Papers by Roberto Pantani
In this work infrared (IR) spectroscopy was adopted to study the hydrolytic degradation of films ... more In this work infrared (IR) spectroscopy was adopted to study the hydrolytic degradation of films of aliphatic polyesters in alkaline environment. A relevant increase with the time of immersion of the absorbance of the peak centered at about 1570 cm -1 was observed. The analysis of the IR spectra showed that the integrated peak area in that region can be used to quantify changes in the concentration of degradation product and thus to provide indications regarding the kinetic constant of the hydrolysis reaction. It was found that the hydrolysis of ester bounds proceeds linearly with time, and this result suggests that the controlling mechanism is the chemical reaction rather than water diffusion. Results also show that degradation rate increases on increasing polydispersity.
In this work, FTIR spectroscopy was applied to the determination of crystalline content in an ali... more In this work, FTIR spectroscopy was applied to the determination of crystalline content in an aliphatic polyester. To this goal, a range of wavelengths (from 560 cm-1 to 680 cm-1), non conventional for this kind of polymers, was selected by analyzing spectra collected during isothermal crystallization tests. A deconvolution of the IR spectrum in that range showed the presence of three peaks sensitive to the amorphous content and one sensitive to the crystal content. An analysis of the time evolution of the absorbances allowed to determine the parameters needed to determine the absolute crystallinity degree. The time evolution of crystallinity during isothermal tests at different temperatures was successfully compared with results obtained by DSC. The procedure was also favorably compared with the result obtained by WAXD on a solid sample at room temperature.
Aliphatic polyesters are readily degradable polymers, hydrolysis being the dominant mechanism of ... more Aliphatic polyesters are readily degradable polymers, hydrolysis being the dominant mechanism of degradation. On one side, this makes them extremely interesting for industrial applications in which degradability is required. On the other side, they present considerable processing problems due to their sensitivity to process and stocking conditions. In this work, the degradation of two aliphatic polyesters was studied in the molten state by analysing the rheological properties with the aim of defining the significance of previous thermal history and of residence time at a given temperature. Rheological measurements were adopted as a mean of analysis for degradation kinetics because rheological properties are strongly dependent on molecular weight. In particular, the change in complex viscosity (at constant frequency) as a function of time at different temperatures was measured. The experimental results show that a significant reduction of viscosity takes place during the isothermal tests for all the materials analyzed. This reduction was ascribed to the hydrolysis reaction. Indeed, a dried sample showed only a marginal viscosity reduction. After this initial decrease, an increase in viscosity (more pronounced at higher temperatures) was found for all the materials and at all the temperatures investigated. This phenomenon was ascribed to the inverse reaction (esterification) taking place in the absence of water. The dried sample showed, in fact, a much faster increase in viscosity with respect to the undried one. The degradation kinetics was modeled considering both forward and reverse reactions. The relative rate of the two reactions depends on the moisture content, and thus the water evaporation from the sample was kept into account in the rate equations.
The current goal in the simulation of injection molding is the description of material morphology... more The current goal in the simulation of injection molding is the description of material morphology. The path to reach this goal passes through the prediction of molecular orientation and strain, namely the molecular conformation. To obtain this information, the viscoelastic nature of the polymer must be taken into account. The aim of this paper is to check if a simple, recently proposed, non-linear dumbbell model is sufficiently accurate to quantitatively describe birefringence distribution in injection molded PS samples. To this goal, a series of rheological measurements were performed in a parallel plate rheometer, measuring in the meantime the birefringence.
