Modified phenolic resin for low-odor anti-net-sticking conductive paste and preparation method of modified phenolic resin

Abstract

The invention relates to modified phenolic resin for low-odor anti-net-sticking conductive paste and a preparation method of the modified phenolic resin, and belongs to the technical field of electronic materials. The method comprises the following steps: S1, reacting thermosetting phenolic resin with an amino-containing end-capping reagent in the presence of a solvent and a catalyst; s2, adding a hydrophobic silane coupling agent into the system in S1 for grafting reaction; s3, adding a hydrophilic polyether compound into the system in S2 to carry out copolymerization reaction; and S4, removing the solvent to obtain the modified phenolic resin. Through three-step reaction, the modified resin has low free formaldehyde (less than or equal to 0.1%), high contact angle (more than or equal to 65 degrees) and alkaline water washability (less than or equal to 5 min), and the three problems that traditional phenolic resin is large in smell, sticky to a net and difficult to clean are solved. The conductive paste containing the resin can be cured at 150 DEG C for 30 minutes, can resist 260 DEG C reflow soldering, can stably work for a long time at 100 DEG C, and can effectively replace the traditional phenolic resin in the existing production line.

Description

Technical Field

[0001] This invention belongs to the field of electronic materials technology and relates to a modified phenolic resin for low-odor, non-sticky conductive paste and its preparation method. Specifically, it relates to a method for modifying a thermosetting phenolic resin for use in precision electronic conductive pastes, and a conductive paste prepared from the modified resin. Background Technology

[0002] Thermosetting phenolic resins are widely used as binder phases in conductive pastes due to their excellent adhesion, heat resistance (long-term operating temperature can reach above 100℃), and electrical properties. For example, patent CN102585125B discloses a method for preparing thermosetting phenolic resins and their application in conductive pastes. The curing conditions are 150℃ × 30min. After curing, the product can withstand reflow soldering (peak temperature 240-260℃) and operate continuously below 100℃, finding applications in strain gauges, thick-film circuits, and other fields.

[0003] However, in actual mass production, especially in precision screen printing, traditional phenolic resins have three inherent defects: 1. Environmental and health problems caused by high residues: Free formaldehyde and phenol (typically >0.4%) remaining during resin synthesis continuously release irritating odors during slurry preparation and curing, deteriorating the working environment and failing to meet increasingly stringent occupational health and safety (EHS) and environmental regulations. From a polymer chemistry perspective, the release of free formaldehyde originates from unreacted formaldehyde monomers during phenolic resin synthesis, which escape from the resin system during subsequent processing and use.

[0004] 2. Distortion of Precision Patterns Due to Poor Printability: Phenolic resin molecules are highly polar and easily wet the screen, leading to "screen sticking," "stringing," and "burrs" at the edges of the patterns during printing. This makes it difficult to control linewidth accuracy (typically only ±10μm), directly affecting the consistency of the final product's electrical performance and failing to meet the ±3μm accuracy requirements of high-end sensors. According to surface chemistry principles, polar molecules interact strongly with the hydrophilic screen surface, resulting in excessive resin adhesion to the screen, thus causing the aforementioned printing problems.

[0005] 3. Low production efficiency due to cleaning difficulties: Uncured phenolic resin has extremely strong adhesion to metals (stainless steel, rolls) and is insoluble in conventional alcohol or water-based cleaning agents. After production, equipment cleaning requires the use of highly volatile organic solvents such as acetone, which is costly and poses significant safety hazards. This is because the chemical structure of phenolic resin causes it to form strong physical or chemical adsorption on metal surfaces, and conventional cleaning agents cannot effectively destroy this adsorption.

[0006] Existing technologies have explored various improvement methods, such as using melamine to end-cap phenolic resins to reduce free formaldehyde. However, these methods only address the odor issue, offering minimal improvement to printability and cleanability. Other technologies improve release properties by adding external lubricants (such as waxes), but these lubricants are prone to migration, affecting the long-term stability and electrical properties of the paste, and may decompose and generate bubbles at reflow soldering temperatures.

