Roberto Pantani | University of Salerno (original) (raw)
Papers by Roberto Pantani
Polymer Engineering and Science, 2018
Microinjection molding is one of the most efficient processes for wide-scale production of thermo... more Microinjection molding is one of the most efficient processes for wide-scale production of thermoplastic polymer micro-parts. The injection molding process with a cold mold suffers from drawbacks, i.e. the premature solidification, caused by the small thickness and large temperature difference between the surface of the mold and the incoming polymer. In this work, an original and versatile system to dynamically control the local temperature of the cavity surfaces in microinjection molding was developed. The system was used to investigate the effects of rapid variations of the temperature of a 200 μm thick cavity on the reachable flow length and study the morphology of microinjection molded parts of poly(lactic acid). For the rapid mold temperature control, electrical resistive thin components are located very near the surface of the impression to change the temperature by some tens of degrees per second. This study also demonstrates that the system is able to influence the final morphology of the part with a semi-crystalline thermoplastic material. Changing the annealing time and temperature, the effects on the morphology of the part, in the mold and immediately after the injection, were investigated by microscopy analysis and X-ray diffraction.
Polymer Engineering and Science, 2017
Although Polylactic acid (PLA) possesses many desirable properties, above all biodegradability, i... more Although Polylactic acid (PLA) possesses many desirable properties, above all biodegradability, its heat deflection temperature is too low for many desirable applications. Similarly to any other polymers, also for PLA the physical and mechanical properties in the solid state depend on the morphology and crystallinity degree, which in their turn are determined by the thermomechanical history experienced during solidification. A large crystallinity degree is highly desirable to increase the heat resistance of PLA but is rather difficult to reach during injection molding due to the very slow crystallization kinetics of this material. In this work, the crystallization kinetics of an injection molded PLA grade was assessed in function of the thermal history by using calorimetric analysis. The cold crystallization kinetics (starting from the amorphous glassy sample) turned out to be faster than melt crystallization kinetics. Following the indications gained from crystallization kinetics, some samples were injection molded imposing different thermal histories. The effect of molding conditions on crystallinity was determined. This finding was adopted to develop a post-molding stage which allows obtaining crystalline samples in times much shorter (of a factor about two) with respect to samples injection molded in a hot mold kept at temperatures close to the maximum crystallization rate.
The effect of molding conditions on crystallinity was determined to develop a post-molding stage obtaining crystalline samples in times much shorter with respect to samples injection molded in a hot mold.
Journal of Materials Processing Technology, 2016
In conventional injection molding, the mold temperature control is obtained by a continuous cooli... more In conventional injection molding, the mold temperature control is obtained by a continuous cooling method, in which a coolant with constant temperature is circulated in the cooling channels to cool the mold and the polymer melt. During the filling stage, this causes an abrupt polymer solidification close to the mold surface, which reduces the section open to flow and, due to the viscosity increase, causes a decrease of the ability of the polymer melt to fill the cavity. This issue is particularly significant for micro-injected parts in which high aspect ratios are precluded because of premature solidification. In this work, a system for rapid surface temperature control was designed, built and applied to a cavity for micro-injection molding. The system consists in an electrical resistive thin component and an insulation layer and can increase the mold surface temperature of some tenths of a degree Celsius in a time of the order of one second. The system is versatile enough to allow the control of thermal histories during the whole process and at different positions inside the cavity. Injection molding tests were then carried out with this system by using a general purpose isotactic polypropylene and a cavity 200 μm thick in order to check the effect of surface heating on reachable flow length and morphology of the molded parts. The effect of mold temperature on the flow length was as expected dramatic on both the flow length and the obtained morphology: the samples molded with a high mold temperature presented a spherulitic morphology in the whole cross-section, while with a low surface temperature the spherulitic morphology was detectable only at the positions close to the midplane whereas the layer closer to the surface present a very oriented structure due to flow taking place at low temperature.
Polymer, 2016
The effect of flow on the crystallization kinetics of polymers is well known, at least from a qua... more The effect of flow on the crystallization kinetics of polymers is well known, at least from a qualitative point of view. Quantitative determinations are difficult to obtain, due to the experimental difficulties of obtaining direct measurements of nucleation and growth rates during flow. In this work, step shear tests were conducted in the Linkam device adopting a new protocol in which the flow is applied when the crystalline structures have already reached dimensions easily detectable by an optical microscope. This delayed protocol allowed to achieve a more accurate experimental analysis of the effect of flow on spherulite growth rate with respect to what already reported in the literature. The new results obtained for the growth rate were interpreted, with the equilibrium melting temperature function of the shear rate. Also the nucleation rate could be assessed and the existence of a correlation between nucleation rate and growth rate was confirmed.
