Friday, August 26, 2011

Diary of a Summer Intern

If ever this young chap darkens your doorway:

just think about this line from the song "Toes" by the Zach Brown Band
"Life is good today, life is good today"
A truly fantastic intern, about to set sail into his senior year of chemical engineering and then off to grad school. With no hesitation, I can firmly state that he will be quite successful in whatever he endeavors and then some. Somehow I managed to luck out getting him for the summer - Somehow Aspen Research managed to luck out getting him for the summer. He has made huge contributions not only to my projects, but also projects that were given to him by people that don't think interns should have choice projects but rather the most undesirable grunt work imaginable (how about some ISO 17025 documentation?) and everything in between.

Somebody in the near future will be blessed with him in their lab in the near future; may it be you.

Thursday, August 25, 2011

Ancient Egyptian Rheologists

The identity of the world's first rheologist is most likely lost to the annals of time. Whether it was Ugg, Grugg, or their wives that were dragged by their hair into their caves after being clubbed over the head, we will never know who they were nor what they developed. A commonly cited candidate for the first rheologist is Heraclitus the Obscure, who stated "Everything flows" (Panta Rei), thus giving the Society of Rheology its motto.

But Heraclitus lived about 500 B.C., some thousand years after Amenemhet, who made a 7 degree change in the drain line of a water clock in order to compensate for the density differences in the water between day and night, and this was all done about 1600 B.C., although I'd argue that studying Newtonian fluid mechanics does not a rheologist make.

But there now appears a new claimant to the throne, an unidentified individual who back in 3500 BC developed a hair gel used by Egyptians. (Links for the research article, and a Nature News report (open access)). Being gels, they easily qualify as rheological materials, and hence there was a rheologist behind them, although who he/she was, we'll likely never know. (This also proves that human vanity has existed for ~ 5500 years).

All this raises the question of whether we'll ever seen an older example of rheology. Mummies don't get much older than this. Archaeological remains, however, do go back much further, but then trying to determine if a given artifact has rheological implications gets to be far more challenging. It is fun to think about though, isn't it?

Wednesday, August 24, 2011

Review: "Social Marketing to the Business Customer"

The last time I did a book review was probably in the 7th grade, so don't expect much from this effort.

"Social Marketing to the Business Customer" [1] is a timely book focused on how to use all the available social media - blogs, Facebook, Twitter, help with marketing you business. I say timely, because there certainly is a need for this. We've all seen way to many efforts by businesses [2] that fell flat. I also say timely as a pun, because given the rapid pace of change in social media, many parts of the book are already obsolete. Google+ for instance, has arrived on the scene. While in a sense this changes some of the technical details, in other ways, the overall message of the book doesn't. If you understand that message, incorporating Google+ and whatever else is yet to come will not be a problem.

As for what that overall message is, here are several themes that I found repeated over and over:
  • The website is king. Everything you are doing should be aimed at getting people to your website, regardless of how popular Twitter, Facebook, etc. are. The website is where you can completely control your message and where you can convert prospects to customers
  • Getting results will take lots of
    • time
    • energy, and
    • courage, yes courage
  • Nobody has this whole Web 2.0 figured out yet. It is one of the greatest open-ended experiments of all time, so you have to learn as you go, and at the same time, copy the success of others
  • You will fail if you are too "corporate"

The first point doesn't need much further comment, and it is one of those things that is rather obvious when you think about it. But with this frame of mind, you can then see that just setting up a Twitter feed with canned messages once a day isn't going to cut it. Why would anyone follow that? How does that drive anyone to your website or do business with you?

As for the second point, I know from experience with this blog that it takes a long time to build a following, and seeing the sitecounter show extremely small numbers forever-and-a-day is quite discouraging. Granted, polymers isn't as controversial a topic as politics, but still, there are hundreds of thousands of people around the world working with polymers, so the fact that I am only getting a 500 visitors a week shows that I still have a long way to go. The mention of "courage" is important, because you will see plenty of failures along the way, and you need to face up to it. You and your corporation also need courage because the legal department will be very leery of your efforts - the messages going out are not controlled, whitewashed and cleared by every committee available.

I can also expand on the last point, as that is a variation of what I just mentioned: if you let the legal/marketing communications/sales/... departments preapprove everything, you will come across as very "corporate" and fail. As is said in the book, people want to interact with a company, not a brand. One of my favorite examples is what that megacorporation, IBM, does with its employees and blogging - aggregates links to them on a corporate page. This clearly lets you see that the blogs are not the corporate message, but it also humanizes the corporation by letting you see the thoughts of individuals working there.

