Accepted Manuscript

Synthesis and characterization of waterborne polyurethane dispersions with dif-ferent chain extenders for potential application in waterborne ink

Liang Lei, Li Zhong, Xiaoqiong Lin, Yuanyuan Li, Zhengbin Xia

PII: S1385-8947(14)00619-6DOI: http://dx.doi.org/10.1016/j.cej.2014.05.044Reference: CEJ 12138

To appear in: Chemical Engineering Journal

Received Date: 3 January 2014Revised Date: 1 May 2014Accepted Date: 12 May 2014

Please cite this article as: L. Lei, L. Zhong, X. Lin, Y. Li, Z. Xia, Synthesis and characterization of waterbornepolyurethane dispersions with different chain extenders for potential application in waterborne ink, ChemicalEngineering Journal (2014), doi: http://dx.doi.org/10.1016/j.cej.2014.05.044

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1

Synthesis and characterization of waterborne polyurethane

dispersions with different chain extenders for potential

application in waterborne ink

Liang Lei, Li Zhong﹡, Xiaoqiong Lin, Yuanyuan Li, Zhengbin Xia

School of Chemistry and Chemical Engineering, South China University of

Technology, Guangzhou, 510640, China

﹡Corresponding author. Tel: +86-20-87113773, Fax: +86-20-87112093,

E-mail: celzhong@scut.edu.cn

2

Abstract

Waterborne polyurethane (WPU) dispersions with different amine chain

extenders, including ethylene diamine (EDA), diethylene triamine (DETA), and

triethylene tetramine (TETA), were synthesized, and the effects of these chain

extenders and the NCO/OH molar ratio on the properties of WPU dispersions were

investigated. The results revealed that the chain extension mainly occurred on the

particle surface and the actual maximum chain extension degree was approximately

60%. All the synthesized WPUs had a lower degree of crystallinity, and the post-

chain extension promoted phase separation between soft and hard segments in

polyurethane. The trifunctional DETA chain-extended WPU had better thermal

stability and adhesion strength, which indicates that DETA is an effective chain

extender that can be used for the performance improvement of WPU. Moreover, the

chain-extended WPU by EDA and DETA with a mole ratio of 2:3 showed excellent

properties. It was also revealed that the NCO/OH molar ratio of 1.6 was appropriate

for the preparation of stable WPU dispersion with solid content of about 40 wt% and

organic solvent content of 5 wt%. The obtained WPU dispersion could be directly

used as a waterborne ink binder without removing the organic solvent, which is

conducive to the industrialized production of WPU dispersion and avoids the cost of

organic solvent recovery.

Key words: waterborne polyurethane, chain extension, diethylene triamine, binder,

waterborne ink

3

1 Introduction

As the versatile environmentally friendly materials, waterborne polyurethanes

(WPUs) have gained increasing interest in a broad range of applications owing to

their excellent elasticity, abrasion resistance, flexibility, and broad substrate suitability

[1-4]. For instance, WPUs have been widely used in adhesives for numerous flexible

substrates, such as plastic film, leather, textile, paper, and rubber [5, 6]. In the field of

flexible packaging printing ink, WPU offers an efficient alternative to the solvent-

based polyurethane binder, which is still widely used in the market.

However, most WPUs are linear thermoplastic polymers and have a relatively

low average molecular weight. Therefore, some properties of WPUs, such as water

resistance, solvent resistance and mechanical property, are inferior to that of solvent-

based polyurethanes [7, 8]. Crosslinking modification is one of the most important

methods used in improving these properties of WPUs. Crosslinking modification

involves one-component and two-component crosslinking. For the two-component

crosslinking system, a crosslinker as the second component is added to WPUs just

before the application to react with the active groups in WPU molecular chains at

room temperature. The common crosslinkers for WPUs mainly include carbodiimide

[9, 10], polyaziridine [8, 11], and isocyanate [12-14]. Two-component crosslinking

can greatly improve the properties of WPUs. However, the package, transportation,

and construction of the two-component crosslinking system are not convenient and its

pot-life is limited [15]. By comparision, the one-component crosslinking system is

highly advantageous during the production and application of WPUs. So, it is

extensively studied and applied in practical products.

