Protective clothing and protective materials: how they work. An introduction from Explain that Stuff!

Fire and flames

Humans are believed to have used fire more than a million years ago, but it was not until the 20th century that people perfected the art of protecting themselves against the destruction that fire could bring. The widespread use of electricity and petroleum products in the last century or so has made fire protection all the more important.

Fire means intense heat and temperatures, so fire-protective materials must be capable of resisting heat for some time without themselves catching fire. Some plastics and composites are naturally heat- and fire-resistant. Polyvinyl chloride (PVC), for example, is used as an insulator in domestic electrical wiring, because it is difficult to set on fire and prevents flames from spreading. A composite of silica and alumina, two naturally nonflammable rock materials, is used in industrial buildings such as power stations to offer heat protection up to 2200°F (1200°C). Another promising use for this composite is in airplanes, whose combustible plastic interiors give off pound-per-pound as much energy as petroleum when they catch fire. In 1998, Geopolymer, a lightweight, nonflammable fabric laminate based on the alumina-silica composite, became the first material to withstand arduous Federal Aviation Authority (FAA) tests based on an airplane fuel fire. Materials such as this can be expensive, and an alternative approach is to wrap flammable materials in flame-retardant ones. For example, airplane seat cushions are typically made from flammable polyurethane, but can be made safer by coating them in fire-resistant Kevlar, a tough composite made from aramid fibers.

Aluminum fire fighting suits

Photo: These firefighters in protective aluminum clothing are practicing what to do in the event of a helicopter crash. Picture by Staff Sgt. James H. Christopher III courtesy of US Army and Defense Visual Information Center.

People who work close to intense heat, such as firefighters, welders, and vulcanologists (scientists who study volcancos), wear protective clothing made from lightweight, fireproof materials. Aluminum-based fabrics are used in heavy-duty, silver-colored fire-fighting suits, while waterproof fire clothing may be made from a tightly woven plastic or composite material coated with a flash-fire and chemical proof outer layer made from Neoprene (a synthetic rubber). Firefighting visors and airplane smoke hoods are made from Kapton coated with Teflon, two tough fireproof composites used in the outer layers of Apollo space suits.

Bullets and blows

If the human body is ill-designed to withstand fire, it has even less chance where bullets, knives, and bombs are involved. But revolutionary materials that act like a protective outer skin can turn a fatal gunshot into little more than a bruise.

Five times stronger than steel (by weight) and much lighter, the composite Kevlar has revolutionized body protection. It consists of fibers made from long molecular chains of a polymer (plastic material) called polyparaphenylene terephthalamide. Woven tightly together, the fibers effectively knit into a tough protective grid than can withstand knife blows and cuts and some types of gunfire. Effectively, the grid of fibers absorbs and dissipates the energy of a knife blow or bullet before it can damage the body beneath it. Kevlar is also chemical and flame resistant. All this makes it the material of choice in flak jackets, bulletproof vests, anti-mine boots, and chainsaw-protective clothing. An even tougher material called Spectra Fiber is made from a polyethylene composite, in which the fibers are woven at right angles to one another in a flexible resin and coated with a laminate film. This material is ten times stronger than steel, yet extremely light, and provides better protection than woven composites such as Kevlar against automatic weapons.

Piece of Kevlar damaged by a missile

Photo: This is a piece of Kevlar after being hit by a projectile. You can see a dent (coming up toward the camera)—but you can't see a hole. Picture courtesy of US Army.

It is not always practical for people to wear bulletproof clothing; sometimes it is more convenient to protect them by armoring the vehicles in which they travel. Armor-plated cars have long been used by heads of state and celebrities and used to feature heavy duty steel reinforcement built between the vehicle's bodyshell and chassis. Today, composite materials provide the same protection as anti-ballistic steel but add only a third of the weight to the vehicle. Armored vehicles are typically also fitted with bulletproof "glass" (really a tough sandwich of glass and polycarbonate, a transparent thermoplastic).

