Phenolic resin, due to its low cost and excellent cold resistance, chemical corrosion resistance, and electrical insulation properties, is the preferred resin for producing weather-resistant and cold-resistant adhesives. However, pure phenolic resin has low toughness, high brittleness, and difficulty in synthesis at lower temperatures, which affects the application performance of materials. To meet relevant requirements, phenolic adhesive needs to be modified. Introducing epoxy groups into the structure of phenolic resin is a simple and effective method. The experiment synthesized thermoplastic phenolic resin with a certain molecular weight, then introduced epoxy groups into its molecular structure for modification. The resulting adhesive showed stable performance when used for a long time at 250℃.
**Experimental Section**
1.1 Experimental Reagents and Major Instruments
Reagents: Phenol, formaldehyde, epichlorohydrin, NaOH, Na2SO3, dilute hydrochloric acid (all chemical grade); Ethanol, acetone (both industrial products); Diluent, curing agent (both self-made). Instruments: FA2004 electronic balance, vacuum oven, JJ-1 electric stirrer, self-made glass reactor, WMZK-10 automatic temperature controller, acid-base burette, three-necked flask, etc.
1.2 Experimental Steps
1.2.1 Synthesis Process of Thermoplastic Phenolic Resin
A certain proportion of formaldehyde and phenol was placed in a three-necked flask, stirred at a certain temperature for a certain period, adjusted pH with hydrochloric acid, heated and reacted for a certain time, measuring aldehyde content every 0.5 hours. After lowering the temperature, maintaining it, and vacuuming, a light yellow viscous liquid was obtained.
1.2.2 Introduction Process of Epoxy Groups
To the thermoplastic phenolic resin from the first step, a certain amount of epichlorohydrin was added, heated and refluxed for a certain time. A certain amount of NaOH solution was slowly added until the system changed from rust-colored turbid liquid to light white turbid liquid. After cooling and vacuum dehydration, more NaOH solution was continuously added, followed by heating, maturation, washing, pressurized distillation of epichlorohydrin, and vacuum drying in an oven to obtain a light yellow transparent viscous liquid.
1.2.3 Determination of Aldehyde Content
The amount of residual formaldehyde in the system was determined by titrating Na2SO3 with NaOH.
1.2.4 Product Application Properties
(1) Bonding Method: Prepared adhesive with curing agent was applied to clean aluminum sheets, cured by segmented temperature increase.
(2) Shear Strength Measurement: Performed according to GB7124-1986.
(3) Gel Time Measurement: Performed according to GB12007.7-1989.
(4) Thermal Weight Loss Measurement: Samples before complete curing were placed in a crucible, weighed, then placed in a muffle furnace. Weighed every 2 hours to calculate weight loss.
**Results and Discussion**
2.1 Factors Affecting Phenolic Resin Synthesis
2.1.1 Influence of Phenol to Aldehyde Ratio
Under acidic conditions, when synthesizing phenolic resin, if the molar ratio of phenol to aldehyde 1. The molecular weight of phenolic resin increases as this ratio increases. Experimental results are shown in Table 1.
If the phenol/ aldehyde ratio is too small, the molecular weight is very large, leading to low bonding strength; if the ratio is too large, the reaction becomes intense, making gel formation difficult. Through repeated experiments, it was found that when the ratio is around 0.7, there is both sufficient bonding strength and stable reaction.
2.1.2 Influence of pH Value
Using hydrochloric acid as a catalyst to adjust the system's pH value for phenolic resin synthesis, the experimental reaction temperature was 80℃. Observing the relationship between aldehyde content and reaction time at pH=1 and pH=5 shows that the reaction speed at pH=1 is much higher than at pH=5. Under these conditions, the reaction reaches the desired level after about 2 hours.
2.1.3 Reaction Temperature and Time
Adjusting the system pH=1, with the molar ratio of phenol to aldehyde being 1:0.7, observing the relationship between aldehyde content and time at different reaction temperatures shows that the reaction speed is fastest at 80℃. Further increasing the temperature leads to intense reactions and easy gel formation.
