bonding of composite materials

Bonding of Composite materials

Compiled By

pak de jongko

duraposita chem.

Table of content

Part I General understanding and rules

Part 2The various chemical types of Adhesives for

composites, epoxy adhesives

Part 3 Various types of adhesives for composites: Acrylics,

urethanes-acrylates and Polyesters

Part 4 Various types of Adhesives for Composites:

Polyurethanes, Heat stable adhesives, Syntactics, etc…

Part I General understanding and rules

Definition of composites and scope of these chapters

In this chapter, we will study the assembly and the adhesive bonding of 2 kinds of composites:

• primary composite materials are those made of polymer matrixes reinforced with fibers such as glass, carbon or aramid fibers,

• secondary composites are those made by assembly of the primary composite materials, such as sandwich panels composed of composite skins bonded to various core materials such as honeycombs, or complete composite parts including a composite material + metal fixtures / attachments.

Different methods of assembly of the composite materials

The primary composite materials are made by different techniques: lay out, impregnation, molding, infusion, filament winding, pultrusion… All these methods consist in impregnating fibers with a polymer resin ( epoxy, polyester, etc…), the manufacturer may start with prepregs where the fibers are already impregnated with resin, shaping the parts in various ways and then curing the polymer by heat and pressure.

Co-curing

In most composites, the impregnation resin is by itself an adhesive resin, because it has to bind strongly the fibers together. For instance, epoxy matrixes are also excellent adhesives. Therefore it may be used to bond 2 composite parts together at the same time as the cure of the composite matrix occurs. This is called co-curing. No other adhesive is needed, although sometimes a special film of adhesive is laid between the 2 parts. Refer to figure 1.

Figure 1: Monolithic primary composite parts, made by single operation: curing or co-curing for all these simple parts, there are no assembly operations

This technique is widely used in aircraft construction to make, in one single operation, a solid part which can be molded in a press or an aut°Clave. We will not discuss this technique because it belongs to the composites manufacture ( except when an adhesive film is added ).

Adhesive bonding

EWhen the part is hollow, co-curing cannot be used, the 2 halves of the part must be manufactured separately and then they have to be assembled, either by mechanical fixtures or by using inserts, or by adhesive bonding. This chapter will be devoted only to the adhesive bonding of composite materials and parts. Refer to figure 2

Figure 2: Examples of composite bonded parts

Wet tabbing

We will also mention a technique which is frequently used in naval construction: this is the "wet tabbing " (figure 3 ): in this technique, the cured composite parts are aligned closely and then a prepreg ( reinforcing fibers impregnated with the raw, non cured resin ) is tabbed or laid onto the mating parts and then cured usually at room temperature.

Figure 3: Schematic diagram of tabbed stringers in naval construction Several piles of impregnated fibers are applied on the corner between hull and stiffeners.

This method has been used for years for shipbuilding, to join fiberglass reinforced polyester parts by using also fiberglass mats impregnated with polyester resins. However it is now often replaced by resin injection or by true adhesive systems, as we will see later, in next sections.

To summarize, figure 4 shows the various techniques of assembly of GRP in shipbuilding for instance.

Figure 4: Different techniques used in ship building for the assemblies of composite parts

(Source: Scott Bader)

Inserts

A last technique is the use of inserts ( figure 5 ) : here metal inserts such as studs, plates… are placed into the premix in the mold before curing, and the cure of the matrix will bl°Ck the insert into the hardened resin. Here we use the adhesive properties of the polymer resin, and also the possibility of bolting, screwing the parts together, and mechanical engineers, who are accustomed to use mechanical fasteners, like and rely with confidence on this technique. However we will not study this technique here because it is a mechanical assembly.

Figure 5: The 3 steps of the use of metal inserts in the final assembly on large GRP part

(Source: BIGHEAD bonding fasteners Ltd., UK)

In any case, the composite parts must be assembled onto a frame, and this frame is very ofter made of metal. This is true for automotives, aircrafts or construction panels. Therefore it is mandatory to have some kind of metal fasteners, attached to the composite part, which is then attached to the frame.

Figure 6 shows how the speed breakes of the F15 aircraft fighter are molded, including bonding of titanium ribs and actuators. Here both techniques are mixed: co-curing of the graphite-epoxy skins over honeycomb core, bonding of titanium ribs and then final positioning and curing of the graphite-epoxy skins.

Figure 6: F-15 composite speedbrake OML: outer moldline; IML: inner moldline;

HC: honeycomb care (Source: SAMPE 8th Conference)

Adhesives, as we know are able to assemble different materials, for instance composites to metal, metals to metals, plastics to metals, … Epoxy adhesives, which are very frequently used for the composite matrixes, are also an evident choice to bond composites based on epoxy matrix. Compatibility between resin and adhesive is evident. In the same fashion, polyester adhesives and putties are an evident choice to bond glass fiber reinforced plastics.

Advantages and drawbacks of adhesive bonding for composites

Compared to mechanical fasteners, adhesive bonding provides many advantages:

• mechanical fasteners require drilling holes in the parts, and this weakens the composites because it cuts through the reinforcing fibers and also creates weak points. Figure 7 shows the many failure modes that may occur in a bolted joint. Bonding improves tensile resistance ( figure 7 ).

• Bonded joints exhibit lower stresses concentrations than mechanical joints when holes are needed, and thus provides increased static strength,

• Risks of cracks propagation are reduced, • Bonded joints provide always 10 to 25 % weight savings in primary and

secondary structures, • Bonded joints enable the design of smooth external structures, • For large surfaces bonding costs less than mechanical assembly, because

it needs less manpower ( although there are now huge riveting machines which may place automatically thousands of rivets on the fuselage of large aircrafts in a short time, but these machines have of course a very high cost ),

• Adhesive may join together all kinds of materials: metals, composites, plastics, wood etc…

• Adhesives can join very thin materials which could not be riveted or bolted,

• Adhesives can join dissimilar materials without the risk of galvanic corrosion,

• Adhesives may be flexible or rigid according to their formulation, • Adhesives have an excellent resistance to fatigue.

However, adhesives bonding has also some drawbacks:

• elevated temperature creep resistance is fair or even poor for some structural adhesives. In the Aircraft construction, there are for instance several classes for the heat resistance of the structural adhesives: resistance to 80 °C, 120 °C… and only some very sophisticated and expensive adhesives can resist to a service temperature of 200-250 °C which is required for some parts or military fighters. This means that

adhesives cannot be used in or near to the motors of aircrafts or automotive.

• We will see that adhesives do not resist to peel stresses, and this is a drawback compared to welding for instance,

• Bonded parts cannot be dismantled easily, • Bonding requires specific design so that the parts will be stressed only in

shear mode ( refer to figure 7), • Bonding requires an excellent and specific surface preparation of the

materials immediately before bonding, • Bonded joints are difficult to inspect in a non destructive manner, although

there are several NDT such as X rays, ultrasonic inspection, shearography, and others,

• Structural bonding requires an accurate mating of the parts because adhesives do not give high performances in thick joints, ( will be discussed in the section: Design and calculation of bonded joints)

• Water resistance of adhesives are often only fair, • Durability of bonded joints must be assessed by difficult laboratory

accelerated aging tests.

