Rheological and Mechanical Gradient Properties of Polyurethane Elastomers for 3D-Printing with Reactive Additives [original]

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Appl. Rheol. 2019; 29 (1):162–172

Research Article

Peng Wang*, Dietmar Auhl, Eckart Uhlmann, Georg Gerlitzky, and Manfred H. Wagner

Rheological and Mechanical Gradient Properties

of Polyurethane Elastomers for 3D-Printing with

Reactive Additives

https://doi.org/10.1515/arh-2019-0014

Received Sep 24, 2018; accepted Jun 11, 2019

Abstract: Polyurethane (PU) elastomers with their broad

range of strength and elasticity are ideal materials for ad-

ditive manufacturing of shapes with gradients of mechan-

ical properties. By adjusting the mixing ratio of different

polyurethane reactants during 3D-printing it is possible to

change the mechanical properties. However, to guarantee

intra- and inter-layer adhesion, it is essential to know the

reaction kinetics of the polyurethane reaction, and to be

able to influence the reaction speed in a wide range. In

this study, the effect of adding three different catalysts and

two inhibitors to the reaction of polyurethane elastomers

were studied by comparing the time of crossover points

between storage and loss modulus G′and G′′ from time

sweep tests of small amplitude oscillatory shear at 30∘C.

The time of crossover points is reduced with the increasing

amount of catalysts, but only the reaction time with one in-

hibitor is significantly delayed. The reaction time of 90%

NCO group conversion calculated from the FTIR-spectrum

also demonstrates the kinetics of samples with different

catalysts. In addition, the relation between the conversion

as determined from FTIR spectroscopy and the mechani-

cal properties of the materials was established. Based on

these results, it is possible to select optimized catalysts

and inhibitors for polyurethane 3D-printing of materials

with gradients of mechanical properties.

Keywords: Polyurethane, oscillatory shear, crosslinking

kinetics, infrared spectroscopy, 3D-printing

PACS: 61.41.+e, 82.35.-x, 83.10.Tv, 83.80.Va, 83.80.Jx

*Corresponding Author: Peng Wang:

Department of Polymer

Materials and Technologies, Technische Universität (TU) Berlin,

Ernst-Reuter-Platz 1, D-10587 Berlin, Germany;

Email: peng.wang.1@tu-berlin.de

Dietmar Auhl, Manfred H. Wagner:

Department of Polymer Mate-

rials and Technologies, Technische Universität (TU) Berlin, Ernst-

Reuter-Platz 1, D-10587 Berlin, Germany

1Introduction

Polyurethane (PU) elastomers with their broad range of

strength and elasticity have found many applications in

the past few decades, such as resins for automotive, med-

ical, and aerospace industry. On the other hand, additive

manufacturing is one of the fastest developing technolo-

gies and is challenging traditional manufacturing. In re-

cent years, many different techniques of additive manufac-

turing have been developed for different polymers. For ex-

ample, ABS, PLA and PC are commonly used materials for

fused deposition modeling (FDM), while PA and PEEK are

typical polymers for selective laser sintering (SLS) [1]. How-

ever, there are few researches on additive manufacturing

of polyurethane based elastomers because only very few

commercial devices can print polyurethane.

Elsner [2] has set up an inkjet printing prototype for

additive manufacturing with piezoelectric nozzles, which

can precisely adjust the ratio of two or more pre-polymer

components by ejecting tiny amount of liquid. Müller et

al. [3] studied the printability of polyisocyanate and poly-

ols with this 3D-printing prototype. They demonstrated

that by adjusting the mixing ratio of the polyurethane re-

actants it is possible to change the mechanical properties

of each voxel, and established a technique of printing gra-

dient materials. Uhlmann et al. [4, 5] investigated the print-

ability of polyurethane further by optimizing the print-

ing parameters, and managed to print small polyurethane

cylinders by use of this prototype 3D-printing device (Fig-

ure 1).

