ZHANG Kang, ZHENG Yanping, LIN Yi, ZHOU Mi, ZHU Puxin,WU Dacheng, CHENG Fei＊

(1. Textile Institute, Sichuan University, Chengdu 610065, China; 2. School of Chemistry and Chemical Engineering, Hunan Institute of Science and Technology, Yueyang 414000, China; 3. Key Laboratory of Leather Chemistry and Engineer (Sichuan University), Ministry of Education, Sichuan University, Chengdu 610065, China)

Abstract: Waterborne elastic polyester (WEP) with different content of hard polyester was applied in the maize starch (MS) based composites (MS/WEP) via solution casting method. The effects of WEP with different contents of hard polyester on the structure and properties of starch were studied by Fourier transform infrared, X-ray diffraction, ultraviolet-visible, tensile test, differential scanning calorimeter, thermogravimetric analysis and moisture measurement. The experimental results show that the addition of WEP does not change the crystalline type of starch, and only reduces the crystallinity of starch. And the structure and properties of MS/WEP are related to not only the content of starch but also the microstructure of WEP or the content of hard polyester in WEP. Waterborne elastic polyester with 30wt% hard polyester (WEP30) has the best modification effect on the maize starch among all the WEPs. For example, MS/WEP30 film has the optimum toughness,aging resistance and transmittance, the lowest crystallinity and glass transition temperature among all the MS/WEP films, and the lower moisture content. It is related to the compatibility between starch and WEP, resulting from the number of physical crosslinking points in WEP. .

Key words: maize starch; waterborne elastic polyester; hard polyester; compatibility

1 Introduction

Plastic products have many favorable properties,such as light weight, good air and water resistance, and excellent mechanical properties, which have been widely used in all aspects of life[1]. Most of plastic products are hard to degrade, resulting in the aggravation of environmental pollution[2]. And the petroleum-based materials used to make plastic products are not renewable,which is not consistent with the concept of sustainable development[3]. Biodegradable materials, especially starch, can effectively solve the above two problems[4-6].However, pure starch film is fragile and sensitive to ambient humidity, which does not meet the same criteria as plastic products[1]. In order to fulfill certain requirements of products, more researchers are focusing on blending starch with biodegradable polymers, including polylactic acid (PLA)[7-9], polycaprolactone (PCL)[10-13],poly(butylene succinate) (PBS)[14,15], and poly(butylene adipate-co- terephthalate) (PBAT)[16,18].

Compared with pure polymers, biodegradable polymers/starch composites can not only reduce the cost, but also improve the stiffness and the biodegradability[1,19]. Compared with pure starch products, biodegradable polymers/starch composites can improve the mechanical properties and water resistance[1,19]. But hydrophobic biodegradable polymers and hydrophilic starch are incompatible in thermodynamics, which causes a sharp decrease in the mechanical properties of composites when the content starch exceeded 25%[20-22].In general, reducing the hydrophilicity of starch[23,24]and increasing the hydrophilicity of biodegradable polymers[22,25,26]are two common methods used to improve the compatibility between biodegradable polymers and starch. In addition, starch blending with waterborne polymers, such as waterborne polyurethane(WPU)[27,28]and polyester (WPET)[29], should be a direct and effective way to prepare composites with good compatibility and mechanical properties. Caoet alfound that there was no obvious phase separation, even if the starch content was 80% in starch/WPU composites[28]. And the mechanical properties of starch/WPU composites were related to not only the content of starch but also the microstructure of WPU or hard-segment order in WPU[28]. Waterborne polyurethane has good compatibility with starch, but it is expensive,which limits its wide application. Therefore, inexpensive WPET may be more suitable for preparing starch matrix composites than WPU.