We will study a number of these factors and properties in the following sections of this chapter.

Figure 7 (a): Basic failure modes in a

double- lap bolted joint, the holes weaken the composite

Figure 7 (b): Various modes of breaking into an adhesive bonded composite part submitted to shear stresses

Materials to be bonded

• thermosetting matrixes ( epoxy, polyester, phenolics…) reinforced with glass, carbon, graphite, aramid fibers,

• thermoplastic matrixes ( polyolefins, PEEK, nylon, polycarbonate…) reinforced with the same fibers es above,

• various types of honeycombs: aluminium, Nomex, plastics, • metal parts ( steel, aluminum, titanium in the aircrafts, metal alloys ), • various types of sandwich panels with composite skins.

Surface preparation before bonding

Composite materials are made with polymer matrixes and the surfaces to be bonded are made of these polymers. Therefore the required surface preparation is the same as it is used for the pure polymer or plastic.

Cleaning:

Prior to bonding, parts must be cleaned, the demolding release agents used during curing must be completely eliminated by washing with detergents or solvents. Washing must be followed by a perfect rinse and drying with clean air.

Abrasion:

In order to eliminate pollution, mold release agents and obtain a slightly rough surface for a better wetting, the surface may be gently abraded, for instance with

abrasive pad, but the abrasion must be very light so that it never goes down to the reinforcing fibers, because these fibers must always be protected by a layer of the polymer and embedded in the polymer. After abrasion, dust and pollution must be eliminated by cleaning and rinsing thoroughly.

Tear plies:

This is the best technique to get a clean surface on which the adhesive will bond readily. Refer to figure 8. During the manufacture of the composite part, a special textile ( such as Dacron or other ) is applied on the surface of the polymer before curing. Immediately before bonding, this peel ply or tear ply is peeled away, and it leaves underneath a perfectly clean resin surface.

Figure 8: Structural reinforced plastic laminate with tear ply to obtain perfectly clean bonding surfaces.

Chemical surface preparation

There are several chemical surface preparations which are specifically adapted to the various polymers. Table 1 lists these surface preparations.

Chemical and Physical surface preparation Plasma

Polymers None

Cleaning with

detergent or

solvent

Light abrasion Atmospheric

Pressure Low

pressure

Flame treatment Silination Sulfochromic

Toluene sulfonic

acid Other

treatments

Acrylonitrile - butadiene - styrene (ABS)

Epoxy resins Peel ply

Melamine and Urea- Formaldehyde resins

Peel ply

Phenolic resins Peel ply

Polyacetal Polybutyleneterephthalate (PBT)

Polycarbonate (PC) (Alcohol)

Polyester (Thermosetting) (Ketone)

PET: Polyester (Thermoplastic)

Titanate primer, NaOH

Polyetherether Ketone PEEK

Polyolefins: PE, PP (Ketone)

Corona treatment

Polyimide (Ketone) NaOH

Polyamide (PA) (Ketone) Resorcinol

PPO= Polyphenylene oxide

(Alcohol)

Polysulfone (PS) (Alcohol)

Polyurethane (PU) thermosetting

Table 1: Chemicals and Physical surface preparation for plastics, polymers and composites

1. When bonding with epoxy adhesives, the part may be up to 80°C and the adhesive is then affected while parts are still hot. 2. Laser may be also used now on thermosetting reinforced plastics.

In the next sections, we will introduce the different chemical types of adhesives used for structural bonding of composites and their technical characteristics.

And after that we will study the main end uses of adhesives bonding in many industries such as Automotive, Aircraft industry, transportation, shipbuilding and others

Part 2 The various chemical types of Adhesives for composites,

epoxy adhesives

Introduction

In this section, we will study the adhesives which are used for bonding the most important composite materials:

• thermosetting composites based on epoxy matrixes, the main market being those used for aircraft parts,

• thermosetting composites based on polyester matrixes, mainly used in naval construction and transportation equipment,

• other thermosetting composites such as those based on PU matrixes, used in automotive exterior parts,

• thermoplastic composites based on various chemical compounds: polyolefins, PPS,polyamides, etc...

• high heat resistant composites such as those based on cyanate esters, bis maleimides, epoxy-phenolics, etc...

As we will see, when choosing an adhesive for a given composite material, the first idea that comes to our mind is to select a chemical type which is the same as the resin used as the matrix. Epoxy adhesives are of course totally compatible with epoxy matrixes, therefore they are frequently used because they are the best choice for carbon-epoxy and Kevlar-epoxy composites.

Epoxy adhesives, general knowledge, technical characteristics

The base of epoxies adhesives formulation is explained on figure 1

Figure 1: The box of epoxy resins formulations

Epoxy adhesives have a unique combination of useful properties:

• excellent adhesion to many materials, not only on epoxy matrixes but also on many other polymers including thermosetting polyesters, thermoplatic polymers, metals, glass, wood, concrete, etc…so that different materials can be bonded together, for instance a composite to a metal.

• very large possibilities of formulation, with many different types of hardeners : aliphatic or aromatic amines, polyamides, polyaminoamides, amino adducts, acid anhydrides, substituted imidazoles and others, refer to table 1

Curing agents Recommended Parts/100 parts

Liquid resin

Curing Temperature

°C

Curing agent Supplies and rade Names

Aliphatic amines

diethylene triamine (DETA, DETA epoxy adduct)

8-10 R.T.-150

D.E.H. (Dow chemical Company), Pacific Anchor

triethylene tetramine (TETA, TETA adduct)

10-13 R.T.-150

Amicure (Pacific Anchor Chemical Co.), Dow Chemicals

Aminoethyle piperazine (AEP) 20-23 R.T.-150

Epo-Tuf (Reichhold Chemicals)

Other aliphatic amines depends of the formulation pacific Anchor, Huntsman, BASF Aromatic amines methylene dianiline 15-55 175 (2 hr) Curithane

(Dow) 4, 4-diaminodiphenyl Sulfone

30-34 175 (2 hr) Eporal (Huntsman)

MDA/MPDA eutectics 100

Amicure (Air Products and Chemicals, Inc.) Ancamine (Pacific Anchor) Epon (Shell Chemical)

Cycloalphatic amines

wide vareity of modified products from major curing agent suppliers

dependson formulation R.T.-150

Amicure (Air Products) Ancamine (Pacific Anchor) Epo-Tuf (Reichhold)

Versamine (Henkel)

Polyoxypropylene amines Jeffamine

(Texaes) Specialty amines Hiils

Polyamines

LAROMINE (BASF) ARADUR (Huntsman)

Polyamines adducts

ARADUR (Huntsman advanced materials)

Polyamides

wide variety of polyamides curing agent with range of molecular weights

R.T. to 2 hr @ 100

Henkel (Versamide) Unirez (Union Camp) Ancamide (Pacific Anchor) Huntsman

Polyamido amines, Polyamido amines adducts

depends on

the formulations

Ancamides (Pacific Anchor) Aradur (HUNTSMAN)

Heated activated curing agents and catalysts benzyl dimethylamine (BDMA)

2-4 150 (2 hrs) Air Product and Chemical, Inc.

