2.2.1. Temperature and Time Effects
Investigations into the combined effect of temperature and time on the room temperature strength of GF began a number of decades ago. Sakka [23
] was one of the first to report results on a laboratory-produced fibre with high silicon, boron, sodium and potassium oxides content. Fibres were heated for one hour, cooled in air to room temperature and then tensile tested. The results showed that for this glass composition, thermal conditioning at temperatures in the range of 200–650 °C led to a reduction in fibre strength whose magnitude was linearly dependent on the conditioning temperature. Thomas [24
] took a systematic approach to both the manufacture and testing of E-glass fibres. In one experiment, he measured the effect of a 4-h thermal conditioning procedure at a selection of temperatures between 100 and 600 °C. A fairly consistent decrease in strength was shown, with a suggestion that the slope becomes steeper beyond a conditioning temperature of 300 °C. Compared to the strength of virgin fibre, a 65% loss in fibre strength was found when heat treating above 400 °C. Additionally, it was observed that strength loss proceeds with the length of the heat conditioning time until a constant minimum value is reached. At higher conditioning temperatures, this minimum value is reached within a very short time. Small sample sizes in these experiments prevented statistically-significant conclusions from being drawn, but these trends were further researched by others [25
Cameron investigated E-glass fibres and presented data showing the strength of heat-treated fibres asymptotically approaching a minimum value with time [25
]. Similar to the results shown by Thomas, this minimum value of retained strength also decreased with increasing treatment temperature. The maximum strength loss was 60% after 65 h of conditioning at 273 °C, and the strength loss after 4 h of conditioning was approximately 40%, which was also very similar to that reported by Thomas [24
]. In further work [26
] Cameron showed that when working with commercially-produced E-glass fibres (rather than laboratory-spun pristine fibre), the effect of thermal conditioning on room temperature strength was equally significant. Using heating times of 40–60 min, he measured a 50% drop in strength after conditioning at 300 °C. Other early research, while not directly comparable to that mentioned above, added to the body of evidence regarding time and temperature effects. Brearley and Holloway [27
] studied commercial soda-lime-silica 3 mm-diameter rods. Their results showed a steep initial decrease in strength with heat treatment (for 1 h) levelling out above 200 °C and reaching a maximum loss of 70% at 530 °C. Piggott and Yokom [28
] studied laboratory-produced 50 micron-diameter silica fibres and found a linear strength decrease with increasing conditioning temperature (of 1 h) with a maximum loss of 65% at 600 °C. Similarly, the work of Aslanova [29
] showed the weakening effect of conditioning time and temperatures of up to 600 °C on several types of GF.
More recently, Dorzhiev et al.
] conducted heat treatment of high-strength magnesium aluminosilicate fibre for 5 h in the temperature range 100–700 °C. Upon tensile testing, they found very little loss of strength in samples treated at 350 °C and below. However, above this temperature, some “threshold” was crossed, and a rapid deterioration in strength was observed, progressing to a maximum loss of 75% of the initial untreated value. Similar experimental data on E-glass fibre have also been reported by Feih et al.
], who observed coupled temperature and time trends in E-glass fibre strength loss very similar to those produced by Thomas [24
]. The differences in material used between these two investigations are, however, significant. Although both E-glass, in Thomas’ research, the fibres were essentially pristine and uncoated, as laboratory-drawn single fibres, and exhibited an unconditioned strength of approximately 3.7 GPa, typical for pristine single E-glass fibres. In the investigation by Feih et al.
, the samples were commercial 300 tex fibre rovings containing fibres coated with an industrial silane-based sizing and a much lower average single fibre strength of 2.3 GPa, which is a typical value for single fibres extracted from commercially-produced rovings.
Lund and Yue also reported results on [32
] laboratory-drawn single E-glass fibres that had not been sized. The pristine nature of these samples meant that the room temperature strength of untreated fibre was very high at approximately 3 GPa. Interestingly and despite performing long 3-h heat treatments, no significant strength loss was reported until a temperature in excess of 300 °C was used. Rigorous explanations for the physical change(s) in GF that can account for the strength loss measured are often absent. The same might be said about this work, but some interesting observations may be made based on the measurements of enthalpy and anisotropy relaxation that are reported alongside the tensile strength data. Firstly, the critical temperature above which strength loss and relaxation of enthalpy both begin to occur is around 300 °C. Secondly, some links might be also drawn between heat-treated fibre strength and anisotropy relaxation. The same E-glass formulation was used to produce both standard drawn fibres and a spun wool fibre (SWF), the production of which is described in [33
]: without fully detailing the SWF production process, it is important to note that it is less ordered than fibre drawing, which could allow greater fibre surface damage to occur. These E-glass spun wool fibres produced with minimal axial stress, hence very low anisotropy, had a measured tensile strength of approximately 1.5 GPa. The anisotropy of continuous E-glass fibres was found to decay to approaching zero after treating at 500 °C for 3 h; this treatment also caused a decrease in strength from around 3 down to 1.5 GPa. Taken individually, either of these correlations could be used to postulate that either: strength loss is related to enthalpy relaxation or strength loss is related to anisotropy. The authors themselves do not claim causation with respect to either of these observations of fibre relaxation. Heat-treated fibres are still brittle materials, and as such, their fracture is tied to the existence of critical surface flaws or cracks. Whether relaxation of either enthalpy or anisotropy could affect this fracture behaviour is not understood.
