Organically modified mesoporous silica as a performance additive in tubular quenched polypropylene films

Abstract

Mesoporous silica nanoparticles were synthesized by base-catalyzed hydrolysis of tetraethyl orthosilicate in the water-acetone medium. For surface modification of mesoporous silica, commercially available silane, 3-(trimethoxysilyl)propyl methacrylate was reacted with mono-thioglycerol via 2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl (TEMPO) aided thiol-ene reaction, and the obtained silane was incorporated on the surface of neat mesoporous silica to graft extended alkyl chains terminating in hydroxyl groups. The prepared MSNs were characterized by thermal gravimetric analysis (TGA), Fourier transform infrared spectroscopy (FT-IR), nitrogen adsorption-desorption analysis, scanning electron microscopy, and dynamic light scattering. These MSNs were compared to conventionally used commercial silica as an additive in tubular quenched polypropylene films (TQPP) for their anti-blocking, thermal, mechanical, optical, and barrier properties. Organic modification of MSNs decreased the blocking force in TQPP films. The mechanical properties including % elongation and tensile strength increased for TQPP films with mesoporous silica, while it decreased for films containing commercial silica. Both modified and unmodified MSNs decreased haze and enhanced gloss in TQPP films, showing improved optical properties as compared to commercial silica particles. In TQPP film with modified MSNs, a reduced value of the water contact angle was obtained, indicating better wettability of the surface of the TQPP film.

Keywords

Antiblocking additive functionalized silica mesoporous silica thiol-ene reaction tubular quenched polypropylene films

Introduction

Plastic films provide several benefits over rigid forms of packaging materials available, as they are lightweight with appreciable barrier properties, and can be reused or recycled. According to the Research and Markets global survey for 2021–2026, the plastic film market is expected to rise from USD 184 billion in 2020 to around USD 220 billion by 2026. Polypropylene (PP) is known for its excellent properties, such as high chemical and corrosion resistant nature, low cost, and easy processability. PP films have good heat resistance and therefore can be sterilized at high temperatures, making it an attractive candidate for packaging films. Apart from this, PP is translucent, possesses good flexibility, tensile strength, and high gloss.^1,2

Nanoparticles have been utilized as additives in preparing packaging films to improve mechanical, barrier, and thermal properties.^3–9 In the last few decades, researchers have worked towards developing high performance coatings or films, by combining the properties or applications of organic polymers with versatility offered by inorganic materials. However, nanoparticles can often lead to aggregation owing to their small size, that can lead to deterioration of inherent properties of the resin. A common concern encountered by manufacturers is the blocking phenomenon, which can be described as sticking of polymer films together due to Van der Waal’s forces of attraction. The adhesive force increases when the distance between two adjacent films is decreased, or the films are pressed together. To counter this issue, anti-blocking additives, such as silica, magnesium silicate, calcium carbonate, aluminum silicate etc. are added to the polymer composite formulation. Silica is known for its abundance in terms of availability as well as for its existence in amorphous, crystalline, and colloidal forms. Silica particles are used commercially as efficient anti-blocking agents as they create roughness on the polymer surface and minimize the contact or adhesion between two adjacent films. It is also popular for its reinforcing properties in elastomers, use in adhesives, cosmetics, as an adsorbent or dehumidifying agent, and in the food industry as an anti-caking agent to control the rheological properties of the product.^10,11 They also display other interesting properties, including good chemical and thermal stability, permeability, low refractive index (similar range as polyolefins), and high surface area.¹² The incorporation of silica particles has proven to improve a wide range of properties of the base polymer, such as, electrical and mechanical properties, weathering and thermal resistance, optical properties, flame retardancy, impact properties, and abrasion resistance.^13–15 The chemical purity conferred by the synthetic silica particles finds applications in food and medicine packaging as it is devoid of any toxins and heavy metals. Literature reports have suggested that the incorporation of silica nanoparticles in PP resins can potentially improve the lifetime of the product under temperature and high-field stresses, as well as insulating and dielectric properties.^16–18 Crosby et al. evaluated the performance of combination of different types of commercially available silica and other anti-blocking agents and analyzed their performance in terms of openability of tubular quenched water polypropylene (TQPP) films, and as clarifying agents. They observed that formulations with anti-blocking agents with higher particle size of silica resulted in lowest coefficient of friction or best anti-blocking property but resulted in poor optical properties, as overall haze in the film increased. On the other hand, formulations with silica of relatively smaller particle size resulted in lower haze or better clarity but were not efficient enough as anti-blocking agents.¹⁹ Bellel et al. deposited thin silica films on polypropylene surface through plasma induced chemical vapor deposition method and observed that the enhanced polarity resulted in its improved wettability.²⁰ Jankong and co-workers observed an increase in the tensile strength of PP films from 23 MPa to 26 MPa at 5% loading of silica nanoparticles but observed phase separation and agglomeration at higher loading of silica. They also observed an increase in oxygen permeation because of the smaller spherulite size of PP after incorporation of silica.²¹ Mesoporous silica has been known to provide an edge in improving the properties of polymer composites as its inherent pore volume provides easy access to the polymer chains. It is used in several wide-ranging applications, including polymerization, drug-release, catalysis, membrane separation etc. The extensively high surface area of mesoporous substrates allows effective surface modification to alter the bulk properties as desired.^22–24