While the focus is on B2B, the same ideas can be applied to anyone wanting to drive visitors to their website, or in my case, this blog, which is basically my website.

[1] The book was provided to me courtesy of the Don Loepp of the PlasticsNews blog. Thanks, Don.

[2] Want proof? Look at what my employer attempted on Twitter.

Tuesday, August 23, 2011

How the Indian Supreme Court Indirectly Impacted PET Film Makers

Way back in earlier December I blogged about the Indian Supreme Court deciding that plastic is more dangerous than tobacco - it outlawed plastic bags that held gutka, a tobacco product. Now the secondary effects of that ban are being felt: Plastic News reported in their August 1 edition that Indian film suppliers have 220 million pound of PET inventory that they can get rid of. Suddenly last year's shortage of PET film has been become a glut on the market as the Indian inventory is being sold at low prices. A legal decision half-a-world away has now impacted the wallets of PET film makers in the US and elsewhere. The glut will eventually disappear, but it would be painful to have to live through it and compete against it.

Plastics are Forever Jewelry

"Plastic is Forever", that battle cry which in the past has been the exclusive domain of environmentalists and artists, now has apparently added a jeweler to the effort.(O.k., fine, they changed the tagline to "Plastics are Forever")

I say apparently, as the remainder of the ad is so tongue-in-cheek that you begin to suspect that this is a gag gift.
You want to get a special gift, something that will last the test of time, something that demonstrates you care, and that you will care forever. Precious, rare, and synthetic, give the gift of plastic garbage to your special someone. Haha. 5 Gyres has partnered with designer Katelin Gibbs a New York jewelry designer who has designed these beautiful pieces to help support our work at 5 Gyres. The pieces are made from pre-production plastic pellets (nurdles) and actual micro-fragments of plastic collecting in the South Pacific Gyre. 25% of sales go directly to support our work fighting plastic pollution in the world's oceans. Thank you so much Katelin! (emphasis added)
Additionally, there is no information on pricing or ordering.

As I've mentioned before, this jewelry will not last forever or anywhere close to it. The oxygen in the atmosphere will degrade the polymers, aided by the sunlight and ozone, leaving the owner with having to explain that plastics really are not forever.

Monday, August 22, 2011

Glad I'm not buying propylene in China

Just look at the first sentence of this news report (courtesy of Plastemart):
"Spot homo-PP prices in China have been trading close to par with spot propylene prices on an FOB Korea basis since the beginning of August, resulting in significantly squeezed margins for non-integrated producers..."(emphasis added)
In other words, some plants have to buy the monomer and sell the processed polymer for nearly the same price. Keeping in mind that the processing includes not only the use of the equipment and the energy associated with it, but also the catalysts, the QC testing, the final packaging, administrative costs, sales costs...Somebody is taking a hit in the wallet and I'm glad it is not me.

Thursday, August 18, 2011

Names for Biobased Polymers

Since bio-based polymers are the rage, we need new terms to properly describe them, right? So here are my suggestions. If you feel like commenting, let me warn you that I already have donned my flameproof suit with self-contained breathing apparatus so I can hold out for quite some time.

  • Yesterday, you saw my suggestion for the block copolymer made from mad cow parts - "block cowpolymers"
  • If a similar approach is taken to chicken feathers, they could be "flock copolymers"
  • Continuing with the barnyard animals, a polymer made from female horses could be a polymare
  • Polymers made from epoxidized mushrooms could be "polyethylene mycol" (mycology is the study of fungi)
  • Polymers made from reacting diols, diisocyanates and urine could be "polyurineates"
  • Polymers made from diamines, diisocyanates could be "polydiarrheas"
  • Polymers made from ethylene glycol, terephthalic acid and birds nests could be "polynesters"
Thank you very much ladies and gentlemen, I'm in town all week. Be sure to tip your servers...

And if anyone is tempted to tell me not to quit the day job...this is my day job - making polymers from a wide range of biomaterials.

Wednesday, August 17, 2011

Mad about Mad Cows

I've mentioned before that polymer chemists are clever enough to make polymers out of pretty much anything [*], with the emphasis clearly being on any sort of bio-based feedstock. Yesterday's announcement pretty much ices the cake. Tell me you don't chuckle a little bit when you read the headline

"University Turning Mad Cow Parts into Plastic"

Before anyone gets too excited about the commercial possibilities, take this whole project as a sign of academic naivete, considering that "about 5,000 tons of cow parts a week are dumped into landfills, estimated [research David] Bressler. That garbage could yield 3,500 tons of raw material for the new plastic." Who is going to develop and commercialize a process that is capped at 3500 tons of plastic a year? What application is that small? Even if you make a large margin of 10 cents a pound (and I am being very generous in citing a margin like that for a low performing plastic), that is a profit of only $70,000/year.

