Handbook of Plastics, Elastomers and Composites Part 13 pdf

To use these materials, the designer must pay a premium in adhesivecost and also be capable of providing long, high-temperature cures.For an adhesive to withstand elevated-temperature ex

displace the adhesive, and result in bond failure These weak boundary layers may comefrom the environment or from within the plastic substrate itself.

Moisture, solvent, plasticizers, and various gases and ions can compete with the curedadhesive for bonding sites The process where a weak boundary layer preferentially dis-

places the adhesive at the interface is called desorption Moisture is the most common

de-sorbing substance, being present both in the environment and within many polymericsubstrates

Solutions to the desorption problem consist of eliminating the source of the weakboundary layer or selecting an adhesive that is compatible with the desorbing material.Excessive moisture can be eliminated from a plastic part by post curing or drying the partbefore bonding Additives that can migrate to the surface can possibly be eliminated by re-formulating the plastic resin Also, certain adhesives are more compatible with oils andplasticizers than others For example, the migration of plasticizer from flexible polyvinylchloride can be counteracted by using a nitrile-based adhesive Nitrile adhesive resins arecapable of absorbing the plasticizer without degrading

weld-ing as well as with adhesives These alternative joinweld-ing processes are discussed in detail inanother chapter The plastic manufacturer is generally the leading source of information

on the proper methods of joining a particular plastic

welded They are easily bonded with many adhesives, some of which have been listed inTable 7.31 Abrasion is generally recommended as a surface treatment

matrix alone may also be used to bond reinforced plastics Surface preparation of forced thermosetting plastics consists of abrasion and solvent cleaning A degree of abra-sion is desired so that the reinforcing material is exposed to the adhesive

rein-Reinforced thermoplastic parts are generally abraded and cleaned prior to adhesivebonding However, special surface treatment such as used on the thermoplastic resin ma-trix may be necessary for optimal strength Care must be taken so that the treatment chem-icals do not wick into the substrate and cause degradation Certain reinforcedthermoplastics may also be solvent cemented or heat welded However, the percentage offiller in the substrate must be limited, or the bond will be starved of resin

pressure-sensi-tive adhesives will collapse thermoplastic foams Water-based adhesives, based on styrenebutadiene rubber (SBR) or polyvinyl acetate, and 100 percent solid adhesives are oftenused Butyl, nitrile, and polyurethane adhesives are often used for flexible polyurethanefoam Epoxy adhesives offer excellent properties on rigid polyurethane foam

7.5.4.1 Vulcanized elastomers. Bonding of vulcanized elastomers to themselvesand other materials is generally accomplished by using a pressure-sensitive adhesive de-rived from an elastomer similar to the one being bonded Flexible thermosetting adhesivessuch as epoxy-polyamide or polyurethane also offer excellent bond strength to most elas-tomers Surface treatment consists of washing with a solvent, abrading, or acid cyclizing

as described in Table 7.18

Elastomers vary greatly in formulation from one manufacturer to another Fillers, ticizers, antioxidants, etc., may affect the adhesive bond Adhesives should be thoroughlytested on a specific elastomer and then re-evaluated if the elastomer manufacturer or for-mulation is changed

metals and other rigid adherends by priming the adherend with a suitable air- or ing adhesive before the elastomer is molded against the adherend The most common elas-tomers to be bonded in this way include nitrile, neoprene, urethane, natural rubber, SBR,and butyl rubber Less common unvulcanized elastomers such as the silicones, fluorocar-bons, chlorosulfonated polyethylene, and polyacrylate are more difficult to bond

heat-dry-However, recently developed adhesive primers improve the bond of these elastomers tometal Surface treatment of the adherend before priming should be according to good stan-dards

Resorcinol-formaldehyde resins are cold-setting adhesives for wood structures formaldehyde adhesives, commonly modified with melamine formaldehyde, are used inthe production of plywood and in wood veneering for interior applications Phenol-form-aldehyde and resorcinol-formaldehyde adhesive systems have the best heat and weatherresistance

Urea-Polyvinyl acetates are quick-drying, water-based adhesives commonly used for the sembly of furniture This adhesive produces bonds stronger than the wood itself, but it isnot resistant to moisture or high temperature Table 7.34 describes common adhesivesused for bonding wood

