Introduction to Advanced Composites

Introduction to Advanced Composites and Prepreg Technology

SM1010/03.12/6

Table of Contents

Para Title Page

1 General ........................................................................................................................................................................ 12 Introduction............................................................................................................................................................... 13 Advantages of Composites ................................................................................................................................. 14 Matrices....................................................................................................................................................................... 2

4.1 Epoxy.................................................................................................................................................................... 24.2 Phenolic .............................................................................................................................................................. 24.3 Bismaleimide (BMI)........................................................................................................................................ 34.4 Cyanate Ester.................................................................................................................................................... 34.5 Polyester ............................................................................................................................................................. 34.6 Vinyl Ester .......................................................................................................................................................... 3

5 Reinforcements........................................................................................................................................................ 35.1 Common Types of Fibre ............................................................................................................................... 3

5.1.1 Carbon ........................................................................................................................................................ 35.1.2 Glass ............................................................................................................................................................ 35.1.3 Aramid ........................................................................................................................................................ 45.1.4 Dynema...................................................................................................................................................... 45.1.5 Zylon............................................................................................................................................................ 4

5.2 Key Fibre Selection Criteria......................................................................................................................... 45.3 Fabric Styles ...................................................................................................................................................... 6

5.3.1 Plain Weave.............................................................................................................................................. 65.3.2 Twill Weave .............................................................................................................................................. 75.3.3 Satin Weave ............................................................................................................................................. 75.3.4 Multiaxial (Non Crimp Fabric - NCF).............................................................................................. 8

6 Prepregs....................................................................................................................................................................... 86.1 Manufacturing Prepreg ............................................................................................................................... 8

6.1.1 Hot Melt Processing.............................................................................................................................. 86.1.2 Solvent Dip Processing.......................................................................................................................10

6.2 Prepreg & Composite Nomenclature...................................................................................................117 Manufacturing with Prepregs .........................................................................................................................12

7.1 Vacuum Bagging and Autoclave Moulding.......................................................................................127.1.1 Vacuum Bag Consumables..............................................................................................................12

7.2 Press Moulding ..............................................................................................................................................137.3 Pressure Bag Moulding ..............................................................................................................................147.4 Filament Winding/Fibre Placement .....................................................................................................147.5 Thermal Expansion Moulding.................................................................................................................14

8 Key Prepreg Processing Parameters ..............................................................................................................149 Sandwich Construction with Prepregs ........................................................................................................16

9.1 Properties of a Sandwich Panel ..............................................................................................................169.2 Core Materials................................................................................................................................................17

9.2.1 Balsa ..........................................................................................................................................................179.2.2 Foam..........................................................................................................................................................179.2.3 Honeycomb ............................................................................................................................................179.2.4 Syntactics and Pre-Impregnated Non-Wovens.......................................................................17

9.3 Sandwich Construction..............................................................................................................................189.3.1 One-Shot Curing...................................................................................................................................189.3.2 Two-Shot Curing ..................................................................................................................................189.3.3 Three-Shot Curing ...............................................................................................................................189.3.4 Notes on Sandwich Panels...............................................................................................................18

10 Umeco Structural Materials Product Range ..............................................................................................1910.1 Materials ..........................................................................................................................................................1910.2 Material Formats ..........................................................................................................................................20

An Introduction to Advanced Compositesand Prepreg Technology

Introduction_C1.fm Page 1 of 20

1 GeneralThis document is an introductory guide to familiarise the reader with advanced composite materials and in particular thermoset prepreg technology.

2 IntroductionThe term ‘composite’ is the generic name for a material manufactured from a fibre reinforcement embedded in a matrix material which is usually a polymer.

An ‘advanced composite’ usually refers to a structure where high performance composite materials and component geometry work in harmony optimising performance.

A prepreg consists of a reinforcement material pre-impregnated with a polymer or resin matrix in a controlled ratio. Prepreg offers the fabricator total control of the manufacturing process.

There are two types of polymer matrix; thermoplastic and thermosetting. Thermoplastics are made up of randomly orientated chains. It is possible to melt these polymers on heating and for them to solidify on cooling. Thermosetting polymers/resins solidify by cross-linking. This creates a permanent network of polymer chains. The cross-linking process is not reversible.

