Influence of Hydroxyl-Terminated Hyperbranched Polymer and Coupling Agent on the Performance of SF/PLA Composites

ABSTRACT

Herein, a hydroxyl-terminated hyperbranched polymer (HPN) with abundant terminal hydroxyl groups was employed to modify straw-plastic composites and studied the effect of HPN concentration on straw fiber reinforced polylactic acid (SF/PLA). Silane coupling agent (CA) was used to improve the interfacial compatibility between SF, HPN and PLA matrix. The mechanical strength, thermal properties and water resistance of different SF/PLA composites were tested and analyzed. When the concentration of HPN is 6%, the HPN-SF/PLA has the best mechanical strength index. HPN modification, CA modification and the combination of both have positive effect on improving mechanical performance. Compared with the UN-SF/PLA, the tensile, flexural and impact strength of HPN+CA-SF/PLA composites were increased by 24.7%,16.6% and 10.8%, respectively. The crystallinity of SF/PLA treated with HPN, CA, and their combination increased from 28% to 29.4%, 36.9%, and 42.3%, respectively. However, low melting point of HPN caused a decrease in the Tg, Tm, Tc, Td5% and Tdmax of SF/PLA. These characteristic temperatures can be enhanced by adding CA for co-modification. In addition, the three modified approach were able to enhance the water resistance of SF/PLA composites due to the reduction of the number of hydroxyl groups on the SF surface and the enhancement of the interfacial bonding properties.

摘要

本文采用末端羟基丰富的端羟基超支化聚合物（HPN）对秸秆塑料复合材料进行改性，研究了HPN浓度对秸秆纤维增强聚乳酸（SF/PLA）的影响. 采用硅烷偶联剂（CA）改善SF、HPN与PLA基体的界面相容性. 对不同SF/PLA复合材料的力学强度、热性能和耐水性进行了测试和分析. 当HPN的浓度为6%时，HPN-SF/PLA具有最佳的机械强度指标. HPN改性、CA改性及二者的结合对提高力学性能有积极作用. 与UN-SF/PLA相比，HPN+CA-SF/PLA复合材料的拉伸、弯曲和冲击强度分别提高了24.7%、16.6%和10.8%. 用HPN、CA及其组合处理的SF/PLA的结晶度分别从28%提高到29.4%、36.9%和42.3%. 然而，HPN的低熔点导致SF/PLA的Tg、Tm、Tc、Td5%和Tdmax降低. 这些特征温度可以通过添加用于共改性的CA来提高. 此外，由于SF表面羟基数量的减少和界面结合性能的增强，三种改性方法能够提高SF/PLA复合材料的耐水性.

KEYWORDS:

• Hyperbranched polymer
• coupling agent
• straw fiber/polylactic acid composite
• mechanical properties
• thermal properties
• water resistance

关键词:

• 超支化聚合物
• 偶联剂
• 秸秆纤维/聚乳酸复合材料
• 力学性能
• 热性能
• 耐水性

Introduction

Environmental issues, concerns about sustainability, the consumption of fossil resources, and the pollution caused by traditional fossil plastics have aroused increasing interest in the development of environment-friendly natural polymers and composites (Mohanty et al. 2018). Bio composite with higher bio-based content made of plants fiber and bioplastic are under continuous development. They have shown potential application in sustainable packaging, furniture and automotive decoration materials (Arif 2022; Asyraf et al. 2022; Ramachandran et al. 2022). Polylactic acid (PLA) is an important sustainable biodegradable polymer made from the fertilization of agricultural raw materials such as corn and wheat. The raw materials are easily available and sustainable. Compared to other biodegradable polymers, PLA has superior mechanical properties and easy processability as thermoplastic. It is a distinctive polymer of natural biological origin with excellent environmental friendliness. However, this promising polymer is not yet widely used in the commercial field, one of the important reasons being the high cost of PLA (Chin-San, Dung-Yi, and Wang 2022; Rajeshkumar et al. 2021). An approach to solving this problem is to incorporate low-cost ingredients, such as natural fibers, starches and minerals into PLA matrix. It not only reduces the PLA cost, but also enhances its mechanical properties or functionality. Natural fibers have been a popular choice for reinforcing polymer composites for a long time, as they are sustainable, easily accessible and possess satisfactory mechanical strength (Nurazzi et al. 2021). These natural fibers include straw, hemp, flax and so on. Table 1 illustrates the physical properties of the most frequently used plant fibers.

