How to physically modify the surface of silica with natural astaxanthin?

 As we all know, silica is a kind of filler and reinforcing agent in the rubber field, which is second only to carbon black in terms of quantity and reinforcing effect. With the global energy crisis and people's attention to environmental protection, the research and development of green tires have become the focus of the tire industry[1-2] . Compared with carbon black, silica-filled and reinforced tread rubber has the advantages of low hysteresis loss, low rolling resistance and high wet grip[3] . However, due to the high specific surface area and activity of silica, it is easy to agglomerate in the rubber matrix, difficult to be dispersed, and secondary agglomeration will occur during the mixing process, which ultimately affects the application performance of the product. In addition, due to the influence of silanol hydroxyl groups on its own surface, it shows a certain acidity and is easy to adsorb the accelerator in the rubber mixing process, which greatly affects the efficiency and cost of tire manufacturing.

 

Therefore, how to improve the compatibility between silica and polymers and the dispersing ability of silica particles in rubber has been a hot research topic in the industry[4] . Among them, there are numerous reports on the surface modification of silica by coupling agents to improve its reinforcing effect on tread rubber[5-7] . However, these synthetic small molecule modifiers have some drawbacks, such as poor processability and storage stability, and the release of toxic substances that can pollute the environment [8-10]. The modification of silica with green natural substances and the investigation of the dispersion of modified silica in the rubber matrix and its effect on the rubber properties have not been reported yet.

 

Natural astaxanthin is a non-vitamin A ketocarotenoid[11] , which is widely found in algae such as Rhodococcus pyrenoidus and in the feathers of aquatic animals such as shrimps, crabs and salmon, as well as in birds[12] . The natural astaxanthin molecule consists of four isoprene double bonds connected at the beginning and the end, with a total of 11 conjugated double bonds [13-14]. There are unsaturated ketone and hydroxyl groups at the end of the chain of conjugated double bonds, in which the hydroxyl group and the ketone group constitute α-hydroxy ketone. This unique structural feature makes it have a more active electronic effect. It can scavenge free radicals by supplying electrons to the free radicals or by attracting unpaired electrons of the free radicals [15], which has a powerful antioxidant effect. [This unique structural feature gives it a more active electronic effect, which can provide electrons to free radicals or attract unpaired electrons from free radicals, thus scavenging free radicals. Currently, natural astaxanthin is used as an excellent antioxidant[16] and coloring agent[17] .

Astaxanthin is mainly used in pharmaceuticals [19-21], cosmetics [22], food [23-24], beverages [25], health care products [26] and aquaculture [27], but its application in rubber has not been reported. In this paper, we investigated the physical modification of silica surface with natural astaxanthin as a macromolecular surface modifier, and studied its effect on the properties of natural rubber (NR).

 

1 Experimental component

1.1 Main raw materials

Natural astaxanthin, Xi'an Lvquan Technology Co., Ltd; natural rubber (SMR10), Qingdao Banghao International Trading Co., Ltd; precipitated silica (1165MP), Rhodia Silica (Qingdao) Co., Ltd; silane coupling agent Si-69, Qingdao Degussa Chemical Co. Ltd.; Ethyl acetate, analytically pure, Tianjin Bodi Chemical Industry Co.

 

1.2 Major equipment and instruments

XSM-500 Rubber and Plastic Compacting Machine, Shanghai Kechuang Rubber and Plastic Machinery and Equipment Co., Ltd; BL-6175 Double Roller Openers, Baolun Precision Testing Instruments Co. GT-RH-2000 Compression Heating Machine, product of Taiwan High Speed Rail Testing Instrument Co., Ltd; HS-100T-FTMO-2RT Flat Plate Vulcanizer, product of Jiaxin Electronic Equipment Technology (Shenzhen) Co; DMA242 Dynamic Mechanical Analyzer (DMA), Germany NETZSCH company; JSM-7500F type scanning electron microscope (SEM), Japan JEOL company; RLH-225 thermal aging test chamber, Sunan ring test power equipment Co.

