The reaction between phenol and formaldehyde differs depending on the catalyst used and the molar ratio. Formaldehyde is a 3-functionality unit, and phenol is a !functionality unit. Therefore, phenol and formaldehyde can fully react to form a polymer with a body structure. In the industrial production of phenolic paper-based copper-clad laminates, the modified phenolic resin is used for polycondensation in two stages. At this stage, the phenol and aldehyde form an oligomer state that can melt and flow during the molding process. This stage is usually completed in resin synthesis, and the degree of deepening it in the drying of sizing is added. In the second stage, the melt-flowable oligomer solidifies during the molding process and transforms into a body-structured high polymer. This stage is completed in compression molding.
Although the phenolic resin produced in the stage is an oligomer, since there are three positions on the phenol that can be substituted by a cow, many isomers can be produced. It has been calculated that if the phenolic resin is composed of 3 phenols on average and there are no branches, 1485 isomers can be produced. If there are branches, the number of isomers can be higher, up to about 12,000. This explains why it is difficult to accurately represent the chemical structure of phenolic resin.
Phenol formaldehyde resin synthesis reaction must be carried out in the presence of acid or alkali catalyst. When an aqueous solution of formaldehyde (containing 37%-40% formaldehyde) and pure phenol are mixed in equal volumes, the resulting solution has a pH of 3-3.1. Even if this phenolic mixture is heated to boiling, it will not react within a few days. . If an acid is added to the above mixture to make the pH value 3, or a base is added to make the pH value 3, the reaction occurs immediately.
When a basic catalyst is added to make the reaction medium pH 7, and the molar ratio of aldehyde to phenol in the reaction mixture is greater than 1, the phenol and aldehyde first undergo an addition reaction to produce a variety of hydroxymethylphenol. When the generated hydroxymethylphenol and its derivatives are heated, they further react to form a resinous product. If the reaction goes deeper, cross-linking occurs and macromolecules with a body structure are obtained, which should be avoided in the stage of resin formation.
Phenol and formaldehyde (excess) are polycondensed in an alkaline medium to produce fusible A-stage thermosetting phenolic. The molar ratio of phenol to formaldehyde is 6:7 (ph=8-11). If NaOH is used as a catalyst, the total reaction can be divided into two steps.
(2) Condensation reaction of hydroxymethyl hydroxycresol can further undergo polycondensation reaction. There are the following two possible reactions:
Although both reactions 1 and 2 can take place, the products in formula 2 are mainly produced under alkaline conditions. In other words, the polycondensates are mainly connected by methine bonds.
When the reaction continues, large hydroxymethyl molecules are formed. It has been determined that the rate of the addition reaction is much greater than the rate of the polycondensation reaction, so the post-reactant has a linear structure and a small amount of the body structure. The unit phenol alcohol, polyhydric phenol alcohol or dimer formed during the reaction continuously undergoes polycondensation reaction during the reaction to increase the average molecular weight of the resin. If the reaction is not controlled, it will eventually form a gel (gel). When the reaction process is suddenly cooled down before the gel point, the various reaction rates are reduced, which can be synthesized into a first-stage resole phenolic resin required for copper clad laminates.
In alkaline media, if the molar ratio of phenol to aldehyde is less than ), it seems that the amount of aldehyde is too small to constitute a three-dimensional structure. But in fact, what is obtained is still polymethylolphenol, because the phenolic aldehyde is stable in alkaline medium, and the reaction rate of methylol and hydrogen on phenol in phenolic is faster than that of paraformaldehyde and phenol on the para and para hydrogen. The reaction rate is small, so it is not easy to further polycondensate between phenol and alcohol, and only dihydroxy and trimethylolphenol can be formed, so that some of them do not react, but exist in the form of "free phenol".
Most of the materials are phenol formaldehyde resin liquids of polycondensates of methylol phenol. This kind of resin composed mostly of hydroxymethylphenol structure has strong hydrophilicity (polarity) and low resin viscosity. It has good properties of dipping and penetration enhancing materials and can be used in the production of copper clad laminates with two passes of gluing process. Resinization again.
In short, according to the reaction mechanism, it can be seen that the development of modified phenolic resin as the main resin of copper clad laminates, how to design its molar ratio of phenol and formaldehyde, what kind of catalyst is used in the design and production The parameters of the process conditions, to control the content of dihydroxy and trimethylol phenol, the formation ratio of o- and p-hydroxy groups, etc., are one of the key technologies for the development of this type of resin. It is also an important means to improve the performance of the resin.
(1) The three stages of the curing reaction process At the beginning of the 20th century, the founder of phenolic resin, American scientist Buckland, divided the thermosetting phenolic resin made of basic catalyst into Buckland 1, according to the reaction process of different degrees of polycondensation. 2. Three stages. Based on the characteristics of these three stages of resin, they are called "fusing phenolic resin", "semi-melting phenolic resin", and "insoluble phenolic resin". This scientific conclusion and appellation have been used to this day. Now, the phenolic group is usually connected by a methylene group, and a thermoplastic resin without a reactive functional group such as methylol is called a novolac resin. The resin containing a methylol or dimethylene ether bond structure and having self-curing properties is called resol phenolic resin.
