Showing posts with label polypropylene. Show all posts
Showing posts with label polypropylene. Show all posts

Thursday, April 30, 2015

Living Hinges

One mechanical entity that is almost exclusively limited to being constructed from polymers is a "living hinge". Trying to describe a living hinge can be difficult, so let me provide a few pictures of living hinges you might have around the house:
Living Hinge - Ketchup bottle
What American home doesn't have a ketchup bottle or two?

Fish sauce bottle

The common feature in all cases is that there is a thin strip of the polymer that provides a flexible, continuous (usually) hinge between two other more substantial parts. Trying to make such a hinge from metal would be pretty challenging (let alone ceramics or other rigid materials).

Living hinges are most commonly produced from polypropylene (PP) and polypropylene copolymers, not only because that material is plenty cheap, but also because when properly made, the hinges can last for years. I have a large polypropylene tub that originally was used for dog food some 12 years ago. It has since been repurposed for storing bird seed in an icy-cold/sultry-hot Minnesota garage. It has been flexed two or three times a week for its entire life, and other than where mice tried chewing through it last winter looking for some free seeds, it still works beautifully.

Last week when helping out with the laundry, I ran into a living hinge that I thought was a little unusual. It's a container for Tide Pods:
Living Hinge - Tide Pods
Tide Pods Container

I say it is unusual because the container is polyester. While polyester (PET) is used for living hinges, it's usually for food packaging such as this:
Living Hinge - Fresh Herb Container
Fresh Mint
or other items at the grocery store. PET is much more expensive that PP, but it is used in food packaging because its clarity shows off the food so well. But that argument doesn't hold up with the PET used for the Tide Pods as it is not clear at all. Additionally, the hinge will be flexed quite a bit - the container holds 72 pods so that is at least 72 cycles of opening and shutting. So why does P & G use PET instead of PP? I have no idea. Perhaps some other aspect of the molding (cycle time for instance) dominates and allows for the use of a more expensive resin.

The term "living" hinge has always bothered me as there is nothing alive about the hinge. It's a hinge. A thin plastic hinge. But misuse of the word "living" in polymer science and engineering is nothing new. Look at "living" polymerizations. In a living polymerization, the polymerization reaction runs until the monomer is depleted, but unlike with more common reactions, the termination reaction never kicks in to permanently halt the polymerization. If more monomer is added to a depleted living polymerization, the reaction can pick up right where it left off, even if the newly added monomer is different that what was used previously. But is the living polymerization still "alive" while it is in that paused, deplete state, or is it more like a dormant state? (Or maybe even a "zombie" state as was discussed yesterday?) I guess polymer scientists and engineers have a hard time distinguishing between "live" and "dead". But then again, I have wondered that about my colleagues themselves at times too, so perhaps the misuse is understandable.



Previous Years

April 30, 2014 - Dow Chemical Keeps Plodding Along and Silencing Critics

April 30, 2012 - Another Set of Rants about a Rejected SBIR Grant

April 30, 2010 - Natureworks

April 30, 2010 - This Changes Everything!

April 30, 2009 - Skip the ER for potential heart attacks and find a spectroscopist instead

April 30, 2009 - This isn't even peer-reviewed


Tuesday, December 16, 2014

About that Night I Spent in Jail...

Ah yes, Christmas break in 1983. I remember it well. But the story starts well before that.

Some 17,000 years ago when the glaciers retreated from Minnesota for good, they had scrapped the Canadian Shield pretty thoroughly, leaving a tremendous number of lakes in the Northeastern part of Minnesota and also the Southwestern parts of Ontario. The Ojibway discovered that the area was fabulous for canoeing. Since the distances between lakes were short it was easy to carry canoes and such between them, allowing for longer trips to be made. European explorers took advantage of the same routes, as did the fur trappers that followed. While most people think that the history of western civilization in North America in the 1600's was mostly along the coasts, there were in fact trappers and others in this area some 2000 miles away from any coast. In pursuit of beaver pelts, they would travel up the St. Lawrence river, through the Great Lakes to an outpost now called Grand Portage, and then head inland via the some of the lakes I just described. The most common route went towards Lake of the Woods and this route is in fact what now forms the eastern portion Minnesota-Canadian border. [*]

Civilization never made too strong an entry into the area and so in 1978, the Boundary Waters Canoe Area Wilderness Act became law which over the course of the next few years increasingly outlawed motors in the area, known as the BWCA for short. Boats were first to be outlawed while snowmobiles could continue to operate through 1983. The locals were plenty upset about having their motorized access taken from them, although over the last decades, things have gotten better. Ely has grown and embraced the numerous tourists that come in every summer and in the other seasons as well.

