Synthesis of Hydrogenated Natural Rubber Having Epoxide Groups Using Diimide.

Epoxidized natural rubber (ENR) with 50% mol of epoxide groups was synthesized using performic acid generated from the reaction of formic acid/hydrogen peroxide in latex form followed by hydrogenation using diimide generated from hydrazine (N2H4) and hydrogen peroxide (H2O2) with boric acid (H3BO3) as a catalyst. The resulting products (hydrogenated epoxidized natural rubber, HENR) were characterized by proton nuclear magnetic resonance spectroscopy (1H-NMR), gel testing, transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). The effects of reaction parameters such as N2H4 amount, H2O2 amount, H3BO3 amount, gelatin amount, reaction time, and reaction temperature on the percentage of hydrogenation degree and gel content were investigated. The transmission electron micrographs of the particles confirmed a core/shell structure consisting of a highly unsaturated concentration region as the core encapsulated by a low carbon-carbon double bond concentration region as the shell, which indicated that the rubber particle seemed to be modified from the outer layer to the center of the rubber particle. Overall, the data showed that an increase in the amount of the individual chemicals, reaction time, and temperature increased the hydrogenation degree. However, a higher level of gelatin retarded an increase in the percentage of hydrogenation degree. As the hydrogenation degree increased, the gel content increased due to the ether linkage and the crosslinking reaction triggered through hydroxyl radicals. From DSC measurements, the glass transition temperatures of hydrogenated products increased above those of original rubbers. The thermal stability of hydrogenated products was improved, demonstrated by a decomposition temperature shift to a higher temperature than ENR, as shown by the results from the thermogravimetric analysis. Therefore, the hydrogenated ENR (HENR) exhibited good thermal stability, which could extend the applications of ENR in the automotive and oil industries.

