We produce foamed polyethylene. Foamed polyethylene: types and methods of thermal insulation based on it

When describing polyethylene foam, regardless of the methods of its production, two definitions are used: “cross-linked” and “non-cross-linked”.

“Cross-linked” PPE is foamed polyethylene, during the production process of which the molecular structure is modified through various methods, and as a result of cross-linking, a so-called cross-linked or network molecular structure is formed. According to manufacturing technology, three types of “cross-linked” polyethylene foam are distinguished: radiation-cross-linked, chemically foamed (both materials are produced by extrusion) and material produced under pressure.

Non-crosslinked PPE is produced by extrusion using a physical blowing agent, which can be freon, propane-butane and isobutane gases. The main difference between this material and cross-linked material is that during its production the molecular structure of the polyethylene itself does not change.
The material obtained from polyethylene foam using a blowing agent is a good heat insulator, waterproofer, and noise absorber; products made from such materials are easy to bend and cut, and are also able to hold a given shape. The material is compatible with almost any building materials - wood, concrete, cement, gypsum, lime, etc.
An important positive property of polyethylene foam is that the material is chemically inert and environmentally friendly. Due to its structure, it has a low moisture absorption coefficient (less than 2%) and is resistant to rotting (the estimated service life of the material is 25 years).
The material is used primarily in construction, however, due to the above qualities, its consumers are enterprises in many other industries, in particular, mechanical engineering, automotive, medicine, the hardware and footwear industries, as well as industries where it is used as packaging.

There are various technologies for producing cross-linked PPE. Let's consider one of them.

The production process can be divided into 4 main steps:
- mixing
- extrusion
- stitching
- foaming

Mixing

raw materials

At this stage, the main components are mixed. These are polyethylene or polypropylene, a nitrogen-based foaming agent (nitrocarbonate), pigments, crosslinkers and various specific additives.

Extrusion

Extrusion is a generally accepted approach for producing products that are not finished (pipes, sheets, profiles, etc.).
The pre-mixed composition is poured into an extruder containing one or two screws. The mixed, heated under pressure substance exits the extruder already containing all the necessary components to carry out the transformation of the solid mass into a flexible foam by subsequent foaming processes.

Stitching
"Cross-linking" means changing the bonds between polymer chains. On the one hand, this improves the physical properties of the polymers themselves (for example, it imparts heat resistance), on the other hand, it becomes necessary to heat the polymer to a sufficient temperature to prevent the destruction of the material during expansion.

Physical "stitching"
Physical cross-linking is a method of obtaining a substance using the electron-radiation method, without the involvement of any chemical additives. The extruded substance is passed through an electronic processing system, where electronic influence is applied on both sides.

Foaming

Three types of foaming are used: horizontal, vertical and block. Let's look at each of these methods.

Horizontal foaming

Horizontal foaming is used in the production of many materials. In the case of chemically cross-linked materials, the polymer is cross-linked at the beginning of the foaming process; physically cross-linked - the material is pre-cross-linked, as described above.

As the oven temperature increases, bubbles form in the polymer matrix and the gas increases pressure, which converts the bubbles into a connected network of cells.

Vertical foaming

Vertical foaming is only used when the polymer has previously been physically cross-linked. After passing through the horizontal section, where the substance is preheated, it enters an almost “free” fall phase, where it foams as the temperature increases. After this, the sheet of foam is cooled and “rolled” into a roll.

Foam sheets coming out of a vertical oven

Block foaming

To produce blocks, a continuous foaming process is required. After passing through the extruder, the material is “cut” into sheets and manually inserted into formers where they are chemically cross-linked. The sheets are then placed under "foaming presses" where the final foaming step takes place.

Being one of the many modifications of polyethylene, light, thin and environmentally friendly polyethylene foam has become a part of our lives. Its production was established relatively recently - in the 70-80s. 20th century, but during its existence the material managed to prove itself well in everyday life and in industry. This unique product is distinguished by its outstanding chemical and physical properties, as well as its low cost, which makes its scope of application almost limitless.

Areas of application

The excellent characteristics of foamed polyethylene allow it to be used:

In the construction industry - for insulating roofs, walls, floors, foundations; ventilation, air conditioning and sewerage systems; door and glass seals; substrates for laminate; insulation of temporary housing, etc.

