Microstructure and Properties of Glass Fiber-Reinforced Polyamide/Nylon Microcellular Foamed Composites

Article Microstructure and Properties of Glass Fiber-Reinforced Polyamide/Nylon Microcellular Foamed Composites

Xiulei Wang 1 , Gaojian Wu 1, Pengcheng Xie 1,2,*, Xiaodong Gao 1 and Weimin Yang 1,2 1 College of Mechanical and Electrical Engineering, Beijing University of Chemical Technology, Beijing 100029, China; [email protected] (X.W.); [email protected] (G.W.); [email protected] (X.G.); [email protected] (W.Y.) 2 State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China * Correspondence: [email protected]

Received: 30 September 2020; Accepted: 13 October 2020; Published: 15 October 2020

Abstract: The automobile and aerospace industries require lightweight and high-strength structural parts. Nylon-based microcellular foamed composites have the characteristics of high strength and the advantages of being lightweight as well as having a low production cost and high product dimensional accuracy. In this work, the glass fiber-reinforced nylon foams were prepared through microcellular injection molding with supercritical fluid as the blowing agent. The tensile strength and weight loss ratio of microcellular foaming composites with various injection rates, temperatures, and volumes were investigated through orthogonal experiments. Moreover, the correlations between dielectric constant and injection volume were also studied. The results showed that the “slow–fast” injection rate, increased temperature, and injection volume were beneficial to improving the tensile strength and strength/weight ratios. Meanwhile, the dielectric constant can be decreased by building the microcellular structure in nylon, which is associated with the weight loss ratio extent closely.

Keywords: lightweight; glass ﬁber reinforced nylon; supercritical ﬂuid; microcellular foaming; injection molding

1. Introduction In recent years, the microcellular foaming composites are increasingly recognized as an important component for improving the structure and properties of polymeric materials due to their remarkable effects such as lightweight [1–3] and low dielectric constant [4,5]. The microcellular foaming process can generate large numerous of micron-sized cells inside the polymer matrix, forming a unique dense surface and core foamed structure [6–8]. This structure could reduce the weight of the material, and it endows the material with higher toughness, impact strength, and dimensional accuracy [9–13], which is meaningful to the requirement of lightweight. Nylon (PA)-based composites are an important engineering material with excellent mechanical properties, thermal stability, and processability [14], particularly glass fiber-reinforced nylon (GF/PA). Adding the glass fiber into the PA matrix could achieve increased load-bearing capacity and mechanical properties because the fibers retard the rapid growth of cracks [15,16]. The interface of each fiber and the matrix can refine the crack and cause it to propagate in multiple directions, thus extending the propagation length. The fracture in the GF/PA changes from a single-matrix fracture to a combined of matrix fracture, fiber pullout, and fiber broken, which helps increase the mechanical toughness [17–19]. GF/PA microcellular foamed composites have the characteristics of high strength and the advantages of being lightweight as well as having a low production cost and high product dimensional accuracy, because the addition of fibers could

Polymers 2020, 12, 2368; doi:10.3390/polym12102368 www.mdpi.com/journal/polymers Polymers 2020, 12, 2368 2 of 11 promote nucleation, refining the cell structure [20]. Therefore, GF/PA has gradually expanded from the initial interior parts to structural parts such as engines, bodies, and bumpers for vehicle application. In addition, GF/PA microcellular foamed composite will also have a positive impact on the development of automobile and aerospace structural parts. At present, researchers have focused on studying PA-based foamed composites. Wang et al. [21] prepared GF/PA6 composites by chemical foaming injection molding and discussed the influence of foaming agent masterbatch on the cell morphology and mechanical properties. However, compared with the chemical foaming method, supercritical fluid foaming has huge potential in the field of lightweight due to the advantages of being environmentally friendly, controllable, and having no chemical residual process [22,23]. Wang et al. [24] and Hwang et al. [25] prepared PA6 micro-sized cells materials by the supercritical fluid physical foaming method and discussed the mechanical properties of the products. Valentina Volpe et al. [26] studied the effect of gas injection pressure and thickness of the parts on the mechanical properties of GF/PA 66 microcellular foams. As a result, a high gas injection pressure and a large thickness of the parts are adopted to obtain homogeneous foamed parts with good mechanical properties. Roch et al. [27] show the relationship between different density reductions and bending stiffness of GF/PA6 foams with breathing mold technology. All studies have shown that foaming can increase the specific strength of composites to a certain extent. However, there are few studies on the effects of critical injection process parameters on the properties of GF/PA microcellular foams based on the supercritical fluid physical foaming method. In this paper, for the first time, we comprehensively studied the microstructure and strength/weight loss rate of GF/PA 610 microcellular foams with various injection rates, temperatures, and injection volumes based on supercritical fluid microcellular foam injection molding. Especially, we analyzed the correlation between microstructure and foam strength in detail in the study. According to the conclusions of this paper, the mechanical properties of GF/PA microcellular foams are adjusted by refining the microstructure to fulfill product requirements and to be more lightweight. In addition, structural parts such as integrated circuit shells and aircraft fuselages must have low dielectric constant characteristics to avoid excessive current passing through composites and damaging internal products at the working surface [28]. Therefore, we have further studied the correlation between dielectric constant and weight loss rate of the foamed material. The work makes up for the deficiency of the current research data on the properties of GF/PA 610 microcellular foams and provides a useful reference for the comprehensive evaluation of GF/PA microcellular foams.

