Bonding properties between TPV and rubber

From the viewpoint of environmental benefits and cost reduction, thermoplastic vulcanizates (TPV) have been widely used in many industrial applications, particularly auto motive weather seal applications. The adhesive properties between TPV and cured rubber are important in this situation. In this paper, we studied the factors contributing to the adhesion between TPV and cured rubber, and report about a new advanced Excelink TPV with excellent adhesion properties.


1. Introduction

Thermoplastic vulcanizates have been widely used in automotive applications due to environmental benefits and cost reduction in the production process. Environmental benefits include weight reduction, recyclability, safer curing agents (no nitrosamines), energy saving in process, etc. Cost reduction is given by its much simpler process and higher productivity than cured rubber. TPV technology has been recognized for enhancing productivity and performance.


Excelink TPVs, based on ethylene-propylene diene rubber (EPDM) and polypropylene (PP), have been developed since the year 2000 as advanced materials mainly used for automotive weather seal applications worldwide, such as glass run channel corners, door seal corners, drip seals, end caps, body welds, etc. Excelink TPVs are characterized by their morphology.

2. Key factors of adhesion

As mentioned above, weather seals using TPVs for the corner parts and EPDM for the straight parts can be produced by insert molding. At this time, TPVs and dense EPDM are bonded by thermal fusion bonding. What is the key factor of adhesion between TPVs and dense EPDM? The TEM micrograph in figure 2 shows the adhesion interface of the TPV and the EPDM. As shown in this figure, it is confirmed that the PP phase exists at the EPDM/TPV adhesion interface. Considering the above, the adhesion of a TPV and dense EPDM seems to be influenced by three factors:

• Properties of the PP

• Morphology of the TPV

• Surface condition of the cured dense EPDM.

Details of the three factors are described in the following.


figure 2

2.1 Properties of the PP

In this section we investigate the effects of the properties of the  poylpropylene, such as the crystallinity and the moleculer weight, on the adhesive characteristics. Table 2 summarizes the sample details. Here, specimens are prepared by insert molding of PP and dense EPDM. The morphology of the inter-face between the PP and the EPDM is observed by TEM. Figure 3 shows the effect of the crystallinity of the PP on the morphology, where the dark area means the EPDM phase and bright area means the PP phase. It is indicated that the crystal lamellas of PP penetrate into the EPDM. Moreover, it is found that the crystallinity of the PP affects the amount of the crystal lamella and the penetrating depth into the EPDM. Low-crystallinity PP shows a larger amount of crystal lamella and deeper penetrating depth than high-crystallinity PP. These results suggest that in an insert molding process the molten low-crystallinity PP penetrates deeper into the dense EPDM than high-crystallinity PP.

Table 2

To sum up, low-crystallinity PP can penetrate into the cured EPDM more easily than high-crystallinity PP, which provides higher adhesion. Figure 4 shows the effect of the molecular weight of low-crystallinity PP on the morphology. The penetrating depth of the crystal lamella does not depend on the molecular weight of the PP. On the other hand, entanglement of PP molecules tends to increase with increasing molecular-weight. This result suggests the increase in entanglement of PP allows the strong adhesion to the cured EPDM.


2.2 Morphology of the TPV

The effect of the morphology of TPV on the adhesive characteristics is investigated. Table 3 summarizes the sample details. TPV 1 shows a coarse dispersion of EPDM particles, and the adhesive strength is weak. TPV 2 shows a finely dispersed morphology, and the adhesive strength is strong. To investigate the effect of the TPV morphology, we observed the interface between TPV and EPDM by AFM (fig. 5). From the AFM micrograph it can be observed that the TPV is adhered to the cured dense EPDM through the PP phase on the interface. It is found that the adhesive area of TPV-2, which has a finely dispersed morphology, is larger and much more uniform than that of TPV-1 having the coarse morphology. This result indicates that a uniform and finely dispersed TPV morphology can provide an increased adhesion in the area of adhesive PP phase between TPVs and cured EPDM interface. Therefore, TPV-2 shows higher and more stable adhesion characteristics.

2.3 Surface condition of the cured dense EPDM

As mentioned above, the surface of the dense EPDM is smoother than the surface of cured EPDM sponge. In this section we investigate the effect of the surface roughness of the EPDM on the adhesive characteristics. Figure 6 shows the surface profiles of the EPDM used in the experiments.


3. Results

We studied the factors contributing to the adhesion of TPV and cured dense EPDM. As a result, it is confirmed that the following points are important to improve the adhesion strength between the TPV and the EPDM

• Crystallinity and molecular weight of PP The molten low-crystallinity PP penetrates deeper into the cured EPDM phase, and also the increase in molecular of PP leads to the strong adhesion with the cured EPDM

• Morphology of TPV

The finely and uniformly controlled morphology of TPV increases the adhesion area with the cured EPDM dense.

• Surface roughness of cured dense EPDM The increase in the number of the pores in the EPDM surface improves the physical anchoring effects.


Based on the results of this study, we have developed Excelink TPV grades with optimized bonding properties to EPDM. There is a three grades line-up with hardness from 40 – 80 Shore A available. Table 4 summarizes key properties of these grades. Figure 8 shows the results of the adhesive test with cured dense EPDM. Adhesive properties of high bonding grades are better than those of general grades. Regarding the melt flow rate, the new high bonding grades provide a much higher melt flow rate than conventional grades. Consequently, the new high bonding grades may be preferably used for complex parts and parts requiring good adhesion to cured EPDM dense, such as automotive weather seal applications.


Source: TPE magazine