Journal of Polymer Research, 2015
The influence of talc incorporation by melt compounding on a commercial grade of poly (lactic aci... more The influence of talc incorporation by melt compounding on a commercial grade of poly (lactic acid) (PLA), and the choice of the optimal compounding conditions were investigated. Two types of talc, having micrometric and submicrometric distribution, were adopted. Since the compounding itself has a dramatic influence on the properties of PLA, a study was carried out aimed at assessing the effect of processing on this resin and choosing the most suitable processing conditions. It was found that the incorporation of talc increases the viscosity during the compounding and helps the stability of the viscosity during the mixing. Furthermore the talc, acting as a nucleating agent, enhances crystallization kinetics, so that crystallization half time can be reduced by one order of magnitude with respect to the pure PLA processed in the same way.
Polymer, 2016
In this work, specific experiments on an isotactic polypropylene are carried out, aiming to inves... more In this work, specific experiments on an isotactic polypropylene are carried out, aiming to investigate the flow induced crystallization and the final morphology. The viscoelastic nature of the polymer is described by a non-linear Maxwell model applied to the conformation tensor. Shear stress evolutions, recorded during step shear isothermal experiments, are satisfactory described considering the molecular stretch, i.e. the difference between the two main eigenvalues of the conformation tensor. In the general model, the effect of temperature, pressure, and crystallinity are taken into account. Furthermore, a modeling framework is proposed to describe flow-induced crystallization of isotactic polypropylene. The spherulitic growth rate is analyzed on the basis of a flow dependent equilibrium melting temperature, using the molecular stretch as the key parameter. A phenomenological correlation of the nucleation rate with growth rate is observed. By combining the morphological models, both for nucleation and growth rate, for flow induced crystallization is possible to explain the effect of shear rate and shearing times in different experimental results, and potentially in the simulation of polymer processing.
Polymer, 2016
When in contact with water, poly(lactic acid), PLA, undergoes severe physical changes. Hydrolysis... more When in contact with water, poly(lactic acid), PLA, undergoes severe physical changes.
Hydrolysis is obviously the most expected and studied one, and it is followed by weight loss.
However, other phenomena occur such as opacification and crystallization. Although these
phenomena are sometimes reported in the literature, apart from hydrolysis which has been
thoroughly studied at several temperatures, the other ones have not been deeply analyzed and
their timescales have not been so far determined. This work aims at studying the physical
changes induced by water sorption to PLA, by analyzing the physical changes taking place in
solid samples immersed in water for different times (starting from a few hours) and at
temperatures close to the glass transition. The obtained results allow determining the
timescales of the different phenomena and thus the possible cause-effect relationship among
them. In particular, it is shown that opacity is the first phenomenon to become evident, on a
timescale consistent with water sorption, and seems to be due to crazes induced by water. The
second phenomenon to become evident is the crystallinity evolution, which is shown to be
consistent with a heterogeneous nucleation on the crazes. These two phenomena take place
before hydrolysis becomes significant, thus raising doubts about the fact that hydrolysis can
be their cause
Polymer Degradation and Stability, 2014
Poly(Lactic Acid), PLA, is an extremely interesting polymer for industrial applications in which ... more Poly(Lactic Acid), PLA, is an extremely interesting polymer for industrial applications in which degradability is required. However, is presents considerable processing issues due to its sensitivity to stocking and processing conditions. In particular, the decrease of molecular weight at high temperature during processing in the molten state induces a reduction of viscosity.
In this work the degradation of a commercial grade of PLA in the molten state was studied through the analysis of the rheological properties as a function of time and drying conditions.
In particular, measurements of the complex viscosity were taken during time at a frequency of 1rad/s in the temperature range at which the polymer is usually processed (from 180 °C to 220 °C) on samples stored at ambient humidity and on samples subjected to drying.
A kinetic model of degradation of PLA which keeps into account the contribution of hydrolysis and thermal degradation was applied to the experimental data and the parameters of the model were obtained as a function of temperature for the commercial grade analysed.
Macromolecular Theory and Simulations, 2014
The linear viscoelastic response, usually described through the moduli G’(ω) e G”(ω), is widely e... more The linear viscoelastic response, usually described through the moduli G’(ω) e G”(ω), is widely employed to investigate the material properties because it is strictly related to the microstructure of thermoplastics. In a semi-crystalline polymeric material both the amount (degree of crystallinity) and the morphology of the crystalline phase strongly influence the polymer rheological behavior. In order to obtain information about the effect of crystallinity on the linear viscoelastic functions, the parameters of a the linear multi-mode Maxwell equation have been determined by fitting literature data of G’(ω) and G”(ω) collected at different crystallinity degrees. The analysis of the resulting spectra, at least in the considered frequency and crystallinity range, clearly shows that, for each considered mode, the relaxation times of all modes increase with crystallinity in the same way. On the other hand, the parameters Gi of faster modes (low relaxation times) do not depend upon the crystallinity, whereas the parameters Gi increase with crystallinity only for the slowest modes (largest relaxation times).
