The term “frozen food” refers to food that has qualifying quality food raw materials that have been processed appropriately, frozen to a temperature of -30 degrees Celsius, and then packaged, stored, and circulated at a temperature of -18 degrees Celsius or better. Due to the fact that the cold chain preservation method is carried out at a low temperature during the entire process, this particular category of food possesses the qualities of having a long shelf life, not being perishable, and being easy to consume. However, this also presents a significant number of obstacles and raises the bar for the requirements that must be met when selecting materials for packaging. At the moment, the majority of frozen food products on the market are packaged using polyethylene film by the majority of manufacturers. Despite this, a significant number of manufacturers have begun to focus their attention on CPP film because of its lack of transparency.
Cast extrusion is the method that is used to manufacture polypropylene film, which is referred to as CPP film. The nature of the procedure has an effect on the characteristics of the CPP film, which include outstanding transparency and smoothness, adequate resistance to heat and moisture, and good resistance to both. Besides being utilised as a single layer, it may also be utilised as a foundation film for composite materials or go through metallization treatment, which gradually demonstrates the growth trend of functional film materials. In addition, it can be utilised as a single layer. CPP films are still in their infancy when it comes to their application in the field of frozen food packaging. One of the most significant challenges is the succession of impacts and requirements that are placed on packing films as a result of the specific characteristics of the environment that is under refrigeration.
Consumption of dry goods, freezing situations, and burning phenomena
The growth and reproduction of microorganisms will be significantly restricted by the use of frozen storage, which will also slow down the rate at which food will go bad. However, as the freezing time continues, certain processes that occur, such as dry consumption, oxidation, and other similar processes, will become more severe. Under the conditions of the freezing chamber, the temperature and water vapour partial pressure shift in the following order: food surface > ambient air > cooler. The heat that is present on the surface of the food is transferred to the air that is surrounding it, which causes the food to cool down even further. On the other hand, the difference in the partial pressure of water vapour that exists between the surface of the food and the air that is surrounding it causes the moisture and ice crystals that are present on the surface of the food to evaporate, sublimate into water vapour, and then merge into the air. Heat is absorbed by the air that contains more water vapour at this point, which causes the density of the air to decrease and causes it to migrate towards the upper section of the freezer. During the process of cooling the air, water vapour will condense into frost and attach to the surface of the cooler. This occurs because the temperature of the cooler is extremely low, and the saturated water pressure at this temperature is also very small. At the same time, the air is being chilled. The air that has been dehumidified will become denser, which will cause it to sink. This will cause it to come into contact with food once more, so repeating the process. The moisture that is present on the surface of the meal is continuously lost throughout this cycle, which results in the food’s weight decreasing. It is referred to as “dry loss” in this procedure. As dry consumption continues, the surface of the meal will progressively become porous, which will increase its contact area with oxygen. This will speed up the oxidation of fats and pigments in the food, which will ultimately result in the browning of the surface and the denaturation of the protein. “Freeze Burn” is the name given to this phenomenon. Since the oxidation reaction of oxygen in the air and the transfer of water vapour are the root causes of the aforementioned phenomena, the CPP film that is used for packaging should have a good performance in blocking the penetration of water vapour and oxygen.
As soon as it is chilled, it becomes brittle.
The CPP film will become brittle and easy to break if it is exposed to a low temperature environment for an extended period of time, and its physical qualities will decline significantly as a result of this exposure. This has been discovered through practical application. The temperature at which plastics become embrittled is typically used to indicate the cold resistance of plastics. Due to the decreased mobility of its polymer molecular chains, plastic becomes brittle and easy to break as the temperature drops. This is because plastic molecules are less mobile. The plastic initiates the process of breaking when it is subjected to a particular external force. It is the “brittleness temperature” at this moment, which is the temperature that represents the lower limit of the usual use temperature of the plastic material. In the event that the cold resistance of the CPP film is inadequate, it is highly probable that the sharp protrusions of the frozen food will penetrate the packaging and cause leakage during the subsequent transportation process, as well as during the loading and unloading process. This will therefore hasten the process of the food going bad.
strategies for improvement
According to the study presented above, in order to enhance the barrier qualities and mechanical properties of CPP film so that it may play an excellent role in the packaging of frozen food, it is necessary to improve the film from the following three aspects:
Include a solidifying agent
There is a category of additives known as toughening agents, which have the ability to lessen the brittleness of materials and increase their resistance to impact. The following are examples of additives that are frequently utilised for the purpose of toughening CPP films: polyolefin plastomers, polyolefin elastomers, linear ultra-low density polyethylene, metallocenes, in addition to linear low density polyethylene. It is possible to provide a concise summary of its operating principle as follows: In the form of a dispersed phase, the elastomer is scattered in the matrix resin. Within the dispersed phase, there is a critical thickness that must be maintained between the elastomer particles. Under the influence of an external force, it will undergo deformation as a result of tension, compression, or impact. The two-phase interface will have a toughening impact if it has a quality of adhesion that is satisfactory. This effect of toughening is dependent on two different factors: To begin, the compatibility of the matrix and the toughening agent is an important consideration. It will be difficult to scatter the dispersed phase if the compatibility between the two is weak. Additionally, the interface between the two phases will have poor adhesion, which will have an impact on the toughening effect. Therefore, in order to create an effective toughening effect, it is necessary for the two properties to be compatible with one another. Second, the amount of the toughening chemical that should be used. In general, the density of the elastomer particles increases as the amount of toughening agent increases. This results in a significant improvement in the dart impact quality, tensile strength, and elongation at break of the film when it is in its original state at lower temperatures. The performance of the film, on the other hand, will remain consistent if the amount of toughener is increased to a particular degree.
Strengthen the manufacturing process
It is vital to note that temperature is a significant component that influences the mechanical properties of CPP films. The phrase “melt temperature” primarily refers to the temperature at which raw materials melt, which is the primary factor that controls the degree of plasticization and the rate at which the polymer cools and sets. The haze diminishes, the plasticization and cooling rate of the polymer accelerate, and the transparency and mechanical qualities increase as the temperature rises. All of these changes occur simultaneously. If, on the other hand, the melting temperature is higher than a particular range, the film’s transverse tensile strength and dart impact quality will fall instead. In order to maintain a reasonable range, often between 240 and 260 degrees Celsius, the temperature of the melt must be managed.
Because polypropylene (PP) is a semi-crystalline polymer, the crystallinity of the material has a tight relationship with its mechanical qualities, transparency, and haze. Increasing the temperature difference between the cooling roller and the melt temperature in the appropriate manner can quickly cause the PP molecular chain segments to lose their mobility and generate amorphous or polymers with small crystal nuclei, thereby significantly improving the mechanics of the film. This occurs after the raw materials have been melted. Transparency and performance are crucial. As a result, it is often agreeable to set the temperature of the cooling roll to approximately 24 degrees Celsius.
Only by beginning with the production process and working to improve the mechanical properties and barrier qualities of the CPP film is it possible to overcome the varied impacts of low temperature. This is due to the fact that the conditions under which frozen food is stored are quite unique. Additionally, the establishment of a historical database and the continued improvement of methods for detecting film quality will be of great assistance in the future for the production of CPP films as well as for the scientific research that will be conducted on them. The food film modification additives offered by COACE have the ability to enhance the performance of the material in terms of impact resistance, barrier qualities, and the preservation of the quality and freshness of packaged food!
