Plastic modification is a method of improving quality of plastics by adding additives, fillers, etc., to change properties of plastics. At the same time, plastic modification is also the most effective way to reduce cost of plastics. Formulation of plastic modification mainly includes: material selection, compounding, dosage, and mixing method. Process of plastic modification is not simple, underlying mechanisms deserve our study and exploration. Four elements interact, either mutually assisting or canceling each other out. Therefore, designing a high-performance, easy-to-process, and low-cost formulation is by no means easy.

 

1. Resin Type and Grade
Selected target resin should have properties closest to target performance to be modified. First, a subject-specific selection is made based on resin type. For example, to improve resin toughness, ABS and SBS are preferred; for transparency modification, three major transparent resins PS, PMMA, and PC should be considered first.
After determining target resin, select resin grade. Specific resin grades have more specific performance indicators, making target selection more certain. When selecting resins, prioritize well-known brands and general-purpose resins to ensure easier resin procurement later.
2. Resin Compatibility
Selected target resin should be compatible with main resin, forming a unified whole without phase separation. Target resin should be able to achieve modified performance target.
3. Resin Viscosity and Flowability of Composite Resin
In formulation, viscosity of selected target resin should be close to that of main resin. If viscosity of selected target resin is too high, composite resin will have a high viscosity, requiring addition of a modifier to reduce viscosity gradient. For example, in PA66 toughening modification, PA6 is added as a modifier in flame-retardant formulation; in PA6 toughening modification, HDPE is added as a modifier in flame-retardant formulation.
Changes in viscosity affect its flowability, thus ultimately affecting processing method, such as injection molding grade, extrusion grade, blow molding grade, calendering grade, etc. Main factors affecting flowability of same resin are molecular weight and molecular chain arrangement, such as linear molecules and three-dimensional molecules. High-flowability resins include: PS, HIPS, ABS, PE, PP, PA, etc.; low-flowability resins include: PC, MPPO, PPS, etc.; non-flowing resins include: polytetrafluoroethylene, UHMWPE, PPO, etc.
Relationship between different processing methods and melt flow index

1. Purposeful Selection of Additives
Different types of additives have different target properties. Additives added to system can fully realize their expected functions and achieve target specifications. These specifications are generally national standards, international standards, or performance requirements proposed by customer. Common properties of modified target resins are as follows:
1) Flame retardancy: Inorganic phosphorus, organohalides, organophosphorus compounds, organosilicon, and nitrides, etc.
2) Reinforcement: Two main categories: fibers and whiskers; glass fiber, asbestos fiber, carbon fiber, whiskers, quartz fiber, graphite fiber, and ceramic fiber, etc.; PAN fiber, polyethylene fiber, PA fiber, PC fiber, PVC fiber, and polyester fiber, etc.; boron fiber and metal whiskers such as aluminum, titanium, and calcium, etc.
3) Toughening: High-impact resins, such as: CPE, MBS, ACR, SBS, ABS, EVA, modified petroleum resin (MPR), etc.; high-impact rubbers, such as: ethylene propylene rubber (EPR), ethylene propylene diene monomer (EPDM), nitrile rubber (NBR), styrene-butadiene rubber, natural rubber, butadiene rubber, chloroprene rubber, polyisobutylene, and butadiene rubber, etc.
4) Wear resistance: Graphite, MoS2, silica, etc.
5) Biodegradability: Starch filler, biodegradable additives, etc.
2. Influence of Additive Morphology on Composite Performance
Same additive can have different morphological distributions, resulting in varying effects on modification. For powdered additives, the key influencing factor is particle size. 1) Smaller particle size is more beneficial to tensile and impact strength of filler material. For example, in terms of impact strength, every 1μm reduction in particle size of antimony trioxide doubles impact strength. 2) Smaller particle size of flame retardants results in better flame retardant effects. For example, adding 4% antimony trioxide with a particle size of 45μm to ABS has same flame retardant effect as adding 1% antimony trioxide with a particle size of 0.03μm. 3) Smaller particle size of colorants results in higher coloring power, stronger hiding power, and more uniform color. However, smaller particle size is not always better for colorants; there is a limit, and this limit varies depending on specific properties. 4) Taking carbon black as an example, smaller particle size facilitates formation of a network of conductive pathways, reducing amount of carbon black needed to achieve same conductivity. However, like colorants, particle size also has a limit; excessively small particles tend to aggregate and are difficult to disperse, resulting in a poorer effect.

Note: Mesh size refers to the number of sieve openings per square inch.
For fibrous auxiliaries, the higher degree of fiber development, the better reinforcing effect. Degree of fiberization of auxiliary can be expressed by aspect ratio (L/D). Molten state is more advantageous than powder state in maintaining aspect ratio and reducing probability of fiber breakage, making it a better addition method.

