Songhong Paper
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Paper-plastic composite products require strong bonding, a clean and smooth surface, and the absence of defects such as foaming or curling. The quality of the composite is influenced by various factors, including both objective and subjective elements. These include environmental factors at the operating site (e.g., indoor temperature, humidity, dust), product-specific factors (e.g., ink type, paper grade, printing method), and the adhesive, film, and process conditions used in paper-plastic composites. Additionally, the operation method, technical performance, and structural design of the laminating machine play critical roles. A comprehensive analysis is necessary to ensure optimal results.
### 01 Bond Fastness of the Composite
The factors affecting the bond fastness of the composite can be summarized as follows: the inherent adhesion strength of the adhesive, its affinity with the film, paper, and ink layer, the condition of the adhesive layer, and the operational process conditions.
1. **Adhesive Strength**
The adhesive strength depends on the raw materials of the glue, the production process formula, and the solid composition of the film adhesive. Some products exhibit good adhesion immediately after lamination but may fail over time. For instance, the film may peel off from the paper with minimal force, or scratches on the film surface cause separation between the film and paper. These phenomena indicate that the adhesive strength has deteriorated under external natural conditions.
2. **Influence of Adhesive Affinity with Film on Composite Fastness**
Polypropylene films used in composites have a non-polar chemical structure with no active groups, leading to poor adhesive affinity and difficulty in achieving effective bonding. To address this, corona treatment is applied to generate unsaturated double bonds and carbon bases on the film surface, enhancing its active properties. This treatment increases surface roughness, facilitating adhesive coating. After corona treatment, the film's surface tension changes from non-polar to polar, improving adhesive affinity. However, the effectiveness of corona treatment diminishes over time (approximately six months). If the surface tension drops below 39 dynes/cm², the treatment is considered ineffective. Two methods for identifying corona-treated surfaces are provided:
- Writing or marking on the film surface with a fountain pen; the side that retains ink easily is the treated surface. If water does not bead and is difficult to erase, the corona treatment remains effective.
- Using a 38Mn/cm² surface tension indicator (54 grams of formamide and 46 grams of ethylene glycol ether); if the indicator does not shrink within two seconds, the corona treatment is deemed successful.
3. **Influence of Adhesive Affinity with Paper on Composite Fastness**
Paper is a porous material made from interwoven plant fibers. Sizing is applied to fill pores and improve water resistance, while coating and calendering enhance surface flatness and smoothness. Various types of paper are used in printed matter, including white paper, coated paper, and glass cardboard. Papers can be classified into two categories: those with tight structures and smooth surfaces (e.g., coated paper, cellophane) and those with loose structures and rough surfaces (e.g., whiteboard paper). The latter exhibits poorer bonding due to uneven contact between the adhesive layer and paper surface. For papers with high surface smoothness, the adhesive layer achieves better contact, improving bond fastness. Conversely, for papers with loose structures, the adhesive may pull away paper fibers during stripping. Therefore, the adhesive quantity should be adjusted according to the paper type to compensate for surface irregularities and enhance bonding.
4. **Influence of Adhesive Affinity with Ink on Composite Fastness**
Water-based film adhesives must bond effectively with the film, paper, and ink layer simultaneously. The affinity between adhesive and ink is more complex than with film or paper. Printing inks typically consist of pigments, connecting materials, and auxiliary materials designed to meet specific printing requirements. Offset printing uses spot color inks with thin layers, while relief printing employs thicker mixed-color inks. The internal structure of the ink layer, particularly its chemical characteristics, affects phenomena such as foaming and bonding fastness. Key considerations include:
- Large pigment particles (e.g., silver and gold inks) result in weaker adhesive-ink layer bonding due to granular attachment. Domestic silver and gold inks exhibit larger particles than imported versions, exacerbating this issue. Additionally, water-based adhesives may reflect background colors when bonding metallic films, necessitating careful selection based on product background.
- Poor paper flatness prevents ink layers from filling indentations, leading to uneven surfaces and reduced bonding fastness. Offset printing produces thinner ink layers with more pronounced roughness compared to relief printing.
- High ink layer smoothness can also hinder adhesive-ink layer bonding, as crystallization may occur over time, forming a hard, glossy surface that weakens adhesion.
- Thick ink layers in large-area prints exhibit slightly weaker bonding compared to paper, affecting overall fastness.
5. **Influence of Adhesive Layer Condition on Composite Fastness**
The condition of the adhesive layer refers to its thickness and uniformity. Thicker adhesive layers generally improve bonding fastness, but excessive thickness can impair active ingredient functionality, reducing ink adhesion. Uniform application is crucial for consistent bonding. Specific considerations include:
- **Thickness**: Optimal bonding requires controlling adhesive thickness based on actual conditions, aiming for the thinnest layer that ensures firm bonding.
- **Uniformity**: Inconsistent adhesive thickness leads to variable bonding fastness. Thin areas exhibit weaker bonding, while excessively thick areas may cause separation. Uneven application can result in paper-ink separation.
- **Causes of Unevenness**: Potential causes include improper adjustment of gluing machine components, damaged gluing mechanisms, or non-uniform machine speed.
### 02 Composite with "Snowflakes" on the Surface
Water-based and oil-based laminating machines operate on different principles, with water-based machines using low temperature and pressure, and oil-based machines employing high temperature and pressure. Modern printing designs often involve full-plate or multi-spot color printing, resulting in thick ink layers. To prevent sticking or accelerate curing, operators may use fast-drying oils, increase ink-oil ratios, or add powder sprays. However, these practices complicate subsequent lamination, leading to snowflake-like defects. Pre-lamination wiping with alcohol or powder-soaked towels can mitigate this issue. Insufficient adhesive coverage or inadequate press roller pressure exacerbates the problem.
### 03 Uneven Crimp in Composites
Uneven crimping in composites may result from:
1. Excessive membrane tension causing stress recovery and shrinkage after slitting.
2. Overly wet paper with insufficient density.
3. Excessive winding tension deforming the film.
4. Extended storage time allowing prolonged glue curing.
### 04 Large Gaps Between Laminated Cardboard Overlaps
Large gaps between overlaps in laminated cardboard may arise from:
1. Excessive winding tension deforming the film.
2. Insufficient press roller pressure.
3. Moisture-induced deformation of cardboard with insufficient density.
4. Mismatched speeds between press rollers and transfer boards.
5. Excessive tension in the film unwinding device.
### 05 Color Changes in Mounted Composite Products
Mounted composite products may experience discoloration due to:
1. Excessive adhesive quantity in tile gluing.
2. Alkaline pH of tile glue.
3. Moisture absorption in tile glue causing chemical reactions and potential delamination.
