Nowadays, the rubber is not only used in technology products but also in household products. In this study, thermochromic rubber was fabricated which exhibit color changing under the ambient temperature. The effects of thermochromic pigment and silica filler content were investigated for the mechanical properties of materials as well as color-changing properties
Vietnam Journal of Science and Technology 57 (3A) (2019) 85-93 doi:10.15625/2525-2518/57/3A/14054 RESEARCH ON PRODUCING TEMPERATURE INDUCED COLOR CHANGE IN FUNCTIONAL RUBBER COMPOUNDS Tran Tan Dat1, *, Nguyen Tran Ha1, Tran Le Hai2, Bui Duc Chi Thien3 Faculty of Materials Technology, Ho Chi Minh City University of Technology – VNU-HCM, 268 Ly Thuong Kiet street, Ward 14, District 10, Ho Chi Minh City Faculty of Chemical Engineering, Ho Chi Minh City University of Technology – VNU- HCM, 268 Ly Thuong Kiet street, Ward 14, District 10, Ho Chi Minh City Institute of Applied Science, Ho Chi Minh City University of Technology (HUTECH), 475A Dien Bien Phu, Ho Chi Minh City * Email: trandat@hcmut.edu.vn Received: 30 July 2019; Accepted for publication: 24 September 2019 Abstract Nowadays, the rubber is not only used in technology products but also in household products In this study, thermochromic rubber was fabricated which exhibit color changing under the ambient temperature The effects of thermochromic pigment and silica filler content were investigated for the mechanical properties of materials as well as color-changing properties The results showed that the introduction of thermochromic pigment of wt.% into the rubber matrix created the novel color-changing property of the rubber compound and did not reduce mechanical properties of the material Furthermore, the color-changing stability of the compound maintains for more than 100 cycles Keywords: rubber materials, thermochromic pigment, vulcanization Classification numbers: 2.9.3, 2.10.3 INTRODUCTION Thermal-burns accident occurs usually in life such as outlet chamber, water pot, hot vehicle machine part, etc., which causes the pain and after-effect, especially for children Thermal-burns usually have a detrimental effect on human health, such as painful cause and scare formation Therefore, making warning signs of high temperature is required to protect people from high thermal ambient or items Fortunately, the products which have color-changing property can meet the requirement Currently, there are some commercial products that can change their color under the effect of temperature However, most of color-changing property have been used in plastic, paint, cloth materials [1, 2] with the purpose of decoration and entertainment; it has not been widely applied in rubber materials Therefore, the development of rubber materials having color-changing characteristic is necessary for our living convenience The thermochromic effect has been studied in different materials so far As an example, M Rubacha [3] incoporated 1-10 wt.% thermochomric pigment in cellulose fibers obtained from the Tran Tan Dat et al spinning process Such modified fibers not only retained sufficient mechanical properties, but also exhibited an intensive color change when the temperature was reached above 32.7-32.9 0C Y Shibahashi et al [4] coated a thermochromic layer on top of an artificial nail Such constructed artificial nail displayed a visible external color change in response to a change in temperature when the thickness of thermochromic layer was at least μm These thermochromic effects have a potential to be applied in many practical applications Therefore, our study on fabrication of color-changing rubber products not only provides humans with safely thermal-warning, but also contributes to the development of rubber manufacturing industry In this study, the thermochomric pigment was introduced into rubber compound to form rubber products which have the ability of color alteration under the effect of temperature The effect of thermochomric pigment content on the mechanical and color-changing properties and the durability of the functioned rubber compound are investigated In addition, silica is also used as a filler in order to examine the influence of silica fillers on the colors changing ability of obtained rubber material as well as the mechanical properties of the rubber material MATERIALS AND METHODS 2.1 Materials Natural rubber (SVR 5L) is provided from Tay Ninh Rubber Joint Stock Company Thermochromic pigment are purchased from Atlanta Chemical Co., USA Other chemical substances including Zinc Oxide (ZnO), Stearic acid and di-2-ethylhexyl phthalate (DOP) are provided from Casumina, Vietnam [5] 2, 6-Bis (1, 1-dimethylethyl)-4-methylphenol (BHT) was supplied from Sigma (USA) and used as aging antioxidants Silica was purchased from Merck CBS, Tetramethylthiuram monosuldide (TMTD) and sulfur (S) are