Experimental Investigations on the Polypropylene Behaviour during [305143]

Experimental Investigations on the Polypropylene Behaviour during

Ultrasonic Welding

T. Chinnadurai1, S. Arungalai Vendan1, E. Scutelnicu2,*

1[anonimizat], Vellore, India

2”Dunarea de Jos” [anonimizat], Romania

E-mail addresses: [anonimizat], [anonimizat],

* Corresponding Author’s e-mail address: [anonimizat]

Abstract: [anonimizat], low cost. This research has focused on the investigation of the behaviour of polypropylene during ultrasonic welding process. The lap welded samples were examined by modern methods such as Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA) and Fourier transform infrared spectroscopy (FTIR). Besides, [anonimizat]. TGA and DSC results showed a negligible difference between the mass losses of the molded and the welded propylene materials. Furthermore, the SEM images revealed the formation of voids in close correlation with vibration frequency. [anonimizat].

Keywords: Polypropylene, ultrasonic, welding, DSC, TGA, FTIR, SEM, analysis.

Introduction

The major phenomena influencing the behaviour of polypropylene (PP), [anonimizat]: [anonimizat], region and range of melting [1]. [anonimizat] [2]. [anonimizat], a key concern in the PP behaviour [3-4]. However, the slightest modifications/[anonimizat]. The crystallisation degree of thermoplastics is governed by the transcrystallisation phenomenon [5]. It is well known that crystallinity during melting involves molecular formations and reorientations. [anonimizat]. Henceforth, it is very important to investigate these aspects before using PP for any practical engineering applications. Polypropylene (PP) [anonimizat] [6-7].

[anonimizat]-elastic heating during ultrasonic welding. [anonimizat] [8]. In the Ultrasonic Welding (USW) [anonimizat]-frequency vibrations of 20-40 kHz are applied to two material parts to be joined by a vibrating tool (horn) which is held at right angle to the contact area in order to enable the complete transfer of longitudinal vibration through the horn. The heat needed for the welding of material layers is developed by the intermolecular friction at the interface of the joint [9-10]. This welding technique is fast, efficient, non-contaminating and no consumables are required.

In this paper, investigations on ultrasonic welding of injection molded PP material and subsequent examinations have been conducted in order to collect and discuss the experimental data in terms of strength, structure and other properties of the PP joint. Researchers worldwide have performed and reported results in terms of the thermal analysis of raw PP material, while extension of the analysis for welded PP material is rarely or not yet reported. However, it is essential to get information on the thermo-mechanical behaviour of the PP material subjected to high vibrational heat and to better understand the effects of the joining process on the base material properties. Hence, DSC method was used to measure the degree of crystallinity corresponding to varying temperatures and TGA method was applied to examine the mass loss. Furthermore, FTIR and SEM characterizations were performed in order to investigate the phenomena developed at the welded PP interface.

Materials and Methods

The polypropylene PP granules, characterised by the melting temperature of 170°C, have been subjected to injection molding in the experimental programme. Moreover, the decomposition temperature range from 205°C to 430°C was considered in the research. The melt flow index of the polymer was 26-30g/10min (2.16 kg/230°C) and the material's density at a temperature of 23°C was 0.905-0.917 g/cm3. A study of PP using resins with different molecular weight by insert injection molding has been reported by Wang J., Mao Q. C, Chen J. N in [11]. In this study an insert injection molding method was developed to process PP components with short cycle time. Figure 1 illustrates the mold design embedded with the energy director design. During the experimental programme, both the injection time and the holding time were set at value of 5s. The PP parts were subjected to injection and holding pressures of 207 and 167 kPa, respectively, and the cooling time was maintained at 30 s. To investigate the influence of temperature on the properties of PP material behaviour, the injection temperature was modified in the range of 170…185°C. Four injection temperature values were set in the experimental tests, as follows: 170, 175, 180 and 185°C. Figure 2 shows the lab joint design and its orientation during welding with energy director design values.

