polyethylene - LDPE
Polyethylene (PE) is made by polymerization of ethylene monomer, nCH2 = CH2 →（—CH2—CH2—)n, while n is implying the degree of polymerization. With the relative molecular weight is above 50000, polyethylene is the most consumed variety among all the plastic packaging films.
According to different polymerization methods, it can be divided into high pressure method, medium pressure method, low pressure method; and if it’s to be divided by density, it can be divided into low density polyethylene (LDPE: 0.910 ~ 0.925g/cm3)、Medium density polyethylene (MDPE: 0.926~0.940g/cm3)、high density polyethylene (HDPE: 0.941~0.965g/cm3).
There are many kinds of LDPE used in packaging, and to meet the needs of processing and use, often used with other polyethylene or other resin blend modified. Polyethylene and its copolymers commonly used in flexible packaging have the following varieties:
1. LDPE blown film
Basic characteristics of LDPE blown film are as following:
(1) Low density, generally at 0.915 ~ 0.925g/cm3, can float on water.
(2) Good transparency, with certain luster, suitable for packaging use.
(3) Low mechanical strength, good softness, high elongation, low surface hardness.
(4) Good low-temperature resistance, brittle temperature is -70℃, good shock resistance at low-temperature.
(5) Low water absorption, excellent moisture resistance, but with high air permeability, poor fragrance and fresh protectiveness performance.
(6) Good chemical resistance, can withstand hydrochloric acid of various concentrations, Sulfuric acid of less than 50% concentration, and nitric acid of less than 40% concentration; Good alkali resistance, resist to general organic solvents at less than 60℃.
(7) Poor heat resistance, the softening temperature is 84℃.
GB/T 4456-1996 is the standard of polyethylene blown film in China, it’s suitable for polyethylene blown film with density less than 0.925g/cm3 and polyethylene blown plastic film containing additives, pigments and stabilizers. The physical and mechanical properties are shown in Table 2-11.
Table 2-11 The physical and mechanical properties of LDPE:
Name of Items | Standard Requirement | |
thickness≥0.05mm | ||
Tensile strength （MPa）（vertical、horizontal） | ≥10 | ≥10 |
Elongation at break（%）（vertical、horizontal） | ≥140 | ≥250 |
Angle tear strength（N/mm） | ≥40 | ≥40 |