"We don't have a name," he laughed. "We don't really have a catchphrase nickname for it." I can at least help with that. Since it is made with blocks of protein and blocks of synthetic materials, how about a "cowpolymer"?

[*] Isn't that sneaky that I can complement both my fellow polymer chemists and myself in a subtle manner?

Tuesday, August 16, 2011

The Ultimate Time Drain

The past few weeks and the next few weeks are already booked out at work. If you look at my calendar however, you will see very few appointments. Instead, I am glued to my computer spending more time writing than you would think possible for someone who normally spends lots of time in the lab.

I am simultaneously engaged in the two most time-sucking activities known to man. If you've never undertaken either of these activities before, you will have that cute naive innocence that says "Oh, that looks easy and shouldn't take too long!" What am I talking about?
  • Writing a grant proposal
which is plenty time-consuming, but in terms of "where did the time go and why is this taking so long and how many more revisions do I have to go through", it's a child's play. No, I am talking about the ultimate time-kill, the one where a whole week can be spent, where endless searching of exotic databases is needed, where you leave the building each day in a mind-dead zombie-like state (where you can't even count to 4!) without having even looked out the windows once to even see if it was a nice day. I'm talking about
  • Writing a patent application
Lest someone complain about grants, grants are an internal driven process. When you are happy with it, you might have a few colleagues look at it for some comments, but with patents, it's back and forth with the lawyers, who have an entirely different (and appropriate) view on everything. Plus there is the almost absolute guaranty that there will be more back and forth in the coming years as the patent office and the patent examiners starts to get into the game.

Monday, August 15, 2011

An Issue on Nomenclature

Yep, one of those again, although in this case, I'm not sure that I know enough to have all the background, so please feel free to enlighten me if you know more.

So today's battle is: "Interpenetrating Polymer Network" vs. "Double Network", or IPN vs. DN. IPN has the presumption of history on its side (I believe), a term used to describe two independent polymer networks (generally crosslinked [1]) that are intermeshed at a molecular level. To assembly such a beast, two different reactions are run, either sequentially or simultaneously, but most importantly, there are no chemical bonds connecting the networks. The end result in either case is two truly entangled networks. Interpenetrating polymer network is a great term to describe them as if is accurate, and in my mind, abstruse enough that you need to know what the term means - you can't fake it nor does it mislead. At 11 syllables however, "interpenetrating polymer network" is a mouthful, although IPN flows nicely as an 3 syllable abbreviation [2].

So now along comes the term "double network" that I first ran into in a paper on hydrogels. The authors of the paper do use IPN to describe their polymers, but seem to prefer using "double network" instead. At first I thought it was just an upstart term, but a search shows that it appears to be limited to just IPN's that interact strongly with water (hydrogels).

So is that the only time DN is used, and if so, what is wrong with IPN?

Part of my problem with this discussion is that I also think "double network" is a pretty good term to use as well. Just like "interpenetrating polymer network", it's accurate and abstruse. It's also only 4 syllables, although DN doesn't quite roll off the tongue as well as IPN does.

This is not like some of the monstrosities that IUPAC has foisted on us, such as replacing "intrinsic viscosity" with "limiting viscosity number", a change that was not asked for or accepted by rheologists and polymer practitioners, but then again, isn't also the case for "double network"?

[1] When only one network is crosslinked, then you have a semi-interpenetrating polymer network.

[2] The worst abbreviation in the world? www, as in world wide web. The acronym has 9 syllables, three times that of what is being abbreviated.

Friday, August 12, 2011

Polymer Drone

The Polymer Drone is a musical act, not a chemist so I'm probably being a little harsh in picking on his chemical structure, specifically,
  1. it's not a polymer
  2. it has two Texas carbons (carbons with more than 4 bonds - name comes from the saying "Everything's bigger in Texas")
  3. the unpaired electrons are shown as being inside the heterocylic ring, not outside as they normally are.