One of the most important and pragmatic one-component crosslinking

modifications for WPUs is the internal crosslinking. The commonly used inner

4

crosslinking agents are those with functionalities greater than two, mainly including

hexamethylene 1,6-diisocyanate trimer (HDI trimer), trimethylolpropane (TMP), and

diethylenetriamine (DETA). HDI trimer and TMP with polyfunctional –N=C=O

(NCO) groups and –OH groups, respectively, are added into the reaction mixtures

during the synthesis of polyurethane prepolymer to form a cross-linked network or

branched-chain structure. In those cases, the obtained prepolymer often has a high

viscosity and is difficult to be dispersed in water. To reduce the viscosity of the

prepolymer, more organic solvents should be added into the reaction mixtures during

the synthesis process, which increases the cost of solvent recovery. In order to resolve

this problem, the WPU with low solvent content or solvent-free WPU has gained

increasing attention [16, 17].

Compared with the internal crosslinking during the synthesis of polyurethane

prepolymer, the post-chain extension and crosslinking with the multifunctional amine

chain extenders after the emulsification of prepolymer in water can decrease the

consumption of organic solvents because of the relatively low viscosity of the

prepolymer. Some studies have been conducted on the effects of post-chain extension

on the properties of aqueous polyurethane [18-26]. Jhon et al. [18] demonstrated that

the amount of residual NCO groups on the particle surface and the required amount of

chain extender for optimum chain extension decreased as the total surface area of

aqueous polyurethane particle decreased. Moreover, the adhesive strength increased

to the point of maximum value of chain extension and then decreased after that point,

which indicated that excessive amounts of chain extenders had an unfavorable

influence on the adhesive strength. Kawk et al. [23] studied the effect of the chain

extender hydrazine monohydrate (HD), ethylene diamine (EDA), and 1,4-butane

diamine (BDA) on the properties of waterborne polyurethane-urea anionomers. The

5

results showed that particle sizes, storage modulus, thermal degradation onset

temperatures, and tensile strength of samples increased in the order of BDA > EDA >

HD. The mechanical strength, water resistance, and acetone resistance of the

waterborne polyurethane film prepared with trifunctional DETA were higher than that

prepared with EDA [26].

However, most studies were mainly related to diamine chain extenders, and little

effort was directed toward the effects of post-chain extenders with functionalities

greater than two on the properties of WPUs. In this study, the WPUs with different

types and amounts of multifunctional post-chain extenders, including EDA, DETA

and triethylene tetramine (TETA), were synthesized (Scheme 1). The properties of

WPUs, such as particle size and distribution, molecular weight, crystallinity, thermal

and mechanical property, steaming and boiling resistance, and adhesion were studied.

The WPU dispersion with organic solvent content of 5 wt% was also studied.

2 Experimental

2.1 Materials

Polycaprolactone diol (PCL, Mw=2000) (supplied by YIP’S CHEMICAL, HK)

was used as macroglycol, and it was dried at 120 °C under vacuum for 2 h to remove

residual water. Isophorone diisocyanate (IPDI) and 2,2-dimethylolpropionic acid

(DMPA) were used without further purification and also provided by YIP’S

CHEMICAL. 1-methyl-2-pyrrolidinone (NMP, Fuchen, China), triethylamine (TEA,

Lingfeng, China), 1,4-butanediol (BDO, Kermel, China), ethylene diamine (EDA,

Lingfeng, China), diethylene triamine (DETA, Fuchen, China), and triethylene

tetramine (TETA, Fuchen, China) were all purchased in AR grade. Deionized water

was used as the dispersing phase.

2.2 Synthesis of the waterborne polyurethane dispersions

6

Waterborne polyurethane dispersions were prepared by the prepolymer mixing

process as shown in Scheme 1. Firstly, PCL and DMPA dissolved in NMP were

added into a four-neck flask equipped with a mechanical stirrer, thermometer,

condenser and nitrogen gas inlet, and heated at 70 °C with stirring for 30 min to

obtain a homogeneous mixture. IPDI was then added into the homogenized mixture

and stirred under nitrogen atmosphere until the amount of residual NCO groups

reached the desired value (determined by dibutylamine back titration). Then, BDO

dissolved in acetone was dropwise added into the reactor for 30 min and the reaction

proceeded at the constant temperature until the theoretical NCO groups content was

reached. Afterwards, the obtained NCO-terminated prepolymer was cooled down to

60 °C and neutralized by TEA (DMPA equiv). Subsequently, the prepolymer solution

was emulsified with a certain amount of deionized water to obtain a waterborne

polyurethane dispersion. The post-chain extension was carried out with EDA, DETA,

and TETA at 35 °C for 30 min, respectively. Finally, The WPU dispersions with solid

content of about 40 wt% were obtained after the acetone was removed.

