Technical

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

Advances in bonding LSR to plastics, metalsExecutive summary

Lord Corp. offers new adhesive solutions that effectively bond platinum-curedliquid silicone rubber to various substrates directly in an injection or compres-sion molding process.

This technology does not require plasma treatment or other complicated andcostly surface preparation steps. In this study, three new adhesive systems weretested to bond LSR to various substrates, including polycarbonate, thermoplasticelastomer, polyamide and stainless steel.

Parts were molded and peel tested. This process and product technology offersa number of benefits compared to existing technology, including enhanced designfreedom, more robust processing, less surface preparation requirements and en-vironmental friendliness.

By Paul A. Wheeler, Tarek A. Agag,Brian P. Carney, Rebecca S. Cowles

and Douglas H. MowreyLord Corp.

Liquid silicone rubber is used to producea wide range of parts for many differentmarkets. Some notable segments includemedical devices, cookware, electronics andpersonal electronic devices.1

Silicone polymers exhibit manyunique properties that other materialscannot achieve, combining rubbery flexi-bility with excellent thermal stability,durability, low surface energy, biocom-patibility, soft feel, etc.1,2

Because of its unique performance andrelative ease of part manufacturing, theglobal LSR (silicone) market has seen rap-id growth that is expected to continue.3

As the market need for LSR expands,product designs are becoming more sophis-ticated and require bonding of silicone toother substrates, which can be challengingdue to its ultra-low surface energy ofaround 20 mN/m and chemical resistance.4

Many different adhesives and primersfor bonding silicones exist on the markettoday. Most of the commercial offeringsare based on alkoxy silanes that containfunctional groups that are appropriatefor the curing mechanism used in thesilicone. In this embodiment, solvatedsilane must be applied to the substratefollowed by solvent evaporation and sub-

This study focuses on the evaluation ofthree adhesive technologies developed toimprove upon existing silane technology.

These adhesives were designed to bonda wide range of platinum-cured siliconesto many substrates, eliminating theneed for complicated surface pretreat-ment, precise environments, and diffi-cult-to-control reaction kinetics.

Experimental section Materials

For this experiment, a representativespread of thermoplastic materials weregathered from various suppliers,and se-lected as substrates. These materialswill be referred to by their generic desig-nations: polycarbonate (PC), flame-re-tardant polycarbonate (PCFR), thermo-

sequent hydrolysis with atmosphericmoisture to create silanols that bond tothe substrate’s surface. In order forbonding to occur, the substrate mustcontain hydroxyl groups.5

The reaction kinetics of hydrolysisand bond formation are highly depend-ent on atmospheric moisture and tem-perature. This environment, especiallyin a manufacturing setting, can be verydifficult to control.

Furthermore, silanes must be appliedas very thin coating (<1 micron) that is dif-ficult to control and measure in a produc-tion environment. Most plastics requireplasma treatment to create hydroxylgroups to promote silane bonding.

Because of all of the shortcomingswith silane adhesive technology, there isa market need for silicone adhesivesthat are easier to use and more effectiveon various substrates.

TECHNICAL NOTEBOOKEdited by Harold Herzlich

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

Fig. 2. The photo below shows a TPE test specimen that has been overmoldedwith LSR before testing for peel strength. The strip shown is 25 mm wide.

Fig. 3. Schematic view of the cross section of a typical test specimen.

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

plastic elastomer (TPE), and polyamide(PA). Stainless steel 304 with polishedand grit blasted surface finishes alsowas included in this experiment.

Three LSRs were chosen. All of themwere 70 durometer hardness siliconeswith very similar mechanical propertiesand curing profiles, and manufacturedby three major silicone producers so thata range of commercial offerings could beevaluated. They are denoted as LSR 1,LSR 2 and LSR 3.

Three proprietary adhesives, devel-oped by Lord Corp., were included inthis test and are denoted as Lord A,Lord B and Lord C. Lord A is an aque-ous hybrid whose primary solvent is wa-ter. Lord B and C are solvent-borne.

Plastic molding Plastic substrates were molded at Lord

in a specially designed injection mold toproduce plaques whose dimensions are 50mm x 87 mm x 1.5 mm. Half of the plaquehas a roughened surface texture that isdefined as VDI-26, while the other half isa machined finish. Parts were injectionmolded on a Sumitomo SE100EV 100 tonelectric injection molding machine.

Normal process conditions were cho-sen based on the mid points of the man-ufacturer’s recommendation. No moldrelease was used. After molding, partswere stored in a typical lab environment(21°C / 50 percent RH).

