Study of the curing of basalt-fibre-reinforced plastic based on a phenol formaldehyde binder

Transcription

1 Plasticheskie Massy, No. 5, 2009, pp Study of the curing of basalt-fibre-reinforced plastic based on a phenol formaldehyde binder I.D. Simonov-Emel yanov, I.P. Miichenko,* N.L. Shembel, A.S. Kuklin, and A.N. Trofimov M.V. Lomonosov Moscow State Academy of Fine Chemical Technology * MATI K.E. Tsiolkovskii Russian State Technological University simonov@mitht.rssi.ru Selected from International Polymer Science and Technology, 37, No. 3, 2009, reference PM 09/05/23; transl. serial no Translated by P. Curtis Abstract A study was made of the kinetics of curing of phenol formaldehyde binders (PFBs) in microplastics based on basalt fibre by mechanical spectrometry and in basalt-fibre-reinforced plastics based on chopped fibre by rotation viscometry. The possibility was shown of increasing the pultrusion rate of rods by reducing the degree of curing of the PFB to in the forming channel with subsequent heat treatment to increase the strength of basalt-fibre-reinforced plastic rods. Long profiled articles of reinforced plastics are being widely used in the building industry owing to their combination of valuable properties. To produce profiles in the form of rods, the pultrusion method is used [1]. The forming of articles by pultrusion leads to the creation of a structure reinforced in one direction (longitudinal reinforcement scheme), which makes it possible to obtain maximum tensile strength of composite materials. Reinforcing fillers in the form of strands and roving of glass and basalt fibres are used as initial components. Here, to manufacture reinforced rods, most suitable are fillers based on basalt fibres, which are characterized by a higher chemical resistance, elastic modulus, and thermal stability by comparison with glass fibre fillers. The chemical nature of the binder and the mechanism of its curing are the main factors determining the rate of the pultrusion process. Liquid resins are generally used as the binder polyester or epoxy resins containing the necessary modifying additives, or fibres of fusible (matrix) thermoplastic polymers are used. The forming of articles based on them proceeds without the formation of volatile low-molecular-weight products, which facilitates considerably the organisation of the pultrusion process and the design of the pultrusion units. The main disadvantage of pultrusion articles based on three-dimensional polyester and epoxy matrices is the insufficiently high level of working temperatures, which does not generally exceed C. For articles intended for operation at higher temperatures (short or long term), combined with other necessary functional properties, it is expedient to use materials based, for example, on phenolic matrices that are produced on the basis of binders that cure by the mechanism of polycondensation with the formation of low-molecular-weight reaction products and comprising solutions of phenolic resins of 55 80% concentration. When using solutions of thermosetting phenolic resins that cure by the reaction of polycondensation with the formation of low-molecular-weight products, after the stages of impregnation of the reinforcing filler by liquid binder, squeezing out of excess binder, preliminary heating of the roving to remove volatile products, and forming of the profile of the article by pulling it through a forming spinneret, an important stage of the process is the most complete removal of solvent and gaseous products and compaction of the workpiece to produce high-quality profiled articles. To this end, different profiling and vacuum devices, making it possible, by gradual heating, compaction, and repeated vacuum treatment of the material in the moving workpiece, to solve the problem of producing dense and strong articles, have 2010 Smithers Rapra Technology T/43

