Isocyanate polymers for in-mold coating of thermoplastic sheet composites.
Synthetic polymers can be divided into two major classes: thermoplastic and thermosetting. Thermoplastic polymers consist of individual molecules, generally having a chain-like core structure, and a molecular weight between several thousand and several million. When heated above a characteristic temperature, the individual molecules are free to flow under the action of an applied force, such that items can be formed by a wide range of fluid flow processes. The ability to process thermoplastic polymers in the fluid state gives them tremendous advantages with respect to part diversity and manufacturing productivity.
When cooled below that same characteristic temperature, thermoplastic polymers become solid and retain the shape that was imparted to them. Retention of shape depends on whether or not the item is subjected to forces over the service life of the part. A significant shortcoming of thermoplastic polymers is that they permanently deform with time under a sustained load. This phenomenon, known as creep, limits thermoplastic polymers to uses demanding no or low forces during use.
Thermosetting polymers are composed of chemically reacted, low-molecular-weight species that form a structure of infinite molecular weight. Instead of individual molecules with a chain structure, thermosetting polymers exist as three-dimensional networks of covalently bonded chemical segments. The length of the segments can vary widely, producing polymers that range from rubbery flexibility to hard rigidity.
Although the low-molecular-weight prepolymers are easy to handle, the crosslinking chemical reaction takes much longer than a simple heating/cooling process. As a result, thermoset parts usually require longer production times than thermoplastic parts. Moreover, the chemical reaction must be controlled to ensure homogeneity within and among parts. Control has not always been easy to achieve, particularly with thick parts or those having a wide range of thicknesses.
Thermosets do have a performance advantage in that, because they are covalently bonded into a network structure, imposed forces are widely distributed throughout the part, and creep is not as prevalent a problem as it is with thermoplastics. As a result, regardless of processing disadvantages, thermosets are more widely used in structural applications. Many fiber reinforced composites use a thermoset polymer as the matrix. Low-set elastomers also rely on vulcanized rubbers rather than thermoplastic elastomers (TPEs).
The processing advantages of thermoplastic polymers have spurred considerable research toward development of thermoplastics to replace thermosets, or thermosets that process similarly to thermoplastics. Thermoplastics, such as polyetheretherketone (PEEK) and polyphenylene sulfide (PPS), have made some inroads in the composites arena. TPEs have replaced many vulcanizable rubbers in conditions where the degree of set is not critical. Thermosetting polyimides have been made from oligomers that have been end-capped with chemical groups that react when heated above a critical temperature, but permit the oligomer to flow easily at lower temperatures.
Clearly, processing advantages are associated with thermoplastic polymers, and performance advantages are associated with thermosetting polymers. This article discusses a family of polymers that is a hybrid combination of the two main classes of polymers. At elevated temperatures, these polymers consist of individual long-chain molecules that flow under the application of pressure or drag. At low temperatures, however, they consist of a covalently bonded three dimensional network.
Polymers that behave as individual long chain molecules at elevated temperatures and have a crosslinked structure at low temperatures are based on the inherent reversible reaction of compounds synthesized from an aromatic molecule containing a labile hydrogen entity, and another molecule having isocyanate functionality. Schematically, this is shown by the following equation:
R-[Phi]-OH + OCNR[prime] [equivalence] R-[Phi]-O-CO-NH-R[prime]
The labile hydrogen entity is italicized in the above equation. Isocyanate compounds are usually highly reactive toward molecules containing an active hydrogen entity. Such active hydrogen sites are readily available in compounds containing hydroxyl, carboxyl, and amine end groups. In most cases, this reaction proceeds in one direction toward the formation of polyurethanes. However, if an aromatic group is present near the active hydrogen entity, the reaction can be reversed at elevated temperatures. This reversibility of covalent bonding occurs when the compound is heated above a characteristic temperature, which depends on the nature of R and R'. Under these circumstances, the covalent bonds break and the two lower-molecular-weight molecules are regenerated. In such a condition, they have low melt viscosities and can be processed as fluids. It is important to note that they recombine covalently when cooled below that same characteristic temperature.
