Formulating and Processing of PVC-An Overview

TOM HAWKINS Carlon, A n Indian Head Company

R G D Laboratory Oklahoma City, Oklahoma 73135

INTRODUCTION he formulating and processing of PVC pipe can

T b e, and often is, a complex process. This paper gives one man’s views and some guidelines on the subject.

FORMULATING OF PVC PIPE COMPOUND When formulating a PVC pipe compound, there

1. Market 2. Processing 3. cost These items will be treated separately, but when

actually formulating, they must be considered as a unit to obtain optimum results.

Market Eight to ten years ago, the pipe market was not a

major consideration for formulations. This was due to a minimum of standards and specifications. In the few standards that governed the pipe market, the material sections were primarily written around a water pipe compound. This is no longer true.

Today, more and more pipe specifications are written for a specific market and the material sec- tions of the specifications reflect pipe properties that are meaningful to the application. For exam- ple, sewer pipe standards are now being based on a pipe stiffness, a property meaningful to a buried, non-pressured pipe. The specifications for above- ground applications, such as certain irrigation sys- tems and electrical conduits, emphasize abuse properties like impact and sunlight stability.

In order for a formulator to do his job properly, he must be aware of the needs of the market, the specifications that govern that market, and what combinations and levels of additives are needed to develop the desired properties while still keeping the formulation balanced and cost-competitive. Furthermore, the formulator needs to be knowl- edgeable about the standard-writing methods and be involved in them. If he is not, he is liable to find new standards appearing within material sections that are not meaningful or have made his current compounds obsolete.

Finally, a formulator cannot have tunnel vision-that is to say, be only aware of material

are three basic considerations:

properties. As our industry gets more and more so- phisticated, a formulator needs to be aware of all as- pects of the pipe application, environmental expo- sure, installation and handling techniques, joining procedures, etc. All parts of the specification and application can be effected by the pipe compound.

Processing When referring to the processability of a com-

pound, we are mainly concerned with its stability and lubrication. This is somewhat of an oversimpli- fication but adequate for this discussion.

Like marketing, formulating for processing has become more complicated in recent years. The complications are due to:

1. Larger pipe with heavier walls. 2. Demands for higher rates. 3. New extruders with different processing

4. Numerous new types and sources of lubri-

The first three items will be considered in the second portion of the paper, which deals with pro- cessing.

Today there are numerous companies producing stabilizers, lubricants, and combinations thereof. Along with the numerous suppliers, there are sev- eral new types of both stabilizers and lubricants. In order to stay on top of the latest technology, it is im- portant to be able to accurately and rapidly evaluate these additives in your formulations. There are sev- eral adequate ways this can be handled. At Carlon our primary tool is the Brabender torque rheome- ter.

The first order of business in evaluating a formu- lation for processing characteristics on the Brabender is to know what is needed. That is to say, have a good idea what your extrusion equipment needs and how these characteristics show up on a Brabender processing curve. Also, know which changes in response are meaningful to the pro- cessing of the formulation and which are not. At Carlon, we have found that fusion time, stability, and stock temperature profile are most meaning- ful. Torques are noted, but are secondary.

The typical way Carlon would look at a lubricant or lubricant system would be: In the two formulas, those with the widest and most critical processing

needs.

cants and stabilizers.

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Formulating and Processing of PVC-An O v e r v i e w

characteristics, the lubricants under evaluation would be substituted at a “best guess” level. The formulations would be run on the Brabender under controlled conditions, and critical characteristics noted. Based on these findings, adjustments are made until the proper processing response is ob- tained or the lubricants are rejected. If the Brabender criteria are satisfied, the next step is production trials.

In evaluating a stabilizer or a stabilizer system, three Brabender tests are run:

1. Ultimate stability test, which is independent

2. Processing response, same as for lubricant.

3. Color hold on adjusted formulations. Again, a stabilizer can be rejected at any point or

passed on for a production trial. It should be noted when any formulation is

changed, ASTM physicals are run and noted. The production extrusion trials are the key to any

evaluation. The Brabender is only a screening de- vice and gives you the assurance you will make pipe.

In the production evaluations, all aspects of pro- cessing are considered: machine response, wall con- trol, plate-out, pipe physicals, appearance, rate, and length of run. At Carlon, we break our produc- tion trials into two parts: a short trial of a few thou- sand pounds where short term processing charac- teristics can be evaluated, i.e.: machine response, rate and pipe physicals, and an extended run of days, weeks, or months. During the short trial, for- mulation adjustments will be made if needed. The extended trial is the last step and is saved for the formulation that offers the greatest advantages. The length of the trial will be determined by the magni- tude of the formulation change and how well the run goes.

Cost Cost is the bottom line for any formulator. With-

out cost considerations, formulating would be a snap. And, from my experience in the pipe busi- ness, 75 percent or more of all formulation changes are for cost-reduction reasons.

When referring to cost, we are not only consider- ing reduction in material prices. The complete cost of what goes into a piece of pipe must be consid- ered: materials, running rate, scrap, wall control, la- bor, and length of run. Also when considering ma- terial cost, the cost referred to is material cost per pound of acceptable pipe, which takes into account specific gravity and wall thickness.

As an exercise, I have provided a table with what I consider are reasonable additive costs for various pipe markets. Only additive costs are provided be- cause of the fluctuations in resin prices. Also, all the costs are corrected to a 1.40 specific gravity so that filled formulations are not artificially low in cost.

of processing characteristics.

Adjustments are made here.

Reasonable Additive Costs $/lb.

