Fillers and Reinforcements for Plastics - ACS Publications

12 Compounding of Fillers STAN

JAKOPIN

Downloaded by GEORGETOWN UNIV on February 19, 2015 | http://pubs.acs.org Publication Date: June 1, 1974 | doi: 10.1021/ba-1974-0134.ch012

Werner & Pfleiderer Corp., W a l d w i c k , N. J. 07463

The several methods available to produce glass-reinforced thermoplastic materials vary greatly in their methods and results. Data obtained on pre-compounded glass-reinforced polypropylene show that physical properties vary signifi cantly with vanous compounding techniques. Proper selec tion of compounding equipment and optimizing equipment parameters can substantially increase mechanical properties of the final product. Methods of selecting the most efficient equipment for a given compounding operation must take into consideration degree of shear required, temperature sensitivity, residence time distribution, and volume to be produced. When dealing with glassfibers,abrasion or cor rosion of the compounding equipment plays a substantial role in economics. The simplicity and accuracy of the com pounding process are also important.

' " p h e d e v e l o p m e n t of filled plastics has r e a c h e d a p o i n t w h e r e p r o p e r t i e s ·*- of r a w m a t e r i a l s ( filler a n d p o l y m e r ) as w e l l as final m o l d i n g p a r a m e ters h a v e b e e n t h o r o u g h l y s t u d i e d a n d s u b s t a n t i a l t e c h n i c a l d a t a h a v e b e e n p u b l i s h e d . O n t h e other h a n d the t e c h n o l o g y i n v o l v e d i n c o m p o u n d i n g fillers a n d p o l y m e r s has not b e e n w e l l p u b l i c i z e d , a n d this c r i t i c a l area d i r e c t l y influences b o t h t o t a l e c o n o m i c s a n d final e n d - p r o d u c t q u a l i t y . T h e v o l u m e of p r e c o m p o u n d e d p l a s t i c pellets u s e d a n n u a l l y has b e e n increasi n g at a r e m a r k a b l e rate. S e l e c t i o n of t h e p r o p e r c o m p o u n d i n g

system,

therefore, is a c r i t i c a l e n g i n e e r i n g f u n c t i o n . A v a i l a b l e processes m u s t b e c a r e f u l l y e v a l u a t e d , t a k i n g i n t o c o n s i d e r a t i o n e n d - p r o d u c t q u a l i t y , degree of a u t o m a t i o n possible, a n d r e a l i s t i c p r o d u c t i o n rates as w e l l as v e r s a t i l i t y . A w r o n g d e c i s i o n here c a n cause a n e v e n t u a l c o m m e r c i a l f a i l u r e . A g o o d e v a l u a t i o n c a n p r o v i d e a c o m p e t i t i v e edge. T o d a y nearly a l l important polymers

are a v a i l a b l e as

filled

pre-

c o m p o u n d e d pellets f o r i n j e c t i o n m o l d i n g a n d extrusion. B e a r i n g i n m i n d 114 In Fillers and Reinforcements for Plastics; Deanin, R., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1974.

12.

jAKOPiN

Compounding

of

115

Fillers

process economics, e q u i p m e n t a v a i l a b i l i t y , a n d preference, the d e c i s i o n as to w h a t c o m p o u n d i n g t e c h n i q u e a n d w h a t c o m p o u n d i n g e q u i p m e n t to u t i l i z e often centers a r o u n d t h e q u e s t i o n of h o w m u c h shear is r e q u i r e d , h o w sensitive the c o m p o u n d

a n d / o r c o m p o u n d i n g i n g r e d i e n t s are to

t e m p e r a t u r e , a n d the v o l u m e to b e c o m p o u n d e d .

Q u a l i t y levels, s p a c e

l i m i t a t i o n s , a n d t h r o u g h p u t r e q u i r e m e n t s m u s t also b e c o n s i d e r e d .

The

processor t h e n c a n select e i t h e r a separate or i n - l i n e system, w h i c h e v e r is most efficient for h i s needs. T h i s a r t i c l e r e v i e w s v a r i o u s m e t h o d s of c o m p o u n d i n g


forced and non-reinforced)

fillers

(rein-

i n t o t h e r m o p l a s t i c or thermoset m a t e r i a l s .

B e f o r e d i s c u s s i n g t h e m e t h o d s of c o m p o u n d i n g , w e c o n s i d e r b r i e f l y the production requirements for compounding

The Compounding The

Production

Processs

e q u i p m e n t for c o m p o u n d i n g

several requirements: (a)

fillers.

fillers

and polymers must fulfill

steady-state r u n n i n g c o n d i t i o n s , ( b )

d u c i b i l i t y of p r o c e s s i n g c o n d i t i o n s , ( c )

