Anti-fogging additives: Improving transparency and performance of polymers

Multiple antifogging additives/mechanisms are available as per the operating environment. To combat fogging, it is crucial to understand the underlying chemistry. Find out the different chemistries of anti-fog agents.

 

Surfactants

Surfactants are amphipathic substances that are soluble in water. The hydrophilic-hydrophobic balance controls the dissolution of surfactants in the aqueous phase. The hydrophobic portion of the surfactant molecules prevents the complete spreading of water droplets. Therefore, the orientation of surfactant molecules can produce either hydrophilic or hydrophobic groups. The arrangement and orientation of surface atoms and functional groups influence the wettability of surfaces exposed to wetting liquid.

These surfactants have been used in various applications for a long time to achieve antifog effects. These polar additive molecules are relatively small and may migrate fast to the polymer surface as for polyolefin. Common non-ionic anti-fog surfactants used in anti-fog formulations are¹:
 

• glycerol esters
• polyglycerol esters
• polyoxyethylene esters
• sorbitan esters
• alcohol ethoxylates

Polymers
 

Synthetic or natural polymers are employed to create anti-fog surfaces. Common synthetic anti-fog polymers are shown in Table 1. Also, the use of natural polymers for anti-fogging purposes is of great interest because it incorporates non-toxic and environmentally friendly materials¹.
 

Inorganic compounds
 

Inorganic materials used in anti-fogging applications can be divided into two categories.
 

1. The first one is integrated by intrinsically hydrophilic materials such as SiO₂, or In₂O₃-SnO₂ in various forms. This includes solid, hollow, or mesoporous nanoparticles, nanoflakes, and nanorods.
2. The second group is constituted by photoactive materials like TiO₂ or ZnO. They become super hydrophilic when exposed to UV or following suitable chemical or physical modifications to visible light.

Other materials
 

The use of electrothermal coatings using graphene has attracted growing interest for antifogging/defrosting purposes. They are used in:
 

• outdoor displays,
• back and side windows of vehicles,
• window defrosters,
• heat retaining windows, etc.
   

The stacking of double-layered hydroxide nanoparticles with sodium poly (4-styrene sulfonate) has shown potential for anti-fogging coatings, particularly those consisting of metal oxides².

 

A lot of different methods are used to create an anti-fog effect on the surface of plastic which minimizes the formation of fog. Methods can be divided into 3 types, i.e., internal additives, external coatings, and surface modification. The choice of a proper anti-fog effect depends on the following criteria:
 

• polymer type
• the thickness of the plastic
• processing circumstances
• physical conditions (e.g., temperature, humidity, etc.)

Internal additives
 

Internal anti-fog additives are tensoactive materials, mainly non-ionic surfactants. These additives are added to plastics mostly in the form of masterbatches. They are added during compounding or manufacturing.

Internal anti-fog additives form a hydrophilic layer on the surface of the plastic when they are migrating towards the surface. Migration from the bulk layer towards the surface of the plastic results from the fact that these additives and polymer matrix are incompatible with each other.

Anti-fog additives are constantly migrated towards the surface of plastic over time. If an anti-fog additive is available in the polymer there will be migration and the wetting properties will improve. The performance of an anti-fog additive is based on its migration from the bulk layer towards the surface of the plastic. Figure 1 illustrates the operational principle of an anti-fogging additive in plastics.

Figure 1: The Operational Principle of an Anti-fogging Additive³

External coatings

Applying an anti-fog coating to the plastic surface prevents water condensation on the surface (Figure 2). Anti-fog coatings do not degrade even after a lot of use, as they are:
 

• translucent,
• abrasion-resistant, and
• long-lasting

External coatings are generally viscose liquids that are diluted with either water or alcohol or are used undiluted. Anti-fog coatings are spread consistently on the surface of plastic with a dip or spray coater. After the external coating has been added, the film usually goes through dryers where the solvent evaporates and the coating cures on the surface of the film. Anti-fog coatings are typically applied off-line. However, in-line applications are more desirable because of cost savings.

