IntroductIon - American Coatings Association

IntroductIon

Since the early 1800s, traditional solvent-based coatings have been manufactured and used extensively in the United States to protect interior woodwork, including commercial and residential floors. However, to meet the growing health and environmental concerns, regulatory agencies throughout the world have begun to reduce the allowable amount of volatile organic compounds (VOC) that coatings can release into the atmo-sphere. These regulations have initiated the transi-tion from solvent-based to water-based coatings.

Early water-based coatings technology did not offer the same level of durability as solvent-based systems, so the use of external cross-linkers emerged. Crosslinkers such as aziridines, isocyanates, and carbodiimides have been and continue to be used to increase the durability of water-based coatings. Each crosslinker brings with it positives and negatives, but they all improve coating properties to some degree. Aziridines and carbodiimides react with carboxyl groups present in most water-based acrylics and urethanes, while isocyanates typically require hydroxyl-functionality to maximize their benefits.

Although aziridines and isocyanates are used extensively in the professional market, carbodi-imides have a very limited commercial presence. Traditionally, they have not been considered a true workhorse capable of delivering the increased level of durability required by the professional market. The objective of the work presented here has been to determine the effectiveness of a specific car-bodiimide and if its performance can approach the durability demanded by the professional market.

Market requirementsInterior clear wood coatings are used to pro-

tect items such as wooden furniture, cabinets, doors, trim, and floors. They are applied by either a professional contractor or the do-it-yourself (DIY) homeowner. Two-component (2K) coatings typically provide elevated levels of durability compared to one component (1K) coatings because they work to increase the crosslink density of the coating. This provides a significant increase in chemical and abrasion resistance resulting in a longer ser-vice life. There are limitations to how significantly a crosslinker can increase the properties of a coating system because the base resin plays a crucial role

Pat Bell and Ben GlowackiDSM NeoResins

September 201032 COATINGSTECH

Resin Resistance Properties

Designation Description Chemical Black Heel Fingernail

U/A Hybrid Modified aliphatic

urethane acrylic hybrid Fair Excellent Excellent

SXL Urethane Self-crosslinking

aliphatic urethane Very Good Very Good Excellent

Aliphatic Urethane Typical aliphatic urethane

Acrylic Emulsion Self-crosslinking acrylic Very Good Fair Fair

table 1—Resin Description

in the final coating properties. Aziridines and isocya-nates are very reactive, so their use is restricted to professional contractors that are aware of the haz-ards associated with these crosslinkers. DIY homeowners are left with coatings that do not uti-lize these reactive crosslinkers.

For many years aziridine and isocyanate crosslinked systems have provided very good 2K coatings for the wood flooring industry. Although they boost coating properties significantly, they also have several negative drawbacks. Due to their reactivity, both crosslinkers can only be used by professionals with proper personal protective equipment. Another significant downside to either crosslinker is the short pot-life associated with the inoculated material. Isocyanate systems are typically useful for three to four hours, and aziri-dine systems are claimed to be good for up to 24 hours. These numbers vary by manufacturer and should be considered as broad generalizations. Some aziridine systems allow for a second re-inoc-ulation but typically at the cost of some reduction in properties. Isocyanate systems usually require disposal of unused material once inoculated.

carbodiimide crosslinkersThe acceptance of carbodiimides for the archi-

tectural market has historically been limited due to their perceived reduced reactivity at room tempera-ture. Recent product offerings have addressed this problem such that some of the current water-based carbodiimides have very good reactivity at ambient temperatures. The specific carbodiimide chosen for this project is multifunctional and reactive at room temperature. This chemistry is effective in crosslink-ing resins that have an active hydrogen found in most carboxyl groups. It is delivered at 40% solids in water and considered VOC-free.

The pot life of a crosslinker is critical for the professional contractor because it creates

a window in which the job must be completed. The short pot life of aziridines and isocyanates is a major limitation, but carbodiimides seem to have a much longer pot life, extending the ap-plication window. Initial testing indicates that this carbodiimide maintains its effectiveness after a three-month pot life, and continues to contribute to crosslinking even after a six-month pot life. Carbodiimide pot life is now being measured in months, where aziridine and isocyanate pot life is still measured in hours.

