Fiber-reinforced composite materials, which combine the properties of fibers and a polymer matrix, have been around for decades. Fiber-matrix composites are stiffer, have a great strength-to-weight performance and have a much lower density than their metal counter parts. This makes them up to 60% lighter than, for example, steel; a very desirable characteristic when it comes to components for the mobility sector and in particular the automotive industry, where the reduction of weight is important to improve fuel efficiency or extend the range of electric cars. Another advantage that makes fiber matrix composites very interesting for the automotive industry is their resistance to corrosion.
Thermoplastic matrix composites reinforced with glass fibers have a higher density and a lower modulus than carbon fiber-reinforced composites but come at a much lower cost, which is an important factor for the automotive industry. Polypropylene (PP) neat material, but also with short and continuous fiber reinforcement is widely used for automotive parts due to its outstanding mechanical properties, moldability and low cost. Applications are, i.e., cases and trays, bumpers, fender liners, interior trim, instrumental panels and door trims. Other positive characteristics of PP are high chemical resistance, good weatherability, processability and impact/stiffness balance, which explains why it is one of the most widely used polymers on the market.
Quasi-isotropic and anisotropic composites
There are different ways to incorporate the fiber into the thermoplastic matrix ‒ randomly oriented fibers, unidirectional continuous fibers or multi-directional fabric, see Figure 1. The orientation of the added fibers plays an important part when it comes to part properties. While randomly oriented fibers increase the strength and stiffness compared to the neat polymer to some extent, the addition of oriented fibers in a preferential direction significantly increases the performance in this part direction. This preferential orientation gives the composite anisotropic properties, i.e. the properties in fiber orientation are dominated by the fiber properties and perpendicular to it, the matrix properties are more pronounced. Knowledge of this anisotropic behavior is required for the design and production of these composite components. Although the anisotropy of the mechanical properties is the first thing on everyone’s mind, the material’s expansion behavior also differs depending on the fiber direction.
When the anisotropy of a material is overlooked, or is not known, it can cause major problems in the final product. For example, plane surfaces can buckle, or even worse, form cracks or break.
Thermomechanical Analysis – a method to determine anisotropy in composites
Using the method of Thermomechanical Analysis (TMA), dimensional changes and therefore the CTE of fiber-reinforced polymers, can be determined in different material directions. For this study, samples were prepared at Neue Materialien Bayreuth. Three layers of a PP-GF UD tape were stacked on top of each other and pre-consolidated in a double belt press in three heating zones from 180-190°C. The blank was then preheated in a convection oven for 10 min and transferred in a hot press with a mold temperature of 80°C. There, a pressure of 10 bar was applied for 5 min during solidification. The resulting thickness is 1 mm. While the tape has an average fiber volume content of 45 vol%, the local variations in the plate were measured between 40-50 vol% GF.
For the TMA measurements at NETZSCH Analyzing & Testing, samples of 25 x 5 mm were cut from the plate in two different directions: 0° in the fiber direction and 90° to the fiber direction.
The samples were measured with the new TMA 402 F3 Hyperion® Polymer Edition. After an initial cooling step, the temperature was increased from -70 to 140°C at a heating rate of 5 K/min. The thermal expansion coefficient was calculated using the mean CTE analysis (m. CTE), which computes the slope between two data points. All measurement conditions are summarized in the following table:
Table 1: Measurement conditions
|Sample holder||Expansion, made of SiO2|
|Sample load||50 mN|
|Gas flow rate||50 ml/min|
|Temperature range||-70…300°C at a heating rate of 5 K/min|
Example: Anisotropy in PP-GF-UD
This material exhibits different CTEs depending on the direction the material is measured. The CTE of these kinds of composites is a mixture between the matrix and the fiber contained in it. Therefore, the CTE of those materials differ considerably depending on direction. The measurement results of the CTE for the PP-GF in the two different fiber directions are shown in the plot below. The red curve depicts the measurement in the fiber direction 0°. The low CTE value is in the range of the CTE of glass and shows that this measurement direction is dominated by the low thermal expansion of the glass fibers. The same material measured 90° to the fiber direction (black curve), is dominated by the polypropylene matrix. It shows a much higher CTE and exhibits the known glass transition (Tg) of polypropylene at -7°C, not observable in the red curve.
In the matrix, the dominated direction of the CTE of a composite follows the rule of mixture:
Where α is the linear thermal expansion coefficient (CTE), v is the volume fraction and the indices f and m denote the fibers and matrix, respectively. Assuming that the measured CTE in 0° fiber direction is the same as αf and the CTE of the polypropylene matrix, αm= 1.6×10-4K-1 (not measured here), the glass fiber volume fraction in the measured composite is calculated as
The study showed the importance of analyzing the coefficient of thermal expansion of high- performance composite materials based on the fiber direction.
If you are interested to find out more about Thermomechanical Analysis and its areas of application, visit www.netzsch.com/tmapolymeredition
About Neue Materialien Bayreuth GmbH
Neue Materialien Bayreuth GmbH is a non-academic research company developing various novel materials for lightweight constructions, from polymers and fiber-reinforced composites to metals, including also the processing. They provide application-oriented solutions by optimizing available materials and production processes.
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