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ELECTRON PROCESSING OF FIBRE-REINFORCEDADVANCED COMPOSITES
AliT SINGH,t CHRIS B. SAUNDERS, JOHN W. BARNARD, VINCE J. LOPATA,WALTER KREMERS, TOM E. McDOUGALL, MINDA CHUNG and
MIYOKO TATEISHIResearch Chemistry Branch. AECL, Whiteshell Laboratories, Pinawa, MB, Canada ROE I LO
Abstract-Advanced composites, such as carbon-fibre-reinforced epoxies, are ~sed. in the ail7raft,aerospace sporting goods. and transportation industries. Though thermal curmg IS the dommantindustrial'process for advanced composites, electron curing of similar composites containing acrylatedepoxy matrices has been demonstrated by OUf work. The,ma~n attraction .ofelectron processin~ technologyover thermal technology is the advantages it offers which mclude ambient temperature curmg. redu~curing limes. reduced volatile emissions. better material handling, and reduced costs. Electron c1!~ng
Radiation processing involves the use of natural andman-made sources of high-energy radiation on anindustrial scale to give products that are safe. practical and beneficial (Silverman, 1981). In comparisonto conventional processing methods. it is an energyconserving and environmentally benign technology.The number of electron accelerators in industrialuse has been steadily increasing (Saunders. 1988;Leemhorst and Miller, 1990; Singh and Silverman,1992), despite the fact that radiation-based technologies face close scrutiny by society and regulators.The growth of this technology has depended onnormal market forces, which restrict the use of industrial electron and gamma processing to applicationsthat either offer significant cost savings or produceunique and useful products. The major applicationsof this technology are based on: (i) polymerization orcuring, e.g. coatings and rubber, (ii) crosslinking, e.g.wire and cable insulation, and plastic film. (iii) scission, e.g. degradation of Teflon and (iv) biologicaleffects. e.g. sterilization of medical disposables(Silverman, 1981; Saunders, 1988; Stannett et al.,1989; Cook, 1990; Schroeder, 1990; Tabata, 1990;Tenorth, 1990).
Radiation processing is a growing industry; in1990, the value added to products by radiationprocessing was estimated to be in the billions ofdollars (Cook, 1990). A large part of the credit forthis growth belongs to the pioneers, who by theirvisionary work laid the foundation of industrial
tAuthor to whom all correspondence should be addressed.
153
radiation processing of polymers, including ,~
Charlesby, Chapiro, Dole, Silverman, Sta~nett andTabata [see Dole (1972, 1973), Kroh (1989), Singhand Silverman (1992), Woods and Pikaev (1994) forrelevant references]. In the polymer industry, accelerated electrons are the main source of high-energyradiation (Saunders, 1988).
Advanced composites, such as carbon-fibrereinforced epoxies, are being used for many applications, primarily because of their high strength-toweight and stiffness-to-weight ratios, corrosionresistance, impact and damage tolerance characteristics, and wear properties (Margolis, 1986~ Dostal,1987). Applications for such thermosetting composites are found in the aircraft, aerospace, sportinggoods, construction. transportation arid automotiveindustries (Margolis, 1986). Though thermal curingof advanced composites is presently the dominantindustrial process, electron curing of similar composites containing acrylated epoxy matrices has beendemonstrated (Saunders e/ af., 1988a, b; Saundersand Singh, 1989; Beziers and Capdepuy, 1990; Saunders et al., 1991a, c, 1992; Singh and Saunders, 1992;Singh e/ at., 1993). Radiation curing of advancedcomposites offers many advantages over thermalcuring. Because of these advantages, radiation curingis emerging as an alternative to the currently popularthermal curing process. Though the focus of ourwork has been the aircraft industry, electron curingcan also be used to make composites for the otheruses mentioned above. Electron processing is compatible with the manufacture of composite productsusing traditional fabrication methods, includingfilament/tape winding, pultrusion and hand lay-up.
154 Ajil Singh et at.
c.
The basis of the radialion polymerization andcrosslinking reactions is the formation of ionic andfree radical intermediates in the polymeric substrates(Charlesby, 1960; Chapiro, 1962; Williams, 1968;Wilson, 1974; Woods and Pikaev, 1994)
Vinyl monomers N+ Cations, Anions,
Free Radicals~ Polymer (I)
Polymer N+ Free radicals_
Crosslinked polymer (2)
Of course, the main reaction path is accompanied byadditional side reactions which produce scission,unsaturation, hydrogen, low molecular weight produelS, etc. (Dole, 1972, 1973; Wilson, 1974).
In this paper we review our work on radiationcuring of carbon fibre-reinforced advanced composites. This work was started in late 1986 as a partof AECL's business development initiatives in thefield of applications for 10 MeV electrons (Singh andSaunders, 1992). Our work was to support the development of markets for 10 MeV industrial electronaccelerators (Kerluke and McKeown, 1993). Similarwork has also been done by Aerospatiale in France,though their work was primarily focussed onfilament-wound products and has remained largelyproprietary (Beziers and Capdepuy, 1990).
Advantages of electron curing
Compared to thermal curing, many advantageshave been identified for using electron curing (Saun·ders and Singh, 1989), as outlined below.
(1) Curing at ambient temperature. Tooling materialsare required to control the dimensions and shapeof a composite product. During the thermalcuring cycle, the tool expands and contracts,often at a different rate, and to a different extent,than the composite product. During cooling, thematrix usually contracts to a different extent thanthe reinforcing fibre. These combined effects canchange the dimensions of the product and createinternal stresses, adversely affecting the com·posite properties (Weeton el al., 1987). In principle, electron curing at ambient temperature canreduce these dimensional changes and the internal stresses in the produced composite. Wehave demonstrated reduced. internal stresses inelectron-cured composites (Saunders et at.,1993b).
(2) Reduced curing times. A typical electron-curablecarbon-fibre-reinforced acrylated epoxy laminatecan be cured with a dose of about 100 kGy(Saunders el al., 1993a). A commercially available 50 kW electron accelerator, whose typicalavailability exceeds 95%, can provide this dose toabout 900 kgfh ofmaterial, assuming tbat 50% ofthe accelerator's electron beam energy is absorbed by the product being irradiated. By com-
panson, a typical autoclave can cure about200 kg/h of material. Thus, the throughput for a50 kW accelerator is much greater than that of atypical autoclave used for thermal curing. This isdespite the products being cured one at a timeduring electron curing, compared to large batchesin autoclaves.
(3) Improved resin stability. Most electron-curablematrix resins do not readily auto-cure at roomtemperature, making low-temperature storageunnecessary.
(4) Reduction o/volatiles produced. Thermal curing ofcomposites often produces volatile degradationproducts that can be hazardous and require appropriate controls (Weeton el al., 1987). Electroncuring drastically reduces the production ofdegradation produclS (Iverson el aI., 1992),though very small amounts of familiar industrialgases, such as hydrogen, carbon dioxide andmethane, may be produced..
(5) Better malerial handling. Two factors that contribute to more efficient material handling duringelectron curing are: (i) the ability to handle theresins at room temperature makes it easier tofabricate composite products and (ii) the abilityto electron cure each product as it is fabricatedreduces the storage requirements for the uncuredproducts. In the case of electron curing, productswith different resins requiring different doses canbe processed one after the other. In thermalcuring, all the contents of the autoclave musthave the same curing cycle.
(6) Better material compatibility. Products containingtemperature-sensitive materials, e.g. foam or honeycomb containing low melting point polymers,or mixtures of fibres (such as mixed laminateswith carbon fibre, polyetbylene fibre, or aramidfibre reinforcement) can be easily electron curedwithout dimensional distortions. Thick laminates(up to 15 em) have also been produced by electron curing (Saunders el al., 1994). Thermalcuring of such products, where possible, is challenging.