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Paul A. Wheeleris the technologyleader for in-moldbonding productsat Lord Corp.’sfacility in Erie,Pa.

Before joiningLord, Wheeler wasa technology lea-der at Ascend Per-

formance Materials, where he managedAscend’s portfolio of nylon 66 com-pounds for the automotive market.

Before working at Ascend, Wheelerwas a materials development managerat Hybrid Plastics, where he was re-sponsible for creating several nanotech-

nology productsfor thermoplasticand thermosetmaterials, whichreceived two re-search and devel-opment 100 Awards.

Wheeler gothis bachelor’s de-gree in plasticsengineering tech-nology from PennState Erie and a

doctorate in polymer science from theUniversity of Southern Mississippi.

Tarek A. Agag is a staff scientist atLord and has been with the firm forfour years, developing new chemistriesfor bonding dissimilar substrates.

Prior to joining Lord, Agagwas research associate profes-sor at Case Western ReserveUniversity in Cleveland.

Agag received his doctoratefrom Toyohashi University ofTechnology in Japan. He haspublished many peer-reviewedpapers, review articles andbook chapters related to highperformance polymers and re-lated materials.

Brian P. Carney has 27 years

of experience atLord in the Elas-tomer, Adhesivesand Coating Tech-nology division.

Currently, he isa senior staff sci-entist responsiblefor developing bothaqueous and sol-vent-based adhe-sives, primers, and intermediates forrubber-to-metal bonding applications.

Carney has authored and co-authoredseveral technical papers.

He graduated from Penn State Uni-versity with a bachelor’s of science de-gree in 1988.

Rebecca S. Cow-les is a staff scien-tist at Lord’s facil-ity in Erie. Shehas 25 years of ex-perience in prod-uct developmentand is one of thelead technologistsresponsible forthe developmentof In Mold Bond-ing adhesives.

Cowles holds a bachelor’s degree inchemistry from Gannon University.She has been published in Rubber andPlastics News previously and has beenawarded several patents related to ad-hesives and coatings.

Douglas H. Mowrey is a fel-low at Lord who has beenwith the company for 35years, developing rubber tosubstrate adhesives.

Mowrey holds bachelor de-grees in chemistry/biologyfrom Gannon University andin biology education from Ed-inboro University. He has re-ceived several patents relatedto adhesives and coatings inhis career at Lord.

The authors

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

Technical

Wheeler Carney

Mowrey

Agag Cowles

Table 1. Adhesion results and failure modes of three adhesive systems that wereused to bond three LSRs to polycarbonate.

Fig. 5. This chart shows peel strength and rubber retention of three adhesives withthree silicones, all on polycarbonate.

Adhesive application The substrate was masked with Kapton

tape such that only half of the test plaquewould receive adhesive. In the case of theplastic substrates, adhesive was applied tothe section with VDI-26 finish.

Adhesives were applied with a BinksModel 95 siphon feed HVLP spray gun to athickness of 10 to 15 microns. Immediatelyafter spraying, parts were dried in a hot airconvection oven at 50°C for 15 minutes. Af-ter removing from the oven, parts werestored at 21°C / 50 percent RH for up tofour hours before overmolding with LSR.

LSR molding LSR silicones are supplied as two sep-

arate reactive components that aremixed before molding. Both sides of sili-cone were mixed in a Kitchen Aid mixerat low speed for five minutes. The sub-strate was overmolded with LSR in aWabash MPI G30H-18-BX compressionpress that was heated to 125°C.

In each cycle, three pieces of substrate

were placed into a 150mm x 150mm x5mm mold, along with 150 grams of LSR.The parts were then molded at 20 tons ofpressure for five minutes. Fig. 1 shows anexample of the test pieces used and themold used to create LSR test specimens.

After five minutes, parts were careful-ly removed and allowed to cool to roomtemperature before being separatedwith a utility knife. Strips 25 mm widewere cut into the LSR for peel testing.

Peel testing At least 24 hours after molding, parts

were peel tested at 180°C at 300 mm/minin an Instron Model 3365 tensile tester uti-lizing a 1 kN load cell.

Peel strength and failure modes wererecorded. An example of a molded testspecimen is shown in Fig. 2.

Discussion Failure modes

In addition to peel strength, failureSee LSR, page 18

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Technical

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

LSRmode analysis is the key to understand-ing product performance and bondingmechanism.

