Branching Polypropylene Research Articles

Non‐isothermal Crystallization Kinetics, Thermal, and Rheological Behavior of Linear/Branched Polypropylene Blends

AbstractThis work aims at evaluating the rheological and thermal properties and non‐isothermal crystallization kinetics of blends of linear homopolymer polypropylene (HPP) and branched polypropylene (BPP). Two linear polypropylene's of different melt flow index (MFI) are used: H301 (10 g per 10 min) and H604 (1.5 g per 10 min); and one BPP (2 g pr 10 min). The rheological result shows that the H301/BPP blends have an increase in complex viscosity proportional to the addition of BPP amount, while for the H604/BPP blends, it is observed a higher influence of H604 on rheological properties. With the addition of 25 wt% of BPP, the strain hardening behavior is observed in the extensional rheology tests of the polymer blend. The DSC results show that the melt temperature and the crystallinity content in blends with BPP are affected by the molecular weight (MW) of the linear polymer. Results suggest that Pseudo‐Avrami/Jeziorny and Mo models can be used to predict the experimental results of crystallization kinetics of the blends with sufficient precision for all systems studied, regardless of MW. Therefore, it is possible to use the blends of HPP/BPP in processes that demand a combination of rheological properties, such as high strain hardening, and fast crystallization.

Lightweight investigation of long chain branching polypropylene/cellulose nanofiber composite foams

AbstractLong chain branching polypropylene (LCBPP)/cellulose nanofiber (CNF) composite foams were prepared by short shot foam injection molding method and their morphological, mechanical, and thermal properties were also investigated. The cellular structure of LCBPP/CNF composite foams was improved with weight reduction (WR) ratios increasing. The cell densities of LCBPP/CNF composite foams were dramatically increasing with WR ratios rising. More, the fine and uniform cellular structures were obtained due to the incorporation of CNF. The highest cell density, specific flexural strength, and modulus could achieved 20 × 103 cell/cm2, 65 MPa/(g/cm3) and 2.7 GPa/(g/cm3), respectively. Furthermore, the specific Charpy impact strengths were also higher than the ones of solid samples. At last, the thermal insulation properties were discussed accordingly.

Evolution of double crystal melting peak in polypropylene foam assisted by β‐nucleating agent and supercritical CO2

ABSTRACTPolypropylene (PP) foams with double crystal melting peak structure were prepared using supercritical CO2 as blowing agent. Such structure was induced by the β‐nucleating agent (NA) and dissolved CO2 in a lab‐scale autoclave system. From the dynamic rheological behaviors, one can see that long chain branching polypropylene (LCBPP) possess better melt elasticity than linear polypropylene (LPP). In order to improve the foamability, LPP was blended with LCBPP. The results of differential scanning calorimeter and wide‐angle X‐ray diffraction indicated that NA was helpful for the introduction of β‐crystal in LPP, but not for LCBPP and PP blends. Therefore, the foamability and the content of β‐crystal of various PP foams can be effectively controlled by adjusting the ratio of LPP and LCBPP. Meanwhile, both the cooling rate and the content of NA affected the formation of β‐crystal in various PP samples. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46007.

Development of high thermal insulation and compressive strength BPP foams using mold-opening foam injection molding with in-situ fibrillated PTFE fibers

Polymer foam has become an important thermal insulation material due to its outstanding features, while its fabrication is still very challenging by foaming injection molding technology. Herein, we reported the successful fabrication of low-density branched polypropylene (BPP) foam with an expansion ratio of up to 25-fold and fine cellular structures using mold-opening foam injection molding (MOFIM) technology. To improve foaming ability, polytetrafluoroethylene (PTFE) nanofiber reinforced BPP composite was prepared in an in-situ way through a simple twin-screw blending technology. The DSC results show that PTFE nanofibers can promote crystallization of BPP effectively, while the rheological measurements demonstrate that PTFE fibers can improve the viscoelastic behavior of BPP. Furthermore, the MOFIM experimental results show that the cell size is reduced by one order of magnitude while the cell density is increased by four orders of magnitude. Interestingly, PTFE nanofibers endow BPP/PTFE foams a unique cell wall structure with plenty of micro-holes and micro/nano fibrils. Moreover, BPP/PTFE foams show markedly improved thermal insulation and compressive mechanical performance with a thermal conductivity of as low as 32.4 mW·m−1·K−1. Thus, this unique low-density BPP/PTFE foam shows a promising future in high-performance thermal insulation applications, particularly considering the used scalable, versatile, facile and cost-saving processing technology.

