Coating systems based on tricarbamate crosslinkers derived from triaminonane

Progress in Organic Coatings 34 (1998) 27–38

Coating systems based on tricarbamate crosslinkers derived from triaminonane H.P. Higginbottom a, G.R. Bowers a, L.W. Hill a, J.F. Courtier b ,* a

Specialty Resins Division, Monsanto Company, 730 Worchester Street, Springfield, MA 01151, USA b Monsanto Europe SA., Avenue de Tervurn, 270–272 B.1150 Brussels, Belgium Received 15 July 1997; revised version received 20 January 1998; accepted 10 April 1998

Abstract Trifunctional carbamates of the type that could also be called alcohol-blocked isocyanates were evaluated as crosslinkers for hydroxylfunctional polyester or acrylic coreactants. Commercial and developmental tin catalysts of varying structure were used to catalyze cure. Some of the catalysts were observed to be sufficiently active to permit extensive crosslinking of model coating systems at temperatures currently used for powder coatings or higher temperature cure thermoset liquid coatings. Either liquid or solid carbamate crosslinkers were obtained by purposeful variation in the structures of the alcohol portion of the carbamate. Carbamates were prepared directly by a patented non-phosgene process from 4-aminomethyl-1,8-diaminooctane, which is informally called triaminononane (TAN). In a related patented process, TAN was also converted to the corresponding aliphatic triisocyanate, called TAN triisocyanate (TTI). The availability of TTI facilitated synthesis of blocked isocyanates using common blocking agents such as methyl ethyl ketoxime, caprolactam and dimethylpyrazole. The performance of alcohols as blocking agents could therefore be compared directly to performance of blocking agents in current use. Furthermore, properties obtained from blocked and unblocked TTI could also be compared. Cure response was determined by cure profile studies involving hardness, solvent resistance and impact resistance determinations on panels cured in a gradient oven. Dynamic mechanical analysis was used to determine glass transition temperature and crosslink density of selected cured films.  1998 Elsevier Science S.A. All rights reserved Keywords: Blocked isocyanates; Crosslinkers; Tricarbamates

1. Introduction The carbamate products which have been studied can be formed by a patented process involving the reaction of primary amine groups with carbon dioxide in the presence of nitrogenous bases to form carbamate salts. This process is referred to as activated carbon dioxide chemistry (ACDC) [1,2]. Carbamate salts can be further reacted with various alkyl halides to form blocked isocyanates (ACDC-1) [1], or in the presence of dehydrating agents, the salts can be converted to the corresponding isocyanates (ACDC-2) [2] as shown in Fig. 1. The tri-primary amine starting material is obtained as a product in Monsanto’s Fiber Intermediates

* Corresponding author.

0300-9440/98/$19.00  1998 Elsevier Science S.A. All rights reserved PII S0300-9440 (98 )0 0037-X

Division from a patented acrylonitrile electrodimerization process. The purified triamine consists almost entirely of 4aminomethyl-1,8-diaminooctane, but is referred to as triaminononane (TAN). The trifunctional, aliphatic isocyanate obtained from TAN is called TAN-triisocyanate (TTI). TTI has also been prepared from TAN by the usual phosgene process [3]. The carbamate products can also be formed from TTI by reaction with the appropriate alcohol. In order to form certain carbamates it was often easier to react an alcohol with TTI than to obtain the corresponding alkyl halide for use in the ACD-1 process. TTI is a unique and potentially a very useful triisocyanate which has been studied and reported upon previously [4]. The present study was initiated to examine the potential utility of the carbamates of TTI as blocked isocyanate crosslinking agents.

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H.P. Higginbottom et al. / Progress in Organic Coatings 34 (1998) 27–38

removed under vacuum. Catalysts were avoided or used at very low levels. Conversion and absence of free isocyanate was confirmed by Fourier transform infrared spectroscopy (FTIR). 2.2. Test formulations

Fig. 1. Formation of TTI or TAN-tricarbamate by monsanto ACDC nonphosgene process.

Two standard test formulations were prepared using various blocked TTI samples as crosslinkers. Chloroform was selected as a fast evaporating non-interacting solvent to minimize the effect of the solvent on cure profile results. Also, chloroform is an effective solvent for dissolving most of the TAN-tricarbamates and the solid polyester and acrylic resins used in this study. Solids concentrations of just under 40% in chloroform were used in most cases. Formulation A below is essentially a powder composition in solution form. Evaluation of powders by liquid application has been described elsewhere [5].

2. Experimental 2.3. Formulation A 2.1. Materials McWhorter Technologies 30-3016 solid hydroxyl functional polyester (e.w. = 1400) and S.C. Johnson Joncryl 587 solid hydroxyl functional acrylic (e.w. = 598) were used as coreactant resins. Catalyst type and source are identified in Tables 2 and 3. The Fascat catalysts are from Elf Atochem while the Stann catalysts are from Sankyo Organic Chemical Co. and the Formate TK-1 is available from Takeda USA, Inc. (Elf Atochem version is Fascat 4215). Monsanto Modaflow 2100 was used as a flow control agent. Huls B1530 and BF1540 were used as blocked IPDI powder crosslinkers. Tinuvin 144 (HALS) and Tinuvin 900 (UVA) were obtained from Ciba. Carbamates 1, 2 and 5 in Table 1 were prepared by activated carbon dioxide chemistry [1] (ACDC-1). All of the other carbamates and blocked TTI materials listed in Table 1 were prepared by adding a stoichiometric or excess level of blocking group to TTI in a rotary evaporator and heating to effect conversion. Excess blocking group, if present, was

