UDK 678.7:66.017
Professional article/Strokovni članek
ISSN 1580-2949 MTAEC9, 46(6)695(2012)
THE EFFECTS OF MICRO AND NANO CaCO3 ON THE RHEOLOGICAL AND PHYSICO/MECHANICAL BEHAVIOR OF AN SBS/CaCO3 COMPOSITE
VPLIV MIKRO- IN NANODELCEV CaCO3 NA REOLOŠKO IN FIZIKALNO/MEHANSKO VEDENJE KOMPOZITA SBS/CaCO3
Mohsen Sadeghi1, Amirhossein Esfandiari2
1Department of Polymer Engineering, Faculty of Technical and Engineering, Post- Graduate Center, Islamic Azad University, South Tehran
Branch, Tehran, I. R. Iran
2Department of Textile Engineering, Faculty of Technical and Engineering, Islamic Azad University, South Tehran Branch, Tehran, I. R. Iran
ah_esfandiari@azad.ac.ir, a.h.esfandiari@gmail.com
Prejem rokopisa — received: 2012-04-03; sprejem za objavo - accepted for publication: 2012-08-27
Different particle sizes of CaCO3 were used and this was confirmed with the X-ray diffraction method. The nano CaCO3 was added, from mass fraction (w) 1 % to 5 %, in the styrene butadiene styrene elastomer (SBS). Elastomer nanocomposites were compounded on a two-roll mill and molded in a compression-molding machine. The mechanical properties such as the tensile strength, the elongation at fracture, the modulus at 300 % elongation, the hardness, the specific gravity, the swelling index, and the flame retardancy were studied. The results were compared with a commercial CaCO3 (|im) filled SBS. There was an improvement in the properties of the elastomer nanocomposites because of the uniform dispersion of the nano CaCO3 particles in the matrix that intercalates the elastomer chains. Hence, the degree of cross-linking increases multifold in comparison with the commercial CaCO3.
Keywords: swelling index (SI), flame retardancy, vulcanized, dicumil peroxide (DCUP), MFI and nano CaCO3
Uporabljeni so bili delci CaCO3 različne velikosti, potrjeni z metodo rentgenske difrakcije. Elastomeru stiren butadien stiren (SBS) je bilo dodano od 1 % do 5 % masnih deležev (w) nanodelcev CaCO3. Nanokompozitni elastomer je bil izdelan z dvovaljčnim mlinom in ulit na tlačnem stroju. Preizkušene so bile mehanske lastnosti, kot so natezna trdnost, raztezek pri porušitvi, modul pri 300-odstotnem raztezku, trdota, specifična gostota, indeks nabrekanja, zadrževanje širjenja plamena. Rezultati so bili primerjani s komercialnim SBS, polnjenim z mikrodelci CaCO3. Ugotovljeno je bilo izboljšanje lastnosti elastomernega kompozita zaradi enakomerne razpršenosti nanodelcev CaCO3 v osnovi, kar omogoča vrinjenje verig elastomera. Zato je mogoča večkratno povečana stopnja prepletanja v primerjavi s komercialnim CaCO3.
Ključne besede: indeks nabrekanja (SI), zadrževanje širjenja plamena, vulkanizirano, dikumil peroksid (DCUP), MFI in nanodelci CaCO3
1 INTRODUCTION
In recent years, combinations of inorganic nanoparti-cles and polymers have received a great deal of interest1-6 for the reason that they provide a means of improving the strength together with the toughness of the polymer matrix, which is almost impossible with conventional filled polymers. The performance of the polymer nanocomposites is strongly dependent on the final morphology of the nanoparticles dispersed in the polymer matrix. Various methods7-10 have been used to prepare polymer nanocomposites, but most of them are complicated and costly1-4.
The effect of mineral fillers on the elastic modulus of polymers has been widely studied and there are many theoretical models available for predicting the behavior of a composite in the elastic zone6. Thermoplastic elastomers are materials that combine the characteristics of the good processing ability of thermoplastics with the physical properties of vulcanized rubber.
The mechanical and thermal properties of polymers and composite structures can be improved through the use of various kinds of fillers. Micron-sized fillers usu-
ally cause a decrease in strength, impact resistance, and processability. The application of nanotechnology in elastomer nanocomposites shows a significant improvement in the modulus, strength, toughness, and resistance to chemical attack, gas impermeability in polymer com-posites1-5.
