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Influence of different tempering conditions on the adhesion properties of thermoplastic/liquid silicone rubber combinations.


Since their development in 1970 [1], liquid silicone rubbers' demand has rapidly increased [2], due to outstanding performance characteristics. In 2010, LSR sales reached 11% in the world market of silicone elastomers [3], This trend is due to its distinguished thermal and mechanical properties including fast curability, excellent temperature stability, and good resilience [4], Therefore, LSR can be used as medical tubes, soothers, baking molds or seals in automotive applications [5, 6].

LSR is a high temperature curing silicone that consists of two components, A and B. Component A usually contains a polydimethyl siloxane with vinyl groups and a platinum catalyst. Component B consists of a polydimethyl siloxane with functional groups and a crosslinking agent. After mixing both components in a 1:1 ratio, the composition can quickly be cured at elevated temperatures (120[degrees]C-200[degrees]C) via hydrosilyzation [7], The platinum-catalyzed addition reaction takes place between the double bond of the two polydimethyl siloxanes which are functionalized with vinyl groups and the crosslinking agent, which contains Si--H groups [8].

LSR is processed by liquid injection molding (LIM). The components A and B are mixed with a static mixer. The mixture is then injected at room temperature (20[degrees]C) in a heated tool (120[degrees]C-200[degrees]C).

The combination of thermoplastics and LSR is also increasingly deployed in showerheads (consumer product) and as rain sensors in automotive industry. These examples show that there is no limit in the application of LSR as a soft component, since LSR materials possess an unique range of properties such as high temperature stability or physiological harmlessness.

The joining of the thermoplastic and LSR components can additionally be achieved via two-component injection molding. Compared to the commonly known production of fullthermoplastic hard/soft-parts (e.g., thermoplastic/TPE), the processing of thermoplastic/LSR-combinations is more complex [9]. The hot thermoplastic melt is injected into a cold mold (60[degrees]C-80[degrees]C) and in contrast the cold LSR mixture (20[degrees]C) is injected in a heated mold (120[degrees]C-200[degrees]C) [10]. The process starts by filling the first mold cavity with hot thermoplastic. Then when the carrier plate is quickly cooled down, the mold opens and rotates by 180[degrees]. The mold then closes and the LSR is injected on the side of the carrier plate. After a sufficient cross-linking time, the part is demolded. Besides the process handling with two different mold temperatures, it is coercively necessary for the production of this compound to consider the weakened adhesion between LSR and thermoplastic material [11]. To improve the adhesion between LSR and thermoplastic, an adhesion promoter can be added [12], This bonding agent has a chemical link for the LSR and chemical groups which are compatible with the thermoplastic. The bond strength is due to chemical adhesion and the formation of covalent bonds. Therefore, not every thermoplastic polymer is suitable to combine with LSR; for instance polyolefines possess no functional group to form chemical bindings [13]. Polar thermoplastics, i.e., polybutylene terephthalate (PBT) or polyamide (PA), exhibit functional groups for a possible bonding to the adhesion promoter and thus to the LSR.

To improve the mechanical properties of LSR, it is possible to undergo a post-treatment step such as tempering [14], with which it's post-crosslinking increases. Moreover, some volatile polymer components are removed. Usually, LSR products are tempered at 200[degrees]C for 4 h [15]. Because it is used with a thermoplastic, the tempering has to be adapted to the thermal properties of the carrier material. For standard thermoplastic materials (i.e., PA), a temperature of 200[degrees]C is too high, as at 200[degrees]C most polymers are no longer dimensionally stable. Therefore, the post-treatment temperatures have to be reduced to less than 150[degrees]C. Due to these mechanisms, the question arises if it is necessary to temper a thermoplastic/liquid silicone rubber compound. With the LSR containing an adhesion promoter, annealing can possibly lead to an increased effect of the bonding agent triggered by a higher temperature supply, which is also positive for the adhesion between the two materials. Ronnewinkel and Pohmer studied the effect of tempering on semicrystalline polyamide [2, 10], but not for amorphous materials. Furthermore, the combinations were only tempered at 140[degrees]C for 4 h so that the influence of lower temperatures and different tempering times is not clear. This article thus examines with influence of tempering the combined materials at elevated temperatures (80[degrees]C, 100[degrees]C, and 120[degrees]C) and varying times (1 h, 3 h, 6 h, and 9 h) on their adhesion mechanism and adhesion quality between the polymer and the LSR. Furthermore, the thermal and mechanical properties of the single components are to be examined.

