Antifog additives are used to prevent the formation of fog on the inside of a plastic film, common uses are in agricultural and food packaging applications.
In the case of agricultural or horticultural films, the fog can have many detrimental effects. It will reduce light transmission, resulting in slower plant growth, delayed crop maturity and reduced crop yield per plant. Light and heat can be focused on delicate plant tissues with water droplets acting as lenses, resulting in the burning pf plant tissue or crop spoilage. The coalescence of small droplets into larger ones will create dripping, which can contribute to an increase in plant disease and plant damage.
In case of food packaging films, the fog can be detrimental to the quality of the food and also reduce the clarity of the packaging, which may reduce its shelf appeal for the consumer. These effects can be prevented or delayed by incorporation antifog additives into the plastic film.
Water droplets are typically spherical due to surface tension which acts to reduce the surface area. Droplets on a film develop a half-spherical shape which in turn, determines the effects on light scattering or reflection. This causes the light incident on the film to reflect internally, and reduces the clarity of the film.
Antifogs are typically molecules with a polar portion and a non-polar portion. Having this polar group on the surface allows better wetting of the film. The smaller the difference between these surface tension, the better the wetting will be of the plastic surface by water. In other words, a liquid wets a solid surface better when contact anglemade by the liquid with the solid surface is very low.
The contact angel is defined as the angle between the solid surface and the tangent line at the point of contact of the liquid droplet with the solid surface. The contact angle is 90 degrees for a perfectly half-spherical droplet and it approaches 0 degrees for a perfectly flat droplet. The contact angle of zero can only be achieved when the surface tension of the liquid is equal to, or lower than, the surface tension of the solid surface; this represents the ideal situation. The role of an antifog additive is to lower difference between the surface tension of the water and that pf the film so that the contact angle of water droplet woth the plastic surface is as low as possible. This reduces the multiple internal reflections of incident light inside water droplets and increases the clarity of the plastic film.
As the antifog additive comes to the surface with time, it may eventually be washed off along with the water layer. But internal antifog additives, distributed uniformly throughout the film thickness, act as a reservoir and increase the useful life of films for a given concentration of the antifog additive.
For food packaging applications, the antifog additives need to be FDA acceptable.
Antifog molecules consist of polar head groups such as hydroxy groups and a non-polar tail consisting of a hydrocarbon chain. The polar groups are needed for hydrogen bonding with water molecules. The length of the hydrocarbon chain governs the rate of diffusion/migration of the additive through the polymer matrix. The rate of diffusion of an antifog additive to the surface of a polymer film will depend on several other factors such as: the concentration of the additive; the thickness of the film; the chemical nature of the antifog as well as the polymer; crystallinity of the polymer; the solubility of the additive in the polymer, etc. The bulkiness/size of the antifog molecule may also control the long-term performance of the additive.
The performance of antifogs can be tested by actually measuring the contact angle of the water droplet with the polymer surface. Another method is a Corbi modified version of ICI’s cold fog test. According to this method, the test film is highly wrapped around a beaker containing 200 ml water. The beaker is placed in a refrigerator maintained at 4C. The appearance of the film is observed over a period of time (up to 20 days or more). The following scalar rating criteria is used to record the antifog performance:
– 1 = completely opaque film
– 8 = presence of four droplets
– 9 = presence of two droplets
– 10 = completely clear film
– all other intermediate ratings are subjective.
A rating of eight or higher is considered good for antifog performance.