In this study, three different sizes of CaCO3 were used as filler in styrene butadiene styrene (SBS) and compared with commercial CaCO3 composites of SBS/CaCO3. The mechanical properties and physical properties were studied and compared with other fillers6-8. The effect of mineral fillers on the elastic modulus of polymers has been widely studied and there are many theoretical models available for predicting the behavior of a composite in the elastic zone9-11. Thermoplastic rubbers are materials that combine the characteristics of the good process ability of thermoplastics with the physical properties of vulcanized rubber. SEBS is an elastomer that has been subjected to a hydrogenation process, through which the polybutadiene chain is eliminated. This new rubber has a high resistance to environment, temperature, UV radiation, etc12. This is without losing the properties of a thermoplastic, so making them
useful in applications where a standard SBS is not useful. SBS is a thermoplastic material that successfully combines the properties of an elastomer (rubber) with the low costs of processing thermoplastics.
2 EXPERIMENTAL PROCEDURE
2.1	Materials
The SBS elastomer used in this study was a commercial TPE grafted with maleic anhydride function, 2600 S-40B supplied from RTP Co., USA. The stearic acid, nano-sized calcium carbonate with an average particle size of 50-70 nm, with a bulk density of 0.4 g/cm3 and a pH of 9.2 and commercial CaCO3 with a bulk density of 0.81 g/cm3 and a pH 8.6 was supplied from Omya Co., Austria. The DCUP (Bis (1-methyl-1-phenylethyl) peroxide) for the curing agent was supplied from MERCK Co., Germany.
2.2	Preparation of the composite
At the beginning the nanometer and macrometer (commercial) calcium carbonate particulates were first dried at a temperature of 75 °C for 2 h, then premixed with the SBS elastomer together with different nCaCO3 and mCaCO3 loadings (w = 1-5 %) and with DCP and physically mixed in a mixer with a rotor speed of 75 r/min at 30 °C for 30 min (before the materials for making every master-batch with a 0.1 balance were weighed). This procedure was performed in order to achieve a homogeneous dispersion of nanoparticles. The temperature cannot be more than 30 °C because the DCP may change the solid state to a liquid phase and lead to the aggregation of CaCO3. The resulting master-batch (all the master-batches were 100 g) was then extruded on a Brabender twin-screw extruder at 60 r/min and (140, 160, 180, 200) °C for each sample. The extruder compounds were fed into a compression set mold to make a molded kind of specimen shape for the tensile and other tests (Dumbbell and circular plate) using ASTM standards (D638 and D395).The formulation of the samples is given in Table 1.
Table 1: Sample specification Tabela 1: Specifikacija vzorcev
Sample	SBS	nCaCO3	|CaCO3	DCP
Neat (S 1)	99	0	0	1
1 % Nc-HPC (S 2)	97	1	1	1
3 % Nc-HPC (S 3)	93	3	3	1
5 % Nc-HPC (S 4)	89	5	5	1
2.3 Characterization method
Scanning electron microscopy (SEM) was performed with a Tescan 130 VEGA-II apparatus equipped with an energy beam of 20 kV. The prepared samples were cryo-genically fractured in liquid nitrogen and then coated
with gold by vapor deposition using a vacuum sputtering machine before the SEM observation. Therefore, the SEM images were all obtained by inspecting the cryo-genically fractured surfaces of the samples.
The mechanical properties tests were carried out on a TCS-2000 Universal Testing Machine (GOTECH, Taiwan) at a crosshead speed of 500 mm/min and 23 °C according to the ASTM D 412 method, and the stressstrain curves were drawn simultaneously. The hardness tests were carried out with a Durometer Hardness tester (TECLOOCK, Japan) (shore) in accordance with ASTM D 2240 at 23 °C. The MFI test was carried out on a GT-7100-MI (GOTECH, Taiwan) with ASTM D 1234 at 220 °C, 5 kg. The specific gravity carried out per ASTM D792. The shrinkage carried out per ASTM D 955.
