Research on producing temperature induced color change in functional rubber compounds

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

RESEARCH ON PRODUCING TEMPERATURE INDUCED

COLOR CHANGE IN FUNCTIONAL RUBBER COMPOUNDS

Tran Tan Dat1, *, Nguyen Tran Ha1, Tran Le Hai2, Bui Duc Chi Thien3

1

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

2

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

3

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 1 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

1 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 1 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

2 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 2 min Thermochromic pigment and reagents were then added into the rubber compound for 30 min 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

(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 2 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

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 1 Composition of rubber-rolled rubber mixture for mixing rubber changing colors

3 RESULTS AND DISCUSSION 3.1 Effect of thermochromic pigment on the rheological property of natural rubber compound

(% according to the proportion of rubber)

Figure 1. Vulcanization curves of rubber compounds: 0 wt.% and 1 wt.% of thermochromic pigment.

Tran Tan Dat et al

The rheological property of rubber compounds characterized via vulcanizing curves is shown in Figure 1 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 2 Results of color-changing property of rubber compounds with various pigment content

Colour

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

Colour-changing

Original-color

recovery speed

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

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 2 As can

be seen the results from the Table 2, the rubber compound containing thermochromic pigment of

1 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 0 wt.% and 1wt.% are investigated and shown in Table 3 Regarding the testing results

from the Table 3 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 1 wt.% in rubber compound not only provides the color-changing ability, but also improves the mechanical properties of the rubber compound [7, 8]

Table 3 Results of mechanical properties of rubber compounds with various pigment content

0 wt.%

Pigment

Tensile strength ASTM D412:2004 Die C 19.17 ± 1.42 22.07 ± 1.22 MPa Strain at break ASTM D412:2004 Die C 1751.62 ± 51.34 1719.88 ± 73.17 %

Stress at 100 % ASTM D412:2004 Die C 0.64 ± 0.1 0.67 ± 0.2 MPa Stress at 300 % ASTM D412:2004 Die C 1 32 ± 0.14 1.42 ± 0.37 MPa Tear resistance ASTM D624:2004 4.66 ± 0.13 5.34 ± 0.24 kN/m Abrasion ASTM D430 2.89 ± 0.09 2.87 ± 0.13 cm3/1.61 km

Figure 2 Graph comparing the rate of crack development of rubber samples: 0 wt.% and 1 wt.% of

thermochromic pigment

Additionally, bending durability and crack-increasing rate are essential factors in application of thermal-warning rubber products Figure 2 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 1 wt.% thermochromic pigment is found to have a crack at 2000 folding cycles Moreover, the crack-increasing rate of the rubber compound with 1 wt.% thermochromic pigment is also faster than that with 0 wt.% thermochromic pigment It demonstrates that the introduction of the

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 4 illustrates the mechanical properties of rubber compounds with 1 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 1 wt.% thermochromic pigment exhibited lower tensile strength than that with

no thermochromic pigment It reveals that the rubber compound with 1 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 4 Results of mechanical properties of rubber compounds before and after aging

0 wt.%

Pigment

Tensile

strength

ASTM D412:2004

Die C

Not aging 19.17 ± 1.42 22.07 ± 1.22

MPa Aging at 70 oC in 72 hrs 20.41 ± 0.67 17.73 ± 1.97 Strain at

break

ASTM D412:2004

Die C

Not aging 1751.62 ± 51.34 1719.88 ± 73.17

% Aging at 70 oC in 72 hrs 1544.97 ± 34.32 1426.91 ± 70.53 Stress at

300 %

ASTM D412:2004

Die C

Not aging 1 32 ± 0.14 1.42 ± 0.37

MPa Aging at 70 oC in 72 hrs 1.66 ± 0.4 1.66 ± 0.3

3.5 Effect of silica fillers on the color-changing property of rubber compound

Figure 3 describes the color of the bare rubber compound with 1 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 1 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 5 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]

Figure 3 The color of rubber samples with and without silica fillers at room and high temperatures Table 5 Some assessments of the effect of fillers on the ability of the pigment powder

Nature Rubber sample added by silica filler Rubber sample without silica filler

Color Paler than the original color of

thermochromic pigment

Similar to the original color of thermochromic pigment

Color-changing

speed

Change rapidly as exposing to high

temperatures

Change more slowly, the color of the sample fades from the outer to the inside

Original-color

recovery

speed

Slower (about 3 minutes and 34 seconds

after isolating from heat and cooling at

room temperature

Faster (about 2 minutes and 15 seconds after isolating from heat and cooling at room temperature

3.6 Durability of the color-changing functioned rubber compound

The rubber compound with 1 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 6

Table 6 Result of investigating color-changing durability of the rubber compound

Red (t o room )  no color(  45 o C)

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]

No fillers, at room temperature

Fillers, at room temperature

No fillers, ≥ 45 0 C

Fillers, ≥ 45 0 C

Tran Tan Dat et al

The color-changing durability at different temperatures and time are also evaluated and the results are shown in Table 7 As can be seen from the Table 7, the rubber compound with 1 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 7 Results of color changing durability at different temperatures and operating time

Time

Temperature

15 min

30 min

1 hrs

2 hrs

4 hrs

8 hrs

16 hrs

24 hrs Number of cycles

The rubber compound with 1 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 4 Some rubber products possess thermal warning property

4 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 1 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 1 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.

REFERENCES

1 Aitken D., Burkinshaw S M., Griffiths J and Towns A D - Textile Applications of

Thermochromic Systems, Coloration Technology 26 (1996) p 8

2 Tanaka K., Kawamoto M., Temperature-Sensitive Color-Changeable Composite Fiber, U.S Patent No 5,153,066 6 Oct 1992

3 Rubacha M - Thermochromic cellulose fibers, Polymers for Advanced Technologies 18

(2007) 323-328

4 Shibahashi Y., Nakasuji N.- Thermochromic Artificial Nail, US Patent No 4,920,9911 1 May 1990

5 Nguyen Xuan Hien, Rubber Technology, Viet Rubber Co., 1987, p.350

6 Bamfield P - Chromic Phenomena - Technological Applications of Colour Chemistry, The Royal Society of Chemistry, 2010, p.561

7 Do Thanh Thanh Son - Rubber Experiment Guide, Publishing National University of Ho Chi Minh City, 2005, p 120 (in Vietnamese)

8 Simpson R B - Rubber Basics, Rapra Technology Limited, 2002, p.213

9 Glover, Leon C., and Eugene F Lopez - Thermochromic Paint for Use on Plastic Substrates U.S Patent No 4,105, 583 8 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 3 (2) (2010) 5143-5168