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PLASTICIZERS IN POLYMER BLENDS
12
Plasticizers in Polymer Blends 12.1 PLASTICIZER PARTITION BETWEEN COMPONENT POLYMERS Simulations of partition coefficient of small amounts of monomer, solvent, or plasticizer in the two-phased polymer system were conducted based on the theoretical calculations of thermodynamical quantities, such as free energy of mixing, enthalpy and entropy of mixing.4 Partition coefficients of low molecular weight compound between polymethylmethacrylate and polyurethane forming interpenetrating network were varied between 0.1 (10 times more of low molecular substance in polymethylmethacrylate phase) and 10 (10 times more of low molecular substance in polyurethane phase). Free energy of mixing, enthalpy and entropy were calculated for each selected value of partition coefficient. The minimum of free energy of mixing occurs at partition coefficient equal unity (uniform distribution of low molecular weight compound in both phases). It was also observed that the enthalpy of mixing was constant and close to zero, and much smaller than corresponding entropy of mixing. This means that process of distribution of low molecular weight substances among the components of interpenetrating network is governed by entropic considerations which demand that low molecular weight component is evenly distributed among participating polymer. Similar results can be expected in the case of polymer blends. The above results of simulation imply that low molecular substances are distributed uniformly in most systems (different polymers and plasticizers). Potential exception occurs when one polymer of a blend has good compatibility with plasticizer and the other is incompatible. In such a case the entropic considerations become important.4 Figure 12.1 shows relationship between the composition of chlorinated polyethylene, CPE, and di-(2-ethylhexyl) phthalate, DOP, mixtures and glass transition temperature.5 Regression lines are very close for both relationships. This means that CPE behaves in a similar manner when tested in a simple mixture with DOP and in ternary blend containing also PVC. Figure 12.2 shows similar relationship for PVC which also behaves in a way similar to CPE. It can be noted that PVC relationship shows cusp at about 30 wt% of DOP as typical of PVC behavior (see more on this in Section 11.52.4 and Figures 11.53 and 11.54). The observed behaviors seem to suggest that addition of DOP to the system leads to its distribution between components. Two phases are maintained (two glass transition temperatures), which means that initially DOP is used for the formation of two separate blends those of PVC-DOP and CPE-DOP. Figure 12.3 shows that for a blend PVC/CPE having the same amounts of both components at least 55 wt% is required to transform
486
Plasticizers in Polymer Blends
PVC-DOP CPE-PVC-DOP
CPE-DOP CPE-PVC-DOP
300
400
y = 185.1 + 93.7x R= 0.993 y = 187.0 + 89.8x R= 0.993
280
350
g
220
250 200
200 180
300
o
T , C
240
g
o
T , C
260
0
0.2
0.4 0.6 0.8 CPE fraction
1
Figure 12.1. Glass transition temperature of CPEDOP compositions measured in the blends of CPE and DOP and in the ternary blends containing CPE, PVC, and DOP. [Data from Champagne M F; Prud'homme R E, J. Polym. Sci.: Polym. Phys. Ed., 32, No.4, March 1994, p.615-24.]
150
0
0.2
0.4 0.6 0.8 PVC fraction
1
Figure 12.2. Glass transition temperature of PVC-DOP compositions measured in the blend of PVC and DOP and in the ternary blends containing CPE, PVC, and DOP. [Data from Champagne M F; Prud'homme R E, J. Polym. Sci.: Polym. Phys. Ed., 32, No.4, March 1994, p.615-24.]
blend to a uniform mixture as determined by the presence of one glass transition temperature. This amount of DOP depends on the incompatibility of both polymers forming blend. For example, for blend of PVC and chlorinated PVC only 20 wt% is required to form one phase material (one Tg). The experimental data discussed here support results of theoretical simulation which were previously reported. It is important to note that two separate phases are present until a certain concentration of DOP is reached which is large enough to counterbalance the repulsion forces Figure 12.3. Phase diagram of ternary CPE-PVC-DOP blend. Open circles are for blends having one glass tran- between two immiscible polymers.5 When sition temperature, Tg, closed circles are for blends having two Tgs (of PVC-DOP and CPE-DOP). The number this concentration of DOP is reached on each dotted line is the value of interaction parameter homogeneous ternary phase is formed. of PVC and CPE used in calculations. The experimental This behavior is common of ternary blends curve is represented by a solid line. [Adapted, by percontaining a third component which is mismission, from Champagne M F; Prud'homme R E, J. Polym. Sci.: Polym. Phys. Ed., 32, No.4, March 1994, cible with both polymers which are immisp.615-24.] cible when used alone. It can be anticipated that exceeding this critical concentration leads to dissolution of the binary phases (e.g., PVC-DOP and CPE-DOP) into a homogeneous ternary blend.5
12.1 Plasticizer partition between component polymers
487
This behavior can be expected from systems in which entropic factors can be neglected. ABS/PMMA blend plasticized with a mixture of ethylene and propylene carbonates is an example of system where entropic factors play a role. PMMA is plasticized in polymer electrolytes to increase ionic conductivity.1 Addition of plasticizer in the amount sufficient to achieve required conductivity decreases mechanical performance of gel to the level that it needs to be reinforced. ABS is added as reinforcing polymer and sufficient mechanical properties are obtained. Two phases are obtained: plasticizer reach phase giving pathway for ion transport and ABS-rich phase which acts as matrix increasing mechanical performance. ABS is miscible with PMMA forming transparent blends if no plasticizer is added. Addition of plasticizer results in phase separation (opaque materials) because of incompatibility between ABS and plasticizer. This is example of system in which plasticizer is not uniformly distributed among participating polymers but plasticizer is found in PMMA-rich phase.1 The following equation was proposed to calculate glass transition temperature of polymer blend and to account for solvent quality of plasticizer by the use of parameter “b”:7 T g = w 1 T g + w 2 T g + b ( T g – T g )w 1 w 2 1
2
2
1
[12.1]
where: Tg glass transition temperature w weight fraction 1,2 indices of plasticizer and polymer, respectively b parameter characteristic of solvent quality of plasticizer.
