OR I G I N A L AR T I C L E

Composite reinforcement: Recent development of continuousglass fibers

Hong Li1,* | Thibault Charpentier2 | Jincheng Du3,* | Sandeep Vennam1

1Fiber Glass, Glass Business andDiscovery Center, PPG Industries, Inc.,Pittsburgh, Pennsylvania2NIMBE, CEA, CNRS, Universit�e Paris-Saclay, Gif-sur-Yvette Cedex, France3Department of Materials Science andEngineering, University of North Texas,Denton, Texas

CorrespondenceHong LiEmail: hli@ppg.com

Funding informationNational Science Foundation (NSF) DMRCeramics Program, Grant/Award Number:1105219, 1508001; Department of Energy(DOE) Nuclear Energy University Pro-gram, Grant/Award Number: DE-NE0000748

AbstractLight weight, glass fiber-reinforced composites have gained a broad, global accep-

tance in commercial markets with a total of more than 7 billion of US dollars in

revenue since its first commercial production in US in mid 1930s. This article

briefly reviews recent development of continuous glass fibers with a focus on

high-performance glass fibers. With accelerated commercial demands on high-per-

formance glasses and/or glass fibers, there is a growing realization of fundamental

needs in decoding nature of glass structures or “genes” of glass structure building

blocks and establishing their relationships to properties of glasses or glass fibers.

The related database development can enable researchers shortening the number

of product development cycles to bring new fiber products to the market. A spe-

cial section is, therefore, provided illustrating recent progress in characterizations

of glass structures by using techniques of nuclear magnetic resonance spec-

troscopy, Raman spectroscopy, and molecular dynamics simulations.

KEYWORD S

composites, composition, fiber drawing, fiber Property, liquidus temperature, mechanical properties

1 | INTRODUCTION

Continuous silicate and borosilicate glass fibers have beenwidely used as reinforcements in plastic composite materi-als for commercial applications, including automobile,wind turbine blades, chemical storage tanks and trans-portation pipes, printed circuit board substrate, infrastruc-ture etc.1 At present, a total world consumption of glassfibers well surpasses 5 million metric ton annually,1

among which boron-containing E-Glass fibers and grow-ing volume of boron-/fluorine-free, acid-resistant E-CRGlass fibers2 dominate the markets. Historical develop-ment of the glass fibers and their classifications were dis-cussed elsewhere.1,3,4 In this article, for simplicity, wegroup E-Glass and E-CR Glass fibers under a category ofgeneral commercial applications, whereas specialty glassfibers refer to those exhibiting unique properties to meet

special application requirements, yet significant less involume by a market share, covering S-Glass fibers withsignificantly greater strength for aerospace and ballisticprotection,5,6 R-Glass fibers with significantly greaterYoung’s modulus for longer wind turbine blades,3,6

D-Glass fibers with significantly lower dielectric constantand dielectric loss factor for high-frequency circuit boardsof high-speed communications7,8, AR-Glass fibers withsignificantly greater resistance to strong caustic corrosionfor concrete infrastructures.9,10

This article provides a brief review on continuous glassfiber development, focusing on the specialty fibers, cover-ing (i) glass compositions, (ii) fiber processing, (iii)mechanical property of glass fibers and fiber-reinforcedcomposites, (iv) characterizations of glass structures focus-ing on the use of magic angle spinning nuclear magneticresonance (MAS NMR) spectroscopy, Raman spectroscopy,and molecular dynamics simulation (MD), and (v) chal-lenges and trend of future fiber glass development. A*Member, The American Ceramic Society.

Received: 22 August 2016 | Accepted: 28 December 2016

DOI: 10.1111/ijag.12261

Int J Appl Glass Sci 2017; 8: 23–36 wileyonlinelibrary.com/journal/ijag © 2017 The American Ceramic Societyand Wiley Periodicals, Inc

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comprehensive review on fiber glass technology is beyondthe scope of this article, rather we intend to provide anupdate capturing the current state of research and develop-ment for specialty reinforcement glass fibers.

