Influence of Processing Parameters on Mechanical Properties and Degree of Crystallization of Polylactide

This work attempts to assess the influence of process parameters on the change of mechanical properties and the degree of crystallinity of polylactide (PLA). PLA is a biodegradable material that has been widely used in various areas—from packaging, through medicine, to 3D printing, where it is used to produce prototypes. The method of processing is important, because the technological process and its parameters have a significant impact on the quality of the finished product. Their appropriate selection depends on quality and mechanical properties. The process parameters have an impact on the structure of PLA, specifically on the share of the crystalline phase, which is also important from the point of view of the functional properties of the finished product. This work assessed the impact of the technological parameters of the injection process on the final properties of the obtained samples. The obtained results of static tensile strength, hardness and differential scanning calorimetry (DSC) analysis confirm that changing these parameters affects the material properties.


Introduction
The widespread use of plastics has resulted in a negative impact on the environment.These include long decomposition times, lingering in landfills and a significant carbon footprint.On the one hand, polymeric materials have become synonymous with progress, but on the other hand, they have become a global problem.One of the activities limiting these negative impacts is the development of biodegradable polymers that decompose in the natural environment.Biodegradable polymers are an alternative to traditional plastics.
A representative of this group is polylactide (PLA).It is a thermoplastic material that is completely biodegradable within six months to two years.These include aliphatic polyesters.Its good mechanical and processing properties enable it to be formed using basic methods such as injection, extrusion, thermoforming and 3D printing [1][2][3][4][5].
PLA, due to its thermoplastic and processing capabilities on typical machines, as well as good mechanical properties, biodegradability, and non-toxicity of both the material itself and its decomposition products, has a very wide range of applications, from packaging to medicine.It is used as a biodegradable material for bioresorbable surgical threads, clips, staples, and surgical meshes, as well as for bone fixation screws and capsules that release a predetermined dose of the drug at a specific time.Products made of PLA also include membranes that accelerate the healing of extensive wounds, hygiene products, surgical masks, and medical clothing [6][7][8][9][10][11].
In agriculture, it is used for protective foils when growing plants, for tunnels to protect flowers and vegetables, and to protect young trees against pests and negative temperatures.
After the end of their useful life, these products remain in the soil and biodegrade, thus enriching it further.PLA is also used as input material for 3D printers.It is expected that the scope of use of PLA will systematically expand [21][22][23][24][25].
Biodegradation itself is a process in which the polymer material decomposes under the influence of environmental factors.This process is favored by appropriate ambient humidity and temperature as well as living organisms such as yeast, bacteria and fungi that are found in the surroundings of the material [26,27].The polymer material may undergo complete biodegradation with the release of e.g., carbon dioxide, ammonia, methane or water, or only partial biodegradation, for instance of one of the components of the material [28][29][30][31][32][33].
However, the rate of biodegradation, in addition to humidity and heat, is determined by the shape of the object, its geometry and thickness.Low crystallinity, low molecular weight, chemical groups susceptible to the action of appropriate enzymes, and water absorption by the polymer are important factors.PLA naturally undergoes a crystallization process, and, in this respect, it is like PET, but the crystalline phase is very poorly biodegradable [34][35][36][37].
The degree of crystallinity of PLA can be very high, up to 60%.The melting point of this material ranges from 170 to 180 • C, and the glass transition temperature is approximately 65 • C. It crystallizes fastest at 110 • C. It is a relatively high-density material, ranging from 1.2 g/cm 3 to 1.3 g/cm 3 .It is possible to control the share of the crystalline phase through appropriate machine settings, and thus, the time and speed of PLA decomposition can be adjusted [38][39][40].Control of the share of the crystalline phase in PLA can be achieved by regulating the parameters of the production and technological process, such as melting temperature, cooling speed, and processing time.Additives to PLA may also influence the share of the crystalline phase.Therefore, by adjusting these parameters, it is possible to regulate the time and speed of PLA decomposition by controlling the share of the crystalline phase.However, this process requires a thorough understanding of the PLA crystallization mechanisms and adaptation of the process conditions to specific application requirements [41][42][43][44][45].
PLA can exist in various isometric (or polymorphic) forms that differ in their crystal system and physical properties.Isomerization refers to the change from one crystalline form to another, which can be triggered by various factors such as temperature, time, pressure, processing method, etc. PLA can crystallize in several different forms, including α (alpha), β (beta) and γ (gamma).Each of these forms has a different crystal system and properties.α-form (alpha): This is the most stable and common crystalline form of PLA.It is characterized by an orthorhombic system.β (beta) form: This is less common and can be induced under certain processing conditions such as high temperature stretching.γ (gamma) form: This is rare and occurs under specific processing conditions.Each isometric form of PLA has different mechanical and thermal properties.For example, the α form tends to have a higher melting point and greater thermal stability than other forms.The β form may have increased mechanical strength in the tensile direction [30][31][32][33][34][35][41][42][43][44][45].
The aim of this work is to carry out the sample injection process with various machine settings (temperature, pressure, speed) and to assess the impact of these parameters on the mechanical properties and degree of crystallization of PLA.This research will make it possible to attempt to explain the phenomenon of PLA crystallization in the injection molding process.It is also an attempt to control the degree of crystallinity and mechanical properties of the product by selecting appropriate technological parameters of the manufacturing process.The innovativeness of the work lies in a holistic approach to the analysis of processing processes and their impact on the properties of PLA products.This technological and process approach can bring significant benefits to the production of products with the desired characteristics.
To confirm the thesis stated in the introduction, this study used tests to determine strength in a tangential test and hardness, as critical parameters for this material, to assess the mechanical properties.The degree of crystallinity was determined based on the obtained differential scanning calorimetry (DSC) curves.

