Bioplastic Poly Research Articles

Isothermal crystallization of polyhydroxyalkanoate (PHA) utilizing Raman spectroscopy to follow chain packing as well as molecular motion

Raman spectra of bioplastic poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHx at 13.8 % Hx) were recorded between −250 cm−1 and 3200 cm−1 during isothermal crystallization at 25⁰C after quenching from the melt in liquid nitrogen. At room temperature the crystallization proceeds slowly, so spectra were recorded over a 14-hour period. While there are spectral changes throughout the spectrum, the focus was on interpretable bands known to be sensitive to crystalline form. These bands included the carbonyl band that sharpens and shifts, a pair of bands on the high energy side of the carbon-hydrogen stretch, and a low frequency band that we assign to the molecular phonon in the crystal unit cell. After appropriate pre-processing of the spectra, they were further analyzed by 2D-COS (two-dimensional correlation spectroscopy) that provides determination of the order in which the polymer functional regions assemble into the crystalline state. According to this analysis one of the methyl CH’s interacts with the carbonyl bond to produce a line at 3000 cm−1. Following that, multiple changes appear in the carbonyl region, the strong CH band at 2930 cm−1 of the crystalline phase grows, then the 80 cm−1 phonon band, and the splitting of the methyl CH only appears after the phonon. From this sequence one can derive a picture of how the polymer unit locks into the crystal form. This can be of interest to commercialization of the materials because mechanical properties are intimately controlled by the crystallinity of the material. By understanding how the crystallization process proceeds, it can be engineered to be “fit for purpose” for a polymer targeted for a specific use.

Chemicals Affecting Cyanobacterial Poly(3-hydroxybutyrate) Accumulation: 2-Phenylethanol Treatment Combined with Nitrogen Deprivation Synergistically Enhanced Poly(3-hydroxybutyrate) Storage in Synechocystis sp. PCC6803 and Anabaena sp. TISTR8076

Various photoautotrophic cyanobacteria increase the accumulation of bioplastic poly(3-hydroxybutyrate) (PHB) under nitrogen deprivation (-N) for energy storage. Several metabolic engineering enhanced cyanobacterial PHB accumulation, but these strategies are not applicable in non-gene-transformable strains. Alternatively, stimulating PHB levels by chemical exposure is desirable because it might be applied to various cyanobacterial strains. However, the study of such chemicals is still limited. Here, 19 compounds previously reported to affect bacterial cellular processes were evaluated for their effect on PHB accumulation in Synechocystis sp. PCC6803, where 3-(3,4-dichlorophenyl)-1,1-dimethylurea, methyl viologen, arsenite, phenoxyethanol and 2-phenylethanol were found to increase PHB accumulation. When cultivated with optimal nitrate supply, Synechocystis contained less than 0.5% [w/w dry weight (DW)] PHB, while cultivation under -N conditions increased the PHB content to 7% (w/w DW). Interestingly, the -N cultivation combined with 2-phenylethanol exposure reduced the Synechocystis protein content by 27% (w/w DW) but significantly increased PHB levels up to 33% (w/w DW), the highest ever reported photoautotrophic cyanobacterial PHB accumulation in a wild-type strain. Results from transcriptomic and metabolomic analysis suggested that under 2-phenylethanol treatment, Synechocystis proteins were degraded to amino acids, which might be subsequently utilized as the source of carbon and energy for PHB biosynthesis. 2-Phenylethanol treatment also increased the levels of metabolites required for Synechocystis PHB synthesis (acetyl-CoA, acetoacetyl-CoA, 3-hydroxybutyryl-CoA and NADPH). Additionally, under -N, the exposure to phenoxyethanol and 2-phenylethanol increased the PHB levels of Anabaena sp. from 0.4% to 4.1% and 6.6% (w/w DW), respectively. The chemicals identified in this study might be applicable for enhancing PHB accumulation in other cyanobacteria.

