Prepreg, its test method, and its manufacturing method

Abstract

A prepreg comprising a fiber having a gap therebetween, and a structural layer containing a curable thermosetting resin impregnated within the structural layer and present within the gap, wherein the prepreg comprises a test region that does not contain the curable thermosetting resin; A method for testing a prepreg, comprising: a) performing a measurement test on a test area of the test region, and then b) inferring the properties of the prepreg from the results of the measurement; A method for manufacturing a prepreg, comprising: providing a structural layer containing fibers having a first surface and a second surface, and a first impregnating layer containing a curable thermosetting resin; contacting the first surface of the structural layer with the first impregnating layer; compressing the structural layer and the first impregnating layer together such that the curable resin impregnates the fiber layer such that the curable resin is present between the gaps between the fibers, thereby forming the prepreg, wherein the first impregnating layer includes a region where impregnation does not occur, thereby generating the test region of the prepreg that does not contain the curable resin.

Description

Technical Field

[0001] The present invention relates to a prepreg including a test area for enabling accurate measurement of the areal weight of the prepreg, a test method performed on the test area, and a method for manufacturing the prepreg, particularly but not limited thereto.

Background Art

[0002] Composite materials have well-documented advantages over conventional construction materials, particularly in terms of providing excellent mechanical properties at extremely low material densities. As a result, the use of such materials has become widespread, and their applications range from "industry" and "sports and leisure" to high-performance aerospace components.

[0003] Prepregs, which include an arrangement of fibers or fabrics impregnated with a thermosetting resin such as an epoxy resin, are widely used in the production of such composite materials. The resin can be combined with the fibers or fabrics in various ways. The resin may be adhered to the surface of the fiber material, but more commonly, the resin partially or completely impregnates the gaps between the fibers. Although it is generally known how much fiber and resin are introduced into the prepreg during its manufacture, since the manufacturing process causes physical and chemical changes, it is necessary to perform tests on the formed prepreg to determine its physical properties.

[0004] Various methods for manufacturing prepregs have been proposed, and one of the preferred methods is to impregnate a moving fiber web with a liquid, molten, or semi-solid uncured thermosetting resin.

[0005] Once manufactured, typically, multiple plies of such prepregs are "stacked" (laid up) as desired, and the resulting prepreg stack, i.e., laminate or preform, is typically cured by exposure to elevated temperatures to produce a cured composite structure. Curing can be carried out within a vacuum bag that can be placed in a mold for curing. Alternatively, the stack can be formed and cured directly in the mold.

[0006] However, to ensure that the prepreg meets the requirements for its final application, various quality control measurements are typically performed on the prepreg prior to curing. One of the fundamentally important measurements is to accurately determine the relative weights of the resin and fibers in the prepreg. The amount of fiber is defined as the "fibre areal weight" (FAW), which is the weight of the fiber per unit area of the prepreg without the impregnated resin. The amount of resin is defined as the "resin content" (RC), which is the weight of the resin per unit area of the prepreg. Since the prepreg has fibers impregnated with resin, these are difficult to measure. Therefore, various methods have been devised for this calculation.

[0007] One commonly used method for measuring FAW and RC is the "washout" test. This test involves taking a 100 cm 2 sample of the prepreg, weighing the sample, and then removing all the resin from the sample using a solvent (e.g., methylene chloride). The sample is then placed in an oven to remove the solvent, and the sample is weighed again to determine the FAW. The RC can then be estimated by subtracting the FAW from the total weight. This method is accurate enough for quality control purposes but has various drawbacks. This method is described in ASTM D3529.

[0008] The solvents used, especially methylene chloride, pose many environmental, health, and safety concerns and risks. Moreover, despite taking precautions, there is always a small amount of resin that is not removed by the solvent, and there is always a suspicion that some of the sizing on the fibers can be removed by the solvent, which can lead to inaccuracies in the test methods, as has been shown by experience.

[0009] Therefore, alternative methods to the washout method have been developed, such as the Eddycus™ CF Inline FAW test solution provided by Suragus, which uses a sensor that utilizes eddy current technology to measure the FAW of carbon fibers.

