Synthesis of semi-crystalline polyester resins for outdoor high-level powder coatings

Abstract: The semi-crystalline polyester resin used to synthesize powder coatings under different monomer, ratio and process conditions is studied, and the fluidization properties of semi-crystalline polyester resin are discussed.
the crystallization and melting point of semi-crystalline polyester resins are indicated by XRD and DSC, and the effects of various factors on the crystallization performance of polyester resins are discussed, and the performance of the prepared powder coating is examined.
foreword
powder coatings after years of development has made considerable progress, market share and application areas expand year by year, but powder coatings themselves also have structural defects

.
The most fundamental reason is due to the powder coating of the most critical component polyester resin caused by the powder coating is currently mainly using ambient polyester resin, a prominent feature of ambient polyester resin is that its glass temperature and molten viscosity show a positive correlation, that is, the greater the viscosity, the higher the glass temperature.
For powder coatings, polyester resins are expected to have a higher glassy temperature to ensure storage stability, but also hope that polyester resins have a lower melting viscosity, so that when the powder coating curing polyester resins have better wetting properties on pigments, resulting in a coating with better leveling performance.
low melting viscosity at the same time can reduce the curing temperature, which can reduce the energy consumption required for curing. Traditional ambient polyester resins cannot meet the requirements of higher glassing temperatures and lower melting viscosity at the same time.
Therefore, people draw on the physical and chemical properties of semi-crystalline polymers, the synthesis of semi-crystalline polyester resin, effectively solve the problem that amorphous polyester resin can not meet the higher glass temperature and lower melting viscosity at the same time, thus giving powder coating better performance.
Eastman disclosed a research patent for crystalline polyester resins in 1981, which focused on the synthesis of end hydroxyl semicrystalline polyester resins.
Zhang Xinhua and Princess Yan reviewed the basic formulation and synthesis process of semicrystalline polyester resins for synthetic mixed powder coatings, polyurethane powder coatings, polyester/TGIC powder coatings, polyester/HAA powder coatings and UV-cured powder coatings, and quickly provided technicians with a large amount of reference information for synthetic semicrystalline polyester resins.
And with the progress of modern analytical characteristicization technology, the understanding of material structure is more in-depth, Li Ying commented on the advantages and disadvantages of using DSC method, WAXD method, density determination method and infrared absorption assay to express polymer crystallization.
DSC method, density method and infrared absorption method are all data of crystallization by comparing the material parameters of the test sample correlation with the material parameters of the fully crystalline sample.
But in the field of polyester resin used in powder coatings, it is difficult to define and obtain a complete crystallization sample, and XRD peaking method is the simplest and most convenient, do not need a sample, but the peak method to calculate crystallization there is a certain randomness, generally only suitable for relative comparison.
, He Manjun and others introduced the structure and morphological analysis of polymer aggregation, which researchers can refer to according to the needs of their work.engineering and technical personnel have done a lot of research on how to synthesize semi-crystalline polyester resin, but rare articles on the effect of materials, ratios, processes, etc. on the crystallization properties of semi-crystalline polyester resins.
this paper provides a reference for technicians to synthesize polyester resins with a certain crystalline degree by studying the effects of raw material types, ratios and processes on the crystallization properties of semicrystalline polyester resins.
1 Experimental Part
1.1 Raw Materials
1.2 Main EquipmentHomemade 8L Small Stainless Steel Reactor; Conical Viscosity Meter (REL/UK); X-ray Diffraction Meter (TD-3500/Dandong Tongda Instruments Co., Ltd.); STA6000 Synchrotron Thermal Analyzer (Perkin Elmer/UNITED States).
1.3 semicrystalline polyester resin is The ratio of polyols, polyic acids and catalysts are added to the 8L reactor, and in N2 atmosphere, the temperature is heated to 235 degrees C (235 degrees C can be determined as needed to add polyols);
sampled acid values reach 8-15mg KOH/g after acidic soytics are added for acidic solution, sealing (or, depending on the process needs, acid). After the value is up to standard, vacuum is first added to the acid antidote for acid dissolution, sealing end), the acid value reaches 40-50 mg KOH/g after vacuum condensation to lift the vacuum, cooling to
200 degrees C;
1.4 crystallization test
TD-3500 X-ray diffraction meter, starting angle 10-40 degrees, scanning speed 0.04 degrees /s, tube voltage 35kv, tube current 25mA.
the melting point of a semi-crystalline resin with a value of 1.5 STA6000 Synchronous Thermal Analyzer (Perkin Elmer), nitrogen flow 20ml/min, heating 20K/min.