Though silica nanoparticles have been extensively explored as additives to improve various properties of PP films, mesoporous silica hasn’t been studied for PP film formulations. In this paper, we have synthesized mesoporous silica particles and analyzed its performance as an additive on thermal, mechanical, and optical properties of tubular quenched polypropylene (TQPP) films as compared to the commercial silica grade used commonly in industries for preparing TQPP films. To improve the dispersion of mesoporous silica in PP matrix and decrease agglomeration, its surface was modified to graft an extended alkyl chain terminating in hydroxyl groups. The incorporation of non-modified (control sample) and modified mesoporous silica particles improved the tensile strength as well as % elongation of TQPP films, while commercial silica deteriorated its mechanical properties. Enhancement in gloss and clarity (low haze) was also observed with both kinds of mesoporous additives, while commercial silica increased haze of TQPP films owing to agglomeration. Surface modification of mesoporous silica also decreased the blocking force in TQPP films as compared to control mesoporous silica. To the best of our knowledge, an optimum balance of optical, mechanical, barrier properties as well as anti-blocking ability hasn’t been reported with mesoporous silica particles in PP films.

Experimental section

Materials

PP Homopolymer grade PP1100FS, was received from Indian Oil Corporation Limited, Panipat Refinery, India in powder form without containing any additives. Commercial grade silica grade Sylobloc 45H was received from W.R. Grace. Tetraethyl orthosilicate (TEOS, 98%) and 3-(trimethoxysilyl)propyl methacrylate were supplied by TCI, India. Methanol (HPLC grade) and liquor ammonia (30%) were obtained from Fischer Scientific. Cetyltrimethylammonium bromide (CTAB, 98%) was procured from Spectrochem, India. Mono-thioglycerol was procured from Sigma-Aldrich. Solvents used included ethanol (99.9%; Analytical CS reagents), anhydrous toluene (99.5%; from SRL), and acetone (from Merck). All the chemicals were used as received.

Synthesis of control and organically modified mesoporous silica

Synthesis of control mesoporous silica

Control mesoporous silica (CMS) was synthesized via base-catalyzed hydrolysis of silane precursor TEOS in a water-acetone solution mixture using CTAB as a template. A typical procedure involved the dissolution of 35 mmol of CTAB in 640 mL of distilled water, followed by the addition of 370 mL of acetone and 200 mL of liquor ammonia solution. The reaction mixture was stirred at 30 °C for a duration of 20 min. When CTAB was completely dissolved, 250 mmol of TEOS was added to the reaction which resulted in immediate precipitation of silica particles. The reaction mixture was stirred for another 2 h at the same temperature. The silica particles obtained were precipitated by centrifugation at 7000 rpm, followed by 3–4 times washing with distilled water and twice with ethanol. The precipitated silica particles were collected and dried in a hot air oven at 80 °C. After drying, silica powder was collected and dried at a temperature of 160 °C for 2 h, followed by calcination at 550 °C for 8 h for complete removal of the surfactant CTAB. The product yield post-calcination was around 13 g.

