Abstract
This paper reports work on extrusion foaming of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) with a chemical blowing agent based on sodium bicarbonate and citric acid and calcium carbonate nucleation agent. It includes investigations in the effects of rheological behaviour of the polymer, blowing agent, nucleation agent and processing conditions on the foam density and morphology. The poly(3-hydroxybutyrate-co-3-hydroxyvalerate) is a natural biodegradable polyester with high crystallinity, low melt viscosity and slow crystallisation rate and high sensitivity to the thermal degradation at temperatures above its melting point, making it particularly difficult to control the foaming process. Use of negative gradient temperature profile was found beneficial to minimise the thermal degradation and achieve necessary melt strength to stabilise the cell structure. Solidification of the super-cooled polymer melt occurring at the die was discussed in relation to the selection of the temperature profile and rheological behaviour and solidification of the poly(3-hydroxybutyrate-co-3-hydroxyvalerate) characterised by rotational rheometry. In addition to extrusion foaming conditions, effect of the blowing and nucleation agents on rheology of the polymer, the cell refinement on foam density and morphology were discussed. The poly(3-hydroxybutyrate-co-3-hydroxyvalerate) was extruded with a twin screw extruder fitted with a strand die yielding up to 60% density reduction with uniform fine cell structure.
Keywords
Introduction
Biopolymers can be polymers synthesised by plants and other living organisms or polymers derived from renewable resources. Their potentials as alternative to petrochemical-based materials have been recognised worldwide and many experts have predicted that the 21st century will herald the rise of the carbohydrate economy. 1 Furthermore, there is also a realistic possibility that the biopolymers could increase their market share from <1% to about 10% of global plastic consumption representing 20 million tonnes per annum. The market for bioplastics has already grown from a small base of 262 kt in 2007 to 505 kt in 2009 and is set to reach 885 kt by 2011. 2
PHBV (poly(3-hydroxybutyrate-co-3-hydroxyvalerate) is a biodegradable copolymer of polyhydroxy butyrate and polyhydroxyvalerate produced commercially using biotechnology methods.3,4 It possesses exceptional gas barrier properties and rigidity and hence has high potential in advanced packaging such as modified atmosphere packaging of red meat. It has better oxygen barrier property than poly(ethylene terephthalate) (PET), but poorer than polypropylene (PP5) and the water vapour resistance of PHBV is as good as PET and much better than other biopolymers like polylactic acid (PLA).5,6 Like many biopolymers, however, the material cost is typically three or four times as high as conventional plastics. Therefore, creating lightweight structures by foaming is considered to be one of the effective ways to reduce the material consumption and associated emissions.
Extrusion sheet foaming followed by thermoforming is the most common processing route for food packaging containers. Physical foaming using gas injection 7 on tandem extruders is widely used in sheet foaming. Solid state foaming where gas is saturated into the material at high pressure in an autoclave and released to create a foamed structure during a subsequent heating and forming stage has also been developed. 8
The use of chemical blowing agent (BA) on a conventional sheet extrusion line has the advantage of low capital investment and hence this work focuses on the chemical foaming method. As the applications are focused on food packaging, the choice of BA must conform to the regulations for packaging in contact with foods.
PHBV foams have been studied and prepared by many researchers by using different techniques and often blended with other polymers. Richards et al. 8 used a temperature soak batch foaming technique for preparation of PLA/PHBV foams. Nascimento et al. 9 patented the preparation of polyurethane/PHBV foamed with isocyanate, whereas the use of methyl formate to produce foamed PHBV beads was reported by Handa. 10 Extrusion foaming of starch/PHBV with water acting as a BA is reported by Willett and Shogren. 11 However, the extrusion foaming of PHBV itself has not been well studied.
In this work, the extrusion foaming using food-approved chemical BA is reported. This includes effect of material formulations and extrusion conditions on the rheological and foaming behaviour of PHBV and the resulting foam structures.
Experimental details
Materials
ENMAT™ Y1000P, a PHBV with 3 mol% hydroxyvalerate content was manufactured by Tianan Biologic Material Co. (Ningbo, P.R. China) in yellowish white pellet form with density of 1.24 g/cm3. The melting and crystallisation temperature of PHBV was determined from differential scanning calorimeter (DSC) (model 2000, TA Instruments, USA) using a heating rate of 10℃/min under nitrogen atmosphere. The melting peak was found at 168℃ and the crystallisation peak was found at 125℃ as shown in Figure 1.
DSC curves of the PHBV during heating (red) and cooling (blue) at a rate of 10℃/min under nitrogen atmosphere showing the peaks of melting and crystallisation temperatures.
