The Difference Between the Parker-51 and 21

[ATisch]

[Hello to all! It’s been over 5 years since I’ve posted an essay on fountain pens. Hopefully, a few of you will recall some of my past contributions describing the technical side of pens. This piece is no exception.]

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Obviously the biggest difference between the two Parker offerings was (and is) price. The Parker-21 (P-21) was sold at a fraction of the price of the Parker-51 (P-51). While this difference may seem trivial, it’s of paramount importance to understanding the technical differences.

 

There are a number of places where it’s easy to (literally) see some of the cost-cutting measures. The most significant of these is the much simpler feed and collector, which is much more complex and intricate in the P-51. Other differences can be seen in the nib itself (14 K gold versus steel), the filler and in the hood design.

 

However, the biggest difference isn’t to be seen by the eye at all. This is in the nature of the plastic from which they both were made. They are both made of the same plastic, polymethylmethacrylate (PMMA)*. PMMA is the “acrylic” polymer made from the monomer, methylmethacrylate (MMA). The first and most obvious question that comes to the mind of anyone not well-versed in polymer chemistry, is how can they be made out of the same polymer and yet be so different? If they are both made out of the same polymer, why has the P-51 plastic proven to be very durable over time, while the P-21 has not? The answers, while intentionally simplified for general understanding, follow.

 

The differences in the two PMMA’s, like the more obvious differences we can see, are also driven by production cost considerations. Let’s start by describing the factors that lead to higher quality PMMA, like in the P-51. Possibly the single biggest determining factor in the quality of the PMMA is the average molecular weight. The higher the average molecular weight, the higher the overall strength and quality. In theory, the highest quality PMMA would result when all of the monomer is converted via a single chain reaction into a single molecule of the polymer. Such a polymer is impractical, if not impossible. It would be the result of an extremely slow (and costly) reaction. In practice, time is money, and the best solution is to find the best compromise between molecular weight, time and cost. The production of high-quality PMMA, is achieved by starting with extremely pure MMA, addition of a very small amount of “initiator” (a source of free radicals that start individual chain reactions) and temperature regulation or cooling, as the reaction is very exothermic. The high purity of the starting MMA is a major cost itself. If the MMA is not very pure, one gets poor control over the polymerization, which results in unacceptable batch to batch reproducibility. Limiting the amount of initiator, coupled with cooling is needed to control the reaction, which is in turn also required for a reproducible reaction and batch to batch consistency.

 

To make a much less expensive PMMA, as used in the P-21, the three above-mentioned variables are compromised much further. It starts with less expensive (and less purified) MMA. It is polymerized much faster (and in much larger batches), by adding much more radical initiator and allowing a much higher reaction temperature. This, of course, results in a much lower average molecular weight, which translates into inferior polymer strength and integrity.

 

There is one further problem that arises from the much hotter reaction that ensues. It allows for numerable microscopic hot-spots in the reaction that result in forming microscopic bubbles where the monomer, MMA actually vaporizes. These bubbles add to the inherent weakness and quicker ‘aging’ of the resultant polymer.

 

There are a few words that should be mentioned in Parker’s defense. Firstly, they were trying to fulfill a demand, which the P-21 certainly did. Many of the (cracking) problems we see today, were not as apparent when the plastic was new. While the PMMA that was used in the P-21 was known to be inferior (but sufficient), it has become much more so over time. Despite considerable variability from batch to batch, almost all P-21s lasted several years before they started to crack and fail. However, it’s also this batch variability that leads to the state of P-21’s today. There are still some that are holding up. However, most have already started to fail (crack). I believe that one conclusion that can be safely made is that any P-21 part that has already cracked just isn’t worth trying to fix. It will most certainly crack again and again. Some knowledgeable pen folks are of the opinion that all P-21s will soon fail they haven’t already. This is a reasonable philosophy, but it is possible that some batches will continue to hold up.

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*There is some confusion over PMMA versus the trade names “Plexiglas” and “Lucite”. Both Plexiglas and Lucite are high quality PMMA. Rohm & Haas was the first to bring PMMA to market in 1933 under the trade name Plexiglas. DuPont soon followed up with its own PMMA under the trade name of Lucite. The main (at least initially) difference between them is that Plexiglas was literally polymerized between two sheets of plate glass to make a product that was optically pure. Lucite was thermally extruded to give a plastic of similar quality, but without the optical purity. [This difference explains why only Plexiglas was used for airplane canopies during WWII.] Apparently, DuPont was able to market Lucite because Rohm & Haas chose not to patent Plexiglas. They reasoned—correctly—that it would take other companies much longer to work out the details of making optically pure PMMA than a patent would have provided.

 

[Jerome Tarshis]

Thank you so much for taking the trouble to write and post this. Welcome back.

 

When you write "21," do you refer to the Parker 21 only, or also to the Super 21? AFAIK, the Super 21 had a tubular Octanium nib similar to the gold nib of the 51, and a similar feed and collector design, though the collector was not quite identical.

 

I have a sense that the earliest Super 21s were, for all intents and purposes, leftover 41s with a cheaper cap than was used on the 41. During the run of the Super 21 it had several different cap clutch and clutch ring configurations; I've owned two different ones myself, and have seen others that I've not owned.

 

Was the plastic of the 41 the same as the plastic of the 21? The Super 21? I am an ignorant and easily confused man. Frank Dubiel wasn't right about everything technical, but I seem to remember posts to acpp in which he asserted that the Super 21 was made of stronger plastic than the original 21's plastic, and was less given to cracked hoods.

