Assessments of classical dry-blending/moulding and solution casting PVC plasticization effectiveness – a comparative study

doi:10.21203/rs.3.rs-2015602/v1

Plasticizers are the most popular and indispensable plastic additives, facilitating their processability and providing suitable flexibility. Plasticizing efficiency is a term that is used to compare between different types of plasticizers. Plasticizing efficiency can be assessed using specimen fabricated through either a three-stage method consisting of mixing, extruding and pelletizing, or a solution casting method. In this paper, we aim to compare the results of plasticization efficiency assessed for specimen fabricated through both methods, to reveal their applicability and possible limitations. As an experimental model, we used poly(vinyl chloride) plasticized with three common plasticizers, namely bis(2-ethylhexyl) terephthalate, bis(2-ethylhexyl) phthalate and tris(2-ethylhexyl) trimellitate. Plasticization efficiency assessment was based on mechanical, thermal and migrability properties, including elongation at break, tensile strength, exudation and leaching in n-hexane, thermal stability, and glass transition temperature. The results revealed inconsistencies in the assessment of plasticization efficiency depending on plasticization method. For instance, the migration resistance studies showed that plasticizer release was much faster from solution casted specimens than from the moulded ones. On the other hand, higher tensile strength and elongation at break values were obtained for samples prepared by a solution casting method. The dependency of the results on the method of specimens’ plasticization highlights it as a factor that overestimates/underestimates plasticizer efficiency, and could affect the process of selection of plasticizers for industrial practice.

plasticizers

plasticization assessment

plasticizing efficiency

PVC

dry-blending

PVC is one of the most common plastics in the world. Bare poly(vinyl chloride) (PVC) is a hard and brittle material. It contains about 57% chlorine by weight, which provides polar regions in the chain making it compatible with a wide range of additives. Tensile strength of a PVC plastics decreases with increasing plasticizer content, hence the role of a plasticizer is to decrease the tensile strength enough to make the material more flexible and easier to process^1–6. The plasticization mechanism could be explained by three main plasticization theories: gel theory, lubricity theory and free volume theory. The most useful is the free volume theory proposed by Fox and Flory. The free volume can be understood as a gap between molecules and atoms. The addition of a plasticizer increases the free volume, which implies enhanced motion of polymer chains⁷. The effectiveness of plasticizer depends on its structure, for instance plasticizers that are not enough long molecules are not able to separate the PVC chains completely, so the interactions between PVC chains result in higher tensile strength⁸. Increased flexibility of plasticized materials leads also to the rise of elongation at break that is often expressed in percentage terms⁹.

In industrial practice the most common PVC plasticizers are phthalates. Under increasing environmental awareness and social pressures, however, phthalate plasticizers have become a subject of legal restrictions in many countries, mainly because of their adverse impact on human health and the environment^1–3. Therefore, several attempts have been made to develop new compounds that might act as plasticizers in PVC formulations. So far, researchers have been investigating bio-based compounds, branched and hyperbranched compounds, internal plasticizers (chemically bound to PVC chains) etc^7,10,11. One of the most important properties of newly developed plasticizers is their plasticization efficiency, relating the desired properties of plastics with the amount of plasticizer required to achieve this effect. According to the latest literature reports, to assess the plasticizing efficiency three main methods of material preparation are used: a) mould extrusion¹², b) calendering^13,14, c) solution casting^15–17.

In most cases, researchers prepare films using a solution casting method. Solution casting was developed in 19th century and is the oldest technology to make plastic films¹⁸. It is a laboratory low temperature process, in which a film with any thickness can be obtained. In this process, PVC and plasticizer are dissolved in tetrahydrofuran (THF) using a magnetic stirrer at elevated temperature. Then, the solution is casted into a Petri dish or another flat surface and dried to remove the solvent. According to Jia at al.^19,20 3–5% THF remains in the film and is difficult to remove. Nguyen et al.⁴ state that the residual solvent is at 2–3%. Regardless, this method is the easiest and does not require specialistic equipment, so it is widely used in R&D facilities.

