How to Change the Performance of UHMW-PE
Compared with other engineering plastics, UHMW-PE has the disadvantage of low surface hardness and thermal deformation temperature, bending strength and poor creep performance. This is due to the molecular structure and molecular aggregation of UHMW-PE, which can be improved by filling and crosslinking.
1. Filling
Using glass beads, glass fiber, mica, talcum powder, silicon dioxide, molybdenum disulfide, aluminium oxide, carbon black etc. to fill modification of UHMW-PE, which can improve the performance of surface hardness, stiffness, creep, bending strength, thermal deformation temperature. After treatment with coupling agent, the effect is more obvious. If filling glass beads, the thermal deformation temperature performance of UHMW-PE will raise to 30℃ Filling material such as glass beads, glass fiber, mica and talcum powder can improve hardness, rigidity and temperature resistance performance; Molybdenum disulphide, silicon oil and special wax can reduce the friction factor to further improve the self-lubricity; Carbon black or metal powder can improve anti-static and electrical conductivity and heat transfer. However, the impact strength of the filler has decreased slightly, and the high molecular weight polyethylene (UHMW-PE) still has high impact strength if the content is contained within 40%.
2. Processing
The molecular chain of UHMW-PE resin is long and easy to be fractured by shear force, and it can be degraded by heat. Therefore, lower processing temperature, shorter processing time and reduced shear are necessary.
In order to solve the problem of UHMW-PE processing, it's not only has to the special design on common molding machinery, but also to improve the resin formula: blending or join with other resin flow modifier in a normal extrusion machine and injection molding machine processing, that is lubricating extrusion (injection).
3. Blending
It is the most effective, simplest and most practical way to improve the melt fluidity of UHMW-PE. The technology in this area is more in the patent literature. The second component of blend is mainly refers to the low melting point, low viscosity resin, low density polyethylene (PE - LD), high density polyethylene (PE - HD), polypropylene (PP), polyester, etc. There are a lot of them are used in the molecular weight of PE (molecular weight 400,000 ~ 600,000) and low molecular weight PE molecular weight (< 400,000). When the blending system is heated to the melting point, UHMW-PE resin will be suspended in the liquid phase of the second component resin, forming the extruded and injectable suspension materials.
3.1 Blending with low and medium molecular PE
UHMW-PE compounding with low molecular weight PE - LD (molecular weight 1,000 ~ 20,000; 5,000 ~ 12,000 is the best) blending can make its molding process ability get improved significantly, but make the mechanical properties such as tensile strength, flexural elastic fell. PE - HD can also significantly improve the processing fluidity of UHMW-PE, but causes the performance of impact strength and friction resistance decline. For UHMW-PE mechanical properties of the blend system at a higher level, an effective compensation method is to join PE nucleating agent, such as benzoic acid, benzoic acid, stearic acid salt, salt adipic acid salt, etc. Have pointed out that patent in UHMW-PE and PE - HD blending system with very small amounts of tiny nucleating agent wollastonite (range 5 ~ 50 nm, particle size of the surface area of 100 ~ 400 m²/g), can be well compensated the decrease of mechanical properties.
3.2 Blend morphology
The chemical structure of UHMW-PE similar to other varieties of PE, but in general the melt mixing equipment and conditions, their blending polymer can't form a uniform form, this may be related to viscosity difference between components. The two components of UHMW-PE/PE - LD combined with ordinary single screw mixing are respectively crystallized and cannot form eutectic. UHMW-PE is basically dispersed in PE - LD matrix with packing form. The melting body has been used for a long time and used the double roll plasticizer. The function of the two components has been strengthened, and the performance has been further improved, but it still cannot form a eutectic form.
Vadhar found that when the two-step blending method was adopted, UHMW-PE was fused at high temperature and then reduced to low temperature to join the linear low-density polyethylene PE-LLD for blending, which could be used to obtain the common mixture of eutectic. Vadher mixed with solution was also obtained to form a eutectic UHMW-PE/PE - LLD blend polymer.
3.3 The mechanical strength of blends
To non-additional nucleating of UHMW-PE agents/PE system, the cooling process will form larger spherules, and there is an obvious interface between spherules. In these interfaces, there are internal stresses caused by different molecular chains which lead to cracks. Compared with the matrix polymer, the tensile strength of the blend decreased. When subjected to external force impact, the crack will rapidly develop along the spherical interface, resulting in the final crushing, thus causing a decrease in impact strength.
