Transparent fiber glass reinforced composites

Composites Science and Technology 77 (2013) 95–100

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Transparent fiber glass reinforced composites D.J. Krug III a,b, M.Z. Asuncion a, V. Popova a, R.M. Laine b,c,⇑ a

Mayaterials Inc., Ann Arbor, MI 48108-2297, United States Macromolecular Science and Engineering, University of Michigan, Ann Arbor, MI 48109-2136, United States c Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109-2136, United States b

a r t i c l e

i n f o

a b s t r a c t

Article history: Received 10 August 2012 Received in revised form 5 November 2012 Accepted 18 December 2012 Available online 9 January 2013

We present here studies targeting the processing of cross-woven, fiber-glass reinforced epoxy–resin composites with 50 wt.% loadings of 0/90 cross-woven fiber glass mats of potential use as low-cost, high strength, light weight materials for safety/sports goggle, motorcycle helmet or window armor applications. Epoxy–resin systems were developed that match the refractive index (RI) of S-glass (1.521–25) fiber mats with woven densities of 0.012 g/cm2. The fiber glass mats were impregnated with the RI matched matrix and cured under pressure to produce composites with high transparencies, up to 84%. Moreover their mechanical properties are superior to those reported for traditional fiber glass reinforced composites exhibiting tensile strengths of up to 333 ± 12 MPa and flexural strengths of up to 436 ± 28 MPa. The improved values likely result from the elimination of large surface flaws through the use of very smooth mold surfaces. The current composites exhibit slightly blue or yellow chromatic dispersion resulting from incomplete RI matching at all visible wavelengths. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: A. Hybrid composites A. Nanocomposites A. Polymer–matrix composites (PMCs) Transparent composites A. Glass fibers

of the glass fiber (1.46–1.56) making transparent FRCs per Fig. 1. Here, the polymer matrix consists of a photocured mixture of polythiopropylsilsesquioxane and an allyl compound, reactions (1) and (2). The transmittance of the resulting polymer is shown in Fig. 1c. Although Matsukawa has yet to measure the mechanical properties of these materials, it seems likely that, based on the Table 1 data, they will outperform polycarbonate (PC) and related materials.

1. Introduction It is well recognized that composite materials can offer excellent high-strength-to-weight properties. For example, glass and carbon fiber reinforced epoxy resin (FRC) composites offer significantly better properties than pure epoxy resins. While epoxy resins do not offer the properties of polycarbonates (Table 1), epoxy FRCs offer superior properties. Unfortunately, glass fiber RCs are not normally transparent. However, recent work at the Osaka Municipal Technical Research Institute (OMTRI) by Matsukawa provides an unexpected solution to making transparent, glass fiber RCs [1]. These materials are expected to be superior to most all-polymer transparent composite materials. Matsukawa finds that by tailoring the properties of the matrix material, the refractive index (RI) can be tailored to match the RI

H+/H2O SH

(EtO)3Si

S

N

O HS

SiO1.5

n

+

O

O N

N

HS

SiO1.5

n

ð1Þ

SiO1.5 n

O

N

Uv Curing

N

-EtOH

N

RI = 1.56

ð2Þ

with Initiator

O

⇑ Corresponding author at: Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109-2136, United States. Tel.: +1 734 764 6203. E-mail address: [email protected] (R.M. Laine). 0266-3538/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compscitech.2012.12.010

O

Earlier work in this area is limited to publications from several groups from Missouri S & T, one group from Japan, and Boeing [4–10]. The objective in these studies was to make high strength,

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Table 1 Properties of commercial matrices, polymer and glass fibers, and glass epoxy FRCs [2]. Glass fibers

Density (g/cc)

Tensile strength (GPa)

% Elongation at break

Elastic moduli (GPa)

CTE (lm/°C)

RI

Izod strength (J/cm)

Tg (°C)

