Measurement of gas solubility in linear/branched PP melts

Foam 3D Printing of Thermoplastics: A Symbiosis of Additive Manufacturing and Foaming Technology

Due to their light-weight and cost-effectiveness, cellular thermoplastic foams are considered as important engineering materials. On the other hand, additive manufacturing or 3D printing is one of the emerging and fastest growing manufacturing technologies due to its advantages such as design freedom and tool-less production. Nowadays, 3D printing of polymer compounds is mostly limited to manufacturing of solid parts. In this context, a merged foaming and printing technology can introduce a great alternative for the currently used foam manufacturing technologies such as foam injection molding. This perspective review article tackles the attempts taken toward initiating this novel technology to simultaneously foam and print thermoplastics. After explaining the basics of polymer foaming and additive manufacturing, this article classifies different attempts that have been made toward generating foamed printed structures while highlighting their challenges. These attempts are clustered into 1) architected porous structures, 2) syntactic foaming, 3) post-foaming of printed parts, and eventually 4) printing of blowing agents saturated filaments. Among these, the latest approach is the most practical route although it has not been thoroughly studied yet. A filament free approach that can be introduced as a potential strategy to unlock the difficulties to produce printed foam structures is also proposed.

Determination of CO2 solubility in semi-crystalline polylactic acid with consideration of rigid amorphous fraction

Due to phase heterogeneity in semi-crystalline polymers, accurate determination of gas solubility has been a challenge. In this regard, PLA/CO₂ was used as a case study to investigate the parameters governing formation of the rigid amorphous fraction (RAF) and its effect on the gas sorption behavior of the polymer. Six samples with different degrees of RAF were prepared through varying PLA tacticity and thermal history. Then, a gravimetric method involving a magnetic suspension balance and an in-house PVT visualization system was employed to experimentally determine the CO₂ solubility at 70 °C under a pressure of 4.5 MPa. Furthermore, a theoretical CO₂ solubility was calculated based on the Simha-Somcynski equation of state and was used in conjunction with the two-phase and three-phase models to describe the phase dependency of the gas solubility. The conventional two-phase model that considered the bulk amorphous phase consistently over-approximated the CO₂ solubility compared to the measured data. On the other hand, the three-phase model that distinguished the rigid and the mobile amorphous phases well represented the experimental result. The analysis yielded CO₂ solubility coefficients of 0.0375 g_gas/g_poly for the RAF and 0.0817 g_gas/g_poly for the mobile counterpart.

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.

Application of a constant hole volume Sanchez-Lacombe equation of state to mixtures relevant to polymeric foaming

A variant of the Sanchez-Lacombe equation of state is applied to several polymers, blowing agents, and saturated mixtures of interest to the polymer foaming industry. These are low-density polyethylene-carbon dioxide and polylactide-carbon dioxide saturated mixtures as well as polystyrene-carbon dioxide-dimethyl ether and polystyrene-carbon dioxide-nitrogen ternary saturated mixtures. Good agreement is achieved between theoretically predicted and experimentally determined solubilities, both for binary and ternary mixtures. Acceptable agreement with swelling ratios is found with no free parameters. Up-to-date pure component Sanchez-Lacombe characteristic parameters are provided for carbon dioxide, dimethyl ether, low-density polyethylene, nitrogen, polylactide, linear and branched polypropylene, and polystyrene. Pure fluid low-density polyethylene and nitrogen parameters exhibit more moderate success while still providing acceptable quantitative estimations. Mixture estimations are found to have more moderate success where pure components are not as well represented. The Sanchez-Lacombe equation of state is found to correctly predict the anomalous reversal of solubility temperature dependence for low critical point fluids through the observation of this behaviour in polystyrene nitrogen mixtures.

High-Pressure Preform Foam Blow Molding

Recently, several companies have started to use the foaming technology in blow molding processes, primarily in extrusion blow molding. Despite the design complexity involved in the preform blow molding method, substantial advantages result when microcellular foaming and blow molding are combined. In preform and extrusion blow molding, the preform (i. e., the parison) undergoes significant biaxial stress during the inflation stage. Since either extensional or shear stress can dramatically improve cell nucleation, an externally applied stress can cause small-scale, local pressure variations throughout the sample, thus reducing the energy barrier for cell nucleation. So, unlike the current low-pressure foam blow molding technology, where cell nucleation occurs before inflating the preform/parison, we used a high-pressure system to prevent premature foaming in the shaping stage. Consequently, cell nucleation was induced after biaxial stresses were created to induce a higher cell density.

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.

A new approach designed for improving flame retardancy of intumescent polypropylene via continuous extrusion with supercritical CO2

scCO₂-aided IFR dispersion of PP/IFR composites and their improved flame retardancy.

Study of volume swelling and interfacial tension of the polystyrene-carbon dioxide-dimethyl ether system

We investigated the interaction of blended carbon dioxide (CO2) and dimethyl ether (DME) with polystyrene (PS) through volume swelling and interfacial tension. The experiments were carried out over a temperature range of 423-483 K, and the pressure was varied from 6.89 MPa to 20.68 MPa. With an incremental concentration of DME in the blend, the volume swelling increased while the interfacial tension between the PS/blend gas mixture and the blend gas decreased. The validity of the Simha-Somcynsky (SS) equation of state (EOS) for the ternary system was established by comparing experimentally measured volume swelling to that obtained via SS-EOS.

