4 Energy Conservation Through Silicone Liquid Polymer Processing System J. L. ELIAS, M. T. MAXSON, and C. L. LEE Research Department, Dow Corning Corporation, Midland, MI 48640
Fabricated silicone rubber parts are traditionally made from high consistency silicone gum stock. Because of the stiffness of the gum stock, the material must be worked with rubber masticating equipment and preformed before fabrication. A new fabricating process using a low consistency liquid silicone rubber was introduced by Dow Corning Corporation recently (1,2,3). This process is called liquid polymer system (LPS). Prior to the introduction of the LPS process, the low consistency liquid silicone rubber was not considered for use in fabricated parts because of the inadequate physical properties. Recent advancements in the low consistency silicone elastomer technology, however, have led to the development of high strength material. The property profile of silicone rubber thus obtained is now comparable to that of high consistency silicone rubber. This is shown in Table 1. The LPS process offers a variety of manufacturing advantages over the conventional fabricating process. Primarily, benefits are seen in the reduction of capital investment and labor costs, (1,2) pollutants, waste material and energy requirements. The energy conservation by the LPS process is especially important and attractive in view of today's worsening energy situation. The purpose o f t h i s paper i s t o demonstrate a f a b r i c a t o r ^ energy savings through the use o f the LPS process compared t o a conventional process using a high consistency s i l i c o n e gum. Results and D i s c u s s i o n I n j e c t i o n Molding. LPS i s a completely automated process. Once the l i q u i d components are introduced t o the feed system, the m a t e r i a l i s pumped, mixed, molded, demolded and c o l l e c t e d without manual handling. Energy i n t e n s i v e equipment such as m i l l s , ext r u d e r s and heavy duty presses normally used i n rubber p r o c e s s i n g are r e p l a c e d with a i r pumps, motionless mixers and l i g h t w e i g h t i n j e c t i o n molding machines. This r e s u l t s i n considerable energy savings. Schematics i l l u s t r a t i n g the LPS system and a conventional 0-8412-0509-4/79/47-107-037$05.00/0 © 1979 American Chemical Society
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high c o n s i s t e n c y molding operation are shown i n Figure I and respectively. TABLE 1.
II
Comparison Of T y p i c a l L i q u i d S i l i c o n e Rubber With A T y p i c a l High Consistency S i l i c o n e Rubber L i q u i d S i l i c o n e Rubber Silastic' Silastic Q3-9595 Q3-9590
5
High Consistency S i l i c o n e Rubber Silastic® 4515U
As Supplied Property S o l i d Content (%) Viscosity Physical Properties
100 (a) Pourable Pumpable
100 (b) Pumpable
100 Gumlike
(c)
Durometer (Shore A) T e n s i l e Strength [M Pa ( p s i ) ] E l o n g a t i o n (%) Tear,Die B,KN/M(ppi)
35 5.5(800) 450 15.8(90)
52
48 7.6(1,100) 350 31.6(180)
7.6(1,100) 540 17.5(100)
Note: (a) 1,000 poise (b) E x t r u s i o n Rate at 90 p s i through 1/8" orifice=150 g/min (c) V u l c a n i z a t i o n C o n d i t i o n 5 min at (a) Silastic® Q3-9590 and S i l a s t i c ' 4515U 150° C + 4 hrs post cure at 250°C (b) S i l a s t i c * Q3-9595 : 5 mins at 150°C 5
1
To demonstrate the a c t u a l savings r e a l i z e d by a f a b r i c a t o r , the energy r e q u i r e d to mold a 9 gram spark plug boot by the LPS process and by the conventional process i s compared. In t h i s c a l c u l a t i o n , i t i s assumed t h a t 100% acceptable p a r t s are produced by both processes. In p r a c t i c e , the r e j e c t i o n r a t e o f LPS i s much lower than the conventional process. The r e s u l t s o f c a l c u l a t i o n are summarized i n Table I I . The r a t i o o f energy r e q u i r e d by the LPS process to t h a t o f the conventional process i s almost 1:4. The energy cost o f m a t e r i a l s i n KWH per p a r t can be misleading i f the u s e f u l l i f e o f the products being compared i s s i g n i f i c a n t l y d i f f e r e n t . Because o f the environmental c o n t r o l s placed on the automobile i n d u s t r y , the under hood temperature have r i s e n significantly. At these high temperatures, which can reach 450°F, the s e r v i c e l i f e o f a s i l i c o n e spark plug boot i s at l e a s t two and one h a l f times that of a comparable organic boot. The f a c t t h a t a s i l i c o n e spark plug boot has t h i s longer s e r v i c e l i f e would make i t a b e t t e r choice than the boot made from organic elastomers. For example, assume that an organic boot (a) has an energy r e q u i r e ment equivalent to that o f conventional process, i . e . , 0.0536 KWH per boot and l a s t s 40,000 mile (4 y e a r s ) . A s i l i c o n e boot molded from a high c o n s i s t e n c y gum (b) has an energy cost o f .0536 KWH per boot, while a boot molded by LPS process (c) has an energy