2 | FIBER GLASS COMPOSITIONS

Compositions of glass fibers for both general and specialtyapplications are summarized in Table 1, including the most

TABLE 1 Chemical compositions (wt%) of continuous glass fibers for reinforcement1,3,5-10

Fiber Glass SiO2 Al2O3 MgO CaO SrO BaO B2O3 R2O F2 ZrO2 Others

E, E-CRa 52-62 12-16 0-5 16-25 — — 0-10 0-2 0-2 — Fe2O3

0.05-0.8TiO2

0-1.5

R 55-60 23-28 4-7 8-15 — — 0-0.35 0-1 0-0.3 — Fe2O3

0-0.5

New R 60-62 14-17 6-8.75 14-17.5 — — 0-2 <1 — 0-2 TiO2

0-1

New R 60-65.5 14.5-20.5 9.4-10.9 11-13.2 — — 0-1 0-4 — — TiO2

0-2

S7 64-66 24-25 9.5-10 0-0.2 — — — 0-0.2 — — Fe2O3

0-0.1

New S 56-61 16-23 8-12 6-10 — — — 0-2 — — <2

New S 51-65 12-19 0-12 0-16 — — — 0-2 — — TiO2

0-3RE2O3

>0.05

New S 50-60 23-27 10-20 — — — 0-4 0-1 — — TiO2

0.5-8

New S 60-71 10-25 12-20 (MgO≥5, CaO≥6) — 0-3 — — —

AR 55-75 0-5 1-10 CaO 0-8 11-21 0-5 1-18 TiO2

0-12Fe2O3

0-5

AR 57-64 0-1 RO (MgO, CaO, SrO, BaO)plus ZnO: 0.2-8

0 Li2O0.5-3Na2O11-15K2O1-5

0 19-24 TiO2

0.5-5

New AR20 67-72 <1 CaO: 3-9RO (MgO, SrO, BaO): <1as impurities

— 11-17Li2O0-5.5K2O 2.5-5.5

0 5-9.5 —

ARb 60�0.8 <1 — 4.5�0.5 — — — Na2O12.5�0.8K2O2.5�0.3

14.5�0.8 Fe2O3

<0.5

D 72-75 0-1 — 0-1 — — 21-24 0-4 — — Fe2O3

0-0.3

New D 52-60 10-18 0 4-8 — — 20-30 Trace 0-2 — —

New D 60-68 9-15 8-15 0-4 — — 7-12 0-2 0-1 — <1

New D 50-60 10-18 1-6 2-5 1-9 1-5 14-20 <1 0-2 — —

New D 60-77 9-15 5-15 0-11 — — 5-13 0-4 0-2 — —

aASTM D 578-00 Standard Specification for Glass Fiber Strands, ASTM, West Conshohocken, PA, US (2000).bAlkaline-Resistant Glass Marbles, JC719-1990, China Standard Publisher, Beijing, (2001) p. 236-237.

24 | LI ET AL.

recent representative glass fibers.1,3,5-10 In general, a major-ity of the glass systems for fiber reinforcement applicationscontain two common major oxides, Al2O3 and SiO2. Bydifferent chemical and structural modifications of the glassnetwork, various fiber glass categories have been created:high alkali (primarily Na2O) vs alkali-free, boron vs boron-free, single alkaline earth (MgO or CaO, primarily) vsmixed alkaline earth (MgO, CaO primarily), zirconia vs zir-conia-free, etc. Exceptions were only found for original D-Glass and AR-Glass fibers, in which the presence of Al2O3