These results are very relevant to the rheology evolution during solidification: it is not sufficient to analyze only one viscoelastic function during crystallization, the relaxation time for instance; also the moduli change, and their increase seems concentrated to the modes having the largest relaxation times
Advances in Polymer Science, 2018
In this work, a review is presented concerning hydrolytic degradation and biodegradation of Poly(... more In this work, a review is presented concerning hydrolytic degradation and biodegradation of Poly(lactic acid) (PLA). Hydro- lytic degradation, that induces morphological and compositional changes, is considered the most important step in determining the bio-degradation. The main factors influencing the hydrolytic degra- dation, such for instance temperature, pH, sample morphology, mo- lecular weight, are considered and analyzed. An overview on the bi- odegradation in composting conditions is also reported. Finally, the chapter analyses also the possibilities of modulating degradation and biodegradation rates in function of the expected lifetime of the ob- jects made in PLA. This can be considered a frontier of research in this field.
Journal of Polymers and the Environment, 2015
The control of biodegradation rate is even more important than the characteristic of biodegradabi... more The control of biodegradation rate is even more important than the characteristic of biodegradability itself. This is the reason why it is critical to find additives able to modulate the biodegradation rate of biodegradable polymers in relationship to the expected lifetime. This paper reports the effects of silver nanoparticles (AgNP) on the biodegradation behavior of the Poly(lactic acid) (PLA). Different amounts of AgNP (0.01; 0.07; 0.12 wt%) were melt mixed with a commercial PLA in order to analyze their effect on the composites properties. The composites were submitted to biodegradation tests in controlled composting conditions, according to ASTM D 5338 and ISO14855 standards. Thermal properties of PLA and composites were analyzed at different biodegradation times. The aim was the tuning of the biodegradation rate of PLA by the addition of silver nanoparticles.
Polymer Testing, 2015
The aim of this work is to describe in detail an experimental approach to quantitatively investig... more The aim of this work is to describe in detail an experimental approach to quantitatively investigate water vapor transport properties in Poly(lactic acid), PLA. The approach is based on in situ time resolved Fourier Transform Infrared Spectroscopy, FTIR, in transmission mode, and provides a mean for monitoring accurately the penetrant sorption kinetics. FTIR spectra were analyzed by a deconvolution procedure, and the area of a specific peak (at 3651 cm-1) was directly correlated to the amount of water inside the sample. The time evolution of this area was first normalized by using a peak uninfluenced by the amount of water and then interpolated to obtain the diffusivity parameter. The results were in good agreement with those obtained by a more classical, and more complex microgravimetric methods.
Journal of Rheology, 2015
A rotational rheometer equipped with an optical module is adopted to characterize the evolution o... more A rotational rheometer equipped with an optical module is adopted to characterize the evolution of rheological parameters and at the same time the morphology development during isothermal crystallization. This allowed the determination of the effect of crystallinity on the viscoelastic functions of an iPP. A linear multi-mode Maxwell model was then applied to obtain the modulus and relaxation time of each mode as a function of the measured crystallinity degree. It was found that at low crystallinity levels the increase of moduli with crystallinity is about the same for all the modes whereas, when crystallinity degree rises, the increase of moduli is higher for the modes characterized by higher relaxation times. Concerning the relaxation times, it was found that the increase due to crystallinity is about the same for all the modes and reaches a factor of about 10 when relative crystallinity degree is close to 10%. The effect of crystallinity on complex viscosity was also assessed by determining a shift factor due to crystallinity. It was observed that the data collected at the lowest frequencies need higher shift factors with respect to the other ones. This was ascribed to the fact that more than one shift factor is needed to describe the effect on moduli, and low frequencies stir modes with higher relaxation times, which undergo a moduli increase larger than modes with shorter relaxation times.
European Polymer Journal, 2017
Two injection molding tests conducted on an isotactic polypropylene were used as case studies to ... more Two injection molding tests conducted on an isotactic polypropylene were used as case studies to validate the predictions of a software code developed at the University of Salerno (UNISA Code) which is able to describe the evolution of the main process variables while considering the evolution of crystallinity and morphology. The results of the simulation and the comparison with experimental results, allowed the clarification of some unexpected behaviors of the experimental data. In particular, by considering the effect of flow on crystallization kinetics (Flow Induced Crystallization – FIC) the software was able to describe the presence of smaller spherulites at the midplane of the molded part, whose presence cannot be explained if quiescent crystallization kinetics is adopted.
Materials and Design, 2017
The production of polymeric components with functional structures in the micrometer and sub- micr... more The production of polymeric components with functional structures in the micrometer and sub- micrometer range is a complex challenge for the injection molding process, since it suffers the use of low cavity surface temperatures that induce the fast formation of a frozen layer, thus preventing accurate replication of micro and nano-features.
