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主题:聚酯PBT和PET:结晶造成差异

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聚酯PBT和PET:结晶造成差异  发帖心情 Post By:2012-10-06 14:51:00 [只看该作者]


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PBT and PET Polyester: The Difference Crystallinity Makes

By Michael Sepe from Michael P. Sepe LLC

 

聚酯PBTPET:结晶造成差异

 

上海北京顺德(翻译和评述)

 

 


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  发帖心情 Post By:2012-10-06 14:53:00 [只看该作者]

 

Last month we profiled similarities and differences between different chemistries in the acetal polymer family. This month we will begin making a similar comparison between different commercially available polyesters known as PBT and PET. Many years ago, while working for a custom injection molder, I worked closely with one of our customers to convert a series of parts from 30% glass-fiber reinforced PBT to a PET with the same level of filler. It served as a practical application of the differences between the materials on both processing and performance levels.

 

上个月,我们曾经归纳了POM家族的不同化学结构的相似性和差别。这个月,我们将开始对不同化学结构的聚酯,如PBT何PET进行比较。许多年前,在一个注塑模塑公司那里工作时,我与我的一个客户一起工作,加工一系列部件,材料包括30%玻纤增强PBT到具有同样填料含量的的PET。它们用于不同的应用,因此加工和性能也有所不同。

 

Fundamentally, the chemistry of PET and PBT is very similar. Polyesters are synthesized by reacting an organic acid, in this case terephthalic acid, with an alcohol. In the case of PBT the alcohol is generically referred to as butylene glycol while in PET it is ethylene glycol. The resulting polymers are known, therefore, as polybutylene terephthalate (PBT) and polyethylene terephthalate (PET).

 

PET和PBT的化学结构基本是很相似的。聚酯是由一种有机酸(此处为对苯二甲酸)与一种醇合成的。对于PBT,其醇单体成为丁二醇,而PET是乙二醇。由此所得的聚合物是众所周知的—PBT和PET。

 

The differences in the materials are best understood by examining the chemical structure of the repeating unit that makes up the polymer chains, as shown in Fig. 1. The essential feature that makes the materials distinctive is the ester group that lends its name to this family of materials. Other polymers such as PTT and PCT are also members of this chemical family that display slight variations on these structures.

 

理解PBT和PET差异的最好方式就是检查构成聚合物分子链的重复单元的化学结构,如图(1)所示。造成PBT和PET区别于别的材料的基本特征是对苯二甲酸酯基团,这类材料的名字就来源于这里。其它的聚合物,如PTT何PCT,也是这个化学家族的成员,只是在它们的结构上显示了轻微的差别。

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  发帖心情 Post By:2012-10-06 14:53:00 [只看该作者]

 

Another key feature of the chemistry in this material is the six-sided ring that appears at regular intervals in the backbone. Known as a phenyl ring or more generally as an aromatic ring, this element provides stiffness to the polymer chain. This influences several important properties, including the glass-transition temperature, a region where polymers lose a significant percentage of their load-bearing properties.

 

这类材料的另一个关键的化学特征是有六元环按照规则的间隔出现在聚合物主链上。这种称为苯环的结构单元,一般也称为芳香环,它为聚合物分子链提供刚性。这将影响几个重要性质,包括玻璃化转变温度,高于这个温度范围,聚合物就会失去大部分的承载负荷的性能。

 

It is not evident from a two-dimensional rendering(透视图) of the chemical structure, but a 3D view would show that while many of the chemical groups in a polymer chain project into or out of the page, the aromatic ring sits in a plane. It also constrains the natural tendency of the other groups in the chain to rotate and vibrate. This is part of the stiffening effect of this ring structure. The reduced mobility and the bulky nature of the ring also influence the ability of the polymer to crystallize as it cools. PBT, with greater spacing between the aromatic rings, crystallizes more efficiently than PET. But PET, if it is successfully crystallized, provides for better mechanical properties, including strength, stiffness, and performance at elevated temperature.


