Review: Natural rubber – Improvement of properties

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The article will be focused on methods of chemical modification of NR and the use of improved traditional fillers such as carbon black (CB) and silica. Modification reactions include epoxidation, hydrogenation, and grafting vinyl monomers onto NR molecules. Some methods of surface modification of NR film were also reviewed. Thanks to modification the NR degree of unsaturatiton is decrease that leads to its enhanced aging resistance.

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Cite this paper: Vietnam J. Chem., 2023, 61(3), 269-283 Review article DOI: 10.1002/vjch.202200225 Review: Natural rubber – Improvement of properties Tran Vinh Dieu1, Bui Chuong1*, Dang Viet Hung1, Nguyen Huy Tung1, Nguyen Pham Duy Linh1, Doan Thi Yen Oanh2 1 Hanoi University of Science and Technology, 1 Dai Co Viet, Hai Ba Trung, Hanoi 10000, Viet Nam 2 Publishing House for Science and Technology, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Hanoi 10000, Viet Nam Submitted December 11, 2022; Revised February 7, 2023; Accepted March 20, 2023 Abstract Natural rubber (NR) is valuable material of natural origination with growing demand worldwide. Thus, the methods for NR property improvement are continuously developed. The article will be focused on methods of chemical modification of NR and the use of improved traditional fillers such as carbon black (CB) and silica. Modification reactions include epoxidation, hydrogenation, and grafting vinyl monomers onto NR molecules. Some methods of surface modification of NR film were also reviewed. Thanks to modification the NR degree of unsaturatiton is decrease that leads to its enhanced aging resistance. Besides, grafted functional groups may impart new, never met before properties to NR. Application of modified traditional fillers obviously increase their reinforcement possibility, particularly mechanical. Also, cellulose as a potential reinforcing filler is considered due to its ability to meet requirement of environment protection while keep the mechanical properties of NR products at acceptable high level. Keywords. Natural rubber, modification, carbon black, silica, cellulose. ABBREVIATION AAm – Acrylamide. CB – Carbon black. CNT – Carbon nanotube. CSDPF – Carbon-silica dual phase filler. DVB – Divinyl benzene. DAAm – Diacetone acrylamide. 2-EHA – 2-Ethylhexyl acrylate. IPN – Interpenetrating network. MMA – Methyl methacrylate. NR – Natural rubber; DPNR – Deproteinized NR; ENR – Epoxidized NR. NRL – NR latex; LNR – Liquid NR; LENR – Liquid ENR; HNR – Hydrogenated NR. PS – Polystyrene. SCA – Silane coupling agent. SMA – Stearyl methacrylate. TEM – Transmission electron microscopy; EFTEM – energy filtering TEM. TEOS – Tetra ethoxy silane. TESPT – bis(triethoxy silylpropyl) pentasulfide. TEPA – Tetraethylene pentaamine. Tert-BHPO – Tert-butyl hydroperoxide. 1. INTRODUCTION origination has commercial value for hundred years due to its advantages such as high tensile strength, Natural rubber (NR) - the polymer material of natural elasticity, good dynamic property, and low head build 269 Wiley Online Library © 2023 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH
25728288, 2023, 3, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/vjch.202200225 by Readcube (Labtiva Inc.), Wiley Online Library on [02/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Vietnam Journal of Chemistry Bui Chuong et al. up. Until now, NR still is irreplaceable elastomer causes NR disadvantages such as low thermal and inspire of abundance of synthesis rubber with variety ozone resistance, may be useful for many of properties (table 1).[1] modification reactions which can improve NR Together with consumption on big quantity, the properties.[2] With chemical modification, many NR widening NR application field become more and derivatives with new functional groups and new more actual issue. High chemical activity of NR, that properties may be formed. Table 1: World rubber consumption 2015-2020 1000 ton Year 2015 2016 2017 2018 2019 2020 Natural 12,140 12,685 13,217 13,767 13,640 12,702 rubber Synthesis 14,457 14,790 15,195 15,309 15,155 14,233 rubber Total 26,597 27,475 28,412 29,076 28,795 26,925 Fillers have a particularly important role in cyclization or crosslinking. reinforcement of NR. With fast development of Chemical modification of NR may be classified science and technology, traditional fillers such as as follow:[2,6] carbon black (CB) and silica are permanently - Change in molecule structure or reduce improved, and their reinforcement efficiency is molecule mass without introducing in NR raising. Besides, organic filler begins attracting chain new atoms or molecules. Examples of attention for developing environmentally friendly this kind of reaction are cyclisation, rubber materials. isomerization or depolymerization. All mentioned above have active contribution to - Functionalization of NR by introducing into improvement of NR properties and widening of NR chain atoms or functional groups with applications of this valuable material. specific physico-chemical characters. In this article, some recent results in NR - Modification of allylic carbon by grafting properties improvement will be discussed, focusing other monomers or polymers. on chemical modification of NR molecules, and using Examples of reactions of first group are reinforcing fillers. An important and fast developing cyclisation and chain decomposition. Cyclisation area, namely NR nanocomposites, will not be reaction has been carried out successfully in the first discussed in this article. Interested readers may find half of the XX century, and cyclized NR has had some information about NR nanocomposites in special commercial applications. However cyclized NR has reviews.