Polyurethane materials are mainly obtained from a

polyaddition reaction between di-isocyanates and poly-

ols [6–8]. The basic reaction equation is shown as follows:

In this reaction, the hydroxyl groups of polyol lose a hydro-

Eckart Uhlmann, Georg Gerlitzky:

Department of Machine Tools

and Factory Management, Technische Universität (TU) Berlin, Pas-

calstraße 8-9, 10587 Berlin, Germany

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Properties of Polyurethane Elastomers for 3D-Printing with Reactive Additives |163

Figure 1: Prototype of 3D-printer and specimen of a 3D-printed test shape

gen atom and form an oxygen anion to connect with the

carbon of isocyanate groups. The main effect of catalysts

in this reaction is to help hydroxyl groups lose hydrogen

ions. Common catalysts are amine-based 1,4-Di-Aza-Bi-

Cyclo-(2,2,2)-Octane (DABCO), and tin-based Di-Butyl-Tin-

Di-Laurate (DBTDL) [9]. Many studies [10–16] have already

investigated the effects of these two types of catalysts

onto the polyurethane formation reaction. The catalytic

mechanism of DABCO is shown as follows for example:

The function of an inhibitor is opposite to the action of

catalysts, relying mostly on Lewis acids to hinder the pro-

ton transfer to the isocyanate groups. Commonly used

inhibitors include Benzoyl-chloride and para-Toluene

Sulfonic Acid (p-TSA) [17, 18], and these materials play

an essential role in the pre-polymer preparation of

polyurethane. However, it has also been noticed that the

reaction can be accelerated by adding an excess of strong

organic acids [19].

For the printing of polyurethanes, it is important to

know the reaction time when the soft material is able to

maintain a stable shape during the crosslinking reaction,

because it determines when the next layer can be printed

on the previous layer. Therefore, rheological characteriza-

tion is an essential technique. By measuring the crossover

point of storage modulus G′and loss modulus G′′ with con-

stant amplitude and oscillatory frequency, this time can be

determined and guaranteed precisely. Winter and Cham-

bon et al. [20–25] have made great efforts in characterizing

the gel point by rheological measurements for crosslinking

polymers, including polyurethane. Moreover, Hu et al. [26]

applied this method to bismaleimide/cyanate ester. They

demonstrated that this method is practicable to compare

the kinetics of a crosslinking reaction.

To characterize the overall polyaddition process

of polyurethane, many studies have applied Fourier-

transform infrared (FTIR) spectroscopy [27–30]. Particu-

larly with the help of in-situ FTIR, one can observe the de-

lay of the peak of the isocyanate group, and by comparison

of isocyanate peak area and reference peak area, the con-

version aof the NCO group can be calculated as shown in

the following equation [31, 32]:

a= 1 −A1(t)/A2(t)

A1(0)/A2(0) (1)

A1(0) and A1(t) are the peak areas of the isocyanate group

at the beginning of the reaction and at time t, respectively;

A2(0) and A2(t) are the reference peak areas at the begin-

ning and at time t, respectively.

In this work, we investigate the effects of various reac-

tive additives to the polyaddition reaction of polyurethane

elastomers. For 3D-printing of reactive polymers like such

polyurethanes, it is essential to be able to adjust the reac-

tion rate, because the possible printing speed, especially

in the vertical z-direction, relies on the reaction time un-

til high viscous polymer is formed. By conducting time-

dependent measurements with small-amplitude oscilla-

tory shear (SAOS), it is possible to evaluate a processing

time window from the dynamic mechanical response. Fur-

thermore, the reaction process can be monitored also from

conversion of chemical groups by FTIR spectroscopy. In ad-

dition, the mechanical properties of polyurethanes at dif-

ferent conversion stages are reported.

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164 |P. Wang et al.

Table 1: Physical properties of Lupranol L1100, L3300, and Lupranat M20S according to the materials data sheet

Hydroxyl value

[mg KOH/g]

-N=C=O value

[%]

Viscosity (25∘C)

[mPa·s]

Water Content

[%]

Density (25∘C)

[g/cm3]

Lupranol L1100 104 - 155 ≤0.05 1.00

Lupranol L3300 400 - 373 ≤0.10 1.05

Lupranat M20S - 31.5 210 - 1.24

Table 2: Weight fraction of basic reactants with different catalysts and inhibitors

Reaction batch M20S L1100 L3300 DBTDL Stannous octoate DABCO Benzoyl chloride p-TSA

[wt.%]

