Introduction

2. An insight into Current Prepreg Applications and Commercial Prepreg Manufacturing Market

2.2. Prepreg Market Highlight

Recent market research highlighted that the current global market size of prepreg is forecast to reach USD 25.69 billion by 2031[48]. The report further emphasized that there could be a rapid growth of carbon fibre prepregs, which is currently dominating the current prepreg market representing over 84% of the total market value. This prediction is based on the fact that automotive manufacturers are forced to utilize carbon fibre composites and reduce the overall weight of automobiles which helps to reduce carbon emissions and increase fuel efficiency.[48]. Based on a market report published by Lin highlighted the gradually increasing demand for carbon fibre reinforced polymer (CFRP) composites from 2008 to 2025 [49]. It is predicted that CFRP demand will be increased to 285 kilotonnes in 2025. The prepreg layup, pultrusion, and winding processes represent over 50% of the total global CFRP demand by process (Figure 5a). When compared to other composite manufacturing processes, the pultrusion and winding process has the least material waste, thus increasing the demand for CFRP production [50].

Figure 5. Market overview showing (a) CFRP production by process and (b) main prepreg manufacturers in the world [49].

Figure 5. Market overview showing (a) CFRP production by process and (b) main prepreg manufacturers in the world [49].

Composite manufacturers use highly expensive autoclaves to cure components made using prepregs. Also, requires clean rooms, vacuum bagging equipment, etc which causes the cost of CFRP per kilogram to be about £6.38/kg, while that of steel and aluminum remain around £0.30/kg and £1.36/kg, respectively [51,52]. All the above factors along with the high energy consumption and elevated manual labor for prepreg layup have made prepreg manufacturing more expensive than the manufacturing of ordinary composites starting from dry fibres and resins [52]. Market research conducted by IndustryArc emphasized that the prepreg market is mainly governed by technology launches, acquisitions, and research and development (R&D) activities [53]. Because of these limitations along with massive R&D activities, and limited availability of resin formulations, the global prepreg industry has been confined to several giant composite manufacturers as seen in Figure 5b.

3. Resin Matrix for Epoxy Prepreg Manufacturing

3.1. Key Parameters for Epoxy Resin Matrix Development

3.1.1. B-Stage Control

Most of the commercially available prepregs are made with thermosetting resins such as epoxies. However, there can be thermoplastic prepregs and hybrid-type prepregs for specific applications. The thermoset resins can be crosslinked using various types of curing agents. The crosslink density in a thermoset resin can be determined using the degree of cure, based on which, any thermoset composite can be categorized into three major stages: A-stage, B-stage, and C-stage (Figure 6). A-stage refers to the initial stage when the epoxy and curing agent molecules exist as separate components with zero crosslinks, whereas C-stage refers to the resin system with high cross-link density. The C-stage is achieved by the post-curing process [55]. The B-stage occurs in between the A and C-stages where the thermoset is partially cured with fewer crosslinks so that resin viscosity can be still controlled by temperature [55,56]. Most of the commercially available epoxy-resin matrices are in B-stage condition. In general, B-staged resins do not cure at room temperature and can be stored as solid composites for longer periods [56]. However, the shelf life of prepreg material is highly dependent on the type of curing agent in the matrix, and hence further cross-linking could occur if the matrix contains a low-temperature curing agent, especially during transportation and storage.

Control of B-stage or degree of cure is vital when developing epoxy-prepregs as it accounts for viscosity and tack; two other important parameters for prepreg resin development. Further, the optimum B-stage is crucial for maintaining drape, tack, and optimum handling of prepreg at different temperatures. If the degree of cure is too low, it may result in excellent handling and drape properties, but it could lead to insufficient tack. On the other hand, a high level of degree of cure may result in poor handling of prepreg [57]. There are different approaches to controlling B-stage and the most common method is to expose the resin mixture for a specific time at room temperature. In addition, some studies have used elevated temperature while some have used different types of hardeners to control B-stage [57,58]. Table 3 summarizes some of the recent studies with different B-stage control strategies.