 

Figure 2: Mechanism of Anti-fog Coatings⁴

Anti-fog coatings can be divided into 3 categories according to their chemical nature. These categories include organic, inorganic, and inorganic-organic.
 

• Organic coating: This involves depositing thin films of natural or synthetic water-soluble polymers bearing hydrophilic functionalities. It is one of the simplest and most effective ways to endow any transparent material with anti-fogging performance.

• Inorganic coating: They are more durable and resistant to scratching, abrasion, and chemical degradation than organic coatings. This makes them suitable for use where the coated surface is exposed to harsh conditions or frequent cleaning. Common additives include metal oxides, metal salts, and porous materials (zeolites and silica aerogels).

• Organic-inorganic coating: Another winning strategy for preparing anti-fog coatings involves hybrid materials. Here, organic and inorganic components are blended at the nanometric level. The inorganic nanomaterials are mixed with polymers to build nanostructured 3D networks. This is the core concept of designing anti-fogging films that display the qualities of both materials.

External anti-fog coatings surpass internal additives
 

Internal anti-fog additives are used generally with polyolefins. However, their suitability with esters is worse. The compatibility of PET with internal agents is in most cases too good, thus, migration towards the surface turns out to be very slow. In such cases, it is common to use external anti-fog coatings. The processing temperatures of PET are higher than those of polyolefins. Thus, internal additives may degrade during the melt process.

The use of anti-fog coatings is also common when the polymer layer is very thin. The thin polymer layer includes a smaller amount of anti-fog additive than the thicker layer which causes the rapid decrease of anti-fog effect. Anti-fog coatings include surfactants similar to internal anti-fog additives. In the case of liquid coatings, the used surfactants are generally anionic or non-ionic.

 

Surface treatment

A lot of surface treatment methods are available to process polymer surface properties. These are cost-effective alternatives for creating anti-fog properties in plastics. The chemistry of polymer surfaces can be changed, for example, by using plasma, corona, flame treatments, grafting, etc.
 

• Plasma treatment takes place in air or oxygen and is the most designated technique when oxidizing surfaces of polymers. The depth of oxidation in air plasma treatment is >10 nanometers.
• Equally used techniques are corona and flame treatments. In flame treatment, surface combustion of polymers occurs where hydroperoxide and hydroxyl radicals are formed on the surface. The depth of oxidation in flame treatment is 5-10 nanometers.

All plasma, corona, and flame treatments effectively process polymer surfaces resulting in improved wettability. However, the efficiency of treatment depends on the polymer that is being treated. Polar groups that are formed during surface oxidation do not remain on the surface of the polymer. They tend to penetrate the polymer bulk when the plastic is in contact with air for a long time. The polar groups remain better on the surface if the surface is in contact with the polar environment⁵.

 

Anti-fogging surfaces have found many applications in a wide range of fields. The large-scale industrial and professional ones are by far the most relevant. The main sectors of activity where anti-fogging technology is applied include the:
 

• Automotive industry (e.g., windshield glass and rearview mirrors),
• Optical industry (e.g., lenses, cameras, telescopes, and sensors),
• Architectural sector (e.g., windows and mirrors),
• Solar industry (e.g., photovoltaic modules),
• Food industry (e.g., food packaging), and
• Medicine (e.g., goggles and endoscopes)
   

Food packaging and greenhouse films
 

Some concerns caused by fogging in the food industry can be found in food packaging and greenhouses. In the horticultural sector, the undesirable effects of condensation arise from the reduction by more than 50% of the light transmission through the greenhouse claddings and damage provoked by dripping water¹.

In addition, droplets can cause further injury to plants, as they behave as an array of convex lenses that focus the sun's rays on them (lens effect) (Figure 3). For vegetable growers, these detrimental effects result in delayed crop maturity. Hence, a late delivery date of poor-quality produce. The use of anti-fogging plastics in greenhouses is thus highly recommended, as better light transmission enhances photosynthesis and improves crop yield.

