The Guest Editor of this special issue on additive manufacturing of polymeric materials, Prof. Dr.-Ing. Dr. h. c. Mult. Klaus Friedrich, passed away at the end of May 2021. His death is undoubtedly a great loss for the international community of composites.

As a famous scientist of composite materials, Prof. Friedrich published vast amounts of papers and books and book chapters as well, with which he became one of the mostly cited researchers worldwide. In 2005 he was recognized as a “World Fellow” of the International Committee on Composite Materials. The most fascinating part that I learned from him for years was his spirit of continuous exploration and innovation, which kept him active in the latest academic frontiers. In addition to the outstanding scientific achievements, Prof. Friedrich was also well known for his management and leadership abilities. He made tremendous contribution to the international reputation of the Institute for Composite Materials. (IVW) at the University of Kaiserslautern, Germany, when he served as Research Director for Materials Science from 1990 to 2006, and emeritus professor as of 2006.

In my memory, Prof. Friedrich was a very nice and generous gentlemen, always ready for caring others. During my research stay in IVW in 1990s, for example, he gave me full help in every aspect, which made me feel very warm. Since that time, I had continually got valuable advices from him, for which I am always grateful. In 2017 he kindly agreed to prepare a high quality review article entitled “Polymer composites for tribological applications” (https://doi.org/10.1016/j.aiepr.2018.05.001) for the inaugural issue of the journal Advanced Industrial and Engineering Polymer Research, which was critically important for the smooth growth of the new journal. Last year he promised to organize the present special issue by inviting leading experts in the area of 3D printing from different countries, in order to timely reflect one of the cutting-edge progresses of polymer engineering. Even in late March 2021, he communicated with the editorial office on certain technical issues. His work enthusiasm, personality and charm live on.

Unfortunately, Prof. Friedrich couldn't see the publication of the special issue, but it comes out as scheduled under the strong support of his friends and colleagues. There are six invited papers and two contributed papers in the collection. The topics cover preparation of feedstock for 3D printing, development of new fabrication approaches, effects of processing conditions, and applications in various aspects, which are presented in the form of research articles and reviews. Readers may build a panoramic understanding of the exciting field within a relatively short time.

Lastly, I would like to take the opportunity to thank the authors, and dedicate the present issue to Prof. Friedrich in token of gratitude and remembrance.

Continuous lattice fabrication is a newly introduced method for additive manufacturing of fiber-reinforced thermoplastic composites that allows to deposit material where it is needed. The success of this technology lies in a printing head in which unconsolidated continuous fiber-reinforced composite is pulled through a pultrusion die before the material is extruded and deposited out of plane without the use of supporting structures. However, state-of-the-art composite feedstock like commingled yarns shows limits in achievable material quality and part dimensions due to the underlying fiber architecture where thermoplastic fibers are mingled with reinforcement filaments. Hybrid bicomponent fibers overcome these constraints because each individual reinforcement filament is clad in a thermoplastic sheath. This results in absence of time-consuming fiber impregnation steps that would negatively effect void content and material quality.

This study compares the material quality of pultrudates made from hybrid bicomponent fibers to that of commercially available commingled yarns at various processing conditions. Experiments are reported in which polycarbonate composite profiles with a diameter of 5 mm containing 50 vol% to 60 vol% E-glass fibers are pultruded at different die filling degrees, mold temperatures and pultrusion speeds. The results show that the pultrudates obtained from hybrid bicomponent fibers have lower void content than those manufactured under the same conditions from commingled yarns. We assess this to be caused by the difference in consolidation mechanism which in the case of the hybrid bicomponent fibers is dominated by coalescing of the thermoplastic sheaths compared to the Darcian flow-dominated consolidation of commingled yarns.

The dramatic situation with the environmental impact of plastics waste we observe nowadays is related with their inherent peculiarity as materials – they do not change substantially their properties after converting into waste even into litter. Just this their peculiarity could help for partial solution of the problem – to convert the plastics waste via recycling into valuable materials and thus to reduce the amount of plastics litter. As a possible technology of polymer recycling the concept of microfibrillar composites (MFCs) is discussed when blends of two non-miscible polymers are melt blended, extruded, cold drawn and further compression- or injection molded for manufacturing of polymer-polymer composites. The MFC based on PET from Coca Cola bottle and low-density polyethylene are characterized by superior mechanical properties achieved via cold drawing.

It is stressed that the recycling does not solve the problem of the negative impact of plastics on the environment, it only postpones this solution. This is because after the end-of-life of the recycled plastics they are converted again in waste or litter.

Short-fiber-reinforced thermoplastic composites (SFRTCs) and particle-filled thermoplastic composites (PFTCs) are widely used in various industrial sectors, from automotive to electronic appliances, from building constructions to domestic equipment. Their environmental impact could be significantly reduced through both open and closed loop recycling, which allows a reduction of the usage of new resources and limit the energy consumption and CO₂ release associated to their production. The main challenges and opportunities in the recycling of SFRTCs and PFTCs are currently represented by i) reprocessing (also called mechanical recycling), use of recycled matrices ii) use of recycled fibers, iii) use of waste composites, iv) chemical recycling. Opportunities and current limitations in the recycling strategies and technologies for SFRTCs and PFTCs are here reviewed.

