氏名　ORAPHIN CHAIKUMPOLLERT
学位の種類　博士（工学）
学位記番号　博甲第600号
学位授与の日付　平成23年8月31日
学位論文題目　Study on Structure and Morphology of Natural Rubber and Its Vulcanizates　（天然ゴムおよびその加硫物の構造とモルフォロジーに関する研究）
論文審査委員
　主査　准教授　河原　成元
　副査　教授　五十野　善信
　副査　教授　塩見　友雄
　副査　教授　佐藤　忠久
　副査　准教授　竹中　克彦
　副査　准教授　前川　博史

• 論文目次
• 論文要旨
• 論文審査の結果の要旨

［平成23（2011）年度博士論文題名一覧］ ［博士論文題名一覧］に戻る．

Table of Contents

CHAPTER 1
Introduction
1.1　Inherent structure of natural rubber　p.2
1.2　Degradation of natural rubber after production process　p.5
1.3　Purification of natural rubber　p.6
1.4　Crosslinking juntions of vulcanized natural rubber　p.7
1.5　Outline of this thesis　p.10
1.6　References　p.13

CHAPTER 2
Preparation and characterization of protein-free natural rubber
2.1　Introduction　p.15
2.2　Experimental　p.17
　2.2.1　Material　p.17
　2.2.2　Deproteinization of natural rubber　p.18
　2.2.3　Charaterizations　p.19
2.3　Results and Discussion　p.22
　2.3.1　Effect of surfactant　p.22
　2.3.2　Effect of polar organic solvent　p.24
　2.3.3　Effect of solvent concentration　p.26
　2.3.4　DPNR characterization　p.27
　2.3.5　Morphology　p.32
　2.3.6　Mechanical propertives of the natural rubber　p.35
2.4　Conclusion　p.36
2.5　References　p.37

CHAPTER 3
Morphology and properties of natural rubber with nanomatrix of non-rubber components
3.1　Introduction　p.39
3.2　Experimental　p.41
　3.2.1　Sample preparation　p.41
　3.2.2　Characterizations　p.42
3.3　Results and Discussion　p.43
　3.3.1　Characteristis of natural rubber　p.43
　3.3.2　Morphology　p.44
　3.3.3　Physical propertives of the natural rubber　p.46
3.4　Conclusion　p.49
3.5　References　p.49

CHAPTER 4
Effect of decelerated fermentation of natural rubber ;atex on morphology and mechanical properties of the rubber
4.1　Introduction　p.51
4.2　Experimental　p.53
　4.2.1　Material　p.53
　4.2.2　Preparation of concentrated natural rubber latex　p.53
　4.2.3　Characterizations　p.54
4.3　Results and Discussion　p.55
　4.3.1　Morphology　p.62
　4.3.2　Mechanical properties of the natural rubber　p.66
4.4　Conclusion　p.69
4.5　References　p.69

CHAPTER 5
Low temperature degradation and characterization of natural rubber
5.1　Introduction　p.71
5.2　Experimental　p.73
　5.2.1　Material　p.73
　5.2.2　Effect of potassium persultfate（KPS）　p.73
　5.2.3　Vulcanization　p.74
　5.2.4　Characterizations　p.74
5.3　Results and Discussion　p.75
　5.3.1　Characterization of natural rubber as a source　p.75
　5.3.2　Degradation of natural rubber with potassium persulfate　p.82
　5.3.3　Tensile strength of the rubber　p.88
5.4　Conclusion　p.89
5.5　References　p.89

