J. L. Thomason et aL/Composites: Part A 61 (2014) 201-208 average roo te fibre tensile strength of ● Silane sized water sized and silane sized E-glass fibres was decreased by up 78 to 70% by only15 min conditioning at 600C Silane sized fibre exhibited a much higher initial strength than water sized fibres due to the protective influence of the silane surface coating. Silane sized fibres also exhibited relative stability in average tensile strength up to 250C. However, conditioning above this tempera ture resulted in a precipitous drop in room temperature strength. The water sized fibres exhibited an approximately linear decrease in average room temperature tensile strength with increasing con- d i Further analysis of the single fibre strength distributions using Weibull methods indicated that the strength distribution of the Fibre Age( Log10 Days) water sized fibres could be well represented by a unimodal, three-parameter, Weibull distribution which included the lower Fig 10. Change of glass fibre modulus with age(a water sized, APS sized) strength limit imposed by the minimum fibre strength required in order to be able to prepare a test sample. The strength distribu- tions of the sized fibres were more complicated and required the shown to increase in the temperature range 200-550C in silica use of a partially concurrent bimodal Weibull distribution. How- glass [32] implying the likelihood of increasing Si-0-Si structural ever, the use of a simple analysis of the cumulative fibre strength bond breakage with increasing conditioning temperature in the probability resulted in more useful material science understanding temperature range of this investigation. han the Weibull analysis. The observed loss in fibre strength wa Many researchers engaged in elucidating the complex struc- attributed, in part, to the thermal degradation of the organic part of an nh e relationships in composite materials operate un- the silane coating and the consequential increase in sensitivity to inforcement fibres is fixed and invariable. Consequently the the parallel changes in strength of the uncoated fibres may imply source of environmentally driven composite performance changes that there are also other changes taking place in the glass fibre is often sought in the polymer matrix or the fibre-matrix interface. structure at these relatively low processing temperatures. These results bring up the interesting possibility that long-term Fibre strength regeneration was investigated using acid surface exposure of glass fibres to an uncontrolled humid environment activation and post-silanisation of the heat-treated glass fibres. Lit- ay result in a significant reduction of fibre modulus. Since the fi- tle significant change in fibre strength was observed for a range of bres used in this investigation have also been the subject of a num- acid and silane post-treatments of heat conditioned fibres. Several the room possibile explanations were discussed including the density of su temperature modulus of these fibres over a period of approxi- face hydroxyl groups and potential changes in the fibre surface mately five years since their production. These results are shown composition. These aspects were further investigated through in Fig. 10 as a plot of average modulus versus Log(time). There silanisation of unsized glass fibres. The initial results suggest that is some scatter in the data, possibly due the uncontrolled storage recoating glass fibres with typical silanes may not be an attractive nvironment and to the large number of different investigators in- route to regenerating the value of heat conditioned glass fibres. olved in the generation of the data over this long time period. More work is required to further investigate the potential of Nevertheless, there is a clear trend visible in the data showing a post-silanisation in strength regeneration of thermally recycled time dependent lowering of the fibre modulus of both the water glass fibres. In contrast to the fibre strength, the fibre modulus sized and silane sized glass fibres. Fig. 10 also shows the expected was found to increase with increasing thermal conditioning tem- value of fibre modulus for these samples after 10 and 25 years. At perature. This was discussed in terms of the possible effects of ab- 25 years the fibre modulus could be below 62 GPa, which is more sorbed water on fibre properties. It was found that the modulus of than 20% reduction from the initial modulus of an Advantex glass these fibre decrease logarithmically as a function of age. It was sug- fibre. If this phenomenon is associated with slow moisture diffu- gested observed that this effect could be of significance for the sion into the glass fibres then clearly one might expect an acceler- long-term performance of glass fibres reinforced composites. ation of the effect at higher temperature and humidity. Consequently, we consider that this effect could be of great impor- tance for the long term performance of glass fibres (and their com- Acknowledgements posites)in humid environments such as offshore wind turbine blades, or the short term performance of glass fibres(and compos The authors would like to thank Owens Corning-Vetrotex for es)that experience hot-wet conditions such as composites used providing the glass fibres used in this study. The authors would in automotive cooling systems. Clearly these considerations merit also like to acknowledge the financial support from Engineering further controlled investigations and we have initiated a dedicated and Physical Sciences Research Council under the Recover and study of time-dependent changes in the modulus of E-glass fibres TARF-LCv projects and the assistance of the Advanced Materials stored in controlled environments. This will be reported at a later Research Laboratory of the University of Strathclyde with the mechanical testing. 