Effect of azodicarbonamide on microstructure,cure kinetics and physical properties of natural rubber foam

Abstract

The effect of azodicarbonamide as chemical blowing agent on the morphology, cure kinetics and physical properties of natural rubber foam is investigated. From the morphology, when the amount of chemical blowing agent increases from 3 to 4 phr, the bubble size in the rubber matrix slightly decreases due to the increase of vulcanization reaction rate from the presence of amine fragment species as by-products from the decomposition of azodicarbonamide. The coalescence between bubbles is observed in the specimen with 5 and 6 phr of azodicarbonamide owing to high gas content in the rubber matrix. Moreover, the scorch time slightly reduces and cure rate increases as a function of azodicarbonamide content. The autocatalytic model can be used to explain the curing reaction and mechanism of this natural rubber foam. Furthermore, the activation energy (E_a) directly relates to the bubble size and microvoid structure of natural rubber foam. When compared with the vulcanized natural rubber without adding chemical blowing agent, it is found that the bulk density of natural rubber foam significantly decreases and the volumetric expansion ratio of natural rubber foam increases at high content of chemical blowing agent. Moreover, natural rubber foam at 4 phr of azodicarbonamide exhibits the lowest thermal expansion coefficient due to the smallest bubble size with less coalescence.

Keywords

Chemical foaming/blowing agent thermoset foams cell growth cross-linked foam decomposition

Introduction

Natural rubber foam (NRF) has been used in many applications because of its light weight, good thermal insulation and sound absorption.^1–5 There are several methods for producing NRF.⁶ Generally, NRF is produced by latex process^7–12 and dried rubber process.^1–5,13 In the automotive industries, NRF is always fabricated by an addition of chemical blowing agent using dried rubber process which is suitable for producing NRF with complex shape. There are many types of chemical blowing agent such as sodium bicarbonate, p-toluenesulfonyl semicarbazide, 5-phenyl tetrazole, 4,4-oxydibenzenesulfonyl hydrazide (OBSH), dinitroso pentamethylene tetramine (DPT) and azodicarbonamide.^{2,3,5,6,13–16} Among them, azodicarbonamide is an interesting chemical blowing agent because it shows closed-cell foam and is decomposed at low temperature. However, the chemical blowing agent technique still has problems because when natural rubber (NR) is compressed and removed from the mold, it considerably expands in the air, resulting in the oversized dimension.

To control the size and properties of NRF specimen produced by this technique, the balance between two reactions that occurred concurrently during compression molding process, cross-link reaction of rubber molecule and thermal decomposition of chemical blowing agent, is important. The cure kinetics of vulcanization reaction is the crucial factor used to control the degree of cross-link because it represents the rate of the cross-link reaction. If the cross-link reaction is faster than the gas expansion from the chemical blowing agent decomposition, small bubbles can be formed.

Rubber industries generally use sulfur vulcanization system or peroxide system for cross-linking the rubber molecule;^6,17–21 however, in this research, NRF is produced by sulfur vulcanization system with high ratio of accelerator to sulfur content in order to limit the post cross-link reaction at the post cure period^17,21 and prevent the cross-link reaction during the mixing process. Several authors studied the effects of content and type of chemical blowing agent in elastomer.^{4,5,13,14,22,23} Although Wimolmala et al.⁵ and Sombatsompop et al.²² studied the effect of azodicarbonamide content in the cellular NR system, the effect of azodicarbonamide content on the cure kinetics, kinetic parameters, thermal and physical properties of NRF has not been investigated yet.

In this research, the effect of content of azodicarbonamide as a chemical blowing agent ranging from 0 to 6 phr on the cure kinetics, morphology, and properties of NRF is investigated. The cure kinetics and kinetic parameters are investigated from a torque measurement by a moving die rheometer (MDR) using the theoretical model fitted with the experimental data.