Highly-toughened and dimensionally-stable TEMPO cellulose nanofiber/bio-PBSA nanocomposites fabricated via Pickering emulsion process

Bioplastic poly(butylene succinate-co-adipate) (PBSA) has several advantages because of its flexibility and high biodegradability. PBSA, however, possesses poor mechanical properties and dimensional instability. To overcome this, PBSA composites with TEMPO-oxidized cellulose nanofibers (TOCN/PBSA) were fabricated via the Pickering emulsion method. Porous bio-nanocomposite TOCN/PBSA mediums were made by the Pickering emulsion method, followed by the subsequent lyophilization to obtain TOCN/PBSA films by compression molding. It was confirmed that the Pickering-emulsion condition was a key factor to control the TOCN dispersion state in composite films. The Young's modulus of TOCN/PBSA composite films increased to 305 MPa at the TOCN concentration of 5 wt%, which was almost twice as high as that of pure PBSA. The studies on the morphology and tensile testing also revealed that the dispersion state of TOCNs was important to achieve excellent reinforcement effects and to suppress the fracture of composite films caused by stress concentration. The coefficient of thermal expansion at the TOCN concentration of 5 wt% was reduced to ∼25 × 10−5 K−1, half the value of pure PBSA. It was concluded that compounding TOCNs via the Pickering emulsion method was a very effective reinforcement method to improve the mechanical properties and the dimensional stability of PBSA.

Heterologous phasin expression in Rhodopseudomonas palustris CGA009 for bioplastic production from lignocellulosic biomass

Rhodopseudomonas palustris CGA009 is a metabolically robust microbe that can utilize lignin breakdown products to produce polyhydroxyalkanoates (PHAs), biopolymers with the potential to replace conventional plastics. Our recent efforts suggest PHA granule formation is a limiting factor for maximum production of the bioplastic poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) by R. palustris. The Phap1 phasin (phaP1) from the PHB-producing model bacterium Cupriavidus necator H16 was expressed in R. palustris with the aim of overproducing PHBV from the lignin breakdown product p-coumarate by fostering smaller and more abundant granules. Expression of phaP1 yielded PHBV production from R. palustris aerobically (0.7 g/L), which does not occur in the wild-type strain, and led to a significantly higher PHBV titer than wild-type anaerobic production (0.41 g/L). The 3HV fractions were also significantly increased under both anaerobic and aerobic conditions, which boosts thermomechanical properties and potential for application. Thus, heterologous phasin expression in R. palustris provides flexibility for industrial processing and could foster compositional changes in copolymers with better thermomechanical properties compared to PHB alone.

Improvement of Interfacial Adhesion between Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) and Silica Particles

To improve the mechanical properties of biodegradable bioplastic poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH), a filler made up of silica particles (SiO2) is mixed with PHBH; however, SiO2 ...

Rational flux-tuning of Halomonas bluephagenesis for co-production of bioplastic PHB and ectoine

Ectoine, a compatible solute synthesized by many halophiles for hypersalinity resistance, has been successfully produced by metabolically engineered Halomonas bluephagenesis, which is a bioplastic poly(3-hydroxybutyrate) producer allowing open unsterile and continuous conditions. Here we report a de novo synthesis pathway for ectoine constructed into the chromosome of H. bluephagenesis utilizing two inducible systems, which serve to fine-tune the transcription levels of three clusters related to ectoine synthesis, including ectABC, lysC and asd based on a GFP-mediated transcriptional tuning approach. Combined with bypasses deletion, the resulting recombinant H. bluephagenesis TD-ADEL-58 is able to produce 28 g L−1 ectoine during a 28 h fed-batch growth process. Co-production of ectoine and PHB is achieved to 8 g L−1 ectoine and 32 g L−1 dry cell mass containing 75% PHB after a 44 h growth. H. bluephagenesis demonstrates to be a suitable co-production chassis for polyhydroxyalkanoates and non-polymer chemicals such as ectoine.

