International Journal of Engineering & Technology IJET-IJENS Vol:14 No:01 135 Synergic Effect of a combined use of two Chemical Blowing Agents on the Foaming Efficiency of an Extruded Rigid PVC Compound Abdelmalek DOUIBI *, Djafer BENACHOUR, Abdelhak HELLATI Laboratoire L.M.P.M.P; Département de Génie des Procédés, Faculté de Technologie; Université Ferhat ABBAS, Sétif 1 (19000), Algérie. *Email: douibi_a@yahoo.com. Abstract-- This work deals with the effect of the addition of a combination of two chemical Blowing Agents (BA); AzoBisFormAmide (ABFA) and/or Sodium BiCarbonate (SBC), on the foaming efficiency and cell structure of a rigid Poly(Vinyl Chloride) (PVC), using a laboratory scale extruder and optical microscope. It was found that the foaming efficiency increased with BA loading to an optimum value, which is characterized by a Critical Concentration (CC) of BA. This CC was noticed to be higher in the case of SBC. On the other hand, the foaming efficiency (expressed in term of diameter of foamed extrudates) showed a synergistic effect according to some mixing ratio of the two BA, which were based on their CC. Index Term-- Poly(vinyl chloride), azobisformamide, sodium bicarbonate, cell structure, expansion, extrusion, decomposition, foaming efficiency, coalescence and synergism. continuous PVC matrix. These ideas have been written upon at some length in prior work [5-7]. In order to provide an adequate product with the highest foaming efficiency, a considerable amount of work has been done reporting foaming efficiency with BA level; but they were restricted to the use of only one BA [4, 8]. A combined use of two BA was developed by K.U. Kim & col [9], where the mixture of the BA was not based on their CC. In their study, they focused their works on ABFA / SBC ratios limited to five ratios (0/10; 1/9, 5/5, 9/1, and 10/0). The results showed no synergistic effect in term of foaming efficiency (i.e. all the values of the mixture of the BA are being restricted between those of each BA when used separately) (see Fig.1). INTRODUCTION As plastic materials go up in cost, the idea of using less material to do the same job becomes more and more enticing [1]. Cellular PVC technology has had an impressive growth rate and continues to have commercial importance [2]. Recently, the rigid one holds a unique place in the field of structural thermoplastic foams and in the wood replacement market [3]. To that effect, this product possesses several adequate properties, which can not be achieved neither by other ways nor by traditional compact products. To investigate the effect of addition of two BA on rigid PVC compound properties, the extrudates were examined in terms of foaming efficiency, cell structure and surface state. Theory Foaming in cellular PVC is generally accomplished by adding a chemical BA to the formulation in sufficient concentration to give the desired density. The degree of foaming depends on the viscosity and melt elasticity of the PVC formulation, the gas pressure evolved and the degree of solvation of the gas in the melt. Ideally the BA will not produce any gas until after the PVC blend has totally fused and can form a melt seal. This prevents gas from escaping out the extruder hopper [4]. During processing, the BA will decompose quickly, but small percentage remains unreacted at the die exit to provide nucleation sites for bubble formation. After exiting from the die, the supersaturated gas-resin mixture will expand with the pressure drop forming the foam as discrete bubbles in a Fig. 1: Foam density as a function of volumetric flow rate at different ABFA /SBC ratios [9]. The synergism between ABFA and SBC in the extrusion of rigid PVC foam was reported by N.L. Thomas & col using a three-dimensional plot (see Fig.2) [10]; which was largely attributed to the complementary properties of the gaseous decomposition products N 2 (evolved from ABFA) and CO 2 (evolved from SBC) [10].
