Influence of Non-Reactive Epoxy Binder on the Permeability and Friction Coefficient of Twill-Woven Carbon Fabric in the Liquid Composite Molding Process
Round 1
Reviewer 1 Report
Introductions needs to be elaborated. There are a lot more articles in the field of permeability measurement of fabrics. I have not seen any specific permeability results from any other work in the article, so it is hard to tell, where the results score compared to others. Moreover, half of the used references are general ones and are referenced in a group (1-7.). Volume fraction of 50% was used for the test of the out-of-plane direction, why was it not used for the in-plane direction? What was the reason behind picking these 3 specific fiber volume fractions? How many samples have been tested? Why are the minimum and maximum values shown in the figures not the standard deviation?
Author Response
Reviewer :
1. Introductions needs to be elaborated. There are a lot more articles in the field of permeability measurement of fabrics. I have not seen any specific permeability results from any other work in the article, so it is hard to tell, where the results score compared to others. Moreover, half of the used references are general ones and are referenced in a group (1-7.).
> In accordance with your comments, the existing introduction [1-7] was more specifically divided into [1-4] and [5-7]. References about Liquid Composite Molding (LCM) process were added in [1-4], and references related to fiber preforming in LCM were introduced in [5-7]. In addition, new references representing results of the fabric permeability under similar conditions to our work were added ([6][13][14][15][16][23]) and presented in the introduction section.
2. Volume fraction of 50% was used for the test of the out-of-plane direction, why was it not used for the in-plane direction?
> After processing the fiber preform with vacuum bag under vacuum pressure (Figure 1), the fiber volume fraction (calculating through thickness) is close to 50%. After that, adjust the thickness of preform using a tool (Figure 2) that presses the preform to set the desired fiber volume fraction. However, in the measurement of the permeability in the in-plane direction, the preform is not sufficiently pressed since it is already 50% V.F., as previously mentioned, so that a gap between the top plate and the preform is generated. We cannot obtain the reliable results for 50% V.F since the test fluid fill the gap first before impregnation of preform. So, we decided not to add results of 50% V.F. to avoid reader’s confusion. Instead, the tendency of the 55, 60% V.F. results and the permeability results in out-of-plane direction would be enough to make our point.
3.What was the reason behind picking these 3 specific fiber volume fractions?
> In general, it is known that the composite product processed using prepreg has a 65% V.F. whereas in the case of Liquid composite molding process (LCM) such as Vacuum assisted resin transfer molding (VARTM) or Resin transfer molding (RTM), the products are processed with 50% to 60% V.F. [Chuang, K. C., Criss, J. M., Mintz, E. A., Shonkwiler, B., & McCorkle, L. S. (2011). Composite Properties of RTM370 Polyimide Fabricated by Vacuum Assisted Resin Transfer Molding (VARTM)]. Accordingly, we tried to investigate the effect of binder on the permeability in the three fiber volume fraction (50%, 55%, 60%) used in the LCM process.
4. How many samples have been tested?
> All cases of permeability measuring test were performed 5 times in each case, and the average value was shown in this study. The number of test specimen information was added to section 2.3.1 and 2.3.2.
5. Why are the minimum and maximum values shown in the figures not the standard deviation?
> As described above, each permeability measuring test was conducted 5 times. Since the number of samples was small, we tried to show the range of test values by representing the maximum and minimum values.
Reviewer 2 Report
the present manuscript deals with the analysis of the effect of usage of binders during preforming process on the permeability of fibrous reinforcement in LCM processes. Authors investigated how in-plane and out-of-plane permeability, and friction coefficient of the textile varied when binder is used and if the treated preform is subject to a heat treatment.
The topic is worth of interest and the results from the authors’ work could be useful for researches and companies involved in the study of the manufacturing of composite materials. However, the present manuscript presents several weaknesses that must be addressed before recommending it for the acceptance.
Below the comments are listed.
- the introduction needs to be revised. The analysis state of the art is not enough detailed. The state of the art about the evaluation of permeability of fabrics are far beyond the one depicted by the authors. Tons of papers were published in the last two decades exploring the measurements of permeability and the factor that could affect it (see Gutowsky T., Advanced Composite manufacturing, cap 10 as starting point). The same can be said about the analysis of the binders on the manufacturing of composites, not only on the impregnation but also on the curing phase (see for example M. Tonejc Advanced Manufacturing: Polymer & Composites Science 2019).
- Ref 1-7: some references are not consistent with the statement.
- Page 2: “There have been many studies on the effect …in LCM”. As for the previous comment, this point should be better debated.
- please add more details on the reinforcement. Is the textile balanced? differences in warp and weft affect the value of permeability in the in-plane directions
- please add the value of viscosity of working fluid.
- add detail on the heat treatment. is it conducted applying a compaction of the preform? It could affect the compaction of the plies, because the binder could act as lubricant enhancing the wetting compaction of fabric and the nesting of the fibers.