[0007] Therefore, the industry urgently needs a modification method that can simultaneously improve odor, printability, and cleaning issues at the molecular structure level, while fully preserving the original resin's 150°C curing window, reflow soldering resistance, and long-term working reliability at 100°C. Summary of the Invention

[0008] The purpose of this invention is to overcome the shortcomings of the prior art and provide a modified thermosetting phenolic resin and its preparation method. This method, through a three-step molecular-level design, enables the prepared modified resin to simultaneously possess low odor, excellent anti-sticking properties for screen printing, and easy water-based cleaning characteristics, while retaining the original resin's curing behavior at 150°C, its resistance to reflow soldering, and its long-term stability up to 100°C.

[0009] Another object of the present invention is to provide a conductive paste containing the modified resin to meet the mass production requirements of high-precision screen printing.

[0010] The objective of this invention can be achieved through the following technical solutions: In a first aspect, the present invention provides a method for preparing a modified phenolic resin that is low in odor, non-sticky, and easy to clean, comprising the following steps: S1. In the presence of a solvent and a catalyst, thermosetting phenolic resin is reacted with an amino-containing capping agent to consume free formaldehyde and active hydroxymethyl groups in the resin. S2. Add a hydrophobic silane coupling agent to the system obtained in step S1 to carry out a grafting reaction and introduce hydrophobic groups. S3. Add a hydrophilic polyether compound to the system obtained in step S2 to carry out a copolymerization reaction and introduce hydrophilic segments; S4. Remove the solvent to obtain the modified phenolic resin.

[0011] Preferably, the capping agent in step S1 is at least one of melamine or urea. Taking melamine as an example, its amino group undergoes a nucleophilic addition reaction with free formaldehyde in the resin to first form hydroxymethyl melamine, as shown in the following reaction formula: Subsequently, the hydroxymethyl groups in hydroxymethyl melamine further condense with the hydroxymethyl groups on the phenolic resin molecules, forming stable heterocyclic or cross-linked structures, thereby permanently locking the free aldehyde and consuming some of the hydroxymethyl groups. In this reaction, p-toluenesulfonic acid acts as a catalyst, providing protons to promote the nucleophilic addition and condensation reactions between the amino group and formaldehyde and the hydroxymethyl group, accelerating the reaction process. The amount of the end-capping agent is 5-15 parts by weight relative to 100 parts by weight of thermosetting phenolic resin (solids), and the amount of the catalyst is 0.5-1 parts by weight. The reaction temperature is 60-70°C, and the reaction time is 2-3 hours. This step aims to reduce the free formaldehyde content to below 0.1%.

[0012] Preferably, the hydrophobic silane coupling agent in step S2 is phenyltrimethoxysilane. Under heating conditions, the alkoxy group of phenyltrimethoxysilane is first hydrolyzed to a silanol group, as shown in the following reaction formula: The silanol group undergoes a condensation reaction with the residual hydroxyl group on the resin molecular chain to form a stable silicon-oxygen bond, thereby introducing a hydrophobic phenyl group onto the resin molecular chain. The reaction formula is as follows: The dosage is 3-8 parts by weight relative to 100 parts by weight of thermosetting phenolic resin. The reaction temperature is 80-90℃, and the reaction time is 1.5-2 hours. According to the Young equation γLVcosθ=γSV-γSL (where γLV is the liquid-gas surface tension, γSV is the solid-gas surface tension, γSL is the solid-liquid surface tension, and θ is the contact angle), the introduction of hydrophobic phenyl groups changes the chemical composition and structure of the resin surface, reduces the surface energy, and increases the contact angle with the screen. This step aims to increase the contact angle between the slurry and the screen to over 60°.

[0013] Preferably, the hydrophilic polyether compound in step S3 is polyethylene glycol glycidyl ether, with a number-average molecular weight of 400-600. The epoxy groups of polyethylene glycol glycidyl ether undergo ring-opening addition reactions with active hydrogens such as phenolic hydroxyl groups and hydroxymethyl groups on the resin molecular chain, achieving block copolymerization of the polyether segments. The amount used is 2-5 parts by weight relative to 100 parts by weight of thermosetting phenolic resin. The reaction temperature is maintained at 80-90°C, and the reaction continues for 1-1.5 hours. The polyether segments are hydrophilic and, under alkaline conditions, may interact with the alkali (such as forming salts), increasing the resin's solubility in alkaline water, thereby imparting alkaline washability to the resin.

