Mechanical properties and fracture mechanisms of Novatein thermoplastic protein and blends with core–shell particles (CSPs) have been examined. Novatein is brittle with low impact strength and energy‐to‐break. Epoxy‐modified CSPs increase notched and unnotched impact strength, tensile strain‐at‐break, and energy‐to‐break, while tensile strength and modulus decrease as CSP content increases. Tg increases slightly with increasing CSP content attributed to physical crosslinking. Changes to mechanical properties are related to the critical matrix ligament thickness and rate of loading. Novatein control samples display brittle fracture characterized by large‐scale crazing. At high CSP content a large plastic zone and a slow crack propagation zone in unnotched and tensile samples are observed suggesting increased energy absorption. Notched impact samples reach critical craze stresses easily regardless of CSP content reducing impact strength. It is concluded that the impact strength of thermoplastic protein can be modified in a similar manner to traditional thermoplastics.

Novatein thermoplastic protein was extrusion blended with poly(butylene adipate-co-terephthalate) (PBAT) in the presence of dual compatibilizers to produce blends with greater energy absorbing properties than pure Novatein. Compatibilizer pairs were Joncryl ADR-4368 (glycidyl methacrylate-functionalized) with 2-methylimidazole (2MI), and poly-2-ethyl-2-oxazoline (PEOX) with polymeric diphenyl methane diisocyanate (pMDI). Uncompatibilized Novatein/PBAT blends had decreased tensile mechanical properties, attributed to phase separation, and poor interfacial adhesion. PBAT became finely dispersed in both compatibilized systems, but PEOX/pMDI blends showed embrittlement and large Novatein domains, which acted as stress concentrations. Tensile strength and elongation at break for Joncryl/2MI blends did not decrease compared with Novatein, even at 10 wt % PBAT, and impact strength increased threefold. Dynamic mechanical analysis and solvent extraction showed that PBAT coalesced in all systems, at compositions as low as 2 wt %. It was concluded that using Joncryl/2MI as a dual compatibilizer system can successfully produce a morphology that enhances energy absorption during fracture.

Novatein thermoplastic protein was blended with modified polyethylene (containing either epoxy, carboxylic acid functionalities partially neutralised to produce zinc carboxylate salts, or maleic anhydride functionalities) to alter blend morphology and to manipulate thermal and mechanical properties. Up to 40 pph Novatein polyethylene (PE) was blended with Novatein by extrusion and injection moulding. Using zinc ionomer resulted in optimal properties and was compatible with a finely dispersed morphology at high content; high interfacial tension (σ) and a viscosity ratio (λ) of ~1 was observed. Unmodified blends and those containing epoxy functionalities showed co-continuity at low PE content. Whilst co-continuity appeared to increase impact resistance, other mechanical properties decreased due to lack of phase interaction. Maleic anhydride-grafted-polyethylene blends showed a finely dispersed PE phase, yet was less compatible. Zinc ionomer was deemed to be the most appropriate for modification of mechanical properties in Novatein. K E Y W O R D Sbiopolymers, blends, morphology, thermoplastic protein How to cite this article: Smith MJ, Verbeek CJR. The relationship between morphology development and mechanical properties in thermoplastic protein blends.

Differential scanning calorimetry (DSC) was used to study the curing reaction of bisphenol A diglycidyl ether (DGEBA) and Novatein thermoplastic protein (NTP). NTP is made from bloodmeal and is ∼60% protein. Activation energy (E α ), pre-exponential factor (ln A o ) and order of reaction (m + n) were calculated using the Kissinger, model-free isoconversional and autocatalytic models. Curing kinetics were almost independent of concentration and the addition of plasticizers and protein denaturants to bloodmeal caused a decrease in E α of ∼50%. Increasing protein denaturants had little effect on the reaction; however, inclusion of salt proved detrimental. At NTP's maximum processing temperature (∼450 K), using an equal molar ratio of epoxy groups to reactive amino acids, 75% conversion can be reached in 3 min based on the modeling done here.

Novatein thermoplastic protein was blended with 10 wt% poly(butylene adipate-coterephthalate) (PBAT) compatibilized with Joncryl ADR-4368 and 2-methylimidazole (2MI). Morphology was tailored for favorable impact strength through changing viscosity ratio (λ) and interfacial tension (γ 12 ). For uncompatibilized blends, λ decreased and γ 12 increased with increasing Novatein water content, whereas compatibilizers caused a decrease in both λ and γ 12 . PBAT continuity was high when uncompatibilized, but dispersion improved with decreasing λ and increasing γ 12 . The dispersed domain size decreased in all compatibilized blends; PBAT continuity was lowest in samples with the smallest λ. Compatibilized blends had higher impact strength than Novatein and uncompatibilized blends through improved interfacial adhesion, smaller domain size, and increased dispersion. By altering λ and γ 12 , and with appropriate chemical interaction, a morphology can be created for improved impact strength. Increasing PBAT content showed further increases in impact strength; however, a cocontinuous morphology formed, demonstrating that composition can override the effect of λ and γ 12 . K E Y W O R D Sblends, impact resistance, morphology development, polyester, thermoplastic protein