Mohan et.al., developed an in-house unidirectional carbon/epoxy prepreg for aerospace applications by modifying an existing prepreg manufacturing process [20]. The authors employed Methyl tetrahydrophthalic anhydride (MTHPA) as the curing agent to cure the resin. However, the drapability, which solely depends on the viscosity of the resin was visually inspected. This is a major drawback of this study as the control of viscosity plays a major role in the epoxy prepreg manufacturing process.

Banks et al., conducted a case study on the development of glass/epoxy prepreg for marine and civil infrastructure applications [57]. The degree of cure was varied from 1 to 57% by holding the prepreg at room temperature for different times in order to determine the optimum degree of cure for better handling, drape, and tack properties of the developed prepreg. It is worth noting that the term ‘rotational speed’ in the context of oscillation performance is not accurate. This is because rotational mode may be used to determine a partially cured resin matrix as it could destroy the crosslinks leading to fault results. As per the results, the complex viscosity is increased with increasing the degree of cure. Furthermore, it is interesting to note the typical Newtonian behaviour of resin mixture below 30% of the degree of cure as the viscosity is independent of the rotational frequency. However, when the degree of cure is further increased from 30% to 57%, the viscosity changes with the rotational speed revealing the non-Newtonian behaviour of the resin matrix (Figure 7). The overall results suggested that 30% of cure could be the best for prepreg properties with adequate resin viscosity.

Recently, several researchers attempted manufacturing different types of prepregs by using different resin formulations and B-staging conditions [59,60,61]. Surprisingly, all these studies lack information as to how the B-stage was determined. Two types of natural fibres were used to produce epoxy prepregs by Dalla Libera Junior et al., in two different studies [59,61]. However, the effect of fibres on B-staging has not been discussed or analysed. Similarly, Karakurt et al., studied the effect of poly(amidoamine)(PAMAM) functionalized graphene oxide on the electrical and thermal properties of carbon/epoxy prepregs [60]. Based on the published literature, the addition of particles could enhance the viscosity, therefore affecting the B-stage conditions [62,63,64]. Although there was a significant amount of PAMAM functionalized graphene oxide amount (up to 15 wt%) added to the epoxy matrix, the lack of determination of the degree of cure or viscosity is a major drawback of this study. Pouladvand et al., presented a different approach to control the degree of cure by changing the amount of low-temperature curing agent along with temperature [58]. Interestingly, they used off-stoichiometric levels of low-temperature hardener and presented the linear relationship between the amount of hardener and degree of cure. The study suggested that the approach can be used to produce prepregs with tailored tack and drape. Budelmann et al., evaluated the effect of B-staging on prepreg tack [62]. The cure enthalpy values obtained from DSC analysis were used to calculate the B-stage using Eq 1,

$$\alpha =1-{\displaystyle \frac{H_{res}}{H_R}}$$

(1)

where, $\alpha$ is the degree of cure, $H_{res}$ is the residual heat enthalpy, and $H_R$ is the total reaction enthalpy [62]. (More details about this study can be found under section 3.1.2).

It is important to notice that most of the studies discussed above have used low-temperature curing agents and hence the control of B-state is crucial. However, prepreg production through a hotmelt resin impregnation approach can be used to avoid the drawbacks of B-stage control and is discussed in later sections in this review.

3.1.2. Viscosity and Flow

Viscosity and flow of the resin matrix are some of the key factors to consider when manufacturing prepregs as they directly affect the resin impregnation, drape, tack, and final properties of prepregs. The ability to resist the flow is defined as the viscosity of a substance. Flow viscosity is important for determining the viscosity of neat resins, whereas complex viscosity plays a major role in setting up the resin impregnation temperature and, pre or post-curing cycles of prepregs. The resin matrix viscosity of prepregs is highly dependent on a few factors including the type of resins, impregnation temperature, curing agents, and tougheners used in prepregs. Some of the processing conditions especially in thermoset prepregs including applied pressure and curing temperature profiles solely rely on minimum viscosity and gelation point [65]. There is a high chance of getting improper fibre wetting resulting in very low interactions between the resin and fibre if the minimum viscosity is very high [65]. It is further highlighted that maintaining viscosity is crucial during the Automated Fibre Placement (AFP) as improper resin viscosity may result in slippage and bridging during the fibre layup process [65].

