PROCESSING OF LOW-VISCOSITY CBT THERMOPLASTIC COMPOSITES: HEAT TRANSFER ANALYSIS

Transcription

1 PROCESSING OF LOW-VISCOSITY CBT THERMOPLASTIC COMPOSITES: HEAT TRANSFER ANALYSIS Dr. Adrian Murtagh, Siora Coll and Dr. Conchúr Ó Brádaigh Comosites Research Unit Det. of Mechanical & Biomedical Engineering, National University of Ireland, Galway, Ireland ABSTRACT: Cyclic butylene terahthalate (CBT) thermolastics offer significant advantages in terms of comosite rocessing. Primarily, their inherent low viscosity means that fibre wet-out and imregnation can occur easily and raidly, offering otential reductions in rocessing time and cost. CBT comosites can offer the benefits of thermolastics, including toughness and recyclability, without the high viscosities normally associated with these materials. These advantages are articularly significant for the manufacture of largescale comosite structures, for examle, wind turbine blades. Existing technology using thermosets mean that it is getting more and more difficult to rocess large quantities of comosite materials cost-effectively. However, there are certain issues eculiar to CBT thermolastics that must be overcome before large-scale rocessing can occur. High temeratures and raid heating rates are unique to these materials when comared to traditional thermosets. The nature of the heat transfer mechanism for CBT materials heated this way is not very well understood. Previous exerimental work in this area has rovided some degree of understanding. The effect of different forms of material (re-reg and ower-imregnated dry glass fibres) has been investigated. The heating mechanism of the various material forms is discussed and analysed. The temerature-deendent thermal conductivity of the various material forms is quantified using a guarded comarative longitudinal heat flow aaratus. A one-dimensional transient heat transfer model is obtained which will assist in determining if the necessary rocessing conditions are met before CBT rocessing is carried out. KEYWORDS: Cyclic butylene terahthalate (CBT) comosites, resin film infusion, owder imregnation, comosite tooling, heating mechanism, thermal conductivity. 157

2 INTRODUCTION CBT TM (cyclic butylene terahthalate) is a relatively new material that has been develoed by Cyclics Cor. CBT is based uon using the cyclic oligomer form of PBT, a conventional engineering thermolastic. PBT s natural roerties mean that it has very good otential to be used in a range of thermolastic comosite alications [1]. Processing of CBT occurs when a catalyst is added to the material and when subsequently heated, the short-chain oligomer chains oen and bond together to form the long-chain olymeric PBT-form of the material. The inherent low initial viscosity (circa. 150 mpa.s), resulting from the use of shorter chain oligomers, means that it is eminently suitable for raid rocessing of thermolastic comosites. Previous alications for thermolastic comosites have been restricted by having to use much higher temeratures and ressures in order to wet-out reinforcing fibres using the much higher viscosity olymer-form of the thermolastic. CBT-comosites may be rocessed in a number of ways; however, two methods are most ertinent to the work that is discussed in this aer: resin film infusion / re-reg and owder imregnation [2] (Figure 1). Resin film infusion (RFI) uses continuous carbon or glass fibre reinforcement that has been re-imregnated with the CBT-oligomer form of the material rior to being rocessed. A secial catalyst is added to the CBT material that is not activated until a set temerature is reached. In essence, the CBT forms a resin film mainly on one side of the reinforcement fabric that only fully wets-out the fibres when secondary rocessing occurs at ressure. A more aroriate term to use would be semi-reg rather than rereg as only one side of the glass fabric is comletely coated with CBT during the alication stage. Plies may be stacked in a conventional manner and de-bulk significantly when heat and ressure are alied during rocessing. Powder-imregnated rocessing of CBT involves using dry continuous fibre reinforcements that are sandwiched together, with an intervening layer of CBT dry owder between reinforcement layers. The owder is laid down during the lay-u rocess. Although more difficult to handle, owder imregnation does offer otential in terms of cost reduction (no need for additional re-reg creation stage) and allows easier maniulation of the dry fabric layers as they are much more flexible comared to the boardy and rigid RFI material. The higher bulk density of the owder also means that there is a more significant amount of debulking and thickness reduction that occurs for owder-imregnated CBT. CBT resin film Reinforcement CBT owder layer Figure 1 Simle schematic of RFI and owder-imregnated CBT/glass fibre forms 158