Cyanobacterial production of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) from carbon dioxide or a single organic substrate: improved polymer elongation with an extremely high 3-hydroxyvalerate mole proportion

The bioplastic poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) changes material properties upon altering mole ratio (mol%) of the two monomers: 3-hydroxybutyrate (HB) and 3-hydroxyvalerate (HV). Biotechnology for producing PHBV with a wide range of HV mol% using low amount of organic substrate is required. Here, 20 evolutionarily divergent cyanobacteria were examined for PHBV production. Under photoautotrophy with carbon dioxide (CO2) supply, one strain produced PHBV. But under heterotrophy using a medium containing only one type of organic substrate, 4, 15, and 13 strains were found to accumulate PHBV when supplied with acetate (C2H3O2), propionate (C3H5O2), and valerate (C5H9O2), respectively. Hence, these cyanobacterial PHBV productions required only CO2 or a single type of organic substrate. The obtained PHBV showed a broad range of HV mole ratios from 4 to 96 mol%. The cells producing PHBV with high HV mole ratios (31–96 mol%) exhibited low cellular acetyl-CoA levels. The PHBV with a 96.2 mol% HV and 3.8 mol% HB from Nostoc microscopicum elongated at 98 ± 15% (length/length), a 19.6-fold higher elongation than that of commercial poly-3-hydroxybutyrate containing 100 mol% HB. The results demonstrated the economical utility of cyanobacterial conversion of CO2 and a single organic substrate to synthesize PHBV with improved elongation quality.

Development of Inherently Flame—Retardant Phosphorylated PLA by Combination of Ring-Opening Polymerization and Reactive Extrusion

In this study, a highly efficient flame-retardant bioplastic poly(lactide) was developed by covalently incorporating flame-retardant DOPO, that is, 9,10-dihydro-oxa-10-phosphaphenanthrene-10-oxide. To that end, a three-step strategy that combines the catalyzed ring-opening polymerization (ROP) of L,L-lactide (L,L-LA) in bulk from a pre-synthesized DOPO-diamine initiator, followed by bulk chain-coupling reaction by reactive extrusion of the so-obtained phosphorylated polylactide (PLA) oligomers (DOPO-PLA) with hexamethylene diisocyanate (HDI), is described. The flame retardancy of the phosphorylated PLA (DOPO-PLA-PU) was investigated by mass loss cone calorimetry and UL-94 tests. As compared with a commercially available PLA matrix, phosphorylated PLA shows superior flame-retardant properties, that is, (i) significant reduction of both the peak of heat release rate (pHRR) and total heat release (THR) by 35% and 36%, respectively, and (ii) V0 classification at UL-94 test. Comparisons between simple physical DOPO-diamine/PLA blends and a DOPO-PLA-PU material were also performed. The results evidenced the superior flame-retardant behavior of phosphorylated PLA obtained by a reactive pathway.

Fabrication of Poly(lactic acid)/Silkworm Excrement Composite with Enhanced Crystallization, Toughness and Biodegradation Properties

This study investigated and analyzed the mechanical and thermal properties as well as biodegradability of bio-plastic Poly(lactic acid) (PLA) and PLA/silkworm excrement (PLA/SE) composites fabricated by injection molding. Differential scanning calorimetry (DSC) results demonstrated that SE enhanced the crystallization ability of PLA represented by the increased crystallinity. The tensile strength of PLA/SE composites did not enhance when adding SE due to SE particles became drawbacks while stretching as well as the lower tensile strength of SE than PLA. However, the impact strength of PLA/SE 10% was enhanced approximately 30.1%, comparing with neat-PLA. Biodegradation experiment showed that PLA/SE composites had faster and obvious disintegration in 7 weeks, especially PLA/SE 20% composite, which indicating SE enhanced the biodegradability of PLA. Additionally, neat-PLA and the PLA in PLA/SE composites can be fully biodegraded.

Disruption of cyanobacterial \u03b3-aminobutyric acid shunt pathway reduces metabolites levels in tricarboxylic acid cycle, but enhances pyruvate and poly(3-hydroxybutyrate) accumulation

