Deeply dissecting the curling factors of cigarette pack liner paper—grasping these key points is crucial!

Deeply dissecting the curling factors of cigarette pack liner paper-grasping these key points is crucial!

Cigarette liner paper is the inner layer of soft or hard boxes for cigarette packaging, and is an important type of packaging material. The liner paper used for cigarettes serves as the initial packaging layer, with its main functions being fragrance retention, moisture retention, light blocking, and barrier, while also showcasing the cigarette's aesthetic features. With the continuous development of the tobacco industry, the production processes of packaging materials are increasing, the variety is increasing, and the quality is constantly rising. Lining paper is classified by material into aluminum foil composite liner paper, direct-plated liner paper, direct-plated transfer liner paper, composite transfer liner paper, and aluminum-free liner paper; By color, they are divided into gold liner paper, silver liner paper, and white liner paper; According to embossing process, it is divided into ordinary liner paper and transfer liner paper; By weight, it is divided into 52g/m2 liner paper, 57g/m2 liner paper, 62g/m2 liner paper, 80g/m2 liner paper, and 90g/m2 liner paper.

In cigarette production, the adaptability of the liner paper directly affects the quality and production efficiency of cigarette products. Smooth and smooth paper flow are important indicators for assessing the adaptability of the liner paper for machine loading. How to produce smooth liner paper with strong machine adaptability has been a continuous focus for all lining paper manufacturers.

Analysis of curling factors of lining paper

The most basic raw materials for liner paper are liner paper, aluminum foil, adhesives, ink, and straight-plated aluminum or laser straight-plated aluminum. The simplest lining paper consists of four parts: backing paper, adhesive, aluminum foil, and ink, as shown in Figure 1.

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Figure 1: Cross-sectional diagram of lining paper

The main component of backing paper is ultrafine wood fiber. During the copying process, fillers are added, and adhesives are used to bond the fibers and fillers together, forming the liner paper through multiple pressings. During pressing, there are tiny pores between the wood fibers, fillers, and adhesives, with some moisture remaining between them. As environmental temperature and humidity change, these pores absorb and release moisture into the air. Under normal circumstances, the moisture content of the liner paper is 5.0% ± 1.5%. If the moisture is too high, the liner paper will wrinkle when rewinding; If the moisture is too low, the lining paper's elongation and breakage rate decrease, making it easy to break during use.

Aluminum foil is the surface layer of the lining paper, smooth with certain metallic ductility and metallic luster, with a thickness of 0.006mm, excellent ink adhesion, and minimal influence from environmental temperature and humidity. Ink is a resin liquid that disperses pigment powder, divided into water-soluble and fat-soluble types, printed on the surface of aluminum foil to enhance the aesthetic appeal of the liner paper.

The main function of adhesives is to bond the backing paper and aluminum foil into one unit, and it is divided into wet composite adhesive and dry composite adhesive. The main component of wet laminated adhesive is acrylic emulsion, while dry-type laminating adhesive mainly consists of the base material, curing agent, thinner, and other additives. The base material is usually made from polymers such as polyurethane, polyester, and acrylic, which have good adhesion and adhesion. Curing agents undergo chemical reactions with functional groups in the base material, causing the adhesive to cure into a film, which has a significant impact on the curing speed, degree, and strength of the adhesive. Thinner is mainly used to adjust viscosity and improve flowability, enhancing the penetration and wettability of adhesives.

In the composition of liner paper, adhesives, aluminum foil, and ink are relatively stable and less affected by environmental factors; Backing paper is greatly affected by the environment, with significant changes. Moisture changes in the lining paper are the main cause of curling of the lining paper, followed by slitting tension.

01/ Moisture changes in the backing paper

The main component of backing paper, fibers, is sensitive to changes in environmental humidity. When a single fiber absorbs water and expands or shrinks after losing water, the fiber shape changes, causing variations in paper size. For fibers, changes in humidity cause dimensional changes in the fiber length direction of less than 1%, but can reach 20%~30% radially.

The liner paper absorbs moisture, the backing paper surface expands, and the aluminum foil surface remains relatively stable. According to the principle of bimetallic sheets, the liner paper bends toward the aluminum surface, as shown in Figure 2; The lining paper releases moisture, the lining surface shrinks, and the liner paper bends toward the backing surface, as shown in Figure 3. Moisture is the main cause of lateral curling of the lining paper.

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Figure 2: Transverse aluminum surface bending

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Figure 3: Horizontal bending of the paper surface

02/ Slitting tension

If the lining paper is slitted under excessive tension, the whole is stretched and loses tension, the aluminum foil remains unchanged, the liner paper shrinks, and the liner paper curls longitudinally to the paper surface, as shown in Figure 4.

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Figure 4: Longitudinal bending of the liner paper

Measures to prevent curling of lining paper

The curling of the cigarette liner paper is unavoidable and can only be reduced, not completely eliminated. Having been engaged in lining paper production for over 30 years, I will share five methods to prevent curling of liner paper.

01/ Strictly control moisture

During production, the moisture content of the liner paper is tested in advance, and then, based on equipment performance and environmental temperature and humidity, the oven temperature, machine speed, glue application amount, and ink application amount are strictly controlled to ensure stable production speed, ensuring the finished product moisture is 5%±2%, effectively controlling the curling of the lining paper. This measure is low-cost, but when environmental temperature and humidity change, it becomes difficult to maintain moisture continuously.

02/ Add a backing coating

During production, add a back coating to seal the back of the product and prevent the exchange of moisture between the product and the environment. This method is relatively costly, but the product quality is stable, with no significant changes in moisture caused by changes in storage environment. The product quality is stable and the machine adaptability is good.

03/ Added embossing processes

During product design and development, unless otherwise specified, the embossing process should be used whenever possible to alter the physical properties of local fibers and reduce the product's shrinkage rate. Embossing shapes usually include regular mesh patterns, mesh patterns + text, and patterns. After the product undergoes embossing, the internal fibers of the liner paper are compressed and deformed, causing misalignment and rearrangement. The raised parts form a mesh wall, while the recessed parts form a mesh. The raised parts act like reinforcing ribs, increasing strength and preventing product curling. The physical properties also change accordingly. After the pressure is removed, the compressed parts are difficult to return to their original state, greatly reducing deformation, improving shaping, decreasing curling severity, and ensuring stable product use.

Embossing has low production costs, stable product characteristics, minimal influence from environmental temperature and humidity, and is widely used; over 95% of lining papers use this process.

04/ Reduce slitting tension

When slitting the liner paper, the winding tension of the liner paper increases, causing the aluminum foil layer, ink layer, adhesive layer, and liner layer to be stretched simultaneously. When the backing layer is stretched, the fibers elongate and become thinner, losing tensile strength, the fibers shrink longitudinally, and the backing paper returns to its original shape. The aluminum foil layer lengthens and thins under stretching, giving the aluminum foil ductility; after losing tensile strength, the foil remains in its original state.

Lining paper is used in cigarette rolling machines, usually cut every 125mm. Cutting can be considered a process of losing tension: when the lining paper loses its tension, the inner lining paper shrinks longitudinally, the aluminum foil remains unchanged, and the lining paper curls toward the paper surface. Therefore, reducing slitting tension can effectively control the curling of the lining paper.

05/ Use sealed packaging

Packaged in sealed bags, ready to use immediately. If not used promptly, it should still be sealed and stored, shortening the product's contact time with air, reducing the exchange of moisture between the lining paper and moisture in the air, and minimizing curling of the liner paper. This method lowers production costs but requires strict control of the production process to ensure products are flat and not curled before packaging. ...