Mould extrusion of dry blends is a method used by manufacturers, e.g. in wires and cables production²¹. First, a plasticizer is absorbed in the polymer matrix at higher temperature (about 50 to 80°C) during mixing. Later, also another additives can be added to the mixture, like inert fillers, stabilizers and others. The product is a dry powder that does not differ much from the initial polymer. Extrusion is a process in which a powder is converted into a molten viscous fluid in the extruder barrel by mechanical shearing and heating. In the second step, the extrudate is cooled (using water or air) and it can be pelletized^22–24. The most commonly used additives are: antidegradants, colorants, fillers, reinforcements etc.²¹. Calcium carbonate is the most widely used PVC formulations filler that is added mainly to reduce the price of the final product, which are often cables and wires²⁵.

Calendering is a process in which the hot thermoplastic is formed in rolled sheets using a system of heated counter-rotating rolls. The melt is fed to the rotating rolls (usually 3–6 rolls are used in industrial calenders) and then to cooling drums^26,27.

A comparison of two methods of preparing PVC plastic (solution casting and precipitation) without additives with the original PVC was performed in 1969 by Malac et. al.²⁸ Already then, it was noticed that the results of the studies vary depending on the plastic preparation method. The results were related to different densities of the samples caused by reduction in the ordered arrangement of polymer chains. The differences in sample properties were also caused by the presence of residual THF, which is impossible to remove completely, even at high temperatures for a long time.

There are several methods to evaluate the plasticizing efficiency and its usefulness in particular applications. Mechanical properties like elongation at break and tensile strength²⁹, shore hardness³⁰ and elastic modulus³¹ are the most commonly used and described in literature. Another study that can characterize a plasticized PVC material is the determination of a glass transition temperature. In this case, two methods are used: differential scanning calorimetry³² and dynamic mechanical analysis³³. The permanence of a plasticizer in a polymer matrix can be evaluated by migration studies. There are four main methods of plasticizer release to the environment: leaching, migration into solids, exudation and volatilization³⁴. The term migration stability is often used in each of the above phenomenon and is usually described as a change of weight in time than exposed to particular environment, e.g. air, liquids or solids. Apart from the environment contamination, the migration of plasticizers deteriorates the physical properties of a plastic.

In this paper, a comparison between two plasticized PVC preparation methods is presented: solution casting with THF (used mainly in laboratories), and the traditional method that consists in preparing a dry blend, extruding, pelletizing and pressing (used by manufacturers). The aim of the work is to evaluate the usefulness of these methods to estimate the properties of a newly proposed plasticizer incorporated in a PVC material. To the comparison, materials with three commonly used plasticizers were used: bis(2-ethylhexyl) terephthalate, bis(2-ethylhexyl) phthalate and tris(2-ethylhexyl) trimellitate in the amount of 50 parts per hundred resin (phr).

2.1 Materials

Bis(2-ethylhexyl) phthalate DEHP (99% purity), tris(2-ethylhexyl) trimellitate TOTM (96% purity), bis(2-ethylhexyl) terephthalate DEHT (98% purity) were provided by Grupa Azoty ZAK S.A. PVC type S-70 was purchased from Anwil S.A. THF with 99,9% purity was purchased from Honeywell. N-hexane was purchased from Avantor VWR. Thermal stabilizer – Baeropan R 8890 KA/2 was purchased from Baerlocher. Calcium carbonate used as a filler was purchased from Piotrowice Sp. z o.o. Low density polyethylene for exudation studies LOTRENE ® FD 0474 was purchased from Qatar Petrochemical Company Ltd. All the materials were used as received.