came from impurity only.Focusing on recent development of the specialty fibers,

the use of mixed alkaline earth oxides (MgO and CaO)were considered in designing new S-Glass fibers to signifi-cantly lower liquidus temperature (TLiq) by more than150°C as compared to the traditional S-Glass fiber contain-ing single alkaline earth, MgO. To recover losses of fiberstrength and Young’s modulus by introducing CaO, rareearth oxides (RE2O3) were selectively chosen for designingnew S-Glass fibers.6 For new R-Glass, efforts seemed tofocus on tuning CaO/MgO ratio, plus adding a smallamount of Li2O, to improve batch melting and fiber draw-ing.3 For new D-Glass fibers, both alkaline earth oxidesand alumina have been introduced at the expense of SiO2

or B2O3 or both3,7,8 In turn, microscale phase separation ofthe traditional D-glass has been suppressed during fiberdrawing. In designing AR-Glass compositions a common,key feature is that nearly all of Al2O3 has been replaced byZrO2.

9-11

Compositions of the above specific mixtures drive thechanges of glass properties, such as density, viscosity, liq-uidus temperature, chemical durability, mechanical prop-erty, dielectric property, thermal property, etc. As anexample, Figure 1 depicts the measured composition effectson TF and fiber Young’s modulus, respectively, for E-Glass-based compositions. Clearly, melt viscosity (or TF)and Young’s modulus of glass fiber increase as Al2O3

increases at the expense of other oxides; whereas increasein SiO2 alone raises melt viscosity or TF, but lowers theYoung’s modulus of glass fiber. It becomes self-evidentthat a conflict of interests often exists in glass designrequirements between fiber performance and fiber process-ing. It is also not uncommon in raw material selections thatglass designers need to address tradeoff between cost ofraw materials and cost of productions and fiber glass per-formance as a complete technology package.

In glass design, it is commercially critical to balance/op-timize properties to meet requirements of glass fiber pro-cessing, product performance, and product costs.Utilization of statistical composition—property models canbe adventurous to aid glass design.3 Attention is growingon decoding glass “genome”12 or identifying “genes” ofglass structural building blocks (g-GSBB). The recent

developed topological approach has proved to be a usefulsupplementary approach in optimizing glass composi-tions.12 The use of statistical composition—property mod-els for designing glasses with target properties can offerdistinctive advantages, shortening the design cycle.3,13 Onefurther step explored in glass design to achieve multipletarget properties was reported in an attempt developing sta-tistical g-GSBB—property models.14 The success of usingthe above approaches for glass design highly depends onavailability and generation of reliable glass property data-base, including g-GSBB, relevant to composition spaces ofthe commercial interests.

3 | FIBER GLASS PROCESSING

Fiber glass-manufacturing processes cover a broad spec-trum, beyond the scope of this article, and their generalsteps were described elsewhere.1,15 As illustrated in Fig-ure 2, high-performance glass fibers typically demand forhigher processing temperatures. The requirements have ledto advances in fiber glass furnace technology, including

FIGURE 1 Composition effects on E-Glass: (A) fiber drawingtemperature and (B) fiber Young’s modulus as a function of singleoxide change. [Color figure can be viewed at wileyonlinelibrary.com]

LI ET AL. | 25

electrical boost, oxy-fuel firing, and most recently theadoption of crown firing technology. Selection and qualitycontrol of raw materials are vital to maintain the processstability and hence, higher production efficiency. Types ofraw materials and variations impact batch-to-melt conver-sion rate, melt foaming, melt fining, iron oxidation states inglass, melt volatility, production energy, under glass tem-peratures in furnace, fiber production yield, furnace refrac-tory service life, etc.

Microcrystals in molten glass often leads fiber breakagein a fiber drawing process. Glass with lower liquidus tem-perature (TLiq) likely makes fiber processing more robust asa result of minimizing glass crystallization tendency, espe-cially in some areas along Forehearth where temperaturecan be significantly lower than the nominal controlled tem-perature. For fiber glass design, the difference between TF

and TLiq (or DT) should be kept greater than 55°C. TLiq issensitive to variations in many of the key glass oxides; anexample in Figure 3 shows a nonlinear effect of MgO/CaOratio on TLiq for E-CR Glass fibers.

