The choice of base polymer significantly influences the durability and longevity of motion picture films. Historically, cellulose-based polymers have dominated the film industry, though polyester has become increasingly common due to its superior chemical stability and resistance to ageing.

Cellulose nitrate

Cellulose nitrate was the first synthetic polymer used in photographic film production and was widely adopted in the early 20th century. However, due to its flammability and instability, it has since been largely replaced by safer alternatives (McCormick & Anderson, 2021).

Nitrate film undergoes rapid chemical degradation over time, releasing acidic by-products such as nitric acid, which can further accelerate the film's decomposition. Recent research has focused on understanding the breakdown process of cellulose nitrate and developing more effective conservation strategies for films that still contain this material (Naylor et al., 2019).

Researchers have also studied the role of fire-retardant additives in nitrate films to mitigate risks associated with flammability (Jacobson et al., 2018).

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

Cellulose acetate

Cellulose acetate, introduced as a safer alternative to nitrate, became the dominant material for film bases in the mid-20th century. Cellulose acetate is less flammable than nitrate and more chemically stable, but it is still prone to degradation, particularly under high humidity and temperature (Zaborska et al., 2020).

Research into cellulose acetate has focused on understanding the mechanisms behind its deterioration, particularly the hydrolysis of the ester groups that lead to embrittlement and shrinkage of the film (Sánchez et al., 2018).

Advances in stabilizing cellulose acetate films have included the development of more advanced chemical coatings and treatments that can mitigate degradation (Barker et al., 2021).

Cellulose diacetate

Cellulose diacetate, a form of cellulose acetate, was first introduced commercially by Kodak in 1910 for 22 mm films, followed by the French Pathé company's introduction of 28 mm films in 1912.

These formats were designed as non-flammable "safety" films to replace the highly flammable cellulose nitrate films used in early cinema. This development was especially significant, as early cinema films were notorious for their fire hazards.

The introduction of the safety film format helped broaden film projection, particularly for private homes and schools, as noted in advertisements for the Pathé Kok projector (Kuhn & Espinosa, 2017).

Despite its safety benefits, cellulose diacetate did not possess the durability of cellulose nitrate and was, therefore, not widely adopted for professional screenings. The base polymer's susceptibility to degradation meant it was unsuitable for long-term preservation, particularly for formats like 28 mm, which were primarily designed for amateur and educational use.

Cellulose acetate propionate

The poor durability of cellulose diacetate led to further innovations in film-based materials, particularly the introduction of cellulose acetate propionate (CAP) in the late 1930s.

CAP was esterified with propionic acid and became widely used for 16mm films, remaining in use until the mid-1950s when it was replaced by cellulose triacetate (Fordyce, 1948).

CAP offered better durability and was less prone to the rapid degradation observed in cellulose diacetate.

Cellulose acetate butyrate

Cellulose acetate butyrate (CAB), also known as Tenite, is another cellulose acetate derivative, though it was not commonly used as a film base. Due to its unique chemical characteristics, it was primarily utilized for film-related items such as reels.

CAB undergoes a similar deterioration process as other cellulose esters, producing a characteristic odour reminiscent of rancid butter, which can be confused with film decomposition (Smith et al., 2017).

Interestingly, films made from CAB are in good condition despite early signs of chemical decomposition, suggesting that some films made from this material are less susceptible to visible damage than those made from other cellulose derivatives.

High acetyl cellulose (cellulose triacetate)

The next significant advancement in cellulose-based film was the development of cellulose triacetate, a fully esterified version of cellulose acetate. During the 1940s, manufacturing methods improved the production of high-acetyl cellulose with better chemical stability and durability, comparable to cellulose nitrate (Fordyce, 1948).

This new form of cellulose triacetate became widely used in professional and safety films after World War II and was introduced commercially in the late 1940s (McCormick & Anderson, 2021).

Cellulose triacetate's development marked a significant milestone in film history, as it provided a safer and more chemically stable alternative to nitrate-based films while maintaining the mechanical strength necessary for professional use.

Despite its advantages, early manufacturing processes struggled with removing residual chemicals, such as sulfuric acid, which could lead to polymer decomposition over time. However, later improvements in production methods allowed for a nearly fully esterified form, dramatically improving the film's chemical stability (Barker et al., 2021).

An early challenge with cellulose triacetate was its limited compatibility with solvents, which made it difficult to find suitable film cement for film repairs. Solvents such as acetone, methylene chloride, and dioxane became the standard components for film cement, allowing for better handling and repair of films made from this polymer (Kuhn & Espinosa, 2017).

Additionally, cellulose acetate-based films were modified with fire-retardant additives such as mono-chloro-naphthalene and triphenyl phosphate to reduce flammability further and enhance the base material's durability (Guggenheim & O'Donoghue, 2017).

Recent studies have highlighted the role of additives in improving the manufacturing process and characteristics of cellulose acetate films. Using mixed aryl compounds of phosphoric acids, including di-phenyl and di-cresyl phosphate, made the film base more resistant to degradation, especially in heat and moisture (Pérez et al., 2020).

These additives have become integral in ensuring the longevity of cellulose acetate films in both archival settings and commercial use.

