Blowing Agent Content Research Articles

Achieving high impact toughness in injection-molded SMMA foams via the synergistic effects of cell size and SBS

Optimizing the cellular structure of polymeric foams has long been considered one of the most economical and effective methods for enhancing their impact properties. However, the relationship between cell size and impact strength has rarely been systematically studied. In this study, styrene-butadiene-styrene (SBS) was used as a toughening agent to improve the toughness of Poly(styrene-co-methyl methacrylate) (SMMA). The expansion ratio of pure SMMA foams and their blended counterparts were controlled by fixing the mold-opening distance, while the cell size was adjusted by changing the blowing agent content and packing time. Notably, the impact strength of the optimized SMMA/SBS blend foam reached 15.5 kJ/m2, representing a significant increase of 1400 % compared to that of the pure SMMA foam. In addition, the impact strength of pure SMMA foams did not change significantly as the cell size increased from 29.7 μm to 278.9 μm. However, for SMMA/SBS blend foams, an optimal cell range (100−150 μm) was identified, where the impact strength was approximately twice that of foams with smaller cell sizes (1−25 μm). Then, examination of the impact-fractured surfaces at different cell sizes revealed that the synergistic effects of an appropriate cell size (100−150 μm) and the presence of SBS particles promoted craze initiation and expanded the plastic deformation area during crack initiation, leading toenhanced impact toughness in the blended foam. This work not only systematically elucidates the relationship between cell size and impact strength but also provides new insights into the synergistic effects of rubber particle and cell size on the mechanical properties of polymeric foams.

The influence of viscosity buildup on the foaming dynamics of rigid polyurethane foams

AbstractThe viscoelastic properties development during the reactive foaming of four standard polyurethane formulations was monitored using a flooded parallel plate rheometer. The measurements in situ demonstrated how foams of low catalyst content took longer than 10 min to increase their viscosity from 1 to 103 Pa s, while foams with higher catalysts reached this modulus in less than 4 min. This polymerization speed deeply impacted the evolution of the cellular structure during foaming, which was demonstrated through in situ x‐ray imaging and computer simulation. Foams with high catalyst stabilized in their final cell size in 4–5 min, concurring with the stage of largest viscosity buildup. In contrast, the foams of low catalyst content underwent strong degeneration as a result of the slow gelification and stiffening of the matrix. Simulation results also revealed that stabilization of the cellular structure happens faster when the catalyst and blowing agent contents are increased.

Prediction of the Bubble Growth Behavior by Means of the Time-, Temperature-, Pressure- and Blowing Agent Concentration-Dependent Transient Elongational Viscosity Function of Polymers.

Bubble growth processes are highly complex processes, which are not only dependent on the foaming process parameters (temperature, pressure and blowing agent concentration) but also on the type and structure of the polymer used. Since the elongational viscosity at the bubble wall during bubble growth also depends on these influencing factors, the so-called transient elongational viscosity plays a key role in describing the gas bubble growth behavior in polymer melts. The model-based description of the transient elongational viscosity function is difficult due to its dependence on time, Hencky strain and strain rate. Therefore, representative viscosities or shear viscosity models are usually used in the literature to predict the bubble growth behavior. In this work, the transient equibiaxial elongational viscosity function at the bubble wall during bubble growth is described holistically for the first time. This is achieved by extending the so-called molecular stress function (MSF) model by superposition principles (temperature, pressure and blowing agent concentration) and by using the elongational deformation behavior (Hencky strain and strain rate) at the bubble wall during the initial, and thus viscosity-driven, bubble growth process. Therefore, transient uniaxial elongational viscosity measurements are performed and the non-linear MSF model parameters of the two investigated polymers PS (linear polymer chains) and PLA (long-chain branched polymer chains) are determined. By applying the superposition principles and by changing the strain mode parameter to the equibiaxial case in the MSF model, the transient equibiaxial viscosity master curve is obtained and used to describe the bubble growth process. The results show that the extended MSF model can fully predict the transient equibiaxial elongational viscosity function at the bubble wall during bubble growth processes. The bubble growth behavior over time can then be realistically described using the defined transient equibiaxial elongational viscosity function at the bubble wall. This is not possible, for example, with a representative viscosity and therefore clearly demonstrates the influence and importance of knowing the transient deformation behavior that prevails at the bubble wall during bubble growth processes.

