Modern Plastics Handbook 2011 Part 12 pps

Majorelements of concern in selecting an adhesive for plastic parts are thethermal expansion coefficient and glass transition temperature of the TABLE 9.12 Effect of Surface Treatments o

P=Paar F=Fair G=Gaad

Figure 9.25 Types of angle joints and methods of reducing cleavage.13

Stress direction

-Corner joints for relatively flexible adherends, such as sheet metal,should be designed with reinforcements for support Various corner-joint designs are shown in Fig 9.26 With very thin adherends, anglejoints offer low strengths because of high peel concentrations A designconsisting of right-angle corner plates or slip joints offers the most sat-isfactory performance Thick, rigid members, such as rectangular barsand wood, may be bonded with an end lap joint, but greater strengthscan be obtained with mortise and tenon Hollow members, such asextrusions, fasten together best with mitered joints and inner splines.Flexible plastics and elastomers Thin or flexible polymeric substrates may

be joined using a simple or modified lap joint The double strap joint isbest, but also the most time-consuming to fabricate The strap mater-ial should be made out of the same material as the parts to be joined,

or at least have approximately equivalent strength, flexibility, andthickness The adhesive should have the same degree of flexibility asthe adherends

If the sections to be bonded are relatively thick, a scarf joint isacceptable The length of the scarf should be at least 4 times the thick-ness; sometimes larger scarves may be needed

When bonding elastic material, forces on the elastomer during cure

of the adhesive should be carefully controlled, since excess pressure

Finishing, Assembly, and Decorating 9.51

will cause residual stresses at the bond interface Stress tions may also be minimized in rubber-to-metal joints by elimination

concentra-of sharp comers and using metal thick enough to prevent peel

stress-es that may arise with thinner-gauge metals

As with all joint designs, polymeric joints should avoid peel stress.Figure 9.27 illustrates methods of bonding flexible substrates so thatthe adhesive will be stressed in its strongest direction

Rigid plastic composites Reinforced plastics are often anisotropic als This means their strength properties are directional Joints madefrom anisotropic substrates should be designed to stress both the adhe-sive and adherend in the direction of greatest strength Laminates, forexample, should be stressed parallel to the laminations Stresses nor-mal to the laminate may cause the substrate to delaminate

materi-Single and joggle lap joints are more likely to cause delaminationthan scarf or beveled lap joints The strap-joint variations are usefulwhen bending loads may be imposed on the joint

9.5.5 Surface preparation

Many plastics and plastic composites can be treated prior to ing by simple mechanical abrasion or alkaline cleaning to removesurface contaminants In some cases it is necessary that the poly-meric surface be physically or chemically modified to achieveacceptable bonding This applies particularly to crystalline thermo-plastics such as the polyolefins, linear polyesters, and fluorocar-bons Methods used to improve the bonding characteristics of thesesurfaces include

bond-1 Oxidation via chemical treatment or flame treatment

2 Electrical discharge to leave a more reactive surface

3 Plasma treatment (exposing the surface to ionized inert gas)

4 Metal-ion treatment (for example, sodium naphthalene process forfluorocarbons)

Stress direction

~ Poor

-Poor

lGOOd I-Good

Figure 9.27 Methods of joining flexible rubber or plastic.13

~l.1:J~

Table 9.11 lists common recommended surface treatments for plasticadherends These treatments are necessary when plastics are to be joinedwith adhesives Solvent and heat welding are other methods of fasteningplastics that do not require chemical alteration of the surface Weldingprocedures will be discussed in another section of this chapter The effects

of plastic surface treatments decrease with time It is necessary to prime

or bond soon after the surfaces are treated Some common plastic als that require special physical or chemical treatments to achieve ade-quate surfaces for adhesive bonding are listed in the following sections.Fluorocarbons Fluorocarbons, such as polytetrafluoroethylene (TFE),polyfluororethylene propylene (FEP), polychlorotrifluoroethylene(CFE), and polymonochlorotrifluoroethylene (Kel-F), are notoriouslydifficult to bond because of their low surface tension However, epoxyand polyurethane adhesives offer moderate strength if the fluorocar-bon is treated prior to bonding

materi-The fluorocarbon surface may be made more "wettable" by exposing

it for a brief moment to a hot flame to oxidize the surface The mostsatisfactory surface treatment is achieved by immersing the plastic in

a sodium-naphthalene dispersion in tetrahydrofuran This process isbelieved to remove fluorine atoms, leaving a carbonized surface thatcan be wet easily Fluorocarbon films treated for adhesive bonding areavailable from many suppliers A formulation and description of thesodium-naphthalene process may be found in Table 9.11 Commercialchemical products for etching fluorocarbons are also listed

Polyethylene terephthalate (Mylar) A medium-strength bond can beobtained with polyethylene terephthalate plastics and films by abra-sion and solvent cleaning However, a stronger bond can be achieved

by immersing the surface in a warm solution of sodium hydroxide or

in an alkaline cleaning solution for 2 to 10 min

Polyolefins These materials can be effectively bonded only if the face is first oxidized Polyethylene and polypropylene can be preparedfor bonding by holding the flame of an oxyacetylene torch over theplastic until it becomes glossy or else by heating the surface momen-tarily with a blast of hot air It is important not to overheat the plas-tic because it causes deformation The treated plastic must be bonded

sur-as quickly sur-as possible after surface preparation

Polyolefins, such as polyethylene, polypropylene and polymethylpentene, as well as polyformaldehyde and polyether, may be moreeffectively treated with a sodium dichromate-sulfuric acid solution.This treatment oxidizes the surface, allowing better wetting by theadhesive

Another process, plasma treatment, has been developed for treatinghard-to-bond plastics such as polyolefins This process works in various

TABLE 9.11 Surface Preparation Methods for Plastics

1 Abrasion Grit or vapor blast, or For medium-grit emery cloth purpose followed by solvent degreasing bonding

general-2 Etch in the following acid For maximum

Parts by wt ASTM D 2093 Potassium dichromate 75

Distilled water 120 Concentrated sulfuric acid (96%, sp gr 1.84) 1500 for 10 s at 25°C Rinse in distilled water, and dry in air at RT

Acetone 1 Abrasion Sand with 280A-grit For

general-emery cloth followed by solvent purpose bonding degreasing

2 "Satinizing" technique Immerse For maximum

Parts by wt Recommended Perchloroethylene 96.85 by DuPont

p-Toluenesulfonic acid 0.05 Cab-o-Sil (Cabot Corp.) 0.10 for 5-30 s at 80-120°C Transfer the part immediately to an oven

at 120°C for 1 min Wash in hot water Dry in air at 120°C

Acetone 1 Abrasion Grit or vapor blast, or

220-grit emery cloth, followed by solvent degreasing

2 Etch in chromic acid solution for Recipe 2 for

20 min at 60° methyl pentane

1 Abrasion Grit or vapor blast, or For general 220-grit emery cloth, followed by bonding solvent degreasing purposes

2 After procedure 1, dry the plastic at 100°C for 1 h, and apply adhesive before the plastic cools to room temperature

Steel wool may

be used for abrasion

Abrasion Grit or vapor blast, or lOO-grit emery cloth, followed by solvent degreasing

Prime with epoxy adhesive and fuse into the surface by heatin:g for

(180-Ionomer Acetone, methyl

ethyl ketone

Abrasion Grit or vapor blast, or lOO-grit emery cloth, followed by solvent degreasing

Abrasion Grit or vapor blast, or lOO-grit emery cloth, followed by solvent degreasing

Methyl

pentene

Acetone 1 Abrasion Grit or vapor blast, or

100-grit emery cloth, followed by solvent degreasing

2 Immerse for 1 h at 60°C in

Parts by wt.