Influence of different functionality degree grafting monomers on polypropylene melt radical branching reaction and their structure

Long‐chain–branched polypropylene (LCBPP) is one of polypropylenes (PPs) with high melt strength and good melt elasticity. Recently, due to its outstanding properties, LCBPP have been attracted increasingly attention in the field of development and characterization by the researchers all over the world. In this study, LCBPP was prepared by the melt radical branching reaction in a torque rheometer. The influences of various acrylate monomers with different functionality degrees on the structure and melt performance of PP products were investigated. The results indicated that grafting monomers with different functionality degrees made diverse influences on the branching density and branching chain length of branching PP products. With the increase of the functionality degree of grafting monomers, the branching level of PP products increased gradually and the “multiplicity” of branches became increasingly obvious. Besides, a higher reactivity of pentaerythritol triacrylate with hydroxyl than the similar molecular structured pentaerythritol tetraacrylate was confirmed. Furthermore, due to the high reactivity of dipentaerythritol penta(hexa)acrylate, branching and crosslinking reaction occurred simultaneously during the reaction process. As a result, the gel content increased and finally formed highly star branching structures with a shape of “dense and short.”

The morphological evolution and β-crystal distribution of isotactic polypropylene with the assistance of a long chain branched structure at micro-injection molding condition

Long chain branching polypropylene (LCBPP) has good application prospect but the synergistic effects of the long chain branching (LCB) structure and complex flow field on the crystallization behavior of LCB polymers are still elusive. In the present work, microparts of isotactic polypropylene (iPP)/LCBPP blends were prepared by microinjection molding (MIM). The effect of the LCB structure on the morphology evolution and the β-crystal distribution in microparts were investigated. Interestingly, adding LCBPP facilitates the formation of oriented β-crystals along the shear direction in the core layer of the microparts but unexpectedly, decreases the relative content of β-crystals due to the heterogeneous nucleation effect of LCBPP. Moreover, the introduction of LCBPP can also improve the orientation degree of the molecular chains and shrink the thickness of the core layer thereby inevitably suppressing the “skin–core” structure which might optimize the properties of MIM parts.

Influence of molecular structure on the foamability of polypropylene: Linear and extensional rheological fingerprint

The foaming structure and rheological properties of four different isotactic homo-polypropylenes with various molecular weights and an isotactic long chain branched polypropylene were investigated to find a suitable rheological fingerprint for PP foams. The molecular weight distribution and thermal properties were measured using GPC-MALLS and differential scanning calorimetry, respectively. Small amplitude oscillatory shear data and uniaxial extensional experiments were analyzed using the frameworks of van Gurp-Palmen plot (δ vs. | G*|) and the molecular stress function model, respectively. These analyses were used to find a correlation between the molecular structure, rheological properties and foaming structures of linear and long chain branching polypropylenes. Two linear viscoelastic characteristics, | G*| at δ = 60° and | η*| at ω = 5 rad/s were used as criteria for foamability of these polymers, where decreasing of both parameters by increasing the long chain branching content results in smaller cell size and higher cell density. The molecular stress function model was able to quantify the strain hardening properties of long chain branching blends using small amplitude oscillatory shear data and two nonlinear material parameters, 1 ≤ β ≤ 2.2 and 1 ≤ [Formula: see text] ≤ 600, where the minimum and maximum values of these parameters belong to the linear and long chain branched polypropylene, respectively. Increasing the long chain branched polypropylene content of the PP blends increased strain hardening, and therefore improved the foaming characteristics significantly by suppressing the coalescence of cells. Dilution of linear PP with only 10 wt% of long chain branched polypropylene enhanced the cell density from 5.7 × 106 to 2.7 × 107 cell/cm3 and reduced the average cell diameter from 58 to 26 µm, respectively, while their volume expansion ratio remained in the same range of 2–3. Increasing of long chain branching to 50 and 100 wt% enhanced the V.E.R. to 6.2 and 7.8, respectively.