In this work, a thin heating device allowed reaching and maintaining the desired temperature on the cavity surface, by joule effect. A nickel insert with micro and nano-features in relief was located on the cavity surface, and the replication by isotactic polypropylene of the features was analyzed, by Atomic Force Microscopy, under several process parameters. The increase of holding pressure enhanced the replication, but accurate replication was achieved only increasing the cavity surface temperature. A heating time comparable with cavity filling time was sufficient to obtain accurate replication, with adequate surface temperatures.
In the case of nano-features, the replication accuracy was affected by the morphology developed on the molding surface, that is aligned along the flow direction with dimensions comparable with the dimension of the nano-features. Therefore, their formation on the surface reduced the accuracy of replication in the direction orthogonal to the flow front.
International Polymer Processing, 2018
It is quite well known that the morphology of an injection molded part made by a semicrystalline ... more It is quite well known that the morphology of an injection molded part made by a semicrystalline polymer presents several layers. In particular spherulitic structures are found in the core region, a layer characterized by highly oriented fibrillar morphology (the shear layer) usually follows and a skin layer is often observed at the sample surface. The thickness of the fibrillar layer deeply influences the mechanical properties of the part. In this work, a criterion to predict the thickness of the fibrillar layer is proposed and verified. The criterion is essentially based on the amount of viscous work done when the molecular stretch is higher than a critical value: the molecular stretch should be above a critical value while a critical amount of viscous work is accumulated. In order to tune the parameters, and to validate the criterion, a well characterized polypropylene was chosen as test material, and four different injection molding conditions were analyzed. The criterion is verified by comparing some experimental results with the prediction of the UNISA code (an injection molding software developed at the University of Salerno), good comparison between software predictions and experimental data confirms the suitability of the criterion.
International Polymer Processing, 2016
UNISA code, a software for the analysis and modeling of injection molding, was born at the Univer... more UNISA code, a software for the analysis and modeling of injection molding, was born at the University of Palermo in Italy in the 1980s. Afterwards, in the 1990s, it was rewritten and expanded at the University of Salerno (Italy) and continuously improved over the years. It is a study code, aimed at understanding rather than simulating. It has the unique characteristic of describing, since the early versions, the morphology of the molded samples. Furthermore, it always implemented the interrelationships among the different material properties (crystallinity, viscosity, density). In this work, the evolution of the software is reviewed, placed in the background, underlining the contribution given to the understanding of polymer processing and morphology evolution. Eventually, the future challenges of modeling are presented.
Polymer Engineering and Science, 2018
Foam injection molding is a process by which complex, thick parts with a cellular core and a comp... more Foam injection molding is a process by which complex, thick parts with a cellular core and a compact solid skin can be manufactured. The addition of a physical blowing agent under high pressure allows the decrease of the viscosity and the glass transition temperature of the gas/polymer melt. These features of the foam injection molding are very beneficial for biodegradable polymers, in which the processability window is very narrow. In this study, morphology, rheological and mechanical properties of parts with a complex shape, obtained by foam injection molding, were analyzed. In particular, two commercial grades of poly(lactic acid) having different rheological behavior were adopted to obtain foamed parts by injection molding process with nitrogen as a physical blowing agent. For both resins, the effect of the mold opening on morphology and mechanical properties was assessed.
Industrial Crops and Products, 2018
The high cost of the Poly(lactic) Acid when compared to its durable competitors is a significant ... more The high cost of the Poly(lactic) Acid when compared to its durable competitors is a significant drawback that inhibits its diffusion for many industrial applications. A common solution is the compounding with other polymers that preserve biodegradability, and thermoplastic starch (TPS) is a conventional choice. The innovation proposed in this work is to replace into the starch plasticization process part of the glycerol with a sunflower seed oil resulting from the frying process in a fast food. The so plasticized TPS was compared to the TPS plasticized with only glycerol and then mixed to the PLA. The replacement of a certain percentage of glycerol with fried edible sunflower oil as plasticizer for starch results in an improvement in material properties of the TPS and does not change the properties of the PLA/TPS blends. Plasticization of starch with fried oil has been proven to be the most environmentally sound solution also from a Life Cycle Assessment.
Journal of Applied Polymer Science, 2018
This work reports the preparation and characterization of composites based on EVA and iron based ... more This work reports the preparation and characterization of composites based on EVA and iron based powder produced by an injection molding machine specifically designed to host an electromagnet connected to a power supply which generates a magnetic field during the forming phases. The magnetic field allows the repositioning of the particles along the magnetic field lines leading to an anisotropic structural reinforcement. Thermogravimetric analyses show that the addition of iron powder to the EVA allows thermal stabilization, delaying the first degradation step ascribed to the loss of acetic acid. Mechanical characterizations show that the samples present a higher tensile modulus in the direction of the magnetic field with respect to the same property measured in the direction perpendicular to the magnetic field and considerably higher than the modulus of the samples obtained without the application of magnetic field. Furthermore, the samples obtained in the presence of magnetic field present a sensitivity to the application of an external magnetic field.
These results demonstrate that the application of a magnetic field during the injection molding process of EVA/Fe composite induced an alignment of the particles, which therefore induce peculiar properties to the samples.
