Multi-component injection mol-ding technologies gaining continuously increasing interest and market importance. They combine different materials and functionalities in one product and offer different part and design options, a high degree of automation, and high output rates in constant quality [1, 2]. Multi-component products include versatile material combinations such as plastic-metal, plastic-glass, and plastic-plastic. Hard/soft combinations based on rigid thermoplastic and one or more soft thermoplastic elastomers (TPE) are omnipresent in daily products and play a key role (Fig. 1).
Fig. 2: 2K test specimen for peel tests according to VDI 2019 (left) and test trolley with clamping arrangement of specimen (right).
Several technologies are available to produce multi-component products based on substance-to-substance bonding mechanism of the components, e.g. bi-injection molding, sandwich molding or multi-shot molding [2-4]. For hard/soft combinations, most relevant way is the multi-shot technology which means that a rigid thermoplastic is first injection molded and then directly overmolded by one or more soft materials. The multi-shot injection molding process is very complex and influenced by both material properties and processing parameters. Many publications deal with the influence of processing conditions on the adhesive strength of hard/soft combinations [4-9]. One very important parameter is the delay time which defines the delay before injecting the soft material [3, 10]. A high delay time means a large gap between the first and the second shot and promotes strong cooling of the first component. The higher the delay time the more challenging is a high adhesive strength between the hard and the soft component. The worst case but one of the cheapest ways to produce hard/soft products is the so called cold insert molding where a (cold) rigid part is inserted and overmolded by the soft material.
Testing hard/soft combinations
Several methods exist to test the adhesive strength of multi-components. Most of them have been developed for testing adhesive joints, e.g. DIN EN 1939 or DIN EN 1463. For testing the adhesive strength of hard/soft combinations, one important guideline is the VDI 2019 which describes the measurement of peel force as a function of peel path in a very detailed way . In this guideline the soft component is subjected to tensile stress using a constant test speed until either the bond at the interface or the material itself fails. The peel-off angle of the soft component is 90°. The force required and the distance traversed are recorded continuously during the peel test. A peel resistance can be calculated from the peel force and the width of the interface. Fig. 2 shows the characteristic 2K test specimen as well as the clamping arrangement of the specimen. In this paper a standard tensile test machine from Zwick ZN20 with a test speed of 50 mm/min was used for the peel tests. In addition the test trolley is equipped with a distance sensor which is recording the trolley movement. This makes it possible to measure force/peel path curves without superposition of the TPE lengthening. The 2K test specimens were stored for 24 h at standard conditions (23 °C and 50 % relative humidity) before testing.
Three main failure pattern are described in the VDI 2019, namely two types of cohesive break and adhesive break or peeling. Cohesive break means that the adhesive strength of the interface is higher than the intrinsic strength of the TPE. In this case the peel force increases continuously to a maximum followed by a sudden drop to zero, as shown in Fig. 3(d). The second way of cohesive break could appear during peeling with a fracture inside of the TPE layer. This leads to TPE residue on the substrate. Adhesive failure means that the adhesive strength of the interface is lower than the intrinsic strength of the TPE. In this case the soft TPE layer is continuously peeled off from the rigid substrate without TPE residue. The adhesive failure can result in different force/displacement curves: The first case in Fig. 3(a) describes a constant peeling reflected by a plateau of the peel force. This means that the adhesive strength is the same throughout the whole interface. The second case in Fig. 3(b) describes an increasing or decreasing peeling reflected by an increase or decrease in peel force. Increasing peel force means an increasing adhesive strength along the interface. Decreasing peel force describes the opposite. The third case in Fig. 3(c) defines a non-constant peeling indicated by a fluctuating peel force along the peel path. This type is often observed for hard/soft combination with TPE-U. The fluctuation is an indication for alternating adhesion in the interface which in turn can be the result of the specific flow behavior of the TPE or slip-stick effects between the two materials and/or the mold wall.
New PA adhesion-modified TPE-V
Substance-to-substance bonding of non-polar TPE such as styrene-based thermoplastic elastomers (TPE-S) or thermoplastic vulcanizates (TPE-V) on engineering plastics such as polyamide (PA) is very difficult. One important reason is the large difference in their polarity, surface tension, and thermal properties (e.g. heat conductivity, crystallinity). PA is polar and has a high surface tension whereas conventional TPE-V is non-polar with a rather low surface tension. Consequently, proper modification of the TPE-V is necessary to achieve substance-to-substance bonding to PA. One way includes in-line surface activation processes such as plasma treatment or flame treatment [12-14]. In addition, automated in-line application of adhesives or primers is another option . However, both approaches are costly and result mostly in higher production times. Blending the non-polar TPE-V with the rigid polymer (e.g. PA) is another and more efficient way to improve the adhesion [12, 15]. Nonetheless, certain amount of PA is required to improve the bonding performance. Due to its thermoplastic nature, PA will have negative influence on the typical elastomeric properties of the TPE-V such as compression set. Also extra addition of compatibilizers is mostly necessary due the incompatibility between TPE-V and PA. Therefore, a more productive way to improve the compatibility with and adhesion to PA is blending the non-polar TPE-V with specific polymeric bonding agents such as functionalized copolymers .
