CC BY
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ISSN 2522-1841 (Online) AZERBAIJAN CHEMICAL JOURNAL № 2 2023 ISSN 0005-2531 (Print)
UDC 541.68
CRYSTALLIZATION OF MODIFIED COMPOSITES BASED ON ALUMINUM HYDROXIDE AND POLYETHYLENE MIXTURE F.A.Mustafayeva1, N.T.Kakhramanov1, O.M.Guliyeva2
institute of Polymer Materials, Ministry of Science and Education of the Republic of Azerbaijan
2Azerbaijan Medical University
Received 16.11.2022 Accepted 09.02.2023
The article presents the results of a dilatometric study of the crystallization of modified with structure-forming agent composites based on aluminum hydroxide and a mixture of high and low density poly-ethylenes. The influence of aluminum hydroxide concentration on the regularity of the change in the dependence of the specific volume, as well as the free specific volume on temperature, has been established. The crystallization onset temperature and the approximate values of the glass transition temperature of composite materials are determined. The studies were carried out in the process of step-wise cooling of the samples at a load of 5.3 kg on an IIRT-1 device converted into a dilatometer. The concentration of aluminum hydroxide varied within 1-30 wt.%. A mixture of high and low density polyethylene was taken in a 50/50 ratio. Titanium dioxide was used as a structure-forming agent in an amount of 1.0 wt.%. It has been established that with an increase in the concentration of aluminum hydroxide, the crystallization onset temperature of modified composites based on a mixture of polyethylenes remains unchanged and is equal to 1050C. It is shown that with an increase in the content of aluminum hydroxide in the polymer mixture, a regular increase in the density of the composites is observed.
Keywords: high density polyethylene, low density polyethylene, aluminum hydroxide, titanium dioxide, structure-forming agent, crystallization, dilatometry, specific volume.
doi.org/10.32737/0005-2531-2023-2-69-77 Introduction
The study of composites crystallization regularities is one of the most interesting areas in the field of physical-chemical analysis of polymers [1-3]. During the processing of polymers, crystallization usually proceeds under rather difficult conditions associated with the mixing of the mixture components in the melt, the geometry of the forming part of the mold, etc. [4]. Time and sudden temperature changes play an important role in the formation of the final structure and physical properties of polymer composite materials.
More than 50% of the thermoplastic polymers used worldwide are semi-crystalline, which makes crystallization part of their cooling process in the mold of the injection molding unit. Crystallization in polymers affects mechanical, thermal, electrical, optical, and other properties. The mechanical and optical characteristics, as well as the long-term stability of the
final products, depend mainly on three factors: the structure of the polymer, the type and amount of filler and structure-forming agent, and the conditions for obtaining test samples [5, 6].
Polyethylene is a semi-crystalline solid which properties are highly dependent on the relative content of the crystalline and amorphous phase, i.e. crystallinity degree [7, 8]. It is known that low density polyethylene (LDPE) has a degree of crystallinity in the range of 50-60%. High density polyethylene (HDPE) is characterized by a high degree of crystallinity, equal to 75-75%, which makes it possible to improve its properties such as strength, hardness, impact resistance while maintaining. The polymer crystallization process affects the basic properties of all semi-crystalline polymer materials, which make up approximately 75% of all commercially produced plastics. Thus, it is important to understand the role of fillers in the formation of the phase composition of a semi-crystalline polymer matrix. Therefore, understanding the process of
nucleation, crystallization, and morphology of polymers is fundamental to tailor their properties in relation to structure.
Crystallization in polymer blends greatly affects the overall properties of the composite material. Therefore, crystallization in multi-component polymer systems is more complex than in a single-component one. In multiphase polymer systems, the crystallization kinetics depends on the degree of miscibility of the components, processing conditions, size, shape, and degree of dispersion of the filler particles. Taking into account the above and the high demand for fire-resistant composite materials, the purpose of this work was to study the crystallization process of multiphase systems, modified composites based on aluminum hydroxide and a mixture of polyethylenes.
