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Thermal stability of epoxidized soybean oil and its absorption and migration in poly(vinylchloride).

INTRODUCTION

Poly(vinylchloride) (PVC) is widely used to fabricate various articles for different applications across various market segments. In many cases, the PVC compositions are plasticized to make them suitably soft and flexible for end-use applications, such as the insulation or jacket of cable constructions (1). The plasticizers used to impart flexibility to the compositions are largely phthalates (2), but higher molecular weight plasticizers (3) (primarily trimellitates) are used for applications where plasticizer loss and migration need to be minimized, particularly to retain good properties after aging at test temperatures as high as 136[degrees]C.

Important considerations in the manufacture of flexible PVC are the absorptive characteristics of plasticizers in PVC powder (grains), which happens to be porous (4). Typical factors that influence plasticizer uptake in PVC resins, in terms of dry blend times, have been described in Ref. (5) The porous suspension-polymerized PVC grains are irregularly shaped with an average diameter of about 140 microns, consist of clusters of primary particles roughly 1 micron in diameter, and are surrounded by a thin (0.5-5 micron) pericellular membrane (6, 7). The compatibility of plasticizers with PVC also needs to be considered (6). Solubility parameters are often used to predict PVC/plasticizer interactions (8). Depression in the glass transition temperature ([T.sub.g]) of PVC can also be used to assess plasticization efficiency (9). Other tests used to investigate PVC/plasticizer interactions are torque rheometer tests, plasticizer absorption tests, and solution temperature tests (10-18).

In recent times, there has been keen interest to develop bio-based plasticizers (derived from renewable resources) as more sustainable alternatives to the incumbent phthalates and trimellitates (19-23). One of the most readily available bio-plasticizers is epoxidized soybean oil (ESO), which has typically been used as a secondary plasticizer for PVC. This has been due to historical concerns about its thermal, hydrolytic, and photo-oxidative instability, which in turn lead to long-term incompatibility in the polymer composition, although use of ESO having reduced iodine value (from 13 to 1 g [I.sub.2]/100 g) and increased oxirane oxygen content (from 6.0 to 6.6%) has been reported to result in significant improvement in compatibility (24-29). Thus, more recent literature has been promoting the possibility of using ESO as a primary plasticizer for PVC (30-33).

This article presents results on the thermal stability of ESO along with the solubility and transport characteristics of ESO in PVC, with a view to assessing its utility as the single plasticizer for this polymer.

EXPERIMENTAL MATERIALS AND PROCEDURES

Materials

The resin used was Oxy Vinyls 240F suspension-grade PVC homopolymer powder which, according to the technical data sheet from the manufacturer (Oxy Vinyls, LP), has K-value of 70, inherent viscosity of 1.02 dl/g, relative viscosity of 2.37, and porosity of 0.35 [cm.sup.3]/g.

The other materials used were Polyfil 70 calcined kaolin clay (as electrical insulation filler), Baeropan MC 9754 KA calcium-zinc mixed metal soap (as the primary heat stabilizer), Irganox 1076 (as antioxidant), and Plas-Chek 775 ESO (primarily as plasticizer, but also as a secondary stabilizer). The latter had an iodine value of 0.7 g [I.sub.2]/100 g (measured in accordance with ASTM D5554-95), oxirane oxygen content of 7.2% (measured in accordance with ASTM D1652-04), density at 25[degrees]C of 0.993 g/ml (measured in accordance with ASTM D1298) and APHA color of 123 (measured in accordance with ASTM D1209).

Thermal Characterization of Plasticizer

Thermogravimetry (TG) was conducted in platinum pans under nitrogen (at flow rate of 100 [cm.sup.3]/min) by raising the temperature from 30 to 900[degrees]C at a rate of 1 WC/min, to determine the mass loss of plasticizer as a function of temperature.

Differential scanning calorimetry (DSC) was conducted using a closed pan with ~2 mg of plasticizer under nitrogen using the following procedure: equilibrate at 30[degrees]C for I min; first heat at 10[degrees]C/min to 100[degrees]C (to melt any solid fractions); isothermal for 2 min; cool at 5[degrees]C/min to -85[degrees]C (to detect the temperatures at which any crystallization occurred); isothermal for 3 min; second heat at ST/min to 100[degrees]C (to measure the melting points of any solid fractions). In another variation, DSC was conducted using the conditions described above, except that only one heating ramp was used (from 30 to 370[degrees]C at 10[degrees]C/min) and cooling was not investigated.

