Author: Maria Esteva
Date:Fall 2001
Class: LIS 392.P 5 Technology and Structure of Records Materials
Corrugated
Polypropylene: Properties and its Use in Conservation
Paper
Outline
Introduction
Purpose of the project:
* To understand the chemical and physical properties of polypropylene
focusing on the form of polypropylene that is used to manufacture corrugated
polypropylene. The paper focuses on the brand Coroplast because it is
broadly used and documented. However, the kinds of matters that are discussed
about the product can be asked, with more time, in relation with other
brands.
* To determine how the manufacturing process to produce corrugated PP
changes the basic properties of PP.
* To learn how corrugated PP is marketed and used in the preservation
field.
* To corroborate if the properties of corrugated PP are adequate for its
use in conservation and to find out under which conditions the material
is better used.
The Chemistry of Polypropylene
Polymer formation
Molecular weight and melt flow rate
Molecular weight distribution
Morphology of Polypropylene
Properties of Polypropylene
Commercial Forms of PP
Additives
Provides an overview of the type of additives that are and could be included
in the PP resin or in PP final products.
PP as a Barrier
Flammability
Water
Permeability to gases and vapors
Odors
Migration
Manufacturing Process of Corrugated PP
The use of PP in Preservation of Heritage Materials
Marketing
Coroplast
Conservator's experiences and concerns in the use of PP
Enclosure design
Conclusions
Behavior of corrugated PP during disasters
Corrugated PP as a long-term storage material
About the pros and cons of the product
About the adequate uses of the product
About the expectations of conservators in relation to the product
****** Editor's
Note: Numbers appearing in the text refer to endnotes cited after the
Works Cited ******
Introduction
Corrugated plastic sheet is made of polypropylene copolymers, or high-density
polyethylene. This material has been developed in the industry to build
boxes and is also suitable to produce inexpensive printed signs that resist
outdoor conditions more than the paper or board signs. By 1994 fluted
plastic sheet was widely used in museums to construct boxes, trays, and
folders to house heritage materials. That year the Canadian Institute
for Conservation (CCI) published the "Technical Bulletin 14: Working
with Polyethylene Foam and Fluted Plastic Sheet,"1 in which various
techniques to work with corrugated plastic sheets are described.
Since then, cultural institutions use corrugated plastic sheets and pre-cut
corrugated plastic boxes as a sturdier alternative material to house bulk
storage, for protection of materials when moisture in the environment
is a problem, for transportation during inclement conditions, and for
use during water-originated disaster recovery activities. Other important
conservation uses of this material include: construction of transportation
enclosures and framing supports for paintings and works of art on paper,
and construction of customized size boxes for objects storage and transportation.
Conservators have discussed the advantages and disadvantages of polypropylene.
There are those who think adversely about because of the high flammability,
and its ability to melt over the materials housed inside at not so high
temperatures. On the other hand, conservators have observed that during
a water disaster the corrugated PP keeps the materials inside dry for
a longer period of time and that resistance of the material to water allows
for a better transportation of wet items to the freezing or drying facilities.
Other concerns relate to how different heritage materials - that in general
have a primary housing or support, - behave inside of corrugated plastic
enclosures that allow a reduced and slow exchange of water vapor and pollutants
with the outside environment.
This paper explores the physical and chemical characteristics of polypropylene
and focuses especially in those of corrugated polypropylene in order to
better understand the performance of the material as a "conservation
quality material." It will not focus on high-density polyethylene
(HDP), the other resin that is used to fabricate corrugated plastic sheets,
because the subject was too broad to efficiently study both materials
in one semester. However, some insight about this material will be discussed
in comparison with polypropylene.
In order to arrive to a better knowledge of the limitations and possibilities
of the material, besides from the review of the current literature on
plastics and packaging a number of interviews with corrugated plastic
manufacturers, conservators, and conservation product vendors had been
conducted.
Polypropylene Chemistry
Polymerization 2
Polypropylene (PP) is a thermoplastic material produced by addition polymerization
of propylene, a gaseous by product of petroleum refining in the presence
of a catalyst under carefully controlled heat and pressure. PP is an unsaturated
hydrocarbon, containing only carbon and hydrogen atoms. In the PP polymerization
reaction, many propylene monomers-molecules are added sequentially through
a reaction between the metallic functional group and the unsaturated bond
of the propylene monomer. The catalyst used for the reaction to occur
is an organometallic transition metal that provides active polymerization
sites by holding the growing polymer chain and the propylene monomer in
close proximity to each other so that they can react. A long linear polymer
chain of carbon atoms is formed with methyl (CH3) groups attached to every
other carbon atom of the chain.
The catalysts used in this reaction are essential to determine the structure,
properties, and future uses of the resulting PP resin. The catalysts used
in the polymerization of propylene are Ziegler- Natta and metallocene
catalysts. The PP polymerization reaction that this catalysts produce
is highly sterospecific, which means that depending on the chemical and
crystal structure of the catalyst, propylene molecules add to the polymer
chain only in a particular orientation to form a polymer chain of regular
and repeating three dimensional structure. According to the orientation
of the pendant methyl groups attached to alternate carbon atoms, PP can
be isotactic, syndiotactic, and atactic.
