Thinking and Practice of Composite Rapid Prototyping Technology (I): Problems of HP-RTM

 

Composite materials, especially carbon fiber composite materials, have low molding efficiency and high production and processing costs, which have always been the core issues restricting their large-scale use. Taking carbon fiber as an example, in 2021, the domestic carbon fiber production capacity was only 30,000 tons. In the past two years, under the stimulation of wind power, by 2023, the domestic production capacity has soared to 100,000 tons, and the demand for wind power has shrunk seriously.

 

There is nowhere to go for excess production capacity, and the carbon fiber industry is facing the dilemma of not being able to sell before 2021. In such a market environment, it is particularly important to expand the application of carbon fiber through low-cost rapid prototyping technology of composite materials. The author has been thinking and practicing to solve this problem for ten years. This article will share the author’s achievements with readers.

 

1. The Origin of Thinking and Practice

During my work at Times New Materials, the author was responsible for the development of composite leaf springs. At that time, the composite leaf spring used a prepreg molding process, and the prepreg used Guangwei Composites’ 1600-gram unidirectional glass fiber prepreg, and the production time was more than 2 hours. The production rhythm and cost of this process obviously cannot meet the needs of the automotive market. At this time, Kangde Composites was established and introduced HP-RTM technology from Germany. The author then joined Kangde Composites to closely study whether HP-RTM technology could meet the needs of the market.

 

During the Kangde Composites period, the author believed that HP-RTM could not meet the needs of the market through his understanding of HP-RTM, so he embarked on the road of self-development of low-cost composite material molding technology.

 

2. The Problem of HP-RTM

The core problem of HP-RTM is that HP-RTM is too expensive. In terms of equipment cost, the minimum cost of a fully imported HP-RTM production line injection molding machine is more than 20 million; in terms of mold cost, a set of fully imported HP-RTM molds from Austrian ALPEX at that time cost more than 6 million.

 

Now the HP-RTM production line and molds have been domestically produced. The national production line can be less than 10 million yuan, and the mold cost has also dropped to the range of hundreds of thousands of yuan per set, but it is still too expensive compared to the fragmented market of composite materials. Therefore, it is difficult to see any other applications of HP-RTM in China except for the glass fiber battery box cover.

 

Observing the domestic HP-RTM production line, it can be seen that it is different from the German production line. The German production line uses epoxy resin, while the domestic production line basically uses polyurethane. This is because HP-RTM is essentially a polyurethane high-pressure foaming machine. The material is heated in a low-pressure cycle, and after secondary pressurization by the booster pump, it is atomized and sprayed from the L or V-shaped self-cleaning gun head. Relying on the high-activity collision and mixing of the material, it is sprayed into the mold at one time for filling.

 

Unlike the high-pressure foaming machine, the high-pressure foaming machine fills the empty mold, while the HP-RTM fills the mold with fiber reinforcement. This filling process will cause back pressure, so a large-tonnage press is needed to lock the mold. For high-pressure foaming machines, the viscosity of the conventional polyurethane resin reaches 3000mpa·s, which is the limit. The viscosity of the epoxy resin used in HP-RTM is as high as 8000-10000mpa·s. Conventional high-pressure foaming machines may suffer from metering pump wear and inaccurate metering when using such high-viscosity materials.

 

Therefore, since the domestic production line is directly modified from the polyurethane high-pressure foaming machine, the direct use of polyurethane resin has become the best choice both in terms of habit and feasibility.

 

 

Figure 3 Applicable Viscosity of Dieffenbacher HP-RTM Polyurethane Mixer and Epoxy Mixer

 

 

Figure 4 Hexion HP-RTM Special Epoxy Resin

 

In addition to being too expensive, another problem with HP-RTM is that because it uses a sprayed atomized resin mass instead of a continuously injected resin flow, and the high-activity material itself has a short shelf life, the resin’s impregnation effect on the fiber in the HP-RTM process is not particularly ideal.

