Two-Component Weatherable Aliphatic Polyurethane Topcoat, Performance-Based

PAINT SPECIFICATION NO. 36 – Two-Component Weatherable Aliphatic Polyurethane Topcoat, Performance-Based Coating

Polyurethane Topcoat Application

1. Scope

1.1 This specification covers the requirements for a high-performance two-component UV-stable polyurethane topcoat (ASTM D 16, Type V polyurethane). It is intended as a topcoat that provides excellent color and gloss retention, not as a thick-section elastomeric coating.

1.2 Coatings meeting the requirements of this specification are generally suitable for exposures in environmental zones 1A (interior, normally dry), 1B (exterior, normally dry), 2A (frequently wet by fresh water, excluding immersion), 2B (frequently wet by salt water, excluding immersion), 3B (chemical exposure, neutral), and 3C (chemical exposure, alkaline).

1.3 The specified coating is intended for application by brush, spray, or roller. It is generally applied over a primer or intermediate coating.

2. Description
2.1 This coating is typically based on a hydroxyl-functional polyol resin and an aliphatic polyisocyanate co-reactant.
2.2 WEATHERING LEVELS: This specification contains three levels of accelerated and South Florida weathering performance (see Section 6.2).

3. Referenced Standards
3.1 The latest issue, revision, or amendment of the refer­enced standards in effect on the date of invitation to bid shall govern, unless otherwise specified. Standards marked with an asterisk (*) are referenced only in the Notes or the Appendix, which are not requirements of this specification.
3.2 If there is a conflict between the requirements of any of the cited referenced standards and this specification, the requirements of this specification shall prevail.

3.3 SSPC STANDARD:

3.4 ASTM INTERNATIONAL STANDARDS:
1D 16 Standard Terminology for Paint, Related Coatings, Materials, and Applications
D 523 Standard Test Method for Specular Gloss
* D 562 Standard Test Method for Consistency of Paints Measuring Krebs Unit (KU) Viscosity Using a Stormer-Type Viscometer
* D 968 Standard Test Methods for Abrasion Resistance of Organic Coatings by Falling Abrasive
D 1014 Standard Practice for Conducting Exterior Exposure Tests of Paints and Coatings on Metal Substrates
* D 1296 Standard Test Method for Odor of Volatile Solvents and Diluents
* D 1308 Standard Test Method for Effect of Household Chemicals on Clear and Pigmented Organic Finishes
* D 1475 Standard Test Method for Density of Liquid Coatings, Inks, and Related Products
* D 1535 Standard Practice for Specifying Color by the Munsell System
* D 1640 Standard Test Methods for Drying, Curing, or Film Formation of Organic Coatings at Room Temperature
D 1849 Standard Test Method for Package Stability of Paint
D 2244 Standard Practice for Calculation of Color Tolerances and Color Differences from Instrumentally Measured Color Coordinates
* D 2369 Standard Test Method for Volatile Content of Coatings
* D 2371 Standard Test Method for Pigment Content of Solvent-Reducible Paints
* D 2621 Standard Test Method for Infrared Identification of Vehicle Solids from Solvent-Reducible Paints
* D 2794 Standard Test Method for Resistance of Organic Coatings to the Effects of Rapid Deformation (Impact)
* D 3359 Standard Test Methods for Measuring Adhesion by Tape Test
* D 3719 Standard Test Method for Quantifying Dirt Collection on Coated Exterior Panels
D 3925 Standard Practice for Sampling Liquid Paints and Related Coatings
* D 4214 Standard Test Methods for Evaluating Degree of Chalking of Exterior Paint Films
D 4587 Standard Practice for Fluorescent UV-Condensation Exposures of Paint and Related Coatings
* D 5402 Standard Practice for Assessing the Solvent Resistance of Organic Coatings Using Solvent Rubs
* D 5590 Test Method for Determining the Resistance of Paint Films and Related Coatings to Fungal Defacement by Accelerated Four-Week Agar Plate Assay

ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959. Standards available online from http://www.astm.org.

3.5 AMERICAN NATIONAL STANDARDS INSTITUTE (ANSI) STANDARD:

³Z129.1 Hazardous Industrial Chemicals – Precautionary Labeling

3.6 AMERICAN ASSOCIATION OF STATE HIGHWAY AND TRANSPORTATION OFFICIALS (AASHTO) STAN­DARD:³

TP 67-03 Standard Method of Test for Analysis of Structural Steel Coatings for Isocyanate Content

4. Composition Requirements
4.1 RESIN REQUIREMENT: The curing agent shall be an aliphatic polyisocyanate. The total coating binder solids shall be made up of at least 17% polyisocyanate by weight. The method and formula for determining isocyanate content, developed by AASHTO as TP 67, is shown in the Annex.
4.2 VOC CONTENT: See Note 10.1.