Isotactic PP is the type most used in commercial form. Due to the repeating
arrangement of the methyl group it has a high degree of crystallinity,
which results in good mechanical properties such as stiffness and tensile
strength. Syndiotactic PP is less stiff but has a better impact strength
and clarity. The Atactic form has an irregular structure that makes the
PP to have low crystallinity resulting in a sticky amorphous material
used mainly for adhesives and tars. Atactic and isotactic forms can be
combined to produce a PP with different characteristics. Mostly all commercial
PP resins are isotactic. Because of the different types of PP resin that
can be manufactured, the properties of the resin cover a substantial range
and thus, the applications of PP are quite diverse. Molecular weight,
molecular weight distribution, and melt flow index.3
PP has an average molecular weight that ranges from 220,000 - 700,000
g/mol. Since molecular weights are difficult to measure, the melt flow
rate test (MFR) was developed to estimate the molecular weight of PP.
The MFR provides an estimate of the average molecular weight of the PP
polymer in an inverse relationship; high melt flow indicates a lower molecular
weight. PP has MFR numbers ranging from 0.2 to 45 and they correspond
to molecular weight averages from 1,000,000 down to 100.000.
Viscous materials with low MFR - less than 2- values are used in extrusion
processes such as sheet and blow molding that require high melt strength.
Resins with MFR values from 2-8 are used in film and fiber applications.
Corrugated sheets are made with low MFR PP. As the molecular weight average
increases so do the impact resistance and the melt strength of the PP,
while the strength, the crystallization temperature, and the heat distortion
temperature decreases.
A PP resin is composed of numerous chains of varying lengths with varying
molecular weights. Thus, to characterize the polymer, it is better to
talk about molecular weight distribution (MWD), which indicates the variation
of molecular weight in a particular formulation. The MWD is narrow if
most molecular chains are approximately the same length, and broad if
the chains vary widely in length. Different manufacturing processes require
a resin with a particular MWD. The MWD of PP ranges between 2.0 and 6.0
depending on the catalyst used.
General properties of PP are:
* PP is a very poor conductor of electricity and makes a very good insulating
material that ranks better than most other plastics. At the same time
this has the side effect that the material builds up static electrical
charges on the surface.
* Because it is composed of carbon and hydrogen atoms and non-polar atoms
such as oxygen, polypropylene is non-polar. Because non-polar molecules
are generally soluble in non-polar solvents, PP is resistant to attack
by polar chemicals such as soaps, wetting agents, and alcohols.
* Due to its non- polar nature PP is water repellent and is not affected
in dimensions or properties by changes in RH.
Morphology of PP4
PP polymer crystallization occurs when melted material solidifies. When
cooled to temperatures below the melting point, PP molecules associate
to form supra-molecular structures. At the crystallization temperature
molecules begin to arrange themselves into crystals, and ordered crystalline
regions are formed along with disordered amorphous regions.
PP is a semi-crystalline polymer and its degree of crystallinity has mainly
to do with the stereo-chemical structure of the polymer at the molecular
level. However, the presence of additives, and the processing or crystallization
conditions such as pressure, temperature, and cooling rate will also determine
crystallinity. Because of all this variables, PP has different morphologies
that will affect the melting point, the mechanical properties, and the
haze of the resulting product. Some of the properties that are related
to the morphology of the PP are the next:
* Melting points: decrease with lower crystallinity. The melting point
of an isotatic PP resin can range from 160-166 C (320-331 F). Compared
to other plastic resins PP has a high melting point, which provides resistance
to softening at elevated temperatures.
* Mechanical properties of PP depend on its crystallinity. Increasing
crystallinity increases stiffness but decreases toughness and impact strength.
* Transparency in semi-crystalline polymers is related to crystallinity.
The refractive index of crystalline regions is higher than that one of
the amorphous regions, thus a semi-crystalline polymer has more haze than
an amorphous resin.
Forms of PP 5
There are three main commercial forms of PP and those have different properties.
PP that only contains propylene monomers is called Homo-polymer PP (HPP).
Homo-polymers contain both crystalline and non-crystalline regions. The
non-crystalline regions are comprised of both isotactic and atactic PP.
PP that contains ethylene as a co-monomer is called random copolymer (RCP).
RCP are ethylene/polypropylene copolymers that are made co polymerizing
propylene and small amounts of ethylene. This changes the properties of
the polymer significantly and results in thermoplastic products that have
better impact properties, improved clarity, decreased melting point, and
enhanced flexibility.
PP that contains a mixture of HPP and RCP is called Impact Copolymer (ICP).
Impact copolymers are used when impact resistance is needed especially
at very low temperature. PP corrugated plastic sheets are made of RCP.
Additives 6
In order to stabilize the material during and after processing or to modify
the properties of the PP resin, various additives are used. These are
added in the resin before or after processing or are applied to the finished
surface of the product. The selection of the proper additive will depend
on the performance required during processing, and on the application
requirements. Thus, a great variety of additives can be present in PP
products. The types of additives present in Coroplast will be discussed
in the section devoted to the product. Additives used in PP are for the
next purposes:
* Antioxidants: PP is highly susceptibility to oxidation and it can begin
to decompose almost immediately after formation. PP is usually stabilized
immediately after polymerization through antioxidants that inhibit the
oxidation reactions.
