Impact of organic peroxide on a moderate molecular weight homo-polypropylene vis breaking,and mechanism of interaction

Abstract

In this study, we show that organic peroxide is a useful tool for breaking the viscosity or chain of polypropylene during melt processing to provide a regulated rheology product. Reactive extrusion is used to crosslink peroxide and combine it with polypropylene (PP). To achieve end-use applications with performance targets, stabilizers are required to preserve the polymer’s initial strength, flexibility, and toughness properties. Other additives are added to PP in addition to stabilization in order to enhance or change certain of its properties. With the addition of varying levels of organic peroxide [2,5-Dimethyl-2,5-di (tert-butyl peroxy) hexane]. The use of peroxide in the manufacturing process of polypropylene is a method of breaking in the polymer chains, which can affect its properties, including its MFI. It is possible that increasing the amount of peroxide used leads to a higher degree of branching or cross-linking, which in turn leads to a higher MFI value. However, it is important to note that the relationship between the amount of peroxide used and the resulting MFI values may not be linear and may depend on other factors as well. In addition to the MFI, other properties of the polypropylene were also measured, including shear and melt flow index, melting and crystallization temperatures, flexural and tensile moduli, and yield stress. These properties are important for understanding the mechanical and thermal behavior of the polymer and can be used to optimize its performance for specific applications.

Keywords

Polypropylene peroxide extrusion compounding plastic antioxidants extrusion and polymer degradation plastic additives

1 Introduction

There are numerous formulations of polymers available, and the underlying polymer type and additions will affect the qualities. These polymers have improved resistance to high temperatures, oxidation, impact loads, flammability, UV, surfactants, radiation, as well as changes in other properties. Compounding is the process of adding essential additives or components to polymers in order to improve them. However, tailoring a basic polymer’s inherent properties to a particular application is frequently impracticable. In contrast, other polymers, such as polypropylene, can have a range of properties based on their molecular weight, tactility, and molecular weight distribution. Compounding provides a practical method of modifying specific attributes for other polymers with a narrower range of qualities.

As previously said, the modification to create these properties is normally carried out by mixing a polymer with extra polymeric components to fit the intended end-use.

To achieve a particular set of desired final attributes, compounds must pass through a system involving the primary polymer and chosen additions. Blending at two important levels during compounding, distributive and dispersive mixing levels must be carried out.

The amount of shear that is utilized to mix the polymer and additives influences the dispersive mixing level. Different viscosities, melting points, chemical compatibilities, and other differences must be taken into account during dispersive mixing. The improper dispersion of polymer molecules will produce domains as a result of insufficient mixing of all components. The dispersive mixing level focuses on the short-range mixing of the compound, whereas the distributive mixing level is more concerned with the overall homogeneity of the combination. Depending on the apparatus being utilized, distributed mixing imposes a number of restrictions on the blending process. It is essential that every ingredient that is added to a polymer throughout the compounding process is kept in the compound in a way that guarantees functioning.

Due to both the intense shear action and the thermal degradation brought on by high temperatures, several additives may degrade. The method must be developed and validated using a functional analysis of the final product. Procedures for mixing must be continuously controlled to guarantee consistency in quality. Variations in the compound’s quality may have a negative effect on components made from it in a number of different ways.

359 million tons of plastic were manufactured worldwide in 2018, which represents an increase of roughly 3% from 2017. There are numerous unique uses for various plastic products. Packaging (39.9% of the market), building and construction (19.8%), and automotive (9.9%) constituting the top three markets for plastic products. These three industries use polypropylene (PP) frequently for goods like banknotes, food packaging, hinged caps, microwave containers, pipelines, and other items. When low-density polyethylene (PE-LD) and high-density polyethylene (PE-HD) are divided into two separate groupings, PP (with a percentage of 19.3%) is the most important polymer for producing plastic goods in 2018 [1].

The advantages of PP, such as its remarkable chemical resistance, high melting point, high tensile modulus of elasticity, high stiffness, low density, and low price, are what give it its significance. Furthermore, it has excellent flexural fatigue resistance, is straightforward to make using injection moulding, extrusion, and spinning, and can be stabilised to provide good thermal ageing stability [2, 3].

However, PP is challenging to use in processes including thermoforming, film blowing, blow moulding, extrusion coating, and foaming because it lacks melt strength and strain hardening, linear, or unmodified [4].

2 Materials and methods

2.1 Materials and instruments

All reagents were purchased from Merck and were used as received without any further purification.