[0014] Post-processing: After the reaction, the solvent was removed by vacuum distillation to obtain the modified phenolic resin. The gelation time reflects the time it takes for the resin to transform from a liquid state to a gel state with a certain elasticity, and it is closely related to the crosslinking rate of the resin. Since the three-step modification is carried out on the side chains or end groups without destroying the main chain structure and thermal crosslinking mechanism, the change rate of the gelation time of the modified resin at 150℃ compared with that before modification is controlled within ±15%, ensuring that the curing window remains unchanged.

[0015] Secondly, the present invention provides a precision printing conductive paste, characterized in that it comprises the following components in parts by weight: Conductive functional phase: 70-85 parts; The conductive functional phase plays the role of conducting current in the slurry. Different types of conductive functional phases (such as silver powder, copper powder, carbon nanotubes, etc.) have unique electrical and physical properties, which, together with the modified resin, determine the conductivity of the slurry.

[0016] The modified phenolic resin prepared by the method described in the first aspect: 10-20 parts; as a binder phase, it binds the conductive functional phase particles together to form a stable conductive network structure, and its modified properties help improve the printing and cleaning performance of the paste.

[0017] Reactive diluent: 2-5 parts; the reactive diluent can reduce the viscosity of the paste and improve its printability. It participates in the cross-linking reaction of the resin, affects the cross-linking density of the resin, and thus affects the mechanical properties and electrical properties of the cured paste, such as hardness and flexibility.

[0018] Thixotropic modifier: 0.5-2 parts; The thixotropic modifier can improve the rheological properties of the paste. During the printing process, when subjected to shear force, the paste viscosity decreases, making printing easier; after the shear force disappears, the viscosity recovers, preventing sagging and ensuring the accuracy of the printed image.

[0019] Curing accelerator: 0.3 - 1 part (optional, adjust according to curing speed requirements); The curing accelerator can accelerate the curing reaction of modified phenolic resin, shorten the curing time and improve production efficiency without changing the curing temperature.

[0020] The conductive paste is prepared by mixing, dispersing, rolling, and degassing the components.

[0021] Preferably, the conductive paste is fully cured at 150℃ for 30 min, and after curing, the product undergoes three reflow solderings at 260℃ (peak temperature, 10 s) without any decrease in adhesion.

[0022] The beneficial effects of this invention are: (1) This invention reduces the free formaldehyde content from the traditional >0.4% to below 0.1% by chemically capturing melamine / urea, thereby reducing the irritating odor from the source.

[0023] (2) By grafting phenylsilane, the present invention forms a micro-hydrophobic layer on the resin surface, thereby increasing the contact angle between the slurry and the wire mesh from 35° to more than 65°, thus improving the problems of "sticking to the mesh" and "stretching".

[0024] (3) This invention enables the resin to be washable in alkaline water through block polyether segments. The residual slurry can swell and detach within 5 minutes in a 5% NaOH aqueous solution at 40-50℃. The cleaning time is shortened by more than 80% compared with traditional organic solvent cleaning, eliminating hazardous chemicals such as acetone, and reducing production costs and safety risks.

[0025] (4) The three-step modification of this invention is carried out on the side chains or end groups of the resin without destroying the main chain structure and thermal crosslinking mechanism of the phenolic resin. Tests showed that the gelation time of the modified resin at 150℃ and the glass transition temperature (Tg>120℃) after curing were basically the same as before modification. The slurry can be completely cured under the original 150℃×30min process, and the cured product can withstand reflow soldering at 260℃ and can work stably for a long time at 100℃. Detailed Implementation

[0026] To further illustrate the technical means and effects of the present invention in achieving its intended purpose, the following detailed description of the specific implementation methods, structures, features, and effects of the present invention, in conjunction with embodiments, is provided below. Example 1

[0027] 1. A modified phenolic resin preparation method comprising the following steps: S1. In a reactor equipped with a stirrer, thermometer, and reflux condenser, add 1000g of thermosetting phenolic resin solution (equivalent to 600g of solids, with a free formaldehyde content of 0.48%), and add anhydrous ethanol to adjust the solid content of the system to 50%. Start stirring, add 60g of melamine and 4.8g of p-toluenesulfonic acid, raise the temperature to 65℃, and maintain the temperature for 2.5 hours.

