Polyethylene vs. Aramid pair - comparison of technologies and properties

Manufacturers of high-strength polyethylene (PE) and aramid fibers declare the fantastic properties of their products, focusing on their strength, many times superior to the strength of steel. These fibers compete in the market in the field of cut-resistant products, ropes and ballistic materials. This article evaluates the strengths and weaknesses of aramid and polyethylene fibers, as well as discusses their production technologies.

Aramid fibers

Карозерс , Wallace, inventor of polyamides (nylons) in 1930-In the 1990s, he has already postulated some prerequisites for the production of high-strength and high-modulus polymer fibers, namely: the presence of long chains of polymer molecules located parallel to the fiber axis. However, all synthetic polymers, such as polyethylene (PE), polyamides (PA), polyesters (PET), etc. They are flexible polymer molecules that can take any shape and, in the state of melting, are strongly entangled. How can we put these chains together by building them in parallel in the fiber? DuPont in the 1960s began to investigate the molecular chains of a solid rigid polymer, which are much easier to align and orient in the fiber. The polymer PPTA (poly-para-phenyleneterephthalamide) has become the base material for Kevlar fiber (Fig. 1).

Fig. 1. Aramid fiber spinning, PPTA molecule structure

The synthesis of PPTA (PPTA), especially in an industrial plant, is complex, the reaction between monomers proceeds very quickly, and the polymer solution obtained in the reactor cannot be used as a spinning solution. PPTA must be reconstituted and re-dissolved in a suitable solvent. Usually concentrated sulfuric acid is used as a solvent for PPTA, but the result is a solution of very high viscosity.

Kvolek, Stephanie made a big breakthrough when she discovered that the viscosity of the solution sharply decreases in the concentration range of 15-20%, at which PPTA chains are arranged in domains resembling logs floating in a river stream. Spinning from these liquid crystal (nematic) PPTA solutions in sulfuric acid occurs on the principle of dry-wet molding (through an air gap into water). Upon entering the aqueous phase, sulfuric acid dissolves in water, and the PPTA molecules line up parallel to the fiber axis. Next, the finished fiber is heated and wound.

In 1970, AkzoNobel in the Netherlands engaged in the production of aramid fibers. A major patent problem began, which was partially solved based on the fact that the HMTP solvent used by DuPont turned out to be carcinogenic, while Akzo Nobel found a unique combination of solvents, NMP + calcium chloride. Thus, the Twaron paraaramide fiber was born. Teijin took over the management of this business in 2001. Currently, the global market share of DuPont and Teijin for paraaramides is more than 70%, while the rest of the market belongs mainly to South Korean (Kolon, Hyosung) and Chinese companies.

Polyethylene fibers

Spinning from melt

Linear PE (HDPE) melt spinning in combination with hot drawing of spun fibers was first proposed by Professor Ian Ward from the University of Leeds/UK in the 1970s and has been commercialized for some time by the Italian company SNIA under the brand Tenfor. Currently, melt-molded fibers are manufactured by Toyobo (Japan) and Huvis (South Korea) under the names, respectively, Tsunooga and Duraron. These fibers are characterized by a tensile strength of up to 1.5 GPa and elastic modulus of approx. 70 GPa. These figures are impressive, but they are much lower than the theoretical predictions discussed below.

Forecasts regarding the ultimate strength of polyethylene fibers

Calculations show that a single fully stretched PE molecule breaks at 20 GPa with a corresponding modulus of elasticity of 180 GPa, which is close to the modulus of elasticity of steel (200 GPa). However, these calculations are far from reality, since in practice we are dealing with polymer chains with a length of no more than 10 microns in a fully elongated state. Therefore, when testing a PE fiber, the response will depend on the voltage transfer between the chains only at weak Van der Waals forces ( Van der Waals forces). Therefore, very long chains are needed to create a sufficient amount of lateral interaction (friction) between individual molecules. Polyethylene fibers obtained by molding from a melt are based on low molecular weight polyethylene (about 100,000 g/mol), and a much higher molecular weight is required for high strength. On the other hand, aramid fibers have a stronger inter-chain hydrogen bond, and a molar mass of about 50,000 g/mol is sufficient to create the necessary properties of the fiber.

