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Polyethylene v. Polypropylene

Started by Foolkiller,

52 posts in this topic

AussieRuss

AussieRuss

Posted
Posted
Hows this....

 

 

Why polyethylene rather than polyproylene?

 

 

--------------------------------------------------------------------------------

 

 

AussieRuss, I hope you didn't write this article because it is a crock of nonsense. Its accuracy is every bit as careful as its proofreading.

 

I hardly know where to start. Let's just concentrate on a few paragraphs.

 

 

In more chemistry-speak, propylene (CH3CH:CH2) is is a sub-stratum of ethylene. Ethylene is the purest form of the base chemical and thus the purer better of the two. Consider this example: Suppose you were looking for the perfect gene to clone a tough guy. Now, would you rather have the genes from the son of a tough guy or fom the original tough guy, in this case, the son's father? Obviously, you want the purest tough guy genes, so you would go with the father. Polyethylene is like the father; polypropylene is like the son. See definitions below.

 

Propene (propylene) used to make polypropylene is every bit as pure as ethene (ethylene) used to make polyethylene. If the monomers are not pure, the catalysts have a very short lifetime. Propene being a "sub-stratum" (What??) of ethene has nothing to do with purity. Genes from the son of a tough guy. (What???)

 

 

Due to its rigitity, polypropylene sleeves are more prone to tear, especially at the seams. Try this experiment if you don't believe this. Take an ordinary piece of writing paper (rigid) and a common paper towel (pliable). Try to gently tear each one. The paper tears readily, while you have to give the towel a little more of a tug to tear it. It actually kind of resists tearing by having some flexibility. The same principle applies for propylene vs. ethylene. The ethylene base provides more "give" and thus stronger seams and less likelyhood of tearing.

 

 

Does anyone here really think that it's easier to rip a sheet of writing paper compared to a paper towel, or that the analogy has anything to do with polyethylene vs. polypropylene?

 

 

Polyethylene costs more. Simple logic dictates that it should be a superior product and it is. It costs more because it is a product of higher purity (100% Virgin) in addition to the advantages already presented, and will outlast and outperform any derivative product. Polyethylene provides better protection for both the short and long term.

 

"Product of higher purity?" Higher cost is a guarantee of higher performance?

 

One reason that mylar (PET, polyethylene terephthalate) provides better archival protection for paper goods is that its oxygen permeability is much lower than PE or PP, down in in the 1% range (hard to find an exact number without knowing the exact specs of the material in use).

 

The wikipedia articles on polyethylene, polypropylene and PET (mylar) are pretty good. Those would be a much better starting point than this claptrap. Notice that there are many different kinds of PE and PP -- the chain length and exact properties depend on the manufacturing process.

 

Jack

 

 

Hello Jack.

 

No I did not write this.

 

I was just trying to find some information on Mylar to share with the group.

 

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AussieRuss

AussieRuss

Posted
Posted

From Wikipedia.

 

MYLAR:

Biaxially-oriented polyethylene terephthalate (boPET) polyester film is used for its high tensile strength, chemical and dimensional stability, transparency, gas and aroma barrier properties and electrical insulation.

 

A variety of companies manufacture boPET and other polyester films under different trade names. In the US and Britain, the most well-known trade names are Mylar and Melinex.

 

 

*boPET film is used in bagging comic books, in order to best protect them during storage from environmental conditions (moisture, hot and cold) that would otherwise cause paper to slowly deteriorate over time. This material is used for archival quality storage of documents by the Library of Congress

 

 

*Conservation of comics

Comics, being a printed medium, should be stored in cool, dark places, as sunlight can bleach the pages, and heat and moisture can also damage them. Sunlight can also react with the paper, causing it to "yellow", as well as having a bleaching effect on the inks used within the comic. Some collectors advise against storing comics in cardboard boxes, or using backing boards, as these are both sources of acid which can react with the fibers of the paper of comics, eventually destroying a comic. If these products are used to store comics, these collectors advise using products marked as acid free.

