Preparation, characterization and decomposition test on Tapanuli organoclay reinforced cellulose acetate/cellulose acetate butyrate blend composites

The need for biodegradable composites has increased for many applications in recent years. Cellulose acetate (CA) and cellulose acetate butyrate (CAB) are relatively easy and cheap to fabricate, as well as relatively easy to decompose compared to other polymers. These materials are transparent and lightweight with low tensile properties. In this current study, the effect of Tapanuli clay addition on tensile and decomposition properties of CA and CA–CAB systems were investigated. Tapanuli organoclay was prepared by a cation exchange treatment using hexadecyltrimethylammonium bromide (HDTMA-Br) surfactant to Na-bentonite. Prior to the treatment, the Tapanuli clay was subjected to purification from organic and carbonate compounds and to balance the cations by homogenizing them into Na+. The basal spacing of Tapanuli clay increased from 1.52 nm up to 1.98 nm. CA and CA −5 wt% CAB composites were then synthesized using a solvent casting method. It was found that the addition of both 5 wt% CAB and 7 wt% organoclay in CA decreased the tensile strength and reduced the mass loss by 70%. After 45 days of the decomposition test, it was indicated that the presence of 5 wt% CAB in CA reduced the mass loss of the system by about 50%. These findings were con-firmed by the Scanning Electron Microscope (SEM) images which showed different patterns of as-synthesized and decomposed materials. In conclusion, the presence of 1 wt% Tapanuli organoclay slightly increased the decomposed mass of CA film and enhanced the tensile strength of CA-co-CAB.


Introduction
Polymer materials have been developed greatly and continuously for extensive application because of their novel and tailor-made properties.They are commonly used as packaging materials due to their unique features.Plastics usually possess flexibility, specific strength, and are inexpensive compared to other materials.However, the use of these polymeric materials or plastics has created serious problems being caused by plastic waste and disposal in the environment.Researchers have worked to overcome this problem for the last two decades [1,2].
One way to reduce plastic waste in the environment is to use both biodegradable and natural polymers.Biodegradable polymers are beneficial materials that are produced from natural resources [3].It offers some advantages such as biocompatibility, less environmental toxicity and easy of degradation.Biopolymers have been widely used in some sectors such as pharmaceuticals, food, and environmental sectors.Unfortunately, it is a fact that biodegradable and natural polymers possess poor mechanical and thermal properties compared to synthetic polymers.A number of biodegradable polymers, such as poly (lactic acid) (PLA), polycaprolactone, cellulose, polyhydroxy butyrate (PHB), and starch have been in research interest [4,5].
One technique to enhance these properties of biodegradable polymers is to add nanoparticles or nanofibers as fillers or additives to the polymers [6][7][8] or to produce polymer blends [9].Nanocomposites play an essential role in this research field.It improves materials properties by combining two or more different materials to produce a hybrid material [10][11][12].Polymeric nanocomposites usually consist of inorganic solids like clays or oxides in nanometre scale and polymer mixtures.The combination of these materials produces new materials with special effects and special properties that can accommodate some requirements for specific applications [13].
Indonesia is known for its abundance natural resources which are distributed in all region of the country.Some of the materials are commonly used for invaluable goods and materials [14,15].Among these are bentonite, that usually used as concrete mixture [16,17], cat litter, sealing and waterproofing, adsorbent [18], and drilling mud in oil and petroleum industry [ref].To enhance the value of bentonite, several strategies are needed to employ.Tapanuli organoclay, particularly montmorillonite or Na-bentonite, are frequently utilized as nano fillers.Montmorillonite possesses swelling properties due to its hydrophilic nature.However, unmodified clay cannot interact with polymers, necessitating a modification of its layers to render it organophilic.This modification process, achieved through cation exchange reactions using cationic surfactants, results in what is commonly termed as organo layered silicate or organoclay [19].
Composite materials consist of two phases; a matrix and reinforcement(s).Matrix materials are used to maintain the framework, whereas the filler materials become a part of the matrix phase.Numerous studies related to biodegradable plastic have been conducted [20].Some of them are focused on the formula optimization for the bioplastics and others are focused on the performance.They observed various kinds of things such as mechanical properties, the effect of plasticizers and their tensile strength.
Modified natural polymers such as cellulose acetate (CA) become more popular as a semi-synthetic polymer [21].Cellulose acetate is produced from natural wood through acetylation reactions.It has some favorable characteristics such as good impact resistance, transparency, and its hardness.Cellulose acetate is also considered as eco-friendly material [22], so it is inherently degradable in the natural environment.
To the best of our knowledge, the actual sustainability of biocomposite materials is being neglected [23].In this research, a biodegradable plastic using nano bentonite as a reinforcement filler and cellulose acetate as a matrix polymer was developed.Besides the advantages, some studies show that cellulose acetate has a lack of elasticity.So, in this study, due to that limitation, for the flexibility of packaging application, Cellulose Acetate Butyrate (CAB) was introduced to the matrix to create more elastic materials.