The BA (grade BA.F4.E MG) was supplied in white pellet form by Adeka-Palmarole (Saint Louis, France). The BA masterbatch contains 40 wt% active agent, a mixture of sodium bicarbonate and citric acid, in a low density polyethylene (LDPE) carrier. It had an onset decomposition temperature at 150℃. The calcium carbonate RLO 7999 with a density of 2.71 g/cm3 was supplied by Imerys Minerals Ltd (Par, UK) and its particle size distribution is shown in Figure 2.
Particle size distribution of the calcium carbonate RLO 7999.
The extrusion foaming process
Temperature profile along the extruder.
Rheological characterisation
Sample preparation
Rheological characterisation was performed on the as-received PHBV pellets and extruded foams. The extruded foam strands were first grinded by using a Retsch SM2000 cutting mill fitted with a sieve with 1.50 mm trapezoid apertures. Both pellets and grinded foams were dried for 2 h at temperature of 100℃ in a vacuum oven prior to the measurements.
Complex viscosity measurement
The sample polymer was then loaded onto the bottom plate of a rotational rheometer (ARES, TA Instruments, USA) to be melted and compressed by adjusting the top plate to a designated gap of 1.8 mm. Excess material was then removed so that the polymer could fill the volume between the plates. No air bubbles entrapment was observed. Viscosity measurements were performed at a range of temperatures from 145 to 180℃. To maintain similar thermal history, each sample was molten for 4 min at temperature of 180℃ in the rheometer prior to the testing. Heating time was increased to 6 min for the extruded foams in order to decompose any residual BA. To minimise the effect of thermal degradation at temperatures above the melting point, the angular speed sweep was conducted in the range from 1 to 512 rad/s for all tests, since the use of frequencies lower than 1 rad/s could result in much longer measurement times per point and potentially lead to thermal degradation and considerable decrease in viscosity. The commanded strain was set to 1% for all the tests.
Foam characterisation
A Zeiss Supra 35VP field emission scanning electron microscope (SEM) was used to observe the foam morphology. Extruded foams were quenched in liquid nitrogen and fractured. The fracture surfaces perpendicular to the direction of extrusion were used for the observations. The fracture surfaces were then sputter coated with gold prior to the SEM examination. The average cell size and cell population density (N) were analysed from SEM images. The cell population density was calculated using equation (1)
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Results and discussion
Rheological behaviour of the as-received PHBV
Complex viscosity as functions of shear rate at different temperatures for the as-received PHBV is shown in Figure 3. The result reveals that the PHBV exhibits shear thinning behaviour when subjected to increasing shear rate (directly related to frequency or angular speed, ω). The complex viscosity at 180℃ and 1 rad/s was about 1300 Pa s. This is nearly four times lower when compared with crystalline polylactic acid which has a viscosity of about 5000 Pa s at the same condition.
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Lowering the temperature led to considerable increase in viscosity and at 145℃ the viscosity at 1 rad/s increased to about 3000 Pa s. No apparent Newtonian plateau was observed, possibly resulted from the limitation on the selected shear speed and thermal degradation during testing.
Complex viscosity of the as-received PHBV measured as a function of angular speed at different temperatures.
When tested at a fixed angular speed (at 1 rad/s) at different temperatures, the complex viscosity of the PHBV melt changed over time as shown in Figure 4. At temperatures above melting point (170 and 180℃), viscosity was continuously decreasing, indicating that the polymer was undergoing chain scission. At temperatures below melting point, however, viscosity could be seen to increase over time. This can be attributed to crystallisation of the super-cooled PHBV melt. Such increase in viscosity is strongly dependant on the degree of super cooling. The lower the temperature, the faster the viscosity increase and eventually leads to solidification and slippage at the surfaces of the plates, resulting in a plateau.
Complex viscosity of the as-received PHBV as a function of time at fixed angular speed of 1 rad/s at different temperatures.
Figure 5 presents time at G′/G″ crossover for the super-cooled PHBV, where G′ is storage modulus and G″ is loss modulus. The crossover point is when G′ = G″. Prior to the crossover point, the molten polymer is predominantly viscous and after the crossover, the melted polymer becomes more elastic. Increase in melt elasticity over time is indicative of crystallisation which eventually results in solidification. This phenomenon was observed during extrusion foaming of PHBV and will be discussed later.
G′/G″ crossover times for the super-cooled PHBV (as-received) melt at different temperatures.
