Raw Materials - AMT Composites

Raw Materials
AMTS-SWP-0002-A-2008
AMTS STANDARD WORKSHOP PRACTICE
_________________________________________
Raw Materials
Reference Number:
AMTS_SWP_2_2008
Date:
December 2008
Version:
A
1
Raw Materials
AMTS-SWP-0002-A-2008
Contents
1 Technical Terms ................................................................................... 3
2 Scope ..................................................................................................... 3
3 Primary References .............................................................................. 3
4 Typical Composite Materials ............................................................... 3
4.1 Structural fibres and fabrics .........................................................................3
4.1.1 Glass fibres ....................................................................................4
4.1.2 Carbon fibres..................................................................................5
4.1.3 Aramid (KevlarTM) fibres.................................................................5
4.1.4 Other fibre types.............................................................................6
4.1.5 Types of fabrics..............................................................................6
4.2 Sandwich structure core materials ..............................................................8
4.2.1 Foams ............................................................................................8
4.2.2 Honeycombs ..................................................................................8
4.2.3 Woods ............................................................................................9
4.3 Resin systems..............................................................................................9
4.3.1 Epoxy resin systems ....................................................................10
4.3.2 Polyester resin systems ...............................................................10
4.3.3 Polyurethane resin systems.........................................................11
4.3.4 Vinyl-ester resin systems.............................................................11
4.4 Hardeners ..................................................................................................12
4.5 Applications of resin systems ....................................................................12
4.5.1 Laminating resins .........................................................................12
4.5.2 Bonding resins .............................................................................12
4.5.3 Gelcoats .......................................................................................12
4.5.4 Casting resins ..............................................................................13
4.6 Resin additives (fillers)...............................................................................13
4.6.1 Cotton flocks ................................................................................13
4.6.2 Carb-o-sil......................................................................................14
4.6.3 Micro balloons ..............................................................................14
4.7 Prepregs.....................................................................................................15
5 Processing of Composites................................................................. 15
6 Logistics .............................................................................................. 16
6.1 Procurement of Raw Materials ..................................................................16
6.1.1 Vendor Evaluation........................................................................16
6.1.2 Selecting Raw Material ................................................................17
6.1.3 Selecting Consumables ...............................................................17
6.1.4 Recording of actions – taking into stock......................................17
6.1.5 Quality Control of Materials .........................................................17
6.2 Storage and Issuing of Raw Materials.......................................................17
2
Raw Materials
AMTS-SWP-0002-A-2008
1 Technical Terms
Hygroscopic:
Affinity for moisture
2 Scope
The purpose of this document is to outline different composite materials and their use as well as
guide composite part manufacturers in the procurement of these raw materials.
It covers the following:
• Typical composite structural fibres, resin systems, additives as well as core materials
used in sandwich structures.
• Special processing requirements of resin systems
• Basic logistics concerning raw materials
3 Primary References
J.S.U. Jonker & J.P. Schümann, Training Manual – Composites, Jonker Sailplanes CC, 2007.
A.C. Marshall, Composite Basics, Marshall Consulting, 1994.
4 Typical Composite Materials
Typically, basic composite parts or components are made up of at least two parts – a
reinforcement material substrate e.g. fibreglass, carbon fibres or aramid (Kevlar™) fibres and a
resinous binder. The relatively brittle and firm resin matrix transfers forces acting on the part to
the load-capable flexible fibres. The low weights of both resin and fibres lend these parts
extremely high strength-to-weight ratios. Furthermore, simple composite “skins” may be fixed to
the top and bottom of so called “core materials” to form sandwich structures, e.g. composite
beams, still holding true to exceptional strength characteristics of these parts.
4.1 Structural fibres and fabrics
3
Raw Materials
AMTS-SWP-0002-A-2008
When compared to an unreinforced cured resin system, the mechanical characteristics of
reinforcement fibres are tremendously higher. The mechanical performance of a composite part
is therefore dictated in most part by that of the structural fibres.