Scheme 1

2.3 Preparation of WPU films

The films were obtained by pouring the WPU dispersions onto a Teflon disk to

dry at room temperature for 7 d and then at 50 °C in a vacuum dry oven for 24 h to

remove the solvent completely, and then stored in a desiccator to avoid moisture.

2.4 Characterization

The average particle size and distributions of WPU dispersions were measured

with a Malvern Nano-ZS laser particle sizer (UK).

The molecular weight of WPU dispersions was measured using a Polymer

Laboratories PL-GPC50 gel permeation chromatograph (GPC, UK). All the samples

7

were dissolved in tetrahydrofuran (THF) at a constant concentration 0.1 wt%. The

flow rate of THF solvent was 1.0 ml/min and the sample injection volume was 10 µl.

The chemical structure of WPUs was analyzed with a Perkin Elmer 2200 Fourier

transform infrared spectrophotometer (FTIR, USA).

The crystallinity of WPU film was analyzed by a Bruker D8 advance X-ray

diffractometer (XRD, Germany). A scanning of 2θ angle was from 4 to 70°. The

scanning speed was 0.1 s/step and the every scanning step was 0.02°.

The dynamic mechanical properties of WPU films were measured at 1 Hz using

a Netzsch 242E dynamic mechanical analyzer (DMA, Germany) at a heating rate of

3 °C/min in the temperature range of -100 to 100 °C.

The thermal property of WPU films was measured using a TA Instruments Q20

differential scanning calorimeter analyzer (DSC, USA). 3-8 mg of the WPU films

hermetically sealed in an aluminium pan were heated up to 150 °C with a heating rate

of 10 °C/min and kept for 3 min to keep a consistent thermal history for the melting

process. And then, the samples were cooled to -80 °C at a cool rate of 10 °C/min. The

non-isothermal measurement was scanned from -80 to 200 °C with a heating rate of

10 °C/min.

The thermal stability of WPU films was measured using a Netzsch STA-449C-

Jupiter thermogravimetric analyzer (TGA, Germany) at a constant heating rate of

10 °C/min over a temperature range of 30 to 600 °C and under nitrogen atmosphere

with a gas flow rate of 20 cm3/min.

T-peel strength was determined using a Labthink XLW tensile tester (China) at

23 ± 2 °C and at 50 ± 5% relative humidity. The tests were performed using a 90°

angle and a crosshead speed of 300 mm/min. The polyethylene terephthalate (PET)

film, coated with the WPU binder first and then with the polyurethane adhesive, was

8

laminated with the casting polypropylene (CPP) film to obtain a laminated film (Fig.

1), which was used to measure the T-peel strength. The peel strength value was the

average pull value obtained during the peeling of the tape and the initial peel values

were disregarded.

Fig. 1

The solids content of WPU dispersions were obtained by difference in weight

before and after water evaporation. About 1 g WPU dispersion was placed in an

aluminium container and the water was evaporated at 120 °C in an oven until constant

weight was reached. The solids content was calculated as the average of five

experimental determinations.

Water resistance and blocking resistance were measured according to Chinese

Standard GB/T1733-1933 and GB/T13217.8-2009, respectively. The storage stability

was measured by heating the WPU dispersions sealed in a plastic can at 60 °C for 2

weeks to observe whether sediments and gels appeared. Steaming and boiling

resistance was measured by steaming and boiling the test bag obtained by heat-sealing

the above prepared laminated film (Fig.1) at 121 °C for 30 min to observe if there was

a wrinkle or damage.

