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Chapter 15: Problem 43

Which of the following polyethylene thin films would have the better mechanical characteristics: (1) formed by blowing, or (2) formed by extrusion and then rolled? Why?

Short Answer

Expert verified
Answer: The blowing process generally results in better overall mechanical characteristics for polyethylene thin films, as it produces balanced and more uniform properties in both the machine and transverse directions. However, the extrusion and rolling process might be more suitable for specific applications that demand higher strength and stiffness in the machine direction.

Step by step solution

01

Understanding the blowing process

In the blowing process, a thermoplastic material, in this case polyethylene, is melted and extruded through an annular die to form a continuous tube. The tube is then inflated with air to expand it to the desired size and shape, followed by cooling and solidification. This process produces films with a uniform thickness and balanced material properties in both the machine (longitudinal) direction and the transverse direction.
02

Understanding the extrusion and rolling process

In the extrusion and rolling process, polyethylene is melted and extruded through a flat die, producing a much thicker sheet. The sheet is then passed through a series of heated rollers where it gets progressively thinner until it reaches the desired thickness. This process can produce very thin films with a high degree of accuracy and consistency due to the control provided by the rollers. However, the material properties can be somewhat different in the machine direction and transverse direction due to orientation and processing stresses.
03

Comparing mechanical characteristics

In the blowing process, the film is more balanced in its mechanical properties due to the simultaneous stretching in both directions. This means it has better strength, elongation, and toughness in both the machine and transverse directions. On the other hand, the extrusion and rolling process may result in a film having higher strength and stiffness in the machine direction, but lower toughness and elongation in the transverse direction. This anisotropy may be undesirable in certain applications.
04

Conclusion

Based on the comparison between the two processes, we can conclude that the polyethylene thin film formed by the blowing process would generally have better overall mechanical characteristics since it has balanced and more uniform properties in both directions. However, it is essential to consider specific application requirements as the extrusion and rolling process might be more suitable for applications that demand higher strength and stiffness in the machine direction, at the cost of reduced toughness and elongation in the transverse direction.

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Most popular questions from this chapter

Which of the following would you expect to be elastomers and which thermosetting polymers at room temperature? Justify each choice. (a) Linear and highly crystalline polyethylene (b) Phenol-formaldehyde (c) Heavily crosslinked polyisoprene having a glass-transition temperature of \(50^{\circ} \mathrm{C}\) \(\left(122^{\circ} \mathrm{F}\right)\) (d) Lightly crosslinked polyisoprene having a glass-transition temperature of \(-60^{\circ} \mathrm{C}\) \(\left(-76^{\circ} \mathrm{F}\right)\) (e) Linear and partially amorphous poly(vinyl chloride)

For each of the following pairs of polymers do the following: (1) state whether or not it is possible to determine whether one polymer has a higher melting temperature than the other; (2) if it is possible, note which has the higher melting temperature and then cite reason(s) for your choice; and (3) if it is not possible to decide, then state why. (a) Branched polyethylene having a numberaverage molecular weight of \(850,000 \mathrm{g} / \mathrm{mol}\); linear polyethylene having a number-average molecular weight of \(850,000 \mathrm{g} / \mathrm{mol}\) (b) Polytetrafluoroethylene having a density of \(2.14 \mathrm{g} / \mathrm{cm}^{3}\) and a weight-average molecular weight of \(600,000 \mathrm{g} / \mathrm{mol}\); PTFE having a density of \(2.20 \mathrm{g} / \mathrm{cm}^{3}\) and a weight-average molecular weight of \(600,000 \mathrm{g} / \mathrm{mol}\) (c) Linear and syndiotactic poly(vinyl chloride ) having a number-average molecular weight of \(500,000 \mathrm{g} / \mathrm{mol} ;\) linear polyethylene having a number-average molecular weight of \(225,000 \mathrm{g} / \mathrm{mol}\) (d) Linear and syndiotactic polypropylene having a weight-average molecular weight of 500,000 g/mol; linear and atactic polypropylene having a weight- average molecular weight of \(750,000 \mathrm{g} / \mathrm{mol}\)

Demonstrate, in a manner similar to Equation \(15.4,\) how vulcanization may occur in a chloroprene rubber.

The tensile strength and number-average molecular weight for two poly(methyl methacrylate) materials are as follows: $$\begin{array}{cc}\hline \begin{array}{c}\text { Tensile Strength } \\\\\text { (MPa) }\end{array} & \begin{array}{c}\text { Number-Average } \\ \text { Molecular Weight (g/mol) }\end{array} \\\\\hline 50 & 30,000 \\\150 & 50,000 \\\\\hline\end{array}$$ Estimate the tensile strength at a numberaverage molecular weight of \(40,000 \mathrm{g} / \mathrm{mol}\).

List two important characteristics for polymers that are to be used in fiber applications
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