The PP material was subjected to ultrasonic welding which was made with a Dukane iQ Series i220 ultrasonic welding system characterised by a frequency of 20 kHz and power levels from 1200 to 2400 W. The equipment offers advanced features such as process limits, 0.5 ms sample rate and patented trigger by power [12] apart from time, energy and distance provisions. Figure 3 shows the welded samples and the region of the energy director melting.

The investigations were conducted by using modern methods such as Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA), Fourier Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscope (SEM) examination.

The DSC method is a current technique used for characterization and for determining the thermal transition of a polymer such as crystallisation, glass transition temperature, degradation and melting [13]. A DSC 200F3 Maia instrument was used to measure the thermal transitions of PP samples which were subjected to heating from -70°C to 220°C and subsequently cooled from 220°C to -70°C. The second heating started at -70°C up to 200°C, in the presence of nitrogen, with a heating rate maintained at 10 K/min in aluminium crucible with lid and sealed conditions. Thermal degradation was expected in nitrogen with negligible thermo-oxidation in the presence of a small residual amount of absorbed oxygen. Henceforth, in polypropylene the degradation of nitrogen has to typically proceed in an identical pattern and the temperature difference of the degradation onset is considerably small [14]. Using DSC analysis, the melting temperature (Tm), melting enthalpy (ΔHm) and the degree of crystallinity may be determined. Applying equation (1), the degree of crystallinity (% cryst) of the samples is estimated based on the melting enthalpy results (ΔHm) of each sample, where PP is the mass fraction of the PP material in the sample, ΔHm is the experimental melting enthalpy and ΔHm0 is the melting enthalpy for 100% crystalline PP material (207.1 J/g) [13].

(1)

The DSC thermoanalytical technique enables the measurement of transmission and reaction enthalpy. Glass transition is a reversible transition when amorphous material is subjected to heating or cooling in a temperatures range [15]. Changes in glass transition alter the specific heat capacity, the mechanical modules, the coefficient of thermal expansion and the dielectric constant [16]. It is well known that the cold crystallisation process occurs below the glass transition. As the temperature increases beyond Tg, no formation of cold crystallisation occurs. So, it is obvious that the cold crystallisation or crystallisation formation is inversely proportional to the temperature [17]. Information on the crystalline structure is generally obtained by estimating the cristallysation or melting peaks. The melting peak delivers the melting enthalpy of the sample and the crystallinity can be forecasted. The peak temperature determines the average size of the crystallites and the peak shape can be transformed to crystallites size distribution. Crystallisation curve, peak temperature and corresponding enthalpy of the crystallisation are determined while the peak temperature can be expressed as a function of the cooling rate. Kissinger method is used to determine the degradation of crystallisation kinetics by equation (2) [13].

(2)

where α is the fraction rate, A – frequency factor, E – activation energy, T – temperature and n – reaction order.

The activation energy of crystallisation with heating rate is obtained from the Kissinger equation (3):

(3)

where: Φ is the cooling rate, Tp – temperature at which the maximum conversion rate occurs in the curve.

The TGA of the PP material was performed with a STA 449 F3 Jupiter instrument whose working temperature is up to 1000°C. Nitrogen atmosphere and heating rate of 10 K/min were considered in the experiments. TG-DSC was the sample carrier and TG/DSC Alumina with lid was the sample crucible. TG analysis is a technique that measures the mass of a sample while it is heated, cooled or held isothermally at a defined atmosphere. This mass loss is related to the loss of moisture content, solvents, monomers and it also occurs due to the polymer decomposition. Sometimes, the mass loss may be caused by the final residuals and the combustion of the carbon block. The mathematical model which has been developed to describe the kinetics of a system undergoing chemical change is expressed by the mathematical equation (4) [18]. As polymer degradations are often chain reactions, f(X) represents the net result of a series of elementary steps and the rate constant k changes with absolute temperature, according to the Arrhenius equation.

(4)

The sample was also analysed through the FTIR method, in the laboratory of VIT University, Vellore. Infrared light source generates wavelength from 4000 to 400 cm-1 32 times per sample, Fourier transforming instrument keeping air as reference. The spectra are obtained for the polymers absorbance as a function of the wave number. The variations in absorbance before and after irradiation are compared and the peak analysis is done to the variation in position and relative intensity of the bands. Some existing bands could disappear and new bands could emerge. The nature of a group frequency is identified by the region in which it is located [19-20].