Update: Sure, I can count to 4. Heavens, what an embarrassment. Blame it on the patent application

Swirling Wine Clockwise and Counterclockwise

Here's a good laugh for a Friday. It's too early to start drinking, but we can certainly plan ahead for a few experiments with this evenings imbibitions. As far a bogus science goes, this might be the limit. Fortunately, no one is getting hurt from the claims (just maybe a little tipsy), so it's "no harm, no foul".
"Like all living things wine cells have a magnetic polarity, just like humans and the Earth. The positive pole is more highly charged, just like the North Pole of the Earth, which is why there are Northern Lights in the Arctic Circle, but not Southern Lights in the Antarctic."(Sorry, this is completely false.) "This polarity tends to keep wine cells generally upright, (Why? The magnetic field is vertical only at the magnetic poles) spinning on their axis when they are being swirled. This magnetic action within a liquid is commonly demonstrated in laboratories. Because plant molecules are mostly liquid, when they form they are also subject to the electromagnetic forces that are a component of the rotation of the Earth. As a result, the pores on the surface of the molecules (Pores on the surface of molecule?) develop based on that rotation, like the shingles on a roof.

When you swirl the wine counter-clockwise you are pushing against the molecules nap
(So somehow the molecules inherently pick up a clockwise spin. How? Why? Do Australian wines spin the opposite direction?) , just like stroking the fur of a cat the wrong way, this dislodges anything on the surface. Since the flavor from the barrel is introduced fairly late in the wine's development it tends to concentrate in the outer layers. When you swirl the wine counter-clockwise it dislodges that flavor, while at the same, pushing liquid into the pores, inhibiting the fruit flavors that are inside the cell from coming out.

In comparison, when you swirl the wine clockwise the pressure of the surrounding fluid forces the fruit flavors out through the pores. It also pushes any flavors concentrated on the surface down onto the skin of the molecule.
(Well if a molecule can have pores, it can have a skin too.) The fact that the wine is alive, electrically charged, and still changing is why this happens."
Oh, and there's a followup article too!
"Someone quite rudely took exception with my use of the word cell, which is in fact incorrect. The proper term would be molecule or even atom. Everything has a polarity right down to the atomic level, and when put into suspension in a liquid it rotates in relation to that pole. Because we are on a planet that has both a polar system and a consistent rotation, everything forms with a pole and a circular patterning. Wind it one way and it tightens and wind it the other and it unwinds.

Honestly this is just basic physics related to molecular science and plant chemistry, something which herbalists and herbal researchers deal with all the time. A pretty sober group of people."
(emphasis added)
It's also basic physics that the thermal energy in a wine of drinking temperature is more than enough to rapidly randomize any orientation that could possibly exist as a result of the weak magnetic field felt here on Earth.


Hat tip to Stuart Cantrill for the lead (Twitter @stuartcantrill)

Thursday, August 11, 2011

Why We Need to Sort Plastics for Recycling

Here's a question from another site:
"Dear EarthTalk: Why can’t plastics of all types, instead of being initially sorted, simply be melted together to be separated later?"
Overlooking the simple issue that each individual polymer has separate melting/softening temperatures and just shooting for the highest temperature needed would degrade the lower melting materials, there are thermodynamic issues as well involved that challenge all polymer scientists and engineers that are looking to make a compatible blend between two polymers (never mind attempting to blend all 6 different versions of purified streams available after sorting by hand).

Compatible blends of polymers are very much an exception and not the rule. Small differences in polymers can have a tremendous impact on solubility. The most extreme example that I'm aware of is deuterated polymers phase separating from their non-deuterated equivalents. The only possibility more extreme would be a homopolymer separating from itself - and even that isn't that remote of a possibility at times (such as in field-flow fractionation).

Polymer compatibility is challenging and it is all because of the high molecular weight of the materials involved. Little help is provided from entropy (the number of molecules per unit volume is so much smaller than for a low molecular weight material) and so much is therefore requested from the enthalpic interactions which occur at every repeat unit. If the interactions aren't right, you will not have a soluble mixture since there are so many of them.

It would be great if these constraints didn't exist and all plastics could be thrown into a big gemisch, but that won't happy anytime soon and probably not ever.

Wednesday, August 10, 2011

A Sceptical Chemist

Not sure what I was sceptical [sic] about, but a recent email interview with me was published in the Sceptical Chymist blog from Nature Chemistry.

Tuesday, August 09, 2011

Monomers and Residues

One of the few (make that exceedingly few!) ways in which I think biochemists have better vernacular than chemists is when dealing with macromolecules and the materials that are used in their biosynthesis. Specifically, the use of the word "residue".