Synthesis of modified mesoporous silica with alkyl chain terminating with hydroxyl groups

The first step of surface modification of mesoporous silica involved a reaction of 17 mmol of 3-(trimethoxysilyl)propyl methacrylate with 18 mmol of mono-thioglycerol in 15 mL of methanol. The reaction was stirred for 16 h at 35 °C. The formation of modified silane was confirmed using ¹H NMR spectroscopy. Further, 30 g of CMS was dispersed in 300 mL of anhydrous toluene, followed by addition of modified silane solution in methanol (approximately 21 mL). The reaction mixture was then refluxed at 120 °C for 48 h. The modified mesoporous silica with grafted alkyl chain terminating in hydroxyl groups (abbreviated as MS-OH) was collected post-precipitation via centrifugation and washed sufficiently with toluene and ethanol. The obtained MS-OH was dried for around 16 h in a hot air oven at 90 °C. The synthetic scheme for the synthesis of CMS and MS-OH has been given in Scheme 1.

Scheme 1.

Synthetic route for (A) control mesoporous silica (CMS) and (B) modified mesoporous silica with grafted alkyl chain with terminal hydroxyl groups (MS-OH).

Compounding of TQPP with silica and preparation of films

The composition of all the batches of compounded TQPP (containing 10.5 Kg of TQPP powder), consisted of the same ratio of antioxidants and slip agent erucamide, as shown in Table 1. Control mesoporous silica (CMS), modified mesoporous silica (MS-OH), and commercial silica (CS) each were added at 2750 ppm level in respective formulations. The additives in the respective batches were uniformly dispersed in a high-speed mixer, followed by their processing in a twin-screw extruder. The temperature profile of the barrel zones in the extruder have been given in Table 2.

Table 1.

Additive formulation for compounding of TQPP for preparation of films.

S. No.	Composition	TQPP (kg)	AO1010 (ppm)	AO168 (ppm)	Calcium stearate (ppm)	Erucamide (ppm)	Silica (ppm)
1	Neat TQPP powder	10.5	500	1000	500	2000	-
2	TQPP – Control mesoporous silica	10.5	500	1000	500	2000	2750
3	TQPP – Modified mesoporous silica	10.5	500	1000	500	2000	2750
4	TQPP – Commercial silica	10.5	500	1000	500	2000	2750

Table 2.

Temperature profile of barrel zones in twin-screw extruder during TQPP compounding.

Barrel zone	Z1	Z2	Z3	Z4	Z5	Z6	Z7	Z8	Z9	Die
Temperature (^oC)	140	160	180	185	190	190	195	200	200	200

The TQPP films listed above were prepared using blown-film extrusion process. The compounded homopolymer granules were fed in an extruder, and the extrudate coming out of the die was blown by air pressure and consequently quenched using water maintained at 15 °C.

Characterization techniques

Thermogravimetric analysis

Thermogravimetric analysis (TGA) for neat TQPP and TQPP-silica films was performed on Q-500 TA instrument. A sample weighing approximately 5–6 mg was analyzed at a heating rate of 10 °C/min in a nitrogen environment. All samples were dried at 90 °C overnight in a hot air oven prior to analysis.

Differential scanning calorimetry

Thermal characterization of prepared TQPP films by Differential scanning calorimetry (DSC) was carried out on Temperature Modulated Mettler Toledo DSC one star system instrument under a nitrogen atmosphere. A known sample weight of 5–6 mg was heated from 30 to 200 °C, held at 200 °C for 5 min to remove prior thermal history, and then cooled down to 30 °C. Cooling as well as a heating rate of 10 °C/min was used to record DSC thermograms.

Fourier transform infrared spectroscopy

Fourier transform infrared spectroscopy (FT-IR) spectra for the prepared mesoporous silica samples were recorded using a Thermo Nicolet IR 200 spectrometer within the spectral range of 4000-400 cm⁻¹. KBr pellets for testing were prepared as circular disc of 1 mm diameter using a hydraulic press.

Dynamic light scattering

Dynamic light scattering (DLS) measurements of mesoporous silica, as well as commercial silica, were conducted on Malvern Zetasizer NANO ZS 90. Nearly 2 mg of the sample was dispersed in 40 mL of ethanol by probe-sonication for 5 min. The reported hydrodynamic diameter is an average of at least three measurements.

Field emission scanning electron microscopy

The morphology of all the silica samples and their particle size were analyzed on JEOL JSM-6610LV at 10 kV.

Nitrogen adsorption-desorption analysis

The surface area of mesoporous silica samples and commercial silica was determined using AutosorbiQ-XR-AG-AG Viton Quantachrome BET surface analyzer. The samples were dried for approximately 12 h in a hot air oven at 90 °C. Samples were degassed in vacuum and by heating at 100 °C prior to the experiment.

