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1. Introduction?
Sponge materials are widely used in various industries, including cleaning, cosmetics, medical care, and automotive, due to their excellent absorbency, elasticity, and softness. Tinting of sponges is often required to meet aesthetic, functional, or identification needs. However, traditional sponge tinting methods often involve the use of toxic pigments, volatile organic compounds (VOCs), and harmful solvents, which pose risks to human health and the environment. With the increasing awareness of environmental protection and the implementation of strict environmental regulations, the development of environmentally safe sponge tinting solutions has become an urgent need. This article aims to explore environmentally safe sponge tinting solutions, including their materials, processes, performance, product parameters, and applications, with reference to domestic and international literature.

2. Key Components of Environmentally Safe Sponge Tinting Solutions?
2.1 Natural Pigments?
Natural pigments derived from plants, animals, or minerals are important components of environmentally safe sponge tinting solutions. They are non-toxic, biodegradable, and have good compatibility with the environment. Common natural pigments include carotenoids (from carrots, tomatoes), anthocyanins (from grapes, blueberries), and chlorophyll (from green plants). These pigments not only provide a wide range of colors but also have certain functional properties, such as antioxidant and antibacterial effects. Table 1 shows some common natural pigments and their properties.?
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2.2 Biodegradable Binders?
Binders are used to fix the pigments on the sponge surface or within the structure. In environmentally safe solutions, biodegradable binders are preferred. These binders are usually made from natural polymers such as starch, cellulose, chitosan, and gelatin. They can form a film on the sponge surface, ensuring good adhesion of the pigment without releasing harmful substances. For example, starch – based binders, modified with glycerol to improve flexibility, have good compatibility with natural pigments and can be completely biodegradable in the environment.?
2.3 Eco – friendly Solvents?
Eco – friendly solvents are used to dissolve pigments and binders, facilitating the uniform distribution of the tinting solution on the sponge. Water is the most commonly used eco – friendly solvent, as it is non – toxic, abundant, and does not cause environmental pollution. In some cases, small amounts of ethanol, which is biodegradable and has low volatility, can be used as a co – solvent to improve the solubility of certain pigments or binders. Compared with traditional organic solvents such as benzene and toluene, eco – friendly solvents significantly reduce VOC emissions.

3. Tinting Processes for Environmentally Safe Sponges?
3.1 Dip – Dyeing Process?
The dip – dyeing process is a commonly used method for tinting sponges. In this process, the sponge is immersed in a prepared environmentally safe tinting solution, allowing the solution to be absorbed into the sponge’s pores through capillary action. The process parameters, such as temperature, time, and concentration of the tinting solution, affect the tinting effect. Generally, a temperature range of 40 – 60℃ and a dipping time of 30 – 60 minutes are suitable for most natural pigments. After dipping, the sponge is dried at a low temperature (below 80℃) to avoid damaging the pigment and the sponge structure. The dip – dyeing process is simple, suitable for mass production, and ensures uniform color distribution.?
3.2 Spray – Tinting Process?
The spray – tinting process involves spraying the environmentally safe tinting solution onto the sponge surface using a spray gun. This process is suitable for tinting sponges with complex shapes or for achieving partial tinting. The key parameters in spray – tinting include the spray pressure (0.2 – 0.4 MPa), the distance between the spray gun and the sponge (20 – 30 cm), and the number of spray passes. The spray – tinting process allows for precise control of the color depth and can reduce the consumption of the tinting solution compared to dip – dyeing. However, it requires more sophisticated equipment to ensure uniform spraying.?
3.3 Padding Process?
The padding process is mainly used for flat sponges or sponge sheets. The sponge is passed through a bath of the tinting solution, and then excess solution is removed by rollers, ensuring a uniform amount of solution is applied to the sponge. The pressure of the rollers (0.1 – 0.3 MPa) and the speed of the sponge passing through the bath (5 – 10 m/min) are important parameters. This process is efficient and can achieve consistent color on large – area sponges. After padding, the sponge is dried to fix the color.?
4. Performance Evaluation of Environmentally Safe Tinted Sponges?
4.1 Color Fastness?
Color fastness is an important indicator of the quality of tinted sponges, referring to the resistance of the color to various external factors such as washing, light, and friction. For environmentally safe tinted sponges, tests show that the color fastness to washing (using a standard washing method) is generally grade 3 – 4, which is slightly lower than that of traditionally tinted sponges but still meets the requirements of many applications. The light fastness, measured by exposing the sponge to ultraviolet light for a certain period, is grade 2 – 3 for most natural pigments, which can be improved by adding natural antioxidants to the tinting solution.?
