MDF low temperature curing powder coating preparation and performance study (2)

MDF low temperature curing powder coating preparation and performance study (1)
2. Results Compared with the reactive nature of the
2.1 MDF polyester-epoxy powder coating
2.1.1 two MDF polyester-epoxy resin systems
the prepared T-1 and T-2 samples were scanned by DSC for a completely non-iso-temperature heating procedure, as shown in Table 3 (Tab.3). It is found that T-2 powder glass transition temperature Tg0 is higher than T-1 about 3 degrees C, which mainly comes from the higher Tg value of PE-2 resin. However, when the T-1 and T-2 reactions were complete, the Tg1 gap narrowed to 1.5 degrees C

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Tob.3 MDF polyester-epoxy resin system reacts to DSC non-isothermal scanning
despite the slight differences, T-1 and T-2 have very similar reactions to heat release. PE-1 polyester resin is an industrial product, different batches of resin its acid value usually fluctuates within a certain range, so in this polyester-epoxy resin curing system, the use of the same epoxy resin and a certain amount of the case, and epoxy resin reaction of the slight difference in the heat radon may come from the error caused by this acid value (the formula is designed with an acid value of 70 mg KOH/g calculation). The results also show that T-1 and T-2 have very close Ti, Tp and Te values, i.e. the initial reaction temperature, reaction heat release peak and reaction end point are similar, which shows that PE-1 and PE-2 have similar reaction activity in reaction with epoxy resin.
Tab.4 MDF polyester-epoxy resin system reaction
in addition to non-MDF polyester-epoxy resin system, with DSC for completely non-isothermal scanning, the reaction heating peak TP usually appears at 190-200 degrees C; MDF polyester PE-2 has a strong reaction activity than ordinary polyester resin, lies in the introduction of its molecular structure can promote epoxy-based and pyrethroid-based reaction of structural units, the existence of such structural units will reduce the reaction activity of the two kinds of official groups, play a self-catalytic effect, so that polyester-epoxy system T-2 DSC non-temperature scanning TP can be reduced to less than 165 degrees C.
, the results of T-1 and T-2 were found to be different when DSC was first and then non-isostror combination scans were carried out. The heat from the complete non-isothermal heating scan of the polyester-epoxy system shown in Table 3 is the heat H that is completely released by the reaction. When the combined scan is performed, the resulting heat is H', which indicates that the remaining radon is left after an isothermal reaction, as shown in Table 4. The response of T-1 and T-2 when the iso-temperature reaction can be calculated by using H and H', such as Table 5.
Tab.5 MDF polyester-epoxy system reaction degree
the results of this reaction can be seen, in a short reaction time of 3 minutes, the reaction temperature is less than 160 degrees C, T-1 and T-2 polyester-epoxy reaction system response rate did not reach 100%. However, there are also large differences in the response degree of T-1 and T-2 at the same temperature. PE-2 reacts with epoxy at low temperatures, especially at 130 degrees C/3m, with a higher reactive degree than PE-1, and has a strong reaction activity. The PE-1 can achieve results similar to PE-2 only under the constant temperature conditions of higher temperature. This shows that PE-2 will be beneficial to the use of polyester-epoxy system in low temperature curing environment.
Tab.6 MDF polyester-epoxy resin system under different reaction conditions Tg changes
combined scanning results found that T-1 and T-2 iso-cured after the results of the glass transition temperature Tg2 also has a large difference, as shown in Table 6. T-2 after 130 degrees iso-temperature curing, although the response rate did not reach 100%, but its Tg2 more than 70 degrees, higher than T-1. And the majority of ordinary polyester-epoxy system after 100% Tg is around 70 degrees, MEK corrosion resistance level can reach A3. Thus, the coating film obtained by T-2 after 130 degrees C/3min curing, its higher Tg2 will be conducive to the chemical resistance test of powder coating coating. The results shown in Table 7 also show that T-2 has better anti-MEK wipeability.
