Jun Young Cheonga,
Jaehwan Ahna,
Mintae Seoa and
Yoon Sung Nam*ab
aDepartment of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon, 305-701, Republic of Korea. E-mail: yoonsung@kaist.ac.kr
bKAIST Institute for NanoCentury (KINC CNiT), Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon, 305-701, Republic of Korea
First published on 13th July 2015
Flame-retardant, flexible polymer thin film and coating materials are in large demand for various applications. Three approaches have been attempted: inherently fire-retardant polymers; chemically modified polymers; and the addition of fire retardants as additives for polymers. The first two approaches are based on specific polymers, limiting their wide applications. The last approach provides great flexibility in designing materials with multifunctional properties. Herein, we report the fabrication of a flexible vermiculite–polymer hybrid film with very low flammability through photo-cross-linking of polyethylene glycol network incorporating micronized vermiculite particles. Vermiculite is a thermally insulating agent that can withstand flame up to about 1200 °C. The film fabrication process is very simple, time-efficient, and thickness-controllable. Despite quick processing of the film, vermiculite particles are uniformly distributed within the polymer network. Direct fire testing proves that, at a vermiculite concentration of about 75 wt%, the films of 20 μm thick can withstand actual fire for more than a minute. Without vermiculite, the polymer film is burnt out immediately when in contact with flame. This study demonstrates that a vermiculite–polymer hybrid film, though it is relatively thin and highly flexible, can suppress the heat flow without decomposition for more than just a brief moment when in direct contact with flame.
With these limitations, inorganic materials have also been investigated. Vermiculite ((MgFe,Al)3(Al,Si)4O10(OH)2·4H2O), a hydrous silicate mineral, has been known for its inherent flame-retarding properties.8,9 It has a triclinic structure and forms as a layered structure of minerals and water molecules. One important characteristic of bulk vermiculite is its exfoliated structure: when bulk vermiculite is heated at an elevated temperature (e.g., >200 °C), exfoliation occurs through loss of water in the sheet interlayers and adapt to the elevated heat.10 The main flame retardation mechanism of vermiculite lies in the self-intumescing properties through exfoliation that passively inhibits thermal transfer when it is exposed to the heat flow.11,12 Due to its resistance to structural decomposition, soil-compatible characteristics, and relatively low-cost, vermiculite has also been used in a variety of applications for the past 10 decades, such as a natural sorbent for heavy metals,13 thermal energy storage materials,14 a siloxane remover from biogas,15 a soil conditioner,16 brake pads,17 anti-bacterial materials,18 cements,19 and an enhanced dechlorinator.20 Among the different applications, it was used predominantly as a soil conditioner, in accordance with its biocompatible properties with the soil. Since soil is composed of different kinds of minerals, including silicate minerals and generally has a melting temperature above 1500 °C, it demonstrates inherent thermal stability. As vermiculite is one of the natural minerals found in soil,21–23 its thermal stability is also somewhat analogous with that of soil.
With its heat-resistant characteristics, vermiculite has been proven to be an effective flame-retardant material and applied to flame-resistant lignocellulosic-mineral composites,24 siloxane-based composites,25 polypropylene composites,26–28 and water-based acrylic fire retardant coating formulations.29 Until now, vast majority of research dealt with either making composite materials and coating formulation or fabricating a hybrid film by multi-step and time-consuming procedures (longer than six hours). As a result, there is still limited research in the manufacturing of a thin, thickness-controllable (20–140 μm), and easily processed vermiculite film. Constant demand for such a thin, fast and efficiently processed film is present, ranging from energy harvesting material to simply preventing certain materials from catching on fire.