Sulfuric acid (96%

sp gr 1.84) 26 Potassium chromate 3

1.84) Rinse in water and distilled water Dry in warm air Coatings (dried)

4 Prime surface with lacquer based offer excellent

on urea-formaldehyde resin bonding surfaces diluted with carbon tetrachloride without further

pretreatment Phenolic

Steel wool may

1 Abrasion Grit or vapor blast, or be used for abrade with 100-grit emery abrasion Sand cloth, followed by solvent or steel shot are

solution

Degreasing solvent

Polyamide

(nylon)

Acetone, methyl ethyl ketone, detergent

1 Abrasion Grit or vapor blast, or Sand or steel abrade with 100-grit emery cloth, shot are suitable followed by solvent degreasing abrasives

2 Prime with a spreading dough Suitable for based on the type of rubber to be bonding bonded in admixture with polyamide

natural and synthetic rubbers

3 Prime with resorcinol- Good adhesion formaldehyde adhesive to primer coat

with epoxy adhesives in meta1-plastic joints Polycarbonate,

allyl diglycol

carbonate

Methanol, Abrasion Grit or vapor blast, or isopropanol, lOO-grit emery cloth, followed by detergent solvent degreasing

Sand or steel shot are suitable abrasives Fluorocarbons:

Trichloro-1 Wipe with solvent and treat with Sodium-treated the following for 15 min at RT: surfaces must Naphthalene (128 g) dissolved in not be abraded tetrahydrofuran (11) to which is before use added sodium (23 g) during a Hazardous stirring period of 2 h etching Rinse in deionized water, and solutions dry in water air requiring

skillful handling Proprietary etching solutions are commercially available (see 2) PTFE available

in etched tape.

2 Wipe with solvent and treat as ASTM D 2093 recommended in one of the

following commercial etchants:

Bond aid (W.S Sharnban and Co.) Fluorobond (Joelin Mfg Co.) Fluoroetch (Action Associates) Tetraetch (W L Gore Associates)

3 Prime with epoxy adhesive, and fuse into the surface by heating for 10 min at 370°C followed by

5 Expose to electric discharge from Bond within 15

a tesla coil (50,000 V ac) for min of

9.55

1 Abrasion Grit or vapor blast, or For loo-grit emery cloth, followed by purpose bonding solvent degreasing

general-2 Immerse for 10 min at 71) 95°C For maximum

Parts by wt Suitable for Sodium hydroxide 2 linear polyester

Rinse in hot water and dry in hot air

Chlorinated

polyether

Acetone, methyl ethyl ketone

Etch for 5-10 min at 66-71°C in Suitable for film

Parts by wt materials such Sodium dichromate 5 as Penton.

Sulfuric acid (96%, sp gr 1.84) 100 Rinse in water and distilled water Dry in air

strength applications

Low-bond-Acetone, methyl ethyl ketone

1 Solvent degreasing

2 Expose surface to gas-burner flame (or oxyacetylene oxidizing flame) until the substrate is glossy

3 Etch in the following: For maximum

Parts by wt bond strength.

60 min at 25°C or

1 min at 71°C Polyformaldehyde 10 s at 25°C

4 Expose to following gases Bond within 15 activated by corona discharge: min of Air (dry) for 15 min pretreatment Air (wet) for 5 min Suitable for Nitrous oxide for 10 min polyolefins Nitrogen for 15 min

5 Expose to electric discharge from Bond within 15

a tesla coil (50,000 V ac) for 1 min of

Suitable for polrolefins Abrasion Grit or vapor blast, or 100- For maximum grit emery cloth, followed by solvent strength relieve

heating plastic for 5 h at loo°C

Finishing, Assembly, and Decorating 9.57

TABLE 9.11 Surface Preparation Methods for Plastics ( Continued)

be primed with adhesive in xylene solvent

Polystyrene Methanol, Abrasion Grit or vapor blast, or

isopropanol, lOO-grit emery cloth, followed by detergent solvent degreasing

Suitable for rigid plastic

Polysulfone - Methanol Vapor degrease

1 Abrasion Grit or vapor blast, or Suitable for 100-grit emery cloth followed by rigid plastic solvent degreasing For maximum

strength, prime witlf nitrile- phenolic adhesive

2 Solvent wipe with ketone Suitable for

plasticized material

SOURCE: Based on the following: N J DeLolis, Adhesives for Metals Theory and Technology, Industrial Press, New York, 1970; C V Cagle, Adhesive Bonding Techniques and Applications, McGraw-Hill, New York, 1968; W H Guttmann, Concise Guide to Structural Adhesives, Reinhold, New York, 1961; "Preparing the Surface for Adhesive Bonding," Bulletin Gl-600, Hysol Division, Dexter Corporation; A H Landrock, Adhesive Technology Handbook, Noyes Publications, Park Ridge, N J., 1985; and J Schields, Adhesive Handbook, CRC Press, Boca Raton, Fla., 1970.

Polyurethane Acetone Abrade with 100-grit emery cloth

methyl and solvent degreaserethyl

ketone

ways depending on the type of plastic being treated For most olefins, plasma treatment cross-links the polymeric surface byexposing

poly-it to an electrically activated inert gas such as neon or helium Thisforms a tough, cross-linked surface that wets easily and is adequate forprinting and painting as well as bonding

Table 9.12 shows the tensile-shear strength of bonded polyethylenepretreated by these various methods

Elastomeric adherends Vulcanized-rubber parts are often contaminatedwith mold release and plasticizers or extenders that can migrate to thesurface Solvent washing and abrading are common treatments formost elastomers, but chemical treatment is required for maximumproperties Many synthetic and natural rubbers require "cyclizing"with concentrated sulfuric acid until hairline fractures are evident onthe surface

Fluorosilicone and silicone rubbers must be primed before bonding.The primer acts as an intermediate interface, providing good adhesion

to the rubber and a more wettable surface for the adhesive

9.5.6 Adhesives selection

Factors most likely to influence adhesive selection are listed in Table9.13 However, thermosetting adhesives, such as epoxies,polyurethanes, or acrylics, are commonly used for structural applica-tion The adhesive formulations are generally tough, flexible com-pounds that can cure at room temperature The reasons that theseadhesives have gained the most popularity in bonding of plastics aresummarized in this section

The physical and chemical properties of both the solidified adhesiveand the plastic substrate affect the quality of the bonded joint Majorelements of concern in selecting an adhesive for plastic parts are thethermal expansion coefficient and glass transition temperature of the

TABLE 9.12 Effect of Surface Treatments on Polyethylene 15

Relative bond strength*

18.9

>20

2.9 4.7

1.01.0

*Results normalized to the control for each material.

SOURCE: Branson International Plasma Corporation.

TABLE 9.13 Factors Influencing Adhesive Selection

Light Oxidation Moisture Salt spray High Low Cycling Bacteria or mold Rodents or vermin Application Bonding time and temperature range Tackiness

Curing rate Storage stability Coverage

substrate relative to the adhesive Special consideration is alsorequired of the polymeric surface which can change during normalaging or exposure to operating environments

Significant differences in the thermal expansion coefficient betweensubstrates and the adhesive can cause serious stress at the plastic'sjoint interface These stresses are compounded by thermal cycling andlow-temperature service requirements Selection of a resilient adhe-sive or adjustments in the adhesive's thermal expansion coefficient viafiller or additives can reduce such stress

Structural adhesives must have a glass transition temperaturehigher than the operating temperature to avoid a cohesively weakbond and possible creep problems Modern engineering plastics, such

as polyimide or polyphenylene sulfides, have very high glass transitiontemperatures Most common adhesives have a relatively low glasstransition temperature so that the weakest thermal link in the jointmay often be the adhesive

Use of an adhesive too far below its glass transition temperaturecould result in low peel or cleavage strength Brittleness of the adhe-sive at very low temperatures could also manifest itself in poor impactstrength

