With the widespread use of PP plastic woven bags, the production volume of PP woven bags is increasing, leading to a surge in the amount of waste bags. Recycling these waste bags is an effective measure to reduce production costs, protect the environment, and fully utilize resources. In recent years, many manufacturers have conducted research in this area.
This discussion focuses on the recycling of PP woven bags. Waste materials refer to PP plastic waste suitable for producing PP woven bags. This is a single-variety waste utilization method with high requirements; it cannot be mixed with other types of plastics, and it cannot contain mud, sand, impurities, or mechanical impurities. Its melt flow index must be within the range of 2-5 (not all PP plastics are suitable). Its sources are mainly twofold: waste materials from the PP woven bag production process and recycled waste PP bags, such as fertilizer bags, feed bags, salt bags, etc.
2. Recycling Methods
There are two main recycling methods: melt pelleting and extrusion granulation, with extrusion granulation being the most common. The processes for both methods are as follows.
2.1 Melt Granulation Method
Waste material -- selection and washing -- drying -- cutting into strips -- high-speed granulation (feeding -- heat shrinking -- water spraying -- granulation) Discharge and packaging.
2.2 Extrusion Granulation Method
Waste material -- selection -- washing -- drying -- cutting into strips -- heated extrusion -- cooling and pelletizing -- packaging.
The equipment used in the extrusion method is a self-made two-stage extruder. To remove the gas generated during waste material extrusion, a vented extruder can also be used. To remove impurities from the waste material, an 80-120 mesh screen must be used at the extruder discharge end. The process conditions for recycled extrusion are shown in the table.
The temperature of the extruder must be properly controlled, neither too high nor too low. Excessive temperature easily causes the material to age and yellow, or even carbonize and turn black, which will seriously affect the strength and appearance of the plastic; insufficient temperature causes poor plasticization, low extrusion rate, or even no material output, and is particularly prone to damaging the filter screen. The appropriate recycled extrusion temperature should be determined based on the melt flow index results of each batch of recycled waste sampled and tested.
3. Utilization of Recycled Materials and Their Impact on PP Bag Performance: Thermal aging during plastic processing significantly affects performance, especially for recycled PP woven bags which have undergone two or more thermal processes. Combined with UV aging during use before recycling, performance deteriorates noticeably. Therefore, PP woven bags cannot be reused indefinitely. If recycled materials are used alone to produce PP bags, they can only be recycled a maximum of three times. Since it is difficult to determine the number of times recycled waste has been processed, to ensure PP bag quality, even for bags with lower requirements, a mixture of virgin and recycled materials should be used in production. The ratio of the mixture should be determined based on actual measurement data of the two materials. The amount of recycled material used directly affects the quality of the PP bag flat yarn. The quality of woven bags hinges on the relative tensile strength and elongation of the flat yarns. The national standard (GB8946-88) specifies a flat yarn strength of >=0.03 N/denier and an elongation of 15%-30%. Therefore, in production, approximately 40% recycled material is typically added. Depending on the quality of the recycled material, this can sometimes be increased to 50%-60%. While adding more recycled material lowers production costs, it compromises bag quality. Therefore, the amount of recycled material added should be appropriate, ensuring quality. 4. Adjustments to the drawing process based on recycled material utilization: Due to repeated heat processing and UV aging during long-term use, the melt index of recycled PP increases with each processing cycle. Therefore, when adding a large amount of recycled material to virgin material, the extruder temperature, die head temperature, and stretching and setting temperature should be appropriately lowered compared to virgin material. The adjustment amount should be determined by testing the melt index of the new and recycled material mixture. On the other hand, because recycled materials undergo multiple processing steps, their molecular weight decreases, resulting in a large number of short molecular chains, and they have also undergone multiple stretching and orientation processes. Therefore, in the production process, the stretching ratio must be lower than that of the same type of virgin material. Generally, the stretching ratio of virgin material is 4-5 times, while after adding 40% recycled material, it is generally 3-4 times. Similarly, due to the increased melt index of recycled material, the viscosity decreases, and the extrusion rate increases. Therefore, under the same screw speed and temperature conditions, the drawing speed should be slightly faster. In the blending of new and old raw materials, it is important to ensure uniform mixing; at the same time, raw materials with similar melt indices should be selected for blending. Large differences in melt indices and melting temperatures mean that the two raw materials cannot be plasticized simultaneously during plasticizing extrusion, which will seriously affect the extrusion stretching speed, resulting in a high scrap rate, or even making production impossible.
As mentioned above, the recycling and reuse of PP woven bags is entirely feasible with careful material selection, appropriate process formulation, and reasonable and accurate process condition control. It will not affect product quality, and the economic benefits are very significant.
Post time: Nov-13-2025