Paint & Coatings Industry - February 2009 - (Page 36) 1K Polyurethane Dispersion for Conventional 2K Applications affected by other components in the waterborne polymer or in the wood, such as tannins. The crosslinking can be very effective but is regarded as somewhat slow unless high amounts of drier are used with a high content of “fast-drying” unsaturated groups, which can give the final dispersions and coatings a high degree of color.6-8 Moreover, the side-chain unsaturated functional groups attached to the polymer backbone are typically C16-based esters, which make the coatings much more brittle than typical polyurethanes due to less-effective polymer alignment and packing. This is analogous to the effect of the side chains in poly(meth)acrylate type polymers. Carbonyl-amine crosslinking has also seen significant use in waterborne polymer crosslinking and can provide significant improvement in resistance properties of polyurethanes, however, it has been noted that the chemistry suffers from sub-par water resistance in the cured coatings.8 The reaction of carbonyls with amines is triggered by a drop in pH and loss of water; an abbreviated depiction of the reaction is also shown in Figure 1 and involves several intermediates (not depicted) to get to the final imine or azomethine product. A third option for crosslinking waterborne polymers depicted in Figure 1 is through acetoacetoxy functionality either by enamine formation via reaction with polyamines or Michael reaction by nucleophilic addition to a Michael acceptor such as an acrylate group. These reactions can be a rapid and very effective means of crosslinking, however, issues revolve around hydrolytic stability of the ester, premature crosslinking and the potential to obtain undesirable color in the dispersion and applied coating.9, 10 The formation of the chromophore responsible for the color is likely related to the reaction of various amines with the acetoacetoxy groups,10, 11 some of which are typically present in polyurethane synthesis. TABLE 1 | Chemical resistance of 1K SCL PUD versus 2K commercial controls after 7 days at room temperature (0 = highly effected/damaged, 5 = no effect/damage). Test Water (16 hr) 409 Cleaner (1 hr) Spic & Span (1 hr) Windex (1 hr) 1.4% Ammonia (1 hr) Vinegar (1 hr) Grape juice (1 hr) 50% Clorox bleach (1 hr) Olive oil (16 hr) Mustard (6 hr) 50% Ethanol (1 hr) 70% IPA (1 hr) Red Wine (6 hr) Overall Total 36 Experimental Formulation The Lubrizol 1K self-crosslinking (SCL) PUD was formulated with 13.7% diproplylene glycol n-butyl ether versus polymer solids and reduced to 30% solids; the final dispersion has a VOC of ~269 g/L and is a film former at 4 °C on uncoated paper (porous substrate). The commercial 2K PUDs were purchased from a local contactor supply outlet and used as is; the containers indicated the VOCs were <350g/L. Chemical Resistance Testing followed ASTM method D 1308. Coatings are tested by applying three coats of similar weight to maple veneer panels that are cured for the designated time (either 1 or 7 days) at ambient conditions before testing. One-hour spot tests are run on the surface of the finish. Sections of cotton facial pads are soaked in the test agent on the surface of the finish. After removal of the pad and test agent, the surface of the coating is blotted dry and observed for damage. The tested surface is re-evaluated after a one-hour recovery. Ratings are: 5 = no effect on finish; 4 = finish is slightly swelled or surface of finish is permanently changed; 3 = finish surface is changed and wood color has been impacted; 2 = more severe damage than 3; 1 = very severe damage to film or discoloration of wood; and 0 = finish is removed from the substrate. Tests for specific applications or customer end uses may dictate longer exposure times. Chemical resistance evaluations run for longer than an hour are covered, using either watch glasses or a similar device to keep the test agent from evaporating. The rating system used above is also used for this testing. Chemical test agents that are typically evaluated for longer exposure times are: water, water/alcohol combinations, red wine, coffee, hand creams, and strong acids and bases. Black Heel Mark, Mar and Scuff Resistance The apparatus consists of a pivoting pendulum arm with a 2-lb weighted piece of hard, black rubber (such as that of a hockey puck) at the end meant to simulate a hard shoe sole capable of producing black heel markings. This is the striking surface to the test panel. The total length of the pendulum device from the arm pivot to the striking surface is ~38”. The test consists of striking the coated maple panel six times on different areas and direction on the panel with the pendulum black heel marking device. Striking is done by raising the pendulum arm to an angle of 90 – 94° of the coated panel, which is placed on the floor, and then releasing the arm under the force of gravity to strike the coated panel at the designated area. Results are expressed by visual assessment of the panel (scratching, scuffing, and black markings/discoloration) and gloss measurements. The test was evaluated and supported by statistical experimental analysis to other methods for predicting real floor wear resistance. Coating test samples were prepared as above for chemical resistance testing. 1K SCL PUD 5 5 5 5 5 5 5 5 5 (fades to pass) 2K NCO 5 5 5 5 4 5 5 5 5 (fades to pass) 2K Thermoplastic Aziridine PUD 5 5 5 5 5 5 5 5 5 (fades to pass) 5 4 4 4 4 5 5 5 5 (fades to pass) 3 3 3 3 4 4 5 61 4 4 5 60 5 4 5 62 1 1 5 51 FE BRUARY 2009 | W W W . P C I M A G . C O M http://WWW.PCIMAG.COM
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