* Acid scavengers: used to neutralize acidic catalysts residues present
in the resin after manufacture.
* Antistatic agents: for several applications of PP the use of antistatic
agents is required. However, this material has less tendency to build
up a static charge than polyester.
* Anti-block and slip agents: these agents provide surface lubrication
to PP sheets during and after processing so that the plastic does not
stick to itself.
* Flame-retardants: large amounts of flame-retardants are used to improve
the material's resistance to fire. Those can increase brittleness and
interfere with the mechanical performance of the PP.
* Catalyst/metal deactivators: used to deactivate the metal residues that
are present in the resin due to catalyst residue, impurities in additives
such as lubricants pigments, stabilizers and fillers.
* Light stabilizers UV absorbers: un-stabilized PP deteriorates in the
presence of sunlight, resulting in cracking, brittleness, chalking, discoloration,
and the loss of mechanical properties such as impact strength, tensile
strength and elongation. Stabilization of the resin can occur by the use
of additives that absorb UV radiation and prevent its absorption by molecules
in the PP resin. The most accurate test of UV stability is use of the
material in its intended end-use environment over a period of time
* Pigments: dyes and pigments are used to impart color to PP.
* Nucleating agents: used to improve clarity and alter mechanical properties
that modify crystallization.
* Clarification agents: added to improve clarity in the resin that otherwise
has haze.
* Plasticizers: It could be said that the co-polymer form of PP has what
is called "internal plasticizer" which means that the molecular
structure of polymer has been modified with the incorporation of a co-monomer
that provides greater flexibility. The addition of ethylene placed randomly
in the molecule, prevents the high values of crystallinity obtained for
isotactic PP.
* Fillers and reinforcements could also be used in PP applications
Polypropylene resin is a very stable and non-reactive product. However,
the additives included in the resin can be a concern when PP is used as
a housing material for preservation purposes.
PP as a Barrier
The barrier properties of materials indicate their resistance to diffusion
and sorption of substances. Different manufacturing processes such as
the catalyst involved in the creation of the polymer, and the fiber orientation
during manufacture will influence the behavior of PP as a barrier.
Fire
PP is a combustible material. It ignites spontaneously at about 360 C
and can be ignited at around 345 C. It burns with a faint luminous flame
that melts and produces droplets that have the potential to spread the
fire.
Water absorption and water vapor transmission
In PP the water absorption is very low and less than that one of most
other plastics with the exception of high-density polyethylene. The percentage
of water absorption of PP copolymer is of 0.03, while that one of the
high-density polyethylene is 0.02. Polycarbonate has a % of water absorption
of 0.15. At 90% RH, Polypropylene's water vapor transmission in gm/mm/m2/day
is of 0.3, while that one of high density polyethylene is 0.2
Gases
PP is definitively permeable to gases. Within the polymer there are amorphous
and crystalline regions. Gases, vapors and other low molecular weight
substances can dissolve in polymers, diffuse through them and travel to
a contacting substance. All mass transport through semi-crystalline materials
can be considered to occur through the amorphous regions. As the temperature
increases, the gas permeability and water vapor transmission rate also
increases. The orientation of the product will influence permeability
to gases and water vapor. For example, for PP, the oxygen transmission
rate is 90 cm3/mm/m2/atm/day, and that one for carbon dioxide is of 250
cm3/mm/m2/atm/day. Compared with polycarbonate, polystyrene and polyethylene,
PP is a better gas barrier. Permeability is also related to the effects
of the polymer's functional group. PP's functional product CH3 is a non-polar
molecule, thus the material is more permeable. PVC for example has a strongly
polar functional group (CL), which makes the material be less permeable
to gases.
Migration
Transfer of substances originally present in the plastic material can
migrate into the product that is inside the PP package. Even if virgin
resins contain very low level of additives, migration of substances might
happen. This can occur if recycled and regrind resins are used, if contaminated
equipment and lines are used to process the resin, or if oxidation is
not stabilized during resin manufacture.8
Microbiological activity
On account of its resistance to water and water vapor PP in its pure state
is not susceptible to microbiological attack. PP is not a material with
any nutritional value.
Light stability
PP is very susceptible to light damage by exposure to UV radiation. Unless
the PP is stabilized, UV radiation will deteriorate the surface of the
PP turning it into a chalky friable material of low strength. Semi-crystalline
structures are more resistant than amorphous PP to deterioration produced
by light. An accurate way to test light stability is to submit a sample
of the product to light and observe how much time it takes for the material
to deteriorate.
Toxicity
PP is accepted as a non-toxic and non-carcinogenic material. Within the
industry, it is acknowledged that the basic PP resin that is manufactured
couples with the FDA requirements. Any concern about the toxicity of PP
is due to the inclusion of additives during the manufacturing processes
of products.
Material Safety Data Sheet9
It should be noticed that PP can emit volatile organic compounds during
its processing. The information related to safety concerns is mainly related
to the procedures involved during the product's manufacture.