supplied from Merck [6] 2.2 Analytical methods 2.2.1 Method for preparing thermochromic rubber Rubber (SVR 5L) was mixed and reacted with vulcanization reactants and fillers through a Brabender instrument In the first state, the compound was mixed at 150 °C with 50 rpm for Thermochromic pigment and reagents were then added into the rubber compound for 30 The torque data were recorded by Brabender software and exploited as an indicator of the extent of mixing Following, the obtained rubber compound was stored at room temperature for 24 hours before testing the rheological property and sample processing The recipe of rubber compound is presented in Table (1) equation (1) shows the vulcanization extent of rubber compounds (Mlh) which is determined via the pre-vulcanization time (T10) and the optimal vulcanization time (T90) 2.2.2 Mechanical and morphological characterization Dumbbell specimens with a thickness of mm were molded using a die (ASTM D412) These specimens were evaluated at a strain rate of 500 mm.min−1 The reported data are recored based on the average of five samples testing Tests were performed on the M350-10CT measuring device at room temperature 86 Effect of thermochromic pigment on the rheological property of natural rubber compound The samples with dimensions of 5×10×2 mm were prepared for color-changing analysis Color-changing property of rubber compounds with various pigment content (0.5-2 wt.%) is investigated with the temperature above and below 45 oC Table Composition of rubber-rolled rubber mixture for mixing rubber changing colors Composition Phr (% according to the proportion of rubber) Raw materials SVR 5L 100 ZnO Stearic acid SPP BHT 0.5 Silica 10 PEG 4000 0.8 Nyxflex 222B Thermochromic pigment 0–2 DM 1.5 TS 0.2 DPG 0.4 S Total 125.4 – 127.4 g RESULTS AND DISCUSSION 3.1 Effect of thermochromic pigment on the rheological property of natural rubber compound Figure Vulcanization curves of rubber compounds: wt.% and wt.% of thermochromic pigment 87 Tran Tan Dat et al The rheological property of rubber compounds characterized via vulcanizing curves is shown in Figure It can be seen that the presence of thermochromic pigment does not affect to the rheological properties of the rubber compounds [7] Particularly, both rubber compounds have wide vulcanization tray which indicate the good thermal resistance of the rubber compounds The thermal resistance plays an important key in the application of rubber compounds in thermal caution because they are usually working with high temperature [8] 3.2 Effect of thermochromic pigment content on the color-changing property of the rubber compound Table Results of color-changing property of rubber compounds with various pigment content Pigment content 0.5 wt.% wt.% wt.% The colour is slightly pale and difficult to observe the colour alteration The colour is clear, and easy to observe the colour alteration The colour is clear, and easy to observe the colour alteration Slow Quick Quick After isolating from heat, the color recovery of the rubber sample from pale yellow to original red color is rapidly After isolating from heat, the color recovery of the rubber sample from yellow to original red color is regularly After isolating from heat, the color recovery of the rubber sample is sluggish and need a low temperature source to alter yellow to original red color Colour Colour-changing speed Original-color recovery speed The thermochromic pigment has an operating temperature at 45 oC Therefore, the rubber compound sample introduced thermochromic pigment exhibited the red color, which is the same color of thermochromic pigment The color of the rubber sample changes to the color of the rubber phase as elevating the temperature higher than 45 oC [9] The influence of the pigment content on the color-changing property of the rubber compound is illustrated in Table As can be seen the results from the Table 2, the rubber compound containing thermochromic pigment of wt.% shows a good color-changing property, which is suitable for heat alert application because of its rapid color-alteration and regularly original-color recovery with the temperature above and below 45 oC [10] 3.3 Effect of thermochromic pigment on the mechanical properties of rubber compound The mechanical properties of the rubber compound with different thermochromic pigment content of wt.% and 1wt.% are investigated and shown in Table Regarding the testing results 88 Tran Tan Dat, Nguyen Tran Ha, Tran Le Hai, Bui Duc Chi Thien from the Table It can be seen that the rubber compound containing thermochromic pigment exhibits tensile strength of 22.07 MPa, which is higher than that of no thermochromic pigment Besides, it exhibits a hardness of 37 shore A and a tear resistance of 5.34 kN/m which are also higher than the comopund without thermochromic pigment (35 shore A and 4.66 kN/m respectively) Meanwhile, the elongation and the abrasion of the rubber compound embedded with thermochromic pigment is comparable with that one with no thermochromic pigment It proves that the introduction of thermochromic pigment of wt.