Long chain polymers are produced by having functional groups on both sides of the R-[Phi] (OH functionality) and R[prime] (NCO functionality) moieties. Providing multiple isocyanate functionality at the end of the R[prime] group produces a three-dimensional crosslinked structure when the material is at a low temperature. The actual properties of both the fluid and solid depend on the chemical structure of R and R[prime]. These groups may consist of segments such as bisphenol A polycarbonate, polyphenylene sulfide, and polyphenylene oxide. Such groups increase the temperature at which reversibility occurs. The result is a polymer that is fully crosslinked up to the critical disassociation temperature, thus exhibiting good mechanical properties up to rather high temperatures. It is important to know that this polymer processes as a thermoplastic after disassociation into a fluid at elevated temperatures.
In-Mold Coating Formulation Development
Toughness and low temperature impact strength are decided advantages for a surface coating on an automobile. Development of the formulation was guided by those requirements. Of particular interest was the case in which R was a mixture of bisphenol A polycarbonate and the bis-hydroquinone ester of isophthalic acid. A composition based on this mixture was investigated for use as an in-mold coating to provide a class "A" finish to glass mat reinforced thermoplastic composites produced by a high speed stamping process.
TABLE 1. Formulation of In-Mold Coating Reaction Oligomer. Chemical specie Concentration, wt% Phenol end-capped PC 37.20 Polycaprolactone diol 26.88 4,4' diphenylmethane diisocyanate 25.53 HQ/IPA/HQ 9.41 Trimethylol propane 0.83 P-phenyl phenol 0.15
Thermoplastic sheet composites provide the mechanical properties of thermosetting sheet composites, but with the added advantage of fast-cycle part forming. A critical shortcoming of thermoplastic sheet composites is that their surface texture is unacceptably coarse for use as exterior automobile panels - a large-volume application in which visual aesthetics are important. Even if the surface of the original sheet material has the desired surface smoothness, the process of forming complex parts causes the glass fibers and polymer to flow at different rates, resulting in surface roughness. This also happens during the processing of thermosetting sheet compounds (sheet molding compound, or SMC, in particular).
An in-mold coating process has overcome this shortcoming in SMC processing. In the process, the mold is opened part way through the curing cycle and injection of liquid polyester resin between the mold and curing part. The mold is then closed again, causing the polyester to flow within the small gap between the mold and part. The liquid resin covers surface blemishes and cures with the polyester in the composite. The final part has a surface of pure resin - which replicates the quality of the mold surface - and is covalently bound to the structural part. The process works quite well because the coating resin and composite resin are of the same type and are able to co-cure. Because the coating is a liquid, it easily flows between the mold surface and the part when the mold is open. The elevated temperature of the mold that is used to cure the composite resin also serves to cure the coating resin.
TABLE 2. Shear Strength of In-Mold Coating on Thermoplastic Sheet Composite Substrates. Substrate Lap shear strength, MPa 40% glass fiber-reinforced PP 0.41 40% glass fiber-reinforced PET 0.33
In the molding of a thermoplastic sheet composite, the situation is very different. The composite is preheated prior to entering the mold. The material flows inside the mold because its residual heat permits it to remain viscoelastic for the duration of the molding portion of the cycle. The mold surfaces are cooled to promote rapid solidification of the part. A liquid coating, used in a manner similar to the in-mold coating process described for thermosetting composites, would not be exposed to sufficient temperatures for the time necessary to effect a cure. In addition to this concern, the adhesiveness of such coatings to polypropylene (PP) and polyethylene terephthalate (PET) is uncertain.
