polyethylene linear low density
4, process technology for the quality of LLDPE
Process development of new technologies not only improve product performance and lower manufacturing costs and promote competition between the polymer and interchangeable. Catalyst system, the total catalyst, comonomer, the reactor, polymerization media, and other aspects of change, affecting the polymer molecular structure, crystallinity of the resin, branched-chain degrees, comonomer distribution, and density, molecular weight, molecular weight distribution (MWD) and so on. These structural factors also determine the final properties of polymers, including mechanical strength, optical properties, purity, rheological behavior (workability), stability (heat, ultraviolet, etc.), thermal and electrical properties.
Bimodal with a low pressure process such as the production of a wide MWD and LLDPE copolymers and terpolymers, can be similar to the traditional high-pressure processing and properties of LDPE resin.
LDPE has more branched structure, in which the dominant long-branch chain, and only a short branched LLDPE, and their number determines the polymer crystallinity and density. Will help improve the processing properties of LDPE LLDPE applications to expand access to those previously due to performance (such as transparency, melt strength, etc.) differences and failed to enter the field.
In recent years, some of LLDPE production of new technologies, in addition to bimodal technology, the most prominent is the metallocene, single-center non-metallocene catalyst technology, has allowed easy processing, high performance LLDPE in large numbers. Application of these new technologies developed LLDPE resin, known as the second generation of LLDPE resins.
From the above we can see whether or not the advanced production technology, product quality, cost has a decisive influence. At present, China basically is the introduction of LLDPE production technology abroad, until the early 90s of last century's technology, combined with the digestion and absorption is not enough to produce the product grade and quality is not high, less varieties, the product of impurities, uneven quality, processing performance also poor. Thus, the domestic processing enterprises would rather spend a lot of high-priced foreign imports of materials, rather than with the domestic like product.
(D), LLDPE Analysis of the quality of
Molecular weight distribution of LLDPE, the average molecular weight and density of the critical nature of the final product.
1, Structure and Properties
Structure of different kinds of polyethylene, the main difference is the type of side chain, number and distribution.
High-pressure low density polyethylene (H P-LDPE) both the long chain branches have short branched-chain, the length of the long chain branches can even reach the length of the main chain. The number of long chain branches per 1,000 main chain carbon atoms of 0.5 to 5. The number of short branched-chain main-chain carbon atoms per 1,000 15 to 30. Main chain was branched.
High-density polyethylene (HDPE) no long chain branches, only a few short side chain. The number of short branch chains in the main chain of carbon atoms per 1000 less than 10 months (short-branched-chain homopolymer than the copolymer less). Main chain was linear. Because HDPE has little short-short branched-chain, so the density, crystallinity is high.
Linear low density polyethylene (LLDPE) and high density polyethylene, have no long chain branching, but its ratio of short-branched HDPE both short and long, in the main chain of carbon atoms per 1,000 10 to 35 Short chain. It can also be through the introduction of different types (from propylene to 1 - octene) and varying amounts (5% to 20%) of comonomer, and it's short length and number of branched-chain adjustable to change its molecular structure, to obtain the desired properties of the resin. It is also the main chain similar to a linear type of HDPE.
The main factors affect the performance of polyethylene are: the type of side chain, number and distribution; molecular weight and molecular weight distribution. Particularly among the type and number of branched-chain effect on performance even more.
(1) The crystallinity of polyethylene is a crystalline polymer. Crystallinity of polyethylene of different densities is not the same. Crystallinity and density is linear polyethylene their significant impact on many properties.
In view of the short branched-chain polyethylene presence would interfere with the crystallization of the main chain, thus increasing the damage will be short-branched-chain crystallization and lower density. The high density polyethylene homopolymer contains very few short branched-chain, so it is the crystallization of high density high.
LLDPE and HDPE, although both belong to linear polyethylene, but is completely LLDPE ethylene and Î±-olefin copolymer formed. Contained in the LLDPE copolymer of comonomer than the high density of more linear LLDPE main chain and thus there are a lot of short branched-chain, resulting in its low crystallinity and density; again because of the short branched-chain type and number of with different monomers is varied, if the number of carbon atoms comonomer more and more content in the copolymer, the copolymer decreased the density is also large.
(2) thermal properties of polyethylene after heating, with the temperature, the crystalline part of the gradual decrease in some completely disappeared when the crystal, the polyethylene to melt, when the temperature is the melting point. Increased density polyethylene, crystallinity increased, also will increase its melting point, so the different densities of polyethylene, its melting point is different. LLDPE melting point of 120 ~ 125 â„ƒ, between H P-LDPE and HDPE between. Different comonomer LLDPE, with its low melting point comonomer increase or decrease the carbon atoms changes, increased melting point increased the number of carbon atoms. Since the melting point of LLDPE than H P-LDPE high, so the model products can be stripping at a higher temperature, but fast and clean. Due to the melting point of LLDPE range narrower than the H P-LDPE, LLDPE film so good sealing performance, high heat sealing strength.