(7) Reducing the processing costs. Estimates suggestthat the energy required for electron curing couldbe lower by a factor of five or more, with overallcost savings of the order of 30% (Saunders et at.,1993b).
Challenges for electron processing
Electron processing of composites also faces somechallenges, as listed below:
(1) Availability ofelectron-curable matrix resins. Theepoxy fonnulations currently being used in theaircraft industry have been optimized for thermalcuring and are not appropriate for electron curing. The thermal-curable epoxy formulationscontain proton scavenging curing agents, such as
Electron processing of composites 155
amines. Because they are typically kept refrigerated to prevent auto-eure, they also tend toabsorb moisture during composite fabrication.Water, amines and other proton scavengers caninterfere with the cationic polymerization ofepoxies, during irradiation (Dickson and Singh,1987). However, acrylated epoxy resins that areelectron-eurable via free radical mechanisms, arecommercially available. The same range of properties for composites can be obtained using acrytated epoxy matrices, as with epoxy matrices(Beziers and Capdepuy, 1990; Saunders er al.,1991a). Work is also being done to develop resinsthat undergo cationic curing on irradiation(Lapin, 1986; Crivello er aI., 1992; Walton andCrivello, 1994). However, the total number andvariety of commercially available electron-eurable resins remains much smaller than for thethermally cured resins.
(2) Qualification procedures. Extensive testing is required to develop electron-cured matrices foradvanced composites that art truly equivalent tothe conventional thermally cured matrices. Therequired criteria and qualification procedures forthe aircraft industry are time consuming andexpensive (Laramee, 1987; Traceski, 1987; Filaand Fews, 1993). However, this constraint maynot apply to other uses of composites, e.g. in thetransportation and automotive industries.
KADIATION PROCFSSING
Whiteshell irradiator
The Whiteshell irradiator pilot scale facility, suitable for curing of small to quite large parts but at alow throughput, is shown in Fig. 1. This faciJity hasbeen described previously (Barnard and Wilkin, 1987;Barnard and Stanley, 1989; Saunders er al., 1991a). Inour 10 MeV, 1kW accelerator, a beam of electrons is
accelerated horizontally, energy analyzed in a 2700bending magnet and projected downwards through ascanner which disperses the beam across the productas it passes by on a conveyor. The accelerator roomis large enough to accommodate quite large parts forcuring. However, most products are presented to thebeam on a conveyor which accesses the acceleratorroom by way of a shielding maze.
The operations section of the facility is containedon a single basement level. Shielding of the workersin the accelerator control area is provided by pouredin-place concrete. Shielding to the out-of-doors isprovided mainly by an earth berm 3.5 m thick. Awarehouse facility with loading bay is located atstreet level. The conveyor extends all the way fromthe warehouse onto the basement leveJ, through themaze to the accelerator and back out through themaze and back to the warehouse. Dosimetry andquality assurance laboratories, and offices are alsoprovided on the basement level. An AECL Gammacell 220 provides gamma irradiations for comparativedose rate studies.
Radiation protection
In general, the radiation protection objectives arethe same for all types of accelerators used in radiationcuring, namely, to provide protection to the workerand public from direct exposure to the beam orintense X-rays which might cause immediate harm,and to keep radiation doses as low as reasonablyachievable (the ALARA principle). These objectivesare achieved by providing sufficient shielding tomaintain dose rates at negligible levels during operation, insuring that a set of administrative proceduresfor operating the facility safeJy are in place, andinstalling a set of fail-safe redundant interlocks toprevent personnel from inadvertent exposure to highradiation levels (IAEA, 1982, 1992).
Conveyor system -Earth fill
AECL 1-1011electronaccelerator
Concrete Lead entrancedoor
Controlroom
Shippingarea
Fig. I. Diagram of the 1-10/1 accelerator facility.
156 Ajit Singh et al.
Beyond these objectives are radiation protectionconsiderations which are dependent on the type ofradiation curing to be carried out and the type ofequipment required. If thin composites are to beproduced, low energy electrons are sufficient to penetrate the part. For thicker parts, higher energies willbe required to penetrate the part or X-rays may beneeded to achieve a uniform cure. As the energy isincreased to improve penetration, the amount ofshielding required to protect against X-rays and thelikelihood of producing radioactivity either in theproduct being cured or the equipment also increases(Swanson, 1979).
The electron beam
Even for low power electron accelerators exposureof personnel to the direct electron beam is harmful.The strict adherence to procedures and a functioninginterlock system are the primary protection againstthis hazard. However, for high power accelerators,the amount of heat generated by the impact of thebeam is sufficient to melt metal components, explosively spall concrete and do other structural damageto the facility. Therefore, water-cooled beam dumpsare required to protect building components. For10 MeV beams with power greater than 20 kW, theintensity of X-rays produced from completelystopped electron beams in a beam stop may besufficient, depending on the geometry of the beamstrike area, to cause overheating of the shielding.Beam stops must be made thick enough to stop theelectrons as well as partially attenuate the X~ray
intensity. In such situations, damage could resultfrom loss of coolant flow. The accelerator shouldtherefore be interlocked to cease operation on lowcoolant flow in the beam stop.
X-rays
The tenn X·ray used here describes both theconsequences of ejection of a bound electron by ahigh energy electron and "bremsstrahlung", the photons emitted by deceleration of the fast electrons bythe nucleus of the target atoms/molecules (Swanson,1979). For an practical purposes, as far as designingthe shielding is concerned, it is the "bremsstrahlung"that needs to be protected against. The intensity ofX-rays and, hence, the thickness required for adequate shielding increases with beam intensity and theatomic number of the target material. For a to MeVelectron beam, for example, a lead target is about 3times more effective in producing X-rays than analuminum target (Swanson, 1979).
Energy also detennines how effectively X-rays canpenetrate materials and the thickness of the shieldingrequired to provide protection. When electrons impinge on a target, the X-ray spectrum produced iscontinuous from a few tens of keV (the minimumenergy of X-rays that can escape a thick target) up toa cutoff corresponding to the maximum energy of theelectron beam. The average X-ray energy from a
5 MeV electron accelerator is about I MeV; theseX-rays have about the same penetration ability as6OCO gamma-rays. The average X-ray energy from a10 MeV beam is about 4 MeV. The X-ray absorptioncoefficient (a measure of loss of energy to the absorber) decreases up to about 10 MeV and thenincreases as the probability of pair production increases with energy. However, the increased effectiveness of shielding of higher energy X-rays is offset bythe onset of "showers" or "cascades", whereby increased X-ray intensity is produced from electron-positron pairs created in the target or theshielding (NCRP, 1977; Swanson, 1979).
Direct penetration of shielding is not the onlyconsideration in designing shielding for worker protection. X-rays can scatter along mazes provided forproduct transport to the beam. Mazes must be longenough and provide enough scatters for X-rays toafford comparable attenuation to the shielding provided for the direct penetration path. Service portsfor cabling and ventilation duets must also be tortuous in order to prevent beams of X.rays from beingscattered from the irradiation zone into the workingareas.
A number of materials are available for shieldingagainst X.rays. The shielding material of choice isconcrete because it is easy to fonn and pour to shape,is relatively cheap, is widely available, and presentsreasonable attenuation for the amount of shieldingweight required. Lead is a good choice for localshielding where the most reduction for the amount ofvolume available is required. However, lead is veryheavy, has a low threshold for photo-disintegrationand, when shielding higher energy beams, contributesto neutron and proton production by way of the(y, n) and (y, p) reactions (NBS, 1973). While contributing to neutron production, lead itself is a relatively poor material for shielding neutrons (NCRP1977; Swanson, 1979).
