Introduction to Advanced Composites and Prepreg Technology

Introduction to Advanced Composites and Prepreg Technology

SM1010/03.12/6

Table of Contents
Para Title Page
1 General ........................................................................................................................................................................ 1
2 Introduction............................................................................................................................................................... 1
3 Advantages of Composites ................................................................................................................................. 1
4 Matrices....................................................................................................................................................................... 2
4.1 Epoxy.................................................................................................................................................................... 2
4.2 Phenolic .............................................................................................................................................................. 2
4.3 Bismaleimide (BMI)........................................................................................................................................ 3
4.4 Cyanate Ester.................................................................................................................................................... 3
4.5 Polyester ............................................................................................................................................................. 3
4.6 Vinyl Ester .......................................................................................................................................................... 3
5 Reinforcements........................................................................................................................................................ 3
5.1 Common Types of Fibre ............................................................................................................................... 3
5.1.1 Carbon ........................................................................................................................................................ 3
5.1.2 Glass ............................................................................................................................................................ 3
5.1.3 Aramid ........................................................................................................................................................ 4
5.1.4 Dynema...................................................................................................................................................... 4
5.1.5 Zylon............................................................................................................................................................ 4
5.2 Key Fibre Selection Criteria......................................................................................................................... 4
5.3 Fabric Styles ...................................................................................................................................................... 6
5.3.1 Plain Weave.............................................................................................................................................. 6
5.3.2 Twill Weave .............................................................................................................................................. 7
5.3.3 Satin Weave ............................................................................................................................................. 7
5.3.4 Multiaxial (Non Crimp Fabric - NCF).............................................................................................. 8
6 Prepregs....................................................................................................................................................................... 8
6.1 Manufacturing Prepreg ............................................................................................................................... 8
6.1.1 Hot Melt Processing.............................................................................................................................. 8
6.1.2 Solvent Dip Processing.......................................................................................................................10
6.2 Prepreg & Composite Nomenclature...................................................................................................11
7 Manufacturing with Prepregs .........................................................................................................................12
7.1 Vacuum Bagging and Autoclave Moulding.......................................................................................12
7.1.1 Vacuum Bag Consumables..............................................................................................................12
7.2 Press Moulding ..............................................................................................................................................13
7.3 Pressure Bag Moulding ..............................................................................................................................14
7.4 Filament Winding/Fibre Placement .....................................................................................................14
7.5 Thermal Expansion Moulding.................................................................................................................14
8 Key Prepreg Processing Parameters ..............................................................................................................14
9 Sandwich Construction with Prepregs ........................................................................................................16
9.1 Properties of a Sandwich Panel ..............................................................................................................16
9.2 Core Materials................................................................................................................................................17
9.2.1 Balsa ..........................................................................................................................................................17
9.2.2 Foam..........................................................................................................................................................17
9.2.3 Honeycomb ............................................................................................................................................17
9.2.4 Syntactics and Pre-Impregnated Non-Wovens.......................................................................17
9.3 Sandwich Construction..............................................................................................................................18
9.3.1 One-Shot Curing...................................................................................................................................18
9.3.2 Two-Shot Curing ..................................................................................................................................18
9.3.3 Three-Shot Curing ...............................................................................................................................18
9.3.4 Notes on Sandwich Panels...............................................................................................................18
10 Umeco Structural Materials Product Range ..............................................................................................19
10.1 Materials ..........................................................................................................................................................19
10.2 Material Formats ..........................................................................................................................................20

An Introduction to
Advanced Composites
and Prepreg Technology
1 General
This document is an introductory guide to familiarise the reader with advanced composite
materials and in particular thermoset prepreg technology.
2 Introduction
The 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 Composites
Composites 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.
Introduction_C1.fm Page 1 of 20

