Structure And Properties Of Polyethylene

Polyethylene (Polyethylene , referred to as PE) is a thermoplastic resin produced by polymerization of ethylene monomer. In industry, also includes ethylene and a small number of a-olefin copolymers. Polyethylene odorless, non-toxic, feel like wax, with excellent low-temperature resistance (the lowest use temperature up to -100 ~ -70 ° C). Good chemical stability, because the polymer molecule through the carbon - carbon single bond connection, can resist most of the acid and alkali erosion (not resistant to oxidizing properties of the acid). Insoluble in common solvents at room temperature, low water absorption, excellent electrical insulation.

Structure of polyethylene

The general formula for the structure of each type of polyethylene (PE) can be expressed as follows:

Its composition is only two atoms of carbon and hydrogen, and it has the simplest structure and the smallest chain links among the polymer carbon and hydrogen compounds. It is essentially a paraffin wax of high relative molecular mass, i.e., a fatty long-chain polymer. Due to the monomer molecular ethylene completely symmetric, and thus PE structural unit in the molecular chain bonding mode is basically only one. C-C single bond is σ bond, its electron cloud distribution has axisymmetric, is the smallest polarity in the carbon chain polymer compounds, intramolecular inter-atomic interactions are very small, the degree of internal rotation is very low, the internal rotation barriers are not large, and the number of possible conformations is large.

The Van der Waals force and hydrogen bonding force of intermolecular interaction of PE is also the smallest, the cohesion energy is 260J/cm3, the molecular chain is soft and easy to be deformed, and other macromolecules lower than 293 J/cm3 are usually used as rubber, only PE is an exception, which belongs to the typical flexible macromolecule chain.

PE chemical composition and linear low-density polyethylene (LLDPE) with different polymerization conditions, there are high-density polyethylene (hdpe), low-density polyethylene (LDPE), the main chain has a different number of different lengths of branched side groups, and even a small number of different types of double bonds, there is still a certain amount of carbonyl and ether group in the LDPE. Different varieties of dosage on the size of the number of branches in order of LDPE > LLDPE > HDPE, the more branches of its resistance to photodegradation and oxidation of the ability to deteriorate. HDPE only a few short branches, the Department of Linear macromolecules, macromolecular chains are not connected to the bond, so soft and elastic; LDPE is a long, short branched linear macromolecules, branched so that the distance between the molecular chain increases, the macromolecules stacked loose, low density, low crystallinity, low density, low crystallinity. LDPE is a linear macromolecule with long and short branched chains, the branched chains increase the distance between the macromolecular chains, the macromolecules are loosely stacked, the density is low, the crystallinity is low, and it is softer, so the hardness, strength and heat resistance of LDPE are lower.

The configuration of polyethylene molecule (the geometrical arrangement of atoms or groups in the space of the macromolecule fixed by chemical bonds) is a random line group in the free state, and it is serrated after stretching by external force, the bond length of C-C single bond is 0.154 nm, the bond angle is 109.3°, and the pitch of teeth is 0.253 nm.

The crystallinity of different types of polyethylene is different, LDPE is about 65%, HDPE is about 80%~90%, LLDPE is about 65%~75%. With the increase of crystallinity, the density, rigidity, hardness and strength of PE products improve, but its impact properties decline. Polyethylene varieties are not only different crystallinity, crystallization form and crystal parameters are not the same.

The crystalline form of polyethylene includes spherical crystals and single crystals. The former is obtained after the melting of polyethylene, i.e., the crystalline aggregates obtained by the growth of nuclei dispersed in all directions; the latter is obtained by the cooling of dilute solutions of polyethylene. Table 1-2 shows the crystallinity of PE obtained by different methods.

The density of PE is closely related to the crystallinity xc, and the relationship between the two is:

where d is the measured density of the sample; d1 and d2 are the densities of fully crystallized and fully amorphous PE, respectively. Generally, the density of the crystalline phase of unbranched PE is 1.014 g/cm3 and the density of the amorphous phase is 0.84 g/cm3 at 25℃.

