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Microstructure analysis of brominated styrene-butadiene rubber.

INTRODUCTION

The use of synthetic polymeric materials has increased sharply over the past few decades. A great concern regarding their flammability and reducing their flammability were noted and methods reducing their potential hazard were investigated. Some of that concern was directed to the combustion of synthetic polymers and some others were directed to the elastomeric products [1-7]. Styrene-butadiene rubber (SBR) is one of the elastomers that is widely employed in the tire industry, underground coal mining, rugs, carpet underlay, and car floor mats. Great attention has been paid to modify SBR's combustion characteristics. Several approaches have been made to decrease the burning tendency of elastomeric materials [8-13]. One of these is the compounding of the rubber with flame-retardant additives, such as metal hydroxides, phosphorous containing additives, and chlorinated compounds. Another approach is to incorporate a small percentage of special monomers that contain in their structures one or two elements which are known to impart a degree of flame-retardation. A third approach has involved the treatment of the elastomers with specific reagents. However, the first approach, using flame-retardant additives, is most widely used in practically because of its ease of application. One factor of great importance to be taken into consideration when formulating a flame-retardant rubber is to compromise between the original physical and mechanical properties of the rubber and the modified combustion characteristics.

The earlier published results on reactivity ratios of 1,2-vinylic, 1,4-cis, and 1,4-trans double bonds in bromination reactions is ambiguous and several authors have reported different values [14, 15]. In most cases, spectroscopic methods such as IR spectroscopy would not be very useful for the estimation of copolymer composition because of the similarity in the comonomer units. However, NMR spectroscopy offers simple and rapid evaluation of copolymer composition compared to the other techniques.

In this work, much attention has been focused on evaluating the elastomeric characteristics of SBR, which reacts with different ratios of bromine to acquire flame retardancy characteristics. The effect of reaction progress on the positions of next additions sites and some of the physical and thermal properties have also been investigated.

EXPERIMENTAL

Materials

Styrene-butadiene rubber (commercial product) was supplied from Bandar Imam Petrochemical Co (Iran). SBR 1502 is a random styrene-butadiene copolymer with 23 and 77% of styrene and butadiene content, respectively. IR (film): 3070.5 (s), 2925.8 (s), 2669.3 (w), 1743.5 (sh), 1703 (m), 1641 (m), 1600 (m), 1494.7 (m), 1442.6 (s), 968.2 (m), 912.2 (m), 759.9 (m), 698.1 (m). [.sup.1.H] NMR (CD[Cl.sub.3], TMS): [delta] 1.23 (d, 2H), 1.82-2.02 (s, br, aliphatic hydrogens), 4.91-5.03 (t, =[CH.sub.2]), 5.36-5.40 (d, br, -CH=CH-), 5.51 (m, -CH=[CH.sub.2]), 7.11-7.24 (m, 5H) ppm.

Equipments

Proton nuclear magnetic resonance ([.sup.1.H] NMR) 400 MHz spectra were recorded on a Bruker Avance NMR spectrometer. Multiplicities of proton resonances are designated as singlet (s), doublet (d), triplet (t), quarted (q), doublet of doublet (dd), and broad (br). IR spectra were recorded on shimadzu FTIR-4300 spectrophotometer using films of 50 mm thickness obtained from chloroform solutions on KBr window. Vibrational transition frequencies are reported in wave number ([cm.sup.-1]). Band intensities are assigned as weak (w), medium (m), shoulder (sh), strong (s), and broad (br). Differential scanning calorimetry (DSC) data were recorded on a DSC-PL Thermal Analyzer, under [N.sub.2] atmosphere at a rate of 20[degrees]C/min in the temperature range from ambient to 400[degrees]C and -100[degrees]C to room temperature.