The photoautotrophic cyanobacterium Synechocystis sp. PCC 6803 assimilates carbon dioxide as the sole carbon source, and a major portion of the assimilated carbon is metabolically consumed by the tricarboxylic acid (TCA) cycle. Effects of partial interference of TCA cycle metabolic activity on other carbon metabolism have yet to be examined. Here, the γ-aminobutyric acid (GABA) shunt, one of the metabolic pathways for completing TCA cycle in Synechocystis, was disrupted via inactivating the glutamate decarboxylase gene (gdc). Under normal photoautotrophic condition, cell growth and the level of the TCA cycle metabolites succinate, malate and citrate were decreased by 25%, 35%, 19% and 28%, respectively, in Δgdc mutant relative to those in the wild type (WT). The cellular levels of glycogen and total lipids of the Δgdc mutant were comparable to those of the WT, but the intracellular levels of pyruvate and bioplastic poly(3-hydroxybutyrate) (PHB) were 1.23- and 2.50-fold higher, respectively, in Δgdc mutant. Thus, disruption of the GABA shunt pathway reduced the TCA cycle metabolites levels, but positively enhanced the bioaccumulation of pyruvate and PHB. The PHB production rate in Δgdc mutant was 2.0-fold higher than in the WT under normal photoautotrophy.

Fed-Batch Cultivations of Rhodospirillum rubrum Under Multiple Nutrient-Limited Growth Conditions on Syngas as a Novel Option to Produce Poly(3-Hydroxybutyrate) (PHB).

Syngas from gasified organic waste materials is a promising feedstock for the biotechnological synthesis of the bioplastic poly([R]-3-hydroxybutyrate) (PHB) with Rhodospirillum rubrum. In a first approach, growth studies were carried out with this strain in gas-tight serum vials. When syngas (40% CO, 40% H2, 10% CO2, and 10% N2 v/v) was diluted with N2 to 60%, a 4-fold higher biomass production was detected compared to samples grown on 100% syngas, thus indicating a growth inhibitory effect. The best performing syngas-mixture was then used for C-, C,N-, and C,P-limited fed-batch fermentations in a bioreactor with continuous syngas and acetate supply. It was found that C,P-limited PHB productivity was 5 times higher than for only C-limited growth and reached a maximal PHB content of 30% w/w. Surprisingly, growth and PHB production stopped when N, as a second nutrient, became growth-limiting. Finally, it was concluded that a minimal supply of 0.2 g CO g−1 biomass h−1 has to be guaranteed in order to cover the cellular maintenance energy.

Effects of chain-extending stabilizer on bioplastic poly(lactic acid)/polyamide blends compatibilized by reactive extrusion

The immiscible bio-plastic blend of poly(lactic acid) (PLA) and polyamide11 (PA11 or Nylon11) was compatibilized, using a catalyzed the ester-amide interchange chemical reaction during reactive extrusion. The effects of screw configuration on mixing and reaction optimization were explored. It was found that effective mixing and minimized degradation were achieved when kneading elements were installed only near the die end of the extruder rather than near the beginning of the melt section. However, significant degradation of PLA could not be avoided during processing, which led to a decrease in mechanical strength of blend. To prevent the molecular weight reduction, tris(nonylphenyl) phosphite (TNPP) stabilizer was introduced to improve the thermal stability of PLA matrix. Improvement to both tensile strength and elongation were achieved in the resulting PLA/PA11/TsOH/TNPP blend.

Efficient production and secretion of oxaloacetate from Halomonas sp. KM-1 under aerobic conditions

The alkaliphilic, halophilic bacterium Halomonas sp. KM-1 can utilize glucose for the intracellular storage of the bioplastic poly-(R)-3-hydroxybutyric acid (PHB) and extracellular secretion of pyruvate under aerobic conditions. In this study, we investigated the effects of sodium chloride concentration on PHB accumulation and pyruvate secretion in the KM-1 strain and, unexpectedly, observed that oxaloacetate, an important intermediate chemical in the TCA cycle, glycogenesis, and aspartic acid biosynthesis, was secreted. We then further analyzed oxaloacetate productivity after changing the sodium chloride additive concentration, additive time-shift, and culture temperature. In 42-h batch-cultivation experiments, we found that wild-type Halomonas sp. KM-1 secreted 39.0 g/L oxaloacetate at a rate of 0.93 g/(L h). The halophilic bacteria Halomonas has already gained attention for industrial chemical-production processes owing to its unique properties, such as contamination-free culture conditions and a tolerance for high substrate concentrations. Moreover, no commercial scale oxaloacetate production was previously reported to result from bacterial fermentation. Oxaloacetate is an important intermediate chemical in biosynthesis and is used as a health food based on its role in energy synthesis. Thus, these data provided important insights into the production of oxaloacetate and other derivative chemicals using this strain.

