Interventive treatments should only be considered if they are absolutely necessary to minimise further deterioration or to preserve the significance of an object. Approach the treatment of plastic materials with utmost caution. Before undertaking any treatment, determine the type of plastic in the object. Because of the sensitivity of many plastics to solvents, cleaning agents and adhesives and the difficulty involved in identifying them, contact a conservation professional for advice rather than attempting to treat objects yourself. There is a significant risk of damage to objects otherwise.
Reasons for cleaning plastic objects include the removal of surface degradation products, the improvement of unsightly appearances and the removal of dust which can enhance degradation. Few reports have been made about cleaning and stabilising degraded plastic and rubber because of their delicate nature, susceptibility to further damage and relative impermeability.
Physical cleaning may involve either brushing, gentle wiping with soft, lint-free and/or microfibre cloths, vacuuming or blowing air to remove adherent particles. Great care must be taken if physical methods are used because of the risk of damage to the plastic surface.
Although aqueous cleaning may be problematic for polymers that are susceptible to hydrolytic breakdown (such as semi-synthetic polymers, polyesters and polyester-based polyurethanes), some plastics may be safely washed with warm water containing small amounts (3 - 5 %) of a non-ionic detergent (for example, Lissapol, Teric N9, Arkopal N090) and a soft cloth or brush. Objects should never be immersed in water however. To avoid the possibility of swelling dry washed objects immediately after wet cleaning with either a soft cloth or tissue. This approach has been successfully used on plasticised PVC (Huys and van Oosten 2005) and on a polyether-based polyurethane sculpture (Winkelmeyer 2002). It is most important to use soft, lint-free swabs that minimise surface abrasion when applying the detergent and during subsequent rinsing and drying.
Do not use water on (Morgan 1992):
- vulcanite (hard rubber) that is beginning to degrade;
- surface-dyed casein;
- degrading cellulose nitrate or cellulose acetate where the surface is cracked or crazed; and
- objects which contain metal components which may corrode.
The use of non-aqueous solvents is not generally recommended due to the increased possibility of damage occurring to the polymer (Shashoua 2008). If it is necessary to use a non-aqueous solvent either as a cleaner, to remove an adhesive or to reverse an earlier repair for instance, it is best to consult Hildebrand solubility parameters (as modified by Hansen 2007) to ensure that the solvent used will not damage the polymer. Once a potential solvent has been selected, test it on an inconspicuous part of the object before use because the solubility properties of the polymer may have changed as it degraded.
Some collectors have used commercial acne cleaners to remove green copper staining from doll’s PVC skin and commercial polishes to remove scratches to improve the appearance of plastic objects (Shashoua 2008). As most commercial polishes are silicone-based, the latter treatment is likely to be irreversible. In addition these treatments have not been evaluated to determine any possible long-term effect on polymer preservation.
Stabilising Deteriorating Objects
There have only been a few descriptions of attempts to stabilise deteriorating polymeric objects.
Despite the reservations expressed above (Morgan 1992), deteriorating cellulose nitrate beads were washed in water to remove acid and then soaked in very dilute sodium carbonate solution to neutralise any remaining acid and to buffer future acid formation. This approach was successful in stabilising these objects. It would be unlikely to be effective in the treatment of large objects due to the lack of porosity of cellulose nitrate (Green and Bradley, 1988).
The consolidation and repair of a pair of severely degraded rubber bathing shoes has also been described (Maltby 1988). The shoes had areas of differing degradation, soft and slightly tacky in some areas, hard and brittle in other areas and supple and undegraded elsewhere. Specialised techniques were used to reshape the shoes, fill lost areas and strengthen fragile areas. A fitted padded support was constructed from ethafoam and a specially designed display mount was prepared. As the shoes were extremely fragile and could not be strapped in place, a tin plate insole was inserted in each shoe and magnets fixed under the acid-free mount. Removal of the magnets allowed the shoes to be ‘freed’. To enhance the longevity of the shoes they were stored in a nitrogen atmosphere.
Repairs and In-Painting
Before embarking on the repair or treatment of plastics there is a number of factors that must be considered. These include the:
- basic nature of the plastic including the extent of degradation and surface tension;
- solvent and plastic interaction;
- properties of possible adhesives including coefficients of expansion, flexibility, ageing characteristics and refractive indices; and
- necessity for pre-treatments to improve adhesion
To repair or even in-paint a plastic object effectively, it is necessary for a bond to form between the applied adhesive or in-painting medium and the substrate. If this is not the case then there will be no effect as the adhesive will not hold and the paint will fall off. Implicit in this process is the use of a solvent which will not adversely affect the plastic in some way.
While water-based adhesives would seem to be the least likely to damage most polymers, large differences between the surface tensions of these materials limits the effectiveness of these adhesives. Although information is available which matches adhesive types with specific plastics and descriptions are given of other joining techniques including heat and solvent-welding, it is strongly recommended that a conservator is contacted before attempting this type of treatment (Blank 1988, Shashoua 2008).
Consolidating, impregnating and filling plastics are highly specialised tasks. While successful treatments have been applied to crumbling, cracking polyurethane foams and crazing cellulose nitrate objects, these types of treatments require specialist knowledge (Shashoua 2008).
If labelling of plastics is needed, use either a soft pencil on a discrete surface (suitable for polyethylene and PVC) or an ink based on an acrylic dispersion medium with carbon black or rutile titanium white as pigments (Shashoua 2008). While tie-on labels with cotton tape can be safely used, there is always a risk of detachment or loss of the label. Avoid applying a barrier layer between the plastic and the ink and also avoid the use of solvent-based inks and paints, rubber bands or adhesive tapes.