2. Experimental

2.1. Materials PA 610 (3RILSAN SMVO-F, Arkema (Suzhou) Polymer Materials Co., Ltd., Feng Huang Town, China) with the melt point of 222.5 ◦C and the relative viscosity of 2.56 was used in the experiment. GF/PA 610 composite material (SLL/G30) containing 30 wt % glass ﬁber was prepared by the Aerospace Research Institute of Materials and Processing Technology, Beijing, China. The supercritical nitrogen with 99.99% purity was purchased from Beijing Li Synthetic Gas Co., Ltd. (Beijing, China).

2.2. Experiment Procedure Figure1 illustrates the GF /PA microcellular foams molding process for this experiment.

2.3. Preparation of GF/PA Microcellular Foams A 160-ton modiﬁed injection molding machine (MA1600IIS, Haitian, China) equipped with a supercritical ﬂuid injection unit was used to prepare GF/PA microcellular foams. The gas compression and control equipment were supplied by Beijing Chn-top Moldplastech Co. Ltd. (Beijing, China). The PA pellets were dried for at least 6 h at 80 ◦C in the dehumidifying dryer to remove moisture before use. The microcellular foamed GF/PA 610 composites were manufactured based on the Polymers 2020, 12, 2368 3 of 11 orthogonal experimental method by controlling the injection volume, temperature, and injection rate, as shown in Table1. According to the ASTM D638-03 standard, the specimen for tensile strength test was prepared, and the 2 mm thickness disc samples were prepared for the characterization of dielectric properties. Among them, the trial with an injection rate of 100-100% (B5) was unable to prepare a completePolymers 2020 sample, 12, x FOR because PEER REVIEW the exorbitant pressure drop resulted in a large amount of gas escaping.3 of 11

Figure 1. The molding flow ﬂow chart of glass glass fiber ﬁber-reinforced‐reinforced nylon (GF/PA) (GF/PA) microcellular foams.

2.3. Preparation of GF/PA MicrocellularTable 1. FoamsThe experiment factors and levels. Factor A Factor B Factor C A 160‐ton Levelmodified injection molding machine (MA1600IIS, Haitian, China) equipped with a Injection Rate/% Temperature/ C Injection Volume/cm3 supercritical fluid injection unit was used to prepare◦ GF/PA microcellular foams. The gas compression and1 control equipment 20-20 were supplied 240 by Beijing Chn‐top 30.2 Moldplastech Co. Ltd. (Beijing, China). 2 50-50 250 32.7 3 80-80 260 35.2 The PA pellets4 were dried 50-80 for at least 6 h at 270 80 °C in the dehumidifying 37.7 dryer to remove moisture before use.5 The microcellular 100-100 foamed GF/PA 280 610 composites were 40.2 manufactured based on the orthogonalNotes: The injection experimental rate is divided method into two by stages, controlling for example, the “50-80”injection refers volume, to the first temperature, half of the volume and (0–50%) injection rate,being as shown injected atin 50% Table of maximum 1. According injection rate,to the while ASTM the second D638 half‐03 of thestandard, volume (50–100%) the specimen is injected for at 80%. tensile 3 strengthThe maximum test was injection prepared, rate is 148 and cm / s.the The temperature2 mm thickness refers to the disc maximum samples barrel temperature.were prepared The injection for the volume was determined by the actual screw injection distance, and the screw diameter of the injection molding characterizationmachine is 40 mm. of dielectric properties. Among them, the trial with an injection rate of 100‐100% (B5) was unable to prepare a complete sample because the exorbitant pressure drop resulted in a 2.4.large Characterization amount of gas escaping. Microstructural and macroscopic properties are important indicators to evaluate the quality of microcellular foam productsTable [6]. In 1. The this experiment work, a scanning factors and electron levels. microscope (Quanta FEG 650, FEI Company, Hillsboro, OR,Factor USA) A was used toFactor observe B the micro-sizedFactor cells C structure. The tensile Level strength of the standardInjection specimen Rate/% was measuredTemperature/°C using an Injection electronic Volume/cm universal3 testing machine (CMT5205, Meters1 Industrial20 Systems-20 Co., Ltd.,240 Suzhou, China). The30.2 weight of the sample was measured using an2 electronic50 balance-50 (ME3002, METTLER250 TOLEDO32.7 Instruments (Shanghai) Co., Ltd., Shanghai, China).3 The dielectric80-80 constant was260 evaluated in the X-band35.2 (8.2–12.4 GHz) using the rectangular waveguide method. The dielectric constant measurement and analysis instrument were 4 50-80 270 37.7 modified by the Aerospace Research Institute of Materials and Processing Technology, Beijing, China. 5 100-100 280 40.2 To compare the mechanical properties of the foamed GF/PA composite with the mechanical Notes: The injection rate is divided into two stages, for example, “50‐80” refers to the first half of the properties of un-foamed products, this paper defines the “tensile strength/weight ratio N” with volume (0–50%) being injected at 50% of maximum injection rate, while the second half of the reference to the concept of specific strength [29]. The N was used to analyze the tensile properties of volume (50–100%) is injected at 80%. The maximum injection rate is 148 cm3/s. The temperature refers to the maximum barrel temperature. The injection volume was determined by the actual screw injection distance, and the screw diameter of the injection molding machine is 40 mm.