% in rubber compound not only provides the color-changing ability, but also improves the mechanical properties of the rubber compound [7, 8] Table Results of mechanical properties of rubber compounds with various pigment content Results Tensile strength Strain at break Hardness Stress at 100 % Stress at 300 % Tear resistance Abrasion Test method Pigment wt.% Pigment wt.% ASTM D412:2004 Die C 19.17 ± 1.42 22.07 ± 1.22 ASTM D412:2004 Die C 1751.62 ± 51.34 1719.88 ± 73.17 ASTM D2240:2004 35 ± 37 ± ASTM D412:2004 Die C 0.64 ± 0.1 0.67 ± 0.2 ASTM D412:2004 Die C 32 ± 0.14 1.42 ± 0.37 ASTM D624:2004 4.66 ± 0.13 5.34 ± 0.24 ASTM D430 2.89 ± 0.09 2.87 ± 0.13 Unit MPa % Shore A MPa MPa kN/m cm3/1.61 km Figure Graph comparing the rate of crack development of rubber samples: wt.% and wt.% of thermochromic pigment Additionally, bending durability and crack-increasing rate are essential factors in application of thermal-warning rubber products Figure shows the crack-increasing rate of rubber compound with and without thermochromic pigment The investigation is performed based on the ASTM-D430:2004 The results revealed that a crack was observed at 4000 folding cycles for the rubber compound with no thermochromic pigment Whereas, the rubber compound with wt.% thermochromic pigment is found to have a crack at 2000 folding cycles Moreover, the crack-increasing rate of the rubber compound with wt.% thermochromic pigment is also faster than that with wt.% thermochromic pigment It demonstrates that the introduction of the 89 Tran Tan Dat et al thermochromic pigment into the rubber compound decreases the cracking resistance of the rubber product 3.4 Effect of thermochromic pigment on thermal-aging resistance of rubber compound Table illustrates the mechanical properties of rubber compounds with wt.% thermochromic pigment and without pigment before and after aging at 70 oC in 72 hrs The results showed that despite having higher tensile strength before aging, the rubber compound introduced with wt.% thermochromic pigment exhibited lower tensile strength than that with no thermochromic pigment It reveals that the rubber compound with wt.% thermochromic pigment owns the less aging resistance compared to that with no thermochromic pigment That's due to the fact that there are several spironolactone groups that mix thermochromic pigment The spirolatone groups can easily react with active hydrogen atoms of polymer chains to form hydro peroxides (ROOH) which are rapidly decomposed to alkoxy and hydroxyl groups Thus, the radicals are spread on the polymer chain and accelerate the aging process in the rubber phase [11] Table Results of mechanical properties of rubber compounds before and after aging Results Test method Conditional test Pigment wt.% Pigment wt.% 19.17 ± 1.42 22.07 ± 1.22 20.41 ± 0.67 17.73 ± 1.97 Unit Tensile strength ASTM D412:2004 Not aging Die C Aging at 70 oC in 72 hrs Strain at break ASTM D412:2004 Not aging 1751.62 ± 51.34 1719.88 ± 73.17 % Die C o Aging at 70 C in 72 hrs 1544.97 ± 34.32 1426.91 ± 70.53 Stress at 300 % ASTM D412:2004 Not aging Die C Aging at 70 oC in 72 hrs 32 ± 0.14 1.66 ± 0.4 1.42 ± 0.37 1.66 ± 0.3 MPa MPa 3.5 Effect of silica fillers on the color-changing property of rubber compound Figure describes the color of the bare rubber compound with wt.% thermochromic pigment and that added by 10 wt.% silica filler It is found that the silica filler has a significant effect on the color changing property of the rubber compound with wt.% thermochromic pigment Regarding appearance, an addition of fillers into the rubber compound functionalized with thermochromic pigment could alter apparently the original color of the heat sensitive rubber compound Table shows the original-color recovery speed of color-changing functionalized rubber compound added by filler is considerably slower than that not added by filler Therefore, the addition of silica fillers is unnecessary for the rubber compound in thermal warning application [12] 90 Tran Tan Dat, Nguyen Tran Ha, Tran Le Hai, Bui Duc Chi Thien No fillers, at room temperature No fillers, ≥ 45 C Fillers, at room temperature Fillers, ≥ 45 C Figure The color of rubber samples with and without silica fillers at room and high temperatures Table Some assessments of the effect of fillers on the ability of the pigment powder Nature Rubber sample added by silica filler Color Paler than the original thermochromic pigment Colorchanging speed Change rapidly as exposing to high Change more slowly, the color of the temperatures sample fades from the outer to the inside Originalcolor recovery speed Slower (about minutes and 34 seconds Faster (about minutes and 15 seconds after isolating from heat and cooling at after isolating from heat and cooling at room temperature room temperature color Rubber sample without silica filler of Similar to the original color thermochromic pigment of 3.6 Durability of the color-changing functioned rubber compound The rubber compound with wt.