Molecular weight of polymer and certain concentration of plasticizer determine the uniformity blend. If molecular weight of PVC used in CPE-PVC-DOP blend is doubled the concentration of DOP needed to obtain homogeneous mixture is increased from 55 wt% to 70 wt% of DOP.5 A statistical thermodynamic approach was developed8 for mixing of polymers, small molecules (plasticizers), and holes which are different in sizes. Plasticizer efficiency is determined by polymer-plasticizer interaction and plasticizer segment size. Deuterated and no-deuterated plasticizer (benzyl butyl phthalate) was added to bilayer of thin films of polystyrene and polymethylmethacrylate.9 addition of plasticizer improved compatibility between polymers and increased degree of entanglement at polymer interface. This resulted in improved interfacial strength.9 Interesting case was reported for the ability of plasticized PVC to sorb drugs.10 The amount of sorbed drug was not only linked to the amount of plasticizer in the tubing and to the solubility of the drug in the plasticizer alone but it also depended on specific interactions between plasticizer and PVC.10 This shows that any other component of the mixture may influence partition of plasticizer, not just another polymer, but any one of many frequently added additives (some of them are sometimes also having polymeric structure).
488
Plasticizers in Polymer Blends
12.2 INTERACTION OF PLASTICIZERS WITH BLEND COMPONENTS Interaction of plasticizer and matrix polymer was described in Section 5.2 as being caused by: • polarization of charges in molecular fragments • hydrogen bonding • Lewis acid-base interactions. The previous section suggests that the presence of another polymer in most cases is not likely to affect interactions because plasticizer is evenly distributed and acts in individual domains of component polymers. Section 11.11.4 gives an example of diblock PCL/PS copolymer whose crystallization is affected because PS block solidifies first and retards motion of PCL block which is responsible for crystallization of copolymer. Selective plasticization of PS block remedies the problem. In the previous section, the example of plasticization of PMMA/ABS blend shows that only one component (PMMA) is plasticized. These two cases show that the presence of more than one polymer may complicate structure in the presence of plasticizers. Potential reasons for these complications are as follows: • incompatibility with component polymer(s) • retarded mobility of a segment or component • heterogeneous dependence of dynamic moduli2 • selective effect on amorphous phase12 • suppression of lamellar thickening by inclusion of small molecules in the amorphous fold surfaces12 • effect on size of spherulites12 • effect on crystallization rate12 • effect on conformation of chain segments interacting with plasticizer or in close neighborhood • amount of one polymer may be used to control crystalline structure of the other component polymer.12 The above list shows that plasticizer interaction may selectively affect a component of blend and influence its morphology and thus mechanical performance of blend. A few contributions2,11,12 available so far are not sufficient to make a judgement on the magnitude of these effect or even completeness of the above list. Larger concentration of glycerin in starch/PVAl blends causes phase separation of blend components.13 Phase inversion was observed in EPDM/PP blend when amount of plasticizer was increased.14 Epoxidized soybean oil increased compatibility of PVC/NBR similar to its mixture with DOP.15 Interphase transfer of di-2-ethylhexyl adipate between ethylene-propylene copolymer and polyisobutylene in the laminated sheets was temperature controlled. DOA moved to EPR from PIB at −20°C and from PIB to EPR at 40°C.16
12.3 Effect of plasticizers on blend properties
489
12.3 EFFECT OF PLASTICIZERS ON BLEND PROPERTIES Plasticizers may affect properties of polymer blends such as: • compatibility5,28,29 • miscibility33 • glass transition temperature3,5,17 • morphology23,31,33 • rheology33 • surface properties • optical properties • electrical properties22,24,28,34,35 • thermal stability35 • material durability • mechanical performance3,17,19,22,25,34 Some details on these effects are discussed below. In addition to the mechanisms of miscibility between PVC and CPE and CPVC discussed in Section 12.1, increased levels of DOP decrease interfacial tension between the two phases and improve their interpenetration.5 This lowering of the physical barrier results eventually in formation of truly ternary phase. Decrease in the interfacial tension between another immiscible pair of polyvinylchloride and poly(3-octylthiophene) by addition of DOP is also credited for formation of co-continuous phase.28 Figures 12.1 and 12.2 show the effect of plasticizer addition on the glass transition temperatures of component polymers.5 Because polymers are plasticized on their own, separate domains exist, and there is nothing that distinguishes behavior of polymer in blend from the behavior of polymer mixed with plasticizer. There is no direct literature on the effect of plasticizers on morphology but some of these influences may be anticipated from the effect of solvents which have plasticizing effect on dissolved polymers.23,31 Figure 12.4 shows that there is a substantial difference in morphologies of PMMA/PS 50/50 blends obtained without and with the presence of supercritical carbon dioxide. Much coarser PMMA domain sizes are obtained when supercritical carbon dioxide Figure 12.4. Morphology of PMMA/PS blends in pres- is present because its addition lowers melt ence and without supercritical CO2. Blends were viscosity and interfacial tension, and obtained either by mixing in high pressure mixer or in increases free volume and chain mobility. twin-screw extruder. [Data from Elkovitch M D; Lee L J; Tomasko D L, Antec '99. Volume II. Conf. proc., SPE, This allows for more effective blending of New York City, 2nd-6th May 1999, p.2811-5.] components. Viscosity decrease of polymer
490
Plasticizers in Polymer Blends
Conductivity, S cm
-1
10
-1
10-3 10-5 10
-7
10
-9
10-11 10-13 0
Figure 12.5. The structure of DOP near polyaniline, PANI (a) and near polystyrene, PS, as a result of molecular simulation. [Adapted, by permission, from Zilberman M; Siegmann A; Narkis M, Polym. Adv. Technol., 11, No.1, Jan.2000, p.20-6.]