Silicone
4. ELIAS ET AL.
Liquid
Polymer
Processing
Aro Drum Pumps (Transfer As Supplied L i q u i d S i l i c o n e Rubber from Drums i n t o Meter Mix)
Techcon Meter Mix (blends A and Β component i n a 1:1 r a t i o )
Boy S e r i e s 15/7 I n j e c t i o n Molding Machine
Figure 1.
Liquid injection molding for a two component system
Raw Base Stock
Two-Roll M i l l (24") used t o s o f t e n stock
2V
Extruder, used t o preform stock
75 Ton Press
F i n i s h e d Part Figure 2.
Transfer press molding for a high consistency silicone system
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TABLE I I .
C O N S E R V A T I O N ΓΝ
TEXTILE
A N D POLYMER
PROCESSING
Case No. 1 Energy Consumption For S i l i c o n e Spark Plug Boot Production, LPS Process vs Conventional Process
LPS Process Equipment
Conventional Process Energy (KWH)
2 Aro Drum Pumps 130cc/CFM a t 80 p s i
.001
1 Meter Mix Pump
.001
Boy I n j e c t i o n Molding Machine S e r i e s 15/7 Motor 6.6 Heater(50% duty) 1.0 Total 7.602 Spark Plug Production Rate (4 c a v i t y mold, 24 sec c y c l e time) .= 600 p a r t s / h r Energy comsumption per Spark plug boot (KWH) .01267
Energy (KWH)
Equipment 24" 2 - R o l l M i l l (.425 hrs o p e r a t i o n )
9.724 4.833
2.5" Extruder (30 l b / h r )
75 ton Krass T r a n s f e r Mold Press 7.000 Motor 4.180 Heater(50% duty) 25.737 Total Spark plug production r a t e (40 c a v i t y mold, 5 min c y c l e time) = 480 p a r t s / h r .0536
TABLE I I I . Case No. 2 Energy Consumption For Three Gram S i l i c o n e P a r t s , LPS Process vs Conventional Process Conventional Process
LPS Process Equipment
Energy (KWH)
2 Aro Drum Pumps 130 cc/CFM a t 80 p s i .0004 1 Meter Mix Pump
.0004
Boy I n j e c t i o n Molding Machine S e r i e s 15/7 Motor . 6.60 Heater(50% duty) 1.00 Total 7.6008 Production Rate 4 c a v i t y mold, 720 p a r t s / h r 20 sec c y c l e time Energy Consumption per p a r t (KWH)
.01055
Energy (KWH)
Equipment 2 - R o l l Mill(14.92 5 min/8 hr s h i f t
KW) .1554
T r a n s f e r Mold Press Motor Heater(50% duty) Total
59 c a v i t y mold 5 min c y c l e time
6.3840 4.5000 11.0394
708 p a r t s / h r
.01559
cost o f .01267 KWH. Both s i l i c o n e boots have a u s e f u l s e r v i c e l i f e o f 100,000 miles (10 y e a r s ) . Thus (a) r e q u i r e s .0134 KWH per