A D E R S S-2

2.44 2.11 2.58 ± 0.04 2.54 2.48 2.46

3.3 2.4 3.4 4.1 4.6 4.9

4.8 4.6 4.8 4.8 5.4 5.7

69 52 74 86 86 87

9.0 2.5 5.0 3.3 5.2–5.6 1.6

1.538 1.465 1.558 1.546 1.525 1.521

– – – – –

– – – – –

Polymer fibers Spectra 1000 Polystyrene

0.97 1.06

(MPa) 300 42–52

– 1.5–2

172 3.1

– 66

– 1.58

– 0.19

– 120

Matrices PC PC/PMMA alloy OG/Epon862/Epikure3300

1.19 1.20±.05 –

55–65 34–60 23.8

50–130 4.3–40 3.48

2.1 1.6–2.7 –

65–70 94 –

1.573–86 – –

3.2–8.5 0.5–7.2 –

150 – –

FRCs PC PMMA PC/PMMA alloy

1.49 1.52 1.49

150 121 124

3 1 2

10 13 12

34 – –

40 40 40

1.5 0.5 0.9

– – –

Fig. 1. (a) Glass cloth, RI = 1.56. (b) Cloth impregnated with (HSCH2CH2SiO1.5)n/[NC@O(allyl)]3 and photocured. (c) Transmittance of photocured matrix [3].

optical quality composites for aircraft canopies/windows. All efforts listed involved the researchers first making their own glass fibers prior to making uniaxially reinforced composites. The Missouri researchers produced fiber glass from a specialty BK10 optical glass (RI = 1.498 at 589 nm) with average fiber diameters of 12 lm and used PMMA (RI = 1.492 at 589 nm) as the polymer matrix. They reported that uniaxially aligned composites with 30 vol.% fiber offered 20% transmission; whereas fibers with average diameters of 50 lm and the same vol.% loading exhibited 45% optical transmission. In somewhat more recent work, Kagawa et al. [9] described processing fiber glass reinforced epoxy resin composites using uniaxially aligned glass fibers with average fiber diameters of 18, 37 and 50 lm. These fibers were fabricated from bulk glass (Corning 1724) with an RI = 1.542 at 589 nm. Volume fractions of fiber ranged from 0 to 45 vol.%. At 45 vol.% and 589 nm, light transmission averaged 50–55% for all fiber diameters. As the reader might imagine, from a practical perspective, the need to fabricate fibers from bulk glass means that this approach is really more of an academic study than one with any particular practicality. Furthermore, the use of uniaxial fiber alignment means that the transverse mechanical properties will be those of the matrix; whereas the axial mechanical properties will be highly dependent on the volume fraction of the fibers.

In our efforts, we began with the objective of using commercial 0/90 cross-woven fiber-glass mats of S-glass targeting composites with at least 50 wt.% loadings of fiber-glass and transparencies of >80%. Our approach focused primarily on the development of matrices based on polyfunctional T8, T10 and T12 silsesquioxane SQs [(RSiO1.5)8,10,12] Fig. 2a–c, and in particular, in this report, Q8 cage compounds (Fig. 2d). SQs and Q cages represent versatile classes of highly symmetrical, three-dimensional organosilicon compounds with well-defined nanometer size structures. These compounds are extremely useful as platforms for assembling hybrid nanocomposites with properties intermediate between those of ceramics and organics, because these materials possess an ideal combination of a rigid, thermally stable silica core and a more flexible, modifiable organic shell [11–28]. Because they can be assembled nanometer by nanometer at nanometer length scales, they offer the potential to provide materials with highly reproducible global properties and the opportunity to precisely predict and tailor those properties [11]. There is also the potential to generate new properties on the nanoscale not available in the bulk that can then be used to create entirely new materials by ordering over large length scales or simply using them as is [11–28]. We have previously reported the synthesis of a variety of functionalized Q8s, including the octaglycidyl cage, OG (Fig. 3a) and its bifunctional (Janus) analog (TGTSE, Fig. 3b) [11,29].

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RMe2SiO RMe2SiO

OSiMe2R Si O OSiMe2R Si O

O

Si

O O Si O

RMeO 2SiO O Si O RMe2SiO

O Si OSiMe2R O Si OSiMe2R

O

Fig. 2. Idealized structures of (a) T8, (b) T10, (c) T12 SQs and (d) Q cage compounds [11–13].

O O O

O

O

O

O SiMe 2

SiMe2

O

O O

O

Si O O Si O

SiMe 2 O

O (EtO)3Si Me 2Si

Si

O

Me2 Si Si O O O

O O Si Si Me2 O Si O O

O

O

Si

Si O

O

O

SiMe 2 O

O

O Si Me 2

O

O

SiMe2

O

O

Si(OEt)3

SiMe 2

Me2 Si

O

(EtO)3Si

Si

O O Si O

O O Si Si Me 2 O Si O O

(a)

Si

O

O

Si

Si O

O

O

Me2 O Si Si O O O Si Me 2

O

O O

SiMe 2

O Si(OEt)3

SiMe2

O

O

O

O

O

(b)

Fig. 3. Structures of (a) octaglylcidyl SQ (OG) and (b) tetraglycidyl Janus SQ (TGTSE).

