How should the plastic material polyethylene be classified

The wire and cable industry is a heavy industry, the main material is copper, but it also involves the application of many non-metallic materials, and plastic materials in non-metallic materials are the top priority, can not be ignored
.
Among the many plastic materials, wire and cable design engineers most frequently used plastic materials non-polyethylene materials, this article only from the physical properties, structural properties of a brief introduction to the classification of polyethylene materials, to provide you with a macroscopic and easy-to-understand memory model
.

Low Density Polyethylene (LDPE)

Abbreviation (LDPE) is usually produced by high-pressure method (14.
17~196.
2MPa), so the crystallinity is low (4%~65%), the density is small (0.
910~0.
925), light weight, flexibility, low temperature resistance, good impact resistance
.

High-density polyethylene (HDPE)

Abbreviation (HDPE) is mainly produced by low pressure, so it is also known as low pressure polyethylene
.
HDPE molecule has few branched chains, high crystallinity of 85%~95%), high density (0.
941~0.
965}, high use temperature, hardness, mechanical strength and chemical resistance
.
It is suitable for hollow blow molding, injection molding and extrusion of various products (hard), such as various containers, nets, strapping belts, and can be used as cable cladding, pipes, profiles, sheets, etc
.

Linear Low Density (LLDPE)

LLDPE is a new type of polyethylene that has been newly developed and rapidly developed, which is a copolymer of ethylene and olefin
.

Since LLDPE is copolymerized with ethylene and a-olefin under the action of a highly active catalyst with coordination structure by low-pressure method, the polymerization method is basically the same as HDPE, so like HDPE, its molecular structure is straight chain.

However, due to the introduction of a-olefin, there are many short and regular branched chains on the molecular chain, and the number of branched chains depends on the number of moles of the comonomer, and there are 10~35 short branched chains per 1000 carbon atoms on the general molecular chain, and the length of the branched chains is determined
by the number of carbon atoms of the olefin.
However, the length of LLDPE branches is generally larger than that of HDPE branches, and the number of branches is also large
.
Compared to LDPE, it does not have the long branch chain
that is unique to LDPE.
The molecular chain of LLDPE is a structure with short branched chains, and its molecular structure regularity is between LDPE and HDPE, so the density and crystallinity are also between HDPE and LDPE, and closer to LDPF
.
In addition, the relative molecular mass distribution of LLDPE is narrower than that of LDPE, and the average relative molecular mass is larger, so the melt withering degree is larger than that of LDPE, the processing performance is poor, and the melt rupture phenomenon is easy to occur
.
It is precisely because of the structural characteristics of LLDPE that its performance is similar to LDPE and has the characteristics of
HDPE.

LLDPE has high melt viscosity during extrusion, and the extruder must be equipped with a motor with larger power, the power is usually 25%~30% larger than when extruding LDPE, and thrust bearings with higher strength grades should be selected, and screws
with smaller diameter and deeper screw groove should be selected.
If the length-to-diameter ratio of the screw cannot be changed, a short metering section can be used as compensation
.
Using a screw designed in this way can reduce its driving torque and give the melt the best processing viscosity limit, which is not prone to melt breakage
.
The deepening of the screw groove in the metering section also facilitates the control of the melt temperature
.
LLDPE is prone to melt rupture, so LLDPE film is produced with a blow molding film head that processes ordinary LDPE, and the product is prone to fish skin phenomenon
.
In addition to designing the extruder screw according to the above requirements, the method of overcoming also needs to increase the die gap
of the machine head.
Generally, the gap between the mouth die of LDPE film is 0.
5~0.
9mm, and the gap of the mouth die of the processed LLDPE film should be increased to 1.
3~1.
8mrn
.
The gap between the mouth and die is increased, so that the shear action on the melt is reduced, and the excessive pressure of the machine head can be avoided
.
LLDPE has a higher melting point, extrusion and temperature are also higher, usually about 200~215 °C, and adopt a relatively stable temperature distribution
along each section of the screw to the machine head.
LLDHE melt has a very low tensile viscosity after extrusion of the mold, and blow molding is produced

The stability of the film is poor, and if the processing temperature is increased, this tendency becomes stronger, so the method of increasing the temperature to reduce the viscosity of the melt is limited
.
LLDPE melt has good elongation performance, can use high-speed traction device, and is also suitable for processing sheets and containers
.
However, its melt strength is low, the elongation is large, and the control of membrane bubbles and parislets and pipe shaping are more difficult
.
LLDPE has high toughness, cutting tools are easy to wear, and hardened tools
are required.
The shear rate of injection LLDPE is higher than that of extrusion and has a higher viscosity than LDPE, so it is necessary to appropriately increase the injection temperature and injection pressure
.
If LLDPE with a large melt flow rate is selected, low injection pressure molding can also be selected, even if the melt flow rate is slightly larger than LDPE, a product
with satisfactory strength can be obtained.
In addition, LLDPE has a high melting point and high rigidity, and the product can be demolded at a higher temperature, so the molding cycle is short
.

Ultra-low density polyethylene (VLLDPE)

A new linear structural resin
made of ethylene and polar monopoly, such as ethyl acetate, acrylic acid or methyl acrylate vinegar.
The minimum density of this copolymer resin is 0.
912g/cm3
.
Due to its low density, it has the softness and compliance that other types of PE cannot match, but still has the mechanical and thermal properties
of high-density linear polyethylene.
Western Europe and Japan have developed such polyethylene, and their grades such as DFDA-1137 and DFDA-1138
.
The melt characteristics of VLLDPE are similar to LLDPE, and the processing equipment of the two can be used universally
.

VLLDPE can be used to manufacture hoses, bottles, vats and carton linings, caps, shrink and stretch films, coextruded films, wire and cable coatings, play with them, etc
.

Ultra-high molecular weight polyethylene (UHMW-PE)

UHMW-PE is also a linear polymer
.
The relative molecular mass is 50~5 million (generally M100-1.
5 million polyethylene is more suitable for UHMW-PE), the main chain is long and entangled with each other, and the crystallinity (65%~85%) and density (0.
92~0.
94g/cm3) are low
.
Due to the high relative molecular mass, the melt viscosity is very large, it is difficult to flow in a high elastic state, and the melt index is close to zero, which is difficult to process
.
In addition to the performance of general HDPE, UHMW-PE also has outstanding wear resistance, low friction coefficient and self-lubrication, excellent stress crack resistance, high temperature creep resistance and low temperature resistance (even at -269 °C can be used), excellent tensile strength, extremely high impact strength, and does not drop at low temperature, good noise damping, at the same time, has excellent chemical stability and fatigue resistance, no surface adsorption force, excellent electrical insulation performance, non-toxicity and other excellent
。

UHMW-PE has a very low shear rate and is processed
by cold-pressing sintering method, type and other methods.
The melt index is extremely low, the viscosity is very high, the fluidity is extremely poor, and the criticality is not suitable for processing by general thermoplastic molding processing methods
.
It can also be processed by twin screw and plunger extruder extrusion, injection molding and other methods
.