The mechanical properties (e.g., stiffness or tensile strength) of the single components were analyzed via tensile tests in order to quantify the influence of tempering. Thermal characterization was additionally conducted by differential scanning analysis (DSC), thermogravimetric analysis (TGA), and dynamic mechanical analysis (DMA). These measurements show the influence of annealing on the molecular structure and its potential to develop volatile components in the LSR. Gas chromatography coupled with mass spectroscopy gives insight to the chemical groups in the gaseous mixture. Finally, the mechanical properties of the thermoplastic/LSR material were assessed with adhesion tests according to VDI Richtlinie 2019. The adhesion test results categorize the adhesion bond and its quality of the material.



Starting from a two-component commercial grade system Elastosil 3070/50 A and B (Wacker-Chemie GmbH) with a viscosity of 270,000 mPas [16], these two components were mixed in a 1:1 ratio with a batcher mixer (TOP 3000 S from Elmet) and cross-linked at a temperature of 150[degrees]C. Two different thermoplastics were selected as carrier material: a semicrystalline polybutylene terepthalate (Celanex 2402, Ticona) and an amorphous polyamide 12 (Grilamid TR90, EMS-Chemie). These materials were selected, as PBT and PA 12 differentiate regarding their functional groups and their molecular structure. Thus, the influence on adhesion of the materials' crystallinity and different covalent bondings were examined.

Two-Component Injection Molding

The test specimen in Fig. 1 was produced via two-component injection molding on a KraussMaffei Multinject CXV 65-180/ 55. The standard three-zone screw of the vertical injection unit (thermoplastic) has a diameter of 20 mm and a horizontal unit (LSR) of 25 mm. The thickness of the thermoplastic as well as the LSR was 2 mm.

The thermoplastic carrier material is produced by classical injection molding (see Table 1). The two component (A and B) system has to be pumped from a dose station in a 1:1 ratio into a cooled (20[degrees]C) static mixer for good homogenization quality. The LSR mixture is pumped into the cooled (20[degrees]C) injection unit and is then injected into a hot mold (150[degrees]C). Figure 2 illustrates schematically two-component injection molding of a thermoplastic in combination with LSR. Table 1 shows the adjusted parameter values during the two-component injection molding.

Tempering of Test Specimen

The single components (LSR and thermoplastics) as well as the thermoplastic/LSR combinations were tempered in a convection oven (Binder) at three different temperatures (80[degrees]C, 100[degrees]C, and 120[degrees]C) for 3 h or at constant temperature (100[degrees]C) for various durations (1, 3, 6, or 9 h). Although 200[degrees]C is commonly used as the tempering temperature, it is not suitable for every thermoplastic so the standard temperature was halved to see the influence of 100[degrees]C. To see the influence of lower and higher temperatures on the material behavior or adhesion, 20[degrees]C were added or subtracted from this 100[degrees]C tempering temperature. The adjusted tempering parameters are summarized in Table 2.

Tensile Test in Accordance with DIN EN ISO 527

The LSR was produced via LIM and the test specimens were created out in accordance with DIN EN ISO 527-2 in the geometry S2. The tensile tests were performed on a universal testing machine (Zwick Z050) with a traverse speed of 200 mm/min. The tension bars of PBT and PA 12 (according to DIN EN ISO 527-2-geometry 1A) were produced by classic injection molding and examined on a universal testing machine (Zwick 1485) with a traverse speed of 50 mm/min. For the LSR and thermoplastic tensile test, 7 test specimens were examined.

Dynamic Mechanical Analysis

For the dynamic mechanical analysis, which was performed on an RSA II from Rheometrics, 600 [micro]m thick plates were pressed at 150[degrees]C and DMA test specimen (8 mm X 20 mm) were punched. For the examination, the elongation was set to 2% and the frequency to 1 Hz. The measurements were performed isothermally at 80[degrees]C, 100[degrees]C, and 120[degrees]C for 3 h and at 100[degrees]C for 1 h-9 h (see Table 2). The chosen temperatures and times were suited to the tempering conditions.

Differential Scanning Calorimetry

To observe the influence of tempering on the thermoplastics, DSC (TA Instruments Q 1000) measurements were conducted. Each time 5 mg-10 mg of material was tested using a heating rate of 10 K/min. First the samples were heated from 25[degrees]C to 250[degrees]C, then cooled down to 25[degrees]C, and finally heated again. For further discussion, the focus lies upon the first heating step to identify the direct effect of tempering on the material behavior.

Thermogravimetric Analysis

During the tempering of LSR, volatile components are released thus resulting in a mass loss. To examine that process as well as the actual mass loss, thermogravimetric analysis measurements were conducted isothermally with a Mettler Toledo TGA/SDTA 85 le. The test specimens weighed between 15 mg and 20 mg. The heating rate was 20 K/min, and the measurements were done in a nitrogenic atmosphere. The used temperatures and times were identical to the tempering conditions (see Table 2).

