Building with Composites: Chemistry

In building structures, it is important that the materials we use satisfy all of our needs, and that the material has all of the properties that we want.  However, in a world where there are only metals, ionic compounds, and molecular compounds with no in between, this isn’t possible.  Almost everything today, from consumer products to spacecraft hulls, would not be possible without composites, multi-phase materials formed from a combination of materials which differ in composition or form.  In combining these different materials that have different properties, completely new materials with completely new properties are formed.  How does this happen you may ask? Well, it strongly depends on what kind of composite we’re talking about.

Fibrous Composites

There are two elements to a fibrous composite: the matrix and the fibers.

Fig 1. A simplified model of a fibrous composite

The matrix, in most cases, is a low stiffness, ductile polymer, usually epoxy, vinyl ester, or polyester thermosetting plastic.  Fibers are typically higher strength materials, including glass, plexiglass, carbon, basalt, and  aramid (strong, heat resistant, synthetic fibers made from long carbon chains with high IMF) just to name a few.  The overall concept of a fibrous composite is to increase the fracture strength of a material by utilizing the strength of matrix while avoiding failure due to brittleness through fibers.  However, if the fibers of a composite are aligned with the direction of the stress (loading direction), then stiffness and strength are only weighted averages based on the volume fractions of the matrix and fibers.  Thus, by crossing fibers in the matrix in a weave as seen in Fig. 2, a whole new level of strength is gained, along with new properties.

Fig 2. Crossed fibers weaved together

Fig 3. Chemical makeup of aramid fibers; their high tensile strength being due to strong IMF between long carbon chains due to hydrogen bonding and LDF

The most common type of fibrous composite is fiber reinforced plastics (FRPs), utilizing an epoxy matrix with traditional fiber materials such as fiberglass and aramid.  A preliminary step in manufacturing FRPs is the preparation of the fibers by popular techniques such as weaving, braiding, knitting, and stitching, creating multiple sheets of fibrous material.  Next, it must undergo some sort of forming process for the fibers to be set in the matrix, most commonly utilizing a mold or a curing via a vacuum bag to remove flux gases from the laminate.

Recently however, carbon fiber reinforced plastics (CFRPs) have been especially popularized in industry due to their desirable and advantageous traits; they are lightweight (.010 pounds less per cubic inch with the same fiber weight), stronger, and can even be conductive if need be.  The lightweight of CFRPs is extremely important; by comparison, a sheet of aluminum (notorious for being one of the most lightweight metals available) with the same strength of an equivalent sheet of carbon fiber weighs 1.5 times the mass of that of the carbon fiber sheet.

Carbon Fiber Cloth

Fig 4. Weaved carbon fiber cloth used as fiber in CFRPs

 

Another emerging type of fibrous composite has been ceramic matrix composites (CMCs), a subgroup of composite materials and a subgroup of technical ceramics.  They are essentially ceramic fibers embedded in a ceramic matrix, thus creating a ceramic fiber reinforced ceramic (CFRC).  One CFRC that is extremely common and highly used in aerospace engineering is reinforced carbon-carbon (C/C), which utilizes carbon fibers in a graphite matrix.  Due to its functionality at high temperatures, it is a common material used on the nose cones of rockets, which is where most of the heat is focused during re-entry.  Other important commercially available CMCs include  C/SiC, SiC/SiC and Al2O3/Al2O3.  Overall, CMC-based heat shield systems offer  a number of advantages, such as reduced weight, higher load carrying capacity, reusability for several re-entries, and better steering during re-entry with CMC flap systems.

Fig 5. Fracture surface of a CJRC composed of SiC fibers and SiC matrix . The fiber pull-out mechanism shown is the key to CMC properties.

Laminar Composites

Laminar composites can be seen as a “composite squared”, as it is comprised of layers of other composites that are melded together through intense heat and pressures to form a plate of material with properties completely separate of the initial materials. When you cut into a plate made of any kind of laminar composite, you will find a layering similar to that of a piece of plywood. This layering produce an interesting set of properties that can change depending on which direction you view the material at.

http://www.emeraldinsight.com/content_images/fig/1560140501024.png

Fig 6. A simplified model of a laminar composite

The overall laminar exhibits one of four behaviors: It either behaves anisotropically, orthotropically, isotropically, or quasi-isotropically. Each individual layer either behaves orthotropically or transversely isotropically. This determines how the material reacts when a force is applied to it from a certain direction. Consider that there are three men attempting to break a series of laminar composite materials. One man tries to smash it apart with a downward motion (along the z-axis), one man tries to pull the layers apart (along the x-axis) and one man attempts to bend the material until it snaps.

