Waterborne coating technology focuses on achieving and exceeding the performance of solventborne systems. Crosslinking is an important component in improving the properties of waterborne resins. The following discussion will review the coating market's requirements of appearance, durability and handling in three market areas: interior clear wood, pigmented interior/exterior wood, and pigmented metal applications. The development and performance of one-package crosslinking polymers, as well as polymers that can work successfully with current commercially available crosslinking technologies, are highlighted.

General Coatings Market Requirements

Appearance, durability and handling summarize the requirements of the coatings market. Appearance is viewed as clear non-yellowing films, durability as good chemical and wear resistance to protect the substrate and handling refers to fast dry and safe easy cleanup. To be successful, a formulation must meet all of these criteria, as well as the price constraints of the market.

Meeting Market Needs with Waterborne Technology

In the early civilizations of Egypt and Greece, coating producers used natural polymer products such as milk, egg white, pitch and beeswax to produce simple but effective coatings. Since that time, coating technology has made many advances.

Today waterborne technology consists of a variety of different systems. Standard acrylics and urethanes have many benefits. The formulations are low in VOCs, environmentally safe and user friendly. Standard waterborne coatings do have limitations. They are less chemical, corrosion and wear resistant, and generally are not as glossy as solventborne systems. Two-pack crosslinking systems address these drawbacks. These systems have improved chemical and wear resistance, low color, and can cure at room temperature. They also have limitations such as the need to use caution in handling and a short pot life. The new one-pack self-crosslinking systems demonstrate additional improvements. They have good chemical and wear resistance, are user friendly, environmentally safe, crosslink at room temperature and have a shelf life of greater than 12 months. The deficiencies of these systems are fewer reactive sites and possibly higher cost. While one-pack self-crosslinking systems generally do not perform as well as the two-pack crosslinking systems, they are superior to non-crosslinked systems.

Figure 1

Crosslinking Review

Polymer systems can be either crosslinked or non-crosslinked. Non-crosslinking resins contain long unconnected polymer chains and are easily solvent soluble. Crosslinked resins have long attached polymer chains that form a "chain-link fence" type network (see Figure 1). These systems are only slightly solvent swellable, and have improved chemical and wear resistance.

There are many types of crosslinking systems. One-pack melamine containing systems cure by heating. Epoxies, water dispersible isocyanates and aziridines are two-pack systems that cure at room temperature. The new one-pack self-crosslinking systems also cure at room temperature. Crosslinking improves the performance of each of these resin systems.

Figure 2
An example of a two-pack system is NeoCryl CX-100 combined with a waterborne resin. This material is a versatile multifunctional ambient temperature cure aziridine crosslinker. The crosslinking mechanism is the reaction of the carboxylic acid along the backbone of the resin combining with the aziridine to form an aminoester (see Figure 2).

The aziridine in the formulation reacts with water to form an aminoalcohol in the unused portion. The pot life of the system is relatively long (48-72 hours). Re-inoculating the system by adding more aziridine and resin restores the crosslinking capabilities at the end of the pot life.

Figure 3
NeoCryl CX-100 crosslinking of a waterborne polyurethane improves the final coating performance. For example, crosslinking stiffens the film, thus the elongation is lower. Also, the 200% modulus is larger due to the increase in the network strengthening the polymer (see Figure 3).

The need for special handling with NeoCryl CX-100 is recognized. To avoid contact with the liquid or overspray, users must wear personal protective equipment. Good housekeeping during use is also recommended.

As seen, the aziridine is an effective two-pack crosslinking system that improves the performance properties of waterborne formulations. New one-pack waterborne self-crosslinking systems are striving to meet the performance of these traditional aziridine systems.

Figure 4

Market Area: Clear Interior Wood Coatings

One area where self-crosslinking resins show promise is the clear interior wood coatings market such as brushing and flooring varnish. The customer wants a clear film appearance, specifically less yellow, with the toughness of an oil-modified urethane. Handling must include safety for contractors, do-it-yourselfers and factory workers.

The NeoResins product line includes a variety of waterborne resins for the interior clear wood market, including standard NeoCryl acrylics, NeoRez urethanes and NeoPac copolymers. NeoCryl CX-100 can be used to crosslink with the carboxylic acid in all these products. An air oxidizing self-crosslinking resin, NeoRez R-9403, exhibits excellent performance properties for this market. Other new one-pack self-crosslinking resins that cure at room temperature are NeoCryl XK-12 and NeoPac E-125.

Most formulators today are blending resins to maximize cost vs. performance. Can blending with one-pack self-crosslinking resins increase film performance? To answer this question, an experimental blend study examined the performance of one-pack self-crosslinking resins when blended with themselves and non-crosslinking resins. The focus was on NeoCryl XK-12, an acrylic emulsion, and NeoPac E-125, a urethane/acrylic copolymer. These resins were developed specifically for the clear wood coatings market. To evaluate the performance benefits of blends of one-pack self-crosslinking resins, experimental samples of these same resins without the self-crosslinking mechanism were prepared (see table).