化学结构的2D透视图不是很明显,但是3D透视图显示,不管聚合物链上的许多化学基团能够投影到一个纸面或者不能投影到一个纸面,芳香环都位于一个明面内。芳香环也因此限制了分子链上别的基团的旋转和振动的本真性能。这是芳香环结构硬化效应的一部分。苯环的这种减少的活动性和其个头庞大的性质也将影响聚合物在冷却结晶时的能力。PBT具有较大的芳香苯环的间隔,结晶比PET更加有效。但是,PET如果能更有效地结晶,那将提供更好的力学性能,包括强度、刚性和高温下的性能。


Most consumers are familiar with PET in the containers that holds their bottled water or soft drinks. This type of PET is amorphous and is engineered to prevent crystallization. If bottle-grade PET did crystallize it would become cloudy and, more importantly, it would lose its impact resistance. So while there are probably a lot of parts molded in crystalline PET polyester under the hood of your car, where they encounter elevated temperatures and aggressive chemical environments; the vast majority of the PET in the world is consumed in packaging, where it is amorphous, unreinforced, and incapable of handling such rigorous environments.


许多消费者熟悉装瓶装水和软饮料的容器的PET材料。这种类型的PET是无定形的,工程上设法阻止其结晶。如果瓶用PET牌号结晶了,那它将变得模糊不透明,更重要的是,它将失去其耐冲击性能。所以,尽管在发动机罩下可能有许多部件是用结晶PET模塑的,这里会存在高温和苛刻的化学环境;而世界上的大多数PET消费于包装,这里它是无定形的、非结晶强化的,不能应对上述苛刻的环境。


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  发帖心情 Post By:2012-10-06 14:54:00 [只看该作者]

 

The type of PET that we will be discussing in Part 2 of this article is semi-crystalline and almost always contains high levels of glass fibers and/or mineral fillers. PBT polyester, however, can be provided in its semi-crystalline form both filled and unfilled. In fact, because PBT crystallizes faster than PET, it is not possible under normal processing conditions to produce PBT parts that are amorphous. The polymer crystallizes efficiently enough to always achieve some level of organization in its structure. The stiffness of the ester groups and the aromatic rings is balanced by the flexibility and mobility of the butylene group. But in PET the shorter ethylene group makes crystallinity optional. We can have amorphous PET if we cool it rapidly or we can have semi-crystalline PET if we cool it slowly.


本文的第二部分我们要讨论的PET类型是无定形的,并且总是包含高比例的玻璃纤维和/或矿物填料。但是,聚酯PBT是按照它的的结晶形式提供的,既可以是填充型的,也可以是非填充型的。事实上,由于PBT结晶比PET快,在通常加工条件下,PBT不可能生产出无定形PBT部件。如果聚合物足够有效地结晶,它们在结构上都可以达到一定的组织水平。聚酯基团和芳香环的刚性与丁基基团的柔性和活动性达成平衡。但是,在PET中,较短的乙烯基使得结晶具有可选择性。如果我们快速能却PET,我们就能得到无定形PET;否则,我们慢慢冷却它,就得到半结晶PET。


Most PET bottles start out as injection molded preforms. They are clear and tough and relatively thick-walled in order to allow for the thinning effect that the wall will undergo when the preform is reheated and stretched to form the bottle. If you have worked in a bottle manufacturing plant you know that if, during the preheat cycle, the preforms get too hot they turn cloudy—a sign of crystallization. (In fact, if you look closely at the gate area of a preform you will see a small amount of haze in this area due to the extra heat generated in this area of the part). If you try to blow a bottle from this cloudy, partly crystallized material, it will have reduced impact performance. The preform may even shatter during the blowing process if enough crystallinity develops. So the trick is to keep the material above its glass-transition temperature, but below its crystallization temperature. This temperature window may not be very wide, as can be seen in Fig. 2.


许多PET瓶开始时是通过注塑预成型瓶坯。这些瓶坯是透明的,壁厚较厚,目的是为了防止在后续的瓶坯重新加热并拉伸成瓶时经历的稀化效应。如果你在制瓶厂工作过,你就知道,在预热阶段,如果瓶坯变得过热,它们将变得模糊不透明—这是结晶的标志。(事实上,如果你凑近瓶坯的浇口,你将看到在这个区域存在少量的雾斑,这是由部件在这个部分过热导致的)。如果你试图用这种不透明的部分结晶瓶坯吹塑瓶子,那将减少冲击性能。如果有足够的结晶形成,瓶坯有可能在吹塑加工时甚至破碎。所以,诀窍是保持材料在其玻璃化温度以上和结晶温度以下。这个温度窗口可能不是很宽,如图(2)所示。



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  发帖心情 Post By:2012-10-06 14:54:00 [只看该作者]