Polymer Engineering and Science, 2018
Microinjection molding is one of the most efficient processes for wide-scale production of thermo... more Microinjection molding is one of the most efficient processes for wide-scale production of thermoplastic polymer micro-parts. The injection molding process with a cold mold suffers from drawbacks, i.e. the premature solidification, caused by the small thickness and large temperature difference between the surface of the mold and the incoming polymer. In this work, an original and versatile system to dynamically control the local temperature of the cavity surfaces in microinjection molding was developed. The system was used to investigate the effects of rapid variations of the temperature of a 200 μm thick cavity on the reachable flow length and study the morphology of microinjection molded parts of poly(lactic acid). For the rapid mold temperature control, electrical resistive thin components are located very near the surface of the impression to change the temperature by some tens of degrees per second. This study also demonstrates that the system is able to influence the final morphology of the part with a semi-crystalline thermoplastic material. Changing the annealing time and temperature, the effects on the morphology of the part, in the mold and immediately after the injection, were investigated by microscopy analysis and X-ray diffraction.
Polymer Engineering and Science, 2017
Although Polylactic acid (PLA) possesses many desirable properties, above all biodegradability, i... more Although Polylactic acid (PLA) possesses many desirable properties, above all biodegradability, its heat deflection temperature is too low for many desirable applications. Similarly to any other polymers, also for PLA the physical and mechanical properties in the solid state depend on the morphology and crystallinity degree, which in their turn are determined by the thermomechanical history experienced during solidification. A large crystallinity degree is highly desirable to increase the heat resistance of PLA but is rather difficult to reach during injection molding due to the very slow crystallization kinetics of this material. In this work, the crystallization kinetics of an injection molded PLA grade was assessed in function of the thermal history by using calorimetric analysis. The cold crystallization kinetics (starting from the amorphous glassy sample) turned out to be faster than melt crystallization kinetics. Following the indications gained from crystallization kinetics, some samples were injection molded imposing different thermal histories. The effect of molding conditions on crystallinity was determined. This finding was adopted to develop a post-molding stage which allows obtaining crystalline samples in times much shorter (of a factor about two) with respect to samples injection molded in a hot mold kept at temperatures close to the maximum crystallization rate.
The effect of molding conditions on crystallinity was determined to develop a post-molding stage obtaining crystalline samples in times much shorter with respect to samples injection molded in a hot mold.
Journal of Materials Processing Technology, 2016
In conventional injection molding, the mold temperature control is obtained by a continuous cooli... more In conventional injection molding, the mold temperature control is obtained by a continuous cooling method, in which a coolant with constant temperature is circulated in the cooling channels to cool the mold and the polymer melt. During the filling stage, this causes an abrupt polymer solidification close to the mold surface, which reduces the section open to flow and, due to the viscosity increase, causes a decrease of the ability of the polymer melt to fill the cavity. This issue is particularly significant for micro-injected parts in which high aspect ratios are precluded because of premature solidification. In this work, a system for rapid surface temperature control was designed, built and applied to a cavity for micro-injection molding. The system consists in an electrical resistive thin component and an insulation layer and can increase the mold surface temperature of some tenths of a degree Celsius in a time of the order of one second. The system is versatile enough to allow the control of thermal histories during the whole process and at different positions inside the cavity. Injection molding tests were then carried out with this system by using a general purpose isotactic polypropylene and a cavity 200 μm thick in order to check the effect of surface heating on reachable flow length and morphology of the molded parts. The effect of mold temperature on the flow length was as expected dramatic on both the flow length and the obtained morphology: the samples molded with a high mold temperature presented a spherulitic morphology in the whole cross-section, while with a low surface temperature the spherulitic morphology was detectable only at the positions close to the midplane whereas the layer closer to the surface present a very oriented structure due to flow taking place at low temperature.
Polymer, 2016
The effect of flow on the crystallization kinetics of polymers is well known, at least from a qua... more The effect of flow on the crystallization kinetics of polymers is well known, at least from a qualitative point of view. Quantitative determinations are difficult to obtain, due to the experimental difficulties of obtaining direct measurements of nucleation and growth rates during flow. In this work, step shear tests were conducted in the Linkam device adopting a new protocol in which the flow is applied when the crystalline structures have already reached dimensions easily detectable by an optical microscope. This delayed protocol allowed to achieve a more accurate experimental analysis of the effect of flow on spherulite growth rate with respect to what already reported in the literature. The new results obtained for the growth rate were interpreted, with the equilibrium melting temperature function of the shear rate. Also the nucleation rate could be assessed and the existence of a correlation between nucleation rate and growth rate was confirmed.