2.4. Characterization Microstructural and macroscopic properties are important indicators to evaluate the quality of microcellular foam products [6]. In this work, a scanning electron microscope (Quanta FEG 650, FEI Company, Hillsboro, OR, USA) was used to observe the micro‐sized cells structure. The tensile strength of the standard specimen was measured using an electronic universal testing machine Polymers 2020, 12, 2368 4 of 11 the product, and it was calculated as shown in Equation (1). The larger the N value, the higher the tensile strength of the microcellular foam product with the same density or weight loss ratio.

1 [(σ0 σ) σ0] N = − − ÷ (1) 1 [(m m) m ] − 0 − ÷ 0 where “σ0” is the tensile strength of unfoamed products, “σ” is the tensile strength of foamed products, and “m0” and “m” are the quality of unfoamed products and foamed products. The ImageJ software was used to analyze the cell information of the microcellular foams based on SEM images. According to the number of closed cells counted in a unit area (1 cm2), we use the 3 following formula to calculate the cell density N0 (cells/cm )[8]:

3/2 N0 = (n/A) (1/∅) (2) × where “n” is the number of cells in a unit area (1 cm2), “A” is the average unit area (1 cm2) of the micrograph, and “∅” is the weight retention rate.

3. Results and Discussion

3.1. Cellular Morphology The cell structure is a key factor affecting the final properties of microcellular foamed composites [30]. Figure2 shows the comparison of SEM photos of GF /PA microcellular foam products with different injection rates, temperatures, and injection volumes. Glass fiber and micro-sized cells were uniformly distributed in foamed composite without obvious manufacturing defects, as shown in Figure2. The actual structure in the figure is similar to the theoretical deduction structure [ 22]; that is, the injection rate, temperature, and injection volume are key points that affect the cell structure. Therefore, the samples could be used to evaluate the influence of process parameters on product performance. The injection rate is closely related to the cell nucleation during the microcellular foam injection molding process. According to classical theory, the increased injection rate could enhance the melt pressure drop rate; thereby, more nucleation points were formed inside the matrix [31]. However, as shown in Figure2a,b, the high injection rate (A3) results in the formation of larger cells, smaller cell numbers, and irregular cell structure. The number of cells in the composite shown in Figure2a is about 5.35 107 cells/cm3, and the number of cells in the composite shown in Figure2b is × about 13.58 107 cells/cm3. Furthermore, the finer cell structure was achieved with the condition of the × lower injection rate (A4) in the initial stage, which is that the pressure in the mold cavity (outside the melt front) is close to atmospheric pressure, while the pressure inside the melt is the injection pressure in the initial injection stage. The cell wall rupture that was caused by the melt strength decreased as did the strain hardening [32] due to the extreme pressure difference and pressure drop between the inside and the outside of the melt front when the injection rate is extremely high in the initial stage. Then, the cells of the melt front will merge and collapse as the cell wall ruptures. However, decreasing the injection rate in the initial stage could reduce the pressure difference between the melt front and the cavity, avoid the melt strength decrease and strain hardening for the melt front; then, the finer cell structure was achieved. This paper has illustrated the correlation between the initial injection rate and the cell structure at the melt front, as shown in Figure3. The mold cavity pressure gradually increases during the filling process. The increased injection rate in the second stage could increase the rate of the melt pressure drop and cell nucleation, satisfying the classical nucleation theory. Therefore, the “slow–fast” injection rate could lead to a finer cell structure and avoid cells merging and collapsing when molding GF/PA foamed composites. According to the classical nucleation theory, the increased melt temperature was also beneficial to improve the cell nucleation rate and thus achieve a finer cell structure [33,34]. As shown in Figure2c,d, the cell structure was improved when the temperature was increased by 20 ◦C. The temperature Polymers 2020, 12, 2368 5 of 11 increased is conducive to the mixture of the supercritical fluid and the polymer melt, and it is conducive to filling the cavity with the GF/PA. The temperature increase could slow down the cooling rate of the melt and help more bubble nuclei overcome the energy barrier to grow up and form cells, therebyPolymers 2020 increasing, 12, x FOR the PEER number REVIEW of cells and reducing their size [35]. The number of cells in the composite5 of 11 shown in Figure2c is about 4.52 107 cells/cm3, and the number of cells in the composite shown in × Figure2d is about 17.20 107 cells/cm3. Therefore, the paper proves that the temperature increase stage could increase the ×rate of the melt pressure drop and cell nucleation, satisfying the classical withinnucleation the rangetheory. that Therefore, does not decomposethe “slow–fast” the polymer injection is rate also could beneficial lead to to refining a finer the cell cell structure structure and of theavoid GF cells/PA 610merging foams. and collapsing when molding GF/PA foamed composites.