Plasticizer Uptake by Centrifuge Plasticizer Absorption Test

The determination of plasticizer absorption in PVC powder was conducted using variations of the centrifuge plasticizer absorption test, which is conventionally conducted only for 15 min at room temperature (34), in accordance with ASTM D 3367-09, ISO 4608-98 (E), and GB/T 3400-2002. In the present study, the temperature was varied from 40 to 120[degrees]C, and the absorption time ranged from 5 to 720 min. Centrifuge tubes, made of glass and with conical bottoms pierced by a hole of about 0.8 mm diameter, were used for these experiments (11). The experimental procedure employed was as follows, using an analytical balance (accurate to 0.1 mg) for weighing the materials: (a) Pharmaceutical-quality cotton wool (100 [+ or -] 2 mg) was placed at the bottom of the centrifuge tube and packed slightly; (b) 1.000 [+ or -] 0.010 g of the PVC powder was placed in the tube, on top of the cotton wool; (c) A small quantity of plasticizer was heated separately for at least 30 min in an oven set at the desired temperature for absorption measurements; (d) A plastic burette was used to add 2 ml (~2g, but weighed accurately) of plasticizer to the tube containing PVC powder and cotton wool; (e) The tube containing plasticizer, PVC powder and cotton wool was kept in the oven at a specified temperature (40, 80, or 120[degrees]C) for a stipulated length of time; (f) At various temperature--time conditions, the tube was removed from the oven, inserted into a sheath, and the whole assembly was placed in one of the compartments of a centrifuge; (g) The centrifuge was run at its maximum speed of 4000 revolutions per minute (rpm) for 60 min, which resulted in unabsorbed plasticizer draining into the sheath of the tube; (h) The tube was removed from its sheath, carefully wiped to remove any plasticizer on the outside, and weighed.

The plasticizer absorption, expressed as g/100 g of resin or parts of plasticizer absorbed per 100 parts of resin (phr), was calculated from Eq. 1:

X = [([m.sub.3] - [m.sub.0]) - [m.sub.2]/ [m.sub.2]) - [m.sub.1]] x 100 (1)

where, [m.sub.0] is the mass, in grams, of plasticizer absorbed by the cotton wool in the blank test (conducted without PVC); [m.sub.1] is the mass, in grams, of the centrifuge tube plus cotton wool; [m.sub.2] is the mass, in grams, of the centrifuge tube plus cotton wool and PVC powder; [m.sub.3] is the mass, in grams, of the centrifuge tube plus cotton wool, PVC powder and absorbed plasticizer (after centrifuging).

Scanning electron microscopy (SEM) was also conducted on the PVC powder before and after absorption of plasticizer, at 100x and 2000x magnifications.

Plasticizer Absorption by Haake Rheomix Test

Plasticizer uptake was also studied using a Haake rheomix torque rheometer (in a variation of ASTM D-2396) and a composition comprising 68.2 wt% OxyVinyls 240 F, 29.2 wt% ESO, 2.3 wt% Baeropan MC 9754 KA, and 0.3 wt% Irganox 1076. Nitrogen environment was not used. The primary heat stabilizer (Baeropan MC 9754 KA) and antioxidant (Irganox 1076) were first mixed with the PVC resin at room temperature, and the plasticizer (ESO) was also kept at room temperature before use. Next, the solids mixture and plasticizer were added to a mixing bowl equipped with twin-screw rotors (screw diameter: 16 mm; RID of 25: 1), at set metal temperature (80[degrees]C or 100[degrees]C or 120[degrees]C) and fixed revolutions per minute (15 or 30 or 60 rpm), and the time-dependent changes in torque and compound temperature were recorded.

Dry Blending and Melt Mixing of Flexible PVC Composition

Various samples of the following flexible PVC compositions were made: 63.9 wt% OxyVinyls 240 F, 27.3 wt% ESO, 6.4 wt% Polyfil 70, 2.1 wt% Baeropan MC 9754 KA, and 0.3 wt% Irganox 1076. That is, the loading of ESO was 43 part-per-hundred resin (phr). The plasticizer (ESO) was heated in the oven to 60[degrees]C for at least 30 min before use. The following procedure was used: (a) Dry blends (1000 g batches) were made by mixing all the ingredients, except Polyfil 70, in a high-speed mixer at about 1200 rpm and about 50[degrees]C for 3 min (the temperature tended to increase during mixing, because of friction between the materials and the rotor, and so water in the mixer barrel was used to control the temperature at 50[degrees]C) until all the plasticizer had been absorbed, followed by addition of the Polyfil 70 and mixing for another 3 min at this temperature; (b) Melt blends were made by subsequently mixing the entire composition in a two-roll mill for 5 min at 165 [+ or -] 5[degrees]C; (c) The melt blended compositions were placed in molds (which had been preheated at 180[degrees]C for 10 min, in a press machine) and compression molded at this temperature and 10 MPa for 10 min, followed by compression molding at ambient temperature for 5 min in another press; (d) Specimens of 50 [+ or -] 1 mm diameter and 1 [+ or -] 0.1 mm thickness were cut from the molded compositions, and a micrometer, accurate to 0.01 mm, was used to measure the dimensions of the specimens.