In my youth, I was active in the Boy Scouts and would go wilderness camping as much as possible. Since the Boundary Waters was about 5 hours from home, my brothers and friends would go there often. Mostly in September. Due to the University of Minnesota not starting until the last week of September, we could quite our summer jobs and go canoe for a week or 10 days. Since it was after Labor day, the area was practically abandoned and best of all - no mosquitoes!

During our trip in the fall of 1983, I was with my friend Eric on Lake Kekekabic. He suggested that we ditch the canoes and walk up the hills on the south shore of the lake to see how progress was going on the new Kekekabic hiking trail. We did this and by sure luck stumbled up the Kekekabic ranger cabin. It was empty and unlocked, but that got the wheels turning...We marked the spot on our maps and began plotting. A group of us had been wanting to go camping in the winter in the Boundary Waters, so we saw this a great opportunity. The cabin was about a 1-1/2 day trip from the nearest road end, so we figured we could ski in, spend a few days at the cabin and ski back out.

So on Boxing Day, 1983, me, two of my brothers (Mark and Paul) and 3 friends (Mike, Eric and Bill) set off for a 6-day trip, planning to come back out on New Year's Eve. We had all done winter camping before but only for short trips or 2 or 3 days. This was a much bigger undertaking. The map below shows our intended route: put in at Moose Lake, work our way over the portages to Knife Lake and then down to Kekekabic. One bonus of this route was that we could stop in to see "Knife Lake Dorothy", someone we had never met but we knew of her legend. She had run a resort on Knife Lake for decades. When the BWCA was formed, her cabin was "grandmothered" in and she was allowed to stay put until her death. (The Wikipedia article shows how much authorities kept trying to push her out, but she fought on and never conceded.) She was famous as the root beer lady, selling homemade root beer to canoeists in the summer. So we stopped by on the way out, sharing some ice cream with her that we had brought on the trip. (Yes, it was cold enough that winter to keep ice cream frozen.) She told us she wasn't overly concerned that snowmobile access ended later that week. She had lived there so long that she really didn't really rely too much on people from Ely providing supplied via snowmobile. And besides, she had a battery-powered 2-way radio if she ever got into real trouble.

We said goodby, headed on to the ranger cabin, and moved in. We got the woodburning stove going and got comfy even if the lack of electricity made it pretty dark. Breaking trail through waist-deep snow on the portages and dealing with slush on the lakes had made the trip to the cabin take the all of 3 days, so we were happy for some creature comforts. Since the trail was already broken, we figured we could make it back to the car in 2 days so we planned to break camp on the 30th. That morning, we fired up water on the camping stove for making a big batch of oatmeal. Unfortunately, in the poor light and somewhat cramped conditions, I knocked the water pot off the stove and onto the leg of Mike. Mike was only wearing his long underwear at the time...

Time to get technical and bring plastics into the discussion. Polypropylene was used back in 1983 and is stilled used to make terrific long underwear for active lifestyles as it doesn't absorb water (unlike cotton). It's also is good and insulative. If you are winter camping, that is a good thing, except when you have a couple of quarts of boiling hot water on one side and human flesh on the other. Mike got the underwear off ASAP but he had already gotten blisters and was in a good deal of pain.

After about an hour or so, we realized that Mike would have had a difficult time skiing out, so we decided Mark, Eric and Bill would ski back to Dorothy's and use her radio so that we could get Mike out on a snowmobile. I stayed behind with Mike and my brother Paul. Within a surprisingly short period, the absolute silence of the wilderness was spoiled by the sound of snowmobiles. The group had made it to Dorothy's and as luck would have it, 2 forest rangers were there talking with Dorothy and stocking her up with supplies. They took our well broken trail and got to the ranger cabin in a snap. But they also looked at Mike's leg and realized that in the rough terrain over the portages, he was not going to be well off so they made the decision to fly him out. They called in a Forest Service ski plane from Ely which arrive in a short time. The plane had room for 3 passengers, so Paul and I hopped on as well.

We got to Ely and Mike went off to the clinic for treatment. And that is when we realized we had a problem. The other three people weren't going to be coming out until tomorrow and yet Mike, Paul and I needed a place to stay that night. Ely in 1983 was not setup for overnight guests, particularly in late December. There was no room at the inn as there was no inn open for business. Afterall, who would want to stay in a motel in Ely in late December? The car was 20 miles away and even if we got there, we didn't have the keys (they other guys did). And so they put us up in the only spot they could - the jail.

