Petar Doshev - Academia.edu (original) (raw)

Papers by Petar Doshev

Research paper thumbnail of POLYPROPYLENE WITH LIVING HINGE PROPERTIES

Research paper thumbnail of Crystallinity-based product design: Utilizing the polymorphism of isotactic PP homo- and copolymers

Research paper thumbnail of Heterophasic Polypropylene Resin with Long Chain Branching

Research paper thumbnail of Heterophasic propylene copolymers with stiffness/impact/flowability balance

Research paper thumbnail of Propylene/1-hexene copolymer composition with broad sealing window

Research paper thumbnail of Chain regularity of isotactic polypropylene determined by different thermal fractionation methods

Journal of Thermal Analysis and Calorimetry, 2014

The chain regularity of isotactic polypropylene (iPP) homo-and random copolymers was characterize... more The chain regularity of isotactic polypropylene (iPP) homo-and random copolymers was characterized by different thermal fractionation methods in this study. Different stepwise temperature programs were applied in a calorimeter (DSC), in order to develop a method which is significantly faster than SIST and provides reliable information about the chain regularity of iPP. Our studies prove that self-seeding accelerates the crystallization process during annealing in SSA-DSC experiments (successive self-nucleation and annealing). Consequently, the time of isothermal steps can be shortened significantly in the SSA-DSC method. On the other hand we found that step time should not be too short if the goal of the measurement is the determination of average chain regularity. Our results clearly indicate that both the experimental conditions and the evaluation technique influence the obtained results. A standard experimental procedure is proposed for reliably determining the average chain regularity of iPP. The length of the SSA-DSC temperature program developed in this study is much shorter compared to that of the conventional SIST (stepwise isothermal segregation technique) measurements used recently for such experiments. The proposed SSA-DSC program makes the reliable characterization of a large number of samples on an acceptable timescale possible.

Research paper thumbnail of The role of solubility and critical temperatures for the efficiency of sorbitol clarifiers in polypropylene

RSC Adv., 2014

ABSTRACT The optical properties of polypropylene (PP) were modified by nine different sorbitol ty... more ABSTRACT The optical properties of polypropylene (PP) were modified by nine different sorbitol type clarifiers available commercially or synthesized in the study. The solubility of the clarifiers in PP was estimated by thermodynamic model calculations. The results showed that the solubility of these additives in PP is small, a few 1000 ppm at most. Solubility is determined by the chemical structure of the sorbitol, and the heat of fusion of the latter changes solubility by at least one order of magnitude. Solubility can be estimated reasonably by the Flory-Huggins lattice theory. The morphology of most sorbitols transforms at a temperature much below their melting point upon heating. This transformation, which is accompanied by crystal perfection, seems to influence melting and solubility. A fibrillar structure forms upon the cooling of molten sorbitols, but the diameter of the fibrils is much larger than those forming in the polymer melt. The nucleating effect of the clarifier depends on solubility, but also on processing conditions. Nucleus density is related to the amount of dissolved clarifier. A close correlation was found between the Flory-Huggins interaction parameter of sorbitols and the smallest achievable haze, which can be explained by the effect of solubility and nucleus density.

Research paper thumbnail of Heterogeneous Ziegler-Natta, metallocene, and post-metallocene catalysis: Successes and challenges in industrial application

MRS Bulletin, 2013

ABSTRACT The success of polyolefins is governed to a large extent by the development of robust an... more ABSTRACT The success of polyolefins is governed to a large extent by the development of robust and versatile catalysts offering excellent morphology control. This review highlights the major evolution steps made in the polyolefin catalyst systems in terms of productivity and possibilities to control the molecular architecture of both polypropylene and polyethylene. Starting from the initial TiCl3-types, the continuous improvement of the Ziegler-Natta catalysts in terms of performance and cost is the major factor behind their wide market penetration. On the other hand, metallocene and the other “single-site” catalysts enable an unprecedented fine-tuning of chain microstructure by ligand design. In this article, special emphasis is placed on the influence of catalyst type on polymer structure characteristics such as molecular weight distribution, stereoregularity, and comonomer distribution and, ultimately, on the end-use properties of polyolefins. It is the excellent balance among price, performance, and processability that will further strengthen the position of polyolefins as a dominant class of materials in the polymer industry.

Research paper thumbnail of Effect of molecular architecture on the crystalline structure and stiffness of iPP homopolymers: Modeling based on annealing experiments

Journal of Applied Polymer Science, 2013

Five PP homopolymers were selected and their molecular structure was thoroughly characterized to ... more Five PP homopolymers were selected and their molecular structure was thoroughly characterized to determine the effect of molecular architecture on their annealing behavior and on the ultimate stiffness achieved by heat treatment. Molecular mass and its distribution were characterized by rheological measurements, while chain regularity was determined by calorimetry, by the stepwise isothermal segregation technique (SIST). The samples were annealed in two different ways. Tensile bars ere treated i a o e at °C for i reasi g ti es to deter i e ha ges i stiff ess. Various defe ts developed during the annealing of tensile specimens that did not allow the reliable determination of modulus by direct measurement. On the other hand, the second approach, the annealing of small samples in a DSC cell clearly showed the changes occurring in crystalline structure and also the effect of nucleation and molecular architecture on them. The large molecular weight fraction used to facilitate nucleation hinders crystal perfection, while the presence of a heterogeneous nucleating agent increases overall crystallinity, but does not influence recrystallization during annealing. Melting traces were transformed into lamella thickness distributions, from which average lamella thickness was derived. Lamella thickness and crystallinity, the independent variables of the empirical equation used for the calculation of modulus, were extrapolated to infinite annealing time to predict maximum stiffness. The value obtained, 3.5 GPa, is very far from the theoretically predicted 40 GPa of oriented crystals, which cannot be achieved under practical conditions.