Migration Test on Flexible PVC Compositions

The migration test was conducted in accordance with ISO 176-2005 ("Determination of loss of plasticizers--activated carbon method"). A metal can container of cylindrical form (ca. 100 mm in diameter and 120 mm in height), having a non-airtight lid with a small vent hole of 3 mm diameter, was used for the experiments. Activated carbon, with a grain-size of about 4 to 6 mm (i.e., free of powder) was used as the medium. The following was the test procedure: (a) The molded test specimens were conditioned for 48 h at 23[degrees]C and 50% relative humidity in accordance with ISO 291-2008 ("Plastics--standard atmospheres for conditioning and testing"); (b) Each test specimen was weighed to the nearest 0.001 g and its mean thickness determined to the nearest 0.01 mm; (c) About 120 [cm.sup.3] of activated carbon was spread on the bottom of the container and one of the test specimens was placed on the carbon layer. About 120 [cm.sup.3] more of activated carbon was spread over this specimen. Two additional specimens were placed in the container, each covered by 120 [cm.sup.3] of carbon, as depicted in Fig. 1; (d) The lid was put on the container, and the entire assembly was placed in an oven (set at 40[degrees]C, 80[degrees]C, or 120[degrees]C) for varying lengths of time; (e) After the stipulated time, the container was removed from the oven and cooled at room temperature for 30 min; (e) The specimens were removed from the container, carefully brushed free of any trace of carbon particles and reconditioned under the same conditions as those to which they were subjected before the original weighing (i.e., 48 h at 23[degrees]C and 50% relative humidity); (f) Each specimen was re-weighed to the nearest 0.001 g.

The mass change, [DELTA]M expressed as a percentage, was computed from Eq. 2:

[DELTA]M = [M.sub.0] - [M.sub.1]/[M.sub.0] x 100 (2)

where [M.sub.0] is the initial mass, in grams, of the test specimen after conditioning for 48 h at 23[degrees]C and 50% relative humidity; [M.sub.1] is the mass, in grams, of the test specimen after treatment in the oven and reconditioning.

The order of the various steps involved in the preparation and evaluation of the flexible PVC compositions was as follows: dry blending, melt blending, compression molding, specimen preparation, and property testing. In addition to mass loss, the other tests conducted on the plasticized (molded) specimens were measurements of hardness at 23[degrees]C (Shore D, average of six values, in accordance with ASTM D2240) and glass transition temperature ([T.sub.g]). The [T.sub.g] was measured by DSC by conducting a temperature sweep from room temperature to 120[degrees]C at 5[degrees]C/min under air. The [T.sub.s] of PVC powder (unplasticized) was also measured using this procedure.

RESULTS AND DISCUSSION

In DSC analyses of the ESO, no changes were detected upon heating of this plasticizer from 30 to 100[degrees]C. However, several peaks were evident upon cooling to--85[degrees]C and subsequent reheating to 100[degrees]C. These thermal transitions were probably indicative of the triglycerides composition of ESO, specifically the significant amounts of saturated fatty acid glycerides (35) that are known to melt at higher temperatures than their unsaturated fatty acid counterparts. Most of the thermal transitions occurred below 0[degrees]C, although a sharp and minor transition was also evident around 30[degrees]C. Since, the plasticizer uptake and PVC compounding experiments in this study were conducted by preheating the ESO to at least 40[degrees]C, it is reasonable to expect that no solid fractions were present in the ESO at test conditions.

In a variation of DSC, the ESO was heated from 30 to 370[degrees]C. Interestingly, as shown in Fig. 2, exothermic activity started around a temperature of 260[degrees]C and reached a maximum around 345[degrees]C, with the onset of a second exotherm being detected around 360[degrees]C. These trends were consistent with those reported previously in Ref. 25, which were attributed to oxirane ring-opening reactions, except that the first exotherm in the present study started and peaked at considerably higher temperatures. This observation may possibly be accounted for by the different grades of ESO used in the two separate studies.

The TG of ESO is shown in Fig. 3. The onset of mass loss started around 240[degrees]C, with 5% mass loss being recorded around 345[degrees]C (same as the first exothermic peak detected by DSC). The differential thermogravimetry (DIG) plot shows that significant mass loss started around 310[degrees]C and peaked around 390[degrees]C. Another minor peak was observed in the DIG plot at about 450[degrees]C.