In this work, we introduce a simple method to fabricate a fire-retardant vermiculite–polymer hybrid film through photo-initiated polymerization of polyethylene glycol macromers mixed with micronized vermiculite. For a vermiculite sample, South African vermiculite was used, devoid of asbestos contamination controversy as in the case of some other kinds.10,21,30–33 To create a simple fabrication process of a film, poly(ethylene glycol) (PEG) macromers were mixed with vermiculite in ethanol and cross-linked into a polymer network by ultraviolet (UV) light-initiated polymerization with a photoinitiator.34,35
Sample # | Vermiculite | PEGDMA | Ethanol | Darocur 1173 | Weight percentage of vermiculite |
---|---|---|---|---|---|
S1 | 100 mg | 80 mg | 110 mg | 10 mg | 52.6% |
S2 | 200 mg | 80 mg | 110 mg | 10 mg | 69.0% |
S3 | 260 mg | 80 mg | 110 mg | 10 mg | 74.3% |
Fig. 2 SEM image of (A) raw material of commercial vermiculite and (B) vermiculite particles after sieving. |
As the S3 sample was expected to exhibit the best fire-retardant effects, the surface morphology of the S3 sample was analyzed using SEM. To confirm the accuracy of the morphology, different set of SEM images was taken in multiple different points, with front and backsides of the film, as shown in Fig. 3. The observation between front and back side of film indicates that no significant difference in surface form exists between the front and back side of the film, supporting the case that cross-linking of PEGDMA in the process of UV curing using Darocur 1173 was successfully done. SEM images of randomly chosen two points (point #1 and point #2) showed that no significant difference in surface morphology was observed. In general, the surface morphology of the film resembled the top view of the chocolate crunched balls, as some of vermiculite particles stick out of the film as some particles have a diameter between 20 μm and 50 μm.
Fig. 3 SEM image of the front and back sides of randomly chosen two points of the vermiculite–PEG hybrid film (scale bar = 10 μm). |
To more clearly observe the morphology of the sieved vermiculite particles and their dispersion in the hybrid film, TEM images were taken, as shown in Fig. 4. Fig. 4A shows the overall morphology of sieved vermiculite particles, and Fig. 4B and C show their layered structure. Fig. 4D shows the dispersed sieved vermiculite particles in the vermiculite–PEG hybrid film, where the size of vermiculite particles is much smaller than that in Fig. 4A. From the TEM images, it can be concluded that sieved vermiculite particles maintain the well-known layered structure of vermiculite and were successfully dispersed within the inorganic-polymer film.
To determine whether a new chemical bonding was formed between PEG and vermiculite within vermiculite–PEG film, FTIR analysis was conducted. The FTIR spectra of PEG film, vermiculite, and vermiculite–PEG film are presented in Fig. 5. PEG film has two prominent absorbance peaks at 1093 cm−1 and 2866 cm−1; vermiculite has two prominent absorbance peaks at 402 cm−1 and 947 cm−1; and vermiculite–PEG film has four prominent absorbance peaks at 402 cm−1, 946 cm−1, 1097 cm−1 and 2868 cm−1. Such absorbance peak location suggests that no new chemical bonding was formed between vermiculite and PEG. No new absorbance peaks or significant peak shifts occurred in vermiculite–PEG film compared to the absorbance peaks of PEG film and vermiculite, indicating that no direct chemical bonding between vermiculite and PEG was formed.36,37 Nevertheless, minor peak shifts of PEG and vermiculite in vermiculite–PEG hybrid film indicate that cations exposed on the surface of vermiculite could mediate reactions with PEG through metal–polymer coordination, as previous studies demonstrated that PEG has coordination reactions with different cations such as Mg2+, Fe2+, and K+, elements of which are present in vermiculite.38–41
Fig. 5 FTIR spectrum of PEG (black, dots), vermiculite (blue, dash), and vermiculite–PEG film (S3, thickness = 20 μm, red, straight). |
The mechanical properties of vermiculite–PEG film (S2 and S3) and PEG film (control) were determined more precisely in the stress–strain curve (Fig. 6). From the stress–strain curve, the measured ultimate tensile strengths (UTS) of PEG film, S2, and S3 were 0.74 MPa, 3.64 MPa, and 4.58 MPa, respectively. Both S2 and S3 samples had higher maximum strain values before deformation, higher UTS, and longer elongation than PEG film. The vermiculite particles were well-dispersed in the cross-linked PEG network, allowing the vermiculite–PEG hybrid film also to be flexible (Fig. 7A). Slight change in color was observed among the samples of vermiculite–PEG hybrid films due to the different weight ratios of vermiculite in hybrid films (Fig. 7B). However, on a larger scheme, the SEM image of cross-section of the film shows that such stick-out is not vertically sharp. The approximate thickness of the film measured from Fig. 7C is ranged from 25 μm to 30 μm, which is slightly higher than the thickness measured on a Vernier caliper. Nevertheless, the thickness was measured on the Vernier caliper for its easy measurement and distinct unit difference (by 10 μm).