Table 1
Polypropylene resin, Homopolymer polypropylene as virgin polymer (semi-crystalline) as shown in Fig. 1

Properties Identification

Synonyms: Polypropylene; Polipropene; Polypropene;

Propene polymers; Propylene polymers;

1-Propene; [-CH₂-CH(CH₃)-]_n

Density 0.91 g/cm³

Melting density 0.7389 0.91 g/cm³

Cas no. 9003-07-0 (C₃H₆)_n

Melt point 165°C

Properties	Identification
Synonyms:	Polypropylene; Polipropene; Polypropene;
	Propene polymers; Propylene polymers;
	1-Propene; [-CH₂-CH(CH₃)-]_n
Density	0.91 g/cm³
Melting density	0.7389 0.91 g/cm³
Cas no.	9003-07-0 (C₃H₆)_n
Melt point	165°C

Fig. 1

Structure of polypropylene Homopolymer.

Table 2

2,5-Dimethyl-2,5-di(tert-butyl peroxy) hexane,is an efficient peroxide for the degradation of polypropylene

Properties	Identification
CAS number	78-63-7
Active oxygen	≥10.14%
Appearance	Clear liquid
Assay	>92%
Color	≤50 Pt-Co / APHA
Viscosity	20°C 6.4 mPa.s
Density	20°C 0.870 g/cm₃

Fig. 2

Structure of 2,5-Dimethyl-2,5-di(tert-butyl peroxy) hexane.

2.2 The experimental approach

The sample preparation according to the reactive extrusion as the follow: The Collin single screw extruder was used for the reactive extrusion process. This type of extruder is commonly used in the processing of polymers and other materials, The length/diameter (L/D) ratio of the extruder is 25. The L/D ratio is an important parameter that affects the processing of the material and the resulting properties of the product. A higher L/D ratio generally allows for better mixing and homogenization of the material during processing, The extruder has 3 independent heating zones: feeding zone, extrusion zone, and die zone. These heating zones are used to control the temperature of the material during processing, which can affect its flow and properties, The screw has a channel depth of 1 mm and a diameter of 30 mm. The screw geometry is an important factor that affects the processing of the material, including its mixing, shearing, and compression, The screw speed can go up to 160 rpm, which is a measure of the rotational speed of the screw. The screw speed affects the throughput and the shear rate of the material during processing, The hopper has a capacity of up to 19 liters, which is the volume of material that can be fed into the extruder at one time.

The reactive extrusion was carried out with a screw speed of 77 rpm at 230°C to simulate industry-related requirements. According to Giles et al. [5], the calculation of the shear rate (γ) in an extruder screw is done by using Equation (1), where D is the screw diameter in mm, N is the screw speed in rpm and h is the channel depth in mm. Therefore, according to the above-mentioned parameters a shear rate of 121 s^–1 in the screw channel of the extruder was calculated. $γ = (π \times D \times N) / (60 \times h)$ (1)

The various peroxide concentrations were combined directly with the virgin PP powder and added to the extruder. The same amount of polypropylene powder was used in each composition, but the peroxide concentrations varied as follows Table 3.

Table 3

The different concentrations of virgin pp with peroxide

Sr	Sample name	Concentration of peroxide in ppm	Weight of sample
1	0.1 pp	100	1 kg
2	0.2 pp	200	1 kg
3	0.3 pp	300	1 kg
4	0.4 pp	400	1 kg
5	0.5 pp	500	1 kg

For further sample preparation, the five PP composites were pelletized with a cutting machine “Standard pelletizer GSG 171/1, Dr Collin, Germany).

2.3 Mechanical testing

According to ISO 1873-2 And U17310. The test specimens for tensile test were prepared by injection molding machine ENGLE (victory 200/80 focus, Austria). The injection molding parameters were set with an 80°C mold temperature and a clamping force 790 KN. According to (ISO 527) the tensile testing machine (smar Tens 005,KARG Industrietechnik, Germany) was used with a test speed of 2 mm/min and was Supplied with a 5 kN load cell and an extensometer. The flexural test specimens (60 mm×10 mm×4 mm) were prepared by injection molding under the same conditions. And according to (ISO 178) the flexural modulus was got by the same device of tensile test [6].

2.4 Characterization of melt mass-flow-rate (MFR) and the melt volume-flow-rate (MVR)

By forcing molten material through a die with a predetermined length and diameter under predetermined temperature and load conditions, the melt mass-flow rate (MFR) and melt volume-flow rate (MVR) are calculated.