2.2 Mixing, extrusion, pelletizing and pressing of PVC blends

For preparing a plastic similar to plastics produced by manufacturers, the following formulation was used: PVC – 100 phr, plasticizer – 50 phr, CaCO₃ – 10 phr, thermal stabilizer – 4,5 phr. A heating and cooling mixer (High Power Jacketed High-Speed Laboratory Mixer from LabTech Engineering Company LTD) was used to prepare PVC dry-blends. The polymer and thermal stabilizer were mixed at 2500 rpm until the temperature of the blend reached 80°C. At this temperature, mixing speed was decreased to 1800 rpm and the plasticizer was added. In 95°C carbon dioxide was added, and the blend was mixed until it reached 110°C. After that the blend was poured into a cooling bowl and cooled using water at 750 rpm during 10 min. The whole cycle lasted about 25 min. The dry blend was subjected to seasoning at room temperature for 24 h. An intermeshing twin-screw extruder (ZAMAK Mercator, screw diameter: 24 mm, screw length: 40 D) with cooling bath and a pelletizer was used for extrusion. The extruder with nine heating zones was operated at a rotation speed of the screws of 35 min^− 1 and an operating temperature varying from 40°C (hopper) to 160°C (die head). Pellets were pressed in a bench top hydraulic press (Labtech, The Micro Scientific Bench Top Hydraulic Presses Type LP30-B) to obtain sheets of 2 ± 0,042 mm thickness (170°C, 16 MPa during 3,5 min followed by 4 min cooling). Additionally, for comparison with the solution casting prepared specimens the moulded samples containing only PVC and DOTP plasticizer (50 phr) were prepared. The preparation routine was the same as in the case of samples with all ingredients.

2.3 PVC/plasticizer solution casting

To investigate the mechanical properties and migration resistance of the studied materials containing plasticizers, the materials were prepared as films by solution casting^4,35. PVC powder (7,66 g) and plasticizer (3,83 g) were dissolved in THF (100 mL) to give solutions of 50 phr. The solutions were stirred at 50°C for about 6 hours until they became homogeneous. The prepared solutions were poured into clean Petri dishes and dried at ambient pressure in room temperature to remove most of THF. The solid films were taken out and kept in vacuum oven for 24 h at 35°C. Transparent thin films with thickness of 0,5 ± 0,012 mm were obtained. Finally, films were packed in aluminium foils and kept in desiccators for further testing. Similarly, for comparison with the dry-blending/moulded specimens, a solution casting samples containing also calcium carbonate filler and thermal stabilizer were prepared using analogues protocol.

2.4 Mechanical properties determination

Based on the standard SIST EN ISO 527-2:2012³⁶ the specimens for mechanical studies were printed into dumbbell shapes (type 1BA) using a pneumatic press (Zwick Roell, The 7108 Series). The mechanical properties (tensile strength, elongation at break) of the PVC samples were evaluated using universal testing machine (Zwick Roell 10kN Allround table-top universal testing machine) at room temperature. The specimen bars were tested at a strain rate of 50 mmˑmin^− 1 with a 10 000 N load cell (for extruded plastic) and with a 100 N load cell (for solvent-casted films). All values are reported as the average of at least five samples.

2.5 Thermal properties determination

For the thermogravimetric analysis (TGA), a TGA-2 Mettler Toledo instrument was used. A ~ 10 mg sample (pellet or foil) was heated from 25 to 650°C at a heating rate of 10°Cˑmin^− 1 under N₂ with a flow rate of 50 mLˑmin^− 1. Differential scanning calorimetry test were performed using DSC-3 Mettler Toledo calorimeter. Approximately 5–12 mg samples were weighed in aluminium pans (40 µL) and were subjected to two heating-cooling cycles from − 60°C to 120°C at 10°Cˑmin^− 1 under nitrogen (flow rate 50 mLˑmin^− 1). All values are reported as the average of at least five samples.