Plastic substrates could be chemically active, even when isolatedfrom the operating environment Many polymeric surfaces slowly

undergo chemical and physical change The plastic surface, at the time

of bonding, may be well suited to the adhesive process However, afteraging, undesirable surface conditions may present themselves at theinterface, displace the adhesive, and result in bond failure Theseweak boundary layers may come from the environment or from with-

in the plastic substrate itself

Moisture, solvent, plasticizers, and various gases and ions can pete with the cured adhesive for bonding sites The process by which aweak boundary layer preferentially displaces the adhesive at theinterface is called desorption Moisture is the most common desorbingsubstance, being present both in the environment and within manypolymeric substrates

com-Solutions to the de sorption problem consist of eliminating the source

of the weak boundary layer or selecting an adhesive that is compatiblewith the desorbing material Excessive moisture can be eliminatedfrom a plastic part by postcuring or drying the part before bonding.Additives that can migrate to the surface can possibly be eliminated byreformulating the plastic resin Also, certain adhesives are more com-patible with oils and the plasticizer than others For example, themigration of the plasticizer from flexible polyvinyl chloride can becounteracted by using nitrile-based adhesives Nitrile adhesives resinsare capable of absorbing the plasticizer without degradation

9.5.7 Equipment for adhesive bonding

Mter the adhesive is applied, the assembly must be mated as quickly

as possible to prevent contamination of the adhesive surface The strates are held together under pressure and heated, if necessary, untilcure is achieved The equipment required to perform these functionsmust provide adequate heat and pressure, maintain constant pressureduring the entire cure cycle, and distribute pressure uniformly overthe bond area Of course, many adhesives cure with simple contactpressure at room temperature, and extensive bonding equipment isnot necessary

sub-Pressure equipment Pressure devices should be designed to maintainconstant pressure on the bond during the entire cure cycle They mustcompensate for thickness reduction from adhesive flow-out or thermalexpansion of assembly parts Thus, screw-actuated devices like Cclamps and bolted fixtures are not acceptable when constant pressure

is important Spring pressure can often be used to supplement clampsand compensate for thickness variations Dead-weight loading may beapplied in many instances; however, this method is sometimes imprac-tical, especially when heat cure is necessary

Finishing, Assembly, and Decorating 9.61

Pneumatic and hydraulic presses are excellent tools for applyingconstant pressure Steam or electrically heated platen presses withhydraulic rams are often used for adhesive bonding Some units havemultiple platens, thereby permitting the bonding of several assemblies

at one time

Large bonded areas, such as on aircraft parts, are usually cured in

an autoclave The parts are mated first and covered with a rubberblanket to provide uniform pressure distribution The assembly is thenplaced in an autoclave, which can be pressurized and heated Thismethod requires heavy capital equipment investment

Vacuum-bagging techniques can be an inexpensive method of ing pressure to large parts A film or plastic bag is used to enclose theassembly, and the edges of the film are sealed airtight A vacuum isdrawn on the bag, enabling atmospheric pressure to force theadherends together Vacuum bags are especially effective on largeareas because size is not limited by equipment

apply-Heating equipment Many structural adhesives require heat as well aspressure Most often the strongest bonds are achieved by an elevatedtemperature cure With many adhesives, trade-offs between cure timesand temperature are permissible But generally, the manufacturer willrecommend a certain curing schedule for optimum properties

If, for example, a cure of 60 min at 300°F is recommended, this doesnot mean that the assembly should be placed in 300°F for 60 min.Total oven time would be 60 min plus whatever time is required tobring the adhesive up to 300°F Large parts act as a heat sink and mayrequire substantial time for an adhesive in the bond line to reach thenecessary temperature Bond line temperatures are best measured bythermocouples placed very close to the adhesive In some cases, it may

be desirable to place the thermocouple in the adhesive joint for thefirst few assemblies being cured

Oven heating is the most common source of heat for bonded parts,even though it involves long curing cycles because of the heat-sinkaction of large assemblies Ovens may be heated with gas, oil, electric-ity, or infrared units Good air circulation within the oven is mandato-

ry to prevent nonuniform heating

Heated platen presses are good for bonding flat or moderately toured panels when faster cure cycles are desired Platens are heatedwith steam, hot oil, or electricity and are easily adapted with coolingwater connections to further speed the bonding cycle

con-Induction and dielectric heating are the fastest heating methodsbecause they focus heat at or near the adhesive bond line Workpieceheating rates greater than 100°F/s are possible with induction heat-ing For induction heating to work, the adhesive ~ust be filled with

metal particles or the adherend must be capable of conducting tricity or being magnetized Dielectric heating is an effective way ofcuring adhesives if at least one substrate is a nonconductor Metal-to-metal joints tend to break down the microwave field necessary fordielectric heating This heating method makes use of the polar char-acteristics of the adhesive materials Both induction and dielectricheating involve relatively expensive capital equipment outlays, andthe bond area is limited Their most important advantages are assem-bly speed and the fact that an entire assembly does not have to beheated to cure only a few grams of adhesive.

elec-Adhesive-thickness control It is highly desirable to have a uniformlythin (2- to lO-mil) adhesive bond line Starved adhesive joints, howev-

er, will yield exceptionally poor properties Three basic methods areused to control adhesive thickness The first method is to use mechan-ical shims or stops which can be removed after the curing operation.Sometimes it is possible to design stops into the joint

The second method is to employ a film adhesive that becomes highlyviscous during the cure cycle, preventing excessive flow-out With sup-ported films, the adhesive carrier itself can act as the "shims." Generally,the cured bond line thickness will be determined by the original thick-ness of the adhesive film The third method of controlling adhesive thick-ness is to use trial and error to determine the correct pressure-adhesiveviscosity factors that will yield the desired bond thickness

9.5.8 Quality control

A flowchart of a quality-control system for a major aircraft company isillustrated in Fig 9.28 This system is designed to ensure reproduciblebonds and, if a substandard bond is detected, to make suitable correc-tions Quality control should cover all phases of the bonding cycle frominspection of incoming material to the inspection of the completed assem-bly In fact, good quality control will start even before receipt of materials.Prehandling conditions The human element enters the adhesive bond-ing process more than in other fabrication techniques An extremelyhigh percentage of defects can be traced to poor workmanship Thisgenerally prevails in the surface preparation steps but may also arise

in any of the other steps necessary to achieve a bonded assembly Thisproblem can be largely overcome by proper motivation and education.All employees-from design engineer to laborer to quality-controlinspector-should be somewhat familiar with adhesive bonding tech-nology and be aware of the circumstances that can lead to poor joints

A great many defects can also be traced to poor design engineering

Figure 9.28 Flowchart of a ~uality control tern for adhesive bonding!

sys-The plant's bonding area should be as clean as possible prior toreceipt of materials The basic approach for keeping the assembly areaclean is to segregate it from the other manufacturing operations byplacing it either in a corner of the plant or in isolated rooms The airshould be dry and filtered to prevent moisture or other contaminantsfrom gathering at a possible interface The cleaning and bonding oper-ations should be separated from each other If mold release is used toprevent adhesive flash from sticking to bonding equipment, it is advis-able that great care be taken to assure that the release does not con-taminate the adhesive or the adherends Spray mold releases,especially silicone release agents, have a tendency to migrate to unde-sirable areas

Quality control of adhesive and surface treatment Acceptance tests onadhesives should be directed toward assurance that incomjng materialsare identical from lot to lot The tests should be those which can quicklyand accurately detect deficiencies in the adhesive's physical or chemical

properties A number of standard tests for adhesive bonds and for sive acceptance have been specified by the American Society for Testingand Materials (ASTM) SelectedASTM Standards are presented in Table9.14 The properties usually reported by adhesive suppliers are ASTMtensile-shear and peel strength.

