Storage Materials: Plastics

From SPNHC Wiki
Jump to: navigation, search
This content is excerpted from Preventive Conservation: Collection Storage (2019), Elkin and Norris, eds.

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.

Error creating thumbnail: Unable to save thumbnail to destination



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).

Error creating thumbnail: Unable to save thumbnail to destination

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.

Error creating thumbnail: Unable to save thumbnail to destination

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.

SIDEBAR 1 - How to Interpret the Data in Table 3

Table 3 lists data for plastics that have been used for storage applications. Some of these plastics are no longer recommended for storage applications.

The data has been collected from many sources and has been “normalized” to three levels of sensitivity (low, medium, and high) for exposures corresponding to daylight exposure through glass or artificial lighting with more than recommended UV content. The table lists the light and UV-radiation sensitivities of plastics that are commonly used for storage products. Many data for light and UV radiation susceptibility are obtained from accelerated aging experiments at conditions that are more extreme than experienced in museums. This data has been normalized to museum conditions by comparing the accelerated aging data to natural aging of museum and consumer objects in ambient conditions similar to typical storage room conditions of 300 lux × 60 hours per week. Where normalization leaves doubt, the sensitivity has been listed at the higher value.

Many assumptions were made, with the biggest being that the order of sensitivity under extreme conditions will be the same as for exposure under moderate conditions. For example, if after aging outdoors in Florida, the order of sensitivity from high to low is A B C D and in another experiment using daylight under glass the order is A B E C, then the overall order is listed as A B E C D. This ranking has been tempered by observations and reports of light damage to objects in museums, such as

  1. What occurs under folds
  2. What differences are seen between exterior and interior surfaces
  3. What differences are observed when labels are removed
  4. What differences are observed between lighted and shadowed areas on the same object or group of identical objects in different lighting environments


Another confounding factor is that most of the light and UV sensitivity is probably due to photo- oxidation. In general, the ranking of sensitivity under light and UV is the same as under thermal oxidation, and this may be the reason. It is generally impossible to separate the phenomena unless objects kept in the dark are compared to objects in the light. It may also be true that most of what is ascribed to thermal oxidation could be photo oxidation, using the same argument. Nevertheless, there are separate columns for thermal oxidation and light and UV because these are dealt with by different strategies within the museum, or lead to different choices depending on how a problem is approached. These arguments lead to confidence that the sensitivities are listed in the correct order, but they might be a bit too severe – some plastics at the border between 2 and 3 are probably listed as 3 when they are actually 2; that is, under museum conditions, the plastic would not be as sensitive as listed in the table.

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.

SIDEBAR 2 - Photodegradable, Biodegradable, and Compostable Plastic Products

Biodegradable, compostable, photodegradable, and bio-based plastic products are made with polymers that are inherently susceptible to oxidation, hydrolysis, and photochemical reactions, are food for microorganisms, or have additives like photosensitizers that build susceptibility into normally unreactive polymers. Because we want long lifetimes for our storage products, such degradable plastics are not good choices for long-term preservation applications because they do not have the inertness and longevity required. An example of a very poor choice is some packaging peanuts made from starch or starch derivatives that form a sticky mass when contacted by water and provide a food source for insects – properties that are very hazardous to collections.

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).

SIDEBAR 3 - Fibrous, Porous, and Monolithic Plastic Films and Sheets

Plastic films (also called membranes or sheets) can be described in terms of their permeability and porosity as monolithic, permeable, and porous. Permeability is the property of a material to transmit gases and liquids by passing into one surface and out at another surface by diffusion and sorption processes within the bulk of the material. Porosity is the property of a material to allow the passage of gases, liquids and solids in through one surface and out at another surface via very fine continuous holes without sorbing or diffusing in the material. Permeability and porosity depend on the thickness of the film. Porosity depends on the pore sizes, the pore size distribution, and the number of tortuous paths of interconnecting micropores through the thickness of the film.

Monolithic films are nonporous, solid polymer films that allow the passage (or not) of gases due to the permeability (or lack of permeability) of the plastic to the gas. Porous films have holes of varying sizes ranging from microporous (submicrometer) to macroporous (supramicrometer). Microporous films will pass gases but not liquid (e.g., water vapor but not liquid water, as in waterproof breathable fabrics). Pores can be created from monolith films by punching holes, stretching monolithic films containing particulate fillers until tiny fissures occur where plastic separates from the filler particles, or by creating a film from fibers by weaving, matting, or entanglement of molten fibers in various processes to make nonwoven fabrics, with pores between the fibers.