PU-0 43.00 18.81 38.19 - - - - -

PU-L-0.01 42.99 18.81 38.19 0.01 - - - -

PU-L-0.1 42.96 18.79 38.15 0.1 - - - -

PU-L-0.2 42.92 18.77 38.11 0.2 - - - -

PU-O-0.01 42.99 18.81 38.19 - 0.01 - - -

PU-O-0.1 42.96 18.79 38.15 - 0.1 - - -

PU-O-0.2 42.92 18.77 38.11 - 0.2 - - -

PU-A-0.01 42.99 18.81 38.19 - - 0.01 - -

PU-A-0.1 42.96 18.79 38.15 - - 0.1 - -

PU-A-0.2 42.92 18.77 38.11 - - 0.2 - -

PU-C-0.1 42.96 18.79 38.15 - - - 0.1 -

PU-C-2.5 41.95 18.35 37.26 - - - 2.44 -

PU-S-0.05 42.98 18.80 38.17 - - - - 0.05

PU-S-0.1 42.96 18.79 38.15 - - - - 0.1

PU-S-1 42.57 18.62 37.81 - - - - 1

2Experimental

2.1 Materials

Two polyols (Lupranol L1100 and Lupranol L3300, BASF

Polyurethanes GmbH) and one polyisocyanate (Lupranat

M20S, BASF Polyurethanes GmbH) were used as the

main reactants and diluted by anhydrous toluene (Sigma-

Aldrich). Lupranol L1100 is a difunctional polyol, which is

used for production of foams and elastomers, and Lupra-

nol L3300 is a trifunctional polyol based on glycerine,

which is used for the production of rigid foams, but also

for non-foamed polyurethane. Lupranat M20S is a solvent-

free polyisocyanate, containing oligomers and isomers,

which is mainly used for high-density rigid foams as well

as for packing and casting materials. Lupranat M20W was

used in a previous study [3], but is not commercially avail-

able any more, and therefore, Lupranat M20S is used as

replacement with comparable properties. Table 1 presents

the typical properties of the three main reactants.

Catalysts used as accelerating agents are Di-Butyl-

Tin-Di-Laurate (DBTDL, Sigma-Aldrich), Tin(II)2-Ethyl-

Hexanoate (also called Stannous Octoate, Sigma-Aldrich),

and 1,4-Di-Aza-Bi-Cyclo-(2,2,2)-Octane (DABCO, Carl Roth

GmbH), while inhibitors used are Benzoyl-Chloride

(abcr GmbH) and para-Toluene-Sulfonic Acid (p-TSA, MP

Biomedicals). The total amounts of each additive added to

the reaction are shown as weight fractions in Table 2.

The molar ratio (MR) between the NCO group and the

OH group as well as the polyol ratio (PR) has a significant

effect on the reaction rate, and MR and PR are defined as

MR =αNCO

αOH

×100 (2)

where αNCO and αOH are number of moles of NCO and OH,

respectively;

PR =mL1100

mL1100 +mL3300

(3)

where mL1100 and mL3300 are mass of L1100 and L3300, re-

spectively.

In this work, the molar ratio was chosen to be 105,

which is higher than 100, in order to release the rich of an

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Properties of Polyurethane Elastomers for 3D-Printing with Reactive Additives |165

excess of isocyanate to react with water in air, forming al-

lophonates [3]. From the results of tests conducted by time

sweep tests in the rheometer for different polyol ratios, PR

with 0.33 was chosen as the main polyol ratio in order to

compare the effects of different reactive additives. All the

reactants and tools were thoroughly dried in advance in a

vacuum oven at a temperature of 50∘C for 24 hours before

preparation of samples. All compositions of reacting com-

pounds investigated are shown in Table 2.

To prepare polyurethane reaction compounds, which

can be printed with piezoelectric nozzles, Lupranol L1100,

Lupranol L3300 and Lupranat M20S were firstly diluted by

Toluene of 30 wt.%, 32 wt.% and 25 wt.%, respectively [3].

Then Lupranol L1100 and Lupranol L3300 were dropped in

a neat beaker according to the mass fraction intended, and

finally mixed. For rheological measurements, Lupranat

M20S was poured into a disposable cup used under curing

conditions. The polyol mixture was dropped into the poly-

isocyanate within 1 min. The reactants were stirred for 1

min and then transferred to the instrument. This is defined

as the pure PU sample without catalysts or inhibitors.