repro-

ease of c l e a n i n g , a n d ( d ) v e r s a -

t i l i t y to a d a p t to n e w f o r m u l a t i o n s . T o a c h i e v e o p t i m u m m a t e r i a l q u a l i t y , the e q u i p m e n t s h o u l d h a v e : ( a ) the a b i l i t y to generate sufficiently h i g h i n t e r n a l shear stresses to f a c i l i t a t e g o o d d i s p e r s i o n of t h e a d d i t i v e s ( b ) the c a p a b i l i t y to expose e a c h p a r t i c l e to short a n d e q u a l stresses ( c ) exact t e m p e r a t u r e c o n t r o l to r e g u l a t e a n d m i n i m i z e heat h i s t o r y Compounding Methods. B a s i c a l l y , w e c a n differentiate t w o types of c o m p o u n d i n g : ( 1 ) d i s c o n t i n u o u s system a n d ( 2 ) c o n t i n u o u s system. T h e d i s c o n t i n u o u s system is f a i r l y o l d a n d i n most cases refers to B a n b u r y ( R ) i n t e n s i v e mixers or r o l l m i l l s . T h r o u g h p u t s for these systems r a n g e f r o m 500 to 10,000 l b s / h r , a n d s i z a b l e investments are r e q u i r e d . H o w e v e r , a n efficient p r o c e s s i n g system w i l l a l l o w the c o m p o u n d e r to operate econ o m i c a l l y at h i g h v o l u m e . O n the o t h e r h a n d , c o n t i n u o u s c o m p o u n d i n g systems h a v e capacities u p to 7000 l b s / h r . B e c a u s e of e c o n o m i c s a n d t h e large v o l u m e r e q u i r e m e n t s for

filled

p l a s t i c s , c o n t i n u o u s systems

are

u s u a l l y p r e f e r r e d . F o r q u a l i t y , c o n t i n u o u s systems offer better u n i f o r m i t y of p r o d u c t w i t h less b a t c h - t o - b a t c h v a r i a t i o n t h a n a d i s c o n t i n u o u s system. I n most cases, the p r o p e r p r o c e s s i n g c o n d i t i o n s f o r

compounding

can be filled a d e q u a t e l y w i t h s i n g l e - s c r e w c o m p o u n d i n g t y p e extruders. The

advances of s c r e w d e s i g n t e c h n i q u e s a n d n e w devices that a i d i n

l o c a l i z e d a n d c o n t r o l l e d i n t r o d u c t i o n of shear are n o w a v a i l a b l e to a p o i n t w h e r e , i n p r o b a b l y t w o out of t h r e e cases, s i n g l e - s c r e w extruders are a d e q u a t e . A relatively n e w approach i n designing a single-screw compounding e x t r u d e r is r e p r e s e n t e d b y a n e w m e d i u m shear t y p e c o m p o u n d e r c a l l e d

In Fillers and Reinforcements for Plastics; Deanin, R., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1974.

116

FILLERS

AND

REINFORCEMENTS FOR

PLASTICS

t h e T r a n s f e r m i x . T h i s is a c o n t i n u o u s , e n f o r c e d o r d e r , stepless extensive, v a r i a b l e intensive m i x e r , a viscous heat exchanger, a n d v e n t e d extruder. T h e T r a n s f e r m i x consists of t w o opposite h a n d e d s c r e w s — a r o t o r t u r n i n g i n s i d e the stator. I n the n a r r o w l a n d e d extensive m i x i n g stages, the g r o o v e d e p t h of the r o t o r decreases f r o m m a x i m u m to m i n i m u m w h i l e , i n the stator, it increases f r o m m i n i m u m to m a x i m u m . S h e a r rate c a n b e a d j u s t e d f r o m 30 to 3,000 r e c i p r o c a l seconds b y speed v a r i a t i o n a n d a r u n n i n g clearance adjustment. F o r some c o m p o u n d i n g operations, t w i n - s c r e w c o m p o u n d i n g


are the most effective

approach.

systems

If p r o p e r l y d e s i g n e d , t w i n - s c r e w ex-

t r u d e r s p r o v i d e m a x i m u m process controls, e s p e c i a l l y w i t h respect shear a n d stock t e m p e r a t u r e .

Also w i t h twin-screw compounders,

to

large

q u a n t i t i e s of volatiles c a n b e r e m o v e d . Types of Fillers. T h e m a n y fillers u s e d i n the plastics i n d u s t r y t o d a y c a n b e separated i n t o t w o (b)

non-reinforcing

g e n e r a l categories:

fillers.

B o t h categories

reinforcing

fillers,

are a p p l i c a b l e to

(a)

either

t h e r m o p l a s t i c or thermoset resins. C h o p p e d or r o v i n g glass is the most common

t y p e of r e i n f o r c i n g

filler.

Others commonly

u s e d are

cotton,

asbestos, F y b e x , a n d so f o r t h . T h e s e c o n d category contains fillers s u c h as c l a y , C a C 0 , talc, w o o d flour, a n d p i g m e n t s . 3

T h e t y p e of

filler

i n f o r c i n g or n o n - r e i n f o r c i n g ) d r a s t i c a l l y affects t h e c o m p o u n d i n g

(re-

process.

C o m p o u n d i n g of n o n - r e i n f o r c i n g fillers u s u a l l y r e q u i r e s the h i g h e s t d e g r e e of dispersion—e.g., c a r b o n b l a c k i n L D P E .

Therefore, equipment

s h o u l d b e a b l e to generate h i g h shear stresses to separate the a g g l o m e r ates, p a r t i c u l a r l y since these fillers u s u a l l y h a v e v e r y s m a l l p a r t i c l e sizes. I n c o m p o u n d i n g r e i n f o r c i n g fillers, the opposite a p p r o a c h is t a k e n : l o w shear c o m p o u n d i n g m u s t be u s e d to p r e v e n t d a m a g e to the main

consideration

is to

wet

the

filler

fillers.

