Is there a difference between Mylar/Prolar Polyester/Generic Polyethylene Terephthalate holders?

airplanenut

I asked a question about currency sleeves a little while back, so apologies for starting a new topic. I know that Mylar/Melinex are DuPont brand names. Frame-A-Coin sleeves are advertised as being made of this material. Other brands, namely Safgard and Supersafe, list Prolar Polyester or Polyethylene Terephtalate as the material. In a broad sense, these should be just about the same material if not the same. If they all are the same (or any differences are negligible) I'll just buy what's most economical. If true Mylar is the best of the best for whatever reason, I'm willing to invest more for the best product. I've found some discussions of the materials when searching, but nothing conclusive. Can anyone here provide some guidance?

Steve_in_Tampa

It’s possible that Prolar Polyester and Polyethylene Terephtalate currency sleeves are completely safe and a bit more affordable, but in the 20 years or so of collecting paper money, Mylar has been my preferred material.

Member @sellitstore supplied my sleeves for many years, maybe Russ will see this thread and weigh in.

airplanenut

@Steve_in_Tampa said:
It’s possible that Prolar Polyester and Polyethylene Terephtalate currency sleeves are completely safe and a bit more affordable, but in the 20 years or so of collecting paper money, Mylar has been my preferred material.

And this is where it gets confusing. Looking at Wizard's descriptions...

Safgard: the package says PET, but the description says Mylar: "Safgard sleeves use the same great, inert, pure 4 mil Mylar as is used for Saflips to safely preserve currency, large coins, postcards and other similar items."

The E&T Kointainer website (https://www.vcoins.com/en/stores/et_kointainer/173/product/safgard_tm_inert_sleeves__large_currency___50pack____made_in_usa/312155/Default.aspx) goes a step further, but also doesn't say Mylar specifically:

Archival quality, heavy duty, inert, 4 mil. plastic. The Best Made

We certify that all sleeves are made from film that adheres to the Library of Congress Spec. 500-500, "Polyester Film for the Storage of Artifacts". The film was also tested an approved in the "Photographic Activity Test" (PAT)), ISO 18916. This is the best quality film produced in the USA
This film is acid-free and there are no internal additives such as plasticizers, antioxidants, slip agents, anti-block agents, or UV inhibitors, all of which can migrate into stored materials. This film is completely inert.

Frame-A-Coin: Description says Mylar and Melinex (it seems there's a slight difference, but I'm not sure if it's significant): "Frame-A-Coin DuPont Mylar Currency Holders are made from DuPont Melinex polyester. Melinex is one of a very few films that have been tested by the Library of Congress and found to be satisfactory for use in encapsulation of paper artifacts. We are aware of no other brand of currency sleeves that uses a film found to be acceptable by the Library of Congress."

Supersafe has a description that says Prolar Polyester, and I'm okay ignoring this option due to it being more generic.

I hope my confusion is at least a bit justified... I just want to be sure I'm doing right by my collection.

sellitstore

Yes, your confusion is justified and I can't give you the absolute definitive answer that a chemist may be able to provide.

My understanding is that all three films that you mention are equivalents, at least in terms of safety for archival storage, and that's what really matters. Mylar is the Dupont trade name for their plastic, but the others are essentially the same.

There are differences in the thickness used for these holders. Most use 4 mil but there are thinner, cheaper versions done in 3 mil and sturdier ones that I've seen in 5 mil. Most are sealed on three sides but ones that are just folded at top and bottom, with open ends are much cheaper (under half the price) as they only require folding, not hand sealing during manufacturing. The labor is the main cost in producing the sealed (not folded) ones.

airplanenut

@sellitstore said:
Yes, your confusion is justified and I can't give you the absolute definitive answer that a chemist may be able to provide.

My brother, a chemist/bio engineer who during their temporary merger to trade business units before splitting off as separate companies again, worked for Dow DuPont was wholly unhelpful when I posed the question to him. Specifically, I asked about Mylar vs. PET vs. Prolar, and he wasn't sure of what specifically differentiated them beyond ownership of a brand name (and surely the consistency of a single DuPont product versus a generic plastic that could be sourced from different places between batches).

My understanding is that all three films that you mention are equivalents, at least in terms of safety for archival storage, and that's what really matters. Mylar is the Dupont trade name for their plastic, but the others are essentially the same.

There are differences in the thickness used for these holders. Most use 4 mil but there are thinner, cheaper versions done in 3 mil and sturdier ones that I've seen in 5 mil. Most are sealed on three sides but ones that are just folded at top and bottom, with open ends are much cheaper (under half the price) as they only require folding, not hand sealing during manufacturing. The labor is the main cost in producing the sealed (not folded) ones.

Very helpful--thank you! I'm pretty sure I've handled all the various styles of holder in terms of thickness and seal type. While I like the thicker ones from a protection standpoint, I've had instances where it's hard to free a bottom corner of a note from their grip or open the holder enough for the corner to go in, and especially with fragile paper (such as colonials or tattered notes) that can greatly increase the risk of tearing when removing a note or scrunching it when putting it back. I think the 4 mil holders are the right ones for my needs--the notes can live sandwiched between something sturdy to ensure they don't get bent--and it's really just been a question of choosing the right material to ensure the holder won't cause damage over the long term.

sellitstore

For notes that are tough to get in or out of a holder with three sealed sides, try trimming off one of the shorter seals, leaving an "L" seal. This is usually better for notes that are too fragile or a tight fit.

airplanenut

@sellitstore said:
For notes that are tough to get in or out of a holder with three sealed sides, try trimming off one of the shorter seals, leaving an "L" seal. This is usually better for notes that are too fragile or a tight fit.

That's a good idea I'll keep in mind. In the past I haven't had a ton of trouble as long as the sleeve isn't too thick (especially for a colonial going into a large holder, you can usually open up the holder with a finger pretty easily), but I will remember this trick. It's kind of like a mix of the folded sleeves that don't have a seal on either side and the standard sleeve.

Serial_no_8

My understanding is that all three films that you mention are equivalents, at least in terms of safety for archival storage, and that's what really matters. Mylar is the Dupont trade name for their plastic, but the others are essentially the same.

That's my understanding too @sellitstore. I've seen several threads & even posts from chemists who all state the similar takes. See this thread on NUMISTA

Most are sealed on three sides but ones that are just folded at top and bottom, with open ends are much cheaper (under half the price) as they only require folding, not hand sealing during manufacturing. The labor is the main cost in producing the sealed (not folded) ones.

These large open-sided currency sleeves have really grown on me because I find them easy to open & they almost snap close once you have handled a few often enough. I always place my large sized world notes in these without the note bending, etc.

I also attended an RCNA workshop on banknote preservation hosted by a museum curator. She stated that all of today's PVC-free "museum safe" plastics are safe but always be on the look out for the old (poor) holders (discard them). Also she emphasized (several times) to be sure to get a desiccant (water absorber) for your safe &/or keep notes in a dry space.