Fabrication of High Expansion Microcellular Injection-Molded Polypropylene Foams by Adding Long-Chain Branches

Herein, 10-fold expansion ratio foams were successfully fabricated for the first time by a core-back foam injection molding (FIM) technique using a newly developed long-chain branching polypropylene (LCBPP). Different rheological curves were investigated to distinguish the strain hardening behavior and melt strength of the LCBPP from isotactic polypropylene (iPP). Fast scanning chip calorimetry (FSC) results revealed that the long-chain branches could increase the crystallization temperature even at fast cooling rates. The FIM of LCBPP could notably not only reduce the cell sizes but also increase the cell densities through the capability of LCBPP to stabilize cells and prevent cells from coalescence. Furthermore, the foam processing window of manufacturing 10-fold expansion foams was significantly broadened for LCBPP foams. This study revealed that the introduction of long-chain branches is a good approach to fabricate high expansion microcellular foams for different potential applications such as constr...

Relationship between molecular structure, crystallization behavior, and mechanical properties of long chain branching polypropylene

The crystallization behaviors and mechanical properties of both linear isotactic polypropylene (iPP) and long chain branching polypropylene (LCBPP) were investigated with differential scanning calorimetry, polarized optical microscopy, and wide-angle X-ray diffraction. The results indicated that the long chain branching (LCB) structure could act as nucleating agent accelerating the nucleation but impeding the crystal growth of LCBPP, and eventually increased the crystallization peak temperature and decreased the spherulite size of LCBPP. Moreover, with the presence of LCB structure, β-form polypropylene was generated in LCBPPs under injection molding conditions, and the content of β-form increased with the increasing of LCB structure content. Benefiting from the combined effect of the LCB structure and β-form crystal, the LCBPPs revealed not only outstanding toughness but also desired stiffness. The impact strength, flexural modulus and tensile strength of HMSPP3 were increased by, respectively, 260.0, 54.7, and 17.2 % than those of linear iPP. These results are helpful to develop new types of high performance polypropylene.

Shear-induced β -form polypropylene in long chain branching isotactic polypropylene

Long chain branching polypropylene (LCBPP) with different long chain branching (LCB) contents were prepared by reactive extrusion in the presence of styrene and benzoyl peroxide, and their shear-induced crystallization behaviors were investigated. The results indicated that the LCB structure extended the relaxation time of LCBPP in the molten state, which led to the formation of β-form polypropylene under shear and high cooling rate. The content of β-form (Kβ) increased with the increase of LCB content, shearing rate and cooling rate. The Kβ value of LCBPP3 whose weight average molecular weight was 920,000 g mol−1 could be up to 52.0% with a shear rate of 60 s−1 associated with a cooling rate of 280°C min−1. This study is expected not only to have a deeper understanding of the shear-induced crystallization behavior of LCBPP, but also provide a new strategy to obtain high level β-form polypropylene. POLYM. ENG. SCI., 56:240–247, 2016. © 2015 Society of Plastics Engineers

The rheological property and foam morphology of linear polypropylene and long chain branching polypropylene

The rheological behavior, thermal properties and foam morphology of linear polypropylene and long chain branching polypropylene prepared through UV irradiation reactive extrusion were studied by rheological test, melt index test, DSC and supercritical carbon dioxide foaming technology. Rheological test and melt index test confirmed that under UV irradiation and extrusion, adding photo-initiator and crosslinking agent could achieve the purpose of branching, thus improved the melt strength of polypropylene effectively. The DSC results revealed that with the introducing of long chain branching, the melting range of the polypropylene broadened and the crystallization temperature increased. Owing to the introduction of long chain branches, polypropylene exhibited higher melt strength and strain hardening behavior. Compared with linear polypropylene, the foam morphologies of long chain branching polypropylene were more uniform.

Preparation of long-chain branching polypropylene and investigation on its foamability

In this work, zinc N,N-dimethyldithiocarbamate (ZDMC) was used to mediate the melt modification of polypropylene (PP) with 1,6-hexanediol diacrylate (HDDA) in an internal mixer in the presence of dicumyl peroxide (DCP). Fourier transformed infrared spectroscopy analysis revealed that HDDA was directly grafted onto PP backbone. The dependence of torque on processing time indicated that the presence of ZDMC restrained the degradation of PP, and the end-torque value increased with the addition of ZDMC. Dynamic rheological measurement indicated that the modified PP possessed higher G′ and lower tan δ at low frequency, displaying an increase in η* and disappearance of Newtonian plateau in η*–ω plot, as well as larger radius of semicircle in Cole–Cole plot. All the rheological characterizations, together with the decreased gel content with the increase of ZDMC, confirmed the formation of long-chain branching. Subsequently, the foamability of the modified samples was investigated by one-step compression–molding process. The cellular structure and morphology of the obtained foams were observed by scanning electron microscopy, and the results showed that the addition of ZDMC decreased the cell size, increased the cell density, and brought about well-defined closed cell structure. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers

