In "Advanced Composite Materials: An Overview", the basic nature, properties, and applications of advanced composite materials (ACMs) were introduced. ACMs are structures made of a combination of specific fibers bound into a solid matrix. In "Advanced Composite Materials: A Closer Look at Fibers", specific materials for the production of fibers, and some characteristics of fibers used in ACMs were examined. In the present article, the investigation of ACMs will continue with a closer look at some specific matrix materials and applications of those amazingly versatile structures.

Recall that an ACM consists of two different parts: specific fibers, and a matrix material in which those fibers are physically bound. The specific strength characteristics and behaviour properties of an ACM are determined by the absolute and relative orientations of layers of fibers bound within the matrix, and to a lesser extent by the rigidity and toughness of the matrix material.

Functions and Properties of the Matrix

An ACM is a curious thing. The principles that determine its strength lie entirely in the fibers, yet those fibers are entirely pointless unless they are bound into a tough, solid matrix. In actuality, the matrix serves two distinct purposes. First and foremost, the matrix must hold the fibers securely in place so that forces are transmitted only along the length of the fibers (compression and tension, or squashing and stretching). This is the only direction in which the fibers of an ACM have strength.

The second function of the matrix is to protect the fibers from contact with forces and influences acting from outside. Since fibers in an ACM have no strength to resist forces acting across their length, it becomes the task of the matrix to counter those forces as much as possible. To accomplish this, a matrix must be solid or very resilient, tough, thermally stable, and chemically stable. A matrix material must not enter into chemical reactions with the materials that compose the fiber portion of an ACM. It  must not react with any of the materials with which it will come into contact, nor can it allow those materials to migrate or pass through so that they can come into contact with the fibers.

These are tough limitations. Matrix materials therefore tend to be rather application-specific. This is actually just some applied logic. For example, if the ACM will be in contact with, say, water, as in the hull of a boat, then obviously the matrix material must be waterproof. An ACM that will be in contact with a solvent liquid such as oil or alcohol must be unaffected by those solvents.

Thermal stability works the same way. A matrix material should have consistent properties throughout the range of temperatures to which it will be exposed. It also stands to reason that since there is a huge number of possible situations in which ACMs may be used, there should be a correspondingly huge variety of possible matrix materials. This is indeed the case, and the number of matrix materials that are available today far exceeds the number of fiber materials currently being used. Matrix materials for ACMs range from very simple plastics through to very complex plastics, and even include metals. (The new Airbus A-380 passenger transport  will make extensive used of Glare, an ACM composed of glass fibers bound into an aluminum matrix.)

Thermoplastic and Thermosetting Polymers

Matrix plastics come in one of two forms, either 'thermoplastic' or 'thermosetting', each of which sees its own particular range of applications. Both types begin as simpler compounds that undergo polymerization to form a particular matrix material.

Thermoplastics are not what one would call 'thermally stable'. As the name suggests, their properties change significantly with temperature. As temperature increases, they become softer and more flexible (more plastic), until they melt and turn into liquids. As temperature decreases, thermoplastics become stiffer and more brittle. At a certain temperature, known as the 'glass transition temperature' (Tg), a thermoplastic material changes from being plastic in character to being brittle and glass-like in character. At temperatures below Tg, the material will shatter and fracture like glass, while at temperatures above Tg the material will absorb the energy of impact by deforming. This basic characteristic of thermoplastics places some strict limitations on the applicability of any particular thermoplastic material. Generally, and for use in ACMs, thermoplastics are only useful within relatively narrow temperature ranges, in which their properties and behaviour are consistent and well understood.

