Effect of β-nucleating agents on crystallization and melting behavior of isotactic polypropylene

Enhancing Crystallization and Toughness of Wood Flour/Polypropylene Composites via Matrix Crystalline Modification: A Comparative Study of Two β-Nucleating Agents

Incorporation of short wood fillers such as wood flour (WF) into polypropylene (PP) often results in a marked reduction of toughness, which is one of the main shortcomings for WF/PP composites. This research reports a facile approach to achieve toughening of WF/PP composites via introducing self-assembling β-nucleating agents into PP matrix. The effect of two kinds of nucleating agents, an aryl amide derivative (TMB5) and a rare earth complex (WBG II), at varying concentrations on the crystallization and mechanical properties of WF/PP composites was comparatively investigated. The results showed that both nucleating agents were highly effective in inducing β-crystal for WF/PP, with β-crystal content (k_β) value reaching 0.8 at 0.05 wt% nucleating agent concentration. The incorporation of TMB or WBG significantly decreased the spherulite size, increased the crystallization temperature and accelerated the crystallization process of WF/PP. As a result of PP crystalline modification, the toughness of composites was significantly improved. Through introducing 0.3 wt% TMB or WBG, the notched impact strength and strain at break of WF/PP increased by approximately 28% and 40%, respectively. Comparatively, although WF/PP-WBG had slightly higher K_β value than WF/PP-TMB at the same concentration, WF/PP/TMB exhibited more uniform crystalline morphology with smaller spherulites. Furthermore, the tensile strength and modulus of WF/PP-TMB were higher than WF/PP-WBG. This matrix crystalline modification strategy provides a promising route to prepare wood filler/thermoplastic composites with improved toughness and accelerated crystallization.

The Influence of Melt Status and Beta-Nucleation Agent Distribution on the Crystallization of Isotactic Polypropylene

Although being investigated extensively in past decades, the factors affecting β-crystallization in β-nucleating agent/iPP composites have not been identified completely. In this study, β-crystallization in mesomorphic melt and free melt...

Effect of rare earth nucleating agent on supercritical CO2 foaming behavior of block copolymerized polypropylene

The rare earth nucleating agent was used to modify block copolymerized polypropylene (PPB) in foaming process. The results show that the crystallization of PPB and the melting temperature of β-crystal increased gradually with increased β-crystal nucleating agent content. The total crystallinity decreased with amount of addition increasing, and the relative content of β-crystal increased first and then decreased. When β-crystal nucleating agent content was 0.4 wt%, the relative β-crystal content reached the maximum value of 95.27%, and the final crystal grain refinement significantly. The addition of rare earth β-crystal nucleating agent has a good effect on improving the uniformity of foam cells. Under the same content of β-crystal nucleating agent and pressure, the average cell diameter and expansion ratio increased with the saturation temperature increasing. After the foaming temperature reaches 155°C, the expansion ratio began to decrease, which was also consistent with the changed trend of relative β-crystal content. At the same content of temperature and relative β-crystal, as the foaming pressure increased, the cell diameter decreased gradually, and the expansion ratio increased first, and then decreased.

Mechanical, Thermal, and Morphological Properties of Graphene Nanoplatelet-Reinforced Polypropylene Nanocomposites: Effects of Nanofiller Thickness

In this work, polypropylene (PP) and graphene nanoplatelet (GNPs) composites are routed through twin screw mixing and injection moulding. Two types of GNPs with a fixed size of 25 µm with surface areas ranging from 50–80 m2/g (H25, average thickness 15 nm) and 120–150 m2/g (M25, average thickness 6–8 nm) were blended with PP at loading rates of 1, 2, 3, 4, and 5 weight%. Mechanical properties such as tensile, flexural, and impact strengths and Young’s modulus (Ε) are determined. The X-ray diffraction (XRD), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA), field emission scanning electron microscopy (FESEM), and polarised light microscopy (PLM) techniques are used to understand the crystallisation, thermal, dynamic mechanical, and structural behaviour of the prepared composites. The improvement of mechanical strength is observed with GNP loading for both grades. Decreasing the GNP thickness decreases the impact strength and on the other hand improves the tensile and flexural strengths and Young’s modulus. Maximum tensile (≈33 MPa) and flexural (≈58.81 MPa) strength is found for the composite carrying 5 wt% M25. However, maximum impact strength (0.197 J) is found for PP-5 wt% H25. XRD analysis confirms GNPs have an induction effect on PP’s β phase crystal structure. The PP-GNP composite exhibits better thermal stability based on determining the TD (degradation temperature), T10 (temperature at 10% weight loss), T50 (temperature at 50% weight loss), and TR (temperature at residual weight). Enhancement in melt (Tm) and crystallisation temperatures (Tc) is are observed due to a heterogeneous nucleation effect. The FESEM analysis concludes that the GNP thickness has a significant effect on the degree of dispersion and agglomeration. The smaller the thickness, the better is the dispersion and the lower is the agglomeration. Overall, the use of thinner GNPs is more advantageous in improving the polymer properties.

Crystalline modification of a rare earth nucleating agent for isotactic polypropylene based on its self-assembly

In this paper, the crystalline modification of a rare earth nucleating agent (WBG) for isotactic polypropylene (PP) based on its supramolecular self-assembly was investigated by differential scanning calorimetry, wide-angle X-ray diffraction and polarized optical microscopy. In addition, the relationship between the self-assembly structure of the nucleating agent and the crystalline structure, as well as the possible reason for the self-assembly behaviour, was further studied. The structure evolution of WBG showed that the self-assembly structure changed from a needle-like structure to a dendritic structure with increase in the content of WBG. When the content of WBG exceeded a critical value (0.4 wt%), it self-assembled into a strip structure. This revealed that the structure evolution of WBG contributed to the K_β and the crystallization morphology of PP with different content of WBG. In addition, further studies implied that the behaviour of self-assembly was a liquid-solid transformation of WBG, followed by a liquid-liquid phase separation of molten isotactic PP and WBG. The formation of the self-assembly structure was based on the free molecules by hydrogen bond dissociation while being heated, followed by aggregation into another structure by hydrogen bond association while being cooled. Furthermore, self-assembly behaviour depends largely on the interaction between WBG themselves.