Waterborne elastic polyester (WEP) is a kind of WPET with good water solubility and high elasticity as WPU[30]. It is worth studying whether WEP has good compatibility with starch as WPU. In our previous study, a series of WEP with different compositions or soft/hard segment ratios were prepared via chain-extending[30]. We found that elastic recovery rate of WEP also depended on hard polyester content in WEP[30]. In this study, the starch/WEP composite was prepared by solution casting method. The effects of WEP content and compositions on the properties of the starch/WEP composites, especially the mechanical performance,were studied.

2 Experimental

2.1 Materials and chemicals

Normal maize starch granule was purchased from Guowei Starch Co., Ltd., (Xi’an, China). Its molecular weight is 1.53×107calculated by viscosity method and its amylose content is 25%[29]. The synthesis method of waterborne elastic polyester (WEP) was as reported by our group[30], and the detailed process was as follows.Soft polyester (SP) and hard polyester (HP) with varied weight ratios (SP/HP, 90/10, 80/20, 70/30, 60/40,50/50) were added into three-neck flask. Then moisture in polyester was removed at 150 ℃ for 10 min by vacuum. Finally, isophorone diisocyanate (IPDI) was added and reacted at 170 ℃ for 10 min. The products were separately named as WEP10, WEP20, WEP30, WEP40 and WEP50, of which number average molecular weights were 7312, 6413, 6166, 6284, and 3898, respectively. The ultrapure water used in the experiment was made by YL-100BD type Laboratory Ultrapure Water Machine (Ereeran Water Treatment Equipment Co., Ltd., Shenzhen, China).

2.2 Preparation of maize starch films with WEP

Maize starch granule of 4wt% and deionized water were added into three-neck flask, and then WEP with varied content (0, 10wt%, 20wt%, 30wt%, or 40wt%, based on the weight of starch) was added into three-neck flask. Then the mixture was stirred at 150 r/min and 95 ℃ for 60 min. Then the starch pasting of 90 mL was poured onto the glass dish of 15 cm × 21 cm and dried at room temperature. The film peeled off was placed in a container with relative humidity (RH) of 68% at room temperature for 3 days. According to the type of WEP added, the starch film was named as MS,MS/WEP10, MS/WEP20, MS/WEP30, MS/WEP40,and MS/WEP50, respectively.

2.3 Structural characterization of MS and MS/WEP films

The functional groups of MS film and MS/WEP films with 30wt% WEP were recorded by FTS3000 FTIR Spectrum Scanner (Hercules, USA). The scanning mode was attenuated total reflection (ATR) and each sample was scanned 32 times. Wavenumber ranged from 650 cm-1to 4 000 cm-1with a resolution of 2 cm-1. The crystalline structure of MS film and MS/WEP films with 30wt% WEP stored for 3 and 60 days were recorded by Philips X’Pert Pro Multi-purpose Diffraction System (Netherlands) in the scanning 2θrange from 5° to 40° at a step length of 0.026°.

2.4 Transmittance of MS and MS/WEP films

Transmittance of MS film and MS/WEP films with 30wt% WEP were recorded by Jena Specord S600 Ultraviolet-visible Spectrophotometer (Germany) in the wave range from 200 nm to 1 000 nm. Each sample was scanned 3 times and averaged.

2.5 Mechanical properties of MS and MS/WEP films

The MS and MS/WEP films were cut into strips with the size of 10 mm × 150 mm, and then put into a container with RH of 68% for 3 days. Tensile strength of MS/WEP films with varied WEP content were measured by YG061-1500 Electronic Strength Tester(Laizhou, China) at room temperature, and were the average of 10 times. The stretching speed and clamping distance were 100 mm/min and 100 mm, respectively.

Tensile strength of MS/WEP films with 30wt%WEP stored for varied days (3, 7, 15, 30, and 60 days)at RH of 68% and room temperature were measured in the way mentioned above.