Glass adhesives are generally transparent, heat-setting resins that are water-resistant tomeet the requirements of outdoor applications Adhesives generally used to bond glass,and their physical characteristics, are presented in Table 7.35

7.6 Effect of the Environment

For an adhesive bond to be useful, it not only must withstand the mechanical forces acting

on it; it must also resist the service environment Adhesive strength is influenced by manycommon environments, including temperature, moisture, chemical fluids, and outdoorweathering Table 7.36 summarizes the relative resistance of various adhesive types tocommon environments

All polymeric materials are degraded to some extent by exposure to elevated temperatures.Not only are physical properties lowered at high temperatures, they also degrade due tothermal aging Newly developed polymeric adhesives have been found to withstand 500 to600°F continuously To use these materials, the designer must pay a premium in adhesivecost and also be capable of providing long, high-temperature cures.

For an adhesive to withstand elevated-temperature exposure, it must have a high ing or softening point and resistance to oxidation Materials with a low melting point, such

melt-as many of the thermoplmelt-astic adhesives, may prove excellent adhesives at room ture However, once the service temperature approaches the glass transition temperature ofthese adhesives, plastic flow results in deformation of the bond and degradation in cohe-sive strength Thermosetting materials, exhibiting no melting point, consist of highlycross-linked networks of macromolecules Many of these materials are suitable for high-temperature applications When considering thermoset adhesives, the critical factor is therate of strength reduction due to thermal oxidation and pyrolysis

tempera-Thermal oxidation initiates progressive chain scission of molecules resulting in losses

of weight, strength, elongation, and toughness within the adhesive Figure 7.30 illustratesthe effect of oxidation by comparing adhesive joints aged in both high-temperature air andinert-gas environments The rate of strength degradation in air depends on the temperature,the adhesive, the rate of airflow, and even the type of adherend Certain metal–adhesive in-terfaces are capable of accelerating the rate of oxidation For example, many structural ad-hesives exhibit better thermal stability when bonded to aluminum than when bonded tostainless steel or titanium (Fig 7.30)

High-temperature adhesives are usually characterized by a rigid polymeric structure,high softening temperature, and stable chemical groups The same factors also make these

TABLE 7.34 Properties of Common Wood Adhesives (from Ref 36)

Resin type used

Resin solids

in glue mix, % Principal use applicationMethod of Principal property Principal limitation Urea

formaldehyde

23–30 Wood-to-wood interior

Spreader rolls Bleed-through-free;

formaldehyde

68–72 Wood-to-wood, splicing, patching, scarfing

Sprayed, combed Adhesion, color,

durability

Relative cost; poor washability; needs heat to cure

Melamine

urea 1/1

55–60 End and edge gluing exterior

Applicator Colorless, durability

Spreader rolls Cold sets durability Cost, odor

Phenol-resorcinol

10/90

50–56 Wood-to-wood exterior (laminating)

Spreader rolls Warm-set durability Cost, odor

Polyvinyl acetate

emulsion

45–55 Wood-to-wood interior

Brushed, sprayed, spreader rolls

Handy Lack of H 2 O and heat

resistance

adhesives very difficult to process Only epoxy-phenolic-, polyimide-, and zole-based adhesives can withstand long-term service temperatures greater than 350°F.

be-low 300°F Figure 7.31 illustrates the aging characteristics of a typical epoxy adhesive atelevated temperatures Certain epoxy adhesives are able to withstand short terms at 500°Fand long-term service at 300 to 350°F These systems were formulated especially for ther-mal environments by incorporation of stable epoxy co-reactants, high-temperature curingagents, and antioxidants into the adhesive

One successful epoxy co-reactant system is an epoxy-phenolic alloy The excellentthermal stability of the phenolic resins is coupled with the adhesion properties of epoxies

to provide an adhesive capable of 700°F short-term operation and continuous use at 350°F.The heat-resistance and thermal-aging properties of an epoxy-phenolic adhesive are com-pared with those of other high-temperature adhesives in Fig 7.32

TABLE 7.35 Commercial Adhesives Most Desirable for Glass (from Ref 37)