3 6 9 12 DOP content, wt%
15
Figure 12.6. Electrical conductivity of PS+DOP/PANI 80/20 blends. [Data from Zilberman M; Siegmann A; Narkis M, Polym. Adv. Technol., 11, No.1, Jan.2000, p.20-6.]
solution was found to increase phase separation when thermodynamically unstable system was left without mixing.31 Studies on electrical conductivity suggest possible interaction between each component polymer and plasticizer. Molecular simulation data in Figure 12.5 show that DOP molecule may locate itself between two polymers with aromatic group and two ester groups near PANI molecule and aliphatic chains near PS.21 Glass transition temperature studies support results of this simulation because after addition of PANI to PS plasticized with DOP, the glass transition temperature of PS was increased by 5oC, which suggests that some DOP was taken out of PS domain into its interphase with or by PANI itself.21 This finding can be taken as an interesting example of possible mechanism of compatibilization of two immiscible polymers. Also, Figure 12.6 shows that conductivity rapidly increases to begin levelling above 10 wt% DOP. This is understandable in view of the fact that only 0.5-1% DOP migrates to interphase between PS and PANI therefore larger concentrations of DOP are not utilized to increase conductivity. These three independent observations seem to confirm interesting mechanism of compatibilization of two immiscible polymers by plasticizer which interacts with both partners. The above study is further confirmed by an invention of conductive material which is composed of intrinsically conductive polymer salt incompatible with matrix polymer, conductive phase polymer partially compatible with both polymer salt and matrix polymer, and plasticizer that is capable compatibilizing the conductive phase polymer and intrinsically conductive polymer salt which makes both to form concentrated conductive phase.20 Sulfonamide and phosphate plasticizers, used alone or in combination are useful in particular combination of polymers used in invention.20 Conductivity is also increased with addition of DOP to PVC/poly(3-octylthiophene) blend which is compatibilized by plasticization. In this case, the authors explain compatibilizing action of plasticizer by lowering surface tension between two incompatible polymers.24 It should be noted that
12.4 Blending to reduce or to replace plasticizers
491
this last explanation may also relate observed changes to migration of plasti700 cizer to interphase as discussed above. PVC/PU blends were compounded 600 from rigid and plasticized PVC (plasticized 500 PVC contained 36 phr DOP).3 Figure 12.7 shows that elongation behavior of two 400 blends is dramatically different. It is sus300 pected that plasticizer helps in compatibilization of blend components. The blend 200 with plasticized PVC appears to be uniform 100 on the macroscopic scale.3 addition of DOP 0 to ABS/PVC blend improved impact 0 20 40 60 80 100 strength and elongation but hardness, tenPolyurethane concentration, wt% sile strength, and Young modulus were reduced.12 Figure 12.7. Elongation of PVC (plasticized with 36 wt% DOP and unplasticized) and polyurethane, PU, Addition of poly(ethylene glycol), blend vs. PU concentration. [Adapted, by permission, PEG, (molecular weight of 400 daltons) to from Petrovic Z S; Javni I; Zhang W, Polyurethanes polyvinylalcohol/methylcellulose blend Expo '99. Conf. proc., American Plast. Council, Alliance for the Polyurethanes Ind., Orlando, Fl., 12th-15th involves interesting synergistic mechanism. Sept.1999, p.79-82.] PEG helps to bring water to the system and water acts as plasticizer of polyvinylalcohol while PEG plasticizes methylcellulose. This synergistic plasticization improves mechanical performance of blend and compatibilizes its components.18 When a plasticizer is added to the polyamide/poly(isobutylene-co-p-methylstyrene) blend elastomer continuity development and cocontinuity shifts to higher concentrations. When plasticized PA is the dispersed phase, however, these two phenomena oppose each other. Addition of 5 and 10 wt% chitosan acted as a plasticizer to starch matrix, increasing the elongation at break (from 35 to 56%) and decreasing tensile strength and elastic modulus.38 The above review of existing literature shows that a little is known about fundamental reasons which determine properties of plasticized blends.32 With a few exceptions papers record properties rather than to analyze principles of their formation.32 Elongation at break, %
plasticized rigid
12.4 BLENDING TO REDUCE OR TO REPLACE PLASTICIZERS For more than 30 years properties of plasticized PVC have been improved by blending with nitrile rubber, NBR.19 Figure 12.8 shows that it is possible to increase elongation of material with reducing amount of DOP. All materials presented in Figure 12.8 have the same shore A hardness. Figure 12.9 shows that plasticizer migration is substantially reduced by addition of NBR. Almost linear reduction of plasticizer migration with improvement of elongation shows that NBR has supplementary plasticizing action. Plasticizers such as for example DOP are required to improve aging resistance of blends of PVC and epoxidized natural rubber.22 Further addition of NBR helps to retain
492
Plasticizers in Polymer Blends
y = 7.54 - 0.19x R= 0.997
y = 804.4 - 7.919x R= 0.99998
8 7 Weight loss, %
Elongation at break, %
580
540
500
460 30
34 38 42 DOP concentration, wt%
Figure 12.8. Elongation at break vs. DOP concentration in the following PVC/DOP/NBR blends: 57.5/ 42.5/0, 54/36/10, 50/30/20. [Data from Thomas N L; Harvey R J, Progress Rubber Plast. Technol., 17, No.1, 2001, p.1-12.]
