何謂硫化Vulcanization？？？www.tool-tool.com
在高分子化學中，硫化（Vulcanization）指的是橡膠膠料通過生膠分子間交聯，生成具有三維網路結構的硫化膠的過程。含有雙鍵的彈性體在工業上多採用硫或有機硫化合物來進行硫化交聯，因此在橡膠工業中，硫化與交聯是同義詞。交聯的目的是為了使膠料具備高強度、高彈性、高耐磨、抗腐蝕等優良性能，消除永久形變，使橡膠在變形之後，能迅速並完全地恢復原狀。因為最早發現的交聯劑是硫磺，故得名硫化。
一般需經過硫化的橡膠品種有丁二烯、氯丁二烯、異戊二烯的1,4-聚合物——順丁、異戊、氯丁橡膠，以及共聚物丁苯、丁基和丁腈橡膠等。
具體過程
二烯烴類化合物在經過聚合後， 主要生成的是線形的高分子長鏈。這樣的橡膠通常性能較差，不易成型，受熱變軟，遇冷變硬變脆，容易磨損和老化。硫化是對橡膠性能進行改良的一種過程。在這 個過程中，線性結構的大分子發生交聯生成具有三維立體網狀結構的分子，穩定了分子的立體結構，從而使橡膠的彈性、強度等諸多性能都得到增強。
單以硫作為二烯烴聚合物的交聯劑時，實驗表明自由基引發劑、阻聚劑都對反應沒有影響，電子順磁共振也未檢測出自由基，而相反，有機酸鹼和介電常數大的溶劑卻能加速硫化的過程，從而說明該硫化過程是一個離子型連鎖反應。一般認為，硫化過程的第一步是聚合物的雙鍵與極化後的硫或硫離子對反應，形成一個環狀的鋶離子。鋶離子從聚合物鏈奪取氫原子，使後者生成烯丙基碳正離子。該碳正離子先與硫反應，然後再與大分子的雙鍵加成，從而產生交聯。之後再發生一個氫轉移，繼續與大分子反應，從而再生出碳正離子，推動反應一直進行下去。硫化後的分子結構大致如下：
加速硫化
用硫單獨對聚二烯烴進行硫化時，硫化速度相當慢，通常需要幾小時才能完成，效率也較低，只有40～50％的硫能有效地達到交聯的目的。而其他的硫，主要浪費在大分子的分子間相鄰雙交聯（實際上只起到單交聯的作用），以及分子內的硫化成環上。所以實際生產時，常常需要加入促進劑，來加快硫化的速率以及提高硫化的效率。促進劑主要是有機硫化合物，特別是二硫化物和多硫化物，例如四甲基秋蘭姆二硫化物、二甲基二硫代氨基甲酸鋅、2-巰基苯並噻唑和苯並噻唑二硫化物。少數不含硫的化合物，如二苯胍類，也可作為促進劑。
促進劑一般還需要與金屬氧化物、脂肪酸等活化劑連用，使效率達到最大。最常用的活化劑是氧化鋅和硬脂酸。
雖 然加速硫化的具體機理目前還不清楚，但加速硫化的效果是顯著的。原來用硫需要用數小時才能完成的硫化，在促進劑和活化劑的作用下幾分鐘內就可以完 成。反應的效率也有所提高，硫的浪費程度大大降低，有些時候促進劑和活化劑體系的交聯效率可達到每交聯略少於2個硫原子，大多數交聯是單硫鍵和雙硫鍵，甚少相鄰雙交聯和硫環。
Vulcanization or vulcanisation is a chemical process for converting rubber or related polymers into more durable materials via the addition of sulfur or other equivalent "curatives". These additives modify the polymer by forming crosslinks (bridges) between individual polymer chains.[1] The vulcanized material is less sticky and has superior mechanical properties. A vast array of products are made with vulcanized rubber including tires, shoe soles, hoses, and hockey pucks. The process is named after Vulcan, Roman god of fire. Hard vulcanized rubber is known as ebonite or vulcanite and is used to make hard articles such as bowling balls and clarinet mouth pieces.