Plastics films or membranes are frequently used as gas barriers to water vapor or oxygen (anoxic storage), as liquid barriers (separators between object and wet poultices during cleaning or relaxing treatments), as dust and water barriers in the form of covers and drapes, and as interleaves between sticky components (inks on prints or pages, oily accretions on objects). Their function in these applications depends critically on their permeability or porosity. In addition to the hazards posed by the plastics, the permeability and porosity of the films must be considered when selecting film products for these applications. Permeability of a plastic for different gases varies with the type of gas. A plastic film that may have low permeability for water may have high permeability for oxygen, and vice versa for a different plastic (Burke 1996).

Another factor to consider is the fibrous nature of some sheets. If the object has protrusions that can be snagged or surface accretions that could flow into interstices between fibers of a sheet, then a nonfibrous film should be chosen.



Williams Table 04.png

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.

Error creating thumbnail: Unable to save thumbnail to destination



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.

SIDEBAR 4 - Colored Plastics

Plastics are colored by the addition of pigments, dispersed as particles in opaque plastics, or dyes dissolved at the molecular level in transparent plastics. Pigment particles are insoluble in the plastic. Both the pigment particles and dye molecules are completely encapsulated by the plastic, even very near the surface of the plastic. Pigments and dyes can only bleed or be extracted from the plastic by a solvent that dissolves the plastic. Because water is not a solvent for plastics, pigments and dyes cannot bleed from the plastic when exposed to water. Therefore, bleeding of color from plastics will not occur even during a flood. However, bleeding of pigments and dyes may happen during nonaqueous solvent exposure if the solvent dissolves the plastic. In storage, this could happen if solvent-based cleaners are used and come in contact with plastics.

Colored material may be transferred from a plastic to an object in contact if abrasion occurs. If degradation of the plastic is manifested as a weakened surface, or even as a chalky mate- rial on the surface, then loose colored material may be transferred. This is not bleeding of color by solvent action but mechanical transfer by mechanical processes such as abrasion.

Aside from providing aesthetic qualities of color, pigments are also added as UV absorbers and blockers. For example, carbon black is a strong absorber of UV radiation. UV radiation that falls on a plastic object with carbon black is absorbed very near the surface of the plastic and does not penetrate the body of the plastic. Photochemical reactions, if they occur, occur only at the surface, and the plastic is protected against photochemical reaction in the body of the plastic. This greatly increases the longevity of the plastic. When selecting plastic products, if transparency is not required, such as to enable viewing of contents then opaque plastic may be a wise choice.

