Flexibilizing epoxies

Flexibilized epoxy resins are important structural adhesives [69]. Liquid functionally terminated nitrile rubbers are excellent flexibilizing agents for epoxy resins. This liquid nitrile rubber can be reacted into the epoxy matrix if it contains carboxylated terminated functionalities or by adding an amine terminated rubber. The main effects produced by addition of liquid nitrile rubber in epoxy formulations is the increase in T-peel strength and in low-temperature lap shear strength, without reducing the elevated temperature lap shear. 

Similar to composite propellants, flexibilized epoxy or novolac epoxy resins reinforced with fillers or fibers are used for inhibition of fuel-rich propellants. 

Diluents are higher-MW components than solvents that are also added to the epoxy adhesive formulation to lower the viscosity and modify processing conditions. The primary function of a diluent in an epoxy resin formulation is to reduce its viscosity to make it easier to compound with fillers, to improve filler loading capacity, or to improve application properties. Solvents, certain curing agents, and flexibilized epoxy resins can also lower the viscosity of epoxy adhesive formulations, but this is not their primary function. The effect of various diluents on the initial viscosity of a diglycidyl ether of bisphenol A (DGEBA) epoxy resin is illustrated in Fig. 6.3. 

Flexibility is primarily characterized by a material s elongation. Flexibilizers in epoxy systems work by allowing the material to deform under stress. In this way stresses on the joint are distributed over a larger area. The flexibilized epoxy resin has the capability of compensating for differences in thermal expansion or elastic moduli of the substrates. 

Another common method of flexibilizing epoxy adhesives is by blending the primary epoxy resin with other, more elastic polymers. Epoxy-nylon, epoxy-polysulfide, and to a certain extent epoxy-urethane hybrids use such a mechanism to provide flexibility. These flexibilizers are important additives for epoxy adhesives even though they may reduce certain... 

FIGURE 11.5 Effect of a flexibilized epoxy on tensile shear and peel strength of epoxy adhesive formulation.19... 

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. 

Flexibilized epoxy adhesives have moderate strength on flame and corona treated polyolefin substrates. Elevated cure temperature results in better adhesion because of more efficient wetting of the substrate surface. Table 16.13 shows a starting formulation for an epoxy adhesive that develops high peel strength to many difficult-to-bond substrates such as polyethylene, thermoplastic rubber, and polyester film. 

Figure 45-8 in a modern food piant, a properiy designed and laid floor tile (8 in. x 3 / in. X 1 /i6 in.) floor, using a A in. thick glass cloth-reinforced hot asphalt membrane and a furan resin bed and grout. Note the flexibilized epoxy expansion joint. 

Polymers of approximately 400-500 poises are used most frequently in sealants and adhesives, while the lower viscosity polymers are most frequently used for flexibilizing epoxy polymer casting compounds. 

In each instance of nitrile elastomer modification - whether rubber is added to the epoxy portion or to the hardener portion -the level of rubber largely determines whether a toughened or a flexibilized epoxy results. The former is characterized by little loss in thermal/mechanical properties. The latter shows a dominant influence of the added rubber. 

Zielinski, J., Vratsanos, M.. Laurer, J., and Spontak. R.. Phase separation studies of heat-cured ATU-flexibilized epoxies. Polymer, 37, 1, 75 84 (1996). 

Plasticized epoxy resins can be produced by reacting low molecular mass epoxy resins with dimeric fatty acids. These resins (e.g., Epikote 872, Shell) generally exhibit better substrate wetting than the unmodified epoxy resins. This reaction is, however, associated with the formation of ester bonds, and the resins therefore have a poor resistance to alkaline solutions. Instead of fatty acids, carboxy-functional polybutadiene-acrylonitrile elastomers can also be used to flexibilize epoxy resins. 

Thermoplastic rubber block copolymers, with completely new adhesive performance, were developed in 1965 [21]. The first commercial product was Shell Chemical s Kraton 101, of styrene polybutadiene-styrene composition. This development led to the carboxy-terminated nitrile (CTBN) rubber modifiers used to flexibilize epoxy and other brittle resin adhesives in the late 1960s. Today, the thermoplastic rubber block copolymer adhesives are used in hot melt-, solvent- and water-based adhesives, and as hot melt- and solvent-based sealants. Major applications are as pressure-sensitive adhesives, construction adhesives and sealants, and general assembly adhesives. 

