Practical Aspects of Plastics Found in Archives

Practical Aspects of Plastics Found in Archives

by ALAN CALMES

Introduction

At the end of the nineteenth century, cellulose nitrate, as a filler for pyroxlin bindings and as a transparent substrate for photographic film, marked the advent of human-made plastics as an information carrier. During the first half of the twentieth century, vinyl, in the form of phonograph records, replaced wax cylinders and joined film as a plastic material for holding information. Since 1950, more and more plastics have been used in record-keeping practices. Sound recordings, for example, have gone through a number of formats with different plastic materials, from physically embossed grooves on Dictaphone® belts to magnetically charged particles on tape and, more recently, to laminated laser-produced digitally encoded disk recordings. Each involves a complex combination of plastics.

The great variety of plastics began to significantly affect archives by the mid-twentieth century. The desire to have quick copies of paper documents led to the development of a variety of wet and dry and sometimes heat-processed coated papers during the 1940s and 1950s. Some of these processes involved plastics. Beginning in 1960, the plain paper electrostatic photocopier produced an image using copolymcrs mixed with carbon black and fused to the paper surface. Early conservation methods used plastics for the lamination of fragile documents. The document, tissue paper and sheets of cellulose acetate were heat-pressed into a melt. This method of preservation was developed at the Virginia State Library and the US National Archives in the 1940s. Plastic enclosures and boxes have been used in archives, and plastic parts are found in information-recording equipment. Additionally, paints containing plastics have been used on surfaces of shelves and on containers, and plastic adhesives have been used to hold boxes and envelopes together.

Records are usually 20-30 years old by the time they reach the care of an archivist. Before records are transferred to an archives, they may have been stored in harsh environmental conditions and subjected to rough handling. Archival materials made of plastic often arrive in need of special attention.

For example, cellulose nitrate film may arrive in an advanced state of deterioration and be highly flammable and in need of special handling, packaging and storage; Dictaphone belts may arrive cracked and broken. An archivist needs to know the aging characteristics of plastics, conservation measures suitable for plastics, and copying techniques.

Some of the most important historical information of the second half of the twentieth century will require special conservation and duplication efforts to preserve the memory of humanity. A partial list of important events recorded on plastics would include: political debates and presidential addresses and news conferences on radio and television; satellite mapping and environmental observations; and motion pictures of historic events. An example of valuable information on a plastic medium is the sound-recording of the Nuremberg trials, which were recorded on a long loop of cellulose acetate film. The embossed grooves and bumps have near])' disappeared, as the plastic material has gradually returned to its pre-embossed smooth stale. A special machine had to be built with a special tracking stylus to play the loop.

Definitions

Plastics are modifications of naturally occurring, complex organic compounds or synthetic polymers, usually consisting of long chains, produced by polymerization. A polymer consists of linked units called monomers. Polymerization is the uniting of many monomers into a single polymer molecule. Paper, which is a naturally produced polymer, is excluded from this discussion, as are other polymers found in nature, such as wool, parchment or natural resins. Rubber bands, made of natural and/or synthetic rubber, must be mentioned, however, since their deterioration has been especially noted in archives.

The majority of plastics found in archives are thermoplastic materials, such as:

• cellulose nitrate

• cellulose acetate

• polycarbonate

• polymethyl methacrylate (PMMA or acrylic)

• nylon

• polyvinyl chloride (PVG)

• polystyrene

• polyethylene terephthalate (PETP or polyester) • polyolefms, such as polypropylene

and some thermosetting materials, such as:

• polyurethane •epoxy

• melamine

and phcnolics, such as phenol-formaldehyde and Bakelite.

Examples

Examples of plastics found in archives arc:

• cellulose nitrate film

• various cellulose acetate films

• vinyl phonograph disks

• polyester encapsulation

• polyoiefin shrink-wrapping

• polyethylene boxes and envelopes

• polyurethane binders on magentic tape

• acrylic sheets and blocks used in exhibits

• epoxy adhesives and coatings

• nylon gears.

Many product names are used by nonspecialists as generic names of plastics, such as DuPont's Mylar for poly(ethylene terephthalate), Rohm & Haas' Plcxiglas for poly (methyl methacrylate) and General Electric's Lexan for polycarbonate.

Additives

Plastic are seldom used in a pure state. Polyester used for encapsulation is generally described as a simple polyester, but even it may contain some byproducts left over from the manufacturing process, such as lubricants and some silica compounds to prevent blocking. Polypropylene and polyethylene have antioxidants added so that they can be melted and formed into sheets or poured into molds without undergoing oxidation. Additives increase the complex nature of plastic materials. Lubricants in magnetic tape, for example, can ooze out of the tape onto its surface. The reading-head of a tape-drive will collect the oozed-out lubricants and can cause the reading-head to cither fail

to read the data or gouge into the surface of the magnetic tape, and destroy the recorded information.