Materials and Methods
In this study, polylactide (PLA) from NatureWorks (Minneapolis, MN, USA) under the name Ingeo Biopolymer 3251D was used for research; it is intended for processing using injection technology.Table 1 provides data for the polylactide Ingeo Biopolymer 3251D [46].To prepare samples for testing, a UT90 horizontal screw injection molding machine from Ponar Żywiec ( Żywiec, Poland) of the UT series for thermoplastics was used, with a five-point, double, lever mold closing system and direct drive of the screw with a hightorque hydraulic motor.Peripheral devices used in the process include an injection mold with replaceable inserts for paddles and bars, a thermostat used to maintain a constant temperature of the injection mold, a DARwag electronic scale, and a KC 100/200 dryer.
The parameters of the paddle injection technological process are given in Table 2, where the temperature of the technological process has changed.In this way, three groups of material with different processing temperatures were obtained: 180 However, the change in the injection pressure and mold temperature settings for the preparation of the bars and the assessment of the change in the impact of these settings on the change in the degree of crystallinity determined in the DSC tests are given in Table 3.To properly prepare the material for testing, PLA was dried before the processing process to remove moisture, as it could have a negative impact on the technological process and the quality of the obtained products.The treatment was carried out for 8 h at a temperature of 80 • C in accordance with the material manufacturer's recommendations.
For mechanical tests, three series of samples in the form of paddles were made changing the processing temperature according to the parameters included in Table 2.However, samples in the form of bars for DSC tests were made in six series, changing the mold temperature and injection pressure in accordance with Table 3.In order to check the injection process, mass monitoring was carried out, and each time the samples were weighed after the injection process.Table 4 shows the average weight of the injected samples.
A small standard deviation indicates that the technological process was stable, and the obtained injection mass values are closely concentrated around the average mass value of the tested samples.Before commencing the actual tests, all samples were air-conditioned at a constant temperature of 23 • C and 50% humidity for 120 h.The examination of strength characteristics in a static tensile test was carried out in accordance with the guidelines contained in the ISO 527-1 [50] and ISO 527-2 [51] on the Fu1000e testing machine from Heckert (Chemnitz, Germany) with a measuring head up to 10 kN.The measurement consisted of static stretching of standardized samples at a constant speed of 2 mm/min.During the test, the change in force and elongation at break were recorded.At least five repetitions were performed for each series so that the measurement error was within 20%.Samples that broke in the machine holders or outside the extensometer were discarded.
Hardness was determined using the ball indentation method in accordance with the ISO 2039-1 standard [52] on a 101 Kudi Gnehm plastic hardness tester (Basel, Switzerland).Ten measurements were made on each batch of samples, and then the average hardness value was determined, and the measurement uncertainty was estimated in accordance with the procedure adopted in the standard.
DSC tests were carried out using a DSC Q2000 differential scanning calorimeter (TA Instruments, New Castle, DE, USA) in accordance with ISO 11357-1 [53].The device was connected to a computer equipped with Q Series software (https://www.tainstruments.com/support/software-downloads-support/downloads/, accessed on 17 June 2024) for processing data obtained during measurements.
DSC studies were performed using a Q2000 flow calorimeter (TA Instruments, New Castle, DE, USA).Indium was used for calibration as a standard substance.Measurements were carried out at temperatures from −10 to 250 • C, with a constant temperature increase of 10 • C/min, in a nitrogen flow of 50 mL s −1 .The system was heated, cooled, and heated again.
Then, the calorimeter was programmed by entering the required data into the program, such as sample weight, maximum heating and minimum cooling temperatures, and temperature jump (every 10 • C/min).Two measurements were made on one sample.First, the sample was quickly cooled to −10 • C, then heated, cooled and heated again.This treatment allows you to get rid of the stresses left in the sample after the processing process.The entire measurement took about an hour and a half.The process proceeded according to the following scheme: 1.
Marking the end of cycle 1, 4.
End of measurement.
From each series of samples produced with the given technological parameters, three samples were taken and tested.In this way, DSC thermograms were obtained for six samples for which the technological parameters were changed.

