Reactive Liquid Rubber

Formulations have been developed where small rubber domains of a definite size and shape are formed in situ during cure of the epoxy matrix. The domains cease growing at gelation. After cure is complete, the adhesive consists of an epoxy matrix with embedded rubber particles. The formation of a fully dispersed phase depends on a delicate balance between the miscibility of the elastomer, or its adduct with the resin, with the resin-hardener mixture and appropriate precipitation during the crosslinking reaction. 

The advantage of this toughening process is that the high modulus and Tg of the epoxy resin are not sacrificed. The Tg of the product is indicative of the epoxy resin used as the matrix, although the precipitated domain has a Tg of about -40 to -50°C. 

The butadiene-acrylonitrile tougheners that have been especially successful in both DGEBA and DGEBF types of epoxy resins are 

CTBN and ATBN are the most commonly used in structural epoxy adhesive formulations. CTBN (Fig. 8.6) is generally the elastomer of choice because of its miscibility in many epoxy resins. These tougheners were originally developed by BF Goodrich (now Noveon, Inc.) under the tradename Fly car. 

The degree of toughness is determined by the crosslink density of the matrix, the elastomer particle size and size distribution, the volume fraction of the elastomeric phase, and the degree of adhesion between the epoxy matrix and the particle. The formulating procedure was found to have as strong an effect on the fracture toughness as the materials themselves.16 

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Thermosets are generally used in advanced composites due to their excellent thermal and dimensional stability, high modulus, and good mechanical properties. Because thermoset resins are inherently brittle, however, some applications require improved fracture resistance. Toughening of thermosets has been achieved through various methods, such as incorporation of reactive liquid rubber [1-9], elastomer [10], or rigid thermoplastics [11-25], and IPN formation with ductile component [26]. 

High temperature epoxy resins are brittle materials, and one method of improving their fracture properties is to incorporate reactive liquid rubbers in the formulations In situ phase separation occurs during cure the cured rubber-modified epoxy resins consist of finely dispersed rubber-rich domains ( 0.1-S pm) bonded to the epoxy matrix. TTT diagrams can be used to compare different rubber-modified systems. 

The polyester resin used in this study, MR 13006 (Aristech Corporation), was supplied as a 60-wt% solution in styrene monomer. The epoxy resin, a digly-cidyl ether of bisphenol A (Epon 828), was obtained from Shell Chemical Company. The reactive liquid rubber, an amino-terminated butadiene-acrylonitrile copolymer (ATBN 1300 x 16), was provided by the BFGoodrich Company. The resin was mixed with additional styrene monomer to maintain the ratio of reactive unsaturation in the polyester-to-styrene monomer at 1 to 3. We added 1.5 wt% of tert-butylperbenzoate initiator to the solution, which we then degassed under vacuum. The mixture was poured between vertical, Teflon-coated, aluminum plates and cured under atmospheric pressure at 100 °C. In the modified compositions, the rubber was first dissolved in the styrene monomer, and then all the other components were added and the solution cured as described. In all the compositions, the ratio of the amine functions with respect to the epoxy functions was kept at 1 to ensure complete cure of the epoxy. 

Fracture-Energy Testing. Table I gives the recipes and the fracture energies measured under slow and fast rates of test, for the elastomer-modified VER and for ETBN additions to the elastomer-modified VER. Also given are the total amounts of reactive liquid rubber, from both ETBN addition and CTBN reacted directly into the VER, for each recipe. For reference, the unmodified VER has a slow GIc. of 0.11 kj/m2. Finally, the Tg obtained from differential scanning calorimetric measurements is given for each recipe. 

Impact Performance of Epoxy Resins with Poly(/i-butyl acrylate) as the Reactive Liquid Rubber Modifier... 

Baldwin and Lees (18) disclosed that VTBN with THP methacrylate can serve to optimize a set of properties based on shear (collar/pin and lap joint) and impact strengths. This balance appears to occur at reactive liquid rubber levels of 7.5-10.0%. These improvements occurred without detriment to storage stability and subsequent curability of the composition. Catena (19) essentially verifies that comparable activity is obtained when VTBN serves as a 10% monomer replacement in anaerobic formulations based on tetraethylene glycol dimethacrylate. 

The attempt to blend natural rubber with epoxy resins resulted from the abundance of natural rubber and that it was a renewable resource. Nevertheless, interfacial adhesion between natural rubber and epoxy resins was weak due to the hydrophobic nature of natural rubber. Thus, it was an interesting experiment to blend the toughened epoxy resins with synthetic reactive liquid rubber. In order to achieve an efficient stress transfer between rubber and the... 

This was ground and functionalized with amine-reactive liquid rubber, to improve its compatibUity with carboxylated LLDPE [142]. 

More recently Crosbie and Philips [85,86] investigated the toughening effect of several reactive liquid rubbers (carboxyl terminated butadiene-acrylonitrile, vinyl terminated butadiene-acrylonitrile, hydroxyl terminated polyether, polyepichlorohydrin) and an unspecified experimental reactive liquid rubber developed by Scott Bader Ltd. on two different polyester resins a flexibilized isophthalic-neopentyl glycol polyester resin, PVC compatible and an epoxy modified polyester resin, which is preaccelerated. The results of these studies are summarized as follows ... 

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