Bromination Reaction

The bromination reactions were carried out in chloroform solution at 0[degrees]C. The amount of bromine needed for a certain reaction was added dropwise as chloroform solution into the reaction vessel containing the polymer solution. After the consumption of bromine, the reaction mixture was stirred for 30 min at room temperature. Then it was continued for 15 min at 50[degrees]C. The brominated random copolymers were precipitated by acetone through a solvent-nonsolvent process. The modified copolymers were used for subsequent IR, [.sup.1.H] NMR, and DSC analysis.

Entry 1. Brominated SBR with 5% Modifications. IR (film): 3072.4 (s), 2935.1 (s), 2840.9 (s), 2673 (w), 1664.4 (w), 1641.3 (m), 1602.7 (m), 1492.8 (m), 1444.6 (s), 964.3 (s), 912.2 (m), 759.9 (s), 700.1 (s), 547.7 (m). [.sup.1.H] NMR (CD[Cl.sub.3], TMS): [delta] [.sup.1.H] NMR (CD[Cl.sub.3], TMS): [delta] 1.23 (d, 2H), 1.82-2.02 (s, br, aliphatic hydrogens), 4.25 (br, >CH-Br), 4.91-5.03 (t, =[CH.sub.2]), 5.36-5.40 (d, br, -CH=CH-), 5.51 (m, -CH=[CH.sub.2]), 7.11-7.24 (m, 5H) ppm.

Entry 2. Brominated SBR with 10% Modifications. IR (film): 3072.4 (s), 2937.4 (s), 2837.1 (s), 2675.1 (w), 1639.4 (m), 1600.0 (m), 1492.8 (m), 1442.6 (s), 966.2 (s), 912.2 (m), 759.9 (s), 700.1 (s), 545.8 (m). [.sup.1.H] NMR (CD[Cl.sub.3], TMS): [delta] 1.23 (d, 2H), 1.82-2.02 (s, br, aliphatic hydrogens), 4.25 (br, >CH-Br), 4.91-5.03 (t, =[CH.sub.2]), 5.36-5.40 (d, br, -CH=CH-), 5.51 (m, -CH=[CH.sub.2]), 7.11-7.24 (m, 5H) ppm.

Entry 3. Brominated SBR with 15% Modifications. IR (film): 3076.2 (s), 2912.3 (s), 2831.1 (s), 2677.0 (w), 1641.3 (m), 1600.0 (m), 1492.8 (m), 1444.6 (s), 966.2 (s), 912.2 (m), 759.9 (s), 700.1 (s), 547.7 (s). [.sup.1.H] NMR (CD[Cl.sub.3], TMS): [delta] 1.23 (d, 2H), 1.82-2.02 (s, br, aliphatic hydrogens), 4.25 (br, >CH-Br), 4.91-5.03 (t, =[CH.sub.2]), 5.36-5.40 (d, br, -CH=CH-), 5.51 (m, -CH=[CH.sub.2]), 7.11-7.24 (m, 5H) ppm.

Entry 4. Brominated SBR with 20% Modifications. IR (film): 3076.2 (s), 2912.3 (s), 2820.0 (s), 1641.3 (m), 1602.7 (m), 1492.8 (m), 1444.6 (s), 968.2 (s), 912.2 (s), 757.9 (s), 700.1 (s), 549.6 (m). [.sup.1.H] NMR (CD[Cl.sub.3], TMS): [delta] 1.23 (d, 2H), 1.82-2.02 (s, br, aliphatic hydrogens), 4.05 (br, [CH.sub.2]-Br), 4.25 (br, )CH-Br), 4.91-5.03 (t, =[CH.sub.2]), 5.36-5.40 (d, br, -CH=CH-), 5.51 (m, -CH=[CH.sub.2]), 7.11-7.24 (m, 5H) ppm.

