Polyethylen (PE)

Polypropylen (PP)

Polyethylene (PE)

Polyethylene is the world's most widely used plastic.

Polyethylene is primarily used for films and packaging. It is also known by the abbreviation PE. It is the most widely used plastic in terms of volume.

With a share of around 30% of all plastics, polyethylene is the world's most widely used plastic. Polyethylene is a semi-crystalline and non-polar thermoplastic. PE, or polyethylene for short, is primarily used for packaging and films. By selecting the conditions during production, known as polymerization, very different variants can be produced. The various PEs and their properties sometimes differ significantly. Typically, four main types are produced, the predominant distinguishing feature of which is density.

The types are:

  • High-density polyethylene, abbreviated as PE-HD or HDPE. HD stands for high density.
  • Medium-density polyethylene, abbreviated as PE-MD or MDPE
  • Low-density polyethylene, abbreviated as PE-LD or LDPE
  • Linear low-density polyethylene, abbreviated as PE-LLD or LLDPE

The main reason for the difference in density is the degree of branching of the molecular chains. The higher the degree of branching, the lower the density of the polyethylene. HDPE has only weakly branched polymer chains and thus a high density. LDPE, on the other hand, has a high degree of branching, which is why its density is low. LLDPE also has many branches, but they are very short.

Other important types are:

  • High molecular weight polyethylene (HMWPE): The polymer chains are longer than in other types. The molecular weight ranges between 500 and 1000 kg/mol.
  • Ultra-high molecular weight polyethylene (UHMWPE): This is a PE-HD with an even higher average molecular weight of up to 6000 kg/mol.
  • Cross-linked polyethylene (XPE): In this case, the polymer chains are additionally cross-linked, which changes the properties of the polyethylene.

Production

Various processes are used in the production of polyethylene. The choice of process allows the degree of branching of the molecular chains and thus the density to be determined to a certain extent. Special catalysts allow the production of polyethylene with a narrow molecular weight distribution or very low density.

Polyethylenes are generally produced in the presence of radicals using high-pressure processes (radical polymerization) or with the aid of catalyst systems using medium- and low-pressure processes (anionic polymerization). The processes can also be divided into suspension, solution, gas-phase, and bulk polymerization. Highly branched polymers (LDPE) are synthesized using the high-pressure process, while linear polymers (HDPE, MDPE, LLDPE) are synthesized using the medium- and low-pressure processes.

High-pressure process

LDPE is produced using the high-pressure process, either batchwise in stirred tanks or continuously in tubular reactors from ethylene (CH2 = CH2) at a pressure of 1000 to 3000 bar and temperatures of 150 to 275 °C. 0.05 to 0.1% oxygen or peroxide is used as a catalyst. The result is a highly branched polyethylene with branches of varying lengths. The density of the polymers produced is 0.915 to 0.935 g/cm3, and the average molecular weight is up to 600 kg/mol. With high-performance catalysts, LDPE plants can be converted to produce LLDPE.

Medium- and Low-Pressure Processes

HDPE is produced using the medium-pressure (Phillips method) or low-pressure (Ziegler method) processes. Both are suspension processes. The Phillips method uses a pressure of 30 to 40 bar and a temperature of 85 to 180 °C. Chromium oxide is used as the catalyst. The average molecular weight is approximately 50 kg/mol. The Ziegler process uses a pressure of 1 to 50 bar and a temperature of 20 to 150 °C. Titanium halides, titanium esters, and aluminum alkyls as catalysts, achieving average molecular weights of 200 to 400 kg/mol. HDPE is only very weakly branched and therefore has a higher density than LDPE. It is approximately 0.94 to 0.97 g/cm3.

UHMW-PE, with a density of 0.93 to 0.94 g/cm3, is produced using special catalysts in a low-pressure process. The average molecular weight is approximately 3000 to 6000 kg/mol.

LLDPE is produced using high-performance metal complex catalysts using four different processes: a low-pressure process from the gas phase, from solution or suspension, and a modified high-pressure process. Copolymerization of ethylene with 1-olefins such as butene-1 or hexene-1 produces short-chain branches.