Melt flow index

The melt flow rate for neat TQPP and TQPP compounded with all three types of silica in consideration was determined using Gӧttfert MI 4. The reported Melt flow index (MFI) values were recorded against a load of 2.16 kg at 230 °C.

Contact angle measurements

Specimens of dimensions 2 × 2 cm² were used to estimate the wettability of all the prepared TQPP films via sessile drop method using Drop Shape Analyser (DSA100 E). For the experiment, 2 μL drop of ultra-pure water was carefully deposited on the film surface and the angle between water and film surface was determined using Kruss Advance software. The contact angle values are reported as the mean of 20 measurements for each sample.

Optical properties

Surface Gloss was determined with BYK Gardner photometric unit of model Micro TRI/Gloss. The measurements were recorded at 60° angle as per the ASTM D-523 procedure. The % Haze in the TQPP films was estimated using Konica Minolta CM36DG Haze meter and the experiment was done as per ASTM D1003 standard.

Blocking force

Blocking Force between the adjacent film surfaces was quantified in Gram force units and the measurements were recorded according to ASTM D3354 standard using Dynisco D9047/230V Instrument.

Mechanical properties

The tensile strength and % elongation of all the prepared TQPP films was estimated using LLyods LR10 K Plus UTM instrument via procedure described in ASTM D882. Five specimens were tested for each sample and average values have been reported.

Barrier properties – water vapor transmission rate (WVTR) and oxygen transmission rate (OTR)

WVTR of all the prepared TQPP films was determined using Mocon-Permatran-W-3/33 MG Plus by procedure described in ASTM F1249 at 38 °C at 90% relative humidity. Oxygen Transmission Rate (OTR) of the films was examined using OTR Mocon 2/22H as per ASTM D3985 at 23 °C at 0% relative humidity.

Results and discussion

Characterization of mesoporous silica nanoparticles

As described above, base-catalyzed hydrolysis of TEOS in a water-acetone solution yielded CMS. In the first step of the post-modification procedure, 3-(trimethoxysilyl)propyl methacrylate was treated with mono-thioglycerol via thiol-ene chemistry using TEMPO as an initiator.^24–28 The reaction mixture was continuously stirred until the disappearance of NMR signals corresponding to vinyl protons. The synthesis of modified silane was confirmed using ¹H NMR, as shown in Figure 1. ¹H NMR (400 MHz, CDCl₃), δ ppm 4.08 (t, 2H), 3.7 (m, 1H), 3.62 (d, 2H), 3.51 (s, 9H), 2.81 (m, 1H), 2.72 (m, 2H), 2.53 (m, 2H), 1.7 (m, 2H), 1.2 (d, 3H), 0.65 (m, 2H).

Figure 1.

¹H NMR spectrum of modified silane formed after reaction of 3-(trimethoxysilyl)propyl methacrylate with mono-thioglycerol.

Thermal stability of all the silica samples was analyzed by TGA, as shown in Figure 2. The weight loss of around 17.7% for the modified mesoporous silica sample was due to the organic group grafted on the surface of mesoporous silica. The onset degradation temperature for MS-OH was around 343 °C which makes it suitable to process with TQPP. The initial dip in the weight loss observed in MS-OH is due to the water trapped in the pores of silica. The weight loss observed in the case of commercial silica can be attributed to the loss of water molecules adsorbed on the surface followed by condensation of surface hydroxyl groups.

Figure 2.

TGA curves of control mesoporous silica, modified mesoporous silica, and commercial silica.

FT-IR analysis was carried out to identify the functional groups incorporated on the surface of MSNs post organic functionalization (as shown in Figure 3). The characteristic absorption bands of silica present in both neat and modified silica particles include a broad band at 1090 cm⁻¹ due to Si-O-Si asymmetric stretching, 954 cm⁻¹ corresponding to silanol Si-OH stretching, 810 cm⁻¹ due to Si-O-Si symmetric stretching and 455 cm⁻¹ because of Si-O-Si asymmetric bending. Additionally, a broad band at 3400 cm⁻¹ due to OH stretching and around 1630 cm⁻¹ corresponding to OH bending was observed. The organic functionalization on neat mesoporous silica particles was confirmed by a distinct band around 1720 cm⁻¹ due to carbonyl stretching, 1460 cm⁻¹ due to CH₂ bending vibrations, and CH asymmetric and symmetric stretching at 2958 cm⁻¹ and 2861 cm⁻¹, which were absent in CMS.

