Alkali resistance of glass fiber
Did you know that the properties of glass fibers are greatly reduced when they come into contact with alkaline substances?
This time, I would like to take a look at glass fiber, which is a typical reinforcing fiber for FRP, from the perspective of its alkali resistance.
The civil engineering and construction industry takes the lead in evaluating the alkali resistance of glass fiber
In the civil engineering and construction industry, there is a term called GRC.
This is an abbreviation for Glass Reinforced Cement, which mainly refers to cement products reinforced with glass fiber.
Below is one example (in Japanese only).
You may feel that the properties of composite materials, including FRP, are well incorporated, including their tenacity.
Cement, which is the matrix in a composite material called GRC, exhibits strong alkalinity due to calcium hydroxide (Ca(OH)2) produced by the reaction between the minerals it contains and water.
This fact has highlighted the need to evaluate the alkali resistance of glass fibers, which are sometimes used as reinforcing fibers in GRC.
Evaluation of alkali resistance of glass fiber
The fact that glass fibers are damaged by acids has been introduced in the following columns.
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According to the evaluation results listed in the heading, glass fiber is weak against alkalis.
This time, I would like to introduce the results of evaluating the resistance of glass fibers to alkali, with reference to the following paper by Professor Uomoto. In addition to glass fiber, the evaluation targets include carbon fiber and aramid fiber.
The evaluation in this document was very thorough and easy to understand.
The motivation for the evaluation seems to be related to the application of FRP rods, rather than the GRC mentioned above.
In addition to cement, I have mentioned several times the use of FRP as rods, so I would like to introduce them as well.
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Evaluation is based on monofilament tensile properties after immersion in alkali
The basic alkali is sodium hydroxide, glass fibers are immersed in a strong alkaline aqueous solution with a pH of 14, and carbon fibers and aramid fibers with twice the concentration.
The maximum soaking period is 56 days.
Data on elastic modulus and strength are obtained by removing a single fiber from the monofilament, i.e., roving or strand, before and after soaking and pulling it.
The test standard is JIS R7601, but this standard has now been discontinued and is now JIS R7606.
The video below will give you a better idea of what the exam is like (in Japanese only).
The task of extracting a single fiber from a strand of thin fibers, especially 5-6 μm class carbon fiber, is extremely difficult, and I had a lot of trouble with it myself.
This is because static electricity is generated, which makes it difficult to install it in the desired location.
The key is to use a magnifying glass, brighten the lighting at hand, and use tape to temporarily secure the area.
As a result of the evaluation, as shown in Table 2 in the paper, after 28 days of immersion in a strong base, the elastic modulus and strength of the carbon fiber remain almost unchanged, and the aramid fiber also shows only a slight decrease in physical properties, but the glass transition is close to 90%. This decrease in physical properties has been observed.
If you want to understand this visually, it may be better to use the graph shown in Figure 2-4 in the paper, where the vertical and horizontal axes are the probability of failure and the fiber strength, respectively.
It can be seen that only for glass fiber, the plot data clearly shifts to the left, and the slope of the regression line approximated by the data increases rapidly.
This shows that the fibers are more likely to break at low stress levels.
T glass is used for evaluation
The glass fiber used for evaluation is T-glass. This fiber has higher strength, elastic modulus, and heat resistance compared to general E-glass.
You can see an overview of T-glass on this page.
The composition of T glass is characterized by a higher proportion of silicon dioxide and alumina (aluminum oxide) than E glass.
Deterioration of glass fiber due to alkali
It may be easier to understand how T-glass deteriorates due to alkali by looking at Photo 3 in the above paper.
It looks like mussels are clustered together in a rocky area.
Looking at Photo 2, you can see that the diameter of the threads becomes smaller in some places, which shows that the plate-like structure shown above is peeling off due to deterioration and alkali is penetrating inside.
The reaction model used to simply capture this phenomenon is shown in Figure 5 of the same paper.
It is based on the idea of using the diffusion coefficient D according to the classical theory of Fick’s law.
Fick’s law is a very versatile theory and is also used to predict moisture absorption and water absorption behavior of FRP.
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This model is based on circular coordinates that represent the cross section of the fiber, and the alkali gradually penetrates into the interior from the outermost layer.
Furthermore, based on the premise that alkali reacts with silica components (silicon dioxide), we set the assumption that the alkali will continue to penetrate until it reaches the silica-containing components, taking into account the reaction ratio with the silica component.
The basic idea seems to be that when the alkali reaches a saturated state in all regions, diffusion stops, that is, it becomes rate-limiting.
The diffusion coefficient is calculated using a simple formula, assuming that a decrease in strength corresponds to a decrease in cross-sectional area.