The SEM examination of the weld was performed with a Hitachi scanning microscope (Model S-450, Japan). Firstly, the samples were etched in a solution of KMnO4-H2SO4-HNO3-H3PO4 and forwards the surface morphology and interface integrity were examined by field emission SEM, operating at 20 kV.

Results and Discussion

The thermal analysis of the injection molded PP surface is shown in figure 4. It can be noticed that the presence of endothermic peaks in the 1st and 2nd phases of the heating curves enables the energy absorption by molecules leading to transitions and diffusion. Eventually, these actions lead to bonding among the PP grains. Gradually, the material flow pattern is enabled, causing adhesion among the PP grains. Contrary to this observation, exothermic peaks occur during the cooling phase which is initiated at a temperature of 123.3°C temperature. During this phase, an excessive amount of energy is released wherein there is a transition of amorphous to semi-crystalline nature. The area under the curve is proportional to the amorphous content in the material.

Melting is an exothermic process with a greater energy release and occurs at a defined temperature for pure substances. The energy is responsible for weakening the molecular bond, facilitating good adhesion among the molecules of identical materials during molding and welding. While cold crystallisation is absent at glass transition temperature, the melting peak corresponds to broading and relaxation, creating energy bonding between melted layers whose viscosity undergoes substantial mitigation. As the materials exhibit greater amorphous nature, the glass transition temperature, Tg, occurs during the heating-cooling cycle. The material hardens during the cooling process and this could cause the embrittlement of the material. Due to the vibrational heat, the specimens subjected to ultrasonic joining process could undergo a considerable decrease of grains size. The joint interface reveals higher hardness after cooling, comparing to the PP base material which is considerably softer. During the first heating, the glass transition (Tg) starts from the temperature of -1.1°C , reaches the mid temperature of 3.6°C and ends at a temperature of 8.7°C, with 0.200J/Kg heat capacity (Cp). During the second heating, the Tg onset value is 4.3°C, the mid value is 7.5°C and the end value is the temperature of 10.5°C, for a corresponding heat capacity of 0.188 J/Kg. Comparing these values to the PP base material characteristics (Cp=0.46J/kg, Tg=0°C), lower thermal values are noticed during the heating process due to localized heat loading on the energy directors with the application of the vibrational energy of the sonotrode. The glass transition also exhibits enthalpy relaxation, which is evident from the overlapping of endothermic peaks. The second heating curve illustrates that the Tg is smaller for the first heating curve wherein the amorphous material content is lower than the crystalline content. That could be interpreted as a remarkable increase of the crystallinity degree [21-22]. Mechanical modules and dielectric constant are detected at steep curves (Tg) which reveal a change of the specific heat capacity (Cp) during the melting stage. Due to the glass transition change which occurs between the first and the second heating, the heat capacity decreases by 6%, from 0.200J/Kg to 0.188 J/Kg, respectively.

The cooling time curve shows the following temperature values: 120.3°C (onset), 123.3°C (peak) and 126.8°C (end). Cold crystallisation is an exothermic process occurring below the Tg value, whereas the molecular mobility is restricted and the crystallisation is hindered. Above the Tg value, the crystallisation process is developed at a relatively higher temperature. Crystallinity of the PP material is proportional to the rate of PP chains interchanges. It is generally impossible to observe relative molecules between the polymers chains. Wunderlich illustrated the fact that semi-crystalline polymers reveal reversible and irreversible melting. The reversible portion during melting corresponds to recrystallisation, re-melting and melting cycles at the laminar surface, while the non-reversible portion points to the complete melting of the laminar surface. In the reference [22] the authors reported an increase in the magnitude of the reversible contribution as temperature increases. However, temperature modulation is a value adding parameter.