SUMMARY OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

[0010] Therefore, improvements in this field are highly desirable.

MEANS FOR SOLVING THE PROBLEMS

[0011] In a first aspect, the present invention relates to a prepreg comprising fibers having a gap therebetween, and a structural layer impregnated within the structural layer and containing a curable thermosetting resin present within the gap, wherein the prepreg includes a test region that does not contain the curable thermosetting resin.

[0012] Therefore, the test region of the prepreg is made of only fibers and does not contain any curable resin, so there is no need to remove anything to test the fibers of the prepreg. Therefore, for example, a test can be performed on the test region to simply and accurately determine the fiber areal weight (FAW).

[0013] The term "not containing a curable resin" means that there is no measurable amount of curable resin present. This does not include the "sizing" that may be present on the surface of certain fibers that exist as a result of fiber manufacturing.

[0014] The test area of the prepreg can take on a number of different sizes depending on the type of test envisioned. Preferably, the test area is 1 - 1000 cm 2 , preferably 50 - 500 cm 2 , more preferably 50 - 200 cm 2 in surface area. Generally, the larger the patch, the more waste of the prepreg occurs, but there is a possibility that more accurate sampling of the prepreg can be achieved. A convenient balance between these results in a convenient size of 100 cm 2 .

[0015] For the test area to more accurately represent the entire prepreg, the test area is preferably surrounded by an area impregnated with a curable thermosetting resin of the prepreg.

[0016] Preferably, the prepreg can include a second test area that does not contain a curable thermosetting resin. The prepreg can also include a third test area that does not contain a curable thermosetting resin. Having multiple test areas can be particularly useful when the test is destructive or when it is desirable to test multiple areas of the prepreg. Thus, preferably, when multiple test areas are present, they are gathered together to form a test cluster area, and in the test cluster area, the multiple test areas are spaced such that each test area samples a different area of the prepreg within the test cluster. Advantageously, the test areas are spaced in the width direction of the prepreg, which is particularly useful when the fibers are unidirectional and parallel to the length of the prepreg so that different unidirectional fibers can be sampled across the entire width.

[0017] Prepregs are typically manufactured as a continuous web of material that has a length that is greater than, typically much greater than, its width, as described later. Such prepregs are generally manufactured as prepreg rolls, the length of which is given by the width of the prepreg. A backing sheet is generally provided to enable the prepreg roll to be unrolled during use, taking into account the adhesiveness of the prepreg.

[0018] The fibers may be in the form of a fabric or may be formed from separate fiber tows. However, generally, fabrics are more resistant to the physical changes resulting from the prepreg formation process. Thus, the present invention is more usefully applicable when the fibers are separate and not woven together. Preferably, the fibers are arranged parallel to each other and are unidirectional in that they are generally parallel to the length of the prepreg. The fibers may include randomly discontinuous or continuous fibers that are cracked (i.e., stretch-broken).

[0019] The fibers can be made from a variety of materials such as carbon, glass, graphite, metallized polymers, metal-coated fibers, and mixtures thereof. Carbon and glass fibers are preferred.

[0020] Typically, the fibers in the structural layer generally have a circular or substantially circular cross-section with a diameter in the range of 3 to 20 μm, preferably 5 to 12 μm.

[0021] An exemplary layer of unidirectional fibers is made from HexTow™ carbon fibers available from Hexcel Corporation. Suitable HexTow™ carbon fibers for use in making many unidirectional fiber layers include IM7 carbon fibers available as fibers containing 6,000 or 12,000 filaments and having weights of 0.223 g / m and 0.446 g / m, respectively; IM8 - IM10 carbon fibers available as fibers containing 12,000 filaments and having weights in the range of 0.446 g / m to 0.324 g / m; and AS7 carbon fibers available as fibers containing 12,000 filaments and having a weight of 0.800 g / m.

[0022] The prepreg of the present invention is mainly composed of a thermosetting resin and structural fibers, but other materials such as a curing agent or other additives often exist. Typically, the prepreg contains 25 to 50% by weight of the curable resin. Further, the prepreg typically contains 45 to 75% by weight of the structural fibers.

