SPE Vinyltec 2012, “Versatile Vinyl Plastic: Formulating for the future”, Chicago, October 22-24, 2012

New bio-based Mirel rubber copolymers for PVC modification

Y. Kann, R. Engle, Metabolix, kann@metabolix.com, 617-949-0287 Abstract

Blends of biobased polyhydroxyalkanoates (PHAs) with PVC have been developed and demonstrated very interesting and 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. The presentation will discuss the background to this work, features and properties enabled by these blends and future prospects.

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- butadiene-styrene (ABS), where one phase, e.g. methyl methactylate would be miscible with PVC and provide good adhesion to the matrix and another phase, e.g. n-butyl acrylate or butadiene rubber would provide the toughening. In this case, the performance of toughened PVC would strongly depend on production method, the structure of preformed core-shell particles, their size and dispersion in the matrix [3,4,5 ]. The art of impact modification becomes very challenging and has higher success in shear intensive processes. The formation of entangled

PVC/ rubber networks could ease the compounding and achievement of well dispersed rubber segments, which might be a better approach to impact modification. 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 relationship between changes in these properties and plasticizers content depends on the nature of the particular plasticizer. There are many approaches attempting to explain plasticization mechanisms in PVC, but the simplified picture of plasticized PVC consists of fairly uniform matrix of polymer molecules held apart to degrees depending on the concentration and nature of the plasticizer. The plasticizer molecules act as shields between strong polymer dipoles known to provide high concentration of the secondary valence forces and rigidity to PVC. Reduction of the dipole bonding therefore eases restriction of deformation of PVC molecules and increases flexibility and flow. 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 and staining. 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. PHA copolymers Poly(3-hydroxybutyrate) or PHB is the most well-known polymer belonging to the family of poly(hydroxyalkanoates), PHAs. It is aliphatic polyester that can be produced by many types of microorganisms and has the following structure:

The high molecular weight isotactic PHB serves as a reserve carbon and energy source for the microorganisms, much like fat is for animals or starch is for plants [6]. In addition to producing PHB homopolymer, special bio-fermentation techniques are capable of producing random copolymers of PHB and other hydroxyalkanoates. These PHB copolymers are the focus of this presentation.

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). 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 PHB based co-polymers that are made by microbial fermentation of sugars or

O CH

CH3

CH2

C

O

n

vegetable oils. Metabolix produces a range of PHA based materials that are marketed as Mirel™ biopolymers.

Fig. 1 Intracellular accumulation of PHA:

PHA copolymers are receiving tremendous attention for employment in various thermoplastics applications such as injection molding, extruded sheet, film (blown and cast), extrusion coating, monofilaments, etc.

PVC/PHA blends

Previous publications on PVC/PHA blends “Miscibility of P(3hydroxybutyrate-co-3hydroxy valerate) and PVC blends” [9] have focused on using PVC as an additive to crystalline PHAs to improve the mechanical and processing properties of the PHA. 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.

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 (labeled PHA30 in the results below), a crystallizable medium copolymer (labeled PHA11), homopolymer PHB, and their blends marketed by Metabolix as MirelTM M4100, M4200 and M2100. 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 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 loading 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 330-350 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 Tg 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. Results and Discussion

Miscibility

The calculated solubility parameters (VanKrevelen method) of PVC, PHAs 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 PHAs 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.

Table 1 VanKrevelen solubility parameters

Total Polar Non-polar

vinyl chloride 21.2 24.1 16.1

3-hydroxybutyrate 20.4 27.3 17.1

4-hydroxybutyrate 19.8 27.3 16.1

3-hydroxyvalerate 19.6 27.3 16.9

methyl methacrylate 19.5 27.3 16.7

DIDP plasticizer 17.6 27.3 16.3

Levulinic ketals 20.6 31.9 16.2

According to this data, the polar and non-polar group contributions in PHAs 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.

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 DMA data confirms that the PVC blends with up to 28 phr of PHA30 as well as up to10 phr of PHA11 are miscible (single Tg on the loss modulus curve, Fig.2).

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 PHA30 blend, which is shifted to the lower temperature compared to the PVC control. We do not observe any β-transitions at about 0 °C, believed to be responsible

for the impact strength of unplasticized PVC, but this peak is known to disappear in plasticized blends.

