Poly(dimethylsiloxane) Block Copolymers - ACS Publications

Downloaded by NORTH CAROLINA STATE UNIV on January 3, 2013 | http://pubs.acs.org Publication Date: March 10, 2003 | doi: 10.1021/bk-2003-0838.ch028

Chapter 28

Synthesis and Properties of Poly(butylene terephthalate)-Poly(dimethylsiloxane) Block Copolymers David A . Schiraldi KoSa, P.O. Box 5750, Spartanburg, SC 29304

Abstract

Silanol-terminated poly(dimethyl siloxanes) were demonstrated to copolymerize into PBT under normal polyester polymerization conditions. The resultant polymers exhibited enhanced chemical resistance, and only modest loss of molded mechanical properties up to 15% PDMS content in the copolymer. At higher PDMS levels, the polymer exhibited no cohesive properties in molded parts. Model studies suggest that the actual mode of chemical reactivity is for the silanol end groups to form silanol ethers with the excess diol present in the reaction system, or more likely, with hydroxyl end groups of growing polyester chains. These silanol-diol ethers can serve effectively as chain extenders, and reduce the overall polymerization times.

© 2003 American Chemical Society

In Synthesis and Properties of Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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330

Starting around 1990, several reports of polyester-polysiloxane copolymers began to appear in the literature. These efforts appear to have been focused on the production of materials possessing improved antistick/antiblock and mold release properties (1-5), improved thermal properties (6), and improved fiber properties (7). In each of these initial studies, the authors' intent was to combine the useful mechanical properties of polyesters with the extremely low surface energies of polysiloxanes, for extruded and/or injection molded applications. In order to effectively incorporate the polysiloxane groups into the polyester backbone, the most common approach was to endcap the polysiloxane with terminal hydroxyalkyl groups, thereby creating Si-O-C linkages in the final polymer products. These bonds were found to be somewhat susceptible to hydrolysis, and further modification of the polymer with phenolic monomers was employed to reduce problematic hydrolysis (8). Alternatively, end capping of the polysiloxane with an epoxy group was employed to once again generate a Si-O-C linkage in the synthesis of polyester-polysiloxane copolymers. The objective of current work was to directly produce polyester-polysiloxane block copolymersfromsilanol-terminated polysiloxanes. This effort was undertaken with the knowledge that silanols would not be expected to be highly reactive. If successful in producing the desired materials, evaluation of mechanical properties and solvent resistance of these polymers would be undertaken, and comparisons to the analogous copolyesterether elastomers, Figure 1, could be made.

Experimental Materials All reagents were used without further purification. Laboratory Polymerizations In order to test the viability of these polyester-polysiloxane copolymerizations, laboratory batches were produced using 4.0 moles dimethyl terephthalate (DMT), 9.0 moles 1,4-butanediol, silanol-terminated poly(dimethyl siloxane) (PDMSDS), and tetrabutyl titanate catalyst. Laboratory polymerizations proceeded in a typical manner, except that polycondensation times to the desired melt viscosities decreased with increasing PDMSDS levels. Given that no problems were encountered, several of these polymers were scaled up in a pilot plant batch autoclave.



^ 2

2

2

^ 2

2

2

2

2

r

2

2

2

C0 CH CH CH CH 0-J- —fSi(Me) 04_J-

Figure 1. Copolyesterester elastomer and corresponding copolyestersiloxane copolymer