Fiber cools from above 1000°C to nearly room tempera-ture at a typical rate of 0.59106°C/s16 and glass viscositychanges by several orders of magnitude. Glass with a char-acter of “strong” liquid17 favors continuous fiber attenua-tion process, whereas one with more “fragile” liquidcharacter17 favors spinning or cascading process, makingdiscontinuous fibers or wool fibers. Figure 4 shows temper-ature (normalized)—viscosity relationship of “strong” and“fragile” liquids, which can be numerically described byfragility index, m, defined by the change in viscosity(log10g) over temperature (Tg/T) at glass transition temper-ature (Tg).

17 Fibers of E-, E-CR, R-Glass exhibit similarmelt fragility between the two extremes.

Iron oxidation state Fe2+/Fetotal in the melt is critical tofiber-forming stability because of its impact on glass struc-ture, hence, viscosity,18,19 fiber-cooling rate, fiber-formingtension, and strength of final fiber products.20,21 In com-mercial fiber glass production, oxidation states of iron orstability of Fe2+/Fetotal or Fe2+ concentration are typicallycontrolled and/or adjusted by selectively using oxidizingagents (sodium sulfate, sodium nitrate, cerium oxide, etc.)and reducing agents (coal, graphite, etc.) based on chemicaloxygen demanding (COD) of the glass batch and CODvariability (increase or decrease), which can fluctuate overtime from raw materials, such as clay, limestone, dolomite,colemanite, etc. Calcined limestone, or quicklime (CaO),has been used to aid batch melting, especially under highfiber production throughput conditions. Quicklime suppliesoften contain different residual sulfur levels and differentreactivity to moisture water, depending on the source of fir-ing (natural gas, oil, and coal) used and the duration of the

FIGURE 3 Effect of mixed alkaline earth (MgO/CaO) onliquidus temperature of E-CR Glass compositions

FIGURE 4 Viscosity as a function of reduced temperature,illustrating glass as “strong”, “fragile” glass and characteristics ofcommercial fiber glasses.3

FIGURE 2 Melt viscosity-temperature relationships for typicalcompositions of continuous glass fibers for general reinforcement andfor PCB applications. [Color figure can be viewed atwileyonlinelibrary.com]

26 | LI ET AL.

calcination processes. Understanding of the effects ofquicklime usage on iron redox stability and melt foamingover the furnace-fining area (or hot spot) is important formaintaining a stable commercial operation.

4 | FIBER GLASS AND FIBER-REINFORCED COMPOSITEPERFORMANCE

Benchmarking with E- and E-CR Glass fibers, mechanicalproperties of some of specialty glass fibers are illustrated inFigure 5. New R-Glass fiber with improved Young’s mod-ulus meets the requirements for longer wind turbine bladesand new S-Glass fiber offers a low processing temperaturesolution to achieve high strength and Young’s modulus foraerospace and ballistic protection applications. Yttriumoxide (Y2O3) in the new S-Glass fibers benefits mechanicalproperties (Figure 6) and fiber processing (not shown);6,22

the observed benefits of using Y2O3 appeared to correlatewith creation of fivefold coordinated Al, that is, [AlO5], atthe expense of fourfold coordinated Al, that is, [AlO4], inthe new S-Glass fibers when high ionic field strength (IFS)Y3+ replaced low IFS Ca2+ (cf. Section 5.1)

New D-Glass fibers23 showed significant improvementon tensile strength beyond traditional D-Glass fibers,

(A)

(B)

FIGURE 5 Single-fiber pristine strengths (A) and Young’s moduli (B) (one standard deviation was within 10% and 5% of the averagedvalues for the strength and modulus, respectively. Pristine strength means that the values were derived by testing individual fresh, uncoated fibersin tension right after the fiber samples were drawn in ambient condition, plus the laboratory relative humidity controlled between 30 and 40%. Tominimize air dust in contact with fiber surfaces, fiber drawing and testing were conducted in the same “clean” room)