A perfect combination of resin viscosity and flow is important to control the defects in prepregs in addition to the layup process. A mini-review focused on manufacturing defects of complex shape prepreg-based composites discussed that resin distribution is one of the key factors that lead to layup defects during the manufacturing of prepregs [16]. The study further highlights that low resin viscosity could cause inter-laminar defects. For instance, the work has shown that the low viscous resin could cause more resin loss during the complex shape manufacturing processes. The study further concludes that the resin viscosity is the main factor to influences the percolation flow.

Effect of Type of Resin and Their Composition on Resin Viscosity

There is a wide range of epoxy resins and mainly bisphenol-A epoxy resin is commonly used to produce prepregs for most commercial applications. Most of the published research on prepreg resin development has used bisphenol-A epoxy resins in liquid states [57,58,59,66,67]. In this case, the viscosity of the matrix is controlled by the curing agent itself. However, Honda et al., and Kim et al., have used a different approach to control viscosity by employing a solid bisphenol-A resin along with a liquid bisphenol-A resin, and to make the final resin mixture to optimize the viscosity and tackiness of their epoxy-prepreg [68,69]. The patent published by Honda et al., highlighted the epoxy resin composition for carbon-fibre prepregs with superior flame retardance and mechanical properties that can be used to manufacture electrical and electronic equipment [70]. In this patent, the authors have recommended both upper and lower limits of viscosity of resin that are essential to maintain during the resin impregnation process. Herein, they highlight that at 60 ℃, the resin viscosity has to be between 10 to 700 Pa.s range. They further claimed that if the viscosity is below 10 Pa.s at 60 ℃, the resin would flow down to the bottom fibre layers resulting in low tack on the surface. Further, this may increase the resin flow during molding resulting in irregular surfaces in the final product. If the resin viscosity is beyond 700 Pa.s at 60 ℃, it may be difficult to impregnate the resin into fibre resulting in prepregs with patchy surfaces. This may further affect the mouldability of prepregs.

A recent study published by Kim et al., also highlighted the importance of maintaining viscosity when manufacturing prepregs [69]. They have mixed different ratios of two types of Diglycidyl ether of bisphenol-A (DGEBA) based epoxy resins (having two epoxy equivalent weights) to bring the viscosity of the resin mixture to the range between 10 to 700 Pa.s at 60 ℃ as highlighted in the previous patent. They used a mixture of solid and liquid types of resin and measured the viscosity of mixtures with different resin ratios with respect to temperature in order to determine the best resin ratio (Figure 8) [69].

Based on the recommended viscosity at 60 ℃, the authors have selected EA60/EB40 as the best resin ratio for prepreg development [69]. Although the recommended viscosity has been achieved through an epoxy mixture, the reason for using solid and liquid state resins is still unclear. In general, the solid resins could provide more tackiness and strength due to their high epoxy equivalent weights (EEWs) while the liquid resin with lower EEWs could enhance the flow properties, which are necessary for better impregnation of resin into fibre [71,72,73]. Further, liquid resins are more suitable for B-stage curing [71]. This could be the reason for the usage of solid and liquid DGEBA-based epoxy resin mixture.

The Effect of Curing Agent (Hardener) on Resin Viscosity

The use of curing agents or hardeners has been the most common practice to control the resin viscosity in epoxy-based prepreg. Few studies on epoxy prepreg development have employed hardeners to control the viscosity and tack of the resin during the resin impregnation process [57,58]. In these studies, complex viscosity has been measured as curing agents that have been used to control the degree of cure. However, as discussed above, Honda et al., and Kim et al., have used only latent curing agents, which do not affect the degree of cure during the resin impregnation process. As such, they have controlled the viscosity and tack of the resin by employing a resin mixture with different EEWs as highlighted in the previous section. This is an important strategy to suppress the crosslink reaction and enhance the storage life of prepregs. More details on latent curing agents for prepregs and the hot-melt process are discussed in later sections.