Addition of Modifying Agents vs Basic Functional Properties of Moldings Made of ABS

Unconventional injection methods include i.e. blowing injection that allows for production of moldings with a lower weight while maintaining mechanical properties of detail. Due to the fact that most polymers are dyed at the processing stage, it is important to examine the simultaneous effect of blowing and colorbatch, as well as variable processing conditions, on the functional properties of moldings made of the most popular construction material, i.e. ABS. Samples were made of ABS polymer without and with the addition of a colorbatch containing brown pigment on ABS matrix, and blowing agent. Variable processing conditions were: mold and injection temperature, holding pressure and time. Other parameters were constant. Samples were tested for basic functional properties such as color, gloss, mass, density and thickness. Moldings produced with a higher blowing agent content and a higher injection temperature were characterized by lower mass. No significant influence of processing parameters and content of colorbatch on mass of samples was found. Blowing agent has no significant impact on thickness of moldings, regardless of processing conditions. Addition of a colorbatch influenced samples’ gloss. Increase in colorbatch and blowing agent content and mold temperature allow to obtain moldings with a higher surface gloss. Colorbatch also changed color of samples – a decrease in luminance L even by about 40% and great changes in parameters “a” and slight changes in parameter “b”. It was found that blowing agent content had the greatest influence on density of moldings. Injection temperature and colorbatch, do not significantly affect density of moldings.

Dissolution Mechanism of Physical Blowing Agent into the Polymer in Low-pressure Physical Foam Injection Molding Process (Part 1) Effect of Molding Conditions on Physical Blowing Agent Concentration

Dissolution Mechanism of Physical Blowing Agent into the Polymer in Low-pressure Physical Foam Injection Molding Process (Part 1) Effect of Molding Conditions on Physical Blowing Agent Concentration

A Novel In-Line Measurement and Analysis Method of Bubble Growth-Dependent Strain and Deformation Rates during Foaming.

Bubble growth processes are highly influenced by the elongational viscosity of the blowing agent-loaded polymer melt. Therefore, the elongational viscosity is an important parameter for the development of new polymers for foaming applications, as well as for the prediction of bubble growth processes. Thus, knowledge of the initial expansion and deformation behavior in dependency on the polymer, the blowing agent concentration, and the process conditions is necessary. This study presents a novel method for the in-line observation and analysis of the initial expansion and deformation behavior within the bead foam extrusion process. For this purpose, nitrogen as the blowing agent was injected into the polymer melt (PS and PLA) during the extrusion process. The in-line observation system consists of a borescope equipped with a camera, which was integrated into the water box of an underwater pelletizer. The camera is controlled by a developed trigger by means of angular step signal analysis of a rotary encoder on the cutter shaft of the underwater pelletizer. Thus, images can be taken at any time during the foaming process depending on the cutter position to the die outlet. It is shown that the developed method provides reliable results and that the differences of the initial expansion and deformation behavior during bubble growth can be analyzed in-line in dependency on real foaming process conditions and the type of polymer used.

A study on GCCM (geosynthetic cementitious composite mat) performance improvement by structural composition of woven geotextiles

By applying ‘yarn inlay’ technology using reinforcing fibers, a double raschel GCCM that improved the structural elongation control and reinforcement ability of knitted fabrics was manufactured. In order to improve the cross-sectional reduction of GCCM due to curing shrinkage of concrete, the effects of blowing agent content and concrete filling rate were analysed as a result of the bending strength test, Double raschel GCCM showed the improved brittleness compared to that of general concrete. In addition, it was confirmed that the durability of Double Cell GCCM did not decrease significantly even after 10 days of concrete curing. Finally, the bending strength in special environments such as seawater, freeze-thaw, acidic and alkaline solutions was analysed, and the durability of double raschel GCCM in application environments was confirmed.

Behavior Characteristics and Thermal Energy Absorption Mechanism of Physical Blowing Agents in Polyurethane Foaming Process.