Solution molding (gel spinning) of UHMWPE

Currently, grades of polyethylene with a molecular weight of > 3 million g/mol are available - the so-called ultrahigh molecular weight polyethylene (UHMWPE). The main disadvantage of UHMWPE is that it is not processed by standard methods, such as screw extrusion, due to the excessively high viscosity of the melt. An easy way to facilitate recycling is to use a solvent, and in the fiber world, solvents are used in well-known dry and wet spinning technologies.

In the 1980s, the company DSM in the Netherlands studied the molding of UHMWPE solution. UHMWPE was dissolved in a decalin solvent in a very low concentration, approx. 1%, and fibers were formed. After that, the formed fibers were pulled at elevated temperatures, while the solvent evaporated. This technology made it possible to obtain fibers with a tensile strength > 3 GPa (30 sN/dtex) and the corresponding modulus of elasticity (stiffness) >100 GPa (1000 sN/dtex). Initially, this observation was not unexpected, since it is well known in this field of technology that polymer chains separate and unravel in solution, which leads to a lower viscosity, which makes the difficult-to-process UHMWPE suitable for processing. During the subsequent stretching of the spun fibers, the solvent acts as a plasticizer, facilitating the alignment of long polymer chains in the fiber. What came as a surprise was that after removing the solvent before pulling, the fibers, after molding, still retained the ability to pull. Apparently, when the freshly formed fiber is cured in a water quenching bath, a favorable topology of polymer chains is achieved, making it possible to super-stretch and align the chains.

At that time, the DSM company did not produce fibers, and the Akzo Nobel company, whose headquarters was also located in the Netherlands, was not interested in the resulting product, which they nicknamed as candle wax fiber. Thanks to the interest from Toyobo (Japan) and Allied Spectra (now Honeywell) (USA), the project survived, and Dyneema fiber was born. Currently, DSM is the leading manufacturer of Dyneema (Fig.2), followed by Honeywell (Spectra) and Toyobo (Izanas), as well as many Chinese companies.

Fig. 2. Dyneema process diagram. The suspension of UHMWPE in decalin, approx. 10%, is fed into a twin-screw extruder with parallel rotation, in which the solution is produced. The filaments are cooled in a water bath, and then drawn out in an oven with simultaneous removal of the solvent by evaporation

Chineema

Spinning UHMWPE in solution is a simple process that is therefore easy to copy, and there are currently many Chinese manufacturers of polyethylene fiber - about 50 at the time of writing, who call their product Chineema. Instead of decalin, Chinese manufacturers almost exclusively use paraffin oil, which is a less volatile and less flammable solvent, but cannot be removed by evaporation during drawing, so it must be extracted and restored, as shown in Figure 3.

Fig.3. The Chineema Production plant – extruded polyethylene filaments containing ~10% UHMWPE are assembled in boxes with a perforated bottom (left). During storage (1 or 2 days), part of the paraffin oil "erodes" (syneresis) and can be reused. Then the semi-dry filaments are extracted (by heptane) and stretched in a number of heated furnaces

Processing of UHMWPE into high-performance tapes without the use of solvent

The so-called untangled state, which can be obtained by dissolving UHMWPE followed by gelation (crystallization), can also be achieved at present during polymerization in the reactor. During polymerization of UHMWPE, polymer chains grow from catalytic centers, and if these centers are sufficiently separated, the growing chains practically do not intertwine, and, consequently, the result is an untangled UHMWPE reactor powder. These powders can be processed exclusively in the solid state by compacting the UHMWPE powder into a sheet using a belt press, followed by calendering to make the sheets thinner, followed by slow super-stretching, close to the melting temperature, but below it. Ultra-thin sheets are cut into small ribbons. Currently; These high-performance tapes are produced by Teijin Aramid (Endumax)* and DuPont (Tensylon). DSM produced the BT10 tape, but it seems to have stopped and is now pre-marketing an improved version with greater durability.

