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Chapter 33: Plastic Storage Products

Scott Williams, Ottawa, Ontario, Canada

A brief description of polymers, plastics and polymerization is given. Criteria for suitability of plastic storage products are discussed and related to the hazards that plastics have, initially before aging, and develop, after aging. The types and severity of hazards, including off-gassing, exudation, and loss of function, before and after natural aging in museum environments, are described and tabulated for specific plastics. The sensitivity of plastics to degradation agents in museum environments and how this degradation affects the hazard type and severity of plastic degradation then ultimately the suitability of the plastic for storage products are described. Several tables are presented, including tables of sensitivity of plastics to environmental degradation agents, and types and degree of severity of hazards. The risk that a selected plastic storage product will cause harm to objects depends on the types of hazard presented by the product and the probability that the object will suffer adverse effects from that hazard. With these tables and their knowledge of the types of objects being stored, collection managers can determine the risk of using specific plastic products to store the different objects in their collection.

Introduction

Examples of plastic storage products include containers and bags to separate, isolate, and organize collections; gas barriers for anoxic storage of specific objects; fabrics for bags for organization of parts, and dust and light protection; foams for padding and cushioning; sheets, panels, rods, and bars for supports, glazing, and containers; extruded profiles of rubber and foam; and sealants for gaskets. These plastic storage products are the subjects of this chapter. Plastics found in objects as part of their com- position, plastics used in treatments such as adhesives and consolidants, and plastics in coatings are not discussed.

Storage products can be made of a whole range of materials including, but not limited to paper, glass, metal, wood, and plastics. Why choose plastic storage products? Plastic products tend to be tough, light weight, unaffected by water immersion during floods, noncorroding, readily available in many forms, cost effective due to mass production and raw material cost, and, if properly chosen, unreactive toward stored collections. Plastics products can be impermeable to water (Tupperware containers), or permeable (Tyvek, various meshes and nets).

Choosing between storage products made of plastic or other materials involves careful “risk-benefit” analysis. For example, consider the responses of various storage materials to fire. Plastics usually melt before they burn thus presenting less surface area to support combustion. By contrast, paper burns and is destroyed by water immersion. Glass is heavy and fragile but not flammable; metal does not burn but is heavy and may corrode, particularly when exposed to water. Wood burns, and some woods can offgas. So one needs to understand the collection vulnerabilities and potential risks in making any collection-preservation decisions.

Basic Terminology

An array of terms is used, often interchangeably, when describing plastics. For example, the term “grade” is often thought to indicate purity, but it is more often related to use, as in food grade, structural grade, high-viscosity grade, and low-emission grade. Grade can also refer to a unique product. A search of the Universal Selector by Omnexus/ SpecialChem (a database containing technical data sheets for more than 116,950 plastic products) shows there are 24,240 polyamide/Nylon products in the database, each with a specific product name. The manufacturer refers these to as grades, and purity is not implied by any of these grades. Each of these may have different ingredients, that is, different formulations. To avoid confusion, I will use the following definitions for the terms “class,” “grade,” “product,” and “formulation.”

• class is determined by the plastic’s base polymer, for example, polyethylene (as in Ethafoam)
• grade is the designation for a specific plastic with unique properties, for example, Ethafoam 220, a specific grade of Ethafoam with a density of 2.2 pounds per cubic foot, with unique properties that are different from other grades such as Ethafoam 400 and Ethafoam 600
• manufacturers sometimes refer to their materials as products; I will call these grades, and reserve the term “products” for the things made from plastics, for example, containers, foams, and sheets
• formulation is the recipe or list of ingredients or complete chemical composition of the plastic; for example, polyalphaolefin copolymer with isobutene blowing agent and glycerol monostearate permeation control agent (as in Ethafoam) (this is usually proprietary and seldom known in detail)

International standards for technical terminology in the plastics industry are defined by ASTM (2012), along with acceptable contractions of these terms (ASTM, 2014).

Suitability Criteria for Plastic Storage Products

Plastic storage products, just like products made from any other materials, must meet some minimum criteria to be suitable for use in collection storage.

