Degradable plastics, as a class of polymers that are currently attracting more attention, are still in their infancy in product application, production process and formulation research.

The performance of the polymer mainly depends on its relative molecular weight. Generally, the higher the molecular weight, the better the performance. The molecular weight of the polymer is mainly determined by the polymerization process and conditions.

However, in the face of application needs in different occasions, the requirements for materials are not only limited to the size of molecular weight, but also need to consider the control of product quality indicators such as crystallinity, impurity content, moisture content, dust content, and require these Indicators should be as uniform as possible.

Therefore, by implementing a series of conditioning measures in the finished product section of the material, the deficiencies of the upstream polymerization process can be compensated, and the product quality can be improved. The quenching and tempering measures for finished products mainly include crystallization, degassing, drying, blending and powder removal, etc. For different degradable materials and their different application directions, the finished quenching and tempering process will be one or a combination of several.

Figure 1 Production process of degradable material finished product section

1. Crystallization and drying

Polymer degradation materials such as PGA, PLA, PBAT, etc., are highly crystalline and partially crystalline polymers. These plastics are not crystallized after a short process of cooling and pelletizing from the melt state, and a special crystallization process needs to be designed in the finished product section to complete the crystallization.

Different types of degradable materials have different crystallization temperature and crystallization time. The pre-crystallization process can increase the softening temperature of the polymer, so that the material can use a higher drying temperature during the drying process, thereby achieving a better drying effect.

In addition, the dehydration effect of the pelletizing process has a great impact on the energy consumption and time of the crystallization process.

Figure 2 Crystallization and drying process flow

The pre-crystallized chips are transported to the drying tower for drying. The choice of drying temperature is subject to the softening point temperature of the resin. During the drying process, moisture, residual monomers, solvent volatiles, etc. migrate from the interior of the chips to the surface and require a certain mass transfer time. and affected by dynamic factors. The higher the drying temperature and the lower the moisture content of the drying medium, the faster the drying speed and the shorter the required drying time. The drying time required for different degradable materials usually takes 3 to 24 hours, and the drying time is different according to different drying processes, drying medium conditions and drying effects.

After the slices are dried in the drying tower, they need to be cooled with low dew point air to a temperature suitable for packaging (usually below 60°C). The dry slice cooling process is actually accompanied by a non-isothermal crystallization process. Too fast and too rapid cooling process is not conducive to crystallization, and the slices may not be completely cooled. When the slices are cooled, the temperature of the cooling medium rises. The air from the cooling chamber recovers the heat released when the slices are cooled. This part of the hot air is recovered for drying or pre-crystallization in the previous stage as all or part of the heating source (depending on the specific heating load). ), the heat of the entire drying and cooling process can be recycled, and the energy consumption of the process will be significantly reduced.

2. Blending

Fluctuations in process variables during polymerization can cause polymer molecular weights to fluctuate with production time as well. In order to make the final product appear macroscopically uniform in density, viscosity, melting point and crystallinity, the dried product needs to be blended.

The blending amount and blending times are closely related to the plant capacity, blending equipment selection and downstream packaging equipment selection. When the blending equipment adopts multi-tube or wall-mounted gravity blending, the self-circulation for 3 to 5 times can achieve a uniformity of more than 95%.

Figure 3 Blending process flow

3. Selection of pneumatic conveying form

The transportation of degradable material products generally adopts closed pneumatic transportation. The closed conveying system can effectively prevent external impurities from entering the system and causing pollution. Using dehumidified low-dew point air as the conveying medium can avoid the secondary moisture absorption of the dried slices and the increase in moisture content.

According to the physical properties of the degradable material itself and product requirements, dilute-phase pneumatic conveying or dense-phase pneumatic conveying can be selected as needed. Dilute phase pneumatic conveying slices are suspended in the pipeline, the conveying speed is fast, and dust is easily generated, which is easy to cause "drawing" for low melting point materials. Dense-phase pneumatic conveying slices move in an intermittent "plug flow" shape in the pipeline, and less dust is generated during the conveying process and will not cause "drawing" phenomenon.