For the sample preparation with additives, the cata-

lysts were added after mixing of the three reactants, and

then stirred for another 1 min before transfer to the instru-

ment. When inhibitors were added, a certain amount of in-

hibitor needed to be dissolved in the polyol mixture in ad-

vance to form an acid liquid, and then it was added with

the polyols to the reaction.

2.2 Rheological testing

The storage and loss modulus G′and G′′ as well as the

crossover time were measured by using an Anton Paar

MCR-301 rheometer with a CTD-450 oven chamber.

The time sweep tests were performed by plate and

plate geometry with a diameter of 25 mm at a temperature

of 30∘C. To investigate the appropriate strain amplitude

in linear viscoelastic region, amplitude sweep tests were

first conducted for a strain range between 1% and 100% at

an angular frequency of 0.5 rad/s. Time sweep tests were

performed in controlled strain mode with 1% strain and

at a frequency of 1 Hz. Rheological measurements were

stopped much later than the crossover times, and repeated

at least twice for every type of sample.

2.3 Fourier-Transform Infrared (FT-IT)

Spectroscopy

The FTIR measurements were conducted on Thermo Nico-

let 380 spectrometer with ATR mode to follow the conver-

sion reaction. To maintain a reaction temperature of 30∘C,

a small oven was placed on the top of the ATR unit. After

sufficient preheating for thermal equilibrium, the reaction

liquid was dropped onto the diamond window measure-

ments repeated with a certain time interval. This method

mimics an in-situ measurement.

For the FTIR measurements, the samples were pre-

pared with similar procedures as for the rheological test-

ing. After the samples were stirred homogeneously, they

were transferred with a pipette to the ATR crystal, and mea-

sured every 2 min for the first 10 min, then every 10 min for

the first hour, every 20 min for the second hour, every 30

min for the third hour, every 60 min for the following time

period until the absorbance of the NCO peak is lower than

a relative intensity of 0.05.

2.4 Mechanical testing

The samples for the mechanical tests were prepared in a

polytetrafluoroethylene (PTFE) mold, which was designed

based on the international standard ISO 527-2 (Figure 2). Af-

ter homogenously mixed, the reactants without additives

were slowly casted into the mold to avoid the formation of

bubbles. Then the mold with liquid reactants was shifted

into a freezer with a temperature of about 5∘C and kept

there for 24 hours. This step slows down the crosslinking

reaction and decreases the formation of bubbles. When

taken out from the freezer, the samples have already be-

come high viscous liquids and were subsequently stored

in an oven at a temperature of 30∘C.

When the samples had obtained a stable shape, they

were demolded and kept at 30∘C for further reaction. Be-

fore the tensile testing, three different points on the spec-

imen were tested by FTIR to confirm its homogeneity in

conversion (Figure 2), and the Shore A hardness was mea-

sured by a durometer. Afterwards, the samples were longi-

tudinally stretched at a rate of 1 mm/min for the determina-

tion of the tensile modulus, and then by 100 mm/min for

the remaining stress-strain diagram. The relationship be-

tween reaction conversion and corresponding mechanical

properties was studied.

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166 |P. Wang et al.

Figure 2:

Tensile test specimen and corresponding FTIR spectrum at

three different points

3Results and discussion

3.1 Crosslinking behaviour from time sweep

tests

Figure 3 shows for PU with different PRs the complex vis-

cosities and dynamic moduli with changes in the G′and

G" crossover points. The curves show some characteristic

regions and onset times, at which they grow with different

rates. In Figure 3, the viscosities decreases with increasing

amount of L1100 in the reaction, because L1100 has a low

viscosity and can react with isocyanate, forming longer lin-

ear chains and networks with a low density of crosslinks.

L1100 dilutes the reaction system and hinders the reaction

between L3300 and isocyanate to form a 3-dimensional

network structure. Therefore the sample with PR=0.66

needed the longest time to reach the crossover point. The

compound with PR=0 has the highest growth rate of the

three samples both for the viscosity and the moduli, be-

cause there are no diols in the reaction, which means that

the resulting structure has more crosslinks than the other

compounds. However, the resulting high viscosity slows

down the reaction. Therefore, PR=0 needed more time

to reach the crossover point than PR=0.33. By adding 33

wt.% of L1100, isocyanate forms more linear chains than

PR=0 resulting in a lower viscosity, which in turn results

in faster crosslinking with L3300. PR=0.33 is therefore the

compound which reached the crossover point after the

shortest reaction time. Since the compound with PR=0.33

has an earlier crossover point than other two compounds,

it was chosen as the test compound in order to compare

the effects of different catalysts and inhibitors.