3 RESULTS AND DISCUSSION 3.1 Rheological characterization
3.1.1 Isothermal curing behavior
Figure 1 illustrates the rheographic profile of neat SBS, SBS/nCaCO3 and SBS/^CaCO3 blends containing mass fractions (1, 3 and 5) % at (140, 160, 180 and 200) °C. From each of the curves shown in Figure 1, some characteristic parameters were determined, as listed in Table 2. These parameters included Tx and To as the maximum and minimum torques during the curing process; the time during which the torque begins to rapidly increase as the onset of the curing and also the whole curing time. It is evident that at a given nCaCO3 loading, an increase in the isothermal curing temperature drives the curing reaction forward, as well as decreasing the curing time. The reason for such an observation could be correlated with the availability of more thermal energy in addition to the lower viscosity of the compound, which facilitates the formation of the cross-linking networks13. A closer view of the experimental data showed that nCaCO3 nanoparticles introduced to the system seem to act as accelerators with respect to the curing reaction times. This acceleration effect should be related to the fact that the stearic acid layer on the nCaCO3 surface possesses strong acid sites to activate the electrophiles effectively at the curing temperature14. Therefore, it can be inferred that there is a relatively high concentration of curing agents on the surface of the nanofiller at the beginning, which then advances into the unreacted zone in the nCaCO3 modified SEBS blends. Interestingly, it can be observed that the higher the nCaCO3 loading, the shorter the curing time for the SBS systems. In other words, the addition of treated nCaCO3 increases the thickness of the stearic acid layer on the nanofiller surface, which provides extra regimes for the cross-linking reaction; and thus a further catalyzing effect occurs during the curing reaction. However, one cannot ignore the contribution of the large surface area of the nCaCO3 particles, which ensures the proper, sufficient and increased dispersion of the radical reactant. It is well known that
the maximum torque of the rheometer curves is the most relevant factor with respect to the cross-link density of the cured systems13,14. The retractive force to resist a deformation is proportional to the number of network-supporting polymer chains per unit of thermoset and the higher number of junctures increases the number of supporting chains15. In this regard, T« is expected to increase at a higher value of the network chain density. However, as can be observed in Table 2, the variation of the minimum torque concerning the curing temperature and the composition of the sample is not so appreciably related to the maximum torque.
Hence, the difference T = T«, -To is normally used to analyze the experimental data in order to exclude the effect of To. As is clear from the obtained AT values, there is a tendency to increase this value as the amount of nCaCOs content increases up to 3 %. In contrast, dissimilar behavior was observed for the effect of the curing temperature on the torque value. In this case the availability of more thermal energy helps the segmental and diffusional chain motion, which leads to a lower torque value1617.
3.1.2 Isothermal cure kinetics
The torquemeter test results under isothermal conditions yielded valuable information about the amount of torque versus time that could be used to evaluate the kinetic parameters by using a series of mathematical expressions. In this concern, the degree of curing, 3, which was employed to indicate the extent of the resin cross-linking, might be estimated from the time dependency of the torque values using the following equation18:
3 =
T - T
1 (t) 1 o
T - T,
(1)
where T(t) is the torque at a given time of the curing process. There are two primary mechanisms describing most thermoset curing, including the nth-order and the autocatalytic19. The nth-order model assumes that the reaction rate is proportional to the concentration of the unreacted material (1 - 6), as shown in Eq. (2), where n is the reaction order20:
d3
- = *(1-e)n
(2)
