Morphology and mechanical behavior of polypropylene hot plate welds

Polypropylene tensile bars were hot plate welded and analyzed by microscopy and mechanical tests. The welding process caused significant modifications to the microstructure and shape of the bars at the weld zone. The weld morphology was affected by both the welding parameters and the thermal history of the components. Oriented textures, coarse spherulites, voids or sharp notches occurring at the beads are the key morphological factors that affect the weld quality. Weld performance is dependent upon the type of testing method used. A tensile impact test has been shown to be potentially useful for quality control, as the fracture behavior is affected by the overall morphoiogy.     

Vibration welding of thermoplastics. Part III: Strength of polycarbonate butt welds

In vibration welding of thermoplastics, frictional work done by vibrating two parts under pressure, along their common interface, is used to generate heat to effect a weld. The main process parameters are the weld frequency, the amplitude of the vibratory motion, the weld pressure, and the weld time or weld penetration; The effects of these parameters on weld quality were systematically studied by first butt-welding polycarbonate specimens under controlled conditions over a wide range of process parameters, and by then determining the strengths and ductilities of these welds by tensile tests, A significant result is the apparent existence of a weld-penetration threshold above which high weld strengths are attained, but below which the strength drops off. Under the right conditions, the strengths of polycarbonate butt welds are shown to equal the strength of the virgin polymer.     

The impact strength of butt welded vibration welds related to microstructure and welding history

A study has been made of vibration butt welds between plaques of polypropylene. The quality of the welds, as determined by impact tests, has been examined as a function of the welding variables: pressure and vibration amplitude. In addition, the microstructure of the welds has also been examined, classified, and correlated with the welding variables and weld quality. Penetration as a function of time shows three distinct regimes and It is shown that the impact strength of the welds is independent of time once the third regime is reached. The time required to reach the third regime decreases as pressure or amplitude increases and is more sensitive to amplitude of vibration than to pressure. The highest quality welds were produced at low pressure and low amplitude with corresponding long times to reach regime three and exhibited a unique, readily identifiable microstructure.     

The microstructure of polypropylene welds as a guide to processing history

The well known occurrence of different crystalline forms in polypropylene and their easy recognition in the polarising microscope has provided a method of analysing the behaviour of this material during the production of hot plate welds. The different thermal behaviour of the α- and the β-forms of crystalline polypropylene allows estimation of the thermal history of the polymer in the region of the weld. In particular it is possible to estimate the amount of material actually melted by contact with the hot plate. The effect of the original crystalline microstructure of the components being welded on the microstructure of the recrystallised material forming the weld bond has also been explored. It is possible to characterise the general crystalline texture in the weld bead in terms of the shear and thermal history of the material. The influence of pre- and post-welding annealing of the polypropylene has been investigated and a difficulty is encountered in explaining an apparent ‘memory’ of β-form crystallisation which conflicts with established x-ray diffraction results.     

Raman study of chain deformation in polypropylene

Summary Raman spectroscopy was used to study the transfer of energy that occurs among the internal vibrations of crystalline isotactic polypropylene as it is compressed. The group was found to play a significant role in the subsequent chain deformation.     

The hot-tool and vibration welding of acrylonitrile-butadiene-styrene

Thermoplastic window frames are made by hot-tool welding mitered, extruded profiles. The effects of weld process conditions on the strengths of hot-tool butt joints are investigated for a grade of acrylonitrile-butadiene-styrene that was specially developed for window-frame applications. Vibration-weld strength data, obtained on a research machine in which all the process variables can be independently controlled, are used to benchmark strengths of hot-tool welds made on a commercial welding machine. Process differences between hot-tool butt welding and the hot-tool welding of mitered, extruded profiles are discussed.     