The control formulation A. was composed of McWhorter 3016 polyester/TANtBzC/[Fascat 4102]/Modaflow 2100 at 88:12:(0.2):0.5 solids ratio. Changes were made to this formulation by substituting other blocked crosslinkers for TANtBzC but keeping the stoichiometric ratio of blocked group/hydroxyl near 1:1 unless noted otherwise. Catalyst level used in this paper is always expressed as parts of Sn per 100 parts resin. The catalytic effects of different catalysts were compared at 0.2 phr Sn unless noted otherwise. The wt.% Sn for each catalyst used is noted in Tables 2 and 3. Formulation A was cured at predetermined profile temperatures for a constant 30 min. 2.4. Formulation B The control formulation B was composed of Joncryl 587 acrylic/TANtBzC /[TK-1]/Modaflow 2100 at a 75:25:(0.2):0.5 solids ratio. The same comments about changing crosslinkers, stoichiometric ratio and catalysts

Table 1 Blocked TTI properties #

TTI blocking group

Product appearance

Property

Equivalent weight

COT (°C)a

1 2 3 4 5 6 7 8 9 10

Ethanol (EtOH) n-Butanol (n-BuOH) 90:10 Ethanol/butanola (EtOH/BuOH) 2-Butoxyethanol Benzyl alcohol (BzOH) Tertiary butanol (t-BuOH) 3,5-Dimethylpyrazole (DMP) Phenol Methylethyl ketoxime (MEKO) Caprolactam (CapL)

White crystals White crystals Clear, colorless liquid Viscous, colorless liquid White crystal Colorless, viscous liquid Faint yellow, viscous liquid Colorless semi-solid Colorless, viscous liquid Colorless, viscous liquid

m.p. = 44–46.5°C m.p. = 60–61°C 65000 cps – Tm = 108.7°C (by DSC) – – – 3500 cps @ 82% nv in Dowanol PM –

130 158 133 202 192 158 180 178 171 197

140 140 140 140 135 177 95 110 120 156

a

TTI reacted with 90:10 molar mixture of ethanol/butanol. Product = TANt(Et/Bu)C.

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H.P. Higginbottom et al. / Progress in Organic Coatings 34 (1998) 27–38 Table 2 Catalyst screening, polyester/carbamate system a cure profiles, variable bake temperature, 20 min Formulation A, McWhorter 3016/TANtBzC/C(catalyst), 88/12(0.2 Sn)a Film property Baked Film

a

Bake temperature (°C) 149

156

163

Control: catalyst = Fascat 4102 butylintris(2-ethylhexonate), %Sn = 19.6 Hardness – H H MEK double rubsb 15 200(M) 200+ Impact resistance (F)c – 75 160 Catalyst, Formate TK-1 tetra-n-butyl-1,3-diacetoxy-distannoxane, %Sn = 39.5 Hardness – – – MEK double rubsb – – 8 Impact resistance (F)c – – – Catalyst, Stann OMF (Sankyo) di-n-octyltin maleate polymer, %Sn = 25.9 Hardness – – B MEK double rubsb – – 5 Impact resistance (F)c – – – Catalyst, Stann BM(N) dibutyltin maleate polymer, %Sn = 34.2 Hardness – – B MEK double rubsb – – 5 Impact resistance (F)c – – 10 Catalyst, Fascat 4200 dibutylin dilaurate, %Sn = 18.0 Hardness – – – MEK double rubsb – – 5 Impact resistance (F)c – – – Catalyst Faxcat PE, 1032 monobutyltin experimental catalyst (Elf Atochem), %Sn = 21.9 Hardness H 2H 2H MEK double rubsb 120 200+ 200+ Impact resistance (F)c 40 160 160 Catalyst, Fascat 4200 dibutyltin diacetate, %Sn = 33.8 Hardness – – – MEK double rubsb – – 10 Impact resistance (F)c – – – Catalyst, Fascat 4100 hydrated monobutyltin oxide, %Sn = 44.8 Hardness – H 2H MEK double rubsb 50 150 200+ – 80 160 Impact resistance (F)c

177

191

204

2H 200+ 160

2H 200+ 160

2H 200 160

B 200 (50%) ,5

HB 200+ 160

H 200 160

H 25 20

2H 200+ 160

2H 200 160

B 20 20

H 200+ 160

H 200 160

– 15 –

H 200+ 160

2H 200+ 160

2H 200+ 160

2H 200+ 160

2H 200 160

,B 40 ,10

2H 65 160

2H 150 160

2H 200+ 160

2H 200+ 160

2H 200 160

a

Catalyst level at (0.2) parts tin based on 100 parts resin. Formulation A is at 36% solids and formulation B at 42% solids in chloroform. Formulations A and B were draw coated on 22.5 × 4.0 inch cold rolled steel panels and cured in a gradient oven for 20 or 30 min, respectively, to give 1.0–1.2 mil dft cured films. b MEK double rubs stopped at 200. 200+ Indicates no marring or film loss at stopping point. (M) Indicates some form of gloss loss, scratching or maring at 200 rub stopping point. c Gardner forward (F) impact resistance in inch-lbs.

made for formulation A also apply to B. Chloroform continued to be used as a non-reactive, fast solvent for catalyst comparison purposes but the acrylic formulation B can be used with many standard coating solvents including ketones, esters, glycol ethers etc. Formulation B was cured at predetermined profile temperatures for a constant 20 min. 2.5. Film formation and cure Cure profile comparisons were made in most cases using a BYK Chemie Gradient oven. ACT cold rolled steel test panels (4 × 22.5 × 0.032 inch, B1000, P60, DIW, polished) were used as substrates. A 3-inch wide film was draw coated the length of the test panel using a doctor blade and the BYK Chemie Applicator Tool. Blade size was selected to give a cured film of 1.0–1.2 mil thickness on the test panel. The

wet films after draw coating were air dried at room temperature for 30 min prior to cure in the Gradient oven. The Gradient oven was programmed to give four constant temperature cure zones 4 inches in length for each cure cycle. Each cure zone was separated by a 0.75-inch buffer zone at 40°C. When more than one set of four cure zones had to be used to define a cure profile, the end cure zones of each separate panel had the same temperature. In some cases, cure profiles were also measured on films cured in a circulating air electric oven by baking separate panels at a series of temperatures. 2.6. Powder coating preparation and cure Solid powder versions of formulation A were prepared by melt blending components in a twin screw lab extruder at 110°C. Fascat 4102 liquid catalyst was introduced on silica