Production of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) under photoautotrophy and heterotrophy by non-heterocystous N2-fixing cyanobacterium

The photoautotrophically grown cyanobacterium Oscillatoria okeni TISTR 8549 was found to produce bioplastic poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV). This PHBV production occurred under nitrogen deprivation (–N) that yielded PHBV accumulation of 14±4% (w/w DW) in which 3-hydroxyvalerate accounted for 5.5mol%. The heterotrophically grown (–N condition with acetate supplementation) cells under light showed no increase of PHBV storage, but under dark condition these cells increased PHBV accumulation to 42±8% (w/w DW) with 6.5mol% of 3-hydroxyvalerate. Compared to poly-3-hydroxybutyrate (PHB), the PHBV from O. okeni had a lower melting temperature by 5–7°C, a higher % elongation at break by 4–7times and a greater Young’s elastic modulus by 2.3–2.5times.

Assessment of models for anaerobic biodegradation of a model bioplastic: Poly(hydroxybutyrate-co-hydroxyvalerate)

Kinetic models of anaerobic digestion (AD) are widely applied to soluble and particulate substrates, but have not been systematically evaluated for bioplastics. Here, five models are evaluated to determine their suitability for modeling of anaerobic biodegradation of the bioplastic poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV): (1) first-order kinetics with and without a lag phase, (2) two-step first-order, (3) Monod (4) Contois, and (5) Gompertz. Three models that couple biomass growth with substrate hydrolysis (Monod, Contois, and Gompertz) gave the best overall fits for the data (R2>0.98), with reasonable estimates of ultimate CH4 production. The particle size limits of these models were then evaluated. Below a particle size of 0.8mm, rates of hydrolysis and acetogenesis exceeded rates of methanogenesis with accumulation of intermediates leading to a temporary inhibition of CH4 production. Based on model fit and simplicity, the Gompertz model is recommended for applications in which particle size is greater than 0.8mm.

Efficient production and secretion of pyruvate from Halomonas sp. KM-1 under aerobic conditions.

The alkaliphilic, halophilic bacterium Halomonas sp. KM-1 can utilize both hexose and pentose sugars for the intracellular storage of bioplastic poly-(R)-3-hydroxybutyric acid (PHB) under aerobic conditions. In this study, we investigated the effects of the sodium nitrate concentration on PHB accumulation in the KM-1 strain. Unexpectedly, we observed the secretion of pyruvate, a central intermediate in carbon- and energy-metabolism processes in all organisms; therefore, pyruvate is widely used as a starting material in the industrial biosynthesis of pharmaceuticals and is employed for the production of crop-protection agents, polymers, cosmetics, and food additives. We then further analyzed pyruvate productivity following changes in culture temperature and the buffer concentration. In 48-h batch-cultivation experiments, we found that wild-type Halomonas sp. KM-1 secreted 63.3 g/L pyruvate at a rate of 1.32 g/(L·h), comparable to the results of former studies using mutant and recombinant microorganisms. Thus, these data provided important insights into the production of pyruvate using this novel strain.

Improvement of the mechanical behavior of bioplastic poly(lactic acid)/polyamide blends by reactive compatibilization

ABSTRACTPolymer blends are of significant interest for reinforcing bioplastics via addition of a second polymer and a blend compatibilizer or in situ reaction. However, increased costs associated with additional materials and extra processing steps can limit the viability of this solution. Here, a simple, continuous reactive extrusion processing method was examined for producing tougher bioplastic blends. p‐Toluenesulfonic acid (TsOH) catalyst was added to two immiscible biobased polymers, poly(lactic acid) (PLA) and polyamide11 (PA11), to induce ester‐amide exchange reaction. The mechanical properties of PLA were improved through mixing with PA11 by introducing copolymers at the interface thereby reducing interfacial tension. The morphology, chemical structure analysis, and tensile testing supported that copolymerization reaction occurred resulting in improved bonding between PLA and PA11 with 0.5 wt % TsOH catalyst in the batch mixing, but depolymerization dominated at higher shear stress (2000 rpm) and catalyst loading (over 2 wt %). The PLA/PA11 blend with 0.5 wt % TsOH catalyst displayed around 50% improvement in elongation at break in twin‐screw extruder blending (around 5 min mixing time) at 250 rpm screw speed, which was similar to the improvement using batch mixing (20 min mixing time). © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43350.

