Applying Aliphatic Polyurethane Topcoat
PAINT SPECIFICATION NO. 38 – Single-Component Moisture-Cure Weatherable Aliphatic Polyurethane Topcoat, Performance-Based
1. Scope
1.1 This specification covers the requirements for a high-performance single-component moisture-curing UV-stable polyurethane topcoat (ASTM D 16, Type II polyurethane). It is intended to be used as a topcoat that provides good color and gloss retention.
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 single-package moisture-curing polyurethane coating is characterized by the presence of free polyisocyanate groups capable of reacting with atmospheric moisture in order to form a film.
2.2 WEATHERING LEVELS: This polyurethane topcoat specification contains three levels of accelerated and South Florida weathering performance that are discussed in 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, 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:
Guide 13 Guide for the Identification and Use of Industrial Coating Materials in Computerized Product Databases
3.4 AMERICAN SOCIETY FOR TESTING AND MATERIALS (ASTM) STANDARDS:1
* D 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 Units (KU) Viscosity Using the Stormer-Type Viscometer
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 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 Test Method 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 2697 Standard Test Method for Volume Nonvolatile Matter in Clear or Pigmented Coatings
D 2794 Standard Test Method for Resistance of Organic Coatings to the Effects of Rapid Deformation (Impact)
D 3925 Standard Practice for Sampling Liquid Paints and Related Pigmented Coatings
D 4214 Standard Test Methods for Evaluating Degree of Chalking of Exterior Paint Films
D 4541 Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers
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
G 154 Standard Practice for Operating Fluorescent Light Apparatus for UV Exposure of Nonmetallic Materials
1 ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959. Standards available online from http://www.astm.org.
2 American National Standards Institute, 1819 L Street, NW, Suite 600, Washington, DC 20036. Standards available online from http://www. ansi.org.
3 American Association of State Highway and Transportation Officials, 444 North Capitol Street, NW, Suite 249, Washington DC 20001, 202624-5800 (http://www.transportation.org/aashto/home.nsf).
3.5 AMERICAN NATIONAL STANDARDS INSTITUTE (ANSI) STANDARD:2
Z129.1 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 resin used in the coating shall be a polyisocyanate-based product that cures through a chemical reaction with atmospheric moisture. Use of other resins or modifiers shall be minimal and limited to products necessary for formula ingredient compatibility and to improve recoating properties. Manufacturer’s formulations of single-component moisture-curing weatherable polyurethane topcoats normally contain levels of polyisocyanate in vehicle solids above 3.5% 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 12.1 for information on VOC content.
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 thinning requirements and special application requirements shall be followed (see Note 12.2). Following the manufacturer’s directions, the coating shall be easily applied by brush, roller, or spray. The coating shall have no streaking, running, or 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. [NOTE: Commercial test sites in North America conforming to this requirement are located in southern Florida and Arizona.] Other test sites may be used if agreed upon by the contracting parties.
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 with a curing duration as mutually agreed upon by the manufacturer and the contracting parties.
- Apparatus – Fluorescent UV/condensation apparatus complying with ASTM G 154.
- Procedure –Test according to ASTM D 4587 Cycle 2 procedure. This eight-hour duration comprises one test cycle. Continue cycle testing to achieve the exposure hours defined in the required performance levels (as shown in Table 1).
6.1.2 South Florida Weathering: South Florida weathering shall be performed in accordance with ASTM D 1014 (45˚ south exposure, washed with mild detergent before readings are taken.) Test panels shall have a primer/intermediate coat as recommended by the manufacturer.
6.1.3 Coating application shall be according to the manufacturer’s written recommendations. A minimum of three panels shall be exposed. Each replicate panel must meet the minimum acceptance criteria. Clear coatings must be tested over the intended basecoat. Initially, and at the end of the exposure period for each level, tristimulus color values shall be measured according to ASTM D 2244, and 60 degree gloss shall be measured according to ASTM D 523. Table 3 summarizes the tests of Section 6 as well as the minimum acceptance criteria. See Note 12.3 regarding test results for darker colors.
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. Laboratory Physical Tests of Applied Coatings
7.1 RECOATABILITY: Intercoat and substrate adhesion properties of the polyurethane topcoat shall be determined by measurement of adhesion values between:
- a) primer and the substrate,
- b) polyurethane topcoat and the primer or intermediate coat, and, if applicable
- c) the second coat of polyurethane topcoat and the same previously applied topcoat.