4 Matrices
4.1 Epoxy
3
Density (kg/m )
3 )
4.2 Phenolic
Wood
Composites
& Plastics
Concretes
Figure 1: Comparison of Several Material Characteristics
An Introduction to
Advanced Composites
and Prepreg Technology
Aluminium
100 1000 10000
Tensile Modulus (GPa) (GPa)
Tensile Strength (MPa)
Concretes
Plastics
Plastics
Wood
glass aramid
carbon
boron
Wood
glass
Light Alloys
Composites
Concretes Aluminium
In 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.
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.
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.
Page 2 of 20 Introduction_C1.fm
aramid
Composites
carbon
boron
Titanium
1 10
100 1000
10 100
1000 10000
Steel
Titanium
Steel
Steel

An Introduction to
Advanced Composites
and Prepreg Technology
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
4.5 Polyester
Cyanate ester resins can retain their mechanical properties at extremely high temperatures (up
to 350�C), 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.
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 Reinforcements
The 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
5.1.2 Glass
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.
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.
Introduction_C1.fm Page 3 of 20

5.1.3 Aramid
5.1.4 Dyneema
5.1.5 Zylon
An Introduction to
Advanced Composites
and Prepreg Technology
Known by the trade names Kevlar TM , Tecnora TM or Twaron TM . 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.
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.
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.
3
2.5
2
1.5
1
0.5
0
Hig h Modulus
Carbon
High Strength
Carbon
E-glass S-g lass Aramid Polyethylene
Figure 2: Relative Properties - Density
Page 4 of 20 Introduction_C1.fm

An Introduction to
Advanced Composites
and Prepreg Technology
350
300
250
200
150
100
50
0
400
350
300
250
200
150
100
50
0
High Modulus
Carbon
High
Modulus
Carbon
High Strength
Carbon
High
Strength
Carbon
E-glass S-glass Aramid Polyethylene
Figure 3: Cost Ratio
E-glass S-glass Aramid Polyethylene
Figure 4: Relative Properties - Modulus GPa
Introduction_C1.fm Page 5 of 20

5.3 Fabric Styles
Figure 5: Relative Properties - Tensile Strength MPa
An Introduction to
Advanced Composites
and Prepreg Technology
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
5000
4500
4000
3500
3000
2500
2000
1500
1000
500
0
High Modulus
Carbon
High Strength
Carbon
E-glass S-glass Aramid Polyethylene
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.
Plain Weave
Figure 6: Schematic of a Plain Weave Fabric
Page 6 of 20 Introduction_C1.fm

An Introduction to
Advanced Composites
and Prepreg Technology
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.
5.3.3 Satin Weave
2 x 2 Twill Weave
Figure 7: Schematic of a 2 x 2 Twill Fabric
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.
Satin Weave (5HS)
Figure 8: Schematic of a 5 Harness Satin Weave Fabric (5HS)
Introduction_C1.fm Page 7 of 20

5.3.4 Multiaxial (Non Crimp Fabric – NCF)
An Introduction to
Advanced Composites
and Prepreg Technology
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.
6 Prepregs
Figure 9: Schematic of a Multiaxial Fabric
A 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
0�
90�
+30 >
+ 60�
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.
Page 8 of 20 Introduction_C1.fm
90�
-30 >
- 60�
90�
+30 >
+ 60�
90�

An Introduction to
Advanced Composites
and Prepreg Technology
Reinforcement
Release film
Step 1
Step 2
Matrix
Release film
Reinforcement
Release film
Knife
Matrix
Coating head
Matrix film
Heating
Heating
Heating
Film recovery
Consolidation
Release film
Release film
Release film
Figure 10: Schematic of Both Stages of the Hot Melt Process
Prepreg
Matrix film
Prepreg
Introduction_C1.fm Page 9 of 20

6.1.2 Solvent Dip Processing
The solvent dip method can only be used to produce fabric prepregs.
An Introduction to
Advanced Composites
and Prepreg Technology
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.
Reinforcement
Matrix bath
Nip rollers
Figure 11: Schematic of the Solvent Dip Process
Prepreg
Page 10 of 20 Introduction_C1.fm
Oven
Release film
Release film

An Introduction to
Advanced Composites
and Prepreg Technology
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 (V f): 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 (T g ): 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.
Introduction_C1.fm Page 11 of 20