Introduction

Natural rubber (NR) obtained from Hevea brasiliensis is a renewable material possessing excellent characteristics such as high tensile strength due to its ability to crystallize upon stretching.[1] However, the disadvantages of NR are its low heat, oxygen, and oil resistance, mainly because of its unsaturated chain structure.[2] In addition, it does not perform well when exposed to oils and hydrocarbon solvents due to its non-polar character,[3] having limitations in demanding applications. Epoxidation of NR (ENR), using peracid produced in situ, is the reaction to convert the unsaturated units into epoxide rings, which have turned out to be an attractive material. It has high polarity due to the epoxide groups in the chain, bringing about an improved resistance toward oils and hydrocarbon solvents[4] while retaining superior strength and fatigue properties due to undergoing strain-induced crystallization like that of NR. As the percentage of the epoxide group content increases, the glass transition temperature (Tg) increases, thus resulting in decreasing resilience and air permeability. Two grades of ENR, ENR-25 (25% epoxidation) and ENR-50 (50% epoxidation), have attained commercial importance. ENR-25 can be utilized for tire products because of its good wet-grip and rolling resistance.[5] In contrast, the oil resistance property of ENR-50 was comparable to that of nitrile-butadiene rubber (NBR) with 34% of the acrylonitrile group.[6] Natural rubber latex was epoxidized with performic acid generated from a formic acid and hydrogen peroxide reaction under various reaction conditions by Heping et al.[7] They reported that the glass transition temperature (Tg), thermal degradation temperature, and activation energy of thermal degradation of the ENRs increased with the extent of epoxidation. Tanrattanakul and co-workers[8] synthesized an in situ epoxidized natural rubber (NR) from 20% dry rubber content NR latex with performic acid at a temperature of 50 °C and systematically studied the oil resistance and mechanical properties of the synthesized ENR. They illustrated that the ENR showed tensile properties and tear resistance as good as NR and improved resistance to oils and solvents such as petroleum ether, ASTM no. 3 oil, and automobile oils. Chuayjuljit et al.[3] synthesized ENR via in situ epoxidation from NR latex with performic acid using different reaction times. They found that the Tg increased as the epoxide group content increased. The prepared ENRs were compounded and vulcanized. They reported that as the epoxide group content increased, the hardness and oil resistance increased. After aging, all rubbers showed a deteriorated tensile strength and lower elongation at break but higher hardness. The oil resistance of ENRs was significantly better than that of NR, especially the oil resistance of ENR with 63.9% epoxide group content, which was comparable to that of nitrile-butadiene rubber. However, He et al.[9] examined the thermal oxidative degradation of ENR with 50% epoxide group content (ENR-50) with TGA and DTA. They have reported that the thermal stability of ENR-50 is worse than that of NR, thus limiting its use in outdoor applications. The primary drawback of ENR, like NR, is its poor heat aging properties. Hydrogenation is one of the most efficient methods used to reduce the degree of unsaturation in polymers by adding hydrogen atoms to the unsaturated units, thus enhancing the thermal stability of the polymer. The hydrogenation of ENR is expected to yield a product with improved heat resistance due to the conversion of carbon–carbon unsaturated units to saturated units. There are several methods to hydrogenate polydienes that involve catalytic and non-catalytic procedures. In the catalytic hydrogenation process, polydienes dissolved in an organic solvent are hydrogenated selectively under high pressures of hydrogen at elevated temperatures using a transition metal catalyst.[10] Magnetic Fe3O4 nanoparticles embedded in graphene oxide behave as a highly efficient and reusable heterogeneous nanocatalyst for alkene hydrogenation in ethanol at 80 °C temperature using hydrazine hydrate as the hydrogen source to deliver the corresponding alkanes in good to excellent yields together with a 4–20 h reaction time.[11] Generally, transition metal ions are not effective as catalysts in latex systems because the metal ions may be isolated from the reactants in the aqueous phase. In the case of non-catalytic hydrogenation, diimide (N2H2) is widely used as a hydrogen-donating agent. Diimide is a short-lived intermediate that can be generated by various methods. An efficient method was developed for the hydrogenation with diimide, generated in situ from hydrazine hydrate by oxidation with oxygen. The hydrogenation process proceeded for 24–48 h with excellent yields. This procedure offers synthetic advantages over metal-catalyzed hydrogenation.[12] He et al.[13] studied the conditions for the hydrogenation of styrene-butadiene rubber (SBR) latex via the diimide reduction process. They pointed out that the particle surface is an important parameter in controlling the hydrogenation degree. They also found that the gel fraction of SBR latex increased after the hydrogenation. Samran et al.[14] investigated the hydrogenation of NR and various ENRs using diimide generated in situ from the thermal decomposition of p-toluenesulfonylhydrazide in o-xylene solution at 135 °C. They showed that the Tg of the hydrogenated rubbers was increased by about 10–20 °C compared with the starting rubbers. The effects of reaction conditions for hydrogenated styrene-butadiene rubber (HSBR), such as reaction time and temperature, pH, and concentrations of hydrogen peroxide, hydrazine, and catalysts, have been reported by De Sarkar et al.[15] The reaction conditions directly affect the hydrogenation degree and give rise to an increase in the Tg with an increasing hydrogenation degree due to the development of crystalline segments. Lin et al.[16] investigated the hydrogenation of nitrile-butadiene rubber latex via utilization of diimide, generated by the oxidation of hydrazine (N2H4/H2O2). They claimed that the hydrogenation efficiency when a copper ion, silver ion, or ferrous ion was used as a catalyst was lower than the hydrogenation efficiency in boric acid. The reaction conditions directly affect the hydrogenation degree and give rise to an increase in the glass transition temperature (Tg) with an increasing hydrogenation degree due to the development of crystalline segments. Unfortunately, hydrogenated unsaturated rubber synthesized via diimide hydrogenation in latex form is prone to a crosslinking side reaction, producing gelled or crosslinked hydrogenated rubber latex particles.[17] It is commonly known that a high gel content has poor mechanical properties and is not beneficial to milling hydrogenated unsaturated rubber when used as dried rubber because extra energy will be consumed to break the crosslinking structures.[18] Roy et al.[19] prepared the hydrogenation of ENR (HENR) with a homogeneous catalyst at 323 K in a solution phase. They reported that HENR reduced both the green strength, a rubber’s resistance to deformation and fracture before vulcanization, and the modulus at 300% elongation, whereas the elongation at break was increased marginally. They also reported that HENR showed a better thermal stability and oil resistance. Saengdee and co-workers[20] studied the preparation condition of modified NR by prior epoxidation using performic acid followed by hydrogenation using diimide in latex form. Compared to the NR, they reported that HENR showed improved thermal and mechanical properties and oil and ozone resistance. They suggested that HENR can overcome drawbacks of NR, which could extend the applications of NR.

