The Cochineal: An Online Student Journal and Repository of Conservation, Preservation, and Cultural Studies

 

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.


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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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