FRAGMENTATION ANALYSIS OF GLASS FIBRES RECOVERED FROM HYDROLYSIS PROCESSES

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1 THE 19 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS FRAGMENTATION ANALYSIS OF GLASS FIBRES RECOVERED FROM HYDROLYSIS PROCESSES Y.T. Shyng*, O. Ghita, 1 College of Engineering, Mathematis and Physial Sienes, University of Exeter, North Park Road, Exeter, EX4 4QF, UK * Corresponding author (Y.Shyng@exeter.a.uk) Keywords: Glass Fibres, Hydrolysis, Reoating, Fragmentation Test, Interfae, Miromehanis 1 Introdution Composite materials have been extensively used in various industries for past deades with signifiant suesses due to their mehanial strength, design flexibility, redued weight and low system ost [1], [2]. Based on their benefits, use of omposite materials is expeted to inrease in speifi setors suh as transport, where fuel effiieny is a key requirement. An inreased use of omposites will automatially lead to inreased waste, through endof-life (End-of-Life Diretive) and from manufaturing waste. The reovery and reuse of these heterogenous materials is a onern for all industries, industries suh as automotive being under onsiderable pressure to reyle omposite materials due to End-of-Life legislation and inreased landfill osts. [3] [4]. Majority of omposites onsist of strong fibre reinforement (glass or arbon fibres) and thermoplasti or thermoset resins. Various tehniques have been reported in the past for reovery of the fibrous reinforement the most ommon methods being mehanial granulation and fluidised bed [4] [5]. Mehanial granulation is the most ommon and ost-effetive way to proess and reyle omposites. Powerful breaking and grinding proess are applied whih results in a partiulate end produt that an be used as filler [6]. Despite the low-ost proess, the reylate loose its reinforement properties being too damaged to retain any meaningful performane hene the less desired filler appliation. Fluidised bed or in a broader term, inineration, is another well-adopted proessing method. The omposite an be proessed between 5 ºC and 1 ºC, either with or without oxygen. Previous results showed that it is possible to retain between 5% and 9% of the fibre original mehanial property although the setup is relatively expensive and the proess is known to generate toxi gases in some ases [7] [8]. Hydrolysis proesses using subritial or superritial water to break down thermoplasti and even stronger thermoset resins has been developed in reent years [7] [9] [11]. As water requires high temperature and high pressure to reah superritial, one at suitable ondition resins an be broken down and dissolved, whih results in reyled fibres in an aqueous resin solution [7]. This allows the fibres to be reyled without experiening signifiant mehanial fore (mehanial granulation) and extreme temperatures (inineration). Although superritial water an also damage the fibre, various studies pointed out the loss in mehanial property is less omparing to onventional methods. Several reent studies reported fibres proessed by hydrolysis ould retain approximately 7% to 9% of its original mehanial property [7] [9]. Series of studies at University of Exeter investigated the mehanial properties of reyled glass fibres reovered through different reyling methods for potential use as further omposite reinforement. [7] [1] [11]. This study aims to strengthen the previous researh results by investigating the interfaial properties of glass fibres embedded inside thermoset epoxy matrix through fragmentation test

2 rather than the pull-out test. Single fibre omposites onsisted of virgin, reovered and silane reoated reovered glass fibres were prepared for fragmentation tests. Critial fragment length and the orresponding fibre strength are obtained in aordane to Weibull model. This enables the alulation of interfaial shear strength (IFSS) and therefore better understanding of fibre mehanial properties for reuse as omposite reinforements. 