The

uniformly, devolatilize,

and

discharge. Residence Time and Residence Time Distribution. T h e t w o essential elements i n a successful c o n t i n u o u s s y s t e m are absolute c o n t r o l over r e s i d e n c e t i m e a n d residence t i m e d i s t r i b u t i o n . T o m i n i m i z e heat h i s t o r y , the r e s i d e n c e t i m e m u s t b e short a n d u n i f o r m d u r i n g the entire process. R e s i d e n c e t i m e for thermoset resins, for e x a m p l e , s h o u l d n o t e x c e e d 60 sec. T h i s is also t r u e for m a n y heat- a n d shear-sensitive t h e r m o p l a s t i c m a t e r i a l s . R e s i d e n c e t i m e is p r i m a r i l y a f u n c t i o n of m a c h i n e d e s i g n ,

screw

R P M , a n d t h r o u g h p u t . A s i n a l l continuous m a c h i n e s , extruders d o not h a v e a n e x a c t l y defined r e s i d e n c e t i m e b u t r a t h e r a residence t i m e spect r u m . U n i f o r m i t y of a c o n t i n u o u s o p e r a t i o n is i l l u s t r a t e d b y the t y p e of spectrum.

W i t h s c r e w m a c h i n e s i n g e n e r a l w e c a n differentiate

four

p r i n c i p a l types of residence t i m e spectra ( F i g u r e 1 ). T h e t w o i d e n t i f y i n g characteristics of a residence t i m e d i s t r i b u t i o n are the d i s t a n c e w b e t w e e n the points of i n f l e c t i o n a n d the o v e r a l l w i d t h b of t h e d i s t r i b u t i o n c u r v e .


12.

Compounding

jAKOPiN

of

Fillers

117

a

J


w

-

>*

—

ft

1

TIME

Figure

1.

Four typical residence time curves for extruder processes (see text for discussion)

A s m a l l distance b e t w e e n t h e points o f i n f l e c t i o n indicates l i t t l e b a c k a n d f o r t h m i x i n g ; a great distance i n d i c a t e s a greater l o n g i t u d i n a l m i x i n g . T h e o v e r a l l w i d t h b of t h e d i s t r i b u t i o n c u r v e is i n f l u e n c e d b y s o - c a l l e d d i s t r i b u t i o n tails w h i c h i n d i c a t e t h e c l e a n i n g efficiency of t h e m a c h i n e . L o n g d i s t r i b u t i o n tails i n d i c a t e p o o r s e l f - c l e a n i n g efficiency.


118

FILLERS

AND

REINFORCEMENTS FOR

PLASTICS

T h e best c h a r a c t e r i z a t i o n of s e l f - c l e a n i n g is the s e l f - c l e a n i n g t i m e , s, defined as: s = U s i n g average

b — w. As s increases, t h e c l e a n i n g efficiency decreases. residence t i m e , w e

can obtain a similar

dimensionless

v a l u e for s e l f - c l e a n i n g c h a r a c t e r i z a t i o n w i t h v a r i o u s processes a n d average residence times: s _ b

w

—

~t

Γ

E x t r u d e r s w i l l a l w a y s h a v e a v a l u e greater t h a n 1. L i k e the t e r m s, as the


v a l u e increases, the s e l f - c l e a n i n g characteristics deteriorate.

Average

residence t i m e is defined as the t i m e i n w h i c h h a l f of the particles i n t h e residence t i m e s p e c t r u m pass t h r o u g h the m a c h i n e .

When

average

residence t i m e cannot be d e t e r m i n e d f r o m a residence t i m e s p e c t r u m , it c a n be c a l c u l a t e d b y :

where V = c = ψ =

free v o l u m e degree of fill v o l u m e t r i c flow p e r u n i t t i m e

C u r v e a i n F i g u r e 1 is t y p i c a l for a m a c h i n e w i t h little l o n g i t u d i n a l m i x i n g a n d p o o r s e l f - c l e a n i n g characteristics. C u r v e b is t y p i c a l for a m a c h i n e w i t h greater l o n g i t u d i n a l m i x i n g b u t s t i l l p o o r

self-cleaning

characteristics. T h e s e t w o curves are t y p i c a l for s i n g l e - s c r e w extruders a n d for t w i n - s c r e w extruders w i t h o u t a s e a l i n g profile. C u r v e s c a n d d s h o w o n l y short r e s i d e n c e t i m e tails w h i c h i n d i c a t e g o o d s e l f - c l e a n i n g characteristics.

T h e latter curves v a r y o n l y i n the

a m o u n t of b a c k a n d f o r t h m i x i n g a n d are t y p i c a l of t w i n - s c r e w extruders w i t h a s e a l i n g profile. I n m a c h i n e s w i t h g o o d s e l f - c l e a n i n g , n o p a r t i c l e s r e m a i n excessively l o n g i n t h e u n i t w h e r e t h e y m i g h t be subjected

to

severe heat. T h e r e are no d e a d corners w h e r e m a t e r i a l c o u l d a c c u m u l a t e . T h e s e are c r i t i c a l factors, e s p e c i a l l y w h e n p r o c e s s i n g thermoset m a t e r i a l s . A g o o d e x a m p l e of a n e x t r u d e r w i t h a s e a l i n g profile is the t w i n - s c r e w intermeshing and co-rotating compounder.

T h e w o r k i n g p r i n c i p l e of this

u n i t is d e s c r i b e d b e l o w . Twin-Screw

Intermeshing

and

Co-Rotating

Compounder.