Experimental part
Materials
The objects of study were industrial HDPE and LDPE, which are characterized by a variety of supramolecular structures. LDPE is characterized by the following properties: ultimate tensile stress 15.0 MPa, elongation at break 764 %, melt flow index 8.9 g/10 min. HDPE is characterized by the following properties: ultimate tensile stress 30.0 MPa, elongation at break 50 %, melt flow index 17 g/10 min.
Filler - aluminum hydroxide Al(OH)3 (GOST 11841-76, OKP 631887, repackaged by ZAO VEKTON). Colorless solid, insoluble in water, has amphoteric properties, is a constituent of many bauxites. Amorphous aluminum hydroxide has a variable composition AhO3-nH2O. When heated above 180-2000C, it decomposes.
Titanium dioxide TiO2 - amphoteric oxide of tetravalent titanium (GOST 9808-84, OKP 26 3421 2070 00, repackaged by ZAO VEKTON) was used as a structure-forming agent. Inorganic finely dispersed crystalline substance with a melting point of 18430C.
Preparation of composites
Composite materials based on a mixture of LDPE / HDPE (50/50), with a fixed concentration (1 wt. %) of titanium dioxide and 1, 3, 5,
10, 20, 30 wt. % of the amount of aluminum hydroxide were obtained by mixing the components on laboratory rollers at a temperature of 1600C within 8-10 minutes. The mixing of the components was carried out in stages. The structure-forming agent was added to the melt of the polymer mixture, and then the filler. At a pressing temperature of 160-1700C, plates were molded, from which the corresponding samples were cut out for testing. The choice of the 50/50 ratio of the LDPE/HDPE polymer mixture was due to the fact that it is when using an equal amount of mixture components that phase inversion and the best technological compatibility and miscibility are achieved [9].
Dilatometric study of composites
The study of the dilatometric characteristics of the composites was carried out in the process of stepwise cooling in the temperature range from 1800C to room temperature and a load of 5.3 kg on an IIRT-1 device converted into a dilatometer. The principle of the method is to measure the change in the volume of materials depending on temperature [10-12]. The dilatometric method of analysis is a highly accurate and sensitive method for assessing the first-order phase transition and the kinetic regularities of crystallization in the Avrami coordinates [13-15].
Results and discuss
Figure 1 shows the results of studying the dependence of specific volume on temperature for composites based on LDPE/HDPE+1 wt% TiO2 at various contents of aluminum hydroxide. The concentration of aluminum hydroxide was varied within 0-30 wt %. Comparing the curves in this figure, it can be established that as the concentration of aluminum hydroxide in the LDPE/HDPE + 1 wt% TiO2 mixture increases, a regular decrease in the specific volume of the composites is observed.
The results obtained are of great importance from a technological point of view [16], since they allow us to evaluate the state of the composite depending on the temperature in the solid and viscous state.
Fig. 1. Influence of aluminum hydroxide concentration on the regularity of change in the dependence of specific volume on temperature for composites based on LDPE/HDPE+TiO2+Al(OH)3, in wt.%: 1(x)- initial mixture of LDPE/HDPE +1 wt.% TiO2, 2(*)-1, 3(o)-3, 4(^)-5, 5(^)-10, 6( A)-20, 7(A)-30 Al(OH)3.
As can be seen from this figure, with an increase in the concentration of aluminum hydroxide, a regular decrease in the specific volume is observed, i.e. increase in the density of the composite. Such a noticeable change in the specific volume of the samples indicates that aluminum hydroxide particles and their concentration affect the processes of orientation and nucleation.
