Plastic films are produced using completely different technologies than paper and cardboard. An example of the production of unplasticized or crucible rigid PVC at Klockner Pentaplast illustrates with an inset how the properties of the diaphragm are set on a high temperature calender.
The production of diaphragm materials is an established area of ​​polymer processing because such diaphragms can be used in a myriad of different applications. Whether as a semi-finished product or a finished product, the scope for their vastly improved possibilities is to help them meet the preconditions for most of the different branches of the industry. A variety of different process technologies have been developed for the production of plastic strip materials, which can produce suitable diaphragms for many different applications. These techniques refer to extrusion molding or blow molding using a slit die, coating, injection molding, and calendering.
Unplasticized PVC Diaphragm Rolling (Twilight)
Calendering is a processing method that forms an endless ribbon-like material of high viscosity polymer formulation by pressure in a gap between two or more rollers at a defined processing temperature. The set gap width will determine the resulting film thickness. The resulting gap pressure results from the geometry of the gap and the rheological properties of the material being calendered. Therefore, the calender should be regarded as a processing machine that is fully used for forming purposes.
The first machine similar to the calender was designed before 1800 for the smooth processing of textile surfaces. The first patent was awarded in 1836 for the calenders used to apply rubber and apply rubber to textiles. The requirements for higher production speeds and tighter tolerances have further promoted the development of calenders and expanded their scope of application. In addition to rubber, they can also be used in polyvinyl chloride (plasticized and unplasticized PVC) Processing. The first calender design for unplasticized PVC, introduced in 1937, required heating to 220°C. Around 1960, high-activity stabilizers were introduced, which allowed for the addition of an improved recipe for the previously used cryogenic process (LT) and a high-temperature process (HT) based on a higher temperature in the calender.
Over the past few decades, technology configurations and equipment have remained virtually unchanged. The only real change is the widening of the calendering rolls to increase production or to facilitate the specialization of diaphragm production. The established technology for the production of unplasticized PVC film is based on HT (High Temperature Process), which is between 1,800 mm and 3,000 mm in width. The advantage of a high temperature process is that it has more options for improving the properties of the diaphragm, and a wider machine will also make the output larger. The materials used are mainly S-PVC and M-PVC with a K value (degree of polymerization) of about 58-63, which can produce films with high transparency, good deep-drawing characteristics and a very large thickness.
Unplasticized PVC
In terms of production volume and consumption, PVC is ranked third in the most commonly used standard polymer list with an annual capacity of 28.6 million tons, ranking behind polyethylene (57 million tons) and polypropylene (35 million tons). In the past approximately 60 years, the ever-increasing consumption from the initial 10,000 tons per year to the current nearly 30 million tons reflects the continuing importance of PVC to industrial development. Due to compatibility with all additives and media, PVC has established and maintained a very broad range of applications, from window frames and diaphragms to pastes and coatings.
In Germany and other parts of Europe, the processing of unplasticized PVC is about twice that of plasticized PVC. Unplasticized PVC film accounts for about 15% of the total PVC consumption. The packaging film accounts for about 60% of the total film production. More important applications are technical diaphragms and printed diaphragms. 20% of unplasticized PVC is calendered and its most important market area is packaging and technical applications.
Vinyl chloride was first produced by Henri Victor Regnault in 1835. The first industrial-scale production began in IG Farben in Germany in 1938, and production began in the United States at the same time as unio Carbide and DuPont. It is now produced by a method of reacting chlorine and ethylene in one or two stages. PVC with 57% chlorine content requires less mineral oil than any other polymer and is industrially produced in one of three ways:
* Emulsion Polymerization (E-PVC) - for paste and membrane applications;
* Suspension Polymerization (S-PVC) - the main process for all applications;
* Bulk Polymerization (M-PVC) - Mainly used for rigid PVC applications.
The choice of a particular PVC is subject to further processing requirements and purchase prices. S-PVC and M-PVC are very versatile and interchangeable. M-PVC is often used to make transparent products because of its purity.
Diaphragm characteristics
PVC can be distinguished not only by its production process but also by its material and processing characteristics (Figures 2 and 3). The international standard name is PVC-U (for unplasticized or rigid PVC) and PVC-P (plasticized PVC). As a variant of the membrane suitable for printing, rigid PVC shows the following characteristics of the selection:
* High mechanical strength, rigidity and hardness,
* The unmodified morphology is sensitive to impact at low temperatures.
*Transparency has changed,
*Good electrical characteristics in low and low frequency range,
* Good resistance to chemical attack,
* Can extinguish itself after removing the fire source.
