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flame retardant properties
Journal Pre-proof Natural rubber composites reinforced with basic magnesium oxysulfate whiskers: Processing and ultraviolet resistance/flame retardant properties Xi Chen, Tian Qiu PII:
S0142-9418(19)31769-6
DOI:
https://doi.org/10.1016/j.polymertesting.2019.106271
Reference:
POTE 106271
To appear in:
Polymer Testing
Received Date: 30 September 2019 Revised Date:
13 November 2019
Accepted Date: 29 November 2019
Please cite this article as: X. Chen, T. Qiu, Natural rubber composites reinforced with basic magnesium oxysulfate whiskers: Processing and ultraviolet resistance/flame retardant properties, Polymer Testing (2020), doi: https://doi.org/10.1016/j.polymertesting.2019.106271. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Natural
rubber
composites
reinforced
with
Basic
Magnesium oxysulfate whiskers:Processing and ultraviolet resistance/flame retardant properties Xi Chen, 1,3* Tian Qiu2 1
College of Chemistry and Material science, Longyan University, Longyan, Fujian 364000,
China; 2
College of Physical Education, Longyan University, Longyan, Fujian 364000, China;
3 Fujian Provincial Colleges and Unversity Engineering Research Center of Soild Waste Resource Utilization, Longyan University, Longyan, Fujian 364000, China.
Abstract The Magnesium sulfate whiskers (MOSw) were first modified by Stearic acid or Si69, and Natural rubber(NR)/modified-MOSw composites were prepared by blending the modified-MOSw with natural rubber latex. By adding modified-MOSW into NR, the mechanical properties, the antiultraviolet aging property, flammability, and thermal stability of composites were improved obviously. The mechanical properties, crosslink density and thermal stability of composites reach the highest value at 4 wt% Si69-MOSW. The composite with MOSW addtion had a higher retention rate after ultraviolet irradiation and the MOSW could improve the anti-ultraviolet aging property of rubber matrix. The modified MOSW can effectively improve the oxygen index and the flame retardant grade of rubber composites.
Keywords Natural rubber; Magnesium oxysulfate whiskers; composites
1 Introduction Natural rubber latex (NRL) is a kind of macromolecule material cut from rubber trees and has been widely applied in different field. [1-3] As a natural environmental protection material, natural latex is widely used in various fields of production and life. Because of its superior
*
Corresponding author.
E-mail addresses:[email protected]
resiliency, larger elongation, permeability, it is mainly used to make pillow, mattress, medical catheter and mats gloves and condoms. [3-5] For many years, latex products have been confronted with the problems of bad tensile strength and no fire protection.[6] These all restrict the application of natural rubber latex. With the development of science and technology, people have put forward higher requirements for NRL products.[7] Especially in the military field, the military NRL products such as military latex gloves and military latex ground pads need high mechanical strength, good thermal stability and excellent flame retardancy. [8-10] Therefore, it is necessary to find new materials to improve these performance for NRL products. In recent years, magnesium oxysulfate whisker has been widely used in polymer fire-proof materials. Magnesium oxysulfate whisker has attracted more and more researchers'interest for their excellent reinforcement and fire resistance properties. [11-12] Magnesium oxysulfate whisker is a novel, one-dimensional structure, and exists in the form of needle-shaped or sector–shaped. [13] Magnesium oxysulfate whiskers are recognized for their superior properties,such as high strength and high modulus of rupture, nonflammability. Basic magnesium oxysulfate (5Mg(OH)2 MgSO43H2O, abbreviated as 513MOS) whiskers is one of the many types of magnesium oxysulfate whiskers. On the basis of the literature surveys, it is often used as reinforcing agent and fire retardant of polymer composites. [14] Up to date, MOSw has been widely used because of enhancement property and fireresisting property, especially in the field of plastic and ceramics. [15-18] For rubber field, there are few related researches on MOSw as additives for rubber composites. [19] Because the rubber mixing of the traditional dry rubber process destroy the structure of MOSw, and it prevents the application of MOSw in rubber field. In order to avoid the above phenomenon, we abandon traditional dry rubber process and adopt wet rubber latex technology to make NR/MOSw composites with high mechanical strength and good fire protecting performance. However, unmodified MOSw has high surface activity and strong hydrophilicity which causes its aggregation and nonuniform dispersion in the rubber matrix. [19] Thus, surface modification of pure whiskers must be carried out. In the first place, the pure MOSw were modified by Stearic acid and Si69, and then the Stearic acidMOSw and Si69-MOSw were made into the the Stearic dispersion liquid by ultrasonic dispersion method. Finally, these two dispersion liquid respectively were added into NRL matrix to manufacture NR/Stearic acid-MOSw composites and NR/Si69-MOSw composites. [20-21]