[0023] The resin includes a thermosetting resin and can be selected from those conventionally known in the art such as phenolic formaldehyde, urea-formaldehyde, 1,3,5-triazine-2,4,6-triamine (melamine), bismalemide, epoxy resin, vinyl ester resin, benzoxazine resin, polyester, unsaturated polyester, cyanate ester resin, or a mixture of these resins. Epoxy resin is particularly preferred. A curing agent and optionally an accelerator can be included as desired.

[0024] The thermosetting resin is preferably an epoxy resin and can include one or more monofunctional, difunctional, trifunctional, and / or tetrafunctional epoxy resins. Such resins may become brittle upon curing, and thus, in order to impart durability, a reinforcing material may be included in the resin, but this can also increase the viscosity of the resin. The reinforcing material can be provided as a separate layer such as a veil.

[0025] When the reinforcing material is a thermoplastic polymer, the reinforcing material should be insoluble in the resin at room temperature and at the elevated temperature at which the resin cures. Depending on the melting point of the thermoplastic polymer, the thermoplastic polymer can melt or soften to various degrees during the curing of the resin at an elevated temperature and re-solidify as the cured laminate cools. Suitable thermoplastic materials include thermoplastic materials that should not dissolve in the resin, such as polyamide (PAS), polyethersulfone (PES), and polyetherimide (PEI). Polyamides such as nylon 6 (PA6), nylon 11 (PA11) or nylon 12 (PA12) and / or mixtures thereof are preferred.

[0026] (One or more) test areas can be usefully employed to perform various tests on the prepreg, and it is advantageous for there to be no resin in the test areas. These tests can include, for example, the following. · Measuring the thickness of the test area · Fiber damage through the process o Signs of fuzz o Content of broken filaments o Or other signs of damaged fibers · Measuring fiber spread and looking for cracks · Measuring areal weight uniformity · Obtaining individual tow tension measurements from a test area where the tows are exposed · Individual tow MPUL measurement · Measuring fiber tension as a result of fiber relaxation and / or length differences · Measuring sizing o Content o Quality o Sizing progression · Measuring the progression of resin from the compaction roller · Calculating RC by looking at the difference between the test area and the impregnated patch adjacent thereto · Fiber surface property evaluation o Gloss o Entanglement (tangle) o Twist and ecta · Measuring the degree of fiber alignment to predict mechanical properties · Also measuring the degree of uniformity of tow overlap to predict Cpt

[0027] Thus, in a second aspect, the present invention relates to a method of testing a prepreg as described herein, the method comprising a) performing a measurement test on a test area of the test area, and then b) inferring the properties of the prepreg from the results of the measurement.

[0028] As described, the test area is particularly useful for measuring the fiber areal weight (FAW) of the prepreg. Thus, the method preferably comprises: a) cutting a test sample of fibers from a test area of the test area; b) weighing the test sample of fibers; c) calculating the weight per unit area of the test sample; and d) presuming that the fiber weight per unit area of the test sample is a measurement of the fiber weight per unit area of the prepreg.

[0029] The test can be carried out with the prepreg under tension to reflect the tensions experienced during manufacture, as appropriate for the test being carried out.

[0030] The prepreg according to the present invention can be manufactured in a continuous process that typically includes passing thousands of fibers forming a fibrous structural layer through a series of impregnation steps, guided by rollers that typically act to impregnate a resin within the structural layer. The point where the fibers, which are usually in sheet form, meet the resin is the start of the impregnation step.

[0031] Accordingly, in a third aspect, the present invention relates to a method for the manufacture of a prepreg as described herein, the method comprising providing a structural layer comprising fibers having a first surface and a second surface, and a first impregnating layer comprising a curable thermosetting resin; contacting the first surface of the structural layer with the first impregnating layer; and compressing the structural layer and the first impregnating layer together such that the curable resin impregnates the structural layer such that the curable resin is present between the gaps between the fibers, thereby forming the prepreg, wherein the first impregnating layer comprises regions where impregnation does not occur, thereby producing a test area of the prepreg that does not contain the curable resin.

