A Yarn of Chemistry: All Ewe Knit to Know-Part 2

In part 1, I dealt with the chemistry of animal fibers: wool and silk. But animals are not the only source of fibers that we use for making yarns. Another source is from plants; these are termed "cellulosic fibers," as they are all primarily comprised of cellulose. I plan on covering cotton, linen, and hemp in this section. (The photo on the left is me in a yarn shop in Heidelberg, Germany.)

To begin, let's talk about the structure of cellulose. It is a polysaccharide. This means that it is a polymer (made of many units) and the repeating units are "saccharides" also called sugars, or carbohydrates. In cellulose there is only one sugar that is repeated to make it a polysaccharide. That sugar is glucose. Now this might surprise some people because we use glucose all the time: it is in starches, and refined table sugar, and the bowl of candy on my desk, all of which we eat and digest. But we cannot digest cellulose. And yet the part of the potato we can digest is made of the same stuff that the part of the potato that  we peel off because we can't digest. This has to do with how the glucose units are strung together. 

The picture on the right shows two molecules of glucose put together: the bond between glucose molecules is called the glycosidic bond and is highlighted in red. On the top is an "alpha" bond-this is how glucose that we can digest is arranged, so like starch. On the bottom is a "beta" bond-this is how cellulose is put together. Most animals do not possess an enzyme that allows them to break beta bonds and therefore they cannot digest cellulose. By having beta bonds in place of alpha bonds, the chain of glucose molecules becomes more linear and more rigid. This is why cellulose is used in plant cell walls: the rigidity gives the plant strength. 

Probably the most important cellulosic fiber is cotton. This particular fiber, unknown in Europe until the Middle Ages, is most associated historically with the growth of slavery and the industrial revolution. The fruit of the cotton plant produces bolls, in which the seeds are wrapped up in a mass of cotton fibers. The cotton plant requires long, hot summers, well drained soil, moisture, and no frost: this is why Canada has never been known for its quality cotton. There are many countries around the world that manufacture cotton, but the conditions that it is grown in can impact the properties of the fiber. The cellulose chains in cotton long and linear, due to the beta bond, discussed vide supra; this allows many chains to pack closely together and interact with each other through the formation of hydrogen bonds, making it highly crystalline.These cellulose fibrils are then arranged in essentially three layers that are spiraled together, resulting in the high strength that cotton is known for. This is important to know when working with cotton yarn because they are less stretchy than wool, and will show any mistakes or irregularities in your work. This can be quite frustrating for new crocheters or knitters. The other thing I have found when working with cotton is that the individual plies of the cotton yarn don't stick together as well as they do in wools, making really easy to put your hook through the strand. There are different standards of cottons. The longer the length of the fibers, the softer and nicer (and consequently more expensive) the cotton is. Egyptian cotton fibers are between 25-65mm, this is what makes them so lovely. American cotton is between 10-25mm. Another interesting property is that the fibers are actually stronger when wet.

Cotton is easy to wash, breathable, absorbent, and less of an allergen than wool. It also dyes very well, meaning that it can be found in all sorts of great colours. One of the most common projects for cottons is dish clothes and towels, like the set I made on the left. Because you are going to be pretty hard on dish cloths, you don't want to use high quality cotton, go with shorter, rougher cotton. Now when making something for a baby, cotton is not a bad choice. I most recently used cotton in a baby blanket. Anything you are making for a baby, you want to make sure that it washes easily because it WILL get dirty and they aren't going to be gentle with it. Here is a great project for a nicer cotton. The longer, softer fibers make a nice blanket. 

The next type of cellulose fibers are called "bast fibers". These are ones that are derived from the stem of the plant. Unlike cotton, these fibers are part of the structural make up of the plant, and the job of holding it up requires a lot of reinforcements, meaning that these cellulose fibers are mixed with a bunch of other things like: pectins, gums, waxes, lignins, and hemicelluloses. 

Linen has to be one of the oldest, if not the oldest, cloth fibers. Seriously, hop in your time machine and head to Egypt in 8000 B.C. and you will find linens.  This prized cloth is made of fibers isolated from the flax plant. Flax fibers are found at the surface of the stem and run the whole length. Since the stem is about a metre in length, you can see how flax fibers are longer than cotton fibers, ranging from 6-65mm (average length is 20mm). Flax fibers are also stronger than cotton fibers; actually this is one of the strongest naturally occurring fibers. Like cotton, linen is light-weight and absorbs water readily. It is easily laundered and takes dye well. It is a good conductor of heat, which is why it is so nice to wear in hot climates. As a dense fiber, it drapes well, but it also wrinkles super easy. Look at it the wrong way and it will wrinkle. Its stiffness can make it a challenge to work with, especially if you are just learning, but the history makes it an interesting choice too. 