Copolyestersiloxane copolymer

__J_OC

2

C0 CH CH2CH CH20-^-[—fOCH CH CH CH204^-

Copolyesterether elastomer

—[-OC


tu


332

Pilot Scale poly(butylene terephthalate)-poly(dimthylsiloxane) Polymerizations Given in Table 1 are formulations for PBT-PDMS copolymers produced in a 10 gallon, 316-ss pilot plant autoclave. For each batch, Dimethyl terephthalate (DMT, KoSa), 1,4-butanediol (ISP), PDMSDS (Huels), and tetrabutyltitanate (DuPont, 70.0 gm) were charged to the reactor under nitrogen. The reaction mixture was heated to approximately 170°C, at which time methanol evolution commenced. The reaction temperature was allowed to slowly increase with time; the theoretical methanol was removed over approximately 90 min, with a typical batch temperature of 210°C at that point. The batch temperature was then increased to 250°C, and vacuum was applied. An ultimate reactor pressure of ca. 1 Torr was obtained for each batch. The progress of the polymerization reaction was followed by monitoring electrical current required to maintain an agitation rate of 10 rpm (current being proportional to viscosity) - when the target viscosity was reached, the polymer was extruded under nitrogen pressure into a water trough, and cut into cylindrical pellets for further processing. Injection Molding and Testing of Polymers The pilot plant produced polymers were injection molded using a Boy 22-S molding machine, operating with an average melt temperature of 250°C. A "family" mold containing one ASTM type IV tensile bar and an ASTM flexural bar was used in the molding machine - the mold temperature was maintained at approximately 80°C using a recirculating oil supply. After a 48 hour period during which tensile and flexural bars were allowed to equilibrate under laboratory temperature and humidity, the bars were tested in tension, and 3pointflexuralbending using a Tineous-Olsen universal tester, and were manually tested for Rockwell D hardness using a needle-penetration type tester. For solvent resistance studies, tensile andflexuralbars were immersed in the appropriatefluidsin sealed glass jars held at room temperature. Sample bars were removed at prescribed times, blotted with a paper towel, allowed to dry in a laboratory hood for 1 hour at room temperature, then were tested on the universal tester.

Results and Discussion

Polymer Synthesis The formulations tested on a pilot scale are given in Table 1, and the synthetic scheme used is given in Figure 2. Brilliant white polymers were obtained in all cases, in polymerizations that proceeded in a manner typical for PBT (except for


333


Table 1. Pilot Plant Scale Syntheses of PBT-PDMS Copolymers DMT (lb)

1,4-BDO (lb)

PDMS (lb)

MW PDMS

Poly Time (min)

13.3 15.7 17.0 16.9 13.3 12.8

9.2 10.9 11.8 11.8 8.8 8.8

2.4 3.0 0.94 1.0 8.0 8.4

1750 4200 1750 4200 1750 4200

40 55 55 100 15 15

SAMPLE/ Wt % PBT (1) 85 (2) 85 (3) 95 (4) 95 (5) 62 (6) 62

Figure 2. PBT-PDMS Synthetic scheme



334 polymerization times). Polymerization times necessary to achieve the desired melt viscosity decreased monotonically with increasing PDMSDS content. Unlike systems which contain monomers with functionalities greater than two, these polyester-polysiloxane polymers flowed well and appeared to be tough and ductile. These polymers were found to be completely insoluble in all solvents tested, so it was not possible to obtain nmr spectra of the PBTpolydimethyl siloxane copolymers. Given that chemical resistance of polymers is generally a desirable attribute, we took the insolubility of these polymers in nmr solvents such as trifluoroacetic acid to be positive, if annoying properties. Analysis of reactor overheads indicated only trace amounts of polysiloxane had been removedfromthe reactor. The T values, given in Table 2, are consistent with the expected melting point depressions that would result from full incorporation of the polysiloxane. Soxhlet extraction of ground polymer samples with refluxing chloroform resulted in negligible reduction in weight, so we conclude that the PDMSDS was in fact incorporated into the PBT copolymer. This differs from a literature report that suggested only afractionof PDMS, terminated with hydroxyalkyl groups, are incorporated into polyesters (9). m

Injection Molding and Polymer Properties Samples 1-6 and a PBT control, 7, were all injection molded into ASTM flexural and tensile test bars. The PBT control and PBT-PDMS copolymers containing 85-95% PBT all produced molded bars that exhibited toughness, and appeared suitable for mechanical testing. The molded parts producedfromthe 62% PBT copolymersfibrillatedupon bending, and could be separated into strands. It is our belief that PBT and polysiloxane phases are incompatible with one another, and this phase separation led to loss of mechanical integrity. Because these 62% PBT samples lacked cohesiveness, they were not further tested. Tensile,flexural,and hardness properties of samples I-4and 7 are given in Table 2. Noteworthy among these mechanical tests is that (i) the molecular weights of PDMSDS used (1750 and 4200) generally had little effect on polymer properties, and (ii) tensile strength,flexuralmodulus, and hardness all declined monotonically with polysiloxane content. A comparison of tensile strengths for these reactor products with those produced by another group using reactive extrusion with epoxy-terminated PDMS and PBT is given in Figure 3 (5). As can be seen in this comparison, the reactive extrusion products appear to have higher tensile strengths, measured using a similar test method. It is not readily obvious what differences in structures were obtained using the reactive extrusion and melt polymerization processes, though it is possible that the former produced extremely high molecular weights via chain extension, thereby leading to increased tensile strength with added bisepoxy additive levels.


