Engineering with Rigid PVC

Processability and Applications

I. Luis Gomez, I. Luis Gomez

• Publisher: CRC Press
• Year: 2017
• ISBN:
  9781351450881
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

This comprehensive, long-needed reference provides the thorough understanding required tomodify and manipulate rigid PVC's thermal/shear sensitivity and rheological properties, helpingyou utilize rigid PVC most effectively in manufacturing applications as diverse as pipes, house siding, bottles, window frames, and packaging films.With complete, up-to-the-minute coverage in one convenient source, Engineering with RigidPVC encompasses rheological principles, resin properties, and additive modification, as wellas polymer preparation, melt processing, and forming techniques... major conversion operationsand their manufacturing applications-including actual commercial formulations andprocesses... quality control procedures necessary to monitor compounding processes...aspects of processability critical for product development and improvement... and muchmore.International in scope, this time- and money-saver is an eseential daily resource for all professionalsinvolved in Engineering with Rigid PVC, including plastics engineers, polymer chemists, process engineers, and plastics processors and technicians. Furthermore, the volume isideal for training programs and professional seminars, and is an outstanding supplement forstudents in polymer chemistry, materials science, and plastics engineering.

1

Introduction

I. LUIS GOMEZ /Monsanto Company, Springfield, Massachusetts

1.1 Background

1.2 Rigid PVC Conversion Operations

1.3 Polymer Preparation

1.4 Rigid PVC Rheology

1.5 Rigid PVC Extrusion

1.6 Extrusion Blow-Molding of Rigid PVC Containers

1.7 Injection Molding of Rigid PVC

1.8 Injection Blow-Molding of Rigid PVC Containers

1.9 Calendered Rigid PVC Products

1.10 Thermoforming of Rigid PVC Sheet

1.1 BACKGROUND

Nonflammability, weatherability, chemical resistance, low gas permeability, rather wide processability range, and low cost are six of the outstanding advantages of rigid PVC. Also, because 56.8% of its weight is chlorine, a nonpetroleum product, the effect of petroleum price increases is not as drastic as it has been on 100% petroleum-based polymers. Consequently, PVC also shows an advantage in cost stability. In summary, it is not pretentious to state that there is no other commodity polymer in the field with such a balance of processability, properties, cost, and cost stability. This explains the vital role it plays in the economy of the developing and underdeveloped countries, since it is the polymer that also offers the broadest spectrum of end uses.

Engineering with Rigid PVC covers the series of conversion techniques carried out on this polymer to increase the utilization of its desirable properties. The technical aspects of the PVC polymer conversion industry normally include machine and equipment design; plant design and operations, with emphasis on streamlined operations, from the incoming resin and other formulation ingredients to the finished products; product development, including auxiliary product line to complement principal products; and the analysis of PVC conversion operations. In this book, emphasis is placed on polymer processing rather than on the machine and equipment aspects of the process. Polymer processing consists of the polymer preparation, melt processing, and forming techniques.

The machinery and equipment used in compounding and processing rigid PVC have been the subject of several books and many technical articles. Extruder screws, for example, have been described in hundreds of patents. No attempt will be made to review or interpret the extensive literature. Frequent references to processing equipment and the polymer behavior in this equipment will be made throughout this book.

Because deformation and flow are involved in all the polymer conversion operations, rheological data are frequently used in this book. Also, polymer modifications via selective additives will be covered. Reference to key components and to formulations that, because of their use, are becoming classic in the industry, will also be used.

Tailoring the resin to the process and to the finished product is, perhaps, the first step in engineering with PVC. For example, if the resin is to be used in a dry blend extrusion process, a resin with a large amount of porosity and high bulk density is needed. If the resin is for calendering of rigid PVC film and sheet, a lower molecular weight (K value 55 to 60) is normally used. Also, for food contact applications, the level of residual vinyl chloride monomer in the resin is more critical than that for, e.g., siding, gutter, or furniture.

In the last 25 years, a great number of significant developments have brought the PVC industry to a very advanced stage. The days of frequent in-house tooling improvisation, due to the lack of reliable commercially available equipment, are almost gone, and complete machine systems are now readily available. A visit to any of the international fairs, like the Interplas in Birmingham, England, and Kuntststoff in Düsseldorf, West Germany, held every four years, will give the reader a good idea of the great profusion of processing equipment now available. To bring together machine manufacturers and PVC processors was not an easy task and it took a lot of time and frustration. It was not until the personnel involved realized that the many and varied problems associated with PVC conversion operations could be solved only by systematic research and a team effort approach by resin and additives suppliers, equipment manufacturers, and PVC processors that the development of processing machinery kept pace with the expansion and progress achieved in the resin and compound fields.