Automotive industry - for insulating the car interior, including in combination with non-woven materials.

In light industry - as an element of sporting goods and recreational items (backpacks, rugs, life jackets, protective accessories, etc.)

As protective packaging when transporting various goods.

For thermal insulation of industrial and domestic refrigeration equipment.

For the needs of the defense industry - as packaging for navigation equipment and ammunition, as well as heat and sound insulation of military equipment.

In the shipbuilding industry - as an insulating material for finishing cabins.

Thermal insulation made of polyethylene foam deserves high marks, because thanks to its low thermal conductivity coefficient (0.037 - 0.038 W/mK), the material is an effective insulation material. In addition, polyethylene foam is durable - its service life is about 80-100 years without loss of its original physical and chemical properties. The properties of the product include its high vapor barrier properties. The use of polyethylene is possible at temperatures from -80 to +100 °C

The emergence of safe polyethylene foam has made it possible in most cases to abandon the use of traditional thermal insulation materials, especially in Western countries. Having a closed-cell structure, the material has excellent heat, noise and waterproofing properties, is easy to bend and cut, retains its given shape, is resistant to aggressive building materials, and also does not support combustion and is non-toxic in fire conditions. In addition, polyethylene foam can be used in combination with other building materials - concrete, cement, wood, etc. Extruded polyethylene is also used for anti-corrosion protection of pipes.

Types of foamed polyethylene

Today there are three types of foamed polyethylene:

Chemically cross-linked, with a modified molecular structure. It is obtained using chemical reagents, which contribute to the formation of a network molecular structure.

Physically cross-linked polyethylene foam also has a modified cross-linked structure, but its production is associated with exposure to radiation.

Uncrosslinked (or gas-filled), which is mainly used for packaging. For its production, physical gas generators are used (freon, propane-butane and isobutane), and the main difference from its cross-linked “brother” is the preservation of the integrity of the molecular structure of the material.

Material production technology

To produce foamed polyethylene, processed high-density polyethylene is used, which is subjected to physical foaming or direct extrusion. The material production technology includes a number of stages:

At the first stage, granules of thermoplastic low-density polyethylene are fed into the hopper of injection molding equipment, where they are melted at a temperature exceeding the melting point of polyethylene - 115°C.

After the molten mass is formed, liquefied gas (carbon dioxide or nitrogen) is supplied to the chamber. It is the very foaming agent due to which the structure of the future product is formed. The creation of a gaseous environment is carried out in two ways: chemical or physical.

Thus, chemical gas generators are various substances that are capable of releasing gas under the influence of high temperature. Depending on the type of material used and the desired properties of the resulting polyethylene, their compounds can be very different. The use of chemical foaming agents is possible on standard equipment, and no special fire safety measures are required.

Physical gas formers are liquids with low boiling points - they release gas during evaporation. Despite the fact that from an economic point of view, the use of physical additives is more profitable, the process of producing foamed polyethylene becomes explosive and fire hazardous. This, in turn, requires strict adherence to preventive measures and the use of specialized equipment.

As a result of the continuous rotation of the hopper, the polymer mass acquires a homogeneous structure, including at the molecular level. The fluidity of the melt increases almost 2 times compared to the initial indicators, while the fluidity temperature decreases. Depending on the degree of pressure and temperature in the chamber, the size of the material cells changes.

The final stage of polyethylene production involves injecting a liquid mass into an injection mold and its subsequent cooling. This avoids shrinkage and possible deformation of the finished material when removed from the molds.

Polyethylene foam is most often produced with a one-sided or double-sided coating, which is used as foil, metallized film or lavsan. Polyethylene foil foam, which is commonly used for insulation, is also called reflective insulation.

The form of production of foamed polyethylene products can be very different - sheets, plates, films, threads, tubes, etc. The density of such products ranges from 5 to 800 kg/cub.m., and the cell size ranges from 0.05 to 15 mm.

As a rule, the production of foamed polyethylene is based on the use of polyethylene waste, which reduces the cost of the work process and at the same time avoids serious environmental problems. Of course, processing of secondary raw materials imposes a number of restrictions on its use. For example, if a material created as a result of primary processing can be used as packaging for various goods, then polyethylene, which has gone through several cycles of processing, can only be used as a covering garden film.