3 Advantages of CompositesComposites offer engineers a new freedom to optimise structural design and performance. Composites have several advantages over conventional metallic structures. The most significant of these are:

• Low density leads to high specific strength and modulus. Very strong and stiff structures can be designed, with substantial weight savings.

• Fibre can be orientated with the direction of principle stresses, increasing structural efficiency.

• Exceptional environmental and corrosion resistance.

• Improved vibration and damping properties.

• The ability to manufacture complex shapes and one offs from low cost tooling.

• Very low and controllable thermal expansion.

• Excellent fatigue resistance, carbon fibre composites can be designed to be essentially fatigue free.

• Potential for energy absorbing safety structures.

• Damaged structures can be easily repaired.

A comparison of several key material characteristics is shown in Figure 1. It can be seen that advanced composites offer reduced weight, greater strength and stiffness.

An Introduction toAdvanced Composites

and Prepreg Technology

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Figure 1: Comparison of Several Material Characteristics

4 MatricesIn a composite, the matrix supports and bonds the fibres, transferring applied loads and protecting the fibres from damage. The matrix also governs the maximum service temperature of a composite. This section describes some of the key thermosetting resins used as matrices in composites.

4.1 Epoxy

Epoxies are available in many different forms and can be processed using numerous techniques. They offer excellent mechanical performance, high toughness and good environmental resistance.

4.2 Phenolic

Phenolics are used where fire resistance/low smoke and toxicity outweigh all other criteria (e.g. aircraft interiors). Phenolic resins are relatively cheap but can be difficult to process and compared to epoxies, have poor mechanical properties.

Density (kg/m3)

Wood

Composites& Plastics

Concretes

Aluminium

100 1000 10000

Titanium

Steel

boron carbon aramid glass

Tensile Strength (MPa)

Wood

Composites

Concretes Light Alloys

10 1000 10000

Plastics

Steel

100

Tensile Modulus (GPa)

carbon aramid

Wood

Composites

Concretes Aluminium

1 100 1000

Plastics Steel

10

Titanium

glass boron

Density (kg/m3)

Tensile Modulus (GPa)

Tensile Strength (MPa)



4.3 Bismaleimide (BMI)

These are relatively expensive systems, but they have excellent mechanical properties at elevated service temperatures. Bismaleimide resins are difficult to process due to their high cure temperatures and the low viscosity achieved during curing.

4.4 Cyanate Ester

Cyanate ester resins can retain their mechanical properties at extremely high temperatures (up to 350C), but they are also expensive. These systems can absorb water, which can cause problems with blistering. Processing is similar to that used for epoxy resin systems.

4.5 Polyester

Polyester resins are low cost but lack the performance of epoxy resins. They are often used in structures where only moderate mechanical and thermal performance is required. Prepreg polyesters can be formulated without styrene and so do not pose the same health and safety issues as their wet lay-up counterparts.

4.6 Vinyl Ester

Vinyl ester offers a balance of epoxy and polyester performance and cost. Vinyl ester is superior to polyester in terms of performance generally being tougher and offering higher thermal resistance. Prepreg vinyl ester does not generally contain styrene.

5 ReinforcementsThe most commonly used fibre reinforcements in composites are glass, carbon and aramid. There are also a number of other fibres that are used for specialist applications.

Fibres are processed as bundles of continuous filaments, referred to as rovings (glass) or tows (carbon). These can be woven or stitched to produce a fabric. A summary of some of the most common types of fabric used in prepregs is given in section 5.3. A unidirectional (UD) prepreg is produced using tows or rovings directly from the manufacturer.

5.1 Common Types of Fibre

5.1.1 Carbon

Fibres are available in high strength, intermediate modulus, high modulus, and ultra high modulus grades. They are used in high strength, high stiffness applications where the benefits of weight saving are more critical than any additional material costs.

5.1.2 Glass

Most commonly used is E-glass, with S-glass and Quartz used in specialist applications such as ballistics and where dielectric properties are important. Glass is much lower in cost but denser than carbon and has lower strength and stiffness values.




5.1.3 Aramid

Known by the trade names KevlarTM, TecnoraTM or TwaronTM. Aramid is normally used where there is a likelihood of impact damage. Aramid has the ability to absorb and dissipate energy and has excellent abrasion resistance but suffers from poor compression performance.