1.6 powder coatings and coating preparation According to the basic formula of Table 2 preparation powder coating, the process is: ingredients →mixed→ extruded →pressure→sieve→sieve→ products, the prepared powder coating with electrostature spraying, 180 degrees C / 10 to 15min curing to get the coating, coating and coating performance testing.
1.7 coating and coating performance Reactive and gelding time is measured by GB 16995-1997, shock is measured by GBT1732-1993, gloss is measured by GBT 9754-2007, and pencil hardness is measured by GBT 6739-2006.
2 results and discussion of the effect of
2.1 polyols on the crystallization performance of polyester resins the effect of polyic acids and total ratios in experimental fixed formulations, polyols were replaced by isomorphs, and the effects of BDO, 1,5-PDO and HDO polyols on the crystallization properties of polyester resins were studied. Table
3 is a data on the viscosity, melting point and crystallization of semi-crystalline polyester resins synthesized using three polyols, which are generally related to the regulation, symmetry, and intermolecal forces of polymer structure units.
From Table 3, it can be known that BDO is the easiest to crystallize, and the crystallization is the highest, 1,5-PDO crystallization is the lowest, indicating that saturated fat binary alcohol in synthetic powder coating polyester, carbon chain shorter alcohol BDO synthetic resin has a higher degree of regulation, better crystallization performance.
1 (a) is the fluid curve and melting point of three semicrystalline polyester resins, and the amorphotic polyester resin referenced is a common outdoor polyester.
From Figure 1 (a), it can be seen that the melting viscosity of polyester resin decreases gradually with the increase of temperature, the viscosity of semicrystalline resin is significantly lower than that of amorphous resin, and the viscosity of amorphous resin decreases significantly higher than that of amorphous resin.
Generally speaking, the lower the molten viscosity of the resin, the fuller the flow, the better the leveling performance of the prepared powder coating in the curing process, so the powder coating prepared with semi-crystalline polyester resin is significantly better than the amorphous polyester resin.
the flow curve of the semi-crystalline polyester resin synthesized with BDO can also be seen to have the best crystallization performance of BDO, but the semi-crystalline polyester melting point synthesized with BDO is as high as 180.1 degrees C and the melting point is too high.
powder coating processing temperature is generally not more than 120 degrees C or so, powder coating post-processing difficulties, so the full use of BDO synthetic semi-crystalline polyester resin is not suitable.
and HDO crystallization performance is good, rheodient performance is moderate, can be used alone to synthesize semicrystalline polyester or HDO and BDO together.
2.2 alicic acid ratio R1 has a significant effect on the crystallization The experiment uses two-step method to synthesize semi-crystalline polyester resin, that is, the first step to synthesize the hydroxyl pre-polymer resin, the second step is to add acid inflying agent for acid solution and sealing, the total acid-alcohol ratio of this experiment fixed formula, change the first step alcohol-acid ratio, to investigate the effect of aolic acid ratio on the crystallization performance of polyester resin.
table 3 is data on the effect of alcoholic acid ratio on the crystallization performance of polyester resins, and Figure 1 (b) is a fluidized graph and melting point of different alcoholic acid ratio synthetic polyester resins.
can be seen from Table 3, with the increase of the alcohol-acid ratio, the viscosity increases, the regulation of the molecular chain decreases, and the crystallization decreases. The melting point and crystallization show a positive correlation, because the molecular structure is basically the same, the higher the crystallization, the more rules of the molecular chain arrangement, the higher the energy is required to melt, so the melting point is higher.
Figure 1 (b) is a fluidization curve with different alicic acid ratios, as can be seen from Figure 1 (b), the higher the alcohol-acid ratio, the worse the fluidization performance of polyester resins, so the formulation design can choose the appropriate aerolate ratio according to the performance needs of the product.
2.3 The effect of 2.3 acid antidotes on the crystallization performance of polyester resins On the basis of having the same end hydroxyl pre-polymer in the first step, the second step uses different polyic acid monomers for acid desealing and sealing, and studies the effect of different polyic acid monomers as an acid dissolver on the crystallization properties of polyester resins.
table 3 is data on the effect of different acid antidotes on the crystallization performance of polyester resins, and Figure 1 (c) is the fluid curve and melting point of polyester resins synthesized by different acids.