ExpErIMEntal

Water-Based Flooring resinsTo assess the effect of the carbodiimide cross-

linker, three types of flooring systems were chosen for the evaluation: a urethane-acrylic modified hy-brid, a self-crosslinking urethane, and a standard urethane combined with an acrylic emulsion as a cold blend. These resin systems were chosen to give a broad overview of technologies that can be used to determine where a carbodiimide crosslinker can have the greatest effect on coating properties (Table 1). The following is a detailed de-scription of the three different resin systems:

U/A Hybrid: A high solids, modified, aliphatic urethane-acrylic hybrid with a high ratio of urethane to acrylic for enhanced scratch and wear resistance.SXL UretHane: A self-crosslinking, aliphatic urethane designed to provide exceptional abrasion resistance while providing very good chemical resistance.U+A blend: A standard aliphatic urethane blended with a self-crosslinking acrylic emul-sion at a 30% urethane and 70% acrylic ratio to represent a typical commercial DIY product.

September 2010COATINGSTECH 33

Resin Designation Resin Solids Starting Gloss Formula

% Solidsa

VOC NMP Film Quality

U/A Hybrid 45 35 275 No Excellent

SXL Urethane 35 30 210 No Excellent

Aliphatic Urethane 36 30%b

Acrylic Emulsion 45 70%b

31 235 Yes Excellent

(a) Solids before addition of crosslinker

(b) Percent on resin solids

table 3—Formulation Description

allowed to sit for another 10 minutes before applica-tion to the test substrate. All of the preparation was done at standard lab condi-tions of 40–50% humidity and 70–75ºF.

test panel preparationThree coats were ap-

plied on maple veneer panels using a normal spread rate with light sanding after the first coat, and allowed to dry for six hours between coats. In the case of crosslinked systems, each coat was crosslinked before application. After the final coat, the boards were allowed to air dry at standard lab conditions for seven days before testing on the eighth day. Property development was also tested at various cure intervals of less than seven days to determine the rate of cure.

test proceduresThe coated panels underwent a series of tests

designed to approximate conditions that can be encountered by a hardwood floor in a residential or commercial setting (Table 4). This list of tests is not all-inclusive of what can be done, but does highlight different attributes that are inherent with each resin system. Coatings manufacturers typi-cally identify resins with the desired properties and work to minimize shortcomings that might exist, or possibly further emphasize specific resin strengths.

Chemical resistance testing utilizes a standard method, but the selection of chemical reagents can vary and has been noted below. Black heel mark resistance (BHMR) testing is less common because shoe manufacturers have developed non-marking materials. However, the test provides an indication of a resin’s resistance to flow during impact where high amounts of heat are generated in a short

Each resin was formulated and tested as a sin-gle component system, and then with the addition of an aziridine or carbodiimide crosslinker to assess the effect on resistance properties (Table 2). The urethane-acrylic cold blend was tested as a system of the two resins, and no individual testing was done on each separate polymer. They were chosen to work as a system—one that would be considered a typical formulation for a DIY brushing varnish.

FormulationsCoatings were formulated using a basic combi-

nation of glycol ethers for coalescing, surfactants for wetting, defoamer for foam control, and ammonia where needed to prevent coagulation during solvent addition. These systems were not optimized for commercial use but instead constructed to allow for film formation and testing. Solids were deter-mined based on film formation and viscosity con-trol to avoid the use of rheology modifiers. The VOC was targeted for 275 g/L or less depending on the solvent demand of the resin, which resulted in a range of about 200–275 g/L.

After the formulations were prepared, each was blended with the appropriate crosslinker for each battery of testing (Table 3). Mechanical mixing using a lab mixer was used to achieve consistent incorporation throughout the sample preparation process, but hand mixing can be used in the field. Formulations were mixed for 10 minutes and then

Description % Active Carrier VOC

Carbodiimide Poly-functional carbodiimide

40% 60% Water None

Aziridine Poly-functional aziridine

100% None None

table 2—Crosslinker Parameters


period of time. Scuff resistance is derived from the same impact point if there is very little black marking left behind, and a reduction in gloss can be determined. Fingernail mar resistance (FNMR) is by far the most subjective test in this battery, but experience has shown it to be a quick indicator of what can be expected when a floor is subjected to damage by rocks, sand, or animal nails. This method leaves room for interpretation but is in-cluded as an indication of scratch resistance.

testing overviewTo assess the effect of the carbodiimide on the

water-based formulations, the study was divided into three phases, each phase driving the testing of the next phase:PHase I: Initial Screening—focused on the perfor-

mance gain of the three resin systems when com-bined with the carbodiimide at 3% and 6% (active).

