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Epoxy Resin Adhesives
T.M.Goulding
Consultant,Johannesburg,South Africa
I.INTRODUCTION
Epoxy or epoxide resins are a group of reactive compounds that are characterized by the prence of the oxirane group
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They are capable of reacting with suitable hardeners to form cross-linked matrices of great strength and with excellent adhesion to a wide range of substrates.This makes them ideally suited to adhesive applications in which high strength under adver conditions is a prerequisite.Their unique characteristics include negligible shrinkage during cure,an open time equal to the usable life,excellent chemical resistance,ability to bond nonporous substrates,and great versatility.Although they were haile
d as wonder products whenfirst introduced,it has now been accepted that they will not do everything.They have,however, clearly established niches,especially in high-technology applications,and have shown steady growth,generally ahead of the industry average.Sales of epoxy resins in Europe, for example,totaled101,000metric tons in1980,150,000metric tons in1985,and205,000 metric tons in1990.
Although work on epoxy resins started in the mid-1920s,thefirst commercially uful epoxy resins appeared during World War II.The were bad on the diglycidyl ether of bisphenol A(usually referred to as DGEBA resins),and today the resins,in a range of molecular weights,constitute the majority of all epoxy resins ud.By contrast, however,hardeners come in a variety of shapes and sizes,including amines and amides, mercaptans,anhydrides,and Lewis acids and bas.Choice of hardener depends on the application requirements,and the wide range of hardeners available increas the versa-tility of adhesives bad on epoxy resins.
II.CHEMISTRY OF EPOXY RESINS
Epichlorhydrin is capable of reacting with hydroxyl groups,with the elimination of hydro-chloric acid.The most widely ud epoxy resins are the family of products produced by the reaction between epichlorhydrin and bisphenol A.
This reacts with additional epichlorhydrin to produce a molecule of general structure王哲林年薪
Commercially uful grades are relatively low-molecular-weight products in which n ranges from0to about4.When n is between0and1,the product is a liquid,and this is the most uful product for adhesi
恐怖睡前故事ve applications.As n increas,the product moves toward a brittle solid.Solid gradesfind application principally in paints.Regardless of molecular weight,the resulting resin has two epoxy groups per molecule.Resins of greater functionality can be produced from polyols having more than two hydroxyl groups per molecule.Thus phenol novolac resins,having the general structure can be reacted with epichlorhydrin to produce epoxy novolac resins.
咳白痰是怎么回事The products may have much greater functionality,although stearic considera-tions limit the uful size of the molecule.Becau of their higher functionality,epoxy novolacs have greater cross-link den
sity,generally yielding better temperature resistance at the expen of incread brittleness.They are thus ldom ud on their own,but make uful modifiers of the properties of DGEBA resins.激荡30年
Other products that may be epoxidized in this way include dihydric and trihydric phenols,aliphatic polyols such as glycerol,and simple alcohols such as butanol or allyl alcohol.The products,especially the monofunctional glycidyl ethers,are ud at rela-tively low percentages to modify properties of DGEBA resins,particularly to achieve lower viscosities.
Epoxy groups may also be produced by oxidation of olefinic unsaturation within animal and vegetable oils.The resulting products have too low a functionality for u as resins in their own right,but are added to DGEBA resins to introduce a measure of flexibility.Major manufacturers of epoxy resins include Shell,Dow and CIBA-GEIGY.
The resulting epoxy resin is capable of reacting with various products,or itlf,to form a solid,infusible product of considerable strength.The fact that the reactions generally occur without the production of low-molecular-weight by-products means that shrinkage during cure is negligible.This reduces stress in the cured structure,contribut-ing to the strength of the cross-linked matrix and eliminating the need for sophisticated clamping techniques.
The two cross-linking reactions are external,by reaction of the oxirane group with active hydrogen,and internal,by homopolymerization through the oxirane oxygen.The former is typical of cross-linking by hardeners and the latter of catalyzed cross-linking. Both hardeners and catalysts are referred to as curing agents.The classic epoxy curing mechanism is illustrated by the reaction between a primary amine and an epoxy group:
生姜汤This product can react with an additional epoxy group to continue the cross-linking process.
This reaction is characteristic of hardeners having active hydrogens available,including amines,amides,and mercaptans.The reaction is catalyzed by hydroxyl groups,especially phenolic hy
droxyls and tertiary amines.Becau of the bulk of the substituent groups involved,steric factors have a major influence on the reaction rate.Thus low-molecular-weight hardeners tend to react more vigorously and produce more cross-linked structures, while hardeners of high molecular weight tend to react more sluggishly.Hardeners may
thus be lected to produce highly exothermic reactions or reactions that take place only under the influence of external heat.Similarly,DGEBA resins having both epoxy groups at the ends of the molecule will react more readily with hardeners than will epoxy novolacs or other types of epoxies in which one or more of the functional groups may be hindered by the rest of the molecule.
