Advances in Bonding LSR to Plastics, Metals

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16 Rubber & Plastics News ● July 25, 2016 www.rubbernews.com Technical Advances in bonding LSR to plastics, metals

By Paul A. Wheeler, Tarek A. Agag, plastic elastomer (TPE), and polyamide Brian P. Carney, Rebecca S. Cowles Executive summary (PA). Stainless steel 304 with polished and Douglas H. Mowrey and grit blasted surface finishes also Lord Corp. Lord Corp. offers new adhesive solutions that effectively bond platinum-cured was included in this experiment. Liquid silicone rubber is used to produce liquid silicone rubber to various substrates directly in an injection or compres- Three LSRs were chosen. All of them a wide range of parts for many different sion molding process. were 70 durometer hardness silicones markets. Some notable segments include This technology does not require plasma treatment or other complicated and with very similar mechanical properties medical devices, cookware, electronics and costly surface preparation steps. In this study, three new adhesive systems were and curing profiles, and manufactured personal electronic devices.1 tested to bond LSR to various substrates, including polycarbonate, thermoplastic by three major silicone producers so that elastomer, polyamide and stainless steel. a range of commercial offerings could be TECHNICAL NOTEBOOK Parts were molded and peel tested. This process and product technology offers evaluated. They are denoted as LSR 1, Edited by Harold Herzlich a number of benefits compared to existing technology, including enhanced design LSR 2 and LSR 3. freedom, more robust processing, less surface preparation requirements and en- Three proprietary adhesives, devel- Silicone polymers exhibit many vironmental friendliness. oped by Lord Corp., were included in unique properties that other materials this test and are denoted as Lord A, cannot achieve, combining rubbery flexi- Lord B and Lord C. Lord A is an aque- bility with excellent thermal stability, sequent hydrolysis with atmospheric This study focuses on the evaluation of ous hybrid whose primary solvent is wa- durability, low surface energy, biocom- moisture to create silanols that bond to three adhesive technologies developed to ter. Lord B and C are solvent-borne. patibility, soft feel, etc.1,2 the substrate’s surface. In order for improve upon existing silane technology. Because of its unique performance and bonding to occur, the substrate must These adhesives were designed to bond Plastic molding relative ease of part manufacturing, the contain hydroxyl groups.5 a wide range of platinum-cured silicones Plastic substrates were molded at Lord global LSR (silicone) market has seen rap- The reaction kinetics of hydrolysis to many substrates, eliminating the in a specially designed injection mold to id growth that is expected to continue.3 and bond formation are highly depend- need for complicated surface pretreat- produce plaques whose dimensions are 50 As the market need for LSR expands, ent on atmospheric moisture and tem- ment, precise environments, and diffi- mm x 87 mm x 1.5 mm. Half of the plaque product designs are becoming more sophis- perature. This environment, especially cult-to-control reaction kinetics. has a roughened surface texture that is ticated and require bonding of silicone to in a manufacturing setting, can be very defined as VDI-26, while the other half is other substrates, which can be challenging difficult to control. Experimental section a machined finish. Parts were injection due to its ultra-low surface energy of Furthermore, silanes must be applied Materials molded on a Sumitomo SE100EV 100 ton around 20 mN/m and chemical resistance.4 as very thin coating (<1 micron) that is dif- For this experiment, a representative electric injection molding machine. Many different adhesives and primers ficult to control and measure in a produc- spread of thermoplastic materials were Normal process conditions were cho- for bonding silicones exist on the market tion environment. Most plastics require gathered from various suppliers,and se- sen based on the mid points of the man- today. Most of the commercial offerings plasma treatment to create hydroxyl lected as substrates. These materials ufacturer’s recommendation. No mold are based on alkoxy silanes that contain groups to promote silane bonding. will be referred to by their generic desig- release was used. After molding, parts functional groups that are appropriate Because of all of the shortcomings nations: polycarbonate (PC), flame-re- were stored in a typical lab environment for the curing mechanism used in the with silane adhesive technology, there is tardant polycarbonate (PCFR), thermo- (21°C / 50 percent RH). silicone. In this embodiment, solvated a market need for silicone adhesives silane must be applied to the substrate that are easier to use and more effective Fig. 2. The photo below shows a TPE test specimen that has been overmolded followed by solvent evaporation and sub- on various substrates. with LSR before testing for peel strength. The strip shown is 25 mm wide.

Fig. 1. An example of a plastic test specimen is shown on the left, and the compression mold is shown on the right.

Fig. 4. Examples of three failure modes are shown. Failure mode is determined by the tested area and quantified based on a percentage of the test area.

