MACROMOLECULAR COMPOUNDS AND POLYMERIC MATERIALS

1983

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2011, Vol. 84, No. 11, pp. 1983−1987. © Pleiades Publishing, Ltd., 2011. Original Russian Text © R.G. Zhitov, V.N. Kizhnyaev, V.V. Alekseenko, A.I. Smirnov, 2011, published in Zhurnal Prikladnoi Khimii, 2011, Vol. 84, No. 11, pp. 1898−1902.

Bitumen–Rubber Composite Binders for Production of Asphalt Concretes

R. G. Zhitov, V. N. Kizhnyaev, V. V. Alekseenko, and A. I. Smirnov

Irkutsk State University, Irkutsk, Russia

Received February 11, 2011

Abstract—The dissolution of vulcanized rubber in bitumen in the presence of a devulcanizing additive and the formation of bitumen–rubber composites, which are promising binders for the production of asphalt concretes, were studied.

DOI: 10.1134/S1070427211110255

Binders based on oxidized petroleum bitumens, commonly used in the production of asphalt concretes, do not always ensure the required service characteristics and durability of the asphalt pavement. This is caused by intrinsic physicochemical characteristics of the bitumen material [1]. The pavement quality can be considerably improved by using composite materials based on bitumen, e.g., polymer–bitumen binders (PBBs) [2]. Compounding of bitumen with a polymer (rubber) allows the physicomechanical characteristics of the binder, namely, its heat and frost resistance and adhesion properties, to be substantially enhanced. However, the use of expensive rubbers as polymeric additives makes the PBB cost several times higher than that of petroleum road bitumens (BND).

It is more attractive from the economical viewpoint to use as modifi er for bitumen binders rubber wastes, and primarily the rubber from spent car tires. Rubber waste is a valuable source of cheap high-quality synthetic rubbers. Furthermore, owing to the presence of special chemical substances (antioxidants, antiaging agents), tire rubber is resistant to oxidation and to various aggressive media, which favors improvement of the service characteristics of materials produced with the addition of tire rubber. Therefore, the development of bitumen–rubber composite binders (BRCs) will allow solution of two urgent problems at once: improvement of the road quality, on the one hand, and utilization of rubber wastes, on the other

hand. However, the BRC development involves problems with compounding of bitumen with vulcanized rubber, which is a diffi cultly soluble material.

Thus, the problem of compounding of bitumen with vulcanized rubber consists in the breakdown of the three-dimensional structure of the rubber and in the dissolution of the degradation products in the bitumen. Such a process can be performed either at high temperatures (above 300°С) or in the presence of special additives promoting the rubber devulcanization at lower temperatures [3]. However, in the fi rst case, not only disulfi de bridges but also rubber macromolecules can degrade, which negatively affects the quality of the resulting products. In the second case, the product dissolving in the bitumen is relatively high-molecular-weight devulcanized rubber, which favors preservation of the advantages of polymeric bitumen modifi ers. The following devulcanizing additives are used: aromatic fractions from oil refi ning [4], SUNTEX aromatic oils [5], and aromatic or heteroaromatic amines [6]. As a rule, these are expensive chemicals, which considerably increases the cost of the fi nal product, BRC.

In this study we examined the possibility of using coal tar as dissolving additive for bitumen–rubber blend and evaluated the properties of the BRCs obtained and of asphalt concretes based on them. We considered two modes of BRC preparation: preliminary dissolution of the

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rubber in coal tar and blending of the binary composite with bitumen, or dissolution of the rubber directly in the ternary system rubber–bitumen–coal tar.

EXPERIMENTAL

In our experiments we used tire rubber crumb of size 5–10 mm, prepared by mechanical crushing of cordless tires or tires with cellulose cord, a commercial naphthalene fraction of coal tar (NFCT) (Zarinsk, Russia), and BND of 90/130 grade (Angarsk, Russia).

