J O U R N A L O F M A T E R I A L S S C I E N C E(2005)
MuCell R technology for injection molding:A processing method for polyether-urethane scaffolds
S.LEICHER,J.WILL,H.HAUGEN,E.WINTERMANTEL
Central Institute for Medical Engineering ZIMT,Technische Universit¨at M¨unchen, Boltzmannstras11,D-85748,Garching bei M¨unchen,Germany
MuCell R injection molding technology which takes supercritical carbon dioxide as blowing agent was ud as processing method to generate porous samples made of thermoplastic polyether-urethane.The influence of processing parameters on the morphology of the porous structures was examined by mercury intrusion porometry and image analysis.An increa in injection speed and content of CO2in the polymer melt decread pore sizes whereas an increa in the percentage of weight reduction incread pore sizes.
Polyether-urethane samples with porosities of70%and pore sizes from184to1102µm were obrved.Interconnective pore sizes ranged from40to275µm.
C 2005Springer Science+Business Media,Inc.
1.Introduction
lkjhgfdsaasasPrent approaches for fabricating porous polymer structures for biomedical applications include partic-ulate leaching methods[1–3],pha paration[4], emulsion freeze-drying[5],and combinations of the. The named techniques u organic solvents which may be left in the scaffold and might cau inflammatory reactions[6].Newer approaches like carbon dioxide expansion avoid the solvents[6].Nevertheless,all afore mentioned methods are limited in number and size of the samples due to laboratory handling.No ideal scaffold processing technique suited for industrial pro-duction of porous scaffolds is yet known.
Haugen[7,8]developed a polymer foaming method for injection molding which us water as non toxic blowing agent combined with a salt leaching technique. Although this method is simple and is able to produce scaffolds with high porosities and pore sizes in the de-sired range,the major drawback of the process is the time-consuming preparation of the polymer and the necessary leaching step.Also,the hydrolysis of the chon thermoplastic polyether-urethane at high tem-peratures in the prence of water[9]is a disadvantage. In this study,the MuCell R injection molding tech-nology which us benign gas as blowing agent was obrved concerning its ability to work as a large scale scaffold processing method for biomedical engineer-ing.The parameters of the process
and their influ-ence on pore morphology were examined.As there are a large number of dependent process parameters,it was necessary to focus on key parameters.Fixed pro-cess parameters were the cooling time,plasticize pres-sure and microcellular process pressure(MPP),tem-perature of the heating bands and plasticizing rotation whereas the changeable parameters were CO2weight gain,percentage of weight reduction,injection speed, and temperature of the mold.The choice of the change-able parameters was done bad on the knowledge given by nucleation theory and literature data[7,10]. Rodeheaver and Colton[11]found that an incread foaming temperature is an important factor to achieve a high content of open pores.Therefore,the highest possible temperature for the chon material was t on the heating bands.Samples were produced by vary-ing one parameter while the others were held constant. Thus,the influence of each processing parameter on the pore morphology could easily be obrved.After pro-cessing,the samples were subquently analyzed using mercury intrusion porometry,optical analysis and me-chanical tests.
2.Materials and methods
2.1.Polymer processing
Thermoplastic polyether-urethane(TPU)(Texin R 985,Bayer Plastics Division,Pittsburgh,PA,USA) wa
s chon to process the samples.An injection mold-ing machine(KM125C2,Krauss Maffei GmbH, Munich,Germany)with a mold temperature control-ling device(90S,Regloplas GmbH,Munich,Germany) was ud for the mass production of the porous sam-ples.The injection molding machine was equipped with a MuCell R package by the manufacturer.This package contains a special plasticizing unit(Sp220MuCell R plasticizing unit with a screw diameter of25.0mm and an adaptor for Sp520plasticize unit,Krauss Maf-fei GmbH,Munich,Germany)with one supercriti-calfluid(SCF)injection valve(25mm ries II in-jector,Trexel Inc.,Woburn,MA,USA).It also in-cludes a SCF metering system that consists of a SCF
0022-2461C 2005Springer Science+Business Media,Inc. DOI:10.1007/s10853-005-0853-y
T A B L E I Variable andfixed parameters while processing Variable parameters Examined range CO2weight gain 1.6–6.3wt% Percentage of weight reductionthx是什么意思
(compared to bulk part)
四级高频词汇
25–59% Injection speed30–150mm/s Fixed parameters Value
Cooling time65s
fineart
Injection pressure1500bar Plasticize pressure200bar
MPP200bar
随着社会的发展Dwell pressure200bar Beginning dwell pressure5mm
Duration of dwell pressure0.5s
Clamp tonnage200kN Temperature of the heating bands180–210◦C Temperature of the mold40◦C Plasticizing rotation60min−1
delivery system(TE-3ries II,Trexel Inc.,Woburn, MA,USA)and piping as well as instrumentation.The ud mold was capable to produce six toroid-shaped samples with an outer diameter of31mm and a thick-ness of10mm at one shot.Table I gives an overview over thefixed parameters and shows the analyzed range of the variable parameters that were ud during the experiments.