1. The initial intrinsic properties of the plastic storage product must be appropriate for its intended function without being hazardous to objects. For example, products used for gas-barrier pouches must be impermeable to the gas of interest (e.g., blocking oxygen for anoxic treatments). Foams meant to cushion should be neither too stiff, nor too soft, given the object weight they support.
2. The chemical, physical, and mechanical properties of the plastic storage product must not change so much over time as to become unsuitable for the intended function. For example, foams for cushioning must not suffer too much compression set or become so brittle as to lose their cushioning function (data concerning compression set can be found within the technical data sheets for the product). An extreme case of compression set occurs when polyethylene foam is placed under heavy objects or gaskets in cabinets lose their ability to seal when stressed by the closure pressures.
3. Substances initially present, or that develop over time in the plastic (due to reaction with their environment or inherent vice) must not damage objects in contact or nearby.

Objects are at risk of damage from plastic storage products if any of the suitability criteria are not met. Some hazards are inherent properties due to initial chemical composition or physical structure of the plastic products. Other hazards are produced by degradation influenced by environmental factors.

Hazards Associated with Plastic Storage Products

Plastic storage products that do not meet the suitability criteria pose three main hazards (potential damage) to collections:

1. Volatile substances: Volatile (gaseous) substances in the plastic migrate from the body of the plastic to the surface of the plastic. They then evaporate and are offgassed into the storage environment where they diffuse and react with or deposit on objects causing damage. Examples include corrosion of silver by sulfur from some plastics and rubbers, loss of strength of textiles by reaction with acidic degradation products, or creation of greasy or crystalline deposits by condensation of plasticizers or other plastic additives.
2. Nonvolatile exudations: Nonvolatile liquid or solid substances in the plastic such as plasticizers, stabilizers, and other additives migrate from the body of the plastic to the surface where they remain as exudations or blooms without evaporating. These exudates may then damage objects with which they come into contact by staining and corrosion. The damage only occurs at points of contact. They can also pose a health hazard if handled without appropriate gloves (typically, nitrile is best).
3. Loss of function upon aging: Over time, products may become unacceptable for their original intended use due to chemical degradation and physical processes like diffusion of components. Examples include yellowing of plastic glazing that obscures objects, embrittlement of plastic containers causing containers to break and objects to be lost, development of tackiness causing object to adhere to product, fracture of plastic creating sharp or abrasive edges, or gas-barrier films becoming permeable.

The Nature of Plastics

By nature, plastics have complex chemical compositions. There are tens of thousands of plastic grades based on about 50 classes of plastics (Shashoua 2008). In addition to the base polymer, that gives the plastic its class name (e.g., polyethylene (PE), polystyrene (PS), poly(vinyl chloride) (PVC), phenol-formaldehyde (PF)), plastics contain additives (e.g., plasticizers, ultraviolet (UV) absorbers, heat stabilizers, antioxidants, colorants, slip agents, and mineral and organic fillers) that modify the inherent properties of the base polymer to give suitable end-use properties to the plastic (Williams 1993, Hiorns et al., 2012). Different plastics made from the same polymer type can have different properties, depending on variations in the polymer, including molecular size (e.g., molecular mass, degree of polymerization) and mo- lecular shape (e.g., linear, branched, crosslinked) (Baker 1995, Shashoua 2008, Horie 2010). Additional discussions of plastics, polymers, and polymerization in the conservation context are given by Mills and White (1987), Shashoua (2008), Horie (2010), and Fenn and Williams (2017).

Chemical and physical changes to the plastic, caused by reactions of the polymer and the additives with environmental factors, create the potential for damage to collections by plastic storage products.

Table 1 lists classes of polymers, abbreviations, and examples of polymers commonly used in plastic storage products.

Polymerization

A polymer is a substance composed of very large molecules, called macromolecules, each having a chain or network of monomeric repeating units joined together by covalent bonds during a chemical reaction called polymerization. The chain of repeating units is the backbone of the macromolecule. The basic repeating units are monomers, the chemical compounds that join to form the macromolecule (e.g., ethylene monomers react with other ethylene monomers to form PE). The number of monomer molecules that joined to form the macromolecule is the degree of polymerization (DP) of the macromolecule, and can range from hundreds to thousands. DP is a measure of the size or length of the macromolecule.