According to the given dilatometric curves, the temperature of the onset of crystallization or the first-order phase transition of the modified composites based on aluminum hydroxide and a mixture of polyethylenes practically does not change and is equal to 1050C. It has been established that the interactions between the structure-forming agent particles and the polymer components are the decisive factors for the redistribution and agglomeration of aluminum hydroxide particles. Previously, in [17], it was shown that, in the absence of a structure-forming agent, the concentration of aluminum hydroxide affects the composites crystallization onset temperature in a mixture of high and low density polyethylene, which is expressed in the mixing of the first-order phase transition temperature. It is known that the process of melt
crystallization occurs in three stages [18]: 1) primary nucleation, 2) secondary nucleation leading to crystal growth, and 3) secondary crystallization. Probably, the loading of a structure-forming agent into the composition of composites containing aluminum hydroxide in a polymer mixture contributes to an increase in the number of nucleation centers that contribute to maintaining a stable phase transition temperature.
Table-1 presents the results of studying the density of samples at room temperature. From the data obtained, it can be seen that with an increase in the concentration of aluminum hydroxide, a noticeable increase in the density of the composites is observed at a constant value of the phase transition temperature.
Firstorder phase transitions are characterized by temperature constancy and volume changes. A sharp jump in the change in specific volume (Figure 1), corresponding to the temperature of the first-order phase transition, showed that for samples with 1, 3, 5, 10, 20, 30 wt.% aluminum hydroxide content, this indicator changed in the following sequence: 0.1526; 0.149; 0.1086; 0.101; 0.101; 0.0814 cm3/g.
Table 1. Influence of aluminum hydroxide concentration on the onset temperature of the crystallization process and the density of composites modified with titanium dioxide based on a mixture of low and high density polyethylene
№ Composite composition, wt % Crystallization onset temperature, 0C Density at room temperature, g/cm3
1 LDPE/HDPE+1 TiO2 105 0.939
2 LDPE/HDPE+1 TiO2+1 Al(OH)3 105 0.899
3 LDPE/HDPE+1 TiO2+3 Al(OH)3 105 0.953
4 LDPE/HDPE+1 TiO2+5 Al(OH)3 105 1.012
5 LDPE/HDPE+1 TiO2+10 Al(OH)3 105 1.055
6 LDPE/HDPE+1 TiO2+20 Al(OH)3 105 1.077
7 LDPE/HDPE+1 TiO2+30 Al(OH)3 105 1.230
Based on the data obtained, it can be argued that as the filler content increases and, accordingly, the proportion of the polymer matrix decreases, the crystallization process becomes more difficult.
The method of dilatometric measurements allows you to determine the approximate value of the glass transition temperature. This is done by determining the intersection point of the extrapolation of the upper and lower branches of the dilatometric curves (Figure 1) [9, 19]. According to the data obtained, the glass transition temperature of the initial LDPE/HDPE+1 wt.% TiO2 and filled composites with 1, 3, 5, 10, 20, 30 wt.% aluminum hydroxide content, respectively, changes as follows: -1130C; -1630C; -1370C; -870C; -740C; -630C; -490C. Thus, an undulating change in the glass transition temperatures of composites filled with aluminum hydroxide based on a mixture of polyethylenes is observed. While a small amount of filler leads to a decrease in the glass transition temperature, with relatively large amounts of aluminum hydroxide (>5 wt.%), on the contrary, the glass transition temperature increased. At a concentration of aluminum hydroxide in the range of 1-3 wt.%, the glass transition temperature becomes lower than that of the initial mixture of LDPE/HDPE+1 wt.% TiO2. This means that at a filler concentration of 1-3 wt.%, the latter have a structure-forming effect.
As noted above, at low filler concentrations (1-3 wt.%), the particles mainly participate in the formation of heterogeneous nuclea-tion centers, which, during cooling, are transformed into crystallization centers with the
formation of a fine spherulite structure. In this case, a supramolecular structure is created, which favorably affects the formation of a strong structure. At such a filler concentration, the polymer matrix retains high strength and plasticity with a simultaneous decrease in the glass transition temperature. A further increase in the filler concentration leads to the appearance of brittleness of the composite and, as a consequence, an increase in its glass transition temperature.