Cyclic deformation and relaxation characteristics in polypropylene

Cyclic deformation and stress relaxation after cyclic preloading in polypropylene were investigated by using an electrohydraulic, servocontrolled testing machine. The cyclic deformation tests were performed under various sets of strain rate, number of cycles, and strain amplitude in the as-received sample and a quenched sample. The stress relaxation tests were made after a cyclic preloading in both samples. The distinctive shape of the hysteresis loop, termed a propeller-like shape, is characteristic of the as-received polypropylene with large spherulites, in marked contrast to the behavior of metals. The curves at strain amplitudes from ± 1.5% to ± 5% indicate a propeller-like shape; these loops change into the steady state response as the number of cycles (N) is increased up to N = 30 to 50. The drop of stress in relaxation tests for the quenched samples is smaller than that for the as-received samples at the same strain levels. This stress drop behavior reflects the difference of spherulite structure. The stress relaxation behavior depends on the morphology, the predetermined strain amplitude, and the process of the tests.     

Viscoelastic-plastic deformation behavior of polypropylene after cyclic preloadings

Stress-relaxation behavior is studied in polypropylene samples subjected to different cyclic preloadings and to simple tension. The relaxation tests are performed under different sets of strain amplitude, number of cycles, and strain rate, using a closed-loop, electrohydraulic, servocontrolled testing machine. The calculated stress-strain curves are determined from a constitutive equation based on an overstress theory in which an equilibrium stress and a viscosity function are treated. The calculated results agree well with the experimental ones. It is concluded that the overstress theory explains the nonlinear viscoelastic-plastic behavior of polypropylene.     

A constitutive theory for polypropylene in cyclic deformation

The shape of the hysteresis loop has been studied for polypropylene subjected to different cyclic loadings. Cyclic tests were performed under different sets of cyclic strain amplitude and number of cycles, using a closed-loop, electrohydraulic, servocontrolled testing machine. A constitutive theory is developed for the interpretation of loop shape behavior in terms of elastic, perfectly plastic materials of different yield strengths. The constitutive equation is based on a simple overlay model of bar elements. The calculated results agree well with experiments, which indicates the distictive shape of the hysteresis loop, termed a propelled-like shape. The truss model reasonably explains the cyclic deformation behavior in polypropylene.     

A deformation induced order-disorder transition in isotactic polypropylene

In this study, isotactic polypropylene (i-PP) has been deformed by uniaxial compression to draw ratios up to 16×, and at draw temperatures from 30°C to 140°C. An order-disorder transition in the crystals is observed at draw temperatures well above the stability limit, 70°C, reported for the disordered phase; Furthermore, this disordered phase (called smectic) is found to induce ductility and improve the efficiency of draw. The deformation induced smectic phase has been characterized using WAXS, DSC, and on-line compression load versus draw ratio measurements. In consequence, a set of process conditions are offered to optimize draw. A mechanism for plastic deformation of i-PP is also suggested, using the smectic phase as a probe.     

Plastic deformation of polypropylene in relation to crystalline structure

The tensile drawing behavior of quenched and annealed films of isotactic polypropylene is investigated as a function of draw temperature and strain rate. A strain‐induced structural change from the smectic to the monoclinic form is observed for the quenched films. A kinetic interpretation is proposed for the phenomenon. Data of thermal activation volume at the yield point indicate two regimes of plastic flow for the quenched sample, between 25 and 60°C, but only one regime for the annealed sample. Homogeneous and heterogeneous crystal slip processes are proposed to account for these regimes in relation to the nucleation and propagation of screw dislocations. The basic mechanism of molecular motion in the polypropylene crystal is suggested to be a wormlike motion of conformational defects along the 3/1 helix chains that allows a 120° rotation and a c/3 translation. The occurrence of the smectic form as a transitory state in the deformation pathway is discussed in terms of plasticity defect generation. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1873–1885, 1999     

From stiff plastic to elastic polypropylene: Polymorphic transformations during plastic deformation of metallocene-made isotactic polypropylene