Polyethylene at elevated temperatures to increase liquidity and when the load changes, mainly by the molecular weight of. The determination of melt flow rate of polyethylene easier than the determination of molecular weight, which usually melt index (MI), or melt flow index (MFI) to represent the molecular characteristics of polyethylene. In the molten state, the molecular weight polyethylene is a function of melt viscosity, which increased with the increasing of molecular weight. When the same molecular weight, temperature increases the melt viscosity. Polyethylene at room temperature with different density have different flexibility. Polyethylene at low temperature flexibility and good nature, and its crisp analysis of low temperature, which related to its molecular weight. When higher molecular weight polyethylene, its brittle temperature decreased, the limit is -140 â„ƒ.
In the case of the same molecular weight, linear structure of the LLDPE and HDPE in the melt viscosity than the non-linear structure of the H P-LDPE large. In the case of the same melt index, H P-LDPE LLDPE significantly lower melt viscosity and HDPE, therefore, the former melt flow during processing significantly better than the latter two, the screw load is small, the heat is also small.
(3) environmental stress cracking resistance and creep resistance of polyethylene resin from the practical point of view, resistance to environmental stress cracking (ESCR) performance is an important indicator of the physical properties. Branched-chain polyethylene ESCR performance is due to the increase of density reduction and much improved. In 3 different polyethylene resin, LLDPE many of the performance between the H P-LDPE and HDPE between, but the ESCR performance is among the highest ranks. 6 and 8 high-carbon carbon carbon Î±-olefin copolymer LLDPE, because of its side chain increases, the ESCR value is better than C 4 copolymer LLDPE.
Another increase of short branched-chain, reduce the impact of the performance of the density creep resistance or the ability to withstand load. The performance of the use of the polymer is also very important. That as long as the density decreased slightly, creep resistance to be greatly improved. It can be said to increase the short branched-chain ethylene, ethylene reduced the density of the greatest benefit is to increase the ESCR performance and creep resistance.
Exhibit 7: different density polyethylene melt index value of ESCR
Density (g / cm 3)
Melt Index (g/10min)
High-density (low pressure)
High-density (in the pressure method)
(4) thermal aging and light aging properties of polyethylene because of its molecular structure and the polymer contained in the internal trace impurities, and by forming the atmospheric environment and the impact of external conditions, will produce oxygen thermal aging and light aging . The aging reaction by free radical reaction mechanism of touch, resulting in degradation of polyethylene occurs mainly irreversible chemical reaction, and its performance deterioration and even complete loss of use value.
Polyethylene in the presence of oxygen when heated prone to thermal aging effects, this thermal aging process has a catalytic effect automatically, so when the higher temperature, accelerated oxidation, which can deteriorate electrical insulation properties of polyethylene. In addition, ESCR, and other properties will be reduced elongation and increased brittleness, serious will happen when the special smell odor. The impact of oxidation on the length of time with the heat, for example, containers made of high density polyethylene by the short time to heat, there is no decrease of its value in use, the cable if it made a long time in the 60 â„ƒ heat, its power insulation is significantly reduced.
Polyethylene by ultraviolet radiation in sunlight and the role of oxygen in the air, so molecules of carbonyl oxygen content increased aging effect of light occurs, this light the role of oxidative aging is carried out at room temperature, it will enable the solution of polyethylene molecules together, and generate part of the body structure, side chain.
Therefore, in order to prevent or slow down the aging effects of light oxygen should be added in the polyethylene has a masking effect of light stabilizers such as carbon black or UV absorbers. Polyethylene in the heat molding process, especially a lot of air in contact with the case, for example, the process of rolling or extrusion, injection molding, due to the heat leaving oxide reduces the mechanical properties of polyethylene, although the addition of antioxidants can be part of prevention, but still can not completely avoid, and thus improve the polymerization process forming method, and the use of modified method can improve the stability of polyethylene by external effects.
(5) Polyethylene dielectric properties of pure polyethylene non-polarity genes, so it has good dielectric properties. The molecular weight polyethylene dielectric properties of its effect does not occur, but if the polyethylene contains impurities, such as catalysts, the metal elements present in ash and polar groups (hydroxyl, carbonyl), etc., then the dielectric properties such as dielectric constant, dielectric loss (dielectric loss tangent), and other adverse effects occur.
In the current frequency is 50 ~ 1 Ã— 109Hz range, polyethylene dielectric constant and dielectric dissipation factor and the current frequency independent, so suitable for high-frequency insulation. Dielectric properties of polyethylene 8,9 data as shown in the chart.