Bond characteristics

Strength, lb/in 2 Type of failure

Weathering quality

Res–N–Glue, du Pont 5459 Cellulose vinyl 1000–1200 Adhesive Fair

Anhydride curing agents give unmodified epoxy adhesives greater thermal stabilitythan most other epoxy curing agents Phthalic anhydride, pyromellitic dianhydride, andchlorendic anhydride allow greater cross-linking and result in short-term heat resistance to450°F Long-term thermal endurance, however, is limited to 300°F Typical epoxy formu-

ni-trile-phenolic blend has the best resistance to shear at elevated temperatures Nitrile nolic adhesives have high shear strength up to 250 to 350°F, and the strength retention onaging at these temperatures is very good The nitrile phenolic adhesives are also extremelytough and provide high peel strength

Their chief application is in nonstructural applications such as high-temperature sensitive tape

pressure-Attempts have been made to incorporate silicones with other resins such as epoxies andphenolics, but long cure times and low strength have limited their use

Figure 7.30 The effect of 500°F aging in air and nitrogen

on an epoxy-phenolic adhesive (HT-424).39

Figure 7.31 Effect of temperature aging on typical

epoxy adhesive in air Strength measured at room

temperature.40

7.6.1.4 Polyaromatics. The most common polyaromatic resins, polyimide and benzimidazole, offer greater thermal resistance then any other commercially available ad-hesive The rigidity of their molecular chains decreases the possibility of chain scissioncaused by high temperatures The aromaticity of these structures provides high bond-dis-sociation energy and acts as an “energy sink” to the thermal environment.

poly-Polyimide. The strength retention of polyimide adhesives for short exposures to

endur-ance of polyimides at temperatures greater than 500°F is unmatched by other cially available adhesives

commer-Polyimide adhesives are usually supplied as a glass-fabric-reinforced film having a

necessary for optimal properties High-boiling volatiles can be released during cure, whichcauses a somewhat porous adhesive layer Because of the inherent rigidity of this material,peel strength is low

Figure 7.32 Comparison of (a) heat resistance and (b) thermal aging of

high-temperature structural adhesives 41

Polybenzimidazole. As illustrated in Fig 7.32, polybenzimidazole (PBI) adhesivesoffer the best short-term performance at elevated temperatures However, PBI resins oxi-dize rapidly and are not recommended for continuous use at temperatures over 450°F.PBI adhesives require a cure at 600°F Release of volatiles during cure contributes to aporous adhesive bond Supplied as a very stiff, glass-fabric-reinforced film, this adhesive

is expensive, and applications are limited by a long, high-temperature curing cycle

The factors that determine the strength of an adhesive at very low temperatures are (1) thedifference in coefficient of thermal expansion between adhesive and adherend, (2) theelastic modulus, and (3) the thermal conductivity of the adhesive The difference in ther-mal expansion is very important, especially since the elastic modulus of the adhesive gen-erally decreases with falling temperature It is necessary that the adhesive retain someresiliency if the thermal expansion coefficients of adhesive and adherend cannot be closelymatched The adhesive’s coefficient of thermal conductivity is important in minimizingtransient stresses during cooling This is why thinner bonds have better cryogenic proper-ties than thicker ones

Low-temperature properties of common structural adhesives used for cryogenic cations are illustrated in Fig 7.33

appli-Epoxy-polyamide adhesives can be made serviceable at very low temperatures by theaddition of appropriate fillers to control thermal expansion But the epoxy-based systemsare not as attractive as some others because of brittleness and corresponding low peel andimpact strength at cryogenic temperatures

Epoxy-phenolic adhesives are exceptional in that they have good adhesive properties atboth elevated and low temperatures Vinyl-phenolic adhesives maintain fair shear and peelstrength at –423°F, but strength decreases with decreasing temperature Nitrile-phenolicadhesives do not have high strength at low service temperatures, because of rigidity.Polyurethane and epoxy-nylon systems offer outstanding cryogenic properties Poly-urethane adhesives are easily processable and bond well to many substrates Peel strengthranges from 22 lb/in at 75° to 26 lb/in at –423°F, and the increase in shear strength at

Figure 7.33 Properties of cryogenic structural adhesive systems.41

–423°F is even more dramatic Epoxy-nylon adhesives also retain flexibility and yield

Heat-resistant polyaromatic adhesives also have shown promising low-temperatureproperties The shear strength of a polybenzimidazole adhesive on stainless-steel sub-

applicability of polyaromatic adhesives on structures seeing both very high and low peratures

