Laboratory Testing

Standard laboratory tests were performed as part of a routine comparison of the two sealant binders and to augment the exterior and accelerated weathering data. Results are presented in Table 5 and Fig. 12.

Adhesion and Joint Movement Performance-The acrylic sealant has excel­lent wet and dry peel adhesion to mortar, with no adhesive failure and peel strengths substantially higher than the 5 lbf (22.2 N) required by ASTM c920- 05. The polyurethane sealant also passes ASTM C920-05 peel adhesion require­ments but with somewhat lower peel strengths. The acrylic sealant passes ±25 % ASTM C719-93 joint movement testing on concrete mortar with no adhesive or cohesive failure. The polyurethane sealant unexpectedly fails this test in the first room temperature cycle. Since the manufacturer’s technical data sheet (TDS) clearly indicates that this sealant passes ±25 % joint movement testing to mortar, the premature failure noted in this evaluation may be due to the fact that these sealants were tested without the use of primer. The adhesion and

TABLE 5—Adhesion, joint movement, hardness, and tensile properties.

Properties

Acrylic

Polyurethane

Adhesion to mortar (ASTM C794-06) Dry, lbf (N)

Wet, lbf (N)

24 (106) C 15 (66) C

8 (35) C 10 (44) C

±25 % joint movement to mortar (C 719-93)

Pass, no failure

Fail, 1st RT cycle

Durometer hardness (ASTM C661-06)

29±2

16±2

Tensile properties at 2.0 in./min (5.1 cm/min) Stress at 25 % elongation, psi (MPa) Maximum stress, psi (MPa)

Elongation to break (%)

36±1 (0.25±0.01) 143±1 (0.99±0.01) 647±9

13± 1 (0.09 + 0.01) 84±3 (0.58 + 0.02) 297±12

joint movement properties in Table 5 and in the polyurethane TDS are consis­tent with the El Paso warehouse exposures where mortar adhesion is excellent and where the joint movement capabilities of the sealants are clearly adequate for the movement encountered.

Hardness and Tensile Properties—The acrylic sealant has substantially greater elongation than does the polyurethane sealant, a property generally associated with higher performance and greater joint movement capability. The measured hardness and stress values of the acrylic sealant are also higher than those of the polyurethane sealant but well within the ranges that are typical for

image285

FIG. 12—The effect of rate of testing on stress at 25 % elongation of the acrylic and polyurethane sealants.

ASTM C920-05 Class 25 formulations. These measured differences are appar­ent in the field, where samples of acrylic sealant pulled from a joint are slightly harder and stiffer than pulled samples of the polyurethane sealant.

When comparing the tensile properties of sealants based on different chem­istries, it is essential to do so with an understanding of the differing viscoelastic natures of these chemistries. Acrylic sealants are typically more viscoelastic than are more heavily cross-linked sealants based on reactive chemistries such as silicones and polyurethanes. Because of this, the mechanical properties of acrylic sealants are more strain rate dependent than are those of sealants based on reactive chemistries. When deformed quickly (such as at the rates typically found in the laboratory), acrylic sealants are often harder and stiffer (with higher stress or modulus values) than their reactive chemistry counterparts. However, when deformed slowly (such as at the rates typically encountered outside), the properties of acrylic sealants fall in line with those of alternative chemistries.

The differences in the strain rate response of the tested acrylic and poly­urethane sealants are illustrated in Fig. 12, where stress at 25 % elongation (a measure of sealant stiffness) is plotted against the rate of tensile testing. Tensile measurements are generally done at rates of testing which are convenient for generating data in a timely manner. ASTM D412-06a, the commonly referenced “Standard Test Methods for Vulcanized Rubber and Thermoplastic Elastomers—Tension” [10] specifies that tensile testing be done at 20 in. (508 mm)/min. The author’s laboratory routinely uses 2 in. (51 mm)/min for tensile testing as a more reasonable compromise between timely data generation and real world deformation rates. However, both of these testing rates are several orders of magnitude greater than the rate of deformation in ASTM C719-93 joint movement testing (2 X 10-3 in.(5 X 10-2 mm)/min) and the rates of joint movement likely to be encountered in exterior low rise masonry buildings (2-5 X 10-4 in.(5-13 X 10-3 mm)/min) [11,12]. These high rates of tensile testing over emphasize the differences in mechanical properties between strain rate independent elastomeric sealants and more strain rate dependent vis­coelastic sealants.

When the rate of sealant testing is reduced from 2.0 in. (51 mm)/min to a more appropriate rate of 0.02 in (0.5 mm) /min, the stress of the acrylic sealant at 25 % elongation converges on that of the polyurethane sealant, minimizing the perceived differences between the two sealants.

Conclusions

The El Paso warehouse exposure provides a unique, side by side, comparison of a high performance acrylic sealant to a commercial two part polyurethane seal­ant. After 3 years of exterior exposure the polyurethane sealant continues to function as a sealant, with good adhesion and adequate joint movement capa­bility for the application. However, the polyurethane sealant exhibits consider­able crazing, chalking, and softening as the result of exposure, and the function of at least one sealant joint appears to have been compromised by crazing through to the underlying backer rod. Accelerated weathering data generally support these observations.