6 5 4 3
0
5 10 15 20 NBR concentration, wt%
Figure 12.9. Migration loss to rigid PVC after 30 days at 70oC vs. DOP concentration in the following PVC/ DOP/NBR blends: 57.5/42.5/0, 54/36/10, 50/30/20. [Data from Thomas N L; Harvey R J, Progress Rubber Plast. Technol., 17, No.1, 2001, p.1-12.]
aging resistance of blend, eliminate low molecular weight plasticizer, and improve elastomeric properties of composition. Several materials were blended with PVC and properties of blends were compared with effect of plasticization by DOP.36 These included nitrile rubber, NBR, poly(ethyleneco-vinyl acetate), EVA, and poly(ethylene-co-vinyl acrylate-acrylate), Ac. Increase in plasticizer concentration (DOP) normally reduces torque and final melt temperature of blend but this is not the case when NBR replaces plasticizer. EVA behaves in similar manner to NBR but Ac reduces torque required for mixing. Only DOP rapidly decreases the glass transition temperature. Blending with other polymers has much smaller influence on the glass transition temperature. Changes in mechanical properties by blending with the above polymers and DOP addition are very similar as measured by ultimate tensile strength, Young modulus, and elongation at break. This study shows that processing properties are more improved by low molecular weight plasticizers but mechanical performance can be successfully improved by blending with various polymers.36 General assessment of studies on plasticizers use in polymer blends shows that large amount of work is required before results of blending will become predictable. So far blending is more based on trial-and-error mixing of components than on fundamental research data.
REFERENCES 1 2 3 4 5
Xinping Hou; Kok Siong Siow, Polymer, 41, No.24, 2000, p.8689-96. Zarraga A; Pena J J; Munoz M E; Santamaria A, J. Polym. Sci.: Polym. Phys. Ed., 38, No.3, 1st Feb.2000, p.469-77. Petrovic Z S; Javni I; Zhang W, Polyurethanes Expo '99. Conf. proc., American Plast. Council, Alliance for the Polyurethanes Ind., Orlando, Fl., 12th-15th Sept.1999, p.79-82. Mishra V; Thomas D A; Sperling L H, J. Polym. Sci.: Polym. Phys. Ed., 34, No.12, 15th Sept.1996, p.2105-8. Champagne M F; Prud'homme R E, J. Polym. Sci.: Polym. Phys. Ed., 32, No.4, March 1994, p.615-24.
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Hansen C M, Hansen Solubility Parameters, CRC Press, Boca Raton, 2000. Jenckel E; Heusch R, Kolloid Z., 130, 1953, p.89-105. Dong Z; Fried J R, Comput.Theor. Polym. Sci., 7, 1997, p.53. Sivaniah E, Jones R A L, Higgins D, Macromolecules, 42, 8844-50, 2009. Al Salloum, H; Aymes-Chodur, C; Barakat, H; Yagoubi, N, Int. J. Pharmaceutics, 496, 664-75, 2015. Karpukhin A A; Ledeneva I N; Aleksandrov V I, Kauchuk Rezina (USSR), No.4, 1995, p.10-2. Marentette J M; Brown G R, Polymer, 39, Nos.6-7, 1998, p.1415-27. Sreekumar P A, Al-Harthi M A, De S K, J. Appl. Polym. Sci., 123, 135-42, 2012. Sahhbikian S, Carreau P J, Heuzey M-C, Ellul M D, Nadella H P, Cheng J, Shirodkar P, Polym. Eng. Sci., 51, 2314-27, 2011. Ghiou N, Benaniba M T, Int. J. Polym. Mater., 59, 463-74, 2010. Kahukongkiat, N; Wachteng, V; Nobukawa, S; Yamaguchi, M, Polymer, 78, 208-11, 2015. Belhaneche-Bensemra N; Bedda A, Macromol. Symp., 176, Nov. 2001, p.145-53. Jun-Seo Park; Jang-Woo Park; Ruckenstein E, J. Appl. Polym. Sci., 80, No.10, 31st May 2001, p.1825-34. Thomas N L; Harvey R J, Progress Rubber Plast. Technol., 17, No.1, 2001, p.1-12. Cooper A I, J. Mater. Chem., 10, No.2, Feb.2000, p.207-34. Zilberman M; Siegmann A; Narkis M, Polym. Adv. Technol., 11, No.1, Jan.2000, p.20-6. Ishiaku U S; Lim F S; Ishak Z A; Senake Perera M C, Polym. Plast. Technol. Eng., 38, No.5, 1999, p.939-45. Elkovitch M D; Lee L J; Tomasko D L, Antec '99. Volume II. Conf. proc., SPE, New York City, 2nd-6th May 1999, p.2811-5. US Patent 5,908,898. US Patent 5,834,543. US Patent 5,747,588. Deanin R D; Barry C M; Kourtis S, Antec '98. Volume III. Conf. proc., SPE, Atlanta, Ga., 26th-30th April 1998, p.3327-9. Ljungqvist N; Hjertberg T, Synthetic Metals, 71, Nos.1-3, 1st April 1995, p.2251-2. Kalkar A K; Parkhi P S, J. Appl. Polym. Sci., 57, No.2, 11th July 