Experimental

Materials

The air-dried natural rubber sheet (M_n = 201,268 g/mol, M_w = 701,263 g/mol, and Mooney viscosity at 100℃ (ML₁₊₄ = 66.77)) was purchased from Rayong province, Thailand. The commercial grades of steric acid, zinc oxide, and sulfur were purchased from TSL chemical Co., Ltd., Thailand. 2-Morpholinothiobenzotiazole (MBS) accelerator and azodicarbonamide (decomposition temperature about 148±3℃) were kindly provided by Sunny World Chemical Co., Ltd., and AF Goodrich chemical Co., Ltd., Thailand, respectively.

Sample preparation

The air-dried natural rubber sheet was masticated in an internal mixer (Charoen TUT Co., LTD, MX105-D40L50, Thailand) at 80℃ and 60 r/min of rotor speed for 2 min. The softened rubber was further mixed with stearic acid as co-activator, zinc oxide as activator, MBS as accelerator, azodicarbonamide as chemical blowing agent, and sulfur as a cross-linking agent for 10 min. MBS accelerator which is a sulfenamide accelerator was chosen because it has scorch delay behavior.^17,21,24,25 The delay in scorch time can increase the mixing time and ensure that the cross-link reaction did not occur during the mixing process. The composition of rubber compound is shown in Table 1. The rubber compound was stored at room temperature overnight in order to release the remaining stress of polymer chains after the mixing process. The rubber compound was subsequently compressed at 155℃ and 20 bars to obtain the samples with dimensions of 17 × 100 × 5 mm³.

Table 1.

Composition of rubber compound with different chemical blowing agent contents.

Sample name	NR (g)	ZnO (phr)^a	Steric acid (phr)	MBS (phr)	Sulfur (phr)	Azodicarbonamide (phr)
NR	40	5	2	2.5	0.5	0
NRF-BA3	40	5	2	2.5	0.5	3
NRF-BA4	40	5	2	2.5	0.5	4
NRF-BA5	40	5	2	2.5	0.5	5
NRF-BA6	40	5	2	2.5	0.5	6

Part per hundred of rubber.

Characterization

About 4–5 g of the rubber compound was characterized by a MDR (Alpha’s technologies, TechPro, RheoTechMD+, USA) to determine the amount of necessary torque to rotate the rubber-filled die when the molecules were cross-linked by sulfur under the specific temperature. The degree of cure was obtained from the measured torque by equation (1), where X, T_t, T_max, and T_min refer to degree of cure, torque at time of measurement, maximum torque, and minimum torque, respectively.

X = \frac{T_{t} - T_{min}}{T_{max} - T_{min}}

(1)

Cure rate ( $\frac{dX}{dt}$ ) was calculated using the three-point differential numerical method. The plot between the degree of cure (X) and the cure rate ( $\frac{dX}{dt}$ ) was used to predict the kinetic parameters in terms of the cure rate constant (K) and the reaction order (m, n) via the curve fitting method using MathLab. The activation energy (E_a) was obtained by using Arrhenius equation. This parameter relates to the cure rate constant (K) of the cross-link reaction and explains how easily the cross-link reaction can be occurred.

The morphology of NRF was observed by scanning electron microscopy (SEM) (JEOL, JSM-5400, Tokyo, Japan). The specimen was dipped in liquid nitrogen and fractured. It was bound onto the substrate using carbon paint and coated with gold by an ion sputtering machine in order to create a conductive surface layer. The accelerating voltage was set at 10 kV.

The physical and thermal properties including bulk density, volumetric expansion ratio, and thermal expansion coefficient were investigated. The bulk density was measured by weighing the samples in both air and distilled water. After that, the bulk density was calculated from the weight difference of the sample. The expansion ratio was calculated in terms of volumetric change of the NRF specimen compared to the volume of vulcanized natural rubber without chemical blowing agent (NR) from the compression process after being stored overnight to cool it down and released the heat history from the vulcanization process. The dimension of the mold used to compress the rubber specimen was 17 × 100 × 5 mm³. The thermal expansion coefficient of NRF was evaluated by a thermo mechanical analyzer (Perkin Elmer, Pyris diamond TMA, Boston, USA). The specimen was cut into 0.5 × 0.5 × 0.5 cm³, and the temperature was set between 30℃ and 120℃ with a heating rate of 5℃/min. The specimen was pressed with 5 mN force during the test.