 

3. Compatibility between Additives and Resin
Good compatibility between resin and additives is a crucial foundation for ensuring optimal performance of additives, guaranteeing good durability, extraction resistance, migration resistance, and precipitation resistance during use. Good compatibility is a basic requirement; even barrier formulations require additives to be layered within resin. Main methods to improve resin compatibility are:
1) Addition of compatibility additives, such as surfactants, with amount added to achieve optimal efficacy.
2) Surface treatment: Using compatibility agents or coupling agents as surface treatment agents improves compatibility between resin and additives. Common coupling agents include silanes, titanates, and aluminates; compatibility agents include maleic anhydride graft polymers corresponding to resin. Additives requiring surface treatment include inorganic additives and fiber-based additives.
4. Amount of Additives Added
Appropriate additive amounts not only improve target resin to suitable performance but also maintain low cost based on economic considerations. Different additives have different requirements for dosage: 1) Flame retardants, toughening agents, magnetic powders, barrier agents, etc. While more is generally better from a performance perspective, cost must be considered; 2) Conductive additives generally only need to form a circuit path; 3) Antistatic agents only need to form a charge-dissipating layer on the surface; 4) Coupling agents only need to form a surface coating.
5. Relationship between Resin and Additives
Addition of additives should not cause a change in resin's inherent properties. For example, lead- and copper-containing additives should not be added to PPS, and antimony trioxide should not be used in PC, as these can lead to depolymerization. Simultaneously, acidity or alkalinity of additive should be consistent with that of resin; otherwise, a reaction may occur.
6. Relationship between Additives and Other Components
In a formulation, multiple different additives are needed to simultaneously achieve multiple objectives. Interactions between additives are very complex, mainly summarized as follows: 1) Independent and without mutual influence; 2) Synergistic effect: Multiple additives in a formulation promote each other, making the overall effect higher than average of individual additives; 3) Mutual elimination: Effect of adding two or more additives together is lower than average of adding them individually. For example, in anti-aging plastic formulations, sulfide-based antioxidants and HALS-based light stabilizers should not be used simultaneously because acidic components generated by sulfide-based antioxidants inhibit light-stabilizing effect of HALS.

Uniform mixing is a basic requirement for plastic formulation modification, and it also requires additives to be evenly distributed in resin formation. Uneven distribution of additives not only fails to improve original resin performance, but may also result in performance worse than pure resin due to uneven filler distribution. For example, when using fillers, uneven dispersion can significantly reduce mechanical properties and processability of materials, affecting both mechanical properties and processability. Common methods to improve uniform mixing of additives include: 1) Rationally ordering added fillers, such as adding filler first during coupling treatment, followed by heating and dehydration, then adding coupling agent; 2) Adding components to system in a primary-secondary order. Large quantities of fillers can be added in multiple batches to facilitate uniform mixing. For coupling agent treatment, it is generally necessary to spray in three stages to ensure uniform dispersion and good coupling effect.
IV. Formulation Selection
Formulation processability refers to processability of modified plastic formulation, which ensures that modified plastic can be molded into finished products. Main indicators of formulation processability are: good heat resistance of additives, no evaporation loss, decomposition and inactivation; minimal wear and corrosion to equipment, and no release of toxic gases. These can all be used as evaluation criteria.
1) Flowability of Modified Plastics
Flowability of plastics is a major factor affecting their processing performance and an important basis for selecting appropriate processing method. Generally, addition of inorganic fillers will reduce flowability, requiring addition of flow modifiers for optimization. Common organic additives include: decabromodiphenyl ether and tetrabromobisphenol A.
2) Heat Resistance: Heat resistance is considered from two aspects: 1) Lowering resin processing temperature, ensuring it does not exceed decomposition temperature of additive, thus ensuring stability of additive and demonstrating its modifying properties; 2) Adding a certain amount of antioxidant additive to prevent thermal decomposition and its impact on resin performance. Generally, most organic dyes have low decomposition temperatures and are unsuitable for high-temperature processing of engineering plastics; fragrances also have low decomposition temperatures, generally below 150℃, requiring use of low-processing-temperature resins such as EVA as carriers.
3) Environmental Friendliness of Formulation: Environmental friendliness of formulation includes: harmlessness to humans, equipment, environment, and users. Human hygiene—resin and selected additives should be absolutely non-toxic, or their content should be controlled within specified range. Environmental pollution—selected components should not pollute environment.
4) Cost and Source: Plastic modification formulations aim for the lowest possible price. During implementation, use of additives follows these principles: 1) Prioritize low-priced raw materials; 2) Prioritize existing raw materials: no need for procurement, stable source, and clear performance; 3) Prioritize domestic raw materials: price fluctuations are less affected by foreign exchange and trade policies; 4) Prioritize nearby raw materials: reduce inventory costs; 5) Prioritize general-purpose raw materials: widely available and relatively stable performance.