Research paper thumbnail of Effect of the Molecular Structure of the Polymer and Nucleation on the Optical Properties of Polypropylene Homo- and Copolymers

ACS Applied Materials & Interfaces, 2014

Two soluble nucleating agents were used to modify the optical properties of nine PP homo- and ran... more Two soluble nucleating agents were used to modify the optical properties of nine PP homo- and random copolymers. The ethylene content of the polymers changed between 0 and 5.3 wt %. Chain regularity was characterized by the stepwise isothermal segregation technique (SIST), while optical properties by the measurement of the haze of injection molded samples. Crystallization and melting characteristics were determined by differential scanning calorimetry (DSC). The analysis of the results proved that lamella thickness and change in crystallinity influence haze only slightly. A model was introduced which describes quantitatively the dependence of nucleation efficiency and haze on the concentration of the nucleating agent. The model assumes that the same factors influence the peak temperature of crystallization and optical properties. The analysis of the results proved that the assumption is valid under the same crystallization conditions. The parameters of the model depend on the molecular architecture of the polymer. Chain regularity determines supermolecular structure and thus the dependence of optical properties on nucleation.

Research paper thumbnail of Heterophasic copolymers of polypropylene: Development, design principles, and future challenges

Journal of Applied Polymer Science, 2013

ABSTRACT Initially developed to overcome the limited low‐temperature impact resistance of polypro... more ABSTRACT Initially developed to overcome the limited low‐temperature impact resistance of polypropylene (PP) and allow its application in subzero temperatures, the PP/elastomer blends became a very important and industrially relevant material class. Within the last 40 years, a whole range of PP impact copolymers from multireactor processes have been developed that are capable of covering wide application areas. This review summarizes their developmental history and presents options for the in‐reactor design of these polymers and their modification by compounding with elastomers and fillers or reinforcements after the polymerization. An overview of the application range is also presented together with a glimpse into the future of this material class. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3028–3037, 2013

Research paper thumbnail of POLYPROPYLENE WITH LIVING HINGE PROPERTIES

Research paper thumbnail of Crystallinity-based product design: Utilizing the polymorphism of isotactic PP homo- and copolymers

Research paper thumbnail of Heterophasic Polypropylene Resin with Long Chain Branching

Research paper thumbnail of Heterophasic propylene copolymers with stiffness/impact/flowability balance

Research paper thumbnail of Propylene/1-hexene copolymer composition with broad sealing window

Research paper thumbnail of Chain regularity of isotactic polypropylene determined by different thermal fractionation methods

Journal of Thermal Analysis and Calorimetry, 2014

The chain regularity of isotactic polypropylene (iPP) homo-and random copolymers was characterize... more The chain regularity of isotactic polypropylene (iPP) homo-and random copolymers was characterized by different thermal fractionation methods in this study. Different stepwise temperature programs were applied in a calorimeter (DSC), in order to develop a method which is significantly faster than SIST and provides reliable information about the chain regularity of iPP. Our studies prove that self-seeding accelerates the crystallization process during annealing in SSA-DSC experiments (successive self-nucleation and annealing). Consequently, the time of isothermal steps can be shortened significantly in the SSA-DSC method. On the other hand we found that step time should not be too short if the goal of the measurement is the determination of average chain regularity. Our results clearly indicate that both the experimental conditions and the evaluation technique influence the obtained results. A standard experimental procedure is proposed for reliably determining the average chain regularity of iPP. The length of the SSA-DSC temperature program developed in this study is much shorter compared to that of the conventional SIST (stepwise isothermal segregation technique) measurements used recently for such experiments. The proposed SSA-DSC program makes the reliable characterization of a large number of samples on an acceptable timescale possible.

Research paper thumbnail of The role of solubility and critical temperatures for the efficiency of sorbitol clarifiers in polypropylene

RSC Adv., 2014

ABSTRACT The optical properties of polypropylene (PP) were modified by nine different sorbitol ty... more ABSTRACT The optical properties of polypropylene (PP) were modified by nine different sorbitol type clarifiers available commercially or synthesized in the study. The solubility of the clarifiers in PP was estimated by thermodynamic model calculations. The results showed that the solubility of these additives in PP is small, a few 1000 ppm at most. Solubility is determined by the chemical structure of the sorbitol, and the heat of fusion of the latter changes solubility by at least one order of magnitude. Solubility can be estimated reasonably by the Flory-Huggins lattice theory. The morphology of most sorbitols transforms at a temperature much below their melting point upon heating. This transformation, which is accompanied by crystal perfection, seems to influence melting and solubility. A fibrillar structure forms upon the cooling of molten sorbitols, but the diameter of the fibrils is much larger than those forming in the polymer melt. The nucleating effect of the clarifier depends on solubility, but also on processing conditions. Nucleus density is related to the amount of dissolved clarifier. A close correlation was found between the Flory-Huggins interaction parameter of sorbitols and the smallest achievable haze, which can be explained by the effect of solubility and nucleus density.

