Modification of PVC with bio-based PHA rubber. Part 2.

Transcription

1 Modification of PVC with bio-based PHA rubber. Part 2. Yelena Kann, PhD, Metabolix Inc., Lowell, MA Abstract Blends of biobased polyhydroxyalkanoates (PHAs) with PVC have been developed and demonstrated very unique properties when added between 5 and 30 phr. These blends promise to improve both mechanical and environmental performance of PVC. The breakthrough is based on the miscibility of PHA and PVC resins and similar processing windows. Based on the miscibility and performance requirements, specific compositions of PHA copolymers were created to improve plasticization, impact and processing modification. In impact modification, PHA rubber copolymers outperform the best available MBS core/shell impact modifiers and do not compromise PVC transparency and UV stability. In plasticization, PHA copolymers perform as high molecular weight, readily dispersible plasticizers and enable formulation of compounds with low additive migration, low extractables, volatile loss and staining. As a processing aid, the metal adhering properties of PHA copolyesters promote homogeneous shear melting of PVC particles and prevent overheating and degradation. It will be shown that due to their multifunctional performance, the PHA modifiers could significantly simplify the formulation of PVC compounds and reduce the overall amount of required additives. The PHA rubber copolymers are commercially biosynthesized by fermentation technology from renewable resources. They satisfy requirements on sustainability and biodegradability. Introduction Impact modification of PVC Poly (vinyl chloride) (PVC) is a brittle thermoplastic with a serious limitation for its application due to propensity for brittle fracture, particularly in the presence of sharp notches or cracks, and at low temperatures or high deformation rates. A method to enhance fracture toughness under these conditions is to incorporate a rubbery phase [1, 2]. Less polar elastomers, such as polybutadiene, natural rubber, butyl rubber, styrene butadiene rubbers are not useful in PVC modification due to their incompatibility and phase separation with PVC. The traditional way to solve the incompatibility issues is to use grafted core-shell impact modifiers, such as methacrylate-butadiene-styrene (MBS), all-acrylics impact modifiers (AIM), or acrylonitrile- butadienestyrene (ABS), where one phase, e.g. methyl methacrylate would be miscible with PVC and provide good adhesion with the matrix and another phase, e.g. n-butyl acrylate or butadiene rubber, which would absorb and dissipate an impact. In this case, the performance of toughened PVC would strongly depend on dispersion quality, the structure and size of preformed core-shell particles [3,4,5 ]. The shear intensive processes are more likely to produce the most impact resistant materials. The formation of miscible entangled PVC/ rubber networks could be a better approach to achieve impact modification in less shear intensive processes and milder conditions, which would improve the overall thermal stability of PVC. Plasticization of PVC Plasticization of PVC is almost always done in order to achieve flexibility. In addition to producing flexibility, practically every other property of PVC is changed by the introduction of plasticizers [10]. The impact strength and tensile toughness are usually lost with plasticization. Miscibility between PVC and plasticizer is essential for good performance. If the plasticizer is not fully miscible and has low molecular weight it will migrate to the surface of the PVC article and form a separate layer. It will also be extractable, responsible for high volatile loss, staining and lost properties upon aging. The development of polymeric plasticizers eliminated some of these deficiencies, but there are only a few commercially important high molecular weight plasticizers and they are quite inefficient, expensive and difficult to homogenize into compounded PVC. It will be shown in this presentation that miscible with PVC PHA act as very efficient polymeric plasticizers, which could improve performance of plasticized PVC very significantly. PHA copolymers Poly(3-hydroxybutyrate) or PHB is the most wellknown polymer belonging to the family of poly(hydroxyalkanoates), PHA. It is an aliphatic polyester that can be produced by many types of microorganisms and has the following structure: CH 3 O O C C C H H2 n

