Polypropylene

A few years ago polypropylene was added to the list of "acceptable"
plastics, joining polyester and polyethylene. I had never read anything
about how to specify polypropylene, in order to get a type which was OK
for use in archives.  So I called the ever-helpful Tuck Taylor earlier
this month.  He said that we'd be looking for a biaxially oriented
polypropylene film, without antioxidants, slip agents, or UV inhibitors.
Mr. Taylor also put me in touch with a manufacturer.

Last week I spoke with the packaging film division of this east coast
manufacturing company.  They used to make polypropylene which was used
for photo albums and sheet protectors.  But the selling price for the
film is so low (less than $1/pound) that the film is now being supplied
by overseas firms (Japan, Korea, and Turkey).  [You can't make money
when the price is so low.]  He believes that most or all of the finished
products (sleeves, sheet protectors, etc.) are also imported.

Their company continues to make biaxially oriented polypropylene, but it
is slip modified (for potato chip bags, candy bar wrappers, 2 liter
bottle labels), electrical grade, or otherwise specialized.

A few other things I learned:

    *   Biaxial orientation is what gives stiffness and clarity to the
        film.

    *   Polypropylene's dimensional stability is not affected by
        humidity or temperature.

    *   Polypropylene is not affected by acids or alkalis.

    *   Polypropylene has three times the water vapor transmission rate
        that polyester has, for the same thickness.  So polypropylene is
        a more "breathable" film.  And polyester absorbs water vapor,
        while polypropylene doesn't.

Cathy Atwood
Local Records Program
Missouri Secretary of State Office

                                  ***
                  Conservation DistList Instance 8:77
                  Distributed: Tuesday, March 21, 1995
                        Message Id: cdl-8-77-004
                                  ***