Improvement of mechanical and physical performance of poly (lactic acid) biocomposites by application of surface silanization for huntite

Abstract

Silanization process was applied to huntite–hydromagnesite (HH) mineral in for improve its adhesion to poly (lactic acid) (PLA) matrix. Surfaces of HH samples were examined by infrared spectroscopy and energy-dispersive X-ray spectroscopy analysis. HH was compounded with PLA using melt-blending technique. Modified HH incorporations led to nearly 19% and 20% increase in tensile strength and modulus of PLA, respectively. The maximum improvement for tensile strength value is obtained for 15 wt% of modified HH containing composite sample. On the other hand, percentage strain of PLA decreases nearly 22% with the addition of HH. Silane-modified HH-containing composites yield higher impact energy compared to untreated HH. Impact performance of composites increases nearly 18% with the increase in loading ratio of HH. Silane-modified HH displays higher storage modulus than HH, and glass transition temperature of PLA shifts to higher levels with the inclusion of HH. HH-filled composites exhibit no significant difference with melt flow rate value of PLA. Water resistance of silane-treated HH-containing composites are found to be higher than untreated HH attribitued to hydrophobicity of silicon-rich surface. Accordingly, composites that include modified HH are suitable for outdoor applications. Morphological investigations confirm that dispersion homogeneity and strong adhesion are achieved due to the improvement of surface interactions between HH and PLA phases after silane treatment process.

Keywords

Poly (lactic acid)biocomposites melt compounding surface silanization

Introduction

Researchers from scientific and industrial fields focused on environmentally-freindly materials due to ecological problems in recent years. Development of biocomposite materials that consist of renewable and natural sourced components exhibited increasing trend for their possible replacement with petroleum-based products. Recyclability, weight reduction, and practical productions are the major advantageous points of biocomposites. For these reasons, these kind of materials became favorable in various industrial areas such as packaging, construction, medical, and transportation.^1–4 However, achievement of compatible interface between two phases in the composite system plays a key role for biocomposites to obtain desired properties in their applications.^5–7 Surface silanization is the most cost-effective and conventional method to donate compatible surface between reinforcement and polymer matrix.^8–10

Poly (lactic acid) (PLA) is a commercial and widely used biopolymer, especially on the fabrication of biodegradable packaging films.^11–13 Reinforcement of bio-based polymers with natural additives provides sustainable and biodegradable products. However, poor toughness, low melt strength, and narrow processing window of that kind of materials are the main limitations for manufacturers.¹⁴

Minerals serve as reinforcing agents for bioplastics since they have low cost and naturally occurring characteristics.¹⁵ Huntite–hydromagnesite (HH) is the mixture of huntite and hydromagnesite minerals which refer to magnesium calcium carbonate and hydrated magnesium carbonate, respectively. Huntite mineral does not respond to silane treatments due to the inertness of magnesium calcium carbonate, whereas hydromagnesite mineral can be treated with silane coupling agents since hydrated magnesium carbonate has the ability to react with silane coupling agents.^16,17 HH is a mineral that is mostly used to achieve effective fire performances for polymers. HH has been studied as an additive for several polymeric materials including polyethylene,^18,19 polypropylene,^20,21 polyester,²² polyvinylchloride,²³ polyurethane,^24–26 and ethylene vinyl acetate copolymer.^27–29 Based on the literature survey, PLA-based composites filled with HH mineral has not been studied yet. This study is a novel research work regarding the investigations on properties of HH-loaded PLA composites first time.

The main target of this current study is the improvement of surface adhesion between the PLA matrix and the mineral phase by applying surface silanization process. Surface properties of neat and treated HH samples were examined by infrared spectroscopy and scanning electron microscopy (SEM) integrated elemental energy-dispersive X-ray spectroscopy (EDS) analysis. Melt compounding and injection molding processes were used for the fabrication of biocomposites. Characterizations of PLA and its composites were performed based on mechanical, thermomechanical, water absorption, melt flow, and morphological tests. Findings in the case of mechanical and physical performance of HH mineral-filled PLA biocomposites provide novel scientific and technical data in industrial fields including mainly packaging, medical, and outdoor applications.

Experimental

Materials

The commercial PLA with a trade name of Ingeo biopolymer 6100D was supplied from Natureworks LLC, Minnetonka, Minnesota, USA. It has density of 1.24 kg m⁻³ and relative viscosity of 3.1 according to CD Internal viscotek method. HH (Ultracarb LH15) was purchased from Likya Mining (Turkey). It is composed of nearly 60 wt% hydromagnesite (Mg₅(CO₃)₄(OH)₂·4H₂O) and 40 wt% huntite (Mg₃Ca (CO₃)₄). HH has density and surface area of 2.5 g cm⁻³ and 11–13 m² g⁻¹, respectively. Reagent-grade ethanol and silane coupling agent (3-aminopropyltriethoxysilane) were obtained from Merck AG (Germany).