The effects of viscoelastic properties on the cellular morphology of silicone rubber foams generated by supercritical carbon dioxide

Silica content, saturation temperature and pressure all have an effect on silicone rubbers' viscoelastic properties, which further has a close connection with the cellular structure.

Simha-Somcynsky Equation of State Modeling of the PVT Behavior of PP/Clay-Nanocomposite/CO2 Mixtures

The Pressure-Volume-Temperature (PVT) property of polymer nanocomposite (PNC)/gas solutions is an important fundamental property in the foaming of PNC. However, accurate data have not yet been reported. We examined the PVT behaviors of polypropylene (PP) and PP/organoclay polymer nanocomposite (PP-PNC) by monitoring the swelling changes of the polymer melt in supercritical carbon dioxide (scCO2). A model was adopted that describes the PVT behaviors of PP-PNC with and without dissolved gas. Based on the model, a PNC consists of two sections: a hard section (a nanoparticle surrounded by solidified polymer) and a soft section (neat polymer). It was observed that an infusion of nanoparticles decreased the swelling. It seems that the hard section had a minimal free volume in which to dissolve the blowing agents, and that the number of hard sections increased with the infusion of nanoparticles. As a result, the total gas absorption capacity of the system decreased, and consequently, the swelling also decreased.

Lightweight polypropylene/stainless-steel fiber composite foams with low percolation for efficient electromagnetic interference shielding

Lightweight polypropylene/stainless-steel fiber (PP-SSF) composites with 15-35% density reduction were fabricated using foam injection molding. The electrical percolation threshold, through-plane electrical conductivity, and electromagnetic interference (EMI) shielding effectiveness (SE) of the PP-SSF composite foams were characterized and compared against the solid counterparts. With 3 wt % CO2 dissolved in PP as a temporary plasticizer and lubricant, the fiber breakage was significantly decreased during injection molding, and well-dispersed fibers with unprecedentedly large aspect ratios of over 100 were achieved. The percolation threshold was dramatically decreased from 0.85 to 0.21 vol %, accounting for 75% reduction, which is highly superior, compared to 28% reduction of the previous PP-carbon fiber composite foam.1 Unlike the case of carbon fiber,1 SSFs were much longer than the cell size, and the percolation threshold reduction of PP-SSF composite foams was thus primarily governed by the decreased fiber breakage instead of fiber orientation. The specific EMI SE was also significantly enhanced. A maximum specific EMI SE of 75 dB·g(-1)·cm(3) was achieved in PP-1.1 vol % SSF composite foams, which was much higher than that of the solid counterpart. Also, the relationships between the microstructure and properties were discussed. The mechanism of EMI shielding enhancement was also studied.

Foaming behavior of cellulose fiber-reinforced polypropylene composites in extrusion

This paper investigates the effect of polypropylene type and cellulose content on the foaming behavior of cellulose fiber-reinforced polypropylene composites in extrusion. Two types of polypropylene (linear and branched structures) were used as a polymer matrix. The thermal properties of the composites were characterized by a differential scanning calorimeter, and the viscosity of the composites was evaluated by a rotational rheometer. The foaming behavior of the composites was examined using an extrusion foaming system, in which carbon dioxide was used as a physical blowing agent for foams. The results suggested that the cell density increased with the increase of cellulose content. On the other hand, the void fraction decreased with the addition of cellulose, but the void fraction at the 40 wt% cellulose was higher than that at the 20 wt% cellulose. The results also indicated that the two types of polypropylene had a minimal effect on the foaming behaviors of the cellulose fiber composites.

The foamability of low-melt-strength linear polypropylene with nanoclay and coupling agent

In this work, the feasibility of producing microcellular foam by using homopolymer linear polypropylene with an melt flow index of 18 g/10 min, a high melt flow index coupling agent (G3003, melt flow index of 380 g/min), and different nanoclay (Cloisite 20A) content was investigated. A twin-screw extruder was used to prepare the nanocomposite compounds, and an X-ray diffraction machine was used to characterize the intercalation and exfoliation of nanoclay within the matrix. A rheology test was undertaken to investigate the melt shear viscosity of the samples. A single-screw extruder was employed to produce foam by using 5% supercritical CO2 at various die temperatures. Scanning electron microscopy was used to explore the morphology of the foamed samples, and cell density was calculated by scanning electron microscopy images. Density measurement data was used to calculate the expansion ratio of the foamed samples. Extrusion foaming produced foams with high expansion ratios of about 20 and a high cell density of about 108−9 cells/cm3. The crystallinity behavior of the foamed and unfoamed linear polypropylene and linear polypropylene nanocomposites was also investigated. A high-pressure differential scanning calorimetry was used to investigate the dependency of crystallization behavior on high pressure CO2.

Synergistic Effects of Modification with Nanoclay and Polystyrene on the Foaming Behavior of a Random Copolymerized Polypropylene

Microcellular polypropylene/polystyrene/nanoclay (PP/PS/nanoclay) composites were processed in a continuous extrusion foaming system using supercritical CO2 as the foaming agent. Neat PP foam showed quite a broad distribution of cell sizes and thick cell walls. Under the same foaming conditions, nanoclay and PS were introduced in the foaming process of PP. Consequently, foams with uniform cell size distribution, reduced cell size of 10~30 μm and enhanced cell-population density of 2.16×108 cells/cm3 were obtained. The heterogeneous nucleation of PS and nanoclay dramatically increased the nucleation rate, decreased the nucleation time interval, and hence facilitated an almost instantaneous growth of cell size. This method also provided new insight into the mechanism of improving the foaming performance of PP.