Table 1. Physical properties of selected plant fibers.

	Modulus of elasticity(Gpa)	Tensile sterength(MPa)	Density (g/cm^−3)	References
Straw	2–7	270–450	0.9–1.65	(Sakil et al. 2019; Mahmud et al. 2020)
Hemp	35–65	690	1.5	(Faruk et al. 2012)
Flax	30–70	400–1000	1.4	(Ku et al. 2011)
Jute	25–50	210–750	1.3	(Bledzki and Gassan 1999)
Kenaf	53	925	1.4	(Mahmud et al. 2021)
Sisal	10–20	125–800	1.5	(Kalia, Kaith, and Inderjeet 2009)
Coir	53	180	1.3–1.5	(Ramachandran et al. 2022)
Cotton	6–12	300–480	1.6	(Wambua, Jan, and Ignaas 2003)
Bamboo	12–16	150–220	0.8–1.1	(Ramachandran et al. 2022)

S.F.K. Sherwani et al. combined 1–10 mm sugar palm fibers and PLA by a twin-screw extruder and found that composites with 30% sugar palm fiber content showed the best performance in terms of flexural and tensile strengths of 26.65 MPa and 13.70 MPa, respectively. As the sugar palm fiber content increased, the densities, thickness swellings, and water absorption properties of composite increased (Sherwani et al. 2021, M. R. M.; Asyraf et al. 2021). Roberto J. Aguado et al. prepared hemp fiber-reinforced PLA by power mixing, extrusion, milling, and injection molding steps, and showed that the tensile modulus increased linearly with an increasing volume fraction of the dispersed phase. PLA/Hemp biocomposites showed a higher Young’s modulus compared to that of glass fiber-reinforced polypropylene (GF/PP) composites (Aguado et al. 2023). Arif Mahmud explored the possibility of using coir fibers as thermal insulators and reinforcements for biocomposites, indicating that coir fibers, as reinforcements, can significantly improve the mechanical, interfacial, and thermal properties of PLA composites (Mahmud et al. 2023). Mohsin Ejaz prepared biodegradable green composites using jute and flax added to polylactic acid (PLA). The results of the study showed that the mechanical properties of the composites were improved with the reinforcement of jute, flax, and blended jute/flax fibers in the PLA matrix. The mechanical properties of the composites reached their highest level when 40% of natural fiber reinforcement was added to the PLA matrix (Mohsin et al. 2020). Guili Li explored the non-isothermal crystalline characteristics and mechanical strength of PLA/Ramie fiber composites through experiments and numerical simulations. The findings showed that the crystallinity of PLA rose from 4.5% to 16.2% with the incorporation of 20% Ramie fibers. Furthermore, the tensile strength and Young’s modulus of PLA composites were increased without compromising their thermal stability, mechanical, thermal and crystalline properties of PLA resins (Guili et al. 2022). Table 2 provides a list of mechanical properties of PLA biocomposites using various natural fibers as reinforcements { (Rajeshkumar et al. 2021) #1078}.

Table 2. Mechanical properties of natural fiber/PLA composites.

Fiber type	Fiber(wt.%)	Tensile strength (MPa)	Tensile modulus (GPa)	Flexural strength (MPa)	Flexural modulus (GPa)	Impact strength (kJ/m^2)
Kenaf	20	-	-	115	-	3.1
Banana	20	57	1.61	104	5.6	-
Hemp	40	54.6	8.5	112.7	-	-
Jute	30	64.13	3.39	97.74	7.36	-
Coir	30	13.96	4.35	24.6	3.96	-
Nettle	50	50.82	3.34	36.89	-	28.76
Elephant grass	20	66	2.54	113	6.95	5.45
Coconut	30	56	3.975	91	3.618	8.6

Straw is one of the main agricultural wastes with rich sources, which is a broad term for the stalks and leaves of crops that have reached maturity, such as wheat, rice, corn, potatoes, oilseed rape, cotton, sugarcane, and others. Although China’s current straw utilization rate has reached 80%, the proportion of straw used as industrial raw material is relatively low, with low value-added. Straw is used comparatively less often in its raw form, but it still has many applications, such as making paper, fiberboard, biomass fuels, handicrafts, and biomass carbon, as seen in Figure 1.