A study of the crystallization process of syndiotactic polystyrene is carried out, with particula... more A study of the crystallization process of syndiotactic polystyrene is carried out, with particular emphasis on the effect of previous melt annealing on the subsequent crystallization kinetics and polymorphism, with a combination of thermal analysis, infrared spectroscopy, and X-ray diffraction. An effort is made to obtain quantitative results in terms of the relative crystallinity content of the α and β phases after a given thermal treatment. A kinetics of melting for the crystalline memory is proposed, which enables the determination of the time at a given temperature that ensures the complete fusion of the α crystals. This leads to a generalization of the effects of the time–temperature couple on the memory effect. A particular protocol is identified, which upon solidification from the melt induces the development of just one crystalline phase, either α or β. This allows the determination of the enthalpy of crystallization, the maximum attainable crystal volume fraction, the crystallization half-time as a function of temperature, and the Avrami index for each of the two crystalline phases. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 196–207, 2007
A modelling of injection moulding process able to predict morphology of fmal samples is of enormo... more A modelling of injection moulding process able to predict morphology of fmal samples is of enormous strategic importance, but still not available. In this work an injection moulding test was performed using an isotactic polypropylene. The resulting morphology (in terms of distribution along tltickness of crystallinity degree, molecular orientation and crystals structure and dimensions) was characterised by adopting different experimental techniques, and analysed with reference to tlte solidification conditions. Samples morphological characteristics were compared witlt predictions of a simulation code developed at University of Salerno. In internal layers, calculations provide a satisfactorily description of data.
... The amount of β-form was found to increase by effect of high shear rates [8]. A wide analysis... more ... The amount of β-form was found to increase by effect of high shear rates [8]. A wide analysis on the effect of processing conditions on the morphology of injection molded iPP was conducted by Viana et al. [9] and, more recently, by Mendoza et al. ... In particular, Mendoza et al. ...
This study aims at exploring the effect of a commercial organoclay montmorillonite (MMT) on the f... more This study aims at exploring the effect of a commercial organoclay montmorillonite (MMT) on the final properties of syndiotactic polystyrene (sPS) injection-molded samples. To this goal, injection-molded specimens made from neat sPS and commercial MMT modified with various organic compounds were prepared in different molding conditions. Dispersion of clay was attained via melt blending, directly in the injection chamber of the injection-molding machine. The obtained specimens were analyzed by IR spectroscopy, X-ray diffraction, thermogravimetry, and differential thermal analysis, with the aim of elucidating the effect of clay on the microstructures of the samples. Results clearly show that, depending on the organic modification, the presence of clay can induce strong effects on final crystallinity. This behavior can be attributed mainly to the role played by clay on the kinetics of the crystallization process. Eventually, it was found that the addition of a small percentage of clay (1%) in sPS can substantially widen the processing window of the material. POLYM. ENG. SCI. 46:1768–1777, 2006. © 2006 Society of Plastics Engineers.
The effect of pressure on viscosity is an important but often overlooked aspect of the flow prope... more The effect of pressure on viscosity is an important but often overlooked aspect of the flow properties of polymeric materials. Generally, an exponential dependence (the so-called Barus equation: η=η0exp(βP)) can be adopted to describe this effect. In this work two polymers (an atactic and a syndiotactic Polystyrene) were characterized as far as the effect of pressure on viscosity is concerned by analyzing the non-linearities in the so-called Bagley plots. The results obtained show that for both materials the average value of β is in the range 1–3×10-8 Pa-1. No relevant effects of temperature and shear rate were detected in the range analyzed. The data obtained were also described by means of a Cross-Vogel model, which reproduces the main features of experimental data.
Poly(lactic acid) (PLA) is an emerging material mainly because it can be synthesized from renewab... more Poly(lactic acid) (PLA) is an emerging material mainly because it can be synthesized from renewable resources and is thus environmentally and ecologically safe. The mechanical properties, above all the thermal resistance of PLA are determined by the crystalline content: the heat deflection temperature of crystalline PLA can reach 100 C, whereas amorphous PLA loses mechanical properties at temperatures slightly higher than 60 C. However, PLA has a low crystallization rate, so that after processing it remains mostly amorphous. This characteristic heavily limits the use of PLA for commercial applications. Many studies have been recently published on the crystallization kinetics of PLA. The effect of processing on this feature is however often neglected. In this work, the significance of processing on the crystallization kinetics of a commercial PLA was investigated. Two processing methods were explored: extrusion and injection moulding. The obtained materials, and the starting pellets of virgin polymer, were analyzed by calorimetry in order to obtain the crystallization kinetics. Two protocols were adopted to determine the crystallization rates during cooling from the melt or heating from the solid. The parameters of a kinetic equation were determined for all the materials and protocols adopted and it was thus possible to describe the evolution of crystallinity during heating and during cooling.
