International Materials Reviews ISSN: 0950-6608 (Print) 1743-2804 (Online) Journal homepage: https://www.tandfonline.com/loi/yimr20 A review of Long fibre-reinforced thermoplastic or long fibre thermoplastic (LFT) composites Haibin Ning, Na Lu, Ahmed Arabi Hassen, Krishan Chawla, Mohamed Selim & Selvum Pillay To cite this article: Haibin Ning, Na Lu, Ahmed Arabi Hassen, Krishan Chawla, Mohamed Selim & Selvum Pillay (2019): A review of Long fibre-reinforced thermoplastic or long fibre thermoplastic (LFT) composites, International Materials Reviews, DOI: 10.1080/09506608.2019.1585004 To link to this article: https://doi.org/10.1080/09506608.2019.1585004 Published online: 11 Mar 2019. Submit your article to this journal Article views: 1 View Crossmark data Full Terms & Conditions of access and use can be found at https://www.tandfonline.com/action/journalInformation?journalCode=yimr20 INTERNATIONAL MATERIALS REVIEWS https://doi.org/10.1080/09506608.2019.1585004 A review of Long fibre-reinforced thermoplastic or long fibre thermoplastic (LFT) composites Haibin Ning a,NaLub, Ahmed Arabi Hassenc, Krishan Chawlaa, Mohamed Selima and Selvum Pillaya aDepartment of Materials Science and Engineering, Materials Processing and Applications Development (MPAD) Centre, University of Alabama at Birmingham, Birmingham, AL, USA; bLyles School of Civil Engineering, School of Materials Engineering, Purdue University, West Lafayette, IN, USA; cManufacturing Demonstration Facility (MDF), Oak Ridge National Laboratory (ORNL), Knoxville, TN, USA ABSTRACT ARTICLE HISTORY Long fibre-reinforced thermoplastic or long fibre thermoplastic (LFT) composites possess Received 8 August 2018 superior specific modulus and strength, excellent impact resistance, and other advantages Accepted 15 February 2019 such as ease of processability, recyclability, and excellent corrosion resistance. These KEYWORDS advantages make LFT composites one of the most advanced lightweight engineering Long fibre thermoplastics; materials and enable their increasing use in various applications. This review paper composites; properties; fibre summarises the research and development work that has been conducted on LFT orientation distribution; fibre composites since their initial development. Different aspects of LFTs, such as process length distribution development, fibre orientation distribution (FOD), fibre length distribution (FLD), and their effects on the mechanical properties of LFT composites are described. The characterisation of the FOD and FLD in the LFT composites using advanced imaging technology such as high- resolution 3D micro-CT scanning technique is summarised. Research and development of LFT hybridisation and LFT additives are also discussed. Finally, conclusions are made and the future outlook of LFT composites is given.. Introduction Long fibre-reinforced thermoplastics (LFT) are compo- high-performance engineering polymers. The thermo- site materials comprised of the thermoplastic polymer plastic polymers used in LFT include polypropylene matrix and discontinuous reinforcement fibres with a (PP) [19–24], high-density polyethylene (HDPE) length to a diameter aspect ratio greater than the criti- [25,26], nylon 6 or polyamide 6 (PA6) [27–29], cal aspect ratio. These composites have been widely nylon 66 or polyamide 66 (PA66) [5,6,30], polylactic used in various applications as a result of their superior acid (PLA) [31], polymethylmethacrylate (PMMA) mechanical properties, excellent processability, low [32], poly butylene terephthalate (PBT) [33], poly- density, recyclability, low cost, excellent corrosion ethylene terephthalate (PET) [34], thermoplastic resistance, good vibration damping, and infinite shelf polyurethane (TPU) [35], polyoxymethylene (POM) life. Compared to short fibre-reinforced thermoplastic [36,37], polyphynelenesulfide (PPS) [38–41], polyary- or short fibre thermoplastic (SFT) composites, LFTs letherketone (PAEK) [42], and polyetherketoneether- offer better mechanical properties [1–16], which, in ketoneketone (PEEKEK) [43]. It is estimated that conjunction with their ease of processability, have 65% of the LFT market is polypropylene-based. enabled their use as advanced lightweight engineering Polyamide-based LFTs have about 20% of the market materials, particularly within the automotive sector. share and LFTs with other polymer systems comprise As a result, LFT production has experienced rapid the remaining 15% [17]. Though single polymer sys- growth since year 2000. The global market grew from tems are most commonly used as the matrix in 108 to 245 million kg from 2004 to 2009 with an annual LFTs, multiple polymer systems have also been growth rate of 18% [17]. A market survey published in studied such as TPU + POM [44], PP+ polystyrene Jan 2017 estimated that the LFT global market will (PS) [45], TPU + acrylonitrile butadiene styrene grow from USD 3.28 billion to USD 5.55 billion by (ABS) [46], PBT + PET [47,48], polycarbonate + PBT 2022 at an annual growth rate of 9.29% [18]. The [49], TPU + styrene acrylonitrile (SAN) [50], PA66 increasing