Figure 1. Wheat straw fibers and their applications.

Studies have been conducted that combine straw and PLA as fillers. Zhongyu Qi studied the tensile and flexural characteristics of PLA composites reinforced with corn stover of varying particle sizes and fractions. The results indicated that the tensile strength and elongation at break of SF/PLA composites increased with the decrease of corn stover fiber particle size, with the maximum values of 54.08 MPa and 4.60% being achieved when the particle size of SF was larger than 120 mesh (Qi et al. 2022). Jorge R. Robledo-Ortíz and his colleagues examined the effect of varying sugarcane straw content on polylactic acid. When 20% straw was added, the tensile modulus was 1675 MPa; for 30%, 1523 MPa; and for 40%, 1832 MPa (Robledo-Ortiz et al. 2021). Aside from fiber content, the hydrophilicity of the fibers is a factor that influences the properties of natural fiber reinforced composites. Straw is mainly composed of cellulose, hemicellulose, and lignin. It is essential to bear in mind that the hydrophilic functional group (−OH) of these components has a weak interaction force with the PLA matrix, which does not promote adhesion, thus diminishing force dispersion and weakening the mechanical properties of the composites. Additionally, the higher hydrophilic straw increase water absorption, creating voids around the fibers and further decreasing the bonding strength between the fibers and the matrix. To achieve better interfacial properties with PLA, it is essential to adapt the surface chemical properties of the straw fiber surface.

Fiber surface modification is a relatively common means of improving compatibility between fibers and polymer matrixes. In order to improve the compatibility and adhesion between the plant fibers and the polymer matrix, the treatment strategy can be broadly divided into coupling agents (Asyraf et al. 2021; Chen et al. 2021), enzyme treatment (Hýsková et al. 2020) and nano-material coating (Zhang et al. 2022). In addition, there are physical methods such as steaming, blasting and grinding to refine the straw in order to change the morphology and surface activity of the straw fibers, which often require large equipment or consume large amounts of energy. Hyperbranched polymer are highly branched dendrimers, which have many branching points, and a large number of active end groups, such as epoxide, amines and hydroxyls. It is worthy of attention for improving the interface compatibility between plant fiber and polymer matrices (Sun 2019). Lou (Luo et al. 2018)used functionalized end-amino hyperbranched polymer modification to treat sisal fiber nanocrystals to enhance the interfacial compatibility performance between sisal fiber and epoxy resin. Sun etc (Sun, Fengqiang, and Zhao 2018; Zhao, Sun, and Tang 2022). adopt hyperbranched polyamide to improve the interface bonding between sisal fiber and epoxy resin, and the results showed that hyperbranched polyamide can greatly increase interface bonding performance. However, there are few literature studies on hyperbranched polymer modified straw fibers. Investigating the biocomposites of straw that have been altered by hyperbranched polymer is essential.

In this paper, different concentration (2%,4%,6%,8%,10%) of hydroxy terminated hyperbranched polyamide (HPN) were added to the straw fiber/polylactic acid composite (SF/PLA). Then the modification mechanism was studied by testing the mechanical strength and observing it using SEM. Based on the optimum weight content of HPN, SF was treated with HPN, CA and a combination of both, and then prepared into SF/PLA composites. The mechanical strength, thermal properties and water resistance of the different SF/PLA composites were measured. Herein, the effects of different modification strategies on the above properties of SF/PLA composites were analyzed, which is beneficial to the development of bio composites.

Experiment and methodology

Materials

Hyperbranched polyamide (HPN-202) was purchased from Wuhan Hyperbranched Resin Technology Co., Ltd., the properties are listed in Table 3. Polylactic acid(4032D) was purchased from American Nature Works Company. Straw fiber, with a length of 5–10 mm and a width of 1–3 mm was provided by Anhui Shangyuan Household Materials Co., Ltd. Silane coupling agent (KBM-403) was provided by Shin-Etsu Chemical Co., Ltd., Japan. Ethanol was provided by Shanghai Macklin Biochemical Technology Co., Ltd. All chemical reagents were used as is without further purification.

Table 3. Properties of hydroxyl-terminated hyperbranched polyamide HPN202.