The polymer matrix is characterized by the presence of "free" and occupied volumes. The "free" volume is created mainly due to the interspherulitic amorphous region. In polymer composites, it can be considered as the volume unoccupied by macrochains and filler particles. Experimental determination of the "free" specific volume is a difficult task, since gaps occur at the molecular level. The simplest and most frequently used method for calculating the "free" specific volume in a polymer is the dilatometry method, which is as follows [9, 19]:
Vf=Vi-Vo
where, Vf (cm3/g) free specific volume; Vi (cm3/g) specific volume; Vo (cm3/g) occupied specific volume. With approaching absolute zero (Figure 1), composites are characterized by the presence of only occupied volume. Thus, knowing the specific volume at any temperature and the occupied volume, the "free" specific volume is determined from the difference.
Figure 2 shows the dependence of the "free" specific volume of the modified composites on the concentration of aluminum hydroxide and temperature.
Fig. 2. Influence of A1(OH)3 concentration on the regularity of change in the dependence of free specific volume on temperature for composites based on LDPE/HDPE+TiO2+Al(OH)3, in %: 1(x)- initial mixture of LDPE/HDPE +1 wt % TiO2, 2(^)-1, 3(o)-3, 4(^)-5, 5(^)-10, 6( A)-20, 7(A)-30 Al(OH)3.
An increase in the "free" specific volume in the region of the viscous-flowing state with the loading of 1-3 wt % aluminum hydroxide is due to the fact that homogeneous and heterogeneous crystallization centers are simultaneously formed in the melt, which makes it difficult to form a relatively dense structure. With a subsequent increase in the amount of filler (5-30 wt %), on the contrary, a regular decrease in the value of the "free" specific volume of composites in a viscous-fluid state is observed. The decrease in the "free" volume can be interpreted by the fact that the filler particles are embedded in the free volume of the interspherulite space. Nevertheless, for the solid state region, it can be said that the values of the "free" specific volume of the composites at room temperature are practically equal. However, previous studies have shown [17] that with an increase in the amount of aluminum hydroxide in composites based on a polyethylene mixture, the values of the "free" specific volume increase, which contributes to the loosening of the material and thereby the deterioration of the physical-mechanical properties. Most likely, the modifi-
cation of the polymer mixture with a structure-forming agent contributes to the uniform distribution of the filler in the volume of the polymer matrix and to the enhancement of interaction at the phase boundary, which leads to equalization of the values of the free specific volume of the composites at room temperature. According to Avrami's model, we have studied the kinetic regularities of the crystallization process in the region of the first-order phase transition. The studies carried out in this direction have repeatedly shown the applicability of this model for polymeric materials [20]. Based on this theory, the crystallization process proceeds in accordance with the equation:
(p = e
-Kt"
(1)
where y - is the part of the polymer that has not yet undergone transformation into a crystalline phase; K - generalized crystal nucleation and growth constant; n - constant, ranges from 1-4 and depends on the mechanism of nucleation and the shape of growing crystals.
After taking the double logarithm of Av-rami's equation, the following expression is obtained:
lg(- ln^) = lg K + n lg t
(2)
This dependence is a straight line in double logarithmic coordinates lg(-lny) from lgt. On the basis of the conducted studies, the applicability of this theory for studying the mechanism of crystallization of the nanocomposites under consideration was proved.
