Alexei Volkov
Plastic packaging is prone to cracking due to various factors, including exposure to temperature changes, incompatibility with packaged products, and exposure to solar radiation. These factors can cause molecular degradation, making the plastic brittle and susceptible to cracking. Additionally, plastic materials often produce a distinct crackling sound when crushed or crumpled due to their unique elastic properties. This phenomenon is observed in thin materials that can bend without breaking, forming creases and storing elastic energy. Understanding the causes of plastic cracking is essential for effective product design and packaging storage.
| Characteristics | Values |
|---|---|
| Cause of crackling sound | The plastic sheet bends and buckles into a different configuration, making a cracking noise and often a permanent crease. |
| Factors that increase the likelihood of plastic cracking | Exposure to solar radiation, high humidity, and high temperature |
| Factors that catalyze the process of cracking | Chemical substances and elevated temperature |
| Ways to prevent stress cracking | Avoid continuous load, use a suitable material, and reduce stress and temperature |
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What You'll Learn
Plastic degradation
Plastic is a highly versatile material, but it can crack or break. Plastic degradation is a significant concern due to the material's accumulation in the environment and its adverse effects on human health and ecology. The specific surface degradation rate (SSDR) is a metric used to harmonize disparate types of measurements and extrapolate half-lives for different plastics. However, the degradation rates reported in peer-reviewed literature and popular media vary significantly, with some media reports claiming that plastics do not degrade at all.
Plastics can degrade through four primary mechanisms: photodegradation, thermo-oxidative degradation, hydrolytic degradation, and biodegradation by microorganisms. Photodegradation, or the breakdown of plastic by light exposure, often leads to thermo-oxidative degradation, which is influenced by temperature and oxygen levels. Biodegradation by microorganisms plays a crucial role in ecological health, and different bacterial enzymes contribute to the degradation of various polymers. For example, PET hydrolase acts on PET, while PCL-cutinase targets PCL.
The development of biodegradable polymers has gained attention, but it is challenging to create biodegradable polymers that can compete with conventional synthetic plastics in terms of material properties and widespread application. This is because the very characteristics that make conventional plastics popular, such as physical and chemical stability, also hinder their ability to degrade. Additionally, the degree of crystallization in polymers influences their degradability, with higher crystallization resulting in limited degradation.
The presence of plastic additives and plasticizers can also impact degradation efficiency. Pretreatments have been explored to enhance biodegradation and reduce toxic plastic pollution. While there is potential for employing various bacterial strains for efficient plastic degradation, further research is needed to thoroughly understand the specific mechanisms involved.
In conclusion, plastic degradation is a complex and multifaceted issue that requires further investigation. The development of biodegradable polymers and improvements in biodegradation efficiency hold promise for mitigating the environmental impact of plastic accumulation. However, addressing the challenges associated with plastic degradation is crucial for ensuring ecological health and human safety.
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Repairing cracks with glue
Plastic is a versatile material, but it can crack or break. Repairing cracked plastic can seem like a difficult task, but it is relatively cheap and easy. The key to fixing cracks in plastic is knowing which method is best to repair the crack. Smaller cracks can usually be fixed with glue, hot water, or plastic slurry, while larger cracks may require some welding with a soldering iron.
If you are repairing cracks with glue, the first step is to select the right adhesive. Plastic glue is often used for making models, so it is easy to use and safe to handle for adults. You can also use most super glues, but check the packaging to make sure it can be applied to plastic. Some adhesives, such as the Loctite Plastics Bonding System, are formulated to bond difficult materials like polyethylene, polypropylene, and Teflon. You will also need to consider whether your adhesive is needed for interior or exterior applications and how you use the object you want to repair.
Before you begin, make sure you have enough glue to connect the whole crack. You don’t want to have to stop after you’ve started applying strong glue! Work in a well-ventilated area or wear a facemask when handling the glue. Wear gloves to prevent it from getting onto your skin and becoming painful to remove. Protect your tabletop or workspace with a drop cloth or paper covering in case you spill some glue. Clean the surfaces you’re bonding. You might need to lightly sand very smooth or shiny surfaces so the glue can adhere better. Apply an activator (if required) and allow it to dry according to the manufacturer’s directions.
Now you are ready to apply the glue. Squeezing the tube gently, spread a layer of glue over the edges you will reconnect. Keep a cloth or wipe nearby in case you pour too much and dab any excess glue immediately to prevent it from hardening. Glue dries fast, so be sure you are ready to connect the cracked plastic before you start applying! With the glue spread on the edges, push them together to connect them. Line up the edges carefully. Apply pressure to the reconnected plastic for 1 minute to allow it to set and then gently release the pressure. You can use a C-clamp to hold the plastic together.
Finally, allow the glue to fully cure. Different glues have different drying times, so check the packaging for specific instructions. Wait at least 1 hour before using the repaired plastic.
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Repairing cracks with water
Plastic is a versatile material, but it can crack or break. Luckily, repairing cracked plastic is relatively cheap and easy. Smaller cracks can be fixed with glue, hot water, or plastic slurry, while larger cracks may require welding with a soldering iron.
To repair cracks in plastic using water, follow these steps:
- Prepare the Water: Fill a bowl or container with hot water. The water should be hot but not boiling. You will also need a separate container of cold water to set the plastic after repairing the crack.
