SURFACE TREATMENT OF SANTOPRENE FOR ADHESIVE BONDING P. Hoobin, W.S. Gutowski, M. Morehouse Polymer Surface Engineering Group CSIRO Division of Building, Construction and Engineering Melbourne / Australia Postal Address: PO Box 56, Highett, Victoria, 190/Australia E-mail Address: Voytek.Gutowski@csiro.au 1. INTRODUCTION Thermoplastic elastomers (TPE s) are increasingly important materials in the automotive industry. Their main applications are weatherseals for static and dynamic sealing applications such as trim seals, glass encapsulation, primary foam layers for doors and windows, glass run channels, hood and trunk seals, engine air duct systems and many others. TPE s exhibit highly elastic properties resembling those of thermoset rubbers. Unlike rubbers, however, TPE s can be processed using conventional thermoplastics processing equipment, e.g. extruders. They can also be combined with other thermoplastic materials commonly used in automotive industry, e.g. thermoplastic olefins (TPO s) such as polypropylene blends. Another important advantage of TPE s is their recyclability and ease of re-processing. Polyolefin-based thermoplastic vulcanizates (TPV s) such as Santoprene are one of the most important categories of materials amongst thermoplastic elastomers. This class of TPE s is based on finely dispersed fully cross-linked EPDM (Ethylene Propylene Diene Monomer) rubber in the matrix of polypropylene. The material is foamed in order to further control the flexibility and specific density of the final product. The latter can be reduced from 0.97 to as low as 0. g/cm. The major drawback of Santoprene and other types of TPV s is that the final products, e.g. gaskets, manufactured from these materials are very difficult to bond. One of the main reasons for this is the fact that both ingredients of TPV s, i.e. EPDM and PP (PP typically constitutes 0 to 0% of the total composition), are chemically inert and hence non-bondable without special surface treatment. Although PP can be surface treated by processes such as 1
flame or corona discharge, the dominant EPDM phase is virtually non-treatable due to outstanding resistance of this material to oxidation. The use of primers is recommended for improving bondability of Santoprene. However, the authors of this paper are not aware of the availability of an effective primer that enables successful bonding of TPV s with the use of typical adhesives used in the automotive or building products assembly. This paper presents a new technology, Santos currently under development, that facilitates effective surface treatment of Santoprene for adhesive bonding with the use of commodity adhesives. Substrates exhibiting 55 to 87 Shore A hardness were used in experiments. Adhesives used were flexible (commodity type) polyurethanes adhesives and sealants, eg Lord 740 and Betaseal 5870. Bonded assemblies were exposed to ambient dry environments well as water immersion prior to testing. Excellent bondability has been achieved by the materials surface treated with the use of the new process, as demonstrated by a high percentage (up to 100%) of cohesive failure within the Santoprene substrate.. EPDM s AND THERMOPLASTIC VULCANAZITES IN AUTOMOTIVE APPLICATIONS Currently, EPDM s are the most important elastomers in automotives for non-tyre applications. It is estimated that 65% of EPDM in automotive applications is represented by extruded weather seals, 5% by coolant hoses and 10% by mouldings and rubber metal parts. In European passenger cars the weight of EPDM components amounts to approximately 1 kg. It is now broadly recognised that current EPDM weather seal systems have reached limitations and further improvements are limited. In the manufacture of EPDM components, the extrusion, vulcanisation and finishing are very complex and labour intensive and amount to approximately 65% of the total product cost. Thermoplastic vulcanizates (TPV s) such as Santoprene offer significant system cost reduction in comparison to EPDM, particularly in the field of weather seals and other applications. One advantage of using TPV s is the weight reduction due to the lower material density. TPV s have about 5% lower density in comparison to a standard solid EPDM profile formulation.another advantage is in more flexible design and the possibility of coextrusion with rigid TPO s eg. Polypropylene. This allows for the functional integration of TPV sealing lips with the rigid elements of the seal (PP segment) that are designed to be snapped onto the relevant car body channel.. EXPERIMENTAL.1 Surface treatment Santos : a novel surface treatment technology for Santoprenes The novel surface treatment for Santoprenes enables an on-line and continuous surface modification at a controlled speed within the range of 1 to 40 m/minute. The process can be
integrated with an extruder, eg for flexible gasket manufacture thus enabling an on/off treatment depending on the type of the gasket material. During the treatment, a range of reactive graft chemicals is permanently attached to the surface of the product. The type of the chemical is chosen to provide the optimum chemical reactivity with the adhesive or coating to be applied to the gasket surface. Corona discharge treatment Corona discharge treatment was carried out with the use of the following equipment: Tantec EST System HV 010 Power output 1 kw (maximum) Treatment speed 0.1 to 70 m/min. The distance between the substrate surface and electrode was maintained at.5 mm in this work. The energy output was controlled, as required. Flame treatment Flame treatment was carried out with the commercially available equipment manufactured byaerogen/uk. The type of unit used in this work was an Aerogen FT Lab system equipped with a 00 mm AT 5 burner providing 5 kw (10,000 BTU/hr) energy output. Flame treatment was carried out at 1.0% excess oxygen in the air/propane mixture, unless indicated otherwise. The treatment distance (between the flame tip and substrate surface) can be adjusted from 5 10 mm. In this work the standard distance was maintained at 10 