Assessment of the melt flow of injection-moulding phenolic plastics

Transcription

1 Plasticheskie Massy, No. 7, 2004, pp Assessment of the melt flow of injection-moulding phenolic plastics I. D. Simonov-Emel yanov, N. L. Shembel, V. V. Lisitsa, L. G. Lyutova, and D. Yu. Kuznetsov M. V. Lomonosov Institute of Fine Chemical Technology, Moscow Selected from International Polymer Science and Technology, 31, No. 10, 2004, reference PM 04/07/06; transl. serial no Translation submitted by P. Curtis Interest has recently been shown in injection-moulding thermosetting plastics and electrical engineering articles produced by injection moulding. Injection-moulding phenolic plastics fall into the category of multitonnage polymer composites. The combination in these materials of valuable mechanical, physicochemical, and processing properties enables them to be used for a wide range of general- and special-purpose electrical engineering parts [1]. The base grades of injection-moulding phenolic plastics O250 (O SK) and O251 (O SKT) produced by the TOKEM Joint Stock Company according to TU [2] largely have fair processability, but, in the factory production of thin-walled articles of complex configuration, considerable difficulties are being encountered and the volume of substandard production is rising. There are three reasons for this: the inadequate flow and time of the viscoplastic state of the injection-moulding phenolic plastic, the worn-out state of the injection moulding machines and moulds, and the low level of organisation of the production of articles. For the manufacture of thin-walled parts of complex configuration, mould filling, and the production of highquality parts, the melt flow of the phenolic plastic is particularly important. It is determined by the composition of the phenolic plastic and by the physical and chemical processes taking place in the material under the action of temperature, pressure, and shear strains. At a prescribed processing temperature, the melt flow of the phenolic plastic changes with time. To begin with, as the material is heated up, its viscosity decreases and its flow increases. However, simultaneously with heating and the increase in temperature in the material, chemical reactions occur that result in the formation of a three-dimensional structure, and here the viscosity increases while the flow decreases. The two competing physicochemical processes lead to the formation of maxima on the dependence of the melt flow of the phenolic plastic on the heating time at constant temperature and on the dependence of flow on the temperature with an identical heating time. The construction of the article, the injection moulding system, and the equipment and the injection moulding parameters also affect the flow of melts in the mould and its filling. The flow of the injection-moulding thermosetting plastic should be sufficient for the complete filling of the given injection mould within an established time. The level of flow and the time of the viscoplastic state should ensure the production of a high-quality article in the specific production process within the time of the moulding cycle. The higher the flow and the greater the time of the viscoplastic state of the thermosetting plastic melt, the easier it is to organise the process of producing high-quality articles. To assess the processing properties of the thermosetting plastics, use is made of different standard and non-standardised methods of capillary and rotation viscometry, the disc deformation method, the moulding of typical articles, etc. [3]. In the work, the processing properties of homeproduced phenolic plastics of grades O250 and O251, and also of imported material produced by the Bakelit company in Germany, were assessed from the following indices: the time of the viscoplastic state and the coefficient of viscosity at a temperature of 120 C T/26 International Polymer Science and Technology, Vol. 32, No. 1, 2005