PE Properties

The low density of LDPE grades results, as mentioned, from the high proportion of short- and long-chain branches, which prevents the polymer chains from clustering together tightly. Therefore, LDPE grades have a lower degree of crystallization, at approximately 40 to 50%, than HDPE grades, which have 60 to 80%. They are also more translucent, which can even reach transparency in thin films. The higher degree of crystallization is also the reason for HDPE's higher melting temperature.

The strength, hardness, and stiffness of polyethylene are lower than those of most other thermoplastics. However, high elongation and low-temperature impact resistance, as well as good sliding friction behavior, are among the properties of polyethylene. Grades with high molecular weights are used for injection-molded parts, at which point UHMW-PE is no longer thermoplastically processable. Polyethylene can also be stretched into very strong reinforcing fibers. This strength is based on the extremely high crystallinity achieved through the processing method. The maximum continuous service temperature, depending on the grade, is approximately 60 to 85 °C; for short periods, temperatures of 80 to 120 °C are possible, and UHMW-PE even 150 °C.

Polyethylene has good electrical insulating properties and exhibits good chemical resistance to a wide range of acids, bases, oils, and greases. While LDPE has only very limited resistance to hydrocarbons, HDPE can also be used for fuel tanks. Such containers are often additionally equipped with barrier films or a plasma coating because polyethylene has high gas permeability (permeation). Strong oxidizing agents such as highly concentrated inorganic acids and halogens attack polyethylene. PE is flammable and not weather- and UV-resistant. Therefore, additives such as flame retardants and UV absorbers are required.

Chemical Resistance

The resistance of polyethylene increases with increasing density. Therefore, PE-HD is generally more resistant than PE-LD or PE-LLD. Creep tests on PE-HD show a significant difference in durability in water and air. However, the resistance to stress cracking is somewhat lower for PE-HD than for PE-LD and PE-LLD.

PE-LD and PE-LLD are resistant to water, dilute acids, alkalis, solutions of inorganic substances, solvents, alcohols, aliphatic hydrocarbons, and gasoline. Both exhibit minimal swelling in fats and oils. PE-LD and PE-LLD are not resistant to strong acids and oxidizing agents, oxidizing acids, esters, ketones, and aromatic and chlorinated hydrocarbons [1].

HDPE is resistant to hot water, air, mineral acids, alkalis, solutions of inorganic salts, aliphatic hydrocarbons, alcohols, esters, ketones, mineral oils, amines, organic acids, fats, and oils. HDPE is not resistant to oxidizing acids, aromatic hydrocarbons, fuels, and, depending on the type, detergents.

Oxidizing media, such as sodium hypochlorite, nitric acid, and chromic acid, trigger degradation reactions. Once a crack has formed in the degraded layer, it often migrates into undamaged material due to the notch effect [2]. Organic substances have a more or less pronounced swelling effect on polyethylene. This manifests itself in an increase in weight and a reduction in the yield strength or modulus of elasticity. The swelling depends on the chemical nature of the organic substances. Aliphatic and aromatic hydrocarbons have a strong swelling effect. However, if they contain functional groups such as alcohol, acid, aldehyde, or amino groups, the swelling effect is significantly lower, depending on the structure of the compound. Chlorinated hydrocarbons, on the other hand, have a strong swelling effect.

Toxicity and Sustainability

Polyethylene is considered harmless to human health. For example, the North Rhine-Westphalia Consumer Protection Agency classifies polyethylene as non-toxic [3]. Other plastics, such as PVC, are often criticized for the release of plasticizers or other substances. This is not the case with polyethylene. Since polyethylene is not considered toxic, it is often used for food packaging. Polyethylene generally burns without releasing toxic gases. This produces CO2 and water.

However, polyethylene has been criticized with regard to environmental protection. Since polyethylene is used extensively for packaging, a large proportion of the plastic waste found in the ocean and the rest of nature also consists of this material. Polyethylene, like other plastics, has also been criticized in connection with the microplastics problem.

Polyethylene is a plastic that is relatively easy to recycle. It can be mechanically processed in appropriate facilities and then reprocessed into granules. However, this requires separate separation of the materials. This is often hampered by the multilayer films used for packaging. They often consist of several different plastics and are usually not easy to separate in the recycling process. However, with appropriate collection systems and appropriate product design, polyethylene could be recycled on a large scale.