3 A heat transfer analysis will be esecially imortant going forward for CBT rocessing in order to design aroriate heated tooling and to allow rediction of the temerature rofiles, heat transfer rates, etc. throughout the thickness of CBT-comosites. This will be essential for very large thermolastic comosites structures e.g. wind turbine blades, where thick root section thicknesses (greater than 100mm) and use of sandwich construction in the aerofoil sections greatly comlicates the heat transfer issue [3]. HEAT-UP EXPERIMENTAL ANALYSIS It is useful initially to carry out a direct comarison of 3 different material forms as regards heat-u characteristics during rocessing dry glass fibres (reference), RFI and owderimregnated glass fabric. To do this, a series of simle heat-u exeriments was carried out. This involved lacing a tyical 6-layer lay-u on the surface of a heatable metal laten within a vacuum bag, attaching thermocoules at different oints throughout the thickness and alying heat to a tyical set-oint of 195 C for CBT. The initial thickness of the dry glass fibre lay-u (using Saertex UD non-crim glass fabric, 950 gsm) when vacuum is alied was 4.5 mm. Results for the dry glass only are shown in Figure 2. As exected, the temeratures within the glass fibres lag the laten temerature by u to 20 C. However, the alication of vacuum ressure ensures that intimate contact is made between the glass fibre layers and heat transfer occurs quite efficiently by conduction. Figure 3 shows the heat-u curve for a CBT-resin film infused glass fabric lay-u using the same tye of glass reinforcement in terms of reinforcement tye (unidirectional E-glass, 950 gsm). The initial stack thickness is significantly greater than with the dry glass, aroximately 7.5 mm. This time, there is a much more significant temerature lag initially. The RFI layers are boardy and inflexible, even under vacuum ressure. Thermal contact resistance is much higher comared to dry glass due to the semi-rigid nature of the RFI form and non-intimate contact between the layers. U to 120 C, the temerature of the to layer lags the bottom layer by u to 40 C. However, as the temerature of the stack aroaches and asses 150 C, the heat transfer mechanism is significantly altered by a change in state of the CBT. It begins to melt rior to olymerisation taking lace at higher temeratures. This hase change increases the heat transfer rate suddenly as the layers come into much more intimate contact as the liquid form of CBT removes any gas between the layers and de-bulking occurs causing the thickness of the stack to reduce significantly. This sudden increase in heat transfer is seen as a kink uwards on the time-temerature curve at around 130 C. The kink is somewhat reversed between 140 C and 150 C (decrease in heat transfer rate) due to the latent heat of fusion effect as all of the CBT melts. Above 160 C, the through-thickness temeratures converge raidly until the difference is only aroximately 5-6 C at 190 C. 159

4 Temerature ( C) 4.5mm Insulation a b c Time (s) Figure 2 Heat-u of dry glass fibres Insulation Temerature ( C) 7.5 mm a b c Time (s) Figure 3 Heat-u of RFI/CBT-glass fibre layers For the owder-imregnated form, the glass fibre layers (unidirectional E-glass, 950 gsm) were laced on the laten, with the CBT owder sandwiched in between. The amount of CBT owder alied was the same amount as that used for the RFI-form in order to roduce a final glass fibre volume fraction of 50%. Five identical owder layers were alied between the six glass fibre layers. When vacuum ressure was alied, the much greater bulk density of the CBT-owder meant the initial stack thickness was 10.4 mm, a 35% increase in thickness. It might therefore be exected that the owder imregnated form would exhibit the oorest heat transfer erformance due to the greater thickness. The time temerature heat-u curve is shown in Figure 4. Again, the through-thickness temeratures lag the laten temerature significantly. However, comared to Figure 3 (RFI-form), the heat transfer erformance is significantly imroved. 100 C is reached at Thermocoule A (to of the stack) after aroximately 500s (8.3 C/min) comared to 720s for the RFI-form (5.8 C/min). This 160

5 may be attributed to a much greater intimate contact between the layers for the owderimregnated form, allowing heat transfer by conduction to occur more readily, desite the increased thickness. Again, the latent heat of fusion effect as the CBT melts is seen as a kink downwards in the sloe of the grah at about C. After this however, the temeratures converge raidly as heat transfer occurs more readily through the fully wettedout glass fibre layers. Temerature ( C) 10.4 mm Insulation a b c Time (s.) Figure 4 Heat-u of owdered CBT/glass fibre layers THERMAL CONDUCTIVITY MEASUREMENT The behaviour of CBT comosites is quite comlex as regards heating behaviour. It has been demonstrated how the resence of CBT (as either a resin film or owder) can initially insulate and revent heat flow, then as the CBT begins to melt, heat flow is increased significantly. A detailed analytical study incororating a non-linear model of hase change effects on the heat transfer is beyond the scoe of this work. Instead, an alternative aroach is taken in which an account is made of temerature-variant thermal conductivity with the lay-u during heating this is a reasonable assumtion to make if we assume that the lay-u stack including all of the glass fibre and CBT behaves as a solid continuum during heating. In order to measure thermal conductivity, an aaratus designed and built by Lee [4] was used. This aaratus uses the guarded comarative longitudinal heat flow technique to measure k values for different materials comared to a reference block of known conductivity. The design is based on a standard test method (ASTM E ). Although Lee only used his aaratus for measuring the thermal conductivity of dry fibre stacks, it was considered feasible to use it also for CBT RFI re-reg and CBT-owder imregnated glass fibre. 161