Proteins are synthesized from amino acids, but the process is a condensation reaction which removes a molecule of water from each amino acid. As a result, the amino acid that enters the chain is similar, but not in fact exactly the same as what is after it is part of the chain. The amino acid monomer is now referred to as a "residue", since it is the residue of the polymerization. This picture of cysteine and the cysteine residue should make it clearer.Strangely, I don't think I've seen that language used when speaking of synthetic monomers and polymers. Ever heard of "ethylene residue" to refer to a repeat unit of polyethylene ?[*] Sure, that is an addition reaction, but is it any better for condensation reactions? Every heard of "terephthalic acid residue"?

I'm not going to start a one-man effort to change the world, but it this is something that we should all be aware of, and consider the use of "residue" if appropriate in discussion of polymers and their monomers.

[*]...which completely overlooks that the repeat unit in the final product is actually methylene - CH2

Monday, August 08, 2011

What's in a Name? What's in a Number?

If you've ever worked with certain polymer additives such as antioxidants, UV absorbers, photoinitiators, etc., you probably have noticed a pattern. Take for instance, Irganox 1010, a product of Ciba.
As an temporary aside which will later not be an aside, it's a pretty complicated molecule until you look at the structure a little bit. First, you can see that there are four identical arms. As such, this imagesimplifies the matter. Looking at the left end, you may recognize the butylated hydroxytoluene segment, aka BHT: a well known antioxidant. So know we can see that Irganox 1010 is simply 4 molecules of BHT bonded to a central molecular segment (I suspect that it is pentraerythritol.) BHT by itself is too volatile and mobile to be very useful in durable plastic applications, but when modified like this, it will be much more effective in the long run.

But getting back to my original intent, I don't know for sure that Ciba first starting marketing this product, but you will see numerous products from other companies that have their own name for the product, but kept the 1010 number with it, such as Plaox 1010, BNX 1010, Songnox 1010 and Westco 1010. I've no experience with trademark laws, but it seems as if Ciba, or whoever first developed these products, could protect their name - Irganox - but not the number 1010 (nor the combination of name and number). This pattern exists with more than just the 1010 molecule. It clearly is more than a coincidence.

In some ways, this is a good things as it simplifies the sourcing of alternate materials, but it also oversimplifies the situation by implying equivalence. Since we do a lot of work here at Aspen Research on degradation of polymers, we've had the opportunity to examine in details many of these "equivalent" products and found them to be not so "equivalent". The biggest culprit is that the reaction binding the BHT to the central framework is not as well completed as it should be - i.e., there may only be 3 BHT moieties instead of 4. That 4th BHT molecule is actually in there, but it is footloose and fancy free to disappear rather than stick around where it is needed. The bottom line is beware of implied equivalence - analyze and test the materials to prove to yourself the equivalence.

Thursday, August 04, 2011

Sometimes You Can Sew A Silk Purse From a Sow's Ear

Graphene is now the wonderchild of the material science world, although obtaining it has been very expensive. This is not only because of the slow throughput and expensive capital equipment, but also because the carbon sources used in its production have been of very high purity.

A new paper (open access) shows that high quality carbon sources are in fact not needed. Look at the labels of the Raman spectra and try not to laugh too hard - this is serious science.Need a few specifics on the materials (I do - "Plastic"?! Seriously, they labeled it just "Plastic"?)
"Six different carbon sources were used: Girl Scout Cookie (the Girl Scouts of America Troop 25080 from Houston, Texas, provided the cookies, shortbread flavor), chocolate (Chocolate Kennedy Half Dollar Gold Coins), grass (Ophiopogon picked at Rice University), plastic (Fisherbrand polystyrene Petri dishes, catalog # 08-757-12), dog feces (Miniature Dachsund) and a cockroach leg (American cockroach caught in a house). The grass and the dog feces were dehydrated in a vacuum oven (102 Torr) at 65 °C for 10 h before being used in the growth process."
Where they got the nerve to try some of those options, I can't imagine. Technology Review has estimates that a single box of the cookies can product $15 billion dollars worth of graphene, so maybe order a few extra boxes next year when the Girls Scouts are at your door.

Hat tip to Neil Withers (twitter - @neilwithers) for the lead

Wednesday, August 03, 2011

Less Toxic Chemicals?

I really have to be skeptical of this report - that chemists, with a little bit of training, should be able to design less toxic chemicals.

Let's modify that statement a little bit to see if we can get any insight. How about this - that with a little bit of training, chemists should be able to design better drugs. (This isn't much of a stretch since we all know Paracelsus's decree - it's the dose that makes the poison.)