2 been proposed and used [2, 3]. After the drying stage, the formed and compacted workpiece arrives for the curing stage, where processes of removal of solvent residues, softening (conversion to the viscous flow state) of the binder, and its curing take place, with the formation of a polymer matrix of threedimensional structure. The pultrusion regimes are determined by the composition of the composite material: the temperature and time regimes are determined by the type of binder used, and the force and speed regimes are determined by the type of binder and reinforcing filler. The reinforcing filler primarily determines the force with which the workpiece is pulled through the forming spinneret, while the polymer binder determines indices of the production regimes such as the temperatures in the various zones of the pultrusion unit and the time spent by the workpiece at certain temperatures, i.e. geometric parameters of elements of the unit. In the study, the reinforcing filler used was roving of complex basalt threads with an elementary thread diameter of 13 µm and a linear density of 2520 tex on oil KV-02, with thread wound round and a number of twists of 60 per metre of roving, produced to specifications TU ). The roving was type NRB KV-02(0-60). The specific tensile load for roving with an elementary thread diameter of 13 µm amounts to at least 500 mn/tex (gf/tex). Roving treated with oil KV-02 is compatible with epoxy, phenolic, epoxy phenol, melamine, and acrylate resins. In the study, use was made of three main grades of phenol formaldehyde resins (PFRs): LBS-20, SFZh-301, and BZh-3, the properties of which are presented in Table 1. The kinetics of curing of binders of different nature and composition on basalt fibres was studied by mechanical spectrometry (free torsional oscillations). The viscoelastic properties of systems of microplastics based on basalt fibre impregnated with phenol formaldehyde binder were determined from the change in the mechanical loss tangent and mechanical loss modulus during heating of the system at the specified rate in the temperature range C, and also the kinetics of curing at specified temperatures (120, 140, 160, and 180 C) [4, 5]. The microplastics consisted of vol% binder and 65 70% fibre. Figures 1 to 3 show the temperature dependences of the mechanical loss tangent tg d of the reinforced plastic. On the curve there are several characteristic peaks that Figure 2. Temperature dependence of tg d: binder SFZh-301 Figure 1. Temperature dependence of tg d: binder LBS-20 Table 1. Characteristics of PFR ( Karbolit OJSC, Orekhovo-Zuevo) Resin grade SFZh-301 LBS-20 BZh-3 Batch number Solvent Ethyl alcohol Ethyl alcohol Water Viscosity, s 7/(2 12) Viscosity, MPa s 1460/( ) 1013/( ) Mass fraction of dry residue, % 84.9/ /(71 78) Conditions of determination: temperature 100 ± 3 C 100 ± 3 C time 45 min 2 h 20 min Mass fraction of free phenol, % <9.0/ 9.0 <10.0/ /(8 16) Mass fraction of free formaldehyde, % 0.7/ 2.0 Gelation time at 150 ± 2 C, s 77/(80 110) 123/( ) Mass fraction of water, % 12.3/ 19 Bakelisation losses, % (T = 180 ± 5 C, 1 h) 27.2/ 30 Alkali content, % 0.3 Note. Numerators actual quality indices of obtained resins; denominators standard quality indices according to current specifications. For LBS-20, the standard quality indices for higher-grade lacquer are indicated T/44 International Polymer Science and Technology, Vol. 37, No. 8, 2010

3 Figure 3. Temperature dependence of tg d: binder BZh-3 correspond to different physicochemical processes during the heating of PFR. Removal of the solvent is observed in the C range, and removal of water at 100 C. The curing of phenolic binders proceeds in several stages, which can be seen from the data in Figures 1 to 3. For binder LBS-20, curing begins in the C range and proceeds at maximum rate at C (with maximum tg d at 135 C). For binders SFZh-301 and BZh-3, the curing process is most intense at temperatures of C (most intense tg d peak), and completion of the process occurs at temperatures of C. The peaks on the tg d T curve at temperatures above 180 C indicate completion of curing processes and the start of thermo-oxidative degradation of the three-dimensional phenolic polymer. Figure 4 shows the temperature dependence of the mechanical loss modulus lg G for basalt fibre impregnated with resin BZh-3. It can be seen that, at 120 C, the modulus increases sharply. This indicates an increase in the curing rate of PFR under dynamic heating conditions, and at 160 C the modulus practically settles at a constant level. Small fluctuations in the modulus at temperatures above 160 C are connected with the occurrence of post-curing processes, and above 200 C by the start of processes of thermo-oxidative degradation of the three-dimensional PF polymer. Figures 5 to 7 show time dependences of tg d obtained at a constant temperature of 160 C, from which it can be seen that, to complete the process of curing of PFR, holding for min, depending on the composition of the binder, is sufficient. Thus, for a composition based on binder LBS-20, curing ends within min holding at 160 C; for a composition based on SFZh-301, within 20 min; for a composition based on BZh-3, within 15 min. In this way, as a result of the conducted investigations, the temperature and time regimes have been determined by mechanical spectrometry for the main stages of the pultrusion process: removal of the solvent; the curing process: the start and end, and the temperature of the maximum curing rate; the time parameters of the curing process at optimum final temperatures for each type of binder. Figure 4. Temperature dependence of lg G: binder BZh-3 Figure 5. Dependence of tg d on the heating time for binder LBS-20 at a constant temperature T = 160 C Figure 6. Dependence of tg d on the heating time for binder SFZh-301 at a constant temperature T = 160 C It was of interest to study the processes of curing of basalt-fibre-reinforced plastics on PFR by Kanavets method. To conduct the experiment, filled basalt-fibrereinforced plastics with a content of short basalt fibres 2010 Smithers Rapra Technology T/45