Fig. 6 Stress–strain curves of PEG film (black, dot), S2 (blue, dash) and S3 (red, straight) vermiculite–PEG hybrid films. |
The EDX spectra of vermiculite–PEG hybrid film (S3) is presented in Fig. 8A. It was observed that earth-abundant elements such as Mg, Al, and Si were derived from the vermiculite. A small amount of C can be attributed to the presence of carbon atoms in the cross-linked PEG network. TG analysis shows that the film (S3) did not decompose wholly even at 800 °C, which is the usual temperature of the center of candle flame, and that pristine vermiculite particles undergo very small weight decomposition up to 800 °C (less than 6 wt%) (Fig. 8B and S1†). The weight loss slightly occurred from 20 °C to 200 °C (stage 1); critical weight loss then occurred from 300 °C to 450 °C (stage 2); and after that, the weight% is generally maintained until 800 °C (stage 3). These three stages can be explained in the following ways: in stage 1, slight decomposition occurred as some residues of ethanol capped inside the vermiculite–PEG film and some water contents in vermiculite disappeared. Relatively sharp weight decrease of pristine vermiculite can be shown in Fig. S1† at 90 °C to 120 °C, suggesting the disappearance of minute water contents in vermiculite. For residues of ethanol, some ethanol remained as it was kept inside the gelling structure of PEG as it underwent polymerization soon before ethanol had enough time to evaporate between two glass slides. Some of ethanol was also capped inside the PEG and layered structure of vermiculite, making it difficult to be evaporated when exposed to the air condition.
In stage 2, a majority of PEG melted, as the melting point of the PEG is higher than 200 °C (Fig. 8B). The general weight loss patterns of both PEG film and S3 sample are in good agreement, suggesting that the weight loss in stage 2 is mainly attributed to the melting of PEG in a regular pattern. Such observation is also apparent in DSC curve (Fig. S2†), where no melting point was observed up to 240 °C. Upon reaching 450 °C (in stage 3), almost all of PEG polymers were melted, and only the vermiculite particles remained, probably in a layered structure. Because of its complex layered structure, it remained thermally stable even at 800 °C. If one were to compare the remaining weight percentage of the film (sample S3) at 800 °C (about 71 wt%) and the theoretical weight of vermiculite (74.3 wt%), the experiments are in good agreement with the fact that most of vermiculites remained thermally stable.
The combustion behaviors of two different samples (S2 and S3), which are about 60 μm thick, are shown in Fig. 9A. The fire flared up as the films were ignited due to the residue of ethanol inside. As expected, the S2 sample could not maintain its original structure 14 s after ignition; on the other hands, the S3 sample endured inside of flame over 120 s. Fig. 9B shows fire endurance data of three different samples (S1, S2, and S3). Fire endurance time was defined as the time period from the beginning of the test to the point when the weight loss of sample reached 32 wt% (remaining weight of the sample: 68% of the original weight of the sample before fire testing). Fire endurance time was greatly affected by the weight percentage of vermiculite added. Slight difference was observed between S1 and S2 and much larger difference in fire endurance time was observed between S2 and S3 under 60 μm in thickness. The result clearly proves that the sample with the maximum concentration of vermiculite exhibits very favorable, thermally stable properties. On the other hand, even for the S2 sample, as the film gets thicker, fire endurance time significantly increases, especially from 80 μm to 100 μm and from 100 μm to 120 μm. The results suggest that depending on the desired thickness of the film, one does not always need to have the maximum amount of vermiculite in a solution to create a fire-retardant film.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra08382f |
This journal is © The Royal Society of Chemistry 2015 |