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Unit_8_-_Plastic_Packaging (5)

Unit_8_-_Plastic_Packaging (5) - this process is what we...

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Remember that when we talked about the manufacturing of paper, the fibers tend to line up in the machine direction, and that consequently the MD and CD properties of the paper differ from each other. The same thing happens when we process plastics. Because the plastic molecules are long and skinny, the forces exerted on them when they flow tend to line them up in the direction of the flow, just like it happens with paper fibers; however, plastic molecules are not as rigid as the paper fibers, so when the motion stops, they tend to curl up again, and lose their orientation. The mobility of the molecules is strongly affected by temperature, as energy is required to do this “curling up” (or relaxation, as it is called). Also, with plastic, unlike with paper, we can get the molecules to stretch out by literally stretching the solid material;
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Unformatted text preview: this process is what we usually mean when we say orientation. Biaxial orientation is orientation along two perpendicular directions. Uniaxial orientation is orientation in one direction. If we want uniaxially oriented film, we stretch it in the machine direction (MD). That causes the polymer molecules to line up in the direction of the stretch, and results, for example, in an increase in MD tensile strength. However, we pay a price – the CD strength will decrease. So, if we want to improve strength (or other properties) in both the MD and CD, we can use biaxial orientation. Then, the molecules tend to line up in the plane of the film, and strength improves in both these directions. Film Casting...
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