Gas Chromatography Coupled with Mass Spectrometry

To identify the volatile components of the LSR, a gas chromatography coupled with mass spectrometry measurement was done. The test specimens were tempered in hermetic bags to catch the volatile components. The gaseous mixture was injected in the gas chromatograph and cooled down to fluidify the chemical components. The liquid composition was then heated, so that the single substances could be separated due to their respective boiling points. This analysis delivered a gas chromatography diagram with characteristic peaks of the sought substances. With the help of the mass spectrometry, these peaks could be assigned to chemical bonds and thus specific molecules.

Peel-Test According to VDI Richtlinie 2019 [17, 18]

The peel-test was performed on a universal testing machine (Zwick Z2.5) to determine the peel resistance, adhesion quality and fracture patter between the thermoplastic and LSR. The test specimen was clamped in a test slide which was fixed via a tension rod. Then the soft component (LSR) was pulled off at a 90[degrees] angle. The haul-off speed was 100 mm/min [17]. The outcome of the peel-test is the average force F (within an interval of 5 N), which also represents the adhesive force. Once the average force F has been determined, the peel resistance Ws (see Eq. 1) can be obtained [17], in which h is the width of the soft component (LSR).

[W.sub.s] = F/b (1)

For every peel-test series, five samples were examined.


The Effect of Tempering Temperature on the Single Components

First the influence of the tempering temperature on the mechanical and thermal properties of the single components (thermoplastic and LSR) was examined. If there is already an effect on the carrier material (e.g., postcrystallization or thermal damage) and the soft component (for example, post-crosslinking), an impact on the characteristics of the composite is anticipated.

In Fig. 3, the influence of the tempering process on the young's modulus of PBT and PA is shown.

The temperature treatment clearly indicated an impact on the young's modulus of the semicrystalline thermoplastic because the young's modulus increased around 9% after tempering. The amorphous carrier materials showed a different behavior; these materials barely indicated a variation in stiffness after tempering. This shows that semicrystalline and amorphous materials respond differently to tempering. It is, thus, possible the tempering induced a postcrystallization at PBT, which means a change in the crystal structure.

Reviewing the assumption of a possible postcrystallization, the thermal properties of PBT and PA 12 were examined via DSC measurements so the previous results could be confirmed. In Fig. 4, the influence of tempering on the thermal behavior of PBT (semicrystalline) and PA 12 (amorphous) is demonstrated for a tempering temperature of 100[degrees]C.

The peaks in Fig. 4A showed a small widening. Equally, the crystallinity degree for PBT increased from 25% to 32%. On the other hand the glass transition temperature of PA 12 was not affected by the tempering process.

As previously mentioned, the influence of tempering on the liquid silicone rubber must also be examined. The first step included tension tests of LSR to investigate the mechanical properties and establish a postcrosslinking. Table 3 summarizes the tension values at 300% elongation for the different tempering temperatures. It is evident that the tempering process influences the measured tension of LSR. The tension at 300% elongation increases around 12% with increasing tempering temperature, which might be a result of a post-crosslinking of LSR.

To confirm these results, dynamic mechanical analysis measurements were performed, where the storage modulus E' was considered. With these investigations, the post-crosslinking is illustrated by an increase of the storage modulus from 5.9-[10.sup.6] Pa at room temperature to around 7.9 x [10.sup.6] Pa at 120[degrees]C (see Table 4), which is an increase of 26%. However, the postcrosslinking is not complete after the tempering at different temperatures and for a tempering time of 3 h because the storage modulus is still increasing. In case of a complete crosslinking, the storage modulus would remain constant.

After confirming a post-crosslinking of the LSR, it was necessary to examine if any volatile components had been released during the tempering process. Therefore, the tempering was simulated by isothermal thermogravimetric analysis measurements. Table 5 indicates that after-treatment by temperature induces a mass loss, which increases in speed and amount of lost mass with increasing temperature.

The mass loss was then investigated more closely via gas chromatography coupled with mass spectroscopy in order to draw conclusions about the gaseous composition of the volatile components. In Fig. 5, the distribution of the individual ingredients is shown. The gas chromatography diagram provides five peaks, which could be assigned to separate chemical bonds with the help of mass spectroscopy. The volatile components mainly consist of silicon-oxygen connections, such as siloxanes, silanes, and silanols.

