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Volume 22/05
Table Of ContentsPage NumberIntroductionDescription Of Adhesives Cyanoacrylate Adhesives Epoxy Adhesives Hot Melt Adhesives Light Curing Acrylic Adhesives Polyurethane Adhesives Silicone Adhesives No-Mix and Static Mix Acrylic Adhesives Why Bond Elastomers With Loctite Brand Adhesives?How To Use The Rubber and TPE Bonding ChaptersHow to Use The Adhesive Shear Strength TableRubber and TPE Bonding Chapters Butyl Rubber (IIR) Chlorosulfonated Polyethylene (CSM) Copolyester TPE Epichlorohydrin Rubber (CO, ECO, GCO, GECO) Ethylene Acrylic Rubber (EEA) Ethylene Propylene Rubber (EPM, EPDM) Ethylene-Vinyl Acetate Copolymer (EVA) Fluorocarbon Rubber (FKM) Fluorosilicone Rubber (FVMQ) Halogenated Butyl Rubber (BIIR, CIIR) Hydrogenated Nitrile Rubber (H-NBR, HSN) Melt Processible Rubber (MPR) Natural Rubber (NR) Neoprene Rubber (CR) Nitrile Rubber (NBR, XNBR) Polyether Block Amide (PEBA) Polyacrylate Rubber (ACM) Polyisoprene Rubber (IR) Polyolefin Elastomers (POE) Poly(propylene oxide) Rubber (GPO) Polysulfide Rubber (TM) Silicone-Modified EPDM Silicone Rubber (MQ, VMQ, PMQ, PVMQ) Styrene-Butadiene Rubber (SBR) Styrenic TPE (S-B-S, S-I-S, S-EB-S) Thermoplastic Vulcanizates (TPV)Adhesive Joint DesignTest MethodologyIndex of Trade NamesAcknowledgementsDisclaimerThe Loctite Design Guide for Bonding Rubbers and TPEs, Volume 4648505254565860626466697376761
The ProblemAdhesive InformationFrom the discovery of natural rubber to the developmentof modern day thermoplastic elastomers (TPEs),elastomeric materials have found a wide variety ofuses that make them an integral part of an industrialsociety. In a diverse variety of products ranging fromautomobile tires to lifesaving implantable medicaldevices, their unique ability to be greatly deformedand return to their original shape fills an importantniche in the world of engineering materials. It wouldbe difficult to identify a manufacturing process whichdoes not use elastomers in one form or another.Elastomeric materials have achieved widespreadacceptance due to the virtually limitless combinationsof elastomer types, fillers and additives which can becompounded at relatively low costs and processed bya wide variety of methods. This gives end-users theability to develop specific formulations with propertiestailored to their needs. By properly selecting the baseelastomer, additives and fillers, as well as the appropriatecure method, the physical, chemical and thermalproperties of an elastomer can be made to meet orexceed the performance requirements of almost anyapplication requiring elastomeric properties.However, while the limitless variety of elastomers is aninvaluable asset to a designer selecting an elastomer,it is the designer’s biggest limitation when selecting anadhesive. The countless adhesives available, coupledwith the virtually limitless elastomer formulationspossible, make it highly unlikely that there will bebond strength data for the specific adhesive/elastomercombination in the designer's application.The SolutionBond Strength InformationThis guide is designed to indicate the bondability of the26 most commonly used families of elastomers. Thiswas accomplished using two approaches. The majorityof elastomers which were evaluated were compoundedspecifically to determine the effect different additivesand fillers had on the bondability of these materials.Once the designer identifies the elastomer formulationscontaining the same fillers and additives that aredesired to be bonded, the adhesives can then bepinpointed which performed best on those materials.Although this will probably not tell the designer theexact bond strength that will be achieved by thatspecific adhesive on that specific material, it will givethe designer a general idea of what bond strengths canbe achieved. For the other elastomers, bond strengthtesting was performed on commercially availablegrades which were selected to represent each majorcategory of the elastomer based on the major end-useapplications, and/or the chemical structure of thatelastomer.2An adhesive cannot be selected for an applicationsolely on the basis of bond strength information. Otherfactors such as the cure speed, environmentalresistance, thermal resistance, and suitability forautomation of an adhesive will play a critical role indetermining the best adhesive for a specific application.To give a designer insight into these design parameters,an in-depth description of the seven adhesive types,namely cyanoacrylates, no-mix/static mix acrylics, hotmelts, epoxies, polyurethanes, silicones and light curingacrylics, has been included in this guide. Theseadhesive sections contain a general description of eachadhesive, a detailed discussion of the chemicalstructure and cure mechanism of each adhesive, andthe benefits and limitations of using each adhesive.Elastomer InformationManufacturers may have the flexibility to select theelastomers which are best suited for their applicationsin terms of performance and