	Molecular mass, g/mol	Hydroxyl number/mol	Hydroxyl value, mg KOH/g	Acid value, mg KOH/g	Melting point,°C
HPN202	2700	12	250	<80	100–120

Straw fiber treatment process

Firstly, straw fiber was washed with clean water for 4–5 times to remove the ash and impurities, and then dried in an oven at 70°C for 12 hours. Then, ethanol solutions of HPN202 with mass fraction of 2%, 4%, 6%, 8%, and 10% were prepared. The straw fiber was soaked in the HPN202 solution for 12 hours for HPN treatment. After the surface anhydrous ethanol evaporated, they were placed in an oven and dried at 70°C for 12 hours. Additionally, straw fiber treated with 2% KH403 ethanol solution (2 wt.%) for 1hour. The straw fiber soaked in 6% HPN202 were treated with a coupling agent as co-modification. Different straw fiber treatment approaches flow as shown in Figure 2.

Figure 2. HNP treatment and HNP/CA co-treatment process.

Preparation of straw/polylactic acid composites

The prepared polylactic acid film and straw fiber were laminated, wherein the weight ratio of straw fiber to polylactic acid was 2:8. Preheating for 5 min at 180°C, and then pressing for 5 min under the pressure of 4MPa, the composite material was taken out after natural cooling to complete the preparation. The thickness of the laminated composite was 4 mm. The preparation process is shown in Figure 3.

Figure 3. Preparation process of SF/PLA composites.

Characterization

Mechanical properties test

An electronic universal testing machine (WCW-20, Jinan Tianchen Testing Machine Manufacturing Co., Ltd, China) was used for tensile and three-point bending testing. According to ISO 527–4:1997, the tensile speed was 2 mm/min, and the tensile specimen size was 150 × 10 × 4 mm. According to ISO 178–2010, the three-point bending strength was measured. The span ratio of the sample was 16:1, the specimen width was 10 mm, the specimen length was 20% longer than the span, and the test speed was 2 mm/min. According to ISO 179–1:2006, the impact strength was obtained with a Charpy impactor (XJJ-50S, Jinan Hengsi Shengda Instrument Co., Ltd, China). The energy of the pendulum was 7.5 J, the speed was 3.8 m/s, and the impact specimen size was 80 × 10 × 4 mm. All the values were calculated by taking the average of five samples.

Differential scanning calorimetry (DSC)

A DSC analyzer (DSC Q2000, TA Instruments, USA) was used to study the thermal history at a rate of 10°C/min over a temperature range of 30–200°C at a 20 ml/min N₂ flow. The glass-transition temperature (T_g), crystallization temperature (T_c), melting temperature (T_m), crystallization enthalpy (ΔH_c), and melting enthalpy (ΔH_m) were identified from the second heating process. By considering the enthalpy of fusion (ΔH_o) of a PLA crystal of infinite size as 93.6 J/g, and χPLA as the PLA content, the degree of crystallinity (χ_c) can be calculated as follows:

(1) $\chi \mathrm{c}=\frac{\left(\mathrm{\Delta }\mathrm{H}\mathrm{m}-\mathrm{\Delta }\mathrm{H}\mathrm{c}\right)\times 100\mathrm{\%}}{\mathrm{\Delta }\mathrm{H}\mathrm{o}\times {\chi }_{PLA}}$(1)

Thermogravimetric analysis (TGA)

A thermogravimetric analyzer (TGA 550, TA Instruments, USA) was used to determine the thermal stability of the unmodified and modified composites. The test was performed in an N₂ environment, in the temperature range of 30–800°C, at a heating rate of 10°C/min, and at an N₂ flow rate of 50 ml/min.

Fracture SEM analysis

A bench top scanning electron microscope (S-4800, Hitachi, Japan) was used to observe the cross-sectional morphology of the composite samples. After 20 s of gold sputtering by ion ejection, the samples were scanned and observed at an accelerating voltage of 10.0 kV.

Water absorption and thickness expansion test

According to the requirements of GBT 1034–2008, the sample completely immerse in water. The soak time were 0 h, 24 h, 48 h, 72 h, 96 h, 120 h, 144 h and 168 h. After each sampling, the average value of water absorption and thickness change percentage of at least three samples shall be taken as the final water absorption and expansion rate of the sample.