1995, p.233-43. Mayer J M; Elion G R; Buchanan C M; Sullivan B K; Pratt S D; Kaplan D L, J. Macromol. Sci. A, A32, No.4, 1995, p.775-85. Roeder T, Morgenstern B, Kammer H W, Polym. Networks Blends, 3, 4, 19993, p.203-6. Bhadane P A, Tsou A H, Cheng J, Ellul M, Favis B D, Polymer, 52, 5107-17, 2011. Aouachria K, Belhaneche-Bensemra N, Polym. Testing, 25, 1101-8, 2006. Souza F G, Soares B G, Siddramaiah, Barra G M O, Herbst M H, Polymer, 47, 7548-53, 2006. Rajendran S, Sivakumar M, Subadevi R, Mater. Lett., 58, 641-49, 2004. Pena J R; Hidalgo M; Mijangos C, J. Appl. Polym. Sci., 75, No.10, 7th March 2000, p.1303-12. Tsuchiya M; Ishii M; Kojima T; Oohira Y, J. Thermal Analysis Calorimetry, 56, No.3, 1999, p.1163-6. Mendes, J F; Paschoalin, R T; Carmona, V B; Sena Neto, A R; Marques, A C P; Marconcini, J M; Mattoso, L H C; Medeiros, E S; Oliveira, J E, Carbohydrate Polym., 137, 452-8, 2016.
Plasticizers in Polymer Blends 12.1 PLASTICIZER PARTITION BETWEEN COMPONENT POLYMERS Simulations of partition coefficient of small amounts of monomer, solvent, or plasticizer in the two-phased polymer system were conducted based on the theoretical calculations of thermodynamical quantities, such as free energy of mixing, enthalpy and entropy of mixing.4 Partition coefficients of low molecular weight compound between polymethylmethacrylate and polyurethane forming interpenetrating network were varied between 0.1 (10 times more of low molecular substance in polymethylmethacrylate phase) and 10 (10 times more of low molecular substance in polyurethane phase). Free energy of mixing, enthalpy and entropy were calculated for each selected value of partition coefficient. The minimum of free energy of mixing occurs at partition coefficient equal unity (uniform distribution of low molecular weight compound in both phases). It was also observed that the enthalpy of mixing was constant and close to zero, and much smaller than corresponding entropy of mixing. This means that process of distribution of low molecular weight substances among the components of interpenetrating network is governed by entropic considerations which demand that low molecular weight component is evenly distributed among participating polymer. Similar results can be expected in the case of polymer blends. The above results of simulation imply that low molecular substances are distributed uniformly in most systems (different polymers and plasticizers). Potential exception occurs when one polymer of a blend has good compatibility with plasticizer and the other is incompatible. In such a case the entropic considerations become important.4 Figure 12.1 shows relationship between the composition of chlorinated polyethylene, CPE, and di-(2-ethylhexyl) phthalate, DOP, mixtures and glass transition temperature.5 Regression lines are very close for both relationships. This means that CPE behaves in a similar manner when tested in a simple mixture with DOP and in ternary blend containing also PVC. Figure 12.2 shows similar relationship for PVC which also behaves in a way similar to CPE. It can be noted that PVC relationship shows cusp at about 30 wt% of DOP as typical of PVC behavior (see more on this in Section 11.52.4 and Figures 11.53 and 11.54). The observed behaviors seem to suggest that addition of DOP to the system leads to its distribution between components. Two phases are maintained (two glass transition temperatures), which means that initially DOP is used for the formation of two separate blends those of PVC-DOP and CPE-DOP. Figure 12.3 shows that for a blend PVC/CPE having the same amounts of both components at least 55 wt% is required to transform
486
Plasticizers in Polymer Blends
PVC-DOP CPE-PVC-DOP
CPE-DOP CPE-PVC-DOP
300
400
y = 185.1 + 93.7x R= 0.993 y = 187.0 + 89.8x R= 0.993
280
350
g
220
250 200
200 180
300
o
T , C
240
g
o
T , C
260
0
0.2
0.4 0.6 0.8 CPE fraction
1
Figure 12.1. Glass transition temperature of CPEDOP compositions measured in the blends of CPE and DOP and in the ternary blends containing CPE, PVC, and DOP. [Data from Champagne M F; Prud'homme R E, J. Polym. Sci.: Polym. Phys. Ed., 32, No.4, March 1994, p.615-24.]
