Preparation and Characterization of Melamine-Formaldehyde Crosslinked Acrylic Resins

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Abstract

This lab involves the synthesis of an acrylic polymer. The reagents of hydroxyethyl acrylate, styrene, and butyl acrylate were reacted in a semi-batch process The product was then stored for one week before being crosslinked with a melamine-formaldehyde resin and a catalyst. The resulting coating was applied to an Al panel, cured, and tested according to ASTM D5402 for solvent resistance.

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This process was repeated and the formulation adjusted until a formulation exceeded 200 MEK double-rubs. The formulation was then created in a larger batch and applied to Al, steel, and free-film substrates by drawdown bars. These samples were then cured and characterized using various ASTM methods based on mechanical and chemical properties.

Keywords

Melamine-Formaldehyde, Acrylic Resins, Crosslinking, Characterization, Semi-Batch

Introduction

Acrylic resins were first used in the 1950s in the automotive industry.2 They have since increased their durability and practicality. Acrylic resins are formed by radical chain polymerization. This process has three stages: initiation, propagation, and termination. The formation of a radical is what begins this polymerization. That radical species is then able to react with monomer to produce a carbon radical on the monomer. This new radical can then react with other monomer creating a longer and longer chain of monomer. This polymer continues to grow being driven forward by the reactivity of the radical until it is terminated by either another radical or by other means.2

The objective of this lab is to successfully synthesize an acrylic resin with favorable PDI, then crosslink the polymer with melamine-formaldehyde. This reaction looks to create a coating that will be able to be tested using numerous characterization techniques following ASTMs.

Experimental

The first step in this experiment was the synthesis of the acrylic resin. A semi-batch process was used in this synthesis. Xylene (100.0g) was added to the resin kettle and began heating to 90?°C. A nitrogen flow was then started to blanket the entire apparatus. Special care was taken to not have an excessive flow to preserve the solvent. While the solvent is heating, hydroxyethyl acrylate (15.0g), styrene (82.6g), butyl acrylate (52.5g), and Vazo 67 (3.75g) was mixed in an Erlenmeyer flask until the initiator, Vazo 67, was fully dissolved. This solution was then added to an addition funnel that ran into the kettle of xylene. The monomer solution began being deposited into the heated solvent dropwise. The temperature was monitored constantly. The temperature goal of the addition period was 90-95?°C. To control the temperature, the mantle was turned off and lowered to cool and raised and turned on to heat. This back-and-forth heating and cooling cycle created a predictable and steady heating throughout the addition. The entire addition lasted 56 minutes. Once the entire monomer solution was added, the batch was held at the 90-95?°C for an additional 30 minutes. A chaser of xylene (2.5g) and Vazo 67 (0.5g) was added directly into the kettle after this waiting period. The temperature was then maintained for an additional hour. This complete synthesis was followed by cooling the resin and storing for future use in a sealed jar.

One week after the synthesis, formulation with a crosslinker was performed along with characterization. The first test was determining percent solids according to ASTM D2369. The percent solids was determined to be 51.88%. With this information, a formulation can be calculated with the crosslinker, Cymel 303. The formulation of synthesized resin (10.00g), Cymel 303 (1.30g)(25%), and pTSA (0.026g)(0.5%) was prepared with a drop of the flow aid, Byk 301 in solvent. This was then drawn down on an Al panel that was chemically cleaned with solvent. The drawdown was conducted with a 5 mil bar. The panel was left to flash dry for 15 minutes before being put in a 160?°C oven for 20 minutes. To test the cure, MEK double rubs were conducted according to ASTM D5402. If the coating passed more than 200 rubs without failing, then the formulation passes and can be used for further testing. The initial formulation failed at 50 rubs. The formulation was adjusted twice more with identical formulation as stated above except the amount of catalyst, pTSA, was increased with every new formulation. The final successful formulation contained the synthesized resin (20.09g), Cymel 303, (2.63g)(25%), and pTSA (0.21g)(2%). This formulation successfully exceeded 200 rubs and was used to coat Al and steel panels. A glass panel was also used to attain a thin film and for gloss measurements. These Al and steel panels were subjected to numerous ASTM test methods explained in the Results and Discussion section.

Results and Discussion

The acid number was calculated to be 0.55 using Equation (1).

(Acid Number)=(mL of KOH)(Normality)(56.1)/((g of Sample) ) (1)

The hydroxyl equivalence was calculated to be 0.010 using equation (2).