Bis (2-ethylhexyl) maleate Butyl oleate Mica PEG-6 trimethylolpropane flexibilizer, acrylate coatings Bis (4-vinyl oxy butyl) hexanediyibiscarbamate flexibilizer, coatings PPG-4 diacrylate Stearyl acrylate flexibilizer, epoxies... 

Rigids (silica-filled) Flexibilized epoxies Liquid elastomers Rigid polyurethane... 

In the first section, a review is given on the synthesis of crosslinked pol3nners that might be suitable for low-temperature applications. In the second part, some mechanical properties of polymers at low temperatures are discussed, followed by our own results on the relaxation behaviour and material properties of different flexibilized epoxy resin systems at temperatures down to 77 K. 

The crosslinking density of the flexibilized epoxy resin systems is variable over a wide range, and the chemical incorporation of the flexibilizer precludes the migration frequently observed with plasticizers. Figure 19 shows part of an idealized network structure for epoxy-polyester copolymers. 

The good low-temperature properties of the flexibilized epoxy resin systems are due to the increased mobility of the segments from the main relaxation down to the y relaxation. Epoxy resin pol3nners modified with Empol(dimerized fatty acid) or Hycar (butadiene-acrylonitril copolymer, terminated by carboxyl groups) show notable strength, impact resistance, and temperature-shock resistance down to 20 Yet none of these systems is ductile... 

A comparison of the yield stress as a function of temperature for different flexibilized epoxy resin systems is given in Fig. 6. The crystalline EP exhibit lower yield stresses than the conventional flexibilized EP. The crystalline system, based on sebacic acid even shows ductile behaviour at -80 C, whereas the adipic acid system reaches its critical yield stress limit at -70°C, and brittle failure at lower temperatures is found. Measurements of Hartwig also showed brittle fracture at cryogenic temperatures. But there also seems to be no evidence of ductile behaviour in the case of the sebacic acid polyester segments. The lowest yield stress at -100°C is exhibited by the rubberlike polymer based on sebacic acid polyester. Here too, the adipic acid polyester shows higher yield stresses, but achieves ductile behaviour at -100°C. The more rapid increase in yield stress is due to the absence of crystallinity. These results show that good low temperature flexibility may be... 

The flexibilized epoxy A has a low modulus and high ultimate strain at room temperature, but a much higher modulus and lower ultimate strain at 4 K, about 0.4%. The filled epoxy B has the highest modulus, but a relatively low ultimate strain, 0.32% at 4 K. The low-viscosity epoxy C has a low modulus at 4 K, but the highest ultimate strain, about 0.8% at 4 K. 

Note the substantial change in properties of the flexibilized epoxy A on cooling to 4 K. At liquid helium temperature, its elongation is no better than the filled epoxy B. By contrast, the low-viscosity epoxy C has, by far, the greatest elongation at 4 K. The comparative differences in elongation at 4 K are the most important property for the purposes of this study. 

J.O. Turner, "Flexibilized Epoxy Formulation, Unfilled, and Its Use in Vacuum Impregnation of Magnet Coils," LBL Specification M20C, Lawrence Berkeley Laboratory, Berkeley, California (1970). 

Table 14.3 contains data that compares the results of plasma treatment and sodium etching for four fluo-ropolymers. Peel strengths of untreated and treated samples were measured by bonding them into T-peel specimen using the flexibilized epoxy adhesive Scotch-Weld 3553 (available from 3M Corp.). The laminates were cured for several hours at 70°C and peel tested at 12.5 cm/min pull rate. Polytetrafluoroethylene does not accept plasma treatment as well as PEA and FEP, as indicated by its relatively low peel strength. Sodium etching is the only effective method of modifying the surface of PTFE. 

Long chain aliphatic epoxy resins based on a poly glycol or a vegetable oil fatty acid, when reacted with epichlorohydrin, are used as additives to flexibilize epoxy resin adhesives. They are not used alone because of their water sensitivity and overall lack of toughness. They serve as modifiers for Bisphenol A based epoxy resins. An idealized structure for flexible epoxy resins is shown below ... 

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