5.1.4 Dyneema

Dyneema is an ultra high molecular weight polyethylene. It is mainly used for applications that require impact resistance. Dyneema fibres offer good dielectric properties and have a low density. However, they have poor temperature resistance and like, aramid, exhibit poor compression performance.

5.1.5 Zylon

Zylon (PBO) fibres demonstrate superior tensile strength to aramid fibres. They exhibit excellent impact resistance and high temperature stability. Their weaknesses include poor compressive strength and poor UV resistance.

5.2 Key Fibre Selection Criteria

Factors governing fibre selection include; density, cost, strength and modulus. Figures 2 to 5 give comparisons of these factors for a range of fibre types.

Figure 2: Relative Properties - Density

0

0.5

1

1.5

2

2.5

3

H ig h Modulus

Carbon

High Strength

Carb on

E-glass S-g lass Aramid Polyethylene



Figure 3: Cost Ratio

Figure 4: Relative Properties - Modulus GPa

0

50

100

150

200

250

300

350

High ModulusCarbon

High StrengthCarbon

E-glass S-glass Aramid Polyethylene

0

50

100

150

200

250

300

350

400

HighModulusCarbon

H ighStrengthCarbon

E-glass S-glass Aram id Polyethylene




Figure 5: Relative Properties - Tensile Strength MPa

5.3 Fabric Styles

Reinforcement fibres can be woven into fabrics. Fibres running along the length of a roll are referred to the warp fibres, and those across the width, weft fibres. There are several different fabric styles which are commonly used in the composites industry.

5.3.1 Plain Weave

Warp fibres are interlaced each time they cross weft fibres, as shown in Figure 6. The resulting fabric is very stable but difficult to drape around sharp profile changes. Plain weave fabrics can be woven with a heavy balance of fibres in the warp direction giving a near unidirectional format.

Figure 6: Schematic of a Plain Weave Fabric

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

High ModulusCarbon

High StrengthCarbon

E-glass S-glass Aramid Polyethylene

Plain Weave



5.3.2 Twill Weave

The fibres pass over and under a number of fibre bundles. A 2 x 2 twill fabric has fibres passing over two bundles and then under two bundles, as depicted in Figure 7. Subsequent fibre intercepts are offset by one fibre bundle creating a diagonal, ‘herring bone’ pattern. Twill weave fabrics have a much more open weave, readily draping and conforming to complex profiles.

Figure 7: Schematic of a 2 x 2 Twill Fabric

5.3.3 Satin Weave

A fibre bundle passes over a number of fibre bundles and then under one fibre bundle (e.g. 5 harness - 4 over, 1 under, see Figure 8). This produces a much flatter fabric that can be easily draped to a complex surface profile. However, due to the construction, satin weaves are unbalanced (fabric with one side consisting of mainly warp fibres whilst the other is mainly weft). The resultant imbalance must be accounted for in a laminate construction and it is normal practice to invert the plies around the neutral axis of the laminate.

Figure 8: Schematic of a 5 Harness Satin Weave Fabric (5HS)

2 x 2 Twill Weave

Satin Weave (5HS)




5.3.4 Multiaxial (Non Crimp Fabric – NCF)

A multiaxial consists of orientated layers of unidirectional fibre (e.g. 0, +45, -45, 0 stitched together so it can be handled in the same way as a woven fabric, as shown in Figure 9. Multiaxial fabrics can be manufactured to produce heavier areal weights that are neither practical nor economic to achieve in a woven format. Multiaxials readily conform to complex shapes with the added advantage of rapid laminate thickness build up. However, care has to be taken to ensure the laminate is balanced and very heavy fabrics can be difficult to tailor around fine details.

Figure 9: Schematic of a Multiaxial Fabric

6 PrepregsA prepreg consists of a reinforcement material pre-impregnated with a resin matrix in a controlled ratio. The resin can be partially cured (referred to as B-staged) and in this form, is supplied to the fabricator who can use it to lay-up a part.

The reinforcements used in a prepreg can either be a fabric, (as described in section 5) or unidirectional (all fibres in one direction).

6.1 Manufacturing Prepreg

There are two main methods of producing prepreg; hot melt and solvent dip.