We investigated injection molding polypropylene and polyethylene terephthalate resin into a partially opened and partially heated mold under high pressure. Thus, many of the advantages of fast compression molding of thermoplastic sheet composites are lost. The ideal in-mold coating material for a thermoplastic composite is one that can be injected as a liquid into a cold mold, to provide fast surface coverage and favorable fill of surface depressions. Then, when the part cools to room temperature, the coating solidifies into a solid that replicates the surface of the mold and has good adhesion to the substrate. The thermally reversible isocyanate polymer chemistry discussed above provides materials with exactly those characteristics.
Preparation of the in-mold coating material with the desired thermal reversible processing characteristics required stoichiometric amounts of isocyanate and labile hydrogen end groups. Each of the components required reactive linkages at either end of their respective oligomers.
Bisphenol A polycarbonate provided one of the bases for these properties. Our keeping the degree of polymerization of polycarbonate at a level low enough so that its oligomers would melt prior to reaching their degradation temperature, yet high enough so that the bisphenol A polycarbonate segments could provide the necessary toughness and low temperature impact resistance, was a key element of the polymer synthesis. Bisphenol A is a desirable source of labile hydrogen because of its phenolic hydroxyl end groups, which were ensured by using a controlled excess of bisphenol A during the phosgene-bisphenol A reaction that produced the polycarbonate oligomer. The oligomeric structure of this compound is shown below:
[Mathematical Expression Omitted]
In the compounds prepared for this study, n was on the order of 7, but could range between 1 and 20. This polycarbonate oligomer had a crystalline melting point of 182 [degrees] C.
The bis-hydroquinone/isophthalic acid compound also provided high temperature stability and strength, but added stiffness in place of toughness to the final compound. A balance between the two materials determined the relative toughness and stiffness of the compound. In a manner similar to that of the polycarbonate oligomer, an hydroxy end-capped oligomer of bishydroquinone and isophthalic acid (referred to as HQ/IPA/HQ) was prepared, having the following formula:
In this molecule, the isophthalic acid is flanked by the bishydroquinone to ensure hydroxyl functionality on either end. This compound has a crystalline melting point of 215 [degrees] C.
Also added to the in-mold coating composition was a low-melting-point aliphatic polyester polyol. The addition of this compound further modified toughness and flexibility. The aliphatic polyester polyol used in this study was polycaprolactone diol with an average molecular weight of 530. This compound had the following chemical formula:
In addition to the three polyol components, one or more isocyanate components were needed. Dual functionality polyisocyanates produce linear polymers; trifunctional isocyanates produce the desired crosslinked structure. Polyisocyanates can be either aromatic, aliphatic, or a mixture of both, and may contain up to 100 carbon atoms in the oligomer. In the formulation investigated in this project, the linear isocyanate was 4-4' diphenylmethane diisocyanate.
In essence, the molecular structure had three basic components: an isocyanate linking group, a high performance aromatic oligomeric backbone group, and a toughening aliphatic oligomeric group. The isocyanate linking group included a trifunctional isocyanate crosslinker, which preferentially reacted with the aromatic oligomer backbone group. The trifunctional compound used was trimethylol-propane, and the resulting triisocyanate had the following structure:
[Mathematical Expression Omitted]
By controlling the concentration of trifunctional isocyanate, the crosslink density was controlled. The formulation investigated in the study is given in Table 1.
In relatively low concentrations, p-phenyl phenol was added to the mixture as an end-capper to limit linear molecular weight buildup to approximately 100,000. The end caps also ensured nonfunctional polymer chain ends.
The structure of p-phenyl phenol is [Phi][Phi]-OH.
Compound Preparation Procedures
The polycarbonate oligomer, bishydroquinone isophthalic acid diester, polycaprolactone diol, and p-phenyl phenol were melt mixed at 210 [degrees] C. After these ingredients were mixed into a homogeneous melt, the mixture was allowed to cool to 100 [degrees] C. When the mixture reached this temperature, the isocyanate components were added, and the mixture was heated to 150 [degrees] C. The amount of isocyanate added was such that the NCO groups balanced the OH groups.