2 Fragmentation Test Fragmentation test was first developed by Kelly and Tyson in 196s and has been widely used for interfaial adhesion analysis between reinforement and matrix in a omposite system [12] [13]. A single fibre omposite, or model omposite, with a dumbbell shape has to be prepared and subjeted to an axial tension. The purpose of this axial tension is not to break the sample but to indue enough strain so the interfae an be effetively disrupted. This normally requires a resin matrix of higher maximum strain than the fibrous reinforement whih results in the fibre breaking into small segments under the axial deformation. Figure 1 shows as the strain gradually gets higher, the embedded fibre breaks into inreasingly smaller piees. However, after a ertain level of strain, the fibre fragments stop getting smaller, the fragment beoming too short to transfer enough stress to the fibre, hene no further fibre breakage takes plae [13]. At this stage although the sample is not broken there is no requirement to ontinue the tensile test further as the strain is already at saturation level. Throughout the tension proess the fragmentation of embedded fibre is losely monitored by optial mirosope and at the end several key data are reorded, namely total amount of fragments and respetive lengths. For interfaial shear strength ( ) alulation a simple fore balane approah is used as the following equation shows [14]: ( L ) d 2L (1) where d is the fibre diameter, L is the ritial fragment length and is the fibre strength at the ritial fragment length. As a general desription, ritial fragment length an be onsidered as the fragment length prior to the strain saturation therefore indiates the minimum length that is effetive for stress transfer [15]. A more detailed desription in the alulation of ritial fragment length an be found elsewhere [15] but in short the ritial fragment length (l ) an be expressed as: 4 (2) 3 L L F. avg where L F.avg is the average fragment length at the end of axial tension. The is alulated in aordane to a two-parameter Weibull model, whih has been widely used in various studies for analysing mehanial properties of brittle fibrous materials suh as glass fibres [15] [16]. The model is given by the following equation: P 1 exp L L m (3) where P is the umulative probability of failure at stress when the fibre length is L. represents the average fibre strength obtained at gauge length L, whereas Weibull modulus, m, desribes the flaw density and an be related to the variation in fibre strength. The higher the m value the less fibre strength variation, suggesting the fibre has better onsisteny. Equation (3) an then be rearranged to allow alulation of the Weibull modulus, m: 1 L ln ln mln( ) (ln mln ) 1 P L (4) It should be noted that the probability of failure for the i th fibre strength, P i, is deided by following equation [16]: i. 5 P i (5) N

3 FRAGMENTATION ANALYSIS OF GLASS FIBRES RECOVERED FROM HYDROLYSIS PROCESSES where N is the total amount of the sample tested for eah type of the glass fibre. After obtaining average fibre strength at gauge length L and Weibull modulus (m), ritial fibre strength ( ) at the ritial fragment length (L ) an be alulated through the equation: ( L ) ( L ) L 1/ m L (6) whih enables the alulation of interfaial shear strength in equation (1). 3 Experimental Details 3.1 Glass Fibres and Hydrolysis Proess Glass fibres used for this study were supplied by AHLSTROM in the form a reinfored E-glass fabri. Single glass fibres (virgin fibres) were arefully extrated from the fabri. Samples were leaned with weak solvent suh as isopropanol, rinsed with deionised water and then dried for later use [11]. Same E-glass fabri was also used for omposite preparation for subsequent hydrolysis reyling proess. Composite panels onsisted of unsaturated polyester resin and multi-ply E-glass fabri were prepared and then ut into piees at proper size for hydrolysis proess [7]. The hydrolysis proess was arried out by Institut Catholique d Arts et Métiers (ICAM) with a labsale reator of 5 ml apaity. The reation is a deliate balane between temperature, pressure, atalyst and time. Detailed desription an be found in earlier studies [7] [11]. Composite samples at 4 x 4 mm and distilled water were brought to 3 C for 3 minutes. After the reation glass fibres (reyled fibres) were dried and olleted for subsequent sample testing. 