FIGURE 6 Young’s modulus (A) and pristine strength (B) ofsingle fibers of new S-glass, illustrating effects of Y2O3 on theproperties and fivefold coordinated [AlO5] determined by 27Al MASNMR method21 (density values are shown in the modulus plot next toeach point and a start represents different host glass chemistry thanthe rest). [Color figure can be viewed at wileyonlinelibrary.com]

LI ET AL. | 27

(2415 MPa5) as appreciable amounts of MgO and Al2O3

were introduced into the original binary system of B2O3

and SiO2. Besides its intended primary application inprinted circuit board (PCB), some of the new D-Glassfibers with higher strength and lower density can beconsidered for reinforcement applications, competing withE-Glass or E-CR Glass fibers.22 Their improved dielectricproperties may be explained in terms of their borate

network structure, predominant threefold boron, that is,[BO3], which is less polarizable as compared with fourfoldboron, that is, [BO4] (cf. Figure 10).

Better mechanical performance of commercially pro-duced new R-Glass fiber strands (a bundle of fibers) hasbeen demonstrated over E-, E-CR, and R-Glass counterparts (Figure 7), whereas low-forming temperature, new S-Glass fiber strands have similar tensile strength and

(A)

(B)

FIGURE 7 Mechanical properties of glass fiber strands

(A)

(B)

FIGURE 8 Mechanical properties of UD laminate composites (epoxy resin)

28 | LI ET AL.

Young’s modulus as high-forming temperature S-Glassfiber strands. The UD laminate composite samples rein-forced by using new S-Glass fibers6 (made at temperature,about 150°C lower than S-2 Glass� Fiber) exhibitedmechanical performance close to the properties (tensilestrength=1710 MPa, Young’s modulus=56 GPa) for thehigh-temperature S-Glass fiber, that is, S-2 Glass� Fiber,reinforced UD laminate composites (AGY Technical Pro-duct Guide-Rev.11, www.agy.com), which outperforms ofUD laminate composites made of both E-, E-CR, and newR-Glass fibers (Figure 8).

5 | RECENT PROGRESS ONSTRUCTURE CHARACTERIZATIONSOF GLASS AND GLASS FIBER

Continuous random network (CRN) of oxide glasses byZachariassen24 has been widely used to describe glassstructure in general and associated isotropic properties. Ahalf century later, modified random network (MRN) ofglass was proposed by Greaves.25 It has been well acceptedthat the network structure of glass lies between CRN andlong-range ordered crystalline structures. Due to the pagelimitation, we briefly provide few examples from wealthyliteratures on glass structure characterizations, focusing onthree techniques, nuclear magnetic resonance (NMR),Raman spectroscopy, and molecular dynamics simulations(MD).

5.1 | Nuclear magnetic resonancespectroscopy (NMR)

Several decades of technological and methodological devel-opments have firmly established high-resolution solid-stateNMR as one of the most powerful tools to probe glassstructure at an atomic scale,26-28 including recent advancesin coupling NMR with Molecular Dynamics simulations.29

NMR is an atomic selective spectroscopy: each atomic spe-cie with a nucleus possessing a nonzero spin (the nuclearspin I) is, therefore, a potential probe of the structure, pro-vided that the nuclei of interest has favorable NMR proper-ties. Information from NMR study essentially provides afingerprint of the local atomic environment. In the contextof fibers (cf. Table 1), useful nuclei for NMR are networkformer cations (29Si (I=1/2), 27Al (I=5/2) and 11B (I=1/2))and network modifier cations 23Na (I=3/2), 43Ca (I=7/2),etc. Anions such as 17O (I=5/2) play a special role in oxidematerials and provide very rich information; 19F (I=1/2)can also be a useful probe.

