Blends and alloys based on polycarbonate

Transcription

1 Plasticheskie Massy, No. 11, 2003, pp Blends and alloys based on polycarbonate T.I. Andreeva, A.E. Chalykh and Yu. K. Godovskii Scientific Research Institute for Plastics (NIIPM), the Institute of Physical Chemistry of the Russian Academy of Sciences, and the L. I. Karpov Scientific Research Institute of Physical Chemistry Selected from International Polymer Science and Technology, 31, No. 4, 2003, reference PM 03/11/17; transl. serial no Translation submitted by P. Curtis INTRODUCTION Despite of the extensive grade range, some of the shortcomings of polycarbonate (PC) are not eliminated by traditional methods of modification through the introduction of stabilisers, lubricants, fillers, glass fibre, fireproofing agents, etc. These shortcomings are: sensitivity of impact strength to the thickness of the article and to the size and shape of the notch; reduction in impact strength as a result of thermal ageing and subzero temperatures; cracking under static load and in the presence of organic solvents; high melt viscosity. These shortcomings can be overcome, while retaining most of the advantages of PC, by mixing with other amorphous thermoplastics (ABS, ASA, SMA, MBS, PMMA) and crystallising thermoplastics (PE and its copolymers, PETP, PBTP, PA, PTFE, LCPs), and also with elastomers (acrylate, butadiene, and ethylene propylene rubbers). Depending on the position of the figurative point of the system in the temperature concentration field of the constitution diagrams, mixing of the polymers can be accompanied with their complete dissolution with the formation of a single-phase system or with their partial dissolution with the formation of a heterogeneous dispersed system. From the thermodynamic viewpoint, the behaviour of the blends is connected with change in the Gibbs free energy of mixing [1 4]. For a polymer blend, the expression for the thermodynamic potential of mixing G m has the form: ktv ϕ G V x L x L 1 ϕ2 m = nϕ1 + nϕ2 + χϕϕ 1 2 (1) s 1 2 where ϕ 1 and ϕ 2 are the volume fractions of the polymers, x 1 and x 2 are the degrees of polymerisation of the components, χ is the parameter of interaction, V is the total volume of the blend, V s is the molar volume of a monomer unit, and k is Boltzmann s constant. It can be seen that the thermodynamic compatibility of the polymers depends considerably on the molecular weight, the ratio of the components, the parameter of interaction, and the temperature. A single-phase system (solution or alloy) is formed if the mixing process is accompanied with loss of the free energy of mixing of the system, i.e. G m < 0. The phase structure of the polymer blends, like the compatibility, is not a constant quantity for the given pair of polymers and depends on the selected ratio of components, the rheological behaviour of each of the components, the interphase interaction, and the mixing conditions. ALLOYS BASED ON POLYCARBONATE AND POLYALKYLENE TEREPHTHALATES (PATPS) Figure 1 presents the phase diagram of the alloy PC/PBT, plotted on the basis of a study of the concentration profiles of the distributions of components in the diffusion zones of mixing of the polymers, their formation, and change in the regimes of increase and reduction in temperature. From the diagram it can be seen that, with increase in temperature, the compatibility of the components is International Polymer Science and Technology, Vol. 31, No. 8, 2004 T/31