Homopolymerization occurs readily in the prence of catalysts,especially at elevated temperatures.
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Again this reaction is accelerated by hydroxyl groups or tertiary amines.This is also the predominant reaction with anhydrides.In fact,reactions with resin and hardener or catalyst are very much more co
mplex than the idealized reactions,and both reactions as well as a number of side reactions probably occur to varying extents in any cross-linking mechanism.Major suppliers of curing agents include Anchor Chemicals,Dow,Shell,and Cray Valley Products.
III.PROPERTIES OF EPOXIES
A.Resins
轶闻趣事Epoxy resins react with hardeners in stoichiometric quantities.Thus a knowledge of the number of reactive sites is needed in order to calculate correct ratios.For the resin this is given by the epoxide equivalent weight(EEW),which is the quantity of resin required to yield one epoxy group.For a DGEBA type in which n¼0,the molecular weight is340. Since there are two epoxy groups per molecule,the EEW is thus170.Typically,the pure liquid DGEBA resins commercially available for adhesive applications have EEW in the range180to310,usually190to210,while for paints or special applications,EEW may reach2000or more.The epoxy novolacs usually have EEW in the range150to250, usually around180.
The viscosity of a DGEBA resin is dependent primarily on molecular weight.Even at low molecular weight,viscosity is typically in excess of6,000cP,while at EEW190 viscosity is usually around12,000c
P.For applications requiring low viscosity it is thus necessary to include other types of epoxy resin or to u reactive or nonreactive diluents to achieve the desired viscosity.
B.Hardeners
Stoichiometric ratios can be calculated similarly for hardeners.In principle,each active hydrogen will react with one epoxy group.Thus a low-molecular-weight aliphatic poly-amine such as diethylene triamine(DETA)has a molecular weight of103andfive active hydrogens.The hydrogen equivalent is thus20.6.The stoichiometrically correct ratio with an epoxy resin of EEW200would thus be100parts resin to10.3parts of DETA.In practice there is always a percentage of homopolymerization,especially at the temperature
of reaction,and smaller amounts of DETA will still cau a complete cure,at the expen of increasing brittleness.
In general,suppliers of proprietary hardeners do not furnish detailed chemical descriptions.Instead,they supply data on recommended mix ratios,and from this the formulator can calculate the correct quantities.With catalysts,stoichiometry is not criti-cal,in theory.In practice,however,the quantity ud will affect both the rate of cure and the cured properties.Thus wit
h catalysts,in practice,the mix ratio is sometimes more critical than is the ca with hardeners.
C.Mixed Product
During cure of epoxies,especially systems with a short pot life or large mixes,considerable heat is evolved.This accelerates the cure,leading to even greater heat evolution.Mixes larger than5kg can reach excessive temperatures even with systems that have relatively long pot lives in quantities of100g.In addition to shortening the pot life dramatically, exothermic reactions can push the peak temperature to the point where thermal degrada-tion occurs,or at least to a level that creates excessive stress in the curing matrix,causing it to crack on cooling.Except in certain circumstances,peak exotherm temperature should be limited by formulation to150 C or preferably less in the mix quantities ud.Cured epoxy resins may be formulated to be extremely hard,with Shore D hardeners of80or more,or soft,flexible products that barely produce a reading on the Shore A scale.When cured at approximately stoichiometric ratios and unmodified with diluents or plasticizers, however,they are generally hard and tough to brittle,especially DGEBA and epoxy novolac types.
Heat distortion temperature(HDT)or deflection temperature(DT)is a measure of the tendency of cure
d product to soften when heated.It is a feature of the inherent thermo-plasticity in cured epoxy compounds as a result of the relatively low cross-linking density, and may be any value from below50 C to about250 C,depending on formulation and cure cycle.Resins and hardeners of high functionality tend to have higher HDT. Postcuring at elevated temperature can increa HDT significantly.
IV.FORMULATING EPOXY ADHESIVES
Epoxy resins offer a unique combination of properties for adhesive applications.The include the ability to formulate liquid systems without solvents or carriers,the ability to convert the systems to cured products without the production of low-molecular-weight by-products,the ability to bond dissimilar or nonporous surfaces,and the ability to produce thick ctions without subquent stress cracking due to shrinkage.
Although epoxy resin and hardener may be ud in unmodified form in adhesive systems,most systems will consist of components that have been modified by incorpora-tion of various additives to achieve specific effects.Formulators will add catalysts or blend hardeners to obtain a specific usable life of the mix and to control the curing temperature. Reactive diluents may be added to modify visco
sity orflexibility.Fillers impart improved compression strength and reduce shrinkage and cost.Solvents may be ud to reduce viscosity or improve adhesion.Various additives may be added,usually at a low percen-tage,to reduce aeration,improve adhesion to difficult surfaces,or minimize ttlement offillers.Depending on the application,particular properties such asflame retardency,