Thermosetting materials, on the other hand, are very thermally stable. They typically begin as simple liquid or semi-liquid materials that turn into very tough solids through polymerization processes. Once set, a thermosetting polymer is unaffected by changes in temperature. They essentially do not have a Tg, and since their hard nature is a molecular structure property rather than a bulk material property, the effects of temperature decrease are quite small. But temperature increase is a different story. Again because of their molecular structures, thermosets maintain their strength and rigidity without softening or melting. At some point, however, the temperature will become high enough to begin breaking down and destroying the chemical bonds within the material. At that point the material suffers irreversible damage, and the structure has been effectively compromised. The temperature and conditions to which thermosets will be exposed thus become important engineering considerations.

Thermoplastics and thermosets are both produced by processes of polymerization. For the use of thermosets in ACM structures, the polymerization process must be an integral part of the procedure or process by which the structure is made. But because thermoplastics can be melted and heat-formed, the formation of ACMs from these materials can be, and usually is, a two-stage process.

Polymerization:

All polymerization process, including those involved in the formation of the most advanced ACM matrices, begin with just one or two simple starting materials called a 'monomer' or a 'prepolymer'. This is referred to in industry as a resin or a resin mixture. The monomer molecules may bond to each other in a repetitive manner to produce molecules thousands of times larger. The large molecules so produced are easily recognized as polymers by the repeated sub-units that make up the overall molecular structure. A very simple example of this process is the formation of polyethylene from ethylene.

Polyethylene, and the vast majority of such 'linear' polymers are thermoplastic in nature.

Each ethylene molecule in this example has only two atomic centers that can take part in bond formation. If one begins instead with a starting material that has more than two active atomic centers, the resulting polymer molecules are much more complex. In these cases each individual molecule can form new bonds to two or more different molecules at once. This results in the formation of highly branched polymer molecules, in which each branch is itself a long, branched polymer chain. Such 'cross linking' has the potential to turn an entire tank truck full of resin into a single large polymer molecule (although the likelihood of that actually happening is exceedingly small!). It is also what gives thermosetting materials their intrinsic strength and other characteristics as described above. The extent to which cross linking occurs can be controlled somewhat during the polymerization process, imparting greater or lesser toughness and resilience to the finished polymer.

In general terms, polymerization is a process of repeated bond formations. The bonds themselves can be formed by several different mechanisms, although any reliable process relies on a single mechanism rather than a combination of processes. Bonds can also be formed between many different types of molecules, and it is the variety of copolymer combinations that gives rise to the great number of possible matrix materials for use in ACM structures. 

Each bond formation reaction can be described as either an addition reaction (in which the component molecules simply add together as bonds form between them), or as condensation reactions (in which a bond forms by 'condensing' the components of a small molecule such as HCl or H₂O from the two molecules that then become bonded together). For example, in the reaction between ethylene diamine and carbonyl chloride, a molecule of hydrogen chloride is produced for each bond that forms between an amine nitrogen atom and a chloride carbon atom.

Processes for working with ACMs to form composite structures must provide for the elimination of such extraneous and undesirable materials from the final product.

There are literally millions of  possible polymeric materials that can be formed by addition and condensation reaction processes. One of the major goals of research and development programs in this area is the engineering of matrix materials having specific and reliable properties for use in ACM structures. In the course of such programs, resins are often developed that are themselves quite complex. An example of such polymers are the special epoxy resins used in aerospace applications,  an example of which is shown here:

More detailed discussions of the processes and products of polymerization reactions, a huge topic,  would fill a good sized library, and is well beyond the scope of this article. Hopefully, this very brief introduction to polymerization will be sufficient to ground the reader in the nature of ACM matrix polymers.

ACM Structure Formation:

In addition to thermal stability, the polymers (and other materials) used  to produce an ACM matrix must have dimensional stability. That is, the structure should retain its designed shape and dimensions through any environmental changes. Thermoplastic matrix materials are generally not suitable for use in applications that call for strict dimensional stability or the retention of strength properties. Their ready deformability and changes of strength properties   effectively define the conditions under which thermoplastics can be used.  Thermoplastics can be melted or heat-formed into an ACM structure after the polymer has already been formed, or the structure can be formed by allowing a base resin to polymerize in place. This allows a wide variety of processes to be employed in the production of ACM structures using thermoplastics: spray forming, injection molding, and hot processing are all applicable methodologies. Many structures can also be 'laid up' by hand using fiber fabrics and liquid resins.