From figure 5 it can be noticed that, compared to the molded PP base material, the melting temperature tends to decrease by about 15% on the welded surface due to the preheating which the material undergoes during moulding, leading to change in the crystalline nature. The concentration of the heat flux is only on the tip of the energy director which accommodates the discharge of the heat for non energy director regions. On the other hand, the melting peaks, corresponding to the first heating and the second heating of the welded material, are merged together. The major change occurs in terms of the Tg values which are increasing in the case of the molded PP material, while they are decreasing for the welded PP material, due to the slow cooling time which is attributed to the absence of any cooling mechanism which may induce stress cracks. The first heating curve reveals that the melting onset and peak values are 132.6°C and 162.3°C, respectively, while the end temperature is 167.1°C. In this case, the Tg values are 15.3°C at onset, 21.0°C mid value and 26.6°C at the end of temperature transition, corresponding to 0.313J/Kg heat capacity. As figure 5 shows, crystalline melting onset value of 132.5°C, mid value of 162.3°C and end value of 166.8°C are recorded during the second heating. The glass transition values achieved at the second heating are the following: 5.6°C (onset), 8.5°C (mid) and 11.2°C (end), corresponding to Cp of 0.265J/Kg (Fig. 5). A decrease of 16% of Cp values is recorded between the first and the second heating which may be attributed to the formation of oxygen bonds during oxidation, yielding flakes on the surface.

As regards the cooling curves – specific to the PP material, non-welded and welded – a comparison between the charts illustrated in figures 4 and 5 shows an insignificant change of the peak areas and of the shape of the curves. Multiple exothermic peaks observed during the cooling process may be attributed to different types of crystal nucleation, as it demands different energy barriers causing nucleation process.

Due to the ultrasonic vibration, higher molecular frictional heat is developed during the welding of the PP material and, consequently, the melted amorphous phase grows while the crystalline phase decreases. In comparison with the crystalline materials, which have a narrow transition temperature range, the amorphous materials have a relatively wide transition in properties near the glass transition temperature (Tg) and, therefore, the materials properties are less affected. Increasing the surface energy at the crystal / amorphous phase interfaces, the crystallinity stability decreases and henceforth the melting point, too. The crystals thickness is governed by different chains length that relies on the crystallisation conditions. The crystallisation process has a significant influence on the amorphous phase behaviour. On the other hand, the cooling time is dictated by the response of the PP material to welding. For instance, higher crystalline structure of the material demands higher welding vibration time and also higher energy to melt the crystalline phase. The absorption / release of the energy in different ranges is based on the crystalline nature and orientation of the crystalline structure, as well as on the amorphous phase. During fast cooling, because of insufficient time for proper crystal growth, the melting point decreases and imperfect crystals are finally formed [23]. This imperfect crystalline nature will considerably improve the weld strength, since the amorphous structure increases and the crystalline structure decreases. The increase of the crystallinity degree determines the increase of melting enthalpy, too. Higher cooling rate leads to the formation of crystallites with imperfections and smaller size, causing the diminishing of melting and crystallisation temperatures in comparison with the crystals formed from highly long propylene chains. A slight increase of the crystallinity degree is clearly observed for a lower cooling rate, which may be attributed to the formation of ordered crystalline structure. During welding, an excessive amount of energy is released wherein there is a transition of the ultrasonically heated portion from amorphous to semi-crystalline nature. The area under the curve is proportional to the amorphous content in the material. The frequency of the ultrasonic vibrations creates electronic torsions, spin and vibrations, leading to the generation of molecular frictional heat which is slowly released during the cooling curve. This also enables the conversion from amorphous to the semi-crystalline state of the welded portion of the material. The presence of increasing amorphous content after welding indicates the occurrence of reversible transition Tg during the heating-cooling cycle. This reversible transition alters the material hardness and is likely to increase the brittle nature. This may be caused by the considerable reduction of the grain size which is altered during the ultrasonic joining process by the vibrational heat. The joint interface reveals higher hardness after cooling in comparison with the PP base material which is considerably softer.