As fibers being essentially silicate materials, 29Si MASNMR would be expected to provide key information. How-ever, the complexity of multicomponent glasses significantly

affects resolution of the different tetrahedral Qn units (Q:tetrahedral of T-O-T where T=Si, Al), which can be clearlyobserved in simple binary systems.30,31 The frequencyrange of 29Si MAS NMR spectra covers between �70 ppmand �120 ppm, which mainly corresponds to the distribu-tion of the isotropic chemical shift diso. For alkali and alka-line earth silicates, typical values are reported for Q4

(�110 ppm to �100 ppm), Q3 (�95 ppm to �90 ppm),Q2 (�90 ppm to �80 ppm), Q1 (�76 ppm to �68 ppm),and Q0 (�62 ppm to �66 ppm), but bonding with alu-minum or boron atoms generally results in a larger chemi-cal shift dispersion. For example, each Al substitution forSi in linkages Si-O-T shifts diso by 4-5 ppm in average.32

More sophisticated techniques exist for a direct edition of

FIGURE 9 27Al MAS NMR spectra and its simulation (R202:La2O3-MgO-CaO-Al2O3-SiO2 fiber glass with 4.5 mol% La2O3) (A)

23

and Two-dimensional MQMAS spectrum showing the clearseparation between AlO4 and AlO5 units (B). In the MAS spectrum,only the central transition +1/2↔�1/2 is shown and contributions ofthe so-called satellite transitions (ST, that is, all other transitionswhich spread overs several MHz) to the center band are indicated.Data were acquired at a magnetic field of 17.6 T. [Color figure canbe viewed at wileyonlinelibrary.com]

LI ET AL. | 29

Qn(mAl) units (a Qn tetrahedron bonded to m AlOx units),but their applications have been so far limited to simplesystems and require29Si isotopic enrichment (99%).33

The 27Al MAS NMR has certainly found its largest fieldof application in the context of aluminosilicate glasses. Thesalient feature of 27Al MAS NMR is its capability to detectwith very high sensitivity (few %) the different coordina-tion numbers [AlO4], [AlO5], and [AlO6].

34-37 Similarly,11B MAS NMR has become an essential tool, probingboron in borosilicate glass structures,38,39 in which [BO4]and [BO3] units can be resolved. Both are so-calledquadrupolar nuclei (nuclear spin I>1/2) and the spectra aretherefore subjected to a quadrupolar broadening (resultingfrom the coupling of the nuclear quadrupolar moment tothe local electric field gradient) even under MAS,40 which

in turn often limits the spectrum resolution. In the lattercase, a two-dimensional technique named MQMAS (Multi-ple Quantum MAS) has to be employed.22 Typical spectraare shown in Figure 9 and Figure 10 for 27Al and 11B,respectively.

Concerning 27Al MAS NMR, high-coordination units[AlO5,6] are generally in low quantity for alkaline and alka-line earth in peralkaline compositions so that high magneticfields (typically 17.6 T) or/and the two-dimensionalMQMAS technique are required for its observation andquantification. In contrast, high IFS cation (such as RE)largely favors formation of high-coordination units.36,41

Typical 27Al MAS NMR frequency ranges are 50-70 ppmfor [AlO4], 30-40 ppm for [AlO5], and 0-20 ppm for[AlO6].

42 The NMR spectra are also dependent on charge

(A)

(B)

FIGURE 10 11B MAS NMR experimental and simulated spectra of a new D-Glass fiber (before and after thermal treatment) (A) and two-dimensional MQMAS spectra (B). Data were acquired at a magnetic field of 11.7 T. [Color figure can be viewed at wileyonlinelibrary.com]

30 | LI ET AL.

compensation mechanisms through the quadrupolar interac-tion : the larger cation field strength results in largerquadrupolar interactions.43,44 27Al MAS NMR in alumi-nosilicate has been particularly employed to investigate theeffect of thermal history or pressure: an increase in thepopulation of high-coordination states units [AlO5,6] is gen-erally observed with higher quench rate and pressure.45,46