Figure 3 shows the kinetic regularity of crystallization of composites based on LDPE/HDPE+TiO2+Al(OH)3 at the temperature of the first order phase transition. Analyzing the kinetic regularity of crystallization of composites from time, it can be established that with an increase in the filler content, there is a tendency to a decrease in the angle of inclination of the curves to the abscissa axis. This circumstance is interpreted by the fact that with an increase in the concentration of aluminum hydroxide, the crystallization rate of the composites decreases. The difference is manifested in the fact that the loading of 1.0 wt.% aluminum hydroxide, on the contrary, contributes to a certain increase in the crystallization rate. Apparently, in this case it would be correct to state that at 1.0 wt.% content of aluminum hydroxide
in the composition of the composite, the latter exhibits structure-forming properties. Practically up to 3.0 wt.% of the filler content, the crystallization rate does not undergo significant changes. With an increase in the content of aluminum hydroxide in the composition of the composite within: 0, 1.0, 3.0, 5.0, 10, 20, 30 wt %, the value of K changes in the sequence: -0.3, -0.5, -0.7, -1.0, -.1.5, -1.7, -2.0, and the value of n respectively changed as follows: 2.68, 2.75, 2.62, 2.24, 1.75, 1.42, 1.2. For the initial polymer mixture and samples with 1.0-3.0 wt.% aluminum hydroxide content, the value of n is 2.75-2.62, which corresponds to the three-dimensional growth of spherical crystals with the continuous formation of crystallization centers. Samples with a concentration of particles in the composition of 5-10 wt.%, the value of n is 2.24-1.75, which corresponds to a lamellar two-dimensional growth. For composites with a value of n equal to 1.4-1.2, a linear one-dimensional type of crystal growth takes place with the continuous formation of crystallization centers.
If we substitute the values of n and K into dependence (2), then for each type of composite we obtain the following equations.
Fig. 3. Mechanism and kinetic regularity of crystallization of composites based on LDPE/HDPE+TiO2+Al(OH)3 in logarithmic Avrami coordinates in the region of the first-order phase transition at different concentrations of Al(OH)3, in wt %: 1(x)- initial mixture of LDPE/HDPE+1 wt % TiO2, 2(^)-1, 3(o)-3, 4(^)-5, 5(d)-10, 6(A)-20, 7(A)-30 Al(OH)3.
- initial LDPE/HDPE+1. 0 wt %TiO2 mixture
- LDPE/HDPE+Ti02+I.0 wt % Al(OHb
- LDPE/HDPE+Ti02+3.0 wt % Al(0H)3
- LDPE/HDPE+Ti02+5.0 wt % Al(0H)3
- LDPE/HDPE+Ti02+10 wt % Al(0H)3
- LDPE/HDPE+Ti02+20 wt % Al(0H)3
- LDPE/HDPE+Ti02+30 wt % Al(0H)3
1- lg(-ln^) = -0.3+2.68 lgr
2- lg(-ln^) = -0.5+2.75 lgT
3- lg(-ln^) = -0.7+2.62 lgT
4- lg(-ln^) = -1.0 + 2.1 lgT
5- lg(-ln^) = -1.5+1.75 lgT
6- lg(-ln^) = -1.7+1.40 lgT
7- lg(-ln^) = -2.0+1.20 lgT
Given the crystallization time, it is possible to determine the proportion of non-crystallized material (9) in the nanocomposite using the presented equations. The results of the study can be useful for specialists in assessing the approximate temperature regime for cooling a product in the process of processing polymers by injection molding and extrusion.
Conclusions
Thus, based on the above, we can come to the following conclusions:
The loading of a structure-forming agent into a mixture of polymers led to an intensification of the interaction at the interface between the phases of two polymers, due to which this modification made it possible to obtain composites with an improved set of performance characteristics.
The process of crystallization of modified composites based on aluminum hydroxide and a mixture of high and low density polyethylenes has been experimentally studied by dilatometry. It has been established that the interactions between the particles of the structure-forming agent and the components of the polymer composite are the decisive factors for the distribution and agglomeration of aluminum hydroxide particles.
The loading of aluminum hydroxide into the composition of the initial LDPE/HDPE+ 1 wt % TiO2 mixture leads to a significant increase in the density of the composite.
With an increase in the concentration of aluminum hydroxide in the composition of HDPE / LDPE + TiO2 + Al(OH)3, the three-dimensional crystal growth is replaced by a two-dimensional lamellar, and it is replaced by
a linear type of growth of crystalline formations.
