J. L. Thomason et aL/Composites: Part A 61 (2014) 201-208 The effect of chemical treatments on the strength of 450"C conditioned glass fibre. ment (h) type strength(GPa) confidence 06 04 0.0020.40.60.81.012141.61.8 APS: SPS 0.81 004 Fibre Strength(GPa) Fig. 7. Cumulative failure probability for water sized single fibres after thermal nditioning(◆23°口250°0300°x380°+450 00°C) iture dependent structural effects observed with the water sized fibre data will also increasingly affect the results for the APS sized fibres conditioned at increasingly higher temperatures 3.4. Infiuence of chemical post-treatment on the glass fibre strength The effects of various chemical post-treatments on the strength of 450C heat-conditioned glass fibres are summarised in the Table 2. Direct silanisation with either a single silane(Aps or GPS) or the APS: SPs silane blend did not result in any significant increase in the average fibre strength of heat-treated fibres. the expectation for restoring fibre strength through simple silanisation originated from the critical role that silane coupling agents play in the manufacture of glass fibre reinforcements. It is well known that these silanes have the ability to help maintain fibre strength[16.It has been recently demonstrated that strength retention caused by Fibre Strength(GPa) silane deposition on glass fibres during manufacture can be ex- plained from surface protection viewpoint [16. However, it has Fig 8: Cumulative failure probability for Aps sized single fibres after thermal also been suggested by some authors that silanes can'healsurface flaws by patching the crack tips, mitigating stress concentration water-sized fibres is then also reduced in greater degree w during loading 21, 22 It is reasonable to suggest that polysiloxane bonds with high cross-link density need to be formed between increasing heat conditioning temperature possibly indicating the deposited silane molecules and the glass surface if any significant presence of some temperature dependent structural change in effect is to be achieved through such a healing mechanism. one these fibres 12, 18, 19). of possibilities for the failure of direct silanisation in strength In the case of the APs sized fibres the data in Fig. 8 indicate regeneration of the heat-treated glass fibre may be the deactiva somewhat different trends in PE. The as received fibres show a tion of glass surface by exposure to elevated temperatures. It has broad strength distribution that appears to be split approximately been shown with both amorphous silica and glass fibre that severe 50: 50 in low and high strength regions. Initially heat conditioning dehydration can occur when subjected to thermal treatment above appears to affect the high strength part of this distribution, shifting 400C in air [23, 24]. While it has been suggested that complete it to lower values with increasing heat conditioning from 250C to rehydration of such calcinated surfaces may be very difficult.it 980C. The lowest 30% of these three distributions appear to be has been reported that acid treatment with HCl can successfully re- unaffected. Heat conditioning at 450C and above, appears to shift store hydroxyl groups on the surface of glass fibre heat-treated at all fibres to lower strengths and removes t the appearance of bimo- 450C [24, 25]. In the present work, we also attempted this acid dality. Building on the discussion of the water sized fibre results reactivation of the fibre surface prior to a further attempt at silani- this could be interpreted as indicating that the APS coating protects sation. The results in Table 2 indicate that HCl treatment itself does the fibres from the possibility of mechanical damage during the not significantly change the strength of heat-treated fibres and a handling of the fibres in the process of heat conditioning. Conse- similar lack of significant changes was found for samples silanised quently there is no reduction in strength of the fibres in the low after the acid reactivation treatment, as shown b les g-K. trength region of the distribution. Nevertheless, heat conditioning Several possible explanations exist which may account for this lack will degrade the organic part of the silane coating [10, 12, 18]. It of strength improvement. Although it has been reported that E- seems likely that the high strength fraction of the fibres are ini- glass fibre calcinated at 450 C can be reactivated with a significant tially well protected by the silane and so it is the high strength increase of surface silanol groups using acidic conditions, the num- raction of the distribution that is initially affected by the thermal ber of silanol groups may be still insufficient to form a tight net conditioning at lower(250-380C) temperatures. At 450C and work in the vicinity of the critical flaws. In fact, one may we above it has been shown that the organic part of the silane is fully question whether such a strong flaw repair patch could ever be degraded resulting in a further reduction in the strength of the formed with relatively large silane molecules. Other possibilitie high strength fibres [18]. It can also be supposed that the temper- may be associated with compositional change in the glass fibrewater-sized fibres is then also reduced in greater degree with increasing heat conditioning temperature possibly indicating the presence of some temperature dependent structural change in these fibres [12,18,19]. In the case of the APS sized fibres the