What is the ‘fibre’ used to make bulletproof jackets?

Question

UnlockIAS · Accepted Answer

Correct — D, Kevlar. Kevlar is DuPont's trade name for a para-aramid — poly(p-phenylene terephthalamide), repeat unit [-CO-C6H4-CO-NH-C6H4-NH-]n — developed by Stephanie Kwolek at DuPont in 1965 and sold in its modern form from 1971. Two structural features make it ballistic. Its backbone is aromatic and the amide links sit in the para position, so the chains are stiff and rod-like and align almost perfectly along the fibre axis when the liquid-crystalline dope is spun; and the amide groups hydrogen-bond sideways between neighbouring chains, locking them into sheets. The spun fibre reaches a tensile strength of about 3,000 MPa and, weight for weight, is roughly five times stronger than steel, and it does not melt — it decomposes. That combination is what a soft vest needs, because a bulletproof jacket is not a plate but many layers of woven Kevlar cloth: the round is caught by the crossing yarns, its energy is spread across a wide area of fabric and used up stretching them, and the bullet is deformed and decelerated instead of punching through. DuPont sells grades made for exactly this — K129 (higher tenacity for ballistic use) and K149 (highest tenacity, for armour and aerospace) — alongside industrial K29 and high-modulus K49 for cables and ropes. The same fibre goes into helmets, cut-resistant gloves, racing tyres, sails, mooring lines and brake linings. Twaron, developed by Akzo in the 1970s and in commercial production from 1986, is the chemically identical para-aramid competitor.

Why the other options are wrong — (a) The most defensible wrong answer. Nylon is the aliphatic polyamide Wallace Carothers developed at DuPont in the 1930s, and ballistic nylon genuinely was the fibre of the flak jackets that preceded modern body armour — the US National Institute of Justice programme that produced the police vest (Lester Shubin and Nicholas Montanarelli) was explicitly a project to replace nylon with Kevlar. But nylon's flexible aliphatic chain cannot pack and hydrogen-bond like a para-aramid, so its tenacity is a fraction of Kevlar's; nylon flak jackets stopped shell splinters, not bullets.  (b) Terylene is Imperial Chemical Industries' trade name for polyethylene terephthalate (PET) fibre — the polyester patented by John Rex Whinfield and James Tennant Dickson in 1941 and sold in the United States as Dacron after DuPont bought the rights in 1946. It is the crease-resistant apparel fibre blended with cotton and wool (terrycot, terrywool) and is chemically the same polymer as a drinking-water bottle. Strong enough for clothing; nowhere near a ballistic fibre.  (c) Tweed is not a fibre at all, which makes it the quickest elimination on the page. It is a rough woollen fabric of soft, open texture, woven in plain, twill or herringbone patterns and associated with Scottish and Irish outerwear (Harris Tweed). The stem asks for a fibre; tweed names a cloth, and the wool it is woven from is a natural protein fibre with no ballistic application whatever.

Concept: Fibres are classified first as natural (cotton, wool, silk, jute) or man-made, and man-made fibres again as regenerated (rayon, made from cellulose) or fully synthetic (polyamides, polyesters, acrylics, polyolefins). What separates an ordinary synthetic from a ballistic one is specific strength — strength divided by density — together with a high modulus and the ability to absorb energy in microseconds. Ordinary polymer chains are coiled and flexible, so a fibre spun from them stretches easily. High-performance fibres are built from chains that are stiff, extended and packed in register: para-aramids such as Kevlar and Twaron, ultra-high-molecular-weight polyethylene such as Dyneema and Spectra, and PBO (Zylon). Aramid simply means aromatic polyamide; the para isomer gives the straight rod-like chain Kevlar needs, while the meta isomer gives Nomex, which is not ballistic but is flame-resistant and is used for firefighters' and racing drivers' suits. Hard armour meant to stop rifle rounds is a different construction again: a ceramic strike face of boron carbide or alumina, backed by an aramid or polyethylene composite.

State PCS 2023 Q11 — Science & Technology PYQ with Answer

State PCS 2023 Q11 — Science & Technology PYQ with Answer