Analysis of Processing Methods of High Density Polyethylene

High density polyethylene (HDPE), also known as low-pressure polyethylene, is a kind of translucent with high crystallinity and non-polar surface to a certain extent.

Development History of HDPE

A revolutionary progress has taken place in the field of pipeline in this century, namely "replacing steel with plastic". Today, plastic pipes are no longer mistaken as "cheap substitutes" for metal pipes. In this revolution, polyethylene pipes are favored and increasingly shine. They are widely used in gas transportation, water supply, sewage discharge, agricultural irrigation, mine fine particle solid transportation, oil field, chemical industry, post and telecommunications, especially in gas transportation.

High density polyethylene (HDPE) is a thermoplastic polyolefin produced by ethylene copolymerization. Although HDPE was introduced in 1956, this plastic has not yet reached the mature level. This general-purpose material is still developing its new uses and markets.

HDPE is usually manufactured by Ziegler Natta polymerization, which is characterized by no branched chain on the molecular chain, so the molecular chain is arranged regularly and has high density. In this process, ethylene is used as raw material and oxygen or organic peroxide as initiator in tubular or kettle low-pressure reactor.

HDPE is an environmentally friendly material, which can be recycled after heating to the melting point. It should be noted that plastic raw materials can be divided into two categories: "thermoplastic" and "thermosetting". Thermosetting becomes solidified after heating to a certain temperature, and its state cannot be changed even if heating continues. Therefore, products with environmental protection problems are "thermosetting" products (such as tires), Not "thermoplastic" products (such as plastic pallet note: pallet is called "plywood" in Hong Kong and Macao), so not all "plastics" are environmentally friendly.

Characteristics of HDPE Materials

Basic Characteristics

HDPE is an opaque white waxy material with a specific gravity of 0.941 ~ 0.960, which is lighter than water. It is soft and ductile, but it is slightly harder than LDPE and can be elongated. It is non-toxic and tasteless.

Combustion Characteristics

It is flammable and can continue to burn after leaving the fire. The upper end of the flame is yellow and the lower end is blue. It will melt during combustion, with liquid dripping and no black smoke. At the same time, it gives off the smell of paraffin burning.

Key Advantages

Acid and alkali resistance, organic solvent resistance, excellent electrical insulation, and can still maintain a certain toughness at low temperature. The surface hardness, tensile strength, rigidity and other mechanical strength are higher than LDPE, close to PP and tougher than PP, but the surface finish is not as good as PP.

Main Disadvantages

Poor mechanical properties, poor air permeability, easy deformation, easy aging, easy brittleness, brittleness lower than PP, easy stress cracking, low surface hardness and easy scratching. It is difficult to print. During printing, surface discharge treatment is required, electroplating is not allowed, and the surface is dull.

Application

Used for extruding packaging film, rope, woven bag, fishing net, water pipe; Injection molding low-grade daily necessities and shell, non load-bearing components, rubber box, turnover box; Extrusion blow molding containers, hollow products, bottles.

Injection Molding Process

HDPE has countless applications, ranging from reusable thin-walled beverage cups to 5-gsl cans, consuming 1 / 5 of HDPE produced in China. The grade of injection molding is generally melt index 5 ~ 10, with low toughness and high flowability. Uses include daily necessities and food thin-walled packaging; Tough and durable food and paint cans; High resistance to environmental stress cracking applications, such as small engine fuel tanks and 90 one gal waste cans.

General HDPE has a melting point of 142 ℃ and a decomposition temperature of 300 ℃; The adjustable range of injection temperature is large. During injection molding, the general service temperature is 180 ℃ - 230 ℃; Because it is an olefinic plastic, it does not absorb water and does not need to be dried during production. However, for the sake of product quality, it can be dried at 60 ℃ for 1hr to discharge floating water; The melt viscosity of polyethylene is high, the flow length ratio is small, and the thin-walled products may lack glue. Therefore, the gate and runner are relatively large; The product is easy to carry static electricity and the surface is easy to absorb Angstrom. The shrinkage rate is 16 ‰; The overflow value is 0.05mm.

