Acoustic Characteristics of Microcellular Foamed Ceramic Urethane

Noise pollution critically degrades the quality of human life, and its effects are becoming more severe due to rapid population growth and the development of industry and transportation. Acoustic wave aggregation in the 30–8000 Hz band can have a negative impact on human health, especially following continuous exposure to low-frequency noise. This study investigates the acoustic performance of microcellular foams made of a mixture of brittle and soft materials and their potential use as absorption materials. It is common to use porous materials to improve acoustic properties. Specimens prepared by mixing ceramic and urethane were made into microcellular foamed ceramic urethane by a batch process using carbon dioxide. The specimens were expected to exhibit characteristics of porous sound-absorbing materials. After measuring the acoustic characteristics using an impedance tube, a significant sound-absorption coefficient at a specific frequency was noted, a characteristic of a resonance-type sound-absorbing material. However, the sound-absorption properties were generally worse than those before foaming. Differences based on the size, shape, and structure of the pores were also noted. It will be necessary to check the effects of cellular morphological differences on the absorption properties by controlling the variables of the microcellular foaming process in a future study.

Cell characteristics of epoxy resin foamed by step temperature-rising process using supercritical carbon dioxide as blowing agent

A temperature-rising batch foaming process with supercritical carbon dioxide (ScCO2) as blowing agent was used to prepare epoxy resin foams consisting of diglycidyl ether of bisphenol A and m-xylylenediamine. The dissolution and diffusion behaviors of CO2 in pre-cured epoxy resin were investigated, as well as the parameter effect of CO2 saturation step and foaming step on the cell characteristics. It was proved that closed-cells could be generated for CO2 unsaturated samples and the cell characteristics with the same dissolved CO2 concentration were similar. The merged and cracked bubble morphologies were usually obtained for CO2-saturated epoxy resin samples. With increasing CO2 concentration from 0.021 g CO2/g epoxy resin to 0.061 g CO2/g epoxy resin in the unsaturated samples, the cell size increased from 170.2 µm to 262.6 µm and the cell density decreased from 6.8 × 105/cm3 to 3.1 × 105/cm3. Bubble nucleation and growth occurred simultaneously with curing reaction in temperature-rising step. As the final foaming temperature increased from 60℃ to 120℃, the cell size of samples with dissolved CO2 concentration of 0.021 g CO2/g epoxy resin increased from 172.7 µm to 369.0 µm, while the cell density first increased from 6.8 to 7.3 and then decreased to 3.5. The cell size of samples with CO2 concentration of 0.031 g CO2/g epoxy resin increased from 145.3 µm to 180.5 µm with foaming time from 5 min to 20 min, but changed slightly when curing reaction almost finished and CO2 was depleted after 20 min.

Tensile and impact behavior of polystyrene microcellular foams with bi-modal cell morphology

Bi-modal PS foams with various volume fractions of large cells ( fL), cell sizes and densities were prepared to investigate the effect of cell structures on the tensile and impact behaviors. The tensile results showed that for the similar density, the tensile strength and modulus decreased with the increase of fL, unless the cell size of large ones is smaller than 25 µm. Similarly, the impact experimental results showed that the impact strength decreased with increasing fL, unless the fL is in the range of 25–32%. It indicated that the bi-modal cell structure could lead to the better properties than that of uniform one, when the cell morphology was proper ( fL in the range of 25–32% and the cell size of large ones smaller than 25 µm). The SEM images of impact-fractured surface of bi-modal foams further confirmed that the cell morphology with fL of 32% was more favorable to the absorption of impact energy during the fracture process.

Effects of Blend Morphology on the Foaming of Polypropylene/ Low-density Polyethylene Blends during a Batch Foaming Process

The effect of blend morphology on the foaming of polypropylene (PP)/low-density polyethylene (LDPE) blends with different blend ratios has been studied in this work. The results showed that the cell morphology of a foamed blend was significantly impacted by the blend morphology and foaming conditions. When the foaming temperature was close to the melting temperature of LDPE, cells were nucleated at the interfaces of PP droplets and LDPE matrix in the 10/90 and 25/75 PP/LDPE blends and grew in the LDPE matrix. Also, cells could only grow in the LDPE phase in the 90/10 and 75/25 PP/LDPE blends and consequently their sizes were determined by the sizes of the LDPE droplets. However, when the foaming temperature was close to the melting temperature of PP, cells with bimodal cell size distributions were generated in the pure PP, 90/10 and 75/25 PP/LDPE foams. In particular, fine open-cell structure was formed in the co-continuous 50/50 PP/LDPE blend.