Chemical Resistance of Oligomeric Acrylates

Transcription

1 Chemical Resistance of ligomeric Acrylates Sartomer Company, Inc. Exton, Pennsylvania USA Sartomer Company, Inc. aklands Corporate Center 52 Thomas Jones Way Exton, PA Tel: SARTMER ( ) Fax: Web: /8

2 ABSTRACT Chemical resistance of a series of UV cured oligomeric acrylate-based coatings was evaluated. Acids, bases, organic solvents, and industrial fluids were employed in the study. Gloss, yellowing index, tensile strength, and tensile elongation were measured at different time intervals and related to the backbone structures of these oligomers. INTRDUCTIN UV curing involves the rapid polymerization of acrylic monomers and oligomers in the presence of photo-active initiators. In most cases, the monomers and oligomers are multifunctional in nature, resulting in the formation of a highly crosslinked thermoset polymeric film. Compared to thermoplastic polymers, which are often used in solvent-based coating applications, the UV cured thermoset films offer distinct advantages such as chemical resistance, high gloss, dimensional stability and mechanical integrity. This new technology is quickly expanding into a wide spectrum of industrial sectors and replacing the out-dated solvent-based process. Coatings with improved chemical resistance and other desired properties are being required to survive and function in harsh environments such as automotive, marine, and aeronautical applications. A generally accepted view in the radcure (UV/EB) industry is that for any parent resin (oligomer), i.e., epoxy, urethane, polyester, silicone, acrylated acrylics etc., the typical properties of the parent backbone are more or less carried through to the radiation curable analogue 1. In this study, oligomers account for more than eighty percent of the overall formulation, thus the film properties are largely determined by the oligomer used. Previous investigation by others has shown that oligomers based on polybutadiene have exceptional resistance to hydrolysis, but oxidative stability was identified as a deficiency. Polyether based oligomers afforded coatings with fair oxidative and hydrolytic stability. Polyester based oligomers provided good oxidation resistance but poor hydrolytic stability 2. Results of recent work from this lab have revealed that aliphatic urethane acrylates are the best oligomers in the 3 hours QUV accelerated weathering test. Whereas in the QUV chamber, short wavelength UV and high humidity are the major degradation factors, the oligomers are expected to behave differently in a chemical exposure environment. Based on the results from this chemical resistance study and the ongoing weathering study, oligomers with superior chemical and weathering resistance will be identified. The findings will also help design novel oligomers with higher performance properties for demanding environments. This work is only an initial investigation of the influence of the structures of seven selected oligomers on the chemical resistance of UV cured coatings. The effect of monomers, photoinitiators, and additives, as well as adhesion and temperature, will be the topic of future presentations. EXPERIMENTAL Formulation and Materials Seven oligomers were used in this study. The general back-bone structure and physical properties of these oligomers are listed in Table 1. 3

3 Table 1 Structure and Physical Properties of ligomers Group I Group II Group III ligomer 1 Backbone MW M=1, 6 C CN964 Ester M 21, CN966 Ester M 7, CN981 Ester/Ether - 1, CN982 Ester/Ether M 9,375 CN986 Ether M 45 Pro1154 Ether M 26,5 CN31 2 PBD - 4,175 1 All oligomers are aliphatic urethane acrylates (AUA) except CN31. 2 CN31 is polybutadiene dimethacrylate (PBDMA). The typical formulation for all oligomers studied is as follows: Component Weight% ligomer 8% Monomer 2% Photoinitiator 5% (on resin) The monomer used is HDDA, hexanediol diacrylate. The photoinitiator is Esacure KIP1F 1, oligo[2-hydroxy-2-me-thyl-1-[4-(1- methylvinyl)phenyl]propanone]. This is an a-hydroxyacetophenone type unimolecular photo-initiator. All materials are commercially available from Sartomer Company. Wet Film Preparation The 5 mil wet films were cast on mill finished aluminum Q-Panels by using zero drawdown rods with double tapes. Great care was taken to obtain films with uniform thickness. This is very important because film thickness is a very critical factor in determining a coating s performance. UV Curing System A Fusion Model F44 UV curing unit was used. For gloss and yellowing measurements, 5 mil samples were cured by passing one 3 W/in D lamp followed by one 4 W/in H lamp at a conveyor speed of 5 fpm (feet per minute). Each sample was exposed twice using this curing condition. For mechanical property measurements, 5 mil wet films were cast on aluminum Q-Panels and initially cured under one 3 W/in H lamp at 2 fpm. The semi-cured films were then cut into.5 inch wide strips. Finally, fully cured films were 1 available from Lamberti USA, Inc obtained by passing the strips under one 3 W/in D lamp followed by one 4 W/in H lamp at a conveyor speed of 5 fpm. Again, a total of two passes was used. Prior to testing, all the coated panels were stored in the dark after curing for 3 days. This was done to ensure a uniform environmental history. Chemical Resistance Test The test procedure followed ASTM method D Chemical reagents were as follows: 5% H 2 S 4 1% H 2 S 4 5% NaH 1% NaH Ethanol Toluene Skydrol, 5B-GL Tap water Skydrol is a proprietary fire resistant hydraulic fluid which is formulated by Monsanto Company. Chemically, Skydrol belongs to the phosphate ester group and has performance additives. For gloss and yellowing measurements, the covered spot method was used. The test was conducted at 23 ± 2 C and 5±5% relative humidity. A 5-mL pipet graduated in.1 ml was used to pipet onto the horizontal UV coated aluminum panel 1 ml of the reagents. The reagent was immediately covered with a watch glass. After an interval, the spot was wiped and washed clean and examined immediately. The 4

