Text/POLYMER

Today, there are two production methods for thermoplastic polyurethane elastomer (TPU): one is a batch synthesis method, other is a continuous reaction synthesis method, and continuous synthesis method is a relatively advanced synthesis method at present. In TPU continuous synthesis method, dosing, transportation, homogenization, reaction and granulation of TPU synthesis raw materials are carried out continuously in a filling machine, a twin screw reaction extruder and an underwater granulator. raw materials reacted completely.

1. TPU twin screw reactive extrusion

This process usually includes three parts, namely raw material pre-treatment and dosing module (isocyanate, polyester polyol and chain extender), twin screw reaction extrusion module, and underwater granulation module.

Fig. 1. Twin screw reaction extruder[1]

[1] Shang Yetao. China Polyurethane Industry Association. Proceedings of seventeenth annual conference of China Polyurethane Industry Association.

1.1 Raw material pretreatment and dosing module

Taking MDI-BDO-PBA as experimental case, diisocyanate index R = 1, an extrusion reaction experiment was carried out.

Table 1-1 Raw material processing methods

At present, dosing and conveying module has automatic weight loss dosing technology and automatic pouring high-speed mixing technology. Because TPU and thermoplastics have some similarity in certain properties, weight loss measurement method is generally used to control flow of raw materials.

1.2 Twin Screw Reactive Extrusion Module

As a reactor, twin screw extruder has unique advantages for high viscosity material systems and applications with high heat and mass transfer requirements. It has characteristics of high conveying efficiency, high dispersing and mixing ability, good self-cleaning performance, and even distribution of material residence time in machine.

Barrel temperature and screw speed are most important debugging points when setting parameters. Barrel temperature mainly affects degree of polymerization of product, and then affects molecular weight distribution, hardness and uniformity of product. The barrel of a twin screw extruder can be divided into a feeding section, a conveying and mixing section, an extrusion section, and a machine head. The temperature of each section can be set independently, and in each temperature range, there are also inextricable correlations: screw rotation speed. is key to controlling extrusion performance of extruder, and increasing screw speed increases extrusion performance.

Barrel temperature: according to actual production experience, generally set temperature at front of feeding section to about 155°C, and temperature at end of extrusion section to about 220°C. The actual temperature should determine most suitable setting method according to particular type of raw material and equipment parameters. In addition, approximate temperature range can also be known depending on required hardness of product. The relationship between hardness and melting point is shown in Table 1-2 below.

Table 1-2 Relationship between hardness and melting point

Auger speed: by planning experiments and comparing degree of reaction of raw materials, we determine a reasonable screw speed. If speed is too high or too low, there will be additional requirements on metering pump, so you need to reasonably adjust screw speed based on actual production.

Figure 2. Screw speed versus degree of reaction (MDI/BDO/PBA system)

It can be seen from figure 2 that degree of reaction does improve at lower screw speeds, but in terms of overall production efficiency and product throughput, screw speed is preferably 250-300 rpm.

1.3 Underwater granulation module

In terms of underwater pelletizing, equipment includes all imported underwater pelletizing system and domestic underwater pelletizing system. Manufacturers can choose appropriate pelleting system according to rigor of product quality.

2. Conclusion

Thermoplastic polyurethane elastomer is a new type of environmentally friendly material due to its excellent physical and chemical properties. Wherever PVC is used, TPU can replace PVC. At present, my country's research and development capabilities for production of high-end injection molding machines still need to be improved. In this article, we hope to provide some production theory and experienced support to manufacturers of reactive injection molding machines for twin screw extrusion.

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