Hybrids: composites, foams, and natural materials

Composites are one of the great material developments of the 20th century. Those with the highest stiffness and strength are made with continuous fibers of glass, carbon, or Kevlar (an aramid) embedded in a thermosetting resin (polyester or epoxy). The fibers carry the mechanical loads, whereas the matrix material transmits loads to the fibers, provides ductility and tough­ness, and protects the fibers from damage from handling or the environment. It is the matrix material that limits the service temperature and processing conditions. Polyester-glass composites (GFRPs) are the cheapest, epoxy-carbon (CFRPs) and Kevlar-epoxy (KFRPs) the most expensive. A recent innovation is the use of thermoplastics as the matrix material, using a coweave of poly­propylene and glass fibers that is thermoformed, melting the PP.

If continuous fiber CFRPs and GFRPs are the kings and queens of the composite world, the ordinary workers are polymers reinforced with chopped glass or carbon fibers (SMC and BMC) or with particulates (fill­ers) of silica sand, talc, or wood flour. They are used in far larger quantities, often in products so ordinary that most people would not guess that they were made of a composite: body panels of cars, household appliances, fur­niture, and fittings. It would, today, be hard to live without them.

So composites have remarkable potential. But the very thing that creates their properties—the hybridization of two very different materials—makes them near-impossible to recycle. In products with long lives, made in rela­tively small numbers (aircraft, for instance), this is not a concern; the fuel energy saved by the low weight of the composite far outweighs any penalty associated with the inability to recycle. But it is an obstacle to their use in high-volume, short-lived products (small cheap cars, for example).

Foams are made by variants of the process used to make bread. Mix an unpolymerized resin (the dough) with a hardener and a foaming agent (the yeast), wait for a bit, and the agent releases tiny gas bubbles that cause the mixture to rise in just the way bread does. There are other ways to make foams: violent stirring, like frothing egg white, or bubbling gas from below in the way you might make soap foam. All, suitably adapted, are used to make polymer foams. Those made from elastomers are soft and squashy, well adapted for cushions and packaging of delicate objects. Those made from thermoplastics or thermosets are rigid. They are used for more seri­ous energy-absorbing and load-bearing applications: head protection in cycle helmets and cores for structural sandwich panels. And because they are mostly trapped gas, they are excellent thermal insulators.

Their ecocharacter, however, is mixed. The blowing agents used in the past—CFCs, chlorinated and fluoridated hydrocarbons—cause damage to the ozone layer; these have now been replaced. Some can be recycled, but only 1-10% of the foam that has to be collected, transported, and treated is real material (the rest is space), so you don’t get much for your money.

Natural materials: wood, plywood, paper, and card. Wood has been used for construction since the earliest recorded time. The ancient Egyptians used it for furniture, sculpture, and coffins before 2500 BC. The Greeks at the peak of their empire (700 BC) and the Romans at the peak of theirs (around 0 AD) made elaborate buildings, bridges, boats, and chariots and weapons of wood and established the craft of furniture making that is still with us today. More diversity of use appeared in Medieval times, with the use of wood for large-scale building, and mechanisms such as carriages, pumps, windmills, even clocks, so that, right up to end of the 17th century, wood was the prin­cipal material of engineering. Since then cast iron, steel, and concrete have displaced it in some of its uses, but timber continues to be used on a mas­sive scale, particularly in housing and small commercial buildings.

Plywood is laminated wood, the layers glued together such that the grains in successive layers are at right angles, giving stiffness and strength in both directions. The number of layers varies, but is always odd (3, 5, 7 … ) to give symmetry about the core ply; if it is asymmetric it warps when wet or hot. Those with few plies (3, 5) are significantly stronger and stiffer in the direc­tion of the outermost layers; with increasing number of plies the properties become more uniform. High-quality plywood is bonded with synthetic resin. The data listed here describe the in-plane properties of a typical five-ply.

Papyrus, the forerunner of paper, was made from the flower stem of the reed, native to Egypt; it has been known and used for over 5000 years. Paper, by contrast, is a Chinese invention (105 AD). It is made from pulped cellulose fibers derived from wood, cotton, or flax. Paper making uses caustic soda (NaOH) and vast quantities of water—a bad combination if released back into the environment. Modern paper-making plants now release water that (they claim) is as clean as when it entered.

Profiles for 12 hybrid materials follow, in the order in which they appear in Table 12.1.