Uncured natural rubber is sticky, easily deforms when warm, and is brittle when cold. In this state it cannot be used to make articles with a good level of elasticity. The reason for inelastic deformation of unvulcanized rubber can be found in its chemical structure: rubber is composed of long polymer chains. These chains can move independently relative to each other, which enables the material to change shape. Crosslinking introduced by vulcanization prevents the polymer chains from moving independently. As a result, when stress is applied the vulcanized rubber will deform, but upon release of the stress, the article will revert to its original shape.
Vulcanization is generally irreversible, similar to other thermosets and in contrast to thermoplastic processes (the melt-freeze process) that characterize the behavior of most modern polymers. The cross-linking is usually done with sulfur, but other technologies are known, including peroxide-based systems.
The main polymers subjected to vulcanization are polyisoprene (natural rubber) and styrene-butadiene rubber (SBR), which are used for most passenger tires. The "cure package" is adjusted specifically for the substrate and the application. The reactive sites - "cure sites" - are allylic hydrogen atoms. These C-H bonds are adjacent to carbon-carbon double bonds. During vulcanization, some of these C-H bonds are replaced by chains of sulfur atoms that link with a cure site of another polymer chain. These bridges contain between one and eight atoms. The number of sulfur atoms in the crosslink strongly influences the physical properties of the final rubber article. Short crosslinks give the rubber better heat resistance. Crosslinks with higher number of sulfur atoms give the rubber good dynamic properties but with lesser heat resistance. Dynamic properties are important for flexing movements of the rubber article, e.g., the movement of a side-wall of a running tire. Without good flexing properties these movements will rapidly lead to formation of cracks and, ultimately, to failure of the rubber article.
"Vulcanization curve" showing the increase in viscostiy of the polymeric material during crosslinking. The steepness of the curve is strongly affected by the nature of the accelerators and other additives.
Vulcanization methods
A variety of methods exist for vulcanization. The economically most important method (the vulcanization of tires) uses high pressure and temperature. A typical vulcanization temperature for a passenger tire is 10 minutes at 170 °C. This type of vulcanization is called compression molding. The rubber article is intended to adopt the shape of the mold. Other methods, for instance to make door profiles for cars, use hot air vulcanization or microwave heated vulcanization (both continuous processes).
Four types of curing systems are in common use. They are:
- Sulfur systems
- Peroxides
- Urethane crosslinkers
- Metallic oxides
Vulcanization with sulfur
By far the most common vulcanizing methods depend on sulfur. Sulfur, by itself, is a slow vulcanizing agent and will not vulcanize synthetic polyolefins. Even with natural rubber, large amounts of sulfur, as well as high temperatures and long heating periods are necessary and one obtains an unsatisfactory crosslinking efficiency with unsatisfactory strength and aging properties. Only with vulcanization accelerators can the quality corresponding to today's level of technology be achieved. The multiplicity of vulcanization effects demanded cannot be achieved with one universal substance; a large number of diverse additives, comprising the "cure package," are necessary.
The combined cure package in a typical rubber compound consist of sulfur together with an assortment of compounds that modify the kinetics of crosslinking and stabilize the final product. These additives include accelerators, activators like zinc oxide and stearic acid and antidegradants. The accelerators and activators are catalysts. An additional level of control is achieved by retarding agents that inhibit vulcanization until some optimal time or temperature. Antidegradants are used to prevent degradation of the vulcanized product by heat, oxygen, and ozone.[2]
Vulcanization of silicones
"Room-temperature vulcanizing" (RTV) silicone is constructed of reactive oil base polymers combined with strengthening mineral fillers. There are two types of room-temperature vulcanizing silicone:
RTV-1 (One-component systems)
RTV-1 hardens directly under the action of atmospheric humidity. The curing process begins on the outer surface and progresses through to its core. The product is packed in airtight cartridges and is either in a fluid or paste form. RTV-1 silicone has good adhesion, elasticity and durability characteristics. The Shore A hardness can be varied between 18 and 60. Elongation at break can range from 150% up to 700%. They have excellent aging resistance due to superior resistance to UV radiation and weathering. Industrial RTV-1 products are referred to as CAFs.