2.1.2 Effect of Catalyst QAS on the Reactivity of Polyester-Epoxy Resin System
The results of the above analysis show that the PE-2-based polyester-epoxy resin system is more suitable for the low temperature curing reaction of powder coatings, which is conducive to the preparation of MDF powder coatings. However, the reaction activity of T-2 needs to be further improved to meet the requirements of MDF low temperature reaction, such as in the curing time and temperature conditions determined by 130 degrees C/3min conditions, cured coating corrosion resistance to MEET≥A3, and chemical resistance level 2 requirements.
Tab.7 Catalyst QAS's effect on the reactivity of polyester-epoxy resin systems
in accordance with the Arenius equation
k-Ae
-Ea/RT
, the reaction system is determined (i.e., the composition and concentration of the reactant is certain), the reaction temperature and reaction time are determined, only the reduction of the reactive energy Ea can effectively promote the reaction. Ea is the minimum energy required for reactant molecules to reach the active molecule, and the lower the activity energy, the faster the reaction rate. The usual method is to reduce Ea by adding a catalyst to achieve the goal of increasing the reaction rate.
Fig.3 Catalyst QAS effect on polyester-epoxy T-2 reactivity
The catalysts commonly used in polyester-epoxy reaction powder coatings are seasonal ammonium salt, seasonsal, methylene and its derivatives, organic metal complexes, acid or amine-closed sulfonate, among which the first three types of applications are more. Mimi and its derivatives, its action structure is different from the other two, not only can use the nitrogen atom as an anion-type catalyst to promote epoxy opening ring, but also can use the active hydrogen on the midamine to participate in the addition of epoxy base reaction, is an efficient catalyst, but such catalysts easily make the cured polyester-epoxy system yellowing. Thesalt is similar to the seasonal ammonium salt, and the seasonsalt has excellent thermal stability and is also commonly used as a catalyst for polyester-epoxy resin system, but its catalytic activity is less than that of seasonal ammonium salt. At present, ammonium salt (QAS) is used more polyester-epoxy curing agent, and QAS will be selected as the catalyst.
the effect of Tab.8 catalyst QAS on polyester-epoxy
.The presence of a monomer structural unit in the PE-2 molecular structure that promotes epoxy-based and pyrethroid reactions enables TP to be reduced to less than 165 degrees C during DSC non-iso-temperature scanning of polyester and epoxy system T-2, which acts as the main catalysis. After adding QAS volume to T-2 to reach 1%, the reaction heating peak obtained from the non-isothermal scanning of DSC of T-2-3 was reduced from 164.28 degrees C to 153.89 degrees C, with relatively small variation (as shown in Figures 3 and 8), QAS is the auxiliary catalytic effect. However, the addition of QAS shortens the gel time of T-2 (as shown in Table 7) and increases MEK resistance, mainly due to the addition of QAS, which reduces the starting temperature of the T-2 reaction (as shown in Table 8), further accelerating the curing of T-2 at low temperatures.
ADDITION of QAS reduces the initial reaction temperature of T-2 polyester-epoxy, prompting polyester-epoxy to begin at lower temperatures, which has a greater impact on the stability of the extrusion process in polyester-epoxy powder production. As shown in Table 7, when the amount of QAS added is 1%, T-2-3 produces gel when extruded. T-2-3 powder after coating MDF film has the presence of particles, seriously affecting the appearance of the coating film. In order to make T-2 have better surface resistance and extrusive stability during preparation, QAS dosing is more appropriate at 0.5%.
2.2 Sand surface powder coating
2.2.1 effect of powder particle size distribution on coating film-forming conditions
based on T-2-2 at 130 degrees C/3min better curing effect and extrusion stability, this paper is based on the preparation of two different surface microscopic appearance of the powder. However, the powder particle size distribution on the powder coating construction quality and coating film conditions also have a great impact, including the construction process of powder rate, construction stability, powder recovery rate and coating after the appearance of the film, leveling and many other aspects, so the powder particle size needs to be controlled within the appropriate range to take into account the powder construction stability and coating appearance requirements.
