1 Introduction

Nowadays, the increasing pressure about the preservation of natural resources and political awareness on exploring green alternatives to decreasing global fossil fuel emission [1]. Vegetable fibers arise from natural resources and they fulfill the renewability and marketing apse requirements. In the last four decades, thermoset composites with vegetable fibers have been extensively studied are mainly applied in the automotive interior components [2]. There is an increasing demand for applying natural composites in other areas, such as in civil structures due to their low density, good processability, low abrasion, and high resistance to corrosion [3]. However, the application is restricted to low-medium components and they cannot yet substitute com synthetic fiber composites, such as glass fiber structures. Seeking at minimizing these drawbacks and aiming to broaden the range of applications, there are increasing studies in hybrid synthetic/vegetable fiber-reinforced composites. When the fibers are adequately combined, the resulting properties can be comparable to pure synthetic fiber composite and hence replace synthetic fibers, depending upon the application [4-10].

Hybrid composites are tailorable materials, which are driven by the role of fibers arrangement in the composite system. The behavior of hybrid composites considers the contribution of each reinforcing phase (the type of reinforcement, geometry, etc.), in which the composite can be manufactured aiming at taking the best characteristics of each fiber. There are innumerous studies regarding glass/vegetable hybrid composites that can be found in the literature in different databases [4, 10–12]. Often, the chosen research topic is not immediately found in the performed searches, and when a review on this particular chosen topic is carried out, several times, it is identified that previous studies on the same topic have already been done, invalidating the approach. In this context, a systematic review is a powerful tool that helps the readers to rapidly find the lacks and most studied topics of a specific subject.

2 Systematic Review

Different than a conventional literature review, a systematic review aims to identify, evaluate, and summarize the findings of all relevant individual studies over a defined issue, making the available evidence more accessible to decision-makers [13]. Primarily, the protocol is employed based on a severe and rigorous scientific search allowing anticipate potential problems and indicating gaps in knowledge, which can be used as a starting point for developing new researches. An important characteristic is that some granting agencies require a systematic review to justify the planned research [14]. Another important characteristic is preventing arbitrary decisions (by respecting inclusion criteria and extraction of data), then, it follows a protocol.

Systematic reviews are very widespread in the area of medicine and health care. Some systematic reviews were published regarding low health literacy and health outcomes [15], insomnia, complementary medicine [16], and renal cancer [17]. In the polymeric fields, very few studies were released following the PRISMA protocol [18, 19]. However, until now, in the composites field, there is no systematic review that follows PRISMA methodology focusing on their mechanical, thermal, and dynamic mechanical properties, especially when it comes to vegetable fibers/cellulose chemically modified/epoxy composites.

Hence, the main aim of this book chapter is to compile data between 2016 and 2020, about composite laminates with epoxy resin as matrix and hybrid glass/vegetable fibers as the reinforcing phase. We focus on thermal, mechanical, and dynamic mechanical properties, as well as the manufacturing process to verify a future trend or gaps in this area. Moreover, we list the obtained data regarding the reinforcement type and processing method.

3 Systematic Review Methodology

The methodological guidelines outline the Transparent Reporting of Systematic Reviews and Meta-Analyses (PRISMA)1 in which a determined protocol is followed before carrying out this systematic review. This protocol aims to rationale, hypothesize, and plan the review, being a guide. In this book chapter, three different literature databases are considered, and only research papers focused on recent studies in glass/vegetable fiber hybrid composites using epoxy resin as a matrix are taken into consideration. More details are presented next.

Papers are selected using Scopus (www.scopus.com), Web of Science (www.webofknowledge.com), and SciFinder (https://sso.cas.org/) databases. The terms of search are: ([epoxy] AND [hybrid] AND [composites] AND [natural] AND [glass] AND [fiber]). We choose the word “natural” instead “vegetable” because no paper was found with this term, despite being a well-accepted term in the field. The results are limited to papers published from 2016 to 2020. The identified articles have their titles and abstracts assessed independently by two reviewers (Neves, R.M., and Ornaghi Jr, H.L.) to screen their allocation in the systematic review.

