I think it is nice to write about the science of something around us. I wrote a few posts about how did I made pendants and decoration about heat shrink plastic, I am going to understand how it works :)

我覺得靈感是從自己的身邊開始的。我發了幾個用熱縮膠片做裝飾的帖子，就來探討一下熱縮膠片的科學吧！

Heat shrink plastic is a crafting material to making "jewelry" and decorations. For people who do not know what heat shrink plastic is, let me demonstrate to all of you.

熱縮膠片是個用來做首飾和裝飾的手工材料。如果你不熟悉熱縮膠片的話來看看我的示範吧！

1

Draw the image you like on a clear heat shrink plastic using sharpies

用水性筆在透明的熱縮膠片上畫上你喜歡的圖案。

2

Heat it up using a hot air gun or in an oven, then flaten it up using a flat object.
We can see the plastic curls at the beginning and then flattens itself.

用熱風槍或烤箱加熱，用平的東西把熱縮膠片壓平。
我們可以看到，在加熱後熱縮膠片卷曲了起來再變平了。

3

Here is the product (which does not look so good...) It was kind of fail in this attempt but I used up my shrink plastic so I cannot do it again. (An the look is not the main point of this demonstration) The plastic became smaller and thicker than before.

這就是製成品(雖然不太好看)。 這次做的裝飾是失敗的，但是我的熱縮膠片用完了，不能重做了。熱縮膠片比之前變得更小更厚。

Why?

為甚麼會這樣呢？

Let me give you guys an introduction to heat shrink plastic first.

我來為大家做個小小的介紹。

Plastics are man-made polymers, which mean their molecules are long chains of a repeated unit (monomer). Heat shrink plastic is plastic No. 6, which is polystyrene (PS).

塑膠是人造的聚合物，即是它的分子用很多相同的重覆單位(單體)構成。熱縮膠片是6號塑膠，聚苯乙烯(PS)。

(Image credit: http://selektaspektra.com/process.html)

The monomer of PS is styrene, here is its molecular structure of PS

PS的單體就是苯乙烯，以下是PS的分子結構。

(Image credit: http://www.essentialchemicalindustry.org/polymers/polyphenylethene.html)

PS is everywhere around us. It is commonly used as food container and packaging. Styrofoam is also consist of PS. The lid of the coffee bought is also made of PS.

在我們的生活中可以常常用到PS. PS廣泛在食品容器和包裝。保麗龍也是PS來的。我買來的咖啡的杯蓋也是PS。

Heat shrink plastic is a thermopalsatic, which mean it can be heated, melted then molded repeatedly. Thermoplastics have a property that they turn into a rubbery state (become soft and bendable) when they are heated between their glass transition points and melting points.

熱縮膠片是種熱塑性塑膠。熱塑性塑膠是可以重覆加熱，溶化，然後再倒模的。在加熱至玻璃轉移温度和溶點之間的熱塑性塑膠會處於像膠態，變成柔軟和可屈曲。

To make a heat shrink pastic sheet, the raw materials of PS are put in a heated mixing drum, pass the plastic through a slot die to make it into a plastic sheet . To make the plastic shrinkable, when the plastic is still warm, compress the plastic into a thinner sheet and let the sheet cool down, the plastic maintains the stretched state, a heat shrink plastic is done! When the temperature exceed the glass transition point, the molecules of the plastic able to move and return to their original curled configuration, i.e. return to the size before compression.

熱縮膠片的做法是首先把材料放到已加熱的混合爐，把膠通過狹縫做出一塊膠片。在膠片還熱的時候，把膠片加壓，讓分子拉直，讓膠片得更薄。冷卻後熱縮膠片就完成了。當熱縮膠片到達玻璃轉移温度, 熱縮膠片的份子會復原至壓縮之的卷曲狀態，膠片就會變小和厚了。

I have learnt a lot when researching the information of heat shrink plastic. I think it is good for us to question how the things around us :)

在資料搜集的過程我學到很多。希望大家多多留意身邊事物的原理吧！

Source:
http://polymerdatabase.com/polymer%20classes/Polystyrene%20type.html
http://www.madsci.org/posts/archives/1999-02/920255002.Ch.r.html
http://entertainment.howstuffworks.com/shrinky-dink3.htm
https://www.creativemechanisms.com/blog/polystyrene-ps-plastic