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First, why the paper honeycomb products moisture so-called moisture, adhesive structure is the ability to adapt to environmental factors. It is precisely the product's ability to withstand moisture, moisture, and temperature. Paper is the base material of honeycomb products, and its main components are cellulose and hemicellulose. According to the structure of cellulose, using Fedors' method and Lee's formula, the surface tension of cellulose is calculated as rc=72.4 dyne/cm, which is almost equal to the surface tension of water, so cellulose is easily wetted by water. The surface of the paper is also porous, and water vapor easily penetrates into the interior of the paper and reaches the bonding surface between the adhesive and the fiber, gradually causing the bonding interface to break down and forming a weak interface layer.
The porosity, wettability, and permeability of the paper substrate show a weak interface in a wet environment, resulting in a reduction in the rigidity of the honeycomb structure and a decrease in strength, and this decrease is exacerbated by increasing humidity. When cellulose absorbs moisture and expands its own volume, its expansion ratio is longitudinal: transverse: thickness = 1:2:50, and as a result, structural strength is lost. Machines that do not go through a dry cell board or a rain-dried honeycomb product do not have any strength and are worthless when used.
Tests have shown that moisture damage to bond strength is more severe than water, especially in coastal areas where salt water and salt fog are more destructive, because the molecules of water in moisture are smaller and penetrate into the interior of the paper.
The decrease in structural strength is also proportional to the ambient temperature, and high temperatures and high humidity can cause structural strength to decrease. Under the effect of high temperature and high humidity, the paper honeycomb product first appeared a weak interface layer on the honeycomb adhesive surface, and then appeared cohesive failure, and then produced interface failure. In general, this damage is irreversible.
To make honeycomb paperboard, it is possible to use high-strength moisture-proof paper with low water absorption, and use water-resistant heat-resistant adhesive. When the water molecules invade the bonding interface, they cannot replace the secondary bonds formed by the water-resistant cellulose macromolecules, thereby preventing the formation of a weak interface layer, and avoiding internal destruction and surface destruction, so that the environmental resistance of the products is greatly enhanced.
Second, the paper honeycomb moisture-proof technology is the first choice of scientific use of moisture-proof paper as a substrate. Most of China's paper honeycomb products use cardboard, corrugated paper, wood pulp noodle paper. The national standards GB5034 and GB13024-91 only specify 7-11% and 6-14% of the moisture content of the delivery, and there are no regulations in the standards on moisture protection. Some domestic companies, such as the Beijing Fulcrum Company and Suzhou Shengbao Company, have established the fixed-point paper suppliers by setting the standards for the paper board's water absorption rate for paper honeycombs.
In the standard for GB 13024-91 cardboard, there is a requirement for water absorption of Class A (export boxboard) not to exceed 35/50 (g/m 2 ). Grades B and C water absorption only specify positive: 40/-60/, but no requirement for negative.
Judging from the current standards for corrugated base paper and linerboard in China, strictly speaking, it is not ideal as a base material for paper honeycomb paper.
Japan's corrugated board industry standard JISZ1574 specifies three indexes: hydrophobic, water-repellent, and water-resistant. Water barrier indicators are 24h, 48h impermeable two. Anti-water refers to water-impermeable contact with water for a long time. The water resistance index is further subdivided into several grades such as R8, R9, and R10 in JISZ1573. Water-resistant paper is made by adding synthetic resins (urea, melamines, epoxies).
The scientific and technical personnel of the paper industry in China have devoted themselves to successfully researching various water-repellent agents or waterproofing agents (water-proof and moisture-proof), which can be used for secondary processing of paper, that is, adding hydrophobic materials such as paraffin, fatty acid derivatives, synthetic resins, and chromium salts. Complexes, etc., technology is mature, the effect is also obvious.
Paper honeycomb products, especially those used in the field of force, such as building materials, pallets, packaging boxes, cushioning materials, etc., have been accepted and valued by many manufacturers in recent years for their moisture-proof performance, but there are still some today. Paper honeycomb manufacturing companies use whole recycled paper and pure straw pulp fiber paper to make honeycomb paper products. This is the main reason why a large number of manufacturers of honeycomb products have fallen into difficulties in previous years.