Figure 3.

FT-IR spectra of control mesoporous silica and modified mesoporous silica.

Field emission scanning electron microscopy (FE-SEM) of the mesoporous silica samples and commercial silica was carried out to study the shape and morphology of particles. The silica particles of CMS and MS-OH have spherical morphology, as shown in Figure 4(a) and (b), and lie in the range 400–700 nm. Commercial silica can be seen as large clusters of particles having irregular shape and size, as seen in Figure 4(c). Figure 4(d) shows a highly magnified FE-SEM image of commercial silica, where it can be seen clearly that the silica particles of extremely small size are aggregated together to form large clusters.

Figure 4.

FE-SEM images of (A) Control mesoporous silica (X 25,000) (B) Modified mesoporous silica (X 25,000) (C) Commercial silica (X 15,000) (D) Commercial silica (X 100,000).

Nitrogen adsorption-desorption analysis was done to determine the surface area and average pore size of all the three kinds of silica in consideration. As seen in Figure 5, both CMS and MS-OH exhibited Type IV adsorption isotherms, which is typically shown by all mesoporous substrates. It is also evident that the mesoporous structure was not altered post-surface modification of mesoporous silica. The BET surface area and pore diameter of all three kinds of silica have been listed in Table 3. The surface area for CMS and MS-OH was 1446 m²/g and 912 m²/g respectively. The surface modification of CMS to yield MS-OH resulted in decrease in the pore diameter as well, i. e. from 2.31 nm to 1.95 nm. Reduction in the surface area and pore diameter indicated incorporation of organic moiety on the surface of mesoporous silica. These observations are consistent with the literature reports.^29,30 Commercial silica exhibited Type V adsorption isotherm, that is shown by porous materials displaying weak adsorbent-adsorbate interactions at low pressures, hence convex to the relative pressure axis. The surface area of commercial silica was found to be 416 m²/g.

Figure 5.

BET adsorption isotherms of control mesoporous silica, modified mesoporous silica, and commercial silica.

Table 3.

Parameters obtained from BET (surface area and pore diameter) and DLS (particle size and Z-average size).

Silica	BET surface area (m²/g)	BET pore diameter (nm)	Particle size by DLS (nm)	Z-average by DLS (μm)
Control mesoporous silica	1446	2.31	527	2.14
Modified mesoporous silica	912	1.95	429	1.03
Commercial silica	416	–	45	13.24

The particle size distribution in the silica samples was studied by dynamic light scattering (DLS). The average particle size and Z-average size (refers to the average size of the aggregate present in the solution) of CMS, MS-OH, and commercial silica have been listed in Table 3. The size of aggregated molecule decreased after surface modification in mesoporous silica, which indicated better dispersion of silica particles in the test solution. The commercial silica particles had the smallest size of individual particle that could be the reason for the largest size of aggregates or clusters present. The results obtained from DLS were found to be consistent with the FE-SEM images shown above (Figure 4).

The process of creating tubular water quenched PP film (TQPP) includes extrusion of film from an annular die that yields a tube through which air is passed into its interiors. Contact is created between the tube and the water bath underneath the die. The tube is then flattened into a film and subjected to drying and winding onto a roll. The conventional formulation for preparing a TQPP film includes PP resin with antiblock/slip additives. All the TQPP films prepared were analyzed for their thermal, optical, barrier, and mechanical properties. The blocking force as well as wettability of each film was also examined.