% thermochromic pigment is evaluated by the changing the operating temperature above and below 45 oC (40-45 oC) The experiment is repeated more than 100 cycles as illustrated in Table Table Result of investigating color-changing durability of the rubber compound Temperature Number of color-changing cycles Red (toroom) no color( 45 oC) 45 oC >100 The result shows that the color-changing durability of the rubber sample is maintained after more than 100 cycles and the original color of the rubber compound is not changed compared to before the durable test [13] 91 Tran Tan Dat et al The color-changing durability at different temperatures and time are also evaluated and the results are shown in Table As can be seen from the Table 7, the rubber compound with wt.% thermochromic pigment exhibits an excellent color-changing ability and durability at high temperature in long-time performance without altering the original color and the color-changing property Table Results of color changing durability at different temperatures and operating time Time 15 Temperature o 60 C o 80 C o 100 C 30 hrs hrs hrs hrs 16 hrs 24 hrs Number of cycles > 100 > 100 > 100 The rubber compound with wt.% thermochromic pigment is exploited to fabricate some products suitable for thermal warning such as coasters, cups and spoons for children, caution signs (Figure 4) Figure Some rubber products possess thermal warning property CONCLUSIONS The thermal warning rubber compound functionalized by introducing thermal-sensitive pigment is successfully fabricated The effect of pigment content and silica fillers on the mechanical and color changing properties were investigated It indicates that the introduction of thermochromic pigment of wt.% does not affect the mechanical property and provide the novel color-changing function of the rubber compound Silica filler is found to reduce the original color and the color-changing property of the compound Moreover, the rubber compound with wt.% thermochromic pigment exhibits an excellent color-changing ability and durability at high temperature in long-time performance without altering the original color and the color-changing property with more than 100 color-changing cycles Acknowledgements This work was supported in part by Ho Chi Minh City University of Technology – VNU-HCM under Grant To-CNVL-2019-26 92 Tran Tan Dat, Nguyen Tran Ha, Tran Le Hai, Bui Duc Chi Thien REFERENCES Aitken D., Burkinshaw S M., Griffiths J and Towns A D - Textile Applications of Thermochromic Systems, Coloration Technology 26 (1996) p Tanaka K., Kawamoto M., Temperature-Sensitive Color-Changeable Composite Fiber, U.S Patent No 5,153,066 Oct 1992 Rubacha M - Thermochromic cellulose fibers, Polymers for Advanced Technologies 18 (2007) 323-328 Shibahashi Y., Nakasuji N.- Thermochromic Artificial Nail, US Patent No 4,920,9911 May 1990 Nguyen Xuan Hien, Rubber Technology, Viet Rubber Co., 1987, p.350 Bamfield P - Chromic Phenomena - Technological Applications of Colour Chemistry, The Royal Society of Chemistry, 2010, p.561 Do Thanh Thanh Son - Rubber Experiment Guide, Publishing National University of Ho Chi Minh City, 2005, p 120 (in Vietnamese) Simpson R B - Rubber Basics, Rapra Technology Limited, 2002, p.213 Glover, Leon C., and Eugene F Lopez - Thermochromic Paint for Use on Plastic Substrates U.S Patent No 4,105, 583 Aug 1978 10 Hali Industrial Co., Material Safety Data Sheet, Thermochromic Pigments, 2010 11 Seeboth A., Klukowska A., Ruhmann R and Lotzsch D - Thermochromic Polymer Materials, Chinese Journal of Polymer Science 25 (2007) 123-135 12 Seeboth A., Ruhmann R., Lotzsch D and Mühling O - Thermochromic Polymers Function By Design, Chemical Reviews 114 (2014) 3037-3068 13 Seeboth A., Ruhmann R., and Mühling O - Thermotropic and Thermochromic Polymer Based Materials for Adaptive Solar Control, Materials (2) (2010) 5143-5168 93 ... important key in the application of rubber compounds in thermal caution because they are usually working with high temperature [8] 3.2 Effect of thermochromic pigment content on the color- changing property... excellent color- changing ability and durability at high temperature in long-time performance without altering the original color and the color- changing property Table Results of color changing durability... excellent color- changing ability and durability at high temperature in long-time performance without altering the original color and the color- changing property with more than 100 color- changing cycles