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How to get the best foaming effect and elastic recovery energy?

How to get the best foaming effect and elastic recovery energy?
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  • Categories:Industry news
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  • Time of issue:2020-11-30 11:17
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(Summary description)

How to get the best foaming effect and elastic recovery energy?
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(Summary description)

  • Categories:Industry news
  • Author:
  • Origin:
  • Time of issue:2020-11-30 11:17
  • Views:
Information
Using nano clay as nucleating agent and azodicarbonamide (AC) as chemical foaming agent, TPU was foamed by injection. It was found that different nucleating agent contents and foaming agent contents in the foaming system had influence on the morphology, structure and mechanical properties of TPU foaming products. The results showed that the addition of nanometer clay was beneficial to improve the foaming effect and resilience of TPU injection foam samples. When the mass fraction of nano clay and AC foaming agent is 0.6 % and 3% respectively, the best foaming effect and elastic recovery energy can be obtained.


TPU is a kind of elastomer that can be plasticized by heating and dissolved by adding solvent. It is composed of hard and soft segments and has a two-phase microstructure. It is a kind of block polymer, also known as thermoplastic polyurethane rubber. TPU has excellent physical and mechanical properties such as high strength, high toughness and high elasticity, as well as excellent comprehensive properties such as corrosion resistance, abrasion resistance, oil resistance and shock absorption. Because of its good processing performance, it is widely used in national defense, medical, automobile, food, clothing and other industries.

 

Due to the wide hardness range, high hardness and high density of TPU material, its application is limited. In order to solve these problems, domestic and foreign scholars have done a lot of research on the foaming molding of TPU material to reduce the density of foaming material and improve the resilience energy of samples. The light microporous TPU foaming materials were obtained by extrusion foaming molding, injection foaming molding and batch foaming molding.

 

In the research of TPU injection foaming molding, the predecessors mostly adopted the physical foaming method, while the chemical foaming method was less studied, and the foaming effect was not ideal. In order to study the influence of foaming system on TPU injection foaming molding, nanometer clay was selected as the nucleating agent and azodicarbonamide (AC) as the foaming agent, which was added into the injection molding machine. Through chemical foaming injection molding, the influence of foaming system on the shape, structure and physical and mechanical properties of the product was studied.

 

1. Experimental Part

 

1. 1 Main raw materials

 

TPU: Elastollan 1180A10, from BASF, Germany; Nanometer clay: 1 250 mesh, Guanteng Mineral Processing Plant, Lingshou County; Azodicarbonamide (foaming agent) : AC180, purity 99.9%, Qingdao Shengda Xinchuan Industry and Trade Co., LTD. Liquid paraffin: Chemical purity, Sinopac Chemical Reagent Co. LTD.

 

1. 2 Main equipment and instruments Injection molding machine: TTI-90F2, Donghua Machinery Co., LTD.; Multi-function density tester: AR-300VP, Dongguan Hongtuo Instrument Co., LTD. - Hongda Meituo; Scanning electron microscope (SEM) : S-4700, Hitachi, Japan; Universal material testing machine: KXWW-20C, Chengde Kebiao Monitoring Instrument Manufacturing Co., LTD. Rubber impact elasticity testing machine: MZ-4065, Jiangsu Mingzhu Testing Machinery Co., LTD.

 

Table 1 shows the injection process parameters of the injection molding machine in the sample preparation stage. In the experiment, self-made compression molds were used in the laboratory, and the molded samples were tensile splines conforming to GB /T 1041.1 -- 2006. TPU was first dried at 100 ℃ for 4 h, then mixed with azodicarbonamide (mass fraction 2%), then mixed evenly with nucleating agent nano clay with different mass fraction, and then added into the injection machine for injection foaming molding.

 

After the optimal content of nanometer clay was obtained through the above experiments, the TPU dried under the same conditions was mixed with the nanometer clay with the above content, and then mixed evenly with different content of AC foaming agent respectively. After that, it was added into the injection machine for injection foaming molding.

 


1. 4 Measurement and characterization of apparent density of foam samples: The apparent density of multiple foam samples was directly measured using a multifunctional density tester with accuracy of.001. Characterization of surface morphology of foaming samples: the foaming samples were placed in liquid nitrogen and cooled sufficiently before brittle fracture, and the section was sprayed with gold. SEM observation of the section was used to obtain pictures. The number and diameter of bubble holes in SEM images obtained were counted by using Image J software, and then the average value was taken. The formula for calculating the foam hole density is shown in Formula (1) : in the formula, the foam hole density of N0-foaming products is per cm3; N - The number of bubble holes in the statistical area; S-electron microscopy selected statistical area, cm2; The density of - unfoamed specimen is g /cm3. Determination of mechanical properties: the tensile properties of the injection foam samples were tested using a universal material testing machine. The tensile samples were cut from the splines. The standard was GB /T 528-2009. The elastomer impact elasticity testing machine was used to test the springback rate of the injection molded foam samples. The springback samples were cut by the spline made, and the standard was GB /T 1681 -- 2009.