3 Results and discussion

3.1 Chain extension degree

3.1.1 Particle size and distributions

After emulsification of polyurethane prepolymer, the post-chain extension

reaction mainly occurs on the latex particle surface [18, 21]. The chain extension

degree (CED) is defined as the percentage of the mole ratio of amino groups of chain

extenders to the theoretically residual NCO groups of WPU dispersions, and the mole

of theoretically residual NCO groups is the mole of NCO groups of IPDI minus the

9

mole of -OH groups of PCL, DMPA, and BDO. The actual maximum CED is related

to the average particle size of WPU dispersion. In general, the smaller the average

particle size of WPU dispersion is, the more the particle number is. As a result, the

total surface area of particles and the amount of NCO groups on the particles surface

increase, and the actual maximum CED correspondingly increases. In this work, the

synthetic formula and process of polyurethane prepolymer are uniform, so the average

particle size and residual NCO groups content of WPU dispersions before post-chain

extension are almost the same. Fig. 2 shows the effect of different post-chain

extenders on the particle sizes and distributions of WPU dispersions. It is found that

the average particle size of WPU dispersion without post-chain extension is about 82

nm, and increases after post-chain extension. The increase of particle sizes reveals

that the chain extension mainly takes place on the particle surface, which is consistent

with previous studies [18, 21]. A chain extender molecule has two or multi-amino

groups and can react with NCO groups located both on the surface of single particle

or different particles. So, the single particle size may increase and the particles may

also aggregate together because of the bridging effect of the multifunctional chain

extenders. For WPU-DETA, its particle size distribution is a multiple distribution and

some large particles are even detected. It means that trifunctional chain extender

DETA promotes the aggregation of particles. But, there are no large particles for

WPU-TETA, which may be because of the low reaction extent of TETA.

Fig. 2

3.1.2 Molecular weights

It is known that chain extension can increase the molecular weight of WPU.

Hence, the variations in molecular weight of WPU can be used to show the actual

extent of chain extension. The number average molecular weight (Mn) and weight

10

average molecular weight (Mw) of EDA chain extended WPU dispersions with

different chain extension degrees are given in Fig. 3. It can be clearly found that both

Mn and Mw of WPU first increase sharply and then increase very slowly with the

increase of CED. When the CED is larger than 60%, there is no significant change in

the molecular weights of WPU dispersions. That is to say, the actual maximum CED

is approximately 60% in this study. The result is in agreement with the result reported

by John et al, in which when the average particle size is about 80 nm, the actual

maximum CED is approximately 57% [18]. So, it is suggested that not all residual

NCO groups can react with the chain extenders. The reason may be that NCO groups

located in the inside of particles hardly react with chain extenders.

Fig. 3

The molecular weights of chain extended WPUs with different post-chain

extenders based on 60% chain extension degree are shown in Fig. 4. It can be found

that the molecular weights of WPU are increased because of the post-chain extension

reaction between the prepolymer and the post-chain extenders. Moreover, the

molecular weights of WPU-DETA and WPU-TETA are higher than that of WPU-

EDA due to that the multifunctional DETA and TETA react with prepolymer to form

a three-dimensional structure. Between them, the molecular weights of WPU-DETA

are the higher, indicating that the reaction extent of DETA is higher than that of

TETA.

Fig. 4

3.2 Effects of types of chain extenders on the properties of WPU dispersions

The effects of chain extender EDA, DETA, and TETA based on 60% chain

extension degree on the properties of WPU dispersions were investigated.

3.2.1 Structure of the WPUs

11

Fig. 5 shows the IR absorption spectra of WPU dispersions. It can be found that

the FTIR spectra of the samples are quite similar, and only some differences in the

relative intensities and positions of several bands are distinguished. In general, the

free N-H stretching band is at 3447-3600 cm–1

. However, the N-H stretching peak of

WPU in our present study shifts to 3360 cm–1

, indicating that the hydrogen bonds are

formed between N-H group and carbonyl group (C=O). In addition, the intensity of

the N-H stretching peak (3360 cm-1

) of WPU-EDA and WPU-DETA is higher than

that of WPU-0 and WPU-TETA, which may be attributed to the formation of more

polar urea groups. Compared with EDA and DETA, the longer molecular chain of

TETA may diminish its diffusion ability. Therefore, for WPU-TETA, the chain

extension reaction extent and amount of urea groups formed are relatively low.

