Talking about the substitution of polyester resin curing agent HAA for TGIC

Abstract
This paper introduces the difference between the molecular chemical structure of the curing agent HAA and TGIC, the confusion caused by the different curing ratios in the replacement process, briefly describes the practical problems and coping measures encountered in the HAA alternative TGIC, and puts forward the improvement plan and considerations for the yellowing and thick coating pinholes of the HAA system coating.
00
With the rapid development of powder coatings in China in recent years, powder coating production has surpassed that of other regions such as North America and Europe, and is growing by nearly 10% every year. At present, the curing agents used in outdoor pure polyester powder coatings are isocyanate triglycerides (usually abbreviated as TGIC) and β-hydroxyalinelamide (usually HAA), and other types of curing agents are rarely . Among them, TGIC caused workers skin allergy health problems are more and more concerned, but also because of the toxicity of genetic mutation in many countries and regions have been restricted or banned from use, and with the recent TGIC manufacturers maintenance, parking, market supply is tight, raw material prices rose significantly overall, the increase is close to 70% of the price has skyrocketed from more than 30 yuan per kilogram all the way up to more than 50 yuan, many powder mills face TGIC shortage, rising costs pressure, and even the emergence of individual customers due to TGIC shortage, unable to normal production and stop orders, the market panic. As a result, most people have turned their attention to HAA curing agents. HAA curing agents will be developed more rapidly in the future and will become the main curing agent for outdoor pure polyester powders. Today and we discuss several issues in the process of using the curing agent HAA, hoping to help the correct use of HAA curing agent, to promote the healthy and stable development of the industry.
01 resin selection
often used to TGIC type polyester resin, TGIC type polyester resin will be taken directly to cure with the curing agent HAA, in fact, this is a mistake. HAA curing system to choose HAA-specific polyester resin (also known as Primid polyester resin), this polyester resin acid value is slightly lower than TGIC type, about 20-25 mgKOH/g, with HAA curing speed is slow, leveling will be better. However, due to its molecules in the β bit of hydroxyl, affected by N atoms, its activity is very high, if the direct selection of TGIC polyester resin, the powder coating level is mostly not too good.
02 Yellow change/pinhole problem
First look at HAA, whose chemical structure is as shown in (1):
(1)
and polyester resins are cured by their four hydroxyl and pyrethroid reactions, i.e.:
In R'OH-HOOC-R"→R'OOC-R" and H₂O(2)
(1), there are 2 exposed nitrogen atoms in the molecular structure of the HAA curing agent, making it more susceptible to oxidation and loss of stability, which is the gene that causes high temperature yellowing. In the preparation of white or other light-colored powder coatings, it is recommended to add 0.5% to 1% antioxidants to improve its yellowing; The curing reaction begins at 30 degrees C, the laboratory tests the same polyester resin, and using HAA at a curing temperature of 180 degrees C can achieve the same curing performance of TGIC at 200 degrees C, so the appropriate reduction of curing temperature can also improve its yellow denaturation.
type (2), HAA curing agent and pyride polyester resin curing reaction is a contraction reaction, the product of which has H₂O production, the release of reaction by-products easy to make the coating film to produce pinholes, should not be thick coating. In polyester resin testing, by appropriately adding degassing agent in the formulation, spray thickness of less than 120 m, generally there will be no pinholes, "mirror frame" "fat edge" situation.
03 curing ratio is different
in powder coating curing, if the curing agent is insufficient, coating film curing is not complete, coating film can not achieve the best chemical and chemical performance; Polyester resin and curing agent is generally the amount of the same substance (equivalent equivalent) chemical reaction, but taking into account the acid value of polyester resin and curing agent purity and other issues, generally in a certain range is variable.
the amount of curing agent required for every 100g polyester resin curing can be calculated by type (3) when preparation of powder coatings.(3)
:
W - for curing agent dosage, g
E for epoxy of curing agent, eq/100g
Ape - for acid value of polyester resin, mgKOH/g
100 - for quality of polyester resin g
56100 - indicates (equivalent) conversion of mgKOH coefficient
to calculate the curing ratio, curing agent HAA is generally calculated according to molecular weight 320, equivalent to 82g/eq, TGIC according to molecular weight 297, equivalent to 110g/mol. Well, that is to say, when using the same polyester resin, the use ratio of the two curing agents is not the same, the amount of curing agent on the impact strength of the coating film is more obvious, the curing agent dosage is low, the impact strength is not good. For everyone's convenience, by calculation, a approximate ratio is given for reference (except in exceptional cases):
polyester acid value
mgKOH/g
TGIC curing ratio
HAA curing ratio
18-24
to 1 96/4
-96.5/3.5
26-30
-94/6
-96/4
30-36
93/7
-95/5
48-55
-90/10
-93/7
70-80
-88/8 12
to 90/10
04 Catalyst Problem
Because there are 4 ernatic groups in the HAA molecular structure, compared to 3 official groups in the molecular structure of TGIC, there is a stronger reaction activity or popular call reaction participation opportunity, in the absence of catalysis, HAA cures much faster than TGIC.