3.2 Silane Preparation and Re-Coating Proess Additional silane oating was applied to the glass fibres olleted from the hydrolysis proess (reoated fibre). Silquest A-174NT silane oupling agent supplied by ACC Siliones was used for fibre reoating. The Silquest A-174NT silane is designed to form hemial bonding between fibre surfae and resin matrix [7]. Firstly an ethanol-water mixture was prepared in 7:3 ratio. Aeti aid was then added to adjust ph value to 4.5, whih is important for the silane ativation. Finally the silane was added at 2.5% by weight and the solution was vigorously stirred for at least 2.5 hours before immediate use. A bundle of reyled fibres was added to the solution for 2 minutes to ensure an even oating. The bundle, now onsisting of reoated glass fibres, was then olleted and washed with ethanol to remove exessive silane solution. In order to hek the presene of silane oating on the reovered glass fibres,.1g rhodamine B was added to 1ml ethanol and used as a drop test solution on the bundle of the oated fibres. The presene of red spots on the surfae of the oated fibres after immediately washing the fibres with warm water indiates that vinylsilane or metharylsilane is present and therefore a suessful oating [7]. All surfae observations were arried out with optial mirosope and images aptured for later omparison. The reoated glass fibres were dried for 24h prior to any further testing. 3.3 Fibre Morphology (SEM) All glass fibres (virgin, reyled and reoated) were subjeted to SEM imaging for detailed observation on fibre surfae morphology. Gold oating was applied to avoid build-up of surfae harge and all samples were examined by Hitahi S-32N sanning eletron mirosope. Images were saved digitally for further monitoring of fibre diameter variations aused by both hydrolysis and silane reoating proesses. 3.4 Single Fibre Mehanial Property Test Single fibre tensile tests were arried out on all glass fibres based on the method desribed by ASTM-D Paper frames, as shown in figure 2, were ut for 1 mm gauge lengths and single straight glass fibre was then ut and fixed to the paper frame 3

4 using yanoarylate adhesive. The sample was then mounted in a single-fibre deformation rig equipped with 1N load ell and preision LDVT driving system, as shown in figure 3. All samples were arefully examined under optial mirosope prior to the test for diameter measurements, as well as for removing samples with lear surfae defet or damage. The edges of paper frame were ut off after the sample was fixed to the grips to avoid exessive load to the fibre. All fibres were tested at 2mm/min for a detailed strain-stress reording and minimum of 2 samples were tested for eah type of the glass fibres. 3.5 Fragmentation Test Single fibre omposite samples onsisting of individual glass fibre and epoxy resin were prepared in aordane to the shemati in figure 4. A silione mould was used for sample preparation and the hosen matrix was the LY552 supplied by Mouldlife, UK. This speifi epoxy resin system was hosen for its old-uring features as well as its low visosity and slow uring rate, whih makes it easier to degas and manipulate the omposite during its preparation. The resin mixture was first degased in a vauum hamber for removing air ontent and then gently poured into the silione mould until it was half full. One the resin beame semi-ured a single glass fibre was plae at the entre of the gauge length, followed by more resin mixture to fill the mould. The sample was then left to ure at room temperature for 48 hours and then oven ured at 5 ºC for 24 hours. One the omposite was fully ured it was arefully removed and polished to the designed dumb-bell shape. Fragmentation test was arried out on a miniaturised material tester designed and manufatured at Exeter Advaned Tehnologies (X-AT), University of Exeter, as shown in figure 5. Tensile deformation was applied in steps at.25% strain and stopped at 3.% strain sine the purpose is to indue fragments instead of destroy the sample. The sample omposite was losely monitored by stereomirosope, whih allowed the surfae strain to be applied inrementally by measuring the distane between the two surfae markers. After reahing maximum strain (3.%) fragments were then measured and ounted under optial mirosope in order to determine their lengths and the total number of fibre fragments. The findings were then used for interfaial shear strength alulation aording to the equations desribed in setion 2. 