What is polyethylene

Polyethylene (PE) is one of the earliest large-scale and most common polymer materials. It would not be an exaggeration to say that polyethylene is known to almost all people and this very concept in everyday life is synonymous with plastic as such. Non-experts often call many materials polyethylene that have nothing in common with it.

PE is the simplest of polyolefins, its chemical formula is (–CH2–)n, where n is the degree of polymerization. The main types of PE are low-density polyethylene (HDPE), also known as high-density polyethylene (PVP, PEHD, HDPE) and high-density polyethylene (LDPE), also known as low-density polyethylene (PELD, LDPE). Next we will look at these and other types of PE in more detail.

Polyethylene is a synthetic polymer, it is obtained by polymerization of ethylene (chemical name - ethene) according to the free radical mechanism. Large-scale synthesis of LDPE and HDPE is produced by almost all of the world's leading oil and gas concerns. In Russia, polyethylene is produced at the petrochemical plants of Rosneft, Lukoil, Gazprom, SIBUR, Kazanorgsintez and Nizhnekamskneftekhim. In the countries of the former USSR, the polymer is produced in Belarus, Uzbekistan, and Azerbaijan. Serial grades of polyethylene are produced in the form of granules measuring 2-5 mm, but there are also grades in powder form, for example, this is how ultra-high molecular weight polyethylene (UHMWPE) is produced for sale.

Fig.1. Polymer in granules

History of PE

Polyethylene has been around for over 100 years. It was first obtained by German engineer Hans von Pechmann in 1899; since then he has been considered the inventor of this polymer. But, as often happens, an important discovery did not immediately find application. It came only towards the end of the 1920s, and in the 1930s the production of polyethylene was finally established, in which engineers Eric Fawcett and Reginald Gibson played a major role. Initially, they synthesized a low molecular weight paraffin product, which can be called a polyethylene oligomer. As a result of a lot of work, in 1936, the engineers’ research to develop a high-pressure unit ended in obtaining a patent for LDPE (LDPE). In 1938, the production of commercial polyethylene started. Initially it was intended for the production of telephone cable sheaths and, somewhat later, for the production of packaging.

Technology for the production of high-density polyethylene (HDPE) also began to be developed in the 1920s. A major role in the production of this material was played by Karl Ziegler, a well-known inventor in the plastics industry of ion-coordination polymerization catalysts, the most important of which was later named after Ziegler-Natta. The process of obtaining HDPE was finally fully described only in 1954, and at the same time a patent was issued for it. Industrial production of new polyethylene with properties higher than LDPE started a little later.

Production of polyethylene

Let us briefly describe the production technology of both main types of polyethylenes.

1. LDPE

This polyethylene, as the name implies, is synthesized at elevated pressure. The synthesis is usually carried out in a tubular reactor or autoclave. Synthesis occurs under the influence of oxidizing agents - oxygen, peroxides, or both. Ethylene is mixed with a polymerization initiator, compressed to a pressure of 25 MPa and heated to 70 degrees C. Typically, the reactor consists of two stages: in the first, the mixture is heated even more, and in the second, polymerization is carried out directly under even more stringent conditions - temperatures up to 300 degrees C and pressure up to 250 MPa.

The standard residence time of the ethylene mixture in the reactor is 70-100 seconds. During this period, 18-20 percent of ethylene is converted into polyethylene. The unreacted ethylene is then recycled, and the resulting PE is cooled and granulated. The polyethylene granules are again cooled, dried and sent for packaging. Low-density polyethylene is produced in the form of undyed granules.

1. HDPE

HDPE (high density PE) is produced at low pressure in the reactor. Three main types of polymerization technical processes are used for synthesis: suspension, solution, gas-phase.

For the production of PE, a solution of ethylene in hexane is most often used, which is heated to 160-250 degrees C. The process is carried out at a pressure of 3.4-5.3 MPa during the contact time of the mixture with the catalyst for 10-15 minutes. The finished HDPE is separated by evaporating the solvent. Granules of the resulting polyethylene are steamed at a temperature above the melting temperature of PE. This is necessary to transfer low molecular weight fractions of PE into an aqueous solution and remove traces of catalysts. Like LDPE, finished HDPE is usually colorless and shipped in 25 kg bags, less often in big bags, tanks or other containers.