Entry 5. Brominated SBR with 25% Modifications. IR (film): 3070.5 (s), 2937.4 (s), 2673.1 (w), 1641.3 (m), 1602.7 (m), 1492.8 (m), 1443.0 (s), 964.3 (s, br), 912.2 (s), 757.9 (s), 700.1 (s), 551.6 (m). [.sup.1.H] NMR (CD[Cl.sub.3], TMS): [delta] 1.23 (d, 2H), 1.82-2.02 (s, br, aliphatic hydrogens), 4.05 (br, [CH.sub.2]-Br), 4.25 (br, )CH-Br), 4.91-5.03 (t, =[CH.sub.2]), 5.36-5.40 (d, br, -CH=CH-), 5.51 (m, -CH=[CH.sub.2]), 7.11-7.24 (m, 5H) ppm.

Entry 6. Brominated SBR with 30% Modifications. IR (film): 3072.4 (s), 2927.9 (s), 1641.3 (m), 1600.5 (m), 1492.8 (m), 1444.6 (s), 968.0 (s, br), 912.2 (s), 757.9 (s), 700.1 (s), 549.6 (m). [.sup.1.H] NMR (CD[Cl.sub.3], TMS): [delta] 1.23 (d, 2H), 1.82-2.02 (s, br, aliphatic hydrogens), 4.05-4.25 (br, -[CH.sub.2]-Br and >CH-Br), 4.91-5.03 (t, =[CH.sub.2]), 5.36-5.40 (d, br, -CH=CH-), 5.51 (m, -CH=[CH.sub.2]), 7.11-7.24 (m, 5H) ppm.

Entry 7. Brominated SBR with 35% Modifications. IR (film): 3072.4 (s), 2923.9 (s), 1641.3 (m), 1602.7 (m), 1492.8 (m), 1444.6 (s), 964.3 (s, br), 912.2 (s), 757.9 (s), 700.1 (s), 547.9 (m). [.sup.1.H] NMR (CD[Cl.sub.3], TMS): [delta] 1.23 (d, 2H), 1.82-2.02 (s, br, aliphatic hydrogens), 4.05-4.25 (br, -[CH.sub.2]-Br and >CH-Br), 4.91-5.03 (t, =[CH.sub.2]), 5.36-5.40 (d, br, -CH=CH-), 5.51 (m, -CH=[CH.sub.2]), 7.11-7.24 (m, 5H) ppm.

RESULTS AND DISCUSSION

Characterization of Primary SBR

The microstructure of the primary SBR copolymer was ambiguous. Several techniques were used to determine the structural properties of the primary unmodified copolymer, such as FTIR, [.sup.1.H] NMR, and DSC analysis. According to the FTIR data (Fig. 1), the polybutadiene domain shows two characteristic absorption bands at 910 and 965 [cm.sup.-1] corresponding to 1,2 (vinylic) and 1,4-trans structural units, respectively. The absorption band at 714 [cm.sup.-1] corresponding to 1,4-cis structural units was not observed for the preliminary SBR sample. The double bonds of polybutadiene regions are either in the main chain because of the 1,4-addition (trans only) or as -CH=[CH.sub.2] in side chains due to 1,2 addition to the butadiene monomer. These two types of butadiene structural units have been distributed randomly in a polybutadiene domain of SBR.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

According to [.sup.1.H] NMR spectrum (Fig. 2), this part of polymer contains 80.38% 1,4-trans units and 19.62% 1,2-vinyl units.

[GRAPHIC OMITTED]