*- by the time the article was published on the website of Teijin Aramid (the Netherlands), the following information appeared: "We hereby inform you that Teijin Aramid has decided to cease production and sale of Endumax.After a thorough evaluation of this product line, both from a market point of view and from a financial point of view, we have come to the conclusion that the release of Endumax cannot be continued in Teijin Aramid.

Are aramid and polyethylene fibers really stronger than steel?

Manufacturers of aramid and polyethylene fibers claim that their fibers are stronger than steel. Currently, the strength of aramid fibers reaches 3.5 GPa, and the modulus of elasticity is approaching 150 GPa. UHMWPE (UHMW-PE) fibers currently have a tensile strength in the range of 3->4 GPa and corresponding elastic modules in the range of 130-160 GPa. Tapes made of ultra-high molecular weight polyethylene have a very high modulus of elasticity, approaching 200 GPa, and the strength obtained in laboratory conditions is >4 GPa, but commercial grades such as Endumax have much lower strength, <2.5 GPa.

The steel has a rigidity of approx. 200 GPa, and the yield strength is in the range of 0.25-0.7 GPa. It is even more impressive when specific values of strength and modulus of elasticity are considered (Fig. 4).

The density of polyethylene fiber is 950 kg/m³ (lower than that of water), and aramid fiber is about 1400 kg/m³ compared to steel 7800 kg/m³ and glass 2400 kg/m³.Before you get excited about this data, please read the additional information that follows below.

Fig.4. Specific strength σspec (N/tex) depending on the specific modulus of elasticity (N/tex) at ambient temperature Ar = aramid fibers (N/tex = HPa/ρ ; ρ in g/cm³)

Influence of temperature and time

In the world of plastics, the question is often not what "if" the material or product will fail, but what "when?». In this respect, aramid fibers are an exception. Aramid fibers have a very high melting point, estimated at more than 500 °C, but begin to decompose at a temperature of 400 °C. An increase in temperature reduces modulus, tensile strength and elongation at break, but this becomes a serious problem only at temperatures exceeding 150 °C for extended periods of time. In addition, aramid fibers are not afraid of the so-called creep, irreversible deformation when a constant load is applied. Aramid fibers subjected to loads in the range of 1-1.5 GPa are preserved for many decades.

On the contrary, the properties of PE fibers strongly depend on temperature and time. The melting point of the polyethylene fibers is about 150°C (which is actually not the real melting point). However, at a temperature that is significantly lower than the melting point and slightly above 70 °C, PE fibers begin to lose their properties.

Figure 5 shows typical stress-strain curves for aramid and polyethylene fibers, respectively.

Fig. 5. Load vs. voltage. Comparison of aramid and polyethylene fibers at room temperature

The curve of aramid fibers shows a slight bulge, while the curve of PE fibers has a concave characteristic, and, moreover, at a lower test speed, the curve decreases. At very low test speeds, the PE fiber begins to withstand already <0.5 GPa. In an experiment with creep under static load, a polyethylene fiber cannot withstand any load and will "crawl" to destruction (plastic flow). From a scientific point of view, polyethylene fiber can be considered as a one-dimensional (1D) liquid, albeit with a very high viscosity. At high testing speeds, the polyethylene fiber behaves like an elastic solid, and at very low testing speeds it behaves like a liquid. This concept is well known in polymer rheology (Deborah number).

Applications of aramid and polyethylene fibers

Due to the limited heat resistance of polyethylene fibers, they cannot be used in applications such as rubber reinforcement: for example, in tires and conveyor belts, but they compete with aramids in other areas.

Cut-resistant gloves

Both aramid and polyethylene fibers are used in cut-resistant gloves and provide very good cut resistance, which, however, is not associated with the high strength and high rigidity of the fibers. The resistance to cuts of PE fibers molded from a melt having 50% less strength and rigidity, as in Tsunooga, is similar to PE fibers based on ultra-high molecular weight polyethylene, such as Dyneema and Chineema, shows that the deformation mode during cutting is completely different from the deformation under tensile load. The market for the production of cut-resistant gloves is easy enough to enter, it is non-technological, and currently it is dominated by Chinese manufacturers. DSM has improved the cut resistance in its Dyneema-Diamond technology by incorporating mineral wool fibers into polyethylene threads (mineral wool fibers blunt the knife).