This equation assumes that the densities of the crystalline and amorphous phases in a partially crystallized polymer (i.e., the sample to be tested) are equal to the densities of the fully crystalline and fully amorphous phases, respectively. In fact, it is impossible for any PE to be 100% crystalline or completely amorphous.

The relative molecular mass of PE is often described by its average degree of polymerization (picture), weight average relative molecular mass (picture) or number average relative molecular mass (picture), and the distribution of relative molecular mass is expressed by the distribution curve and the distribution width index (picture).The relative molecular mass of PE and its distribution have the same effect on the performance as the degree of PE branching and the degree of unsaturation. Due to the different polymerization methods and operating conditions, the relative molecular mass can be varied in a wide range, such as from the critical relative molecular mass of 10,000 to tens of thousands, hundreds of thousands, or even millions. The relative molecular mass distribution also varies with different polymerization conditions, especially for low-pressure PE with carrier Ziegler catalyst, the relative molecular mass distribution can be from quite narrow to quite wide. The relative molecular mass of ordinary PE is 40,000~120,000, and that of UHMWPE is 1,000,000-4,000,000. The higher the molecular weight, the better the mechanical properties of the resin, such as tensile strength, low-temperature embrittlement property, resistance to environmental stress cracking, etc., but the processing performance deteriorates.

In addition to the above parameters, the size of PE molecule can be expressed by the melt flow rate (MFR) to indirectly illustrate the size of the relative molecular mass, the smaller the MFR, the higher the relative molecular mass, and vice versa, the lower the relative molecular mass. For LDPE, the MFR is 20 ~ 50 g/10 min, for HDPE 4 ~ 15 g/10 min, and for LLDPE 3 ~ 10 g/10 min.

For LDPE, the MFR and the number average relative molecular mass picture have the following approximate relationship:

The size of relative molecular mass and its distribution play an important role in the usability and processing performance of plastics, the above relationship just illustrates the effect of the size of the polymer molecule on the processing performance, because the level of the MFR is a physical quantity that characterizes the size of the melt viscosity, which is a measure of the processing fluidity, and there is also the following approximate relationship between the MFR and the apparent viscosity (η) of the melt:

HDPE due to the lower MFR, more than the viscosity (a relative measure of relative molecular mass) expressed. Industrial PE dissolved in tetrahydronaphthalene or decahydronaphthalene, its mass fraction in solution: C is 0.5%, high and low density PE were measured at 120 ℃ 75 ℃ under the conditions of its solution viscosity η, the following formula is calculated than the concentration of viscosity [ η ' ]

where η0 is the solvent viscosity, Pa-s.

MFR does not reflect the relative molecular mass distribution, in fact, the relative molecular mass distribution has a great influence on its fluidity, as the relative molecular mass distribution broadens, the melt fluidity increases, in which the low relative molecular mass part is equivalent to the plasticizer of the high relative molecular mass PE. For PE with the same average relative molecular mass, the PE with wider distribution has better flowability. In addition, the density of PE also has a great influence on the viscosity of its melt, small density, viscosity is also small. Therefore, the melt flow rate MFR is not suitable for evaluating the relative molecular mass of PE with different densities. In short, the relative molecular mass of small, wide distribution of LDPE is favorable to its processing fluidity, but most of the application properties, especially the mechanical properties are unfavorable, so the relative molecular mass of PE and its distribution and other structural parameters of PE are the same as the PE is an important factor affecting the final performance of PE.

Properties of Polyethylene

1) Physical Properties

The physical properties of PE of various densities are shown in Table

PE has a low permeability to water vapor but a high permeability to organic compounds. Water absorption is small, about 0.03%.

2) Mechanical properties

The mechanical properties of PE are lower than those of common plastics, such as hardness, modulus of elasticity and strength, except for the better toughness and impact strength, which is decided by the nature of PE flexible chain, and the mechanical properties of PE are also related to the relative molecular mass of PE, its distribution, crystallinity and density, etc. The effect of density on the strength is obvious.

