Cardboard, a broad term for thick, paper-based materials, is fundamentally a matrix of plant fibers. Its chemical identity is more complex than simple wood pulp, consisting of naturally occurring polymers, intentionally added functional chemicals, and trace compounds from manufacturing or recycling. This blend is engineered to meet specific application demands, such as providing strength, barrier properties, and printability.
The Primary Structural Components
The foundational chemical structure of all cardboard originates from lignocellulosic biomass, primarily wood pulp or recovered paper fiber. The main structural polymer is cellulose, a long chain of glucose units linked together, which provides the material’s inherent tensile strength and fibrous nature. Cellulose typically accounts for over half of the total chemical makeup of corrugated cardboard, providing the framework for the entire structure.
Associated with cellulose are hemicellulose and lignin, the other two major natural components. Hemicellulose consists of shorter, complex chains of various sugars like xylose and mannose, helping to bind the fibers together. Lignin is a complex, non-carbohydrate phenolic polymer that acts as the natural glue in wood, embedding the cellulose fibers and giving the material its characteristic stiffness.
For high-quality or white cardboard, much of the lignin must be removed through chemical pulping and bleaching, as its presence causes the material to yellow over time. Retaining a higher percentage of lignin, common in traditional corrugated boxes, contributes to the brown color and overall rigidity. The performance of the cardboard also depends on its water content, which is managed during high-pressure forming to ensure a dense, tightly compressed board with maximum compression resistance.
Additives for Strength and Structure
Manufacturers introduce various chemicals during the pulping and forming stages to enhance the mechanical properties of the board. Improving water resistance is achieved through sizing agents, which are added directly to the fiber slurry. These compounds reduce the material’s natural tendency to absorb moisture, preventing structural compromise.
Adhesives are significant additions, particularly in corrugated cardboard where multiple layers are laminated together. The most common adhesive is a simple, plant-derived starch glue, often corn starch, used to bond the fluted medium to the linerboards. For specialized or heavy-duty packaging, synthetic resins are sometimes employed to achieve higher bonding strength.
Chemical enhancers, such as urea-formaldehyde resin, can be applied to boost compression strength significantly. These treatments allow for the use of lighter-weight board while maintaining the stacking strength required for shipping. Other additives may be included to impart fire-retardant properties or to improve fiber-to-fiber bonding strength.
Coatings, Inks, and Surface Treatments
The outermost layers of cardboard frequently receive chemical treatments to enhance aesthetics, barrier function, or print quality. Surface coatings often contain mineral pigments, such as kaolin clay and calcium carbonate, which fill microscopic cavities in the paper surface. This creates a smoother, brighter surface better suited for high-resolution printing.
Polymer coatings, like polyethylene, are applied to create a moisture barrier for use in food packaging or environments where liquid contact is a concern. Waxes, such as paraffin, are also utilized in topcoat formulations to provide resistance to water and grease. While crucial for product protection, these barrier layers can complicate the recycling process.
Printing inks are complex chemical formulations consisting of pigments for color, and resins and binders to fix the ink to the surface. Various solvents maintain the correct viscosity for printing, such as ethanol and ethyl acetate, which must evaporate or be cured. Specialized photoinitiators may be included in the ink to allow for rapid curing under ultraviolet light.
Unintended Chemicals and Trace Contaminants
A separate category of chemicals includes those unintentionally present, often due to the material’s history, especially in recycled cardboard. A major concern in packaging, particularly for food contact, is the presence of Mineral Oil Saturated Hydrocarbons (MOSH) and Mineral Oil Aromatic Hydrocarbons (MOAH). These compounds originate primarily from mineral oil-based printing inks used in old newspapers and magazines that are part of the recycled fiber mix.
MOSH and MOAH can migrate from the packaging into food through direct contact or by gas-phase transfer. While MOSH compounds accumulate in organs like the liver, certain MOAH fractions are more concerning because they are suspected to have toxic properties. Recycled materials inherently contain a higher proportion of these mineral oil compounds compared to cardboard made from fresh fiber.
Other trace contaminants include heavy metals such as lead, cadmium, and zinc, which are often residues from past or current pigments and dyes. These metals are frequently associated with colored inks and can accumulate in the recycled pulp. Furthermore, Volatile Organic Compounds (VOCs), which include residual solvents from inks and curing agents from adhesives, can be emitted from the finished board, particularly when the chemicals have not fully set.