In the present study, we attempt to prepare hydrogenated epoxidized natural rubber (HENR). First, ENR-50 was synthesized by in situ epoxidation of natural rubber (NR) using performic acid generated from the formic acid/hydrogen peroxide reaction in the latex stage. The ENR with 50% epoxide group content was then hydrogenated by diimide reduction in a water system using hydrazine reacted with hydrogen peroxide and boric acid as a catalyst. The effects of the amount of the individual chemicals (N2H4, H2O2, H3BO3, and gelatin) and hydrogenation conditions such as reaction time and temperature on the percentage of hydrogenation degree (% HDresidual double bond) characterized by proton nuclear magnetic resonance spectroscopy (1H-NMR) were investigated. The possibility of crosslinking (gel content) during a hydrogenation reaction was also mentioned. The thermal properties of the resulting product were examined by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA).

Experimental Studies

Materials

The commercial high-ammonia natural rubber latex comprises almost entirely cis-polyisoprene, with 60% by weight dry rubber content (DRC). It was produced by Yala Latex Industry Co., Ltd. (Yala, Thailand). Aqueous hydrogen peroxide (50 wt %), formic acid (85 wt %), and hydrazine hydrate (85 wt %, N2H4·H2O) were purchased from Sigma-Aldrich. Polyoxyethylene styrenated phenyl ether (C24H22O2) used as a surfactant under the trade name Emulvin WA was purchased from Chemical and Materials Co., Ltd., Thailand. Gelatin was purchased from J R F & B Co., Ltd. (Bangkok, Thailand). Methanol (commercial grade), 99.5 wt % boric acid (H3BO3), 30 wt % ammonium hydroxide solution (NH4OH), and an antifoaming agent (silicone oil) were bought from Facobis Co., Ltd. (Bangkok, Thailand). All chemical reagents were used as received. Deionized water was used throughout the work.

Preparation of In Situ Epoxidized Natural Rubber

Diluted natural rubber latex (500 g) with 20% DRC was mixed with 100 g of 10 wt % aqueous solution of Emulvin WA surfactant in a 1 L three-neck flask mounted with an overhead agitator. After stirring for 1 h, 0.3 mol of formic acid and 0.9 mol of hydrogen peroxide were added dropwise into the reactor slowly and were stirred under a constant reaction temperature of 40 °C for 12 h. The latex was left for 24 h at room temperature to obtain the partially epoxidized natural rubber (ENR) latex, which was used as the starting reactant for the hydrogenation reaction. Then, the pH of ENR latex was adjusted to pH 10 by ammonium hydroxide to convert the residual formic acid to formate ions so not to disturb the following step of hydrogenation. ENR latex was precipitated using methanol to form the coagulated rubber and washed three times with deionized water. The gross polymer was recovered and dried to constant mass in a vacuum oven at 40 °C. The percentage of epoxide group content (% EP) was determined by 1H-NMR spectroscopy.

Hydrogenation of Epoxidized Natural Rubber (HENR)

ENR latex (about 15%) prepared by the above method providing about 50% mol epoxidation was put into a 1 L four-neck round-bottom flask. After stirring, hydrazine hydrate and dissolved boric acid and gelatin were dropped into the latex. A water bath was used to maintain the desired temperature. Aqueous hydrogen peroxide was added dropwise using a peristaltic pump at 12 mL/min. Slowing down the addition rate of H2O2 can help in achieving a higher efficiency of diimide generation and has a beneficial effect on gel reduction.[17] During the addition of hydrogen peroxide, two to three drops of silicone oil were added to reduce foaming if too many bubbles were formed. When hydrogen peroxide was added, the reaction was left to proceed under a constant stirring rate. The product latex was precipitated using methanol to form the coagulated rubber. The product was washed three times with deionized water. The gross polymer was recovered and dried to constant mass in a vacuum oven at 40 °C. The percentage of hydrogenation degree (% HDresidual double bond) was determined by 1H-NMR spectroscopy.