bondability. To aid thedesigner, an in-depth discussion of each of theelastomer types is included. Information coveredincludes a general description of the elastomer and itsproperties, as well as a list of trade names, suppliersand typical applications.Cure Process EffectsFor thermoset rubbers, the ultimate bond strength mightbe improved by stopping the vulcanization processbefore all cross-linking sites available have beenconsumed. Stopping the vulcanization process beforethe rubber has achieved its ultimate modulus will leaveunreacted cross-linking sites on the rubber backboneand form a polymer with a lower cross-link density. As aresult, the unreacted cross-linking sites may improvebond strength by reacting with the adhesive. In addition,the lower cross-link density of the rubber may facilitateadhesive penetration of the rubber polymer network. Toinvestigate this phenomenon, each of the thermosetrubbers, except the PEBA, was cured to 80% of theirultimate modulus (noted as T80 cure in the adhesiveshear strength tables), and tested for bond strength.These results were then compared with those of thecontrol for statistically significant differences.Bond Design InformationFinally, a manufacturer may have a design in which hedesires to incorporate an adhesively bonded joint. Toeffectively design that joint, the designer must knowwhich parameters are critical to the bond strengthsachieved by a bonded joint and the effect thatchanging these parameters will have. A bond designsection which reviews the basics of designing anadhesively bonded single lap joint is included in anattempt to give the designer insight into this area.Although most “real world” bond geometries are morecomplex than single lap joints, this information can beextrapolated as a general indicator of the effectscaused by changing bond geometries.The Loctite Design Guide for Bonding Rubbers and TPEs, Volume 2
CyanoacrylateAdhesivesGeneral DescriptionCyanoacrylates are one-part, room-temperature-curingadhesives that are available in viscosities ranging fromwater-thin liquids to thixotropic gels. When pressed intoa thin film between two surfaces, cyanoacrylates curerapidly to form rigid thermoplastics with excellentadhesion to most substrates.One of the benefits cyanoacrylates offer is theavailability of a wide variety of specialty formulationswith properties tailored to meet particularly challengingapplications. For example, rubber-toughenedcyanoacrylates offer high peel strength and impactresistance to complement the high shear and tensilestrengths characteristic of cyanoacrylates. Thermallyresistant cyanoacrylates are available which offerexcellent bond strength retention after exposure totemperatures as high as 250 F for thousands of hours.Moreover, “Surface-insensitive” cyanoacrylates offerrapid fixture times and cure speeds on acidic surfaces,such as wood or dichromated metals, which could slowthe cure of a cyanoacrylate. In some cases, the use of ageneral-purpose cyanoacrylate adhesive was hamperedby the appearance of a white haze around the bondline. This phenomenon is known as “blooming” or“frosting” and occurs when cyanoacrylate monomervolatizes, reacts with moisture in the air, and settles onthe part. To eliminate this problem, “Low Odor/LowBloom” cyanoacrylates were developed. They have alower vapor pressure than standard cyanoacrylates andtherefore are less likely to volatize. Ultraviolet curing(UV) cyanoacrylates are the latest advancement incyanoacrylate technology. UV cyanoacrylates utilizeproprietary photoinitiators to allow cyanoacrylates tosurface cure in seconds when exposed to ultraviolet orvisible light of the appropriate wavelength. Light CureTechnology makes cyanoacrylates cure even faster,overcome blooming, and limiting or eliminating stresscracking. While advances in cyanoacrylate formulatingtechnology have played a key role in offering additionalbenefits to the end user, there have also beenimportant developments in cyanoacrylate primer andaccelerator technology.Accelerators speed the cure of cyanoacrylate adhesivesand are primarily used to reduce cure/fixture times, tocure fillets on bond lines and/or excess adhesive.Accelerators consist of an active ingredient dispersedin a solvent. The accelerator is typically applied to asubstrate surface prior to the application of theadhesive. Once the carrier solvent has evaporated, thecyanoacrylate can immediately be applied and its cureinitiated by the active species that the accelerator hasleft behind. Depending on the particular solvent andactive species present in the accelerator, the solventcan require 10 to 60 seconds to evaporate, and theactive species can have an on-part life ranging from 1minute to 72 hours. Accelerator can also be sprayedover a drop of free cyanoacrylate to rapidly cure it. Thistechnique has been widely used for wire tacking in theelectronics industry.Another benefit offered by cyanoacrylates is theavailability of primers which enable them to form strongbonds with polyolefins and other