Moisture can affect adhesive strength in two significant ways Some polymeric materials,

notably ester-based polyurethanes, will revert—i.e., lose hardness, strength, and (in the

worst cases) turn fluid during exposure to warm, humid air Water can also permeate theadhesive and preferentially displace the adhesive at the bond interface This later mecha-nism is the most common cause of adhesive-strength reduction in moist environments The rate of reversion or hydrolytic instability depends on the chemical structure of thebase adhesive, the type and amount of catalyst used, and the flexibility of the adhesive.Certain chemical linkages, such as ester, urethane, amide, and urea, can be hydrolyzed.The rate of attack is fastest for ester-based linkages Ester linkages are present in certaintypes of polyurethanes and anhydride-cured epoxies Generally, amine-cured epoxies offerbetter hydrolytic stability than anhydride-cured types Figure 7.34 illustrates the hydro-lytic stability of various polymeric materials determined by a hardness measurement Re-version is usually much faster in flexible materials because water permeates more easily.Structural adhesives not susceptible to the reversion phenomenon are also likely to loseadhesive strength when exposed to moisture The degradation curves shown in Fig 7.35are typical for an adhesive exposed to moist, high-temperature environments The mode offailure in the initial stages of aging is usually truly cohesive After aging, the failure be-comes one of adhesion It is expected that water vapor permeates the adhesive through itsexposed edges and concentrates in weak boundary layers at the interface This effect isgreatly dependent on the type of adhesive and substrate

Figure 7.34 Hydrolytic stability of potting compounds Materials showing rapid hardness loss will soften similarly after two to four years

at ambient temperatures in a high-humidity, tropical climate.42

Stress accelerates the effect of environments on the adhesive joint Little data are able on this phenomenon because of the time and expense associated with stress-agingtests However, it is known that moisture, as an environmental burden, markedly decreasesthe ability of an adhesive to bear prolonged stress Figure 7.36 illustrates the effect ofstress aging on specimens exposed to relative humidity cycling from 90 to 100 percent andsimultaneous temperature cycling from 80 to 120°F The loss of load-bearing ability of acertain flexibilized epoxy adhesive (Fig 7.36) is exceptional The stress on this particularadhesive had to be reduced to 13 percent of its original strength for the joint to last a littlemore than 44 days in the test environment.

The most detrimental factors influencing adhesives aged outdoors are heat and humidity.Thermal cycling, ultraviolet radiation, and cold are relatively minor factors The reasonswhy warm, moist climates degrade adhesive joints were presented in the previous section.When exposed to weather, structural adhesives rapidly lose strength during the first sixmonths to one year After two to three years, the rate of decline usually levels off, depend-ing on the climate zone, adherend, adhesive, and stress level Figure 7.37 shows the weath-ering characteristics of unstressed epoxy adhesives to the Richmond, VA, climate

Figure 7.35 Effect of humidity on adhesion of two structural adhesives to stainless steel.43

Figure 7.36 Time to failure vs stress for two adhesives in a warm, high-humidity environment: (a) adhesive = one-part, heat-curing, modified epoxy; (b) adhesive = flexibilized, amine-cured epoxy.44

The following generalizations are of importance in designing an adhesive joint for door service:

out-1 The most severe locations are those with high humidity and warm temperatures

2 Stressed panels deteriorate more rapidly than unstressed panels

3 Stainless-steel panels are more resistant than aluminum panels because of corrosion

4 Heat-cured adhesive systems are generally more resistant than cured systems

room-temperature-5 With the better adhesives, unstressed bonds are relatively resistant to severe outdoorweathering, although all joints will eventually exhibit some strength loss

MIL-STD-304 is a commonly used accelerated-exposure technique to determine the fect of weathering and high humidity on adhesive specimens Adhesive comparisons can

ef-be made with this type of test In this procedure, bonded panels are exposed to alternatingcold (–65°F), dry heat (160°F), and heat and humidity (160°F, 95 percent RH) for 30 days

Figure 7.37 Effect of outdoor weathering on typical aluminum joints made with four different two-part epoxies cured at room temperature 25

The effect of MIL-STD-304 conditioning on the joint strength of common structural sives is presented in Table 7.37.