Characterization

Determination of the Percentage of Epoxide Group and Hydrogenation Degree

The percentage of epoxide group and hydrogenation degree was examined by proton nuclear magnetic resonance (1H-NMR) spectroscopy. The sample was dissolved in deuterated chloroform (CDCl3) at room temperature. After the sample has been dissolved, it was transferred directly to an NMR tube by allowing the solution to pass through a cotton filter. The spectra were recorded on a Bruker 300 MHz spectrometer (Bruker BioSpin Corp., Massachusetts, USA). The qualitative and quantitative analyses of the functional groups of NR and modified natural rubbers were identified by 1H-NMR spectroscopy, as shown in Figures and 2. The chemical shift was reported in parts per million (ppm). The percentage of epoxide group content before hydrogenation was calculated, as shown in eq .[21] The conversion of residual carbon–carbon double bond content after epoxidation can be used to calculate the percentage of hydrogenation degree using eq (22)where I is the signal intensity and the subscripts represent a value of the chemical shift.

Figure 1

1H-NMR spectra of NR and ENR-50.

Figure 2

1H-NMR spectra of ENR-50 and HENRs.

Determination of Gel Content

A gel test is the ASTM standard test method for determining the gel content (insoluble fraction) for the polymer’s crosslinking linkages, according to ASTM D3616. Approximately 0.4 ± 0.05 g of each sample was weighed and placed on a screen rack (50 mesh) in a borosilicate bottle, and 100 cm3 of toluene was added into the bottle. The sample was soaked in toluene for 20 h. According to eq , the liquid was pipetted to determine the gel contentwhere A is the mass of the original samples and B is the residual mass after solvent evaporation. A test result was regarded as the average of two determinations.

Morphology Study

Osmium tetroxide (OsO4) is the most used chemical for staining unsaturated sections to increase the contrast and gradient of the particles. The rubber surface was observed using a JEM-1230 transmission electron microscope (TEM) with a magnification of 30,000 at an accelerating voltage of 80 kV. The diluted rubber latex was dropped on microscope grids and was exposed to OsO4 vapor in glass-covered dishes at room temperature for 24 h before observation. The relationship between the reaction conversion and particles morphology was studied.

Thermal Property Analysis

The degradation temperature was obtained from thermogravimetric analysis (TGA) and differential thermal analysis (DTA) (Perkin–Elmer Pyris Diamond, TGA/DTA). The mass of the sample was recorded continuously since the 10 mg sample was placed on a platinum pan. At the same time, the heat was applied to increase the temperature at a constant rate from room temperature to 650 °C under nitrogen at a flow rate of 50 mL/min and heating rate of 10 °C/min. Weight losses occur when volatiles adsorbed by the polymer are driven off and at higher temperatures when degradation of the polymer occurs with the formation of volatile products.[23] The initial, final, and decomposition temperatures at weight loss of 5% (Td5) and 95% (Td95) and maxima of the weight loss (Tmax) of samples were measured. The glass transition temperature (Tg) was measured by differential scanning calorimetry (DSC, Perkin Elmer Instrument DSC 8500). A sample mass of 10 mg encapsulated in an aluminum pan was measured for the Tg at a temperature of −90 °C to 50 °C at a heating rate of 10 °C/min.

Results and Discussion

1H-NMR Characterization

The in situ epoxidation of the NR latex using hydrogen peroxide and formic acid also yields epoxidized natural rubber (ENR). The following reaction scheme is proposed for the epoxidation of the NR latex by the performic acid method:

The characteristics of the NR latex were analyzed based on proton nuclear magnetic resonance (1H-NMR). As for NR, three signals characteristic of methyl (a), methylene (b), and unsaturated methine protons (c) of cis-1,4-isoprene units appeared at 1.68, 2.05, and 5.10 ppm, respectively. After epoxidation, the other three signals appeared at 1.29, 1.58, and 2.70 ppm, which were assigned to methyl (d), methylene (f), and oxirane methine protons (e) of the resulting epoxide group. The results reveal that the increasing signals at 2.70 resulted from the increment of the epoxide group, as shown in Figure . This provides evidence that a part of the carbon–carbon double bonds of NR is converted to epoxide groups. The percentage of epoxide group content of the starting reactant ENR-50 was 49.51%.