Hemp is another bast fiber that is used in textiles. But for those of you who would like to use hemp as a reason to legalise marijuana, I hate to inform you, but that is a different plant. While they are of the same genus, the hemp cultivar only contains a small amount of THC. The amount of cellulose in hemp fibers is lower than in cotton, and it tends to have lignin in it. This makes it rougher and stiffer than cotton. But being long, at a typical 15mm length, and strong, it lends itself well to the production of ropes. I have never seen a yarn made of hemp in the shops I frequent, but I am positive there are some out there. 

A relatively newer cellulosic yarn is that derived from bamboo. These fibers are quite long at 38-76mm. Bamboo is super absorbent. The fibers tend to be smooth and round, leading to the soft feel of the yarn, as well as its low irritability, making it ideal for projects for anyone with sensitive skin, like babies. It also has a lovely sheen to it.

Coming up in Part 3-synthetic yarns!

References:

Stoller, D. Stitch'N'Bitch: The Knitter's Handbook 2003, Workman Publishing Company, Inc. New York, NY.

Crowfoot, J. Ultimate Crochet Bible 2010, Sterling Publishing Co. New York, NY.

Pratt, C. W.; Cornely, K. Essential Biochemistry 2004, John Wiley & Sons, Inc. Hoboken, NJ.

Mather, R. R.; Wardman, R. H. Chemistry of Textile Fibers 2011 Royal Society of Chemistry.

Le Couteur, P.; Burreson, J. Napoleon's Buttons 2003 Penguin Group, New York, NY.

  

Winter Tires: Don't Tread the Snow

Well winter has arrived in Edmonton. It is currently -17, with a windchill that makes it feel like -25 C. Over 15 cm of snow has fallen in 72 hours. The roads have become a delightful mix of ice and snow, making driving difficult. And it is not just here in Edmonton that citizens have been hit with a mound of snow and freezing temperatures. Calgarians are currently praying for their next chinook. So how can chemistry help you survive winter? With the science of winter tires! Why are winter tires mandatory in Quebec? Why are some Albertans lobbying for the same law in this province? Are winter tires that important? Well, anyone I have asked have all stated that they love winter tires and are shocked at the difference it has made. The difference all comes down to glass transition temperature (Tg). 

Take a look around your home. I am sure that you can find numerous examples of different types of plastics. Some are rubbery, some are hard, some are fiberous. These characteristics are going to determine how different polymers (plastics are a type of polymer) are going to be used. Now think of a plastic bucket. The kind that you may have used as a kid to build sandcastles. That thing was indestructible during the summer, but leave it outside in Edmonton right now and drop it, that same bucket would shatter into a million pieces. What we are observing is a change in "state" of the polymer. Now this might sound odd, considering it is still solid, and the states of matter are solid, liquid, and gas. So how can we be seeing a change in state? Enter the glass transition.

Polymers can have two solid states: they can be glassy; these are hard plastics, like cellphone cases and water bottles; or they can be rubbery; these are flexible plastics, like rubber balls, or tires. The glass transition temperature (Tg) is the temperature at which a polymer switches between the glassy state and the rubbery state. If a polymer is used BELOW its glass transition temperature, it will be glassy or hard. If a polymer is used ABOVE its glass transition temperature, it will be rubbery or flexible. The polycarbonate water bottle on my desk is an example of a plastic that I am using BELOW its glass transition temperature, while the flexible silicone spatula I used to make my breakfast is an example of a plastic I am using ABOVE its glass transition temperature. Going back to the plastic bucket example: in the summer, the bucket is above its Tg, so there is some flexibility to it and therefore, doesn't break easily. In the winter time, the bucket is below its Tg, making it glassy, and more fragile, so it breaks. 

At cold temperatures, rubber tires are also going to go through this change. Rubber tires were such a great advancement (thank you John Boyd Dunlop) in the tire because these air-filled rubber tires absorbed shock, had more contact area with the road surface, and consequently, gave more traction. The more a tire interacts with the road, the more traction a vehicle has. In the snowy, icy winter, we need all the traction we can get. To get a nice, flexible tire that has lots of contact with the road, it needs to be used above its Tg. However, in Canada, our winters are going to push that. Our -40 C winter days are going to bring a regular tire down to, if not crossing, its Tg. This will make it more rigid, and therefore, it will have less contact with the road surface, which will decrease the traction, precisely at a time when drivers want MORE traction. Also, the treads on the tire will become less flexible, allowing for snow to build up in them, further reducing traction.

Winter tires are made of a type of rubber that has a much lower Tg than summer tires or all season tires. This means that even as the mercury drops, the tire will not approach the Tg, and will stay flexible, resulting in more road contact, less snow build up, and MORE TRACTION. More traction means less sliding, smaller stopping distances, and safer driving. Enjoy safer winter driving thanks to the chemistry of polymers and the glass transition temperature. Get yourself some winter tires.    

For further winter survival reading check out a previous entry: Careful of the Icy Patch

Want more on winter survival through chemistry? Be sure to leave your questions and comments.