In some cases it is possible to save costs onshielding materials by taking advantage of the existing terrain (by recessing into the side of a hill, forexample) and use earth benns as shielding. On a perunit weight basis, compacted or undisturbed earth isabout as effective as concrete. However, if the lay ofthe land can not be exploited and earth has to behauled to provide the shielding, it usually provesmore costly than concrete.
A variety of other materials, including water, canbe used as shielding materials (NCRP, 1977). Typically, steel or lead with steel stiffening is used forshielding doors. Shielding curves for many materialsare available from several sources (NCRP, 1977;Swanson, 1979; Nucleon Lectern, 1984). These refereoees also give rules of thumb and computationalmethods for estimating scattering or streaming alongmazes and conduits. For low atomic number rna·terials, it is generally acceptable to derive shieldingcurves from a material by scaling according to itsdensity, from the shielding data of a material of I
I Electron processing of comJX>sites 157
\
similar atomic number. However, care must be takenfor converting shielding curves where the materialsdiffer widely in atomic number. because it is theelectron density that determines the effectiveness of amaterial for shielding against X-rays (Swanson,1979).
Neutrons
X-rays from eJectron beams produce both neutronsand protons by photo-disintegration (Y. nand y, Preactions). Since protons are heavy charged particles,their linear energy transfer to the medium is high andthey lose energy very rapidly as they traverse materials. Most do not escape the target materials inwhich they are formed. Neutrons, however, are neutral particles and are much more penetrating. Sincethey can be produced by accelerated electron beams,attention to shielding for neutrons is also required(Swanson, 1979).
Except for some notable cases, the threshold en~
ergy for most photo-disintegration reactions in lowatomic number materials lies above to MeV. Theseexceptions are deuterium (threshold energy:2.23 MeV), beryllium (threshold energy: 1.67 MeV)and "c (threshold energy: 4.9 MeV) (NBS, 1973).Beryllium, a very toxic metal, is seldom encounteredunder normal electron-curing circumstances and theabundances of deuterium and J3C are very low innature. However, if electron-curing is conducted atenergies higher than 10 MeV to enhance penetration,care must be taken in considering what extra shielding may be needed to protect against the neutronsthat may be produced. As energies are increasedabove 10 MeV, morc and more common elements oflow atomic weight will photo-disintegrate, increasingneutron production and induced radioactivity. Noneof the above mentioned reactions should present aproblem in electron-curing of plastics and compositesup to 10 MeV in energy.
In general, the best neutron shielding materials arehigh in hydrogen because they effectively absorbneutron energy in elastic scattering reactions. Again,concrete is excellent because of its high water content(NCRP, 1977). Paraffin wax, oils, and organicsolids also make good neutron shielding material.Their disadvantages are that they are generallyftammable.
A general rule of thumb in shielding facilities usingelectron beams up to 20 MeV is that if the shieldingis sufficient for the XMrays produced, it will also besufficient for the neutrons produced. However, if ashielding maze is provided for product transport inand out of the irradiation room, then the shieldingmay not be sufficient because most shielding materialsbackscatter neutrons more efficiently than X.rays(Swanson, 1979). In such cases an analysis of scattering of both X-rays and neutrons along the maze(NCRP, 1977) must be carried out to show that theprotection against scattered radiation afforded by themaze is comparable to the protection the bulk shield-
ing provides against directly penetrating radiation,and that together, the two components of leakageradiation satisfy the design criteria for work placeradiation fields established to meet the national standards for worker radiation protection.
Radioactivity
The production of neutrons usually accompaniesthe production of induced radioactivity. Many of thereaction products of photo-disintegration are radioactive. Also, the neutrons are absorbed by materialsin the shielding and targets to create 'activationproducts by way of the n, y reaction.
At low atomic weights most of these radioactivespecies are short-lived and decay away with half-livesranging from seconds to days. However, in the timebetween production and decay, activation and photodisintegration products in targets, beam lines andproduction materials can be the source of significantradiation fields. There is also the potential for inhalation or ingestion of radioactivity if materials thathave been activated by the beam are machined orground before the radioactivity has been allowed todecay. Swanson (1979) provides complete tables forestimating saturation activities as a function of energy and beam power for electron beam incident ona variety of target materials including concrete, steel,air, water and various elements. These tables alsoprovide half-lives and the threshold energies for theproduction of each radioactive product.
The air of the facility can also be activated. Thethreshold for "N(y, n)l3N reaction is 10.55 MeV. Thehalf-life for the decay of "N is 9 min. In general, thisshould not present a hazard for workers since thehighest concentrations are produced in the beamstrike area where the X-radiation is most intense, andunder normal circumstances will be vented awayimmediately following shutdown of the accelerator.However, it could contribute significantly to theworker's dose as an external source of radioactivityin which the worker is immersed for very brief periodsfollowing accelerator shutdown, provided the timegap between the accelerator shut down and theworker entry is short. The time for safe re-entryfollowing shutdown of the electron beam depends onthe rate of air exchange, the beam energy, the beampower and the duration of the beam as well as the halflife of l3N. Swanson (1979) provides tables for estimating maximum permissible concentration in accelerator room air, from UN as well as some higherenergy photo-disintegration and activation produCts,based on the recommendations of the InternationalCommission on Radiological Protection in ICRPPublication 2 (ICRP, 1960).
Although induced radioactivity in products exposed to 10 MeV electrons, or X·rays produced fromthem, would be negligible, it may be useful tocheck the product handling equipment in the targetarea, periodically, for any induced radioactivitybuild-up.
158 Ajit Singh el al.
Radia/ion protection programs
Radiation protection programs at an electron accelerator facility should be drawn up in advance, tomonitor and protect the worker from the anticipatedradiation hazards. This means that limits on processing energy and the maximum beam power should beestablished in the planning stages. The degree ofhazard based on the consequences of credible accidents should be estimated in advance. Mitigatingmeasures and safety procedures should be implemented prior to first processing. A period ofcommissioning should also be provided with operational tests planned in advance and carried out, todemonstrate the safe operation of the facility aspredicted in the safety analysis.
A good radiation protection program will containthe following elements (IAEA, 1982; IAEA, 1992): (I)workplace monitoring for radiation fields and radioactivity, (ii) worker dose monitoring, (iii) regularreview and assessment of the sources of worker dosewith corrective action to maintain doses as low asreasonably achievable, (iv) training in operationalsafety and safety equipment use, (v) a quality program which controls the risk at a very low ornegligible level, and (vi) accident and near-miss investigation to prevent injury or its recurrence.
Industrial safety
A number of industrial safety hazards associatedwith electron curing are readily evident. These includetoxic gases. machinery, fire, electrical equipment,pressurized systems, and flooding. Provision must bemade to protect the worker from these hazards aswelt as from ionizing radiation. The following sections discuss three of the most commonly encountered hazards. Any or all of them may be present inan electron accelerator facility.
Machinery
One of the principal advantages of electron-curingis process speed. This makes the use of producthandling equipment a necessity. Equipment such ashigh speed winders and conveyors offer the opportunity for entanglement of worker clothing and hair orthe capturing and pinching of extremities. All processequipment should be designed to current nationalsafety standards. The design of unique custom-madeequipment for the facility should be analyzed separately as part of the safety analysis to identify inadvance, and mitigate wherever possible, potentialfor injury in its use.
Toxic gases
There are two principal sources of toxic gases andvapours in an electron processing facility: volatilesfrom the product and toxic gases generated by ionizing radiation in air.