Table 1 Selection table of two forms of pneumatic conveying

Fig. 4 Dense phase conveying of PBAT sending tank

(Photographed at the scene of the PBAT dense phase conveying case in Keri, Guangzhou)

Fig. 5 Dense-phase conveying of PBAT rotary valve

(Photographed at the scene of the PBAT dense phase conveying case in Keri, Guangzhou)

Slices are conveyed by dilute phase pneumatics, and the dust content at the end will be as high as 300~800ppm. In order to reduce the dust content of the finished slices before packaging to the control requirements, it is necessary to remove powder for the slices. The dust removal equipment used includes an elutriator that uses convective air to strip dust at the conveying end and a special dust remover installed before the packaging machine at the bottom of the packaging bin. The special dust remover eliminates the static electricity on the surface of the slices before the convective air strips the dust. The electrostatic adsorption effect, and then the slicing and powder removal are more thorough.

Table 2 Comparison of two powder removal schemes

Figure 6 Powder removal process at the end of slice conveying

4. Summary

Degradable materials have gained unprecedented attention at present, but their production process and finished products still need further research and development. By implementing a series of conditioning measures on the finished slices after melt granulation, the product quality can be significantly improved.

The crystallization drying process requires proper partition design, which can effectively improve the process energy efficiency while improving the process effect;
Blending the finished chips can eliminate the index deviation in the production process time and improve the product uniformity;
The selection and design of finished pneumatic conveying play a decisive role in reducing particle breakage and wire drawing. The dilute phase pneumatic conveying has a fast conveying speed, and the conveying process is easy to cause chip breakage and wire drawing. The dense-phase conveying speed is not easy to cause particle breakage and slicing and drawing, especially suitable for the conveying of PBAT materials with low melting point;
In order to reduce the dust content of the finished chips, the chips can be dedusted before entering the packaging. The professional powder remover can control the dust content of the finished chips within 50ppm, but the investment is relatively high.

About Guangzhou Kerishi Technology Co., Ltd.

Focus on automated solid material handling equipment and systems

Corey is China's leading complete system service provider in the field of material handling and a national high-tech enterprise. It provides one-stop engineering design, special equipment and digital production management system for customers in various degradable plastics industries, and realizes the application of many innovative technologies and products in the industry.

company's product

Corey's Pneumatic Conveying Technology

Corey has developed and successfully applied 14 types of pneumatic conveying technologies, using positive or negative pressure, dilute phase or dense phase to convey a wide variety of solid materials. In view of the unique conveying requirements of degradable materials, we successfully applied DensePLUGTM dense phase conveying technology to the finished product conveying of PBAT polymerization device and achieved ideal conveying effect.

Corey's Pneumatic Conveying Technology Matrix

性能特点：

1. Strong stability; 2. Clean and dust-free; 3. Easy to clean; 4. Low conveying unit consumption; 5. Smaller wear of pipes and elbows;

PolyDRYTM solid phase heat treatment

Quenching and tempering technology

For the crystallization and drying of PLA and PGA; the removal and drying of volatile organic compounds (such as THF) of PBAT and PBS polyesters; . The finished product can be quenched and improved in the shortest possible time with the lowest energy consumption, and the quality of the product can be improved.

Features:

Processing capacity: Max. 30t/h;
Applicable materials: PLA, PBAT, PA, POM, TPU, PBT, PET, etc.;
Preheat dryer: fluidized bed/drying tower;
Drying temperature: 80℃~220℃;
Drying air: ① circulating dehumidification hot air, ② circulating nitrogen;
Drying and tempering time: 3h~24h;
Drying and tempering effect: ①Moisture content is low at 300ppm, ②Viscosity, crystallinity, and content of key ingredients reach the expected target.