depolymerization of pp review article

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Journal of Polymer & Composites ISSN: 2321-2810(online), ISSN: 2321-8525(print)

Volume 4, Issue 2

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Depolymerization of Polypropylene: A Systematic Review

Parag Kulkarni*, Ruchika Pache Department Polymer and Surface Engineering, Institute of Chemical Technology, Mumbai-400019,

Maharashtra, India

Abstract Polypropylene have become an indispensable ingredient of human life and most widely used

thermoplastic in commodity applications. Polymerization of propylene is carried out by using

Ziegler-Natta or Metallocene catalysts which results in high molecular weights and

crystalline polypropylene which are not required for several applications. Reduction in

molecular weight of polypropylene in conventional reactors is uneconomical and complicated.

Therefore to overcome this problem researchers and technologists proposed a post reactor

operation to reduce molecular weight (depolymerization) of polypropylene. To induce

modification in polypropylene different techniques of depolymerization are proposed. In this

review article, we have discussed various types of polypropylene depolymerization methods

with their mechanisms like oxidative, thermal, high energy radiation and chemical (free

radical induced) depolymerization along with their effect on thermal, rheological, crystalline

and chemical properties of the polypropylene.

Keywords: Polypropylene, molecular weight, depolymerization, thermal depolymerization,

rheology

*Author for Correspondence E-mail: [email protected]

INTRODUCTION Polyolefins are the most widely used polymers

for manufacturing of commodity plastics

article. They are manufactured usually by

using transition metal catalysts and have very

high molecular weights. Polypropylene is an

enormously adaptable member of the

polyolefin family, which is due to the

prochiral nature of the propylene monomer.

Highly crystalline polypropylene is built up by

the chain growth polymerization of propylene,

a gaseous compound obtained by the thermal

cracking of ethane, propane and butane or

naptha fraction of petroleum. In 1951

scientists at Phillips Robert L Banks et al.

found that one of their processes of converting

propylene into gasoline led to the formation of

whitish substance called crystalline

polypropylene. Ziegler modified the

equipment and prepared polypropylene in

1954. In 1954 Natta managed to synthesize

polypropylene, followed shortly by Ziegler.

The first industrially important crystalline,

high molecular weight polypropylene was

synthesized by Natta in 1955 from organo-

metallic catalysts based on titanium and

aluminum [1]. Polypropylene was developed

by scientists at Phillips, Hoechst, Montecatini,

Hercules, Farbwerke-Hoechst, by Ziegler and

Natta and in separate development by standard

oil of Indiana, Du Pont in 1957 [2].

Polypropylene has been widely used as

injection molding plastics as well as in many

extruded forms including film, fiber and split

yarns [3].

Polypropylene is one of the world’s most

important thermoplastic materials and it is

used in numerous applications in the plastics

industry. It has desirable properties such as

low-cost, high-melting point, low density,

appropriate process ability, high-strength, high

stiffness, hinge properties, easy to handle and

excellent chemical resistance [4]. Its

applications have expanded continuously for

the last decade. Since commodity plastics do

not require extremely high mechanical

properties, molecular weights and crystallinity

always, highly crystalline isotactic

polypropylene has been reported to be having

very high melt viscosity with consequently

poor flow properties and deficient in impact

strength at low temperatures, mainly owing to

its relatively high Tg and large spherulite

dimensions. It’s moderately elevated glass

transition temperature results in it being too

Depolymerization of Polypropylene Kulkarni and Pache


brittle and hard at application temperatures

below 0ºC. These high molecular weights of

polypropylene give resistance to flow at

elevated temperatures and hence operations of

shaping by compression molding or injection

molding or extrusion are unnecessarily

complex. The toughness and rheological

properties of polypropylene can be enhanced

by reducing the molecular weight, molecular

weight distribution, introduction of branching

in polymer backbone and controlling average

size of the spherulites in depolymerization.

Control of molecular weight in the reactor,

during the process of polymerization is

difficult. The process parameters are set and

any modification in the process like addition

of a chain transfer agent would result in

resetting the process, which in turn is very

tedious, difficult and economically not

feasible. To achieve the diversity in

polypropylene grades suitable for the different

applications the molecular weight and

molecular weight distribution (MWD) must be

tailor-made to fit the performance

requirements of each application.

DEPOLYMERIZATION OF

POLYPROPYLENE Depolymerization leads to changes in

molecular properties such as molecular

weight, molecular weight distribution, thermal

properties, and crystallisability, etc. which are

considered to be more beneficial in mechanical

properties. This foregoing defects and

deficiencies can be overcome by

depolymerization process for reducing the

molecular weight of the polypropylene.

Polypropylene is depolymerized to achieve

easy processing for high-quality products in

injection molding and fiber spinning with

enhancing mechanical properties [5–7].