In Scheme 1 the peaks at 5.36 and 5.40 ppm are related to two trans hydrogens of butadiene region resulting from 1,4-addition. Also peaks at 4.91, 4.95, and 5.03 ppm show the presence of two geminal hydrogens of pendant vinylic groups. [H.sub.A] and [H.sub.B] are not chemically equivalent. [H.sub.X] ([delta] = 5.51 ppm) has been deshielded and splitted during coupling with [H.sub.A] ([J.sub.AX] = 18 Hz) and [H.sub.B] ([J.sub.BX] = 11 Hz). [H.sub.A] is splitted with [H.sub.X] and [H.sub.B] by [J.sub.AX] and [J.sub.AB] equal to 18 and 2 Hz, respectively. [H.sub.B] is also splitted with [J.sub.BA] = 2 Hz and [J.sub.BX] = 11Hz by [H.sub.A] and [H.sub.X], respectively. So, overlapping of the two doublet of doublets of [H.sub.A] and [H.sub.B] show three broad peaks at 4.91, 4.95, and 5.03 ppm. Integral of the peaks at 5.36 and 5.40 ppm correspond to the two hydrogens and total integral of the peaks at 4.91, 4.95, and 5.03 ppm are considered for two hydrogens too. Therefore, the percentage of 1,2-addition and 1,4-addition could be calculated as given below:

[FIGURE 3 OMITTED]

1, 2 - vinylic % = [[A.sub.4.91+4.95+5.03]/[[A.sub.5.36+5.40] + [A.sub.4.91+4.95+5.03]]] x 100

= 19.62%

1, 4 - trans % = 100 - 1, 2 - vinylic % = 100 - 19.62

= 80.38%

The broad peak at 2.53 ppm corresponds to the styrenic CH proton and its integral relative to phenylic protons is 1:5, which confirms the above assignments. A singlet peak at 2.02 ppm is related to the [CH.sub.2] protons in 1, 4-trans butadiene segments.

An overview onto the [.sup.1.H] NMR spectrum of the original SBR shows that there are more unexpected peaks at different regions. In the expanded region of 0.8-2.5 ppm, there could be observed several broad and multiple peaks around 1.41, 1.57, 1.69, 1.96, and 2.35 ppm, which correspond to the different existing microstructures for the SBR copolymer (Fig. 3).

[GRAPHIC OMITTED]

According to the integral of the characteristic peak for each microstructure, one of them has the majority and the others are in low abundance. The probable microstructures resulting from [.sup.1.H] NMR spectra have been revealed in Schemes 2-4. The basis of the difference in chemical shifts returns to the sequence positions in copolymer chain. The peaks appearing at 1.69 and 1.96 ppm are related to the -[CH.sub.2]- protons from styrene and trans-1,4-butadiene units (Scheme 2). The -CH[.sub.2]- group of trans 1,4-butadiene neighboring to the styrenic group is desheilded to 2.24 ppm because of the anisotropic effect of the phenyl ring relative to the other methylene ones. But this peak is not worthy because of its overlapping with the -CH- protons of 1,2-vinylic units (vice versa).

The presence of a multiplet peak at 1.57 ppm corresponds to the frontier -[CH.sub.2]- protons of styrene and 1,2-vinyl units (Scheme 3). The anisotropic effect of phenyl ring shifts these protons to up-fields relative to the other methylene, which exists in main chain.

[GRAPHIC OMITTED]

The presence of 1,4-trans butadiene in vicinity of the other isomer (i.e. 1,2-vinilic) caused to appear some new peaks at 1.41 and 1.96 ppm relevant to two -[CH.sub.2]- protons (right hand of Scheme 4). Displacement of double bond between 4,5-position in 1,2-vinyl units with 3,4-position in the 1,4-trans units desheilded -CH- protons to 2.35 ppm because of the anisotropic effect of C=C (left hand of Scheme 4).

It was expected that the DSC curve of unbrominated SBR shows two clear transitions, one below zero for polybutadiene parts and the other above zero for polystyrene parts. But in practice, only one transition about [T.sub.g] = -52.52[degrees]C relating to polybutadiene was observed and no trace of the second one corresponding to the glass transition temperature of the polystyrene was perceived. These observations indicated that some few styrene monomers are linked together to form the polystyrene domain between the long chain of butadiene.

