Storage-Stable Polyurethane Prepregs via Blocked Isocyanates
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Solution Overview
Problem
The production of polyurethane prepregs with 2-component (2-C) PUR systems is limited by their short processing time due to rapid viscosity increase and gelling, making them unstable for several hours, let alone days, which complicates handling and processing into composite materials.
Innovation Solution
The use of liquid resin components, preferably polyols, with a glass transition temperature (Tg) below 25°C and internally blocked di- or polyisocyanates as curing agents, allows for the creation of storage-stable polyurethane prepregs that remain stable for several days to weeks at room temperature, enabling easier handling and processing into composite components.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If 2-component polyurethane systems are used for prepreg production, then mechanical properties and toughness are improved, but processing time is limited due to rapid viscosity increase and gelling
Solution Approach 1:
The polyurethane system is segmented into two separate components: a polyol component and an isocyanate component. These components are kept separate during storage and only mixed immediately before processing. This segmentation prevents premature reaction, allowing the prepreg to maintain processability while still achieving the mechanical properties of polyurethane systems when cured.
Solution Approach 2:
The polyol is pre-applied to the reinforcing fibres during prepreg manufacturing, but the actual polyurethane formation is postponed until the curing stage. This preliminary action allows the prepreg to be stored and handled without the viscosity problems that would occur if the full polyurethane system were present from the start.
2Strength
If 2-component polyurethane systems are used, then material toughness is improved, but storage stability deteriorates due to rapid gelling
Solution Approach 1:
The system is divided into stable polyol and isocyanate components that do not react until mixed. The polyol component alone remains stable during storage, preventing gelling and viscosity increase, while still providing the base for forming the tough polyurethane matrix when cured.
Solution Approach 2:
The polyol acts as an intermediary substance that can be stored stably on its own but will ultimately form the polyurethane matrix when combined with isocyanate. This intermediary state allows for stable storage while maintaining the capability to achieve the desired tough final material.
3Ease of operation
If liquid resin components with low Tg are used, then processability and handling are improved, but risk of blistering during pressing increases
Solution Approach 1:
The glass transition temperature of the polyol is carefully selected to be below 25°C, ensuring it remains liquid and processable at room temperature and during prepreg handling. However, during the pressing and curing process, temperatures are raised above this Tg threshold, which changes the viscosity characteristics and allows for proper consolidation without blistering, while still maintaining ease of handling during storage and preparation.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach results in prepregs with improved processability, reduced risk of blistering or surface disruption during pressing, and superior material properties, including good mechanical properties, weather resistance, and balanced rigidity and flexibility, suitable for various industrial applications.
Implementation Method 1
The two parts are only mixed together shortly before processing. Thereafter the chemical curing takes place by polyaddition with formation of a network of polyurethane or polyurea.
Implementation Method 2
The crosslinking is effected at elevated temperatures by the polyaddition of the NCO groups with the functional groups of the polymer or by thermal decomposition of the blocked isocyanates with subsequent polyaddition.
Data Source
AI summary
Provided is a storage-stable polyurethane prepegs and mouldings produced therefrom composed of polyurethane compositions with liquid resin components, preferably liquid polyols. A prepeg contains(A)a fibrous support and (B)a matrix material containing a reactive polyurethane composition. The polyurethane composition contains a mixture of a liquid resin component(b)having a functional group reactive towards an isocyanate and having a glass transition temperature(Tg)or melting point below 25° C., with a hydroxyl number of 50 to 2000 mg KOH/gram, and a curing agent(a)comprising a di- or polyisocyanate internally blocked or blocked with a blocking agent. In addition, the polyurethane composition contains 0.6 to NCO equivalents or 0.3 to 1 uretdione group in (a) for each functional group of (b).