Depolymerization is the reversion of a

polymer to a low molecular weight polymer

fragments or splitting of polymers into

molecules [8]. Reverse of the propagation step

in chain polymerization is depolymerization or

unzipping, which is characterized by reduction

in molecular weight of the polymer to low

molecular weight polymer fragments [9].

Depolymerization of polymer may be brought

about by physical factor such as heat, light,

mechanical stress, high energy radiation, and

ultrasonication or by chemical agent such as

initiator, oxygen, ozone etc. Depolymerization

of the polymer is subdivided according to its

various modes of initiation. There are

oxidative, thermal, high energy radiation and

chemical (free radical induced) modes for

depolymerization of polypropylene. Lanrence

has patented a process which gives a

procedure of controlled thermal

depolymerization of a crystalline polymer of

an aliphatic mono-a-olefin which comprises

treating the polymer as slurry with water,

containing oxygen and including free radicals,

at a temperature of 60 to 90oC and recovering

from the slurry, a polymer of increased melt

index [10]. The depolymerization of 16.7

weight % slurry of isotactic polypropylene

dispersed in water was carried out in presence

of free radical initiator and it was found that in

the presence of 0.2% benzoyl peroxide at

95°C, the melt flow index increased to 13 from

9 after 1 h of reaction. Similarly in the

presence of 0.3% AIBN the melt flow index

increased from 0.1 to 1.2 after 2 h of reaction.

It is important to note that this reduction in

polymer is brought about by molecular chains

scission of polypropylene. Hans et al. gave a

process for reduction in border viscosities of

the polymer, by treating polypropylene in

fluidized bed, in presence of oxygen, within

temperature range of 100–130°C. Powdered

polypropylene, of border viscosity 6, was

treated in an eddy furnace for 40 min and at

temperature of 130°C. The resulting polymer

had a border viscosity of 3 [11]. The Dow

Chemical company has patented a process for

reduction in molecular weight of isotactic

polypropylene to improve its proccessability

[12]. The process comprises of thermally

depolymerizing polypropylene at temperatures

of 160 to 300°C, in the presence of small

amount of depolymerizing agent such as

bromine or bromine containing compound (to

result in a bromide radical). The treated

polypropylene had a melt index improved by

2.2. Davis et al. subjected polypropylene

samples to various degrees of thermal

depolymerization [13]. The molecular weight

distributions of these samples were

experimentally determined and compared with

those expected theoretically for random

scission of the polymer chains. The

comparison confirmed that the chain breakage

was predominately random and also indicated

that determination of molecular weight by

viscosity average molecular weight were

Journal of Polymer & Composites

Volume 4, Issue 2

ISSN: 2321-2810(online), ISSN: 2321-8525(print)


adequate for use in evaluating hypothesized

depolymerization mechanisms. The most

common types of depolymerization occur

through chemical reactions that alter the

molecular weight of the polypropylene,

leading to a change in its mechanical, thermal,

chemical and rheological properties. Such

reactions include (a) chain scission, (b) cross-

linking, (c) modification of branched chains,

or (d) a combination of all of these reactions.

The agent(s) initiating the depolymerization

process defines the type of depolymerization.

A review of the different types of

depolymerizations and their initiating agents

are given in Table 1.

Table 1: Initiating Agents and

Depolymerization Methods.

Initiating Agent Depolymerization Type

Heat Thermal

Oxygen, Ozone Oxidative

X-rays, γ-rays, electron

beam radiation High energy radiation

Free radical induced Chemical

The chemical reactions in depolymerization

process are comparatively similar, with barely

trivial differences owing to variations in the

initiation mechanism [14].

Depolymerization Mechanism of

Polypropylene Depolymerization of polypropylene is usually

done by means of free radical chain reaction

mechanism shown in Figure 1 consisting of

the steps of: (i) initiation (Eq. (1)), (ii)

propagation (Eq. (2)) and (iii) termination

(Eq. (3)). Throughout the steps, heat-

facilitated hydrogen abstraction from the

tertiary carbons may lead to the formation of

tertiary alkyl radicals shown in Figure 1. In

PP, tertiary radicals are formed mostly, due to

the poorer dissociation energy of a tertiary C-

H bond (ca. 373 kJ.mol-1 at 25ºC), compared

to that of a secondary C-H bond (ca.

394 kJ.mol-1 at 25ºC) [19]. During the later

propagation step, formed tertiary radicals will

follow separate reaction paths. The reaction

paths followed by tertiary radicals are depicted

in schemes.

Termination of polymer reactions involving

radicals or macroradicals may take place [20].

Recombination is influenced by cage effects,

steric control, mutual diffusion and the

molecular dynamics of the polymer matrix

[21–23].