[3-5] almost no application at present because there are synthesis polymers for its replacement. 2. CHEMICAL MODIFICATION OF NR Chain decomposition form liquid NR (LNR) with low molecular mass, easy to dissolve and processing. One of the biggest disadvantages of NR is low In LNR may be functional groups containing oxygen resistance to ageing, ozone and saturated due to reaction of oxygen with radicals formed during hydrocarbons. The reason is the high chemical chain scission.[6] LNR may be use as sealant or for activity of isoprene unit in NR molecule. To viscosity controlling agent during processing.[2,7,8] overcome this disadvantage, an effective technique – Some modification reactions of two other groups chemical modification - is applied. are discussed below. In the molecule structure of NR at every five carbon atoms in chain there is one olefin double bond. 2.1. Epoxidation of NR NR has good chemical activity of this double bond is similar one in alkene, therefore chemical reactions In this reaction, epoxidation agent interacts with that may occur to alkene also have place with NR, double bond C=C to form oxirane group. The such as electrophilic addition at C=C bond or epoxidation agent may be hydroperoxide, organic substitution at allylic position. The difference of NR peracids.[6,9] Organic peracid also maybe formed in- in comparison with alkene is the double bond next to situ by reaction of H2O2 and suitable organic acid.[10- 12] reacted one may enter in other reaction, for example, Epoxidation of NR occurs as follow.[6] © 2023 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 270
25728288, 2023, 3, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/vjch.202200225 by Readcube (Labtiva Inc.), Wiley Online Library on [02/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Vietnam Journal of Chemistry Review: Natural rubber – Improvement of properties C C + R C O O OH C C + R C OH O O O The existence of epoxy group in NR chain make have been compounded with three accelerator NR properties improved, such as oil or aging systems effective, semieffective and peroxide. resistance, damping. However, this effect is notable Compared properties were mechanical, dynamical, only with epoxy content high enough: ENR10 with thermal, and curing. Results showed, both factors - epoxy content 10%mol is almost no different from modification and accelerator systems - have NR. That why commercially used mostly ENR with remarkable effect on considered properties. However, epoxy content 25%mol and more for many modification has a more notable effect on dynamic applications.[2] and thermal properties than accelerator. In another Many research works focused on epoxidation work, ENR have been partly hydrogenated to HENR1 reaction of NR, particularly in of NR latex and HENR2 with degree of hydrogenation 27%mol (NRL).[10,11,13,14] For example, NRL is stabilized by and 25%mol, respectively. Thanks to decreasing of non-ionic surfactant in acetic acid medium, and then double bonds amount, thermal properties, and ozone suffered to reaction with peracetic acid formed in-situ resistance of HENR1 and HENR2 are higher than that from acetic acid and H2O2.[10] Similarly, when of NR while tensile strength and elongation at break epoxidize NRL by performic acid formed in-situ with stay at high level.[21] Hydrogenation of ENR was also H2O2/isoprene unit ratio 0.4-4.0 mol/mol and studied, for example, in [22,23]. H2O2/HCOOH ratio 3-13 mol/mol, the high epoxy Modification of NR also may be carried out with content till 60%mol may be reached.[15] By using LNR.[6,24] LNR may be epoxidized, followed by suitable chain scissor[16] or surfactant,[10] the hydrogenation to form LENR and LENR-OH. Epoxy epoxidation process may be controlled, and epoxy content of LENR may be very high - till 98%; epoxy content may be regulated in the range of 5-70 %mol. group may further hydrolyze and open ring to form By hydrobromination of epoxidized NR (ENR) it was diol and carbonyl groups. It is expected that LENR proved the epoxy groups distributed randomly on and LENR-OH might be used as compatibilizer for ENR molecular chain.[17] NR/PS blends.[24] Note, in epoxidation process, some side reactions may occur. These reaction tent to form groups 2.2. Hydrogenation containing oxygen: tetrahydrofuran, hydroxyl, carbonyl ether.[6,9] The higher reaction temperature, Addition of hydrogen atoms into C=C bonds makes the more side reaction, and when reaction unsaturated degree decrease, that results in temperature exceeds 25oC, side reactions may improvement of thermal stability and ozone overpass reaction of epoxidation.[9] resistance of NR ENR has improved properties in comparison with The most popular hydrogenation agent is NR. For example, ENR adhesion to steel or to nylon hydrogen gas in presence noble metal catalysts (Pt, is higher than that of NR about 40%.