The experimental results of the investigation on the PP base material mass loss using TGA technique have been plotted and presented in figure 6. As it can be noticed, the degradation of the polymer occurs in a single stage process. Two peaks based on the endothermic reactions have been recorded during heat flow change as function of temperature. The first peak corresponds to the melting of the polypropylene material; it starts at 154.8°C and proceeds up gradually to 174.7°C with the dissociation of weight loss. The primary weight loss in the composite material is caused by the moisture absorbed in the presence of filler particles [24]. The second peak is associated to the decomposition reaction. The mass change is discussed by analysing the weight derivative versus temperature plot. The temperatures during the initiation and the end reactions, respectively, reveal the phase of maximal reaction rate owing to the higher rate of molecular combinations and dislocations associated to the domain wall breakages. The sample weight remains almost stable in the temperature range from 165°C to 390°C and it rises steeply above 400°C up to 503.5°C. Furthermore, increasing the heating rate, the thermograms shift right, confirming the rapid decomposition produced at melting temperature. The weight loss which occurs within the temperatures range from 410.5°C to 503.5°C may be attributed to the thermal degradation of PP main chains. This thermal degradation is caused by the oxidation phenomenon where the molecular bonds of polymer are altered by oxygen molecules.

The DSC curve presented in figure 6 illustrates the mass loss stages, where the first peak indicates the initiation phase of mass loss and the second stage depicts the complete mass loss. The TGA results reveal a less ordered crystalline nature that leads to the attainment of double melting peaks, while a higher ordered crystalline nature shows a defined deposition of up and down helix in the unit cell. The mechanism of the homogenic nature is observed for less ordered crystalline scenarios while a more ordered crystallisation formation tends to yield a heterogenic nature mechanism at higher temperatures.

Because of the absence of moisture and gas bubble infiltration, there is no mass loss during the welding process, as it is observed from the TGA chart (Fig.6). Also, due to the lesser rate of the reaction, which does not significantly alter the molecular integrity and the domain wall locations, no prominent properties change occurs during the heating-cooling cycle. The ultrasonic vibration has negligible implications on the material nature, but changes in terms of thermal behaviour have been clearly noticed. Higher crystalline samples, showing greater elastic modules, exhibit less elastic behaviour for deformations of small ranges. This observation can be interpreted as an alteration of the PP material during the welding process.

The FTIR analysis, presented in figure 7, provides information on the behaviour of the PP material subjected to ultrasonic welding. The investigations have revealed the following recorded characteristic peaks assigned to different vibrational modes or other factors: 2916.37 cm-1 (–CH2 symmetric stretching), 2949.16 cm-1 (–CH3 asymmetric stretching), 1375.25 cm-1 (–CH3 symmetric bending), 1460.11 cm-1 (–CH3 asymmetric bending), 1166.93 cm-1 (characteristic peak specific to CHCl3) and 997.20 cm-1 due to the helical chains of the PP base material.

Subsequent to the closeness of double bonds, the propylene substance is evaluated by the extent of weld portions subjected to CH3 and CH2 bending bands at 1375.25 cm−1 and 1460.11 cm−1, independently. On the other hand, the ethylene substance is measured by extent of the locale bunches at 719.45 cm−1 and 1166.93 cm−1, due to the proximity of single bonds and uneven stretching out of particles. The peak at 997.20 cm−1 occurs in view of the reiterating monomer units in the crystalline triple helical structure of PP. The absorption at 972.12 cm−1 is a result of the short helix segments resulting from the sub-nuclear vibrations at the crystalline and amorphous locale of PP, independently. The relative intensity of these two bands may be adopted for estimating the level of PP crystallinity in the material. Characteristic of the polyethylene crystallinity is the band at 719.45 cm−1 created from continuous methylene sequences. The band intensity rises with the content of crystallisable ethylene sequences. The dominant bands observed are the characteristic bands of the control PP sample which depict the isotactic nature of the polymer. The presence of the same main bands in the welded PP reveals that the isotactic nature of PP is not distorted by the vibration.