Baron trifluoride amine complexes 2-4 150 (2 hrs)

Ancaflex (Pacific Anchor) Allied chemicals

Polyurethome amines 2-5 150-160°C (2

hrs) Aradur (HUNTSMAN)

Polymercaptan, polysulfide 2-5 150-160°C (2

hrs) Aradur 90 (HUNTSMAN)

Table 1: Curing Agents for Epoxy Resins Adhesives

Also, mixed compositions of polymers are very interesting, such as flexibilised epoxy-nylon , heat resistant epoxy-phenolic, toughened epoxies which are flexibilised by addition of elastomers,etc... Epoxies can be cured and coreacted with many different resins. Therefore there are many different epoxy formulations, with quite different technical characteristics,Epoxy formulations are studied elsewhere in Specialchem4adhesives site. We will mention here only the products and informations which are useful for composites bonding.

• good to excellent heat resistance according to the formulation: ranging from 70°C for simple 2 components formulations, up to 150°C service temperatures and more for heat curing formulations used in aircraft construction, refer to Table 2

• very high cohesion, high modulus, and sometimes they are brittle and this causes low peel strength, so that it has been necessary to give them some flexibility by adding elastomers to the formulations, this is called: Toughened epoxies. We study it hereunder. Refer to Table 2 for modulus and elongation.

• Very high tensile shear resistance, as it is indicated on table 2, ranging from 25 to 35 MPa ar ambient temperature. Table 2 also provides the tensile lap shear resistances at other temperature such as 93°C and - 55°C this latter temperature being the low temperature reached on exterior parts when the aircraft reaches the cruise altitude.

• intrinsic water resistance is good, but the sides of the bonded parts should be protected against water ingress to the bond line, because this water could penetrate between the epoxy adhesive and the substrates and damage the bond.

Click here for Table 2

We will now review the different types of epoxy adhesives which may be used for composite bonding

One component, heat curing epoxies

Here the hardeners is usually dicyandiamide because this product is a solid which becomes miscible with epoxy resin only when the temperature reaches 140°C. The curing takes place at temperatures ranging from 140 to 180°C, during times from 20 to 60 minutes. Other one components epoxy adhesives are the epoxy-phenolic and epoxy-nylon.

Co curing:

In the aircraft construction, these adhesives can be cured at the same time that the composite part itself (prepregs ) . This is called cocure. In this technique, the

http://www.specialchem4adhesives.com/documentsas/indexables/editorial/articles/1623/table-2.htm

parts prepregs are stacked together with the film epoxy adhesive, between the mold and a vacuum bag for instance ( refer to figure 3 ), the whole system is loaded into an autoclave where the prepregs and the adhesive are cured simultaneously. This means that the curing conditions ( temperature, time and pressure ) must be the same.One component, heat curing adhesives may have different forms:

Figure 2: Manufacturing and cocuring honeycomb sandwich

parts, with prepregs and film adhesives

• film adhesives, supported by a carrier, that must be stored at low temperatures -10 to -20°C to prevent the curing reaction to start, can be stored only for a few months. table 3 presents the properties of HEXCEL REDUX film adhesives, many of them are epoxy based, these film adhesives are mostly used in Aircraft construction, very often by cocuring technique. These film adhesives are usually expensive, because they are high performance adhesives designed for aircraft construction. But it should be possible to formulate cheaper formulations for other applications such as Transportation equipment and Automotives. Figure 4 shows the manufacture of sandwich parts with carbon-epoxy facings and epoxy film adhesives.

• or paste adhesives. ( refer to suppliers catalogs: HUNTSMAN, HEXCEL, HENKEL Aerospace...)

Applications Product Performance

Product Composite Bonding

Metal to Metal

Bonding Honeycomb

Bonding

Maximum service

temperature (°F/°C)

Typical cure

temperature (°F/°C)

Lap shear at R.T.

*1(MPa/psi)

Honeycomb climbing

drum peel at R.T.*1

(N/76mm)

Flatwise tensile at

R.T.*1 (MPa/psi)

Tg Dry (°F/°C)*2

Key Features

and Applications

Epoxy Film Adhesive

Redux® 610 185/85 250/120 30/4300 240/27 7/1000 230/110

Flame retarded . Co-cures with 120°C prepregs. Panel bounding for rail interiors, marine, building applications.

Redux® 335 195/90 250/120 40/5800 750/84 8/1200 175/80

Outstanding peel performance, ideal for motorsport.

Redux® 312 210/100 250/120 40/5800 650/73 9/1300 220/105

Short cure cycle:30 minutes at 120°C for a wide range of applications.

Redux® 330 - 275/135 350/175 40/6000 N/A 6.9/900

255, 365/125,

195

Ideal for composite to composite bonding and lightning strike applications.

Redux® 319 300/150 350/175 36/5200 600/68 9/1300

275, 390/135,

200

High peel performance for wide range of applications, particularly automotive and aerospace (engine nacelles, flaps, aileron bonding).

Redux® 641 300/150 350/175 40/6000 620/70 12.5/1800

250, 385/120,

195

High performance adhesive for

industrial markets. Exceptional honeycomb bonding.

Redux® 322 350/175 350/175 20/3000 240/27 8/1200 390/200

Very high temperature performance. For military, engine nacelles, missile bonding, aerospace, motorspace and high temperature industrial applications.

Redux® 340SP 350/175 350/175 42/6100 550/62 N/A

295, 390/145,

200

Low weight film adhesives for space applications.

BMI Film Adhesive

Redux® HP655 445/230

375+post cure/

190+post cure

26/3800 200/23 5/700 430/220

Very high temperature performance. Good co-cure with BMI prepregs.

Cyanate Ester Film Adhesive

Redux® A54 - - 320/160 350/175 23/3300 N/A 4/600 310/155

Low out-glassing and moisture absorption

Table 3: Redux® Film Adhesives Source: HEXCEL

*1 Room Temperature= 77°C/25°F *2Tg are for standard cure cycle by DMTA , log E

Two components, RT or elevated temperature cure

Table 2 lists a number of 2 components paste adhesives designed for composites bonding in aircrafts, and provides their technical characteristics. These adhesives cure at RT in a few hours or days, or at moderate temperatures such as 80°C, in one hour, they usually have good peel strength, for instance 2 components epoxy-polyamide have excellent peel strength, and also good tensile lap shear strength. Prices are quite lower than the epoxy film adhesives.

They have usually lower heat resistance than the one component heat curing epoxies.Table 4 provides the technical characteristics of 2 components epoxy adhesives from HUNTSMAN

Let us mention that recently 2 components epoxy paste adhesive have been developed for cold curing in aircraft construction: this is for instance the EPIBOND 1590 A/B of Vantico now HUNTSMAN.