2 Figure 1. Phase diagram of PC/PBT alloy. Molecular weight of PBT: ; ; ; Molecular weight of PC Plotted by optical interferometry method improved. The upper critical solubility temperature (UCST) varies as a function of the molecular weight of the components from 220 to 300 C. The critical composition of the blend in accordance with equation (1) is determined by the ratio of molecular weights of PBT and PC. As follows from the diagram, at 260 C the alloy PBT/ PC = 80:20 is in the form of a two-phase melt, while 50:50 and 20:80 alloys are in the form of single-phase melts. This may lead to a situation where, with sudden cooling of the composite to room temperature, incomplete phase separation of the blend may occur, which manifests itself in a corresponding displacement of the glass transition temperatures of the homopolymers. Specific to PC/PATP blends are reactions of interchain exchange between the polymers being mixed, which occur in the melt, both in the production and in the processing of composites, and can change significantly the phase state of the binary blends and, consequently their properties. On the thermograms of heating of blends of PC (MW ) and PBT (MW ) in the concentration range 20:80 80:20, the following transitions were identified: two glass transition temperatures T g, a peak of cold crystallisation of PBT, and a peak of its melting (Figure 2). In the initial blends, one of the glass transition temperatures T g is slightly higher than the T g of the PBT (T g1 ) and changes little with composition, while the second lies in the region of glass transition of the PC (T g2 ). As the PC content of the blends decreases to 30%, T g2 decreases roughly by 20 K, and with further reduction in the PC content it again increases practically to the T g of the homopolymer. The Figure 2. Dependences of glass transition temperature (1, 2), melting temperature (3), temperature of cold crystallisation (4), and degree of crystallinity a (3, 4 ) of alloy PC/PBT on PBT content (differential scanning calorimetry) existence of the two glass transition temperatures indicates the heterogeneity of the system. PBT crystallises in the entire region of compositions investigated. Here, the degree of crystallinity 3 and the melting temperature 3 change little with increase in the PC content. Increase in the mixing time leads to a change in the dependences of the transition temperatures on composition. In a system with excess PBT there are two glass transition temperatures that are lowered with increase in the mixing time by comparison with the initial blend of the same composition. The melting temperature T m will also decrease sharply, but the degree of crystallinity hardly changes. In a 50:50 PC/PBT blend, after mixing for 30 min, the two glass transition temperatures are retained but T g2 is lowered by 40 K, while T g1 increases by 20 K compared with the initial blend. After 1 h, one T g is observed in the blend, which with further mixing hardly changes (Figure 3). The degree of crystallinity of PBT in 50:50 blends and with excess PC decreases with increase in the mixing time, and after mixing for 1 h the blends become amorphous. The pattern of change in the transition temperatures as a function of the composition of the blends at a mixing temperature of 260 C and a mixing time of 1 h is the same as for a mixing temperature of 280 C and a mixing time of 30 min. The data obtained indicate that increase in the mixing temperature is similar to increase in its duration. T/32 International Polymer Science and Technology, Vol. 31, No. 8, 2004

3 Figure 3. Phase and relaxation transitions of 50:50 PC/PBT alloy as function of mixing time at 260 C In the case of mixing in a melt, the occurrence of two processes capable of resulting in changes in the transition temperatures is possible, firstly the interchain interaction of polyesters, which leads to the formation of block and statistical copolymers, and secondly the degradation of the components, accompanied with a reduction in their molecular weight, on account of which there may also be an improvement in compatibility and interphase interaction (Figure 1). Figure 4 presents IR spectra of films of PC and a 50:50 PC/PBT blend as a function of the mixing time. In the IR spectrum of a PC film produced from a methylene chloride solution there is a band with a 1774 cm 1 maximum. An identical spectrum was obtained for a 50:50 PC/PBT blend after mixing for 5 min. Increase in the mixing time leads to the appearance of a new band in the spectrum of the soluble part of the specimen at 1716 cm 1 that is characteristic of C=O vibrations in PBT. With further increase in the mixing time, the intensity of the band with a maximum at 1716 cm 1 increases. With prolonged holding of a blend at 260 C, the specimen becomes completely soluble in methylene chloride. The IR spectrum of the film undergoes qualitative changes: the maximum of the band characteristic of PC shifts to 1762 cm 1 ; the frequency of the maximum of the carbonyl group of PBT, on the other hand, increases, reaching 1721 cm 1 ; a new band appears with a maximum at 1740 cm 1, characteristic of aromatic ethers. With increase in the melt temperature to 280 C, the rate of the reaction of interchain exchange increases appreciably. Holding of a 50:50 PC/PBT blend at a temperature of 280 C for 30 min is sufficient for the final formation of a block copolymer, while during holding of the blend for 1 h at 280 C a completely soluble statistical copolymer is formed. The rheological investigations confirmed the conclusion concerning the influence of the composition of the blend Figure 4. IR spectra of PC and 50:50 PC/PBT films: 1 PC; :50 PC/PBT: 1, 2 5 min; 3 30 min; 4 1 h; 5 6 h and the processing time on the kinetics of interchain exchange and its relation with the process of degradation of the components. Blends with excess PC are more stable compared with blends where the PBT phase predominates. It is the degradation of PBT that initiates the exchange reactions between PC and PBT. In a study of the thermal behaviour of a binary PC/PBT blend by isothermal thermogravimetry, it was established that the most tangible difference between the experimental and calculated values of the weight losses is observed with a PBT content in the blend of wt.%. Since processing is carried out in the temperature region where processes of dispersion (constant particle surface renewal), dissolution of particles by a diffusion mechanism, and chemical reactions between the monomeric units of PC and PBT occur simultaneously, it can be assumed that they are all realised within the diffusion zone arising in the matrix of the thermoplastic close to the particle. Figure 5 shows the proposed scheme of the structure of the transition zone of such a particle. Depending on the time and temperature, a different structure of organisation of the material can arise during processing. With short times, the dissolving particle exists as an independent phase formation, about which occurs a diffusion zone of mixing of components with the phase boundary at which a sudden change in concentrations is observed. The magnitude of this sudden change is determined by the composition of the coexisting phases, information about which can be obtained from the constitution diagrams. Cooling of such a system either fixes the morphology described or leads to an additional phase composition and a diffusion zone in accordance with the change in solubility with reduction in temperature. A prolonged residence time of such a particle in the International Polymer Science and Technology, Vol. 31, No. 8, 2004 T/33

