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Nature of Carbon Black Reinforcement of Rubber: Perspective on the Original Polymer Nanocomposite

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Nature of Carbon Black Reinforcement of Rubber: Perspective on the Original Polymer Nanocomposite polymers Review Nature of Carbon Black Reinforcement of Rubber: Perspective on the Original Polymer Nanocomposite Christopher G. Robertson 1 and Ned J. Hardman 2,* 1 Endurica LLC., Findlay, OH 45840, USA; [email protected] 2 Monolith Materials, Monolith Technical Center, Lincoln, NE 68522, USA * Correspondence: [email protected] Abstract: Adding carbon black (CB) particles to elastomeric polymers is essential to the successful industrial use of rubber in many applications, and the mechanical reinforcing effect of CB in rubber has been studied for nearly 100 years. Despite these many decades of investigations, the origin of stiffness enhancement of elastomers from incorporating nanometer-scale CB particles is still debated. It is not universally accepted whether the interactions between polymer chains and CB surfaces are purely physical adsorption or whether some polymer–particle chemical bonds are also introduced in the process of mixing and curing the CB-filled rubber compounds. We review key experimental observations of rubber reinforced with CB, including the finding that heat treatment of CB can greatly reduce the filler reinforcement effect in rubber. The details of the particle morphology and surface chemistry are described to give insights into the nature of the CB–elastomer interfaces. This is followed by a discussion of rubber processing effects, the influence of CB on crosslinking, and various chemical modification approaches that have been employed to improve polymer–filler interactions and reinforcement. Finally, we contrast various models that have been proposed for rationalizing the Citation: Robertson, C.G.; CB reinforcement of elastomers. Hardman, N.J. Nature of Carbon Black Reinforcement of Rubber: Keywords: carbon black; elastomers; hyperelastic material; stress–strain behavior; polymer–filler Perspective on the Original Polymer interactions; reinforcement Nanocomposite. Polymers 2021, 13, 538. https://doi.org/10.3390/ polym13040538 1. Introduction Academic Editor: Juan Lopez Valentin The term reinforcement can have a variety of meanings when considering the effects of adding nanometer-scale particles such as carbon black (CB) to an elastomer. Without fillers, Received: 12 January 2021 crosslinked rubbers are generally weak materials. Reinforcing filler particles enhance the Accepted: 9 February 2021 tensile strength, tear resistance, and abrasion resistance of rubber—to varying degrees based Published: 12 February 2021 on the particle size and shape features and nature of elastomer–filler interactions—while also increasing static and dynamic stiffness characteristics. All these mechanical property Publisher’s Note: MDPI stays neutral improvements can be considered reinforcement. Our focus in this review is on the influence of with regard to jurisdictional claims in CB on the tensile stress–strain curve, in particular the stress upturn (strain stiffening) behavior published maps and institutional affil- in the medium to large strain regime. iations. The most prevalent filler type in demanding rubber applications such as automobile tires and mechanical rubber goods is carbon black, which has many grades encompassing a large range of particle sizes (surface areas) and nano-structured aggregate morphologies for producing different property balances. There is ongoing research interest in employing Copyright: © 2021 by the authors. other carbon allotropes (e.g., graphene, carbon nanotubes) for reinforcing elastomers [1–3], Licensee MDPI, Basel, Switzerland. and the use of precipitated silica in rubber applications is growing [4]. However, carbon This article is an open access article black is by far the dominant filler in rubber products. The global production of carbon black distributed under the terms and is 15 million metric tons per year, and 93% of this material goes into rubber applications conditions of the Creative Commons (automobile tires (73%) and non-tire rubber products (20%)), with the remaining 7% used in Attribution (CC BY) license (https:// paints, coatings, inks, and plastics compounding (Figure1) [ 5]. This industrial importance of creativecommons.org/licenses/by/ carbon black provides motivation for our review on the origin of the mechanical performance 4.0/). Polymers 2021, 13, 538. https://doi.org/10.3390/polym13040538 https://www.mdpi.com/journal/polymers Polymers 2021, 13, x FOR PEER REVIEW 2 of 28 Polymers 2021, 13, 538 2 of 28 industrial importance of carbon black provides motivation for our review on the origin of enhancementsthe mechanical provided performance by carbon enhancements black in rubber provided compounds by used carbon in these black challenging in rubber com- applications.pounds used in these challenging applications. FigureFigure 1. 