As can be seen from Table 3, ADA and DDA with long chains with a good symmetrical structure have the best crystallization performance, while the synthetic polyester resin crystallization of non-symmetric aromatic polyic acid IPA as an acid antidote can also reach 41.77 percent.
although the crystallization of semi-crystalline polyester synthesized with IPA is not low, its fluidity can be seen from Figure 1 (c) to be the worst of the four acidic antidotes.
of the four polyic acids shown in Table 3, IPA has the lowest cost and the best weather resistance, and IPA can be used as an acid antidote to synthesize polyester resins with a certain crystallization in cases where weather resistance is high.
effect of 2.4 chemical agents on the crystallization performance of polyester resins The introduction of an appropriate content of the branching agent in the formulation can improve the degree of branching of the resin and help to increase the cross-link density of the powder coating, but the introduction of the branching agent will destroy the degree of molecular regulation of polyester resin, resulting in a decrease in the crystallization of the system.
the effect of TMP on the crystallization of polyester resin was studied by adding a moderate amount of TMP after the temperature of the reactor reached 235 degrees C. Table 3 is data on the effect of TMP (as a percentage of total alcohol) on the crystallization performance of polyester resins, and Figure 1 (d) is the fluid curve and melting point of synthetic polyester resins after a certain amount of TMP is added.
from Table 2, the crystallization of polyester resin after the introduction of TMP decreased rapidly, the viscosity of the system increased, and the fluidity performance decreased.
2.5 vacuum process has a significant impact on the crystallization performance of polyester Compared with amorphotic polyester resin, semi-crystalline polyester resin has a more regulated molecular structure, and this experiment compares the effects of one-step vacuum method (vacuum after acidization) and two-step vacuum method (vacuum first, then acid-based antidote and then vacuum) on polyester resin crystallization performance.
table 3 is data on the effect of different vacuum processes on the crystallization performance of polyester resins, and Figure 1 (e) is the fluid curve and melting point of polyester resins synthesized in different vacuum processes.
From Table 3 and Figure 1 (e), it can be seen that the two-step vacuum synthesis of polyester resin crystallization is high, low melting point, but the two-step vacuum method and one-step vacuum synthesis of semi-crystalline polyester resin fluidization performance is not much different, most likely due to two-step vacuum resulting in a more uniform distribution of molecular weight to improve the system's regulation.
2.6 powder coatings are more important than Generally speaking, the lower the viscosity of the resin, the better the leveling performance of the prepared powder coating, but due to the increase of the soft monomer, the glass transition temperature of the resin will be reduced, while the semicrystalline polyester resin has the advantages of higher glassing temperature and lower viscosity, thus providing the possibility for the preparation of high level powder coating.
performance of
semi-crystalline polyester resin is indicated by the preparation of powder coatings, and Table 4 is the main performance comparison between semi-crystalline polyester resin preparation powder coatings and conventional amorphotic powder coatings.
As can be seen from Table 4, when semi-crystalline polyester/amorphotic polyester is 1:1, the leveling performance of the coating is significantly improved relative to amorphotic polyester, with horizontal fluidity of 32mm, coating gloss to 93.8%, and excellent leveling performance.
can be seen at the same time, after adding semi-crystalline polyester resin, the softness of the resin becomes better and the hardness of the powder coating decreases. Through the introduction of semi-crystalline resin, it provides a good way to prepare high-gloss high-level powder coating.
3 Conclusion
(1) Polyester resins with different crystallization degrees can be obtained using BDO, HDO, 1,5-PDO, with the highest crystallization of resins synthesized with BDO. The addition of the branching agent will reduce the crystallization of the system. (2) using 2 vacuums can help improve the crystallization of the system, but it has little effect on the fluid performance of the system. (3) semi-crystalline polyester resin with amorphotic polyester resin can be prepared high-gloss high-level powder coating, while the prepared coating has better storage stability. This paper, by studying the crystallization and melting point changes of synthetic polyester resin under different conditions, synthesizes a semi-crystalline polyester resin with excellent fluidization performance, and the powder coating prepared with semi-crystalline polyester resin has better leveling and mechanical properties than conventional amorphotic polyester resin preparation, which can effectively improve the quality of powder coating.
semi-crystalline polyester resin has many advantages over amorphous polyester resin, but there are still many difficulties in the scale promotion of semi-crystalline polyester resin, with the deepening of research, various problems will be solved, semi-crystalline polyester resin applications will be more and more extensive.