Results and Discussion
Table 5 summarizes the average results obtained in the static tensile test.Strength and elongation at break were analyzed.The obtained results show that a change in the processing (injection) temperature influences the change in stress and strain at break.It is not clearly large, but observable.The course of all stretching curves is similar, and straight.After reaching the maximum stress, the samples experienced brittle fracture.The hardness measurement results are presented in Table 6.The values given are averages from a series of measurements.As the process temperature increases, a slight decrease in the hardness of the material is observed.The processing temperature may affect the rheology of the material, i.e., its behavior during the process.These changes may lead to micro and macro changes in the structure, which in turn may affect its hardness.The obtained values, similarly to the results obtained in the static tensile test, oscillate within the error limit.However, this does not exclude the fact that a slight decrease in hardness was observed.Table 6.Average hardness of samples depending on the processing temperature.In order to prepare samples for determining the degree of crystallinity, a fragment of previously produced "bars" (Figure 1) weighing approximately 5 mg was taken.

PLA-I-180
Materials 2024, 17, x FOR PEER REVIEW 6 of 11 lead to micro and macro changes in the structure, which in turn may affect its hardness.The obtained values, similarly to the results obtained in the static tensile test, oscillate within the error limit.However, this does not exclude the fact that a slight decrease in hardness was observed.In order to prepare samples for determining the degree of crystallinity, a fragment of previously produced "bars" (Figure 1) weighing approximately 5 mg was taken.In this way, DSC thermograms (a graph of temperature versus heat flow) were obtained for six samples.All calculations were performed on the first measurements.The analyzed process curve on the graphs obtained from the device was marked as a solid line, as shown in Figure 2 for the sample marked PLA25C0B (Table 3).
DSC measurements of the obtained samples were performed.Examples of DSC curves obtained for the PLA25C0B sample are shown in Figure 2. The DSC curves presented in Figure 2 show the changes that the PLA samples underwent during the measurements.The first, low-temperature transformation is the process of glassing the sample, which is associated with a step change in heat capacity.In the first measurement, this transformation is superimposed by the B-relaxation process.In this way, DSC thermograms (a graph of temperature versus heat flow) were obtained for six samples.All calculations were performed on the first measurements.The analyzed process curve on the graphs obtained from the device was marked as a solid line, as shown in Figure 2 for the sample marked PLA25C0B (Table 3).
Materials 2024, 17, x FOR PEER REVIEW 6 of 11 lead to micro and macro changes in the structure, which in turn may affect its hardness.The obtained values, similarly to the results obtained in the static tensile test, oscillate within the error limit.However, this does not exclude the fact that a slight decrease in hardness was observed.In order to prepare samples for determining the degree of crystallinity, a fragment of previously produced "bars" (Figure 1) weighing approximately 5 mg was taken.In this way, DSC thermograms (a graph of temperature versus heat flow) were obtained for six samples.All calculations were performed on the first measurements.The analyzed process curve on the graphs obtained from the device was marked as a solid line, as shown in Figure 2 for the sample marked PLA25C0B (Table 3).
DSC measurements of the obtained samples were performed.Examples of DSC curves obtained for the PLA25C0B sample are shown in Figure 2. The DSC curves presented in Figure 2 show the changes that the PLA samples underwent during the measurements.The first, low-temperature transformation is the process of glassing the sample, which is associated with a step change in heat capacity.In the first measurement, this transformation is superimposed by the B-relaxation process.The DSC curves presented in Figure 2 show the changes that the PLA samples underwent during the measurements.The first, low-temperature transformation is the process of glassing the sample, which is associated with a step change in heat capacity.In the first measurement, this transformation is superimposed by the B-relaxation process.For the second heating cycle, B-relaxation is not visible.At a temperature of approximately 100 • C, an exothermic transformation is visible, associated with the crystallization of the amorphous phase present in the sample.The last high-temperature transformation visible on the DSC curve is the melting of the crystalline phase.The obtained DSC results are presented in Table 7.In the program (TA Universal Analysis), on ready-made thermographs, calculations were made of the increase in heat capacity in the process of glass transition, crystallization, and melting, as well as calculations of extrapolated temperatures (onset temperatures) of these transitions.The onset temperature was determined at the intersection of the tangent edge of the peak and the baseline.