 

1.3 Experimental formulations

SMR10, 100 (mass fraction, below); 1165MP, 50; stearic acid (SA), 2; ZnO, 5; accelerator NS, 1.5; S, 2.2; Si-69, 4; natural astaxanthin, variable (0, 0.5, 1).

 

1.4 Silica modification

Accurately weighed 0, 0.5 and 1 part of natural astaxanthin into three three flasks, and then measured 300mL of acetate solution into three flasks, protected by nitrogen, 25 ℃ constant temperature water bath mechanical stirring for 2h, and then add the above formula calculated in accordance with the above silica, continue to protect the nitrogen, warming up to 45 ℃ and continue to mechanical stirring after 1h and then stop, and poured into the petri dish. After warming up to 45℃, continue mechanical stirring for 1h and then stop, pour into petri dish, and then dry in 30℃ vacuum drying oven until constant weight. The silica not modified with natural astaxanthin was used as the control group, labeled as 1#, and the silica modified with natural astaxanthin was used as the experimental group, labeled as 2# and 3#, respectively.

 

1.5 Sample Preparation

The preparation of the rubber is divided into two mixing stages. The first stage of mixing is carried out in the compactor, the initial temperature is set at 90℃, the rotational speed is 70r/min, firstly, the molded natural rubber is added, after 2min of mixing, the Si-69 non-natural astaxanthin-modified silica/natural astaxanthin-modified silica is added, and then the mixing is carried out for 4min, and finally, the small materials other than sulfur are added, and the mixing is carried out for 3min, and then the rubber is discharged. The second mixing is carried out in the opener, set the roll pitch to 1mm, roll temperature to 50℃, roll speed to 20r/min, the rubber after mixing is wrapped in the opener, after wrapping the roll, add sulfur yellow, and then use the "left and right three knives" method to make its mixing uniform, adjust the roll pitch to 0.2mm and then thin pass to hit the triangle package for 6 times, then adjust the roll pitch to 1.6mm to exhaust the air. Adjust the roller pitch to 1.6mm exhaust under the piece, under the piece of time is about 1min. Mixed rubber in the plate vulcanizing machine vulcanization, vulcanization conditions for 150 ℃ / 10MPa × t90.

 

1.6 Performance Characterization

(1) Vulcanization characteristics Vulcanization characteristics are tested according to GB/T 16584-1996 "Determination of Vulcanization Characteristics of Rubber by Non-Rotor Vulcanizer".

(2) Mechanical properties Tensile properties shall be tested on electronic tensile machine according to GB/T 528-2009 "Determination of tensile stress-strain properties of vulcanized rubber or thermoplastic rubber"; tear strength shall be tested on right-angle shaped specimens, and shall be tested on electronic tensile machine according to GB/T 529-2008 "Determination of tearing strength of vulcanized rubber or thermoplastic rubber (trouser, right-angle and crescent-shaped specimens)". The tearing strength is tested on electronic tensile machine according to GB/T 529-2008 "Determination of tearing strength of vulcanized rubber or thermoplastic rubber (trouser, right-angle and crescent-shaped specimens)".

(3) Abrasion resistance DIN abrasion amount according to GB/T 9867-2008 "Rotary roller abrasion machine method" in the GT-7012-D type abrasion machine test.

(4) Hot air aging performance Hot air aging test according to GB/T 3512-2001, the test conditions are at 100 ℃ specimens were aging 24h and 48h, to reach the specified time after taking out the specimen to stop overnight after the test.

(5) Compression heat generation test The compression heat generation performance is tested on the compression heat generation tester according to GB/T 1687-1993. The test conditions are: stroke 4.45mm, load 1.0MPa, compression frequency 30Hz, temperature 55℃.