The following factors determine the ultimate mechanical properties of a cured composite part:
• Basic mechanical characteristics of the reinforcement fibres
• Bond / surface interaction between resin system and fibres
• Amount of fibres per volume in the composite (fibre count)
• Orientation of fibres in cured part
Note:
The bond between fibres and resin can be improved by surface treatment. This
is especially important when bonding composite parts. See SWP 12 on
Adhesive Bonding.
As mentioned above, the orientation of fibres play an important role. In almost all cases
reinforcement fibres are available in different types of weaves, making up a fabric. The
following are typical commercially available reinforcement fibres and weaved fabrics:
4.1.1 Glass fibres
Many unique chemical compositions of glass fibres are manufactured worldwide. These
different compositions are designated by an alphabet letter and each display varying
mechanical and chemical properties.
The most common are:
• E-glass
• C-glass
• S-glass
• H-glass.
The E denotes high Electrical resistance, the C Chemical resistance, S high Strength
characteristics and the H stands for Hollow glass (extremely light fibres).
Many more types of glass fibre exist, too numerous to all be mentioned here. Refer to a
supplier’s catalogue and datasheets for fibre-specific information. As an example the properties
of the already mentioned E-, C- and S-glass are listed below:
Property
E-glass
C-glass
S-glass
2540 kg/m3
6.5
2490 kg/m3
6.5
2480 kg/m3
6.5
3.447 GPa
2.620 GPa
1.724 GPa
72.395 GPa
3.309 GPa
68.948 GPa
4.585 GPa
4.447 GPa
2.413 GPa
85.495 GPa
Physical
Density
Hardness
Mechanical
Tensile strength 25°C
Tensile strength 350°C
Tensile strength 550°C
Young’s Modulus
4
Raw Materials
AMTS-SWP-0002-A-2008
Chemical Composition
Silicon oxide
Aluminium oxide
Iron oxide
Calcium oxide
Magnesium oxide
Sodium oxide
Potassium oxide
Boron oxide
54.3%
15.2%
17.2%
4.7%
0.6%
8.0%
64.6%
4.1%
13.2%
3.3%
7.7%
1.7%
4.7%
64.2%
24.8%
0.21%
0.01%
10.27%
0.27%
0.01%
Table 4.1: Properties of single-fibre E-, C- and S-glass
4.1.2 Carbon fibres
Carbon fibres are usually made by taking strands of poly-acrylanitrile (PAN) in the form of multifilament yarn and then oxidizing, carbonizing and graphitizing it to form carbon fibre filaments.
These are then usually given a surface oxidation treatment to promote bonding with resins.
Other properties of carbon fibre include:
• High thermal conductivity
• Conducts electricity
• Electrically opaque to radio waves
4.1.3 Aramid (KevlarTM) fibres
Also a relative newcomer, aramid fibre has many of the characteristics of carbon fibre but sets
itself apart from carbon and glass with its unique degree of toughness. Several types of aramid
fibres can be found commercially.
Some benefits include:
• Electrically non-conductive
• Heat resistant
• Transparent to radio waves
One big drawback of using aramid fibre in a composite part is that it cannot be sanded after
curing. Be sure to leave gaps between the edges of aramid fabrics and part ends.
Important:
Special shears, sharpened at a specific angle are needed in order to cut aramid
fabrics correctly.
Aramid fibres are hygroscopic (absorbs moisture from the atmosphere) and UV sensitive.
Fibres should therefore be stored in a dry, darkened storage room. Moisture will adversely
affect its bonding properties with resins.
It should also be noted that aramid only bonds satisfactorily with epoxy and vinylester resin
systems. Using aramid fabrics with polyester resins is not recommended – poor interlaminated
bonding can be expected.
5
Raw Materials
AMTS-SWP-0002-A-2008
4.1.4 Other fibre types
Many exotic, experimental and classified fibres are in existence around the world, each having
unique properties still being explored and tested. Many of these fibres are either very rare or
very expensive and are therefore beyond the scope of this manual.
