1. Raw Materials: The Foundation of Paper Quality

Pure Wood Pulp > Mixed Pulp > Recycled Pulp

Raw materials are the foundation of kraft paper quality, directly affecting fiber strength and stability:

• 100% Pure Wood Pulp: Long fibers (average length 1–3 mm), strong toughness, excellent tear resistance and tensile strength — a hallmark of high-quality kraft paper. Commonly used for industrial packaging, heavy-duty bags, and other high-strength applications.
• Mixed Pulp (Wood Pulp + Straw/Bamboo Pulp): Lower cost, performance between pure and recycled pulp. Moderate fiber strength, suitable for general packaging, handicrafts, and applications with moderate strength requirements.
• Recycled Pulp (Reprocessed Waste Paper): Fibers become shorter and more brittle after multiple processing cycles; contains more impurities (such as ink residues and dust). The paper is prone to breakage and has low tensile strength — typically used for temporary or low-strength packaging.

2. Physical Properties: The Core Metrics of Performance

Physical parameters provide objective, data-driven measures of paper quality and functionality:

(1) Grammage (GSM)

Definition: Weight per square meter of paper (unit: g/㎡), directly related to thickness and cost.

Usage: Not simply “the heavier, the better.” Must match application needs (e.g., 120–250g/㎡ for cartons, 200–300g/㎡ for paper bags). High-quality paper has low weight deviation (within ±3%) and consistent thickness.

(2) Water Absorption Height

Definition: The vertical height water rises in paper through capillary action after absorption.

Usage: Reflects capillary absorption capability of the internal fiber structure.

• Densely packed, fine pores = stronger capillary force = higher water rise (e.g., refined kraft paper)
• Loosely arranged, coarse pores or impurities = weaker capillary force = lower water absorption height (e.g., recycled pulp paper)

(3) Tensile Strength

Definition: Maximum force the paper can withstand before breaking under tension (unit: kN/m).

Usage: Critical for tear resistance — e.g., for heavy-duty bags or tape base paper, usually >50 kN/m.

(4) Bursting Strength

Definition: Maximum pressure paper can withstand vertically (unit: kPa).

Usage: Indicates resistance to crushing and impact — a key factor for courier boxes and cement bags (industrial grade usually >300kPa).

(5) Tear Resistance

Definition: The force required to tear the paper (unit: mN).

Usage: Important for repeated folding or lifting scenarios (e.g., shopping bags, document envelopes). Kraft paper made from pure wood pulp has significantly higher tear resistance than recycled pulp.

3. Appearance: Quality in the Details

(1) Surface Smoothness

High-quality kraft paper has no wrinkles, pulp clumps, or holes. It feels smooth (or appropriately rough depending on the application).

Low-quality paper may have noticeable bulges, dents, or rough edges.

(2) Color and Uniformity

Natural kraft paper has a uniform brown color (due to incomplete lignin removal in wood pulp). Bleached kraft paper has consistent whiteness.

Yellowing, dark spots, or blotches may indicate poor raw materials or subpar processing.

(3) Dimensional Stability

High-quality paper has clean cuts, no frayed edges, and maintains shape in varying humidity.

Poor-quality paper may shrink or wrinkle significantly when exposed to moisture due to uneven fiber distribution.