This study included three additional controls: NeoCryl A-633, an acrylic emulsion with good hardness and chemical resistance; NeoRez R-9637, an aliphatic urethane with excellent wear and chemical resistance; and NeoRez R-9403, an aromatic air oxidizing self-crosslinking urethane alkyd that cures at room temperature.

The simple formulation used to evaluate these resins consisted of resin, cosolvent (ethylene glycol monobutyl ether) and leveling aid (FC-1201 at 1% active). All formulations were at 30%-solids by weight. The substrates used included coated maple panels (three coats brush-applied, natural spread rate) and Leneta drawdowns (3 mil wet). After coating, the samples were air dried for eight hours and then baked at 60degC for 30 minutes. The drying schedule completes film formation, accelerates aging and facilitates testing.

A variety of tests are used to assess the degree of crosslinking, since one test may not give the full picture. Household chemical resistance, alcohol rubs, scuff development, dynamic mechanical analysis (DMA) and solvent swell evaluate the sample durability.

The first test, household chemical resistance, is a simple spot test. The test begins by placing the chemical on the coated maple panel and allowing it to remain for one hour. The film is then tested for softness. The degree of softening is given a rating of 10 (no effect) to 0 (dissolved). This test uses household cleaner 409 (see Figure 4). The first group, one-pack self-crosslinking standards, shows slight softening. As expected, the non-crosslinked controls are much worse. The blends of NeoCryl XK-12 crosslinking/NeoPac E-125 non-crosslinking and NeoCryl XK-12 non-crosslinking/ NeoPac E-125 crosslinking are more chemical resistant than the non-crosslinking controls. The performance of the NeoCryl XK-12 non-crosslinking/NeoPac E-125 crosslinking is better due to the urethane crosslinking component. The first control, NeoRez R-9403 did not soften. The NeoCryl A-633/NeoRez R-9637 blend by itself is quite soft, but when combined with NeoCryl CX-100 improves significantly.

Figure 5
The next test is alcohol rubs. Placing a small amount of alcohol (IPA 70%) on the Leneta drawdown and rubbing for 10, 20 and 30 times initiates the testing. After removing the alcohol, the 60deg gloss was measured (see Figure 5). The graph key orders the samples as they appear at 30 rubs.

The NeoRez R-9403 and the NeoCryl A-633/NeoRez R-9637 with NeoCryl CX-100 are both excellent. As expected, the non-crosslinking samples are poor. Adding the crosslinking resins increases the performance as noted in the key of Figure 5 as "Good."

The Ladder-Pendulum Apparatus measures the scuff development (see Figure 6). The black heel attached to the pendulum allows the device to scuff the substrate. Raising the pendulum to 90? and dropping it onto the coated maple panel creates the scuff footprint. After scuffing, a cleaner is used to remove the black mark from the substrate. Assessing the difference between the original gloss and the after-scuff gloss gives a visual reading (5 = no effect, 0 = black scuff can not be removed).

Figure 7
The visual reading for NeoCryl A-633/NeoRez R-9637 is 2, but when crosslinked with NeoCryl CX-100 gives a visual reading of 5 (see Figure 7). The NeoRez R-9403 reading is 5, no scuff. After scuffing, the blended samples visually read 0 to 4.5 (see Figure 8).

Figure 8
Following scuffing, the controls exhibit excellent resistance to scuff (see Figure 9). The one-pack self-crosslinking resins exhibit good results with a slight remaining scuff. The blends containing the one-pack self-crosslinking resins also produced good results, confirming the previous chemical test results.

Figure 9
DMA measures the drying film resistance to flexing. A small amount of formulation is coated onto a fiberglass braid. The test equipment heats the braid to a constant temperature and applies mechanical stress. Modulus over time measures the resistance to flex from resin crosslinking. At 240 minutes, the standard samples and the blends show higher modulus than the non-crosslinking samples (see Figure 10).

Figure 10
Of the reference controls, NeoCryl A-633/NeoRez R-9637 exhibits lower modulus than the same blend crosslinked with NeoCryl CX-100. In each case, as the crosslinked network increases, the modulus increases.

Figure 11
The last test is solvent swell. A micrometer in the eye piece of a microscope measures a small piece of dry film. Then, sample exposure to methyl ethyl ketone (MEK) swells the film. After a set period of time, the film is again measured and the percentage swell is calculated. As seen in the non-crosslinking group, the NeoCryl XK-12 without crosslinking has tremendous swelling of 780% (see Figure 11). All of the non-crosslinked samples swell more than their standard counterparts or the blends. The reference sample NeoRez R-9403 exhibits very little swelling. The blend of NeoCryl A-633/NeoRez R-9637 swells less when crosslinked with NeoCryl CX-100.