The work presented here details our efforts to find combinations of Q8 systems, especially OG used in conjunction with traditional epoxy resins and amine curing agents in an effort to first match RI values with samples of 0/90 cross-woven S-glass fiber mats and then to learn to process fully dense and transparent FRCs. After overcoming a number of problems outlined below we are now able to make good to excellent FRCs yet some problems remain, as addressed in a second paper [30].

New Castle, DE). Samples (5–10 mg) were loaded in alumina pans and ramped to 1000 °C while heating at 10 °C/min. The N2 or air flow rate was 60 mL/min.

2.1.2. Differential Scanning Calorimetry (DSC) Calorimetry was performed on materials using a DSC Q20 (TA Instruments, Inc., New Castle, DE). The N2 flow rate was 60 mL/ min. Samples (10–15 mg) were placed in a pan and ramped to 400 °C (5 °C/min/N2) without capping.

2. Experimental 2.1. Analytical 2.1.1. Thermal Gravimetric Analyses (TGA) Thermal stabilities of materials under N2 or air were examined using an SDT Q600 simultaneous DTA-TGA (TA Instruments, Inc.,

2.1.3. Fourier-Transform Infrared Spectroscopy (FTIR) Attenuated total reflectance (ATR) Fourier transform spectra were recorded on a Nicolet iS10 Series ATR-FTIR spectrometer equipped with Smart Orbit diamond ATR accessory 30,000– 200 cm1 (Thermo Scientific, Inc., Madison, WI).

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2.1.4. Gel permeation chromatography All GPC analyses were done on a Waters 440 system equipped with Waters Styragel columns (7.8  300, HT 0.5, 2, 3, 4) with RI detection using Optilab DSP interferometric refractometer and THF as solvent. The system was calibrated using polystyrene standards and toluene as reference. Analyses were performed using PL Caliber 7.04 software (Polymer Labs, Shropshire UK).

3. Results and discussion

Octakis[[3-(2,3-epoxypropoxy)propyl] dimethylsiloxy] Q8 silsesquioxane (OG) was synthesized in-house at Mayaterials. EPON™ Resin 862 (Diglycidyl Ether of Bisphenol F) and EPIKURE™ 3300 cycloaliphatic amine were purchased from Momentive Specialty Chemicals, Inc. ANCAMINEÒ 2286 was purchased from Air Products, Inc. All other solvents were purchased from Fisher or Aldrich and used as received. All reactions were conducted in the presence of air. S-glass fiber mats were purchased from Fibre Glast, Corp. with weave densities of 3.4–5.8 oz/yd2 (0.0115–0.0197 g/cm2).

Transparent resins with high RIs and Abbe numbers are important as adhesives for joining optical glass fibers or for mending broken glass objects, for example [30]. Alternately, matching the RI of a resin to S-glass fibers (Table 2) offers the potential to fabricate transparent fiber-reinforced composites (FRCs) for high strength/ low weight applications as suggested in Section 1. We have performed detailed studies mapping synthesis, processing and properties relationships for a wide variety of SQ and Q composite materials that set the stage for the current studies [11,12]. In these previous studies, we found that we could tailor mechanical properties, CTEs and thermal stabilities over a wide range of values. However, given that the mechanical properties of FRCs rely directly on fiber volume fraction, the objectives of our current work focused simply on developing transparent matrices with controlled RIs and with Tg’s either well above normal use temperatures or systems without noticeable Tg’s. Further constraints include the need to use non-aromatic amine curing systems to avoid commonly observed yellowing, as well as the need to use systems that are liquid at or near ambient temperatures for impregnation. A further unexpected issue that arose during our studies was the discovery that most commercial grade fiber glass (as opposed to optical fibers) processed in volume contains bubbles or ‘‘seeds’’ within the fibers that scatter light and make it very difficult to obtain true transparency. However, some fiber glass producers offer low seed S-glass products providing more suitable woven mats for our studies. As our experience in processing increased, it also became obvious that having optically finished molds made a considerable difference in transmission. A further result of this is that by introducing high quality mold surfaces, mechanical properties greatly improved as surface flaws were greatly reduced. Table 2 provides basic properties data for PMMA FRCs produced at Missouri. As can be seen, volume fractions of 33% gave flexural strengths of 580 MPa and moduli of 18 GPa with transmission at 589 nm of 20%. At 4 vol.%, these values drop to 170 MPa and 3.5 GPa but transparencies increase to >50%. These values are somewhat better than pure PMMA, which has a transmittance of 92%.