Most organic adhesives tend to be susceptible to chemicals and solvents, especially at vated temperatures Standard test fluids and immersion conditions are used by adhesivesuppliers and are defined in MMM-A-132 Unfortunately exposure tests lasting less than

ele-30 days are not applicable to many requirements Practically all adhesives are resistant tothese fluids over short time periods and at room temperatures Some epoxy adhesives evenshow an increase in strength during aging in fuel or oil This effect is possibly due to apost-curing or plasticizing of the epoxy by oil

Epoxy adhesives are generally more resistant to a wide variety of liquid environmentsthan other structural adhesives However, the resistance to a specific environment isgreatly dependent on the type of epoxy curing agent used Aromatic amine, such asmetaphenylene diamine, cured systems are frequently preferred for long-term chemical re-sistance

There is no “best adhesive” for universal chemical environments As an example, mum resistance to bases almost axiomatically means poor resistance to acids It is rela-tively easy to find an adhesive that is resistant to one particular chemical environment Itbecomes more difficult to find an adhesive that will not degrade in two widely differingchemical environments Generally, adhesives that are most resistant to high temperatureshave the best resistance to chemicals and solvents

maxi-The temperature of the immersion medium is a significant factor in the aging properties

of the adhesive As the temperature increases, more fluid is generally absorbed by the hesive, and the degradation rate increases

ad-From the rather limited information reported in the literature, it may be summarizedthat

1 Chemical-resistance tests are not uniform in concentrations, temperature, time, erties measured

prop-TABLE 7.37 Effect of MIL–STD–304 Aging on Bonded Aluminum Joints (from Ref 45)

Adhesive

Shear, lb/in2, 73°F

Shear, lb/in2, 160°F

2100 1640 Failed 1730 3120‘

920 1970 2350 3400 3900

2700 1700 720 220 3300 3300 1600 2900 4100 3070

1800 6070 Failed 80 2720 1330 1560 2190 3200 2900

2 Generally, chlorinated solvents and ketones are severe environments.

3 High-boiling solvents, such as dimethylformamide and dimethyl sulfoxide, are severeenvironments

4 Acetic acid is a severe environment

5 Amine curing agents for epoxies are poor in oxidizing acids

6 Anhydride curing agents are poor in caustics

The ability of an adhesive to withstand long periods of exposure to a vacuum is of primaryimportance for certain applications Loss of low-molecular-weight constituents such asplasticizers or diluents could result in hardening and porosity of cured adhesives or seal-ants

Since most structural adhesives are relatively high-molecular- weight polymers,

or other degrading environments may cause the formation of low-molecular-weight ments that tend to bleed out of the adhesive in a vacuum

days at room temperature However, polyurethane adhesives exhibit significant outgassing

High-energy particulate and electromagnetic radiation, including neutron, electron, andgamma radiation, have similar effects on organic adhesives Radiation causes molecular-chain scission of the adhesive, which results in weakening and embrittlement of the bond.This degradation is worsened when the adhesive is simultaneously exposed to elevatedtemperatures and radiation

Figure 7.38 illustrates the effect of radiation dosage on the tensile-shear strength ofstructural adhesives Generally, heat-resistant adhesives have been found to resist radiationbetter than less thermally stable systems Fibrous reinforcement, fillers, curing agents, andreactive diluents affect the radiation resistance of adhesive systems In epoxy-based adhe-sives, aromatic curing agents offer greater radiation resistance than aliphatic-type curingagents

7.7 Processing and Quality Control of

Adhesive Joints

Processing and quality control are usually the final considerations in the design of an hesive-bonding system These decisions are very important, however, because they alonemay (1) restrict the degrees of freedom in designing the end product, (2) determine thetypes and number of adhesives that can be considered, (3) affect the quality and reproduc-ibility of the joint, and (4) affect the total assembly cost

When a multiple-part adhesive is used, the concentration ratios have a significant effect onthe quality of the joint Strength differences caused by varying curing-agent concentrationare most noticeable when the joints are tested at elevated temperatures or after exposure to

water or solvents Exact proportions of resin and hardener must be weighed out on an curate balance or in a measuring container for best adhesive quality and reproducibility.The weighed-out components must be mixed thoroughly Mixing should be continueduntil no color streaks or density stratifications are noticeable Caution should be taken toprevent air from being mixed into the adhesive through overagitation This can causefoaming of the adhesive during heat cure, resulting in porous bonds If air does becomemixed into the adhesive, vacuum degassing may be necessary before application.

