Why Do Black Take-Out Boxes Scratch Easily?
author: Iris
2025-12-11
I. Basic Characteristics and Scratch Resistance of PP Material
1.1 Physical and Mechanical Properties of PP Material
PP (polypropylene), as a common thermoplastic, possesses unique physical characteristics. Its density is 0.90-0.91 g/cm³, the lowest density among all synthetic resins, approximately 60% of the density of PVC. This low density makes the PP material lightweight, facilitating use and transportation. The melting point of PP material is 164-176℃, the heat distortion temperature is 114℃, the Vicat softening point is greater than 140℃, and the operating temperature range is -30℃ to 140℃.
In terms of mechanical properties, the PP material exhibits good overall performance. Its tensile strength is generally 21-39 MPa, bending strength is 42-56 MPa, bending elastic modulus is 980-1560 MPa, and elongation at break is as high as 200%-400%. These data indicate that PP material has good strength and toughness, but also reveal its shortcomings in terms of hardness.
The hardness of PP material is usually measured by Rockwell hardness, generally in the range of Rockwell hardness R 95-105. Specifically, block copolymer PP with a low melt index has a Rockwell hardness of approximately 75, while high-hardness PP materials can reach 105-120. In comparison, PP material has a relatively low surface hardness among the five major general-purpose plastics, only slightly higher than PE, and significantly lower than PVC, PS, and ABS.
In terms of mechanical properties, the PP material exhibits good overall performance. Its tensile strength is generally 21-39 MPa, bending strength is 42-56 MPa, bending elastic modulus is 980-1560 MPa, and elongation at break is as high as 200%-400%. These data indicate that PP material has good strength and toughness, but also reveal its shortcomings in terms of hardness.
The hardness of PP material is usually measured by Rockwell hardness, generally in the range of Rockwell hardness R 95-105. Specifically, block copolymer PP with a low melt index has a Rockwell hardness of approximately 75, while high-hardness PP materials can reach 105-120. In comparison, PP material has a relatively low surface hardness among the five major general-purpose plastics, only slightly higher than PE, and significantly lower than PVC, PS, and ABS.
1.2 Analysis of the Scratch Resistance of PP Material
PP material has poor scratch resistance, which is determined by its inherent characteristics. According to professional tests, in a scratch comparison experiment of seven mainstream plastic materials, PP showed the worst scratch resistance, with the most obvious scratch marks. This is mainly because PP material is most prone to compressive yielding. Even adding mineral fillers to improve tensile strength only partially addresses fracture and yielding issues in certain areas, while the compressive properties remain largely unchanged. Therefore, the scratch resistance of filled PP is still not ideal.
The scratching process can be divided into three stages: the pressure application stage, the sliding stage, and the observation stage. In the pressure application stage, when the pressure on the material exceeds its compressive yield strength, irreversible plastic deformation occurs, leaving an indentation. In the sliding stage, the material is simultaneously subjected to compressive force, friction force, and tensile force. Due to its low compressive yield strength, PP material is prone to significant scratches under the combined action of these forces.
From the perspective of the material's molecular structure, the hardness of the PP material is affected by its crystallinity. PP material has high crystallinity (up to 95%), but this high crystallinity does not significantly increase its surface hardness to a level comparable to PVC, PS, and ABS. The relatively low surface hardness of PP material makes it easily scratched by sharp objects, which is one of the fundamental reasons why it is prone to scratching in household use.
The scratching process can be divided into three stages: the pressure application stage, the sliding stage, and the observation stage. In the pressure application stage, when the pressure on the material exceeds its compressive yield strength, irreversible plastic deformation occurs, leaving an indentation. In the sliding stage, the material is simultaneously subjected to compressive force, friction force, and tensile force. Due to its low compressive yield strength, PP material is prone to significant scratches under the combined action of these forces.