In an attempt to develop a novel custom-tailored epoxy prepreg system, the researchers have employed diethylenetriamine (DETA) as the low-temperature curing agent to control the viscosity of resin soon after the impregnation process [58]. Therein, an off-stoichiometry ratio of DETA to DGEBA has been used to precure the prepreg along with dicyandiamide (DICY) as the latent curing agent and 1,1-dimethyl, 3-(3’,4’-dichlorophenyl) urea (commonly known as Diuron) as the accelerator. The chemical structures and physical properties of these three substances are shown in Table 4.

When determining the B-stage of resin, it is important to notice the linear relationship between the degree of cure and the amount of curing agent due to the polyaddition curing reaction between DGEBA and DETA. The degree of this reaction mainly depends on the availability of functional groups. Therefore, by limiting the amount of curing agent, the degree of cure can be suppressed. The researchers have predicted the degree of cure with respect to the amount of DETA and have confirmed the results by the DSC (Differential Scanning Calorimetry) test. Although the authors have tried to relate the viscosity of resin by conducting probe tack and drape tests, the lack of determination of resin viscosity and flow is a considerable drawback of this study [58].

The Effect of Temperature on Resin Viscosity in Prepregs

Temperature is one of the key factors that control the viscosity and flow of resin and plays a major role in the epoxy resin impregnation process. Moreover, the control of the B-stage is often carried out by changing the resin temperature (Table 3). A general curve that shows the changes in resin viscosity and degree of cure with respect to the temperature (curing cycle) of an epoxy prepreg is shown in Figure 9 [74]. At the start, the viscosity drops with increasing temperature and once the temperature reaches to curing temperature, the epoxy starts making chemical crosslinks. This results in a rapid increase in viscosity followed by a plateau indicating the maximum number of crosslinks in the epoxy resin.

Belnoue et al., highlighted the effect of the viscosity of resin on the fibre waviness using predictive numerical models [75]. Accordingly, the authors have used temperature as the parameter to vary the viscosity and summarized the correlation between the resin viscosity and the wrinkle severity as depicted in Figure 10. The results showed a clear increase in excess length and wrinkle amplitude at lower viscosity levels. As such, the control of temperature is very important to control the defects in prepregs especially due to resin viscosity variations.

Kim et al., used employed the hot melt-impregnation technique to impregnate resin into carbon fibres. Therein, the viscosity of the resin is reduced by increasing the temperature to achieve better wettability and complete impregnation during the impregnation process. In order to determine the optimum impregnation temperature, the complex viscosity of the resin matrix is measured at different isothermal curing temperatures between 50 to 140 ℃ on a rheometer (Figure 11). The outcome showed that the temperatures lower than 70 ℃ had no significant increase in viscosity (or curing), and hence, the optimum temperature is set between 60-70 ℃.

Few studies have indicated that viscosity and temperature are two major parameters that affect interplay friction, which leads to the formation of wrinkle-effect through the ply slippage in complex shaped prepregs [64,76,77,78]. Wang highlighted the correlation between temperate dependent viscosity of the resin matrix and dynamic friction through Eq 2, where $\tau$ shear stress, $\gamma$ is shear rate, $\eta \left(T\right)$ is temperature-dependent resin viscosity, $h$ is the thickness of the viscous fluid layer, and $\nu$ is lateral velocity.

$$\tau =\gamma \eta \left(T\right)={\displaystyle \frac{\nu }{h}}\eta \left(T\right)$$

(2)

As indicated by Equation 1, processing temperature can significantly affect the dynamic friction among prepreg plies as it accounts for controlling viscosity and flow during the post-curing stage.