RTV-2 (Two-component systems)
RTV-2 elastomer are two-component products that, when mixed, cure at room-temperature to a solid elastomer, a gel, or a flexible foam. RTV-2 remains flexible from -80 °C to +250 °C. Break down occurs at temperatures above 350 °C leaving an inert silica deposit that is non-flammable and non-combustible. They can be used for electrical insulation due to their dielectric properties. Mechanical properties are satisfactory. RTV-2 is used to make flexible moulds, as well as many technical parts for industry and paramedical applications.
History of vulcanization of rubber
Although vulcanization is a 19th century invention, the history of rubber cured by other means goes back to prehistoric times. The name "Olmec" means "rubber people" in the Aztec language. Ancient Mesoamericans, spanning from ancient Olmecs to Aztecs, extracted latex from Castilla elastica, a type of rubber tree in the area. The juice of a local vine, Ipomoea alba, was then mixed with this latex to create an ancient processed rubber as early as 1600 BC [3] In the western world, rubber remained a curiosity although it was used in the production of waterproofed products such as Mackintosh rainwear.
Goodyear's contribution
Charles Goodyear (1800–1860) invented vulcanization of rubber when he heated a mixture of rubber and sulfur. The Goodyear story is one of either pure luck or careful research. Goodyear insisted that it was the latter, though many contemporaneous accounts indicate the former. Goodyear claimed that he discovered vulcanization in 1839 but did not patent the invention until June 15, 1844, and did not write the story of the discovery until 1853 in his autobiographical book Gum-Elastica. Meanwhile, Thomas Hancock (1786-1865), a scientist and engineer, patented the process in the UK on November 21, 1843, eight weeks before Goodyear applied for his own UK patent.
Here is Goodyear's account of the invention, taken from Gum-Elastica. Although the book is an autobiography, Goodyear chose to write it in the third person, so that 'the inventor' and 'he' referred to in the text are in fact the author. He describes the scene in a rubber factory where his brother worked:
... The inventor made some experiments to ascertain the effect of heat on the same compound that had decomposed in the mail-bags and other articles. He was surprised to find that the specimen, being carelessly brought into contact with a hot stove, charred like leather.
Goodyear goes on to describe how his discovery was not readily accepted.
He directly inferred that if the process of charring could be stopped at the right point, it might divest the gum of its native adhesiveness throughout, which would make it better than the native gum. Upon further trial with heat, he was further convinced of the correctness of this inference, by finding that the India rubber could not be melted in boiling sulfur at any heat ever so great, but always charred. He made another trial of heating a similar fabric before an open fire. The same effect, that of charring the gum, followed; but there were further indications of success in producing the desired result, as upon the edge of the charred portion appeared a line or border, that was not charred, but perfectly cured.
Goodyear then goes on to describe how he moved to Woburn, Massachusetts and carried out a series of systematic experiments to optimize the curing of rubber.
... On ascertaining to a certainty that he had found the object of his search and much more, and that the new substance was proof against cold and the solvent of the native gum, he felt himself amply repaid for the past, and quite indifferent to the trials of the future.
Goodyear did not profit from his invention.
Later developments
Whatever the true history, the discovery of the rubber-sulfur reaction revolutionized the use and applications of rubber, and changed the face of the industrial world.
Up to that time, the only way to seal a small gap between moving machine parts, such as between a piston and its cylinder in a steam engine, was to use leather soaked in oil. This was acceptable up to moderate pressures, but above a certain point, machine designers had to compromise between the extra friction generated by packing the leather more tightly and greater leakage of precious steam.
Vulcanized rubber offered the ideal solution. With vulcanized rubber, engineers had a material which could be shaped and formed to precise shapes and dimensions, and which would accept moderate to large deformations under load and recover quickly to its original dimensions once the load was removed. These, combined with good durability and lack of stickiness, are the critical requirements for an effective sealing material.
Further experiments in the processing and compounding of rubber by Hancock and his colleagues led to a more repeatable and stable process.