Fig. 3. Schematic view of the cross section of a typical test specimen. P017_RPN_20160725.qxp 7/21/2016 3:22 PM Page 1

www.rubbernews.com Rubber & Plastics News ● July 25, 2016 17 Technical

Adhesive application were placed into a 150mm x 150mm x The substrate was masked with Kapton 5mm mold, along with 150 grams of LSR. The authors tape such that only half of the test plaque The parts were then molded at 20 tons of would receive adhesive. In the case of the pressure for five minutes. Fig. 1 shows an Paul A. Wheeler of experience at plastic substrates, adhesive was applied to example of the test pieces used and the is the technology Lord in the Elas- the section with VDI-26 finish. mold used to create LSR test specimens. leader for in-mold tomer, Adhesives Adhesives were applied with a Binks After five minutes, parts were careful- bonding products and Coating Tech- Model 95 siphon feed HVLP spray gun to a ly removed and allowed to cool to room at Lord Corp.’s nology division. thickness of 10 to 15 microns. Immediately temperature before being separated facility in Erie, Currently, he is after spraying, parts were dried in a hot air with a utility knife. Strips 25 mm wide Pa. a senior staff sci- convection oven at 50°C for 15 minutes. Af- were cut into the LSR for peel testing. Before joining entist responsible ter removing from the oven, parts were Lord, Wheeler was for developing both stored at 21°C / 50 percent RH for up to Peel testing Wheeler a technology lea- aqueous and sol- Carney four hours before overmolding with LSR. At least 24 hours after molding, parts der at Ascend Per- vent-based adhe- were peel tested at 180°C at 300 mm/min formance Materials, where he managed sives, primers, and intermediates for LSR molding in an Instron Model 3365 tensile tester uti- Ascend’s portfolio of nylon 66 com- rubber-to-metal bonding applications. LSR silicones are supplied as two sep- lizing a 1 kN load cell. pounds for the automotive market. Carney has authored and co-authored arate reactive components that are Peel strength and failure modes were Before working at Ascend, Wheeler several technical papers. mixed before molding. Both sides of sili- recorded. An example of a molded test was a materials development manager He graduated from Penn State Uni- cone were mixed in a Kitchen Aid mixer specimen is shown in Fig. 2. at Hybrid Plastics, where he was re- versity with a bachelor’s of science de- at low speed for five minutes. The sub- sponsible for creating several nanotech- gree in 1988. strate was overmolded with LSR in a Discussion nology products Rebecca S. Cow- Wabash MPI G30H-18-BX compression Failure modes for thermoplastic les is a staff scien- press that was heated to 125°C. In addition to peel strength, failure and thermoset tist at Lord’s facil- In each cycle, three pieces of substrate See LSR, page 18 materials, which ity in Erie. She received two re- has 25 years of ex- search and devel- perience in prod- Table 1. Adhesion results and failure modes of three adhesive systems that were opment 100 Awards. uct development used to bond three LSRs to polycarbonate. Wheeler got and is one of the his bachelor’s de- lead technologists gree in plastics responsible for engineering tech- the development nology from Penn of In Mold Bond- Agag State Erie and a ing adhesives. Cowles doctorate in polymer science from the Cowles holds a bachelor’s degree in University of Southern Mississippi. chemistry from Gannon University. Tarek A. Agag is a staff scientist at She has been published in Rubber and Lord and has been with the firm for Plastics News previously and has been four years, developing new chemistries awarded several patents related to ad- for bonding dissimilar substrates. hesives and coatings. Prior to joining Lord, Agag Douglas H. Mowrey is a fel- was research associate profes- low at Lord who has been sor at Case Western Reserve with the company for 35 University in Cleveland. years, developing rubber to Agag received his doctorate substrate adhesives. from Toyohashi University of Mowrey holds bachelor de- Technology in Japan. He has grees in chemistry/biology published many peer-reviewed from Gannon University and papers, review articles and in biology education from Ed- book chapters related to high inboro University. He has re- performance polymers and received several patents related lated materials. to adhesives and coatings in Brian P. Carney has 27 years Mowrey his career at Lord.