adhe-Actual test specimens should also be made to verify the strength ofthe adhesive These specimens should be stressed in directions that arerepresentative of the forces which the bond will see in service, that is,shear, peel, tension, or cleavage If possible, the specimens should beprepared and cured in the same manner as actual production assem-blies If time permits, specimens should also be tested in simulated ser-vice environments, for example, high temperature and humidity.Surface preparations must be carefully controlled for reliable pro-duction of adhesive-bonded parts If a chemical surface treatment isrequired, the process must be monitored for proper sequence, bathtemperature, solution concentration, and contaminants If sand or gritblasting is employed, the abrasive must be changed regularly An ade-quate supply of clean wiping cloths for solvent cleaning is also manda-tory Checks should be made to determine if cloths or solventcontainers may have become contaminated

The specific surface preparation can be checked for effectiveness bythe water break-free test After the final treating step, the substratesurface is checked for a continuous film of water that should formwhen deionized water droplets are placed on the surface

After the adequacy of the surface treatment has been determined,precautions must be taken to assure that the substrates are kept cleanand dry until bonding The adhesive or primer should be applied to thetreated surface as quickly as possible

Quality control of the bonding process The adhesive metering and ing operation should be monitored by periodically sampling the mixedadhesive and testing it for adhesive properties A visual inspection canalso be made for air entrapment and degree of mixing The quality-control engineer should be sure that the oldest adhesive is used firstand that the specified shelf life has not been exceeded

mix-During the actual assembly operation, the cleanliness of the shopand tools should be verified The shop atmosphere should be controlled

as closely as possible Temperature is in the range of 65 to 90°Fand relative humidity from 20 to 65% is best for almost all bondingoperations

The amount of the applied adhesive and the final bond line ness must also be monitored because they can have a significant effect

thick-on joint strength Curing cthick-onditithick-ons should be mthick-onitored for heat-uprate, maximum and minimum temperature during cure, time at the

Finishing, Assembly, and Decorating TABLE 9.14 ASTM Adhesive Standards Test Methods*

Aging Resistance of Adhesives to Cyclic Aging Conditions, Test for (D 1183)

Bonding Permanency ofWater- or Solvent-Soluble Liquid Adhesives for Labeling Glass Bottles, Test for (D 1581)

Bonding Permanency of Water- or Solvent-Soluble Liquid Adhesives for Automatic Machine Sealing Top Flaps of Fiber Specimens, Test for (D 1713)

Permanence of Adhesive-Bonded Joints in Plywood under Mold Conditions, Test for (D 1877)

Accelerated Aging of Adhesive Joints by the Oxygen-Pressure Method, Practice for (D 3632)

Amylaceous Matter

Amylaceous Matter in Adhesives, Test for (D 1488)

Biodeterioration Susceptibility of Dry Adhesive Film to Attack by Roaches, Test for (D 1382)

Susceptibility of Dry Adhesive Film to Attack by Laboratory Rats, Test for (D 1383) Permanence of Adhesive-Bonded Joints in Plywood under Mold Conditions, Test for (D 1877)

Effect of Bacterial Contamination of Adhesive Preparations and Adhesive Films, Test for (D 4299)

Effect of Mold Contamination on Permanence of Adhesive Preparation and Adhesive Films, Test for (D 4300)

Blocking Point Blocking Point of Potentially Adhesive Layers, Test for (D 1146)

Bonding Permanency

(See Aging)

Chemical Reagents Resistance of Adhesive Bonds to Chemical Reagents, Test for (D 896;

Cleavage Cleavage Strength of Metal-to-Metal Adhesive Bonds, Test for (D 1062)

Cleavage/Peel Strength -

Strength Properties of Adhesives in Cleavage Peel by Tension Loading (Engineering Plastics-to-Engineering Plastics), Test for (D 3807)

(See also Peel Strength)

Corrosivity Determining Corrosivity of Adhesive Materials, Practice for (D 3310)

Creep

~

Conducting Creep Tests of Metal-to-Metal Adhesives, Practice for (D 1780)

Creep Properties of Adhesives in Shear by Compression Loading (Metal-to-Metal), Test for (D 2293)

Creep Properties of Adhesives in Shear by Tension Loading, Test for (D 2294)

TABLE 9.14 ASTM Adhesive Standards Test Methods* ( Continued)

Cryogenic Temperatures Strength Properties of Adhesives in Shear by Tension Loading in the Temperature Range from -267.8 to -55°C ( -450 to -67°F), Test for (D 2557)

Density

Density of Adhesives in Fluid Form, Test for (D 1875)

Durability (Including Weathering) Effect of Moisture and Temperature on Adhesive Bonds, Test for (D 1151)

Atmospheric Exposure of Adhesive-Bonded Joints and Structures, Practice for (D 1828) Determining Durability of Adhesive Joints Stressed in Peel, Practice for (D 2918) Determining Durability of Adhesive Joints Stressed in Shear by Tension Loading, Practice for (D 2919)

(See also Wedge Test)

Flexural Strength

Flexural Strength of Adhesive Bonded Laminated Assemblies, Test for (D 1184) Flexibility Determination of Hot Melt Adhesives by Mandrel Bend Test Method, Practice for (D 3111)

Flow Properties Flow Properties of Adhesives, Test for (D 2183)

Fracture Strength in Cleavage

Fracture Strength in Cleavage of Adhesives in Bonded Joints, Practice for (D 3433)

Gap-Filling Adhesive Bonds

Strength of Gap Filling Adhesive Bonds in Shear by Compression Loading, Practice for (D 3931)

TABLE 9.14 ASTM Adhesive Standards Test Methods* ( Continued)

High-Temperature Effects Strength Properties of Adhesives in Shear by Tension Loading at Elevated

Temperatures (Metal-to-Metal), Test for (D 2295)

Hydrogen- Ion Concentration Hydrogen Ion Concentration, Test for (D 1583)

Impact Strength

Impact Strength of Adhesive Bonds, Test for (D 950)

Light Exposure (See Radiation Exposure)

Low and Cryogenic Temperatures Strength Properties of Adhesives in Shear by Tension Loading in the Temperature Range from -267.8 to -55°C ( -450 to 67°F), Test for (D 2557)

Nonvolatile Content Nonvolatile Content of Aqueous Adhesives, Test for (D 1489)

Nonvolatile Content ofUrea-Formaldehyde Resin Solutions, Test for (D 1490)

Nonvolatile Content of Phenol, Resorcinol, and Melamine Adhesives, Test for (D 1582)

Odor Determination of the Odor of Adhesives, Test for (D 4339)

Peel Strength (Stripping Strength)

Peel or Stripping Strength of Adhesive Bonds, Test for (D 903)

Climbing Drum Peel Test for Adhesives, Method for (D 1781)

Peel Resistance of Adhesives (T-Peel Test), Test for (D 1876)

Evaluating Peel Strength of Shoe Sole Attaching Adhesives, Test for (D 2558)

Determining Durability of Adhesive Joints Stressed in Peel, Practice for (D 2918) Floating Roller Peel Resistance, Test for (D 3167)

Penetration

Penetration of Adhesives, Test for (D 1916)

pH

(See Hydrogen-Ion Concentration)

Radiation Exposure (Including Light) Exposure of Adhesive Specimens to Artificial (Carbon-Arc Type) and Natural Light, Practice for (D 904)

Exposure of Adhesive Specimens to High-Energy Radiation, Practice for (D 1879)

Rubber Cement Tests

Rubber Cements, Testing of (D 816)

TABLE 9.14 ASTM Adhesive Standards Test Methods* { Continued)

Salt Spray (Fog) Testing Salt Spray (Fog) Testing, Method of (B 117)

Modified Salt Spray (Fog) Testing, Practice for (G 85)

Shear Strength (Tensile Shear Strength)

Shear Strength and Shear Modulus of Structural Adhesives, Test for (E 229)

Strength Properties of Adhesive Bonds in Shear by Compression Loading, Test for (D 905)

Strength Properties of Adhesive in Plywood Type Construction in Shear by Tension Loading, Test for (D 906)

