Plastic wrap
Plastic wrap is a thin, flexible, transparent plastic film primarily used to seal containers and cover food items, thereby preserving freshness by restricting oxygen exposure and moisture loss. It is most commonly manufactured from low-density polyethylene (LDPE) for general food-safe applications or polyvinyl chloride (PVC) enhanced with plasticizers for stronger cling adhesion.[1][2] The material originated from polyvinylidene chloride (PVDC), discovered accidentally in 1933 by Dow Chemical lab worker Ralph Wiley during equipment cleaning, when a residue proved resistant to solvents; this led to the development of Saran Wrap, first commercialized for industrial use in 1949 and households in 1953.[3][4] Widely applied in kitchens for wrapping leftovers, steaming vegetables, and preventing freezer burn, plastic wrap has enabled efficient food storage and reduced spoilage-related waste, though its persistence in the environment raises concerns over microplastic pollution and wildlife entanglement; empirical assessments nonetheless show that such films lower net ecological footprints by curbing food waste emissions relative to less durable alternatives.[1][5][6]History
Invention and early development
In 1933, Ralph M. Wiley, a laboratory assistant and college student at Dow Chemical Company in Midland, Michigan, accidentally discovered polyvinylidene chloride (PVDC) while cleaning glassware used in experiments related to dry cleaning solvents and adhesives.[7][8] Wiley observed a clear, film-like residue coating the inside of a flask that resisted dissolution in common solvents, prompting further analysis which revealed it as a novel polymer formed from vinylidene chloride monomers.[9][4] This serendipitous finding marked the initial identification of PVDC, distinct from earlier cellulosic films like cellophane, which Swiss chemist Jacques E. Brandenberger had invented in 1908 as a transparent, regenerated cellulose sheet for packaging but lacked inherent adhesiveness and superior chemical stability.[10][11] Early evaluations highlighted PVDC's unique properties, including strong clinginess due to its tacky surface, exceptional resistance to oils, greases, and corrosive chemicals, and high impermeability to gases and moisture—qualities that surpassed cellophane's limitations in barrier performance and durability under varied conditions.[7][8] These attributes stemmed from PVDC's chlorinated structure, which provided denser molecular packing and lower permeability compared to cellophane's more porous, hygroscopic cellulose base.[12] Dow chemists recognized its potential for thin-film applications, initiating laboratory-scale polymerization studies to control the radical-initiated reaction of vinylidene chloride, yielding consistent, flexible sheets rather than brittle residues.[4][8] Through the late 1930s, pre-World War II research at Dow focused on refining PVDC synthesis, including copolymerization techniques to enhance processability and reduce crystallinity for extrudable films, laying the groundwork for scalable thin-film production without immediate commercial intent.[7][8] By 1940, Dow formally registered the polymer, solidifying its foundational role in developing durable, cling-forming wraps superior to wax papers or early cellophane variants in laboratory sealing and containment tests.[8][4]Commercialization and material shifts
Dow Chemical Company commercialized Saran Wrap in 1949 for industrial applications, followed by its national household launch in 1953 as the first dedicated cling film designed specifically for food preservation. Formulated from polyvinylidene chloride (PVDC), the product offered exceptional adhesion and moisture/oxygen barrier properties, enabling effective sealing of fresh foods and leftovers without refrigeration dependency. Within five years of the consumer rollout, over 200 million rolls were sold, establishing it as a staple in post-World War II kitchens and transforming routine food storage practices.[4][7][8] By the early 2000s, formulations began shifting from PVDC and polyvinyl chloride (PVC) dominance due to escalating processing challenges and regulatory scrutiny of chlorine-containing polymers. In 2004, S.C. Johnson & Son, then owner of the Saran brand, replaced PVDC with low-density polyethylene (LDPE), emphasizing environmental benefits like reduced chlorine emissions during production, though the change compromised original cling strength, heat tolerance, and gas barrier efficacy without substantiating direct health risks from PVDC residues in approved food contact uses. This transition prioritized manufacturability and compliance costs over maintaining superior performance metrics.[13][14][3] Post-2000 adaptations accelerated globally toward non-chlorinated options such as LDPE or linear low-density polyethylene (LLDPE) for broader cost savings in extrusion and recyclability, particularly in North American markets where chlorinated variants faced stricter disposal regulations. Outside the United States, PVC-based films retained prevalence in cling wrap due to their inherent tackiness from plasticizer formulations, sustaining demand in regions with less emphasis on chlorine phase-outs. These material evolutions reflected pragmatic responses to supply chain economics and policy incentives rather than uniform performance mandates.[3][15]Composition and Production
Primary materials and formulations
Plastic wrap formulations primarily utilize polyvinyl chloride (PVC) as the base polymer, valued for its inherent flexibility, clarity, and cling properties derived from its linear chain structure with polar chlorine atoms.[16] Polyvinylidene chloride (PVDC) serves as a key alternative or blend component, offering superior gas barrier performance with an oxygen permeability coefficient of 1.2 cm³·mm/(m²·24 h·MPa).[17] Low-density polyethylene (LDPE) and linear low-density polyethylene (LLDPE) are employed in non-PVC variants for their chemical inertness and compatibility with recycling streams.[18] To achieve desired cling and processability, PVC-based formulations incorporate plasticizers such as di(2-ethylhexyl) adipate (DEHA) at concentrations ranging from 22% to 28.3% by weight.[19][20] These additives reduce the glass transition temperature, enabling thin-film extrusion while maintaining adhesion without natural tackiness in pure LDPE or PVDC polymers.[16] Formulations are typically tuned for film thicknesses of 10 to 30 micrometers, balancing tensile strength and barrier efficacy; modern blends often employ co-extrusion of PVC outer layers with PVDC cores to enhance oxygen impermeability while preserving overall flexibility.[21][22]Manufacturing processes