Heat generated by the beam and from the exothermic curing reactions induced during irradiation tends
to elevate the temperature of the product and driveoff the solvents and the uncombined monomers remaining after the cure is completed. These are released into the air of the irradiation room as airbornepollutants. Most of these pollutants may be toxic tosome degree, the liver being the principal organ atrisk, and may have adverse effects on the health ofworkers. Threshold limit values for chemical substances and physical agents in work place air arerecommended by the American Conference of Governmental and Industrial Hygienists (ACGIH, 1993),and its counterparts in most countries.
Ventilation is the best protection against thesetoxic gases. For materials irradiated under vacuum,the evolution of volatiles directly from the part beingprocessed is restricted by the vacuum envelope. However, it is good industrial hygiene practice, in thiscase, to discharge vacuum pumps directly to theexhaust ventilation duct of the target room. Normallythe levels are not high enough to warrant environmental abatement in the form of charcoal or resinabsorbers. However, for large scale production,workplace and emuent monitoring of off-gases andvapours from the product should be undertaken, atleast initially, to determine if further action is warranted.
The two principal toxic gas species produced byionizing radiation in air are nitrogen oxides andozone (Brynjolfsson el al., 1971; Swanson, 1979). TheG values for nitrogen oxide and ozone are 4.8 and10.3 moiecuies/IOO eV respectively. The thresholdlimit values (TLV) for nitrogen oxide and ozone are3 and 0.1 pg/g respectively. Because ozone is moretoxic by an order of magnitude, and produced ingreater quantities, it is the one that is generallytargeted for control ofworkplace exposure (Swanson,1979).
Ventilation is the best protection against the toxicgases produced. The facility ventilation should bebalanced so that the net flow of supply air is alwaysinto the irradiation room and the total facility exhaust is out of the irradiation room. For very highpower electron accelerators, the concentrations ofozone in air outside the facility may also represent ahazard to personnel or the environment. This may bemitigated by dilution with fresh air from outsidebecause ozone decomposes back to molecular oxygenwith a half-life of about 1 h, or ozone can be reducedback to 0, before discharge by the use of recombiners. Guidance is given by Swanson (1979) forcalculating the production and airborne concentrations of 0 3 in electron accelerator facilities.
Fire
The injection of large quantities of energy from theelectron beam into products consisting of flammableresins and combustible reinforcing materials presentsthe potential for setting of fire. If the product beingcured is large, the fire can rapidly become intense,fuelled by the resins under cure and supplied with
Electron processing of composites 159
oxygen from the ventilation air. The presence ofhighly oxidizing ozone will tend to accelerate combustion. The heat can become intense with the fireconfined in a relatively small area in the irradiationroom. or may be driven along the ductwork by theventilation air to break out elsewhere in the facility.Normally, the combustion of the part in process willresult in heavy smoke and airborne toxic vapour inthe target room which can quickly overcome personnel attempting to respond to the fire. Self containedbreathing apparatus should be provided. easily accessible near the facility entrance. Personnel should betrained in its 'use.
Therefore. fire detection systems should be designed to monitor for both smoke and temperaturerise rate. Fire detection is further complicated in thehigh radiation environments of the accelerator roombecause some fire detection instrumentation. particularly smoke detectors, are subject to deterioration inradiation fields. It may be necessary to replace somecomponents on a regular basis or ensure in someother way (hy shielding, perhaps) that the protectionthey afford is always available. From this point ofview, continued surveillance of the target room,particularly the target area, with a TV camera (whichis common at many accelerator sites) assumes animportant safety role.
The accelerator operation and the curing processshould always be monitored by an operator on dutyto insure that the product movement is not interrupted. The beam cannot be permitted to rest in onearea long enough to induce burning. Nonnally theproduct handling system is interlocked to trip theaccelerator if the product stops moving in the beam.or power to the product handling system is lostduring production.
PREPARATION OF COMPOSITES
Materials
The carbon fibre types used in our work are; AS4,IM6. and 1M?, in the following weaves: unidirectional. plain, and 5-harness satin. all from Hercules Inc. The resins used in our work are: CNI04and C-3000 (epoxy diacrylate oligomer), CNI14(epoxy acrylate), CN964 (acrylated urethane), S-297(1,3-butylene glycol dimethacrylate), S-399 (dipentaerythritol monohydroxy pentaacrylate), S-604(polypropylene glycol monomethacrylate), C-2000(C,.-C" diol diacrylate), C-5000 (polybutadiene diacrylate) and C-9503 (aliphatic urethane diacrylate)from Sartomer Inc; FW3 (blend of epoxy acrylates,methacrylates and acrylates), and BMII and BMI4(blends of epoxy acrylates and bismaleimides) fromApplied Poleramic Inc.; and electron-curable adhesives from Loctite Corporation and Union Carbide. A proprietary acrylated isocyanate couplingagent was prepared by us.
Laminate fabrication
We fabricate laminates using two methods. prepregand resin transfer moulding (RTM).
Prepreg preparation. We have used two of thecommonly used methods. wet-layup and solventprepregging, for the fabrication of the laminates(Dostal, 1987).
Wet-layup me/hod. In this method the fabric, withthe desired resin poured on the fabric. is laid betweentwo sheets of Vac-Pak HS-8171 bagging film (Richmond Aircraft Products). If the resin is not excessively viscous, it is manually spread throughout thefabric. When the resin is too viscous. the resin/fabricis placed on a heating pad and the resin viscosity isadjusted by controlling the temperature of the pad.Once the viscosity of the resin has been sufficientlyreduced, the resin is spread manually throughout thefabric.
Solvent me/hod. In this method, the fabric is placedin a polyethylene tray of the same size as the desiredlaminate panel. For routine laboratory scale work.we usually place? layers of fabric in one tray. Theresin to be used is dissolved in an appropriate solvent.In the case of acrylated epoxies. acetone is a good solvent to use; for the BMI resins, chlorofonn works well.The resulting solution is then poured over the fabricin the tray and the solvent is then allowed to evaporate. In both cases the tray is placed in a heated ovenat ?O°C. This is to ensure that all the solvent is drivenoff, and that there is no moisture incorporated in thelaminate during the solvent evaporation process.
Bagging prepreg. Once made, the prepreg with thefibre in the desired orientation is placed on aluminumplates which have been treated with a release agent.Most common mould release agents can be used forthis purpose, up to a dose of 200 kGy. A sealant tape(e.g. Tacky Tape 5126-2, Schnee-Morehead Inc.) isplaced around the edge of the aluminum plate. Oncethe prepreg has been laid to the desired thickness. arelease fabric such as B-4444 (Richmond AircraftProducts) or perforated release film such as A5000(Richmond Aircraft Products) is placed over theprepreg. A breather cloth such as RC-3000-20 (Richmond Aircraft Products) is placed over the releasefilm or cloth. A vacuum port is then incorporated inthe setup. This consists of a Swagelok 1/4 inchquick-disconnect coupler. A vacuum bag film (e.g.Vac-Pak HS-8171, Richmond Aircraft Products) isthen placed over the laminate lay up. A vacuumsystem is then connected to the vacuum port and thebag evacuated. The laminate is evacuated for approximately one hour to provide some consolidationof the fabrics as well as to remove any air between thefabric layers. This also helps remove any traces of thesolvent and moisture remaining in the prepregs. Thelaminate is then rolled with a rolling pin to providefurther consolidation. Once this procedure is complete, the laminate is irradiated under vacuum usingthe electron accelerator.
160 Ajit Singh el al.
Resin transfer moulding. In order for RTM to beused, a resin with a viscosity of I Pa s or less isrequired. In some cases, where the resin viscosity istoo high, the resin is heated to decrease its viscosity.The fabric to be used is cut to the specified dimensions and placed in the tool to the thickness and fibreorientation required. The tool is then closed, thevarious hoses for resin transfer and vacuum accattached, and the tool is evacuated, which removesair. solvents, and any traces of water. Once thepressure is down to approximately 1 kPa, the resin isintroduced either under its own flow or under pressure. Care is taken to ensure that no air enters thesystem. The resin is allowed to flow continuously intothe tool until the resin exits on the vacuum sidewithout air bubbles. At this point, the resin pressureis reduced. The resin and vacuum lines are thensealed. The tool is taken to the accelerator and thepanel cured.