4 intervals used in this study were 1, 8, 24, and 12 hours. For tensile measurement, film strips were fully immersed in the testing reagents in an amber bottle. The strips were exposed using the same intervals as those for the covered spot test. The strips were washed and dried before testing. Gloss Measurement 6 gloss was measured using a Micro-Tri- Glossmeter from BYK-Gardner. Yellowing Measurement The yellowing property of a film is expressed by a yellow index which is calculated by the following equation: YI=[(1.28*X-1.6*Z)/Y]*1 where X, Y, and Z are the tristimulus coordinates. The values for X, Y, and Z were obtained from a Hewlett Packard 8452-A Diode Array UV-Vis Spectrometer equipped with a Reflectance Spectroscopy Accessory from LapSphere. The UV-Vis is operated by a Window Color software program which gives Tristimulus, Hunter Lab, CIELAB, and PLAR Coordinates. Mechanical Properties Test Tensile strength, elongation, and tensile modulus were determined with a Thwing-Albert tensile tester with a strain rate of.5 in/min. For each sample, at least 5 specimens were tested and an average value was reported. RESULTS AND DISCUSSIN ligomers: the Structural Aspects As shown in Table 1, all oligomers are aliphatic urethane acrylates (AUA) except CN31. The aliphatic urethane acrylates can be further divided into three different groups based on the structure of the polyol utilized to synthesize the acrylate oligomer. CN981 and CN982 (Group II) are aliphatic urethane acrylates, with ester/ether polyol as the backbone. Extended with branched polyester polyol, CN964 and CN966 (Group 1) have identical stoichiometries with the exception of the polyester polyol molecular weight. CN986 and PR1154 (Group III) can be identified as an oligomer group where the backbone is pure polyether. CN986 is characterized as strong and tough. PR1154 imparts extreme flexibility to the film. I: Polyester Aliphatic Urethane Acrylate (AUA) Group II: Polyester/ether Aliphatic Urethane Acrylate (AUA) CN31 is a polybutadiene terminated with methacrylates (PBDMA). With the high hydrophobicity of the polybutadiene chain, CN31 is expected to exhibit exceptional resistance to acids and bases. n Group III: Polyether Aliphatic Urethane Acrylate (AUA) n m Polybutadiene Dimethacrylate (PBDMA) n n 5

5 Among these three groups of oligomers, CN982, CN964, and CN986 have different polyol backbones, but the molecular weight of the polyol are the same. This provides a good base to study the influence of polyol structure on the coatings properties. For all the formulations tested in this study, the oligomer is the only variable. The properties measured should be characteristic of the specific oligomer. Acid Resistance Both highly concentrated (5%) and diluted (1%) sulfuric acid were used to test acid resistance. The chance for a coating to encounter 5% sulfuric acid in its service life is probably extremely rare. Thus, 5% sulfuric acid is only used in this study to demonstrate the ultimate performance of the coatings. Figure 1. Effect of sulfuric acid on tensile strength of polyester AUA Tensile Strength vs Time, in H 2 S 4 CN966, 1% CN964, 1% CN966, 5% CN964, 5% Figures 1-6 plot tensile strength and elongation vs. time for these seven oligomers after exposure to both 1% and 5% sulfuric acid. The general observation is that all oligomers perform well in 1% sulfuric acid. In 5% sulfuric acid, however, a dramatic reduction in tensile strength and tensile elongation was detected for all oligomers except CN31, the polybutadiene dimethacrylate. Figure 2. Effects of sulfuric acid on tensile strength of Polyester/ether AUA Figure 3. Effect of sulfuric acid on tensile strength of Polyether AUA and PBDMA Tensile Strength vs Time, in H 2 S 4 CN981, 5% CN982, 1% CN981, 1% CN982, 5% Tensile Strength vs Time, in H 2 S 4 Figure 4. Effect of sulfuric acid on elongation of Polyester AUA Pro1154, 5% Pro1154, 1% CN31, 1% CN31, 5% CN986, 5% Tensile Elongation vs Time, in H 2 S 4 CN964, 1% CN966, 1% CN966, 5% CN964, 5% CN986, 1% 6