The manufacturing of plastic wrap primarily relies on extrusion processes, where thermoplastic polymer resins, such as polyvinyl chloride (PVC) or polyethylene (PE), are melted and shaped into continuous thin films. In the standard flat-die extrusion method, resin pellets are fed into a screw extruder, heated to temperatures between 150°C and 250°C to achieve a molten state, and then forced through a narrow flat die to form a thin sheet of uniform thickness, typically ranging from 5 to 20 micrometers. The extruded film is immediately cooled on chilled rollers to solidify it, preventing crystallization and maintaining clarity and flexibility, before being slit into narrower widths and wound onto rolls for further processing or packaging.[23][24] For enhanced barrier properties, co-extrusion techniques are employed to produce multi-layer films by simultaneously melting and extruding different polymer formulations through a multi-manifold die, allowing precise layering of materials like PVC with polyvinylidene chloride (PVDC) or PE to optimize cling, puncture resistance, and gas permeability. This process supports high-throughput production, with modern lines capable of outputs exceeding 500 kg per hour per extruder, facilitating scalability for industrial demands. Blown film extrusion variants, particularly for PVC cling wrap, involve extruding molten polymer through a circular die to form a tube, which is inflated with air, stretched, and cooled to achieve biaxial orientation and superior cling characteristics.[25][26] Post-extrusion quality controls include corona treatment, where the film surface is exposed to a high-voltage electrical discharge in air to generate ozone and reactive species, increasing surface energy from around 30-35 dynes/cm to 38-42 dynes/cm or higher, thereby improving wettability for printing, coating, and lamination without altering bulk properties. Uniformity is ensured through automated gauging and edge trimming, while the thin gauge of the films results in relatively low energy consumption per unit length, often under 5 MJ per kg of output due to efficient heat transfer and minimal material use in high-speed lines. These steps enable consistent production of defect-free rolls suitable for end-use.[27][28]Physical and Chemical Properties
Key mechanical and barrier properties
Plastic wrap demonstrates tensile strength ranging from 10 to 30 MPa, permitting the film to endure applied stresses during stretching without fracturing.[29] Its elongation at break commonly exceeds 300%, facilitating extensive deformation prior to rupture and contributing to conformability around irregular shapes.[29] Cling adhesion, essential for self-sealing, derives from triboelectric charging in unmodified films or incorporation of tackifying agents, quantifiable via peel force per ASTM D5458 standards.[30] Barrier performance features low oxygen transmission rates, with PVDC formulations achieving values below 1 cc/100 in²/day at 23°C and 0% RH, substantially outperforming waxed paper or uncoated alternatives that exhibit higher gas permeability and thus reduced efficacy against oxidative spoilage.[31] [32] Water vapor transmission rates remain minimal, typically under 5 g/m²/day for common thicknesses, enhancing moisture retention compared to porous paper substrates even when waxed.[33] Thermal stability in PVC-based wraps supports integrity up to 80-100°C, beyond which softening or decomposition risks emerge due to plasticizer volatility and polymer chain scission.[34] Optical properties include high visible light transmittance exceeding 90%, ensuring product visibility while minimizing haze for clarity in packaging inspection.[35]Variations across material types
Polyvinyl chloride (PVC) films exhibit high cling properties due to inherent electrostatic adhesion, superior stretchability, and flexibility, with tensile strength typically around 20-30 MPa and elongation at break exceeding 200%. These attributes enable thin gauges while maintaining form-fitting coverage, though PVC shows moderate barrier performance with water vapor transmission rates (WVTR) of 5-10 g/m²/day and oxygen transmission rates (OTR) of 500-600 cc/m²/day at standard conditions.[36][37][38] Polyvinylidene chloride (PVDC), frequently incorporated as a coating or layer in multilayer films, delivers the highest barrier efficacy among common plastic wrap polymers, achieving WVTR values below 1 g/m²/day (often 0.05-0.2 g/m²/day for 25 µm thickness) and OTR under 2 cc/m²/day. This stems from its dense, crystalline structure, which restricts gas and moisture permeation more effectively than PVC or polyethylene, albeit with reduced flexibility and higher processing demands.[39][40] Low-density polyethylene (LDPE) and linear low-density polyethylene (LLDPE) prioritize mechanical robustness, with LLDPE offering tensile strengths of 20-40 MPa, superior puncture resistance, and elongation comparable to PVC but lower cling reliant on additives. Barrier properties are inferior, registering WVTR of 10-30 g/m²/day and OTR of 2000-8000 cc/m²/day, often requiring increased film thickness (e.g., 50-100% thicker than PVC equivalents) to approximate similar protection levels.[36][41][42]| Material | WVTR (g/m²/day) | OTR (cc/m²/day) | Tensile Strength (MPa) | Elongation at Break (%) |
|---|---|---|---|---|
| PVC | 5-10 | 500-600 | 20-30 | >200 |
| PVDC | <1 | <2 | 40-50 | 100-200 |
| LDPE/LLDPE | 10-30 | 2000-8000 | 20-40 | 300-600 |