Materials
The materials used in this research were Bentonite -from Tapanuli, North Sumatera, Indonesia, Hexadecyltrimethylammonium bromide (HDTMABr) surfactant, Cellulose Acetate [Mn ∼ 30,000], and Cellulose Acetate Butyrate [Mn ∼ 30,000] (Pro Analysis) that were purchased from Sigma Aldrich, Germany; Sodium Hydroxide (NaOH) and Glacial Acetic Acid that were purchased from Merck, Germany.A 5-kg sack of commercial fertilizer for decomposition test, was directly used without any treatment.

Preparation of Tapanuli organoclay (TOC)
The Tapanuli organoclay was prepared in four stages, using an intercalated method.Firstly, a sedimentation step was carried out.A certain amount of bentonite, which was previously dried at 105 °C for 2 h, was dispersed in deionized water and stirred for 6 h.The bentonite suspension was dried at 105 °C.The second step was the purification.This dry bentonite was dispersed into a buffer acetate solution with a pH of 6.The suspension was stirred for 32 h, centrifuged, and dried at 105 °C.Thirdly, the preparation of Na-bentonite was conducted.A certain amount of purified bentonite was dispersed in NaCl, stirred for 6 h, and centrifuged.The bentonite was then washed using deionized water until the conductivity of the solution was 0.01 mS.The synthesis of organoclay was the fourth step.A certain amount of Na-bentonite was dispersed in deionized water and stirred for 1 h.The HDTMABr surfactant was added slowly [19], and this mixture was stirred for 1 h at 60 °C and was then ultra-sonicated for 3 min.The organoclay was finally dried at 105 °C and sifted using a 100 μ sieve and was identified as Tapanuli Organoclay (TOC).

Preparation of cellulose acetate-co-cellulose acetate butyrate/organoclay nanocomposites
The nanocomposites were synthesized using a casting method.A mixture of cellulose acetate (CA) and acetone was stirred for 1 h at room temperature.Then, 5 wt% of cellulose acetate butyrate (CAB) was added slowly into the mixture and stirred for 1 h.At the same time, TOC (1, 3, 5, 7 wt%) was dispersed into acetone and stirred for 1 h.This TOC solution was added slowly into the CA/CAB mixture and this suspension was ultra-sonicated for 2 min.This mixture was then poured into petri-dishes and the temperature was maintained at 40 °C for 30 min.The CA/CAB-TOC films were then collected.The TOC contents varied from 1, 3, 5, and 7 wt%.The same procedure was carried out to obtain CA and CA-TOC films.Figure 1(a) shows the biocomposite samples.

UV-vis Spectrophotometry
Light transmission and transparency of films were measured using UV-visible spectrophotometer (T92+, PG Instruments).The films were cut into 10 mm × 35 mm and then placed in a spectrophotometer cell at wavelengths of 200-800 nm.

Mechanical characterization 2.5.1. Tensile test
The tensile test was conducted on a Universal Testing Machine (Shimadzu) at room temperature, with a rate of 1 mm min −1 , based on ASTM D822.The dimension of the specimen is 90 mm × 20 mm and the thickness of each specimen was measured before the tensile test.One measurement consisted of 5 specimens.

WVTR
The WVTR (Water Vapour Transmission Rate) of the films was determined gravimetrically.Deionized water was filled in glass bottles.The mouth of the bottles was covered with films and tightened with tape.The weight of the bottles was measured, and then the bottles were placed in an oven for 24 h at 40 °C and weighed as the final weight.The WVTR values (g/m 2 h) were calculated using equation (1).
where; A is the area of the circular mouth of a glass bottle (m 2 ); t is 24 h; W i is the initial weight of glass bottle (g); and W f is the final weight of glass bottle (g).