Chart 8: The dielectric properties of polyethylene
10 3 Hz
10 6 Hz
3 Ã— 10 7 Hz
Dielectric loss tangent
10 3 Hz
10 6 Hz
3 Ã— 10 7 Hz
Volume resistivity, âˆ© Â· cm
Dielectric strength, kV / mm
2.28 ~ 2.32
2.28 ~ 2.32
6 Ã— 10 15
2.34 ~ 2.36
2.34 ~ 2.38
6 Ã— 10 15
Figure 9: The density and dielectric constant of polyethylene
Density, g / cm 3
Dielectric constant (ASTM D150)
(6) chemical stability of polyethylene with the chemical properties of saturated aliphatic hydrocarbons, so it is highly stable and inert. Different density polyethylene contained in the number of double bonds and branched-chain number of different crystallinity is not the same, so they have slightly different chemical stability. For example, low density polyethylene can be dissolved in boiling benzene, and high-density polyethylene in benzene under the same conditions only swelling.
(7) physical and mechanical properties of the physical and mechanical properties of polyethylene and its crystallinity (density) and molecular weight (melt index), and therefore different density polyethylene, or a different melt index of the same density polyethylene, the physical and mechanical performance is also different, as shown in Table 10.
Exhibit 10: The density polyethylene and the relationship between physical and mechanical properties
With the temperature increased performance density
With the density increase performance and reduce
Permeability (including permeability, moisture permeability and oil resistance)
Environmental stress cracking resistance
Polyethylene is the relationship between stiffness and density: With increasing density polyethylene, the crystallinity also increased, to increase rigidity. In the three kinds of polyethylene, HDPE high density, the stiffness is the highest. However, almost rigid rapidly decreased with the decreased density polyethylene, such as density 0.960g/cm3 of HDPE, its rigidity is 1000MPa, when the density drops 0.940g/cm3, its rigid quickly dropped to 550MPa. But the same density of HD-LDPE and LLDPE, which rigid than the former large, therefore, the same thickness of film, LLDPE film rigidity better than the H P-LDPE.
The impact strength and density of polyethylene is the relationship: density increased, the crystallinity increased, impact strength decreased. The impact strength of LLDPE film greatly influenced by the comonomer, and 1 - butene copolymer LLDPE film, the impact strength and H P-LDPE film equivalent, but with 1 - hexene and 1 - octene copolymer LLDPE film , the impact strength is significantly improved.
Permeability and density of polyethylene is the relationship: the barrier to increase the crystal density, permeability decreases. Compared with other plastic film, polyethylene film on the nitrogen, oxygen, carbon dioxide permeability is greater, in particular, the permeability of low density polyethylene film than polystyrene, polyvinyl chloride, polyethylene terephthalate, etc. permeability of the film are great. Various media for the permeability of ethylene, ethylene in its solubility in a great relationship, in general, the permeability of non-polar material is greater than the permeability of polar substances.
Elongation of polyethylene and the relationship between density polyethylene: density decreased, the increase in non-crystalline components, so that become more of polyethylene plastic, which will soon increase its elongation.
With the increasing density polyethylene, the crystalline component also increased, the crystallization region is also expanded. Exist in the crystalline region spherulite structure, grain size when the ball over the visible wave, the reflection of visible light present as white, thereby reducing the transparency of polyethylene, turbidity increases.
Other physical and mechanical properties of polyethylene factors affecting the molecular weight distribution (MWD). Representation of the molecular weight distribution, the weight average molecular weight (M w) of the number average molecular weight (Mn) ratio, that is, Mw / Mn. The most important feature of molecular weight distribution is sensitive to shear or cut features to that of the melt properties. The wider the molecular weight distribution, the greater the shear sensitivity. Shear sensitivity of the decline in tandem with the rise in the melt index, it should be the basis of the melt index up relatively the same shear sensitivity. Some of the uses for polyethylene, the high shear sensitivity (broad MWD) of polymers most likely processing, and can get out of a high rate. Some solid-state mechanical properties of polyethylene is also dependent on the molecular weight distribution. Homopolymer of polyethylene in the melt index value must be wider when the molecular weight distribution, the density, the crystals are more complete. The value of ESCR, and particularly environmental stress cracking resistance of copolymer properties, in a certain melt index, its value is often resistant to environmental stress cracking due to the broadening molecular weight distribution becomes larger.
The following table shows the same density and H P-LDPE LLDPE melt index, molecular weight and molecular weight distribution, and comparison of their physical and mechanical properties.
Chart 11: LLDPE and H P-LDPE compared the physical and mechanical properties
Density, g / cm 3
Melt index, g/10min
Molecular weight M w
Number average molecular weight M n
Molecular weight distribution M w / M n
High shear viscosity
Ultimate Tensile Strength, MPa
Tensile yield strength, MPa
Flexural modulus (2% s), MPa
Melting point, â„ƒ
As can be seen from the table, resin and LLDPE resins A and D B (both high molecular weight and the corresponding lower melt index) than the resin D is a high-pressure low density polyethylene, with a long chain branches and thus the lower molecular weight, the lower the melt index. Entanglement of long chain branching at higher shear rates, little effect on the viscosity, this time in the production of blown film is common.
Resin C and Resin D, as compared to when the low molecular weight, have very high melt index and molecular weight distribution closer to resin D, the structure of the resin melt index of 6.0 C, melt index than 0.3 D processing of resin viscosity higher.