In each of the tests studied, one pack self-crosslinking resins alone, or in blends with non-crosslinking resins, improve the durability and achieve the best cost/performance balance. The formulator can vary the polymer type and crosslinking capability to meet the needs of the clear interior wood market.

Figure 12

Market Area: Pigmented Interior/Exterior Wood Coatings

Interior/exterior pigmented wood coatings is the second market area where one-pack self-crosslinking resins show promise. The market needs include high gloss, colorfastness, chemical and block resistance, fast dry, and safe handling. NeoCryl XK-982 is a new one-pack self-crosslinking acrylic resin for this market.

Hand lotion resistance is one of the most interesting aspects of durability testing. When closing or opening doors and windows, hand lotion can build up and cause coatings to soften. A one-hour spot test assesses the effect of hand lotions on different paint formulations. Both NeoCryl XK-98 and a typical acrylic are in identical formulas at a VOC of less than 150 g/l. A variety of lotions are applied to the coating for one hour (see Figure 12). After exposure, the lotions are removed and the effect on the coating is evaluated. The XK-98 not only out-performs the typical acrylic, but exhibits excellent resistance to the various lotions.

Figure 13
Block resistance is also an important performance requirement for this market. Doors and windows often stick when newly painted. For this test, XK-98 and the typical acrylic are drawn down on Leneta charts. The films are allowed to dry at room temperature.

Cutting the charts into strips and placing the coated surfaces face to face allows the positioning of the 1,000 grams/square inch weight on each square. The samples sit at room temperature for 4 hours. Pulling the squares apart permits the evaluation of the block resistance (see Figure 13). The XK-98 shows good initial block resistance after 4 hours developing excellent block resistance at 16 hours. This is due to continued self-crosslinking as the polymer dries.

Figure 14
Block resistance at elevated temperature, 52degC, for one hour indicates performance on hot days (see Figure 14). At both 24 hours and 7 days air dry, the NeoCryl XK-98 exhibits good performance when compared with the typical acrylics, whose performance was poor.

With XK-98, one-pack self-crosslinking improves the performance of paints for the pigmented interior/exterior wood coatings market in air-dry systems.

Figure 15

Market Area: Pigmented General Metal Coatings

Self-crosslinking resins are available for many market areas, as are polymers to work with current commercial crosslinkers. The last market area, general metal coatings, is such an example. The market needs are high gloss, no microfoam, chemical and impact resistance, hardness development, low VOC, rapid drying and ambient cure. There is, as of today, no one-pack self-crosslinking resin for this market area. NeoResins has designed a new polymer, NeoCryl XK-100, a hydroxy functional acrylic for two-pack systems that can be crosslinked with a water-dispersible isocyanate. Key features are room temperature curing and low carbon dioxide development thus low microfoam formation.

Formulators vary the isocyanate (NCO) level needed to develop the properties of gloss, MEK and impact resistance. At each NCO to OH ratio, XK-100 exhibits the same level of performance (see Figure 15). This allows the use of the lowest level of NCO to OH ratio to achieve the best properties. By using the lower level of isocyanate, manufacturers achieve cost savings.

Figure 16
Hardness is also an important property for industrial pigmented metal coatings. Increasing the NCO to OH ratio increases the Konig hardness level (see Figure 16). To test hardness, the formulation was drawn down on a glass plate and allowed to cure at room temperature for a week.

Increasing the NCO to OH ratio increases the hardness. The additional urea linkages contribute to the hardness development.

When crosslinked, NeoCryl XK-100 shows excellent flexibility combined with hardness. Films also show less development of microfoam in thick film applications.


New developments in crosslinking technology are increasing the performance of waterborne resins. The most interesting approach is one-pack self-crosslinking resins that are designed for specific market areas. They are safe to handle, cure at ambient conditions and meet the diverse market requirements. In addition, many opportunities exist to design resins that are compatible with commercially available crosslinkers. Both of these systems can balance the cost/performance requirements necessary to achieve commercial success.


I would like to thank the following people for their contributions to this article: Dr. Richard Coogan, Dave Fortier, Dr. Robert Seidewand, Tracy Bergeron, Gail Pollano, Greg Stopher, Kim Besse, Jim Brown, Dave Lenz, Pieter Meulemans, Dr. Richard Brown, Nico Visscher, Nathalie Balch, Jan Bouman, Sheena Killy and Ankie VanGorkum.

This article is based on a paper presented at ICE 2000, Chicago.

For more information on crosslinking, contact Patricia Bell, phone 978/658.6600; e-mail Patricia.Bell@Avecia.com; visit www.NeoResins.com; or Circle Number 135.