Controlling the Transition of Long- and Short-Chain Branching Polypropylene

To control the length of branched chains on polypropylene under different quantities of monomer and free radical capture agent, trimethylol propane triacrylate (TMPTA) was chosen as monomer, and tetraethylthiuram disulfide (TETDS) was chosen as free radicals capture agent, also known as co-agent. Samples were modified by melting reaction in the presence of DHBP, 2,5-Dimethyl-2,5(tert-butylperoxy) hexane, in an internal mixer. From the results of the experiment, it was concluded that controlling of transition from long-chain branching PP to short-chain branching PP was achieved via the quantity variation of TMPTA and TETDS.

A novel strategy for the preparation of long chain branching polypropylene and the investigation on foamability and rheology

A novel strategy for introducing long chain branches (LCB) on commercial polypropylene (PP) was described using an extender agent, poly (hexamethylendiamine-guanidine hydrochloride) (PHGH), to couple with glycidyl methacrylate-grafted PP (PP-GMA). The presence of LCB was confirmed by GPC and melt strength measurements in two modified PP samples, MPP-1 and MPP-2. The foamability of LCB PPs and commercially linear PP (EPS) was investigated by foam extrudate. The results showed that the foamability of LCB PPs was significantly improved. To find out the difference in foamability of EPS and LCB PPs, some rheological properties were investigated. In the dynamic shear measurement, it was found that the elastic response of LCB PPs at low frequencies was distinctly enhanced in comparison with that of EPS, implying a presence of a long relaxation time mode that was not revealed in linear PP. The elongational rheology results showed that the strain hardening behaviors of LCB PPs were still intense even at higher strain rate of 1 s −1; moreover, MPP-2 could sustain larger elongational stress and deformation, which helps to improve the foamability. In step-shear stress relaxation measurements, the fast and slow relaxation processes were observed over the entire ranges of strains, indicating the presence of two different relaxation time modes. Also, the strain dependence of slow damping function became weaker because of the introduction of LCB on linear PP. In addition, it was also found that the rheological behaviors characterizing the long relaxation time mode were further enhanced with the increase of the concentration of PHGH in the melt grafting reactions.

Supercritical carbon dioxide-assisted reactive extrusion for preparation long-chain branching polypropylene and its rheology

An environmental benign process, which uses supercritical carbon dioxide (ScCO 2) as a processing aid, is developed in this work to prepare long chain branching polypropylene (LCB-PP). Results from the oscillatory shear rheology, melt elongational behavior and Fourier transformed infrared spectroscopy (FTIR) show that long chains have been linked as branches to the original linear PP chains using scCO 2-assisted reactive extrusion in the presence of cumene hydroperoxide and 1,6-hexanediol diacrylate. Compared to the initial linear PP, the branched samples show higher storage modulus ( G′) at low frequency, distinct strain hardening of elongational viscosity, lower melt flow rate, increased crystallization temperature and improvement of the melt strength. ScCO 2 can improve the branching efficiency of modified PPs. The elastic response, melt strength and strain hardening parameter of the modified PPs increase with increasing scCO 2 concentration, which is ascribed to scCO 2 acting as a plasticizer for reducing PP viscosity and a carrier for active chemical species.

Structure and melt rheology of long-chain branching polypropylene/clay nanocomposites

Long-chain branching polypropylene (LCB-PP)/clay nanocomposites were prepared by melt blending in a twin-screw extruder. The microstructure and melt rheology of these nanocomposites were investigated using x-ray diffraction, transmission electron microscopy, oscillatory shear rheology, and melt elongation testing. The results show that, the clay layers are intercalated by polymer molecular chains and exfoliate well in LCB-PP matrix in the presence of maleic anhydride grafted PP. Rheological characteristics, such as higher storage modulus at low-frequency and solid-like plateau in tan-ω curve, indicate that a compact and stable filler network structure is formed when clay is loaded at 4 phr (parts per hundred parts of) or higher. The response of the nanocomposite under melt extension reveals an initial decrease in the melt strength and elongational viscosity with increasing clay concentration up to 6 phr. Later, the melt strength and elongational viscosity show slight increases with further increasing clay concentration. These results might be caused by a reduction in the molecular weight of the LCB-PP matrix and by the intercalation of LCB-PP molecular chains into the clay layers. Increases in the melt strength and elongational viscosity for the nanocomposites with decreasing extrusion temperature are also observed, which is due to flow-induced crystallization under lower extrusion temperature. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Interfacial tension of linear and branched PP in supercritical carbon dioxide

