High Density Polyethylene (НDPE, HDPE, Low Pressure Polyethylene)
Basic physicochemical properties
Polyethylene (PE) [–CH2 – CH2–] n exists in two modifications that differ in structure and, therefore, in properties. Both modifications are obtained from ethylene CH2 = CH2. In one form, the monomers are connected in linear chains with a joint venture, usually 5000 or more; in the other, branches of 4–6 carbon atoms are randomly attached to the main chain. Linear polyethylenes are produced using special catalysts; polymerization proceeds at moderate temperatures (up to 150 ° C) and pressures (up to 20 atm).
A polyethylene molecule is nothing more than a long chain of carbon atoms, each of which has two hydrogen atoms attached. Depending on the manufacturing method, macromolecules with different degrees of branching and different densities are obtained. Therefore, PE is divided into two main groups:
1. Low density polyethylene
Low Density Polyethylene (LDPE) – PE with a relatively highly branched macromolecule and low density (0.916–0.935 g / cm³). The process of its manufacture proceeds at a very high pressure from 100 to 300 MPa and a temperature of 100–300 ° C, therefore it is designated in the same way as high-pressure polyethylene (LDPE).
2. High Density Polyethylene
High Density Polyethylene (HDPE) – PE with a linear macromolecule and relatively high density (0.960 g / cm³). This is polyethylene, also called low-pressure polyethylene (HDPE), it is obtained by polymerization with special catalyst systems.
Linear polyethylenes form crystallinity regions that strongly affect the physical properties of the samples. This type of polyethylene is commonly called high density polyethylene; it is a very solid, durable and rigid thermoplastic widely used for injection and blow molding of containers used in the household and industry. High density polyethylene is stronger than low density polyethylene.
Table. Properties of high density polyethylene
The linear structure, which was mentioned earlier, is characteristic of PE obtained at low pressure, side chains are formed, but they are short and their number is small. Ethylene copolymers, for example with butene-1, are also obtained at low pressure in order to introduce a controlled number of branches into a linear, in essence, molecule. The density of the copolymers is 0.945-0.950 g / cm3, while the linear homopolymers are 0.960 g / cm3.
HDPE-based films are more rigid, durable, and less waxy to the touch than LDPE-based films. They can be obtained by extrusion blowing or through a flat slot (with irrigation on a cooled roll or water cooling). Blown extrusion, however, produces a more cloudy, translucent film.
The softening point of HDPE (121 ° C) is higher than that of LDPE, so it can withstand steam sterilization. Frost resistance is approximately the same as that of LDPE.
The tensile and compressive strength is higher than that of LDPE, and the impact and tear resistance is lower. Due to the linear structure, HDPE molecules tend to orient in the flow direction, and the tear resistance in the longitudinal direction of the films is much lower. Differences in tear resistance in the longitudinal and transverse directions can be increased during orientation, and the properties of ribbons working on tear will be inherent in the film.
The permeability of HDPE is lower than that of LDPE, about 5-6 times, and it is an excellent barrier to moisture.
Among ordinary films, HDPE is inferior in moisture permeability only to films based on copolymers of vinyl chloride and vinyl idenchloride.
In terms of chemical resistance, HDPE is also superior to LDPE, especially in resistance to oils and fats.
With increasing density, the solubility in organic solvents decreases, as does the permeability with respect to solvents.
HDPE is susceptible to cracking under the influence of the medium, as is LDPE, but this effect can be reduced by using high-molecular grades of PE in which this disadvantage is absent.
PROPERTIES OF PND PIPE COMPOSITIONS
Density = 0.948-0.964 kg / cm3 (according to GOST 15199-69).
Tensile yield strength = not less than 21.6 MPa (according to GOST 11262-80).
Elongation at break = at least 700% (according to GOST 11262-80).
Flexural modulus = 680-750 MPa (according to GOST 9550-81).
Melting point = 125-132 ° C (polarizing microscope).
Softening point = 120-125 ° С (according to Vika).
Thermal coefficient of linear expansion = (1.7-2.0) • 0.0001-41 / ° С (according to GOST 15173-70).
Heat conductivity coefficient = 0.41-0.44 W / m • ° С.
Electric strength (sample thickness 1 mm at a frequency of 50 Hz) = not less than 40 kV / mm (according to GOST 6433.3-7).