 

Figure 3: Effect of Anti-fogging Additives Incorporated Agricultural Film for Plant Growth and Yield⁶

 

For example, the incorporation of a 2% active level of Cargill anti-fog additive (Atmer™ 103) into LDPE agricultural films leads to the following benefits:
 

• Improved light transmission through the film by almost 50%. This spreads condensed water droplets into a thin transparent layer of water on the surface of the film.
• Almost a 60% reduction in fruit damage as compared to the control sample.
• The ripening rates of the tomatoes also increased significantly each day (Figure 4). Over 25 days, there was a 26% increase in the amount of ripe fruit collected compared to the previous season.

Figure 4: Increased Ripening Rates of Tomatoes when Using 2% Atmer™ 103 in LDPE Greenhouse Film⁶

In general, commercial additives used in greenhouses are also employed in food packaging. Following are some examples:
 

• Armofog 151-XE35 by AkzoNobel, Netherlands, for PE and EVA,
• Atmer™ 7373 by Cargill Polymer Additives, UK,
• PGE 907 by Danisco Emulsifiers, Denmark,
• PPM12927 by Techmer PM, USA, and
• ONCAP™ by Avient Corporation, (formerly PolyOne Corp), USA, for PP,
• LOXIOL® A 4 SPEZIAL by Emery Oleochemicals, Malaysia, for PVC, and
• VF 152 by Polyvel, Inc. USA, for LDPE

The application of anti-fogging films in food packaging is on the rise and under perpetual development¹. When freshly prepared foods are packed and exhibited in a chiller cabinet in supermarkets, moisture usually tends to condense on the inner surface of the packages. This thereby reduces the consumer's ability to see the product. When produce is not aesthetically and hygienically displayed, the condensation may result in food spoilage.

Using polymeric films with anti-fogging performance increases the shelf life of produce. The 'see-through' property provides a more attractive display of freshly chopped vegetables or meats. Figure 5 shows plastic film with and without anti-fog additives.

 

Figure 5: a) Plastic Film with Anti-fog Property; b) Plastic Film without Anti-fog Property³

By adding a suitable anti-fogging additive to food wrap film, condensed water droplets are spread into a thin continuous layer. This improves the transparency of the packaging and the durability of the contents. For example, Cargill anti-fog additive Atmer™ 7373 is specially formulated for polypropylene. Having been incorporated into the polymer at 3.75%, Atmer™ 7373 dramatically reduced the amount of fog formation on the packaging. Over 95% of packages showed complete clarity and no droplet buildup. The water droplets spread evenly in a continuous layer across the film when compared to glycerol-based anti-fogs. Also, polypropylene film incorporated with Atmer™ 7373 showed uniform performance irrespective of position within the cabinet as compared to a glycerol-based anti-fog⁶.

Multi-layer co-extruded films are modern, long-lasting agricultural films. They commonly use a co-extruded structure to further enhance their properties. Typically, these structures are employed to allow the use of the core layer of the structure to achieve a controlled release effect of the anti-fogging additive to the surface of the film (Figure 6). This layer will be highly loaded, usually around 5% active with a lower 1% active used in the inside skin layer.

 

Figure 6: Cross Section of Three-layer Co-extruded Film⁶

Automotive industry

The automotive industry has been a major driver of anti-fogging technology development. Windshield fogging is a significant safety hazard, especially in adverse weather conditions. Many automakers have implemented hydrophobic coatings on windshields. This repels water and prevents droplet formation. These coatings often utilize nanotechnology to create a micro-textured surface that mimics the self-cleaning properties of lotus leaves.

VOC-free anti-fog additive formulation-based coating is used to prevent fog accumulation on PC and PMMA sheets and panels. Such additives provide a long-term antifogging activity while avoiding the challenges of integrating them into the product itself. The short drying and curing cycle of the anti-fog additives is advantageous for in-line application⁷.