Can be seen from the table, LLDPE and H P-LDPE, the tensile yield strength and flexural modulus is a function of resin density, the absolute value but also because of the same density and very close. For the H P-LDPE and LLDPE, the ultimate tensile strength are with weight average molecular weight increases, however, LLDPE ultimate tensile strength, and H P-LDPE melt index of considerable circumstances, than H P-LDPE is much higher.
LLDPE resins C on behalf of an example of the series, that is at a high melt index, makes the ultimate tensile strength decreased. A similar relationship exists in the melt between the index and ultimate elongation.
Comparison of resin A and B, can be argued that broad molecular weight distribution of the resin B, if A and the resins have the same melt index, its ultimate tensile strength and ultimate elongation is relatively low.
2, product quality control
Production of linear polyethylene of various methods are used to join the chain transfer agent methods to adjust the product weight, by adding more chain transfer agent, the smaller molecular weight, the greater the melt index MI. Commonly used chain transfer agent is hydrogen, and some processes such as reaction temperature solution method can be used to adjust the molecular weight, temperature, molecular weight decreases, the melt index increases. Gas law and the requirements of slurry temperature stability, temperature regulation can not weight.
Density of the product is used to change the amount of the feed monomers to control. The same kind of process, product density, the lower the required feedstock containing more comonomer. Different comonomer, the density to get the same product, the amount of comonomer used is different. The UCC of the gas-phase, for example, the product is adjusted to density 0.91 ~ 0.94g/cm3, different amount of comonomer following table.
Exhibit 12: the ratio of comonomer and ethylene
(Product density: 0.91 ~ 0.94g/cm3)
Content of copolymer
Comonomer materials in gas /
Ethylene, the molar ratio
1 - butene
1 - pentene
1 - hexene
1 - octene
3.0 ~ 10
2.5 ~ 7.0
2.0 ~ 6.0
1.0 ~ 5.0
0.8 ~ 4.5
0.2 ~ 0.9
0.2 ~ 0.7
0.15 ~ 0.45
0.12 ~ 0.4
0.10 ~ 0.35
3, determination of product quality
The production of polyethylene of various methods to determine molecular weight (characterized by melt index), the molecular weight distribution and density of the same methods, but different physical properties with the product to determine the conditions under which to be a little adjustment.
Determination of molecular weight average molecular weight of resin (the weight) can be determined by gel chromatography, but this is not easy to carry out the production process control, which is characterized by melt index of average molecular weight. With the increasing molecular weight, melt viscosity, flow variation (or vice versa). Determination of unit time, to a certain load, the melt flow through the tiny hole the weight of resin melt flow can be measured, thus we know the size of the melt viscosity, reflecting the molecular weight resin.
Determination of Melt Index is based on ASTMD-2238 condition E, or ISO1133 conditions, the provisions of 4, at 190 â„ƒ or so (high density slightly higher temperatures), the vinyl-like material in the standard load of 2.16kg, 10min adopted within the model head, out of the melt index is the number of grams, the unit is g / min.
When the molecular weight to a certain extent, MI less than 0.1, the load increased to 5kg or 21.6kg, the value measured at this time is expressed as HLMI, and should indicate the heavy load of 5 or 21.6, this value is a flow rate.
Molecular weight molecular weight distribution can be measured molecular weight ratio and number average molecular weight (Mw / Mn) to represent; also can make use of the sensitivity of the melt to characterize the shear weight distribution, which adopts the melt flow ratio (MFR) - -HLMI and MI to reflect the molecular weight distribution ratio; also can stress exponent (SE) to represent the molecular weight distribution.
This 3 is the relationship between representation: Mw / Mn greater, MFR greater, SE greater; the wider the molecular weight distribution.
Density per unit volume density of the material when the temperature t â„ƒ quality, usually expressed marked t laboratory temperature for 20 â„ƒ, 23 â„ƒ and 27 â„ƒ, the density in units of g / min.
According to the provisions of ISO, the sample shape is not measured simultaneously by different methods. There are impregnation, the proportion of bottle law, buoys and density gradient sedimentation column method.