[18] In [9] showed Pd, Ni).[25-27] It is also may be molecules such as that damping properties of ENR rise along with epoxy N2H2.[28,29] For example, when the diimide reduction content while mechanical properties such as tensile technique is applied, NRL may be hydrogenated into strength, tear resistance and compression set copolymer ethylene-propylene.[28] Some latex remained in a level of that of NR. Also, oil resistance, stabilizers, e.g. sodium dodecyl sulfate or gas permeability or abrasion resistance of ENR hydroquinol may decrease degree of remarkable enhanced in comparison with NR.[1,11-14] hydrogenation.[29] In contrary, grafting small quantity ENR may also continue to be modified for of monomer such as methymethacrylate (MMA) may reducing unsaturated degree and enhance some promote hydrogenation.[30] Some of authors point out properties.[17,19-23] Ruksakulpiwat et al. have carried the fact that when deproteinized NR latex (DPNR) is out the hydrogenation of ENR three times that used, the grafting efficiency of hydrogenation as well allowed raise the degree of hydrogenation from 6.5% as another grafting reaction onto NR latex remarkable mol to 67%mol.[19] Thank to high degree of increase.[25-27] hydrogenation, thermal resistance of material is much Thermal stability of hydrogenated NR (HNR) higher in comparison with unmodified ENR. increases according to hydrogenation degree. Using Thitithammawong et al. have compared NR, ENR suitable catalyst system, it is possible to achieve and chlorinated ENR properties.[20] These elastomers hydrogenation degree of DPNR 97% with activation © 2023 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 271
25728288, 2023, 3, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/vjch.202200225 by Readcube (Labtiva Inc.), Wiley Online Library on [02/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Vietnam Journal of Chemistry Bui Chuong et al. energy 169 kJ/mol in temperature range of 80-90 protein in latex may partly inhibit formed radicals. o [25] C. Inoue and Nishio have reached hydrogenation When grafting styrene monomer into NRL with degree of 100% with catalyst RhCl(PPh)3.[31] redox system, including tert-butylhydroperoxide Interestingly, hydrogenation reaction strongly (tert-BHPO) activated by tetraethylenepentamine decrease double bond amount but has almost no effect (TEPA), the interpenetrating (IPN) and/or semi- on molecule mobility of HNR - at hydrogenation interpenetrating network (semi IPN) between NR and degree 90% the Tg of HNR is about -57oC.[2] The PS are formed.[35] Although PS at small content (10 elastic characteristics of HNR are in the range of NR, phr) may reduce strain-induced crystallization of NR and strain induced crystallization of HNR occurs until but this is compensated by reinforcement effect of PS hydrogenation degree of 40.6%.[32] at high contents (20-30 phr). As a result, the tensile strength of copolymer NR/PS increase remarkable. 2.3. Grafting vinyl monomers With the same catalyst system but when DPNR latex is used instead of NRL, grafted styrene reached 70- Vinyl monomers type CH2=CHX is an important 90%.[36-37] DPNR-g-PS has tensile strength four times agent for grafting into NR molecules. Grafting higher and storage modulus twenty-five times higher reaction may be carried out in solution, e.g., grafting than that of origin DPNR. Such high rise of stearyl methacrylate,[33] or in latex.[34] Afterward mechanical properties is assigned to formation of grafting reaction are preferably carried out in latex, unique nanomatrix - core-shell structure with the core and it was showed that water-soluble initiators is NR and shell is NR-g-PS. (persulfates, organic hydroperoxide) have higher Core-shell structure is an interesting result of efficiency for grafting reaction of vinyl monomers grafting vinyl monomer on NR in latex. According to into NR.[2] Arayapranee W. et al.[34] grafting process in latex One of the features of grafting in NRL is the involves two steps. In first step, monomers grafted ammonia being in latex as preservative agent may slow directly on NR molecules on surface of latex down the reaction.[2] To overcome this effect, the redox particles, while homopolymerization of free catalyst system is applied. In redox system, monomers in aqueous medium is going to form hydroperoxide initiator is activated by polyamine. polymer particles. By regulating the reaction Hydroperoxide is mixed with monomers and conditions such as surfactant, emulsifier, introduced in latex. Then after penetration of monomer/rubber ratio etc. it is possible to control monomer/hydroperoxide mixture into latex particles, grafting efficiency. The core-shell structure size is polyamine is introduced for catalytic activation and the defined by TEM image: the core is rubber particle of reaction occurs at ambient temperature. Both NRL 0.5 µm; and shell is layer of grafted polystyrene of latex and DPNR latex can be used for grafting reaction, thickness of 50-60 nm (Fig. 1).[38,39] but NRL latex has lower grating efficiency because Figure 1: TEM image of PS-NR nanomatrix[38] Thanks to combination of hard, brittle PS shell material rise dramatically, and mechanical loss tg layer and elastic NR core, the storage modulus of decrease. If the PS shell layer is replaced by soft © 2023 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 272
