Composites take flight

Many vacations begin with an airplane flight. Chemistry is essential in lifting every jet off the ground. Most airplanes, though, are made of the same materials that were in vogue at the dawn of the jet age.

But novel new materials, such as lightweight polymer composites, will soon rule the sky. The Boeing 787 Dreamliner, for example, consists of nearly 50% composites by mass, which helps it reduce carbon dioxide emissions and save fuel.

Ultrastrong composites, like those in the Dreamliner, combine a polymer matrix derived from thermosetting resins and reinforcements made of materials such as carbon fibers, glass fibers, or aramid fibers (such as Kevlar).

Credit: Drach, A. et al., The 19^th International Conference on Composite Materials (2013)

Polymer composites used in airplanes are made of a polymeric material reinforced by carbon or glass fibers. The fibers provide strength and stiffness, while the polymer serves as the “glue” holding the fibers together.

Despite their advantages, composites are limited by operational temperatures, explains Boris Bulgakov of Lomonosov Moscow State University. “For most common materials in aerospace based on epoxy resins, operational temperatures cannot be higher than 120 °C; the usual range is −50 to 50 °C,“ he says. Some models of aircraft and parts, such as low-pressure compressor blades, can reach up to 350 °C.

Bulgakov and his team are developing alternative polymer matrices composed of bis-phthalonitriles, organic compounds with high aromatic content. Thermosetting plastics, like phthalonitrile resins, can be molded into various shapes then hardened with heat—but only once. Reheating the resins won't make them pliable again. That means phthalonitrile resins can be exposed to high operating temperatures without damage.

Although these thermosetting plastics have been around since the 1980s, they haven't taken off because the monomers' high melting points limited their use. Attempts to decrease the resins' melting points resulted in a decrease in thermoset properties, Bulgakov explains.

Now, his team has produced low-melting-point phthalonitrile resins with strong thermoset qualities. “Our resins can withstand temperatures up to 450 °C,” he says. “Also, the matrices possess the highest flame-retardant properties among the known plastics.”

Another perplexing issue for composites is bonding. Composites are built from multiple layers of material. And while they often have higher rigidity and greater strength, they are also subject to catastrophic failure.

A team from the Department of Aeronautics & Astronautics at Massachusetts Institute of Technology, working with Swedish aerospace company Saab, has devised a surprising solution: stitching composites with carbon nanotubes. It achieved this ultrastrong stitching by vertically aligning billions of carbon nanotubes per square centimeter of composite and bonding them with polymers.

The idea of boarding a plane that's “sewn together” may be unnerving for some travelers. But the team found that their carbon nanotube stitches were 30% stronger than traditional bonding methods. “Carbon nanotubes have an extremely high surface area—1,000 times more than carbon fiber,” explains team member Roberto Guzman, now a researcher at FIDAMC, the Foundation for the Research, Development & Application of Composite Materials in Madrid. “The greater the surface area, the higher the energy required to break the material.“

These new resins and bonding strategies may lead to composites that can be used in more and more extreme aerospace environments. That should lead to lighter-weight airplanes with lower emissions and fuel consumption. Travelers may not notice that from the (relative) comfort of seat 22C, but they may see it in lower ticket prices, while the more eco-minded passengers will also be cognizant of the smaller carbon footprint they're leaving.

Pooling our health

Richardson will never forget her first dive, off West Palm Beach, Fla. “It was incredible seeing the fish, the coral reefs, and all the color,” she says. “I instantly fell in love.”

Many divers take that first plunge on vacation. With 70% of Earth's surface covered in water, there is a smorgasbord of diving hot spots to pick from, ranging from the Amalfi Coast in Italy to the Great Blue Hole in Belize or Barracuda Point in Malaysia.

Despite that enviable experience, Richardson first earned her scuba diving stripes in the safety of a nearby swimming pool. For others, pools offer the opportunity to simply cool off, float down a resort's lazy river, or get soaked at a splash park. That means sharing the water with other vacationers. With all those people using them, how do public pools stay clean?

Credit: Saqib Qayyum/Wikimedia Commons

Built over 5,000 years ago, The Great Bath of Mohenjodaro in Pakistan is the earliest known non-natural pool. Scholars believe The Great Bath was used for purification and religious ceremonies.