Polymer composites have proved to be an effective measure of material innovation, which combine the advantages of inorganic fillers (e.g., particulates, fibers, etc.) and organic polymer matrices. This is especially true for modern polymer engineering, where less and less brand new synthetic polymers are transformed from laboratory scale preparation to industrialized production.

“Advanced Industrial and Engineering Polymer Research” is an international academic journal primarily targeted at both industrial and academic researchers on all aspects of polymer studies and practical applications, in hopes of bridging the gap between these groups. Polymer composites are chosen as the main theme of the inaugural issue, which conforms to the vision of the editorial board, i.e. promoting positive hybrid effects of diverse knowledges or materials.

Here, the papers written by worldwide eminent scientists from different angles are collected. In spite of the limited space, a broader spectrum of interesting topics is covered to the best of the editors' ability. The readers may figure out the state-of-the-art and developing trend in this area accordingly.

In general, properties of matrix polymers greatly affect processability and performance of composites. Any change in matrix species would result in a new family of polymeric composites. The review article by Karger-Kocsis and Sibikin systematically discusses synthesis, modification, and manufacturing of lactam-based polymers and their composites. The high performance composite materials and the related processing techniques are promising for versatile applications.

Composites interface in another important subject for academia and industry. The work by Mäder and her co-workers compares the influences of geometrical and physical factors as well as dada analysis on interfacial properties of fiber reinforced epoxy and nylon-6,6, respectively. They successfully find out the most reliable method of quantification of interfacial interaction, which lays the foundation for precise characterization of future investigation. In the case of thermoplastic composites, interfacial crystallization, i.e. transcrystallization, is closely related to their mechanical strength. It in turn becomes a useful parameter for tuning property. The group of Goda reports the relationship between transcrystalline layer in injection-molded ramie/polyamide and tensile strength. The former appears to more prominently enhance the composite as compared to heat treatment. Meanwhile, McNally et al. study the issue in terms of isothermal and non-isothermal crystallization kinetics. Their results about multi-walled carbon nanotubes filled polypropylene reveal the crystal growth behavior, which may be directly correlated to optimization of processing.

With respect to polymer nanocomposites, which have attracted intensive interests since the invention of clay/nylon 6 hybrid in Toyota in 1985, the over thirty-year researches so far indicate inorganic nanoscale building blocks are hard to be dispersed in an organic polymer matrix using existing techniques. In this context, Fakirov proposes the concept of “converting instead of adding”. Two types of polymer nanocomposites, nanofibrillar polymer-polymer and nanofibrillar single polymer composites, are described in his review article. It shows good examples of thinking differently.

Besides mechanical properties, functional properties is also a main concern of polymer composites. Friedrich comprehensively reviews traditional applications of polymeric tribo-component, and recent achievements in imparting additional functionalities (e.g., electroconductivity and self-healability) to polymer composites for tribological applications. Moreover, additive manufacturing methods for friction and wear loaded polymer parts, and the composition of polymer composites under friction at cryogenic temperature conditions are included. On the other hand, Chung provides a general overview of thermoelectric polymer composites, which serve as candidates for energy materials. Her focus is laid on composition, structure and method of preparation of nonstructural and structural composites, which are critical for conversion of thermal energy to electrical energy.

In the paper by Sun et al., strategies for improving dielectric properties of epoxy composites are explored. By taking advantage of various fillers (i.e. carbon black, carbon black@TiO₂ core-shell particles and CB-SiO₂ hybrid particles), dielectric properties of the epoxy composites are improved due to interfacial polarization or micro-capacitor effect. Interfacial polarization loss, dipolar relaxation loss and conduction loss are found to account for the dielectric loss in the composites.

Last but not least, it is worth noting that polymer composites are advancing towards intelligent materials for the next-generation industry. The group of Michaud summarizes the current applications of shape memory alloy wires in high performance fiber reinforced polymer composites. They indicate that the composites can be coupled with improved damping response, impact resistance, crack closure capability and shape morphing.

The progress in polymer composites is bringing about changes day after day. This issue could only present a timely report of a few developments in the field. It is believed that many more interesting results will be reported in the near future.

To conclude, may we take the opportunity to express our sincere gratitude to all the authors for their highly qualified contribution to this inaugural issue and to all the colleagues who agreed to serve as reviewers for their help and efforts in ensuring a strong collection of papers.

Special thanks are due to Mr. Zhimin Yuan, chairman of the board of Kingfa Sci. & Tech., for his initial proposal of the journal and continuous support.

Ultra-high molecular weight polyethylene (UHMWPE) possesses distinctive properties, but has an extremely low melt flow rate (MFR) of about zero, which makes it unsuitable for processing by standard methods for polymers. The aim of this paper was to investigate the tribological properties of two-component UHMWPE-based composites with different content of isotactic PP. The composites were fabricated by three methods: a) hot pressing of the powder mixtures; b) hot compression of granules; and c) 3D printing (FDM). It was shown that the UHMWPE-based composites obtained by extrusion compounding (hot compression of granules and 3D printing) in terms of the mechanical and tribological properties (wear resistance, the friction coefficient, Young's modulus, and yield strength) were superior to the ones manufactured by hot pressing of the powder mixtures. The most effective was the ‘UHMWPE+20% PP’ composite in terms of maintaining high tribological and mechanical properties and the necessary melt flow rate (MFR) in a wide range of loads. It was recommended as a feedstock for additive manufacturing of complex-shaped products (joint components) for friction units in orthopedics.