Decomposition test
The decomposition was carried out in a soil burial test method that was adapted from Siddiquee et al [24] using a commercial fertilizer with composition as listed in table 1 and used directly without further treatment.This method was applied because the soil burial has a similar condition to actual waste disposal.The samples were prepared in a coin shape with a diameter of 22 mm and were placed in sample holders that were specially designed for this purpose, as seen in figure 1(b).On the 15th, 30th, and 45th days of burial at ambient temperature, the specimens were weighed to study the mass loss over time.The samples were dug out, cleaned up from the soil, and weighed to determine the weight loss.Two commercial 'easy-degradable' plastics were also decomposed for comparison.Three specimens were prepared for each sample.To study the effect of TOC addition to the decomposed polymer surface, specific specimens -CA, CA −7 wt% TOC, CA/ CAB, and CA / CAB −7 wt% TOC -were observed using SEM (FEI Quanta FEG) before and after 45th days of burial [25,26].
The SEM observation was carried out at different magnifications.The sample was cut into 1 cm × 1 cm and dried in the oven prior to SEM analysis.

Physicochemical and mechanical characterization 3.1.1. ATR-FTIR Analysis
Figure 2 shows the ATR-FTIR spectra of the films with different TOC loadings.The appearance of the CA characteristic bands at 1734 cm −1 , 1368 cm −1 , 1212 cm −1 , 1030 cm −1 corresponding to carbonyl stretching, methyl bending, stretching of ether groups, and ester stretching, respectively, were observed in the ATR FTIR spectrum.This result is in accordance with previous results [27,28].The peak height of 1700 cm −1 , 1200 cm −1 , and 1030 cm −1 bands are presented in table 2. Furthermore, there was no significant difference between the FTIR spectra of CA/CAB film and its composites.This was due to the small fraction of CAB in CA.It was observed that a broad band at 3750 cm −1 -3100 cm −1 (-OH stretching band), and 2900 cm −1 −2700 cm −1 (-CH stretching band) corresponds to -CH 2 groups.The board band at 3750 cm −1 -3100 cm −1 relates the hydrogenbonded molecular water to the -OH group.The CA −3% TOC film has the highest peak at 3500 cm −1 .This indicates that the highest water absorption occurred in the CA −3% TOC.In contrast, the increase in TOC loading reduces the water absorption.Interestingly, the peak from CA/CAB film is higher than the peak of CA.
The CA/CAB film is more polar than the CA film.This relates to CA/CAB film absorbing more water.3.1.3.X-ray diffraction analysis (XRD) X-ray diffraction analysis was conducted to determine the crystallinity and typical diffractogram peaks possessed by materials.The raw bentonite was conducted with chemical treatments such as sedimentation, purification, and Na-exchange bentonite.XRD analysis of the samples before and after chemical treatment have a similar pattern because bentonite has high stability in chemical treatment.Figure 4(a) shows the XRD diffractogram from raw bentonite, bentonite sedimentation, bentonite purification, and Na-bentonite [30], in their study, it was stated that montmorillonite has typical peaks of at 2θ of 5°, 19°and 34°and impurities of quartz at 2θ of 21°and 26°12 Based on the XRD analysis, it can be seen that raw bentonite and bentonite that have any treatment such as sedimentation, purification, and synthesis of Nabentonite still have the same characteristic peak.Organoclay intercalated surfactant HDTMA Br also has been characterized by using XRD [31].which is related to 19.76 Å, indicating that the surfactant of HDTMABr has been intercalated successfully into bentonite increases the basal spacing of organoclay.It indicates that the surfactant HDTMABr has been intercalated successfully into bentonite.The intercalation process could not damage the structures of bentonite.Figures 4(c) and (d) show the XRD pattern of CA and CA/CAB biocomposites [32].There are no diffraction peaks associated with the ordered structures of the CA and CA/CAB biopolymer.It indicates that the matrices of CA and CA/CAB have amorphous structures and the organoclay intercalated HDTMABr were well distributed in the matrix without forming organoclay aggregate order.