Polymerization occurs primarily by two mechanisms. During addition polymeriza- tion (polyaddition), monomers are linked together without splitting off water or other simple molecules. PE, polypropylene (PP), and PS are examples of addition polymers produced by addition polymerization of ethylene, propylene, and styrene respectively. In condensation polymerization (polycondensation) monomers are linked together by reactions that produce water or other simple molecules as byproducts. For example, the polycondensation reaction of hexamethylene diamine with adipic acid produces poly(hexamethylene adipamide)(polyamide 66 or Nylon 66), plus water. Polyesters, cellulose esters, polyurethanes (PURs), and polyamides (PAs) are typical condensation polymers.

The stability of the polymer in various environments is affected by its polymerization mechanism. For example, in high relative humidity, condensation polymers have a tendency to react with water/moisture and revert to the initial monomer state, whereas addition polymers are not affected in this way by atmospheric moisture.

Thermoplastics and Thermosets

Plastics can be separated into two broad categories, thermoplastic and thermoset, based on thermal properties, which are determined by extent of crosslinking. When heated, thermoplastics will soften and melt before charring or burning, enabling them to be reshaped, but thermosets will char and burn before softening, so cannot be re- shaped. Table 2 shows the contrast in several properties between thermoplastics and thermosets. This classification scheme gives some insight into the long-term behavior and suitability of plastic storage products. Those made from thermoplastics may distort over time due to creep if exposed to stress. For example, a gasket that initially functions properly because it presses firmly against both sides of the closure may eventually fail because the plastic creeps and suffers compression set under the constant stress in the closure. Eventually, it no longer presses against the sides and thus fails to seal. Joints made with thermoplastic adhesives to hold heavy pieces together may fail if the temperature rises in the storage environment sufficiently to soften the thermoplastic adhesive, especially if it passes through its glass-transition temperature (Tg).

Degradation of Plastics

As for all materials, plastics will degrade over time when exposed to moisture, oxygen, pollutants, UV radiation and short-wavelength light, and physical forces. Chemical re- activity of the polymer is related to chemical composition, particularly the chemical functional groups in the polymer. Functional groups are specific combinations or arrangements of atoms in a compound that are responsible for the characteristic chemical reactions of that compound (Mills and White 1987). Plastics degrade differently, reacting at varying rates, and exhibiting a wide range of effects from degradation. Plastic Storage Products 759

Plastics degradation data is available from accelerated aging tests and from weathering studies in outdoor environments with sunlight, precipitation, varying RH, and temperatures ranging from below freezing to greater than 100°C, conditions that are much more extreme than those encountered in museum storage. Observations during condition surveys of plastics that have been displayed and stored in museums for several decades provide important data concerning plastic degradation – see POPART: Preservation of Plastic ARTefacts in Museum Collections as an example of one such study(POPART, n.d.).

Chemical Degradation

Chemical degradation affects the chemical, physical, and mechanical properties of different plastics in different ways. Upon aging, properties of plastics may change due to chemical reactions with atmospheric oxygen, water, or pollutants, possibly abetted by light and UV radiation or elevated temperature exposure. They may also change via physical reactions such as recrystallization or by migration of components. Chemical degradation of plastics is primarily by oxidation in the presence of oxygen and hydrolysis in the presence of moisture. Light and UV radiation initiates and catalyzes oxidation, and elevated temperatures increase rates of reactions.

Oxidation

Materials react with oxygen by the chemical process called oxidation. Oxidation can be initiated and catalysed by light and UV radiation of sufficient energy (wavelength), in which case the chemical process is called photo-oxidation. Oxidation in the absence of light and UV radiation is called thermal oxidation. All plastics are subject to thermal oxidation, to varying degrees depending on the plastic. Although oxidation occurs at room temperature, the rate always increases as temperature increases. Plastics for storage products should be chosen from those with slow rates of oxidation at room temperature (or the temperature of intended use) so that the product will function adequately for the longest possible time.