2.6 Migration tests

The leaching of plasticizers from the films was determined by the extraction of the samples in n-hexane. The films (10x10 mm squares;) were weighed (270 ± 10 mg for compression moulded samples and 63,5 ± 3,0 mg for solution casted films) and immersed into 50 mL of solvent for different periods of time (0,1 h; 0,5 h; 1 h; 2 h; 4 h; 8 h; 24 h; 48 h), at room temperature. After that time, the samples were removed from the solvent and dried at room temperature for about 24 hours until the weight was constant. The migration was evaluated by the content of the extractable part (w_e), calculated according to Eq. (1):

\({w}_{e}= {w}_{0}-{w}_{t}\) Eq. (1)

where w₀ and w_t represent, respectively, the weight before and after extraction.

The extraction degree (e_df) of solution casted films was evaluated according to Eq. (2):

\({e}_{df}= \frac{{w}_{e}\times 100}{{w}_{0}\times \frac{50}{150}}\) Eq. (2)

The extraction degree (e_dt) of moulded samples was evaluated according to Eq. (3):

\({e}_{dt}= \frac{{w}_{e}\times 100}{{w}_{0}\times \frac{50}{\text{164,5}}}\) Eq. (3)

The exudation tests were performed according to ISO 177:2016(E) standard³⁷. Migration was evaluated through an exudation test consisting in measuring the weight loss of flexible PVC specimens, being in contact with rigid LDPE sheets under pressure, at 70°C. The discs of ø 50 mm were weighed (5,57 ± 0,55 g for compression moulded samples and 1,1 ± 0,1 g for solution casted films) The weight measurements were performed at different times: 1, 2, 4; 5; 7; 10; 15 and 30 days. The migration was calculated according to Eq. (1)³⁸ and the migration degree was evaluated according to Eq. (2) and Eq. (3) for solution casted films and traditionally prepared plastic respectively.

3.1 Fabrication of plasticized PVC specimens

Despite there are several ways to prepare flexible PVC plastic, there is an urgent need to meet the expectations of manufacturers in studying of plasticizing efficiency and, therefore, the usefulness of new compounds applicable for plasticization of PVC. Among two fabrication methods compared in this study, one is preferred by the manufacturers (a three stage method), and the other by the researchers (a solution casting method). The solution casting method does not need specified laboratory equipment: the plastic is prepared in a flask with a magnetic stirrer. THF is used as a solvent that dissolves PVC chains and enables plasticizer to diffuse into polymer matrix. After evaporating the solvent, the plasticizer remains in the polymer matrix making it soft and flexible.

In case of a dry-blending/moulding method, PVC is mixed until its temperature reaches 80°C, i.e. temperature above a PVC glass transition temperature T_g. At this point the polymer chains are relaxed and the plasticizer is added to the mixture, so it can easily diffuse into the polymer matrix. After 24-hour seasoning, the dry-blend is extruded and the plastic is pelletized. The pellets are than pressed to prepare plastic sheets for further studies. To run the process, a special equipment is needed: mixer, extruder, pelletizer and press.

In the solution casting method, the most important challenge is to remove the most of THF. Samples that are dried only at atmospheric pressure and room temperature contain too much solvent after drying what results in large differences in migration and leaching results and overestimates the results of mechanical tests.

For comparison of the studies, additional attempts of material preparation were carried out. A dry blend without additional carbon dioxide and thermal stabilizer was prepared. Unfortunately, the mixture was not dry enough and therefore it caused problems during extruding the blend - the suspended wet powder made bridges in the hopper, so the extrusion process was hard to conduct. Calcium carbonate acts as a dryer that makes the blend dry which is necessary for further extrusion. Without additives the colour of the extruded melt was darker than for samples containing thermal stabilizer, supposedly the thermal degradation of the plastic occurred in some extent. Additional exposure of the sample to elevated temperatures during pressing caused significant darkening of the sample. To limit the time of pressing and therefore the exposure to elevated temperature in case of specimens without additives, their thickness was reduced to 0,4 mm instead of 2 mm thick moulders.

To prepare the solution casted samples with four ingredients, i.e. PVC, plasticizer, calcium carbonate and thermal stabilizer, the already ground and homogenized mixture of calcium carbonate and thermal stabilizer was added to the PVC and plasticizer solution in THF. As the inorganic component did not dissolve in THF, it sedimented during the casting and drying into Petri dish. Therefore, further testing of the samples was not performed.