Moreover, the dashed line at 3163 cm-1 indicates that the N-H absorption bands of

WPU-EDA and WPU-DETA are broader and shift to lower frequencies because of

the formation of more hydrogen bonds [27-29]. The absorption peaks centered around

1724 cm–1

, 1535 cm–1

, and 1350 cm

–1, are assigned to the C=O, C-N, and N-H groups,

respectively. The stretching bands of urea groups (1700-1640 cm–1

) are not obvious

because the amount of urea groups is too small to be detected.

Fig. 5

3.2.2 Crystallinity

Wide angle X-ray diffraction (WAXD) measurement was used to investigate the

crystallinity of the samples. The results are depicted in Fig. 6. As can be observed

from the figure, all the diffractograms are similar with a large broad diffraction peak

at around 19.8° and a small broad diffraction peak at around 42.1°, indicating that

they have a low degree of crystallinity. The intensity order of the diffraction peaks

around 19.8° is as follows: WPU-0 > WPU-TETA > WPU-EDA > WPU-DETA,

12

implying that the crystallinity gradually decreases. The crystallinity is commonly

deemed to the ordered structure of chain segments occurring in polyurethane. The

decrease of the crystallinity may be due to the formation of more polar urea groups in

the structure and more interchain hydrogen bonds formed between the

macromolecular chains, which can induce disorganization of polyurethane chain

structure [19]. For WPU-DETA and WPU-TETA, DETA and TETA with more than

two amino groups can react with residual NCO groups to form a three-dimensional

network structure, which restricts the movement of chain segments and destroys the

regularity of chain segments, hindering the crystallization of soft segments in

polyurethane. Moreover, the intensity of diffraction peak of WPU-DETA is lower

than that of WPU-TETA because of the higher reaction extent of DETA.

Fig. 6

3.2.3 Thermal and mechanical properties

The thermal properties of all samples were studied by differential scanning

calorimetry (DSC). The DSC thermograms and data are shown in Fig. 7 and Table 1,

respectively. It is found that for all samples there is only a single glass transition

temperature of soft segment (Tgs) in the range from -49.1 to -47.4 °C. The melting

endothermic peaks are not detected, which indicates that the crystallization is very

slow [30]. Furthermore, the Tgs is almost the same, and this is probably attributed to

the same amount of the soft segment PCL.

Fig. 7

Table 1

The loss modulus and glass transition temperature (Tg) obtained from the DMA

measurements are shown in Fig. 8 and Table 2, respectively. For all samples, two loss

13

modulus peaks Tgs and Tgh (glass transition temperature of hard segment) can be

observed. Moreover, their difference, ∆Tg (Tgh- Tgs), is related with the extent of

microphase separation between soft and hard segments in polyurethane. It can be

found that Tg values from DMA and DSC are different owing to their measuring

principles, but the Tgs values of the samples determined by these two methods are

close to each other. It is also found that the ∆Tg of chain extended WPU dispersions is

higher than that of WPU without chain extension (WPU-0), indicating that the chain

extension promotes microphase separation between soft and hard segments in

polyurethane. The ∆Tg of WPU-EDA, WPU-DETA, and WPU-TETA is close to each

other, implying that the extent of microphase separation of soft and hard segments in

these polymers is almost the same. The Tgh of WPU-DETA is higher than that of the

other samples, probably due to its higher crosslinking density and molecular chain

rigidity, which confine the motion of its molecular chains.

Fig. 8

Table 2

Fig. 9 shows the thermogravimetric (TG) curves of WPU films and Table 3 lists

the characteristic thermal decomposition data. It is known that the hard segment is

more prone to thermal decomposition than the soft segment in polyurethane. As a

consequence, the thermostability of WPU is mainly dependent on the thermal stability

of its hard segment. T5% represents the temperature at which weight loss reaches to

5%, which is generally assigned to be the decomposition temperature. It is found that

the T5% of the samples decreases in the order of WPU-DETA (267.1 °C) > WPU-

TETA (261.5 °C) > WPU-EDA (255.0 °C) > WPU-0 (242.5 °C). It indicates that the

post-chain extension improves the thermal stability of WPU. The chain extension

14

forms more urea groups, which are more thermally stable than urethane groups [31].