4.1.5 Types of fabrics
Different fabrics are mainly recognised by varying strand thickness and type of weave. Consult
with manufacturers on their unique fabrics with different yarn thicknesses and weaves for
specific purposes.
In general, the following types of woven fabric can be found commercially:
Plain weave
Twill weave
6
Raw Materials
AMTS-SWP-0002-A-2008
Satin weave
Basket weave
Leno weave
Mock Leno weave
7
Raw Materials
AMTS-SWP-0002-A-2008
4.2 Sandwich structure core materials
Sandwich structures are made by “sandwiching” a core material between two skins. The basic
idea behind these structures is to significantly increase a structure’s bending stiffness whilst
only marginally increasing the overall weight. With this in mind there are 2 basic characteristics
a core material should display:
• Low weight
• High volume
This basically means any core should have a low density (other than the skin material consisting
of heavier resin and fibres). Several compounds are suitable as cores and can be placed under
3 main categories:
• Foams
• Honeycombs
• Woods
Refer to SWP 18 on Sandwich Structures, where core materials are discussed in greater
detail.
4.2.1 Foams
Foamed plastic materials are affordable and easy to use as cores. The mechanical and
physical properties of different foams vary greatly and their specific datasheets should be
consulted for more detail.
Examples are:
o
o
o
o
o
o
o
Important:
PVC foam
Polystyrene foams
Polyurethane foams
Polymethyl methacrylamide foams
Styrene acrylonitrile (SAN) co-polymer foams
Metallic foams
Other thermoplastics
Polystyrene “bead foam” is not suitable for use as a sandwich core material.
This is foam made by exposing polystyrene granules to steam which then
expand in a mould. The bonds between these beads are weak and varied. Air
might also become trapped in the structure. These factors make this type of
foam unusable.
4.2.2 Honeycombs
Composite honeycombs are made from a variety of materials. Used mostly in the aerospace
industry, honeycombs can also be found in stage flooring sandwich structures, marine vessels.
Of all the core materials, honeycomb has the best compressive strength (next to balsa, see
section 4.2.3)
8
Raw Materials
AMTS-SWP-0002-A-2008
Examples are:
o
o
o
o
o
o
Aluminium honeycomb
Nomex honeycomb
Thermoplastic honeycomb
Glass fibre / plastic honeycombs
Carbon fibre / Kevlar honeycombs
Stainless steel, titanium and super-alloy honeycombs
4.2.3 Woods
Balsa wood offers good strength whilst having a very low density. If the grain is orientated
perpendicular to the sandwich skins, balsa wood’s compressive strength is better than most
honeycombs.
Examples are:
o
o
o
o
o
o
o
o
Balsa (most common because of low density)
Cedar
Spruce
Mahogany
Redwood
Pine
Fir
Many others
Woods are normally cheaper than foams, but prone to the attack from insects, mildew and will
deteriorate when exposed to moisture. Proper sealing and treatment is therefore necessary
where woods are used.
4.3 Resin systems
Typically material composites include at least two parts – a reinforcement material substrate
e.g. fibreglass, carbon fibres or aramid (Kevlar™) fibres and a resinous binder. The sole
purpose of the hardened (cured) resin system is to keep the fibres in place and along their
correct orientation. Thus a resin should also be able to chemically connect to the different
layers of material.
Resin systems fall in the thermosetting plastic category can be classified under the following
groups, according to their chemical composition:
• Epoxy resin
• Polyester resin
• Vinyl-ester resin
• Polyurethane resin
Other binding materials categories (for information purposes) are:
9
Raw Materials
•
•
•
•
•
AMTS-SWP-0002-A-2008
Thermoplastic (e.g. Perspex)
Phenolics
Unsaturated polyesters
Silicones
Polyimides
In this SWP only the thermosets will be discussed.