 

This graph shows the behavior of amorphous PET polyester, an unfilled clear material that is used to make parts that require toughness and transparency but do not need to withstand elevated temperatures. As the material is heated from room temperature, the first notable event is the glass transition. This appears as a step change in the heat content of the material, and in this compound that process is complete at 75 C (167 F). At this point the material has lost the rigidity it possessed at room temperature and is soft and pliable. As the temperature is increased, the viscosity of the softened polymer will decrease until it reaches a temperature near 110 C (230 F). This is the temperature at which the baseline of the scan starts to rise rapidly, and the gap between 75 C and 110 C represents the window of opportunity for blowing the bottle.


这个图线显示了无定形PET聚酯的行为,这种材料是一种未填充的透明材料,用于制造要求韧性和透明性但是不要求耐高温性能的部件。当这种材料从室温开始加热时,第一个显著的现象是玻璃化转变。这似乎是材料热焓变化的一个台阶,在这种材料中这个过程是在75℃完成的(167F)。在这点,材料丧失了其在室温下拥有的刚性,变得柔软和柔顺起来。随着温度升高,软化的聚合物的粘度开始下降,一直到接近110℃(230F)。这就是扫描基线开始快速增加的温度,75℃和110℃之间的间距代表吹塑PET瓶的机会窗口。


Most PET bottle plants that I have visited are running a preheat temperature near 100 C (212 F). Once the crystallization process has started, the material begins to turn cloudy. It will also begin to regain some of the stiffness that it lost as it went through the glass transition. If this process goes far enough, the polymer becomes crystallized at about 140 C (284 F). At this point the material will be opaque and brittle and will stay that way until the crystal structure melts at about 245 C (473 F). So PET can go either way—amorphous or semi-crystalline—depending on how we treat it.


我参观过的许多制瓶厂都使用接近100℃(212F)的预热温度。一旦结晶过程启动,材料开始变得模糊。这种材料就将开始重新获得它经过玻璃化温度时失去的一些刚性。假如这个过程发展下去,聚合物将在140℃(473F)结晶。所以,PET可以两种形态存在—无定形或半结晶—取决于如何处理它。


But PBT is always semi-crystalline under normal commercial circumstances. So to properly understand the differences in performance between PET and PBT we need to compare apples to apples—the semi-crystalline forms of each polymer. Because PET crystallizes very slowly, producing parts with a semi-crystalline structure requires the help of chemicals known as nucleating agents, as well as the presence of solid particles of fillers and reinforcements. Thus commercial semi-crystalline PET polyesters are always sold filled or reinforced, and to make a fair comparison of PET vs. PBT performance we therefore need to compare the materials with an equivalent level of the same type of filler. We will do this in Part 2 of this investigation and also discuss the differences in processing that molders will encounter as they work with these two polymer families.


但是,在通常的商业条件下,PBT总是半结晶的。所以,为了正确理解PET和PBT之间的性能差别,我们需要按照一个合理的标准来比较每个聚合物的结晶形成。由于PET结晶很慢,使用这类半结晶结构的材料制造部件要求借助于某些化学物质,如成核剂,以及一些固体粒子,如填料和增强剂。所以,商用半结晶PET总是固体填充或增强的。为了对PET与PBT性能作公平的比较,我们因此需要用相同填充比例的同种类型的填料的材料进行比较。在本研究的第二部分,我们将做这件事,并将讨论模塑者在使用这两种材料时面临的加工差异。


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  发帖心情 Post By:2012-10-07 16:21:00 [只看该作者]

期待下文继续,辛苦了不得

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  发帖心情 Post By:2012-11-10 11:18:00 [只看该作者]

学习了!谢谢廖工

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  发帖心情 Post By:2013-03-16 8:55:00 [只看该作者]

正要学习这方面的知识,看到不少配方,不论是PBT还是PET 总发现工程师们喜欢互相参着用,所以想了解一下其中的缘由呢

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  发帖心情 Post By:2013-03-16 8:56:00 [只看该作者]

发现差不多是半年前的帖子了,廖老大怎么不更新了,或者把文献发我一份我看看,外文文献我找不好


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  发帖心情 Post By:2013-03-16 15:36:00 [只看该作者]

辛苦廖工了,
PC与PBT共混会发生酯交换,那PBT与PET共混也应该会发生酯交换吧。

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