Plastic bags, a widely used packaging material in daily life, experience significant changes in their flexibility at low temperatures. This phenomenon is closely related to the plastic's molecular structure, the types of additives, and the ambient temperature. Plastic bags are primarily composed of polymers such as polyethylene (PE), polypropylene (PP), or polyvinyl chloride (PVC). These materials exhibit excellent flexibility and ductility at room temperature. However, as the ambient temperature drops, the thermal mobility of the molecular chains weakens and intermolecular forces intensify, causing the material to gradually harden and become brittle, significantly reducing its flexibility.
At a molecular level, low temperatures inhibit the mobility of plastic molecular chains. At room temperature, polymer chains can absorb external impacts through segmental motion, maintaining the material's flexibility. However, at low temperatures, segmental motion is hindered, making it difficult for the chains to dissipate energy through deformation, making the material more susceptible to breakage when subjected to external forces. For example, the molecular mobility of polyethylene plastic bags is significantly reduced at -10°C. If the temperature drops further to -20°C or even lower, the molecular chains may enter a glassy state, completely losing their flexibility and becoming hard and brittle.
The plasticizers added to plastic bags have a significant impact on their low-temperature flexibility. Plasticizers improve the material's flexibility by reducing intermolecular forces and increasing the fluidity of the molecular chains. However, at low temperatures, the plasticizers may harden or precipitate, losing their lubricating properties and reducing the bags' flexibility. For example, PVC plastic bags containing plasticizers are soft and flexible at room temperature, but at -15°C, the plasticizers may partially crystallize, causing the material to become hard and even brittle. Therefore, the type and content of plasticizers must be optimized based on the temperature range of the intended use.
The material type of plastic bags is also a key factor in determining their low-temperature flexibility. Different plastic bags exhibit significant differences in their performance at low temperatures. For example, high-density polyethylene (HDPE) plastic bags, due to their densely packed molecular chains, are more brittle at low temperatures. Low-density polyethylene (LDPE) plastic bags, however, are relatively flexible due to their more branched molecular chains, but they still become harder as the temperature drops. Polypropylene (PP) plastic bags have poor low-temperature performance and can become brittle at temperatures as low as -10°C. In contrast, some specially formulated plastic bags can significantly improve their low-temperature flexibility by adding cold-resistant agents or using copolymerization modification technology.
Low temperatures can have multiple impacts on the physical properties of plastic bags. In addition to reduced flexibility, low temperatures can also cause plastic bags to shrink and deform. Due to the large coefficient of thermal expansion of plastic, plastics experience significant dimensional changes with temperature fluctuations. If plastic bags are poorly designed or made of inappropriate materials, shrinkage at low temperatures can compromise the packaging seal, allowing oxygen, moisture, and other substances to penetrate, affecting the quality of the products inside. Furthermore, low temperatures can reduce the barrier properties of plastic bags, making them more susceptible to environmental influences.
In practical applications, the changes in the flexibility of plastic bags at low temperatures can directly affect their performance. For example, in cold chain transportation or frozen storage, ordinary plastic bags can rupture due to low-temperature embrittlement, leading to leakage of their contents. Using plastic bags with improved cold resistance, such as PE or PP copolymers with added cold resistance agents, can effectively extend their service life and ensure safe storage of products in low-temperature environments.
Plastic bags' flexibility decreases significantly at low temperatures, a change determined by their molecular structure, additives, and material type. To improve the low-temperature performance of plastic bags, optimize the material formulation, add cold resistance agents, or select materials with improved cold resistance. In actual applications, the appropriate type of plastic bag should be selected based on the specific environment and requirements to ensure reliability and safety in low-temperature environments.