Oxidative Depolymerization Free radical depolymerization of

polypropylene cannot be controlled, to avoid

such problems in depolymerization; oxidation

on neat polymer is to work with solutions and

initiator begun by Dulog, Radlman and Kerns

[24]. In oxidative depolymerization, the

formation of free radical sites on the chain

backbone by attack of molecular oxygen,

ozone etc. is done [9]. The free-radical chain

reactions depicted in Figure 2 describe

oxidative depolymerization of polypropylene

[20, 25], consisting of individual steps of

initiation, propagation and termination.

During the initiation stage of oxidative

depolymerization, initiating agent stimuli such

as heat or UV radiation are significant for the

growth of radicals. Cleavage of a polymer

chain is done, due to the abstraction of a

hydrogen atom from the polymer backbone

(reaction 2.1).

Fig. 1: Depolymerization Mechanism of Polypropylene.



Polymer chain scission is frequently caused by

severe deformation of a polymer. In the

propagation steps, the radicals formed during

initiation, react with molecular oxygen and are

changed into macro-alkyl peroxyl radicals

(reaction 2.2). Then abstraction of a hydrogen

atom from another polymer molecule gives

macro-hydroperoxides and more alkyl radicals

(reaction 2.3). Hydroperoxides formed through

this step of reaction might be cleaved

homolytically to give up radical. These kinds

of radicals are capable to abstract hydrogen

form adjacent polymer chains ensuing to the

growth of more radicals accountable for

initiating depolymerization.

Free radicals produced through the first two

steps may also go through further reactions.

Such reactions are identified as β-scission

(reaction 2.5), reduce the molecular weight of

chains and also persuade the crystallinity of

polymers [26]. Considering as amorphous

regions of semi-crystalline polypropylene

depolymerized faster than the crystalline phase

[27–29], the scission of polymer backbone,

dependable for relating two adjoining

crystalline areas, will cause a rapid devastation

of the physical and mechanical properties of a

polypropylene. Elevated mobility of oxygen in

non-crystalline material leads to propagation

within amorphous regions. The termination

reactions are reliant on the molecular structure

of the polymer as well as the existing

depolymerization conditions and termination

(reactions 2.7, 2.8) prevails [20]. Though, in

the case of oxygen starvation alkyl radical

dominate and bimolecular termination

reactions are of superior importance. This

leads to cross-linking which is evidenced by a

rise in molecular weight. Oxidative cycle of

polypropylene is shown in Figure 3 [4].

Fig. 2: Oxidative Free Radical Depolymerization of Polypropylene.


Volume 4, Issue 2



Sunggyu et al. have patented a process for

selectively depolymerizing isotactic PP in the

oxidative depolymerization pilot plant system,

at 383°C and 232 atm pressure. The batch

charge varied from 0.5 to 20 gm of isotactic

PP, with oxygen flow rates varying from 50 to

2000 scc/min [30]. Upadhyaya et al. patented

a method for depolymerizing polyolefins like

amorphous polypropylene, Fischer-Tropsch

waxes, petroleum waxes, and mixtures.

The process comprises of reacting the molten

polymers, at atmospheric pressure, in the

absence of catalysts at a temperature between

about 130 and about 215oC. The molten

material was constantly stirred as it was

sparged with air [31]. Jansson et al.

hypothesized that hydroperoxides formed

during the ageing step, decompose in the

subsequent processing step, thereby causing

faster degradation of the materials.

Hydroperoxide formation and decomposition

into radicals are known to play a key role in

polymer auto-oxidation [32].

Thermal Depolymerization Thermal depolymerization of polypropylene is

molecular reduction as a result of overheating.

At high temperatures the components of the

long chain backbone of the polypropylene can

begin to separate the molecular scission it

involves change in molecular weight,

molecular weight distribution. The

conventional model for thermal

depolymerization involves the major steps of

depolymerization such as initiation (4.1, 4.2),

propagation (4.3), branching (4.4) and

termination (4.5) shown in Figure 4 [33].

Fig. 3: Oxidative Cycle of Polypropylene [4].

Fig. 4: Mechanism of Thermal Depolymerization.



Polypropylene is very susceptible to thermal

depolymerization even at normal temperature

which causes chain scission; the reduced chain

length reduces molecular weight. This can

considerably change the mechanical

properties, thermal properties and crystallinity.

Ying et al. explained the mechanism of

thermal oxidative and thermal mechanical

degradation of isotactic polypropylene (PP) in

the presence of incorporated organic peroxide.

The change in MWD was explained by a

random chain scission mechanism and

mechanical chain scission [34]. Polypropylene

subjected to multiple extrusion shows that the

chain scission processes during thermo-

mechanical depolymerization causes the

reduction in molecular weight. This indicates

that the probability of chain breaking is

dependent on the depolymerization and the

molecular weight of the chain [35].

Free Radical Induced Depolymerization

Free-radical initiators are chemical substances

that, under certain conditions, initiate chemical

reactions by producing free radicals as cause

of homolytically cleavage [36]. Radicals are

reactive chemical species possessing a free

electron and ions. Initiator-derived radicals are

very reactive chemical intermediates and

generally have short lifetimes [37], i.e., half-

life times less than 10-9. The decomposition

rate of these peroxides depends on the class of

peroxide as well as on the type of alkyl group.