6 Further examination reveals that Group III oligomers with pure ether backbones, i.e., CN986 and PR1154, suffered the most in tensile reduction. CN964 and CN966 (Group I), the oligomers with pure ester backbones, had the least reduction whereas the mixed ester/ether backbone oligomers (Group II) lay somewhere between the I and III groups. This conclusion would be more valid if one compares CN964, CN982, and CN986 which have the same molecular weight but different polyol backbones. It is highly possible that in the presence of this powerful oxidizing acid (5% H 2 S 4 ) the labile hydrogens along the ether and ester backbone have been oxidized. The oxidation and consequent reactions may have resulted in the partial breakage of the polymer network structure, which was reflected by the reduction in both tensile strength and elongation of the coatings. Figure 5. Effect of sulfuric acid on elongation of polyester/ether AUA Tensile Elongation vs Time, in H 2 S 4 CN982, 5% CN981, 1% CN981, 5% CN982, 1% Figure 6. Effect of sulfuric acid on elongation of polyether AUA and PBDMA Tensile Elongation vs Time, in H 2 S 4 CN986, 5% CN986, 1% CN31, 1% Pro1154, 1% CN31, 5% Pro1154, 5% The α hydrogen in an ether is more prone to oxidation than the hydrogen in an ester. Thus, the oligomers with the most ether linkages suffered more from oxidation. This was confirmed by the fact that CN986 and PR1154 (Group III) started yellowing after only one hour immersion in 5% sulfuric acid. The color development began after eight hours for CN981 and CN982 (Group II). Group III & II coatings broke after two and eight hours in a covered spot test, respectively. The test was stopped when a film was broken. The yellowing of CN964 and CN966 (Group I) was only observed in the later stage of the covered spot test ( Figure 7). The data also suggested that within the same oligomer group, the higher the molecular weight of the oligomer, the more the reduction in tensile strength and elongation. This can be easily explained by the difference in crosslink density and the number of oxidizable hydrogens. n a volume basis, a low molecular weight oligomer will have a higher acrylate concentration than its high molecular weight counterpart. As a result, a higher crosslink density was achieved for CN964 and CN986 in Group I and III, respectively. At the same time, CN964 and CN986 also possess less oxidizable hydrogens than CN966 and PR1154, which have substantially longer ester and ether polyol backbones. Both factors also contributed to the more pronounced color development for CN966. 7

7 Figure 7. Effect of 5% sulfuric acid on the yellowing of AUA and PBDMA Excellent gloss and color retention were observed in 1% sulfuric acid as shown in Figures 9 & 1. Yellow Index Yellow Index vs Time, in 5% H 2 S 4 8 hr 1 hr 8 hr 1 hr hr 12hr For CN31, excellent retention of tensile, elongation, gloss, and color were observed in both 1% and 5% sulfuric acid. The hydrophobic nature of the polybutadiene backbone is the major reason for the superior acid and oxidation resistance of this remarkable oligomer. The polybutadiene building block also makes this oligomer highly flexible (7% elongation with Tg of -7 C). -1 CN964 CN966 CN981 CN982 CN986 Pro1154 CN31 Figure 1. Effect of 1% sulfuric acid on yellowing of AUA and PBDMA It is interesting to note that all the coatings had a very high initial gloss level. More importantly, for the oligomers (coatings) which did survive the test, the high gloss of the coatings was not affected by this oxidation process (Figure 8). Figure 8. Effect of 5% sulfuric acid on Gloss of AUA and PBDMA Yellow Index Yellow Index vs Time, in 1% H 2 S 4 hr 12hr Gloss Gloss Retention, in 5% H 2 S 4 8 hr 8 hr 1 hr 1 hr CN964 CN966 CN981 CN982 CN986 Pro1154 CN31 hr 12 hr -2 CN964 CN966 CN981 CN982 CN986 Pro1154 CN31 Figure 11. Effect of NaH on tensile strength of polyester AUA 3 Tensile Strength vs Time, in NaH Figure 9. Effect of 1% sulfuric acid on Gloss of AUA and PBDMA Gloss Gloss Retention, in 5% H 2 S 4 8 hr 8 hr 1 hr 1 hr CN964 CN966 CN981 CN982 CN986 Pro1154 CN31 hr 12 hr CN964, 1% CN966, 5% CN964, 5% CN966, 1% 8