- Submerge the Plastic: Place the cracked plastic in the hot water. You may not need to fully submerge the entire piece of plastic, especially if you are only repairing a crack. Leave the plastic in the water for at least 30 seconds.
- Test Malleability: Take the plastic out of the water periodically to test its malleability. It is ready when it becomes soft enough to mould with your fingers.
- Reconnect the Cracked Plastic: When the plastic is malleable, remove it from the water. Press the cracked edges together to reconnect them, being careful not to squish the plastic so it doesn't appear uneven. You can use a C-clamp to hold the crack together.
- Set the Plastic: Fully submerge the repaired plastic in the container of cold water. Do not move it around and allow it to sit for at least 30 seconds.
- Let it Harden: Remove the plastic from the water and check to make sure it has hardened completely before using it.
Other Repair Methods
In addition to the water method, there are several other ways to repair cracks in plastic:
- Glue: Plastic glue, super glue, or epoxy can be used to reconnect cracked plastic. Clean and sand the surfaces, apply the glue, and hold or clamp the pieces together until the glue sets.
- Soldering Iron: For larger cracks, a soldering iron can be used to melt the edges of the crack so they can be easily reconnected without melting the rest of the plastic.
- Plastic Slurry: A plastic slurry can be used to fill in cracks. It will quickly bond with the plastic and start to harden.
- Plastic Welding: Plastic welding involves applying heat to the edges of two separate pieces of plastic to melt them together.
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Environmental stress cracking
Amorphous polymers exhibit ESC because of their loose structure, which makes it easier for fluid to permeate into the polymer. Amorphous polymers are more prone to ESC at temperatures higher than their glass transition temperature (Tg) due to the increased free volume. The result is a decrease in yield stress and Tg, as well as a plasticisation of the material, which leads to crazing at lower stresses and strains.
There are several methods used to evaluate a polymer's resistance to ESC. One common method is the use of the Bergen jig, which subjects the sample to variable strain during a single test. The Bell Telephone test is another widely used test, where bent strips are exposed to fluids of interest under controlled conditions.
ESC is considered the most frequent mode of failure of plastic materials. It was estimated in 1996 that 20% of plastics failed because of ESC. ESC occurs when materials in contact with a fluid are subjected to stress. It is accelerated by higher temperatures, cyclic loading, increased stress concentrations, and fatigue.
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Plastic design and stress
Plastic is a versatile material with a wide range of applications. However, it is susceptible to cracking and degradation over time, especially when subjected to stress and certain environmental factors. This phenomenon is known as stress cracking, and it can be influenced by various design and material considerations.
One crucial aspect of plastic design and stress is the choice of plastic type. Amorphous plastics, such as ABS, PS, PMMA, and PC, are more sensitive to continuous load and chemicals compared to semi-crystalline plastics or polymers. These amorphous materials are more prone to stress cracking, especially when exposed to elevated temperatures and chemical substances. Therefore, it is essential to select the appropriate plastic type based on the anticipated stress levels and environmental conditions.
The mechanical properties of the plastic play a significant role in its resistance to stress cracking. When designing plastic components, it is crucial to consider the maximum stress the component will encounter. By doubling the strength of the material in critical areas, the stress level can be reduced by half, thereby decreasing the likelihood of stress cracking. Additionally, the thickness of the plastic material should be consistent, as considerable variations in thickness can induce internal stress and lead to cracking.
Another factor to consider is the effect of time, load, and temperature on stress cracking. Plastic components subjected to prolonged stress, heavy loads, and elevated temperatures are more susceptible to cracking. Environmental stress cracking (ESC) occurs when chemicals and high temperatures catalyze the cracking process. Therefore, it is essential to design plastic products that can withstand the anticipated stress levels, loads, and temperature conditions to prevent premature failure.
To mitigate stress cracking, competent plastic product design is necessary. This includes selecting the appropriate plastic type, considering mechanical properties, and accounting for environmental factors. Additionally, proper stress testing during the design phase can help identify potential weaknesses. By subjecting the plastic component to slightly elevated stress and temperature conditions, one can evaluate its sensitivity to stress cracking. However, it is important not to exaggerate these conditions, as extremely high stress and temperature will inevitably lead to breakage.
In summary, plastic design and stress are intricately linked, and a comprehensive understanding of the material's behaviour under stress is essential for successful product development. By considering the plastic type, mechanical properties, environmental factors, and conducting thorough stress testing, designers can minimize the occurrence of stress cracking and prolong the service life of plastic products.
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Frequently asked questions
Plastic contains carbon and hydrogen and therefore degrades over time. As plastic ages, it gets weaker until it can no longer sustain a load and cracks.
Fixing a cracked plastic cover is relatively cheap and easy. You can use plastic glue, super glue, or epoxy to reconnect the cracked pieces. First, clean the surfaces you are bonding and lightly sand very smooth or shiny surfaces so the glue can adhere better. Apply an activator if required, then apply the glue to the edges of the crack and hold or clamp the pieces together until the glue sets.
When plastic is crushed, it first bends and then suddenly buckles into a different configuration, making a cracking noise and often a permanent crease. This process repeats many times on smaller and smaller scales, producing a stream of crackles.
To prevent your plastic cover from cracking, avoid exposing it to chemical substances and elevated temperatures, as these catalyze the process.