Pressure Pipe 0.337

Tele/Utility Duct 0.178 Conduit 0.552

Sewer Pipe 0.191

PROCESSING OF PVC PIPE The term processing refers to the complete oper-

ation of converting a PVC compound, and for the purposes of this discussion, a dry-blended com- pound, to a piece of “spec.” pipe. This is a complex subject, so this paper will be limited to basic con- siderations and some opinions.

The following are the items that must be consid- ered for a good processing operation:

1. Convert the compound to pipe with a mini- mum power consumption.

2. Maximize the rate of production. 3. Control the pipe wall to a specification mini-

mum, both circumferentially and linearly. 4. Minimize scrap. 5. Optimize properties. 6. Keep manpower to a minimum. If all of the above are accomplished, conversion

costs will be reasonable. The best way to accom- plish these goals is to thoroughly understand your operation, set good conversion standards, and live by these standards.

The processing of PVC pipe can be broken down into three parts:

1. The extruder. 2. The die. 3. Downstream equipment.

The Extruder

The extruder is the heart of the processing opera- tion; the bulk of the money is in the extruder, it op- timizes properties, controls power consumption and rate, and plays the biggest role in material conservation-therefore it deserves the most attention.

The first consideration is to select the right ma- chine for the job. There are two considerations: one, the type of extruder; and two, the size of ex- truder. In selecting the type of extruder there are two basic choices, multiple-screw and/or single- screw. Within each of these two basic categories there are several choices, but I won’t go into these other than to say there are all sorts of ways to skin a cat, and there are several good extruder manufac- turers. If you deal with a reputable manufacturer and understand the principles of their equipment, they will do the job.

The choice of multiple-screw versus single- screw has been almost hammered to death. The ex- truders operate on two differentprinciples. The single-screw generates high shear and high tem- perature with a minimum amount of residence time. The single-screw depends on shear heat,

JOURNAL OF VINYL TECHNOLOGY, JUNE 1983, VOL. 5, NO. 2 67

Tom Hawkins

back flow, and mixing sections in the screw to do its job.

The multiple-screw extruder is a low-shear, high-volume, positive pump. It relies on a rolling, kneading action along with inductive heat to do its job.

The following are, in my opinion, the advantages of each type machine:

Advantages of Single and Multiple-Screw Extruders

Single-Screw 1. Lower capital cost. 2. Easier mainte- nance. 3. Lower cost barrel and screw.

4. .Develops better properties

5. Quicker turnaround time.

Muleiple-Screw 1. Longer runs, less heat to material. 2. More forgiving, easier to operate. 3 . Because oflower tempera- tures, only machine for large-diameter, heavy-wall products. 4. Manufacturers are more aware of latest PVC pipe technology.

From my viewpoint, if you are going to be mak- ing larger-diameter pipe with a limited product line, and long runs, multiple-screw machines are the way to go. If you have a broad product line which is primarily smaller-diameter and high lev- els of fillers, select a single-screw.

Once the type of extruder has been selected, three principles must be followd:

(1) Size the extruder to be job being done. An ex- truder operates most efficiently when it runs on the top end of its motor rating; don’t lug a big machine with a small, low-output product. And certainly don’t put a product that can run at a thousand-plus pounds per hour on a ma- chine that top-sides at seven hundred pounds per hour.

(2) Maintain your equipment. Do the necessary preventive maintenance. Know how to detect when problems start and take care of them be- fore costly breakdowns occur. And finally, know when wear has taken its toll and be ready to change barrels, screws, or whatever.

(3) Stay on top of the latest technology. I am not referring to “fruit salad,” but the latest, proven equipment, proportional controls, new screw designs, slotted feed boxes, micro- processors, etc. Use whatever will make your Cadillac run like a Rolls Royce.

The Die One of the worst things a pipe producer can do is

hang the wrong head on the right machine. The wrong head can strangle a machine or create pro-

cessing problems that are literally impossible to solve.

The basic principles of pipe dies are the same for single and multiple-screw. The areas of concern are:

(1) Material flow-must be streamline without

(2) Residence time, material inventory. (3 ) Area of spider to area of die opening, compres-

sion ratio. (4) Land lengths. ( 5 ) Methods of heating and cooling, both external

Single-screw heads and dies are relatively uncomplicated. The high temperature of the .melt going into the head dictates the head be simple, allowing for a minimum of residence time. Also, be- cause single-screw pipe heads and dies have been around for a long time, they are well-proven; most commercial dies do a good job. At Carlon, we find our major concerns are with the die opening, land lengths (must be properly sized to give good wall control and the desired drawdown), and the spider opening(must be fully relieved to obtain maximum rates).

Multiple-screw heads and dies are considerably more complicated. The reason for this is that a great deal more work is done on the material in a multiple-screw head and the results can determine resulting pipe properties.

Because different type multiple-screw machines deliver an extrudate in different states of flux, the head must be designed to handle this extrudate. In general, the extruder manufacturer will supply a head that will work best on their machine or direct you to a head that will.

stagnate areas.

and internal.

DOWNSTREAM EQUIPMENT The term downstream equipment refers to every-

thing in front of the die on a pipe line, sizers, pull- ers, saws, bellers, etc. It is my experience that this is the most neglected area in most pipe plants. A company will spend hundreds of thousands of dol- lars on an extruder and die and put obsolete, im- properly sized and poorly maintained equipment downstream. The results are significant scrap, wasted manhours, lost production time, and field problems. T h e answer is to treat downstream equipment just like you do the extruder and the head, size the equipment to the product being made, have an effective preventive maintenance program, and stay abreast of the latest technology.

One area that needs lots of attention is automatic belling equipment. With today’s faster rates and higher wages, it is essential that automatic bellers be used to keep costs down and reduce field fail- ures. Time must be spent to select an automatic beller for your operation that will do the job and will withstand the rigors of the day-to-day opera- tion.

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