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Source: Conservation of Plastics: Materials Science, Degradation, and Preservation. Yvonne Shashoua. Elsevier, 2008.

•Joining plastics usually irreversible: partial dissolution, melting, disruption to surfaces

•Methods: mechanical fastening, adhesive bonding, thermal-welding, solvent-welding.

•Mechanical fastening:

strong flexible plastics

Screws, clips, nuts, stitching with synthetic thread (synthetic thread mimics environmental dimensional changes).

Mechanical Theory: describes the surface profile of a plastic as a maze of microscopic peaks/troughs. Adhesive must penetrate and replace air caught in troughs. Prepare surface mechanically to increase cleanliness/reactivity/surface area.

Adsorption theory: adhesion is a process resulting from contact at a molecular level between two materials. Adhesive must first wet plastic and spread over surfaces.

Need liquid adhesive with high wettability - spreads more over surfaces to increase adhesion. Adhesive should have surface tension less than that of plastic. After molecular contact is made through good wetting, adhesion is result of electrostatic forces - need good contact because this bonding is weaker than covalent bonding. *Difficult to find adhesives with lower surface tensions than the plastic substrates (esp. polyethylene/polytetrafluoroethylene) even dilute adhesives don't work because of the water surface tension.

Achieving Good Wetting: modify chemically or mechanically to increase surface tension. Plastics most often requiring modifications: polyethylene, polyesters, polypropylene, polytetrafluoroethylenes. Must select correct modification types for specific plastics. Do not use mechanical abrasion on polyethylene - will introduce more air pockets. Should oxidize polyethylene by heating rapidly in hot air/oxyacetylene flame - improves adhesion by toughening material (strengthens surface tension). *Careful when applying heat - check melting/deformation point. Plasticized PVC - pre-treat with acetone to clean surface to prevent migrating adhesive. Abrade surface with emery, then clean with alcohol for: polystyrene, nylon, melamine formaldehydes, polyesters.

**All pre- surface treatments should be done a short time before treatment because the effectiveness of all surface treatments decreases rapidly with time.

•Choosing an adhesive:

1) Structural or non-structural material required?

Structural: high load bearing, stable to heat/radiation/weathering/solvents/exhibit high toughness and flexibility. Thermosetting adhesives, epoxies, polyurethanes, acrylics.

Non-structural: for holding light-weight components in place. Pressure sensitive/contact/hot-melt adhesives.

2) Consider thermal coefficients of expansion, changes in physical and chemical properties with time: Large difference in thermal coefficients of expansion between plastic and adhesive introduce stress. Consider surface degradation, may change surface or displace adhesive. Adhesive may be only compatible with the cleaned surfaces, not contaminants.

3) Refractive index of adhesive should be similar to adherend.

Note: RI's of polymers change because plastics yellow with time.

4) Thinning solvents in adhesives should not cause environmental stress cracking or promote degradation of substrate

epoxy, UV curing acrylics, solvent drying acrylics are good

Testing for chlorides in plastics: if present, will offgas HCl when plastic degrades

-heat a copper wire in flame

-rub hot wire on plastic in question to pick up a small sample

-put wire back in flame

-Epoxy: adhesive created by the chemical reaction between two ingredients. Most adhesives set through the evaporation of solvents (ex: B-72, white immulsions) no chemical rxn involved. Sets in 1-2 days.

-Conservation grade, does not degrade as fast, yellows less.

-Glass/porcelain repairs:

     •Refractive index close to glass. Makes glass repairs less noticeable because light passes through glass and epoxy the same way. used on objects with vitrous surfaces where B-72 will not hold well. Can be removed from glass/high fire wares with solvents. CANNOT be removed from low fire wares once it has soaked into pores.

-Preparing the epoxy:

     •mixing by weight:

          >tare the container

          >3 parts A (resin)

          >1 part B (hardener)

     •mixing: try not to mix many bubbles in (especially for glass repair where it has to be transparent). Can use a vacuum to prevent bubbles. Mix until there are no more 'threads' in the epoxy. Mix all the way, because it will not set well if it is not mixed thoroughly.

     •Bulking: pour epoxy onto mixing surface, then add bulking agent. Bulking allows hold to be less slippery, makes the mixture thicker, makes it less glossy and more porcelain-like, helping it blend better. Bulking agents: Cabosil (fumed silica), glass microballoons (tiny spheres of glass). *BE CAREFUL not to inhale the bulking agents - VERY SMALL PARTICULATES, will catch in lungs*

     •Add pigment to color match

-Applying the epoxy:

     •apply to the break surface, run nail over edges to test evenness, do not readjust too much. Hold with tape or microcrystalline wax where possible.

     •Use bulked epoxy as fill material

-Cotton: collapsed lumen (Hollow space in fiber). Very characteristic of cotton

-Hair: thicker and darker medula (middle area). Medulas can be compared to determine if samples came from same animal/human/species.

The polarized light microscope uses a polarizer to direct lightwaves in one consistent direction. The light is refracted as it passes through the object under the microscope. The way that the light is transmitted through an object under these circumstances characterizes different materials.

-ISOTROPIC: does not allow cross polarized light through

-Ceramics are generally impervious to solvents

-B-72 is used on low fire vessels (ONLY) because it can soak into the pores and bind the shards together. Will not work on porcelain/other high fire vessels because the surface is vitrious and has no pores. Takes a few hours to dry v. epoxy which takes a few days

-Best technique is to do it all at one time, making sure not to lock pieces out

-Always do a dry run, understand what order you will do it in

-Holding pieces together for drying:

     •tape: electrical tape can be stretched from one shard to the other while attaching so that the mend is constantly pulling toward each other. Hold down ends of electrical tape with blue painters tape - is removed easily. *DO NOT use tape is surface is friable*

     •Sandbox: place attached shards in sandbox so that gravity pulls the top one toward the bottom one.

     •Rubber bands

-Correct lighting is important. Most labs use daylight/simulated daylight. Some simulate gallery lighting

-Paint = pigment in a medium

-Acrylics:

     •Acrylic immulsion paints are water soluble, no need for solvents. Useful for painting on fill material on ceramics. Ex: Liquitex (dries darker than on pallet, is glossy)

     •Acrylic (nonimmulsion) paints, best for outdoor use.

     •B-72 is an acrylic, dissolved in a solvent

     •White glue (Elmer's) is an acrylic with added immulsifiers to help it stay mixed, no seperation. Irreversible treatment because immulsifiers cross-link when dried, not used on objects. When immulsifiers are cross-linked, they cannot be dissolved in water, will only swell.

-Colors:

     •Earth colors

          >umbres, siennas, black, white

          >raw = cooler, burnt = warmer

     •White:

          >titanium white = opaque, absorbs UV light, helps inpainting treatment last longer

          >zinc white = also opaque

          >calcium carbonate = translucent

     •Blacks:

          >bone, lamp, etc. All kinds are organic materials burned until black, resulting in subtle differences in color

     •Blue:

          >Ultramarine blue is the basic blue

     •Red:

          >cadmium red is the purest pigment

-Mixing colors:

     •pallet should be nuetral or white

     •use as few colors as possible. many colors are synthetic and already contain a complex mixture of pigments. the more colors added, the muddier the color becomes.

     •Liquitex dries darker and is glossy

     •Fumed silica can be added to get a matte finish

     •use more medium or water to make colors translucent

-ONLY PAINT FILL MATERIAL, NOT OBJECT SURFACE