Low-density Polyethylene Bags

Content
1. What are low-density polyethylene bags?
    1.1 What is Low-density Polyethylene (LDPE)?
    1.2 Low-density polyethylene production method
    1.3 Production characteristics of Low-density polyethylene bags
    1.4 Low-density polyethylene product properties
2. Features of low-density polyethylene bags
3. How to buy low-density polyethylene bags?

Low-density Polyethylene (LDPE) is a kind of high molecular weight polyolefin material. Like all polyolefins, LDPE is non-toxic and non-polluting and has excellent fracture resistance. It is lighter than water, easily tolerates a variety of common laboratory chemicals, and has a milky translucent appearance. Therefore, low-density polyethylene bags are also often used in laboratory scenarios.

Low-density polyethylene (LDPE) is a thermoplastic obtained by free radical polymerization of ethylene under high pressure. LDPE is the oldest member of the polyethylene family of resins and was first commercially produced in the early 1940s as an electrical wire jacket. LDPE combines several good properties: transparency, chemical affinity, good sealing ability, and ease of molding and processing. This defines LDPE as one of the most widely used materials in the polymer industry today.

LLDPE is a kind of resin with good film-forming properties and low-temperature resistance, which is made by polymerization of ethylene. LLDPE bags are a kind of packaging product formed by LLDPE raw materials through the blow molding process, which can be used for food packaging and pharmaceutical packaging.

High-pressure low-density polyethylene (high pressure-low density polyethylene, HP-LDPE, abbreviated as LDPE) has been industrialized for more than 70 years. Although with the discovery and development of olefin polymerization catalysts, the varieties and production of polyethylene have made great progress, high-pressure polyethylene still occupies an important position. Ethylene is polymerized into high molecular weight polyethylene by a free radical mechanism at high temperature and pressure with the following characteristics.

a. The process of the ethylene polymerization reaction is strongly exothermic. The heat of polymerization of ethylene is about 93.5kJ/mol (or 3.3kJ/g), and the specific heat capacity of ethylene at 235MPa, 150~300℃ is 2.51~2.85J/(g-℃), if the reaction heat cannot be removed in time, every 1% of ethylene polymerization will increase the temperature by 12~13℃. If the temperature is too high, it will also lead to the decomposition of ethylene.

b. Under high pressure, ethylene has been compressed to a gastight phase state with a density of 0.5g/mL, which is close to a liquid that cannot be compressed anymore. At this time, the distance between ethylene molecules is significantly shortened, thus increasing the probability of collision between free radicals and ethylene molecules, so it is easy to the polymerization reaction.

c. The free radical activity of the growing chain is high at high temperatures, the chain transfer reaction is easy to occur, and the resulting polymer is a linear structure with more branched chains. Usually, it contains 20~30 branched chains per 1000 carbon chain atoms, with a crystallinity of 45%~65% and a density of 0.910~0.925g/ml. The carbon chain atoms are light, flexible, and have good low temperature resistance and impact resistance.

According to the polymerization method, LDPE can be divided into high-pressure methods and low-pressure methods. The reactor type can be divided into the kettle method and tube method. Ethylene is used as raw material and fed into the reactor. The polymerization reaction is carried out under the action of the initiator with high-pressure compression. The material coming out of the reactor, after removing unreacted ethylene by a separator, is granulated by melt extrusion, dried, blended, and sent to packaging.

Both LDPE and LLDPE have good rheology or melt flow, and LLDPE has less shear sensitivity because of its narrow molecular weight distribution and short branched chains. In shear processes, linear low-density polyethylene maintains a greater viscosity and is, therefore, more difficult to process than LDPE with the same melt index. In extrusion, the lower shear sensitivity of LLDPE results in faster stress relaxation of the polymer molecular chains and the resulting reduced sensitivity of the physical properties to changes in the blow-up ratio. In melt extension, LLDPE typically has lower viscosity at all strain rates. This means that it will not strain or harden on stretching as much as LDPE. As the deformation rate of polyethylene increases, LDPE shows a striking increase in viscosity, which is caused by molecular chain entanglement. This phenomenon is not observed in LLDPE because of the lack of long branched chains in LLDPE to keep the polymer from entangling. This property is extremely important for thin film applications. The rheological properties of LLDPE can be summarized as "rigidity in shear" and "softness in extension". When replacing LDPE with LLDPE, film extrusion equipment, and conditions must be modified. And provide higher melt temperature and pressure. The mold gap must be widened to avoid yield reduction due to high back pressure and melt breakage.