 

There is no problem with making a 1.2mm thick glass fiber battery box cover, but there may be impregnation problems when it comes to carbon fiber or thicker glass fiber composites. According to the author’s experience, the maximum thickness of HP-RTM infusion is about 50mm, and thicker products are not suitable for HP-RTM.

 

 

Figure 5 Viscosity-Temperature Curve of Hexion HP-RTM Epoxy Resin

 

 

Figure 6 Wanhua Chemical HP-RTM Special Polyurethane Resin

 

 

Figure 7: Wetting Problems When HP-RTM Infused a 50mm Thick Glass Fiber Composite Leaf Spring

 

Epoxy Resin Issues

As mentioned in (I) above, the viscosity of epoxy resin used in HP-RTM is as high as 8000-10000mpa·s, while the viscosity of commonly used injection epoxy is only 1-2000mpa·s, and the viscosity after mixing with amine curing agent is generally 2-300mpa·s. Why does HP-RTM need to use epoxy resin with such a high viscosity? And why is it necessary to extend the curing time to more than two hours when using epoxy resin, and heat and pressurize in stages? The core of these problems lies in that during the curing process of epoxy resin, if the heating rate is too fast, the resin will accumulate heat and it will easily explode. In order to avoid explosion, it is necessary to avoid the occurrence of heat accumulation. Therefore, the conventional practice is to slowly heat the epoxy resin to the gel state and then heat and pressurize, which is the curing curve recommended by various epoxy resin manufacturers.

 

 

 

 

Figure 9 Common Epoxy Resin Curing Curve

 

Then let’s look at the epoxy resin used in HP-RTM. It can be seen that the resin used in HP-RTM can be considered as E24 epoxy resin in terms of epoxy value, which is much lower than the E44 and E51 resins used in conventional processes. Therefore, this E24 epoxy resin uses less curing agent during the curing process, and the proportion of reactive groups that react is lower, so the self-exothermic heat of the reaction will be lower than that of the E44 and E51 epoxy resins used in conventional processes. In addition, the atomization injection method of HP-RTM makes the resin more dispersed than the conventional liquid infusion process, and the degree of heat collection will be lower. Under the combined effect of these two aspects, the self-heating of the epoxy resin is not enough to cause violent polymerization, and rapid curing can be achieved through various other means.

 

Carbon Fiber Problem

 

For glass fiber composite materials, the problem of epoxy resin is not a problem. After all, glass fiber composite materials mainly use unsaturated resins and epoxy resins are used less; but for carbon fiber, the problem of epoxy resin is very fatal.

Because the carbon fiber composite materials with thermosetting matrix on the market are basically all epoxy resin-based, which leads to the high processing cost of carbon fiber composite materials, and it is difficult to achieve rapid prototyping except using HP-RTM process.

So why do almost all carbon fiber composite materials use epoxy resin as the resin matrix, instead of unsaturated resin and polyurethane like glass fiber composite materials?

This is because the surface activation energy of carbon fiber is much lower than that of glass fiber, and even after ionization sizing treatment, it is less than one-eighth of that of glass fiber.

Therefore, glass fiber can be impregnated with various resins, while the affinity of carbon fiber and resin is subject to the sizing agent on its surface.

At present, the practical carbon fiber surface treatment process relies on various methods to oxidize oxygen-containing groups, and these oxygen-containing groups also have a certain affinity with epoxy resin.

Therefore, the sizing agent on the surface of commercial carbon fiber is almost all epoxy resin system, which leads to that in the production of carbon fiber composite materials, only epoxy resin can well impregnate the carbon fiber with epoxy resin sizing agent on the surface.

Given the limited performance of epoxy resin itself, it is not a wise choice to continue to use epoxy resin to achieve rapid prototyping of carbon fiber composites. It is a more practical approach to use other systems of resin to composite with carbon fiber.

 

The Problem of Unsaturated Resin

 

As the most used resin in glass fiber composite materials, unsaturated resin will naturally be used to produce carbon fiber composite materials.