Timed segments of the extrudate are weighed and utilised to determine the extrusion rate, in grammes per 10 minutes, for MFR measurement (process A).

For the purpose of measuring MVR (process B), the piston’s movement over a predetermined distance in a predetermined amount of time is recorded and utilized to determine the extrusion rate in cubic centimeters per ten minutes. If the melt density of the material at the test temperature is known, MVR can be converted to MFR or MFR to MVR.

NOTE At the test temperature and pressure, the melt must have the specified density. The values obtained at the test temperature and ambient pressure in practice are sufficient because the pressure is low.

The melt mass-flow rate (MFR) and melt volume-flow rate (MVR) of the various samples were characterized by Melt Flow Indexer (Tinuis Olsen, USA, MP 1200, 230°), and under 2.16 kg weight according to the DIN EN ISO 1133 method A, in g/10 min [7].

3 Mechanism of interaction

A wide molar mass distribution (MMD) distinguishes traditionally made ex-reactor polypropylene. A following in-line melt processing phase in an extruder creates so-called “controlled rheology” grades with decreased molar mass and dispersity. These grades have better processability and the precise rheological behavior needed for a variety of applications, such as thin-wall injection molding and fiber. The procedure, which often involves radical-induced chain-scissioning or vis-breaking, has been thoroughly documented in studies addressing reactive extrusion processing of polyolefins. The compounds that contain organic peroxide, such as cumyl peroxide, are those that are most frequently used as sources of radicals, di-tert-butyl peroxide, 2,5-dimethyl-2,5-di-tert-butylperoxyhexane (DHBP; also known under trade names such as Luperox™ 101 or Triganox™ 101), di(tert-butylperoxyisopropyl)benzene and other peroxides. However, there are a number of disadvantages to using these organic peroxides in this situation, chief among them the production of organic volatiles such tert-butanol and acetone in the case of DHBP (Scheme 1). This has spurred a large effort to create additional initiators for use in the procedure. Cyclic peroxides, a variety of nonperoxide initiators, such as certain azoalkanes and alkoxyamines, and UV photoinitiators are some of these [8].

Scheme 1

Mechanism of thermal decomposition of 2,5-dimethyl-2,5-di-tert-butylperoxyhexane (DHBP). [H] is a hydrogen atom source, e.g., polypropylene.

The typical explanation of the vis-breaking mechanism involving organic peroxides is depicted in Scheme 2. In the procedure, tertiary radicals (α to the polypropylene methyl) are formed on the backbone and then split into an unsaturated chain end and a polypropylene propagating radical by b-scission, Bertin et al. [9] have put out a really distinct mechanism. Surprisingly, they discovered that there was a slight increase in molar mass for trials conducted under an inert atmosphere when they investigated polypropylene breakdown in solution at 140–165°C. It should be emphasised that both the commercial procedure for generating regulated rheology polypropylene and the conditions used in the current study (∼280°C, polymer melt), differ markedly from those used by Bertin et al. [9] (140–165°C, solution). They discovered that oxygen was necessary for effective chain scissoring. On the basis of density functional calculations, they also suggested that, in contrast to predictions made on the basis of relative bond strengths, hydrogen abstraction by tert-butoxy (and other tert-alkoxy radicals) Since the activation entropy of polypropylene methyls is more favourable, radical should arise preferentially from these compounds. The principal radicals so produced primarily terminate by combination rather than easy -scission [9]. They hypothesised that oxygen’s presence and the resulting methylperoxy and hydroxy radicals’ production were responsible for the efficiency of chain scission seen in the commercial vis-breaking method, and that these radicals should exhibit less selectivity.

Scheme 2

Proposed mechanism of chain scission.

4 Results and discussions

4.1 MFR and MVR

Table 4 shows that adding organic peroxide to virgin polypropylene powder caused an increase in melt volume and melt flow rates. This information can be used to calculate the die’s shear rate on MFR equipment, the following Equation (2) can be used [5, 10]. In this study, the virgin PP with an MFR of 6.99 g/10 min has a shear rate value of approximately 13 s^–1.