 

2 Results and discussion

 

As can be seen from FIG. 1 and FIG. 2, with the increase of the mass fraction of nanometer clay, the pore density first increased and then decreased, and the pore diameter first decreased and then increased. The foaming effect of the sample was extremely poor without the addition of nanometer clay. The average diameter and density of the foam holes were 285. 5 m and 4. 6× 104 /cm3, respectively. When the mass fraction of nanometer clay was 0.6%, the foaming effect was the best, and the density of foam holes was 3.1 ×105 /cm3, which was 6.7 times of that without the addition of nanometer clay. The average diameter of foam holes was 134.6 m, which decreased by 52.9%. When the mass fraction of nanometer clay exceeds 0.6 %, the pores merge, the diameter of the pores increases, and the density of the pores decreases. It can be seen that the addition of nano-clay plays a positive role in the injection foaming molding of TPU, but the content should not be too high. If the nano-clay is continued to be added, the density of bubble holes will decrease.


As can be seen from Figure 3, the influence of the content of nanometer clay on the tensile strength and elongation at break of TPU foaming samples is similar. When no nucleating agent is added, the foaming effect of the sample is poor, and its tensile strength and elongation at break are both high. At this time, the tensile strength of the TPU foaming sample is 18.35mpa, and the elongation at break is 588.6%. When the mass fraction of nanometer clay increased from 0% to 1.0 %, the tensile strength was 12.1mpa, which decreased by 34.1%, and the elongation at break was 431.4 %, which decreased by 26.7%. The addition of nanometer clay increased the number of bubbles in the sample, reduced the compactness of the sample, and reduced the tensile property.
As shown in Table 2, with the increase of nano clay content, the rebound of TPU foaming samples first increased and then decreased. Without the addition of nanometer clay, the sample's rebound rate was 26.5%, and when the mass fraction of nanometer clay was 0.6%, the maximum rebound rate was 32.1%, which was due to the large density and uniform distribution of bubble holes in the sample, and the uniform gap between bubble holes provided support for the sample's resilience. If the nanometer clay is added continuously, the internal foam holes will merge and collapse, and the foam holes will be uneven. The foaming effect will be reduced, and the resilience of THE TPU foam sample will also be reduced, indicating that the resilience of the sample is inhibited.

When the mass fraction of nano clay is 0.6 %, the foaming quality of TPU foam samples is higher. At this time, the bubble density is the largest, the average bubble diameter is the smallest, and the rebound rate is the best. The optimum content of nano clay in the foaming system was determined to be 0.6 %.

 

2.2 Influence of foaming agent content on the structure and performance of TPU foaming sample

FIG. 4 shows THE SEM photographs of the injection sample cross-section when different amounts of foaming agent (nanometer clay content is 0.6%) are added in TPU. FIG. 5 shows the influence of foaming agent content on average hole diameter and hole density of TPU foaming sample. As can be seen from the figure, with the increase of foaming agent content, the bubble hole density first increased and then decreased, and the bubble hole diameter first decreased and then increased. When the foaming agent mass fraction was 1%, the bubble density was 1.3 ×104 /cm3, and the average bubble diameter was 343.8 m. When the mass fraction of foaming agent reached 3%, the bubble holes were the most uniform. At this time, the density of bubble holes was 3.2 ×105 /cm3, which was 24.7 times that of the mass fraction of foaming agent 1%. The diameter of bubble holes was 11.3 m, decreased by 67.9%. However, when the foaming agent content continues to increase, the diameter of the foam hole becomes larger, the foam hole appears to break and merge, the distribution of the foam hole is not uniform, and the foaming effect is not ideal.

 

As shown in FIG. 6, when no foaming agent was added, the tensile strength was 34 MPa and the elongation at break was 590%. When the foaming agent content in the system increases gradually, the tensile strength of the sample decreases gradually, and the breaking elongation first decreases, then increases, and finally decreases. With the increase of foaming agent, the number of bubble holes in the product increases gradually, and the toughness of sample decreases gradually. When the mass fraction of foaming agent rose to 4%, the tensile strength was 12.65 MPa, which decreased by 62.8 %, and the elongation at break was 521.6 %, which decreased by 11.6 %.

As shown in Table 3, with the increase of foaming agent content, the springback of the sample first increased and then decreased. Combined with Figure 5 and Figure 6, it can be seen that the increase of foaming agent content increases the bubble hole density in the sample and improves the elasticity of the sample.

 

When the foaming agent content is 0, the springback rate of the sample is 25%. When the foaming agent mass fraction is 3%, the springback rate reaches the maximum value of 33.5 %, increasing by 25.4 %. After that, continue to increase the content of foaming agent, due to the combined effect of bubble hole, the rebound rate will be reduced. When the foaming agent mass fraction is 3%, the foaming quality of the TPU foaming sample is higher. At this time, the bubble hole density is the largest, the average bubble hole diameter is the smallest, and the best recovery rate is also obtained. The optimum foaming agent mass fraction in foaming system was determined to be 3%.

 

3 conclusion

 

The addition of nanometer clay is beneficial to improve the foaming effect and resilience of TPU injection foam samples. When the mass fraction of nanometer clay is 0.6 %, the samples with the best foaming effect and elastic recovery energy can be obtained. Increasing the foaming agent content in the foaming system is conducive to improving the foaming effect and resilience of TPU injection foaming samples. When the mass fraction of AC foaming agent is 3%, the samples with the best foaming effect and resilience energy can be obtained.

 

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