Antioxidants are added to the polymer during the production of the plastic to protect against thermal oxidation at elevated temperatures during thermal processing such as molding. They are also added to protect against slow thermal oxidation at ambient temperature during the lifetime of the product. Some antioxidants are consumed as they function and eventually are exhausted so that the plastic is no longer protected against oxidation. The duration of protection is unpredictable because it depends on the amount of antioxidant initially present, the exposure time of the plastic, and the nature of the antioxidant.

Oxidation usually occurs at C=C double bonds or at tertiary carbons in branched polymer backbones leading to bond scission, which causes a decrease in molecular mass of the polymer. Carbonyl functional groups (aldehydes, ketones) and carboxylic acids are produced on oxidation and often these are colored (chromophores) causing yellowing of the product. Because strength decreases as molecular mass decreases, the storage product becomes weaker and more prone to fracture during use. Usually chain scissions occur in the middle of polymer backbones rather than at the ends so the carbonyls produced are attached to high-molecular-weight polymer fragments and are not volatile compounds. Thus, the primary effects of oxidation of hydrocarbon polymers are yellowing and loss of strength, but not emission of volatile compounds. The number of acid groups attached to the polymer increases, but because they are attached to the backbone, this does not tend to increase the amount of extractable acidity present. So, although midchain oxidation of a plastic makes it weaker, this usually does not contribute volatile pollutants to the storage atmosphere. In other words, the oxidation of hydrocarbon polymers might not be a primary issue if strength is not required of the storage product and if the yellowing does not impair its functionality.

Table 3 lists the thermal oxidation sensitivities at room temperature of plastics that are commonly used for storage products. Sidebar 1 provides some background for better interpreting the data in the table.

Moisture and hydrolysis

As mentioned, polymers formed by polycondensation are formed from two different monomers that combine to make the polymer and split off a small molecule, usually water. To complete the polycondensation reaction with the maximum quantity of polymer produced, the water is removed during polymerization. This is a reversible reaction. If the polycondensation polymer is exposed to water, then the polymer may react with the water to reverse the condensation reaction and produce the initial monomer. This is the basis of hydrolytic degradation.

Hydrolysis always breaks a bond. For example, an ester bond breaks to produce an acid group and an alcohol group. If the ester bond that breaks is in the middle of the backbone of a large molecule then two smaller molecules, each of which is still quite large, will be formed. No volatile material will be produced. If the ester bond is at the end of the polymer molecule backbone or is a pendant group attached to the polymer backbone, then a small molecule of low molecular weight and high volatility will be produced. This is the problem with cellulose esters like CA. The acetate ester groups are pendant to the cellulose backbone and when hydrolysis occurs, low molecular weight volatile acidic acetic acid is produced, which migrates throughout the storage area and will damage nearby materials. Storage products made from plastics that produce acids when hydrolysed, such as ester-type polyurethane and urea-formaldehyde (an adhesive in some wood products), are hazardous to objects and unacceptable for use in storage.

Polymers that contain hydrolysable ester and amide functional groups in the polymer backbone such as poly(ethylene terephthalate) (PET), and PA tend to be less susceptible to hydrolysis because water is less accessible to the ester bond. If they do hydrolyse, the alcohol and acid groups produced are on large molecular fragments and, therefore, not volatile. Molecular weight decreases because backbone chain cleavage occurs.

Table 3 lists the hydrolysis sensitivities at room temperature of plastics that are used for storage products.

UV Radiation and Short Wavelength Light

Most plastics photo-degrade when exposed to visible light and UV radiation. In the presence of oxygen, photo-degradation is predominantly by photo-oxidation, whereby plastics become discolored, embrittled, and fractured. Plastics that are susceptible to photo-degradation have functional groups called chromophores that absorb incident radiation and are elevated to reactive higher energy states, from which they undergo degradation reactions that change the properties of the plastic. Common chromophores are aromatic groups, carbonyl groups, and chains with alternating carbon-carbon single and double bonds (conjugated bonds). Plastics containing these functional groups are susceptible to photo-oxidation and include materials such as PS and rubber.

Sunlight and fluorescent lamps are the most common sources of light and UV ra- diation in museums. Collection storage rooms should be designed to reduce light levels to prevent damage to the objects so these controls will also reduce damage to plastic products (see Himmelstein, Rosenfeld and Weintraub, chapter 11, this volume).

