range from 3 hours at 60°C to 20 minutes at 100 C. ... 

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)... 

The second means of transforming a liquid adhesive entirely into a solid without the loss of a solvent or dispersion medium is to produce solidification by a chemical change rather than a physical one. Such reactive adhesives may be single-part materials that generally require heating or exposure to electron beam or UV or visible radiation (see Radiation-cured adhesives) to perform the reaction, and which may be solids (that must be melted before application), liquids or pastes. The alternative two-part systems require the reactants to be stored separately and mixed only shortly before application. The former class is exemplified by the fusible, but ultimately reactive, epoxide film adhesives and the latter by the two-pack Epoxide adhesives and Polyurethane adhesives and by the Toughened acrylic adhesives that cure by a free-radical Chain polymerization mechanism. 

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. 

Single-component solventless reactive liquid adhesives where the diisocyanate group is the key bonding element in a prepolymer base. 

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

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