Flexibilization and toughening

Future chapters describe the other raw materials that contribute to the epoxy adhesive formulation (curing agents and catalysts, Chap. 5 solvents and diluents, Chap. 6 hybrid resins, Chap. 7 flexibilizers and tougheners, Chap. 8 fillers, Chap. 9 and adhesion promoters, Chap. 10). Complete adhesive formulations are then discussed in subsequent chapters. 

Chapters 4 through 10 describe the basic raw materials that are commonly employed in formulating epoxy adhesives. These include the epoxy resins, curing agents and catalysts, solvents and diluents, resinous modifiers, flexibilizers and tougheners, fillers, and adhesion promoters. 

Chlorinated polyethylene (CPE) flexibilizes and toughens PVC more by a miscibility mechanism especially when the chlorine content is above 42% [Donbe and Walsh, 1979]. However, partial miscibility occurring at chlorine levels of 36% leads to higher toughening effects. CPE offers also weatherability advantage, which is the major reason for its commercial use. 

Chem. Descrip. Polyoxyall lene diamine Uses Epoxy curing agent with flexibilizing and toughening props. also in polyamide, polyurea, modified urethane resins, in adhesives, elastomers, foam formulas intermediate for textile and paper treatment chemicals Properties Nearly colorless liq. m.w. 148 sp.gr.1.0154 (25/4 C) vise. 8 cSt flash pt. (PMCC) 265 F distort, temp. 44-47 C (264 psi)... 

Most epoxy formulations contain diluents, fillers or reinforcement materials, and toughening agents. Diluents may be reactive (mono- and diepoxides) or nonreactive (di-n-butyl phthalate). Toughening (flexibilizing) agents such as low-molecular-weight polyesters or... 

Elastic adhesives deform when exposed to such internal stress and are less affected by shrinkage. Thus, flexibilizers or tougheners may be added to the formulation to distribute the stresses over a larger area (Chap. 8). 

The modification of epoxy structural adhesives by the addition of flexibilizing and/or toughening agents has greatly increased the utility of these products. Most structural adhesives based on DGEBA or cycloaliphatic resins cure to give products which are very strong but brittle. Such adhesives perform well when loaded in tensile and shear, but poorly when... 

Tougheners Tougheners eliminate brittleness from epoxy resin to prevent cracks and decrease distortion. Unlike flexibilizers, they do not reduce crosslink density. 

The catalyst does not make up part of the final epoxy network structure or have a significant effect on the final properties of the cured resin. Thus, the final cured properties of the epoxy system are primarily due to the nature of the epoxy resin alone. Homopolymerization normally provides better heat and environmental resistance than polyaddition reactions. However, it also provides a more rigidly cured system, so that toughening agents or flexibilizers must often be used. In adhesive systems, homopolymerization reactions are generally utilized for heat cured, one-component formulations. 

Functionalized, liquid polybutadiene derivatives have also been developed as hybrid flexiblizers for epoxy resins. Carboxyl-terminated butadiene/acrylonitrile polymers, butadiene homopolymers, and maleic anhydride-amino acid grafted butadiene homopolymers have been used as flexibilizers to impart good low-temperature strength and water resistance to DGEBA-based epoxy adhesives. An epoxy system toughened by polybutadiene with maleic anhydride is claimed to provide a hydrophobic backbone, low viscosity, softness, and high tensile strength and adhesion (Table 7.10). 

The toughening mechanism of elastomer modified epoxy systems is different from that of flexibilized epoxy systems and can be used in combination with them. Flexibilized epoxy systems reduce mechanical damage by a reduction in modulus or plasticization of the adhesive. This allows stress to be relieved through distortion of the adhesive, but it also generally results in a lowering of the adhesive s glass transition temperature with an accompanying reduction in heat and chemical resistance. 

Heat Distortion Test. Two remaining bars from the cleavage tests are used to determine heat distortion temperature in accordance with ASTM D-621 test methods. These thermal-mechanical tests are necessary to determine whether a brittle resin has been truly toughened or whether it has been merely flexibilized. The morphology of the resin also effectively describes a true toughening situation and can aid immeasurably in explaining departures from true toughening. 

To a lesser extent, amine-terminated polybutadiene/acrylo-nitrile (ATBN) liquids are also used by epoxy formulators. This polymeric disecondary amine is employed in admixture with amines, amldoamlnes or fatty polyamides of choice (10, 11). Thus, one admixes chemical and polymeric amines to create a novel toughening or flexibilizing hardener package. 

Uses Abrasion resist, aid, toughener, flexibilizer, hardener, gloss aid in water-based coatings (automotive OEM, refinish, primers, basecoats, topcoats), adhesives for wood/plastic/metal substrates, inks binder in waterborne inks, bonding and laminating applies. 

Chem. Descrip. Urethane diacrylate oligomer Uses Flexibilizer, toughener for coatings, inks, UV/EB adhesives (pressure-sensitive tapes) applied to flexible substrates Features Imparts high flexibility and elasticity, adhesive peel sir., toughness high elong. 120%... 

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