All spectral bands of highly coupled non-localized factor-group mode reveal the presence of an appreciable concentration of polymer segments that confirm the structure of the regular chain, i.e. helical segments in the control PP films. The key portion of these bands has a rise in their intensity and a few disappeared, since validation disorder is introduced because of the damage by the energetic electrons. The symmetric and asymmetric stretching, bending and wagging of CH3 and CH2 group frequencies are observed in the welded PP material. The asymmetric stretching of CH3 and CH2 groups is observed at the wave number 2953 cm-1 and 2916 cm-1, respectively, because of conjugations in the molecular bonds. The CH3 symmetric stretching is observed at 2870 cm-1 in the welded samples which is explained by the increase in amplitude of the molecular bond vibrations. The symmetric and asymmetric scissoring vibrations of the methyl group are observed at 1456 cm-1 and 1375 cm-1, respectively which are attributed to the differential wavelength and differential energy that cause stretching. The absorption bands representing the helix structure of PP are observed in the welded PP at the value of 1166.93 cm-1 and the value of 972 cm-1, indicating that the helix structure has been altered by the high vibration energy.

The images achieved by SEM method, representing the welded PP surface in correspondence with the vibrations frequency applied during the USW process, are shown in figure 8. It can be noticed the melting flow direction which is marked on the SEM images. A comparative analysis of the images illustrated in figure 8 (a), (b), (c) reveals more voids and a lower rate of melting when low and medium frequency systems are used. During the ultrasonic welding of high frequency, fewer voids are developed and, consequently, good adhesion and weld integrity at the interface are achieved. Hence, weld strength and weld quality are better. It is evident that the melting rate of the energy director and the strength of the welded joint increase with increasing the ultrasonic vibrational frequency.

Figure 8 (a) shows less melting because of smaller vibration frequency leading to poor weld quality and weld strength. During smaller vibrations, melting is non-uniform along the entire energy director, irrespective of the applied pressure, and the heat flow associated with the material in the welded region is inadequate thereby affecting the weld strength and quality. SEM images clearly exhibit the differences between the variation of frequency and the melting of energy directors.

Conclusions

This study focused on the weldability investigation of the Polypropylene (PP) material by different and modern investigations techniques. The following key points are summarized from this research study:

The change in the thermal behaviour of the moulded and welded PP material was observed when analysed by DSC technique. Increase of glass transition temperature (Tg) values and decrease of crystalline melting temperature have been noticed during the acquisition and processing of data. These phenomena are caused by the molecular alignment which occurs when the specimens are subjected to ultrasonic vibrations. However, these changes observed during the heating and cooling curves do not prominently alter the basic nature and the fundamental structure orientation ratios of polypropylene.

TGA-DSC analysis shows marginal difference of 2-3 percentage mass loss between the moulded PP and the welded portion of the specimen. This is an important observation, as any welding process identified for polymer joining is expected to create minimal mass loss to ensure retaining of originality. Hence, the feasibility of ultrasonic welding is also established for the PP material.

FTIR analysis has revealed that the bond strength has undergone minimal change while there are a few small peaks visible in the finger print region which indicates negligible alterations in material composition.

SEM analysis has illustrated that higher vibration frequency causes fewer voids, compared to small and medium frequency vibrations. The weld strength and bond integrity appears to be higher for higher frequency vibrations, emphasizing that the lap joint interface strength is higher for minimal or no voids. However, voids have to be within certain limits whose standards are yet to be evolved.

Future investigation involves the welded specimens to be subjected to multiple heating temperatures during DSC analysis to determine the delta change in the exothermic and endothermic curves during cooling and heating phases. These results would help understand the fracture behaviour of the ultrasonic welded polypropylene. Finite element analysis is to be carried out to estimate the temperature distribution occurring along the material.

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Fig. 1. Butterfly type mold design parts with energy director

Fig. 2. Lap joint design incorporated, with energy director and without energy director design

Fig. 3. Molded PP and welded PP material with energy director melting condition

Fig. 4. Injection molded PP – DSC thermal analysis

Fig. 5. Injection molded PP after welding – DSC thermal analysis

Fig. 6. Mass loss analysis of PP base material using TGA technique

Fig. 7. FTIR analysis of PP base material after welding

(a)

(b)

(c)

Fig. 8. SEM images: (a) low amplitude vibration melting of energy director, (b) medium amplitude vibration melting of energy director, (c) high amplitude vibration melting of energy director