Recycling of pp woven bags
1 Introduction With the widespread use of "" plastic woven bags (hereinafter referred to as "bags"), the production of "" bags is increasing, and it is accompanied by more and more such discarded bags. Recycling these waste bags for recycling is an effective measure to reduce production costs, protect the environment, and make full use of resources. In recent years, many manufacturers have conducted research in this area.
What is discussed here is the "recycling of bags", which refers to "plastic waste" that is suitable for the production of "bags". This is a single type of waste utilization, the requirements are relatively high, can not be mixed with other varieties of plastic, can not contain sediment, debris, mechanical impurities, etc., its melt index is required to be in the range of 2--5 (not any Pp plastics are available). There are two main sources, one is "the waste in the bag production process; the second is the recycling of discarded pp bags, such as fertilizer bags, feed bags, salt bags and so on.
2 Regeneration method It mainly has two kinds of molten pellet method and extrusion granulation method, and most of them adopt extrusion granulation method. The flow of the two methods is as follows.
2.1 Melt agglomeration method Waste - material selection and washing - drying - cutting - high-speed pellets (feeding - heat shrinking - water spray - pellets) discharge packaging.
2.2 Extrusion granulation method Waste - material selection - water washing - drying - cutting - heating extrusion - cooling pelletizing - packaging.
The equipment used in the extrusion process is a self-made two-stage extruder. To exclude the gas generated in the extrusion of the waste, a vented extruder can also be used. In order to eliminate the debris in the waste, a screen of 80-120 mesh must be used at the discharge end of the extruder. The process conditions for regenerative extrusion are shown in Table 1.
The temperature of the extruder should be controlled appropriately and should not be too high or too low. If the temperature is too high, it will cause the material to age and yellow, and even carbonization and blackening, which will seriously affect the strength and appearance of the plastic. If the temperature is too low, the plasticization will be poor, the extrusion rate will be low, and even the material will not be discharged, which is particularly likely to damage the filter. Suitable regenerative extrusion temperatures should be determined based on the results of each batch of recycled waste samples tested for their melt index. The temperature conditions in Table 1 are general rules summarized based on production practice experience.
3. The use of recycled materials and the impact on the performance of pp bags due to the thermal aging of plastic processing has a considerable impact on performance, especially the recycled "" bag after two or even multiple thermal processes, plus in use before recycling The UV aging has a significant decrease in performance. The test data of Table # can illustrate this point.
It can be seen that the recycling of the "" bag cannot be repeated indefinitely. If the pp bag is produced by using the recycled material alone, it can only be regenerated up to three times. Since the recycled waste is difficult to judge the number of times it is recycled, therefore, in order to ensure the quality of the pp bag,
Even the less demanding bags should be mixed with new and recycled materials in production. The proportion of the mixture should be determined based on the measured data of the two materials. The amount of recycled material directly affects the quality of the pp bag, and is a set of comparative data in production practice.
The quality of the woven bag is the same as the relative tensile strength and elongation of the flat yarn. The national standard (gb8946--88) has a flat yarn strength of >=0.03N/denier and an elongation of 15%--30. .% Therefore, in production, 40% of recycled materials are generally added. Depending on the quality of the regrind, sometimes 50% - 60% can be added. The more the recycled material is added, although the production cost is lower, the quality of the bag cannot be guaranteed. Therefore, the added amount of the recycled material can only be appropriately added under the premise of ensuring the quality.
4. Utilization of recycled materials Adjustment of wire drawing process Due to heat aging of pp plastics and heat aging of long-term use, the melt index of pp recycled materials increases with the number of processing (see Table 2). . Therefore, after adding a large amount of recycled materials in the new material, in the control of the process temperature, the temperature of the extruder, the temperature of the head, and the temperature of the stretching and setting should be appropriately adjusted relative to the new material. The adjustment amount should pass the test. The melt index of the mixture is determined.
On the other hand, since the recycled material is processed a plurality of times, the molecular weight is lowered, a large number of short molecular chains are present, and a plurality of stretching orientations are performed. Therefore, in the production process, the draw ratio is also lower than the same new material. Generally, the draw ratio of the new material is 4--5 times, and after adding 40% of the recycled material, it is usually 3--4 times. Also, due to the increase of the melt index of the recycled material, the viscosity is lowered and the extrusion rate is increased, so the drawing speed of the wire drawing is also slightly accelerated under the same screw rotation speed and temperature conditions. In the compounding of new and old raw materials, it is worth noting that it should be mixed evenly;
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