Tensile testing
Figure 5 shows the tensile strengths of CA and CA/CAB films with the variation of TOC loadings.From figure 5, it can be seen that the tensile strength of CA films is 37 MPa, which is in the range of 31-55.2MPa [33].The addition of TOC reduced the tensile strength of CA films.Furthermore, the addition of 5 wt% CAB, as seen in figure 5, also reduced the tensile strength of CA films by about 67%.It is interesting that the addition of 1 wt% TOC in the CA/CAB blend increased the tensile strength significantly, by about 150%.However, the tensile strength of this CA/CAB −1 wt% TOC film was still slightly lower than that of the pristine CA film.The higher the TOC loading, the lower the tensile strength of the CA/CAB composites.
It was expected that the organoclay contributed enhancing the tensile strength and modulus of polymers by distributing the tensile load through the nanofiller as the reinforcement phase, especially in the exfoliated structure.In general, these films were quite brittle so the addition of TOC produced the films even more brittle.It indicated that the bonding between the organoclay particle and polymer matrix was weak.In order to increase the tensile strength, it is considered that either plasticizer is added to these systems to produce more ductile materials or the composition of CAB in CA is altered [27].

WVTR Analysis
Figure 6 shows the water vapor transmission rate (WVTR) of CA, CA-TOC, CA/CAB, and CA/CAB-TOC.It can be seen that the addition of 1-7 wt% TOC does not change the WVTR values significantly.It is clear that the addition of CAB reduced the WVTR value, from 25 g m −2 h −1 (CA) to 15 g m −2 h −1 (CA/CAB).In this case, as hydrophobic material, CAB hindered the moisture that passed the CA/CAB film.This result was supported by Zhao et al (2021), in which WVTR decreased due to increasing hydrophobicities.Interestingly, by adding TOC up to 7 wt%, the WVTR values are increased.This occurred because organoclay allowed the moisture that passed the CA/CAB films.Packaging films should have the ability to block the moisture entering the film.Films with low values of WVTR are considered suitable for food packaging applications.The WVTR values of PVA/ nanocellulose/Ag composite films were in the range of 14-43 g m −2 h −1 [34].The WVTR values of the current research are in the range of 15-35 g m −2 h −1 , which is in the range of previous results.

Soil degradation test
Figure 7 shows the decomposed mass versus burial time as the result of decomposition test for CA, CA/CAB, their composites, and two commercial plastics [35].In general, the longer the burial time, the higher decomposed mass for all samples.
It is clear that pristine CA film lost its mass by 18% and the presence of 1 wt% TOC contributes about 20% to the decomposition of CA film after 45 days of burial.The presence of organoclay in the degradable polymers did not affect the decomposition rate because this nanofiller acted as a load transfer agent as well as the organoclay content was very low.It is evident that the organoclay loading of 3 wt% and 7 wt% did not change the percentage of CA decomposed mass.Figure 7 shows that the presence of 5 wt% CAB in CA films decreased the decomposed mass significantly, by about 48%.The CAB and CA formed more cross-linked bonding and the blend was more difficult to degrade.Similar to the decomposition pattern of CA, the presence of TOC did not significantly affect CA/CAB decomposed mass.In relation to the tensile strength of CA/CAB in figure 5, the presence of crosslinking decreased the polymer chain mobility and this resulted in the reduction of the tensile strength [27].