Albis Plastic has developed new adhesion-modified TPE-V called Alfater XL 4PA0010. They are characterized by very good bonding to PA6 and PA66 when using 2-shot molding as well as cold insert molding. The PA adhesion-modified TPE-V is based on polypropylene (PP), cross-linked ethylene propylene diene monomer rubber (EPDM), and a specific bonding agent system. The new bonding TPE-V is currently available in Shore A55, Shore A70, and Shore A85. Tab.1 shows typical properties.
Conventional 2-shot molding and cold insert molding trials have been carried out by SKZ Das Kunststoff-Zentrum Würzburg on a multi-component injection molding machine from Wittmann-Battenfeld TM1300/525 + 130l. Different PA6 and PA66 have been tested as hard component including non-reinforced and 30 wt.% glass fiber reinforced grades as well as impact-modified grades with 30 wt.% glass fibers.
The new PA adhesion-modified TPE-V show basically two main peel curve patterns as well as a high reproducibility of the peel behavior indicating an effective and stable bonding to PA. The left hand graph of Fig. 4 is the result of cohesive break of the TPE-V. It is characterized by continuous increase in peel force to a maximum followed by sudden drop to zero. At the maximum the TPE-V ruptures meaning that the adhesive strength of the interface is higher than the intrinsic strength of the TPE-V. The obtained peel paths shown in the graph therefore only reflect the elongation of the TPE-V until it breaks. Contrary, the right hand graph shows adhesive failure which is characterized by continuous peeling. In this case the adhesive strength of the interface is lower than the intrinsic strength of the TPE-V. It can be seen that the peel force decreases steadily along the peel path indicating that bonding is better close to the gate. The interfacial or contact temperature plays a very important role for effective bonding.
Higher interfacial temperatures generally improve the bonding between the hard and the soft component [4, 7, 8]. Especially adhesion-modified TPE-V or TPE-S require sufficiently high contact temperatures. Due to high cooling rates at the surface, the mass temperature of the injected TPE-V melt is higher close to the gate than far away from the gate. Thus, the interfacial temperature also decreases fast with increasing flow length or distance from the gate leading to slightly reduced bonding (lower peel forces) at the flow end.
Excellent bonding to different PA
The PA adhesion-modified TPE-V series provides very good bonding to non-reinforced, glass fiber reinforced as well as impact-modified PA6 and PA66. This is exemplarily shown for the soft Alfater XL A55I 4PA0010 in Fig. 5. All combinations show cohesive failure for the molding conditions investigated. The peel paths therefore only reflect the elongation of the TPE-V until rupture. Despite the good adhesion results, one has to point out that high glass fiber contents (> 30 wt.%) will of course negatively affect the bonding. High glass fiber contents reduce the thermoplastic PA content and promote glass fiber agglomerations at the part surface (Fig. 6). These agglomerations disturb effective chain interdiffusion in the interface between the hard and soft material. The negative effect of high glass fiber contents is also found by Neuer  for hard/soft combinations based on adhesion-modified TPE-S and PA6. Commercial (Fig. 6) impact modifiers for PA are mostly functionalized copolymers, rubbers or TPE including glycidyl methacrylate (GMA) grafted EPDM or MAH grafted ethylene acrylate copolymers [18-20]. These functionalized impact modifiers are able to interact with the specific bonding agent system used in the TPE-V and can therefore support the bonding.
As the PA market is much customized and lots of PA suppliers are available it is worthwhile for the molder to know how this will affect the bonding capability of the TPE-V. Tab. 2 shows therefore the bonding of the new TPE-V to different PA6 from two suppliers.