Figure 3:

Time sweep results for storage and loss modulus G

′

and

′′

(left) as well as complex viscosity (right) for polyurethanes with

PR of 0, 0.33, 0.66 without solvent

In the primary test for samples with different PRs, the

pure reactants were used in bulk polymerization. As a re-

sult, the reaction heat was difficult to control, and the reac-

tion heat accelerated the reaction between isocyanate and

water in the air, forming a few bubbles. To decrease the

viscosity for printing, the three reactants were diluted by

toluene. In addition, toluene can absorb part of the heat

produced by the reaction and isolates the water in the air

from the reaction. Therefore, fewer bubbles were produced

by the diluted reaction, and a higher G′/G" crossover point

was obtained, but the time to reach the crossover point was

extended to 612 min (black curve in Figure 4a-c) because

of the decreased concentration of the reactants. Measured

data of G′and G′′ at the start of the reaction were not re-

liable due to the extremely low viscosity, and they are not

shown here.

From the diagrams, it can be seen that all the G′curves

have a similar shape and reach the level of 106Pa, which

means the crosslinking density of all samples are almost

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Properties of Polyurethane Elastomers for 3D-Printing with Reactive Additives |167

Figure 4:

Time sweep tests for polyurethane compounds with no

catalyst and with the different accelerating catalysts

identical, irrespective of the amount and type of catalyst

used. However, the curves of G′′ obviously differ after the

crossover point. The samples with concentrations of 0.1

wt.% and 0.2 wt.% of catalyst show a pseudo-plateau (or

Figure 5:

Time and value of G

′

and G

′′

at crossover point as a func-

tion of mass fraction of each catalyst

slowly increasing) region with lower G′′ values compared

to the sample without catalyst. For example, G′′ of the neat

sample after the crossover point is approximately 105Pa,

but the loss modulus of the catalyzed samples reaches only

104Pa. This may indicate that the three catalysts are ef-

ficient in producing a more perfect crosslinked network

with fewer dangling ends.

Comparing the time to reach the crossover point in

Figure 5, it is obvious that the time of samples with dif-

ferent amounts of catalyst L and A obey a power law,

and L is more effective than A. This is because catalyst

A is an amine-based catalyst, which not only improves

the reaction between isocyanate and hydroxyl, but also

the isocyanate-water reaction [19]. Stannous octoate (cat-

alyst O) is a Tin-based complex compound. The results re-

veal that catalyst O has the lowest efficiency when a small

amount (0.01 wt.%) is added, but is much more effective at

higher concentrations. For the sample with 0.2 wt.% of O,

the lowest crossover time is obtained.

Considering the value of G′and G′′ at the crossover

point, the trend is first a decrease and then a plateau for

catalyst L and O, while catalyst A still follows a power law.

The decrease of the modulus values at the crossover point

with increasing catalyst concentration is due to the fact

that the catalysts create an efficient three-dimensional net-

work.

To delay the curing time of this polyurethane, as typ-

ical inhibitors, Benzoyl chloride (inhibitor C) and para-

Toluene-Sulfonic Acid (inhibitor S), were applied and their

effects on the dynamic modulus are shown in Figure 6. Ben-

zoyl chloride reacts with polyol of the system to form HCl.

From the results, it is obvious that the time to reach the

crossover point for the sample with 0.1 wt.% Benzoyl chlo-

ride is almost the same as for the sample without inhibitor,

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168 |P. Wang et al.

Figure 6:

Time sweep tests for polyurethane reactions with different

inhibitors Benzoyl-Chloride (a), and para-Toluene-Sulfonic Acid (b)

compared to the sample without inhibitors

which means that such a small amount has only little effect

on the reaction. If the amount of inhibitor is increased to

2.5 wt.%, it can delay the reaction more significantly. How-

ever, only poor reproducible results can be obtained for

this amount because the reaction between Benzoyl chlo-

ride and polyol seems difficult to control.

