Controlling foamability of polypropylene/γ-irradiated ethylene acrylic elastomer blends by extent of crosslinking and domain microstructure of elastomer

Foams of polypropylene/elastomer blends can often lead to softer foams which may not be desirable every time. Incorporating rigidity to the foams can often be made possible by preferentially crosslinking the elastomer phase prior to blending. Although foamability of polypropylene/elastomer blends has been understood in the scientific community, the influence of the extent of crosslinking in the elastomer phase is not yet understood well. The purpose of this investigation is to identify the influence of the extent of elastomer crosslinking and the blend morphological attributes (achieved by varying screw speed during melt mixing) on foamability of polypropylene/partially crosslinked elastomer blends. Crosslinking of ethylene-acrylic elastomer is carried out using gamma radiation with several doses (0, 12.5, 25, 50 kGy) before melt blending and, subsequently, 10 wt.% of the irradiated elastomers (prior optimized) are mixed with polypropylene in a micro-compounder at three different screw speeds. The microstructure development in blends is characterized by scanning electron microscopy. Frequency sweep rheological analysis is done for selected blends to identify the ease of foamability among the series of blends. Foaming of blends is done with supercritical carbon dioxide in batch mode at three different temperatures. The density reduction along with the microcellular morphology development of blends with foaming is analyzed with the screw speed, the extent of crosslinking, and foaming temperature; furthermore, the individual input parameters (the elastomer domain size, controlled by the screw speed and the extent of crosslinking, controlled by gamma radiation dose) are optimized based on the foam morphology. A uniform and good foamability were achieved at 155 and 160°C for blends with elastomers, irradiated at 12.5 and 25 kGy radiation doses. The lowest density foam (0.37 g/cc) was obtained for polypropylene with 12.5 kGy irradiated crosslinked elastomer mixed at 200 rpm at 160°C foaming temperature. The final elastomer domain dispositions within the foam morphologies are characterized and the plausible foaming mechanism is proposed.

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.

Analysis of the Foaming Window for Thermoplastic Polyurethane with Different Hard Segment Contents

A series of thermoplastic polyurethanes (TPUs) with different amounts of hard segments (HS) (40, 50 and 60 wt.%) are synthesized by a pre-polymer method. These synthesized TPUs are characterized by Shore hardness, gel permeation chromatography (GPC), differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD), dynamic mechanical thermal analysis (DMTA), and rheology. Then, these materials are foamed by a one-step gas dissolution foaming process and the processing window that allows producing homogeneous foams is analyzed. The effect of foaming temperature from 140 to 180 °C on the cellular structure and on density is evaluated, fixing a saturation pressure of 20 MPa and a saturation time of 1 h. Among the TPUs studied, only that with 50 wt.% HS allows obtaining a stable foam, whose better features are reached after foaming at 170 °C. Finally, the foaming of TPU with 50 wt.% HS is optimized by varying the saturation pressure from 10 to 25 MPa at 170 °C. The optimum saturation and foaming conditions are 25 MPa and 170 °C for 1 h, which gives foams with the lowest relative density of 0.74, the smallest average cell size of 4 μm, and the higher cell nucleation density of 8.0 × 109 nuclei/cm3. As a final conclusion of this investigation, the TPU with 50 wt.% HS is the only one that can be foamed under the saturation and foaming conditions used in this study. TPU foams containing 50 wt.% HS with a cell size below 15 microns and porosity of 1.4–18.6% can be obtained using foaming temperatures from 140 to 180 °C, saturation pressure of 20 MPa, and saturation time of 1 h. Varying the saturation pressure from 10 to 25 MPa and fixing the foaming temperature of 170 °C and saturation pressure of 1 h results in TPU foams with a cell size of below 37 microns and porosity of 1.7–21.2%.

