Barrier Brittle

Permeable seal/flow barrier BRITTLE DUCTILE... 

Evaporation Retardants. Small molecule solvents that make up the most effective paint removers also have high vapor pressure and evaporate easily, sometimes before the remover has time to penetrate the finish. Low vapor pressure cosolvents are added to help reduce evaporation. The best approach has been to add a low melting point paraffin wax (mp = 46-57° C) to the paint remover formulation. When evaporation occurs the solvent is chilled and the wax is shocked-out forming a film on the surface of the remover that acts as a barrier to evaporation (5,6). The addition of certain esters enhances the effectiveness of the wax film. It is important not to break the wax film with excessive bmshing or scraping until the remover has penetrated and lifted the finish from the substrate. Likewise, it is important that the remover be used at warm temperatures, since at cool temperatures the wax film may not form, or if it does it will be brittle and fracture. Rapid evaporation occurs when the wax film is absent or broken. 

Since this bloom is brittle, it is broken by flexing. Therefore, waxes only protect under static conditions. For serving conditions which involve continuous flexing, /j-phenylenediamines (A, A -alkyl-aryl derivatives) can be added. These chemical antiozonants scavenge the ozone before it reacts with the rubber. A barrier of ozonized products is created which protects both the rubber and antiozonant from further attack. However, p-phenylenediamines are staining compounds. Whenever colour is an important concern, blends of elastomers can be used elastomers loading should be higher than 30 phr to provide sufficient effectiveness. 

Similarly it seems that retained austenite may be beneficial in certain circumstances , probably because the austenite acts as a barrier to the diffusion of hydrogen, although in high concentrations (such as those obtained in duplex stainless steels) the austenite can also act as a crack stopper (i.e. a ductile region in the microstructure which blunts and stops the brittle crack). 

Non-flammable plasticisers, such as tri-tolyl phosphate, tri-xylene phosphate, or a number of different brominated plasticisers, produce a dense hard brittle carbon char after initial combustion which then acts as a barrier to exclude oxygen. 

In electronic applications, where it is common to deposit copper and/or copper alloy and tin in sequence, with a nickel diffusion barrier layer, 0.5 fim thick, between the layers present, no failure occurs. Without the nickel layers between bronze/-copper/tin layers themselves, for instance, intermetaUic brittle layer(s) and Kirkendall voids are formed, leading eventually to separation of the coated system and substrate. 

As we have seen, mobility of the molecules is one of the sources of ductility. However, if the mobility is obstructed by some barrier, an internal crack may form, and initiate crack propagation. In this way a ductile solid may become brittle. 

Edible moisture barriers usually include hpids. Because of their apolar nature, these hydrophobic substances are capable of forming a water-impervious structure and reduce efficiently the water transfer. However, lipid-based materials are most of the time brittle so they are frequently combined with proteins and/or polysaccharides to improve their mechanical and structural properties (Wu et al. 2002). Several reviews focussing specifically on edible moisture barriers (Debeaufort et al. 2000 Koelsch 1994) and/or lipid-based edible films have been published (Baldwin et al. 1997 Callegarin et al. 1997 Greener and Fennema 1992 Hernandez 1994 Quezada-Gallo et al. 2000). The most recent review on lipid-based moisture barriers is that of Morillon et al. (2002). 

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