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What are the types and characteristics of low shrinkage agents

Oct 31, 2018

Unsaturated polyester resin has good processing performance and is suitable for a variety of molding processes, such as SMC molding, BMC injection, extrusion, RTM, hand lay-up and pouring.But in the production process, there are such as resin phase produced cracks;Quality defects on the surface;Size loss.The main cause of these problems is that the resin units are tightly packed and nucleated during the polymerization, which leads to aggregation shrinkage and thermal contraction of the resin.The internal stress caused by shrinkage will lead the resin from the surface to the inner depression, which will be transferred from the polymer surface to the center gradually, releasing the internal stress until the balance, which directly leads to the occurrence of defects.The function of low shrinkage additive (LSA) is to release the internal stress compensation polymerization contraction through local relaxation, so as to reduce the shrinkage.


Non-polar low shrinkage additive

Represented by polystyrene (PS), also including polyethylene (PE), polypropylene (PP) and other solid powder low shrinkage additives.This kind of low shrinkage additive is a two-phase system with the resin before curing.This kind of low shrinkage additive has good inner coloring, but the low shrinkage is general.


Non-polar low shrinkage additive


Weak polarity low shrinkage additive

With polymethyl methacrylate (PMMA) as representative, also including cellulose butyl acetate, polyvinyl chloride (PVC polarity strong too, though, but due to its glass transition temperature Tg at 87 ℃, the viscous flow temperature as high as 165 ~ 190 ℃, split phase effect is not very good, poor effect of low shrinkage), etc.This kind of low shrinkage additive improves the stability of the resin due to the enhancement of polarity.


PVAc is a powder or round particle


Polar low shrinkage additive

Polar low shrinkage additives include polyvinyl acetate (PVAc), saturated polyester, polycaprolactone, polyurethane, etc.This kind of low shrinkage additive has similar polarity to unsaturated polyester and is compatible with the resin. Before curing, it is a homogeneous system with the resin. After curing, the shrinkage of the resin can be controlled to the highest degree.However, this kind of low shrinkage additives also has the problem of uneven coloring, and does not meet the requirements of inner coloring products.


Polar low shrinkage additive

Combined low shrinkage additive

The combined low shrinkage additives include grafted core/shell polymers, segmented polyacetate-styrene (pvac-st), modified lightly crosslinked polystyrene, inorganic modified polar low shrinkage additives, etc.This kind of low shrinkage additive has excellent comprehensive properties, including excellent internal coloration, excellent low shrinkage, good microscopic interface, good mechanical properties and resistance to small molecule permeability.

The disadvantages are high cost due to factors such as preparation technology.


Comparison of properties of various types of low shrinkage reagent

Compatibility with resin:

Polar low shrinkage reagent (PVAc) > weak polarity low shrinkage reagent (PMMA) > non-polar low shrinkage reagent (PS)


Low shrinkage:

Polar low shrinkage reagent (PVAc) > weak polarity low shrinkage reagent (PMMA) > non-polar low shrinkage reagent (PS)



Non-polar low systolic agent (PS) > weak polar low systolic agent (PMMA) > polar low systolic agent (PVAc)


In the actual production, according to different products to the low shrinkage and coloring requirements, select appropriate low shrinkage agent to achieve the best effect.





Quanzhou NEWECO polymer material co., LTD.


NEWECO is one of the leading unsaturated polyester resin manufacturers and  suppliers,widely used in automobiles, ships, rail transit, wind power  generation, pipe cans, heat and corrosion protection, building  materials, stone repair, sanitary ware, artificial stone and other  industries. Suitable for hand lay-up, spray, pultrusion, winding,  molding, vacuum introduction / RTM, casting and other molding processes. NEWECO Resin is committed to excellence and focuses on the application  of composite materials to provide customers with comprehensive  solutions.

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