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Article Dynamic Moduli of Terephthalate Reinforced in High-Temperature Environments

Carmelo Gómez 1 , Jorge Mira 1, F.J. Carrión-Vilches 2 and Francisco Cavas 3,*

1 Doctorate Program in Industrial Technologies, International School of Doctorate, Technical University of Cartagena, 30202 Cartagena, Spain; [email protected] (C.G.); [email protected] (J.M.) 2 and Metallurgical Engineering Group, Materials Engineering and Manufacturing Department, Technical University of Cartagena, 30202 Cartagena, Spain; [email protected] 3 Department of Structures, Construction and Graphical Expression, Technical University of Cartagena, 30202 Cartagena, Spain * Correspondence: [email protected]; Tel.: +34-968-338856

Abstract: The aim of this work was to show the evolution over time of the dynamic moduli in components made of Polybutylene Terephthalate reinforced with glass fiber when they are held to temperatures close to the temperature over time. For this purpose, PBT samples reinforced with short, glass fibers of Ultradur® material with 0%, 20%, and 50% in weight content were tested. Dynamic moduli showed an increment with glass fiber content showing a nonlinear behavior with the temperature. The evolution of storage modulus was depicted by means of a modified law of mixtures with an effectiveness factor depending on temperature and fiber content, whereas the evolution over time was obtained with a time–temperature transformation generated   with the TTS Data Analysis software of TA-instruments for a given temperature. Storage modulus showed a linear relationship with glass fiber content when components were held to temperatures Citation: Gómez, C.; Mira, J.; near to their respective glass transition temperature, obtained from the maximum of loss modulus Carrión-Vilches, F.J.; Cavas, F. curve with temperature. In summary, the value and evolution of dynamic moduli of PBT samples Dynamic Moduli of Polybutylene Terephthalate Reinforced improved with glass fiber content, allowing us to increase the durability of components when they in High-Temperature Environments. are submitted to high-temperature environments. Materials 2021, 14, 483. https:// doi.org/10.3390/ma14030483 Keywords: composites; storage modulus; glass fiber; high temperature

Academic Editor: Ana González-Marcos Received: 28 December 2020 1. Introduction Accepted: 18 January 2021 Overcoming new technological challenges and greater social awareness of environ- Published: 20 January 2021 mental protection are the ideal basis for investigating new materials that improve the properties of existing ones, but by putting natural resources to better use. In recent years, Publisher’s Note: MDPI stays neutral considerable efforts have been carried out to develop biodegradable materials [1–8] and with regard to jurisdictional claims in composites [9,10], allowing the reduction of contamination and optimizing com- published maps and institutional affil- posite components and structures. materials have shown a strong growth iations. due to their good mechanical properties and possibility to be recycled [11]. Furthermore, properties can be amended by using additives or blending different . Polybutylene terephthalate (PBT) is a thermoplastic polymer with excellent dimensional stability, is very rigid, and offers good thermal resistance, electric isolation, and few dielec- Copyright: © 2021 by the authors. tric losses. The increasing use of this material is related mainly with the electronic sector, Licensee MDPI, Basel, Switzerland. particularly with the incorporation of new functionalities into the automotive sector by the This article is an open access article integration of connectors and sensors due to its good performance in micro-molding pro- distributed under the terms and cesses [12] and 3D printing technology with good thermal and mechanical properties [13,14]. conditions of the Creative Commons PBT offers extremely good versatility when mixed with other materials, which has Attribution (CC BY) license (https:// enabled additives to be incorporated, such as flame retardants, and particularly more creativecommons.org/licenses/by/ 4.0/). enviro-friendly, nonhalogen additives to improve their properties against fire [15–21].

Materials 2021, 14, 483. https://doi.org/10.3390/ma14030483 https://www.mdpi.com/journal/materials Materials 2021, 14, 483 2 of 13

