Toughening methods

Further details of the above techniques to improve the transverse fracture toughness of composites by controlled interfaces of various nature and modifying materials are discussed in the following sections. The effectiveness of these toughening methods on transverse fracture toughness and strength in relation to the controls are summarized in Table 7.1. 

Table 1.1. Comparison among various toughening methods for ceramics [50]...

There are fewer limitations with this approach, although it is not the best toughening method for solution-based adhesives because of the possible settling out of the polymeric dispersion with time. These materials can be readily dispersed in both hot melt and paste adhesives great care has to be taken, however, that the powder is efficiently and uniformly distributed throughout the matrix. 

In this way, a total of 144 combinations (= 2 [toughening methods] x 3 [bond conditions due to cavitation] x 3 [elastic moduli] x 4 [bonding at the crack tip] x 2 [plane stress/strain]) are possible from the different conditions without taking into account the dispersion, leading to various morphologies. 

Creating compressive stress around modifier particles as a toughening method... 

The versatihty of the powder-toughening method appears particularly attractive for high-temperature thermosetting resins due to the possibility of combining relatively insoluble or infusible products. The initial problem lies, however, in preparing a polymer powder with a particle size distribution suitable for reinforced composite fabrication, namely, below interfiber spacing, which is typically in the xm range. 

Acrylonitrile—Butadiene—Styrene. ABS is an important commercial polymer, with numerous apphcations. In the late 1950s, ABS was produced by emulsion grafting of styrene-acrylonitrile copolymers onto polybutadiene latex particles. This method continues to be the basis for a considerable volume of ABS manufacture. More recently, ABS has also been produced by continuous mass and mass-suspension processes (237). The various products may be mechanically blended for optimizing properties and cost. Brittle SAN, toughened by SAN-grafted ethylene—propylene and acrylate mbbets, is used in outdoor apphcations. Flame retardancy of ABS is improved by chlorinated PE and other flame-retarding additives (237). 

Nonpowder Synthesis. Many ceramic composites (qv) under iavestigation utilize reinforcing ceramic whiskers or fibers to achieve toughening (19). Whiskers (17,19,20) are produced by vapor-synthesis techniques. SiC whiskers can be produced by the rice hull or vapor—soHd (VS) method whereby rice hulls are pyrolyzed to produce a mixture of carbon, C, and Si02, and whiskers are produced by directional growth by vapor... 

Whisker reinforcement is a viable method of toughening composites. However, health considerations associated with the aspiration of fine, high-aspect-ratio whiskers raise serious concern about their widespread use. 

Liquid organic rubbers with reactive functionality can be prepared by several methods. End-functional oligomers are preferred. Chains attached to the network at only one end do not contribute as much strength to the network as those attached at both ends [34], Urethane chemistry is a handy route to such molecules. A hydroxy-terminated oligomer (commonly a polyester or a polyether) can be reacted with excess diisocyanate, and then with a hydroxy methacrylate to form a reactive toughener [35]. The methacrylate ends undergo copolymerization with the rest of the acrylic monomers. The resulting adhesive is especially effective on poIy(vinyl chloride) shown in Scheme 2. 

Crivello, J.V., Electron beam curable epoxy compositions, US Patent 5,260,349, 1993. Janke, C.J., Dorsey, G.F., Havens, S.J. and Lopata, V.J., A toughened epoxy resin system and a method thereof, US Patent 5,726,216, 1998. 

Nelsen, S., Golder, M. and DeStio, P., Method and compositions for toughening polyester resins, US Patent 6020414 (to Hoechst Celanese Corporation, Somerville, NJ), 2000. 

In Toughened Composites, ASTM STP 937, (N.J. Johnston ed.) ASTM, Philadelphia, PA, pp. 179-196. Wu, H.F. and Claypool, C.M. (1991). An analytical approach of the microbond test method used in characterizing the fiber-matrix interface. J. Mater. Sci. Lett. 10, 260-262. 

Toughening Composite Modifying % increase in % decrease in Test method... 

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]. 

Rubber-toughened polystyrene composites were obtained similarly by polymerising the dispersed phase of a styrene/SBS solution o/w HIPE [171], or a styrene/MMA/(SBS or butyl methacrylate) o/w HIPE [172], The latter materials were found to be tougher, however, all polymer composites had mechanical properties comparable to bulk materials. Other rubber composite materials have been prepared from PVC and poly(butyl methacrylate) (PBMA) [173], via three routes a) blending partially polymerised o/w HIPEs of vi-nylidene chloride (VDC) and BMA, followed by complete polymerisation b) employing a solution of PBMA in VDC as the dispersed phase, with subsequent polymerisation and c) blending partially polymerised VDC HIPE with BMA monomer, then polymerisation. All materials obtained possessed mixtures of both homopolymers plus some copolymer, and had better mechanical properties than the linear copolymers. The third method was found to produce the best material. 

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