Thermal And Thermo Oxidative Degredations Of Natural Rubber Biology Essay

Published:

Abstract: Deproteinization of natural rubber was achieved in the latex stage. The structure of deproteinized natural rubber (DPNR) was characterized by fourier transform infrared spectroscopy (FTIR). The thermo degradation of DPNR was studied by thermogravimetry analysis (TG) under air atmosphere and nitrogen atmosphere. The kinetic parameters apparent activation energies (Ea) of the thermal decomposition reaction been calculated from the TG curves using the method described by Broido. And the results were compared with the thermo degradation of natural rubber (NR) under the same conditions. The effect of proteins in natural rubber latex on thermal/ thermo-oxidative stability of NR was discussed. The results show that: the absorptions of the proteins in DPNR at 1546 ㎝-1, compared to NR, become significantly weaker, nearly disappear, which indicates most of proteins has been removed from NR. The thermo degradation of DPNR in nitrogen atmosphere is a one-step reaction. The initial degradation temperature (T0) 、the maximum degradation temperature(Tp) and the final degradation temperature(Tf)as well as the Ea of DPNR are higher than those of NR, which indicates that DPNR represents a better thermal stability than NR under nitrogen atmosphere. Thermo-oxidative degradation of DPNR and NR are two-step reaction. The characteristic temperatures (T0, Tp and Tf) of DPNR are lower than those of NR. The Ea during the First Step of Thermooxidative Degradation of DPNR are also lower than those of NR. These results prove that the thermo-oxidative stability of DPNR is worse than that of NR. Protein is the key role to the thermal stability of natural rubber.

INTRODUCTION

Lady using a tablet
Lady using a tablet

Professional

Essay Writers

Lady Using Tablet

Get your grade
or your money back

using our Essay Writing Service!

Essay Writing Service

It has been reported that natural rubber (NR) latex particles are stabilized by complex layer of proteins and lipids[1] .The presence of proteins in NR has been assumed to exert influence on some undesirable properties such as poor creep and stress relaxation, increasing storage hardening and moisture adsorption [2]. Recently, Latex-allergy mediated by Typeâ… immunoglobulin E (IgE) has been counseled to be hazardous for human beings, because its antigen was confirmed to have several proteins having molecular weight between 14 and 30 kD, present in natural rubber latex [3-4]. Patients frequently exposed to products prepared from natural rubber latex show intra-operative anaphylactic reactions[5],which sometimes causes them to die just after exposing them to latex surgical gloves or condoms and it is recommended these proteins should be removed[6].

Removal of proteins from natural rubber (NR) may be essentially concerned with methods on how to control interactions between the rubber and proteins in the latex stage. Tanaka and coworkers have effectively eliminated proteins from NR latex employing a enzyme/ surfactant combination[7-8]. In recent years Yamamoto removed proteins from NR latex with urea as a denaturant in the presence of a surfactant. The nitrogen content was significantly reduced to 0.02 wt%, which was less than 0.38wt% for the untreated natural rubber. Many other works were focused on the preparation[9-12], structure[13],properties[14-17] and Modification[18-20] of DPNR.

However, there are few reports about the effect of removal proteins from natural rubber on the thermal degradation behavior of DPNR. In present work, the thermal stability of DPNR was studied by thermogravimetry analysis (TG). The kinetic parameters apparent activation energies (Ea) of the thermal decomposition reaction been calculated from the TG curves using the method described by Broido. And the structures of DPNR were analyzed using Fourier transform-infrared (FTIR),

EXPERIMENT

Preparation Samples

Deproteinized natural rubber prepared by Enzymatic(E-DPNR)[21]:The NR latex was deproteinized by incubation of the latex with 0.33% Novo_Alcalase proteolytic enzyme (Alcalase, Novozymes, Denmark) in the presence of 0.125wt % SDS and 0.019% ammonium laurate for 20 h at 308k followed coagulated the latex with 5% acetic acid. The nitrogen content of E-DPNR was 0.092wt%;

Deproteinized natural rubber prepared by Urea/SDS combination (US-DPNR)[22]: The NR latex was deproteinized by incubation of the latex with 0.1% urea at 303k for 2h, then treated with 0.15 wt% SDS for 6 h followed coagulated the latex with 5% acetic acid. The nitrogen content of US-DPNR was 0.089wt%ï¼›

NR: Taken the same batch of NR latex coagulated the latex with 5% acetic acid. The nitrogen content was 0.43wt%ï¼›

FTIR analysis

The DPNR and NR latex was coated onto a clean glass plate directly. After having been air dried, the film was peeled carefully and scanned with the FT-IR spectrometer. The determination was performed on a Perkin-Elmer Spectrum GX-1 FT-IR spectrometer. The FT-IR spectra were recorded in the wave number range of 4000~400 cm-1 with a spectral resolution of 4 cm-1, and the scanning frequency is 64.