PBT components are mainly manufactured by injection methods in which short, glass fibers are incorporated as reinforcement. The reinforced material obtained can be shredded and injected in a further process maintaining good mechanical properties [22]. PBT components show good strength and stiffness properties but also poor performance as regards impact and fracture mechanisms. Material properties can be improved adding new reinforced materials [5,23] or blended with other polymer materials. Incorporating elastomeric materials increases fracture tenacity, performance upon impacts, and components’ surface resistance by, in turn, increasing the retention and recovery capacities of the material’s properties, even after aging processes [24–27]. The improvement is attributed to local deformation zones being produced, which act as energy dissipaters and would, thus, increase the plasticity of the analyzed samples. Improving the properties linked with impacts makes resistance to flexion and stor- age modulus worse. Zhang et al. [28] ran tests with thermoplastic polyurethane (TPU). Dynamic mechanical analysis (DMA) indicated that the addition of TPU increases tensile and notched Izod impact strength with a small reduction in flexural strength and modulus. Other authors have demonstrated that when other materials were also added, such as PTW poly(ethylene-butylacrylate-glycidyl methacrylate ), they allowed further increments in the storage modulus and the loss factor tangent, although performance worsened after a certain percentage [29]. Strength and stiffness of polymers’ materials depend on the strain rate. Abdo et al. [30,31] and Isogai et al. [32] incorporated thermoplastic (TTPE) into short, glass fiber-reinforced PBT samples in order to investigate the strain speed dependence. The results indicated that ultimate tensile strength is sensitive to strain rate. Both damage initiation and crack growth were affected by blending with TTPE. Stiffness of polymer materials linearly increases with the strain rate logarithm [33]. This is because polymer chains are less prone to mechanisms. The material is less ductile and offers greater resistance and a higher modulus of , while properties worsen with temperature and presence of [34]. Incorporating other polymer materials also allows dynamic performance to increase, and, hence, the fatigue properties of the obtained also increase. Varga and Bartha [35] demonstrated improved dynamic properties for short, glass fiber-reinforced PBT after incorporating cyclic oligomers (CBT-100), and the fatigue performance analysis indicated differences with conventional -type materials. Eftekhari et al. [36] tested fatigue with variable amplitude, and a rigidity process in the first load cycle was observed. Schaaf et al. [37] developed a method to estimate the life cycle of short, glass fiber-reinforced polymers, whereas Mbyniec and Uhl [38] proposed a failure criterion that depended on both aging time and fibers’ alignment. The aim of these proposals was to analyze the durability and resistance of materials. Durability of polymers is strongly dependent on thermal aging, with action of elevated temperatures being a key factor in electrical components submitted to high-temperature environments during long periods of time. Hashemi [39] investigated the behavior of PBT reinforced with short, glass fibers in samples between 23 ◦C and 100 ◦C. The results showed that tensile strength and elastic modulus decreased nonlinearly when temperature was increased. Mortazavian and Fatemi [33] analyzed the performance of glass fiber-reinforced PBT and PA6 samples. A Ramberg–Osgood-type relationship was used to relate stress and strain. Time–temperature superposition principle with a shift factor of Arrhenius types was applied to obtain tensile strength data at different temperatures. PBT is an ideal candidate in the growing number of electronic and electrical compo- nents in the transport sector, such as batteries and components for chargers, due to its excellent properties as an electric insulator. These components could be submitted to high-temperature environments during long periods of time, and stiffness and energy absorption are key factors due to dimensional stability and durability requirements. Materials 2021, 14, x FOR PEER REVIEW 3 of 13

These components could be submitted to high-temperature environments during long periods of time, and stiffness and energy absorption are key factors due to dimen- sional stability and durability requirements. Materials 2021, 14, 483 This work aimed to analyze the influence of glass fiber content on the evolution3 of 13 with temperature and frequency of the dynamic moduli. By means of a time–temperature transformation, the evolution with time of dynamic moduli was obtained. For this pur- pose,This temperatures work aimed that to analyzecame close the influence to the ma ofterial’s glass fiber glass content transition on the temperature evolution with were considered.temperature and frequency of the dynamic moduli. By means of a time–temperature transformation, the evolution with time of dynamic moduli was obtained. For this purpose, 2.temperatures Experimental that came close to the material’s glass transition temperature were considered.

2.1.2. Experimental Materials and Sample Preparation 2.1. MaterialsPBT samples and Sample were manufactured Preparation with three variants of Ultradur® material obtained from PBTthe samplescompany were BASF manufactured (Ludwigshafen, with three Germany). variants B of 4520 Ultradur variant® material was unreinforced, obtained whereasfrom the the company variants BASF B 4300 (Ludwigshafen, G4 and B4300 Germany). G10 contained B 4520 20% variant and 50% was of unreinforced, glass fiber rein- forcementwhereas the in variantsweight equivalent B 4300 G4 andto 11.7% B4300 and G10 34.7% contained percentage 20% and in volume, 50% of glass respectively. fiber Toreinforcement manufacture in weightsamples, equivalent a DEU to250H55 11.7% andmini 34.7%-VP injection percentage machine in volume, (Barcelona, respectively. Spain) wasTo manufacture employed by samples, applying a DEU12 MPa 250H55 injectio mini-VPn pressure injection and machine9 MPa maintenance (Barcelona, Spain)pressure. Temperaturewas employed at by which applying samples 12 MPa were injection injected pressure was set and at 230 9 MPa °C, maintenancewith an initial pressure. mold tem- ◦ peratureTemperature of 55 at °C. which The maintenance samples were time injected was 15 was s. The set atinjection 230 C, process with an was initial based mold in one ◦ centraltemperature injection of 55 point.C. The maintenance time was 15 s. The injection process was based in one centralDue to injectionmaterial’s point. shrinkage, samples showed differences from nominal values in the valuesDue of tothe material’s transversal shrinkage, width and samples thickness showed (Figure differences 1). Dimensions from nominal were values reduced in the with values of the transversal width and thickness (Figure1). Dimensions were reduced with glass fiber content. Width values measured showed values between 3.8 mm and 4.2 mm, glass fiber content. Width values measured showed values between 3.8 mm and 4.2 mm, while thickness values showed values from 3.1 mm to 3.9 mm in all samples injected, while thickness values showed values from 3.1 mm to 3.9 mm in all samples injected, where wheresamples samples of three of materials three materials were considered. were consid Afterered. injection After process, injection samples process, were samples trimmed were trimmedin order to in be order adapted to be to adapted the dynamic to the mechanical dynamic mechanical analysis (DMA). analysis For this(DMA). purpose, For this only pur- pose,the 30-mm only the central 30-mm part central of samples part was of samples considered. was Dependingconsidered. on Depending the test analysis on the carried, test anal- ysisout onecarried, or two out ends one wereor two clamped ends were in DMA clamped equipment. in DMA equipment.

