Low density polyethylene and high-density polyethylene

The choice in the packaging field is very typical. Last year, when designing cling film for a food factory, LDPE had a significant advantage in ductility: it could still maintain a fracture elongation of over 160% at a thickness of 0.03mm, which HDPE could not achieve. However, the chemical barrel project conducted during the same period must use HDPE, which has a tensile strength of over 30MPa and can withstand a 50kg weight drop test. Our laboratory has compared HDPE samples with a density of 0.95g/cm ³, and found that its resistance to environmental stress cracking time is more than three times longer than LDPE.
Temperature control during processing is a key experience in production line debugging. The melt flow rate (MFR) of LDPE is usually controlled within the range of 2-8g/10min, and the actual injection molding temperature of the barrel needs to be maintained at 160-190 ℃. However, HDPE production encountered the problem of melt fracture, which was later resolved by raising the temperature to 210-230 ℃ and increasing the mold cavity pressure. Here is a lesson: when the MFR is below 0.3g/10min, the screw speed must be adjusted to improve fluidity. Last quarter, a packaging factory neglected this parameter, resulting in a 15% decrease in yield and a loss of over 200000 yuan.
The chemical resistance test data further illustrates the problem. In the 80 ℃ oil immersion experiment, the weight change rate of HDPE was only 0.8%, while LDPE reached 3.5%. Therefore, HDPE must be used for car fuel tanks, but LDPE is required for infusion bags in the medical field. The accident of a company using the wrong material last year, which resulted in excessive drug adsorption, is still a warning. It is worth noting that the emergence of linear low-density polyethylene (LLDPE) has partially bridged the performance gap, with a puncture strength 300% higher than LDPE, and has gradually replaced traditional materials in the field of winding film.
The differences in the recycling process are often overlooked. The data from the processing plant shows that the impact strength retention rate of HDPE recycled material is still 75% after three cycles of regeneration, but the tensile performance of LDPE deteriorates by 40% after two cycles of recycling. So HDPE drainage pipes in municipal engineering explicitly require the addition of 30% recycled materials, while LDPE cling film rarely uses recycled materials. However, the blending technology of the two is developing, and a modification factory has increased the melt index stability of the recycled mixture by 50% by adding compatibilizers.
Cost accounting needs to be combined with the lifecycle. Although the price of HDPE raw materials is about 800 yuan higher per ton, a certain household appliance enterprise has reduced the wall thickness by 0.5mm after switching to HDPE shell, saving 11% of material costs per piece and saving over one million yuan in annual raw material costs. But the flexible packaging field is still dominated by LDPE, which has a unit area cost one-third lower than HDPE film.
In terms of future trends, the mPE prepared by metallocene catalysts is reshaping the landscape. A certain enterprise uses mLLDPE to produce stretch film with a thickness reduced to 8 μ m while maintaining its original strength, which reduces transportation consumables by 20%. However, the position of high-pressure LDPE in the field of ultra-thin medical catheters is still difficult to shake, and its processing accuracy below 0.1mm is difficult to achieve with other processes.

Material selection ultimately returns to the essence of application. The advice for new engineers is to prioritize LDPE for flexibility and transparency, and HDPE for strength and rigidity. But the reality is often more complex. Last year, a certain agricultural greenhouse project adopted a three-layer co extrusion structure - the outer layer of HDPE is UV resistant, the middle layer of LDPE insulation, and the inner layer of EVA is anti fog. This composite solution extends the life of the greenhouse film to more than five years. With the development of catalytic technology, new materials that combine the advantages of both may emerge in the future, but understanding their essential differences is still the foundation for making good products at this stage.