2.6 Moisture content and sensitivity

The moisture content and sensitivity of MS film and MS/WEP films with 30wt% WEP were measured according to our previous report[31]. The detailed process was as follows. All the samples were dried at 105°C for 48 h and then weighed (m0). Then, they were placed in containers with varied RHs (11%, 33%, 44%,68%, and 80%) for 15 days and weighed (m1). The moisture content (Mc) and sensitivity (Ms) of MS and MS/WEP films were calculated by equations (1) and (2),respectively.

where,Mc11andMc80represent the moisture contents at RH of 11% and 80%, respectively.

2.7 Thermal properties of MS and MS/WEP films

The differential scanning calorimeter (DSC)analysis of MS film and MS/WEP films with 30wt%WEP were performed by DSC214 (Netzsch, Germany)with temperature from -50 ℃ to 200 ℃ at a heating rate of 10 ℃/min, where the purge gas was nitrogen.The thermogravimetric analysis (TGA) of MS film and MS/WEP films with 30wt% WEP were measured by TG209F3 (Netzsch, Germany) from 50 ℃ to 600 ℃and heating rate was 10 ℃/min. The whole process was protected by nitrogen.

3 Results and discussion

3.1 FTIR analysis of MS and MS films

FTIR spectrums of MS and MS/WEP films are illustrated in Fig.1. The peaks at 3 285 cm-1, 1 145 cm-1, and 997 cm-1are -OH stretching vibration peak,C-O of C-O-H stretching vibration peak, and C-O of C-O-C asymmetric stretching vibration peak, respectively, which are the characteristic absorption peak of starch[28]. Compared with the FTIR spectrum of MS, the FTIR spectrums of MS/WEP films have a new characteristic absorption peak at 1 720 cm-1, which is C=O of ester and carbamate groups stretching vibration peak[30].The peak at 1 640 cm-1is related to the water in the amorphous region of the starch. The strength of peak at 1 640 cm-1in the FTIR spectrums of MS/WEP films is weaker than that of MS film. It reflects that the addition of WEP reduces the moisture absorption of starch film.There are two reasons for this. On the one hand, the polar groups on the WEP could form hydrogen bond with the -OH groups on the starch molecules. Moisture absorption of starch film would weaken because some-OH groups on the starch molecule were blocked[31]. On the other hand, the moisture absorption of WEP in MS/WEP composite was weaker than that of starch[10].

Fig.1 FTIR spectra of MS and MS/WEP films

3.2 XRD analysis

XRD patterns of MS and MS/WEP films stored for 3 days are shown in Fig.2, and its crystallinity is listed in Table 1. It can be seen from the XRD pattern of MS film in Fig.2 that the peaks appear at 5.6°, 14.9°,19.7°, 22.1°, and 23.8°, which are the typical B-type crystal[32]. Compared with the XRD pattern of MS film,the position of these peaks in the XRD pattern of MS/WEP films do not change, but the strengths of peaks at 5.6°, 14.9°, 19.7°, 22.1°, and 23.8° weaken or disappear.It indicates that the addition of WEP do not change the crystalline type of starch, but only reduce the crystallinity of starch as shown in Table 1. For all the MS/WEP films with 30wt% WEP stored for 3 days, the crystallinity of MS/WEP30 film is the lowest. It suggests that the addition of WEP30 could inhibit the recrystallization of starch during the film-forming process[33].The recrystallization of starch during the film-forming process includes rapid crystallization of amylose (or short-term retrogradation) and crystallization of partial amylopectin, while WEP30 mainly reduces the crystallization proportion of amylopectin[32].

Fig.2 XRD patterns of MS and MS/WEP films stored for 3 days

Table 1 The crystallinity of MS and MS/WEP films stored for 3 and 60 days

The crystallinity of starch would change during storage due to the recrystallization of amylopectin (or long-term retrogradation)[34], resulting in the deterioration of its mechanical properties[10]. It can be seen from Table 1 that the crystallinity of MS/WEP30 film at 60 days was also the lowest among all the MS/WEP film at 60 days, implying that WEP30 could inhibit the recrystallization of starch during storage process, namely the crystallization of amylopectin[35]. It may be related to the better compatibility or stronger interaction between WEP30 and starch[29].