150
0
0.2
0.4 0.6 0.8 PVC fraction
1
Figure 12.2. Glass transition temperature of PVC-DOP compositions measured in the blend of PVC and DOP and in the ternary blends containing CPE, PVC, and DOP. [Data from Champagne M F; Prud'homme R E, J. Polym. Sci.: Polym. Phys. Ed., 32, No.4, March 1994, p.615-24.]
blend to a uniform mixture as determined by the presence of one glass transition temperature. This amount of DOP depends on the incompatibility of both polymers forming blend. For example, for blend of PVC and chlorinated PVC only 20 wt% is required to form one phase material (one Tg). The experimental data discussed here support results of theoretical simulation which were previously reported. It is important to note that two separate phases are present until a certain concentration of DOP is reached which is large enough to counterbalance the repulsion forces Figure 12.3. Phase diagram of ternary CPE-PVC-DOP blend. Open circles are for blends having one glass tran- between two immiscible polymers.5 When sition temperature, Tg, closed circles are for blends having two Tgs (of PVC-DOP and CPE-DOP). The number this concentration of DOP is reached on each dotted line is the value of interaction parameter homogeneous ternary phase is formed. of PVC and CPE used in calculations. The experimental This behavior is common of ternary blends curve is represented by a solid line. [Adapted, by percontaining a third component which is mismission, from Champagne M F; Prud'homme R E, J. Polym. Sci.: Polym. Phys. Ed., 32, No.4, March 1994, cible with both polymers which are immisp.615-24.] cible when used alone. It can be anticipated that exceeding this critical concentration leads to dissolution of the binary phases (e.g., PVC-DOP and CPE-DOP) into a homogeneous ternary blend.5
12.1 Plasticizer partition between component polymers
487
This behavior can be expected from systems in which entropic factors can be neglected. ABS/PMMA blend plasticized with a mixture of ethylene and propylene carbonates is an example of system where entropic factors play a role. PMMA is plasticized in polymer electrolytes to increase ionic conductivity.1 Addition of plasticizer in the amount sufficient to achieve required conductivity decreases mechanical performance of gel to the level that it needs to be reinforced. ABS is added as reinforcing polymer and sufficient mechanical properties are obtained. Two phases are obtained: plasticizer reach phase giving pathway for ion transport and ABS-rich phase which acts as matrix increasing mechanical performance. ABS is miscible with PMMA forming transparent blends if no plasticizer is added. Addition of plasticizer results in phase separation (opaque materials) because of incompatibility between ABS and plasticizer. This is example of system in which plasticizer is not uniformly distributed among participating polymers but plasticizer is found in PMMA-rich phase.1 The following equation was proposed to calculate glass transition temperature of polymer blend and to account for solvent quality of plasticizer by the use of parameter “b”:7 T g = w 1 T g + w 2 T g + b ( T g – T g )w 1 w 2 1
2
2
1
[12.1]
where: Tg glass transition temperature w weight fraction 1,2 indices of plasticizer and polymer, respectively b parameter characteristic of solvent quality of plasticizer.
Molecular weight of polymer and certain concentration of plasticizer determine the uniformity blend. If molecular weight of PVC used in CPE-PVC-DOP blend is doubled the concentration of DOP needed to obtain homogeneous mixture is increased from 55 wt% to 70 wt% of DOP.5 A statistical thermodynamic approach was developed8 for mixing of polymers, small molecules (plasticizers), and holes which are different in sizes. Plasticizer efficiency is determined by polymer-plasticizer interaction and plasticizer segment size. Deuterated and no-deuterated plasticizer (benzyl butyl phthalate) was added to bilayer of thin films of polystyrene and polymethylmethacrylate.9 addition of plasticizer improved compatibility between polymers and increased degree of entanglement at polymer interface. This resulted in improved interfacial strength.9 Interesting case was reported for the ability of plasticized PVC to sorb drugs.10 The amount of sorbed drug was not only linked to the amount of plasticizer in the tubing and to the solubility of the drug in the plasticizer alone but it also depended on specific interactions between plasticizer and PVC.10 This shows that any other component of the mixture may influence partition of plasticizer, not just another polymer, but any one of many frequently added additives (some of them are sometimes also having polymeric structure).