4. ELIAS ET AL.
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Polymer
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year per boot, (b) .0054 KWH per year and ( c ) has an energy cost of .0013 KWH per year per boot. T h i s i s a f o u r f o l d energy savings over high c o n s i s t e n c y s i l i c o n e and a t e n f o l d savings when compared to high c o n s i s t e n c y o r g a n i c s . Table I I I shows an a c t u a l case h i s t o r y f o r molding a three gram rubber a r t i c l e . Even though the conventional process i n v o l v e d l i t t l e mechanical performing, the data s t i l l shows an energy savings by u s i n g the l i q u i d polymer i n j e c t i o n molding concept. A d d i t i o n a l l y , the amount o f waste was reduced from 48% i n the conventional system t o 10% i n the l i q u i d polymer system. E x t r u s i o n Coating - The use o f l i q u i d s i l i c o n e rubber i n c o a t i n g a p p l i c a t i o n r e q u i r e s l e s s energy than conventional d i s p e r s i o n c o a t i n g processes. S o l v e n t l e s s m a t e r i a l , r a p i d cure time, absence o f by-products and the e l i m i n a t i o n o f m u l t i p l e passes a l l c o n t r i b u t e t o the reduced energy demand o f the l i q u i d polymer system. One area i n which t h i s technology i s making inroads i s i n the manufacture o f conductive i g n i t i o n core. Schematics comparing t h i s system f o r producing a s i l i c o n e conductive core ( F i g u r e 3) to the s o l v e n t d i s p e r s i o n method t r a d i t i o n a l l y used ( F i g u r e 4) are shown. The f o l l o w i n g c r i t e r i a were used i n determining the energy requirements needed to f a b r i c a t e a s i l i c o n e conductive c o r e . 1. A c t u a l production runs have shown t h a t the footage o f core produced with the LPS system i s double t h a t o f the d i s p e r s i o n system. 2. A c t u a l energy c a l c u l a t i o n s are based on a p i l o t s c a l e v e r t i c a l oven which i s s i x f e e t i n l e n g t h . The maximum footage produced i n t h i s system was 50 f e e t per minute using the l i q u i d s i l i c o n e conductive core m a t e r i a l . Prod u c t i o n runs exceed t h i s f i g u r e by a l a r g e percentage. 3. The diameter o f the uncoated core i s 75 m i l s and the t h i c k n e s s o f the c o a t i n g i s 5 m i l s . A c t u a l energy (KWH) used i n the manufacture o f one m i l l i o n f e e t o f conductive core are shown i n Table IV. I f one takes i n t o c o n s i d e r a t i o n energy r e q u i r e d t o v a p o r i z e the s o l v e n t i n the d i s p e r s i o n process, f u r t h e r energy savings can be r e a l i z e d . T h i s i s t h e o r e t i c a l l y c a l c u l a t e d u s i n g the f o l l o w i n g data: (a) core diameter = 75 m i l s , (b) c o a t i n g t h i c k n e s s = 5 m i l s (c) d i s p e r s i o n composition = 22% s o l i d , 78% xylene, (d) s p e c i f i c heat o f s i l i c o n e = 0.3 cal/g/°c and heat o f v a p o r i z a t i o n o f xylene = 93.4 c a l / g , (e) output speed: 120 f t / m i n by LPS process and 60 f t / m i n by d i s p e r s i o n process and ( f ) cure temperature: 110°C by LPS process and 204°C by d i s p e r s i o n process r e s p e c t i v e l y . R e s u l t s o f c a l c u l a t i o n are shown i n Table V. A 20-fold energy savings could be r e a l i z e d i n a system designed e x c l u s i v e l y f o r the LPS system.
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CONSERVATION
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A N D POLYMER
PROCESSING
Motionless Mixer
CD-
F i n a l Product
-Hot A i r V u l c a n i z i n g Oven
Cross head ( a p p l i e s uniform c o a t i n g t o core) Core Stock Pneumatic Frame and Motor Conductive L i q u i d S i l i c o n e Rubber A and Β components as s u p p l i e d (Silastic® LSR Q3-9593) Figure 3.