Thermal analysis

Lady using a tablet
Lady using a tablet

Comprehensive

Writing Services

Lady Using Tablet

Plagiarism-free
Always on Time

Marked to Standard

Order Now

Thermogravimetric analysis was performed with a Perkin-Elmer TGA-7 thermogravimetric analyzer (Norwalk, CT). The mass of each sample was 5~6 mg. The carrier gas were air and nitrogen respectively, at a flow rate of 50 mlï¹’min-1.The samples were heated from 50 to 650°C at 10℃﹒min-1 for the recording of the thermogravimetry (TG) and differential thermogravimetry (DTG) curves.

RESULTS AND DISCUSSION

FTIR analysis

The FTIR spectra of DPNR and NR are shown in Fig.1. The absorptions at 2921cm-1, 2856cm-1, 1448cm-1 and 1375cm-1 are attributed to the methyl and methene in NR. The absorption at 1661 cm-1, 1087cm-1and 836cm-1 are carbon-carbon double bond in NR. The absorption at 1546cm-1 is acidamide II of proteins in NR[24]. The FTIR spectra of DPNR are similar to those of NR. Their differences are in the absorptions of acidamide II of the proteins, and the absorption in DPNR at 1546 cm-1 is weaker, nearly disappear, which indicates most of proteins has been almost removed from NR.

Fig. 1. FTIR spectra of DPNR and NR

Thermal degradation of DPNR

The TG and the corresponding DTG curves of the thermal degradation of DPNR and NR in nitrogen are presented in Figure 2. The TG curve was a smooth weight loss curve with only one step. The DTG curve showed a single peak. This indicated that the degradation consisted of one weight loss step. The characteristic temperatures for the thermal degradation of DPNR and NR are given in Table 1.

Fig. 2 TG and DTG curves of thermal degradation of DPNR and NR

Table. 1 Characteristic Temperatures for Thermal Degradation of DPNR and NR

Sample

T0/℃

Tp/℃

Tf/℃

NR

361.31

389.76

420.24

US-DPNR

363.91

391.16

422.38

E-DPNR

364.22

392.14

423.04

It can be seen from Table 1, T0、Tp and Tf of DPNR are higher than those of NR, which indicates that in the absence of oxygen, the thermal stability of DPNR is better when compared to that of NR. This is because when protein and other non-rubber material removed, the bonding between rubber molecules became more closely. Thus DPNR behaved a better heat resistance in nitrogen. Moreover, the T0、Tp and Tf of E-DPNR were slightly higher than those of US-DPNR.

The apparent activation energies (Ea) for the thermal degradation of DPNR and NR in nitrogen are calculated from the TG curves using the method described by Broido [23]. Broido method is based on eq. 8. Plots of ln{ln[1/(1-a)]} versus 1000/T for the temperature range of 330~470℃ are shown in Figure 3. It's cleared that the thermal degradation reaction can be decided into two distinct periods. The Ea obtained for the two degradation periods are reported in Table 2.

Fig. 3 The relationships between ln{ln[1/(1-a)] and 1000/T

in thermal degradation of DPNR and NR

Table. 2 Activation Energies during Thermal Degradation of DPNR and NR

Sample

Ea(330~400 ℃)/

(kJ·mol-1)

Ea(400~470 ℃)/

(kJ·mol-1)

Total Ea(330~470 ℃)/

(kJ·mol-1)

NR

188.81

81.76

270.57

US-DPNR

191.65

82.92

274.57

E-DPNR

193.01

83.56

276.57

Table 2 shows that between the temperature range of 330~470℃, Ea of DPNR is higher than that of NR. This indicates that the thermal stability of DPNR is better than that of NR. This conclusion is well corresponding with T0 、Tp and Tf obtained from TG and DTG curves. Moreover, the Ea of E-DPNR was slightly higher than that of US-DPNR. When proteins were decomposed by proteolytic enzyme, the products amino acids and other components,which may catalyze the condensation reaction of molecules[25]. The condensation reactions of aldehyde groups enhance the rigidity and polarity of NR molecules. In this case, the degradation temperature and thermal stability of NR increased. Thus E-DPNR behaved a best heat resistance in nitrogen.

