剑桥大学多学科设计项目数据库：实用工程资源宝典

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Limitations

Temperature resistance: Adhesives are drawn from the

class of materials which we know as ‘polymers’, ‘plastics’

or ’synthetic resins’. They have the limitations of that class.

They are not as strong as metals. (the difference is offset

by the increased surface contact area provided by the

bonded joints). With increasing temperature the bond

strength decreases, and the strain properties of the

adhesive move from elastic to plastic. This transition is

usually in the temperature range 70 – 220

C: the transition

temperature depends on the particular adhesive.

Chemical resistance: The resistance of bonded joints to

the in-service environment is dependent on the properties

of the polymer from which the adhesive is made. Possible

exposure of the bonded structure to oxidising agents,

solvents, etc., must be borne in mind when selecting the

adhesive type to use.

Curing time: With most adhesives maximum bond

strength is not produced instantly as it is with mechanical

fastening or with welding. The assembled joint must be

supported for at least part of the time during which the

strength of the bond is building up. The quality of the bond

may be adversely affected if, in the bonding process, the

surfaces are not readily wetted by the adhesive.

Process controls: Ensuring consistently good results may

necessitate the setting up of unfamiliar process controls. A

badly made joint is often impossible to correct.

In service repair: Bonded assemblies are usually not

easily dismantled for in-service repair.

Modern adhesives: types and main

characteristics

Modern adhesives are classified either by the way they are

used or by their chemical type. The strongest adhesives

solidify by a chemical reaction. Less strong types harden

by some physical change. Key types in today’s industrial

scene are as follows.

Anaerobics: Anaerobic adhesives harden when in contact

with metal and air is excluded, e.g. when a screw is tight

in a thread. Often known as ‘locking compounds’ or

‘sealants’, they are used to secure, seal and retain turned,

threaded, or similarly close-fitting parts. They are based on

synthetic resins known as acrylics. Due to the curing

process, anaerobic adhesives do not have gap-filling

capability but have advantage of relatively rapid curing.

Cyanoacrylates: A special type of acrylic, cyanoacrylate

adhesives cure through reaction with moisture held on the

surfaces to be bonded. They need close-fitting joints.

Usually they solidify in seconds and are suited to small

plastic parts and to rubber. Cyanoacrylate adhesives have

relatively little gap-filling capability but can be obtained in

liquid and thixotropic (non-flowing) versions.

Toughened Acrylics/Methacrylates: A modified type of

acrylic, these adhesives are fast-curing and offer high

strength and toughness. Supplied as two parts (resin and

catalyst), they are usually mixed prior to application, but

specialised types are available which are applied by

separate application: resin to one bond surface, catalyst to

the other. They tolerate minimal surface preparation and

bond well to a wide range of materials. The products are

available in a wide range of cure speeds and as liquids or

pastes which will gap-fill up to 5mm.

UV curable adhesives: Specially modified acrylic and

epoxy adhesives, which can be cured very rapidly by

exposure to UV radiation. Acrylic UV adhesives cure

extremely rapidly on exposure to UV but require one

substrate to be UV transparent. The UV initiated epoxy

adhesives can be irradiated before closing the bondline,

and cure in a few hours at ambient temperature or may be

cured at elevated temperature.

Epoxies: Epoxy adhesives consist of an epoxy resin plus a

hardener. They allow great versatility in formulation since

there are many resins and many different hardeners. They

form extremely strong durable bonds with most materials.

Epoxy adhesives are available in one-part or two-part form

and can be supplied as flowable liquids, as highly

thixotropic products with gap-filling capability of up to

25mm, or as films.

Polyurethanes: Polyurethane adhesives are commonly

one part moisture curing or two-part. They provide strong

resilient joints, which are resistant to impacts. They are

useful for bonding GRP (glassfibre-reinforced plastics) and

certain thermoplastic materials and can be made with a

range of curing speeds and supplied as liquids or with

gap-filling capability of up to 25mm.

Modified Phenolics: The first adhesives for metals,

modified phenolics now have a long history of successful

use for making high strength metal-to-metal and metal-to-

wood joints, and for bonding metal to brake-lining

materials. Modified phenolic adhesives require heat

pressure for the curing process.

The above types set by chemical reactions. Types that

are less strong, but important industrially, are as

follows:

Hot Melts: Related to one of the oldest forms of adhesive,

sealing wax, today’s industrial hot melts are based on

modern polymers. Hot melts are used for the fast

assembly of structures designed to be only lightly loaded.

CMI-MDP Programme

Plastisols: Plastisol adhesives are modified PVC

dispersions which require heat to harden, The resultant

joints are often resilient and tough.

Rubber adhesives: Based on solutions of latexes, rubber

adhesives solidify through loss of solvent or water. They

are not suitable for sustained loading.

Polyvinyl Acetates (PVAs): Vinyl acetate is the principal

constituent of the PVA emulsion adhesives. They are

suited to the bonding of porous materials, such as paper

or wood, and general packaging work.

Pressure-sensitive adhesives: Suited to use on tapes

and labels, pressure-sensitive adhesives do not solidify but

are often able to withstand adverse environments. They

are not suitable for sustained loading.

No one company supplies all these types of adhesives.

Each supplier specialises in particular types.

Huntsman Advanced Materials supplies many

industries with epoxy, polyurethane, modified phenolic,

toughened methacrylate and UV curable acrylic

adhesives under the tradenames Araldite

, Epibond

Epocast

and Uralane

Designing a bonded joint

It is important that bonded articles are designed with

bonding in mind, rather than simply bonding a design

made for welding or mechanical joining. When designing

bonded joints the considerations include:

• Joint geometry

• Adhesive selection

• Mechanical properties of adhesive and adherent

• Stress in the joint

• Manufacturing conditions

Bonded joints may be subjected to tensile, compressive,

shear or peel stresses, often in combination. (See Figure

3). Adhesives are strongest in shear, compression and

tension. They perform less effectively under peel and

cleavage loading. A bonded joint needs to be designed so

that the loading stresses will be directed along the lines of

the adhesive’s greatest strengths.

To indicate the performance of an Araldite, Epibond,

Epocast or Uralane adhesive, the Huntsman Advanced

Materials Instruction

Sheet for the particular adhesive quotes the shear

strengths and peel strengths obtained by standard test

methods. For example, the standard test method for shear

(ISO4587) uses a simple lap joint made from metal sheet,

usually an aluminium alloy, 25mm wide with 12.5mm

overlap. The mean breaking stress at room temperature

will be in the range 5 to 45 N/mm

depending on the

adhesive. At the top end of this breaking stress range,

joints made from aluminium alloy sheet of up to 1.5mm

thickness will yield or break in the metal. (The lap joint is

only one of sveral different types of bonded joint).

Tension Compression Shear Cleavage Peel

stress stress stress stress stress

component component component component component

Fig.1 Loading conditions

A bonded joint can be loaded in five basic ways (shown in the diagram). Cleavage and peel loading are the most taxing: they concentrate the applied force into

a single line of high stress. In practice a bonded structure has to sustain a combination of forces. For maximum strength, cleavage and peel stresses should be

as far as possible designed out of the joints.

CMI-MDP Programme