Fig.2 PVC/28 phr PHA30 blend

Fig. 3 Plasticized PVC/PHA30 blend

71.67°C

29.87°C

Loss Modulus

Storage Modulus

Tan Delta

0.2

0.4

Tan

Delta

0

50

100

150

200

250

Los

s M

odul

us

(MP

a)

0

1000

2000

3000

4000

5000

Sto

rage

Modul

us

(MP

a)

-60 -40 -20 0 20 40 60 80 100

Temperature (°C)

Sample: PVC Blend 20Size: 17.7000 x 13.6600 x 1.9100 mmMethod: Temp Ramp/Single Freq

DMAFile: T:...\DMA\2012\YK\PVC Blend 20.001Operator: IDCRun Date: 18-Jul-12 09:26Instrument: 2980 DMA V1.5B

Universal V3.9A TA Instruments

-8.63°C

PHA30

PVC control

60.80°C

PVC/10 phr PHA30 blend

54.12°C

0.0

0.2

0.4

0.6

0.8

Tan

Del

ta

-80 -60 -40 -20 0 20 40 60 80 100

Temperature (°C) Universal V3.9A TA Instruments

The AFM also confirms formation of a single phase system in the PVC/28 phr PHA30 blend (Fig.4) and of the plasticized PVC/15 phr PHB/PHA11/PHA30 blend (25 phr of the DIDP, Fig.5):

Topography image Phase image

Fig.4 Unplasticized PVC/28 phr PHA30 blend

Topography image Phase image

Fig.5 Plasticized PVC/15 phr PHA11/PHA30 blend

The plasticized sample provided more contrast and it could be suggested that PVC and PHA form the interpenetrating networks on the segmental level, which could explain impact strength improvement and toughness of the blends.

The Tg of the blends is also found to be lower than predicted by the Fox equation, once again confirming miscibility:

Fig.6 Tg of PVC/PHA blends

Impact modification of PVC

We have found that PHA copolymers, being miscible with PVC and containing 25-40 % 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 165-185 °C, even at high molecular weight of PHA. The physical properties of the PVC/28 phr PHA30 miscible blend are presented in Table 2 along with the semi-rigid blend containing 18 phr of DIDP (also miscible). Table2 Semi-rigid compounds with plasticized and not plasticized PVC blends

PVC 

Control

MBS imp. 

modifier (B‐22)PHA30 

rubber

modifier phr 0 10 10 18 28

DIDP phr 18 18 18 18 0

Tg, DMA, °C 35.3 47.7 and ‐51.1 25.6 23.1 32.3

tan δ, max, °C 60.8 62.8 and ‐51 53.1 50 70.3

Fracture brittle ducti le ducti le ducti le ductile

Tensile Modulus, MPa 1298 1451 469 184 1407

Tensile stress, break, MPa 44 29.4 32.1 27.7 30.2

Tensile strain, break, % 152 130.4 167.47 187.6 102

Flex modulus, MPa  952 865.1 336 149 1202

Lowest T brittleness, 5 °C failed failed ok ok ok

Impact, Izod, ft lb/inch 1.25 1.93 3.1 7.42 >18

Hardness, shore D 58 62 50 66

PHA30 Rubber

As could be concluded from Table 2, a very tough material could be made with 28 phr of PHA30 copolymer without any low molecular weight plasticizer. The PHA rubber modifies both impact strength and flexibility (provides plasticization). It is quite remarkable that the sample prepared from this PVC/PHA blend could not be broken by 18 ft lb/inch impact, while an MBS containing sample broke at much lower impact. The PVC/PHA sample demonstrated ductile fracture with pronounced crazing at the impact point that is responsible for the increased impact strength. The sample also had high stiffness and good low temperature performance. This compound could be used to make impact resistant shields, boxes, bottles, not possible with other existing technology. A secondary but very important discovery is that PHA copolymers do not reduce transparency of clear PVC, as do many core-shell impact modifiers. As the polymers are miscible there is no additional scattering produced at their phase interface. Interestingly enough, based on FTIR results, the copolymers containing up to 10 phr of crystallizable segments also remain amorphous in PVC matrix. Furthermore, UV stability of PHA copolymers is known to be very good: aliphatic polyesters do not have UV-VIS light absorbing chromophores and cannot induce photo-degradation of PVC. Plasticization of PVC The PHA copolymers (across a range of copolymer composition) could be very useful in plasticization as they reduce Tg (Fig.6), increase the flexibility of PVC and do not phase separate. A rubbery copolymer offers enhanced impact together with the addition of flexibility. Fig. 7 shows the change in hardness in plasticized PVC/PHA blends: the incorporation of PHA could, if not completely replace the monomeric plasticizer, reduce its loading very significantly. This would improve resistance to migration, extraction and volatilization.