High density polyethylene

Production Process of HDPE

PE can be manufactured in a wide range of different processing methods. With ethylene as the main raw material and propylene, 1-butene and hexene as copolymers, under the action of catalyst, slurry polymerization or gas phase polymerization process is adopted. The obtained polymer is flashed, separated, dried and granulated to obtain a finished product with uniform particles. These include, for example, sheet extrusion, film extrusion, pipe or profile extrusion, blow molding, injection molding and rotational molding.

HDPE is suitable for various molding processes of thermoplastic molding, with good molding processability, such as injection molding, extrusion, blow molding, rotary molding, coating, foaming process, thermoforming, thermal sealing welding, thermal welding, etc.

Extrusion

Grades used in extrusion production generally have a melt index of less than 1 and a medium to wide MWD. In the processing process, low MI can obtain appropriate melt strength. Wider MWD grades are more suitable for extrusion because of their higher production speed, lower die pressure and reduced melt fracture trend.

PE has many extrusion applications, such as wire, cable, hose, pipe and profile. Pipe applications range from small cross-section yellow pipes for natural gas to thick wall black pipes with a diameter of 48 inches for industrial and urban pipelines. The use of large-diameter hollow wall pipes as substitutes for concrete rainwater drainage pipes and other sewer pipelines is growing rapidly.

Plate and thermoforming: the thermoforming lining of many large picnic refrigerators is made of PE, which has toughness, light weight and durability. Other sheet and thermoformed products include mudguards, tank liners, pan guards, transport boxes and tanks. A large number of rapidly growing sheet applications are plastic film or pond bottom village, which is based on MDPE with toughness, chemical resistance and impermeability.

Blow Molding

More than 1 / 3 of HDPE sold in the United States is used for blow molding. These range from bottles of bleach, oil, detergent, milk and distilled water to large refrigerators, car fuel tanks and cans. The characteristic indexes of blow molding grade, such as melt strength, es-cr and toughness, are similar to those used for sheet and thermoforming applications, so similar grades can be used.

Injection blow molding is usually used to make smaller containers (less than 16oz) for packaging drugs, shampoos and cosmetics. One advantage of this process is that the edges and corners of the bottle are removed automatically, and there is no need for post-processing steps like general blow molding. Although some narrow MWD grades are used to improve surface finish, medium to wide MWD grades are generally used.

Rotational Molding

The material processed by this method is generally crushed into powder to melt and flow in the thermal cycle. Rotational molding uses two types of PE: general and crosslinkable. General purpose MDPE / HDPE usually has a density range from 0.935 to 0.935 945G / cc, with narrow MWD, so that the product has high impact and minimum warpage, and its melt index range is generally 3-8. Higher Mi grades are generally not applicable because they do not have the impact and environmental stress cracking resistance expected of rotational molded products.

Film

PE film processing generally uses ordinary film blowing processing or flat extrusion processing. Most PE is used for thin films, and general low density PE (LDPE) or linear low density PE (LLDPE) are available. HDPE film grade is generally used where superior tensile property and excellent impermeability are required. For example, HDPE film is commonly used in commodity bags, grocery bags and food packaging.

Main Uses of HDPE

Injection molding products: turnover boxes, bottle caps, barrels, hats, food containers, plates, dustbins, boxes, and plastic flowers

Blow molding products: hollow molding products, such as various series of blow molding barrels, containers, bottles, containing detergents, chemicals, cosmetics, gasoline tanks, daily necessities, etc. There are also film blowing products, such as food packaging bags, miscellaneous shopping bags, chemical fertilizer lining films, etc.

Extruded products: pipes and pipe fittings are mainly used for gas transportation, public water and chemical transportation, such as building materials drainage pipes, gas pipes, hot water pipes, etc.; sheets are mainly used for seats, suitcases, handling containers, etc.

Rotary molding: injection molding products such as large containers, storage tanks, barrels, boxes, etc.

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