Table 4
MFR and MVR of virgin powder and the different concentrations of powder and peroxide

Sample name Melt Flow Rate Melt volume rate

Virgin powder 6.99 g/10 min 10.28 cm³/10 min

0.1 pp 11.79 g/10 min 17.35 cm³/10 min

0.2 pp 15.2 g/10 min 22.35 cm³/10 min

0.3 pp 22.75 g/10 min 33.46 cm³/10 min

0.4 pp 28.71 g/10 min 42.33 cm³/10 min

0.5 pp 31.33 g/10 min 46.08 cm³/10 min

Sample name	Melt Flow Rate	Melt volume rate
Virgin powder	6.99 g/10 min	10.28 cm³/10 min
0.1 pp	11.79 g/10 min	17.35 cm³/10 min
0.2 pp	15.2 g/10 min	22.35 cm³/10 min
0.3 pp	22.75 g/10 min	33.46 cm³/10 min
0.4 pp	28.71 g/10 min	42.33 cm³/10 min
0.5 pp	31.33 g/10 min	46.08 cm³/10 min

$Die shear rate = 1.85 * MFR$ (2)

4.2 Mechanical properties

By the successive increase of the organic peroxide to the polypropylene virgin powder and carrying out the reactive extrusion, then doing some mechanical tests like tensile strength, elongation, and flexural modulus, it is noted that the higher the peroxide concentration the lower the tensile strength and the lower the flexural modulus and the higher the elongation Table 5.

Table 5
The mechanical properties result against the corresponding samples

Virgin powder Flexural Modulus Tensile Strength Elongation at Yield

(Eb) Mpa (σ M) Mpa Strength (ɛ y) %

Results Error (±) Results Error (±) Results Error (±)

Sample 1 1325 3.975 33.58 0.03358 10.73 0.01073

Sample 2 1372 4.116 33.28 0.03328 10.59 0.01059

Sample 3 1357 4.071 33.06 0.03306 10.79 0.01079

Sample 4 1385 4.155 33.07 0.03307 10.68 0.01068

Sample 5 1321 3.963 33.28 0.03328 10.93 0.01093

Average 1352 4.056 33.25 0.033 10.74 0.0537

Virgin powder	Flexural Modulus	Tensile Strength	Elongation at Yield
Sample 1	1325	3.975	33.58	0.03358	10.73	0.01073
Sample 2	1372	4.116	33.28	0.03328	10.59	0.01059
Sample 3	1357	4.071	33.06	0.03306	10.79	0.01079
Sample 4	1385	4.155	33.07	0.03307	10.68	0.01068
Sample 5	1321	3.963	33.28	0.03328	10.93	0.01093
Average	1352	4.056	33.25	0.033	10.74	0.0537

Fig. 3

(a) Tensile strength, Elongation at yield strength, (b) Flexural modulus of corresponding samples.

Table 6

The mechanical properties results against the corresponding samples with 0.1 pp samples

0.1 pp sample	Flexural Modulus		Tensile		Elongation at Yield Strength
	(Eb) Mpa		Strength		(ɛ y) %
	Results	Error (±)	Results	Error (±)	Results	Error (±)
Sample 1	1270	1.27	32.11	0.03211	10.93	0.02186
Sample 2	1264	1.264	32.93	0.03293	11.13	0.02226
Sample 3	1260	1.26	32.69	0.03269	10.96	0.02192
Sample 4	1281	1.281	32.79	0.03279	10.81	0.02162
Sample 5	1266	1.266	32.27	0.03227	10.99	0.02198
Average	1268	1.26	32.55	0.0325	10.96	0.1096

Fig. 4

(a) Tensile strength, Elongation at yield strength, (b) Flexural modulus of corresponding samples 0.1 sample.

Table 7

The mechanical properties results against the corresponding samples with 0.2 pp sample

0.2 pp sample	Flexural modulus		Tensile strength		Elongation at yield strength
	(Eb) Mpa		(σ M) Mpa		(ɛ y) %
	Results	Error (±)	Results	Error (±)	Results	Error (±)
Sample 1	1255	1.27	32.38	0.03238	11.22	0.00561
Sample 2	1235	1.264	32.48	0.03248	11.31	0.00566
Sample 3	1255	1.26	32.58	0.03258	11.21	0.00561
Sample 4	1216	1.281	32.74	0.03274	11.24	0.00562
Sample 5	1235	1.266	32.44	0.03244	11.3	0.00565
Average	1239	1.268	32.52	0.033	11.25	0.0056

Fig. 5

(a) Tensile strength, Elongation at yield strength, (b) Flexural modulus of corresponding samples 0.2 sample.

