Barrier Core Function in Laminated Cosmetic and Personal Care Tubes

Laminated cosmetic and personal care tubes typically consist of a five-layer to seven-layer structure comprising an outer polyethylene film for print protection, a tie resin, a barrier core of ethylene-vinyl alcohol copolymer or aluminium foil, a second tie resin, and an inner polyethylene contact layer. Total wall thickness ranges from 250 µm to 400 µm for standard tubes, with the barrier core occupying 5% to 12% of the total cross-section in plastic barrier laminates and 3% to 6% in aluminium barrier laminates. The function of the barrier core is to limit oxygen ingress, water vapour transmission, carbon dioxide loss, and volatile fragrance compound permeation across the laminate wall. Oxygen permeation data are commonly generated according to ASTM D3985-17 or ISO 15105-2 using a coulometric sensor at 23 °C and a defined relative humidity; water vapour transmission rate is determined according to ASTM F1249-20 or ISO 15106-3 at 38 °C and 90% relative humidity. The barrier core also contributes to axial compression resistance and heat-seal integrity, but its primary role is defined by the permeation coefficient of the selected core material under the humidity and temperature conditions encountered during filling, storage, and consumer use.

Why Does EVOH Require Polyolefin Encapsulation in Tube Laminate Design?

Polyolefin encapsulation of EVOH is required because ethylene-vinyl alcohol copolymer absorbs moisture from the product or the external environment, and moisture plasticises the amorphous regions of the copolymer to reduce its oxygen barrier by an order of magnitude or more. Published data for EVOH grades with ethylene content between 27 mol% and 44 mol% show oxygen transmission rates at 0% relative humidity that are typically below 0.2 cm³/(m²·day·atm) for a 15 µm layer, but values at 65% relative humidity can rise to 1.0 cm³/(m²·day·atm) or higher depending on grade and temperature. The polyolefin skins alone do not provide adequate moisture protection; the functional encapsulation is achieved by positioning the EVOH core between continuous tie layers and polyolefin layers with controlled moisture vapour transmission rate. The tie layers are typically maleic anhydride grafted linear low-density polyethylene, with a graft level of 0.8 wt% to 1.2 wt% and a thickness of 4 µm to 10 µm in tube laminates, providing reactive adhesion to the EVOH core during coextrusion. In a standard five-layer blown-film line with a 300 µm total thickness, the structure may be low-density polyethylene 80 µm, tie 8 µm, EVOH 15 µm, tie 8 µm, and linear low-density polyethylene 189 µm; the barrier core is therefore not directly exposed to the tube contents. Moisture uptake in EVOH during storage at 75% relative humidity can exceed 2 wt%, and the resulting oxygen barrier loss is partially reversible after drying to 0.1 wt% moisture in a desiccant dryer at 80 °C for 4 h to 6 h. The encapsulation also protects the EVOH from acidic formulations and surfactants in cosmetic emulsions that can promote stress cracking and interlayer failure at the core/tie interface. Without continuous polyolefin skins, flexural stress on the rigid EVOH core would propagate cracks through the laminate after fewer than 50 flex cycles under ASTM F392-93(2018), whereas the encapsulated structure typically survives 200 to 500 flex cycles before pinhole formation, depending on core thickness and tie resin selection. This is the primary reason that EVOH is not used as a monolayer barrier tube in cosmetic applications.