Bromination of SBR

Bromine reacts with the polybutadiene double bonds during addition,

and for each bromine molecule addition, a double bond disappears. All of the brominated copolymers were soluble in common organic solvents such as THF, chloroform, benzene, etc. irrespective of the bromination percent; therefore, the bromination reaction of SBR is not accomplished by crosslinking side-reactions. The variations in the copolymer structure were investigated by [.sup.1.H] NMR spectroscopy. In this article, talking about the polybutadiene microstructure of brominated SBR is in fact with respect to the microstructure of the butadiene structural units, which have not reacted with bromine molecules. Three probable structures could be observed by bromination of SBR: (i) addition to the 2,3-double bonds of the butadiene in the backbone, (ii) addition to the 1,2-double bonds of the butadiene in the pendant group, and (iii) addition to both the 2,3- and 1,2- double bonds. The structures of the brominated SBR through each of the above modifications have been shown later (Scheme 5).

[GRAPHIC OMITTED]

The [.sup.1.H] NMR spectra of microstructures demonstrated in Schemes 5 and 6 reveal the presence of each proton at corresponding chemical shift. The saturation of the double bond during bromination reaction causes the appearance of new peaks at 1.21, 1.79, 2.06, 4.05, and 4.25 ppm (Schemes 5 and 6). Among them, the peaks at 4.05 and 4.25 ppm are the characteristic peaks, which is the basis of our study to follow the chemoselectivity of addition reaction. The others are of with less significance because of their overlapping with other unbrominated peaks. By addition of bromine from 0 to 15% (W/W) the peak at 4.25 ppm (trans -CHBr- protons) appears and grows up gradually by increasing the amount of bromine relative to the phenylic protons as an internal standard (Scheme 5). When the bromine content exceeds up to 20%, a new peak is observed at 4.05 ppm relating to -[CH.sub.2]Br- that comes from 1,2-addition of bromine (Scheme 6). Above 30% (W/W) of added bromine to SBR, the peak at 4.25 ppm is broadened and overlapped with the peak at 4.05 ppm.

[GRAPHIC OMITTED]

Since the chemical shift around 7.15 ppm, of the phenylic ring of styrene structural units is a reproducible peak, this was chosen as internal standard. Figure 4 shows the relationship between [A.sub.4.05+4.25]/[A.sub.7.15] versus the amount of bromine added in each reaction, which [A.sub.4.05+4.25] and [A.sub.7.15] correspond to peak intensities of the geminal hydrogens bonded to the brominated carbons (-CH(Br)s and [CH.sub.2]Br) and phenylic hydrogens, respectively. Figure 4 reveals that the addition reaction of bromine to SBR is divided into three steps. The first step occurs below 15% conversion. The second is between 15 and 30% conversion. The third one is above 30%. The first and third steps have approximately identical slopes and this shows that they obey from the same mechanisms. Hence, 1,2-vinylic and 1,4-trans characteristic peaks were considered to determine the mechanism of this addition reaction.

To find out the extent of 1,4- and 1,2- addition, the area of each characteristic peak relative to the internal standard (phenylic protons) were plotted versus bromination percent. In Fig. 5, [A.sub.5.36+5.40]/[A.sub.7.15] versus each bromination addition percent was designated. [A.sub.5.36+5.40] and [A.sub.7.15] correspond to peak intensities of 1,4-trans double bond hydrogens (-CH=CH-) and phenylic hydrogens, respectively.

[GRAPHIC OMITTED]

It is observable that by addition of bromine from 5-15%, [A.sub.5.36+5.40]/[A.sub.7.15] is reduced remarkably. This illustrates that most of the bromine molecules are added to the 1,4-trans double bonds. By addition of bromine to 20-30%, the slope of the graph relating to 1,4-addition is reduced and becomes approximately constant. It is noteworthy that after 30% of bromine added to the reaction mixture, [A.sub.5.36+5.40]/[A.sub.7.15] is reduced with considerable rate. This is obvious from the corresponding curve. It should be noted that the reproducibility of the above data were checked for several times.