[0028] S2. Add 30g of phenyltrimethoxysilane to the above reaction system, add 5g of deionized water dropwise to promote hydrolysis, raise the temperature to 85℃, and keep the reaction at this temperature for 1.5 hours.

[0029] S3. Add 18g of polyethylene glycol glycidyl ether (number average molecular weight 500) to the above reaction system and continue the reaction at 85°C for 1 hour.

[0030] S4. After the reaction is complete, the system temperature is lowered to 50℃, and vacuum distillation is started (vacuum degree -0.09MPa, temperature 50-60℃) to remove anhydrous ethanol, and a light yellow transparent solid modified phenolic resin is obtained.

[0031] 2. Preparation of a conductive paste, comprising: Prepare conductive paste according to the following weight ratio: Modified phenolic resin prepared in Example 1: 15 parts Silver powder: 80 parts Benzyl alcohol: 3 parts Fumed silica: 1.2 parts 2-Methylimidazole: 0.5 parts Preparation process: Mix modified phenolic resin and benzyl alcohol evenly, add 2-methylimidazole and stir to dissolve, then add silver powder and fumed silica, and premix in a planetary mixer for 10 min; transfer the premix to a three-roll mill and grind it 3 times until the fineness is ≤5μm; vacuum degas for 30 min to obtain conductive slurry. Example 2

[0032] 1. A modified phenolic resin preparation method comprising the following steps: The reaction was basically the same as in Example 1, except that: 72g of urea was used as the capping agent; S1, reaction temperature 70°C, time 2h; S2, reaction temperature 80°C, time 2h; S3, reaction temperature 80°C, time 1.5h.

[0033] 2. Preparation of a conductive paste, comprising: Same as Example 1. Example 3

[0034] 1. A modified phenolic resin preparation method comprising the following steps: The method is basically the same as in Example 1, except that: the capping agent is a mixture of melamine and urea (mass ratio 1:1), with a dosage of 60g; the dosage of phenyltrimethoxysilane is 24g; and the dosage of polyethylene glycol glycidyl ether is 12g. S1, reaction temperature 60℃, time 3h; S2, reaction temperature 90℃, time 1.5h; S3, reaction temperature 90℃, time 1h.

[0035] 2. Preparation of a conductive paste, comprising: Same as Example 1. Example 4

[0036] 1. A modified phenolic resin preparation method comprising the following steps: The modified resin is the same as in Example 1.

[0037] 2. Preparation of a conductive paste, comprising: In the slurry formula, silver powder is replaced with copper powder, while everything else remains the same.

[0038] Comparative Example 1 Thermosetting phenolic resin was prepared according to the method described in patent CN102585125B: 40 g of phenol, 41.4 g of 37% formaldehyde, 0.85 g of sodium hydroxide, and 1.81 g of barium hydroxide were added to a reaction flask and reacted at 60°C for 5 hours. Then, 27.6 g of 37% formaldehyde was added, and the reaction was continued at 60°C for another 5 hours. 4.17 g of 50% sulfuric acid was added for a 10-minute neutralization reaction. Next, 60 g of cyclohexanone and 20 g of toluene were added for extraction, followed by two washes with water to remove the aqueous phase. The solution was transferred to a new reaction flask and subjected to vacuum desolventizing at 115°C to obtain the thermosetting phenolic resin.

[0039] Preparation of conductive paste: The modified phenolic resin in the slurry formulation is replaced with the above-mentioned thermosetting phenolic resin, while everything else remains unchanged.

[0040] Comparative Example 2 Only step S1 of Example 1 was performed, without steps S2 and S3. The solvent was removed by direct vacuum distillation after the reaction. The slurry formulation and preparation process were the same as in Example 1.

[0041] Comparative Example 3 Perform steps S1 and S2 of Example 1, but skip S3. After the reaction, remove the solvent by vacuum distillation. The slurry formulation and preparation process are the same as in Example 1.

[0042] Comparative Example 4 Perform steps S1 and S3 of Example 1, but skip S2. After the reaction, remove the solvent by vacuum distillation. The slurry formulation and preparation process are the same as in Example 1.

[0043] Comparative Example 5 Thermosetting phenolic resin, melamine, phenyltrimethoxysilane, polyethylene glycol glycidyl ether, and p-toluenesulfonic acid were added to the reactor in one step. The mixture was reacted at 65°C for 2.5 hours, then heated to 85°C and reacted for another 2.5 hours. The solvent was removed by vacuum distillation. The slurry formulation and preparation process were the same as in Example 1.