Para-Toluene-Sulfonic Acid can be added directly to

the reaction to vary the pH value. From Figure 6, it is seen

that 0.05 wt.% has little effect as the time to the crossover

point is nearly the same as without inhibitor. The most ef-

fective inhibitor amount is 0.1 wt.%, which results in a time

of 634 min. However, when the amount is increased to 0.2

wt.%, the inhibitor may work as a catalyst, which reduces

the time strongly to 312 min [19].

Figure 7:

FTIR spectrum measured for the pure PU sample with

PR=0.33 at different reaction times (a) and transformed into a 3-

dimensional spectrum (b)

3.2 Degree of conversion from FTIR

spectroscopy

Figure 7a shows the FTIR spectrum of the pure

polyurethane PR=0.33 sample. The NCO group absorbance

occurs between 2200-2350 cm−1with a peak at 2280 cm−1

and as the reaction proceeds, the absorbance gradually

drops. Figure 7b is the 3-dimensional spectrum trans-

formed from Figure 7a. From this plot, it is obvious that the

reaction rate of NCO is fast during the first 200 min, and

then is reduced progressively because of the spatially hin-

dering network structure and the high viscosity limiting

the further reaction between functional groups. With the

help of the reference peak at 2960 cm−1for C-H stretching

vibration in the CH2group of a polyether, the decrease of

the peak area can be used to calculate the conversion of

NCO group by use of equation 1 (Figure 8).

From Figure 8, it can be seen that the conversion of

NCO group for polyurethanes with different reactive addi-

tives increases also sharply at the beginning of the reaction

and then enters a plateau, which is similar to the pure PU

as shown in Figure 7b. Compared to the starting point of

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Properties of Polyurethane Elastomers for 3D-Printing with Reactive Additives |169

Figure 8:

Conversion of NCO group for polyurethane with different reactive additives calculated from the FTIR spectrum (a-c). Reaction times

to reach 90% conversion as a function of catalyst concentration (d)

the pure PU sample without catalyst, the starting points

of PU with catalysts all have already a higher conversion,

and with increasing amount of catalyst, the conversion is

higher at comparable reaction times. This is because there

is a certain loading time between adding the catalyst to the

reaction compounds and the start of the measurements.

This indicates that the catalysts have an obvious catalytic

function during this loading time.

The catalysts L and O are both tin-based, but the effi-

ciency is obviously different when compared at the same

mass fractions. When 0.01 wt.% of L is added, the reaction

time decreases to 815 min, which means that the reaction

rate almost doubles. However, when the same amount of

catalyst O is added, the reaction time is 1568 min, which

is almost similar or even slightly longer than the time for

the pure PU sample. This reveals that catalyst L is more ef-

fective than O at low mass fractions. Combined with the

results from Figure 4b, it can be said that 0.01 wt.% of O is

effective at the beginning of the reaction, but has no signif-

icant effects later during the reaction.

When the mass fraction is increased from 0.1 wt.% to

0.2 wt.%, the reaction time with addition of L is only re-

duced from 466 min to 341 min, which indicates that it is

nearly saturated for this reaction. In contrast, when the

mass fraction of catalyst O is increased to 0.1 wt.%, the

reaction time decreases more than 10 times to 124 min,

and when it is increased to 0.2 wt.%, the reaction time de-

creases almost another 10 times to 19 min. That means, for

mass fractions above 0.1 wt.%, catalyst O is more effective

than L. This may be explained in terms of the molecular

structure, because O is sterically smaller than L, and there-

fore may diffuse much easier through the PU network to

catalyze functional groups.

The addition of catalyst A with 0.01 wt.% results only

in a small acceleration of the curing reaction, whereas with

0.1 wt.% and 0.2 wt.%, the reaction times are reduced to

136 min and 100 min, and possibly with 0.2 wt.% a satura-

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170 |P. Wang et al.

Figure 9:

Tensile properties of PU with different degrees of NCO-conversion in stress-strain diagram (a), tensile modulus diagram (b), and

characteristic peak of toluene in the FTIR spectrum (c), and Shore A hardness as a function of NCO-conversion (d)

tion concentration is already reached (Figure 8c). The sum-

mary of all reaction times of three accelerating catalysts is

shown in Figure 8d.

