Toughening agent

Many commercial heat curing paste adhesives contain additives which allow for the separation of microscopic rubber particles before or during the cure cycle. Medium to high molecular weight butadiene-acrylonitrile copolymers were first used but these additives caused high viscosity and stringiness before cure. 

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Mechanical Properties. Properties of typical grades of PBT, either as unfiUed neat resin, glass-fiber fiUed, and FR-grades, are set out in Table 8. This table also includes impact-modified grades which incorporate dispersions of elastomeric particles inside the semicrystalHne polyester matrix. These dispersions act as effective toughening agents which greatly improve impact properties. The mechanisms are not fiiUy understood in all cases. The subject has been discussed in detail (171) and the particular case of impact-modified polyesters such as PBT has also been discussed (172,173). 

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. 

Interestingly, this same effect has been observed for the addition of a rubber toughening agent to ethyl cyanoacrylate-based adhesives, as was reported previously. The rubber must contain enough latent acid functionality on the polymer backbone or in an additive to inhibit the thermally activated decomposition of the alkyl cyanoacrylate adhesive polymer. 

Sulfones - Although thermosets as exemplified by the cured epoxies, cyanates and blsmaleimides are high modulus materials, they are brittle when void of toughening agents (e.g. rubbers or thermo-... 

Most epoxy formulations contain diluents, fillers or reinforcement materials, and toughening agents. Diluents may be reactive (mono- and diepoxides) or nonreactive (di-n-butyl phthalate). Toughening (flexibilizing) agents such as low-molecular-weight polyesters or... 

In the same way, ABS acts as a toughener for vinyl ester resins (34). Assessment of the mechanical properties suggest that a chemical reaction may have occurred between the constituents of the blends. The blends are intended as a toughening agent for interlayer toughened vinyl ester/glass composite materials. 

Semicrystalline polyamide fine powders have been used as toughening agents for epoxy networks. The powders can be obtained by grinding granules, or directly by anionic polymerization of lactams, 6 or 12, in an organic solvent from which the formed semicrystalline polymer precipitates. Microporous powders with an average particle size in the range of 10 pm and a narrow particle-size distribution, are obtained. 

MISCELLANEOUS TOUGHENING AGENTS 13.6.1 Use of Liquid-Crystalline Polymers (LCP)... 

Zhan, G.D., Kuntz, J.D., Wan, J. et al., Single-wall carbon nanotubes as attractive toughening agents in alumina-based nanocomposites, Nature Mater, 2003, 2 38. 

SEM image of the fracture surface of 5.7 vol% SWCNT-Fe-Al203 composite densified by spark plasma sintering (SPS) of a mixture of nanometric alumina and ropes of SWCNTs. Reprint from Nature Materials, No. 2, 2002, pp. 38-42, Zhan G.-D., Kuntz J.D., Wan J. and Mukherjee A.K., Single-wall carbon nanotubes as attractive toughening agents in alumina-based nanocomposites, with the permission of Nature Materials and of the authors (http // www.nature.com/nmat/index.html). 

The technical and commercial success of high impact polystyrene (HIPS) and acrylonitrile-butadiene-styrene (ABS) has led to a widespread research program on the use of rubbers as toughening agents for plastics. There is now an impressive 11st of rubber-toughened polymers including both amorphous and... 

The catalyst does not make up part of the final epoxy network structure or have a significant effect on the final properties of the cured resin. Thus, the final cured properties of the epoxy system are primarily due to the nature of the epoxy resin alone. Homopolymerization normally provides better heat and environmental resistance than polyaddition reactions. However, it also provides a more rigidly cured system, so that toughening agents or flexibilizers must often be used. In adhesive systems, homopolymerization reactions are generally utilized for heat cured, one-component formulations. 

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 ... 

ATBN liquid toughening agents are synthesized by reacting a CTBN polymer with an amine. For example, n-aminoethylpiperazine (AEP) will give a low-molecular-weight,... 

Various kinds of engineering thermoplastics such as polyether sulfone, polyether imide, polyaryl ether ketone, and polyphenylene oxide have been studied as toughening agents for... 

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. 

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