An analysis of the mechanical properties and polymorphic transformations occurring during plastic deformation of isotactic polypropylene with variable stereoregularity, containing only rr stereo-defects, is presented. Thermoplastic materials showing high stiffness, or high flexibility, or elastic properties can be produced depending on the concentration of defects. Relationships between the different mechanical behavior and the different observed polymorphic transformations occurring during tensile deformation are discussed. The elastic behavior of the poorly isotactic samples and the values of the corresponding mechanical parameters are related to the structural transformations occurring during stretching. The γ-form present in the unstretched sample transforms by stretching into the α-form, which in turn, transforms into the mesomorphic form at very high deformations. The mesomorphic form transforms back into the crystalline α-form upon releasing the tension and elastic recovery is observed. The crystallization of the mesomorphic form into the α-form upon releasing the tension is not observed in the case of flexible or stiff-plastic samples, which do not show elastic behavior. This indicates that in the elastomeric samples elasticity is probably partially due to the enthalpic contribution associated with the crystallization of the mesomorphic form into the α-form.     

Structural dependence of the deformation characteristics of polypropylene film fibre in stress relaxation

The deformational rigidity increases due to relaxation of stresses in oriented PP fibres. Elongation of PP fibres is accompanied by mechanical failure in all stages, which is a distinctive feature of deformation processes in these materials. Ordering of the structure of amorphous regions, manifested by an increase in the deformational rigidity of these fibres, is the result of stress relaxation in oriented PP fibres. Based on proceedings of the memorial conference dedicated to the 100th birthday of Z. A. Rogovin. __________ Translated from Khimicheskie Volokna, No. 1, pp. 32–35, January–February, 2006.     

Effect of mean strain on the cyclic deformation and stress relaxation in polypropylene

The influence of mean strain Changes in the range from 0 to 4% on the cyclic deformation and stress-relaxation properties is investigated using rod specimens of polypropylene. An extensometer measures and controls the axial strain in a closed-loop, electrohydraulic, servocontrolled mechanical testing machine. The hysteresis loop at different numbers of cycles N of N = 35, 45, and 50 are irregular in shape, and the tensile portion decreases in size as the number of cycles was increased. The stress value of the tensile portion for three mean strains of ?_m = 1.0%, 2.0%, and 4.0% remains constant, but the value of the compressive portion decreases as the mean strain was increased. The stress level at a strain width of 3% changes little with mean strain. For the strain widths of 5% and 7%, however, the minimum stress levels at N = 35, 45, and 50 increase with increasing mean strain, in contrast to the behavior of maximum stress level. The stressrelaxation tests show that the drop of stress decreases with an increase in number of cycles. The discrepancy in the results of relaxation tests is due to the effect of the difference in strain rates.     

Dependence of the orientation strengthening of polypropylene fibers on temperature and rate of deformation

Conclusions The mean deformation rate rather fully characterizes the kinematics of polypropylene fiber stretching. It can be used as a norming parameter to obtain generalized temperature-rate characteristics of this process.The degree of orientation in stretching polypropylene fibers, like that for previously studied polymers, is determined by the magnitude of the accumulated highly elastic deformation.Translated from Khimicheskie Volokna, No. 4, pp. 24–25, July–August, 1983.     

Superstructural description of deformation process in uniaxial extension of preoriented isotactic polypropylene

The strains on uniaxial extension of preoriented isotactic polypropylene are separated over the whole range of deformation into three processes: crystallite boundary slip (A₁ process), uniform shear deformation of crystallites (A₂ process), and restoration of molecular orientation from the sheardeformed state (ā₂ process). They are separately evaluated by the use of the parameters of superstructure varying in deformation. Both the A₁ and A₂ processes predominate before the A₂ → ā₂ transition takes place. Wide- and small-angle x-ray goniometries detect this transition. After the A₂ → ā₂ transition takes place, the specimen cannot be deformed by the A₁ or A₂ process, but by a molecular process such as pulling out the isolated extending chains from the folded chain crystals followed by their refolding. From the quantitative evaluations of the strains associated with the A₁ and A₂ processes it is deduced that the initial shear compliance for the A₁ process is smaller than that for the A₂ process.     