 

PET film (trade name Mylar), polyethylene or polypropylene storage bags are popular, and allow a comic to be "bagged" in a contained environment, and have become the traditional way of storing comics. Most comic shops now sell comics already in bags, although the quality of the bag can vary. It should be noted that even some plastic bags are not considered "archival safe" because various plastic compositions may contain elements or have other properties that could harm the comic in years to come. Only Mylar is truly considered archival safe as polyethylene and polypropylene will eventually break down. It is debatable whether these bags are sufficient to protect comics from the acids of cardboard storage boxes as mentioned above.

 

Corrugated plastic boxes, preferred by some collectors, offer greater protection against acid while also offering better protection against moisture damage and vermin damage. Original art can also be better protected against direct sunlight as well as acid deterioration by using an archival quality frame coupled with glass which has been treated to protect against ultraviolet rays1.

 

 

 

 

 

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AussieRuss

AussieRuss

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And this is what the Library of Congress states on its website.

 

 

Update on Acceptance of Polyester Film Products for Use in Encapsulation of Paper Artifacts

Revised October 22, 2004)

 

Over the past several months, we have tested several biaxially oriented polyethylene terephthalate (PET) polyester films and found them to be satisfactory for use in encapsulation of paper artifacts.

 

To the best of our knowledge, all of the tested films, with the exception of Melinex type KL and type KM films, are coated for slip on one or both sides for ease of handling. The two uncoated films are available in very small thicknesses, 92 gauge for Melinex® type KL and 48-200 gauge for type KM, which conservators may find limiting for many applications (100 gauge equals approximately 1 mil or one thousandth of an inch). While we would generally prefer uncoated stock to eliminate any chance of an unforeseen interaction with the artifacts being encapsulated, we do realize that most processors find slip coating to be essential for ease of handling. Moreover, most such coatings are highly stable.

 

While we have found the films mentioned above to be generally acceptable for use in paper conservation, the ultimate test for acceptability of any polyester film must be the ease and consistency with which these films can be sealed or joined with the equipment that is available to the individual user.

 

We are pleased to share information about product testing with the preservation community, with the understanding that information concerning favorable test results in no way implies product endorsement. Vendors are strictly prohibited by law from implying or claiming that the Library of Congress endorses any specific product, service, or material.

 

An essential cautionary note: Research in our laboratories has clearly shown that, over the long-term, encapsulation of papers without an alkaline reserve is likely to hasten their degradation. The real and immediate gain in protecting fragile paper from physical handling needs to be weighed against the equally real risk of faster aging. However, clear and practical solutions are available to conservators to avoid this balancing act. With the many options available for deacidification today, most papers can be deacidified before encapsulation. Where this option is not available or desirable for whatever reason, a sheet of alkaline paper inserted within the polyester envelope will also overcome the effects of trapped acids formed in the aging of paper, and actually help extend the life of the encapsulated artifact.

 

 

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AussieRuss

AussieRuss

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Posted

And one more reference.

 

 

Mylar

 

Images and text by Garry Harrison,

Head, Circulating Collections Conservation,

Indiana University Libraries Preservation Department

 

 

--------------------------------------------------------------------------------

 

Mylar is mostly used for the encapsulation of flat materials. It is available in a variety of thicknesses, which are measured in "mils". The mil is not a metric unit of measure; one mil equals .001" (1/1000" or one one-thousandth of an inch). Thus, a "mil" is the same as a "point", the unit of measure used for the thickness of card and folder stock. The thickness range of the mylar we use is 2 mil to 5 mil. 2 mil is normally used for the encapsulation of small materials, such as 8.5" x 11" documents. 5 mil is used for items such as large maps and posters. Mylar can be bought in rolls, but cutting from the roll can be time-consuming, especially when dealing with large items (a full roll weighs approximately 125 lbs.). So we also keep pre-cut sheets in the following gauges/sizes:

 

 

5 mil: 50" x 50"

3 mil: 18" x 30"

2 mil: 9" x 12"

 

 

 

With all of the above 3 posts to go by and the lack of information stating otherwise. It would appear that I will stick with Mylar, Buffered backing boards & Microchamber paper for my books.

 

I would be interested in seeing some references to institutions that do advise using Mylar as a preferred storage medium for paperwork.

 

Russ...

