22. Polypropylene technology

General aspects (PP)

The most recent of the so called commodity plastics

Broad selection of different grades

In technically usable form PP has regular molecular structure

…isotactic

Polypropylene is polymerized with coordination or Ziegler-Natta

catalysts using with either slurry or gas-phase process.

Development of polypropylene polymerization technology has

been rapid and it has combined breakthroughs in both

fundamental chemistry and catalysts development for technical

development steps.

General aspects (PP)

In bulk or mass polymerization of polypropylene liquid propylene

is used as a medium inside the reactor. Thus high polymerization

rate is achieved and simple process engineering is attained. Gas-

phase technology competes with bulk technology. When new

production facilities for PP are built, impact-strength the so called

block copolymers are polymerized exclusively in gas-phase

reactors.

Reasons for the rapid increase in polypropylene utilization have

been highly versatile product in combination with low price, low

weight, low energy content, and good chemical and long-term

properties.

General aspects (PP)

The following properties are important for PP:

Adequate impact strength in the whole operating temperature

range

Preservation of mechanical and visual properties also after

exposure to UV radiation

No stress-cracking caused by fuels

Good processing properties for large batches

Low weight and costs

Possibility for material recycling

Low energy content

Shape stability at elevated temperatures

Polymerization catalysts

Catalyst technology is in key position…

Typical properties of PP catalysts:

• High stereospecificity

• Very high activity

• Catalyst may remain in the product

• Catalyst must be so stereospecific that extraction of atactic

polymer is not required

• Catalyst activity should last long

• Polymer grows outwards from the active centers on the surface of

catalyst

…

Polymerization catalysts

…

• The shape of catalyst particles defines the shape of polymerparticles

• Catalyst deactivates

• Various different kinds of active centers

• Desired molecular weight distribution (MWD) must be obtainedfor product

• Product should have narrow MWD, high bulk density and goodplastic yield

Production process

Production process for homopolymer (selection criteria):

Product properties

Operating cost

Safety

Enviromental issues

Usability

Support for research and development

Investment cost

Maturity of technology

Licence issues

Production process

Three basic process types are used in polypropylene production:

Bulk process

Gas-phase process

Suspension

Solution and slurry processes are already out of date concerning

new investments. However, old slurry-based plants are still in

use.

Production processDesign parameters of polypropylene process:

Production process

Structures of polypropylene processes:

Production process

Comparison of

polypropylene processes:

Traditional slurry process

Traditional slurry-based production process for PP:

Bulk process

In bulk process polymerization occurs in liquid propylene. Solid

polymer precipitates further as powdery substance. Bulk process

is usually realized in a stirred tank reactor or by using technology

based on the so called loop reactor.

Usually the extraction of generating heat from the reactor

becomes a problem.

In the case of stirred tank reactors it’s possible to use an external

heat exchanger through which slurry is pumped out and returned

to the reactor. Loop reactors are inexpensive in terms of heat

transfer.

Gas-phase process

In gas-phase process the production costs are even lower than in

bulk-based production process. However, it should be taken into

account that temperature control inside gas-phase reactor is

probably harder than in a liquid-phase reactor.

Several technical solutions exist for gas-phase polymerization,

e.g., BASF has vertical mixed-bed reactor, Amoco has horizontal

mixed-bed reactor and Union Carbide’s Shell process is based on

the use of a fluidized bed.

Gas-phase process

BASF’s Novolen PP production process:

Gas-phase process

Montell’s Spheripol process for the production of homo-PP and

copolymer. Spheripol process is a combined gas-phase and bulk

process (the basic idea was originally developed by Himont):

Copolymers

Copolymers with ethylene have big importance in PP technique.

Copolymers can be either random copolymers or block

copolymers.

Random copolymers can be produced quite easily either by using

liquid- or gas-phase process with the addition of ethylene.

Copolymers from both processes act as homopolymers. Ethylene

content can be as high as 4 %.

Block copolymers of polypropylene are not random block

copolymers. They are rather referred to as heterophasic

copolymers. They consist of a reactor blend of isotactic

polypropylene and ethylene-propylene rubber.

Copolymers

Advantage of heterophasic grades is the improved impact strength.

Good impact strength is achieved only when the properties of the

rubber phase, such as particle size, their distribution and adhesion,

are at the right level.

Heterophasic copolymer:

Copolymers

Quality of a copolymer is dependent on, e.g., the following factors:

Type of homopolymer matrix, isotacticity, melt index etc.