2 Experimental section 2.1 Chemicals and apparatus NRL with solid content of 61.7 wt% was obtained from Chinese Tropical Agriculture Academy (Hainan, China). The bis-(y-triethoxysilypropyl)-tetrasulifide (Si69) and Stearic acid were supplied by LongKom chem Co. Ltd (Guangzhou, China). MgSO4 whiskers (MOSW) with length-to-diameter ratio of 15-25 were supplied by Shanghai Muhong Industrial Co. Ltd (Shanghai, China). Thermo Nicolet IS 10 (Thermo Fisher Science, USA) was used for Fourier transform infrared spectroscopy (FTIR). X-ray diffraction (XRD) spectra were recorded Bruker D8 Advance (Bruker AXS, Germany) . Water contact angle was carried out on a SL-150E (Kenuo industries, America) . The scanning electron microscope (SEM) image were performed with a Hitachi S-3400N (Hitachi, Japan). The Instron IX3365 Universal materials testing machine (Instron, America) measures the mechanical properties of the samples. The oxygen indexes measurement was performed with an YG-813 oxygen indexes determinator (Changzhou Erfang Precision Machinery Co., Ltd, China). The flame retardant performance testing were tested through an M601 vertical inflaming retarding determinator (Changzhou Erfang Precision Machinery Co., Ltd, China). The characteristic decomposition temperature were recorded on Mettler-Toledd TGA/DSC. (Mettler-Toledd, Switzerland) 2.2 Preparation of Stearic acid-MOSW, Si69-MOSW, NR/Stearic acid-MOSW composites and NR/Si69-MOSW composites Firstly, stearic acid and Si69 respectively were dissolved in ethanol. At 80 °C, under highspeed stirring for 1h, Stearic acid-ethanol and Si69-ethanol respectively were added into 10 wt% unmodified MOSW aqueous solution. Then they are filtered and dried and the resulting finished product with Stearic acid-MOSW and Si69-MOSW were obtained. After that, At 70 °C, under intermediate-speed stirring, the Stearic acid-MOSW and Si69-MOSW ultrasonic aqueous dispersion respectively were added into NRL with different mixing ratios (NR/modified-MOSw = 99/1~6, w/w) to obtain uniform NRL/modified-MOSw. Respectively, film Stearic acid-MOSW/NRL and Si69-MOSW/NRL on a glass plate and dry them at 80 °C. Finally through drying, the composites were formed. (see Figure 1)
Figure 1. Preparation process of NR/Stearic acid-MOSW composites and NR/Si69-MOSW Composites 2.3 Characterization The modified whiskers of 15g was added into distilled water and fully stirred and mixed for 10min. After standing, they are filtered and dried. The formula for calculating activation index is as follows: [10]
The samples (1cm × 1cm× 2cm), was immersed in toluene for 48h and the solvent on the sample surface was wiped clean. These samples were then dried in an oven for 10 hours and weighed. The formula for crosslink density is as follows: [22-23]
,
3 Results and discussion 3.1 Effect of surface modifation on MOSW In order to reduce the surface energy of MOSW, we must treat the surface of MOSW particles. Surface modification of MOSW, was carried out with Si69 and stearic acid as modifiers. It is evident from Table 1 and Figure 1, pure MOSW is completely soluble in water and presents
hydrophobicity. With the increase of the amount of modifier, more and more modified whiskers float on the water. When the dosage of modifier is 4%, almost all whiskers float on the water.The modifiers dosages were more than 4%, and the activation index of Si69-MOSW and Stearic acidMOSW remain relatively unchanged. Therefore, the surfactant dosage of 4% was best for modifier dosage. (see Figure 2)
Figure 2. Activation index of MOSW Table 1. Activation index of MOSW The surfactant dosage (wt%)
0
2
3
4
5
Activation index of Stearic acid-MOSW (%)
3.7
39.7
83.4
97.5
97.3
Activation index of Si69-MOSW (%)
3.7
35.8
80.2
98.2
98.1
Figure 3. SEM and WCA images of Unmodified-MOSW (a), Stearic acid-MOSW(b) and Si69MOS The MOSW with a diameter of 0.3~1.0 and a length of 40~80 is showed in Figure 3. The morphology of MOSW were needle-like or fan-shaped crystal fiber. The MOSW before and after modification were examined by SEM to determine the change of dispersion. There was essentially no difference among Unmodified-MOSW, Stearic acid-MOSW, and Si69-MOSW from SEM examination. Figure 3 shows that the morphology of the modified MOSW has not changed significantly. The unmodified-MOSW appear aggregation to some extent, while Stearic acidMOSW and Si69-MOSW exhibited limited agglomeration, indicating better dispersion. We can also see from Figure 3a, the surfaces of Unmodified-MOSW is complete hydrophilicity. Can be
seen in Figure 3b and 3c, the WCA of Stearic acid-MOSW is 110.18 °C, more than 90 °C, and it is oleophobic property. Si69-MOSW has the greater WCA of 125 °C than Stearic acid-MOSW. At this time, Si69-MOSW shows excellent hydrophilic and hydrophobic properties.
Figure 4. X-ray diffraction (XRD) curves, of Unmodified-MOSW(a), Stearic acid-MOSW(b) and Si69-MOSW(c) Figure 4 shows XRD patterns of Unmodified-MOSW, Stearic acid- MOSW and Si69-MOSW. These three curves present the same diffraction peaks at 2ɵ around 13°, 17°, 23°, 30°, 34°, 40° and 46°, assigned to (201), (202), (203), (403), (601), (114) and (513) crystal faces, respectively. According to the peaks retrieved from JCPDS card No. 7-415, the samples are orthorhombic 5Mg(OH)2•MgSO4•3H2O. The result of XRD characterization indicate that the modified whiskers does not alter its crystal shape and have high quality single crystal structure. It maintains a high degree of crystallinity.