The Effect of Tougheners on Resin Viscosity in Prepregs

Tougheners are often used along with epoxy resins to improve fracture toughness and improve damping properties in high-performance aerospace-grade prepregs [15,79,80]. The commonly used tougheners in epoxy resins are high molecular weight thermoplastics including polyethersulfones (PES) and polyimides (PEI) owing to the ability to form two-phase morphology in the epoxy resin matrix thus improving the crack propagation [62,63,64,81,82,83,84]. In addition, several reviews were conducted on a wide range of toughening materials including core-shell rubber particles, liquid rubbers, dendritic polymers, block copolymers, rigid particles, and soluble thermoplastic fibres [79,80]. Although the use of tougheners brings important benefits, their effect on resin viscosity and tack could be problematic, especially during the resin impregnation, prepreg layup, and post-curing [80]. Budelmann et al., incorporated PES into epoxy prepolymer; tetrafunctional tetraglycidyl-4,4’ -methylenedianiline (TGMDA) by mixing PES powder with preheated resin [62]. The results showed that rise in complex viscosity and glass transition temperature with an increase in PES content from 10 to 30%. The authors claimed that the system with 10% PES content and the B-stage level of 20% showed similar values for a commercially available aerospace prepreg in terms of their tack. Galledari et al., fabricated a solid acrylonitrile-butadiene rubber (NBR) toughened epoxy/glass prepreg by hotmelt method [85]. The amount of NBR increased from 0 to 5% and the rheological results indicated an increasing trend of both gel times due to increasing viscosity.

Having difficulty to extract the resin from commercially available prepregs (due to the B-staged condition of prepreg resin matrix) and mixing tougheners, several researchers have fabricated the tougheners on the prepreg surface to make interlaminar toughened composites [15,64,86,87]. However, the main drawback of this process is the limited use of toughener amounts owing to the increase of resin viscosity and enhanced particle agglomerations [15,64]. Laberg et al., evaluated the interplay friction force of four unidirectional carbon/epoxy prepregs, one (T700/M21) having a resin layer with toughened particles [78]. The friction coefficient force was calculated at similar viscosity ranges, which resulted from different temperature ranges. The results indicated a significant increase in the friction coefficient of the prepreg with toughened particles compared to the other three prepregs as shown in Figure 12.

The above-discussed studies clearly outline that the type and the quantity of tougheners are some of the key factors that affect the final viscosity, gel time, and tack of resin matrix in prepregs.

Resin Viscosity Measuring Techniques

Researchers have employed different techniques to determine the viscosity of resin in prepregs. The quantitative analysis of prepreg viscosities by probe, peel, and tension or compression tests is highlighted in a few studies [17,58,88,89,90,91]. Among these three methods, probe and tension/compression methods follow the same principle as they use a load separation process to determine the viscosity of prepreg [91]. During the probe test, the prepreg surface is pressurized with a defined force by a probe head for a certain period, and subsequently, the probe head is released at a constant rate while measuring the load-displacement force. The probe test device and the model are depicted in Figure 13 (a and b).

The probe test is also used to measure the tack of prepregs, which will be discussed in a later section of this review. It is interesting to note that the peel method provides more reliable results as the tests are conducted through a laid prepreg. Moreover, the peel method provides more benefits over other methods owing to its ability to measure the bending stiffness and adhesive forces between prepreg layers [92,93,94]. By considering these benefits, Zu et.al., compared the probe and peel two tests using a commercially available prepreg [91]. A peel test device was employed to measure the load displacement during the peeling process. Subsequently, the authors established a relationship between the two tests through a peel simulation study.

Kim et al., utilized a parallel plate rheometer to determine the viscosity of epoxy resin [69]. Therein, the steady shear viscosity of neat epoxy as a function of temperature and at different shear rates was determined to assess the flowability of epoxy resin mixtures. The dynamic rheological properties and complex viscosities of epoxy resin mixtures with curing agents were determined by oscillatory shear mode in the rheometer (Figure 14). Complex viscosity studies are extremely important to highlight the curing behaviour of prepregs.