Preparation and performance of melamine-formaldehyde rigid foams with high closed cell content

Melamine-formaldehyde (MF)rigid foams with high closed cell content were prepared via oven heating process, using MF prepolymer prepared from melamine and paraformaldehyde as a matrix, cyclohexane as the foaming agent, dimethyl silicon oil as the foam stabilizers, hydrochloric acid as the catalyst. The effect of MF prepolymer viscosity, foaming temperature, amount of catalyst on morphology, closed cell content, apparent density, water absorption and compressive strength of MF rigid foams were systematically studied. The optimized foaming conditions are as follows: the viscosity of MF prepolymer ranges from 35 Pa·s to 45 Pa·s, the foaming temperature is 125°C and the content of the catalyst is 0.65 wt%. The as-prepared MF foams showed the best comprehensive performance with closed cell content of 83.5%, apparent density of 62 kg·m−3, water absorption of 12.0%, compressive strength of 292kPa, thermal conductivity of 0.033 W m−1 K−1 and limiting oxygen index (LOI) of 36%. Compared to conventional organic foams, MF rigid foams possess low water absorption, excellent thermal insulation and flame retardancy due to high closed cell content, and can be expected to be used as thermal insulation material for building exterior walls.

Chemistry, Processing, Properties, and Applications of Rubber Foams

With the ever-increasing development in science and technology, as well as social awareness, more requirements are imposed on the production and property of all materials, especially polymeric foams. In particular, rubber foams, compared to thermoplastic foams in general, have higher flexibility, resistance to abrasion, energy absorption capabilities, strength-to-weight ratio and tensile strength leading to their widespread use in several applications such as thermal insulation, energy absorption, pressure sensors, absorbents, etc. To control the rubber foams microstructure leading to excellent physical and mechanical properties, two types of parameters play important roles. The first category is related to formulation including the rubber (type and grade), as well as the type and content of accelerators, fillers, and foaming agents. The second category is associated to processing parameters such as the processing method (injection, extrusion, compression, etc.), as well as different conditions related to foaming (temperature, pressure and number of stage) and curing (temperature, time and precuring time). This review presents the different parameters involved and discusses their effect on the morphological, physical, and mechanical properties of rubber foams. Although several studies have been published on rubber foams, very few papers reviewed the subject and compared the results available. In this review, the most recent works on rubber foams have been collected to provide a general overview on different types of rubber foams from their preparation to their final application. Detailed information on formulation, curing and foaming chemistry, production methods, morphology, properties, and applications is presented and discussed.

Foaming Performance and FTIR Spectrometric Analysis of Foamed Bituminous Binders Intended for Surface Courses

This study explores the effects of foaming on three selected bituminous binders: 50/70 paving grade bitumen, 45/80-55 polymer modified bitumen and 45/80-80 HiMA binder. The first part of the investigations included the evaluation of the foaming performance in terms of foaming temperature and foaming water content with the utilization of desirability functions and based on the equality of maximum expansion ratio and bitumen foam half-life. The second part of the study investigated the effects of foaming on the chemical structure of the binders using Fourier-transform infrared spectroscopy. The results of the spectroscopic measurements permitted calculation of structural indices specific to functional groups associated with bitumen oxidation, as well as those indicative of elastomeric modification. The results have shown that the different types of bitumen exhibited different foaming characteristics, which was most evident in bitumen foam half-lives, with the HiMA binder performing the best. The spectrometric measurements did not show any significant effects of foaming on the chemical structure of the evaluated binders related to oxidative stress, neither were any major changes in the PmB-specific regions found.