2 Table 1. Viscoplastic properties of injection moulding phenolic plastics Grade of phenolic plastic Time of viscoplastic state, s Coefficient of viscosity, MPa s O250 90/ /0.01 O251 70/ /0.01 Imported phenolic plastic Note. Numerators experimental data; denominators values of time of viscoplastic state (minimum) and coeffi cient of viscosity (maximum) according to TU and a shear rate of 15 s 1, which were determined from GOST (form A) on a Polimer R-1 rotation plastometer. The data are given in Table 1. The results given in Table 1 indicate that homeproduced grades of phenolic plastic are inferior to the imported material, both in the time of the viscoplastic state and in the magnitude of the coefficient of viscosity, which characterises them as melts of lower flow that present difficulties in the manufacture of thin-walled injection-moulded articles of complex configuration. To assess the melt flow of injection-moulding grades of phenolic plastic directly under injection moulding conditions, the present authors, together with the Kashin Electrical Apparatus Works (Kashin), used the spiral injection moulding procedure developed earlier for thermoplastics at the Department of Plastics and Polymer Composites Processing Technology of the Lomonosov Institute of Fine Chemical Technology, Moscow [4]. According to this procedure, the melt flow of an injection-moulding thermosetting plastic is assessed by two principal parameters: the length of the spiral and the value of the limiting shear stress τ m on the wall of the spiral injection mould on cessation of melt flow of the material in the mould at prescribed (constant) values of injection pressure and mould and material temperatures. In the absence at factories of expensive instruments for assessing the melt flow of injectionmoulding thermosetting plastics, the given procedure makes it possible to obtain reliable data on the flow of any materials directly on the injection moulding machine under real injection moulding conditions using a modified spiral mould. The data obtained take into account all features of injection moulding, which distinguishes the given procedure favourably from methods using different instruments on which it is not possible entirely to model the conditions and behaviour of the melt during flow in the injection mould. Calculation formulae are given below to determine the value of the limiting shear stress on the wall of the spiral mould on cessation of melt flow into the mould without allowance for (1) or with allowance for (2) the process of curing of the material during filling of the mould: e τ m = PR 1 MPa 2 L, (1) or 05. PR { 1 ( t / t ) }, MPa 2L 1 e inj cur τ m = From the value of the limiting shear stress on the wall of the spiral mould, as calculated by means of equation (1) or (2), it is possible to determine the pressure losses during filling of any mould, ΔP mould, with a known maximum length of the melt flow path within it, L max, and thickness of the article, h, by means of the formulae P or P mould L = 2 max τmax, MPa h 2L τ max max h 1 ( t / t ) = mould { 05. inj cur }, MPa where P 1 is the pressure of the material at entry into the spiral mould (MPa), R e is the equivalent radius of the spiral (0.25 cm), L is the length of a good-quality spiral (cm), and t inj and t cur are respectively the injection time and the curing time of the material (s). The value of t max for O250 grade injection-moulding phenolic plastic, determined by the given procedure, amounted to 0.25 MPa, which makes it possible to calculate the pressure losses during filling of the injection mould for any article. A shortcoming of the given procedure can be considered to be the comparatively large consumption of materials, time, and energy resources for conducting the experiments, which is holding back its application in the development of new grades of injection-moulding phenolic plastics and in the carrying out of investigations. The need to create competitive high-flow injectionmoulding phenolic plastics for thin-walled electrical (2) (3) (4) International Polymer Science and Technology, Vol. 32, No. 1, 2005 T/27

3 engineering parts of complex configuration in comparison with foreign materials required the development of a rapid laboratory method for determining the melt flow. To this end, we used a standard IIRT constant-pressure capillary viscometer that can be employed to determine the melt flow index (MFI) of thermoplastics. To determine the optimum conditions and the parameters for conducting the experiments from assessment of the melt flow of injection-moulding phenolic plastics, the load was varied from 5 to 25 kg, the cylinder temperature was varied from 100 to 150 C, and the time of preheating of the material in the cylinder from the start of loading to the start of melt outflow (plug removal) was varied from 20 to 150 s. The flowrate of the material through a standard capillary of 2 mm diameter and 8 mm length was determined by sectioning the extrudates every 20 s from the moment of plug removal until practically complete cessation of melt flow. The melt flow was assessed from the mass of extrudate flowing out within 20 s, and the dependence of melt flow of the phenolic plastic on the heating time in the cylinder of the instrument at constant temperature was plotted (Figure 1). Figure 1 shows typical material flow heating time curves obtained on the IIRT instrument. The higher the temperature, the earlier and the more pronounced is the flow curve maximum due to the physicochemical processes occurring in the phenolic plastic melts under the action of temperature with time. Figure 2 shows the dependence of the melt flow of phenolic plastic O250 on the weight of the load at 120 C and a preheating time of 90 s. It was established that, with increase in the weight of the load over 18 kg, the flow begins to be determined principally by effects of slippage of the phenolic plastic melt (change in the mechanism of flow from volumetric to plug) in relation to the capillary walls and varies little (it does not exceed 10%). Therefore, a test load weight of kg (ram weight kg and two loads each weighing 9.1 kg) was adopted as the optimum. Figure 3 gives the time dependence of the melt fl ow of phenolic plastic O250 at 120 C and different preheating times. Increase in the preheating time in the cylinder under static conditions from 20 s (curve 1) to 60 s (curve 2) leads to displacement of the curves in time and to higher-quality heating of the material, which is accompanied at the initial instant of time with an increase in fl ow, and then with a decrease in fl ow as a three-dimensional structure is formed and curing of the phenolic plastic takes place. The maximum fl ow is achieved at a preheating time of 90 s (curve 3), and at 120 s (curve 4) and 150 s (curve 5) a reduction in fl ow is observed. Figure 4 shows the dependence of maximum melt fl ow of phenolic plastic O250 on the preheating time at a temperature of 120 C, from which it follows that the fl ow has a maximum value after Figure 1. Influence of heating time on melt flow of phenolic plastic O250 at different temperatures Figure 2. Dependence of maximum melt flow of phenolic plastic O250 on magnitude of load at 120 C and preheating time of 90 s Figure 3. Time dependence of melt flow of phenolic plastic O250 at 120 C at different preheating times: 1 20 s; 2 60 s; 3 90 s; s; s heating for 90 s. On the basis of the results obtained, the preheating time amounted to 90 s. Figure 5 shows the dependence of the maximum melt flow of phenolic plastic O250 on the cylinder temperature at a preheating time of 90 s, from which it can be seen that the maximum flow is observed at 120 C, which is T/28 International Polymer Science and Technology, Vol. 32, No. 1, 2005