3.2 Mechanical properties

The main aim of incorporating a plasticizer into PVC polymer is to enhance its flexibility and softness as well as to improve its processability. The most valuable parameters to estimate the flexibility of a plasticized PVC material are two mechanical properties: elongation at break and tensile strength. Tensile strength σ can be determined from the applied force and the cross-section of the sample from the following Eq. 3⁹:

Eq. (4)

where:

F_max – force at which the film breaks, N

t – initial film thickness, m

w – initial film width, m

A relationship between tensile strength and elongation is described by Young’s modulus (E) and is often determined when testing a new plasticizer^40,41. Values of Young’s modulus are calculated from the slope of a stress–strain curve. The lower the Young’s modulus value, the better the plasticizing efficiency⁴². In this paper, a comparison of tensile strength and elongation at break between differently plasticized/prepared PVC samples is presented in Fig. 1.

In each case, the values of elongation at break and tensile strength are clearly higher for solution-casted films than for traditionally prepared plastic. The differences in tensile strength are 7,7 (+ 45.8%), 7,5 (+ 41.0%) and 6,1 (+ 39.4%) MPa for DEHT, TOTM and DEHP plasticizer, respectively. The elongation at break values are 84–114 percentage points higher for solution-casted plasticized samples. In DEHT plasticized samples, the differences of tensile strength and elongation at break are the most prominent. Probably a completely removal of THF solvent was the most difficult in DEHT plasticized material. This could be indicated by two factors: big differences in elongation at break values (residual THF may act as plasticizer) as well as differences in resistance against migration to the low density polyethylene LDPE for samples that were not dried enough In such case the weight loss was significantly higher for samples with big amount of residual THF which also migrates to LDPE sheets and gives a wrong overestimated result.

3.3 Migration studies

Under given conditions, the permanence of a plasticizer in a flexible polymer product depends on several factors, namely its structure, chemical composition, molecular weight, and polarity. Usually, a plasticizer molecule is not chemically bonded to a polymer chain, therefore it can be released during manufacturing of a polymer or, later, during its everyday use. The plasticizer migrability has been a heavily investigated topic^{4,17,35,43−48} and n-hexane is the most commonly used extraction medium. However, the test conditions reported in literature often varied in details. While monitoring the weight changes of a sample soaked in different liquids, there are two possible observations: a) weight increase when small molecules of the liquid diffuse into the polymer matrix, and b) weight reduction when some additives (in this case plasticizer) are extracted from the sample. These two phenomena can occur simultaneously or separately⁴⁹. When using n-hexane in migration studies, the extraction of plasticizer dominates, so the sample weight decreases^50,51. As summarized in Table 1, the leaching of plasticizer from DEHP (also known in literature as DOP) plasticized specimens has been investigated in the temperature range from 23 ^oC to 50 ^oC, for the time from 2 h up to 168 h, for the plasticizer content from 30 to 100 phr, and for the specimen of thickness between 0.1 mm and 0.5 mm. Because of the high variety of experimental conditions, the extent of leaching for DEHP plasticizer was between 21.6% and 93.9%, and for DEHP between 8.45% and 22%.

The weight loss always depends on the plasticizer content in the PVC plastic as well as the medium that is in contact with the plastic. Obviously the thermal and pressure conditions also play an important role in determining the plasticizer migration³⁴.