[0044] Performance testing: Free formaldehyde content: determined according to GB / T 14074-2006.

[0045] Contact angle: Using a contact angle meter, the slurry to be tested is uniformly coated on the surface of a stainless steel wire mesh, and the contact angle of deionized water on the coating surface is measured. Each sample is measured 5 times and the average value is taken.

[0046] Cleaning time: Apply the uncured slurry evenly to a stainless steel plate (10cm×10cm) with a thickness of 0.1mm. Comparative samples 1-3 are placed in acetone, and other samples are placed in a 5wt% NaOH aqueous solution at 45℃. Gently shake and record the time it takes for the slurry to completely fall off.

[0047] Gelation time: Take about 1g of resin sample and place it on a 150℃ constant temperature hot plate. Stir continuously with a thin glass rod and record the time from when the resin melts to when it can no longer be drawn into threads. Repeat 3 times and take the average value.

[0048] Glass transition temperature (Tg): Measured by differential scanning calorimetry (DSC) after curing, under nitrogen atmosphere, heating rate 10℃ / min, scanning range 30-200℃.

[0049] Adhesion: Tested according to ASTM D3359 standard, cross-cut test, using 3M 600 tape for peel, rating from 5 (best) to 0 (worst).

[0050] Volume resistivity: Measured using a four-probe resistance meter. The paste was printed on a glass substrate (line width 1mm, line length 50mm, thickness 20μm) and tested after curing at 150℃ for 30min.

[0051] Reflow soldering resistance: Place the cured sample in a reflow oven at a peak temperature of 260℃ for 10 seconds, repeat 3 times, and test the adhesion after reflow soldering.

[0052] Long-term thermal stability: After curing, the sample was placed in a constant temperature oven at 100℃ and the volume resistivity was measured at 0h, 100h, 500h and 1000h respectively, and the rate of change was calculated.

[0053] *Note: Acetone is required for cleaning comparative examples 1-3, and the time should be >30 min; the cleaning solution for other samples is a 5% NaOH aqueous solution at 45℃.

[0054] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for preparing a low-odor, non-sticky, and easy-to-clean modified phenolic resin, characterized in that, Includes the following steps: S1. In the presence of a solvent and a catalyst, thermosetting phenolic resin is reacted with an amino-containing capping agent to consume free formaldehyde and active hydroxymethyl groups in the resin. S2. Add a hydrophobic silane coupling agent to the system obtained in step S1 to carry out a grafting reaction and introduce hydrophobic groups. S3. Add a hydrophilic polyether compound to the system obtained in step S2 to carry out a copolymerization reaction and introduce hydrophilic segments; S4. Remove the solvent to obtain the modified phenolic resin.

2. The preparation method according to claim 1, characterized in that, The capping agent in step S1 is at least one of melamine and urea.

3. The preparation method according to claim 1, characterized in that, The hydrophobic silane coupling agent mentioned in step S2 is phenyltrimethoxysilane.

4. The preparation method according to claim 1, characterized in that, The hydrophilic polyether compound mentioned in step S3 is polyethylene glycol glycidyl ether, with a number-average molecular weight of 400-600.

5. The preparation method according to claim 1, characterized in that, It contains, by weight: 100 parts thermosetting phenolic resin, 5-15 parts end-capping agent, 3-8 parts hydrophobic silane coupling agent, and 2-5 parts hydrophilic polyether compound.

6. A modified phenolic resin prepared by the method according to any one of claims 1-5.

7. A conductive paste, characterized in that, It includes the modified phenolic resin as described in claim 6.

8. The conductive paste according to claim 7, characterized in that, By weight, it includes: Conductive functional phase: 70-85 parts; Modified phenolic resin: 10-20 parts; Reactive diluent: 2-5 parts; Thixotropic modifier: 0.5-2 parts; Curing accelerator: 0.3 - 1 part.

9. The conductive paste according to any one of claims 7-8, characterized in that, The curing conditions for the slurry are 150°C for 30 minutes.

10. A method for preparing the conductive paste according to any one of claims 7-8, characterized in that, This includes steps such as mixing, dispersing, rolling, and degassing the components.

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