The eight classes of organic peroxides that are

produced commercially for use as initiators are

listed as follows Diacyl peroxides, Dialkyl

peroxydicarbonates, tert-Alkyl peroxyesters,

OO-tert-Alkyl O-alkyl monoperoxy

carbonates, Di (tert-alkylperoxy) ketals, Di-

tert-alkyl peroxides, tert-Alkyl

hydroperoxides, Ketone peroxides.

Fig. 5: Depolymerization of PP in Presence of Free Radical Initiators.


Volume 4, Issue 2



The Saule et al. studied of the modification of

polypropylene based on the decomposition of

unsaturated peroxides which helped to

decrease the molecular weight of the polymer.

Mechanisms for the branching and the

depolymerization of the polypropylene were

also proposed [38]. To optimize desired

processing capabilities and the end use

properties the controlled degradation of

polypropylene (PP) using peroxides is

proposed by Iedema et al.

The author performed a series of controlled

degradation experiments on a twin-screw

extruder with polypropylene of varying MW

under addition of various amounts of initiator

and developed a model which was adapted to

the geometry of the extruder entrance and the

peroxide feed practice. Effect of thermal

heterogeneities, residence time distribution,

micromixing on molecular weight distribution

was studied [39]. Balke et al. presents a new

kinetic model for the free radical initiated

degradation of polypropylene in the reactive

extrusion. The peroxide used was 2,5-

Dimethyl-2,5-di(t-butylperoxy) hexane. The

resulting product is known to provide superior

processing properties; this is an example of the

production of a specialty polymer by chemical

modification of a commodity polymer [40].

Possible reactions mechanism of

polypropylene in presence of free radical

initiators or organic peroxide is shown in

Figure 5 [41].

There are several common processes for

supplying the energy required to variety

radicals from initiators thermal and radiation

[36]. Formerly produced, radicals experience

two basic types of reactions, propagation

reactions and termination reactions. The

initiator molecule is denoted by I and the free

radical produced is shown by I*. In a

propagation reaction, a radical reacts to form a

covalent bond and to create a new radical. The

three most ordinary propagating reactions are

atom abstraction, ß-scission, and addition to

carbon–carbon double bonds or aromatic

rings. The polypropylene radicals produced

will further react with other polymer

molecules. The main importance of shift to

polymer is chain branching which frequently

occurs in two ways such as:

i. Hydrogen might be abstracted from sites

on the backbone thus transferring radical

movement and initiating a long chain

branch.

ii. Short chain branches possibly will be

fashioned by intra-molecular atom transfer

(backbiting). In this case the propagating

radical abstracts backbone hydrogen as of

a close by carbon.

In a termination reaction, two radicals

interrelate in a mutually vicious reaction in

which both radicals fashioned covalent bonds

and reaction ceases. Termination of the

polymer radicals can also be done by

recombination, where two polymer radicals

formed would merge.

i. In which combination of two tertiary

radicals would effect in cross-linking,

which is slightest liable in case of

polypropylene.

ii. Amalgamation of two secondary, or a

secondary and tertiary radical would give

a polymer molecule or by

disproportionate.

In the provisions of the free radical

depolymerization theory, intermolecular

transfer of the radical was found to be the

leading process. An arch linking the degree of

depolymerization to the intrinsic viscosity was

obtained from calculations based on a random

depolymerization. With this arch, intrinsic

viscosities measured as a function of

depolymerization time could be used to gain

the time dependence of the degree of

depolymerization [13].

Depolymerization by High Energy

Radiation Depolymerization of polypropylene can be

done by means of irradiation methods, such as

electron beams, ultrasonic, ultraviolet (UV)

and γ-rays, which have been favored to modify

the structure and properties of polypropylene

besides using chemical initiators and free

radicals.

Electron beams, ultrasonic, ultraviolet

radiation and γ-rays energy which possibly

will cause bond scission by free radical

formation for higher doses than 60 kGy for

long period depolymerization occurs. The



depolymerization of polypropylene resulting

from experience to high energy radiation is an

important subject for many reasons. The

mechanism of high energy induced

depolymerization is a free radical, one similar

to the general mechanism of depolymerization

as shown in Figure 6.

Fig. 6: Mechanism of High Energy Induced Depolymerization PP.

