THE EFFECT OF HARD SEGMENT CONTENT AND POLYOL TYPE ON THERMAL AND MECHANICAL PROPERTIES OF TPU/PC BLENDS
UTJECAJ UDJELA TVRDOG SEGMENTA I VRSTE POLIOLA NA TOPLINSKA I MEHANIČKA SVOJSTVA PU/PC MJEŠAVINA
Emi Govorčin Bajsić1,Tanja Trbojević2,Vesna Rek1, Mirela Leskovac1
1Faculty of Chemical Engineering and Technology, University of Zagreb, Marulićev trg 19, 10000 Zagreb, Croatia
2 Pliva Hrvatska d.o.o., Prilaz baruna Filipovića 25 , 10000 Zagreb, Croatia
Abstract
The miscibility of polyurethane / polycarbonate ,PU/PC, blends prepared by mixing of PU elastomers based on polyether and polyester polyol as a soft segments and different hard segments content with an amorphous PC was investigated.
Neat thermoplastic polyurethane (PU), polycarbonate (PC) and PU/PC blends with different weight ratios prepared in a Brabender mixer were investigated by dynamic mechanical analysis (DMA) and tensile testing.The results indicated partial miscibility of PU/PC blends. The miscibility obtained with polyester-type PU and with higher hard segment content was higher than that observed with polyeter-type PU, what was attributed to better interactions between ester groups from soft segments and urethane groups from the hard segments. In the blends with 70 wt. % of PC the formation of PU–PC copolymer by –OH of PC and ~NCO groups of PU interchange reaction was observed.
Keywords:polyurethane elastomers, polymer blends, polycarbonate, thermal properties, mechanical properties
Ključne riječi: poliuretanski elastomeri,polimerne mješavine,polikarbonat,toplinska svojstva,mehanička svojstva
Introduction
Blending has been widely and effectively used to modify or control the properties of polymer by appropriately compounding miscible polymers. Miscible polymer blends can create new materials with completely different properties, and fabricated articles may possess good mechanical properties (1). Because the physical properties of polymer blends are influenced strongly by the blending conditions and processes that, in turn, affect the level of mixing of the blends, there is a growing interest in studying the miscibility and phase behaviour of polymer blends.
Polyurethanes (PU) are versatile materials used in a large number of application (2,3).Segmented polyurethanes are composed of alternating soft (low glass transition) segments and more rigid, polar urethane (hard) segments. The soft segments are generally polyethers (4-6) or polyesters (7,8) and the hard segments are formed from the extension of a diisocyanate terminated prepolymer with a low molecular weight diol.Depending on the soft segment or hard segment concentration the polymer has a continuous hard phase with a dispersed soft phase, a continuous soft phase with dispersed hard phase (9,10). The morphology developing during production, the miscibility of the phases, the size of the dispersed particles and their characteristics affect strongly the final properties of the polymer.Polycarbonate (PC) is a tough, transparent, impact-resistant thermoplastic polymer with a very wide range of service temperature. However, it suffers from its high melt viscosity, which causes difficulties in processing(e.g., injection molding), and its low fatigue strength, which generates cracks at points of concentrated residual stress. For these resons, blends of PU and PC are expected to usefully combine the properties of the parent polymers.The objectives of the present workis the study of the thermal and mechanical behaviour of neat polyurethane (PU),neatpolycarbonate (PC) and their blends PU/PC.
Experimental
Synthesis of PUR prepolymers
The PUR prepolymer was prepared at 80oC under a nitrogen atmosphere in a stirred-glass reaction kettle. The diisocyanates (MDI) was charged into the kettle first and heated up to 80oC. Then an appropriate amount of polyol (PTMO) was added, NCO/OH ratios of 2/1 (35 % NCO/OH) and 4/1 (52% NCO/OH) were employed. The reaction was considered competed when the experimental value of the NCO concentration and the calculated value, were within one percent. The NCO content of isocyanates and prepolymers was measured by the di-(n-butyl) amine titration method.
Preparation of polyurethane elastomers
A chain extender 1,4-butanediol (1.4-BD) was added to the prepolymer with intensive mixing at a temperature of 90oC. After 60 s of mixing, the reaction mixture was immediately poured intoa preheated teflon-coated aluminiummould This mixture was then heated to 100oC in a Carver hydraulic platen press for about 30 min. The elastomers were postcured in an oven for 24h at 105oC immediately after molding.
Preparation of polyurethane /polycarbonate blends
The neat polymers (TPU and PC) and their blends (TPU/PC blends) were prepared in a Brabender mixer using a temperature of 210 ; 180 and 190oC for PC, PU and PU/PC blends, respectively with a screw speed of 45 rpm. Then, the specimens were compression moulded in a hydraulic press (Dake Model 44-226) at 230 oC .