335


Table 2. Polymer/Molded Part Properties for PBT-PDMS Copolymers MWPDMS

T °C

Tensile Strength (KPsi)

Flexural Modulus (KPsi)

Elongation Break (%)

Hardness Rockwell D

1750 4200 1750 4200

221.2 220.3 221.4 221.3 223.4

8.3 8.7 10.6 10.6 12.4

202 202 232 224 252

70 50 40 260 80

70 71 77 76 80

m

SAMP LE/Wt % PBT (1) 85 (2) 85 (3) 95 (4) 95 (7) 100

—

Figure 3. Comparison of Reactor and Reactive Extrusion Product Tensile Strengths (Kpsi) as a Function of % PBT in PBT-PDMS Copolymers


336


Solvent Resistance of PBT-PDMS Copolymers A major objective of this work was to determine whether relatively low levels of PDMS copolymerized into PBT would significantly improve its solvent resistance. Molded bars of PBT and of the 95% PBT/5% PDMS 1750 copolymer were immersed in a wide spectrum of solvents, with samples withdrawn for tensile andflexuraltesting after 7 and 14 days immersion. These results, given in Tables 3 & 4, show that neither polymer is significantly affected by immersion in common alcohols or hydrocarbons, and that the PBTPDMS copolymer is less susceptible to attack by ketones and chloroform. This small improvement in solvent resistance of PBT which results from copolymerization of PDMS blocks, is consistent with the increased resistance to nmr solvents noted earlier. In general, semicrystalline polyesters are resistant to most solvents, with ketones and halogenated solvents posing the most significant challenge to these polymers. The incorporation of PDMS blocks did improve resistance in this one area of weakness. An interesting question not answered in this study is whether incorporation of PDMS blocks would significantly improve solvent resistance in amorphous and low crystallinity polyesters; amorphous polyesters are extremely susceptible to attack by many families solvents, with low crystallinity polyesters being somewhat more resistant (10)

Silanol Reactivity Analysis of the PBT-PDMS copolymers was severely hampered by our inability to find a suitable nmr solvent. A model monomer was copolymerized into PBT, in hopes of obtaining a structure which was soluble in nmr solvents, and would demonstrate the presence of silanol esters in the backbone of the PBT copolymers. l,4-bis(dimethyl silanol)benzene (BDSB) was copolymerized into PBT, following the same procedures used for laboratory synthesis of PBTPDMSDS copolymers. When equimolar amounts of BDSB and DMT were polymerized in the presence of 2.5 molar excess 1,4-butanediol, the resultant polymer, which was soluble in CDC13, showed a 1:2:1 ratio of terephthalate:butanediol:BDSB, with the butanediol resonances shifted from their normal locations in PBT. We believe that this ratio and nmr chemical shift difference suggests formation of a butanediol bisether of BDSB, which then acts as a diol to condense with DMT, as is illustrated in Figure 4. The new resonances were observed at 4.25, 3.76,1.88 and 1.65 ppm (vs. IMS) and coincide with the patterns/chemical shifts predicted by simulation software.