The DSC and TG findings indicate that the grade of ESO used in this investigation should remain stable even when exposed for short lengths of time to temperatures as high as ~ 240-260[degrees]C in air-free environments). However, questions do arise about the viability of using ESO as a plasticizer in PVC compositions, since acid catalyzed oxirane ring opening of ESO is well documented (36) and PVC is known to undergo dehydrochlorination under the influence of heat, resulting in the formation of conjugated double bonds, with discoloration occurring when the sequence length exceeds four to five units (37-39). In this context, it was previously found that ESO dissolved in vinyl chloride polymers, in which acid generation can occur, did not undergo rapid decomposition until temperatures well in excess of 200[degrees]C were attained (with peak decomposition temperatures ranging from 215 to 244[degrees]C, depending on ESO concentration) (25). It is also known that, when ESO is used as a plasticizer in PVC, it can form network structures when the compositions are aged at elevated temperatures (e.g., days at 100[degrees]C), but only in the absence of heat stabilizers (metal soaps) (30). In the present investigation, only the plasticizer absorption studies were conducted with compositions that did not contain heat stabilizers, at a maximum temperature of 120[degrees]C and absorption times at this temperature of 2 h or less, which would have minimized any potential network formation of the ESO. All other experiments to evaluate the efficacy of ESO as a plasticizer for PVC were done using formulations that contained mixed metal soap as heat stabilizer, where it is reasonable to suppose that extensive degradation of ESO and consequent network formation would not have occurred. Furthermore, since flexible PVC compounding, and the melt processing of such compounds to fabricate end-use articles, is typically conducted with heat stabilizers at temperatures below 200[degrees]C in closed-systems (like extruders), where air is only present at low concentrations, it is expected that significant decomposition of ESO would also not take place under those conditions when it is used as a plasticizer.

The uptake of ESO in PVC powder, after 10 and 20 min at temperatures ranging from 40 to 120[degrees]C, is shown in Fig. 4a. It is known that, in the manufacture of plasticized PVC, liquid plasticizer first fills the voids or pores in the PVC grains rapidly during powder mixing and it is only after sufficient time at elevated temperatures that any plasticizer in excess of the void volume is completely absorbed into the PVC matrix, resulting in what is ostensibly a "dry blend" (5), (6), (40). The rate-determining process in dry blending is the diffusion of plasticizer within the polymer and the mechanism is the same below and above the glass transition temperature of the polymer (41). The pericellular membrane surrounding the PVC granule generally does not impede uptake of plasticizer, due to pore openings in the topographical skin (16). In the standard centrifuge plasticizer absorption test, the resin (powder) is saturated with plasticizer at room temperature for 15 min and then centrifuged at 4500 rpm for 60 min to remove any unabsorbed plasticizer (34). Under these conditions, the plasticizer is fully absorbed in the pores of the PVC powder, with little or no absorption in the PVC matrix, such that the absorbed volume of plasticizer correlates very well with the internal pore volume of PVC powder measured by a mercury intrusion porosimeter or other means (12), (14), (15), (34). Hence, the amount of ESO absorbed after 10 min at 40[degrees]C can be deemed to be a reasonable measure of the pore volume of the PVC resin used (Fig. 4a). This value was 28.6 g ESO/100 g PVC powder, which translates to 28.8 (cm.sup.3)/100 g. That is, the pore volume of the resin was--0.3 c(m.sup.3)/g, which was in the range of values reported in the literature for similar resins (4), (12), (14-16), (34) and close to that reported in the technical data sheet of the PVC resin (see "Materials" section). By adjusting for the amount of plasticizer absorbed in the voids, the actual amount of plasticizer absorbed in the matrix polymer can be computed, as shown in Fig. 4b. Clearly, longer time and higher temperature both yielded increased plasticizer uptake, but it is not clear from these results if equilibrium concentrations had been achieved at any temperature.

Figure 5 shows the time-dependent absorption of ESO in the PVC resin, and the corrected uptake in the matrix polymer, at temperatures of 80 and 120[degrees]C. It is worth noting that the test temperature of 80[degrees]C was close to the glass transition temperature of suspension-grade PVC reported in Ref. 42, and measured in the present study to be 83[degrees]C. Three regions were clearly evident in the sorption curve at the lower temperature. The first was an induction period that lasted for the first 10 min, corresponding to the rapid filling of the voids in the polymer, characterized by very little uptake of plasticizer in the polymer matrix (Fig. 5b). As the plasticizer diffused through the pericellular membrane and into the primary particles that constitute the PVC grain, it would progressively have decreased the glass transition temperature of the polymer and, when a [T.sub.g] below the test temperature of 80[degrees]C was attained, diffusion of the plasticizer would have accelerated (accounting for the second region of rapid increase in plasticizer uptake after 15 min). Thereafter, there was a third region in which the uptake of plasticizer approached an asymptotic value representative of the equilibrium solubility of ESO in PVC. Thus, the equilibrium solubility was calculated to be 72 g/100 g PVC at 80[degrees]C and was largely attained within 30 min at this temperature. At a test temperature of 120[degrees]C (which was well above the starting [T.sub.g] of the polymer), no induction period was evident and ESO absorption occurred much faster, with equilibrium solubility of 163 g/100 g PVC (much higher than that at 80[degrees]C). Clearly, the time to reach equilibrium solubility at 40-C would be extremely long, given the large difference from the starting [T.sub.g], of the polymer.