PHase II: Carbodiimide Optimization—utilized the U/A Hybrid resin system from Phase I and ex-amined the effect of reduced carbodiimide levels on performance.PHase III: Verification—compared the U/A Hybrid

(2K carbodiimide) vs. the Aziridine crosslinker vs. a commercial (2K aziridine) system.

rEsults and dIscussIons

Initial screening results (phase 1)The initial run of experiments was structured

to identify different polymers that can potentially benefit from the addition of the carbodiimide crosslinker. The three resin systems tested in this series were the U/A Hybrid, the SXL Urethane, and the U+A Blend (Table 5). Three percent aziridine and 3% and 6% active carbodiimide were used as the initial reference levels.

Test Method Description Based on ASTM

Chemical Resistance

Resistance to a variety of chemicals:

— Water and alcohols (ethanol & IPA)

— Household cleaners (Fantastic,

Formula 409 & Windex)

— Stains (mustard & cordovan shoe polish)

ASTM D 1308

1 hr covered spot test, with a

1 hr recovery, evaluating

softening, discoloration and loss

of gloss using a 1–10 rating

where 10 = Best (no effect)

BHMR

Determination of the degree of black

marking of coatings by the pendulum

apparatus utilizing a rubber block

Based on original ASTM

Method D 3714-82 where

5 = Best

FNMR Determination of the degree of marking

by hitting the surface with the edge of a

fingernail

Subjective physical test,

ref. Koleske, J.V., Paint &

Coating Testing Manual, ASTM

International, p 582 (1995),

where 5 = Best

table 4—Test Method Description

Crosslinker (% Active)

1K Carbodiimide Aziridine Testing

U/A Hybrid ∗ 3% and 6% 3%

SXL Urethane ∗ 3% and 6% 3%

U+A Blend 3% and 6% 3%

Chemical

resistance

BHMR &

FNMR

∗

table 5—Phase 1 Samples


The U/A Hybrid demonstrated a significant improvement in alcohol resistance, both IPA and ethanol, at the 3% level of carbodiimide. Although the 6% carbodiimide showed a slight improvement, the difference was very small given the increase in carbodiimide level. The carbodiimide provided excellent resistance properties approaching the aziridine, exceeding the expected results of the carbodiimide. Resistance to household cleaners and stains was also improved, but not as dramati-cally as the alcohol resistance. See Figure 1 for results of chemical resistance of U/A Hybrid.

The SXL Urethane system did not show the same improvement in chemical resistance, al-though the resistance properties were already higher than those of the U/A Hybrid as a 1K sys-tem (Figure 2). The internal crosslinking mecha-nism of the SXL Urethane provided added chemical resistance properties, therefore the benefits of the carbodiimide addition were small. The addition of the aziridine crosslinker improved resistance prop-erties but also not significantly.

The U+A Blend benefited from the addition of the carbodiimide and aziridine crosslinkers,

0

2

4

6

8

10

Ethanol, 50% IPA, 70% Cleaners Stains

Rating 1

-10, 10=

Best

SXL Urethane

+ 3% Carbodiimide

+ 6% Carbodiimide

+ 3% Aziridine

Figure 2—Chemical resistance of SXL Urethane (1K and 2K).

Figure 3—Chemical resistance of the U+A Blend (1K and 2K).

0

2

4

6

8

10


Ra

tin

g 1

-10

, 1

0=

Be

st

U+A Blend

+ 3% Carbodiimide

+ 6% Carbodiimide

+ 3% Aziridine

0

2

4

6

8

10


Rating 1

-10, 10=

Best

U/A Hybrid

+ 3% Carbodiimide

+ 6% Carbodiimide

+ 3% Aziridine

Figure 1—Chemical resistance of U/A Hybrid (1K and 2K).


but again, not as dramatically as the U/A Hybrid. The acrylic emulsion used in the U+A Blend is a self-crosslinking acrylic emulsion that offers very good chemical resistance properties, but is further improved with the addition of the carbodiimide or aziridine crosslinker (Figure 3).

The initial screening results indicated that the greatest increase in chemical resistance occurred with the U/A Hybrid, a resin that does not have an internal self-crosslinking mechanism. The carbodi-imide increased the crosslink density significantly, and provided a noticeable benefit to the overall system in terms of chemical resistance properties. The SXL Urethane and the U+A Blend both have a significant self-crosslinking mechanism within the resins already and showed only a modest improve-ment in chemical resistance.

