Mylar is DuPont’s brand name for biaxially-oriented polyethylene terephthalate (BoPET), a polyester film prized for its high tensile strength, chemical stability, and ultra-low moisture vapor transmission rate. Because the word “Mylar” is often used generically, engineers and buyers can confuse it with lower-grade PET films that look similar but perform very differently under thermal, electrical, or mechanical stress.
This article dissects how Mylar differs from commodity polyester films, from competing high-performance polymer films, and from metallized or coated variants you will encounter in real procurement cycles. Each section gives measurable data, cost implications, and field-tested selection tips so you can specify the right sheet or roll on the first try.
Core Chemistry: Why Mylar’s Molecular Alignment Outperforms Standard PET
During Mylar manufacture, molten PET is stretched first in the machine direction and then transversely while still above its glass-transition temperature. This biaxial stretching forces the polymer chains into a near-crystalline lattice that doubles tensile modulus and triples dielectric strength compared to cast, unstretched PET of the same thickness.
Standard “polyester film” from regional extruders is often mono-axially stretched or simply quenched, leaving an amorphous matrix that creeps at 120 °C. Mylar’s crystallinity keeps its shrinkage below 1 % after 30 min at 150 °C, letting it survive solder-reflow profiles that would wrinkle generic PET.
X-ray diffraction shows Mylar’s orientation factor of 0.87 versus 0.42 for off-brand films, explaining its 30 % higher yield stress and 40 % lower thermal expansion coefficient. If your flex-circuit application sees repeated 100 °C swings, that difference translates into via reliability and no micro-cracking after 1 000 cycles.
Practical Tip: Verifying Orientation in the Warehouse
Hold a 25 µm strip against an LED panel; Mylar shows uniform brightness with no mottled streaks, while low-grade film displays cloudy bands where crystallinity is spotty. You can also fold a 1 cm strip 180 °C; Mylar snaps back elastically, whereas generic PET develops a white crease that lowers dielectric strength by 15 %.
Thickness Tolerance and Yield: How Micron-Level Variance Hits Your Budget
Mylar is sold to ±5 % thickness tolerance at 95 % confidence, a spec that cheaper PET rolls often meet only at ±12 %. A 12 % variation forces you to over-specify thickness to guarantee electrical clearance, driving up material usage by 8–10 % on million-unit runs.
Roll-length yield is inversely proportional to thickness. A 2 % over-thickness on a 500 mm wide, 3 000 m roll of 50 µm film adds 1.5 kg of resin you never planned to buy. Multiply by 20 rolls per month and you have 360 kg of hidden resin cost per year.
DuPont’s laser gauging system records every meter and tags out-of-spec sections for removal before winding. Generic suppliers spot-check every 500 m, so a single thick band can propagate 50 m into your slit reels and cause capacitor winding jams.
Quick Cost Calculator
Request the supplier’s raw gauge profile data, not just the COA average. Plug the standard deviation into a Monte Carlo model to see how often you will need to scrap pouches or laminate stacks that fail Z-direction thickness limits.
Thermal Class and UL Recognition: Why 105 °C vs. 150 °C Matters
Mylar 454 is UL-listed for Class E (120 °C) insulation; Mylar 851 ups the rating to Class F (155 °C) thanks to proprietary nucleating agents that stabilize the crystallites. Generic PET film usually carries only a 105 °C rating, forcing motor manufacturers to add extra safety margins that increase slot fill loss.
Over a 20 000 h motor lifetime, every 10 °C drop in hotspot temperature doubles insulation life. Using 155 °C Mylar instead of 105 °C PET lets designers shrink motor size by 8 % while meeting IEC 60034-1 thermal class limits.
UL file numbers are searchable online; cross-check that the exact grade on the reel matches the file, not just the brand. Some distributors relabel generic PET as “Mylar-type” and count on buyers never auditing the UL card.
Moisture Barrier: Metallized Mylar vs. EVOH vs. AlOx-Coated Films
A 12 µm metallized Mylar pouch delivers a water-vapor transmission rate (WVTR) of 0.02 g m⁻² day⁻¹ at 38 °C/90 % RH, rivaling 25 µm aluminum foil. Clear AlOx-coated Mylar hits 0.3 g, still ten times better than uncoated PET at 3.0 g.