Exhibit 13: linear low density polyethylene (enterprise standard) grade DNDA-DNDA-DNDB-DNDC-DNDC-DEX-DFDA-DFDA-DFDA-DEX-DEX-based resin 7145 7146 7143 7,150,714,883,027,047 7,068,707,582,188,220 project DGL-DGL-DGM-DGM-DGM-DGM-DGM-DGM-DGM-DGM-DMG-2612 2612 3440 3928 H 3450H 1810 1810H 1850 2670H 2610H 2610H melt index g/10min 12 12 4 2.8 5 3 1 1 0.5 0.7 1 Density g / cm 3 0.926 0.926 0.934 0.939 0.934 0.934 0.918 0.918 0.918 0.926 0.926 Melt Flow vs 30 30 30 3,030,303,030,303,030 bulk density kg / m 2 480 480 480 480 480 480 480 480 480 480 480 hexane extract rate% 5.3 5.3 5.3 5.3 5.3 5.3 5.3 5.3 5.3 5.3 5.3 metal content mg / kg 7 7 7 7 7 7 7 7 7 7 7 Yield strength MPa 11 11 14.5 17 14.5 14.5 8.3 8.3 8.3 8.3 8.3 Tensile strength MPa 9 9 12 1,312,261,726,202,524 secant modulus MPa 270 270 340 390 340 360 200 200 200 380 380 properties tear kN / m 130 400 150 Impact Strength MJ / m 2 7 7 10 12 15 85 35 105 50 90 75 film appearance -10 -10-10-10-10 brittle temperature â„ƒ -60 -60 -60 -60 -60 anti-environmental cracking F 50, h 10 10 200 300 250 131 712 turbidity% gloss (45 o ) 494 055 dielectric constant use of vessels made of plastic sheeting
Continued on the table linear low density polyethylene (enterprise standard) grade DFDA-DFDA-DFDA-DFDA-DFDA-DFDA-DFDA-DFDA-DNDA-DNDA-DNDA-based resin 6080 7340 7064 7,081,704,270,437,510 7,540,714,471,471,077 project DFM-DFH-DGH-DGM-DGM-DGM-DFH-DFH-DGL-DGL-DGC-2076 2076 2685H 1810 1820 2230 2076 2076 2420 2650 3100 melt index g/10min 0.76 0.76 0.9 1 2 3 0.76 0.76 20 50 100 Density g / cm 3 0.92 0.92 0.926 0.918 0.918 0.922 0.926 0.92 0.924 0.926 0.931 Melt flow ratio 65-85 65-85 3030303065-85 65-85303030 bulk density kg / m 2 480 480 480 480 480 480 480,480,480,480,480 hexane extraction rate% 5.3 5.3 5.3 5.3 5.3 5.3 5.3 5.3 5.3 5.3 5.3 metal content mg / kg 15 15 7 7 7 7 15 15 7 7 7 Yield strength MPa 9 9 11 8.3 8.3 8.3 9 9 11 10.3 9 Tensile strength MPa 25 17 12 8 9 9 secant modulus MPa 220 220 270 200 200 200 200 260 270 300 properties tear kN / m 350 130 100 Impact Strength MJ / m 2 20 20 80 35 20 15 20 20 5 53 film appearance grade -10-10-10-10 brittle temperature â„ƒ -60 -60 -60 -60 -60 -60 -60 anti-environmental cracking F 50, h 200 200 200 200 2.5% 17,714 turbidity 16 Gloss (45 o) 40 65 45 40 bottles of the dielectric constant re-use of extruded transparent film packaging film packing film extrusion cable household goods
Product Description English name Linear low density polyethylene (LLDPE) production methods, linear low density polyethylene to ethylene as the main raw material, 1 - butene or 1 - hexene as comonomer, in the catalyst bed under the gas phase polymerization . Material reflected by granulation, drying, and sent packing. Performance of linear low density polyethylene is non-toxic, tasteless, odorless white particles, density of 0.918 ~ 0.935g/cm3. It is compared with the LDPE has a higher softening temperature and melting temperature, there is strength, toughness, rigidity, heat, cold and good, but also has good resistance to environmental stress cracking resistance, impact strength, resistance to tear strength and other properties. And can be acid, alkali, organic solvents. Use for the production of plastic sheeting, packaging films, composite films, tubes, hollow containers, wire, cable insulation layer. Main Grade Grade: 7042 (tablets) Grade: 7047 (tablets) Grade: Grade 7042 Powder: 7047 Powder Grade: Tianlian 9020 aggregates, aggregates 9085, 1820 pink, 1875 pink packaging and storage precautions delivered products installed on the heavy bag of polyethylene film bag, outer polypropylene woven bag, net weight 25kg. Storage warehouses should be kept clean, dry, cool and well ventilated. Available trains, cars, ships, aircraft and other transportation. Non-dangerous goods transport. Vehicles should be kept clean, dry, nails and other sharp objects are prohibited. Storage and transportation process should pay attention to fire, water, sun, dust and pollution prevention and so on. Shall not be used when handling hook. Data-CH2-CH2n-called third generation of polyethylene linear low density polyethylene (LLDPE) resins, except with the general performance of polyolefin resin, its tensile strength, tear strength, resistance to environmental stress cracking , low temperature resistance, heat resistance and puncture resistance, particularly superior, won the attention of the development. Although LDPE and LLDPE with the last part of the same - the density range, but because of the molecular structure is different between them, melt rheological behavior is different, so their physical properties and forming characteristics of the basic is not the same. Wide narrow molecular weight distribution LLDPE LDPE melting point (â„ƒ) 110 ~ 125 05 ~ 115 relative to the size of 1.5 to 1.75 against the tension of the relative elasticity size 1 1.80 1 1.4 good resistance to environmental stress cracking poor heat resistance and good oil a little bit weak poor