Toward Subtle Manipulation of Fine Dendritic β-Nucleating Agent in Polypropylene

Dendritic β-nucleating agent (β-NA) can readily manipulate the formation of dendritic β-crystal with a unique toughening effect on polypropylene (PP) to drastically enhance the ductility. However, by the current method, the geometric size is too large to fully perform the nucleating efficiency. In this study, by comparatively investigating the effect of molecular weight of PP and diffusion of β-NAs in a PP melt, we proposed a novel carrier strategy that selective enrichment of β-NAs in a PP carrier was followed by directed migration into polymer matrix. Accordingly, the growth of NAs was controlled by the release from the PP carrier, which decreased the available amount of β-NAs during the growth stage. In this case, the viscosity difference between PP carrier and matrix determined the interfacial movement of β-NAs. When the PP carrier and matrix had same molecular weight, the diffusion and release became favorable to facilitate the formation of the dense and fine dendritic aggregates. As a result, the relative content of β-crystals reached 92%, with a drastic increase of ∼82% in the optimal condition compared to the directed compounded PP/β-NAs sample. This study can open a new avenue to tailor the topologies of β-NAs and the ensuing β-crystals for high-performance PP products.

Effects of surface modification of halloysite nanotubes on the morphology and the thermal and rheological properties of polypropylene/halloysite composites

Effects of unmodified halloysite nanotubes (HNTs) and hexadecyl trimethyl ammonnium bromide treated halloysite nanotubes (CTAB-HNTs) on the morphology as well as the thermal and rheological properties of the HNT-filled polypropylene (PP) composites were investigated. The composites were melt-blended with a novel vane mixer dominated by extensional deformation. Fourier transform infrared spectroscopy well demonstrated that ammonium molecules were successfully interacted with halloysite groups. Compared with unmodified HNTs, the modified HNTs had a better dispersion in the PP matrix. The degree of crystallinity increased with the introduction of HNTs into the PP matrix. Thermal analyses revealed that CTAB-HNTs can improve the composites’ thermal stability compared with unmodified HNTs. As for dynamic shear rheology, the PP/CTAB-HNT composites showed higher viscoelaticity than neat PP in most frequency regimes, and that the raw HNT/PP composites presented the opposite phenomenon. The relaxation time of PP melt and the availability of heterogeneous PP domains increased with the presence of CTAB-HNTs.

Structural Evolution and Toughening Mechanism of β-Transcrystallinity of Polypropylene Induced by the Two-Dimensional Layered Interface during Uniaxial Stretching

The structure and morphology of β-crystals of isotactic polypropylene (iPP) are of great significance because β-crystals can improve the toughness and ductility of iPP. Toughening of β-spherulites, which was ascribed to phase transformation, has been extensively investigated. However, the toughening mechanism of other β-crystals with special structures and morphologies is not clear. In this study, β-transcrystallinity (β-TC), which showed a greater toughening effect than that of β-spherulite, was constructed through microlayered coextrusion. During uniaxial stretching, β-TC preferred to transform into an α-crystal, whereas β-spherulite preferred to transform to a smectic mesophase. The transformation degree of β-TC was much higher than that of β-spherulite. More importantly, the lamellar fragments from β-TC gradually rearranged along the stretching direction, accompanied by continuous absorption of energy. The special β-α phase transformation, high transformation rate, and rearrangement of lamellar fragments led to the highly improved toughness of the layered samples.

Root-like natural fibers in polypropylene prepared via directed diffusion and self-assembly driven by hydrogen bonding

Constructing root-like natural fibers in polypropylene via hydrogen bonding-driven diffusion and aggregation of self-assembling molecules.

Facilely assess the soluble behaviour of the β-nucleating agent by gradient temperature field for the construction of heterogeneous crystalline-frameworks in iPP

Nucleating agent (NA) species with solubility and self-assembly abilities can readily and effectively manipulate the crystalline morphology of semicrystalline polymers through the construction of heterogeneous frameworks prior to the primary crystallization of basal resins. However, the solubility of NA species is difficult to assess by the current traditional methods. In this study, gradient temperature field (g-T field) was utilized for the first time to ascertain the dissolution and self-assembly behaviors of β-NA in the melts of isotactic polypropylene (iPP). The g-T field technique can facilely assess the soluble behavior of β-NA by determining the transformation between several NA frameworks, namely the needle-, flower- and dendrite-like supramolecular structures. Clarifying the soluble behavior of β-NA is of great significance to guide the formation of various crystalline frameworks under the homo-temperature fields and control the resultant crystalline morphology of β-modified iPP. Some interesting findings are summarized as follows: (1) an in situ observation under the g-T field clearly indicates the sequential occurrence of various nucleation and crystallization events in the same observed window, and proves the migration of well molten β-NA, (2) the exact correlation between Tf and framework type reveals that an abrupt transformation (over the narrow temperature range of 1 °C) occurred between needle-like and dendrite-like frameworks, (3) the primary crystallization of iPP is strongly dependent on the construction mode of the β-NA framework.

Polymer blend nanocomposites based on poly(l-lactic acid), polypropylene and WS2 inorganic nanotubes

The overall thermal and mechanical properties of PLLA/PP_PP-g-MAH/INT-WS₂ confirm the high-performance of these novel biopolymer blend nanocomposites, which opens new possibilities for use in biomedical applications.