4 Figure 6. Micrographs of cleavage faces of surface of fracture of 50:50 PC/PBT specimens with different mixing times: 1 5 min; 2 30 min; 3 60 min; min 80:20 PC/PBT alloy (transmission electron microscopy) Figure 5. Particle morphology and scheme of structure and transformation of phase particle at stage of displacement and cooling melt should lead to chemical reaction of the components and, consequently, to increase in the mutual solubility of the thermoplastics being mixed. Such a process can be referred to as chemical dissolution. It is evident that the process of dissolution is realised mainly in the diffusion zone, and growth of this zone is connected with diffusion within it of macromolecules of both components. Cooling of such a system fixes the chemical transformations, while the material is a dispersion of particles that is devoid of a phase boundary but has a developed transition zone. Within the framework of this model, the interaction of the components can be prevented not only by inhibition of the elementary reactions but also by introducing a component that acts as an encapsulating agent. Structural investigations showed that the introduction of PBT into PC leads to the appearance of dispersed particles, the size of which depends on the composition of the system. On specimens containing wt.% PBT, the phases are formed insufficiently distinctly, and each particle has about itself a zone of different chemical structure connected, probably, with the reaction of transesterification. In the region with a PBT content of 60 wt.% or more, a structure appears that is characteristic of dispersed systems and with which a dispersed PC phase is distributed in the matrix. In the middle region, structures of the matrix type are observed, i.e. two continuous phases (Figure 8). The mechanical properties of PC/PBT blends with change in the temperature and time of mixing depend on the composition since PBT is more prone than PC to thermal degradation. In the initial blends the elastic modulus is slightly higher than in the homopolymers and depends little on the ratio of the components in the blend. Increase in the PBT content in the blends leads to an increase in the breaking elongation compared with pure PC. The blends are characterised by a lower level of impact strength compared not only with PC but also with PBT. Increase in the mixing time and temperature leads to strengthening of the dependence of the mechanical properties on the composition, especially in the region rich in PBT. Table 1 presents the mechanical properties of a 60:40 PC/PBT blend as a function of the holding time of the material in the cylinder of the injection moulding machine: with increase in the holding time τ from 6 to 18 min there T/34 International Polymer Science and Technology, Vol. 31, No. 8, 2004