The dissolution of tire rubber crumb in NFCT was performed using a modified household microwave oven. For this purpose, a 750-ml fl at-bottomed fl ask was charged with a weighed portion of tire rubber crumb, and a defi nite amount of NFCT preheated to 70–80°С was added. The fl ask was placed in a microwave oven equipped with a blade stirrer and a sensor for measuring the temperature in the mixture. The mixture was heated with continuous stirring to 220–230°С. The binary system became homogeneous within 30 min.

Dissolution of tire rubber crumb in a mixture of bitumen with coal tar under thermomechanical treatment was performed in a specially designed laboratory installation. Its main part was a heated metal reactor equipped with a blade stirrer and a thermocouple and combined with a worm device. The reactor was charged with the required amounts of rubber crumb, NFCT (preheated to 70–80°С), and BND of 90/130 grade (preheated to 100–120°С). The mixture was heated with continuous stirring to 220–230°С, and this temperature was maintained for 2–3 h. Then the mixture was passed three times through the worm device without interrupting heating.

For the production of asphalt concretes based on BND of 90/130 grade and BRCs prepared by various procedures, we used a mineral mixture consisting of 28% crushed diabase stone of size 5–20 mm, 30% crushed diabase stone siftings, 35% siftings from gravel crushing, and 7% dolomite fl our. The mineral component thus obtained was compounded at 180°С with the binder (BND or BRC) taken in an amount of 5.7–6.5% relative to the mineral component. The asphalt concrete mix thus prepared was mixed at 180°C for 1 h.

The water absorption of the binding materials was studied by sorption from saturated water vapor at 25°С. The water sorption was monitored by the weight gain of the sample, measured at defi nite time intervals. The

brittle point was determined according to GOST (State Standard) 11507–78, Method for Determination of the Fraas Brittle Point, using KP-125А Fraas apparatus (Dorpribor Public Joint-Stock Company). The softening point was determined according to GOST 11506–73, Method for Determining the Ring-and-Ball Softening Point. The adhesion of bitumen to the mineral material was determined according to GOST 11508–74, Methods for Determining Adhesion of Bitumen to Marble and Sand, using marble stone and crushed diabase stone. The extensibility of binders was measured according to GOST 11505–75, Method for Determining Extensibility. The physicomechanical characteristics of asphalt concrete mixes were determined in accordance with GOSTs 31015–2002 and 12801–98 using checked and certifi ed equipment at the laboratory of the Road Service of the Irkutsk Oblast. The 1Н NMR spectra of solutions of BRCs in CCl4 were recorded with a Varian VXR-500 spectrometer.

Heating of a mixture of NFCT with tire rubber crumb at 200–250°С for 3–4 h leads to the rubber dissolution and formation of a homogeneous mass [7]. The homogenization is considerably accelerated when the temperature is maintained using microwave radiation. The rubber crumb of size 5–10 mm fully dissolves in coal tar (as judged from the absence of inclusions visible with the unaided eye) at 220–230°С within less than 0.5 h. Apparently, the use of microwave radiation accelerates the dissolution owing to bulk heating of tire crumb, because it contains carbon black particles which are centers of simultaneous heating throughout the rubber bulk. An important advantage of microwave heating, compared to common thermal heating, is the possibility of dissolving coarser rubber particles (5–10 against 1–3 mm). The presence of cellulose cord in the tire rubber crumb does not affect the dissolution process.

The assumption that thermal homogenization of the tire rubber crumb–coal tar system involves the devulcanization, followed by dissolution of rubber macromolecules in the coal tar, is confi rmed by NMR spectroscopy. In the 1Н NMR spectra of solutions of the binary composite in CCl4, we recorded signals in the ranges δ 4.9–5.6 and 1.7–2 ppm, characteristic of the monomeric unit of butadiene-containing rubber (–H2C–CH=CH–CH2–) and absent in the spectrum of the initial NFCT. Thus, the very fact of the detection of rubber macromolecules in solution indicates that the crumb dissolved owing to rubber devulcanization (vulcanized

