Improving Toughness

Since direct aspect ratio information could not be obtained for the Si3N4 systems, toughness correlations are, thus, based upon the measured mean deflection angles. As a reference required for comparison, the highest toughness at a median 

The experimental evidence correlates well with the guidelines developed for toughening brittle materials by crack deflection processes. 

Because of the resistance of the particles, the crack bows between them, causing the stress intensity, K, at the particle to increase, while, in the bowed segment of the crack, a softening effect occurs, i.e., K decreases along this segment. Bowing 

Lange [36] has analyzed the effect of the pinning points and expressed the fracture energy (i.e., the energy/unit area required to initiate fracture) as  

As may be seen, each of the series resulted in a linear plot. The slope of each plot equals the line energy of the crack front. The effect of the particle size may be incorporated into Eq. (8.64) as  

Two-component, room temperature curing epoxy adhesives have evolved over three distinct generations, from brittle to flexible to toughened. The quest for toughening is due to the inherent lack of peel and impact strength properties in brittle epoxy adhesives and 

TABLE 11.19 Typical Formulations and Properties of Epoxy-Terminated and Nonterminated Polysulfide-Epoxy Adhesives26 

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. 

Carboxy-terminated curative, such as CTBN, provides excellent toughening in part due to its miscibility in many epoxy resins. Phase separation during cure is required to obtain toughening, and generally the phase separation requires an elevated-temperature cure. However, by prereacting the CTBN with a portion of the epoxy to obtain an adduct, a room temperature curing toughened epoxy is possible. Adduction reduces the likelihood of early phase separation and maintains the solubility of the elastomer in the uncured resin system. 

One of the disadvantages of CTBN epoxy adhesives has been their high viscosity, which limits formulation options. Recently new adducts, such as EPON 58003 and RSM-2577 from Resolution Performance Products, have been introduced that have significantly lower viscosities.28 In addition, lower concentrations of these new CTBN epoxy adducts are generally required to achieve equivalent adhesive performance. New lower-viscosity CTBN adducts have also resulted in formulations where a greater concentration of cost-reducing filler, such as calcium carbonate, can be used. 

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The cured polymers are hard, clear, and glassy thermoplastic resins with high tensile strengths. The polymers, because of their highly polar stmcture, exhibit excellent adhesion to a wide variety of substrate combinations. They tend to be somewhat britde and have only low to moderate impact and peel strengths. The addition of fillers such as poly (methyl methacrylate) (PMMA) reduces the brittleness somewhat. Newer formulations are now available that contain dissolved elastomeric materials of various types. These mbber-modifted products have been found to offer adhesive bonds of considerably improved toughness (3,4). 

Polypropylene polymers are typically modified with ethylene to obtain desirable properties for specific applications. Specifically, ethylene—propylene mbbers are introduced as a discrete phase in heterophasic copolymers to improve toughness and low temperature impact resistance (see Elastomers, ETHYLENE-PROPYLENE rubber). This is done by sequential polymerisation of homopolymer polypropylene and ethylene—propylene mbber in a multistage reactor process or by the extmsion compounding of ethylene—propylene mbber with a homopolymer. Addition of high density polyethylene, by polymerisation or compounding, is sometimes used to reduce stress whitening. In all cases, a superior balance of properties is obtained when the sise of the discrete mbber phase is approximately one micrometer. Examples of these polymers and their properties are shown in Table 2. Mineral fillers, such as talc or calcium carbonate, can be added to polypropylene to increase stiffness and high temperature properties, as shown in Table 3. 

In addition to the semicrystalline nylons, which comprise the vast majority of commercial resins, nylon is also available in an amorphous form that gives rise to transparency and improved toughness at the expense of high temperature properties and chemical stress crack resistance. Table 2 shows the properties of some different polyamide types. 

Polypropylene. PP is a versatile polymer, use of which continues to grow rapidly because of its excellent performance characteristics and improvements in its production economics, eg, through new high efficiency catalysts for gas-phase processes. New PP-blend formulations exhibit improved toughness, particularly at low temperatures. PP has been blended mechanically with various elastomers from a time early in its commercialisation to reduce low temperature brittleness. 

Nylon. Nylons comprise a large family of polyamides with a variety of chemical compositions (234,286,287). They have excellent mechanical properties, as well as abrasion and chemical resistance. However, because of the need for improved performance, many commercial nylon resins are modified by additives so as to improve toughness, heat fabrication, stabiUty, flame retardancy, and other properties. 

Materials are also blended with VDC copolymers to improve toughness (211—214). VinyHdene chloride copolymer blended with ethylene—vinyl acetate copolymers improves toughness and lowers heat-seal temperatures (215,216). Adhesion of a VDC copolymer coating to polyester can be achieved by blending the copolymer with a linear polyester resin (217). 

Plasticizers. Plasticizers (36—38) are often added to a binder to reduce cross-link density and increase flexibiHty. Plasticizers improve toughness, springback, and flexibiHty, but degrade overall green strength. Additionally, plasticizers can increase the sensitivity of a binder system to moisture. 