The salient feature of 11B MAS NMR is its capacity toresolve unambiguously the two [BO3] and [BO4] specia-tion, even at moderate magnetic field (in practice >11.7T),as shown in Figure 10. These results also illustrate the very

high sensitivity of [BO4]/[BO3] speciation to thermal his-tory: an increase in [BO4] is generally observed for lowerquench rate, as discussed in several papers.47,48 Such abehavior is also observed under pressure in aluminoborosil-icates46,49 and E-Glass50 where uniaxial pressure werecompared to isostatic pressure. Among the modifier cations,NMR studies of 23Na,30,51,52 43Ca,53-56 and 25Mg56-58 insilicate glasses can be found in literature.

5.2 | Raman spectroscopy

Raman spectroscopic technique has been widely used instudying structures of minerals, glasses, and melts.59-61 Fig-ure 11 shows an example of Raman study of new S-Glassfibers, illustrating effect of RE2O3 on the glass structure.22

Relative to the baseline host glass (MgO-CaO-Al2O3-SiO2),RE2O3 depolymerizes the glass network, reducing thevibration band peaked near 450 cm�1 and pre-existing Q3

peaked near 1050 cm�1 and at the same time, create Q2(RE)

and Q1(RE) associated with RE ions.22 Speciation reaction

of the silicate network can be qualitatively deduced fromthe Raman measurement, which explains why RE ions withdifferent IFS affect melt viscosity or fiber drawing tempera-ture differently.23

Among E-Glass compositions, the degree of the glassnetwork polymerization can be qualitatively compared interms of their Raman low-frequency band between 450 and

FIGURE 11 Raman spectroscopic study of new S-Glass fibers: (A) Baseline Glass and (B) Raman Difference Spectrum (RDS) Analysis ofthe Fibers Containing Rare Earth Oxides (2 and 4.5 mol%)22. [Color figure can be viewed at wileyonlinelibrary.com]

FIGURE 12 Relationship between dielectric constant andRaman low-frequency band intensity of E-Glass samples withsimultaneous changes of SiO2, B2O3, Al2O3, CaO, and MgO

LI ET AL. | 31

500 cm�1, which represents stretching vibrational mode ofbridging oxygen, T-O-T where T=Si, Al, and B. Thegreater the low-frequency band is, the polymerized the net-work becomes; and in turn, lower dielectric constant (Dk)of the glass will be expected, which is in good agreementwith experiments shown in Figure 12.

5.3 | Molecular dynamics simulations (MD)

Since the first MD simulations of silica glass by Angel andcoworkers62 four decades ago, MD has been widelyaccepted as a critical method to understand the structureand properties of oxide glasses and other amorphous mate-rials.63 MD is an atomistic simulation method based on thesolution of Newton’s equation of motion of an assembly ofatoms or molecules that represent the structure of a system,

a glass, crystal, or a melt. One of the most important inputsin MD simulations is the empirical interatomic potential;the quality and reliability of the potential to a large extentdetermine the results and thus the quality of MD simula-tions.63-66 The cooling rate of the simulated glass is typi-cally in the order of 1012 K/s, significantly higher than thatexperienced in commercial glass processing (1-100 K/s)but closer to fiber drawing (105�7 K/s). The effect of cool-ing rate on the structure of sodium silicate glasses wasexamined.67

Molecular dynamics simulations have been used tostudy sodium aluminosilicate glasses for both peralkali andperaluminous compositions with some similar to fiberglasses (Table 1).65,68 Aluminum were found to be mainlyfourfold coordinated with sodium being charge compen-sators of [AlO4]

� tetrahedrons.66 Oxygen tri-clusters (oneoxygen bond to three aluminum or two aluminum and onesilicon) were also observed as a charge compensationmechanism.66 MD simulations have also been used tounderstand RE ion-doped silica, silicates, and aluminateglasses.67–73. Du studied Y2O3-Al2O3-low SiO2 glasses andfound that [AlO4] species were dominant with a smallamount of [AlO5] and [AlO6] and additional SiO2 furtherconverted [AlO5] or [AlO6] to [AlO4],