Figure 1. Sample mold’s nominal dimensions. Figure 1. Sample mold’s nominal dimensions. The material injected was observed by means of scanning microscopy (SEM, Hitachi,The Chiyoda,material injected Tokio, Jap wasón). observed showed by mean a randoms of scanning and distributed electron microscopy . (SEM, In Hitachi,the following Chiyoda, figures, Tokio, photographs Japón). Fibers of samples showed of PBTa random G4 (left and side) distributed and PBT G10dispersion. (right In theside) following are shown figures, in three photographs different amplification of sample levelss of PBT (Figure G4 2(left). side) and PBT G10 (right side) are shown in three different amplification levels (Figure 2).

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FigureFigure 2.2. ScanningScanning electron microscopy (SEM) (SEM) of of samp samplesles of of PBT PBT B B 4300 4300 G4 G4 (left (left side) side) and and PBT PBT B B 4300 G10 (right side) with magnitude amplification (500×, 1000×, and 2000×). 4300 G10 (right side) with magnitude amplification (500×, 1000×, and 2000×).

2.2.2.2. DynamicDynamic MechanicalMechanical AnalysisAnalysis (DMA)(DMA) andand SampleSample PreparationPreparation TestsTests werewere carried carried out out in in Q800 Q800 equipment equipment (TA (TA instrument, instrument, New New Castle, Castle, DE, DE, USA). USA).

2.2.1.2.2.1. DynamicDynamic ModuliModuli and and Glass Glass Transition Transition Temperature Temperature Evolution Evolution with with the the Temperature Tempera- ture The evolution of both the storage and loss moduli was studied in the multifrequency modeThe by maintainingevolution of frequencyboth the storage constant and and loss provoking moduli was a temperature studied in gradient.the multifrequency The glass transitionmode by temperaturemaintaining was frequency determined constant with theand maximum provoking loss a temperature modulus. Single gradient. cantilever The configurationglass transition was temperature considered andwas subjected determined to a with 0.1% strainthe maximum increment lo andss modulus. 1-Hz frequency. Single ◦ ◦ cantileverTests wereconfiguration carried outwas atconsidered a range ofand temperatures subjected to froma 0.1% 25 strainC to increment 225 C with and a1- ◦ 3HzC/minute frequency. temperature gradient, which allowed us to measure the evolution of both the storageTests and were loss carried moduli out for at this a range whole of temperature temperatures range. from 25 °C to 225 °C with a 3 °C/mi- nute temperature gradient, which allowed us to measure the evolution of both the storage 2.2.2. Multifrequency Assays and loss moduli for this whole temperature range. Multifrequency assays were performed between a range of 0.1 and 10 Hz at constant ◦ ◦ ◦ temperature.2.2.2. Multifrequency A work rangeAssays between 25 C and 145 C was used with 3 C intervals. Sam- ples built in on one side were considered with a 10-µm amplitude and a 5-min temperature Multifrequency assays were performed between a range of 0.1 and 10 Hz at constant stabilization time. The variation in the glass transition temperature with frequency was evaluatedtemperature. by theA work maximums range between of the loss 25 °C modulus and 145 curves, °C waswhich used with were 3 generated°C intervals. for Sam- the distinctples built frequencies in on one byside the were TA considered Universal Analysiswith a 10- 2000μm softwareamplitude (TA and instrument). a 5-min tempera- Mas- terture curves stabilization were generated time. The with variation the TTS in Datathe glass Analysis transition (TA instrument), temperature and with the frequency applied