ac-Only enough adhesive should be mixed that can be used before the adhesive begins tocure Working life of an adhesive is defined as the period of time during which an adhesiveremains suitable for use after mixing with catalyst Working life is decreased as the ambi-ent temperature increases and as the batch size becomes larger One-part, and some heat-curing, two-part, adhesives have very long working lives at room temperature, and appli-cation and assembly speed or batch size are not critical

For a large-scale bonding operation, hand mixing is costly, messy, and slow, and ability is entirely dependent on the operator Equipment is available that can meter, mix,and dispense multicomponent adhesives on a continuous or shot basis

The selection of an application method depends primarily on the form of the adhesive: uid, paste, powder, or film Table 7.38 describes the advantages and limitations realized inusing each of the four basic forms Other factors influencing the application method arethe size and shape of parts to be bonded, the total area where the adhesive is to be applied,and production volume and rate

liq-Figure 7.38 Percent change of initial tensile-shear strength caused by nuclear

radia-tion dosage.46

7.7.2.1 Liquids. Liquids, the most common form of adhesive, can be applied by a riety of methods Brushes, simple rollers, and glue guns are manual methods that providesimplicity, low cost, and versatility Spray, dipping, and mechanical roll coaters are gener-ally used on large production runs Mechanical roller methods are commonly used to ap-ply a uniform layer of adhesive to a continuous roll or coil Such automated systems areused with adhesives that have a long working life and low viscosity Spray methods can beused on both small and large production runs The spray adhesive is generally in solventsolution, and sizable amounts of adhesive may be lost from overspray Two-component ad-hesives are usually mixed prior to placing in the spray-gun reservoir Application systemsare available, however, that meter and mix the adhesive in the spray-gun barrel This isideal for fast-reacting systems.

reproducible adhesives to apply These systems can be troweled or extruded through acaulking gun Little operator skill is required Since the thixotropic nature of the paste pre-vents it from flowing excessively, application is usually clean, and little waste is generated

1 They may be sifted onto a preheated substrate The powder that falls onto the substratemelts and adheres The assembly is then mated and cured according to recommendedprocesses

2 A preheated substrate could also be dipped into a fluidized bed of the powder and thenextracted with an attached coating of adhesive This method helps to ensure an evendistribution of powder

3 The powder can be melted into a paste or liquid and applied by conventional means Powder adhesives are generally one-part, epoxy-based systems that require heat andpressure to cure They do not require metering and mixing but often must be refrigeratedfor extended shelf life

1 High repeatability—no mixing or metering, constant thickness

2 Easy to handle—low equipment cost, relatively hazard-free, clean operating

3 Very little waste—preforms can be cut to size

4 Excellent physical properties—wide variety of adhesive types available

Films are limited to flat surfaces or simple contours Application requires a relatively highdegree of care to ensure nonwrinkling and removal of separator sheets Characteristics ofavailable film adhesives vary widely, depending on the type of adhesive used

Film adhesives are made as both unsupported and supported types The carrier for ported films is generally fibrous fabric or mat Film adhesives are supplied in heat-acti-vated, pressure-sensitive, or solvent-activated forms Solvent-activated adhesives are madetacky and pressure-sensitive by wiping with solvent They are not as strong as the othertypes but are well suited for contoured, curved, or irregularly shaped parts Manual sol-

sup-vent-reactivation methods should be closely monitored so that excessive solvent is notused Solvent-activated films include neoprene, nitrile, and butyral phenolics Decorativetrim and nameplates are usually fastened onto a product with solvent-activated adhesives.

After the adhesive is applied, the assembly must be mated as quickly as possible to preventcontamination of the adhesive surface The substrates are held together under pressure andheated, if necessary, until cure is achieved The equipment required to perform these func-tions must provide adequate heat and pressure, maintain the prescribed pressure during theentire cure cycle, and distribute pressure uniformly over the bond area Of course, manyadhesives require only simple contact pressure at room temperature, and extensive bond-ing equipment is not necessary

con-stant pressure on the bond during the entire cure cycle They must compensate for ness reduction from adhesive flow or thermal expansion of assembly parts Thus, screw-actuated devices like C-clamps and bolted fixtures are not acceptable when a constantpressure is important Spring pressure can often be used to supplement clamps and com-pensate for thickness variations Dead-weight loading may be applied in many instances;however, this method is sometimes impractical, especially when heat cure is necessary.Pneumatic and hydraulic presses are excellent tools for applying constant pressure.Steam or electrically heated platen presses with hydraulic rams are often used for adhesivebonding Some units have multiple platens, thereby permitting the bonding of several as-semblies at one time

thick-Large bonded areas such as on aircraft parts are usually cured in an autoclave The partsare mated first and covered with a rubber blanket to provide uniform pressure distribution.The assembly is then placed in an autoclave, which can be pressurized and heated Thismethod requires heavy capital-equipment investment