25728288, 2023, 3, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/vjch.202200225 by Readcube (Labtiva Inc.), Wiley Online Library on [02/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Vietnam Journal of Chemistry Review: Natural rubber – Improvement of properties butylacrylate (PBA) shell (storage modulus of PBA is prevulcanized. However, prevulcanization reduces lower than that of NR), they have got DPNR-g-PBA NR chemical activity. For example, during with storage modulus lower than that of DPNR.[40] epoxidation of NR latex it is defined only 6% of Core-shell structure is also observed where vinyl double bonds participated in modification reaction. monomers such as MMA, styrene butyl acrylate, Even so, damping properties and abrasion resistance cyclohexyl methacrylate are grafted in NRL. Owing of modified NR obviously increase.[45] to this structure, grafted NR have obviously higher Prevulcanized NR film may be treated by plasma tensile strength than that original NR.[41] In the case before modification. Peroxide prevulcanizated ENR when grafting reaction is carried out under - film is treated by o-plasma then dipped in acrylamide radiation, the tensile strength is still higher due to solution (AAm). The properties of NR, ENR and additional crosslinking by radiation.[41,42] AAm modified ENR films were compared. The It is possible grafting reaction of two monomers results show, in comparison with NR, ENR film simultaneously, e.g. stearyl methacrylate (SMA) and reduces oxygen transmission rate 22-35%, water divinyl benzene (DVB)[33] or 2-ethyhexylacrylate vapor transmission rate increase 116-170%, Young (2EHA) and methacrylic acid (MAA).[43] In the case modulus increase 56-138 %. These parameters of grafting SMA and DVB, these two monomers are change quite little when ENR film is replaced by grafted randomly on surface of latex particles while AAm modified ENR. That means plasma acts only on create some degree of crosslinking. This crosslinking surface and grafting AAm reaction occurred only on network make NR-g-SMA/DVB insoluble in film surface.[46] petroleum ether and THF.[33] Wang Xueyuam et al. Grafting efficiency will be higher if UV- pointed out the formation of grafting layer on NR activation is used together with plasma treatment. In particles surface and the double crosslinking network this case, UV sensitive agents are introduced into – inside NR particles and between particles.[43] Due to monomer before grafting reaction. Thanks to this double crosslinking network the tensile strength, method, various monomers may be grafted into NR, thermal stability, and solvent resistance of NR-g- depending on the required properties of the film. For 2EHA/MAA obviously increased. In another work example, graft polyethyleneglycol for rising copolymer MMA and diacetone acrylamide (DAAm) hydrophility, reducing protein adsorption and platelet was synthesized with emulsifier O-carboxy methyl adhesion of medicine instrument,[47] graft butyryl chitosan. Copolymer MMA/DAAm in the form of derivatives of chitosan,[48] N-vinylpirolidon[49] or crosslinked latex particles of about 100 nm dimension MMA,[50] which obviously decrease hydrophility of are absorbed on surface of NRL particle forming modified films. core-shell structure: the core is NR and the shell is Another surface modification technique is layer- cross-linked MMA/DAAm. Such modified NR owns by-layer deposition of polymeric particles on films improved tensile strength, ageing and solvent surface.[51] NR films firstly is treated by Ar-plasma resistance in comparison with NR. The film from this and UV-sensitive acrylamide. Then PMMA particles modified NR also has higher compactness. of nanometer dimension are deposit on surface by Thereby, the formation of core-shell structure is electrostatic reaction. After that, UV radiation is one of factors assisting improvement of NR applied to form chemical bonding between PMMA properties by grafting of vinyl monomers. particles and NR film surface. Thanks to the layer of separate PMMA particles on surface (Fig. 2), NR 2.4. Surface modification films has higher hardness, roughness, and reduced surface friction while mechanical properties stay Many NR articles are used in the form of film, for almost unchanged. Also by chemical bonding with examples gloves or balloons. Therefore, surface NR film surface, PMMA particles are kept fixed on treatment is an important technique for modification surface even after stretching to 300%. Note, layer-by- of NR film. layer technique may be applied for both For stability during modification, NR film is often unvulcanized[51] and vulcanized films.[52] © 2023 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 273