5 is a gradual reduction in the strength properties and heat resistance, and, with increase in τ to 30 min, brittle failure of the specimens occurs, both in tensile tests and in impact strength tests, as well as a considerable reduction in heat resistance. Table 2 presents data of mechanical tests on special specimens of a 60:40 PC/PBT blend as a function of the holding time in the injection moulding machine cylinder τ. As can be seen from the data presented, the change in the phase composition of the blend with increase in the heating time has an adverse effect on the properties of the blend in injection moulded specimens with a weld line. Thus, the need for inhibition of transesterification in PC/PBT mixtures is obvious. Inhibition of the transesterification process is observed when a number of alkyl, aryl, and alkylaryl phosphites are introduced [5]. The inhibiting effect of phosphorus-containing compounds is connected [6, 7] with blocking of residues of organotitanium catalysts by the formation of insoluble salts of phosphorus-containing acids. Furthermore, some organophosphorus compounds (for example, triphenyl phosphite) can act as a chain extender for PBT [8], which has a positive influence on the properties of the polymer blend. To achieve an inhibiting effect, use was made of organophosphorus compounds, which were introduced during the mixing of PBT and PC in a melt. Figure 7 presents data on the change in the content of gel fraction in the initial and in the stabilised 60:40 PC/ PBT blend as a function of the time of thermal action. It was shown that the introduction of organophosphorus compounds leads to stabilisation of the phase composition Figure 7. Influence of heating time on gel fraction content for 60:40 PC/PBT alloy (2) and 60:40 PC/PBT alloy + stabiliser (1) Table 1. Mechanical properties of 60:40 wt.% PC/PBT blend Holding time in cylinder of injection moulding machine, min Yield point in elongation σ y Tensile stress causing failure σ t Breaking elongation ε b, % Unnotched impact strength, kj/m n.b* n.b* n.b* 27 V icat softening point, C n.b* - no break Table 2. Composition Strength properties of PC/PBT blend with different mould filling methods Holding time in cylinder of injection moulding machine, min σ y Injection from two sides Injection from end σ t ε b, % σ y σ t ε b, % Makrolon % Vestodur PC-2S % PBT International Polymer Science and Technology, Vol. 31, No. 8, 2004 T/35

6 and mechanical properties of the PC/PBT blend. Similar investigations were carried out on a PC/PETP blend in the 20:80 90:20 concentration range. Figure 8 presents the phase diagram of the PC/PETP alloy. The given system is characterised by an UCST positioned in the temperature range C. With increase in the molecular weight of the components, the UCST increases. It was shown that the phase structure of a PC/PETP alloy changes during annealing: in accordance with the mechanism described above, the reaction of interchain exchange occurs at the interface between the components, leading to the formation of copolymer systems. Slower crystallisation of PETP compared with PBT results in a greater importance of the problem of controlling this process in blends with PC. The introduction of structureforming agents into a PC/PATP blend leads to an increase in the crystallisation rate and temperature compared with unmodified blends, but the heat and temperature of melting in this case are lowered, which indicates the defectiveness of the crystallites formed. The investigations carried out made it possible to develop a range of constructional materials based on binary PC/PATP blends that are characterised by high chemical resistance and cracking resistance, both in the presence of solvents and under the action of a static load, and also high-impact composites additionally containing impact modifiers. REFERENCES 1. A. A. Tager, Physical chemistry of polymers, Khimiya, Moscow, A. E. Chalykh et al., Constitution diagrams of polymer systems, Yanus-K, Moscow, V. N. Kuleznev, Polymer blends, Khimiya, Moscow, D. Pol and S. Newman (Eds.), Polymer blends, Mir, Moscow, J. Devaux et al., Polym. Engng Sci., 22, No. 4, 1982, pp H. Zemmerman, Faseforsch. und Textiltechn., 13, No. 11, 1962, pp B. M. Kovarskaya et al., Thermal stability of heterogeneous polymers, Moscow, 1977, pp S. M. Anakoni and C. F. Forbers, J. Polym. Sci., 24, No. 6, 1986, pp Figure 8. Phase diagram of PC/PETP alloy; molecular weight of PC: ; ; (No date given) T/36 International Polymer Science and Technology, Vol. 31, No. 8, 2004