Figure 5. FTIR spectra of Unmodified-MOSW(a), Stearic acid-MOSW(b) and Si69-MOSW(c) Figure 5 shows the infrared spectra of Unmodified-MOSW, Stearic acid-MOSW and Si69MOSW. The characteristic band of (SO42-), surface hydroxyl (OH) and crystal water (H2O) at 2117cm-1 (SO4 2-), 1117 cm-1 (SO4 2-), 3549 cm-1 (OH), 3660 cm-1 (H2O), and 1622cm-1 (H2O) were found in the infrared spectra of Unmodified MOSW. (see figure 5a) Compared to pure MOSW, Stearic acid-MOSW and Si69-MOSW have developed new characteristic peaks of CH3 and CH2 at 2914 cm-1 (CH3) and 2842 cm-1 (CH2);(see figure 5b and 5c) In addition, comparing the the infrared spectra of Stearic acid-MOSW and Si69-MOSW, carboxyl groups were confirmed by the new appearance of weak characteristic peak at 2355 cm-1 since the C=O stretching vibration in figure 5c. 3.2 Mechanical properties, crosslink density and morphology of Pure NR and NR/MOSW composites Table 2. Mechanical properties and Crosslink density of Pure NR and NR/MOSW composites with different Contents Tensile strength/ MPa 16.81±0.06
Tear Strength/ (kN·m-1) 30.52±0.06
Modulus at 300%/ MPa 1.70±0.05
Elongation at break (%) 820±10
ν mol/
m 0 (g)
md (g)
3.829
0.3697
0.2495
NR/UnmodifiedMOSW(3wt%)
20.82±0.07
31.81±0.05
1.92±0.07
845±13
4.120
0.3851
0.2650
NR/Stearic acidMOSW(3wt%)
21.94±0.04
33.32±0.05
2.03±0.05
879±12
4.514
0.3998
0.2817
NR/Si69MOSW(3wt%)
24.45±0.05
35.93±0.04
2.12±0.03
903±15
6.308
0.4051
0.3098
Samples Pure NR
m3
NR/UnmodifiedMOSW(4wt%)
21.51±0.06
33.22±0.03
1.95±0.04
867±11
5.407
0.3658
0.2696
NR/Stearic acidMOSW(4wt%)
26.84±0.04
36.53±0.06
2.23±0.03
910±13
6.729
0.3566
0.2768
NR/Si69MOSW(4wt%)
28.91±0.06
38.44±0.05
2.34±0.06
924±13
8.460
0.3485
0.2845
NR/UnmodifiedMOSW(5wt%)
20.53±0.08
31.02±0.07
1.86±0.02
832±13
4.3
0.3701
0.2575
NR/Stearic acidMOSW(5wt%)
25.51±0.06
34.96±0.04
2.02±0.04
888±15
4.310
0.3670
0.2555
NR/Si69MOSW(5wt%)
26.91±0.04
37.25±0.03
2.23±0.03
918±10
5.933
0.3578
0.2697
Figure 6. SEM images of NR/Unmodified-MOSW composites (a), NR/Stearic acid-MOSW composites (b)and NR/Si69-MOSW composites (c) As seen from table 2 and figure 6a, the mechanical properties and crosslink density of NR/modified MOSW composites are better than those of NR/Unmodified-MOSW composites; The unmodified MOSW and rubber are completely incompatible, and there is a big gap between the two-phase. The unmodified MOSW can not allow full play to the function of reinforcing rubber. From table 2, we find that the mechanical properties and crosslink density of NR/Si69MOSW composites are superior to those of NR/Stearic acid-MOSW composites. This is principally because different methods of modifying MOSw. The MOSw modified with Stearic acid, belongs to physical adsorption, and the MOSw modified with Si69, belongs to chemisorption. The MOSw was modified with stearic acid. The hydrophilic group of stearic acid reacts with the hydroxyl group on the MOSw surface. The reaction mechanism between stearic acid and MOSw surface is as follows:
Compared to the unmodified MOSw, the compatibility between Stearic acid-MOSw and rubber matrix was improved obviously. But the stearic acid is easy to fall off the surface of MOSw. This leads to a little gap between the MOSw and the rubber matrix. From figure 6b, we can see that there is not perfect fusion between two-phase. It brings about reduce the mechanical properties and crosslink density of NR/Stearic acid-MOSW composites. The MOSw is modified using Si69 as coupling agent. The hydrophilic group of Si69 reacts with the hydroxyl group on the surface of MOSw. During coupling reaction, The X group is hydrolyzed into the silanol group, which reacts with the hydroxyl group on the surface of MOSw. This leads to the formation of-SiO-G (G denotes the surface of MOSW). At the same time, the silane alcohols of silane molecules bind to each other to form a network structure of the membrane. covering the MOSw surface, which makes the surface of the MOSw is organic.The membrane is coated around the surface of MOSw, which produces its surfcae organic. The reaction mechanism is as follows: [10]
The four sulfur atoms of Si69 are involved in the vulcanization of rubber
[2,7]
Moreover, the
polysulfide group in Si69 can crosslink with the rubber, which effectively improves the compatibility between Si69-MOSW and the rubbers matrix. This plays an important role in strengthening rubber matrix by whisker. From figure 6c, we can see that there is perfect fusion between the MOSw and the rubber matrix, and the interspace of two-phase is non-existent. The mechanical properties and crosslink density reach the maximum value at 4 wt% Si69-MOSW. Compared with pure NR, the tensile strength, tear strength, modulus at 300% ,elongation at break and crosslink density of the NR/Si69-MOSW(4 wt%) composites are increased by 72.0%, 25.9 %, 35.3%, 12.7%, 54.7%, respectively. When MOSW content is 5 wt%, the properties of the composites decreased in different degrees. This is mainly because a great quantity of MOSW tend to agglomerate in rubber. 3.3 Mechanical properities and Rate of change of Mechanical properities of pure NR and NR/MOSW composites after ultraviolet irradiation
Figure 7. Tensile strength (A), Tear strength (B), Modulus at 300% (C) and Elongation break (D) of pure NR and NR/MOSW composites after ultraviolet irradiation; Rate of change of Mechanical properities of pure NR and NR/MOSW composites (E). After 3 hour of ultraviolet light exposure, the mechanical properties of pure NR and NR/MOSW composites were tested. From Figure 7A, 7B, 7C, 7D, we can see that the mechanical properties of pure NR and NR/MOSW composites have obviously decreased after ultraviolet irradiation. As shown in Figure 7E, the change rate of mechanical properties of pure NR after ultraviolet irradiation is greater than thoseof NR/MOSW composites. The results showed that the