2.2.Porometry
Mercury intrusion porometry(AutoPore IV9500,Mi-cromeritics Instrument Corporation,Norcross,GA,
USA)was applied to determine pore size distribution and porosity of the samples.The porous polymer sam-ples were placed in a solid penetrometer with5ml bulb volume(Solid5cc,Pen Stem.38cc,Micromerit-ics Instrument Corporation,Norcross,GA,USA).The intrusion chamber was thenfilled with mercury and the T A B L E I I Processing conditions to produce the samples
CO2content
Percentage
of weight
reduction
Injection
speed Varying the injection speed 1.8%25%–
Varying the percentage of
weight reduction
2.5%–150mm/s Varying the content of CO2–57%150mm/s Overall porosity 6.3%59%150mm/s
samples were penetrated with mercury until a maxi-mum pressure of110MPa,where the total intrusion volume reached a plateau.A curve resulting from a measurement consists of120measure points,the lines in Fig.1are drawn to guide the eyes only.The peak of a curve gives the mean pore size diameter.Mercury in-trusion measurements were done at samples produced with an injection speed of30and150mm/s,a per-centage of weight reduction of27and59%,and a con-tent of carbon dioxide of1.6respectively6.3%.The processing conditions that were ud to produce the samples for mercury intrusion porometry are shown in Table II.
2.3.Image analysis
庐舍Scanning electron microscope(SEM)(S-3500N, Hitachi Science Systems,Tokyo,Japan)was ud for obrvation of the internal pore morphology.The porous samples were sliced with a scalpel and then coated with gold by using a sputter-coater(SCD005, BAL-TEC AG,Balzers,Lichtenstein).The morphol-ogy of the pores and the connections was evaluated by using image-processing analysis(Cellenger4.0, Definiens AG,Munich,Germany)bad on SEM mi-crographs.The rule ts were created especially for this application and are explained in the following.The area of an image object is the number of pixels forming
学校消防
it. Figure1Pore size distribution as a function of the injection speed(top),the percentage of weight reduction and the content of carbon dioxide (bottom).
T A B L E I I I Parameters for the quasi-static tensile-test Parameters Value
Ambient medium Air,room temperature Initial load5N
Initial load speed50mm/min
Test speed500mm/min
Load cell 2.5kN
Derived from the object area is the pore diameter which gives the diameter of a round pore with the same area. The average diameter of all pores that can be found in one image is the mean pore diameter.In the follow-ing this value is referred to as median pore diameter and describes the size of the interconnections of adja-cent voids.Due to the fact that the pores shown in the micrographs are two-dimensional projections of three-dimensional objects,their maximum diameter may not be reprented in the image.To consider this,a factor of0.616was ud to determine the maximum spherical diameter,called corrected median pore diameter,from the measured median pore diameter.
Image analysis was done at samples produced with an injection speed of30to150mm/s in increments of30mm/s,a percent-age of weight reduction of27and59%,and a content of carbon dioxide of1.6respectively6.3wt%.Table II gives the processing conditions that were ud to pro-duce the samples for mercury intrusion porometry and image analysis.
stuck in my heart2.4.Mechanical analysis
Ring shaped samples went through a quasi-static tensile-test with a material testing machine(TC-FR 2.5TS.D09,Zwick GmbH&Co.,Ulm,Germany) under conditions defined by DIN53504.Table III lists the chon experimental parameters.The tested samples were procesd with an injection speed of 150mm/s,a gas content of2.15wt%(±0.35%)and a percentage of weight reduction of25respectively 59%.
3.Results
ceibs3.1.Porometry
Fig.1shows the influence of the injection speed,the percentage of weight reduction,and the content of car-T A B L E I V Average porosity and Standard deviation.A molded part consists of six toroid-sha
ped samples,the sprue and the runner system Tested samples Average porosity One toroid(n=7)70.4±3.8% One moulded part(n=6)69.6±2.9% Series of moulded parts(n=6)69.5±2.7% Overall porosity(n=19)69.9±3.2%
bon dioxide on pore morphology,measured by mercury intrusion porometry.At a speed of30mm/s the mean pore size diameter(top ction of Fig.1)was at300µm with a porosity of77%.An increa of the injec-
tion speed to150mm/s shifted the mean pore size to45µm with a porosity of72%.The pore size distribution of the samples procesd with150mm/s was narrower than the distribution obtained at the injection speed of 30mm/s.The mean pore size diameter at59%weight reduction(middle ction of Fig.1)was at345µm with a porosity of68%.At a percentage of weight reduction of27%the mean pore size was at60µm with a poros-ity of66%.At59%weight reduction a very broad pore distribution with veral peaks of mercury intrusion was visible,whereas at the lower percentage of weight reduction the pore size distribution was narrower.The most common pore size for samples procesd with 1.6wt%carbon dioxide(bottom ction of Fig.1)was 165µm with a porosity of74%.A gas content increa to6.3wt%shifted the mean pore size diameter to12µm with a porosity of75%.The pore size distribution of both curves was of nearly similar shape.Table IV displays the ave
rage porosities and standard deviations of the tested samples.The porosity of one molded part was70.4±3.8%.Samples taken from six different toroids of one shot showed an average porosity of69.6±2.9%.The porosity of samples taken from a ries of molded parts was69.5±2.7%.This leaded to an overall porosity of69.9±3.2%.
