Cellophane, a thin, transparent film, does not shrink significantly when exposed to heat like commercial heat-shrink materials. When subjected to moderate heat, cellophane maintains its physical structure. Excessive heat exposure will cause the material to degrade, dry out, and eventually char or burn. This difference in thermal behavior is rooted in cellophane’s distinct molecular composition compared to synthetic films.
Understanding Cellophane’s Structure
Cellophane is an organic material made from regenerated cellulose, a polymer derived from plant fibers like wood pulp or cotton linters. The chemical process creates a highly crystalline structure where long cellulose molecules are held together by strong hydrogen bonds. These extensive bonds form a rigid, three-dimensional network that provides the film with structural stability.
Cellophane is classified as a non-thermoplastic material because of this rigid, crystalline arrangement. Unlike plastic films that soften and melt when heated, cellophane does not possess a true melting point. Instead, excessive heat causes the cellulose to first dry out, then degrade through thermal decomposition.
This structural difference means cellophane lacks the ability to store mechanical stress. When heated, the film may become slightly more pliable, but the molecular network remains fixed. Cellophane maintains its structural integrity up to temperatures around 150°C, but higher temperatures lead to browning and eventual combustion rather than controlled shrinkage.
The Science Behind Polymer Shrinkage
The ability of a material to shrink tightly upon heating relies on the scientific principle of polymer memory. This effect is engineered into synthetic films through a precise manufacturing technique involving stretching and cooling. The polymer material, such as polyolefin or PVC, is first heated above its glass transition temperature (\(T_g\)), where its molecules become mobile and rubbery.
While in this soft state, the film is mechanically stretched, a process known as polymer orientation. Stretching forces the long, coiled polymer chains to straighten and align parallel to the direction of the force. The film is then rapidly cooled below its \(T_g\) while still under tension, which locks the stretched chains into a temporary, high-energy state.
When the oriented film is reheated, the temperature rises past the \(T_g\), allowing the polymer chains to regain mobility. The chains immediately attempt to revert to their original, coiled, low-energy state, effectively “remembering” their initial shape. This molecular relaxation causes the film to rapidly shrink, generating the tight, conforming seal characteristic of commercial heat-shrink packaging.
Cellophane vs. Commercial Heat-Shrink Films
The confusion between cellophane and heat-shrink films often occurs because consumers incorrectly use the term “cellophane” for any thin, clear wrapping material. True commercial heat-shrink films are made from synthetic thermoplastic polymers like polyolefin (POF), polyvinyl chloride (PVC), or polyethylene terephthalate (PET). These materials are specifically designed to utilize the polymer memory effect.
Unlike cellophane, which is used for its breathability and biodegradability in applications like candy wrapping, synthetic heat-shrink films are preferred for industrial packaging. POF film seals securely around products to provide tamper evidence, moisture protection, and a professional appearance. The heat required for these films to shrink is typically applied using a heat gun or a heat tunnel.
Cellophane’s rigidity prevents it from achieving a tight, uniform seal. The primary sealing method for cellophane packaging, often seen on cigarette boxes or perfume containers, is called over-wrapping. This process involves folding the film and sealing it with a localized, momentary burst of heat, which differs significantly from the full-film shrinkage seen in synthetic wraps.