The

p r o c e s s i n g section consists of t w o i n t e r m e s h i n g screws, r o t a t i n g i n the same d i r e c t i o n a n d at the same s p e e d i n t h e b a r r e l . T h e screws are selfc l e a n i n g a n d w i p e e a c h other w i t h a m i n i m u m clearance. B e c a u s e of this s e a l i n g profile, d e a d spaces w h e r e m a t e r i a l d e g r a d a t i o n c o u l d o c c u r are m i n i m i z e d , a n d a n e v e n t o r q u e d i s t r i b u t i o n is assured ( F i g u r e 2 ) .



12.

jAKOPiN

Compounding

of

119

Fillers

Figure 2. Twin-screw compounding extruder with co-rotating intermeshing screws. The screws are self-cleaning and wipe each other with a minimum clearance. Screws a n d barrels are b u i l t u p i n a b u i l d i n g b l o c k p r i n c i p l e . T h e s c r e w elements consist of different lengths a n d pitches a n d s p e c i a l k n e a d i n g elements of v a r i o u s w i d t h s w h i c h are i n t e r c h a n g e a b l e .

T h e screw

elements a n d k n e a d i n g b l o c k s are s e c u r e d o n t h e shaft b y a k e y .

The

s c r e w elements are h e l d o n the shaft b y the s c r e w t i p . B y v a r y i n g t h e s c r e w elements a n d k n e a d i n g b l o c k s , the s c r e w c o n f i g u r a t i o n c a n t a i l o r e d to the shear i n t e n s i t y r e q u i r e d b y t h e specific m a t e r i a l . s c r e w b a r r e l consists of i n d i v i d u a l b a r r e l sections.

When

be The

processing

t h e r m o p l a s t i c m a t e r i a l s , t h e b a r r e l b e f o r e t h e d i s c h a r g e is u s u a l l y the v e n t e d one w h e r e v o l a t i l e constituents c a n b e r e m o v e d f r o m the m e l t . E v e r y b a r r e l has c o o l i n g cores, so t h a t close t e m p e r a t u r e c o n t r o l c a n b e


120

FILLERS

o b t a i n e d w i t h w a t e r or o i l c o o l i n g . trically.

AND

REINFORCEMENTS FOR

PLASTICS

I n m a n y cases, h e a t i n g is d o n e elec-

T h i s b u i l d i n g b l o c k p r i n c i p l e m a k e s i t p o s s i b l e to d e s i g n the

p r o c e s s i n g section exactly as r e q u i r e d to o b t a i n o p t i m u m p r o c e s s i n g c o n ditions. A h i g h degree of v e r s a t i l i t y is also o b t a i n e d t h r o u g h the a b i l i t y to v a r y the l e n g t h a n d c o n f i g u r a t i o n of the screws a n d k n e a d i n g elements. T h u s , the r e q u i r e d shear stresses c a n b e a d j u s t e d to m e e t processing needs. T h e p r o c e s s i n g features a r e :


( a ) T h e e n e r g y r e q u i r e d to m e l t a n d h o m o g e n i z e the r e s i n w i t h the a d d i t i v e s c a n b e c r e a t e d b y f r i c t i o n w i t h i n the m a c h i n e . T h i s results i n excellent d i s p e r s i o n a n d h o m o g e n i z a t i o n ( r a t i o w:b is v e r y h i g h ) . ( b ) E n e r g y c a n be c r e a t e d i n a v e r y short t i m e a n d a v e r y short m a c h i n e l e n g t h b y h i g h e n e r g y i n p u t k n e a d i n g elements. A l l this results i n a v e r y short average r e s i d e n c e t i m e — i n m o s t cases, b e l o w 30 seconds.

S e l f - w i p i n g a n d s e l f - c l e a n i n g characteristics of

screw geometry prevent any deposition.

E v e n i n long-term

the

operation,

u n i f o r m m e l t c o n v e y a n c e a n d u n i f o r m p r o d u c t q u a l i t y are m a i n t a i n e d . T h e s e p r o c e s s i n g characteristics are e s p e c i a l l y c r i t i c a l for heat-sensitive materials where

controlling

the

m a t e r i a l t e m p e r a t u r e a c c u r a t e l y w h i l e e n s u r i n g that a l l p a r t i c l e s

are

exposed

successful

compounding

depends

on

to a preset t e m p e r a t u r e for the same t i m e .

Since they

these r e q u i r e m e n t s , c o n t i n u o u s h i g h i n t e n s i t y c o m p o u n d e r s

fulfill

with small

free v o l u m e a n d r e l a t i v e l y short residence t i m e h a v e g a i n e d w i d e ceptance.

ac-

T h e t e c h n i q u e s u s e d to c o m p o u n d fiber glass w i l l i l l u s t r a t e the

considerations

involved i n selecting c o m p o u n d i n g

equipment

and

the

influence of v a r i o u s e q u i p m e n t o n final properties. Glass Fiber Reinforced Thermoplastic

Polymers

T h e r e are s e v e r a l m e t h o d s to p r o d u c e glass-reinforced t h e r m o p l a s t i c m a t e r i a l s , a n d t h e y v a r y greatly i n t h e i r t e c h n i q u e s a n d results. obtained on precompounded

glass-reinforced p o l y p r o p y l e n e s h o w

physical properties v a r y significantly w i t h various c o m p o u n d i n g niques.

P r o p e r selection of

compounding

equipment

Data that tech-

and optimizing

e q u i p m e n t parameters c a n s u b s t a n t i a l l y increase m e c h a n i c a l p r o p e r t i e s of the final p r o d u c t .