The first transformation is the glass transition process (Figure 3).To determine the desired values, the baseline lines before and after the glass transition process were marked on the graph.After determining the beginning and end points of the process, the end temperature of the transformation (63.  7. In the program (TA Universal Analysis), on ready-made thermographs, calculations were made of the increase in heat capacity in the process of glass transition, crystallization, and melting, as well as calculations of extrapolated temperatures (onset temperatures) of these transitions.The onset temperature was determined at the intersection of the tangent edge of the peak and the baseline.
The first transformation is the glass transition process (Figure 3).To determine the desired values, the baseline lines before and after the glass transition process were marked on the graph.After determining the beginning and end points of the process, the end temperature of the transformation (63.46 °C), the onset temperature (64.38 °C) and the change in heat capacity (0.6755 J/g * °C) were determined.3).The arrows indicate the beginning and end of the measurement section.
The second transformation is crystallization (Figure 4).Characteristic parameters are determined in the same way as for glass transition.The final transformation temperature is 102.40 °C, the final temperature is 90.53 °C, and the change in heat capacity is 27.97 J/g.  3).The arrows indicate the beginning and end of the measurement section.
The second transformation is crystallization (Figure 4).Characteristic parameters are determined in the same way as for glass transition.The final transformation temperature is 102.40 • C, the final temperature is 90.53 • C, and the change in heat capacity is 27.97 J/g. Figure 3. Glass transition curve for PLA25C0B (Table 3).The arrows indicate the beginning and end of the measurement section.
The second transformation is crystallization (Figure 4).Characteristic parameters are determined in the same way as for glass transition.The final transformation temperature is 102.40 °C, the final temperature is 90.53 °C, and the change in heat capacity is 27.97 J/g.  3).3).
The last transformation to take place was the melting process (Figure 5).In this case, the characteristic parameters were also determined in the same way as in the case of glass transition.The final transformation temperature was 169.65 • C, while the final temperature was 164.58 • C, and the change in capacity was 43.30J/g.The last transformation to take place was the melting process (Figure 5).In this case, the characteristic parameters were also determined in the same way as in the case of glass transition.The final transformation temperature was 169.65 °C, while the final temperature was 164.58 °C, and the change in capacity was 43.30J/g.  3).
All thermograms for all samples included in Table 3 were analyzed in the same way.Then, the degree of crystallinity of individual PLA samples was determined and the impact of changing process parameters (Table 3) on this degree was determined.
The degree of crystallinity of the polylactide samples was calculated based on their enthalpy of melting.The determined melting enthalpy should be corrected for the cold crystallization process visible on the DSC curve.The degree of crystallinity of polylactide was determined using Formula (1).
The enthalpy of the melting process of a 100% crystalline sample of polylactide is 93 J/g [22].The summary of DSC results is presented in Table 7, while the calculated degree of crystallinity of the tested samples is given in Table 8.   3).
All thermograms for all samples included in Table 3 were analyzed in the same way.Then, the degree of crystallinity of individual PLA samples was determined and the impact of changing process parameters (Table 3) on this degree was determined.
The degree of crystallinity of the polylactide samples was calculated based on their enthalpy of melting.The determined melting enthalpy should be corrected for the cold crystallization process visible on the DSC curve.The degree of crystallinity of polylactide was determined using Formula (1).
where ∆H t -fusion enthalpy; ∆H t -crystallization enthalpy; ∆H t100% -enthalpy of the melting process of a 100% crystalline sample of polylactide.

