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2014-04-21    来源:网络    【      美国外教 在线口语培训

Science and Technology 科技

Plastics 塑料

There and back again 旧法新用

An old idea may help solve the problem of plastic waste

PLASTICS were once regarded as wonder-materials. They are still ubiquitous, but find less favour than they used to because of the very stability and persistence that won them plaudits in the first place. Persistence is not a quality to be desired in something that gets thrown away, and so much plastic is used in packaging, and in articles that are disposable, that many people now see conventional petrochemical plastics as a nuisance and a threat.

The search is on, then, for biodegradable alternatives. One possibility has recently been explored by David Schiraldi of Case Western Reserve University, in Ohio, and his colleagues. They propose to reach back into history and revive the use of a feedstock that was used to make some of the first plastics invented: milk.
此后,寻找生物可降解替代品的研究一直在持续。最近,俄亥俄州凯斯西储大学的David Schiraldi及其同事已开辟了一条潜在途径。他们追溯历史并改良了塑料在发明之初时所使用的原料---牛奶的使用方法。

What they actually suggest is using casein, the principal protein found in milk. The curds (of "curds and whey") are mostly made of this protein. In 1889 a French chemist called Jean-Jacques Trillat discovered that if casein is treated with formaldehyde the result is a hard, shiny substance that does well as a substitute for materials like ivory and tortoiseshell. So widespread was the enthusiasm for the new material that Queen Mary herself ordered several pieces of jewellery made from it. However, casein-based plastic of this sort is too brittle for general use. It was eventually superseded by the modern, petrochemical variety, and manufacture stopped altogether by the 1970s.
事实上,他们的建议就是使用牛奶中的主要蛋白质---酪蛋白,凝乳(凝乳及乳清)中的大部分成分都是由酪蛋白组成。1889年,法国化学家Jean-Jacques Trillat发现被甲醛处理过的酪蛋白会生成一种坚硬而有光泽的物质,并可以作为铁及龟甲类材料的替代品。之后,这种新材料受到了广泛的热衷以至于玛丽女王自己都定制了一些用这种材料制作的珠宝。但是,这类酪蛋白塑料由于过于脆弱而无法成为一种通用材料。其最终被现代石油化工衍生物所超越,并在20世纪70年代全面停产。

The thought of reviving it, though, has never quite gone away, and these days the fact that it is made mostly of protein, and could thus be chewed up by bacteria, is regarded as a virtue—if only the structural weakness could be overcome. Dr Schiraldi's approach does this by using a silicate clay called sodium montmorillonite as a skeleton that holds the plastic together.

Sodium montmorillonite can be freeze-dried into a spongelike material known as an aerogel. Aerogels are famously fragile. But that is because they are mostly empty space. Indeed, they are sometimes nicknamed "solid smoke". This fragility disguises an underlying stiffness. Filling the pores in the aerogel with plastic should remove its fragility and, conversely, the network of clay molecules in the aerogel will stop the plastic cracking. So the researchers reckoned that if they mixed casein with the clay and added glyceraldehyde (which substitutes for the poisonous formaldehyde used in the original plastic), they might be able to make something really rather useful.
蒙脱土可以被冷冻干燥成一种叫做气凝胶的海绵状材料。因为主要是空心,所以气凝胶是一种有名的易碎材料。事实上,它们有时候也被戏称为"固态烟"。 其实,他们的易碎性掩盖了他们坚强的一面。如果将酪蛋白塑料灌入气凝胶的细孔,那么气凝胶将不再易碎,相反,气凝胶中粘土分子的网状系统同样会防止酪蛋白塑料破裂。所以,研究者们认为如果他们将酪蛋白与粘土混合在一起并添加甘油醛(用来取代原先塑料中所使用的含毒性的甲醛),将形成一种非常有用的材料。

To test their ideas the team mixed a solution of casein with glyceraldehyde and sodium montmorillonite and vigorously stirred the result to get rid of the bubbles before freezing it at 80°C below zero. Once frozen, the material was placed in a freeze-dryer that removed all the water from it over the course of four days. It was then cured for 24 hours in an oven at 80°C above zero.

The researchers report in Biomacromolecules that their new material matches the stiffness, strength and compressibility of expanded polystyrene, a common packaging material that is the bane of many a rubbish dump. However, unlike polystyrene, it goes away once it has been dumped. An initial experiment suggested that 20% of it vanished within 18 days in a dump-like environment. A comedown, perhaps, from being a queen's brooch. But far more useful.

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