A typical MSDS for PP indicates the next special hazard precautions, specially
to be considered in manufacturing settings: "Combustible, acute:
particulate may scratch surface/cause mechanical irritation to eyes. Vapors
and/or aerosols that may be formed at elevated temperatures may be irritating
to eyes and respiratory tract. Minimal toxicity if swallowed."
Other important information has to do with the fire and explosion hazard
data that indicate that the extinguishing media is water spray used to
cool the fire exposed surfaces and to protect people. In terms of the
Reactivity Data, it is stated that temperatures over 480 F may cause the
resin to degrade, and that oxygen-lean conditions may produce carbon monoxide
and irritating smoke. Enclosed is the MSDS for the PP resin Besell SV2
58 that is used by Coroplast to fabricate corrugated plastic board.
Fabrication Processes of Corrugated PP10
If during manufacture the hot polymer is subjected to an external stress
the chains align in the direction of the external stress. This makes the
formation of the crystalline structure easier Fiber orientation has an
impact in the resulting properties of the material. As an example, fiber
orientation reduces the permeability and increases the tensile strength
of the resulting product. There is always some type of orientation during
the melt process.
The fabrication of corrugated board involves an oriented extrusion process11.
The whole process is described in the book " Polypropylene Handbook,"
and is quoted in this paper, "Corrugated polypropylene sheets are
done in four steps: extrusion, forming, annealing, and cutting. In the
first step, the molten12 PP polymer is extruded through a custom-designed
die with a ladder cross section. The purpose of extrusion is to deliver
a molten polymer uniform in temperature, molecular weight, and output
rate, and free of contaminants or faults such as bubbles or un-melted
polymer, to the forming die. In the second step the molten extruded board
is cooled as it passes through a vacuum former. The vacuum is applied
to the top and bottom platens that hold the vertical dimension of the
board while the sheet and flutes solidify into the final shape. The vacuum
applied to the sheet is controlled to get acceptable flatness and crush
resistance, which are obtained when the internal flutes are perfectly
perpendicular to the top and bottom surfaces. The third step is annealing
the sheet in an oven to release induced stresses and insure flatness.
The sheet is cut into its final dimension in the last step. Low MFR Homo-polymer
PP grades are suitable to make corrugated PP because it is required that
the melt retains its shape for some time before crystallization occurs."
Another ways to manufacture corrugated board are directly using extruded
profiles that are assembled by laminating together three separate sheets,
the two liners and the inner fluted layer. On account of the twin-wall
structure and the orientation of corrugated PP, its permeability to water
vapor is considerably reduced in comparison with other PP products.
Marketing of PP13
Conservation products vendors started to include corrugated plastic materials
in their catalogues around 1995. Between 1995 and 1998, the major conservation
materials vendors in the United States added the product to their catalogue.
Asked about the reasons why they started to market the product, University
Products, Gaylord and Metal Edge said that customers where asking for
corrugated board in the first place, and also because they wanted to compete
with other vendors that were including it in their catalogues. Corrugated
PP is also known in the USA and Canada by its trademark Coroplast. Gaylord
and University Products sell Coroplast "archival quality corrugated
sheet" and Metal Edge sells other corrugated polypropylene manufactured
in Canada.
According to vendors, corrugated plastic is a popular product. Reasons
why clients buy the product are: for boxmaking, for frame backing and
for shelf lining. Pre-cut corrugated PP boxes are used for bulk storage,
for long-term storage, for transportation, and instead of acid/free board
boxes when moisture is a problem14. Even though there have been no complaints
on the side of the customers, all the vendors acknowledge that the product
has its pros and cons, and some vendors seem to know more about the product
than others. None of the vendors have submitted samples of the material
for a Photographic Activity Test.
Cartons made of corrugated PP are three times more expensive than acid
free corrugated
board cartons but they last more time than those so customers like them
because they save money in the long term. Commercial corrugated board
ranges from 2 mm to 16 mm in thickness.
Coroplast15
As it has been discussed, PP can be processed to have different properties.
Since Coroplast is the corrugated plastic mostly used in conservation
purposes, it is important to consider the particular characteristics of
the product. Coroplast produces an archival quality corrugated board that
is sold for conservation purposes. This boared is composed by the PP copolymer
resin SV2 58 produced by Basell Resins16. The pellets of resin are run
through the extruder and Coroplast do not include any extra additive into
the product. However, the resin itself does have antioxidants and co-stabilizers.
Both components are necessary to protect the resin from oxidative deterioration,
and to neutralize any active remains of the polymer catalyst. The additives
used are antioxidants of 1st. or 2nd. grade but since they are proprietary
the manufacture from Basell resin cannot give the information without
having the informant sign a contract of confidentiality17. According to
the resin manufacturers, SV2 58 is a very clean and neutral product. Due
to the simplicity of the chain composed of hydrocarbons with no double
bonds, the reactivity of the product is extremely low. The basic additives
inside of the resin do not tend to leach and the Food and Drug Administration
(FDA) has approved them. The shelf life of the resin is indefinite providing
that it is kept in-doors at a stable room temperature. If it is submitted
to light and it remains out-door, its life expectancy will be much shorter.
Until the Coroplast sheet reaches a temperature of approximately 300C
(aprox. 600 F), it will not release flammable low molecular weight hydrocarbons.