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was evaluated by the maximums of the loss modulus curves, which were generated for Materials 2021, 14, 483 the distinct frequencies by the TA Universal Analysis 2000 software (TA instrument).5 Mas- of 13 ter curves were generated with the TTS Data Analysis (TA instrument), and the applied temperature was that at which the maximum loss modulus appeared. Double cantilever temperatureconfiguration was was that considered at which for the test maximum development. loss modulus appeared. Double cantilever configuration was considered for test development. 3. Analyzing and Discussing the Results 3. AnalyzingThe results and performed Discussing in the the Results characterization tests of the Ultradur® material in the studiedThe configurations results performed are shown in the characterizationbelow, considering tests the of following the Ultradur notations:® material in the studied• Ultradur configurations® B 4520: arePBT shown GF0, below, considering the following notations: •• UltradurUltradur®® B 4520:4300 G4: PBT PBT GF0, GF20, and •• UltradurUltradur®® B 4300 G4:G10: PBT PBT GF20, GF50. and • Ultradur® B 4300 G10: PBT GF50. 3.1. Evolution of Dynamic Moduli with Temperature 3.1. EvolutionThe storage of Dynamic modulus Moduli showed with a Temperaturenonlinear behavior with temperature and drastically droppedThe storageat around modulus the glass showed transition a nonlinear temperature behavior (Tg), with as can temperature be observed and in drastically Figure 3, droppedwhich agrees at around with results the glass shown transition by other temperature authors [40–42]. (Tg), asAt cantemperatures be observed above in Figure Tg, rel- 3atively, which large agrees segments with results of 10 to shown 50 repeat by otherunits can authors move [40 freely–42]. by At changing temperatures their confor- above Tmation.g, relatively Below large Tg, these segments polymers of 10 have to 50 many repeat properties units can associated move freely with by crystalline changing mate- their conformation.rials, while above Below Tg T, gmaterials, these polymers behave havelikely many elastomers properties. In all associated ranges of with temperatures crystalline materials,analyzed, whilethe behavior above T improvedg, materials with behave glass likely fiber elastomers. content. The In allreason ranges is that of temperatures glass fibers’ analyzed,addition modified the behavior the structural improved behavior with glass of fiber the material content. that The reasonbehaved is like that a glass composite fibers’ additionmaterial modified[33]. Near theto the structural polymer behavior oftemperature the material the that reinforcement’s behaved like effectiveness a composite materialwas reduced. [33]. NearThis behavior to the polymer can be melting attribut temperatureed to the fact the that reinforcement’s storage modulus effectiveness depended wason polymer’s reduced. This properties, behavior reinforcement’s can be attributed prop to theerties, fact and that interface’s storage modulus properties depended between on polymer’smaterials. properties,Final value reinforcement’s reached depended properties, on mate andrials’ interface’s percentage properties and temperature. between materi- Near als.melting Final temperature, value reached interface depended properties on materials’ were percentage weakened, and working temperature. the materials Near melting sepa- temperature,rately again. interface properties were weakened, working the materials separately again.

FigureFigure 3.3. VariationVariation inin thethe storagestorage modulusmodulus withwith temperaturetemperature (f(f= = 11 Hz).Hz).

MortazavianMortazavian andand FatemiFatemi [[33]33] distinguisheddistinguished amongamong threethree evolutionevolution intervalsintervals forfor thethe storagestorage modulusmodulus withwith temperaturetemperature inin aa simplifiedsimplified manner.manner. InIn thisthis work,work, forfor thethe ragerage ofof temperaturetemperature analyzed,analyzed, thethe storagestorage modulusmodulus evolutionevolution waswas depicteddepicted byby meansmeans ofof polyno-polyno- mialmial functionsfunctions consideringconsidering aa modifiedmodified mixturemixture lawlaw andand consideringconsidering glassglass fiberfiber content,content, accordingaccording toto thethe followingfollowing equation.equation.

Φ (%)  Φ (%)  E0 = η· f ·E + 1 − f ·E0 (Units MPa) (1) 100 f 100 PBT

where E’(MPa) represents the storage modulus of composite material, Ef (MPa) represents

the glass fiber modulus, E’PBT (MPa) represents the storage modulus of the PBT matrix, Materials 2021, 14, x FOR PEER REVIEW 6 of 13

(%) (%) E =η· ·E +1− ·E′ (Units MPa) (1)