a. A series of equipment around the polymerization unit, such as compressors, reactors, separators, piping, pumps, and other equipment, are required to be able to operate at ultra-high pressures of 100 MPa or more. Even for the equipment for the separation and recovery processes, some instruments require operation at 100-350 MPa. Therefore, whether from the equipment or the operation, the whole process there are many difficulties.

b. The heat of polymerization of ethylene is much higher than the heat of polymerization of other monomers. In the polymerization reaction, the polymerization rate reaches 10%-20% or even 30%-40% in one instant. Therefore, the removal of polymerization heat becomes an important issue in the process and is one of the keys to improving the one-way conversion rate and reducing energy consumption.

c. The viscosity of the polymerization products in the reaction system is very large, and the inner walls of the kettle reactor in the kettle method process and the tube reactor in the tube method process tend to accumulate polymer.

d. There are also difficulties in transporting polymers in the molten state. Both reaction pressure and temperature affect the viscosity of the product, which requires great care to control the temperature and pressure.

e. Low molecular weight polyethylene waxes are contained in the recycled ethylene coming out of the high-pressure separator. The reactor type can be divided into two categories: tube method process and kettle method process. The main feature of the tubular reactor is that the flow is plunger-like in the tube and there is no re-mixing; the reaction temperature varies along the length of the reaction tube, so the reaction temperature has the highest peak, so the molecular weight distribution of the resulting polyethylene is wider. In contrast, in the kettle-type reactor, the materials can be fully mixed, so the reaction temperature is uniform, and it is also possible to operate in partitions so that each reaction zone has different temperatures, thus obtaining polyethylene with narrow molecular weight distribution.

a. The film is slightly opalescent and transparent and soft. The strength is smaller than HDPE, while the impact strength is greater than HDPE.

b. Resistant to cold, low temperature, and higher temperatures. The thicker film can withstand the disinfection process by immersion in hot water at 90°C.

c. Moisture resistance is relatively good. Chemical properties are stable and insoluble in general solvents.

d. There is a large air permeability, so when used as easily oxidized food packaging, the contents of the storage period should not be too long.

e. Poor grease resistance, products can be slowly dissolved up. Packaging containing greased food, long storage will make the food odor.

f. It will be aged by ultraviolet light and heat for a long time, which will affect its physical properties and dielectric properties.

g. The melting point is 110~115℃, and the processing temperature is 150~210℃. If in the inert gas, the temperature can reach 300℃ and still be stable. However, the melt is susceptible to degradation by contact with oxygen.

Low-density polyethylene is lighter in mass, and softness, ductility, and good processability, are suitable for film products, so low-density polyethylene bags are not easy to tear. Low-density polyethylene bags also inherit the electrical insulation, chemical stability, and low moisture absorption of LDPE. LDPE bag itself is not easy to absorb water and is basically insoluble in acid and alkaline aqueous solutions. LDPE bags also have excellent transparency, it is easy to see the material form when doing packaging. Of course, LDPE bags also have corresponding disadvantages, their moisture, and the gas barrier is poor, so the need for a strict plastic sealing process, LDPE heat resistance, aging resistance, and light resistance is also poor, so in the use of the process to avoid being placed in high-temperature environments and outside exposure to the sun.

At present, low-density polyethylene bags are mainly used as food packaging and pharmaceutical packaging, such as dried fruit packaging in supermarkets, vegetable packaging, fresh milk bags, etc. Pharmaceutical packaging is often used as hospital pharmaceutical bags, powder packaging bags, etc.

If you are interested in our low-density polyethylene bags or have any questions, please write an e-mail to [email protected], we will reply to you as soon as possible.