The manufacturers advertise the product in their web-site saying that
"Compared to other thermoplastics, polypropylene copolymers normally
generate little smoke. The compounds of combustion of polyolefin plastics
are not highly toxic, except for carbon monoxide given off by burning
any organic matter. Carbon monoxide given off by burning Coroplast is
less than for cardboard or hardboard.18 When Coroplast burns, it does
not have an uncontrollable flame spread rate like some acrylics or styrene
and responds very much like paper. Any type of extinguisher can be used
to fight the fire. Water is very successful because it cools and damps
down the fire."
Even though Coroplast is advertised as "archival," the manufacturers
have not performed any aging test on it to determine the long-term stability
of the material. Coroplast can be found in many forms, with anti-static,
flame retardants and UV protection. It is important to make sure that
the vendor has purchased the right type of Coroplast.
Enclosures design19
The design of the enclosures also plays an important part in the protection
of heritage material. In general, institutions buy the pre-cut corrugated
cartons. The design of these has improved in the last years to provide
the tightest enclosure possible that will take advantage of the corrugated
PP water vapor properties. Thus conservation materials suppliers offer
boxes with interlocking flaps, snap lock design, and double walled bottom
to assure rigid construction and tight lid closure. However, none of these
boxes are a tight seal unless they are molded like Tupperware. Because
they are folded up, there are small gaps at these points where air intrudes.20
Corrugated PP is not easily glued. In the publication of the Canadian
Conservation Institute21 a number of fastening and hot-melt gluing techniques
are shown to build custom size boxes. However, these techniques do not
work very well and depending on their size, the boxes do not hold themselves
very well because if the sidewalls are large and short (like the ones
in the large textile boxes), they tend to bend. To make corrugated PP
boxes, conservators work with templates and have been experimenting with
different hot-melt adhesives. According to Hugh Phibbs from the National
Gallery of Art, the 3M 3797 electrical grade hot melt that is chemically
safe works well providing that the joints do not receive sharp blows that
will make the glue crack and fail.
Corrugated PP is also sold in sheets to line drawers, to make supports
for objects, and as a backing material for framing purposes when there
are concerns about humidity located in the walls in which the works of
art are hanged.
How do conservators view and use the product?22
Since the publication of CCI's Technical Bulletin corrugated PP has also
been recommended in the book "Storage of Natural History Collections:
Ideas and Practical Solutions," edited by Carolyn Rose and Amparo
Torres23. However, nothing in depth about the material's chemical and
physical properties and long-term stability has been published and only
a few discussions about the material are recorded in the archives of the
Conservation Discussion List.
In countries like Argentina, where there is no local manufacture of conservation
quality board boxes, corrugated plastic has been used for long-term housing
of heritage collections of all types of materials since 199124. Other
countries such as Brazil, Venezuela, Chile, and Mexico have also been
using corrugated PP boxes instead of good quality conservation board boxes.
It is frequently the case that in this countries corrugated plastic boxes
with heritage materials inside are placed in storage spaces with no environmental
control (temperature, relative humidity and pollution), and with little
or no precautions against disasters. Conservators recommend that the boxes
should be used under the basic premise that the materials placed inside
are clean, free from mold and mildew, and dry (as opposed to slightly
humid). Like in the USA, in all these other countries, local companies
fabricate corrugated PP board with resin that is bought from the resin
manufacturer.
In general, conservators have the concerns about PP in the next areas:
Its behavior as a water vapor barrier25: Since corrugated PP is not totally
impermeable it slows down the transmission of water vapor but it will
not stop it. However, PP has better water vapor barrier properties than
most plastics and taking into consideration the orientation of the process,
and the double wall of corrugated PP, it could be said that if the enclosure
design is relatively tight the exchange of humidity with the outside is
not very significant. Besides, corrugated PP will definitively buffer
the effects of fluctuating temperature and RH. On the flip side, because
the material does not constitute a total barrier whenever a complete impermeable
enclosure is needed to retain specific environmental conditions, conservators
use Marvelseal instead of corrugated PP.
Some of the same concerns related with the use of plastic sleeves to house
photographic materials can be considered when thinking about the use of
corrugated PP. In the Conservation Dist List email discussion, Lauren
Charles Pigniolo26 commented the next about polypropylene sleeves "There
are pros and cons to putting images in plastic. On the con side is the
possibility of dreaded condensation inside the sleeve if you reach the
dew point... .On the pro side is that the plastic can create a microclimate
inside the sleeve that buffers rapid changes in relative humidity. I have
noticed this phenomenon when working at an institution with no climate
control on a very dry Santa Ana condition day. When prints were in the
plastic sleeves, they were relatively flat. When removed they rapidly
desiccated and curled. Quite a stress on the object."
Gas exchange and build up of off gassing products in corrugated PP boxes27:
Conservators have been always concerned with the types of microclimates
that are created inside of containers. In terms of plastics, it has often
been discussed the fact that non-permeable materials allow for a build
up of deterioration off products. In the Conservation Dist List Discussion,
referring to plastic sleeves to house photographic negatives, Loren Charles
Pigniolo indicated that materials such as nitrate and acetate films "might
benefit from a more breathable enclosure than plastic."