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ALÜMiNiUM HiDROKSiD VO POLiETiLEN QARI§IGI OSASINDA OLAN MODÎFÎKASÎYA OLUNMU§
KOMPOZiTLORiN KRlSTALLA§MASI
F.O.Mustafayeva, N.T.Qahramanov, O.M.Quliyeva
Maqalada quruluçamalagatirici ils modifikasiya olunmuç yuxari va açagi sixliqli polietilen qançigi va alüminium hidroksid asasinda olan kompozitlarin kristallaçmasinin dilatometrik tadqiqinin naticalari taqdim olunmuçdur. Alüminium hidroksidin miqdarinin xüsusi hacmin, sarbast xüsusi hacmin temperaturdan asililiginin dayiçmasinin qanunauygunluguna tasiri tadqiq olunmuçdur. Kompozit materiallarin kristallaçmasinin baçlangic temperaturu va çûçalaçma temperaturunun taxmini qiymatlari müayyan edilmiçdir. Tadqiqatlar IIRT-1 cihazinda asasinda dûzaldilmiç dilatometrda 5.3 kq yük altinda nümunalarin pillali soyudulmasi prosesinda apanlmiçdir. Alüminium hidroksidin miqdari 1-30 kütla % araliginda dayiçdilirmiçdir. Yuxari va açagi sixliqli polietilenlar qançigi 50/50 nisbatinda götürülmüijdür Titan dioksidin 1.0 kütla % miqdari quruluçamalagatirici kimi istifada edilmiçdir. Müayyan edilmiçdir ki, alüminium hidroksid midannin artimi ila polietilen qançigi asasinda olan modifikasiya olunmuç kompozitlarin kristallaçmasinin baçlama temperaturu dayiçmaz qalib 1050C-a barabar olmuçdur. Polimer qançigin tarkibinda alüminium hidroksidin miqdarinin artmasi ila kompozitlarin sixliginin müntazam artmasi mü^ahida olunmuçdur.
Açar sözlar: yuxari sixliqli polietilen, a§agt sixliqli polietilen, alüminium hidroksid, titan dioksid, qurulu§3m3l3g3tirici, kristalla§ma, dilatometriya, xüsusi hscm.
КРИСТАЛЛИЗАЦИЯ МОДИФИЦИРОВАННЫХ КОМПОЗИТОВ НА ОСНОВЕ ГИДРОКСИДА
АЛЮМИНИЯ И СМЕСИ ПОЛИЭТИЛЕНОВ
Ф.А.Мустафаева, Н.Т.Кахраманов, О.М.Гулиева
В статье приводятся результаты дилатометрического исследования кристаллизации модифицированных структурообразователем композитов на основе гидроксида алюминия и смеси полиэтиленов высокой и низкой плотности. Установлен влияние концентрации гидроксида алюминия на закономерность изменения зависимости удельного объема, а также, свободного удельного объема от температуры. Определены температура начала кристаллизации и приближенные значения температуры стеклования композитных материалов. Исследования проводились в процессе ступенчатого охлаждения образцов при нагрузке 5.3 кг на приборе ИИРТ-1 переделанном под дилатометр. Концентрацию гидроксида алюминия варьировали в пределах 1 - 30% масс. Смесь полиэтиленов высокой и низкой плотности было взята в соотношении 50/50. В качестве структурообразователя использовали диоксид титана в количестве 1.0 % масс. Установлено, что с увеличением концентрации гидроксида алюминия температура начала кристаллизации модифицированных композитов на основе смеси полиэтиленов остается неизменным и равен 1050С. Показано, что с повышением содержания гидроксида алюминия в полимерной смеси наблюдается закономерное повышение плотности композитов.
Ключевые слова: полиэтилен высокой плотности, полиэтилен низкой плотности, гидроксид алюминия, диоксид титана, структурообразователь, кристаллизация, дилатометрия, удельный объем.