Table 1 N-hexane leaching data collected from literature

Plasticizer	Temperature, ^oC	Time, h	Amount of plasticizer, phr	Film thickness, mm	Additional info	Extent of leaching, %	Reference
DEHP	50	168	60	0,4	stirred at 100 rpm	77*	⁴⁴
DOP	23	24 h	60	no data	-	19*	¹⁷
DEHP	50	2 h	60	0,1	-	83*	³⁵
DEHP	50	2 h	60	0,25	stirred at 100 rpm	93,9	⁴³
DOP	50	2 h	60	0,1	-	22*	⁴⁵
DEHP	50	2 h	60	0,2	-	36,0	⁴⁶
DEHP	50	2 h	60	0,2	-	21,6	⁴⁷
DOP	hot	2 h	100	no data	-	14	⁴
DOP	30	24 h	30	0,5	-	8,45	⁴⁸
* - values estimated from figure

Therefore, in our study we have made the efforts to keep the experimental conditions constant, to make it easier to compare plasticizer migration in samples fabricated by different methods. The ratio of plasticizer migration between three studied plasticizers is presented in Fig. 2.

In each case the percentage loss of plasticizer was higher for the solution casted samples than for the moulded ones. The leaching process from solution casted samples was much faster and the equilibrium is reached about 8 hours (Fig. 2B). For moulded samples, the equilibrium is observed after two days of leaching (Fig. 2A). Extent of migration of each tested plasticizer from a solution-casted film reached about 70% after 8 hours while at the same time the plasticizer loss from moulded samples was from about 30% (DEHP) to about 60% (DEHT). Probably it is because of the possible migration of plasticizer outside the material sample, which was more difficult for thicker materials than for thin films, because of the longer diffusion patch from the inner part of samples. Finally, after longer time the extent of migration in case of moulded samples is lower about 10 percentage points that for solution casting prepared specimen. Due to that, the migration resistance for manufactured plastics cannot be estimated on the basis of the results of plasticizer migration from the solution-casted samples because it will be always overestimated. On the other hand, comparing the studied plasticizers it seems that regardless the sample’s preparation method, the order of migration resistance of plasticizer is the same, that it allows to infer which plasticizer shows the highest migration resistance and which present the lowest. Bis(2-ethylhexyl) terephthalate in both cases (solution-casted films and traditionally prepared plastic) exhibits the highest migration. Comparing the leaching resistance of novel plasticizers with the commonly used products from solution casted films enables to estimate qualitatively the extent of leaching in a real product, for example if the leaching resistance is higher than for DEHT but lower than for DEHP, in traditionally prepared plasticized PVC materials the relationship should be similar. Because the maximum difference between the extent of migration depends on a plasticizer type, estimating of plasticizer leaching from the traditionally plasticized material based on studies of solution casting films will not always be relevant.

Leaching studies in n-hexane were also conducted for samples prepared by dry-blending/moulding but without a filler and thermal stabilizer and plasticized with DEHT (Fig. 2C). As it can be deduced from the graph, the extent of migration of solution casted specimen and moulded without additives is very similar in both cases and visibly higher than for plasticized PVC sample that was moulded with calcium carbonate and thermal stabilizer additives. It indicates that calcium carbonate reduces the migration of a plasticizer from the plastic. The filled composite polymer material forms complex structure at which all components can contribute to the overall migration resistance.

Exudation tests are important for plasticizers applicable e.g. in flooring manufacturing and cables industry, where the pressure applied to the material is higher. Migration tests into solid materials are widely described in literature. Different polymers are used as an “absorbing” material into which the plasticizer will migrate under pressure, e.g. acrylonitrile butadiene styrene (ABS)¹⁷, rigid PVC³⁸, polyethylene terephthalate⁴⁴, filter paper^52–54, poly(methyl-methacrylate)⁵⁵. The tests could be done with⁵⁵ or without^52–54 pressure. The exudation tests in this study were performed using a low density polyethylene and the results are shown in Fig. 3. It is important to ensure the adequate absorption capacity of used solid absorber, especially for longer measurement time. If the absorption capacity is too low the deference between the weight loss of specimen and gain in weight of solid absorber will appear.