composite with MOSW addtion had a higher retention rate after ultraviolet irradiation and the MOSW could improve the anti-ultraviolet aging property of rubber matrix. 3.4 Flammability of pure NR and NR/MOSW composites Table 3. Oxygen index and the vertical burning of pure NR and MOSW/NR with different Contents
Samples
Oxygen index (LOI)
The vertical burning (UL-94)
Smoke situation
Molten drop fall
Pure NR
23.2
FV-1
Heavy smoke, Black
heavy
NR/Unmodifiedmuch smoke 24.9 FV-1 much MOSW(4wt%) Only a little somke NR/Stearic acid-MOSW(4wt%) 26.5 FV-0 barely no little somke NR/Si69-MOSW(4wt%) 28.7 FV-0 no The oxygen index and the vertical burning are often used to evaluate macroscopic combustion performance of composites. The combustion performance of pure NR and NR/MOSW composites are shown in Table 4. From table 4, the oxygen index of pure NR is the lowest. At this time, the combustion of pure rubber produces very black heavy smoke, accompanied by a large number of molten drop falling. Compared with pure rubber, the oxygen index of whiskers composites increased obviously, and the burning smoke and molten drop were obviously less. Especially NR/modified MOSW composites, not only the oxygen index of composites have been significantly improved and the vertical burning of composites have been raised from FV-1 to FV0 , but also composites are basically smoke-free and no molten drop in the combustion process. The MOSw molecule contains crystalline water, which is decomposed by heat and dehydrated when it burns. It absorbs a large amount of external thermal energy to lower the temperature of the rubber. The steam produced can not only dilute the concentration of the reaction gas in the flame zone, but also absorb the flue gas. MOSw has much larger heat capacity than NR/MOSw composites. Its decomposition temperature is high, and its thermal stability is good. It can play a flame-retardant effect. At the same time, MOSw can play a certain barrier inside the rubber matrix, which slows down and hinders the migration of small flammable molecules produced by the degradation of rubber molecules to the combustion interface during the combustion process. It delays the penetration of oxygen into the material, and the oxidation reaction at the combustion interface is difficult to fully carry out, thus playing a role in preventing
combustion. It also delays outside oxygen to infiltrate into the material and its oxidation reaction is difficult to put in effect on the combustion interface. As a result, the combustion of composites is prevented. In addition, the migration of MOSw to the combustion interface forms a protective carbon layer on the surface of the rubber, limiting the outward diffusion of the volatile decomposition products, isolating the internal flame and oxygen from the outside of the polymer to achieve the flame retardant effect. It insulate the interior of composites from external flame and oxygen, and it reaches the effect of flame retardancy. The results show that modified-MOSW is a good flame retardant for rubber composites. 3.5 Thermal stability of Pure NR and NR/MOSW composites
Figure 8. TG and DTG curves for Pure NR and NR/MOSW composites in nitrogen atmosphere
Samples
To/oC
Tp/oC
Tf/oC
Pure NR
357.5
379.6
406.6
NR/Unmodified-MOSW(4wt%)
359.0
381.9
411.5
NR/Stearic acid-MOSW(4wt%)
359.1
383.0
412.6
NR/Si69-MOSW(4wt%)
361.6
385.6
417.8
Table 4. Thermal degradation temperatures of composites prepared with Pure NR and NR/MOSW composites in nitrogen atmosphere Thermal stability is a very important property for NRL products. Materials with poor thermal stability are not suitable for use in NRL products. Figure 8, each TG curves has only one smooth step, and the corresponding DTG curves has only one degradation peak. Both Pure NR and composites have similar thermal degradation processes. By adding MOSW, the thermal stability of NR was improved obviously. Especially NR/Si69- MOSW(4wt%), the To, Tp, and Tf of NR/Si69-
MOSW composites markedly increase 4.1°C, 6°C, and 11.2°C, respectively, compared to NR (see Table 4).
4 Conclusions By adding modified-MOSW into NR, the mechanical properties, the anti-ultraviolet aging property, flammability, and thermal stability of composites were improved obviously. The mechanical properties, crosslink density and thermal stability of composites reach the highest value at 4 wt% Si69-MOSW. Compared with pure NR, the tensile strength, tear strength, modulus at 300%, elongation at break and crosslink density of the NR/Si69-MOSW(4 wt%) composites are increased by 72.0%, 25.9 %, 35.3%, 12.7%, 54.7%, respectively. The composites with MOSW addtion had a higher retention rate after ultraviolet irradiation and the MOSW could improve the anti-ultraviolet aging property of rubber matrix. The modified MOSW can effectively improve the oxygen index and the flame retardant grade of rubber composites.
Acknowledgements The authors gratefully acknowledge supports by the Science and Technology planning Project in Longyan city (2018LYF8008), Research Projects of Undergraduate Education and Teaching Reform in Fujian province (FBJG20190119) and National Innovation and Entrepreneurship Project for College Students (201911312011)
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Highlights 1.
The Magnesium sulfate whiskers (MOSw) were first modified by Stearic acid or Si69,
and Natural rubber(NR)/modified-MOSw composites were prepared by blending the modified-MOSw with natural rubber latex. 2.
By adding modified-MOSW into NR, the mechanical properties, the anti-ultraviolet
aging property, flammability, and thermal stability of composites were improved obviously. 3.
The composite with MOSW addtion had a higher retention rate after ultraviolet
irradiation and the MOSW could improve the anti-ultraviolet aging property of rubber matrix. 4.
The mechanical properties, crosslink density and thermal stability of composites reach
the highest value at 4 wt% Si69-MOSW. 5.
The modified MOSW can effectively improve the oxygen index and the flame retardant
grade of rubber composites.