Quality assurance of computer controlled hot-tool welding for mass production

The hot-tool welding process is commonly used for welding plastics, but high seam quality can be obtained only by optimizing weld parameters. Because of demand for better quality, the importance of quality control is increasing. At present, quality control is mainly performed by inspecting the end product, resulting in high scrap rates. An effective quality control system must therefore be able to recognize errors as they occur during the manufacturing process. For this, an FMEA (Failure Mode Effect Analysis) should be performed prior to mass production, and statistical quality control should be implemented during and after the process. This paper describes a quality control system for computer controlled hot-tool welding that is based on an understanding of the physics of the process.     

Microstructure transformation and stress-strain behavior of isotactic polypropylene under large plastic deformation

The macroscopic stress-strain behavior of monoclinic polypropylene samples was investigated at 70°C under uniaxial tension and simple shear by means of a special videometric testing system that gives access to the constitutive equation of plastic behavior at constant strain rate up to large deformation. At several levels of plastic strain, the microstructural evolution of the material was characterized by means of X-ray scattering, densitometry and viscoelastic analysis. It appears that the strain hardening is high in tension, whereas it is nearly zero in shear. This behavior is associated with the development of a fiber texture in tension, which differs drastically from the planar crystalline texture developed in shear. Furthermore, it is shown that structural damage takes place as the plastic deformation proceeds in tension, while only little damage is recorded in shear. A viscoplastic model has been developed that specifically tales into account the various slip systems activated in the polypropylene crystallites and the elastic interactions of the lamellae through a self-consistent scheme. Simulations based on this model reproduce correctly the contrasting strain-hardening in tension and in shear and the different crystalline textures induced for these two loading paths.     

Vibration welding of thermoplastics. Part IV: Strengths of poly(butylene terephthalate), polyetherimide,and modified polyphenylene oxide butt welds

In vibration welding of thermoplastics, frictional work done by vibrating two parts under pressure, along their common interface, is used to generate heat to effect a weld. The main process parameters are the weld frequency, the amplitude of the vibratory motion, the weld pressure, and the weld time or weld penetration.; The effects of these parameters on weld quality were systematically studied by first butt welding thermoplastic specimens under controlled conditions, over a wide range of process parameters, and by then determining the strengths and ductilities of these welds by tensile tests. The three thermoplastics investigated are poly (butylene terephthalate), polyetherimide, and modified polyphenylene oxide. Changes in the weld pressure are shown to have opposite effects on the strengths of polyetherimide and modified polyphenylene oxide welds; Also, the weld frequency is shown to have a significant effect on the weldability of polyetherimide. The weldability data for these three thermoplastics are compared with data for polycarbonate. Under the right conditions, the strengths of butt welds in these materials are shown to equal the strength of the virgin polymer.     

Novel polypropylene microporous membranes via spherulitic deformation – Processing perspectives

The processing boundaries for initiating intra-/inter-spherulitic deformation to create microporous polypropylene membranes by lamellar separation are delineated. The processing parameters are: annealing temperature, extension ratio, stretching rate, and stretching temperature. A fixed set of extrusion conditions was chosen for producing precursor films having similar spherulitic morphologies. The morphological changes indicating the occurrence of intra-/inter-spherulitic transition on a spherulitic scale can also be detected on a lamellar scale by WAXS examination via analysis of the α-form orientation index. Membrane porosity measurements showed a consistent correlation with the observed values of the α-form orientation index. Increasing the extension ratio did not change the microstructure in the non-annealed sample; however, the lamellae can be further oriented in the annealed samples. Inter-spherulitic deformation became obvious at slow stretching rates; intra-spherulitic deformation was favored at fast stretching rates. The DSC thermal analysis of the precursor films showed two significant endothermic discontinuities (T₁ at 0 °C and T₂ at 40 °C) in both non-annealed and annealed precursor films; T₁ is believed to be the conventional T_g of polypropylene whereas T₂ appears to originate from the rigid-amorphous fraction trapped within the lamellar “wells” where the amorphous phase is surrounded by the R-lamellae and the T-lamellae. The lamellae could break down or slip from the lamellar knots as stretching temperatures are high enough to minimize the influence of the rigid-amorphous fraction, and the annealed lamellae can still be oriented without a catastrophic cold-drawn deformation.