After 3 years of identical exposure, the acrylic sealant also continues to perform as a sealant, with good adhesion and adequate joint movement capa­bility. The acrylic sealant has comparable dirt pickup to the tested polyurethane sealant and better exterior durability (i. e., little crazing and no chalking or softening). Laboratory test results and accelerated weathering data support and confirm these results. The lack of plasticizer in the acrylic sealant formulation eliminates plasticizer migration and dirt pickup of coatings applied over the sealant joint. Feedback from the moisture-proofing contractor suggests that the properties of the wet acrylic sealant require minimal adjustment for optimal application and that the water cleanup of the acrylic sealant is a distinct ad­vantage from convenience, safety, and environmental points of view.

The data presented herein represent the results of 3 years of exterior and laboratory testing of a high performance acrylic sealant and a commercial two part polyurethane sealant. The results, of course, pertain to the sealants tested and are not necessarily representative of the performance of all high perfor­mance acrylic and polyurethane sealants. Certainly there are commercial ure­thane products on the market, which will out perform the product tested in this evaluation. However, the combined results of this side by side comparison clearly demonstrate the adhesion, durability, and aesthetic benefits of the tested acrylic sealant. While these results cannot necessarily be extrapolated to all high performance acrylic sealants, they do suggest that these products can be highly suitable for use in low rise industrial applications such as tilt-up ware­houses.

[1]FlackTech, Inc., 1708 Highway 11, Bldg. G, Landrum, SC 29356.

[2]SURA Instruments GmbH, Jena, Germany.

[3]DELO Industrial Adhesives, Windach, Germany.

[4]Rocatec™-Pre, Rocatec™-Plus, ESPE™ SIL: 3M Deutschland GmbH, Neuss, Germany.

[5]Plasmatreat GmbH, Steinhagen, Germany.

[6]LM = Low Modulus; 25: ±25% movement capability according to ISO 9047; G = Glazing applications.

[7]Test condition: irradiation energy= 180 W/m2 (300-400 nm), black panel temperature at 63 ° C, water spray for 18 min within 120 min weathering cycle, SUGA TEST INSTRU­MENTS SX-120.

[8]Test condition: black panel temperature at 63 ° C, water spray for 18 min within 120 min weathering cycle.

[9]All of the compared materials, silicone, polysulfide, and STPE, are commercial products of low modulus two-part sealant in Japan.

[10]Test condition: panel angle = 60°, direction = south, at Takasago, Hyogo, Japan.

[11]Test method of oil resistance: a piece of cured polymer (2 mm thickness) was immersed in certain oil at certain condition exhibited on Table 4; then the sample was taken out, swiped on the surface, and the weight change from the original was measured at room temperature.

[12]Silicone sealant was tested in the same condition for IRM 903 oil. After the test, bleed­ing out of the oil was observed on the surface of test piece. Such a phenomenon was not observed for STPA.

[13]JASS-8-2008 shows appropriate choice of sealant materials for each type of construc­tion application, and STPE and PU are not suitable for glass glazing use.

[14]This assumption neglects inertia dominated cases like bomb blast loading and related high speed phenomena.

[15]Fast curing mortar according to the manufacturer’s description.

[16]Nominal 50 mm/min.

[17]In addition, one should note that in the numerical model, the interfaces are geometric boundaries with stepped properties, while in nature, interfaces at least of the glass sur­faces show a different behavior on the micro-scale level [10].

[18]In this case, only load transfer along the symmetry axis is considered.

[19]An adequate flange length Lf is assumed.

[20]Certain commercial materials and equipment are identified in this paper in order to specify adequately the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply necessarily that these items are the best available for the intended purpose.

[21]ISO 11600-F-25LM; ASTM C920, Type S, Grade NS, Class 35, Use NT, M, A, G, and 1; Canadian Specification CAN/CGSB-19.13-M87, Classification MCG-2-25-A-N.

[22]JIS A 1414-1: Water-Spray rate 0.4 L/(min-m2), Maximum test-pressure: 2303 Pa, Minimum test-pressure: 49 Pa.

[23]ASTM E331-00: Water-spray minimum rate of 3.4 L/(min/m2), Test-pressure of at least 137 Pa.

[24]Konica Minolta Sensing Americs, Inc., Ramsey, New Jersey 07446, USA. The Minolta Model CR-231 Chroma Meter color analyzer has a 25 mm diameter measuring area, 45° illumination angle, and 0° viewing angle. Illuminant: D65. Color measurement according to ISO 7724. Color-coordinates: CIELAB.

[25]Atlas Material Testing Technology, Chicago, Illinois 60613, USA. The Ci65A Xenon Weather-Ometer has a 6500 W water cooled xenon arc lamp and a total exposure area of 11 000 cm2.

[26]Q-Laboratory Corporation, Cleveland, Ohio, 44145, USA.

[27]PVC is the fractional volume of a pigment in the total volume solids of a dry paint film.

[28]Tinius Olsen Testing Machine Co., Inc., Horsham, PA 19044, USA.