CHAPTER 6
Analysis of Crosslinking Junctions of Vulcanized Natural Rubber and Its Mechanical Properties
6.1　Introduction　p.91
6.2　Experimental　p.94
　6.2.1　Vulcanization of natural rubber in the latex stage　p.94
　6.2.2　Vulcanization of natural rubber in the solid stage　p.94
　6.2.3　Characterizations　p.96
6.3　Results and Discussion　p.98
　6.3.1　Crosslinking junctions of the vulcanized natural rubber　p.98
　6.3.2　Crosslinking junctions of the CV, EV and SemiEV rubbers　p.108
　6.3.3　Mechanical properties of the vulcanized natural rubber　p.116
　6.3.4　Possible crosslinking junctions of vulcanizesd natural rubber　p.118
6.4　Conclusion　p.119
6.5　References　p.120

CHAPTER 7
Conclusion　p.123

List of Publications and Presentations

Acknowledgements

Structure of natural rubber may be a primary factor to control properties of the rubber, although effects of molecular structure such as terminal units, non-rubber components and crosslinking junctions are not rationally elucidated. It is, thus, quite important to investigate a relationship between the structure and the properties of natural rubber in term of various techniques. Recently, some analytical techniques have been developed for the study on the molecular structure of natural rubber. Using the techniques, we may obtain a deeper understanding on the effective structural units of natural rubber, which dominantly govern the outstanding properties of the rubber. Moreover, morphology of natural rubber may play an important role in the properties of the rubber; for instance, nano phase structure was found to be associated with outstanding mechanical properties of the rubber. In the present work, effects of both the molecular structure and the morphology of natural rubber on the mechanical properties of the rubber were investigated in term of the analysis of the non-rubber components, the terminal units and the crosslinking junctions, in order to understand the relationship between the structure and the properties of natural rubber.
First, the effect of the non-rubber components such as the proteins was studied by removing the proteins from natural rubber. A novel method was proposed to remove the proteins completely. The protein-free natural rubber was prepared by incubation of the natural rubber latex with urea and polar organic solvent in the presence of surfactant. Under an optimum condition, total nitrogen content and amount of extractable proteins of deproteinized natural rubber （DPNR） were 0.000 w/w% and 0.00 μg/ml, respectively, which were less than those of natural rubber deproteinized with proteolytic enzyme or urea in the presence of surfactant. Dramatic decrease in the tensile strength of the protein-free natural rubber was associated with the removal of the all proteins from natural rubber.
The effect of the higher order structure of the non-rubber components on the properties of natural rubber was also studied in the present work. The nanomatrix structure of the non-rubber components was found to be inherently formed in natural rubber, which was a key factor influencing the outstanding mechanical properties of natural rubber. The nanomatrix structure of the non-rubber components disappeared after deproteinization of natural rubber with urea. The mechanical properties and viscoelastic properties of the natural rubber were dependent upon the nanomatrix structure of the non-rubber components. The nanomatrix structure of the non-rubber components was decomposed by decelerated fermentation of the natural rubber latex with sodium hydroxymethylglycinate （SHMG）. Bacterial decomposition of the nanomatrix structure of the non-rubber components resulted in the decrease in the mechanical properties of the rubber. Chain scission of natural rubber through oxidative degradation with potassium persulfate （K2S2O8） was studied to control the viscosity of natural rubber in order to accomplish a good processability of the rubber. Molecular weight and gel content of the degraded natural rubber were about one-half as low as those of the source rubber. The degraded natural rubber was found to contain carbonyl and formyl groups as an evidence of the oxidative degradation. Tensile strength of the vulcanized natural rubber, prepared from the degraded natural rubber, was the same as that prepared from the source rubber, even though the gel content and the molecular weight of the degraded rubber were distinguished from those of the source rubber.
The crosslinking junctions of the vulcanized natural rubber were analyzed by solid-state NMR spectroscopy equipped with a new field gradient-fast-magic angle spinning （FG-MAS） probe. The signal at 40 and 44 ppm were assigned to C4 of trans-1,4-isoprene units and secondary carbons adjacent to carbons linking to S atom, respectively, and the signals at 58 ppm were to the tertiary and quaternary carbons linking to sulfur atoms. Furthermore, the 13C-signals at 58 ppm were distinguishably correlated to the 1H-signals group and at 3.4 and 4.2 ppm, which were assigned to the -C-CH-Sx group and =C-CH-Sx group, respectively. Consequently, the crosslinking junctions of the vulcanized natural rubber were found to be not only tertiary carbons but also secondary and quaternary carbons. The effect of the crosslinking junctions on the mechanical properties was investigated by preparing the vulcanized natural rubbers through conventional vulcanization （CV）, efficient vulcanization （EV）, and semi-efficient vulcanization （SemiEV）. Even though the crosslink density of the vulcanized natural rubbers was the same as each other, the mechanical properties were dependence upon the vulcanization system; that is, the mechanical properties of the CV rubber were the most excellent among three. Based on the 13C-NMR, APT and DEPT measurements, the crosslinking junctions of the CV and SemiEV rubbers were not only tertiary carbon but also quaternary carbon, whereas those of the EV rubber were not detected. The most excellent mechanical properties of the CV rubber were attributed to the quaternary carbon of crosslinking junctions.
In this thesis, the outstanding mechanical properties of natural rubber were associated with not only the molecular structure but also the morphology of the rubber. It was concluded that the hierarchal structure of the terminal unit, the non-rubber components and the crosslinking junctions were important factors to control the outstanding mechanical properties of natural rubber.

