In PP woven bag production, precise control of the drawing temperature to minimize brittle breakage requires coordinated optimization of extrusion, drawing, cooling, and equipment maintenance. Extrusion temperature is the primary control point. The PP raw material must be fully melted and plasticized in the extruder. Excessively low temperatures can lead to insufficient melt fluidity, resulting in the appearance of crystals and impurities on the film surface. These defects act as stress concentration points, causing brittle breakage during the subsequent drawing process. Excessively high temperatures can cause thermal degradation of the PP, leading to molecular chain breakage and a decrease in mechanical properties, also increasing the risk of brittle breakage. Therefore, the extrusion temperature must be stably controlled within an appropriate range to ensure uniform melt plasticization and avoid thermal degradation.
Matching the drawing temperature with the draw ratio is crucial for minimizing brittle breakage. The drawing temperature should be set within an appropriate range based on the raw material characteristics. For virgin material, it is typically adjusted between 110-130°C. For recycled material, the temperature may be raised to 130-150°C due to the reduced molecular chain integrity. If the drawing temperature is too low, the PP molecular chains will be insufficiently mobile, leading to internal stress concentration during drawing and causing the flat yarn to break. If the temperature is too high, the molecular chains will relax excessively, reducing orientation and resulting in insufficient tensile strength. The draw ratio must be controlled in conjunction with the temperature. For virgin material, the draw ratio should be kept between 5-6.5 times, while for recycled material, a lower value should be used to avoid flat yarn breakage caused by excessive draw ratios. For example, by adjusting the drawing temperature and draw ratio to ensure orderly molecular chain alignment during drawing while maintaining sufficient elongation at break, the brittle fracture rate can be significantly reduced.
Controlling the cooling water temperature is crucial to the physical properties of the flat yarn. PP is a crystalline polymer. Excessively high cooling water temperatures can lead to excessive crystallization, excessive crystal nuclei growth, and reduced tensile strength. Excessively low water temperatures can make the film brittle, prone to cracking, and result in a high rate of yarn breakage during drawing. In actual production, the cooling water temperature is typically set between 40-60°C. Slow cooling promotes the formation of a dense crystalline structure in PP, improving the tensile strength and toughness of the flat yarn. For example, one company successfully addressed the issues of low flat yarn strength and yarn breakage during stretching by increasing the cooling water temperature from 40°C to 55°C, significantly improving the strength of PP woven bags.
Equipment maintenance is fundamental to temperature control. Excessive screw temperature can lead to melt degradation. Circulating cooling water is necessary to maintain the water temperature between 30-40°C to ensure that the temperatures in all screw zones remain stable within the process requirements. Regular die cleaning is essential. The high-temperature molten material increases friction with the inner wall of the die cavity, driving impurities out of the die opening. This prevents die blockage, resulting in splitting or missing coating, and reduces brittle fracture caused by impurities. Furthermore, the surface roughness of the drafting rollers must be kept to a low level to prevent scratches on the flat yarn surface caused by excessive friction, which could become a source of brittle fracture.
The compatibility of the raw material properties of PP woven bags with temperature parameters also affects the quality of the flat yarn. The addition of filler masterbatch must be strictly controlled within the range of 8%-12%. Excessive addition can disrupt the continuity of the PP molecular chain and reduce the tensile strength of the flat yarn. Due to molecular chain degradation and high impurity content, recycled materials require a drawing temperature of 130-150°C, and increased screw speed to enhance melt mixing and reduce localized brittle fractures caused by uneven raw material. For example, one company reduced the brittle fracture rate of flat yarns from 3.2% to 0.8% by optimizing the drawing temperature and screw speed of recycled materials, significantly improving product quality.