6.1.1 Hot Melt Processing

The hot melt method can be used to produce unidirectional (UD) and fabric prepregs. This requires two processing stages. In the first stage, heated resin is coated onto a paper substrate in a thin film. The reinforcement (unidirectional fibres or fabric) and the resin film are then brought together on the prepreg machine. Impregnation of the resin into the fibre is achieved using heat and pressure from nip rollers. The final prepreg is then wound onto a core. A schematic diagram of the process is given in Figure 10.

9090

90

900

+30 >+ 60

-30 >- 60

+30 >+ 60



Figure 10: Schematic of Both Stages of the Hot Melt Process

Release film

Release film

Reinforcement

Prepreg

Knife

MatrixHeating

Heating

Release film

Release film

Reinforcement

PrepregHeating

Release film

Release film

Matrix

Coating head

Step 1

Matrix film

Matrix film

Consolidation

Film recoveryStep 2




6.1.2 Solvent Dip Processing

The solvent dip method can only be used to produce fabric prepregs.

In this technique, resin is dissolved in a bath of solvent and reinforcing fabric is dipped into the resin solution. The solvent is evaporated from the prepreg in a drying oven. This can be horizontal or vertical.

A schematic of this technique, showing a vertical drying oven is given in Figure 11.

Figure 11: Schematic of the Solvent Dip Process

Reinforcement

Matrix bathPrepreg

Oven

Nip rollers

Release film

Release film



6.2 Prepreg & Composite Nomenclature

The following are terms commonly encountered when discussing prepregs, (in alphabetical order).

Cure: This is the time duration and temperature needed for the resin in the prepreg to harden.

Debulking: The application of vacuum pressure at specific points in the lay-up sequence to ensure full consolidation of the prepreg plies.

Fibre Volume Fraction (Vf): Percentage of fibre in the prepreg (by volume).

Flow: The ability of the resin to move under pressure allowing it to wet out all parts of a laminate.

Fibre Areal Weight (FAW): The weight of fabric used in a prepreg (gsm).

Glass Transition Temperature (Tg): Temperature at which a phase change occurs in the matrix. This gives an indication of the maximum end use temperature.

Lay-Up: The number of plies and their orientation needed to produce a given part.

Out Life: Period of time that a prepreg remains usable at workshop temperature. Out life is lost progressively each time the prepreg is defrosted. Manufacturers normally state out life at a standard temperature, usually 21°C.

Ply: A layer of prepreg.

Resin Weight (%RW): Percentage of resin in the prepreg (by weight).

Shelf Life: The length of time the prepreg can be stored under specified conditions and remains usable.

Tack: Measurement of the capability of an uncured prepreg to adhere to itself or to the tool.

Tack Life: Period of time at a given temperature that the prepreg has sufficient tack.

Vacuum Bagging Technique: This refers to the arrangement of vacuum bagging materials used when moulding a part via vacuum or autoclave processing.

Viscosity: A measure of the flow characteristics of a resin with respect to time, temperature and heat up rates.

Void Content: This is the measure, by volume, of voids within a cured composite. Voids are air pockets trapped within the resin. They can be caused by a number of factors and reduce the performance of the composite.




7 Manufacturing with PrepregsThe production processes for the manufacture of advanced composite components with prepreg requires two elements:

• Pressure to consolidate the laminate.

• Heat to initiate and maintain the curing reaction.

7.1 Vacuum Bagging & Autoclave Moulding

Vacuum bagging techniques have been developed for fabricating complex shapes, double contours and relatively large components. The technique is employed to remove air and volatiles and consolidate the lay-up during cure. Vacuum bagging utilises a flexible membrane under which a vacuum is drawn applying an even pressure up to 1 bar (14psi) to the lay-up in the mould tool. Recent developments in prepreg technology have seen wider use of low pressure vacuum bag processing (often referred to as oven curing or ‘Out-of-Autoclave [OoA]), even in areas such as aerospace.

In autoclave moulding, the part is placed in a vacuum bag to achieve initial consolidation and then loaded into the autoclave. An autoclave is a large, heated pressure vessel, which is used to help consolidate the part by subjecting it to an additional pressure, up to 7bar (100psi), whilst heat is applied to cure the resin.

The pressure exerted on the lay-up is normally within the range 3 to 7 bar (45 to 100psi). The autoclave moulding process produces laminates of high quality with minimum void content, and control of laminate thickness is much better than that achieved by the vacuum bag moulding method. The capital equipment costs are high, however, and the output relatively low, which restricts the use of the autoclave moulding process to higher cost markets where high quality is essential.