Polyurethane rigid foam is a widely used insulation material, and the behavior characteristics and heat absorption performance of the blowing agent used in the foaming process are key factors that affect the molding performance of this material. In this work, the behavior characteristics and heat absorption of the polyurethane physical blowing agent in the foaming process were studied; this is something which has not been comprehensively studied before. This study investigated the behavior characteristics of polyurethane physical blowing agents in the same formulation system, including the efficiency, dissolution, and loss rates of the physical blowing agents during the polyurethane foaming process. The research findings indicate that both the physical blowing agent mass efficiency rate and mass dissolution rate are influenced by the vaporization and condensation process of physical blowing agent. For the same type of physical blowing agent, the amount of heat absorbed per unit mass decreases gradually as the quantity of physical blowing agent increases. The relationship between the two shows a pattern of initial rapid decrease followed by a slower decrease. Under the same physical blowing agent content, the higher the heat absorbed per unit mass of physical blowing agent, the lower the internal temperature of the foam when the foam stops expanding. The heat absorbed per unit mass of the physical blowing agents is a key factor affecting the internal temperature of the foam when it stops expanding. From the perspective of heat control of the polyurethane reaction system, the effects of physical blowing agents on the foam quality were ranked in order from good to poor as follows: HFC-245fa, HFC-365mfc, HFCO-1233zd(E), HFO-1336mzzZ, and HCFC-141b.

Preparation and Characterization of Natural Rubber Foam Filled with Bagasse Fiber

The properties of natural rubber foam filled with bagasse fiber of varying content (0-50 phr) and different blowing agent content (5 and 8 phr) were investigated, with rubber compounds and vulcanized rubber also examined. Rubber compound properties included measurements of scorch time, cure time, minimum torque, maximum torque, density, and percentage of expansion in the mold. When bagasse fiber and blowing agent content increased, the maximum torque value of the compounded rubber increased. For rubber compound with blowing agent loading at 5 phr, cure time increased with an increasing bagasse fiber content of 40-50 phr, while scorch time and density of the rubber compound remained unchanged. The percentage of rubber compound expansion in the mold decreased with increasing bagasse fiber content. The cell size of natural rubber foams was inspected using an optical microscope. Small and homogeneous cell size was found in natural rubber foam with blowing agent content of 8 phr and higher bagasse fiber content. The incorporation of bagasse fiber enhanced the compressive strength of the natural rubber foam. Vulcanized rubber properties such as the modulus at 100% strain, tensile strength, and strain at break were also studied. The modulus increased, whereas stress at break and strain at break decreased with increasing bagasse fiber content. On the other hand, tensile strength and strain at break increased with increasing blowing agent loading.

Feasibility Study of Bio-Composite Foam Preparation from Poly(Butylene Succinate) with Spent Coffee Grounds Using Compression Molding

Plastic foam is widely used in varying industries due to its light weight, high strength, and good heat insulation. However, most plastic foams are produced from petroleum-based polymers which cannot be naturally degraded and can release aromatic pollution to the environment when they are molten or burned. Therefore, poly(butylene succinate) (PBS) and KMnO4-treated spent coffee grounds (SCG), which are biopolymer and bio-filler, are used to prepare the bio-composite foam in this research by using azodicarbonamide (ACDA) as a chemical blowing agent. The 10, 20, and 30 phr of the treated SCG and the 6 and 10 wt% of blowing agent are compounded with PBS resin using a two-roll mill and foamed by compression molding machine to investigate the possibility of the batch foam production. All bio-composite foams are investigated for both physical and mechanical properties including morphology, compressive strength, abrasion resistance, bulk density, and water absorption. All foams were successfully prepared by a two-step technique in compression molding to melt the compounded PBS pellets first at 160°C, 90 bar and then decompose the ADCA blowing agent to generate foam cells at 200°C, 120 bar. The appearance and morphology of the obtained foams showed that the cells were smaller and more even distribution with the treated SCG addition. The compressive and abrasion resistant properties decreased as the treated SCG increasing, excepted the bio-composite foam with 30 phr of the treated SCG. whereas the addition of ADCA showed an ambiguous trend. Both filler and blowing agent contents caused a somewhat decrease bulk density and increase water absorption.

In-Line Monitoring of the Physical Blowing Agent Concentration by Transmission Near-Infrared Spectroscopy with High-Pressure Resistance Fiber Optic Probes for Foam Injection Molding Processes

In-Line Monitoring of the Physical Blowing Agent Concentration by Transmission Near-Infrared Spectroscopy with High-Pressure Resistance Fiber Optic Probes for Foam Injection Molding Processes

A study of the cell morphology and mechanical properties of bumper material PP-T-EPDM composite foam

In this study, cellular polypropylene based composite foams were prepared using an universal injection moulding machine. The chemical foaming agent was added to neat polypropylene (PP) polymer, talc filled polypropylene (PP-T) composite and talc filled polypropylene/ethylene-propylene-diene blend (PP-T-EPDM) composite materials at the ratio of 1% and 2% by weight. The influence of foaming agent content on the mechanical and cellular properties of both neat PP polymer and PP composites was investigated. The results showed that the tensile strength, tensile modulus, impact strength, hardness, cell diameter, foam density and viscosity values and skin layer thickness decreased while volume expansion ratio increased with the increment in chemical blowing agent content.