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1985BITUMEN–RUBBER COMPOSITE BINDERS

rubber is incapable of dissolution).By dissolving the rubber in coal tar under the action

of microwave radiation, we obtained a bitumen-like compound with the characteristics similar to those of BND (Table 1). The binary composite, insignifi cantly differing from bitumen in the softening point, at increased content of the dissolved rubber is characterized by lower brittle point compared to bitumen but is inferior to bitumen in the extensibility. Therefore, by homogenization of the mixture of the binary composite (40%) with BND 90/130 bitumen (60%) in the molten state, we prepared a ternary composite containing 12% dissolved rubber. Thus, by successive dissolution of the rubber in coal tar and compounding with bitumen, we were able to prepare a BRC with the characteristics given in Table 1.

The BRC obtained met the requirements of GOST 22245–90, Viscous Petroleum Road Bitumens, with respect to all the physicochemical characteristics and was tested as binder for asphalt concrete. However, the tests showed that the BRC prepared by the above procedure, when used as binder, did not exert the expected positive effect on the asphalt concrete quality as compared to the bitumen binder.

To develop binders surpassing in properties BND bitumens, we examined the possibility of dissolving tire rubber crumb with bitumen directly in the course of the rubber devulcanization in the presence of NFCT. This approach allowed the amount of the dissolved rubber to be considerably increased and the content of the coal tar in the fi nal composite to be decreased.

The rubber devulcanization in the ternary system rubber crumb–NFCT–bitumen occurs in the same

temperature interval as the rubber dissolution in coal tar (220–230°С), but with certain specifi c features. First, because of low capability of bitumen to absorb microwave radiation, application of this mode of maintaining the required temperature to the ternary mixture appeared to be ineffi cient. It was found experimentally that the optimal NFCT amount favoring the rubber dissolution in the ternary system is about 30% of the tire rubber crumb weight, which corresponds to no more than 10% of the total fi nal weight of the compound. A decrease in the NFCT content made it necessary to subject the ingredient mixture to an additional mechanical treatment to ensure homogenization of the ternary mixture by the procedure described in the patent [8]. In this case, the rubber dissolution is accompanied by devulcanization, and rubber macromolecules dissolve already in a mixture of bitumen and coal tar to form BRC. The presence of rubber macromolecules in solutions of the composite in СCl4 was detected by NMR. The dissolution of the tire rubber crumb in bitumen with the addition of NFCT under thermomechanical treatment took 3–4 h, i.e., a considerably longer time compared to the dissolution in coal tar under the action of microwave radiation. However, the BRC prepared by the second procedure contains up to 30% dissolved rubber crumb at a lower NFCT content (no more than 10%), which positively affects the properties of the composite.

Figure 1 shows the dependence of the brittle and softening points of BRC on the content of the dissolved rubber crumb. These temperature parameters are very important service characteristics of asphalt concrete binders: The larger the difference between the brittle and softening points, the higher the quality of the bitumen

Table 1. Physicochemical characteristics of the binary composite rubber–NFCT and ternary mixture (BRC) rubber (12%)–NFCT (28%)–BND (60%)

Rubber content in composite, wt %

Penetration at 25°C, mm Softening point, °C Brittle point, °C Extensibility at 25°C,

cm

according to GOST

BND 90/130 98 47.8 –18.0 >65

10 119 46.7 –21.1 56

25 105 46.5 –26.3 38

30 108 47.9 –32.3 32

40 126 46.0 –31.3 49

Ternary mixture 120 46.4 –30.0 >65

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binder. For the BRC prepared by thermomechanical treatment, the optimal content of the dissolved rubber is 20–25%. At this rubber content, the working temperature interval (difference between the brittle and softening points) of BRC is 95–97°С, at the lower limit (brittle point) of –30 and upper limit (softening point) of 72°С.