Declaration of interest statement
We declare that we have no conflict of interest.
Xi Chen，Tian Qiu
S0142-9418(19)31769-6
DOI:
https://doi.org/10.1016/j.polymertesting.2019.106271
Reference:
POTE 106271
To appear in:
Polymer Testing
Received Date: 30 September 2019 Revised Date:
13 November 2019
Accepted Date: 29 November 2019
Please cite this article as: X. Chen, T. Qiu, Natural rubber composites reinforced with basic magnesium oxysulfate whiskers: Processing and ultraviolet resistance/flame retardant properties, Polymer Testing (2020), doi: https://doi.org/10.1016/j.polymertesting.2019.106271. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Natural
rubber
composites
reinforced
with
Basic
Magnesium oxysulfate whiskers:Processing and ultraviolet resistance/flame retardant properties Xi Chen, 1,3* Tian Qiu2 1
College of Chemistry and Material science, Longyan University, Longyan, Fujian 364000,
China; 2
College of Physical Education, Longyan University, Longyan, Fujian 364000, China;
3 Fujian Provincial Colleges and Unversity Engineering Research Center of Soild Waste Resource Utilization, Longyan University, Longyan, Fujian 364000, China.
Abstract The Magnesium sulfate whiskers (MOSw) were first modified by Stearic acid or Si69, and Natural rubber(NR)/modified-MOSw composites were prepared by blending the modified-MOSw with natural rubber latex. By adding modified-MOSW into NR, the mechanical properties, the antiultraviolet aging property, flammability, and thermal stability of composites were improved obviously. The mechanical properties, crosslink density and thermal stability of composites reach the highest value at 4 wt% Si69-MOSW. The composite with MOSW addtion had a higher retention rate after ultraviolet irradiation and the MOSW could improve the anti-ultraviolet aging property of rubber matrix. The modified MOSW can effectively improve the oxygen index and the flame retardant grade of rubber composites.
Keywords Natural rubber; Magnesium oxysulfate whiskers; composites
1 Introduction Natural rubber latex (NRL) is a kind of macromolecule material cut from rubber trees and has been widely applied in different field. [1-3] As a natural environmental protection material, natural latex is widely used in various fields of production and life. Because of its superior
*
Corresponding author.
E-mail addresses:[email protected]
resiliency, larger elongation, permeability, it is mainly used to make pillow, mattress, medical catheter and mats gloves and condoms. [3-5] For many years, latex products have been confronted with the problems of bad tensile strength and no fire protection.[6] These all restrict the application of natural rubber latex. With the development of science and technology, people have put forward higher requirements for NRL products.[7] Especially in the military field, the military NRL products such as military latex gloves and military latex ground pads need high mechanical strength, good thermal stability and excellent flame retardancy. [8-10] Therefore, it is necessary to find new materials to improve these performance for NRL products. In recent years, magnesium oxysulfate whisker has been widely used in polymer fire-proof materials. Magnesium oxysulfate whisker has attracted more and more researchers'interest for their excellent reinforcement and fire resistance properties. [11-12] Magnesium oxysulfate whisker is a novel, one-dimensional structure, and exists in the form of needle-shaped or sector–shaped. [13] Magnesium oxysulfate whiskers are recognized for their superior properties,such as high strength and high modulus of rupture, nonflammability. Basic magnesium oxysulfate (5Mg(OH)2 MgSO43H2O, abbreviated as 513MOS) whiskers is one of the many types of magnesium oxysulfate whiskers. On the basis of the literature surveys, it is often used as reinforcing agent and fire retardant of polymer composites. [14] Up to date, MOSw has been widely used because of enhancement property and fireresisting property, especially in the field of plastic and ceramics. [15-18] For rubber field, there are few related researches on MOSw as additives for rubber composites. [19] Because the rubber mixing of the traditional dry rubber process destroy the structure of MOSw, and it prevents the application of MOSw in rubber field. In order to avoid the above phenomenon, we abandon traditional dry rubber process and adopt wet rubber latex technology to make NR/MOSw composites with high mechanical strength and good fire protecting performance. However, unmodified MOSw has high surface activity and strong hydrophilicity which causes its aggregation and nonuniform dispersion in the rubber matrix. [19] Thus, surface modification of pure whiskers must be carried out. In the first place, the pure MOSw were modified by Stearic acid and Si69, and then the Stearic acidMOSw and Si69-MOSw were made into the the Stearic dispersion liquid by ultrasonic dispersion method. Finally, these two dispersion liquid respectively were added into NRL matrix to manufacture NR/Stearic acid-MOSw composites and NR/Si69-MOSw composites. [20-21]