The effect of density on strength is very obvious, there are many and dense long and short branched chains in the main chain of WPE, which change the symmetry and regular sequential arrangement of PE chain, thus lowering the crystallinity of PE, weakening the density of molecular chain, affecting the macroscopic mechanical properties of PE, but the impact strength and elongation at break are higher.

3) Thermal properties

The ignition temperature of PE is about 350 ℃, and the ignition temperature of PE dust is 450 ℃. the low temperature resistance (often referred to as the low temperature brittle breaking point) of PE is very good, and it becomes better with the increase of the relative molecular mass, and the narrower the distribution of the relative molecular mass, the better the resistance to low temperature.

4) Chemical properties

PE is a nonpolar saturated fatty long-chain polymer, inert to water and various chemical reagents, almost insoluble in general organic solvents at room temperature, except for a few solvents. However, fat diameter, aromatic light, halogenated by long time contact with PE, can make it dissolved. When the temperature exceeds 60 ℃, may be benzene, toluene, amyl acetate, turpentine, petroleum ether, mineral oil and paraffin, etc. Gradually dissolved in PE can be dissolved in tetrahydronaphthalene and decahydronaphthalene. Its solubility is related to crystallinity and relative molecular mass, and decreases with the increase of crystallinity and relative molecular mass.

At room temperature, PE is resistant to dilute sulfuric acid, nitric acid, hydrochloric acid, phosphoric acid, hydrofluoric acid, formic acid, acetic acid, ammonia and amine, and various alkali and salt solutions, but it is not resistant to strong oxidizing acids, such as concentrated sulfuric acid, nitric acid, acid and sulfuric acid mixture. When the temperature is high, about 90 ~ 100 ℃ of sulfuric acid, nitric acid can quickly destroy PE Different densities of PE resistance to oxidation of different capabilities, LDPE structure in the branched chain more and longer, tertiary carbon atom density, oxidation resistance than HDPE poor. In addition, the role of gaseous chlorine and fluorine on PE, with the increase in temperature and aggravated. PE chemical structure of inertia and the surface of the nonpolar, determines its compatibility with other polymers is poor, but also difficult to bonding and printing. Only after treatment with strong oxidizing agents, flames and electrical discharges is it possible to improve the adhesion and printability.

5) Electrical Properties

PE macromolecule chain is made of many atoms connected by covalent chain, valence electrons are in stable low energy state, the structure itself is not easy to generate conductive ions, coupled with the symmetry of PE macromolecule, non-polar and other structural characteristics, determines its hydrophobicity, thus enhancing the electrical insulation properties.PE plastics must be added additives are also very small in number, so the electrical insulation properties compared with the commonly used plastics are also excellent.

The electrical insulation performance of PE has little to do with density, and its relative dielectric constant and dielectric strength are related to relative molecular mass, ambient temperature, humidity, oxidation degree and other factors. At room temperature in the range of (1~ 50) X 109Hz, PE's relative dielectric constant and dielectric loss factor is independent of frequency, so it is particularly suitable for high-voltage electrical Insulation Materials.

6) Environmental stress cracking resistance

The environmental stress cracking resistance of PE has a greater relationship with its density, of which HDPE is an extremely sensitive plastic varieties of environmental stress cracking. In the role of lower stress, the normal does not crack, but in the environmental reagents and stress at the same time under the conditions prone to brittle cracking.

PE ESCR test is carried out according to the national standard GB 1842-80. The method is the same class of multiple specimens immersed in a certain temperature of the environmental reagents, when the breakage rate of 50% of the time F50 (h), that is, the specimen in the medium of the ESCR, the results of the processing according to the logarithmic probability of the coordinates of the graphing method.

7) Hygienic

PE is non-toxic to the body and can be used in the food industry as a packaging material in direct contact with food. However, attention should be paid to the toxicity of the additives (additives) used.