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H.P. Higginbottom et al. / Progress in Organic Coatings 34 (1998) 27–38

Table 3 Catalyst screening, acrylic/carbamate system B cure profiles, variable bake temperature, 30 min Formulation B: Joncryl 587 Acrylic/TANtBzC/(catalyst), 75/25/(0.2 Sn)a Film property Baked film

a

Bake temperature (°C) 150

195

140

Control, catalyst = Formate TK-1 tetra-n-butyl-1,3-diacetoxydistannoxane, %Sn = 39.5 Hardness – H H MEK double rubsb 60 200(M) 200 Impact resistance (F)c – 30 50 Catalyst, Fascat 4102 butyltintris (2-ethylhexaoate), %Sn = 19.6 Hardness – – – MEK double rubsb – – 8 Impact resistance (F)c – – – Catalyst, Fascat 8213 Di-n-octyltin maleate polymer, %Sn = 25.9 Hardness – H H MEK double rubsb – 200(M) 200+ Impact resistance (F)c – 10 20 Catalyst, Fascat 4213 dibutyltin maleate polymer, %Sn = 34.2 Hardness – H H MEK double rubsb – 200(M) 200(M) Impact resistance (F)c – 10 40 Catalyst, Fascat 4208 dibutyltin bis(2-ethylhexanoate), %Sn = 22.9 Hardness – H 2H MEK double rubsb – 200(M) 200+ Impact resistance (F)c – 10 20 Catalyst, Fascat PE 1023 dibutyltin experimental resin (Elf Atochem), %Sn = 25.6 Hardness – 2H 2H MEK double rubsb – 200(M) 200+ Impact resistance (F)c – 10 50 Catalyst, Fascat 4200 dibutyltin diacetate, %Sn = 33.8 Hardness – 2H 2H MEK double rubsb 20 200(M) 200+ Impact resistance (F)c – 10 40 Catalyst, Fascat 4100 hydrated monobutyltin oxide, %Sn = 44.8 Hardness – – – MEK double rubsb – 8 10 – – – Impact resistance (F)c

149

165

177

2H 200(M) 70

2H 200+ 60

2H 200 70

– 10 –

– 20 –

B 200 ,5

H 200+ 50

2H 200+ 50

2H 200 50

2H 200+ 40

2H 200+ 50

2H 200 50

2H 200+ 60

2H 200+ 30

2H 200 60

2H 200+ 60

2H 200+ 50

2H 200 50

2H 200+ 60

2H 200+ 50

2H 200 50

– 10 –

– 15 –

– – –

a

Catalyst level at (0.2) parts tin based on 100 parts resin. Formulation A is at 36% solids and formulation B at 42% solids in chloroform. Formulations A and B were draw coated on 22.5 × 4.0 inch cold rolled steel panels and cured in a gradient oven for 20 or 30 min, respectively, to give 1.0–1.2 mil dft cured films. b MEK double rubs stopped at 200. 200 + Indicates no marring or film loss at stopping point. (M) Indicates some form of gloss loss, scratching or maring at 200 rub stopping point. c Gardner forward (F) impact resistance in inch-lbs.

as a 66% free flowing powder. The cooled extruded formulation was coarse ground in a Waring blender and pulverized in a centrifugal mill. The pulverized material was classified in an Alpine Jet air sieve so as to pass through a 200 mesh screen. This powder was electrostatically applied to one side of a 10 × 15 cm, 24 gage polished, treated, cold rolled steel panel (Bondrite37). 2.7. Film test procedures Solvent resistance was measured by the methyl ethyl ketone (MEK) double-rub test. MEK was placed in a hollow-barreled felt tip marker and one back and forth stroke made across the film per second. A reported value less than 200 is the number of double rubs required to remove the coating down to the substrate within the stroke path. The test was stopped at 200 double rubs even if the coating was still

present. If there was not film removal or marring of the film at the stopping point, it was reported as 200 + . If there was some film marring, scratching or gloss loss at the stopping point, it was reported as 200(M). Impact resistance was measured with a Gardner Impact Tester as described in ASTM D 2794. Gloss was measured as described in ASTM D 523. Distinctness of image (DOI) was measured using a Hunter Dorigon Meter D47-R6. Pencil hardness was measured according to ASTM D 3363. 2.8. Dynamic mechanical analysis Dynamic mechanical analysis (DMA) was used to augment cure profile determinations for assessing extent of cure. DMA was carried out on free films at 11 Hz oscillating frequency with a temperature scan from 10 to 190°C at 2°/ min on an Autovibron instrument (Imass). This DMA

H.P. Higginbottom et al. / Progress in Organic Coatings 34 (1998) 27–38

method and interpretation of the resulting DMA plots have been described in detail [6]. Dynamic viscosity results were obtained on a Rheometrics mechanical spectrometer (RMS 800EH) in an oscillatory shear mode at 1 rad/s and 15% maximum strain amplitude. Parallel plates with a diameter of 2.5 cm were utilized, with a gap of about 1.5 mm. All powder samples were compression molded at 90°C, in situ in the instrument. Initial run temperature was 90°C and the temperature ramp rate was programmed at 5°C/min.