Atmosphere. Acrylated epoxies cure via free radicalmechanisms (Dickson and Singh, 1987). Thus, asexpected, oxygen inhibits the gel formation in anacrylated epoxy formulation on gamma irradiation(Saunders et al., 199Ib). However, the inhibition isnegligible on electron irradiation (Saunders et aI.,199Ib). For all practical purposes, our work onelectron curing of composites has been done underinert atmosphere. Either the samples are vacuumbagged, or the samples are under mechanical pressurebetween metal plates which prevents diffusion ofoxygen into the bulk of the sample.
RESIN AND COMPOSITE TESTING
Several testing methods have been used to evaluatethe resins and composites, as described below.
Sample preparation. Resin samples for gel fractionand size exclusion chromatography (SEC) wereplaced in sealed, evacuated pyrex test tubes. Samplesfor dynamic mechanical analysis (DMA) were prepared by pouring the resin into aluminum mouldsand curing them under inert atmosphere. All sampleswere irradiated using either the Gammacel1 (doserate: 7.8 kGY/h) or the 1·10/1 electron accelerator(dose rate: 5.4 MGy/h) as described earlier (Singhand Saunders, 1992; Saunders et al., 1993a). Thepreparation of carbon fibre prepregs has been described above. Thick composite samples (> 70 ply)have also been prepared using the CNI04 acrylatedepoxy resin, to study possible variations in bothabsorbed dose and properties through the thicknessof the cured laminates (Saunders et al., 1994), in asimilar manner. The samples for electron curing andX-ray curing were consolidated using standard vacuum bagging techniques, and external pressure wasapplied as needed. Samples were irradiated to dosesfrom 0.1 to 1000kGy, as required.
Gel fraction/size exclusion chromatography. Curedpolymer samples (I g) were crushed and placed in apyrex glass tube containing tetrahydrofuran (THF).
In the case of the composites, 2 g samples were cutand placed in pyrex glass tubes containing THF. Thesample was heated overnight at 65°C, and the liquidextract was collected for SEC. A fresh aliquot ofTHFwas added to the sample for another 24 h extractionat 65°C to remove any residual soluble fraction. Afterthe second extraction in THF, the liquid was drainedoff, the resin sample was dried overnight undervacuum at 100°C, cooled and weighed, to determinethe gel fraction. For SEC of the THF extract, themobile phase used was THF at a flow rate ofI ml/min. A 10 pi sample was injected using a WatersWISP 710B Injection System. Separation of thedifferent molecular weights was done using Phenogel5 500A (500-10,000 Dalton) and Phenogel 5 100A(50-1000 Dalton) Phenomenex 300 x 78 mmcolumns. Two different detectors were used in thiswork; a laser light scattering detector (Dawn F,Wyatt Technologies) and an index of refraction detector (Hewlett Packard 1047A). The signals fromboth the detectors were sent to a personal computer.Wyatt Technologies ASTRA software was used forthe data acquisition and EASI software was used forthe data analysis. The response of the SEC columnswas calibrated using polystyrene molecular weightstandards (800-233,000 Dalton) and biphenyl (154Dalton). Soluble (Sol) fractions were calculated fromthe areas under the SEC curves for each dose. Figure2 shows a typical SEC chromatogram obtained usingthe laser light scattering detector and the index ofrefraction detector.
Mechanical testing. Mechanical testing of electroncured polymers and composites is done according tothe ASTM testing methods (ASTM, 1984; tensileproperties, D638M; flexural properties, D790M;compressive properties, D695). These tests were carried out at an aircraft manufacturer's testing laboratory.
Dynamic mechanical analysis. This method givesinformation about the rheological properties of ma-
10' ,.,- .,
2!10'.. fI
.l! 10' i!
J j10' I!
10'40 4Z 44 40 40
Eluted Volume, rnL
Fig. 2. Typical SEC chromatogram of polymer extractables.The molecular weight curve is obtained from the laser lightscattering detector; the relative intensity curve is obtainedfrom the index of refraction detector. The refractive indexcurve suggests that there are two major products and at least
one minor product.
•
•
Electron processing of composites 161
.....~
fDJm
I~...,.. ,.. 10'
Fig. 4. Void size distribution for FW3 acrylated epoxy RTMcomposite using unsized, plain weave, 3 K, AS4 fibre,
obtained by mercury intrusion porosimetry.
large number of resin fonnulations. If the resinpolymerizes on irradiation in the Gammacell, theresulting temperature rise curve provides the induction dose (gel point), the dose at 50% cure, and anindication of the polymerization process (single ormulti-step curing process, slow or fast polymerization), From this information, one can calculate theelectron beam curing dose for the neat resin and thefibre-reinforced composite, using an empirical formula derived during the development of this method(see later).
For the calorimetry measurements (Fig. 5), theresin to be investigated is placed in a Pasteur pipetwhich has been sealed at one end. The pipet is placedin an insulated jacket which can hold up to 9 samples.Beside each resin sample a K-type (chromel-alumel)thennocouple is placed, approximately I em from thebottom of the sample tube. In every run, one of thesamples is a reference which consists of a cured resinin an identical vial, to monitor nonnal temperaturefluctuations in the Gammacell. The thennocouplesare wired into a multichannel datalogger which isconnected to a computer. The computer stores thevoltages from the thermocouples as a function oftime. Once the samples are setup, they are placed inthe Gammacell 220 chamber and lowered into theradiation field. As the resin polymerizes, heat isevolved, causing an increase in temperature, and thevoltage which is related to the temperature. When theexothermic polymerization reactions seem to havebeen completed, the samples are taken out of theGammacell. Analysis of the thermal data is carriedout using a spreadsheet program.
The signal from the reference resin is subtractedfrom that of the sample being investigated and is thenconverted to temperature. Figure 6 shows the temperature rise curve for S-297 (l,3-butylene glycoldimethacrylale), obtained by this method. From thecurve, two values are used to determine the electronbeam curing dose. One value is the gel point, the doseat which the temperature begins to rise. The secondvalue is the dose at which the maximum temperaturerise occurs. By applying equation (3), which we have
10" 0.'
E*,E'
10' 0.'
l10'En
0.'
t €Ii
110'
0.2 ~
10' 0.1
Curing dose determination
We have detennined the curing dose for resins andcomposites by three methods: (I) gamma calorimetry,(2) DMA, and (3) Barcol Hardness (ASTM D2583;ASTM, 1984). The first two methods can be used forboth resins and composites, while the third method isused only for composites.
Gamma calorimetry. We have developed an empir~
ical method to quickJy screen the curing dose of a
terials. We have used this method extensively, particularly for the composites and resins. The nonnalmode is to measure the flexural modulus as thetemperature is increased or cycled (Fig. 3; Rheometries, 1990; Mayer er al., 1990). The standard tool usedis the dual cantilever with zero stress and strain. Theresults obtained from the scan give flexural modulus,glass transition temperatures (tan lJ, loss modulus),and service temperature for the material being stud~
ied. The service temperature is defined as the temperature at which the flexural modulus is 50% of theroom temperature value.