2.3. Synthetic methods

3.1. Mechanical properties

2.3.1. Typical S-glass composite processing method OG (9.6 g) was added to EPIKURE™ Curing Agent 3300 (6.9 g) and EPON™ Resin 862 (5.2 g) with light heating (60 °C) to reduce the viscosity and thoroughly mixed. The resin was poured into a polypropylene bag containing woven S-glass fiber mats and partially cured at 70 °C/15–30 min until tacky. The fiber glass preform was then removed and sandwiched between two 10  15 cm PMMA plates and allowed to cure at RT/24 h under constant pressure (1.3 MPa).

Table 3 provides similar data for our S-glass composites. The mechanical properties listed were measured by the Composite Materials Laboratory at Michigan State University according to ASTM standards D3039 and D790-03. As can be seen, for the same volume fractions of fiber glass, transmittance is significantly increased from 18% to 77%. Transmittance was further increased to 84% at slightly higher fiber volume (37%) as molding techniques were improved to impart optical finishes. The higher transmittance is attributed to matching the

2.1.5. % Transmission measurements Transmission data was collected on a Shimadzu UV-1601 UV– vis transmission spectrometer equipped with solid sample holder. 2.1.6. Refractive index and Abbe number measurement RIs and Abbe numbers of samples were measured with an ATAGO Multi-Wavelength Abbe Refractometer DR-M2. RIs were measured at wavelengths of 486, 589, and 656 nm from which the Abbe number (mD) was calculated. The system was calibrated with distilled water at ambient temperature. 2.1.7. Mechanical property testing Mechanical properties were measured by the Composite Materials Laboratory at Michigan State University. Tensile testing was conducted according to ASTM D3039 on specimen cut to 0.500  500 . Flexural properties were determined in three-point bend testing according to ASTM D 790-03 where the span to depth ratio was 24:1 and the strain rate was 0.01 (in./in.)/min. 2.2. Materials

Table 2 Mechanical and optical properties of borosilicate glass fiber/PMMA composites [5–8]. Sample Missouri MS & T Borosilicate glass/ PMMA Borosilicate glass/ PMMA Borosilicate glass/ PMMA PMMA matrix

Vol.%

Fiber dia. (lm)

Tensile strength (MPa)

Elastic modulus (GPa)

Flexural strength (MPa)

Flexural modulus (GPa)

%T @589 nm

Thickness (mm)

33

12

–

–

580 ± 350

18 ± 6

18

10

4

12

–

–

170 (±50)

3.5 ± 1

50

10

5.2 (±0.5) –

17

–

5.8 ± 0.6

243 ± 13

–

84

1.1

–

–

3.8 ± 0.6

126 ± 13

1.2

92

10

99

D.J. Krug III et al. / Composites Science and Technology 77 (2013) 95–100 Table 3 Mechanical and optical properties of 0/90 S-glass epoxy resin FRC composites. Sample

Vol%

Fiber dia. (lm)

Tensile strength (MPa)

Elastic modulus (GPa)

Flexural strength (MPa)

Flexural modulus (GPa)

% T @589 nm

Thickness (mm)

Mayaterials – OG-based system 5 Ply S-glass (rough surface) 5 Ply S-glass (optical surface) 6 Ply S-glass 10 Ply S-glass (rough surface) 10 Ply S-glass (optical surface)

