J.L. Thomason et aL/Composites: Part A 61(2014) 201-208 range 9-12] which is typical of the many different potential grP the treatment solutions, one or two hours for acid treatment, cycling processes. Similar behaviour has also been observed in 15 min for silane solution, at room temperature and then dried silica and basalt reinforcement fibres 13, 14 We are currently en- in an oven at 110C for another 15 min gaged in research projects where the ultimate goal is the genera Single fibres were meticulously separated from the glass fibre ion of the fundamental knowledge to enable cost-effective strands avoiding fibre-fibre interactions or excessive fibre bending regeneration of the mechanical properties of glass fibres produced as much as possible. Individual fibres were glued onto a card tabs from thermal recycling of glass reinforced thermoset composites with a central window cut out to matched the desired gauge length such as wind turbine blades. In this paper we report on the influ- for the test Card frames were cut from 250 g/m grade card and ence of thermal conditioning, at temperatures typical for GRP recy- single fibres were fixed to the card at both sides of the window ling up to 600C, on the properties of water sized and silane sized using Loctite TM Gel Superglue. A Nikon Epiphot Inverted optical E-glass fibres. We also report initial results of a study on the use of microscope was used at 200x magnification to obtain a digital acid and silane treatments on the strength of heat-treated fibres. photo of each fibre. The cross-sectional area was calculated from individual average fibre diameters measured at five points along gauge length During handling of the fibre in the microscope, care was taken to avoid fibre damage through contact with the microscope objective. Single fibre tensile properties we Boron free E-glass fibres supplied by Owens Corning-Vetrotex mined following ASTM C1557-03 using an Instron 3342 vere investigated in this work. The fibre rovings were produced testing machine equipped with a 10N load cell. Sampl on a pilot scale bushing and were received as 20 kg continuous sin- length was 20 mm for both fibre types and approximately 75 fibres gle end square edge packages. The rovings had a nominal tex of were tested at each condition. The tensile testing strain rate used 1200 g/m and a nominal fibre diameter of 17 um. No sizing was ap- was 1.5% min and all the tests were carried out at room tempera plied to the water finished fibres which had only been sprayed ture and 50% relative humidity. Only the tests where the sample using the normal water prepad cooling sprays under the bushing, broke along the gauge length at a distance greater than 3 mm from these samples are referred to as water sized or unsized. The APs the clamps were used for further data processing coated fibres were coated with a normal rotating cylinder sizing plicator containing a 1% volume r-aminopropy APS) hydrolysed solution in deionized water. All fibre packages 3 Results and discussion were subsequently dried at 105C for 24 h. The fibres were used as received from the manufacturer. heat treatment of both fibre 3.1. fibre diameter distribution types was conducted simultaneously to obtain samples with iden- ical thermal history. 300 mm lengths of silane sized and water Over the course of the investigation the diameters of more than sized fibre strand with no visible damage were removed from the 1200 individual glass fibres were measured using optical micros- inside of the roving packages. The glass fibre strands were sus- copy. The results are summarised in Table 1 which reveals the rel- pended on a specially constructed jig preventing any contact with, atively large distribution fibre diameters present in and therefore damage to, the fibres(see Fig. 1). Heat conditioning commercially produced glass fibre reinforcements. Fibre diameter ras carried out in a Carbolite LHT6 high temperature oven for is an important parameter in defining the final performance of fi- 15 min at temperatures in the range 250-600C The jig was then bre reinforced composites and is related to several parameters removed from the oven and the samples allowed to cool naturally including the internal diameter of the bushing tip, the velocity of to room temperature. molten glass flow, and attenuation rate 15. For a given glass-fibre For chemical post-treatment ACS reagent 37% hydrochloric acid manufacturing configuration it is the variation of the molten gla (HCI). APS. y-glycidyloxypropyltrimethoxysilane(GPS)and merca- flow that mainly accounts for fibre diameter distribution. The va ptopropyltriethoxysilane(sPs)were supplied by Sigma Aldrich in iation in the viscosity of glass melt is believed to be caused by the the UK. 10 v% HCl was prepared by diluting concentrated HCl with temperature distribution across the bushing 15 In the deionised water. 1% volume silane solution was prepared by mix- gle fibre strength and modulus these fibre diameters are used to ing the silane with deionised water. APS was hydrolysed without calculate the fibre cross section. Examining the values for mini- PH adjustment, all other silanes were hydrolysed in water whose mum and maximum fibre diameter in Table 1 it can be seen that pH value had been adjusted to 5. 