Results and discussions

Effect of HPN concentration on the mechanical strength of HPN-SF/PLA

Figure 4 reveals the tensile, bending, and impact mechanical strength of HPN-SF/PLA with concentrations of 2%, 4%, 6%, 8%, and 10%. The best mechanical properties based on static strength were achieved when the concentration of hyperbranched polyamide was 6%. Compared with SF/PLA (untreated by HPN), its tensile, bending and impact strength were increased by 14.1%, 14.4% and 8%, respectively. The abundance of hydroxyl groups in hyperbranched polymers will cause hydrogen bonding with the hydroxyl groups in cellulose and create intermolecular network connections (Shaorong et al. 2013). In addition to network construction, the enhancement of these properties may be due to intermolecular networks generated by hydrogen bonding between the HPN and the straw surface. Due to the hydroxyl-terminated hyperbranching having more active groups, it can form more hydrogen bond structures with the hydroxyl groups on the surface of the straw fiber and in the PLA. These factors lead to increased interfacial compatibility (Zhao, Sun, and Tang 2022).

Figure 4. Effect of HNP concentration on the mechanical strength of HNP-SF/PLA.

The mechanism of interfacial enhancement by hyperbranched polymers has been discussed by many academics, but the effect of hyperbranched polymer concentration on performance has rarely been considered. We found that the static mechanical strength of HPN-SF/PLA decreased after the concentration of HPN exceeded 6%. This implies deterioration of the interface properties. It can be evidenced by Figure 5, there are significant interface breaks appear on the untreated, 8% and 10% HPN-SF/PLA, as indicated by the arrows. HPN increases the length of the molecular chain between the straw fiber and the PLA and forms a flexible interface (Shuiping et al. 2016; Shuiping et al. 2016. However, when excessive amounts of HPN202 will form a separate phase (Lu et al. 2013), this will lead to stress concentration and reduce its mechanical properties when subjected to external forces.

Figure 5. SEM images of tensile fracture sections of HNP-SF/PLA with different HNP concentrations: (a).0% (b).2% (c).4% (d).6% (e).8% (f).10%.

Effect of treatment methods on the mechanical strength of SF/PLA

From the Figure 6, the tensile, bending and impact properties of straw fiber after three kinds of modification are 10.3MPa, 9.6MPa and 10.6MPa respectively, which is an increase of 21.2%, 12.9% and 24.7% respectively compared to the unmodified straw fiber. The bending strength is 26.1MPa, 26.9MPa and 27.4MPa, and the bending performance increases by 11.1%, 14.4% and 16.6% respectively. The impact strength is 1.40 KJ/m², 1.41 KJ/m² and 1.44 KJ/m², and the impact performance is increased by 7.7%, 8.4% and 10.8%, respectively. These treatment methods all contribute to improving the mechanical properties. The mechanical properties of the co-modified composites were the best, mainly due to the coupling reaction between the terminal hydroxyl group of hyperbranched polyamide and silane coupling agent. Under the same conditions, HPN can graft more silane coupling agent, thus reducing the polarity of the straw fiber surface, thus further improving the interfacial compatibility. Within the appropriate amount of hyperbranched polymers, the hydrogen bonding cross-linked network structure plays a dominant role. After combined treatment of hyperbranched polymer and coupling agent, due to the existence of hyperbranched polymer, HPN on the surface of straw fiber can be grafted with more silane coupling agent to reduce the polarity of straw fiber surface. At this time, silane coupling agent and polylactic acid react to form a new covalent bond. As a result, the synergistic effect of hyperbranched polymers and coupling agents enhances their interfacial strength. The schematic as depicted in Figure 7.

Figure 6. Effect of treatment methods on the mechanical strength of SF/PLA.

Figure 7. Schematic diagram of co-modification of HNP and CA.