The Effect of Tempering Time on the Single Components

To examine the influence of the tempering time on the mechanical and thermal properties of the single components, the tempering temperature was set to 100[degrees]C and the time was changed from 1 h to 9 h. In Fig. 6, the stiffness was examined in detail.

The tempering time can clearly be seen to has an influence on the young's modulus of the semicrystalline material (PBT), while the young's modulus of PA 12 remains the same. After 1 hour of tempering at 100[degrees]C, the material undergoes a postcrystallization and after 3 h this phenomenon stops. In direct comparison with the tempering temperature, the temperature has more influence on the semicrystalline material than the time does. So, the impact on the degree of crystallinity is also higher when varying the temperature. The DSC measurements show the same trend (see Fig. 7). The degree of crystallinity of PBT is raised from 25% to 31% and remains nearly constant after 1 h.

The tempering time has also less influence on the tension value at 300% elongation of LSR than the tempering temperature. The value of the tension as well as the crosslinking-level remain the same after 3 h (see Table 6).

Through DMA measurements, the influence of the tempering time on the storage modulus was examined. Therefore, the tempering time of 1 h is too short for a complete crosslinking (see table 7). After 6 h, the storage modulus is much higher than after 1 h, but the crosslinking is not yet finished. More time is needed for a complete crosslinking of the LSR at 100[degrees]C.

During tempering, volatile components are released. The amount of volatile components increased during varying tempering times, but their chemical composition remained the same while altering the temperature.

The Effect of Tempering Temperature and Time on the Thermoplastic/LSR Combination

As already mentioned above, the tempering temperature has an influence on the single components. It is also necessary to prove this influence on the thermoplastic/LSR combinations. Therefore, the combinations were produced via two component injection molding and tempered for 3 h at different temperatures (80[degrees]C, 100[degrees]C, and 120[degrees]C). Then the adhesive force, which is illustrated in Fig. 8, was determined by use of an adhesion test according to the German standard "VDI Richtlinie 2019."

In Fig. 8A, the temperature influence on the adhesion between the carrier material and LSR as soft component is shown. It is clearly seen that the adhesion strength between the LSR and PBT decreased by about 30% when the test specimen was tempered. On the other hand, the polyamide-LSR combination behaves differently; the adhesion does not decline before a tempering at 120[degrees]C. There could be two different reasons for a deterioration of the adhesion between the thermoplastic and LSR: the semicrystalline material undergoes a postcrystallization, or the volatile components could diffuse to the interface and dilute the adhesion. The main cause for the adhesion reduction after tempering the combinations seems to be the carrier material and not the volatile components. This could be traced to the different behavior of the PBT/LSR and PA/LSR combinations after the temperature treatment. If the volatile components were the principle reason for the adhesion deterioration, both combinations would show a similar trend. But the adhesion strength between the amorphous PA 12 and LSR decreases merely at 120[degrees]C. Thus, it could be assumed that the volatile components do not show an influence at temperatures below 120[degrees]C. Until 120[degrees]C, the carrier material is essential during the tempering and the volatile components play a minor role.

By varying the tempering time nearly the same trend could be received. The adhesion between the PBT and LSR is decreased already after 1 h (see Fig. 8B). The adhesion remains approximately constant, so it could be assumed that the postcrystallization takes place during the first hour and deteriorate so the adhesion between LSR and PBT during this time. After the first hour it does not matter if the tempering process continues for 3, 6 or 9 h. On the other hand, the adhesion between LSR and PA 12 shows the same trend as when the temperature was changed; it remains roughly steady, because polyamide is amorphous. Therefore, the adhesion is disturbed by the molecular structure of the carrier material during varying the tempering time at 100[degrees]C.


In this study, the influence of tempering on the adhesion of hard/soft combinations were shown. As carrier materials, a semicrystalline polybutylene terephthalate and an amorphous polyamide 12 were used. The soft component was an adhesion-modified standard liquid silicone rubber. In case of using silicone elastomers, a tempering process is often used to improve the mechanical properties through specific post-crosslinking, but at the moment no sufficient studies concerning the impact of tempering of thermoplastic/LSR combinations exist. So it is not clear if and how the tempering temperature and time influence the adhesion between the two materials. For this purpose the hard/soft combinations, which consisted of the already mentioned materials, were tempered at 80[degrees]C, 100[degrees]C, and 120[degrees]C for 3 h, and at 100[degrees]C for 1, 3, 6, and 9 h. The temperatures were chosen to be under 150[degrees]C, because the amorphous polyamide loses its dimension stability around 155[degrees]C.