Two steps achieve the diimide hydrogenation reaction: (1) the reaction between hydrazine and hydrogen peroxide to produce diimide [eq ] and (2) the reaction between diimide and carbon–carbon double bonds to form hydrogenated ENR [eq ]

Figure shows the 1H-NMR spectra of ENR-50, HENR-14 (60.50% HDresidual double bond), HENR-06 (67.61% HDresidual double bond), and HENR-24 (88.48% HDresidual double bond). After hydrogenation of ENR, new signals appeared around 1.30–1.40 ppm, 1.10 ppm, 0.84 ppm, and 3.70 ppm, which were assigned to the methylene protons (h), methine proton (i), methyl proton (g), and ether unit (k), respectively, whereas the signals at 2.70 and 5.10 ppm decreased as the hydrogenation proceeded.[21,24] The percentages of the epoxide group content and hydrogenation degree were calculated from the integrated peak area of these signals based on the 1H-NMR spectrum according to eqs and 2, respectively. The percentage of epoxide group content of ENR used in the present study was 49.51% (ENR-50); hence, the estimated residual carbon–carbon double bond content was 50.49%.

Morphology of HENR Particles

The morphology of NR, ENR-50, and HENR-25 (89.19% HDresidual double bond) particles was observed from the TEM micrographs, as shown in Figure . NR, ENR-50, and HENR-25 are spherical and have smooth surfaces. Since the OsO4 staining agent can only stain at the carbon–carbon double bonds, the lightly colored domain indicates a region of low carbon–carbon double bond content. NR showed a relatively sharp particle edge because of the high OsO4 amount inside the particle. For ENR-50, the contrast between the core and the shell of the modified NR particle was quite different, as shown in Figure b. The dark color domain indicated the particle region with a high carbon–carbon double bond content as the core, while the lighter color domain at the outer layer as the shell indicated a low carbon–carbon double bond content for OsO4 staining. HENR-25 exhibited a much lighter color domain due to the small carbon–carbon double bond content, as shown in Figure c. The epoxidation and hydrogenation reaction occurred from the outer surface to the center of the rubber particle, suggesting a heterogeneous structure (core/shell morphology).

Figure 3

TEM micrographs of (a) NR, (b) ENR-50, and (c) HENR-25 (magnification 30,000×).

Effect of Reaction Time on the Hydrogenation of ENR

The reaction time usually plays a significant role in a chemical reaction. The effect of reaction time on the percentage of hydrogenation degree and gel content was studied over the range of approximately 0 to 10 h while keeping the concentration of all other reagents constant under a reaction temperature of 40 °C, as shown in Table . The residual double bond conversion and gel content were sharply increased and almost unchanged for a reaction time greater than 8 h. According to eq , it can be seen that diimide was formed as an active species at the surface of the particle and was consumed toward the surface of the unsaturated region of the latex particles. Then, the residual double bond content was reduced due to the percentage of hydrogenation degree, as shown in eq . Thus, the fast increment in the percentage of hydrogenation degree happened during the first period of the reaction. According to the layer model for diimide hydrogenation reported by Sakorn et al.,[25] the hydrogenation occurred from the outer layer and then into the inner particle. The gel content increased as the percentage of hydrogenation degree increased due to the linking between the rubbery chains. Unfortunately, gel formation was found during the progress of the reaction with time. It has been reported that two polymeric chains carrying epoxide groups can easily be crosslinked through the formation of ether linkage because the oxirane is easily ring-opened in alkali media, as shown in eqs and 9.[26,27] Referring to Figure , a new signal around 3.70 ppm during hydrogenation was attributed to the ether unit. This signal is not observed in the case of ENR. As a result, the gel content of the resultant rubber increased by increasing the percentage of hydrogenation degree. For reaction times above 8 h, the diffusion of the diimide species was retarded in the transformation of carbon–carbon double bonds to carbon–carbon single bonds due to a thicker saturated layer and the linking between the rubbery chains that acted as a barrier, thus resulting in a limited hydrogenation level.