7.1.1 Vacuum Bag Consumables

There are many consumable materials used in a vacuum bag, each for a specific purpose.

A summary of the consumables is provided in this section. A schematic diagram of a typical vacuum bag, indicating each consumable is given in Figure 12.

Figure 12: Schematic of a Typical Vacuum Bag

Image courtesy of Richmond Aerovac

Release Film

Release AgentPeel Ply (optional)

Vacuum Port Bagging Film

Breather

Sealant Tape

Prepreg Stack

Tool



Release Agent: Allows release of the cured component from the tool.

Peel Ply (optional): Light weight fabric (polyester or nylon) applied and moulded onto the component surface. This protects the surface and when removed can provide a surface for secondary bonding. Peel ply can be dry or impregnated with the same resin as the laminate. NB dry peel ply will take resin from the laminate so care must be taken to not starve the laminate of resin which can lead to voids.

Release Film: Allows removal of the vacuum consumables from the laminate. It can be solid or perforated (pin pricked or punched) with various hole patterns to control resin bleed during de-bulk or cure. Solid release films do not allow any resins or gases to escape. Pin pricked films allow only gas or very small amounts of resin (if viscosity allows) to escape. Punched release films allow more resin to bleed out of the laminate.

Breather: Allows the free passage of air under the vacuum bag over the laminate to ensure the level of vacuum is equalised over the whole surface. Various grades (weights) are available.

Bagging Film: Polymer film, usually nylon, sealed over the laminate to form the bag and allow removal of air.

Sealant Tape: Mastic tape used to seal the vacuum bag to itself or to the surface of the tool.

Additional Air Extraction: Glass tows, strips of glass fabric or peel ply can be placed around the periphery of the lay-up to provide air paths under the release film and into the breather (these are not shown on the diagram).

7.2 Press Moulding

In press moulding, prepreg is laid into a tool, or pre-formed in a separate process to facilitate rapid loading in a hot tool.

The tools are usually manufactured from machined or cast metal, and are produced as matched male and female halves, the space between them defining the shape and wall thickness of the component being made.

The prepreg is constrained within the tool and consolidation pressure is generated hydraulically. The tool may be heated directly by electric cartridge heaters installed within the mould or oil or steam, can be pumped through galleries built into the tool. When moulding small components the heat may be supplied via the heated platens of the press.

Cure cycles can be very accurately controlled and high degree of automation can be achieved. The process can produce components of very high quality and consistency to very high dimensional tolerances.

Due to the high costs of capital equipment and tooling, this method is best suited to high volume production. Less expensive nickel electro-formed, glass fibre or sprayed metal tooling can be used for short production runs.




7.3 Pressure Bag Moulding

A flexible bag, often manufactured from silicone rubber, is placed inside the structure and inflated to apply consolidation pressure. The part can be cured in an oven with temperature and pressure applied for consolidation. This method is often applied to simple hollow sections such as tubes, but it should be noted that the tooling must be rigid enough to withstand the internal pressure without distortion.

7.4 Filament Winding/Fibre Placement

In its simplest form, continuous, narrow, UD tape (or impregnated fibre bundles known as towpreg) is wound onto a rotating mandrel. The tape or towpreg is fed via a translating head with an accurately controlled fibre feed angle to the axis of the rotating mandrel. Consolidation pressure is achieved through tensioning the fibres as they are wound onto the mandrel.

With the growth of automation this basic process is now being used to produce non circular components by utilising multi-axial robot placement which allows UD prepreg to be placed very accurately onto the tool surface. The process is then referred to Automated Fibre Placement (AFP) or Automated Tape Placement (ATL).

7.5 Thermal Expansion Moulding

Thermal expansion moulding is generally used to mould integrally stiffened structures with complex forms. Prepreg layers are wrapped over blocks of rubber or foam and the lay-up then is restrained in a tool. The assembly is then heated. As the temperature increases, a high differential thermal expansion takes place between the tool and rubber generating very high pressures which consolidate the lay-up.

This method requires very little capital equipment, and the tooling is simple and low cost. Components with very complex shapes can be moulded in a single cure cycle, thus reducing the number of joints and parts, and therefore significant weight and production cost savings can be achieved.

