Approach Toward a Performance-Related Material Identification and Deduction of Mechanical Characteristics by DMA Methods

Earlier Development of DMA Methods—Although dynamic-mechanical ma­terial analysis is a very common tool in polymer technology (even in the explo­ration of visco-elastic materials), and in spite of the predominant influence of shear loads on the material performance in this specific field of adhesive appli­cation, its adaption in the field of SSG sealant materials for the performance – based identification, quality protection, and further material exploration is still

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Performance-oriented System Design

■ Improved Construction and Design supported by numerical analysis • Optimized Technological methodology

FIG. 3—Schematic of proposed methodology for a complementary approach.

in an early-development stage. In a recent review of silicones in industrial applica­tions [2] the application of DMA to determine the frequency-dependent material characteristic of silicone materials for electronic applications is reported. Temper­ature regions with fundamental mechanical phase transition are also reported for silicone-based coating materials in electronic technologies [3]. In recent years, the potential of dynamic-mechanical material analysis for sealant characterization is increasingly exploited. Its suitability for a comparative exploration of temperature-dependent visco-elastic properties of epoxy, acrylate, and silicone sealants is reported by Weller and Nicklisch [4] as one of the latest research trends. Gordon et al. [5] extend the use of DMA into the field of fatigue investiga­tions, taking the importance of cycling shear loading into account. Based on our own experiences in the field of joint sealing compounds for pavements [6,7] this contribution builds on these suggestions for an extended dynamic-mechanical material test methodology—not only for material identification—but also for tech­nological as well as durability aspects. These considerations were substantially advanced by the latest improvements in measurement technology.

More Recent Developments—Technical developments in measurement tech­niques and device development over recent years extended the range of use of DMA not only for the testing of solids but also permitted superimposing both mechanical (even multi-axial) and climate loading (as illustrated in Fig. 4). Examples of these technical developments are:

• rotational motor with speed ranges over nine decimal orders of magni­tude and friction-free air bearing with high stiffness (torsional moment up to 300 mNm),

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FIG. 4—Schematic illustration of dynamic-mechanical material testing.

• optical encoder for deformation measurement with high resolution (~0.1 irad), and

• additional independent force control for applied normal forces.

Using the advantages of such a measurement technique, a comprehensive material characterization can be obtained for structural sealants in every mate­rial phase or phase of working life (e. g., wet, pre-cured, during cure, post-cure, in solid state, under fatigue, artificially aged, etc.). This permits investigations of technical characteristics of sealant materials when wet and soft (prior to being applied), thereafter determining the curing behavior and kinetics of cure, and allows extension to the analysis of the cured sealant at different life-cycle stages as well. Such evaluation may also include an analysis of the material characteristics of already-loaded or damaged material.

Apart from the characterization of the mechanical properties of sealant materials, this approach also permits determining the morphology of the poly­meric structure. Encouraged by our previous experiences in the development of a performance-related identification and classification of various polymers used in roadway construction [6-8], it is proposed that the same measurement tech­nique and methodology may be suitable for the development of methods adapted to the characterization of SSG sealant products.

The methodological approach for a characterization of SSG sealant prod­ucts and—based on the performance-based material exploration—in a second step, the extension to the exploration of system behavior is to be developed within a new research project, sponsored by the German Federal Ministry of Ec­onomics and Technology (BMWi) over the next 3 years as a cooperative effort between various industrial partners and the BAM Federal Institute for Materials Research and Testing, Berlin. This paper provides information on preliminary investigations on the use of DMA measurement methods adapted for silicone- sealant materials for SSG systems. In addition, an outlook on the complemen­tary system test methodology is provided to present the comprehensive charac­ter of the new approach.

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Experimental Studies

Basics—It is common practice in mechanical polymer analysis to describe the complex viscoelastic material behavior as shown in Eqs 1-3:

storage modulus: G’ = — • cos d

У0

[Pa]

(1)

loss modulus: G" = — • sin d

у0

[Pa]

(2)

G”

loss angle: d = arc tan [ G’

°]

(3)

shear deformation (SRF): у = (<c1,X b

(C2 X

if[39] [40] %

(4)

where s = shear stress and у = shear deformation (see also Fig. 4; for a more detailed definition of these parameters and their response to stress, deforma­tion, or temperature see Ref [9]). These characteristics subdivide the complex shear modulus G* into its viscous and elastic components (see Eq 5):

|G*|[41] = G’2 + G"2 (5)

The methodological tool for exploring these characteristics is the dynamic – mechanical analyzer (DMA) (see also Fig. 4). In addition to the mechanical ex­ploration of materials in various states and exposed to different loading histor­ies as offered by this method, it also provides a direct link to the assessment of the material’s inner structure (morphology). A modern dynamic spectrometer, such as the MCR 501 by Anton Paar GmbH2 used in this study, completed by special equipped climatic loading devices, is the experimental basis for the ma­terial analysis in different test modes (see Fig. 5).

The development of adapted performance-related material characterization using DMA and first investigations to verify this methodological concept on typ­ical SSG materials was done within the following categories:

1. identification of the complex mechanical material behavior,

2. response to technological characteristics such as curing kinetics and processing, and

3. response to fatigue and ageing effects.

Material—To transfer and validate this material test approach, specimens of three types of silicone-sealant products (characteristics are shown in Table 1)

FIG. 5—DMA-test modes in the universal spectrometer Physica MCR 501 by Anton Paar (for more detailed information, see Ref [9]).

were cast and cured in controlled laboratory conditions (curing time >20 days at + 23°C; relative humidity (RH) ~45%). The filler content was determined in accordance with the procedure laid out in the EOTA recommendation [1] with the help of thermogravimetric analysis (heating rate 10 K per minute).

Bar-shaped specimens with dimensions of 25 x 10 x 3 mm3 (length x width x thickness of test geometry between the clamps) were cut from labora­tory cured sheets of the sealants (3 weeks of cure) for the exploration of the complex dynamic-mechanical behavior in the solid state. For this study, the solid rectangular fixture was used for this DMA test mode. The technological and curing characteristics were investigated within this study by the plate-plate fixture on, respectively, soft and wet samples of the sealants. Analysis and inter­pretation of the measured parameters are discussed in detail by Mezger in his textbook [9].