From the perspective of the material's molecular structure, the hardness of the PP material is affected by its crystallinity. PP material has high crystallinity (up to 95%), but this high crystallinity does not significantly increase its surface hardness to a level comparable to PVC, PS, and ABS. The relatively low surface hardness of PP material makes it easily scratched by sharp objects, which is one of the fundamental reasons why it is prone to scratching in household use.
II. The Impact of Black Colorants on the Performance of PP Material
2.1 Addition and Dispersion of Carbon Black Colorant
Black PP black Chinese take-out boxes use carbon black as the main colorant, usually added at a concentration of 2%-5%. Carbon black not only provides coloring but also enhances the material's UV resistance, preventing aging caused by long-term exposure. The type and amount of carbon black directly affect the coloring effect of plastic products. High-pigment carbon black can provide extremely high blackness and gloss, giving plastic products a deep black and shiny appearance.
However, the addition of carbon black also brings some negative effects. As the carbon black content increases, the strength and hardness of the composite material will increase, but its toughness and elastic modulus may decrease. This change in performance is closely related to the dispersion of carbon black in the PP matrix. If the carbon black is unevenly dispersed, it may form local defects, which become stress concentration points and are more likely to produce scratches when subjected to scratching.
The particle size of carbon black also has an important impact on its performance in PP material. Studies show that reducing carbon black particle size and increasing its content helps improve its coloring ability in talc powder, thus significantly improving scratch resistance. Experimental data show that samples with 0.4% carbon black A and 0.8% carbon black B (by weight) had ΔL values of 10.8 and 3.2, respectively, indicating that different types and amounts of carbon black have a significant impact on coloring effect and scratch resistance.
However, the addition of carbon black also brings some negative effects. As the carbon black content increases, the strength and hardness of the composite material will increase, but its toughness and elastic modulus may decrease. This change in performance is closely related to the dispersion of carbon black in the PP matrix. If the carbon black is unevenly dispersed, it may form local defects, which become stress concentration points and are more likely to produce scratches when subjected to scratching.
The particle size of carbon black also has an important impact on its performance in PP material. Studies show that reducing carbon black particle size and increasing its content helps improve its coloring ability in talc powder, thus significantly improving scratch resistance. Experimental data show that samples with 0.4% carbon black A and 0.8% carbon black B (by weight) had ΔL values of 10.8 and 3.2, respectively, indicating that different types and amounts of carbon black have a significant impact on coloring effect and scratch resistance.
2.2 Influence of Carbon Black on the Hardness and Toughness of PP Materials
The influence of carbon black on the hardness of PP materials shows a complex, non-linear relationship. Within a certain range, the amount of carbon black added is positively correlated with the hardness of the PP material. Experiments found that when the amount of carbon black N990 was increased from 0% to 10% (by weight), the Rockwell hardness of the PP product could be increased by 20%-30%. This increase in hardness is mainly because when carbon black particles are uniformly dispersed in the PP matrix, they act like tiny reinforcing particles, restricting the movement of plastic molecular chains, making the material more difficult to deform under external force.
However, when the amount of carbon black added is too high, negative effects occur. Excessive carbon black may lead to poor processing performance of the plastic, and may also cause the increase in hardness to no longer follow a linear pattern due to problems such as carbon black agglomeration, and may even lead to a decrease in the toughness and impact resistance of the plastic product. This decrease in performance is unfavorable for the use of Christmas take-out boxes, because reduced toughness means that the material is more likely to crack or break during scratching.
Carbon black, as an effective reinforcing agent, can significantly improve the mechanical properties of polypropylene materials, including tensile strength, hardness, and wear resistance. However, this reinforcing effect is conditional and requires good dispersion of carbon black in the PP matrix. If the carbon black is poorly dispersed, it will not only fail to provide reinforcement, but will also become a weak point in the material, making it prone to scratches and damage starting from these points during use.
However, when the amount of carbon black added is too high, negative effects occur. Excessive carbon black may lead to poor processing performance of the plastic, and may also cause the increase in hardness to no longer follow a linear pattern due to problems such as carbon black agglomeration, and may even lead to a decrease in the toughness and impact resistance of the plastic product. This decrease in performance is unfavorable for the use of Christmas take-out boxes, because reduced toughness means that the material is more likely to crack or break during scratching.