Types of polyethylene

In addition to the HDPE and LDPE described in detail in this article, the industry produces and uses numerous other types of polyethylenes, the main groups of which are:

LDL, LLDPE - linear low density polyethylene. This type is gaining more and more popularity. The properties of this polyethylene are similar to LDPE, but it surpasses it in many respects, including strength and product resistance to warping.

mLLDPE, MPE - metallocene LLDPE.

MDPE - medium density PE.

HMPE, HMWPE, VHMWPE - high molecular weight.

UHMWPE, UHMWPE - ultra-high molecular weight.

EPE - foaming.

PEC – chlorinated.

There are also a large number of copolymers of ethylene with various other monomers. The best known of these are copolymers with propylene, which are produced under the general names random or static copolymer and block copolymer. In addition to them, copolymers of ethylene with acrylic acid, butyl and ethyl acrylate, methyl acrylate and methyl methyl acrylate, vinyl acetate, etc. are produced. There are also ethylene-based elastomers, they are designated by the abbreviations POP and POE.

Properties of polyethylene

Speaking about the characteristics of PE, you need to understand that the properties of different types of this polymer are very different. Let us consider, as in the case of synthesis, indicators of the two most common types.

1. High pressure PE (LDPE)

The molecular weight of LDPE ranges from 30,000 to 400,000 atomic units.

MFR, depending on the brand, varies from 0.2 to 20 g/10 minutes.

The degree of crystallinity of PVD is approximately 60 percent.

The glass transition temperature is minus 4 degrees C.

The melting point of grades of material is from 105 to 115 degrees C.

Density about 930 kg/cub.m.

Technological shrinkage during processing is from 1.5 to 2 percent.

The main property of the structure of high-density polyethylene is its branched structure. This results in its low density, due to the loose amorphous-crystalline structure of the material at the molecular level.

1. Low pressure PE (HDPE)

The molecular weight of HDPE ranges from 50,000 to 1,000,000 atomic units.

The MFR, depending on the brand, varies from 0.1 to 20 g/10 minutes.

The degree of crystallinity of HDPE ranges from 70 to 90 percent.

The glass transition temperature is 120 degrees C.

The melting point of grades of material is from 130 to 140 degrees C.

Density is about 950 kg/cub.m3.

Technological shrinkage during processing is from 1.5 to 2.0 percent.

1. General properties of polyethylenes

Chemical properties. PE has low gas permeability. Its chemical resistance depends on the molecular weight and density of the polymer. PE is inert to dilute and concentrated bases, solutions of all salts, some strong acids, organic solvents, oils and greases. Polyethylene is not resistant to 50% nitric acid and halogens such as pure chlorine and bromine. Moreover, bromine and iodine have the property of diffusion through polyethylene.

Physical characteristics. Polyethylene is an elastic, fairly rigid material (LDPE is much softer, HDPE is harder). Frost resistance of polyethylene products - up to minus 70 degrees C. High impact strength, strength, good dielectric characteristics. The water and vapor absorption of the polymer is low. From the point of view of physiology and ecology, PE is a neutral, inert substance, odorless and tasteless.

Performance properties of polyethylene. The destruction of PE in the atmosphere begins at a temperature of 80 degrees C. Polyethylene without special additives is not resistant to solar radiation and most of all to ultraviolet radiation, and is easily subject to photodestruction. To reduce this effect, stabilizers are added to PE compositions, for example, carbon black for light stabilization. Polyethylene does not release chemicals harmful to health and nature into the environment, and it decomposes on its own very slowly - the process takes decades. PE is quite flammable and supports combustion; this fact must be taken into account when using it.

Application of polyethylene

Polyethylene is the most popular polymer in the world. It is easy to recycle and is perfectly reusable. It is possible to obtain polyethylene products using almost all plastic processing methods developed today. It is not demanding on the quality and design of equipment and accessories; PE does not require special preparation before processing, such as drying. The industry of concentrates and polymer additives produces a huge number of masterbatch pigments for PE and polyethylene. In many cases, they are applicable for bulk coloring of products not only from other polyolefins, but also from other polymers.

