Epoxy resins fracture toughness

Table 19.1 Comparisons of increases in epoxy resin fracture toughness achieved using liquid reactive rubbers and core-shell paitides at equivalent volume fractions of particles...

Composite Particles, Inc. reported the use of surface-modified rubber particles in formulations of thermoset systems, such as polyurethanes, polysulfides, and epoxies [95], The surface of the mbber was oxidized by a proprietary gas atmosphere, which leads to the formation of polar functional groups like —COOH and —OH, which in turn enhanced the dispersibility and bonding characteristics of mbber particles to other polar polymers. A composite containing 15% treated mbber particles per 85% polyurethane has physical properties similar to those of the pure polyurethane. Inclusion of surface-modified waste mbber in polyurethane matrix increases the coefficient of friction. This finds application in polyurethane tires and shoe soles. The treated mbber particles enhance the flexibility and impact resistance of polyester-based constmction materials [95]. Inclusion of treated waste mbber along with carboxyl terminated nitrile mbber (CTBN) in epoxy formulations increases the fracture toughness of the epoxy resins [96]. 

Our interest in PDMS as an epoxy modifier lies partly in its low Tg relative to the ATBN and CTBN modifiers. Up to this time, however, improvements in K,c through copolymerization of dimethyl siloxane with TFP and DP siloxane require raising the Tg of the siloxane modifier above that of PDMS, as shown by Table 1. It is hoped that increased understanding and control of the synthesis and morphologies of siloxane-modified epoxies will make it possible to retain the low Tg of the modifier while raising the fracture toughness of the resin. The true value of this objective could eventually be shown by measurement of Klc at temperatures below ambient. 

A variety of toughening agents have been used to modify epoxy adhesives to improve peel strength and fracture toughness without significantly affecting other properties of the epoxy base resin. Generally these modifiers can be classified into three types ... 

Some inorganic particulate fillers have also been considered as toughening agents for epoxy materials. Glass beads, fly ash, alumina trihydrate, and silica were used early on to improve the toughness of filled epoxy resins. Various studies, however, have demonstrated that the fracture energy of filled epoxies reaches a maximum at a specific filler concentration. 

Chemical functionalization of the thermoplastics was found to improve toughness without such detractions. High-molecular-weight resins based on amine-terminated PES oligomers or chain extension of bismaleimide resin with the same amine-terminated PES were found to have improved fracture resistance and reduced thermal shrinkage.14 Also a mechanism was found to toughen cyanate esters by incorporating epoxy resins, which can react with the ester.15... 

Mode I and II interlaminar fracture toughness was investigated with unidirectional CF/epoxy and ALF/ epoxy laminates in RT-air and 77K-LN2- A common epoxy matrix was used for both laminates. Mode I fracture toughness of the neat resin was also measured for comparison. The results are summarized as follows ... 

The ratio of the mode I interlaminar fracture toughness in 77K-LNj to that in RT-air was 1.8 for ALF/epoxy laminates. This ratio was decreased to 1.2 to 1.3 for CF/epoxy laminates. Thus, the increase of the matrix toughness was directly translated into the increase of the interlaminar fracture toughness for ALF/epoxy laminates. The effect was smaller for CF/epoxy laminates. Fractographic observation indicated foe contribution of resin is larger for ALF/epoxy laminates. Dominant interfacial fracture concealed the effect of resin for CF/epoxy laminates. 

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