Void content. Qualitative analysis of the void con~
tent is carried out by using the C~scan method(Henneke, 1987). This non-destructive method showsareas of porosity in the composites. By comparing theoverall attenuation of the signal from the componentbeing tested with the signal from a known samplewith zero voids, relative overall porosity of thecomponent can be determined. For more accuratedetennination of porosity our laboratory uses mercury intrusion porosimetry to detennine the voidcontent of composites. The instrument used is anAutopore II 9220 (Micrometries), which can measure4 samples at a time in 4 h. It gives the pore sizedistribution and the total void content for the composite being tested. Figure 4 shows a typical outputshowing the void size distribution for a compositesample. The instrument is capable of measuring poresizes between 360 and 0.003 micrometers in diameter.
derived empirically, one can determine the approximate dose required to cure the resin. Generally, thecomposite curing dose is about 30 kGy more thanthat for the neat resin, for unsized fibre. For sizedfibre, the curing dose is even higher (another 30 kGyor more, depending on the resin).
D,oo = DGP + 10(10,DTM - DCP» (3)
where DulO = dose at 100% cure, DTM = dose atmaximum temperature rise (50% cure), DGP = doseat gel point.
Table 1 shows a comparison between the estimatedcuring dose from gamma calorimetry and the actualelectron curing dose determined by DMA. With aslight modification to the sample chamber, thismethod can also be used to study the radiation curingof composites.
Dynamic mechanical analysis. For the determination of the curing dose, the flexural modulus andthe glass transition temperatures derived from theDMA curves are important (Fig. 3). The temperatures at the peaks are the glass transition temperatures, T,(E") and T,(b). This data allows us to plotthe T, values vs dose. The dose at which the glasstransition temperature plateaus is considered thecuring dose for the system being studied (Rheometries, \990). This method can be used to determine thecuring dose for both resins and composites. Themethod is viewed as one of the more accurate waysof determining the curing dose of a system. However,
100,---------------,..........
.l.o...:l£...~~~~__=-_:':--,Jo 2 4 • 10 12
-kGy
Fig. 6. Gamma calorimetry: temperature variation for 8-297(1,3-butylene glycol dimethacrylate), as a function of dose.
this method is time consuming because each specimenmust be analyzed separately. Depending on the T. ofthe specimen, each analysis can take up to 3 h tocomplete. Figure 7 shows a typical plot of glasstransition temperatures vs dose.
Barcol hardness. This method (ASTM, 1984;D2583) is a qualitative test to determine if a composite has been adequately cured. In this method, theBarcol Impressor is used to make measurements overa large area of the composite. Typically, one makes10-100 hardness measurements over an area of100-1()()() cm2, though it is easy to make an evenlarger number of measurements, over the desired areaof the composite. Normally, if the hardness readingis 65 or greater, the dose received by the compositeindicates that the part has been fully cured. Figure 8shows a comparison of Barcol hardness with Tg
values for FW3 acrylated epoxy composite as afunction of dose. For more accurate detennination,the hardness for each resin system employed at fullcure needs to be determined.
Resin shrinkage
As monomers and oligomers polymerize, shrinkageoccurs with the formation of chemical bonds. Thenormal Van der Waals spacing between molecules isapproximately 3.4 A. With polymerization, the spacing is reduced to approximately 1.34 A, the distancebetween the C-e atoms (Sadhir and Luck, 1992).The degree of shrinkage (density increase) depends onthe monomer size. The larger the original monomer,the smaller the overall shrinkage.
Shrinkage of the resin is one of the main contributors to internal stresses in composites. In thermallycured systems, it has been concluded that up to 70%of a resin's shrinkage will occur below the gel point(Luck and Sadhir, 1992). Below the gel point, theresin still has mobility to allow the resin to shrinkwithout incurring any internal stresses. Above the gelpoint, the resin becomes increasingly immobile andthe stresses from shrinkage are then locked in thestructure.
We have determined how the radiation-cured resinsystems behave as compared to the thermal-curedresins. Figures 9-11 show the effect of dose on the
Resin
5297C3000S604FW3BMl4
Electron processing of composites
Table I. Comparison of estimated with actual curing dose
Gel Dose at Estimated Actualpoint temp. max. curing dose curing dose(kGy) (kGy) (kGy) (kGy)
temperature rise and density changes for three resins;8-297 (1,3-butylene glycol dimethacrylate), 8-604(polypropylene glycol monomethacrylate) and BMI4(acrylated bismaleimide). The data show that increases in the density and temperature due to polymerization occur at similar doses. From other studies,the gel point (dose at which crosslinking starts)results show that the onset of the temperature riseand crosslinking occur at the same dose. This indi·cates that, as expected, the density starts to increaseas crosslinking begins. The maximum polymerizationand crosslinking as denoted by the highest densityvalues occurs at a much higher dose than the dose forthe maximum temperature rise. It has been determined, using gel fraction measurements, that themaximum temperature rise occurs at ...., 50% gelfraction point (Figs 12 and 13). Further irradiationwas required to achieve full polymerizationjcrosslinking of the resin. Comparjng the density andtemperature curves, we conclude that no significantshrinkage occurred below the gel point.
For the density measurements on the resins S-297,604 and BMI4, 20 ml of monomer was placed in glassvials and capped. The samples were irradiated in theGammacell at various doses. The dose rate was110 Gylmin. The densities of the liquid resin andcomposite samples were detennined by the ASTM0-792 procedures (A8TM, 1984).
Temperature rise during composite cure
As expected, there is also a rise in the temperatureof the fibre· reinforced acrylated epoxy compositesamples during radiation curing (Saunders et al.,1991, 1994). The actual temperature rise depends onthe resin being used, the resin loading, and thethickness of the composite. However, the temperature
Fig. 7. Effect of dose on the glass transition temperature forthe composite laminate with FW3 acrylated epoxy matrix.
rise can be reduced if the required electron dose isgiven in several fractions, allowing the sample to coolbetween the irradiations (Saunders et 01., 1991b). Inmore recent work on thick samples (up to 15 emthick; Saunders et 01., 1994), the temperature rise wasfound to be up to 132"C for a 2.5 em thick samplewhen cured with a single electron dose of 50 kGy;however, the temperature rise was much lower (40°C)on X-ray curing (which takes a much longer time dueto its lower dose rate and thus allows the heatgenerated to dissipate).
Our work has shown that multiple passes do notaffect the properties of the material. Figure 14 showsthe DMA curves for the same acrylated resin, curedto the same dose in a single pass and in multiplepasses. Figure l4{a) shows the curves where thecuring dose has been delivered in a single pass of70 kGy. Figure 14(b) shows the same resin with thedose delivered in 7 passes of 10 kGy each. In this casethe flexural moduli and glass transition temperaturesare identical for both methods of delivering the dose.
Dose rate effect
Dose rate affects the kinetics of free radical polymerization reactions (Chapiro, 1962; WiIliams, 1968).From a practical point of view, the higher the doserate, the lower the time of irradiation. Therefore, itis important to detennine whether the curing characteristics of the various resins that can be used as thematrix materials for fabricating composites are dependent on the dose rate. We have examined the dose
250 70~T.
~20080
I!
~ 50
f 150 40 J~
1'00 30 I~
20
50j '0e>
0 00.1 '0 100
Do.., kGy
Fig. 8. Comparison of glass transition temperatures andbarcol hardness for the composite with FW3 acrylated
epoxy matrix.
164 Ajit Singh el ai.
20 7 50 10
6 6
" 15 '" " 40
'"j ~5 .. I 6 ..
II: ,- oj 30 i! ! 010
~ 3()
~ ~f 20 • f1 ~ '-E 5 • & 0&{! {! 1.