Surface treatment of HH

Silanization process was applied to HH samples according to similar silane modification routes in the literature.^30–33 HH powders were mixed continuously in 2 wt% of silane coupling agent/ethanol solution for 2 h at room temperature. After mixing, samples were dried at 70°C in vacuum oven for 4 h. Neat and silane-modified HH samples were named as HH and HH (S), respectively.

Production of composites

PLA-based composites were fabricated using lab-scale 15 cm³ DSM Xplore micro-compounder at 210°C for 5 min. The mixing speed was 100 r min⁻¹. The unfilled PLA was mixed with the same parameters and named as PLA. Loading ratios of HH and HH (S) in PLA were 5, 10, 15, and 20 wt%. Dog-bone-shaped test samples with dimensions of 80 × 7.5 × 2.5 mm³ were prepared by micro-injection molding machine of Daca Instruments (Santa Barbara, California, USA). The barrel and mold temperatures were 215°C and 60°C, respectively.

Characterization methods

Infrared spectroscopy in attenuated total reflectance mode was conducted to examine the surface characteristics of neat and silane-treated HH powders by Bruker Optics (Billerica, Massachusetts, USA), 66/S series. Tensile tests were done using Lloyd LR 30 K universal tensile testing machine with 5 kN load cell and a crosshead speed of 5 cm min⁻¹. Tensile strength, percentage strain, and tensile modulus parameters were recorded as the minimum of three samples with standard deviations. Impact test was performed using Coesfeld material impact tester with the 4 J pendulum. Dynamic mechanical analysis (DMA) studies in dual cantilever bending mode were carried out using Perkin Elmer DMA 8000 (Waltham, Massachusetts, USA). Temperature ranged from −150°C to 200°C during DMA test with a heating rate of 10°C min⁻¹ under nitrogen atmosphere. Melt flow rate (MFR) values were measured by Meltfixer LT, Coesfeld (Germany). The tests were performed with the standard load of 5 kg at 210°C. During water absorption study, test samples were conditioned and immersed in water bath at room temperature. They were taken out from water and weighted periodically. The surfaces of HH powder and composites fractured after impact test were photographed by FEI (Hillsboro, Oregon, USA) Quanta 400F FESEM scanning electron microscope (SEM). Elemental compositions of HH surfaces were studied by EDS analysis applied in indicated spots of SEM images.

Results and discussion

Surface properties of HH samples

SEM images of neat and surface-treated HH samples can be seen in Figure 1. Elemental analysis data investigated in indicated spots of SEM images are listed in Table 1. According to EDS data, oxygen and silicon compositions of HH show slight increment after surface silanization stem from the existence of Si and O containing silane coupling agent.

Figure 1.

SEM images of HH samples: (a) HH and (b) HH (S).

Table 1.

SEM/EDS results of huntite samples.

Samples	O Wt%/At%	Mg Wt%/At%	Ca Wt%/At%	Si Wt%/At%
HH	68.92/77.96	26.24/19.80	4.58/2.07	0.26/0.17
HH (S)	69.35/78.25	26.01/19.56	4.16/1.88	0.48/0.31

SEM: scanning electron microscopy; EDS: energy-dispersive X-ray spectroscopy; O: oxygen; Mg: magnesium; Ca: calcium; Si: silicon; HH: huntite–hydromagnesite; HH (S): silane-modified huntite–hydromagnesite.

IR spectrum of HH and HH (S) samples and indicative peaks due to specific functional groups are displayed in Figure 2. The characteristic peaks of huntite portion can be seen from the 600, 710 and 850 cm⁻¹ wave numbers. These bands are indicative peaks of O–C–O stretching vibrations of carbonate $({CO}_{3}^{2 -})$ ion into HH.^34–37 These characteristic carbonate-related bands are seen for both HH and HH (S) samples with equal intensities. Additionally, the intense peak at approximately 1410 and 1500 cm⁻¹ wave numbers corresponds to the vibration of carbonate ion existence in huntite part of HH.^38–41 It can be obtained from the spectra of HH (S) that there are shoulder peaks around 800 cm⁻¹. The shoulder peaks at 950 and 1200 cm⁻¹ wave numbers in IR spectrum of HH (S) arises from the Si–O–Si stretching vibrations of siloxane groups.^42–44 These peaks can be seen only for HH (S) sample which are attributed to Si–O vibrations. This finding implies that the formation of aminosilane–oxide layer on the interface of HH after silane treatment.^45,46 The peaks located in the region around 3500 cm^–1 wave numbers assign to oxygen-containing vibrations due to the hydroxyl group.^47–50 It can be clearly seen that neat HH sample gives higher intensities compared to HH (S). The presence of the peaks around 1620, 2800, and 3700 cm⁻¹ in IR spectrum of HH (S) sample correspond to vibrations of C=C, C–H and N–H groups, respectively. These bands are observed after silane treatment since methylene and amino functional groups exist in silane coupling agent.^51–54 These results indicate that neat HH surface was covered by silane coupling agent after silanization process. Hydromagnesite portion in HH is responsible to form siloxane bonds during silane treatment since hydrated magnesium carbonate can react with silane coupling agent.^16,17

Figure 2.