2.2 Factors Affecting the Introduction of Epoxy Groups
2.2.1 Reaction Material Ratio
In theory, epichlorohydrin should react with phenol in a molar ratio of 1:1, but due to side reactions consuming epichlorohydrin and its tendency to hydrolyze, this reagent must be in excess. Experiments roughly determined the molar ratio of epichlorohydrin to phenol to be around 1.18. Too much epichlorohydrin may cause excessive cross-linking density, making the resulting elastomer unsuitable as an adhesive. Excessive epichlorohydrin will hydrolyze into glycerol, significantly affecting product performance and causing waste.
2.2.2 NaOH Amount and Dropping Rate
NaOH's function is to remove chlorine atoms, as chlorine atoms are weak points in large molecules, hard to remove at high temperatures. NaOH can remove them while introducing epoxy groups, greatly enhancing product heat resistance. If NaOH is insufficient, some chlorine cannot be removed, resulting in poor heat resistance. In main dechlorination, the amount of NaOH should match the molar quantity of epichlorohydrin. The dropping rate of NaOH solution should match the generation rate of HCl, ensuring timely removal of chlorine atoms and minimizing side reactions. If NaOH is added too quickly, it might cause phenolic hydroxyl groups to react with epoxy groups, forming elastomers.
This study adopted a two-step alkali addition method: the first step using 80% of the required NaOH amount, preparing a 40% concentrated solution (which reduces the hydrolysis of epichlorohydrin and facilitates polycondensation), slowly added, evenly distributed, ending with vacuum dehydration; the second step using the remaining NaOH solution prepared as a 7.5% diluted solution (ensuring uniform and thorough reaction throughout the system).
2.3 Maturation Time Determination
A shorter maturation time ensures sufficient reaction of NaOH, thoroughly removing chlorine, improving product quality. Through experiments, the maturation time was determined to be 2 hours.
2.4 Product Infrared Absorption Spectrum Analysis
Infrared spectrum analysis of the product revealed a strong absorption peak at 3400 cm-1, indicating -OH group stretching vibration absorption peaks, showing the presence of unreacted hydroxyl groups. Possible reasons include low molecular weight thermoplastic phenolic resin containing -CH2OH groups or incomplete introduction of epoxy groups leaving hydroxyl groups. There is significant absorption at 925 cm-1, the characteristic absorption peak of epoxy groups; weaker absorption at 1100 cm-1, C-O-C chain absorption peak; and clear absorption at 1245 cm-1, indicating C-O bonds connected to benzene rings. This confirms the occurrence of epoxy reaction and ether bond formation. Significant absorption around 800 cm-1 indicates the presence of phenolic and epoxy resin structures. Combined, these confirm the product as epoxy-modified phenolic resin.
2.5 Evaluation of Product Application Performance
The adhesive synthesized under optimal conditions showed shear strength of 28.8 MPa, gel time of 82 seconds, and thermal weight loss test showing minimal change in the first 8-16 hours, gradually stabilizing, indicating stable product performance. Around 20 hours, large molecules had mostly been removed, allowing long-term use at 250℃.
**Conclusion**
(1) Linear phenolic resin synthesis conditions: Molar ratio of phenol to formaldehyde around 1:0.7, reacting for 2 hours at pH=1 and 80℃, followed by lowering temperature and pressurized water removal.
(2) Introduction of epoxy groups: Adding epichlorohydrin at 1.18 times the molar quantity of phenol, heating to reflux, reacting for a certain time, then adding a certain amount of NaOH solution. The molar quantities of NaOH and epichlorohydrin are equal, with 80% of NaOH (40% concentration) added over approximately 2 hours until the system turns light yellow turbid liquid, followed by pressurized dehydration. Continuing to add the remaining NaOH solution (7.5% concentration) and refluxing, cooling, taking approximately 0.5 hours, then maturing for 2 hours. Separating the turbid liquid, washing with distilled water several times to remove salts, and removing water in a vacuum oven, resulting in a light yellow transparent viscous liquid.
(3) Products obtained under optimal synthesis conditions show shear strength approaching 30 MPa, gel time less than 100 seconds, able to withstand 250℃ for extended periods with stable performance, indicating reliable performance as a temperature-resistant adhesive, with potential for industrial production.
Related Articles:
Analysis and Construction Methods for Cracks and Seepage in Concrete Basement Walls
Application of Epoxy Vinyl Ester Resin Corrosion Protection (Part 1)