Omar Khalid
Thick plastic can crumble due to several factors, including the type of plastic, environmental conditions, and the passage of time. Different types of plastics such as Bakelite, cellulose acetate, polyurethanes, and early polyethylenes are known to deteriorate, with some becoming brittle, discoloured, or deformed. Environmental factors like temperature, humidity, and ventilation can also influence the degradation process. For instance, storing plastic items in extreme temperatures can accelerate deterioration. Additionally, the age of a plastic item plays a role, as chemical reactions causing degradation occur slowly over time.
| Characteristics | Values |
|---|---|
| Plasticizer evaporation | Over time, plasticizers in the plastic can evaporate, causing the plastic to become brittle and crumble |
| Thickness | Thicker pieces of plastic are more susceptible to deterioration |
| Absorption of moisture | Plastic can absorb moisture, causing stress that leads to physical damage and crumbling |
| Low relative humidity and temperature | These factors slow down the chemical reactions causing degradation |
| Material type | Certain types of plastic, such as Bakelite, cellulose acetate, polyurethanes, and early polyethylenes, are more prone to deterioration |
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What You'll Learn
Plasticizer evaporation
Plasticizers are added to polymers to enhance their flexibility, toughness, and processability. However, plasticized polymers are susceptible to degradation during storage and service due to plasticizer evaporation and diffusion. The loss of plasticizers leads to a decline in mechanical properties and eventual contamination of the surrounding environment. This is particularly prominent in plasticized poly(vinyl chloride) (PVC) products, where plasticizers such as diisodecyl phthalate (DIDP), dioctyl phthalate, and benzylbutyl phthalate can evaporate, especially at elevated temperatures. The rate of evaporation depends on temperature and the initial concentration of the plasticizer.
The concentration profiles of plasticizers within the polymer provide insights into the dominant migration mode. Flat concentration profiles indicate evaporation-controlled systems, while clear gradients towards the surface suggest diffusion-controlled systems. Additionally, the thickness of the polymer samples influences the migration process, with thicker samples potentially hindering the escape of gaseous acids formed during degradation, as observed in thicker plastic artifacts in museums.
The loss of plasticizers can be quantified using techniques such as FTIR spectrophotometry, gravimetry, and isothermal thermogravimetry. These methods help in understanding the kinetics of plasticizer evaporation and the factors influencing it. The kinetic parameters, such as rate constants and activation energies, can be mathematically defined and modeled to predict the behavior of plasticizers in different temperature and concentration conditions.
Understanding plasticizer evaporation is crucial for developing strategies to mitigate the degradation of plasticized polymers and preserve artifacts made from these materials. The preservation of plastic artifacts in museums, for instance, requires careful consideration of environmental factors, such as relative humidity, temperature, and ventilation, to slow down the degradation process and prevent the spread of degradation to unaffected items.
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Absorption of moisture
Plastic degradation is a common issue that museums and collectors face when preserving artefacts and older plastic items. Several factors contribute to the deterioration of plastics over time, including the absorption of moisture.
The absorption of moisture by plastics can lead to physical damage and crumbling. This is particularly true for thicker plastic items. Plasticizers used in processing thicker plastic items, such as combs and mirror handles, can become trapped within the material. Over time, these plasticizers can migrate to the surface and emit a gaseous acid. This process can cause the plastic to crumble and fall apart. Additionally, the absorption of moisture can cause stress within the plastic material, leading to cracking and deformation.
The environment in which plastic items are stored plays a crucial role in their preservation. Storing plastic items in controlled environments with low relative humidity and temperature can slow down the chemical reactions that lead to degradation. However, when exposed to varying temperatures, plastics can absorb moisture, leading to deterioration. For example, a sculpture made from cellulose nitrate and stored in an airtight case remained intact until the case was opened, after which it immediately crumbled due to the release of nitric acid.
Different types of plastics are affected by degradation in unique ways. For instance, Bakelite, the first totally synthetic plastic, is known to fade and change colour over time, while early polyvinyl chloride (PVC) and polyethylene (PE) plastics can shrink, become brittle, and deform. Polyurethane (PU) is another plastic that can harden, crack, and crumble over time. The degradation of plastics is a significant concern, especially for museums and collectors aiming to preserve historical artefacts and artwork created using these materials.
