Foaming behavior of microcellular foam polypropylene/modified nano calcium carbonate composites

Density graded polymer composite foams

This article reviews the main developments associated with density graded polymer composite foams. After a short introduction, a discussion is made on both density graded polymer-based composites and foams separately to better understand the challenges when both structures are combined together to produce composite foams. In all cases, the basic concepts, fabrication methods, main properties and general applications are presented. Opening for future works are also presented to conclude.

Extensional rheology and CO2 foaming of thermoplastics vulcanizates: Influence of the crosslinking chemistry

The objective of this study is to investigate the ability of thermoplastic vulcanizates (TPVs) (materials based on PP/EPDM blend) to foam under CO2 batch conditions. The EPDM phase, which is dispersed into the PP phase, was dynamically crosslinked either by a phenolic resin (Resol) or by a radical peroxide (dicumyl peroxide). The results show an influence of the crosslinking chemistry on the extensional viscosity of the TPV. Regarding radical chemistry, the peroxide induces polypropylene degradation by β-scission reaction during the dynamic crosslinking process. As a result, the ability of the TPV to deform under extensional flow (Hencky deformation at break <0.5) is greatly reduced. On the contrary, the Resol-based TPV has demonstrated a non-linear viscosity behaviour (strain hardening) and a great ability to deform (Hencky deformation at break >1.5). This unexpected result for a non-homogeneous system can be explained by the confinement of the PP phase between EPDM nodules which gives to the PP chains a gel rheological behaviour. In addition, the influence of the addition of carbon black filler has also been studied. Finally, the relationship between extensional viscosity and physical foaming has been investigated. As for a homogeneous polymer, the extensional viscosity has been proved to be a key factor to estimate the foaming behaviour of complex systems like TPV. Hence, the importance of non-linear viscosity for a multi-phasic polymer to ensure foaming ability has been demonstrated.

Effects of inorganic particles on the crystallization, mechanical properties and cellular structure of foamed PP composites in the IMD/MIM process

Foamed polypropylene (PP) composites containing inorganic particles were prepared using a combined in-mold decoration and microcellular injection molding (IMD/MIM) method, and supercritical fluid-nitrogen was used as the physical blowing agent. Calcium carbonate (CaCO₃) and montmorillonite (MMT) with particle size that varied from nanometer to micron were selected as reinforcement particles. The effect of inorganic particle type and size on the crystallization, mechanical properties, and cellular structure of the parts were investigated. The results showed that the polymer chains were not well inserted into the MMT layer, and there was almost no increase in the interlayer spacing. As the particle size decreased, the relative amount of β-crystal and mechanical properties increased. The addition of CaCO₃ was more effective than MMT at increasing the volume fraction of the β-crystal form when the particle size was reduced to the nanometer level. In the vertical and parallel section, the average cell diameter of the composites decreased and the cell density increased as the particle size decreased. As the particle size of the inorganic filler decreased, the deformation degree of cells gradually increased.

As the particle size decreased, the relative amount of β-crystal and mechanical properties increased, while the thickness of the core layer gradually decreased.

Investigation on Foamed PP/Nano-CaCO3 Composites in a Combined in-Mold Decoration and Microcellular Injection Molding Process

A combined in-mold decoration and microcellular injection molding (IMD/MIM) method has been used in this paper. The foamed PP/nano-CaCO₃ composites were prepared to investigate their mechanical properties, cellular structure, and surface quality. The content of nano-CaCO₃ varied from 0 to 10 wt %. The results showed that nano-CaCO₃ acted as a reinforcing phase and nucleating agent, which help to improve the mechanical properties of foamed composites. The cellular structure and mechanical properties were optimum when the nano-CaCO₃ content was 6 wt %. In the vertical section, the cell size and density of transition layer on the film side was bigger than that on the non-film side. In the parallel section, the cell ratio of length to diameter of transition layer on the film side was smaller than that on the non-film side, and the cell tile angle was larger than that on the non-film side. With nano-CaCO₃ content increasing, the surface quality showed a trend of decreasing first and then increasing.