In case of PP irradiation it has been

established that the PP undergoes essentially

chain scission. Polymers are increasingly more

exposed to radiation in their service where

depolymerization may cause [42]. Radiation-

induced depolymerization has been well

reviewed [43–46]. High energy radiation is

defined as all forms of radiation with energies

much higher than those of chemical bonds. It

includes both electromagnetic radiation (x-

rays, γ-rays, etc.) and particulate beams (α-

and β-particles, electrons, neutrons, etc.). The

series of energies is exceptionally broad and

superior to chemical bond energies. Radiation

is absorbed by interaction with atomic nuclei

and electron clouds. He et al. developed UV

initiation based reactive procedure for

depolymerization of polypropylene for the

production of controlled rheology

polypropylene, benzophonene has been used

as the photo initiator. At low-irradiation time

and low-levels of photo initiator molecular

weight distribution was noticeably tailored.

After depolymerization, MFR improved with

drop off in viscosity and elasticity with

benzophonene concentration as predictable.

Crystallinity levels and crystallization

temperatures of the tailored polypropylene

were lower than those of virgin polypropylene

because of the reduced molecular weight and

narrower MWDs [47]. Radiation induced

depolymerization caused by chain scission was

successfully used. Controlling the degree of

degradation, uniform molecular weight is

friendly process are the beneficial effects of

using radiation technology. When polymeric

materials are irradiated by ionizing radiation,

they are divided into two types, degradation

(chain scission) and chain link (crosslinking).

Studying the effect of hydrogen peroxide

and/or γ irradiation on the degradation process

of Na-alginate was investigated. It was found

that the molecular weight of the polymer

decreases by using gamma radiation or H2O2.

However, combining both γ radiation and

H2O2 accelerates the degradation rate [48]. The

influence of three hindered amine light

stabilizers (HALS) and two EVA copolymers

on the radiation degradation processes in

isotactic polypropylene (iPP) was investigated

by Zimek et al. The process consequences

degradation, oxidation, release of harmful

gaseous products, etc. as the polymer is known

as one of the most degradable upon experience

to ionizing radiation. Polypropylene properties

are changed even before exposure to electron

beam due to structural modifications caused

due to chain scission initiated by additional

components [49]. The range of energies is

extremely broad and higher than chemical

bond energies. Radiation is absorbed by

interaction with atomic nuclei and electron

clouds. The radiation interaction with the

electron clouds of molecules consequences in

the transfer of energy to the molecules to form

ions, with the elimination of a secondary

electron [50, 51]. Further ionization and

excitation of nearby molecules is caused by

these secondary electrons having enough

kinetic energy. The immediate effect of

absorption of high energy radiation is

construction of energetic or excited species,

including trapped electrons, ions, and radical

ions, results in fragmentation to provide free

radicals. In polypropylene, scission of main-

chain C-C bonds gives radical pairs. The

scission of C-H bonds leads to comparatively

stable radicals and formation of molecular

hydrogen. The primary radical formed by

irradiation might not be stable. For example,

irradiation of polypropylene with γ-radiation at

doses of 25, 50, 100 and 150 kGy produces the

secondary radical [52]. In the absence of

oxygen, carbon-centered radicals might


Volume 4, Issue 2



undergo cross-linking, by recombination,

chain scission, β-elimination depending on the

radical structure. Polymers can consequently

be divided into two main groups, those which

react to radiation by cross-linking and those

which drop molecular weight by chain scission

[53]. Ultrasonic irradiation has been recently

looked upon as a new technique for

degradation of polymer compounds, mainly

due to the fact that the reduction in the

molecular weight is simply by splitting the

most susceptible chemical bond without

causing any changes in the chemical nature of

the polymer. Detailed analysis of cavitations

generated ultrasound radiation used for

molecular weight reduction of polypropylene

has been done by Desai et al. The effect of

various operating parameters including initial

concentration of the polymer, power density

into the system and type of solvent, on the

extent of depolymerization of polypropylene

are investigated. Cavitations results in liquid

turbulence associated with liquid shear and

generation of highly reactive free radicals. The

shear force leads to the rupture of chemical

bonds of the polypropylene, ultrasonic sound

waves produce a permanent reduction in

viscosity [54].

CHARACTERIZATIONS TO STUDY

DEPOLYMERIZATION As it has been mentioned before,

polypropylene has extreme importance

commercially, but regrettably it is also less

susceptible to attack by heat, oxygen,

radiation, and free radical initiator. It is very

essential to study the depolymerization process

i.e., to set up the mechanisms throughout

oxidative depolymerization, the products

formed as well as the influence of molecular

weight and structure on the properties of

material. A full understanding of

depolymerization is of great significance,

since without this, successful stabilization

approaches and superior methods of lifetime

prediction would not have been possible. It is,

consequently, essential to consider the

characterization techniques employed to study

the depolymerization process. Analytical

chemistry field possesses a conventional

techniques that have been used for the purpose

of studying polypropylene depolymerization,

some of which include high temperature gel

permeation chromatography (GPC), Fourier

transform infrared spectroscopy (FTIR),

differential scanning calorimetry (DSC) and

viscometry. A brief overview on the use of

some of these techniques for the purpose of

studying polyolefin depolymerization will now

be presented.

