There are three large-volume types of polystyrene. These are general purpose (GPPS), sometimes called straight polystyrene; medium impact MIPS), containing about 5% rubber; and high impact (HIPS), containing up to 0.15 lb rubber/lb styrene. From Table 2E-2 it can be seen that around 50% of polystyrene produced is rubber-modified. However, at present the GPPS market is growing at a higher rate.9 Therefore 60% of the product will be GPPS, 20% will be MIPS, and 20% will be HIPS.
Polystyrene Storage and Shipping
The product will be shipped in bulk by truck and hopper cars as well as in 50 lb bags, 200 lb fiber cartons, and 1,000 lb Since there are many small customers, it will be assumed that 30% is shipped in 50 lb bags, and 15% each in 200 lb cartons and 1,000 lb boxes. Most of these small customers will not have a large storage facility. To provide good service, a 60-day supply of each product stored in bags, cartons, and drums will be specified. A 25-day storage facility for bulk materials will be constructed.
Physical Properties
The physical properties of polystyrene depend upon the specific reaction compo- nents, the mass ratios of the components, and the conditions at which the reaction occurs. These will be discussed later. The impurities remaining in the polystyrene also affect the properties. For instance, the heat distortion temperature may be as low as 70°C if there is unreacted styrene present. It is normally between 90 and 95°C. Therefore the maximum percentage of styrene that will be allowed in the product is 0.01%. Careful drying is also necessary if the polystyrene is to be extruded. For this application the polystyrene must contain a maximum of 0.03- 0.05% water.” We will set 0.03% as the maximum amount of water allowed. The specifications for the polystyrene are given in Table 3E-1. Different types of
rubbers may be used for making impact We shall use
polybutadiene.
Table 3E-1
Chemical Composition of Product
Polystyrene Rubber Water Styrene Other impurities GPPS 99.96% 0.0% < 0.03% < 0.01% <O.Ol% MIPS 5 % < 0.03% <O.Ol% < 0.03% HIPS 88% 12% < 0.03% <O.Ol% < 0.03%
Case Study: Scope 7 3
Operating Hours
The number of operating hours per year will be assumed to be 8,300. This plant is large and the technology is well developed.
Styrene Storage
The styrene will be obtained by barge from Louisiana (approx. 1,250 miles away). For GPPS, assuming a 3% loss in processing, the amount of styrene needed per hour is:
150,000,000(1b/yr) X 1.03 = 1 8,600 lb/hr 8,300 (hr/yr)
Barges carry between 1,000 and 3,000 tons On large rivers a single tow may consist of up to 12 barges. A barge containing 1,000 tons of styrene will last:
1,000 tons X 2,000 (lb/ton) (1 day/24 hr) = 4 5 days 18,600 (lb/hr)
A 3,000-ton barge load will last 13.5 days. It is assumed that an agreement can be made with a supplier to ship the styrene within 5 days after the order is received and that after leaving Louisiana a shipment will take 10-15 days to reach the plant. Under ideal conditions it will take 1 day after the order is received to obtain a barge and load it. This means it will take a minimum of 11 days between sending the order and delivery of the styrene. If the order arrived at the styrene plant on the Saturday of a 3-day weekend and the maximum order delays occurred (5 days before a barge was loaded and the trip took 15 days) then the shipment would arrive 23 days after the order was sent. The difference between these times is 12 days. This means that when a large barge is used a storage capacity of 12 + 13.5 or 26 days is required.
For a smaller load of 1,000 tons the storage capacity should be 17 days. Since a barge shipment would be needed every 4.5 days, undoubtedly automatic ordering procedures would be instituted and some of the possible time delays could be eliminated. See Chapters 10 and 11 for methods to determine which size barge shipment is best. A 17-day styrene storage capacity will be assumed.
The Suspension Process
There are many different ways of making polystyrene using the suspension process. Most producers use a batch process, although there are no technical reasons why a continuous process could not For this study a batch
processing scheme will be used. In the suspension process a number of small styrene drops 0.15-0.50 mm in diameter are suspended in water. The reaction occurs within these drops. To aid in the formation of the proper size drops a suspending agent is used, and to keep them at that size a stabilizing agent is added. A catalyst is used to control the reaction rate.
James gives the following general information about the suspension process. Some typical suspension agents are methyl cellulose, ethyl cellulose, and polyacrylic acids. in addition lists polyvinyl alcohol, sulfonated polys- tyrene, and polyvinylpyrollidone). Their concentration in the suspension is be- tween 0.01 and 0.5% of the monomer charged. The stabilizing agents are often insoluble inorganics such as calcium carbonate, calcium phosphates, or bentonite clay. in addition lists barium sulfate, calcium oxalate, and aluminum hydroxide). These are present in smaller amounts than the suspending agents. The catalysts are usually peroxides. The most common ones are benzoyl, diacetyl, lauroyl, caproyl, and tert-butyl. Their concentration varies from 0.1 to 0.5% of the monomer charged. The ratio of monomer to dispersing medium is between 10 and 40%. The parts per 100 parts of monomer that are typical for a polystyrene system are given in Table 3E-2 along with the temperature and cycle time.
Table 3E-2
Typical Formulations Used for the Batch Suspension Process for Polystyrene
P a r t s Styrene Water Calcium phosphate Tricalcium phosphate M e t h y l c e l l u l o s e Dodecylbenzene sulfonate Diacetyl peroxide Benzoyl peroxide Reaction temperature (° F) Cycle time (hours)
Smith* 100 4 0 0 0.2 Church** 100 68 0.11 0.5 0 . 0 0 2 5 6 0.3 194° 6.5 0 . 2 0 4 190-200° 3-4
* Smith, W.M.: Manufacture of Plastics, Reinhold, New York, 1964, p. 410 - as obtained from Grim patent issued to Koppers (U.S. Patent 2,715,118), Aug. 1955.
** Church, J.M.: “Suspension Polymerization,” Chemical Engineering, Aug. 1, 1966, p. 79.
states the reaction time varies from 6 to 20 hours, depending on the desired product. Obviously more than one recipe is successful. Some companies produce over 10 different kinds of polystyrene. None will reveal their reaction mix or conditions.