4.3.1 Epoxy resin systems
Epoxy resins are nearly transparent after curing. They are commercially available in hardware
stores for small scale repairs as well as in large quantities (resin and different hardeners) for
aerospace and marine applications.
Epoxies are used as either a structural matrix material reinforced with fibres (glass, carbon,
aramid, boron) or as a structural adhesive.
Some properties of epoxy resin systems:
•
•
•
•
•
•
•
Resin-to-hardener ratio is usually between 1:1 and 5:1
Excellent chemical- and corrosion resistance
Excellent thermal properties
Better mechanical properties compared to polyester resins
Good gap filling properties when mixed with additives (see section 4.4)
Offers excellent adhesive properties (including to polyester resin surfaces)
Low shrinkage compared to Polyester resins.
•
•
Gelcoat does not readily adhere to epoxy surfaces
Deteriorates when exposed to UV light
4.3.2 Polyester resin systems
Polyester resins may have a slightly yellow, transparent colour and are also known as
thermosetting plastics (will set at high temperatures.) Because of their sensitivity to UV light
and degradation over time, polyester resins are often coated with a protective layer.
Properties of polyester resin systems:
•
•
•
•
The hardener and accelerator agents are pre-mixed in the resin - the system only
requires a catalyst to set off the reaction.
MEKP (Methyl-ethyl-ketone-peroxide) is used as the abovementioned catalyst
Usually requires only 2% catalyser by weight
Offers good resistance against chemicals, corrosion and exposure to the environment
10
Raw Materials
•
•
•
•
•
•
•
AMTS-SWP-0002-A-2008
Flame retardant (self-extinguishes)
Very easily processed in low cost equipment
Usually cheaper than epoxy systems
Typical shelf life of less than 6 months
Storage containers for these resins must be tightly closed to slow down the natural
hardening process
Hygroscopic (draws in moisture from surrounding air)
High shrinkage compared to epoxies.
4.3.3 Polyurethane resin systems
Polyurethane is widely used in flexible and rigid foams, heavy duty adhesives and sealants,
fibres and hard plastic parts. Products containing polyurethane are often referred to as
"urethanes,” but should not be confused with the specific substance, urethane (ethyl
carbamate).
Polyurethane system properties:
•
•
•
•
•
Offers excellent thermal insulation
Resists the spreading of flames
Results in parts with high strength-to-weight ratios
Easily processed
Usually cheaper than epoxies
4.3.4 Vinyl-ester resin systems
Vinyl-ester resins may have a coloured tint, ranging from green to blue to purple. They are also
slightly more transparent then polyester resins and flows more easily.
A Vinyl-ester system is a good alternative to a polyester or epoxy resin system, having inferior
characteristics to those of epoxies, but better compared to those of polyester.
Properties:
•
•
•
•
•
•
•
Vinyl-ester resins are more flexible than polyester resins
Also catalyzed with MEKP, at a similar mixing ratio
Better corrosion and temperature resistance
Better strength properties
Resists water absorption
They degrade faster than polyester resins
Shelf life less than three months
11
Raw Materials
AMTS-SWP-0002-A-2008
4.4 Hardeners
Hardeners are substances or a mixture added to a plastic composition to promote or control the
curing action by taking part in it. Resins are sometimes referred to as “Part A” while hardeners
are referred to as “Part B”.
The reaction can normally not be controlled by modifying the mixing ratios. Mixing ratio must be
used as per the manufacturer’s datasheets.
Different hardeners can be found for specific types of resins. As explained above in section 4.3,
some resin systems only require a catalyzing agent.
Furthermore different hardeners (as in epoxy systems) differ normally only in respect to the pot
life of each different hardener-resin mixture.
4.5 Applications of resin systems
The different applications for which specific resins can be used allow us to reclassify different
sub-types of resin systems. Therefore common applications will be listed and described in the
following sections, along with a few examples of resins available on the market.
4.5.1 Laminating resins
Laminating resins easily wet any cloth, due to their low viscosity. They also chemically connect
to the weave, resulting in a strong composite material.