5. Requirements of Liquid Coating
5.1 PACKAGE STABILITY: Package stability shall be tested in accordance with ASTM D 1849. Storage conditions shall be 30 days at 52 ± 1 °C (126 ± 2 °F). A change in consis­tency of greater than 10 Krebs units or noncompliance with the application requirements shall be cause for rejection.
5.2 APPLICATION PROPERTIES: All guidance provided by the manufacturer regarding mixing of multicomponent products, thinning requirements, induction times, and special application requirements shall be followed. Following the manufacturer’s recommendations, the coating shall be easily applied by brush, roller, or spray. The coating shall show no streaking, running, sagging, or other defects during applica­tion or while drying.

6. Weathering Requirements
6.1 WEATHERING RESISTANCE: Two weathering test procedures are specified below. Test data may not be avail­able for the desired color and gloss specified. Performance levels are established using whites and light colors. It is left to the discretion of the specifier whether to accept Florida data from a similar color or to use UV-A data in lieu of Florida weathering data.
6.1.1 Accelerated Weathering: Accelerated weathering shall be performed in accordance with ASTM D 4587, Cycle 2. Test panels shall have a primer/intermediate coat as recom­mended by the manufacturer.
6.1.2 South Florida Weathering: South Florida weath­ering shall be performed in accordance with ASTM D 1014 (45 degrees south exposure, washed with mild detergent). Test panels shall have a primer/intermediate coat as recommended by the manufacturer.
6.1.3 Application shall be according to manufacturer’s written recommendations. A minimum of three panels shall be exposed. Clear coatings must be tested over the intended basecoat. Initially, and at the end of each level period, 60 degree gloss shall be measured according to ASTM D 523 and tristimulus color shall be measured according to ASTM D 2244. Table 3 summarizes the tests of Section 6 as well as the minimum acceptance criteria.
6.2 Performance Levels: Tables 1 and 2 specify the amount of time the coating must perform before noticeable change in order to achieve the defined performance level. Polyurethane topcoats are available in a wide range of color and gloss. Procurement documents shall state the desired level of performance, exposure method (in accordance with Tables 1 and 2), color, and gloss. For example, a specifier may require Level 2 Florida exposure, initial gloss greater than 80, matched to a specific color. A certain level of accelerated weathering does not necessarily correspond to a particular level of atmospheric weathering and they need to be specified independently. These are two independent complementary tests for measuring coating performance. If no level is speci­fied, Level 3 will be assumed.

7. Material Quality Assurance
7.1 Tests may be used to determine the acceptability of a lot or batch of a qualified coating (see Note 10.3.)

8. Labeling
8.1 Labeling shall conform to ANSI Z129.1.
8.2 Technical data shall be provided for at least all data elements categorized as “essential” in SSPC-Guide 13. The specifier may require additional data to satisfy quality assur­ance or local regulatory requirements.

9. Disclaimer
9.1 While every precaution is taken to ensure that all infor­mation furnished in SSPC standards and specifications is as accurate, complete, and useful as possible, SSPC cannot assume responsibility nor incur any obligation resulting from the use of any materials, coatings, or methods specified herein, or of the specification or standard itself.
9.2 This specification does not attempt to address problems concerning safety associated with its use. The user of this specification, as well as the user of all products or practices described herein, is responsible for instituting appropriate health and safety practices and for insuring compliance with all governmental regulations.

10. Notes
Notes are not requirements of this specification.
10.1 VOC CONTENT: Each coating, after recommended thinning, must conform to published government regulations regarding volatile organic compound (VOC) content. VOC information should be supplied on the label or the technical data sheet. Various governmental agencies may have different VOC limits or use different methods of testing. The owner may modify this specification as necessary to specify a particular VOC content limit consistent with local regulations. Coatings meeting the composition and performance requirements of this specification usually have a VOC level less than 420 g/L (3.5 lb/gal).
10.2 Test results for color and gloss of deeper colors can be expected to vary significantly due to differences in pigment weathering properties. If a dark color is specified, the user is cautioned to verify that color and gloss retention meet specifi­cation requirements for the desired level of performance. The coating manufacturer should be contacted if the user desires a higher degree of color stability (less than 2.0 ∆E* C.I.E.1976 L*A*B*) for a specific application. Choice of color (including metallics) outside the range of this specification may require consideration of additional procedures (such as clear coating) to meet the requirements of Section 6.2.