Banks et al., measured the complex viscosity of prepreg at different levels of cure using a parallel plate rheometer [90]. They further investigated the effect of oscillating frequency on the complex viscosity of epoxy-based prepreg. The surface friction is considered a crucial factor for prepreg layup and is mainly determined by the viscosity of the partially cured resin in the prepregs [64]. As such, some researchers employed parallel plate rheometer to determine prepreg surface friction considering its ability to control temperature and force precisely [64,95]. However, this technique measures the rotational friction force which resulted from variable rotation speed and fibre orientation, and cannot be separately analysed from the effect of temperature. This is considered a critical shortcoming of the use of a rheometer for surface friction analysis [64].

3.1.3. Tackiness of Prepregs

The tackiness of a prepreg is another important factor that can be mainly achieved by the careful selection of resin matrix. The tackiness or stickiness is important, especially during the layup process of uncured prepregs [16]. The right amount of tack of a prepreg would lead to easy handling and excellent laminate properties. Lack of proper tack properties could lead to bonding defects during the layup process including bridging and wrinkling, and ultimate material loss [17]. Prepreg tack is mainly governed by adhesive and cohesive interactions and is not attributed to any chemical reaction taking place during the curing of prepregs [17]. The aforementioned intrinsic interactions are greatly affected by the viscosity of resins, processing parameters, and environmental factors [17,91].

A recent review published by Budelmann et.al., on the effect of prepreg tack during the automated layup processes discussed the factors affecting prepreg tack in detail [17]. As per the review, the influential factors are classified into two categories: environmental aspects during production (extrinsic) and prepreg material properties (intrinsic). The intrinsic factors are more important to control tack during the early stages of prepreg development whereas, the extrinsic factors are important during the layup or moulding process. Table 5 summarizes the intrinsic properties that affect prepreg tack.

The intrinsic parameters are extremely important to develop tailor-made prepreg systems and can be mainly characterized by rheological, thermal, and microscopic analysis during the early stages of prepreg development. Studer et.al., demonstrated the use of tackiness to join B-staged carbon/epoxy composites without an adhesive layer [96]. The study uses a kinetic model to describe the co-curing of B-staged components and finally highlights the possibility of using this technique for combined manufacturing processes such as resin infusion, prepregs, conventional resin transfer moulding, or compression resin transfer moulding.

Figure 15. Concept of co-curing and joining of B-staged components using resin tack [96].

Figure 15. Concept of co-curing and joining of B-staged components using resin tack [96].

Banks et al., highlight the reduction of tack properties with decreasing degree of cure [57]. (See section 3.1.1 for more detailed discussion). The effect of resin formulation, B-staging, and toughening on epoxy resin tack has been recently studied by Budelmann et al.[62]. The study used different epoxy prepolymers including tetrafunctional tetraglycidyl-4,4’-methylenedianiline (TGMDA), a trifunctional triglycidyl p-aminophenol (TGAP), and a bifunctional Bisphenol A diglycidyl ether (DGEBA) to highlight the effect of prepolymer on resin tack properties (Figure 16a). Based on previously published results by the same researchers, it was concluded that the tack of all A-staged prepolymers is 3 to 4 times higher than that of commercial prepregs [97,98]. Degree of cure results indicated that the tack-bell curves were moved towards higher temperature regions with decreasing tack, which may be attributed to the reduction of resin viscosity at higher temperatures (Figure 16b). Moreover, the tack was reduced by increasing the toughener (high-molecular polyethersulfone) content as shown in Figure 16c.

Tack Measurement Techniques

Until the introduction of the standard test method for the characterisation tack of prepregs by ASTM D8336 in 2021, there was no standard method to test tack in prepregs. Prepreg developers often used trial-and-error methods or previous knowledge rather than proper technical data. The information provided by commercial suppliers on the prepreg tack was very little, which could be a possibility due to the large number of influential factors on tack. Most of the available tack measuring techniques were linked to pressure-sensitive adhesives (PSA) technology. Due to the adhesive nature of prepregs, scientists have tried to use similar PSA standard tack measuring techniques, namely probe tack test and peel test to determine the tack of prepregs [17,89,90,94]. However, the recently introduced ASTM standard test for the characterisation of prepregs tack uses the peel test method and is discussed in detail below [99].

4. Important Insights in Epoxy Resin Matrix Development for Prepregs

5. Conclusions and Outlook

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