6 Aluminium late Reference material Secimen Reference material Heaters Thermocoules Insulation Vacuum bag Aluminium late V Figure 5 Schematic of thermal conductivity measurement aaratus A schematic of the thermal conductivity test aaratus is shown in Figure 5. The test secimen (in our case the CBT/glass fibre combinations) was laced between the reference blocks. In order to minimise contact resistance between the to and bottom layers and the reference material surfaces, a layer of thermally conductive aste was alied. A vacuum bag was alied circumferentially around the stack. When vacuum was alied, this had the effect of comressing the stack, similar to that seen on the heated laten from the revious set of exeriments. A set-oint temerature was secified for both the to and bottom heaters and a steady-state temerature gradient was allowed to develo over a eriod of time. The k value for the secimen material was calculated by comaring to a known conductivity for the Macor glass ceramic reference blocks at a articular temerature. Results are shown in Figure 6. The k values for the dry glass fibres (GF) were taken from [4]. The thermal conductivity for the RFI rereg and the owder imregnated glass was measured at 90 C and 120 C. Due to wattage restraints on the equiment, it was not ossible to measure thermal conductivity at higher temeratures. This is unfortunate as it is above 120 C that the thermal roerties of CBT begin to vary significantly as it changes from a solid to a liquid. Figure 6 Thermal conductivity for material forms 162

7 1-D TRANSIENT HEAT TRANSFER MODEL In general terms, the heating of a flat laminate with initial temerature T, thickness x, much smaller than its length and width, can be described as a simle 1-D heat conduction roblem. Certain assumtions need to be made such as constant temerature boundary conditions, homogeneous make-u of the temerature slab, initially uniform slab temerature, etc. For our uroses, it will be useful to have a model which correlates exerimental data with the redictive model using thermal conductivity data obtained in the revious section. The standard law of thermal conservation of energy alies and the heat transfer equation may be written as [5]: T ( c T ) = k Q ρ (1) t x x where ρ = material density, c is the secific heat, k is the thermal conductivity and Q reresents the heat loss due to the latent heat of fusion uon melting. It is assumed for the initial model that k is a function of temerature and that ρ and c are relatively constant with temerature. Also, we will only be modelling the heat transfer u to 120 C, rior to the hase change of the material, therefore Q can be considered to be zero. A more comrehensive model will be develoed as art of future work to incororate heating to higher temeratures. Eqn. (1) is a artial differential equation and may be solved using the finite difference technique: T t = k ( T ) 2 ρc x (2) Where ( T) = (T i+1 T i ) - (T i T i-1 ) = T i+1-2 T i + T i-1 δt = T j+1 T j, with i indexing across the node and j indexing over time. A numerical algorithm develoed by Pafko [6] was adated for use here. Written in Visual C++, thermal conductivity, density, & heat caacity can all be functions of temerature, time, and osition. Aroriate values were inserted into the algorithm for CBT using data obtained by exeriment and modelled using a simle curve fit (thermal conductivity) and from material data sheets and the literature for density and secific heat [7]. A fixed temerature boundary condition exeriment was carried out to hel verify the model. A stack of six CBT-RFI / glass fibre layers was inserted between two latens re-heated to 120 C. The temerature rofile through the thickness was recorded. 163

8 Temerature ( C) Time (s) Figure 7 1-D exerimental vs. model data The exerimental data is lotted against the model data in Figure 7. There aears to be good agreement for both the mid-oint node and also at a node one layer in from the laten surface. As stated reviously however, this model is only adequate for the initial heating stage u to 120 C and needs to be exanded to include hase change effects between C, and subsequently, the heat transfer in the melt hase u to 195 C, the recommended olymerisation temerature for CBT. REFERENCES 1. R. Eder. S. Winckler, Processing of Advanced Thermolastic Comosites using Cyclic Thermolastic Polyester, Proceedings of the 22nd SAMPE Euroe International Conference, CNIT Conference Centre, Paris, S.M. Coll, A.M. Murtagh, C. O Bradaigh, Resin Film Infusion of Cyclic PBT Comosites: A Fundamental Study, Proc. 25 th SAMPE Euro. Conf., Paris, D. A. Griffin, Comosite Technologies for Large Wind Turbine Blades, Sandia National Laboratories Reort, Blade System Design Studies Volume 1, C. Lee, An Investigation into the Transverse Thermal Conductivity of Fibre Beds, Ph.D. Thesis, University of Limerick, F.P. Incroera, D.P. De Witt, Fundamentals of Heat and Mass Transfer, John Wiley & Sons, W. Pafko, htt:// online article, U. I. Thomann, M. Sauter, P. Ermanni, A Combined Imregnation and Heat Transfer Model for Stam Forming of Unconsolidated Commingled Yarn Preforms, Comosites Science and Technologies, 64 (10-12,