Carbon black, as an effective reinforcing agent, can significantly improve the mechanical properties of polypropylene materials, including tensile strength, hardness, and wear resistance. However, this reinforcing effect is conditional and requires good dispersion of carbon black in the PP matrix. If the carbon black is poorly dispersed, it will not only fail to provide reinforcement, but will also become a weak point in the material, making it prone to scratches and damage starting from these points during use.
2.3 Carbon Black Agglomeration and Surface Defects
The agglomeration of carbon black in PP materials is an important factor affecting its scratch resistance. Carbon black has a small primary particle size (large specific surface area), which, while improving the blackness, coloring power, UV resistance, and electrical conductivity of the plastic, also increases the viscosity of the plastic, reduces the dispersibility of the carbon black, and increases the water absorption of both the carbon black and the plastic. These characteristics make it difficult to achieve uniform dispersion of carbon black in a PP matrix.
When carbon black aggregates, it forms localized high-concentration regions within the material, increasing both the hardness and brittleness of these regions. During scratching, these aggregates may detach from the matrix, forming tiny pits, or act as stress concentration points, leading to crack initiation and propagation. Furthermore, carbon black aggregation affects the surface finish of the material, making the surface rougher and more prone to accumulating dirt and scratches.
Studies have also found that the dyeing ability of carbon black is related to the content and mesh size of other additives (such as talc). Higher talc content leads to a greater amount of exposed talc after scratching, and increasing the talc content and mesh size reduces the carbon black's dyeing ability, resulting in poorer scratch resistance. This indicates that in the formulation design of black PP black Chinese take-out boxes, the interactions between various additives have a significant impact on the final product's scratch resistance.
When carbon black aggregates, it forms localized high-concentration regions within the material, increasing both the hardness and brittleness of these regions. During scratching, these aggregates may detach from the matrix, forming tiny pits, or act as stress concentration points, leading to crack initiation and propagation. Furthermore, carbon black aggregation affects the surface finish of the material, making the surface rougher and more prone to accumulating dirt and scratches.
Studies have also found that the dyeing ability of carbon black is related to the content and mesh size of other additives (such as talc). Higher talc content leads to a greater amount of exposed talc after scratching, and increasing the talc content and mesh size reduces the carbon black's dyeing ability, resulting in poorer scratch resistance. This indicates that in the formulation design of black PP black Chinese take-out boxes, the interactions between various additives have a significant impact on the final product's scratch resistance.
III. The Impact of Home Use Environment on Scratching of PP Black Lunch Boxes
3.1 The Impact of Cleaning Tools
The cleaning tools used in home cleaning are one of the main reasons for scratches on black PP lunch boxes. Steel wool, as a powerful cleaning tool, can effectively remove stubborn stains, but it leaves noticeable scratches on the surface of PP lunch boxes. These scratches not only affect the appearance but also become breeding grounds for bacteria, and long-term use can affect food safety.
The cleaning mechanism of steel wool involves the physical scraping of the object's surface by its hard metal wires and sharp edges. This scraping force is destructive to materials with relatively low surface hardness, such as PP. Even using a scouring pad, if it contains abrasive particles or is made of a hard material, it will damage the surface of the PP lunch box. Therefore, when cleaning PP lunch boxes, a soft sponge with a neutral detergent should be used for cleaning, and they can be soaked in white vinegar for disinfection once a week, avoiding the use of steel wool or hard brushes.
It is worth noting that some consumers use abrasive scouring pads to clean burnt-on food from lunch boxes in pursuit of better cleaning results. Although abrasive scouring pads cause less wear and tear on lunch boxes than steel wool, they still cause noticeable scratches. The correct cleaning method is to use a soft scouring pad with a neutral detergent. This method achieves the cleaning effect without damaging the surface of the lunch box.