International Journal of Engineering & Technology IJET-IJENS Vol:14 No:01 136 Block diagram: experimental setup Fig. 2: Three dimensional plot of foam density as a function of ABFA and SBC concentration [10]. Experimental The ingredients were combined with PVC in kitchen-mixer at full mixing speed for ten minutes to give a dry white free flowing powder. This powder was extruded using a laboratory scale single screw Controlab type extruder (screw diameter = 30 mm an L/D = 20), through a circular die (D = 4mm), without any calibrator, i.e. free foaming extrusion is taking place. The following processing conditions were hold constant (temperature profile: 150, 165 and 175 C screw speed: 40 and 50 rpm) to ensure a maximum foaming efficiency. The resulting circular extrudates were cut to desired lengths after solidification and measurement of their diameters and were carried out. For the examination of their corresponding cell structure using an optical microscopy, the extrudates were microtomed at room temperature with a tungsten carbide blade to produce thin sections (Thin section from 5 to 10 μm was collected). N.B: All samples showed cells distortion toward the edges of the extrudates, not only due to the cell expansion, but also to the deformation during microtoming. Raw material The materials employed in this work are: 1. Poly (vinyl chloride): A suspension grade PVC was used, whose properties are as follows: Trade name: 4000 LL (ENIP Skikda-Algeria). K-value: 55-57 Degree of polymerization: 570-620 Apparent density: 0.52-0.57 2. Lubricants: Dicarbonic-Acid-Ester was used as an internal lubricant and non-polar Hydrocarbon-Wax as an external one 3. Blowing agents: -Azobisformamide (VINSTAB AZ3-HEBRON) was used as an organic BA (exothermic decomposition reaction). Average particle size (µm): 125 -Sodium bicarbonate (NORMAPUR) was used as an inorganic BA (endothermic decomposition reaction). Average particle size (µm): 280 Formulation The formulation suggested in our study can be seen in table 1. Table I: Formulations PVC resin 4000 LL :K 100 parts (per value 57 weight) Thermal stabilizer (Kicker) Internal lubricant External lubricant Blowing agent (ABFA) Blowing agent (SBC) Dibasic Lead Phosphate Dibasic Lead stearate Dicarbonic acid-ester Non-polar hydrocarbonwax VINSTAB AZ-3 (HEBRON) NORMAPUR 1.9 phr 1 phr 0-0.8 phr 0-1.2 phr
Expansion (%) International Journal of Engineering & Technology IJET-IJENS Vol:14 No:01 137 The mixture of BA was prepared taking into account the CC of each BA. For instance the total concentration of the mixture corresponded to CC of the BA, when 100% of BA in question is used. Therefore, the total concentration of the mixture varied according to the BA proportion within the mixture from the CC of SBS to that ABFA. DISCUSSIONS In all cases, foaming efficiency was found to increase with ABFA loading (see Fig.3). This can be attributed to the fact that increasing BA level will provide higher cell number (see Fig.4 & Fig.5), hence, volume of evolved gas and higher pressure. however at higher BA level, foaming efficiency will pass through an optimum value, corresponding to the equilibrium between gas pressure inside cells and the deformability of cell walls, in other words, the polymeric matrix is saturated with gas (cells).this equilibrium is characterized by CC of the BA (see Fig.3). Beyond it surface changed from smooth to rough state (stage I and stage II on Fig.3). Therefore, the coalescence of neighboring cells is attained resulting in the escaping of the gas to the atmosphere upon leaving the die, hence foaming efficiency is reduced. These results were in an agreement with works done by F. Ide & col [8]. Fig. 4. a- Optical micrograph (x 9) of unfoamed rigid PVC extrudate. 60 50 40 I 40 rpm 50 rpm II 30 20 10 Fig. 4. b- Optical micrograph (x 45) of extrudate sample boarder of unfoamed rigid PVC 0 0.0 0.2 0.4 0.6 0.8 ABFA (phr) Fig. 3. Foaming efficiency as a function of ABFA concentration
Extrudate diameter (mm) International Journal of Engineering & Technology IJET-IJENS Vol:14 No:01 138 10.0 9.5 I II 9.0 8.5 40 rpm 50 rpm 8.0 7.5 7.0 6.5 0.0 0.2 0.4 0.6 0.8 1.0 1.2 SBC (phr) Fig. 6. Foaming efficiency as a function of SBC concentration Fig. 5. a- Optical micrograph (x 9) of foamed rigid PVC extrudate (0.4 phr ABFA) at 50 rpm. It is noted, owing to the optical micrograph examination of the SBC foamed extrudates (see Fig.7) that present a random cellular structure where the cells partially communicate. This could be explained by the particle size distribution of the SBC which is characterized by a major fraction of particles with a diameter larger than 280 µm (broad distribution) that gave rise to large cells partially communicating (average cell size is about 280 µm). Concerning the ABFA, the optical micrograph examination of the extrudates (see Fig.5) showed that they present a structure characterized by small and uniform cells (average cell size is about 30 µm). This is also explained by the particle size distribution which showed that the entire particles of ABFA have a diameter 0f 125 µm. The mixture of the two BA was prepared by taking into consideration two parameters, namely: The CC for optimum expansion for each BA alone. The state of the surface of the extrudate. Fig. 5. b- Optical micrograph (x 45) of extrudate sample boarder of foamed rigid PVC (0.4 phr ABFA) at 50 rpm On the other hand it was observed that the CC of SBC was higher than that of ABFA, which were respectively 0.6 and 0.4 phr (see Fig.6 & Fig.3). This can be explained by the fact that water vapor is one of the resulting gas when SBC is decomposed. It contributes to the expansion at the processing temperature (T=175 C). However, it will have an opposite effect when condensing during cooling (T<100 C), while PVC is still deformable above its glass transition temperature [11]. Furthermore, SBC exhibits an endothermic decomposition reaction, which will result in an increase in PVC melt strength (higher viscosity); hence, more cells are required to reach the CC. According to the previous results, it was found that the critical concentration (at 40 and 50 rpm) was 0.4 phr for the ABFA and 0.6 for the SBC (see Fig.6 & Fig.3). At these concentrations the extrudates presented a limit of the smooth external surfaces of the extrudates. Illustration of these compositions is given in table 2.