Results and discussion

Morphology of natural rubber foam

The effect of chemical blowing agent content on the gas bubble and microvoid structure in NRF is investigated by SEM as seen in Figure 1. NRF at 6 phr of chemical blowing agent (NRF-BA6) exhibits the coalescence between bubbles due to high amount of gas produced from the thermal decomposition of azodicarbonamide. Nevertheless, the thermal decomposition of azodicarbonamide, which produced hydrazodicarbamide, urazol and ammonia as by products, can accelerate the sulfur vulcanization reaction, resulting in inhibiting the expansion of bubbles in the rubber matrix. The possible mechanism of azodicarbonamide decomposition is proposed in equation (2).^26–28

\binom{{3 H}_{2} NCON = {NCONH}_{2} Δ}{\to H_{2} {NCONHNHCONH}_{2} + NHCONHCONH + {NH}_{3} + N_{2} + 2 CO Azodicarbonamide Hydrazodicarbamide Urazol Amonia}

(2)

In Figure 1, NRF-BA3 exhibits large bubble size because it has long expansion time in the rubber matrix than other NRF specimens. When the amount of the chemical blowing agent increases to 4 phr (NRF-BA4), the bubble sizes in the specimen reduce with no coalescence because it has faster cross-link reaction than that of NRF-BA3 as a result of the amine by-products. Although NRF-BA5 and NRF-BA6 have high amine fragment species, the high amount of gas in rubber matrix causes the coalescence between bubbles (inserted circle in Figure 1), resulting in the non-uniform bubble size in the NRF specimen. The generation of gas bubble from azodicarbonamide also affects the cross-link reaction rate by sulfur in NRF, and thus the cure kinetics is the important information to explain the curing behavior in our system.

Cure characteristics of natural rubber foam

The torque measured from the MDR from Figure 2 shows the cure characteristics of vulcanized NR and NRF as well as the scorch time. All rubber compounds were tested at least three times to acquire the average data. The definition of scorch time (t_s1) is the increase of torque around 1 unit of measurement. In our system, the sulfur vulcanization can be accelerated by a sulfenamide accelerator that degraded into the amine fragment species and coordinated with the activator forming complex species.^17,25 It is found that the vulcanized NR without chemical blowing agent (NR) has longer scorch time with highest torque from rheograph because it does not have the gas and amine fragment species from azodicarbonamide. The gas in the rubber matrix reduces the torque due to low cross-link reaction of rubber, and the amine fragment species can accelerate the vulcanization reaction with the same behavior as MBS accelerator. The results also show that the presence of the chemical blowing agent leads to a slight reduction in scorch time caused by the decomposition of azodicarbonamide. The presence of the chemical blowing agent in NRF system reduces the maximum torque due to high amount of gas inside the rubber matrix, reducing the shear force.^4,13 NRF-BA5 and NRF-BA6 systems exhibit slight difference in the maximum torque which is consistent with the unchanged microvoid structure as can be seen in Figure 1. In addition, the torque of all rubber compounds after complete sulfur vulcanization reaction is almost constant, which is a characteristic of the normal cure. For the characteristics of the normal cure, most of the cross-link networks form single or double sulfur linkages with high bonding energy, resulting in a strong cross-link network that cannot degrade easily.²⁹

During the curing of rubber compound, the measured torque increases due to the formation of three-dimensional networks of polymer molecules with sulfur. Steep slope of the torque is observed in Figure 2, implying the fast cross-link reaction rate.^25,30–35 The curing period is used to calculate the degree of cure and the cure rate to analyze the kinetic parameters of NRF. Although an addition of chemical blowing agent results in high gas volume in the rubber matrix, the presence of bubbles does not affect the rate of rubber vulcanization reaction. On the other hand, high gas volume in the rubber matrix affects the scorch time and the maximum torque or strength of rubber after complete reaction.