3. Results and discussion 3.1. Formation and properties of TAN-tricarbamates and blocked TTI A variety of TAN-tricarbamate materials were prepared either using the ACDC-1 synthesis method [1] or by reacting TTI directly with selected aliphatic alcohols. TTI was also reacted with 3,5-dimethylpyrazole, methyl ethyl ketoxime, caprolactam and phenol to obtain TTI blocked by other known blocking agents for comparison purposes. The properties of some of these blocked TTI materials are summarized in Table 1. The TAN carbamates are either viscous liquids or crystalline solids depending on the alcohol portion. In the case of crystalline carbamates, by randomly mixing the alcohol portion of the carbamate with two different alcohols, crystallization can be retarded and a liquid product obtained. Liquid (non-crystallizing) carbamates are preferred for formulation into stable liquid coating systems. For example, a random mixed TAN-tri(90:10 mole ethyl/nbutyl)carbamate shows a strong resistance to crystallization while the pure ethyl and butyl TAN tris-carbamates are crystalline solids (Table 1; 1 and 2 vs. 3). 3.2. Catalyst screening and comparisons of blocked TTI Formulation A described in Section 2 is essentially a powder reference system in solution form. It consists of a solid hydroxyl functional polyester (McWhorter 30-3016) combined with a TAN-tricarbamate at a 1:1 equivalence ratio in solution. Formulation B is a hydroxyl functional acrylic (Joncryl 587) combined with a TAN-tricarbamate at a 1:1 equivalence ratio in solution. Different tin catalysts were screened with formulations A and E at a constant 0.2 parts Sn per 100 parts resin unless indicated otherwise. Gradient oven cures were run on each formulation and cured film properties at each predetermined temperature zone allowed the determination of reproducible cure profiles which could be correlated with catalyst activity. This gradient method of screening catalyst activity and cure behavior was preferred because of the excellent correlation that it has with actual baked coating performance. The gradient oven screening of catalyst activity employed chloroform as the primary solvent for both formulations A and B. Chloroform is not a typical coating solvent but was used

31

to increase the testing reproducibility because it rapidly evaporates and eliminates variable solvent effects and interactions during blocked isocyanate cure. Cure properties measured in the gradient temperature zones were pencil hardness, MEK double rubs and forward impact resistance. As indicated in the discussion, correlations were also made with films cured in a conventional circulating air oven. A large number of catalysts, both commercial and experimental, were screened using formulations A and B. Most of these alkyl tin catalysts would fit the general formula RnSnX4 − n where R is an alkyl group, X is an anion and n = 1–4. Table 2 shows the cure profile data of several different catalysts with the hydroxyl functional polyester formulation (A) and Table 3 shows profiles obtained using different catalysts with the hydroxyl functional acrylic formulation (B). In general, our results agree with the conclusions of Seshadri et al. [7] that blocked isocyanate–acrylic polyol systems cure fastest with the dialkyltin diacid catalysts (n = 2) while the blocked isocyanate–polyester systems cure fastest with the monoalkyltin triacid catalysts (n = 1). Stannoxane catalysts like the tetra-n-butyl-1,3-diacetoxy distannoxane (Formate TK-1) performed very similar to the dialkyltin diacid catalysts with acrylic polyols and sometimes were marginally better. The control system for the acrylic polyol formulation B uses Formate TK-1 at 0.2 Sn per 100 parts resin while the polyester formulation A control uses butyltin tris(2-ethyl-hexanoate) at 0.2 Sn per 100 parts resin as a control. Note from the data in Tables 2 and 3 that the use of the control catalyst in the opposite formulation gives poorer cure results. None of the aliphatic alcohol based carbamates studied showed any evidence of uncatalyzed cure below 200°C. This finding indicates a catalyst is absolutely necessary. With catalysis the carbamates can clearly get in the cure range of caprolactam blocked materials and with the catalyzed acrylic polyol system can give good solvent resistance development near the 150°C cure range. Blank [9] and Jones [8] have reported similar cure response with other alcohol blocked carbamates derived from different isocyanate backbones. An exception was found to the above general effects of catalyst type on cure of hydroxy: functional polyesters. A Rucote 121 polyester was catalyzed more effectively with TK-1 catalyst and less effectively with the Fascat 4102 when compared with the control polyester McWhorter 3016. It appears that the specific composition of the hydroxyl end units in the polyester may be influencing catalysis. In model studies, an ethylene glycol terminated polyester shows a different cure response with Fascat 4102 than does a hexamethylene dial terminated polyester. This effect needs to be studied further. Recent papers [7–9] summarize some of the extensive background information and mechanism theories that exist about blocked isocyanates. Seshadri et al. [7] propose that the catalyst first complexes or coordinates with the polyol. The complex is considered to be the active form of the catalyst. It functions by insertion between the blocking

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H.P. Higginbottom et al. / Progress in Organic Coatings 34 (1998) 27–38

Fig. 2. Effect of catalyst level on cure profile. Joncryl 587/TAN-tBzC 75:25 with variable TK-1. Baked for 30 min at indicated temperature. Circulating air oven.

agent and isocyanate. The proposed mechanism does not characterize the next step which can be either a concerted or stepwise reaction to form the urethane linkage. This proposed mechanism appears reasonable. It supports the concept that the structure of the polyol, catalyst and blocked isocyanate can all influence the course of the reaction which the results of the present study support. More catalyst is not better. It was also observed that with most of the tin catalysts studied, increasing the catalyst concentration beyond a certain level had negative effects on film properties and did not accelerate cure further. Fig. 2 illustrates the cure profiles obtained with different levels of TK-1 catalyst in the acrylic polyol formulation B. Maximum cure benefits were observed as the catalyst concentration approached 0.2 phr Sn. Above 0.2 phr Sr increasing catalyst actually appears to retard property development at the cure onset and definitely increases the possibility of deleterious overbake effects including yellowing and ester bond breaking. The effect of higher catalyst level on undesirable overbake effects were especially noticeable with the polyester formulation A at cure temperatures approaching 200°C. The cure profiles shown in Fig. 2 are for test samples baked in a circulating air oven for 30 min at different temperatures and the measured ‘cure onset temperatures’ (COTs) are about 10° higher than in the case of the gradient oven bake used to compare catalyst types summarized in Tables 2 and 3. The film heat up rates and heat transfer efficiency are higher in the gradient oven than in the conventional oven. The differences between the forced air oven and gradient oven in cure profiles emphasize the fact that the actual cure environment and conditions will be a key factor in the rate and extent of cure. However, the trends predicted by the gradient oven are consistent with other cure conditions.

