The obvious polarity difference between the carbon black (CB) and the natural rubber (NR) causes the CB hard to be dispersed in the NR matrix when the addition amount is large. In this paper, polyethylene glycol (PEG) was grafted onto the surface of CB by the liquid phase. The grafted carbon black (GCB) was prepared and applied to reinforce NR. The main physical and mechanical properties of NR were improved because of the better compatibility between GCB and NR. The Mullins effect of the vulcanizate was calculated by the cyclic stress-strain experiment. The results showed that the Mullins effect both existed in the virgin NR system and filled NR system. The degree of Mullins effect was increased with the increase of the filler addition, but that was different for CB and GCB. When the filler addition was below 20 phr, the Mullins effect of NR/GCB was stronger than that of NR/CB. However, when the filler addition was over 30 phr, the Mullins effect of NR/CB was stronger than that of NR/GCB. The Mullins effect was affected by the heat treatment temperature and time. The mechanisms of the Mullins effect were analyzed.
Because the rubber materials have high elasticity, damping, and other excellent properties, they have been widely used in tires, electronics, military, aerospace, and other fields [
Chen [
The required stress at the same certain strain was decreased after cyclic tension treatment for several times in the vulcanized rubber composites, which was called the strain softening effect or Mullins effect [
Some explanatory models were proposed to explain the Mullins effect. Bueche [
Schematic diagrams of Mullins effect mechanisms.
Models | Schematic diagrams |
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Detachment of the molecular chains [ | |
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Slippage of the molecular chains [ | |
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Disentanglement of the rubber molecular chains [ | |
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Destruction of the filler aggregates [ | |
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Rubber shell model [ | |
In this work, in order to reduce the polarity difference between CB and NR and improve the dispersion of CB in the NR matrix, CB was graft modified with PEG and applied to reinforce NR. The comprehensive performance of NR has been increased with the addition of GCB. More importantly, the Mullins effect mechanisms were deduced by comparing the effects of GCB and CB on the Mullins effect of NR under different conditions.
Natural rubber (NR, type of 3L) was purchased from Guangzhou Beishite Company, China. Carbon black (CB, type of N330, the average diameter size of 25~30 nm, the specific surface area of 103 m2/g) was obtained from Cabot Corporation, USA. Concentrated nitric acid, toluene, thionyl chloride, dibutyl dilaurate, zinc oxide (ZnO), stearic acid (SA), and polyethylene glycol (PEG, number-average molecular weight of 400) were analytically pure and purchased from Sigma Aldrich Company, USA. Poly(1,2-dihydro-2,2,4-trimethyl-quinoline (antioxidant RD), 2,2′-dibenzothiazole disulfide (accelerator DM) and sulfur (S) were purchased directly from the market and used as received.
50 g of CB and 350 ml of concentrated nitric acid were added to a 1000 ml three-neck flask, the samples were reacted at 60°C for 2 h using the mechanical stirring, and then the reaction product was filtered, washed with deionized water, dried at 95°C to obtain the intermediate A. 50 g of the intermediate A, 12.5 g of thionyl chloride, and 350 ml of toluene were placed in a 1000 ml three-neck flask, and the samples were reacted at 0°C for 30 min and then at 120°C for 30 min using the mechanical stirring. The reaction product was rotary evaporated at 80°C to remove the unreacted thionyl chloride; the intermediate B was prepared. Subsequently, 50 g of the intermediate B, 25 g of PEG, 0.1 ml of dibutyl dilaurate, and 350 ml of toluene were added to a 1000 ml three-neck flask; the samples were reacted 0°C for 30 min and then at 120°C for 1 h using the mechanical stirring. The reaction product was filtered, washed with deionized water, dried at 95°C to obtain the grafted carbon black, denoted as GCB.
The roller gap of two-roll mill was adjusted to about 2 mm, NR was added and plasticated for 8 minutes, and then CB (or GCB), ZnO, SA, RD, DM, and S were added to the two-roll mill in turn, followed by 5 times side cuts on the left and right, and then a triangle package was implemented 7 times and thin-passing for 8 times. Subsequently, the compounds were stored for 24 h and were cured by a press vulcanizer at 150°C for (t90+2) min. The mass ratio of NR, ZnO, SA, RD, DM, and S was 100: 5: 2: 1: 1: 2. The filler contents were varied from 0 to 60 phr.