Credit: Shutterstock

Manmade communal baths, the precursor to modern pools, have their roots in ancient Greece and Rome. Ancient baths were boisterous places. Besides cleaning themselves, bathers shaved, socialized, ate, and drank. It fell to slaves to remove debris and refresh the water.

Credit: Boston Public Library Tichnor Brothers collection #62844/Wikimedia Commons

Public baths rose in popularity in the U.S. during the early 1900s. By the post-World War II era, they had become mainstream. Early pools were cleaned by filtering out debris and regularly changing the water. A student at Brown University, John Wymond Miller Bunker, is credited with making the first successful attempt at chemical cleaning of a pool. It's believed he did it in 1911 using chlorine.

Credit: Maksym Kozlenko/Wikimedia Commons

Chlorine and bromine kill algae, bacteria, and pathogens by breaking down their cell walls and destroying the proteins inside. They also clean the water of contaminants, such as dirt, oil, and sweat; chlorine oxidizes them while bromine ionizes them. Chlorine has long been the most popular pool sanitizer in the U.S. However, as bromine is more stable in warm water, it is popular with hot-tub fans.

Credit: Shutterstock

Ozone systems clean pool water by splitting oxygen molecules, O₂, into oxygen atoms using UV light; the oxygen atoms then combine with other O₂ molecules to form ozone, O₃.

Initially, pools were cleaned by filtering out debris and regularly changing the water. That, of course, was hard, inefficient work. Brown University student John Wymond Miller Bunker swam to the rescue in 1911 in what is believed to be the first chemical attempt at cleaning a pool with chlorine. Inspired by city waterworks, where engineers were introducing chlorine and filtration systems to sterilize water, Bunker tried adding chlorine to a swimming pool. His results were sparkling and refreshing: Bacterial counts dropped from 700 per cubic centimeter to zero in just 15 minutes.

Chlorine-based cleaning chemicals like hypochlorous acid kill pathogens by breaking down their cell walls and destroying the proteins inside. It remains the most popular method of keeping pools sparkling clean.

There are now other options: Lovers of hot tubs can splash in some bromine, which works similar to chlorine but remains more stable in warm water. For a complete nonchemical solution, some pools are installed with high-intensity ultraviolet light filtration systems, which blast microorganisms with UV beams, destroying their DNA.

Tanks for the memories

Growing up in the heyday of Jacques Cousteau TV shows, Karl Shreeves became so captivated by the wonders of the underwater world that he became a certified diver by 13—the youngest age allowed at the time. “They say you'll never forget your first kiss,” says Shreeves. “But I found that while you do forget your first kiss, you won't forget the first time you breathe underwater.” Today, he dives all over the world, both on vacation and in his role as Education and Content Development Executive at the Professional Association of Diving Instructors.

For much of diving, divers rely on regular compressed air, a mixture of 21% oxygen and 79% nitrogen. But once a diver descends past a certain depth, that nitrogen can become toxic. Under greater pressure, the nitrogen dissolves into body tissues. “Nitrogen gets in your lungs and in your bloodstream,” Shreeves explains.

Once the pressure is removed by ascending, the absorbed nitrogen can form bubbles and cause decompression sickness, or “the bends.“ This is prevented by using dive computers that tell divers how long they can stay at various depths before stops are required to keep dissolved nitrogen within accepted limits while ascending.

An avid diver himself, Shreeves has some advice for those who want to dive at greater depths. To help divers stay longer, they can use enriched air mixtures (also called “nitrox”). “Enriched air commonly has 32% or 36% oxygen,” Shreeves explains. “We do this not for more oxygen, but less nitrogen.” This enables longer dive times without decompression stops.

Although it's not difficult to breathe nitrox and other gases used for much deeper diving, divers must understand the underlying chemistry. “Oxygen starts to become toxic under increasing pressure beyond certain depths,” says Shreeves. “So divers learn how oxygen-based limits work and why.“

Sunshine and a bright clear bay

The sun's UV rays beat down on land and scatter into water. Looking to shield themselves, vacationers lather on sunscreen. But in saving their skins, these travelers may be damaging marine environments.