Figure 2. MicrocellularMicrocellular structure structure of of GF/PA GF/PA foams foams at at different diﬀerent factor factor levels: levels: (a) (Thea) The injection injection rate rate is 80– is 80–80%80% in the in thecase case of A3, of A3, (b) (theb) the injection injection rate rate is 50–80% is 50–80% in the in the case case of A4, of A4, (c) (thec) the temperature temperature is 250 is 250 °C◦ inC 3 inthe the case case of ofB2, B2, (d ()d the) the temperature temperature is is 270 270 °C◦C in in the the case case of of B4, B4, ( (ee)) the the injection injection volume is 35.2 cm 3 in 3 the case of C3, and (f) the injection volume isis 40.2 cmcm3 in the case of C5.

The injection volume had a direct impact on the cell growth space, which is associated with the According to the classical nucleation theory, the increased melt temperature was also beneficial ﬁnal structure. As shown in Figure2e,f, the foams at small injection volume had a large amount of to improve the cell nucleation rate and thus achieve a finer cell structure [33,34]. As shown in Figure cell collapse and a greater number of cells. The number of cells in the composite shown in Figure2e 2c,d, the cell structure was improved when the temperature was increased by 20 °C. The temperature increased is conducive to the mixture of the supercritical fluid and the polymer melt, and it is conducive to filling the cavity with the GF/PA. The temperature increase could slow down the cooling rate of the melt and help more bubble nuclei overcome the energy barrier to grow up and form cells, thereby increasing the number of cells and reducing their size [35]. The number of cells in the composite shown in Figure 2c is about 4.52 × 107 cells/cm3, and the number of cells in the composite shown in Figure 2d is about 17.20 × 107 cells/cm3. Therefore, the paper proves that the Polymers 2020, 12, x FOR PEER REVIEW 6 of 11

temperature increase within the range that does not decompose the polymer is also beneficial to refining the cell structure of the GF/PA 610 foams.

PolymersThe2020 injection, 12, 2368 volume had a direct impact on the cell growth space, which is associated with6 of the 11 final structure. As shown in Figure 2e,f, the foams at small injection volume had a large amount of cell collapse and a greater number of cells. The number of cells in the composite shown in Figure 2e is about 6.63 6.63 × 10107 7cells/cmcells/cm3, 3and, and the the number number of ofcells cells in the in the composite composite shown shown in Figure in Figure 2f is2 fabout is about 2.78 × 2.78× 107 cells/cm107 cells3. /Ascm 3a. result, As a result, the injection the injection volume volume decreased decreased equal to equal the increased to the increased cell growth cell space growth in × spacethe same in the cavity same volume, cavity thus volume, resulting thus resultingin an increased in an increasedrisk of cell risk wall of rupture cell wall after rupture the increased after the increasedcells volume, cells the volume, thinner the cell thinner walls, cell and walls, the decreased and the decreased strength strengthof the cell of walls. the cell The walls. injection The injection volume volumecould be could adjusted be adjusted by the by pressure the pressure transfer transfer point point in the in the injection injection molding molding process. process. The The injection volume decrease hadhad anan adverseadverse eeffectﬀect onon thethe foamfoam performance.performance.

Figure 3. Schematic diagram of the correlation between the initial injection rate and the cell structure in the melt front.