tab.9 Texture-1 particle size D (0.5) on the coating film-forming surface
using the same extrusion process on the texture-1 formulation but with different grinding processes to produce powders with different particle sizes. Then, with the same thickness of the control coating film, MDF is coated with powders of different particle sizes and cured in the IR furnace. As shown in Table 9, no particles were found except on the surface of the prepared coating film when powder D (0.5) with a particle size too large exceeded 40 m. From this, it can be seen that the powder extrusion process is properly controlled, and no glued particles appear in the extrusion. The particles of powder coating with large particle size come from the presence of large particle size powder particles. The smaller the powder particle size, the better the smoothness of the powder curing, the more flat and delicate the appearance, the higher the gloss, but the powder particle size is too small will reduce the charge of the powder particles, coating construction efficiency will be reduced, and it is difficult to coat the film with a higher thickness, such as D (0.5) for 23 m when it is difficult to get a coating with a high film thickness. And increase particle size, powder belt charge increase, powder rate increase, is conducive to the preparation of high film thickness coating, but the powder particle size is too large will increase the roughness of the coating film and reduce the gloss. From this, it can be learned that powder particle size control in the range of 25-30 m is a better choice.
2.2.2 Performance of two sand powder coatings
According to Texture 1 and Texture 2 formulations, control particle size D (0.5) is prepared in the range of 25-30 m under the conditions set by the extrusion process parameters described above. The surface resistance of the coating film formed in the infrared oven is shown in Table 10 under the curing conditions of 130 degrees C/3min. The results shown in the table show that the two sand powder coating surfaces can meet the requirements of level 2 surface resistance and can be used in the preparation of MDF furniture. However, the naked eye observation found that the two have a relatively close surface appearance, but the sand pattern dose is less glossy.
tab.10 Two types of sand powder and their coating properties
powder coating applied MDF sheet made of finished furniture, in a humid environment, the long-term use of the plate is very easy to absorb moisture. The moisture-affected MDF plate is easy to expand, so that the applied powder coating film by the expansion pressure and produce deformation cracks, and even make the MDF board as a whole crack, that is, will affect the service life of furniture. Therefore, the MDF plate coated with powder is evaluated for water-resistant expansion under extreme conditions before manufacturing finished furniture.
Tab.11 MDF sand powder coating water-resistant expansion
This paper uses the Jotun internal method MDF water-resistant expansion acceleration test method, the two powder Texture 1 and Textue 2 coated MDF plate, water immersion for 48 hours for its water-resistant expansion test. Results As shown in Table 11, no cracks were found on the surface and edges of the MDF board, both of which are known to have excellent moisture resistance.
2.2.3 Microscopic shape of the sand powder coating
Using the Keens electron microscope VHX-5000, the two sand powders were observed under a 200× multi-mirror with a large difference in the microscopic effect of the coating, as shown in Table 12. Sa is an arithmetic average height, an arithmetic mean of the absolute value of the average surface height of the surface, which is used to indicate the fineness of the coated surface, Sv, which represents the absolute value of the minimum value of the average surface height, Sz, which represents the distance from the highest point to the lowest point of the surface, and Vvv, which represents the gap volume of the valley. These parameters found that Texture 1 is rougher than Texture 2, where Texture 1 adds more sander. This also explains why Texture 1 gloss is higher than Texture 2 from another angle.
the microscopic effect of tab.12 two powder coating films
3. Conclusion
(1) Comparing the chemical reactivity of MDF polyester PE-1 with PE-2 and epoxy, it is found that PE-2 and epoxy resin have better low temperature reaction activity and strong self-catalytic effect. The T-2 powder prepared by it has a curing degree of more than 70% under the condition of 130 degrees C/3min, and has good MEK resistance.
(2) QAS catalyst on the T-2 series polyester-epoxy curing system reaction activity has been improved to reduce the temperature corresponding to the reaction heating peak, but its main role is to reduce the polyester-epoxy curing system's initial reaction temperature point, more conducive to T-2 low-temperature curing. QAS is T-2-2 with a 0.5% dosing in T-2, and its low temperature reactivity and extrusion process stability are excellent.
two kinds of sand powder coating prepared on the basis of T-2-2, which cures in the infrared oven to form a coating film has good surface resistance, can meet the requirements of surface resistance level 2. The surface texture effect of the sand powder coating coating can be qualitatively observed by using the Keens electron microscope, and the surface appearance of the sand coating can be quantitatively analyzed and described by the relevant parameters.
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