3.3 Tensile properties and shore hardness

The changes of tensile properties of polyurethane elas-

tomers are shown in Figure 9. It is found that the pure

PU sample at crossover time of 612 min corresponds to a

NCO group conversion of approximately 0.7. Thus, the first

mechanical tests started at the conversion of 0.7. It can be

seen that the elastomer with the lowest conversion 0.7 is ex-

tremely soft and weak. However, the stress-strain slope in-

creases with higher degrees of conversion, while the strain

at break has a trend to firstly increase and then to decrease.

Two samples, which were post-cured for one week and two

weeks, respectively, have significantly increased stresses

at break (22 MPa and 36 MPa), but decreased strains at

break (8% and 6%) compared to not fully reacted samples

shown in Figure 9a.

Figure 9b shows the increase of tensile modulus for

materials with increasing conversion. The increase is only

small until a conversion of 0.85 is reached, but then in-

creases significantly. The two post-cured samples have a

much higher tensile modulus of 300 MPa and 396 MPa, re-

spectively. This may also be largely due to the evaporation

of toluene, because the characteristic peak of toluene in

the FTIR spectrum (Figure 9c) decreases and finally disap-

pears with continued post curing.

The Shore hardness of polyurethanes was measured at

different degrees of conversion before tensile testing. How-

ever, hardness can be reliably measured only after passing

the crossover point when the elastic response is dominat-

ing, i.e. when the conversion is larger than about 0.7. It is

obvious that polyurethane turns harder subsequently be-

cause more crosslinks are created when the reaction pro-

ceeds. The growth trend can be fitted to a first approxima-

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Properties of Polyurethane Elastomers for 3D-Printing with Reactive Additives |171

tion with a linear function (Figure 9d). If the function is

extrapolated to full conversion of NCO, the result agrees

relatively well with the Shore A hardness value of about

89 from previous work [3].

4Conclusions

In this work, the effect of adding different reactive addi-

tives to the polyaddition reaction of polyurethane elas-

tomers were studied by monitoring the crossover time of

G′and G′′ in time sweep tests at 30°C. The crossover time is

typically reduced with an increasing amount of accelerat-

ing catalyst, whereas the reaction time of some inhibitors

is found to be delayed. The reaction progress was studied

by FTIR spectroscopy for the NCO group conversion. In ad-

dition, the relation between the 90% NCO conversion as

determined from the FTIR spectrum and the mechanical

properties of the materials was established.

Concerning acceleration of the reaction rate, all three

accelerating catalysts have good catalytic effects, which

rise with increasing amount of catalysts. The two tin-based

catalysts L and O are more efficient than the amine-based

catalyst A. For mass fractions higher than 0.2 wt.% the cat-

alyst O is the most effective additive.

The two inhibitors C and S have only a limited effect on

delaying the reaction. Only S extends the crossover time

to 634 min at a mass fraction of 0.1%. It seems that the

acidic environment is able to inhibit the curing reaction

of polyurethane to some extent.

Based on these results, it is possible to select opti-

mized recipes with catalysts or inhibitors for 3D-printing

with polyurethanes within large limits of reaction rate.

With a certain ratio of reactants and additives, the printed

layers can be adjusted to the printing time with improved

or guaranteed intra-layer adhesion as well as to the time

interval for inter-layer adhesion. After printing several lay-

ers, the load carrying capacity of the support layers also

needs to be considered. Moreover, when the printing pro-

cedure is finished, the conversion diagram can be used to

determine the time for removing the printed object.

With increasing conversion, characteristic mechani-

cal properties such as hardness and tensile modulus of

the polyurethane materials increased. These data are es-

sential to set up process parameters for 3D-printing of

polyurethane shapes, especially also with gradients in me-

chanical properties.

Thus, it is possible to optimize the 3D-printing process

with respect to processing conditions as well as end-use

properties. Yet, further inhibitors and recipes should be in-

vestigated in further detail in the future.

Acknowledgement: The authors would like to thank Ger-

man Research Foundation (DFG) for financial support of

this project by grants DFG Uh100/117-2, DFG Wa 668/332

and the Open Access Publication Fund of TU Berlin.

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