2 Experimental section 2.1 Chemicals and apparatus NRL with solid content of 61.7 wt% was obtained from Chinese Tropical Agriculture Academy (Hainan, China). The bis-(y-triethoxysilypropyl)-tetrasulifide (Si69) and Stearic acid were supplied by LongKom chem Co. Ltd (Guangzhou, China). MgSO4 whiskers (MOSW) with length-to-diameter ratio of 15-25 were supplied by Shanghai Muhong Industrial Co. Ltd (Shanghai, China). Thermo Nicolet IS 10 (Thermo Fisher Science, USA) was used for Fourier transform infrared spectroscopy (FTIR). X-ray diffraction (XRD) spectra were recorded Bruker D8 Advance (Bruker AXS, Germany) . Water contact angle was carried out on a SL-150E (Kenuo industries, America) . The scanning electron microscope (SEM) image were performed with a Hitachi S-3400N (Hitachi, Japan). The Instron IX3365 Universal materials testing machine (Instron, America) measures the mechanical properties of the samples. The oxygen indexes measurement was performed with an YG-813 oxygen indexes determinator (Changzhou Erfang Precision Machinery Co., Ltd, China). The flame retardant performance testing were tested through an M601 vertical inflaming retarding determinator (Changzhou Erfang Precision Machinery Co., Ltd, China). The characteristic decomposition temperature were recorded on Mettler-Toledd TGA/DSC. (Mettler-Toledd, Switzerland) 2.2 Preparation of Stearic acid-MOSW, Si69-MOSW, NR/Stearic acid-MOSW composites and NR/Si69-MOSW composites Firstly, stearic acid and Si69 respectively were dissolved in ethanol. At 80 °C, under highspeed stirring for 1h, Stearic acid-ethanol and Si69-ethanol respectively were added into 10 wt% unmodified MOSW aqueous solution. Then they are filtered and dried and the resulting finished product with Stearic acid-MOSW and Si69-MOSW were obtained. After that, At 70 °C, under intermediate-speed stirring, the Stearic acid-MOSW and Si69-MOSW ultrasonic aqueous dispersion respectively were added into NRL with different mixing ratios (NR/modified-MOSw = 99/1~6, w/w) to obtain uniform NRL/modified-MOSw. Respectively, film Stearic acid-MOSW/NRL and Si69-MOSW/NRL on a glass plate and dry them at 80 °C. Finally through drying, the composites were formed. (see Figure 1)
Figure 1. Preparation process of NR/Stearic acid-MOSW composites and NR/Si69-MOSW Composites 2.3 Characterization The modified whiskers of 15g was added into distilled water and fully stirred and mixed for 10min. After standing, they are filtered and dried. The formula for calculating activation index is as follows: [10]
The samples (1cm × 1cm× 2cm), was immersed in toluene for 48h and the solvent on the sample surface was wiped clean. These samples were then dried in an oven for 10 hours and weighed. The formula for crosslink density is as follows: [22-23]
,
3 Results and discussion 3.1 Effect of surface modifation on MOSW In order to reduce the surface energy of MOSW, we must treat the surface of MOSW particles. Surface modification of MOSW, was carried out with Si69 and stearic acid as modifiers. It is evident from Table 1 and Figure 1, pure MOSW is completely soluble in water and presents
hydrophobicity. With the increase of the amount of modifier, more and more modified whiskers float on the water. When the dosage of modifier is 4%, almost all whiskers float on the water.The modifiers dosages were more than 4%, and the activation index of Si69-MOSW and Stearic acidMOSW remain relatively unchanged. Therefore, the surfactant dosage of 4% was best for modifier dosage. (see Figure 2)
Figure 2. Activation index of MOSW Table 1. Activation index of MOSW The surfactant dosage (wt%)
0
2
3
4
5
Activation index of Stearic acid-MOSW (%)
3.7
39.7
83.4
97.5
97.3
Activation index of Si69-MOSW (%)
3.7
35.8
80.2
98.2
98.1
Figure 3. SEM and WCA images of Unmodified-MOSW (a), Stearic acid-MOSW(b) and Si69MOS The MOSW with a diameter of 0.3~1.0 and a length of 40~80 is showed in Figure 3. The morphology of MOSW were needle-like or fan-shaped crystal fiber. The MOSW before and after modification were examined by SEM to determine the change of dispersion. There was essentially no difference among Unmodified-MOSW, Stearic acid-MOSW, and Si69-MOSW from SEM examination. Figure 3 shows that the morphology of the modified MOSW has not changed significantly. The unmodified-MOSW appear aggregation to some extent, while Stearic acidMOSW and Si69-MOSW exhibited limited agglomeration, indicating better dispersion. We can also see from Figure 3a, the surfaces of Unmodified-MOSW is complete hydrophilicity. Can be
seen in Figure 3b and 3c, the WCA of Stearic acid-MOSW is 110.18 °C, more than 90 °C, and it is oleophobic property. Si69-MOSW has the greater WCA of 125 °C than Stearic acid-MOSW. At this time, Si69-MOSW shows excellent hydrophilic and hydrophobic properties.
Figure 4. X-ray diffraction (XRD) curves, of Unmodified-MOSW(a), Stearic acid-MOSW(b) and Si69-MOSW(c) Figure 4 shows XRD patterns of Unmodified-MOSW, Stearic acid- MOSW and Si69-MOSW. These three curves present the same diffraction peaks at 2ɵ around 13°, 17°, 23°, 30°, 34°, 40° and 46°, assigned to (201), (202), (203), (403), (601), (114) and (513) crystal faces, respectively. According to the peaks retrieved from JCPDS card No. 7-415, the samples are orthorhombic 5Mg(OH)2•MgSO4•3H2O. The result of XRD characterization indicate that the modified whiskers does not alter its crystal shape and have high quality single crystal structure. It maintains a high degree of crystallinity.