In the adhesives area, thermoplastic, fatty polyamides are used in hot-melt and heat-seal adhesives for leather, paper, plastic and metal. Blends of EDA- and DETA-based polyamides are suggested for use in metal can seam sealants with improved toughness (234) pressure sensitive adhesives have been formulated with DETA-based polyamides (235) and anionic and cationic suspensoid adhesives are used as heat-seal coatings in paper converting (236). PIP and certain PIP derivatives are used with EDA in some appHcations (237). 

Coa.ting S. CR has been used to coat a variety of substrates, from cloth for rainwear to concrete decks for protection against salt water. A sol-type latex is preferred to ensure good adhesion to concrete decks. A crystalline polymer latex is preferred where added durabUity is needed. The compound includes a nonionic surfactant to improve its chemical stabUity. A number of thin coatings are appUed to the surface to allow better coverage and facUitate drying. A similar formulation could be used to coat the interior of tanks, but an accelerator is needed to improve toughness. 

The combination of better processing to give smaller flaws with alloying to improve toughness is a major advance in ceramic technology. The potential, not yet fully realised, appears to be enormous. Table 19.1 lists some of the areas in which ceramics have, or may soon replace other materials. 

The very low density materials (VLDPEs) introduced in the mid-1980s are generally considered as alternatives to plasticised PVC (Chapter 12) and ethylene-vinyl acetate (EVA) plastics (see Chapter 11). They have no volatile or extractable plasticisers as in plasticised PVC nor do they have the odour or moulding problems associated with EVA. Whilst VLDPE materials can match the flexibility of EVA they also have better environmental stress cracking resistance, improved toughness and a higher softening point. 

Plasticisers are comparatively uncommon but plasticised grades are supplied by some manufacturers. Plasticisers lower the melting point and improve toughness and flexibility, particularly at low temperatures. An example of a plasticiser used commercially in Santicizer 8, a blend of o- and p-toluene ethyl sulphonamide (Figure 18.18). 

Blends or alloys of polyacetals with polyurethane elastomers were first introduced by Hoechst in 1982, who were then followed by other manufacturers. The key features of these materials are their improved toughness with little change in other important properties. There are two aspects with respect to the impact toughness ... 

As with poly(ethylene terephthalate) PBT-based copolymers have been introduced to overcome some of the deficiencies of the homopolymer. For example, the rather low notched impact strength of unreinforced grades has been overcome by partial replacement of the terephthalic acid with a longer chain aliphatic dicarboxylic acid. Improved toughness has also been obtained by grafting about 5% of ethylene and vinyl acetate onto the polyester backbone. 

The homopolymers, which are formed from alkyl cyanoacrylate monomers, are inherently brittle. For applications which require a toughened adhesive, rubbers or elastomers can be added to improve toughness, without a substantial loss of adhesion. The rubbers and elastomers which have been used for toughening, include ethylene/acrylate copolymers, acrylonitrile/butadiene/styrene (ABS) copolymers, and methacrylate/butadiene/styrene (MBS) copolymers. In general, the toughening agents are incorporated into the adhesive at 5-20 wt.% of the monomer. 

In order to improve toughness many rubbers and other soft polymers may be used as additives to modify the compound. Some copolymers based on vinyl chloride are available of which the most important are the vinyl chloride-vinyl acetate materials used in gramophone records, flooring compositions and surface coatings. 

Cross-linking produces some dimensional stability and improves toughness in a noncrystalline type of plastic above the Tg, but... 

Since that time much work has been done in the area of siloxane-imide systems, especially in industrial laboratories. Therefore most of the available information is enclosed in the patent literature 168 175) and, unfortunately, description of the actual polymerization chemistry is very vague. A great majority of these applications utilized disiloxanes in high concentrations in order to obtain soluble polymers with improved toughness. 

Due to the low level of branching, there is little to hinder the crystallization of high density polyethylene. We routinely observe crystallinity levels in excess of 60%, which translate into densities ranging from approximately 0.94 to 0.97 g/cm3. High density polyethylene is the stiffest of all the polyethylene types. In some cases we incorporate small amounts of a comonomer, such as 1 -hexene, which reduce the crystallinity level. This improves toughness at the expense of stiffness. 

Polycarbonate is blended with a number of polymers including PET, PBT, acrylonitrile-butadiene-styrene terpolymer (ABS) rubber, and styrene-maleic anhydride (SMA) copolymer. The blends have lower costs compared to polycarbonate and, in addition, show some property improvement. PET and PBT impart better chemical resistance and processability, ABS imparts improved processability, and SMA imparts better retention of properties on aging at high temperature. Poly(phenylene oxide) blended with high-impact polystyrene (HIPS) (polybutadiene-gra/f-polystyrene) has improved toughness and processability. The impact strength of polyamides is improved by blending with an ethylene copolymer or ABS rubber. 

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