1.Global Global annual annual production production of carbonof carbon black black and an itsd distribution its distribution in the in indicated the indicated end-use end-use industrialindustrial application application areas, areas, based base ond reportedon reported data data [5]. Representative [5]. Representative examples examples of carbon of carbon black black (CB)(CB) aggregates aggregates are are shown shown in the in transmission the transmission electron electron microscopy microsco (TEM)py images (TEM) for images N234 and for N660 N234 and typesN660 to types show to the show nano-scale the nano-scale morphologies morpho that arelogies key that to reinforcement. are key to reinforcement. ScientificScientific literature literature on on CB CB reinforcement reinforcement of rubber of rubber has been has steadily been steadily amassing amassing since since thethe early early 20th 20th century, century, producing producing over over 10,000 10,000 publications publications including including nearly 100nearly papers 100 papers with “carbon black”, “reinforcement”, and “rubber” in the article title. This sea of literature with “carbon black”, “reinforcement”, and “rubber” in the article title. This sea of litera- makes it a challenge to gain a conclusive scientific understanding of why carbon black is so ture makes it a challenge to gain a conclusive scientific understanding of why carbon black successful at giving high degrees of reinforcement in industrial rubber applications. Toward gainingis so successful a clearer picture, at giving we willhigh first degrees review of some reinforcement key experimental in industrial observations. rubber This applications. set ofToward phenomenological gaining a clearer facts provides pictur ae, consistency we will first check review for any some proposed key experimental rationalization observa- oftions. CB-induced This set stiffness of phenomenological and strength increases facts provides in rubber, a includingconsistency the check various for theories any proposed thatrationalization have been offered. of CB-induced stiffness and strength increases in rubber, including the var- iousPerhaps theories the that simplest have waybeen to offered. look at the influence of CB on the mechanical behavior of elastomersPerhaps is to the compare simplest unfilled way and to filledlook rubberat the influence the standard of CB tensile on the stress–strain mechanical test behavior conductedof elastomers at room is to temperature. compare unfilled Examples and of fill suched stress–strain rubber in the curves standard from Medaliatensile stress–strain and Kraus [6] are reproduced in Figure2, wherein the large impact of CB on the response can be test conducted at room temperature. Examples of such stress–strain curves from Medalia noted immediately. One measure of filler influence on the stress–strain curve is to compare valuesand Kraus of reinforcement [6] are reproduced index (RI) in with Figure and 2, without wherein filler, the where large RIimpact = M300/M100. of CB on Thisthe response ratiocan be of thenoted stress immediately. at 300% strain One (M300) measure to the of stressfiller atinfluence 100% strain on the (M100) stress–strain has a value curve is to ofcompare 3.78 for thevalues CB-filled of reinforcement material shown index in Figure (RI)2 which with isand significantly without greater filler, thanwhere RI = RIM300/M100. = 1.45 for the This unfilled ratio rubber. of the Ifstress the carbon at 300% black strain particles (M300) are heatto the treated, stress then at 100% the strain RI(M100) of the has resulting a value rubber of 3.78 composite for the is CB-filled only slightly material higher shown (RI = 1.61) in Figure than the 2 which unfilled is signifi- material.cantly greater The remarkable than RI = extent 1.45 for of reinforcement the unfilled rubber. from CB If can the be carbon nearly extinguishedblack particles by are heat hightreated, temperature then the annealing RI of the of resulting the filler. rubber composite is only slightly higher (RI = 1.61) than the unfilled material. The remarkable extent of reinforcement from CB can be nearly extinguished by high temperature annealing of the filler. Brennan, Jermyn, and Boonstra [7] showed how reinforcement and other rubber properties were affected when comparing heat-treated carbon blacks with their untreated CB counterparts. They heat treated two grades of CB at 3000 °C for one hour, and a sum- mary of results in styrene–butadiene rubber (SBR) formulations is given in Table 1. The reductions in tensile properties (M300, tensile strength, etc.) for heat-treated CB compared to untreated reference CB are consistent with the results in Figure 2 already discussed. A Polymers 2021, 13, x FOR PEER REVIEW 3 of 28 substantially lower bound
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