DIFFERENCE BETWEEN PARA-ARAMID AND HIGH-PERFORMANCE POLYETHYLENE
One significant difference, though: high-performance polyethylene fibres are almost always engineered in unidirectional layers if they are to be used for ballistics, while para-aramid fibres can be in either unidirectional layers or as woven fabrics.
Unidirectional layers begin with a strongly parallelized sheet of yarn that is bonded to the second sheet of yarn aligned at a 90-degree angle to the first. The bonding is achieved by means of an adhesive matrix. To ensure optimal performance, the unidirectional layers are usually enclosed within an outer protective film.
The advantages of each material must be evaluated on a case-by-case basis and in accordance with set priorities. In many instances, the process differences required to make the protective inserts (woven fabric or unidirectional layers) count for much more than the differences in properties of the fibres themselves.
Strength & Durability
HPPE hat das höchste Verhältnis von Zugfestigkeit zu Masse aller verfügbaren Fasern. Mit einer Zugfestigkeit von etwa 3,8 bis 3,9 N/tex ist es 15-mal stärker als Stahl (im Gegensatz dazu hat Para-Aramid eine Zugfestigkeit von 2,03 bis 2,06 N/tex). Die molekulare Zusammensetzung von HPPE macht das Material im Vergleich zu Para-Aramidfasern wesentlich haltbarer und widerstandsfähiger gegen abrasive Kräfte.
HPPE has the highest tensile strength-to-mass ratio of any fibre available. It is 15 times stronger than steel, with a tensile strength of about 3.8 to 3.9 N/tex (in contrast, para-aramid has a tensile strength of 2.03 to 2.06 N/tex). The molecular composition of HPPE makes the material much more durable and resistant to abrasive forces in comparison to para-aramid fibres.
Both HPPE and para-aramid have long been used for protective vests—over 15 years in the case of HPPE and more than 30 for para-aramid.
With reference to the requirements of the TR protective vests used by German police, the difference between unidirectional layers and para-aramid fabrics can be simply spelt out as follows: The packages with unidirectional layers tend to be a bit lighter than comparable protective constructions using para-aramid fabrics; however, the fabric constructions are more flexible and therefore better able to conform to the body shapes of different wearers while at the same time allowing good freedom of movement for all.
Back-face signature (BFS) test results and so-called “multi-hit performance” tend to be better for unidirectional layers. This is due to the layers' higher degree of stiffness. However, a disadvantage of unidirectional layers is they are relatively easy to crush and bend which, in extreme cases, can lead to permanent creasing and delamination.
It is often argued that high-performance polyethylene, being a thermoplastic material, is highly flammable and that this poses a danger to police officers who wear ballistic protective products made from the material.
As a rule, however, the potential for such a danger is minimized because the ballistic panel is encased within the vest cover. If a covert vest is worn, the ballistic panel is positioned under a layer of clothing. Still, if greater peace of mind is desired, the ballistic panel can be nested within a flame-retardant outer cover.
Performance potential of high-performance Polyethylene
The performance potential of high-performance polyethylene and para-aramid must be assessed as a function of the projectiles used. Certain projectiles tend to be stopped more effectively with para-aramid while others are more easily halted with high-performance polyethylene.
In addition to para-aramid and high-performance polyethylene, another high-strength fibre PBO (brand name: Zylon®) was used for protective vests. PBO had its moment in the sun a few years ago but has since almost completely disappeared from the ballistic-protection market.