3.2. Tensile Strength/Weight Ratios 3.2. Tensile Strength/Weight Ratios The tensile strength and weight loss ratio are important criteria for evaluating the macroscopic The tensile strength and weight loss ratio are important criteria for evaluating the macroscopic properties of microcellular foams. The tensile strength determines the use range of the GF/PA foams, properties of microcellular foams. The tensile strength determines the use range of the GF/PA foams, and the weight loss ratio reflects the lightweight effect of the GF/PA foams. More important, the tensile and the weight loss ratio reflects the lightweight effect of the GF/PA foams. More important, the strength/weight ratio comprehensively determines the degree of performance of the GF/PA foams with tensile strength/weight ratio comprehensively determines the degree of performance of the GF/PA the same density. foams with the same density. Figure4 shows the tensile strength of GF /PA microcellular composites with different injection Figure 4 shows the tensile strength of GF/PA microcellular composites with different injection rates, temperatures, and injection volumes. The tensile strength of foams shows an overall upward rates, temperatures, and injection volumes. The tensile strength of foams shows an overall upward trend as the injection rate increased. However, the high speed (A3: 80-80) in the entire stage led to trend as the injection rate increased. However, the high speed (A3: 80‐80) in the entire stage led to the decreased tensile strength, which was related to the escaping gas and cell collapse in the melt the decreased tensile strength, which was related to the escaping gas and cell collapse in the melt front for the excessive pressure difference in the initial stage. As a result, the tensile strength was front for the excessive pressure difference in the initial stage. As a result, the tensile strength was significantly improved without cavity pressure control after reducing the initial injection rate. The results significantly improved without cavity pressure control after reducing the initial injection rate. The prove that the “slow–fast” injection rate is beneficial to increase the tensile strength of the foamed results prove that the “slow–fast” injection rate is beneficial to increase the tensile strength of the composite. As shown in Figure4, the tensile strength has a linear positive correlation with the injection foamed composite. As shown in Figure 4, the tensile strength has a linear positive correlation with volume and temperature, which is that the temperature increase could improve the nucleation rate, the injection volume and temperature, which is that the temperature increase could improve the promote melt filling into the cavity, optimize the micro-sized cells structure, and thus enhance the nucleation rate, promote melt filling into the cavity, optimize the micro‐sized cells structure, and tensile strength. Furthermore, the injection volume increase contributed to reducing the porosity of thus enhance the tensile strength. Furthermore, the injection volume increase contributed to the product and avoiding the phenomenon of cell collapse. The uneven cell structure in the foamed reducing the porosity of the product and avoiding the phenomenon of cell collapse. The uneven cell materialstructure caused in the foamed by cell collapse material will caused lead by to stresscell collapse concentration will lead and to reducestress concentration the tensile strength and reduce of the materialthe tensile [35 strength]. In addition, of the the material positive [35]. correlation In addition, coeffi cientthe positive (slope) controlledcorrelation by coefficient the temperature (slope) iscontrolled significantly by the bigger temperature than the positiveis significantly correlation bigger coeffi thancient the controlled positive by correlation the injection coefficient volume. Therefore,controlled by the the temperature injection volume. increase Therefore, is more conducive the temperature to rapidly increase increasing is more the conducive tensile strength to rapidly of GFincreasing/PA foams the than tensile the strength injection of volume GF/PA increase foams than when the there injection is only volume one variable. increase when there is only one variable.Figure5 shows the comparison of the weight loss ratio of GF /PA microcellular foams with different injection rates, temperatures, and injection volumes. The injection volume had the more significant impact on the weight loss ratio of the product than the injection rate and temperature; that is, the injection volume has the more obvious impact on lightweight. The injection volume increase was equivalent to the increase in the volume of melt entering the cavity; thereby, the growth space Polymers 2020, 12, x FOR PEER REVIEW 7 of 11

Polymers 2020, 12, 2368 7 of 11

of the cells reduced, and the weight loss ratio reduced in the same cavity volume. The temperature and injection rate had limited impact on the weight loss ratio compared with the effect of injection volume, as shown in Figure5. The temperature and injection rate indirectly a ffected the weight loss ratio through single-phase melt formation and the cavity-filling process. Therefore, decreasing the Polymers 2020, 12, x FOR PEER REVIEW 7 of 11 injection volume was the main measure to enhance the lightweight of foamed composites.

Figure 4. Tensile strength of GF/PA foams at different factor levels.

Figure 5 shows the comparison of the weight loss ratio of GF/PA microcellular foams with different injection rates, temperatures, and injection volumes. The injection volume had the more significant impact on the weight loss ratio of the product than the injection rate and temperature; that is, the injection volume has the more obvious impact on lightweight. The injection volume increase was equivalent to the increase in the volume of melt entering the cavity; thereby, the growth space of the cells reduced, and the weight loss ratio reduced in the same cavity volume. The temperature and injection rate had limited impact on the weight loss ratio compared with the effect of injection volume, as shown in Figure 5. The temperature and injection rate indirectly affected the weight loss ratio through single‐phase melt formation and the cavity‐filling process. Therefore, decreasing the injection volume was the main measure to enhance the lightweight of foamed composites. FigureFigure 4. 4.TensileTensile strength strength of of GF/PA GF/PA foams atat didifferentﬀerent factor factor levels. levels.