Processing

Polyethylene can be processed by injection molding, extrusion, and blow molding, among other methods. Compression molding, fiber spinning, and foaming of polyethylene are also used industrially. Polyethylene is not critical to processing. Due to the great variability of the different types, they cover a wide range of processing conditions. Special types with specific processing characteristics are used for certain applications and processes.

In injection molding, the melt temperatures for LDPE are 160 to 260 °C, and for HDPE 260 to 300 °C. The mold temperatures are 50 to 70 °C and 30 to 70 °C, respectively. Free-flowing types are used for mass production. The density and thus the shrinkage of the molded parts of these semi-crystalline plastics are highly dependent on the temperature control.

Rapidly cooled molded parts exhibit low crystallinity, minimal molding shrinkage, but also high post-shrinkage due to post-crystallization at elevated temperatures. The consequences can be warpage and stress cracking due to frozen-in stresses. Polyethylene grades with a low melt flow rate perform better in this regard. Increasing the melt temperature and using molding compounds with very high flow rates help combat so-called gate brittleness caused by high molecular orientation.

UHMW-PE can only be processed using special grades. Due to their poor flowability, they require machines with high injection pressures of approximately 1100 bar, the absence of non-return valves, grooved feed zones where possible, and short flow paths. Processing temperatures range from 240 to 300 °C, and the mold temperature is between 70 and 80 °C.

Blow molding requires polyethylene grades with higher melt viscosity and sufficient melt strength to prevent preforms from tearing under their own weight. Melt and mold temperatures are 140 °C for LDPE and 160 to 190 °C for HDPE. LLDPE is less suitable for blow molding due to its narrow molecular weight distribution and the associated higher melt viscosity, but is well-suited for rotational molding.

In the extrusion process, LDPE is processed at melt temperatures of 140 to 210 °C (films, pipes), 230 °C (cable sheathing), and 350 °C (coatings). For HDPE, the temperatures are 20 to 40 °C higher. These molding compounds are used, among other things, to produce sheets and monofilaments. Profiles are manufactured from PE-UHMW using high-pressure plasticization (at 2000 to 3000 bar) with cyclically operating twin extruders. Co-rotating twin-screw extruders operating at approximately 10 rpm are also suitable for this purpose. The melt temperature in these extruders is 180 to 200 °C.

Extrusion of LLDPE on extruders designed for LDPE results in a 20 to 30% lower output. This disadvantage results from a required reduction in screw length from 30 D to 25 to 20 D and a 50% reduction in speed. Measures to compensate for this throughput loss include the use of larger-diameter screws, increasing the screw flight depth, and enlarging the die gap. The optimal melt temperature is 210 to 235 °C, and for film extrusion, 250 to 280 °C.

By batch-controlled, 30-fold stretching of staple fibers under conditions that lead to an almost single-crystal-like alignment of the crystallites, extremely strong reinforcing fibers are obtained. They exhibit strengths of 1 to 5 GPa, a Young's modulus of 50 to 150 GPa, and elongations at break of approximately 5%. By separating polyethylene from solutions under shear, cellulose-like, fibrillated fibers, so-called fibrids, can also be produced.

In the Neopolen process, pre-foamed particles obtained from a melt containing a blowing agent by hot die-cutting are steam-sintered into molded blocks or molded parts using a particle foaming process. The result is expanded polyethylene (EPE).

Simple molded parts are produced by compression molding at a pressure of 2 to 5 bar. Electrically conductive compounds can be quickly heated by passing an electric current through them. PE-HD and PE-LD can be pressed at temperatures of 105 to 140 °C.

In powder technology (rotational molding, fluidized bed sintering), powders with a grain diameter of 30 to 800 µm are used, made from polyethylene grades with bulk densities of 0.92 to 0.95 g/cm3 and low volume flow rates. Variants with better flow properties are used for carpet backings and for iron-on fabrics. Unprecipitated polyethylene powders with uniform grain sizes of approximately 50 µm are suitable for electrostatic coating of metals or fabrics. Even finer particles of 8 to 30 µm can be dispersed, for example, in paper processing or in printing inks.

Areas of Application

Polyethylene is primarily used for films and packaging. It is used to produce canisters produced by blow molding, simple injection-molded parts, and thermoformed containers. The plastic is also used for pipes and cable insulation. Polyethylene powders are used to coat textiles or paper. Polyethylene is used as foam for packaging linings and transport protection, for example.