Table 1

Effect of Reaction Time on the Hydrogenation of ENRa

samples	time (h)	% HDresidual double bond	gel content (%)
ENR-50	0	0.00	27.54
HENR-01	2	34.43	30.52
HENR-02	4	61.53	35.99
HENR-03	6	73.52	45.65
HENR-04	8	77.38	62.11
HENR-05	10	77.59	63.63

Conditions: N2H4·H2O, 0.22 mol; H2O2, 0.22 mol; H3BO3, 11 mmol; gelatin, 0.22 g; T, 40 °C.

Effect of Hydrazine Hydrate on the Hydrogenation of ENR

The influence of hydrazine (N2H4) used as the source for diimide production on the hydrogenation of ENR was studied over the range of 0.15–0.44 mol with a fixed amount of all other reagents under a reaction temperature of 40 °C for 6 h. With an increasing hydrazine hydrate, the percentage of hydrogenation degree was slightly increased. The main reaction is the N2H4/H2O2 reaction to produce the diimide species attached to the unsaturated ENR chains, as shown in eqs and 7. Table shows that more diimide molecules were generated from the redox system with an increasing hydrazine amount, thus leading to an increment in the percentage of hydrogenation degree. Above 0.22 mol of N2H4, the amounts of H2O2 were consumed to generate diimide molecules that were needed to hydrogenate the unsaturated units. Consequently, the amounts of H2O2 were insufficient to decompose more hydroxyl radicals. Thus, the gel content decreased with the presence of excess amounts of hydrazine hydrate.

Table 2

Effect of Hydrazine Hydrate on the Hydrogenation of ENRa

samples	N2H4 (mol)	% HDresidual double bond	gel content (%)
HENR-06	0.15	67.61	40.11
HENR-07	0.18	68.35	44.91
HENR-03	0.22	73.52	45.65
HENR-08	0.29	74.27	43.07
HENR-09	0.44	77.69	40.44

Conditions: H2O2, 0.22 mol; H3BO3, 11 mmol; gelatin, 0.22 g; T, 40 °C; time, 6 h.

Effect of Hydrogen Peroxide on the Hydrogenation of ENR

The effect of hydrogen peroxide on the percentage of hydrogenation degree was determined over a range of 0.11–0.33 mol. The amount of H2O2 is a crucial factor in diimide generation. From Table , it can be seen that the percentage of hydrogenation degree increased as H2O2 increased. Both percentages of hydrogenation degree and gel content increased with an increase in the H2O2 amount up to 0.22 mol, and then the percentage of hydrogenation degree and gel content increased slightly. Therefore, the increasing trend may be due to the high amount of diimide generation and the high possibility of reacting with more carbon–carbon double bonds. When the amount of H2O2 was over 0.22 mol, the percentage of hydrogenation degree increased slightly. It may be due to the side reaction of H2O2, as shown in eq . The gel content increased obviously with an increase in the amount of H2O2. The excessive H2O2 may be dissociated into hydroxyl radicals (HO•) that attacked the allylic proton of the cis-1,4-polyisoprene unit, forming a polymeric radical unit. The polymeric radicals can interact with each other to produce a crosslinking reaction, resulting in gel formation, as shown in eqs –13.[20,28]

Table 3

Effect of H2O2 on the Hydrogenation of ENRa

samples	H2O2 (mol)	% HDresidual double bond	gel content (%)
HENR-10	0.11	47.73	32.84
HENR-11	0.16	60.07	35.35
HENR-03	0.22	73.52	45.65
HENR-12	0.27	74.71	49.93
HENR-13	0.33	77.36	60.11

Conditions: N2H4·H2O, 0.22 mol; H3BO3, 11 mmol; gelatin, 0.22 g; T, 40 °C; time, 6 h.