Williams Table 06.png

Table 6 Notes

1. Some volatile antioxidants such as butylated hydroxytoluene (BHT) can be emitted from some PE products. The BHT can be absorbed by objects and then react with atmospheric nitrogen oxides to create colored reaction products on the object causing discoloration of the object.
2. Aged PE foam has been noted to produce acrid odors and to become brittle, probably due to production of a variety of aliphatic aldehyde, ketones, and carboxylic acids by oxidation of the PE matrix. Chemical damage to artifacts attributed to the degradation products has not been reported. Embrittlement and loss of strength of foams usually accompanied by complete loss of support and cushioning function has been reported (Williams 2002a).
3. Polyolefins like PE and PP usually contain so-called slip agents or antiblocking agents, typically composed of alkyl amides like oleamide and ethylene bisstearamide, which are present in thin, sometimes only monomolecular, layers on the plastic surface and may be transferred to objects. In general, very little material if any is transferred. Wilhelm and Brower (1993) show this to be not a problem for photographs. Slip agents have been implicated for tarnishing of mirror bright polished silver and copper alloy surfaces. Glycerol monostearate cell-size-control agents in PE foam have been implicated as one of the causes of contact corrosion during Oddy tests where foam was in contact with copper at 60°C and 100% RH.
4. PE and PP are not susceptible to hydrolysis but are susceptible to oxidation especially in the presence of light and UV, usually causing yellowing and embrittlement with decreased strength. Oxidation is much more pronounced for LDPE than for HDPE because of greater branching. For most PE and PP applications, yellowing causes no damage to objects, although it may obscure viewing of objects in containers. Embrittlement may cause PE and PP to break and thus be a risk to objects through physical failure of support. Containers for fluid-preserved collections could fail resulting in loss of preservative and desiccation or rotting of specimens.
5. There are differences in the aging behavior of the various PS polymers and copolymers, but in general, these differences are not sufficient to assign different copolymers to different damage potentials.
6. Claims of hazards from offgassing of residual unreacted monomers from plastics are exaggerated. Modern manufacturing practices and regulations indicate that residual monomer concentration will be low. Furthermore, since monomers tend to be volatile, most residual monomer will be gone by the time a consumer uses the product. In the case of PS, the residual monomer styrene is a volatile hydrocarbon. It is unlikely to react with any object. Claims that offgassed styrene will dissolve, soften, or swell varnishes and other coatings are unfounded for the amount of styrene that could possibly be emitted by modern PS storage products.
7. PS is very chemically stable at room temperature. PS objects stored in museums for decades continue to show glossy surfaces and no distortion, both of which are indicators of chemical stability. Gloss is usually one of the first properties to change when chemical deterioration occurs. Distortion occurs when there are changes of volume due to loss of material or physical changes such as crystallization.
8. Some grades of PMMA and PS are susceptible to environmental stress cracking and may be affected by cleaning agents. This information is readily available from the manufacturers and suppliers.
9. Extensive studies for photographic and microlithographic computer chip manufacturing applications show that PET has great chemical and dimensional stability.
10. Odors of vinegar (acetic acid from cellulose acetate) and vomit or strong cheese (butyric acid from cellulose butyrate) are frequently detected and indicate hydrolytic degradation of CA and CB.
11. Some regenerated cellulose films contain plasticizers, usually substances that absorb water (humectants), to impart flexibility to the film. These humectants can migrate. Fibers like rayon do not have humectants.
12. The greatest hazard from PVC is transfer of plasticizer. Rigid PVC has no plasticizer so does not have this hazard. PVC that is made flexible by copolymerization with, for example, PVAC, which attaches directly to the polymer backbone, does not have volatile or mobile plasticizers, and does not suffer from exudation of plasticizers. When exposed to UV or high temperatures, PVC degrades by dehydrochlorination to produce hydrochloric acid. All PVC products contain stabilizers to prevent the dehydrochlorination reaction when the polymer is processed into products. Some stabilizers are consumed as they perform their stabilizing function and eventually become exhausted, leaving the plastic unprotected and liable to dehydrochlorination. It is unclear if unplasticized rigid PVC will degrade and produce HCl at room temperature in the absence of UV. Several rigid PVC foam boards pass the Oddy test.
13. Some PVDC films contain plasticizers that exude.
14. PVDF is an engineering plastic, seldom found in consumer products, except as a tough, stain-resistant coating, such as on some specialty high-pressure laminate sheets (Arborite, Formica) and aluminum sandwich panels(aluminum/PE/aluminum laminates such as Alucobond and Dibond).
15. Susceptible to photo degradation with loss of strength (bad for nylon strings suspending objects in lit display cases). Although susceptible to hydrolysis, nylon is extensively used for ship hawsers and fishing nets, which have shown long useful lifetimes in sunlit wet conditions.
16. There is a great variety in type, grades, and composition of PUR, so one term cannot be used to designate all. The term “consumer grade” is used to distinguish the commonly available grades (especially foams) from less commonly available special engineering grades manufactured to tighter and more well-defined engineering specifications. Data from manufacturers must be consulted to ensure that engineering plastics have appropriate values for properties of concern in conservation.
17. Ester groups in ester-type PUR are susceptible to hydrolysis at high RH and in water, but are not so susceptible to oxidation. Adipic acid crystals have been observed in aged ester-type PUR. PUR with aromatic groups are susceptible to UV and light degradation but those with only aliphatic groups are not.
18. Ether groups in ether-type PUR are susceptible to oxidation leading to degradation of PUR often producing a sticky mass.
19. Most consumer-grade RTV silicone cures by condensation polymerization, which releases a small volatile substance like acetic acid, methanol, or amines. Acetic acid vapors are hazardous (Hazard 2), amine vapors may be hazardous (Hazard 1-2), and methanol vapors are probably innocuous (Hazard 1).
20. The softness (“durometer”) of some silicones is achieved by addition of silicone oils. These exude and transfer to objects causing staining, but no chemical reaction (Hazard 2). Softness can also be controlled by polymerization conditions without the need for silicone oils (Hazard 1).
21. Room-temperature vulcanized (RTV) silicones that cure by release of acetic acid tend to have much greater longevity (some silicone caulks have 50-year warranties). Those that release amines or methanol tend to have much short lifetimes, often losing strength and becoming yellow.
22. The term “manufactured product” is used to designate solid products, such as tubing, gaskets, and cookware, made in factories under controlled conditions and distinct from RTV silicone products applied in the field as liquids or gels and polymerized in place under uncontrolled temperature and RH conditions.