Fig. 7 Change of hardness in plasticized PVC/PHA blends The Table 3 shows the improvement of tensile toughness and tear resistance with incorporation of PHAs.

Table 3 Plasticized PVC/PHA blends

Processing Aid (Melt stability and Flow Promotion): PVC exhibits a non-Newtonian flow even at low shear rates. According to the rheological, GPC and TGA data (not shown here), PHA copolymers were found to have no effect on acceleration of thermal degradation of PVC. The data presented in Fig.8 shows the melt strength (elastic modulus at low frequency), which was found to reduce in MBS modified PVC, but not in PVC/28 phr PHA30 blend. The melt strength is needed for a number of processes including thermoforming and blow molding.

Fig. 8 Melt strength (elastic modulus at 0.25 rad/s), 160 °C (preheating at 180 °C) Conclusion: We have demonstrated that newly developed biobased PHA copolymers have a very beneficial effect on the processing and modification of PVC. The following PVC limitations could be improved:

 PA‐20, 

acrylic 

proc. aid

PHA30 

Rubber

PHA 

blend 

4100

PHA 

blend 

4200

PHA 

blend 

2100

PHA 

blend 

4100

PHA 

blend 

4200

PHA 

blend 

2100

modifier, phr 5 5 5 5 5 15 15 15

DIDP, phr 36 36 36 36 36 25 25 25

Tg, DMA, C ‐1.4 ‐1.84 ‐5.9 ‐5.9 ‐4.21 15.1 14.1 14.9

tan δ, max, C 53.02 41.33 40.98 37.2 42.88 44.8 43.5 46.5

Tensil  

toughness, J1.17 1.72 3.82 4.14 4.12 4.21 4.1 4.14

Tensil  stress 

at 100% 

elongation, 

Mpa

19 15 19 19 20 22 22 22

tear, N/mm 31 30 51 50 51 64 58 60

Hardness, 

Shore D44 37 37 36 35 42 42 41

PVC drawbacks PHA contribution Brittleness and loss of flexibility; poor low temperature performance

Effective polymeric plasticization and impact modification in a single additive solution. Ability to reduce overall use of modifiers.

Poor melting, fluxing, gloss Improved melting and flow promotion Plasticizer migration. Volatile loss. Poor oil/fuel/solvent resistance, staining. Safety concerns with phthalates

Polymeric plasticization inhibits migration. Inherent polyester chemical resistance Reduced overall use of migratory plasticizers

Feeding & handling issues with viscous polymeric plasticizers

Easy to handle easy flowing pellets

UV instability and Discoloration from additives

Inherent UV stability PVC miscibility protecting transparency. Ability to reduce overall use of stabilizers.

We believe that the blends could provide a step change in impact modification, plasticization and processing modification of PVC. Additionally, the biobased PHA copolymers would improve the environmental image of PVC as their manufacturing is based on 100% renewable carbon feedstock. Acknowledgements

Special thanks to the AlphaGary Corporation and particularly to Dr. Eugene Globus for support in this research and providing required resources.

Referenced literature: [1]G. Wu, et al, Eur.Polym. J., 40 (2004) 2451–2456

[2] Bucknall CB. Toughened plastics. London: Applied Science Publication; 1977.

[3] Cho K, Yang J, Park CE. Polymer 1998;39(14):3073–81.

[4] Paul DR, Bucknall CB. Polymer blends, Volume 2: Performance. New York: A Wiley-interscience Publication; 1999. 155.

[5] Polymer blends and alloys, edited by G.O. Shonaike, Marcel Dekker Inc., 1999

[6] Thermoplastic Elastomers, 2d edition, edited G. Holden et al, Hanser, 1996

[7] O. P. Peoples and A. L. Sinskey, United States Patent 5,229,279 (July 20th, 1993).

[8] O. P. Peoples and A. L. Sinskey, United States Patent 5,245,023 (Sept 14th, 1993).

[9] S. Choe and Y-J. Cha, Polymer, Vol 36, Number 26, pp 4977-4982, 1995.

[10] PVC: production, properties and uses. G. Matthews, The Institute of Materials, 1996