Effect of Boric Acid on the Hydrogenation of ENR

The effect of boric acid on the percentage of hydrogenation degree was studied by varying the amount of H3BO3 over the range of 5.5–27.5 mmol, as shown in Table . So, the perusal of the results indicates that the percentage of hydrogenation degree and gel content increased rapidly with an increase in the H3BO3 amount up to 22.0 mmol. Then, the percentage of hydrogenation degree and gel content increased marginally. This may be due to the boric acid accelerating the diimide formation and activating the reaction of gel formation. The addition of boric acid as a catalyst could improve the disassociation of hydrazine and reduce the diimide side reactions [eqs and 15].[29]

Table 4

Effect of Boric Acid (H3BO3) on the Hydrogenation of ENRa

samples	H3BO3 (mmol)	% HDresidual double bond	gel content (%)
HENR-14	5.5	60.50	35.93
HENR-03	11.0	73.52	45.65
HENR-15	16.5	75.82	52.08
HENR-16	22.0	77.36	53.11
HENR-17	27.5	77.49	54.65

Conditions: N2H4·H2O, 0.22 mol; H2O2, 0.22 mol; gelatin, 0.22 g; T, 40 °C; time, 6 h.

Effect of Gelatin on the Hydrogenation of ENR

The influence of the presence of gelatin was studied by using the variation in the gelatin amount over the range of 0.11–0.33 g, as shown in Table . The percentage of hydrogenation and gel content increased with an increase in gelatin amount from 0.11 to 0.22 g. The addition of gelatin to the system can help to stabilize the catalyst on the rubber particle surface, thus increasing the percentage of hydrogenation degree. The percentage of hydrogenation degree and gel content decreased at a higher gelation amount. The presence of excess amounts of gelatin inhibited the diimide diffusion at the rubber particle surface, resulting in a decreasing percentage of hydrogenation degree.

Table 5

Effect of Gelatin on the Hydrogenation of ENRa

samples	gelatin (g)	% HDresidual double bond	gel content (%)
HENR-18	0.11	49.74	35.12
HENR-19	0.16	65.75	38.00
HENR-03	0.22	73.52	45.65
HENR-20	0.27	62.36	37.48
HENR-21	0.33	60.72	35.12

Conditions: N2H4·H2O, 0.22 mol; H2O2, 0.22 mol; H3BO3, 11 mmol; T, 40 °C; time, 6 h.

Effect of Reaction Temperature on the Hydrogenation of ENR

According to the Arrhenius equation, the reaction temperature usually plays an important role in hydrogenation. A series of experiments were carried out from 30 to 70 °C. Table indicates the effect of the reaction temperature on the percentage of hydrogenation degree and gel content. The percentage of hydrogenation degree and gel content increased as the hydrogenation temperature increased up to 50 °C. As the reaction temperature elevated, both the activity of the reactant molecules and the probability of particle collision increased, increasing the percentage of hydrogenation degree. With an increase in temperature, H2O2 tended to decompose and produce hydroxyl radicals causing the crosslinking reaction triggered through hydroxyl radicals, resulting in gel formation, as shown in eqs and 13. When the temperature was above 50 °C, the percentage of hydrogenation degree slightly increased because of the formation of gas bubbles during the addition of hydrogen peroxide. The gel content remained almost unchanged.

Table 6

Effect of Temperature on the Hydrogenation of ENRa

samples	temperature (°C)	% HDresidual double bond	gel content (%)
HENR-22	30	43.28	30.99
HENR-03	40	73.52	45.65
HENR-23	50	86.34	62.62
HENR-24	60	88.48	63.63
HENR-25	70	89.19	63.64

Conditions: N2H4·H2O, 0.22 mol; H2O2, 0.22 mol; H3BO3, 11 mmol; gelatin, 0.22 g; time, 6 h.

In this study, the percentage of gel content increased from 30.52 to 63.64 when the percentage of hydrogenation degree increased from 34.43 to 89.19. A similar observation was reported by Saengdee and co-workers[20] who also found that hydrogenated ENR samples (27 mol % degrees of hydrogenation and 17 mol % degrees of epoxidation and 25 mol % degrees of hydrogenation and 28 mol % degrees of epoxidation) were low in gel content (44.3% and 41.7%, respectively). Yusof et al.[28] suggested that materials with gels could be used differently as a lubricant (low gel content) or adhesive (high gel content).