FigureFigure 5. 5. TheThe weight weight loss loss ratio ratio of of GF/PA GF/PA foamsfoams at at di differentfferent factor factor levels. levels. Figure6 shows the tensile strength /weight ratio (N) of GF/PA microcellular foams with different Figure 6 shows the tensile strength/weight ratio (N) of GF/PA microcellular foams with injection rates, temperatures, and injection volumes. The trends of the curves in Figures4 and6 different injection rates, temperatures, and injection volumes. The trends of the curves in Figure 6 are similar, and the slope of the curve in Figure6 is obviously a ffected by the slope of the curve andin Figure5 4. Theare similar, injection and rate the and slope temperature of the curve had littlein Figure e ffect 6 (slope is obviously0) on affected the weight by changethe slope of of ≈ themicrocellular curve in Figure foams 5. inThe Figure injection5. Therefore, rate and the temperature N value trend had was little associated effect (slope with ≈ tensile0) on strengththe weight changeclosely. of For microcellular example, the foams increased in Figure temperature 5. Therefore, was beneficial the N value to promote trend thewas tensile associated strength with within tensile strengththe temperature closely. rangeFor example, of 40 ◦C, whichthe increased also made temperature the N value increasewas beneficial with the to injection promote temperature. the tensile strengthFurthermore, within the the curve temperature of the N value range decreased of 40 °C, as a resultwhich of also the curvemade of the the tensileN value strength increase decreasing with the injectionwhen the temperature. injection rate Furthermore, is at Level 3 (80-80). the curve As aof result, the N the value tensile decreased strength as is a decisiveresult of factor the curve affecting of the tensilethe N strength value when decreasing the variables when are the the injection temperature rate is and at injectionLevel 3 rate.(80‐80). Mo reover,As a result, the N the value tensile at strengthdifferent is injection a decisive rate factor levels wasaffecting basically the consistent N value with when the the trend variables of tensile are strength, the temperature which further and verified that reducing the initial injection rate at high-speed levels could decrease the loss of tensile Figure 5. The weight loss ratio of GF/PA foams at different factor levels. strength with the same GF/PA composite density, indirectly indicating that the finer microcellular cell structure could increase the strength of the foams. Figure 6 shows the tensile strength/weight ratio (N) of GF/PA microcellular foams with different injection rates, temperatures, and injection volumes. The trends of the curves in Figure 6 and Figure 4 are similar, and the slope of the curve in Figure 6 is obviously affected by the slope of the curve in Figure 5. The injection rate and temperature had little effect (slope ≈ 0) on the weight change of microcellular foams in Figure 5. Therefore, the N value trend was associated with tensile strength closely. For example, the increased temperature was beneficial to promote the tensile strength within the temperature range of 40 °C, which also made the N value increase with the injection temperature. Furthermore, the curve of the N value decreased as a result of the curve of the tensile strength decreasing when the injection rate is at Level 3 (80‐80). As a result, the tensile strength is a decisive factor affecting the N value when the variables are the temperature and Polymers 2020, 12, x FOR PEER REVIEW 8 of 11 injection rate. Moreover, the N value at different injection rate levels was basically consistent with the trend of tensile strength, which further verified that reducing the initial injection rate at high‐speed levels could decrease the loss of tensile strength with the same GF/PA composite density, indirectly indicating that the finer microcellular cell structure could increase the strength of the foams. The injection volume had a significant effect on the weight loss ratio compared with the injection rate and injection temperature. As shown in Figure 6, the curve growth trend of the N value controlled by the injection volume was obviously slower than the curve growth trend of the tensile strength controlled by the injection volume in Figure 4 due to the influence of the weight loss ratio. In other words, the weight loss ratio increased with the decreased injection volume, which also led to the increased risk of cell collapse; then, that decreased the tensile strength. That is, the increasing extent of N value was lower than the tensile strength due to the weight loss ratio increasing faster when comprehensively considering the effect of the weight loss ratio. Therefore, the single change of tensile strength could not reflect the effect of injection volume on the GF/PA microcellular foams. In addition to considering the lightweight effect, it is necessary to adjust the weight loss ratio in accordance with the requirements of mechanical properties to obtain a higher N value in actual process applications. As shown in Figure 6, the slope controlled by the injection volume was lower than the slope controlled by the injection rate and temperature. Therefore, the tensile strength decreasePolymers of2020 GF/PA, 12, 2368 foams caused by the decreased injection volume could be compensated by8 of using 11 the “slow–fast” injection rate or improving the temperature.