High Temperature Gel Permeation

Chromatography According to hydrodynamic volume high

temperature gel permeation chromatography

(HTGPC) separates molecules and is the

preferred method for the determination of

molecular weight and molecular weight

distribution. During the depolymerization of

polypropylene, changes in average molecular

weight and molecular weight distribution

(MWD) are often observed [55–57].

Depending on the polymer and the

depolymerization methods, many radical

reactions can take place. In the case of

polyolefin’s, branching and recombination

reactions predominate at lower temperatures,

yielding a long chain branching, which leads

to an increase in the average molecular weight

of the material [58]. In case of polypropylene,

depolymerization almost exclusively by means

of chain scission, leads to a reduction of its

average molecular weight [59]. Chain scission

during depolymerization consequences in

continuous breaking of polymer chains

yielding chains of shorter lengths. The average

result is that the number of short chains

increases with depolymerization time and is

accompanied by a broadening of the molecular

weight distribution.

The molecular weight curve obtained from

GPC measurements, as a result, shifts towards

the region of lower molar mass when chain

scission is the dominant depolymerization

mechanism [60–62]. As depolymerization

proceeds and chain scissions continue, the

molecular weight curve will exhibit bimodality

with the portion of highly depolymerized short

molecules appearing as a narrow distribution

on the lower molecular weight side of the

original material. David et al. proposed an

Eq. (A) for determining the number of chain

scissions in degraded samples [63], by shaping

the numerical average of the initial and final

number-average molecular weights.



𝑛𝑅 =𝑀 𝑛𝑜

𝑀𝑛𝑓− 1 (A)

Where,

𝑛𝑅is the number of chain scission,

𝑀 𝑛𝑜 is initial number average molecular

weight,

𝑀𝑛𝑓is final number average molecular weight.

High temperature gel permeation

chromatography can be used to record

molecular weight changes over a cross-section

of a thick polymer sample by means of

microtoming [64] or layer-by-layer milling

[65]. Computer-aided study method called

molecular weight distribution computer

analysis (MWDCA) has also proven to be

useful for the comparison of depolymerization

rates in polypropylene [66–68]. This technique

uses molecular weight data, obtained by

evaluation of experimental GPC molecular

weight distribution profiles with computer

simulations for determining the scission or

crosslink. Shyichuk investigated chain scission

and crosslink concentrations in photo-

degraded LDPE, polypropylene and ethylene-

propylene copolymer.

Differential Scanning Calorimetry Differential scanning calorimetry is an

additional technique frequently used to obtain

information on the depolymerization behavior

of polymers [69, 70]. Profile of melt

endotherms and changes in the peak

temperatures and as well as the glass transition

temperature, can provide information on the

susceptibility of different crystalline phases or

arrangements to depolymerization [71]. DSC

is very useful in determining the oxidative

induction time (OIT) [72–74], oxidative

temperature (Tox) [75] and thermal stability

degree of depolymerization of polymers under

high temperature conditions of polymers. Rosa

et al. investigated the influence of several

parameters on DSC statistics and found that

sample preparation (shape and size), oxygen

flow and heating rate of the experiment had a

considerable influence on the data obtained

[70].

Fourier Transform Infrared Spectroscopy For studying out the chemical changes brought

about by polymer depolymerization IR

spectroscopy is one of the most accepted

techniques. It has been used for qualitative as

well as quantitative characterization of

depolymerization products [76–80]. The

depolymerization route creates the formation

of different functional groups which are

strongly reliant on the chemical structure of

the polymer. The main chemical species

detectable by infrared spectroscopy are

hydroxyl and carbonyl groups [81]. The

configuration of these groups usually leads to

visible changes in infrared spectrum,

appearing in the regions of 1850–1550 and

3700–3200 cm-1

, respectively [82–84].

Moreover, change in the FTIR absorption of

depolymerization products in PE and PP can

also be experimental and used for studying the

difference in their oxidative depolymerization

mechanisms [85]. From the hydroxyl and

carbonyl groups, depolymerization products

such as peroxides, alcohols, can be identified

by means of derivatisation reactions [86].

Derivatisation methods for this point were first

applied to polyolefins by Carlsson et al. [87].

FTIR has also been sensible in determining

changes in unsaturation of PE during

depolymerization [88]. The degree of

depolymerization of polypropylene is

determined on the basis of their carbonyl index

[89–91]. The bands at 1892, 974, 2720 and

840 cm-1

have all been used as reference bands

for determining the carbonyl index in

polypropylene [89–92].

Viscometry

Viscometry is a useful technique for

determining the polymer molecular weight.