A Novel Hybrid Foaming Method for Low-Pressure Microcellular Foam Production of Unfilled and Talc-Filled Copolymer Polypropylenes

Unfilled and talc-filled Copolymer Polypropylene (PP) samples were produced through low-pressure foam-injection molding (FIM). The foaming stage of the process has been facilitated through a chemical blowing agent (C₆H₇NaO₇ and CaCO₃ mixture), a physical blowing agent (supercritical N₂) and a novel hybrid foaming (combination of said chemical and physical foaming agents). Three weight-saving levels were produced with the varying foaming methods and compared to conventional injection molding. The unfilled PP foams produced through chemical blowing agent exhibited the strongest mechanical characteristics due to larger skin wall thicknesses, while the weakest were that of the talc-filled PP through the hybrid foaming technique. However, the hybrid foaming produced superior microcellular foams for both PPs due to calcium carbonate (CaCO₃) enhancing the nucleation phase.

Effect of acrylonitrile–butadiene–styrene terpolymer on the foaming behavior of polypropylene

To improve the cellular foam structure of common polypropylene (PP), acrylonitrile–butadiene–styrene terpolymer (ABS) and compatibilizer were used to blend with PP, and the foaming behavior of PP/ABS blends was investigated. The solid and foamed samples of the PP/ABS blend with different component were first fabricated by melt extrusion followed by conventional injection molding with or without a blowing agent. The mechanical properties, thermal features, and rheological characterizations of these samples were studied using the tensile test, dynamic mechanical analyzer, differential scanning calorimetry, scanning electron microscopy, X-ray diffraction, and torque rheometry. The results suggest that ABS is a suitable candidate to improve the foamability of PP. The effect of ABS and compatibilizer on the foamability of PP can be attributed to three possible mechanisms, that is, the weak interaction between phases that facilitates cell nucleation, the improved gas-melt viscosity that prevents the escape of gas, and the influence of crystallization behavior that helps to form a fine foaming structure.

Effect of Compatibility on the Foaming Behavior of Injection Molded Polypropylene and Polycarbonate Blend Parts

To improve the foaming behavior of a common linear polypropylene (PP) resin, polycarbonate (PC) was blended with PP, and three different grafted polymers were used as the compatibilizers. The solid and foamed samples of the PP/PC 3:1 blend with different compatibilizers were first fabricated by melt extrusion followed by injection molding (IM) with and without a blowing agent. The mechanical properties, thermal features, morphological structure, and relative rheological characterizations of these samples were studied using a tensile test, dynamic mechanical analyzer (DMA), scanning electron microscope (SEM), and torque rheometer. It can be found from the experimental results that the influence of the compatibility between the PP and PC phases on the foaming behavior of PP/PC blends is substantial. The results suggest that PC coupling with an appropriate compatibilizer is a potential method to improve the foamability of PP resin. The comprehensive effect of PC and a suitable compatibilizer on the foamability of PP can be attributed to two possible mechanisms, i.e., the partial compatibility between phases that facilitates cell nucleation and the improved gas-melt viscosity that helps to form a fine foaming structure.

Effects of Nano-CaCO₃ Content on the Crystallization, Mechanical Properties, and Cell Structure of PP Nanocomposites in Microcellular Injection Molding

Using supercritical nitrogen as the physical foaming agent, microcellular polypropylene (PP) nanocomposites were prepared in microcellular injection molding. The main purpose of this work is to study effects of content of nano-CaCO₃ on the crystallization, mechanical properties, and cell structure of PP nanocomposites in microcellular injection molding. The results show that adding nano-CaCO₃ to PP could improve its mechanical properties and cell structure. The thermal stability and crystallinity enhances with increase of nano-CaCO₃. As a bubble nucleating agent, adding nano-CaCO₃ to PP improves the cell structure in both the parallel sections and vertical sections. The mechanical properties increase first and then decrease with increase of nano-CaCO₃. The mechanical properties are affected by the cell structure, as well. The mechanical properties and cell structure are optimum when the content of nano-CaCO₃ is 6 wt %.