Since plastics may burn, melt and spread a fire rapidly, flame-retardants

arc sometimes added to plastic products. Sonic local fire codes require the addition of flame-retardants in plastic materials found in household and office furniture and, therefore, indirectly in plastic items that might be found in abundance inside any building. Some flame-retardants, however, can evaporate in small quantities and affect materials in contact with them. Some halogen:,, for example, may produce, oxidants that can react with the silver halides of photographic film.

Changes in Composition

Plastics have evolved rapidly since 1950. Chemical formulas have been replaced or modified to achieve desired results. New applications or improved materials brought about the obsolescence of one form of plastic in favor of another. As a result, it is difficult to identify old plastics found in archives without conducting laboratory tests.

Manufacturing processes have changed. Most plastics originally served more immediate needs and were not designed for long life. There was almost an assumption in our society that plastic products were disposable, and if one desired continued use of the product, it would have to be replaced by a new and better one. This philosophy is changing, as manufactures are beginning to produce engineering plastic, with substantial durability and environmental resistance. Paradoxically, there are now environmental concerns that plastics are here forever; this fear has led to the development of biodegradable plastics for throwaway products.

The need to periodically duplicate information on plastic media extends to all formats: sound recordings, video recordings, microfilm, motion picture film, and computer tapes. How often this is done depends on how the medium is stored and handled and on the characteristics of the plastics used.

Physical. Characteristics

Plastics can be rigid or flexible, soft or hard. Plastics can he molded to almost any shape. During manufacturing processes, plastics are malleable and can be stretched as well as molded. These attributes, convenient for making any shaped item, can cause problems later. With sufficient heat, plastics can return to the malleable state and change shape. When stretched during a forming process, plastics can be made into thin films; however, the polymer will con-

tinue to have a memory of an earlier state, and try to revert back to a previous condition. Many plastic films will relax back to their umtretched dimensions if they are heated even briefly above a transition temperature that varies from one plastic to another.

Plastics can be made with such a smooth surface that, when placed in contact with another smooth surface, such as a photograph, pressed and then removed, the photograph will he left with a shiny surface. This process is sometimes called ferrotyping (a term borrowed from photography, referring to the process of transferring an image directly onto the smooth surface of a specially prepared iron plate). Such plastic sheets should not be used in albums. Another deleterious process, offsetting, is when plastics adhere to the ink and (oner of paper documents or to the imaging layer of photographic prints. PVC plastic sheets have this quality because of its plasticizer (dioctyl phthalate), which acts like a solvent in dissolving' and attracting the copolymcr ingredients of (oners and dyes used in electrostatic copies and photographic images. Fur-

thermore, PVC should not he used lor albums or interfiled with papers because

itis an unstable plastic that dehydrochlorinates to produce hydrogen chloride (HCl) and, with a little water, hydrochloric acid.

Plastieizers are solutes placed in such plastics as PVC, which arc normally glass-like, to make them soft and pliable. Such materials are not chemically bound to the polymer and, over time, may come out of solution and be found on the, surface of the plastic from which they might evaporate if heated or be rubbed off. Eventually, plastics that arc plasticized dry out, shrink and crack, like the dashboard of a car. Other additives, such as oils, lubricants, antioxi-ants and cyanamides may also ooze out onto the surface of the plastic material. The white powder found on the surfaces of old films and tapes consists of a number of additives that have come out of solution in the plastic support and/ or recording layer.

Aging Characteristics

Cellulose nitrate

Originally, almost all black-and-white 35-mm motion picture film was on cellulose nitrate film. It was used exclusively for studio work from the 1920s to the 1950s. Cellulose nitrate was not used for color film, or for 16-mm and 8-mm home-movie film.

When ignited, cellulose nitrate burns very rapidly. Nitrate film decomposes with the emission of nitrogen oxides. The reactions are highly exothermic an

are responsible for the self-ignition of cellulose nitrate. Despite the hazards,

commercial film makers preferred cellulose nitrate film over cellulose acetate safety film because it was easy to handle and produced a very clear, sharp image when projected. By 1950, however, after many theater fires, fire codes mandated the use ofsatefy film. During the past 10 years, after several devastating and dangerous cellulose nitrate film vault explosions and fires, archives and libraries have copied most cellulose nitrate film images onto safety film (a cellulose acetate or polyester film) and disposed of the cellulose nitrate film. There remains, however, some spliced-in cellulose nitrate film within reels of safety film. Nitrate-based film stock can be identified by feel; it is softer and more supple than cellulose acetate or polyester film. When degrading, its appearance may be deceiving. It is safer to have a laboratory test it.