Examples of laminating resins include:
•
•
•
•
•
SP Systems Ampreg 20, 22,
Axon Epolam 2015, 2020, 2022
Axon Epolam 2080 (high temperature epoxy)
Hexion L285 (LBA aircraft certified system)
NCS and Scott Bader Polyester resins
4.5.2 Bonding resins
Bonding materials have to have a higher viscosity to prevent the material from flowing off the
area being bonded.
A few examples:
•
•
•
SP Spabond 345
Axson H 9940
Laminating resins (mixed with cotton flocks and carb-o-sil, see section 4.4)
4.5.3 Gelcoats
12
Raw Materials
AMTS-SWP-0002-A-2008
Gelcoats can be divided into spray-paintable gelcoats and mould surface gelcoats. Whereas
the mould surface gelcoat is usually black or some darker colour, the spray-painting Gelcoat
can have any colour, although it is frequently white.
Spray-paint Gelcoat:
Properties: Thick when mixed according to standards but can be diluted.
Examples:
•
•
•
•
SP 127
Hexion, T 35
Azko Nobel, Schwabellack
NCS Ultragel P1075
Mould surface Gelcoat:
Properties: Develops a hard and durable surface.
Examples:
•
•
•
Axon GC1050
Hexion F 200/F 15 (polishable surface)
Hexion F 260/F 16 (non- polishable surface)
4.5.4 Casting resins
Casting materials can be used for both high and low density foams or to form polyurethanebased rubbers.
Examples:
•
•
•
Axson F16
Axson 3034
Axon 5056
4.6 Resin additives (fillers)
By augmenting a resin-hardener mixture with a variety of additives, a wide range of different
properties can be obtained.
These additives work well when mixed with epoxy resin systems
4.6.1 Cotton flocks
This additive is made from natural cotton and appears as fine fibres. A mixture of cotton fibres
and epoxy is referred to as “flox”. Other names: “Micro fibres”, “Cotton flakes”
13
Raw Materials
AMTS-SWP-0002-A-2008
The mixture is used in structural joints and in areas where a very hard, durable build-up is
required.
Preparation:
Flox is normally mixed with 1 part cotton and 1 part carb-o-sil to one part resin.
Effects:
Turns resin into a tough bonding compound
Examples where used:
• Bonding of parts
• Filler material
• To create chamfers when needed
4.6.2 Carb-o-sil
Carb-o-sil or
names are:
•
•
•
fused quartz is a non-crystalline form of silicon dioxide, also called silica. Other
Aerosil (German)
Fused silica
Colloidal Silica (SP Systems)
Carb-o-sil can be used to reduce the flow of epoxies on vertical surfaces, as well as for filling
pinholes.
Uses:
• Carb-o-sil can be mixed with epoxies or gelcoats to modify the flow characteristics
• Carb-o-sil can be mixed with microballoons or cotton flocks to give non-sag properties to
fillers.
Effects:
It decreases the viscosity of resin.
Examples where used:
• Sealant in water tanks
• As a bonding adhesive when mixed with laminating resin and other fillers (see 4.3.1)
• A filler material, when combined with microballoons.
• Sealing paste on vacuum bag edges
4.6.3 Micro balloons
Micro balloons are hollow spheres made from either glass or phenolic. The differences are:
Other names
Colour
Particle Size
Glass Micro balloons
Phenolic Micro balloons
Microballoons
White
40-80 microns
Glass bubbles
Reddish/brown
50 microns
14
Raw Materials
Density
Sandability
Waterproof
Cost
AMTS-SWP-0002-A-2008
250g/litre
Good
Moderate
Expensive
200g/litre
Moderate
Good
Less expensive
Also known as glass bubbles, micro balloons are used in composites to fill polymer resins for
specific characteristics such as weight, sandability and sealing properties.