337

Table 3. Effect of Solvents on PBT and PBT-PDMS Copolymers - Tensile


Solvent

None IPrOH MeOH EtOH Acetone MEK EtOAc Hexane Toluene Motor Oil Cloroform

Flexural

Strength

(Psi)

PBT

PBT

PBT-PDMS

PBT-PDMS

7 days

14 days

7 days

14 days

8,400 8,000 8,400 7,200 7,500 7,700 8,300 8,100 8,500 5,600

6,700 6,600 7,100 7,200 6,000 6,500 6,700 7,200 6,900 7,200 7,200

8,300 8,400 8,300 8,400 7,300 8,000 7,900 8,400 8,100 8,300 5,400

6,800 6,900 6,900 5,800 6,200 6,500 7,200 6,800 7,200 7,300

Table 4. Effect of Solvents on PBT and PBT-PDMS Copolymers - Flexural Solvent

None IPrOH MeOH EtOH Acetone MEK EtOAc Hexane Toluene Motor Oil Cloroform

Flexural

Strength

(Psi)

PBT

PBT

PBT-PDMS

PBT-PDMS

7 days

14 days

7 days

14 days

12,400 12,900 10,800 11,100 11,100 11,700 10,500 12,500 11,000 11,300 5,800

12,900 10,300 11,200 11,000 10,800 11,300 12,300 10,800 11,300 4,700

10,600 11,000 9,500 9,600 8,500 9,600 8,800 10,600 9,300 9,700 4,300

11,000 9,300 9,500 8,700 8,900 8,500 10,800 8,900 9,600 3,300



Figure 4. Modci Silanol Polymerized in the presence of Terephthalate



339 Heating neat BDSB with excess 1,4-butanediol in the presence of tetrabutyl titanate produced a polymeric substance, which we have tentatively identified as the linear polyether of these two reactants, Figure 5. Hence, we believe that the mode of reactivity of silanol-terminated poly(dimethyl siloxane) in polyester synthesis is to form ether links with the diol solvent; these hydroxy-terminated polysiloxane oligomers then copolyrnerize into the polyester backbone. Assuming that the ether formation reaction is comparable to that of polycondensation reactions under polymerization conditions, the disilanols can perhaps be thought of as chain extenders, and this may be the root of the reduced polymerization times observed when PDMSDS is added to PBT polymerization reactions.

Conclusions Silanol-terminated poly(dimethyl siloxanes) were demonstrated to copolyrnerize into PBT under normal polyester polymerization conditions. The resultant polymers exhibited enhanced chemical resistance, and only modest loss of molded mechanical properties up to 15% PDMS content in the copolymer. At higher PDMS levels, the polymer exhibited no cohesive properties in molded parts. Model studies suggest that the actual mode of chemical reactivity is for the silanol end groups to form silanol ethers with the excess diol present in the reaction system, or more likely, with hydroxyl end groups of growing polyester chains. These silanol-diol ethers can serve effectively as chain extenders, and reduce the overall polymerization times.

References (1) Haken, J. K.; Harahap, N.; Burford, R. P. J. Chromatography, 1990, 500, 367. (2) Ginnings, P. R. U.S. Patent 4,496,704, 1995. (3) Fleury, E.; Michaud, P.; Tabus, L.; Vovelle World Patent 9318086, 1993. (4) Young, D. J.; Murphy, G. J.; Deyoung, J. J. U.S. Patent 5,132,392 1992. (5) Nakane, T., Hijikata, K.; Kagayama, Y. Takahashi, K. U.S. Patent 4,927,895, 1990.


340

l,4-bis(dimethyl silanol) benzene CH

'C H ,

3

I Si

In,

CH


OH

excess 1,4-butanediol, cai. Ti(OBu)|

CH,

CH, - X . ]i

/f^S\

ι CH

x ^ y 3

Si

OCHjCfyCfyC^O

ι tH

J —

» 3

Figure 5. Model Silanol Polymerized in the Absence of Terephthalate

(6) Matsukawa, K.; Inoue, H. J. Polym. Sci., Polym. Lett., 1990, 28, 13. (7) Ostrozynski, R. L.; Greene, G. H.; Merrifield, J. H. U.S. Patent 4,766,181, 1988. (8) Yamamoto, N.; Mori, H.; Nakata, Α.; Suehiro, M . U.S. Patent 4,894,427, 1990. (9) Mikami, R.; Yoshitake, M.; Okawa, T. U.S. Patent 5,082,916, 1992. (10) Schiraldi, D. Α., J. Ind. Eng. Chem., submittedfor publication.


Poly(dimethylsiloxane) Block Copolymers - ACS Publications

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