Nevertheless, the nth-order model seems to be incapable of describing the progress of the entire reaction because several simultaneous reactions may occur during the curing process21. For an isothermal reaction, the nth-order mechanism predicts the maximum reaction rate at time = 0. However, this is not the case for autocata-lytic curing processes, in which the final products of the curing reaction can catalyze the subsequent reaction between the resin and the hardener. On the other hand, in an autocatalytic model, the conversion rate is proportional to the concentration of both the unreacted and the reacted material:
d3
^ = K (1- 3)n 3
(3)
where m is also a reaction order. In both the autocatalytic and nth-order models, K represents the temperature-dependent reaction rate constant, obeying the well-known Arrhenius equation as22:
K = A exp
-E RT
(4)
where A is a frequency factor corresponding to the incidence of molecular collisions that should be obtained to produce a chemical reaction. In addition, E, R and T are
Figure 1: Torque (T) as a function of curing time for neat and filled systems containing 5 % nCaCO3 and | CaCO3 at the analyzed temperatures
Slika 1: Navor (T) kot funkcija časa do uravnoteženja za čist in polnjen sistem s 5 % nanodelcev CaCO3 in mikrodelcev CaCO3 pri temperaturah preizkušanja
Table 2: To and obtained from the rheometer curves for nCaCO3 (Figure 1), for all of the samples at the analyzed temperatures Tabela 2: To in Tx, dobljena iz rheometrskih krivulj za nanodelce CaCO3 (slika 1), za vse vzorce pri temperaturah preizkušanja
Sample	Curing temperature (°C)	T0/(N m)	T„/(N m)	AT	Onset of cure (s)	Curing time (s)
Neat	140	0.235	3.83	3.595	23	34
	160	0.315	6.412	6.097	20	36
	180	0.466	6.807	6.341	13	37
	200	0.575	8.845	8.27	10	42
1 % Nc-HPC	140	0.545	6.976	6.431	37	18
	160	0.615	9.215	8.6	29	24
	180	1.126	10.694	9.568	19	42
	200	1.356	12.396	11.04	16	50
3 % Nc-HPC	140	1.075	8.795	7.72	24	30
	160	1.49	12.327	10.837	20	37
	180	1.356	12.741	11.376	16	39
	200	0.854	14.365	13.511	11	44
5 % Nc-HPC	140	0.47	8.236	7.766	19	24
	160	1.045	11.7	10.655	15	26
	180	1.425	12.431	11.006	13	34
	200	0.912	15.883	14.971	10	38
Table 3: To and Tx obtained from the rheometer curves for ^CaCO3 (Figure 1), for all of the samples at the analyzed temperatures Tabela 3: To in Tx, dobljena iz rheometrskih krivulj za mikrodelce CaCO3 (slika 1), za vse vzorce pri temperaturah preizkušanja						
Sample	Curing temperature (°C)	T0/(N m)	T„/(N m)	AT	Onset of cure (s)	Curing time (s)
Neat	140	0.06	2.67	2.61	26	28
	160	0.27	5.14	4.87	25	30
	180	0.73	6.34	5.61	14	33
	200	0.05	7.19	7.14	13	37
1 % Nc-HPC	140	0.38	4.81	4.43	42	16
	160	0.27	7.16	6.89	32	21
	180	0.09	9.21	9.12	22	36
	200	1	10.91	9.91	18	42
3 % Nc-HPC	140	1.6	7.23	5.63	26	21
	160	2.43	9.85	7.42	24	28
	180	2.29	11.40	9.11	18	34
	200	2.24	12.36	10.12	10	39
5 % Nc-HPC	140	1.24	7.91	6.67	20	23
	160	0.88	9.53	8.65	14	25
	180	1.63	11.84	10.21	11	33
	200	0.28	13.68	13.40	10	38
Table 4: Combination of the curing kinetic model parameters determined from the curve fits of d9/dt versus 9 and the values of the nth-order and autocatalytic mechanism activation energies for all of the samples at the analyzed temperatures for nCaCO3
Tabela 4: Kombinacija parametrov modela kinetike uravnoteženja, določenih iz krivulj d9/dt proti 9 in vrednosti n-tega reda ter aktivacijske energije avtokatalitičnega mehanizma za temperature preizkušanja za nanodelce CaCO3
Sample	Curing temperature (°C)	K1/ (x10-4 s-1)	K2/ (x10-2 s-1)	m	N	R2	E1/ (K J/mol)	E2/ (K J/mol)
Neat	140	1.23	1.04	1.17	1.23	0.99		
	160	7.03	4.14	1.46	1.26	0.98		
	180	11.15	4.87	1.53	1.32	0.99		
	200	13.05	5.21	1.49	1.54	0.98	15.31	48.79
1 % Nc-HPC	140	1.13	1.18	1.33	0.90	0.98		
	160	6.42	4.26	1.24	1.17	0.99		
	180	11.75	5.49	0.98	1.28	0.99		
	200	13.48	6.18	1.33	1.34	0.98	17.67	53.44
3 % Nc-HPC	140	2.37	2.57	1.29	1.24	0.99		
	160	8.79	6.41	1.67	1.33	0.98		
	180	12.07	8.69	1.74	1.38	0.98		
	200	14.48	9.31	1.69	1.41	0.98	19.21	68.74
5 % Nc-HPC	140	3.51		1.54	1.15	0.99		
	160	8.89		1.33	1.23	0.99		
	180	12.44		0.97	1.35	0.98		
	200	14.56		1.24	1.44	0.99	23.66	89.77
Table 5: Combination of the curing kinetic model parameters determined from the curve fits of AO/At versus 0 and the values of the n^-order and the autocatalytic mechanism activation energies for all of the samples at the analyzed temperatures for ^CaCO3