Table 2 Moisture content (Mc) and sensitivity (Ms) of MS and MS/WEP films

3.3 Mechanical properties of MS/WEP films

Mechanical properties of different MS/WEP films with varied WEP content are illustrated in Fig.3. With the increase of WEP content, elongation at break of MS/WEP films increases, and tensile strength of MS/WEP films decreases. The increase in elongation at break of MS/WEP film is due to the better interfacial adhesion between WEP and starch[24]. In other words,there is the better compatibility in thermodynamics between WEP and starch. The detailed illustration is stated in the section of Transmittance of MS/WEP films and DSC. When the same content of WEP is added,WEP30 has the best plasticizing effect on the starch.This might also indicate indirectly that WEP30 has the best compatibility with starch among all the WEPs.The mechanical properties of the material depend on its crystallinity[36]. For all the MS/WEP films, MS/WEP30 film has the lowest crystallinity.

Fig.3 Mechanical properties of MS/WEP films varied with WEP content: (a) Elongation at break; (b) Tensile strength

When the addition amount of WEP30 is 40%based on the weight of starch, the elongation at break and tensile strength of MS/WEP30 are 15.5±4.8%and 13.9±0.9 MPa, respectively. It is reported that the elongation at break and tensile strength of composites(55wt% thermoplastic starch and 45wt% modified PBAT) are 6.09% and 6.05 MPa, respectively[25]. And the elongation at break of 70wt% starch/30wt% WPU composites is 12%[27]. It indicates that WPE30 has the better compatibility with starch than WPU and modified polyester. Considering the cost and performance of MS/WEP, the addition amount of WEP is 30% based on the weight of starch.

The mechanical properties of the conventional thermoplastic starch/polyester composites tended to deteriorate during storage, which limited the use of starch[10]. Starch molecules were rearranged as a result of plasticizer migration or crystallization. The mechanical properties of MS/WEP films with 30wt% WEP varied with storage time are illustrated in Fig.4. It can be seen from Fig.4 that the elongation at break of MS/WEP films decreased slowly and tensile strength of MS/WEP films also increased slowly. Compared with the elongations at break of films stored for 3 days, the elongations at break of MS/WEP10, MS/WEP20, MS/WEP30, MS/WEP40, and MS/WEP50 films stored for 60 days decrease by 28.6%, 46.5%, 10.0%, 32.9%, and 18.4%, respectively. It suggests that MS/WEP30 film has the optimum aging resistance. It might be related to the stronger interaction or better compatibility between starch and WEP30. In Ortega-Toro’s research about starch/PCL composites, the elongation at break of 90wt% starch/10wt% PCL composites at 5 weeks decrease by 50% compared with the elongation at break at 1 week[10].

To sum up, MS/WEP30 film has good toughness and anti-aging properties, which makes potential use of starch/WEP instead of starch/WPU and starch/biodegradable polyester.

3.4 Transmittance of MS/WEP films

Transmittance of materials was related to many factors, such as its composition and crystallinity. It is reported that the transmittance of composite materials can also indirectly reflect the compatibility between different components[37]. UV-Vis curves of MS and MS/WEP films with 30wt% WEP are illustrated in Fig.5.The transmittance order of MS and MS/WEP films is as follows: MS ＞ MS/WEP30 ＞ MS/WEP40 ＞ MS/WEP20 ＞ MS/WEP10 ＞ MS/WEP50. The transmittance of MS/WEP30 film is slightly lower than that of MS film, and higher than that of other MS/WEP film.It indicates that WEP30 has better compatibility with starch than other waterborne elastic polyesters[37].