cure temperature point where certain cure criteria are met. For formulation A the cure criteria is ≥160 MEK double rubs and ≥160 inch-lbs forward impact resistance while the cure criteria for formulation B was ≥160 MEK double rubs and .20 inch-lbs forward impact resistance. The COT values of some alcohol blocked TTI carbamates are compared with TTI blocked with some known blocking agents which have been studied in the past with other isocyanates These measurements were made using the acrylic formulation B with and without TK-1 catalyst at a 0.2 phr Sn content. The reference blocking groups are 3,5-dimethyl pyrazole (DMP), phenol, methyl ethyl ketoxime (MEKO) and caprolactam (CapL). The alcohol blocked TTI samples referenced in Fig. 3 are benzyl alcohol, a random blocked 90:10 blend of ethanol/n-butanol and t-butyl alcohol. The reference blocking groups all gave COTs below 200°C uncatalyzed but varied widely from 3,5-dimethylpyrazole at 135°C to caprolactam at 177°C. All of the reference blocking groups showed at least a 40°C decrease in COT value when catalyzed except for caprolactam blocked TTI which only dropped about 10°C catalyzed. The very moderate effect of catalyst on the COT of caprolactam blocked TTI was also observed with some of the commercial caprolactam blocked isophorone diisocyanate based crosslinkers. The order of reactivity of the reference blocking groups on TTI (catalyzed appears to be generally consistent with coating cure data reported elsewhere but not necessarily consistent with measured thermal deblocking temperature measurements [10–13]. 3.4. DMP . Phenol . MEKO . CapL Almost all of the conventional aliphatic alcohol type blocked TTI carbamates which we studied fell in reactivity between the MEKO and CapL when catalyzed with TK-1. None of them cured below 200°C without catalyst. This included a number of functionally modified alcohols. For example, ethanol, n-butanol, 2-butoxyethanol, 3-methoxy propanol and some mixtures of these all gave COT values of 140°C under the test conditions. Also, the mixed 2- and 3-

3.3. The effect of TTI blocking group on COT Using the cure profile data obtained with different formulations and catalysts, a COT is defined as a minimum

Fig. 3. COT of blocked TTI crosslinkers using acrylic formulation B with and without catalyst.

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H.P. Higginbottom et al. / Progress in Organic Coatings 34 (1998) 27–38

temperatures. Changes in dynamic properties with increasing extent of cure are well established [6]: Tg increases, tan dmax decreases, and storage modulus in the rubbery plateau, E′min, increases. Substantial changes in the expected directions are observed in all three dynamic properties for films A, B. and C. The inability of MEK rubs (and other paint tests) to distinguish between the early stages of cure at 149°C bake and advanced cure at 177°C is the basis for selection of the terminology ‘cure onset temperature’ (COT) in the cure profile studies. Often in papers on cure of thermoset coatings, observation of 200 + MEK double rubs is quoted as evidence for rather complete cure, but our results suggest that 200 + double rubs can be observed while crosslinking, still has a long way to go. Film D differs from the others in that dynamic properties as well as paint test results show little difference for the two cure temperatures. The reason is transparent. Film D contains unblocked isocyanate so we expect complete reaction even at the lower cure temperature. Comparison of dynamic properties in Table 5 for unblocked TTI (D(1) or D(2)])and benzyl alcohol blocked TTI at the higher cure temperature (A(2)) shows remarkable similarities. It appears that the structures of the networks formed from TTI and blocked TTI are very similar. This similarity is clearly shown in Fig. 4. The storage modulus plots (Fig. 4A) superimpose nearly perfectly in the transition region and in the rubbery plateau region. Only in the glassy region is there evidence that the blocking agent has resulted in any difference in network structure relative to that formed from unblocked TTI. The crosslink density, for example, can be calculated from E′min [6], and it is obvious that crosslink density is the same within response variation. Fig. 4B shows that tand plots are indistinguishable throughout the temperature scan. Repeat runs on the same film could hardly be expected to give closer overlap. Comparison of properties of films E(1) and E(2) with

hydroxypropyl carbamate formed by reacting TAN with propylene carbonate gave a COT of 140°C when catalyzed with TK-1. Benzyl alcohol blocked TTI gave a marginally lower COT of 135°C probably because of slightly greater acidity of the benzyl alcohol compared with the aliphatic alcohols. The t-butyl alcohol blocked TTI showed a significantly higher COT at 177°C, possibly because of steric inhibition of active catalyst insertion between the t-butyl blocking group and isocyanate. Interestingly, the difference in boiling point of the leaving alcohol blocking group did not alter the COT under the test conditions. Utilizing an experimental catalyst Fascat PE-1023 at 0.4 phr Sn the COT of the tris(Et/Bu) carbamate was dropped to 135°C which appeared to match the cure of the benzyl carbamate (Fig. 2, striped bar). However, actual measurements of crosslink density discussed below suggest that cure development is less than with the benzyl carbamate. The screening of a large variety of experimental catalysts at different levels did not disclose any significant effect at lowering the COT of the TAN carbamates further. 3.5. Baked film properties related to COT and crosslink density Selected Joncryl 587 acrylic formulations with different carbamate crosslinkers derived from TTI are summarized in Table 4. For comparison the acrylic is also formulated with a blocked IPDI crosslinker and with unblocked TTI. These formulations were oven baked under different conditions to give cured films. Cure profile results and dynamic mechanical properties of the cured films are given in Table 5. For most of the formulations, MEK rubs hardness and impact resistance are quite similar for 149 and 177°C bakes. Based on paint tests alone, it would appear that cure is nearly complete at 149°C. Dynamic properties, in contrast, indicate substantial differences in properties for the two bake Table 4