The cleaning mechanism of steel wool involves the physical scraping of the object's surface by its hard metal wires and sharp edges. This scraping force is destructive to materials with relatively low surface hardness, such as PP. Even using a scouring pad, if it contains abrasive particles or is made of a hard material, it will damage the surface of the PP lunch box. Therefore, when cleaning PP lunch boxes, a soft sponge with a neutral detergent should be used for cleaning, and they can be soaked in white vinegar for disinfection once a week, avoiding the use of steel wool or hard brushes.
It is worth noting that some consumers use abrasive scouring pads to clean burnt-on food from lunch boxes in pursuit of better cleaning results. Although abrasive scouring pads cause less wear and tear on lunch boxes than steel wool, they still cause noticeable scratches. The correct cleaning method is to use a soft scouring pad with a neutral detergent. This method achieves the cleaning effect without damaging the surface of the lunch box.
3.2 Frequency of Use and Physical Wear
The service life of PP lunch boxes is affected by five factors: frequency of use, cleaning methods, high-temperature exposure, physical wear, and material aging. In home use, PP black lunch boxes usually require frequent use and cleaning, and this high frequency of use accelerates surface wear.
Physical wear mainly comes from contact and friction between the lunch box and other items. In a home environment, PP lunch boxes may rub against metal cutlery, ceramic tableware, other plastic lunch boxes, etc. Especially when these items have rough surfaces or hard parts, scratches are more likely to occur on the surface of the PP lunch box. In addition, the lunch boxes will also experience wear and tear due to friction during stacking and storage.
An aged or scratched PP surface is not only more prone to bacterial growth but may also release more harmful substances (such as microplastics). Therefore, proper use and maintenance methods are crucial for extending the lifespan of PP black Chinese take-out boxes. It is recommended to avoid using aged or scratched containers, wash them by hand with a soft sponge, air dry them naturally, and avoid high-temperature storage.
Physical wear mainly comes from contact and friction between the lunch box and other items. In a home environment, PP lunch boxes may rub against metal cutlery, ceramic tableware, other plastic lunch boxes, etc. Especially when these items have rough surfaces or hard parts, scratches are more likely to occur on the surface of the PP lunch box. In addition, the lunch boxes will also experience wear and tear due to friction during stacking and storage.
An aged or scratched PP surface is not only more prone to bacterial growth but may also release more harmful substances (such as microplastics). Therefore, proper use and maintenance methods are crucial for extending the lifespan of PP black Chinese take-out boxes. It is recommended to avoid using aged or scratched containers, wash them by hand with a soft sponge, air dry them naturally, and avoid high-temperature storage.
3.3 Effects of High-Temperature Exposure
High-temperature exposure is another important factor affecting the performance of PP black Chinese take-out boxes. Although PP material has good heat resistance and can be used continuously at 110-120℃, in household use, if the temperature exceeds 65℃, the plastic container may deform, increasing the possibility of harmful substance migration.
High temperatures not only cause material deformation but also accelerate the aging process of the material. Under high-temperature conditions, the movement of PP molecular chains intensifies, and the intermolecular forces weaken, leading to a decrease in the hardness and strength of the material. This decrease in performance makes the food container more susceptible to scratches. In addition, high temperatures may also lead to the migration and aggregation of colorants (carbon black), further affecting surface performance.
It is particularly noteworthy that some black recycled plastics may contain impurities such as brominated flame retardants. At high temperatures of 200℃-300℃, these impurities may migrate into food, and long-term contact poses a risk of endocrine disruption. Therefore, when choosing black PP black Chinese take-out boxes, it is recommended to prioritize white, transparent, or light-colored products, and try to buy branded products with traceability, avoiding the use of unknown, all-black "unlabeled products."
High temperatures not only cause material deformation but also accelerate the aging process of the material. Under high-temperature conditions, the movement of PP molecular chains intensifies, and the intermolecular forces weaken, leading to a decrease in the hardness and strength of the material. This decrease in performance makes the food container more susceptible to scratches. In addition, high temperatures may also lead to the migration and aggregation of colorants (carbon black), further affecting surface performance.
