Document DMDKYrE8x2eGgRDd3wZB9ambo

Journal of Thermal Analysis, Vol. 52 (1998) 355-361 COMPARATIVE STUDY OF EFFICIENCY OF NUCLEATING AGENTS IN PA-6 I. Mudra and G. Baldzs Department of Plasties and Rubber Technology, Technical University of Budapest, H-1521 Budapest Muegyetem rkp, 3, Hungary (Received March 10,1997; in revised form June 5, 1997) Abstract The isothermal and anisothermal crystallization of nucleated polyamide-6 (PA-6) was in vestigated by DSC, A comparative study was made of twelve potential nucleating agents, in cluding some commercial products for PA-6 and polypropylene. The amide wax processing aid lubricant originally introduced into the polymer was found to exhibit a marked nucleation ability. . Keywords; crystallization, DSC study, nucleating agents, PA-6 Introduction Polyamide-6 (FA-6) is a widely used, relatively cheap crystalline engineering thermoplastic which has a moderate crystallization rate similar to that of polypropylene (PP). Nucleating agents are mostly used in these two polymers. These additives normally increase the rate of crystallization, which shortens the cycle time in in jection moulding, improving the productivity of the process, decreases the spherulite size and results in the development of a more uniform structure with enhanced mechanical properties [1-4]. Thousands of different materials have been studied as potential nucleating agents in the case of PP, but a literature search reveals that less attention has been paid to the nucleation of PA-6, Nevertheless, a patent dealing with the fillers of PA-6 more than 50 years ago claimed an appreciable nucleating effect [2], Nu merous commercial nucleated PA grades are currently on the market, e.g. several materials of Allied Signal Co.; Adell; EMS; Nylon Corp,; Texapol; etc. [5], In ba sic monographs, several materials are mentioned as efficient nucleating agents in PA-6: - kaolin -- lead(II) phosphate 1418-2874/98/$ 5,00 1998 Akademiai Kiado, Budapest Akademiai Kiado, Budapest Kluwer Academic Publishers, Dordrecht 356 MUDRA, BALAZS: PA-6 - sodium phenyl phosphinate - polyethylene terephthalate - polyamide 6,6 -colloidal silica - molybdenum(IV) sulfide - iron sulfide - titanium(IV) oxide -talc The nucleating effects of some new materials have been verified; -poly(4,4-diphenylsulfone terephthalamide) [6] - polyvinylidene fluoride [7] - montanates [8] -phthalocyanine [9] -fatty acid amide [10] - metal diary 1 phosphates [11] - surface-treated montmorillonite [12] An on-line literature search demonstrated that a great number of materials may be used as nucleating agents in PA-6, but their mechanisms are not known, and extensive studies to compare tens or hundreds of potential nucleating agents have not been made at all for PA-6. The main aims of the present study were: - a comparative study of some potential nucleating agents, with a commercial grade PA-6 as base polymer - a check on the activity of some nucleating agents of PP in PA-6 (since more information is available on the mechanism and efficiency of nucleating agents in PP). Experimental The base polymer used was an injection moulding grade material produced by ` Viscosa RT (Hungary), containing 0.5 wt% amide wax lubricant. The nucle ating agent content was 0.1, 0.5 or 1.0 wt%. The additives were introduced into the polymer with a Brabender ZSK42-7 twin-screw extruder at 250C. The nucleating agents studied and their abbreviations are listed below; - Two different grades of calcium stearate (CAN and CAS) - Polyethylene terephthalate (PET) -Wyoming bentonite with high montmorillonite content (NAB) -CaCOs with high surface area, precipitated (Socal, Solvay Co.) (SU) - Colloidal silica, normal grade (Degussa Co.) (A200) -Colloidal silica, surface-modified (Degussa) (R972) -Talc (Luzenac Co.) (LT) i. Thermal Anal, 52, 1998 MUDRA, BALAZS: PA-6 357 ~ A phosphorus compound, highly effective in PP (Palmarole) (NA11) - An organic aluminium compound, a nucleating agent in PP (Sandoz AG) (S4030) - Bruggolen, a commercial nucleating agent in PA-6 (Bruggeman AG) (VKB22) - A commercial nucleating agent in PA-6 (Viba SpA) (VRF6) Crystallization of the samples was followed by DSC, Anisothermal crystal lization runs were carried out on a ~5 mg sample in