Figure 5. FTIR spectra of Unmodified-MOSW(a), Stearic acid-MOSW(b) and Si69-MOSW(c) Figure 5 shows the infrared spectra of Unmodified-MOSW, Stearic acid-MOSW and Si69MOSW. The characteristic band of (SO42-), surface hydroxyl (OH) and crystal water (H2O) at 2117cm-1 (SO4 2-), 1117 cm-1 (SO4 2-), 3549 cm-1 (OH), 3660 cm-1 (H2O), and 1622cm-1 (H2O) were found in the infrared spectra of Unmodified MOSW. (see figure 5a) Compared to pure MOSW, Stearic acid-MOSW and Si69-MOSW have developed new characteristic peaks of CH3 and CH2 at 2914 cm-1 (CH3) and 2842 cm-1 (CH2);(see figure 5b and 5c) In addition, comparing the the infrared spectra of Stearic acid-MOSW and Si69-MOSW, carboxyl groups were confirmed by the new appearance of weak characteristic peak at 2355 cm-1 since the C=O stretching vibration in figure 5c. 3.2 Mechanical properties, crosslink density and morphology of Pure NR and NR/MOSW composites Table 2. Mechanical properties and Crosslink density of Pure NR and NR/MOSW composites with different Contents Tensile strength/ MPa 16.81±0.06
Tear Strength/ (kN·m-1) 30.52±0.06
Modulus at 300%/ MPa 1.70±0.05
Elongation at break (%) 820±10
ν mol/
m 0 (g)
md (g)
3.829
0.3697
0.2495
NR/UnmodifiedMOSW(3wt%)
20.82±0.07
31.81±0.05
1.92±0.07
845±13
4.120
0.3851
0.2650
NR/Stearic acidMOSW(3wt%)
21.94±0.04
33.32±0.05
2.03±0.05
879±12
4.514
0.3998
0.2817
NR/Si69MOSW(3wt%)
24.45±0.05
35.93±0.04
2.12±0.03
903±15
6.308
0.4051
0.3098
Samples Pure NR
m3
NR/UnmodifiedMOSW(4wt%)
21.51±0.06
33.22±0.03
1.95±0.04
867±11
5.407
0.3658
0.2696
NR/Stearic acidMOSW(4wt%)
26.84±0.04
36.53±0.06
2.23±0.03
910±13
6.729
0.3566
0.2768
NR/Si69MOSW(4wt%)
28.91±0.06
38.44±0.05
2.34±0.06
924±13
8.460
0.3485
0.2845
NR/UnmodifiedMOSW(5wt%)
20.53±0.08
31.02±0.07
1.86±0.02
832±13
4.3
0.3701
0.2575
NR/Stearic acidMOSW(5wt%)
25.51±0.06
34.96±0.04
2.02±0.04
888±15
4.310
0.3670
0.2555
NR/Si69MOSW(5wt%)
26.91±0.04
37.25±0.03
2.23±0.03
918±10
5.933
0.3578
0.2697
Figure 6. SEM images of NR/Unmodified-MOSW composites (a), NR/Stearic acid-MOSW composites (b)and NR/Si69-MOSW composites (c) As seen from table 2 and figure 6a, the mechanical properties and crosslink density of NR/modified MOSW composites are better than those of NR/Unmodified-MOSW composites; The unmodified MOSW and rubber are completely incompatible, and there is a big gap between the two-phase. The unmodified MOSW can not allow full play to the function of reinforcing rubber. From table 2, we find that the mechanical properties and crosslink density of NR/Si69MOSW composites are superior to those of NR/Stearic acid-MOSW composites. This is principally because different methods of modifying MOSw. The MOSw modified with Stearic acid, belongs to physical adsorption, and the MOSw modified with Si69, belongs to chemisorption. The MOSw was modified with stearic acid. The hydrophilic group of stearic acid reacts with the hydroxyl group on the MOSw surface. The reaction mechanism between stearic acid and MOSw surface is as follows:
Compared to the unmodified MOSw, the compatibility between Stearic acid-MOSw and rubber matrix was improved obviously. But the stearic acid is easy to fall off the surface of MOSw. This leads to a little gap between the MOSw and the rubber matrix. From figure 6b, we can see that there is not perfect fusion between two-phase. It brings about reduce the mechanical properties and crosslink density of NR/Stearic acid-MOSW composites. The MOSw is modified using Si69 as coupling agent. The hydrophilic group of Si69 reacts with the hydroxyl group on the surface of MOSw. During coupling reaction, The X group is hydrolyzed into the silanol group, which reacts with the hydroxyl group on the surface of MOSw. This leads to the formation of-SiO-G (G denotes the surface of MOSW). At the same time, the silane alcohols of silane molecules bind to each other to form a network structure of the membrane. covering the MOSw surface, which makes the surface of the MOSw is organic.The membrane is coated around the surface of MOSw, which produces its surfcae organic. The reaction mechanism is as follows: [10]
The four sulfur atoms of Si69 are involved in the vulcanization of rubber
[2,7]
Moreover, the
polysulfide group in Si69 can crosslink with the rubber, which effectively improves the compatibility between Si69-MOSW and the rubbers matrix. This plays an important role in strengthening rubber matrix by whisker. From figure 6c, we can see that there is perfect fusion between the MOSw and the rubber matrix, and the interspace of two-phase is non-existent. The mechanical properties and crosslink density reach the maximum value at 4 wt% Si69-MOSW. Compared with pure NR, the tensile strength, tear strength, modulus at 300% ,elongation at break and crosslink density of the NR/Si69-MOSW(4 wt%) composites are increased by 72.0%, 25.9 %, 35.3%, 12.7%, 54.7%, respectively. When MOSW content is 5 wt%, the properties of the composites decreased in different degrees. This is mainly because a great quantity of MOSW tend to agglomerate in rubber. 3.3 Mechanical properities and Rate of change of Mechanical properities of pure NR and NR/MOSW composites after ultraviolet irradiation
Figure 7. Tensile strength (A), Tear strength (B), Modulus at 300% (C) and Elongation break (D) of pure NR and NR/MOSW composites after ultraviolet irradiation; Rate of change of Mechanical properities of pure NR and NR/MOSW composites (E). After 3 hour of ultraviolet light exposure, the mechanical properties of pure NR and NR/MOSW composites were tested. From Figure 7A, 7B, 7C, 7D, we can see that the mechanical properties of pure NR and NR/MOSW composites have obviously decreased after ultraviolet irradiation. As shown in Figure 7E, the change rate of mechanical properties of pure NR after ultraviolet irradiation is greater than thoseof NR/MOSW composites. The results showed that the
composite with MOSW addtion had a higher retention rate after ultraviolet irradiation and the MOSW could improve the anti-ultraviolet aging property of rubber matrix. 3.4 Flammability of pure NR and NR/MOSW composites Table 3. Oxygen index and the vertical burning of pure NR and MOSW/NR with different Contents
