Purpose of Modification

Plastic surface modification falls into two categories: modification for direct application and modification for indirect application.
1. **Modification for direct application**: It directly endows plastic surfaces with properties including gloss, hardness, wear resistance, anti-ageing, flame retardancy, conductivity and barrier performance. Surface barrier modification has developed rapidly and occupies a vital position in this field.
2. **Modification for indirect application**: It serves as pre-treatment for subsequent processing. Its core is to raise surface tension and improve adhesion, printing and lamination performance.
Example: For plastic electroplating, only ABS can achieve qualified coating adhesion without surface treatment. Polyolefin plastics have extremely poor coating adhesion, so surface modification is mandatory to improve bonding strength before electroplating.

What Properties Can Be Modified?

Different application requirements correspond to different modification solutions. Below are the 8 most common modification types.

01 Density Adjustment

- Density reduction: Foaming modification, adding lightweight fillers, blending lightweight resins.
Foaming is the most effective method to reduce density. Adding lightweight fillers or blending lightweight resins can only cut density slightly (max ~50% reduction, minimum relative density ~0.5). Foamed plastics feature a wide density range, with relative density as low as 10⁻³.
- Density increase: Adding heavy fillers or blending high-specific-gravity resins.
Common heavy fillers: metal powder, heavy mineral fillers. Blending heavy resins brings limited density increase (max ~50%). Suitable for light resins such as PE, PP, PS, EVA, PA1010 and PPO. Typical heavy resins: PTFE, FEP, PPS, POM.

02 Transparency Improvement

Principle: Control crystallization characteristics. Plastic transparency depends on crystallinity and crystal structure. Optimizing micro-morphology can greatly improve transparency.
Key evaluation indicators: Light transmittance and haze for light transmission capacity; refractive index, birefringence and dispersion for light quality. Premium transparent plastics require balanced and outstanding indicators.
Common approaches:
① Control crystallization: crystal form, spherulite content, crystal size and regularity;
② Add high-refractive-index additives without sacrificing transparency;
③ Reduce processing orientation to lower birefringence;
④ Add small-molecule additives to boost transparency;
⑤ Add nucleating agents (most efficient): convert homogeneous nucleation to heterogeneous nucleation, increase crystal nuclei and refine grains;
⑥ Incorporate high-refractive-index inorganic materials;
⑦ Add birefringence-reducing substances;
⑧ Add anti-fog agents.

03 Hardness & Flexibility Tuning

1. Improve bulk hardness by adding rigid inorganic fillers or fibers.
2. Surface hardness modification: Only enhance the outer surface hardness while keeping inner hardness unchanged. It is cost-effective for housings, decorative parts, optical components and daily goods. Methods include coating, plating and surface treatment.
3. Hardness improvement via blending & lamination
- Blending: Mix low-hardness resins with high-hardness resins (PS, PMMA, ABS, MF) for PE, PP, PA, PTFE substrates.
- Lamination: Coat low-hardness products with a high-hardness resin layer, suitable for extruded sheets, films and pipes.
4. Add plasticizers to gain flexibility: Plasticizers reduce processing temperature, improve melt flow and endow products with softness. Suitable substrates: PVC, PVDC, CPE, SBS, PA, ABS, PVA, chlorinated polyether.

04 Processability Optimization

Thermoplastics and thermosets behave differently during processing. The targets of process modification: raise thermal decomposition temperature, lower melting temperature, improve melt flow and optimize melt characteristics.
Common additives: plasticizers and lubricants. Plasticizers increase polymer plasticity. Lubricants reduce friction between materials and between materials & equipment, lowering melt viscosity, preventing sticking and improving surface finish.

05 Reinforcement

Reinforcement mainly uses reinforcing fillers and fibers. Ordinary fillers usually reduce tensile strength, while reinforcing fillers can slightly boost tensile performance, though their effect is weaker than fiber reinforcement.
Fiber reinforcement is the most widely used and effective solution. Fiber reinforcements account for over 90% of all reinforcing materials.
- Inorganic: Glass fiber, carbon fiber, whisker, quartz fiber, graphite fiber, ceramic fiber;
- Organic: PAN fiber, polyethylene fiber, PA fiber, PC fiber, PVC fiber, polyester fiber;
- Metallic: Boron fiber, metal whiskers of Al, Ti, Ca.

06 Toughening

The mainstream toughening method is blending with elastomers.
Elastomer toughening materials:
High-impact resins: CPE, MBS, ACR, SBS, ABS, EVA, modified petroleum resin (MPR);
Rubbers: EPR, EPDM, NBR, styrene-butadiene rubber, natural rubber, polybutadiene rubber, neoprene, polyisobutene.

07 Thermal & Flame Retardant Performance

Thermal indicators: Heat deflection temperature, Martens heat resistance, Vicat softening point; low-temperature performance is evaluated by brittleness temperature.
Most inorganic mineral fillers improve heat resistance, e.g. calcium carbonate, talc, wollastonite, mica, calcined clay, bauxite. Mica can withstand up to 1000°C.
Fiber reinforcement delivers better heat resistance than mineral filling. Glass fiber, carbon fiber and whiskers have melting points above 1500°C.
Most plastics are flammable. Flame retardants are added to stop ignition or slow flame spread, often combined with smoke suppressants.

08 Cost Reduction

Common methods: Adding mineral fillers and blending low-cost resins. Cost reduction must be achieved on the premise of meeting required material performance.

Conclusion

Plastic modification generally covers these 8 categories. Actual formulation development is complex. Multiple modification techniques are often combined to balance customer performance requirements and production costs.