where E’(MPa) represents the storage modulus of composite material, Ef (MPa) represents the glass fiber modulus, E’PBT (MPa) represents the storage modulus of the PBT matrix, Φf Materials 2021, 14, 483 represents the fiber volume as a percentage, and η is a nondimensional parameter that6 of 13 denotes the reinforcement effectiveness factor. The effectiveness factor depends on fibers’ length, their degree of dispersion, and the effect of temperature on the adhesion force be- tween fibers and matrix, which worsens with temperature. This work contemplated a tem- Φ represents the fiber volume as a percentage, and η is a nondimensional parameter that perature-dependentf effectiveness factor based on a five-order polynomial, in which the denotes the reinforcement effectiveness factor. The effectiveness factor depends on fibers’ parameters defining this polynomial presented linear dependence with the fiber volume length, their degree of dispersion, and the effect of temperature on the adhesion force expressed as a percentage. between fibers and matrix, which worsens with temperature. This work contemplated a temperature-dependent effectiveness factor based on a five-order polynomial, in which the parameters defining this polynomialη(Φ,T presented) =F linear(Φ) ·T dependence with the fiber volume(2) expressed as a percentage. i=5 i F(Φ)η=A(Φf,T·Φ) =(%)∑ +Fi B(Φ (if) ·=T 1 to 5) (3)(2) i=0 The values of the constants for a test run at a 1-Hz frequency appear in Table 1, where Fi(Φf) = Ai·Φf(%) + Bi (i = 1 to 5) (3) Ai, Bi, and Fi (i = 1 to 5) are expressed in the same units. The storage modulus evolution for glassThe fiber values weight of the percentage constants of for 10% a test and run 30% at ain 1-Hz accordance frequency with appear these in parameters Table1, where is reflectedAi,Bi, andin Figure Fi (i = 3. 1 to 5) are expressed in the same units. The storage modulus evolution for glass fiber weight percentage of 10% and 30% in accordance with these parameters is Tablereflected 1. Constants in Figure defining3. the effectiveness factor (f = 1 Hz).

ConstantsTable 1. forConstants defining the effectiveness factor (f = 1 Hz). A0/B0 Polynomial A5/B5 (°C−5) A4/B4 (°C−4) A3/B3 (°C−3) A2/B2 (°C−2) A1/B1 (°C−1) Constants for Polynomial (-) CoefficientsA /B (◦C −5)A /B (◦C−4)A /B (◦C−3)A /B (◦C−2)A /B (◦C−1)A /B (-) Coefficients 5 5 4 4 3 3 2 2 1 1 0 0 A value 2.37 × 10−13 −1.55 × 10−10 3.62 × 10−8 −3.59 × 10−6 1.17 × 10−4 1.19 × 10−3 A value 2.37 × 10−13 −1.55 × 10−10 3.62 × 10−8 −3.59 × 10−6 1.17 × 10−4 1.19 × 10−3 B value B value6.74 × 10−12 6.74− ×4.64 10×−1210 −9−4.641.17 × 10×−910 − 1.176 ×− 101.31−6 ×−101.31−4 × 105.31−4 × 5.3110− ×3 10−3 1.56 1.56× 10× 10−1 −1

Loss modulus showed a bell shape evolution with temperature, as can be observed Loss modulus showed a bell shape evolution with temperature, as can be observed in in Figure 4. Maximum value was obtained at glass transition temperature. In all ranges of Figure4. Maximum value was obtained at glass transition temperature. In all ranges of temperatures analyzed, the behavior improved with glass fiber content. This factor can be temperatures analyzed, the behavior improved with glass fiber content. This factor can be attributedattributed to toan an increase increase in ininner inner friction, friction, allowing allowing us us to toimprove improve the the energy energy dissipated dissipated in in eacheach strain strain cycle. cycle.

Figure 4. Variation in the loss modulus with temperature (f = 1 Hz). Figure 4. Variation in the loss modulus with temperature (f = 1 Hz). Maximum peak and its amplitude increased with glass fiber content. This fact can be attributed to inhibited relaxation mechanisms on the interface between fibers and polymer matrix. Fibers induced constraints, imposing mobility restrictions in polymer chains at relaxation temperatures. Percentage of polymer chain immobilized increased with fiber content increasing the glass transition temperature obtained. With unreinforced material, the obtained values correlated well with other values found in the literature [43]. In this work a linear relationship between the glass transition temperature, considering maximum Materials 2021, 14, x FOR PEER REVIEW 7 of 13

Maximum peak and its amplitude increased with glass fiber content. This fact can be attributed to inhibited relaxation mechanisms on the interface between fibers and polymer matrix. Fibers induced constraints, imposing mobility restrictions in polymer chains at Materials 2021, 14, 483 relaxation temperatures. Percentage of polymer chain immobilized increased with fiber7 of 13 content increasing the glass transition temperature obtained. With unreinforced material, the obtained values correlated well with other values found in the literature [43]. In this work a linear relationship between the glass transition temperature, considering maxi- values of loss modulus, and the volumetric glass fiber percentage was considered (Table2), mum values of loss modulus, and the volumetric glass fiber percentage was considered according to the following equation: (Table 2), according to the following equation: ◦ TTg((°CC) )=A+B·Φ= A + B·Φ(%)f(% ) (4)(4)

◦ TableTable 2. Defining 2. Defining parameters parameters of ofTg Tevolutiong evolution (°C) ( C)with with the the glass glass fiber fiber volume. volume.