Tabela 5: Kombinacija parametrov modela kinetike uravnoteženja, določenih iz krivulj dO/dt proti 0 in vrednosti n-tega reda ter aktivacijske energije avtokatalitičnega mehanizma za temperature preizkušanja za mikrodelce CaCO3
Sample	Curing temperature (°C)	K1 (x10-4 s-1)	K2 (x10-2 s-1)	m	N	R2	E1/ (K J/mol)	£2/ (K J/mol)
Neat	140	3.42	1.13	1.17	1.23	0.99		
	160	7.12	6.42	1.46	1.26	0.98		
	180	8.85	11.75	1.53	1.32	0.99		
	200	9.76	13.48	1.49	1.54	0.98	14.12	39.24
1 % Nc-HPC	140	1.13	3.51	1.33	0.90	0.98		
	160	6.42	8.89	1.24	1.17	0.99		
	180	11.75	12.44	0.98	1.28	0.99		
	200	13.48	14.56	1.33	1.34	0.98	16.33	47.35
3 % Nc-HPC	140	1.04	1.23	1.29	1.24	0.99		
	160	6.41	7.03	1.67	1.33	0.98		
	180	8.69	11.15	1.74	1.38	0.98		
	200	9.31	13.05	1.69	1.41	0.98	17.42	63.57
5 % Nc-HPC	140	4.26	3.25	1.54	1.15	0.99		
	160	5.49	8.46	1.33	1.23	0.99		
	180	7.18	12.03	0.97	1.35	0.98		
	200	10.57	14.22	1.24	1.44	0.99	21.04	83.66
the activation energy, the gas constant and the absolute temperature, respectively. Commonly, the isothermal curing of the thermoset material may be the result of more than one type of chemical reaction23. This combination of reactions can be represented by the generalized expression given by Kamal and Sourour:
M dt
= A1 exp
-Ei
yRTy
(-0)n + A2 exp
-E
RT
j - 0)n 0m (5)
The first term in this model corresponds to an nth-order reaction and the second one is attributed to an autocatalytic reaction occurring during the curing reaction of the material under study24 25.
All the values were rounded up to two decimal places according to the error bands associated with 95 % confidence limits. A careful inspection of the calculated m and n values reveals that there is no trend for the systematic variation of either m or n with temperature during the isothermal curing of samples with the nCaCO3 loading. As already reported by many authors, this conclusion is expected to be reached theoretically, since m and n do not depend on the curing temperature and the nanofiller content.
In addition, the curing kinetic characterizations (Tables 2 to 5) show a direct proportionality between both the nth-order and the autocatalytic reaction-rate constants and also the value of the curing temperature. However, for all the samples, the K2 value was higher than Kj, which suggests the autocatalytic mechanism was more favorable than the other one. A variety of reasons might be given for the large K2 value, including the fact that the reaction mixture is very viscous. Indeed, because of the high viscosity, upon completing the initial uncatalyzed reaction, the reactants cannot move away;
they would rather sequester together. As a result, they are more prepared for subsequent catalyzed reactions. It is interesting to note that the increment of the nCaCO3 content increases the rate constant obtained for the systems, which seems to be connected with the higher SBS chains absorbed on the stearic treated nanofiller surface rather than the catalytic effect of the nCaCO3. In addition, the value of the n^-order and autocatalytic activation energies (E and E2) can be determined from the slope of the linear relationship between (K1) and (K) versus 1/T (plots not shown here). The numerical values calculated for the above-mentioned parameters are represented in Table 3 for all of the samples. Interestingly, both Ei and E2 are sharply decreased with the increment of the nCaCO3 content. This observation is consistent with the discussion previously mentioned, that the curing rate of SBS systems increases when raising the nCaCO3 content. Consequently, a lower amount of energy for curing, together with a shorter curing time for the nCaCO3-containing SBS samples over the neat blends, could offer the coating's formulators an excellent approach to meeting the requirements of today's challenges in the coating market. The curing results are shown in Tables 2 and 3.
3.2 Physical properties
3.2.1 Flame Retardancy (FR) and TGA analyses
The thermal decomposition was verified using 10 mg samples in an aluminum holder under a nitrogen or air flow (50 cm3 min1), heated from 25 °C to 600 °C at different heating rates of (5, 10, 20 and 40) °C min1.