Click here for Table 4

Toughened epoxies

There are several ways to flexibilise epoxy adhesives systems. One way is to use HYCAR carboxyl terminated reactive liquid polymers at 10 to 25 parts of this rubber for 100 parts of epoxy resin. This increases impact strength, peel strength and fracture surface energy with little loss of modulus and heat resistance. HYCAR polymers are acrylonitrile-butadiene copolymers ( CTBN ) bearing free carboxyl groups at the polymer end of the chain or distributed along the chain. Their structure is shown on figure 5. They react with epoxy rings as indicated on figure 6 and this gives flexible links.

Figure 5: Structure of HYCAR CTBN

Figure 6: Chemical reaction between epoxies and carboxyl groups of the HYCAR CTBN

http://www.specialchem4adhesives.com/documentsas/indexables/editorial/articles/1623/table-4.htm

The interesting effects of toughening is indicated on figures 7 and 8. Toughened epoxies are very often used when the bonded parts are subjected to peel or clivage efforts and fatigue, for instance for aircraft and automotive parts.

Figure 7: Demonstates that for all test temperature, tensile energy increases as concentration of Hycar®

CT - RLP is increased to 20 phr

Table 2 provides technical characteristics of some toughened epoxies used in aircrafts construction.

Figure 8: Show that tensile strength is significantly improved by increasing concentration of Hycar® CT-

RLP at all strain rates tested

Epoxy-polymers alloys

These alloys are formulated with epoxy resins modified by the addition and co-reaction of other polymers:

• epoxy-phenolics provide high heat resistance up to 250°C, • epoxy-nylon give excellent flexibility and peel strength, • epoxy-polysulphide are more flexible than standard epoxies.

All are one component heat curing adhesives that require high temperature for the cure.

In the next section we will study the other types of adhesives for composites, namely the structural acrylics, polyurethanes, polyesters, heat stable adhesives and related products such as Syntactics.

In following sections later we will describe all the main applications / end uses of adhesives in the various industries that use large amounts of composites, such as Aerospace, Automotive and Transportation equipment, windmills and naval construction.

Part 3 Various types of adhesives for composites: Acrylics,

urethanes-acrylates and Polyesters.

Introduction

In this section, we will study several types of structural adhesives which are used for the assembly of composites: Acrylics, Polyesters, and others.

Acrylics and methacrylates:

These are also called "engineering acrylics" or "structural acrylics", in order to differentiate it from the non structural acrylics which are used for Pressure sensitive adhesives or Construction and decoration applications.

There are several types but all are based on the same chemistry:

The formulations include methacrylate monomers plus a dissolved rubber polymer added as a toughener, cure accelerator and a free radical generator, sometimes also crosslinking agents. It may be one part adhesive with an activator applied on one of the substrates or 2 part adhesives. These adhesives cure at room temperature by a redox reaction of the accelerator with a free radical source - a peroxide -. This gives free radicals that open the double bonds.

Accelerators may be N-phenyl 2-propyl 3,5 diethyl 2 dihydropyridine, or aromatic amine such as NN dimethyl para toluidine. Tougheners may be ABS polymers, acrylonitrile rubbers and other soluble and miscible rubbers. These toughening agents provide flexibility, impact and crack propagation resistance.

Standard 2 components RT curing methacrylates:

This is the main family of structural acrylics. They display very interesting properties for the assembly of composites:

• excellent adhesion to a variety of substrates: GRP, other reinforced plastics, several plastics, metals, wood... Table 1 indicates the tensile strength that may be obtained on steel ( grit blasted ) tested according to ASTM D 1002. It is common to obtain from 15 to 30 MPa , according to the substrates, in tensile shear strength on these materials.

• These adhesives are much more flexible than epoxies and offer excellent peel strength: 150 N for a 25 cm width. They give also a good resistance to impact ( 25 J in the ASTM D 3163 test ). Therefore they are often used

for the construction of transportation equipment in cars, buses for instance.

• Very high resistance to fatigue, as indicated on figure 1: this is very important for naval construction where the boats have to withstand hundreds of thousands repeated shocks on the waves during life time.

Figure 1: Fatigue data for Adhesively bonded Lap Shear

(ASTM D 1002) joints Source: ITW- PLEXUS

• Their flexibility provides a better distribution of loads and also reduces noise and vibrations, and allows to bond dissimilar materials for instance metals to GRP.

• They have also an excellent resistance to humidity, so that they are used in naval construction,

• Heat resistance may reach 100 to 120 oC for continuous service temperature, depending on the formulation,

• Mixing ratios vary from 1 to 1 to 10 to 1, according to the type of formulation. In some cases it is not necessary to premix the resin and the hardener but the resin may be applied on one substrate and the hardener on the other. Application is easy with different types of applicators such as twin pump mixing equipment equipped with static mixers, when the mixing ratio is close to 1/1 .

• They cure at room temperature: full cure takes 1 to 6 hours, but we can define a "fixture time": it is the length of time between the combining of adhesive and activator or hardener and the development of sufficient strength to permit handling of the bonded parts without deformation or rupture of the bond. Fixture times vary from 15 minutes for the faster grades to 4 hours for the slower.

• An important feature is the fact that they can fill the gaps between the parts to be bonded, with some formulation it may fill gaps up to 10 mm or even 20 mm thickness. This is very useful in naval construction where the

hulls and decks may have wide dimensional tolerances resulting from demoulding of the GRP large parts.

• Also important is that they usually require little or no surface preparation.

Table 1 provides the technical characteristics of the ITW PLEXUS range of structural acrylics, as an example.

Adhesive Description/ Substrates

Mix Ratio (By

Volume) Viscosity (cps)

Working Time (min)

Fixture Time (min)

Tensile1 Strength

(psi)

Tensile Elongation

(%)

Shear2 Strength

(psi)