use of LFTs in the automotive and electrical +PP[51,52], PP + PBT [53], and polyvinyl chloride and electronics industries is expected to drive the (PVC)+PP [54]. LFTs with multi-matrix systems can growth of the LFT market [18]. gain the benefit of both the matrix polymers. For Various thermoplastic polymers have been used as example, LFTs with a PP + PA66 matrix can benefit LFT matrices, ranging from commodity polymers to from the low water absorption and excellent CONTACT Haibin Ning [email protected] Department of Materials Science and Engineering, Materials Processing and Applications Development (MPAD) Centre, University of Alabama at Birmingham, 35294, Birmingham, AL, USA © 2019 Institute of Materials, Minerals and Mining and ASM International Published by Taylor & Francis on behalf of the Institute and ASM International 2 H. NING ET AL. processing properties of PP and the high mechanical strength and temperature resistance of PA66 [51]. Glass fibre is predominantly used as the reinforce- ment material in LFT composites due to its low cost and superior mechanical properties. The most studied LFT is glass fibre-reinforced polypropylene (glass/PP) [14,20,22,54–63], driven by its popularity in the auto- motive industry. Other fibres such as carbon fibre [27,29,35,39,64], basalt fibre [37,65,66], aramid fibre [67,68], PP fibre [69], polyimide fibre [33], and PET fibre [70] are also used as reinforcement. Although Figure 1. The change of the fibre tensile stress, σ and inter- natural fibre-reinforced polymer matrix composites facial shear stress, τ with fibre aspect ratio, (l/d). The fibres have not been traditionally considered to be LFTs, an in LFT composites have the aspect ratio equal to or greater l/d increasing number of composites reinforced with than ( )c, critical aspect ratio [1]. long natural fibres have been developed and studied [71]. Long natural fibre composites include chicken tensile strength. LFTs generally have a fibre aspect ratio fi fi / fi feather bre/PLA [72], keratin feather bre/polyethy- equal to or greater than (l d)c, and therefore, the bres lene [73], flax fibre/PP [74,75], viscose fibre/PP [76], are used to their maximum capacity. When the fibre fi fi / sisal bre/PP [77], pita bre/polyhydroxybutyrate aspect ratio is below (l d)c, as in SFTs, the maximum [78], rayon fibre/PP [79], hemp fibre/PP [80], and stress on the fibre is below the fibre tensile strength jute fibre/PLA [31,81]. In addition, more than one and fibre pullout is expected to occur, indicating that type of fibre reinforcement may be used within a single the fibres are not used to their maximum capacity. LFT. For example, Lee [82] used both glass and carbon Consequently, SFTs generally exhibit lower mechanical fibres in a PP matrix LFT composite to study the elec- properties than LFTs. trical percolation and the hybrid effect of multiple fibre If the values for the fibre diameter, interfacial bond- types. It was found that the percolation threshold point ing strength, and fibre tensile strength are known, the is between the loading of 6 and 10 vol.-% carbon fibre. critical fibre length may be determined and the critical As the loading of carbon fibres increases, the modulus fibre aspect ratio can be calculated. For instance, the increases while the strength reduces [82]. Wood pulp critical fibre length for kenaf fibres in a PP matrix fibres have been added to jute/PP LFT as an impact was calculated to be 2.4 mm for a fibre tensile strength modifier to achieve higher impact performance [83– of 374.5 MPa, fibre-matrix interfacial shear strength of 85]. Synthetic fibres are also used with natural fibres 4.9 MPa, and an average diameter of the kenaf elemen- together in LFTs [86,87]. The synthetic fibres offer tary fibres of 62.2 μm[88]. The critical fibre aspect superior mechanical properties, while the natural ratio is approximately 39. The critical fibre length fibres can provide recyclability and sustainability. and critical fibre aspect ratio can be different if the The incorporation of 10% glass fibres into a jute fibre same fibre is used in a different polymer matrix due LFT was found to improve the flexural and tensile to changes in the interfacial bond strength. For strength of jute fibre-reinforced PP composites by example, the critical fibre length in glass/PP LFTs is about 60–74% and impact strength by almost one mag- between 3.2 and 4.4 mm [89,90], whereas the critical nitude [87]. fibre length for glass/PA66 is about 1.24 mm [91], Although the fibres used as reinforcement in LFTs which results in a critical fibre aspect ratio of approxi- are discontinuous, these composites possess excellent mately 213–293 for the glass/PP LFT and 83 for the mechanical properties owing to high fibre length to glass/PA66 LFT, respectively. Surface treatment on diameter ratio or fibre aspect ratio which are higher the fibres or the addition of coupling agents in the than the critical fibre aspect ratio. The critical fibre matrix may result in different critical fibre lengths / fi fi fl aspect ratio, (l d)c,isde ned as Equation (1) [1].