In high-water-fraction formulations above 60% water, the inner polyethylene layer does not prevent long-term moisture migration into an EVOH core; published steady-state calculations indicate that a 150 µm low-density polyethylene inner layer with a water vapour transmission rate of 4 g·mm/(m²·day) to 6 g·mm/(m²·day) permits sufficient moisture uptake to reduce EVOH oxygen barrier within 30 to 90 days of storage at 40 °C. The rate of barrier loss depends on the solubility and diffusion coefficient of water in the inner polyethylene, and on the temperature dependence of the EVOH moisture sorption isotherm. To maintain oxygen barrier in such formulations, the barrier core is either shifted to aluminium foil or the EVOH layer is placed between thicker polypropylene layers with lower water vapour transmission, an approach that raises the laminate stiffness and complicates tube shoulder sealing. Field experience on production-scale blown-film lines indicates that oil-free screw configurations with a compression ratio of 2.8:1 to 3.5:1 and a barrel length-to-diameter ratio of 24:1 to 30:1 are preferred for EVOH processing, while grooved feed sections designed for high-density polyethylene can generate excessive shear heating and degrade the EVOH at the core layer. The extrusion temperature profile is normally set with feed zone temperatures of 160 °C to 180 °C, transition zone temperatures of 190 °C to 210 °C, and die temperatures of 210 °C to 225 °C; melt temperatures above 240 °C produce gels, black specks, and pinholes in the core. A stable bubble in blown-film coextrusion is obtained at a blow-up ratio of 2.0:1 to 3.0:1 and a frost line height of 2 to 4 die diameters; higher frost line heights promote EVOH crystallisation before the bubble is fully stretched, producing thickness fluctuations of ±2 µm in the core and intermittent barrier defects.

Oxygen Ingress and Fragrance Loss Through the Barrier Core

Oxygen ingress through the barrier core is governed by the steady-state transmission rate, which is normalised to layer thickness and partial pressure difference. The oxygen transmission rate of a 12 µm aluminium foil core is below the detection limit of standard coulometric instruments, typically reported as less than 0.0005 cm³/(m²·day·atm) at 23 °C, while a 15 µm EVOH core at 0% relative humidity provides 0.05 cm³/(m²·day·atm) to 0.20 cm³/(m²·day·atm), and at 65% relative humidity the value may increase to 0.8 cm³/(m²·day·atm) to 2.5 cm³/(m²·day·atm) depending on ethylene content. Fragrance loss through plastic barrier cores is not adequately described by oxygen transmission rate alone; the diffusion coefficient of low-molecular-weight esters, terpenes, and aldehydes through polyethylene is orders of magnitude higher than oxygen, and the barrier improvement provided by EVOH depends on the polarity and molecular diameter of the fragrance compound. Limonene, for example, permeates non-polar polyethylene rapidly, but the polar EVOH core reduces its transport by a factor of 50 to 200 relative to low-density polyethylene at equal layer thickness, according to published permeation studies using isostatic concentration cells. The remaining permeation path is dominated by the tie layers and polyolefin skins, with the core functioning as the rate-limiting layer only when its permeability is at least one order of magnitude lower than that of the surrounding layers. Aroma barrier is typically assessed at 40 °C by gas chromatography coupled with flame ionisation detection after a fixed exposure interval; comparative testing of tube laminates with and without an EVOH core can demonstrate a 60% to 90% reduction in total volatile peak area after 12 weeks, although published data for specific fragrance combinations and cosmetic emulsions is limited. Water vapour transmission through the core is also relevant because water loss can cause emulsion separation and viscosity change; a 9 µm to 12 µm aluminium foil core yields water vapour transmission rates below 0.001 g/(m²·day) at 38 °C and 90% relative humidity, while a 15 µm EVOH core in a polyethylene-rich laminate may show 0.5 g/(m²·day) to 2.0 g/(m²·day) under the same conditions because the polyethylene skins dominate the moisture transport. Barrier core selection therefore requires simultaneous evaluation of oxygen, water, and volatile organic compound transport, rather than reliance on a single gas transmission standard.