In a recent paper from researchers at Owens Corning, the effect of thermal condition on the room temperature strength of a range of fibre reinforcements was examined. Korwin-Edison et al.
] studied the room temperature strength of S-glass fibres, E-glass fibres, silica fibres and basalt fibres, after 1 h of heat treatment in air over a range of temperatures of 100–800 °C. All fibres exhibited a strength decrease with temperature treatments above 200 °C. A direct comparison of the results after 650 °C heat conditioning showed that the S-glass fibres had the highest absolute strength and that the basalt fibres had the poorest level of strength retention. However, when compared to the original values, all fibres showed a strength loss of 70%–80% after treatment at 650 °C, although the basalt fibre was still ranked lowest with a 97% relative strength loss. Militly et al.
also reported loss in the strength of basalt fibres after a 1-h thermal conditioning in air [35
]. At conditioning temperatures up to 200 °C, the fibre strength remained approximately constant; however, when heated above 200 °C, the room temperature strength of the fibres dropped precipitously with a 90% reduction in strength after treatment at 500 °C. Jenkins et al.
compared the thermally-induced fibre strength loss of commercial, epoxy-compatible, basalt and boron-free E-glass fibres [36
]. Both fibre types exhibited a threshold value for strength loss of approximately 300 °C. Above this temperature, both fibre types also exhibited an approximately linear strength loss with temperature. After conditioning at 600 °C (for 25 min), the basalt had lost 80% of its original strength compared to the boron-free E-glass, which lost 65%. Sabet et al.
studied the effect of thermal conditioning on the bundle strength and single fibre strength of basalt fibres [37
]. Heat treatment of the basalt rovings was carried out over a 300–500 °C temperature range for times from 5 to 20 min. Significant strength loss was recorded at all conditioning temperatures, with the magnitude of the loss increasing with increasing conditioning temperature. Furthermore, the basalt single fibre strength loss was relatively constant for 5–15 min treatment times, but increased significantly when the time was increased to 20 min. Interestingly, the strength dependence on treatment temperature exhibited a threshold behaviour for 5-min treatments, but was an approximately linear relationship for 20-min treatments. The basalt fibres exhibited a strength loss of 90% after 20 min at 500 °C.
and Table 1
show an overview of data selected from some of the papers reviewed here and illustrate the level of the loss in room temperature fibre strength as a function of heat conditioning temperature. Figure 1
illustrates the typical steep loss in the strength as a function of conditioning temperature for E-glass and basalt fibres. Table 1
shows the maximum fibre strength loss reported for a wide range of different silica-based fibres. The scale of the challenge for the reuse of recycled reinforcement fibres is clearly illustrated by the data in Figure 1
and Table 1
2.2.2. The Effect of Heating Atmosphere
The effect of the atmosphere in which the fibre heat conditioning occurs has also been considered as a variable, and this was studied in detail by Cameron during the 1960s. Some of his early thesis work [38
] tentatively suggested that no difference in retained strength of E-glass occurred when heating in either air or argon. However, he later reported data comparing heating in an oven with vents closed (to air particle contamination) or open [25
]. Some of these results appeared to show that after lengthy heating times (>10 h), more strength was retained in fibres heated in the “vents closed” furnace. A possible explanation of dust particle contamination being a strength-reducing factor, also postulated by Brearley and Holloway [27
], was later shown to be unlikely by Cameron himself [25
] when he performed shorter duration heat treatments on E-glass under either laboratory air or argon with a very high purity (and exceptional dryness). Up to a temperature of 300 °C, no significant differences in retained strength were measured between the two atmospheres.
More recent investigation of the heating atmosphere effect has generally yielded similar negative conclusions. In work on spun basaltic wool fibres (discontinuous-at-formation vitreous fibre), Lund and Yue [32
] found no difference in the retained strength of fibres thermally conditioned for 3 h in either air or nitrogen. Similarly, Feih et al.
], in work using silane-coated E-glass, reported no difference between treatment in ambient air or nitrogen in most cases. However, when thermally conditioning fibre at 450 °C for only 30 min, they observed greater strength retention when using a nitrogen atmosphere compared to either dry or ambient air. It appears from this body of research that, for uncoated fibres, neither dust contamination nor atmospheric moisture attack during thermal conditioning can provide an explanation to the strength loss measured in GF. What is less certain is what may occur when the normal commercial organic surface coatings have been applied to the fibres. The data of Feih et al.