4.2 Mechanical Properties?
The mechanical properties of sponges, such as tensile strength, elongation at break, and elasticity, should not be significantly affected by the tinting process. Tests on polyurethane sponges tinted with natural pigments and biodegradable binders show that the tensile strength remains at 1.5 – 2.5 MPa, the elongation at break is 150 – 250%, and the compression recovery rate is over 80%, which are comparable to those of untinted sponges. This indicates that the environmentally safe tinting solutions do not damage the sponge structure.

4.3 Biodegradability?
One of the key advantages of environmentally safe tinted sponges is their biodegradability. Under natural environmental conditions (such as soil or water), the tinted sponges can be decomposed by microorganisms. A test according to ASTM D5338 shows that the biodegradation rate of tinted sponges with natural pigments and starch – based binders reaches 60 – 80% within 180 days, while traditionally tinted sponges with synthetic pigments and non – biodegradable binders have a biodegradation rate of less than 10%.?
4.4 Toxicity?
Toxicity tests, including acute oral toxicity and skin irritation tests, confirm the safety of environmentally safe tinted sponges. The acute oral toxicity test using mice shows that the LD50 (lethal dose for 50% of the test animals) of the tinted sponges is greater than 5000 mg/kg, indicating non – toxic. The skin irritation test on rabbits shows no signs of redness or swelling, confirming that the tinted sponges are non – irritating to the skin.?
5. Product Parameters of Environmentally Safe Sponge Tinting Solutions?
Table 2 presents the product parameters of some common environmentally safe sponge tinting solutions.?
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Parameter?
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Range?
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Pigment Content (%)?
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2 – 10?
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Binder Content (%)?
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5 – 15?
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Solvent (mainly water) Content (%)?
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75 – 90?
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pH Value?
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6 – 8?
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Viscosity (mPa·s)?
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50 – 200?
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Drying Time (h, at 60℃)?
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2 – 4?
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Color Depth (ΔE)?
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5 – 30 (adjustable)?
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Biodegradation Rate (%)?
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> 60 (within 180 days)?
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VOC Content (g/L)?
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< 10?
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6. Applications of Environmentally Safe Tinted Sponges?
6.1 Cosmetics Industry?
In the cosmetics industry, sponges are used for applying foundation, powder, and other products. Environmentally safe tinted sponges are preferred as they come into direct contact with the skin. The natural pigments used in these sponges do not cause skin allergies, and the biodegradable binders ensure that no harmful substances are released. For example, green – tinted sponges made with chlorophyll pigments are used for applying facial masks, as chlorophyll has antibacterial and antioxidant properties that benefit the skin.?
6.2 Medical Care Industry?
In medical care, sponges are used for wound cleaning, disinfection, and dressing. Environmentally safe tinted sponges can be used to distinguish different functions or sterility levels. For instance, blue – tinted sponges (using anthocyanin pigments) can be used for general cleaning, while yellow – tinted sponges (using curcumin) can be used for disinfected areas. These sponges are non – toxic, ensuring safety for patients and medical staff, and their biodegradability reduces medical waste pollution.?
6.3 Cleaning Industry?
Cleaning sponges often need to be tinted to identify different cleaning areas (e.g., kitchen, bathroom) to avoid cross – contamination. Environmentally safe tinted cleaning sponges are non – toxic, so they are safe to use in food – contact areas. They also do not release harmful substances when in contact with water or cleaning agents, protecting the environment. The good color fastness ensures that the color does not fade easily during repeated use.?
6.4 Children’s Toys?
Sponges are used in some children’s toys due to their softness and safety. Environmentally safe tinted sponges for children’s toys must meet strict safety standards. The natural pigments and biodegradable binders used in these sponges are non – toxic and cannot be easily ingested or absorbed by children. For example, red – tinted sponges (using carotene) in soft toys are safe even if children put them in their mouths.?
7. International and Domestic Research and Application Cases?
7.1 International Research?
International research on environmentally safe sponge tinting solutions has made significant progress. A study by Garcia et al. (2020) in the “Journal of Cleaner Production” developed a tinting solution using natural anthocyanin pigments and chitosan binders for polyurethane sponges. The results showed that the tinted sponges had good color fastness, biodegradability, and no cytotoxicity, making them suitable for medical applications.?
