Pressure-ruptured microcapsules

Microencapsulation is also useful in enhancing the performance of lubricating oils when used at the application site. Pressure-ruptured microcapsules containing lubricating oil are used in powder metallurgy. The requirements for the microcapsule shell wall is that it should have a high abrasion resistance, be able to withstand... 

The CB sheet is coated on the back with microcapsules 5 to 10 pm in diameter, in which leuco dye solution in a nonvolatile solvent is contained. The CF sheet is coated on the front with an acidic coreactant such as reactive clay, zinc salt of salicylic acid derivatives, zinc-modified phenolic resin, etc. On the application of pressure, the microcapsule is ruptured in the area delineated by the pressure pattern, and the leuco dye solution is thereby transferred to the CB sheet to bring about a color-forming reaction with acidic coreactant resulting in a distinct image on the surface of the CF sheet. By inserting a middle sheet (or sheets) called the CFB sheet, which is coated on the front and back with acidic coreactant and microcapsules, respectively, multiple copies can be obtained. 

In pressure-sensitive printing technology the color former is dissolved in a solvent and encapsulated.44 The use of pressure (pen, typewriter key) ruptures microcapsules containing the color former, which generates color upon... 

Even under high pressure, the microcapsules can swell. However, the changes in pressure and volume were not sufficient to rupture the matrix resin. Therefore, both the expansion of the microcapsules and the softening of the matrix resin are indispensable for the expansion of the adhesive. 

Mathiowitz [35 0] realizes reservoir-type delivery systems recurring to a photochemical control. Microcapsules, built up by interfacial polymerization of polyamide, also contain azobisisobutyronitrile, a substance that emanates nitrogen due to a photochemical action. Accordingly, after exposition to light, microcapsules internal pressure increases (as a result of nitrogen release) until membrane rupture and consequent contents release. 

The abundance of natural and man-made polymers provides a wider scope for the choice of shell material, which may be made permeable, semi-permeable or impermeable. Permeable shells are used for release applications, while semi-permeable capsules are usually impermeable to the core material but permeable to low molecular-weight liquids. Thus, these capsules can be used to absorb substances from the environment and to release them again when brought into another medium. The impermeable shell encloses the core material and protects it from the external environment Hence, to release the content of the core material the shell must be ruptured by outside pressure, melted, dried out dissolved in solvent or degraded under the influence of light (see Chapter 7). Release of the core material through the permeable shell is mainly controlled by the thickness of the shell wall and its pore size. The dimension of a microcapsule is an important criterion for industrial applications the following section will focus on spherical core-shell types of microcapsules (Fig. 1.8). 

The reason for choosing polyamide as the encapsulating polymer is that it has low permeability to nitrogen, is transparent to near-UV light, and is chemically stable to the photoeliminator used. The mechanism of microcapsule rupture is due to AIBN, on UV irradiation, producing N2 gas which leads to a build-up of pressure within the capsule. 

Carbonless Copy Paper, in carbonless copy paper, also referred to as pressure-sensitive record sheet, an acid-sensitive dye precursor, such as crystal violet lactone or AT -benzoylleucomethylene blue, is microencapsulated with a high boiling solvent or oil within a cross-linked gelatin (94,100,101) or in synthetic mononuclear microcapsules. Microcapsules that have a starch binder are coated onto the back of the top sheet. This is referred to as a coated-back (CB) sheet. The sheet intended to receive the image is treated on the front (coated-front (CF)) with an acid. When the top sheet is mechanically impacted, the dye capsules rupture and the dye solution is transferred to the receiving sheet where the acid developer activates the dye. 

Pressure-indicating film can be used to verify the amount of load applied on the package. The film contains tiny microcapsules of colored dye. These microcapsules rupture depending on the amount of pressure applied on the film, producing a pressure footprint. The results are not extremely precise, but can be used to estimate the amount of load applied, as a first pass approximation. 

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