To explain above observations, the variation of [A.sub.4.91+4.95+5.03]/[A.sub.7.15] ([A.sub.4.91+4.95+5.03] is the intensity of 1,2-vinylic hydrogens) versus the amount of bromine added in each reaction was plotted in Fig. 6.

No considerable variation in [A.sub.4.91+4.95+5.03] was observed from 0-15% of the added bromine. This implies that no 1,2-addition occurs up to 15% but the addition of bromine to 1,2-vinylic double bonds begins from 20%. The comparison between the slopes of above two curves (Figs. 4 and 5) between 15 and 30% shows that the bromination reaction on vinylic groups in SBR has chemoselectivity depending upon the progress of reaction. That is, 1,4-addition is being performed with about 100% selectivity below 15% of bromine. In the range of 20-30%, the addition reaction occurs on both sites and with preference to 1,2-vinylic double bonds. Above 35%, the addition reaction was observed on 1,4-double bonds mainly; with the probable reason that the concentration of 1,2-vinylic groups decreases relative to 1,4-trans double bonds.

[FIGURE 4 OMITTED]

Thermal Properties

Thermal behaviors of the modified SBRs and investigations on variation of [T.sub.g] data were studied by DSC. As the bromination reactions were performed on butadiene segments, the variation in [T.sub.g] of the corresponding region had specific interest. The comparison between the obtained [T.sub.g] for unmodified and modified samples versus degree of bromination has been plotted in Fig. 7.

[FIGURE 5 OMITTED]

The trend for increase in [T.sub.g] values is completely logical. Decrease at the curve during 15-30% of bromination is related to the addition reaction being occurred on 1,2- double bond, which is in conformation with the previous results. 1,4-Addition reaction causes the saturation of double bonds existing in the backbone of the polymer and transition from elastomeric to plastic properties. This makes the increase in [T.sub.g] of the polymer. In the case of 1,2-addition, the reaction will take place at the pendant double bond. Thus its effect on [T.sub.g] will not be sensed remarkably and the increasing trend could not be observed in this range. Accordingly, the existing ambiguous problem could be explained by earlier discussions [15].

[FIGURE 6 OMITTED]

One factor of great importance to be taken into account when formulating a flame retardant rubber is to compromise between the original physical and mechanical properties of the rubber and the modified characteristics. However, the degree of acceptance of the change, which might occur in the physical or mechanical properties of a rubber, is determined by the type of the application for which the modified rubber will be used. For example, in the tire industry, where the mechanical properties of the rubber is the main concern, a change of about 10-20% in the modulus of elasticity is considered a significant change, which may not be as significant in other applications. This could counteract the advantage of imparting a degree of flame retardancy to the product. Figure 7 shows the effect of various bromine concentrations on the elasticity properties of SBR. The results indicate that addition of B[r.sub.2] up to 10% to the rubber will decrease the elasticity. However, the increase in added bromine more than 15% enhances both flame retardancy and elasticity properties.

[FIGURE 7 OMITTED]

CONCLUSIONS

The structural characteristics of brominated SBR were fully investigated by [.sup.1.H]-NMR spectroscopy. For this purpose, bromine reacts with the polybutadiene double bonds via addition reaction. All of the brominated copolymers were soluble in common organic solvents such as chloroform irrespective of the bromination percentage and the bromination reaction of SBR is not accomplished by cross-linking side-reactions.

ACKNOWLEDGMENTS

We thank Dr. Mahdavian and his group, Iran Polymer and Petrochemical Institute (IPPI), Tehran, for recording DSC thermograms.

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Sepideh Khoee, Maedeh Sorkhi

Department of Chemistry, Faculty of Science, University of Tehran, Tehran 14155-6455, Iran

Correspondence to: S. Khoee; e-mail: khoee@khayam.ut.ac.ir

Contract grant sponsor: Research Affairs Division Tehran University, Tehran.
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Date:Feb 1, 2007
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