Chemists have had rule of thumb for drugs (Lipinski's Rule of 5 is a terrific example) and yet numerous drugs defy those rules and even with that guidance, new drug introductions are falling at a horrifying rate. QSAR was once held out as a great hope but has failed to deliver as widely as expected. (If only God had left the design manual around for humans, we would be in much better shape in pharmaceutical design!) If you doubt any of this, just start rolling around in Derek Lowe's blog to see the challenges facing drug designers.

Toxicology and pharmaceuticals both are at the interface between chemistry and biology. Given the extreme difficulty in designing drugs, I can't believe that designing "low toxicity" chemicals will be any less challenging.

A Good Week for Plastics

The broad outlook for plastics had some positive news this last week.

On the home front (i.e, in the US), the Corporate Average Fuel Economy (CAFE)standards for automobiles is increasing to 54.5 miles per gallon (mpg) from the current value of 24.1 mpg. [1] That is a tremendous increase, some of which will undoubtedly be met by increasing the number of hybrid vehicles, but also by decreasing the weight of vehicles, and this, in no uncertain terms means increased use of plastics in cars. I've mentioned some of the more innovative ideas in the past (polycarbonate windows, oil pans and mufflers, and carbon-fiber composites) so it is largely a matter of getting these pieces in place, rather than having to invent them anew. While the standards only directly affect the US, the results will certainly be applied around the world to at least some degree.

Elsewhere around the world, capital investment in polymers (and other petrochemicals) is increasing, being led first and foremost by Dow and its partners. Dow and Saudi Aramco are building a $20 billion plant in Saudi Arabia for producing a wide range of basic chemical feedstocks. The project will comprise 26 separate(sub)plants, and is the largest chemical plant ever built.

Dow is also looking beyond the petrochemical paradigm and announced the formation of a joint venture in Brazil with Mitsui to create plastics from sugarcane. It's a much smaller investment, (the number $2 billion seems to be tossed around), but that doesn't surprise me much. [2] Regardless of the feedstock, the investment is still in plastics, a material that won't be disappearing from the planet anytime soon. [3]

[1] This as an average value, actually a harmonic mean, across the entire set of cars sold by a manufacturer, rather than a minimum standard that has to be met by all cars. I won't get into all the details as there are too many of them and they are unimportant here. Whether or not cars hit the new targets or fall short, the bottom line is still the same - car manufacturers will be increasing the gas mileage in their cars.

[2] I've always maintained that we will still be using large amounts of petroleum for far longer than most people would guess - there's too much infrastructure in place and too many people have too much to lose by converting to "green" feedstocks. Since the transition will occur around the world in the 197 or so independent countries, it can't be legislated, so it will occur gradually, not instantly.

[3] I can't believe I just said that. Isn't that too much like "Plastic is Forever"?

Tuesday, August 02, 2011

Strange Graphical Abstract

In general, I like graphical abstracts in the Table of Contents for journals. although I think most of them are not too helpful. Looking at a plot - specifically trying to decipher the small labels on the axis to deduce what relationship is being shown - is not quicker than just reading/scanning the title of the article. The graphical abstract shown below however, bothers me in an entirely different manner.
Cartoon hands?

I haven't read the article (reading the abstract does not convince me it's worth the effort) but the authors found a relationship between the persistence length of their polymer family and the extinction coefficient. That's fine and nice. So are they trying to show with the cartoon drawing that these two variables go "hand-in-hand"?

Monday, August 01, 2011

Adding More Crosslinker to Avoid Crosslinking

What probably draws me the most to polymer science and engineering is the (apparent) contradictions that show up so often, whether it is rod climbing (the Weissenberg effect),polymer demixing, or any of an endless number of other unexpected pleasures.

I recently ran into a new one: to avoid crosslinking in a polymer, an excess of crosslinker is added. I won't get into the details too much, but it involves a 1-part, moisture-cured, hydroxy-terminated low-molecular-weight polymer that reacts with a tri-functional crosslinker. Now if one group of the crosslinker reacts with a polymer segment, the molecular weight and viscosity of the material increases only slightly, an entirely acceptable outcome. If two groups react, the molecular weight has now doubled. And if three groups react, the a networked is formed and the entire material has set up, an entirely undesirable result. Keep in mind that there will be distributions of reactions in the container, with some of the crosslinkers being unreacted, mono-reacted,... So in order to decrease the odds of a crosslinker reacting with three polymers, extra crosslinker is added.