T h e s i m p l i c i t y a n d a c c u r a c y of the

compounding

process are also i m p o r t a n t . W h e n d e a l i n g w i t h glass fibers, a b r a s i o n or c o r r o s i o n of the c o m p o u n d i n g e q u i p m e n t also p l a y a s u b s t a n t i a l r o l e i n t o t a l economics. T h e p r o p e r t i e s of the c o m p o s i t e a r e of course i n f l u e n c e d b y the glass fiber c o n c e n t r a t i o n , the strength of the fibers, a n d t h e effectiveness of the s i z i n g agent. I n the c o m p o u n d i n g o p e r a t i o n , h o w e v e r , the q u a l i t y of the r e i n f o r c e d p o l y m e r is d i r e c t l y affected b y ( a ) the l e n g t h of the glass fibers i n the e n d p r o d u c t , ( b )

t h e u n i f o r m i t y of the glass d i s t r i b u t i o n i n the

p o l y m e r , a n d ( c ) u n i f o r m w e t t i n g of the glass fibers b y the p l a s t i c m e l t .


12.

jAKOPiN

Compounding

of

121

Fillers

T h e r e are t w o b a s i c w a y s of m a k i n g a g l a s s - r e i n f o r c e d t h e r m o p l a s t i c part: ( 1 ) m a t e r i a l c a n b e f e d d i r e c t l y i n t o a n i n j e c t i o n m o l d i n g or e x t r u d i n g m a c h i n e as a p r e b l e n d m a d e p r i o r to m o l d i n g , or ( 2 ) M a t e r i a l c a n b e p u r c h a s e d as p r e - c o m p o u n d e d pellets, w h i c h c a n b e f e d i n t o a n i n j e c t i o n m o l d i n g or e x t r u d i n g m a c h i n e . Single-Screw Extruders. T h e most c o m m o n l y u s e d system for c o m p o u n d i n g glass r e i n f o r c e d t h e r m o p l a s t i c s i n s i n g l e - s c r e w extruders i n volves the use of c h o p p e d glass fibers w h i c h are p r e b l e n d e d w i t h p o l y m e r a n d f e d into the extruder. T h e glass is c o n v e y e d together w i t h t h e p o l y Downloaded by GEORGETOWN UNIV on February 19, 2015 | http://pubs.acs.org Publication Date: June 1, 1974 | doi: 10.1021/ba-1974-0134.ch012

m e r t h r o u g h a l l three stages of s i n g l e - s c r e w e x t r u s i o n : c o n v e y i n g , c o m pression, a n d m e t e r i n g . D u r i n g c o m p r e s s i o n , w h e n the p o l y m e r m e l t s , the glass fibers are exposed to h i g h shear stresses, a n d most of t h e r e d u c t i o n i n glass fiber l e n g t h takes p l a c e at this t i m e . A f t e r m e l t i n g , u s u a l l y a v e n t i n g section is u s e d to r e m o v e the v o l a t i l e s . T h e c o m p o s i t e is t h e n pushed through a die. C o m p o u n d i n g i n a single-screw extruder depends l a r g e l y o n h e a d pressure w h i c h g r e a t l y influences t h e glass fiber l e n g t h i n the

final

product.

I n c r e a s i n g h e a d pressure damages

the glass

fibers

w h i c h results i n c o n s i d e r a b l y decreased i m p a c t strength. T h e r e f o r e , h e a d p r e s s u r e m u s t b e as l o w as possible. I n s t e a d of u s i n g a p r e m i x , components c a n be f e d s e p a r a t e l y i n t o the extruder.

G l a s s fiber c a n be i n c h o p p e d or r o v i n g f o r m ; h o w e v e r ,

the

s c r e w s h o u l d rotate fast e n o u g h to p r e v e n t b u i l d - u p of m a t e r i a l i n the feed

pocket—so-called

starve f e e d i n g — o t h e r w i s e

the material might

" a r c h " across the f e e d p o c k e t a n d segregate because of v i b r a t i o n s . G l a s s damage d u r i n g melting can be reduced b y using a screw geometry w h i c h p e r m i t s g r a d u a l m e l t i n g b y e x t e r n a l heat a n d not s t r i c t l y b y m e c h a n i c a l e n e r g y i n p u t . W e a r c a n b e e x p e c t e d a l l a l o n g the s c r e w section, p a r t i c u l a r l y i n the u p s t r e a m section. C o m p o u n d i n g of glass fibers c a n also b e d o n e w i t h the T r a n s f e r m i x system. T h e glass fiber l e n g t h c a n b e c o n t r o l l e d to a c e r t a i n extent b y c h o o s i n g the r i g h t shear rate for a p a r t i c u l a r p o l y m e r i n o r d e r n o t to o v e r w o r k the glass fibers. T h e m a c h i n e is u s u a l l y f e d w i t h a p r e m i x of glass feed

fibers-polymer throat.

downstream.

o r b y m e t e r i n g t h e c o m p o n e n t s separately i n t o the

V o l a t i l e s are r e m o v e d Glass compounding

t h r o u g h t h e v e n t section

with

single-screw extruder compounding,

located

the T r a n s f e r m i x is s i m i l a r

b u t i t offers

to

improved versatility

a n d c o n t r o l of fiber l e n g t h . Twin-Screw Extruders. A recent d e v e l o p m e n t i n the p r o d u c t i o n of glass fiber r e i n f o r c e d t h e r m o p l a s t i c c o m p o u n d s intermeshing,

i n v o l v e s the use of the

co-rotating twin-screw compounder.

