Research On Microcellular Foaming Process Of Thermoplastic Polyurethane And Their Mechanical Properties

Thermoplastic Polyurethane(TPU)is a type of multi-block copolymer composed of soft and hard segments,and is one of the most widely used thermoplastic elastomers.The hydrogen bonding interaction between the polar groups of the soft and hard segments causes the hard segments to be distributed in the form of cross-linking points within the soft segments.The thermodynamic incompatibility between the two segments results in TPU exhibiting a microphase-separated state at room temperature.This unique structure gives TPU excellent properties such as high wear resistance,recoverability,and processing performance,making it widely used in fields such as footwear,construction,and medical industries.With the increasing maturity of foaming technology,thermoplastic elastomer foam products have been widely used in many industries such as furniture,automotive,sportswear,and packaging.The commonly used foaming methods mainly include chemical foaming and physical foaming.Among them,physical foaming has advantages such as being green,pollution-free,low-cost,and having superior foam product performance,which has a broader application prospect.CO2 and N2,as the most widely used blowing agents,have been tried in various foaming processes of elastomers.However,using CO2 as a physical blowing agent to prepare TPU foam requires more stringent conditions,and the size stability of the foam remains a huge challenge.When using N2 as the blowing agent,there is often a problem of low expansion ratio of TPU foam.Both of these factors limit the furtherproduction and application of TPU foam.On the basis of this,we propose a new strategy for preparing TPU foam with a cellular structure using a one-step foaming method with scCO2 as the blowing agent by regulating the ratio of soft and hard segments.The impact of hard segment content on the shrinkage and expansion ratio recovery performance of TPU foam was demonstrated through data analysis.The cellular structure was observed and analyzed using scanning electron microscopy,and the expansion ratio,cell size,and cell density of TPU foam prepared with different hard segment contents were studied with temperature and pressure variations.The results showed that increasing the hard segment content shifted the foam foaming process window to a higher temperature range.When preparing foam with a higher expansion ratio,an increase in hard segment content significantly reduced foam shrinkage and increased foam recovery ability.The foaming rate of the prepared TPU-93A foam can reach 16.5 times,and the shrinkage ratio is only 37.8%.Secondly,in response to the shrinkage of foam,TPU foam was prepared by depressurization foaming method by introducing N2 and forming mixed blowing agents with CO2.The study showed that the mixed blowing agents played an important role in maintaining the size stability of TPU foam.TPU-93A foam prepared with a 2:8(N2:CO2)mixed fblowing agents had a expansion ratio of up to 20.5 times and a shrinkage ratio of only 8.4%.Foam with final expansion ratios of 16.5 times and 16.7 times,respectively,could be prepared using CO2 and 4:6(N2:CO2)mixed blowing agents.However,compared with foam prepared with pure CO2,the shrinkage ratio of foam prepared with the mixed blowing agents was reduced from 37.8%to 6.3%.In addition,the compression performance of the foam prepared with the N2CO2 mixed blowing agents was significantly improved.The average compression stress of TPU foam prepared with a 2:8(N2:CO2)mixed blowing agents was 152.6%higher than that of foam prepared with CO2,demonstrating superior compression performance.This provides a wider application prospect for the preparation and production of thermoplastic elastomer foam.Secondly,based on CO2-N2 mixed blowing agents,TPU foam was prepared using a twostep method under low-temperature saturation conditions.The effects of saturation temperature,saturation pressure,saturation time,and mixing ratio of blowing agents on the solubility of blowing agents in TPU were investigated.Similarly,the effects of foaming temperature and solubility on TPU foaming behavior were studied.The results showed that the solubility of blowing agents increased with increasing pressure and CO2 fraction,and decreased with increasing saturation temperature.In the subsequent foaming experiments,the effects of mixed blowing agents on the morphology and expansion ratio of TPU-93A foam were further investigated,and it was found that the solubility of gas was the main influencing factor on foam foaming behavior.TPU-93A foam with a final expansion ratio of 7.2 was prepared using pure CO2 and 2:8(N2:CO2)mixed blowing agents,and the foam prepared with mixed blowing agents exhibited better elasticity and higher compression strength,with an average compression strength 42.5%higher.Through the above research,this paper prepared TPU foam with anti-shrinkage,lightweight,and high elasticity using CO2 and N2 mixed blowing agents by depressurization foaming method(one-step method)and heating foaming method(two-step method),and achieved control over the dimensional stability of TPU foam.The mechanism of foam shrinkage was systematically studied and high expansion ratio foam with reduced shrinkage ratio was successfully prepared,providing a wider space for the production and application of TPU foam.