Principles of soft PVC formulations
I. Introduction
The production of PVC from Vinyl chloride (VC) was first documented in the early nineteenth century. Large-scale production of PVC began in the USA in 1928 and in Germany in 1930. By the end of World War II, PVC was the most widely produced plastic globally. With a molar mass chlorine content of 56.7%, PVC is a valuable byproduct of chlorine production. PVC materials have a favorable energy balance and carbon dioxide footprint due to their low hydrocarbon content.
Polyvinyl chloride (PVC) is a material that is sensitive to heat and light. It degrades through dehydrochlorination and oxidation, which is evident by the color change in PVC. The formation of conjugated double bonds in chemical terms causes this color change. PVC compounds undergo heat exposure during various processes such as mixing cycles, extrusion, molding, embossing, thermoforming, laminating, and scrap rework. Exposure to weathering leads to the formation of oxidation products. Stabilizers play a crucial role in delaying heat degradation, allowing the compound to be processed into a product before degradation occurs. Stabilizers achieve this by absorbing hydrogen chloride, displacing active chloride atoms, scavenging free radicals, disrupting double bond formation, deactivating degradation by products, decomposing peroxides, and absorbing ultraviolet energy.
II. PVC resin
Sustainability has become a global trend, leading both manufacturers and consumers to prioritize products that are durable, safe, and environmentally friendly. Polyvinyl chloride (PVC) resins have gained attention for their versatility in producing a wide range of products. Advanced suspension polymerization processes have made PVC resins recyclable and eco-friendly.
PVC resins come in various molecular weights, or 'K values,' suitable for different production processes. By selecting PVC resins with specific K values and adding appropriate additives, manufacturers can meet their requirements and safety standards. These high-quality, recyclable resins offer high purity, improving manufacturing productivity and reducing defects in various products, promoting energy and resource efficiency for a sustainable future.
Polyvinyl Chloride Copolymer is a white, free-flowing resin produced through suspension polymerization. Compared to PVC homopolymer, it offers better processability, increased productivity, lower torque requirements, and reduced processing temperatures. Its unique characteristics enhance additive compatibility, improving the mechanical properties of the final product. Additionally, in plasticized formulations, the material's increased flexibility can reduce the need for plasticizer content. The Non-Bisphenol A Series PVC resins offer good fisheye properties, initial coloration, thermal stability, and low contamination levels, making them a reliable choice for various applications.
PVC resins (Figure 1. SEM of PVC granules (100micron) used for dry-blend applications) with low K values demonstrate fast fusion behavior, high melt flow rates (MFR), and low contamination levels. High K value PVC resins exhibit excellent plasticizer absorption, drying capabilities, high strength, elasticity, and superior mechanical performance in finished products.
Figure 1. SEM of PVC granules (100micron) used for dry-blend applications
The High Flow Series PVC resins have high melt flow rates and fast fusion properties, facilitating easier polymer flow in injection molds and uniform mixtures in extruders before die-casting. Known for enhancing manufacturing productivity and reducing defects, the High Flow Series is a preferred choice for many applications.
III. Plasticizer
The plasticizer acceptance by Polyvinyl chloride (PVC) in hot-process dry blending is influenced by factors such as granule porosity, resin molecular weight, and synthesis recipe in PVC production through the suspension process. To achieve dry blending, where PVC resin forms a free-flowing powder when mixed with a plasticizer under hot-processing conditions, the resin granules must be porous due to interstices between primary PVC particles. An increase in primary particle agglomeration at a given granule porosity can negatively impact dry blend performance. Dry-blend performance is primarily determined by granule porosity for resins manufactured with a specific recipe and constant molecular weight. A higher granule porosity is required with increasing resin molecular weight to maintain an equivalent dry-blend time (DBT). The DBT is directly related to granule porosity and inversely related to molecular weight for most suspending agent recipes. However, if the suspending agent is a rapid film former, dry-blend performance with varying molecular weight depends on the agent's concentration rather than granule porosity or resin molecular weight. An interfacial film-forming suspending agent enhances fusion of primary PVC particles at the suspension granule-water interface, increasing the granule's "pericellular membrane" thickness. This membrane, a PVC skin, has minimal impact on dry-blend performance with low or intermediate viscosity plasticizers and can hinder blending rates with high-viscosity, poorly solvating plasticizers. Dry blending occurs below the glass transition temperature (Tg) of PVC with low-viscosity plasticizers and above Tg with high-viscosity modifiers. The dry-blend process is diffusion-controlled, with the rate dependent on pore penetration and plasticizer diffusion into the PVC matrix.