The cure characteristics of NR and NRF are displayed in Figure 3. The autocatalytic reaction, which is the reaction of the prior product with a reactant to form a new product, is observed. Therefore, it can be concluded that the kinetic reaction of this reaction is controlled by the scorch delay mechanism as a result of the reaction between persulfernyl radicals and sulfurating species (the formation species of sulfur, activator and accelerator) before the cross-link reaction occurred. Moreover, increasing the azodicarbonamide content increases the rate of reaction. Therefore, the decomposition of azodicarbonamide produces the amine fragment species in the system which can accelerate the cross-link reaction of NRF similar to sulfenamide accelerator.

Kinetic parameters of natural rubber foam

The cure kinetics of NR and NRF at processing temperatures ranging from 155 to 170℃ was measured using a MDR to determine the effect of temperature on the cure characteristics of the rubber compound.^1,4,33 The effect of the processing temperature in NRF-BA3 system is shown in Figure 4. The similar trends of torque development are observed in the vulcanized NR and all rubber foam systems. The increase of processing temperature reduces the scorch time and increases the cross-link reaction rate, corresponding to the Arrhenius’ law, in which increasing temperature results in an increased cure rate constant (K). In addition, the increase of processing temperature changes the post cure behavior of the NRF. In Figure 4, the characteristic of post cure of NRF-BA3 indicates the reversion characteristics when temperature increases because the given energy is high enough to break the bond of the single and double sulfur linkages. In this study, only the cure time period is concerned because it is used to evaluate the effect of chemical blowing agent content on the kinetic parameters.

Figure 5 shows the cure rate as a function of degree of cure for NRF-BA3. The increase of processing temperature results in the increase of cure rate. Cure kinetics and kinetic parameters such as the cure rate constant (K) and the reaction order of autocatalytic reaction (m, n), are calculated by fitting with theoretical model. As we know, the autocatalytic reaction can be represented in equation (3)^{25,30,36–41} where $\frac{dX}{dt}$ , X, m, n, and K(T) refer to cure rate of reaction, degree of cure, reaction order, and cure rate constant, respectively.

\frac{dX}{dt} = {K (T) X}^{m} (1 - {X)}^{n}

(3)

K, m and n values are calculated by plotting data between the degree of cure and the cure rate. This model is determined to have high accuracy in curve fitting. After the cure rate constant is obtained via the curve fitting method, it is used to predict the activation energy (E_a), which is a slope of linear plot between lnK(T) and 1/T as shown in Figure 6.

The cure characteristics and kinetic parameters of NR and NRF are summarized in Table 2. When the processing temperature increases, the scorch time (t_s1) and cure time at 90% of maximum torque (t_c90) decrease with high reaction rate constant (K). The scorch times of NRF-BA5 and NRF-BA6 are longer than those of others due to low cross-link network as can be seen from the maximum torque in Figure 2. The m and n orders of the cross-link reaction are shown in Table 2. The presence of a chemical blowing agent in NRF does not affect the n order of the cross-link reaction at higher temperature than 155℃; however, the m order of the cross-link reaction decreases when the amount of chemical blowing agent increases. From the scheme of autocatalytic reaction model as shown in equation (3), the term X^m means the rate of reactant disappearance and product formation. The term (1 − X)ⁿ refers to the formation of hindrance product or intermediate retarding reaction.^42,43 Thus, m order might be related to cross-link network of NRF. The increase of chemical blowing agent content reduces the m order as a result of low cross-link network. From the kinetics study, the n order would be the reaction between persulfenyl radical reacted with sulfurating species to cross-link precursor in sulfur vulcanization system. This mechanism caused the autocatalytic behavior of vulcanization reaction with sulfenamide accelerator system, which Coran AY²⁴ and Ghosh et al.²⁵ have also discussed. Furthermore, the azodicarbonamide can be degraded and amine fragment species can be formed which can accelerate this mechanism. Thus, the more amount of amine fragment species from the faster degradation of azodicarbonamide at high temperature condition, the faster reaction between the persulfunyl radical and sulfurating species could be presented resulting in unchanging of n order at high temperature from very fast reaction. The K value also increases with an additional chemical blowing agent because of the formation of amine fragment species from the decomposition of chemical blowing agent. NRF-BA3 shows low activation energy (E_a) compared to the compound without a chemical blowing agent (NR). When the amount of chemical blowing agent increases to 4 and 5 phr, the E_a value slightly increases caused by high gas volume in the rubber matrix. However, E_a significantly decreases at 6 phr of chemical blowing agent. A decrease in activation energy appears to be triggered by the change in microvoid structure, bubble size, and amine fragment species caused by the decomposition of azodicarbonamide.