] suggest that, over the shorter time-scales of interest when considering recycling applications, an unreactive nitrogen atmosphere may lead to greater fibre strength retention. One of the possible mechanisms suggested is a reduction of the degradation of the organic sizing material, and indeed in the same work, a thermo-gravimetric analysis (TGA) comparison using both nitrogen and air shows a small, but measureable difference, with less mass lost at above 300 °C under the nitrogen atmosphere.
2.2.3. The Effect of Heat Treatment under Stress
Bartenev and Motorina [43
] published some early results about the effect of applied stress on fine glass fibres of both alkaline and non-alkaline compositions during thermal conditioning. Of particular interest are the outcomes of the tests on the alkaline fibres as these were benchmarked against values for fibres that were thermally conditioned in a stress-free state. With a fairly low initial strength of little more than 1 GPa, the authors presented a roughly linear decrease in fibre strength with stress-free heating, reaching a maximum 70% loss after conditioning at 500 °C. On the other hand, fibres heated under stress showed improved strength retention. With an application of a load equalling 2% of the average ultimate fibre strength, a moderately higher strength was obtained throughout the temperature range. Application of 70% of ultimate pre-stress produced significantly improved results with only minor strength loss at 400 °C and up to a 15% strength increase above the initial non-heat treated value in the temperature range 100–350 °C.
Cameron also conducted some experiments on the heating of tensioned fibres working with largely flaw-free E-glass [25
]. This difference in fibre condition, pristine versus
the aged fibres of Bartenev and Motorina, has a significant effect on the strength values obtained. Cameron thermally conditioned his fibres for 2 h, either stress-free or under a pre-load, which he estimated to equal between 2% and 20% of the fibre room temperature strength of around 3.8 GPa. All heat treatment led to a decrease in retained room temperature strength. However, the conditioning temperature above which significant strength loss began was approximately 250 °C for the stressed fibres, but as low as 150 °C for the fibres conditioned stress-free. Furthermore, the magnitude of the maximum fibre strength loss at 450 °C was smaller (37%) for the stressed fibres than for the stress-free condition fibres (54%). Consequently, the tensioned fibres retained significantly more of their strength during the thermal conditioning. In the discussion of the results of both of these papers, the reduced weakening effect was attributed to alterations to the geometry around cracks or flaws on the fibre surface. Bartenev and Motorina discussed elastic and plastic deformation, and Cameron “inelastic flow”, of material in the vicinity of cracks. In both cases, they assume that this stress-forced flow leads to a less critical crack geometry, and hence, reduced stress concentrations develop when tensile testing is conducted.
More recently, Lezzi et al.
] reported on work using silica glass fibre, of an approximate diameter of 125 µm. Single fibres were loaded in tension at stresses of up to 60% of the breaking stress and treated for 60 s with a hot gas stream at temperatures between 100 and 500 °C while under tension. After heat treatment, the fibres were unloaded and tensile tested at room temperature and humidity. They reported that fibres heat treated while under tension retained more strength than those treated in a stress-free state. The degree of improvement of strength retention was related to the applied stress during the heat treatment. For example, after 60 s at 500 °C in the stress-free state, the fibre strength was approximately 1.3 GPa, but when a 1-, 2- or 3-GPa applied stress was applied, the retained strengths were 1.8, 2.5 and 3.5 GPa, respectively. The explanation given for this phenomenon was that a thin residual compressive stress layer forms on the surface of fibres when they are heated under tension while exposed to water vapour. The investigation was also extended to 100 µm-diameter E-glass fibres [45
], which were conditioned under stress and humidity for various times. Stress was induced by placing fibres in a two-point bending configuration inside silica tubes, and testing was correspondingly carried out using a two-point bending method. Holding fibres under bending loads of 1 and 2 GPa led to an increase in the failure strain of fibres beyond that of the as-received value. Again, the explanation given for this observed strength increase was surface residual stress formation by surface stress relaxation.
The full implications that this work may have for RGF obtained from composite recycling are open to discussion. These experiments were all carried out using unsized fibres, whereas in practical applications, it is almost certain that some silane-based sizing will have been applied to the fibre surface. Although an accepted plausible hypothesis, it is not absolutely certain that physical cracks or flaws in the GF surface are always the source of critical stress concentrations leading to the failure of thermally-conditioned GF. For example, agglomerations of sizing or matrix material or foreign particles that have become bonded to the fibre surface could act as sufficient stress raisers to cause fibre failure. Whether or not this further level of complexity regarding the surface coating would affect the ameliorating conditioning-under-tension effect described is unknown at this time.