The jail had 4 cells each with one bed. I still remember this horrible revulsive feeling while being shown my cell by the officer. They didn't lock the doors, but still just that feeling from seeing the bars and the toilet and the bed...I hope I never feel it again.

The next day, Mark, Eric and Bill came out off the wilderness and bailed us out picked us up. They had had a terrible last day coming out because that day, December 31st, was not only the last day that snowmobiles were ever allowed into the BWCA, but it was also the opening day for lake trout fishing, and so everyone was taking advantage of the situation. They were all heading in while our group was trying to head out.

The trip home was thankfully uneventful. Mike was ok as the burns were limited to 2nd degree and just required regular cleaning and topical antibiotics. I've seen him since and he jokes about getting into hot water fights. I still feel bad about my flying elbow causing so many problems for all of us. I've not been back to the "scene of the crime" since, but my brother Mark has. The cabin is now padlocked and there is a "No Trespassing" sign.

No surprise there. No surprise at all.



[*] The western portion of the border is a different story. The original border was based on a faulty map (it's not the first time that's happened) that showed the Mississippi river starting up in Canada when in fact it starts quite a bit further south. The border was supposed to go along the voyageurs route and then go due west from the northwestern corner of Lake of the Woods until it hit the Mississippi. The error was soon discovered and so they changed the border by drop south from that same corner of the lake to the 48th parallel, and then proceeded west to the Pacific. As a result, Minnesota is the northernmost state in the lower 48. Also as a result, there is part of the state that you cannot access by land except by going through Canada.


Previous Years

December 16, 2103 - What does EPDM stand for?

December 16, 2101 - The Real Danger in Polycarbonate (it's not BPA!) and a Solution for it

December 16, 2100 - PLA that acts like ABS

Tuesday, November 11, 2014

e-Polypropylene - "And now I've lost it."

Crystalline polypropylene (PP) has always been one of the more interesting polymers because it is polymorphic - there are multiple crystalline phases that can form. Without any special processing, the α- phase dominates. This is the one that melts at about 170 oC or so. In the past few decades, nucleating agents for the β-phase have become commercialized. I've not worked with any β-PP, but it according to literature reports, it melts at about 150 oC or so. If the crystals are drawn during processing, they will revert to the α-phase, so you need to be careful about that. I'm not aware of any commercial applications, but there is probably something out there. There is also the the γ-phase, of which even less is known.

Those three phases have been known for about 20 years and are at least mentioned in passing on most books on PP. So imagine my surprise last week to find that they have now discovered the ε-phase! Wait - I didn't even know about the δ-phase Did I miss something?

Apparently I did. Or maybe not. I'll let you decide. A frantic Google search for the delta phase turned up nothing, but one of the introductory paragraphs of the ε-phase article mentions that the δ-phase occurs for copolymers of propylene and hexene or pentene when the comonomer content is great than about 10%. So is that really a polypropylene crystalline phase? Before you decide, consider what we know about the ε-phase.

Just like with the δ-phase, the ε-phase doesn't occur with stereotactic, defect-free polypropylene, but requires specially prepared material, where "...the polymers produced have progressively lower melting transition temperature, lower homosteric pentad sequence population, and higher solubility in low-boiling solvents, indicating frequent stereochemical inversion in monomer enchainment." In other words, this polypropylene has some significant "issues". ("Normal" polyproplyene has a high melting temperature as noted above, high pentad regularity, and is insoluble in low-boiling solvents.) Somehow the defects are not enough to make this completely amorphous. So again I ask, is this really a polypropylene crystalline phase?

My opinion (which comes with a full-money-back guarantee if not completely satisfied): No, the δ- and ε-phases are not part of the polypropylene polymoprhs. For "normal" polypropylene, you can make the α-, β-, and γ-phases from the same starting material, but you can't make the δ- and ε-phases and so they are not part of the set.

Before I finish, consider this example of life imitating art:
"Unfortunately, both the details of the material and the material itself were lost when our laboratory moved to its new location."
Que up the theme from "The Medicine Man". I can only imagine the scene within the new lab: "...now I've lost it. Haven't you ever lost anything doctor Bronx? Your purse? Your car keys? Well, it's rather like that: Now you have it and now you don't." That would be pretty painful to discover and hats of to the researchers for being upfront about it.


Previous Years

November 11, 2011 - Birefringence in Polycarbonate

November 11, 2011 - Flashing Labels

November 11, 2010 - Olefin Metathesis