In contrast, ACM structures that make use of a thermosetting matrix polymer can only be made by allowing the base resin system to polymerize in place. Heat-based methods of forming the polymer to a shape can not be used on thermosets after they have polymerized. The use of these materials, though widespread, is therefore somewhat limited, and is restricted to labour-intensive hand lay-up techniques carried out by skilled, trained technicians. The base resin system of a thermoset polymer is generally a liquid, or can be made liquid, before polymerization occurs. and industrial processes have been developed in which the resin system is injected into a preformed fabric lay-up mold.

The polymerization or 'curing' process itself requires a certain amount of time to proceed. This length of time can be 'tailored' to produce a specific result or property in the finished matrix by controlling the rate at which the polymerization reaction occurs. It is during the curing stage or 'cure cycle' that the structure attains its final shape, as the liquid system conforms to a mold surface and sets into solid form. Thus, the vast majority of ACM applications using a thermosetting matrix employ procedures that provide the greatest amount of control over the shape and the curing process. In a typical lay-up procedure, the trained technician arranges several layers of specific fiber fabrics into a mold, as specified in the particular engineering documentation, and then applies the prepared resin. The entire 'stack' is then enclosed and consolidated into a more compact mass by pressure. Small lay-ups are generally sealed under a sheet of plastic and connected to a vacuum pump to remove unwanted gases and to squeeze the stack together using the force of atmospheric pressure. Large lay-ups are prepared in a similar manner, but may be processed in an autoclave to make use  of higher pressures to consolidate the layers of fiber and resin in the curing process.

Consolidation of the stack occurs as the external pressure squeezes the fibers together within the liquid matrix resin. This also acts to distribute the resin evenly throughout the fibers. Heat is used to facilitate the process, first by causing the resin to flow more easily through the fiber mass, allowing excess resin to be expressed, and secondly, by determining the rate of polymerization and driving off unwanted by-products as gases. When the curing process is completed, the resin will have set up into a tough, solid polymer matrix, in which the fiber mass has become securely and permanently bound. The 'bag layers' are stripped off, the piece is removed from the mold, and it is then put through any trimming or finishing processes that may be required.

Additional Structural Materials

In many cases, the lay-up may be prepared using fiber fabrics that have been pre-impregnated with a resin system. Such 'pre-preg' fabrics save a great deal of extra labour during the preparation of a lay-up and produce a very consistent matrix in the finished structure. But they also require some specialized materials handling, such as freezer storage.

Other materials may also be used in the preparation of an ACM structure by these procedures. An added requirement as placed on the matrix material of large. thin structures that must exhibit both rigidity and low weight. These are often prepared by including special filler materials or "cores" in the lay-up stack. Balsa wood is used for certain core applications, typically to meet a requirement to fill space while adding as little excess weight as possible. The most commonly used core materials, however, are specially manufactured "honeycomb" fillers. Due to their honeycomb structures, these cores are exceptionally resistant to flexing or twisting and to compression that might otherwise deform the panel. Such core materials are included in the fiber stack, but they are not part of it. They act rather to extend some strength properties of the matrix into the fiber stack.

Core materials, fiber fabrics, and matrix materials together make up a finished ACM structure. Each structure has its own specific properties provided by the combination of these components. At the same time, each particular combination of components has its own unique characteristics that can be used in the design of new applications of ACMs. It is a kind of 'vicious cycle' of development and discovery in which the possibilities are endless, and each one adds a whole new dimension to the field of advanced composite materials.

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For further information you may view the following useful links:

Renaissance Aeronautics Associates Inc.
www.raacomposites.com

SeaWind Aircraft Company
www.seawindsna.com