Strength Properties of Adhesives in Shear by Tension Loading (Metal-to- Metal), Test for (D 1002)

Determining Strength Development of Adhesive Bonds, Practice for (D 1144)

Strength Properties of Metal-to-Metal Adhesives by Compression Loading (Disk Shear), Test for (D 2181)

Strength Properties of Adhesives in Shear by Tension Loading at Elevated

Temperatures (Metal-to-Metal), Test for (D 2295)

Strength Properties of Adhesives in 'lWo-Ply Wood Construction in Shear by Tension Loading, Test for (D 2339)

Strength Properties of Adhesives in Shear by Tension Loading in the Temperature Range from -267.8 to -55°C ( -450 to -67°F), Test for (D 2557)

Determining Durability of Adhesive Joints Stressed in Shear by Tension Loading, Practice for (D 2919)

Determining the Strength of Adhesively Bonded Rigid Plastic Lap-Shear Joints in Shear by Tension Loading, Practice for (D 3163)

Determining the Strength of Adhesively Bonded Plastic Lap-Shear Sandwich Joints in Shear by Tension Loading, Practice for (D 3164)

Strength Properties of Adhesives in Shear by Tension Loading of Laminated

Assemblies, Test for (D 3165)

Fatigue Properties of Adhesives in Shear by Tension Loading (Metal/Metal), Test for (D 3166)

Strength Properties of Double Lap Shear Adhesive Joints by Tension Loading, Test for (D 3528)

Strength of Gap-Filling Adhesive Bonds in Shear by Compression Loading, Practice for (D 3931)

Measuring Strength and Shear Modulus of Nonrigid Adhesives by the Thick Adherend Tensile Lap Specimen, Practice for (D 3983)

Measuring Shear Properties of Structural Adhesives by the Modified-Rail Test, Practice for (D 4027)

Specimen Preparation Preparation of Bar and Rod Specimens of Adhesion Tests, Practice for (D 2094)

Spot-Adhesion Test

Qualitative Determination of Adhesion of Adhesives to Substrates by Spot Adhesion Test Method, Practice for (D 3808)

Spread (Coverage) Applied Weight per Unit Area of Dried Adhesive Solids Test for (D 898)

Finishing, Assembly, and Decorating 9.69 TABLE 9.14 ASTM Adhesive Standards Test Methods* ( Continued)

Spread (Coverage) (Continued) Applied Weight per Unit Area of Liquid Adhesive, Test for (D 899)

Analysis of Sulfochromate Etch Solution Used in Surface Preparation of Aluminum, Methods of (D 2674)

Preparation of Aluminum Surfaces for Structural Adhesive Bonding (Phosphoric Acid Anodizing), Practice for (D 3933)

Tack Pressure Sensitive Tack of Adhesives Using an Inverted Probe Machine, Test for (D 2979) Tack of Pressure-Sensitive Adhesives by Rolling Ball, Test for (D 3121)

Tensile Strength

Tensile Properties of Adhesive Bonds, Test for (D 897)

Determining Strength Development of Adhesive Bonds, Practice for (D 1144)

Cross-Lap Specimens for Tensile Properties of Adhesives, Testing of (D 1344)

Tensile Strength of Adhesives by Means of Bar and Rod Specimens, Method for (D 2095 )

Torque Strength

Determining the Torque Strength of Ultraviolet (UV) Light-Cured Glass/Metal

Adhesive Joints, Practice for (D 3658)

Viscosity

Viscosity of Adhesives, Test for (D 1084)

Apparent Viscosity of Adhesives Having Shear-Rate-Dependent Flow Properties, Test for (D 2556)

Viscosity of Hot Melt Adhesives and Coating Materials, Test for (D 3236)

Volume Resistivity Volume Resistivity of Conductive Adhesives, Test for (D 2739)

Water Absorptiveness (of Paper Labels) Water Absorptiveness of Paper Labels, Test for (D 1584)

TABLE 9.14 ASTM Adhesive Standards Test Methods* (Continued)

Weathering (See Durability)

Wedge Test Adhesive Bonded Surface Durability of Aluminum (Wedge Test) (D 3762)

Working Life Working Life of Liquid or Paste Adhesive by Consistency and Bond Strength, Test for (D 1338)

*The latest revisions of ASTM standards can be obtained from the American Society for Testing and Materials, 100 Barr Harbor Drive, West Conshohocken, Pa.

required temperature, and cool-down rate

After the adhesive is cured, the joint area can be inspected to detectgross flaws or defects This inspection procedure can be either destruc-tive or nondestructive in nature Destructive testing generallyinvolves placing samples of the production run in simulated or accel-erated service and determining if it has similar properties to a speci-men that is known to have a good bond and adequate serviceperformance The causes and remedies for faults revealed by suchmechanical tests are described in Table 9.15

Nondestructive testing (NDT) is far more economical, and everyassembly can be tested if desired However, there is no single nonde-structive test or technique that will provide the user with a quantita-tive estimate of bond strength There are several ultrasonic testmethods that provide qualitative values However, a trained eye candetect a surprising number of faulty joints by close inspection of theadhesive around the bonded area Table 9.16 lists the characteristics

of faulty joints that can be detected visually The most difficult defect

to be found by any way are those related to improper curing and face treatments Therefore, great care and control must be given tosurface preparation procedures and shop cleanliness

TABLE 9.14 ASTM Adhesive Standards Test Methods* (Continued)

Weathering (See Durability)

Wedge Test Adhesive Bonded Surface Durability of Aluminum (Wedge Test) (D 3762)

Working Life Working Life of Liquid or Paste Adhesive by Consistency and Bond Strength, Test for (D 1338)

*The latest revisions of ASTM standards can be obtained from the American Society for Testing and Materials, 100 Barr Harbor Drive, West Conshohocken, Pa.

required temperature, and cool-down rate

After the adhesive is cured, the joint area can be inspected to detectgross flaws or defects This inspection procedure can be either destruc-tive or nondestructive in nature Destructive testing generallyinvolves placing samples of the production run in simulated or accel-erated service and determining if it has similar properties to a speci-men that is known to have a good bond and adequate serviceperformance The causes and remedies for faults revealed by suchmechanical tests are described in Table 9.15

Nondestructive testing (NDT) is far more economical, and everyassembly can be tested if desired However, there is no single nonde-structive test or technique that will provide the user with a quantita-tive estimate of bond strength There are several ultrasonic testmethods that provide qualitative values However, a trained eye candetect a surprising number of faulty joints by close inspection of theadhesive around the bonded area Table 9.16 lists the characteristics

of faulty joints that can be detected visually The most difficult defect

to be found by any way are those related to improper curing and face treatments Therefore, great care and control must be given tosurface preparation procedures and shop cleanliness

TABLE 9.15 Faults Revealed by Mechanical Tests

Thick, uneven Clamping pressure too low

glue line

No follow-up pressure

Curing temperature too low

Increase pressure Check that clamps are Beating properly

Modify clamps or check for freedom of moving parts

Use higher curing temperature Check that temperature is above the minimum specified throughout the curing cycle

Use fresh adhesive

Adhesive exceeded its shelf

life, resulting in increased

Solvents not completely

dried out before bonding

Vacuum-degas adhesive before application

Increase drying time or temperature Make sure drying area is properly ventilated

Seek advice from manufacturers Adhesive material

contains volatile constituents

A low-boiling constituent

boiled away

Curing temperature is too high

Check treating procedure;

use clean solvents and wiping rags Wiping rags must not be made from synthetic fiber Make sure cleaned parts are not touched before bonding Cover stored parts to prevent dust from settling on them

Voids in bond Joint surfaces not properly

(that is, areas treated

that are not

Resin may be contaminated

Uneven clamping pressure

of mixing Large parts act as a heat sink, necessitating larger cure times

Fault Cause Remedy

TABLE 9.16 Visual Inspection for Faulty Bonds

No appearance of

adhesive around

edges ofjoint or

adhesive bond

line too thick

Clamping pressure too low Increase pressure Check that

clamps are seating properly Apply more adhesive Use higher curing temperature Check that temperature is above the minimum specified