Dynamic Mechanicalmeasurements
Dynamic Mechanical measurements were performed on a Dynamic Mechanical Analyser DMA 983, TA Instruments, at a frequency of 1Hz. The changes of loss (E’’) moduli were measured over the temperature range of -100 to 250°C at a heating rate of 5°C/min. The sample length between the clamps was approximately 25 mm. All the samples were cooled to –100°C using liquid nitrogen. The miscibility of the PU/PC blends is characterized also by employing DMA technique.
Mechanicalmeasurements
Mechanical measurements were performed on a Zwick Universal testing machine (model 1445) in uniaxial tension mode at 23oC and 65% of relative humidity.
Tests were performed at a crosshead speed of 50.00 mm / min.At least five test specimens of 30.00 mm x 13.0 mm x 3.5 mm were tested for each blend and average values were calculated.
RESULTS AND DISCUSSION
Dynamic mechanical analysis of the blends
DMA spectra on binary blends can provide information on the degree of polymer-polymer interaction and interphase mixing.
The DMA curves for the loss modulus versus temperature (E''/T) of neat PU elastomers ,neat PC and PU/PC blends prepared from the polyether (PU ET) and polyester (PU ES) as the soft segment with varying isocyanate content (NCO/OH 2/1 and 4/1) and with various compositions (PU/PC 30/70; 50/50 and 70/30) are shown in Figures 1 and 2, the DMA results are summerized for various compositions in Table 1.
Fig.1. Curves of loss (E'') versus temperature Fig.2. Curves of loss (E'') versus temperature
for PU/PC blends. for PU/PC blends
The soft segment Tg in neat PU elastomers was found below 0oC anddepends on the hard segment content and type of the soft segment.
The Tg value for the polyether PU elastomers with lower hard segment content (PU ET II) remains the same value as that of the polyether PU elastomer with higher hard segment content (PU ET IV) -37.0oC. Thiswas attributed to the restricted chain mobility of the soft segments with the increasing NCO/OH ratio (11). The weak interactions between theether oxygen in the soft segment and the urethane groups in the hard segment of the polyurethane may lead to the phase separation between the hard and soft segments in the polyurethane. On the other hand, the interactions between the urethane groups and the ester carbonyl groups in the polyester polyurethane (PU ES) are higher and therefore the Tg increases, also Tg increased with increasing content of the hard segments (PU ES IV). As we can see in Figures 1 and 2 is a relaxation of neat PC at 154.4 oC while neat PU ET II, ET IV, ES II and ES IV show a Tg at -37.1 , -37.7 , -14.6 and 2.2 °C, respectively, as shown by the temperature dependence plots of the loss modulus E''. The curve of PU/PC blends show primary relaxations intermediate between those of neat components. As we can see in Table 1, there is a systematic shift of the Tg of the blends with the composition, the Tg values increases with increasing NCO/OH and PC contentin the PU/PC blends of polyether (PU ET) and polyester (PUES) soft segments compared toneat polyether and polyesterPU.
The Tg’s maxima of the PU broadened and decreased its intensity more in polyether than in polyester PU (Figure 1).ThePC glass transition temperature in all investigated PU/PC blends were shifted to lower temperature. In general, the shift in the polyurethane Tg is more significant than the shift for the polycarbonate (Table1).According to the obtained results the changes are much higher for PU/PC blends with polyester polyurethane (Fig. 2, Table 1). One possible explanationcan be that urethane groups of PU may interact with carbonyl groups from PC (12). The Tg’s maxima of the PU and PC phases shift towards to each other as the composition changes. This shift of the Tgs gives an indication of possible partial miscibility of the PU and PCchains.
Table 1. Dynamic mechanical properties of neat components and PU/PC blends.
PU elastomers /(°C) /
(°C) /
(MPa) /
(MPa)
PU ET II
PU ET IV / -37.1
-37.7 / 255.8
376.7
PU ES II
PU ES IV / -14.6
2.2 / 693.1
459.8
PC 100 / 154.4 / 1001
PU/PC Blends
PU ET II
PU/PC 70/30
PU/PC 50/50
PU/PC 30/70 / 151.8
152.9
150.1 / -32.1
-33.2
-35.7 / 71.07
227.7
369.9 / 417.8
461.5
304.1
PU ET IV
PU/PC 70/30
PU/PC 50/50
PU/PC 30/70 / 152.8
147.8
148.7 / -18.4
-7.9
-16.3 / 79.79
144.4
316.3 / 395.4
300.7
216.4
PU ES II
PU/PC 70/30
PU/PC 50/50
PU/PC 30/70 / 147.5
150.1
148.3 / -7.1
-4.0
-4.2 / 56.67
121.9
154.4 / 527.3
465.8
214.4
PU ES IV
PU/PC 70/30
PU/PC 50/50
PU/PC 30/70 / 151.4
151.2
149.1 / 3.4
0.0
3.1 / 118.7
198.4
209.5 / 471.0
307.3
333.1
The new relaxation maxima around 80oC was detected only in DMA curves of PU ET IV 30/70; PU ES II 30/70 and PU ES IV 30/70 blends. This new relaxation maxima was attributed to interactions (chemical) at the interface of PC-PU and partial miscibility of the PU and PC. In this PU/PC blends the PC-PU copolymer was result of reaction between the –OH groups from aromatic of PC and the –NCO– groups of PU. The sequence of reactions that may take place is summarized in Sheme 1.