The thermogravimetric analyzer (TGA, 209 F3, Netzsch, Germany) was used to test the graft ratio of PEG on CB. The temperature range was 30~850°C, the heating rate was 50 K/min, and the test atmosphere was nitrogen. In order to remove the PEG adsorbed on the surface of CB through physical adsorption, GCB was purified by reflux condensation for 0.5 h with toluene and then dried.
The electronic tensile testing machine (GT-TCS-2000, Gaotie, China) was used to test the tensile and tear properties. The tensile strength was characterized according to GB/T 528-2009, the tensile speed was 200 mm/min, the thickness was 2.0 mm, and the sample shape was dumbbell-shaped. The tear strength was tested according to GB/T 529-2008; the sample shape was right angle. The Shore A hardness was determined according to GB/T 531-1999. The wear resistance was measured according to GB/T 9867-2008. The apparent crosslink density was tested by the swelling method. About 1 g of the vulcanizate was weighed and cut into small strips about 2 mm wide and 1 mm thick and the treated samples were stored in a container with toluene for 7 d at room temperature. After completing the swelling process, these strips were taken out, and then the toluene adsorbed on the surface of these strips was removed using the filter paper; subsequently, these strips were weighed. The crosslink density was calculated according to the Nory-Rehner formula [
The Mullins effect of the vulcanizate was calculated by the cyclic stress-strain experiment [
Due to the large difference in polarity between CB and NR, it is difficult for CB to be uniformly dispersed in the rubber matrix, especially when the amount of CB is large, thereby affecting the physical and mechanical properties of the rubber. The difference in polarity between CB and NR can be reduced by the graft modification of CB. Hence, the dispersibility of CB in the rubber matrix is improved significantly, so the physical and mechanical properties of the rubber are improved. The graft ratio of PEG on CB was measured through the TGA curves of CB and GCB (Figure
TGA analysis of carbon black before and after modification.
Figures
Physical and mechanical properties of NR/CB and NR/GCB.
For the NR/GCB composites, with the increase of the addition amount of GCB, the modulus at 300%, tensile strength, tear strength, and shore A hardness were increased gradually. Unlike the NR/CB composites, the optimum values of the modulus at 300%, tensile strength, tear strength, and shore A hardness were shown when the addition amount of GCB was 60 phr. In addition, the modulus at 300%, tensile strength, elongation at break, and tear strength of NR/GCB were always bigger than that of NR/CB at the same filler content. Compared with the NR/CB composites, the increase rates of the modulus at 300%, tensile strength, elongation at break, and tear strength of NR/GCB were 55.6%, 28.3%, 33.8%, and 42.6%, respectively, when the filler content was 50 phr. It indicated that the addition of GCB could greatly improve the comprehensive physical and mechanical properties of the rubber. Since GCB can be better dispersed in the rubber matrix, it is less likely to form filler aggregation, and the interaction between GCB and NR was stronger; thereby the physical and mechanical properties were improved.
The wear resistances of NR/CB and NR/GCB with the different filler content were shown in Figure
The apparent crosslink density is a representation of the degree of vulcanization, including the chemical crosslinks (polysulfidic, disulfidic, and monosulfidic), as well as the physical crosslinks (rubber-filler interaction and filler-filler network) [
Apparent crosslink densities of NR/CB and NR/GCB.
The effect of filler content on Mullins effect of NR/CB and NR/GCB composites was shown in Figure
Effect of filler content on Mullins effect of NR/CB and NR/GCB.
In addition, the degree of Mullins effect was increased as the increase of filler addition amount whatever CB or GCB was added to the rubber matrix, but the increase rate in the Mullins effect of adding CB or GCB was different. When the filler addition was below 20 phr, the Mullins effect of NR/GCB was stronger than that of NR/CB. This is because when the amount of filler is smaller than 20 phr, the filler particles are only dispersed in the rubber matrix in isolation and they are unlikely to form a filler-filler 3D network structure [
However, when the filler addition was over 30 phr, the Mullins effect of NR/CB was stronger than that of NR/GCB. Particularly, when the filler addition was 60 phr, the degree of Mullins effect of NR/GCB was only 47% of that of NR/CB. This is because when the amount of CB is over 30 phr, the filler-filler 3D network structures as a new factor affecting the Mullins effect began to appear, and when the amount of CB reached to 60 phr, these filler-filler 3D network structures were already intense. The filler-filler 3D network structure can be destroyed during the stretching process, thus leading to obvious Mullins effect [
Moreover, when the amount of CB was increased from 40 phr to 50 phr, the Mullins effect rose sharply from 33% to 60%. Some authors called this phenomenon a percolation transition of filler reinforcement [
Studying the changes in the Mullins effect during the storage is helpful for further understanding the mechanism of the Mullins effect. According to formula (
Effect of storage on Mullins effect of NR/CB and NR/GCB.