The choice between cut-resistant gloves based on polyethylene and aramid depends on the application. For thermal protection and fire resistance, aramid fiber is the winner, it does not drip and does not melt when exposed to open fire. PE fibers have a remarkable property, namely, high thermal conductivity (40 W/ mK) and, consequently, fabrics based on polyethylene fibers are pleasant to the touch. Cut-resistant gloves based on UHMWPE have softness and smoothness to the touch, and due to thermal conductivity are very comfortable in hot climates.

Ropes and cables for mooring and anchoring

The above statement that polyethylene fibers, such as Dyneema, can be considered as one-dimensional liquids sounds quite frightening if we consider polyethylene fibers in ropes for mooring ships or attaching floating wind turbines to the seabed. However, the load on the ropes is intermittent for mooring cables, and the polyethylene cables used for anchoring experience a cyclic load (wave motion). Moreover, some creep does not affect the properties of the fiber and is currently considered an advantage. The load on various elements that are under tension during anchoring, for example, when attaching wind turbines to the bottom of the sea, after some time is leveled and compensated by the creep property of PE fibers, but this cannot be achieved using steel or aramid cables.

Another important advantage is that the density of polyethylene fibers is lower than that of water, they float in water and have no weight when fixed, and, as a bonus, water does not affect the properties of PE fiber, unlike aramid fibers. In addition, temperatures are relatively low in deep water areas, and the creep problem is negligible. The problem of creep is more important when using products at elevated temperatures in hot climates, where the temperature can rise to 50 °C, and in ropes due to friction even up to 70 °C and above.

On its website and in several documents, Teijin Aramid claims that aramid ropes are much safer than HMPE ropes at elevated temperatures. These statements may be quite true, but what kind of material does Teijin Aramid mean by HMPE? Chineema manufacturers are forced to buy UHMWPE on the open market from a limited number of suppliers. They use paraffin oil (white oil) and to obtain a homogeneous polymer solution, extruders operate at high temperatures - up to 300 °C. At such high temperatures, UHMWPE decomposes, and, in particular, fragmentation of long-chain molecules occurs due to mechanical shifts. Surprisingly, the short-term properties, namely the tensile strength of 3-3.5 GPa and the corresponding modulus of elasticity of 130-150 GPa, remain at a high level, but the long-term properties, such as creep and fatigue, deteriorate.

DSM produces feedstock from ultra-high molecular weight polyethylene on its own and can fine-tune the molecular weight distribution, as well as the chemical structure, for example, by producing UHMWPE copolymers. In addition, they use decalin as a solvent at operating temperatures of <200 °C. Currently, DSM produces DM-20 grade based on a low-creep UHMWPE copolymer (Fig. 6).

Fig. 6. Deterioration of properties due to creep in typical seawater conditions for standard Dyneema SK75 and SK78 brands compared to low creep DM20

In conclusion, it is necessary to understand that HMPE is a common name, but fibers can be completely different. A detailed discussion of temperature restrictions in ropes is beyond the scope of this article, and the reader is referred to experienced manufacturers of ropes and cables, such as Samson, Lankhorst and Fibremax - companies with extensive experience and databases on both aramid and polyethylene fibers.

Ballistics

DuPont Kevlar has become an ideal fiber for ballistic applications in bulletproof vests, helmets and other ballistic protection products. The ballistic protection of a material depends on its ability to absorb energy locally, as well as on the efficiency and speed of transmission of absorbed energy, which is realized in the so-called unidirectional (UD) laminates either in the form of "soft" liners or compressed into solid sheets. Kevlar in ballistics can be replaced with the material Twaron, since the properties of the fibers are the same, but the adoption of a new fiber by the market; that's another question. Allied Signal (now Honeywell) invented polyethylene fiber for ballistics in the 1990s, and DSM Dyneema is currently leading the field with its SK99 brand, which has a strength of >4 GPa.