The thermal stability of the films was evaluated by TGA analysis (Figure 6). As seen in Table 4, the addition of ppm level of all three types of silica particles improved the thermal stability of PP. The onset degradation temperature (T_onset) was increased from 379 °C to upto 409 °C for modified mesoporous silica as well as commercial silica. The organic functional group grafted on the inorganic framework of silica in modified mesoporous silica led to an increase in T_onset for modified mesoporous silica as compared to the control mesoporous silica. The crystallization behavior of TQPP films was examined via DSC analysis (as shown in Figure 7) and the corresponding parameters have been listed in Table 4. The melting point of all the films remained constant, while the peak crystallization temperature (T_c) increased for all the TQPP-silica films as compared to the neat polymer. Maximum effect of heterogenous nucleation was observed for TQPP film with modified mesoporous silica. The improved dispersion of silica particles and increased interaction of organic group on silica surface with the PP chains increased the nucleating ability of silica particles in the matrix. As observed from FE-SEM and DLS results, particles in commercial silica sample are present in agglomerated form having size ranging in 5–13 microns. Increased particle size and lower surface area of commercial silica particles in comparison to mesoporous silica sample can be the possible reason for not enough rise in T_c for commercial silica sample.²¹ Similar conclusions were drawn by Borysiak et al., where improved dispersion, higher porosity and reduced particle size resulted in higher nucleating ability of silica particles in PP composites.³¹

Figure 6.

TGA curves of prepared neat TQPP and TQPP-silica films.

Table 4.

Thermal parameters obtained from DSC - melting point (T_m), peak crystallization temperature (T_c), heat enthalpy of fusion (ΔH), as well as onset degradation temperature (T_onset) determined by TGA.

Film composition	T_m (^oC)	T_c (^oC)	ΔH (J/g)	T_onset (^oC)
Neat TQPP	166	114	103.5	379
TQPP – Control mesoporous silica	166	117	104.6	404
TQPP – Modified mesoporous silica	166	119	102.2	409
TQPP – Commercial silica	165	116	107	409

Figure 7.

DSC thermograms depicting (a) cooling curves and (b) melting curves of neat TQPP and TQPP-silica films.

TQPP films for commercial applications must be easy to open for the operator and should have good gloss and clarity. A typical TQPP film consists of over 2000 ppm each of anti-blocking agent as well as slip additive. Blocking refers to the adhesion between two adjacent layers of a film and is therefore a common concern for the manufacturers of polyolefin coatings and films. Van der Waal’s forces of attraction between the relatively amorphous portions of the polymer matrix are the primary reason for the adhesion between adjacent layers of film. Certain low molecular weight compounds present in the resin also contribute to blocking. Anti-blocking additives are therefore added during polymer processing, as they tend to create asperities by microscopically protruding from the film surface. This results in minimizing the surface contact between films and hence combat blocking or decrease the value of blocking force. Both organic and inorganic anti-blocking additives are utilized commercially. Organic anti-blocking additives are migratory in nature and reduce the adhesion between film layers by crystallizing on the surface of the film. Erucamide, a primary amide, is a well-known anti-slip additive used by industries and is even added in this TQPP compositions prepared for making films. Inorganic anti-blocking additives are non-migratory in nature and because of their increased thermal stability, they are utilized in high-temperature applications. They are relatively inexpensive as compared to organic additives and are therefore more popular for commercial applications. Silica, talc, and clay are some of the commonly used anti slip-additives in polyolefin films. Synthetic silica particles have an edge over other anti-blocking agents since their porous structure offers a larger number of particles per unit weight, leading to higher efficiency.³² However, a larger surface area of particles could also lead to the adsorption of the slip agent, thus reducing its efficiency.¹⁹ For a similar reason, blocking force was found to increase for the TQPP film with control mesoporous silica (as seen in Table 5). The spherical morphology or nearly aspect ratio of unity in the case of mesoporous silica particles can also add to the blocking force. The surface modification of mesoporous silica resulted in decrease of surface area, as discussed above (BET analysis). This explains the lower adsorption of slip agent as compared to control mesoporous silica. Thus, lower blocking force for TQPP film with MS-OH as compared to CMS is a combined effect of relatively lower surface area and improved dispersion of silica particles, thus making it a better anti-blocking additive in comparison to unmodified mesoporous silica. Commercial silica displayed the lowest value of blocking force in all the three types of silica particles in consideration. The major reason could be the large particle size owing to aggregation and the irregular shape of agglomerates, creating more roughness at the surface of polymer.¹⁰ Moderate porosity and lower surface area as compared to CMS and MS-OH also added to its efficiency as an anti-blocking additive.

Table 5.

Blocking Force values and optical parameters including gloss and haze of neat TQPP and TQPP-silica films.