The structural organization of blends of isotactic polypropylene (iPP) and linear low-density pol... more The structural organization of blends of isotactic polypropylene (iPP) and linear low-density polyethylene (LLDPE), with different compositions, was studied and correlated with the thermal history followed by the samples during solidification from the molten state. The materials were cooled at two extreme controlled rates: 0.1 and 500°C/s. The resulting structure was investigated both in the crystalline and the amorphous phases. In particular, attention was focused toward the analysis of the diffusion parameters of dichloromethane vapors, and the morphological organization of the amorphous phases was interpreted using models that consider them (in terms of resistance to diffusion) combined in series and in parallel. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2237–2244, 2001
An experimental study of shrinkage in injection molded products is presented in this paper and do... more An experimental study of shrinkage in injection molded products is presented in this paper and documented with all information needed for any further analysis. In particular, the effect of holding pressure, holding time and mold geometry on product shrinkage was investigated for amorphous polystyrene. It turned out with increasing holding pressure the in-plane shrinkages varied from 0.6% to 0.1%, while the product thickness increased from about 1% to 10%. The holding time only affected product shrinkage for settings less than gate freeze-off time. Shrinkage usually increases along the flow path unless back flow sets in at the end of the holding stage. It was also found that if a constraint prevents in-mold shrinkage to take place, final shrinkage may decrease if holding pressure and time are small. The effects of all the variables analyzed were described by a thermomechanical model recently proposed.
Injection molding is one of the most widely employed methods for manufacturing polymeric products... more Injection molding is one of the most widely employed methods for manufacturing polymeric products. The final properties and the quality of an injection molded part are to a great extent affected by morphology. Thus, the prediction of microstructure formation is of technological importance, also for optimizing processing variables, in order to cut down on the expensive costs of tooling and
Gate solidification time is an important topic in injection molding technology, as it determines ... more Gate solidification time is an important topic in injection molding technology, as it determines cycle time, which itself is an important issue in the balance of the production process. In this work, a study of the effect of both gate and cavity geometries on gate solidification time was conducted, using a commercial polymer, injection molded with constant holding pressure into a rectangular cavity. Three cavity lengths were used, and for each, two cavity thicknesses were adopted. Special dies containing different gates were assembled in the mold. Gate thickness was found to be the most important factor determining gate sealing time. However the cavity geometry is also quite important. A clear indication on gate solidification could be drawn by analyzing time evolution of pressure distribution inside the mold. The solidification phenomenon leading to gate sealing was analyzed by a simple model, which also takes into account the effect of cavity geometry, by comparing the heat flow through the gate walls and the energy required to solidify the packing flow rate. Model results satisfactorily describe the main features of the experimental data.
The effect of pressure on viscosity is an important but often overlooked aspect of the flow prope... more The effect of pressure on viscosity is an important but often overlooked aspect of the flow properties of polymeric materials. In this work, two polymers (an atactic and a syndiotactic Polystyrene) were characterized to determine the effect of pressure on viscosity. In particular, a device was adopted to increase the exit pressure of a standard capillary rheometer, thus obtaining data of viscosity under high pressure and high shear rates. The Simha-Somcynsky equation of state was applied to the pressure–volume–temperature experimental data of both materials to obtain the dependence of free volume on temperature and pressure. The Doolittle equation was eventually employed to verify the dependence of viscosity on free volume. It was found that, for both materials, a linear relationship holds between the logarithm of zero-shear-rate viscosity (at several temperatures and pressures) and the inverse of free volume.
