heat reactive polymers
Shape memory polymer strips (image by Inventables)
Heat Reactive Materials
Heat reactive materials change state, shape and/or color when exposed to temperatures above ambient. Naturally, many materials change state, eg. melt, at high temperatures. What’s special about some of them that their state, shape and/or color can be altered at relatively low temperatures (provided through hot water, body heat, hair dryers, ambient heaters, ovens, or just a hot summer day), making them easy to use and suitable for DIY projects. In this post I’ll go over polymorph, shape memory polymers and heat-shrink materials.
Polymorph, aka polycaprolactone, is a biodegradable polyester with a low melting point of around 60ÂºC (140ÂºF). It can be heated with just hot water then molded by hand or cast. Once it cools to room temperature, polymorph becomes a hard, nylon-like plastic, which can be reheated and reshaped any number of times.
“Molding a Handle” tutorial by Inventables
Polymorph is extremely easy to use. Start by filling a container with very hot water. Add some polymorph granules and wait until they turn clear and cluster together. At this point, the polymorph is ready to be shaped. Scoop it out of the hot water bath (with tongs or something like that) and shape it by hand or press it into a mold (see video above). Once molded let the polymorph cool completely – you’ll know it’s ready when it turns back to solid white. You can also melt polymorph with a hair dryer or a heat gun, but avoid using flames (such as a lighter) as this will blacken the material. Powdered pigments such as this one can be used to color polymorph.
Shape Memory Polymers (SMP) can be re-shaped when exposed to heat and will retain this new shape after cooling down. But once exposed again to the change-over temperature the polymer will revert back to its original shape. The physical properties, behavior and change-over temperature vary greatly from SMP to SMP. According to Wikipedia:
SMPs can retain two or sometimes three shapes, and the transition between those is induced by temperature. In addition to temperature change, the shape change of SMPs can also be triggered by an electric or magnetic field, light or solution. As well as polymers in general, SMPs also cover a wide property-range from stable to biodegradable, from soft to hard, and from elastic to rigid, depending on the structural units that constitute the SMP. SMPs include thermoplastic and thermoset (covalently cross-linked) polymeric materials.
Shape memory plastic sheet (image by Inventables)
Shape memory polymers have been finding several industrial applications, such as CRG’s Smart Mandrels:
When heated above the transition temperature, the mandrel becomes elastic and can easily be molded into a desired shape. Once cooled, the material will become rigid and retain the new shape. The mandrel can then be filament wound and the resulting part cured on the mandrel. Heating the mandrel above its transition temperature after the part is cured makes the mandrel elastic again and easily extractable from the part. Because of the mandrel’s shape memory properties, it can be returned to its original tubular shape and reused.
Heat-Shrink Tubing is manufactured from a thermoplastic (such as nylon or polyolefin) which shrinks when exposed to heat. It’s used mostly to insulate wires, connections, joints and terminals in electrical engineering. According to Wikipedia:
According to the exact material used, there are two ways that heat shrink may work. If the material contains many monomers, then when the tubing is heated the monomers polymerise. This increases the density of the material as the monomers become bonded together, therefore taking up less space. Accordingly, the volume of the material shrinks. Heat shrink can also be expansion-based. This process involves producing the tubing as normal, heating it to just above the polymer’s crystalline melting point and mechanically stretching the tubing (often by inflating it with a gas); finally, it is rapidly cooled. Later, when heated, the tubing will relax back to the un-expanded size. The material is often cross-linked through the use of electron beams, peroxides, or moisture. This cross-linking helps to make the tubing maintain its shape, both before and after shrinking. For external use, heat shrink tubing often has a UV stabilizer added.
To use simply run the wires, or whatever you wish to enclose/insulate, through the heat-shrink tubing and then apply heat with a heat-gun or lighter, this will cause the tubing to shrink and mold itself around the wires. This shape change is irreversible, i.e. once shrank it’s not possible to revert the tubing back to its original shape.
Heat-Shrink Thread, which is made of polyester, looks and sews just like regular thread but when exposed to heat (176ÂºC/350ÂºF) shrinks 10 to 30% (depending on composition). To use, start by stitching normally and then apply heat with a household iron. See Erica’s Craft and Sewing Center for detailed instructions (clicking on the image of the thread will open a PDF with instructions)
Heat-shrink thread and textile perfboard (image by Plug & Wear)
Erica’s Craft and Sewing Center (US): heat-shrink thread
Inventables (US) :: shape memory polymers (sheets and strips), hand moldable plastic (aka polymorph)
Mindsets (UK): polymorph, shape memory polymer
Plug & Wear (Italy): heat-shrink thread
* Polymorph and heat-shrink tubing are common crafts and electronics materials and can be found in a variety of online stores.
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