12 Polymer composites
When looking at Wikipedia we find the following description for composites:
Composite materials (also called composition materials or shortened to composites) are materials made from two or more constituent materials with significantly different physical or chemical properties, that when combined, produce a material with characteristics different from the individual components. The individual components remain separate and distinct within the finished structure.
The new material may be preferred for many reasons: common examples include materials which are stronger, lighter or less expensive when compared to traditional materials.
Analyzing this definition more closely we can highlight certain keywords which are typical for all composites:
made/engineered
two or more constituents
different physical or chemical properties
a material with different characteristics from the individual components
the individual components remain separate and distinct.
Typical engineered composite materials are cements, concrete, reinforced plastics, metal and ceramic composites. Applications range from building construction to high tech. A nice illustration is the use of composites in the new Boeing 787 Dreamliner (see Figure 12.1)
Figure 12.1:Illustration of the use of composites in the Boeing 787 Dreamliner.
12.1Composition of a composite¶
Considering a two component composite one can draw a very naive picture, see Figure 12.2.
Figure 12.2:Naive picture of a two component composite. Light blue is the matrix, dark green is the dispersed phase and light green is the interphase.
Three clear distinct phases result:
matrix/continuous phase (light blue)
dispersed phase (dark green)
interphase (light green)
The matrix or continuous phase contains the dispersed phase. The dispersed phase is distributed in the continuous phase in a discrete manner and in most cases is a stronger phase when compared to the continuous phase that is why it sometimes is also referred to as the strengthening phase. The last phase is the interphase and is the zone in between the matrix and the dispersed phase.
12.1.0.1Matrix phase¶
This phase in general consists of a polymer, metal, or ceramic and is continuous. This layer is responsible for the load transfer in the complete composite. Preferably the matrix material is a though material, this to prevent localization and as a result matrix failure. Sometimes a matrix material is added to protect the fragile dispersed phase against damage.
12.1.0.2Dispersed phase¶
The dispersed phase comes in a huge variety of shapes and materials. Some examples used in polymer composites are glass and carbon fibers. Also super polymer fibers are used as a dispersed phase, e.g., UHMWPE (Dynema®) or Aramid (Kevlar®, Twaron®), resulting in an all polymer composite.
Looking at the microstructure different influences need to be taken into account like concentration, size, shape distribution and orientation, see Figure 12.3 and Figure 12.4.
Figure 12.3:Influence of (a) concentration, (b) size, (c) shape, (d) distribution, and (e) orientation.
Figure 12.4:Influence of (a) particles and random, (b) discontinuous fibers unidirectional, (c) discontinuous fibers random, and (d) continuous fibers unidirectional.
12.1.0.3Interphase¶
The interaction between the phases is in the interphase and is either of chemical, physical or mechanical nature, or a combination of these. This phase is basically responsible for proper stress transfer between the matrix and dispersed phase. Sometimes the surface of the dispersed phase is mechanically and/or chemically modified to improve adhesion and as a result a better interphase is created.
12.1.1Classification based on matrix material¶
Composites can be classified based on different properties, most frequently they are classified based om their matrix material, see Figure 12.5.
Figure 12.5:Classification based om matrix material.
For polymer composites three material classes are identified, i.e. thermosets, thermoplasts, and rubbers. Polymer composites are an interesting class of composites mainly because of their high modulus-weight and strength-weight ratios. Thermoplasts are used in e.g., paints, crash cones of F1-cars, airplane parts, thermoplasts are used in e.g., plastic cups, and the main application area of rubber composites is tires.
A new area which should be added to the polymer classification are bio-polymer composites. In general you will not find this class of materials in textbooks about composites.
In the lecture we will show two techniques to model polymer composites, one based on the methods described in Micro-macro coupling and Structure quantification using the micro-macro coupling, using tires as an example, the other using a homogenization technique. The latter makes use of ingredients presented in Hyperelasticity, Viscoplasticity, and Structure quantification and has as applications coatings and cheese (bio-polymer composite).