Rheological Behavior of Warm Mix Asphalt Modified with Foaming Process and Surfactant Additive

Surfactants are frequently used to improve the engineering performances of foamed bitumen. Additionally, the foaming process can also perform a significant influence on the foam characteristics and rheological properties of foamed bitumen. However, rare research investigates the synergistic effect of both surfactant and foaming process on the engineering properties of foamed bitumen. To fill the gap, this research investigated the synergistic effect of surfactant and foaming process on the foaming characteristics and rheological properties of foamed bitumen. Based on the experimental results, the synergistic effect shows a significant effect on improving the half-life of foamed bitumen, which reached up to 69 s when 6% foaming Evotherm-DAT content was used. In addition, the foaming temperature also has a significant effect on the foaming characteristics. This study shows that the best foaming conditions can be achieved when the foaming temperature and Evotherm-DAT content are 170 °C and 8%, respectively. Based on the study of synergistic effect, the engineering performances of surfactant foamed bitumen were further characterized in this research, for instance, the enhancement in high-temperature performance and fatigue resistance, and the improvement in workability. Generally, the results of this study have greatly promoted the application of surfactant foam bitumen in the engineering practice.

Solid-State Foaming Process Optimization for the Production of Shape Memory Polymer Composite Foam

This research examined the optimization of the sustainable manufacturing process for polyester-based polymers/Fe3O4 nanocomposite foaming. The foamed structure was achieved by using a solid-state foaming process, where the prepared foams were tested in order to ascertain the optimum foaming parameters with the highest foaming ratios and the lowest foaming densities. The foaming parameters used in this research were the polymer type, nanoparticle percentage, packing pressure, holding time, foaming temperature, and foaming time. Two levels were selected for each factor, and a Taguchi plan was designed to determine the number of experiments required to reach a conclusion. Further characterization techniques, namely, differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD) were used with the original samples to gain a better understanding of their structure and chemical composition. The data analysis showed that regardless of the parameters used, a high foaming ratio resulted in a low density. The introduction of nanoparticles (NPs) to the polymer structure resulted in higher foaming ratios. This increment in foaming ratio was noticeable on Corro-Coat PE Series 7® (CC) polymer more than Jotun Super Durable 2903® (JSD). The optimum parameters to prepare the highest foaming ratios were as follows: CC polymer with 2% NPs, compressed under a pressure of 10 K lbs. for a 3 min holding time and foamed at 290 °C for 15 min in the oven.

Open porous morphology evolution in poly (butylene succinate)/chitin nanocrystal nanocomposite foams

In recent decades, biodegradable polymeric open-porous foams have been engaging increasing interests owing to their biodegradability, porosity and biocompatibility. In this work, biodegradable poly (butylene succinate) (PBS)/chitin nanocrystals (ChNCs) nanocomposite foams with open porous structure were successful fabricated by a solid-state batch foaming method. ChNCs were obtained from chitin by using sulfuric acid treatment and then introduced into PBS. The incorporation of ChNCs had a positive effect on the crystallization behaviors, melt viscoelasticity and thermal stability of diverse PBS specimens. Compared with the change of foaming temperature, the addition of ChNCs would cause a prominent influence on the open porous structure of diverse PBS foams. The probable explanation was that during the foaming process, the spherulites and/or ChNCs as the hard region in PBS could be served as pore wall and the amorphous area as the soft region was acted as pore, leading to open porous PBS foams. The reported strategy in this work could provide the guidelines to regulate and control open porous foams in other semi-crystalline polymer matrices.

Compression Molding of Thermoplastic Polyurethane Foam Sheets with Beads Expanded by Supercritical CO2 Foaming

Expanded thermoplastic polyurethane (ETPU) beads were prepared by a supercritical CO2 foaming process and compression molded to manufacture foam sheets. The effect of the cell structure of the foamed beads on the properties of the foam sheets was studied. Higher foaming pressure resulted in a greater number of cells and thus, smaller cell size, while increasing the foaming temperature at a fixed pressure lowered the viscosity to result in fewer cells and a larger cell size, increasing the expansion ratio of the ETPU. Although the processing window in which the cell structure of the ETPU beads can be maintained was very limited compared to that of steam chest molding, compression molding of ETPU beads to produce foam sheets was possible by controlling the compression pressure and temperature to obtain sintering of the bead surfaces. Properties of the foam sheets are influenced by the expansion ratio of the beads and the increase in the expansion ratio increased the foam resilience, decreased the hardness, and increased the tensile strength and elongation at break.