How do you assess archaeological objects made of rubber and plastic and decide upon a conservation treatment?

Material Characteristics

Plastics include objects composed of both synthetic and natural materials. All plastics are composed of polymers, which are made up of a large number of smaller molecules called monomers. Polymers can be grouped into three broad categories: 1) Natural (amber, gutta percha, and other resins); 2) Semi-synthetic (cellulose nitrate, cellulose acetate) and 3) Synthetic (polyethylene, polyvinyl chloride, and polyester). Most of the plastics manufactured today are made from synthetic polymers, but examples of semi-synthetic objects still exist. Plastics can be grouped into two categories based on how they were manufactured. Thermosetting materials are heated and stay in a fixed shape even after heating. Thermoplastic materials are heated, become firm when they are cold, and soften again upon heating. Additives to the polymers help to form the final shape and characteristic of the plastic. Fillers, colorants, fibers, anti-oxidants, and dyes are all added to modify the plastic and obtain the desired result.

Plastics appear in the archaeological record in a number of forms ranging from toys and personal items to buttons and jewelry. Fragments of rubberized fabric, typically from buckets, tents and ground cloths, are often found in 19th century contexts.

Deterioration:

Although plastics often seem eternal, these polymers break down and deteriorate with age like other materials. Several factors contribute to the decomposition of plastic objects including the environment in which they were used, their composition, and how they were manufactured. Temperature, relative humidity, ultraviolet (UV) radiation and pollutants are a few of the factors that can cause chemical and physical changes in plastics.

High temperatures cause chemical reactions to proceed at a faster rate and may also cause mold spores and fungal colonies to begin to grow; excess heat causes polymer chains to break effecting the mechanical properties of the material

Fluctuations in relative humidity can be harmful to plastics. Large fluctuations in RH can cause materials to crack, split and warp. Excess moisture can cause chemical changes

Ultra violet radiation causes plastic materials to change color (usually manifested as increased yellowing) and will also cause the polymer to cross-link (two or more polymer chains join by a chemical bond) causing the material to become inflexible and brittle, resulting in physical breaks

Pollutants such as sulfur dioxide, nitrogen dioxide and ozone are harmful to plastics and may cause off-gassing of vulnerable materials, leading to their further degradation

Physical damage can result to fragile plastics if dropped, handled improperly or packed improperly for transit or long term storage

The degradation of polymers can result in obvious changes to plastic materials. These changes occur for a variety of reasons and may occur slowly in stages over time or quite dramatically overnight. One indication that plastic materials are degrading is that they often produce a very discernible odor. Cellulose acetate, for example, produces acetic acid upon deterioration, which smells like vinegar. Other materials, such as polyvinyl chloride, produce hydrochloric acid upon deterioration, which has a very sharp and acidic smell. Some other common problems to look for with plastics are listed below:

Many plastics change color indicating they are undergoing a chemical change. The most common form of this change is when clear plastics are exposed to ultra violet radiation causing the items to turn yellow over time. This can occur in a variety of materials, but it is most noticeable in polyvinyl chlorides, nylon and plastic adhesives such as epoxy resins and cyanoacrylates. Other color changes can exist where clear and colored objects become more opaque. Occasionally a white bloom may be seen on the surface of the plastic. This is usually the result of plasticizers or other additives leaching to the surface of the material. This is common with both cellulose acetate and cellulose nitrates.

Decaying cellulose nitrate with active corrosion at the contact points between the metal and plastic

Some plastics may become chemically distorted over time. The appearance of small bubbles, often trapped beneath the surface, may be an indication that degradation has occurred. These bubbles are most often seen in cellulose acetate and cellulose nitrate materials.

Physical damage to plastics can occur in a variety of forms. If the plasticizer off-gasses or leaches from the object over time, softer, more flexible plastics such as rubbers and resins will become inflexible and brittle over time. This can result in physical breaks and cracks. In more rigid plastics such as polystyrene or polymethyl methacrylate, cracks may also be the result of either internal stress in the materials from the manufacturing process or the result of an outside force (i.e. dropping the object).

Crumbling of materials, such as polyurethane foams, is the result of chemical breakdown due to the exposure of oxygen and ozone. Other damage resulting in physical changes may include warping, flaking of layered materials, and fraying of fibers.

Many plastic materials begin to “weep” as they degrade. This term refers to the formation of tiny droplets of liquid on the surface of the materials. The droplets, often acidic in nature, are sticky and oily. They are a result of the loss of the plasticizer in the material and are most commonly associated with polyvinyl chloride, which produces fine droplets of phthalates on the surfaces. These droplets are not only harmful to the material, but can also be harmful to those handling the artifacts.