Ethylene/propylene ratio

Homopolymer/copolymer ratio

Distribution and size of phases

Copolymers

AMOCO/Chisso gas-phase process for PP block copolymers:

Copolymers

BASF-Norchem production process for E-P copolymer:

Polymerization processes of

homopolymer PP:

Process Bulk ( liquid pool ) Dilute slurry Gas phase

Corporation Himont /

Mitsui

Dart / El Paso

Montedison

Sumitomo

AMOCO

Montedison

Sumitomo

Union Carbide

BASF / ICI / USI AMOCO

Reactor Loop CSTR CSTR Fluidized-

bed

vertical

mixed-bed

horizontal

mixed-bed

Solvent - - n-hexane

n-heptane

- - -

Catalyst Ti support Ti support or

supportless

Ti support or

supportless

Ti support Ti support or

supportless

Ti support of

supportless

Yield ( kg / g kat. ) 20-40 20-30 20-40 3-15 3-10 3-10

Tacticity max. 99 % max. 98 % max. 98 % max 98 % max. 98 % max. 98 %

Temperature ( °C ) 60-80 60-75 60-80 60-80 70-90 70-90

Pressure ( atm ) 30-40 30-40 15-20 20 20 20

Residence time ( h ) 1-2 2 3-4 3-5 4 4

Grade change-

loss per week

4 % 6 % 10 % 12 % 12 % 3-4 %

Molec. weight ( MI ) 0,1-800 0,1-800 0,1-100 0,1-750 0,1-100 0,1-100

MWD narrow-wide-

Polymerization processes of

homopolymer PP:Process Bulk ( liquid pool ) + second phase "Suspension" Solution Multistep gas

Corporation Himont /

Mitsui

Sumitomo Dart /

El Paso

Montedison /

Dutral

Dupont

Exxon

Goodrich

Uniroyal

Sumitomo

Union

Carbide

BASF / ICI /

USI

AMOCO /

Chisso

Reactor Loop-

fluidized-

bed

CSTR-

CSTR

CSTR-

mixed-bed-

(horizontal)

CSTR CSTR Fluidized-

bed x 2

- Mixed-bed

(vertical) -

Mixed bed

Mixed-bed

(vertical) x 2

Solvent - - - Monomers and

diluents

different

solvents

- - -

Catalyst Ti

support

Ti

support or

without

Ti

support or

without

Ti/V Ti/V Ti

support

Ti support or

unsupported

Ti support

T ( °C ) 60-80 60-75 40-75 0-20 30-60 60-80 70-90 70-90

Paressure (atm)

Phase 1 30-40 30-40 30-40 5-20 10-20 20 20 20

Phase 2 20 30-40 20 - - 20 20 20

Residence time (h) 1-2 2 2-5 1 0,1-1 3-5 4 4

Comonomers ethylene +

others

ethylene ethylene Ethylene,

butylene,

dienes

Ethylene,

dienes

Ethylene +

others

Ethylene +

others

Ethylene +

others

Objectives for PP process:

Wider selection of copolymers and terpolymer products

New catalysts and processes for polar monomers and dienes

Improved control over copolymer composition, composition

distribution and crystallinity

More efficient adjustment of MWD

Improved control over catalyst activity profiles

Improved control over the shape of polymer particles

Shorter residence times inside reactor, which leads to higher

productivity and shorter grade change times

Modern on-line process control systems for evaluation of polymer

properties

Borstar (PP)

• Borstar PP is based on the earlier-developed Borealis Borstar PEprocess that was originally developed to produce bimodal MWD.

• Borstar is a modular process. Production facility consists of aloop reactor and a large gas-phase reactor.

• It can be used to produce both homo- and copolymers.

• Significant advantages in comparison to traditional productionmethods of polypropylene:

– Fast start-up and grade change

– High monomer concentration

– High catalyst activity

– More efficient use of propylene

– The basic properties of a product are generated already in thereactor lower production costs

Borstar (PP)

• Significantly higher temperature range (80 - 92 °C) can be used

than with traditional production methods for PP (70 °C).

• Higher operating temperature enables also the production of

polypropylene grades with very high melt index and very wide

molecular weight distribution

• In Borstar PP processes ethylene can be used as a comonomer as

high as 10 % concentrations, when producing random

copolymers.

Borealis Borstar production

process for PP