The type of transition between additive and polar low shrinkage additive is polymethyl methacrylate (PMM low shrinkage additive)
LPA is a substance added to reduce the volume of the contraction surface during curing.LPA is an important key raw material in SMC/BMC production.Although there are many types of LPA, they can be roughly divided into two types according to their different molecular structures and properties: plastic LPA, which mainly includes polyethylene, polyfluoroethylene, polystyrene, polymethyl methylbenzoate, polycaprolactone, polyvinyl acetate, and copolymer dominated by their monomers.The other is the elastomer LPA, which mainly includes butadiene-butadiene-polyethylene block copolymer, polyurethane, saturated polyester and sub-flexible block copolymer.
2. Development stage of low shrinkage additives for plastics
The research and development of plastic LPA has gone through the following four stages:
1. Non-polar low-shrinkage additives, represented by polystyrene (PS), have poor compatibility with the resin. Before curing, they are two-phase system with the resin.
2. Non-polar low-shrinkage additives (A) are represented. As A result of the increase in polarity, the stability of such low-shrinkage additives in tree esters is improved.
3. Low shrinkage additive, polarity with polyvinyl acetate (PVAc) as the representative, such low shrinkage additive has good compatibility with resin, resin curing system with LPA for a photograph, before after curing evenly split phase, low shrinkage effect is very good, can make the fiberglass products shrinkage rate of 0105% or below zero contraction, the disadvantage is that products of mechanical strength.
4. Combined LPA, such low shrinkage additives are represented by block acid-styrene polymers, grafted core-shell polymers, etc., which have good comprehensive performance in terms of coloring, low shrinkage and mechanical strength.
The appearance and development of low shrinkage additives for elastomers
Since the mid-1980s, with the use of plastic LPA in the morning and some defects in performance, the use of elastomer LPA has been increasing.Compared with plastic LPA, elastomer LPA has more advantages:
1. Elastomer LPA can not only reduce solidification shrinkage, but also increase toughness, which greatly increases impact strength, fracture capacity and fracture elongation.
2. The elastomer LPA has good fluidity, and its forming pressure need not be high.This is especially beneficial when large - scale products are pressed into shape.
3. It can be used to make inner colored products or lacquered products, which is difficult to achieve with plastic LPA.
Research progress of low shrinkage additive in unsaturated polyester resin
Unsaturated polyester resin (UP) due to its good manufacturability, suitable for a variety of molding process, such as between 130 ℃ ~ 130 ℃ SMC moulded, BMC molding injection, pultrusion, RTM of room temperature curing, hand lay-up and casting resin, etc.But in the process of production, there are common problems: such as resin phase produced cracks, which greatly reduced the mechanical properties of products;There are quality defects on the surface, such as subsidence spots and surface ripples;Size out of control, such as warping, deformation.These problems are mainly caused by the compact molecular accumulation and the increase of density caused by the nucleation of the polymer unit during resin polymerization, resulting in the shrinkage and thermal contraction of resin polymerization. The volume shrinkage rate of unsaturated polyester is generally 6~10%. Adding appropriate low shrinkage additives or low profile additives (LPA) into UPR can significantly reduce the shrinkage rate of products.
UP USES the general mechanism of action of LPA :(1) the material is heated to start thermal expansion;(2) when the temperature rises, the initiator is decomposed and the curing process starts.(3) when the compatible hot plastics become incompatible, they begin to form two phases;(4) unreacted styrene and UP are collected in hot plastics;(5) as the polymerization goes on, the temperature and degree of polymerization increase. As the temperature rises UP phase shrinks, the volume occupied by LPA and unreacted monomers increases to compensate for the polymerization contraction.(6) the monomers in the LPA phase begin to react, and the stress caused by polymerization produces micro-cavitation;(7) finally, begins to cool down, in the above UPR phase of Tg, the thermal expansion coefficient of LPA and UPR phase roughly, under below Tg of UPR phase, phase of the UP cold contraction than LPA phase of cold much smaller, the difference is caused by cold contraction LPA phase micro holes and micro holes due to peripheral stress LPA phase, until reach the LPA Tg.
The research progress of low shrinkage additive in phenolic resin
General phenolic resin (PF) products have poor surface quality, low dimensional accuracy, internal cracks and pores, etc., which are mainly caused by the condensation polymerization of PF during high temperature curing and the release of low molecules.At present, the varieties and dosage of low shrinkage additives widely used in other composite materials have been fully studied, while there are few effective low shrinkage additives (LP A) used in P F. At present, only modified P F can achieve better low shrinkage.
Based on our data review, two kinds of linear P U prepolymers are currently used for low shrinkage additives applied to phenolic resin. The experimental results are as follows :(1) both end hydroxyl and end carboxyl P U can improve curing contraction of P F.The contractile control effect is related to the mass fraction of P U.The low shrinkage performance of end carboxyl group PU was more significant. When the addition amount was between 5< wpu-2 < 10, P F/ PU complex reached zero shrinkage.(2) the shrinkage effect is related to the molecular weight of P U prepolymer.The higher the molecular weight, the better the low contraction effect.
6. Research status of low shrinkage additives
LPA was developed to reduce the shrinkage of UP moulded products.After more than 30 years of development, the LPA varieties studied have reached dozens.LPA varieties are available in molded FRP products of different surface grades and resin systems.In the high temperature molding process where low profile or coloring is required unilaterally,LPA already has a more mature variety series.For example, polyvinyl acetate series of united carbon corporation from lp-40a, lps-60, lp-60, lp-90 to 1990s xlp-7015, xlp-7020, viscosity from 5000cps to 50cps, applicable to the molding process of FRP without internal coloring.Ashland's aropolq-701-c (pm-ma), AROPOL q-6514 (PVAc), URALLOY lp85-05 (saturated polyester), URALLOY LP2035 and URALLOY lp-ntlp100 (polyurethane) are all LPA with low shrinkage effect but no inner coloring.The SBS low shrinkage additive Kraton D of shell chemical company, Diarex hf-77 of Mitsubishi chemical company and em-116 of China are all LPA with good internal coloring but poor contraction effect.
In order to give full play to the comprehensive advantages of composite materials and adapt to the market demand, new varieties of LPA with excellent comprehensive performance are being vigorously developed internationally.In addition, previous studies have focused on the use of LPA in SMC/BMC.In the medium and low temperature moulding process, such as RTM process, the selection of such LPA is limited, and the research and development is still in the initial stage.The expanded application of vinyl ester resin system with the requirement of matching LPA.There are few domestic and foreign researches on this aspect.
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