The molecular weight obtained by this

technique is the viscosity averaged molecular

weight. The increase in the viscosity imparted

by the macromolecules in solution is a direct

function of the hydrodynamic volume and,

hence, the molecular weight of the

macromolecule. The relationship between the

intrinsic viscosity and the viscosity average

molecular weight is given by the semi-

empirical Mark-Houwink Eq. (B) [9]. [𝜂] = 𝐾(𝑀𝑉)⍺ (B)

Where η is intrinsic viscosity, K and ⍺ are

constants for a polymer-solvent system at a

given temperature and Mv is viscosity average

molecular weight. The η, values for the

polypropylene solution were calculated by the

one point intrinsic viscosity Eq. (C).

[𝜂] =(2(ηsp−ln ηr))

0.5

𝐶 (C)


Volume 4, Issue 2



Where ηr and ηsp are relative and specific

viscosities respectively, C is concentration of

polymer solution [93]. The viscometric studies

of polymer solutions as a means of molecular

characterization of polymers are well

recognized and widely practiced because of

simplicity in terms of experimental approach

and the apparatus needed. Dilute solution

viscosity can be conveniently measured in

capillary viscometers such as Ostwald type or

the Ubbelohde type [9]. Shenoy et al.

dissolved the depolymerized polypropylene

samples in xylene at 100°C and the viscosity

measurement at 85°C was carried out by using

the Ubbelohde viscometer. Relative (ηr) and

specific viscosities (ηsp) respectively were

calculated using standard formulae to

determine the viscosity average molecular

weight [94]. For PP-xylene system at 85°C the

Mark Houwink constants are K=9.6×10–3

and

α=0.63 [100].

EFFECTOF MOLECULAR WEIGHT

REDUCTION Melt Flow Index

The melt flow index (MFI) of the samples is

considered as a critical parameter in polymer

processing and industrial designs. MFI of a

polyolefin resin refers to the rate at which it

extrudes from a capillary die under a standard

set of conditions. The MFI is reflected by the

average dimensions of the molecules in a resin

and their entanglements with one another so it

depends on molecular characteristics (Mw and

MWD) and branching characteristics, short

chain branching (SCB), of the sample. Fazeli

et al. have showed that the MFI of the samples

is related to their branching characteristics.

With increasing the degree of branching

(SCB/100°C), the MFI of the sample goes

through a minimum. The increase in SCB will

cause more entanglements between the chains

(inter-molecular entanglement) and therefore

will impede the flow [101]. However, after the

minimum point, increase in the SCB/100°C

will cause the chains to have a more compact

molecular profile (more intra-molecular

entanglements instead of inter-molecular

ones), so the chains will cause less hindrance

to the flow of other chains. In other words, the

degree of branching (number of branching per

100°C atoms of main chain) and the amount of

branching (the number of chains which have

the same degree of branching), have the same

effect on MFI. The melting point of a-form

iPP is strongly influenced by the

stereoregularity [102, 103]. Melting points in

the 160–168°C range are typical for

commercial homopolymer samples under

normal analysis conditions.

Melt Strength

The melt strength of a polymer is defined as

the maximum force at which a molten thread

can be drawn under standard conditions before

it breaks. High values of MS are desired in

processes where the material is stretched in its

molten state, such as in film blowing,

thermoforming, or foaming. Lagendijk et al.

have shown how the melt strength is enhanced

by the presence of strain hardening in

elongational viscosity [104]. Increasing

average molecular mass of a polymer results in

higher shear viscosity, as well as higher melt

strength. The melt strength also increases

when the molecular mass distribution (MWD)

becomes broader. Ghijsels et al. have

demonstrated how the melt strength increases

much more dramatically than shear viscosity

upon the addition of long chain branches on

the polymer backbone [105–107]. De Maio

and Dong have studied the effect of chain

structure on melt strength of polypropylene.

Comparison among several linear and

branched polypropylenes obtained by electron

beam irradiation has shown that the melt

strength of branched can be up to 10 times

higher than that of linear PP with the same

MFI [108]. Gotsis et al. found that the

enhancement of the melt strength is related to

the increase of the weight average number of

long chain branches per molecule [109].

Rheology The molecular architecture of the polymer i.e.

short and long chain branching and MWD

affects the rheology of the melt. In a thorough

review on the effect of long chain branching

(LCB) on the linear viscoelasticity of

polyolefins, Vega et al. showed that the

introduction of LCB induces higher levels of

elasticity than broadening of the MWD of a

melt of linear chains of similar molecular

weight [110]. On the other hand, it is current

understanding that short chain branching

(SCB) cannot cause large increases in



elasticity. Yet, Vega et al. reported that SCB

resulted in higher zero shear rate viscosities,

higher relaxation times, higher values of the

elastic storage modulus and higher activation

energies of flow compared to linear polymers

[111]. Those authors surmised that the number

of entanglements per branch is a decisive

parameter that influences shear and

elongational behavior. Much less research has

been done on the extensional properties of PP

melts because appropriate samples have been

difficult to obtain. Hingmann and Marczinke

found that branched PP samples showed

distinct strain hardening and concluded that a

few branches on the chain had an enormous

effect on the extensional behavior of the melt

[112].