The synthetic ingredients many sunscreens use to filter and absorb UV can pose a hazard to ocean wildlife. Several studies, in fact, found that these chemicals can contribute to bleaching and deformities in coral reefs. Hawaii passed a bill this year to ban sunscreens containing oxybenzone and octinoxate. Some manufacturers, meanwhile, now offer natural mineral alternatives that protect revelers and reefs.

Sunscreen protects humans from the damaging effects of the sun, but some sunscreen ingredients harm coral reefs. In particular, two active ingredients that serve as UV absorbers, oxybenzone and octinoxate, promote coral bleaching at lower temperatures, damage coral DNA, act as endocrine disruptors, and reduce the survival rates of coral larvae and whether coral colonies recruit them. Overall, these compounds make the coral reefs more vulnerable to the effects of climate change. Sunscreens may only contain 3% to 6% of oxybenzone and octinoxate, but these amounts add up since 6,000 to 14,000 tons of sunscreen lotion are released into reef areas each year.

A unit of doped titanium dioxide. Titanium atoms are the blue spheres; oxygen atoms are the red spheres. The dopant is a transition metal shown as the brown sphere.

Mineral-based sunscreens are generally considered safer for coral reefs. These products contain zinc oxide or titanium dioxide particles that block or scatter UV rays. However, these ingredients may still cause problems if the particles are small enough for corals to ingest.

Lifeguard Brian Guadagno founded the sunscreen line Raw Elements in his kitchen with the aim of producing eco-safe sunscreens. His mineral formulations use zinc oxide and protect the skin in a different way. Whereas chemical sunscreens, such as those using oxybenzone and octinoxate, are absorbed into the skin, mineral sunscreens physically block UV rays by sitting on the skin's surface, reflecting the rays.

Zinc oxide is a photocatalyst, and UV exposure can produce free radicals that may damage living cells. Therefore, Guadagno uses formulas which are rendered inert and are not photocatalysts. And, unlike nanoparticles, they cannot be ingested by coral or absorbed into the human bloodstream.

And critically, these more eco-conscious sunscreens will not leave beachgoers any more exposed, as mineral-based sunscreens can effectively protect skin from the whole UV spectrum.

Keeping it clean and green

After relaxing in the sun, few things feel better than showering off the sand, drying with a fluffy towel, then falling asleep in crisp white sheets. Hotels offer such indulgences, but the resultant laundry operations use lots of resources. In fact, they account for approximately 16% of a given property's water use, according to the U.S. Environmental Protection Agency.

One of the simplest and most effective ways of reducing a hotel's energy, water, and detergent consumption is by persuading guests to reuse towels and sheets. To that end, the EPA launched the WaterSense H₂Otel Challenge in 2014 to help hotels and guests conserve water. “According to the American Hotel & Lodging Association, a typical 300-room hotel can reduce its yearly water usage by 51,840 gallons and detergent usage by 346 gallons with a towel and linen reuse program,“ notes EPA spokesperson Enesta Jones.

Approximately 94% of hotels have implemented some form of reuse program, according to the American Hotel & Lodging Association. The Radisson Hotel Group (RHG) has even managed to link its reuse program to a bonus social benefit: access to safe drinking water. For every 500 towels reused, RHG makes a donation to the international water charity Just a Drop. Over 3 million towels have been reused to date.

Source: American Hotel & Lodging Association

Source: American Hotel & Lodging Association

Source: American Hotel & Lodging Association

Source: American Hotel & Lodging Association

Unpacking the chemistry of souvenirs

Sadly, all vacations must end. Before then, though—no matter how they spend their downtime—most vacationers will pick up souvenirs. In Richardson's case, her favorite souvenir was something she found on a riverbed: a nonfossilized horse jaw complete with three molars. “My curiosity got the best of me,” Richardson says. “I had to have it dated, since I thought it was from the Revolutionary War.” She sent it to the Woods Hole Oceanographic Institute, which confirmed her suspicions with mass spectrometry.

Richardson's unconventional souvenirs have also included “log dogs”—pieces of metal with a loop that 18^th-century early settlers used to hook trees together and float them downriver. However, once exposed to air, these fragile artifacts start to disintegrate. Her diving buddy Jimmy, an accountant and self-taught electrochemist, stepped in to save the day. “He made his own electrochemistry rig to de-rust our log dogs and preserve them,” she says.

Chemistry has not only helped Richardson make vacation memories; the science has also enabled her to keep them for years to come.