Table 3 lists the light and UV radiation sensitivities of plastics that are commonly used for storage products.

Offgassing and Outgassing

Volatile compounds emitted by plastics at ambient conditions (a process called offgassing) may react with objects in proximity to, but not in contact with, the plastic. Offgassed compounds may be present initially in plastic (intrinsic compounds, e.g., residual monomer, additives) or they may be produced upon aging (degradation products). Previous discussions of plastic-pollutant issues have identified offgassing of monomers as a significant issue but, due to improved manufacturing processes and greater concerns about health effects from these residues, monomer concentration in plastics is now very low, and this author believes this is not a significant concern for new products. Offgassing issues are not unique to plastics, but are important concerns for wood, paper, and textiles as well (see Hatchfield, chapter 31, and France, chapter 32, this volume). The amount of material offgassed, the chemical nature of the offgassed material, and the potential of the offgassed material to react detrimentally with stored objects must be assessed.

Emission of volatile substances from products under vacuum at elevated temperature (a process called outgassing) has been studied extensively in the aerospace industry. This is because condensation and reaction of outgassed materials on sensitive electronic and optical components can compromise spacecraft function. Standard test methods have been developed to monitor and evaluate levels of outgassing.

For aerospace applications, ASTM E595 (ASTM 2007) is a standard test to determine the total mass lost (TML), the amount of collected volatile condensable materials (CVCM) on a cold collector plate, and the water vapour regained (WVR) calculated after exposure of the specimen to 125 °C for 24 hours in a vacuum and condensing volatile material on a cold plate. Products are considered safe for aerospace application if CVCM <0.1% and TML <1%, or if CVCM <0.1% and TML-WVR <1%. In the auto industry tests, SAE J-1756, DIN 75201 and others involve incubating a specimen/sample at 100°C at which point the amount of material condensed onto a plate at 25°C is measured after 3 hours by a reflectometric method or after 16 hours by a gravimetric method (Pratt 2011).

Certainly, materials that meet outgassing requirements under the extreme conditions of the ASTM, SAE, and DIN standards would also meet the low outgassing criterion for collection storage applications. Under these conditions, samples are tested at elevated temperature and/or in a vacuum – conditions much more extreme than those encountered in a museum. Subsequently, the test will detect materials that may not ever be volatile at ambient conditions. It is important to note that the tests do not indicate whether the emitted substances will react chemically with other objects or surfaces, or whether the products will exude nonvolatile substances that will stain on the product surface. These tests show which products do not emit at extreme conditions and there- fore will not produce products that will react with objects at ambient conditions.

Databases for outgassing have been created by NASA and ESA, and one can find outgassing data for many products used in collection storage there (NASA 2008 and ESA 2015). Table 4 shows results from the NASA Database for outgassing from some plastics used in conservation and collection care. These data show low outgassing values that correlate with the general acceptability of these products in conservation applications as shown by years of use without damage. Many suppliers of plastic products will also provide outgassing data (for example, Poron PUR foam recommended for gaskets in electro-optical equipment (Rogers Corporation 2003, 2015).

In conservation, accelerated aging tests, including those referred to as the Oddy Test, are used to detect products that will offgas substances that will corrode metal coupons, typically copper, lead, and silver. The sample and coupons are incubated for about a month at high temperature (60°C) and high relative humidity (RH) (100% RH) at near ambient pressure (Thickett and Lee 2004 and Hatchfield, chapter 30, this volume). This test assesses the reactivity of emitted substances only with specific metals. It does not test for reactivity with anything else. Results are based on subjective quantification by visual observation of corrosion and expressed as “P” (suitable for permanent use, no corrosion), “T” (suitable for temporary use – up to six months only, slight corrosion) and “U” (unsuitable for use, obvious corrosion). Note that the “T” rating implies a level of acceptable damage for some exhibition situations (short term) but would not be acceptable for collection storage (long term).

Another test developed for conservation applications is the photographic activity test (PAT) that assesses the stains caused by substances emitted from products on a specific silver photographic emulsion dosimeter when incubated in contact with the dosimeter at 70°C and 86% RH at near ambient pressure for 15 days (Image Permanence Institute 2015 and Hatchfield, chapter 30, this volume). Incubation with acid- base indicators, such as A-D strips, is also used to detect products that outgas acidic compounds (see Coughlin, Storage at a Glance: Plastics, this volume).