MA300 High strength, all-purpose adhesive 1:1

A: 40,000~60,000 B: 40,000~60,000

4~6 12~15 3000~3500 15~25 3000~3500

MA310 High strength adhesive for difficult-to-bond plastics

1:1

A: 40,000~60,000 B: 40,000~60,000

15~18 30~35 4000~4500 5~15 3000~3500

MA330 Gray version of MA310 for difficult-to-bond plastics

1:1

A: 40,000~60,000 B: 40,000~60,000

15~18 30~35 4000~4500 5~15 3000~3500

MA320 Excellent to low-temp and toughness properties

10:1

A: 135,000~175,000B: 40,000~70,000

8~12 25~30 2000~2500 100~140 1500~2000

AO420 All-purpose adhesive; Fast curing

10:1

A: 100,000~125,000B: 50,000~70,000

4~6 15~18 2700~3000 100~125 1750~2250

MA422 All-purpose adhesive; Medium open time

10:1

A: 100,000~125,000B: 40,000~60,000

17~24 35~40 2000~2500 75~100 1500~1800

MA425 All-purpose adhesive; Long open time

10:1

A: 100,000~125,000B: 40,000~60,000

30~35 80~90 2000~2500 120~140 1500~1800

MA550 Excellent marine adhesive; UV Stable 10:1

A: 130,000~160,000B: 40,000~60,000

40~45 70~75 1750~2000 30~45 1300~1800

MA556

For boat-building; Low exotherm adhesive for bond line to 1" thick

10:1

A: 190,000~220,000B: 40,000~60,000

40~45 110~120 2500~3000 140~160 1250~1500

MA557 Low exotherm adhesive for bond line to 1.5" thick

10:1

A: 180,000~220,000B: 40,000~60,000

80~90 180~220 2000~2500 120~160 1250~1550

MA820 Primerless adhesive for metal bonding; Fast cure

10:1

A: 80,000~110,000B: 50,000~70,000

4~6 15~18 3300~3700 75~100 2000~2400

MA920 Low-odor; all-purpose adhesive; Fast cure

10:1

A: 100,000~125,000B: 50,000~70,000

4~6 15~18 2700~3000 80~100 1500~2000

MA922 Low-odor; all-purpose adhesive; Medium open time

10:1

A: 100,000~125,000B: 40,000~60,000

17~24 35~40 1800~2200 75~100 1400~1700

MA925 Low-odor; all-purpose adhesive; Long open time

10:1

A: 100,000~125,000B: 40,000~60,000

30~35 80~90 1700~2200 100~120 1400~1700

MA1020 Low-shrink/odor; Fast cure, Multi purpose

10:1

A: 100,000~125,000B: 50,000~70,000

4~6 15~20 1750~2000 90~110 1250~1600

MA1025

Low-shrink/odor/exotherm for bond lines to 1" thick

10:1

A: 180,000~220,000B: 40,000~60,000

20~25 40~45 1750~2000 90~110 850~1200

Table 1: Typical characteristics of structural, 2 component engineering acrylic adhesives Source: (ITW- PLEXUS USA)

1: Tensile strength of coat adhesive 2: shera strengthon git blasted sheet

Applications:

• Naval construction, for all bonding jobs of deck to hull, partitions to hull, stiffeners to hulls.

For these applications, structural methacrylates tend to replace polyester adhesives and wet tabbing operations,

• Automotives and transportation equipment:

Figure 2 shows the many composites parts which may be bonded in a modern bus. Here all the excellent properties of the structural acrylics mentioned above are very interesting, mostly the adhesion to these composites, the high fatigue and shocks resistances, the gap filling properties, the ease of application in the plants where there are no large manufacturing series for buses, and the room temperature cure.

Figure 2: Schematic of a bus showing composite parts that may

be bonded with adhesives

These structural acrylics may also be used in automotive body jobs, for instance to bond composite parts such as fenders, stiffeners to door panels, doors, sliding roofs. Note that their cure could be accelerated by mild heating at 60°C for instance in order to adapt to the production line speed. There will be no deformation of the composite parts at these low temperatures.

One interesting application is for bonding plastic bumpers in USA: these bumpers are made of XENOY alloy based on polycarbonate and polyester. These acrylic adhesives are used to bond the face bar or fascia to the back bar or reinforcement.

12 - 2 components structural methacrylates resisting to low temperatures ( - 25°C ) These adhesives also display a good adhesion to metal oily surfaces and are used in automotive body work.

13 - There are also hybrid epoxy-acrylates that provide high heat resistance up to 150°C service temperatures, and urethane-acrylates.

Main suppliers of engineering / structural acrylics and methacrylates are: HENKEL- LOCTITE, ITW PLEXUS USA , LORD Corporation USA , PERMABOND ( USA and Europe ), and others.

PLEXUS Adhesive MA 320 MA 425 MA 550 MA 555

Typical Use

Helm and console; General-purpose bonding

Stringer; Liner; Deck and hull bonding (12' to 24')

Helm and console; Radar arch; General-purpose bonding

Stringer; Liner; Deck and hull bonding (16' to 80')

Working Time (mins) 8~12 30~35 40~45 40~45

Fixture Time (mins) 25~30 80~90 70~75 110~120

Mix Ratio 10:1 10:1 10:1 10:1 Mixed Viscosity (cps x 1000)

135~175 100~125 130~160 115~130

Lap Shear Strength (psi) 1500~2000 1500~1800 1300~1800 1250~1500

% Elongation 100~140 120~140 35~45 140~160 Gap Filling yes yes Methacrylates Adhesive are recommended for bonding:

• Polyesters • Vinyl ester • FRP • ABS

• Nylon • PVC • SMC

Table 2: Properties of structural, 2 component methacrylates adhesives recommended for Naval constructor

Source: ITW- PLEXUS, USA

Urethane-acrylates:

Here a urethane component is fully reacted into the molecular backbone, it provides the excellent adhesion properties of polyurethanes without the hazards associated with isocyanates, and the molecules include acrylic sections which can be crosslinked by addition of styrene, in order to obtain a crosslinked thermoset adhesive.

These adhesives have many excellent features:

• excellent adhesion to composites, Glass reinforced plastics, cured laminates, glass and carbon fibers,

• toughness, resilience, • flexibility and superior fatigue resistance compared with polyesters,

• high mechanical strength even in thick sections, they fill gaps up to 25 mm thickness, to bond decks to hulls for instance, or very large parts such as windmills blades,

• a range of curing conditions to fit the various industries and jobs,

Curing conditions:

These adhesives need the addition of accelerator and catalyst, for instance Cobalt/MEKP or amine/BPO curing systems that will initiate the cross linking reaction between styrene monomers and the acrylates sections, in a way rather similar to the cure of polyesters.

Table 3 provides an overview of the CRESTOMER range of products from SCOTT BADER, UK, and table 4 gives the bond strength of several of these CRESTOMER adhesives.

Crystic Crestomer

Product Description Approvals

*Gel Time

(mins)

Tensile1 Strength

(MPa)

Tensile1 Modulus

(MPa)

Elongation at Break

(%) Performance

Characteristics

1152PA

High Performance Structural Adhesive

Lloyds Acceptance DNV

*50 26 500 100

Structural adhesive for demanding applications

1181A

Multi Purpose Structural Adhesive Amine Accelerated


**120 14 800 6

High strength gap filling adhesive with extended gel time

1186PA

Multi Purpose Structural Adhesive


*35 14 800 6 High strength gap filling adhesive

1196PA

Structural Core Bonding Adhesive


*50 18 1300 4

Low density adhesive specifically developed for demanding core bonding applications

Advantage Adhesive

High Performance Structural Adhesive Amine Accelerated For Bulk Application


25 26 500 100

Structural adhesive for demanding applications

Advantage 30

High Performance Structural Adhesive pre-packed in cartridges


30 32 500 120

High performance structural adhesive for convenience and flexibility

Table 3: Product range of urethane-acrylic adhesive Source: CRESTOMER OF SCOTT BADER UK

* 2% Butanox® M-50 at 25°C ** 2% Perkadox® BT-50 at 25 °C

It shows that these adhesives can be used everywhere in marine and transportation industries, for bonding FRP, GRP, metals, wood ( teck for decks ), when high adhesion ,gap filling properties, toughness, flexibility and fatigue resistance are needed together.