Extrudate diameter.(mm) International Journal of Engineering & Technology IJET-IJENS Vol:14 No:01 139 Table 2: Total concentration variation of the mixture of blowing agents A B F S B C ABFA Concentration (phr) SBC Concentration (phr) C Tot (phr) A (%) (%) 100 0 0.4 100/100 0.6 0/100 0.40 = 0.40 = 0 70 30 0.4 70/100 0.6 30/100 046 = 0.28 = 0.18 50 50 0.4 50/100 0.6 50/100 0.50 = 0.20 = 0.30 30 70 0.4 30/100 0.6 70/100 0.54 = 0.12 0 100 0.4 0/100 = 0 = 0.42 0.6 100/100 = 0.60 0.60 Fig. 7. a- Optical micrograph (x 9) of foamed rigid PVC extrudate (0.6 phr SBC) at 40 rpm. NB: It is to be noted that the total concentration (C Tot ) varies depending on the individual proportions of the two BA; contrarily to the works of K. U. KIM and al who maintained the total concentration constant [9], and do not take into account the critical concentration of each BA when they are added separately. Based on this, it was noticed that the foaming efficiency of the mixture of BA exhibited higher values than additivity line (synergism), when ABFA level was higher than 50% whereas, below this percentage, lower values were obtained (antagonism) (see Fig.8). 10.5 10.0 40 rpm 50 rpm Syn Fig. 7. b- Optical micrograph (x 45) of extrudate sample boarder of foamed rigid PVC (0.6 phr SBC) at 40 rpm. 9.5 additivity line According to the previous results, it was found that the critical concentration (at 40 and 50 rpm) was 0.4 phr for the ABFA and 0.6 for the SBC (see Fig.6 & Fig.3). At these concentrations the extrudates presented a limit of the smooth external surfaces of the extrudates. Illustration of these compositions is given in table 2. 9.0 8.5 Ant Ant 8.0 0 10 20 30 40 50 60 70 80 90 100 ABFA ( %) Fig. 8. Foaming efficiency as a function of ABFA percentage at 40 and 50 rpm The synergism (at 70% ABFA) can be explained by the following interrelated factors: - At 70% ABFA, the higher the proportion of ABFA in the mixture the greater the amount of heat released (10 Kcal/mole at 230 C) when decomposing. This will lead to a decrease in polymeric matrix viscosity and, therefore, to a better expansion resulting from a better efficiency of the gases released.