- “the fiber volume fraction of preform was determined by fixing the thickness using the upper and lower jigs.” please explain better. it is not clear how the infusion was conducted. is it the vacuum bagging resin infusion, the vacuum assisted resin infusion or others? the type of process, indeed, influences the aspects of the process. In addition, the control of the volume fraction, especially in the short range reported in the table 1, is difficult in vacuum bagging infusion if any other parameters (number of plies, areal density of fibers, driving pressure gradient, etc.) are kept constant.
- Section 2.3.1: it is not clear how the permeability was evaluated. Please describe in detail the approach adopted: infusion tests made using steady-state pressure or flow rate , evaluation by using the ratio between the ellipse axes, etc. Consider that using the flow media (depicted in fig 1) could affect the observation, so you see an apparent permeability (see for example Rubino F., Polymers 2018 to cite but a few).
- As for the previous comment, it is not clear how the measurement of out-of-plane permeability was conducted. it requires a dedicated equipment different from the ones used for in-plane measurements (see D. May, The Journal of The Textile Institute 2019 and D. Becker, Composites: Part A 2015 to cite but a few). As presented, the equation is the solution of Darcy's law for one-dimensional flow assuming that not multiscale impregnation phenomena occurred during the infusion (see Carlone, International Journal of Advanced Manufacturing Technology, 2018)
- Section 2.4: a scheme of the equipment could be useful to accompany the text
- Section 3.1: the discussion on the results is weak and trivial. The mechanism of how the binders affect the impregnation is not clear. Is it related to an enhanced compaction of the preform, an alteration of the viscosity of the working fluid? Please enhance.
- Section 3.3: the section is unconnected with the rest of the paper. the data are clearly reported, and the behavior is understandable. however, the relation of the variation in friction coefficient and fiber impregnation must be better and more thoroughly investigated.
- Section 3.4: the section should be placed before the friction test results. moreover, the discussion is weak and limited to trivial comments. the connection between the microstructural observations and the data on permeability is not clear. Please enhance.
- “In the two types of binder treated fabric preform… in the out-of-plane directional permeability.”. the sentence is more related to the data on permeability than to the what it can be observed in the micrograph. it is hard to catch how the shape in the binder droplets affect the impregnation. Please revise.
Author Response
Reviewer :
1. the introduction needs to be revised. The analysis state of the art is not enough detailed. The state of the art about the evaluation of permeability of fabrics are far beyond the one depicted by the authors. Tons of papers were published in the last two decades exploring the measurements of permeability and the factor that could affect it (see Gutowsky T., Advanced Composite manufacturing, cap 10 as starting point). The same can be said about the analysis of the binders on the manufacturing of composites, not only on the impregnation but also on the curing phase (see for example M. Tonejc Advanced Manufacturing: Polymer & Composites Science 2019).
> Following your comments, the existing introduction [1-7] was more specifically divided into [1-4] and [5-7] according to the reference’s topic. References [1-4] include advance research related to liquid composite molding (LCM) process, references [5-7] contain research related to the fiber preform. In addition, recent permeability studies with binder usage were also added [15,16], and permeability researches according to the measurement method [20,21], according to the fluid type [23] were shown in the introduction section.
2. Ref 1-7: some references are not consistent with the statement.
> The first paragraph of the existing introduction was modified to match the references to the stated contents. The second paragraph also revised and added references, and the changes are marked in red in the text.
3. Page 2: “There have been many studies on the effect …in LCM”. As for the previous comment, this point should be better debated.
> In the section after Page 2: “There have been many studies on the effect …in LCM” we added and modified the contents of the references as answered in above 1 and 2. The revised content is marked in red.
4. please add more details on the reinforcement. Is the textile balanced? differences in warp and weft affect the value of permeability in the in-plane directions
> In this study, we use balanced twill woven fabric. The information of reinforcement was added to section 2.1 materials.
5. please add the value of viscosity of working fluid.
> All cases of permeability measurement test were performed at room temperature (25℃), and silicon oil having a viscosity of 340 cp at room temperature was used. The information of silicon oil was added to section 2.1 materials.
6. add detail on the heat treatment. is it conducted applying a compaction of the preform? It could affect the compaction of the plies, because the binder could act as lubricant enhancing the wetting compaction of fabric and the nesting of the fibers.
> To keep the compaction condition constant, all test specimens were used after holding for 30 min. under vacuum pressure (1atm) using a vacuum bag at 110℃ in the convection oven. To further explain for better understanding, the test specimens made using above method is less than 50% V.F., and the target V.F. was made by adjusting the thickness of specimens using a jig described in answer 7 below.
7. “the fiber volume fraction of preform was determined by fixing the thickness using the upper and lower jigs.” please explain better. it is not clear how the infusion was conducted. is it the vacuum bagging resin infusion, the vacuum assisted resin infusion or others? the type of process, indeed, influences the aspects of the process. In addition, the control of the volume fraction, especially in the short range reported in the table 1, is difficult in vacuum bagging infusion if any other parameters (number of plies, areal density of fibers, driving pressure gradient, etc.) are kept constant.