[0032] Before the fibers contact the resin and reach the impregnation zone, the fibers are typically arranged in a plurality of tows of unidirectional fibers, each tow containing thousands or more, for example 12,000 filaments. These tows are mounted on a bobbin and are first subjected to a combing unit to ensure uniform separation of the fibers.

[0033] To improve the handling of the resin, it is common to support the resin on a backing material such as paper. Then, the resin is typically supplied from a roll so that the resin contacts the fibers, and the backing material remains in a predetermined position outside the contact area of the resin and the fibers. During subsequent impregnation processes, the backing material serves as a useful outer material for applying pressure to achieve uniform impregnation of the resin.

[0034] In one configuration of the process, the region where impregnation does not occur is the region of the impregnation layer that does not contain the curable resin. For example, this can be achieved by a region of the backing material that supports the resin and does not contain resin. For example, this can be achieved by placing a label on the impregnated resin layer and removing the label to which the resin has been transferred to create a region of the backing material that does not contain resin.

[0035] In an alternative configuration of this process, the region where impregnation does not occur is the region that includes a barrier sheet adhered to the first impregnation layer that prevents the curable resin from impregnating into the structural layer. Thus, the resin may be present, but due to the presence of the barrier sheet, it is prevented from moving into the test region. Preferably, the barrier sheet includes an adhesive for adhering the barrier sheet to the first impregnation layer to assist in positioning and subsequent removal of the barrier sheet.

[0036] It should be understood that some of the resin may flow past the sides of the barrier sheet, and thus the dimensions of the test region may be slightly smaller than the dimensions of the barrier sheet.

[0037] If there are problems caused by resin flowing over the side of the barrier, both solutions can be combined, that is, preparing an area of the backing material without resin can be combined with placing a barrier sheet adhered to the backing material without resin.

[0038] The barrier sheet can be made from various materials, especially materials that can withstand temperatures from 60 to 150 °C and can withstand the pressure of impregnation. Examples of suitable barrier sheets are vellum or polyethylene terephthalate (PET).

[0039] Typically, the process includes the next step of removing the barrier sheet to expose the test area of the prepreg for testing. This can include cutting any backing material that supports the resin.

[0040] Preferably, a second impregnation layer containing a thermosetting resin is provided, the second surface of the fiber layer is brought into contact with the second impregnation layer before compression, and the second impregnation layer includes an area where impregnation does not occur and is arranged to align with the area where impregnation does not occur in the first impregnation layer, thereby generating a test area of the prepreg that does not contain curable resin.

[0041] To facilitate the impregnation of the resin into the fibers, this is typically carried out at an elevated temperature, for example, from 60 to 150 °C, preferably from 100 to 130 °C, so that the resin viscosity decreases. This is most conveniently achieved by heating the resin and the fibers to the desired temperature before impregnation, for example, by passing the resin and the fibers through an infrared heater. After impregnation, there is typically a cooling step to reduce the tackiness of the formed prepreg. This cooling step can be used to identify the end of the impregnation stage.

[0042] Subsequent processing steps such as lamination, slitting, and separation may follow.

[0043] Once prepared, the prepreg may be wound so that it can be stored for a certain period of time. Subsequently, the prepreg can be unwound and cut as desired. For example, a test can be performed on a part of the prepreg containing the test area either before or after winding.

[0044] Once a prepreg is made by the method of the present invention, typically, a plurality of prepregs are stacked together to form a stack of prepregs or a preform. Then, the stack of prepregs or the preform can be cured by exposure to an elevated temperature, where the thermosetting resin cures. This is typically done under elevated pressure by known methods such as autoclave or vacuum bag techniques.

[0045] Here, the present invention will be illustrated by way of example only with reference to the following figures.

