Protective Mechanism of Sulfur-Free Paper

In the fields of cultural relic protection, precious metal storage, and precision electronic component packaging, sulfur-free paper serves as a special functional material with irreplaceable protective effects. Unlike ordinary paper, sulfur-free paper achieves effective protection for items vulnerable to sulfidation, oxidation, and acid corrosion through special manufacturing processes and strict composition control.

Basic Characteristics of Sulfur-Free Paper

Sulfur-free paper, also known as sulfur-free coated paper or acid-free paper, is a special paper with low acidity, no sulfur, and no acidic substances. It is obtained through special treatment of paper to reduce or eliminate damage to items preserved on it.
Traditional paper may contain acidic substances such as lignin and acidic sizing agents. These acidic substances react with moisture in the air, releasing acidic compounds, causing the paper to yellow, become brittle, or even decompose. Sulfur-free paper uses alternatives to acidic substances in the production process to achieve an acid-free effect.

Main Characteristics of Sulfur-Free Paper

Neutral or slightly alkaline pH: The pH value of sulfur-free paper is usually between 7.0-8.5, showing neutral or slightly alkaline properties, in sharp contrast to ordinary paper (pH usually between 4.5-6.5).
Free of sulfur and sulfides: Through strict control of raw materials and production processes, the paper contains no sulfur elements or compounds, preventing sulfidation reactions.
Free of acidic substances: Removes components like lignin and hemicellulose from wood pulp that easily produce acidic substances, and adds alkaline substances like calcium carbonate as buffers.
High chemical stability: Does not easily release harmful gases, does not react chemically with the surrounding environment, and has long-term stable chemical properties.

Chemical Principles of Sulfur-Free Paper Protection

1. Prevention of Sulfidation Reactions
Sulfidation reactions are oxidation-reduction reactions that many metals (especially silver, copper, and other precious metals) undergo in sulfur-containing environments. Taking silver as an example, when silver products are exposed to sulfur-containing environments, the following reaction occurs:
2Ag + H₂S + 1/2O₂ → Ag₂S + H₂O
This reaction leads to the formation of black silver sulfide (Ag₂S) on the silver surface, commonly known as "silver tarnishing." Since sulfur-free paper contains no sulfur elements or compounds, it cuts off the possibility of sulfidation reactions at the source, effectively preventing sulfide corrosion on metal surfaces.
2. Inhibition of Acid Hydrolysis
The cellulose in ordinary paper undergoes hydrolysis under acidic conditions, producing glucose monomers, causing the paper to lose strength, become brittle, and yellow. This process not only damages the paper itself but also releases acidic substances, causing secondary damage to surrounding items. Sulfur-free paper inhibits acid hydrolysis through the following mechanisms:
Removes components like lignin and hemicellulose from paper pulp that easily produce acidic substances
Adds alkaline substances like calcium carbonate (CaCO₃) as pH buffers
Uses high-purity cellulose like pure cotton and linen as raw materials
These measures ensure that sulfur-free paper does not produce acidic substances during long-term storage, thereby protecting cultural relics, archives, and other items in contact with it.
3. Blocking Environmental Pollutants
Sulfur-free paper has a certain physical barrier effect, blocking pollutants in the environment (such as sulfur dioxide, nitrogen oxides, ozone, etc.) from damaging protected items. Its neutral or slightly alkaline properties can also neutralize acidic gases in the environment, such as:
CaCO₃ + 2H⁺ → Ca²⁺ + H₂O + CO₂
This reaction consumes hydrogen ions (H⁺) in the environment, reducing the risk of acidic substances corroding protected items.
4. Prevention of Photocatalytic Oxidation
Components like lignin and hemicellulose in ordinary paper easily undergo photocatalytic oxidation reactions under light, producing free radicals, triggering a series of oxidation reactions, causing the paper to yellow and become brittle. Sulfur-free paper significantly reduces the risk of photocatalytic oxidation by removing these easily oxidized components, improving the stability of protected items.

Through its neutral or slightly alkaline pH value, absence of sulfur and sulfides, and absence of acidic substances, sulfur-free paper effectively prevents harmful processes such as sulfidation reactions, acid hydrolysis, environmental pollutant damage, and photocatalytic oxidation, providing reliable protection for cultural relics, precious metals, precision electronic components, and artwork.