In an isotropic composite, each of the three men will have the exact same success in breaking the material. Isotropic composites exhibit the same properties no matter which direction the force is applied. In an orthotropic composite, each man will have an easier or more difficult time breaking the material than his fellows. This is because orthotropic materials exhibit different qualities depending on which direction force is applied much like trying to break a piece of wood with the grain rather than perpendicular to it. Quasi-isotropic materials will have the same properties along an entire plane, but may have different qualities outside of that plane. The difference between orthotropic materials and anisotropic materials are that for a composite to be orthotropic, the differences in properties have to be exhibited along perpendicular axes, such as the wooden log, while anisotropic materials can exhibit different properties along any set of axes, perpendicular or otherwise.

Now how does this affect the usefulness of the overall laminar? And what makes the laminar acquire these interesting properties? The secret is in the creation process.

Laminates of any kind are created through a layering process, where material is applied in uniform layers and then melded together somehow to form the overall product. In the case of the laminar composites, the materials are usually epoxies, alumina, aluminum, titanium, or polyimides, and the materials are melded through a combination of intense heats and pressures.The layering and initial materials all affect the outcome of the final composite. If the layers were uniform matrices, the final composite will exhibit strong quasi-isotropic properties, as each matrix will resist certain directions of force better than others. In our example of the three men, the man pulling the layers apart will have a much easier time than either of the others. Laminars made of less uniform layers will begin to exhibit more isotropic qualities and more uniformity of the layers will produce more orthotropic properties.

While laminars themselves are not used in the TPS of NASA spacecraft, they are nonetheless extremely important in daily life. Plywood is a common example of a laminar composite, as well as plates used in the construction of aircraft. The layering of the composite allows it to be formed into a smoother shape than most composites, ideal for streamlining aircraft.

Particulate Composites

Particulate composites can be seen as a type of composite made from embedding particles of one material in another. The embedded particles can be extremely fine particles, even down to less than .25 microns. These particles provide reinforcement to the matrix material, strengthening it, and provide specific properties for different applications. An example of this type of composite is binging course rock and gravel in a matrix of cement to provide stiffness and strength.

Fig 7. Basic conceptual drawing of particulate composites

There are advantages to using particulate composites. For instance, many composites of this type have high tensile (or pulling) strength at high temperatures, and high toughness. They also have a high strength to weight ratio, meaning these composites are less dense, and lighter, than other materials that can withstand the same amount of force.

Common applications of this composite include metals such as aluminum alloy, polymers such as rubber, and ceramics such as concrete. Due to its inherent strength, and cheap construction, particulate composites are commonly used in small appliances like cell phone casings, and helmets.

Fig 8. a microscopic view of the particles in the matrix of a metal matrix of a composite

However, the particulate does not have to be a metal, as it is above. In any cases, the particulate is chopped fibers, platelets, hollow spheres, or even new materials such as buckyballs or carbon nanotubes.

In conclusion…

Composite materials are surprisingly relevant, not just in aeronautics and space flight, but in everyday uses as well. Plywood, most likely found in cheap building projects and temporary wooden structures, is a chaotic laminar composite, sidewalks are made out of concrete, one of the most common particulate composites, and the fiberglass insulation that most likely resides inside the walls of your house or apartment could indeed be a simple matrix composite. There is a reason for composites showing up in human history from 4000 BC to modern times, and that reason is that composites as a building material are historically better.

In the last blog post, we praised ceramics as being a huge improvement over simple materials. Composites are to ceramics as ceramics are to simple materials. Composites show a blending of different materials, much like ceramics, but due to the more ordered nature of composites, a similar effect can be achieved with even fewer drawbacks. Many ceramics feature a chaotic blend of material, similar to a solution. Composites are a more orderly mixture, with each material remaining separate from the whole while still contributing its properties to the final material. In spaceflight, this results in more advanced materials that can stand up to the rigorous requirements of space even better than before.

In short: If composite technology continues to improve, they may well be the future of material science.

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