25728288, 2023, 3, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/vjch.202200225 by Readcube (Labtiva Inc.), Wiley Online Library on [02/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Vietnam Journal of Chemistry Bui Chuong et al. (a) (b) Figure 2: Particle PMMA deposited on surface of NR film[52] (a) Before stretching, (b) After stretching 300% 3. APPLICATION OF IMPROVED FILLERS groups, the dispersion of CB enhanced but its reinforcement effect get worse.[56] Filler is one of the most important components of An effective method for enhancing CB rubber mixture. In many cases, rubber could not be reinforcing effect is combination of CB and other applied without filler. Therefore, the role of filler as fillers – so called hybrid fillers.[57-60] It worth to note reinforcing component, implying mechanical hybrid filler itself does not only enhance reinforcing reinforcement, attracts fixed attention, particularly ability of each filler in system but also optimize the from 1960 years until now. reinforcement by advantage of each one. For Two important fillers which are used most example, filler systems from two types of CB with popularly in rubber industry are carbon black (CB) different particle size may improve compression - and silica. That is why the number of research works elastic properties thanks to big particles, while small on these fillers gain very high portion in the works on particles impart improvement of tensile properties. reinforcing fillers. Also, filler of natural origin begins Combination of CB mark N220 and nanobarite to be interested due to growing requirements for (particle size 40-50 nm) enhance a number of environmental protection. properties as tensile and tear strength, abrasion resistance.[57] Combination CB/CNT or 3.1. Carbon black (CB) and its derivatives CB/conductive CB not only improve mechanical properties but also electric or thermal CB is produced from petroleum products by burning conductivity.[58,59] In its turn, CB may intensify the or thermal decomposition in oxygen shortage influence of nanofiller in their hybrid system: conditions. Therefore, on CB surface exist some nanofiller content in CB/nanofiller system is lower functional groups containing oxygen, such as than if only nanofiller is used at the same hydroxyl, carbonyl, pirone etc. During 1960x years, reinforcement effect.[60,61] In CB/nanoclay filler it was proposed that rubber-CB interaction was network is formed at clay content of 3.1 phr, while if mainly the chemical bonding of these groups with only nanoclay is applied, the same network is rubber molecules. So, for enhancing CB activity, the revealed at clay content of 5.9 phr.[61] CB surface was modified by oxidation with strong An important derivative of CN is carbon-silica oxidation agents, e.g. H2O2, ozone, concentrated dual phase filler (CSDPF). CSDPF is obtained by HNO3.[53] CB was also modified both physical e.g. reaction of CB with organic silicon that forms two surfactant adsorption, or chemically, e.g. oxidation, phases in filler particles: CB phase and silica phase halogenation, grafting some polymer. These doped in each other. Silica is distributed rather modifications are reviewed in.[54,55] regularly inside particles and between aggregates as However, investigations afterward show well.[62] CSDPF surface has polarity much lower than interactions of CB and rubber, particularly non-polar that of silica and almost the same of CB, that why this ones, e.g. NR, BR etc. not only, but major part are not filler has rubber-filler interaction better than silica has chemical interactions of oxygen containing groups while keep the advantages of silica fillers.[63] [56]. Moreover, when short molecules were grafted Surface chemistry show the functional groups on on CB surface, e.g. by esterification of carboxyl CSDPF surface have better interaction with silane © 2023 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 274
25728288, 2023, 3, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/vjch.202200225 by Readcube (Labtiva Inc.), Wiley Online Library on [02/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Vietnam Journal of Chemistry Review: Natural rubber – Improvement of properties coupling agent, higher content of active hydrogen in of reinforced NR. In the case of comparison with CB.[64] Thorough investigation by bis(trioethoxysilylpropyl)tetrasulfide (TESPT) NMR reveal that CSDPF reduce rubber chain modification the covalent bonds are formed between mobility, increase crosslinking density, and promote filler particle - rubber (Fig. 4). This interaction the formation of polysulfide bonds in rubber.[65] reduced rubber molecule mobility, that results in However, modified CSDPF may facilitate the limitation of strain - induced crystallization of rubber formation of more monosulfide bonds in network.[66] compound. As a result, the rise of tensile strength Owing to two phase in one structure, CSDPF coming from reinforced filler is compensated by reduce filler-filler interaction, increase rubber-filler reducing effect due to limitation of strain-induced interaction that results in higher reinforcement effect crystallization. Totally the tensile strength increases in rubber, particularly in tire compounds: higher tear little in comparison with compound with unmodified and abrasion resistance, improved rolling and wet CSDPF. On the contrary, when CSPDF is modified skid resistance at the almost same tensile strength and with 1-allyl-3-methyl imidazolium chloride, there are elongation at break in comparison with compound only hydrogen bonds between filler particle and containing only CB.