It is particularly noteworthy that some black recycled plastics may contain impurities such as brominated flame retardants. At high temperatures of 200℃-300℃, these impurities may migrate into food, and long-term contact poses a risk of endocrine disruption. Therefore, when choosing black PP black Chinese take-out boxes, it is recommended to prioritize white, transparent, or light-colored products, and try to buy branded products with traceability, avoiding the use of unknown, all-black "unlabeled products."
IV. Comparative Analysis of PP Black Chinese Take-Out Boxes
4.1 Performance Comparison of PP and PET Black Chinese Take-Out Boxes
PET (polyethylene terephthalate) black Chinese take-out boxes outperform PP black Chinese take-out boxes in some aspects, but have a significant disadvantage in heat resistance. PET material has a higher tensile strength (40-70 MPa), significantly higher than PP (20-40 MPa), and also has excellent impact resistance, making it less prone to breakage, especially suitable for cold food packaging.
In terms of surface hardness, PET has a superior surface hardness compared to PBT, with a Shore D hardness of 80-85 and a Mohs hardness of approximately 3-4, which can resist a certain degree of surface scratching. In contrast, PP material has a relatively lower surface hardness, giving PET Christmas take-out boxes an advantage in scratch resistance. PET black Chinese take-out boxes also feature high transparency, resulting in good product display and good recyclability.
However, the biggest drawback of PET black Chinese take-out boxes is their poor heat resistance, generally withstanding temperatures below 60℃, making them unsuitable for hot food or microwave heating. This characteristic limits the scope of use for PET black Chinese take-out boxes, especially in household settings where food needs to be heated. In contrast, PP black Chinese take-out boxes have excellent heat resistance, withstanding temperatures up to 120-130℃, making them suitable for microwave heating, which is a significant advantage for household use.
In terms of surface hardness, PET has a superior surface hardness compared to PBT, with a Shore D hardness of 80-85 and a Mohs hardness of approximately 3-4, which can resist a certain degree of surface scratching. In contrast, PP material has a relatively lower surface hardness, giving PET Christmas take-out boxes an advantage in scratch resistance. PET black Chinese take-out boxes also feature high transparency, resulting in good product display and good recyclability.
However, the biggest drawback of PET black Chinese take-out boxes is their poor heat resistance, generally withstanding temperatures below 60℃, making them unsuitable for hot food or microwave heating. This characteristic limits the scope of use for PET black Chinese take-out boxes, especially in household settings where food needs to be heated. In contrast, PP black Chinese take-out boxes have excellent heat resistance, withstanding temperatures up to 120-130℃, making them suitable for microwave heating, which is a significant advantage for household use.
4.2 Performance Comparison of PP and PS Black Chinese Take-Out Boxes
PS (polystyrene) black Chinese take-out boxes are characterized by high hardness, with a hardness range of 70-105, good structural strength, and low cost. The hardness of PS material is significantly higher than that of PP, giving PS black Chinese take-out boxes an advantage in scratch resistance. PS black Chinese take-out boxes are rigid, not easily deformed, and maintain their shape well.
However, the fatal drawback of PS material is its brittleness and poor impact resistance; it is prone to cracking, especially in low-temperature environments. PS black Chinese take-out boxes may crack or even shatter with slight impact, so they are mostly suitable for single use or short-term storage and are not suitable for repeated use in household settings.
From an environmental perspective, PS black Chinese take-out boxes have poor recyclability, and foamed PS is difficult to collect, resulting in high recycling costs. This has led to PS black Chinese take-out boxes gradually losing market share in modern households that prioritize environmental protection. In contrast, although PP black Chinese take-out boxes are also traditional plastics and non-biodegradable, they have better durability and recyclability, making them more suitable for long-term household use.
However, the fatal drawback of PS material is its brittleness and poor impact resistance; it is prone to cracking, especially in low-temperature environments. PS black Chinese take-out boxes may crack or even shatter with slight impact, so they are mostly suitable for single use or short-term storage and are not suitable for repeated use in household settings.