 

I would trust Graham Oliver's posts about Parker pens more easily than I would trust Frank Dubiel's, but offhand I don't remember his addressing the plastic of the 41 or the Super 21.

[psfred]

Hmmm. I was under the impression that the 21 was made not of acrylic but of polystyrene, like the contemprary Sheaffer TD and Snorkel pens (and lots of other injection molded stuff). Low grade methymethacrylate would certainly reduce the cost, but I don't think it would be that much different.

 

Parker 21s solvent weld nicely with cyclohexanone, while "51" barrels are completely untouched by it.

 

Peter

[ATisch]

Jerome,

 

I wish I could be more specific with regard to the Super 21 and the 41. I confess that what I'm writing here is a combination of my knowledge of acrylics on the one hand and my personal experience with the actual pens on the other. In my experience, the Super 21's and most 41's seem to be of an intermediate quality--most of mine have held up much better than the regular 21's. The pastel 41's seem to be better than the darker colors.

 

Allan Tischler

The Fountain Pen Chemist

[ATisch]

They are definitely not made of polystyrene.

 

The lower MW acrylic of the P-21 would of course dissolve more easily than the P-51. Polystyrene dissolves almost instantly in acetone and ethyl acetate. Neither solvent does much at all of the P-21 or the P-51.

 

Allan Tischler

The Fountain Pen Chemist

Edited June 2, 2007 by ATisch

[jirish1957]

Just out of curiosity, the P61's also have cracking problems. Were they using a similar plastic to the 21's?

[HesNot]

jirish - I was going to ask the same question. This is particularly interesting since the 61 was in no way supposed to be a low cost, budget or student pen.

[Russ]

Allan, thank you for the informative article! Full of facts and very well-written. Thank you!

[ATisch]

Just out of curiosity, the P61's also have cracking problems. Were they using a similar plastic to the 21's?

 

Good Question! I've also noted a tendecy for some P-61s to crack for more than one would expect, given it's price and positioning in the Parker catalog. They are also an acrylic PMMA product, but I can't quite believe that Parker cut as many corners in it's production compared with the P-21 end of the acrylic spectrum. I can only make a couple of (hopefully) educated guesses. I have noticed that in some cases the hood is thinner than in the P-51 Mach 1 and Mach 2. If so this could explain increased cracking. Another thought that comes to mind--and I'd be curious to hear whar others have experienced here--is that M and NM P-61s seem to hold up significantly better than well-used ones. If so it could very well be that given the P-61s tendency to dry up and stop functioning, owners tended to soak the whole section/filler assembly in harmful detergents and/or solvents, such as alcohols. Such a tendency could explain poorer aging of the plastic over time. Also consistent with this idea is that (in my experience) that the aerometric filling P-61s have held up better as well.

 

ATisch

[J English Smith]

Aha! This is probably the "why" of my cracked 61 barrel...I was shocked to find it that way a month ago...it hasn't had any rough use at all but I did go a long time between fillings. Kind of annoying for a "top of line" pen like this to not have materials that can last...especially since they know how to do it (the 51 etc.)

 

I have a new old stock barrel coming, I sure hope that it will last a while.

[J English Smith]

So, given this discussion of the plastic differences beweeen a 51/21 and 61...

 

I am currently getting a NOS barrel for my Parker 61 to replace the original barrel which suddenly and unexpectedly cracked. (Hoping that it fits ok...)

 

So, the question for chemists - are there some inks that will be safer to use than others to prevent new plastic fatigue? I have been using Pelikan Violet in this pen, I also have on hand older Skrip blue-black, black Quink, brown Pelikan, and two colors of Private Reserve (Midnight Blues and Sherwood Forest).

 

I probably will just stick with the Pelikan inks for the 61 if they are ok to use...?

[psfred]

The ink will likely have very little effect on the plastic.

 

Parker 61's are injection molded, not turned as were the 21 and "51", and the plastic is much more like that of the 21 (again, I am under the impression it is a polystyrene or similar, NOT acrylic). Whatever it is, it is both somewhat thinner and more brittle than the acrylic used in the "51". It is also under some pressure whenever the pen is capped as the cap clutch fingers bear on the shell, not the thin "clutch ring", with the result that most of them have "dents" where the cap clutch has caused the plastic to flow.

 

Shrinkage and embrittlement are rather typical of plastics with age on them. UV light, heat, and ozone all accelerate the process, with UV exposure also causing the pigments to fade (my Rage Red 61 has a red ocher colored barrel while the hood is still bright red). Exposure to chemical vapor can also cause them to get brittle.

 

The acrylic Parker used in the "51" appears to have been optical grade with good UV protection, and has been remarkably stable. So is the polystyrene Sheaffer used in the TD and Snorkes of the same era -- very rare to see on in bad shape or distorted badly (although the large o-ring Sheaffer used causes the barrel to bulge there sometimes). No cracks, though, except at the clip.

 

Peter

Edited June 12, 2007 by psfred

[FranklinChristoph]

Thanks for the post Allan. We get questions all the time about the type of methacrylate we use, and it is so hard to explain - but your post was very well crafted.

 

Scott Franklin

Franklin-Christoph

[JimStrutton]

Allan,

 

Thanks for posting such an informative article, when you describe it it all makes perfect sense, even to me

 

Please post more of you stuff as it makes fascinating reading.

 

Jim