Development of Rigid Polyurethane Foams Based on Kraft Lignin Polyol Obtained by Oxyalkylation Using Propylene Carbonate

This study aimed to develop new rigid polyurethane foams (RPUFs) for thermal insulation based on kraft lignin, the main by-product of the pulp and paper industry. Crude lignin-based polyol (LBP) was obtained via the oxyalkylation of kraft lignin using propylene carbonate (PC). A design of experiments (DoE) was used to evaluate the effect of the isocyanate (NCO)-to-hydroxyl (OH)-group’s ratio, the content of crude LBP, the blowing agent (BA), and catalyst on the thermal conductivity and density of RPUFs. Statistical analysis revealed that the increase in crude LBP and BA content in the formulation decreases the thermal conductivity and density of the foams. In addition, the fact that LBP is a viscous polyol containing PC-oligomers appears to affect the cellular structure of RPUFs, and consequently reduces their mechanical and thermal properties. The main novelty of this study consisted in the careful optimization of the formulation, namely, with regard to the type of blowing agent and with the high content of crude LBP obtained from the oxyalkylation of LignoBoost kraft lignin without purification to obtain good quality RPUF that meets market requirements for insulation materials.

A design tool for core‐back timing in foam injection molding

AbstractA visual observation experiment mimicking the pressure history in foam injection molding was utilized to infer the bubbles formation in core‐back operations. The different stages of bubble nucleation, dissolution, and re‐nucleation, corresponding to injection, packing, and core‐back opening, respectively, were studied. A reduction of the number density of re‐nucleated bubbles with respect to first nucleation was observed. In order to understand this observation, the evolution of the blowing agent concentration around the dissolved bubbles was analyzed. A characteristic time was introduced, required to achieve blowing agent uniformity or, equivalently, required to cancel the mass transport‐mediated memory of the system. A numerical model, assuming Fickian behavior, was derived and validated with the experimental results. The study allowed for the development of a predictive tool to design core‐back operations in foam injection molding.

Effect of blowing agent content on the structure and flame‐retardant properties of rigid polyurethane foam/expanded graphite composites

AbstractDriven by global environmental issues, the development of green building materials has become an immediate focus. In this work, n‐pentane was used as an environmentally friendly blowing agent to prepare flame‐retardant rigid polyurethane foam (RPUF) with the addition of expandable graphite (EG) successfully, and the effect of n‐pentane content on flame retardancy and compressive properties of RPUF/EG composites was investigated through limit oxygen index (LOI), vertical burning test (UL‐94), scanning electron microscope (SEM), thermogravimetric analysis (TGA), and compressive properties test. SEM results show that the content of EG and n‐pentane causes a change in the cell structure of RPUF. The change of the n‐pentane content has also an obvious effect on the thermal stability, flame retardancy, and specific compressive strength of RPUF/EG composites. In addition, the cell structure of RPUF matrix has an obvious influence on the distribution of EG in the composites, which indirectly affects the flame‐retardant efficiency of EG. This research explores the conditions for the application of environmentally friendly RPUF and expands its application prospects.

In-line measurement and modeling of temperature, pressure, and blowing agent dependent viscosity of polymer melts

Abstract During processing and bubble growth processes, the melt viscosity changes with temperature, pressure, and blowing agent concentration. Therefore, measurement and prediction methods for viscosity characterization in terms of temperature, pressure, and blowing agent dependency are needed. This study demonstrates the applicability of in-line viscosity measurements during the foam injection molding process and a model for viscosity superposition and prediction. In the present study, polystyrene and a modified polylactide for foaming applications with nitrogen as blowing agent are investigated. By changing the injection speed, temperature, and blowing agent concentration, the process conditions are varied, and thus the resulting pressure drops within the in-line measurement die. The calculated shear rates and viscosities are shifted to a master curve by the application of superposition principles. The viscosity dependency on temperature is described by the Arrhenius equation, the pressure by the Barus equation, and for the blowing agent concentration, a novel Barus-like equation was derived and applied. The prediction of the master curve viscosity function was achieved by the power-law model in combination with the superposition principles and showed good agreement with the shifted in-line data. Finally, the in-line measurements and viscosity predictions are validated by comparing them to rotational and capillary rheometer measurements.