This book chapter focuses on recent studies in the hybrid glass/vegetable fiber composites field. The type of vegetal fiber, manufacturing process, and thermal, mechanical, and dynamic mechanical properties are listed excluding microscopies (e.g. scanning electron microscopy—SEM, and optical microscopy) and chemical analyzes (e.g., FTIR, XRD, NMR) as well other results such as flammability, aging, wettability, and water absorption.

4 Results of Data Collection

The study selection can be better visualized in the flowchart (Fig. 1), in which the records for every database are identified as well the exclusion criteria and selected papers by eligibility. The total search is of 285 studies, including all chosen databases. In this step, conference papers, review studies, studies not presented in English language, book chapter, editorial, letters, and notes are not considered. With only research papers remaining, some studies which do not account for the present systematic review methodology are excluded: studies with other matrices other than epoxy resin [12 studies], no hybrid composites (7 studies), hybrid composites other than epoxy glass/fiber/vegetable fibers (14 studies), hybrid composites with epoxy matrix but without vegetable fibers as reinforcement [15], vegetable hybrid composites with epoxy matrix but without glass fiber as reinforcement [67], and composites that add another material as a third component [38] are excluded. After this step, 132 studies remained. From these, 84 are excluded for duplicity on the databases. After all, 48 meet all selected criteria, then composing the current systematic review.

Fig. 1
figure 1

Systematic review flowchart following PRISMA protocol

Full size image

The type of glass fiber (chopped, unidirectional, and woven), natural fiber (chopped, continuous mat, woven, and powder), and composite are presented in Fig. 2. Highlighting the frequency of appearance in the selected research papers.

Fig. 2
figure 2

Frequency of the glass fibers, natural fiber, and type of composites found on the systematic review

Full size image

5 Results and Discussion

Table 1 presents all results of the 48 selected papers according to PRISMA flowchart (Fig. 1) summarized in the systematic review search. The type of fiber, processing manufacturing, and the description of thermal, mechanical, and dynamic mechanical properties are described in Table 1.

Table 1 Thermal, mechanical, and dynamic-mechanical properties of the hybrid composites
Full size table

6 Conclusion and Future Perspectives

In this study, a systematic review on glass/vegetable hybrid composites between 2016 and 2020 was performed. The type of reinforcements and manufacturing processes were included. Thermal, mechanical, and dynamic mechanical properties were also targeted. The systematic review proved to be a useful tool for both young and the experienced researchers as a guideline in a specific(s) subject(s). It easily indicates the trends and lacks in a searched topic. This study showed an enormous lack of thermal and dynamic mechanical properties that can be explored for further research. The main studied aspect is at the mechanical properties point of view, which include impact, flexural, compressive, and tensile tests, independently of the reinforcement. Hand lay-up is the most employed manufacturing process, which is certainly associated to the low cost of this process. Finally, the most employed type of both glass and vegetal fiber was woven fabric, being the interleaved composite the most used. Consequently, the fast and easy identification of studies and the possibility of new combinations of reinforcements/manufacturing processes and tested properties make the systematic review a powerful tool in any scientific field.

Vegetable fibers will be continuously employed, and the studies evaluated here pointed out that there is a growing trend for different reasons given ecological appeal and environmental benefits or combination with synthetic fibers for different applications. The weight reduction, easy formability, low cost and ease-processing are some of the many advantages promoted by the vegetal fibers even when combined with synthetic fibers. As a result, glass/vegetable hybrid composites will keep being applied in interior automotive components to add an eco-friendly character while still meeting design requirements [67]. In addition, the possibility to chemically/physically modify vegetable fibers aiming to improve their physical–chemical properties is attractive, although it is still ineffective from the mass production point of view. Other promising applications included load-bearing structural members for application in civil applications, including concrete elements, roofing components, bridges, decks, among many others. Other applications include textile and yarn industries, furniture, housing (door, panels, roofing sheets), sporting goods (tennis racket, snowboarding), window frame, fencing, bicycle frame, mobile cases, insulations, bags, flush door shutters, mirror casing, filling material for upholstery, among others [68].