Databases of results of Oddy tests and PATs are available online (AIC Wiki 2015, British Museum 2015, National Archives of Australia 2015).

Exudation and Bloom

Outgassing and offgassing data address the issue of emission of volatile materials from the plastics, but do not address the issue of migration of nonvolatile components to the surface of the plastic. Exudations and blooms on the surface of plastics could interact with objects in direct contact with the plastic. Exudation poses a different (perhaps re- duced) level of severity or type of concern compared to offgassed materials because there must be direct contact between the plastic and the object for damage to occur. For example, products made of flexible PVC should not be used because oily plasticizers can exude from the PVC and stain objects in contact.

Migration and extraction of substances from products has been studied extensively in the food, medical, electronic, and cosmetic industries where substances migrated into or extracted into products produce tainted odors, tastes, compromised medicinal properties, corrosion, or other detrimental effects. Standardized tests have been developed in these industries to monitor migration and evaluate effects of migrated sub- stances and the results of these standard tests reported in product literature could be applied to collection storage products (Crompton 2007).

Another factor to consider is the interaction of the storage object with the storage product. Is something oozing from the object? Plasticizers exuding from PVC are very good solvents and can dissolve polystyrene boxes; oils, fats, and waxes can be absorbed by some polyethylene; stress cracking can be induced in acrylic by ethanol and other organic vapors. If the substance coming from the object can be identified, then chemical compatibility or chemical resistance charts and databases can be consulted to determine if the product is suitable for the object. (Plastics International, n.d., Cole Parmer, n.d.).

Elemental and Functional Group Types

A major concern is how a plastic will be affected by environmental factors. Because chemical reactivity of the plastic is related to chemical composition, it is reasonable to classify plastics on the basis of their chemical composition, particularly the chemical functional groups in the polymer. Plastics sorted according to their elemental and functional group content are listed in Table 1.

Specific types of reactions characterize each functional group. On the basis of the reactivity of the functional groups in their constituent polymers, the level of stability and type of degradation products produced as they age can be predicted for many plastic-storage products. Polymers containing ester (CHO-type, e.g., CA) and amide (CHON-type, e.g., Nylon) functional groups are polar and therefore induce hygroscopicity into a plastic so that these plastics absorb atmospheric water found in high RH environments. High moisture content promotes hydrolysis, a chemical reaction by which ester bonds break to produce acids and other products. Hydrocarbons such as PE, PP, and PS, which contain only carbon and hydrogen (CH-type), are nonpolar, not hygroscopic, not sensitive to water, and do not hydrolyse. Some functional groups such as C=C double bonds and carbons at back- bone branch points, frequently found in hydrocarbons, are susceptible to oxidation by atmospheric oxygen causing the plastic to become discolored and embrittled. For example, cellulose acetate (CA) ester functional groups hydrolyse to produce cellulose and volatile acetic acid. Acetic acid can diffuse through the storage space and react with organic materials and metals in collections. The reversion of the CA to cellulose and the exudation of additives, especially plasticizers, which may no longer be compatible, results in sticky surfaces and volume change that eventually causes cracking of the object. PAs and some PURs react in a similar way.

Similar assessments can be made for the other groups in the classification scheme. For instance, sulfur gases tarnish silver and chlorides corrode copper. So, the sulfur-(S) containing plastics (CHS- and CHOS-type) in products like the sulfur-vulcanized rubbers found in some gaskets, rubber mats, or carpeting should be avoided when storing silver. Chlorine-containing plastics (CHX-type) such as PVC used in gaskets or pad- ding on wires and rods should be avoided around copper alloys. Other generalizations can be made as shown by the hazards listed in Table 6.

Damage Potential for Plastic Storage Products

Plastic products can be ranked for suitability for collection storage based on the likelihood or potential for the specific plastic product to damage collection items. Table 3 lists the sensitivity of each class of plastic to degradation by thermal oxidation, hydrolysis, and visible light and UV radiation. Sensitivity is also a measure of longevity of the plastic. This table does not list what form the degradation will take. As described previously, if degradation occurs then the plastic is hazardous due to outgassing, exudation, or loss of properties. These hazards are not produced equally for all plastics or for all environmental factors. Different plastics have different levels of severity of effect or potential for damage for each of the three types of hazard.