Coating manufacturers shall provide acceptable window (time frame) ranges between application of coatings in a multi-coat system. Unless the purchaser specifically requests a variance, the maximum dry film thickness and the minimum curing time (with reported conditions for temperature and humidity) as stated on the manufacturers product data sheets will be used for coating application prior to adhesion testing. All testing shall be performed on a minimum of three steel test panels each with minimum dimensions of 75 x 150 x 5 mm (3 x 6 x 1/4 inches). The panels shall be prepared to the minimum surface preparation recommendations of the coating manufacturer.
7.2 ADHESION
7.2.1 Primer Adhesion to Steel Substrate: Apply the primer to the steel panel and cure according to the manufacturer’s recommendations. Test adhesion according to ASTM D 4541, and identify the type of adhesion tester used (Type II, III, IV, or V). The minimum adhesion acceptance value of the primer to the substrate shall be 4.1 MPa (600 psi).
7.2.2 Polyurethane Topcoat Adhesion to the Primer: Apply topcoat to panels (previously primed as described in 7.2.1) and cure according to the manufacturer’s recommendations. Test the intercoat adhesion between topcoat and primer or intermediate coat in accordance with ASTM D 4541, and identify the type of adhesion tester used. Intercoat adhesion is defined as the force required to separate the topcoat from the underlying coat. The minimum intercoat adhesion acceptance value of the topcoat to the primer shall be 4.1 MPa (600 psi).
7.2.3 Polyurethane Topcoat Adhesion to (Same) Previously Applied Topcoat: Apply a second coat of topcoat, and cure according to the manufacturer’s recommendations. Test the intercoat adhesion in accordance with ASTM D 4541, and identify the type of adhesion tester used. The minimum intercoat adhesion shall be 4.1 MPa (600 psi).
7.3 Polyurethane topcoat cohesion (bond strength within a single layer of coating) shall be equal to or greater than the minimum acceptable intercoat adhesion.
7.4 Properties required of the polyurethane topcoat are described in Table 4, which includes the corresponding ASTM test methods.
8. Material Quality Assurance
Tests may be used to determine the acceptability of a lot or batch of a qualified coating (see Note 12.4).
9. Labeling
9.1 Labeling shall conform to ANSI Z129.1
9.2 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.
10. Inspection
10.1 All coatings supplied under this polyurethane topcoat specification are subject to timely inspection by the purchaser or an authorized representative. The purchaser shall have the right to reject any materials supplied that are found to be defective under this specification (see Note 12.5). In case of dispute, unless otherwise specified, the arbitration or settlement procedure established in the procurement documents shall be followed. If no arbitration procedure is established, a procedure mutually agreed upon between the purchaser and supplier shall be used.
10.2 Samples of paints may be requested by the purchaser and shall be supplied upon request along with the manufacturer’s name and identification, and batch number for the materials. Samples may be requested at the time the purchase order is placed or may be taken from unopened containers at the job site.
10.3 Unless otherwise specified, the sampling shall be in accordance with ASTM D 3925.
11. Disclaimer
11.1 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 specifi ed herein, or of the specification or standard itself.
11.2 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 ensuring compliance with all governmental regulations.
12. Notes
Notes are not requirements of this specification.
12.1 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)
12.2 These coatings may be susceptible to pinholing if applied under adverse conditions such as high humidity. The degree of pinholing depends on formulation variables. Consult the coating manufacturer’s recommendations for formulation additives that may be used to address this problem.
12.3 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 color stability less than 3.0 ΔE* 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 6.1.3.
12.4 QUALITY ASSURANCE TESTS: Quality assurance tests are used to determine whether the supplied products are of the same type and quality as those originally tested. The selected tests should rapidly and accurately 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.
12.4.1 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 interlaboratory precision of the test methods used. The specifier may require additional technical data to satisfy QA or local regulatory requirements.
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.
12.4.2 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), volume solids (ASTM 2697), dry time (ASTM D 1640), percent pigment (ASTM D 2371), gloss (ASTM D 523), color (ASTM D 1535, ASTM D 2244 ), and odor (ASTM D 1296), and AASHTO TP 67.
12.5 The procurement documents should establish the responsibility for samples, testing, and any required affidavit certifying full compliance with the specification.