Joncryl 587 acrylic formulations with TAN carbamates versus TTI and blocked IPDI controls Componenta

Joncryl 587 (J587) TAN-tris-benzyl carbamate (TANtEtC) TAN-tris-ethyl carbamate (TANtEtC) TAN-tris-isocyanate (TTI) TAN-tris (90:10 Et/Bu)C (TANt(EtBu)C) Huls B 1530 (H1530) blocked IPDI adduct Modaflow 2100 (phr) Formate TK-1 catalyst (phr Sn) Fascat PE-1023 catalyst (phr Sn) Dibutyl tin dilaurate catalyst (phr Sn) Tinuvin 144 HALS Tinuvin 900 UVA X-linker/acrylic-OH equivalent ratio

Formulation (parts solids)

Equivalent weight

A

B

C

D

E

F

G

75 25

75 25

81

88

70

80 20

82

19 12 18 0.5 0.2

0.5 0.2

0.5 0.2

0.5

30 0.5

0.01

0.1

0.99

1.03

0.5 0.2

610 191.9 129.8 83.76 133 270

0.5 0.4

1.06

1.0 1.0 1.06

1.10

0.80

1.01

Formulations from Table 4. Values in parenthesis are cure conditions: (1) = 149°C, 30 min; (2) = 177°C, 30 min; (3) = 163°C, 30 min; (4) = 140°C, 30 min. Films draw coated on B37 CRS 24 gauge polished panels with cured film dft of 1.2–1.4 mils. b MEK rubs, double rubs stopped at 200 max. 200+ Indicates no wear or marring at 200 double rub stop point. a

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H.P. Higginbottom et al. / Progress in Organic Coatings 34 (1998) 27–38

Table 5 Properties, acrylic formulations with carbamates versus TTI and blocked IPDI controls Formulation (cure condition)a

A(1) A(2) B(1) B(2) C(1) C(2) D(1) D(2) E(1) E(2) F(1) F(2) G(4) G(1) G(3)

Descriptiona

Physical propertiesb

J587/TANtBzC 75:25 J587/TANtBzC 75:25 LJ587/TANtBzC 75:25 stabilized LJ587/TANtBzC 75:25 stabilized J587/TANtEtC 81:19 J587/TANtEtC 81:19 J587/TTI 88:12 J587/TTI 88:12 J587/H1530 70:30* J587/H1530 70:30* J587/TANtBzC 80:20 J587/TANtBzC 80:20 J587/TANt(EVBu)C 82:18 J587/TANt(EVBu)C 82:18 J587/TANt(EVBu)C 82:18

DMA properties

MEK rubs (double)

Pencil hardness

Gardner impact (inch-lbs)

Tg, °C

Tand (max)

E′min (dynes/cm2)

200+ 200+ 200+ 200+ 200+ 200+ 200+ 200+ 180 200(M) 200(M) 200(M) 100 200(M) 200+

3H 3H 3H 4H 4H 3H 3H 3H 2H 2H 3H 4H H 2H 2H

40 30 40 50 20 40 40 40 20 30 40 40 ,5 30 30

97.8 102.1 92.0 105.8 89.8 106.1 105.9 103.9 106.2 130.2 93.9 105.9 – 84.9 103.2

1.334 1.245 1.437 1.149 1.571 1.222 1 224 1.262 1.851 1.163 1.443 1.338 – 1.78 1.30

0.95 1.42 0.88 1.25 0.86 1.07 1.30 1.40 0.29 1.02 0.74 0.99 – 0.51 1.09

× × × × × × × × × × × ×

108 108 108 108 108 108 108 108 108 108 108 108

× 108 × 108

Formulations from Table 4. Values in parenthesis are cure conditions: (1) = 149°C, 30 min; (2) = 177°C, 30 min; (3) = 163°C, 30 min; (4) = 140°C, 30 min. Films draw coated on B37 CRS 24 gauge polished panels with cured film dft of 1.2–1.4 mils. b MEK rubs, double rubs stopped at 200 max. 200+ Indicates no wear or marring at 200 double rub stop point.

a

those for A(1) and A(2) indicate differences in cure response based on the blocking agent. The crosslinker in E is the caprolactam blocked isocyanurate oligomer of IPDI (Huts B1530). Differences in both MEK rubs and dynamic properties for 149 versus 177°C bake are much larger for caprolactam as blocking agent than with benzyl alcohol as blocking agent. This finding suggests that the benzyl alcohol blocked crosslinker has the advantage in cure response. The rigid nature of the cycloaliphatic ring in IPDI as well as the rigid nature of the isocyanurate ring from oligomerization are believed to contribute to the very high Tg observed for film E(2). The importance of stoichiometric balance for high crosslink density is evident in the E′min value for film A(2), which is near stoichiometric (see Table 4), versus that for film F(2) which has a 0.80 ratio of functional groups (blocked isocyanate/-OH). The imbalance results in a 30% reduction in crosslink density. Interestingly, the Tg increases as crosslink

density decreases for this case. This Tg increase is probably related to a higher content of the higher Tg component, J587. Compared with other crosslinkers, those based on TAN have rather low Tg due to numerous methylene groups. Films G(1), G(3) and G(4) represent a cure temperature study. Paint test results show a large difference between 140 and 149°C cure. This comparison could not be made using dynamic properties because the film baked at 140°C, G(4) was too brittle for DMA. If an acrylic has a rather high Tg, it will lacquer dry to a hard film, but it will be very brittle. The impact result for G(4) is consistent with this behavior. For the 149 and 163°C bakes, paint tests no longer distinguish well between films, but now dynamic properties do distinguish well. The effect of bake temperature is also shown in Fig. 5. Differences are large and in the expected directions. The crosslink density at 149°C bake is obviously much lower than at 163°C bake, but as the temperature scan approaches the high end, more crosslinking is believed to

Fig. 4. Acrylic formulation with TTI versus TANtBzC. Full cure 177°C.