Film composition	Blocking force (GF)	Gloss (G. U.)	Haze (%)
Neat TQPP	64	141	0.41
TQPP – Control mesoporous silica	76	145	0.43
TQPP – Modified mesoporous silica	62	147	0.55
TQPP – Commercial silica	58	138	0.85

For packaging applications, the visual appeal of the films enclosing the articles, i. e. high clarity, low haze, and high gloss, is another crucial factor for both manufacturers and customers. The refractive index of synthetic silica (1.46) is quite similar to that of PP (1.49), which makes it a suitable additive for maintaining or improving the optical characteristics of polymer films. These optical properties are mainly a function of antiblocking aid, slip additive and internal haze arising in the crystalline structure. It has been observed that the gloss and clarity in films is compromised with higher loading of anti-blocking agents or slip additives.³³ Therefore, it is extremely important to obtain an appreciable balance of openability and optical properties of films. Haze is referred to the % of incident light which is scattered by more than 2.5 degrees by the surface of the film, while gloss is the ability of the film surface to reflect the incident light in any direction. Stehling et al. observed that the primary cause of haze in blown films was found to be the roughness of the film surface.³⁴ Bheda and co-workers also studied haze as a function of surface roughness or surface scattering, which correlates to the kind of anti-blocking additive used.³⁵ As seen from the values of haze and gloss in Table 5, commercial silica particles displayed poor optical properties in comparison to neat TQPP film. The large particle size of commercial silica particles (presence of agglomerates) lead to relatively more surface roughness and decreased the gloss of TQPP films and increased the internal haze development to more than double that observed for neat TQPP.^19,36 The addition of control mesoporous silica showed no significant change in haze values as well as increased the gloss of the film. The purity of synthetic mesoporous silica particles reflected better optical properties as compared to commercial silica particles. Modified mesoporous silica particles increased the surface gloss due to improved dispersion in the matrix, while slightly increasing haze value as compared to neat TQPP because of increased anti-blocking property. This value (0.55%) was still much lower than that observed with commercial silica (0.85%).

Another important parameter of packaging films is their resistance against stretchability and the extent of elongation. Literature reports have suggested that the well-ordered structure and extensively high surface area of mesoporous substrates improved dispersion and mechanical properties in PP as compared to the non-porous additives.^37,38 The stress versus strain plot for neat TQPP and TQPP-silica films has been given in Figure 8 and the tensile parameters have been listed in Table 6. Even with the addition of such a small quantity of silica, the tensile strength for TQPP film increased up to 37.5% and 28% with CMS and MS-OH respectively, in comparison to neat TQPP. The extent of elongation is an important parameter to consider as it is a direct indicator of ductility, which is in turn an essential requirement in the case of packaging films.³⁹ As seen in the values listed in Table 6, the % elongation of the film was also enhanced up to 28% with CMS and 15% with MS-OH, indicating an increase in the ductile behavior of prepared TQPP-mesoporous silica films, which can also be clearly seen in Figure 8. There was absolutely no change in tensile properties with the addition of commercial silica particles in TQPP film. However, the Young’s modulus and yield stress of TQPP film decreased in the case of commercial silica as additive, while it nearly remained the same for neat TQPP and TQPP-mesoporous silica films. It is possible that the surface roughness due to agglomerates of particles in commercial silica samples is resulting in the crack generation and therefore counteracting the reinforcing properties of silica.⁴⁰

Figure 8.

Stress versus strain curves of neat TQPP and TQPP-silica films.

Table 6.

Parameters from tensile testing; tensile strength, % elongation, yield stress, and Young’s modulus in the machine direction (MD) for neat TQPP and TQPP-silica films.

Film composition	Tensile strength (MPa)	% elongation	Yield stress (MPa)	Young’s modulus (MPa)
Neat TQPP	32.7 ± 1.6	826 ± 42	17.6 ± 0.8	722 ± 76
TQPP – Control mesoporous silica	44 ± 2.6	1065 ± 52	16.6 ± 0.8	717 ± 47
TQPP – Modified mesoporous silica	41 ± 3	951 ± 55	16.3 ± 0.3	700 ± 52
TQPP – Commercial silica	31 ± 2	827 ± 34	15.7 ± 1.1	572 ± 16

Application of coating, inks, or adhesives requires control of the surface properties of plastics. The ability of a liquid to spread and adhere to the surface, referred to as wettability, is a parameter to determine the printability of these plastic films. PP is a hydrophobic material and thus has poor wettability. To examine the change caused in the wetting properties of TQPP films with the addition of silica, contact angle measurements were carried out. With the addition of CMS and commercial silica particles, the contact angle decreased slightly with respect to neat TQPP due to the surface hydroxyl groups. In the case of TQPP film with modified mesoporous silica, the contact angle decreased from 96° to 86° (as shown in Table 7) due to increased hydrophilicity and thus higher surface energy. The increase in hydrophilicity was possibly due to a higher degree of hydrogen bonding because of grafted organic moiety consisting of more oxygen atoms in the pendant group (in the form of esters) and terminal hydroxyl groups.^41,42

Table 7.