Journal of Polymer Research, 2015
The influence of talc incorporation by melt compounding on a commercial grade of poly (lactic aci... more The influence of talc incorporation by melt compounding on a commercial grade of poly (lactic acid) (PLA), and the choice of the optimal compounding conditions were investigated. Two types of talc, having micrometric and submicrometric distribution, were adopted. Since the compounding itself has a dramatic influence on the properties of PLA, a study was carried out aimed at assessing the effect of processing on this resin and choosing the most suitable processing conditions. It was found that the incorporation of talc increases the viscosity during the compounding and helps the stability of the viscosity during the mixing. Furthermore the talc, acting as a nucleating agent, enhances crystallization kinetics, so that crystallization half time can be reduced by one order of magnitude with respect to the pure PLA processed in the same way.
Polymer, 2016
In this work, specific experiments on an isotactic polypropylene are carried out, aiming to inves... more In this work, specific experiments on an isotactic polypropylene are carried out, aiming to investigate the flow induced crystallization and the final morphology. The viscoelastic nature of the polymer is described by a non-linear Maxwell model applied to the conformation tensor. Shear stress evolutions, recorded during step shear isothermal experiments, are satisfactory described considering the molecular stretch, i.e. the difference between the two main eigenvalues of the conformation tensor. In the general model, the effect of temperature, pressure, and crystallinity are taken into account. Furthermore, a modeling framework is proposed to describe flow-induced crystallization of isotactic polypropylene. The spherulitic growth rate is analyzed on the basis of a flow dependent equilibrium melting temperature, using the molecular stretch as the key parameter. A phenomenological correlation of the nucleation rate with growth rate is observed. By combining the morphological models, both for nucleation and growth rate, for flow induced crystallization is possible to explain the effect of shear rate and shearing times in different experimental results, and potentially in the simulation of polymer processing.
Polymer, 2016
When in contact with water, poly(lactic acid), PLA, undergoes severe physical changes. Hydrolysis... more When in contact with water, poly(lactic acid), PLA, undergoes severe physical changes.
Hydrolysis is obviously the most expected and studied one, and it is followed by weight loss.
However, other phenomena occur such as opacification and crystallization. Although these
phenomena are sometimes reported in the literature, apart from hydrolysis which has been
thoroughly studied at several temperatures, the other ones have not been deeply analyzed and
their timescales have not been so far determined. This work aims at studying the physical
changes induced by water sorption to PLA, by analyzing the physical changes taking place in
solid samples immersed in water for different times (starting from a few hours) and at
temperatures close to the glass transition. The obtained results allow determining the
timescales of the different phenomena and thus the possible cause-effect relationship among
them. In particular, it is shown that opacity is the first phenomenon to become evident, on a
timescale consistent with water sorption, and seems to be due to crazes induced by water. The
second phenomenon to become evident is the crystallinity evolution, which is shown to be
consistent with a heterogeneous nucleation on the crazes. These two phenomena take place
before hydrolysis becomes significant, thus raising doubts about the fact that hydrolysis can
be their cause
Polymer Degradation and Stability, 2014
Poly(Lactic Acid), PLA, is an extremely interesting polymer for industrial applications in which ... more Poly(Lactic Acid), PLA, is an extremely interesting polymer for industrial applications in which degradability is required. However, is presents considerable processing issues due to its sensitivity to stocking and processing conditions. In particular, the decrease of molecular weight at high temperature during processing in the molten state induces a reduction of viscosity.
In this work the degradation of a commercial grade of PLA in the molten state was studied through the analysis of the rheological properties as a function of time and drying conditions.
In particular, measurements of the complex viscosity were taken during time at a frequency of 1rad/s in the temperature range at which the polymer is usually processed (from 180 °C to 220 °C) on samples stored at ambient humidity and on samples subjected to drying.
A kinetic model of degradation of PLA which keeps into account the contribution of hydrolysis and thermal degradation was applied to the experimental data and the parameters of the model were obtained as a function of temperature for the commercial grade analysed.
Macromolecular Theory and Simulations, 2014
The linear viscoelastic response, usually described through the moduli G’(ω) e G”(ω), is widely e... more The linear viscoelastic response, usually described through the moduli G’(ω) e G”(ω), is widely employed to investigate the material properties because it is strictly related to the microstructure of thermoplastics. In a semi-crystalline polymeric material both the amount (degree of crystallinity) and the morphology of the crystalline phase strongly influence the polymer rheological behavior. In order to obtain information about the effect of crystallinity on the linear viscoelastic functions, the parameters of a the linear multi-mode Maxwell equation have been determined by fitting literature data of G’(ω) and G”(ω) collected at different crystallinity degrees. The analysis of the resulting spectra, at least in the considered frequency and crystallinity range, clearly shows that, for each considered mode, the relaxation times of all modes increase with crystallinity in the same way. On the other hand, the parameters Gi of faster modes (low relaxation times) do not depend upon the crystallinity, whereas the parameters Gi increase with crystallinity only for the slowest modes (largest relaxation times).