　本論文は、「Study on Structure and Morphology of Natural Rubber and Its Vulcanizates（天然ゴムおよびその加硫物の構造とモルフォロジーに関する研究）」と題し、7章より構成されている。
　第1章では、天然ゴムの構造解析に関する従来の研究の概要を示すとともに、非ゴム成分の役割が示され、そのゴム中での存在状態を解明することの重要性、および、本研究の目的と範囲を述べている。天然ゴムがパラゴムの樹（Hevea brasiliensis）の中では水に分散した直径約1 μmの粒子であることに着眼し、ラテックスを凝固して得られる天然ゴムが、厚さ数～数十nmの非ゴム成分のマトリックスに直径約 μmのゴム状高分子が分散したナノマトリックス構造を形成すると予測している。また、天然ゴムの物性におけるナノマトリックス構造および架橋点の効果とそれらの解析法が述べられている。
　第2章では、天然ゴムからタンパク質を完全に除去する新規精製方法と得られたタンパク質フリー天然ゴムの構造と物性の関係を述べている。タンパク質フリー天然ゴムは、これまでに開発されてきた脱蛋白質化天然ゴムとは異なり、天然ゴムよりも物性が劣ることが示され、天然ゴムの物性にはタンパク質が重要な役割を果たしていることを示している。
　第3章では、天然ゴムは非ゴム成分のナノマトリックス構造を形成していること、および、このナノマトリックス構造が天然ゴムの物性が特異的に優れている原因であることが述べられている。天然ゴムにおけるタンパク質の量を増やすことにより、物性が向上することを示している。
　第4章では、天然ゴムラテックスの腐敗に着眼し、腐敗させることによるナノマトリックス構造の変化とその物性との関係が述べられている。腐敗を遅らせることにより、ナノマトリックスの厚さを制御でき、物性を制御できることを示している。
　第5章では、天然ゴムをラテックスの状態で分解することにより、粘度が制御できることを述べている。核磁気共鳴（NMR）法により、分解前後における天然ゴムの構造を示し、分解天然ゴムは原料として工業利用に利用できることを示している。
　第6章では、磁場勾配高速マジック角回転プローブを用いた固体NMR法における加硫ゴムの新規構造解析法の開発、および、この解析法により決定された架橋点の構造と物性の関係を述べている。
　第7章では、天然ゴムの一次構造、架橋構造および非ゴム成分が形成するナノマトリックス構造が総合的に天然ゴムの優れた物性の発現に重要な役割を果たしていることを総括している。
　以上より、本論文は工学上及び工業上貢献するところが大きく、博士（工学）の学位論文として十分な価値を有するものと認める。