Furthermore, the urea groups take part in more intermolecular chain association and

improve the thermal stability [19]. For WPU-DETA and WPU-TETA, their molecular

chains have a three-dimensional network structure, favoring the increase of thermal

stability [32, 33]. The higher thermal stability of WPU-DETA than that of WPU-

TETA is probably attributed to the higher crosslinking density and molecular weight

of WPU-DETA. It is worth noting that the thermal stability of WPU-TETA is similar

to that of WPU-0 when the weight loss is greater than 10%. It may be ascribed to their

almost the same amount of soft segments and the lower crosslinking density of WPU-

TETA due to the lower reaction extent of TETA. However, the thermal stability of

WPU-EDA is apparently inferior to that of WPU-0 when the weight loss is greater

than 5%, which is theoretically abnormal. It may be due to that the weight of WPU-

EDA film for testing is much less than that of WPU-0 film, which results in that the

measured TG curve of WPU-EDA shifts to low temperature. Therefore, the measured

thermal stability of WPU-EDA is inferior to its actual thermal stability. Actually, the

thermal stability of WPU-EDA should not be inferior to that of WPU-0, due to more

amounts of the urea groups and higher molecular weight of WPU-EDA.

Fig. 9

Table 3

3.2.4 Adhesion performance

The adhesion performances of all samples were obtained by the T-peel tests. Fig.

10 depicts the T-peel strength values. As can be seen in Fig.10, the T-peel strength

values of WPU-EDA, WPU-DETA, and WPU-TETA are higher than that of WPU-0,

which shows that the post-chain extension improves the adhesion performance of

15

WPU. As mentioned previously in this paper, these chain extenders react with NCO

groups to form polar urea groups, which enhance the intermolecular interaction of

WPUs and the interaction between WPUs and PET films. Moreover, it is worth noting

that the WPU-DETA possesses the greatest T-peel strength, which may be due to its

highest crosslinking density.

Fig. 10

3.3 Properties of WPU dispersions with EDA and DETA

3.3.1 Effects of EDA/DETA molar ratio

From the above study of the effects of different chain extenders on the properties

of WPUs, it can be obtained that that WPU-DETA had better thermal stability and

adhesion strength. The water resistance and solvent resistance of WPU prepared with

DETA are also better than those prepared with EDA [26]. Therefore, it can come to a

conclusion that DETA is an effective chain extender to improve the properties of

WPU dispersions. But, if only DETA is used as a chain extender and crosslinker, the

storage stability of WPU dispersion probably declines because of the excessive

crosslinking. Therefore, EDA and DETA as the combined chain extenders were used

simultaneously and the influences of the molar ratio of EDA to DETA on the

properties of WPU dispersions were also investigated. The results are summarized in

Table 4.

Table 4

It can be found that the average particle size of WPU dispersion gradually

increases and its appearance accordingly changes from translucent to opalescent with

the increase of DETA content. At the same time, the storage stability declines. In

addition, the water resistance and blocking resistance are gradually improved because

16

of the increase of crosslinking degree. The T5% and adhesion strength are relatively

high when the molar ratio of EDA to DETA is 2:3. To sum up, the chain extenders

EDA and DETA with a mole ratio of 2:3 are appropriate for WPU dispersions with

relatively good overall performance in this work.

3.3.2 Effects of NCO/OH molar ratio

The WPU dispersion with low organic solvent content (5 wt% with respect to the

WPU dispersion) does not require removal of the organic solvent, which is conducive

to the industrialized production of WPU dispersion and avoids expenditure on solvent

recovery. In order to prepare a stable WPU dispersion with high solid content and low

organic solvent content, it is necessary to decrease the viscosity of the polyurethane

prepolymer, which is mainly determined by the NCO/OH molar ratio of polyurethane

prepolymer. In general, an increase in the NCO/OH molar ratio decreases the

molecular weight and viscosity of the polyurethane prepolymer [34]. The effects of

NCO/OH molar ratio on some properties of waterborne polyurethanes with the same

DMPA content and organic solvent NMP content of 5 wt% are given in Table 5. It

can be seen that with the increase of NCO/OH molar ratio from 1.4 to 1.7, the average

particle size of the dispersions increases from 102.7 to 132.3 nm, the appearance

changes from translucent to opalescent and the pot-life also gradually decreases. The

smaller NCO/OH molar ratio produces the higher molecular weight and viscosity of

prepolymer. Hence, the emulsification of the prepolymer would require more water,

which leads to low solid content in the WPU dispersion. With the increase of

NCO/OH molar ratio, the molecular weight and viscosity of the prepolymer gradually

decrease, and less water can emulsify the prepolymer and the solid content of WPU

dispersion increases. However, a larger NCO/OH molar ratio produces more

remaining NCO groups. The NCO groups will react with water to form more

17

hydrophobic urea groups during the dispersion process, which decreases the

dispersion of the prepolymer in water, thus requiring more water and resulting in the

decrease of solid content. Moreover, during the post-chain extension process, the

reaction between residual NCO groups and amine chain extenders increases the

entanglement extent of the polyurethane chain on the surface of WPU dispersion

particles, which results in the increase of particle size and the decrease of pot-life. The