These results are very relevant to the rheology evolution during solidification: it is not sufficient to analyze only one viscoelastic function during crystallization, the relaxation time for instance; also the moduli change, and their increase seems concentrated to the modes having the largest relaxation times
Advances in Polymer Science, 2018
In this work, a review is presented concerning hydrolytic degradation and biodegradation of Poly(... more In this work, a review is presented concerning hydrolytic degradation and biodegradation of Poly(lactic acid) (PLA). Hydro- lytic degradation, that induces morphological and compositional changes, is considered the most important step in determining the bio-degradation. The main factors influencing the hydrolytic degra- dation, such for instance temperature, pH, sample morphology, mo- lecular weight, are considered and analyzed. An overview on the bi- odegradation in composting conditions is also reported. Finally, the chapter analyses also the possibilities of modulating degradation and biodegradation rates in function of the expected lifetime of the ob- jects made in PLA. This can be considered a frontier of research in this field.
Journal of Polymers and the Environment, 2015
The control of biodegradation rate is even more important than the characteristic of biodegradabi... more The control of biodegradation rate is even more important than the characteristic of biodegradability itself. This is the reason why it is critical to find additives able to modulate the biodegradation rate of biodegradable polymers in relationship to the expected lifetime. This paper reports the effects of silver nanoparticles (AgNP) on the biodegradation behavior of the Poly(lactic acid) (PLA). Different amounts of AgNP (0.01; 0.07; 0.12 wt%) were melt mixed with a commercial PLA in order to analyze their effect on the composites properties. The composites were submitted to biodegradation tests in controlled composting conditions, according to ASTM D 5338 and ISO14855 standards. Thermal properties of PLA and composites were analyzed at different biodegradation times. The aim was the tuning of the biodegradation rate of PLA by the addition of silver nanoparticles.
Polymer Testing, 2015
The aim of this work is to describe in detail an experimental approach to quantitatively investig... more The aim of this work is to describe in detail an experimental approach to quantitatively investigate water vapor transport properties in Poly(lactic acid), PLA. The approach is based on in situ time resolved Fourier Transform Infrared Spectroscopy, FTIR, in transmission mode, and provides a mean for monitoring accurately the penetrant sorption kinetics. FTIR spectra were analyzed by a deconvolution procedure, and the area of a specific peak (at 3651 cm-1) was directly correlated to the amount of water inside the sample. The time evolution of this area was first normalized by using a peak uninfluenced by the amount of water and then interpolated to obtain the diffusivity parameter. The results were in good agreement with those obtained by a more classical, and more complex microgravimetric methods.
Journal of Rheology, 2015
A rotational rheometer equipped with an optical module is adopted to characterize the evolution o... more A rotational rheometer equipped with an optical module is adopted to characterize the evolution of rheological parameters and at the same time the morphology development during isothermal crystallization. This allowed the determination of the effect of crystallinity on the viscoelastic functions of an iPP. A linear multi-mode Maxwell model was then applied to obtain the modulus and relaxation time of each mode as a function of the measured crystallinity degree. It was found that at low crystallinity levels the increase of moduli with crystallinity is about the same for all the modes whereas, when crystallinity degree rises, the increase of moduli is higher for the modes characterized by higher relaxation times. Concerning the relaxation times, it was found that the increase due to crystallinity is about the same for all the modes and reaches a factor of about 10 when relative crystallinity degree is close to 10%. The effect of crystallinity on complex viscosity was also assessed by determining a shift factor due to crystallinity. It was observed that the data collected at the lowest frequencies need higher shift factors with respect to the other ones. This was ascribed to the fact that more than one shift factor is needed to describe the effect on moduli, and low frequencies stir modes with higher relaxation times, which undergo a moduli increase larger than modes with shorter relaxation times.
European Polymer Journal, 2017
Two injection molding tests conducted on an isotactic polypropylene were used as case studies to ... more Two injection molding tests conducted on an isotactic polypropylene were used as case studies to validate the predictions of a software code developed at the University of Salerno (UNISA Code) which is able to describe the evolution of the main process variables while considering the evolution of crystallinity and morphology. The results of the simulation and the comparison with experimental results, allowed the clarification of some unexpected behaviors of the experimental data. In particular, by considering the effect of flow on crystallization kinetics (Flow Induced Crystallization – FIC) the software was able to describe the presence of smaller spherulites at the midplane of the molded part, whose presence cannot be explained if quiescent crystallization kinetics is adopted.
Materials and Design, 2017
The production of polymeric components with functional structures in the micrometer and sub- micr... more The production of polymeric components with functional structures in the micrometer and sub- micrometer range is a complex challenge for the injection molding process, since it suffers the use of low cavity surface temperatures that induce the fast formation of a frozen layer, thus preventing accurate replication of micro and nano-features.
In this work, a thin heating device allowed reaching and maintaining the desired temperature on the cavity surface, by joule effect. A nickel insert with micro and nano-features in relief was located on the cavity surface, and the replication by isotactic polypropylene of the features was analyzed, by Atomic Force Microscopy, under several process parameters. The increase of holding pressure enhanced the replication, but accurate replication was achieved only increasing the cavity surface temperature. A heating time comparable with cavity filling time was sufficient to obtain accurate replication, with adequate surface temperatures.