4 Results 4.1 Glass Fibre Surfae Charaterisation Figure 6 shows the SEM images of virgin glass fibres and the reovered glass fibre after hydrolysis proess. Although it was reported hydrolysis is a less violent method, a signifiant fibre diameter redution an be learly identified. This is believed to have a diret influene on the fibre mehanial property. The effetiveness of hydrolysis is also demonstrated in figure 6(b). Small piees of resin residual were found on the surfae of reovered fibres, whih agrees with earlier studies on the resin removal is likely to be approximately 8% [7]. As explained in setion 3.2, reovered glass fibres were than reoated with silane with expetation of improved interfaial bonding. Figure 7(a) shows the SEM image of reoated fibre. It appears the reoated fibre had smooth surfae and an inreased diameter, whih are most likely due to the additional layer of silane oating. To ensure the evenness of silane oating the reoated fibres were exposed to rhodamine B solution and the result an be seen in figure 7(b). Sine rhodamine B reats to the presene of silane, good onsisteny in olouration and its thikness indiates the silane oating was evenly distributed along the fibre surfae. 4.2 Single Fibre Mehanial Property Results of tensile measurements for all glass fibres an be found in table 1. Previous results by Kao et al [7] are also inluded sine all fibres were obtained

5 FRAGMENTATION ANALYSIS OF GLASS FIBRES RECOVERED FROM HYDROLYSIS PROCESSES from the same soure and treated under same onditions in the hydrolysis proess. The test results of both virgin and reovered glass fibres agreed well with previous findings. The loss in fibre strength was signifiant and in general agreed with measurements arried out in previous work [7]. As to the reoated fibres, diret omparison was not appliable sine previous work used a different silane solutions however ertain amount of strength reovery was found in both ases. 4.3 Fragmentation Test Single fibre omposite samples were subjeted to axial tension to indue fibre fragmentation. The ourrene and propagation of fragments were observed and reorded via optial mirosope. Figure 8(a) shows the typial result of suh fragmentation proess. Multiple fragments an be easily identified and the software-generated sale allows aurate alulation of atual fragment lengths. Moreover, under a higher magnifiation, the end of individual fragment an be observed in better detail for the visual evaluation of the effetiveness of interfaial bonding. As seen in figure 8(b), the spike-like feature indiates the bonding was strong enough for the resin to be raked during the fragmentation. The amount of fragments and the distribution of the lengths were reorded at the end of the test as shown in figure 9. As a general desription it is thought that weak interfae tends to have long fragments whereas more but short fragments indiate strong bonding [13] [14] [15]. The rather sattered tail setion of reovered fibre is thought to be indued by its inferior mehanial property. Although both virgin and reoated fibres showed similar distribution, reoated fibre appeared to be more onentrated with less sattering. Silane oating is thought to be the ause sine the presene of oating inreased the diameter and therefore the surfae area, whih ould have had a positive influene on interfaial bonding. As a qualitative desription it appeared that after reoating the reovered glass fibres were able to restore the bonding to the level similar to the virgin glass fibre. For a more detailed omparison interfaial shear strength (IFSS) were alulated so the differenes ould be quantified. 4.4 Interfaial Shear Strength (IFSS) As desribed in setion 2, Weibull model was used for the alulation of Weibull modulus, m, and the results an be found in figure 1. Following equation 6, the ritial fibre strength ( ) an be obtained based on from table 1 and L from figure 9. The interfaial shear strength (IFSS) was the alulated aording to equation (1). The results an be found in figure 11. The interfaial shear strength of a bare glass fibre was found at 1.3 MPa, whih in general agrees with previous findings [17]. As to the reovered fibre, the IFSS was found to be muh lower, 6.12 MPa and most likely due to its inferior tensile strength. Interestingly, the silane reoated fibre was found at 6.89 MPa, whih showed 12.6% improvement in terms of interfaial bonding but still muh less than the virgin glass fibre. This proves the importane of preserving fibre strength as it was found to be the dominant fator for any signifiant interfaial adhesion. As to the silane reoating, its presene did improve the bonding although learly it had a muh weaker influene on the matrix-fibre interfae. On the other hand, both reovered and reoated fibres reahed 6% of the IFSS of virgin glass fibres. Suh results suggest that the reovered glass fibre, despite its inferior tensile strength, an improve its adhesion properties and depending on the appliation be reused. Conlusions Results of fragmentation tests and subsequent Interfaial shear strength (IFSS) analysis showed that for reovered glass fibres, the fibre strength was the most important fator regardless of the silane 5