Fig.5 UV-Vis curves of MS and MS/WEP films with 30wt% WEP

3.5 DSC analysis

DSC curves of MS and MS/WEP films with 30wt% WEP are illustrated in Fig.6. It can be seen from Fig.6 that there is no additional glass transition behavior except the one attributed to starch. It indicates that 30wt% WEP in MS/WEP films is miscible with starch at molecular level. In other words, there is no phase separation in MS/WEP films[38]. For all the MS/WEP films except MS/WEP30, the glass transition temperature (Tg) of MS/WEP films increases with the increase of the hard polyester content in WEP. This is because the hard polyester has the relatively highTg[30].It is worth noting that MS/WEP30 film has the lowestTgin all the MS/WEP films. It might be related to the number of physical crosslinking points in elastic polyester[39]or to the interaction among starch molecule,waterborne elastic polyester and water.

Fig.6 DSC curves of MS film and MS/WEP films with 30wt%WEP

3.6 TG analysis

TG and DTG curves of MS and MS/WEP films are illustrated in Figs.7(a) and 7(b), respectively. In Fig.7(a), there are two stages of weight loss in TG curve of MS film. The first stage of weight loss is below 150 ℃, which is attributed to the evaporation of water (including free water and bound water) in the starch film[29]. The second stage of weight loss is at 250-350 ℃, which is due to the degradation of starch.TG curves of MS/WEP films could be viewed as three stages of weight loss. The first stage of weight loss in TG curves of MS/WEP films is also due to the loss of water. But all the weight losses of water in MS/WEP films are lower than that in MS film. It suggests that the moisture absorption of MS/WEP films are weaker than that of MS film. The second stage of weight loss in TG curves of MS/WEP films is from 220 ℃ to 320℃, which is the degradation of starch. The degradation temperature of starch in MS/WEP films is lower than that in MS film, resulting from destruction of crystalline structure of starch in MS/WEP films[40]. The third stage of weight loss in TG curves of MS/WEP films is range from 320 ℃ to 350 ℃, which is resulted from the degradation of WEP.

Fig.7 (a) TG and (b) DTG curves of MS film and MS/WEP films with 30wt% WEP

3.7 Moisture absorption of MS and MS/WEP films

Application of starch in many areas, especially in packaging material, was restricted because of its strong hygroscopicity. Therefore, it is very important to reduce the moisture absorption and sensitivity of starch to humidity for the application of starch-based composites. Moisture content and sensitivity of MS and MS/WEP films are shown in Table 2. The values of MS/WEP films are lower than those of MS film. This is consistent with the results from the sections of FTIR analysis and TG, which also indicates less moisture absorption by MS/WEP. And the detailed reason is discussed in the section of FTIR. Generally speaking,at the same relative humidity, with the increase of hard polyester content in WEP, the moisture content of MS/WEP film decreases. In terms of moisture sensitivity of MS/WEP films, MS/WEP40 film is the weakest and MS/WEP30 film is secondary. This suggests that the moisture sensitivity of MS/WEP film is related to not only the content of hard polyester in WEP, but also the interaction between starch and WEP[29].

4 Conclusions

Maize starch/waterborne elastic polyester (MS/WEP) composite films were prepared by solution casting method, and the effects of WEP compositions on the properties and compatibility of MS/WEP films were studied. When the content of WEP ranged from 10wt%to 40wt% based the weight of starch, elongation at break of MS/WEP films increased, and tensile strength of MS/WEP films decreased. The addition of WEP did not change the crystalline type of starch, but only reduced the crystallinity of starch. WEP could improve the mechanical properties of starch film, but the properties of MS/WEP were not positively related to the WEP composition. Among the specimen, MS/WEP30 film had the optimum toughness, aging resistance and transmittance, and the lowest crystallinity and glass transition temperature. In addition, WEP reduced the moisture content and sensitivity of starch film in varying degrees. MS/WEP40 film had the lowest moisture content and the weakest moisture sensitivity, and MS/WEP30 film followed. In short, MS/WEP30 film worked best when the same content of WEP was added.The reason may be related to the stronger interaction or the better compatibility between starch and WEP,which was certified by differential scanning calorimeter and transmittance of MS/WEP films.