Fig.2. HDPE pipes

In the case of polyethylene processing by extrusion, a film is obtained that is used at every step, both in pure form and in the form of bags in packaging, packaging, and agriculture; PE pipes for water supply and gas; cable sheaths; sheets; foam profiles, etc..

Polyethylene injection molding produces numerous packaging products, such as lids and stoppers, and jars. Casting is also used to produce medical products, household goods, stationery, and toys.

Polyethylene can be processed by extrusion blow molding, injection blow molding, rotomolding, calendering, and pneumatic or vacuum forming from sheets.

Rarer, specialized types of polyethylene, such as cross-linked, chlorosulfonated, ultra-high molecular weight, are used in many industries, but most of all in construction. For example, ultra-high molecular weight PE is included in compositions for the production of fiber optic cable sheaths. Reinforced polyethylene, unlike pure polymer, can be a structural material. Products made of PE lend themselves well to welding by any methods: thermal contact, gas, using a filler rod, friction, etc.

Ecology and recycling of polyethylene

In recent years, polyethylene has come under serious pressure due to its supposed environmental friendliness. In fact, this material is one of the safest. The problem with PE is that it is the main polymer used for the production of films, including thin ones, and bags from them. Without adequate policies for separate waste collection, many underdeveloped countries are dumping huge amounts of PE waste, which leads to polyethylene entering and contaminating the environment and water resources.

Fig.3. Garbage bags – a typical application for recycled PE

Moreover, in the case of proper collection and sorting of waste, polyethylene waste becomes a valuable resource and an excellent secondary raw material. Already, a fairly large number of enterprises in the countries of the former USSR purchase polymer waste for processing into recyclable materials, producing granules and subsequent use in their production or selling recycled PE on the market. Thus, polyethylene pollution of the planet should soon disappear.

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Foamed polyethylene is a modern building material that perfectly retains heat by 70%, insulates from noise and moisture, and tolerates vibrations and mechanical shocks. The minimum service life and wear resistance is from 50 to 80 years, due to its durability and composition it is not subject to rotting. Used in many areas of construction and repair.

This unique material has good value and excellent qualities, and is safe for human health and life. Foamed polyethylene is successfully used in construction, medicine, mechanical engineering, and in the manufacture of leather haberdashery and footwear.

Where to buy foamed polyethylene? Here is one recommendation - we recommend buying polyethylene foam from manufacturers or trusted sellers. For example, at http://www.tecsound.com.ua/products_category/vspenenyi-polietilen-i-lenty/, representative offices of the Spanish company “TEXSA” in Ukraine.

Technical characteristics of foamed PE:

1. Foamed polyethylene has the properties of flammability and melting at temperatures exceeding + 102 0 C.
2. When the air temperature drops to - 60 0 C, foamed PE will retain its strength and elasticity.
3. The material has low thermal conductivity, which enhances thermal insulation many times over.
4. Even when burning, the product is non-toxic.
5. It does not require much effort in transportation and installation, and is light in weight.
6. The material has good load resistance.
7. Minimum amount of waste during production.
8. Environmentally friendly material, without a specific odor.

Polyethylene foam production technologies at different enterprises differ little and have similar operating characteristics.

Methods for producing foamed polyethylene

The production process of foamed polyethylene occurs by injection molding or extrusion. During production, the same qualities of various ethylene polymers remain the same - water resistance, resistance to temperature changes, plasticity, non-toxicity

Production technologies for foamed PE:

1. The production of non-crosslinked polyethylene foam in the process of physical foaming does not require a chemical reaction to preserve the molecular structure of the primary substance - foamed polyethylene. The process is divided into two stages: the first, when the raw material is melted in granules. The second is that isobutane, propane or freon gas is supplied to the chamber where the mass of the polymer substance is mixed.
2. Chemically cross-linked polyethylene foam using chemical reagents. The structure of polyethylene changed at the molecular level becomes a network type. To start the reaction, granules plus mixing and foaming reagents, special additives and pigments are mixed. Afterwards, to give the desired shape, the molten mass passes through an extruder (melting and foaming).
3. Physically cross-linked foamed PE during production requires modification of the structure at the molecular level. The production process technology involves foaming additives and radiation. The heated molten mass of polyethylene moves through an electron accelerator.

A significant drawback is that the material is highly flammable; fire retardants are added during manufacturing.

