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THERMOPLASTICS AND THERMOPLASTIC COMPOSITES
By Michel Biron
William Andrew
Copyright © 2013 Michel Biron and Odile Marichal
All right reserved.
ISBN: 978-1-4557-3035-3
Chapter One
Outline of the Actual Situation of Plastics Compared to Conventional Materials
No engineer or designer can be ignorant of plastics, but the decision to use a new material is difficult and important. It has both technical and economical consequences. It is essential to consider:
• the actual penetration of the material category in the industrial area
• the abundance or scarcity of the material and the process targeted
• the functionalities of the device to be designed
• the characteristics of the competing materials
• the cost
• the processing possibilities
• the environmental constraints.
The goal of the facts and figures that follow is to help clarify quickly the real applications for thermoplastics and thermoplastic composites and the relative importance of the various material families and processes involved.
1.1 Polymers: The Industrial and Economic Reality Compared to Traditional Materials
1.1.1 Plastic and Metal Consumption
Usually, material consumption is considered in terms of weight (Table 1.1), but it is also interesting to examine:
• the consumption or production in terms of volume (Table 1.2), which is the most important for fixed part sizes
• the consumption linked to the rigidity of the engineering materials (Table 1.3).
In this last case, if the reference material, of unitary section area and unitary length, is M0 (volume V0 = 1) with Young's modulus E0, it can be replaced with material M1 with unitary length, section area S1, and Young's modulus E1. For the same tensile stress:
S1*E1 = 1*E0
So:
S1 = E0/E1
The volume of M1 with the same rigidity as M0 is:
V1 = S1*1 = V0*E0/E1
Therefore,
V1*E1 = V0*E0
Table 1.3 compares the rigidity-modified data for consumption expressed as volume (million m3) x Young's modulus (GPa). The elastic tensile modulus is arbitrarily fixed at 2 for plastics, 200 for steel, and 75 for aluminum.
The annual consumption of plastics is:
• intermediate between those of steel and aluminum in terms of weight, that is, roughly a fifth of the consumption of steel and more than 10 times the consumption of aluminum for recent years
• higher than those of steel and aluminum in terms of volume in recent years: roughly 1.4 times the consumption of steel and about 30 times that of aluminum
• lower than those of steel and aluminum if we reason in terms of equal rigidity: plastic consumption is equivalent to roughly 1% of the steel consumption and half that of aluminum.
• Affected by the global economic crisis in 2008/ 2009.
The average annual growth rate over the past 40 years is:
• 5.4% for plastics
• 2.2% for steel.
• 2.2% for aluminum.
Over the 25 years from 1985 to 2010, the average annual growth rates are confirmed for plastics and steel (Table 1.4). Polymer composites also show a progression exceeding that of metals. For the period 2005–2010, plastics slow down slightly and steel slightly accelerates but the gap between annual growth rates is not significantly affected.
Figure 1.1 displays these normalized changes in world consumption.
1.1.2 Mechanical Properties
1.1.2.1 Intrinsic Mechanical Properties
Expressed as Vickers hardness, the hardnesses of the engineering materials cover a vast range, broader than 1 to 100. The handful of example figures in Table 1.5 does not cover the hardnesses of rubbers, alveolar polymers, and flexible thermoplastics.
Figure 1.2 visualizes the hardnesses of a broad range of materials.
Table 1.6 indicates the tensile characteristics of some traditional materials (metals, glass, wood) and polymers in various forms:
• unidirectional (UD) composites, highly anisotropic
• sheet molding compound (SMC), two-dimensional (2D) quasi-isotropic
• long fiber-reinforced thermoplastics (LFRT), more or less quasi-isotropic
• short fiber-reinforced plastics, 3D isotropic
• neat polymers, 3D isotropic
• alveolar polymers.
The indicated figures are examples and do not constitute exhaustive ranges.
Figures 1.3 and 1.4 and Tables 1.6 and 1.7 show that:
• UD composites in the fiber direction can compete with existing metals and alloys. However, it is necessary to moderate this good classification by taking into account the high anisotropy of these composites, with low resistance and modulus in the direction perpendicular to the fibers,
• the highest performance engineering plastics compete with magnesium and aluminum alloys.
1.1.2.2 Specific Mechanical Properties
The specific mechanical properties take account of the density and consider the performance to density ratio (performance/density).
Due to the high densities of metals, the resulting classification (Table 1.7 and Figures 1.5 and 1.6) is different from that of the mechanical properties alone.
Figures 1.5 and 1.6 and Table 1.7 show that:
• UD composites in the fiber direction can compete with existing metals and alloys and some have the highest performances. However, it is necessary to moderate this good classification by taking account of their high anisotropy, with low resistance and modulus in the direction perpendicular to the fibers,
• the best of the other engineering plastics cannot match the high performance of the magnesium and aluminum alloys in terms of rigidity.
1.1.3 Thermal and Electrical Properties
Metals are characterized by their low coefficients of thermal expansion and their strong thermal and electric conductivities, whereas wood (except where there is excessive moisture) and neat polymers have high coefficients of thermal expansion and are electrical and thermal insulators.
The loading or reinforcement of the polymers changes these characteristics:
• the coefficients of thermal expansion decrease
• carbon fibers (CFs), steel fibers, and carbon blacks lead to more or less conducting polymer grades.