FTIR spectra of HH samples.

Tensile test

Tensile test parameters of PLA and composites are listed in Table 2, and the characteristic stress–strain curves are presented in Figure 3.

Table 2.

Tensile test results.

Samples	Tensile strength (MPa)	Tensile modulus (GPa)	Elongation at break (%)
PLA	58.4 ± 1.6	1.0 ± 0.1	11.6 ± 0.5
PLA-HH 5	61.4 ± 1.3	1.0 ± 0.1	9.0 ± 0.3
PLA-HH 10	63.5 ± 1.4	1.1 ± 0.1	9.2 ± 0.2
PLA-HH 15	64.9 ± 1.2	1.1 ± 0.1	9.3 ± 0.4
PLA-HH 20	64.6 ± 1.5	1.1 ± 0.1	8.9 ± 0.3
PLA-HH(S) 5	65.2 ± 1.0	1.1 ± 0.1	8.9 ± 0.5
PLA-HH(S) 10	67.7 ± 1.2	1.2 ± 0.1	8.8 ± 0.3
PLA-HH(S) 15	69.3 ± 1.3	1.2 ± 0.1	8.2 ± 0.4
PLA-HH(S) 20	68.9 ± 1.1	1.2 ± 0.1	8.7 ± 0.2

PLA: poly (lactic acid); HH: huntite–hydromagnesite; HH (S): silane-modified huntite–hydromagnesite.

Figure 3.

Stress–strain curves of composites.

Tensile strength of unfilled PLA improved as the loading ratio of HH increases. This increasing trend reaches to maximum at the optimum concentration of 15 wt%. Further additions of HH and HH (S) led to a slight reduction in tensile strength values due to formation of agglomeration at the highest loading level. Nearly 6% increase in tensile strength values are observed for HH (S)-containing composites compared to PLA-HH composites at their identical concentrations. Enhancement of interfacial adhesion between the HH surface and the PLA matrix yields higher strength values with the help of silane treatment.^55,56 In contrast, elongation of unfilled PLA decreases with the addition of HH, regardless of neat or treated samples. Percantage strains of composites are found to be reduced nearly 20% with respect to percantage strain of PLA. As a difference from the similar studies in which remarkable decrease for modulus of polymer matrix obtained,^22,24 additions of HH powders cause a slight increase in tensile modulus of unfilled PLA. Surface-treated HH-containing composites display relatively higher modulus values than untreated HH.

Impact test

Impact strength values of PLA and its composites are demonstrated in Figure 4. Inclusions of HH and HH (S) cause reduction for impact strength of unfilled PLA in the range of 20–35%. However, HH (S) additions give higher impact performance compared to the untreated HH-filled PLA. Silane-treated HH-containing composites exhibit about two points higher impact strength values than PLA-HH samples at their same filling ratios. These findings are related with the improvement of adhesion between the two phases, thanks to the formation of silane layer on the HH surface after treatment.^57,58

Figure 4.

Impact test results of composites.

Dynamic mechanical analysis

Storage modulus and tan δ curves with respect to temperature are illustrated in Figure 5(a) and Figure 5(b), respectively. It can be observed from Figure 5(a) that storage modulus curves decrease nearly 70°C immediately which indicates the glass transition temperature (T_g) of PLA. Silane-modified HH-containing composites show higher storage modulus relative to untreated HH-containing samples. The highest storage modulus is observed for PLA-HH (S) 5 among the samples. This composite displays the higher result than unfilled PLA in terms of storage modulus.

Figure 5.

DMA test results of composites: (a) storage modulus and (b) tan δ.

The peak temperature of tan δ curve indicates the T_g of PLA as presented in Figure 5(b). According to tan δ curves, T_g value of unfilled PLA increases with the inclusions of HH and HH (S) because of the restriction of chain motions.^59–61 The highest improvement for T_g value is found for PLA-HH (S) 5 sample among the composites. The height of the tan δ signal is related with the ability of the material to convert the exposed vibrational energy into the heat form.^16,61 HH (S)-loaded composites give slightly higher tan δ values compared to PLA-HH. Based on this finding, it can be said that silane-modified HH-containing composites exhibit more vibration damping behavior compared to unmodified HH-containing ones.