In addition, when the filler content is from 10 phr to 60 phr, with the increase of the filler content, the Mullins effect was increased gradually during the high temperature treatment process. But the values of
The phenomenological theory is a research method of rubber elasticity theory. The Mooney-Rivlin equation is one of the most commonly used expressions in the phenomenological theory [
In formula (
According to the formula (
Mooney curves of NR/CB and NR/GCB at different conditions.
The Mooney curve of the NR/CB sample being directly stretched to break (Figure
The trend of the Mooney curves of the NR/GCB samples under the three different stretching conditions (Figure
Combining the previous analyses and drawing on Roozbeh’s viewpoint [
Filler reinforcement and Mullins effect mechanisms.
(1) For the virgin polymer vulcanizate system with no filler, the strength of the vulcanizate is derived from the orientation and fracture of the polymer permanent crosslink network and the crystallization owing to the orientation of the polymer chains during the stretching process [
(2) For the polymer vulcanizate system with low filler content, the orientation and fracture of the polymer permanent crosslink network and the crystallization can result in the strength improvement; the filler-rubber interaction is another factor to lead to the strength improvement of the vulcanizate. The external force acting on the rubber matrix is transferred to the rigid filler through the filler-rubber bonding points, so the strength of the vulcanizate is improved. The filler-rubber bonding points are including the filler-rubber physical bonding points connected through the noncovalent bonds and filler-rubber chemical bonding points connected through the covalent bonds. The Mullins effect should be mainly attributed to the destruction of the filler-rubber physical bonding points and chemical bonding points and the slippage of the polymer chains on the surface of the fillers (Figure
(3) For the polymer vulcanizate system with high filler content, the more filler-rubber interaction points are formed because of the bigger filler content; thus the strength is improved. But when the addition amount of the filler is too big, the filler-filler 3D network structure will be formed in the rubber matrix, thus causing the filler agglomeration. The agglomeration of the filler in the rubber matrix causes the stress concentration, thereby resulting in a decrease in the tensile strength. This is why the tensile strength of the rubber is lowered when the amount of CB is increased from 50 phr to 60 phr. The Mullins effect at this time should be mainly ascribed to the destruction of the filler-filler 3D network structure during the stretching process (Figure
(1) About 20% polyethylene glycol of the total mass was grafted on the surface of the carbon black, thus the dispersion of the carbon black in the rubber matrix and the compatibility with the rubber matrix were improved, thereby the comprehensive performance being increased.
(2) The degree of Mullins effect was increased with the increase of filler. However, when the filler addition was below 20 phr, the Mullins effect of NR/GCB was stronger than that of NR/CB. When the filler addition was over 30 phr, the Mullins effect of NR/CB was stronger than that of NR/GCB. The degree of Mullins effect of NR/GCB was only 47% of that of NR/CB when the filler addition was 60 phr. The Mullins effect was increased gradually during the high temperature treatment process because of the re-entanglement of the rubber molecular chains, regeneration of the filler-rubber interaction and filler-filler 3D network structure.
(3) For the virgin polymer vulcanizate system with no filler, the Mullins effect should be mainly attributed to the disentanglement of the polymer chains during the stretching process. For the polymer vulcanizate system with low filler content, the Mullins effect should be mainly ascribed to the destruction of the filler-rubber interaction during the stretching process. For the polymer vulcanizate system with high filler content, the Mullins effect at this time should be mainly resulted from the destruction of the filler-filler 3D network structure during the stretching process.
The data used to support the findings of this study are included within the article.
The authors declare that they have no conflicts of interest regarding the publication of this paper.
This work was supported by the Natural Science Foundation of Guangdong Province (2017A030310663, 2018A030307018, and 2018A0303070003), the Foundation of Guangdong Province Rubber/Plastic Materials Preparation & Processing Engineering Technology Development Centre (2015B090903083), Distinguished Young Talents in Higher Education of Guangdong (517053), Maoming Science and Technology Project (517055), and Guangdong Research Center for Unconventional Energy Engineering Technology (GF2018A005).