While plastic is known for its durability, it is not indestructible, and the absorption of moisture is one of the factors that can accelerate its deterioration. Conservators and curators face ethical dilemmas when dealing with crumbling plastic artefacts, weighing the options of replacing parts or displaying the pieces as they are, accepting the natural deterioration of the material over time.
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Prolonged exposure to heat
Additionally, the plasticizer used in processing thick plastic items can become trapped and may eventually rise to the surface, emitting a gaseous acid. This process can be expedited when plastic is exposed to heat, causing the plastic to crumble and fall apart.
The effects of heat on plastic can also be observed in plastics like Bakelite, which can change color and fade when exposed to heat over time. Cellulose acetate, an early form of plastic, is also susceptible to heat, causing it to discolor and warp as it ages.
Furthermore, heat can cause plastic to become brittle and more susceptible to cracking or shattering. This is due to the evaporation of plasticizers, which are added to make the plastic softer. Over time, these plasticizers can evaporate, leaving the plastic hard and brittle.
To prevent or slow down the deterioration of plastic due to heat, it is important to store plastic items in a cool, well-ventilated environment. Low relative humidity and temperature can help slow the chemical reactions that lead to degradation. However, it is important to note that sudden changes in temperature, such as removing an item from an airtight case, can also cause plastic to crumble, as observed in the case of a cellulose nitrate bust.
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Chemical reactions
Plastic degradation is a significant issue in conservation efforts, with even recently produced plastics showing signs of deterioration. Chemical reactions play a crucial role in understanding why thick plastic crumbles over time.
One key factor is the presence of plasticizers, which are added to plastics like ABS to make them softer and more flexible. However, some plasticizers, like dinonylphthalate, are volatile and can evaporate over a couple of years, causing the plastic to become brittle and prone to cracking or shattering. This issue can be mitigated by remelting the plastic and adding new plasticizers or using more stable plasticizers discovered in recent times.
The plasticizer used in processing thick plastic items can also become trapped within the material. As the plasticizer migrates to the surface over time, it can emit a gaseous acid, causing the plastic to crumble and affecting nearby objects as well. This phenomenon is known as "the disease" in the context of plastic degradation.
Cellulose nitrate, an early form of plastic, is known to cause corrosion in metals and the breakdown of other materials. Curators in museums have observed cellulose nitrate attacking other objects, such as causing celluloid beads in necklaces to corrode and the string holding them to break down.
Additionally, some plastics absorb moisture, which can lead to physical damage due to the stress it causes. Low relative humidity and temperature can slow down the chemical reactions responsible for degradation. For instance, a bust sculpted from cellulose nitrate crumbled immediately when removed from an airtight case, as the nitric acid had remained stable in the enclosed environment.
The degradation of plastics is a complex issue, and while some measures can be taken to slow down or reverse specific types of deterioration, the overall challenge of preserving plastics over extended periods remains a concern for museums and conservationists alike.
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Proximity to cellulose nitrate
Cellulose nitrate has been observed to cause metals to corrode and string to break down. This degradation can spread to other materials, causing a chain reaction of deterioration. This is especially true when cellulose nitrate and cellulose acetate are stored together, as this can accelerate their deterioration. In these cases, separation is essential to preservation.
The degradation of cellulose nitrate can also have a detrimental impact on human health, as even short-term exposure to the decomposition chemicals of cellulose nitrate can provoke respiratory issues.
To prevent the deterioration of materials caused by cellulose nitrate, facilities storing large quantities of negatives made of cellulose nitrate film may be required to comply with the National Fire Prevention Association Standard 40 (NFPA 40). This standard outlines the storage and handling procedures for nitrate film and can help to preserve the negatives and prevent the spread of deterioration to other materials.
Overall, the proximity to cellulose nitrate can have far-reaching consequences, causing damage to a variety of materials and even impacting human health. It is important to be aware of the potential risks and take the necessary precautions to preserve historical artefacts and other materials.
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Frequently asked questions
Thick plastic can crumble due to the degradation of plasticizers used in processing. Over time, the plasticizer evaporates, and the polymer becomes brittle. This can be remedied by remelting the plastic and adding new plasticizers.
Discolouration, warping, melting, hardening, and cracking are all signs of plastic degradation.
Low relative humidity and temperature can slow the chemical reactions that cause degradation.
Yes, several types of plastics are known to cause preservation problems, including Bakelite, cellulose acetate, polyurethanes, and early polyethylenes.
