Chain Branching Polypropylene Research Articles

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.

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.

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.

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.

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 Crystal Morphology of Linear/Long Chain Branching Polypropylene Blends

Strain hardening and crystallization behaviors, crystal structure and morphology of linear/long-chain-branching polypropylene blends (LPP/LCBPP) was studied by means of Rheotens, DSC, XRD, and POM. Upon incorporation of LCBPP into LPP, the melt strength and the strain rate durability of the latter were obviously enhanced, the nucleation and overall crystallization rates were increased and crystallization temperature elevated. At some middle fraction of LCBPP, the most remarkably effects were observed. The content of the branched architecture and the entanglement density were considered responsible for these influences.

The preparation and rheology characterization of long chain branching polypropylene

In order to study the rheological behavior of long chain branching (LCB) polypropylene (PP), linear polypropylene was modified by melt grafting reaction in the presence of 2,5-dimethyl-2,5( tert-butylperoxy) hexane peroxide and pentaerythritol triacrylate (PETA) in mixer. The transient torque curves and Fourier transformed infrared spectroscopy (FTIR) results indicated that macroradical recombination reactions took place and PETA had been grafted onto PP backbone. Various rheological plots including viscosity curve, storage modulus, loss angle, Han plot, Cole–Cole plot were used to distinguish LCB PP from linear PP. On the other hand, to quantify the LCB level in modified PPs, a new method was suggested on the basis of macromolecular dynamics models. The results showed that the level of LCB was in the range of 0.025–0.38/10 4 C . Moreover, the length of the branched chains and the content of the branched component increase with PETA concentration. Furthermore, the LCB efficiency of monomer can also be calculated, less than 20% of grafting monomers was used to form branch structure.