8 Figure 12. Effect of NaH on tensile strength of polyester/ether Figure 14. Effect of NaH on elongation of polyester AUA Tensile Strength vs Time, in NaH CN981, 1% CN981, 5% CN982, 1% CN982, 5% Figure 13. Effect of NaH on tensile strength of polyether AUA and PBDMA Tensile Elongation vs Time, in NaH CN966, 1% CN964, 5% CN964, 1% CN966, 5% Figure 15. Effect of NaH on elongation of polyester/ether AUA Tensile Elongation vs Time, in NaH Tensile Strength vs Time, in NaH 5 CN982, 5% CN982, 1% CN986, 5% CN986, 1% CN31, 1% Pro1154, 1% Pro1154, 5% CN31, 5% Base Resistance All oligomers were quite resistant to base. In 1% and 5% sodium hydroxide solutions, all the coatings retained their original tensile, color, and gloss. Again, CN31 kept its tensile properties fairly constant while other oligomers exhibited observable fluctuations during the course of the 5- day test. Figures show the impact on tensile strength and elongation with time. Figures 17 & 18 show the color and gloss change after exposure to 5% sodium hydroxide solution CN981, 1% CN981, 5% Figure Figure 16. Effect of NaH on elongation of polyether AUA and PBDMA Tensile Elongation vs Time, in NaH CN986, 5% CN31, 5% CN986, 1% Pro1154, 1% CN31, 1% Pro1154, 5% 9

9 Figure 17. Effect of 5% NaH on gloss of AUA and PBDMA Figure 19. Effect of solvent on tensile strength of polyester AUA Gloss vs Time, in 5% NaH Tensile Strength vs Time, in Solvent Gloss CN964 CN966 CN981 CN982 CN986 Pro1154 CN31 hr 12 hr Tensile Strength psi CN964, Ethanol CN964, Toluene CN966, Toluene CN966, Ethanol Figure 18. Effect of 5% NaH on yellowing of AUA and PBDMA Yellow Index 9 4 Yellow Index vs Time, in 5% NaH hr 12hr It is interesting to note that the tensile strength of CN966, with a higher polyester molecular weight, was barely affected by the solvents, while CN964 experienced a dramatic reduction. However, both CN964 and CN966 retained a constant elongation. The reason for this unusual behavior remains unclear. Again, the oligomer with the best property retention was CN CN964 CN966 CN981 CN982 CN986 Pro1154 CN31 Figure 2. Effect of solvent on tensile strength of polyester/ether AUA Solvent Resistance As shown in Figures 19-24, all oligomers behaved similarly in ethanol and toluene. The initial conclusion is that the ether backbone provides better solvent resistance. This is evident when the tensiles and elongations of oligomers with the same molecular weight (CN964, CN982, and CN986) are compared. Group I and II oligomers were swelled and plasticized by the small solvent molecules. This contributed to a significant reduction in tensile strength. However, the tensile elongation of the coatings remained almost unchanged even after 5 days of immersion Tensile Strength vs Time, in Solvent CN982, Ethanol CN981, Ethanol CN982, Toluene CN981, Toluene 1

10 Figure 21. Effect of solvent on tensile strength of polyether AUA and PBDMA Figure 22. Effect of solvent on elongation of polyester AUA Tensile Strength vs Time, in Solvent Pro1154, Ethanol CN31, Ethanol Pro1154, Toluene CN986, Toluene CN986, Ethanol CN31, Toluene Tensile Elongation vs Time, in Solvent CN966, Ethanol CN964, Toluene CN964, Ethanol CN966, Toluene Figure 23. Effect of solvent on elongation of polyester/ether AUA Figure 24. Effect of solvent on elongation of polyether AUA and PBDMA Tensile Elongation vs Time, in Solvent Pro1154, Toluene CN986, Ethanol CN986, Toluene CN31, Toluene Pro1154, Ethanol CN31, Ethanol The covered spot test was not performed for ethanol and toluene. High rates of evaporation of the solvents and poor wetting made the test impractical. Skydrol and Water Resistance Figures 25-3 illustrate the response of the coatings based on seven oligomers to water and Skydrol. Water resistance of all the oligomers was generally good, which is probably due to the high crosslink density of the UV cured coatings. However, variation in tensile properties did occur for certain oligomers such as CN986 and PR1154. It is proposed that the hydrophilicity of the pure polyether backbone renders the coatings interactive with water. After long exposure, water might function as a plasticizer. verall, no detectable damage was done to the coatings. 5 Tensile Elongation vs Time, in Solvent Figure 25. Effect of water and Skydrol on tensile strength of polyester AUA CN982, Ethanol CN981, Ethanol CN982, Toluene CN981, Toluene Tensile Strength vs Time CN966, Water CN964, Water CN964, Skydrol CN966, Skydrol 11