FigureFigure 6. 6.TheThe ratio ratio of of tensile tensile strength strength retention retention rates to qualityquality retentionretention rates rates of of GF GF/PA/PA foams foams at at differentdiﬀerent factor factor levels. levels.

The injection volume had a significant effect on the weight loss ratio compared with the injection 3.3. Dielectric Constant rate and injection temperature. As shown in Figure6, the curve growth trend of the N value controlled byThe the injectionfoaming volumeprocess was is a obviously common slower method than to the make curve foamed growth materials trend of the with tensile low strength dielectric constantscontrolled [4,5]. by Figure the injection 7 shows volume the correlation in Figure4 due between to the influencedielectric of constant the weight and loss weight ratio. loss In other ratio of GF/PAwords, microcellular the weight lossfoaming ratio composites increased with with the different decreased injection injection volumes. volume, The which weight also retention led to the rate increasedincreased and risk the of cellporosity collapse; gradually then, that decreased decreased as the the tensile injection strength. volume That increased. is, the increasing Figure extent7 shows thatof the N value trend was of lowerdielectric than constant the tensile is strength highly duesimilar to the to weightthe trend loss of ratio weight increasing retention faster rate. when For example,comprehensively the curve considering of the weight the e ffretentionect of the weightrate in lossthe ratio.case Therefore,of 35.2 cm the3 exhibited single change oscillations of tensile on accountstrength of couldthe fluctuation not reflect of the gas effect injection of injection amount. volume Furthermore, on the GF/PA the microcellular curve of the foams. dielectric In addition constant to considering the lightweight effect, it is necessary to adjust the weight loss ratio in accordance with also exhibited oscillations in the case of 35.2 cm3, which proves that the porosity of foams is a key the requirements of mechanical properties to obtain a higher N value in actual process applications. factor affecting the dielectric constant [36]—that is, the porosity increase could effectively reduce the As shown in Figure6, the slope controlled by the injection volume was lower than the slope controlled dielectric constant compared with GF/PA 610 with a dielectric constant above 3.8 produced by the by the injection rate and temperature. Therefore, the tensile strength decrease of GF/PA foams caused traditional injection molding process. Therefore, the dielectric constant of foams could be changed by the decreased injection volume could be compensated by using the “slow–fast” injection rate or by improvingadjusting the the temperature.injection volume, which is highly related to the porosity. At the same time, the paper inferred that the local area of the foams was also consistent, indicating3.3. Dielectric that the Constant local porosity of the foams was a key factor affecting the dielectric constant of the correspondingThe foaming position. process Therefore, is a common the injection method rate to makeand temperature foamed materials closely with related low dielectricto the local constants [4,5]. Figure7 shows the correlation between dielectric constant and weight loss ratio of GF/PA microcellular foaming composites with different injection volumes. The weight retention rate increased and the porosity gradually decreased as the injection volume increased. Figure7 shows that the trend of dielectric constant is highly similar to the trend of weight retention rate. For example, the curve of the weight retention rate in the case of 35.2 cm3 exhibited oscillations on account of the fluctuation of gas injection amount. Furthermore, the curve of the dielectric constant also exhibited oscillations in the case of 35.2 cm3, which proves that the porosity of foams is a key factor affecting the dielectric constant [36]—that is, the porosity increase could effectively reduce the dielectric constant compared with GF/PA 610 with a dielectric constant above 3.8 produced by the traditional injection molding process. Therefore, the dielectric constant of foams could be changed by adjusting the injection volume, which is highly related to the porosity. At the same time, the paper inferred that the local area of the foams was also consistent, indicating that the local porosity of the foams was a key factor affecting the dielectric constant of the corresponding position. Therefore, the injection rate and temperature closely related to the local porosity and cell structure will also affect the dielectric constant of the foamed composites. The influence of injection rate and temperature on the dielectric coefficient of the material will be small on average, but the Polymers 2020, 12, x FOR PEER REVIEW 9 of 11 porosityPolymers and2020 cell, 12, 2368structure will also affect the dielectric constant of the foamed composites.9 of 11 The influence of injection rate and temperature on the dielectric coefficient of the material will be small on average,influence but on the dielectricinfluence uniformity on the dielectric of different uniformity positions of of the different product positions will be significant. of the product That is, thewill be significant.“slow–fast” That injection is, the rate“slow–fast” and the temperature injection rate increase and could the temperature optimize the uniformityincrease could of the product’soptimize the uniformitydielectric of constant.the product’s dielectric constant.