(6) RPA test The rubber processing analyzer was used to carry out strain scanning test on the compound and vulcanized rubber in shear mode. The test conditions are temperature 60℃, frequency 1Hz, strain range 0.2%~100%.

(7) DMA analysis The dynamic mechanical property analysis of vulcanized rubber was carried out on DMA242 produced by NETZSCH Company in Germany, under the following conditions: stretching mode, frequency 10Hz, temperature range -80~100℃, and warming rate 3℃/min.

(8) SEM Characterization of Tensile Sections The surface structure of the samples was characterized by a JSM-7500F field emission scanning electron microscope manufactured by Nippon Electronics Corporation, and the samples were sprayed with gold before testing.

 

2 Results and Discussion

2.1 Vulcanization characteristics

The effects of natural astaxanthin-modified silica on the vulcanization properties of natural rubber are shown in Table 1. In the vulcanization properties, the minimum torque ML reflects the plasticity of the rubber at a certain temperature, the maximum torque MH reflects the modulus of the vulcanized rubber, and MH-ML indicates the maximum cross-linking degree of the rubber. As shown in Table 1, the ML and MH of the natural astaxanthin-modified silica/NR adhesive increased compared to the non-natural astaxanthin-modified silica/NR adhesive, indicating a decrease in the plasticity and an increase in the modulus and cross-linking degree of the adhesive. At the same time, the scorch time t10 and the process vulcanization time t90 of the rubber were significantly shortened, indicating that natural astaxanthin can promote the vulcanization process of rubber.

It was found that silanol hydroxyl groups on the surface of silica are acidic and adsorb the vulcanization accelerator, and even react with alkaline accelerator, which reduces the content of accelerator in the rubber and lowers the rate of vulcanization reaction and the degree of vulcanization. After the modification of silica by natural astaxanthin, the polar hydroxyl group and ketone group in astaxanthin molecule and silanol hydroxyl group on the surface of silica form hydrogen bonding and other interactions, which weaken the adsorption of silanol hydroxyl group on accelerator, and at the same time, the longer molecular chain of astaxanthin has hydrophobicity, which has shielding effect on silanol hydroxyl group, and it can also reduce the absorption of sulfurization accelerant, so that the rate of vulcanization reaction is accelerated, and the degree of sulfurization is improved.

 

2.2 Physical and mechanical properties

The effects of natural astaxanthin-modified silica on the physical and mechanical properties of natural rubber are shown in Table 2. As can be seen from Table 2, compared with the natural astaxanthin-modified silica, the tensile strength and elongation at break of the natural astaxanthin-modified silica/NR vulcanized rubber basically remained unchanged, and the constant tensile stress and hardness were almost the same, while the tear strength was slightly reduced; what is interesting is that the rebound value of the natural astaxanthin-modified silica/NR vulcanized rubber was increased and its elasticity was improved under the same dosage of silica, which may be due to the weakening of the surface polarity of silica modified by natural astaxanthin, the degree of aggregation of nanoparticles with each other to a certain extent. This may be due to the weakening of the surface polarity of silica modified with natural astaxanthin, the degree of aggregation of nanoparticles with each other was weakened to some extent, and the dispersion in the rubber matrix was increased.

 

2.3 Abrasion resistance

The effect of natural astaxanthin-modified silica on the abrasion resistance of natural rubber is shown in Figure 1. From Fig. 1, it can be seen that the DIN abrasion volume of natural astaxanthin-modified silica/NR vulcanized rubber decreased significantly compared with that without natural astaxanthin-modified silica, and the vulcanized rubber prepared with 1 part of natural astaxanthin had a better abrasion resistance than that with 0.5 part of modified silica, with a decrease in DIN abrasion volume of 19.2% and 23.6%, respectively, and a significant improvement in abrasion resistance. This may be attributed to the fact that the distribution and dispersion of silica in the rubber are not only improved after modification by natural astaxanthin, but also the interfacial bonding between silica particles and the rubber matrix is increased, so that the resistance to abrasion damage is improved.