••• ..• • 0 6 6 1. • • 3 0
~kOy ~kOy
Fig. 9. Gamma calorimetry: effect of dose on the temperature rise and density change of 8-604 (polypropylene glycol
monornethacrylate) resin.
rate effect on the curing characteristics of some of theresins used in our work. In the case of the epoxydiacrylate based formulations, it was found that toreach a similar gel fraction, a much higher dose wasneeded on gamma irradiation, than on electron ir~
radiation (e.g. 8 vs 60 kGy, for 90% gel fraction inC3000; Saunders et al., 1991c). However, in the caseof S-297 and S-604, the dose required for gammairradiation is lower, as shown by the data in Figs15-19. In the case of BMIl, the gel fraction of theneat resin is not affected by the dose rate; however,the gel fraction formation in the composite is moreefficient on electron curing (Fig. 20). As mentionedearlier, at lower dose rates, the temperature rise in theirradiated samples is lower (Saunders et al., 1994).
Crosslink density
The crosslink density (v) and the average molecularweight between crosslinks (M~) can be estimatedfrom the equilibrium elastic modulus (G;) of thecured resin. This modulus is assumed to be theminimum value of the flexural modulus for thepolymer, above its Til' According to the kinetictheory of rubber elasticity (Ferry, 1970)
G; =gnRTv =gnRTp/M~ (4)
35 16
30 10P ,. '"I 25
~ f'- ,.i
206 ()
15
i5E 10{! 0 &
5 2
•0 • 0 6 6 ,.DoH,kGy
Fig. 10. Gamma calorimetry: effect of dose on the temperature rise and density change of S·297 (1,3-butylene glycol
dimethacrylate) resin.
Fig. II. Gamma calorimetry: effect of dose on the temperature rise and density change of BMI4 (acrylated bis
maleimide) resin.
where G; = equilibrium elastic modulus (dynes/cm2 ).
gn = a numerical factor (1.0083), R = gas constant[8.31 x 10' dynes/(cmKmol)], T~absolute temperature (K), v = crosslink density (mol/em'),p = resin density (g/em'), M, = molecular weight between crosslinks (glmol). The crosslink densities andthe molecular weights between the crosslinks can alsobe determined for the composites of these resins(Pater et aI., 1991) by using the following formula:
T.= T.. +k,v ~ T.. + k,M;' (5)
where Tg = glass transition temperature, TgO = a con·stant, k[ = crosslink density constant. k2 = molecularweight constant, v = the crosslink density in crosslinks per emJ
• M~ = the molecular weight betweencrosslinks.
From the plots of the T"
using tan(e5), vs crosslinkdensity or molecular weight between crosslinks, thecrosslink constants for the resin can be derived andthose for FW3 are given in Table 2.
Radiation effects on FW3 resin and its composites
The data on the effect of electron treatment on thegel fraction, the Til and the molecular weight distribution of the extractables from FW3 resin, cured todoses ranging from 0.1 to 1000 kOy is shown in Fig.
120 so
1000 ()
'" • •<! ao j0
~so ..
... ao t.. 20co'0 !.0 10
0.10
10 100DoM,kGy
Fig. 12. Comparison of temperature rise with gel fraction ofthe gamma irradiated S-604
Electron processing of composites 165
120 3'
'5030.. P
~ .0 j~ 30 II:
J '0 I!".. !
& •• S.10
~30 ,•0.1 10 ...
Dooe,kOy
Fig. 13. Comparison of temperature rise with gel fraction ofthe gamma irradiated C-3000 resin.
'00.1 kGyJh
•• +10 kGylh
", * 5400 IcGyJh
f,.4.
;I2.
••1 ,. '00
Fig. 15. Dose rate effect on the gel fraction of the S-297(1,3-butylene glycol dimethacrylate) resin.
Fig. 16. Dose rate effect on the gel fraction of the 8-604(polypropylene glycol monomethacrylate) resin.
about 100 kGy. The molecular weight curves areautomatically expanded by the data acquisition software to 100% height; their total amount is equal tothe Sol level (IQO-gel fraction %), at the noted dose.The molecular weight of the extractables is constantabove 100 kGy.
Figure 22 shows the effect of electron treatment onthe gel fraction, the T. and the molecular weightdistribution of the extractables from the AS4 fibre-reinforced composite samples made with FW3 as thematrix resin, cured to doses ranging from 0.1 to1000 kGy. The maximum T. for the FW3 composite(using sized AS4 fibre), based on the tan ~ curve, isabout 185°C, occurring at a dose of about 300 kGy.The gel fraction of the matrix polymer at this dose is-99%, The dose of 300 kGy, using X-ray treatment,also produces a gel content in the matrix polymer of- 99%. The SEC data shows that the molecularweight distribution of the extractables is continuallychanging, up to the maximum dose studied(1000 kGy), although the changes between 200 and1000 kGy are minor.
The crosslink density (v) and the average molecularweight between crosslinks (Me) were also estimatedfor the FW3 polymer, from the equilibrium eJasticmodulus (G;), as described above. A plot of theflexural modulus of the neat polymer, as a functionof temperature and dose (Fig. 23), shows that the G;varies from 6 x J01 to J x 108 Pa over the dose range
'0.
•
10
'00
"'1 kGy/h8. +10 kOyJh
", *1400 kGylh
f8.
4.a2.
•.1
10 o.eE" (0)
8 0.&
1 E" 004<if 8~ 0,3 ~I 7 ~
J 0.2
8 Ton(a) 0.1
• 0.020 50 100 140 180 220 :!GO
Tlmpermure, ac
10 0.'E" (b)
80.&
1 E" OA
1 80.3 {t ..
7
J OJ!
• Ton(a) 0.1
• 0.020 50 100 140 180 220 280
Tempe....ure,OC
21. The maximum Tg for the FW3 polymer, based onthe tan lJ curve, is about 190°C, occurring at a doseof about 150 kGy. The gel fraction curve shows thatthe maximum gel content, greater than 99%, is alsoreached at a dose of about 150 kGy. The SEC datashows that the molecular weight distribution of theextractables is continuaUy changing, up to a dose of
Fig. 14. Comparison ofdelivering the curing dose in a singleor multi-pass protocol, to a proprietary acrylated epoxyresin: (a) 70 kGy in a single pass. (b) 70 kGy in seven passes(10 kGy each)
Ajit Singh et aI.
DoN,kGy
Fig. 17. Dose rate effect on the gel fraction of the CN964(acrylated urethane) resin.
'··1r====~------7.;=*=+
00
'"f ..&
70
000.1 10 100
DoN, kGy
I ••
-1 kQylh
+10 kQy/h
*1400 kQylh
I •
166
I ••
I.
'"15 I.
I 4.&
2.
••1
of 3()-1000 kGy. At room temperature, the typicaltlexural modulus for the FW3 polymer is about2.52 GPa. The density of the cured resin is1.237 gfem'. Figure 24 shows a plot of the crosslinkdensity of the FW3 polymer as a function of theelectron dose. The crosslink density increases withdose, reaching its maximum of about2.5 x 10-' mol/em' at a dose of 100-300 kGy. Thedata in Fig. 24 shows how the molecular weightbetween the crosslinks of the FW3 polymer varieswith dose. The minimum molecular weight betweenthe crosslinks is about 500 Dalton. The molecularweight between the crosslinks for the neat resin andthe composite do not show any difference. In the caseof FW3, the carbon fibre does not appear to affect thecuring of the resin.
Figure 25 shows the data on the flexural modulusof the composite with the FW3 resin, as a function ofdose and temperature. The typical flexural modulusfor the composite is about 35 GPa, at room temperature, with the elastic modulus varying from about14-20 GPa, depending on the dose (3()-1000 kGy).