nitrogen on a Perkin-Elmer DSC-7 instrument. Some samples were crystallized isothermally at 202C. The crystallization temperature was reached at a cooling rate of 40C muT1 after melting the sample at 280C for 5 min. Flexural tests were carried out accord ing to ISO 178 in order to investigate the structure - property relationship. A knowledge of the upper limit of the effect of the best nucleating agent in a given polymer would be very important from both practical and theoretical as pects. The method developed by Fiilon and co-workers mainly for PP [13] gives a possibility for determination of this value. The earlier methods used only one reference point, the crystallization peak temperature of the `non-seeded' poly mer. This value is compared with those measured for nucleated samples. In the Fiilon method, two extreme reference points are applied: the peak temperature of the crystallization of the `non-seeded' polymer (blank melt) gives the lower point, while the completely seeded material, i.e. the base polymer itself, possess ing the maximum number of self-nuclei, but without a considerable crystalline phase content, gives the upper point. This latter state is achieved by limited melt ing of the sample before crystallization. By means of the Fiilon method, the two limiting temperatures of the base polyamide were determined in several DSC runs by varying the upper tempera ture (7 of melting before crystallization. With decreasing T), the crystallization .peak temperature increases continuously as more and more self-nuclei survive the melting process. To verify that an increase in the number of self-nuclei is not accompanied by the presence of crystalline phase, a subsequent melting run is necessary. If an appreciable amount of crystalline phase survives the melting process relating to a give 7V, a very sharp peak appears at high temperature, su perimposed on the main melting peak. DSC runs according to the above method yielded the upper and lower limits as 192.2 and 187.7C, respectively. This means that an increase of less than 5C can be reached with the best nucleating agent in the peak temperature of crystal lization, (The differences are in the range of 20-30C in the case of PP.) Crystallization characteristics obtained from the anisothermal DSC traces of samples containing 1 wt% additives are shown in Fig. 1 (rco=onset temperature; 7'cp=peak temperature; and rct=final temperature of crystallization). The commercial material Bruggolen (VKB22) developed for this purpose is one of the best nucleating agents; the effect of the other commercial product, VRF6, is poorer. However, our data indicate that even the efficiency of Brug- J. Thermal Anal., 52. 1998 358 MUDRA, BALAZS: PA-6 Fig. 1 Crystallization characteristics of PA-6 containing 1 wt% different additives golen is surpassed by that of the cheap inorganic filler talc (mentioned, not sur prisingly, in some newer patents). In the presence of talc, the crystallization in terval is widened significantly, which is important for practical applications. As concerns the other additives, montmorillonite (NAB) (an excellent nucleating agent according to the literature) and the phosphate (NA11) (one of the best ad ditives in PP) exert only minor effects. Similarly, PET and the aerosils (all effec tive nucleators according to some publications) increased the rate of crystal lization to only small extents. Calcium stearate and CaC03 behave as inert com pounds, although these particulate fillers with high specific surface areas are ac tive nucleating agents in PP. The aluminium salt (S4030) strongly decreased the rate of crystallization. We have never met such a retardation of nucleation before, especially not at such a low concentration, which was only 1 wt%. Further investigations were focused on determination of the dependence of the crystallization characteristics on the additive content, and checking of the va lidity of previous observations in isothermal crystallization experiments using only the interesting or promising materials (talc, VKB22, VRF6 and S4030). The results of anisothermal experiments can be seen in Fig. 2, where the charac teristic temperatures of crystallization are plotted against additive content. The basic tendencies