In the case of nano-features, the replication accuracy was affected by the morphology developed on the molding surface, that is aligned along the flow direction with dimensions comparable with the dimension of the nano-features. Therefore, their formation on the surface reduced the accuracy of replication in the direction orthogonal to the flow front.
International Polymer Processing, 2018
It is quite well known that the morphology of an injection molded part made by a semicrystalline ... more It is quite well known that the morphology of an injection molded part made by a semicrystalline polymer presents several layers. In particular spherulitic structures are found in the core region, a layer characterized by highly oriented fibrillar morphology (the shear layer) usually follows and a skin layer is often observed at the sample surface. The thickness of the fibrillar layer deeply influences the mechanical properties of the part. In this work, a criterion to predict the thickness of the fibrillar layer is proposed and verified. The criterion is essentially based on the amount of viscous work done when the molecular stretch is higher than a critical value: the molecular stretch should be above a critical value while a critical amount of viscous work is accumulated. In order to tune the parameters, and to validate the criterion, a well characterized polypropylene was chosen as test material, and four different injection molding conditions were analyzed. The criterion is verified by comparing some experimental results with the prediction of the UNISA code (an injection molding software developed at the University of Salerno), good comparison between software predictions and experimental data confirms the suitability of the criterion.
International Polymer Processing, 2016
UNISA code, a software for the analysis and modeling of injection molding, was born at the Univer... more UNISA code, a software for the analysis and modeling of injection molding, was born at the University of Palermo in Italy in the 1980s. Afterwards, in the 1990s, it was rewritten and expanded at the University of Salerno (Italy) and continuously improved over the years. It is a study code, aimed at understanding rather than simulating. It has the unique characteristic of describing, since the early versions, the morphology of the molded samples. Furthermore, it always implemented the interrelationships among the different material properties (crystallinity, viscosity, density). In this work, the evolution of the software is reviewed, placed in the background, underlining the contribution given to the understanding of polymer processing and morphology evolution. Eventually, the future challenges of modeling are presented.
Polymer Engineering and Science, 2018
Foam injection molding is a process by which complex, thick parts with a cellular core and a comp... more Foam injection molding is a process by which complex, thick parts with a cellular core and a compact solid skin can be manufactured. The addition of a physical blowing agent under high pressure allows the decrease of the viscosity and the glass transition temperature of the gas/polymer melt. These features of the foam injection molding are very beneficial for biodegradable polymers, in which the processability window is very narrow. In this study, morphology, rheological and mechanical properties of parts with a complex shape, obtained by foam injection molding, were analyzed. In particular, two commercial grades of poly(lactic acid) having different rheological behavior were adopted to obtain foamed parts by injection molding process with nitrogen as a physical blowing agent. For both resins, the effect of the mold opening on morphology and mechanical properties was assessed.
Industrial Crops and Products, 2018
The high cost of the Poly(lactic) Acid when compared to its durable competitors is a significant ... more The high cost of the Poly(lactic) Acid when compared to its durable competitors is a significant drawback that inhibits its diffusion for many industrial applications. A common solution is the compounding with other polymers that preserve biodegradability, and thermoplastic starch (TPS) is a conventional choice. The innovation proposed in this work is to replace into the starch plasticization process part of the glycerol with a sunflower seed oil resulting from the frying process in a fast food. The so plasticized TPS was compared to the TPS plasticized with only glycerol and then mixed to the PLA. The replacement of a certain percentage of glycerol with fried edible sunflower oil as plasticizer for starch results in an improvement in material properties of the TPS and does not change the properties of the PLA/TPS blends. Plasticization of starch with fried oil has been proven to be the most environmentally sound solution also from a Life Cycle Assessment.
Journal of Applied Polymer Science, 2018
This work reports the preparation and characterization of composites based on EVA and iron based ... more This work reports the preparation and characterization of composites based on EVA and iron based powder produced by an injection molding machine specifically designed to host an electromagnet connected to a power supply which generates a magnetic field during the forming phases. The magnetic field allows the repositioning of the particles along the magnetic field lines leading to an anisotropic structural reinforcement. Thermogravimetric analyses show that the addition of iron powder to the EVA allows thermal stabilization, delaying the first degradation step ascribed to the loss of acetic acid. Mechanical characterizations show that the samples present a higher tensile modulus in the direction of the magnetic field with respect to the same property measured in the direction perpendicular to the magnetic field and considerably higher than the modulus of the samples obtained without the application of magnetic field. Furthermore, the samples obtained in the presence of magnetic field present a sensitivity to the application of an external magnetic field.
These results demonstrate that the application of a magnetic field during the injection molding process of EVA/Fe composite induced an alignment of the particles, which therefore induce peculiar properties to the samples.