Introduction to linear low density polyethylene (Linear Low Density Polyethylene), the English abbreviation for the LLDPE. Linear low density polyethylene in the structure different from the normal low-density polyethylene, because there is no long chain branches. Linearity LLDPE LLDPE and LDPE depends on the different production process. LLDPE is usually at a lower temperature and pressure, the ethylene and higher olefins, such as a butene, hexene or octene copolymer generated. Copolymerization process of LLDPE polymers have generated more than narrow molecular weight distribution of LDPE, while it has a linear structure with different rheological properties. Linear low density polyethylene (LLDPE), ethylene and a small amount of high Î±-olefin (such as butene -1, -1 hexene, octene -1, methyl pentene -1, etc.) in the catalyst, the high or low pressure polymerization of a copolymer, the density is 0.915 ~ 0.940 g / cc between. But according to ASTM D-1248-84's provisions, 0.926 ~ 0.940 g / cc density range is a medium density polyethylene (MDPE). LLDPE-density expansion of the new generation to its plastic body (0.890 ~ 0.915 g / cc) and elastomer (<0.890 g / cc). But the U.S. Plastics Industry Association (SPI) and the U.S. Plastics Industry Council (APC) only extended to LLDPE plastic body, not including elastomers. 80s of last century, Union Carbide and Dow Chemical Company will sell its early elastic plastic body and is called very low density polyethylene (VLDPE) and ultra low density polyethylene (ULDPE) resin. Usually, LLDPE resin used to characterize the density and melt index. Density of the polymer chain, the concentration of comonomer decision. Determine the concentration of comonomer in the short branched-chain polymer content. Short chain length depends on the type of comonomer. The higher the concentration of comonomer, the lower the density of the resin. In addition, the melt index is a reflection of resin molecular weight, mainly by the reaction temperature (solution method) and added chain transfer agent (gas law) to decide. Independent of molecular weight and molecular weight distribution, which is mainly affected by the catalysts. LLDPE 70 in the 20th century by the Union Carbide Corporation of industrialization, which represents the technology of polyethylene catalysts and the major changes, so that PE significantly expanded range of products. LLDPE with the complex catalyst instead of free radical initiator, and at a lower cost to replace the low pressure gas phase polymerization reactor, the high cost of high pressure, in a relatively short period of time, then its excellent performance and lower cost, in many field has been replaced by LDPE. Production and properties of LLDPE production began in the transition metal catalyst, in particular, Ziegler (Ziegler) or Philips (Phillips) type. Metal derivatives of cyclic olefin-based catalyst LLDPE production of new technology is another option. The actual solution polymerization can be carried out and gas phase reactor. Typically, octene and ethylene in the solution phase copolymerization reactor, butene. Hexene and ethylene in the gas phase polymerization reactor. Reactor in the gas phase LLDPE resin produced in the form of particles, and can be further processed into powder or pellets for sale. For hexene and octene-based LLDPE by a new generation of super-Mobile, Union Carbide, Novacor, and introduced plastic road. These materials have great toughness limit, remove the bag in the automatic application of a new potential. Very low density PE resin (density less than 0.910g/cc.) Also appeared in recent years. VLDPES flexible and has the softness is LLDPE unattainable. Reflected in the general characteristics of the resin melt index and density. Resin melt index reflects the average molecular weight and is mainly controlled by reaction temperature. Average molecular weight and molecular weight distribution (MWD) has nothing to do. Catalyst selection of MWD. Density by the comonomer concentration in the polyethylene chain decisions. Comonomer concentration control the number of short branched-chain (its length depends on the comonomer type) to control the resin density. The higher the concentration of comonomer, the lower the resin density. In the structure of, LLDPE in the number and type of side chain with different LDPE, high pressure LDPE with long side chains, and linear LDPE only has a short side chain. The structure of, LLDPE only in the number of short side chain with HDPE different. HDPE small number of short side chain, therefore, there is a higher density of the material. The physical properties of LLDPE controlled by its molecular weight, MWD and density. Than LLDPE LDPE, ultimately depends on its use. Typically, all applications used in the production of rigid LLDPE more products, though according to ATSM standards of low-density material, LLDPE and LDPE of density between the 0.91-0.925. Crystalline structure of LLDPE to form a higher, because there is no long chain branches. LLDPE produced large high rigid crystalline products. This high crystallinity LLDPE and LDPE are compared to the melting point increase 10 ~ 15 â„ƒ. Higher resistance to tensile strength, penetration resistance, tear resistance and elongation properties increase in LLDPE, making it especially suitable for manufacturing thin film. If instead of hexene or octene comonomer butene even for impact resistance and tear is also available to larger improvements. For the same melt index and density of a given resin, hexene and octene LLDPE resin in the performance impact and tear to 300%. Hexene and octene resin in the chain between the longer side chains serve as "knots" as the role of molecules to improve the toughness of the compound. Metal derivatives with cyclic olefin resin catalyst to produce a unique performance. Narrower MWD, improved comonomer distribution, there is a better film transparency, sealing, and impact strength, these catalysts produced with the use of Ziegler LLDPE similar. On the transparency of this feature, LLDPE and LDPE with similar shortcomings O LLDPE film turbidity and gloss is not good, mainly because it resulted in a higher crystalline surface roughness. The transparency of LLDPE resin can be blended with a small amount of LDPE to improve. Comonomer type according to statistics of, LLDPE mainly divided into three kinds of copolymers: C4 (butene -1), C6 (hexene -1) and C8 (octene -1). Among them, the butene copolymer is the largest global production of LLDPE resin, and the hexene copolymer LLDPE is the fastest-growing varieties. In the LLDPE resin, the typical amount of comonomer is 5% to 10% weight fraction, the average amount of about 7%. Metallocene based LLDPE plastic body (mLLDPE) traditional LLDPE 3 times the average comonomer content. Figure 1 shows the reference to foreign periodicals in the 10 years since the World 3 comonomer LLDPE production