Fig. 5. This chart shows peel strength and rubber retention of three adhesives with three silicones, all on polycarbonate. P018_RPN_20160725.qxp 7/21/2016 3:23 PM Page 1

18 Rubber & Plastics News ● July 25, 2016 www.rubbernews.com Technical

and cured via addition reaction catalyzed of these adhesive technologies, they were ing the surface area and creating areas by platinum. The same process conditions evaluated on multiple substrates, which for mechanically locking the adhesive LSR (125°C for five minutes) were used for all included polycarbonate (PC), flame-retar- onto the substrate. three silicones. Bonding data is shown in dant polycarbonate (PCFR), thermoplas- Lord C provided complete adhesion to Continued from page 17 Table 1 and charted in Fig. 5. tic elastomer (TPE), 304 stainless steel all substrates except for polished SUS mode analysis is the key to understand- The results show that all three adhe- (SUS) with polished and grit blasted sur- and polyamide, in which case they com- ing product performance and bonding sives bond completely to LSR 1 with high face finishes (25 to 50 micron blast pro- pletely failed to bond to the substrate. mechanism. peel strength values of greater than 70 file), and polyamide. In this experiment, For the results reported in this paper, N/cm, at which point the silicone fails. LSR 1 was used on all of the substrate Conclusion the test area is a 25 mm strip that is ap- This sample group did not show any ev- and adhesive combinations and results Three new adhesive technologies were proximately 45 mm long. A schematic of idence of failure to either the substrate or are shown in Table 3 and Fig. 6. tested for bonding platinum-cured liquid the test specimen is shown in Fig. 3. Fail- the silicone, indicating good adhesive per- A rubber retention of 100 percent is the silicone rubber to multiple substrates. ure mode is reported as a percentage of formance. With LSR 2, excellent adhesion best possible result, meaning that the ad- Common, off-the-shelf silicones of 70 duro- this area based on its state after testing is was achieved with adhesive A and C. hesive bond exceeds the strength of the meter were chosen for this study. Of the carried out. Failure modes were divided However, adhesive B showed complete silicone. Lord A gave nearly 100 percent three adhesives, Lord A and C bonded to into four categories; each one is described rubber-to-adhesive failure, indicating no rubber retention to all substrates with all LSRs, while Lord B only bonded to two. below with depictions given in Fig. 4. bonding occurred between the chemistry the exception of a small amount of adhe- Analysis of the LSRs concluded that Thick rubber retention: The entire used in the adhesive and that of the silicone. sive-substrate failure on polyamide. low volatile content and high reactivity thickness of the silicone rubber is intact. Mixed results were achieved when bond- Further unpublished experiments con- are important aspects to maximize bond- This is the most desirable failure mode ing to LSR 3, with adhesive A and C con- ducted at Lord have shown that residual ing performance. Multiple substrates and means that the strength of the adhe- tinuing to achieve full bonding, but adhe- moisture content in polyamide can cause were also evaluated, including PC, PCFR, sive exceeds the strength of the silicone. sive B showed relatively low peel strength this type of failure, so pre-drying the TPE, 304 SUS and polyamide. All of the Thin rubber retention: A thin layer and a large amount of thin rubber failure. polyamide or applying the adhesive in a adhesives were effective on PC, PCFR, of rubber is retained on the substrate. To help understand bonding mecha- dry-as-molded state is critical to achieve a TPE, and grit-blasted 304 SUS. Some of This means that the silicone rubber it- nism and understand why Lord B adhe- good bond. the adhesives had problems bonding to self failed, but failed at a layer that is sive failed to bond to LSR 2, LSR 1 and 2 Lord B fully bonded to all substrates polished 304 SUS and polyamide. close to the adhesive interphase. This were analyzed for chemical differences. except for polished SUS. However, it We can conclude the three adhesive sys- failure mode can be influenced by the Data is outlined in Table 2. had no problems bonding to grit-blasted tems developed were successful in bond- performance of the adhesive and can be It was discovered that LSR 2 con- SUS, which is a common method to pre- ing multiple platinum-cured silicones to caused, for example, if the adhesive retained around 4 percent of low molecu- pare metals for adhesive bonding.6 Grit multiple substrates that do not require tards or interrupts the silicone curing lar weight silicones, while none were de- blasting improves adhesion by increas- See LSR, page 19 mechanism. A small amount (<20 per- tected in LSR 1. Also, LSR 1 contained cent) of thin rubber failure is normal more vinyl and silicon hydride function- Table 2. Molecular weight and functional analysis of LSRs. and unavoidable in this test method. ality than LSR 2. Substrate-to-adhesive failure: The Considering that the LSR adhesive adhesive does not stick to the substrate, technology utilizes functional groups on delaminating from it during testing and the LSR to create covalent bonds, a high- usually sticking to the silicone. er concentration of those groups should Rubber-to-adhesive failure: The ad- theoretically lead to stronger adhesion. hesive does not stick to the silicone and During LSR molding, low molecular remains on the substrate. This failure weight silicone constituents are well mode indicates that reaction between the known to bloom to the surface, which can adhesive and silicone did not occur. further interfere with adhesion. Both of these findings explain why LSR 2 is more Adhesive bonding to multiple LSRs difficult for Lord B to bond. Future work The first test evaluated the effective- includes running this same analysis on ness of each adhesive on three different LSR 3 to validate this theory. representative LSRs of similar durometer from three major silicone producers. Each Adhesive bonding to multiple LSR was mixed, per the manufacturer rec- substrates ommendation, at a 50:50 ratio of A to B To further understand the applicability

Table 3. Silicone adhesives were tested on multiple substrates.