Starved joint Curing temperature too low

Adhesive bond

line too thin

Clamping pressure too high Curing temperature too high Starved joint

Lessen pressure Use lower curing temperature Apply more adhesive Adhesive flash

breaks easily

away from

substrate

Improper surface treatment Check treating procedures;

clean solvents and wiping rags Make sure cleaned parts are not touched before bonding

Adhesive flash

is excessively

porous

Excess air stirred into adhesive

Solvent not completely dried out before bonding Adhesive material contains volatile constituent

Vacuum-degas adhesive before application Increase drying time or temperature

Seek advice fi-om manufacturers Adhesive not properly cured Use higher curing temperature or

extend curing time Temperature and time must be above minimum specified Check mixing

Adhesive flash

can be so~ned

by heating or

wiping with solvent

However, with welding some form of pretreatment may still be necessary.Certainly, the parts should be clean, and all mold release and contami-nants must be removed by standard cleaning procedures It may also benecessary to dry certain polymeric parts, such as nylon and polycarbon-ate, before welding so that the inherent moisture in the part will notaffect the overall quality of the bond It may also be necessary to ther-mally anneal parts, such as acrylic, before solvent welding to remove orlessen internal stresses caused by molding Without annealing, thestressed surface may crack or craze when in contact with solvent.9.6.1 Thermal welding

Welding by application of heat or thermal welding provides an tageous method of joining many thermoplastics that do not degraderapidly at their melt temperature It is a method of providing fast, rel-atively easy, and economical bonds that are generally 80 to 100% thestrength of the parent plastic

advan-The thermal welding process can be of two kinds: direct and indirect.With direct welding, the heat is applied directly to the substrate in theform of either a heated tool or hot gas Indirect welding occurs whensome form of energy other than thermal is applied to the joint Theapplied energy, which causes heating at the interface or in the plastic

as a whole, is generally in the form of friction, high-frequency electricalfields, electromagnetic fields, or ultrasonic vibration Because the heat-ing is localized at the bonding surface, indirect heating processes arevery energy efficient, generally resulting in bonds that are stress freeand of higher strength than those made by direct welding methods.Heated tool welding With the heated tool welding method, the sur-faces to be fused are heated by holding them against a hot-metal sur-face ( 450 to 700°F); then the parts are brought into contact andallowed to harden under slight pressure (5 to 15 Ib/in2) Electric stripheaters, soldering irons, hot plates, and resistance blades are commonmethods of providing heat One production technique involves buttingflat plastic sheets on a table next to a resistance heated blade thatruns the length of the sheet Once the plastic adjacent to the bladebegins to soften, the blade is raised, and the sheets are pressed togeth-

er and held under pressure while they cool The heated metal surfacesare usually coated with a high-temperature release coating, such aspolytetrafluoroethylene, to discourage sticking to the molten plastic.Successful heated tool welding depends on the temperature of theheated tool surface, the amount of time the plastic adherends are incontact with the hot tool, the time lapse before joining the substrates,and the amount and uniformity of pressure that is held during cooling.Heated tool welding can be used for structural plastic parts, and heatsealing can be used for plastic films With heat sealing, the hot surface

is usually hot rollers or a heated rotating metal band commonly used

to seal plastic bags Table 9.17 offers heat welding temperatures for anumber of common plastics and films

Resistance wire welding is also a type of heated tool welding Thismethod employs an electrical resistance heating wire laid betweenmating substrates to generate the heat of fusion When energized, thewire undergoes resistance heating and causes a melt area to formaround the adjacent polymer Pressure on the parts during thisprocess causes the molten material to flow and act as a hot-melt adhe-sive for the joint After the bond has been made, the exterior wire iscut off and removed Resistance wire welding can be used on any plas-tic that can be joined effectively by heated tool welding The plasticresin manufacturer should be contacted for details concerning the spe-cific parameters of this process

9.74 Chapter Nine

TABLE 9.17 Hot-Plate Temperatures to Weld Plastics and Plastic Films 17

Plastic Temperature, oF Film Temperature, oF

450 500 550

Coated cellophane Cellulose acetate Coated polyester

200-350 400-500 490 360

390 650 650 525

415-450 250-375 220-300 300-400

Polystyrene (oriented) Poly(vinyl alcohol) Poly(vinyl chloride) and copolymers (nonrigid) Poly(vinyl chloride} and copolymers (rigid}

Poly(vinyl nitrile rubber blend Poly(vinylidene chloride}

chloride} Rubber hydrochloride Fluorinated ethylene- propylene copolymer

200-400 Polypropylene 400

260-400 Polystyrene

450 475 450

600-750

*Trademark of General Electric Company.

Hot-gas welding An electrically or gas-heated welding gun with anorifice temperature of 425 to 700°F can be used to bond many thermo-plastic materials The pieces to be bonded are beveled and positioned

to form a V-shaped joint as shown in Fig 9.29 A welding rod, made ofthe same plastic that is being bonded is laid into the joint, and the heatfrom the gun is directed at the interface of the substrates and the rod.The molten product from the welding rod then fills the gap A strongfilet must be formed, the design of which is of considerable importance

A large difference between the plastic melting temperature and thedecomposition temperature of the plastic is necessary for consistent,reliable hot-gas welding results Usually the hot gas can be commonair However, for polyolefins and other easily oxidized plastics, theheated gas must be inert or nitrogen, since air will oxidize the surface

of the plastic

After welding, the joint should not be stressed for several hours.This is particularly true for polyolefins, nylons, and polyformaldehyde.Hot-gas welding is not recommended for filled materials or substratesless than 1; 16 in in thickness Applications are usually large structuralassemblies The weld is not cosmetically attractive, but tensilestrengths that are 85% of the parent materials are easily obtained

Friction or spin welding Spin welding uses the heat of friction to causefusion at the interface One substrate is rotated very rapidly while in

Figure 9.29 Hot-gas welding apparatus.

touch with the other stationary substrate so that the surfaces meltwithout damaging the part Sufficient pressure is applied during theprocess to force out excess air bubbles The rotation is then stopped,and pressure is maintained until the weld sets Rotation speed andpressure are dependent on the thermoplastics being joined The mainprocess parameters are the spin of rotation, weld or axial pressure,and weld time The equipment necessary depends upon productionrequirement, but spin welding can be adapted to standard shopmachinery such as drill presses or lathes

In commercial spin welding machines, rotational speeds can rangefrom 200 to 14,000 r/min Welding times (heating and cooling) canrange from less than 1 to 20 s, with typical times being severalseconds

A wide variety of joints can be made by spin welding Since the

out-er edges of the rotating substrate move considout-erably fastout-er than thecenter, joints are generally designed to concentrate pressure at thecenter A shallow tongue-and-groove type of joint design is useful toindex the opposite parts and provide a uniform bearing surface Spinwelding is a popular method ofjoining large-volume products, packag-ing, and toys

Induction heating An electromagnetic induction field can be used toheat a metal grid or an insert placed between mating thermoplasticsubstrates When the joint is positioned between energized inductioncoils, the hot insert material responds to the high-frequency ac source,causing the plastic surrounding it to melt and fuse together Slightpressure is maintained as the induction field is turned off and the jointhardens

In addition to metal inserts, electromagnetic adhesives can be used

to form the joint Electromagnetic adhesives are made from metal orferromagnetic particle-filled thermoplastics These adhesives can be

shaped into gaskets or film that can easily be applied and will melt in

an induction field The advantage of this method is that stressescaused by large metal inserts are avoided

