Paper-aluminum-plastic composite films are widely used in pharmaceutical packaging for powders and granules. Compared to plastic-aluminum-plastic structures common in the market, the paper-aluminum-plastic structure offers three key advantages: high mechanical strength and rigidity for better formed pouch appearance, natural easy-tear properties without requiring tear notches, and strong resistance to shear forces during filling and sealing ??protecting the aluminum foil barrier layer from perforation.
Under the YBB 20062012 pharmaceutical packaging standard (Type IV: Paper, Aluminum, Plastic), key technical specifications include: water vapor transmission ≤1.5 g/m²·24h (38°C, 90% RH), oxygen transmission ≤3.0 cm³/m²·24h·0.1 MPa, inner layer peel strength ≥2.5 N/15mm, heat seal strength ≥12 N/15mm, total solvent residue <5.0 mg/m² (benzene not detectable).
Paper Characteristics Affecting Production
Paper differs significantly from plastic layers in several ways that impact printing, laminating, and end-use: surface roughness (smoothness ≥200S per GB/T 10335.2), moisture content (5.5±1.0%), high liquid absorbency (ink absorption 15–28%), dimensional changes from moisture gain/loss, and poor thermal conductivity.
Gravure Printing on Paper
While flexographic printing handles rough paper surfaces better, gravure dominates soft packaging production with finer screen rulings (150–170 LPI vs. flexo’s typical maximum of 150 LPI). Key gravure considerations:
- Avoid light screen tints: Below 30% dot area transfers poorly on rough paper surfaces. Use solid lines and blocks with deepened engraving.
- Avoid tight registration designs: Paper shrinks as it loses moisture during hot-air drying, causing transverse registration issues. Pre-dry paper before printing.
- No cylinder diameter increment needed: Unlike plastic films that stretch under heat, paper shrinks ??so standard diameter progression is unnecessary.
- Use water-based inks: Solvent-based inks penetrate deeply into paper fibers, causing residual solvent and odor problems. Water-based inks and UV coatings, proven in cigarette packaging, should be adopted for gravure paper printing.
Solvent-Free Lamination: The Preferred Method
Extrusion lamination and solvent-free lamination offer clear environmental advantages over traditional dry lamination. Dry lamination requires coating adhesive on paper, where solvent is absorbed into fibers and difficult to remove.
Adhesive selection: Single-component solvent-free adhesives are ideal for paper-aluminum structures. These adhesives cure via moisture reaction ??the paper’s own water content serves as the reaction medium. Their higher molecular weight and viscosity after coating reduce excessive absorption into paper. Two-component adhesives with lower viscosity are more readily absorbed by paper capillaries, and paper moisture can consume the curing agent, leading to incomplete curing.
Coating method: Apply adhesive to the aluminum foil rather than the paper. Foil-coated adhesive reduces paper absorption and allows lower coating weights (2.0 gsm vs. 4+ gsm needed on paper). However, 7μ aluminum foil is delicate and requires precise tension control.
Coating weight: Only the adhesive layer remaining at the paper-foil interface contributes to bond strength ??adhesive absorbed into paper does not. Higher coating weights need more free moisture for curing, but paper dried at 80°C+ during printing has minimal remaining moisture. This can cause incomplete crosslinking at the foil interface and poor heat resistance.
Process tips: Keep lamination roller temperature low to reduce adhesive flow into paper. Filter and dry incoming air. Maintain ambient RH below 65%. Allow complete curing of the paper-aluminum layer before applying the inner PE heat-seal layer.
Common Defects and Solutions
Heat Seal Delamination
Paper’s poor thermal conductivity requires heat seal temperatures sometimes exceeding 200°C. If delamination occurs only at sealed areas: the adhesive lacks sufficient heat resistance or crosslinking is incomplete ??extend curing time or humidify paper slightly. If delamination is widespread: insufficient adhesive at the interface ??switch to foil-side coating or increase coating weight. Paper interlayer separation indicates weak fiber bonding exacerbated by shear stress ??adjust heat-seal roller gap.
Residual Solvent
Solvent-based inks and coatings on absorbent paper inevitably cause residual solvent problems. The fundamental solution is switching to water-based inks and water-based coatings.
Heat Seal Discoloration and Odor
Discoloration stems from inadequate heat resistance of inks/coatings or insufficient drying. Odor may originate from volatile compounds in the paper itself ??test by heating paper samples in sealed containers and performing GC analysis.
Poor Heat Sealability
Use low-temperature heat-seal PE grades. Control paper grammage (thinner is better for heat transfer). Consider ultra-low-temperature heat-seal materials for the inner layer.
References
- Wikipedia: Pharmaceutical Packaging: Overview of drug packaging materials and standards
- ISO 11607 ??Packaging for Terminally Sterilized Medical Devices: International standard for medical packaging
- FDA: Pharmaceutical Quality Resources: U.S. FDA guidance on drug packaging quality
- ASTM F2096 ??Gross Leak Detection in Medical Packaging: Standard test for package integrity
- Flexible Packaging Association (FPA): Industry resource for flexible packaging standards