Self-healing

For some applications, it is necessary to maintain a hydrophobic surface. If the surface is damaged then the recovery or restructuring is beneficial and is called self-healing. In this sense, PDMS is a smart material, in that it responds constructively to a change in the environment. 

Self-healing is often used in a broader sense, to mean reconstruction of the entire polymer instead of just its surface. A relevant example here is a PDMS elastomer that contains microencapsulated PDMS resin and microencapsulated cross linker. If this type of PDMS is damaged, both capsules rupture and the newly formed elastomer mends the damaged area. 

Microcrack formation and crack propagatiOTi are a common cause of material failure. To overcome this problem, supramolecular polymer networks have been developed that can self-heal on the basis of the dynamic nature of their reversible cross-links [268]. The healing can either occur in an autonomous fashion or upmi exposure to an external stimulus such as heat, light, pressure, or mechanical stress. 

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The wide range of soHd lubricants can generally be classified as either inorganic compounds or organic polymers, both commonly used in a bonded coating on a matching substrate, plus chemical conversion coatings and metal films. Since solid-film lubricants often suffer from poor wear resistance and inabihty to self-heal any breaks in the film, search continues for improved compositions. 

Several theories have appeared in the Hterature regarding the mechanism of protection by -PDA antiozonants. The scavenger theory states that the antiozonant diffuses to the surface and preferentially reacts with ozone, with the result that the mbber is not attacked until the antiozonant is exhausted (25,28,29). The protective film theory is similar, except that the ozone—antiozonant reaction products form a film on the surface that prevents attack (28). The relinking theory states that the antiozonant prevents scission of the ozonized mbber or recombines severed double bonds (14). A fourth theory states that the antiozonant reacts with the ozonized mbber or carbonyl oxide (3) in Pig. 1) to give a low molecular weight, inert self-healing film on the surface (3). 

The relinking (14) and self-healing film (3) theories require chemical interaction between the antiozonant and ozonized mbber. The evidence for these interactions is meager (35,36). Overall, there seems to be no clear evidence in the Hterature for PDA derivatives becoming attached to mbber chains as a result of ozone attack. Much fundamental work in this area remains to be done, however. It seems clear at this point that any antiozonant—mbber interaction must be much less important than the scavenging effect of the antiozonant. In summary, the scavenger model is beheved to be the principal mechanism of antiozonant action. Ozone—antiozonant reaction products form a surface film that provides additional protection (37). 

Shunt power capacitors of the self healing type for a.c. systems having rated voltage up to and including 1000 V 13340/1993 BS EN 608.31 -1/1998... 

Shunt power capacitors of non self-healing types up to and including 1000 V General performance, testing and rating. Safety requirements. Guide for installation and operation 13585-1/1994 BS EN 60931-1/1996... 

Systems prone to frequent voltage variations Capacitor elements are non-self-healing and are generally provided with internal fuses An inductor coil may not be necessary with such capacitors... 

The metallized film capacitors have the characteristic of self-healing. On a small dielectric failure the capacitor element is not rendered completely unserviceable. After clearing the fault, the affected capacitor element returns to the circuit and the capacitor unit functions normally. Only the punctured area is eliminated from the element and causes a negligibly small reduction in its capacitance value. Such a characteristic is termed self-healing and such capacitors are known as the self-healing type. 

For LT capacitor units up to 1000 V For both self-healing and non-self-healing types ... 

Shunt capacitors for a.c. power system rated voltage above 1000 V (non-self-healing type) Endurance testing 13925-2/1994 BS 7264-2/1990... 

Mtiking a ctipacitor element A critictil review of internally protected capacitor units Self-healing capacitors Making a capacitor unit from elements Making capacitoi banks from eapticitor units Rating... 

The ceramics SiC and Si3N4 do not share this problem. They oxidise readily (Table 21.1) but in doing so, a surface film of Si02 forms which gives adequate protection up to 1300°C. And because the film forms by oxidation of the material itself, it is self-healing. 

Self-healing Oxidation Reaction Activated carbon ... 

Braunelle, /. self-heal (Prunella vulgaria). braunen, v.t. brown dye brown bronze burnish. 

Brlih-messer, m. (Leather) barkometer. -wasser, n. water for scalding, briillen, v.i. roar, bellow, howl, brummen, v.i. hum, buzz, low, growl, grumble. Brunelle, /. self-heal (Prunella vulgaris). 

Notwithstanding the large amount of work on pure iron and binary alloys, it remains difficult to translate the results to commercially useful steels. It is believed, on the one hand, that effusion of carbon monoxide can cause non-healing Assures in the scale , and on the other, that silicon creates self-healing layers at the metal interface . ... 

Metals immersed or partly immersed in water tend to corrode because of their thermodynamic instability. Natural waters contain dissolved solids and gases and sometimes colloidal or suspended matter all these may affect the corrosive projjerties of the water in relation to the metals with which it is in contact. The effect may be either one of stimulation or one of suppression, and it may affect either the cathodic or the anodic reaction more rarely there may be a general blanketing effect. Some metals form a natural protective film in water and the corrosiveness of the water to these metals depends on whether or not the dissolved materials it contains assist in the maintenance of a self-healing film. 

A distinction must be made between a thick layer of deposit — whether of calcium carbonate or of other material—and a protective layer. The ideal protection in fact consists of layers of negligible thickness which do not impede water or heat flow and which are self-healing. This is difficult to... 

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