10.3 QUALITY ASSURANCE TESTS: The quality assur­ance tests of polyurethane topcoat are used to determine whether the supplied products are of the same type and quality as those originally tested. The selected tests should accurately and rapidly measure the physical and chemical characteristics of the coating necessary to verify that the supplied material is substantially the same as the previously accepted material. All of the quality assurance tests must be performed on the originally submitted qualification sample. The results of these tests are used to establish pass/fail criteria for quality assurance testing of supplied products.
10.3.1 Establishing Quality Assurance Acceptance Criteria: Many ASTM test methods contain precision and bias statements. Specification developers should be cognizant of the fact that these statements exist. Quality assurance test criteria should not be more stringent than the inter-laboratory precision of the test methods used. Where precision and bias data are not available for a given test method, determine the standard deviation of a minimum of five measurements taken on the originally tested and certified material. The pass/fail criterion is that the measurement of the test sample shall fall within two standard deviations of the target value. The contracting parties must agree on a target value.
10.3.2 Quality Assurance Testing Procedures: Quality assurance tests include but are not limited to: infrared analysis (ASTM D 2621), viscosity (ASTM D 562), weight per gallon (ASTM D 1475), total solids (ASTM D 2369), dry time (ASTM D 1640), percent pigment (ASTM D 2371), gloss (ASTM D 523), color (ASTM D 1535), and odor (ASTM D 1296), and AASHTO procedure TP 67.
10.4 The procurement documents should establish the responsibility for samples, testing, and any required affidavit certifying full compliance with the specification.

Appendix: Optional Laboratory Physical Tests of Applied Films
Typical properties that might be expected of a quality poly­urethane topcoat are described in Table 4 with corresponding ASTM test methods and suggested test requirements.

ANNEX: METHOD FOR DETERMINING ISOCYANATE CONTENT (AASHTO TP 67)

A1. SCOPE
A1.1 This method is designed for the determination of the isocyanate content of the resin system found in polyurethane topcoat paints. The reported isocyanate content is based upon 100% resin solids.

2. APPARATUS AND MATERIAL

3. PROCEDURE

A4. NOTES
A4.1 Methanol and o-xylene used for this titration should be passed through separate columns containing 3A molecular sieve to remove any moisture that may be present. A recom­mended flow rate for passing methanol or o-xylene through a drying column is approximately 100 mL per minute. Dried stock may be collected into amber bottles and preserved by placing a small amount of activated 3A molecular sieve in the bottles.
A4.2 Separation of pigment and resin of one-component materials may be aided by diluting the material up to the maximum volume of the centrifuge tube using dry o-xylene. Seal the centrifuge tube and shake the sample to obtain a homog­enous sample prior to placing sample in the centrifuge.

5. CALCULATIONS

A5.1 Calculation for Relative Centrifugal Force (RCF):

RCF = 0.00001118 • r • N²

Where, RCF = Relative Centrifugal Force r = Rotating radius (cm) N = Revolutions per minute

A5.2 Calculation of weight percent resin solids of the resin solution:

^W2 SR = • 100% ^W1

Where, S_R = Weight percent solids of the resin solution W₂ = Weight of sample after drying W₁ = Initial weight of sample

A5.3 Calculation of the appropriate amount of resin sample to be used:

84 W = NCO • S_R

Where, W = the weight in grams of the resin solution to be used 84 = the equivalent weight of a diisocyanate group NCO = the expected % NCO in the resin system S_R = the percent weight solids of the resin solution

A5.4 Calculation of the weight percent NCO of the sample:

0.042 • N • (B – S) % NCO = • 100% W • S_R

Where, 0.042 = the equivalent weight of an NCO group N = the normality of the hydrochloric acid solution in equivalents per liter B = the amount of titrant used to titrate the blank in milliliters S = the amount of titrant used to titrate the sample in milliliters W = the weight of the resin solution sample in grams SR = the percent weight solids of the resin solution

A6. REPORT
A6.1 Report the percent weight solids of the resin solution to the nearest 0.01%.
A6.2 Report the resin solution sample weight to the nearest milligram, 0.001 g.
A6.3 Report the percent NCO of the resin system to the nearest 0.1%.

PAINT SPECIFICATION NO. 36 – Two-Component Weatherable Aliphatic Polyurethane Topcoat, Performance-Based