FRP Marine Ply Aluminium Stainless

Steel Teak

FRP 10 4 10 10 5 Marine Ply 4 4 4 4 Aluminium 10 10 5 Stainless Steel 12 5

Crystic Crestomer

1152PA

Teak 5

FRP Marine Ply Aluminium Stainless

Steel Teak

FRP 10 4 10 10 5 Marine Ply 4 4 4 4 Aluminium 10 10 5 Stainless Steel 12 5

Crystic Crestomer

1186PA

Teak 5

Balsa uPVC Foam Low Density

uPVC Foam Med Density

uPVC Foam Hi Density

Crystic Crestomer

1196PA FRP 6 3 7 12 Table 4: Bond strength of urethane-acrylic adhesives with different

substrates.

Cohesive Failure Substrate Failure

Polyester adhesives:

Most GRP parts for naval construction or Buildings have been made traditionally with glass fiber reinforced polyester resins. Therefore the paste adhesives that were traditionally used for bonding or wet tabbing of these parts were also polyester based, and are supplied by the same companies as the suppliers of polyester resins: REICHHOLD, SCOTT BADER, ASHLAND, ...

Polyester adhesives cure by the addition of styrene plus accelerator and or catalyst, at room temperature, the gel times vary from 10 minutes to 2 hours, the complete hardening from 3 to 10 hours, the gap filling properties are good enough from .3 to 10 mm maximum.

The shear strength may reach 6 to 15 MPa for FRP to FRP bonds, flexural modulus are in the range 2 to 3 Gpa. These adhesives are always very rigid, not at all flexible, the elongation at break is only 2 to 3 %, and for this reason tend to be replaced by methacrylates or urethane-acrylates.

However they are still used in naval construction in large quantities, for deck to hull bonding, wet tabbing or bonding of stiffeners to the hull etc...and they are cheaper than the other naval adhesives, because polyesters are manufactured in very large quantities for GRP.

Table 5 provides the technical characteristics of the bonding pastes ENGUARD from ASHLAND which are based on polyesters.

Product Description &

Main Characteristics

Marine Building/ Sanitary

Tanks &

SilosTranspose

Viscosity* 2=Low

3,4=High & thrixol

Gel Time

(mins)MEKP-50 %

Hardening Time (hrs)

Typical bond/gap thickness

(mm)

Linear shrinkage

(%)

TH 06

Thixotropic - Low density - Flexible - Easy to sand - Low shrinkage

3 18 2 5 0,3 - 3 < 1

TH 72 All - around bonding paste - short gel time

3 10 2 3 0,3 - 3 2 - 2,5

TH 72 AR

Same as TH 72 with catalyst indicator

3 10 2 3 0,3 - 3 2 - 2,5

TH 75

High thixotropic - containing glass fibres and catalyst indicator - Short gel time

4 10 2 3 3 - 5 2 - 2,5

TH 80

Fire resistance - Catalyst indicator with low shrinkage/exotherm

3 23 2 4 3 - 5 < 0,5

TH 89

High thixotropic - Catalyst indicator - Low shrinkage - Low exotherm

4 18 1 4 3 - 5 < 0,5

TH 95 High thixotropic - Catalyst indicator - Low shrinkage

4 45 2 6 3 - 5 < 0,5

TH 725

Low thixotropic with long gel time - Shrinkage controlled - Good flexibility

3 35 2 5 0,3 - 3 1,5 - 2

Bonding/ Joining/ Fixing

TH 730

Same as TH 725 with low viscosity - High mechanical values

2 35 2 5 0,3 - 3 1,5 - 2

TH 751

High thixotropic containing glass fibres and catalyst indicator - Long gel time

4 35 2 5 3 - 5 2 - 2,5

TH 1725

Thixotropic with long gel time - Low Shrinkage

3 60 1,5 6 0,3 - 3 < 0,5

TH 2000

TH ixotropic - Very long gel time and low shrinkage

3 90 1 6 3 - 5 < 0,5

TH 6000

VER based resin - Using Standard MEKP - Low thixotropic with long gel time

2 45 2 6 0,3 - 10 1,5 - 2

TH 72 All - around bonding paste - short gel time

3 10 2 3 0,3 - 3 2 - 2,5

TH 75

High thixotropic containing glass fibres and catalyst indicator - Short gel time

4 10 2 3 3 - 5 2 - 2,5

TH 86

Thixotropic - Low density - Easy to sand - Short gel time

3 13 2 3 0,3 - 3 1,5 - 2

TH 90 Very flexible - Thixotropic 3 18 2 4 0,3 - 3 < 0,5

TH 97 High thixotropic - Low density 4 60 1,5 6 0,3 - 3 1,5 - 2

TH 724 Low thixotropic with catalyst indicator 2 10 2 3 0,3 - 3 2 - 2,5

TH 725

Low thixotropic with long gel time - Shrinkage controlled - Good flexibility

3 35 2 5 0,3 - 3 1,5 - 2

TH 727 Low thixotropic - Low shrinkage 2 17 1 4 0,3 - 3 < 0,5

TH 751

High thixotropic containing glass fibres and catalyst indicator - Long gel time

4 35 2 5 3 - 5 2 - 2,5

TH 761 Low viscosity - Low density - Low gel time - Easy to sand

2 35 1,5 5 0,3 - 3 1,5 - 2

Sandwich core bonding

TH 1725

Thixotropic with long gel time - Low shrinkage

3 60 1,5 6 0,3 - 3 < 0,5

Table 5: Technical characteristics of polyester bonding parts ENGUARD® (registered symbol) from ASHLAND

In the next section, we will study the other adhesives that may be used for composites bonding, namely Polyurethanes, heat stable adhesives and also syntactics

Part 4 Various types of Adhesives for Composites: Polyurethanes,

Heat stable adhesives, Syntactics, etc…

Introduction

In previous chapters we have studied several types of adhesives for composites: epoxies, polyesters, structural acrylics... We are now studying other types available.

Polyurethanes:

Polyurethane chemistry is a very wide subject, PU enjoy a very versatile chemistry: there are one and 2 components PU, chemically curing, humidity curing, PU reactive hot melts, they may be flexible or rigid adhesives, thermoplastic or thermosetting adhesives, adhesives and/or sealants, semi-structural or non structural adhesives. These different products depend on the formulation, by using various polyols, prepolymers, isocyanate terminated polymers, various isocyanates...

Therefore we must define here the types of PU adhesives which are used for composite bonding: these adhesives are usually 2 components, semi-structural, high performances adhesives. They may cure at room temperature or provide accelerated curing by mild temperature heat curing, at temperature ranging from 60 to 100°C.

Figure 1 shows the typical curing mechanism of 2 components PU adhesives, explained in a short and simple way, because many chemical reactions occur during the cure of polyurethanes.

Figure 1: Curing mechanism of 2C-PUR adhesives. R and R' are polymer chains.

Isocyanate terminated prepolymers can react with water, alcohols, amines and other reactive chemicals. For more details we advice our readers to refer to comprehensive handbooks on PU chemistry, which is very complex, and cannot be explained here in details.