A (°C)A( ◦C) B (°C) B (◦C) RR2 2 50.76 50.760.4 0.4 0.989 0.989

3.2. Glass Transition Temperature Evolution with Frequency 3.2. Glass Transition Temperature Evolution with Frequency Glass transition temperature increased with frequency, as can be observed in Glass transition temperature increased with frequency, as can be observed in Figure5 Figure 5 where an asymptotic behavior in the glass transition temperature with frequency where an asymptotic behavior in the glass transition temperature with frequency was was observed, indicating a limit value of glass transition temperature (Tg). At low frequen- observed, indicating a limit value of glass transition temperature (Tg). At low frequencies, cies, macroscopic deformation times came close to the order of magnitude of the micro- macroscopic deformation times came close to the order of magnitude of the microscopic scopic deformation times, and dissipated energy increased by internal friction. When the deformation times, and dissipated energy increased by internal friction. When the strain strainrate rate was was increased, increased, polymer polymer chains chains were were less less prone prone to these to these relaxation relaxation mechanisms, mechanisms, which whichresulted resulted in those in those chains chains with with shorter shorter effective effective lengths lengths supporting supporting the the load. load. The The material ma- terialshowed showed less less ductility and offeredand offered more more resistance resistance and a higherand a higher storage storage modulus. modulus. Variation Variationin the storage in the storage modulus modulus with frequency with freq presenteduency presented logarithmic logarithmic evolution evolution with the with strain the strain rate, according to several authors [40–42]. Glass transition temperature (Tg) can rate, according to several authors [40–42]. Glass transition temperature (Tg) can be linearly be fittedlinearly to thefitted natural to the logarithm natural logarithm of frequency of frequency (f [Hz]) with (f [Hz]) very with good very regression good regression coefficients, coefficients,according according to the following to the following equation: equation:

T(°C◦ ) = C + D · ln(f) (5) Tg( C) = C + D· ln(f) (5) where the defining parameters were obtained from test curves’ fitting and can be observed in whereTable 3. the defining parameters were obtained from test curves’ fitting and can be observed in Table3.

Figure 5. Variation in the glass transition temperature with frequency. Figure 5. Variation in the glass transition temperature with frequency.

◦ Table 3. Defining parameters of Tg evolution ( C) with frequency.

Material/Parameters C (◦C) D (◦C/ln (Hz)) R2 PBT GF0 47.15 2.18 0.94 PBT GF20 50.15 2.17 0.94 PBT GF50 53.85 1.54 0.81 Materials 2021, 14, x FOR PEER REVIEW 8 of 13

Table 3. Defining parameters of Tg evolution (°C) with frequency.

Material/Parameters C (°C) D (°C/ln (Hz)) R2 PBT GF0 47.15 2.18 0.94 PBT GF20 50.15 2.17 0.94 Materials 2021, 14, 483 PBT GF50 53.85 1.54 0.81 8 of 13

The difference in Tg found in the multifrequency test when frequency equaled 1 Hz, compared to the test with a constant frequency at 1 Hz, can be attributed to the differences The difference in Tg found in the multifrequency test when frequency equaled 1 Hz, in testing conditions. This emphasizes that Tg value was not a unique value. The value compared to the test with a constant frequency at 1 Hz, can be attributed to the differences obtained depended on the technique, testing conditions, and methodology. The values in testing conditions. This emphasizes that T value was not a unique value. The value showed a difference that was increased with glassg fiber content, with a maximum differ- obtained depended on the technique, testing conditions, and methodology. The values ence of 19.5% for the samples reinforced with the 50% glass fiber weight. Glass transition showed a difference that was increased with glass fiber content, with a maximum difference temperature was linear with glass fiber content and the logarithm of the frequency, ac- of 19.5% for the samples reinforced with the 50% glass fiber weight. Glass transition cording to Equations (4) and (5). The values obtained could be combined considering the temperature was linear with glass fiber content and the logarithm of the frequency, according mean values between both equations for the 1-Hz frequency and by maintaining the Tg to Equations (4) and (5). The values obtained could be combined considering the mean ratio for the other frequencies to calculate the mean values. Glass transition temperature values between both equations for the 1-Hz frequency and by maintaining the Tg ratio for could be obtained according to the following equation where parameters were obtained the other frequencies to calculate the mean values. Glass transition temperature could be from Table 4. obtained according to the following equation where parameters were obtained from Table4 . T (°C) =T + K ·Φ(%) + K ·ln(f) ◦ (6) Tg( C) = Tg0 + K1·Φf(%) + K2· ln(f) (6)

Table 4. Defining parameters of Tg evolution (°C) with the glass fiber volume percentage and fre- ◦ quency.Table 4. Defining parameters of Tg evolution ( C) with the glass fiber volume percentage and frequency.