F o r several

good

reasons, t w i n - s c r e w c o m p o u n d i n g extruders h a v e b e c o m e m o r e a n d m o r e i m p o r t a n t for glass fiber c o m p o s i t e p r o d u c t i o n .


122

FILLERS

material feed

chopped glass or roving feed

kneading elements


A N D R E I N F O R C E M E N T S F O R PLASTICS

material feed

vacuum

left handed kneading element

chopped glass or roving feed

m

ρ

(θ)

kneading elements

Figure

neutral kneading element

3.

left handed flights

Typical screw configuration for compounding roving glass downstream into the melt

chopped or

V a r i a b i l i t y of s c r e w d e s i g n is p a r t i c u l a r l y i m p o r t a n t f o r p r o d u c i n g glass fiber r e i n f o r c e d composites since i t is possible to v a r y t h e r e q u i r e d average glass fiber l e n g t h , d e p e n d i n g o n p o l y m e r a n d t h e p e r c e n t a g e of glass used.

P o l y m e r s w i t h h i g h m e l t viscosities or h i g h glass l o a d i n g s

( 4 0 % b y weight or more) require m i l d e r screw configuration than p o l y mers of l o w m e l t v i s c o s i t y o r l o w p e r c e n t glass fiber ( 3 0 % b y w e i g h t o r l e s s ) . F i g u r e 3 shows a t y p i c a l s c r e w c o n f i g u r a t i o n a r r a n g e m e n t f o r t w o Table I.

Effect of Compounding on Mechanical Compounding Single-Screw Extruder

G l a s s fiber, w t % T y p e of fiber Tensile strength, psi Flex, modulus, m psi Izod impact, ft l b / i n c h notched H e a t defl. t e m p . , ° F a t 264 p s i % of F i b e r s smaller t h a n 0.5 m m Remarks

Technique Continuous

Mixer

25 1/8-inch c h o p p e d glass 6100 580

25 1/4-inch c h o p p e d glass 4700 460

1.45

0.7

264

153

glass w a s fed i n t o the feed section


12.

Compounding

jAKOPiN

of

123

Fillers

different p o l y m e r s : ( a ) l o w v i s c o s i t y p o l y m e r ( b ) h i g h v i s c o s i t y p o l y m e r . T h e s m a l l k n e a d i n g elements or s c r e w elements w i t h r e v e r s e d flights c a n b e u s e d to d e t e r m i n e the final fiber l e n g t h d i s t r i b u t i o n as w e l l as the p h y s i c a l properties. T h e effect of a s c r e w c o n f i g u r a t i o n o n p h y s i c a l p r o p erties is s h o w n i n T a b l e I. T h e properties are r e l a t i v e , a n d no s p e c i a l l y t r e a t e d p o l y p r o p y l e n e was u s e d . S i m i l a r to other p r o c e s s i n g m e t h o d s , t w i n - s c r e w i n t e r m e s h i n g extruders c a n also b e f e d w i t h a p r e m i x e d p o l y m e r / g l a s s b l e n d , or c o m ponents c a n be m e t e r e d separately i n t o the f e e d throat. A g a i n , the e q u i p -


m e n t is u s u a l l y starve-fed to p r e v e n t segregation. S i n c e these c o m p o u n d ers u s u a l l y h a v e h i g h s c r e w s p e e d ( 2 0 0 - 3 0 0 r p m ) , there is no d a n g e r of b e i n g l i m i t e d b y the c o n v e y i n g c a p a c i t y of the screws. T o p r e v e n t excess w e a r , t w i n - s c r e w c o m p o u n d e r s

are u s u a l l y not

f e d w i t h glass fibers into the f e e d p o r t b e c a u s e of the abrasiveness of the glass. T h e w e a r to w h i c h the screws a n d the barrels are s u b j e c t e d i n the p l a s t i c i z i n g z o n e m a y b e v e r y severe. T o p r o l o n g the e q u i p m e n t l i f e t i m e a n d to m i n i m i z e t h e glass fiber l e n g t h r e d u c t i o n d u r i n g p l a s t i c i z i n g , glass c a n b e f e d d o w n s t r e a m i n t o the m e l t t h r o u g h the degassing p o r t or via a side feeder

flanged

r o v i n g or c h o p p e d

onto the side of the b a r r e l .

form.

T h e glass c a n b e i n

T h e p o l y m e r is m e t e r e d i n t o the f e e d

port

i n the c o n v e n t i o n a l w a y a n d p l a s t i c i z e d i n the first section of the m a c h i n e b y a s u i t a b l e s c r e w c o n f i g u r a t i o n . I f necessary, other a d d i t i v e s s u c h as flame retardants, p i g m e n t s , p l a s t i c i z e r s , or s t a b i l i z e r s , c a n be t h o r o u g h l y b e f o r e the glass is f e d .

compounded

A l s o , a n y volatiles c a n b e

removed

i n that section. Roving Process. T h e r o v i n g process uses c o n t i n u o u s r o v i n g strands i n t r o d u c e d i n t o the m e l t t h r o u g h a n o p e n degassing a d a p t o r , w i t h o u t Properties of Fiber Glass-Filled Polypropylene Compounding Twin-Screw Compounder 1

Turin-Screw Compounder 2

Technique Twin-Screw Compounder S

25 roving

23 roving

25 roving

4900 600 0.9 203

5800 550 1.1 184 — m o d e r a t e screw after a d d i t i o n of glass

8000 550 1.2 268 29 m i l d screw after a d d i t i o n of glass ( N o . 14)

screw w i t h v e r y s t r o n g sections after a d d i t i o n of glass

Twin-Screw Compounder 4

25 1/8-inch chopped glass 8000 550 1.3 266


124

FILLERS

material feed

AND

REINFORCEMENTS FOR

glass fiber rovings vacuum vent port

strand cutter

strand die

5


Figure

4.