For the results reported in this paper,the test area is a 25 mm strip that is ap-proximately 45 mm long. A schematic ofthe test specimen is shown in Fig. 3. Fail-ure mode is reported as a percentage ofthis area based on its state after testing iscarried out. Failure modes were dividedinto four categories; each one is describedbelow with depictions given in Fig. 4.

Thick rubber retention: The entirethickness of the silicone rubber is intact.This is the most desirable failure modeand means that the strength of the adhe-sive exceeds the strength of the silicone.

Thin rubber retention: A thin layerof rubber is retained on the substrate.This means that the silicone rubber it-self failed, but failed at a layer that isclose to the adhesive interphase. Thisfailure mode can be influenced by theperformance of the adhesive and can becaused, for example, if the adhesive re-tards or interrupts the silicone curingmechanism. A small amount (<20 per-cent) of thin rubber failure is normaland unavoidable in this test method.

Substrate-to-adhesive failure: Theadhesive does not stick to the substrate,delaminating from it during testing andusually sticking to the silicone.

Rubber-to-adhesive failure: The ad-hesive does not stick to the silicone andremains on the substrate. This failuremode indicates that reaction between theadhesive and silicone did not occur.

Adhesive bonding to multiple LSRs The first test evaluated the effective-

ness of each adhesive on three differentrepresentative LSRs of similar durometerfrom three major silicone producers. EachLSR was mixed, per the manufacturer rec-ommendation, at a 50:50 ratio of A to B

and cured via addition reaction catalyzedby platinum. The same process conditions(125°C for five minutes) were used for allthree silicones. Bonding data is shown inTable 1 and charted in Fig. 5.

The results show that all three adhe-sives bond completely to LSR 1 with highpeel strength values of greater than 70N/cm, at which point the silicone fails.

This sample group did not show any ev-idence of failure to either the substrate orthe silicone, indicating good adhesive per-formance. With LSR 2, excellent adhesionwas achieved with adhesive A and C.

However, adhesive B showed completerubber-to-adhesive failure, indicating nobonding occurred between the chemistryused in the adhesive and that of the silicone.

Mixed results were achieved when bond-ing to LSR 3, with adhesive A and C con-tinuing to achieve full bonding, but adhe-sive B showed relatively low peel strengthand a large amount of thin rubber failure.

To help understand bonding mecha-nism and understand why Lord B adhe-sive failed to bond to LSR 2, LSR 1 and 2were analyzed for chemical differences.Data is outlined in Table 2.

It was discovered that LSR 2 con-tained around 4 percent of low molecu-lar weight silicones, while none were de-tected in LSR 1. Also, LSR 1 containedmore vinyl and silicon hydride function-ality than LSR 2.

Considering that the LSR adhesivetechnology utilizes functional groups onthe LSR to create covalent bonds, a high-er concentration of those groups shouldtheoretically lead to stronger adhesion.

During LSR molding, low molecularweight silicone constituents are wellknown to bloom to the surface, which canfurther interfere with adhesion. Both ofthese findings explain why LSR 2 is moredifficult for Lord B to bond. Future workincludes running this same analysis onLSR 3 to validate this theory.

Adhesive bonding to multiplesubstrates

To further understand the applicability

Continued from page 17

Table 3. Silicone adhesives were tested on multiple substrates.

Table 2. Molecular weight and functional analysis of LSRs.

Fig. 6. This chart details the bonding results on multiple substrates.

of these adhesive technologies, they wereevaluated on multiple substrates, whichincluded polycarbonate (PC), flame-retar-dant polycarbonate (PCFR), thermoplas-tic elastomer (TPE), 304 stainless steel(SUS) with polished and grit blasted sur-face finishes (25 to 50 micron blast pro-file), and polyamide. In this experiment,LSR 1 was used on all of the substrateand adhesive combinations and resultsare shown in Table 3 and Fig. 6.

A rubber retention of 100 percent is thebest possible result, meaning that the ad-hesive bond exceeds the strength of thesilicone. Lord A gave nearly 100 percentrubber retention to all substrates withthe exception of a small amount of adhe-sive-substrate failure on polyamide.

Further unpublished experiments con-ducted at Lord have shown that residualmoisture content in polyamide can causethis type of failure, so pre-drying thepolyamide or applying the adhesive in adry-as-molded state is critical to achieve agood bond.

Lord B fully bonded to all substratesexcept for polished SUS. However, ithad no problems bonding to grit-blastedSUS, which is a common method to pre-pare metals for adhesive bonding.6 Gritblasting improves adhesion by increas-

ing the surface area and creating areasfor mechanically locking the adhesiveonto the substrate.