difficult-to-bondplastics such as fluoropolymers and acetal resins. Likethe accelerators, polyolefin primers consist of an activeingredient dispersed in a solvent. Once the carriersolvent has evaporated, the surface is immediatelyready for bonding, and the primer will have an on-partlife ranging from minutes to hours. Depending on theplastic, bond strengths up to 20 times the unprimedbond strength can be achieved.ChemistryCyanoacrylate adhesives are cyanoacrylate esters, ofwhich methyl and ethyl cyanoacrylates are the mostcommon. Cyanoacrylates undergo anionic polymerizationin the presence of a weak base, such as water, and arestabilized through the addition of a weak acid. Whenthe adhesive contacts a surface, the water present onthe surface neutralizes the acidic stabilizer in theadhesive, resulting in the rapid polymerization of thecyanoacrylate.Advantages One-part system Solvent-free Rapid room temperature cure Excellent adhesion to many substrates Easy to dispense in automated systems Wide range of viscosities available Excellent bond strength in shear and tensile mode Primers available for polyolefins and difficult to bondplastics UV/Visible cure formulas availableDisadvantages Poor peel strength Limited gap cure Poor durability on glass Poor solvent resistance Low temperature resistance Bonds skin rapidly May stress crack some plasticsThe Loctite Design Guide for Bonding Rubbers and TPEs, Volume 23
Epoxy AdhesivesGeneral DescriptionEpoxy adhesives are typically two-part systems (resinand hardener) which cure at room temperature,although one-part pre-mixes which utilize a heat cureare also available, as are UV curable one and twocomponent epoxies. The two components reactstoichiometrically, so maintaining proper mix ratio isimportant to ensure consistent performance. Uponmixing, the curing reaction of the epoxy can release agreat deal of heat and result in a significanttemperature rise in the adhesive. In some applications,such as deep section potting, this heat rise can besufficient to char the adhesive. Upon cure, epoxies formtough, rigid thermoset polymers with high adhesion toa wide variety of substrates and good environmentalresistance. The viscosities of epoxy adhesives canrange from a few thousand centipoise to thixotropicpastes.ChemistryThe wide variety of chemical species that can reactwith the epoxide end group and the inherent stability oftwo-part adhesive systems lead to a wide variety ofepoxy formulations available to the end-user. Theperformance properties of epoxies can be tailored tospecific needs through a wide variety of techniques.Epoxy adhesives are typically rigid and formulatingtechniques must be employed to produce flexibleepoxies. These techniques include the use of nonreactive plasticizers, the incorporation of rubber into theepoxy and the use of epoxy resins with flexiblebackbones. The properties of epoxy adhesives are alsovaried through the use of fillers. For example, quartzfillers can impart improved impact resistance, ceramicfillers can offer improved abrasion resistance, and silvercan be used to produce epoxies which are electricallyconductive.Advantages4Epoxy adhesives polymerize to form thermosetpolymers when covalent bonds between the epoxyresin and the hardener are formed through the reactionof the epoxide ring with the ring-opening species onthe hardener. Amines, amides, mercaptans, andanhydrides are some of the types of hardener that arecommonly used. Catalysts can be employed toaccelerate the reaction rate between the epoxy resinand hardener. In addition, heat will also accelerate thereaction. If heat is used to accelerate the cure of theepoxy, the increase in temperature can result in a dropof viscosity and an increased flow of the adhesive. Inaddition, curing the epoxy at a higher temperature willusually result in a stiffer material with a higher crosslink density and glass transition temperature. High cohesive strength High adhesion to a wide variety of substrates Good toughness Cure can be accelerated with heat Excellent depth of cure Good environmental resistanceDisadvantages Two-part systems require mixing One-part systems require heat cure Long cure and fixture times Limited pot life and work time Exotherm may be problematicThe Loctite Design Guide for Bonding Rubbers and TPEs, Volume 2