◦ ◦ ◦ Tg0 (°C)Tg0 ( C) K1 (°C) K1 ( C) K K22 (°C( C/ln/ln (Hz)) 48.95548.955 0.31 0.31 2.04 2.04

The values obtained with the parameters in Table 4 presented an average difference of 5.45The ± 2.6% values and obtained 6.54 ± 3.8% with in the relation parameters to the in test Table values4 presented at the 1-Hz an average frequency difference and the of 5.45 ± 2.6% and 6.54 ± 3.8% in relation to the test values at the 1-Hz frequency and the multifrequency, respectively. multifrequency, respectively.

3.3.3.3. Master Master Curves Curves TheThe Data Data Analysis Analysis software ofof TATA InstrumentInstrument was was used used to to produce produce master master curves curves [44 ]. [44].For For this this work, work, reference reference temperatures temperatures that that were were lower lower but but closeclose to the the glass glass transition transition temperaturetemperature were were selected. selected. Values Values were were obta obtainedined in in the the tests tests run run with with a aconstant constant 1-Hz 1-Hz frequencyfrequency for for each each material material (Figures (Figures 6–8).6–8).

Figure 6. Variation in the storage and loss modulus with time PBT G0 (T = 50.2 ◦C).

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Figure 6. Variation in the storage and loss modulus with time PBT G0 (T = 50.2 °C). Figure 6. Variation in the storage and loss modulus with time PBT G0 (T = 50.2 °C). Storage modulus values showed a loss of stiffness properties over time that was mit- igatedStorage with glassmodulus fiber values content, showed allowing a loss us of to sti improveffness properties storage modulus over time in that all rangeswas mit- of igatedfrequencies with glassanalyzed. fiber Thecontent, results allowing showed us how to improve glass fiber storage content modulus can be inuseful all ranges to guar- of frequenciesantee a minimum analyzed. storage The modulusresults showed value overhow glasstime whenfiber contentcomponents can be are useful submitted to guar- to anteehigh-temperature a minimum environments,storage modulus allowing value overoptimized time whenand reduced components polymer are consumption.submitted to high-temperatureLoss modulus environments, increased with allowing glass fiber op contenttimized inand all reduced ranges of polymer frequencies consumption. analyzed. MaximumLoss modulus value moved increased toward with higher glass fiber frequenc contenties, indecreasing all ranges theof frequencies slope after analyzed.the maxi- Materials 2021, 14, 483 9 of 13 Maximummum, increasing value movedin this way toward the dissipatedhigher frequenc energy.ies, decreasing the slope after the maxi- mum, increasing in this way the dissipated energy.

FigureFigure 7. 7.VariationVariation in inthe the storage storage and and loss loss modu moduluslus with with time time PBT PBT G20 G20 (T (T = =56.3 56.3 °C).◦C). Figure 7. Variation in the storage and loss modulus with time PBT G20 (T = 56.3 °C).

◦ Figure 8. Variation in the storage and loss modulus with time PBT G50 (T = 64.4 C). Figure 8. Variation in the storage and loss modulus with time PBT G50 (T = 64.4 °C). Figure 8.Storage Variation modulus in the storage values and showed loss modu a losslus with of stiffnesstime PBT propertiesG50 (T = 64.4 over °C). time that was mitigatedThe storage with modulus glass fiber value content, showed allowing a linear us tobehavior improve with storage glass modulus fiber content in all when ranges The storage modulus value showed a linear behavior with glass fiber content when samplesof frequencies were held analyzed. at a temperature The results that showedcame close how to glassthe material’s fiber content glass cantransition be useful tem- to samplesperature,guarantee were bearing a minimumheld in at mind a temperature storage its fiber modulus percentage that came value accordingclose over to time the to whenmaterial’s the following components glass Figure transition are 9: submitted tem- perature,to high-temperature bearing in mind environments, its fiber percentage allowing optimizedaccording andto the reduced following polymer Figure consumption. 9: Loss modulus increased with glass fiber content in all ranges of frequencies ana- lyzed. Maximum value moved toward higher frequencies, decreasing the slope after the maximum, increasing in this way the dissipated energy. The storage modulus value showed a linear behavior with glass fiber content when samples were held at a temperature that came close to the material’s glass transition temperature, bearing in mind its fiber percentage according to the following Figure9:

MaterialsMaterials 20212021, 14, ,14 x ,FOR 483 PEER REVIEW 10 10of 13 of 13

Figure 9. Variation in the storage modulus with the percentage of fiber volume depending on the Figure 9. Variation in the storage modulus with the percentage of fiber volume depending on the exposure status. exposure status. The evolution with glass fiber content, expressed as volume percentage, for specimens initiallyThe evolution exposed towith their glass glass fiber transition content, temperatures expressed as and volume after one percentage, year at temperatures for speci- mensclose initially to said exposed temperature to their presented glass transition a linear temperatures evolution given and byafter the one following year at temper- equation, atureswhere close parameters to said temperature were obtained presented from Table a linear5: evolution given by the following equa- tion, where parameters were obtained from Table 5: 0 E = A·Φf(%) + B (Units MPa) (7) E =A·Φ(%) + B (Units MPa) (7)

TableTable 5. 5.DefiningDefining parameters parameters of the of thestorage storage modulus modulus evolution evolution with with the percentage the percentage of glass of glass fiber fiber volumevolume depending depending on onexposure exposure time. time.