Vacuum-bagging techniques can be an inexpensive method of applying pressure tolarge parts A film or plastic bag is used to enclose the assembly, and the edges of the filmare sealed airtight A vacuum is drawn on the bag, enabling atmospheric pressure to forcethe adherends together Vacuum bags are especially effective on large areas, because size

is not limited by equipment

pres-sure Most often, the strongest bonds are achieved by an elevated-temperature cure Withmany adhesives, trade-offs between cure times and temperature are permissible But, gen-erally, the manufacturer will recommend a certain curing schedule for optimum properties

If, for example, a cure of 60 min at 300°F is recommended, this does not mean that theassembly should be placed in a 300°F oven for 60 min It is the bond line that should be at300°F for 60 min Total oven time would be 60 min plus whatever time is required to bringthe assembly up to 300°F Large parts act as a heat sink and may require substantial timefor an adhesive in the bond line to reach the necessary temperature Bond-line tempera-tures are best measured by thermocouples placed very close to the adhesive In somecases, it may be desirable to place the thermocouple in the adhesive joint for the first fewassemblies being cured

Oven heating is the most common source of heat for bonded parts, even though it volves long curing cycles because of the heat-sink action of large assemblies Ovens may

in-be heated with gas, oil, electricity, or infrared units Good air circulation within the oven ismandatory to prevent nonuniform heating.

Heated-platen presses are good for bonding flat or moderately contoured panels whenfaster cure cycles are desired Platens are heated with steam, hot oil, or electricity and areeasily adapted with cooling-water connections to further speed the bonding cycle

(2- to 10-mil) adhesive bond line Starved adhesive joints, however, will yield ally poor properties Three basic methods are used to control adhesive thickness The firstmethod is to use mechanical shims or stops which can be removed after the curing opera-tion Sometimes it is possible to design stops into the joint

exception-The second method is to employ a film adhesive that becomes highly viscous during thecure cycle preventing excessive adhesive flow-out With supported films, the adhesive car-

rier itself can act as the shims Generally, the cured bond-line thickness will be determined

by the original thickness of the adhesive film The third method of controlling adhesivethickness is to use trial and error to determine the correct pressure-adhesive viscosity fac-tors that will yield the desired bond thickness

A flow chart of a quality-control system for a major aircraft company is illustrated in Fig.7.39 This system is designed to ensure reproducible bonds and, if a substandard bond isdetected, to make suitable corrections Quality control should cover all phases of the bond-ing cycle from inspection of incoming material to the inspection of the completed assem-bly In fact, good quality control will start even before receipt of materials

process more than in other fabrication techniques An extremely high percentage of fects can be traced to poor workmanship This generally prevails in the surface-prepara-tion steps but may also arise in any of the other bonding steps This problem can be largelyovercome by proper motivation and education All employees, from design engineer to la-borer to quality-control inspector, should be somewhat familiar with adhesive-bondingtechnology and be aware of the circumstances that can lead to poor joints

de-The plant’s bonding area should be as clean as possible prior to receipt of materials.The basic approach to keeping the assembly area clean is to segregate it from the othermanufacturing operations either in a corner of the plant or in isolated rooms The airshould be dry and filtered to prevent moisture or other contaminants from gathering at apossible interface The cleaning and bonding operations should be separated from eachother If mold release is used to prevent adhesive flash from sticking to bonding equip-ment, it is advisable that great care be taken to ensure that the release does not contaminatethe adherends Spray mold releases, especially silicone release agents, have a tendency tomigrate to undesirable areas

on adhesives should be directed toward assurance that incoming materials are identicalfrom lot to lot The tests should be those that can quickly and accurately detect deficien-cies in the adhesive’s physical or chemical properties ASTM lists various test methodsthat are commonly used for adhesive acceptance Actual test specimens should also be