488
Plasticizers in Polymer Blends
12.2 INTERACTION OF PLASTICIZERS WITH BLEND COMPONENTS Interaction of plasticizer and matrix polymer was described in Section 5.2 as being caused by: • polarization of charges in molecular fragments • hydrogen bonding • Lewis acid-base interactions. The previous section suggests that the presence of another polymer in most cases is not likely to affect interactions because plasticizer is evenly distributed and acts in individual domains of component polymers. Section 11.11.4 gives an example of diblock PCL/PS copolymer whose crystallization is affected because PS block solidifies first and retards motion of PCL block which is responsible for crystallization of copolymer. Selective plasticization of PS block remedies the problem. In the previous section, the example of plasticization of PMMA/ABS blend shows that only one component (PMMA) is plasticized. These two cases show that the presence of more than one polymer may complicate structure in the presence of plasticizers. Potential reasons for these complications are as follows: • incompatibility with component polymer(s) • retarded mobility of a segment or component • heterogeneous dependence of dynamic moduli2 • selective effect on amorphous phase12 • suppression of lamellar thickening by inclusion of small molecules in the amorphous fold surfaces12 • effect on size of spherulites12 • effect on crystallization rate12 • effect on conformation of chain segments interacting with plasticizer or in close neighborhood • amount of one polymer may be used to control crystalline structure of the other component polymer.12 The above list shows that plasticizer interaction may selectively affect a component of blend and influence its morphology and thus mechanical performance of blend. A few contributions2,11,12 available so far are not sufficient to make a judgement on the magnitude of these effect or even completeness of the above list. Larger concentration of glycerin in starch/PVAl blends causes phase separation of blend components.13 Phase inversion was observed in EPDM/PP blend when amount of plasticizer was increased.14 Epoxidized soybean oil increased compatibility of PVC/NBR similar to its mixture with DOP.15 Interphase transfer of di-2-ethylhexyl adipate between ethylene-propylene copolymer and polyisobutylene in the laminated sheets was temperature controlled. DOA moved to EPR from PIB at −20°C and from PIB to EPR at 40°C.16
12.3 Effect of plasticizers on blend properties
489
12.3 EFFECT OF PLASTICIZERS ON BLEND PROPERTIES Plasticizers may affect properties of polymer blends such as: • compatibility5,28,29 • miscibility33 • glass transition temperature3,5,17 • morphology23,31,33 • rheology33 • surface properties • optical properties • electrical properties22,24,28,34,35 • thermal stability35 • material durability • mechanical performance3,17,19,22,25,34 Some details on these effects are discussed below. In addition to the mechanisms of miscibility between PVC and CPE and CPVC discussed in Section 12.1, increased levels of DOP decrease interfacial tension between the two phases and improve their interpenetration.5 This lowering of the physical barrier results eventually in formation of truly ternary phase. Decrease in the interfacial tension between another immiscible pair of polyvinylchloride and poly(3-octylthiophene) by addition of DOP is also credited for formation of co-continuous phase.28 Figures 12.1 and 12.2 show the effect of plasticizer addition on the glass transition temperatures of component polymers.5 Because polymers are plasticized on their own, separate domains exist, and there is nothing that distinguishes behavior of polymer in blend from the behavior of polymer mixed with plasticizer. There is no direct literature on the effect of plasticizers on morphology but some of these influences may be anticipated from the effect of solvents which have plasticizing effect on dissolved polymers.23,31 Figure 12.4 shows that there is a substantial difference in morphologies of PMMA/PS 50/50 blends obtained without and with the presence of supercritical carbon dioxide. Much coarser PMMA domain sizes are obtained when supercritical carbon dioxide Figure 12.4. Morphology of PMMA/PS blends in pres- is present because its addition lowers melt ence and without supercritical CO2. Blends were viscosity and interfacial tension, and obtained either by mixing in high pressure mixer or in increases free volume and chain mobility. twin-screw extruder. [Data from Elkovitch M D; Lee L J; Tomasko D L, Antec '99. Volume II. Conf. proc., SPE, This allows for more effective blending of New York City, 2nd-6th May 1999, p.2811-5.] components. Viscosity decrease of polymer
490
Plasticizers in Polymer Blends
Conductivity, S cm
-1
10
-1
10-3 10-5 10
-7
10
-9
10-11 10-13 0
Figure 12.5. The structure of DOP near polyaniline, PANI (a) and near polystyrene, PS, as a result of molecular simulation. [Adapted, by permission, from Zilberman M; Siegmann A; Narkis M, Polym. Adv. Technol., 11, No.1, Jan.2000, p.20-6.]
3 6 9 12 DOP content, wt%
15
Figure 12.6. Electrical conductivity of PS+DOP/PANI 80/20 blends. [Data from Zilberman M; Siegmann A; Narkis M, Polym. Adv. Technol., 11, No.1, Jan.2000, p.20-6.]