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.

SIDEBAR 5 - Plastic Foams

Plastic foams (or cellular plastics), come in two main forms – closed-cell and open-cell. In closed-cell foams, such as Ethafoam, gas is contained in discrete cells within the matrix polymer and cannot pass from one cell to another, except by slow diffusion through the cell walls. Closed-cell foams are considered gas barriers. In open-cell foams, such as many PUR cushion foams, the walls between adjacent cells are open. Gases and liquids can pass through the foam via the open cell walls. Open-cell foams have high gas permeability and are often used in filter applications. Both types can be used in cushioning applications in storage though not all foams are appropriate for use with collections (Williams, 2002b). The cushioning function of open-cell foam depends on the resilience of the matrix poly- mer; there is a damping effect due to restricted flow of gas from one cell to the next. In closed- cell foams, cushioning is dependent on the compression of the gas within the individual cells. When the cells of a closed-cell foam burst, such as by a sufficiently large impact, the cushioning ability due to compression of gases in discrete cells is no longer present. For some more rigid foams, like PS, crushing of the polymer matrix during the impact dissipates impact forces. These foams do not return to original shape after crushing.

Most PE and PP foams encountered in collection storage, such as Ethafoam, Volara, and Propazote, are closed cell. The most commonly encountered open-cell foams are consumer-type polyether-type polyurethanes (PUR) such as those used in cushions. These are not recommended for storage applications because they are subject to oxidative degradation. Engineering grades of PUR foams have much better properties and may be suitable for use, but their proper- ties must be known and tested for suitability for long-term use in collection storage.

Cellular plastics (foamed plastics) are formed by causing bubbles (cells) of gas to form in molten plastic, which are then frozen in place by cooling the foam. The substances used to create the bubbles are gases called blowing agents, which eventually diffuse out of the foam to be replaced by air. Foams with hydrocarbon blowing agents are stored in ventilated conditions for a time after manufacture to avoid hazardous conditions produced by offgassing of flammable gas into a storage area.

Chemical foaming agents (also called blowing agents) are substances added to the molten plastic that decompose or react with other substances when heated (thermally foamed) to produce a gas and a solid residue. Chemical blowing agents can leave residues that are hazardous to collections. Physical foaming agents are gases or low-boiling liquids that evaporate to create bubbles. Mechanical foaming, where air bubbles are created by whipping or beating, can also be used. Physical and mechanical foaming leave no solid residues that are hazardous to collections. In order to find out how a foam is produced, consult the product’s technical literature.

Foams can have different cell sizes ranging in diameter from a few millimeters to a less than a tenth of a millimeter (often described as microcellular). Cutting a large cell foam can result in rough edges; these should be covered with a smooth microcellular foam, such as Volara, to avoid snagging of fibrous objects.

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).

References


AIC Wiki. 2015. Oddy Tests: Materials Databases. Accessed September 14, 2017.

ASTM 2007. ASTM Designation E595-07. Standard Test Method for Total Mass Loss and Collected Volatile Condensable Materials from Outgassing in a Vacuum Environment. West Conshohocken, PA: ASTM International.

ASTM. 2012. ASTM Designation: D883 – 12. Standard Terminology Relating to Plastics. West Conshohocken, PA: ASTM International.

ASTM. 2014. ASTM Designation: D1600-14. Standard Terminology for Abbreviated Terms Relating to Plas- tics. West Conshohocken, PA: ASTM International.

Baker, Mary T. 1995. “Synthetic Polymers.” In Storage of Natural History Collections: A Preventive Conservation Approach, edited by Caroline L. Rose,Catharine A. Hawks, Hugh H. Genoways, 305–323. New York: Society of the Preservation of Natural History Collections.

British Museum. 2015. “Database of Materials Test Results (Oddy Test Results).” Accessed October 20, 2015.

Burke, John. 1996. “Anoxic Microenvironments: A Simple Guide.” SPNHC Leaflets, Volume 1, Number 1, Spring. Society for the Preservation of Natural History Collections. Accessed November 7, 2015.

Cole Parmer. n.d. “Chemical Compatibility Database”. Accessed 22 March 2019.

