A Systematic Literature Review of Waste Polypropylene Reinforced with Glass Fiber: Mechanical and Thermal Properties in the Context of Sustainability ^†

Abstract

Recycled polypropylene reinforced with glass fiber (rPP-GF) represents a promising sustainable material alternative addressing global imperatives for environmentally responsible manufacturing. This systematic literature review examines the mechanical, thermal, and sustainability performance of rPP-GF composites based on 66 peer-reviewed studies published between 2014 and 2024, following PRISMA guidelines. The evidence demonstrates that glass fiber reinforcement significantly enhances mechanical and thermal performance. Compared to virgin counterparts, rPP-GF composites reduce environmental impact. However, technical challenges, including fiber attrition during reprocessing and a lack of standardized testing protocols, remain significant barriers. Future research should prioritize unified testing frameworks, hybrid filler systems, and advanced recycling technologies for enhanced circular economy applications.

1. Introduction

The worldwide production of plastics exceeds 400 million metric tons annually, with polypropylene (PP) making up nearly 20% of plastic waste due to its use in packaging, automotive parts, and consumer goods [1]. Despite efforts to promote mechanical recycling, only 10–15% of PP waste is reprocessed globally, leaving most to landfills and environmental pollution [2]. PP is valued for chemical resistance, low density, and cost-efficiency, but its relatively low mechanical strength and thermal stability have driven the development of reinforced composites [3]. Recycled PP (rPP) often exhibits degraded strength, thermal instability, and inconsistent melt flow, limiting high-performance applications [4]. Reinforcing rPP with glass fibers (GF) enhances structural integrity while supporting circular economy goals. GF-reinforced PP composites offer a high strength-to-weight ratio, corrosion resistance, and thermal stability, suitable for automotive, construction, and aerospace industries [5]. However, challenges include poor interfacial adhesion, fiber attrition during reprocessing, and inconsistent waste quality [6], affecting mechanical and thermal performance. Blending rPP with 30–50% virgin PP improves tensile strength by up to 20% and thermal stability [7], while hybrid systems combining GF with graphene nanoplatelets further enhance flexural strength [8]. This review critically analyses recent advances in rPP-GF composites, assessing mechanical, thermal, and sustainability performance for circular economy and sustainable engineering applications.

2. Methodology

This systematic review follows PRISMA guidelines to ensure transparency and reproducibility. An extensive search was conducted using Scopus, Web of Science, and Google Scholar with Boolean operators and predefined keywords such as “waste polypropylene” OR “recycled polypropylene,” “glass fiber” OR “fiber reinforcement,” “mechanical properties” AND “thermal properties,” and “sustainability” OR “circular economy.” Specific inclusion criteria were: peer-reviewed articles published between 2010 and 2024, reporting recycling of waste polypropylene with glass reinforcement and addressing sustainability aspects. Exclusion criteria included non-English publications, unrelated studies, and duplicates. The selection involved four phases: identification, screening, eligibility, and inclusion, resulting in 66 final studies from 4,918 initially retrieved articles. This approach ensured high-quality, relevant research.

3. Results and Discussion

3.1. Mechanical Performance Enhancement

The incorporation of glass fibers in recycled polypropylene provides significant reinforcement as shown in

Table 1. Mechanical Performance Summary of rPP-GF Composites.

Fiber Content (%)	Tensile Improvement (%)	Modulus Improvement (%)	Key Findings
20–25	~30–40	~50–60	Optimal strength and ductility with maintained properties [14]
30–35	~40–50	~60–70	Enhanced stiffness with moderate impact on ductility [14]
40–45	~50–70	~70–80	Maximum strength gains but increased brittleness concern [15]

. At 20–40 wt% fiber content, tensile strength typically rises by 30–50%, with elastic modulus gains of 50–70% compared to neat recycled PP [9]. These improvements result from effective load transfer within the matrix, compensating for defects in recycled polymers [9]. Reinforcement levels of 15–45% enhance tensile strength and stiffness, but contents above 40% cause poor dispersion, brittleness, and stress concentrations [10]. Processing conditions also affect properties. Injection Molding aligns fibers for higher longitudinal strength but reduces isotropy [11], while high processing temperatures or shear can degrade the matrix and fiber–matrix interface. Performance can be improved by blending recycled PP with 30–50% virgin PP, restoring crystallinity and yielding tensile properties of virgin PP composites while maintaining sustainability [12]. Hybrid fillers, such as graphene nanoplatelets with glass fiber, further improve stress distribution, impact resistance, and fracture energy, delaying crack initiation while preserving stiffness [13].

