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Introduction

1.1 Outline on Wave-Transparent Composites

Wave-transparent composites are a class of functional composites that can pass through electromagnetic waves. On the one hand, wave-transparent composites can provide electromagnetic windows for the transmission and reception of electromagnetic waves to ensure their efficient operation [1]. On the other hand, they can protect the radar antennas, communication, and microwave systems from the harsh external environment such as heavy rain, strong winds, snow, sand, solar radiation, and salt spray [2], ensuring the stability and reliability of electromagnetic wave transmission. With the rapid development of modern electronic information technology as well as the aviation and aerospace industries, the requirements for comprehensive performance of wave-transparent composites are becoming more and more demanding [3].

As far as matrix classification, wave-transparent composites can be divided into ceramic-based and polymer matrix wave-transparent composites [4]. Ceramic-based wave-transparent composites can meet the electrical performance requirements of radar radomes in the centimeter-band electromagnetic wave range. However, for millimeter-band electromagnetic waves (wavelength in the range of 1–10 mm and frequency in the range of 30–300 GHz), ceramic-based wave-transparent composites have disadvantages such as low strength, thick cover walls, and poor wave-transparent performances, which make it difficult to meet the performance requirements of radar radomes for millimeter wave [5, 6].

Polymer matrix wave-transparent composites have the advantages of lightweight, high strength, low dielectric constant (ε) and dielectric loss (tan δ), and materials/structure/function integration, which have a wide range of promising applications in satellite antennas, aircraft, missiles, 5G ground communication base stations, printed circuit boards, and so on. (Figure 1.1) [7].

This book will describe the wave-transparent mechanism, polymer matrix and reinforced fibers, their two-phase interfaces, molding process, and application prospects of the polymer matrix wave-transparent composites.

Figure 1.1 Application examples of polymer matrix wave-transparent composites.

Source: Polymer matrix wave-transparent composites: A review. Journal of Materials Science & Technology, 2021, 75: 225–251 (Figure 1).

1.2 Composition of Polymer Matrix Wave-Transparent Composites

Polymer matrix wave-transparent composites consist of polymer matrix, reinforced fibers, and two-phase interfaces [8]. Polymers with low ε and tan δ values as the matrix fibers with high strength and modulus as reinforced fibers produce advanced polymer-based composites (Figure 1.2) with both mechanical properties and wave-transparent performances via hot pressing, vacuum bagging, or resin transfer molding [9].

The heat resistance of polymer matrix determines the thermal stability of the composites in this case, and the fibers mainly serve as reinforcement [10]. Because the dielectric properties of different polymer matrices differ substantially. However, the ε value of reinforced fibers is generally larger than that of polymer matrix. Therefore, the selectively reinforced fibers possess excellent mechanical and thermal properties but also wonderful dielectric properties [11].

Figure 1.2 Composition of polymer matrix wave-transparent composites (commonly used polymer matrix and reinforced fibers).

1.2.1 Polymer Matrix

Polymers commonly used in wave-transparent composites mainly include epoxy resins [12], phenolic (PF) resins [13], polyimide (PI) resins [14], bismaleimide (BMI) resins [15], silicone resins, polytetrafluoroethylene (PTFE) resins [16], unsaturated polyester (UP) resins [17–19], and cyanate (CE) resins [20]. Table 1.1 shows the main physical and chemical properties of the common polymer matrix.

Epoxy resins have good flowability, low curing shrinkage, and high thermal decomposition temperatures (300–350 °C), but their high ε and tan δ values limit their application in high-performance polymer matrix wave-transparent composites [21–23]. PF resins have good heat resistance (long-term service temperature at 250 °C), mechanical properties, and weatherability [24]. However, the ε values of PF resins increase significantly with increasing temperatures [25–27]. PI resins have high heat resistance (Tg ≥ 250 °C), ε, and tan δ values that remain stable over a wide range of temperatures and frequencies [28]. At the same time, PI resins have excellent mechanical properties, chemical resistance, and dimensional stability [29–31]. However, PI resins are costly and difficult to process [32, 33]. BMI resins are an ideal polymer matrix for advanced composites due to their good heat resistance, excellent mechanical properties, relatively low ε value, resistance to humidity, chemical reagents, and good processability [34, 35]. However, the relatively high tan δ values of BMI resins limit their wider application to a certain extent [36–38]. Silicone resins have excellent heat resistance and stable ε and tan δ values under a wide range of environmental conditions [39–41], but their poor mechanical strength makes them rarely used alone [42–44]. PTFE resins have the lowest ε and tan δ [45, 46] but are not easy to process and have low bonding properties between PTFE matrix and reinforcements [47–49]. UP resins have better mechanical properties than PF resins and have low ε and tan δ values [50–52], which can be cured at room temperature. UP resins have a simple molding process, making them suitable for large-scale or large radome production [53–55]. However, UP resins have a short storage period, relatively low heat deflection temperature, and large curing shrinkage, which makes them unsuitable for the preparation of polymer matrix wave-transparent composites with high dimensional accuracy requirements [56–58].

Table 1.1 Main physical and chemical properties of the common polymer matrix.

In comparison, CE resins combine the high-temperature resistance of BMI and PI resins with the good processing properties of epoxy resins [59–61]. The highly symmetrical triazine ring structure and low polarity of the cured CE resins also make them low ε (2.8–3.2) [62–64], good heat resistance, and dimensional stability over a wide temperature and frequency range [65]. The structure and properties of commonly used polymer matrix are described in detail in Chapter 3.

1.2.2 Reinforced Fibers

Reinforced fibers for polymer matrix wave-transparent composites mainly include glass fibers [66, 67], quartz fibers [68], Kevlar fibers [69, 70], ultra-high-molecular-weight polyethylene (UHMWPE) fibers [71, 72], and poly(p-phenylene-2,6-benzobisoxazole) (PBO) fibers [73, 74]. Their main physical and chemical properties are shown in Table 1.2.

Table 1.2 Main physical and chemical properties of common reinforced fibers.