Fibrous additives

The Tg value is reduced by the addition of moderate amounts of plasticizers (additive which reduces intermolecular forces) and is sometimes even increased by the addition of small amounts of plasticizers (antiplasticization) and optimum amounts of fillers (usually a relatively inert material used as the discontinuous phase of a composite) and reinforcements (materials such as fibrous additives which give increased strength to a polymer). 

The mixing sections are used for distributing liquid, powdered and fibrous additives. These sections are composed of separate or several successively arranged toothed discs differing, as a rule, in their number, width and tooth form. 

Experience in compounding rubber established many of the principles of compounding plastics (especially PVC), in particular the mechanism of interface adhesion between polymer matrix and particulate and fibrous additives, and the process of compounding materials of differing size, shape, and chemical composition. 

Fibrous additives are used in polymers to improve the mechanical properties, notably the tensile strength, tensile modulus, toughness, creep and resistance to impact. They may themselves be polymeric, carbonaceous, metallic, glassy or ceramic in nature. Their importance derives particularly from their dimensional anisotropy and in many cases also from the anisotropy of their mechanical properties. 

Polymeric additives for concrete have been described (52). These are fabricated from a poly(oxymethylene) (POM) copolymer. POM copolymers can be utilized to form fibrous additives for concrete, i.e., microfibers or macrofibers. The POM copolymers can also include chemical groups, e.g., end groups or pendant groups that can increase the polarity of the POM and thus increase the hydrophilic-ity of the formed fibers, which can improve miscibility of the fibers in wet concrete. The chemical groups of the POM copolymers can bond with components of the concrete or can hydrolyze to form groups that are bonding with some components of the concrete. 

Fibrous additives are generally introduced to the compounding extruder with loss-in-weight single- or double-screw feeders that feed into the side of the compounding extruder at a point the resin is already melted. The feed system must minimize glass movement to avoid fiber attrition. Vibratory trays should not be used for fibrous additives. 

Feeder Operation It is common to sequence the addition of SPS ingredients to the compounding extruder. The pellets, crumb, oil, and powders are added at the feed funnel of the extruder. Glass fiber and fibrous fillers are fed through the side of the extruder barrel at the point the resin is melted and well mixed with the other ingredients that were introduced at the feed funnel. This is done to avoid excessive attrition of the fibrous additives. The amount of material that can be added to the extruder is dependent on the torque capabilities of the extruder. 

The beater additive process starts with a very dilute aqueous slurry of fibrous nitrocellulose, kraft process woodpulp, and a stabilizer such as diphenylamine in a felting tank. A solution of resin such as poly(vinyl acetate) is added to the slurry of these components. The next step, felting, involves use of a fine metal screen in the shape of the inner dimensions of the final molded part. The screen is lowered into the slurry. A vacuum is appHed which causes the fibrous materials to be deposited on the form. The form is pulled out after a required thickness of felt is deposited, and the wet, low density felt removed from the form. The felt is then molded in a matched metal mold by the appHcation of heat and pressure which serves to remove moisture, set the resin, and press the fibers into near final shape (180—182). 

Some common flake-shaped LCMs consist of shredded cellophane and paper, mica (qv), rice hulls, cottonseed hulls, or laminated plastic. These materials He flat across the opening to be sealed or are wedged into an opening such as a fracture. Some are sufficiently strong to withstand considerable differential pressure, whereas others are weak and the seal may be broken easily. Weaker flake materials typically are used near the surface or in combination with fibrous or granular additives. 

The tensile and flexural properties as well as resistance to cracking in chemical environments can be substantially enhanced by the addition of fibrous reinforcements such as chopped glass fiber. Mechanical properties at room temperature for glass fiber-reinforced polysulfone and polyethersulfone are shown in Table 5. 

Fibrous stmctures represent a grain refinement of columnar stmcture. Stress-reHeving additives, eg, saccharin or coumarin, promote such refinement, as do high deposition rates. These may be considered intermediate in properties between columnar and fine-grained stmctures. 

In addition to the three principal polymorphs of siUca, three high pressure phases have been prepared keatite [17679-64-0] coesite, and stishovite. The pressure—temperature diagram in Figure 5 shows the approximate stabiUty relationships of coesite, quart2, tridymite, and cristobaUte. A number of other phases, eg, siUca O, siUca X, sihcaUte, and a cubic form derived from the mineral melanophlogite, have been identified (9), along with a stmcturaHy unique fibrous form, siUca W. 

Historically, strontium metal was produced only in very small quantities. Rapid growth of metal production occurred during the late 1980s, however, owing to use as a eutectic modifier in aluminum—silicon casting alloys. The addition of strontium changes the microstmcture of the alloy so that the siUcon is present as a fibrous stmcture, rather than as hard acicular particles. This results in improved ductility and strength in cast aluminum automotive parts such as wheels, intake manifolds, and cylinder heads. 

Organic clutch materials contain continuous-strand reinforcements in addition to fibrous reinforcements. These include cotton (primarily for processing), other organic yams, carbon—graphite yam, and asbestos yam, and brass wire or copper wire for high burst strength. 

The fundamental goal in the production and appHcation of composite materials is to achieve a performance from the composite that is not available from the separate constituents or from other materials. The concept of improved performance is broad and includes increased strength or reinforcement of one material by the addition of another material. This is the well-known purpose in the alloying of metals and in the incorporation of chopped straw into clay for bricks by the ancient Egyptians and plant fibers into pottery by the Incas and Mayans. These ancient productions of composite materials consisted of reinforcing britde materials with fibrous substances. In both cases the mechanics of the reinforcement was such as to reduce and control the production of cracks in the brittle material during fabrication or drying (2). 

Vibrating-conveyor dryers are suitable for free-flowing solids containing mainly surface moisture. Retention is limited by conveying speeds which range from 0.02 to 0.12 m/s. Bed depth rarely exceeds 7 cm, although units are fabricated to carry 30- to 46-cm-deep beds these also employ plate and pipe coils suspended in the bed to provide additional heat-transfer area. Vibrating dryers are not suit le for fibrous materials which mat or for sticky sohds which may ball or adhere to the deck. 

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