From an environmental perspective, PS black Chinese take-out boxes have poor recyclability, and foamed PS is difficult to collect, resulting in high recycling costs. This has led to PS black Chinese take-out boxes gradually losing market share in modern households that prioritize environmental protection. In contrast, although PP black Chinese take-out boxes are also traditional plastics and non-biodegradable, they have better durability and recyclability, making them more suitable for long-term household use.
4.3 Performance Comparison of PP and PC Black Chinese Take-Out Boxes
PC (polycarbonate) Christmas take-out boxes have excellent overall performance. They have extremely high impact resistance, 250 times that of ordinary glass, and are known as "bulletproof plastic." PC material has high impact resistance, optical transparency, and thermal stability. Although its surface hardness (Shore D hardness 70-75) is slightly lower than that of PET, it can be significantly improved by modification (such as adding silicone, nano-ceramic particles, etc.) to enhance surface hardness and scratch resistance. The outstanding characteristics of PC material are its low compressive yield strength, high tensile strength, and tensile toughness. This results in unique behavior during scratching: the scratches are wide and deep, but the surface remains smooth and less prone to cracking. This property means that although PC black Chinese take-out boxes are easily scratched, the visibility of the scratches is relatively low, unlike those on PP black Chinese take-out boxes.
However, PC material also has some drawbacks. Pure PC has relatively low surface hardness and generally poor scratch resistance. More importantly, PC material may contain bisphenol A (BPA), and long-term or repeated use may release heavy metal antimony in high-temperature or acidic/alkaline environments. Its surface is also easily scratched, making it prone to harboring bacteria. Therefore, special attention should be paid to the usage conditions and cleaning methods of PC black Chinese take-out boxes in household use.
However, PC material also has some drawbacks. Pure PC has relatively low surface hardness and generally poor scratch resistance. More importantly, PC material may contain bisphenol A (BPA), and long-term or repeated use may release heavy metal antimony in high-temperature or acidic/alkaline environments. Its surface is also easily scratched, making it prone to harboring bacteria. Therefore, special attention should be paid to the usage conditions and cleaning methods of PC black Chinese take-out boxes in household use.
4.4 Comprehensive Comparative Analysis
Through a comprehensive comparative analysis of common food container materials such as PP, PET, PS, and PC, it can be seen that each material has its unique advantages and limitations. In terms of scratch resistance, PS > PET > PC > PP. PS black Chinese take-out boxes have the highest hardness and best scratch resistance, but are brittle and not suitable for repeated use; PET black Chinese take-out boxes have good surface hardness but are not heat-resistant; PC take-out bento box can achieve better scratch resistance through modification, but the cost is higher and there are potential health risks; PP black Chinese take-out boxes have the worst scratch resistance, but offer excellent heat resistance, toughness, and cost advantages.
From the perspective of household use, choosing a food container material requires considering multiple factors. If heat resistance and cost control are important, PP black Chinese take-out boxes are the best choice, especially for scenarios requiring microwave heating. If product display effect and transparency are important, PET take-out bento boxes are more suitable, but only for cold food use. If high hardness and scratch resistance are desired, PS black Chinese take-out boxes can be considered, but their brittleness must be accepted. If comprehensive performance is needed, PC black Chinese take-out boxes are a good choice, but it is important to choose BPA-free products and use them correctly.
From the perspective of household use, choosing a food container material requires considering multiple factors. If heat resistance and cost control are important, PP black Chinese take-out boxes are the best choice, especially for scenarios requiring microwave heating. If product display effect and transparency are important, PET take-out bento boxes are more suitable, but only for cold food use. If high hardness and scratch resistance are desired, PS black Chinese take-out boxes can be considered, but their brittleness must be accepted. If comprehensive performance is needed, PC black Chinese take-out boxes are a good choice, but it is important to choose BPA-free products and use them correctly.
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