New Superefficiently Flame-Retardant Bioplastic Poly(lactic acid): Flammability, Thermal Decomposition Behavior, and Tensile Properties

In this study, a superefficiently flame-retardant bioplastic poly(lactic acid) was developed by incorporating gas–solid biphase flame-retardant N,N′-diallyl-P-phenylphosphonicdiamide (P-AA), into PLA matrix. The flame retardancy of PLA/P-AA was investigated by limiting oxygen index (LOI), vertical burning test (UL94), and cone calorimeter test. Surprisingly, it was noted that only 0.5 wt % loading of P-AA increased LOI value of PLA from 20.5 to 28.4 and passed UL 94 V-0 rating at 3.2 mm thickness. In order to understand the effect of P-AA on the thermal decomposition behavior of PLA, a comprehensive study was investigated in this paper, including (i) adopting modified Coats–Redfern method to study the thermal decomposition kinetics of PLA and PLA/P-AA systems, and (ii) characterizing the evolved gaseous products and the residues in the condensed phase by thermogravimetry linked Fourier transform infrared spectroscopy (TGA–FTIR) and variable temperature Fourier transform infrared spectroscopy (VT-FTIR) tec...

Toughening of poly(lactic acid) with the renewable bioplastic poly(trimethylene malonate)

ABSTRACTThe aim of this work was to enhance poly(lactic acid)'s (PLA) flexibility and ductility by blending it with another bioplastic. Poly(trimethylene malonate) (PTM), developed as part of this study, was synthesized from 1,3‐propane diol and malonic acid via melt polycondensation. Blend films of PLA and PTM were prepared by solvent casting from chloroform. Differential scanning calorimetry and thermogravimetric analysis were used to show shifted phase transitions and a single glass‐transition temperature, indicating miscibility of PTM in the blend films. Morphology and mechanical characterizations of the PLA/PTM blend films were performed by atomic force microscopy using a quantitative nanomechanical property mapping mode, tensile testing, and scanning electron microscopy. Miscible blends exhibited Young's modulus and elongation at break values that can significantly extend the usefulness of PLA in commercial applications. The blending of PTM with PLA resulted in films with a 27‐fold increase in toughness compared with neat PLA film. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40888.

Enhancing the Flexural and Impact Properties of Bioplastic Poly(lactic acid) by Melt Blending with Poly(butylene succinate)

Bio-based biodegradable Poly (lactic acid) (PLA) suffers limitations such as brittleness and slow crystallization. This study aims to resolve the brittle nature of PLA by blending with Poly (butylene succinate) (PBS), a more ductile biodegradable polymer with superior toughness and flexural properties. In this research, a series of PLA/PBS blends was prepared at the blend ratios of 100/0, 80/20, 60/40, 40/60, 20/80 and 0/100. FTIR showed that there was no change in the functional groups of the PLA/PBS blends. Thermal stability assessed by TGA revealed that PBS degraded at higher temperature than that of PLA; the decomposition temperature (Td) at 10% weight loss of PLA and PBS were 330.8 and 356.4°C respectively. The Td of all the blends increased gradually with the addition of PBS. The flexural properties in terms of the flexural strength and the flexural modulus of the blends reduced significantly with PBS content. The PLA/PBS specimens with greater PBS content were softened and flexed more easily, thereby requiring a much lower flexural strength. The flexural modulus of the 80/20 and 60/40 blends dropped from 3.5 GPa for neat PLA to 3.2 GPa and 2.1 GPa while the flexural strength also declined from 105.3 MPa to 90.9 MPa and 69.1 MPa respectively. The toughness of all the blends was greater than that of neat PLA; in particular the 60/40 blend exhibited superior impact strength of 48.7 J/m compared with 30.9 J/m of the neat PLA. The microscopic images of all the blends showed two distinct phases; the 60/40 blend consisted of well dispersed small particles of the tough PBS, resulting in greater absorption of energy upon impact.