solution was found to increase phase separation when thermodynamically unstable system was left without mixing.31 Studies on electrical conductivity suggest possible interaction between each component polymer and plasticizer. Molecular simulation data in Figure 12.5 show that DOP molecule may locate itself between two polymers with aromatic group and two ester groups near PANI molecule and aliphatic chains near PS.21 Glass transition temperature studies support results of this simulation because after addition of PANI to PS plasticized with DOP, the glass transition temperature of PS was increased by 5oC, which suggests that some DOP was taken out of PS domain into its interphase with or by PANI itself.21 This finding can be taken as an interesting example of possible mechanism of compatibilization of two immiscible polymers. Also, Figure 12.6 shows that conductivity rapidly increases to begin levelling above 10 wt% DOP. This is understandable in view of the fact that only 0.5-1% DOP migrates to interphase between PS and PANI therefore larger concentrations of DOP are not utilized to increase conductivity. These three independent observations seem to confirm interesting mechanism of compatibilization of two immiscible polymers by plasticizer which interacts with both partners. The above study is further confirmed by an invention of conductive material which is composed of intrinsically conductive polymer salt incompatible with matrix polymer, conductive phase polymer partially compatible with both polymer salt and matrix polymer, and plasticizer that is capable compatibilizing the conductive phase polymer and intrinsically conductive polymer salt which makes both to form concentrated conductive phase.20 Sulfonamide and phosphate plasticizers, used alone or in combination are useful in particular combination of polymers used in invention.20 Conductivity is also increased with addition of DOP to PVC/poly(3-octylthiophene) blend which is compatibilized by plasticization. In this case, the authors explain compatibilizing action of plasticizer by lowering surface tension between two incompatible polymers.24 It should be noted that
12.4 Blending to reduce or to replace plasticizers
491
this last explanation may also relate observed changes to migration of plasti700 cizer to interphase as discussed above. PVC/PU blends were compounded 600 from rigid and plasticized PVC (plasticized 500 PVC contained 36 phr DOP).3 Figure 12.7 shows that elongation behavior of two 400 blends is dramatically different. It is sus300 pected that plasticizer helps in compatibilization of blend components. The blend 200 with plasticized PVC appears to be uniform 100 on the macroscopic scale.3 addition of DOP 0 to ABS/PVC blend improved impact 0 20 40 60 80 100 strength and elongation but hardness, tenPolyurethane concentration, wt% sile strength, and Young modulus were reduced.12 Figure 12.7. Elongation of PVC (plasticized with 36 wt% DOP and unplasticized) and polyurethane, PU, Addition of poly(ethylene glycol), blend vs. PU concentration. [Adapted, by permission, PEG, (molecular weight of 400 daltons) to from Petrovic Z S; Javni I; Zhang W, Polyurethanes polyvinylalcohol/methylcellulose blend Expo '99. Conf. proc., American Plast. Council, Alliance for the Polyurethanes Ind., Orlando, Fl., 12th-15th involves interesting synergistic mechanism. Sept.1999, p.79-82.] PEG helps to bring water to the system and water acts as plasticizer of polyvinylalcohol while PEG plasticizes methylcellulose. This synergistic plasticization improves mechanical performance of blend and compatibilizes its components.18 When a plasticizer is added to the polyamide/poly(isobutylene-co-p-methylstyrene) blend elastomer continuity development and cocontinuity shifts to higher concentrations. When plasticized PA is the dispersed phase, however, these two phenomena oppose each other. Addition of 5 and 10 wt% chitosan acted as a plasticizer to starch matrix, increasing the elongation at break (from 35 to 56%) and decreasing tensile strength and elastic modulus.38 The above review of existing literature shows that a little is known about fundamental reasons which determine properties of plasticized blends.32 With a few exceptions papers record properties rather than to analyze principles of their formation.32 Elongation at break, %
plasticized rigid
12.4 BLENDING TO REDUCE OR TO REPLACE PLASTICIZERS For more than 30 years properties of plasticized PVC have been improved by blending with nitrile rubber, NBR.19 Figure 12.8 shows that it is possible to increase elongation of material with reducing amount of DOP. All materials presented in Figure 12.8 have the same shore A hardness. Figure 12.9 shows that plasticizer migration is substantially reduced by addition of NBR. Almost linear reduction of plasticizer migration with improvement of elongation shows that NBR has supplementary plasticizing action. Plasticizers such as for example DOP are required to improve aging resistance of blends of PVC and epoxidized natural rubber.22 Further addition of NBR helps to retain
492
Plasticizers in Polymer Blends
y = 7.54 - 0.19x R= 0.997
y = 804.4 - 7.919x R= 0.99998
8 7 Weight loss, %
Elongation at break, %
580
540
500
460 30
34 38 42 DOP concentration, wt%
Figure 12.8. Elongation at break vs. DOP concentration in the following PVC/DOP/NBR blends: 57.5/ 42.5/0, 54/36/10, 50/30/20. [Data from Thomas N L; Harvey R J, Progress Rubber Plast. Technol., 17, No.1, 2001, p.1-12.]
6 5 4 3
0
5 10 15 20 NBR concentration, wt%
Figure 12.9. Migration loss to rigid PVC after 30 days at 70oC vs. DOP concentration in the following PVC/ DOP/NBR blends: 57.5/42.5/0, 54/36/10, 50/30/20. [Data from Thomas N L; Harvey R J, Progress Rubber Plast. Technol., 17, No.1, 2001, p.1-12.]
aging resistance of blend, eliminate low molecular weight plasticizer, and improve elastomeric properties of composition. Several materials were blended with PVC and properties of blends were compared with effect of plasticization by DOP.36 These included nitrile rubber, NBR, poly(ethyleneco-vinyl acetate), EVA, and poly(ethylene-co-vinyl acrylate-acrylate), Ac. Increase in plasticizer concentration (DOP) normally reduces torque and final melt temperature of blend but this is not the case when NBR replaces plasticizer. EVA behaves in similar manner to NBR but Ac reduces torque required for mixing. Only DOP rapidly decreases the glass transition temperature. Blending with other polymers has much smaller influence on the glass transition temperature. Changes in mechanical properties by blending with the above polymers and DOP addition are very similar as measured by ultimate tensile strength, Young modulus, and elongation at break. This study shows that processing properties are more improved by low molecular weight plasticizers but mechanical performance can be successfully improved by blending with various polymers.36 General assessment of studies on plasticizers use in polymer blends shows that large amount of work is required before results of blending will become predictable. So far blending is more based on trial-and-error mixing of components than on fundamental research data.
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