Thus, by varying of the content of the dissolved rubber, it is possible to prepare composite binders for various purposes, for production of both frost- and heat-resistant asphalt concrete pavements. In addition, BRC exhibits increased, as compared to the initial bitumen and polymer–bitumen binders, adhesion to any mineral materials. This is favored by functionalization of the composite owing to introduction of NFCT and the dissolved rubber (which itself is a composite). Depending on the ingredient ratio in BRC, the content of oxygen-containing functional groups in the composite varies within 0.12–0.18 mg-equiv g–1 and increases as the dissolved rubber content is increased from 10 to 30%. Owing to the functional groups, the adhesion of BRC to mineral materials is improved.

Because the properties of asphalt concretes can be af-fected by contact with water, it was interesting to compare the water absorption of the standard binder BND and that of BRC with different rubber contents. In addition, we evaluated the water absorption of binders in a mixture with the mineral powder added in an amount of 20% of the binder weight. The results of water vapor sorption by the samples at 25°С (Fig. 2) show that the water absorp-tion of BRC samples is considerably lower than that of bitumen. The BRC and BND samples show opposite trends in changes in the water vapor sorption on adding a mineral powder. Introduction of a mineral additive led to a decrease in the water absorption with BND and to a slight increase in the water absorption with BRC.

Table 2. Properties of asphalt concretes based on BRC

Binder content, wt %Ultimate strength, MPa, at indicated temperature, °C

Water resistance0 20 50

6.1 6.2 4.0 2.2 1.0

5.7 8.0 3.8 1.5 0.95

6.5 6.9 3.6 1.3 1.0

6.5a 7.2 3.9 1.3 0.97

6.5b 9.0 3.8 1.0 0.95a Asphalt concrete based on BRC without mineral powder.b Asphalt concrete based on BND 90/130.

Fig. 1. (1) Brittle and (2) softening points Т of BRC as functions of the dissolved rubber content m.

T, °C

m, wt %

Fig. 2. Weight gain Δm of BND (1) without and (2) with addition of mineral powder and of BRC containing (3, 4) 10 and (5, 6) 20% dissolved rubber (3, 5) without and (4, 6) with addition of the mineral powder, as a function of time t of saturated water vapor sorption at 25°С.

Δm, %

t, days

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The BRC containing 20% dissolved rubber and prepared by the thermomechanical procedure was tested as binder in the production of asphalt concrete mixes of type B (Table 2). A remarkable feature is very high strength, and hence high shear resistance, of the asphalt concrete based on BRC with the optimal binder content at 50°С. The temperature sensitivity of the strength of the asphalt concrete based on BRC is even lower than that of polymer–asphalt concretes for which one of major advantages is heat resistance. Thus, the binder exhibits positive properties owing to compounding of the bitumen and rubber.

CONCLUSIONS

(1) A procedure was developed for preparing bitumen–rubber binders for asphalt concretes, based on thermomechanical compounding of the ternary mixture tire rubber crumb–bitumen–coal tar (naphthalene fraction).

(2) The softening and brittle points of the binder suggested can be controlled by varying the binder composition.

REFERENCES

1. Pechenyi, B.G., Bitumy i bitumnye kompozitsii (Bitumens and Bitumen Compounds), Moscow: Khimiya, 1990.

2. Galdina, V.D., Modifitsirovannye bitumy (Modified Bitumens), Omsk: Sib. Avtodor. Inst., 2009.

3. Saltanov, A.V., Pavlovich, L.B., and P’yankov, B.F., Khim. Inter. Ustoich. Razv., 2001, no. 9, pp. 79–88.

4. Shunin, D.G., Filippova, A.G., and Okhotina, N.A., Zh. Prikl. Khim., 2000, vol. 75, no. 6, pp. 1038–1041.

5. RF Patent 2162475.6. RF Patent 2164927.7. Basova, S.P., Leonenko, V.V., and Safonov, G.A.,

Neftepererab. Neftekhim. (Nacuh.-Tekh. Dostizh. Pered. Opyt), 1999, no. 9, p. 38.

8. RF Patent 2327719.