 

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selegue

selegue

Posted
Posted
Hows this....

 

 

Why polyethylene rather than polyproylene?

 

 

--------------------------------------------------------------------------------

 

 

AussieRuss, I hope you didn't write this article because it is a crock of nonsense. Its accuracy is every bit as careful as its proofreading.

 

I hardly know where to start. Let's just concentrate on a few paragraphs.

 

 

In more chemistry-speak, propylene (CH3CH:CH2) is is a sub-stratum of ethylene. Ethylene is the purest form of the base chemical and thus the purer better of the two. Consider this example: Suppose you were looking for the perfect gene to clone a tough guy. Now, would you rather have the genes from the son of a tough guy or fom the original tough guy, in this case, the son's father? Obviously, you want the purest tough guy genes, so you would go with the father. Polyethylene is like the father; polypropylene is like the son. See definitions below.

 

Propene (propylene) used to make polypropylene is every bit as pure as ethene (ethylene) used to make polyethylene. If the monomers are not pure, the catalysts have a very short lifetime. Propene being a "sub-stratum" (What??) of ethene has nothing to do with purity. Genes from the son of a tough guy. (What???)

 

 

Due to its rigitity, polypropylene sleeves are more prone to tear, especially at the seams. Try this experiment if you don't believe this. Take an ordinary piece of writing paper (rigid) and a common paper towel (pliable). Try to gently tear each one. The paper tears readily, while you have to give the towel a little more of a tug to tear it. It actually kind of resists tearing by having some flexibility. The same principle applies for propylene vs. ethylene. The ethylene base provides more "give" and thus stronger seams and less likelyhood of tearing.

 

 

Does anyone here really think that it's easier to rip a sheet of writing paper compared to a paper towel, or that the analogy has anything to do with polyethylene vs. polypropylene?

 

 

Polyethylene costs more. Simple logic dictates that it should be a superior product and it is. It costs more because it is a product of higher purity (100% Virgin) in addition to the advantages already presented, and will outlast and outperform any derivative product. Polyethylene provides better protection for both the short and long term.

 

"Product of higher purity?" Higher cost is a guarantee of higher performance?

 

One reason that mylar (PET, polyethylene terephthalate) provides better archival protection for paper goods is that its oxygen permeability is much lower than PE or PP, down in in the 1% range (hard to find an exact number without knowing the exact specs of the material in use).

 

The wikipedia articles on polyethylene, polypropylene and PET (mylar) are pretty good. Those would be a much better starting point than this claptrap. Notice that there are many different kinds of PE and PP -- the chain length and exact properties depend on the manufacturing process.

 

Jack

 

Very nice response, Jack.

 

I'm not a chemist but I know BS when I read it. It's just another example of trying to sell "American-made" the wrong way. Why not be truthful: "Buy our American-made bags. The cost just pennies more but they keep American workers employed."

 

--Gary

 

Argh. My long, detailed reply just disappeared into the ozone.

Now I only have time for a quick comment that I agree with you on some points but disagree on others. I'll try to rewrite later.

 

Jack

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selegue

selegue

Posted
Posted
Hows this....

 

 

Why polyethylene rather than polyproylene?

 

 

--------------------------------------------------------------------------------

 

 

AussieRuss, I hope you didn't write this article because it is a crock of nonsense. Its accuracy is every bit as careful as its proofreading.

 

I hardly know where to start. Let's just concentrate on a few paragraphs.

 

 

In more chemistry-speak, propylene (CH3CH:CH2) is is a sub-stratum of ethylene. Ethylene is the purest form of the base chemical and thus the purer better of the two. Consider this example: Suppose you were looking for the perfect gene to clone a tough guy. Now, would you rather have the genes from the son of a tough guy or fom the original tough guy, in this case, the son's father? Obviously, you want the purest tough guy genes, so you would go with the father. Polyethylene is like the father; polypropylene is like the son. See definitions below.

 

Propene (propylene) used to make polypropylene is every bit as pure as ethene (ethylene) used to make polyethylene. If the monomers are not pure, the catalysts have a very short lifetime. Propene being a "sub-stratum" (What??) of ethene has nothing to do with purity. Genes from the son of a tough guy. (What???)