Similarly, in a highly humid environment, bathroom and automobile mirrors have the same problem⁸. Fogging makes them useless for an extended period, as condensation quickly recurs, although a cloth can be used to wipe them dry. In this regard, anti-fogging mirrors with photocatalytic activity (cleaning of whitish stains appearing after wiping) or equipped with heat elements have been very successful in preventing such inconveniences (Figure 7).

 

Figure 7: Automobile Side-view Mirror with Anti-fog TiO₂ Coating (Right) and without Anti-fog TiO₂ Coating (Left)⁸

Optical industry

There are many concrete examples related to eyeglasses or protective eyewear misting. For example, having a steaming cup of coffee or even when opening the dishwasher results in fogged glasses. Physical activities like swimming, skiing, or snowboarding goggles also create favorable conditions for fogging to occur. Besides being a general inconvenience, fogging can also give rise to safety concerns. 

Protective eyewear misting up is proven to be a significant safety issue¹. Surgeons and surgical technicians also experience the frustrating problem of fogging goggles when performing surgery¹. Several studies show how to effectively deal with this problem. For example, the fogging of surgical goggles can be reduced by applying an anti-fogging gel (Figure 8).

 

Figure 8: Anti-fogging Gel on a Treated Half of Goggles⁹

Electronics industry
 

Due to the increase in the use of touchscreens and displays, anti-fogging technology is key for a better user experience.
 

• Anti-fog smartphones: Several smartphone manufacturers have introduced anti-fog coatings on display screens. This prevents smudges and fogging during use in humid conditions.

• Anti-fog industrial displays: In industrial settings, anti-fog technology is essential for clear visibility in harsh environments. Displays used in cold storage facilities or outdoor applications benefit from anti-fog coatings.

Anti-fog coatings on glass

While primarily used for plastics, anti-fog additives can also be applied to glass surfaces in some cases. However, the adhesion and durability of the coating may be different compared to plastics. The antifog coating has a combination of both hydrophilic and water-absorbing functions
 

• The hydrophilic function maintains high visibility in low to medium-humidity environments by absorbing moisture.
• The water-absorbing function forms a water film to maintain visibility in high humidity.

This makes it applicable for use in places where temperature and humidity are highly changeable. For example, a smooth transparent antifogging coating can have little light scattering, with almost no light absorption in the visible light range (Figure 9).


 

Figure 9: Transmission and Reflectance Characteristics of Substrate with and without Anti-fog Coating¹⁰

The growing demand for functional materials with special wetting behavior has led to the development of anti-fogging surfaces with a wider range of applicability. The anti-fogging industry is evolving, driven by technological advancements and changing consumer demands. Some key trends are explained below¹¹.
 

• Sustainable anti-fog solutions: Increasing the use of eco-friendly materials (hydrophilic) to reduce the environmental impact.

• Nanotechnology: The development of innovative synthesis strategies and new morphologies of anti-fogging materials.

• Integration with other technologies: Designing anti-fogging materials with additional features further improves the performance of solar cells. For example, the incorporation of antireflective or self-cleaning properties. Surfaces combining anti-fogging and anti-bacterial characteristics show potential in several applications. For example, dealing with condensation and bacterial growth in the food packaging industry and medical practice.

   

• Smart coatings: The development of coatings that respond to environmental conditions, such as humidity and temperature, is a promising area of research. Also, future development is related to the implementation of one-step manufacturing processes.

Anti-fogging technology is crucial for preventing water condensation on various surfaces, enhancing product performance, and ensuring safety. By understanding the fog formation and the properties of materials, scientists and engineers have developed various methods to combat this issue. These include:
 

• incorporating anti-fog additives into plastics,
• applying specialized coatings, and
• modifying surface properties
   

While challenges such as durability and cost-effectiveness persist, ongoing research and development in nanotechnology and sustainable materials promise to advance anti-fogging capabilities.

The applications of anti-fogging technology span industries from automotive to packaging and electronics. Anti-fog solutions address issues like fogged windshields, condensation on food packaging, and obscured displays. They improve product quality, user experience, and overall efficiency. With evolving consumer demands and environmental concerns, the anti-fog industry is innovating to meet these challenges and create a clearer future.