PU polymers and adhesives have a very interesting balance of strength, flexibility and excellent adhesion to many different substrates, plastics, composites, metals, wood. On the opposite, epoxy adhesives have very high strength but they are very rigid and brittle.

It is interesting to compare 3 types of adhesives which are used for composites, namely PUs, epoxies and toughened acrylics. When bonded samples are pulled, the stress / strain curves look like those in figure 2, according to ASHLAND's PLIOGRIP technical catalog.

Figure 2: Comparison of stress - strain curves of polyurethane, epoxy and

structural adhesives acrylics Source: According to ASHLAND's

PLIOGRIP technical catalog

PU adhesives withstand much higher elongation ( refer to table 1 )

Maximam Cure Time,e

Lap Shear Strength

t Mix Ratio, By Wt./ ByVol.

Viscosity, Components, cP

Mixed Viscosity, cP

Pot life, Min @ 77°F+ (80 g sample)

Handling Strength,b Hrs @ 77°F+

Hrs @ 77°F+

Min @ 140°F+

Min @ 212°F+

psi @ 77°F+

psi @ 180°F+(d)

Tg, °F/°C

Elongation, % at break

Coefficient of Thermal Expansion in/in./°F

Maximam Service Temp.,e °F/°C

SuggestApplicatCommen

e® 100:108/ 100:100

48,000 50,000 50,000 20 10 48 120 30 1,700 380 115/

46 250 8.50x10-5 120/50

Very flexadhesiveoffer exceadhesionpolycarbonylon andplastics.

e® 80:100/ 100:100

50,000 20,000 50,000 15 4 16 120 10 2,800 600 122/50 250 7.50x10-5 140/60

Flexible adhesivebonding avariety ofplastics.Rminimal spreparati

e® 80:100/ 100:100

50,000 20,000 50,000 3 1 6 60 10 2,800 600 122/50 250 7.50x10-5 140/60

Fast-settflexible afor bondiwide varisubstrateRequiresminimal spreparati

e® 0 er 0

100:50/ 100:40

200 20,000 5,000 8-10 4 24 3 30 2,000 600 122/50 30 6.40x10-5 140/60

Low viscoadhesivebonding averiety ofsubstrateExcellentterm environmperforma

e® 0 er 5

80:100/ 100:100

50,000 20,000 50,000 8 2 12 90 10 2,800 600 122/50 250 7.50x10-5 140/60

Fast-settflexible awith goodresistancbonding thermoplaand meta

Table 1: Two-Component of Polyurethane Adhesives Source: VANTICO/HUNTAMAN

TEST METHOD Viscosity: ASTM D-2393 Specific Gravity: ASTM D-792 Pot Life: ASTM D-2471 Lap Shear Strength: ASTM D-1002 (Tested on aluminum) Tg per DMA Elongation ASTM D-638 Ultimate Tensile Strength: ASTM D-638 Coefficient of Thermal Expansion: ASTM E-831 ' Below Tg " Above Tg +77°F = 25°C 104°F = 40°C 140°F = 60°C 180°F = 82°C 212°F = 100°C 248°F = 120°C 302°F = 151°C 356°F= 180°C a = Tested on SMC b = Time to achieve 150 psi lap shear strength c = Time to achieve 1500 psi lap shear strength, in most products d = After room-temperature cure e = Temperature at which bond maintains 1,000 psi lap shear strength, in most products VISCOSITY IN CENTIPOISE CONSISTENCY SIMILAR TO 1 Water 500 #10 Motor Oil 2,500 Pancake Syrup 10,000 Honey 25,000 Chocolate Syrup 50,000 Catsup

This means that the work required to break the bond, which is proportioned to the area under these curves, is higher for the PU adhesives, although the tensile strength of the PU is lower. This fact has been used in the last 15 years by PU adhesives manufacturers to promote these PU adhesives even in structural applications, mostly for automotive and transportation equipment.

Let us consider for instance the bonds of composite parts to metal parts in automotives. Here there are sometimes large deformations of the parts and the bond, because of the relative movements of the car body. PU adhesives and sealants will withstand very well these movements thanks to their flexibility, while rigid epoxies will not. Epoxies are perfect for metal to metal bonding and rigid bonding with very low deformations, but when it comes to bonding of reinforced plastics to metals, semi-structural PU may be a better choice.

PU adhesives have also a high adhesion to many substrates, especially to plastics and composites, as indicated in Table 1. This table also provides the characteristics of utilisation of some PU semi-structural and structural PU adhesives, such as: viscosities, curing conditions, handling strength, and also their performances.

ASHLAND Chemicals offers the PLIOGRIP PU systems, based on polymeric MDI ( methylene diisocyanate ) based prepolymers + hardening resins, which are often used as structural adhesives in the manufacture of trucks and cars body panels. These adhesives provide many advantages:

• gap filling properties, • excellent impact strength, • excellent adhesion to thermoplastic and thermosetting reinforced plastics,

especially to PU body panels: fenders, hoods and trunk lids, roof top and spoilers,...

• allow bonding of reinforced plastic parts to metal frames, because these PU adhesives withstand differential movements of these materials.

Table 2 provides some technical characteristics of a typical PLIOGRIP PU structural adhesive used for cars and trucks assembly.

Single Components Bulk Adhesive

System Properties @ 23C:

Assembly Criteria @ 23C

Prepolymer Resin (Polyol)

Tensile Modulus, MPa 17 Opening-

time, min 5-20

Color Tan Green Young's Modulus, MPa 388

Handling Strength.

min

Viscosity 28.000 cps 12.000 cps

Poisson's Ratio 0.4 @ 23C 15-

60 Density 1.6 kg/l 1.1 kg/l Elongation, % 55 @ 125C 2-8

Hardness, Shore A 60 Volume

Cost, S/cc 0.005

Table 2: Technical characteristics of a typical PLIOGRIP Polyurethane, 2 components

Source: ASHLAND Chemicals

Figure 3 shows the lap shear values of a PLIOGRIP 2 components PU adhesive on SMC reinforced plastic. It is important also to note that the curing may be accelerated by heat, in order to met the requirements of the fast production lines in automotive industry: figure 4 shows the effects of heat for a 2 parts PU adhesive.

EX-1516 Toughened Cyanate Ester Film Adhesive (250°F to 350°F Cure) Meets NASA Outgassing Specifications Excellent Microcrack Resistance Compared to Epoxy Electrical Properties Superior to Epoxy Film Adhesives Product Delivery Forms Unsupported Films (0.006 - 0.030 Lbs/Sq Ft Weights) Supported with Polyester, Fiberglass or Quartz (0.035, 0.045, 0.060 Lbs/Sq Ft Areal Weight) EX-1516 Physical Properties Moisture Absorption 0.6 - 0.7% Dielectric Constant 2.6 - 2.7 Loss Tangent 0.005 - 0.006 Mechanical Poperties: Fiberglass Non-Woven Suppoted (0.045 Lbs/Sq in)

Lap Shear 4310 psi on 6061 T-6 Aluminum

T Peel 23.6 Lbs/In Width Unsupported (0.002" Thickness) -67°F 77°F 180°F 250°F Flat Wise Tensile 2500 psi 2800 psi 2400 psi 1700 psi

Table 3: Specialty Adhesive for Space and Electrical Use Source: CYTEC, USA

Figure 3: Lap shear values of a PLIOGRIP PU

adhesive Source: ASHLAND Chemicals

Figure 4: Heat Accelerated Cure Calculated Conversion

Rate for a PLIOGRIP 2-part PU Adhesive

Note: Some PU reactive Hot melts may be also used for composites bonding.

