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Papermaking is one of the four great inventions of ancient China, representing a significant contribution of Chinese civilization to global cultural development. As early as 105 AD-the first year of Emperor He's reign during the Eastern Han Dynasty-Cai Lun improved upon earlier techniques and successfully developed a method for producing high-quality paper using raw materials such as rags, hemp waste, fishing nets, and tree bark. The resulting product, known as "Cai Hou Paper," was comparable to modern paper in both production principles and texture. The paper industry in Britain did not emerge until the 17th century, while the first paper mill in the United States was established in 1690. In early British and American papermaking, textile scraps were commonly used as primary raw materials. These materials were pulped, diluted, and then transferred onto screens through immersion and vibration, allowing fibers to deposit uniformly. After pressing out excess moisture and drying, the final paper sheets were obtained. With continuous advancements in culture and living standards, demand for paper has grown significantly, surpassing the supply capacity of traditional rag-based raw materials. This necessitated the exploration of alternative cellulose sources, leading to the widespread adoption of wood pulp.
Packaging made from paper and cardboard is collectively referred to as paper packaging. It encompasses various forms, including corrugated boxes, folding cartons, honeycomb board structures, paper bags, paper tubes, paper drums, and other specialized packaging papers. Paper packaging is extensively utilized across industries-not only for general merchandise, textiles, hardware, telecommunications equipment, and household appliances, but also for food, pharmaceuticals, and military supplies.
I. The Role of Paper and Cardboard Among Packaging Materials
Paper and cardboard rank as the most widely used packaging materials globally, accounting for approximately 40% to 50% of total packaging material consumption. Current trends indicate that they will remain dominant in both present and future packaging applications. Paper-based containers possess unique advantages that distinguish them from other packaging materials:
1. Paper and paperboard can be mass-produced with abundant raw material availability and relatively low cost.
2. They are suitable for both mechanized and manual manufacturing processes and exhibit excellent foldability.
3. Containers made from cardboard, particularly corrugated board, offer superior elasticity compared to plastic and other conventional packaging materials.
4. Paper packaging can be customized into diverse structural designs tailored to specific products. It can meet ventilation ("breathing") requirements or be engineered into fully sealed packages, offering hygienic, non-toxic, and pollution-free characteristics.
5. Due to their fibrous structure, paper and cardboard provide excellent ink and coating receptivity, ensuring sharp, durable printing with clear text and graphics.
6. Paper packaging is recyclable and biodegradable, generating minimal waste and posing negligible environmental impact. Consequently, paper-based packaging has gained increasing recognition and support. In recent years, advances in composite paper technology have expanded its application in food packaging. Additionally, specialty papers and paperboards incorporating chemical additives as functional fillers have seen significant progress in China, indicating promising future development.
II. Classification of Packaging Paper and Paperboard
Paper and paperboard are classified based on basis weight (defined as mass per unit area, expressed in grams per square meter) or thickness. Materials with a basis weight below 200 g/m² or a thickness under 0.1 mm are generally categorized as paper. Those exceeding these thresholds are termed paperboard or board. However, certain products-such as white cardstock and drawing paper-with basis weights between 200 and 250 g/m² may still be referred to as paper due to established industry conventions.
In packaging applications, paper is primarily used for wrapping goods, producing paper bags, and printing labels and decorative elements, whereas paperboard is mainly employed in the manufacture of rigid packaging containers such as boxes, cartons, and drums.
Specifications for paper and paperboard vary according to form: flat sheets require defined length and width dimensions, while roll (web) paper only requires specified width. Standardizing these dimensions is crucial for achieving uniformity and compatibility in downstream packaging production, especially for corrugated boxes, folding cartons, and similar items.
Common widths for domestic roll paper used in corrugated box manufacturing include 1940 mm, 1600 mm, 1120 mm, and 940 mm. Imported kraft paper and corrugated base paper are typically available in widths such as 1600 mm, 1575 mm, and 1295 mm. Standard sheet sizes for flat paper include 787 mm × 1092 mm and 850 mm × 1168 mm.
The maximum weight of a roll generally does not exceed one metric ton, with typical weights ranging from 250 to 350 kilograms. For flat sheets, each bundle should not exceed 250 kilograms. Generally, paper weighing less than 250 g/m² is packaged in reams of 500 sheets, with 10 reams constituting one bundle. For heavier paperboard, individual bundles must not exceed 250 kg. However, the number of sheets per bundle or rolls per package may vary depending on the specific grade and weight.