[67,68] Moreover, silane modified rubber. Hence the molecular mobility of rubber is CSDPF may cause synergistic effect with CB at little influenced and strain-induced crystallization of suitable CSDPF/CB ratio. Xin Xiong et al. report NR compound is almost unlimited. In this case, both about synergistic effect at ratio CSDPF/CB = 15/50 tensile and tear strength are remarkable higher than %w – observed dramatic rise of rolling and wet skid that of compound with unmodified CSDPF.[66] resistance.[69] At other studied ratio this effect is not found. In any case, mechanical properties such as tear 3.2. Silica and modified silica resistance of fatigue life of NR compound with CSDPF stay higher than one with CB. Silica (silicon oxide) is an important filler in rubber Interesting that system silane modified CSDPF- industry – just second place after CB. Nevertheless, amine in combination with antioxidant has thermal silica was first used only as CB supplement because stabilization effect for NR compound at high of disadvantages connected with its structure and temperature (150oC) better than that of system CB- surface properties. antioxidant. Meanwhile, CB-antioxidant better Primary silica particles have a size of about 2-20 stabilizes NR compound at lower temperature nm, less than CB ones. These particles often (110oC). It is because activation energy for aggregated to form “string of pearl” with the size of antioxidant release of CSDPF – antioxidant is rather 50-500 nm. The interaction inside aggregate is high - at 110oC the antioxidant hardly released. Only chemical, so “string of pearl” is hardly destroyed by at elevated temperature (150oC) antioxidant from mechanical mixing. Unlike CB, silica surface is rich CSDPF-antioxidant system could be released and of silanol and siloxane groups which are able to link stabilize NR compound.[70] with H2O through hydrogen bonds (Fig. 3). It is Thus, using modified CSDPF-antioxidant system necessary to note beside free H2O that is easily to be may be promising direction for NR stabilization at removed at 100-250oC there is on silica surface bound high temperature. water formed from neighboring silanol groups which The nature of interaction between CSDPF particle might be removed only at very high temperature 900- and modificator has obvious influence on properties 1000oC.[56] Figure 3: Scheme of silica surface[56] © 2023 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 275
25728288, 2023, 3, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/vjch.202200225 by Readcube (Labtiva Inc.), Wiley Online Library on [02/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Vietnam Journal of Chemistry Bui Chuong et al. High moisture content, as well as existence of moments. It was application of silica coupling agent oxygen atoms on surface make filler-filler interaction (SCA) and obtaining silica in-situ.[71] of silica higher and worsen silica dispersion on rubber One of the first works on application of SCA as matrix. As a result, processing, curing as well as silica modification was using TESPT.[72] TESPT is mechanical behaviors of silica filler NR become able to react with hydroxyl groups on surface of silica worse. particles, reducing silica hydrophility. At the same To make silica become reinforcing filler on the time, TESPT may participate in NR network due to level of CB, it is important to note two break-through sulfide links (Fig. 4). Si OH O CH2 Et O Si CH2 S Si OH 3 3 S C O S Si OH CH Et O 3 Si CH2 S 3 O CH2 Si OH OEt Si O CH2 O Si CH2 S S C CH3 3 Si O O Si O O Si CH2 S S CH 3 Si O CH2 OEt Silica Silica Transisional layer Transitional layer Rubber Rubber Figure 4: Scheme of silica – NR interaction through TESPT The nature of NR-silica interaction through regularity of filled NR network,[81,85] that leads to the TESPT has been studied by different methods, for fact that bimodal distribution of NR molecules is example, bound rubber,[73] gas chromatography[74] or blurred, and temperature of beginning destruction of energy filtering transmission electron microscopy material rise 6oC than that of NR.[81] However, if (EFTEM).[75] Specially, EFTEM technique allow to TESPT content in modified silica is too high, curing define directly the transitional layer of TESPT characteristic of filled rubber as well as reinforcement between silica and NR. Silica modification with effect of silica may decrease. It is because excess TESPT continue to be studied until now.[76-84] TESPT may form polysiloxane clusters on surface of TESPT enhances silica dispersion and rubber- silica particles, which are able to adsorb a part of filler interaction. TESPT modified silica raise rubber accelerator in rubber compound.[78] Similarly, when network density, improves cure characteristics,[78-84] surfactant is used for dispersion of modified silica, mechanical properties[78,79,87] or thermal behavior.[81] excess surfactant may prevent dispersion silica in It was shown TESPT modified silica may ameliorate NR.[77] © 2023 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 276