Two different plastics exposed to the same conditions are likely to change at different rates, and produce different degradation products with different potentials to damage objects. These plastics have different damage potentials. For example, when exposed to the same environmental conditions, PE might yellow and embrittle but would not produce much volatile material, whereas CA might produce acetic acid but a lesser amount of discoloration and embrittlement. Offgassing acetic acid causes more damage to more types of objects in a collection than does discoloration and embrittlement, so acetic acid is a greater hazard and, therefore, CA is a more hazardous plastic than PE.

Definitions or descriptions and examples for three levels of damage potential based on the extent of damage caused to objects are given in Table 5.

Damage potentials for specific plastics are assigned by estimating the combined effects of the following:

1. Initial hazard due to chemical and physical properties inherent in the plastic before aging
2. Sensitivity of the specific plastic to degradation by environmental factors and subsequent production of degradation products (rate of degradation)
3. Quantity of degradation products produced (extent and type of degradation)
4. Severity of the damage to collections attributable to changes in the plastic

From this data, the damage potentials due to inherent properties or to degradation by environmental factors for plastics most commonly used for storage products are compiled and presented in Table 6.

Table 6 Notes

Assessing Suitability for Collection Storage

To find a suitable plastic storage product for an object, the susceptibility to damage from offgassing, exudation, and loss of function of a storage product must be deter- mined. By consulting Table 6, plastics with low damage potential for outgassing can be selected.

If a plastic storage product is on hand, one can determine whether it is safe to use for a specific object/material by combining one’s knowledge of the susceptibility of the object to hazards with the damage potentials for each hazard. For example, there is a slight to moderate potential for PE bags to break but only slight potential for them to offgas or exude; hence, storing small beads in such bags is moderately risky because of the possibility of loss of beads when the bag breaks but only slightly risky because of reaction with material offgassed or exuded by the PE. Some food-grade PE may be manufactured with butylated hydroxytolulene (BHT), which can leach out over time and has a tendency to cause PE to yellow. For most applications, yellowing causes no damage to objects, although it may obscure viewing of objects within PE bags and containers. However, in some situations, objects can absorb BHT. It can then react with atmospheric nitrogen oxides to create colored reaction products on the object. When purchasing PE for use in museum storage, be sure to use a trusted supplier.

In general, those storage products that are made of plastics listed in Table 3 with sensitivities of 1 and in Table 6 with damage potentials of 1 are most likely to be suitable for collection storage. Because there are thousands of grades in each plastic class, these classifications of sensitivity and damage potential are general guidelines and some products will not perform as predicted. Storage must be monitored to locate failures. Problems must be published so that confidence in our selections can be maintained.

Conclusion

The use of plastic storage products for collections entails some risks. Selecting suitable plastic products involves assessing degradation susceptibility of the plastic in the storage environment coupled with assessing the types and severity of hazards to objects posed by the initial composition, evolved degradation products, and changes in performance of the degraded product. The most common hazards are offgassing, exudation, and loss of function, caused by intrinsic composition and chemical degradation.

A plastic may be degraded by oxygen, moisture, and light, or a combination of these factors. The sensitivity (susceptibility) of each plastic class to degradation differs for each environmental factor and varies from plastic to plastic. Table 3 shows the sensitivity of each plastic, ranked into three levels of sensitivity, for each environmental factor, but does not show the type of hazard that will be produced.

The potential for damage by each of these hazards depends on the type and the amount (intensity) of the hazard that is produced. Table 6 shows this information, ex- pressed as the damage potential, for each of the hazards produced for each plastic type. The environmental factor that creates the hazard is also indicated.

The actual damage caused to a collection depends on the initial composition of plastic (Table 1, manufacturer’s data), environmental factors that may be present to cause degradation (RH, light, oxygen) (Table 3), the reactivity of degradation products with different types of objects (sensitivity of objects), and the type and proportion of different objects in the collection (composition of collection).

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