This has also produced a unique saying in the domestic modified parts market: dry carbon is good and wet carbon is bad. If epoxy resin is used, the performance of carbon fiber composite materials mainly depends on the fiber volume fraction and the direction of the ply.

There should not be much difference under normal temperature. The source of this gap is that domestic manufacturers of so-called wet carbon use unsaturated resin instead of epoxy resin.

In fact, as early as 2019, the author developed a sizing agent that allows carbon fiber to be impregnated with unsaturated resin, and successfully achieved rapid curing, but the author did not go further in this direction because the problem of unsaturated resin itself made the author feel that it was not worth using. First of all, unsaturated resin is too smelly.

Whether it is styrene or acrylic acid, there are VOC emissions during use and in the product. At a time when environmental protection is becoming more and more stringent, the market space for unsaturated resin will only get smaller and smaller.

Secondly, the toughness of unsaturated resin is too poor. The low toughness of epoxy resin and carbon fiber makes the machining of carbon fiber composites difficult. The toughness of unsaturated resin is even lower than that of epoxy resin.

If carbon fiber composite materials use unsaturated resin as the resin matrix, the problem of low toughness of carbon fiber composite materials is more prominent, which seriously affects the application of carbon fiber composite materials.

Finally, unsaturated resin is not UV-resistant and is easy to age under the action of ultraviolet light. After aging, it will turn yellow or bulge. The appearance of composite products is that the quality is too poor.

 

Polyurethane-based Composite Materials

 

Since the routes of epoxy resin and unsaturated resin have been rejected, the remaining feasible routes are only polyurethane-based composite materials except thermoplastic composite materials. Compared with traditional composite materials, polyurethane-based composite materials appeared very late.

The earliest one appeared on the composite leaf spring produced by HP-RTM process of Volvo XC90. The resin is Henkel’s Loctite MAX2 and MAX3. At the same time, Covestro’s pultruded resin for pultruded window frames and cable trays also appeared.

Now it has developed into polyurethane pultruded resin for photovoltaic pultruded frames. Later, Covestro and Wanhua also launched polyurethane resins for HP-RTM, which is the type used in the current glass fiber battery box cover.

The first time I came into contact with polyurethane-based composite materials was when I was working with Times New Materials.

It happened that Covestro and Times New Materials cooperated to develop polyurethane wind turbine blades.

However, that polyurethane resin was isocyanate-modified acrylate, which was different from the isocyanate and polyol reactive polyurethane used in HP-RTM.

 

 

Figure 10 XC90 Glass Fiber Composite Leaf Spring

 

 

Figure 11 Polyurethane Pultrusion Profile

 

Compared with other resin-based composite materials, polyurethane-based composite materials have two unparalleled advantages. First, the molding efficiency is high.

There is almost no implosion during the curing process of polyurethane. Therefore, the system temperature and the amount of catalyst can be boldly increased as long as the application period meets the equipment process capability.

Second, the toughness is high. The floor of polyurethane toughness is higher than the ceiling of epoxy resin toughness.

No matter how epoxy resin is toughened, it cannot catch up with polyurethane resin. Glass fiber reinforced polyurethane composite materials can be processed and punched at will without special consideration of processing tools and equipment, and even self-tapping screws can be directly driven.

With extremely high toughness, the reliability of polyurethane-based composite materials is much higher than that of other resin-based composite materials.

 

Problems With Polyurethane

 

Although polyurethane resin has great advantages in both efficiency and performance, polyurethane also has its own problems that limit its large-scale use in composite materials.

The first is the problem of wettability. The wettability of polyurethane can be said to be the worst among commonly used thermosetting resins.

As mentioned above, glass fiber can be impregnated with various resins because of its high surface activation energy, but polyurethane is difficult to impregnate glass fiber, so Chongqing International has specially developed polyurethane-specific glass fiber 467W for polyurethane-based composite materials.

 

 

Figure 12 Comparison of Ordinary Glass Fiber and Special Glass Fiber Infused with Polyurethane

 

Another major problem with polyurethane is that it must be used with special equipment, unlike epoxy resin and unsaturated resin which can be operated manually, which brings a threshold for equipment investment.