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H.P. Higginbottom et al. / Progress in Organic Coatings 34 (1998) 27–38

occur as indicted by the rise in E′. Finally, at about 175°C the differences caused by the difference in cure temperature have been swamped out by additional cure during the scan, and the films have essentially the same crosslink density. The scan rate was 2°/min so the film cured at 149°C was at temperatures above the original cure temperature for 20.5 min in the 140–190° interval of the scan. 3.6. QUV behavior of blocked TTI versus TTI and blocked IPDI Some of the formulations in Table 3 were draw coated on white base coat panels, baked and subjected to QUV (QPanel) weatherometer exposure. Fig. 6 shows the 20° gloss retention with time for the formulations cured at 149°C. There is not much gloss change with time or difference for the blocked TTI samples compared with the unblocked TTI and caprolactam blocked IPDI sample except for the TANtBzC formulation A in Table 4 which falls off significantly in gloss after 1000 h. It is believed that the gloss loss is due to the retention of some unreacted aromatic benzyl groups on the crosslinker. This happens because of incomplete cure of TANtBzC at 149°C. Interestingly, the TANtBzC which is under indexer by 20% (Table 3; F), compared with the available hydroxyls, does not show this fall off. Also, the aliphatic ethyl alcohol blocked TTI formulation (Table 3; C) does not show this fall off.

Fig. 6. 20° gloss loss on formulations from Table 4. QUV weatherometer (B313) on films cured for 30 min at 149°C, on white basecoat panels.

The TANtBzC formulation which contains a HALS and UVA (Table 4B) shows excellent stability to gloss loss even though its retention of the benzyl group is similar to A. For films baked at 177°C (not shown), none of the formulations showed significant difference in retention of gloss. Because of extensive cure of the TANtBzC at 177°C, most of the benzyl shrouds have been displaced by crosslinking and consequently do not contribute to gloss loss. When yellowing was measured during QUV exposure by delta b values, TTI and most of the TTI carbamates as well as the blocked IPDI sample all showed similar amounts of yellowing The exception was the TANtBzC system which was cured at 149°C and it had almost double the rate of yellow development. Again, this appeared due to the presence of residual benzyl groups. The use of a HALS and UVA or minimizing the chances for residual benzyl by raising the cure temperature all had a pronounced effect on reducing the yellowing.

4. TAN-carbamates for powder coatings The curing behavior of the TAN-carbamates as indicated above, clearly suggest that these materials should match the performance of caprolactam blocked IPDI materials in powder applications. From the properties of the TAN-Carbamate samples summarized in Table 1, it appears that TAN-tribenzyl carbamate may be the only crosslinker listed with a high enough melting point to be adaptable to powders. The TANtBzC appears to have a low order of toxicity based on preliminary toxicity testing. Table 6 summarizes preliminary toxicity data obtained on TANtBzC. The properties of TANtBzC are compared with two commercial Table 6 Preliminary toxicity data for TANtBzC

Fig. 5. Effect of cure temperature with tANt (Et/Bu)C, G(1) and G(3).

Toxicity test

Test results

Oral LD 50 (mg/kg) Dermal LD50 (mg/kg) Eye irritation Skin irritation

>5000 >2000 Slightly irritating (2.6/110) Practically non-irritating 90.8/8)

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Table 7 Physical property comparison of TANtBzC with commercial caprolactam blocked IPDI powder crosslinkers

Property

Crosslinker TANtBzC

Appearance Tg or (Tm) (°C) Equivalent weight Mn Blocking group

of TANtBzC which is comparable with the liquid formulation A described above using McWhorter 3016 polyester. The liquid Fascat 4102 was used as the catalyst (0.2 phr Sn) by supporting it at 66% active on silica to give a powder catalyst for compounding. For direct comparison, McWhorter 3016 was blended with Huls B1530 blocked IPDI crosslinker with the same level of catalyst. At the 1:1 equivalence level of blocked isocyanate/hydroxyl, about 1;3 less of the TANtBzC is required compared with B1530. 4.1. Properties of TANtBzC powder coatings

McWhorter 24–2400

Huls B1530

White crystalline (108.7) 192

White amorphous 50 240

White amorphous 52 270

576 Benzyl alcohol

>800 Caprolactam

>800 Caprolactam

blocked IPDI type crosslinkers in Table 7. The TANtBzC is a uniform crystalline molecule with a Tm near 109°C. The typical IPDI powder crosslinkers are amorphous, oligomerized materials with Tg near or above 50°C. Benzyl alcohol as a blocking group will not cause oven fouling as is often encountered with caprolactam. However, other aspects of the release of benzyl alcohol as a cure volatile have not been determined. The TANtBzC was studied in a variety of powder formulations. Table 8 summarizes a powder formulation

Table 8 summarizes powder properties and film properties obtained on the two formulations. At the 177°C cure for 30 min both baked powder films have the same solvent resistance, impact resistance and hardness. Both films have high gloss but the carbamate crosslinked film has significantly higher DOI (less orange peel). The lower orange peel of the carbamate system is primarily due to higher flow. The longer inclined plate flow values and longer gel times of the formulated carbamate powder system are consistent with the flow difference. Fig. 7 shows the DMA plots obtained from films prepared with the formulations of Table 7. Results are also given for a third film in which Huls B1530 has been replaced by another type of blocked IPDI based crosslinker, Huls BF1540. The storage modulus plots (Fig. 7A) show that

Table 8 Powder formulations and properties. Direct comparison of TANtBzC versus blocked IPDI crosslinker Powder component