In samples prepared by /moulding, during the exudation process the samples colour changed from nude to dark violet and from transparent to violet in samples prepared using solution casting method (Fig. 4). At the beginning of the exudation process, the migration rate is the highest. The migration phenomenon is much faster for samples prepared by a solution casting method, which is mainly caused by the thickness of the samples and their slightly lower densities (1,27 ± 0,020 gˑcm^− 3 against 1,34 ± 0,033 gˑcm^− 3 for moulded samples) that provide free spaces for facile diffusion path formation of plasticizer. In both groups of samples prepared by different methods, the highest migration under elevated pressure is exhibited by DEHT and the lowest by TOTM, what was also observed in a leaching in n-hexane. The migration tests of solution casted samples must be performed directly after their preparation, otherwise the results will be underestimated. The source of error in case of solution casted specimens, especially at high plasticizer content like 50 phr, is a phenomenon of plasticizer self-exudation without any external forces during their storage, what is not observed for the dry-blended/moulded samples.

In exudation test performed after 7 days. the extent of migration of the sample moulded without stabilizer and carbonate fillers reaches 34,70% (Fig. 3C). The value is higher than for moulded specimens (14,50%) but still clearly lower than for solution casted samples (51,40%), what is different than in n-hexane leaching where the determined extent of migration was very close for moulded without additives and solution casted samples.

3.4 Glass transition and thermal stability

As the temperature decreases, there is a point when all rotations about chain bonds cease entirely so the energy is insufficient to cause neighbouring atoms to twist around one another. The temperature at which chain molecular rotation ceases and the polymer becomes glassy and rigid is the glass transition temperature. In PVC chains, the glass transition temperature is relatively high (e.g. in comparison to polyethylene) because of the size of the side group – chlorine atom, which hinders the chain rotation^56,57. The heating of PVC above the glass transition temperature causes loosening of chain-chain interactions and therefore enables plasticizer to penetrate the polymer matrix in more spots. Differential scanning calorimetry is a commonly used method to achieve a complete miscibility of the plasticizer and PVC. A glass transition temperature (T_g) is described as a temperature in which amorphous regions of a polymer transit from rigid state into flexible state²³. A single T_g on a DSC graph can be understood as a criterion of compatibility of PVC and plasticizer⁵⁸. In DSC studies, a sample is usually heated in two heating cycles. The first heating cycle is needed to remove residual solvents and to erase the thermal history of the polymer while in the second cycle the data about T_g are collected. The lower the T_g the better the plasticizing efficiency. In many studies the T_g of sample with proposed plasticizer is compared to a sample with DEHP and usually the latter presents the lowest T_g, therefore finding better plasticizer than DEHP is difficult^14,35,38,43.

The glass transition temperature was determined using differential scanning calorimetry. The glass transition of plasticized PVC samples is quite weak and barely visible, so the extremum taken from the first derivatives of DSC curves is presented as a temperature of a glass transition instead of onset temperature. The results obtained as an average value from 5 independent measurements are shown in Table 2.

Table 2 Glass transition temperature determined from DSC studies

Plasticizer	Glass transition temperature, T_g °C
Plasticizer	Solution casted	Moulded
DEHT	-14,4 ± 1,21	-16,7 ± 0,28
DEHP	-9,5 ± 0,98	-11,6 ± 0,35
TOTM	-8,1 ± 1,07	-10,4 ± 0,26
DEHT*	-	-8,6 ± 0,46
*sample moulded without additives: thermal stabilizer and carbonate filler

The presence of only one glass transition temperature is a proof of good miscibility in polymer blends⁵⁹. In all cases, the T_g values were lower in case of PVC specimen plasticized in traditional manner by dry-blending/moulding method and the difference was equal around 2 degrees. Along with the calculated standard deviations the T_g study points out that plasticization process is more effective during dry-blending/moulding PVC processing.

An important factor to evaluate properties of a new plasticizer is thermal stability of the plasticized PVC material. In this case thermogravimetric analysis (TGA) is used. The thermal stability is known as the mass loss of a sample during heating. On TGA curve of a plasticized PVC often three steps occur: a) dehydrochlorination of PVC (the highest weight loss) and formation of polyenes, b) formation of aromatic compounds by the cyclization of conjugated polyene, c) degradation and decomposition of the complex structures resulting from an aromatization and formation of low hydrocarbon structures^54,60,61. Dehydrochlorination of PVC causes many problems in plastics products like discoloration and deterioration of mechanical properties⁴³. There are several factors that affect thermal stability. More ester bonds, benzene groups or epoxy rings (that can scavenge evolved HCl) provide a good thermal stability to the material. Also polyesters with high molecular weight can delay elimination of HCl molecules from PVC^14,35,62,63.