33 37 32 34 50

– – – – –

278 ± 9 – – 333 ± 12 –

16.4 ± 1.2 – – 17.6 ± 1.6 –

247 ± 12 387 ± 26 320 ± 16 318 ± 10 436 ± 28

16.8 ± 1.9 21.8 ± 0.4 18.3 ± 1.2 19.5 ± 0.3 26.7 ± 0.5

77 84 71 52 –

0.99 0.65 1.1 1.8 1.5

refractive indices of the matrix and fibers over a wide range of wavelengths of light. The high transparency also provides insight on how and where defects (such as air bubbles) form during our molding process. These defects were minimized and eliminated resulting in mechanical properties superior to those previously reported in the literature. As can be seen by comparing Tables 2 and 3, for similar transmittance (84%) the flexural strength of our 5-ply FRC is 60% higher than previously reported due to the high volume fraction of fiber glass. At similar volume fractions of fiber glass and with optically flat surfaces, the flexural modulus of our FRCs (5-ply, 37 vol.% S-glass optical surface, 10-ply 34 vol.% S-glass rough surface) can be 10–20% higher at 6.5–18% of the thickness compared to the PMMA/Borosilicate FRC (33 vol.%). The previously reported flexural strength is higher than the values obtained with our FRCs however it has a very high error range making it unreliable. Thus, the highly transparent FRCs made with our Q tailored matrices have tensile and flexural properties superior to transparent FRCs previously reported in the literature. 3.2. Optical properties Table 3 also shows that an optical finish is very important in improving transmission properties at 589 nm. Fig. 4 provides several examples of optical transmission for optically finished S-glass FRCs correlating the Table 3 data with a more complete range of wavelengths. As can be seen for the highest fiber loadings, transmittance across the visible wavelength is not quite uniform especially in the 350–450 nm range where the matrix itself is quite uniform. These deviations can be ascribed to differences in the refractive indi-

ces of the S-glass fibers (RI = 1.525 at 589 nm) compared to those of the OG based matrix (RI = 1.524 ± 0.007 at 589 nm). Tailoring the RI leads to composites that offer good transparencies as suggested by Fig. 5; however all of the processed composites show some degree of chromatic dispersion. In a bright light, these composites show either a blue or yellow cast with some haze. The haze is brought about by scattering from small amounts of oligomers formed during processing as well as the slight mismatch in RIs. Gel-permeation chromatograms of the OG starting material (not shown) reveal the presence of 2–4 vol.% oligomers representing dimers, trimers and tetramers for the most part. This is related in part to the synthetic methods used to make OG where removal of excess glycidyl ether requires heating under vacuum which can cause a small amount of ring-opening polymerization. As one might imagine, such oligomers increase crosslink densities in their immediate vicinity during curing and lead to very small concentrations of higher density materials in the matrix. These species will have slightly different RIs leading to some scattering; hence the haze witnessed. An alternate explanation is that the haze simply arises from the slight mismatch between the cured matrix RI and that of the fiber glass. Originally, the observed chromatic dispersion was attributed to the sizing present on the as-received S-glass mats. However, removal of the sizing by heating or solvent dissolution did not improve/eliminate the chromatic aberration. We have since learned that this chromatic dispersion arises because of RI mismatches between the matrix and the fiber at selected wavelengths depending on whether the matrix has an RI slightly above (blue) or below (yellow) that of the S-glass.

Fig. 4. Transmission behavior of optically finished S-glass mat reinforced epoxy resin as a function of wt. fraction.

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Fig. 5. S-glass (55 wt.%) fiber-reinforced composite refractive index matched with the above described Q resin system. The edges of the rectangular composite are approximated with a dashed line above. FRC is 10 cm from background and 30 cm from camera.

While this paper was being prepared and at the time of submission, an article appeared discussing the preparation of glass ribbon composites with similar volume fractions to those shown in Table 2 [31]. The transparency of these composites ranged from 70% to 90% based on wavelength and also loading. These materials exhibit superior mechanical properties to the FRCs noted above and represent a potential breakthrough in this area. However, unlike commercially available fiber-glass mats, these ribbons are at present at the prototype level of production and therefore it is hard to estimate their potential impact at present. 4. Conclusions The work reported here demonstrates the feasibility of making transparent fiber glass reinforced epoxy composites that actually outperform traditional fiber glass reinforced epoxy composites in properties while still providing transparencies that may be suitable for a wide variety of novel materials as listed in Section 1. We are working to solve the last issues of mismatches in RI across the entire visible spectrum to further refine this approach. Acknowledgements Mayaterials would like to thank U.S. Army Soldier Systems Center, Natick for support for the major portion of the work reported here through support under Contract W911QY-08-C-0098 Phase II SBIR Program. Work done at UM including characterization work was supported by Boeing Inc. We also would like to thank Dr. Kimihiro Matsukawa for sharing his efforts with us. References [1] Matsukawa K, Fukuda T, Watase S, Goda H. Preparation of photo-curable thiol– ene hybrids and their application for optical materials. J Photopolym Sci Technol 2010;23:115–9.

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