Utilisations of PU adhesives for composites bonding:

in Car manufacturing : bonding of RRIM injected fenders made of glass fibers reinforced PU resin, to metal frame, bonding of glass fiber reinforced stiffeners to door panels, roof panels, reinforced PU spoilers, ( figure 5 ) hood and fenders assemblies for trucks, some truck's cabs are made with a complete outer shell made of SMC panels , including the front, sides, doors, rear and underbody ,which are bonded to a steel frame with 2 components PU adhesives. Bonding body parts for buses and recreational vehicles ( caravans walls and roof panels ) assembly of front grills and fascia made of reinforced thermoplastics,...

Figure 5: Bonding of 2 parts of a rear door of a car, made of SMC, with a 2

components of PU adhesive, mixed with automatic metering system and applied by a

rolot

in shipbuilding: in the past, deck to hull assembly was made with polyester putties, but now it is done with PU adhesives let us also mention the assembly of deck to hull in jet skis, Bulkheards are also bonded to the GFRP hull with 2 components PU.

Heat stable adhesives:

These are adhesives that withstand fairly high temperatures, higher than the standard epoxies ( which resist only to service temperatures up to 100-150°C )

Bismaleimides

( in short BMI ) may withstand service temperatures up to 230 - 250°C

Up to now, they are used only for aircrafts manufacturing because their price is very high ( 200 euros / M2 for films ) They are used to cure with BMI prepregs. Let us mention here the HYSOL EA 9673 film from HENKEL Aerospace that may withstand a service temperature up to 290°C, and REDUX HP 655 from HEXCEL, maximum service temperature 230°C. Figure 6 describes the chemistry of these resins and adhesives.

Figure 6: Examples of bismaleimides (BMI) monomer and maleimide-terminated resin prepared via Michael-

type addition of 4, 4'-methylenebisbenzeneamine (MBA) to the carbon-carbon double bond of BMI. Thermal

polymerization of oligoimide generates the cross-linked network

Cyanate esters:

These are based on cyanate ester resin, supplied by LONZA for instance, refer to figure 7 showing their chemical base formulation. Cyanate ester resins and adhesives requires high curing temperature up to 300°C. Service temperatures may reach up to 400°C. Of course these products are still quite expensive, and they are used only for special aircrafts and aerospace parts.

Figure 7: Chemical formulation of Cyanate esters

Table 3 provides, as an example, the technical characteristics of a cyanate ester film adhesive.

Other heat stable adhesives:

These include Polyimides ( refer to figure 8 ), phenolic resins, polybensimidazoles.

Figure 8: Thermosetting polyimide resins

Syntactic compounds and adhesives:

Syntactic compounds are resins filled with low density, high strength, glass microspheres.

Syncore from HENKEL Aerospace is supplied as a controlled thickness film, it is used with reinforced thermoset or thermoplastic prepreg face sheets, ( figure 9 ); after assembly the sandwich laminate is vacuum bagged and cured by using the recommended cure cycle of the prepreg itself.

Figure 9: SynCore® Sandwich Construction

Source: HENKEL Aerospace

SYNCORE provides many advantages: it replaces plies of reinforcing fibers, ( figure 10 ) reduces density ( we will see hereunder that there are also expanding syntactics ), it is very simple to use and reduces manufacturing costs since it is co-curable with the prepregs faces, it may be used in thin core sections ( while this is not possible with honeycomb ), it fits to curved and complex shapes, and provides high adhesion to the face sheets.

Figure 10: Comparative Advantages of Syntactic Core vs.

Composite Structural Configuration Source: HENKEL Aerospace, USA

Expanding syntactics are syntactics that expend 2 to 4 times during the heat cure, also sometimes called foaming compounds, they have basically the same uses and advantages as the former syntactics, but with lower specific gravity.

Syntactics are used for thin but strong sandwich construction in aircraft construction, for inserts reinforcements and protection, they seal the sides of the parts against moisture, they provide high stiffness to weight panels with decreased deflection compared to a solid laminate design, close the honeycomb edges and tapers, absorb acoustic vibrations.

Table 4 provides technical characteristics of some Syntactics from HUNTSMAN. HENKEL Aerospace also supplies syntactics.

Product Reference

Specific Gravity

Mix ratio by weight

Pot life at 23°C

Room temperature cure

Accelerated heat cure

Max service temperature

Compressive strength MPa

Features

EPOCAST 1610 0.5 - 30

days - 120°C 1 hr 90°C 17 at 23°C

• long work life at room temperature • ultra low density

EPOCAST 1614ATF 0.75 - 8 hrs -

120°C 90 mins 175°C 1 hr

180°C 117 at 23°C 76 at 120°C 62 at 175°C

• 8 hour work life • good strength up to 180°C • low density • extrudable • self-extinguishing

COLD STORAGE 1 COMPONENT

EPOCAST 927 1.2 - 24

hrs - 120°C 90 mins 175°C 1 hr

200°C 150 at 23°C 90 at 175°C 69 at 200°C

• very high compressive strength • excellent temperature performance up to 200°C • extrudable • self-extinguishing

ROOM TEMPERATURE STORAGE 2 COMPONENTS

EPOCAST 87005A/B-80

0.5 100.50 2 hrs 2 days 50°C 5 hr 90°C 17 at 23°C

• soft paste, non-sag after application • pale cream color suitable for interior • ultra low density

EPOCAST 1637A/B-1 0.5 100.30 50

mins 2 days 50°C 5 hrs 65°C 2 hrs 100°C 17 at 23°C

• ultra low density • extrudable, non-flow after application

ARALDITE 252A/B 0.65 100.40 60

mins 3 days 70°C 2 hr 80°C 35 at 23°C

• low out-gassing • self-extinguishing • good surface finish

EPOCAST 1617A/B 0.7 100.20

60-90 mins

7 days 50°C 5 hr 90°C 38 at 23°C

• soft paste, easy to handle • high strength, room temperature cure

EPOCAST 1631A/B 0.7 100.100 15

mins 5 days 50°C 5 hr 70°C 31 at 23°C

• easy to apply from 1:1 mix cartridges • good compression strent • self-extinguishing • cream color

Table 4: Syntactics from HUNTSMAN We have now finished the study of the different types of Adhesives for Composites, and in the next sections we will study the numerous applications in Aerospace, Aircrafts, Automotive and Transportation equipment, buses and railway equipment, sports goods and other industries.

bonding of composite materials

Documents