Melamine-formaldehyde resin (MFR) is a thermosetting amino resin with good hardness, thermal stability, and transparency. Compared with urea-formaldehyde resin (UF), MF resin has better water resistance and weather resistance. Therefore, it is widely used in outdoor panel coatings, wooden floor decorative layers and veneer products. However, shortcomings such as high brittleness, easy cracking, and short storage period of melamine formaldehyde resin limit its application. MF resin is often used to prepare impregnated adhesive film paper. Plywood and blockboard faced with impregnated adhesive film paper are collectively called ecological boards. They are loved by consumers because of their wear resistance, waterproofness, and various colors.
However, when the environmental humidity changes, the ecological board base material is prone to swelling or shrinkage, resulting in uneven internal stress. The surface layer of MF resin is brittle and has poor flexibility, and is prone to cracking under stress. Therefore, improving the flexibility of MF resin and improving its brittleness problem are the keys to solving the problem of resin cracking and improving the quality of its decorative products. This article mainly summarizes the progress of research on toughening modification of MF resin, with a view to providing theoretical guidance and technical basis for solving the brittleness problem of MF resin.
1. MF structural characteristics
MF resin is a triazine ring polymer compound formed by the condensation of melamine and formaldehyde and connected by methylene or ether bonds. It has stable chemical properties and rigid structure. The formation of MF resin is divided into two stages: the first stage is the addition reaction of melamine and formaldehyde. Under neutral or alkaline conditions, melamine undergoes hydroxymethylation to generate 9 different hydroxymethylmelamines. The second stage is a condensation reaction. Melamine reacts with methylolated melamine, or methylolated melamine undergoes self-condensation to generate oligomers connected by methylene and ether bonds. In an environment with a pH value of 7 to 8, methylene bridge connections dominate; in an environment with a pH value higher than 9, ether bond connections are more likely to form.
After cross-linking and curing of MF resin, a transparent coating that is wear-resistant, acid and alkali resistant can be obtained. Although the cured coating has a high hardness, it is also brittle. The main reason is that the MF resin contains a large number of triazine rings connected with methylene groups. The triazine rings are extremely stable under impact conditions and are difficult to deform and absorb impact energy. Only methylene bonds can absorb most of the energy, causing the methylene bonds to break and exhibit brittleness. Incomplete curing of MF resin will cause resin cracking. Since resin curing mainly occurs between hydroxyl groups, some hydroxyl groups that have not participated in condensation will remain in the resin after curing. These hydroxyl groups have certain hygroscopicity. As the environmental humidity changes repeatedly, the free hydroxyl groups will undergo hygroscopic and desorption cycles and gradually generate stress, causing cracks in the resin coating and seriously affecting the appearance and performance of the product.
The flexibility of MF resin can be effectively improved by reducing the cross-linking density of MF resin and extending its flexible chain length. However, the reduction of cross-linking density means that the rigid structure of the triazine ring in the resin is dispersed. Therefore, the surface rigidity and wear resistance of MF resin will be affected to varying degrees. When performing toughening modification, care should be taken to maintain the hardness of MF resin and not destroy its excellent surface properties. Choosing an appropriate modification method to improve the flexibility of MF resin is of great significance to expanding the application scope of MF resin.
2. Toughening modification method of MF resin
Toughening modification is often used to optimize the strength of rigid cross-linked polymers to improve the impact resistance and fatigue resistance of the polymer without destroying the original stiffness and dimensional stability of the polymer. Toughening modification methods suitable for MF resin can generally be divided into two types, namely external toughening modification and internal toughening modification. External toughening modification refers to adding reinforcing phase groups to the MF resin through blending to form a cross-linked structure with the MF resin to prevent the triazine rings from approaching each other to achieve the purpose of toughening. Commonly used external toughening methods for MF resin include:
1) Add nano-SiO2, nano-cellulose and other nanoparticles;
2) Use flexible long-chain polymers such as polyethylene glycol copolymers to form interpenetrating polymer networks (IPNS) in MF resin;
3) Use fiber materials such as polyvinyl chloride to connect MF resins together to enhance the stability of the resin structure.