Discussion
The effect of TOC addition on the decomposition of biopolymer was studied further using SEM measurement.For significant effect, only samples with 7 wt% TOC addition and 45 days of burial were observed.The SEM images of the as-casted and 45 days burial films are exhibited in figure 8 (a to p).The micrograph of as-casted CA films, as seen in figure 8(a) shows a surface of CA film that is smooth as a typical polymer film.After 45 days in the soil, the top surface changed its contour and col-our, showing big pores and light colour, as indicated in figure 8(d).With a mass loss of 18%, this CA film degraded and; the mineral contents in the soil covered up the surface or reacted with the polymer to create a lighter colour surface compared to the as-casted film.
The presence of TOC altered the morphology of the CA surface figure 8(e).The contour of the CA-TOC surface has a bumpy contour.From figure 7, the decom-posed mass of CA and CA −7 wt% TOC are 18% and 17% respectively.Even if the decomposed mass after 45 days of burial of these two specimens were similar, the average sizes of the pores could be distinguished, they are 0.72 μm for CA (figure 8(d)) and 0.31 μm for CA -TOC (figure 8(h)).Big holes are also observed on the CA −7 wt% TOC surface (figure 8(h)).The minimum and maximum diameters of the big holes are 1.87 μm and 21.18 μm respectively.While these big holes are not seen clearly on the CA specimen (figure 8(d)).It is suggested that 7 wt% of TOC contributed to the pore formation and surface contour of the CA films.
The micrograph of the CA/CAB surface is shown in figure 8(i) and exhibits a typical CA film because the CAB content was very low.After being buried for 45 days, the surface possesses pores and light colour, as seen in figure 8(l).The same explanation to the decomposed CA films, the soil degraded the polymer blend, and the soil minerals covered up the surface.After being buried for 45 days, the CA/CAB surface (figure 8(l)) shows bigger pores compared to the as-casted CA film (figure 8(d)).The average sizes of the pores are 0.81 μm (figure 8(l)).This indicates that CA/CAB-films degrade more to a certain level.However, the amount of the decomposed mass of CA/ CAB films is similar to the CA film after 45 being buried for 45 days, the presence of 7 wt% in CAB gave an effect on the pore size.The SEM images of as-casted and decomposed CA/CAB −7 wt% TOC surfaces are shown in figures 8(m) and 8(p) respectively.Similar to the previous SEM micro-graphs, big pores and light colour are observed on the decomposed samples.The soil minerals covered the surface of CA/CAB −7 wt% TOC after being buried for 45 days and created light colour.The average sizes of the pores are 0.36 μm (figure 8(p)).Comparing the sizes of the pores of CA/CAB (0.81 μm, figure 8(l)) and CA/CAB −7 wt% TOC (0.36 μm, figure 8(p)), the 7 wt% of TOC contributed to the pore formation and surface contour of the CA/CAB films.It is interesting to know that this film possessed the least mass loss (figure 7) and the lowest tensile strength (figure 5), even though there was no clear explanation that the small amount of organoclay would reduce the decom-posed mass.A further study should be considered to determine the types and the quantity of minerals and molecules on the polymer surface.

Conclusions
It can be concluded that the presence of both cellulose acetate butyrate and Tapanuli organoclay decreased the tensile strength of cellulose acetate films and reduced the decomposed mass after 45 days of burial.The presence of small pores and colour change on the samples after 45 days of burial confirmed that degradation occurred in the current soil burial test condition.The addition of 1 wt% Tapanuli organoclay slightly increased the decomposed mass of cellulose acetate film and slightly decreased the decomposed mass of cellulose acetate-cocellulose acetate butyrate film.
However, 1 wt% Tapanuli organoclay enhanced the tensile strength of cellulose acetate-co-cellulose acetate butyrate film.The presence of Tapanuli organoclay did not affect greatly the Water Vapour Transmission Rate (WVTR) values for all samples.However, the presence of organocaly reduced the transparency of both cellulose acetate and cellulose acetate-co-cellulose acetate butyrate films.For further study, the composition of the three components in this bio-nanocomposite should be investigated further to obtain the optimal properties for a wide range of applications.

Figure 3
exhibits the UV-vis spectra of CA and CA/VAB films with 1, 3, 5, and 7% clay contents.The CA and CA/CAB films have the highest transmission level and similar spectra patterns from 250 nm up to 900 nm.The presence of clay reduces the transmission level.In other words, the presence of clay reduces the transparency of CA and CA/CAB films[29].Especially, CA/CAB −7%TOC has the least transparency.

Figure 3 .
Figure 3. UV-vis spectra of CA and CA/CAB films in the presence of TOC (0-7 wt%).

Figure 5 .
Figure 5. Tensile strengths of CA and CA/CAB films in the presence of TOC (0-7 wt%).

Figure 7 .
Figure 7. Decomposed mass of bio-composite films and commercial plastics for different burial times in commercial fertilizer at ambient temperature.

Figure 8 .
Figure 8. SEM images of biocomposite films before and after 45-day burial test in commercial fertilizer at ambient temperature.

Table 1 .
The composition of commercial fertilizer for soil burial test.