the other HAA's official energy group and polyester resin's carboxyl reaction is not yet a more suitable catalyst to choose from, so the general HAA-specific polyester can not introduce catalysts to control the reaction speed. The polyester resin cured with TGIC can add catalysts such as seasonal ammonium salt and season phosphorus salt to the powder coating formulation design to obtain the required reaction speed. Therefore, when using HAA polyester resin mixed WITH HAA and TGIC curing agent testing, it is necessary to add catalysts to meet TGIC curing requirements, in order to achieve better coating performance.
because TGIC activity is low, reaction is slow, HAA activity is high, reaction is fast, so in the selection of two different curing agent reaction powder coating dry mix, lighting effect is better, lower gloss.
05 squeeze out the problem
HAA curing agent melting point is higher, higher than 120 degrees C, and polyester resin compatibness is very poor, so the general extrusion temperature is slightly higher than TGIC, recommended extrusion temperature 110 to 120 degrees C (high reaction activity, extrusion temperature can not be higher than 130 degrees C for a long time, otherwise there is collation phenomenon); Improve, resulting in a reduction in the viscosity of the system, melting mixing effect is worse, so HAA cured polyester resin to do high-gloss product surface easy fog, gloss is not high, the general experience is to increase feeding speed, improve the material in the screw filled to increase screw shearing, improve the melting mixing effect, if not, can also be a secondary extrusion solution.
06 powder suction moisture powder / drop powder problemBecause HAA is a hydrophobic compound, water solubility is better, raw material storage and finished powder storage should pay attention to waterproof, moisture-proof, powder moisture absorption after powder is not good, adsorption force is poor;
At the same time HAA curing agent structure of the unclean structure so that it has a greater polarity, easy friction and charge, and powder coating with electrostitive spraying charge, resulting in sprayed work parts in storage after a period of time in the movement will have the phenomenon of powder drop, can be mixed with a little TGIC to improve the phenomenon of powder off, but can not be completely resolved.
07 Other
TGIC cured polyester resin transfer powder with a small amount of HAA curing agent can improve cross-link density, improve transfer powder tearing paper.
Tg of HAA-cured polyester resin powder coating is higher than TGIC and has better storage stability.
the same HAA curing agent has frictional charge, in the sand or texture powder with a small amount of HAA instead of TGIC, not only can improve the performance of powder, but also save costs.
08 Conclusion
By analyzing several problems in the process of HAA replacing TGIC, we can see that:
1) curing agent HAA can directly replace TGIC use, but pay attention to adjust the curing ratio, otherwise;
2) curing agent HAA itself reaction rate is fast, there is currently no effective catalyst to adjust the haA curing system reaction rate, extrusion more attention to extrusion temperature, to prevent collation

4) Although the curing agent HAA in the process of replacing TGIC there are still the above problems, but its comprehensive performance is no less than TGIC system, due to the small amount of use, relatively low price, but also environmentally friendly products, its application in outdoor powder coatings more and more.
5) finally recommended a white HAA curing formula for your reference:Formula composition
dosage/mass
polyester resin
570
Av: -32mg KOH/g
xysylamide HAA
30
general leveling agent
10
degassing agent
10
high purity, crystalline resting fragrance is better,
can also rest incense compound degassing agent using
antioxidant
5
recommended composite antioxidant B900
titanium dioxide
260
recommended goldstone type titanium dioxide
pigment
115
recommended precipitation sulphuric acid
References:
1) Powder Coatings and Coating Technology/NanRenzhi/2014 P240-244
2) Journal of Powder Coatings and Coatings/Choice of Polyester in Hydroxyamide (T105) Applications/Hu Ningsan, Zhang Yunwei
3) China Powder Coatings and Coatings Growth 30 Years/Liu Zexuan, Nan Renzhi, etc./2013
4) Optical and curing properties of primid base powders/
Focus on Powder Coatings
5) Cure Characterization of powders/G. Belder, D.Y. Perera
6) A new beta-hydroxalklamide powder coatings/European Coatings Journal