A wide set of isothermal and non-isothermal crystallization experiments were carried out in this ... more A wide set of isothermal and non-isothermal crystallization experiments were carried out in this work on an iPP resin. Several experimental techniques were adopted in order to characterize crystallization kinetics and final morphology of the material, also under cooling rates comparable to those encountered during material processing (up to several hundred K/s). The whole set of data was taken as a reference to identify a kinetic model which describes the evolution of the structural organization of iPP (a crystalline phase and mesomorphic phase) as a parallel of two non-interacting kinetic processes competing for the available amorphous volume. Kolmogoroff equation was adopted to describe the crystallization of the a form. Avrami -Evans -Nakamura isokinetic approach was adopted to describe the evolution of the mesomorphic phase. Resulting kinetic model satisfactorily describes the whole set of experimental data including those obtained on samples solidified under high cooling rates, and reveals that a correct description of the evolution of the a phase during solidification can be attained only if the evolution of the competing mesomorphic phase is kept into account. The effect of cooling rate during solidification from the melt on diameters of spherulites, observed on solidified samples, is also satisfactorily described by model predictions. q
Control of volume changes with time has a critical industrial relevance for the production of obj... more Control of volume changes with time has a critical industrial relevance for the production of objects made of thermoplastic materials (obtained, e.g., by injection molding), but this phenomenon is completely disregarded by commercial codes for simulation of processes. In this work, attention is focused on the relevance of thermomechanical history on volume relaxation at room conditions of an amorphous polystyrene. A set of data of volume relaxation of samples obtained in an extremely wide range of thermomechanical treatments was collected. Data were analyzed with the aim of applying a simplified model on the basis of the well-known KAHR model, which describes the postprocessing volume relaxation of amorphous polymers by adopting a minimum number of material parameters. Despite the fact that only two relaxation times are considered, the model satisfactorily describes volume evolution (either contraction or expansion) at room conditions after a given thermomechanical treatment if an appropriate partition of free volume into two fractions is provided. Furthermore, in its present form that neglects the effect of pressure on volume relaxation, the model satisfactorily describes the effect of a given thermal treatment (at room pressure), starting from the melt, on both specific volume and its relaxation rate after treatment. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1526–1537, 2003
In polymer crystallization, the correct determination of the melting temperature is of crucial im... more In polymer crystallization, the correct determination of the melting temperature is of crucial importance since crystallization kinetics mainly depends on the amount of undercooling. In this work, the thermodynamic melting temperature and the reference temperature for crystallization kinetics (also called “zero growth rate temperature”) are calculated by means of alternative procedures with the objective of providing a general discussion on problems occurring in their determination and of identifying the relationship between them. Syndiotactic polystyrene is used as test material due to its very interesting polymorphic behavior and because many data have been recently reported in the literature concerning its melting temperature. It was found that the thermodynamic melting temperature resulting by differential scanning calorimetry heating analysis provides a poor description when applied to crystallization kinetic data. The reference temperature to be adopted in crystallization kinetic equations resulted to be significantly lower. This seems to agree with recent findings and somewhat to contradict conventional understanding.
Obtaining reliable predictions for molecular orientation is currently one of most challenging tar... more Obtaining reliable predictions for molecular orientation is currently one of most challenging targets in the simulation of the injection moulding process, being the starting point toward a better understanding of how crystallisation kinetics and final morphology are influenced by flow fields during processing. Although pressure and velocity distribution can be satisfactorily described by viscous models, the viscoelastic nature of the polymer needs to be accounted for in the description of molecular orientation evolution. In this work, different choices for the dumbbell model are adopted to describe the evolution of molecular orientation by effect of kinematics obtained by a viscous approach. Comparison with literature data of birefringence distributions in injection moulded disks identifies one of the choices which correctly describes main features of data.
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Papers by Roberto Pantani