Research done by IPI focused in paper/plastic sleeves used to house photographic
materials. Concluded that the difference in deterioration rate between
things stored in sealed plastic or metal containers versus those stored
in open or porous containers is not significant. The reason for this might
be that the products of deterioration do not disseminate that much to
adjacent negatives, neither contribute so much to the deteriorating process
that is happening of the same degraded negative. What do have a major
impact on the deterioration processes of early photographic negatives
are the environmental conditions of the storage space.28
PP is not a very efficient gas barrier, which means that oxygen and gaseous
pollutants are exchanged back and forth through the material. Even though
there is data about the amounts of specific types of gasses that are exchanged
within PP, it still remains unclear what happens in relationship to the
materials housed inside of a corrugated PP box. It can be inferred that
on account of both, the characteristics of the corrugated PP and of those
derived from the way in which the boxes are constructed, the level of
gas exchange may actually not be that different than that one of a thick
lined cardboard box. However, this assumption needs to be corroborated.
Long-term stability testing and PAT 29It is often the case that conservators
rely on the testing that conservation vendors do to the materials they
sell. Up to this date, no data about a long-term stability test of corrugated
PP is available. The three main vendors of PP have reported that they
have not submitted corrugated PP for a PAT. At the National Gallery, Hugh
Phibbs reports the same. During the photographic housing project in Argentina
the conservators in charge did not submit samples of the corrugated PP
for a PAT either. However, basic PP in other forms does pass the PAT30.
Other useful comments derived from the Conservation Dist List indicate
that PP is considered by ANSI as an acceptable housing material for photographs.
Static charge/ maintenance: In general PP has less tendency to build up
a static charge than other plastics such as polyester. When corrugated
PP is used as a housing material, the problem is when red-rot books and
photographic albums, brittle and fragile newspapers or textiles and very
friable objects surfaces are placed inside of the box with no other secondary
protection. In those cases it is frequent to see small pieces of material
attached to the interior walls of the boxes. Other inconvenience has to
do with the maintenance of the boxes. When placed in clean and filtered
environments not so much dust is accumulated in the box surfaces. However,
in non- filtered environments the boxes have to be wiped frequently.
Flammability/ melting properties and resistance to water31: Conservators
are mainly concerned about the low melting point and the flammability
of corrugated PP. Hugh Phibbs commented that he uses corrugated PP only
in rooms that have fire suppression systems and when works of arts backed
with corrugated PP are hanged in the wall so that the plastic board will
be the last material to get heated. A major concern in relation to the
use of corrugated PP is its low melting point. Miranda Martin reported
that the performance of Coroplast boxes was observed during the Burn Baby
Burn workshops sponsored by AIC in 1996. In that case, before burning
the plastic melted over the materials the where housed.
In terms of the materials ability to withstand water disasters, conservators
see pros and cons. On the pro side they recognize that PP box holds up
better than a paperboard box and sheds water from above, as long as the
water does not get inside trough a fold over area. PP boxes are more resistant
to biological activity and they withstand the weight of wet materials
in cases when those need to be transported. On the con side, conservators
fear that because the board is impermeable to water it will not absorb
water that can pool inside once it enters through folds. Hugo Gez reported
that during a recent fire disaster in the Museo Nacional Ferroviario in
Buenos Aires, in which firefighters through great quantities of water
over corrugated PP boxes and cardboard boxes, the materials inside of
the PP boxes remained dry, and the ones inside of the cardboard boxes
got all wet while the boxes collapsed. However, it is important to always
check the condition of the materials inside of boxes that are dry on the
outside because if the materials inside are wet and not taken care of
serious problems can arise in terms of mold growth.
Conclusions
Like with every material that is used in conservation, each professional
will have to make a decision based on the specific properties of the product,
the characteristics of the materials that are to be housed, and the particular
circumstances that surround the project - budget, availability, and environmental
conditions.
As a material to be used in humid and fluctuating climates such as Buenos
Aires where few storage areas have climate control, corrugated PP is not
a bad choice because it will allow for a limited water vapor exchange
between the inside and the outside of the box and will make the RH fluctuations
arrive in a slower rate to the materials enclosed. Like all enclosure
materials, corrugated PP's performance is enhanced in a climate-controlled
environment.
So far, corrugated PP has proved to behave well in terms of its chemical
stability. Harmful reactions between corrugated PP with the materials
inside have not been reported. However, the material still needs to be
tested.
The example of the use of plastic sleeves to protect photographic negatives
has been raised to analyze the ability of corrugated PP to form a micro-climate
in which the off products of deterioration of acetate negatives could
promote further deterioration of the negatives. According to investigations
done by the IPI, although some differences have been noticed between porous
and non-porous enclosures, the difference is not significant enough as
to discard the use of plastic envelopes. From there it could be said that
the use of corrugated PP boxes will not necessarily promote more deterioration
to happen in materials that are per-se prone to deteriorate. Overall,
the effect of an inadequate environment is what has the major impact in
those deterioration processes. However, conservators seem to prefer the
use of porous enclosure materials.
Concerns about flammability and PP low melting point are real and as was
pointed by Hugh Phibbs, materials made of PP should not be used in places
where there are no fire suppression systems.