Thermo-oxidative degradation of DPNR

The TG and DTG curves of the thermo-oxidative degradation of DPNR and NR in air are presented in Figure 4. During the early degradation (before 300°C), oxidation, crosslinkage, and chain scission may occur at the same time, and the main reaction may be oxidation and crosslinkage. In this case, no weight loss occurs, or weight loss is slower. During the late degradation (after 300°C), the main reaction is oxidative degradation. It can be seen that there are two steps on the TG curve corresponding to the two peaks on the DTG curves. So the thermo-oxidative degradations of DPNR in air are two-step reactions. The first step occurs in the temperature range of 300~450°C, while the second step is taken place in the temperature range of 450~550°C. Table 3 lists the characteristic temperatures (T0, Tp and Tf ) for the two steps of thermo-oxidative degradation of DPNR and NR.

Lady using a tablet
Lady using a tablet

This Essay is

a Student's Work

Lady Using Tablet

This essay has been submitted by a student. This is not an example of the work written by our professional essay writers.

Examples of our work

Fig. 4 TG and DTG curves of thermooxidative degradation of DPNR and NR

Table. 3 Characteristic Temperatures and Weight Loss Percentage

for Thermooxidative Degradation of DPNR and NR

Sample

NR

US-DPNR

E-DPNR

First step(300~450 ℃)

T0/℃

349.22

342.54

330.89

Tp/℃

381.13

377.81

370.82

Tf/℃

405.55

400.35

399.78

Second step(450~550 ℃)

T0/℃

494.35

493.31

495.23

Tp/℃

523.51

520.76

520.73

Tf/℃

565.09

556.91

546.23

The thermo-oxidative degradation of DPNR consists of two kinds of degradation reactions-thermal degradation and thermal oxidative degradation. In contrast to thermal degradation, the T0, Tp and Tf of DPNR in thermo-oxidative degradation are lower than those of NR. This may be due to that there is about 1-3% natural proteins existing in natural latex, some of which are natural antioxidants[26-27]. For NR most of the proteins are remained during the acid-coagulating process, which enhances the antioxidant ability of NR. But during the removal proteins process, most of the proteins are decomposed or removed, which leads to the reduction of antioxygenic property DPNR. The results of FTIR have proved this point.

The apparent activation energies (Ea) for the thermo-oxidative degradation of DPNR and NR in air can be calculated from the TG curves using the method described by Broido[23]. The first step can be divided into two distinct periods, Period 1 (300~400℃) and Period 2(400~450℃)(Figure 5). The Ea obtained for the two degradation periods are reported in Table 4.

It can be seen from Table 4, the apparent activation energies (Ea) for the thermo-oxidative degradation of DPNR is obviously lower than that of NR. The result indicates that the heat resistance and oxygen resistance of DPNR in air are worse than those of NR. This conclusion is accordant to the results of T0、Tp and Tf obtained from TG and DTG curves.

Fig. 5 The relationships between ln{ln[1/(1-a)]} and 1000/T

in the first step of thermo-oxidative degradation of DPNR and NR

Table. 4 Activation Energies during the First Step of Thermooxidative Degradation of DPNR and NR

Sample

Period 1 Ea

(300~400 ℃)/

(kJ·mol-1)

Period 2 Ea

(400~450 ℃)/

(kJ·mol-1)

Total Ea

(330~450 ℃)/

(kJ·mol-1)

NR

131.15

53.52

184.66

US-LPNR

120.76

36.76

157.52

E-LPNR

116.26

30.61

146.868

CONCLUSIONS

Thermal degradation of DPNR and NR are one-step reactions. The characteristic temperatures (T0, Tp and Tf) of DPNR are higher than those of NR. The corresponding activation energies (Ea) of DPNR are also higher than those of NR, indicating that the thermal stability of DPNR is better than that of NR.

Thermo-oxidative degradation of DPNR and NR are two-step reaction. The characteristic temperatures of DPNR are lower than those of NR. The Ea during the First Step of Thermooxidative Degradation of DPNR are also lower than those of NR. These results prove that the thermo-oxidative stability of DPNR is worse than that of NR.

By compared the thermal and thermo-oxidative degradations of DPNR with those of NR, we can found that proteins make a great influence on the thermal stability of natural rubber.