Fig. 6. This chart details the bonding results on multiple substrates. P019_RPN_20160725.qxp 7/21/2016 4:53 PM Page 1

www.rubbernews.com Rubber & Plastics News ● July 25, 2016 19 ue to expand.” Europe is facing some uncertainty with Henniges the United Kingdom’s recent vote to leave the European Union. However, Williams Continued from page 1 said the decision shouldn’t significantly Rollins said. “We’ve been very detailed affect Henniges’ business. about how we want to do this.” “We don’t see it impacting us directly,” Larry Williams, president and chief fi- Williams said of Brexit. “The only impact nancial officer, said local government is we would see is through the OEMs if providing the refurbishment of the build- they alter any of their manufacturing or ing, with Henniges investing in the production plants as a result of Brexit. equipment to manufacture product. Ford has some exposure there with their However, initially there will be equip- engine plant being in the U.K., but as far ment moving over from its Rehburg facil- as our product, we don’t ship anything ity to allow that plant to relocate some into the U.K. So unless there is some outsourced production under its roof. movement by the OEMs, we don’t see When Prudnik starts to launch the any impact on our business.” new business, which Williams projects Headquartered in Auburn Hills, to be at the end of 2017 at the earliest, Mich., Henniges employs 7,700 world- Henniges will begin investing in addi- wide with facilities in all four major re- tional equipment. gions of the world. It is a subsidiary of The new facility is about 93 miles north China-based Aviation Industry Corp. An artist’s rendering of the refurbished plant Henniges secured in Prudnik, Poland. of its manufacturing operation in Hranice. A wire harness manufacturer previously used the building. Rollins said local government has refurbished the building completely with a new roof, parking lot and in- frastructure specifically for Henniges. “That was one of the things that at- tracted us to this city in Poland,” Williams said. “There was a prior company that was in the facility, so there’s already a work force there that we could tap into as we start to ramp up and re- hire. It gives us the ability to support a city in Poland that was struggling from an economic standpoint. We can help them, and they can help us.” Rollins said Henniges recruited the for- mer plant manager to come work for the firm, and that has allowed it to tap into some of the engineering talent in the area. “They already know a lot of people in the community,” Rollins said. “If you look at it from my perspective, it’s a win- win for the community and for us.” Prudnik will serve the entire European market. Rollins said the facility was neces- sary to make Henniges’ cost structure more competitive in order to continue to grow its business in the region. He added the location allows it to draw technical talent from Poland and the Czech Republic. “I think the European automotive market is at this point stable,” Williams said. “We don’t see significant growth in the market, but we see it being stable, and there’s opportunity for us. There’s a little bit of excess capacity in the market, and so it’s holding prices down. But we do see an opportunity to gain some market share in the region as we contin- LSR Continued from page 18 special surface preparation techniques such as plasma or flame treatment, and do not have the same application limitations as typical silane primers. Some of the limitations found are at- tributed to the composition of the different LSRs used. It is important to note one of the two options that were successful is water- based and constitutes an environmen- tally friendly option.

References 1. “Characteristic Properties of Silicone Rubber Compounds” by Shin-Etsu Co. www.silicone.jp/e/- catalog/pdf/rubber_e.pdf 2. Pachaly, Bernd; Achenbach, Frank; Herzig, Chris- tian; Mautner, Konrad. Silicones. Wiley-VCH, 2005. 3. ‘Significant and Rapid Growth in China, Com- bined with an Improving Economy, Driving Global Demand for Silicones, but Excess Chinese Additions Threaten to Destabilize Market” IHS news release from April 10, 2014. http://press.ihs.com/press- release/aida-jebens/significant-and-rapid-growth- china-combined-improving-economy-driving-glob 4. Owen, M.J. Why Silicones Behave Funny. Dow Corning Corp., 2005. 5. Gelest Co. Literature. Silane Coupling Agents: Connecting Across Boundaries. 2006 6 Wegman, Raymond; Van Twisk, James. Surface Preparation Techniques for Adhesive Bonding 2nd ed. Elsevier, 2013.