Conductive core coating by LPS process
F i n a l Product
Hot A i r V u l c a n i z i n g Oven (removes solvent and v u l c a n i z e s c o a t i n g )
Approximately 7 passes needed t o produce desired thickness o f 5 mils Core Stock
D i s p e r s i o n Vat f i l l e d with gum dispersed i n solvent
Figure 4.
Conductive core coating by solvent dispersion process
4.
ELIAS E T A L .
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Liquid
Polymer
43
Processing
TABLE IV: Comparison Of Energy Consumption For Coating Conductive Core With L i q u i d S i l i c o n e Rubber vs S i l i c o n e Rubber D i s p e r s i o n Required Energy LPS Process 2 Drum Pumps 130 cc/CFM a t 80 p s i HAV Oven Motor Heater* Conveyor Motor T o t a l (KWH) Conductive Core (ft/min) KWH per f t KWH per M f t
(KWH)
D i s p e r s i o n Process
.001 .7 10.0 .37 11.07
.7 10 .37 11.071 50
25
.00369 3,690
.00738 7,380
*50% duty time TABLE V: C a l c u l a t i o n Of Energy Required For Conductive Core Coating, LPS Process vs Solvent D i s p e r s i o n Process LPS Process Weight o f Coated Rubber
32 g/min (120 ft/min)
D i s p e r s i o n Process 17.18 g/min (60 ft/min)
Removal o f Xylene Weight o f xylene (g/min) Energy r e q u i r e d (k cal/min) Energy r e q u i r e d t o cure rubber c o a t i n g (k cal/min) T o t a l Energy Required (K cal/min) Energy Consumption per 100 f t o f core (K c a l )
60.9 8.5 0.8
0.9
0.8
9.4
0.67
15.67
Future Use o f LPS Process - Other p o t e n t i a l areas where the LPS process can be employed are s l e e v i n g , f a b r i c c o a t i n g , r o l l c o v e r i n g , e n c a p s u l a t i o n , t h i n wire c o a t i n g , t u b i n g , and wire j a c k e t i n g and i n s u l a t i o n . As technology develops, energy savings w i l l be r e a l i z e d i n these f a b r i c a t i o n a p p l i c a t i o n s through the e l i m i n a t i o n o f m i l l i n g , preforming, and s o l v e n t and by-product removal. Summary - A means o f saving energy by u s i n g a new f a b r i c a t i n g process, LPS, f o r i n j e c t i o n molding and e x t r u s i o n c o a t i n g s has been presented. A c t u a l data shows a s i g n i f i c a n t energy savings f o r the LPS process over c o n v e n t i o n a l processes.
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A N D POLYMER
PROCESSING
A more b a s i c way o f viewing t h i s energy savings i s by r e l a t i n g the energy saved t o the e l e c t r i c a l energy needed t o operate a p r i v a t e r e s i d e n c e . Assume 100 M spark plug boots were produced i n the United States l a s t year. The use o f the LPS system would save 4,093,000 KWH. Based on f i g u r e s obtained from Consumers Power Company o f Michigan, t h a t ' s enough power t o e l e c t r i c a l l y operate 682 homes f o r one year. Acknowledgement The authors wish t o acknowledge A. Smith, G. Kehrer, J . Godie, V. Johnson and W. Hays o f Dow Corning C o r p o r a t i o n f o r p r o v i d i n g data and c o n s t r u c t i v e suggestions.
Literature Cited 1. Kehrer, G. P., and Hays, W. R., Paper presented at the 16th Annual TLARGE Foundation Tech. Conf. Univ. of South Calf., Los Angeles, Calf., June (1977). 2. Hays, W. R., Kehrer, G. P., and Monroe, C. M., Paper presented at the 112th Meeting of the Rubber Div., Am. Chem. Soc., Cleveland, Ohio, Oct. 4-7 (1977). 3. Kehrer, G. P., and Monroe, C. Μ., Paper presented at the Passenger Car Meeting, Society of Automotive Engineers, Detroit September 26-30 (1977). RECEIVED
February
8, 1979.