Crompton, T. R. 2007. Additive Migration from Plastics into Foods - A Guide for Analytical Chemists. Shrewsbury, UK: Smithers Rapra Technology Limited.

ESA (European Space Agency). “Outgassing Database.” Accessed October 30, 2015.

Fenn, Julia, and Williams, R. Scott. 2017. “Caring for Plastics and Rubbers.” Accessed June 21, 2017.

Hiorns, R. C., R. J. Boucher, R. Duhlev, K.-H. Hellwich, P. Hodge, A. D. Jenkins, R. G. Jones, J. Kahovec, G. Moad, C. K. Ober, D. W. Smith,R. F. T. Stepto, J.-P. Vairon, and J. Vohlídal. 2012. A Brief Guide to Polymer Nomenclature. Version 1.1. International Union of Pure and Applied Chemistry. Accessed November 2, 2015.

Horie, C. V. 2010. Materials for Conservation: Organic consolidants, adhesives and coatings. Butterworths Series in Conservation and Museology. 2nd Edition. London: Butterworths & Co. (Publishers) Ltd.

Image Permanence Institute. 2015. Photographic Activity Test (PAT). Rochester Institute of Technology. Accessed October 30, 2015.

Jenkins, D. P. Kratochvfl, R. F. T. Stepto, and U. W. Suter . 1996. Glossary of basic terms in polymer science (IUPAC Recommendations 1996). International Union of Pure and Applied Chemistry. Accessed November 2, 2015.

Mills, John S., and Raymond White. 1987. The Organic Chemistry of Museum Objects. London: Butterworths & Co. (Publishers) Ltd.

NASA. 2008. “Outgassing Data for Selecting Spacecraft Materials Online.” Accessed October 30, 2015.

National Archives of Australia. 2015. “About the Photographic Activity Test.” Accessed October 30, 2015.

Plastics International. n.d. Chemical Resistance Chart. Accessed 22 March 2019.

POPART. n.d. “POPART Project – the Preservation Of plastic ARTefacts in Museum Collections". Accessed June 17, 2017.

Pratt, Scott. 2011. “Understanding a Fog Testing System.” Application Note ANTCFOGTEST0311. Newington, NH: Thermo Fisher Scientific. Accessed October 30, 2015.

Rogers Corporation. 2003.“PORON 4701-30 Very Soft.” Publication #17-006. Accessed October 30,2015.

Rogers Corporation. 2015. “PORON Microcellular Urethanes for Gasketing and Sealing.” Accessed October 30, 2015. .

Shashoua, Yvonne. 2008. Conservation of Plastics: Materials Science, Degradation and Preservation. Oxford: Butterworth-Heinemann.

Thickett, D. and L. R. Lee. 2004. “Selection of Materials for the Storage or Display of Museum Objects (Oddy Test).” New and completely revised edition. British Museum Occasional Paper 111. London: The Trustees of the British Museum. Accessed October 30, 2015.

Wikipedia. s.vv. “Polymer Degradation.” Last modified September 13, 2017.

Wiles, David M. 1993. “Changes in Polymeric Materials with Time.” In Saving the Twentieth Century: The Conservation of Modern Materials, edited by David W. Grattan, 105–112. Ottawa: Canadian Conservation Institute.

Wilhelm, Henry, and Brower, Carol. 1993. The Permanence and Care of Color Photographs: Traditional and Digital Color Prints, Color Negatives, Slides, and Motion Pictures. Grinell, IA: Preservation Publishing Company.

Williams, R. Scott. 1993. “Composition Implications of Plastic Artifacts: A Survey of Additives and Their Effects on the Longevity of Plastics.” In Saving the Twentieth Century: The Conservation of Modern Materials, edited by David W. Grattan,135–152. Ottawa: Canadian Conservation Institute.

Williams. R. Scott. 2002a. “Care of Plastics: Malignant plastics.” WAAC Newsletter. 24 (1): 10–15. Accessed September 14, 2017.

Williams, R. Scott. 2002b. “Polyolefin foams.” AIC News 27 (1): 26–29, 31–33.

Online Resources

Ch53 Plastics 884-885 (2).pdf

Coughlin, Mary, 2019. Plastics In Preventive Conservation: Collection Storage, Lisa Elkin and Christopher Norris,eds. Society for the Preservation of Natural History Collections, American Institute for Conservation, Smithsonian Institution, George Washington University. pp. 884-885.