NCO/OH molar ratio of 1.6 is appropriate for the preparation of stable WPU

dispersion with solid content of about 40 wt% and organic solvent content of 5 wt%.

The 5 wt% content of residual high boiling point organic solvent NMP can act as a

coalescing agent.

Table 5

3.3.3 Steaming and boiling resistance test

For printing inks applied in the food packaging, the steaming and boiling

resistance is very important. Consequently, the steaming and boiling resistance of

WPU dispersion obtained above was investigated, and the result is shown in Fig. 11.

It is observed that there is not a wrinkle or damage in the test bag, implying that the

WPU dispersion possesses an excellent steaming and boiling resistance. Therefore,

the synthesized WPU dispersion can be potentially applied as a waterborne ink binder.

Fig. 11

4 Conclusions

In this work, a series of waterborne polyurethane dispersions were prepared with

different functional post-chain extenders. The post-chain extension process and the

influences of these chain extenders on the properties of WPU dispersions were

investigated. The results showed that the post-chain extension mainly occurred on the

particles surface and increased the particle sizes of WPU dispersions. GPC indicated

that the molecular weights of WPU initially increased with the increase of chain

18

extension degree, and almost remained stable when the chain extension degree was

larger than 60%. Therefore, the actual maximum chain extension degree was

approximately 60% in this system. XRD and DSC revealed that all the synthesized

aqueous polyurethanes had a lower degree of crystallinity. FTIR showed that the post-

chain extension enhanced the formation of hydrogen bonds. DMA revealed that the

post-chain extension promoted microphase separation between soft and hard segments

in polyurethane, respectively. TGA and T-peel tests revealed that WPU-DETA had

better thermal stability and adhesion strength. Those results showed that the post-

chain extension had an important influence on the properties of WPUs, and

trifunctional DETA was found to be an effective chain extender and crosslinker that

improved the properties of WPU dispersions. The chain-extended WPU dispersion by

EDA and DETA with a mole ratio of 2:3 had excellent properties. The NCO/OH

molar ratio of 1.6 was suitable for preparing stable WPU dispersion with solid content

of about 40 wt% and organic solvent content of 5 wt%. The residual high boiling

point organic solvent can act as a coalescing agent for aqueous ink. The synthesized

process of WPU dispersion with low organic solvent content was conducive to the

industrial production of WPU dispersion and avoided the cost of organic solvent

removal. The obtained WPU dispersion had excellent steaming and boiling resistance,

thermal stability, adhesion strength, blocking resistance, and storage stability, and

could be directly used as a waterborne ink binder without removing the organic

solvent.

References

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23

Figure captions

Scheme 1 Synthesis process for WPU dispersions with different chain extenders

WPU-0: WPU without post-chain extension; WPU-EDA: EDA chain-extended WPU;

WPU-DETA: DETA chain-extended WPU; WPU-TETA: TETA chain-extended

WPU.

Fig. 1 A sketch of PET/CPP laminated film.

Fig. 2 Particle size and distributions of WPU dispersions with different chain

extenders.

Fig. 3 Number average molecular weight (Mn) and weight average molecular weight

(Mw) of EDA chain-extended WPUs with different chain extension degrees.

Fig. 4 Number average molecular weight (Mn) and weight average molecular weight

(Mw) of WPUs with different chain extenders based on 60% chain extension degree.

Fig. 5 FTIR spectra of WPUs with different chain extenders.

Fig. 6 X-ray diffractograms of WPUs with different chain extenders.

Fig. 7 DSC thermograms of WPUs with different chain extenders.

Fig. 8 Loss modulus (E’’) versus temperature (°C) for WPUs with different chain

extenders.