Treatment

Before a conservator treats an object made of plastic, its condition must be assessed to determine several things. First, the plastic must be correctly identified. Many plastics and rubbers appear to be similar but vary in chemical make-up. It is very important to determine the material prior to attempting to treat the object.

There are several types of analytical equipment available to conservation scientists or polymer chemists that can correctly identify the type of plastic and sometimes even the additives present. Additionally, if there is an unknown substance on the surface it should be analyzed to determine what it is prior to removal.

Plastics are difficult to treat because very little attention has been paid to their long term deterioration. Conservators are just beginning to understand how they age and what treatment is best for their long-term preservation. No single solution exists for each material or type of object. The importance of testing the materials before any treatment is performed cannot be overemphasized.

Cleaning and Stabilization

Minimal cleaning and stabilization is all that is necessary unless the object is needed for an exhibition or is deteriorating and is in jeopardy of total loss. All cleaning methods, no matter how small, should be documented and the materials and techniques used should be recorded and kept with the object. Some cleaning of surfaces can be performed using a museum-quality HEPA (high-efficiency particulate air) filtered vacuum cleaner with variable speed control.

Harder plastics, such as polycarbonate, polystyrene and acrylics can be cleaned using a dry, electrostatic cloth. If surface bloom or leaching is evident on the plastic’s surface, it is advisable not to clean the object and to consult a conservator prior to cleaning. Any wet cleaning undertaken on plastics, including cleaning surfaces with water, should be performed only after consulting a conservator. Some porous plastics will be permanently damaged by water. At no time should solvents, detergents, polishes, scratch cleaners, or waxes be used to clean plastics. Solvents in particular can cause dissolution of the plastics.

Repairing Plastics

There has been little success using adhesives to repair broken plastic objects. A variety of adhesives are available and finding the right adhesive for a particular plastic is much more difficult than it appears. Off-the-shelf adhesives are not suitable for the adhesion of historic plastic objects as the solvents used in them may cause further damage to the plastic. Due to the complexity of plastics therefore, a conservator should perform any mending that is needed.

Storage:

Due to the lack of suitable treatment methods available for plastics, the environment in which the object is stored and exhibited is of primary importance. Some simple rules can be applied to all plastics:

Handling Plastics–All plastics should be handled wearing gloves. White cotton gloves or disposable gloves (such as latex or vinyl) should be worn at all times. Plastics are very sensitive to the acids found in a person’s hands and can be damaged if handled too often. If decay products are visible on the plastic’s surface, then disposable gloves should be worn and disposed of immediately after use.

Plastics benefit greatly from being stored and exhibited in a stable environment. Extremes in temperature and relative humidity should be minimized. A drier environment of approximately 35-45% relative humidity is ideal. Plastics should be stored in a well-ventilated area, free from ultraviolet radiation, pollutants, dust, and insects.

Plastics should be stored and supported using only museum approved acid-free materials. Some plastics benefit from storage with oxygen and pollutant scavengers. These scavengers help to reduce and remove chemicals off-gassed from the objects.

Lastly, some plastics cannot be stored in contact with other materials as they will chemically react and lead to further degradation of the objects. No objects should be stored in close proximity to polyvinyl chloride. Other plastics such as polyurethane foam, silicone rubber and cellulose nitrate can break down in storage producing acidic byproducts, which may harm other objects or storage materials.

Additional Resources:

Blank, S (1990) “An Introduction to Plastics and Rubbers in Collections”, Studies in Conservation 35 (1990): 53-63.

Grattan, D., ed. (1993) Saving the Twentieth Century: The Conservation of Modern Materials. Canadian Conservation Institute: Ottawa, Canada.

Morgan, J. (1991) Conservation of Plastics. Museum and Galleries Commission: London.

Quye, A & Williamson, C. (1999) Plastics: Collecting and Conserving. National Museums of Scotland: Edinburgh.

Shashoua, Y & Ward, C. (1995) “Plastics: Modern Resins with Ageing Problems” SSCR Resins: Ancient and Modern, pp. 33-37.

Shashoua, Y (1996) “A Passive Approach to the Conservation of Polyvinyl Chloride” in ICOM-Modern Materials Working Group: 961-966.

Winsor, P. (1999) “Conservation of Plastics Collections”, MGC Fact Sheets: September 1999. Museums and Galleries Commission: London

Young, L. & Young, A (2001) “The Preservation, Storage and Display of Spacesuits”, Collections Care Report Number 5, Smithsonian National Air and Space Museum: Washington, DC.

Sourche from; http://www.sha.org/research_resources/conservation_faqs/treatment.cfm

 

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