From bicycle helmets to space suits

Not everyone has to defend themselves against petroleum fires or automatic weapons, but everyone is exposed to some risk, every day. Urban traffic makes using a bicycle an increasingly dangerous undertaking, but using a bicycle helmet can reduce the risk of injury by around 85 percent. Most helmets consist of a "styrofoam" (expanded polystyrene (EPS) foam) liner and an outer shell made of plastic or composite. In an impact, the liner crushes gradually, reducing and dissipating the energy of the blow and increasing the time it takes for the wearer's head to come to a stop. Meanwhile, the rounded outer shell helps to spread the impact over a larger area, reducing the damage it does in any one place. Find out more in our main article on how bicycle helmets work.

Some people have no choice but to wear protective materials, all the time. Sufferers of a rare genetic condition called xeroderma pigmentosum have skin that lacks natural protection against ultraviolet (UV) light. Without protective clothing, sunlight readily burns their skin and causes skin cancer. To reduce the risk, they typically wear hats and gloves impregnated with a UV-absorbing chemical called benzotriazole, plus sunglasses or masks with UV-resistant visors similar to those worn by astronauts. They also have to apply sun cream to their bodies regularly throughout the day. Sun creams contain chemicals such as paraaminobenzoic acid (PABA), zinc oxide, and titanium dioxide, which absorb or reflect different wavelengths of UV light and prevent them from reaching the skin. Increasing damage to Earth's ozone layer, which reduces the Sun's harmful ultraviolet rays, means protective measures such as this will become more commonplace for everyone in future. Find out more in our main article on sunscreen.

Most at risk from ultraviolet radiation are space-walking astronauts, closer to the Sun and far outside Earth's ozone layer. Astronauts on the Apollo program in the 1960s wore polycarbonate helmets with gold-plated visors, specially designed to protect against ultraviolet and infrared radiation. Many of today's most important protective materials started life on the Apollo program. Indeed, early specifications for Apollo space suits read like a catalog of today's most advanced materials. Those suits contained some 24 different layers designed to protect against extreme temperatures ranging from -250°F to +250°F (-157°C to +120°C), the lack of atmospheric pressure, and attacks from tiny meteorites.

The innermost garment featured a lightweight nylon suit, surrounded by water-cooling vinyl tubes, and a comfortable nylon outer layer. Next came the outer suit, comprising a layer of comfortable Nomex cloth touching the skin, a knitted jersey laminate, a Neoprene (the material used to make wetsuits) rubber joint material, a Neoprene oxygen-retaining bladder to maintain artificial air pressure inside the suit, a nylon outer layer for the bladder, five layers of aluminized Mylar (polyester film) interleaved with four layers of Dacron (polyester fiber) to protect against extreme heat and cold, two layers of Teflon-coated silica bonded to Kapton, two more layers of Kapton for heat insulation, an inner layer of fire-protective beta cloth (Teflon coated with fiberglass), and an outer layer of white Teflon resistant to flames and micro-meteorites.

Protective materials around us

Bullet-proof glass

Bullet-proof glass is neither bullet-proof nor glass. It is actually a laminate (sandwich) of glass between layers of polycarbonate (a plastic composite) that can absorb and dissipate the energy of a speeding bullet. Different grades of the material can stop bullets from handguns and high-powered rifles, but there is no such thing as a truly bullet-proof material.

Motorcycle crash helmet

A motorcycle helmet is designed to reduce head injuries in a crash by spreading the impact (with a hard outer casing) and absorbing the energy (with a soft inner liner). Helmet casings are typically made of injection-molded plastic or fiberglass, while the inner liners are made from styrofoam. Because the inner liner crumples up in an impact, a motorcycle helmet must always be replaced after a crash.

Kevlar helmets

Protective helmets are a compromise between giving maximum visibility and maximum protection. Made from a combination of plastic laminates or Kevlar and tough metal plate, they are designed to withstand high impacts by dissipating energy and protecting the neck and vertebrae from shocks. Tough polycarbonate visors and rear neck protectors give all-round defense against missiles.

Photo: A soldier adjusts the chin strap on her Kevlar helmet. Picture courtesy of US Army.

Protective materials