Table 2.

Summary of the cure characteristics and kinetic parameters of natural rubber foam in a MBS accelerator-azodicarbonamide system.

Sample name	t_s1 (min)	t_c90 (min)	n order	m order	K(T) (min⁻¹)	E_a (kJ/mol)	R²
NR-155	17.01	25.70	1.43	1.18	1.09	85.61	0.999
NR-160	12.75	16.56	1.38	1.19	1.45
NR-165	9.01	11.87	1.36	1.18	1.89
NR-170	6.44	8.68	1.38	1.21	2.48
NRF-BA3-155	6.43	10.48	1.57	0.91	1.34	72.18	0.989
NRF-BA3-160	4.99	7.12	1.29	0.83	1.55
NRF-BA3-165	3.73	5.29	1.31	0.87	2.17
NRF-BA3-170	2.81	3.86	1.19	0.84	2.57
NRF-BA4-155	5.39	8.48	1.46	0.62	1.15	83.41	0.992
NRF-BA4-160	4.25	6.73	1.57	0.65	1.65
NRF-BA4-165	3.19	4.84	1.44	0.58	1.98
NRF-BA4-170	2.39	3.59	1.42	0.63	2.62
NRF-BA5-155	8.52	8.96	1.60	0.63	1.13	83.85	0.997
NRF-BA5-160	4.50	6.31	1.45	0.56	1.42
NRF-BA5-165	3.24	4.48	1.38	0.55	1.86
NRF-BA5-170	2.80	3.45	1.40	0.52	2.50
NRF-BA6-155	8.68	8.74	1.71	0.59	1.28	52.21	0.975
NRF-BA6-160	N/A	6.24	1.53	0.47	1.37
NRF-BA6-165	N/A	4.33	1.41	0.44	1.68
NRF-BA6-170	N/A	3.48	1.40	0.43	2.08

t_s1 = scorch time; t_c90 = cure time.

The bubbles in the NRF-BA3 foam have larger bubble size compared to other systems due to small content of amine fragment species from the decomposition of azodicarbonamide. The thicker microvoid structure of this system (see Figure 1) initiates an easier cross-linking via sulfur than other NRF specimens. The E_a of NRF-BA3 is lower than those with 4 and 5 phr of the chemical blowing agent. The increase of chemical blowing agent means an increase in amine fragment species, but the microvoid structure changes resulting in less sulfur-induced cross-link reactions. Thus, E_a values of these two systems slightly increase. For NRF-BA6, the number of amine fragment species caused by the decomposition of azodicarbonamide is very high and shows the significant effect on reducing E_a of the cross-link reaction.

Properties of natural rubber foam specimen

The bulk density of both vulcanized NR and NRF specimen at different chemical blowing agent contents was measured to determine the amount of gas trapped in the NR matrix as shown in Figure 7. The bulk density of vulcanized NR is around 0.92 g/cm³, which is the density of common NR. When azodicarbonamide is added to the NR compound, the bulk density of the NRF product significantly decreases to 0.65–0.51 g/cm³ as a result of the trapped gas inside the rubber matrix from the thermal decomposition of chemical blowing agent. The higher the amount of chemical blowing agent, the lower the bulk density from high gas content in rubber matrix could be observed. However, the bulk density of NRF-BA3 and NRF-BA4 is equal which could be because the bubble size of NRF-BA4 is smaller than that of NRF-BA3 (see Figure 1). The bulk density directly relates to the volumetric expansion ratio of NRF specimen which is always compared to that of vulcanized NR because the high amount of gas trapped in the matrix refers to the large dimension of NRF specimen. From this reason, the low bulk density at high amount of chemical blowing agent reveals the large expansion of NRF specimen. The expansion ratio in terms of percent of volume increment compared with the volume of vulcanized NR specimen is shown in Figure 8.