The TPE-V exhibits very good bonding to different PA6 using a 2-shot injection molding process. However, it is difficult to give general statements about the bonding because the multi-component injection molding process is very complex and influenced by multiple factors such as part design, processing parameters and last but not least by the variety of PA grades and suppliers. Especially processing additives (e.g. flow aids or demolding aids) often used in PA can negatively affect the bonding performance. They are mostly non-polar and tend to leach to the PA surface during injection molding which will prohibit sufficient adhesion. Jaroschek  found significant reductions in the adhesive strength of PP compounds which included processing aids such as stearates or oils. Neuer  described the negative influence of flame retardants in PA which can plate-out and reduce the bonding performance of adhesion-modified TPE-S. As a result, it is recommended that the molder gets in close contact with the material supplier to identify suitable PA grades and to make trials to identify the optimal processing conditions for each specific hard/soft combination. Another aspect is the hygroscopic nature of PA which can result in substantial moisture absorption. In this case, the bonding agent system of the adhesion-modified TPE-V will interact with the moisture and not with the functional PA groups. Drying of PA is therefore important. The moisture of PA should be less than 0.1 % before overmolding with adhesion-modified TPE-V.
Shore hardness of the TPE-V
Tab. 3 shows the influence of Shore hardness of the adhesion-modified TPE-V on bonding to non-reinforced PA6 and PA66 using 2-shot injection molding with a delay time of 30 s.
All combinations show cohesive failure for the 2-shot molding conditions given in Table 3. However, harder TPE-V grades require basically higher overmolding temperatures to achieve cohesive failure. As can be seen from Fig. 7, the soft adhesion-modified TPE-V has a very broad temperature window in which cohesive failure is achieved whereas harder TPE-V grades respond more sensitive to the overmolding temperature used.
The influence of TPE-V hardness is more clearly seen when cold insert molding is used. For the same molding conditions, softer TPE-V result in better bonding than harder TPE-V (Tab. 4).
Since good bonding requires a high interfacial or contact temperature between the hard and soft component, high delay times will negatively affect the bonding of the adhesion-modified TPE-V. Neuer  also describes this negative influence for adhesion-modified TPE-S. High delay between the first and second shot cause too strong cooling of the injected PA melt resulting in a very low surface temperature (Fig. 8). The delay time influence is more pronounced at low mold temperatures and for harder TPE-V grades.
For cold insert molding, which is the worst case, the delay between first and second shot can be very long. Thus, the surface temperature of the PA insert is mostly close to room temperature. In this case very high mass temperature of the TPE-V melt is required to achieve sufficiently high interfacial or contact temperatures for good bonding to PA. Especially for the harder TPE-V grades it is recommended to use high overmolding temperatures to partly reduce the negative delay time influence. The positive effect of higher overmolding temperatures is shown in Fig. 9 for the TPE-V with Shore A85. At a given delay time of 30 s an increasing overmolding temperature will improve significantly the bonding performance. To avoid any negative effect of too high delay times, 2-shot injection molding processes with minimal delay are highly preferred.
Hard/soft products based on Alfater XL 4PA0010
2K handles for tools and for industrial cutters are typical applications of the adhesion-modified TPE-V from Albis Plastic. Fig. 10 shows cutters from Mure & Peyrot made from hard/soft material combination. The adhesion-modified TPE-V provides high grip, anti-slip properties, soft touch combined with rubber-like feeling, good dampening characteristics, and bonding to PA.
Another successful application of the new TPE-V series are 2K castors for various containers with fixed wheels, for various mobile devices and equipment, and for rollaway beds. Here, the TPE-V is molded onto PA and offers good noise and vibration damping, excellent grip and anti-slip as well as good resistance to chemicals, water, and weather.
This paper is focused on a new adhesion-modified TPE-V series called Alfater XL 4PA0010 and developed by Albis Plastic for high quality PA-based hard/soft combinations. These adhesion-modified TPE-V grades offer good bonding to a broad range of PA6 and PA66 including non-reinforced, glass fiber reinforced, and impact-modified PA. They are suitable for conventional 2-shot injection molding as well as for cold insert molding. In both cases cohesive failure of the hard/soft combinations can be achieved depending on the injection molding conditions, PA grade, and the part design. Cohesive failure means that the adhesive strength of the interface is higher than the intrinsic strength of the TPE-V. Due to the negative influence of high delay times, especially in case of insert molding, it is highly recommended to use 2-shot injection molding processes which offer minimal delays between first and second shot. Beside the injection molding conditions, PA additives can also have considerable influence on the bonding performance. Particularly processing aids or flame retardants can reduce the bonding capability due to leaching or plate-out. It is therefore suggested that customers make trials and get in close contact with the material supplier to identify best processing conditions and the most suitable PA grade for each specific 2K application. Furthermore, high moisture contents of PA will reduce the bonding performance. Drying of PA is always crucial and recommended. The moisture content of PA should be lower than 0.1 % to avoid bonding problems.
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