Cavitation during Tensile Deformation of Polypropylene (original) (raw)
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Colloid and Polymer Science, 2012
The cavitation phenomenon was studied in isothermally and non-isothermally crystallized polypropylene and high-density polyethylene. I found that nano-voids were not present in the crystallized samples, but were formed during their tensile deformation. The process of cavitation was initiated before reaching the yield point. The ellipsoidal voids were initially elongated perpendicularly to the deformation direction, but if the polymer (i.e. HDPE) was able to deform beyond the yield then reorientation of voids into the deformation direction was observed at local strains of 100-200 %. This behaviour was similar to that observed previously in the samples crystallized without an exact control of solidification conditions. The calculations of Guinier's radius showed that voids in deformed PP samples, were characterized by the gyration radii of 28-50 nm. Smaller voids were observed in polyethylene. The scale of cavitation during deformation, studied on the example of polyethylene, depended on the preceding crystallization process and was most intensive for the specimens crystallized at the highest temperature of 125°C.
Cavitation during deformation of polymers on the example of polypropylene
Journal of Applied Polymer Science, 2012
Cavitation during uniaxial deformation of isotactic polypropylene (PP) was studied in samples having different amounts of a and b crystals. It has been shown that deformation is always accompanied by neck formation. Because the very beginning the plastic deformation is accompanied by cavitation. The scale of cavitation, determined with the use of X-ray scattering and volume measurements increases with an increase of the content of b phase in the examined samples. Cavities change their shape with deformation-at first they are elongated perpendicularly to the direction of stretching, next, at the strain of 1.5, they reorient in the direction of deformation, which is connected with changes in the surrounding lamellar structure. The calculations of Guinier's radius showed that two populations of voids are present in deformed PP samples, characterized by the gyration radii of 5 and 13 nm for small deformations. Estimations based on the volume increase and sizes of cavities indicate that typically more than one cavity are present in amorphous layers adhering to lamella surfaces. V
Thermovision studies of plastic deformation and cavitation in polypropylene
Mechanics of Materials, 2013
The relations between tensile mechanical properties, associated cavitation and generation of heat were analyzed for polypropylene, PP, samples differing in thickness and gauge shape. Thermovision camera was applied to measure the surface temperature of samples during deformation. It was shown that a rapid increase of the temperature occurs during plastic deformation and the highest temperature zone is located at the front of neck, propagating through the sample. Thermal effects were larger in specimens which cavitated during drawing. Cavities cause stronger deformation and disruption of nearby lamellae that generate additional heating in cavitating samples, in addition to ongoing regular crystallographic slips. In thick injection molded PP samples the surface temperature increases even to 80-110°C while in thinner samples larger heat dissipation/heat generation ratio does not allow so large temperature increase. Strain rate jump experiments of cavitating samples showed that a plastic flow stress for higher strain rate is lower than for lower strain rate. Simultaneous measurement of sample surface temperature indicated that the effect is caused by the temperature increase. If the ability of a polymer to cavitation was reduced by infusion of a penetrant to the amorphous phase then the strain rate jump experiment showed the expected relation: for higher strain rate one observes higher plastic flow stress. In such a case, the influence of temperature increase is minor and it is the change in the deformation rate of the sample that plays a dominant role.
Cavitation and morphological changes in polypropylene deformed at elevated temperatures
Journal of Polymer Science Part B: Polymer Physics, 2010
Polypropylene thick films were subjected to tensile drawing at various temperatures from the room temperature to 100 o C. Morphological alterations during drawing were followed by WAXS, SAXS, SEM of sectioned and etched samples, volume strain measurement, light transparency measurement at various level of strain. The morphological observations were paralleled with stress-strain determination. Samples drawn at 25 and 40 o C undergo severe cavitation contributing to their volume increase up to 90-95%. The volume increase contributes greatly to the engineering strain. Polypropylene drawn at 70 and 100 o C does not cavitate. At the strain up to 1.2 a high lamellae orientation is observed in SEM while the 2D WAXS patterns show in contrary circular diffraction rings indicating low orientation of crystals. The rotation of lamellae towards drawing direction is associated with reverse rotation of chains in crystals due to fine chain slips. These two rotations in opposite directions counterbalance resulting in a much weaker crystal orientation than expected from the SEM images. Non-cavitating samples retain their translucency up to a high strain.
Plastic Deformation of Crystalline Polymers: The Role of Cavitation and Crystal Plasticity
Macromolecules, 2005
Plane strain compression in a channel die is kinematically very similar to drawing; however, the possibility of void formation is limited due to a compressive component of stress. In drawing, voids were detected by small-angle X-ray scattering (SAXS) and density measurements in poly(methylene oxide) (POM), polypropylene (PP), and high-density polyethylene (HDPE), but no voiding was found in polyamide 6 (PA 6), low-density polyethylenes (LDPEs), and ethylene-octene copolymer (EOC). The slope and shape of the initial elastic part of true stress-true strain curves are similar in tension and in channel die compression. When drawn samples of POM, PP, HDPE, and PA 6 already show yielding, the channel die compressed samples still undergo elastic deformation to a much larger deformation and respond with a much larger stress. Channel die compressed POM, PP, HDPE, and PA 6 exhibit strong and rapid strain hardening up to 400 MPa in contrast to their behavior in tension. The difference in strain hardening is related to preservation of chain entanglement density in channel die compression and disentanglement in tensile drawing. True stress-true strain curves for polymers having crystals with low plastic resistance and not cavitating are very similar in channel die compression and in tension. In tensile drawing there is a competition between cavitation and activation of crystal plasticity. Cavitation occurs in polymers with crystals of higher plastic resistance, while plastic deformation of crystals in polymers with crystals of lower plastic resistance. The necessary conditions for cavitation and for plastic deformation of crystal are defined. They explain why the cavitation is observed in POM, PP, and HDPE but not in LDPEs. In PA 6 negative pressure causes cavitation but the cavities, due to their small sizes and healing action of surface tension, are unstable, close quickly, but leave the traces of a structural damage. A model of plastic deformation of crystalline polymers accounting for cavitation is outlined.