Another property of NRF specimen that should be measured for automotive application is the thermal expansion of specimen when using it at high temperature condition. The thermal expansion coefficients of vulcanized NR and NRF are calculated by a thermo mechanical analyzer (TMA). Because NRF has the normal cure characteristics, the post cure behavior after complete cross-link reaction does not change indicating that the testing temperature does not alter the properties of the NRF. The value of thermal expansion coefficient is calculated in the temperature range of 40℃ and 90℃ because it shows linear behavior as a function of temperature. In Figure 9, the vulcanized NR without chemical blowing agent displays the lowest thermal expansion coefficient because of high cross-link network with no gas bubble inside the rubber matrix. When the chemical blowing agent is added, the thermal expansion coefficient significantly increases due to low degree of cross-link, resulting in easy expansion of specimen. Among NRF specimens, NRF-BA4 shows the lowest thermal expansion coefficient because it has the smallest bubbles with no coalescence between bubbles. The large variation of thermal expansion coefficient of NRF-BA3 and NRF-BA6 relates to their morphology as a result of the non-uniform bubble size. For example, the non-uniform bubble size is due to low gas volume and slow cross-link reaction for NRF-BA3, and some coalescence between bubbles for NRF-BA6 caused by high gas content.

Conclusion

Azodicarbonamide as a chemical blowing agent affects the cross-link reaction and the strength of the NRF product. One of the reasons why the torque of NRF reduces when the amount of chemical blowing agent increases is that the bubbles from the thermal decomposition of the chemical blowing agent change the microvoid structure of the rubber foam system. The rate of vulcanization reaction increases with increasing the chemical blowing agent content because of amine fragment species as by-products from the decomposition of azodicarbonamide. From the autocatalytic reaction model, the amount of chemical blowing agent does not affect the n order at high temperature because the rapid decomposition of chemical blowing agent generates the high amount of amine fragment species that accelerates the reaction between sulfurating species and persulfunyl radical. The high amount of chemical blowing agent significantly affects the m order of the cross-link reaction. The reduction of m order is caused by reducing the cross-link network. Furthermore, NRF-BA3 displays lower activation energy (E_a) than the vulcanized rubber without the chemical blowing agent. However, E_a of NRF-BA4 and NRF-BA5 system slightly increases while E_a of NRF-BA6 decreases. This behavior might be due to the effect of change in microvoid structure, bubble size, and amine fragment species from the decomposition of azodicarbonamide. The bulk density of NRF directly relates to the expansion ratio of specimen due to the high volume of gas in rubber matrix, resulting in high volume of rubber foam specimen. The uniform and small bubbles with no coalescence are observed in NRF-BA4 specimen, resulting in the lowest thermal expansion coefficient when compared with other NRF specimens. In addition, the thermal expansion coefficient in NRF system is higher than that of vulcanized NR due to low cross-link network and soft rubber specimen.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors would like to acknowledge the 90th Anniversary of Chulalongkorn University Scholarship. P. Charoeythornkhajhornchai acknowledges the Royal Golden Jubilee Ph.D. Scholarship (PHD/0264/2553) by Thailand Research Fund for financial support and Dr. Kanet Wongravee from Faculty of Science, Chulalongkorn University, Thailand for useful suggestions. Finally, we would like to thank Sunny World Chemical Co., Ltd., and AF Goodrich chemical Co., Ltd Thailand for providing the chemicals for this research.

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