The morphology of PVC grains before, during and after plasticizer absorption was characterized by SEM (Fig. 6). The changing morphology of PVC particles with increasing amounts of absorbed ESO was clearly evident at a test temperature of 120[degrees]C. The PVC grains were ~150 [micro]m in diameter and composed of aggregates of primary particles of varying sizes, with voids or pores between the particles. The PVC grains were swollen by ESO entering the pores and subsequently absorbing in the polymer matrix, resulting in pronounced sintering (unlike at 80[degrees]C). As discussed earlier, the absorption of ESO in PVC, which depended on temperature and time, was a three step process comprising an "induction period," a "swelling period," and a "saturation period." The "induction period" corresponded to penetration of the pericellular membrane by the plasticizer, filling of the pores, and slight plasticizer absorption in the matrix polymer. This was followed by rapid uptake of ESO in the polymer, resulting in dilation of the particles and agglomerates (i.e., the "swelling period"). Finally, in the "saturation period," the uptake of ESO in PVC approached its asymptotic value and the swelling process was completed.

At a temperature of 40[degrees]C, the absorption of ESO in PVC remained in the induction stage for the entire 720 min of measurement (Fig. 4). When the temperature rose to 80[degrees]C, the induction period prevailed for the first 10 min and the absorption process was well into the swelling period after 20 min, with the saturation stage quite advanced after 60 min (Fig. 5). At a test temperature of 120[degrees]C, which was much greater than the starting [T.sub.g] of the polymer, the swelling stage was underway after 5 min and saturation had largely been completed within 60 min.

The uptake of ESO in PVC was also studied using a rheometer (in a variation of ASTM D-2396) by assessing time-dependent changes in torque and compound temperature at various set metal temperatures and mixer speeds. This test is conventionally run at 63 rpm and a set temperature of 88[degrees]C. Under these conditions, the torque rises for the first few minutes, but then remains largely flat for some time, before it increases sharply again (this point is referred to as "onset of drying") to a peak value and then drops to a minimum value (the latter is classified as the "dry blend point," and said to correspond to the completion of drying) (5), (16), (17), (43). The results of the present study are shown in Figs. 7-10. The conventional torque profile described above was only observed at 60 rpm and 80[degrees]C set temperature. However, parts of the classical torque profile were evident at some other conditions as well (60 rpm and 100[degrees]C; 60 rpm and 120[degrees]C; 30 rpm and 120[degrees]C).

At a set temperature of 80[degrees]C and 15 or 30 rpm, upon charging the composition to the mixer, the torque decreased initially even as the compound temperature dropped (Fig. 7a and b), corresponding largely to wetting of the PVC grains by the plasticizer (6), (43). However, as the ESO was absorbed in the PVC grains, a minimum torque was attained and the trend reversed (i.e., torque started increasing) in spite of the fact that the compound temperature was also rising. This would be accounted for by decreased lubrication action of the ESO due to absorption into the PVC resin. At both these set conditions, the torques approached (or reached) asymptotic values and the resulting materials were largely free-flowing dry powder, although there was slight evidence of sintering of the grains as the compound temperature exceeded 90[degrees]C (Fig. 7b). Raising the mixer speed to 60 rpm (Fig. 7c) initially resulted in the same torque trends as those observed at lower rpm, but subsequently showed rapid increase in torque and then decline, characteristic of completion of the drying process as reported in Ref. 43. Had the composition been removed from the mixer around the dry blend point, it would likely have been close to free-flowing, as the literature does state that PVC grains are able to move more freely past each other at this point (43). However, keeping the mixer running well past the dry blend point resulted in a partially fused composition, presumably due to increased flow, as the final temperature attained was 126[degrees]C (Fig. 7c). At a set temperature of 100[degrees]C (Fig. 8), the trends were similar to those observed at 80[degree]C, except that: (a) sintering was more pronounced at lower mixer speeds; (b) the dry blending characteristics at 60 rpm were not as well defined; and (c) the fusion of the composition ostensibly due to enhanced flow was relatively greater at the highest mixer speed. The set temperature of 120[degrees]C (well above the [T.sub.g] of the polymer) resulted in significantly different torque profiles, with slight sintering evident even at 15 rpm, significant flow and fusion even at 30 rpm, and comparatively advanced flow and fusion at 60 rpm (Fig. 9). An attempt was made to deduce the drying times and temperatures, as best as possible, from the torque curves (Table 1). The lower the set temperatures and/or mixer speeds, the longer were the characteristic times for onset, peak, and completion of dry blending.

TABLE 1. Dry blending characteristics deduced from torque curves.