2 The high molecular weight isotactic PHB serves as a reserve carbon and energy source for the microorganisms, much like fat in animals or starch in plants [6]. In addition to producing PHB homopolymer, special biofermentation techniques are capable of producing random copolymers of PHB and other hydroxyalkanoates. These PHB copolymers are the focus of this presentation and are referred to as PHA. During bio-fermentation, the water insoluble inert polymer accumulates in intracellular inclusion bodies contributing up to 90% of the total dry weight of the bacteria (Fig.1). Fig. 1 Intracellular accumulation of PHA After the biosynthesis, polymer is extracted, purified and can be compounded and processed using conventional plastics converting equipment. Recent discoveries in genetic engineering [7,8] led to the creation of a technology to produce a broad family of PHA that are made by microbial fermentation of sugars or vegetable oils. Metabolix produces a range of PHA based materials that are marketed as Mirel biopolymers. PHA copolymers are receiving tremendous attention for employment in various thermoplastics applications such as injection molding, extruded sheet, film (blown and cast), PVC/PHA blends Today there is considerable activity in both academia and industry to develop new biobased additives for PVC, to either replace volatile additives such as phthalate plasticizers or to improve PVC processing and physical properties. In this study we have evaluated a series of biobased PHA polymers as potential additives for PVC. The PHA copolymers can contain hard crystalline and soft rubbery segments which cannot be separated from each other by extraction or by the laws of thermodynamics. Both phases have good miscibility with PVC and could be expected to improve toughness and plasticization without phase separation leading to poor physical properties and exudation. Experimental PVC resins with K-values of 57 and 70 were blended with PHA copolymers, mainly with a high copolymer rubbery PHA. For the plasticization we used diisodecyl phthalate (DIDP) as the less extractable monomeric phthalate. For heat stabilization we used BaZn carboxylates, both solid and liquid. All transparent samples were stabilized with liquid carboxylates. The impact modification was benchmarked against MBS KaneAce B-22. The acrylic processing aid Kane Ace PA- 20 was used for the benchmarking of the melt fluxing. A typical PVC two roll mill was used for the compounding at 330 ºF. The PHA copolymers added at up to 30 phr presented no processing problems. All PVC/PHA blends released nicely from the rolls and it was possible to produce transparent glossy blends when appropriate PVC heat stabilizers were used. The milled sheet was then compression molded at ºF into the testing samples for the tensile (according to ASTM D638-03), flexural (ASTM D790-03), impact (notched Izod, ASTM D256-06) performance, Shore D hardness (ASTM D2240), low temperature brittleness (ASTM D746) and DMA (measured using a DMA 2980 Dynamic Mechanical Analyzer in a single cantilever mode). The DMA T g was taken as the peak of the loss modulus curve. Melt strength, G, and viscosity, η*, were measured using oscillatory torsional rheometer TA Instruments AR2000 at 180C and 0.25 rad/s. The molecular weight was measured by GPC in THF, using polystyrene as a reference. The TGA data was collected on TA TGA at 5 to 20 ºC/min from 50 to 450 ºC. The AFM images were collected in the tapping mode on the cryomircotomed samples. Miscibility Discussion The calculated solubility parameters (VanKrevelen method) of PVC, PHA and acrylics used most often with PVC are presented in Table 1. It could be assumed that similarly to PVC/PMMA blends, the miscibility between PVC and PHA is attributed to the exothermic mixing arising from the formation of weak hydrogen bonds between the carbonyl groups of PHA and the methine protons of PVC. According to the data presented in Table1, the polar and non-polar group contributions in PHA and PVC are very similar and miscibility could be expected when the chains have sufficient energy and similar mobility. For the comparison, PMMA that is reported being miscible with PVC is also included.

3 Table 1 VanKrevelen solubility parameters Total Polar Non-polar vinyl chloride hydroxybutyrate hydroxybutyrate hydroxyvalerate methyl methacrylate DIDP plasticizer Levulinic ketals Miscibility is a key parameter affecting plasticization, impact modification and flow promotion. If a miscible second polymer reduces overall Tg, and provides an easy anchoring between the glassy matrix and the rubbery domains (consequently leading to uniform crazing and improved impact performance), this would be considered a step change in the current modification technology. The multi frequency DMA data confirms that the PVC blends with up to 28 phr of PHA are miscible (single Tg on the loss modulus curve), Fig.2:. Fig. 3 PVC/10 phr PHA blend containing 18 phr DIDP plasticizer (blue curve) vs PHA (green) and PVC control with 18 phr DIDP (red) The AFM also confirms formation of a single phase system in the PVC/28 phr PHA blend (Fig.4a) and of the plasticized PVC/15 phr PHA blend containing 25 phr of the DIDP (Fig.4b): a b Fig.4 AFM 700x700 nm scans: a) PVC/28 phr PHA, b) PVC/15 phr PHA/25 phr DIDP Fig. 2 Multi frequency DMA, PVC/28 phr PHA30 The plasticized with 18 phr of DIDP blends are also shown to be miscible (Fig.3): a single Tg is observed for the PVC/10 phr PHA blend, which is shifted to the lower temperature compared to the PVC control. We do not observe any PVC β-transitions at about 0 C, believed to be responsible for the impact strength of unplasticized PVC, as this peak is known to disappear in plasticized blends. The plasticized sample provided more contrast and it could be suggested that PVC and PHA form the entangled networks on the segmental level. As some segments in the networks are of elastomeric nature, their presence could explain an improvement in impact strength and toughness. The Tg of the PVC/PHA blends is also found to be lower than predicted by the Fox equation, once again confirming miscibility:

4 Fig.5 Tg of PVC/PHA blends a) Semi-rigid blends, all with 18 phr DIDP b) Flex modulus b) Rigid PVC, no DIDP Impact modification and plasticization of PVC We have found that PHA copolymers, being miscible with PVC and containing % of PHA rubber segments could provide required toughness modification without a need to tightly control the processing conditions required to disperse core-shell impact modifiers. The PVC and PHA resins melt blend very easily at C, even at high molecular weight of PHA. The physical properties of the same semi-rigid (containing 18 phr DIDP) PVC/PHA blends are presented in Fig.6 along with the semi-rigid control. The blends with higher impact also demonstrate higher flexibility due to the simultaneous plasticization. c) Hardness Thermal degradation The multi rate heating TGA/DTG data was analyzed to evaluate thermal degradation of PVC/PHA blends and to calculate activation energy of degradation (E act ). Fig. 7 presents the data for the control semi-rigid PVC (18 phr DIDP), PVC/28 phr PHA blend, no plasticizer (blend #20) and PVC/10phr PHA with 18 phr of DIDP (blend #7). Fig. 6 Semi-rigid PVC/PHA blends physical properties a) Notched Impact data Fig.7 Comparison of TGA and DTG curves (explanation is in the text). We see well described two stage degradation of PVC resin. During the first stage hydrogen chloride is released,

5 leaving behind the unsaturated hydrocarbons containing conjugated double bonds. The other volatile small molecules might be saturated and unsaturated aliphatic or aromatic hydrocarbons formed by the splitting up of PVC chains, in which benzene and toluene are typically found. The second stage involves the formation and volatilization of the products of intramolecular cyclisation of the conjugated sequences. The weight loss data and E act for each stage of degradation are presented in Table 2. The activation energy is calculated at different heating rates from 2.5 to 20 C/min by the Kissinger equation: Rheology The melt viscosities at the processing temperature ( C) of PHA and PVC are very similar. The melt strength increases in the presence of PHA, which should help in applications requiring high melt strength such as thermoforming and blow molding, (Fig.8). The graph also shows the blend containing 10 phr of MBS impact modifier. This blend has lower melt strength. E / RT a 2 n 1 Ea /( RTm) m An (1 ) m e, where α- conversion, E a apparent activation ebergy, kj/mol, A- pre-exponential factor; β- heating rate, C/min, R- general gas constant, J/mol K, T m - temperature at maximum degradation rate, K, n- reaction order. Apparent activation energy is calculated from the slope and the pre-exponential factor from the intercept of the straight line and the y-axis. Table 2 TGA data for PVC and its plasticized and unplasticized blends with PHA 20 C/min Eact1, kj/mol Eact2, kj/mol T5% loss, C Tm1, C Tm2, C PVC control, 18 pphr DIDP PVC/10phr PHA, 18 phr DIDP PVC/28 phr PHA, no plasticizer The data in Table 2 shows that the unplasticized blend with 28 phr of PHA has slightly lower temperature at 5% weight loss than in control sample. The temperature at maximum degradation rate slightly increased in both stages (4 C first stage, 8 C second stage). The activation energy of the degradation has slightly reduced in the unplasticized PVC/28 phr PHA blend and did not change (even increased in the second stage) for the 18 phr DIDP containing PVC/PHA blend. All these observations conform a very slight effect of PHA on thermal degradation of PVC. The same is confirmed by GPC (Table 3) that demonstrates that the molecular weight of PVC does not significantly change in the presence of PHA: Table 3. GPC data, PVC/PHA compounds Mw, PVC, phr PHA g/mol 0 60, , , ,953 Fig. 8 Melt strength of PVC, PVC/MBS and PVC/PHA blends. Conclusions It has been shown that PHA copolymers could be considered as a new class of renewable and very efficient modifiers for plasticization, impact modification and processing of semi-rigid and flexible PVC. References 1. G. Wu, et al, Eur.Polym. J., 40 (2004) Bucknall CB. Toughened plastics. London: Applied Science Publication; Cho K, Yang J, Park CE. Polymer 1998;39(14): Paul DR, Bucknall CB. Polymer blends, Volume 2: Performance. New York: A Wiley-interscience Publication; Polymer blends and alloys, edited by G.O. Shonaike, Marcel Dekker Inc., Thermoplastic Elastomers, 2d edition, edited G. Holden et al, Hanser, O. P. Peoples and A. L. Sinskey, United States Patent 5,229,279 (July 20 th, 1993). 8. O. P. Peoples and A. L. Sinskey, United States Patent 5,245,023 (Sept 14 th, 1993). 9. S. Choe and Y-J. Cha, Polymer, Vol 36, Number 26, pp , PVC: production, properties and uses. G. Matthews, The Institute of Materials, 1996