When you down-gauge from 25 µm foil to 12 µm metallized Mylar, you cut pouch weight by 28 % and eliminate pinhole cracks that plague foil during flexing. Coffee pouch makers report shelf-life extension from 9 to 18 months with no nitrogen flush change.
EVOH coextrusions achieve 0.1 g at 20 µm but lose 80 % of that barrier at 85 % RH because hydroxyl groups plasticize. Mylar’s metallized layer is humidity-independent, making it the safer choice for tropical supply chains.
Seal-Through Test
Run a 180 °C peel seal on metallized Mylar; the metal layer should fracture cohesively, not lift clean. If it lifts, the supplier used a low-cost tie layer that will delaminate during retort.
Dielectric Strength: Capacitor Grade Mylar vs. Polypropylene and Polycarbonate
Mylar 48 CR1 offers 300 kV mm⁻¹ at 25 µm, while polypropylene (PP) hits 600 kV mm⁻¹ but melts at 165 °C versus Mylar’s 255 °C. For DC-link capacitors in 125 °C automotive inverters, Mylar survives solder-dip testing that would warp PP films.
Polycarbonate (PC) film delivers 250 kV mm⁻¹ and superior tear resistance, yet its dissipation factor rises steeply above 100 °C, causing 30 % more self-heating at 10 kHz. Mylar’s tan δ stays flat to 150 °C, letting you shrink capacitor volume by 20 % without thermal runaway.
Capacitor makers often metallize both sides of 6 µm Mylar to achieve 500 VDC self-healing ratings; the same construction in PP requires 8 µm because of lower thermal conductivity. Thinner Mylar offsets PP’s higher breakdown number with better thermal management.
Selection Matrix
Choose Mylar for >125 °C, PP for <100 °C and lowest loss, PC only where tear toughness trumps thermal limits. Never substitute based on single-parameter specs; model the hotspot temperature under worst-case ripple current first.
Optical Clarity and Haze: Display Applications Where Mylar Beats PEN and PETG
Mylar 450D exhibits 1.2 % haze at 50 µm, outperforming polyethylene naphthalate (PEN) at 2.0 % and PETG at 3.5 %. For projection screen substrates, that difference raises center gain from 0.95 to 1.05, a visible brightness bump in side-by-side demos.
PEN offers 25 % higher modulus but yellows under UV-A, shifting b* from 1 to 6 after 500 h at 340 nm. Mylar’s UV-stabilized grade 453 keeps b* below 2 under the same dose, eliminating the need for an extra acrylic UV filter.
Touch-sensor makers laser-pattern 1 Ω sq transparent conductive oxides on 125 µm Mylar because its low thermal expansion (15 ppm °C⁻¹) matches ITO better than PETG’s 55 ppm, reducing micro-crack failures after 50 000 flex cycles.
Mechanical Fatigue: Fold Endurance Testing at 135 °C
MIT fold testers show 6 µm Mylar surviving 60 000 double folds at 135 °C under 0.5 kg tension, while generic PET fails at 18 000 folds. The difference stems from Mylar’s higher molecular weight (IV 0.84 vs. 0.62) and lower oligomer content that initiates crack sites.
In flexible OLED encapsulation, this fatigue margin translates into 5 years of daily folding at 37 °C body temperature before delamination. Designers can therefore specify 50 µm adhesive stacks instead of 100 µm, shaving 0.1 mm from device thickness.
Always test at the service temperature, not room temperature. A film that passes 100 000 folds at 23 °C can lose 70 % of that endurance at 85 °C because viscoelastic damping concentrates stress at fold apexes.
Chemical Resistance: Battery Separator vs. PVDF and NMP Exposure
Mylar’s crystalline zones resist swelling in N-methyl-2-pyrrolidone (NMP), a common cathode solvent, gaining only 0.3 % mass after 24 h at 85 °C. Amorphous PET swells 1.2 %, enough to drop puncture strength by 18 % and trigger internal shorts during cell assembly.
PVDF binder dissolves in NMP and can creep into film pores. Mylar’s higher density (1.39 g cc⁻¹ versus 1.33 g cc⁻¹ for low-crystallinity PET) blocks pore infiltration, keeping separator resistance below 1.0 Ω cm² after 500 h soak.