Diaphragm thickness and thickness fluctuations
The thickness of the diaphragm will be set by the calender roll adjustment device. The thickness of the packaging film usually ranges between 100 and 800 microns. The thickness is measured by means of radiation measurements during the production process, but in the laboratory it is mechanically examined between two gauge surfaces subjected to the specified pressure. Thickness deviations may occur in portrait, transverse, and diagonal directions. The thickness fluctuation in the transverse direction is mainly overcome by the compensation system of the calender. The profile of the thickness can be improved by the means of arching, bending or twisting the center of the calender roll, using a blower system to provide local thickness correction.
Deviations in the longitudinal direction are usually caused by excessive rolling of the calender roller bearings, inaccurate operation, or speed fluctuations behind the last calender roll. Load fluctuations in the calender roll gap are also one of the possible causes.
Another disturbing source is the formation of so-called flow marks and running direction tilt. Changes in light refraction caused by a rise of about 10 microns will affect the optical quality of the diaphragm. The flow mark is caused by the heterogeneity in the kneaded polymer melt that is delivered to the calender roll. The reason for this may in turn be the fluctuation of the output or the difference in temperature.
The limit values ​​for thickness fluctuations currently used for different applications are between 3% and 10% for rigid PVC diaphragms. Our own research shows that the thickness deviation is rarely related to the formula.
Surface characteristics associated with printing
Rigid PVC will be produced with selected glossy, matte and embossed surfaces. The surface of each film will be controlled by the temperature in the corresponding matt or glossy calender roll surface or in additional separate molding modules, matting agents in the formulation, or final calendering rolls and in the discharge device.
Surface uniformity is a particularly decisive factor for printing foils, because this surface is required to exhibit characteristics suitable for subsequent printing processes. Glossy, matt and embossed films are suitable for screen printing and UV offset printing. For traditional offset printing, matte films are the most commonly used substrates, while glossy films are suitable for gravure applications.
Diaphragm contraction
Shrinkage refers to the change in length and width when the diaphragm is exposed to heat. Factors that can affect shrinkage include:
* Control the temperature and speed after the calender
* additives that affect the glass transition temperature,
* additives that affect stretching,
* Reduce loose design measures.
The cause of this shrinkage is the expansion of the diaphragm above the so-called glass transition temperature, which is indicative of a relatively narrow phase change range between hardness and elasticity. This expansion leads to the orientation of the molecular chain, and the molecular chain will be "frozen" in its new state upon subsequent cooling. Later reheating at a temperature above the glass transition temperature will release the freezing stress, returning the molecule to its original structure. The resilience that occurs appears to reverse the original deformation and produce contraction.
transparency
One of the important optical properties of the diaphragm is its transparency (figure 5), which is the degree to which the outline behind the diaphragm (such as the packaged goods or lettering) remains visible and accurately identifiable. Transparency is affected by the formulation, the surface of the last calender roll and the first exit roll, and the temperature control of the two rolls.
The above-mentioned flow mark is included in many of the diaphragm defects that affect transparency, and the thickness fluctuations formed by it will cause optical distortion.
Diaphragm uniformity and flatness
The uniformity of the diaphragm can be understood as the regularity of certain optical and mechanical diaphragm properties that are particularly important for downstream processing such as drawing, printing, or deep drawing.
The temperature difference, whether in the width of the roller or the circumference of the roller, will cause the thickness deviation in the form of a flatness defect. The result is that the single sheet cannot be laid flat, and the reel film will deviate from its correctness when unwinding. The direction of operation. This temperature difference may also be the cause of the expansion in the width direction of the diaphragm, in the same way as the various expansions caused by the misalignment of the roller arrangement or the error of precise operation. These defects can first be displayed on the edges of the strip-like membrane. If the situation is particularly severe, it means that these edges are no longer suitable for further processing.
If the flow of the plasticizer is not smooth or if the residence time caused by the unsmooth flow in the first calender roll gap is too long, the thermal load acting on the diaphragm may also change, resulting in a change in thermal degradation. The intensity fluctuations found when the diaphragm is stretched or bent are proof of the heterogeneity caused by this condition in the molecular structure. In the case of lateral stretching, this condition may cause tears or holes in the diaphragm.
Impact strength and rigidity
In later processing or later use, diaphragms are often exposed to mechanical shock stress. Their ability to withstand this stress is called impact strength and will be graded according to a rating scale to different levels from weak to high impact strength.
The impact strength can be changed with a suitable modifier (such as MBS, CPE, ABS, or acrylate). Their role is to strengthen and increase the K value of the relevant PVC. The optimization of setting and temperature control of the plasticization and calendering processes associated with homogeneity will be equally important for obtaining a particular impact strength.
The stiffness of a diaphragm depends on its elastic modulus and its thickness. For example, in order to obtain the same bending stiffness, the thickness of the PP film must be 1.3 times that of the PVC film.
Temperature stability (Vicat)
If the diaphragm is to be further processed or used in the food and pharmaceutical industry, a key feature is temperature stability, such as being able to heat in a microwave oven or in a sterilization process.