Cavitation during tensile drawing of annealed high density polyethylene
Polymer, 2010
The annealing of semicrystalline polymers usually leads to the crystal refinement and thickening. It appears that also the cavitation during tensile drawing is affected. In the uniaxially drawn high density polyethylene a massive cavitation was detected by X-ray scattering in samples previously annealed at 125 o C. The number of voids depends on the annealing time, while their size and orientation depends on the local strain. The cavitation resulted in 30% increase of volume for annealed samples, strained to 4-5. Cavitation and volume increase were not observed for small and intermediate strains if the polyethylene samples were not annealed. The decrease of drawing rate results in reducing cavitation and also the void stability: at the low strain rate voids were detected during tensile drawing but they disappeared after unloading the sample.
Cavitation during tensile drawing of semicrystalline polymers
Polimery, 2011
The article presents the state of knowledge on the phenomenon of cavitation observed during uniaxial stretching of semicrystalline polymers. Cavitation occurs when the stress at which the amorphous phase breaks is lower than the stress initiating plastic deformation of crystals. The text presents the process of plastic deformation, conditions in which polymers cavitate, with special attention devoted to plastic deformation and cavitation in polyethylene, polypropylene and polyamide. KAWITACJE PODCZAS JEDNOOSIOWEGO ROZCIĄGANIA POLIMERÓW CZĘŚCIOWOKRYSTALICZNYCH Streszczenie W artykule opisany jest stan wiedzy na temat zjawiska kawitacji, obserwowanego podczas deformacji jednoosiowej polimerów częściowokrystalicznych. Do kawitacji dochodzi, gdy naprężenie przy którym rozerwaniu ulega faza amorficzna ma mniejszą wartość niż naprężenie, przy którym zaczyna się deformacja plastyczna kryształów. W tekście omówiono szczegółowo proces deformacji plastycznej i warunki które muszą zostać spełnione aby polimer kawitował. Szczególną uwagę zwrócono na mechanizmy deformacji plastycznej i kawitacji w polietylenie, polipropylenie oraz poliamidzie.
Polymers and Polymer Composites, 2006
This paper aims at identifying the main parameters that govern the tensile-impact strength of injection-moulded polypropylene. A Taguchi Design of Experiments (DOE) analysis has shown that the key parameters in both flow and transverse directions are the polymer melt and mould temperatures and the volumetric flow rate. The differences in high-speed mechanical behaviour have been explained on the basis of an investigation of the processing-induced morphology/tensile-impact behaviour relationship. The microstructure of parts manufactured under two extreme sets of moulding conditions has been analysed through-the-thickness by means of microscopy observations and by measurements of crystallinity, molecular orientation and thermal expansion. The impact brittleness originates from the skin layers, the major influential parameters being the skin/core ratio and the crystalline structure. The crack initiation energy increases with the oriented skin layer thickness, whereas the brittleness increases with the crystallinity level and the spherulite size.
Polymer, 2001
The -crystalline form of isotactic poly(propylene) (PP) has been long recognized to have a greater mechanical absorption capacity than the ␣-crystalline form. This is of major importance for improving impact properties and crack resistance of injection-molding parts. Unfilled PP samples together with calcium carbonate-filled PP samples having various /␣-phase ratios, with nearly constant morphological parameters, have been investigated from the standpoint of ductile crack propagation and impact behavior. The presence of the -crystalline phase turned out to improve both properties. The  spherulites are notably more prone to craze initiation than ␣ spherulites that display a propensity for cracking. Subsequent crack propagation appears to be faster in the latter ones. The plastic zone ahead from the crack tip broadens, and the specific plastic energy increases with increasing -phase content. The lower elastic limit of the  phase is likely to promote the early crazing. However, the suspected higher density of tie molecules in  spherulites provides more numerous and stiffer microfibrils. The impact strength of PP is also improved by the presence of  crystals as a result of greater energy-absorption capabilities. However, filled samples turned out insensitive to the  phase. A discussion is made about the origins of the -phaseinduced improvement of the mechanical properties. The possible role of the  3 ␣ transition is also explained.
Tensile behaviour of isotactic polypropylene modified by specific nucleation and active fillers
European Polymer Journal, 2004
Commercial-grade polypropylene was modified with a specific nucleation agent based on an amide of dicarboxylic acid, which promotes crystallization predominantly in the b-phase. The resulting material was used as a matrix for composites containing 10%, 20%, and 30% (by weight) of different calcium carbonate fillers. These fillers differed in particle size and surface treatment. The b-phase content, morphology and tensile mechanical properties were investigated. A distinct b-nucleation activity was found with surface-treated fillers; nevertheless, to obtain stiff and reasonably ductile composite materials, a matrix containing a critical nucleant concentration (0.03 wt%) was necessary.