PLASTICS

J

Typical

-jjJJ^woterbath

Ψ arrangement for feeding fiber rovings

continuous

glass

Figure 5. Feeding glass fiber roving into a melt down stream through a vent opening of a twin-screw compounder s p e c i a l m e t e r i n g devices.

S t r a n d s are u n w o u n d f r o m the cores c o n t i n u

o u s l y a n d p u l l e d i n t o the m a c h i n e b y t h e r o t a t i n g screws. i n t a k e of glass strands d e p e n d s

Since the

d i r e c t l y o n s c r e w r o t a t i o n , the d e s i r e d

a m o u n t of glass c a n be c o n t r o l l e d b y the n u m b e r of strands a n d the r p m of t h e screws.

T h e a m o u n t of glass p e r h o u r of one s t r a n d c a n a c t u a l l y

b e p l o t t e d as a f u n c t i o n of r p m . I f the f e e d rate of the p o l y m e r is k n o w n , the n u m b e r of strands c a n b e easily d e t e r m i n e d to m a t c h the r i g h t p e r c e n t a g e

of glass l o a d i n g .

F i g u r e s 4 a n d 5 s h o w a t y p i c a l a r r a n g e m e n t for f e e d i n g continuous glass fiber r o v i n g s . T h e p o l y m e r f e e d rate c a n b e r e a d i l y v a r i e d because t w i n s c r e w extruders are g e n e r a l l y starve-fed w i t h n o a d j u s t m e n t r e q u i r e d i n s c r e w speed.

I n other w o r d s , s c r e w s p e e d c a n b e v a r i e d i n d e p e n d e n t l y

of p o l y m e r f e e d rate, to adjust glass fiber feed.

R o v i n g spools c a n b e

set a l o n g the m a c h i n e , o r i n l a r g e r p r o d u c t i o n ( 2 0 or m o r e strands i n t r o -


12.

jAKOPiN

Compounding

125

of Fillers

d u c e d ) the spools c a n b e p l a c e d i n the creels a n d p u l l e d i n t o the m a c h i n e from one end. T h e s c r e w g e o m e t r y i n this p a r t is d e s i g n e d so t h a t the s c r e w threads are o n l y p a r t i a l l y filled w i t h p o l y m e r . T h i s p a r t i a l filling makes i t p o s s i b l e f o r t h e s c r e w to take u p t h e glass fibers a d d e d at t h e f e e d p o i n t , w h i l e b l o c k a g e of the glass fiber f e e d p o r t b y t h e p l a s t i c m e l t is a v o i d e d . T h e s c r e w g e o m e t r y d o w n s t r e a m f r o m the r o v i n g f e e d p o r t is l a r g e l y r e s p o n sible f o r t h e fiber l e n g t h a n d t h e h o m o g e n e i t y of t h e c o m p o u n d .

Glass

fiber is i n endless f o r m a n d m u s t b e c h o p p e d to a c e r t a i n l e n g t h i n t h e


machine a n d homogenized geometry

with polymer.

m u s t b e selected

T o d o this, p r o p e r

to m e e t these r e q u i r e m e n t s .

screw

Rheological

p r o p e r t i e s of p o l y m e r a n d l o a d i n g of glass m u s t b e c o n s i d e r e d , a n d this can b e done b y installing k n e a d i n g components or right- a n d left-hand s c r e w elements i n s u i t a b l e c o m b i n a t i o n s .

A c c u r a c y of t h e glass c o n t e n t

is w i t h i n ± 1 . 5 % o r less as l o n g as t h e p o l y m e r f e e d is constant a n d t h e u n w i n d i n g rate o f t h e r o v i n g r e m a i n s constant. Chopped Glass Process. C h o p p e d r a t h e r t h a n r o v i n g glass c a n also b e f e d d o w n s t r e a m into t h e m o l t e n p o l y m e r , b u t m e t e r i n g e q u i p m e n t is r e q u i r e d . A s m e n t i o n e d , the glass c a n b e f e d b y g r a v i t y t h r o u g h a r e g u l a r v e n t p o r t or s i d e f e d w i t h a d e l i v e r y u n i t w h i c h is flanged t o t h e b a r r e l at a d e s i r e d l o c a t i o n . polymer & additives

-

a

Arrangement

t

vacuum

s chopped glass feed

•m

Figure 6.

t

vacuum

for forced feeding of chopped glass downstream the melt

into

F e e d i n g b y g r a v i t y has l i m i t a t i o n s as f a r as t h r o u g h p u t is c o n c e r n e d . U s i n g 2 - i n c h extruders, f e e d i n g glass b y g r a v i t y is s a t i s f a c t o r y — 7 0 - 1 0 0 l b s / h r c a n b e f e d easily except f o r p o l y m e r s w i t h s h a r p m e l t i n g points (e.g., p o l y a m i d e s o r p o l y e s t e r s ) .