The term “micro” or “micro balloons” was applied to the mixture of solid microspheres and epoxy
early in the development of composite structures. Although microspheres have been replaced
by glass bubbles, “micro” is still commonly used to refer to a micro balloon and resin mixture.
Effects:
Lightens resin and eases its processing and application
Refer to SWP 13 on Composite Repairs for detail on mixing microballoon fillers.
4.7 Prepregs
Prepreg materials can be different weaves or unidirectional fibres pre-impregnated with a
specific resin system. They are available for purchase from any reputable composite materials
supplier and come packaged between plastic films or wax paper.
Importantly, prepregs should be stored in freezers or freeze rooms below -18°C. Storing
prepregs at higher temperatures will severely shorten their shelf life. Also called the cure-by
date, the shelf lives of prepregs differ and are available in their respective datasheets, and they
should be used and cured before/on this date.
Important:
Care must be taken when handling prepregs in order to prevent contamination
and workers should always wear gloves when handling prepregs.
See SWP 8 on Prepregs for further information on the storing, preparation, processing and
uses of prepreg.
5 Processing of Composites
Processing refers to the material preparation, mixing of a resin and its respective hardener as
well as application of the mixture (laminating, bonding etc.) and trimming
Refer to the following SWP for details on the processing
•
SWP 15 on Dimensioning and Cutting of fibres
15
Raw Materials
•
AMTS-SWP-0002-A-2008
SWP 7 on Mixing of Resins, including:
o
Temperature and humidity
o
Mixing quantities
o
Mixing ratios
•
SWP 14 on Wet Lay-up
•
SWP 9 on Curing of Composites
•
SWP 13 on Composite Repairs
6 Logistics
The following section briefly outlines the basic actions involving raw materials. With the
characteristics of different materials known, it becomes a whole new playing field to select,
evaluate, procure and store raw materials.
The figure below illustrates the typical flow of raw material in any organization:
Figure 6-1: Material flow process
6.1 Procurement of Raw Materials
6.1.1 Vendor Evaluation
The importance of selecting reputable vendors that are knowledgeable on the required raw
materials and can provide repeatability on orders regarding certain products should be stressed.
It is good practise to perform a vendor evaluation to determine the following:
1. Reliability
2. Competence and knowledgeable in composite raw materials
3. Stock sufficient quantities
16
Raw Materials
AMTS-SWP-0002-A-2008
4. Storage facilities to enable them to store as per manufactures specification
5. Quality control process in place
6. Ability to provide Technical support
6.1.2 Selecting Raw Material
To select raw materials that will perform as expected and required is crucial. Viewing and
understanding their technical data sheets is expected from resident engineers. The instructions
on handling, preparation and application should be understood by technicians. Their skills
should also be honed to include all materials involved.
6.1.3 Selecting Consumables
Understanding the performance characteristics of consumables and their price vs. performance
is important:
• Selecting consumables that compliment the work being carried out
• Knowing where compromises can be made to save production costs yet maintain quality
(tricks of the trade)
6.1.4 Recording of actions – taking into stock
The following are important points which should be addressed in a composite parts workshop to
ensure organization, traceability and recordkeeping:
• Stocktaking
• Recording of batch numbers, expiry dates etc.
• Real-time stock lists
• Restocking before running out
6.1.5 Quality Control of Materials
Ensure materials received are what were ordered; check that resins have not yet expired when
they arrive, check for serious blemishes on boards etc.
• Check expiry dates
• Dispose of expired materials in an appropriate manner
• Check prepreg ‘out-life’
6.2 Storage and Issuing of Raw Materials
Refer to the manufacturer’s datasheet of the resin system being used for details on storage
conditions and shelf-life. Epoxy resins and hardeners can be stored for at least 12 months in
their original, tightly sealed containers. Prepregs have very strict rules regarding the shelf-life
and storage specification.
Some resin systems may begin to crystallize when stored below a certain temperature. Before
processing it should be heated up and mixed thoroughly to remove crystals.
See SWP 35 on Handling and Storage of Raw Materials.
17