Although the U/A Hybrid has fair chemical re-sistance properties, it was designed to provide me-chanical wear and scratch resistance. The BHMR and FNMR data highlights those properties as compared to the SXL Urethane and the U+A Blend (Figures 4 and 5). The addition of carbodiimide to the U/A Hybrid and SXL Urethane was practically insignificant in developing BHMR and FNMR, but is instead a function of polymer composition. The

exception is that the addition of aziridine to the U+A Blend improved both BHMR and FNMR. Due to the high level of mechanical wear resistance and re-sponse to carbodiimide crosslinking, the U/A Hybrid was chosen for further testing and optimization.

carbodiimide optimization results (phase 2)

The second part of the study was to evaluate reduced use levels of the carbodiimide based on better than expected results at the 3% loading. A ladder study was developed to test chemical resistance at 1%, 2%, and 3% carbodiimide load-ing, and to also test reduced cure times. Chemical resistance was the best indicator of improvement with the U/A Hybrid, specifically alcohol resistance, and was therefore utilized for this carbodiimide op-timization study (Table 6).

Figure 4—Black heel mark resis-tance (1K and 2K resin systems).

Figure 5—Fingernail mar resistance (1K and 2K resin systems).

0

1

2

3

4

5

U/A Hybrid SXL Urethane U+A Blend

Rating 1

-5, 5=

Best

1K

3% Carbodiimide

6% Carbodiimide

3% Aziridine

0

1

2

3

4

5

U/A Hybrid SXL Urethane U+A Blend

Rating 1

-5, 5=

Best 1K

3% Carbodiimide

6% Carbodiimide

3% Aziridine


1K Carbodiimide (% Active) Testing

U/A Hybrid ∗ 1%, 2%, 3%

Chemical

resistance


0

2

4

6

8

10

Ethanol, 50% IPA, 70% Windex

Rating 1

-10, 10=

Best

0% 1% 2% 3% Active

0

2

4

6

8

10

1 Day 2 Days 5 Days

Rating 1-10, 10=Best

0% 1% 2% 3% Active

0

2

4

6

8

10

U/A Hybrid 1K U/A Hybrid + 2%Carbodiimide

U/A Hybrid + 3% Aziridine Commercial 2K

Rating 1

-10, 10=

Best

Day 1 Day 2 Day 3 Day 7

0

1

2

3

4

5

U/A Hybrid 1K U/A Hybrid +Carbodiimide*

U/A Hybrid + Aziridine* Commercial 2K**

* 2% Active

** with supplied crosslinker

Rating 1

-5, 5=

Best

BMHR FNMR

Figure 6—Chemical resistance with var-ied % Carbodiimide in U/A Hybrid System.

Figure 7—Ethanol resistance of U/A Hybrid with varied % Carbodiimide and time.

Figure 8—Ethanol resistance of U/A Hybrid 1K and 2K at one, two, three, and seven days.

Figure 9—Black heel mark resis-tance and fingernail mar resistance of U/A Hybrid 1K and 2K at seven days.


The results, shown in Figure 6, indicated that there is a dramatic improvement in chemi-cal resistance with the addition of as little as 1% carbodiimide. Ethanol resistance was improved by 4 points and IPA resistance went from virtually zero to about 7. The 2% level provided a further improvement of 1–2 points in ethanol and IPA resistance, while the 3% level showed only an incremental improvement. This was rather surpris-ing since the initial thought was that a high level of carbodiimide would be required to observe any appreciable improvement in chemical resistance. Resistance to Windex was rather consistent in the 1K formulation as well as the various levels of carbodiimide, so most of the resistance came from the resin with minimal impact by the carbodiimide.

To assess the rate of performance develop-ment, the samples were tested at one, two, and five days (Figure 7). The results showed that even at the 1% level of carbodiimide, nearly all of the resistance properties are developed after one day, with very minimal increase after two and five days cure. The U/A Hybrid itself did not improve much with time, so the rapid property development may be linked to the resin system. This rapid property development reinforces the need for proper resin selection to optimize crosslinker benefits.

Verification results (phase 3)For the final part of the study, the U/A Hybrid

was tested as a 1K system, a 2K with a carbodi-imide, a 2K with aziridine, and compared to a commercial 2K aziridine system used in the pro-fessional contractor market. This would serve to verify the 2% level of carbodiimide as an optimized, although not maximum, level compared to the com-mercial 2K system. This testing was done at various cure intervals to observe any differences in overall resistance properties and rate of development.