In 1905 George Oenslager discovered that a derivative of aniline called thiocarbanilide accelerated the action of sulfur to rubber, leading to shorter cure times and reducing energy consumption. This breakthrough, although less famous, is almost as fundamental to the development of the rubber industry as that of Goodyear in discovering the sulfur cure. Accelerators made the cure process improved the reliability of the vulcanization process and, although not obvious at the time, enabled vulcanization to be applied to synthetic polymers. One year after his discovery, Oenslager had found hundreds of applications for his additive.
Thus, the science of accelerators and retarders was born. An accelerator speeds up the cure reaction, while a retarder delays it. In the subsequent century, various chemists have developed other accelerators and ultra-accelerators, that make the reaction extremely fast, and are used to make most modern rubber goods.
Recycling and devulcanization
The market for new raw rubber or equivalent remains enormous, with North America alone using over 10 billion pounds (circa 4.5 million tons) every year. The auto industry consumes approximately 79% of new rubber and 57% of synthetic rubber. To date, recycled rubber has not been used as a replacement for new or synthetic rubber in significant quantities, largely because the desired properties have not been achieved. Used tires are the most visible of the waste products made from rubber; it is estimated that North America alone generates approximately 300 million waste tires annually, with over half being added to existing stockpiles. It is estimated that less than 10% of waste rubber is reused in any kind of new product. The United States, the European Union, Eastern Europe, Latin America, Japan and the Middle East collectively produce about one billion tires annually, with estimated accumulations of three billion in Europe and six billion in North America.
The rubber recycling process begins with the shredding. After the steel and reinforcing fibers are removed and a secondary grinding, the resulting rubber powder is ready for product remanufacture. The manufacturing applications that can utilize this inert material are restricted to those which do not require its vulcanization. In the rubber recycling process, devulcanization begins with the delinking of the sulfur molecules from the rubber molecules, thereby facilitating the formation of new cross-linkages. Two main rubber recycling processes have been developed: the modified oil process and the water-oil process. With each of these processes, oil and a reclaiming agent are added to the reclaimed rubber powder, which is subjected to high temperature and pressure for a long period (5-12 hours) in special equipment and also requires extensive mechanical post-processing. The reclaimed rubber from these processes has altered properties and is unsuitable for use in many products, including tires. Typically, these various devulcanization processes have failed to result in significant devulcanization, have failed to achieve consistent quality, or have been prohibitively expensive.
加硫（かりゅう）とは、架橋反応の一種で、ゴム系の原材料（生ゴムなど）を加工する際に、弾性や強度を確保するために、硫黄などを加える行程のことである。
1839年、アメリカの発明家チャールズ・グッドイヤー(C.Goodyear)により発見され、1843年にイギリスの発明家トーマス・ハンコック(T.Hancock)により、反応の仕組みが解明された。
材料の分子内にある多重結合部に反応し、加えられた硫黄を媒介とした分子間結合が、新たに作り出される。この反応により、材料の分子量は増大し、それに伴い、ゴムの弾性や強度が飛躍的に向上する。
ただし、過剰な加硫を行うと、多重結合や分子間の流動性が失われ、弾性が失われる。この性質を利用した材料にエボナイトがある。
加硫剤としては、硫黄のほか過酸化物なども使用され、加える化学物質により、様々な特性を持つゴムを製造することができる。加硫剤の働きを促進させる加硫助剤には、無機系の酸化亜鉛や酸化マグネシウムなど、有機系のステアリン酸やアミン類などが使われる。加硫時間の短縮などの目的で、チアゾール系を中心とした加硫促進剤が添加されることもある。
タ イヤやゴム製品に使用される。スチールラジアルタイヤに入れられている鋼線は鋼とゴムは接着性が良くないので銅めっきが施されている。加硫によってゴム に数％含まれる硫黄と銅が強力なイオン結合を形成する。1970年代のスティールラジアルタイヤのワイヤは銅メッキで、現在はより強度があるブラス(真 鍮)メッキになった。ブラスメッキは銅と亜鉛を陽極に並べてメッキする。最近は鋼線とゴムとの接着をナフテン酸コバルトというものを介在させる界面活性剤 で解決する方法が見付かったが環境に悪影響を与える可能性がある。
Bewise Inc. www.tool-tool.com Reference source from the internet.
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