Mechanism of low‐profile behavior in single‐phase unsaturated polyester systems

Abstract

SynopsisThe mechanism of the low profile behavior in certain single-phase polyester systems has been examined. It appears that as polymerization occurs, crosslinked beads (<lp) of polymer separate from the bulk solution. As the temperature rises, liquid monomer expands, counteracting polymerization shrinkage. As shrinkage surpasses expansion towards the end of polymerization, stresses are induced in the system. These are relieved internally by a series of micro cracks in the continuous matrix of the polymer rather than, as in ordinary systems, through macroscopic shrinkage of the molded part.Industrial interest in "low profile" reinforced polyester systems has been continually increasing over the past years. These types of resin systems provide not only low profile (a highly uniform, smooth surface without imperfections such as fiber pattern, etc.) but also excellent reproduction of mold dimensions and maximum dimensional stability in subsequent use applications. Thus, it is possible to directly mold high-gloss, close-tolerance parts without costly finishing steps. These systems differ from the usual crosslinking polyester compositions in that they incorporate a small amount (10-20%) of a thermoplastic polymer in the formulation to effect the desired low-profile behavior. The mechanism by which this low-profile behavior occurs and the structure of the cured composition has been the subject of several recent publications.'-1° We have reported8 the results of a study on a system which consists of two phases in the uncured liquid state. That study suggests that the low-profile character is the result of releasing polymerization-induced stress through a process of microstress cracking rather than through manifestation as macroscopic contraction or shrinkage. Ehnert et a12 have developed similar data on both the two-phase and a homogeneous system, as well as a pigmentable low-profile resin. These data are in essential agreement with our experiments: but our work with the microscope suggests a more detailed mechanistic interpretation.In the present paper we will discuss the mechanism of low-shrink and lowprofile behavior in the homogeneous, single-phase system. Several different single-phase systems have been investigated, and all seem to show similar characteristics. Some aspects of the behavior differ from those of the twophase system previously reported on: but the principal processes responsible for the desirable low-profile character are very similar.

Effects of poly(vinyl acetate) and poly(vinyl chloride‐ co‐vinyl acetate) low‐profile additives on properties of cured unsaturated polyester resins. I. Volume shrinkage characteristics and internal pigmentability

“…Pattison et al 25,26 have proposed that, as the crosslinking of LPA-containing UP resins proceeds, strain due to polymerization shrinkage develops in the system, * The values in parentheses represent the estimated standard errors for the experimental averages.…”

Nanostructuration of Unsaturated Polyester by All‐Acrylic Block Copolymers, 1 ‐ Use of High‐Molecular‐Weight Block Copolymers

“…[18][19][20][21][22][23][24][25][26][27][28] Although the mechanism is not fully understood, it is generally accepted that the shrinkage compensation is due to stress-related microvoid formation in the LPA phase and in the interfacial regions between LPA and UP resin. The phase separation between both components during curing process is a necessary condition for generating this stress cracking.…”

Effects of reactive low‐profile additives on the volume shrinkage and internal pigmentability for low‐temperature cure of unsaturated polyester

“…3,4,13,14 This is, however, different from the viewpoint proposed for the ST/UP/ reactive LPA system in this work.…”

“…Pattison et al 3,4 have proposed that as the crosslinking of LPA-containing UP resins proceeds, strain due to polymerization shrinkage develops in the system, particularly at the interface of LPA phase (phase R) and crosslinked UP phase (phase P). This strain can increase to the point that stress cracking propagates through the weak LPA phase, relieving this strain, forming microcracks or microvoids or both, and compensating for the overall volume shrinkage by the microcrack or microvoid space.…”