Effect offibre sizing on the curing dose
We have found that the sizing on the fibre has asignificant effect on the curing dose for the acrylated
Fig. 19. Dose rate effect on the gel fraction of the C-3000(epoxy dicrylate oligomer) matrix in a carbon fibre-re·
inforced composite.
resins. The fibres we have used are from Hercules andcome with a G sizing on them. This sizing may be anuncured epoxy resin. We have found that with theFW3 acrylated epoxy or BMI4 acrylated bismaleimide, the dose required to cure is approximately170 kGy. When the same resins are used with unsizedfibre, the dose to cure is approximately 70 kGy. Thesizing appears to interfere with the free radical curingof the matrix; similar interference was also seen whenaramid fibre-reinforced composites were electroncured (Saunders et al., 1991c); however, in the case ofthe aramid fibre, the organic fibre itself appears to bea free radical inhibitor.
Effect ofa surface coupling agent on composite mechanica� properties
Surface treatment of the carbon fibre can play animportant role in achieving acceptable bondstrengths. The treatment of the fibre surface can takemany foons. A coupling agent may be used toenhance chemical bonding between the fibre and theresin. We used a proprietary chemical, an acrylatedisocyanate, which was applied at a I% loading using
• • 1.0 •• "- • o.•
o ."0
0.8 . ,• ....- •'" • .."
f..
10.8.. ,
•&
4. • ;1 0.'
2. 0.2
• O,~, 10 100 1000••1 I. I •• Dooe, kGy
Do.., kGyFig. 20. Gel fraction of BMII on irradiation as neat resin,
Fig. 18. Dose rate effect on the gel fraction of the C-3000 and as matrix in AS4 plain weave unsized fibre-reinforced(epoxy diacrylate oligomer) resin. composite.
Electron processing of composites 167
Table 2. Crosslink constants for electron cured resins
a solvent method. A heating step was used to chemically bond the isocyanate coupling agent and activesites on the fibres. The acrylated resin was thenapplied and cured with a dose of 50 kGy.
Scanning Electron Microscopy (SEM) micrographs for the untreated and treated. fibre surfacesafter fracture have been published (Lopata et al.,1994). The fibres with no coupling agent are clean,showing no adhesion between the fibres and the resin.On the other hand, the fibres treated with the coupling agent have resin adhering to them, illustratingthe enhanced bonding between the fibre and the resinmatrix.
The use of a coupling agent greatly increases themechanical properties of the composite. Table 3shows a comparison of the mechanical properties ofthe composite samples made from the sized andunsized AS4 fibre with CNI04 as the matrix resin.The use of the coupling agent with the sized fibregives the most significant increase in the mechanicalproperties. It was expected that the unsized fibrewould have given the same increase in mechanicalproperties, which was not shown by the results. Thereason for the difference between the sized and unsized fibres can be attributed to the number of activesites on the fibre with which the coupling agent canreact. The sizing on the fibre most likely protectsthese active sites. The use of acetone as a solvent toapply the agent may dissolve the sizing on the fibre
making the active sites available for reaction. In thecase of the unsized fibre. these active sites maydisappear over time due to exposure to the environment, making the application of the coupling agentineffective. Although the application of the couplingagent on the sized fibre gives more acceptable mechanical properties, the dose required to cure the composite is higher, by a factor of as much as 3. It ispossible that if the coupling agent is applied as asizing agent during the manufacturing of the fibre, itwould also protect the active sites on the fibre (orbond with them); the curing dose for the resultingcomposite would then be much lower, and the composite fonned may show better mechanical properties.
Effect of void content on mechanical properties
We have tried to reduce the void content of thee1ectron--cured composites. The sizing was removedby treating the fibre with hot chromic acid. washingwith distilled water and drying in an oven at ISaaCfor 24 h. An additional step was added in the nonnalfabrication of the composites with FW3 or CNI04 asthe resins. The vacuum-bagged composite was heatedto 80a C for 4 h to reduce the viscosity of the resin. At80aC the viscosity for FW3 resin is 0.6 Pa comparedwith 900 Pa at room temperature. The excess resinwas allowed to bleed into a breather cloth. The voidcontent for these composites was lower. This wasdetermined by C-scan (Henneke. 1987) which showedless voids. The void content is -4%, compared toprevious panels whose void content was up to 8%.The mechanical properties obtained for these panelsare shown in Table 4. As the data shows, the mechanical properties have improved with the reduction ofthe void content. Further improvement of the mech-
°l"IG..~-I"'-~~10"'-~~I:':OO:;;-'"~U;I~OOOo-,kGy
• Ton(a)• Loa. Modulu.i:
sr:J 1.0I-----~~----+..
0.8
Fig. 21. Effect of dose on the gel fraction, glass transition temperature, and molecular weight distributionof the THF extract of the electron-cured FW3 acrylated epoxy resin.
1000 10 100,000Molecule' Weight
..'
. "
Ajit Singh el 01.
I
Doo<l~kGY 100
0.00.1
OJI
Pi :: rl_"_Loss_-=--=-M_od_U_IU-,O• Ton(l)
!'1O1'40'! 120
.,001.0 f-------::..-..--::::....- ........
168
Fig. 22. Effect of dose on the gel fraction, glass transition temperature, and molecular weight distributionof the THF extract of the electron-cured FW3 acrylated epoxy matrix in a carbon fibre-reinforced
composite.
10",.------------------, anical properties should occur with the use of thecoupling agent, which is planned.
Fig. 23. Effect ofdose (in kOy, shown by the numbers besidethe curves) on the flexural modulus of the FW3 acrylatedepoxy resin. The curves for 100--500 kGy are very close to
each other.
CONCLUDING REMARKS
During our work on electron curing of carbonfibre-reinforced composites. we have identified various advantages offered by this technology anddemonstrated some of the key advantages, e.g. reduced stress. The empirical method for curing dosedetermination, developed by us. enables a rapidscreening of the curing dose required for a new resinor formulation. The temperature rise during electroncuring can be kept low. if required. either by fractional irradiation or by converting the electron beaminto X-rays and curing the samples with X-rays. Onthe whole. our work on these and other aspects. e.g.
10 4 10'12
1E10 4 10'f ()
c
j§ IJ 10~ 10'E... f110 4 10' ~()
j10 4 10'
0.1 1 10 100 1000o-,kGy
Fig. 24. Effect of dose on the crosslink density and molecular weight between the crosslinks for the FW3 acrylated
Fig. 25. Effect of dose in kGy (given by the numbers besidethe curves) on the flexural modulus of the carbon-tibre-reinforced composite with the FW3 (acrylated epoxy) matrix.
The curves for 150-500 kGy are very similar.
•
Electron processing of composites
Table 3. Effect of acrylated isocyanate coupling agent on the mechanicalproperties of electron-cured composite with the CNI04 resin matrix
Sized fibre Unsized fibreMechanicalproperty No primer Primer No primer Primer
dose rate effect, void content and use of a couplingagent, should help provide better understanding ofthe electron curing technology and thus help itscommercialization.
Acknowledgements-The financial support of part of thiswork by the Defence Research Establishment Pacific, Victoria, BC, is gratefully acknowledged. We thank Professor J.Silverman for helpful comments on the manuscript. We alsothank AECL Accelerators, the manufacturers of theto MeV electron accelerator (IMPELA), for their interestand generous support of this work.
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Barnard J. W. and Wilkin G. B. (1987) Safety considerations for a general purpose industrial accelerator facility.Proc. 20th Midyear Topical Symp. Health Phys. Soc.Health Physics of Radiation Generating Machines, Reno,Nev.
Beziers, D. and Capdepuy, B. (1990) Electron beam curingof composites. Proc. 35th Int. SAMPE Con/., p.1221.Anaheim, Calif.
Brynjolfsson A. and Martin T. G. III (1971) Bremsstrahlungproduction and shielding of static and linear electronaccelerators. Toxic gas production, required exhaust ratesand radiation protection instrumentation. Int. J. Appl.Radiat. Isot. 22. 29.
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