FigureFigure 7. The 7. Thedielectric dielectric constant constant and and quality quality retention retention rates ofof GF GF/PA/PA foams foams with with diff differenterent injection injection volumevolume levels. levels. 4. Conclusions 4. Conclusions The injection rate, temperature, and volume are key factors affecting the cellular morphology and macroscopicThe injection properties rate, temperature, of GF/PA microcellular and volume foams. are This key paper factors uses affecting the N value the to cellular comprehensively morphology and reflectmacroscopic the effects properties of various parameters of GF/PA on themicrocellular macro-mechanical foams. properties. This paper Increasing uses the the injection N value rate to comprehensivelycould enhance thereflect pressure the dropeffects rate of and various cell nucleation, parameters but an on excessively the macro high‐mechanical initial injection properties. rate Increasingwill cause the gas injection overflow rate and thecould cells ofenhance the melt the front pressure to merge anddrop collapse. rate and Therefore, cell nucleation, a reduced initial but an excessivelyinjection high rate and initial “slow–fast” injection injection rate will rate cause could gas diminish overflow the pressure and the di ffcellserence of between the melt the front melt to front merge and the mold cavity, which improved the cell structure and tensile strength. The injection temperature and collapse. Therefore, a reduced initial injection rate and “slow–fast” injection rate could diminish had an effect on the formation of single-phase melt and the cell nucleation rate. The temperature increase the pressure difference between the melt front and the mold cavity, which improved the cell could improve the nucleation rate, promote melt filling into the cavity, optimize the micro-sized structurecells structure,and tensile and thusstrength. enhance The the injection tensile stren temperaturegth. The injection had an volume effect was on the the direct formation factor of singleinfluencing‐phase melt the weightand the loss cell ratio nucleation and porosity rate. of GF The/PA microcellulartemperature foincreaseams. The couldinjection improve volume the nucleationdecrease rate, contributed promote to melt improving filling the into lightweight the cavity, eff ectoptimize of the product the micro rapidly.‐sized However, cells structure, this will and thus reduceenhance the the tensile tensile strength strength. of the Theproduct. injection The volume temperature was the increase direct is factor more influencing conducive than the theweight loss ratioinjection and volume porosity increase of GF/PA to rapidly microcellular increasing thefoams. tensile The strength injection of GF volume/PA foams decrease when there contributed is only to improvingone variable. the lightweight In order to achieveeffect of lightweight the product effects, rapidly. the tensile However, strength this decrease will ofreduce GF/PA the foams tensile strengthcaused of bythe the product. decreased The injection temperature volume couldincrease be compensated is more conducive by using thethan “slow–fast” the injection injection volume increaserate to or rapidly improving increasing the temperature. the tensile Furthermore, strength of the GF/PA porosity foams of foams when is there a key is factor only aoneffecting variable. the In dielectric constant. The injection volume decrease could increase the porosity and reduce the dielectric order to achieve lightweight effects, the tensile strength decrease of GF/PA foams caused by the constant of the foamed composites, and the “slow–fast” injection rate and the temperature increase decreased injection volume could be compensated by using the “slow–fast” injection rate or could optimize the uniformity of the product’s dielectric constant. improving the temperature. Furthermore, the porosity of foams is a key factor affecting the dielectric constant.Author The Contributions: injection Conceptualization,volume decrease X.W. could and G.W.; increase formal the analysis, porosity X.W. and and G.W.; reduce writing—original the dielectric draft preparation, X.W.; writing—review and editing X.W., G.W., P.X. and X.G.; supervision, P.X. and W.Y.; project constantadministration, of the foamed P.X. and composites, W.Y.; funding acquisition, and the “slow–fast” P.X. and W.Y. All injection authors have rate read and and the agreed temperature to the published increase couldversion optimize of the the manuscript. uniformity of the product’s dielectric constant. Funding: Author Contributions:This research Conceptualization, was funded by X.W. the National and G.W.; Key formal Research analysis, and Development X.W. and G.W.; Program writing—original of China (2019YFC1906105), the Science and Technology Major Project of Ningbo (2018B10021, 2018B10082), and the draft Fundamentalpreparation, Research X.W.; writing—review Funds for the Central and Universities editing X.W., (XK1802-3, G.W., P.X. 2312017BHYC04A). and X.G.; supervision, P.X. and W.Y.; projectConflicts administration, of Interest: P.X.The and authors W.Y.; declare funding no conflict acquisition, of interest. P.X. and W.Y. All authors have read and agreed to the published version of the manuscript.

Funding: This research was funded by the National Key Research and Development Program of China (2019YFC1906105), the Science and Technology Major Project of Ningbo (2018B10021，2018B10082), and the Fundamental Research Funds for the Central Universities (XK1802‐3，2312017BHYC04A). Polymers 2020, 12, 2368 10 of 11

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