External humidity at 75% relative humidity causes an EVOH core to lose barrier in a non-linear manner; the oxygen transmission rate at 23 °C for a 15 µm core can increase from 0.1 cm³/(m²·day·atm) at 0% relative humidity to 1.5 cm³/(m²·day·atm) or higher at 75% relative humidity, and this change is difficult to detect in total tube oxygen transmission measurements when the polyethylene skins contribute a small but stable transmission background. The moisture sensitivity of EVOH is particularly relevant for tubes stored in bathrooms with intermittent hot water exposure, where the external surface can experience humidity spikes above 90% while the internal product remains at 45% to 60% relative humidity. Under these conditions, the oxygen barrier of the core is governed by the moisture content at the core/tie interface, which can be modelled using Fickian diffusion with water concentration-dependent diffusion coefficients for polyethylene and EVOH. In practice, accelerating ageing at 40 °C and 75% relative humidity for 90 days is used to evaluate barrier retention, and the oxygen transmission rate after ageing should not exceed 1.0 cm³/(m²·day·atm) for oxygen-sensitive actives such as ascorbic acid or retinoids. A laminate with a 15 µm EVOH core that meets this criterion at 0% relative humidity may fail after exposure to 75% relative humidity because the core moisture content approaches 3 wt%, and the oxygen transmission rate increases by 5 to 10 times. This failure mode is not fully captured by standard oxygen transmission tests performed at a single relative humidity; therefore ASTM F1927-20, which allows controlled humidity on both sides of the specimen, is preferred for barrier core qualification in cosmetic tubes.

When Aluminium Foil Replaces Transparent Barrier Polymers in High-Risk Formulations

Aluminium foil is used as the barrier core when the formulation contains oxygen-sensitive actives, volatile fragrance compounds, or water-in-oil emulsions where plastic barrier cores cannot meet the required shelf-life specification. The foil thickness in cosmetic tube laminates is typically 9 µm, 12 µm, or 20 µm, with 12 µm representing the most common compromise between flex crack resistance and barrier performance. In an aluminium barrier laminate, the foil is laminated to the outer and inner polyolefin films with a continuous adhesive layer or extrusion-coated tie resin; the foil provides an absolute barrier to oxygen, water vapour, and most volatile organic compounds until flexural fatigue creates pinholes or microcracks. ASTM F392-93(2018) is used to assess the flex durability of the foil-containing laminate, and acceptance criteria are typically 0 pinholes after 20 flex cycles for high-risk pharmaceutical or cosmetic tubes, or 0 to 5 pinholes after 50 cycles for general personal care applications. The presence of aluminium foil in a tube laminate introduces a measurable stiffness increase, with the tensile modulus of the laminate rising from approximately 200 MPa for an all-polyethylene structure to 800 MPa or higher when a 12 µm foil is included; this change affects tube handling, dent resistance, and shoulder sealing. Aluminium foil also acts as an electrolyte in contact with low-pH formulations if the inner polyethylene layer is breached, leading to galvanic corrosion that generates hydrogen gas and delamination blisters. For this reason, tubes containing aluminium foil are not recommended for products with pH below 3.5 or above 10.5 unless the inner layer is at least 100 µm of high-purity polyethylene and the tube is leak-tested at 25 kPa to 40 kPa internal pressure according to ASTM D3078-02. The selection of aluminium foil over EVOH is also driven by the need for barrier against carbon dioxide loss in carbonated formulations and against oxygen ingress in products containing 0.1% to 1.0% ascorbic acid, where even a 15 µm EVOH core may permit sufficient oxygen ingress over 24 months to reduce active concentration by more than 5%.