The U/A Hybrid, with and without crosslinker, was tested for ethanol resistance after one, two, three, and seven days cure, and compared to the 2K commercial aziridine system (Table 7 and Figure 8). The data in this series supports earlier results confirming that property development of

the U/A Hybrid is near maximum after one day for both the carbodiimide and aziridine crosslinkers. When compared with the aziridine, the carbodi-imide crosslinker demonstrated only slightly lower resistance properties in the U/A Hybrid system, but very comparable to the commercial 2K aziridine system. Since this was a new data series, the val-ues for the U/A Hybrid vary slightly from the values earlier due to subjective differences in measure-ment between the various test series.

BHMR and FNMR were also maintained at a maximum of 5 with the U/A Hybrid, comparable to the results for the commercial 2K aziridine system. This was primarily a function of the resin system and not necessarily the crosslinker as observed by the 5 rating with the 1K U/A Hybrid system. BHMR and FNMR testing was performed to verify that property development was not degraded in any way by the addition of the carbodiimide, and for comparison to the commercial 2K aziridine system (Figure 9).

dIscussIon/conclusIon

In clear wood floor finishes, the majority of the coating properties are derived from the base resin, but external crosslinkers can be used to further improve certain resistance properties. Although carbodiimides and aziridines can be used to cross-link various resin systems, they have demonstrated the greatest impact on non-self-crosslinking resin systems. Both improve crosslink density to improve chemical resistance properties, especially alcohols like ethanol.

Use levels of carbodiimides were also evalu-ated and found to be lower than expected with better results than initially anticipated. Due to their perceived reduced activity, it was previously believed that much higher levels would be re-quired compared to aziridine crosslinkers. Higher use levels were not required and better than expected results relating to chemical resistance were achieved, in some cases approaching that of aziridine systems.

Crosslinking Testing

U/A Hybrid 1K

U/A Hybrid 2% Carbodiimide (active)

U/A Hybrid 2% Aziridine (active)

Commercial 2K Aziridine (as supplied)

Chemical resistance,

FNMR and BHMR

at 1, 2, 3 and 7 days



In conclusion, this study has demonstrated that carbodiimide crosslinkers can achieve per-formance levels approaching aziridine crosslinked systems. Carbodiimide use levels are similar to aziridines, and higher dosing is not required as was previously anticipated. Although proper resin se-lection is critical in developing an optimized wood floor finish, a 2K carbodiimide system can deliver the resistance properties of commercial 2K aziri-dine systems.

Bibliography

Caldwell, R.A., “Oil-Modified Urethanes for Clear Wood Finishes: Distinction or Extinction?” JCT CoatingsTech., Vol. 2, pp. 30-41 (March 2005).

Coogan, R.G, “Post-crosslinking of Water-borne Urethanes,” Prog. Org. Coat., 32; No. 1-4, 51-56 (1997).

Kurzer, F.and Douraghi-Zadeh, K., “Advances in the Chemistry of Carbodiimides,” Chem. Rev., Vol. 67, No. 2, pp. 107-140 (1967).

Ozone Transport Commission Website, www.otcair.org

Posthumus, W., Derkson, A.J., van den Goorbergh, J.A.M., and Hesselmans, L.C.J., “Crosslinking by Polycarbodiimides,” Prog. Org. Coat., 58, No. 2-3, pp. 231-236 (2007).

South Coast Air Quality Management District, “Revised Rule 1113.” www.aqmd.gov/rules/reg11/r1113.pdg. Amended July 2007.

Taylor, J.W. and Bassett, D.R., “The Application of Carbodiimide Chemistry to Coatings,” ACS Symposium Series, 663, pp. 137-163 (1997).

Wicks, Z.W. Jr., Wicks, D.A., and Rosthauser, J.W., “Two Package Waterborne Urethane Systems,” Prog. Org. Coat., 44, No. 2, pp. 161-183 (2002).

Williams, A. and Ibrahim, I.T., “Carbodiimide Chemistry: Recent Advances,” Chem. Rev., 81, pp. 589-636 (1981).

autHorsPat Bell and Ben Glowacki, DSM NeoResins, 730 Main St., Wilmington, MA 01887-3366; [email protected].

CT


IntroductIon - American Coatings Association

Documents

Transcript of IntroductIon - American Coatings Association