25728288, 2023, 3, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/vjch.202200225 by Readcube (Labtiva Inc.), Wiley Online Library on [02/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Vietnam Journal of Chemistry Review: Natural rubber – Improvement of properties Another SCA, both with and without sulphur also alkoxide silane precursor, mostly tetraethoxy silane used for silica modification, such as mercaptopropyl (TEOS) in present of amine catalyst: triethoxy silane,[86] 3-octanoyl thio-1-propyl triethoxy silane,[87] methyl-, vinyl- and aminopropyl triethoxy Si(OC2H5)4 + H2O  SiO2 + C2H5OH silane.[88] SCA without sulfur, though do not link with Firstly, silica in-situ was dispersed in silicon NR molecule through sulfide bridge, reduce silica rubber. Then this technique was applied for diene hydrophility and enhance filler-rubber interaction. rubber, such as NR, NBR, SBR and BR. These This enhancement leads to improvement of cure investigations till 2000 year are reviewed in [89]. characteristics, mechanical and thermal properties of Silica in-situ may be obtained both in dry rubber filled rubber. and in latex. In the first case, NR is swollen in TEOS, In review article[85] indicated the network then suffered hydrolysis (sol-gel reaction) to obtain structure of hydrophobic (modified) silica/NR system NR filled with silica in-situ. However, crosslinking has higher performance than hydrophilic NR poorly swells in TEOS, hence formed in-situ (unmodified) silica/NR one. That is why some silica is dispersed only on thin layer of NR surface. properties of hydrophobic silica/NR are higher. That means, for crosslinked NR, this technique is Using silanes for modification of precipitated suitable for thin articles. On contrary, TEOS could silica with aggregate size of 50-500 nm is turning penetrate deeply and evenly into uncrosslinked NR, point of industrial application of silica as a so formed in-situ silica distributed evenly in whole reinforcing filler for rubber - it is possible to consider volume.[90] To reach high content of formed silica it modified silica as replacement of CB in tire is important to get high swell degree of NR in TEOS manufacturing thanks to reduced rolling resistance and use suitable catalyst. For example, Poompradub and heat build up. However, dispersion of silica in S. et al. let NR swell in two step – firstly 1 hour at 40 rubber matrix by mechanical mixing is hard and o C and then 24 hours at 25oC. By this way, NR swell consume higher energy than CB. Therefore, degree in TEOS is much higher than one step production of silica in-situ has been studied and swelling. In second step, suitable catalyst such as n- applied from the beginning of 1990 years.[71] This hexylamine or n-heptylamine are added, and silica technique has permitted to obtain fine silica particle, in-situ content reaches 80%.[91] If additionally increase silica dispersion and silica-rubber compatibility as well. introduce a SCA, e.g. -mercaptopropyltrimethoxy Silica is formed in-situ by hydrolysis reaction of silane, silica in-situ will be distributed more evenly in NR matrix (Fig. 5).[92] (a) (b) Figure 5: Distribution of silica in-situ in NR matrix[92] (a) Without silane coupling agent, (b) With silane coupling agent In the case of obtaining silica in-situ in NRL, due to inter-penetration of filler and rubber in latex. TEOS is mixed directly with HA latex.[93] NH3 at high Nevertheless, the limitation of this method is the concentration in latex could be a catalyst for content introduced into latex TEOS is restricted – hydrolysis of TEOS. It is expected that formed silica high TEOS content may cause NRL coagulation. in-situ will be distributed better than in swollen NR Until now, content of formed silica in-situ in NRL is © 2023 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 277
25728288, 2023, 3, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/vjch.202200225 by Readcube (Labtiva Inc.), Wiley Online Library on [02/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Vietnam Journal of Chemistry Bui Chuong et al. only 30-35 phr when using diluted NRL-about 20% has higher storage modulus, tg while solution in of dry rubber content-and suitable stabilizer. Besides rubber matrix of gases (O2, CO2, NO2) decrease about the size of formed silica stays rather high - about 100 1/3 times in comparison with NR.[97] nm. Therefore, the main problem of process of In general, cellulose filler is not yet a reinforcing obtaining silica in-situ in NRL is stabilization of latex one for rubber like CB or silica. However, when there during reaction.[71] is a demand for complex of properties such as low gas permeability, transparency with acceptable 3.3. Cellulose fillers mechanical properties, low cost and biodegradation, then NR/cellulose is promising material. In scientific Due to growing requirements of environmental plan, the research works should be directed to protection, organic fillers of natural origination are unresolved problems as filler-filler and rubber-filler attracting particular attention. In this area, cellulose is interactions that aim at regulation and/or achieving rising as a new potential filler for rubber due to its required properties.[96] huge reserve in nature, renewability and easy degradation. 4. CONCLUSIONS In rubber, cellulose may be used in form of fiber, short fiber and nanofiller. The main cellulose fiber NR is valuable material having long time application. used for rubber are jute, bamboo, coir, bagasse, and Therefore, investigations directed at its improvement sisal. The characters of these fibers and their are always of particular interest. Nowadays, in the influence on NR properties are summarized in.