The production of cross-linked foam polyethylene has a more complex technology, and therefore has advantages over non-cross-linked polyethylene:

1. The microbiological endurance of the material is stronger.
2. Withstands temperature changes and mechanical stress.
3. Resistance to chemical solvents.
4. It withstands vibration well and is resistant to deformation.
5. More than 30% has a dense structure that increases insulation.
6. Heat retention is 20% greater than non-crosslinked PE foam.
7. Due to its high strength it has a long service life.

The advantage of non-crosslinked polyethylene foam is the low price of the material. Unscrupulous sellers often exaggerate the positive qualities of the material when they recommend it as a good soundproofing material used in construction. They are also used as various non-toxic packaging for products and goods.

Application of non-crosslinked polyethylene foam

1. Non-crosslinked polyethylene foam is convenient for packaging and, if necessary, softens pressure during loading. Has an unlimited shelf life. It cannot spoil and is beneficial when packing valuable goods and cargo. In the packaging market, NPE has no equal competitors, and occupies 90% of applications.
2. Used as packaging material for electrical equipment, dishes, furniture, glass products. Perfectly protects the surface from dust and technical debris.
3. NPE is excellent for insulating from moisture, water, steam, condensation and mechanical noise.
4. Subject to minimum quality requirements, it is used in construction as thermal insulation and mechanical engineering. Not suitable for use when there are powerful loads or the air temperature is too hot.
5. It is well used to reduce energy costs while saving heat in the house - thanks to reflective insulation. Widely used to level the surface, put under parquet, laminate, linoleum.
6. It has a variety of release forms - in polyethylene mesh, in rolls, in sheets of different thicknesses. With a laminated or foil base, non-crosslinked polyethylene foam will perform protective functions depending on the requirements and the task at hand.
7. In the European Union, there is a restriction on the use of NPE; it is used only for packaging.

Areas of application of cross-linked polyethylene foam

The material is widely used in various spheres of life: construction, sports, tourism, medicine, mechanical engineering, in the production of children's toys, automobile factories, household items, sanitary products. It has enhanced strength, heat resistance, and a high level of rigidity.

Construction

1. It is used for heating and water supply systems, has good insulation of heat, water, steam and noise. Resistant to high temperatures from -60 0 to +110 0 C, begins to melt at t 0 115-130 degrees C.
2. Used to insulate ceilings, floors, and ceilings between floors. As a reflective heat insulation for heating systems.
3. The material is easy to install and acts as a high-quality, modern sound insulating layer in a “floating floor” device.
4. Insulation from moisture, water for foundations, partitions. Arrangement of basements, warehouses, garages, balconies and loggias. Insulation of electrical cables.
5. Protection for public utility systems and engineering structures.

Medicine

1. Widely used in the manufacture of orthopedic medical products. Insoles for specialized shoes made of cross-linked polyethylene foam.
2. In the production of internal organ prostheses.
3. Elastic elements that are used in medicine and medical equipment.

Package

1. Various types of containers - containers, vessels, canisters, bottles, tanks.
2. Various inserts to prevent product deformation. Padded material for the preservation and transportation of food and industrial goods.

Sports and tourism

1. Punching bags, gloves, pads, helmets.
2. Devices that do not sink in water and act as a fence or marking. Swimming boards, life jackets and safety vests.
3. Rugs and mats for tourism, yoga, fitness and other sports areas.

Mechanical engineering

1. The material is used to install a heat reflector. As a mounting tape, sealant, protection of elements during vibration isolation.
2. Noise and heat insulation of isothermal cabinets of air conditioners and refrigerators.

Automotive industry

1. Covering most parts of a car dealership to insulate heat and noise. Various seals, gaskets.
2. Buffer gaskets for car components and parts.

As a result, all types of polyethylene foam are resilient and elastic materials that have a closed-cell structure and are available in rolls, sheets or as a finished product. They have high properties of resistance to moisture and aggressive environments - alkalis, acids, oils, petroleum products. Easy installation in any structure, environmentally safe when used. The main disadvantages include poor resistance to direct sunlight and ease of ignition. When choosing, you need to know whether the material has the technical characteristics necessary for your case. What are your goals? To avoid unnecessary costs and the purchase of cheap unnecessary material.