Lord C provided complete adhesion toall substrates except for polished SUSand polyamide, in which case they com-pletely failed to bond to the substrate.

Conclusion Three new adhesive technologies were

tested for bonding platinum-cured liquidsilicone rubber to multiple substrates.Common, off-the-shelf silicones of 70 duro-meter were chosen for this study. Of thethree adhesives, Lord A and C bonded toall LSRs, while Lord B only bonded to two.

Analysis of the LSRs concluded thatlow volatile content and high reactivityare important aspects to maximize bond-ing performance. Multiple substrateswere also evaluated, including PC, PCFR,TPE, 304 SUS and polyamide. All of theadhesives were effective on PC, PCFR,TPE, and grit-blasted 304 SUS. Some ofthe adhesives had problems bonding topolished 304 SUS and polyamide.

We can conclude the three adhesive sys-tems developed were successful in bond-ing multiple platinum-cured silicones tomultiple substrates that do not require

See LSR, page 19

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Henniges

LSR

Continued from page 1

Continued from page 18special surface preparation techniquessuch as plasma or flame treatment, anddo not have the same application limita-tions as typical silane primers.

Some of the limitations found are at-tributed to the composition of the differ-ent LSRs used.

It is important to note one of the twooptions that were successful is water-based and constitutes an environmen-tally friendly option.

References1. “Characteristic Properties of Silicone RubberCompounds” 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 GlobalDemand for Silicones, but Excess Chinese AdditionsThreaten to Destabilize Market” IHS news releasefrom 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. DowCorning Corp., 2005. 5. Gelest Co. Literature. Silane Coupling Agents:Connecting Across Boundaries. 2006 6 Wegman, Raymond; Van Twisk, James. SurfacePreparation Techniques for Adhesive Bonding 2nded. Elsevier, 2013.

Rollins said. “We’ve been very detailedabout how we want to do this.”

Larry Williams, president and chief fi-nancial officer, said local government isproviding the refurbishment of the build-ing, with Henniges investing in theequipment to manufacture product.However, initially there will be equip-ment moving over from its Rehburg facil-ity to allow that plant to relocate someoutsourced production under its roof.

When Prudnik starts to launch thenew business, which Williams projectsto be at the end of 2017 at the earliest,Henniges will begin investing in addi-tional equipment.

The new facility is about 93 miles northof its manufacturing operation in Hranice.A wire harness manufacturer previouslyused the building. Rollins said local gov-ernment has refurbished the building com-pletely 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 com-pany that was in the facility, so there’salready a work force there that we couldtap into as we start to ramp up and re-hire. It gives us the ability to support acity in Poland that was struggling froman economic standpoint. We can helpthem, and they can help us.”

Rollins said Henniges recruited the for-mer plant manager to come work for thefirm, and that has allowed it to tap intosome of the engineering talent in the area.

“They already know a lot of people inthe community,” Rollins said. “If youlook at it from my perspective, it’s a win-win for the community and for us.”

Prudnik will serve the entire Europeanmarket. Rollins said the facility was neces-sary to make Henniges’ cost structuremore competitive in order to continue togrow its business in the region. He addedthe location allows it to draw technical tal-ent from Poland and the Czech Republic.

“I think the European automotivemarket is at this point stable,” Williamssaid. “We don’t see significant growth inthe market, but we see it being stable,and there’s opportunity for us. There’s alittle bit of excess capacity in the mar-ket, and so it’s holding prices down. Butwe do see an opportunity to gain somemarket share in the region as we contin-

ue to expand.”Europe is facing some uncertainty with

the United Kingdom’s recent vote to leavethe European Union. However, Williamssaid the decision shouldn’t significantlyaffect Henniges’ business.

“We don’t see it impacting us directly,”Williams said of Brexit. “The only impactwe would see is through the OEMs ifthey alter any of their manufacturing orproduction plants as a result of Brexit.Ford has some exposure there with theirengine plant being in the U.K., but as faras our product, we don’t ship anythinginto the U.K. So unless there is somemovement by the OEMs, we don’t seeany impact on our business.”

Headquartered in Auburn Hills,Mich., Henniges employs 7,700 world-wide with facilities in all four major re-gions of the world. It is a subsidiary ofChina-based Aviation Industry Corp. An artist’s rendering of the refurbished plant Henniges secured in Prudnik, Poland.

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