As one of the first thermoplastics commercially available, unplasticized PVC resin was used to make various articles as early as 1935. The first rigid PVC applications were developed almost exclusively by trial and error, to fulfill explicit needs. Looking back to those years, when minimum design technology was applied, it is fair to say that the users and producers were quite fortunate with polymer performance. As this industry has grown, prediction of the performance of rigid PVC products has received greater attention, and a designer or product engineer now has information available not only to upgrade performance of existing products but also to define the optimum process, formulation, and dimensions for new products.

Rigid PVC residential siding provides a good illustration of the above statement. Before a systematic outdoor weathering program was completed, producers were selling and installing siding with an average 20 years’ service life warranty. This writer recently inspected an installation about 19 years old and, despite the fact that the titanium dioxide pigment content of this formulation is significantly lower than the levels presently used, the siding was found to be in very good condition. Because of the dual ornamental as well as functional purposes of this product, vinyl siding offers a unique example of the approach used by the various manufacturers before a systematic approach to design technology was implemented and some correlation between short-term accelerated testing and outdoor performance was established. Steps followed in those early days were as follows.

Step 1:	Conceive a part as well as its shape and select the extrusion process.
Step 2:	Screen the available rigid PVC, mostly white compounds, on the basis of processability, short-term weatherability, and those engineering properties that relate to performance.
Step 3:	Use lab tests to predict field performance of engineering properties, such as retention of color and impact resistance, surface distortion, and thermal expansion.
Step 4:	Test under actual part use, such as a model home.
Step 5:	Redesign, retest, and tune up compound formulation and the extrusion process to improve engineering properties and to reduce surface gloss and surface distortions, for example.

If part requirements are simple, as with rigid PVC electrical conduits, these steps also become very simple.

Chapter 30 of the Dekker Encyclopedia of PVC, Testing of Rigid PVC Products, Analysis of Test Results, will help familiarize the reader with the mechanical, thermal, and environmental behavior of this polymer. This knowledge will also help the reader to use the engineering properties of this polymer more efficiently.

Vinyl siding and its accessories, replacement window and door frames, and rigid vinyl bottles for liquid, syrup, oil, and salad dressing, were perhaps the last major product lines introduced to the market several years ago. The vinyl chloride monomer (VCM) scare halted, at least in the United States, the production of bottles for food-contact application and also had a very detrimental effect on the development of new products. Obviously, major effort was placed by the industry on VCM reduction. During this period, it seems that the role of designers or product engineers was switched to optimization of existing products, processes, and cost reduction. A good example of a cost reduction program is one that has been in progress in Europe with the 1.5 liter bottle for mineral water. Through bottle design, mainly ribbing in the bottle panel to increase stiffness, bottle weight has come down from about 60 to 42 g. Via conditioning of blow-molded preforms and subsequent stretch blow process, which provides biaxial orientation, the industry is at present achieving 1.5 liter bottle weights as low as 39 g (more on this later).

An illustration of product appearance optimization is offered by vinyl siding. It is known that when siding is extruded in single-screw extruders or at elevated temperatures it tends to have a high level of gloss, which for the most part is not uniform, showing marked die lines and other defects. When the siding is embossed with a wood-grain pattern finish technique, which has been progressively perfected, the surface imperfections are masked, and the resultant product shows a very uniform and attractive appearance. Another good example of product appearance optimization is that of vinyl window and door frames. Originally, the areas where the frame components were welded together were very noticeable. At present, due to improved welding techniques, the vinyl windows and door frames seem to be all of one piece.

It is known that weather-resistant impact-modified PVC could not be made glass-clear because of the haziness of the impact modifiers. E. Röhrl, BASF, was able to demonstrate, with the aid of electron microscope photographs, that MBS and ABS formed particles in the PVC matrix, and that their size was dependent upon the shearing action during fabrication. Chlorinated polyethylene (CPE) and ethylene vinyl acetate (EVA) first act as a glue for the original PVC granules and later change into discrete particles in the PVC melt. Certain elastic polyacrylic esters (PAE), generally polybutylacrylates, form discrete agglomerated spheres, 50 to 200 µm in diameter, which is close to the resin particle size, and are rather stable against dispersion and shearing action. Using this rubber as the basis, BASF recently was able to produce a rigid PVC compound, Vinuran KR3820, which when polyblended in quantities of 5 to 20% with unplasticized PVC, produces a compound that could be fabricated into high-impact, weather-resistant, transparent glass-clear sheets for such uses as cupolas, greenhouses, and window panes.