The current situation of the composite material market is highly fragmented, and correspondingly, many small factories have no motivation to install equipment.

Why must polyurethane use special equipment? This is because the isocyanate in polyurethane will foam when it encounters water, and the other component, the polyether polyol composition, is particularly easy to absorb water.

Therefore, special equipment must be used to control the moisture in the combination material to avoid obtaining a hard foam instead of a resin casting after infusion.

For this reason, polyurethane cannot be used in mold opening processes such as winding and hand lay-up.

In addition, the final curing degree of polyurethane is highly related to the curing temperature during the reaction.

Once the reaction starts, it will not stop. If you want to make a composite material, you must heat it to a high enough temperature.

There is no way to improve the curing degree of polyurethane in the post-curing process.

Therefore, isocyanate and polyol reactive polyurethane resins cannot be made into prepregs.

What can be made into prepregs are isocyanate-modified acrylic resins, which are more like unsaturated resins than polyurethanes.

 

Rapid Prototyping Application Practice of Polyurethane

 

The author has already talked about the thinking of composite rapid prototyping technology above, which can be summarized in four words: use polyurethane.

The following will introduce the author’s practice of applying polyurethane to composite materials, that is, how to solve various problems in the application of polyurethane in composite materials.

The first is the problem of polyurethane wettability. It is difficult for polyurethane to wet glass fiber, and the problem of wet carbon fiber is naturally greater.

When unsaturated resin is applied to carbon fiber composite materials, no matter how good the wetting effect on the interface is, at least the surface can form a continuous resin phase to wrap the carbon fiber, which cannot be seen from the appearance.

The author personally calls this material that does not realize the resin wetting of the fiber inside, but only the resin wraps the fiber a pseudo-composite material.

And polyurethane directly composited with carbon fiber may not even form a pseudo-composite material, and in many cases, direct phase separation does not solidify.

In order to solve this problem, the author spent three years studying the surface treatment of carbon fiber, and finally developed a surface treatment process that allows carbon fiber to be impregnated with polyurethane resin.

As can be seen from Figure 13, carbon fiber that has not been specially treated cannot form an interface with polyurethane resin, and the fracture surface is smooth and non-adhesive; carbon fiber that has been specially treated can form an interface with polyurethane resin, and obvious interfacial layer tearing can be observed on the fracture surface.

In the author’s previous video, there is a sample that breaks very neatly after being impacted.

In fact, it uses exactly the same resin and exactly the same molding process as the impact-resistant sample behind it.

The difference is that one uses ordinary carbon fiber and the other uses specially treated carbon fiber.

 

 

Figure 13 Comparison of Fracture Surfaces of Composite Materials Before and After Carbon Fiber Surface Treatment

 

Then there are polyurethane resins and supporting equipment for carbon fiber rapid prototyping. In order to stop using HP-RTM and reduce fixed asset investment and use costs, the route taken by the author is to reduce the viscosity of the resin to the maximum.

The HY-02 polyurethane resin for infusion that the author usually uses has a viscosity of less than 100mpa·s at room temperature after mixing, and an infusion viscosity of 15-30mpa·s at 40-50℃, which is one hundredth of the viscosity of epoxy resin for HP-RTM. At the same time, the activity of the resin is greatly reduced, and the collision-type mixing injection of the high-pressure foaming machine is no longer used. Instead, a static mixer is used to mix and inject a continuous resin flow. In this way, the pressure required for the entire equipment system is much lower than that of HP-RTM, and the cost is much lower. For example, the 2000*1500 table-sized press in the author’s previous video has a tonnage of only 400 tons, and the corresponding table-sized HP-RTM process requires a press tonnage of at least 1600 tons.

In order to keep up with the production rhythm of one mold every 5 minutes, the author invented an additional self-cleaning injection system to achieve automated continuous production.

The author named the above set of raw materials, equipment and molding process HY-RTM rapid prototyping technology.