Table 8

The mechanical properties results against the corresponding samples with 0.3 pp sample

0.3 pp sample	Flexural modulus		Tensile strength		Elongation at yield
	(Eb) Mpa		(σ M) Mpa		strength (ɛ y) %
	Results	Error (±)	Results	Error (±)	Results	Error (±)
Sample 1	1199	0.01135	31.92	0.03192	11.35	0.01135
Sample 2	1190	0.01137	31.11	0.03111	11.37	0.01137
Sample 3	1185	0.01139	31.77	0.03177	11.39	0.01139
Sample 4	1191	0.0112	31.33	0.03133	11.20	0.0112
Sample 5	1171	0.01128	31.55	0.03155	11.28	0.01128
Average	1187	0.011	31.53	0.032	11.31	0.0113

5 Conclusion

Chain scissioning or vis-breaking polypropylene to create a controlled rheology polymer in a melt extrusion process has been successfully demonstrated to work with aqueous hydrogen peroxide as a reagent. Similar properties of polypropylene are produced by the technique (molar mass, MMD, melt rheology, crystallinity) compared to that made using an organic peroxide (DHBP) nonetheless,

Fig. 6

(a) Tensile strength, Elongation at yield strength, (b) Flexural modulus of corresponding samples 0.3 sample.

the hydrogen peroxide-scissioned polypropylene has the particular benefit that it contains no organic volatile impurities attributable to the employment of a peroxide, in contrast to the usual process, each sample under investigation was examined in light of its mechanical characteristics. And on other hand, Testing the efficiency towards the diabatic, antioxidant, anti-microbial and anticancer effects.

Table 9

The mechanical properties results against the corresponding samples with 0.4 pp sample

0.4 pp sample	Flexural modulus		Tensile strength		Elongation at yield strength
	(Eb) Mpa		(σ M) Mpa		(ɛ y) %
	Results	Error (±)	Results	Error (±)	Results	Error (±)
Sample 1	1166	0.01139	31	0.031	11.39	0.01139
Sample 2	1176	0.01139	31.17	0.03117	11.39	0.01139
Sample 3	1146	0.01121	31.25	0.03125	11.21	0.01121
Sample 4	1153	0.01141	31.11	0.03111	11.41	0.01141
Sample 5	1169	0.01137	31.26	0.03126	11.37	0.01137
Average	1162	0.0113	31.16	0.0312	11.35	0.01135

Fig. 7

(a) Tensile strength, Elongation at yield strength, (b) Flexural modulus of corresponding samples 0.4 sample.

Table 10

The mechanical properties result against the corresponding samples with 0.5 pp sample

0.5 pp sample	Flexural modulus		Tensile strength		Elongation at yield strength
	(Eb) Mpa		(σ M) Mpa		(ɛ y) %
	Results	Error (±)	Results	Error (±)	Results	Error (±)
Sample 1	1151	3.453	30.22	0.03022	11.13	0.02226
Sample 2	1129	3.387	30.71	0.03071	11.24	0.02248
Sample 3	1148	3.444	30.32	0.03032	11.65	0.0233
Sample 4	1151	3.453	30.28	0.03028	11.37	0.02274
Sample 5	1165	3.495	31.03	0.03103	11.77	0.02354
Average	1149	3.446	30.51	0.031	11.43	0.0228

Fig. 8

(a) Tensile strength, Elongation at yield strength, (b) Flexural modulus of corresponding samples 0.5 pp sample.

Table 11

The mechanical properties results against the corresponding samples with comparison with all investigated samples

Sample	Flexural modulus		Tensile strength		Elongation at yield strength
	(Eb) Mpa		(σ M) Mpa		(ɛ y) %
	Results	Error (±)	Results	Error (±)	Results	Error (±)
Virgin powder	1352	4.056	33.25	0.16625	10.74	6.76
0.1 pp	1268	3.804	32.55	0.16275	10.96	6.34
0.2 pp	1239	3.717	32.52	0.1626	11.25	6.195
0.3 pp	1187	3.561	31.53	0.15765	11.31	5.935
0.4 pp	1162	3.486	31.16	0.1558	11.35	5.81
0.5 pp	1149	3.447	30.51	0.15255	11.43	6.76

Fig. 9

(a) Tensile strength, Elongation at yield strength, (b) Flexural modulus of all investigated samples.

Funding

This research received no external funding.

Footnotes

Acknowledgments

This research has no acknowledgment.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have the work reported in this paper.

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