3.2. Thermal Stability and Fire Resistance

Thermal evaluation of PP-GF composites shows substantial improvements over neat, recycled PP. Glass fibers’ non-combustible nature and high melting point benefit the matrix. TGA studies report degradation onset at 300–350 °C and offset above 500 °C, compared to ~270 °C for neat PP. Multiple recycling cycles reduce thermal performance due to fiber shortening and poor fiber–matrix adhesion, causing earlier degradation. Thermal stabilizers, antioxidants, and compatibilizers help retain structural integrity and heat resistance through repeated recycling as illustrated in

Table 2. Thermal Properties Enhancement in rPP-GF Composites.

Property	Neat rPP	rPP-GF 30%	% Improvement
Onset Degradation Temp. (°C)	270	330	~60 [16]
Heat Deflection Temp. (°C)	85	110	~25 [17]
Crystallinity (%)	43	58	~15 [16]
Thermal Conductivity (W/m·K)	0.22	0.45	~100% [16,17]

.

3.3. Processing Method Impact and Industrial Considerations

Processing techniques strongly affect composite properties by influencing fiber orientation, dispersion, and retention. Injection molding, widely used industrially, aligns fibers with melt flow, improving tensile and flexural strength in that direction but reducing transverse performance. Twin-screw extrusion provides better fiber distribution and property consistency than single-screw systems, which often cause fiber breakage from high shear. Compression molding promotes isotropic orientation and balanced strength with lower void content, though throughput and fiber retention limit scalability [18]. Advanced methods such as supercritical fluid-assisted and reactive extrusion enhance homogeneity, bonding, and minimize degradation. Surface treatments, especially silane agents and maleic anhydride grafted polypropylene (MAPP), are also applied to improve interfacial bonding in recycled systems [19].

3.4. Sustainability Assessment and Circular Economy Integration

Life Cycle Assessment (LCA) shows rPP-GF composites reduce greenhouse gas emissions and energy use compared to virgin counterparts. Virgin PP-GF production consumes more energy, making rPP-GF attractive for automotive and construction applications [20]. Using post-consumer plastics also reduces reliance on petrochemicals while maintaining structural performance. rPP-GF allows multiple recycling cycles, though fiber shortening, interfacial debonding, and property losses limit circularity beyond 3–4 loops [17]. Hybrid strategies with natural fibers, Nano-fillers, or biodegradable matrices improve performance and reduce environmental impact but face scalability challenges. End-of-life management is a barrier, as glass fiber recovery suffers from damage and quality loss [21]. Emerging methods such as solvent-assisted separation, pyrolysis, and electrostatic sorting aim to improve recovery efficiency and material quality.

3.5. Research Gap and Future Direction

Several research gaps hinder industrial adoption of rPP-GF composites. Lack of standardized testing and characterization protocols creates inconsistencies in benchmarking. Limited long-term durability data under UV, humidity, and thermal cycling restricts use in extended-service applications. Life cycle assessment (LCA) studies frequently omit phases such as transport, end-of-life, and economics, and industrial pilot validations are few [22]. Future work should establish ASTM/ISO test standards, develop hybrid fillers (glass + bio-reinforcements), and enable industrial-scale implementation via OEMs and SMEs. Scalable, eco-friendly fiber-recovery methods and bio-based polymer matrices (e.g., PLA, PBS) offer sustainability promise, though their thermal/mechanical performance must improve for structural use.

4. Conclusions

This systematic literature review shows that recycled polypropylene reinforced with glass fiber is a sustainable material with significant performance advantages over neat polypropylene. Mechanical improvements of ~30–70% in strength and stiffness, enhanced thermal stability, and a reduction in environmental impact make rPP-GF composites promising candidates for circular economy applications in automotive, construction, and packaging sectors. Despite challenges in standardization, end-of-life processing, fiber recovery, advanced processing, compatibilizers, and hybrid reinforcements offer ways to overcome them. Success requires coordinated efforts among researchers, industry, and policymakers to set standards and scalable recycling, making rPP-GF composites a step toward resilient, circular, and sustainable manufacturing.