Contact angle, values of WVTR permeation, and OTR for neat TQPP and TQPP-silica films.

Film composition	Contact angle (degrees)	WVTR (gm/m².day)	OTR (gm/m².day)
Neat TQPP	96	6.7	375
TQPP – Control mesoporous silica	94	6.8	482
TQPP – Modified mesoporous silica	86	6.5	446
TQPP – Commercial silica	93	8	431

The evaluation of oxygen and moisture barrier properties are other important factors required for preparing packaging films for different purposes. For example, during the packaging of food items, it is necessary to maintain aroma, and flavor inside the packaged film and prevent the migration of undesirable elements from outside into the film. High barrier properties are required for enclosing coffee or roasted items, medicines, or nutrition-based products while low barrier properties are required for frozen fruits and vegetables.^2,43,44 For the last many decades, plastics and polymer films have replaced glass and metal packaging due to the low cost and easy availability of the former. To evaluate the barrier properties of prepared TQPP films, water and oxygen transmission rate was determined and the corresponding values of WVTR and OTR have been reported in Table 7. PP has good water barrier properties due to its hydrophobic nature but is susceptible to the permeability of gases.⁴⁵ Due to the non-polar backbone of PP and the lower loading of silica, the values of water vapor permeability remained mostly constant for the prepared films. There was a slight increase in the WVTR value for TQPP film with commercial silica. This could be because of increased interfacial voids due to the agglomeration of silica nanoparticles. The results were more pronounced in oxygen transmission rate as compared to water vapor transmission. There have been reports published in the literature about an increase in oxygen permeability through PP on the addition of silica, possibly because of disruption in the polymer chain packaging.^21,46–49 As seen from the values of OTR in Table 7, the value increased from 375 to 482 on the incorporation of control mesoporous silica. The possible reason could be the large surface area and easy permeation of gas through the mesopores. For TQPP-MS-OH film, the value of OTR decreased as compared to film with CMS, as the path length for the diffusion of oxygen slightly increased after surface modification of mesoporous silica. OTR values for TQPP-modified mesoporous silica sample were close to that of TQPP film with commercial silica. An increase in the OTR value for the latter in comparison to neat TQPP could be attributed to the voids created by agglomerates of nano silica, thereby increasing the free volume in the polymer matrix for easy gas permeation. The slightly lower OTR value in commercial silica film samples as compared to CMS could be because of the higher aspect ratio of silica particles that help in reducing gas diffusion as compared to silica particles of spherical morphology of aspect ratio nearly as unity.⁵⁰

Conclusion

Mesoporous silica nanoparticles (MSNs) were organically functionalized using silane that was in turn chemically modified by thiol-ene reaction. TQPP films were prepared with control mesoporous silica (CMS), modified mesoporous silica (MS-OH), and commercial silica as additives, and their effect on the anti-blocking force, mechanical, optical, and barrier properties were analyzed. Surface functionalization on mesoporous silica reduced the blocking force in TQPP films. Mesoporous silica particles show superior mechanical properties as compared to commercial silica particles in terms of higher tensile strength and % elongation in the machine direction. Higher clarity (lower haze) and improved gloss values were observed in TQPP-mesoporous silica films in comparison to that of commercial silica. Modified mesoporous silica decreased the water contact angle from neat TQPP film, indicating improved wettability. Control and modified MSNs showed slightly better WVTR values than commercial silica particles. The oxygen transmission rate was found to decrease with MS-OH as compared to CMS.

Footnotes

Acknowledgements

AJ acknowledges the Ministry of Education and Indian Oil Corporation Ltd, India for fellowship. LN acknowledges the Science and Engineering Research Board.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This study is supported by. Department of Science and Technology, India (SB/S3/CE/061/2015) for financial support.

ORCID iD

Leena Nebhani

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