Calculation of the diffusion coefficient generally requires two types of data: the saturated state and the very initial change, but in this case, we have all of these data (in addition to the initial stage of alkali immersion, the glass fibers showed data on the 56th day of immersion. I think this prediction was made because it was determined that the intensity had almost disappeared.
It is natural to say that this is natural since it was combined from actual measurements, but if you look at Figure 7 of the same paper, it generally shows in a good manner the deterioration of the properties of glass fibers due to alkali immersion.
Long-term prediction using Arrhenius equation
Since the diffusion coefficient of alkali penetration into glass fibers was temperature dependent, it seems that predictions using the Arrhenius equation could be made.
According to this, it was predicted that the strength of glass fibers would be halved in about 5 years and that about 90% of the physical properties would be lost after 20 years due to strong alkali immersion exposure like the one evaluated this time.
Of course, the Arrhenius equation is not a panacea, so ideally it would be desirable to examine changes in its characteristics under assumed environmental exposure and apply regression analysis to predict deterioration.
In particular, the Arrhenius equation should be applied with caution to FRP, where many physical properties change at a specific temperature, such as the glass transition temperature.
Alkali-resistant glass also deteriorates due to alkali
The above paper mentions the initial evaluation results of alkali-resistant glass, and as a result, it was observed that the properties deteriorated due to alkali immersion.
As a result of 31 days of immersion in a strong alkaline aqueous solution with a pH of approximately 13.7, which is a 1 mol/L sodium hydroxide aqueous solution, the alkali-resistant glass fiber was found to have a strength decrease of nearly 90% in a similar evaluation. The rate of decline has been suppressed to around 40%.
However, the paper ends with the statement that verification is needed, including consideration of analytical models, as it is still decreasing.
Since the appearance of deterioration progressing from the outer layer (Photo 6 in the paper) is not significantly different from that of T-glass, we believe that a similar model can be used to predict the penetration of alkali into the fibers.
What should we think about from this paper?
CaO and ZrO2are thought to play the main role in making glass fiber alkali resistant
It seems that efforts to improve the alkali resistance of glass fibers were actively made from the 1980s to the 1990s.
One of the papers that may be helpful is the following.
According to this, the mechanism that increases the alkali resistance of glass fibers is the hydration reaction of CaO called CaO-SiO2-H2O and zirconia (zirconium oxide/ZrO2), which results in the formation of a layer containing a large amount of Zr, and these layers become alkali-rich. It is said that this is an example of suppressing erosion due to
It was also revealed that Ca ions released from CaO are deposited on the surface layer of glass fibers and inhibit the silicon elution reaction.
In the GRC Association video introduced at the beginning,
“Zirconia is the key to developing glass fiber’s alkali resistance”
Therefore, it is possible that this characteristic is now achieved through a slightly different mechanism.
In any case, even if glass fiber is alkali-resistant, elution cannot be completely stopped, so caution should be taken when using FRP reinforced with glass fiber in environments where it will be directly exposed to alkali, for example. Is required.
Alkali penetrates the FRP matrix resin and always reaches the reinforcing fibers
As introduced above, FRP is often used in areas where there is a risk of corrosion due to acid or alkali.
In this case, instead of general-purpose unsaturated polyesters such as ortho-based polyesters, we use vinyl esters that incorporate bisphenol into the main chain.
The reason is purely that vinyl esters have higher chemical resistance.
The problem is that long before the matrix resin is actually damaged by hydrolysis due to acid or alkali and can no longer function as a structural member, the acid or alkali moves within the matrix resin due to the phenomenon of diffusion and strengthens it. It means getting to the fibers.
Therefore, for example, it is out of the question to apply glass fiber-reinforced FRP to the surface in contact with alkali, but even if multiple layers of organic fibers, carbon fibers, etc. are applied to the surface in contact with the liquid, there will be glass fibers underneath. It is quite possible that the glass fibers will be damaged by the alkali that has penetrated at this point.
As a countermeasure against this problem, the diffusion coefficient must be properly calculated by taking into consideration the FRP lamination structure that comes into contact with alkali (acid as well), and the amount of time it will take to reach saturation due to the diffusion phenomenon must be understood in advance. It is important to perform a laminate design in
Furthermore, the key point is not to just make it and call it a day; regular inspections and repairs as necessary are the key.
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The main reinforcing fiber in FRP is undoubtedly glass fiber, at least at this stage.
Carbon fiber and aramid fiber, which are only used for special purposes, have not yet reached the phase of glass fiber.
It is also true that glass fiber requires more attention to its chemical resistance than carbon fiber or aramid fiber.
In the future, it is possible that glass fiber-reinforced FRP, or GFRP, will be used in more areas, not only in GRC but also in infrastructure.
In that case, I think it is essential to not only discuss costs and construction schedules, but also to understand the technical characteristics introduced here.