11 Figure 26. Effect of water and Skydrol on tensile strength of polyester/ether AUA CN982, Water Tensile Strength vs Time CN981, Skydrol CN981, Water CN982, Skydrol Figure 27. Effect of water and Skydrol on tensile strength of polyether AUA and PBDMA Pro1154, Water CN986, Skydrol CN31,Skydrol Tensile Strength vs Time CN31, Water CN986, Water Pro1154, Skydrol Figure 28. Effect of water and Skydrol on elongation of polyester AUA Tensile Elongation vs Time CN966, Skydrol CN966, Water CN964, Water CN964, Skydrol An aviation hydraulic fluid, Skydrol seems to have a very high penetrating power. All oligomers suffered tensile and elongation reduction after being immersed in this fluid. Most of the films were obviously weak and showed significant swelling after being taken out of the Skydrol. Among the seven oligomers, CN981 and CN982 were the most resistant to Skydrol s attack. Figure 29. Effect of water and Skydrol on elongation of polyester/ether AUA Tensile Elongation vs Time Figure 3. Effect of water and Skydrol on elongation of polyether AUA and PBDMA CN981, Water CN981, Skydrol Pro1154, Water CN31, Skydrol CN982, Skydrol Tensile Elongation vs Time Pro1154, Skydrol CN986, Skydrol CN986,Water CN31, Water CN982, Water 12

12 Figure 31. Effect of Skydrol on Gloss of AUA Gloss 6 Gloss vs Time, in Skydrol hr 6 12 hr 4 2 CN964 CN981 CN982 In Skydrol, CN964, CN981, and CN982 were the only oligomers that survived the 5-day covered spot test. All others simply delaminated and peeled off the aluminum panels. For the coatings that did survive, gloss and color were retained (Figure 31 and 32). After exposure to water, only a minor change was observed in gloss and color. This is shown in Figures 33 & 34. Figure 34. Effect of water on yellowing of AUA and PBDMA Figure 32. Effect of Skydrol on yellowing of AUA 1 hr Yellow Index vs Time, in Water 1 Yellow Index vs Time, in Skydrol Yellow Index hr Yellow Index hr 12hr -2 CN964 CN966 CN981 CN982 CN986 Pro1154 CN31-1 CN964 CN981 CN982 Figure 33. Effect of water on gloss of AUA and PBDMA Gloss vs Time, in Water 1 Gloss hr 12 hr 2 CN964 CN966 CN981 CN982 CN986 Pro1154 CN31 13

13 CNCLUSINS verall, UV cured coatings based on the oligomers studied show great promise for chemical resistance. The high crosslink density of these coatings protected them from chemical degradation. Resistance of a coating to certain chemicals is largely controlled by the structural nature of the oligomer backbone. Polybutadiene has been identified as the best oligomer in terms of resistance to strong acids, bases, and organic solvents. 5% sulfuric acid caused damage to most oligomers. The effect of 1% sulfuric acid, as well as 1% and 5% sodium hydroxide on all the coatings was minimal. Polyether-based oligomers, as well as polybutadiene dimethacrylate were superior in solvent resistance. All oligomers exhibited good water resistance. Skydrol seems to penetrate almost any coating. Work is underway to further investigate the actual failure mechanism of UV cured coatings following exposure to strong chemicals. ACKNWLEDGMENTS The author wishes to acknowledge all colleagues in Sartomer Company for their support during the course of this work. Special thanks go to Mr. Henry Miller for helpful and creative discussions. REFERENCES 1. Richard J. Holman, Polymers Paint Color Journal, pp 21-27, ctober, James R. Petisce, RadTech North America Proceedings, pp , KEYWRDS UV curing, Chemical resistance, Acid resistance, Base resistance, Skydrol resistance, Water resistance, ligomeric acrylates, Aliphatic urethane acrylates, Polybutadiene dimethacrylate, Gloss, Yellow index, Tensile strength, Tensile elongation The information in this bulletin is believed to be accurate but all recommendations are made without warranty, since the conditions of use are beyond SARTMER Company s control. The listed properties are illustrative only, and not product specifications. SARTMER Company disclaims any liability in connection with the use of the information, and does not warrant against infringement by reason of the use of its products in combination with other material or in any process. 14