69 which explainedexperimental observations of much improved glass forma-bility of the compositions with higher SiO2. Figure 13compares Young’s modulus and shear modulus of alumi-nosilicate glasses as a function of Al2O3 from MD simula-tions; the results agreed fairly well with the measuredvalues.69

In MD simulations of molten glass of the Na2O-CaO-Al2O3-SiO2 system,73 tendency of Na segregation on themelt surface was predicted, implying the existence of higherNBOs near the melt surface over the bulk (Figure 14). MDsimulations of B2O3-containing glasses pose a more signifi-cant challenge, mainly due to the availability of suitablepotentials to reproduce effect of composition on boron distri-bution, [BO3] vs [BO4].

63 Limited successes were reportedso far,72-75 but with more recent development of empiricalpotentials for borosilicate and boroaluminosilicate glasses, itis expected that more compositions close to realistic fiberglasses can be simulated in the near future.76

6 | CHALLENGES AND TREND OFSPECIALTY FIBER GLASSDEVELOPMENT

Besides fiber glass chemistry development, sizing chem-istry is equally critical for composite applications, whichserves multiple functions of protecting fiber surfaces fromcontact damage, coupling with organic resin in application,etc.1 Kinetics of fiber hydrolysis can be accelerated when

FIGURE 13 Mechanical properties of Young’s modulus (A) andshear modulus (B) as a function of Al2O3 in sodium aluminosilicateglasses65. [Color figure can be viewed at wileyonlinelibrary.com]

32 | LI ET AL.

fibers are under tension in applications, resulting in signifi-cant mechanical degredation.77,78 Achieving the highestusable strength of glass fibers (USGF) requires fiber sizingto have certain hydrophobicity. In general, sizing develop-ment proceeds after candidate fiber chemistry being welldefined; therefore, to find suitable sizing chemistry for newfibers for target applications, researchers continuously facechallenges in acquiring specific knowledge of fiber surfacechemistry/structure (can be different from bulk glass)79-81

including distributions of NBOs, borates (BO4 vs BO3), sil-icates (Qn), hydroxyl concentrations, etc.82,83 Glass surfacechemistry research is crucial and greatly needed.

In future fiber glass development, demands for generat-ing new database are growing; the database ought to con-sist of spectroscopically derived g-GSBB, glass propertiesrelevant to processing and application, and finally g-GSBB—property-based statistical models and/or topological mod-els.3,12-14,84-87 The new approach to fiber glass develop-ment can significantly shorten the number of cycles tobring new products to the market.

7 | SUMMARY

Continuous glass fibers for composite reinforcement werebriefly reviewed with a focus on specialty glass fibers, cov-ering composition, processing, and properties of fibers andcomposites. Research and development of high-performance

glass fibers has been putting more efforts on reducing fiber-processing temperature without or with minimum penaltyof product performance. Also covered were examplesillustrating advances in NMR and Raman spectroscopictechniques and MD simulations, which probe glass networkstructures in correlations with glass/glass fiber properties.Database development of the glass structure building block,g-GSBB, including statistical-based property models andtopological models, is going to play a more vital role inshortening the number of cycles bring new fiber glassproduct to the market.

ACKNOWLEDGMENTS

H. Li thanks PPG support for preparing the manuscript andacknowledges Dr. N. Ollier for providing Raman data ofthe fiber samples. J. Du acknowledges financial supportfrom National Science Foundation (NSF) DMR CeramicsProgram (project # 1105219 and 1508001) and Departmentof Energy (DOE) Nuclear Energy University Program(NEUP, project # DE-NE0000748).

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How to cite this article: Li H, Charpentier T, Du J,Vennam S. Composite reinforcement: Recentdevelopment of continuous glass fibers. Int J ApplGlass Sci. 2017;8:23–36.

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