Raw Material Influences on Weatherability

In order to develop high weatherability sealants, a solid understanding of the factors influencing durability and weathering behavior is necessary. Foremost among these factors are the raw materials that comprise the formula of the sealant. In this chapter the raw material influences on sealant durability will be dis­cussed using selected examples of one-component polyurethane (PU-1) and modified silicone [4] (MS-1) sealants. For the MS-1 model formulas discussed in the paper, two kinds of binders were used: a modified silicone polymer with a polypropylene oxide backbone (in the following, referred to as modified silicone) and an acrylic-containing modified silicone polymer (in the following, referred to as acrylic modified silicone or acrylic MS) [5].

The examples given in this paper are meant to illustrate some of the challenges that can arise when formulating weather-resistant materials. In each of these examples, all raw material amounts, except the

TABLE 1—Influence of binder type on weathering behavior of an MS-і sealant

thickness,

mm

weathermeter,

h

backbone)

(acrylate)

(acrylate backbone)

Cracking15

Chalking0

Cracking15

Chalking0

Crackingb Chalking0

0.2

800

5

5

1

1

1 1

1350

5

5

1

1

1 1

1900

1

1

1 1

2950

2

1

1 1

5.0

800

5

5

1

1

1 1

1350

5

5

1

t

1 1

1900

3

5

1 1

2950

3

1

1 1

50 % modified silicone

Sealant Exposure time in Modified silicone (polypropylene oxide)/50 %

film carbon arc (polypropylene oxide acrylic modified silicone Acrylic modified silicone

a54 r/( Prepolymer, 27 % plasticizer, no filler; samples exposed in an open flame carbon arc wealhermeter5.

"Cracking 1: no cracks; 2: very small cracks; 3: small cracks; 4: large cracks; 5: entire surface covered with large cracks. ‘Chalking 1: no change; 2: slight change in gloss; 3: onset of chalking;. 4: chalking: 5: severe chalking.

one being studied, were kept constant; the formulas vary, however, between the examples given in the different tables. Hence, the data only allows for a relative comparison within each group discussed (within each table).

The accelerated weathering data discussed in this paper was generated using two kinds of accelerated weathering sources: open flame carbon arc [6] and xenon arc testing [7]. The accelerated weathering data was used for a relative ranking within each example only. A comparison of the influences of these two irradiation sources for a given sealant formula, however, is beyond the scope of this paper.