3.2.Image analysis
Typical SEM micrographs of samples procesd with different injection speeds can be en in Fig.2.The pictures display that pore size decread with an in-crea in injection speed from30(left)to150mm/s. Further on an orientation of the pores depending on theflow path of the polymer melt was apparent.The morphology of the pores at different percentages
of Figure2Pore morphology at injection speeds of30(left)respectively150mm/s.
Figure 3Pore morphology at 27(left )and 59%of weight
reduction.
Figure 4Pore morphology at 1.6(left )and 6.3wt%of gas content.
T A B L E V Diameters of the connections between neighboring pores in dependence on the variable parameters Parameter Value Median pore diameter Injection speed 30mm/s 275±89µm 150mm/s 53±27µm Percentage of weight reduction 27%40±15µm 59%59±25µm Content of CO 2
1.6wt%85±45µm 6.3wt%
50±19µm
weight reduction is shown in Fig.3.The most obvious difference between the pictures was that at 27%weight reduction (left)the pores had a uniform structure.On the other hand at higher weight reduction (59%)the pores differ much in size.Samples procesd with dif-ferent gas contents are visible in Fig.4.The structure with the gas content of 6.3wt%,shown on the right side,contained more small pores and also emed to be less interconnective.
Fig.5and Table V prent the computational analysis of the SEM micrographs.Fig.5describes the influence of the injection speed,the percentage of weight reduc-tion,and the content of carbon dioxide on the corrected median pore diameter.An increa in injection speed decread pore sizes.At an injection speed of 150mm/s the most uniform structure with median pore sizes of 284±29µm could be found.An injection speed of 30mm/s generated pores with a median diameter
of 1102±406µm.At a percentage of weight reduc-tion of 27%the median pore diameter could be found at 184±37µm.A percentage of weight reduction of 59%generated pores with a median diameter of 370±287µm.At a gas content of 1.6wt%the median pore diameter was at 571±354µm.6.3wt%carbon dioxide in the polymer melt leaded to pores with a pore diameter of 307±214µm.
Table V shows the diameters of the connections be-tween neighboring pores in dependence on the varied parameters.At an injection speed of 30mm/s the pore diameter was 275±89µm.An increa in injection speed to 150mm/s leaded to a connection size of 53±27µm.A variation in the percentage of weight reduc-tion and the content of CO 2produced openings with a size of 40±15µm at 27%and 59±25µm at 59%of weight reduction respectively 85±45µm at 1.6wt%and 50±19µm at 6.3wt%CO 2.
3.3.Mechanical properties
Fig.6shows the mechanical properties of the molded samples.The breaking force for the specimen pro-cesd with 25%weight reduction (n =20)was 623±124N.The elongation at break could be found at 190±39%.Compared to that a percentage of weight reduction of 59%(n =20)leaded to a breaking force of 249±105N and an elongation at break of 76±41%.
Figure5Corrected median pore diameter in dependence on the injection speed,the percentage of weight reduction,and the gas
content.
Figure6Mechanical properties of the samples in dependence on the percentage of weight reduction.
4.Conclusion
It was obrved that the pore morphology depended on veral adjustable processing parameters.The pore size and the pore size distribution were adjustable by the injection speed,the percentage of weight reduc-tion and the content of carbon dioxide in the polymer melt.The porosity of the samples was not much in-fluenced by the parameters.The injection speed that generates the rate of the pressure drop which is a main factor to generate nucleation sites[12,13],had a ma-jor effect on pore size and pore size distribution.An increa in injection speed decread the pore sizes and caud a narrower pore size distribution.When comparing the results of the pore sizes obtained from mercury intrusion porometry and image analysis it can be en that the pores measured by porometry are much smaller than tho measured by image analysis.This was likely caud by the so-called inkstand effect.This means that a small pore with a large void behind ap-pears as many small pores in the pore size distribution curve.The result showed that an increa in injection speed from30to150mm/s decread the corrected median pore diameter from1102to284µm.
The anisotropic orientation of the pores comes about be-cau the mechanical stress generated during foam-ing are not distributed uniformly through the volume