Formulation I

McWhorter 30-3016 TAN-tribenzyl carbamate Huls B1530 Fascat 4102 (66% on silica) Modaflow powder 2000 Measured property Inclined plate flow (mm)a 200°C gel (s)b Powder Tg (°C) Bake temperature (°C) (time (min)) Coating thickness (mils) MEK double rubs Impact resistance, F/R (inch-lb) Pencil hardness Gloss, 20°/60° DOI

II 88 12 – 1.54 0.7

82 – 18 1.54 0.7

261 352 40 177 (30) 2.1 200+ 160/160 2H 95/100 78

167 220 50 177 (30) 2.0 200+ 160/160 2H 90/98 46

Flow measured at 165°C, 15 min and 65° incline using 6 × 13 nm diameter compressed pellet on a glass plate. b Gel point (s) using 0.9 g of powder on a cure plate stroked with a spatula. a

Fig. 7. Crosslinker comparison with PE 30-3016 cured for 20 min at 191°.

H.P. Higginbottom et al. / Progress in Organic Coatings 34 (1998) 27–38

37

Fig. 8. RMS viscosity versus temperature. In situ curing at 5°C/min, 25 × 1.4 mm gap parallel plates at 1R/S and 15% strain, McWhoter 3016 with TANtBxC or McWhorter 2400.

TANtBzC gives a slightly higher crosslink density (E′min = 4.5 × 107 dynes/cm2) than obtained with B1530 or BF1540 (E′min = 3.3 × 107 and 3.1 × 107, respectively). There is little or no additional cure during the scan in this case as indicated by the flatness of the E′ plots in the rubbery plateau. This finding is expected because the cure temperature used here was nearly the same as the highest temperature reached during the scan, i.e. 191 and 190°C, respectively. The similarity of DMA plots for B1530 and BF1540 films is interesting because there are significant structural differences. B1530 is the isocyanurate oligomer of IPDI blocked with caprolactam whereas BF1540 is IPDI oligomerized through uretidone groups and is referred to as ‘internally blocked’ because the uretidione groups regenerate isocyanate during cure [14]. These polyester films of Fig. 7 have lower crosslink density than the acrylic films considered in Table 5 and Fig. 4. This difference is attributed to a lower number of functional groups per molecule for McWhorter 30-3016 than for Joncryl 587 (S.C. Johnson). Tand plots (Fig. 7B) clearly show the higher Tg values for IPDI based crosslinkers (90 and 94°C) versus the TANtBzC crosslinker (80°C). As noted previously, this is attributed to the rigidity of the cycloaliphatic ring in IPDI versus the flexible methylene group chains in TAN. Fig. 7B is somewhat unusual in that tand plots do not go down close to zero after the transition. Approximately 4000 DMA plots have been examined in our lab and this ‘non-zero feature after the transition’ has only been observed a few times. We are unsure of its physical meaning, but it is only seen at high bake temperatures. We speculate that it may indicate the presence a low content of highly condensed material that

is not attached to the main network. In other words, it may represent the beginning of network degradation that could occur when the cure temperature is high enough so that network formation and network degradation do not differ greatly in rate. Fig. 8 shows a plot of complex viscosity versus temperature obtained on a Rheometrics mechanical spectrometer. In this case the TANtBzC is compared in formula 1, where Fascat 4102 catalyst is introduced on silica, with formula 2, where the catalyst is masterbatched into the polyester. These are compared with McWhorter 24-2400 blocked IPDI system which is uncatalyzed (formula 3). Over the entire melt range, the complex viscosity is lower for the carbamate systems than for the blocked IPDI system. The minimum in the viscosity curves just prior to the rapid increase in viscosity translates to higher, longer flow before gelation with the carbamates which results in smoother films. The difference between carbamate curves 1 and 2 is small with the catalyst on silica appearing to be more efficient that masterbatched catalyst. The question of powder stability must be checked when crystalline components are added to powder formulations. The crystalline component may or may not recrystallize in the formulated powder after melt mixing. If recrystallization does not occur, the powder Tg will be lowered and sintering will result. The Tg of the melt blended TANtBzC powder formulation I in Table 8 is about 40°C. The Huls B1530 powder formulation II has a Tg of 50°C. The formulation I fails a 24-h sintering test at 40°C while formulation II passes. If about 1% crystalline TANtBzC is pulverized and sieved with melt blended formulation I, the resulting

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H.P. Higginbottom et al. / Progress in Organic Coatings 34 (1998) 27–38

powder passes the 24-h sintering test at 40°C even though the measured powder Tg is still bout 40°C. It is believed that the added crystalline TANtBzC induces surface crystallization of the powder particles and this leads to sintering stability. The chance of sintering with TANtBzC formulations can also be reduced by using powder formulations with higher Tg polyester coreactants.

5. Conclusions Carbamates of TTI formed using different alcohol blocking groups can function as crosslinkers in coatings if properly catalyzed. The blocked TTI crosslinkers give baked coatings with excellent properties comparable with unblocked TTI cured coatings when fully cured. These crosslinkers can be used in liquid coatings with hydroxyl functional coreactants if bake temperatures near or above 150°C are employed. TAN-tribenzylcarbamate can be used as a stable, low toxicity crosslinker in powder coatings. This crosslinker matches cure and property development of blocked IPDI crosslinkers. Other potential advantages of TANtBzC over IPDI based crosslinkers is lower use level, reduced oven fouling and significantly improved flow and appearance.

Acknowledgements The authors wish to express thanks to Mr. Donald E. Williams of Mansanto Co. for the RMS support work and to Dr. William D. McGhee of Monsanto Corporate Research for providing TTI and some of the TAN carbamate materials. We also extend our appreciation to Dr. Sri Seshadr and Elf Atochem North America, Inc. for providing samples of development catalysts.

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