Fig. 9 TG curves of WPUs with different chain extenders.

Fig. 10 T-peel test results of WPUs with different chain extenders.

Fig. 11 Image of test bag after the steaming and boiling resistance test.

24

CH2 CH2 O C

O

(CH2)5HO OH

CH3

CH2 CH2 C CH2

CH3

COOH

HO OHn

CH3

H3C NCO

PCL

NCO

IPDI DMPA

prepolymerization

ROCN OOCNH R NHCOO CH2 C CH2 OOCNH R(CH2)2NHCOO O

HO-(CH2)4-OH

CH3

COOH

NCO

BDO

ROCN OOCNH R NHCOO CH2 C CH2 OOCNH R NHCOO (CH2)4 OOCNH R NCO(CH2)2NHCOO O

CH3

TEA

Water

COOH

ROCN NHCOO (CH2)2 OOCNHO R NHCOO CH2 C CH2 OOCNH R NHCOO (CH2)4 OOCNH R NCO

CH3

COONH(Et)3

EDA

WPU-EDA

DETA

WPU-DETA

TETA

WPU-TETA

without post-

chain extension

WPU-0

C (CH2)5 n

O EDA: NH2-CH2-CH2-NH2

DETA: NH2-CH2-CH2-NH-CH2-CH2-NH2

TETA: NH2-CH2-CH2-NH-CH2-CH2-NH-CH2-CH2-NH2

R:

CH3

CH2

CH3

H3C :

Scheme 1

25

Fig. 1

26

Fig. 2

27

Fig. 3

28

Fig. 4

29

Fig. 5

30

Fig. 6

31

Fig. 7

32

Fig. 8

33

Fig. 9

34

Fig. 10

35

Fig. 11

36

Table 1 DSC scan results of the samples

Samples WPU-0 WPU-EDA WPU-DETA WPU-TETA

Tg (°C) -49.1 -48.0 -47.4 -48.4

37

Table 2 Glass transition temperature (Tg) of the samples obtained from the DMA

experiments

Samples WPU-0 WPU-EDA WPU-DETA WPU-TETA

Tgs (°C) -41.9 -52.8 -46.5 -47.0

Tgh (°C) -13.5 3.5 10.2 5.8

∆Tg (°C) 28.4 56.3 56.7 52.8

38

Table 3 Main decomposition temperature of the samples

Samples T5%/°C T10%/°C Tonset/°C Tinflection/°C Tend/°C

WPU-0 242.5 302.2 166.3 352.5 487.6

WPU-EDA 255.0 293.9 179.1 347.5 481.7

WPU-DETA 267.1 306.6 223.1 403 488.9

WPU-TETA 261.5 303.3 197.4 350.5 488.9

39

Table 4 Properties of WPUs with different EDA/DETA molar ratios

n (EDA:DETA) 1:0 4:1 3:2 2:3 1:4 0:1

Appearance Translucent Translucent Translucent Translucent Opalescent Opalescent

Particle size/nm 89.2 92.2 97.7 102.3 110.0 114.7

T 5% 255.0 262.7 266.6 275.3 273.3 267.1

Water resistance + ++ ++ +++ +++ +++

Adhesion

strength/(N/15mm) 2.60 2.78 2.90 2.98 2.95 2.91

Blocking resistance + ++ ++ +++ +++ +++

Pot life/month >6 >6 >6 >6 5 3

+++: best; ++: good; +: worst

40

Table 5 Properties of WPU dispersions with different NCO/OH molar ratios

NCO/OH 1.4 1.5 1.6 1.7

Appearance Translucent Translucent Translucent Opalescent

Particle size/nm 102.7 105.3 108.7 132.3

Solid content/% 34.42 38.25 40.02 35.20

Steaming and boiling resistance +++ +++ +++ ++

Pot life/month >6 >6 >6 2

+++: best; ++: good; +: worst

41

Highlights

* Actual maximum chain extension degree is approximately 60%.

* Diethylene triamine (DETA) is an effective chain extender and

crosslinker.

* Chain extenders of ethylene diamine and DETA with a mole ratio of

2:3 are optimal.

* Stable waterborne polyurethane is obtained without removing the

organic solvent.

* Synthesized waterborne polyurethane can be directly used as an

aqueous ink binder.