In this study, sessile drops are imaged in a high-pressure and high-temperature view chamber to determine the density and interfacial tension of linear polypropylene (LPP) and branched polypropylene (BPP) melts in supercritical carbon dioxide (CO 2). The pressure–volume–temperature (PVT) data of polyprophylene (PP)–CO 2 is investigated by monitoring the swelling changes of the polymer melt in supercritical CO 2. The density difference between the polymer/CO 2 mixture and the CO 2 is determined by combining the swelling results with the CO 2 solubility information in the polymer melt. Both the Sanchez–Lacombe (SL) and the Simha–Somcynsky (SS) equations-of-state (EOS) are applied to predict the density of the PP–CO 2 mixture, which is then compared to the density data obtained experimentally. The dependence of interfacial tension on the temperature and pressure of PP in supercritical CO 2 is investigated at temperatures from 180 °C to 220 °C and pressures up to 31 MPa. Effects of long-chain branching on the density and interfacial tension of PP–CO 2 mixtures are discussed.

The melt grafting preparation and rheological characterization of long chain branching polypropylene

To study the rheological properties of long chain branching (LCB) polypropylene (PP), long chain branches (LCB) were grafted onto the linear PP by melt grafting reaction in the presence of a novel chain extender, poly(hexamethylendiamine-guanidine hydrochloride) (PHGH). The branching reactions between the functionalized PP and PHGH were confirmed by transient torque curves and FTIR. By differential scanning calorimetry (DSC) and polarized microscope measurements, the presence of long chain branching structures was further confirmed. Also, the viscoelastic properties of the LCB PP and linear PP under shear flow were investigated for distinguishing LCB PP from linear PP. It was found that the elastic response of LCB PP at low frequencies was significantly enhanced in comparison with that of the linear PP, implying a presence of a long relaxation time mode that was not revealed in linear PP. Moreover, the branching levels of LCB PP were quantified using a detailed method, which was in correspondence with the molar amount of PHGH grafted on PP.

Rheology and melt strength of long chain branching polypropylene prepared by reactive extrusion with various peroxides

AbstractLong Chain Branching Polypropylenes were prepared in an extruder by a melt grafting reaction in the presence of various peroxides and a polyfunctional monomer of 1,6‐hexanediol diarylate. Fourier Transformed Infrared spectra and the rheological characteristics indicated that the grafting reaction added long branched chains to linear polypropylene (PP). In comparison to the initial PP, the branched samples exhibited higher melt strength, lower melt flow index, and enhancement of crystallization temperature. The branching number of the modified samples agreed well with their melt viscoelasticity and the improved degree of their melt strength. The branching level in modified PPs could be controlled by the property and structure of the peroxide used. Peroxides with lower decomposition temperature and more stable radicals after decomposition promoted the branching reaction, leading to the modified PPs with higher branching level and melt strength. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers

Crystallization behavior and crystallization kinetic studies of isotactic polypropylene modified by long‐chain branching polypropylene

AbstractIn this article, we discuss the crystallization behavior and crystallization kinetics of isotactic polypropylene (iPP) modified by long‐chain‐branching (LCB) high‐melt‐strength iPP over a wide composition range, that is, LCB‐iPP from 10 to 50 wt %. Over the entire range we investigated, the presence of LCB‐iPP accelerated crystallization in both the isothermal crystallization process and nonisothermal crystallization process, even when the LCB‐iPP content was as low as 10%, and both crystallization processes were enhanced more significantly as the LCB‐iPP content increased. Hoffman–Lauritzen theory analysis revealed that the fold‐free energy decreased effectively with the occurrence of the LCB structure, although the growth rate of spherulites was depressed, as shown by polarized optical microscopy. Meanwhile, the regime III–regime II transition temperature was about 15° higher for all of the LCB‐iPP compositions than that of iPP because the LCB structure reduced the mobility of the polypropylene chains. Furthermore, the γ‐form crystal structure was favored by LCB compared to the β form, which was supported by wide‐angle X‐ray diffraction. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009