International Journal of Engineering & Technology IJET-IJENS Vol:14 No:01 140 - In addition to that, in the same range (at 70% ABFA), the ratio of kicker/abfa is low in the way of increasing the extent of ABFA proportion within the BA mixture (at a constant kicker concentration within formulated PVC). This will contribute to increase the ABFA decomposition temperature above the processing temperature which will result in a fraction of ABFA to decompose after leaving the die (that is looked for); therefore a higher expansion is obtained (see Fig.9). the extent of kicker concentration within formulated PVC (at a constant ABFA concentration within the BA mixture). This will contribute to decrease the ABFA decomposition temperature below the processing temperature (premature decomposition of ABFA) which will result in some of the evolved gas will escape from the hopper of the extrudate before total fusion of PVC takes place; therefore a lower expansion is obtained (See Fig. 10) Fig. 9. a- Optical micrograph (X 9) of foamed rigid PVC extrudate (70% ABFA of BA mixture based on their optimum concentrations) at 50rpm Fig. 10. a- Optical micrograph (x 9) of foamed rigid PVC extrudate (30% ABFA of BA mixture based on their optimum concentrations) at 50 rpm Fig. 9. b- Optical micrograph (X 45) of extrudate sample boarder of foamed rigid PVC (70% ABFA of BA mixture based on their critical concentrations) at 50 rpm The antagonism can be explained by the following interrelated factors: - Below 40% ABFA, the ratio of kicker/abfa is high in the way of decreasing the extent of ABFA proportion within the BA mixture (at a constant kicker concentration within formulated PVC); we are in the situation as we are increasing Fig. 10. b- Optical micrograph (x 45) of extrudate sample boarder of foamed rigid PVC (30% ABFA of BA mixture based on their critical concentrations) at 50rpm. - In addition to that, in the same range (below 40% ABFA), the higher the proportion of SBS in the mixture the greater the amount of heat absorbed (-227 Kcal/mole at 170 C) when decomposing. This will lead to an increase in polymeric matrix viscosity [12]; hence the released bubbles have a difficulty to expand resulting in a lower expansion. - The antagonism (at 90% ABFA): The ratio of kicker/abfa is too low in the way of increasing the extent of ABFA
International Journal of Engineering & Technology IJET-IJENS Vol:14 No:01 141 proportion within the BA mixture (at a constant kicker concentration within formulated PVC); Therefore a great extent of ABFA will not decompose at all even leaving the die: we are in the situation as the SBC is acting alone with a lower concentration; hence a lower expansion is obtained. [12] DOUIBI A., BENACHOUR D., Intern. J. Polym. Mater., Vol.52, N 10, 2003. Comparison with other studies These results were unlike those obtained by K.U. Kim & col. [9], who found that a combined use of two BA exhibited near additivity according to mixing ratio. This can be explained by that fact that authors maintained constant the total concentration of the mixture of BA without considering their CC. Therefore, at a given concentration between the CC of each BA, different cell structure is taking place. Whereas, in our case, we are dealing within the range of one type of cell structure: The closed one. On the other hand, a combined use of two BA was developed by N.L. Thomas & col [10] even though the mixture of the BA was not based on their critical concentrations, the results showed a synergistic effect in term foaming efficiency (i.e. some of the values of the mixture of the BA are not being restricted between those of each BA when used separately). The synergism effect between ABFA and SBC in the extrusion of rigid PVC foam was largely attributed to the complementary properties of the gaseous decomposition products N 2, and CO 2 : N 2 (evolved from ABFA) has a relatively low solubility but high diffusivity in PVC compared with CO 2 (evolved from SBC) [10]. CONCLUSIONS - Foaming efficiency would rise with increasing ABFA level to an optimum value corresponding to the CC, and then dropped with additional BA. - The CC would shift to higher value when SBC was used. - In all cases surface quality was improved below the CC. - A combined use of two BA, with a high ABFA content (70%), would improve more the foaming efficiency of extrudates REFERENCES [1] WENDEL B.C., Cellular Plastics, may/june, p.178, 1977. [2] NASS I.L. HEIBERGER C.A., Encylopedia of PVC, 2 edition, vol.2, Marcel Dekker Inc, New Jersey,1975) [3] FRADOS J., Cellular Plastics, Plastic; Engineering hand book p550. Van Nostrand Reinhold, New York,1976 [4] PFENNING J. L., ROSS M., 4th International Conference, Brighton, UK., 24-26, April 1990. [5] HANSEN R. H. SPE Journal, 18, p. 80, 1962 [6] DILLEY E. R., Trans. J. Plastics Inst, February, p17, 1966. [7] MORIZ U., Kunststoffe, 73, p.394, 1983. [8] IDE F., OKANO K., Pure and Appl. Chem., 53, p489, 1981. [9] KIM K. U., PARK T. S. and KIM B. C., J. Poly. Eng., Vol. 7, No.1, p5, 1986 [10] THOMAS N. L. and HARVEY R.J., Statistical experimental design to optimize formulations for foam vinyl applications, Foamplas 98, p.55, 1998 [11] DOUIBI A., Phd, Extrusion of foamed PVC: Effects of a combination of two chemical blowing agents(abfa and SBC) on rheological, mechanical and morphological properties Université of Sétif Algeria, 2007