(OH Eq.)=((g of OH Monomer))/((OH Eq.of Monomer)) (2)

The hydroxyl equivalent weight was calculated to be 1501.00g/eq using equation (3)

(OH Eq.Weight)=((g))/((OH Eq.)) (3)

The value of Tg (theoretical) was calculated using equation (4).

1/T_g =W_1/T_g1 +W_2/T_g2 +W_3/T_g3 +?‹?+W_n/T_gn (4)

The value of Tg(theoretical) was calculated to be 16.18?°C of the synthesized resin. The measured Tg using DSC was found to be 35.55?°C. This result are found in Figure 1 The thin-film coating’s Tg was measured using DSC as well and yielded 57.39?°C. This result is found in Figure 2.

Figure 1: Synthesized Resin DSC

Figure 2: Thin-Film DSC

The results of the DSC tests differ than the theoretically calculated ones. This difference is worth noting. The temperature was held constant within the range during the entire experiment (discarding the start-up). The range was 90.0-94.9?°C. A possibility for this variance is the use of older reagents. It is possible that self-polymerization could have began in any of the reagents that were used. This would also explain the variance in the PDI as shown in Table 1. A more likely source of this difference could be directed from the rate of addition. As shown in Table 3, the rate at which the monomer solution was added was not constant at all throughout the addition with a range of rates from 20 to 40 mL per 10 minutes. This would lead to variations in chain length. An advantage of semi-batch synthesis is that one can control the rate of the addition of monomer. The idea is to keep a steady flow throughout the polymerization. This was not maintained, and the results might vary because of it. DMA was supposed to be performed on the thin-film, but the film was too brittle and was impossible to measure.

Table 1: Results of Resin Analysis

Percent Solids MW Mn PDI Tg (theor.) Tg (Actual)

Value 51.88% 17250 10755 1.604 16.18?°C 35.55?°C

SD 0.36%

Table 2: Results of Crosslinked Coating Tests and Characterization

Conical Bend Pencil Hardness Crosshatch Impact (in*lb) Konig Hardness Thickness (?µm) Gloss

Forward Reverse 20?° 60?° 85?°

Value 32% 8H Fail 5B 78.4 62.72 191.67 41.84 164 162.2 114.4

SD 7H Pass 1.25 1.51 7.29 4.66 2.42

Table 2: Data Observed During the Addition of Monomer Solution

Time (min) Amount Added (mL) Temperature (?°C)

0 0 85.1

10 35 91.7

20 25 92.3

30 40 91.3

40 25 91.3

50 20 94.0

56 25 93.0

Despite the temperature maintenance, the conversion rate of the monomer was less than 99%. This is known because of the percent solids value. A percent solids value of 66% would have given a more promising conversion percentage. This value could have been raised by keeping a steady flow and by heating the sample longer and with slightly more initiator. This would increase the conversion rate, but it could lead to other problems with PDI.

This synthesis needs to be run through in one go. The initiation, propagation, and termination are not processes that can simply be paused. If temperature were to drop to room temperature, then termination could end early and leave monomer unreacted or with a low MW. Any loss of solvent over time would also affect the viscosity and make measurements of percent solids more difficult.

GPC was used to find the MW and Mn of the synthesized resin. GPC works by first diluting the resin and passing it through a series of columns. The size of the polymers begin to separate and larger MW polymers do not diffuse through the columns as fast as the lower MW polymers. The amount of polymer that exits the column series over time is measured and compared to a standard polymer.1 It is important to note the limitations of GPC. There are several variables such as the flow rate and elution volume that can give incorrect results.1

As shown in Table 2, this coating had its strengths and weaknesses as far as coating performance goes. The coating could be harder which can be accomplished by increasing the crosslinking. This could be done but not too much to avoid creating an inflexible and brittle coating.

Conclusion

This lab encompassed the synthesis and characterization of an acrylic resin. This resin was then crosslinked with melamine-formaldehyde to form a coating that was then subjected to a solvent resistant test. A correct formulation was found and many panels were created using this coating. The coating was then subjected to numerous tests and processes to learn about its physical and chemical properties.

References

Holding S.R. Gel Permeation Chromatography. Endeavour. Volume 8, Issue 1, 1984, Pages 17-20. (accessed Oct. 2, 2018).

Wicks, Z. W. Organic Coatings: Science and Technology; Wiley-Interscience: Hoboken, NJ, 2007.

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