 

2.4 Hot air aging resistance

The aging of rubber materials under light, heat and oxygen or their combined effects is an autocatalytic process, mainly in accordance with the course of free radical reactions. In order to investigate the hot air aging performance of natural astaxanthin-modified silica/NR rubber compounds, hot air aging tests were conducted. The effect of natural astaxanthin-modified silica on the hot air aging resistance of natural rubber is shown in Figure 2.

As can be seen from Fig. 2, the tensile strength and tear strength of silica/NR vulcanized rubber decreased significantly after hot air aging. However, the tensile strength retention rate and tear strength retention rate of natural astaxanthin-modified silica/NR vulcanized rubber were significantly higher than those without natural astaxanthin-modified silica, indicating that natural astaxanthin-modified silica has significantly improved the hot air aging resistance of natural rubber; and the tensile strength retention rate and tear strength retention rate of the vulcanized rubber modified with 1 part of natural astaxanthin-modified silica/NR vulcanized rubber were higher than those with 0.5 part of natural astaxanthin-modified silica/NR vulcanized rubber, respectively. Moreover, the tensile strength retention rate and tear strength retention rate of NR vulcanized rubber modified with 1 part of natural astaxanthin were higher than those of silica/NR vulcanized rubber modified with 0.5 parts of natural astaxanthin, which indicated that the effect of air aging resistance of NR vulcanized rubber modified with 1 part of natural astaxanthin was obviously better than that of silica/NR vulcanized rubber modified with 0.5 parts of natural astaxanthin. From the analysis of natural astaxanthin molecular structure, due to the unique chemical structure of natural astaxanthin molecule, it has excellent antioxidant ability. Rubber will generate a large number of free radicals during the aging process, and the natural astaxanthin molecule can remove the free radicals generated in the process of aging in the hot air in a timely manner, so that it can effectively slow down the aging process of natural rubber and the higher the content of natural astaxanthin is, the more aging-resistant the effect is.

 

2.5 Compression Thermal Properties

The effect of natural astaxanthin-modified silica on the compression heating performance of natural rubber is shown in Fig. 3. From Fig. 3, it can be seen that the heat of compression of natural astaxanthin-modified silica/NR vulcanized rubber was significantly reduced compared with that of unadopted natural astaxanthin-modified silica, with decreases of 13.1% and 15.4%, respectively. It was analyzed that, due to the enrichment of polar groups on the surface of silica particles, which aggregated with each other in the rubber matrix to form a filler network, the silica filler network was constantly destroyed and rebuilt under dynamic conditions, resulting in energy loss, so the larger the lag of the rubber, the higher the heat generation. After natural astaxanthin modification of silica, the formation of filler network is weakened to a certain extent, the filler-rubber interaction is enhanced, and the resilience of silica/NR vulcanized rubber becomes better, so the hysteresis is reduced, and the amount of heat generation is lowered.

 

2.6 Dynamic Viscoelastic Properties

The dependence of the shear energy storage modulus G' of natural astaxanthin-modified silica/NR vulcanized rubber on the strain amplitude is shown in Figure 4. Generally, the phenomenon that the shear energy storage modulus (G) of filled rubber decreases sharply with increasing strain is called the Payne effect. The difference between G' at low and high strains (ΔG') can indicate the strength of the Payne effect, which can reflect the dispersion of the filler. The smaller △G' is, the weaker the Payne effect is and the better the dispersion of the filler is. From Fig. 4, it can be seen that the overall trend of G' decreases with the increase of strain amplitude, and finally tends to be the same. This is due to the fact that as the strain increases, the packing network is destroyed and the rubber embedded in the packing aggregates is released, thus increasing the effective volume of the rubber and minimizing the effective volume fraction of the packing, which is the most important contributor to the energy storage modulus.5 The effective volume fraction of the packing, which is the most important contributor to the energy storage modulus, is minimized compared to that of the unprocessed packing, which is the most important contributor.