Internal toughening modification changes the molecular structure of MF resin by participating in the curing reaction of MF resin, extending the flexible connection between triazine rings, and reducing the cross-linking density to achieve the toughening effect. The methods of internal toughening include:
1) Introduce flexible long-chain molecules between the triazine ring structures of MF resin to disperse the rigid structure;
2) Use silicone to extend the flexible connection between MF resins;
3) Block the hydroxyl functional group of MF resin and reduce the cross-linking density of the resin. Commonly used internal toughening modifiers include polyvinyl alcohol, polyurethane, caprolactam, etc.
3. Special requirements for MF resin for impregnation
There are many toughening methods for MF resin, but not all of them are suitable for impregnating MF resin. The toughening modification of impregnating MF resin must adapt to the production conditions of adhesive film paper, so it has certain special requirements. When using MF resin for impregnation, we should not only consider the crack resistance, wear resistance and flexibility after curing of the resin, but also pay attention to the fluidity, permeability and pre-curing degree of the modified resin before curing.
The poor fluidity of MF resin will cause dry flowers on the surface of the impregnated adhesive film paper veneer, producing opaque white spots on the surface of the product, forming surface defects. Poor permeability of MF resin will cause insufficient impregnation, resulting in lack of glue or uneven surface on the product surface. Inorganic nano-wear-resistant materials can increase the toughness and wear-resistant properties of MF resin, but at the same time they will increase the viscosity of MF resin and reduce the fluidity of the resin, thereby affecting the impregnation performance of MF resin. We found that the viscosity of MF resin increases with the increase in the amount of wear-resistant filler nano-SiO2 (HTSi-03). The viscosity of the modified MF resin system was 20.9 s (25°C), and the contact angle between the MF resin and the decorative paper increased from 5° to 30°. At the same time, the wettability decreases, so that the MF resin cannot completely fill the gaps in the decorative paper, and the wear-resistant coating cannot fully protect it, resulting in a decrease in wear resistance.
The low pre-curing degree of MF resin will cause high volatilization during the veneer process, causing surface wet flowers to affect surface performance. Polyhydroxyl alcohol modifiers can effectively increase the flexibility of MF resin, but the certain hydrophilicity of polyhydroxyl modifiers is not conducive to the drying of impregnated adhesive film paper, thereby reducing the pre-curing degree of MF resin. This causes problems such as moisture absorption and adhesion of the impregnated adhesive film paper and difficulty in access. At the same time, the impregnated adhesive film paper with low pre-curing degree will discharge a large amount of water during the hot pressing process, causing problems such as volume shrinkage of the adhesive layer and cracking of the resin layer, affecting the performance of the product.
Therefore, considering the actual production problems, the MF resin toughening agent used for impregnation should use a modifier with low molecular weight, low hydroxyl content, and high reactivity to minimize the impact of the modifier on the fluidity and pre-curing degree of the MF resin. At the same time, the toughening agent is required to adapt to the curing conditions of the MF resin used for impregnation, and still maintain the modification effect after hot pressing curing.
Modification of MF resin for impregnation needs to consider many factors at the same time. The ideal modifier will enhance the flexibility of MF resin without negatively affecting the surface wear resistance, fluidity or pre-curing process. Therefore, it is recommended that future research on the modification of MF resin should be carried out from the following three aspects.
1) Based on the structure of MF resin, the brittleness problem of MF resin can be solved by adjusting the cross-linking density of MF resin.
2) Combining different toughening mechanisms, introduce a flexible chain structure with multi-functional groups into MF resin, and increase the flexibility of MF resin through the synergy of physical combination and chemical reaction.
3) From the perspective of simplifying the production process and saving costs, select highly reactive modifiers suitable for industrial production.
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