The main applications of LDPE are packaging films: heavy-duty bag films, shrink films, carrier bags, agricultural films, and water vapor barriers in composite films. Blends with LLDPE produce stretch films with higher stretchability. The material is also used for pipes, thermoformed sheets, telecommunications cable sheathing, steel pipe coatings, flexible containers, bottles, and canisters.

LLDPE is used to produce films that can be stretched to thicknesses of up to 5 µm, offering good optical properties, better low-temperature toughness, tear and puncture resistance, and lower susceptibility to stress cracking than versions made from LDPE. Blended with LDPE, the material is used for blown film.

HDPE is used for household goods, storage and transport containers, garbage bins and containers, bottle crates, gasoline cans, and vehicle tanks, among other applications. Special types are used for pressure pipes and fittings for drinking water supplies and wastewater disposal, and for PE sheets for equipment in the chemical and automotive industries. Fibrids are used as water-repellent but hydrocarbon-binding additives to fillers, for absorbing oils, or as reinforcement for paper, fibers, and highly consolidated reinforcing fibers.

Surfboards up to 5 m long, monofilaments for nets, ropes, and fabrics are often made from PE-HMW. It can also be used to produce packaging films with reduced thickness.

Press-produced blocks made of PE-UHMW exhibit excellent abrasion resistance and are therefore used for linings of hoppers and chutes and for mechanical components such as screw conveyors, pump parts, sliding elements, rollers, gears, sliding bushings, and rolls. Polyethylene sheets are also manufactured from this material. Core-porous sintered spacers made from this material are used in lead-acid batteries, surgical implants, prostheses, and ski sliding coatings.

Polypropylene (PP):

Polypropylene (PP) is a semi-crystalline, non-polar, thermoplastic polymer produced by the polymerization of propylene. Another name for polypropylene is polypropene (PP). As a material, polypropylene is highly adaptable to the desired application, e.g., through the addition of reinforcing materials, fillers, and additives, as well as through the selection and design of processing methods.

Although polypropylene is chemically relatively similar to polyethylene (PE), it is significantly harder, stronger, and has a higher thermal resistance [1]. PE, PP, and polymethylenepentene (PMP) are among the most important representatives of polyolefins, which are produced from alkenes such as ethylene, propylene, 1-butene, or isobutane by chain polymerization.

Production

Polypropylene is produced by chain polymerization of the gas and monomer propene (C3H6), also known as propylene. In this process, the carbon double bonds of propene are broken with the help of catalysts (metallocene or Ziegler-Natta catalysts) and converted into long-chain hydrocarbon chains, in which a methyl side group is formed. The chemical formula for polypropylene is (C3H6)n.

Depending on the orientation of the methyl groups, the following are formed:

  • atactic polypropylene (random orientation of the methyl group, PP-at),
  • syndiotactic polypropylene (alternating orientation of the methyl group, PP-st), or
  • isotactic polypropylene (uniform orientation of the methyl group, PP-it).

Tacticity affects the physical properties of PP. The more regular the distribution of the methyl group, the better the crystallinity (syndiotactic and isotactic: semi-crystalline; atactic: amorphous). The majority of commercially available polypropylene has an isotactic orientation.

Polypropylene is the second most important plastic, accounting for approximately 24% of total production. Global demand for PP was 74 million tons in 2018.

Areas of Application

The special properties of polypropylene lead to an exceptionally wide range of applications. These range from medical technology and vehicle interiors to food and cosmetic packaging and pressure and sewage pipes. Polypropylene is also used as a housing material for small electrical appliances, molded parts for household appliances, and for body parts (bumpers) and molded parts for vehicles. In the construction sector, PP is used not only for pipes but also for hot water tanks and garden furniture.

Packaging

Polypropylene is widely used in packaging, often in the form of films but also as rigid packaging. In the film sector, in addition to packaging, polypropylene can also be used for laminating or lamination films. Isotactic or syndiotactic PP is used almost exclusively. PE and PP, along with polyethylene terephthalate (PET) and polystyrene (PS), are the most commonly used plastic materials in packaging.

In rigid PP packaging, additives, auxiliaries, and fillers are often incorporated in addition to colors to adapt the properties of the polypropylene. Due to its high extensibility and relatively high dynamic load capacity, PP also allows the production of film hinges.[5] Another important packaging application for polypropylene is closures for containers or bottles.