Thermal Stability of the Hydrogenation of ENR

The glass transition temperature is the temperature below which the amorphous domains of a polymer take on the characteristic properties of a glassy state: brittleness, stiffness, and rigidity. The result for the Tg is shown in Figure and summarized in Table . The Tg of ENR with 50% epoxide group content was −19.6 °C. The Tg of HENR slightly increased with an increase in the percentage of hydrogenation degree. Thus, the increment in Tg of the hydrogenated product may result from a decrease in unsaturated units (amorphous segments) by replacing the ethylene-propylene units (crystalline segments) in the polymer chain, which tends to decrease the mobility of the polymer. However, HENR still retains its rubbery behavior.

Figure 4

DSC thermograms of ENR-50, HENR-19, HENR-06, HENR-12, and HENR-25.

Table 7

Glass Transition (Tg) and Decomposition Temperature (Td5, Td95, and Tmax) of ENR-50 and HENR with Varying % HDresidual double bond

			Td (°C)
samples	% HDresidual double bond	Tg (°C)	Td5	Tmax	Td95
ENR-50		–19.6	347.0	399.0	446.7
HENR-19	65.75	–19.7	368.3	405.3	466.3
HENR-06	67.61	–18.2	370.0	421.7	469.3
HENR-12	74.71	–17.6	371.7	439.0	486.3
HENR-25	89.19	–14.3	396.7	442.0	488.3

Figure shows the TGA and DTG curves of ENR-50 and HENR at various % hydrogenations. The thermal degradation temperature of modified NR is listed in Table . The temperatures at 5% (Td5) and 95% (Td95) of its weight losses were obtained from the TGA curve, and the temperature at the maximum weight loss rate (Tmax) was obtained from the peak of the DTG curves. It can be seen that the thermal degradation temperature of hydrogenated ENR samples increased with an increase in the percentage of hydrogenation or reduction of C=C bonds in ENR. The degradation of ENR and HENRs was similar and occurred via an overall one-step reaction and smooth weight loss curves. ENR has a Td5, Tmax, and Td95 of 347.0, 399.0, and 446.7 °C, respectively, while all HENRs exhibit higher degradation temperatures when compared with ENR. The Tmax of HENRs increased from 405.3 to 442.0 °C when the percentage of hydrogenation degree increased from 65.75% (HENR-19) to 89.19% (HENR-25). The thermal degradation curves of HENRs, compared to ENR, shifted toward higher temperatures. It could be concluded that the hydrogenation of epoxidized natural rubber shows enhanced thermal stability and an increase in degradation temperature.

Figure 5

(a) TGA and (b) DTG thermogram of ENR-50, HENR-19, HENR-06, HENR-12, and HENR-25.

Conclusions

Hydrogenated ENR-50 with different hydrogenation degrees was prepared by the latex method. 1H-NMR could determine the percentage of hydrogenation degree. TEM micrographs of the modified natural rubber particles confirmed the carbon double bond reduction. The analysis of the results showed that the increase in hydrazine hydrate, hydrogen peroxide, boric acid, and gelatin, the prolongation of reaction time, and the reaction temperature were favorable to the extent of hydrogenation. This can lead to either the formation of the ether linkage or the crosslinking reaction triggered through hydroxyl radicals. Thus, the gel content increased with an increasing degree of hydrogenation. The hydrogen peroxide amount and temperature appeared to be the main factors resulting in the gel formations. The ENR having 50% epoxide group content without hydrogenation contained around 27.54% of gel; when the percentage of hydrogenation reached 89.19%, the gel content was 63.64%. The thermal stability of the hydrogenated ENR-50, examined by TGA, was improved, and the glass transition temperature of hydrogenated ENR-50 kept increasing, which was higher than that of the original ENR-50. The hydrazine hydrate/hydrogen peroxide/boric acid as a catalyst system could result in the hydrogenation of ENR having 50% epoxide group content under a mild condition because it was carried out at atmospheric pressure and low temperatures.