Reducing Tin and Aminosilane Concentration in Silicone Elastomeric Coating to Improve Its Durability

ABSTRACT: This study evaluated the impacts of tin and amino-functional silane on the long-term durability of a silicone coating material.

An experimental design was applied. Tin and aminosilane’s concentrations were the variables. Laboratory-made samples were tested for initial mechanical properties. Samples were further placed in the QUV accelerated weathering chamber (fluorescent UV and condensation method) and tested periodically for tensile and elongation properties within the durability evaluation.

Based on the observations of the material and the measurements of mechanical properties, the concentration of tin in the formulation has the most influential impact on durability. The higher the concentration of tin, the faster the chalking. The concentration of the aminosilane also showed similar impacts on the durability, but not as significant as tin.

This study suggested that it is feasible to reduce both tin and aminosilane’s amount by 30 % without significant impacts on the material’s properties, and this may improve the coating’s durability by more than 50 %. This finding may also apply to silicone sealant and adhesives.

KEYWORDS: Silicone coating, durability, QUV, mechanical property, tin catalyst, aminosilane

Introduction

Silicone materials have an unusual combination of properties that are retained over a wide temperature range (-100 to 250 °С). They have good low temperature flexibility. They are very stable at high temperature, during oxidation, in chemical and biological environments, and when subject to weathering. Silicones also have good dielectric strength and water repellency. Silicone materials are produced in the forms of fluids, resins, and elastomers. Among them, elastomer applications include sealants, coatings, adhesives, gaskets, tubing, hoses, electrical insulation, and a variety of medical applications [1].

This study evaluated the impacts of two formulation components, tin and amino-functional silane, on the long-term durability of a silicone coating (sealer) material.

Previous studies by Dow Coming indicated the coating might chalk over time after application, and there were several components in the formulation that may affect the coating’s durability. Among them, tin and the aminosilane were the focus in this study.

Experiments

In the study, an experimental design [2] (DOE) was applied. The concentrations of organotin catalyst and aminosilane adhesion promoter were chosen as the two variables in the design. Eight samples were prepared with different tin and aminosilane’s concentrations in the formulation. The silicone coating samples were made using a laboratory mixer. Table 1 lists the tin and aminosilane’s levels in the eight samples.

TABLE 1—Tin and aminosilane levels in the DOE design samples.

Sample ID

Tin (ppm)

Aminosilane (ppm)

1

1800

1800

2

1800

1800

3

2700

2700

4

750

750

5

750

2700

6

1800

1800

7

1800

1800

8

2700

750

In sample #3, the amount of tin and aminosilane were the same as the coatings that had been demonstrated to perform well. So sample #3 represented the normal coating formulation, while other samples had either or both tin and aminosilane levels at lower than normal concentrations. All samples were tested for viscosity and cure rate (using tack free time). Viscosity was measured using a Cammed Rheometer (Model CSL 500). Tack free time was determined using ASTM C 679-03: Standard Test Method for Tack-Free Time of Elastomeric Sealants. Table 2 summarizes the viscosity and cure rate for all the samples.

TABLE 2—Samples ’ initial properties.

Sample ID

Viscosity (poise)

Tack free time (min)

1

620

44

2

588

44

3

631

28

4

587

240

5

732

150

6

624

44

7

615

43

8

525

29

Samples made with the lowest concentration of tin (#4 and 5) need much longer time to cure. Samples # 1, 2, 6, and 7, which had about 33 % less in tin also had a bit longer cure time, but were still within an acceptable range. Aminosilane’s level did not seem to impact cure time, considering that the sample #8 had the same cure time as the normal sample #3. Viscosity results also had some variations, but all in acceptable range. Samples #5 and 8 had viscosities on the high and the low side, respectively, which could be explained by the low levels of tin or aminosilane.

All samples were left on a plastic sheet at ambient temperature, 50 % humidity to cure for 28 days. The cured samples were tested for the mechanical properties (tensile strength and elongation). Then they were put into QUV [3] chamber (fluorescent UV and condensation method) and were periodically checked for chalking and measured for the mechanical properties. The QUV chamber was used as an accelerated weathering tester to reproduce the damage caused by sunlight, rain, and dew. ASTM G 154-04: Standard Practice for Operating Fluorescent Light Apparatus for UV Exposure of Nonmetallic Materials describes the basic principles and operating procedures for using fluorescent UV light and water apparatus. The QUV tests materials by exposing them to alternating cycles of UV light (340 nm lamp) and moisture at controlled, elevated temperatures. The specific weathering cycles employed in this study consisted of 4 h of UV light at 60°C followed by 4 h of condensation at 50°C [3].

In a few days or weeks, the QUV reproduces the damage that occurs over months or years outdoors [3]. Table 3 summarizes the observations on the chalking of the samples. Tables 4 and 5 summarize the tensile and elongation [4] results of the samples during the QUV exposure. The test method employed for determining tensile strength and elongation was ASTM D 412- 98a(2002)el: Standard Test Methods for Vulcanized Rubber and Thermoplastic Elastomers – Tension. In Figs. 1 and 2, the tensile strength and elongation data of the samples are plotted versus QUV exposure hours.

TABLE 3—Chalking observations during QUV study.

Sample ID

Time to chalk (h)

10 000 h QUV

17 000 h QUV

1

~ 10 000

Minor chalk

Chalk

2

10 000-17 000

No chalk

Chalk

3

-3000

Chalk

Chalk badly

4

No chalk up to 17 000

No chalk

No chalk

5

No chalk up to 17 000

No chalk

No chalk

6

– 10 000

Minor chalk

N/A

7

10 000-17 000

No chalk

Minor chalk

8

5000-10 000

Minor chalk

Chalk badly

TABLE 4—Tensile strength tests results during QUV exposure.

Sample

ID

Tensile Strength Data at Different QUV Exposure Hours (psi)

Oh

1000 h

2000 h

5000 h

10 000 h

17 000 h

1

424

427

611

463

470

554

2

406

474

556

445

460

441

3

428

534

590

447

470

364

4

405

504

621

462

490

506

5

356

407

518

415

420

433

6

386

461

554

424

470

N/A

7

417

472

589

445

500

451

8

418

468

614

437

440

461

Sample

Elongation Data at Different QUV Exposure Hours (%)

ID

Oh

1000 h

2000 h

5000 h

10 000 h

Ї 7 000 h

1

200

140

167

168

150

87

2

196

153

148

153

145

87

3

176

142

131

130

66

34

4

249

202

197

193

176

128

5

237

197

191

196

165

118

6

186

151

141

145

140

N/A

7

196

164

158

155

156

96

8

194

126

125

109

105

59

TABLE 5—Elongation tests results during QUV exposure.

—Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 —Sample 6 —Sample 7 —— Sample 8