In the case of silica modified with natural astaxanthin, the Payne effect of natural astaxanthin-modified silica/NR vulcanized rubber was weakened, which indicated that natural astaxanthin-modified silica facilitated the dispersion of silica in the rubber matrix, and the degree of agglomeration was relatively weak. This may be due to the fact that the dissolution of natural astaxanthin molecules in ethyl acetate can be well contacted with silica, and the silanol hydroxyl groups on the surface of silica form hydrogen bonds with hydroxyl and ketone groups in the molecular structure of natural astaxanthin, which weakened to a certain extent the effect of filler network formed by the aggregation of silica particles in rubber matrix, and thus the distribution and dispersion of silica particles were improved and the Payne effect was weakened. The distribution and dispersion of silica particles are improved, and the Payne effect is weakened.

 

2.7 Dynamical properties

The effect of natural astaxanthin-modified silica on the dynamic mechanical properties of natural rubber is shown in Fig. 5. From Fig. 5(a), it can be seen that the loss factor (tanδ) of silica/NR vulcanized rubber increases and then decreases with temperature, and reaches a peak at the glass transition temperature (T). From Fig. 5(b), it can be seen that the T of silica/NR vulcanized rubber without natural astaxanthin modification is -37.5 °C, while the T of natural astaxanthin-modified silica/NR vulcanized rubber is greater than -37.5 °C, which is -36 °C and -35.5 °C, respectively, which indicates that the movement of the molecular chain of the natural rubber has been restricted, and the interaction between silica and rubber molecular chain is strengthened by the modification of natural astaxanthin. This indicates that the interaction force between silica and rubber molecular chain is enhanced after modification of natural astaxanthin. The loss factor at different temperatures has a certain correlation with the rubber properties. Generally, the tanδ value at 0℃ is used to characterize the anti-slip performance of the rubber, and the larger the value is, the better the anti-slip performance is; the tanδ value at 60℃ is used to predict the rolling resistance of the rubber, and the smaller the value is, the lower the rolling resistance is. As can be seen from Figure 5(c), the tanδ value of one natural astaxanthin-modified silica/NR vulcanized rubber at 0℃ was larger than that of the non-natural astaxanthin-modified silica/NR vulcanized rubber, indicating that the anti-slip property of the modified vulcanized rubber was improved; and as can be seen in Figure 5(d), the tanδ values of the natural astaxanthin-modified silica/NR vulcanized rubber at 60℃ were smaller than that of the non-natural astaxanthin-modified silica/NR vulcanized rubber. From Figure 5(d), it can be seen that the tanδ values of natural astaxanthin-modified silica/NR vulcanized rubber at 60℃ were smaller than those of the vulcanized rubber without natural astaxanthin-modified silica/NR vulcanized rubber, which indicated that the rolling resistance of the modified rubber was reduced, and the tanδ values of the vulcanized rubber with 0.5 parts of natural astaxanthin were lower than that of the vulcanized rubber with 1 part of natural astaxanthin-modified silica, which indicated that it had a lower rolling resistance.

 

2.8 Tensile section morphology

Three groups of tensile specimens were characterized by scanning electron microscope, as shown in Figure 6. From Fig. 6(a), it can be seen that the tensile section of the natural astaxanthin-modified silica/NR vulcanized rubber without natural astaxanthin exposed a lot of silica particles, which were aggregated into larger particles, indicating that the filler-filler interaction was stronger and the filler-rubber interaction was weaker; from Fig. 6(b), it can be seen that the silica particles in the tensile section of the natural astaxanthin-modified silica/NR vulcanized rubber modified with 0.5% of natural astaxanthin agglomerated into larger particles, which indicated the filler-filler interaction was stronger and the filler-rubber interaction was weaker. From Figure 6(b), it can be seen that compared with the tensile section of NR vulcanized rubber without natural astaxanthin, the tensile section of NR vulcanized rubber with 0.5 parts of natural astaxanthin-modified silica/ NR vulcanized rubber had a more refined particle size of silica particles, and there were traces of breakage of the rubber matrix by the external force, which indicated that the dispersion of filler was improved and the filler-rubber interaction was stronger. This indicates that the dispersion of the filler is improved and the filler-rubber interaction is stronger.