ExposureExposure Time/Parameters A (MPa) B (MPa) R2 A (MPa) B (MPa) R2 Time/ParametersINITIAL 209 1955 0.99 INITIAL 1 YEAR209 95.8 1955 397.2 0.99 0.99 1 YEAR 95.8 397.2 0.99 The above equation allowed us to choose the fiber content percentage in order to maintainThe above the valueequation of PBT allowed storage us modulusto choose overthe fiber time content at one temperature percentage higherin order to to the maintainglass transition the value temperature of PBT storage of this modulus material. over For time example, at one in temperature the figure above, higher with to the PBT glassreinforced transition with temperature glass fiber of in this a volume material. percentage For example, near in to the 16% figure (equivalent above, towith a weightPBT reinforcedpercentage with near glass to 30%),fiber in a minimuma volume storagepercentage modulus near to greater 16% (equivalent or equal to to PBT a weight without percentagereinforcement near canto 30%), be guaranteed a minimum for onestorag year,e modulus when the greater sample or is equal held to to a PBT temperature without of reinforcement57.2 ◦C (obtained can be with guaranteed Equation for (4)). one year, when the sample is held to a temperature of 57.2 °C (obtained with Equation (4)). 4. Conclusions 4. ConclusionsPBT is a technical plastic that performs extremely well as an electric insulator with veryPBT few is dielectrica technical losses. plastic Its that use hasperforms considerably extremely increased well as thanks an electric to a growinginsulator number with veryof electronicfew and losses. electric Its systems. use has It considerably is normally manufactured increased thanks by incorporatingto a growing number glass fiber of toelectronic improve and mechanical electric systems. properties It is andnorm canally also manufactured be blended withby incorporating elastomeric glass materials fiber to to improveimprove itsmechanical performance properties upon impactsand can andalso fracturebe blended tenacity. with elastomeric Furthermore, materials PBT offers to improveextremely its performance good versatility, upon which impacts has and enabled fracture additives tenacity. to beFurthermore, incorporated, PBT such offers as flameex- tremelyretardants good and, versatility, particularly, which more has enviro-friendlyenabled additives nonhalogen to be incorporated, additives, tosuch improve as flame their retardantsproperties and, against particularly, fire. All more these enviro-friendly advantages make nonhalogen PBT an ideal additives, candidate to improve to develop their new propertieselectrical against components fire. All associated these advantages with charges make or PBT battery an ideal in all candidate sectors and to develop particularly new in electricalthe growing components transport associated sector. with charges or battery in all sectors and particularly in the growingThe results transport of this sector. work showed the evolution over time of dynamic moduli of PBT reinforced with glass fiber when samples were submitted to high-temperature environment.

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The current study demonstrated that glass fibers’ addition allowed us to increase the value of dynamic moduli in all ranges of temperatures analyzed, allowing us to reduce the loss of stiffness over time and increasing the energy dissipated in each strain cycle. The dynamic moduli evolution with time was obtained by means of a temperature– time transformation using the Data Analysis software of TA instrument and considering glass transition temperature obtained from maximum of loss modulus with temperature. Glass transition temperature increased linearly with glass fiber content and logarithm of the frequency, indicating a limit value of Tg with frequency. Dynamic moduli showed a nonlinear evolution with time when samples were hold to temperatures lower but close to their glass transition temperature. The values obtained improved with glass fiber content in all ranges of frequencies analyzed. In this work, a linear fit of storage modulus depending on fiber content, when samples are submitted to their respective glass transition temperature, was proposed, allowing us to choose the fiber content percentage in order to maintain a value greater than or equal to PBT storage modulus over time at one temperature higher to the glass transition temperature of this material.

Author Contributions: Conceptualization, C.G., F.J.C.-V. and F.C.; methodology, C.G. and F.J.C.-V.; software, C.G. and J.M.; validation, J.M. and C.G.; formal analysis, C.G. and F.J.C.-V.; investigation, C.G. and F.C.; resources, F.J.C.-V.; data curation, C.G. and F.C.; writing—original draft prepara- tion, C.G., J.M., and F.C.; writing—review and editing, F.J.C.-V. and F.C.; visualization, C.G. and F.C.; supervision, F.J.C.-V. and F.C. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Data Availability Statement: Data available on request due to restrictions eg privacy or ethical. Conflicts of Interest: The authors declare no conflict of interest.

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