Due to the difference in the degree of progress of the polycondensation reaction, the performance of each stage of the resin is also different. According to Buckland's theory, the thermosetting phenolic resin is divided into three stages of insoluble and infusible state evolution. The three stages of this entire curing process are: A-stage resin, B-stage resin and C-stage resin.
1. The resin obtained after polycondensation, drying and dehydration of phenol and aldehyde of first-order resin can be liquid, semi-solid or solid. It can melt when heated, but as the heating progresses, because the resin molecule contains hydroxymethyl groups and active hydrogen atoms, it can be quickly converted into an infusible state. A-stage resin can be dissolved in the aqueous solution of alcohol, acetone and alkali, it has thermoplasticity. Also known as fusible resin.
2. B-stage resin A-stage resin continues to heat, and -CH2OH on the molecule continuously reacts with each other to crosslink. Its molecular structure is much more complicated than resol phenolic resin. The molecular chain is branched, and phenol is already fully exerting its potential trifunctional role. It is insoluble in alkali solution, and can be partially or completely dissolved in alcohol and acetone. After heating, it can be transformed into an insoluble and infusible product. The thermoplastic is worse than the fusible resin. Also known as semi-melting resin.
3. C-stage resin B-stage resin is further heated, the cross-linking reaction continues to deepen, the molecular weight increases greatly, has a complex network structure, and is completely hardened to remove its thermoplasticity and meltability, it is an insoluble and infusible solid substance. Also called infusible resin. The network (body) structure of the C-stage resin can be shown in Figure 3-3. The curing process changes from the A-stage resin structure to the B-stage and C-stage resin structures, as shown in Figure 3-4. (2) The actual guidance for the production of thermosetting phenolic resin curing reaction process and its mechanism is a very complex problem . So far, some theoretical issues are still being debated in the academic circles of polymer resin synthesis, and it is impossible to obtain a unified understanding. As a worker in the CCL manufacturing industry, there is no need to investigate its more complex reaction mechanism in depth. However, we should have a good grasp and understanding of the above three stages of changes in performance and molecular structure during its curing process, and use this to guide the actual production of copper clad laminates and improve the ability and level of quality control in product processing.
The production of thermosetting phenolic resins (including tung oil modified phenolic resins) in the production of paper-based copper-clad laminates requires the resin reaction to be controlled at the A-stage resin stage. In the late stage of the resin system, when it reacts to the required state of the first-order resin, it is quickly cooled, and a solvent is added to dissolve and dilute it to stop or reduce the reaction to a very slow state. With this resin solution, some can directly impregnate fiber paper to complete the processing of semi-cured gummed paper. Some can be released from the kettle and temporarily stored for use in the resin for impregnation after preparation.
In this resin system, it is very important to control the degree of polycondensation of its first-order resin. The degree of control is deep, reflecting the resin's small gel time and high viscosity. It is beneficial to improve the production efficiency of gummed paper. However, it is not conducive to improving the permeability of the resin to the reinforced fiber paper, nor to improving the manufacturability of the sizing process. The degree of reaction of resol phenolic resin is commonly used in large-scale production practice as the resin gelation time and viscosity index as the means of judging and controlling. Indices such as resin solids and volatile content are also commonly used as indirect judgment and control methods. In addition, by measuring the refractive index of the resin, the content of free phenol, the content of free aldehyde, the degree of solubility in a certain solvent, the molecular weight of the resin, etc., the purpose of research and control of the degree of resin reaction can also be achieved.
The processing of the adhesive paper on the copper-clad board is to impregnate the reinforcing material (impregnated fiber paper) with a-stage resin in the gluing machine, and then enter the drying oven to heat and dry, bake off the solvent, and make the impregnated resin from the linear structure through the processing step by step The transition to part of the branched chain structure, and even very few parts reach the network structure. That is, part of the transition to the B-stage resin, and a small part of the transition to the C-stage resin stage. In addition to obtaining the uniform and consistent glue content of the glue paper required by the process, there is also an important task to deepen the degree of polycondensation of the resin during or after drying the solvent. The control of this deepening degree is based on the pressing processability and reaching some properties of the copper clad laminate. Among the common adhesive paper detection indicators, the fluidity indicator is a comprehensive quality item indicator, which is subject to
Three factors affect the content of glue, the gel time of resin, and the content of soluble resin. If the first two factors are kept under constant conditions, the fluidity directly reflects the degree of transition of the resin in the size paper to the B and C stages after the size processing is heated.
In the two production and processing stages of copper clad laminate gluing and pressing, it has experienced four processes of resin penetration into the reinforcing material: one is the resin penetration during dipping; the second is the resin penetration before entering the drying box after dipping; The third is the penetration of the resin just after entering the drying oven and heating until the solvent is basically evaporated. The fourth is that the semi-finished gummed paper is placed in the press after laminating the distribution board, and the resin is initially heated under pressure penetration. These four resin penetration processes are closely related to the degree of control that most of the resin is in the first-stage resin stage or the second-stage resin stage. It is a key technology to ensure the quality of glued paper and the quality of press-formed base copper clad laminate.