Figure 1 .
Figure 1.View of samples for DSC testing.

Figure 2 .
Figure 2. Example of a DSC thermogram for the PLA25C0B sample with two measurements, dashed line-first heating process, solid line-second heating.

Figure 1 .
Figure 1.View of samples for DSC testing.

Figure 1 .
Figure 1.View of samples for DSC testing.

Figure 2 .
Figure 2. Example of a DSC thermogram for the PLA25C0B sample with two measurements, dashed line-first heating process, solid line-second heating.

Figure 2 .
Figure 2. Example of a DSC thermogram for the PLA25C0B sample with two measurements, dashed line-first heating process, solid line-second heating.DSC measurements of the obtained samples were performed.Examples of DSC curves obtained for the PLA25C0B sample are shown in Figure 2.The DSC curves presented in Figure2show the changes that the PLA samples underwent during the measurements.The first, low-temperature transformation is the process 46 • C), the onset temperature (64.38 • C) and the change in heat capacity (0.6755 J/g * • C) were determined.Materials 2024, 17, x FOR PEER REVIEW 7 of 11 For the second heating cycle, B-relaxation is not visible.At a temperature of approximately 100 °C, an exothermic transformation is visible, associated with the crystallization of the amorphous phase present in the sample.The last high-temperature transformation visible on the DSC curve is the melting of the crystalline phase.The obtained DSC results are presented in Table

Figure 3 .
Figure 3. Glass transition curve for PLA25C0B (Table3).The arrows indicate the beginning and end of the measurement section.

Figure 3 .
Figure 3. Glass transition curve for PLA25C0B (Table3).The arrows indicate the beginning and end of the measurement section.

Figure 4 .
Figure 4.The course of the crystallization process for the PLA25C0B sample (Table3).Figure 4.The course of crystallization process for the PLA25C0B sample (Table3).

Figure 4 .
Figure 4.The course of the crystallization process for the PLA25C0B sample (Table3).Figure 4.The course of crystallization process for the PLA25C0B sample (Table3).

Table 2 .
Parameters of the injection process of samples for strength and hardness tests.

Table 3 .
Values of injection pressure and mold temperatures when preparing samples for testing to determine the degree of crystallinity.

Table 4 .
Average sample injection weight depending on the process temperature.

Table 5 .
Strength and elongation at break depending on the processing temperature.

Table 6 .
Average hardness of samples depending on the processing temperature.

Table 6 .
Average hardness of samples depending on the processing temperature.

Table 7 .
Summary of DSC measurement results for individual samples.

Table 7 .
Summary of DSC measurement results for individual samples.