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Effects of Alkyl-Substituted Polybenzoxazines on Tribological Properties of Non-Asbestos Composite Friction Materials

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Effects of Alkyl-Substituted Polybenzoxazines on Tribological Properties of Non-Asbestos Composite Friction Materials polymers Article Effects of Alkyl-Substituted Polybenzoxazines on Tribological Properties of Non-Asbestos Composite Friction Materials Anun Wongpayakyotin 1,†, Chanchira Jubsilp 2,†, Sunan Tiptipakorn 3,†, Phattarin Mora 1 , Christopher W. Bielawski 4,5 and Sarawut Rimdusit 1,* 1 Research Unit on Polymeric Materials for Medical Practice Devices, Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok 10330, Thailand; [email protected] (A.W.); [email protected] (P.M.) 2 Department of Chemical Engineering, Faculty of Engineering, Srinakharinwirot University, Nakhonnayok 26120, Thailand; [email protected] 3 Department of Chemistry, Faculty of Liberal Arts and Science, Kamphaengsaen Campus, Kasetsart University, Nakhon Pathom 73140, Thailand; [email protected] 4 Center for Multidimensional Carbon Materials (CMCM), Institute for Basic Science (IBS), Ulsan 44919, Korea; [email protected] 5 Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Korea * Correspondence: [email protected]; Tel.: +66-2218-6862 † These authors contributed equally to the work. Abstract: A series of substituted polybenzoxazines was synthesized and studied as binders in non- asbestos friction composite materials. The structures of the polybenzoxazines were varied in a systemic fashion by increasing the number and position of pendant alkyl (methyl) groups and was accomplished using the respective aromatic amines during the polymer synthesis step. By investigat- Citation: Wongpayakyotin, A.; ing the key thermomechanical and tribological characteristics displayed by the composite materials, Jubsilp, C.; Tiptipakorn, S.; Mora, P.; Bielawski, C.W.; Rimdusit, S. Effects the underlying structure-properties relationships were deconvoluted. Composite friction materials of Alkyl-Substituted with higher thermomechanical and wear resistance properties were obtained from polybenzoxazines Polybenzoxazines on Tribological with relatively high crosslink densities. In contrast, polybenzoxazines with relatively low crosslink Properties of Non-Asbestos densities afforded composite friction materials with an improved coefficient of friction values and Composite Friction Materials. specific wear rates. Polymers 2021, 13, 567. https:// doi.org/10.3390/polym13040567 Keywords: brake friction materials; friction and wear; polymer-matrix composites (PMCs); ther- mosetting resins Academic Editor: Pablo Froimowicz Received: 11 January 2021 Accepted: 9 February 2021 1. Introduction Published: 14 February 2021 Over the past 30 years, developments in the automotive industry have provided vehi- Publisher’s Note: MDPI stays neutral cles with impressive acceleration and speed capabilities. To manage such motion, brake with regard to jurisdictional claims in systems are required to slow or stop a moving vehicle, usually via friction and other energy published maps and institutional affil- absorption processes, and remain one of the most important safety components of modern iations. automobiles [1,2]. While non-asbestos organic friction materials are commonly used in automotive brake linings and clutch plates, such materials should satisfy a multitude of requirements, including a stable coefficient of friction, high wear resistance under aggres- sive operating conditions (such as high speeds, high compressive stress, and temperatures that can range up to 370 ◦C[3]), the ability to generate low noise while in use, and ideally Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. be prepared in an environmentally friendly manner [4,5]. Recently, high-performance This article is an open access article polymer composites are being increasingly favored for friction materials and becoming distributed under the terms and competition for conventional materials [6–8]. conditions of the Creative Commons Composite friction materials, particularly those that are used in braking applications, Attribution (CC BY) license (https:// generally contain four major components: binders, reinforcement fibers, fillers, and friction creativecommons.org/licenses/by/ modifiers [9–13]. The binders are a major constituent and are used to maintain system 4.0/). integrity so that the composite friction materials can display the desired performance Polymers 2021, 13, 567. https://doi.org/10.3390/polym13040567 https://www.mdpi.com/journal/polymers Polymers 2021, 13, 567 2 of 12 outcome while also maintaining key tribological properties (e.g., coefficient of friction and wear rate) and thermal stability. The reinforcement fibers contribute to composite strength, resistance wear rate, impact and thermal degradation, and other