4. Conclusions References The results of single fibre tensile testing presented here clearly show that the thermal history likely to be encountered by glass fi- [1I Job S Recycling glass fibre reinforced composites- history and progress. res recycled out of end-of-life composit 250-600C temperature range can potentially cause dramatic low- Composites in the InterAgency Composites Group Department for ovation ering of the room temperature fibre strength and strain at failure November 2009. <www bis. gov uk>.shown to increase in the temperature range 200–550 C in silica glass [32] implying the likelihood of increasing SiAOASi structural bond breakage with increasing conditioning temperature in the temperature range of this investigation. Many researchers engaged in elucidating the complex struc￾ture-performance relationships in composite materials operate un￾der an initial assumption that the modulus of inorganic reinforcement fibres is fixed and invariable. Consequently the source of environmentally driven composite performance changes is often sought in the polymer matrix or the fibre–matrix interface. These results bring up the interesting possibility that long-term exposure of glass fibres to an uncontrolled humid environment may result in a significant reduction of fibre modulus. Since the fi- bres used in this investigation have also been the subject of a num￾ber of other projects it is possible to examine the room temperature modulus of these fibres over a period of approxi￾mately five years since their production. These results are shown in Fig. 10 as a plot of average modulus versus Log (time). There is some scatter in the data, possibly due the uncontrolled storage environment and to the large number of different investigators in￾volved in the generation of the data over this long time period. Nevertheless, there is a clear trend visible in the data showing a time dependent lowering of the fibre modulus of both the water sized and silane sized glass fibres. Fig. 10 also shows the expected value of fibre modulus for these samples after 10 and 25 years. At 25 years the fibre modulus could be below 62 GPa, which is more than 20% reduction from the initial modulus of an Advantex glass fibre. If this phenomenon is associated with slow moisture diffu￾sion into the glass fibres then clearly one might expect an acceler￾ation of the effect at higher temperature and humidity. Consequently, we consider that this effect could be of great impor￾tance for the long term performance of glass fibres (and their com￾posites) in humid environments such as offshore wind turbine blades, or the short term performance of glass fibres (and compos￾ites) that experience hot-wet conditions such as composites used in automotive cooling systems. Clearly these considerations merit further controlled investigations and we have initiated a dedicated study of time-dependent changes in the modulus of E-glass fibres stored in controlled environments. This will be reported at a later stage. 4. Conclusions The results of single fibre tensile testing presented here clearly show that the thermal history likely to be encountered by glass fi- bres recycled out of end-of-life composite by processing in the 250–600 C temperature range can potentially cause dramatic low￾ering of the room temperature fibre strength and strain at failure. The average room temperature single fibre tensile strength of water sized and silane sized E-glass fibres was decreased by up to 70% by only15 min conditioning at 600 C. Silane sized fibres exhibited a much higher initial strength than water sized fibres due to the protective influence of the silane surface coating. Silane sized fibres also exhibited relative stability in average tensile strength up to 250 C. However, conditioning above this tempera￾ture resulted in a precipitous drop in room temperature strength. The water sized fibres exhibited an approximately linear decrease in average room temperature tensile strength with increasing con￾ditioning temperature. Further analysis of the single fibre strength distributions using Weibull methods indicated that the strength distribution of the water sized fibres could be well represented by a unimodal, three-parameter, Weibull distribution which included the lower strength limit imposed by the minimum fibre strength required in order to be able to prepare a test sample. The strength distribu￾tions of the sized fibres were more complicated and required the use of a partially concurrent bimodal Weibull distribution. How￾ever, the use of a simple analysis of the cumulative fibre strength probability resulted in more useful material science understanding than the Weibull analysis. The observed loss in fibre strength was attributed, in part, to the thermal degradation of the organic part of the silane coating and the consequential increase in sensitivity to fibre surface mechanical and environmental damage. Nevertheless, the parallel changes in strength of the uncoated fibres may imply that there are also other changes taking place in the glass fibre structure at these relatively low processing temperatures. Fibre strength regeneration was investigated using acid surface activation and post-silanisation of the heat-treated glass fibres. Lit￾tle significant change in fibre strength was observed for a range of acid and silane post-treatments of heat conditioned fibres. Several possibile explanations were discussed including the density of sur￾face hydroxyl groups and potential changes in the fibre surface composition. These aspects were further investigated through silanisation of unsized glass fibres. The initial results suggest that recoating glass fibres with typical silanes may not be an attractive route to regenerating the value of heat conditioned glass fibres. More work is required to further investigate the potential of post-silanisation in strength regeneration of thermally recycled glass fibres. In contrast to the fibre strength, the fibre modulus was found to increase with increasing thermal conditioning tem￾perature. This was discussed in terms of the possible effects of ab￾sorbed water on fibre properties. It was found that the modulus of these fibre decrease logarithmically as a function of age. It was sug￾gested observed that this effect could be of significance for the long-term performance of glass fibres reinforced composites. Acknowledgements The authors would like to thank Owens Corning-Vetrotex for providing the glass fibres used in this study. The authors would also like to acknowledge the financial support from Engineering and Physical Sciences Research Council under the ReCoVeR and TARF-LCV projects and the assistance of the Advanced Materials Research Laboratory of the University of Strathclyde with the mechanical testing. References [1] Job S. Recycling glass fibre reinforced composites – history and progress. Reinf Plast 2013;57(5):19–23. [2] Technology Needs To Support Advanced Composites in the UK. Written by The InterAgency Composites Group. Department for Business, Innovation & Skills. November 2009. <www.bis.gov.uk>. 60 62 64 66 68 70 72 74 76 78 80 82 2.0 2.5 3.0 3.5 4.0 Modulus (GPa) Fibre Age (Log10 Days) Silane Sized Water Sized 10yr extrapolated 25yr extrapolated Fig. 10. Change of glass fibre modulus with age (N water sized, d APS sized). J.L. Thomason et al. / Composites: Part A 61 (2014) 201–208 207