Plastic wrap, or cling film, is widely used to cover food and containers, creating a barrier against the environment. Many people rely on this thin, flexible film to protect contents from spills, yet confusion exists about its true protective capabilities. Determining whether this material is truly waterproof or merely water resistant requires examining the precise language used in material science. This analysis will look at the film’s structural properties and how it interacts with liquid water and water vapor.
Defining Waterproof and Water Resistant
The terms waterproof and water resistant describe two distinct levels of defense against liquid penetration. Waterproof refers to a material that is completely impervious to water, meaning it does not allow water to pass through it at all, even under pressure or prolonged submersion. This standard is typically measured by resistance to hydrostatic pressure.
Water resistant, by contrast, describes a material that can impede or repel the penetration of water to some degree, but not entirely. Such materials can handle minor exposure, like splashes or brief contact. However, they will eventually allow water to pass through under sustained pressure or extended time, providing temporary protection rather than an absolute barrier.
The Material Science of Plastic Wrap
Most modern consumer plastic wrap is primarily manufactured from low-density polyethylene (LDPE), although some varieties may contain polyvinyl chloride (PVC). These polymers are inherently hydrophobic, meaning their chemical structure causes them to physically repel liquid water. The long chains of hydrocarbon molecules in polyethylene do not bond well with polar water molecules, causing water to bead up on the surface.
While the material resists water, the structural integrity of the film is compromised by its extreme thinness. LDPE film is commonly manufactured at a thickness of around 30 micrometers. This thinness means the material lacks the robust structure needed to maintain total impermeability under stress. The physical structure, rather than the chemical composition, becomes the limiting factor in the film’s barrier properties.
Performance Against Liquid Water
Standard consumer plastic wrap is accurately classified as water resistant rather than waterproof. The film performs excellently in low-pressure, static situations, such as covering a bowl of leftovers or wrapping a sandwich. In these common household applications, the film prevents bulk liquid water from easily passing through the material.
However, the film’s thinness and lack of structural rigidity prevent it from meeting the stringent criteria for being truly waterproof. Any significant mechanical stress, stretching, or sustained hydrostatic pressure will cause the barrier to fail. The manufacturing process of such thin films inevitably introduces microscopic imperfections or pinholes that compromise the seal over time. If a container covered with plastic wrap were submerged, the water pressure would likely force liquid through these weak points.
Beyond Liquid Water: Moisture Vapor Transmission
The interaction of plastic wrap with water changes when considering water in its gaseous form, known as water vapor or moisture. This behavior is quantified by the Water Vapor Transmission Rate (WVTR), which measures how much moisture vapor passes through a material over a set period. Even though the film successfully resists liquid water, it is highly permeable to these smaller, gaseous water molecules.
LDPE, the primary component of many plastic wraps, has a relatively high WVTR compared to other packaging materials. This permeability explains why food stored in plastic wrap can still dry out or develop freezer burn over time. The material blocks the large cluster of molecules that make up a liquid water drop but cannot effectively stop the smaller water vapor molecules from passing through the polymer matrix. While plastic wrap offers a good barrier against bulk liquid water, it is a poor barrier against atmospheric moisture.