 

 

Figure 14 Typical HY-RTM Rapid Prototyping Production Line

 

Using this HY-RTM rapid prototyping technology to produce carbon fiber composites, as shown in the author’s previous video, the production cycle can easily reach 5 minutes per piece. The toughness of the produced carbon fiber composites is much higher than that of epoxy resin-based carbon fiber composites. Depending on the angle of the ply, the composite material’s simply supported beam notch impact strength can reach 124-266KJ/㎡.

 

 

After solving the rapid prototyping of carbon fiber composites, the author applied this technology to aramid fibers and PBO fibers, and successfully achieved a production cycle of one mold every 5 minutes.

 

 

In order to explore the limits of HY-RTM technology, the author used it to re-make a composite leaf spring. The composite leaf spring selected this time is a heavy-duty truck tractor rear spring with a standard load of 8 tons and a full load of 18 tons; the length of the leaf spring after curing and not machining is 1730mm, the maximum thickness is 106mm, and the total weight is 20kg. The final production cycle of the leaf spring is 15 minutes per mold.

 

 

Problems That Need To Be Solved in Polyurethane-based Composite Materials

 

The advantages of polyurethane-based composite materials, namely high molding efficiency and high toughness, have been introduced in detail above.

These two advantages are enough to allow polyurethane-based composite materials to replace most epoxy-based composite materials.

Another advantage that has not been mentioned is that there is almost no VOC in the whole process, which can replace unsaturated resins in the current and future when environmental protection is becoming more and more stringent.

But in addition to the above problems that have been maturely solved by the author, there are still some problems to be solved. The first is the heat resistance of polyurethane resin.

The Tg limit of conventionally used polyurethane is basically 100-120℃, which is similar to the epoxy resin cured by amine curing agent.

Therefore, there is no big problem in directly replacing epoxy-based composite materials in the infusion process, but it is difficult to replace high-temperature resistant epoxy resins that can be used in prepregs.

The author did make a polyurethane resin with a Tg of more than 200℃, but the higher the Tg is adjusted, the higher the initial viscosity, and the worse the processability of infusion and the stability of the reaction.

Improving the heat resistance of polyurethane resin will be a long-term adjustment and improvement direction. Then there is the flame retardancy of polyurethane resin.

If it is just flame retardant, it is not difficult for polyurethane resin to reach UL94-V0. The key lies in the problem of smoke toxicity.

Polyurethane resin itself contains nitrogen, and nitrogen oxides are inevitably produced after combustion. In my own practice, whether it is the combination of tetrabromobisphenol A and triethyl phosphate or the use of chlorine-containing FR300, it is difficult to be satisfactory.

In this regard, if anyone has a suitable flame retardant, I hope to contact me. Finally, there is the problem of polyurethane curing in open space and at room temperature.

Solving this problem is to allow polyurethane to be manually operated like epoxy resin and unsaturated resin.

The key to using it in an open space is to solve the problem of foaming when it comes into contact with water, and the key to curing it at room temperature is to solve the problem of balancing the resin’s applicability period and operating time.

For these problems, the author has made many attempts, but there is no good solution.

 

The Impact Of Polyurethane-based Composite Materials On The Future Market

 

According to data from the China Composite Materials Industry Association and the Prospective Industry Research Institute, from 2014 to 2020, China’s composite material production increased from 3.62 million tons to 7.06 million tons, and the output value increased from US$27.3 billion to US$55.9 billion, with an average annual growth rate of more than 10% for both production and output value.

The market distribution of composite materials is broad but fragmented.

There are as many as 40 listed companies engaged in composite material product business alone, distributed in various fields.

There are more than 5,000 composite material manufacturers registered with industrial and commercial administration nationwide, and only 422 enterprises with annual sales of more than 20 million yuan are above designated size.

Small and medium-sized enterprises account for more than 90%, and the market development of the entire industry is still in a relatively primitive stage.