The results of TGA tests are shown in Table 3. At the beginning, the decomposition of PVC behaves similar: especially DEHP and DEHT exhibit almost equal temperatures in which the weight loss reaches 5% of the sample. The rate of weight loss of moulded samples is faster and the corresponding weight loss thresholds are reached earlier than for solution casted specimens. Because of the presence of additives, i.e. calcium carbonate and thermal stabilizer in moulded samples, the final weight loss of the solution casted samples is significantly higher and reaches about 95% in contrast to ca. 75% for moulded ones. The calculated differences between expected theoretical weight loss during the first dichlorination step of PVC and derived from TGA measurements are relatively low (ca. 0.25 mg) for PVC samples plasticized by dry-blending/moulding method. Otherwise, solution casted samples exhibit 0,8–1,3 mg higher weight loss from theoretical value, what can be connected to the residual solvent (THF) presence in these samples.

Table 3 The weight loss of plasticized PVC specimen derived from TGA analysis

Weight loss, %	Temperature of given weight loss of plasticized PVC samples, °C
	TOTM		DEHP		DEHT
	casted	moulded	casted	moulded	casted	moulded
5	274,6	267,9	243,4	246,0	257,7	258,2
10	289,1	270,9	259,6	262,4	271,5	268,7
25	306,1	280,2	282,4	269,2	290,2	274,2
50	324,4	300,8	299,5	279,1	309,0	289,5
75	362.0	483,7	355,4	477,8	345,8	506,8

In this paper, two methods of preparing plasticized PVC material were compared. The solution casting method is usually used for laboratory testing because there is no need to use advanced equipment. On the other hand, it cannot be used by manufacturers in production process because of some drawbacks of this method, namely significant amounts of toxic solvent, residual of solvent present in the plastic and limited possibility of using another additives. The traditional, three-stage method consisting of mixing a dry blend, extruding, pelletizing, and pressing requires proper equipment but also bigger amounts of all ingredients. Several tests of samples prepared using these two methods were conducted. In all cases, the absolute values of the measured parameters were different, however, the relativities between three studied plasticizers were the same independently of the preparation method. Presented study shows that the solution casting method used commonly in R&D facilities can be used only for a preliminary assessment of the suitability of newly synthesized plasticizer. Substances that pass the "preliminary evaluation" need to be re-evaluated later in a dry-blending/moulding three stage method that is closer to the industrial PVC processing condition.

Acknowledgment

J.C. acknowledges the Ministry of Science and Higher Education, Poland for financial support received in the framework of the “Industrial doctorate” scholarship funding granted under agreement DWD/3/7/2019-09/001 and Silesian University of Technology for partial funding under the project no 04/040/BKM21/0180. R.T. acknowledges Silesian University of Technology for partial funding under the project no 04/040/RGJ22/0200.

Data availability statement

Data generated and analysed during the current study are available from the corresponding author on a reasonable request.

Author contributions statement

Conceptualization: JC, EP, RT; Methodology: JC, RT; Data curation: JC; Investigation: JC, RT; Formal analysis: JC, RT; Writing – original draft preparation: JC, RT; Writing – review and editing: JC, EP, RT; Visualization: JC, RT; Supervision: EP, RT. All authors contributed to the general discussion, reviewed the manuscript and agreed to it published version.

Additional information - Competing interests

The authors declare no competing interests.

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No competing interests reported.

Assessments of classical dry-blending/moulding and solution casting PVC plasticization effectiveness – a comparative study

Status:

Version 1

Abstract

Figures

1. Introduction

2. Materials And Methods