PP has further applications in packaging tapes, bags, splicing fibers, and woven tapes. As a foamed variant of polypropylene, expanded polypropylene (EPP) can also be used in the packaging sector, replacing expanded polystyrene (EPS), for example.

Medical Technology

Many medical products are made of polypropylene or contain PP components. Examples include various disposable items such as syringes, masks, and spatulas, medical sutures, membranes, and medical packaging.

Particularly due to the coronavirus pandemic, particle-filtering respirator masks (FFP masks) and medical surgical masks have become very popular. The fabric material used for these, so-called nonwovens, consists primarily of very thin polypropylene fibers. Medical mask material is sometimes also made from a blend of polypropylene and polyethylene.

Automotive

In the automotive sector, polypropylene is used in both interior and exterior components. Due to PP's attractive price, this standard plastic is increasingly being used as a replacement for engineering polymer materials, such as ABS or polyamide. For this purpose, polypropylene is usually filled with reinforcing or filler materials, such as glass fibers, carbon fibers, or calcium carbonate, to achieve similar properties to the plastics it is replacing.

A typical use of PP in the automotive sector is the addition of glass fibers for reinforcement. A typical material is, for example, polypropylene with a 30% glass fiber content (PP-GF-30).

Construction Industry

In the construction industry, polypropylene is primarily used for pipes, containers, and piping. Reinforcement materials, fillers, and additives are sometimes also used. The latter are particularly important for long-term outdoor use to increase the weather and UV resistance of the applications. However, the weather resistance of PP is lower than that of PE, for example.

Other Applications

Amorphous, atactic polypropylene is a plastic that is flexible to rigid down to -30°C and is used as a coating for paper packaging and carpet tile backing. This material is also used as an automotive insulation material, anti-corrosive bandages, road marking compound, hot-melt adhesive, sealant, bitumen blend, and in age-resistant waterproofing and roofing membranes.

Polypropylene is also used for fiber-reinforced panels, which are hot-pressed into components or building components.

Polypropylene is also used in functional clothing. Polypropylene fibers are lightweight, abrasion-resistant, elastic, and have an insulating effect. Polypropylene fabrics are breathable and absorb little sweat. PP fabrics dry very quickly.

Properties

The stiffness and strength of PP lie between those of PE and "engineering" plastics such as ABS, PA, and others. The dynamic load-bearing capacity of polypropylene is relatively high, making it suitable for a wide range of applications.

With a glass transition temperature of around 0 °C, PP plastics become brittle in cold temperatures. The crystallite melting range is 160 to 165 °C, which is higher than that of PE. Consequently, the maximum service temperatures are also higher, and the temperature resistance of PP is greater than that of PE: PP's short-term temperature resistance is 140 °C, and its long-term temperature resistance is around 100 °C.

Mechanical Properties of Polypropylene

  • Density: 0.895 and 0.92 g/cm³
  • Tensile Strength: 33 N/mm²
  • Elastic Modulus: 1300 and 1800 N/mm²
  • Elongation at Break: 800%

Electrical Properties of Polypropylene

  • Volume Resistivity: >1016 Ω
  • Dielectric Strength: 55–90 kV/mm

Thermal Properties of Polypropylene

  • Thermal Expansion α: 100–200 10−6/K
  • Moisture Absorption: 0.1%
  • Temperature Range: +10°C to +100°C
  • Heat Resistance: 65°C

Chemical Resistance of Polypropylene

Due to its non-polar nature, PP is highly chemically resistant. It is resistant to aqueous solutions of salts, strong acids, and alkalis up to 120°C, and possibly also to washing solutions.

Highly crystalline grades exhibit the best resistance to polar organic solvents, alcohols, esters, ketones, fats, and oils.

Only special grades are resistant to fuels at higher temperatures.

Strong oxidizing agents such as chlorosulfonic acid, oleum, concentrated nitric acid, or halogens attack PP even at room temperature.

Gases, especially CO2, as well as low-boiling hydrocarbons and chlorinated hydrocarbons, diffuse through PP.

PP exhibits minimal water absorption and permeability.

Products approved under food contact law are suitable for hot filling of beverages and other foodstuffs and can be sterilized under heat.

Source: https://www.kunststoffe.de/