[94-96] trend of green chemistry, the problem of NR property One of disadvantages of natural cellulose (called improvement has become more and more actual. cellulose I) is hardly to achieve high fiber content in Chemical modification of NR aiming at reducing rubber compound because of poor compatibility molecular unsaturanity and introducing new between cellulose and NR. To overcome this functional groups into NR molecules is traditional disadvantage, regenerated cellulose including direction but will be kept growing in near future. cellulose II, cellulose III and cellulose IV are applied. Thanks to modification the working properties of NR Cellulose II is obtained by sodium treatment of continue to be improved and widened. cellulose I. In cellulose II intramolecular hydrogen It is worth to that structure change, e.g. core-shell bonds being in cellulose I are destroyed and formed structure as nanomatrix, or surface structure may inter-molecular hydrogen bonds. Hence the have remarkable influence on rubber properties. crystalline structure of cellulose II is different from Using DPNR is also interesting for enhancement graft that of cellulose I and this led to differences of efficiency of grafting reaction onto NR. cellulose I and cellulose II properties. Cellulose III is Using improved filler, particularly CB and quite amorphous and obtained by amine treatment of modified silica may be considered as cellulose I and cellulose II. Cellulose IV is obtained physicochemical modification of NR. Although being by treatment of cellulose II with glycerol at high the most important filler for NR, CB is continuing to temperature.[97] be improved. Combination of CB and another filler, Cellulose – NR interaction is assumed weaker including nanofiller, application of CSDPF could be than NR bonds in vulcanization network. The promising way to enhance NR properties. Similarly, evidence is decreasing of tensile modulus of modification of silica and use silica in-situ are cellulose/NR compound in successive tensile testing: perspective for amelioration of silica filled NR, at 1st loading modulus of cellulose/NR compound is especially in NR nanocomposite preparation and 2.5 times higher than that of corresponding NR application. A promising organic filler is cellulose. Although compound. But after four loadings, both compounds cellulose could not replace CB or silica as reinforcing have almost the same tensile modulus.[98] filler in near future, but NR/cellulose composites Cellulose may replace partly silica to reinforce would be effective when required special properties NR. Composite NR/silica/MFC have a variety of such as low gas permeability, transparency while improved properties such as tear resistance, modulus acceptable mechanical properties, cost and at 300% elongation, hardness, heat buildup.[99] biodegradability are also demanded. Cellulose filler could improve some non-mechanical Finally, it is necessary to point out that research properties of NR. For example, cellulose filler from work on application of reinforcing fillers will bagasse may enhance barrier properties of filled NR contribute to the elucidation of rubber-filler, filler- from some gases and promote degradation in soil.[100] filler interactions and structure changes as well, and NR/cellulose II composite with 25 phr of cellulose © 2023 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 278
25728288, 2023, 3, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/vjch.202200225 by Readcube (Labtiva Inc.), Wiley Online Library on [02/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Vietnam Journal of Chemistry Review: Natural rubber – Improvement of properties eventually help to control the properties of NR peracetic acid, Rubber Chem. Technol., 1985, 58, 86-96. articles. 15. Nguyen Viet Bac, L. Terlemezyan, M. Mihailov. On stability and in-situ epoxidation of NR in latex by REFFERENCES performic acid, J. Appl. Polym. Sci., 1991, 42(11), 2965-2973. 1. Vietnam Rubber (newspaper online) 19/7/2021. 16. Nguyen Viet Bac, L. Terlemezyan, M. Mihailov. 2. Phinyocheep P. Chemical modification of Natural Epoxidation of natural rubber in latex in presence of rubber for improved performance, in book reducing agent, J. Appl. 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25728288, 2023, 3, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/vjch.202200225 by Readcube (Labtiva Inc.), Wiley Online Library on [02/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Vietnam Journal of Chemistry Review: Natural rubber – Improvement of properties nanoparticle morphology on the properties of natural nanocomposites, Express Polym. Letter, 2012, 6(1), rubber based nanocomposites, Eur. Polym. J., 2010, 14-25. 46, 609-620. 100. Abdelwahab N. A., Helaly F. M., Badran A. S. Effect 99. Xu S. H., Gu J., Luo Y. F., Jia D. M. Effect of partial of bagasse on the physico-mechanical properties of replacement of silica with modified nanocrystalline natural and styrene-butadiene rubbers, J. Elastomers cellulose on properties of natural rubber and Plastics, 2008, 40, 347-363. Corresponding author: Bui Chuong Hanoi University of Science and Technology No. 1, Dai Co Viet, Hai Ba Trung, Hanoi 10000, Viet Nam E-mail: buichuong1953@gmail.com. © 2023 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 283