Polymer films fracture stress

In Fig. 13.11, the comparison of experimental and calculated according to the Eq. (13.16) polymer films fracture stress values is adduced, which shows a good correspondence of theory and experiment. 

Figure 77 demonstrates different creep behavior in the narrow deformation intervals for ultimately drawn fiexible-chain and rigid-chain polymers, melt-crystallized and gel-cast UHMWPE films, and poly(paraphenylene terephthalamide) (PPTA) fibers (Kevlar 49, DuPont). Tensile stresses equal to a half of fracture stress CTf were applied. Total deformation to break was equal to 25%, 13%, and 1.32%, respectively, in these samples. To see better a difference in the development of creep process in the investigated polymers, the inserts in Fig. 77 are presented for very narrow deformation intervals. 

For the considered polymer films two sources of decrease can be distinguished mechanical stress concentration at stable crack tip and an-harmonicity local splash owing to material structure modification in pre-fracture zone - ZD. The first factor can be taken into account by stress concentration coefficient A" introduction (see the Eq. (5.10)) [7] ... 

As oriented polymers studies showed [20], for them the value v at drawing ratio growth reached the magnitude -0.425 very fast and further remains practically constant. Therefore, calculation with the Eq. (13.14) using gives the value y = 7.03. Thus, the theoretical estimation of film samples fracture stress o can be fulfilled as follows [21] ... 

The experimental confirmation of this postulate is adduced in Fig. 13.15 for PASF film sample, prepared from polymer solution in methylene chloride (the experimental value = 1.82, the theoretical one = 1.75). As one can see, the fracture stress growth at sharp notch length a enhancement is observed. 

Stress-strain measurements are also a useful tool for studying film formation in polymer films. Such an investigation, in which the process of polymer chain inter-diffusion in n-butyl methacrylate films was followed by monitoring the films work of fracture, has been reported elsewhere [28]. 

Polyamides, like other macromolecules, degrade as a result of mechanical stress either in the melt phase, in solution, or in the soHd state (124). Degradation in the fluid state is usually detected via a change in viscosity or molecular weight distribution (125). However, in the soHd state it is possible to observe the free radicals formed as a result of polymer chains breaking under the appHed stress. If the polymer is protected from oxygen, then alkyl radicals can be observed (126). However, if the sample is exposed to air then the radicals react with oxygen in a manner similar to thermo- and photooxidation. These reactions lead to the formation of microcracks, embrittlement, and fracture, which can eventually result in failure of the fiber, film, or plastic article. 

Tensile Yield Stresses of Cast Films. At room temperature all of the BPFC-DMS polymers investigated (with one exception) reached their yield stresses before fracturing. BPF polycarbonate on the other hand is brittle, breaking at about 11,000 psi. Traces of residual chloroform make the homopolymer ductile however the yield stress decreased linearly with chloroform content. Extrapolation of these results to a dry polymer gives a yield stress of 14,000 psi. 

The discussion of mechanical properties comprises the various contributions of elastic, viscoelastic and plastic deformation processes. Often two characteristic stress levels can be defined in the tensile curve of polymer fibers the yield stress, at which a significant drop in slope of the stress-strain curve occurs, and the stress at fracture, usually called the tensile strength or tenacity. In this section the relation is discussed between the morphology of fibers and films, made from lyotropic polymers, and their mechanical properties, such as modulus, tensile strength, creep, and stress relaxation. 

Lubricants are used with polymers for two main needs external lubrication and internal lubrication. External lubrication reduces friction between the polymer and the extrusion hardware, such as on the internal flow surfaces of the die. For example, lubricants can help eliminate melt fracture of blown film by reducing stress on the polymer as it passes through the die. Additionally, die drool (or die lip buildup) has been reduced by the use of lubricants. Internal lubrication reduces the friction between flowing polymer molecules, effectively reducing melt viscosity. Use of internal lubricants can reduce the power consumption required for polymers that are difficult to process. Some common lubricants are metal stearates and paraffin waxes. 

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