IV. Heat stabilizers
In extrusion, stabilizers are primarily used to facilitate the processing of PVC through the hot and harsh environment of the extruder. While stabilizers do not significantly impact shelf life or weatherability in most cases, they do enhance weatherability in specific "weatherable" compounds. In these compounds, stabilizer levels are higher, typically five to seven times the level in regular pipe compounds, and they positively affect weatherability. Changes in stabilizer levels or suppliers should be carefully evaluated for weatherability before a complete transition is made.
Various types of stabilizers are used for PVC resin, including lead salts, calcium-zinc, barium-cadmium, and tin mercaptides
- Lead salts, mainly organic compounds like sulfates, silicates, phosphites, stearates, phthalates, and maleates, offer good heat stability, excellent electrical properties, and low water absorption. Calcium-zinc stabilizers are suitable for food contact applications and provide compounds with crystal clarity and low odor properties.
- Barium-cadmium stabilizers offer good color stability, light stability, and heat stability but have plate-out tendencies and toxicity concerns.
- Tin mercaptides, commonly used in the U.S., provide heat, light, and color stability, promote fusion, reduce melt viscosity, and are suitable for various PVC applications.
V. Fillers
Fillers, such as metal carbonates, silicates, gypsum, clay, alum, barytes, and sawdust, can be used with PVC to reduce material costs and improve weatherability. Calcium carbonate, a commonly used filler, is available in ground and precipitated grades with different particle sizes. Increasing calcium carbonate content can reduce tensile strength and increase tensile modulus, affecting the overall properties of the compound. Careful consideration of the filler content is necessary to balance cost reduction and product performance.
VI. Pigments
Pigments are essential for achieving opacity, UV protection, and desired colors in PVC compounds. Titanium dioxide is a major pigment used, with other pigments added in small amounts to achieve specific colors. The choice of pigments can impact staining, plate-out, dispersion, and color retention in PVC compounds. Proper selection and testing of pigments are crucial to ensure the desired properties and appearance of the end product.
VII. Processing aids
Processing aids, such as acrylic polymers, help control melt viscosity, frictional heat, and die flow in PVC compounding. Impact modifiers, including acrylic, CPE, MBS, and ABS polymers, are used to enhance impact strength in PVC for various applications. The type and level of impact modification can affect weatherability, chemical resistance, and physical properties of PVC compounds. Lubricants, categorized as external, internal, or external/internal, play a vital role in controlling the melting point and processing characteristics of PVC during extrusion and molding processes. Over-lubrication can lead to extrusion issues and product quality concerns.
VIII. Lubricants
Internal lubricants are typically polar molecules, such as fatty acids, fatty acid esters, or metal esters of fatty acids, that are highly compatible with PVC. They serve to decrease melt viscosity, reduce internal friction, and facilitate fusion. Insufficient lubrication can lead to issues like rough extrudate, adhesion to metal surfaces, melt fracture, rapid fusion, and unusually low barrel temperatures. In the final product, inadequate lubrication can result in burning, plate-out, matte surfaces, and poor impact strength.
External/Internal lubricants, which contain both polar and non-polar chemical groups, are challenging to categorize. They typically consist of long hydrocarbon chains with amide, alcohol, acid, and ester groups. Common examples used in PVC include fatty acid amides and oxidized polyethylene. Some of these lubricants function as external lubricants before melting and internal lubricants after melting, while others exhibit the opposite behavior. Each lubricant must be evaluated for its lubricating properties in a specific compound.
The primary objective of lubrication is to optimize the properties of PVC and ensure a smooth extrusion or molding process at a reasonable cost. Experimentation is essential to achieve these objectives, starting with preliminary compounding using a bench extruder and progressing to commercial-scale equipment for final validation. Avoid excessive compounding to prevent machine issues. Once a compound has been developed and tested successfully, minimal adjustments are usually required.
Before making any changes to the recipe, thoroughly inspect the equipment. Verify the proper connection and calibration of temperature controllers, thermocouples, screw rpm, back pressure instruments, and heat bands. Compare operating procedures with previous successful runs using the same compound. Ensure cleanliness and unobstructed flow through screen packs, nozzles, adaptors, and dies. If all parameters are correct and a problem persists, focus on identifying and addressing specific melt issues rather than surface-level symptoms in the end product. Only after exhausting these checks should adjustments to the recipe be considered.
Overall, the selection and proper use of stabilizers, fillers, pigments, processing aids, impact modifiers, and lubricants are essential for achieving desired properties, performance, and appearance in PVC compounds for different applications. Careful consideration of these additives and their interactions is crucial to ensure the quality and functionality of PVC products.