 

 

Due to its rigitity, polypropylene sleeves are more prone to tear, especially at the seams. Try this experiment if you don't believe this. Take an ordinary piece of writing paper (rigid) and a common paper towel (pliable). Try to gently tear each one. The paper tears readily, while you have to give the towel a little more of a tug to tear it. It actually kind of resists tearing by having some flexibility. The same principle applies for propylene vs. ethylene. The ethylene base provides more "give" and thus stronger seams and less likelyhood of tearing.

 

 

Does anyone here really think that it's easier to rip a sheet of writing paper compared to a paper towel, or that the analogy has anything to do with polyethylene vs. polypropylene?

 

 

Polyethylene costs more. Simple logic dictates that it should be a superior product and it is. It costs more because it is a product of higher purity (100% Virgin) in addition to the advantages already presented, and will outlast and outperform any derivative product. Polyethylene provides better protection for both the short and long term.

 

"Product of higher purity?" Higher cost is a guarantee of higher performance?

 

One reason that mylar (PET, polyethylene terephthalate) provides better archival protection for paper goods is that its oxygen permeability is much lower than PE or PP, down in in the 1% range (hard to find an exact number without knowing the exact specs of the material in use).

 

The wikipedia articles on polyethylene, polypropylene and PET (mylar) are pretty good. Those would be a much better starting point than this claptrap. Notice that there are many different kinds of PE and PP -- the chain length and exact properties depend on the manufacturing process.

 

Jack

 

Very nice response, Jack.

 

I'm not a chemist but I know BS when I read it. It's just another example of trying to sell "American-made" the wrong way. Why not be truthful: "Buy our American-made bags. The cost just pennies more but they keep American workers employed."

 

--Gary

 

I'll try to reconstruct my earlier "brilliant" response that disappeared.

 

I do agree with Gary that storage conditions are overall more important than bag type. Oxidation of the pulp is the main problem, and light-induced fading is the secondary problem.

 

While PURE polyethylene (PE) and polypropylene (PP) are very stable and produce no breakdown products that can harm comic books, most (or all?) PE and PP films contain additives to improve their handling properties: plasticizers to improve flexibility are probably the worst. As far as I know, phthalates, mineral oil and other plasticizers have been used. When these leach out over time, especially when storage conditions are warm, the bags get yellow and tacky, and inks can be transferred from the covers. I've bought bagged books that looked like that. More good info on additives: Numerous chemical modifiers and additives are compounded with polyolefin film extrusion resins. In some grades, the chemical modifiers are added during resin manufacture. These include thermal stabilizers, anti-static agents and slip/anti-block agents.

Additives table.

 

As far as I know, PP usually has better clarity and contains less plasticizers because the intrinsic properties of the polymer are better for film. There's almost no difference between their triboelectric properties (the static charge problem -- "higher charge emitted???" -- that the Downtown Magazine article mentioned).

 

I don't agree with Gary that PE and PE bags are just as good as Mylar. Archival grade polyethylene terephthalate (PET, Mylar) does not contain any plasticizers. It is MUCH more effective than PE or PP at keeping out oxygen, having an oxygen permeability something like 1% of PE or PP. That's why PET is a much better choice for archival storage.

 

I don't agree with Gary that polyvinyl chloride (PVC, Saran Wrap) would do just as good a job if storage conditions are good. PVC is much less stable than PE, PP or PET. Films are loaded with plasticizers and other additives. When PVC breaks down, it releases very acid hydrogen chloride (hydrochloric acid), which drastically accelerates pulp breakdown.

 

I am surprised that Gary said that "Our PP was manufactured in New Jersey and our PE here in Chicago." As far as I know, no polyolefins are manufactured in major cities, including Chicago (but I could be mistaken). Usually the plants are in remote sites. Are you sure that the materials are manufactured there, or are the bags assembled at those sites from PE and PP manufactured elsewhere?

 

FYI, I'm not a polymer chemist but I worked 3 summers on polymers and some of my PhD students have gone on to work in the polymer industry, so I do know something about them.