Hot Melt AdhesivesGeneral DescriptionHot melt adhesives are one-part solvent-freethermoplastic adhesives that are solid at roomtemperature and a low to medium viscosity (750-10,000cP) adhesive at dispense temperatures (typicallygreater than 195 C). After dispense, hot melt adhesivesrapidly cool to form a strong bond. In the cured orcooled state, hot melt adhesives can vary in physicalproperties from soft rubbery and very tacky to hard andrigid. Hot melts have excellent long term durability andresistance to moisture, chemicals, oils, and temperatureextremes.The latest advancement in hot melt technology is thereactive polyurethane adhesive (PUR). PURs initiallybehave like standard hot melts. That is, heat is addedto the soften the urethane prepolymer and it isdispensed hot. Once the PUR cools, it reacts withmoisture to cross-link into a tough thermosetpolyurethane adhesive that cannot be remelted byadding heat.Polyolefin hot melts are specially formulated foradhesion to polyolefin (polypropylene, polyethylene,etc.) plastics. Compared to other chemistries, they havelonger open times and they have excellent resistanceagainst polar solvents.Reactive polyurethanes are supplied as an urethaneprepolymer, behaving much like a standard hot meltuntil it cools. Once the PUR cools, it reacts withmoisture over time (a few days) to cross-link into atough thermoset polyurethane.Advantages One-part, solvent-free Fast fixturing High adhesion to plastics Wide variety of formulations available Low volumetric costChemistryDisadvantagesChemistries include ethylene vinyl acetate (EVA),polyolefin and polyamide based hot melts. EVA hotmelts are the “original” hot melt and are thought of asthe low cost, low performance hot melt. EVAs providegood adhesion to steel aluminum, rubber, and manyplastics. Typical EVA hot melt applications include boxand carton sealing. EVA hot melts can be formulated tocarry a FDA approval for use in food packaging. Out ofall available hot melts, EVAs typically have the pooresthigh temperature resistance. Hot dispense point Operator safety – Hot dispense point Poor adhesion on metals Cools quickly Equipment is required Thermoplastic parts may deform Charring in reservoir Moisture sensitivityPolyamide hot melts are a higher cost, higherperforming adhesive with excellent high temperatureresistance (up to 300 F). Specialty formulations areavailable that carry a UL-94V-0 rating (flameresistance). Polyamide hot melts have a tendency toabsorb moisture from the air and require specialpackaging and storage considerations.The Loctite Design Guide for Bonding Rubbers and TPEs, Volume 25
Light CuringAcrylic AdhesivesGeneral DescriptionLight curing acrylic adhesives are supplied as one-part,solvent-free liquids with viscosities ranging from 50 cPto thixotropic gels. Upon exposure to ultraviolet orvisible light of the proper intensity and spectral output,these adhesives cure rapidly to form thermosetpolymers with excellent adhesion to a wide variety ofsubstrates. The cure times of light curing acrylicadhesives are dependent on many parameters,however, cure times of 2 to 60 seconds are typical andcure depths in excess of 0.5" (13 mm) are possible.Formulations of light curing acrylic adhesives areavailable which vary in cured properties from veryrigid, glassy materials to soft, flexible elastomers.Light curing acrylic adhesives cure rapidly on demand,which minimizes work in progress and offers virtuallyunlimited repositioning time. In addition, the wide rangeof viscosities available facilitates the selection of aproduct for automated dispensing. These qualitiesmake light curing acrylics ideally suited for automatedbonding processes.Advantages Cure on demand Good environmental resistance Wide range of viscosities available Solvent-free Good gap filling One part Dispensing is easily automated Clear bond lines Rapid fixture and complete cure Wide range of physical properties UV/Visible cure systems available Fluorescent dyes can be added to t curing acrylic adhesives are composed of a blendof monomers, oligomers, and polymers containing theacrylate functionality to which a photoinitiator is added.Upon exposure to light of the proper intensity andspectral output, the photoinitiator decomposes to yieldfree radicals. The free radicals then initiatepolymerization of the adhesive through the acrylategroups to yield a thermoset polymer.6When the adhesive is cured in contact with air, thefree radicals created by the decomposition of thephotoinitiator can be scavenged by oxygen prior toinitiating polymerization. This can lead to incompletecure of the adhesive at the adhesive/oxygen interface,yielding a tacky surface. To minimize the possibility offorming a tacky surface, the irradiance of light reachingthe adhesive can be increased, the spectral output ofthe light source can be matched to the absorbancespectrum of the photoinitiator, and/or the adhesive canbe covered with an inert gas blanket during cure. Light must be able to reach bond line Oxygen can inhibit cure Equipment expense for light source Ozone created by high intensity light source mustbe ventedThe Loctite Design Guide for Bonding Rubbers and TPEs, Volume 2
PolyurethaneAdhesivesGeneral DescriptionPolyurethane adhesives are supplied as one and twopart systems which range in viscosity from self-levelingliquids to non-slumping pastes. They cure to formthermoset polymers with good solvent and chemica
Polyurethane Adhesives 7 Silicone Adhesives 8 . water-thin liquids to thixotropic gels. When pressed into a thin film between two surfaces, cyanoacrylates cure rapidly to form rigid thermoplastics with excellent . Epoxy adhesives are typically rigid and formulating. The Loctite .
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