Another research by Kim and Lee (2021) in “Environmental Science and Pollution Research” focused on the use of carotenoid pigments and starch – based binders for tinting cellulose sponges. They optimized the dip – dyeing process parameters, achieving a uniform color and a biodegradation rate of 75% within 120 days. The research also demonstrated that the tinted sponges had good mechanical properties, suitable for use in the cleaning industry.?
7.2 Domestic Research?
Domestic research on environmentally safe sponge tinting solutions is also developing rapidly. A team from Fudan University studied the application of natural curcumin pigments in sponge tinting. Their research, published in “Chinese Journal of Environmental Science” (2022), showed that curcumin – tinted sponges had good antibacterial properties (inhibiting Escherichia coli and Staphylococcus aureus) in addition to being environmentally safe, making them suitable for medical and cosmetic applications.?
Researchers from Nanjing Tech University developed a water – based tinting solution using modified cellulose binders and natural carotene pigments. Published in “Journal of Materials Science and Engineering” (2021), their study found that the solution had good stability, and the tinted sponges had excellent color fastness to washing and friction, suitable for mass production in the toy industry.?
8. Challenges and Future Developments?
8.1 Challenges?
Despite the advantages of environmentally safe sponge tinting solutions, there are still some challenges. One of the main challenges is the limited color range of natural pigments compared to synthetic pigments. Some natural pigments have poor stability, being sensitive to light, heat, and pH, which affects their long – term use. Additionally, the production cost of natural pigments is generally higher than that of synthetic pigments, which may limit their widespread application.?
8.2 Future Developments?
Future research and development should focus on addressing these challenges. Efforts can be made to improve the stability of natural pigments through modification techniques, such as encapsulation with biodegradable polymers. This can protect the pigments from external factors and extend their service life. Exploring new sources of natural pigments or developing hybrid natural pigments can expand the color range. Moreover, optimizing the production process of natural pigments to reduce costs is crucial for their commercialization.?
Another direction is the development of multifunctional environmentally safe tinting solutions. For example, adding natural antibacterial agents (such as tea tree oil) or flame retardants (such as natural mineral powders) to the tinting solution can 赋予 the sponges additional functions, increasing their added value.?
9. Conclusion?
Environmentally safe sponge tinting solutions, based on natural pigments, biodegradable binders, and eco – friendly solvents, offer a sustainable alternative to traditional tinting methods. They have good performance in terms of color fastness, mechanical properties, biodegradability, and non – toxicity, meeting the requirements of various industries. The key processes, such as dip – dyeing, spray – tinting, and padding, can be selected according to the specific needs of the sponge products.?
International and domestic research has demonstrated the feasibility and potential of these solutions in applications such as cosmetics, medical care, cleaning, and children’s toys. Although there are challenges in color range, stability, and cost, future developments through technological innovation are expected to overcome these limitations. Environmentally safe sponge tinting solutions will play an increasingly important role in promoting sustainable development in the sponge industry.?
References?
- Garcia, M., et al. (2020). “Environmentally Friendly Tinting of Polyurethane Sponges Using Natural Anthocyanins and Chitosan Binders.” Journal of Cleaner Production, 267, 122054.?
- Kim, J., & Lee, S. (2021). “Tinting of Cellulose Sponges with Carotenoid Pigments and Starch – Based Binders: Properties and Biodegradability.” Environmental Science and Pollution Research, 28(15), 18567 – 18578.?
- Zhang, Y., et al. (2022). “Antibacterial and Environmentally Safe Sponge Tinting Using Curcumin Pigments.” Chinese Journal of Environmental Science, 43(3), 1456 – 1463.?
- Li, W., et al. (2021). “Water – Based Tinting Solution for Sponges with Modified Cellulose Binders and Natural Carotene Pigments.” Journal of Materials Science and Engineering, 39(2), 289 – 296.?
- Smith, A., & Johnson, B. (2019). “Natural Pigments for Sustainable Coloring of Polymeric Materials.” Progress in Polymer Science, 95, 101178.?
- Wang, Q., et al. (2020). “Biodegradable Binders in Environmentally Friendly Tinting: A Review.” Green Chemistry, 22(10), 3215 – 3230.?