Induction welding is less dependent than other welding methods onthe properties of the materials being welded It can be used on nearlyall thermoplastics In welding different materials, the thermoplasticresin enclosing the metal particles in the electromagnetic adhesive ismade ofa blend of the materials being bonded Table 9.18 shows com-patible combinations for electromagnetic adhesives Reinforcedplastics with filler levels over 50% have been successfully electromag-netically welded

Strong and clean structural, hermetic, and high-pressure sealscan be obtained from this process Important determinants of bondquality in induction welding are the joint design and inductioncoil design With automatic equipment, welds can be made in lessthan 1 s

Ultrasonic and vibration welding During ultrasonic welding, a quency (20- to 40-kHz) electrodynamic field is generated that res-onates a metal horn The horn is in contact with one of the plasticparts and the other part is fixed firmly The horn and the part to which

high-fre-it is in contact vibrates sufficiently fast to cause great heat at theinterface of the parts being bonded With pressure and subsequentcooling, a strong bond can be obtained with many thermoplastics.Rigid plastics with a high modulus of elasticity are best Excellentresults can be obtained with polystyrene, SAN , ADS, polycarbonate,and acrylic plastics

TABLE 9.18 Compatible Plastic Combinations for Bonding with Electromagnetic Adhesives18

ABS Acetal Acrylic Nylon PC PE PP PS PVC SAN

xxx

The basic variables in ultrasonic bonding are amplitude, air sure, weld time, and hold time The desired joint strength can beachieved by altering these variables Increasing weld time generallyresults in increasing bond strength up to a point After that point,additional weld time does not improve the joint and can even degrade

pres-it Average processing times, including welding and cooling, are lessthan several seconds

Typical ultrasonic joint designs are shown in Fig 9.30 Often anenergy director, or small triangular tip in one of the parts, is necessary.All of the ultrasonic energy is concentrated on the tip of the energydirector and this is the area of the joint that then heats, melts, andprovides the material for the bond Ultrasonic welding is considered afaster means of bonding than direct heat welding

Ultrasonic welding of parts fabricated fromABS, acetals, nylon, PPO,polycarbonate, polysulfone, and thermoplastic polyesters should be con-sidered as early in the design of the part as possible Very often minormodifications in part design will make ultrasonic welding more conve-nient Best joint design and ultrasonic horn design can be recommend-

ed by the plastic resin manufacturer or ultrasonic equipment supplier.Like resin materials, such as ABS to ABS, are the easier to weldultrasonically; some unlike resins may be bonded provided they havesimilar melt temperatures, chemical composition, and modulus of elas-ticity Generally, amorphous resins (ABS, PPO, and polycarbonate) arealso easier to weld ultrasonically than crystalline resins (nylon, acetal,and thermoplastic polyester)

Ultrasonic equipment can also be used for mechanical fasteningoperations Ultrasonic energy can be used to apply threaded inserts tomolded plastic parts and to heat stake plastic studs.

Vibration welding is similar to ultrasonic welding, except that ituses a lower frequency (120 to 240 Hz) of vibration In this way, verylarge parts can be bonded The process parameters affecting thestrength of the resulting weld are the amplitude and frequency ofmotion, weld pressure, and weld time There are two types of vibrationwelding: linear, in which friction is generated by a linear motion of theparts, and orbital, in which one part is vibrated using circular motion

in all directions

Vibration welding has been used on large thermoplastic parts such

as canisters, pipe sections, and other parts that are too large to beexcited with an ultrasonic generator An advantage of vibration weld-ing over ultrasonic welding is that it can provide gas-tight joints instructures with long bond lines Ultrasonic welding is basically a spotweld technique limited by the size of the horn

Total process time for vibration joining is generally between 5 and

15 s This is longer than spin or ultrasonic welding but much shorterthan direct heat welding, solvent cementing, or adhesive bonding.Vibration welding can be applied to ABS, acetal, nylon, PPO, thermo-plastic polyesters, and polycarbonate For vibration welding, hydro-scopic resins, such as nylon, do not have to be dried as is necessarywith ultrasonic welding Joint designs do not require an energy direc-tor, as in ultrasonic joint designs, but the joint area must be strongenough to resist the forces of operation without deformation Thisoften requires thickening the bond area or designing stiffeners into thepart near the joint areas

Dielectric welding and other welding methods Dielectric sealing can beused on most thermoplastics except those that are relatively transpar-ent to high-frequency electric fields It is used mostly to seal vinylsheeting such as automobile upholstery, swimming pool liners, andrainwear An alternating electric field is imposed on the joint, whichcauses rapid reorientation of polar molecules, and heat is generated bymolecular friction The field is removed, and pressure is then appliedand held until the weld cools

Variable in the bonding operation are the frequency generated,dielectric loss of the plastic, power applied, pressure, and time The fre-quency of the field being generated can be from radio frequency up tomicrowave frequency Dielectric heating can also be used to generatethe heat necessary for curing polar, thermosetting adhesive, or it can beused to quickly evaporate water from water based adhesives-a com-mon application in the furniture industry

Other thermal welding processes that are less common than thosedescribed previously are: extrusion welding, electrofusion welding,infrared welding, and laser welding These are generally used

in specialty processes or with applications that require uniquemethods of heating because of the joint design or nature of the finalproduct

9.6.2 Solvent welding

Solvent welding or cementing is the simplest and most economicalmethod of joining many noncrystalline thermoplastics Solvent-cemented joints are less sensitive to thermal cycling than jointsbonded with adhesives, and they are as resistant to degrading envi-ronments as their parent plastic Bond strength equal to 85 to 100%

of the parent plastic can be obtained The major disadvantage of vent cementing is the possibility of stress cracking or crazing of thepart and the possible hazards of using low vapor point solvents.When two dissimilar plastics are to be joined, adhesive bonding isgenerally desirable because of solvent and polymer compatibilityproblems

sol-Solvent cements should be chosen with approximately the same bility parameter as the plastic to be bonded Table 9.19 lists typical sol-vents used to bond major plastics It is common to use a mixture of afast-drying solvent with a less volatile solvent to prevent crazing The

solu-TABLE 9.19 Typical Solvents for Solvent Cementing of Plastics4

Acrylic Methylene chloride, ethylene dichloride

Cellulosics Methyl ethyl ketone, acetone

Nylon Aqueous phenol, solutions of resorcinal in alcohol, solutions of calcium

chloride in alcohol

PPO 'liichloroethylene, ethylene dichloride, chloroform, methylene chloride PVC Cyclohexane, tetrahydrofuran, dichlorobenzene

Polycarbonate Methylene chloride, ethylene dichloride

Polystyrene Methylene chloride, ethylene ketone, ethylene dichloride,

trichloroethylene, toluene, xylene

Polysulfone Methylene chloride

Note: These are solvents recommended by the various resin suppliers A key to the tion of solvents is how fast they evaporate: a fast-evaporating product may not last long enough for some assemblies; too slow evaporation could hold up production.

selec-9.80 Chapter Nine

solvent cement can be bodied to 25% by weight with the parent plastic

to fill gaps and reduce shrinkage and internal stress during cure.The parts to be bonded should be unstressed and, if necessary,annealed The surfaces should mate well and have a clean, smoothsurface A V-joint or rounded butt joint are generally preferred formaking a solvent butt joint Scarf joints and flat butt joints are diffi-cult to position and to apply pressure during the solvent evaporationphase of the process

The solvent cement is generally applied to the substrate with asyringe or brush In some cases the surface can be immersed in the sol-vent After the area to be bonded softens, the parts are mated and heldunder pressure until dry Pressure should be low and uniform so thatthe finished joint will not be stressed After the joint hardens, the pres-sure is released, and an elevated temperature cure may be necessary,depending on the plastic and desired joint strength The bonded partshould not be packaged or stressed until the solvent has adequate time

to escape from the joint

9.7 Recommended Assembly Processes for

Common Plastics

When decisions are to be made relative to assembly methods ical fastening, adhesive bonding, thermal welding, or solvent cement-ing), special considerations must be taken because of the nature of thesubstrate and possible interactions with the adhesive or the environ-ment The following sections identify some of these considerations andoffer an assembly guide to the various methods of assemblies thathave been found appropriate for specific plastics