Rheological behaviour of post-foaming PHBV
The complex viscosity of melts from the post-foaming PHBV with different amounts of BA masterbatch is shown in Figure 6. The viscosity is noticeably lower than that of the as-received PHBV (when compared with that at 180℃ in Figure 3), which indicates that the polymer has undergone some thermal degradation during the extrusion foaming process. Moreover, with the increase of the BA masterbatch from 2.5 to 7.5 wt%, further decrease in viscosity was observed. This can be attributed to the hydrolytic degradation caused by water released from the decomposed sodium bicarbonate. Since PHBV is a polyester copolymer, it is susceptible to thermal and hydrolytic degradation by water at high temperatures and pressures.12,13 In this regard, the use of negative temperature profile can reduce the high temperature exposure time and minimise the thermal and hydrolytic degradation.
Complex viscosity of the post-foaming PHBV measured at 180℃ showing reduction in the viscosity with increasing level of the H2O and CO2 generating blowing agent.
The PHBV foaming process
Figure 7 presents the morphologies of extruded foams using the sheet die at the BA masterbatch levels of 1.25, 2.50 and 3.75 wt%, which correspond to 0.5, 1.0 and 1.5 wt% active contents, respectively. The sheets were extruded with temperature profile given in Table 1 at 30 r/min screw speed and flow rate of 3.3 kg/h. The analysis in density reduction, cell size and cell population density determined from these foams were also presented in Figure 8. Sparse cell population of cells averaging 140 µm was observed for foams extruded with low BA concentration (at 1.25 wt% masterbatch) and resulted in a small density reduction (14%). Increasing the masterbatch content to a medium level (2.50 wt%) gave rise to a high density reduction (of 32%) owing largely to the increased cell size to about 190 µm whereas the cell population hardly changed. At higher BA concentration (3.75 wt%), density reduction reached 40% from considerable increase in cell population while the cell size remained similar to that at 1.25 wt%. Clearly, this is attributable to more gas generated at higher BA contents. One would expect the trend to continue to produce lighter foam by using higher concentrations of BA. This was proved not the case as will be discussed later.
SEM images of PHBV foams extruded with the sheet die and BA masterbatch contents (wt %) of: (a) 1.25, (b) 2.50 and (c) 3.75. Effect of the amount of blowing agent on PHBV foam extruded with the sheet die: (a) cell size, (b) cell density population and (c) density reduction.
Figure 9 shows the influence of screw speed increase from 30 to 50 r/min (with corresponding feeding rate from 3.3 to 5.6 kg/h) without changing the other parameters. The higher screw speed resulted in lower density reductions. Higher screw speed generates higher pressure drop at the die exit which should normally enhance cell nucleation and expansion. It also reduces the residence time of the material in the barrels, particularly when a low L/D ratio extruder is used. Sodium bicarbonate is an endothermic BA with slow decomposition rate over wide range of temperatures;
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therefore, this result can be attributed to the shorter residence time that affected the decomposition of the BA. In addition, the shear thinning at high screw speed also reduced the melt viscosity, leading to cell rupture which was observed during the sheet foaming.
Density reduction as a function of screw speed in sheet foam extrusion at BA masterbatch content of 3.75 wt%.
The BA masterbatch content was extended further up to 7.50 wt% in the extrusion process performed using the 7 mm circular die. The foams were extruded with screw speed of 30 r/min and flow rate of 3.3 kg/h. The morphologies of extruded foams are shown in Figure 10 whereas Figure 11 presents analysis of cell size, cell density population and density reduction of the foams. The result shows that the maximum density reduction about 57% was achieved at 5.00 wt% addition of the masterbatch (corresponding to 2 wt% active content). Use of the strand die could result in the improved pressure profile over the sheet die, where premature foaming inside the sheet die was observed. This led to significantly higher cell density population and thinner cell walls as shown in Figure 10. At 5 wt% masterbatch addition, a fine tuning of processing parameters by reduction of the screw speed to 20 r/min with feeding rate to 2.2 kg/h and reduction of temperature in the last zone of the extruder and the die to 140℃ had shown further density reduction to 61.3%.
SEM images of foams extruded with the 7 mm strand die at BA masterbatch contents of (a) 2.5, (b) 5 and (c) 7.5 wt%, respectively. Effect of the amount of blowing agent on (a) cell size, (b) cell density population and (c) density reduction for foams extruded with the 7 mm strand die.
Calcium carbonate particles were expected to act as nucleation sites and also enhance viscosity of the material for cell stabilisation. Figure 12 shows the foam morphologies corresponding to 5, 12 and 20 wt% calcium carbonate loading and Figure 13 presents cell size, cell density population and density reduction, respectively.