Table 1.8 displays some thermal and electrical characteristics of polymers and conventional materials.
1.1.4 Durability
Metals and glass generally support higher temperatures than polymers, which present a more or less plastic behavior under stresses, leading to:
• an instant reduction of the modulus and ultimate strength
• a long-term creep or relaxation.
Polymers are sensitive to thermo-oxidation and, for some, to moisture degradation. Provided they are not subject to moisture degradation, polymers, unlike current steels, are not sensitive to corrosion.
The thermal behavior of the polymers can be characterized:
• immediately, by the heat deflection temperature under a 1.8-MPa load. For the chosen examples (Table 1.9), the values vary between 150 °C and 320 °C;
• in the long term, by the continuous use temperature in an unstressed state. For the examples chosen, the values vary from 130 °C to 320 °C.
Table 1.9 displays some thermal characteristics of polymer and conventional materials. Metals have minimum melting points higher than 400 °C and often higher than 1000 °C, whereas:
• thermoplastics melt in the range of 120 °C for polyethylene to 350 °C for high-performance thermoplastics
• thermosets, because of the cross-linking, cannot melt but decompose without melting as the temperature increases.
Polymers are sensitive to a greater or lesser degree to photodegradation, which can limit their exterior uses. On the other hand, many polymers, including the commodities, are resistant to the chemicals usually met in industry or at home and displace the metals previously used for these applications: galvanized or cast iron and steel for domestic implements, gas and water pipes, factory chimneys, containers for acids, and other chemicals.
Polymers, like other materials, are sensitive to fatigue. Figure 1.7 plots some examples of fatigue test results according to the logarithm of the number of cycles leading to failure.
To compensate for their handicaps in terms of properties compared to the traditional materials, polymers have effective weapons:
• manufacturing in small quantities or large series of parts of all shapes and all sizes, integrating multiple functions, which is unfeasible with metals or wood
• possibility of selective reinforcement in the direction of the stresses
• weight savings, lightening of structures, miniaturization
• reduction of the costs of finishing, construction, assembling, and handling
• aesthetics, the possibilities of bulk coloring or in-mold decoration to take the aspect of wood, metal, or stone, which removes or reduces the finishing operations
• durability, absence of rust and corrosion (but beware of aging), reduction of maintenance operations
• transparency, insulation, and other properties inaccessible for the metals.
1.1.5 Material Costs
Obtaining information on prices is difficult and costs are continually fluctuating. The figures in the following tables and graphs are only orders of magnitude used simply to give some idea of the costs. They cannot be retained for final choices of solutions or estimated calculations of cost price. Usually, the material costs are considered versus weight but it is also interesting to examine:
• the cost per volume, which is the most important for a fixed part size
• the cost linked to the rigidity for the engineering materials.
In the following 1 Euro = 1.3 US$
1.1.5.1 Cost Per Weight of Various Materials
Table 1.10 and the graph in Figure 1.8 demonstrate that plastics and polymer composites are much more expensive than metals, even more specialized ones such as nickel.
1.1.5.2 Cost Per Volume of Various Materials
As for the specific mechanical properties, the high densities of metals modify the classification (Table 1.11 and Figure 1.9) of the various materials.
According to the cost per volume:
• plastics are competitive: only the very high-performance plastics or composites are more expensive than metals
• wood is the cheapest material.
1.1.5.3 (Performance/Cost Per Liter) Ratios of Various Materials
Table 1.12 and Figures 1.10 and 1.11 confirm that polymer composites are more expensive than metals for the same mechanical performances. It is necessary to exploit their other properties to justify their use.
1.2 What Are Thermoplastics, Thermoplastic Elastomer, Thermosets, Composites, and Hybrids?
1.2.1 Thermoplastics
Thermoplastics have the simplest molecular structure, with chemically independent macromolecules (Figure 1.12). By heating, they are softened or melted, then shaped, formed, welded, and solidified when cooled. Multiple cycles of heating and cooling can be repeated without severe damage, allowing reprocessing and recycling.
Often some additives or fillers are added to the thermoplastic to improve specific properties such as thermal or chemical stability, ultra violet resistance, etc.
Composites are obtained by using short, long, or continuous fibers.
Thermoplastic consumption is roughly 80% or more of the total plastic consumption.
Alloys of compatible thermoplastics allow applications to benefit from the attractive properties of each polymer while masking their defects.
Some thermoplastics are cross-linkable and are used industrially in their two forms, thermoplastic and thermoset; for example, the polyethylenes or the vinyl acetate—ethylene (VAE) copolymers (the links created between the chains limit their mobility and possibilities of relative displacement).
Advantages
• The softening or melting by heating allows welding and thermoforming.
• The processing cycles are very short because of the absence of the chemical reaction of cross-linking.
• Processing is easier to monitor because there is only a physical transformation.
• Thermoplastics do not release gases or water vapor if they are correctly dried before processing.
• The wastes are partially reusable as virgin matter because of the reversibility of the physical softening or melting.
(Continues...)
Excerpted from THERMOPLASTICS AND THERMOPLASTIC COMPOSITES by Michel Biron Copyright © 2013 by Michel Biron and Odile Marichal. Excerpted by permission of William Andrew. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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