I enjoy knitting- I have made scarves, hats, and am currently working on the ugliest sweater in the world. It is my first attempt at a sweater, and I knew it was going to be ugly, so I used some free Red Heart yarn that I didn’t know what to do with. I was wondering if there was any biochemistry behind knitting… and what do you know there is! I decided to research the biochemistry behind dying yarn.
The first thing I realized was that dying natural yarn-for example, wool- is WAAAY different than dying acrylic yarn such as Red Heart. In fact, a lot of knitters hate Red Heart with a passion because they say it is scratchy and squeaks when you knit. I was hoping to do half of the post on natural yarn and the other half on acyrlic, but quickly realized that it would turn into the longest post in the world. So this week we are looking just at the biochemistry behind dying acrylic yarn.
The first thing to know about acrylic fiber is that it is made a polymer, which is composed mostly of monomers of polyacetonitrile, or PAN.
Acrylic fiber also contains monomers of vinyl acetate or methyl acrylate.
Acrylic is composed of copolymers; that is, the monomers that create the chain are not all the same. For example, an acrylic copolymer may be composed of 5 monomers of PAN, followed by 1 monomer of methyl acrylate and one monomer of vinyl acetate. Or maybe 3 monomers of PAN, followed by 2 monomers of vinyl acetate. I couldn’t find out exactly what the pattern/arrangement is in Red Heart yarn–it is probably top secret haha. But we do know that most of it is PAN.
Acrylic yarn starts out as a liquid and ends up as a soft string. How does that work? I found a great website that explains how this transformation occurs. Basically, the liquid is squirted through a nozzle that looks like a shower head. Once dried, those strands are combined and twisted into strands of yarn.
Interestingly, the yarn made at Red Heart is dyed while it is still liquid, so the fiber is already colored as it is stretched out and wound. This is of course different from natural fibers, which are typically a white color which is dyed, then stretched out and wound. However, you CAN dye acrylic yarn after it has been made. Since acrylic fiber is very different from natural fiber, different dyes must be used.
Acrylic polymers typically have an overall negative charge. It was really difficult to find out exactly why this is, because most websites just repeated the same explanation using the same wording. When I tried to find some primary sources, I found out that my university does not have access to any of the articles I found interesting 😦 The little information I found explained that the acrylic polymers had anionic (negative) groups attached to them, mostly sulphonate and carboxylate. It seems that persulfate is used to initiate the polymerization of the acrylic in its liquid form. No idea where the carboxylate groups come from.
Whether or not we know where these groups come from, they are present, and they are making the polymers have a negative charge. This property is taken advantage of in order to dye acrylic. Basic, or cationic, dyes have a positive charge and work great to dye acrylic fibers. The positive dye and the negative polymer undergo an ionic interaction-kind of like a magnet.
Before you run out and start dying all your Red Heart yarn, I should tell you that these basic dyes are very toxic. And they PERMANENTLY stain ANYTHING they touch. AND they might be carcinogenic. So I don’t think I will be dying my Red Heart yarn anytime soon!
Here are some links to more information about the creation and dying of acrylic fibers:
- http://www.afma.org/f-tutor/techpag.htm#wet spinning
This week’s post has turned out to be a bit more chemistry than biology, but come back next time for the scoop on dyeing natural fibers!