SEM images of foams extruded with the 5 mm strand die using 5 wt% BA masterbatch and filled with (a) 5, (b) 12 and (c) 20 wt% of the calcium carbonate. Effect of the amount of the calcium carbonate on (a) cell size, (b) cell density population and (c) density reduction for foams extruded with the 5 mm strand die.
Increase in the calcium carbonate loading from 5 to 12 wt% reduced average cell size from 150 to about 100 µm (Figures 12(a) and (b) and 13(a)) but no further change is observed when the loading is increased to 20 wt% (Figures 12(b) and (c) and 13(a)). In contrast, the cell density population increased with the loading of the calcium carbonate and reached the maximum at about 12 wt% and then dropped at 20 wt% as shown in Figure 13(b). The density reductions achieved (based on densities of the CaCO3 filled PHBV) similar level to the unfilled PHBV (Figure 13(c)) for the range of CaCO3 loadings but achieved much finer cell structure (comparing Figure 12 with Figure 10 or Figure 13(a) with Figure 11(a)). The decrease of cell size and increase of cell density population could be explained by the enhancement of heterogeneous nucleation by the CaCO3 particles whereas the reduction of cell density population at high loading may be attributed to insufficient dispersion of CaCO3 agglomerates.
Further attempts were made to increase viscosity of the PHBV in order to increase the pressure drop at the die exit and enhance melt strength to minimise cell rupture. This was achieved using higher degree of super cooling by reduction of temperature in the last zone of the extruder and the die to 140℃ (below the melting point of 168℃). This led to further expansion of the foams and achieved 57% density reduction using 5 wt% BA masterbatch and 20 wt% CaCO3 loading. High level of super cooling, however, can result in buildup of the material in the die as shown in Figure 14. This can be correlated to solidification of the melt under super-cooling conditions as shown in Figure 4. Rate of crystallisation under super-cooling conditions is dependant on temperature. Temperature drops sharply across the narrow die channel and rapid crystallisation could take place at the die surface, resulting in the buildup of a solidified polymer layer. The polymer was slowly accumulating from the wall to the centre and the rate of accumulation depended on temperature (degree of the super cooling) and time under the super-cooling conditions. The accumulation of the solid layer in the die can lead to some changes in processing conditions (e.g. die pressure and flow characteristics) which led to the deterioration of foam quality and if not controlled, it was only possible to extrude quality foams for a limited time. Because this phenomenon took place at the narrow passages of the die, it seems to suggest that the stress-induced crystallisation may have also played a part.
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Further work is thus necessary to identify the extent of the limitation and optimise the processing conditions.
Die buildup associated with extrusion of the PHBV under super-cooled conditions. The outer layer is crystallised PHBV and the core is a purge polymer.
Conclusions
Rheological behaviour of PHBV is sensitive to its thermal history and during extrusion foaming, it can experience both thermal degradation and hydrolytic degradations from a water-generating BA. PHBV foams with density reduction up to about 60% were extruded using negative temperature profile to super cooling the polymer melt using a chemical BA based on sodium bicarbonate and citric acid. The temperature profile was chosen based on rheological characteristics of PHBV and experimental observations during extrusion foaming. The highest density reduction was obtained for extrusion with 5 wt% BA masterbatch. Extruded foams were characterised by predominantly closed cell structure with cell sizes between about 50 and 200 µm. For the selected BA and relatively low L/D ratio extruder, it was shown that lower screw speed gave necessary residence time for the decomposition of the BA and resulted in higher expansion of the foams. Moreover, the addition of calcium carbonate, while increasing overall density of the foams, resulted in much finer cell structure and decreased material costs. Super cooling proved to be beneficial on two sides, viscosity increase and reduction of thermal degradation. A drawback of such processing conditions was potential buildup of solidified polymer due to the crystallised material in the narrow channel of the die. Further refinement of processing conditions is necessary so as to remove premature crystallisation and improve quality of the foams.
Footnotes
Acknowledgements
The authors wish to acknowledge the collaboration from the research team in this project and the financial support from the Technology Strategy Board (TSB) of the UK government. The research consortium incorporates the following industrial and academic partners: Sainsburys Supermarkets Ltd, Nextek Ltd, Sharp Interpack Ltd, Imerys Minerals Ltd, Wells Plastics Ltd, Bangor University and Imperial College London. TSB is a business-led executive non-departmental public body established by the government. Its mission is to promote and support research into, and development and exploitation of, technology and innovation for the benefit of UK business, in order to increase economic growth and improve the quality of life. It is sponsored by the Department for Innovation, Universities and Skills (DIUS).