Plastic wrap, often referred to as cling film, is a common household item used primarily for covering and preserving food. It provides an effective barrier against liquid water, making it highly resistant to spills and splashes. However, the true effectiveness depends on understanding the difference between blocking liquid and blocking moisture in its gaseous form.
The Science Behind Water Repellency
The material’s ability to repel liquid water stems from its chemical composition and molecular structure. Most modern plastic wraps are made from non-polar polymers, such as low-density polyethylene (LDPE) or polyvinyl chloride (PVC), which are classified as hydrophobic. Water molecules are polar, having slight positive and negative charges. Since the non-polar plastic does not strongly interact with the polar water, liquid water beads up and runs off the surface. This creates a physical barrier that liquid water cannot easily penetrate or absorb.
Waterproof Versus Vapor Proof
It is important to distinguish between a material being waterproof and it being vapor-proof. Waterproof refers to stopping liquid water from passing through the film. Vapor-proof describes a material’s capacity to stop water in its gaseous state, such as water vapor or humidity, from permeating the barrier. The permeability of a material to gaseous water is measured by its Moisture Vapor Transmission Rate (MVTR). While plastic wrap is an excellent liquid barrier, common polyethylene films are often highly permeable to water vapor, meaning humidity can slowly escape or moisture from the outside air can enter. Therefore, while the plastic wrap prevents a glass of water from pouring out, it may not perfectly preserve a food item from drying out over several days.
Practical Applications and Limitations
In practical use, plastic wrap’s primary function is to protect surfaces from direct liquid contact, such as covering dishes to prevent food splatter or protecting items from minor splashes. The effectiveness of the wrap as a water barrier is often determined less by the material itself and more by the quality of the seal. If the film is not perfectly adhered to the container, liquid can easily seep through the gap. A loose or torn film allows water to bypass the barrier, meaning the continuous, unbroken physical seal is the limiting factor in achieving a complete liquid block.