Paper is the basis for the production of cellular products. To make products stand on the market, moisture protection should be an important condition that cannot be ignored. As for the classification of different product moisture levels and water, moisture-proof test methods, the industry has yet to further study and discussion.
Third, the paper honeycomb moisture and water adhesives As we all know, paper honeycomb is a typical adhesive material, its bonding strong moisture resistance and Other water quality features, in addition to the impact of the performance of the paper substrate, the second major factor is The quality of the adhesive. Although the weight percentage of the adhesive in the adhesive product is small, its role is very great.
In 1998, an ordnance enterprise in Shanghai purchased a batch of paper honeycomb panels from a honeycomb factory in Beijing and plans to process and produce pallets. Due to the humid climate in Shanghai, the company applied water-proof moisture-proofing agent to the surface of the paper honeycomb panel. It was thought that this would have the effect of preventing moisture, resulting in a large area degumming of the facial paper and the core layer, resulting in a large number of products being scrapped and causing serious losses. A few years ago, we regretted seeing many similar examples. These facts tell us how important the adhesive's moisture and water resistance properties are.
The problem of the water resistance mechanism of adhesives is limited by space limitations. The paper honeycomb product includes a bonding structure. Under the comprehensive action of environmental factors, particularly temperature and moisture vapor, part of the physical properties, chemical properties, and mechanical properties of the adhesive will gradually decrease and cause the bond strength to decrease. The most important factor is humidity and temperature. For example, white latex has a shear strength of 9.8 Mpa at room temperature when the party conditions HR = 65%. When HR = 90% (wood) water content ≥ 17%, the wet strength is only 2.0Mpa, and the water resistance of the adhesive layer is even worse. Therefore, we should follow the following principles when designing and manufacturing paper-cell products, in particular stress and secondary products, and building materials products:
1, the use of high temperature and humidity is the most harmful environment.
2. The moisture resistance and durability of the bonded products are lower than that of the unbonded products.
3. The heat-curing adhesive system is better in resistance to heat and humidity than a system that cures at room temperature.
4, no stress adhesive moisture heat effect is good.
5, the adhesive has different water resistance and moisture resistance.
6, all adhesive systems have moisture, durability limit, the lowest temperature curing.
7, chemical structure affects moisture, durability. The systems with large cross-linking degree in adhesives such as: epoxy systems, phenolic systems, and recently developed Pm series API systems have higher water-resistant durability.
Pm series water-resistant adhesive is the latest product successfully developed. It is a non-toxic, non-hazardous and non-fire-risk green product modified with TDI isocyanate with vinyl unsaturated resin as the main raw material. The product has good film forming property, strong water resistance, excellent aging and biological resistance, and excellent adhesion to porous materials such as fiber, paper and wood. The film is quick-drying, easy to clean after use, and has good process performance. Several domestic production lines use Pm-3 water-resistant glue as honeycomb paper core, the product quality is reliable, normal temperature, soaking water up to 70h. The use of Pm-2 water-resistant glue is good, and the price is only 30% of imported rubber of the same kind. The recently developed ABC (series) emulsion has a water resistance of 48 hours and excellent rheology and aging resistance.
Several production lines in Beijing, Suzhou, Xiamen, and Shandong have been exported to the United States and Japan using adhesives with good water-resistance. Their quality and moisture-proof features have been recognized by people at home and abroad, and the price is only one-third that of similar foreign products. (Han Liquan) ("China Packaging News")
Talk about the moisture-proof of paper honeycomb products
Paper honeycomb products must have a certain moisture-proof function, but how to define the moisture-proof grade of paper honeycomb products, how to produce qualified moisture-proof products, and how to determine the moisture resistance and water resistance of the substrate and a series of deep-seated problems is improved in the paper honeycomb industry. Product quality, enhance product competitiveness and promote the development of the industry's major issues.