To analyze the influence of tempering, the single components were first determined. Hereby, it could be shown that the semicrystalline material (PBT) showed a postcrystallization. This was reflected by the increase of the young's modulus from 2604 MPa to around 2850 MPa and the increase in degree of crystallinity from [alpha] = 25% to [alpha] = 32%. The amorphous polyamide was not significantly influenced by tempering. Furthermore, the LSR indicated a post-crosslinking. The tension at 300% elongation was enhanced around 12% after the thermal treatment. Besides the post crosslinking of the LSR during the tempering, volatile components, which could be identified as siloxanes and silanes as well as silanols, were released.

During analysis of the impact of tempering on the adhesion between the thermoplastic carrier materials and the LSR, the former results could be reflected. The adhesion between the semicrystalline PBT and LSR was already influenced at temperatures from 80[degrees]C. The decrease was around 23% at 80[degrees]C because the PBT undergoes a postcrystallization and some volatile components were released much faster at higher temperatures. When using a polyamide/LSR combination the adhesion force remained constant; only at 120[degrees]C did a decrease of 26% result. By changing the tempering time, the adhesion between PBT and LSR was deteriorated already after 1 h and remained then constant. Thus, tempering time has no influence on the adhesion between PA 12 and LSR.

Therefore, the main reason for the adhesion deterioration with higher tempering temperatures between thermoplastic materials and LSR was the postcrystallization of the carrier material. The influence of the volatile component release on the adhesion was not until temperatures around 120[degrees]C and higher. Thereby, the adhesion was disturbed through the diffusion of the volatiles to the interface.


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C. Baumgart, D. Weiss, V. Altstadt

Plastic-Injection Molding, Neue Materialien Bayreuth GmbH, Bayreuth, Germany

Correspondence to: C. Baumgart; e-mail;

DOI 10.1002/pen.24313

TABLE 1. Adjusted parameter values, which were used for
test specimen Droduction.

                             PBT       PA      LSR 1

Mold ([degrees]C)            80        80       150
Heating time (s)             --        --       300
Processing ([degrees]C)    240-260   250-260    --

TABLE 2. Adjusted tempering parameters.

                                 Tempering conditions

Materials             Temperature ([degrees]C)    Time (h)

LSR 1                       80, 100, 120             3
                                100              1, 3, 6, 9
Thermoplastic               80, 100. 120             3
                                100              1, 3, 6, 9
Thermoplastic/LSR           80, 100. 120             3
  combinations                  100              1, 3, 6, 9

TABLE 3. Influence of the tempering process on the
tension value at 300% elongation of the LSR.

                              Tempering process

                         Without            for 3 h

[[sigma].sub.300]    4.1 [+ or -] 0.1   4.2 [+ or -] 0.1

                              Tempering process

                      100[degrees]C      120[degrees]C
                         for 3 h            for 3 h

[[sigma].sub.300]    4.5 [+ or -] 0.1   4.6 [+ or -] 0.0

TABLE 4. Influence of the tempering process on the
storage modulus of the LSR.

                    Tempering process

               Without            for 3 h

E' (Pa)    5.9 x [10.sup.6]   6.7 x [10.sup.6]

                    Tempering process

            100[degrees]C      120[degrees]C
               for 3 h            for 3 h

E' (Pa)    7.4 x [10.sup.6]   7.9 x [10.sup.6]

TABLE 5. Influence of the tempering process on LSR.

                              Tempering process

                        Without            80 [+ or -] C for 3 h

Mass loss (%)             100                     99.70

                              Tempering process

                 100 [+ or -] C for 3 h   120 [+ or -] C for 3 h

Mass loss (%)              99.59                  99.41

TABLE 6. Influence of the tempering process on the
tension value at 300% elongation of the LSR.

                                   Tempering process

                               Without              1 h

[[sigma].sub.300] (MPa)   4.1 [+ or -] 0.1   4.3 [+ or -] 0.1

                          Tempering process

                                3 h

[[sigma].sub.300] (MPa)   4.5 [+ or -] 0.1

                                 Tempering process

                                6 h                 9 h

[[sigma].sub.300] (MPa)   4.6 [+ or -] 0.0   4.5 [+ or -] 0.0

TABLE 7. Influence of the tempering process on
the storage modulus of the LSR.

                              Tempering process

               Without              1 h                3 h

E' (Pa)    5.9 x [10.sup.6]   6.9 x [10.sup.6]   7.4 x [10.sup.6]

                    Tempering process

                 6 h                9 h

E' (Pa)    7.9 x [10.sup.6]   8.2 x [10.sup.6]


Please note: Some tables or figures were omitted from this article.
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Author:Baumgart, C.; Weiss, D.; Altstadt, V.
Publication:Polymer Engineering and Science
Article Type:Report
Date:Aug 1, 2016
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