mm. Treatment speed was adjusted within the range of 0 to 60 m/min, depending on the burner energy output. The latter was adjusted, as required, by controlling the flow rate of the air/gas mixture supplied to the burner. Materials Substrate types: Santoprene 101 55; 101 71; 11 80; 101 87 Adhesive types: Polyurethane sealant: (i) Betaseal 5870 (Expandite-Essex) Urethane adhesive: (ii) Lord 740 (Lord Corporation). Test Methods Strength determination All bonded specimens were tested in an Instron mechanical tester (Model 5565) at a rate of 10 mm/min. The strength of bonded assemblies was determined through the use of peel specimens tested in 180º configuration. Five specimens per experimental point were used. Adhesive Bonding The quality of adhesion was determined through the use of peel specimens. The configuration of this is illustrated in Figure 1. The specimens involved two 5 mm wide strips of a Santoprene substrate. In order to control the dimensions of adhesive bead, a strip
of double-sided self-adhesive tape (with the protective tape retained on one surface) is attached at both edges of one of the strips of Santoprene. Figure 1. Configuration of a peel specimen A bead of polyurethane adhesive is extruded into this pre-formed channel and evened-up with a spatula. Subsequently, another 5 mm wide strip of Santoprene substrate is placed on the top of the adhesive-filled assembly. The complete assembly is clamped between two pieces of polyethylene, and allowed to cure for a period of days prior to testing. Exposure conditions The quality of initial adhesive bond was determined using dry specimens subsequent to a - days cure. The durability of bonded assemblies was determined by exposing the specimens to 1-week immersion in 60 C water. Substrate Treatment The substrate samples were used in the following condition: 1. Untreated: wipe-cleaned using isopropyl alcohol. Oxidation: flame or corona treatment (speed range: 0 to 40 m/min). Santos process: on-line surface modification involving surface grafting of specialty chemicals onto the substrate surface (speed range: (10 40 m/minute) 4
. Results Table 1 demonstrates the results of comparative trials involving bonding Santoprene 01-87. All substrates were subjected to the following types of treatment: (i) untreated - isopropanol wipe only to remove surface contaminants; (ii) flame treatment - 0 m/min; (iii) corona discharge 0 m/min; (iv) Santos treatment 0 m/min. Lord 740 urethane adhesive (adhesive bead dimensions: 0. x 1 mm) was used for bonding. The specimens were tested in dry condition only, subsequent to -day cure of the adhesive. Table 1. Peel strength [N/cm] and failure mode of Santoprene 01-87/urethane adhesive bonds (adhesion in dry condition only) subsequent to Santos treatment. Treatment Spec. Dry No. Strength [N/cm] Failure Mode Untreated Av. ( spec) - 100% AF Flame Av. ( spec) - 100% AF Corona Av. ( specs) - 100% AF Santos Process 1 CF: AF: 9.4 9.8 9. cohesive failure within Santoprene substrate delamination between Santoprene and adhesive The results in Table 1 demonstrate that untreated Santoprene, or that treated with the use of corona discharge or flame treatment cannot be effectively bonded with Lord 740 urethane adhesive. In contrast with that, the same Santoprene material, when subjected to Santos process at the speed of 0 m/minute, exhibited excellent bondability as demonstrated by 100 % cohesive failure within the substrate. Another set of experiments involved Santoprene 11 80 adhesively bonded with Betaseal 5870 polyurethane adhesive after Santos treatment, as above. For comparison, untreated samples were also bonded. All assemblies were allowed to cure for days. One batch of samples was tested for dry adhesion, whilst another batch was immersed for 1 week in water at 60 C water prior to testing. The results are summarised in Table below. Table. The strength [N/cm] and failure mode of Santoprene 11-80/polyurethane adhesive bonds (dry and wet strength ) subsequent to Santos treatment. Treatment Spec. Dry Wet (1 week@60 C/H O) No. Strength [N/cm] Failure Mode Strength [N/cm] Failure Mode Untreated Av. ( spec) 0 100% AF Not tested N/A Santos Process 1 Santos Process 1 1.0 1. 5.4 54.. 4.0 80% CF 75% CF 17. 16.7 18.8 Exposure in progress CF: cohesive failure within Santoprene substrate or within a polyurethane adhesive 0% CF N/A 5
It is seen from the results in Table that subsequent to surface treatment with the Santos process, Santoprene 11-80 exhibits excellent bondability. Moreover, the specimens exposed to prolonged immersion in hot water retain bond integrity, as shown by 100% cohesive failure of Santoprene substrates or that within a polyurethane adhesive after testing. Table below demonstrates the results of comparative trials involving bonding Santoprene 101-55/S. Betaseal 5870 polyurethane adhesive (adhesive bead dimensions: 0. x 1 mm) was used for bonding. The specimens were tested in dry condition, subsequent to -day cure of the adhesive. Table. Peel strength [N/cm] and failure mode of Santoprene 101-55/polyurethane adhesive bonds (adhesion in dry condition only) subsequent to Santos treatment at various processing speeds. Treatment Dry Strength [N/cm] Failure Mode Untreated 0 100% AF Santos Treatment speed: 0 m/min 6. Santos Treatment speed: 0 m/min 0.0 Santos Treatment speed: 40 m/min 6.7 CF: cohesive failure within the Santoprene substrate or polyurethane adhesive 4. Conclusions 1. It has been demonstrated that the novel surface treatment process for Santoprene, Santos presented in this paper enables successful adhesive bonding of this class of materials with the use of polyurethane adhesives and sealants.. Subsequent to treatment with the Santos process, adhesively bonded Santoprene assemblies retain excellent durability after prolonged immersion in hot water, as demonstrated by 1 week immersion in 60º water.. The Santos process can be conducted in an on-line mode with treatment speeds within the range of 1 to 40 m/minute. This consequently enables the integration of the surface treatment unit with the extrusion line. 6