The results of stress-strain behavior of neat components (PU elastomers and PC) and their blends (PU/PC; 70/30, 50/50 and 30/70) with various NCO/OH ratio and polyether and polyester polyol are listed in Table 2.
Table 2 shows that for the blends, the tensile strength () increase and elongation at break () decrease as the hard segment content is increased compared with the neat PU elastomers (Table 2). The tensile strength was higher and elongation at break was lower for PU/PC polyester blends (PU ES) than those of PU/PC polyether blends (PU ET) for both NCO/OH ratio (2/1 and 4/1), except for PU/PC 70/30 blend. The Young modulus (E)increases as the hard segment content increases in both type of PU elastomers.The obtained results ndicate that the PU/PC blends with higher NCO/OH ratio and with the polyester soft segment shows better miscibility, these mechanical results are in good agreement with those obtained by DMA.
Table2.Tensile properties of neat components and PU/PC blends.
PU elastomers / (MPa) / (%) / E(MPa)PU ET II
PU ET IV / 3.5
14.1 / 36.0
25.0 / 22.92
260.12
PU ES II
PU ES IV / 2.3
13.6 / 38.0
36.0 / 18.10
181.11
PC / 28.3 / 2.0 / 1716.8
PU/PC blends /
PU ET II
PU/PC 70/30
PU/PC 50/50
PU/PC 30/70
PU ET IV
PU/PC 70/30
PU/PC 50/50
PU/PC 30/70 / 3.6
4.8
16.6
13.0
18.7
20.4 / 12.0
6.0
3.0
19.0
4.0
2.0 / 124.10
480.86
1090.79
180.95
692.10
1222.44
PU ES II
PU/PC 70/30
PU/PC 50/50
PU/PC 30/70
PU ES IV
PU/PC 70/30
PU/PC 50/50
PU/PC 30/70 / 12.8
10.8
21.3
13.6
13.3
24.8 / 22.8
3.0
2.0
36.0
5.0
2.0 / 136.08
424.46
950.23
181.11
457.85
1239.11
Conclusions
In this study, the blends of PU and PC were prepared by melt mixing in Brabender mixer.DMA study revealed that higher concentration of the hard segment results in increased Tg and decreased mobility of the soft segment in PU elastomers.
The results also indicate an increase of Tg of polyester -based PU elastomers (PU ES) in comparasion with Tg of polyether- based PU elastomers (PU ET) because of better interactions between the urethane groups and the ester carbonyl groups in the polyester polyurethane.
The results from DMA measurements showed that the PU/PCblends exhibited two separate Tgs which shift towards to each otherdemonstrating that PU and PC formed a partial miscible blend, these results may be explained in terms that urethane groups of PU may interact with carbonyl groups from PC.
The results of mechanical properties measurements showed that the higher hard segment content and PC concentration increased tensile strength but decreased elongation at break of the all investigated PU elastomers (PU/ET and PU/ES) and PU/PC blends (70/30, 50/50 and 30/70).
References
- B.K. Kim, Y.S. Oh, Y.M. Lee, L.K. Yoon and S. Lee, Polymer41 (2000), p. 385.
- D.Randall, S.Lee The polyurethane book, New York, Willey, 2003.
- M.Lee,S.C.HongS.W.Lee,Polym.Eng.Sci. 47(4) (2007) 439.
- S.W. Seymour, S.L.Cooper, Macromolecules, 6 (1973) 48.
- J.W.C. Van Bogart, D.A.Bluemke S.L.Cooper, Polymer, 22 (1981) 1428.
- S.L.Hun, L.H. Shaw, macromolecules, 22 (1989)1100.
- F.Xiao, D.Shen, X.Zhang, S.Hu, M.Xu, Polymer, 28 (1987)2335.
- X.Yuying,W.Dening,H.Chunpu,Y.Shengkang,L.Yunxian, J.Appl.Polym.Sci., 48 (5) (1993)867.
- S.Abouzahr,G.L.Wilkes,Z.Ophir, Polymer, 23 (1982)1077
- K.K.S.Hwang, D.J.Hemker, S.L.Cooper, Macromolecules, 17(1984)307.
- P.N.Lan, S.Corneillie, E.Schacht, M.Davis, A.Shard, Biomaterials, 17(1996) 2273.
12.C.K. Samions, N.K. Kalfoglou, Polymer, 41 (2000) 5759-5767