                 80[degrees]C         100[degrees]C        120[degrees]C
                   and 60 rpm            and 60 rpm           and 60 rpm

            Time   Temperature   Time   Temperature   Time   Temperature
           (min)  ([degrees]C)  (min)  ([degrees]C)  (min)  ([degrees])C

Onset of     6.2           106    3.0           116    0.2           105
drying

Peak         9.1           115    4.6           123    0.7           110
drying

Dry blend   12.7           123    7.2           130    2.0           133
point
                 120[degrees]C
                         30rpm

            Time   Temperature
           (min)  ([degrees]C)

Onset of     3.6           123
drying

Peak       1 1.0           133
drying

Dry blend   12.8           134
point


The interpretation of results above is based on the conventional understanding in the literature. However, there is another hypothesis that is worth considering to explain the torque rheometer results, as proposed below. As the plasticizer is absorbed in the pores of the PVC resin and diffuses into the matrix polymer, and as the compound temperature increases under the influence of heat and shear, the PVC grains start sintering to form a physically bound network. When a critical temperature is reached (referred to above and in Table 1 as "onset of drying"), a sharp transition occurs in the extent of network formation leading to dramatically increased torque. When the generated network reaches a critical stage (corresponding to "peak drying" in Table 1 and the discussion above), it rapidly disintegrates under the influence of shear and temperature and the torque drops drastically until the so called "dry blend point" is achieved. From Table 1, it is interesting to note that the "onset of drying" (or, perhaps more correctly, "critical temperature for sintering") ranged from 105 to 123[degrees]C, which is close to the sintering temperature of plasticized PVC resins reported in Ref. 6. It is also worth noting that the new mechanism proposed above is analogous to that observed in crosslinked polymeric systems, where the torque or modulus undergoes a sharp increase when a critical network (gel content) is achieved, attains a maximum value, and then decays considerably due to mechanical degradation as further mixing leads to mastication of the highly crosslinked composition (44), (45).

In addition to the solubility and absorptive characteristics, another important consideration is the loss of plasticizers due to migration out of plasticized PVC compositions as it can have an impact on aged properties in end-use applications (46-49), such as jacket and insulation of cables (50). In this context, various samples of the flexible PVC compositions were made using ESO as the single plasticizer, and the migration characteristics in contact with activated carbon were studied as a function of time and temperature (40, 80, and 120[degrees]C), as shown in Fig. 11. The mass measurements were conducted after the specimens had been conditioned for 48 h at 23[degrees]C. None of the specimens exhibited evidence of exudation (spew) at the surface, indicating that any material that came out of the specimens was absorbed by the activated carbon. Of course, in addition to ESO, other additives or degradation products could contribute to the mass loss. The measurements at 80 and 120[degrees]C were also extrapolated to 5000 h (as shown in the insert).

It can be observed from Fig. 11 that the migration to activated carbon generally increased with increasing temperature and time. However, at a temperature of 40T, a strong resistance to migration was observed even after 96 h. These observations can likely be explained by the fact that the test temperature of 40[degrees]C was below the [T.sub.g] of the plasticized polymer (which was measured to be 54[degrees]C), whereas the other two test temperatures were well above the [T.sub.g] of the plasticized polymer. That is, below [T.sub.g], no migration occurred, whereas significant migration took place above [T.sub.g]. It is also worth noting that, at a temperature of 80[degrees]C, an induction period was evident for the first 6 h. This phenomenon was not observed at the temperature of 120[degrees]C, indicating that any induction stage at this temperature had duration of less than 6 h.

The color changes of molded specimens of the flexible PVC composition that were used for the migration test are shown in Fig. 12. As stated earlier, it is well recognized that PVC undergoes decomposition by dehydrochlorination at elevated temperatures, resulting in conjugated double bonds in the polymer chains and noticeable color change (from yellow to pink, orange, red, brown, and finally black) when the conjugated polyene sequences contain more than four to five double bonds (38), (39), (51), (52). Such degradation of the polymer is largely mitigated by the use of heat stabilizers, such as calcium-zinc metal soaps and [beta]-diketones, especially when used in combination with epoxides (51), (53-55).

The plasticized PVC composition did not show any obvious signs of degradation after melt blending for 5 min at 165[degrees]C and compression molding for 10 min at 180[degrees]C (i.e., the 0 h pictures in Fig. 12). Furthermore, the specimens aged at 40 and 80[degrees]C did not change color significantly over the duration of the test. In contrast, discoloration did occur at a temperature of 120[degrees]C, starting around 12 h into the test. However, even after 96 h at 120[degrees]C, the specimen had not turned black color (i.e., had not reached severe levels of decomposition). These results indicate that dehydrochlorination of the PVC resin was largely ameliorated by the mixed metal soap and ESO present in the formulation, for a few minutes at 165-180[degrees]C and several hours at temperatures up to 120[degrees]C. However, over a prolonged period of time at 120[degrees]C, the specimens did exhibit some degradation. This degradation might have influenced the migration behavior at 120[degrees]C, as depicted in Fig. 11.

The temperature- and time-dependent changes in hardness of the specimens used for the migration test are shown in Fig. 13. Note that all the hardness measurement were conducted after the specimens had been conditioned at 23[degrees]C for 48 h. The initial Shore D hardness (i.e., after compression molding, 0 h in the migration test) was 49.4 [+ or -] 0.8. The hardness values tended to increase over time at all three temperatures, but the change was only significant after 96 h at 40[degrees]C or 80[degrees]C, and after 12 h at 120[degrees]C. The largest increase in hardness occurred at the latter temperature. These results were generally consistent with the changes in mass and color discussed earlier, and reinforce the conclusion that substantial degradation of the compositions did not occur at the conditions used for the migration test (since the hardness values did not increase dramatically, which would have been evidence of the compositions turning brittle).