In late 1984, when the Union Carbide Corporation introduced the production of hexene LLDPE copolymer, followed by Exxon, Mobil and other companies. Dow Chemical (Dow Chemical Company) solution in its low use of technology in almost all of octene as comonomer, Canada NOVA (Nova Chemicals) is also the solution process in which pressure is mostly octene. Octene copolymer LLDPE resins have slightly better strength, tear resistance and processing performance, hexene and octene copolymer resin copolymer performance is not very different. Currently hexene LLDPE resin producers are ExxonMobil Chemical (ExxonMobil Chemical Company), Eastman Chemical (Eastman Chemical Company), Equistar (such as Star) and Chevron Phillips (Chevron Phillips Chemical Company) and so on. In addition, Dow Chemical (Dow Chemical Company), Basell (Basel company), Innovene (million Solutia Inc.), Samsung Total (Samsung Total Company), also produce hexene LLDPE. With butene comonomer commonly used in comparison to hexene and octene as comonomer LLDPE production has more excellent performance. LLDPE resin is the largest film production purposes, the long-chain Î±-olefins (such as hexene, octene) as a comonomer LLDPE resins produced films and products in tensile strength, impact strength, tear strength, puncture resistance, resistance to environmental stress cracking, and many are better than with the production of butene as comonomer LLDPE resin. Since the 20th century, 90 years, foreign manufacturers and users of PE tends to use alternative hexene and octene-butene. It is reported that with octene as comonomer resin than the hexene copolymer is not necessarily to further improvements, and the price is rather expensive, so the current use of major foreign manufacturers hexene LLDPE instead of the trend is more obvious butene . Currently, there is no large-scale production due to domestic hexene, octene, and the prices of imports more expensive, therefore, the current domestic production of butene LLDPE resin is mainly used as a comonomer. Some domestic enterprises in the production of devices despite the introduction of LLDPE with hexene as comonomer grades, but the end result of domestic production of non-hexene had to give up only a small amount of imports in the car when assessing hexene. China's imports of such products, mostly high-grade LLDPE. The future is expected to 1 - hexene LLDPE demand for the monomer a big increase linear low density polyethylene production process technology types and process types of polyethylene LLDPE production methods are four kinds: high-pressure method, gas law , solution method and the slurry. But now, the world production of LLDPE resin commonly used gas-phase process and solution method. The production process a variety of LLDPE in the process, is introduced in the gas phase and solution method are two main processes. 1. U.S. Union Carbide Corporation (UCC) of the Unipol gas-phase process. The process is similar with BP gas-phase process, but the UCC wide range of products, more varieties, with 4 different catalysts production of the entire density range from narrow to wide molecular weight distribution, melt index 0.91g/10min ~ 125g/10min the a variety of resins. In a variety of processes, UCC widest range of gas-phase products. BP process uses a catalyst to produce full-density polyethylene, melt index 0.35/10min ~ 30g/10min, narrow molecular weight distribution, when the production of broad molecular weight distribution of grades, we should add additives in the extrusion granulation, but the grade less. 2. DuPont Canada in the pressure solution method (Sclairtech) process. The process is the solution method in the production capacity of the largest and fastest-growing one. DuPont Canada in 1960, Estonia established the first set in 11kt / a of the device, to 1990, using the technology, production capacity has reached 720kt / a ~ 780kt / a, the largest production capacity of the reactor 300kt / a. (1) boost ethylene polymerization cycle after the purified monomers and solvent (cyclohexane) went into the cooling absorbers, while mixing in the cooling solution, with the pressure feed pump to the reaction pressure 10.79 ~ 16.67Mpa (110 ~ 170kgf/cm2), a temperature control system to the reaction temperature (100 ~ 300 â„ƒ), with the added amount of Ziegler-type catalyst to control the ethylene conversion rate of 95%, to regulate the melt index of hydrogen. Polyethylene with a comonomer volume control density. With two (or more) reactor, hydrogen at different temperatures and different points of operation in adjusting to join the molecular weight distribution product. Off in the reactor exit with live agent to terminate the reaction, and then heated to the reaction Logistics 300 â„ƒ, through the removal of catalyst residues Al2O3 adsorbent; such as using new and improved catalyst system (ACS) catalyst can be removed off facilities. Then, the reaction material into the flash device in the pressure removal reaction of ethylene, comonomer, and most of the solvent. (2) after removal of monomer melt processing, solvent and other volatile compounds by mixing with the solid additives