Crystallinity Wide angle X-ray scattering (WAXS) patterns

of iPP, sPP, and aPP are shown in Figure 7

[113]. The regular molecular structure of iPP

and sPP readily enables crystallization of the

chains, leading to well defined crystalline

reflections differing in unit cell symmetry. aPP

lacks a regular molecular structure, and does

not crystallize [114]. Brucker et al. have

shown that the isotactic polypropylene can

crystallize into three different crystal forms

depending on the temperature, pressure and

mechanical stress state: monoclinic a-,

orthorhombic γ- and hexagonal ß-forms [115,

116]. The γ-crystals are formed only under

high pressure in high molar mass

homopolymer polypropylene. Polypropylene

samples with low molar mass or low tacticity

and polypropylene copolymers crystallize

partially in γ-form. The degree of crystallinity

varies between 0 for a completely amorphous

material (such as aPP) and 1 for a completely

crystalline material. The degree of crystallinity

plays a critical role in determining properties

of polypropylene. Commonly measured

properties such as; modulus, yield stress,

oxygen-moisture barrier resistance, and

hardness increase with increasing crystallinity.

In addition to tacticity, crystallinity generally

increases with decreasing molecular weight

(increased chain mobility), and is promoted by

slower cooling rates from the melt. Busico et

al. have explained how the density of iPP in

the a-form varies between the limit of 100%

amorphous (ϱa=0.850 to 0.855 g/cm3) and

100% crystalline (ϱc=0.936 to 0.946 g/cm3)

[117].

Fig. 7: Wide-Angle X-Ray Scattering (WAXS) Patterns of iPP, sPP, and aPP [113].


Volume 4, Issue 2



Balta-Calleja and Vonk indicated a method for

evaluating crystallinity as a number by

numerical means from the relationship of the

peak area to the total area [118]. Jinghua et al.

investigated the nonisothermal crystallization

kinetics of linear and long chain branched

polypropylene (LCB PP) by differential

scanning calorimetry (DSC) at various cooling

rates. It was shown that LCB has the role of

heterogeneous nucleating agent and

accelerates the crystallization process of PP.

Also the activation energy of LCB PPs are

higher than that of linear PP, indicating that

the presence of LCB baffles the transfer of

macromolecular segments from PP melt to the

crystal growth surface [119]. Furthermore, the

crystal morphology of linear PP and LCB PPs

was observed through polarized optical

microscopy (POM), and fine spherulites were

observed for LCB PPs. De Nicola et al. and

Wang et al. have independently investigated

the effect of branching on the crystallization

behaviors of PP resins [120, 121]. It was

observed that the introduction of branching in

the PP resins increases crystal nuclei density.

This promotes faster crystallization, and,

hence, higher crystallization temperatures

were observed for branched materials as

compared to linear materials. The cooling rate

was fixed to be 10°C/min. It was observed that

the branching of PP chains significantly

promoted the crystallization kinetics of the PP

resins by increasing the crystallization

temperature about 20°C.

Chemical Properties

iPP is soluble in high boiling aliphatic and

aromatic hydrocarbons at high temperature.

The high chemical resistance of iPP results in

exceptional stain resistance, and has led to the

use of iPP in automobile batteries. iPP has

outstanding resistance to water and other

inorganic environments. iPP resists most

strong mineral acids and bases, but like other

polyolefins is subject to attack by oxidizing

agents including 98% sulfuric acid and 30%

hydrochloric acid at high temperature

(13<100°C), and fuming nitric acid (ambient

temperature) [122]. PP reacts with oxygen in

several ways, causing chain scission and

brittleness that is associated with the loss in

molecular weight. This action is promoted by

high temperatures, light, or mechanical stress.

Several scientists have independently

demonstrated treatment of polypropylene with

peroxides which has led to controlled rheology

resins with reduced molecular weight and

narrow polydispersity relative to polymerized

product from Ziegler-Natta catalysts [123–

125]. These resins are used in some fiber

spinning and injection molding applications.

The creation of radical sites along the polymer

backbone, most often through peroxide-based

initiation, is also an essential condition for

many fictionalization/grafting chemistries.

CONCLUSION It is concluded from the foregone discussion

that understanding about the mechanism can

go a long way in helping the researchers and

the technologists to induce the different types

of depolymerization methods in the

polypropylene. This depolymerization can

further be enhanced by the addition of the

initiator, additives in the polypropylene and by

understanding the various factors such as

mechanical, physical, and chemical which are

responsible for this depolymerization. It is also

concluded from this discussion that

polypropylene depolymerization could be

enhanced its properties with modification in

structure.

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Cite this Article Parag Kulkarni, Ruchika Pache.

Depolymerization of Polypropylene: A

Systematic Review. Journal of Polymer

& Composites. 2016; 4(2): 1–16p.

depolymerization of pp review article

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