data in Fig. 8 indicate somewhat different trends in PF. The as received fibres show a broad strength distribution that appears to be split approximately 50:50 in low and high strength regions. Initially heat conditioning appears to affect the high strength part of this distribution, shifting it to lower values with increasing heat conditioning from 250 C to 380 C. The lowest 30% of these three distributions appear to be unaffected. Heat conditioning at 450 C and above, appears to shift all fibres to lower strengths and removes the appearance of bimo￾dality. Building on the discussion of the water sized fibre results this could be interpreted as indicating that the APS coating protects the fibres from the possibility of mechanical damage during the handling of the fibres in the process of heat conditioning. Conse￾quently there is no reduction in strength of the fibres in the low strength region of the distribution. Nevertheless, heat conditioning will degrade the organic part of the silane coating [10,12,18]. It seems likely that the high strength fraction of the fibres are ini￾tially well protected by the silane and so it is the high strength fraction of the distribution that is initially affected by the thermal conditioning at lower (250–380 C) temperatures. At 450 C and above it has been shown that the organic part of the silane is fully degraded resulting in a further reduction in the strength of the high strength fibres [18]. It can also be supposed that the temper￾ature dependent structural effects observed with the water sized fibre data will also increasingly affect the results for the APS sized fibres conditioned at increasingly higher temperatures. 3.4. Influence of chemical post-treatment on the glass fibre strength The effects of various chemical post-treatments on the strength of 450 C heat-conditioned glass fibres are summarised in the Table 2. Direct silanisation with either a single silane (APS or GPS) or the APS:SPS silane blend did not result in any significant increase in the average fibre strength of heat-treated fibres. The expectation for restoring fibre strength through simple silanisation originated from the critical role that silane coupling agents play in the manufacture of glass fibre reinforcements. It is well known that these silanes have the ability to help maintain fibre strength [16]. It has been recently demonstrated that strength retention caused by silane deposition on glass fibres during manufacture can be ex￾plained from surface protection viewpoint [16]. However, it has also been suggested by some authors that silanes can ‘heal’ surface flaws by patching the crack tips, mitigating stress concentration during loading [21,22]. It is reasonable to suggest that polysiloxane bonds with high cross-link density need to be formed between deposited silane molecules and the glass surface if any significant effect is to be achieved through such a healing mechanism. One of possibilities for the failure of direct silanisation in strength regeneration of the heat-treated glass fibre may be the deactiva￾tion of glass surface by exposure to elevated temperatures. It has been shown with both amorphous silica and glass fibre that severe dehydration can occur when subjected to thermal treatment above 400 C in air [23,24]. While it has been suggested that complete rehydration of such calcinated surfaces may be very difficult, it has been reported that acid treatment with HCl can successfully re￾store hydroxyl groups on the surface of glass fibre heat-treated at 450 C [24,25]. In the present work, we also attempted this acid reactivation of the fibre surface prior to a further attempt at silani￾sation. The results in Table 2 indicate that HCl treatment itself does not significantly change the strength of heat-treated fibres and a similar lack of significant changes was found for samples silanised after the acid reactivation treatment, as shown by samples G–K. Several possible explanations exist which may account for this lack of strength improvement. Although it has been reported that E￾glass fibre calcinated at 450 C can be reactivated with a significant increase of surface silanol groups using acidic conditions, the num￾ber of silanol groups may be still insufficient to form a tight net￾work in the vicinity of the critical flaws. In fact, one may well question whether such a strong flaw repair patch could ever be formed with relatively large silane molecules. Other possibilities may be associated with compositional change in the glass fibre 0.0 0.2 0.4 0.6 0.8 1.0 Cumulative Probability Fibre Strength (GPa) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Fig. 7. Cumulative failure probability for water sized single fibres after thermal conditioning ( 23 C, h 250 C, s 300 C, 380 C, + 450 C, N 600 C). 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Cumulative Probability Fibre Strength (GPa) Fig. 8. Cumulative failure probability for APS sized single fibres after thermal conditioning ( 23 C, h 250 C, s 300 C, 380 C, + 450 C, N 600 C). Table 2 The effect of chemical treatments on the strength of 450 C conditioned glass fibre. Sample Duration of acid treatment (h) Silane type Ave. tensile strength (GPa) 95% Confidence limit A 0 – 0.68 0.05 B 0 APS 0.86 0.07 C 0 GPS 0.73 0.08 D 0 APS:SPS 0.64 0.05 E 1 – 0.58 0.04 F 2 – 0.74 0.04 G 1 APS 0.75 0.05 H 2 APS 0.74 0.04 I 1 GPS 0.8 0.06 J 2 GPS 0.79 0.05 K 1 APS:SPS 0.81 0.04 J.L. Thomason et al. / Composites: Part A 61 (2014) 201–208 205