Observations of Polymer Foam Development during Rotational Moulding

Large scale buoyant structures typical of offshore wave energy generation devices may be produced using the rotational foam moulding process. Due to the challenging environmental conditions these parts experience during their lifetime at sea, it is important that the structures are optimised to ensure acceptable mechanical performance. Foams containing large cells suffer a reduction in mechanical properties; therefore, an optimal foam contains small, evenly sized, and well distributed bubbles in large numbers. Due to the lower pressures used in the process, this can be challenging to achieve. Careful selection of material and processing conditions is required to achieve an optimal structure. A unique bench-based rig was used to observe developing foam structures. A camera was used to record the foam height change at intervals. Foam height change was obtained using digital image analysis. Several parameters were studied such as the polymer particle size or the mould pressure for example. In addition to the foam height change, foam density were analysed. It is found that mould pressure, polymer particle size, chemical blowing agent concentration, and polymer rheology may be used to control the foamed structures.

Controlling Morphology and Physio-Chemical Properties of Stimulus-Responsive Polyurethane Foams by Altering Chemical Blowing Agent Content.

Amorphous shape memory polymer foams are currently used as components in vascular occlusion medical devices such as the IMPEDE and IMPEDE-FX Embolization Plugs. Body temperature and moisture-driven actuation of the polymeric foam is necessary for vessel occlusion and the rate of expansion is a function of physio-chemical material properties. In this study, concentrations of the chemical blowing agent for the foam were altered and the resulting effects on morphology, thermal and chemical properties, and actuation rates were studied. Lower concentration of chemical blowing agent yielded foams with thick foam struts due to less bubble formation during the foaming process. Foams with thicker struts also had high tensile modulus and lower strain at break values compared to the foams made with higher blowing agent concentration. Additionally, less blowing agent resulted in foams with a lower glass transition temperature due to less urea formation during the foaming reaction. This exploratory study provides an approach to control thermo-mechanical foam properties and morphology by tuning concentrations of a foaming additive. This work aims to broaden the applications of shape memory polymer foams for medical use.

Effect of Cell Morphology on Flexural Behavior of Injection-Molded Microcellular Polycarbonate.

The quantitative study of the structure and properties relationship in cellular materials is mostly limited to cell diameter, cell density, skin layer thickness, and cell size distribution. In addition, the investigation of the morphology is generally carried out in two dimensions. Therefore, the interrelation between morphological properties and mechanical characteristics of the foam structure has remained in an uncertain state. In this study, during the physical foaming process, a foam morphology is locally created by using a mold equipped with a core-back insert. The variation in morphology is obtained by modifying the mold temperature, injection flow rate, and blowing agent content in the polymer melt. X-ray microtomography (μCT) is used to acquire the 3D visualization of the cells structure. The Cell Distribution Index (CDI) is calculated to represent the polydispersity in cell size distribution. The relationship between the wide range of morphological qualities and relevant flexural properties is made explicit via a statistical model. According to the results, the morphology, particularly cell shape, characterizes the mechanism of the linear elastic deformation of the closed-cell foams. IR-thermography reveals the bending failure of cellular structures in the tensile region despite the differences in cell diameter.

Effect of blowing agent concentration of ultra‐low density NBR/PVC composite foams on compressive behavior and energy‐absorption characteristics

AbstractUltra‐low density nitrile butadiene rubber (NBR)/polyvinyl chloride (PVC) composite foams that exhibit satisfactory compressive behavior and energy absorption characteristics were prepared by one‐step compression molding. The properties of the NBR/PVC foams, including the density, hardness, cell morphology, compressive behavior, energy absorption characteristics, and shock absorption characteristics, were studied with various concentrations of azodicarbonamide (AC). Further that, the ideality factor and the energy absorption efficiency of the NBR/PVC foams at a low stress for application in compression and energy absorption were determined. The results revealed that the densities of polymeric foams were crucial in determining the uniformity of cell size and energy absorption characteristics of the composite foams. With increasing AC content, the density of the NBR/PVC foams decreased linearly, whereas the uniformity of cell size and the foaming ratio increased; furthermore, the hardness and compression modulus gradually decreased, and the linear elastic region in the compressive stress–strain curves became narrow. When the applied stress was less than 100 kPa, the foams with a high AC content exhibited higher energy absorption compared to foams with a low AC content capacity because of the deformed cell wall and lower elastic modulus; however, the relation between AC contents and energy absorption was complicated above 100 kPa. The optimum springrate ratio was found at the concentration of 25 phr of the AC blowing agent.