The TGA curves and the differential (DTG) curves were measured with a Shimadzu TGA-50 thermo-
Figure 2: TGA curve for: a) SBS/5 % nCaCOs, b) SBS/5 % |CaCOs Slika 2: TGA-krivulja za: a) SBS/5 % nanodelcev CaCOs, b) SBS/5 % mikrodelcev CaCO3
gravimetric analyzer. The apparent activation energy as a function of the degree of decomposition in air and nitrogen atmospheres was calculated by the Ozawa method25.
According to Figures 2a and b, the SBS thermal profile was not affected by the nCaCO3 addition. In Figure 1b, the TGA curves show the degradation steps attributed to the components of the nCaCO3. The flammability values were 2.1 s/mm and 1.8 s/mm, respectively, for the 5 % nanosize CaCO3 and the commercial CaCO3. This means a reduction in the nanosize shows a better improvement in the FR. This might be due to the nano filler forming an effective layer on the surface, which absorbs the heat of burning (Figure 3).
The FR test was carried out in a flame tester (prolific make), as per ASTM-D 4804. The sample was clamped 85 mm above the horizontal screen so that it would not sag out to touch the screen. The free end is exposed to specify the gas flame for 30 s. The sample was clamped
at a 45° angle with the flame trip. The time required for burning and the relative rate of burning were measured.
3.2.2 Swelling Index and MFI
The swelling index (SI), an indirect way of measuring the total cross-link density, which in turn is correlated to the physical properties of the various vulcanized materials was determined by swelling a small piece of sample in the toluene for 24 h at room temperature:
X-Y
SI = — (6)
where X = the weight of the sample after swelling and Y = the weight of the sample before swelling.
There is an increment in SI for all the compositions up to 3 % additions of nano and commercial CaCO3 filler; the SI decreases for all cases (Table 6). At 5 % of filler loading, the SI is 1.43 and 2.44, respectively, for nanometer-size CaCO3 and the commercial CaCO3 filler. The swelling first increases up to 1 % of CaCO3 filled in SBS, and subsequently decreases with an increase in the amount of filler. This is due to greater cross-linking of the SBS, as a uniform dispersion of nano CaCO3 brings the chains closer and keeps them intact with nano-particles; even if the amount of nano addition is less. The MFI test for each sample in Figure 4 shows that the effect of the nano size CaCO3 filler has a better curing effect on the SBS matrix, because with a decrease of MFI, the matrix curing is high.
Table 6: Swelling Index for nCaCO3 and |iCaCO3
Tabela 6: Indeks nabrekanja nanodelcev CaCO3 in mikrodelcev
CaCO3
sample	kind	Initial mass (g)	Swollen mass (g)	Swelling Index
Neat		5	8.8	1.76
5 % Nc-HPC (Mic)	Nano	5	7.7	1.54
	Micron	5	12.55	2.51
5 % Nc-HPC (Nano)	Nano	5	7.15	1.43
	Micron	5	12.15	2.44
The swelling indexes of two kinds of CaCO3 composite are shown in Table 6. The results show that with an
Figure 3: Flame retardancy comparison between the micro and nano samples
Slika 3: Primerjava zadrževanja plamena med mikro- in nanovzorci
Figure 4: MFI Comparison between nCaCO3/SBS and |CaCO3/SBS Slika 4: MFI-primerjava med nanodelci CaCO3/SBS in mikrodelci CaCO3/SBS
Figure 5: Scanning electronic micrographs for: a) 5 % nCaCO3, b) SBS/5 % ^CaCO3
Slika 5: SEM-posnetek: a) 5 % nanodelcev CaCO3, b) SBS/5 % mi-krodelcev CaCO3
increase of mass fractions w(%) nano CaCO3, rather than the w(%) micro CaCO3, the swelling index is decreased. The decrease in the hardness with the reduction in the nano size is due to a greater and uniform dispersion of filler in the matrix, which brings the chains of the matrix closer to reduce the free volume to a greater extent in the cross-linking of chains.
3.3	Morphological characterization
The SEM micrographs, as shown in Figure 5, revealed that the nanoparticles were reasonably well dispersed in the SBS systems with 5 % nCaCO3 and ^CaCO3 loadings after the processing and dispersion steps.
3.4	Mechanical properties
3.4.1 Tensile properties
The tensile strength of the composites with a small particle size is higher than that of the others (Figure 6).