W h e n scaling u p to a 3-inch extruder,

the t o t a l t h r o u g h p u t is l i m i t e d b y the glass feed. A m a x i m u m of 2 0 0 - 2 5 0 l b s / h r of glass c a n b e f e d b y g r a v i t y . T o o v e r c o m e this l i m i t a t i o n , a s m a l l side f e e d i n g u n i t c a n b e a t t a c h e d to t h e side of t h e b a r r e l . T h e glass is f o r c e d i n t o t h e m a c h i n e , as i l l u s t r a t e d i n F i g u r e 6, a n d t h e t h r o u g h p u t rate c a n b e almost d o u b l e d f r o m t h a t of the g r a v i t y - f e d rate.


126

FILLERS

A N DR E I N F O R C E M E N T S F O RPLASTICS

W i t h c h o p p e d glass, s c r e w g e o m e t r y c a n b e q u i t e different f r o m that u s e d w i t h r o v i n g . H e r e , t h e glass fiber has to b e o n l y w e t t e d b y p o l y m e r , the volatiles r e m o v e d a n d d i s c h a r g e d t h r o u g h a d i e . U s u a l l y , s t r a i g h t c o n v e y i n g sections a r e u s e d w i t h v a r i o u s leads. F o r those p o l y m e r s t h a t are difficult to h o m o g e n i z e a s m a l l n e u t r a l k n e a d i n g section is i n c o r p o rated.

D o w n s t r e a m f e e d i n g c a n also b e u s e d f o r other r e i n f o r c i n g o r

n o n - r e i n f o r c i n g fillers, p a r t i c u l a r l y i f t h e y a r e abrasive. Conclusion


To compound

fillers

properly, the equipment used must maintain

steady-state r u n n i n g c o n d i t i o n s a n d m i n i m i z e b a t c h t o b a t c h v a r i a t i o n s . It s h o u l d b e versatile a n d r e a d i l y a d a p t to n e w a n d different f o r m u l a t i o n s . T h e c o m p o u n d i n g process c a n a d v e r s e l y affect e n d - p r o d u c t p r o p e r t i e s . T w o types o f c o m p o u n d i n g m e t h o d s are a v a i l a b l e : d i s c o n t i n u o u s a n d c o n tinuous. C o n t i n u o u s systems are p r e f e r r e d because t h e y m e e t the t h r o u g h p u t , e c o n o m i c , a n d r i g i d p r o d u c t q u a l i t y r e q u i r e m e n t s o f most

com

p o u n d e r s . T h e essential elements i n a c o n t i n u o u s system a r e c o n t r o l o v e r residence t i m e a n d residence t i m e d i s t r i b u t i o n . E a c h process d e s c r i b e d here has its advantages a n d disadvantages. I n m a n y cases, c o m p o u n d i n g r e q u i r e m e n t s c a n b e m e t b y a s i n g l e - s c r e w extruder. I n other cases, w h e r e several p r o c e s s i n g functions m u s t b e p e r f o r m e d i n a single o p e r a t i o n , t w i n - s c r e w systems a r e essential.

T h e intermeshing, twin-screw

com

p o u n d e r w i t h its i n t e r c h a n g e a b l e s c r e w configurations offers t h e processor p r o d u c t i o n v e r s a t i l i t y . I t p r o v i d e s o p t i m u m shear a n d stock t e m p e r a t u r e control and devolatilizing capabilities needed for many of today s complex formulations.

Bibliography

1. Bernardo, A. C., "How to Get More from Glass-Fiber Reinforced HD,PE," SPE J. (Oct. 1970) 26, 39-45. 2. Davis, J. H., "Fundamentals of Fiber-Filled Thermoplastics," Plastics Polymers (April 1971) 137-143. 3. Ross, Guenther, "Glasfaserverstaerktes Polypropylen," Kunststoffe (1970) 60, 924-930. 4. Lees, J. K., "A Study of the Tensile Strength of Short Fiber Reinforced Plastics," Polymer Eng. Sci. (1968) 8 (3). 5. Schlich, W. R., Hagan, R. S., Thomas, J. R., Thomas, D. P., Musselman, Κ. Α., "Critical Parameters for Direct Injection Molding of Glass-Fiber Thermoplastic Powder Blends," SPE J. (Feb. 1968) 24, 43-53. 6. Cessna, L. C., Thomson, J. B., Hanna, R. D., "Chemically Coupled GlassReinforced Polypropylene," SPE J. (1969) 25, 35-39. 7. Krautz, F. G., "Polypropylene Theory & Practice," Intensive Short Course, PIA, Inc., July 21-23, 1971, University of Massachusetts, Amherst. 8. Coneys, Τ. Α., "Coupled Glass Reinforced Polypropylene—A New Struc tural Composite Material," Society of Automotive Engineers Meeting, Detroit, Jan. 13-17, 1969.


12.

jAKOPiN

Compounding of Fillers

127

9. Ehrenstein, G. W., "Glasfaserverstaerkte Thermoplastiche Kunststoffe— Grenze und Anwendungsmoeglichkeiten," Kunststoffe (1970) 60, 917924. 10. Olmstead, B. S., "How Glass-Fiber Fillers Affect Injection Machines," SPE J. (Feb. 1970) 26, 42-43. 11. Filbert, W. C., Jr., "Reinforced 66 Nylon—Molding Variables vs. Fiber Length vs. Physical Properties," SPE J. (Jan. 1969) 25, 65-69. 12. Herrmann, H., "Schneiken Machinen," in "Der Verfahrenstechnik," Springer Verlag, Berlin, 1972.


RECEIVED October 11, 1973.


Fillers and Reinforcements for Plastics - ACS Publications

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