1-5.3 prior to mixing One silane using only a manufacturers nominal fibre diameter or a single blend was created by mixing the APS and SPs solutions in equal average value of the distribution can lead to up to a 78%error mounts. The solution were left for 24 h to ensure full hydrolysis. the value of fibre cross section. This would translate directly into Heat treated glass fibre bundles were completely immersed in errors of similar magnitude in the fibre modulus and strength. This Fig. 1. Illustration of fibre suspension rig and rig positioned in furnace. For interpretation of the references to colour in this figure legend, the reader is referred to the web ersion of thrange [9–12] which is typical of the many different potential GRP recycling processes. Similar behaviour has also been observed in silica and basalt reinforcement fibres [13,14]. We are currently en￾gaged in research projects where the ultimate goal is the genera￾tion of the fundamental knowledge to enable cost-effective regeneration of the mechanical properties of glass fibres produced from thermal recycling of glass reinforced thermoset composites such as wind turbine blades. In this paper we report on the influ￾ence of thermal conditioning, at temperatures typical for GRP recy￾cling up to 600 C, on the properties of water sized and silane sized E-glass fibres. We also report initial results of a study on the use of acid and silane treatments on the strength of heat-treated fibres. 2. Experimental Boron free E-glass fibres supplied by Owens Corning-Vetrotex were investigated in this work. The fibre rovings were produced on a pilot scale bushing and were received as 20 kg continuous sin￾gle end square edge packages. The rovings had a nominal tex of 1200 g/m and a nominal fibre diameter of 17 lm. No sizing was ap￾plied to the water finished fibres which had only been sprayed using the normal water prepad cooling sprays under the bushing, these samples are referred to as water sized or unsized. The APS coated fibres were coated with a normal rotating cylinder sizing applicator containing a 1% volume c-aminopropyltriethoxysilane (APS) hydrolysed solution in deionized water. All fibre packages were subsequently dried at 105 C for 24 h. The fibres were used as received from the manufacturer. Heat treatment of both fibre types was conducted simultaneously to obtain samples with iden￾tical thermal history. 300 mm lengths of silane sized and water sized fibre strand with no visible damage were removed from the inside of the roving packages. The glass fibre strands were sus￾pended on a specially constructed jig preventing any contact with, and therefore damage to, the fibres (see Fig. 1). Heat conditioning was carried out in a Carbolite LHT6 high temperature oven for 15 min at temperatures in the range 250–600 C. The jig was then removed from the oven and the samples allowed to cool naturally to room temperature. For chemical post-treatment ACS reagent 37% hydrochloric acid (HCl), APS, c-glycidyloxypropyltrimethoxysilane (GPS) and merca￾ptopropyltriethoxysilane (SPS) were supplied by Sigma Aldrich in the UK. 10 v% HCl was prepared by diluting concentrated HCl with deionised water. 1% volume silane solution was prepared by mix￾ing the silane with deionised water. APS was hydrolysed without pH adjustment, all other silanes were hydrolysed in water whose pH value had been adjusted to 5.1–5.3 prior to mixing. One silane blend was created by mixing the APS and SPS solutions in equal amounts. The solution were left for 24 h to ensure full hydrolysis. Heat treated glass fibre bundles were completely immersed in the treatment solutions, one or two hours for acid treatment, 15 min for silane solution, at room temperature and then dried in an oven at 110 C for another 15 min. Single fibres were meticulously separated from the glass fibre strands avoiding fibre–fibre interactions or excessive fibre bending as much as possible. Individual fibres were glued onto a card tabs with a central window cut out to matched the desired gauge length for the test. Card frames were cut from 250 g/m2 grade card and single fibres were fixed to the card at both sides of the window using Loctite™ Gel Superglue. A Nikon Epiphot Inverted optical microscope was used at 200 magnification to obtain a digital photo of each fibre. The cross-sectional area was calculated from individual average fibre diameters measured at five points along the gauge length. During handling of the fibre in the microscope, care was taken to avoid fibre damage through contact with the microscope objective. Single fibre tensile properties were deter￾mined following ASTM C1557-03 using an Instron 3342 universal testing machine equipped with a 10 N load cell. Sample gauge length was 20 mm for both fibre types and approximately 75 fibres were tested at each condition. The tensile testing strain rate used was 1.5%/min and all the tests were carried out at room tempera￾ture and 50% relative humidity. Only the tests where the sample broke along the gauge length at a distance greater than 3 mm from the clamps were used for further data processing. 3. Results and discussion 3.1. Fibre diameter distribution Over the course of the investigation the diameters of more than 1200 individual glass fibres were measured using optical micros￾copy. The results are summarised in Table 1 which reveals the rel￾atively large distribution in fibre diameters present in commercially produced glass fibre reinforcements. Fibre diameter is an important parameter in defining the final performance of fi- bre reinforced composites and is related to several parameters including the internal diameter of the bushing tip, the velocity of molten glass flow, and attenuation rate [15]. For a given glass-fibre manufacturing configuration it is the variation of the molten glass flow that mainly accounts for fibre diameter distribution. The var￾iation in the viscosity of glass melt is believed to be caused by the temperature distribution across the bushing [15]. In the case of sin￾gle fibre strength and modulus these fibre diameters are used to calculate the fibre cross section. Examining the values for mini￾mum and maximum fibre diameter in Table 1 it can be seen that using only a manufacturers nominal fibre diameter or a single average value of the distribution can lead to up to a 78% error in the value of fibre cross section. This would translate directly into errors of similar magnitude in the fibre modulus and strength. This Fig. 1. Illustration of fibre suspension rig and rig positioned in furnace. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 202 J.L. Thomason et al. / Composites: Part A 61 (2014) 201–208