The advantage of PE fibers compared to paraaramides is their lower density (by 50%) and higher strength/rigidity (Fig. 4). In principle, polyethylene tapes such as Endumax should work even better than fibers from the point of view of ballistics. The packing density of tapes in UD laminates looks more favorable than the density of fibers. However, the strength of, so far, is quite limited - about 2.5 Gpa, and the impact energy is below the zone of the curve "stress-strain". In this regard, many articles have been published in the academic world that spider silk is tougher than Kevlar, which is actually true, but the breaking stress of such silk is 30% of the breaking stress of Kevlar, and soldiers with a lower body mass index will be pierced through. Fibers such as aramids and polyethylene can absorb a lot of energy at low voltages, causing less "blunt trauma" (damage).

Future prospects

Ultimate rigidity and strength

Figure 7 shows the change in the elastic modulus over time for aramid and polyethylene fibers (tapes). The modulus of elasticity is associated with the maximum alignment of polymer chains in fibers. For aramid fibers, the modulus of elasticity has not improved significantly recently, despite numerous efforts by manufacturers.

Fig. 7. Modulus of elasticity and its dependence on time for aramid and polyethylene fibers and tapes

The alignment of the chains is achieved at a complex stage of coagulation and is crucial for the modulus of elasticity. As for polyethylene fibers, their modulus of elasticity has approached its theoretical limit. However, many researchers believe that its limits have not yet been reached. It is assumed that the strength of PE fibers will increase with a decrease in the diameter of the fiber, similar to glass fibers. By extrapolating the fiber diameter to zero in accordance with this theory, the estimated strength of 19 GPa was determined. However, these ideas are fundamentally wrong. Currently, it is generally recognized that the destruction under tensile load occurs due to the slippage of polymer chains. The upper strength limit of 7 GPa seems to be a more realistic value and has already been obtained on the scale of a pilot plant, but has not yet been translated into trademarks.

Green polyethylene and aramid fibers

"Green" bio-based plastic is gaining popularity due to the negative image of oil-based plastic. DSM started with bio-based Dyneema using ethylene derived from biomass and/or recycled sources, and Teijin Aramid announced a bio-based monomer study for PPTA. These bio-based fibers are identical to standard fibers and remain with us in the environment forever after their service life. Therefore, recycling is important. For example, Teijin recycles its Twaron fibers from used car tires, and uses them as crushed fibers in brake pads (replacing asbestos).

Hybrid use of fibers

Polyethylene fibers are strong, but they lack compressive strength, while carbon fibers are brittle, but have very good tensile properties and off-axis properties. These two types of fibers can be combined into hybrid composites to combine the best of their properties. The same applies to the combination of polyethylene and aramid fibers.

Epilogue

UHMWPE and aramid differ in their strength and overall efficiency, but these two materials have some differences.

UHMWPE

Ultrahigh molecular Weight polyethylene UHMWPE (UHMWPE) fiber is a gel-molded material consisting of extremely long chains of polyethylene. UHMWPE is also called by the name of the Dyneema or Spectra trademarks. The Dyneema trademark was registered by DSM, and the Spectra trademark was registered by Honeywell. After the patent for the production process expired, about 20 years ago, ultra-high molecular weight polyethylene fibers began to enter the market without specifying trademarks, or under their own brands of the companies producing them.

The main applications of UHMWPE:

• ballistic protection
• sports and tourist equipment (ropes and ropes)
• shock-resistant lining of equipment

Aramid

Aramid is a synthetic fiber consisting of many inter-chain bonds, which are then crosslinked with bound hydrogen. A large number of these bonds strengthens the material and creates a strong, versatile fiber with a wide range of applications.

Aramid is suitable for a number of applications in various industries, including:

• fiber optics
• protective clothing and equipment
• ballistic protection
• car belts and hoses

Compared with aramid, the fabric is made of ultra-high molecular weight polyethylene:

• lighter than aramid, so the final product is lighter
• strength is 40 percent stronger when the temperature is below 70 degrees Celsius
• softer and smoother than aramid. This difference makes the UHMWPE fabric more resistant to cuts and abrasion

higher UV resistance. After two full days of exposure to UV radiation, aramid loses up to 25% of the strength of the material, and the fabric of ultra-high molecular weight polyethylene loses up to 5%; which makes the latter the best choice for products that will be used mainly outdoors.