When electrolyte contains 10 % fluoroethylene carbonate, Mylar’s surface energy (44 mN m⁻¹) remains stable, whereas PP separator drops from 30 to 22 mN m⁻¹ and loses electrolyte wetting. The result is 15 % lower ionic conductivity and 8 % capacity fade after 200 cycles.
Spot Test Protocol
Drop 5 µL NMP on the film at 85 °C; measure contact angle every minute. Mylar keeps θ > 50° for 30 min; PP drops to 20° within 5 min, signaling poor chemical compatibility.
Cost Dynamics: When Mylar Pays for Itself Despite Higher $ kg⁻¹
At $4.80 kg⁻¹, Mylar 454 costs 20 % more than generic 105 °C PET. Yet its 15 % higher modulus lets you down-gauge from 75 to 63 µm in motor slot liners, saving 16 % material mass and offsetting the price premium.
Scrap reduction adds another layer of savings. Because Mylar’s thickness profile is tighter, slitters generate 2 % edge trim waste versus 5 % for commodity PET on the same winder setup. On a 10 t month⁻¹ slitting line, that 3 % delta returns 3.6 t yr⁻¹ of usable film worth $17 000.
Inventory carrying cost drops too. One 155 °C Mylar grade can replace dual inventories of 105 °C PET and 130 °C polycarbonate, freeing warehouse space and cutting SKU complexity by 30 %. Finance teams often overlook this soft saving, yet it can exceed the raw material price delta within two fiscal quarters.
Sustainability and Recycling: How Mylar’s Mono-Material Design Eases Circularity
Mylar is 100 % PET, so it can enter existing bottle-to-fiber recycling streams without density separation. Metallized versions need only caustic demetallization, a standard step at recyclers that handle metallized bottle labels.
Multilayer PP-PE pouches, by contrast, require solvent-based delamination that costs €0.35 kg⁻¹ and yields low-grade mixed polymer. Switching to mono-material Mylar structures lets brands claim 90 % recycled content in new film, qualifying for UK plastic tax relief.
Life-cycle analysis shows 12 µm Mylar pouch generating 1.8 kg CO₂-eq kg⁻¹ versus 2.4 kg for foil laminate, chiefly because aluminum smelting energy is avoided. Retailers publishing scope-3 emissions can cut 0.6 kg CO₂ per pouch by switching, a measurable ESG win.
Supply Chain Risk: Geopolitical Feedstock Sources and Lead-Time Hedging
DuPont produces Mylar in Circleville, Ohio and Luxembourg, using PTA and MEG feedstocks sourced under long-term ethane contracts from US Marcellus shale. That feedstock lock shields prices from Asian PX volatility that can swing commodity PET by ±12 % in a quarter.
During the 2021 Texas freeze, generic PET plants saw 30-day force majeure outages, while Mylar’s dual-site strategy maintained 95 % OTD. Buyers who had qualified only single-source Asian PET faced 8-week gaps and airfreight surcharges that erased any raw material savings.
Allocate 20 % of annual volume to a second resin family such as PEN or PI to create a force majeure hedge. Keep the master specification dual-qualified so a single document revision activates the alternate material within days, not months.
Future-Proofing: Nanocomposite Mylar and 5G mmWave Applications
DuPont’s experimental nano-silica filled Mylar reaches tan δ of 0.001 at 10 GHz, half that of standard PET, enabling thinner antenna substrates at 28 GHz. Early trials show 50 µm films handling 5 W cm⁻² continuous power with ΔT < 10 °C, critical for beam-forming networks in small cells.
The same nanocomposite cuts CTE to 8 ppm °C⁻¹, matching low-temperature co-fired ceramic (LTCC) and eliminating solder-joint strain when surface-mounted chips are reflowed onto a Mylar-based antenna panel. Designers can now integrate the antenna and radome into one conformal sheet, saving 0.3 mm z-height in laptop lids.
Qualification timelines are 18 months, so start dialogue with DuPont’s application labs now if your roadmap includes 6 G sub-THz bands above 100 GHz. Early adopters gain access to joint development kits that include laser ablation parameters and plasma pre-treat recipes not published in datasheets.