Figure 6: Mechanical properties comparison between nCaCO3/SBS and ^CaCO3/SBS: a) tensile strength, b) elongation at break Slika 6: Primerjava mehanskih lastnosti med nanodelci CaCO3/SBS in mikrodelci CaCO3/SBS: a) natezna trdnost, b) raztezek pri porušitvi
The tensile strength of 5 % nano CaCO3 (Figure 7) filled SBS (2.4 MPa for nCaCO3) is higher than the commercial CaCO3 (1.9 MPa for ^CaCO3). This means that the nano CaCO3 provides a higher tensile strength than the commercial CaCO3 filled SBS. This increment in tensile strength is due to the uniform dispersion of nano filler into the elastomer matrix that intercalates the elastomer matrix, and hence the degree of cross-linking of the elastomer chains increases. The elongation at the break decreases by an increase of mass fraction w(%) CaCO3, for which this parameter in ^CaCO3 is clearer than the nano size. This decrement is such that the degree of cross-linking of the elastomer chains increases.
3.4.2 Specific Gravity and Hardness
The compression-molded specimens were tested to provide hardness data by using a shore-A hardness tester, as per ASTM D 2240. An analytical balance equipped with a stationary support for an immersion vessel above or below the balance pan was used for the specific gravity measurement, as per ASTM D 792. The corrosion-resistant wire for suspending the specimen and the sinker for an analytical balance equipped with a stationary support for an immersion vessel above or below the balance pan was used for the specific gravity measurement, as per ASTM D 792. A corrosion-resistant wire for suspending the specimen and a sinker for the lighter specimen (specific gravity < 1) were employed. A beaker was used as an immersion vessel, a test specimen of convenient size was weighed in air, and then the specimen was suspended from a fine wire attached to a balance and completely immersed in distilled water. The weight of the specimen in water was determined (with a sinker):
Specific gravity =
(a + w) - b
(7)
where a is the weight of the specimen in air, b is the weight of the specimen (with a sinker) and the wire in
Figure 7: TEM image of as-received CaCO3 nanoparticles Slika 7: TEM-posnetek nanodelcev CaCO3 v dobavljenem stanju
a
Hardness —»—nano CaC03 - - micro CaCO^
12	3	4
%CaC03
Figure 8: Hardness comparison between the nano CaCO3 sample and the micro CaCO3 sample
Slika 8: Primerjava trdote vzorcev nanodelcev CaCO3 in mikrodelcev CaCO3
water, w is in this equation the weight of the totally immersed sinker and the partially immersed wire.
There is a continuous increment in the specific gravity for all the compositions in comparison to the pure SBS (Table 7). The increment in the specific gravity is more appreciable in the case of the nanometer CaCO3 (0.959 at 5 %) than for the commercial CaCO3 (0.948 at 5 %).
Table 7: Specific gravity testing Tabela 7: Specifična gostota
sample	Specific gravity nCaCO3, (g/cm3)	Specific gravity |iCaCO3, (g/cm3)
Neat	0.930	0.930
1 % Nc-HPC	0.943	0.936
3 % Nc-HPC	0.952	0.943
5 % Nc-HPC	0.959	0.948
The hardness of all the compositions increases with an increase in the amount of filler (0-5 %) in the case of the nano and commercial CaCO3 (Figure 8). The nanometer CaCO3 shows a higher value of the hardness than the commercial of CaCO3. The increase in the specific gravity and the hardness with a reduction in the nano size is due to a greater and more uniform dispersion of filler in the matrix, which brings the chains of matrix closer to reduce the free volume to a greater extent in the cross-linking of chains.
4 CONCLUSIONS
This study set out to investigate the effect of stearic-acid-coated CaCO3 nanoparticles and commercial CaCO3 on the physical characteristics and morphology, the curing behavior, and the mechanical properties of the SBS elastomer blend. The morphological studies were performed by XRD, TEM and SEM methods, and the results clearly indicated a suitable dispersion of nCaCO3 in the matrix. The nano CaCO3 shows a drastic improvement in the mechanical properties, the swelling index, the specific gravity, and the flame-retardancy indices than the commercial CaCO3-filled SBS. The reduction to the nanosize gives more enhancements in the properties
due to a uniform dispersion of nanoparticles into the elastomer matrix that interrelates the elastomer chains and increases the degree of cross-linking of the matrix.
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