7 Manufacturing with Prepregs
An Introduction to
Advanced Composites
and Prepreg Technology
The 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.
Release Film
Peel Ply
(optional)
Vacuum Port Bagging Film
Release Agent
Tool
Prepreg Stack
Figure 12: Schematic of a Typical Vacuum Bag
Breather
Sealant Tape
Image courtesy of Richmond Aerovac
Page 12 of 20 Introduction_C1.fm

An Introduction to
Advanced Composites
and Prepreg Technology
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 debulk
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.
Introduction_C1.fm Page 13 of 20

7.3 Pressure Bag Moulding
An Introduction to
Advanced Composites
and Prepreg Technology
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 Parameters
There 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.
Page 14 of 20 Introduction_C1.fm

An Introduction to
Advanced Composites
and Prepreg Technology
Temperature (°C)
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.
200
180
160
140
120
100
80
60
40
20
0
Intermediate Dwell
Heat Heat Up Up 1°C/minute 1°C/minute
100
4 hours at 130°C
Heat Heat Up Up 1°C/ 1°C/
Dwell 2 hours
at 180°C
minute minute
Total cure time: 690 minutes
Cool Cool Down Down Time Time 1°C/minute
1°C/minute
200 300 400
Time (minutes)
500 600 700 800
Figure 13: Example of a Cure Cycle Including an Intermediate Dwell
Introduction_C1.fm Page 15 of 20

9 Sandwich Construction with Prepregs
An Introduction to
Advanced Composites
and Prepreg Technology
Sandwich 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.
Solid Laminate Core Thickness t Core Thickness t
Relative Stiffness 1 7 37
Relative Flexural
Strength
Prepreg skin
Adhesive film
(optional)
Honeycomb
(or foam)
Adhesive film
(optional)
Prepreg skin
t
1 3.5 9.25
Relative Weight 1 1.03 1.06
Figure 15: Comparison of properties of sandwich panels with a monolithic laminate.
Page 16 of 20 Introduction_C1.fm
2t
4t

An Introduction to
Advanced Composites
and Prepreg Technology
9.2 Core Materials
9.2.1 Balsa
9.2.2 Foam
A wide variety of core materials exist, with varying structural properties, temperature resistance
and cost.
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.
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 180�C. 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.
Introduction_C1.fm Page 17 of 20

9.3 Sandwich Construction
An Introduction to
Advanced Composites
and Prepreg Technology
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.
Page 18 of 20 Introduction_C1.fm

An Introduction to
Advanced Composites
and Prepreg Technology
10 Umeco Structural Materials Product Range
10.1 Materials
Umeco 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;
Umeco’s product range:
• Prepregs
• LTM® series – Typical cure temperatures: 20 to 80�C
• MTM® series – Typical cure temperatures: 80 to 135�C
• HTM® series – Typical cure temperatures: 135�C to 190°C
• VTM® series – Typical cure temperatures: 65 to 180�C
• 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.
Introduction_C1.fm Page 19 of 20

10.2 Material Formats
Material Type Description Application
Prepreg
Unidirectional
Surfacing Films
ZPREG®
Full, partial or one sided
impregnation. Woven or non-crimp
fabrics.
Full impregnation. 100% aligned
fibres.
Partially impregnated format for
the production of high class surface
finishes.
Multi-layer and partially
impregnated rapid lay up formats.
All moulding applications.
High stiffness applications.
An Introduction to
Advanced Composites
and Prepreg Technology
Optimised Surfacing products for the production of
high quality finishes.
Rapid lay up of large parts. Partially impregnated
format allows efficient air release during cure for the
production of high quality surfaces and thick
Selectively slit UD prepreg
laminates.
An innovative prepreg system that combines short
DForm® presented in a multi-layer 0/90 fibre conformability with the handling and laminate
stack.
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.
Tooling
Resin systems formatted for
stability at high temperatures.
High accuracy mould tools capable of operating at
high temperatures.
Page 20 of 20 Introduction_C1.fm