MFR measurements

MFR values are measured to investigate the effect of HH addition to melt flow properties of PLA based biocomposites. Figure 6 represents the MFR values of unfilled PLA and its composites. MFR values of all samples are found in a narrow range which indicates that composites are suitable for the large-scale applications. HH additions cause a slight reduction in MFR value of PLA. This decreasing trend continues as the filling ratio of HH increases. However, no obvious difference for MFR values is observed between PLA and composites. HH (S)-filled samples yield slightly higher MFR compared to HH-containing ones due to their better adhesion to the PLA matrix.

Figure 6.

MFR test results of composites.

Water absorption study

According to Table 3, among water uptake values of PLA and composites listed, unfilled PLA absorbs only about 1% water during immersion for time period of 15 days. Composite samples achieve their maximum water uptake values in 6 days and remain constant during the test as demonstrated with water absorption curves in Figure 7.

Table 3.

Water absorption test results.

Samples	Water absorption (%)
PLA	0.9 ± 0.1
PLA-HH 5	1.3 ± 0.1
PLA-HH 10	1.6 ± 0.1
PLA-HH 15	1.8 ± 0.1
PLA-HH 20	2.1 ± 0.2
PLA-HH (S) 5	1.1 ± 0.1
PLA-HH (S) 10	1.3 ± 0.1
PLA-HH (S) 15	1.4 ± 0.1
PLA-HH (S) 20	1.6 ± 0.2

PLA: poly (lactic acid); HH: huntite–hydromagnesite; HH (S): silane-modified huntite–hydromagnesite.

Figure 7.

Water absorption curves.

Addition of HH into PLA causes uptake of higher amount of water. Water absorption of composites increase with the loading level of HH. HH (S)-reinforced composites display remarkably lower water absorption values than HH-filled composites at their identical concentrations. Improvement of weight is related with the diffusion of water into material as suggested by Fick’s law of single-phase diffusion model.⁶² The lowest water uptake is observed for PLA-HH (S) 5 sample among the composites. Water uptake values of the composite exhibit reduction after silanization process which leads to enrichment for the surface hydrophobicity of HH.^63–66

Morphology of composites

SEM images of composites with the lowest (5%) and the highest (20%) filling ratios are given in Figure 8. It can be clearly observed that pristine HH particles stick together and form large agglomerates. Debonding formations between inert HH surface and polymer matrix are also observed for neat HH samples. On the other hand, silane-modified HH particles display homogeneous dispersion inside PLA phase. The surface of HH particles are surrounded by PLA phase, thanks to silanization as confirmed by IR study in previous discussion. Dispersion homogeneity is reached for the highest concentration of HH (S) where particle–particle adhesion is predominant for untreated HH-containing composite for the same concentration. These findings are correlated with the surface properties characterized by IR analysis that enhancement of surface interaction between HH particles and PLA matrix after silanization process.

Figure 8.

SEM images of composites: (a) PLA-HH 5%, (b) PLA-HH (S) 5%, (c) PLA-HH 20%, and (d) PLA-HH (S) 20%.

Conclusion

This study deals with the evaluation of mechanical, physical, and morphological performance of HH-filled PLA biocomposites after surface silanization of HH mineral. IR spectroscopy and EDS analysis reveal that surface of HH is covered by silane layer by applied modification process. Tensile test results display that treated HH-filled composites give higher tensile strength and impact strength values compared to untreated HH-containing samples. The optimum concentration is obtained as 15% for composites in the case of tensile strength values. Tensile modulus increases where percent elongation show reduction trend with HH and HH (S) inclusions. According to DMA results, additions of treated and untreated HH cause a decrease in storage modulus of PLA with the exception of 5% HH (S)-loaded composite. T_g of PLA shifts to higher values with the incorporation of HH (S). MFR values of composites are found nearly identical with that of unfilled PLA which indicates that processing of composites can be accomplished practically. According to water uptake test data, silane-modified HH-containing composites exhibit lower water absorption compared to untreated HH due to the improvement of hydrophobicity of HH after silanization. HH (S)-filled composite samples are found to be applicable in outdoor environments since they gain water resistance. SEM study confirms that HH (S) particles display homogeneous dispersion into PLA phase. These results show that the enhancement of surface interactions between PLA matrix and HH particles lead to higher mechanical and physical performance of composites, thanks to surface treatment of HH with silane coupling agent.

Footnotes

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Ali Sinan Dike

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Davies

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. Seawater ageing of flax/poly(lactic acid) biocomposites. Polym Degrad Stabil 2009; 94: 1151–1162.

Improvement of mechanical and physical performance of poly (lactic acid) biocomposites by application of surface silanization for huntite–hydromagnesite mineral

Abstract

Keywords

Introduction

Experimental

Materials

Surface treatment of HH

Production of composites

Characterization methods

Results and discussion

Surface properties of HH samples

Tensile test

Impact test

Dynamic mechanical analysis

MFR measurements

Water absorption study

Morphology of composites

Conclusion

Footnotes

Funding

ORCID iD

References