 

3 Conclusion

Natural astaxanthin modified silica in ethyl acetate solution can reduce the adsorption of silica to accelerator, thus improving the rubber vulcanization process; the DIN abrasion volume of natural astaxanthin-modified silica/NR vulcanized rubber was decreased, and the abrasion volume was decreased by 19.2% and 23.6% with 0.5 and 1 part of modification, and the abrasion resistance was improved; the compression heat of natural astaxanthin-modified silica/NR vulcanized rubber was decreased significantly, and the decrease reached 13.1% and 15.4% respectively; due to the excellent antioxidant ability of natural astaxanthin, the compression heat of natural astaxanthin-modified silica/NR vulcanized rubber was decreased significantly. The heat of compression of natural astaxanthin-modified silica/NR vulcanized rubber was significantly reduced by 13.1% and 15.4%, respectively; due to the excellent antioxidant ability of natural astaxanthin, the heat-resistant air-aging property of natural astaxanthin-modified silica/NR vulcanized rubber was greatly increased, and the heat-resistant air-aging property was stronger with 1 part of natural astaxanthin than that of natural astaxanthin-modified silica/NR vulcanized rubber modified by 0.5 part of natural astaxanthin; as for the dynamic viscosity-elasticity, the heat-resistant air-aging property was much better than that of the rubber modified by natural astaxanthin without astaxanthin. In terms of dynamic viscoelasticity, compared with the silica modified with natural astaxanthin, the Payne effect of natural astaxanthin-modified silica in the rubber matrix was weakened, and the dispersion of the filler was increased; in terms of dynamic mechanics, the T,of the modified vulcanized rubber was improved, and the rolling resistance of the vulcanized rubber was also significantly reduced.

 

References:

[1]ZHENG Nan, QIU Zumin, LIU Jie, et al. Research progress of application of solid waste in rubber[J].   Chemical Industry and Engineering Progress, 2013, 32(12): 2988-2996.

[2] XU Yunhui, LI Peipei, ZHANG Zhaohong, et al. Application of waste clay in natural rubber[J]. Chemical Industry and Engineering Progress, 2011,30(9): 2035-2040.

[3] CUI Lingfeng, XIONG Yuzhu, DAI Jun, et al. Preparation and characterization of modified silica/natural rubber composite material [J]. Polymer Materials Science and Engineering, 2017, 33(5): 158-163.

[4] LIU Jiwen, XU Haiyan, WU Chifei. Preparation and characterization of the natural rubber composites reinforced by epoxy natural rubber grafted silica with good dispersibility[J]. Acta Polymerica Sinica, 2008 (2): 123-128.

[5] PAN Qiwei, WANG Bingbing, ZHOU Ying, et al. Synthesis of a silane coupling agent and its application in silica / NR composites[J].  Acta Polymerica Sinica, 2014(2): 202-209.

[6] WAHBA L, D'ARIENZO M, DONETTI R, et al. In situ sol-gel obtained silica-rubber nanocomposites: influence of the filler precursors on the improvement of the mechanical properties[J]. RSC Advances, 2013, 3(17): 5832-5844.

[7] QU L, YU G, XIE X, et al. Effect of silane coupling agent on filler and rubber interaction of silica reinforced solution styrene butadiene rubber [J]. Polymer Composites, 2013, 34(10): 1575-1582.