Simultaneously, the tie-layer selection governs peel strength after heat-sealing the laminate to the shoulder, and this interfacial property is measured by T-peel testing according to ASTM F88/F88M-23 at a crosshead speed of 300 mm/min. For EVOH-containing tubes, the peel strength between the inner polyethylene contact layer and the shoulder material should exceed 5 N/15 mm to prevent seal failure during repeated squeezing; values lower than 3 N/15 mm are associated with core delamination and product contamination. On a 7-layer coextrusion line configured for 300 mm diameter lay-flat tube stock at 200 kg/h, increasing the EVOH core thickness from 12 µm to 18 µm has been observed to raise die pressure by 10 bar to 18 bar and decrease line speed by 5% to 10% because of the higher viscosity of the EVOH layer relative to the polyethylenes. The melt viscosity ratio between the EVOH and the adjacent tie layers must be controlled within 2:1 to 4:1 at the die lip to avoid viscous encapsulation and layer thickness non-uniformity; when the viscosity ratio exceeds 4:1, the EVOH core can migrate toward the die wall and produce intermittent barrier-free regions. Layer thickness distribution across the web is measured by microtome cross-section and optical microscopy at 10× to 20× magnification, with a tolerance of ±1 µm for the barrier core in critical applications. These core thickness variations are not detected by total thickness gauges but can be inferred from oxygen transmission measurements on samples taken at 25 cm intervals across the web; variations in oxygen transmission rate of ±20% from edge to centre are commonly attributed to uneven core distribution during blown-film or cast-film coextrusion.

At 40 °C and 75% relative humidity, the assembled tube barrier performance is often lower than the flat laminate value because shoulder sealing and side seams introduce localised stress concentrations and thickness reductions. Side seam lap joints in laminated tubes are produced by hot air or ultrasonic sealing at temperatures between 180 °C and 220 °C, and the barrier core must survive the sealing heat without degrading or thinning. For EVOH cores, side seam sealing at 220 °C for 1.5 s is acceptable if the core is displaced from the sealing surface by at least 20 µm of polyethylene; direct sealing through the EVOH layer can produce localised gel formation and microvoids. In aluminium foil tubes, the side seam is typically sealed with a foil overlap that excludes the foil edge from the inner product contact surface, and seam continuity is verified by dye penetration testing under vacuum at 15 kPa to 25 kPa. The assembled tube oxygen transmission rate is measured on filled and crimped tubes rather than on flat laminate samples because the sealing operations can create breaches that are not present in the extruded film or sheet. Acceptance criteria for oxygen-sensitive cosmetic formulations typically require an assembled tube oxygen transmission rate below 0.1 cm³/(m²·day·atm) at 23 °C and 0% relative humidity when the product contains retinoids, and below 0.01 cm³/(m²·day·atm) when the product contains ascorbic acid at concentrations above 0.5%.

Controlling Thermal Degradation Pathways in EVOH Cores Without Overshear

Thermal degradation of EVOH in the barrier core proceeds through hydroxyl group dehydration, chain scission, and crosslinking reactions that generate water, conjugated double bonds, and discoloured gel particles visible as black specks in the laminate. The degradation rate is strongly temperature-dependent, with acceptable melt temperatures between 190 °C and 225 °C for standard EVOH grades and a maximum residence time of 10 min to 15 min at the upper temperature limit. Production-scale extruders used for barrier cores are typically single-screw machines with a 24:1 to 30:1 length-to-diameter ratio, a compression ratio of 2.8:1 to 3.5:1, and a screw design that avoids high-shear mixing elements in the EVOH melt channel. Melt temperature measurements at the screw tip should not exceed 230 °C, and purge procedures using low-density polyethylene are required before and after processing to displace carbonised residue. If the barrel temperature is set above 240 °C or the screw speed exceeds 80 rpm on a 60 mm extruder, the residence time distribution broadens and local hot spots generate gel particles at a rate that can increase barrier core defects from 2 to 10 per 100 cm². The produced gels are insoluble and are not removed by screen packs with mesh sizes of 80 µm to 100 µm; therefore the only corrective action is to reduce melt temperature and increase purge frequency. In ethylene-vinyl alcohol copolymers with higher ethylene content, such as 44 mol%, the processing window is broader, but the oxygen barrier at low humidity is reduced in proportion to the increased polyethylene-like character. The core layer temperature during lamination to aluminium foil in extrusion lamination is typically limited to 280 °C to 320 °C for the low-density polyethylene melt curtain, and the foil acts as a heat sink that quenches the EVOH core before significant thermal damage occurs; however, prolonged line stoppages can expose the core to heated nip rolls and produce thermal weld lines.