Many questions still need to be answered about the product. For example,
it would be interesting to estimate the real values of gases and water
vapor exchange in corrugated pp and compare those with the cardboard values.
Up to the present, the long-term behavior of corrugated PP has not been
studied neither by the manufacturers no by the vendors.
Due to its properties, corrugated PP could be very useful to store materials
that are extremely sensitive to humidity such as minerals, metal objects
with copper disease, and ceramics with salts that do require an extra
protection against humidity, and do not particularly benefit from buffered
enclosures. Corrugated PP can be specially considered for use in archeological
sites to house materials that have been recently excavated and where a
certain microclimate needs to be kept for a short period of time. Also,
it is a sturdier alternative to corrugated cardboard that can be economic
and useful in Records Centers where archival materials are kept in large
storage spaces before they are taken to the archives or discarded.
The lack of conservation data about corrugated PP reflects the fact that,
at least in this country, it is not really considered a material of importance
in the conservation arena and it is more an alternative than a first choice
material. On the other hand, in countries where it is extensively used,
it reflects the lack of systematic monitoring and serious research on
materials. After so many years of using the material, at least in Argentina,
a sounder knowledge of the material would have been appropriate.
Bibliography
Basell Resins (Ct) Home page (n.d) Retrieved November 20, 2001 from: http://www.basell.com
Coroplast (Ct) Home page (n.d). Retrieved 10/06/01 from: http://www.coroplast.com
Hernandez, R.,J., Selke, S.,E.,M.,& Culter, J., D. ( 2000). Plastics
Packaging: Properties,
Processing, Applications, and Regulations. Hanser
Kissel. J., Han, H. J., & Meyer, J. A. (1999). Characteristics of
Polypropylene. In Harutun G. K.
(Ed.), Handbook of polypropylene and polypropylene composites (pp. 15
- 37). New York: Marcel Dekker
Maier,C. Calafut, T. (1998). Polypropylene: The Definitive User's Guide
and Databook
Plastics Design Library
Martin, Miranda. (06-20-2000). Large Archival Boxes. Message posted to
Conservation Dist
List, archived at http://palimpsest.stanford.edu/byform/mailint-lists/cdl/2000/0767.html
Moore, E.P. (1996). Polypropylene Handbook. New York: Hanser
Nishimura, Douglas (11-15-1999) Storing photographic materials. Message
posted to
Conservation Dist List, archived at http://palimpsest.stanford.edu/byform/mailing-
lists/cdl/1999/1320.html
Pignolo, Lauren Charles (05-15-1993). Photographic sleeves. Message posted
to Conservation
Dist List, archived at http://palimpsest.stanford.edu/byform/mailing-lists/cdl/1993/0269.html
Rose, C., Torres, A. (Eds.)(1993). Storage of Natural History Collections:
Ideas and Practical
Solutions. Society of Natural History
Schlichting, C. (1994). Technical Bulletin:Working with Polyethylene Foam
and Fluted Plastic
Sheet Canada: Canadian Conservation Institute
Personal Communications
Christine, Allie, Gaylord, personal communication, 10/15/01
Dunphy, John., University Products, personal communication, 10/01
Gez, Hugo, personal communication, 10/01
Nishimura, Douglas, personal communication, 11/19/01
Orozco, Monica, Metal Edge, personal communication, 10/17/01
Phibbs, Hughes, personal communication, 10/01
Reese, Steeve, Coroplast, personal communication, 10/01
Technical representative, Basell resins, personal communication, 20/11/01
Wagner, Sarah, personal communication, 11/18/01.
1 Schlichting, C. (1994). Technical Bulletin: Working with Polyethylene
Foam and Fluted Plastic Sheet" Canada: Canadian Conservation Institute
2 The information in this section was extracted from:
Kissel. J., Han, H. J., & Meyer, J. A. (1999). Characteristics of
Polypropylene. In Harutun G. K. (Ed.), Handbook
of polypropylene and polypropylene composites (pp. 15 - 37). New York:
Marcel Dekker
Maier, C. Calafut, T. (1998). Polypropylene: The Definitive User's Guide
and Databook.(pp 1-9) Plastics Design Library
Moore, E.P. (1996). Polypropylene Handbook. New York: Hanser
3 Kissel. J., Han, H. J., & Meyer, J. A. (1999). Characteristics of
Polypropylene. In Harutun G. K. (Ed.), Handbook
of polypropylene and polypropylene composites (pp. 15 - 37). New York:
Marcel Dekker
4 Information for this section has been extracted from:
Maier, C. Calafut, T. (1998). Polypropylene: The Definitive User's Guide
and Databook. Plastics Design Library. Pp 11-25
5 The information for this section has been extracted from:
Hernandez, R. J., Selke, S.E.M., Culter, J. D. (2000). Plastics Packaging.
Munich: Hanser (pp. 102 - 104)
6 The information for this section has been extracted from:
Maier, C. Calafut, T. (1998). Polypropylene: The Definitive User's Guide
and Databook. Plastics Design Library
Hernandez, R. J., Selke, S.E.M., Culter, J. D. (2000). Plastics Packaging.