The Synergy of Double Cross-linking Agents on the Properties of Styrene Butadiene Rubber Foams

Sulfur (S) cross-linking styrene butadiene rubber (SBR) foams show high shrinkage due to the cure reversion, leading to reduced yield and increased processing cost. In this paper, double cross-linking system by S and dicumyl peroxide (DCP) was used to decrease the shrinkage of SBR foams. Most importantly, the synergy of double cross-linking agents was reported for the first time to our knowledge. The cell size and its distribution of SBR foams were investigated by FESEM images, which show the effect of DCP content on the cell structure of the SBR foams. The relationships between shrinkage and crystalline of SBR foams were analyzed by the synergy of double cross-linking agents, which were demonstrated by FTIR, Raman spectra, XRD, DSC and TGA. When the DCP content was 0.6 phr, the SBR foams exhibit excellent physical and mechanical properties such as low density (0.223 g/cm³), reduced shrinkage (2.25%) and compression set (10.96%), as well as elevated elongation at break (1.78 × 10³%) and tear strength (54.63 N/mm). The results show that these properties are related to the double cross-linking system of SBR foams. Moreover, the double cross-linking SBR foams present high electromagnetic interference (EMI) shielding properties compared with the S cross-linking SBR foams.

The simultaneous effect of nucleating and blowing agents on the cellular structure of polypropylene foamed via the extrusion process

The current work presents the preparation of polypropylene (PP) foams by the extrusion process, focusing on the influence of the foaming agent and nucleating agent on the microstructure of the foams. Sodium bicarbonate alone and also its mixture with citric acid were used as the chemical blowing agents. Expanded graphite nanoparticle and talc were also used as the nucleating agents. Great differences were found in terms of the foam structure depending on the type of nucleating and blowing agents. Using expanded graphite nucleating agent instead of talc resulted in foams with higher cell densities and more uniform cellular structures. Moreover, the foams including the mixed blowing agents exhibited higher cell densities and upper expansion ratio.

Current Issues and Challenges in Polypropylene Foaming: A Review

Thermoplastic foams have several advantages in comparison with unfoamed polymers such as lightweight, high strength to weight ratio, excellent insulation property, high thermal stability, high impact strength and toughness, as well as high fatigue life. These outstanding properties lead cellular plastics to various industrial applications in packaging, automotive parts, absorbents, and sporting equipment. Nowadays, polypropylene (PP), because of its outstanding characteristics such as low material cost, high service temperature, high melting point, high tensile modulus, low density, and excellent chemical resistance, is a major resin in the foaming industry. However, foaming of conventional PP is limited by its low melt strength leading to poor cell morphology, cell rupture/coalescence and limited density reduction. To improve PP melt strength, several strategies including particle addition as nucleating agent, introduction of long chain branching, blending with high melt strength polymers and crosslinking have been proposed. In this review, these issues are discussed and analyzed in terms of mechanical, thermal, and rheological characterizations.

Crystallization behavior of functional polypropylene grafted graphene oxide nanocomposite

With an aim to understand a role of grafted graphene oxide (GO) in crystallization process of polymer, isothermal and non-isothermal crystallization behaviors of a functional polypropylene grafted GO nanocomposite were investigated systematically.

Preparation and Crystallization Property of Ternary Composites of WBG/AA-RCC/PP

A series of ternary composites of WBG/AA-RCC/PP were prepared and their crystallization behaviors were investigated by XRD and DSC. The results indicate that WBG and AA-RCC affect the crystallization of PP matrix with a mutual inhibition effect. This effect is enhanced with increasing content of WBG and induces the growth ofβ2crystal. AA-RCC promotes the heterogeneous nucleation ofαcrystal and offers a template forαcrystal growth along a specific lattice plane and promotes the epitaxial crystallization of PP matrix, while the specific arrangedαcrystal could induce the formation ofβcrystal.

Thermoplastic Elastomer Foams Based on Recycled Rubber

In this work, different concentrations (0, 35, 50 and 65%wt.) of recycled rubber (ethylene-propylene diene, EPDM) were blended with virgin polypropylene (PP) to produce thermoplastic elastomer resins via twin-screw extrusion. Then, the samples were pelletized and foamed by injection molding using a chemical blowing agent (azodicarbonamide, ADC). In particular, the molding process was optimized to determine the effect of processing parameters like blowing agent content, mold temperature, and injection conditions (pressure, velocity, etc.). From the samples obtained, a complete morphological (skin thickness, cell size and cell density) and mechanical characterization was performed including density and hardness. The results obtained showed that it is more difficult to produce a good foam structure with increasing recycled rubber content.