LowDensity Polyethylene Blown Film Tech（LowDensity聚乙烯吹塑薄膜技术）.pdfVIP

Low-Density Polyethylene Thomas I. Butler Introduction Low-density polyethylene (LDPE), sometimes referred to as high-pressure, low-density polyethylene, conventional polyethylene, or branched polyethylene, has been commercially produced since its discovery in the 1930s. These polymers are produced in large volumes by either of two free radical polymerization processes, a stirred reactor (autoclave) or a tubular reactor. In the polymerization process, ethylene monomer combines at high pressures and temperatures to form long polymer chains with many branches. Polyethylene is a thermoplastic material particularly well suited for fabrication into many products. The properties that make LDPE attractive as raw material for film applications are as follows: 1. Light weight. 2. Economical cost. 3. Flexibility (low modulus). 4. Toughness. 5. Chemical resistance. 6. Ease of sealability. Polymer Characterization The long polyethylene molecule will have several branches in the carbon backbone as seen in Figure 29-1. These branches consist of short-chain branches (SCB) and long-chain branches (LCB). LDPE polymers are characterized by their average molecular weight (measured as melt index), crystallinity (measured as density), and molecular weight distribution or MWD (measured by gel permeation chromatography or GPC). These three characteristics provide a polymer with its properties during processing and in the end-use applications. Proper control of the polymerization process conditions produces polymers with the desired average molecular weight, density, and MWD. Film extrusion grades of polyethylene range in melt index from 20 to 0.1 g/10 min. increasing the average molecular weight (a lower melt index) of the polymer produces stronger film tensile and impact properties and lower optical properties. However, higher molecular weight polymers will be mor