COMPOSITES
Particle-reinforced composites These are the cheapest and most widely used. They fall in two categories depending onthe size of the particles: • large-particle composites, which act by restraining the movement of the matrix, if well bonded. • dispersion-strengthened composites, containing 10-100 nm particles, similar to what was discussed under precipitation hardening. The matrix bears the major portion of the applied load and the small particles hinder dislocation motion, limiting plastic deformation. Large-Particle Composites Properties are a combination of those of the components. The rule of mixtures predicts that an upper limit of the elastic modulus of the composite is given in terms of the elastic moduli of the matrix (Em) and the particulate (Ep) phases by: Ec = EmVm + EpVp where Vm and Vp are the volume fraction of the two phases. A lower bound is given by: Ec = EmEp / (EpVm…
Earth’s Atmosphere: Composition, Climate Weather
Astronauts aboard the International Space Station took this image showing Earth's atmosphere and moon on July 31, 2011. Earth is the only planet in the solar system with an atmosphere that…
Processing of Plastics
Injection moulding (United States Injection Molding) is a manufacturing technique for making parts from thermoplastic material. Molten plastic is injected at high pressure into a mold, which is the inverse of the desired shape. The mold is made by a moldmaker (or toolmaker) from metal, usually either steel or aluminium, and precision-machined to form the features of the desired part. Injection moulding is very widely used for manufacturing a variety of parts, from the smallest component to entire body panels of cars. It is the most common method of production, with some commonly made items including bottle caps and outdoor furniture. The most commonly used thermoplastic materials are polystyrene (low-cost, lacking the strength and longevity of other materials), ABS or acrylonitrile butadiene styrene (a co-polymer or mixture of compounds used for everything from Lego parts to electronics housings), nylon (chemically resistant, heat-resistant, tough and flexible - used for combs), polypropylene (tough and flexible - used for containers), polyethylene, and polyvinyl chloride or PVC (more common in extrusions as used for pipes, window frames, or as the insulation on wiring where it is rendered flexible by the inclusion of a high proportion of plasticiser). Compression molding is a method of molding in which the molding material, generally preheated, is first placed in an open, heated mold cavity. The mold is closed with a top force or plug member, pressure is applied to force the material into contact with all mold areas, and heat and pressure are maintained until the molding material has cured. The process employs thermosetting resins in a partially cured stage, either in the form of granules, putty-like masses, or preforms. Compression molding is a high-volume, high-pressure method suitable for molding complex, high-strength fiberglass reinforcements. Advanced composite thermoplastics can also be compression molded with unidirectional tapes, woven fabrics, randomly orientated fiber mat or chopped strand. The advantage of compression molding is its ability to mold large, fairly intricate parts. Compression molding produces fewer knit lines and less fiber-length degradation than injection molding. Extrusion moulding is a manufacturing process used to make pipes, hoses, drinking straws, curtain tracks, rods, and fibres.The machine used to extrude materials is very similar to an injection moulding machine. A motor turns a screw which feeds granules of plastic through a heater. The granules melt into a liquid which is forced through a die, forming a long 'tube like' shape. The shape of the die determines the shape of the tube. The extrusion is then cooled and forms a solid shape. The tube may be printed upon, and cut at equal intervals. The pieces may be rolled for storage or packed together. Shapes that can result from extrusion include T-sections,…
Factors that Influence the Mechanical Properties of Polymers
The tensile modulus decreases with increasing temperature or diminishing strain rate. Obstacles to the steps mentioned in strengthen the polymer. Examples are cross-linking (aligned chains have more van der Waals inter-chain bonds) and a large mass (longer molecules have more inter- chain bonds). Crystallinity increases strength as the secondary bonding is enhanced when the molecular chains are closely packed and parallel. Predeformation by drawing, analogous to strain hardening in metals, increases strength by orienting the molecular chains. For undrawn polymers, heating increases the tensile modulus and yield strength, and reduces the ductility - opposite of what happens in metals. Crystallization, Melting, and Glass Transition Phenomena Crystallization rates are governed by the same type of S-curves we saw in the case of metals Nucleation becomes slower at higher temperatures. The melting behavior of semicrystalline polymers is intermediate between that of crystalline materials (sharp density change at a melting temperature) and that of a pure amorphous material (slight change in slope of density at the glass-transition temperature). The glass transition temperature is between 0.5 and 0.8 of the melting temperature. The melting temperature increases with the rate of heating, thickness of the lamellae, and depends on the temperature at which the polymer was crystallized. Melting involves breaking of the inter-chain bonds, so the glass and melting temperatures depend on: • chain stiffness (e.g., single vs. double bonds) • size, shape of side groups • size of molecule • side branches,…
Internal Structure
Very few questions are asked in prelims from this section. You can ignore these concepts if you found them too scientific. Sun · Age => 4.6 billion years · Diameter => 13,91,785…
Deformation of Polymers
Many semicrystalline polymers have the spherulitic structure and deform in the following steps : • elongation of amorphous tie chains • tilting of lamellar chain folds towards the tensile direction • separation of crystalline block segments • orientation of segments and tie chains in the tensile direction The macroscopic deformation involves an upper and lower yield point and necking. Unlike the case of metals, the neck gets stronger since the deformation aligns the chains so increasing the tensile stress leads to the growth of the neck.
Mechanical behavior of polymers
The description of stress-strain behavior is similar to that of metals, but a very important consideration for polymers is that the mechanical properties depend on the strain rate, temperature, and environmental conditions. The stress-strain behavior can be brittle, plastic and highly elastic (elastomeric or rubberlike). Tensile modulus (modulus) and tensile strengths are orders of magnitude smaller than those of metals, but elongation can be up to 1000 % in some cases. The tensile strength is defined at the fracture point and can be lower than the yield strength. Mechanical properties change dramatically with temperature, going from glass-like brittle behavior at low temperatures (like in the liquid-nitrogen demonstration) to a rubber-like behavior at high temperatures .In general, decreasing the strain rate has the same influence on the strain-strength characteristics as increasing the temperature: the material becomes softer and more ductile.
The Inner and Outer Planets in Our Solar System
In our Solar System, astronomers often divide the planets into two groups — the inner planets and the outer planets. The inner planets are closer to the Sun and are…
Common plastics and their typical uses
Polyethylene (PE) wide range of uses, very inexpensive Polypropylene (PP) food containers, appliances Polystyrene (PS) packaging foam, food containers, disposable cups, plates and cutlery Polyethylene terephthalate (PETE) beverage containers Polyamide (PA) (Nylon) fibers, toothbrush bristles, fishing line Polyester (PES) fibres, textiles Polyvinyl chloride (PVC) plumbing pipes, flooring, shower curtains, erotic clothing Polycarbonate (PC) compact discs, eyeglasses Acrylonitrile butadiene styrene (ABS) electronic equipment cases (e.g., computer monitors, printers, keyboards) Polyvinylidene chloride (PVDC) (Saran) food packaging Polytetrafluoroethylene (PTFE) (Teflon) heat resistant, low-friction coatings, used in things like frying pans and water slides Plastarch…
Planets in Our Solar System
There are more planets than stars in our galaxy. The current count orbiting our star: eight. The inner, rocky planets are Mercury, Venus, Earth and Mars. The outer planets are gas giants Jupiter and Saturn and ice giants Uranus and Neptune. Beyond Neptune, a…