Samples
Oxygen index (LOI)
The vertical burning (UL-94)
Smoke situation
Molten drop fall
Pure NR
23.2
FV-1
Heavy smoke, Black
heavy
NR/Unmodifiedmuch smoke 24.9 FV-1 much MOSW(4wt%) Only a little somke NR/Stearic acid-MOSW(4wt%) 26.5 FV-0 barely no little somke NR/Si69-MOSW(4wt%) 28.7 FV-0 no The oxygen index and the vertical burning are often used to evaluate macroscopic combustion performance of composites. The combustion performance of pure NR and NR/MOSW composites are shown in Table 4. From table 4, the oxygen index of pure NR is the lowest. At this time, the combustion of pure rubber produces very black heavy smoke, accompanied by a large number of molten drop falling. Compared with pure rubber, the oxygen index of whiskers composites increased obviously, and the burning smoke and molten drop were obviously less. Especially NR/modified MOSW composites, not only the oxygen index of composites have been significantly improved and the vertical burning of composites have been raised from FV-1 to FV0 , but also composites are basically smoke-free and no molten drop in the combustion process. The MOSw molecule contains crystalline water, which is decomposed by heat and dehydrated when it burns. It absorbs a large amount of external thermal energy to lower the temperature of the rubber. The steam produced can not only dilute the concentration of the reaction gas in the flame zone, but also absorb the flue gas. MOSw has much larger heat capacity than NR/MOSw composites. Its decomposition temperature is high, and its thermal stability is good. It can play a flame-retardant effect. At the same time, MOSw can play a certain barrier inside the rubber matrix, which slows down and hinders the migration of small flammable molecules produced by the degradation of rubber molecules to the combustion interface during the combustion process. It delays the penetration of oxygen into the material, and the oxidation reaction at the combustion interface is difficult to fully carry out, thus playing a role in preventing
combustion. It also delays outside oxygen to infiltrate into the material and its oxidation reaction is difficult to put in effect on the combustion interface. As a result, the combustion of composites is prevented. In addition, the migration of MOSw to the combustion interface forms a protective carbon layer on the surface of the rubber, limiting the outward diffusion of the volatile decomposition products, isolating the internal flame and oxygen from the outside of the polymer to achieve the flame retardant effect. It insulate the interior of composites from external flame and oxygen, and it reaches the effect of flame retardancy. The results show that modified-MOSW is a good flame retardant for rubber composites. 3.5 Thermal stability of Pure NR and NR/MOSW composites
Figure 8. TG and DTG curves for Pure NR and NR/MOSW composites in nitrogen atmosphere
Samples
To/oC
Tp/oC
Tf/oC
Pure NR
357.5
379.6
406.6
NR/Unmodified-MOSW(4wt%)
359.0
381.9
411.5
NR/Stearic acid-MOSW(4wt%)
359.1
383.0
412.6
NR/Si69-MOSW(4wt%)
361.6
385.6
417.8
Table 4. Thermal degradation temperatures of composites prepared with Pure NR and NR/MOSW composites in nitrogen atmosphere Thermal stability is a very important property for NRL products. Materials with poor thermal stability are not suitable for use in NRL products. Figure 8, each TG curves has only one smooth step, and the corresponding DTG curves has only one degradation peak. Both Pure NR and composites have similar thermal degradation processes. By adding MOSW, the thermal stability of NR was improved obviously. Especially NR/Si69- MOSW(4wt%), the To, Tp, and Tf of NR/Si69-
MOSW composites markedly increase 4.1°C, 6°C, and 11.2°C, respectively, compared to NR (see Table 4).
4 Conclusions By adding modified-MOSW into NR, the mechanical properties, the anti-ultraviolet aging property, flammability, and thermal stability of composites were improved obviously. The mechanical properties, crosslink density and thermal stability of composites reach the highest value at 4 wt% Si69-MOSW. Compared with pure NR, the tensile strength, tear strength, modulus at 300%, elongation at break and crosslink density of the NR/Si69-MOSW(4 wt%) composites are increased by 72.0%, 25.9 %, 35.3%, 12.7%, 54.7%, respectively. The composites with MOSW addtion had a higher retention rate after ultraviolet irradiation and the MOSW could improve the anti-ultraviolet aging property of rubber matrix. The modified MOSW can effectively improve the oxygen index and the flame retardant grade of rubber composites.
Acknowledgements The authors gratefully acknowledge supports by the Science and Technology planning Project in Longyan city (2018LYF8008), Research Projects of Undergraduate Education and Teaching Reform in Fujian province (FBJG20190119) and National Innovation and Entrepreneurship Project for College Students (201911312011)
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Highlights 1.
The Magnesium sulfate whiskers (MOSw) were first modified by Stearic acid or Si69,
and Natural rubber(NR)/modified-MOSw composites were prepared by blending the modified-MOSw with natural rubber latex. 2.
By adding modified-MOSW into NR, the mechanical properties, the anti-ultraviolet
aging property, flammability, and thermal stability of composites were improved obviously. 3.
The composite with MOSW addtion had a higher retention rate after ultraviolet
irradiation and the MOSW could improve the anti-ultraviolet aging property of rubber matrix. 4.
The mechanical properties, crosslink density and thermal stability of composites reach
the highest value at 4 wt% Si69-MOSW. 5.
The modified MOSW can effectively improve the oxygen index and the flame retardant
grade of rubber composites.
Declaration of interest statement
We declare that we have no conflict of interest.
Xi Chen，Tian Qiu