Because aluminium foil provides a near-zero moisture barrier, product formulations containing volatile silicones such as cyclopentasiloxane and dimethicone can be packaged in aluminium barrier laminates without the weight-loss failures observed in all-polyethylene tubes; however, the foil core does not prevent loss of volatiles through the shoulder and cap interface, which often dominates the total permeation in an assembled tube. Hot air leak testing at 25 kPa internal pressure according to ASTM D3078-02 identifies gross shoulder seal defects, while gravimetric weight-loss testing at 40 °C and 75% relative humidity over 28 days quantifies the total volatile loss from the assembled package. For an aluminium barrier laminate tube with a 12 µm foil core and a polypropylene cap, the gravimetric weight loss of a cyclopentasiloxane-containing formulation at 40 °C is typically less than 0.2% after 28 days, whereas an all-polyethylene tube may exceed 2% over the same period; published data for specific cap liner materials and torque settings is limited. The dominant residual loss in aluminium foil tubes occurs at the shoulder welding seam, where the foil fold is compressed and can form microcracks if the shoulder sealing temperature exceeds 220 °C or the sealing dwell time is below 1.5 s. Ultrasonic or hot-jaw sealing processes must be qualified by sectioning the seal area and inspecting for complete polyethylene flow and absence of foil breakthrough; acceptance criteria typically require 90% of the seal cross-section to show cohesive failure rather than adhesive delamination.

Retinol Encapsulation Demands an Oxygen Barrier Threshold of 0.1 cm³/(m²·day·atm)

For retinol-containing formulations at 0.1% to 0.5%, shelf-life modelling based on first-order oxidation kinetics indicates that an assembled tube oxygen transmission rate below 0.1 cm³/(m²·day·atm) at 23 °C and 0% relative humidity is necessary to limit retinol degradation to less than 10% over 24 months. This threshold corresponds to a 15 µm EVOH core at 0% relative humidity, but not at 65% relative humidity; therefore aluminium foil or high-barrier EVOH grades with lower ethylene content and thicker protective polyolefin skins may be required. The oxidation of retinol is also light-sensitive, and the barrier core does not provide ultraviolet protection; an additional light barrier is required in the outer laminate or printing ink. Accelerated stability testing at 40 °C and 75% relative humidity for 90 days is used to screen candidate barrier structures, with retinol remaining assayed by high-performance liquid chromatography according to validated methods. For ascorbic acid at 1.0%, the oxygen ingress tolerance is stricter because the degradation pathway involves direct oxidation rather than photoisomerisation; assembled tube oxygen transmission rates below 0.01 cm³/(m²·day·atm) are generally specified, a level that only aluminium foil cores can reliably provide over a 24-month shelf life. Published data for barrier core performance in specific cosmetic emulsion matrices is limited because the emulsion microstructure and antioxidant load alter the effective oxygen demand of the formulation.

Crimp-seal integrity in laminated tubes depends on the barrier core thickness and the polyolefin skins because the folded laminate must deform without cracking the rigid core. A 12 µm aluminium foil core tolerates crimping if the sealing jaw pressure is controlled between 0.3 MPa and 0.5 MPa and the jaw temperature is held at 180 °C to 200 °C; higher pressures concentrate stress at the foil fold and generate microcracks that are detectable only by oxygen transmission testing. An EVOH core above 20 µm similarly increases crimp cracking under axial compression because the high-modulus core resists the compressive strain imposed by the crimp fold. Production-scale tube lines therefore use barrier cores of 9 µm to 15 µm in plastic barrier laminates and 9 µm to 12 µm in aluminium barrier laminates to balance crimp deformation with permeation resistance. The folded crimp area is tested by sectioning at 10× magnification after 500 compression cycles at 25 N axial force; acceptable crimped tubes show no core fracture or delamination longer than 200 µm.