The reason why the composite material market has not yet reached the stage of large-scale mergers is that the production technology of composite materials cannot be automated at low cost and on a large scale, and is still in a state of heavy reliance on manpower.

Take Henry Composites (HRC), the fastest-growing carbon fiber composite material enterprise for automobiles, as an example.

HRC’s exterior modification products mainly rely on the autoclave process, and there are thousands of employees for this, which is really impressive.

 

The advantages of polyurethane-based composite materials, namely high molding efficiency and high toughness, have been introduced in detail above.

These two advantages are enough to allow polyurethane-based composite materials to replace most epoxy-based composite materials. Another advantage that has not been mentioned is that there is almost no VOC in the whole process, which can replace unsaturated resins in the current and future when environmental protection is becoming more and more stringent.

But in addition to the above problems that have been maturely solved by the author, there are still some problems to be solved. The first is the heat resistance of polyurethane resin.

The Tg limit of conventionally used polyurethane is basically 100-120℃, which is similar to the epoxy resin cured by amine curing agent.

Therefore, there is no big problem in directly replacing epoxy-based composite materials in the infusion process, but it is difficult to replace high-temperature resistant epoxy resins that can be used in prepregs.

The author did make a polyurethane resin with a Tg of more than 200℃, but the higher the Tg is adjusted, the higher the initial viscosity, and the worse the processability of infusion and the stability of the reaction. Improving the heat resistance of polyurethane resin will be a long-term adjustment and improvement direction.

Then there is the flame retardancy of polyurethane resin. If it is just flame retardant, it is not difficult for polyurethane resin to reach UL94-V0.

The key lies in the problem of smoke toxicity. Polyurethane resin itself contains nitrogen, and nitrogen oxides are inevitably produced after combustion. In my own practice, whether it is the combination of tetrabromobisphenol A and triethyl phosphate or the use of chlorine-containing FR300, it is difficult to be satisfactory.

In this regard, if anyone has a suitable flame retardant, I hope to contact me. Finally, there is the problem of polyurethane curing in open space and at room temperature. Solving this problem is to allow polyurethane to be manually operated like epoxy resin and unsaturated resin.

The key to using it in an open space is to solve the problem of foaming when it comes into contact with water, and the key to curing it at room temperature is to solve the problem of balancing the resin’s applicability period and operating time.

For these problems, the author has made many attempts, but there is no good solution.

 

The Impact of Polyurethane-based Composite Materials On The Future Market

 

According to data from the China Composite Materials Industry Association and the Prospective Industry Research Institute, from 2014 to 2020, China’s composite material production increased from 3.62 million tons to 7.06 million tons, and the output value increased from US$27.3 billion to US$55.9 billion, with an average annual growth rate of more than 10% for both production and output value.

The market distribution of composite materials is broad but fragmented. There are as many as 40 listed companies engaged in composite material product business alone, distributed in various fields.

There are more than 5,000 composite material manufacturers registered with industrial and commercial administration nationwide, and only 422 enterprises with annual sales of more than 20 million yuan are above designated size. S

mall and medium-sized enterprises account for more than 90%, and the market development of the entire industry is still in a relatively primitive stage. The reason why the composite material market has not yet reached the stage of large-scale mergers is that the production technology of composite materials cannot be automated at low cost and on a large scale, and is still in a state of heavy reliance on manpower.

Take Henry Composites (HRC), the fastest-growing carbon fiber composite material enterprise for automobiles, as an example.

HRC’s exterior modification products mainly rely on the autoclave process, and there are thousands of employees for this, which is really impressive.

 

 

The law of the market is definitely from decentralization to centralization. Therefore, it can be predicted that in the future composite material market, large enterprises will gradually squeeze the living space of small enterprises. However, there is a lack of appropriate technical means to achieve a qualitative change in per capita production capacity. Polyurethane-based composite materials can achieve this qualitative change in performance, but there is a certain threshold to block small enterprises. In addition to the existing market, there are more incremental markets that traditional composite materials cannot meet, which will be directly created by polyurethane-based composite materials. Author: Chen Zongliang