8 Key Prepreg Processing ParametersThere are several key stages during a typical prepreg cure cycle, these are described below and represented graphically in Figure 13.

Heat Up: The heat up rate dictates how quickly the component/tool is brought up to the cure temperature. This is governed by numerous factors: matrix viscosity and reactivity, thickness of laminate, and tool mass and conductivity. For highly reactive matrices and thick laminates, the heat up rate will be low in order to avoid exothermic heat build up

Intermediate Dwell (optional): These are sometimes employed to help the component and tool reach the same temperature before the final cure temperature is achieved. Intermediate dwells are often chosen at a temperature where the resin is at the optimum viscosity for removal of air from the part.

Dwell/Cure: For each prepreg resin system there is a range of options for cure temperature/duration, and there is also a minimum cure temperature. For each given cure temperature there will be a corresponding cure time. The component must reach the given dwell/cure temperature and be held there throughout the specified cure cycle. Thermocouples are generally used to monitor the temperature of the component and tooling.



Cool Down: The cooling rate is controlled to avoid sudden temperature drops that may induce high thermal stresses in the component and/or damage to the tool.

Post Cure: Further curing may be possible after the initial cure to maximise temperature resistance and/or mechanical performance. Post curing is often carried out in an oven following an initial cure in an autoclave to reduce overall manufacturing costs or where low cost tooling has limited the temperature that can be tolerated during the initial cure

Vacuum/Pressure: At specific times throughout the cure cycle, vacuum and pressure (autoclave only) can be applied and removed.

Figure 13: Example of a Cure Cycle Including an Intermediate Dwell

1000

20

40

60

80

100

120

140

160

180

200

Hea

t Up

1°C

/min

ute

Intermediate Dwell 4 hours at 130°C

Dwell 2 hours

Hea

t Up

1°C/

min

ute

at 180°C

Time (minutes)

200 300 400 500 600 700 800

Total cure time: 690 minutes

Cool Dow

n Time 1°C

/minute

Tem

pera

ture

(°C

)

1000

20

40

60

80

100

120

140

160

180

200

Hea

t Up

1°C

/min

ute

Intermediate Dwell 4 hours at 130°C

Dwell 2 hours

Hea

t Up

1°C/

min

ute

at 180°C

Time (minutes)

200 300 400 500 600 700 800

Total cure time: 690 minutes

Cool Dow

n Time 1°C

/minute

Tem

pera

ture

(°C

)




9 Sandwich Construction with PrepregsSandwich construction substantially increases the stiffness of a structure with very little increase in weight. Thin high stiffness laminates such as carbon fibre are bonded to a low density core material giving a similar result to an I-section beam, see Figure 14. Material is optimally placed to provide bending stiffness, or stability under compressive loading.

Figure 14: Schematic Representation of a Honeycomb Sandwich Panel

9.1 Properties of a Sandwich Construction

The stiffness of a composite panel is not only influenced by the fibre/resin content, it is also a function of the geometry of the panel. Figure 15 demonstrates the increase in stiffness that can be achieved with the introduction of a lightweight core material into a monolithic laminate.

Figure 15: Comparison of properties of sandwich panels with a monolithic laminate.

Solid Laminate Core Thickness t Core Thickness t

Relative Stiffness 1 7 37

Relative FlexuralStrength

1 3.5 9.25

Relative Weight 1 1.03 1.06

Prepreg skin

Adhesive film

Honeycomb(or foam)

Adhesive film

Prepreg skin

(optional)

(optional)

t

2t

4t



9.2 Core Materials

A wide variety of core materials exist, with varying structural properties, temperature resistance and cost.

9.2.1 Balsa

Balsa, generally cut ‘end grain’, exhibits high compressive properties, as well as good thermal and acoustic insulation. However, it has a high density and can absorb large quantities of resin if not pre-sealed.

9.2.2 Foam

PU (Polyurethane): Moderate mechanical properties and can experience deterioration at foam/skin interface with time. Commonly used as fill in stringers.

PVC (Polyvinyl Chloride): Exhibits a good balance of static and dynamic properties as well as resistance to water absorption. Appropriate grades must be specified for elevated temperature applications

Acrylic: High mechanical properties and temperature resistance, but expensive. Some grades are prone to water absorption which can cause problems during processing.