Brief Description of the Drawings

[0046]

Figure 1

Embodiments for Carrying Out the Invention

[0047] FIG. 1 shows a prepreg 10 including a structural layer 12 containing fibers, as well as a first impregnating layer 14 containing a curable thermosetting resin and a second impregnating layer 16 containing a curable thermosetting resin. The fibers are non-woven unidirectional carbon fibers aligned parallel to arrow A. The first impregnating layer 14 is supported by a first backing material 18 made from release paper. The second impregnating layer 16 is supported by a second backing material 20 also made from release paper.

[0048] Within the first impregnation layer 14, a first barrier sheet 22 made of bellarm or PET is embedded. The first barrier sheet 22 includes an adhesive 26 for adhering the first barrier sheet 22 to the first impregnation layer 14. Within the second impregnation layer 16, a second barrier sheet 24 made of bellarm or PET is embedded. The second barrier sheet 26 includes an adhesive 28 for adhering the second barrier sheet 26 to the second impregnation layer 16.

[0049] The prepreg 10 is shown as it is immediately after the method for its manufacture has been carried out. In this method, the structural layer 12, the first impregnation layer 14 and the second impregnation layer 16 are passed over a heated roller while under tension and moved in the direction of arrow A so that a part of the thermosetting resin impregnates the fibers in the structural layer 12, thereby forming the prepreg 10 and being compressed.

[0050] However, due to the presence of the first barrier sheet 22 and the second barrier sheet 24, the prepreg includes regions 30 where impregnation does not occur, thereby creating test regions 30 of the prepreg that do not contain the curable resin. Due to the temperature and pressure involved in the formation of the prepreg, some of the curable resin passes under the first barrier sheet 22 and the second barrier sheet 24, and as a result, it is seen that the test regions 30 become smaller than the sizes of the first barrier sheet 22 and the second barrier sheet 24.

[0051] Testing of the prepreg within the test region can perhaps be carried out even before the prepreg manufacturing process is completed as discussed above.

Example

[0052] Example As shown in FIG. 1, a prepreg was manufactured by integrating a layer of unidirectional carbon fibers with an upper resin sheet on a backing paper and also a lower resin sheet on the same backing paper. These three layers were integrated by passing them between and over a plurality of heated rollers in a known manner to impregnate the carbon fibers with resin into the gaps between the fibers.

[0053] A 125 mm × 197 mm bellram sheet having an acrylic resin adhesive on one side was placed on the upper resin sheet so that the bellram sheet prevented the resin from entering the carbon fibers through the carbon fibers. A bellram sheet of the same size was placed on the lower resin sheet and aligned with the bellram sheet on the upper resin sheet.

[0054] In this way, as the prepreg was formed, a test area having dimensions of about 125 mm × 197 mm and not containing a curable thermosetting resin was left in the prepreg. The prepreg including the test area was wound up and stored. The roll of prepreg was transferred to an edging machine and rewound while applying tension to expose the test area for testing.

[0055] A 100 mm × 100 mm area was cut out from the test area, weighed, and the fiber areal weight (FAW) was determined. A conventional washout test according to ASTM D3529 was also performed on an area 1 m away from the test area to determine the fiber areal weight (FAW). The results are shown in Table 1 below, and the values are shown in gsm.

Table 1

[0056] It can be understood that the measurement of FAW from the test area was highly consistent with the washout method.

[0057] Subsequently, this procedure was repeated. The results are shown in Table 2 below, and the values are also shown in gsm representing the prepreg areal weight (PAW).

Table 2

[0058] It can be understood that the measurement of FAW from the test area highly coincided with the washout method.

[0059] Subsequently, this procedure was repeated, but this time, many more samples were taken from different positions on the prepreg. The results are shown in Table 3 below, and the values are shown in gsm, which also represent the same prepreg areal weight, PAW, and resin content (RC).

Table 3-1

Table 3-2

[0060] It can be understood that the measurement of FAW from the test area highly coincided with the washout method. The average FAW of the "dry" test conducted on the test area was 191.31, and the average FAW of the washout test was 192.35. This systematic difference in the measurement may be presumed to be due to the residual resin remaining after the washout method.