Additional analyses of the data discussed above were conducted to determine if the studies run over various ranges of temperature exhibited Arrhenius behavior. Data from Table 1 were excluded from these analyses, since the temperature was not constant during the course of any single experiment. Data from Fig. 5a (uptake of plasticizer by PVC powder at 10, 20, and 40 min vs. temperature) and Fig. 13 (change in Shore D as a function of temperature) were plotted in an Arrhenius plot (Fig. 14). The amounts of plasticizer absorbed in the allotted times (10, 20, or 40 min) were taken as the relative rates of absorption, and the percent increase in Shore hardness (after 96 h) was representative of relative rate of plasticizer loss. Activation energies were computed from the Arrhenius plot by multiplying the negatives of the slopes of the lines by the gas constant (8.314 J [K.sup.-1] [mo1.sup.-1]). Apparent activation energies (deduced from plasticizer uptake) were 21.8 and 22.7 kJ [mol.sub.-1] at 20 and 40 min, respectively. In contrast, the Arrhenius plot of the absorption data at 10 min suggests a change in activation energy with temperature, particularly around the [T.sub.g] of the unplas-ticized PVC. The apparent activation energy from the Shore D hardness measurements was 12.1 kJ [mol.sup.-1], that is, slightly lower for diffusion out of a fully plasticized and molded system than that derived from plasticizer uptake by PVC powder. Furthermore, using the two data points for percent mass loss after 96 h (plot not shown), the apparent activation energy was calculated as 42.3 kJ [mol.sup.-1], higher than that obtained from hardness measurements of the same specimens.

It is interesting to compare the apparent activation energies determined in this work to values reported in the literature for additive diffusion in other plasticized PVC compositions. Park and Van Hoang studied diffusion of dibutyltin dilaurate and dibutyltin bismonobutylmaleate in plasticized PVC (56). Activation energies for diffusion were found to be between 50 and 90 kJ [mol.sup.-1], increasing with higher plasticizer concentration up to about 60 phr plasticizer, then leveling out at higher levels. Griffiths et al. reported activation energies of 50-57 kJ [mol.sup.1] for diffusion of dialkylphthalates at 100 phr in PVC (57). Storey et al. found the activation energy for the initial slow step of absorption of di-n-decylphthalate into glassy PVC to be 255 kJ [mo1.sup.-1], which was attributed to the energy required for the plasticizer to solvate the PVC particle and change it from a glassy to a rubbery state (18). In other work, Storey et al. reported activation energy for diffusion of di-n-hexyl phthalate, which was 92 kJ [mol.sup.-1] above the PVC glass transition and 33 kJ [mo1.sup.-1] below it (58). The higher activation energy above [T.sub.g] was attributed to a cooperative motion of a greater number of consecutive chain segments above [T.sub.g] (58). Additional phthalate plasticizers showed activation energies in a similar range (up to about 121 kJ [mo1.sup.-1]) (58), (59). Gamage and Farid studied diffusion of epoxidized neem oil (10-50 pph as secondary plasticizer) from PVC films containing 30 pph of dioctylphthalate, and obtained activation energies ranging from about 75 to 130 kJ [mol.sup.-1] depending on composition (60). Lundsgaard et al. (61) determined the activation energy for diffusion of ESO out of PVC (50-67 phr as single plasticizer), over a temperature range of 2060[degrees]C, to range from 36 to 44 kJ [mo1.sup.-1]. The reason for the generally lower activation energy for diffusion of ESO in the present study as compared to values obtained for various plasticizers in the literature cited above is not clear.

To determine if the absorption of plasticizer into PVC powder obeyed Fickian diffusion, plasticizer uptake using data from Fig. 5 was plotted versus the square root of time, giving a very nonlinear curve (not shown) indicating non-Fickian behavior. However, mass loss and hardness change data from Figs. 11 and 13, which are from molded specimens of geometry in which edge effects can be neglected, are linear when plotted versus square root of time (see Fig. 15), suggesting that loss of plasticizer from the plaques occurred via Fickian diffusion.

The diffusion coefficient (D) was calculated from the mass loss data using the approach described in Refs. 49, 62, as shown in Eqs. 3 and 4.

