Study On Microcellular Foaming Behavior Of Ramie Fiber Reinforced Poly(Lactic Acid) Composite

Poly（lactic acid）（PLA）is a biodegradable thermoplastic polyester with good processing performance,mechanical properties,biocompatibility,and transparency.It is widely used in packaging,agriculture,automotive,biomedical and other fields.The preparation of porous foamed materials from PLA using supercritical fluids can reduce production costs and broaden the range of applications.However,due to its low melt strength and slow crystallization rate,it is difficult to obtain uniformly distributed and highly expanded foamed materials.How to improve the foaming ability of PLA without damaging its biodegradability has always been the focus of scholars’ attention and research.Therefore,this study proposes to use ramie fiber（RF）to improve the melt strength and crystallization behavior of PLA,and to study the foaming behavior of poly（lactic acid）/ramie fiber（PLA/RF）composites by one-step foaming molding process.Firstly,a green method of adding ramie fiber to PLA to improve its foaming ability was proposed in this article.PLA/RF foam was prepared using a one-step method,and its crystallization behavior,rheological behavior,CO₂ solubility,and foaming behavior were studied.The results showed that the addition of RF was beneficial to the nucleation of PLA crystals but detrimental to crystal growth.With the increase of RF content,the crystallinity of PLA/RF composite materials first increased and then decreased.RF increased the storage modulus（G’）,loss modulus（G"）,and complex viscosity（η*）of PLA,and the melt viscoelasticity increased.The addition of RF affected the solubility of CO₂ in PLA.As the ramie fiber content increased,the solubility of CO₂ first increased and then decreased.For the crystallization behavior of the foamed samples,both the saturation and unsaturation foamed samples had different degrees of improvement in crystallinity compared to unfoamed samples.The crystallinity of PLA/1RF foam samples was higher than that of PLA foam samples,and the crystallinity of PLA/RF decreased with the increase of ramie fiber content.The addition of RF improved the cell morphology and significantly increased the uniformity of cell size and distribution.With the increase of RF content,the diameter of the foam first decreased and then increased and then decreased,the density of the bubble first increased and then decreased,and the foaming times first increased and then decreased.When the ramie fiber content was 1%,a foaming ratio of 10.09 times,a cell size of 24.10 μm,and a cell density of 9.06×108 cells/cm3 were obtained for PLA/RF foam at a foaming temperature of 110℃,a saturation pressure of 15 MPa,and a saturation time of 40 min.Secondly,to regulate the foaming ratio and cell morphology of PLA/RF foam,and to reveal the relationship between process parameters and average cell diameter,cell density,and foaming ratio,the effects of different foaming temperatures and saturation pressures on the crystallization behavior and foaming behavior of PLA/RF composite materials were studied.The results showed that with the increase of foaming temperature,the crystallinity of PLA/RF foam first increased and then decreased.With the increase of saturation pressure,the crystallinity of PLA/RF foam also first increased and then decreased.At low foaming temperature（100-115℃）and low saturation pressure（10-17.5 MPa）,the DSC curve of PLA/RF foam showed a melting double peak,while at high foaming temperature（120℃）and high saturation pressure（20 MPa）,the DSC curve of foam showed a melting single peak.With the increase of foaming temperature,the foaming ratio first increased and then decreased,the cell density decreased,and the cell diameter first increased and then decreased.With the increase of saturation pressure,the foaming ratio first increased and then decreased,the cell density first decreased and then increased,and the cell diameter first increased and then decreased.When the saturation temperature was 112℃ and the saturation pressure was 15 MPa,the maximum foaming ratio of PLA/RF was 10.43 times.Finally,in order to further improve the expansion ratio of PLA/RF foam and improve its foaming ability,the method of pre-isothermal cold crystallization treatment of PLA/RF composite material samples before foaming was used to clarify the influence of pre-isothermal treatment conditions on the crystallization behavior and foaming behavior of PLA/RF composite material samples before and after foaming.The results showed that for the samples treated at low pre-isothermal temperatures（70-90℃）and low pre-isothermal times（10-20 min）,cold crystallization occurred during the heating process.With the increase of pre-isothermal temperature and pre-isothermal time,the cold crystallization peak disappeared.For the untreated samples and PLA/RF foam samples treated at pre-isothermal temperatures of 70-80℃and pre-isothermal times of 10-20 min.there was a melting heat release peak around 148℃.With the increase of pre-isothermal temperature and pre-isothermal time,PLA/RF composite material showed a melting double peak.With the increase of pre-isothermal temperature and pre-isothermal time,the crystallinity of PLA/RF composite material increased.With the increase of pre-isothermal treatment temperature,the cell size first decreased and then increased,the cell density first increased and then decreased,and the foaming ratio first increased and then decreased.When the pre-isothermal time was fixed at 15 min,the maximum foaming ratio of PLA/RF was achieved at a pre-isothermal treatment temperature of 80℃.With the increase of pre-isothermal treatment time,the cell size first decreases and then increases,the cell density first increases and then decreases,and the foaming ratio first increases and then decreases.When the pre-isothermal temperature is fixed at 90℃,the maximum foaming ratio of 18.06 times is reached at the pre-isothermal treatment time of 30 min.