6 reoating. Suh oating did improve the bonding ondition but with only limited effet. Additionally, single fibre fragmentation tensile tests ombined with Weibull model proved to be an effetive approah for analysing interfaial adhesion of individual fibres embedded in epoxy resin. It provides a better way than widely used pull-out test, whih does not always reflet the atual stress transfer proess being more prone to fibre defets during the testing proess. Referenes [1] K. Chawla Composite Materials: Siene and Engineering. Springer (1998) [2] M.P. Groover Fundamentals of Modern Manufaturing: Materials, Proesses, and Systems. John Wiley & Sons (21) [3] J. Brandrup Reyling and Reovery of Plastis. Hanser Verlag (1996) [4] F. Mohammad Speialty Polymers: Materials and Appliations. I K International (27) [5] E.N. Brown., A.K. Davis, K.D. Jonnalagadda, N.R. Sottos Effet of surfae treatment on the hydrolyti stability of E-glass fiber bundle tensile strength. Composites Siene and Tehnology 65, pp (25) [6] J. Petterson, P. Nilsson Reyling of SMC and BMC in Standard Proess Equipment. Journal of thermoplastis Composite Materials 7: pp (1994) [7] C.C. Kao, O.R. Ghita, K.R. Hallam Mehanial studies of single glass fibres reyled from hydrolysis proess using sub-ritial water. Journal of Composites Part A. 43, 3. pp (212) [8] S.J. Pikering, R.M. Kelly, J.R. Kennerley, C.D. Rudd, N.J. Fenwik A fluidised-bed proess for the reovery of glass fibres from srap thermoset omposites. Compos Si Tehnol 6, pp 59 (2) [9] R. Pinero-Hernanz et al. Chemial reyling of arbon fibre reinfored omposites in near ritial and superritial water. Composites A 39, pp 454 (28) [1] J. Palmer, L. Savage, O.R. Ghita, K.E. Evans Sheet Moulding Compound (SMC) From Carbon Fibre Reylate Composites Part A Applied Siene and Manufaturing 41, pp (21) [11] Y.T. Shyng, E. Salter, C.C. Kao, O. Ghita Analysis of reoated glass fibres reovered from hydrolysis proess, Proeeding of ECCM 15, Venie, Italy, Tu (212) [12] A. Kelly, W.R. Tyson Tensile properties of fiberreinfored metals: opper/tungsten and opper/molybdenum. Journal of the Mehanis and Physis of Solids 13, pp (1965) [13] M.R. Kessler Advaned Topis in Charaterization of Composites. Trafford Publishing (26) [14] T.H. Cheng, F.R. Jones, D. Wang "Effet of fibre onditioning on the interfaial shear strength of glassfibre omposites". Composites Siehe and Tehnology 48, pp (1993) [15] S. Feih et al. Testing proedure for the single fibre fragmentation test. Risø National Laboratory, Denmark (24) [16] J.L Thomason, J. Ure, L. Yang, C.C. Kao Mehanial Study on Surfae Treated Glass Fiobres after Thermal Conditioning. Proeeding of ECCM 15, Venie, Italy, Tu (212) [17] J.L. Thomason, L. Yang "The Influene of Thermal Stress on the Interfae Strength of a Fibre-Reinfored Thermoplasti Investigated by a Novel Single Fibre Tehnique". Proeeding of ECCM 14, Budapest, Hungary, Paper ID: 128-ECCM14 (21) Fig. 1 Shemati of fragmentation test [13] Fig. 2 Single fibre sample and paper frame

7 FRAGMENTATION ANALYSIS OF GLASS FIBRES RECOVERED FROM HYDROLYSIS PROCESSES (a) Fig. 3 Fibre sample and test rig (before ut off) Fig. 7 Images of silane reoated fibre (a) SEM surfae feature, (b) optial with rhodamine B Strength (GPa) Fig. 4 Fragmentation test sample Strain (%) Diameter (μm) Virgin 2.8± ± ±.66 Virgin [7] 2.14± ± Reovered 1.3± ± ±.77 Reovered [7] 1.4± ± Reoated 1.4± ± ±.59 Reoated * 1.2± ± * Internal projet update report, different silane solution onentration was used Table 1 Mehanial property of all glass fibres Fig. 5 Fragmentation test setup Fig. 8 Fragmentation images (a) along the fibre and (b) fibre breakage (a) (b) Fig. 6 Images of (a) virgin, (b) reovered glass fibres 7

8 Interfaial Shear Strength (IFSS) (MPa) MPa 6.89 MPa 6.12 MPa Virgin A Reovered B Reoated C L -virgin = μm L -reovered = μm L -reoated = μm Fig. 11 Calulated interfaial shear strength (IFSS) of all glass fibres Fig. 9 Distribution of fragment amounts and ritial fibre lengths of all fibres 1-1 m = 4.91 R 2 =.92 LN((-1)*LN(1-P)) m = 5.25 R 2 =.95 m = 5.3 R 2 = ln (GPa) Virgin Reovered Reoated Fig. 1 Two-parameter Weibull analysis for all glass fibres with m values