[M.sub.t]/[M.sub.[infinity]] = 2[(Dt/[phi][l.sup.2]).sup.1/2] = 4[(D/[phi]).sup.1/2] X ([t.sup.1/2l) (3)

[D.sup.1/2] = ([M.sub.t]/[M.[infinity])/([t.sup.1/2l] x ([[phi].sup.1/2]/4) (4)

where M, is the mass loss at time t; [M.sub.[infinity]] is the mass loss at infinite time; and 2l is the thickness of the specimen. The computations were done for two temperatures (80 and 120[degrees]C). Two different values of Mx were used; one was the measurement at t = 96 h and the other was the extrapolated value at t = 5000 h (from Fig. 11). Table 2 shows the results of the calculations. The computed diffusion coefficients were of the same order as those extrapolated from Arrhenius plots of previous findings with ESO (ranging from 1.6 E-09 to 4.0 E-09 c.[m.sup.2]/s at 80[degrees]C and 7.3 E-09 to 13.6 E-09 [cm.sup.2]/s at 120[degrees]C) (61). The apparent activation energies (deduced from the calculated diffusion coefficients) were 8.0 and 12.6 kJ [mol.sup.1] using [M.sub.[infinity]] from t = 96 h and 5000 h, respectively; of the same order as that computed from Shore D hardness change earlier.

TABLE 2. Diffusion coefficients from migration data.

                               [M.sup.[infinity]] from t         [M.sup.
                                         96 h (measured)     [infinity]]
                                                                    from
                                                              t = 5000 h
                                                          (extrapolated)

                             80[degrees]C  120[degrees]C    80[degrees]C

[M.sub.[infinity]] (%)               0.09           0.39            0.21

Slope of                           0.1027         0.1179          0.0448
[M.sub.t]/[M.sub.[infinity]
versus

[t.sup.1/2]/2l
([h.sup.0.5/mm) plot

[R.sup.2] of trendline               0.73           0.92            0.73

D (c[m.sup.2)/s)                 5.75E-09       7.58E-09        1.09E-09

                            120[degrees]C

[M.sub.[infinity]](%)                0.82

Slope of                           0.0556
[M.sub.t]/[M.sub.[infinity]
versus

[t.sup.1/2]/2l
([h.sup.0.5/mm) plot

[R.sup.2] of trendline               0.92

D ([cm.sup.2)/s)                 1.69E-09


CONCLUSIONS

Thermal analyses of the ESO used in this study indicated that it was stable up to temperatures as high as 240-260[degrees]C (under nitrogen). The short-term plasticizer absorption test conducted at 40[degrees]C revealed that the pore volume of the PVC resin used in this investigation was --0.3 c[m.sup.3]/g.

ESO was found to be sufficiently soluble in the PVC matrix (72 g/100 g PVC at 80[degrees]C and 163 g/100 g PVC at 120[degrees]C) to make it an effective plasticizer for this polymer. The absorption of ESO in PVC grains, which depended on temperature and time, was a three step process comprising an "induction period," a "swelling period," and a "saturation period." In the induction stage, the pores of PVC grains were filled with plasticizer. The swelling stage corresponded to accelerated plasticizer uptake and dilation of the particles and agglomerates. In the saturation stage, the uptake of ESO in PVC approached its asymptotic value and the swelling process was completed.

Torque rheometer studies showed that the higher the mixer temperature and/or speed, the faster was the uptake of plasticizer in PVC and ultimate fusion of the plasticized composition.

Migration studies conducted on plasticized and stabilized PVC compositions indicated that, on the time scale of these experiments, no migration occurred at 40[degrees]C (below the [T.sub.g] of the plasticized polymer), whereas significant migration was observed at 80 and 120[degrees]C (above the [T.sub.g] of the plasticized polymer). Furthermore, there was no evidence of exudation (spew) at the surface of the molded specimens, indicating that any material that came out was absorbed by the activated carbon.

Apparent activation energies, as deduced from temperature-dependent uptake of ESO in PVC and change in hardness of the plasticized PVC, were 22-23 and 12 kJ [mo1.sup.-1], respectively. The diffusion coefficients computed from mass loss of the plasticized PVC ranged from 5.75 E-09 to 7.58 E-09 [cm.sup.2]/s at 80[degrees]C and 1.09 E-09 to 1.69 E-09 [cm.sup.2]/s at 120[degrees]C.

Correspondence to: B.I. Chaudhary; e-mail: bichaudhary@dow.com

DOI 10.1002/pen.23417

Published online in Wiley Online Library (wileyonlinelibrary.com).

[c] 2012 Society of Plastics Engineers

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Bin Sun, (1) Bharat Indu Chaudhary, (2) Chun-Yin Shen, (1) Di Mao, (1) Dong-Ming Yuan, (1) Gan-Ce Dai, (1) Bin Li, (3) Jeffrey M. Cogen (2)

(1) State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, People's Republic of China

(2) The Dow Chemical Company, 727 Norristown Road, Spring House, Pennsylvania 19477

(3) Dow Chemical (China) Company Limited, 936 Zhangheng Road, Shanghai 201203, People's Republic of China
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Author:Sun, Bin; Chaudhary, Bharat Indu; Shen, Chun-Yin; Mao, Di; Yuan, Dong-Ming; Dai, Gan-Ce; Li, Bin; Co
Publication:Polymer Engineering and Science
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Geographic Code:9CHIN
Date:Aug 1, 2013
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