22/23-HS2339 Essentials of Conservation: Polymers in Conservation Flashcards
Describe methods of applying a polymer to an object (7)
- A liquid as a non-setting adhesive. Some adhesives remain as liquids. These are the contact adhesives used in pressure-sensitive tapes and labels
- A product can be applied as a molten liquid which cools, so forming a solid. eg Heat-activated adhesives in laminating systems
- Some materials such as oil paint and epoxy resin are applied as liquid pre-polymers, which set by chemical reaction to form a polymer.
- A solution of polymer sets by evaporation of the solvents. This is the most common method, but it is of declining importance in industry. eg. picture varnish
- The polymer can be applied in a hot solution, which sets by cooling. The swollen solid will then lose its solvent by evaporation, e.g. bone glue
- Some polymers are applied in emulsions or dispersions. An emulsion sets to a film, first by loss of the dispersant, usually water, and then by coalescence of the small polymer particles. In order for this to happen the particles must be soft enough to flow into one another. Poly(vinyl acetate) emulsion ‘white glues’ and wax polishes are examples of this type
- Pre-polymers may be applied in gaseous form and reacted directly in contact with the object.
[Velson Horie.Materials for Conservation: Organic Consolidants, Adhesives and Coatings]
Describe how molecule length relates to strength, and the effect on the properties when substances are prepared into films.
Many materials form films when spread out properly. In general, the longer the molecule, the stronger the material. Where great strength is required, long molecules are used: cellulose for wood, collagen for sinew and polyethene for plastic bags. When wood and sinew are converted into cel-lophane and glue, the long molecules are released from their fibrous state and reduced in size. Films prepared from these derived materials have less strength than the original source materials but more than films prepared from small molecules.
-Film-forming materials that are useful as coatings, consolidants or adhesives all have large molecular size as a common factor. Most film-forming materiials, both natural and synthetic, are based on polymers
[Velson Horie.Materials for Conservation: Organic Consolidants, Adhesives and Coatings]
What is a polymer?
A polymer is a large molecule built up from many small identical units, called monomers. In order for a polymer to be formed from monomer molecules each monomer molecule must be able to join up with two other molecules. Continued reaction of these difunctional monomers produces a straight-chain polymer. If some of the monomers can react with three others, a branched chain results
[Velson Horie.Materials for Conservation: Organic Consolidants, Adhesives and Coatings]
Describe the properties of polymer molecules which are not chemically bonded, and ones where cross linking has formed links between molecules
- As there are no formal chemical bonds attaching one polymer molecule to another, the molecules can flow past one another. They can be dissolved in solvents and will melt when heated; because of this latter property they are called thermoplastics.
- If there are sufficient trifunctional monomers, the chains join with one another. It will then be impossible to separate one chain from another without breaking a chemical bond. The many molecules are joined up into a cross-linked immobile mass. This will not melt with heating, but will only soften; nor will it dissolve in solvents, but will only swell, although it may be broken down by some solvents.
[Velson Horie.Materials for Conservation: Organic Consolidants, Adhesives and Coatings]
Describe the properties of a polymer in a solution
Polymers increase the viscosity of a solution because the larger molecules increase resistance to flow. In a good solvent the molecule is more spread out, and in a poor solvent it contracts, with corresponding changes in viscosity.
[Velson Horie.Materials for Conservation: Organic Consolidants, Adhesives and Coatings]
What is the Glass Transition Temperature, and its relevance to polymers
Polymers go through various stages as they are heated from a very low temperature. All polymers can be cooled to form brittle solids; rubber, for example, can be broken like glass when cooled in liquid nitrogen. As the temperature is raised, the polymer changes from a glassy state to a liquid one. This change is not sharp but occurs over a range of temperature. The temperature of this transition is called the glass transition temperature. It is important in choosing a consolidant, adhesive or coating, and is an accurate indication of the softness of the polymer. This has two aspects. A polymer that is too soft will lead to cold flow in an adhesive or dirt pick-up in a coating. A polymer that is too stiff may crack when stressed or may not be able to respond to movements in the object. Most polymers applied to objects have Tg around room temperature.
[Velson Horie.Materials for Conservation: Organic Consolidants, Adhesives and Coatings]
Describe the relationship between Tg and time
The ability of the polymer chains to move past one another and adjust to a changed situation depends not only on how mobile the chains are, i.e. how much above Tg the polymer is, but also on how fast the change is made. The value of Tg can be lowered by increasing the time scale of the force.
A material with Tg of 30 °C (measured over 1 minute) will have an effective Tg of 12 °C when used on an object for 1.2 years. It is obvious that the Tg of a polymer is lowered considerably over time, with unfortunate results for the attached object. This flow occurs not only with adhesives but also with coatings. When a dust particle lands on a soft surface, the polymer will gradually flow around it. This results in dirt pick-up where the dirt has become part of the polymer film. The dirt cannot be removed without removing the polymer film.
[Velson Horie.Materials for Conservation: Organic Consolidants, Adhesives and Coatings]