Before pursuing this discussion, it is pertinent to state that, although by definition PVC is classified as rigid when the elastic modulus E is ≥100,000 psi (689 MPa) at 23°C and 50% relative humidity (ASTM Method D 883), the rigid PVC products, which will be the subject of this work, are (except for some calendered rigid films and some compounds for injection blow-molding applications) manufactured from compounds with E ≥ 350,000 psi. In fact, for most of the engineering applications that are discussed throughout this work, the higher the elastic modulus, the better the engineering properties, such as heat distortion temperature, impact resistance, and surface distortion.

1.2 RIGID PVC CONVERSION OPERATIONS

Plastics engineers who process rigid PVC products can improve their skills with ease if more attention is given to

1. Resins and raw materials properties

2. Polymer preparation

3. Resin and compound rheology

4. Melt processing and forming techniques

5. Properties of the finished products

When the level of knowledge in these areas is appropriate to their work needs, learning to troubleshoot better, for example, offers plastics engineers the interest, challenge, and satisfaction of any opportunity to use reason and imagination in solving problems. But limited overall know-how or knowledge in an isolated area may confuse and discourage engineers, thus reducing their troubleshooting ability.

Plastics engineers in highly developed countries can improve their skill almost by themselves. There are available many resources to do this, such as technical magazines, professional societies, technical meetings, seminars, and consultants. But others, such as the engineers working in developing and underdeveloped countries, where rigid PVC products are profusely used, mainly for irrigation purposes, are less fortunate. They not only have to wear all sorts of hats, but for the most part do not have access to the needed resources. This situation tends to divert efforts, and emphasis is placed mainly on forming techniques and overall equipment maintenance and not on resin properties, additive modification, or screw design optimization, for example. In countries with problems such as only one resin, limited production of additives, and high import tariffs, the lack of research on resin and additives is justifiable. It is painful to know of a better quality resin when one has to use whatever is produced locally.

The major goals of this work are to synthesize existing knowledge while providing continuity throughout, and to aim all the discussions toward the finished product. Thus, by sequentially blending the discussions on resin and additive properties and rheology with those of polymer preparation, melt processing, and forming techniques, plastics engineers will be helped in their overall knowledge of the process and their troubleshooting ability. To avoid unnecessary labor, we will take advantage of what is known and highly publicized and reduce discussions in these areas.

Extrusion, injection molding, and calendering are the three basic forming techniques of rigid PVC polymer processing covered in this work. These operations, however, are subclassified according to their largest usage, as follows:

Extrusion of pipe and conduit; siding and trim; window profiles, and so on

Extrusion blow-molding of bottles

Injection molding of fittings, electrical junction boxes, and so on

Injection blow-molding of bottles

Calendering of rigid PVC film and sheeting

Thermoforming of rigid PVC film and sheeting

1.3 POLYMER PREPARATION

Chapter 2 is dedicated to polymer preparation, which is paramount in rigid PVC polymer processing. Melt compounding, pelletizing, and some color-compounding techniques are also covered. Since almost every desirable property of rigid PVC products can be modified to a certain extent with additives, the practice of blending modifiers with resin is included. This practice offers great flexibility to processors. The various commercial PVC resin polymerization processes, as well as the effect of these processes on some key resin properties, are also discussed.

1.4 RIGID PVC RHEOLOGY

In all the forming operations mentioned above, PVC must flow in order to be die or mold shaped, and then must solidify while retaining its shape. Fluidity is achieved by heat (conduction from an external source) and/or mechanical shear. Chapter 3 deals with certain aspects of polymer rheology and gives a general outline of how the rheological properties of rigid PVC compounds help us to understand flow behavior in e.g., screw extruders, die adaptors, dies, and injection molds. Data on rheological properties, determined at the proper shear rate range, are useful not only for designing dies and injection molds but also for analyzing the performance of existing ones.

In the single-screw extrusion of viscous polymers like rigid PVC, traversing thermocouples in the metering zone commonly show melt temperatures increasing significantly as one penetrates inward from the extruder barrel. The film of polymer wetting the screw surface is the hottest, and the film near the barrel wall the coldest. The existence of a radial temperature gradient in the melt can be modeled as a composite of multiple conce...