Thermal Evaluation of Polymer
The rheological properties of the synthesized polymer were determined on a Rheometrics Mechanical Spectrometer, model 605. Measurements were made in the oscillatory mode between two parallel plates at 6.28 rad/sec (1 Hz). An oscillatory strain of 10% was used. The complex viscosity, [[Eta].sup.*], was measured between 60 [degrees] C and 210 [degrees] C. Figure 1 shows the resulting data. A glass transition temperature is observed in the neighborhood of 60 [degrees] C. This transition has been attributed to the polycaprolactone component. A plateau region exists between 80 [degrees] C and 160 [degrees] C, in which the polymer behaves as a rubbery material. Above 160 [degrees] C, the polymer depolymerizes into its low-molecular-weight oligomers. Above 200 [degrees] C, the polymer has completely dissociated and its oligomeric constituents have melted. The mixture of oligomers has a melt viscosity of approximately 200 Pa. At this viscosity, the oligomers are easily processed by melt techniques.
Figure 2 shows the infrared absorption scans of the polymer as it is heated from room temperature to 240 [degrees] C. Notice the growth in the NCO absorption band at 2274 [cm.sup.-1]. The absorption band grows very slowly prior to 140 [degrees] C, and is fully noticeable only when the temperature reaches 160 [degrees] C. Figure 3 shows the growth of NCO with increasing temperature. The relationship between the NCO absorption band height and temperature is almost linear between 160 [degrees] C and 225 [degrees] C. The Figure also shows the disappearance of the isocyanate linkages as the temperature goes back down; thus demonstrating the reversibility of this reaction. The reversible reaction is not linear, and shows a hysteresis from the NCO formation curve. This is logical, in that the rate constant for formation of the polymer would be expected to be slower than the rate constant for disassociation.
In-Mold Coating Trials
Several trials were made in which the formulation was melt coated to glass fiber reinforced sheet composites. Two types of sheet composites were used: one based on polypropylene as the matrix, the other based on polyethylene terephthalate. The reversible isocyanate polymer was melted and placed in the center of the preheated composite sheet. This system was then placed in a mold and the mold was closed. Visual observations were made of the coated surface, and adhesion of the coating to the substrate was measured. In both cases, the coating had a smooth glassy appearance that replicated the exterior mold surface. The lap shear adhesion strength of the coatings to both substrates is shown in Table 2, and was considered favorable.
A polymer was developed that has the necessary characteristics to be used as an in-mold coating for thermoplastic sheet composites. This resin has the unusual characteristic of being a thermoplastic material at elevated temperatures and a thermosetting material at use temperatures. Moreover, the viscosity of the polymer at the desired processing temperatures is low enough to provide easy flow within the partially opened mold.
The polymer has these characteristics because of the reversible nature of compounds synthesized from an aromatic molecule containing a labile hydrogen entity and another molecule having isocyanate functionality. The specific formulation that was studied has a glass transition temperature of approximately 60 [degrees] C and a disassociation onset temperature of 140 [degrees] C. The polymer is completely disassociated into its low molecular weight oligomeric species at 200 [degrees] C.
Because of the inherent flexibility in formulating thermosetting compounds, a wide range of composition fine tuning can take place. The formulation presented in this article is by no means the optimum for this application. It has not been rigorously tested for all the performance characteristics that are necessary for an automotive coating material. However, it does represent a good beginning for future work, and more important, shows an encouraging approach for overcoming a significant technical obstacle.
1. R.A. Markle, P.L. Brusky, and G.E. Cremeans, U.S. Patent 5,097,010 (Mar. 17, 1992).
2. D.M. Bigg, Polypropylene: Structure, Blends, and Composites; Vol. 3, p. 263, J. Karger-Kocsis, ed., Chapman & Hall, London (1995).
3. R.A. Markle, J.D. Elhard, D.M. Bigg, S. Sowell, P.L. Brusky, and G.E. Cremeans, U.S. Patent 5,387,667 (Feb. 7, 1995).
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|Author:||Bigg, Donald M.; Markle, R.A.|
|Date:||Sep 1, 1996|
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