Munich: Hanser (pp.135-156)
7 The information for this section was taken from:
Maier, C. Calafut, T. (1998). Polypropylene: The Definitive User's Guide
and Databook. Plastics Design Library (pp 120-134)
Hernandez, R. J., Selke, S.E.M., Culter, J. D. (2000). Plastics Packaging.
Munich: Hanser (pp.77-78/ 328)
8 According to Steve Reese from Coroplast, the resins used to manufacture
their product are not recycled.
9 Maier, C. Calafut, T.(1998). Polypropylene: The Definitive User's Guide
and Databook. Plastics Design Library (pp. 155-158)10 The information
for this section was taken from:
Hernandez, R. J., Selke, Susan E.M., Culter, J. D. (2000). Plastics Packaging.
Munich: Hanser
Moore, E.P. (1996). Polypropylene Handbook. New York: Hanser
11 Information about the orientation was supplied by Steve Reese from
Coroplast.
12 Molten/ liquefied heat, in a state of fusion, melted produced by melting
and casting.
13 The information for this section was obtained through phone and email
interviews with customer's representatives from Gaylord, University Products,
and Metal Edge.
14 It is important to notice that the RESCUBE boxes used to handle water
damage materials are not made from corrugated PP but from HDP. However,
Coroplast boxes can be used to transport water -damaged materials as well.
15 The information for this section was obtained from conversations with
Steve Reese from Coroplast and with the customers representative that
provides technical information at Basell Resins. Also from the Coroplast
Technical Bulletin CSS-001-93 and from various links in the Web site of
Coroplast http://www.coroplast.com
16 More information about Bassell polyolefines can be found in http://www.basell.com
17 The information about the primary and secondary antioxidants can be
found, among others, in the book Polypropylene: The Definitive User's
Guide and Databook pp. 27 - 29. Since the chemistry is quite complex,
I did not think it was necessary to include it here.
18 This information agrees with the one included in the product's MSDS
that could be found in:
Maier, C. Calafut, T. (1998). Polypropylene: The Definitive User's Guide
and Databook. Plastics Design Library (p 155)
19 The information for this section has been discussed by María
Esteva through email with Hugh Phibbs. 10/01
20 Email exchange between María Esteva and Sarah Wagner, 11/17/01
21 Schlichting, C. (1994). Technical Bulletin: Working with Polyethylene
Foam and Fluted Plastic Sheet" Canada: Canadian Conservation Institute
22 The behavior of PP film to house photographic materials has been frequentely
discussed in the Conservation Dist List. Even though the properties of
PP films and boards are different, and the ways in which both are used
are different as well, the information from those discussions has been
incorporated in this section of the paper because they reflect the concerns
that conservators have about plastic. On the other hand, photographic
materials have, for a long time now, been housed with plastic enclosures:
Pigniolo, L. C. Photographic sleeves, Conservation Dist List 05-15-1993.
Retrieved 11/8/01http://palimpsest.stanford.edu/byform/mailing-lists/cdl/1993/0269.html
Nishimura, D. Storing Photographic Materials. Conservation Dist List 11-15-1999.
Retrieved 11/16/01 from http://palimpsest.stanford.edu/byform/mailing-lists/cdl/1999/1320.html
23 Rose, C., Torres, A. (Eds.)(1993). Storage of Natural History Collections:
Ideas and Practical Solutions. Society of Natural History
24 Information obtained during email exchange between María Esteva
and Hugo Gez, photographic conservator in charge of photographic collections
housing projects in Buenos Aires, Argentina from 1989 to 1993. 10/01
25 Information for this section comes from email exchange between María
Esteva and Hugh Phibbs, 10/01. Some discussions found on the Conservation
Dist List concerning polypropylene films have been included as well because
they reflect the concerns that conservators have about plastics.
26 Pignolo, Lauren Charles (05-15-1993). Photographic sleeves. Message
posted to Conservation
Dist List, archived at http://palimpsest.stanford.edu/byform/mailing-lists/cdl/1993/0269.html27
Most of the information on this section was discussed between María
Esteva and Sarah Wagner in an email exchange 11/17/01, and between Maria
Esteva and Douglas Nishimura in an email exchange 11/19/01.
28 Both Sarah Wagner and Douglas Nishimura have stressed that cold storage
is the solution to acetate negatives deterioration, and that other storage
alternatives will not solve nor diminish the problem. Despite of this,
Douglas Nishumura commented that the Association of Moving Archivists
is still trying to design a porous container for their films. Sarah Wagner
commented that for old negatives she recommends the use of paper enclosures,
and only recommends plastic to house new materials that will be susceptible
to handling.
29 Email exchanges between María Esteva with Hugo Gez and Hugh
Phibbs, and with customers representatives of Metal Edge, Coroplast, University
Products and Gaylord. From 10/01
30 Email exchange between María Esteva and Sarah Wagner, 11/17/01.
Exchange between Maria Esteva and Douglas Nishimura 11/19/01
31 Information from this section has been extracted from:
Martin, M. Large archival boxes. Conservation Dist List 06/20/00. Retrieved
11/6/01from http://palimpsest.stanford.edu/byform/mailing-lists/cdl/2000/0767.html
Email exchange between María Esteva and Hugo Gez. 10/0
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