Thermal analysis

Figure 8 and Table 4 shows the DSC curves of SF/PLA composites, in which straw fibers were treated in different approaches. The crystallization peak of SF/PLA composites increased after HPN, CA and their co-treatment. The poor compatibility between untreated SF and PLA led to difficulty in PLA nucleation, while the interfacial compatibility was improved after HPN and CA treatment, which reduced the nucleation barrier (Chen et al. 2021). It can be seen from Table 4, that crystallinity of SF/PLA treated with HPN, CA, and their combination increased from 28% to 29.4%, 36.9%, and 42.3%, respectively. HPN and CA on the surface of SF provided many nucleation sites and played a role of heterogeneous nucleation, thus leading to the improvement of the crystallinity of SF/PLA composites (Zhang et al. 2022). The characteristic temperatures of polymer materials are the temperature at which the size of the deformation or the change in properties caused by the change in temperature when the polymer is subjected to external forces, which are important for the processing and forming of polymer materials. Usually, it refers to the glass transition temperature (T_g), melting temperature (T_m), crystallization temperature (T_c), decomposition temperature (T_d), etc. As can be seen in Table 4, the HPN modification causes a decrease in the T_g, T_c and T_m of SF/PLA. Compared to UN-SF/PLA, the Tg of HPN-SF/PLA decreased from 62.03°C to 48.18°C, Tc decreased from 110.28°C to 97.26°C, and Tm decreased from 158.18°C to 150.05°C. The non-crystalline component of HPN inhibits the crystal growth of PLA, which leads to the formation of small and imperfect PLA crystals and reduces the characteristic temperature of PLA (Lu et al. 2013). In contrast, silane coupling agent is a small molecular component, that can shorten the nucleation cycle during the cooling crystallization of PLA by lowering the nucleation potential, increase the crystallization rate of PLA, promote the development of single crystals, and reduce the defects in the crystallization region, so the characteristic temperature of the silane coupling agent-modified SF/PLA composites is reduced (Chen et al. 2021). Overall, the effects of HPN and CA on T_g, T_c and T_m have opposite trends, and the temperature of the co-modified SF/PLA is slightly reduced.

Figure 8. DSC curves of SF/PLA composites.

Table 4. Thermal transition temperature of co-modified SF/PLA composites.

Sample	Tg (°C)	Tc (°C)	ΔHc (J/g)	Tm (°C)	ΔHm (J/g)	Χc (%)
UN-SF/PLA	62.03	110.28	2.98	158.18	23.97	28.0
HPN-SF/PLA	48.18	97.26	11.86	150.05	33.92	29.4
CA-SF/PLA	61.46	113.65	3.34	158.72	30.96	36.9
HPN+CA-SF/PLA	53.76	109.48	2.45	156.21	34.09	42.3

Thermal stability is an important indicator of the application temperature range of the composite. By comparing the initial degradation temperature (T_d5%), the maximum degradation temperature (T_max) and the yield of carbonaceous residues, the differences in the effects of modification on the thermal stability of SF/PLA can be better understood. Thermogravimetric (TG) and derivative thermogravimetric (DTG) measurements for SF/PLA have been adopted in Figure 9 and summarized in Table 5.

Figure 9. Thermal curves of SF/PLA composites: (a).TG; (b). DTG.

Table 5. Thermal decomposition data of co-modified SF/PLA composites.

	Degradation temperature (°C)
	Td5%	Tdmax	Residual wt.% @ 790°C
UN-SF/PLA	262.96	317.53	1.58
HPN-SF/PLA	260.23	310.33	0.65
CA-SF/PLA	289.26	345.76	1.6
HPN+CA-SF/PLA	274.71	335.47	1.44

It shows three stages of decomposition. The first stages occur at 30–200°C with a slight weight reduction, corresponding to the evaporation of water. The most significant weight loss was observed in the second phase, due to the decomposition of cellulose, hemicellulose and some lignin, at 250–400°C. The third stage of decomposition (400–600°C) consists mainly of the decomposition of lignin and PLA. In the second stage, the T_d5% and T_dmax of the CA-SF/PLA are increased significantly. Specifically, the T_d5% of HPN-SF/PLA increases by 26.3°C, while the T_max increases by 28.23°C compared to UN-SF/PLA. This is attributed to the improved interfacial compatibility of the composite by the silane coupling agent modification, which increases the interdependence of SF and PLA and improves the overall thermal stability (Chen et al. 2021; Lu et al. 2013). HPN-modified SF/PLA showed a slight decrease in T_d5% and T_max because of its inherent poor thermal stability HPN202 (melting point of 100–120°C). This result is consistent with previous studies (Lu et al. 2013; Zhao, Sun, and Tang 2022). However, the T_d5% and T_max of the SF/PLA were enhanced after grafting of HPN with CA. The T_d5% of HPN+CA-SF/PLA increased from 262.96°C to 274.71°C, and the T_max increased from 317.7°C to 335.47°C. It is worth noting that the addition of HPN can significantly reduce the yield of carbonaceous residues.