into the extrusion machine and cutting machine, pellets were recycled water out, dehydrated and then paired with hot water slurry, further wash out the resin in the solvent, and resin into the stripping machine, the steam counter-current stripping, the residual solvent content less than 500mg / L, further drying and mixing with hot air to the hopper and packaging processes. (3) solvent recovery and low pressure flash from the top of prolapse of ethylene, comonomer, and cyclohexane were treated with one, two stage condenser tower into a low boiling and low boiling vinyl top material and then followed by a single tower and the copolymer Body recovery of ethylene and comonomer tower, low boiling to high boiling bottom material and resin stripper tower handled cyclohexane from the recovery of high boiling top, bottom stripper discharge from the resin, oil-like material with low polymerizationã€‘. Add comonomer comonomer type tower, from the tower side of the line is also discharged isomers of 2 - butene. (MED molecular weight distribution) LDPE and LLDPE processing applications have excellent rheological, or melt flow. LLDPE has less shear sensitivity, because it has a narrow molecular weight distribution and short side chain. In the shearing process (eg extrusion), LLDPE maintains greater viscosity, and therefore the LDPE melt index than the same difficult process. In the extrusion of, LLDPE lower sensitivity to shear stress relaxation of polymer chains faster, and the resulting physical properties on the sensitivity of Inflation less than the change in the melt in the extension school, LLDPE at various strain rates generally have lower viscosity. That it will not be the same as LDPE strain hardening in the tension generated. Deformation rate increases with the polyethylene. LDPE show an alarming increase in viscosity, which is caused by the entanglement of molecular chains. This phenomenon is not observed in LLDPE, LLDPE because of the lack of long-branch in the polymer chain does not tangle. Application of this performance on a very important film. LLDPE films because of high strength and toughness while maintaining easy to produce thinner films under the call. LLDPE rheology can be summarized as "shear rigidity" and "soft extended time." When using alternative LDPE LLDPE film extrusion equipment, and when the conditions must be modified. LLDPE's high viscosity requires greater power extruder. And provide a higher melt temperature and pressure. From the mouth to widen the gap mode in order to avoid high back pressure and melt fracture have reduced yields. LDPE and LLDPE clearance from the mouth of the general model are the size of O. 024 ~ 0. 040 in. And 0. 060-0. 10in. LLDPE "extension when soft" characteristics of the blown film process is a drawback. LLDPE blown film LDPE film bubble so not as stable. General single-lip air ring enough to use on the stability of LDPE. LLDPE unique requirements of the membrane bubble lips better ring to stabilize the wind. Cooling air ring with the lips to increase the internal membrane bubble bubble stability, and improve the film under the high productivity of production capacity. In addition to better cooling film bubble, many film and LDPE production plant blending methods used to enhance the solution logically LLDPE, LLDPE in LDPE film extrusion equipment can be done on existing, when the LDPE LLDPE blends in the concentration up to 50%. Process 100% LLDPE with LDPE or LLDPE-rich blends, the use of general LDPE extrusion machine, it is necessary to improve equipment. According to the life of extruder, may be required to improve die mouth widening gap distance, improved air ring, modified the design to better screw extruder, where necessary, increase the motor power and torque. For injection molding applications generally do not need to improve the equipment, but the optimal processing conditions to be reached. LLDPE rotational molding process requires grinding into uniform particles (35 mesh). Process including the use of LLDPE powder filled mold, heating and dual-axis rotational mold to make LLDPE gravitropic uniform. After cooling products removed from the mold. Applications to the polyethylene LLDPE has penetrated most of the traditional markets, including film, molding, pipe and wire and cable. Impermeable film is a newly developed film LLDPE market, a large sheet extrusion, for landfill and waste pond liner to prevent leakage or contamination of the surrounding area. Some of LLDPE film markets, such as production bags, garbage bags, flexible packaging, industrial hub, towels bushing and shopping bags, which are used to improve strength and toughness of the resin after the merits. Transparent film, such as bread bags, has been dominated by the LDPE, because it has better turbidity. However, LLDPE and LDPE blends to improve strength, penetration resistance and stiffness of LDPE film, not significantly affect the transparency of the film. Injection molding and rotational molding are the two largest application of LLDPE. This resin superior toughness and low temperature impact strength theory for waste containers, toys and refrigeration equipment. In addition, LLDPE's high resistance to environmental stress cracking resistance and oil make it suitable for food contact injection molding the cover, rotational waste containers, fuel tanks and chemical tanks. Coated tubes and wires and cables in the application layer of smaller markets, where LLDPE offers high burst strength and resistance to environmental stress cracking resistance can meet the requirements. Currently, LLDPE 65% -70% for the production of films.
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