Specific volume electric resistance = 1 • 1016-1 • 1017 Ohm • cm (GOST 6433.2-71).
Areas of use
The essential properties of all types of polyethylene (HDPE, LDPE, LLDPE):
– low density (lighter than water);
– very good chemical resistance;
– very little water absorption;
– impermeability to water vapor;
– high viscosity, flexibility, extensibility and elasticity in the temperature range from –70 to +100 ° С;
– good transparency;
– easy processability by all methods suitable for thermoplastics;
– very good weldability.
Fields of application of high-density polyethylene, as a rule, coincide with areas that consume low-density material, but the changed properties of the former undoubtedly improve the quality of the products produced. Thus, a film of high density polyethylene will be stronger and more transparent, molded parts may have a smaller cross section, and pipes and fibers will have greater strength. An increase in the melting temperature of new polyethylene allows for steam sterilization. These factors, combined with the ability to control the properties of the products will contribute to the increase in the use of polyethylene produced on surface catalysts. It should be noted that in some cases, the use of high density polyethylene can be limited by cracking during prolonged application of the load.
But the relatively high permeability of polyethylene to oxygen, carbon dioxide, aromatic substances, as well as problems in contact with certain media (for example, wetting solutions), the phenomenon of the so-called cracking due to internal stresses, especially in HDPE, narrow its scope. The different properties of HDPE compared to LDPE are due to its high density. With the same thickness, HDPE products are tougher and harder. The melting point is 20 ° C higher, and due to the denser structure of the molecule, the impermeability to water vapor, oxygen, carbon dioxide and aromatic substances, as well as chemical resistance is better than that of LDPE. High melting point makes it possible to manufacture packages with higher heat resistance (short-term up to 100 ° C).
A successful and rare combination of chemical resistance, mechanical strength, frost resistance, good dielectric properties, resistance to radiation, extremely low gas permeability and moisture absorption, lightness and harmlessness make polyethylene indispensable in a number of applications.
HDPE is processed by almost all the basic methods used in working with thermoplastics – extrusion, blowing, injection molding, rotational molding.
Table. Applications for HDPE
|Пленки||Фасовочный пакет, пакет “майка”, пакет с вырубной ручкой, барьерный слой многослойных упаковочных материалов (ламинаты и коэкструзионные пленки), воздушно-пузырьковая пленка, мусорные пакеты|
|Трубы||Газоснабжение, холодное водоснабжение, защита электросетей, дренаж, внешняя канализация, внутренняя канализация, обсадные трубы для скважин|
|Кабельная изоляция||Изоляция кабелей высокого напряжения|
|Листы, мембраны, мягкие ленты||Листы: гидроизоляция, формование деталей изделий для машиностроения. Мембраны: гидроизоляционные работы. Ленты: конвейерные ленты, геоячейки|
|Сетки||Бытовые, сельскохозяйственные, сетки для армирования дорожных покрытий, сетки для проведения строительных работ, сетки для ограждения зданий и сооружений|
|Пленки||Фасовочный пакет, пакет “майка”, пакет с вырубной ручкой, мусорные пакеты|
|Емкости||Флаконы для косметики, парфюмерии, бытовой химии, канистры, бочки, баки, цистерны|
|Литье под давлением|
|Товары народного потребления||Изделия для цветоводства, изделия для ванной комнаты, изделия для кухни, предметы домашнего обихода, детские товары, садово-огородный инвентарь|
|Крышки||Двухсоставные и односоставные крышки для ПЭТ бутылок, укупорочные изделия для парфюмерии, косметики, бытовой химии, автохими|
|Мебельная фурнитура||Лицевая, декоративная, крепежная, опорные элементы, прочие комплектующие|
|Автокомплектующие||Около 400 наименований изделий для автомобиля|
|Другая продукция||Не будучи приоритетным видом сырья ПЭНД используется при произодстве другой литьевой продукции: мебели, тарных ведер, детских игрушек, фитингов|
|Емкости||Баки, мусорные баки, бочки,|
|Мобильные туалеты||Передвижные туалеты|
|Детские площадки||Детские игровые комплексы (горки, горки-тоннель, городки)|
|Дорожные огрждения||Дорожные блоки, конусы, буферы|
|Колодцы||Колодцы, септики, мусоросборы|
|Эстакады||Эстакады для мойки колес, установки оборотного вод|