Composite materials – is the use of proper definitions important?

Due to the unprecedented progress of science and technology during the last 50 years we observe nowadays a huge variety of materials with new properties. Nano-materials alone, with their unique properties, provide a good example. At the same time, the importance of the formulation of the respective proper terms, definitions and nomenclatures seems to be underestimated.

A good example in this respect could be the incorrect practice in the composite community of using the term “phase” instead of “component” when dealing with composite materials. As has been previously discussed [1][2] and [3], the term “phase” is very well defined in thermodynamics and is frequently used in polymer physics for describing the various phases in one-component systems (the term “component” is also strictly defined in thermodynamics as a chemically-independent constituent of a system). A good illustration in this respect is the polymer poly(vinylidene fluoride) (PVDF), exhibiting five crystalline polymorphic modifications (phases) and one amorphous phase; but PVDF is still a one-component system. The use of the term “phase” instead of “component” would require the definition of another term for describing the phases in the sense of thermodynamics. Obviously, the misuse of the term “phase” is a remnant from the time when the colloid chemistry was formulated (second half of 19th century, i.e. long before the polymer science was defined). Since the polymer solutions, even true ones, behave as colloid systems of low-molecular-weight substances, they were called “lyophilic colloids”; and the terminology applied to them was typical for colloid chemistry – “dispersed phase” [3]!

Another example demonstrating the negative effect of incorrect definition of new materials on their development is the case of single polymer composites. The concept of single polymer composites was formulated some 40 years ago by Roger Porter and demonstrated in his publication with N. Capiati [4]. They used two types of sample of high density polyethylene (HDPE) differing mostly in their melting temperatures. During the last two decades, the interest from academia and industry in this new material, which they called a “one polymer composite” [4], increased immensely due to the steady increasing adverse environmental impact of synthetic, petroleum-based polymers and their glass fiber reinforced composites.

It should be noted that the designation of single polymer composites (SPCs), as they are referred to in this article, has been an issue of some debate. SPCs have also been called:one polymer compositeshomocompositesself-reinforced compositesone-phase compositeshomogeneous composites, or all-polymer composites [1][2][5][6],[7] and [8]. Of course, the terminology is less important as long as it is used consistently.Composite material typically means a (hopefully) synergistic combination of twochemically different materials; often a polymer resin combined with mineral or natural fibers. Polymer–polymer composites are then such composites whose reinforcementand matrix belong to two chemically different materials. At the same time, the reinforcement and matrix in SPCs are chemically identical; indeed they come from the same original starting material.

What about SPCs, which according to their definition are one-component systems, but comprise two different (but chemically identical) materials as matrix and reinforcement? For such cases it was suggested [1] to use the term “constituents”, which will mean chemically identical materials but with differences in some properties, such as melting temperature, physical structure (polymorphic modifications), mechanical properties, and others [1]. Accordingly, in the multi-component composite systems each component has different chemical composition.

The polymer–polymer composites (PPCs) as well as the single polymer composites are distinguished by another characteristic feature, namely the fact that the matrix is always an isotropic material. In dealing with traditional composites this detail is usually not stressed, possibly because such a large class of composites (as the glass fibers reinforced thermosets) are always characterized by an inherent isotropic matrix as well as the injection molded glass fiber reinforced thermoplastics, while the situation with PPCs and SPCs is completely different. The starting material for their manufacturing is always a highly oriented polymer (for SPCs) or polymer blend (for PPCs) and a processing step is needed when at higher temperature one of the constituent (or blend component, respectively) is converted from highly oriented into an isotropic state. In this way the highly oriented material is converted in a typical composite, i.e. an isotropic matrix reinforced with stronger fibrous material. Without this isotropization step we are supposed to consider the highly oriented polymer blends (e.g. textile yarn spun from polymer blends) as a composite material, which could hardly be correct (or if so, which of the drawn components represents the matrix?).

The above definitions have been suggested by Karger-Kocsis and Fakirov [1] and they will be followed strictly in the current article. This means that the commercially available and widely cited [5][6][7] and [8] as SPC type of materials based on homopolymeric polypropylene (PP) (as reinforcement) and the random copolymer of PP (usually with polyethylene (PE)) (as a matrix) do not belong to SPCs because the two components are chemically different, i.e. such composites belong to the category of polymer–polymer composites. The same holds for thermoplastic polyester copolymers reinforced with poly(ethylene terephthalate) (PET) or with liquid crystalline polyester (LCP). Details about these PPCs can be found in the review by Matabola et al. [5] as well as in a recent review of Karger Kocsis et al. [8].

Serious contributions to the development and commercialization of this special type of polymer–polymer composite (they comprise two chemically different polymers, which belong to the same polymer family as stressed by Karger Kocsis [8]) have been made by Peijs and co-workers (e.g. [9]). It seems important to note that their commercial importance would hardly be negatively affected if they will be properly classified as polymer–polymer but not as single polymer composites. The argument that the two chemically different components belong to the same polymer family [8] should not be seriously considered – it blurs the boundary between the two types of composites, the PPCs [11][12][13] and [14] and the SPCs [1][2][3][4][5][7][8] and [10] and makes a clear definition of single polymer composites impossible.

Finally, in order to be able to make use of the unique properties of single polymer composites we should not forget that they are one-component systems where the matrix and the reinforcement are of the same chemical composition. They should not be mixed up with the closely related polymer–polymer composites where the matrix and the reinforcement have different chemical composition. As a matter of fact, the single polymer composites belong to the larger group of one-component (but not: one-phase)composites, e.g. carbon–carbon composites, composites based on polymorphic crystalline or crystalline-amorphous forms of the same metal.

Acknowledgement

The author would like to thank the Foundation for Research Science and Technology of New Zealand for the financial support (Grant No. UOAX 0406).

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