remained the same as in the case of 1 wt% additive con tent. Talc is the best nucleating agent and the A1 salt decreases the rate of crystal lization at every concentration. The effectiveness of talc reaches saturation at about 0.5 wt%; its activity does not increase appreciably at higher concentra tions. Saturation in PP occurs only at about 4 wt%. J. Thermal Anal., 52, 1998 197 y-- 192 l" 187 182 MUDRA. BALAZS: PA-6 359 Fig, 2 Concentration dependence of crystallization characteristics of nucleated PA-6 (Tct Tcp *7c,f1 10 15 20 25 Time (min) Fig. 3 Crystallization isotherms of PA-6 at 202 C. Additive content 1 wt% In isothermal runs (Fig. 3), the effects of nucleating agents on the crystal lization process can be delected as a function of time. The sequence of the addi tives as regards nucleation activity remained the same as in the anisothermal crystallization experiments, but the differences between them were enhanced. J. Thermal Anal., 52, 1998 360 MUDRA, BALAZS: PA-6 The poor performance of some nucleating agents, and above all the surprising retardation effect of the A1 salt, led us to conclude that the lubricant originally present in the system as a processing aid is not inert from the aspect of crystal lization. We received a true `virgin' polymer (absolutely free from additives) from Viscose RT, and the results of our repeated experiments are given in Ta ble 1. Table 1 Crystallization characteristics of the base and the lubricant free grade PA-6 (AT: crystallization interval; AH: heat of crystallization TJC TJC TJ'C ATf'C AH/J g 1 Base polymer 190.3 187.7 184.6 5.7 63,7 Lubricant free polymer182.7174,5168.S14.263.0 Our assumption was confirmed. Under the same conditions, the virgin PA-6 crystallized at a temperature 10 C lower than that for the `base' polymer, so the amide wax lubricant plays a dual role in the system; as a lubricant and as a nucle ating agent. We assume that some physical or chemical interaction occurs be tween the organoaluminium salt (S4030) and the lubricant, which causes a de crease in crystallization temperature for samples containing S4030. Table 2 Flexural modulus of nucleated PA-6 Sample Base polymer Bruggolen (VKB22) Talc(LT) Additive content/wt% - 0.05 0.10 0.50 0.05 0.10 0.50 Flexural modulus/GPa 1.40 1.63 1.68 1.60 1.69 1.66 1.65 In order to check the effects of the nucleating agents on the properties of the products, some mechanical tests were carried out. Table 2 lists the flexural moduli of the samples nucleated by talc and Bruggolen as a function of additive content. In spite of the moderate nucleating effects of these two additives, the in creases in flexural modulus are surprisingly large. Conclusions DSC crystallization experiments have shown that some well-known and strong nucleating agents of PP are inactive in PA-6. Among them, talc is the only J. Thermal Anal, 52, 1998 MUDRA, BALAZS: PA-6 361 additive which exerts a marked nucleating effect. The amide wax lubricant be haves as a true nucleating agent; it can interact with other additives and lose its efficiency as a result. We are convinced that a detailed study of the interaction be tween S4030 and the amide wax lubricant would help towards an understanding of the mechanism of nucleation in PA-6. References 1 R. Gachter and H. Muller, Plastics Additives Handbook, Hanser, Munich 1990. 2 M. I. Cohan, Nylon Plastics, Wiley-Interscience, New York 1973. 3 Encyclopedia of Polymer Science and Technology, 2nd Ed., ed,: F. Mark, N. Bikales, M. Ovcrbcrgcr, H. Menger, Wi ley - Interscience, New York 1989, 4 J. Brandrup and E. H. Immergut, Polymer Handbook, 3rd Ed., Wiley-Interscience, New York 1989. 5 Modem Plastics International, Encyclopedia and Buyer's Guide, 1993. 6 H. H. Wang and W. L. Chen, J. Polym. Sci. Polym. Chem., 27 (1989) 1359. 7 H. Frenseh and B. J. Jungnickel, Colloid Polym. Sci., 267 (1989) 16. 8 P, Piesold, Kunststoffe 82 (1992) 988. 9 G. Ronca, A. Garcia and M. Matliieson, Simp. Lat. Pit., 1992 Caracas, (CA 118:192593). 10 WO 8802763 (Y. Khanna, G. Chomin, Allied Signal Co.). ' 11 J. Pat. 63108063 Adeka-Argus Ltd. 12 J. Pat 62252426 Toyota Centr. Res. Dev. Inc. 13 B. Fillon, J. C. Wittmann and A. J. Thierry, J. Polym. Sci. Polym. Phys., 31 (1993) 1383. J. Thermal Anal, 52, 1998