Liquid plastics compressibility

By compression-molding in heated molds it is possible to process composites containing large amounts of the liquid-phase components (up to 50 wt %). Liquid plasticizers are able to lower the polymer melting point and, consequently, that of the composite processing, together with the compaction pressure due to high fluidity of the composite melt. 

Disposable Infant Diapers. A basic disposable diaper consists of an absorbent core sandwiched between a liquid permeable top-sheet and an impermeable back-sheet (45). The top-sheet, next to the baby s skin, allows urine to flow through it into the core. The back-sheet, made of impermeable plastic, helps keep the bab s clothing dry. The core takes in the liquid, distributes it within the core and holds the liquid under compression from the baby. 

With PBAs the compressed gases often used are nitrogen or carbon dioxide. These gases are injected into a plastic melt in the screw barrel under pressure (higher than the melt pressure) and form a cellular structure when the melt is released to atmospheric pressure or low pressure. The volatile liquids are usually aliphatic hydrocarbons, which may be halogenated, and include materials such as carbon dioxide, pentane, hexane, methyl chloride, etc. Polychlorofluoro-carbons were formerly used but they have now been phased out due to environment problems. 

Figure 12.6 illustrates the outline of such an analysis. An automatic pipette extracts a preset volume of the liquid sample (or solution) from a cup presented to it on a turntable. The measured sample is mixed with the reagents in the appropriate proportions, and propelled through the instrument by the peristaltic proportioning pump. This pump operates by means of moving bars, attached to a chain drive, which sequentially compress the plastic sample and reagent tubes to drive the liquids forward through the instrument. The incorporation of a succession of air bubbles at... 

Total protection from liquids and vapors can be provided by an impervious hood incorporating a glass or plastic window. Unfortunately, these are very hot and may require additional ventilation directly from an air line, or perhaps through a device known as a vortex tube that converts supplied compressed air to either a cool or warm flow as required. 

DSC studies have shown that multifunctional monomers react quickly to form densely crosslinked networks from liquid monomer solutions. However, even a small amount of unreacted monomer can effectively plasticize a crosslinked network, rendering it more pliable. Eor this reason, mechanical analysis was combined with DSC studies to characterize the physical changes occurring in the proposed dimethacrylate system as polymerization proceeds. Static compression tests (Perkin-Elmer, DMA7e) were completed on disks (d = 11.5 mm, t = 1.7) immediately after they were irradiated for varied times. 

A formed laminate is made by relative movement of first and second contoured mould surfaces toward each other to compress a flexible sheet and vertically spaced film with a foamable liquid mixture therebetween that has substantially completed expansion but not yet substantially cured so as to form the flexible sheet and provide a collapsed foam layer bonded to the sheet. The flexible sheet is preferably porous cloth, vinyl or leather. A film which is preferably a nonadherent plastic such as PE is preferably positioned in a vertically spaced relationship with the flexible sheet either above or below the sheet with the foamable liquid mixture received therebetween for the expansion prior to the compression that provides the formed laminate. The formed laminate... 

In pharmaceutical applications, sorbitol is used as a tablet diluent in wet granulation or dry compression formulations. It is commonly used in chewable tablets because of its sweet taste, and it is also used as a plasticizer for gelatin in capsule formulations. Sorbitol is utilized in sugar-free liquid preparations and as a stabilizer for drug, vitamin, and antacid suspensions. When it is used in syrups, crystallization around bottle caps is prevented. 

Rheometric Scientific markets several devices designed for characterizing viscoelastic fluids. These instruments measure the response of a liquid to sinusoidal oscillatory motion to determine dynamic viscosity as well as storage and loss moduli. The Rheometric Scientific line includes a fluids spectrometer (RFS-II), a dynamic spectrometer (RDS-7700 series II), and a mechanical spectrometer (RMS-800). The fluids spectrometer is designed for fairly low viscosity materials. The dynamic spectrometer can be used to test solids, melts, and liquids at frequencies from 10-3 to 500 rad/s and as a function of strain amplitude and temperature. It is a stripped down version of the extremely versatile mechanical spectrometer, which is both a dynamic viscometer and a dynamic mechanical testing device. The RMS-800 can carry out measurements under rotational shear, oscillatory shear, torsional motion, and tension compression, as well as normal stress measurements. Step strain, creep, and creep recovery modes are also available. It is used on a wide range of materials, including adhesives, pastes, mbber, and plastics. 

Urethanes are processed as rubber-like elastomers, cast systems, or thermoplastic elastomers. The elastomer form is mixed and processed on conventional mbber mills and internal mixers, and can be compression, transfer, or injection molded. The liquid prepolymers are cast using automatic metered casting machines, and the thermoplastic pellets are processed like all thermoplastic materials on traditional plastic equipment. The unique property of the urethanes is ultrahigh abrasion resistance in moderately high Shore A (75—95) durometers. In addition, tear, tensile, and resistance to many oils is very high. The main deficiencies of the urethanes are their resistance to heat over 100°C and that shear and sliding abrasion tend to make the polymers soft and gummy. 

A crystalline or semicrystalline state in polymers can be induced by thermal changes from a melt or from a glass, by strain, by oiganic vapors, or by liquid solvents (40). Polymer crystallization can also be induced by compressed (or supercritical) gases, such as C02 (41). The plasticization of a polymer by C02 can increase the polymer segmental motions so that crystallization is kinetically possible. Because the amount of gas (or fluid) sorbed into the polymer is a direct function of the pressure, the rate and extent of crystallization may be controlled by controlling the supercritical fluid pressure. As a result of this ability to induce crystallization, a history effect may be introduced into polymers. This can be an important consideration for polymer processing and gas permeation membranes. 

Another problem in the packaging of oral liquids is lack of cleanliness of the containers before filling. Fibers and even insects often appear as debris in containers, particularly in the plastic containers used for many of these products. Many manufacturers receive containers shrink-wrapped in plastic to minimize contamination from fiberboard cartons, and many manufacturers use compressed air to clean the containers. Vapors, such as oil vapors, from the compressed air have occasionally been found to present problems, and it is a good practice to use compressed gas from oil-free compressors. 

On initiation of deflagrating explosives, local, finite hotspots are developed either through friction between the solid particulates, by the compression of voids or bubbles in the liquid component, or by plastic flow of the material. This in turn produces heat and volatile intermediates which then undergo highly exothermic reactions in the gaseous... 

Most polymer products are offered for recycling in the form of mixed plastic waste (MPW). Compressed gases as solvents may offer advantages compared with liquid solvents in the necessary separations (e.g., to collect families of polymers out of MPW) because such solvents are easy to recover and, furthermore, they make it possible to use pressure as an additional separation parameter or to remove the organic additives of the polymers. Finally, MPW may be added as a polymer solution in a compressed gas to a feed of a Naphtha cracker, and thus be recycled to base chemicals. 

---