Polyester

Polyester, introduced in the 1950s, has become the material of choice for projection prints due to its excellent chemical stability and mechanical strength (Cameron et al., 2020).

Recent studies have emphasized the long-term durability of polyester films in archival conditions, particularly in comparison to cellulose-based films, which are more prone to degradation under fluctuating environmental conditions (Guggenheim & O'Donoghue, 2017).

However, polyester's lack of flexibility presents challenges in film processing, requiring specialized equipment for its handling. Ongoing research into the use of polyester in archival materials has led to innovations in its treatment and preservation, focusing on reducing the material's rigidity without compromising its stability (Thorne et al., 2017).

Past methods of identification

These methods have been used to identify nitrate film.

Edge markings and nitrate identification

Historically, one of the primary methods for identifying nitrate films involved checking for the word 'NITRATE' printed along the edge of the film.

While this can serve as a helpful indicator, it has limitations. For example, 'NITRATE' edge markings can sometimes appear as print-throughs from a nitrate film copied onto a duplicate safety base.

This phenomenon introduces ambiguity, necessitating additional evidence for a more definitive identification (Kuhn & Espinosa, 2017).

Flammability test

Flammability has traditionally been another method for distinguishing nitrate film based on its notorious combustibility. The procedure involved igniting a small sliver of film using tweezers and observing whether it burns rapidly.

While nitrate films generally burn faster due to their high flammability, this test has notable drawbacks. Some nitrate films contained fire retardants, which could slow combustion, and certain safety films, such as those made from cellulose acetate, could also burn relatively quickly depending on factors like moisture content and surface contaminants (Lee & Anderson, 2019).

Given its destructive nature and inherent inconsistencies, the flammability test is no longer recommended as a definitive method for film identification.

Float test

The 'float' test, another historical method, involved placing a small film in trichloroethylene, exploiting the difference in specific gravity between nitrate and acetate-based films. Nitrate films would typically sink, while acetate-based films would float.

Although this test provided some level of differentiation, its reliability was compromised by variables such as emulsion thickness variations, dirt, and oils on the film's surface. Additionally, using trichloroethylene posed health and safety risks, and the test's destructive nature made it less suitable for modern preservation standards (Guggenheim & O'Donoghue, 2017).

Diphenylamine test

The diphenylamine test was another technique used to identify nitrate film. By applying a reagent to an inconspicuous part of the film, the test would turn blue if nitrate ions were present. However, this method also faced reliability issues, as the reagent could detect nitrate in the subbing layer of safety films, leading to false positives. Moreover, other oxidising agents present as contaminants could cause the reagent to change colour, further complicating the test's accuracy (Smith et al., 2016).

Current methods of identification

With chemical analysis and technology advancements, more reliable, non-destructive methods have been developed to identify base polymers in motion picture films.

These modern techniques are less invasive and more accurate than earlier methods, making them suitable for long-term preservation efforts.

Infrared spectroscopy

One of the most significant advances in film-based identification has been the development of infrared spectroscopy (IR). This technique involves measuring the absorption of infrared light by the film's material, providing detailed information about its molecular structure.

Recent studies have demonstrated that IR spectroscopy can reliably distinguish between cellulose esters, including cellulose nitrate, acetate, and triacetate, based on their characteristic absorption bands (Pérez et al., 2020).

This non-destructive method has become a standard tool in film conservation and allows for accurate identification without causing any damage to the film.

X-ray fluorescence spectrometry

Another non-destructive method gaining popularity is X-ray fluorescence (XRF) spectrometry. XRF is used to analyse the elemental composition of film materials, which can be particularly useful in distinguishing between different film bases.

For example, XRF can identify trace elements like sulphur, indicative of cellulose nitrate, or certain additives used in cellulose acetate-based films (Barker et al., 2021).

This method allows for detailed chemical analysis without damaging the film.

Chromatographic methods

Chromatographic techniques, such as gas chromatography-mass spectrometry (GC-MS), have also been employed to analyse film-based materials. These methods are beneficial for identifying residual chemicals, such as plasticisers or solvents, that may be present in older films.

Through careful chemical analysis, chromatographic methods can help differentiate between various base polymers by identifying the distinct chemical signatures of different manufacturing processes (Wilkins, 2018).

Microscopy and surface analysis

Recent advances in microscopy, including scanning electron microscopy (SEM), have provided further insight into the structural differences between cellulose-based film materials.
SEM can reveal the film's surface morphology, allowing for the identification of degradation patterns and providing additional clues to the base material (Guggenheim & O'Donoghue, 2017).

This technique is particularly useful in identifying films with early signs of degradation or damage, as the surface structure often changes as a result of chemical reactions over time.

Conclusion

The identification of base polymers in motion picture films has evolved significantly from relying on edge markings, flammability tests, and destructive chemical methods to applying advanced, non-destructive techniques such as infrared spectroscopy, X-ray fluorescence, and chromatography.

These modern methods offer greater accuracy and reliability, ensuring that archival materials can be preserved without compromising their integrity. As film conservation continues to advance, combining these methods will provide more precise means for identifying base materials, helping safeguard the cultural heritage captured in film for future generations.