4 Figure 7. Dependence of tg d on the heating time for binder BZh-3 at a constant temperature T = 160 C Figure 8. Time dependence of stress. BZh-3 Figure 9. Time dependence of stress. LBS-20 Figure 10. Time dependence of stress. SVZh-301 of 60 vol% were produced. The composites were dried to remove solvent by established regimes. For all composites investigated, curing curves were obtained on a Polimer R-1 rotation plastometer (Figures 8 to 10). The viscosity values of systems and the time of the viscoplastic state were calculated, and the curing time and the time of total completion of the curing process under dynamic conditions were determined (GOST Plastics. Method for determining the viscoplastic properties of thermosetting plastics). The conducted investigations on the curing of basaltfibre-reinforced plastics showed that, for complete curing at a temperature of 160 C, a time of at least 15 min is necessary according to data of mechanical spectrometry, or 5 7 min according to the Kanavets curve. This difference is due to the fact that in the first case the curing is conducted without pressure in a thin layer of binder applied to the surface of the basalt thread, and in the second case the process is conducted in a closed mould under a pressure of 40 MPa, with a thickness of the formed specimen of 3 mm. The reaction of PFR curing proceeds with the release of heat, as a result of which the additional heating up of basalt-fibre-reinforced plastic occurs by an internal heat source, and the time of achievement of maximum strength depends considerably on the heating conditions and the thickness of the article. To produce articles of phenol formaldehyde plastics on presses, complete curing is practically never achieved, and the time of achievement of a shear stress of 5.9 MPa for pressed powders and of 3.5 MPa for injection-moulded phenol formaldehyde plastics according to the Kanavets curve, which amounts to of the maximum value, is taken as the curing time. During storage or service, curing of the polymer in articles continues, and this leads to a gradual increase in their physicomechanical characteristics. The speed of movement of the impregnated basalt rod will depend on its heating rate, the evaporation rate of the solvent and volatile products, and the curing time. These parameters are determined by the geometric dimensions of the heating and curing chamber of the pultrusion unit. It must be pointed out that, with the incomplete removal of solvent at the drying stage, foaming of the material (above 140 C) and the formation of a porous structure may occur at the stage of curing at high temperatures. Therefore, at the start of the chamber, the temperature should be no higher than C for completion of the process of solvent removal, and then it is raised to the specified curing temperature values, and curing of the binder occurs without foaming. On the basis of the data obtained concerning the curing time of resin BZh-3, on an industrial unit with a forming chamber of 3 m length the pultrusion rate should be no greater than 0.5 m/min. T/46 International Polymer Science and Technology, Vol. 37, No. 8, 2010

5 By preliminary calculations and subsequent experiments we established that, with a speed of movement of the impregnated roving of 1 m/min and a length of the curing chamber of 3 m, at 150 C a sufficient degree of curing of rods of 6 mm and 7.5 mm diameter is ensured: the tensile stress causing failure of reinforced rods amounted to over 750 MPa, and the bending stress at failure to 1050 ± 50 MPa. Thus, to increase the productivity of the pultrusion process, the curing time of the basalt-fibre-reinforced plastic can be shortened considerably, and it may amount to of the maximum established experimentally. Here, adequate strength of a rod of round cross-section is achieved by curing a surface shell with a thickness equal to 0.3 of the radius. Subsequent heat treatment makes it possible to increase the strength of articles of reinforced basalt-fibre-reinforced plastics by 10 15%. REFERENCES 1. M.I. Terent eva and V.V. Il in, Manufacture of products from polymer composites by pultrusion, in Equipment, Economics, Information. Ser. Equipment and Technology, No. 1, 1985, pp SU , Cl. B 29 C 55/30, , Byull. No SU , Cl. E 04 C 5/07, , Byull. No S.V. Akimov and M.V. Barkova, Dynamic method for studying polymer systems. Plast. Massy, No. 9, 1969, pp P.G. Babaevskii (ed.), Practical Course on Polymer Material Science. Khimiya, Moscow, 1980, 256 pp Smithers Rapra Technology T/47