[8] OSTAD-MOVAHED S, ANSARIFAR A, SONG M. Effects of different interphases on the mechanical properties of cured silanized silica- filled styrene- butadiene / polybutadiene rubber blends for use in passenger cartiretreads[J]. Journal of Applied Polymer Science, 2010, 113(3): 1868-1878.

[9] DOHI H, HORIUCHI S. Locating a silane coupling agent in silica- filled rubber composites by EFTEM[J].  Langmuir, 2007, 23(24): 12344-12349.

[10] MOVAHED S O, ANSARIFAR A, SONG M. Effect of silanized silica nanofiller on tack and green strength of selected filled rubbers[J]. Polymer International, 2010, 58(4): 424-429.

[ 11] KATSUMATA T, ISHIBASHI T, KYLE D. A sub-chronic toxicity evaluation of a natural astaxanthin-rich carotenoid extract of Paracoccus carotinifaciens, in rats[J]. Toxicology Reports, 2014, 1: 582-588.

[ 12] LI Haoming, GAO Lan. Astaxanthin: chemical structure, biological functions and usage[J]. Fine Chemicals, 2003, 20(1): 32-37.

[13] DUFOSSE L. Microbial production of food grade pigment[J].  Food Technology & Biotechnology, 2006, 44(3): 313-321.

[ 14] MARTIN J F, GUDINA E, BARREDO J L. Conversion of β-carotene into astaxanthin: two separate enzymes or a bifunctional hydroxylase- ketolase protein? [J]. Microbial Cell Factories. 2008, 7( 1): 1-10.

[15] GAO Guiling, CHENG Jiayang, MA Jiong. Research review of Haematococcus pluvialis and astaxathin[J].  Journal of Fisheries of China, 2014, 38(2): 297-304.

[16] LEE S H, MIN D B. Effects, quenching mechanisms, and kinetics of carotenoids in chlorophyll-sensitized photooxidation of soybean oil[J]. Journal of agricultural &food Chemistry, 1990, 38(8): 1630-1634.

[ 17] JOHNSON E A, VILLA T G, LEWIS M J. Phaffia rhodozyma as an astaxantin source in samonid diets[J].  Aquaculture, 1980, 20(2): 123-134.

[18] JOHNSON E A, LEWIS M J, GRAU C R. Pigmentation of egg yolks with astaxanthin from the yeast phaffia rhodozyma[J]. Poultry Science, 1980, 59(8): 1777-1782.

[19] GUERIN M, HUNTLEY M E, OLAIZOLA M. Haematococcus astaxanthin: applications for human health and nutrition[J]. Trends in Biotechnology, 2003, 21(5): 210-216.

[20] ZHANG Huiyuan. Chinese medicine preparation method for treating constipation: CN105664042A[P]. 2016-06-15.

[21] FU Zhengwei, JIN Yuanxiang, WANG HuiJie. Application of phaffia rhodozyma astaxanthin in preparing medicament for preventing and controlling type Ⅱ diabetes: CN101313897A[P]. 2008-12-03.

[22] TODD L R. Method for retarding and preventing sunburn by UV light: US6433025B1 [P]. 2002-08-13.

[23] WANG Hao, WANG Renwei. Antioxidant sesame mixed oil and a preparing method thereof: CN105901165A[P]. 2016-08-31.

[24] FAN Ping, REN Guopu., LIU Jingping, et al. Dairy products containing astaxanthin and their preparation methods: CN 102125093A [P]. 2011-07-20.

[25] XU Lunchao. Astaxanthin health drink with anti-aging effect: CN105029570A[P]. 2015-11-11.

[26] SOON P J, HEE C J, KYUNG K Y, et al. Astaxanthin decreased oxidative stress and inflammation and enhanced immune response in humans[J]. Nutrition & Metabolism, 2010, 7( 1): 18-28.

[27] LIU Jianguo, SU Fang, SHI Kun, et al. Rainbow trout nutrition enhancer and application thereof: CN105831494A[P]. 2016-08-10.