Plastics energy requirements

Substantial work on the appHcation of fracture mechanics techniques to plastics has occurred siace the 1970s (215—222). This is based on earlier work on inorganic glasses, which showed that failure stress is proportional to the square root of the energy required to create the new surfaces as a crack grows and iaversely with the square root of the crack size (223). For the use of linear elastic fracture mechanics ia plastics, certaia assumptioas must be met (224) (/) the material is linearly elastic (2) the flaws within the material are sharp and (J) plane strain conditions apply ia the crack froat regioa. 

Energy required to cause p/asfic deformation up to point of final fracture (plastic work at fracture)... 

Because most plastics may be fabricated in the melt and at quite low temperatures (e.g. 200°C) the energy requirements for processing are low. Since plastics generally have low densities, costs of transportation and general handling are also relatively low. 

Figure 9.3. Stress-strain curves for (a) rigid amorphous plastics material showing brittle fracture and (b) rubbery polymer. The area under the curve gives a measure of the energy required to break the...

Minimum energy required for piloted ignition of wood, melting of plastic tubing... 

Methods employed to determine the impact resistance of plastics include pendulum methods (Izod, Charpy, tensile impact, falling dart, Gardner, Dynatup, etc.) and instrumented techniques. In the case of the Izod test, what is measured is the energy required to break a test specimen transversely struck (the test can be done either with the specimen notched or unnotched). The tensile impact test has a bar loaded in tension and the striking force tends to elongate the bar (Chapter 5, Impact Strength). 

From a practical viewpoint toughness is readily understood, but technically there tends to be no scientific method of measuring it. One definition of toughness is simply the energy required to break the plastic. This... 

In case (4), potentially the full 80-i- MJ/kg which encompasses feedstock and the fairly high energy requirement for producing plastics can be recovered. However, here various complications arise ... 

When describing the effect of an external force, we must first define the force itself. A lay person s definition of a force is the amount of effort to get the desired effect. As scientists, we need a more precise definition of force. With a precise definition we can understand and quantify the effect of an applied force on a polymeric material. The mathematical definition of force is the work (which is a form of energy) required to move an object over some distance. Another way to define a force is in terms of the acceleration it creates when applied to some object of a mass m. In our everyday experiences, the first explanation is a simple idea to relate to. When we push a stalled car we exert a force on it. We could easily quantify the force from the weight of the car, the slope of the hill it is sitting on, and how far we must push it. Once we begin to talk about forces in polymer systems, the ideas become a bit more complicated. For example, the force required to open a bag of candy is defined by the work required to deform the bag until it ruptures by overcoming the intermolecular forces which hold the plastic together. 

The power factor is the energy required for the rotation of the dipoles of a polymer in an applied electrostatic field of increasing frequency. Typical values vary from 1.5 x 10 for polystyrene to 5 x 10 for plasticized cellulose acetate. Values increase at Tg and because of the increased chain mobility gained so that Tg and Tm have been measured using differences in the power factor as temperature is increased. 

Processibility is dependent on the viscosity or plastic flow of the rubber compound,i.e., resistance to flow. Plasticity or viscosity determines the energy requirement of the rubber during milling, calendering or extrusion while the time to the onset of curing, i.e., scorch time, indicates the amount of heat history which can be tolerated before the rubber is converted from the plastic to the elastic state at which time processing becomes virtually impossible. 

The results shown in Table 1 clearly reveal that the fracture energies of even unmodified, simple epoxy polymers, i.e. about 100 to 300 J/m2, are at last one hundred times the energy required to break solely covalent bonds, i.e. lessthan 1 J/m2. This demonstrates that other energy absorbing processes, such as plastic deformation, must take place at the crack tip. 

In order to avoid defluidization in bubbling fluidized beds, the thickness of the fused plastic that coats the inert particles must be minimized by using sand/plastic ratio, which means a serious limitation in the yield of the reactor and a high-energy requirement in order to fluidized and then heat the large amount of sand. 

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