IX. Compounding
PVC resins, whether homopolymers or copolymers, require additives for processing and performance. The modifications needed for PVC are extensive but essential for its versatile applications. The terms 'composition' and 'compound' refer to the material formed by combining PVC resin with additives, and the process is called 'compounding'.
Compounding involves mixing and/or melt-compounding. Mixing can create a solid composition like a powder or a liquid composition like a paste. Melt-compounding (shown in figure 2) combines the constituents with the resin in a molten state, typically resulting in pelletized compounds.
The exceptional mixing capabilities of compounding are due to its unique operating principle, which involves simultaneous rotation and axial oscillation of the mixing and kneading screw shaft. The oscillating screw shaft facilitates intensive material exchange in an axial direction through multiple splitting, folding, and reorientation of the product. This leads to a superior distributive mixing effect and optimal distribution of raw materials. Compounding technology is especially effective for incorporating liquid components or adding a high degree of fillers.
Compounding requirements involve the absorption of plasticizers, stabilizers, additives, fillers, reinforcement materials, and flame retardants into the porous PVC grain. These components need to be thoroughly mixed, dispersed, and agglomerated within the compounding systems while adhering to specific temperature limits.
Furthermore, the effective mixing is achieved through the interaction between the characteristic kneading flights of the screw shaft and the stationary kneading pins. The oscillating motion of the screw shaft allows for a very short processing length.
X. Soft PVC
By incorporating plasticizers and other additives (Figure 3 SEM of plasticizer absorped PVC (50micron)), the properties of PVC materials can be tailored to specific application requirements. From a mechanical perspective, plasticizers can be seen as "hinges" or "spacers" between neighboring macromolecules. Larger plasticizer molecules result in slower migration rates under extreme stress.
Figure 3 SEM of plasticizer absorped PVC (50micron)
It is likely that the processing behavior and final properties of complex polymer systems are influenced by the thermodynamic interactions between their components. Establishing correlations between these interactions and specific processing and performance properties could provide a solid foundation for predicting the behavior of new polymer formulations. While the importance of these correlations has been acknowledged for some time, accurately measuring thermodynamic properties within the appropriate composition and temperature ranges has proven challenging.
Soft PVC (PVC-P) is typically produced through a compounding process involving hot/cold mixing in the powder phase, followed by compounding and pelletizing on a for subsequent processes requiring pellets or granules. The significant presence of plasticizers, stabilizers, and often high filler content necessitates precise and targeted processing control.
To enhance the properties of PVC, other polymers can be alloyed with it during the compounding process. For instance, blending TPU with flexible PVC can improve abrasion, oil, and flame resistance. PVC-Nitrile Rubber blends enhance low-temperature flexibility and chemical resistance. Alloying PVC with nitrile rubber or TPU can enhance hot melt strength for medical applications, as well as improve abrasion resistance and durability. PVC-EVA blends reduce plasticizer volatility and enhance chemical resistance, while PVC-CPE blends improve compression set and low-temperature flexibility.
Compounding allows PVC to optimize its strengths under uniform, moderate, and adjustable shear rates. The processing axis is designed to meet specific requirements, offering low specific energies, intensive mixing processes, volumetric scale-up procedures, and maximum availability within wide operation windows. These advantages have solidified over sixty years of technological and market leadership in compounding plasticized PVC. Some of the soft PVC formulations are given below:
Soft PVC formulations
1. Ca/Zn shrink-wrap film
2. Medical tubingg
3. Wire and cable
4. Applications
PVC is versatile and can be used in a wide temperature range of -50 to 70°C, making it suitable for hoses, plugs, and buffer elements. Soft PVC (PVC-P) is an excellent electrical insulator for wire and cable insulation up to 10 kV. In the construction industry, soft PVC is commonly used for sealing wall joints and windows, as well as for surfacing floors, tables, and walls. In medical technology, PVC is used in sophisticated systems for preserving blood and infusion solutions. The automotive, packaging, and clothing industries also utilize PVC in various applications, with ongoing improvements and developments.
For specialized food and medical applications, it is crucial to use a material that is soft, durable, and performs well in high moisture and high-temperature environments. The material should have a unique combination of high flexibility and rubber-like strength due to its distinctive hard and soft amorphous structure. Key considerations for this material should include:
- Compliance with FDA food contact, USP Class VI, and ISO 10993-5/10 standards
- Adjustable stiffness ranging from 20 to 1000MPa to meet specific performance requirements
- Consistent extrusion behavior for improved repeatability and reduced waste
- Superior snap back memory to prevent deformation
- Excellent thermal stability during assembly and use
- Prevention of gel formation at bonding points
- Availability of high to low durometer grades that can be easily welded and glued together.
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Author
Dr Muralisrinivasan Natamai Subramanian
Trainer, Polymerupdate Academy
