JOURNAL OF COMPOSITE MATERIALS Article Hydrothermal treatment of sisal fiber for composite preparation Juliana da Luz 1 , Marcelo Andre Losekann 1 , Andressa dos Santos 1 , Jean Halison de Oliveira 2 , Emerson Marcelo Girotto 2 , Murilo Pereira Moises 3 , Eduardo Radovanovic 2 and Silvia Luciana Fa ´varo 1 Abstract The influence of hydrothermal carbonization of sisal fibers on the mechanical properties of composites based on recycled polypropylene matrices was investigated in this paper. The fibers were characterized by X-ray diffraction, thermogravi- metric analysis, differential scanning calorimetry, scanning electron microscopy, and Fourier-transform infrared spectros- copy. The composites, with short fibers randomly distributed, processed by extrusion and injection processes, were characterized using quasi-static tensile and impact mechanical tests and scanning electron microscopy. The hydrothermal carbonized sisal fibers, proved by Fourier-transform infrared spectroscopy analysis, besides increasing the material’s crystallinity, caused a larger adherence on the interface fiber/polymer matrix as verified in the scanning electron micro- graphs. The addition of 10% and 20% of 2 h hydrothermal carbonized sisal fibers treatment produced a composite with, respectively, an increase of 17.9% and 32.2%, on the modulus of elasticity, of 9.25% and 52.3% on the resistance to impact and of 19.06% and 29.85% on yield strength, in comparison to the recycled polypropylene. The hydrothermal carbon- ization technique changed the concept of recycling the polypropylene allowing new applications to the produced mater- ials due to its mechanical properties. Keywords Polymer–matrix composites, fibers, recycling, mechanical properties, hydrothermal carbonization Introduction In 2015, the worldwide production of plastic reached the amount of 322 million tons. 1 The first global association to support the interests of recycling indus- tries, Bureau of International Recycling, estimates that the world recycling plastics commerce counts a total of 12 million tons per year. 2 In Brazil, plastic is the most used recycled material, representing 13.5% of the total urban solid residues generated per year. 1 Polypropylene (PP), used in industry for making plastic packages for food, plastic bags, lids, and labels, is one of the most commonly recycled materials. 3–5 Polymeric matrix composite materials may be produced with the matrix coming from this recycling process and can be rein- forced with natural fibers, due to their resistance, low weight, and low cost properties. 6,7 Natural fibers, like sisal fibers, have numberless advantages in relation to synthetic fibers, mainly for being a renewable source and for being biodegradable. 8 They are basically com- posed of cellulose, hemicelluloses, and lignin; therefore, they are of hydrophilic nature and need to be modified to reach an interfacial compatibility with the hydropho- bic polymer matrices. 8–11 The techniques to modify surfaces, classified as bio- logical, physical, and chemical, have become more and more popular in different fields in the last 30 years. 12,13 Diverse chemical treatments have already been applied, 1 Department of Mechanical Engineering, State University of Maringa, Brazil 2 Department of Chemistry, State University of Maringa, Brazil 3 Department of Chemistry, Federal University of Technology, Brazil Corresponding author: Andressa dos Santos, Department of Mechanical Engineering, State University of Maringa, Colombo Avenue, 5790, Maringa 87020-900, Brazil. Email: dossantos.andressa@hotmail.com Journal of Composite Materials 0(0) 1–11 ! The Author(s) 2019 Article reuse guidelines: sagepub.com/journals-permissions DOI: 10.1177/0021998319826384 journals.sagepub.com/home/jcm