PVC compound used for shoemaking (typically for producing transparent soles and decorative strips) represents a carefully formulated blend that balances transparency, flexibility, wear resistance, cost, and processing performance.
The main components are as follows, where the selection of plasticizers and stabilizers is critical for achieving transparency:
1. Base Material – Polyvinyl Chloride Resin (PVC Resin)
· Proportion: Typically measured as 100 parts (with other additives calculated based on this benchmark).
· Requirements: Must use high-purity, low-impurity suspension-type (S-type) resin. Impurities can affect the final product’s transparency and color. Common models include SG-5 or SG-7.
2. Plasticizers – The Core Component Determining Softness
This is the most important component in footwear PVC after the resin itself. Its high concentration directly determines the sole’s softness and hardness.
· Function: Makes rigid PVC resin soft and elastic, facilitating processing.
· Selection Requirements: Must have good compatibility with PVC, be colorless, transparent, low volatility, and resistant to migration and exudation.
· Common Types:
· DOP (Dioctyl Phthalate): A traditional primary plasticizer with good overall performance, but its use is being phased out in some areas due to environmental and health concerns.
· DINP (Diisononyl Phthalate) / DIDP (Diisodecyl Phthalate): Currently the most commonly used plasticizers in footwear materials, serving as replacements for DOP with good performance.
· DOA (Dioctyl Adipate): An excellent cold-resistant plasticizer, often used in combination with DOP/DINP, etc., to improve the flexibility of soles at low temperatures and prevent hardening in winter.
· Epoxy Plasticizers (e.g., ESO): Provide both plasticizing and stabilizing effects, helping to prevent discoloration from light exposure and improving weather resistance. However, they have weaker plasticizing effects and are usually used as secondary plasticizers.
3. Heat Stabilizers – Prevent Decomposition and Yellowing During Processing
PVC is highly prone to decomposition when heated during processing, causing the product to yellow, blacken, and lose transparency. Thus, stabilizers are crucial.
· Selection Requirements: Highly efficient and offer excellent transparency.
· Common Types:
· Organotin Stabilizers: The absolute preferred choice for rigid and transparent PVC products, offering the best transparency, e.g., Methyl Tin, Octyl Tin. However, they are more expensive.
· Liquid Barium-Zinc/Calcium-Zinc Composite Stabilizers: Environmentally friendly, non-toxic stabilizers often used in softer products where slightly lower transparency is acceptable, or used in combination with organotin stabilizers to reduce costs. Their transparency is inferior to organotin stabilizers.
4. Lubricants
· Function: Reduce friction between the melt and the metal surfaces of processing equipment, prevent adhesion, make processing easier, and give the product a smoother surface.
· Types: Divided into internal and external lubricants. Usually used in combination.
5. Fillers
· Function: Reduce costs, increase rigidity, and reduce shrinkage.
· Important Note: When producing transparent soles, fillers are usually not added, or only very minute amounts of ultrafine fillers are used (e.g., nano calcium carbonate). This is because most fillers (like regular calcium carbonate or talc) destroy transparency, making the product opaque or semi-transparent.
6. Colorants
· If a colored transparent effect is desired (e.g., amber, smoky), small amounts of transparent oil-soluble dyes are added instead of highly opaque pigments.
7. Other Additives
· Optical Brighteners: To counteract the slight yellowish tint of the raw materials and make the transparent product appear more “crystal clear,” small amounts of optical brighteners are usually added.
· UV Absorbers: Prevent the shoes from yellowing and aging due to light exposure during wear and storage.
In simple terms, transparent PVC compound for shoes is made by “soaking” PVC resin in a large amount of transparent plasticizer “oil,” adding “preservatives” (stabilizers) to prevent yellowing during heating, along with other additives to improve performance, all compounded and pelletized together. The core technology lies in the selection and ratio of additives to ensure the final shoe material is both transparent, aesthetically pleasing, soft, and wear-resistant.