SAN (Styrene Acrylonitrile): Similar to PVC, but tougher.

PEI (Polyetherimide): Excellent fire resistance and temperature resistance up to 180C. Expensive.

9.2.3 Honeycomb

Aluminium: Provides one of the highest strength/weight ratios of any core material. Low cost. Potential corrosion problem if used in conjunction with carbon skins.

Nomex: Lightweight with high mechanical properties. Good fire resistance but expensive.

Kevlar: Lightweight core offering superior shear performance to Nomex.

9.2.4 Syntactics and Pre-Impregnated Non-Wovens

A syntactic is a microsphere filled resin film (usually supported on a carrier or fabric) which offers very high drape and low density but relatively poor mechanical performance.

Low density non woven mats can be pre-impregnated to form drapable core materials.




9.3 Sandwich Construction

When bonding prepreg laminates to a sandwich core, an adhesive film is generally used. Applying adhesive in this form gives good control of the overall consistency and thickness of the bond line. Adhesives are formulated to offer toughness but also have controlled flow characteristics to ensure that a good bond is formed, especially when bonding to a honeycomb.

When building large sandwich structures, paste materials that are cure compatible with the prepreg, are often used to splice together sections of core, fill very tight corners or fill the edges of panels to help resist damage.

There are a number of methods by which sandwich structures may be produced.

9.3.1 One-Shot Curing

In this process, the facing skins and core are cured and bonded in one cure cycle. This is a fast manufacturing approach, appropriate for flat panels such as aircraft wing sets.

9.3.2 Two-Shot Curing

This is the most commonly used technique for manufacturing complex shapes, such as Formula One car chassis. In this method, the first facing skin is laid-up on the tool, vacuum bagged and cured. The cured skin is retained on the tool and the subsequent components added, i.e. adhesive, core and outer facing skin. The whole assembly is then bagged and cured.

9.3.3 Three-Shot Curing

This method is sometimes employed to manufacture large scale parts, e.g. boat hulls. In this process, the first facing skin is laid-up and cured. The second cure bonds the core to the first facing skin and the third cure cures the outer facing skin.

9.3.4 Notes on Sandwich Panels

• Flat sandwich components can be manufactured using a press.

• Sandwich structures can be made in one operation but there may be practical problems on large structures.

• Moisture in cores and air entrapment are major factors in delamination.

• Cure inhibition of the prepreg can occur with certain resin/foam combinations.



10 Umeco Structural Materials Product RangeUmeco Composites Structural Materials offers a wide range of prepregs and supporting products to service the advanced composites industry.

Web-based product selector guides, accessible via the Group website, offer guidance for materials selection in specific market sectors;

10.1 Materials

Umeco’s product range:

• Prepregs

• LTM® series – Typical cure temperatures: 20 to 80C• MTM® series – Typical cure temperatures: 80 to 135C• HTM® series – Typical cure temperatures: 135C to 190°C

• VTM® series – Typical cure temperatures: 65 to 180C

• Film Adhesives/Resin Films

• Syntactic Films

• Tooling Materials

• Tooling Block and ancillaries

• Tooling prepregs

• Backing structures and ancillaries

For further information on these products, or any of your composite materials requirements, please visit www.umeco.com or contact one of our Technical Sales Representatives.




10.2 Material Formats

Material Type Description Application

PrepregFull, partial or one sided impregnation. Woven or non-crimp fabrics.

All moulding applications.

UnidirectionalFull impregnation. 100% aligned fibres.

High stiffness applications.

Surfacing FilmsPartially impregnated format for the production of high class surface finishes.

Optimised Surfacing products for the production of high quality finishes.

ZPREG®Multi-layer and partially impregnated rapid lay up formats.

Rapid lay up of large parts. Partially impregnated format allows efficient air release during cure for the production of high quality surfaces and thick laminates.

DForm®Selectively slit UD prepreg presented in a multi-layer 0/90 stack.

An innovative prepreg system that combines short fibre conformability with the handling and laminate characteristics of a conventional long fibre composite.

Syntactic/Core Plies

Filled, lightweight resin film products.

Rapid thickness builds up with low weight.

Adhesive Films Toughened resin film. Core bonding and part assembly.

ToolingResin systems formatted for stability at high temperatures.

High accuracy mould tools capable of operating at high temperatures.

Introduction to Advanced Composites

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