4 Figure 4. Dependence of maximum melt flow of phenolic plastic O250 on preheating time at temperature of 120 C Figure 5. Effect of temperature on melt flow of phenolic plastic O250 at preheating time of 90 s Figure 6. Dependence of melt flow of phenolic plastics on preheating time at temperature of 120 C: 1 O251; 2 O250; 3 imported in good agreement with data of other methods and is sufficient ground for recommending a temperature of 120 C as one of the main conditions for comparative assessment of the melt flow of phenolic plastics by the method proposed. The increase in flow with increase in temperature from 100 to 120 C is due to the more complete softening and melting of all components of the phenolic plastic (resin, lubricants, plasticisers, modifying additives), and the reduction in flow with further increase in temperature is due to the rate of the chemical reaction of curing, the rate constant of the chemical reaction, the formation of a three-dimensional structure, and curing, which increases in intensity with increasing temperature. We determined experimentally the optimum parameters (conditions) of assessment of the flow of injection-moulding phenolic plastics on the IIRT capillary viscometer: load kg, preheating time of material in cylinder 90 s, cylinder temperature 120 C, extrudate discharge time 20 s, standard capillary of 2 mm diameter and 8 mm length. Figure 6 shows the dependence of the melt flow of the phenolic plastics investigated by us. The given data are in good agreement with results obtained earlier by other independent methods, for example on a PPR-1 rotation viscometer and given in Table 1. Thus, the minimum time of the viscoplastic state (70 s) and the maximum coefficient of viscosity (0.008 MPa s) are possessed by phenolic plastic O251 (curve 1). The imported material is characterised by the maximum time of the viscoplastic state (120 s), the minimum value of the coefficient of viscosity (0.004 MPa s), and the maximum melt flow (curve 3). Phenolic plastic O250 occupies an intermediate position with respect to all the indices investigated (curve 2). Using the given procedure it was established that the home-produced grades of injection moulding phenolic plastics, O250 and O251, are actually inferior in the most important processing parameters to imported material (Germany), which leads to difficulties in their processing by injection moulding into thin-walled articles of complex configuration. Thus, a reliable, convenient, and cheap procedure has been developed for assessing the melt flow of injection-moulding phenolic plastics on an IIRT capillary viscometer, by means of which reliable data are obtained. It enables a complex assessment to be made of the melt International Polymer Science and Technology, Vol. 32, No. 1, 2005 T/29

5 flow as one of the most important processing indices of injection-moulding phenolic plastics, reflecting the viscoplastic properties and the kinetics of curing, and a study to be made of the influence of different processing factors and modifying additives with minimum feedstock consumption (no more than 5 g per experiment). REFERENCES 1. Phenolic plastics. Catalogue. Cherkassy, 1987, 36 pp. 2. Product catalogue: phenol formaldehyde resins. Phenolic plastics. Textolite. Ion-exchange resins. TOKEM, Kemerovo, 10 pp. 3. V. P. Stavrov et al., Production tests of thermosetting plastics. Khimiya, Moscow, 1981, pp I. D. Simonov-Emel yanov, Procedure for determining melt flow of spiral injectionmoulding thermosetting plastics. IPTs MITKhT, Moscow, 2003, 15 pp. (No date given) T/30 International Polymer Science and Technology, Vol. 32, No. 1, 2005