[0061] Subsequently, this procedure was repeated, but this time, a 300 mm × 300 mm area was cut out from the test area, weighed, and the fiber areal weight (FAW) was determined. The results are shown in Table 3 below.

Table 4

[0062] It can be understood that the increase in the size of the test area from 100 mm × 100 mm to 300 mm × 300 mm had no significant effect.

Claims

1. A prepreg comprising a structural layer containing fibers having gaps between them, and a curable thermosetting resin impregnated within the structural layer and present within the gaps, The prepreg is a prepreg that includes a test area that does not contain a curable thermosetting resin.

2. The aforementioned test area is 1 to 1000 cm ２ The prepreg according to claim 1, having a surface area.

3. The aforementioned test area is 50 to 500 cm ２ The prepreg according to claim 2, having a surface area.

4. The prepreg according to claim 1, wherein the test area is surrounded by an area of ​​the prepreg impregnated with a curable thermosetting resin.

5. The prepreg according to claim 1, comprising a second test area that does not contain a curable thermosetting resin.

6. The prepreg according to claim 5, comprising a third test area that does not contain a curable thermosetting resin.

7. The prepreg according to claim 5, wherein the test areas are brought together to form a test cluster area, and in the test cluster area, the test areas are spaced apart so that each test area samples a different area of ​​the prepreg within the test cluster.

8. The prepreg according to claim 1, wherein the aforementioned fibers are separate and not woven together.

9. The prepreg according to claim 8, wherein the fibers are unidirectional.

10. The prepreg according to claim 1, wherein the fiber is carbon or glass fiber.

11. A method for testing a prepreg according to any one of claims 1 to 10, comprising the steps of: a) conducting a measurement test on the test area of ​​the test region; and b) then estimating the properties of the prepreg from the results of the measurement.

12. A method for testing a prepreg according to claim 11, comprising: a) cutting a test sample of fibers from the test area of ​​the test region; b) weighing the test sample of fibers; c) calculating the weight per unit area of ​​the test sample; and d) inferring that the fiber weight per unit area of ​​the test sample is a measurement of the fiber weight per unit area of ​​the prepreg.

13. The method of performing the test according to claim 11, wherein the test is performed while the prepreg is subjected to tension equivalent to the tension experienced during the manufacture of the prepreg.

14. A method for manufacturing a prepreg according to any one of claims 1 to 10, A method comprising the steps of: providing a structural layer containing fibers having a first surface and a second surface, and a first impregnation layer containing a curable thermosetting resin; bringing the first surface of the structural layer into contact with the first impregnation layer; and compressing the structural layer and the first impregnation layer together to form a prepreg such that the curable resin impregnates the fiber layer so that the curable resin is present in the gaps between the fibers, wherein the first impregnation layer includes areas where impregnation does not occur, thereby generating the test area of ​​the prepreg that does not contain the curable resin.

15. The method according to claim 14, wherein the region in which impregnation does not occur is the region of the impregnation layer that does not contain a curable resin.

16. The method according to claim 14, wherein the region in which impregnation does not occur is a region including a barrier sheet adhered to the first impregnation layer that prevents the impregnation of the curable resin into the structural layer.

17. The method according to claim 16, wherein the barrier sheet includes an adhesive for adhering the barrier sheet to the first impregnation layer.

18. The method according to claim 16, the step of removing the barrier sheet to expose the test area of ​​the prepreg for testing.

19. The method according to claim 15, wherein the barrier sheet is adhered to the region of the impregnation layer that does not contain a curable resin.

20. The method according to claim 14, wherein a second impregnation layer containing a thermosetting resin is provided, the second surface of the fiber layer is brought into contact with the second impregnation layer before compression, the second impregnation layer includes a region where impregnation does not occur and is arranged to align with the region in the first impregnation layer where impregnation does not occur, thereby generating the test region of the prepreg that does not contain a curable resin.

21. The method according to claim 14, wherein the compression includes passing the layer over one or more impregnation rollers.

22. The method according to claim 14, wherein the first impregnation layer and, if present, the second impregnation layer are supported by a backing material.

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