Base polymers and decomposition

Film Base Polymers

Since the introduction of motion picture film in the 1890's the most commonly used base polymers have been esters of cellulose, however polyester is becomming more widely used. Most projection material is now on polyester and it is being more widely used for laboratory stocks.

^{Fig 3.1 General structure of cellulose and a cellulose ester (triacetate)}

Cellulose nitrate
This was the original film base material that was used from 1895 to approximately 1950. Cellulose nitrate was manufactured using cotton linters, a by-product of the cotton industry, that were reacted with a mixture of nitric and sulphuric acids. The resultant polymer was dissolved in solvents, chemicals - such as camphor - added to improve the polymers properties and then "cast" on a highly polished flat bed. This produced a thin layer of polymer in solution. The residual solvents evaporated leaving a thin flexible plastic film.

The structure of cellulose nitrate is very similar to cellulose triacetate (Fig 3.1), the main difference is the acetyl groups are replaced by nitro (NO₂) groups.

Cellulose nitrate used for film is highly nitrated, this means that the polymer is highly combustible. A common plasticiser used in nitrate film, camphor, is also highly flammable further compounding the problem. This was one of the main drawbacks with nitrate motion picture film. Nitrate film is classed as a 'Dangerous Goods' and as such a license is required to store the film and there are restrictions on transportation.

Nitrate film is soluble in a wide range of solvents. Among these are ether, ethyl and methyl alcohol. One test for nitrate film is solubility in methyl alcohol - nitrate film base is soluble, acetate and polyester film base is not. Another tests compares the specific gravity of nitrate versus acetate base but this uses a solvent that is quite hazardous. The flame test can be used but is not always conclusive as there were additives some manufactures used at times to reduce the flammability of nitrate.

Spectrophotometric testing has shown that nitrate film generally absorbs more towards the blue end of the spectrum than acetate, however this might be indicative of a degree of discolouration (yellowing) due to decomposition.

Positive identification of cellulose nitrate motion picture film therefore needs to be a combination of techniques. Film content that indicate a date of production, edge markings (not print through) and small analytical tests, such as solubility, will provide evidence tending towards nitrate or not.

Cellulose acetates
These are produced from a cellulose starting material, the most common being wood fibres. The fibres are reacted with a mixture of acetic compounds and sulfuric acid to connect the acetyl groups to the cellulose skeleton.

Additives and solvents are mixed into the raw polymer and the resulting material is cast or extruded to form the thin flexible film.

As film bases, cellulose acetates were first introduced commercially as cellulose diacetate in the early 1910's for 28mm film. Further development of polymers was spurred on by World War I and manufacturing methods became more efficient. The next important development for cellulose acetate film base was the introduction of the "safety film" 16mm format for domestic use.

This "diacetate" base was still not sufficiently durable for professional use. World War II further improved cellulose acetate polymers and a form, known as "tri-acetate", was able to be manufactured with qualities of durability that were equal to the existing professional nitrate film base. Cellulose triacetate was introduced commercially in the late 1940's as solvents needed for its' manufacture became more readily available after wartime shortages.

Acetate film falls into two broad categories, 'di'-acetate and 'tri'-acetate. As the names suggest there are varying amounts of acetyl groups in each type. To obtain the desirable properties of strength and durability the cellulose chain needs to be fully esterified or the "tri" form. Early manufacturing methods were unable to remove all the residual manufacturing chemicals from the polymer and some of the residual sulfuric acid would be trapped in the structure. This acid would rapidly decompose the polymer. To produce a commercially viable plastic the acid needed to be removed. The process used to do this was to partially hydrolyse the polymer to reduce the number of acetyl groups. This resulted in a plastic with poorer physical characteristics but greater chemical stability. As production methods improved the acetyl percentage was able to be increased.

An early problem with the introduction of cellulose triacetate was finding a suitable film cement as cellulose triacetate has a limited range of solvents. Acetone, methylene chloride and 1-4 dioxane are the major constituents of most current acetate film cements.

Even though c.nitrate was a successful film base the high flammability and cautions required to handle it drove the development of other plastics suitable for film bases. Acetate film is considered a safety or nonflammable base. This is due in part to the lower flammability of cellulose acetate but mainly to the addition of fire retarding agents added during manufacture. Triphenyl phosphate (TPP) is added to cellulose acetate to serve two main functions:

• a manufacturing additive to help remove solvents and other manufacturing additives from the film base after manufacture
• as a fire retardant, even so acetate film will burn if sufficient heat is provided

TPP may also act as a plasticiser and it is commonly thought that this is the reason for it's addition to the film base.

Polyester (Polyethylene terephthalate)
Polyester was developed in the late 1940's. This plastic is created by a condensation reaction between ethylene glycol and terephthalic acid. The ester groups in the polyester chain are polar, with the carbonyl oxygen atom having a slight negative charge and the carbonyl carbon atom having a slight positive charge. The positive and negative charges of different ester groups are attracted to each other (Fig 3.2). This allows the ester groups of nearby chains to line up with each other forming strong linear structures.

Polyesters do not require plasticisers incorporated in them.

The extreme resistance to tearing makes polyester an ideal film base for the rigours of commercial cinema projection. Even so it is only since the mid 1990's that polyester has become the material of choice for the base of release print materials despite being used for some stills photographic film base since the mid 1950's.

Polyester can be readily identified with a simple non destructive test by examining a clear section of film with two polarising filters, one below and one above the film. By rotating one filter a "rainbow" effect can be seen if the material is polyester, other film bases do not show this effect.