ANNEX 1: METHOD FOR DETERMINING ISOCYANATE CONTENT (AASHTO TP 67)5
A1. SCOPE
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.
A2. APPARATUS AND MATERIALS
A2.1 Analytical balance
A2.2 High speed centrifuge
A2.3 Centrifuge tubes, minimum 50 mL heavy wall glass
with vinyl lined screw caps
A2.4 10 mL syringes with Luer slip tip and caps
A2.5 60 mm aluminum weighing dishes
A2.6 Drying oven 110 ± 5 °C
A2.7 Automatic titrator with stirrer (Mettler-Toledo DL 50 or equivalent)
A2.8 pH electrodes
A2.9 Fleaker or other vessel with stopper or cap, 200 mL minimum volume
A2.10 O-Xylene, reagent grade, dried over activated 3A Molecular Sieve (See A4.1)
A2.11 Buffer solutions, pH 4.00 and pH 7.00
A2.12 Dibutylamine, 99% assay
A2.13 Methanol, ACS reagent grade, dried over an activated 3A Molecular Sieve (See A4.1)
A2.14 Hydrochloric acid solution, 1.0 N
A3. PROCEDURE
A3.1 To analyze a one-component material, the resin system must first be separated from the whole paint.
A3.2 Place a 40 mL sample of the whole paint into a high-speed centrifuge tube and cap immediately.
A3.3 Centrifuge the sample to produce a minimum relative centrifugal force (RCF) of 4300 until the pigment and resin have separated. See A5.1 for RCF equation. See A4.2 for products requiring dilution to facilitate separation.
A3.4 No separation is required for multi-component materials, when the resin system is packaged separately. The resin system is generally labeled “curing agent” or “part B.”
A3.5 Determine the percent solids by weight of the resin solution (the resin solution is considered to be either the supernatant of a one-component material or the resin constituent of a multi-component material).
A3.6 Condition the 60 mm aluminum weighing dishes in a drying oven at 110 ± 5 °C for a minimum of 10 minutes and cool to ambient conditions in a desiccator
A3.7 Weigh 0.5 ± 0.1 g to 1.0 mg of the resin solution by difference from a syringe into a conditioned 60 mm aluminum weighing dish.
A3.8 Immediately place the sample into a drying oven at 110 ± 5 °C for 1.0 ± 0.25 hours.
A3.9 Allow sample to cool to ambient conditions in a desiccator.
A3.10 Weigh sample to 1.0 mg.
A3.11 Calculate the percent solids by weight of the resin solution. See A5.2 for percent solids calculation.
A3.12 Calibrate the titrimeter and pH electrodes with standard pH 7.00 and pH 4.00 solutions
A3.13 Place 25 ml of dry o-xylene into an appropriate size fleaker or vessel.
A3.14 Weigh in the appropriate amount of resin solution. See A5.3 for sample weight equation.
A3.15 Record the sample weight to the nearest 1.0 mg
A3.16 Add 20.00 ml of 1.7 N dibutylamine solution. Prepared by diluting 290 ml of dibutylamine to 1.0 L with dried o-xylene.
A3.17 Cap the container immediately.
A3.18 Allow the sample to stir for a minimum of 30 minutes with a magnetic stirrer but no longer than 60 minutes at room temperature.
A3.19 Prepare a blank in the same manner, omitting the resin solution.
A3.20 Wash down the sides of the containers with 100 mL of methanol.
A3.21 Titrate the stirred blank with 1.0 N hydrochloric acid through the end point, which occurs at pH 5.0. A3.22 Record the volume of titrant used to the nearest 0.01 mL.
A3.23 Titrate the stirred sample with 1.0 N hydrochloric acid through the end point, which occurs at pH 5.0.
A3.24 Record the volume of titrant used to the nearest 0.01 mL.
A3.25 Calculate the weight percent NCO. See A5.4 for weight percent NCO calculation.
5 This method is a draft method that was being reviewed by AASHTO, the American Association of State Highway and Transportation Officials as of January 1, 2003. In June 2003 this method was published as AASHTO TP 67-03.
SSPC-Paint 38 April 1, 2006
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.
A5. 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:
W
S= 2 • 100%
R
W1
Where, SR = 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 • SR
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 SR = 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 • SR 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. 38 – Single-Component Moisture-Cure Weatherable Aliphatic Polyurethane Topcoat, Performance-Based