properties of friction materials. Friction modifiers are used to further tune the coefficient of friction exhibited by the composite friction materials. Fillers are added to reduce cost, improve some specific properties, and enhance processability. Phenolic resins are commonly used as binders for composite friction materials in part because they are available at low cost and possess moderate thermal stability at elevated temperatures, typically up to 300 ◦C[11]. Moreover, phenolics show high mechanical properties such as hardness, compressive strength, creep resistance, and suitable ingredient wet-out [14,15]. Although phenolics are widely used as binders, the resins still have a number of disadvantages [16]. These resins are sensitive to heat and humidity and in situ polymerization starts slowly even at ambient temperature, resulting in its poor shelf life and other serious drawbacks and limitations [10,17]. For example, cracking is often observed during the resin curing step because ammonia gas and other toxins are emitted as by-products [18]. It is brittle and is not resistant to high temperature, which often results in wear loss and fade under 350 ◦C for the friction materials [14,19]. In order to overcome these, an alternative polymer based on polybenzoxazine was synthesized in this work and tribo-evaluated to investigate the possibility of replacing the currently used conventional one, i.e., phenolics. Polybenzoxazines are known as an interesting new class of phenolic resin, polymerized by the ring-opening reaction of cyclic benzoxazine resins by thermal cure without the need for a catalyst or curing agent and without eliminating any by-products during thermal polymerization. Moreover, benzoxazine resins could be prepared by the Mannich-like condensation of a phenolic compound, a primary amine, and formaldehyde. The wide molecular design flexibility can be obtained due to the limitless choices of phenolic compounds and primary amines in the synthesis process. The molecular design flexibility of polybenzoxazines has been demonstrated such as in a synthesis of a series of benzoxazine resins with varied arylamine groups [20]. The authors reported that for benzoxazine resins based on m-toluidine and 3,5-xylidine, their ◦ thermal degradation temperature (Td) at 5% weight loss was reported to be about 350 C, which is significantly higher than aniline-based polybenzoxazine, i.e., about 315 ◦C, with no significant effect on the final char yield. The highest glass transition temperature or glass-to- rubber transition, i.e., 238 ◦C, belonged to 3,5-xylidine-based benzoxazine, while the lowest glass transition was of o-toluidine-based benzoxazine, i.e., 114 ◦C. In addition, additional amounts of arylamine Mannich bridges and methylene bridges in the polybenzoxazine show improved mechanical properties, including higher crosslink densities and rubbery plateau moduli. Glass-to-rubber transition of polymers often leads to substantial change in stiffness, strength, and wear performance of composite friction materials as reported by Wu et al. (2012) [21]. They have studied the effect of glass-to-rubber transition of thermosetting polymer matrices, i.e., phenolic, polybenzoxazine, phenolic-polybenzoxazine, and phenolic- benzoxazine-nitrile rubber, on the friction and wear characteristics of friction materials. Dynamic mechanical thermal analysis and friction test results revealed that glass-to-rubber transition of thermosetting polymers influenced significantly the friction and wear behavior of the composite materials. There was a significant increasing tendency in friction coefficient and wear rate values for all composites when braking temperatures increased to 200 or 250 ◦C, accompanying the polymer matrix converted from glassy state to rubbery state. Therefore, in this study, a systematic series of polybenzoxazines outfitted with dif- ferent alkyl groups was studied as binder matrices in friction composite materials. The benzoxazine resins were synthesized from bisphenol A and an aromatic amine (aniline, o-toluidine, m-toluidine, p-toluidine, or 3,5-xylidine). The selected amines are structurally similar yet feature a different number of pendant alkyl (methyl) groups and oriented in different positions (regioisomers). It was expected that the number and position of these pendant methyl groups would affect the thermomechanical properties and crosslink densi- ties exhibited by the corresponding resins [20]. Moreover, deconvolution of underlying Polymers 2021, 13, 567 3 of 13 Polymers 2021, 13, 567 3 of 12 pendant methyl groups would affect the thermomechanical properties and crosslink densities exhibited by the corresponding resins [20]. Moreover, deconvolution of under- lying structure-property relationships may enable the realization of new classes of resins structure-propertythat display high thermal relationships
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