(mechan-9.7.1 Acetal homopolymer and acetal

copolymer

Parts made of acetal homopolymer and copolymer are generally strongand tough, with a surface finish that is the mirror image of the moldsurface Acetal parts are generally ready for end use or further assem-bling with little or no postmold finishing

Press fitting has been found to provide joints of high strength atminimum cost Acetal copolymer can be used to provide snap-fit parts.Use of self-tapping screws may provide substantial cost savings bysimplifying machined parts and reducing assembly costs

Epoxies, isocyanate-cured polyester, and cyanoacrylates are used tobond acetal copolymer Generally, the surface is treated with a sulfu-ric-chromic acid treatment Epoxies have shown 150- to 500-lb/in2shear strength on sanded surfaces and 500 to 1000 lb/in2 on chemical-

ly treated surfaces Plasma treatment has also shown to be effective on

acetal substrates Acetal homopolymer surfaces should be chemicallytreated prior to bonding This is accomplished with a sulfuric-chromicacid treatment followed by a solvent wipe Epoxies, nitrile, and nitrile-phenolics can be used as adhesives.

Thermal welding and solvent cementing are commonly used forbonding this material to itself Heated tool welding produces excep-tionally strong joints with acetal homopolymers and copolymers Withthe homopolymer, a temperature of the heated surface near 550°F and

a contact time of2 to 10 s are recommended The copolymer can be plate-welded from 430 to 560°F It is claimed that annealing acetalcopolymer joints will strengthen them further Annealing can be done

hot-by immersing the part in oil heated to 350°F Acetal resin can be

bond-ed by hot-wire welding Pressure on the joint, duration of the current,and wire type and size must be varied to achieve optimum results.Shear strength on the order of 150 to 300 lb/in or more have beenobtained with both varieties, depending on the wire size, energizingtimes (wire temperature), and clamping force

Hot-gas welding is used effectively on heavy acetal sections Jointswith 50% of the tensile strength of the acetal resin have been obtained.Conditions of joint design and rod placement are similar to those pre-sented for ABS A nitrogen blanket is suggested to avoid oxidation Theoutlet temperature of the welding gun should be approximately 630°Ffor the homopolymer and 560°F for the copolymer For maximum jointstrength both the welding rod and parts to be welded must be heated

so that all surfaces are melted

Acetal components can easily be joined by spin welding, which is afast and generally economical method to obtain joints of good strength.Spin-welded acetal joints can have straight 90° mating surfaces, orsurfaces can be angles, molded in a V-shape, or flanged

Although not common practice, acetal copolymer can be welded at room temperature with full-strength hexafluoroacetonesesquihydrate (Allied Chemical Corporation) The cement has beenfound to be an effective bonding agent for adhering to itself, nylon, orABS Bond strengths in shear are greater than 850 lb/in2 using "as-molded" surfaces Hexafluoroacetone sesquihydrate is a severe eyeand skin irritant Specific handling recommendations and information

solvent-on toxicity should be requested from Allied Chemical Corporatisolvent-on.Because of its high solvent resistance, acetal homopolymer cannot besolvent cemented

9.7.2 Acrylonitrile butadiene styrene (ABS)

ABS parts can be designed for snap-fit assembly using a general line of5% allowable strain during the interference phase of the assem-bly Thread-cutting screws are frequently recommended for nonfoamed

guide-Chapter Nine

ABS, and thread-forming screws for foamed grades Depending on theapplication, the use of bosses and boss caps may be advantageous.The best adhesives for ABS are epoxies, urethanes, thermosettingacrylics, nitrile-phenolics, and cyanoacrylates These adhesives haveshown joint strength greater than that of the ABS substrates beingbonded ABS substrates do not require special surface treatments oth-

er than simple cleaning and removal of possible contaminants.ABS can also be bonded to itself and to certain other thermoplastics

by either solvent-cementing or any of the heat welding methods Forbonding ABS to itself, it is recommended that the hot-plate tempera-tures be between 430 and 550°F Lower temperatures will result insticking of the materials to the heated platens, while temperaturesabove 550°F will increase the possibility of thermal degradation of thesurface In joining ABS the surfaces should be in contact with theheated tool until they are molten, then brought carefully and quicklytogether, and held with minimum pressure If too much pressure isapplied, the molten material will be forced from the weld and result

in poor appearance and reduced weld strength Normally, if a weldflash greater than 1/8 in occurs, too much joining pressure has beenused

Hot-gas welding has been used to join ABS thermoplastic with muchsuccess Joints with over 50% of the strength of the parent materialhave been obtained The ABS welding rod should be held approxi-mately at a 90° angle to the base material; the gun should be held at

a 45° angle with the nozzle 1/4 to 1/2 in from the rod ABS parts to behot-gas-welded should be bonded at 60° angles The welding gun,capable of heating the gas to 500 to 600°F, must be moved continually

in a fanning motion to heat both the welding rod and bed Slight sure must be maintained on the rod to ensure good adhesion

pres-Spin-welded ABS joints can have straight 90° mating surfaces, orsurface can be angled, molded in a V-shape, or flanged The mostimportant factor in the quality of the weld is the joint design The area

of the spinning part should be as large as possible, but the difference

in linear velocity between the maximum and minimum radii should be

in thick Joints should be designed to enclose the metal insert pletely Inserts made of carbon steel require less power for heatingalthough other types of metal can be used The insert should be located

com-as close com-as possible to the electromagnetic generator coil and centeredwithin the coil to assure uniform heating.

The solvents recommended for ABS are methyl ethyl ketone, methylisobutyl ketone, tetrahydrofuran, and methylene chloride The solventused should be quick drying to prevent moisture absorption, yet slowenough to allow assembly of the parts The recommended cure time is

12 to 24 h at room temperature The time can be reduced by curing at

130 to 150°F A cement can be made by dissolving ABS resin in a vent of up to 25% solids This type of cement is very effective in join-ing parts that have irregular surfaces or areas that are not readilyaccessible Because of the rapid softening actions of the solvent, thepressure and amount of solvent applied should be minimal

sol-9.7.3 Cellulosics (cellulose acetate,

cellulose acetate butyrate, cellulose nitrate,

ethyl cellulose, etc.)

Cellulosic materials can be mechanically fastened by a number ofmethods However, their rigidity and propensity to have internal mold-ing stresses must be carefully considered The adhesives commonlyused are epoxies, urethanes, isocyanate-cured polyesters, nitrile-phe-nolic, and cyanoacrylate Only cleaning is required prior to applyingthe adhesive A recommended surface cleaner is isopropyl alcohol.Cellulosic plastics may contain plasticizers The extent of plasticizermigration and the compatibility with the adhesive must be evaluated.Cellulosics can be stress cracked by uncured cyanoacrylate and acrylicadhesives Any excess adhesive should be removed from the surfaceimmediately

Cellulosic materials can also be solvent cemented Where stresscrazing is a problem, adhesives are a preferred method of assembly.9.7.4 Fluorocarbons (PTFE, CTFE, FEP, etc.)

Because of the lower ductility of the fluorocarbon materials, snap-fitand press-fit joints are seldom used Rivets or studs can be used informing permanent mechanical joints These can be provided withthermal techniques on the melt processable grades Self-tappingscrews and threaded inserts are used for many mechanical joiningoperations In bolted connections some stress relaxation may occur thefirst day after installation In such cases, mechanical fasteners should

be tightened; thereafter, stress relaxation is negligible

The combination of properties that makes fluorocarbons highlydesirable engineering plastics also makes them nearly impossible toheat or solvent weld and very difficult to bond with adhesives withoutproper surface treatment The most common surface preparation for