Water absorption and thickness expansion rate

The water absorption of natural fiber reinforced composites is an important argument for their application. Figure 10 compared the water absorption rate and thickness swelling rate of different modified SF/PLA composites with the change of time. Plant fiber reinforced composites in general do not have favorable water resistance. There are two main ways of water absorption in SF/PLA composites. Firstly, because of the large number of hydrophilic hydroxyl groups on the surface of SF, SF itself has a strong water absorption. In addition, there is a certain gap between the compatible interface between SF and PLA, which provides space for water molecules to enter and store (Chen et al. 2021). As can be seen from Figure 10, when the specimens were immersed in water for 72 hours, the increase in water absorption and thickness swelling rate changed significantly, and the increase after 72 hours was very slow. Silane coupling agent modification, hyperbranched polymer modification and combined modification all improve the water resistance of SF/PLA composites. Among them, the combined modification has the most obvious effect. The reaction between the silane coupling agent and the hydroxyl groups on the straw surface reduces the polarity of the straw, resulting in a weaker ability to attract water molecules. At the same time, the coupling agent can improve the interfacial bonding ability between straw fiber and PLA, making it difficult for water molecules to enter the interior of the composite. Although HPNalso reduces the straw polarity and increases the interfacial bonding between straw and PLA through the hydrogen bonding network structure, HPN itself contains more terminal hydroxyl groups, which introduce water molecules. Therefore, compared with theCA-SF/PLA, the water resistance of the HPN-SF/PLA composites is worse.

Figure 10. Comparison of water absorption properties of SF/PLA composites (a). Water absorption rate; (b). Thickness swelling rate.

Conclusions

Herein, the effect of modified SF with different weight contents of HPN on the mechanical properties of SF/PLA was investigated. Based on the optimum weight content of HPN, SF was treated with HPN, CA and a combination of both, and then prepared into SF/PLA composites. The effects of different modification treatments on the mechanical strength, crystalline properties, thermal degradation properties and water resistance of SF/PLA composites were investigated. The study results show that:

• With the increase of HPN concentration, the tensile, flexural and impact strengths of HPN-SF/PLA showed an increasing, then a decreasing trend. When the concentration of HPN was 6%, the best mechanical performance was achieved with a better combination of straw fiber and PLA.

• Both 6% HPN modification and 2% silane coupling agent modification and their combined modifications contributed to the improvement of the mechanical strength of the SF/PLA composites. HPN can form a hydrogen bonding network structure between the straw and the PLA matrix, while the silane coupling agent plays a bridging role. Combined modification can make more hydroxyl groups attached to the fiber surface to form a synergistic effect, and the combined modification enhances the best effect. Compared with the UN-SF/PLA, the tensile strength, flexural strength and impact strength of the HPN+CA-SF/PLA composites were increased by 24.7%,16.6% and 10.8%, respectively.

• The crystallinity of SF/PLA treated with HPN, CA, and their combination increased from 28% to 29.4%, 36.9%, and 42.3%, respectively. HPN and CA provide many nucleation sites for the crystallization of PLA and act as non-uniform nucleation. However, the low melting point of HPN causes a decrease in the Tg, Tm, Tc, Tg5% and Tmax characteristic temperatures of SF/PLA. The characteristic temperatures can be enhanced by adding CA for co-modification.

• Since HPN and CA can reduce the number of hydroxyl groups on the SF surface and enhance the interfacial bonding properties between the straw and PLA, they can enhance the water resistance of the SF/PLA composites. The best effect was enhanced by the combined modification.

Highlights

• Hyperbranched polymer treatment of straw fiber surface

• Improvement of HNP-SF/PLA composites with silane coupling agent treatment

• Mechanical property analysis in terms of tensile, bending, and flexural strengths

• Combined treatment with HNP and CA significantly improve the water resistance of SF/PLA composites

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the Research projects for Undergraduates of Anhui Polytechnic University [2022DZ19]; Anhui Province University Excellent Talent Cultivation Project [gxgnfx2021133]; Central government guide Anhui Province Science and Technology Development Project [202107d06020014]; Scientific Research Projects to Promote Undergraduate Talent Training [FFBK202207, FFBK202322].