The aging of cellulose nitrate is characterized by rapid change once the deterioration process begins. Prior to the onset of deterioration, there i.s no serious shrinkage of the material, image quality is good, and the images can be copied easily. 1 he kinetics of the reaction are such that there appears to be virtually no intermediate stage between the time when the film is in good condition and the time when it is obviously deteriorated. Archivists have seen reels of cellulose nitrate film change from excellent to extremely poor condition in 2 months. When it deteriorates, cellulose nitrate film produces sticky, brownish, powdery and fibrous globs.

Gases emitted from deteriorating cellulose nitrate film can initiate the process of deterioration in neighboring films of all types. Chemically, cellulose nitrate, upon decomposition, produces its own oxidizer and, therefore, once the chemical bonds begin to break down, a rapid autocatalytic reaction sets in. The reaction produces its own heat, which accelerates the initial breakdown and the process can be so fast as to produce fire and, if film is tightly compacted, an explosion. Once degradation of a cellulose nitrate film has been spotted, the achivisl must move quickly to copy the images onto safety film and to dispose of the cellulose nitrate film. The continued usefulness of a reel of cellulose nitrate film depends upon good environmental conditions, such as clean, cool arid dry air.

Cellulose acetate

The same general mechanism of deterioration of cellulose nitrate occurs similarly in other cellulose esters. The same acid hydrolysis occurs in all cellulose materials, but the other esters produce only an acid that catalyzes further acid hydrolysis, not an oxidizer such as nitrogen dioxide in the case of cellulose nitrate. The degradation process of cellulose acetate film takes longer than that

of cellulose nitrate film, but once started, the autocatalytic chemical reaction cannot be stopped. The result of the self-destruction of cellulose acetate film is somewhat different from cellulose nitrate film, in that an intermediate stage of deterioration between a good condition and a powdery condition can be seen. Acetic acid, a product of cellulose acetate degradation, can be detected by its vinegar odor. The presence of acidic gases and particles found in polluted air initiates the degradation process of cellulose acetate film. Unlike nitrate him, however, the image layer is not chemically aflected by the by-products of the decomposition of the acetate substrate.

The first safety-based films were cellulose monoacetale and cellulose diace-tate. The term safety-base was used because acetate films do not burn easily. By the 1970s, triacetate began to replace diacctate as the favored substrate for film.

Manufacturing experience found that the diacctate substrate took diazo salts more readily than the triacetate base and thus the diacctate base was used in diazo films until polyester began to be used in the 1980s.

Researchers at the Image Permanence Institute in Rochester, NY, USA, arc finding little difference between the degradation processes of cellulose diacctate film and cellulose triacetate film. One is not necessarily more stable than the other. The process, however, might take longer with a triacetate than with a diacctate. Within one category there are likely to be variations from one batch to another, depending on formulas, additives and manufacturing processes.1

The long, intermediate stage of deterioration of cellulose acetate film is characterized by shrinkage. When the cellulose acetate film base shrinks, the emulsion layer on top, which does not shrink, is deformed into a mass of wrinkles. Since the image is within the emulsion layer, the image becomes illegible, unless there is a way to copy or transfer the emulsion layer before the wrinkling obscures the image. The choices arc to copy the image before this occurs or to laboriously remove and reapply the image layer after it occurs. The greater the shrinkage, the more difficult it is to recover the information. When motion picture film shrinks, the sprocket holes arc no longer in the right place, making the copying process necessary to save the images difficult and expensive. The shrinkage is uneven; consequently, engineered sprockets with a different spacing may not provide a solution.

Until the late 1950s, cellulose acetate films were used as a very thin base material for sound recording tape for some early video and computer tape. With age - accelerated by elevated temperatures and high relative humidities - thin cellulose acetate tape becomes brittle and breaks easily. Brittleness is an inevitable condition. Plasticizers used during the manufacturing process will ooze out onto the surface of the tape in the form of white droplets that look

like powder. To complicate matters, a recording layer, such as one composed of iron oxides dispersed in polyurethane, has characteristics of degradation different from that of the base material.

A break in cellulose acetate magnetic tape is usually clean, It can be spliced back together again with little loss of information for an analog sound or video recording. This is different from polyester-based magnetic tapes, which stretch considerably before breaking.

Polyester

Poly (ethylene terephthalate), commonly known as polyester or PET, entered the scene in the late 1950s. Because of its strength, even in a very thin film, it was used as the substrate for all computer and video tapes. Soon thereafter, polyester replaced cellulose acetate as the base material for sound recording tape. The switch from cellulose acetate to polyester-based photographic him has been very slow. The demand for a strong, long-lasting microfilm brought about the use of polyester for microfilm in the 1980s, but it coexisted with cellulose triacetate-based microfilm.