III. General Properties of Paper and Paperboard
From an application perspective, the performance of paper and paperboard can be categorized into seven key aspects:
(1) Appearance Properties
Also referred to as visual characteristics, this includes defects observable by the naked eye, such as dirt spots, holes, pinholes, translucent or semi-transparent areas, wrinkles, creases, streaks, mesh marks, fabric impressions, specks, lumps, fish-scale patterns, cracks, curled edges, and color inconsistencies. Acceptance criteria depend on the intended end-use and quality requirements of the packaged product.
(2) Physical and Mechanical Properties
This refers to the strength and durability of paper and paperboard, which are critical for maintaining packaging integrity. These properties are divided into static and dynamic strength. Static strength is measured under slow loading conditions and includes tensile strength, burst strength, fold strength, and tear resistance. Dynamic strength reflects the material's behavior under sudden stress, including air permeability, elongation, rupture work (e.g., for sack paper), crack resistance, folding endurance, puncture resistance, and compressive strength (e.g., for boxboard).
Tensile strength indicates the maximum load a paper sample can withstand before breaking under uniaxial tension and serves as a key indicator of breaking resistance. Burst strength measures the maximum hydrostatic pressure a paper can endure before rupturing and is a vital parameter for packaging materials. Tear resistance quantifies the force required to propagate a tear once initiated. Folding endurance reflects the number of double folds a sample can withstand before failure, typically higher in the machine direction than in the cross direction. Air permeability indicates the presence of pores within the sheet and is closely related to moisture barrier performance.
(3) Absorption Properties
This category includes sizing degree (resistance to liquid penetration), water absorption, ink absorption, and oil absorption. Most packaging papers undergo sizing treatment to control liquid absorption. Papers designed for filtration, parchment production, or impregnation are often unsized to allow controlled absorption. Absorption characteristics can be modified through chemical or physical processing methods.
(4) Optical and Electrical Properties
Optical properties relate to how paper interacts with light and include whiteness (brightness), opacity, transparency, gloss, and color consistency. Surface smoothness and reflectivity influence perceived luster and print quality.
For electrical insulation applications, paper and paperboard must meet stringent performance standards. Key electrical properties include high dielectric strength, low dielectric loss, appropriate dielectric constant, long service life, and thermal stability. These are largely determined by the physical structure and chemical composition of the paper.
(5) Surface Properties
Surface characteristics include smoothness, abrasion resistance, linting tendency, adhesion capability, suitability for corrugating, and surface roughness. These properties significantly affect printing, coating, lamination, and converting processes and must be evaluated using standardized testing instruments.
(6) Printability
Most paper packaging requires printed graphics, branding, or labeling. Regardless of the printing method-offset, flexographic, gravure, or digital-the substrate must exhibit good ink absorption, dimensional stability, surface uniformity, compressibility, opacity, surface strength, and water resistance to ensure high-quality output.
(7) Chemical Properties
These refer to the chemical composition of paper, including fiber content, moisture, ash content, types and quantities of chemical additives, pH value, acidity (acid number), and viscosity. These factors influence the paper's physical, optical, printing, and electrical characteristics. The chemical properties are primarily determined by the type of fiber source, the amount and nature of non-fibrous components added, and the processing conditions during pulping and bleaching.
Specialty packaging papers impose stricter chemical requirements. For example, anti-corrosion (rust-inhibiting) paper must be free of free acids, chlorine ions, or alkaline substances. Electrical insulation paper, resin-impregnated paper, and food-grade packaging paper must contain no toxic substances, maintain a neutral pH, and have controlled acid values and filler content.
Despite their many advantages, paper and paperboard have several limitations as packaging materials:
- They are hygroscopic and prone to moisture absorption, which can lead to dampness, softening, discoloration, or mold growth when humidity causes moisture content to exceed 12%.
- Exposure to dry, high-temperature, or direct sunlight environments may cause rapid dehydration. When moisture content drops below 9%, the material becomes brittle, leading to warping, cracking, or delamination.
- Mechanical strength is relatively low; paper packaging is susceptible to damage from external forces such as dropping, puncturing, or snagging.