Research paper thumbnail of Heterogeneous Ziegler-Natta, metallocene, and post-metallocene catalysis: Successes and challenges in industrial application

MRS Bulletin, 2013

ABSTRACT The success of polyolefins is governed to a large extent by the development of robust an... more ABSTRACT The success of polyolefins is governed to a large extent by the development of robust and versatile catalysts offering excellent morphology control. This review highlights the major evolution steps made in the polyolefin catalyst systems in terms of productivity and possibilities to control the molecular architecture of both polypropylene and polyethylene. Starting from the initial TiCl3-types, the continuous improvement of the Ziegler-Natta catalysts in terms of performance and cost is the major factor behind their wide market penetration. On the other hand, metallocene and the other “single-site” catalysts enable an unprecedented fine-tuning of chain microstructure by ligand design. In this article, special emphasis is placed on the influence of catalyst type on polymer structure characteristics such as molecular weight distribution, stereoregularity, and comonomer distribution and, ultimately, on the end-use properties of polyolefins. It is the excellent balance among price, performance, and processability that will further strengthen the position of polyolefins as a dominant class of materials in the polymer industry.

Research paper thumbnail of Effect of molecular architecture on the crystalline structure and stiffness of iPP homopolymers: Modeling based on annealing experiments

Journal of Applied Polymer Science, 2013

Five PP homopolymers were selected and their molecular structure was thoroughly characterized to ... more Five PP homopolymers were selected and their molecular structure was thoroughly characterized to determine the effect of molecular architecture on their annealing behavior and on the ultimate stiffness achieved by heat treatment. Molecular mass and its distribution were characterized by rheological measurements, while chain regularity was determined by calorimetry, by the stepwise isothermal segregation technique (SIST). The samples were annealed in two different ways. Tensile bars ere treated i a o e at °C for i reasi g ti es to deter i e ha ges i stiff ess. Various defe ts developed during the annealing of tensile specimens that did not allow the reliable determination of modulus by direct measurement. On the other hand, the second approach, the annealing of small samples in a DSC cell clearly showed the changes occurring in crystalline structure and also the effect of nucleation and molecular architecture on them. The large molecular weight fraction used to facilitate nucleation hinders crystal perfection, while the presence of a heterogeneous nucleating agent increases overall crystallinity, but does not influence recrystallization during annealing. Melting traces were transformed into lamella thickness distributions, from which average lamella thickness was derived. Lamella thickness and crystallinity, the independent variables of the empirical equation used for the calculation of modulus, were extrapolated to infinite annealing time to predict maximum stiffness. The value obtained, 3.5 GPa, is very far from the theoretically predicted 40 GPa of oriented crystals, which cannot be achieved under practical conditions.

Research paper thumbnail of Effect of the Molecular Structure of the Polymer and Nucleation on the Optical Properties of Polypropylene Homo- and Copolymers

ACS Applied Materials & Interfaces, 2014

Two soluble nucleating agents were used to modify the optical properties of nine PP homo- and ran... more Two soluble nucleating agents were used to modify the optical properties of nine PP homo- and random copolymers. The ethylene content of the polymers changed between 0 and 5.3 wt %. Chain regularity was characterized by the stepwise isothermal segregation technique (SIST), while optical properties by the measurement of the haze of injection molded samples. Crystallization and melting characteristics were determined by differential scanning calorimetry (DSC). The analysis of the results proved that lamella thickness and change in crystallinity influence haze only slightly. A model was introduced which describes quantitatively the dependence of nucleation efficiency and haze on the concentration of the nucleating agent. The model assumes that the same factors influence the peak temperature of crystallization and optical properties. The analysis of the results proved that the assumption is valid under the same crystallization conditions. The parameters of the model depend on the molecular architecture of the polymer. Chain regularity determines supermolecular structure and thus the dependence of optical properties on nucleation.

Research paper thumbnail of Heterophasic copolymers of polypropylene: Development, design principles, and future challenges

Journal of Applied Polymer Science, 2013

ABSTRACT Initially developed to overcome the limited low‐temperature impact resistance of polypro... more ABSTRACT Initially developed to overcome the limited low‐temperature impact resistance of polypropylene (PP) and allow its application in subzero temperatures, the PP/elastomer blends became a very important and industrially relevant material class. Within the last 40 years, a whole range of PP impact copolymers from multireactor processes have been developed that are capable of covering wide application areas. This review summarizes their developmental history and presents options for the in‐reactor design of these polymers and their modification by compounding with elastomers and fillers or reinforcements after the polymerization. An overview of the application range is also presented together with a glimpse into the future of this material class. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3028–3037, 2013