> First, we made a preform specimen with constant area (300mm * 300mm) under vacuum pressure using vacuum bag at 110℃, 30 min. (these specimens are about 50% V.F.) After that, the preform specimen was pressed to set the desired thickness (fiber V.F.) using controllable upper and lower jig. For better understanding, figure 2 and its description were added to the section 2. Experimental.
8. Section 2.3.1: it is not clear how the permeability was evaluated. Please describe in detail the approach adopted: infusion tests made using steady-state pressure or flow rate , evaluation by using the ratio between the ellipse axes, etc. Consider that using the flow media (depicted in fig 1) could affect the observation, so you see an apparent permeability (see for example Rubino F., Polymers 2018 to cite but a few).
> figure 1. shows the preform process using vacuum pressure before measuring the permeability. To avoid misunderstanding, fig. 1 was revised (deleting ‘resin flow’). After the preform process using vacuum pressure, the fiber preform was made with 50% V.F. or less, and After that, the preform specimen was pressed to set the desired thickness (fiber V.F.) using controllable upper and lower plate and then, the test fluid was injected at a constant pressure to perform the test (Figure 2). For better understanding, the schematic of the in-plane directional permeability measurement system was added to the 2.Experimental section. In the same way as in the reference [17], the flow front was assumed to be elliptical and the permeability was evaluated from the change in the flow front using the ellipse axes.
9. As for the previous comment, it is not clear how the measurement of out-of-plane permeability was conducted. it requires a dedicated equipment different from the ones used for in-plane measurements (see D. May, The Journal of The Textile Institute 2019 and D. Becker, Composites: Part A 2015 to cite but a few). As presented, the equation is the solution of Darcy's law for one-dimensional flow assuming that not multiscale impregnation phenomena occurred during the infusion (see Carlone, International Journal of Advanced Manufacturing Technology, 2018)
> For better understanding, Figure 2 and its description were added to section 2. Experimental, which shows the schematic of permeability measurement method in the out-of-plane direction. Since the out-of-plane directional specimen has not enough impregnation distance compared to the in-plane direction, it is difficult to measure with optical method In-plane, and turbulence occurs due to the effect of the non-rigid dry fabric resulting in invalid data (Reference [23]). Therefore, we tried to measure the saturated permeability coefficient after enough test fluid has flowed to examine influence of binder on out-of-plane directional permeability.
10. Section 2.4: a scheme of the equipment could be useful to accompany the text
> Following reviewer’s comments, the equipment for measuring friction coefficient (Figure 4) and its description were added in section 2.4
11. Section 3.1: the discussion on the results is weak and trivial. The mechanism of how the binders affect the impregnation is not clear. Is it related to an enhanced compaction of the preform, an alteration of the viscosity of the working fluid? Please enhance.
> In preforming process, compaction of all the cases was carried out for the same time (20 min.) with the same vacuum pressure, and working fluid was also injected with a constant viscosity at the same temperature (25℃, section 2.1). In this section, the only influence of epoxy binder in the preform on permeability was investigated and the results of decrease in permeability according to the binder addition were described. A detailed description of the influence of the binder shape attached to the fibers on permeability was shown in section 3.3
12. Section 3.3: the section is unconnected with the rest of the paper. the data are clearly reported, and the behavior is understandable. however, the relation of the variation in friction coefficient and fiber impregnation must be better and more thoroughly investigated.
> In liquid composite molding, the preforming process is carried out before the resin injection to form the complex shape fiber preform using upper and lower molds (or vacuum bag and lower mold). At this time, the deformation of the shear angle varies depending on the coefficient of friction between the fibers and between the fiber and the mold, which affects wrinkle generation. The coefficient of friction and furthermore, the tendency of wrinkle generation can be changed under the influence of the biner used to fix the preform. The correlation between the fiber friction coefficient and the preforming process was added to the introduction.
13. Section 3.4: the section should be placed before the friction test results. moreover, the discussion is weak and limited to trivial comments. the connection between the microstructural observations and the data on permeability is not clear. Please enhance.
> Following reviewer’s comments, for better understanding, section 3.4 was moved to after the measurement results of permeability (section 3.2), and the influence of the binder shape attached to the fibers was further described.
14. “In the two types of binder treated fabric preform… in the out-of-plane directional permeability.”. the sentence is more related to the data on permeability than to the what it can be observed in the micrograph. it is hard to catch how the shape in the binder droplets affect the impregnation. Please revise.
> Following reviewer’s comments, section 3.4 was moved to after the measurement results of permeability (section 3.1, 3.2) to explain the results of permeability decrease from the viewpoint of binder form. Also, the content was supplemented in section 3.3 Microstructure observations.
Round 2
Reviewer 1 Report
Work was enhanced based on the comments, so it can be accepted.
Reviewer 2 Report
Authors carefully revised their manuscirpt and properly replied point-by-point to the referee's comments.
tne manuscript can be accepted in the present form