 

After all that pontificating, my own storage methods aren't so great. My storage space isn't heated or cooled, and not all books are bagged, much less in Mylar. I can't see spending more on packing and storage than the book itself is worth. Only my most expensive books are in Mylar.

 

Jack

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Moondog

Moondog

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Posted

Jack, thanks for the additional info.

 

I just want to make clear that I would never recommend Saran Wrap or any other household product to protect comic books. I used those examples because of my experience over the years buying collections that were protected in that manner.

 

The barrier to oxygen really plays no role in protecting comics since they aren't encapsulated. And as the LOC states, that's probably not a good idea anyway.

 

And the LOC also states that the slip that's added to the plastic in order for it to "slide" off each other can't be good either - and Mylar has that additive too.

 

The purpose of my posts is to inform collectors that they don't need to run out and buy Mylar sleeves or Mylites. They can use a cheaper plastic and still protect their books against storage and handling.

 

And full disclosure: I get no commission from Diamond if you decide to buy ComiCovers.

 

:)

 

Finally, AEP Industries is located in Hackensack, NJ. http://www.aepinc.com/home.html

They made my PP bags (and some PE too). And PPC Industries http://www.ppcind.com/ is in southern Wisconsin. They were my major supplier of PE bags.

 

 

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ShoNuff-migration

ShoNuff-migration

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Posted
And the LOC also states that the slip that's added to the plastic in order for it to "slide" off each other can't be good either - and Mylar has that additive too.

Actually, the LOC simply stated that while they generally prefer uncoated stock to eliminate any possible chance of an unforeseen interaction...that most such slip coatings are highly stable.

 

"While we would generally prefer uncoated stock to eliminate any chance of an unforeseen interaction with the artifacts being encapsulated, we do realize that most processors find slip coating to be essential for ease of handling. Moreover, most such coatings are highly stable."

 

 

The purpose of my posts is to inform collectors that they don't need to run out and buy Mylar sleeves or Mylites. They can use a cheaper plastic and still protect their books against storage and handling.

I don't think anyone is saying that collectors must go out and buy mylars for all of their comics in order to protect their books. Simply that Mylars/Mylites do offer a safer/more stable storage media than the lower cost alternatives if one feels that their books do warrant the extra cost.

 

The added bonus being that comics sure do look purdy in them. :cloud9:

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selegue

selegue

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Posted
Jack, thanks for the additional info.

 

I just want to make clear that I would never recommend Saran Wrap or any other household product to protect comic books. I used those examples because of my experience over the years buying collections that were protected in that manner.

 

The barrier to oxygen really plays no role in protecting comics since they aren't encapsulated. And as the LOC states, that's probably not a good idea anyway.

 

[/quote

 

You're in the business, so you've seen far more examples than I have.But if an oxygen barrier doesn't play a role in pulp preservation, why do comic books brown and turn brittle (pulp degradation) from the edges first? Isn't it because they have maximum air exposure? I think that if someone could afford to store their pulp collection under nitrogen or argon, they'd last almost forever.

 

]

And the LOC also states that the slip that's added to the plastic in order for it to "slide" off each other can't be good either - and Mylar has that additive too.

 

The purpose of my posts is to inform collectors that they don't need to run out and buy Mylar sleeves or Mylites. They can use a cheaper plastic and still protect their books against storage and handling.

 

And full disclosure: I get no commission from Diamond if you decide to buy ComiCovers.

 

:)

 

Finally, AEP Industries is located in Hackensack, NJ. http://www.aepinc.com/home.html

They made my PP bags (and some PE too). And PPC Industries http://www.ppcind.com/ is in southern Wisconsin. They were my major supplier of PE bags.

 

 

Thanks! I looked for PP and PE film manufacturers and came up blank. I see that PPC is rural southern WI, not Chicago. That makes more sense.

 

Jack

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Jayman

Jayman

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Posted
So for the cheaper books, the safe bet is to buy polypropylene (PP) and for the expensive books 4 mil Mylar?

 

John

 

I use polyethylene for the cheapies and 4-mil Mylar for the good ones!

 

As do I. (thumbs u Polypropylene bites! :sumo:

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